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D4

VIEWS: 42 PAGES: 258

									                                                   Project no.
                                                   PL 517574

                                                Project acronym
                                                 PREWARC

                                                   Project title
       Strategic Plan for the Prevention of Regional Water Resources contamination from
               Mining and Metallurgical Industries in Western Balkan Countries

     Instrument: SSA

     Thematic Priority: Integrated and Strengthening the European Research Area, Specific
     Support Actions for Western Balkan Countries


            D4. Report providing all necessary information required for the
             designation of a frame for potential and feasible technological
                   improvements in both western Balkan countries
                                     Due date of deliverable: 15.02.2008.
                                     Actual submission date: 22.02.2008

     Start date of project: 01/01/2006                                            Duration: 2 years


     Organization name of lead contractor for this deliverable: Technical University of Crete,
     TUC


Project co-funded by the European Commission within the Sixth Framework Programme (2002-2006)
                                               Dissemination Level
PU       Public
PP       Restricted to other programme participants (including the Commission Services)
RE       Restricted to a group specified by the consortium (including the Commission Services)
CO       Confidential, only for members of the consortium (including the Commission Services)
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Technical University Crete, PREWARC project PL 517574




                  STEEL




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                     Technical University Crete, PREWARC project PL 517574


    The purpose of this report, in the framework of the PREWARC project, PL 517574, is to present
feasible preventive and remedial technologies that can be adopted by the mining and metallurgical
industries during production as well as during waste management during steel production.


1. General Information
Steelmaking is a dynamic and evolving industry. The process of steelmaking has undergone
several changes in the 20th century based on the political, social and technological
atmosphere.
In the 1950s and 1960s, demand for high quality steel and the need for larger quantities led to
the emergence of large, integrated steel mills, which produced very high quality steel with
high capital costs and limited flexibility. The energy crisis of the 1970s made thermal
efficiency in steel mills a priority. The furnaces used in integrated plants were very efficient;
however, common production practices needed to be improved. In the large integrated plants,
steel was produced in batches where some equipment was idle while other equipment was in
use. For more efficient energy use, continuous casting methods were developed; blast
furnaces were continually fed with iron ore and therefore heat was used more efficiently.
Environmental concerns as well as the more strict regulations of the 1980s and 1990s rose
capital costs by 20-30% in new steel plants. Competition also increased during the period due
to decreasing markets and increasing foreign steel production plants, forcing steelmaking
facilities to reduce expenses and increase quality.
To meet these changing needs smaller plants, or mini-mills, began to replace integrated steel
plants and revolutionize the steel industry. Mini mills are low capital steel mills which
consist primarily of an electric arc furnace (EAF) and rely on abundant, inexpensive steel
scrap as a base material rather than ore. They usually have a narrower production line that
can economically serve small, local markets, and cannot produce the specialty products
produced by integrated plants nor can they usually maintain the tight control over chemical
composition of steel. (Primary Metals 2001)
As certain steel qualities require the use of virgin materials, and as there are constraints on
the supply of economically available steel scrap, both integrated steelmaking and EAF
steelmaking are required and are not direct substitutes for one another. A recent study notes
that some integrated steel companies have adopted production technologies traditionally used
in mini-mills (such as advanced EAFs and thin slab casting), and distinctions between the
integrated and EAF segments of the industry may be blurring. Though the share of steel
produced by the EAF process has steadily increased, expansion of EAF steelmaking capacity
is predicated on the availability of adequate and cost-effective supplies of scrap. The addition
of alternative ironmaking technologies will be essential to facilitating EAF capacity
expansion. (U.S.E.P.A. Energy Trends 2007)
In 2005, of the steel produced worldwide, 65.4% was produced via the integrated route,
31.7% via EAF and 2.9% via the open hearth and other methods. (World Steel in Figures
2006) In Europe, 61% of steel was produced by the blast furnace/ basic-oxygen route, while
39% by the electric arc furnace (EAF) route. (European Confederation of Iron and Steel
Industries)




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                         Technical University Crete, PREWARC project PL 517574




Crude steel production by process
         Electric: 39%




         Oxygen: 61%



Figure 1: European crude steel production by process, 2005 (European Confederation of Iron and Steel
Industries)
Since the oil crisis of 1974-75, steel production has been virtually stagnant worldwide, with
Europe being particularly affected. The entry of the three new Member States −Austria,
Finland and Sweden, brought EU production of crude steel up to 156 million tonnes in 1995.
The total world crude steel production in 2006 amounted to 1,244 million tonnes.




        Production (crude steel equivalent)


          Others comprise:

         Africa                     1.5%   Central and South America      3.7%

         Middle East                1.2%   Australia and New Zealand      0.7%


        Consumption (crude steel equivalent)




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                      Technical University Crete, PREWARC project PL 517574




         Others comprise:

        Africa               1.8%    Central and South America           3.4%

        Middle East          3.2%    Australia and New Zealand           0.7%

Figure 2: Steel production and consumption: geographical distribution, 2006 (International Iron and
Steel Institute)


According to the European Confederation of Iron and Steel Industries, in 2006 EU crude
steel production was 198430 tonnes.


2. Steel Production
Four routes are currently used for the production of steel: the classic blast furnace/basic-
oxygen furnace route, direct melting of scrap (electric arc furnace), smelting reduction and
direct reduction. An overview of the electric steel making process is given in Figure 3:




Figure 3: Crude steel production methods (BREF)



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                    Technical University Crete, PREWARC project PL 517574


This report will focus on electric steelmaking, as this is the production process used by the
Makstil steel plant.
With respect to the end products, distinction has to be made between production of ordinary,
so-called carbon steel as well as low alloyed steel and high alloyed steels/stainless steels. In
the EU about 85% of steel production is carbon or low-alloyed steel.
For the production of carbon steel and low alloyed steels, the following main operations are
performed:
      • Raw material handling and storage
      • Furnace charging with/without scrap preheating
      • EAF scrap melting and refining
      • Slagging
      • Steel and slag tapping
      • Ladle furnace treatments for quality adjustment (secondary metallurgy)
      • Slag handling
      • Continuous casting

The major feedstock for the EAF is ferrous scrap, which may comprise of scrap from inside
the steelworks (e.g. offcuts), cut-offs from steel product manufacturers (e.g. vehicle builders)
and capital or post-consumer scrap (e.g. end of life products). Direct-reduced iron (DRI) is
also increasingly being used as a feedstock due both to its low gangue content and variable
scrap prices. (Mini-mills)
During the charging, stage scrap is introduced into the EAF. The charge can also include
lime or carbon. With electrodes retracted, the furnace roof can be rotated aside to permit the
charge of scrap steel by overhead crane. Some furnaces are charged through a shaft or
continuously charged from a conveyor without the removal of the furnace roof.
The cylindrical, refractory lined EAF is equipped with carbon electrodes to be raised or
lowered through the furnace roof. A strong electric current generates heat between the
electrodes and through the scrap to melt the scrap. Refining of the melt can occur
simultaneously with melting, especially in EAF operations where oxygen is introduced
throughout the heat. During the refining process, impurities such as phosphorus, sulfur,
silicon, and carbon are removed from the steel. These elements react with the oxygen to form
oxides, which then become slag on top of the steel. The slag is typically removed by tipping
the furnace backwards and pouring the slag out through a slag door.
After completion of the heat, the tap hole is opened, and the steel is poured into a ladle for
transfer to the next operation. The molten metal may be treated in the ladle by adding alloys
and/or other chemicals. The treated metal is then poured into molds and allowed to partially
cool under carefully controlled conditions.
When cooled, the castings are placed on a vibrating grid and the sand of the mold and core
are shaken away from the casting. This is a batch process with a cycle time of about two to
three hours.
In the cleaning and finishing process, burrs, risers, and gates are broken or ground off to
match the contour of the casting. Afterward, the castings can be shot-blasted to remove
remaining mold sand and scale. Casting can be a batch (ingots) or a continuous process
(slabs, blooms, billets). Ingot casting is the classical process and is rapidly being replaced by
continuous casting machines (CCM).
 In 1998, 83% of global crude steel production was cast continuously. Continuous casting is a
significantly more energy-efficient process for casting steel than the ingot casting process.

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                     Technical University Crete, PREWARC project PL 517574


Continuous casting uses 0.1-0.34 GJ/tonne of steel, significantly less than the 1.2-3.2
GJ/tonne required for ingot casting.
       New technology has vastly increased EAF productivity. Originally production ranged
from 10-30 tons/hour, but today there are numerous furnaces producing in excess of 100 tons
per hour. The "mini-mill" has grown from a plant producing 250,000 tons per year to plants
producing in excess of 2 million tons per year. Once relegated to producing inexpensive
concrete reinforcing bar, today mini-mills can produce over 80% of all steel products.




Figure 4: EAF Evolution (Steel Industry Technology Roadmap 2001)


Although EAF productivity has significantly increased, steelmakers must still optimize the
EAF with the finishing operations so their production rates and sequencing are the same.




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                      Technical University Crete, PREWARC project PL 517574




Figure 5: Overview of the processes related to electric arc furnace steelmaking (BREF)



3. Raw materials and energy consumption
The steel industry is a highly material and energy intensive industry. In the EAF process the
primary inputs are scrap metal, furnace lining, and graphite electrodes. Also, fluxes and
alloys are added, and may include: fluorspar, dolomite, and alloying agents such as
aluminum, manganese, and others.
Raw materials and operating practices affect EAF efficiency and yield because the product
quality can be influenced by the scrap input due to contaminations from other metals. Also,
use of suboptimal scrap produces more waste and requires more energy to process.
Therefore, the upgrading of purchased scrap and the physical nature of purchased scrap are
topics of central importance to raw material issues.
Today the steel industry is confronting the issue of radioactive material found in ferrous
scrap supply. The presence of spent radioactive materials in the ferrous scrap supply presents
significant health and safety risks to steel workers and to the general public. Highly
radioactive sources encased in metal shields are in widespread use in medical and industrial
applications, with estimated 2.000.000 different sources in use worldwide. During the past
decade numerous incidents have occurred where these shielded sources have been
accidentally mixed with scrap, and subsequently melted or ruptured in the steel melting
process. The clean up costs associated with melting a radioactive source, including
decontaminating a facility, disposing and storing radioactive electric furnace dust, and
shutdown of steel production, can be as high as $24 million per melt.
Radioactive scrap may arrive in the form of:
      • Man-made radioactive materials in devices such as medical equipment, gauges,
          radiographic cameras, and military aircraft instrumentation. Typically they contain
          the radioisotopes cesium-137, cobalt-60, iridium-192, radium-226, and americium-
          241.
      • Naturally occurring radioactive materials (NORM), such as potassium-40, thorium-
          232, and uranium-235 that are encrusted in pipefittings from the oil and water
          industries.
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                     Technical University Crete, PREWARC project PL 517574


      • Alloyed metals and vintage scrap ferrous metals (not previously screened for
        radioisotopes) containing cobalt-60 and thorium-232.
      • Scrap from decommissioned nuclear facilities

How a steel mill is contaminated by radioactive materials depends on the radioactive source.
A radioactive material containing, for example, cobalt-60 or iridium-192 alloys with the
melted metal can contaminate all surfaces that come in contact with the melt. Cesium-137
vaporizes and enters the furnace exhaust, contaminating the dust and air pollution control
systems. Other isotopes, such as radium-266, end up in the slag. (U.S. Geological Survey,
February 1997)

Energy consumption
The energy consumption is considerable. The following topics related to energy are
discussed: chemical/electrical energy input ratios, AC/DC power, and energy load.
The electrical energy is about 65% of the total energy input. The other 35% comes from
chemical energy generated by the exothermic oxidation of carbon and iron and by oxy-fuel
or natural gas burners. The tapped steel and slag require a specific amount of energy
(approximately 70% of the input), regardless of heat time; heat losses to waste gas, cooling
water, and radiation, which are all directly related to heat time and directly account for the
remaining 30%. (Steel Industry Technology Roadmap 2001)




Figure 6: EAF Energy Input/Output (Steel Industry Technology Roadmap 2001)


The specific energy consumption (also includes the consumption of electricity) for the
production of electric arc furnaces steel is about 4.0-6.5 GJ/tonne compared to 19.3 GJ
needed for steel produced via the coke oven/sinter plant/blast furnace route. The electricity
consumption has been multiplied by factor three to make it comparable with primary energy.

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                    Technical University Crete, PREWARC project PL 517574


The expansion of EAF steel production and contraction of integrated steel production has
decreased the overall energy intensity of the steelmaking industry. However, integrated
steelmaking has less demand for electricity than EAF production.


4. Raw materials handling and storage

Raw material handling operations include receiving, unloading, storing, and conveying all
raw materials for the foundry. Raw materials are usually transferred to the site in bulk
carriers by road, rail or water transport, are stored on stockyards or silos and transported to
the individual processing plants, usually by conveyor belt.
Dust emissions occur at several points in the storage cycle, such as material loading onto the
pile, disturbances by strong wind currents, and load out from the pile. The movement of
trucks and loading equipment in the storage pile area is also a substantial source of dust.
Wind-borne dust from the stockyards and conveyor belts, including transfer points, can be a
significant source of emissions. When material including leachable compounds and
hydrocarbons from mill scale or scrap is stored in unpaved stockyards, attention also has to
be paid to soil and ground water pollution and to run-off water. Suspended solids and in some
cases, oil, may be contained in the runoff water from the storage areas. Depending on
weather conditions volatile inorganic and organic compounds may be emitted.
The main scrap storage areas are usually outside in large uncovered and unpaved scrap-
yards, which may lead to soil pollution, but there are also certain plants having covered and
paved scrap-yards.


5. Scrap preparation and preheating
 Scrap preheating has been used for over 30 years primarily in countries with high electricity
costs such as Japan and Europe. For EU, 12 total numbers of 203 AC arc furnaces, 4 DC arc
furnaces and 38 induction furnaces are reported in a Eurostat document (1993). Nine
installations (location not specified) are told to be equipped with scrap preheating units.
Conventional scrap preheating involves the use of hot gases to heat scrap in the bucket prior
to charging the scrap into the EAF. The source of the hot gases can be either off-gases from
the EAF or gases produced by burning natural gas. Scrap preheating can be accomplished by
delivering the hot furnace gases to the scrap-charging bucket by piping the off- gases from
the fourth hole in the EAF to a special hood over the charging bucket. Typically the gases
leave the EAF at about 2200°F (1200"C), enter the bucket at 1500°F (815"C), and leave at
around 400°F (200°C).




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                     Technical University Crete, PREWARC project PL 517574




Figure 7: Schematic diagram of Scrap Preheating in a Charging Bucket (Surface Combustion, 1996)


The amount of preheating depends on the heat transfer to the scrap, which is a function of
scrap size and time at temperature. Typically the scrap is preheated to a range of 600" to
850°F (315" to 450°C).
The advantages for scrap preheating include:
    • Increased productivity.
    • Removal of moisture from the scrap.
    • Reduced electrode consumption.
    • Reduced refractory consumption.
    • Ability to process lower quality of scrap
    • Reduced arc furnace emissions (lb pollutant/ton of melted metal) due to shorter time
        of steel melting

Scrap preheating can save 4-50 kWh/ton and reduce tap-to-tap times by 8 to 10 minutes,
electrode consumption by 0.6 to 0.8 Bison (0.3 to 0.36 kg/mt) and refractory consumption by
2 to 3 lbhon (0.9 to 1.4 kg/mt). All of these advantages can help improve the competitiveness
of individual steel plants; however, the higher capita1 costs and associated operating costs of
scrap preheating systems must be considered.
Some of the disadvantages to conventional scrap preheating include:
    • Inconvenient to operate such as scrap sticking to bucket and short bucket life.
    • Poor controllability of preheating due to cycling of the off-gas temperature and flow
        rate through various EAF operating phases.
    • For tsp-to-tap times less than 70 minutes the logistics of conventional scrap
        preheating lead to minimal energy savings that cannot justify the capital expense of a
        preheating system.
Scrap preheating may also result in higher emissions of aromatic organohalogen compounds
such as polychlorinated dibenzo-p-dioxins and -furans (PCDD/F), chlorobenzenes,
polychlorinated biphenyls (PCB) as well as polycyclic aromatic hydrocarbons (PAH) and
other partial combustion products from scrap which is contaminated with paints, plastics,
lubricants or other organic compounds.
There may be small amounts of NOx, SOx and HCl generated from components in the oils
on the scrap. Emissions from scrap preparation consist of hydrocarbons if solvent degreasing
is used and consist of smoke, organics, and carbon monoxide (CO) if heating is used.
However, since preheated scrap requires less time in the electric arc furnace, the quantity of

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                     Technical University Crete, PREWARC project PL 517574


ozone related gases emitted per ton of steel are reduced. Specifically, the preheating furnace
generates less NOx and volatile organic compounds (VOC) than an electric arc furnace.
(Steel Foundries)




6. Charging
Electric arc furnaces are charged with raw materials by removing the lid, through a chute
opening in the lid, or through a door in the side. The scrap is usually loaded into baskets
together with lime or dololime, which is used as a flux for the slag formation. Lump coal is
also charged at some plants with the result of relevant benzene (as well as toluene and
xylenes) emissions.
The furnace electrodes are raised in top position, the roof is then swung away from the
furnace for charging. To fill the furnace, two or three charging operations are required,
between which the scrap is partially melted. It is normal to charge about 50-60% of the scrap
initially with the first scrap basket; the roof is then closed and the electrodes lowered to the
scrap. Within 20-30 mm above the scrap they strike an arc. After the first charge has been
melted, the remainder of the scrap is added from a second or third basket.
A proprietary available system is known as the shaft furnace, which allows part of the scrap
to be preheated by charging it through a vertical shaft integrated in the furnace roof.
Other new charging systems have been developed. In the Consteel Process the scrap is
continuously fed via a horizontal conveyor system into the arc furnace. But this system is not
generally considered as a proven technique.
Emissions from charging scrap are difficult to quantify because they depend on the grade of
scrap utilized. Scrap emissions usually contain iron and other metallic oxides from alloys in
the scrap metal. (U.S. EPA, 1995).

7. Arc furnace melting and refining

 The direct smelting of iron-containing materials, mainly scrap, is usually performed in
electric arc furnaces. A direct electric arc furnace is a large refractory-lined steel pot, fitted
with a refractory roof through which 3 vertical graphite electrodes are inserted, as shown in
Figure 8. The metal charge is melted with resistive heating generated by electrical current
flowing among the electrodes and through the charge. Melting capacities range up to 10 Mg
(11 tons) per hour. (Steel Industry Technology Roadmap)




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Figure 8: Electric arc steel furnace


Electric arc furnaces cause substantial emissions to air and solid wastes/by-products. The
highest concentrations of furnace emissions occur when the furnace lids and doors are
opened during charging, back charging, alloying, oxygen lancing, slag removal, and tapping
operations. These emissions escape into the furnace building and are vented through roof
vents.




Figure 9: Electric Arc Furnace Steelmaking Flow Diagram (Steel Industry Technology Roadmap 2001)
Over the past four decades oxygen usage in the EAF has increased by an order of magnitude
(1970: 96 ft3/ton, 1980: 352 ft3/ton, 1990: 769 ft3/ton, 1999: 961 ft3/ton). This trend is
expected to continue. With increased oxygen use, the generation of fumes occurs at a greater
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rate. However, this rate increase is offset by the reduction in average heat time and better
scrap preparation. As a result, the generation of dust/ton of steel is expected to decrease by
35 to 30 lbs/ton by 2010.

8. Steel and slag tapping
The molten metal from the furnace is tapped by tilting and pouring through a spout on the
side and into a ladle.
Emissions consist of iron oxides during tapping in addition to oxide fumes from alloys added
to the ladle. During tapping, iron oxide is the major particulate compound emitted. (EPA
1995)

9. Secondary (Ladle) Metallurgy
The processes referred to as secondary metallurgy are considered, in general, as being all the
treatments of the steel melt, after the tapping step up to the beginning of the casting facilities.
The aim of the secondary metallurgy in principle is the adjustment of the desired steel
quality. The variety of steel types that can be produced via the EAF route requires different
treatments of the melt in order to achieve the desired characteristics. The purpose of
secondary metallurgy has also shifted within its development: Initially, homogenisation, fine
adjustment of steel composition, and deoxidation were important aims, whereas nowadays
the goals of the secondary metallurgy processes are:
   • Improvement of the degree of purity,
   • Reduction of non-ferrous inclusions,
   • Reduction of carbon, sulphur, phosphorus, hydrogen, and nitrogen contents, and
   • Adjustment of a suitable temperature for continuous casting

The units generally used for carrying out the secondary metallurgy process are ladle furnaces
or converters. Depending on the steel quality desired, different units, or vessels, are used in
secondary metallurgy, and thus the required inputs and produced outputs (products, releases)
also differ.
Ladle refining can encompass the following techniques:
   • Addition of alloys to the ladle following tapping of the steel from the furnace
   • Electric-arc or plasma-torch heated ladle refining
   • Degassing of the steel in a separate degassing facility or reheating in the ladle or
       stirring
Ladle metallurgy processes also inclue ladle temperature control, composition control,
deoxidation, cleanliness control, and others. Vacuum degassing plants are often operated as
part of ladle metallurgy stations where additional steel refining is conducted.
Due to the variety of options for secondary metallurgy treatments and the related inputs and
outputs, it is not possible to present a table with representative specific inputs and outputs.
The alloys added may be lime, carbon, magnesium, titanium, zirconium, vanadium, boron.
and aluminum. Electric arc heating is generally used in the final refining process. Figure 10
shows possible input/output flows for secondary metallurgy processes in a ladle furnace.




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Figure 10: Inputs, releases, and selected technologies related to the secondary metallurgy processes (BAT
in the Electric Steelmaking Industry)

10. Slag handling
The processing of slag includes cooling down by water spraying resulting in fumes. These
fumes can be highly alkaline if the slag contains free CaO, which is very often the case.
Alkaline depositions from the fumes may cause problems in the neighbourhood. Iron oxides
and oxides from the fluxes are also constituents of slag handling emissions.

11. Forming and Finishing
Most steel follows one of two major routes to final processing. The most common forming
method is continuous casting. In this process, a ladle with molten steel is lifted to the top of
the continuous caster, where it flows into a reservoir, or tundish, and then into the molds of
the continuous casting machine. As the steel passes through the molds and is cooled, a thin
skin forms on the outside of the steel. Various designs of the casters shape the steel as it
continues to flow. The steel is shaped into semifinished products such as blooms, billets, or
slabs, and subsequently into more finished products.
Another forming route, which is not used as frequently as continuous casting, is ingot
casting. Molten steel is poured from the ladle into an ingot mold, where it cools and begins to
solidify. The molds are stripped away, and the ingots are transported to a soaking pit or
reheat furnace where they are heated to a uniform temperature. The ingots are shaped by
rolling into semifinished products, usually blooms, billets, slabs, or by forging. Continuous
casting is the preferred method of semifinished product production because the soaking-
reheating step is eliminated.


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The semifinished products may be further processed by a number of different steps, such as
hot forming, cold rolling, pickling, galvanizing, coating, or painting. Some of these steps
require additional heating or reheating. For example, one type of furnace used for heating is a
tunnel furnace, which has cars that are moved slowly through the furnace. Annealing
furnaces are another example.

12. Emission and consumption levels
        Emissions are generated during each of the five major EAF processes: charging,
melting, refining, tapping, and slag handling. Particulate emissions from melting and refining
account for about 90% of total EAF emissions. The remaining 10% of emissions are
generated during charging and tapping. (Steel Industry Technology Roadmap)
        The most important environmental issues relate to emissions to air and to solid
wastes/by-products. More than half of the mass input becomes outputs in the form of off-
gases and solid wastes/by-products. Although big efforts have been made to reduce
emissions, the contribution of the sector to the total emissions to air in the EU is considerable
for a number of pollutants, especially for some heavy metals and PCDD/F. However, the use
of EAF for steel production provides the single most effective means of reducing CO2
emissions due to the significantly lower energy requirements of melting scrap compared to
smelting ore.
The following emissions of off gases, solid wastes/by-products and wastewater can be
recognised in electric arc furnace steelmaking:


Emissions to air
      • Primary off gases
          o Off gas directly collected from the EAF
          o Off gas directly collected from secondary metallurgy processes
      • Secondary off gases from scrap handling and charging, steel tapping,
        secondary metallurgy with tapping operations and from continuous casting
      • Fumes from slag processing

Solid wastes/by-products
      •   Slags from production of carbon steel/low alloyed steel/high alloyed steels
      •   Dusts from off gas treatment
      •   Refractory bricks


Effluents
      •   Drainage water from scrap-yard
      •   Off gas scrubbing (exceptional)
      •   Continuous casting

      •   Soil contamination
      •   Noise emissions

Figure 11 provides an overview for the input and output of electric arc furnaces.

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                      Technical University Crete, PREWARC project PL 517574




Figure 11: Mass stream overview of an electric arc furnace


Subsequently specific input factors as well as specific emission factors can be calculated.
Such factors are presented in Table 1. The data derive from various sources mentioned in the
footnotes.



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                      Technical University Crete, PREWARC project PL 517574




Table 1: Input/output data for electric arc furnaces for the production of carbon steel compiled from
various references indicated in the footnotes


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Emissions to air
Primary off gases directly collected from the EAF
Primary off gases represent approximately 95% of total emissions from an EAF. During the
EAF process, oxide and other metal forms are volatilized in the presence of intense heat and
turbulence inside the furnace. Also, carbon from the addition of coal, iron, and steel scrap
and graphite electrodes react with injected air or oxygen. The composition of the emissions
can vary depending on the scrap composition and the furnace additives such as fluxes that are
added to aid in slag formation. Particulate matter and gases evolve together during the
steelmaking process. Specifically, emissions from melting furnaces are:

•   Inorganic compounds (metal dusts) with iron oxides as the primary component and heavy
    metal dusts such as zinc, chromium, nickel oxides, lead, and cadmium, mercury, arsenic
•   Sulphur dioxide SO2
•   Volatile organic compounds (VOCs) from the scrap
•   Nitrogen oxides (NOx) and ozone, which are generated during the melting process
•   Carbon monoxide (CO) and carbon dioxide CO2
•   Small quantities of chlorides and fluorides generated by the flux
•   Organic compounds such as the important organochlorine compounds chlorobenzenes,
    Polychlorinated biphenyls (PCB) and Polychlorinated dibenzo-p-dioxins and furans
    (PCDD/F)
•   Polycyclic aromatic hydrocarbons (PAH)

Inorganic compounds/Heavy metals
 Inorganic compounds (metal dusts) with iron oxides as the primary component and heavy
metal dusts such as zinc, nickel oxides, lead, and cadmium may also be present as well as
other metals associated with the scrap, such as hexavalent chromium and arsenic. The two
primary hazardous constituents of EAF emission are lead and cadmium.
Some emissions also show wide ranges. Higher values can be of high environmental
relevance. Zinc is the metal with the highest emission factors. Mercury emissions can
strongly vary from charge to charge depending on scrap composition/quality.

Sulphur dioxide SO2, Nitrogen oxides (NOx) and Carbon monoxide (CO) and carbon
dioxide CO2
The SO2 emissions mainly depend on the quantity of coal and oil input but they are not of
high relevance.
Minor amounts of nitrogen oxides and ozone are generated during melting, that do not need
special consideration; but based on well-known natural gas combustion relationships, NOx
emissions are expected to increase with the use of oxy-fuel burners, though no specific
information supporting this assumption was found. (Mini-mills)
Carbon from the addition of coal, iron, and steel scrap and graphite electrodes react with
injected air or oxygen. Organics on scrap and the carbon additives produce CO emissions.
However EAF steel production is an effective means of reducing CO2 emissions due to the
significantly lower energy requirements of melting scrap compared to smelting ore.

VOC
Scrap may contain volatile organics (VOCs). VOC emissions, especially benzene can be
remarkably high and correspond with the use of coal, which degasses before being burnt off,

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especially when it is added as a ‘nest’ to the scrap basket. It can be expected, that benzene
emissions correlate with the emission of toluene, xylenes and other hydrocarbons deriving
from coal degassing. It was in the nineties that increasing note of organic pollutants was
being taken.

Chlorobenzenes, Polychlorinated biphenyls (PCB) and Polychlorinated dibenzo-p-dioxins and
furans (PCDD/F)
Organochlorine compounds, such as chlorobenzenes, PCB and PCDD/F have been measured.
Chlorobenzenes have been determined in Swedish EAF (1 – 37 mg/t LS). From one German
plant it is known that hexachlorobenzene is present in the emitted off gas.
The measured PCB emissions vary considerably (15 - 45 mg/t LS). They are of
environmental relevance. It is not known yet, whether PCB can be formed de novo during the
process and/or within the off gas devices. PCBs are present in the scrap input which could be
the dominant source for the measured emissions. Especially PCB in small capacitors in
several technical devices like washing machines, (hair) driers, cooker hoods, oil burners,
fluorescent lamps etc. in the (shredded) scrap represent the main PCB input. The so-called
light fraction (if used as an input) can contain up to 140 ppm PCB (sum of all PCB
congeners). One investigation showed that PCB emissions remained unchanged before and
after a bag filter which achieved low residual dust concentrations (< 5 mg/Nm3) as daily
mean value.
Dioxins and furans have become a major concern over the past few years. Dioxins and furans
are combustion by-products and the prevention of these emissions depends strongly on
control of the combustion process. Dioxin emission factors for electric furnace steel plants
depend strongly on operation conditions; if scrap preheating is applied dioxin emissions can
be up to 5 times higher (European Dioxin Inventory). Regarding PCDD/F there are many
measurements available showing emission factors between 0.07 – 9 µg I-TEQ/t LS. Figure
12 presents an example of the distribution of PCDD/F homologues in the raw and cleaned off
gas of an EAF.




Figure 12: Distribution of PCDD/F homologues in the off gas of a twin shell EAF with scrap preheating
before and after abatement


The PCDD/F homologues with four and five chlorine atoms dominate. There is no reliable
information available telling whether the input of PCDD/F or the de novo synthesis mainly


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cause the PCDD/F emissions. With respect to the absolute PCDD/F emissions there is a
positive correlation between off gas temperature (Figure 13) and dust content (Figure 14).




Figure 13: Correlation of PCDD/F emissions and off gas temperature (after abatement in a bag filter) in
the off gas of an EAF


Figure 13 indicates that as long as the clean gas temperature is below 75 °C PCDD/F
emissions will stay below 1 ng I-TEQ/ Nm3. The physical explanation of this pertains to the
decrease of volatility of PCDD/F with decreasing temperature. At low temperatures PCDD/F
increasingly tend to absorb to the filter dust.




Figure 14: Correlation of residual dust content and PCDD/F concentrations (after abatement in a bag
filter) in the off gas of an EAF at temperatures below 85°C


The observation that there is a close connection between dust and PCDD/F emissions has to
be related to off gas temperature. The dust content itself mainly depend on the dimension and

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quality of the bag filter but also on the relative humidity in the off gas which can be high in
case of off gas quenching or location near to the sea (Figure 15).




Figure 15: Correlation of water vapour and residual dust content (after abatement in a bag filter) in the
off gas of an EAF


Polycyclic aromatic hydrocarbons (PAH)
The emission factors for PAH are also relatively high (3.5 – 71 mg/t LS) but there are not
many reported measurements. PAH are also already present in the scrap input but may also
be formed during EAF operation. The expectation that PAH adsorb to the filter dust to a high
extend (also depending on the off gas temperature) could not be confirmed by investigations
in Luxembourg, where PAH emissions remained unchanged before and after abatement in a
bag filter which achieved low residual dust contents (< 5 mg/Nm3) as daily mean value.

Most of the existing plants extract the primary emissions by the 4th hole (in case of three
electrodes) or by the 2nd hole (in case of one electrode) (Figure 16). Thus 85 – 90% of the
total emissions during a complete cycle “tap-to-tap” can be collected. There are still very few
plants, which do not have a 4th hole but only a doghouse. More than 50% of the EAF in the
EU have, in addition to the 4th hole, a system for evacuation the building atmosphere,
especially hoods (see Figure 16).




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Figure 16: Dust collection systems at EAF


In this way also most of secondary emissions from charging and tapping as well as from EAF
leakages during melting can be captured. If secondary metallurgy is carried out in the same
building also these emissions can be collected. Very often the treatment of primary and
secondary emissions are performed in the same device, mostly in bag filters. Table 2
summarises the qualitative efficiencies to collect emissions from the main operations of
electric arc furnace steelmaking.




Table 2: Systems for the collection of emissions from EAF plants



Figure 17 shows the percentages of the four existing emission collection systems in the EU,
indicating that one third of the plants only have a 4th hole for the collection of primary
emissions.




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Figure 17: Percentages of existing dust collection systems in 67 EAF in the EU


Off gas directly collected from secondary metallurgy and casting processes

Information about emissions from secondary metallurgy (mainly dust emissions) is very
limited. [EC Study, 1996] reports dust emission factors before abatement from seven
AOD/VOD refining installations between 6 – 15 kg dust/t LS and a single low figure of 1.35
kg dust/t LS. These seven installations have a de-dusting device independent from the de-
dusting of EAF.
Emission sources in the ladle metallurgy process include the ladle furnace and the ladle
heater. At some facilities, a roof canopy hood is used to capture the emissions, which are
then vented to a baghouse (which may be the same baghouse used by the EAF). (Mini-mills)
The SO2 and VOC emissions are of particular concern in refining as well as in casting
operations. Small amounts of SO2 are emitted during electric arc heated ladle refining. For
new installations, these additional emissions could require controls in order to meet
Prevention of Significant Deterioration requirements. Research into the mechanism of SO2
formation during this ladle refining process may lead to ways to prevent these emissions.
Effluent from the vacuum degassing process also requires better control technology.
Emissions from continuous casting operations consist of steam from the quenching/cooling
section of the continuous caster, Nox from the fuel torches used to cut the steel sections to
length, and dusts from tundish heating and repair. At some continuous casting facilities, the
steam emissions from the quenching and oiling sections of the caster contain VOCs from the
leakage of lubrication systems used in the cooling sections.
The Nox emissions, although minor, are the primary emissions of concern from fuel cutting
torches. Particulate emissions are minimal at continuous caster operations. For facilities
located in ozone non-attainment areas, additional controls may be needed to reduce
emissions of Nox and VOCs.

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Secondary off gases from scrap handling and charging, steel tapping, secondary
metallurgy with tapping operations and from continuous casting

Secondary emissions mainly mean the emissions of dust except fume leakages from EAF,
which may contain all the pollutants described under primary emissions. Generally, 20
pounds of dust per ton of steel is expected, but as much as 40 pounds of dust per ton of steel
may be generated depending on the scrap that is used. The primary off gases contain 14 – 20
kg dust/t liquid carbon/steel or low alloyed steel and 6 –15 kg dust/t in case of high alloyed
steel. The composition of the dust can be seen from the analysis of the dust separated from
the off gas in the bag filters or electrostatic precipitators (ESP). The heavy metals, especially
mercury, which are present in the gas phase, are not associated with particulate matter. Thus,
they cannot be eliminated by filtration or ESP. However most of the heavy metals are mainly
associated with particulate matter and are removed from the off-gas with the separated dust.
The range of dust emission factors after abatement (1- 780 g/t LS) is extremely wide
indicating big difference in collection and abatement efficiency.
In terms of concentration the emissions of most of the plants are around or below 10 mg
dust/Nm3 but there are also plants with about 50 mg/Nm3. Normally these emission factors
or emission concentrations include secondary dust emissions because primary and secondary
emissions are very often treated in the same equipment.
Figure 18 illustrates the refining and casting processes with their major inputs and outputs.




Figure 18: Refining and Casting Flow Diagram

Information about secondary emissions is limited. From charging the EAF usually 0.3 – 1 kg
dust/t LS and from tapping 0.2 – 0.3 kg dust/t LS are emitted (emissions before abatement).
For fume leakages during EAF operation dust emission factors between 0.5 –2 kg dust /t LS
are reported.



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Emission factors as sum of the mentioned three sources (charging, tapping, fume leakages)
are between 1.4 – 3 kg dust/t LS. This can be considered as a confirmation that primary
emissions are about ten times higher than secondary emissions.
With respect to micropollutants like organochlorine compounds, especially PCDD/F the
contamination of secondary off gases (mainly the leakages from EAF) contribute to the
overall emissions. When emission limit values of < 0.5 ng I-TEQ/Nm3 have to be complied
with, secondary emissions have to be taken into consideration.
During refining, the primary particulate compound emitted is calcium oxide from the slag.
Emissions from ladle refining vary depending on the operations. Emissions may include:
    • Particulate emissions from alloy addition practice
    • Particulate and Sox emissions from ladle refining processes
    • Particulate emissions from degassing, ladle reheating and ladle stirring facilities
Increased control of the particulate emissions from these sources may be required as part of
the overall effort to reduce fine particulate emissions.
Iron oxides and oxides from the fluxes are the primary constituents of slag handling
emissions. During tapping, iron oxide is the major particulate compound emitted.
During casting operations, large quantities of particulates can be generated in the steps prior
to pouring. Emissions are produced when molten steel is poured (teamed) into ingot molds.
Such emissions from pouring consist of fumes, CO, VOC and particulates from the mold and
core materials when contacted by the molten steel. Emissions continue as the mold cools. A
significant quantity of particulate emissions is generated during the casting shakeout
operation. The particulate emissions from the shakeout operations can be controlled by either
high efficiency cyclone separators or bag filters. Emissions from pouring are usually
uncontrolled. (EPA Steel foundries)
At some facilities, fugitive particulate emissions may be emitted through a roof monitor
during transfer from the ladle to the tundish and the continuous caster. No information is
available on any control devices employed for these processes.
Other potential sources of emissions, especially NOx and CO, include reheat furnaces,
annealing furnaces, and tunnel furnaces used in the finishing processes. Low NOx burners,
ultra-low NOx burners, flue gas recirculation, or selective catalytic reduction (SCR) are
being used on some of these furnaces to control emissions of NOx. (Steel Technology
Roadmap)
During ingot casting, particulate emissions, such as FeO, Fe2O3, SiO2, CaO, and MgO are
generated when molten steel is poured (teemed) into the molds. Bottom pouring exposes
much less of the molten steel to the atmosphere than top pouring, thereby reducing the
formation of particulate matter (Steel Technology Roadmap 2001).
Certain refining processes, including ladle metallurgy, generate particulate (and Sox if sulfur
bearing compounds are used) emissions.

Fumes from slag processing
The processing of slags includes cooling down by water spraying resulting in fumes. These
fumes can be highly alkaline if the slag contains free CaO. This is very often the case.
Alkaline depositions from the fumes may cause problems in the neighbourhood.




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Solid wastes/by-products
The various solid wastes/by-products from electric arc furnace steelmaking are compiled
together with their specific quantities in Table 3.




Table 3: Kind and specific quantity of solid wastes/by-products from electric arc furnace steelmaking


Slag from production of carbon steel/low alloyed steel/high alloyed steels
The major non-hazardous by-product generated during EAF steelmaking is slag. The primary
components in EAF slag are CaO, SiO2, FeO, MgO and Al2O3. Cooled, solidified slag is
crushed and screened to recover metallics for recycle or reuse, and the lower metallic
aggregate is used in construction applications. Worldwide, about 77% of the slag produced in
EAFs is reused; the remainder is landfilled. (Steel Technology Roadmap)
The specific composition of slag from production of carbon and low-alloyed steel can be
seen from Table 4. In addition, this table contains the slag composition from the production
of stainless steel and from secondary metallurgy (AOD and VOD).
In traces, other elements than mentioned, such as Pb, As, Sb, Hg, Cl, F and hexavalent
chromium may also be present.




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Table 4: Chemical composition of EAF slag from the production of carbon steel/low alloyed steel and
high alloyed steel


In the EU, most of the slag from carbon and low-alloyed steelmaking is still landfilled (Table
5) whereas the percentage of reuse of slag from production of high-alloyed steels is
significantly higher. But still one third is landfilled and stored.




Table 5: Fate of EAF slag (reuse or disposal) in the EU; data from 57 plants producing 2.7 million t/a of
slags (133 kg/t LS)


Also most of the slag from ladle treatment and secondary metallurgy (also AOD and VOD
slag) is landfilled, with respect to the EU about 80 %. The rate of landfilling respectively
reuse varies in the different Member States depending on legal requirements, availability of
landfills, taxes, market situation, costs and possibilities to reuse processed slag.

Dusts from off gas treatment
EAF dust consists of particulate matter and gases produced during the EAF process and
subsequently conveyed into a gas cleaning system. The particulate matter that is removed
from emissions in a dry system is the EAF dust while particulate matter removed from
emissions in a wet system is EAF sludge. The dust (or sludge) removed from EAF emissions
is designated by EPA as a listed hazardous waste - K061. (Steel Technology Roadmap)
Since most of the dusts are collected dry, associated pollution issues generally fall into a non-
wastewater category. The primary hazardous constituents of EAF emission control

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dust/sludge are lead, cadmium, and chromium. EAF dust can vary greatly in composition
depending on both the composition of the scrap charge and the furnace additives used. The
primary components are iron or iron oxides; typical EAF dust contains 24% iron by weight.
It may also contain flux (lime and/or fluorspar), zinc, chromium and nickel oxides (when
stainless steel is being produced) and other metals associated with the scrap. (Steel
Technology Roadmap 2001)
As already mentioned the treatment of off gases (mostly primary off gases together with
secondary off gases) is very often performed in bag filters. The composition of dusts from
production of carbon, low alloyed and high-alloyed steel can be seen from Table 6.




Table 6: Chemical composition of EAF dusts from the production of carbon steel/low alloyed steel and
high alloyed steel


In the EU, about two third of the dust is landfilled (Figure 19).




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Figure 19: Fate of dust collected from primary and secondary off gases of EAF; data from 67 plants
In the single Member States the percentages of dusts which are reused respectively landfilled
is very different depending on legal requirements, availability of landfills, taxes and other
cost aspects. Table 7 indicates that in Austria, Germany and the Benelux States the dust
recycling has achieved high rates whereas they are low in Southern Europe and in the UK.
That means that the data of the EC study (see Figure 19) are no more fully representative for
the actual situation.




Table 7: Percentages of filter dust from EAF (from carbon and low alloyed steel production) treated in
the Waelz process for zinc recovery respectively landfilled in the EU Member States in 1997


The landfills for the dusts are equipped with different sealing systems. In [EC Study, 1996]
the percentages of the various systems are reported (Figure 20).




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Figure 20: Percentages of applied sealing systems for the landfill of filter dusts from EAF in the EU


Refractory bricks
In most cases refractory bricks are put to landfill.
Refining Solid Waste
Wastes resulting from refining processes are very small in comparison to the wastes
generated from iron making and steelmaking. The more common solid wastes generated
include the following:
    • Ladle metallurgy facility and capped argon bubbling APC dust
    • Nozzle block sludges
Baghouse dusts collected from the electric arc or plasma-heated ladle refining furnaces
contain mostly dusts from flux, ore and slags used in the process, and some metal oxides.
(Steel Technology Roadmap 2001)
Casting By-products
The major by-products of continuous casting are scale and sludge. Scale generated during
casting, which is subsequently washed off of the steel, is periodically removed from the
bottoms of scale-collection settling basins. The primary wastes collected in a continuous
caster are spent casting mold flux and coarse scale and sludges from the continuous caster
wastewater treatment process. The coarse scales and cutting swarf, which are normally
dredged from the caster’s mill scale pit, are relatively coarse particulates of essentially pure
iron oxide. They are not hazardous wastes and contain small amounts of water and grease.
    • Fine-grained solids that do not settle in the pits are typically removed by flocculation
        and clarification or by filtration, depending on the level of water treatment required
        and the degree of water recycle practiced. The fine particulate mill sludges collected
        from the caster wastewater treatment system are also not hazardous wastes, but
        contain larger amounts of water, oils, and greases. (Steel Technology Roadmap 2001)




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Effluents
Mini mills generate up to 80 cubic meters of wastewater per metric ton (m3/t) of steel
product. Untreated wastewaters contain high levels of total suspended solids (up to 3,000
milligrams per liter, mg/l), copper (up to 170 mg/l), lead (10 mg/l), total chromium (3,500
mg/l), hexavalent chromium (200 mg/l), nickel (4,600 mg/l), and oil and grease (130 mg/l).
Chrome and nickel concentrations result mainly from pickling operations. The characteristics
of the wastewater depend on the type of steel, the forming and finishing operations, and the
quality of scrap used as feed to the process.
Most electric arc furnaces, however, are operated with dry gas cleaning systems, which have
no process wastewater discharges. The furnace is extensively cooled by water; however, this
water is recycled through cooling towers.

Drainage water from scrap-yard
The main raw material of EAF, the different kinds of scrap are often stored on unpaved
scrapyards. Drainage water can be contaminated, especially in case of oil/emulsion
containing scrap like turnings. There is no information available on quantities and pollution
of drainage water. Usually it is at least treated in an oil separator prior to be discharged.

Waste water from off gas scrubbing
In the EU in some cases the off gases are treated in a wet scrubber. There is no information
available on applied treatment techniques and discharged quantities and its pollution.

Refining effluents
Ladle refining air emissions are controlled by dry collection devices (typically baghouses);
therefore, process water is not normally discharged from these facilities. The exception is
vacuum degassing. The vacuum for this process is normally generated by steam or water
ejectors. The exhaust steam and water is condensed with water and processed to remove the
suspended solids from the vacuum degassing operation.
Vacuum degassing involves direct contact between gases removed from the steel and
condenser water. Most steel contains low concentrations of zinc and lead. These elements are
removed from the steel during the degassing process and end up in the ejector or quench
water. To comply with NPDES permit effluent limitations for these operations, these
suspended solids and metal discharges must be reduced to extremely low levels.
Principal pollutants contained in the effluent include low levels of total suspended solids
(TSS) and metals (particularly lead and zinc, but also chromium, copper, and selenium) that
volatilize from the steel.
Applied water rates for vacuum degassing are typically around 1,250 gallons/ton of steel,
with discharge rates of 25 gallons/ton achieved through high-rate recycle.

Casting effluents
Wastewater results from direct cooling from continuous casting. Usually this wastewater is
treated together with other streams from the rolling mill(s). (Steel Technology Roadmap
2001)
Continuous casters usually include two separate closed-loop, non-contact cooling water
systems for spray and mist cooling. The mold cooling water system is used to cool the mold,
while the machine cooling water system is used to cool all other mechanical equipment.
Direct-contact water systems are used for spray cooling of the steel as it exits the mold; at the


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gas cutting torches to control fume generation; and for flushing mill scale down the flume
beneath the runout table.
Applied water rates for the contact systems are typically about 3,600 gallons/ton of cast
product; discharge rates for the better-controlled casters are less than 25 gallons/ton.
The principal pollutants are total suspended solids, mill scale (1–3 g/l), oil and grease, and
low levels of particulate metals, such as nickel, chromium, lead, and zinc. As with vacuum
degassing, chromium, copper, and selenium may also be found in continuous casting
wastewater.

Soil contamination
In many cases the scrap-yard is unpaved and uncovered. Contamination of soil may arise
from storage of scrap contaminated with mineral oil/emulsions or other compounds. There is
no information available about extent and impact of such soil contamination.
If the yard for slag processing is unpaved and the raw slag is containing free CaO, alkaline
water may enter the soil.

Noise emissions
The following noise sources are dominating in electric arc furnace steelworks:
   • melting shop including EAF
   • scrap yard
   • primary de-dusting
   • roof hood de-dusting
   • water management equipment

The housing is constructed from sound-insulating elements, and reduces the noise level from
the furnace to 20 – 25 dB.
Conventional EAF show average sound levels (melting and treating) of LWA = 118 – 133
dB(A) for furnaces > 10 t and LWA = 108 – 115 dB(A) for furnaces < 10 t; the specific
transformer power determines the level of noise emissions. In electric steelworks sound
levels of up to LWA = 127 dB(A) can appear (measurement includes melting and treating).
The main share of noise emissions is contributed by the melting shop including EAF, the
scrap yard and the primary dedusting. (BREF)

13. Hot and Cold Forming (from the Reference Document on Best Available
Techniques in the Ferrous Metals Processing Industry)
The hot and cold forming part of the ferrous metal processing sector comprises different
manufacturing methods, like hot rolling, cold rolling and drawing of steel. A great variety of
semi-finished and finished products with different lines of production are manufactured.

Hot rolling
In hot rolling the size, shape and metallurgical properties of the steel slabs, blooms, billets or
ingots are changed by repeatedly compressing the hot metal (temperature ranging from 1050
to 1300 Co) between electrically powered rollers. The steel input for hot rolling differs in
form and shape, depending on the process route and on the product to be manufactured.
 Hot rolling mills usually comprise the following process steps:
        • Conditioning of the input (scarfing, grinding).
        • Heating to rolling temperature.

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        • Descaling.
        • Rolling (roughing including width reduction, rolling to final dimension and
        properties).
        • Finishing (trimming, slitting, cutting).

Emissions from hot rolling
Water effluents
Throughout the hot rolling process and linked process steps water is used for cooling and for
technological reasons. Electric motors, re-heating furnaces, control rooms and power
systems, instruments and process control are usually cooled indirectly. The steel, rolls, saws,
cropped ends, coilers and hot run out tables are cooled directly.
Water is also used for scale breaking, flushing scale and for scale transport. Wherever the
water is in contact with the rolled material (process water) and rolling equipment it will be
contaminated with scale, oil and greases.
The polluted cooling and process waters are collected and treated prior to discharge. First
treatment stage is a sedimentation basin in which solids, mainly iron oxides, are allowed to
settle at the bottom of the basin. The sedimented solids are discharged via appropriate
devices (scraper, screw, etc.) and, in the case of integrated steel plants, returned to the
production process via the hot metal route. The oil pollutants floating on the surface are
removed by means of suitable skimming devices and are discharged to the respective
collecting basins.
The pre-cleaned overflow is supplied via pipes to the filter batteries whose number, size and capacity
are designed in conformity with the water volume. In most cases these filters are gravel filters, i.e. the
overflowing water is cleaned by passing through gravel beds.
The pollutants in the gravel filters must be removed by back-washing in order to maintain the
function and separation efficiency of the filters. The purified waste water from the filters is
discharged into the sewage system and/or lakes and rivers.
The sludge-bearing waters (mostly containing iron oxide) from the filter batteries are separated in a
thickener. The overflow is recirculated to the cleaning circuit system. The high-quality feed material
contained in the sludge is further dewatered and disposed off or returned to the steel production
process, provided the appropriate technical equipment is available.
In order to reduce or avoid waste water discharge from hot rolling operations semi-closed – and
closed circuits are implemented.
In semi-closed circuits, as shown in Figure. 21, the water is treated and partly reused depending on
the temperature. The water treatment devices are the same as for open systems, but the filtered
wastewater is not directly disposed off. Instead it is conveyed into a filter water basin and mixed with
cold fresh water, if necessary. Depending on the temperature of the mixed water, the filtered water is
returned to the different consumers in the hot rolling mill and only the overflow is discharged.
Accordingly, the volume of the circulating water depends on the seasons and the geographical
situation.




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Figure 21: Example of semi-closed water circuit
With application of closed water circuits (see Figure 22) the purified water is not discharged, but re-
cooled in cooling towers or heat exchangers to required temperature and is reused in the rolling
process. For plants using cooling towers, water consumption is restricted to additional water (approx.
3 - 5 %) needed to make up for evaporation and for blow down losses.
When heat exchangers are used, large re-circulating volumes of re-cooling water are required.




Figure 22:Example of closed circuit water system
Water feeding- and treatment systems in hot rolling mills are usually very complex, with several,
partly interconnected water loops and multiple-stage use of water. In some cases the hot rolling mill
water circuit is coupled with water feeding systems of other iron and steel production units, as for
example continuous casting. Reasons for this connection are the similarity of the waste water contents
and the proximity of the installations.
The prevention of effluents by using water in loops or in multiple stages is a well-known and
wide spread practice within the steel industry. Hot rolling mills offer a high potential for
reduction in water consumption and wastewater discharge, because of the large quantities of
water needed.




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Figure 23: Use of water loops in a hot rolling mill

Apart from wastewater, hot rolling operations generate different kinds of solid and liquid
byproducts and waste.
   • Metallic waste and by-products.
   • Scarfing scale/swarf.
   • Dusts from scarfing and rolling.
   • Mill scale (oil free and oily).
   • Water treatment and mill scale sludge.
   • Grinding sludge (roll shop)
   • Oil and greases.


Metallic by-products/waste, like scrap, downgrades, crop ends, etc. arising from hot rolling,
is usually rather clean and is easily recycled into metallurgical processes (e.g. BOF).
Oil free scale and low oil-content (< 1 %) scale, is fed back directly to the metallurgical
process, usually via the sinter plant. Oily, ferrous sludges with up to 80 % FeOn content, like
oily mill scale and grinding sludges, obtained from water treatment plants have to be
conditioned before internal recycling.
Scale is also sold for external use (e.g. to cement manufacturers) or it is supplied to an
external company for treatment (usually thermal treatment to burn the oil content). Thermal
treatment plants can yield a product with an iron-content of about 60 - 70 %. If the thermal
treatment plant is fed with oily mill-scale of about 4.5 % no additional energy supply is
required.
Oxide dusts from air cleaning devices, for example from bag filters for oxide removal at the
mill stands, are recyclable to the steel production (e.g. sinter plant) without risks.


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Oil and grease, separated and collected at various stages, are energy sources and can be
utilized as secondary fuels, for example by injection into the blast furnace or in the coke
making process. Dewatering might be a precondition. Alternatively, these materials can be
used at the coke ovens to increase coal bulk density prior to carbonisation.
Spent emulsion from the roll shop or other consumers can be split into components: oil and
water. The separated oil can be thermally utilized or recycled externally.

Cold rolling
In cold rolling the properties of hot rolled strip products, e.g. thickness, mechanical and
technological characteristics, are changed by compression between rollers without previous
heating of the input. The input is obtained in form of coils from hot rolling mills.
The processing steps and the sequence of processing in a cold rolling mill depends on the
quality of the steel treated. Low alloy and alloy steel (carbon steels) processing usually
follows the order: pickling, rolling, annealing, temper rolling/skin pass rolling and finishing.
Cold rolled products are mainly strips and sheets (thickness typically 0.16 - 3 mm) with high
quality surface finish and precise metallurgical properties for use in high specification
products.
A typical cold rolling mill usually comprises:
• Continuous pickling line, where the oxide layer formed during the hot rolling is removed by
pickling with sulphuric, hydrochloric or a mixture of nitric and hydrofluoric acid. A stretcher
leveller or an in-line skin-pass may be used to improve the shape of the strip and provide
mechanical breaking of the oxide layer.
• Cold rolling mill generally consisting of a 4-stand or a 5-stand four-high tandem mill or of a
four-high reversing mill. Cold rolling reduces the initial thickness of the hot rolled strip by
typically 50 to 80 %.
• Annealing facilities to restore the ductility of the steel strip that is lost as the result of work
hardening during the cold rolling.
• Temper mills to give the annealed material the required mechanical properties (prevents the
formation of Luders lines during drawing). The material is subject to a slight skin pass
rolling typically on a four-high skin pass mill. The roughness of the work rolls of the mill is
transferred to the strip by the roll pressure.
• Inspection and finishing lines, here coils with different length may be welded together to
meet the required weight or may be slit to required width. Also coils are cut into sheets with
required length and width. At the same time defective sections of strip can be discarded.
• Packaging lines for coils or sheets according to the destination and/or the means of
transport.
.• Roll shop, where the work rolls and the backup rolls for the cold rolling mill and the
temper mill are prepared.

Waste and By-product Management in Cold Rolling Mills
Waste water from cold rolling operations, which cannot be regenerated or used elsewhere in
the production line, has to be treated prior to discharge.
Acidic wastewater from rinsing or acid regeneration is usually treated by neutralisation with
agents, such as calcium hydroxide or sodium hydroxide. The dissolved metal ions are
precipitated as hydroxides and then separated by sedimentation techniques including
clarification or filtration. Flocculants are sometimes used to assist the process. The sludge is
dewatered, e.g. by filter presses, to reduce the final volume of sludge.

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Where electrolytic chemical pre-pickling processes are used additional effluent treatment
steps may be required. Typically these can incorporate chromium (VI) reduction processes
using sodium bisulphite or iron (II) compounds.
Alkaline waste water streams may be neutralised using HCl, filtered and then discharged.
Spent coolants/lubricants (emulsions) are treated by emulsion breaking, either done thermal,
chemical, mechanical or physical, followed by a separation of water and oily phase.
Cold rolling gives rise to solid residues, such as scrap (cleaning rags, cleaning paper),
sludges from the waste water treatment plants, remainder of packing material and dust.
Scrap is recycled in the works for steel production. The oil-containing sludges from the
wastewater treatment plants may be used in blast furnaces.
Acid regeneration sludges can be recycled in steel plants (EAF and blast furnace) or given to
external recycling firms for the production of iron oxides.
The iron sulphate-heptahydrate from the sulphuric acid regeneration plant can be used:
            o For the production of complex iron-cyan salts.
            o As flocculation agents in waste water treatment plants.
            o For the production of gas adsorption mass.
            o As chemical amelioration agent.
            o For the production of iron oxide pigments, and
            o For the production of sulphuric acid [Com D].
The iron oxide from the hydrochloride acid regeneration can be used in several industries as
high quality input, e.g.:
            - As input for the production of ferromagnetic materials.
            - As input for the production of iron powder, or
            - As input for the production of construction material, pigments, glass and
                ceramics.
Sludges from oil recovery are externally used, either by incineration or for oil recuperation in
specially dedicated plants.
Only a small part of the sludges from waste water treatment is internally recycled, the vast
majority of is landfilled.
Oily wastes (oil, emulsion, greases) arising are internally or externally used by incineration.

13. BAT for Electric Arc Steelmaking
With tighter environmental restrictions expected in the future, it is expected that electric
furnace operations will have to look at environmental concerns in conjunction with furnace
operations. Research into major process changes of steelmaking is needed.
For electric arc steelmaking these are techniques to consider in the determination of BAT:

      • Process-integrated measures
PI.1 EAF process optimisation
PI.2 Scrap preheating
PI.3 Closed loop water cooling system


      • End-of-pipe techniques
EP.1 Advanced emission collection systems
EP.2 Efficient post-combustion in combination with advanced off gas treatment
EP.3 Injection of lignite coke powder for off gas treatment

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EP.4 Recycling of EAF slags
EP.5 Recycling of EAF dusts PI.1 EAF process optimisation

PI.1 EAF process optimisation
The EAF process has been steadily improved in order to optimise it and to increase
productivity which correlates with the decrease of specific energy consumption.
Some of the most important measures/techniques are:
    • (Ultra) High power operation (UHP),
    • Water cooled side walls and roofs,
    • Oxy-fuel burners and oxygen lancing,
    • Bottom tapping system,
    • Foaming slag practice,
    • Ladle or secondary metallurgy,
    • Automation.




Figure 24: Schematic of an EAF with indication of techniques for optimisation –


(Ultra) High power operation:
The efforts to reduce tap-to-tap times led to the installation of more powerful furnace
transformers. Decisive features for (Ultra) high power furnaces are installed specific apparent
power supply, mean power efficiency (≥0.7), and timewise use of the transformer (≥0.7).
UHP operation may result in a higher productivity, reduced specific electrode consumption,
and reduced specific waste gas volume, but also in increased wear of the furnace lining.



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Water cooled side walls and roofs:
Within the last two decades, furnace walls and roofs have been lined with water cooled
panels, providing the opportunity to save refractory material, to use the (ultra) high power
furnace technology, and also to re-use waste heat by the application of measures for energy
recovery. However, it has to be checked on a plant by plant basis, if the recovery of energy is
economically viable. In principle, two cooling systems can be distinguished. So-called cold
or warm cooling draws off power losses by an increase of the cooling water temperature
flowing through the pipe coils. Evaporation cooling works by the evaporation of cooling
water to draw off radiation heat caused by the electric arc process. To protect water cooled
side panels from thermal strain, especially when foaming slag operation (see below) is not
possible, a computer controlled regulation of the melt-down process helps to prevent tears in
the panels caused by mechanical tension and also saves refractory material .

Oxy-fuel burners and oxygen lancing:
Oxy-fuel burners promote a uniform melting of the scrap. It also partially offsets the effect of
maximum demand control on electricity supply. Usually, additional energy input by oxy-fuel
burners and oxygen lancing results in a decrease of total energy input required.

Bottom tapping system:
The practice of bottom tapping is widely adopted nowadays, as it makes possible to minimize
the amount of oxidic slag (carry over) to the ladle during tapping. It also allows cost savings
for the lowering of refractory material needed, for a more rapid tapping, and for reduced
energy losses. Furthermore, it simplifies the capturing of fumes. While some older furnaces
are still equipped with spouts, usually most of the new electric arc furnaces are equipped
with bottom tapping systems.

Foaming slag practice:
Creating a foamy slag within the furnace improves the heat transfer to the charged inputs,
and also protects the refractory material inside the furnace. Because of better arc stability and
less radiation effects, foaming slag practice leads to reductions in energy consumption,
electrode consumption, noise level, and an increase in productivity. It also causes positive
effects on several metallurgical reactions (eg. between slag and melt). The density of
foaming slag is less than common FeO containing EAF slag (1.15-1.5 t/m3 compared to 2.3
t/m3). For this reason, the volume of slag arising during steelmaking is rising and may
require larger slag buckets.
After tapping, the slag partly degasses again. Information on adverse impacts of the foamy
slag practice on the possibilities to use the slag has not been encountered. It has to be noted,
that the use of foaming slag practice for high-grade steelmaking is often not possible.

Ladle or secondary metallurgy:
Some production steps need not be carried out in the EAF itself and can be performed more
efficiently in other vessels (like desulphurisation, alloying, temperature and chemistry
homogenisation). These tasks have been shifted from the EAF to ladles, ladle furnaces, or
other vessels nowadays. The reported benefits of this development are energy savings (net
savings of 10-30 kWh/t), a reduction of tap-to-tap times of about 5-20 minutes, increasing
the productivity, a better control of steel temperature of the heat delivered to the continuous
casting, a possible reduction of electrode consumption (up to 0.1-0.74 kg/t), alloy savings,
and a decrease of the emissions from the EAF itself. A possible drawback of using ladles or
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other vessels with respect to air pollution control is the increase in the numbers of emission
sources, requiring higher investments for air pollution control equipment, as additional fume
capturing devices like hoods are needed.

Automation:
Computer control in electric arc furnace plants has become necessary within recent years, as
the high throughputs require efficient control systems to manage the material and data flows
arising in the raw material selection, EAF, ladle furnace, and continuous caster. Efficient
control systems permit an increase in productivity, a reduction in energy consumption, and
also a decrease in dust emissions.
Main achieved emission levels: Mentioned above (description)
Applicability: The described techniques are applicable both to new and existing plants but
have to be checked on a plant-to-plant basis.
Cross-media effects: Oxy-fuel burners increase the off gas flow but on the other hand it
decreases the overall energy demand.
Water-cooled side walls and roof need additional energy consumption of about 10-20 kWh/t
but may be compensated by advantages in the field of plant availability and maintenance.
Water-cooled side walls and roofs have inter alia provided the opportunity to apply modern
technology like HP or UHP furnaces.
Reference plants: Many plants in the EU are equipped with the described techniques and are
operated with optimised conditions. Table 8 compiles concerned data from nine German
EAF operated under optimised conditions.




Table 8: Data from 9 German plants operating optimised EAF –




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Continuation of Table 8:




Driving force for implementation: The high market competition and the need to increase
productivity/to reduce costs pushed the introduction of the described techniques.
Operational data and economics: Operational data can be seen from Table 8.
Economical data are not available.

BAT for scrap pre-heating
Scrap preheating is considered as BAT in combination with the minimising of organochlorine
compounds, especially PCDD/F and PCB emissions, by means of:
       • appropriate post-combustion within the off gas duct system or in a separate postcombustion
           chamber with subsequent rapid quenching in order to avoid de novo synthesis and/or
       • injection of lignite powder into the duct before fabric filters. in order to recover sensible
           heat from primary off gas
With scrap preheating of part of the scrap, about 60 kWh/t can be saved; in case of preheating the
total scrap amount, up to 100 kWh/t liquid steel can be saved.
The applicability of scrap preheating depends on the local circumstances and has to be proved on a
plant-by-plant basis. When applying scrap preheating it has to be taken care of possibly increased
emissions of organic pollutants.
PI.2 Scrap preheating BAT
Description: The recovery of waste heat from off gases is a well-known approach. In seventies about
twenty plants have been erected to preheat the scrap in the basket prior to its discharge into the
furnace. But all these systems have been taken out of operation, due to technical and emission
problems. New furnace concepts with shaft integrated scrap preheating. With the single shaft furnace
at least 50 % of the scrap can be preheated [Smith, 1992] whereas the new finger shaft furnace
(Figure 25) allows the preheating of the total scrap amount.


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Figure 25: Schematic of an EAF with a shaft equipped with “fingers” in order to retain the scrap (finger
shaft furnace) for preheating

With finger shaft EAF tap-to-tap times of about 35 minutes are achieved which is about 10-
15 minutes less compared to EAF without efficient scrap preheating. This allows a very short
pay back time which is in the order of one year.
Another available process for scrap preheating is the Consteel process (Figure26) but this
system is not generally considered as a proven technique.




Figure 26: Schematic of the Consteel Process




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Main achieved emission levels: With the single shaft furnace up to 70 kWh/t LS of electric
power can be saved. Calculated on the basis of primary energy the savings are about three
times higher because of the low efficiency of electricity supply. In addition the scrap
preheating significantly reduces the tap-to-tap time, which means a considerable increase of
productivity.
The finger shaft furnace allows energy savings up to 100 kWh/t LS which is about 25% of
the overall electricity input. In combination with an advanced off gas treatment (see EP.2)
scrap preheating may play a significant role in optimisation of electric arc furnace
steelmaking not only related to productivity but also minimise emissions.
As a side effect scrap preheating reduces raw dust emissions about 20% because the off gas
has to pass the scrap, which acts as a filter. This reduction correlates with an increase of the
zinc content in the dust which supports its recycling.
Applicability: Applicable both to new and existing plants. In case of existing installations the
local circumstances like space availability or given furnace concept have to be checked on a
plant-by-plant basis.
Cross-media effects: The scrap preheating in a shaft may lead to an increase of organic micro
pollutants and smell, such as PCDD/F unless adequate thermal treatment of the off-gases is
performed. Additional off gas treatment may be necessary which needs additional energy.
But in relation to the energy saving by scrap preheating this additional energy consumption
may be reasonable and acceptable, especially when taking into account that electric power is
generated from thermal energy with a yield of about 35% and for post-combustion natural
gas is used.
Reference plants: EAF with a single shaft: Co-Steel Sheerness, UK-Sheerness EAF with a
finger shaft: Cockerill-Sambre, B-Charleroi; Gerlafingen Stahl AG, CHGerlafingen (this
furnace has been retrofitted with a finger shaft); Twin-shell furnace with integrated
preheating in a shaft: ARES, L-Schifflange; ASW, FMontereau; Nervacero, Spain.
Driving force for implementation: The main driving force is the increase of productivity. In
some cases scarp preheating by means of a finger shaft furnace has been installed in
combination with advanced off gas treatment.
Operational data and economics: not available

PI. 3 Closed loop water cooling system
Description: Generally, water is only used in the EAF steelmaking processes in connection with non-
contact cooling, since wet scrubbing techniques for off gas cleaning are rarely used. The most
relevant use of water considered here is the water used for the cooling of the elements of the furnace.
Additionally, some water may be used for the cooling of waste gas or in the secondary metallurgy
section. The water needed with respect to the cooling elements amounts to 5-12 m3/(m2h).
Modern plants operate with closed cooling systems in the EAF and secondary metallurgy sections
Main achieved emission levels: No discharge of wastewater.
Applicability: Applicable both to new and existing plants.
Cross-media effects: The closed loop system requires additional energy for water pumping and water
re-cooling.
Reference plants: Preussag Stahl AG, D-Peine; BSW, D-Kehl and many other plants in the EU
Driving force for implementation: Legal requirements and limited availability of cooling
water.
Operational data and economics: not available

EP.1 Advanced emission collection systems
Description: The primary and secondary emissions to air are of high relevance.

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The available abatement techniques should be fed with the raw emissions as complete as
possible. Thus the collection of the emissions is important. The combination of 4th hole (in
case of three electrodes) respectively of 2nd hole (in case of one electrode) direct extraction
with hood systems (or furnace enclosure) or total building evacuation are the most favorite
systems.
A 4th, or 2nd hole should collect practically quantitatively the primary emissions generated
during the melting and refining periods. This type of direct extraction technology is state of
the art in modern EAF steelmaking for the collection of primary emissions. It can also be
applied to secondary metallurgy vessels.
In a hood system, one or more hoods over the furnace indirectly collect fumes escaping from
the furnace during charging, melting, slag-off, and tapping steps (up to 90% of primary
emissions and also secondary emissions). Hood systems are commonly used within the
electric arc furnace steel industry. Combined with direct extraction systems, the collection
efficiency of primary emissions and also secondary emissions improves up to 98%. Hoods
are also installed to collect emissions arising at secondary metallurgy vessels, hoppers and
conveyor belts. Furnace enclosures, also called dog-houses, usually encapsulate the furnace,
its swinging roof, and also leave some working space in front of the furnace door. Typically,
waste gases are extracted near the top of one of the walls of the enclosure, and makeup air
enters through openings in the operating floor. More complex handling steps, causing time
losses and possibly higher investments (e.g. need for additional door opening and closing
mechanisms and procedures in order to charge and empty the furnace) are drawbacks of this
type of collection technology. Collection rates of dog-houses are similar or usually slightly
higher to those of hood-complementary hole combinations.
A positive effect of furnace enclosures is a reduction in the noise level, if they are
constructed in a suitable manner. Noise abatement at an EAF plant by sound protecting
enclosures can reduce the average sound pressure level between 10 and 20 dB(A). Furnace
enclosures may also be applied at secondary metallurgy processes but it needs a treatment of
the shop walls to eliminate reverberation.
Another way to collect secondary emissions from the furnace, as well as preceding and
succeeding installations, is a complete enclosure of all plants in one sealed building. It can be
regarded, roughly speaking, as just a larger type of furnace enclosure, mainly containing
more process steps. The erection of such buildings and the additionally required large de-
dusting installations in order to achieve complete de-dusting impose considerable costs on
the operators. For this reason, the costs and benefits need to be weighed up carefully for
every special plant before this option is considered. A positive effect of this measure is a
reduction in the noise level penetrating to the outside. Usually, the pressure in the enclosing
building is below atmospheric pressure to avoid the escaping of fumes through occasional
door openings.
Main achieved emission levels: The combination of direct fume extraction and a hood system
is often used. This combination achieves a collection of about 98% of the primary emissions.
In addition, a significant share of charging and tapping (secondary) emissions can be
collected, too, though this depends on the type and the number of hoods. A combination of a
direct extraction device and a furnace enclosure even achieves collection rates of over 97%
up to 100% of the total dust emissions. Total building evacuation also achieves practically
100% emission collection.
Applicability: Applicable both to new and existing plants.
Cross-media effects: The emission collection systems need energy, especially the fans.


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Reference plants: Many plants in Europe have a combination of direct off gas extraction and
hoods. Following German plants are equipped with dog-houses only systems or with a
combination of dog-house and hole (direct extraction): Benteler AG, D-Lingen; Krupp
Thyssen Nirosta, DBochum; Krupp Thyssen Nirosta, D-Krefeld; Mannesmannrohr GmbH,
D-Bous/Saar; Moselstahlwerk, D-Trier; Stahlwerke Thüringen GmbH, D-Unterwellenborn
Total building evacuation: ARES, L-Schifflange; ProfilARBED, L-Differdange and L-
Belval.
Driving force for implementation: The main driving forces are legal requirements.
Operational data and economics: not available

EP.2 Efficient post-combustion in combination with advanced off gas treatment
Description: The optimisation of EAF operation (see PI.1), especially the increasing use of
oxygen and fuels have increased the amount of chemical energy in the primary off gas (CO
and H2 content). In order to use this energy post-combustion, trials in electric arc furnace
steelmaking were started middle of the eighties and significant progresses had been made.
Post combustion in the furnace is developed to use a maximum of chemical energy of the CO
in the furnace and to improve the energy balance, but CO and H2 are never completely
oxidised in the furnace; for this reason, it needs post combustion. Post combustion in a
combustion chamber aims primarily at the full combustion of CO and H2 remaining in the
off-gas in order to avoid uncontrollable reactions in the gas cleaning equipment. Secondarily,
this post-combustion, when it is well optimised, reduces the emission of organic compounds.
The heat produced by this combustion is generally not recovered unless recovery from
cooling water is possible. Today the optimisation of the post combustion chamber can reduce
organic micro-pollutants, such as PCB or PCDD/F. Figure 27 shows such a plant originally
equipped with post combustion chambers. Because of relevant de novo synthesis of PCDD/F
the heat exchanger been replaced by a quenching tower for rapid cooling of the off gas.




Figure 27: Schematic layout of the treatment of primary off gas from an twin shell EAF



Because of de novo synthesis of PCDD/F in the tubular heat exchanger, this device has been
replaced by a quenching tower for rapid cooling of the off gas
Post-combustion with the additional aim to minimise organic micropollutants needs
necessary retention time, turbulence and temperature (3 T`s). If a separate combustion


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chamber cannot be introduced, suitable post-combustion can also be achieved in the off gas
duct system (Figure 28).




Figure 28: Post-combustion of the primary off gas within the given duct system from an EAF with
subsequent rapid cooling
Recent developments have separate post-combustion chambers with additional burners to
achieve the necessary “3 T`s). To avoid the novo synthesis of PCDD/F, it is necessary to
have a fast cooling of the fumes before filtration in a bag filter. In some cases, this is
obtained by dilution of the secondary circuit; in other cases, as presented in Figure 9.17, a
solution is the water quenching tower.
Main achieved emission levels: With a proper post combustion followed by a rapid cooling
(by dilution or water quenching) emission concentration of PCDD/F lower than 0.5 ng I-
TEQ/Nm3 can be achieved (Table 9).




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Table 9: Performance of post-combustion at four German EAF


In separate post-combustion chambers with additional burners PCDD/F emission
concentration ≤ 0.1 ng I-TEQ/Nm3 are achievable but there are problems in practice to
achieve this level constantly. The reduction of PCDD/F can be considered as a lead
parameter. Thus it can be expected that other organic micropollutants are also destroyed.
But it has to be noted that PCDD/F emissions from secondary off gases (which are not
combusted but mixed with the primary off gas) could significantly increase the emitted
PCDD/F concentration.
Applicability: In principle, post-combustion can be applied both to new and exiting plants
but in existing ones the local circumstances and possibilities (like available space, given off
gas duct system etc.) have to be checked on a plant-by-plant basis.
Cross-media effects: Post-combustion with additional burners consumes considerable
quantities of energy (in the order of 30 kWh/t) or prevents heat recovery. The application of
post-combustion in combination with efficient scrap preheating (see PI.2) may allow a
balanced solution of energy saving and consumption.
Reference plants: ProfilARBED, L-Differdange; BSW, D-Kehl; Gerlafingen Stahl AG,
CHGerlafingen.
Driving force for implementation: The main driving force for the implementation of
postcombustion are stringent emission limit values requiring for PCDD/F emission limits
<0.5 ng ITEQ/ Nm3.
Operational data and economics: The post-combustion units at ProfilARBED, L-Differdange
and at BSW, D-Kehl are operated without significant problems. Investment cost for a
quenching tower is about 1.2 millions Ecu1997. More data about economics are not
available.


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EP.3 Injection of lignite coke powder for off gas treatment
Description: In order to reduce organic micropollutants in the total off gas (primary and
secondary emissions), especially PCDD/F, lignite coke powder can be dosed to the duct
before the bag filters. The necessary amount is in the order of 100 mg lignite coke
powder/Nm3 off gas (see also EP.3 in 4.3). The lignite coke powder is separated in the gas
phase in the subsequent bag filters. Attention has to be paid to sparks and principally possible
glow fires. The explosion risk has been assessed as to be low.
Main achieved emission levels: Residual PCDD/F emission concentrations of < 0.5 ng ITEQ/
Nm3 are achievable in practice; some measurements show values < 0.1 ng I-TEQ/Nm3.
Applicability: Applicable both to new and existing plants
Cross-media effects: The amount of energy for lignite coke powder dosage is not
considerable. The filter dust contains the lignite coke powder and slightly increased PCDD/F
amounts but this does not interfere with dust recycling.
Attention has to be paid to carbon content of the dust mixture abated at the bag filter, which
is about 3% average with local concentrations up to 5% which could be ignitable.
Reference plants: ARES, L-Schifflange; Gerlafingen Stahl AG, CH-Gerlafingen (the lignite
coke dosage went into operation in September 1998 in addition to post-combustion)
Driving force for implementation: The main driving force for the implementation of this
technique are stringent emission limit values requiring for PCDD/F emission limits < 0.5 ng
ITEQ/ Nm3.
Operational data and economics: Investment for the total off gas flow (primary and
secondary off gases) from a EAF plant producing about 1 Mt steel/a is about 300000
Ecu1997.

EP.4 EAF slag recycling
Description: In an EAF the slag amounts to about 100-150 kg per tonne of steel produced.
EAF slag can be regarded as an artificial rock, similar to natural rock, consisting of iron-
oxides (FeO), lime (CaO), silicium-oxide (SiO2), and other oxides (MgO, Al2O3, MnO).
EAF slags are characterised by high strength, good weathering resistance, and also high
resistance against polishing. They also have properties that make them suitable for use in
hydraulic engineering. An important criterion for the use of EAF slag in general is the
constancy in volume, which depends on the presence of free lime. Most of the slags from low
carbon steel grades are relatively low in free lime and are suitable for various applications
like road construction, earthfill, hydraulic engineering.
Slag from EAF steelmaking has historically not been environmentally regulated. However,
U.S.EPA has recently suggested that in some cases and applications, hazardous constituents
in steelmaking slag may be cause for reporting as part of EPA’s Toxic Release Inventory.
The deciding factors with respect to the uses for slag are environmental acceptability and
structural suitability. If the required legal conditions for use in construction are met the EAF
slag has to be crushed, screened, and sized for use. Ferrous slag components are separated
via magnetic separators. The treated slag is used in various construction purposes, also
dependent on the grain size. Figure 29 shows a processing scheme for a German plant for
slag preparation. In 1994, about 90% of EAF slags generated by the production of non- and
medium-alloyed steel in certain EAFs have been used. Slags arising at high-grade steel
production are only used to a limited extent, so far. Possible uses may be also in the road
construction area, after a preparing treatment.



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Figure 29: Processing scheme of a plant for slag preparation


Options to use the wide spectrum of secondary metallurgy slags are limited. Grain size and
constancy of volume are decisive factors for the use of secondary metallurgy slags. They
sometimes may be used in the construction area. But a significant share of the arising slags
has to be landfilled, as hardly an option for prevention, reduction, or utilisation exists.
Main achieved emission levels: Slag from EAF producing carbon or low alloyed steel can be
treated with subsequent reuse in road construction.
Applicability: Applicable both to new and existing plants.
Cross-media effects: The treatment of slags requires energy. Attention has to be paid to
alkaline fumes when the slag is containing free CaO.
Reference plants: BSW, D-Kehl (treatment of slag with subsequent use for construction
purposes); Georgsmarienhütte GmbH, D-Georgsmarienhütte (selling the slag for external
preparation with subsequent use for road construction – slag from EAF and secondary
metallurgy are mixed); Preussag Stahl AG, D-Peine (treatment and use in the construction
sector) ARES, L-Schifflange; ProfilARBED, L-Differdange; ProfilARBED, L-Belval (high
performance road surfacing, hydraulic engineering and other applications).
Driving force for implementation: The main driving forces are limited space for landfilling
and cost aspects like taxes on landfilled wastes.
Operational data and economics: not available

EP.5 EAF dust recycling
Description: Generally, an EAF produces 10 kilograms of dust per metric ton (kg/t) of steel,
with a range of 5–30 kg/t, depending on factors such as furnace characteristics and scrap
quality. In case of very low quality scrap up to 25 kg dust/t steel can occur.
Separated dusts obtained by the gas cleaning facilities usually contain a significant share of
heavy metals. Dusts and sludges from EAF air pollution control facilities have been a
designated hazardous waste (K061) under the U. S. Resource Conservation and Recovery
Act (RCRA) for many years, and standards exist for the acceptable treatment and disposal of
these wastes and their treatment residuals. Nevertheless, the high costs of treatment and
disposal of K061 continue to present challenges for more cost-effective means of treatment

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and recovery. These are toxic and might be leachable, necessitating special care for further
processing and possibly the landfill of the dusts.
Generally, there are several options of handling EAF dust, which can be classified roughly
into three categories:
       • Chemical stabilisation or vitrification (can not be considered as appropriate,
           because other reasonable options do exist)
       • Recycling of dusts by returning them to the EAF,
       • Hydrometallurgical and pyrometallurgical processes for zinc recovery and recovery
           or removal of other heavy metals.
These options are desirable to different degrees according to their potential to satisfy the aim
of prevention and control of environmental pollution. The use of the iron and heavy metal
content of the dust is preferred compared to landfilling.
Recycling of precipitated dusts
Recycling of precipitated EAF dusts for zinc enrichment by returning them to the EAF
results in certain impacts on the steelmaking process. On the one hand, recycling decreases
the volumetric disposal rate of the dust and increases its zinc content (up to 30-40%) and also
the iron content of the dust is returned in EAF process. On the other hand dust recycling
possibly reduces furnace efficiency and raises the consumption of electrical energy (appr. 20-
30 kWh/t).
Technically, the returning of dusts is limited to a certain share of the total dust yield,
depending on each steelmaking facility. Also the method of dust addition to the furnace
affects the performance of the furnace. To improve performance, some form of pretreatment
to agglomerate the dust, like pelletising or briquetting, is usually beneficial, as it reduces the
share of dust that is just blown through the furnace. According to figures in literature, the
zinc content of the dust and the dust loading increase at the filter can vary, depending on the
blow through rate, between 27-32%. For example, a German electric steelmaking plant
recycles 75% EAF dust of an original yield of 20-22 kg/t and has finally to take care of about
50% of the dust with an average zinc content of 35%. Generally, the dust is added at the
beginning of each melting phase. In principle, the feasibility of EAF dust recycling depends
on many factors that may be dissimilar for different plants.
Zinc recovery and removal of heavy metals
Processes for zinc recovery and recovery or removal of other heavy metals are suitable
options for reclaiming valuable resources, which have already been mined and treated, at
least once.
Pyrometallurgical and hydrometallurgical options exist for the recovery of zinc, in principle.
For dusts from production of carbon/low alloyed steel, different techniques exist and are
proved like the Waelz process (which is the most important one), the ESINEX process and
others. For dusts from production of high alloyed steel also processes for their recycling exist
(ScanDust plasma process, B.U.S process).
To conclude, it is considered best practice to recover zinc from EAF dust containing more
than 15% total zinc and to recycle EAF dust to the extent feasible.
Main achieved emission levels: Quantitative reuse of dust can be achieved.
Applicability: Applicable both to new and existing plants.
Cross-media effects: Energy is used for transport and recycling. In case of pelletisation the
dust before transport/recycling additional energy is needed for pelletisation and additional
dust emissions can occur.
Reference plants: Plants recycling the dust to the EAF: Georgsmarienhütte GmbH,
DGeorgsmarienhütte; Plants for dust recycling to external plants: Many plants in the EU
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Driving force for implementation: The main driving forces are limited space for landfilling,
stringent standards for landfilling and cost aspects like taxes on landfilled wastes.
Operational data and economics: not available

For electric arc furnace steelmaking, the following techniques or combination of techniques
are considered as BAT. The order of priority and the selection of the techniques will differ
dependent upon the local circumstances. Any other technique or combination of techniques
achieving the same or better performance or efficiency can also be considered; such
techniques may be under development or an emerging technique or already available but not
mentioned/described in this document.
1. Dust collection efficiency
- With a combination of direct off gas extraction (4th or 2nd hole) and hood systems or
- dog-house and hood systems or
- total building evacuation
98% and more collection efficiency of primary and secondary emissions from EAF are
achievable.
2. Waste gas de-dusting by application of:
- Well-designed fabric filter achieving less than 5 mg dust/Nm3 for new plants and less than
15 mg dust/Nm3 for existing plants, both determined as daily mean values.
The minimisation of the dust content correlates with the minimisation of heavy metal
emissions except for heavy metals present in the gas phase like mercury.
3. Minimising of organochlorine compounds, especially PCDD/F and PCB emissions, by
means of:
- appropriate post-combustion within the off gas duct system or in a separate postcombustion
chamber with subsequent rapid quenching in order to avoid de novo
synthesis and/or
- injection of lignite powder into the duct before fabric filters.
Emission concentrations of PCDD/F 0.1 - 0.5 ng I-TEQ/Nm3 are achievable.


BAT for reduction of fugitive emissions (Best Available Techniques for Foundries
Chapter 5)
BAT is to minimise fugitive emissions arising from various non-contained sources in the process
chain, by using a combination of the following measures. The emissions mainly involve losses from
transfer and storage operations and spills:
   • avoid outdoor or uncovered stockpiles, but where outdoor stockpiles are unavoidable, use
       sprays, binders, stockpile management techniques, windbreaks, etc.
   • cover skip and vessels
   • vacuum clean the moulding and casting shop in sand moulding foundries
   • clean wheels and roads
   • keep outside doors shut
   • carry out regular housekeeping
   • manage and control possible sources of fugitive emissions to water.
Additionally, fugitive emissions may arise from the incomplete evacuation of exhaust gas
from contained sources, e.g. emissions from furnaces during opening or tapping. BAT is to
minimise these fugitive emissions by optimising capture and cleaning. For this optimisation
one or more of the following measures are used, giving preference to the collection of fume
nearest to the source:

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   • hooding and ducting design to capture fume arising from hot metal, furnace charging,
     slag transfer and tapping
   • applying furnace enclosures to prevent the release of fume losses into the atmosphere
   • applying roofline collection, although this is very energy consuming and should only
     be applied as a last resort.

BAT for solid wastes
For solid wastes, the following techniques are considered BAT in descending order of
priority:
   • Minimisation of waste generation
   • Waste minimisation by recycling of EAF slags and filter dusts; depending on local
       circumstances filter dust can be recycled to the electric arc furnace in order to achieve
       a zinc enrichment up to 30%. Filter dust with zinc contents of more than 20% can be
       used in the non-ferrous metal industry.
   • Filter dusts from the production of high alloyed steels can be treated to recover
       alloying metals.
   • For solid wastes, which can not be avoided or recycled, the generated quantity should
       be minimised. If all minimisation/reuse is hampered, controlled disposal is the only
       option.

BAT for effluents
Emissions to water
  • Closed loop water cooling system for the cooling of furnace devices.
  • Wastewater from continuous casting
  • Recycling of cooling water as much as possible
  • Precipitation/sedimentation of suspended solids
  • Removal of oil in shimming tanks or any other effective device.

BAT for raw materials consumption
R&D opportunities regarding raw materials include needs of characterization and
understanding. The heat transfer coefficients for different scrap types and mixes, including
hot metal as a charge, need to be determined.
Better control of feed material quality is needed because many of the undesirable
components contained in EAF dust are contained in the scrap feed to the furnace. Though
scrap selection is primarily an economic consideration, treatment of scrap to eliminate the
transfer of undesirable materials into the EAF will likely become necessary in the future. The
use of lower grade fluxes and additives containing sulphur is also a concern if the sulphur is
not tied up in the slag and instead reports to the offgas stream.
Upgrading of purchased scrap is a way to increase raw material quality by controlling
residuals, including S, P, Sn, and Cu. Several chemical approaches to remove the copper that
is physically associated with junked cars (shredded scrap, #2 bundles) have been developed.
However, physical separation (shaking, magnets) seems preferable to any of the chemical
methods, all of which either create environmental problems (coping with H2S, chlorine) or
require auxiliary operations (molten aluminum bath). (Roadmap Steel Technology)
The problem of accidental melting of radioactive sources contained in scrap, primarily lost or
discarded shielded radioactive gauges, continues to be a concern. Some scrap sorting is
carried out to reduce the risk of including hazardous contaminants. Increasingly, metal
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consumers worldwide are demanding that suppliers provide certification that products are
free of radioactivity. To respond to the possibility of radioactive devices concealed in scrap
supply, steel mills, scrap metal recyclers and suppliers, aluminum processing facilities,
foundries, and even landfill operators and incinerator plants are installing sophisticated
radiation detection systems to monitor all incoming shipments of scrap. These systems are
capable of detecting very low energy, shielded radioactive sources deeply buried and
randomly positioned in a fully loaded truck or rail car of scrap metal.
Monitoring of scrap is typically done on the inbound approach to a weigh scale. Although
other locations are feasible, the installation of the detectors immediately in front of the scale
provides maximum protection to workers, and also the best opportunity to reject the scrap
before it enters the site. Typically, all incoming vehicles pass between portal detectors to
measure the gamma rays. The vehicle's speed is measured as it enters and leaves the
detectors gate allowing for the calculation of vehicle acceleration. Detection occurs when a
gamma photon strikes one of the portal detectors. The detector consists of a scintillator
(constructed of a PVT plastic impregnated with a chemical agent that emits light every time
it absorbs gamma energy) and a scintillation counter. A scintillation counter converts light
flashes into electrical impulses, which are then amplified and converted by a photomultiplier
tube into a signal that can be digitized and counted. What differentiates various commercially
available systems is the exact knowledge of what is being counted. The detector output is
analyzed for signatures characteristic of buried shielded radioactive source. If the system
determines that a source of radioactivity is present, it sounds an audio alarm and displays the
alarm information on a monitor indicating the location of any radioactive source.
Total radiation protection for a steel mill consists of:
    • Scale radiation detector system to detect radiation at the gate.
    • Shredder and conveyor monitors.
    • Furnace charge basket monitors to check scrap before it enters the furnace. Charge
        buckets may be monitored while loading or when fully loaded.
    • Baghouse dust radiation detectors.

Laboratory radiation detectors to analyze levels of radioactivity in products and to accurately
detect the associated level of contamination are used. Radiation detection facilities are
considered a quality control measure to prevent the inclusion of radioactive scrap. In the
United States, the steelmaking mills that use ferrous scrap most, if not all, perform radiation
monitoring of incoming ferrous scrap. (Vocilka 1998)

Sizing of scrap is important to maximizing bucket density and minimizing energy losses.
Proper scrap sizing limits the number of required recharges; thereby saving energy lost
during roof swings, and minimizes refractory damage due to impact of heavy pieces at
charge and flare from uneven charges. (Steel Industry Technology Roadmap 2001)
The physical preparation of scrap is important for efficient preheating and fast melting.
Furnace designs are not being tailored to optimize scrap handling, so a physically
homogeneous charge is desirable. To this extent, manufactured iron units are ideal. Iron
carbide can and possibly must be injected, and rates of several hundred pounds per minute
have been achieved. Although overall quantities may be limited, as injection cannot be used
throughout the entire process, alternate iron forms that can be used in the conventional scrap
buckets will enable regular use in current shops.
Additionally, continuous scrap feeding systems that eliminate top charging and its energy
losses have been developed.
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BAT for Refractory Recycling (Steel Industry Technology Roadmap 2001)
The annual production of the U.S. refractory industry is in excess of 3.3 million tons, worth
more than $2 billion. The single largest consumer of these materials is the steel industry,
which typically purchases about 50% by weight of the refractories produced annually. Most
of these refractories are high-value, non-clay refractories that are used as linings for various
steelmaking vessels.
When vessel linings can no longer be used, the spent lining is demolished and discarded, and
a new one is put in place. Depending on the particular application, refractory material may
last only a few hours or as long as several years. Spent refractory materials are disposed of in
a landfill or are recycled.
Many issues must be considered for the successful recycling and reuse of spent refractory
material. These include the type and quantity of spent refractories, the location of users and
producers of refractories; local, state, and federal regulations; health concerns;
contamination; value of materials (including worth of components and cost associated with
disposal); and economics of separation or beneficiation. In general, refractory recycling may
fit into a company’s overall recycling program and may follow the same progression as all
other recycling.
Many factors are driving the interest in recycling and reuse of spent refractory materials.
Foremost is the need to develop pollution prevention technologies for the iron and steel
industry that will improve efficiency, reduce costs, and ensure compliance with
environmental regulations.
Refractory recycling is complicated by the presence of varying amounts and types of
impurities within the used refractories, problems with sorting and removing unwanted
refractories, and problems with foreign objects being included with the refractories of
interest.
It is possible that the technology necessary for recycling spent refractory materials exists.
Technologies customarily used by the minerals processing industry have been recently
applied to spent refractories in isolated instances. Although these processing techniques are
not new, such economically viable and proven technologies merit further study in their
application to recycling spent refractory materials.
Previous efforts to recycle spent refractory materials have resulted in some high-value
refractory components, such as natural flake graphite, being reused in steelmaking
refractories. Entire or partial ladle linings may be reused many times by mechanically
removing surface slag or metal, then applying a new surface by vibration casting or hot
gunning. New techniques such as slag splashing and monolithic linings have also helped
reduce refractory wear and costs in BOF vessels.



BAT for materials handling and storage
Emissions from the raw materials handling operations are fugitive particulates generated
from receiving, unloading, storing, and conveying all raw materials for the foundry. These
emissions are controlled by enclosing the major emission points and routing the air from the
enclosures through fabric filters.



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Fugitive emissions of particulate are usually controlled by spraying stockpiles with water or
crusting agents and ensuring that roadways and vehicle wheels are kept clean. The water
runoff is usually directed to a wastewater treatment plant.
Inhouse generated scrap can be cut into handleable sizes using oxygen lancing. The scrap
may be loaded into charging baskets in the scrap-yard or may be transferred to temporary
scrap bays inside the melting shop. Other raw materials including fluxes in lump and powder,
powdered lime and carbon, alloying additions, deoxidants and refractories, are normally
stored under cover. Following delivery, handling is kept to a minimum and where
appropriate, dust extraction equipment may be used. Powdered materials can be stored in
sealed silos (lime should be kept dry) and conveyed pneumatically or kept and handled in
sealed bags.
BAT is to have an impermeable surface for scrap storage with a drainage collection and treatment
system. A roof can reduce or eliminate the need for such a system.

BAT for Refining and Casting
Refining
Ladle refining emissions, such as particulate (and Sox if sulfur bearing compounds are used)
emissions are typically collected in baghouses as air pollution control dust.
For vacuum degassing standard treatment includes processing the total recirculating flow, or
a portion of the flow, in clarifiers for TSS removal; cooling with mechanical draft cooling
towers; and high-rate recycle. Blowdowns are usually co-treated with teelmaking and/or
continuous casting wastewaters for metals removal.
Baghouse dusts are normally not hazardous and can be disposed of in a conventional landfill
or processed and recycled. However, particulate wastes collected from a vacuum degassing
operation may contain enough lead to be characterized as hazardous, and, if so, must be
disposed of or recycled as such.
The industry anticipates continued pressure from regulatory agencies to minimize the
generation and disposal of hazardous waste. This pressure, along with shortages of landfill
space and the additional cost of disposing of hazardous wastes, will serve as incentives for
companies to reduce hazardous waste generation and recycle more of these wastes. (Steel
Technology Roadmap 2001)

Casting
The major emissions generated during ingot casting are controlled by collection devices.
Operational changes in ingot casting, such as bottom pouring instead of top pouring, can
reduce emissions.
The particulate emissions from the casting shakeout operations can be controlled by either
high efficiency cyclone separators or bag filters.
Wastewater treatment includes scale pits for mill scale recovery and oil removal, mixed- or
single-media filtration, and high-rate recycle. The wastewater produced from the direct
cooling of continuous casting is usually treated together with other streams from the rolling
mill(s).
Scale from casting usually recycled within the steelmaking facility at integrated mills that
operate sinter plants. Scale may also be landfilled (particularly by stainless steel producers)
or even charged to an electric arc furnace.
Sludge generated during continuous casting is either processed and recycled on-site or
landfilled. Drivers to increase the use of these caster wastes include the following:
    • Increased pressure for waste minimization and pollution prevention programs
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   •   Scarcity of landfill space and increasing cost of landfill disposal. (Steel Technology
       Roadmap 2001)

BAT for hot rolling (Ferrous Metals Processing Industry Part A/Chapter 5)
For storing and handling of raw materials and auxiliaries the following techniques are
considered to be BAT:
• Collection of spillages and leakages by suitable measures, e.g. safety pits and drainage.
• Separation of oil from the contaminated drainage water and reuse of recovered oil.
• Treatment of separated water in the water treatment plant.
In general, the best way to reduce the environmental impact from surface rectification and
conditioning of input is to avoid the need for rectification. The improvement of surface
quality of cast products to reduce the need for surface rectification is therefore considered
BAT.
Furthermore, the following measures were identified as BAT for surface rectification and
conditioning of input:
For machine scarfing:
     • Enclosures for machine scarfing and dust abatement with fabric filters.
There was agreement that this technique constitutes BAT, but there were different opinions
on the associated emission level and the TWG group that worked on the BREF document
recorded a split view. One plant reported achieved dust emission levels of 5 - 10 mg/m³.
Some Member States argued (without supporting data for this type of installation) that fabric
filters in general can achieve below 5 mg/Nm³ and that this is the level that should be
associated with BAT. Others pointed out that < 20 mg/Nm³ is the appropriate level.
     • The use of an electrostatic precipitator, where fabric filters cannot be operated
         because of very wet fume.
There were no dust emission data available for individual plants, but reported current
emission levels ranged from < 20 mg/Nm³ to 20 - 115 mg/m³. Based on information
submitted by TWG members on generally achievable dust levels for electrostatic
precipitators in the application of oxide and dust removal in the FMP sector, an associated
dust level of 15 – 20 mg/Nm³ was proposed by the EIPPCB. Interventions were made by an
industrial NGO that the BAT-associated level was 20 - 50 mg/m³; while Member States
claimed that generally achievable levels of electrostatic precipitators are < 10 mg/Nm³ and
that this should be the BAT-associated emission level. The TWG was unable to reach
agreement on the BAT-associated level and a split view was recorded.
     • Separate collection of scale/swarf from scarfing. The oil-free scale should be kept
         apart from oily mill scale for easier reuse in metallurgical processes.
For grinding:
     • Enclosures for machine grinding and dedicated booths, equipped with collection
         hoods for manual grinding and dust abatement for the extracted air by fabric filters.
There was consensus among TWG members that these techniques constitute BAT, but no
agreement was reached as to what the associated emission level is. Emission data taken from
various sources lead to a reported current dust emission range for grinding of 1 – 100 mg/m³.
Industry reported data for the application of fabric filters with resulting dust levels of < 30
mg/Nm³ and 20 – 100 mg/Nm³ (for different filter types). Taking into account the better
range of the reported emission levels and the information submitted by TWG members on
generally achievable dust levels for fabric filters4 in the application of oxide and dust
removal in the FMP sector, a BAT-associated level of < 20 mg/Nm³ was proposed.

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Some Member States opposed, saying (based on very limited data) that fabric filters in
general can achieve below 5 mg/Nm³ and that this should be the BAT-associated level.

Aditionally, for all surface rectification processes:
   • Treatment and reuse of water from all surface rectification processes (separation of
       solids).
   • Internal recycling or sale for recycling of scale, swarf and dust.

Air emissions from reheating and heat treatment furnaces basically comprise NOx, SO2 and
dust.
For dust, no specific abatement measures are applied. Generally, dust emissions are in the
range of 4 – 20 mg/m³, but figures as low as 2.2 mg/Nm³ have been reported.
For reducing air emissions, especially NOx, from reheating and heat treatment furnaces and
to reduce the energy consumption, the general measures described in Chapter A.4.1.3.1 of
the BREF document on Ferrous Metal Processing should be taken into account at the design
stage. Special attention should be paid to energy efficiency and waste heat recovery, e.g by
adequate furnace insulation, insulation of skids, adequate stock recuperation zone etc, and to
air emission reduction, e.g. by choice of burners and placement of burners.
Additionally, the following measures, which can also be applied to existing furnaces, are
considered BAT for reheating and heat treatment furnaces:
    • Avoiding excess air and heat loss during charging by operational measures (minimum
        door opening necessary for charging) or structural means (installation of multi-
        segmented doors for tighter closure).
    • Careful choice of fuel (in some cases, e.g. coke oven gas, desulphurisation maybe
        necessary) and implementation of furnace automation and control to optimise the
        firing conditions in the furnace. Depending on the fuel used, the following SO2 levels
        are associated with BAT:
             o − for natural gas < 100 mg/Nm3
             o − for all other gases and gas mixtures < 400 mg/Nm3
             o − for fuel oil (< 1 % S) up to 1700 mg/Nm³
There was a split view in the TWG on whether the limitation of < 1 % sulphur content in fuel
oil can be considered as BAT. Some experts considered this limit enough to be BAT, whilst
others expressed the view that the resulting emissions of up to 1700 mg SO2/Nm³ cannot be
regarded as such. They considered a lower S content or the application of additional SO2
reduction measures to be BAT.
        • Recovery of heat in the waste gas-
                o by feedstock preheating
                o by regenerative or recuperative burner systems
                o by waste heat boiler or evaporative skid cooling (where there is a need for
                    steam)

Energy savings of 40 - 50 % can be achieved by regenerative burners, with reported NOx
reductions potentials of up to 50 %. Energy savings associated with recuperators or
recuperative burners are about 25 %, with reported achievable NOx reductions of about 30 %
(50 % in combination with low-NOx burners).
    • Second generation low-NOx burners with associated NOx emission levels of 250 -
       400 mg/Nm³ (3 % O2) without air preheating and reported NOx reduction potential

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       of about 65 % compared to conventional burners. It should be noted that in evaluating
       the efficiency of NOx reduction measures it is important to also pay attention to
       specific emission levels, not only to the achieved concentration. In some cases, NOx
       concentrations may be higher, but the NOx mass emitted may be equal or even lower.
       Unfortunately, the figures available at present for NOx concentrations and specific
       NOx emissions are very limited.
   Reheating furnaces do not operate in stable conditions during start-up and shut-down;
   during these phases the emission levels may increase.
   • Limiting the air preheating temperature.
       Higher NOx concentrations may arise in the case of reheating furnaces operating with
       combustion air preheating. Only very limited data were submitted on NOx
       concentrations in connection with air preheating. The following data, taken from
       available UK reports, give an indication of the NOx emission levels that may be
       expected with increasing air preheating temperature:




Table 10: NOx emissions


With increasing air-preheating temperature, a significant rise in NOx concentrations is
inevitable. Thus, limiting the preheating temperature may be seen as a NOx reduction
measure. However, the advantages of reduced energy consumption and reductions in SO2,
CO2 and CO have to be weighed against the disadvantage of potentially increased emissions
of NOx.
Regarding further NOx reduction measures, information on actual application of SCR and
SNCR at reheating furnaces was received at a very late stage of the work. It was confirmed
that one plant is applying SCR at its walking beam furnaces, achieving below 320 mg/Nm3
with a reduction rate of about 80 % and that another plant has installed SNCR after its
walking beam furnaces achieving NOx levels of 205 mg/Nm3 (~ 70 % reduction rate) and
172 mg/Nm3 (~30 % reduction rate) with an ammonia slip of 5 mg/Nm3.
Based on this information, some members of the TWG stated that these techniques are BAT
for the sector as a whole; while other members thought the available information on technical
details and on economics was not sufficient enough to allow for a final decision on whether
SCR and SNCR are BAT or not and therefore a split view was recorded.
Furthermore, the following measures to minimize the energy requirements are considered to
be BAT:
      • Reduction of heat loss in intermediate products; by minimizing the storage time and
          by insulating the slabs/blooms (heat conservation box or thermal covers) depending
          on production layout.



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      •    Change of logistic and intermediate storage to allow for a maximum rate of hot
          charging, direct charging or direct rolling (the maximum rate depends on
          production schemes and product quality).
For new plants, near-net-shape casting and thin slab casting are considered BAT, to the
extent that the product to be rolled can be produced by this technique. A great variety of
qualities are already produced by these techniques and rapid developments are taking place.

In reducing water and energy consumption, material tracking is considered BAT for
descaling.
Large amounts of heat contained in continuous cast products or in intermediate products are
lost during handling and storage. To reduce unwanted energy loss during transport of rolled
stock from roughing mill to finishing train, coil boxes or coil recovery furnaces and heat
shields for transfer bars are considered to be the best available techniques, although a
potentially higher risk of surface defects (rolled-in scale) and potential damages caused by
curled transfer bars was reported for heat retention shields. Coil boxes may also result in
increased surface defects.
During rolling in the finishing train fugitive emissions of dust occur. Two techniques have
been identified as BAT for the reduction of these emissions:
            • Water sprays followed by waste water treatment in which the solids (iron
                oxides) are separated and collected for reuse of iron content.
            • Exhaust systems with treatment of extracted air by fabric filters and recycling
                of collected dust.
The reported current dust emission level ranged from 2 – 50 mg/Nm3. Taking into account
the better range of the reported emisson levels and the information submitted by TWG
members on generally achievable dust levels for fabric filters7 in the application of oxide and
dust removal in the FMP sector, a BAT associated level of < 20 mg/Nm³ was proposed.
Some Member States opposed, saying (not supported by data) that fabric filters in general
can achieve below 5 mg/Nm³ and that this should be the BAT-associated level. The TWG
was unable to reach agreement on the BAT associated level and a split view was recorded.
For tube mills, collection hoods and fabric filters for fugitive emissions from rolling stands
are not considered BAT, due to low rolling speeds and resulting lower emissions.
For reducing fugitive dust emissions from levelling and welding, suction hoods and
subsequent abatement by fabric filters was identified as BAT. There were no emission data
available for levelling and welding, but following the general approach on what is achievable
by fabric filters (see above) a BAT-associated dust level of < 20 mg/Nm³ was proposed.
Some Member States expressed the view (without supporting data) that fabric filters in
general can achieve below 5 mg/Nm³ and that this should be the BAT-associated level. The
TWG was unable to reach agreement on the BAT associated level and a split view was
recorded.
Transfer bars are considered to be the best available techniques, although a potentially higher
risk of surface defects (rolled-in scale) and potential damages caused by curled transfer bars
was reported for heat retention shields. Coil boxes may also result in increased surface
defects.


Best available operational and maintenance techniques for roll shops are:
   • Use of water-based degreasing as far as technically acceptable for the degree of
       cleanliness required.
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   •    If organic solvents have to be used, preference is to be given to non-chlorinated
        solvents.
    • Collection of grease removed from roll trunnions and proper disposal, such as by
        incineration.
    • Treatment of grinding sludge by magnetic separation for recovery of metal particles
        and recirculation into the steelmaking process.
    • Disposal by incineration of oil- and grease-containing residues from grinding wheels
    • Deposition of mineral residues from grinding wheels and worn grinding wheels in
        landfills.
    • Treatment of cooling liquids and cutting emulsions for oil/water separation. Proper
        disposal of oily residues, e.g. by incineration.
    • Treatment of waste water effluents from cooling and degreasing as well as from
        emulsion separation in the hot rolling mill water treatment plant.
    • Recycling of steel and iron turnings into the steelmaking process.
    • Recycling of worn rolls which are unsuitable for further reconditioning, into the
        steelmaking process or returned to the manufacturer.
For cooling (machines etc.) separate cooling water systems operating in closed loops are
considered BAT.
Hot rolling leads to a large amount of scale- and oil-containing process water. The
minimization of consumption and discharge by operating closed loops with recirculating
rates of > 95 % is considered BAT.
Treatment of this process water and pollution reduction in the effluent from these systems is
considered BAT. The following release levels from the wastewater treatment are associated
with BAT:
SS: < 20 mg/l
Oil: < 5 mg/l (oil based on random measurements)
Fe: < 10 mg/l
Crtot: < 0.2 mg/l (for stainless steel < 0.5 mg/l)
Ni: < 0.2 mg/l (for stainless steel < 0.5 mg/l)
Zn: < 2 mg/l
As the volume and contamination of wastewater from tube mills are quite similar to other hot
rolling operations, it was noted that the same techniques and the same associated BAT levels
apply for tube mills.
Recirculation to the metallurgical process of mill scale collected in water treatment is BAT.
Techniques are described in Chapter A.4.1.13.2.of the BREF Ferrous Metals Processing
document. Depending on oil content, additional treatment may be required. All oily
waste/sludge collected should be de-watered to allow for thermal utilisation or safe disposal.
Throughout the plant the following techniques for prevention of hydrocarbon contamination
of water have been identified and are considered to be BAT:
    • Preventive periodic checks and preventive maintenance of seals, gaskets, pumps and
        pipelines.
    • Use of bearings and bearing seals of modern design for work- and back-up rolls as
        well as the installation of leakage indicators in the lubricant lines (e.g. at hydrostatic
        bearings). This reduces the oil consumption by 50 - 70 %.
    • Collection and treatment of contaminated drainage water at the various consumers
        (hydraulic aggregates), separation and use of oil fraction, e.g. thermally utilized by


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       blast furnace injection. Further processing of the separated water either in the water
       treatment plant or in dressing plants with ultra filtration or vacuum evaporator.


BAT for cold rolling
At the entry side of pickling lines, decoiling of the hot rolled strip leads to fugitive dust
emissions. For the reduction of these emissions two techniques have been identified as BAT:
       • Water curtains followed by waste water treatment in which the solids are separated
           and collected for reuse of iron content.
       • Exhaust systems with treatment of extracted air by fabric filters and recycling of
           collected dust.
There were no emission data available for decoiling, but following the general approach on
what is achievable by fabric filters (see above), a BAT-associated dust level of < 20 mg/Nm³
was proposed. Some Member States expressed the view (without supporting data) that fabric
filters in general can achieve below 5 mg/Nm³ and that this should be the BAT-associated
level. The TWG was unable to reach agreement on the BAT associated level and a split view
was recorded.
To reduce the environmental impact from pickling, general measures to reduce acid
consumption and waste acid generation should be applied as far as possible and be
considered preferably already at design stage, especially the following techniques that are
considered BAT:
       • Prevention of steel corrosion by appropriate storage and handling, cooling etc.
       • Mechanical pre-descaling to reduce the load on the pickling step. If mechanical
           descaling is applied, BAT is a closed unit, equipped with an extraction system and
           fabric filters. For shot blasting, dust emission levels of < 1 mg/Nm³, 2.6 mg/Nm³
           and 4.5 mg/Nm³ have been achieved.
       • Use of electrolytic pre-pickling.
       • Use of modern, optimised pickling facilities (spray or turbulence pickling instead of
           dip pickling).
       • Mechanical filtration and recirculation for lifetime extension of pickling baths.
       • Side-stream ion-exchange or electro-dialysis (for mixed acid) or other method for
           free acid reclamation for bath regeneration.

For HCl pickling, BAT is considered to be:
      • The reuse of spent HCl or
      • The regeneration of the acid by spray roasting or fluidised bed (or equivalent
         process) with recirculation of the regenerate to the pickling process is considered
         BAT.
Depending on site circumstances, the high acid consumption and amounts of waste acid
generated and the savings generally obtained from regeneration may justify the investment in
a regeneration plant. The acid regeneration plant needs to be equipped with an air scrubbing
system as described in Chapter 4, to reduce emissions, especially acid emissions. Achievable
reduction efficiencies of > 98 % were reported. Some sources report achieved HCl
concentrations by applying caustic scrubbing of < 2 mg/Nm³. The TWG agreed that the
following emission levels are associated with acid regeneration (waste gas treatment by
scrubbers or adsorption towers):
Dust 20 -50 mg/Nm³

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HCl 2 – 30 mg/Nm³
SO2 50 - 100 mg/Nm³
CO      150 mg/Nm³
CO2 180000 mg/Nm³
NO2 300 - 370 mg/Nm³
The recovered solid by-product Fe2O3 is a saleable product and is externally reused.
For H2SO4 pickling processes, recovery of the free acid by crystallisation is considered
BAT.
The recovery plant needs to be equipped with air scrubbing devices; emission levels
associated with this process are:
H2SO4          5 - 10 mg/Nm³
SO2            8 - 20 mg/Nm³
For mixed acid pickling, free acid reclamation (e.g. by side-stream ion exchange or dialysis)
or acid regeneration (e.g. by spray roasting or evaporation process) is considered BAT.
While free acid reclamation is applicable to virtually all plants, the applicability of
regeneration processes may be limited for site-specific reasons. The emissions associated
with BAT are:




Table 11: Free acid reclamation emissions
All three processes are equally considered BAT. Despite the disadvantage of higher air
emissions and energy consumption, spray roasting was selected because of its high acid
recovery rate and associated low fresh acid consumption. Furthermore the waste water is
only a fraction of that produced by reclamation processes. Metals are basically bound in a
solid byproduct.
This mixed iron-chromium- nickel oxide can be reused in metal production.
The evaporation process also provides a very high acid recovery rate and thus low fresh acid
consumption, but with much lower energy consumption than spray roasting. The metal
sulphate filter cake, however, needs to be disposed of.
For the reduction of air emissions from the pickling tanks, totally enclosed equipment or
equipment fitted with hoods and scrubbing of extracted air are considered BAT with
associated emission levels:
HCl pickling: Dust 10 - 20 mg/Nm³
HCl 2 – 30 mg/Nm³ (reduction efficiency > 98 %)
H2SO4 pickling: H2SO4 1- 2 mg/Nm³
SO2 8 - 20 mg/Nm³ (reduction efficiency > 95 %)
For mixed acid pickling of stainless steel, in addition to enclosed equipment/hoods and
scrubbing, further NOx reduction measures are required. The following techniques are
considered to be BAT:
      • Scrubbing with H2O2, urea etc.; or
      • NOx suppression by adding H2O2 or urea to the pickling bath; or

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      • SCR.
Emission levels of 200 - 650 mg/Nm³ for NOx (reduction 75 - 85 %) and 2 – 7 mg/Nm³ for
HF (reduction 70 - 80 %) are associated with these techniques. Some sources reported
achievable emission levels for HF of < 2 mg/Nm³, but as there was some recognition of
difficulties in measuring HF, especially at low levels, it was concluded that the BAT-
associated level is the range given above.
As an alternative, implementation of nitric acid-free pickling (e.g. H2O2 based) with
enclosed equipment or equipment fitted with hoods and scrubbing is considered BAT.
However, this technique is not applicable to all applications.
For heating of acids the direct injection of steam is not considered BAT as it leads to
unnecessary dilution of the acid. BAT is indirect heating by heat exchangers or, if steam for
heat exchangers has to be produced first, by submerged combustion.
The following measures have been identified as BAT for the minimization of acidic waste
water:
      • Cascade rinsing systems with internal re-use of overflow (e.g. in pickling baths or
           scrubbing).
      • Careful tuning and managing of the ‘pickling-acid regeneration-rinsing’ system.
           Some sources report a possible waste water-free operation.
      • Ιn any case where acidic water blow-down from the system cannot be avoided,
           waste water treatment is required (neutralisation, flocculation, etc.).
Associated release levels of the waste water treatment are:
SS: < 20 mg/l
Oil: < 5 mg/l (oil based on random measurements)
Fe: < 10 mg/l
Crtot:< 0.2 mg/l (for stainless steel < 0.5 mg/l)
Ni: < 0.2 mg/l (for stainless steel < 0.5 mg/l)
Zn: < 2 mg/l
There was agreemant in the TWG that there are exceptional cases for stainless steel where
the levels of Crtot and Ni cannot be kept below 0.5 mg/l.
For emulsion systems the following techniques are considered to be BAT:
      • Prevention of contamination by regular checking of seals, pipework etc. and
           leakage control.
      • Continuous monitoring of emulsion quality.
      • Operation of emulsion circuits with cleaning and reuse of emulsion to extend
           lifetime.
      • Treatment of spent emulsion to reduce oil content, e.g. by ultrafiltration or
           electrolytic splitting.
During rolling and tempering, fugitive emissions of emulsion fumes occur. To capture and
reduce these emissions the best technique available is the installation of an exhaust system
with treatment of extracted air by mist eliminators (droplet separator). Reduction efficiencies
achieved are > 90 % and associated emission levels of hydrocarbons 5 - 15 mg/Nm³.
For installations operating with a degreasing step, the following techniques are considered
BAT:
      • Implementation of a degreasing circuit with cleaning and reuse of the degreaser
           solution.
      • Appropriate measures for cleaning are mechanical methods and membrane
           filtration as described in Chapter A.4.

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      •   Treatment of spent degreasing solution by electrolytic emulsion splitting or
          ultrafiltration to reduce the oil content. The separated oil fraction should be reused,
          e.g. thermally; the separated water fraction requires treatment (neutralisation etc.)
          prior to discharge.
       • Extraction system to capture degreaser fume and scrubbing of extracted air.
The main environmental issues for annealing furnaces are air emissions from combustion
processes and efficient energy use. The best available techniques to reduce emissions at
continuous annealing furnaces are low-NOx burners with reduction rates of 60 % for NOx
(and 87 % for CO) and with an associated emission level of 250 – 400 mg/Nm³ (without air
preheating, 3 % O2). The NOx emission level for batch annealing furnaces without the
application of low-NOx burners and without air preheating is in the range of 150 – 380
mg/Nm³ (without air preheating, 3 % O2). Generally the emissions levels to expect from
annealing furnaces are:
Batch Furnaces Continuous Furnaces
Dust 5 - 10 10 - 20 mg/Nm³.
SO2 60 - 100 50 - 100 mg/Nm³.
NOx 150 - 380 250 - 400 mg/Nm³.
CO 40 - 100 50 - 120 mg/Nm³.
CO2 200000 - 220000 180000 - 250000 mg/Nm³.
Oxygen reference level 3 %
The best available measures to increase the energy efficiency are:
Combustion air preheating by regenerative or recuperative burners.
Higher NOx concentrations may arise in the case of annealing furnaces operating with
combustion air preheating. No data was submitted on NOx concentrations in connection with
air preheating, but the figures given for reheating furnaces may serve as an indication.
Limiting the preheating temperature may be seen as a NOx reduction measure. However, the
advantages of reduced energy consumption and reductions in SO2, CO2 and CO have to be
weighed against the disadvantage of possible increased emissions of NOx.
       • Preheating of stock by waste gas. Or
       • For finishing, the steel strip may be oiled for protection; this can lead to oil mist
          emissions.
The best techniques to reduce these emissions are:
       • Extraction hoods followed by mist eliminators and/or electrostatic precipitators.
          Data submitted for one plant showed an achieved average oil droplet concentration
          of 3.0 mg/Nm3 applying mist eliminator and electrostatic precipitator. Or
       • Electrostatic oiling.
Further finishing operations, levelling and welding, generate fugitive dust emissions. BAT to
reduce these emissions are extraction hoods with dust abatement by fabric filters. Emission
data available from one plant range from 7 – 39 mg/Nm³; data from another plant (part time
operation) from 5 – 30 mg/Nm³. Taking into account the better range of the reported
emission levels and the information submitted by TWG members on generally achievable
dust levels for fabric filters in the application of oxide and dust removal in the FMP sector, a
BAT-associated level of < 20 mg/Nm³ was proposed. Some Member States opposed, saying
(not supported by data) that fabric filters in general can achieve below 5 mg/Nm³ and that
this should be the BAT associated level. The TWG was unable to reach agreement on the
BAT associated level and a split view was recorded.


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For cooling (machines etc.), separate cooling water systems operating in closed loops are
considered BAT.
For the roll shops of cold rolling mills the same principles as for roll shops in hot rolling
mills are applicable.
Metallic by-products, scrap from cutting, heads and tails are collected at different stages in
the rolling mill.
      • Collection and recirculation into the metallurgical process is BAT.
      • Use dry dust collection methods such as fabric filters.
      • Replace ingot teeming with continuous casting.
      • Develop continuous scrap feeding systems that eliminate top charging and its
          energy losses.
      • Reduce nitrogen oxide (NOx) emissions by use of natural gas as fuel, use low-NOx
          burners • Use acid-free methods (mechanical methods such as blasting) for
          descaling, where feasible.
      • In the pickling process, use countercurrent flow of rinse water; use indirect
          methods for heating and pickling baths.
      • Use closed-loop systems for pickling; regenerate and recover acids from spent
          pickling liquor using resin bed, retorting, or other regeneration methods such as
          vacuum crystallization of sulfuric acid baths.
      • Use electrochemical methods in combination with pickling to lower acid
          consumption.
      • Use hydrogen peroxide and urea in stainless steel pickling baths.
(Mini Steel Mills Pollution Prevention and Abatement Handbook WORLD BANK GROUP
Effective July 1998


                                       REFERENCES
  •    Best Available Techniques Reference Document on the Production of Iron and Steel,
       Integrated Pollution Prevention and Control (IPPC), European Commission, December
       2001
  •    Reference Document on Best Available Techniques in the Ferrous Metals Processing
       Industry Integrated Pollution Prevention and Control (IPPC) European Commission
       December 2001
   •    Prof. Dr. O. Rentz Dipl.-Ing. Stephan Hähre, Dipl.-Wirtschaftsing. Rainer Jochum,
        Dr. Thomas Spengler Karlsruhe Report on Best Available Techniques (BAT) in the
        Electric Steelmaking Industry, Final Draft, French-German Institute for
        Environmental Research, University of Karlsruhe (TH), June 1997 On behalf of the
        German Federal Environmental Agency, Berlin (UBA) in the frame of the Research
        Project 109 05 006
   •    Best Available Techniques For Foundries Chapter 5 Kv/Eippcb/Sf_Bref_Final July
        2004
   •    Lynn Price, Dian Phylipsen, Ernst Worrell, Energy Use and Carbon Dioxide
        Emissions in the Steel Sector in Key Developing Countries, Environmental Energy
        Technologies Division, Ernest Orlando Lawrence Berkeley National Laboratory,
        April 2001
   •    Steel Mini-mills AP42 12.5.1 www.epa.gov/ttn/chief/ap42/ch12/final/c12s0501.pdf -


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•   Iron And Steel Production 12.5-24 (Reformatted 1/95) 10/86 Metallurgical Industry
    http://www.epa.gov/ttn/chief/ap42/ch12/final/c12s05.pdf
•   Vocilka Michael G. Before It Gets Through Radiation Monitoring, Scrap Metal
    Recycling Archives Articles, Recycling Technology Newsletter, Canada Centre for
    Mineral and Energy Technology (CANMET) Mining and Mineral Sciences Laboratories
    December 1998 http://www.nrcan.gc.ca/ms/canmet-mtb/mmsl-lmsm/rnet/scrapare.htm
•   U.S. Geological Survey, Mineral Commodity Summaries, February 1997
•   U.S. Environmental Protection Agency Energy Trends in Selected Manufacturing
    Sectors: Opportunities and Challenges for Environmentally Preferable Energy
    Outcomes Final Report March 2007
•   USEPA. "Profile of the Iron and Steel Industry." EPA/310-R-95-010, U.S.
    Environmental Protection Agency. Washington, D.C., September 1995
•   U.S. Environmental Protection Agency Energy Trends in Selected Manufacturing
    Sectors: Opportunities and Challenges for Environmentally Preferable Energy
    Outcomes Final Report March 2007
•   World Steel in Figures 2006 International Iron and Steel Institute 4 September 2007
•   Primary Metals, North Carolina Division of Pollution Prevention and Environmental
    Assistance (DPPEA), North Carolina Department of Environment and Natural
    Resources (NCDENR), 2001-07-25 www.p2pays.org/ref/01/text/00778/chapter2.htm
•   European Confederation of Iron and Steel Industries www.eurofer.org
•   Surface Combustion, Ferrous Scrap Preheating System Phase III-Final Report,
    Prepared for U.S. Department of Energy, Washington, D.C. May 13, 1996
•   BACKGROUND REPORT AP-42 SECTION 12.13 STEEL FOUNDRIES Prepared for U.S.
    Environmental Protection Agency OAQPS/TSD/EIB Research Triangle Park, NC 27711
•   Steel Industry Technology Roadmap, Industrial Technologies Program, Energy
    Efficiency and Renewable Energy (EERE) U.S. Department of Energy




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                     ZINC




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    The purpose of this report, in the framework of the PREWARC project, PL 517574, is to present
feasible preventive and remedial technologies that can be adopted by the mining and metallurgical
industries during production as well as during waste management during zinc production.

    1. GENERAL INFORMATION
Zinc is the fourth most popular metal in world production-in terms of tonnage produced-
being exceeded only by iron, aluminium, and copper.
It has a relatively low melting point and is brittle at ordinary temperatures but malleable at
100 to 150°C. It is a reasonable conductor of electricity, and burns in air at high red heat with
evolution of white clouds of the oxide.
Zinc is a necessary element for proper growth and development of humans, animals, and
plants; it is the second most common trace metal, after iron, naturally found in the human
body.
Zinc minerals are generally associated with other metals minerals, the most common
associations in ores being zinc-lead, lead-zinc, zinc-copper, copper-zinc, or zinc-silver. Zinc
also occurs alone in ores and the primary source is the mineral sphalerite (zinc iron sulphide,
ZnS), which is the source of about 90% of zinc produced today. Zinc can also be recovered
from six additional minerals, including hemimorphite, smithsonite, zincite, hydrozincite.
Secondary raw materials such as galvanising residues (ashes, skimmings, sludges etc), flue
dust from steel plants and brass processing and die-casting scrap are also sources of zinc.
Zinc is fully recyclable – it can be recycled indefinitely, without loss of its physical or
chemical properties. The recycling of zinc and zinc containing products is a key issue for the
industry.
End-uses include a wide range of applications, the most important being steel protection
against rust for the automobile, appliance and building industries. Centuries before zinc was
recognized as a distinct element, zinc ores were used for making brass (a mixture of copper
and zinc). Zinc alloys (e.g. brass, bronze, die casting alloys) and zinc semis are respectively
the second and third major consumption areas with applications also in the building,
appliance and car industries.
It can be easily applied (metallurgically bonded) to the surface of other metals such as steel
(galvanising) and when it is used as a metal coating, zinc corrodes preferentially as a
sacrificial coating. Zinc is also used in the pharmaceutical, nutrient, construction, battery and
chemical industries.




Table 1: World and European uses of zinc.




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Figure 1: Zinc demand by first use 2003
Zinc is supplied to the market in various qualities; the highest quality is special high grade
(SHG) or Z1 which contains 99.995% zinc, while the lowest quality good ordinary brand
(GOB) or Z5 is about 98% pure. Extrusion products, such as bars, rods and wires (mainly
brass); rolling products such as sheets and strips; casting alloys; and powders and chemical
compounds, such as oxides are produced.




Table 2: Primary zinc grades




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Table 3: Secondary zinc grades
In 2006, demand for refined zinc metal significantly exceeded supply for the third year in
succession. Preliminary figures indicate that the deficit in the Western World was 315kt and
for world as a whole 332kt. The International Lead and Zinc Study Group continues to
project a significant global zinc deficit. China consumes more than 25% of the world’s zinc
each year and experienced a 13% growth for 2005 in zinc consumption.
World zinc mine production rose by 3.5%, driven primarily by increases in China, India and
Kazakhstan. (International Lead and Zinc Study Group)
                     ZINC SUPPLY AND DEMAND 2002- 2006
000 tonnes                2002      2003      2004    2005 2006p Change 2005-
                                                                       06
WESTERN                                                                          %
WORLD
Mine Production*          6465 6652 6500 6691 6724                         33     0.5
Metal Production          6663 6652 6671 6494 6561                         67     1.0
Metal Usage               7108 7123 7420 7093 7382                        289     4.1
TOTAL WORLD
Mine Production*          8892 9520 9733 10110 10462                      352     3.5
Metal Production          9704 9879 10353 10228 10732                     504     4.9
Metal Usage               9368 9837 10654 10640 11064                     424     4.0
 p: preliminary *: zinc content

Table 4: Zinc supply and demand Source: ILZSG


          WESTERN WORLD REFINED ZINC METAL BALANCE
000 tonnes          2002     2003     2004     2005                           2006p
Metal Production    6663     6652     6671    6494                             6561
Net Imports from     760      669      471      258                             477
East
US Stockpile           3        7       32       29                               28
Disposals
Metal Usage         7108     7123     7420    7093                             7382
Balance              318      205     -247     -312                            -315
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p: preliminary \


Table 5: Western World Refined Zinc Metal Balance Source: ILZSG




Figure 2: Zinc consumption per person
Currently base metal prices are low. In many cases, the mineral deposits are relatively
complex from a processing point of view. These two factors, combined with the high labour
costs in Europe, have led to some temporary and some final closures of mines.
Zinc is mined in more than 50 countries with Australia, Canada, China, Peru and the U.S.A.
being the leading producers.
The EU mine output is essentially accounted for by Ireland and Spain; however, production
has fallen as a result of the exhaustion of reserves and the lower ore grades at some mining
operations. The EU is the major consumer area for zinc among the market economy
countries. However, there is a big imbalance between European mine production and the
European consumption of these metals.
The countries from Central and Eastern Europe, including Bulgaria and Romania, account
for 14% of the lead and 13% of the zinc production. Bulgaria’s and Romania’s joining the
EU is expected to slightly decrease the dependence of the Union on the imports of zinc.




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Figure 3: Largest producing and using countries 2002
Identified zinc resources of the world are about 1.9 billion tons. (U.S. Geological Survey
2007)
Approximately 70% of the zinc produced worldwide originates from mined ores and 30%
from recycled or secondary zinc. The level of recycling is increasing each year, in step with
progress in the technology of zinc production and zinc recycling. Today, over 80% of the
zinc available for recycling is indeed recycled. Metal production from secondary sources
accounted for more than 8% of the total EU refined zinc output in 1994.
Zinc is processed through either of two primary processing methods, electrolytic or
pyrometallurgical. However, before either method, zinc concentrate is roasted to remove the
sulfur from the concentrate and produce impure zinc oxide referred to as roasted concentrate
or calcine.




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Figure 4: Zinc Flow-Sheet (The Metallurgical Society Metallurgists Canadian Institute of Mining,
Metallurgy and Petroleum.
In electrolytic zinc processing, the calcine is digested with sulfuric acid to form a zinc sulfate
solution, from which zinc is deposited through electrolytic refining. In pyrometallurgical
processing, calcine is sintered and smelted in batch horizontal retorts, externally-heated
continuous vertical retorts, or electrothermic furnaces.

    2. Zinc mining
Eighty percent (80%) of zinc mines are underground, eight percent (8%) are of the open pit
type and the remainder is a combination of both. However, in terms of production volume,
open pit mines account for as much as 15%, underground mines produce 64% and 21% of
mine production comes from the combined underground and open pit mining. (International
Zinc Association)
Interest in non-sulphide zinc deposits has increased in recent years. Deposits of this type tend
to be fairly high grade.
Zinc ores contain 5 -15% zinc, therefore the volume of solid waste generated, including
tailings from processing, is one of the main pollution concerns in the mining industry.
Removal of overburden to access the ore can pose major problems in storage and
reclamation. The overburden (waste-to-ore) ratio for surface mining of metal ores generally

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ranges from 2:1 to 8:1, depending on local conditions. The ratio for solid wastes from
underground mining is typically 0.2:1.
The primary waste generated, mine development rock, is usually disposed of in waste rock
dumps or used in onsite construction for road or other purposes.
The waste-rock stacked on heaps can have a high environmental impact if it has a net acid
generating potential. The sulphide in tailings and waste-rock can oxidise when water and air
have access and an acidic leachate is generated. This phenomenon is called Acid Rock
Drainage (ARD). ARD is associated with sulphide ore bodies mined for Pb, Zn, Cu, Au, and
other minerals.
The key issues that are the root of these environmental problems are:
     • tailings and/or waste-rock often contain metal sulphides
     • sulphides oxidise when exposed to oxygen and water
     • sulphide oxidation creates an acidic metal-laden leachate
     • leachate generation over long periods of time.
When sulphide minerals come into contact with water and oxygen they start to oxidise. This
is a slow heat generating process (kinetically controlled exothermal process) which is
promoted by:
     • high oxygen concentration
     • high temperature
     • low pH
     • bacterial activity.
The overall reaction rate for a specified quantity of sulphides is also dependant on other
parameters such us, for example, the type of sulphides and the particle size, which also
governs the exposed surface area. When the sulphides oxidise they produce sulphate,
hydrogen ions and dissolved metals.
Tailings and waste-rock consist of the different natural minerals found in the mined rock. In
the unmined rock, often situated deep below the ground level, the reactive minerals are
protected from oxidation. In oxygen-free environments, such as in deep groundwater, the
sulphide minerals are thermodynamically stable and have low chemical solubility.
Deep groundwater in mineralised areas, therefore, often has a low metal content. However,
when excavated and brought to the surface, the exposure to atmospheric oxygen starts a
series of bio-geo-chemical processes that can lead to production of acid mine drainage.
Hence, it is not the content of metal sulphides in itself that is the main concern, but the
combined effects of the metal sulphide content and the exposure to atmospheric oxygen.
The effect of exposure increases with decreasing grain size and, therefore, increased surface
area. Hence the sulphides in the finely ground tailings are more prone to oxidation.
Tailings and waste-rock are normally composed of a number of minerals, of which the
sulphides only constitute one part, if present at all. Therefore, if sulphide oxidation occurs in
mining waste, the acid produced may be consumed by acid consuming reactions in varying
degrees, depending on the acid consuming minerals available. If carbonates are present in the
mining waste, pH is normally maintained as neutral, the dissolved metals precipitate and thus
are not transported to the surrounding environment to any significant degree.
The interaction between the acid producing sulphide oxidation and the acid consuming
dissolution of buffering minerals determines the pH in the pore water and drainage, which in
turn influences the mobility of metals. If the readily available buffering minerals are
consumed, the pH may drop and ARD will then occur.


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The release of ARD to surface- and groundwater deteriorates the water quality and may
cause a number of impacts, such as depletion of alkalinity, acidification, bioaccumulation of
metals, accumulation of metals in sediments, effects on habitats, elimination of sensitive
species and unstable ecosystems.
In mining, ARD may be produced in waste-rock deposits, marginal ore deposits, temporary
storage piles for the ore, tailings deposits, pit walls, underground workings or in heap leach
piles. Historically sulphide containing material has also been used for construction purposes
at mine sites, e.g. in the construction of roads, dams and industrial yards. However,
regardless of where ARD production occurs, the fundamental processes behind the
generation of ARD are the same.
Figure 5 schematically shows some of the most important geochemical and physical
processes and their interaction and contribution to the generation of ARD and the possible
release of heavy metals from mining waste. As can be concluded from the figure, the ARD
and metal release will depend primarily on the sulphide oxidation rate, the potential
immobilisation/remobilisation reactions along the flow path and the water flow. However,
the sulphide oxidation rate is dependant on redox conditions (Eh), pH, and microbial activity.
The pH is, in turn, determined by the sulphide oxidation rate and buffering reactions
(carbonate dissolution and silicate weathering). Furthermore, the potentially metal retaining
immobilisation reactions that can occur along the flow path are dependant on pH, redox
conditions and the sulphide oxidation rate.




Figure 5:Schematic illustration of some of the most important geochemical and physical processes and
their interaction and contribution to the possible release of heavy metals from mining waste
At the field-scale not only are the temporary variations of material characteristics important
for the evolution of the drainage water quality but the spatial variations will also be a factor
to take into account. The resulting drainage characteristics depend on a number of additional
parameters, such as infiltration rate, evaporation rate, oxygen profile in the deposit, height of
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the deposit, and the construction of the deposit. Heterogeneities in the material
characteristics, such as varying mineralogy and degree of compaction, are other parameters
that may affect the drainage water quality. Due to the normally long residence time of the
infiltrating water in the deposit, the influence of various immobilisation reactions
(precipitation and adsorption) can also be significant. The interaction between the tailings
and/or waste-rock and the atmosphere is illustrated schematically in the following figure.




Figure 6: Schematic illustration of the drainage water generation as a function of the interaction between
the tailings or waste-rock in the facility and the atmosphere
Due to ARD, not only is the physical stability of the tailings ponds and dams an issue, but so
is the chemical stability of the acid generating tailings, both during operation and after the
mine closure.
The chemical composition of mine water generated at mines varies from site to site and is
dependent on the geochemistry of the ore body and the surrounding area. Mine water may
also contain small quantities of oil and grease from extraction machinery and nitrates (NO3)
from blasting activities. EPA and the Bureau of Mines reported concentration ranges in mine
waters of 0.1-1.9 mg/L for lead, 0.12-0.46 mg/L for zinc, 0.02-0.36 mg/L for chromium, 295-
1,825 mg/L for sulfate, and pH of 7.9-8.8. ((U.S.E.P.A. Technical Resource Document,
1994)
After the mine is closed and pumping stops, the potential exists for mines to fill with water.
Water exposed to sulfur-bearing minerals in an oxidizing environment, such as open pits or
underground workings, may become acidified.
Where concentration or other processing of the ore is done on site, the tailings generated also
have to be managed. The tailings from base metal mining activities can be characterised as
follows:
     • usually a slurry of 20 - 40 % solids by weight
     • containing metals
     • containing sulphides
     • large amounts produced.
The slurried tailings are managed in ponds. With some underground mines the coarse tailings
are used as backfill material.
Many of the waste materials may be disposed of onsite or offsite, while others may be used
or recycled during the active life of the operation. Waste constituents may include base
metals, sulphides, or other elements found in the ore, and any additives used in beneficiation
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operations. In general, most wastes from beneficiation of lead-zinc ores are disposed of in
tailings impoundments from which water is likely to be reclaimed during the mine’s life. In
addition, other materials typically not considered wastes, such as mine water, may be
managed onsite during the active life of the facility and may ultimately become wastes.

    3. Zinc processing
Due to low zinc content, zinc-bearing ores must be concentrated before processing.
Beneficiation of zinc ore, which usually occurs at the mine to keep transport costs low,
consists of crushing, grinding, and flotation to produce concentrates of 50% to 60% zinc.
To concentrate the ore it is first crushed and then ground to enable optimal separation from
the other minerals.
Flotation can be carried in various ways, e.g., by selective flotation or by bulk/selective
flotation, depending on the characteristics of the ore, the market demands, the cost of
flotation additives, etc.
Two possible options for the same mineral processing plant are illustrated in the figures
below for the Swedish Zinkgruvan mineral processing plant. The Zinkgruvan mineral
processing plant, which was constructed in 1977, is located next to the mine. It operates
continuously with an annual throughput of 850000 tonnes. The choice of process and
technology is based on a large number of test works with the actual zinc and lead ore.
Autogenous grinding in combination with bulk/selective flotation (see Figure 7 below) of the
ore has been chosen as the main process technique and has been used at Zinkgruvan since
1977.




Figure 7: Bulk/selective flotation circuit for Zinkgruvan site




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Figure 8: Possible selective mineral processing circuit for Zinkgruvan site
After the removal of values in the flotation process, the flotation system discharges tailings
composed of liquids and solids. Between ¼ and ½ of the tailings generated are made up of
solids, mostly gangue material and small quantities of unrecovered lead-zinc minerals. The
liquid component of the flotation waste is usually water and dissolved solids, along with any
remaining reagents not consumed in the flotation process. These reagents may include
cyanide, which is used as a sphalerite depressant during galena flotation. Most operations
send these wastes to tailings ponds where solids settle out of the suspension. The liquid
component either is recycled back to the mill or discharged if it meets water quality
standards. The characteristics of tailings from the flotation process vary greatly, depending
on the ore, reagents, and processes used. Lead, zinc, chromium, iron, and sulfate were all
found in the wastewater of the selected facilities.
In general, most wastes from beneficiation of lead-zinc ores are disposed of in tailings
impoundments from which water is likely to be reclaimed during the mine’s life. In addition,
other materials typically not considered wastes, such as mine water, may be managed onsite
during the active life of the facility and may ultimately become wastes.


4. Primary zinc smelting

Over 95% of the world’s zinc is produced from zinc blende (ZnS). Apart from zinc the
concentrate contains some 25-30% or more sulphur as well as different amounts of iron, lead
and silver and other minerals. Before metallic zinc can be recovered, by using either
hydrometallurgical or pyrometallurgical techniques, sulphur in the concentrate must be
removed. This is done by roasting. The concentrate is brought to a temperature of more than
900°C where zinc sulphide (ZnS) converts into the more active zinc oxide (ZnO). At the
same time sulphur reacts with oxygen giving out sulphur dioxide which subsequently is
converted to sulphuric acid – an important commercial by-product.



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Figure 9: Zinc roasting (The Metallurgical Society)
Roasting is an exothermic process and no additional fuel is used; the heat generated is
recovered. The zinc oxide (calcine) passes from the furnace and is collected and cooled.
Roaster gases are treated in hot EPs to remove dust (which is passed to the calcine). Other
dust and volatile metals such as Hg and Se are removed in a gas cleaning train that
incorporates scrubbing systems and wet EPs. The sulphur dioxide is then converted to
sulphuric acid in a conventional recovery system. The zinc is normally recovered from the
concentrate by pyrometallurgical (Imperial Smelting, IS) or hydrometallurgical methods.
Pyrometallurgical methods are used in other parts of the world, like Japan, China and Poland,
but have gradually lost their importance and are not used in EU for simple zinc concentrates.
Determining factors are the need for the extra distillation stage to obtain high-grade zinc and
the relatively low zinc extraction efficiency. The IS process is an energy-intensive process
and thus became very expensive following the rise of energy prices in recent years. This, and
the lower production of bulk concentrates containing significant amounts of lead led to
abandoning more and more the Imperial Smelting process. The major difference of the
hydrometallurgical process and the Imperial Smelting process is that the first produce very
pure zinc directly, whereas the latter produces lower grade zinc that still contains significant
impurities that have to be removed by thermal refining in the zinc refinery. The
pyrometallurgical Imperial Smelting Furnace process (ISF) is however still of importance in
EU because it enables complex lead-zinc concentrates and secondary material to be treated
simultaneously, yielding saleable lead and zinc. It can also consume residues from other
processes.
Zinc produced in the Imperial Smelting Furnace may contain varying amounts of cadmium,
lead, copper, arsenic, antimony and iron and the process uses a refining stage. Zinc from the
ISF is refined by reflux distillation in columns containing a large number of refractory trays
(New Jersey Distillation). The lower ends of the columns are heated externally by natural
gas. The upper ends are not heated and run cool enough to reflux the higher boiling point
metals before vapours pass to a condenser. The New Jersey distillation column is also used
for secondary zinc materials.
Distillation proceeds in two stages; first the separation of zinc and cadmium from lead and
then separation of cadmium from zinc. In the first stage, molten zinc is fed into a column
where all the cadmium and a high proportion of the zinc are distilled. The mixture is
condensed and fed directly to a second column. This column is operated at a slightly lower
temperature to distil mainly cadmium, which is condensed as a zinc-cadmium alloy. The
alloy is transferred to a cadmium refinery. The metal run-off from the bottom of the second
column is high-grade zinc (SHG) of 99.995% purity.
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The run-off metal from the first stage is zinc with lead, tin, arsenic, iron, antimony and
copper impurities. This alloy is cooled to separate lead, which is recycled to the ISF splash
condenser and an inter-metallic compound of iron, zinc and arsenic, which is recycled to the
ISF itself.
The zinc is then treated with sodium to remove residual arsenic and antimony as sodium
arsenides and antimonides, which are also recycled to the ISF. The zinc produced in this way
is of a lower grade (GOB), but free of cadmium, and is used mainly for galvanising.




Figure 10: Diagram of zinc/cadmium distillation
The hydrometallurgical route is used for zinc sulphide (blendes), oxide, carbonate or silicate
concentrates and is responsible for about 80% -90% of the total world output. The majority
of the EU production facilities use the electrolytic process.
Sulphide concentrates are roasted first in fluidised bed roasters to produce zinc oxide and
sulphur dioxide. Leaching of the calcine is then carried out in a number of successive stages
using a gradually increasing strength of hot sulphuric acid. The initial stages do not dissolve
significant amounts of iron but the later ones do. The leaching process is carried out in a
variety of reactors using open tanks, sealed vessels and pressure vessels or a combination of
them.




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Figure 11: Diagram of the zinc hydrometallurgical process major impurity and the iron is precipitated in
3 basic forms; Jarosite, Goethite or Hematite
Leaching may be stopped after the Neutral Leach. The leach residue is sent to an ISF and
added to the sinter feed. Zinc, lead and silver are recovered as metals, sulphur as H2SO4.
Instead of an ISF, a Waelz Kiln may be used but SO2 absorption is necessary in such a case.
The form of these precipitates is used to give the process names. The precipitation stages are:
-
    • As Jarosite, using ammonia and zinc calcine for neutralisation. Up to 3 stages are
        used depending on whether Ag/Pb recovery is undertaken. A single stage process
        known as the “Conversion Process” is also used.
    • As Goethite, using zinc sulphide for pre-reduction, oxygen for re-oxidation and zinc
        calcine for neutralisation.
    • As hematite, using sulphur dioxide or zinc sulphide for pre-reduction and an
        autoclave with oxygen for precipitation. In this case a sulphur residue is produced as
        well as an iron residue.
The main differences in the iron precipitates are their volume and ease of filterability. There
are also significant differences in process capital and operating costs.
The balance of these with the disposal costs of the residue may be influenced by non-process
related costs. The hematite process was thought to be very attractive as the residue volume
was lower and hematite was a potential raw material for iron. The process has not proved to
be viable and the hematite was not acceptable to the iron and steel industry.
It has been reported that the Jarosite process is capable of high zinc recoveries even with
concentrates containing 10% Fe. Similar recoveries with the Goethite process rely on a low
iron content in the calcine (or ZnO) that is used for the precipitation stage.



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Figure 12: Simplified flow sheets of some iron removal processes
Two applications are known where concentrate is leached directly without calcination i.e. at
Korea Zinc and Outokumpu Zinc. At Korea Zinc the iron is left in solution during the
leaching and is then precipitated in a separate step as goethite, whereas at Outokumpu the
iron is precipitated as jarosite simultaneously with the leaching of the sulphides.
The concentrate together with the slurry from the Conversion process and acid from the
electrolyses are fed to the reactors where the leaching takes place by sparging oxygen into
the slurry. The rest of the dissolved iron in the solution from the conversion and the iron
dissolved from the concentrate is precipitated as jarosite:
3ZnS+3Fe2(SO4)3+(NH4)2SO4+9H2O+1.5O2=2NH4[Fe3(SO4)2(OH)6]+3ZnSO4+3H2
SO4+3S
A sulphur concentrate is separated from the slurry by flotation and stored separately from the
jarosite residue. This sulphur concentrate is not used for H2SO4 production and is a
hazardous waste like Goethite and Jarosite. The equipment used in the process is much the
same as conventionally used in zinc hydro-metallurgy. A flow sheet of the process operated
by Outokumpu Zinc is shown below.

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Figure 13: Concentrate leaching process
Whatever residue is produced by the various process options zinc removal is maximised by
washing the residue. Other soluble metals may be treated by precipitation as hydroxides or
sulphides. The residues are stored in landfill areas, usually on or near the site in a manner
that isolates them from ground or surface water. Water from the storage area is normally
recycled back to the process. Developments are taking place to avoid the residues or at least
to render them more inert by fixation.
Slurry from the final leaching stage is settled and the overflow solution is treated to remove
impurities. The solid in the underflow is filtered and washed on a filter. The filter cake is
disposed of and the filtrate is recycled to the process. Different flow sheets are used
depending on factors such as the choice of the iron removal process and the available
integrated impurity recovery processes.
For example, more or less extensive treatment of the leach residue is carried out by further
leaching or physical separation techniques before it is disposed of.
This is reflected in the recovery rates and composition of possible lead or lead/Ag by
products.
Purification of the zinc bearing solution takes place in a number of consecutive stages. The
processes used are dependent on the concentrations of the various metals contained in the
concentrate and vary accordingly. The basic processes involve the use of zinc dust or powder
to precipitate impurities such as Cu, Cd, Ni, Co and Tl. Precipitation of Co and Ni also
involve the use of a second reagent such as As or Sb oxides. Variations in temperature occur
from plant to plant. Other reagents such as barium hydroxide and dimethyl glyoxime may
also be used to remove lead and nickel. The recovery route for the copper by-product can
affect the choice of process.
Hydrogen may be evolved and arsine or stibine occurrence is monitored. Collection and
treatment of the released gases depends on the presence of these gases, local overall
engineering, open air or enclosed building operations can be used but scrubbing the gases
from the reactors using an oxidising solution for arsine removal is reported to be most
effective.
The purified solution passes to a cell house where zinc is electro-won using lead anodes and
aluminium cathodes. Zinc is deposited at the cathodes and oxygen is formed at the anodes,
where sulphuric acid is also generated and is recycled to the leaching stage. Acid mist is
formed during this process and various coverings are used on the cells to minimise this. Cell
room ventilation air can be de-misted and the acid mist recovered. Heat is produced during
electrolysis and this is removed in a cooling circuit, this can be designed to optimise the
water balance of the process but may be a further source of mists.
The cathodes that are produced are stripped automatically or manually and are then melted in
electric furnaces and alloys made. A small part of the zinc produced is made into zinc
powder or dust for the purification stages. These can be produced by air, water or centrifugal
atomisation of a stream of molten zinc or by condensing zinc vapour in an inert atmosphere.

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One of the main issues in the hydrometallurgical process route is the disposal or use of the
precipitated iron. Special containment sites are used for disposal at the moment but pressure
on landfill options is increasing, this factor is discussed later and the options are assessed.
Several approaches are being developed to allow these residues to be used and these are
covered in emerging techniques.
Restricting the process to neutral leaching only is one alternative method that can be used to
avoid the production of these intractable wastes. In this case iron remains in the leach residue
along with a significant portion of the zinc. This residue is used as the feed for a
pyrometallurgical process to recover the zinc, lead, silver, sulphur and to bring the iron into a
slag.

5. Emission and Consumption Levels
The main environmental issues of the zinc and lead industry are air and water pollution and
the generation of hazardous wastes. The facilities generally have their own wastewater
treatment facilities and wastewater recycling is usually practised. Many wastes are reused but
the major item is leach residue that has a high environmental impact.
Some local aspects, like noise, are relevant to the industry. Due to the hazardous nature of
some solid and liquid waste streams, there is also a significant risk for soil contamination.
The following tables give input and output balances for some lead and zinc plants in Europe.




Table 6: Input and output data for an ISP plant (1998)




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Table 7: Input and output data for the ISA Smelt furnace (lay out and preliminary data)




Table 1: Input and output data for the QSL plant (1997)




Table 2: Typical data for a zinc electrolysis plant. Roast - Leach – Purification - Electrolysis




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Table 3: Typical composition of feed and products for a zinc electrolysis plant

Energy consumption
The energy requirement for the different lead and zinc processes varies to a large extent. It
depends on the quality of the feed and the products, the use of latent or waste heat and the
production of by-products. The following table shows the average energy requirements of the
different processes.




Table 4: Energy requirement of various zinc processes

Emissions to air
The emissions can escape the process either as stack emissions or as fugitive emissions
depending on the age of the plant and the technology used. Stack emissions are normally
monitored continuously or periodically and reported.
The main emissions to air from zinc and lead production are: -
   • sulphur dioxide (SO2), other sulphur compounds and acid mists;
   • oxides of nitrogen (NOx) and other nitrogen compounds;
   • metals and their compounds;
   • dust.
   • VOCs and dioxins.
The sources of emissions from the process are: -
   • roasting (Most emissions occur during unscheduled shutdown);
   • other pre-treatment (battery breaking);
   • transport and handling of material;
   • smelting and refining;
   • leaching and purification;

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    •   electrolysis;
    •   casting;
    •   sulphuric acid plant.




Table 12: Significance of potential emissions to air from lead, zinc and cadmium production
Besides process emissions, fugitive emissions occur. The major fugitive emission sources
are:
    • dust from storage and handling of concentrates (10 t/y);
    • leakage from roasters and smelters;
    • dust from the exhaust gases of leaching and purification vessels (1 t/y);
    • exhaust gases of cooling towers of the leaching and purification units (0.7 t/y);
    • exhaust gases of cooling towers of the electrolysis process (08 t/y);
    • dust from the exhaust gases of casting furnaces (1.8 t/y);
    • miscellaneous (0.7 t/y).
Although fugitive emissions are difficult to measure and estimate, there are some methods
that have been used successfully. The following table gives some emission data based on the
upgrading of a lead process from blast furnace to ISA Smelt and illustrates the potentially
high level of fugitive emission.




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Table 13: Significance of plant improvements on fugitive emissions

Sulphur dioxide and other sulphur compounds
The major sources of sulphur dioxide emission are fugitive emissions from the oxidation
stages, direct emissions from the sulphuric acid plant and the emission of residual sulphur in
the furnace charge.
Good extraction and sealing of the furnaces prevents fugitive emissions and the collected
gases from oxidation stages are passed to a gas cleaning plant and then to the sulphuric acid
plant.
After cleaning, the sulphur dioxide in the gas from the sintering, roasting or direct smelting
stages is converted to sulphur trioxide (SO3). The efficiency generally lies between 95 to
99.8% depending on the sulphuric acid plant used (single or double absorption) and the
concentration of sulphur dioxide in the input gas and its variation or stability. SO2
concentrations in the off gas from 200 - 2300 mg/Nm3 can be emitted. A very small amount
of SO3 is not absorbed and is emitted together with the SO2. During start up and shut down
there may be occasions when weak gases are emitted without conversion. These events need
to be identified for individual installations, many companies have made significant
improvements to process control prevent or reduce these emissions.
During electrolysis, emissions of aerosols (diluted sulphuric acid and zinc sulphate) takes
place to the hall. These emissions leave the cell room via the (natural) ventilation or from the
cooling towers. The emission is small compared with the emissions from the sulphuric acid
plant but as they are in the form of an aerosol form, they can be dealt with in mist eliminators
or dust abatement.
Some processes use coverings for the cells such as foam or plastic beads to reduce mist
formation. One plant has been recently modified to improve roasting and to collect fugitive
emissions from the whole of the process. Sulphur dioxide emissions were reduced from 3000
to 1200 g per tonne of metal produced.
Emissions from other processes are shown below.




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Table 14: Sulphur dioxide production from several zinc and lead processes


Nitrogen oxides
The roasting and smelting stages are potential sources of nitrogen oxides (NOx). NOx may
be formed out of nitrogen components that are present in the concentrates or as thermal NOx.
The sulphuric acid produced can absorb a large part of the NOx and this can therefore affect
sulphuric acid quality. If high levels of NOx are present after the roasting stages, treatment of
the roasting gases may be necessary for reasons of product quality and environmental
reasons.
Other furnaces that use oxy-fuel burners can also show a reduction in NOx. The range for all
of the processes is 20 to 400mg/Nm3.

Dust and metals
Dust carry over from the roasting and smelting processes are potential sources direct and
fugitive emissions of dust and metals. The gases are collected and treated in the gas cleaning
processes of the sulphuric acid plant. Dust is removed and returned to the process.
The gases leaving splash condensers in the ISF, from distillation columns and from the
tapping points are also potential sources. Good extraction and abatement is needed at these
points to prevent fugitive emissions.
Slag treatment and quenching also gives rise to dust. The range of dust emissions from these
captured sources is < 1 to 20 mg/Nm3.




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                                                                                             T
able 15: Mass release of metals from some European processes (controlled emissions only)
De-aeration of vessels in the leaching and purification stages can emit dust and metals.
Arsine can be emitted from the purification stages of zinc. Cadmium can be emitted from the
distillation stages and the cadmium plants.can be emitted from the purification stages of zinc.
Emissions of aerosols takes place in the cell room and battery breakers and can contain
metals.
The range of mist and dust emissions from these sources is 0.1 to 4mg/Nm3.
The melting, alloying, casting and zinc dust processes are potential emission sources of dust
and metals. The range of dust emissions is reported to be 200 to 900mg/Nm3 in the crude
gas. Fume collection and abatement systems are used and cleaned gas values are below 10mg
dust/Nm3.
Metals are associated with the dusts emitted, approximately 50% is zinc. Cadmium and lead
are not present when pure zinc is melted, alloyed and cast.

VOCs and dioxins
The formation of dioxins in the combustion zone and in the cooling part of the off-gas
treatment system (de-novo synthesis) may be possible in some processes particularly if
plastic components are included in the secondary materials that are fed into a process.
Dioxins have also shown to present in some dusts from Waelz kilns treating EAF dust.

Emissions to water
Metals and their compounds and materials in suspension are the main pollutants emitted to
water. The metals concerned are Zn, Cd, Pb, Hg, Se, Cu, Ni, As, Co and Cr. Other significant
substances are fluorides, chlorides and sulphates.
   • metals;

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   •   materials in suspension;
   •   chlorides, fluorides, sulphates.

Possible wastewater streams are:
   • Waste water from wet scrubbers;
   • Waste water from wet electrostatic precipitators;
   • Waste water from the mercury-removal step;
   • Wastewater from battery breaking and classification stages;
   • Wastewater from slag granulation;
   • Wastewater from various hydro-metallurgical processes;
   • Anode and cathode washing liquid effluent;
   • Sealing water from pumps;
   • General operations, including cleaning of equipment, floors, etc.;
   • Discharge from cooling water circuits;
   • Rainwater run-off from surfaces (in particular storage areas) and roofs.

Wastewater from the gas cleaning of the smelter and fluid-bed roasting stages are the most
important sources. Other sources are the process effluent from electrolysis, battery breaking
and cleaning plus miscellaneous sources.

Wastewaters from abatement plant
Generally wet gas cleaning systems operate with liquid recycling. A monitored bleed keeps
suspended solids and dissolved salts within certain defined limits. The bleed is either treated
separately or in an integrated water treatment plant to remove solids and dissolved species
before discharge. The destination of the separated material depends on the origin of the
wastewater.
Wet scrubbers after the roasting process are operated with a SO2-saturated acidic solution.
The scrubber removes fluorides, chlorides, most mercury and selenium and the some
particles that pass the mechanical gas treatment. To avoid the build up of contaminants, some
liquid needs to be bled continuously from the scrubber. Dissolved SO2 is removed during
treatment prior to the discharge.
Wet electrostatic filters will also produce an acidic scrubber liquid. This is recycled after
filtering. Some liquid needs to be bled from this circuit to remove build up of contaminants.
This bleed liquor is treated and analysed before discharge.
The mercury-removal step involves a gas-liquid contact tank in which the liquid contains a
reagent that combines with mercury and removes it. Mercury chloride (HgCl2) is frequently
used and reacts with metallic mercury from the gas to form a solid Hg2Cl2-precipitate (so-
called “calomel”). The relatively clean liquid is discharged as wastewater for further
treatment. The solid Hg2Cl2 is sold for mercury recovery or treated to produce mercury
chloride again. The following table provides an indication of the composition of the gas
cleaning liquids before treatment.




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Table 16: Typical gas cleaning effluents

Electrolyte bleed effluent
Electrolyte may be bled from the cells to control the build up of impurities e.g. magnesium,
that may have a detrimental impact on the operation of the electrolytic cells. For zinc
production, the flows in the electrolytic cells belong to the same (closed) water circuit as the
leaching and purification stages. The sulphuric acid formed during electrolysis is fed to the
leaching process and the remaining liquid is purified and fed to the electrolysis.
The effluent bleed of the electrolytic-leaching-purification circuit is strongly acidic and
contains high concentrations of zinc and suspended solids. The volume of the bleed depends
strongly on the composition of the zinc concentrates that are used in the roasting.
Components that tend to build up in the circuit, especially magnesium, will determine the
bleed flow and the treatment required.

Miscellaneous sources
The electrodes used during the electrolysis need to be rinsed periodically to remove
deposited material upon the surface. Manganese dioxide is formed on the surface of the
anodes by the reaction of oxygen with dissolved manganese. After rinsing of the anodes, the
manganese is separated from the rinse water for external re-use.




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Table 17: Typical wastewater analyses
Cathodes are cleaned after removal of the zinc sheets. The anode and cathode washing liquid
effluents are acidic and likely to contain copper, zinc, lead and suspended solids.




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Table 18: Summary Table of Potential Wastewater Sources and Options
Cooling water from the granulation of slag is usually re-circulated in a closed circuit system.



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Process residues and wastes
The production of metals is related to the generation of several by-products, residues and
wastes, which are also listed in the European Waste Catalogue (Council Decision 94/3/EEC).
The most important process specific residues are listed below.
Solid residues derived from various process and abatement stages may have one of three
possible destinations.
    • Recycling in or upstream of the process;
    • Downstream treatment to recover other metals;
    • Final disposal, if necessary after treatment to ensure safe disposal.
The electrolytic production of zinc is one of the main sources of solid waste in the non-
ferrous industry. Relatively large quantities of iron based solids are generated by the leaching
process.
Jarosite and goethite are classified as hazardous waste because of the content of leachable
elements such as Cd, Pb and As. The leaching and purification processes and electrolysis of
zinc generate other metal rich solids. These are usually rich in a specific metal and are
recycled to the appropriate production process.
The ISF or direct smelting furnaces are also significant sources of solid slag. This slag has
been subjected to high temperatures and generally contains low levels of leachable metals; it
may consequently be used in construction.
Solid residues also arise as the result of the treatment of liquid effluents. The main waste
stream is gypsum waste (CaSO4) and metal hydroxides that are produced at the wastewater
neutralisation plant. These wastes are considered to be a cross-media effect of these
treatment techniques, but many are recycled to pyrometallurgical process to recover the
metals. Dust or sludge from the treatment of gases are used as raw materials for the
production of other metals such as Ge, Ga, In and As etc or can be returned to the smelter or
into the leach circuit for the recovery of lead and zinc.
Hg/Se residues arise at the pre-treatment of mercury or selenium streams from the gas
cleaning stage. This solid waste stream amounts to approximately 40 - 120 t/y in a typical
plant. Hg and Se can be recovered from these residues depending on the market for these
metals.

Leaching residues
The production of iron based solids (goethite, jarosite or hematite) accounts for the greatest
volume of waste depending on the process used. The composition is shown in the following
table:




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Table 19: Example compositions of different types of residues
Typically, these residues account for: -
• Jarosite - 0.35 to 0.80 tonnes per tonne of zinc produced.
• Goethite - 0.3 to 0.35 tonnes per tonne of zinc produced.
• Hematite – 0.2 tonnes per tonne of zinc produced.
Hematite processes have been unable to compete in economic terms as the process is
significantly more complex and expensive to operate. In addition, hematite has not proved to
be acceptable as a raw material in other industries.
There are still some leachable metals in slurry after filtering and washing. The residue can be
treated to a less leachable form with neutralisation and sulphide treatment. The disposal of
these residues can be considerable cost as specially constructed, lined ponds or isolated areas
are used to contain the material. Particular care is taken about leakage and these ponds have a
major need to monitor groundwater. There is a significant cross media effect compared to
processes that are capable of producing an inert residue.
As reported earlier, leaching residues can be treated in ISF or Waelz Kiln. Leachable slag
and recoverable metal oxides, problems with contaminant build up have been reported. Other
developments are reported in Emerging Techniques.

Pyrometallurgical slag and residues
Slag from the Blast Furnace, ISF, Direct Smelting and Waelz kiln processes usually contain
very low concentrations of leachable metals. They are therefore generally suitable for use in
construction. The slag output is between 10 and 70% of the weight of the metal produced
dependant on the raw materials used.
Slag from the battery processing plants account for 13 to 25% of the weight of lead
produced.
They may be suitable for construction uses depending on the leachability of the metals they
contain. The leachability is influenced by the fluxes used and the operating conditions. The
use of sodium based fluxes (Na2CO3) to fix sulphur in the slag causes an increase in the
quantity of leachable metals. These slags and drosses from battery recovery processes can
contain Sb. This is normally recovered but storage in damp conditions can cause the emission
of stibine.
A number of standard leachability tests are used by Member States and these are specific to
the Country in question.



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Table 20: Eluate values of granulated IS furnace slag




Table 21: Eluate values for acidic Waelz slag




Table 22: Eluate values for slag from QSL process




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Table 23: Solid Material from the refining of lead bullion
The drosses and solids, removed during the zinc melting and refining stages, contain metals
that are suitable for recovery.




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Other materials
The following tables show the use or treatment options for the residues produced by several
processes.




Table 24: Residues from zinc processes




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6. BAT for zinc primary production
This section presents a number of techniques for the prevention or reduction of emissions
and residues as well as techniques reducing the overall energy consumption. They are all
commercially available. Examples are given in order to demonstrate techniques, which
illustrate a high environmental performance. The Techniques that are given as examples
depend on information provided by the industry, European Member States and the valuation
of the European IPPC Bureau.
The control of furnace operating parameters and the prevention of fugitive emissions from
furnaces and the tapping and pouring processes is very important. Techniques used by other
sectors are also applicable particularly those relating to the use of sulphur recovery systems.
The techniques to consider on a site by site basis are strongly influenced by the raw materials
that are available to a site; in particular the type and variability of the concentrate or
secondary raw materials, the metals they contain can be crucial to the choice of process.
Some processes have a dedicated single source of raw material but the majority of
installations in Europe buy concentrate on the open market and need to maintain flexibility in
processing a range of raw materials. In a similar manner the standard of collection and
abatement systems used worldwide in the industry reflects local, regional and long-range
environmental quality standards and direct comparison of the environmental performance of
process combinations is therefore difficult. It is possible however, to judge how a particular
process can perform with the appropriate, modern abatement equipment.
The processes described above are applied to a wide range of raw materials of varying
quantity and composition and are also representative of those used worldwide. The
techniques have been developed by the Companies in this sector to take account of this
variation. The choice of pyro-metallurgical or hydrometallurgical technique is driven by the
raw materials used, their quantity, the impurities present, the product made and the cost of
the recycling and purification operation. These factors are therefore site specific.
The basic recovery processes outlined above therefore constitute techniques to consider for
the recovery processes when used with appropriate abatement stages.

BAT for materials storage, handling and pre-treatment processes
The raw materials are concentrates, secondary raw materials, fluxes and fuel; other important
materials are products, sulphuric acid, slags, sludges and process residues.
Important aspects are the prevention of leakage of dust and wet material, the collection and
treatment of dust and liquids and the control of the input and operating parameters of the
handling and feeding processes. The issues specific to this group are: -
The potentially dusty nature of concentrates and fluxes means that enclosed storage, handling
and treatment systems may be needed in these instances. The dust generated by some
crushing operations means that collection and abatement may be applicable for this process.
Similarly granulation water may require settlement or other treatment prior to discharge.
Concentrates are mixed with fluxes to produce a fairly constant feed therefore the general
practice is sampling and analysis to characterise the concentrates and store individual
concentrates separately so that an optimum blend can be prepared for smelting.
Feed blends can be prepared from dosing bin systems using belt weighers or loss in weight
systems. Final mixing and homogenisation can take place in mixers, pelletisers or in the
conveying and metering systems. Enclosed conveyors or pneumatic transfer systems are used

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for dusty material. Hot gas rotary dryers or steam coil dryers can be used if the process
requires a dry feed, steam coil dryers use waste heat from other parts of the process provided
that the heat balance allows it. The drier and associated dust abatement stage therefore
depends on sitespecific conditions such as the reliability of the steam supply. Fabric or
ceramic filters achieve better dust removal efficiencies than EPs when used at this stage of
the process.
Acid produced during the process can be stored in double walled tanks or tanks placed in
chemically resistant bunds. The treatment of acid slimes from the sulphuric acid plant and
weak acid from scrubbing systems depends on local processing or disposal requirements
unless there is a local use for the material.
Sludges and other metallic residues that are destined for recovery off site can be stored drums
or other suitable ways depending on the material. Sludges produced during the process that
are destined for on site disposal should be washed free of zinc or other metals and de-watered
as far as possible. Disposal facilities should be totally contained and leak proof, they are
subject to local control and regulation. Water from the sludge containment areas can be
returned to the process.
There are a variety of secondary raw materials used and they range from fine dusts to large
single items. The metal content varies for each type of material and so does the content of
other metals and contaminants. Batteries are a common source of lead and can contain acid,
the storage and handling therefore needs to take account of the acid content and any acid
mists that can be formed. Nickel cadmium batteries are usually dry but other batteries may be
present and leakage of electrolyte is possible, this should be taken into account in the storage
and separation method used. The techniques used for storage, handling and pre-treatment
will therefore vary according to the material size and the extent of any contamination. These
factors vary from site to site and techniques will be applied on a site and material specific
basis.
The following issues apply to this group of metals.
    • The storage of raw materials depends on the nature of the material described above.
        The storage of fine dusts in enclosed buildings or in sealed packaging is used.
        Secondary raw materials that contain water-soluble components are stored under
        cover. The storage of nondusty, non soluble material (except batteries) in open
        stockpiles and large items individually in the open can be used.
    • Pre-treatment stages are often used to produce sinter or to remove casings or coatings
        and to separate other metals. Milling and grinding techniques are used with good dust
        extraction and abatement. The fine dust that is produced may be treated to recover
        other metals, pneumatic or other density separation techniques are used.
    • Fine dusts can be stored and handled in a manner that prevents the emission of dust.
        They are often blended and agglomerated to provide a consistent feed to the furnace.
        Sintering is used to prepare concentrates for some of the smelting processes up draft
        and down draft sintering machines can be used and recent developments of a steel
        belt sintering process may be appropriate. Collection of fume and gases is important
        and the up draft sintering process is inherently easier for fume capture. Gases contain
        sulphur dioxide and will have abatement and sulphur dioxide recovery processes
        down stream. The sulphur dioxide content is usually low and variable and this
        influences the design of the sulphuric acid plant.
    • Zinc concentrates are roasted prior to hydro metallurgical processing. Fluidised bed
        roasters are almost universally used and need good extraction and calcine removal

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       systems. Gases are treated in an integrated abatement and sulphur dioxide recovery
       processes.
There are also several general techniques that are considered to be BAT in preventing
emissions from material storage and handling processes. These techniques are:
    • The use of liquid storage systems that are contained in impervious bunds that have a
        capacity capable of containing at least the volume of the largest storage tank within
        the bund. Various guidelines exist within each Member State and they should be
        followed as appropriate. Storage areas should be designed so that leaks from the
        upper portions of tanks and from delivery systems are intercepted and contained in
        the bund. Tank contents should be displayed and associated alarms used. The use of
        planned deliveries and automatic control systems to prevent over filling of storage
        tanks.
    • Storage areas for reductants such as coal, coke or woodchips need to be surveyed to
        detect fires, caused by self-ignition.
    • Sulphuric acid and other reactive materials should also be stored in double walled
        tanks or tanks placed in chemically resistant bunds of the same capacity. The use of
        leak detection systems and alarms is sensible. If there is a risk of ground water
        contamination the storage area should be impermeable and resistant to the material
        stored.
    • Delivery points should be contained within the bund to collect spilled of material.
        Back venting of displaced gases to the delivery vehicle should be practised to reduce
        emissions of VOCs. Use of automatic resealing of delivery connections to prevent
        spillage should be considered.
    • Incompatible materials (e.g. oxidising and organic materials) should be segregated
        and inert gases used for storage tanks or areas if needed.
    • The use of oil and solid interceptors if necessary for the drainage from open storage
        areas. The storage of material that can release oil on concreted areas that have curbs
        or other containment devices. The use of effluent treatment methods for chemical
        species that are stored.
    • Transfer conveyors and pipelines placed in safe, open areas above ground so that
        leaks can be detected quickly and damage from vehicles and other equipment can be
        prevented. If buried pipelines are used their course can be documented and marked
        and safe excavation systems adopted.
    • The use of well designed, robust pressure vessels for gases (including LPG’s) with
        pressure monitoring of the tanks and delivery pipe-work to prevent rupture and
        leakage. Gas monitors should be used in confined areas and close to storage tanks.
    • Where required, sealed delivery, storage and reclamation systems can be used for
        dusty materials and silos can be used for day storage. Completely closed buildings
        can be used for the storage of dusty materials and may not require special filter
        devices.
    • Sealing agents (such as molasses and PVA) can be used where appropriate and
        compatible to reduce the tendency for material to form dust.
    • Where required enclosed conveyors with well designed, robust extraction and
        filtration equipment can be used on delivery points, silos, pneumatic transfer systems
        and conveyor transfer points to prevent the emission of dust.
    • Non-dusty, non-soluble material can be stored on sealed surfaces with drainage and
        drain collection.

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•   Swarf, turnings and other oily material should be stored under cover to prevent
    washing away by rain water.
•   Rationalised transport systems can be used to minimise the generation and transport
    of dust within a site. Rainwater that washes dust away should be collected and
    treated before discharge.
•   The use of wheel and body washes or other cleaning systems to clean vehicles used
    to deliver or handle dusty material. Local conditions will influence the method e.g.
    ice formation. Planned campaigns for road sweeping can be used.
•   Inventory control and inspection systems can be adopted to prevent spillages and
    identify leaks.
•   Material sampling and assay systems can be incorporated into the materials handling
    and storage system to identify raw material quality and plan the processing method.
    These systems should be designed and operated to same high standards as the
    handling and storage systems.
•   The use of good design and construction practices and adequate maintenance.




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Table 25: Storage, handling and pre-treatment methods for lead, zinc and cadmium



BAT for mining and processing waste management
The environmentally relevant parameters of tailings and waste-rock management facilities
can be subdivided into two categories: (1) operational and (2) accidental. Both have to be
taken into consideration.
The three very important environmental issues which need to be highlighted are:
    1. the generation of acid rock drainage
    2. the occurrence of accidental bursts or collapses
    3. site rehabilitation and after-care


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Acid rock drainage
If an acid-forming potential exists, it is BAT to firstly prevent the generation of ARD. If the
generation of ARD cannot be prevented, to control ARD impact or to apply treatment
options. Often a combination is used.
The management of potentially ARD is a cyclic process and is originally done in the
planning phase of the mine, but is renewed and re-evaluated continuously throughout the
mine life. The assessment process always covers the ‘cradle-to-grave’ concept, i.e. any
preferred option with respect to the management of tailings and waste-rock during the
operational phase of the operation should also include an acceptable closure strategy.
The basis for any preventive measure is the characterisation of the tailings and waste-rock,
together with a comprehensive management plan which identifies and minimises the amount
of tailings and waste-rock that requires special attention.
There are a number of prevention, control and treatment options developed for potentially
ARD generating tailings and waste-rock.
Preventive as well as control measures usually focus on water management, alkaline addition
and special handling. These strategies alone or in combination can substantially reduce or
mitigate generation of acid drainage. Water management can include the following:
    • Active mining operations can incorporate diversions to route surface drainage away
        from pyritic material or through alkaline material.
    • Spoil material can be placed and rough graded to prevent ponding and subsequent
        infiltration.
    • Prompt removal of pit water can lessen the amount and severity of acid generated.
    • Polluted pit water can be isolated from non-contaminated sources (no commingling) to
        reduce the quantity of water requiring treatment.
    • Constructed underdrain systems can be used to route water away from contact with
        acid forming material.
Alkaline placement strategies involve either mixing directly with pyritic material or
concentrated placement to create a highly alkaline environment. Direct mixing places
alkaline materials in intimate contact with pyritic spoil to inhibit acid formation and
neutralize any generated acidity in situ. Alkaline recharge employs trenches loaded with
alkaline material, usually a combination of soluble sodium carbonate and crushed limestone.
The strategy is to charge infiltrating waters with high doses of alkalinity sufficient to
overwhelm any acid produced within the backfill. This approach is highly dependent on the
placement of the alkaline trenches to provide maximum inflow to the acid producing zones.
A third variant of the alkaline placement technique is encapsulation with alkaline material
above and below the acid- producing zone.
       In the BREF document, to prevent ARD formation, these are specific BAT measures:
   •   Water cover and underwater (sub-aqueous)
   •   Dry covers
   •   Oxygen consuming covers
   •   Wetland establishment
   •   Raised groundwater level
   •   Isolation above the Water Table
   •   Depyritisation
   •   Selective material handling


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Other preventive methods aim at minimising the bacterial activity and minimising the
mineral surface area available for weathering.
Water cover and underwater (sub-aqueous) discharge uses a free water cover as an oxygen
diffusion barrier. Submergence relies on several physico-chemical phenomena for success.
Oxygen diffuses very slowly and has limited solubility in water. For this approach to
succeed, a stagnant or no flow condition and relatively thick saturated zone appears critical.
Stagnant flow conditions leading to the development of anoxic (oxygen free) conditions and
a saturated thickness on the order of several tens of feet appear to effectively curtail oxygen
diffusion. This approach is most successful in large mines in flat terrain where ground-water
gradients are low, the saturated zone is thick, and aquifers are of large areal extent.
In general, flooding to prevent AMD is believed to be more successful in below drainage
mines. It is assumed that complete flooding eliminates oxygen and halts or severely curtails
acid generation. Flooding of above drainage mines is also practiced typically through the use
of “wet” seals, which allow water to drain but exclude air entry. However, sealing and
flooding above drainage mines does reduce acid loading but is technically more difficult and
less effective than other methods in AMD prevention.
Dry cover uses a low permeable layer with high water content as an oxygen diffusion barrier.
Oxygen consuming cover uses a low permeable layer with high water content as an oxygen
diffusion barrier. In addition, the low permeable layer has a high content of organic matter
which, when it degrades consumes oxygen and thereby further reduces oxygen transport to
the underlying sulphides.
Wetland establishment as a closure method, uses the same principle as the water cover but
with less water depth as the plant cover stabilises the bottom, and thereby re- suspension of
the tailings can be avoided.
Raised groundwater level maintains the underlying sulphide material constantly below the
groundwater table by retaining water through:
   • increased infiltration
   • reduced evaporation
   • increased flow resistance
   • capillary forces
Depyritisation promotes the separation of pyrite from the tailings and separate discharge of
the pyrite (e.g. under water).
Selective material handling refers to the selective management of various tailings or
wasterock fractions determined by their composition and properties, e.g. separation of
material with ARD generating potential for separate handling.
Isolation above the water table: Placement of pyritic material above a water table is an
attempt to isolate the material from contact with water, and preclude leaching of acid
weathering products. Compaction and capping with clay or other materials may also be
employed to reduce permeability. In practice, it has proven very difficult to completely
isolate spoil materials from water contact. Clay caps and other flow barriers are prone to
leakage, and the sporadic infiltration of rain or snowmelt may periodically leach the spoil.
The capping approach can be extended to complete encapsulation on top, bottom and sides as
a further effort to isolate the materials from water contact. (Office of Surface Mining)
On occasion, despite the application of sound mining and reclamation principles, Acid Mine
Drainage will be formed and must be treated to meet existing standards before it is released
from the mine site. When the weathering reactions cannot be prevented (such as might be the
case during the operational stage of the mine life), the migration of ARD needs to be
controlled.
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Methods of water treatment used to eliminate acid mine drainage from abandoned
underground mines can be grouped into two types, active and passive. The following
techniques are BAT for treating acid effluents:
   • active treatments:
           o   addition of limestone (calcium carbonate), hydrated lime or quicklime
           o   addition of caustic soda for ARD with a high manganese content
   •   passive treatment:
           o constructed wetlands
           o   open limestone channels/anoxic limestone drains
           o   diversion wells.
Active treatment requires constant maintenance and involves neutralizing acid-polluted water
with lime, sodium hydroxide (caustic soda), sodium carbonate (soda ash) or ammonia. This
treatment reduces acidity and significantly decreases iron and other metals, but is expensive
to construct and operate. The laws require treatment as long as mine drainage is produced,
which can be several decades.
Passive treatment involves the construction of a treatment system that is typically designed to
last 20 – 40 years and requires much less maintenance than active treatment. This technology
involves the use of wetlands, ponds, and anoxic limestone drains. Passive treatment systems
are relatively inexpensive to construct and many are very successful
Also, curtain grouting, relief wells and compartmentalized barriers are several of the
techniques suggested for controlling ARD discharges.
By a combination of compaction and sealing of the underlying strata, ARD generation is
minimized and uncontrolled seepage into the ground is avoided. A number of factors dictate
the level of sophistication of the treatment system that is necessary to ensure that effluent
standards will be met.
    These factors include:
   • the chemical characteristics of the Acid Mine Drainage,
   • the quantity to be treated,
   • climate,
   • terrain,
   • sludge characteristics, and
   • projected life of the plant.
The chemicals used for Acid Mine Drainage treatment include limestone, hydrated lime,
soda ash, caustic soda and ammonia.
Cleaning up Acid Mine Drainage from abandoned coal mines is very difficult and expensive.
The least costly and most effective method of controlling ARD is to prevent its initial
formation. Optimal strategies are site-specific and a function of geology, topography,
hydrology, mining method and cost effectiveness.

Accidental bursts or collapses
On the 30th of August the drainage culvert underneath the waste (“tailings”) dump at the Sasa
lead-zinc mine in FYROM collapsed. There was a spill of tailings into the river which had
extremely serious consequences. Bursts or collapses of tailings dams at several other
operations (e.g. in Aznalcollar and Baia Mare) have brought public attention to the
management of tailings ponds and tailings dams. Such collapses of tailings and waste-rock
heaps can cause severe environmental damage.


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The dimensions of either type of tailings management facility can be enormous. Dams can be
tens of metres high, heaps even more than 100 m and several kilometres long possibly
containing hundreds of millions of cubic metres of tailings or waste rock.
At the other extreme are ponds the size of a swimming pool or heaps smaller than a
townhouse. Tailings dams are built to retain slurried tailings. In some cases, material
extracted from the tailings themselves is used for their construction. Tailings dams have
many features in common with water retention dams. Actually, in many cases they are built
as water retaining dams, particularly where there is a need for the storage of water over the
tailings.
Heaps are used to pile up more or less dry tailings or waste-rock.
The collapse of any type of TMF can have short-term and long-term effects. Typical short-
term consequences include:
     • flooding
     • blanketing/suffocating
     • crushing and destruction
     • cut-off of infrastructure
     • poisoning.
Potential long-term effects include:
     • metal accumulation in plants and animals
     • contamination of soil
     • loss of animal life.
Guidelines for the design, construction and closure of safe TMFs are available in many
publications. If the recommendations given in these guidelines were to be closely followed,
the risk of a collapse would be greatly reduced. However, major incidents continue to occur
at an average of more than one a year (worldwide).
An investigation of 221 tailings dam incidents has identified the main causes for the reported
cases of dam failures. The main causes were found to be lack of control of the water balance,
lack of control of construction and a general lack of understanding of the features that control
safe operations. It was found that only in very few cases did unpredictable events, such as
unexpected climatic conditions or earthquakes, cause the bursts.
Chapter 4 of the BREF document on the Management of Tailings and Waste-Rock in Mining
Activities contains generic as well as more specific guidelines on studies and plans that
should be developed in the design of an TMF WRMF (conceptual, preliminary and detailed
design stages) and then maintained throughout the sites operation and closure:
     • site selection documentation
     • environmental impact assessment
     • risk assessment
     • emergency preparedness plan
     • deposition plan
     • water balance and management plan, and
     • decommissioning and closure plan.
The plan contents listed above only represent the minimum requirements. In practice, on a
case-by-case basis there may be additional aspects which need to be included.

Site closure and after-care
When an operation comes to an end, the site needs to be prepared for subsequent use.
Usually, these plans are part of the permitting of the site from the planning stage onwards
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and should, therefore, have undergone regular updating, depending on changes in the
operation and in negotiations with the permitters and other stakeholders. In some cases, the
aim is to leave as little a footprint as possible, whereas in other cases, a complete change of
landscape may be aimed for. The concept of ‘design for closure’ implies that the closure of
the site is already taken into account in the feasibility study of a new mine site and is then
continuously monitored and updated during the life cycle of the mine. In any case, negative
environmental impacts need to be kept to a minimum.
Some sites can be handed over to the subsequent user after a relatively simple reclamation,
e.g. after reshaping, covering and re-vegetation. In other cases, after-care will need to be
undertaken for long periods of time, sometimes even in perpetuity.
It is impossible to restore a site to its original condition. However, the operator, the
authorities and the stakeholders involved have to agree on the successive use. It will usually
be the operators responsibility to prepare the site for this. In order to receive a permit for the
closure, the characteristics of the impounded material should be well determined (e.g.
amounts, quality/ consistency, possible impacts). Avoiding future ARD is a main concern for
the closure design for tailings with a net ARD potential.




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BAT for primary zinc smelting
The New Jersey Distillation column is a technique to consider for the pyro-metallurgical
production of primary zinc used in conjunction with the ISF for mixed lead and zinc
concentrates.




Table 26: Overview of primary zinc smelters
The roasting and sulphur recovery systems and the hydro-metallurgical processes that are
discussed above are all considered being BAT. It is not possible to conclude that a single
production process can be applied to this metal. The specific feed materials that are available
to the operator will influence the final process choice, particularly the way in which iron is
precipitated.
The hydrometallurgical processes are very important in the production of zinc. The specific
feed materials will influence the final process choice. As reported earlier the Goethite
process relies on a low iron content of the calcine (or ZnO) used for precipitation while the
Jarosite process is able to give good zinc recoveries even with a high iron content (up to
10%).
In both cases effective washing of the precipitated iron is needed.
Because the hydrometallurgical processes involve leaching and electrolytic stages, adequate
disposal of leached material and electrolyte bleed needs to be considered. The connection of
reactors and filters to suitable scrubbers or de-misters should be considered to prevent the
emission of aerosols. Techniques to render the Jarosite or Goethite residues inert should be
used if possible.
All the wastewater treatment methods are also techniques to be considered in the
determination of BAT. The best available treatment technique or a combination of the
different treatment methods can only be chosen on a site by site basis by taking into account
the site specific factors. The most important factors to decide, which in a specific case would
be the best solution in order to minimise the amount of wastewater and the concentration of
the pollutants are:
    • The process where the wastewater is generated,
    • The amount of water,
    • The pollutants and their concentrations,
    • The level of clean up required, i.e. local or regional water quality standards,
    • The availability of water resources.



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Electrolyte purification
Particular attention should be paid to assessing the potential of arsine and stibine emissions
during the electrolyte purification stages with the removal of arsine and stibine by scrubbing
gases from the chemical treatment stages with an oxidising agent such as potassium
permanganate. The appropriate techniques to monitor and remove arsine and stibine should
also be considered in conjunction with these processes.

Electro-winning
Electro-winning processes that feature optimised cell dimensions (spacing, number of cells
etc), and use aluminium cathode blanks are techniques to be considered. Mechanised (and
automatic) harvesting and stripping as well as more elaborate short circuit detection is also
worth considering depending on the scale of the operation.
Electro-winning produces gases that are evolved at the anode and will produce an acid mist.
This needs to be collected and removed, extraction and mist elimination are used and
collected mist returned to the process. Scrubbing the collected gases does not allow reuse of
the mist and contributes to wastewater. Cell coverings can be used to reduce the amount of
mist formed. Organic or plastic bead layers can be used.

EXAMPLE 5.02 COLLECTION AND TREATMENT OF ELECTROLYTE MIST
Description: - Collection of cell gases or cell-room ventilation air so that they can be
demisted.
Main environmental benefits: - Removal of acid mist that would be other wise emitted
to the local environment. Improvement in the workplace conditions.
Operational data: - Non available, subjective comparison with un-modified plant shows
a significant improvement inside and outside of the plant.
Cross media effects: - Positive effect, by recovering acid that can be returned to the process.
Energy cost of fans.
Economics: - Not assessed but is economically viable in a number of installations.
Applicability:- All electro-winning processes.
Example plants: - Spain.
Electrolyte cooling should be practiced and the heat recovered if possible. De misting of the
cooling air should be carried out.
The processes and the techniques for control, mist collection and acid gas removal are
suitable for use with new and existing installations. Sealed tank house drainage systems, the
recovery of electrolyte bleed are also techniques to be considered.

BAT for other process stages
The processes that were discussed earlier as techniques to consider are all considered being
BAT. The specific feed materials will influence the final process choice. Special attention
should be paid to the prevention, collection and recovery of cell room mists.




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Table 27: Summary of other process stages considered as Best Available Techniques

BAT for fume/gas collection and abatement
The techniques discussed in section 2.7 and 2.8 of the BREF document for Non Ferrous
Metals Industries on the removal of SO2, VOCs, dioxins and dust are techniques to consider
for the various process stages involved in the production of the metals in this group.
The use of secondary hoods is also a technique to consider. The design of the hooding system
needs to take account of access for charging and other furnace operations and the way the
source of process gases change during the process cycle. This can be achieved by the use of a
system of intelligent control to target fume emissions automatically as they occur during the
cycle without the high-energy penalty of continuous operation.
The fume collection systems used can exploit furnace-sealing systems and be designed to
maintain a suitable furnace depression that avoids leaks and fugitive emissions. Systems that
maintain furnace sealing or hood deployment can be used. Examples are through hood
additions of material, additions via tuyeres or lances and the use of robust rotary valves on
feed systems. An intelligent fume collection system capable of targeting the fume extraction
to the source and duration of any fume will consume less energy.
Best Available Techniques for gas and fume treatment systems are those that use cooling and
heat recovery if practical before a fabric filter except when carried out as part of the
production of sulphuric acid and this is covered below. Fabric filters that use modern high
performance materials in a well-constructed and maintained structure are applicable. They
feature bag burst detection systems and on-line cleaning methods.
The sulphur recovery systems and the associated dust and metal recovery stages are those
described in section 2.8 of the BREF document for Non Ferrous Metals Industries. The
production of sulphuric acid is most applicable technique unless a local market exists for
sulphur dioxide. The gas cleaning stage that is used prior to the sulphuric acid plant will
contain a combination of dry EPs, wet scrubbers, mercury removal and wet EPs. The factors
that affect the processes in this section are described above under the section techniques to
consider in the determination of BAT.
Slag granulation systems need a venturi scrubber or wet EP because of the high steam
loading. The ISF process gases also need to use wet scrubbing so that the gases are cooled
prior to use as a fuel.




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Table 28: Summary of abatement options for components in the off-gas
The abatement systems that are considered to be BAT for the components likely to found in
the off gases are summarised in Table 28:
There may be variations in the raw materials that influences the range of components or the
physical state of some components such as the size and physical properties of the dust
produced, these should be assessed locally.
The use of hoods for tapping and casting is also a technique to consider. Tapping fumes will
consist mainly of oxides of the metals that are involved in the smelting process. The design
of the hooding system needs to take account of access for charging and other furnace
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operations and the way the source of process gases change during the process cycle. An
example of a coincident hood for charging and tapping is shown below.

EXAMPLE 5.04 COLLECTION OF FUME
Description: - Co-incident charging and tapping zone for a rotary furnace.




Figure 14: Co-incident fume collection system
Furnace lining wear may mean that door end tapping holes may not allow all of the
metal to be tapped.
Main environmental benefits: - Easier fume collection from a single point.
Operational data: - Non available.
Cross media effects: - Positive effect - good collection efficiency with reduced power
consumption.
Economics: - Low cost of modification. Several examples are operating viably.
Applicability: - All rotary furnaces.
Example plants: - France, UK, Germany
There are several site-specific issues that will apply and some of these are discussed
earlier in this chapter. The process technologies discussed in this chapter, combined
with suitable abatement are capable of meeting the demands of stringent environmental
protection.




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Table 29: Chemical treatment methods for gaseous components

Emissions to air associated with the use of BAT
Emissions to air comprise the captured/abated emissions from the various sources, plus the
fugitive or uncaptured emissions from these sources. Modern, well operated systems result in
efficient removal of pollutants and the information at the time of writing indicates that that
the fugitive emissions can be the largest contributor to the total emissions.
For all processes the total emissions to air are based on the emissions from:
    • The materials handling and storage, drying, pelletising, sintering, roasting and
        smelting stages.
    • Slag fuming and Waelz kiln processes.
    • Chemical refining, thermal refining and electro-winning stages.
    • Melting, alloying, distillation, casting etc stages.
Fugitive emissions can be highly significant and can be predicted from the process gas
capture efficiency and by environmental monitoring.
The following tables summarise the techniques and the associated emissions.




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Table 30: Emissions to air from primary smelting, roasting and sintering associated with the use of BAT
in the lead and zinc sector




Table 31: Emissions to air from chemical refining, electro-winning and solvent extraction




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Table 32: Emissions to air from materials pre-treatment, secondary smelting, thermal refining, melting,
slag fuming and Waelz kiln operation
The metal content of the dust varies widely between processes. In addition for similar
furnaces there are significant variations due to the use of varying raw materials. It is
therefore not accurate to detail specific achievable concentrations for all metals emitted to air
in this document.
Some metals have toxic compounds that may be emitted from the processes and so need to
be reduced to meet specific local, regional or long-range air quality standards. It is
considered that low concentrations of heavy metals are associated with the use of high
performance, modern abatement systems such as a membrane fabric filter provided the
operating temperature is correct and the characteristics of the gas and dust are taken into


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account in the design. The issue is site specific but the following table gives some indication
of the effects on the content of metals in dust that will be encountered locally.




Table 33: Metal content of some dusts from various lead and zinc production processes

BAT for process control
The principles of process control presented in Table 34 are applicable to the BAT production
processes. Some of the furnaces and processes are capable of improvement by the adoption
of many of these techniques.




Table 34: Typical furnace applications
Particular attention is needed for the temperature measurement and control for furnaces and
kettles used for melting the metals in this group so that fume formation is prevented or
minimised. The processes and the techniques for furnace control and melting temperature
control are suitable for use with new and existing installations.

BAT for wastewater treatment
This is a site-specific issue, existing treatment systems are reported to be to a high
standard.All wastewater should be treated to remove dissolved metals, solids and oils/tars.
The use of sulphide precipitation or combined hydroxide/sulphide precipitation to ensure the
removal of lead and cadmium is particularly relevant to metals in this section. Absorbed acid
gases (e.g. sulphur dioxide, HCl) and should be reused or neutralised if necessary. The
techniques listed in Table 35 are the techniques to consider. In a number of installations
cooling water and treated wastewater including rainwater is reused or recycled within the
processes.




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Table 35: Overview of wastewater streams and their treatment methods
For primary and secondary production of the metals in this group, the total emissions to
water are based on:
    • The slag treatment or granulating system.
    • The waste gas treatment system.
    • The leaching and chemical purification system.
    • The electro-winning process.
    • The wastewater treatment system:
    • Surface drainage.
The following table gives associated emissions to water after effluent treatment. The data
given may not be transposable to all installations.




Table 36: Summary of associated emissions to water for some processes

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BAT for process residues
The use or recycling of slags, slimes and filter dusts is considered to be part of the
processes.The iron precipitation method used (Goethite or Jarosite) depends on local
conditions and the composition of the concentrate. The effective washing and precipitation of
the leachable metalsas sulphides before disposal should be considered. The solubility of the
residue should be monitored using a standard leachate test. Disposal should meet the
requirements set out in the directive on landfill.
The production processes in this sector have been developed by the industry to maximise the
reuse of the majority of process residues from the production units or to produce residues in
form that enables them to be used in other non-ferrous metal production processes. An
overview of the potential end uses for residues is given earlier in this chapter and some
specimen quantities are also given for specific installations.
The quantity of residues produced is strongly dependent on the raw materials in particular the
iron content of primary materials, the content of other non-ferrous metals in primary and
secondary materials and the presence of other contaminants such as organic materials. The
emissions to land are therefore very site and material specific and depend on the factors
discussed earlier. It is therefore not possible to produce a realistic, typical table of quantities
that are associated with the use of BAT without detailing the raw material specification. The
principles of BAT include waste prevention and minimisation and the re-use of residues
whenever practical. The production of arsine and stibine from the action of water or water
vapour on some residues should be taken into account.
The industry is particularly effective in these practices the use and treatment options for
some residues from the production of lead and zinc is given in Table 24.
The processes that were presented in Table 24 as available techniques are all techniques to
consider in the determination of BAT. The specific feed materials will influence the final
process choice. Furthermore, to achieve effective waste minimisation and recycling the
following can be considered:
    • Waste minimisation audits can be conducted periodically according to a programme.
    • The active participation of staff can be encouraged in these initiatives.
    • Active monitoring of materials throughput, and appropriate mass balances should be
        available. Monitoring should include water, power, and heat.
    • There should be a good understanding of the costs associated with waste production
        within the process. This can be achieved by using accounting practices that ensure
        that waste disposal and other significant environmental costs are attributed to the
        processes involved and are not treated simply as a site overhead.


Costs associated with the techniques
Cost data has been compiled for a variety of process variations and abatement systems. The
cost data is very site specific and depends on a number of factors but the ranges given may
enable some comparisons to be made. The data is provided in an appendix to this note so that
costs for processes and abatement systems over the whole of the non-ferrous metal industry
can be compared.




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                                 REFERENCES
•   Reference Document on Best Available Techniques for Management of Tailings and
    Waste-Rock in Mining Activities, European Commission Directorate-General Jrc
    Joint Research Centre Institute for Prospective Technological Studies, Sustainability
    in Industry, Energy and Transport, European IPPC Bureau, July 2004
•   Reference Document on Best Available Techniques in the Non Ferrous Metals
    Industries, Integrated Pollution Prevention and Control (IPPC), December 2001
•   International Lead and Zinc Study Group Press Release, Zinc Review Of Trends In
    2006, 22 February 2007 http://www.ilzsg.org/archives.asp?go=getarchive&num=144
•   U.S. Geological Survey, Mineral Commodity Summaries, January 2007
•   The Metallurgical Society Metallurgists Canadian Institute of Mining, Metallurgy and
    Petroleum. http://www.metsoc.org/virtualtour/processes/zinc-lead/zincflow.asp
•   International Zinc Association, http://www.zincworld.org/production.html
•   Technical Resource Document, Extraction And Beneficiation Of Ores And Minerals,
    Volume 1, Lead-Zinc, U.S. Environmental Protection Agency, (U.S.E.P.A.) Office of
    Solid Waste, Special Waste Branch June 1994




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                    LEAD




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    The purpose of this report, in the framework of the PREWARC project, PL 517574, is to present
feasible preventive and remedial technologies that can be adopted by the mining and metallurgical
industries during production as well as during waste management during lead production.


   1. GENERAL INFORMATION
Lead is the most abundant heavy metal in the earth’s crust and is found in pure sulphide ores
or nowadays, more commonly in complex ores, where it is associated with zinc and small
amounts of silver and copper. The most important lead mineral for the mining industry is
galena (PbS), which can contain up to 1 % silver.
Lead is a soft metal; it has a low melting point and is resistant to corrosion. These properties
give it great functional value, both in its pure form and in alloys or compounds.
Its high resistance to corrosion makes it ideal for weatherproofing buildings and for
equipment used in the manufacture of acids. Lead’s high density has proved effective for
weights and anchors for fishing lines, boats, and later for munitions. This property is now
utilised in lead radiation screening and soundproofing.
However, the most important use of lead today is in the lead-acid battery which provides
power in numerous situations. The electrochemical properties of lead enable it to be used in
storage batteries in all motor vehicles, and for some back-up power supplies. Certain
compounds of lead, particularly brightly coloured lead oxides, and leaded glasses and leaded
glazes on ceramics, have been used for many centuries. There have been however, major
changes in the pattern of lead use over the years. The use of most leaded paints has recently
been phased out, but lead is still an important addition to some glasses and glazes. The
battery industry creates up to 70 % of the demand, which is fairly stable, but other uses for
lead such as pigments and compounds, protection against radiation, rolled and extruded
products for the building industry, cable sheathing, shots and gasoline additives are in
decline.
Some metals, such as lead, are essential as trace elements but at higher concentrations are
characterised by the toxicity of the metal, ion or compounds and many lead compounds are
classified as toxic. General policy is normally to restrict emissions to the lowest practicable
levels given the state of technology, and recycling is normally conducted whenever
appropriate and economic. Most control measures are concerned principally with human
exposure (humans are most affected by lead exposure) although there are certain instances in
which animals can be exposed to environmental lead.
Lead can be recycled as a secondary raw material from lead-acid batteries, from metallic
scrap and from several composite consumer products in conjunction with existing recycling
loops, for example for steel, zinc and copper, at moderate costs. At least three-quarters of all
lead used goes into products which are suitable for recycling. For this reason, lead has the
highest recycling rate of all the common non-ferrous metals.
Lead is classified in terms of the composition of the product; the following table shows the
chemical composition of lead according to the new European Standard.




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Table 5: Lead grades


Refined lead is derived from primary material in the form of lead ores and concentrates, and
secondary material in the form of scrap and residue.
Primary production requires the smelting of lead-bearing ores to produce lead bullion that is
then refined. The economics of primary lead ore production is linked to the silver and zinc
contents of the ore bodies. Lead metal production requires the sulphur content of the ores to
be treated to produce sulphuric acid. Most primary lead smelters have a complex refining
process associated with them and associated processes to recover the silver content as an Ag-
Au alloy. Primary refining is therefore linked to the economics of mining lead-zinc ore-
bodies. The zinc and silver content of the ores are the principal profit makers.
Preliminary figures indicate that, in 2006, there was a close balance between refined lead
metal supply and demand both for the world as a whole and in the Western World. A 13.1%
fall in lead mine output was primarily due to lower production from Australia. This was,
however, more than balanced by a 10.8% increase in Chinese output. Overall, global
production rose by 1.4%.
Global refined lead metal production increased by 5.4% primarily influenced by a 14.1%
increase in China. The level of output in both Europe and the United States was similar to
that in 2005. An increase of 3.2% in world usage of refined lead metal in 2006 was driven by
further increases in Chinese production of both automotive and industrial lead-acid batteries
that resulted in a rise in Chinese demand of 12.9%. In the United States, there was limited
growth of 1.8% whilst in Europe demand declined by 3.4%. (International Lead and Zinc
Group)
                 WESTERN WORLD REFINED LEAD METAL BALANCE
000 tonnes                         2002          2003          2004          2005        2006p
Metal Production                   4928         4783          4592          4788          4840
Net Imports from                    451           491           515           509          624
East
US Stockpile                          6            60            56            36           19
Disposals
Metal Usage                        5340         5319          5423          5462          5481
Balance                              45            15          -260          -130             2
p: preliminary
Table 6 Western World Refined Lead Metal Balance , Source: ILZSG


Although lead ore is mined in many countries around the world, three quarters of the world
output comes from only six countries: China, Australia, US, Peru, Canada and Mexico.The

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EU possesses few lead mine resources but lead production is a large and important industry.
During the last 10 years, EU consumption and production have experienced only modest
growth, resulting in a decrease in the EU’s share in world markets.
Some 58% of the lead in France, Germany, Italy and the UK is used for the production of
lead-acid batteries as the countries rely chiefly on the imports from China and Korea. The
lead deposits in the EU are chiefly used for the production of lead cables and sheets.
Bulgaria’s and Romania’s joining the EU is expected to slightly decrease the dependence of
the Union on the imports of primary raw materials as lead, zinc, aluminium and nickel. The
countries from Central and Eastern Europe, including Bulgaria and Romania, account for
some 20% of the total production of aluminium in the EU, and for 16% of the copper, 14%
of the lead and 13% of the zinc production. Poland accounts for nearly the whole production
of copper and for half the lead output in Central and Eastern Europe. (SeeNews 2007)

        Top Mining Countries                      Largest Lead Producers
 Australia                      654,000      China                        1,533,000
 China                          618,000      USA                          1,338,000
 USA                            464,000      Germany                         352,000
 Peru                           308,000      UK                              338,000
 Mexico                         152,000      Australia                       304,000

         Major Users of Lead
                                                     Main Recyclers of Lead
 USA                          1,488,000
                                             USA                          1,098,000
 China                        1050,000
                                             Germany                         222,000
 Germany                        392,000
                                             Japan                           190,000
 Korea Rep                      342,000
                                             UK                              176,000
 UK                             330,000
                                             Italy                           153,000
(Values shown in tonnes)


Table 3: Key World Statistics 2003 (Lead Development Association International 2001)



    2. Lead mining
Base metal ores usually contain several metalliferous minerals. Often copper, lead and zinc
are mined together. Typically base metals are mined as sulphides. Hence, acid rock drainage
is a major issue in the management of tailings and waste-rock.
Lead is mined almost exclusively in underground operations although a few surface
operations do exist. The decision to use underground or surface mining techniques is
dependent on the proximity of the ore body to the surface and the exact mining method used
is determined by the individual characteristics of each ore body.


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Profitable recovery of lead-zinc ores ranges from as low as 3 percent metal in ore for large,
easily accessed mines, to 6 percent for small, difficult-to-access underground mines, to more
than 10 percent for extremely high-cost, remote areas. (Lead and Zinc 2002))
Due to the high overburden (waste-to-ore) ratio for mining of metal ores the volume of solid
waste generated, including tailings from processing, is one of the main pollution concerns in
the mining industry. Figure 6-3 shows the amount of lead and zinc mined in relation to the
amount of waste materials produced.




Figure 1: Materials Handled for Lead and Zinc Mining
Waste rock is generated during lead mining and is either used as a construction material
around the mine, stored in underground openings or disposed of in unlined heaps onsite.
The waste-rock stacked on heaps can have a high environmental impact if it has a net acid
generating potential. The sulphide in tailings and waste-rock can oxidise when water and air
have access and an acidic leachate is generated. This phenomenon is called Acid Rock
Drainage (ARD). ARD is associated with sulphide ore bodies mined for Pb, Zn, Cu, Au, and
other minerals.
The key issues that are the root of these environmental problems are:
     • tailings and/or waste-rock often contain metal sulphides
     • sulphides oxidise when exposed to oxygen and water
     • sulphide oxidation creates an acidic metal-laden leachate
     • leachate generation over long periods of time.
When sulphide minerals come into contact with water and oxygen they start to oxidise. This
is a slow heat generating process (kinetically controlled exothermal process) which is
promoted by:
     • high oxygen concentration
     • high temperature
     • low pH
     • bacterial activity.
The overall reaction rate for a specified quantity of sulphides is also dependant on other
parameters such us, for example, the type of sulphides and the particle size, which also

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governs the exposed surface area. When the sulphides oxidise they produce sulphate,
hydrogen ions and dissolved metals.
Tailings and waste-rock consist of the different natural minerals found in the mined rock. In
the unmined rock, often situated deep below the ground level, the reactive minerals are
protected from oxidation. In oxygen-free environments, such as in deep groundwater, the
sulphide minerals are thermodynamically stable and have low chemical solubility.
Deep groundwater in mineralised areas, therefore, often has a low metal content. However,
when excavated and brought to the surface, the exposure to atmospheric oxygen starts a
series of bio-geo-chemical processes that can lead to production of acid mine drainage.
Hence, it is not the content of metal sulphides in itself that is the main concern, but the
combined effects of the metal sulphide content and the exposure to atmospheric oxygen.
The effect of exposure increases with decreasing grain size and, therefore, increased surface
area. Hence the sulphides in the finely ground tailings are more prone to oxidation.
Tailings and waste-rock are normally composed of a number of minerals, of which the
sulphides only constitute one part, if present at all. Therefore, if sulphide oxidation occurs in
mining waste, the acid produced may be consumed by acid consuming reactions in varying
degrees, depending on the acid consuming minerals available. If carbonates are present in the
mining waste, pH is normally maintained as neutral, the dissolved metals precipitate and thus
are not transported to the surrounding environment to any significant degree.
The interaction between the acid producing sulphide oxidation and the acid consuming
dissolution of buffering minerals determines the pH in the pore water and drainage, which in
turn influences the mobility of metals. If the readily available buffering minerals are
consumed, the pH may drop and ARD will then occur.
The release of ARD to surface- and groundwater deteriorates the water quality and may
cause a number of impacts, such as depletion of alkalinity, acidification, bioaccumulation of
metals, accumulation of metals in sediments, effects on habitats, elimination of sensitive
species and unstable ecosystems.
In mining, ARD may be produced in waste-rock deposits, marginal ore deposits, temporary
storage piles for the ore, tailings deposits, pit walls, underground workings or in heap leach
piles. Historically sulphide containing material has also been used for construction purposes
at mine sites, e.g. in the construction of roads, dams and industrial yards. However,
regardless of where ARD production occurs, the fundamental processes behind the
generation of ARD are the same.
Figure 5 schematically shows some of the most important geochemical and physical
processes and their interaction and contribution to the generation of ARD and the possible
release of heavy metals from mining waste. As can be concluded from the figure, the ARD
and metal release will depend primarily on the sulphide oxidation rate, the potential
immobilisation/remobilisation reactions along the flow path and the water flow. However,
the sulphide oxidation rate is dependant on redox conditions (Eh), pH, and microbial activity.
The pH is, in turn, determined by the sulphide oxidation rate and buffering reactions
(carbonate dissolution and silicate weathering). Furthermore, the potentially metal retaining
immobilisation reactions that can occur along the flow path are dependant on pH, redox
conditions and the sulphide oxidation rate.




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Figure 1: Schematic illustration of some of the most important geochemical and physical processes and
their interaction and contribution to the possible release of heavy metals from mining waste
At the field-scale not only are the temporary variations of material characteristics important
for the evolution of the drainage water quality but the spatial variations will also be a factor
to take into account. The resulting drainage characteristics depend on a number of additional
parameters, such as infiltration rate, evaporation rate, oxygen profile in the deposit, height of
the deposit, and the construction of the deposit. Heterogeneities in the material
characteristics, such as varying mineralogy and degree of compaction, are other parameters
that may affect the drainage water quality. Due to the normally long residence time of the
infiltrating water in the deposit, the influence of various immobilisation reactions
(precipitation and adsorption) can also be significant. The interaction between the tailings
and/or waste-rock and the atmosphere is illustrated schematically in the following figure.




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Figure 3: Schematic illustration of the drainage water generation as a function of the interaction between
the tailings or waste-rock in the facility and the atmosphere
Due to ARD, not only is the physical stability of the tailings ponds and dams an issue, but so
is the chemical stability of the acid generating tailings, both during operation and after the
mine closure.
The chemical composition of mine water generated at mines varies from site to site and is
dependent on the geochemistry of the ore body and the surrounding area. Mine water may
also contain small quantities of oil and grease from extraction machinery and nitrates (NO3)
from blasting activities. EPA and the Bureau of Mines reported concentration ranges in mine
waters of 0.1-1.9 mg/L for lead, 0.12-0.46 mg/L for zinc, 0.02-0.36 mg/L for chromium, 295-
1,825 mg/L for sulfate, and pH of 7.9-8.8. ((U.S.E.P.A. Technical Resource Document,
1994)
After the mine is closed and pumping stops, the potential exists for mines to fill with water.
Water exposed to sulfur-bearing minerals in an oxidizing environment, such as open pits or
underground workings, may become acidified. Emissions from old mines and works, even
from many centuries ago, continue to be a source of pollution in the environment.

    3. Lead processing
Due to low lead content, lead-bearing ores must be concentrated before processing.
Beneficiation of zinc ore, which usually occurs at the mine to keep transport costs low,
consists of crushing, grinding, and flotation to produce concentrates of 50% to 60% zinc.
Beneficiation begins with milling (crushing and grinding) to enable optimal separation from
the other minerals. Milling is a multi-stage crushing and grinding operation. It involves
coarse crushing followed by wet grinding. Crushing is usually a dry operation that utilizes
water sprays to control dust. Primary crushing is often performed at the mine site, followed
by additional crushing at the mill. The crushed ore is mixed with water and initial flotation
reagents to form slurry. The ore is then ground in rod and ball mills. Hydrocyclones may be
used between each grinding step to separate coarse and fine particles. The coarse particles are
returned to the mill for further size reduction. Milling is carefully controlled to produce the
required particle size. Grinding the ore too fine will produce slimes or very fine particles,
which are difficult to recover in the separation process and are usually lost to tailing.
Following these steps, the ore is further beneficiated by flotation. (DOE 2002)



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Flotation can be carried in various ways, e.g., by selective flotation or by bulk/selective
flotation, depending on the characteristics of the ore, the market demands, the cost of
flotation additives, etc.
Two possible options for the same mineral processing plant are illustrated in the figures
below for the Swedish Zinkgruvan mineral processing plant. The Zinkgruvan mineral
processing plant, which was constructed in 1977, is located next to the mine. It operates
continuously with an annual throughput of 850000 tonnes. The choice of process and
technology is based on a large number of test works with the actual zinc and lead ore.
Autogenous grinding in combination with bulk/selective flotation (see Figure 4 below) of the
ore has been chosen as the main process technique and has been used at Zinkgruvan since
1977.




Figure 4: Bulk/selective flotation circuit for Zinkgruvan site




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Figure 5: Possible selective mineral processing circuit for Zinkgruvan site
After the removal of values in the flotation process, the flotation system discharges tailings
composed of liquids and solids. Between ¼ and ½ of the tailings generated are made up of
solids, mostly gangue material and small quantities of unrecovered lead-zinc minerals. The
liquid component of the flotation waste is usually water and dissolved solids, along with any
remaining reagents not consumed in the flotation process. These reagents may include
cyanide, which is used as a sphalerite depressant during galena flotation. Most operations
send these wastes to tailings ponds where solids settle out of the suspension. The liquid
component either is recycled back to the mill or discharged if it meets water quality
standards. The characteristics of tailings from the flotation process vary greatly, depending
on the ore, reagents, and processes used. Lead, zinc, chromium, iron, and sulfate were all
found in the wastewater of the selected facilities.
In general, most wastes from beneficiation of lead-zinc ores are disposed of in tailings
impoundments from which water is likely to be reclaimed during the mine’s life. In addition,
other materials typically not considered wastes, such as mine water, may be managed onsite
during the active life of the facility and may ultimately become wastes.


4. Primary lead smelting
After the ore has been concentrated at a mill near the mine, the lead concentrates are shipped
to a smelter. Here, in the case of galena, the first step involves removing the sulfur from the
mineral. Roasting in air causes the sulfur to be converted to sulfur dioxide gas and the lead
sulphide to lead oxide.
There are two basic pyrometallurgical processes available for the production of lead from
lead sulphide or mixed lead and zinc sulphide concentrates: - sintering/smelting or direct
smelting. The processes may also be used for concentrates mixed with secondary raw
materials.
Sintering/smelting using the Blast Furnace or Imperial Smelting Furnace
Lead concentrates are mixed with recycled sinter fines, secondary material and other process
materials. After moisture is added, the mixture is pelletised in rotating drums. Pellets are fed

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onto an up draught or down draught sinter machine and ignited. The burning pellets are
conveyed over a series of wind-boxes through which air is blown. Sulphur is oxidised to
sulphur dioxide and the reaction generates enough heat to fuse and agglomerate the pellets.
The sinter product is crushed and screened to the correct size for the furnace. Undersize
material is cooled by mixing with de-watered sludge collected from gas cleaning equipment
and recycled to the feed mixture.
The sulphur dioxide is recovered from the sinter machine off-gases, which are cooled,
cleaned and recovered in the form of sulphuric acid. Cadmium and mercury are also present
and are recovered from the off-gases or from the sulphuric acid that is produced.
Sinter is charged to the blast furnace with metallurgical coke. Air and/or oxygen enriched air,
is injected through the tuyeres of the furnace and reacts with the coke to produce carbon
monoxide. This generates sufficient heat to melt the charge. The gangue content of the
furnace charge combines with the added fluxes or reagents to form a slag.
The carbon monoxide reduces the metal oxides in the charge. Slag and lead collect in the
furnace bottom and are tapped out periodically or continuously. The slag is quenched and
granulated using water, or allowed to cool and is then crushed, depending on its destination
or further use. For smelting bulk lead and zinc concentrates and secondary material, a
specially designed blast furnace is used; the Imperial Smelting Furnace. Here, hot sinter and
pre-heated coke as well as hot briquettes are charged. Hot air, sometimes oxygen enriched is
injected with these raw materials. The reduction of the metal oxides not only produces lead
and slag but also zinc, which is volatile at the furnace operating temperature and passes out
of the ISF with the furnace off-gases. The gases also contain some cadmium and lead.




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Figure 6: Diagram of a typical Imperial Smelting Process for zinc and lead production
The furnace gases pass through a splash condenser in which a shower of molten lead
quenches them and the metals are absorbed into the liquid lead. The resulting alloy is cooled
when zinc floats to the surface and is separated from the lead. The zinc is refined by
distillation and this process is covered later in this chapter. Lead is recycled to the splash
condenser.
After the splash condenser, the low calorific value furnace gases (LCV gas), which contain
carbon monoxide and hydrogen, are cleaned and burned to preheat the air and the coke.
Direct smelting
Several processes are used for direct smelting of lead concentrates and some secondary
material to produce crude lead and slag. Bath smelting processes are used – the ISA
Smelt/Ausmelt furnaces (sometimes in combination with blast furnaces), Kaldo (TBRC) and
QSL integrated processes are used in EU and Worldwide. The Kivcet integrated process is

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also used and is a flash smelting process. The ISA Smelt/Ausmelt furnaces and the QSL take
moist, pelletised feed and the Kaldo and Kivcet use dried feed.
The sintering stage is not carried out separately in this instance. Lead sulphide concentrates
and secondary materials are charged directly to a furnace and are then melted and oxidised.
Sulphur dioxide is formed and is collected, cleaned and converted to sulphuric acid. Carbon
(coke or gas) and fluxing agents are added to the molten charge and lead oxide is reduced to
lead, a slag is formed. Some zinc and cadmium are “fumed” off in the furnace; their oxides
are captured in the abatement plant and recovered.
These processes all produce a slag that is rich in lead but the QSL and Kivcet furnaces
incorporate an integral reduction zone to reduce the lead content of the slag to an acceptable
level, the Kaldo process uses an adjacent slag fuming process. The silica based slag from the
QSL process is accepted as construction material at the time of writing. Heat recovery and
conversion of sulphur dioxide to sulphuric acid is also featured in these processes. Dust
collected in abatement plant is returned to the process and can be washed or leached to
reduce halides and Zn / Cd in the recycled dust . All of these processes have taken some time
to commission properly and achieve the anticipated through put and conversion rates. The
Kaldo is a two stage process and is well established. It is reported that the QSL process has
overcome all of the initial problems and is operating effectively. The ISA Smelt/Ausmelt
process is operating only on the initial smelting phase at the time of writing and has not been
commissioned for the slag reduction phase. The Kivcet furnace has been operating
successfully since 1990.




Table 4 : Direct smelting processes

5. Refining of primary lead
After smelting, lead bullion usually contains varying amounts of nonlead materials, such as
gold, silver, bismuth, zinc, as well as metal oxides such as oxides of antimony, arsenic, tin,
and copper. There are two methods of refining crude lead: electrolytic refining and
pyrometallurgical refining. Electrolytic refining uses anodes of de-copperised lead bullion
and starter cathodes of pure lead. This is a high cost process and is used infrequently.
A pyrometallurgical refinery consists of a series of kettles, which are indirectly heated by oil
or gas. Copper is the first element to be removed and separates as sulphide dross. If the crude
metal is deficient in sulphur more must be added in the form of sulphur powder or pyrite.
The sulphide dross is removed from the metal surface by mechanical skimmers that
discharge into containers.
Arsenic, antimony and tin are removed by oxidation. The usual method, often referred to as
“lead softening”, involves a reaction with a mixture of sodium nitrate and caustic soda,
followed by mechanical skimming to remove the oxide dross. Air/oxygen can also be used as
the oxidising agent. Depending on the crude lead composition, i.e. the amount of impurities,
the molten salt mixture may be granulated in water and the impurities separated
hydrometallurgically.

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De-silvering is carried out by the Parkes process, which makes use of the preferential
solubility of silver in zinc. Zinc is added to the lead at about 470 °C and the mix is then
allowed to cool to 325 °C. A silver-lead-zinc alloy separates and forms a crust on the surface.
The crust is removed and zinc separated from the silver by vacuum distillation. The silver
bullion is further refined using oxygen to produce crude silver. Excess zinc is removed from
the de-silvered lead by vacuum distillation and then by treatment with caustic soda.
Bismuth is removed by treatment with a mixture of calcium and magnesium (the Kroll-
Betterton process). A calcium-magnesium-bismuth alloy is formed as dross on the surface of
the lead and is removed by skimming. The dross is then oxidised using lead chloride,
chlorine gas or a caustic soda / sodium nitrate mixture and the calcium magnesium oxide is
removed by skimming. A bismuth-lead alloy is recovered and undergoes further refining to
produce bismuth. The refined lead will have a purity of 99.90–99.99%.




Εικόνα 2: Figure 7: Diagram of lead refining processes


The pure lead is cast into blocks or ingots. Fume, drosses, litharges and other residues are
usually smelted in a small blast furnace or a rotary furnace to produce lead bullion which is
recycled to the refining circuit.


6. Emission and Consumption Levels
The waste streams from the production of lead, are:
   • Particulate matter which consists of lead/zinc and iron oxides; oxides of metals such
      as arsenic, antimony, cadmium, copper, and mercury are also present, along with
      metallic sulfates.

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    •   Sulphur dioxide (SO2) and Nitrogen oxides (NOx)
    •   Off-gases contain fine dust particles and volatile impurities (VOCs) such as arsenic,
        fluorine, and mercury.
    • Fugitive emissions of sulfur dioxide and volatile substances occur at furnace
        openings and from launders, casting molds, and ladles carrying molten materials,
    • Fugitive particulate emissions such as dust from raw materials handling and
        transport of ores and concentrates contains metals, mainly in sulfidic form, although
        chlorides, fluorides, and metals in other chemical forms may be present.
    • Acid vapors (other acid gases) from leaching processes and in various refining
        processes
    • Products of incomplete combustion (PICs) from refining processes
    • Emissions of arsine, chlorine, and hydrogen chloride vapors and acid mists are
        associated with electrorefining.
    • Wastewaters are generated by wet air scrubbers and cooling water and may contain
        lead/zinc, arsenic, and other metals. Other sources of wastewater include spent
        electrolytic baths, slimes recovery, spent acid from hydrometallurgy processes,
        cooling water, air scrubbers, washdowns, and stormwater. Pollutants include
        dissolved and suspended solids, metals, and oil and grease.
    • The larger proportion of the solid waste is discarded slag from the smelter. Discard
        slag may contain 0.5–0.7% lead/zinc and is frequently used as fill or for
        sandblasting.grease.
    • Residues from leaching processes (the iron rich residues
    • Sludges from effluent treatment
    • Filter dust
    • Odours
Many wastes are reused but the major item is leach residue that has a high environmental
impact. Some local aspects, like noise, are relevant to the industry. Due to the hazardous
nature of some solid and liquid waste streams, there is also a significant risk for soil
contamination. The following tables give input and output balances for some lead and zinc
plants in Europe.




Table 5: Input and output data for an ISP plant (1998)




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Table 6: Input and output data for the ISA Smelt furnace (lay out and preliminary data)




Table 7: Input and output data for the QSL plant (1997)

Energy
The energy requirement for the different lead and zinc processes varies to a large extent. It
depends on the quality of the feed and the products, the use of latent or waste heat and the
production of by-products.
About one-third of the energy consumed for lead is in mining and beneficiation. Looking at
both mining and processing, lead requires 1,293,700 Btu per ton of lead produced. (Lead and
Zinc 2002)
The following table shows the average energy requirements of the different processes.




Table 8: Energy requirement of various lead processes

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Emissions to air
The main emissions to air from zinc and lead production are: -
   • sulphur dioxide (SO2), other sulphur compounds and acid mists;
   • oxides of nitrogen (NOx) and other nitrogen compounds;
   • metals and their compounds;
   • dust.
   • VOCs and dioxins.

Other pollutants are considered to be of negligible importance for the industry, partly
because they are not present in the production process and partly because they are
immediately neutralised (e.g. chlorine) or occur in very low concentrations. Emissions are to
a large extent bound to dust (except cadmium, arsenic and mercury that can be present in the
vapour phase)
The sources of emissions from the process are: -
    • roasting (Most emissions occur during unscheduled shutdown)
    • other pre-treatment (battery breaking)
    • transport and handling of material
    • smelting and refining
    • leaching and purification
    • electrolysis
    • casting
    • sulphuric acid plant




Table 9: Significance of potential emissions to air from lead, zinc and cadmium production
The emissions can escape the process either as stack emissions or as fugitive emissions
depending on the age of the plant and the technology used. Stack emissions are normally
monitored continuously or periodically and reported. The major fugitive emission sources are

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    •   dust from storage and handling of concentrates (10 t/y);
    •   leakage from roasters and smelters;
    •   dust from the exhaust gases of leaching and purification vessels (1 t/y);
    •   exhaust gases of cooling towers of the leaching and purification units (0.7 t/y);
    •   exhaust gases of cooling towers of the electrolysis process (08 t/y);
    •   dust from the exhaust gases of casting furnaces (1.8 t/y);
    •   miscellaneous (0.7 t/y).

Air emissions for processes with few controls may be of the order of 30 kilograms lead or
zinc per metric ton (kg/t) of lead or zinc produced. Although fugitive emissions are difficult
to measure and estimate, there are some methods that have been used successfully (section
2.7 of the BREF document on Non Ferrous metals). The potentially high level of fugitive
emissions is illustrated in the following table which gives emission data based on the
upgrading of a lead process from blast furnace to ISA Smelt:




Table 10: Significance of plant improvements on fugitive emissions


Sulphur dioxide and other sulphur compounds
The major sources of sulphur dioxide emission are fugitive emissions from the oxidation
stages, direct emissions from the sulphuric acid plant and the emission of residual sulphur in
the furnace charge. Good extraction and sealing of the furnaces prevent fugitive emissions
and the collected gases from oxidation stages are passed to a gas cleaning plant and then to
the sulphuric acid plant.
After cleaning, the sulphur dioxide in the gas from the sintering, roasting or direct smelting
stages is converted to sulphur trioxide (SO3). The efficiency generally lies between 95 to
99.8% depending on the sulphuric acid plant used (single or double absorption) and the
concentration of sulphur dioxide in the input gas and its variation or stability. SO2
concentrations in the off gas from 200 - 2300 mg/Nm3 can be emitted. A very small amount
of SO3 is not absorbed and is emitted together with the SO2. During start up and shut down
there may be occasions when weak gases are emitted without conversion. These events need
to be identified for individual installations. Many companies have made significant
improvements to process, control, prevent or reduce these emissions.
Lead sinter and some secondary raw materials contain residual sulphur and sulphates. It has
been reported that 10% of the sulphur content of lead concentrate remains in the sintered
material that is fed into the furnaces. In most cases the sulphur is fixed in the slag or in other
by-products. The extent of fixation depends on the fluxes used and the other metals
associated with process, for example copper matte may be produced when Pb/Cu

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concentrates are treated together. Pb/Fe matte is produced under reducing conditions when
iron turnings are added. In other cases SO2 may be emitted and may need further treatment.
During electrolysis, emissions of aerosols (diluted sulphuric acid and zinc sulphate) are
generated to the hall. These emissions leave the cell room via the (natural) ventilation or
from the cooling towers. The emission is small compared with the emissions from the
sulphuric acid plant but as they are in the form of an aerosol form, they can be dealt with in
mist eliminators or dust abatement. Some processes use coverings for the cells such as foam
or plastic beads to reduce mist formation. One plant has been recently modified to improve
roasting and to collect fugitive emissions from the whole of the process.
Sulphur dioxide emissions were reduced from 3000 to 1200 g per tonne of metal produced.
Emissions from other processes are shown below.




Table 11: Sulphur dioxide production from several zinc and lead processes


Nitrogen oxides
The roasting and smelting stages are potential sources of nitrogen oxides (NOx). NOx may
be formed out of nitrogen components that are present in the concentrates or as thermal NOx.
The sulphuric acid produced can absorb a large part of the NOx and this can therefore affect
sulphuric acid quality. If high levels of NOx are present after the roasting stages, treatment of
the roasting gases may be necessary for reasons of product quality and environmental
reasons. Other furnaces that use oxy-fuel burners can also show a reduction in NOx. The
range for all of the processes is 20 to 400mg/Nm3.

Dust and metals
The roasting and smelting as well as the storage and handling stages, are potential sources
both of direct and fugitive emissions of dust and metals. The gases are collected and treated
in the gas cleaning processes of the sulphuric acid plant. Dust is removed and returned to the
process.

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The gases leaving splash condensers in the ISF, from distillation columns and from the
tapping points are also potential sources. Good extraction and abatement is needed at these
points to prevent fugitive emissions. Slag treatment and quenching also gives rise to dust.
The range of dust emissions from these captured sources is < 1 to 20 mg/Nm3.




Table 12: Mass release of metals from some European processes (controlled emissions only)
De-aeration of vessels in the leaching and purification stages can emit dust and metals.
Emissions of aerosols takes place in the cell room and battery breakers and can contain
metals.
The range of mist and dust emissions from these sources is 0.1 to 4mg/Nm3.
The melting, alloying, casting and zinc dust processes are potential emission sources of
dust and metals. The range of dust emissions is reported to be 200 to 900mg/Nm3 in the
crude gas. Fume collection and abatement systems are used and cleaned gas values are
below 10mg dust/Nm3.

VOCs and dioxins
The formation of dioxins in the combustion zone and in the cooling part of the off-gas
treatment system (de-novo synthesis) may be possible in some processes particularly if
plastic components are included in the secondary materials that are fed into a process.
Dioxins have also shown to present in some dusts from Waelz kilns treating EAF dust.

Emissions to water
Lead and zinc ore beneficiation and the smelting of primary lead produce a number of
wastewaters and slurries. Metals and their compounds and materials in suspension are the


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main pollutants emitted to water. The metals concerned are Zn, Cd, Pb, Hg, Se, Cu, Ni, As,
Co and Cr. Other significant substances are:
   • metals;
   • materials in suspension;
   • chlorides, fluorides, sulphates.

Possible wastewater streams are:
   • Waste water from wet scrubbers;
   • Waste water from wet electrostatic precipitators;
   • Waste water from the mercury-removal step;
   • Wastewater from battery breaking and classification stages;
   • Wastewater from slag granulation;
   • Wastewater from various hydro-metallurgical processes;
   • Anode and cathode washing liquid effluent;
   • Sealing water from pumps;
   • General operations, including cleaning of equipment, floors, etc.;
   • Discharge from cooling water circuits;
   • Rainwater run-off from surfaces (in particular storage areas) and roofs.

Wastewater from the gas cleaning of the smelter and fluid-bed roasting stages are the most
important sources. Other sources are the process effluent from electrolysis, battery breaking
and cleaning plus miscellaneous sources.

Waste waters from abatement plant
Generally wet gas cleaning systems operate with liquid recycling. A monitored bleed keeps
suspended solids and dissolved salts within certain defined limits. The bleed is either treated
separately or in an integrated water treatment plant to remove solids and dissolved species
before discharge. The destination of the separated material depends on the origin of the
wastewater.
Wet scrubbers after the roasting process are operated with a SO2-saturated acidic solution.
The scrubber removes fluorides, chlorides, most mercury and selenium and the some
particles that pass the mechanical gas treatment. To avoid the build up of contaminants, some
liquid needs to be bled continuously from the scrubber. Dissolved SO2 is removed during
treatment prior to the discharge.
Wet electrostatic filters will also produce an acidic scrubber liquid. This is recycled after
filtering. Some liquid needs to be bled from this circuit to remove build up of contaminants.
This bleed liquor is treated and analysed before discharge.
The mercury-removal step involves a gas-liquid contact tank in which the liquid contains a
reagent that combines with mercury and removes it. Mercury chloride (HgCl2) is frequently
used and reacts with metallic mercury from the gas to form a solid Hg2Cl2-precipitate (so-
called “calomel”). The relatively clean liquid is discharged as wastewater for further
treatment. The solid Hg2Cl2 is sold for mercury recovery or treated to produce mercury
chloride again. The following table provides an indication of the composition of the gas
cleaning liquids before treatment.




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Table 13: Typical gas cleaning effluents

Miscellaneous sources
The electrodes used during the electrolysis need to be rinsed periodically to remove
deposited material upon the surface. Manganese dioxide is formed on the surface of the
anodes by the reaction of oxygen with dissolved manganese. After rinsing of the anodes, the
manganese is separated from the rinse water for external re-use.




Table 14: Typical wastewater analyses
Cathodes are cleaned after removal of the zinc or lead sheets. The anode and cathode
washing liquid effluents are acidic and likely to contain copper, zinc, lead and suspended
solids

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Table 15: Summary Table of Potential Wastewater Sources and Options
Cooling water from the granulation of slag is usually re-circulated in a closed circuit system.

Process residues and wastes
The electrolytic production of zinc is one of the main sources of solid waste in the non-
ferrous industry. Relatively large quantities of iron rich solids are generated by the leaching
process.
Jarosite and goethite are classified as hazardous waste because of the content of leachable of
elements such as Cd, Pb and As. The leaching and purification processes and electrolysis of
zinc and the refining stages of lead also generate other metal rich solids. These are usually
rich in a specific metal and are recycled to the appropriate production process.
The ISF or direct smelting furnaces are also significant sources of solid slags. These slags
have been subjected to high temperatures and generally contain low levels of leachable
metals, consequently they may be used in construction.


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Solid residues also arise as the result of the treatment of liquid effluents. The main waste
stream is gypsum waste (CaSO4) and metal hydroxides that are produced at the wastewater
neutralisation plant. These wastes are considered to be a cross-media effect of these
treatment techniques but many are recycled to pyrometallurgical process to recover the
metals. Dust or sludge from the treatment of gases are used as raw materials for the
production of other metals such as Ge, Ga, In and As etc or can be returned to the smelter or
into the leach circuit for the recovery of lead and zinc.
Hg/Se residues arise at the pre-treatment of mercury or selenium streams from the gas
cleaning stage. This solid waste stream amounts to approximately 40 - 120 t/y in a typical
plant. Hg and Se can be recovered from these residues depending on the market for these
metals.
The production of metals is related to the generation of several by-products, residues and
wastes, which are also listed in the European Waste Catalogue (Council Decision 94/3/EEC).
The most important process specific residues are listed below.
Solid residues derived from various process and abatement stages may have one of three
possible destinations.
    • Recycling in or upstream of the process;
    • Downstream treatment to recover other metals;
    • Final disposal, if necessary after treatment to ensure safe disposal.


Leaching residues
The production of iron based solids (goethite, jarosite or hematite) accounts for the greatest
volume of waste depending on the process used. The composition is shown in the following
table




Table 16: Example compositions of different types of residues.
Typically, these residues account for: -
    • Jarosite - 0.35 to 0.80 tonnes per tonne of zinc produced.
    • Goethite - 0.3 to 0.35 tonnes per tonne of zinc produced.
    • Hematite – 0.2 tonnes per tonne of zinc produced.
Hematite processes have been unable to compete in economic terms as the process is
significantly more complex and expensive to operate. In addition, hematite has not proved to
be acceptable as a raw material in other industries.
There are still some leachable metals in slurry after filtering and washing. The residue can be
treated to a less leachable form with neutralisation and sulphide treatment. The disposal of
these residues can be considerable cost as specially constructed, lined ponds or isolated areas
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are used to contain the material. Particular care is taken about leakage and these ponds have a
major need to monitor groundwater. There is a significant cross media effect compared to
processes that are capable of producing an inert residue.
Leaching residues can be treated in ISF or Waelz Kiln. Leachable slag and recoverable metal
oxides , problems with contaminant build up have been reported.

Pyrometallurgical slags and residues
Slags from the Blast Furnace, ISF, Direct Smelting and Waelz kiln processes usually contain
very low concentrations of leachable metals. They are therefore generally suitable for use in
construction. The slag output is between 10 and 70% of the weight of the metal produced
dependant on the raw materials used.
Slags from the battery processing plants account for 13 to 25% of the weight of lead
produced.
They may be suitable for construction uses depending on the leachability of the metals they
contain. The leachability is influenced by the fluxes used and the operating conditions. The
use of sodium based fluxes (Na2CO3) to fix sulphur in the slag causes an increase in the
quantity of leachable metals. These slags and drosses from battery recovery processes can
contain Sb. This is normally recovered but storage in damp conditions can cause the emission
of stibine.
A number of standard leachability tests are used by Member States and these are specific to
the country in question.




Table 17: Eluate values of granulated IS furnace slag




Table 18: Eluate values for slag from QSL process




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Table 19: Solid Material from the refining of lead bullion
The drosses and solids, removed during the zinc and lead melting and refining stages, contain
metals that are suitable for recovery.

Other materials




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The following tables show the use or treatment options for the residues produced by several
processes.




Table 20: Residues from lead processes




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Table 21: Residues from direct smelting lead processes



7. BAT for lead production
This section presents a number of techniques for the prevention or reduction of emissions
and residues as well as techniques reducing the overall energy consumption. They are all
commercially available.
The control of furnace operating parameters and the prevention of fugitive emissions from
furnaces and the tapping and pouring processes are all important. Techniques used by other
sectors are also applicable particularly those relating to the use of sulphur recovery systems.
The techniques to consider on a site by site basis are strongly influenced by the raw materials
that are available to a site, in particular the type and variability of the concentrate or
secondary raw materials, the metals they contain can be crucial to the choice of process.
Some processes have a dedicated single source of raw material but the majority of
installations in Europe buy concentrate on the open market and need to maintain flexibility in
processing a range of raw materials. In a similar manner the standard of collection and
abatement systems used worldwide in the industry reflects local, regional and long-range

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environmental quality standards and direct comparison of the environmental performance of
process combinations is therefore difficult. It is possible however, to judge how a particular
process can perform with the appropriate, modern abatement equipment.
The processes described above are applied to a wide range of raw materials of varying
quantity and composition and are also representative of those used worldwide. The
techniques have been developed by the Companies in this sector to take account of this
variation. The choice of pyro-metallurgical or hydrometallurgical technique is driven by the
raw materials used, their quantity, the impurities present, the product made and the cost of
the recycling and purification operation. These factors are therefore site specific. The
techniques to consider for collection and abatement stages and other aspects of process
operation and control are covered in sections 2.6, 2.7 and 2.8 of the BREF Document for
Non Metal Ferrous Industries.
Environmental legislation has required investment to reduce lead in air emissions. In recent
years several new technologies have been developed and implemented which offer more
efficient methods of smelting lead concentrates. These processes have also reduced
emissions to the environment. Existing processes have been improved by using up to date
control and abatement systems.
Regulations affecting lead fall into three main categories: occupational exposure, emissions
(ambient air quality) and controls on food water and products. Occupational exposure is
addressed under EU directive 82/605/EEC of July 28, 1992 on the protection of workers
from risks related to exposure to metallic lead and its ionic compounds at work. This
directive sets limits on the level of lead in air in the workplace and on certain biological
indicators which reflect the level of exposure of individual workers. The limit values are
complemented by rules on the protection of the workforce providing for the use of protective
clothing, respirators, washing facilities or specifying rules on eating, drinking, smoking, etc.
Lead in the general atmosphere is limited under directive 82/844/EEC of December 3, 1982,
which sets a limit for levels of lead in air throughout the EU. These limit values are currently
being revised. Levels of lead in water are also controlled in a number of directives relating to
water depending on its type and use e.g., water intended for human consumption, water for
bathing, fishing waters, etc.

BAT for materials storage, handling and pre-treatment processes
The raw materials are concentrates, secondary raw materials, fluxes and fuel; other important
materials are products, sulphuric acid, slags, sludges and process residues.
Important aspects are the prevention of leakage of dust and wet material, the collection and
treatment of dust and liquids and the control of the input and operating parameters of the
handling and feeding processes. The issues specific to this group are: -
The potentially dusty nature of concentrates and fluxes means that enclosed storage, handling
and treatment systems may be needed in these instances. The dust generated by some
crushing operations means that collection and abatement may be applicable for this process.
Similarly granulation water may require settlement or other treatment prior to discharge.
Concentrates are mixed with fluxes to produce a fairly constant feed therefore the general
practice is sampling and analysis to characterise the concentrates and store individual
concentrates separately so that an optimum blend can be prepared for smelting.
Feed blends can be prepared from dosing bin systems using belt weighers or loss in weight
systems. Final mixing and homogenisation can take place in mixers, pelletisers or in the
conveying and metering systems. Enclosed conveyors or pneumatic transfer systems are used
for dusty material. Hot gas rotary dryers or steam coil dryers can be used if the process
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requires a dry feed, steam coil dryers use waste heat from other parts of the process provided
that the heat balance allows it. The drier and associated dust abatement stage therefore
depends on sitespecific conditions such as the reliability of the steam supply. Fabric or
ceramic filters achieve better dust removal efficiencies than EPs when used at this stage of
the process.
Acid produced during the process can be stored in double walled tanks or tanks placed in
chemically resistant bunds. The treatment of acid slimes from the sulphuric acid plant and
weak acid from scrubbing systems depends on local processing or disposal requirements
unless there is a local use for the material.
Sludges and other metallic residues that are destined for recovery off site can be stored drums
or other suitable ways depending on the material. Sludges produced during the process that
are destined for on site disposal should be washed free of zinc or other metals and de-watered
as far as possible. Disposal facilities should be totally contained and leak proof, they are
subject to local control and regulation. Water from the sludge containment areas can be
returned to the process.
There are a variety of secondary raw materials used and they range from fine dusts to large
single items. The metal content varies for each type of material and so does the content of
other metals and contaminants. Batteries are a common source of lead and can contain acid,
the storage and handling therefore needs to take account of the acid content and any acid
mists that can be formed. Nickel cadmium batteries are usually dry but other batteries may be
present and leakage of electrolyte is possible, this should be taken into account in the storage
and separation method used. The techniques used for storage, handling and pre-treatment
will therefore vary according to the material size and the extent of any contamination. These
factors vary from site to site and techniques will be applied on a site and material specific
basis.
The following issues apply to this group of metals.
    • The storage of raw materials depends on the nature of the material described above.
        The storage of fine dusts in enclosed buildings or in sealed packaging is used.
        Secondary raw materials that contain water-soluble components are stored under
        cover. The storage of nondusty, non soluble material (except batteries) in open
        stockpiles and large items individually in the open can be used.
    • Pre-treatment stages are often used to produce sinter or to remove casings or coatings
        and to separate other metals. Milling and grinding techniques are used with good dust
        extraction and abatement. The fine dust that is produced may be treated to recover
        other metals, pneumatic or other density separation techniques are used.
    • Fine dusts can be stored and handled in a manner that prevents the emission of dust.
        They are often blended and agglomerated to provide a consistent feed to the furnace.
        Sintering is used to prepare concentrates for some of the smelting processes up draft
        and down draft sintering machines can be used and recent developments of a steel
        belt sintering process may be appropriate. Collection of fume and gases is important
        and the up draft sintering process is inherently easier for fume capture. Gases contain
        sulphur dioxide and will have abatement and sulphur dioxide recovery processes
        down stream. The sulphur dioxide content is usually low and variable and this
        influences the design of the sulphuric acid plant.
    • Zinc concentrates are roasted prior to hydro metallurgical processing. Fluidised bed
        roasters are almost universally used and need good extraction and calcine removal
        systems. Gases are treated in an integrated abatement and sulphur dioxide recovery
        processes.
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There are also several general techniques that are considered to be BAT in preventing
emissions from material storage and handling processes. These techniques are:
    • The use of liquid storage systems that are contained in impervious bunds that have a
        capacity capable of containing at least the volume of the largest storage tank within
        the bund. Various guidelines exist within each Member State and they should be
        followed as appropriate. Storage areas should be designed so that leaks from the
        upper portions of tanks and from delivery systems are intercepted and contained in
        the bund. Tank contents should be displayed and associated alarms used. The use of
        planned deliveries and automatic control systems to prevent over filling of storage
        tanks.
    • Storage areas for reductants such as coal, coke or woodchips need to be surveyed to
        detect fires, caused by self-ignition.
    • Sulphuric acid and other reactive materials should also be stored in double walled
        tanks or tanks placed in chemically resistant bunds of the same capacity. The use of
        leak detection systems and alarms is sensible. If there is a risk of ground water
        contamination the storage area should be impermeable and resistant to the material
        stored.
    • Delivery points should be contained within the bund to collect spilled of material.
        Back venting of displaced gases to the delivery vehicle should be practised to reduce
        emissions of VOCs. Use of automatic resealing of delivery connections to prevent
        spillage should be considered.
    • Incompatible materials (e.g. oxidising and organic materials) should be segregated
        and inert gases used for storage tanks or areas if needed.
    • The use of oil and solid interceptors if necessary for the drainage from open storage
        areas. The storage of material that can release oil on concreted areas that have curbs
        or other containment devices. The use of effluent treatment methods for chemical
        species that are stored.
    • Transfer conveyors and pipelines placed in safe, open areas above ground so that
        leaks can be detected quickly and damage from vehicles and other equipment can be
        prevented. If buried pipelines are used their course can be documented and marked
        and safe excavation systems adopted.
    • The use of well designed, robust pressure vessels for gases (including LPG’s) with
        pressure monitoring of the tanks and delivery pipe-work to prevent rupture and
        leakage. Gas monitors should be used in confined areas and close to storage tanks.
    • Where required, sealed delivery, storage and reclamation systems can be used for
        dusty materials and silos can be used for day storage. Completely closed buildings
        can be used for the storage of dusty materials and may not require special filter
        devices.
    • Sealing agents (such as molasses and PVA) can be used where appropriate and
        compatible to reduce the tendency for material to form dust.
    • Where required enclosed conveyors with well designed, robust extraction and
        filtration equipment can be used on delivery points, silos, pneumatic transfer systems
        and conveyor transfer points to prevent the emission of dust.
    • Non-dusty, non-soluble material can be stored on sealed surfaces with drainage and
        drain collection.
    • Swarf, turnings and other oily material should be stored under cover to prevent
        washing away by rain water.

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    •   Rationalised transport systems can be used to minimise the generation and transport
        of dust within a site. Rainwater that washes dust away should be collected and
        treated before discharge.
    •   The use of wheel and body washes or other cleaning systems to clean vehicles used
        to deliver or handle dusty material. Local conditions will influence the method e.g.
        ice formation. Planned campaigns for road sweeping can be used.
    •   Inventory control and inspection systems can be adopted to prevent spillages and
        identify leaks.
    •   Material sampling and assay systems can be incorporated into the materials handling
        and storage system to identify raw material quality and plan the processing method.
        These systems should be designed and operated to same high standards as the
        handling and storage systems.
    •   The use of good design and construction practices and adequate maintenance.




Table 22: Storage, handling and pre-treatment methods for lead, zinc and cadmium


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BAT for mining and processing waste management
The environmentally relevant parameters of tailings and waste-rock management facilities
can be subdivided into two categories: (1) operational and (2) accidental. Both have to be
taken into consideration.
The three very important environmental issues which need to be highlighted are:
    1. the generation of acid rock drainage
    2. the occurrence of accidental bursts or collapses
    3. site rehabilitation and after-care

Acid rock drainage
If an acid-forming potential exists, it is BAT to firstly prevent the generation of ARD. If the
generation of ARD cannot be prevented, to control ARD impact or to apply treatment
options. Often a combination is used.
The management of potentially ARD is a cyclic process and is originally done in the
planning phase of the mine, but is renewed and re-evaluated continuously throughout the
mine life. The assessment process always covers the ‘cradle-to-grave’ concept, i.e. any
preferred option with respect to the management of tailings and waste-rock during the
operational phase of the operation should also include an acceptable closure strategy.
The basis for any preventive measure is the characterisation of the tailings and waste-rock,
together with a comprehensive management plan which identifies and minimises the amount
of tailings and waste-rock that requires special attention.
There are a number of prevention, control and treatment options developed for potentially
ARD generating tailings and waste-rock.
Preventive as well as control measures usually focus on water management, alkaline addition
and special handling. These strategies alone or in combination can substantially reduce or
mitigate generation of acid drainage. Water management can include the following:
    • Active mining operations can incorporate diversions to route surface drainage away
        from pyritic material or through alkaline material.
    • Spoil material can be placed and rough graded to prevent ponding and subsequent
        infiltration.
    • Prompt removal of pit water can lessen the amount and severity of acid generated.
    • Polluted pit water can be isolated from non-contaminated sources (no commingling) to
        reduce the quantity of water requiring treatment.
    • Constructed underdrain systems can be used to route water away from contact with
        acid forming material.
Alkaline placement strategies involve either mixing directly with pyritic material or
concentrated placement to create a highly alkaline environment. Direct mixing places
alkaline materials in intimate contact with pyritic spoil to inhibit acid formation and
neutralize any generated acidity in situ. Alkaline recharge employs trenches loaded with
alkaline material, usually a combination of soluble sodium carbonate and crushed limestone.
The strategy is to charge infiltrating waters with high doses of alkalinity sufficient to
overwhelm any acid produced within the backfill. This approach is highly dependent on the
placement of the alkaline trenches to provide maximum inflow to the acid producing zones.
A third variant of the alkaline placement technique is encapsulation with alkaline material
above and below the acid- producing zone.
       In the BREF document, to prevent ARD formation, these are specific BAT measures:
   •   Water cover and underwater (sub-aqueous)
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   • Dry covers
   • Oxygen consuming covers
   • Wetland establishment
   • Raised groundwater level
   • Isolation above the Water Table
   • Depyritisation
   • Selective material handling
Other preventive methods aim at minimising the bacterial activity and minimising the
mineral surface area available for weathering.
Water cover and underwater (sub-aqueous) discharge uses a free water cover as an oxygen
diffusion barrier. Submergence relies on several physico-chemical phenomena for success.
Oxygen diffuses very slowly and has limited solubility in water. For this approach to
succeed, a stagnant or no flow condition and relatively thick saturated zone appears critical.
Stagnant flow conditions leading to the development of anoxic (oxygen free) conditions and
a saturated thickness on the order of several tens of feet appear to effectively curtail oxygen
diffusion. This approach is most successful in large mines in flat terrain where ground-water
gradients are low, the saturated zone is thick, and aquifers are of large areal extent.
In general, flooding to prevent AMD is believed to be more successful in below drainage
mines. It is assumed that complete flooding eliminates oxygen and halts or severely curtails
acid generation. Flooding of above drainage mines is also practiced typically through the use
of “wet” seals, which allow water to drain but exclude air entry. However, sealing and
flooding above drainage mines does reduce acid loading but is technically more difficult and
less effective than other methods in AMD prevention.
Dry cover uses a low permeable layer with high water content as an oxygen diffusion barrier.
Oxygen consuming cover uses a low permeable layer with high water content as an oxygen
diffusion barrier. In addition, the low permeable layer has a high content of organic matter
which, when it degrades consumes oxygen and thereby further reduces oxygen transport to
the underlying sulphides.
Wetland establishment as a closure method, uses the same principle as the water cover but
with less water depth as the plant cover stabilises the bottom, and thereby re- suspension of
the tailings can be avoided.
Raised groundwater level maintains the underlying sulphide material constantly below the
groundwater table by retaining water through:
   • increased infiltration
   • reduced evaporation
   • increased flow resistance
   • capillary forces
Depyritisation promotes the separation of pyrite from the tailings and separate discharge of
the pyrite (e.g. under water).
Selective material handling refers to the selective management of various tailings or
wasterock fractions determined by their composition and properties, e.g. separation of
material with ARD generating potential for separate handling.
Isolation above the water table: Placement of pyritic material above a water table is an
attempt to isolate the material from contact with water, and preclude leaching of acid
weathering products. Compaction and capping with clay or other materials may also be
employed to reduce permeability. In practice, it has proven very difficult to completely
isolate spoil materials from water contact. Clay caps and other flow barriers are prone to

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leakage, and the sporadic infiltration of rain or snowmelt may periodically leach the spoil.
The capping approach can be extended to complete encapsulation on top, bottom and sides as
a further effort to isolate the materials from water contact. (Office of Surface Mining)
On occasion, despite the application of sound mining and reclamation principles, Acid Mine
Drainage will be formed and must be treated to meet existing standards before it is released
from the mine site. When the weathering reactions cannot be prevented (such as might be the
case during the operational stage of the mine life), the migration of ARD needs to be
controlled.
Methods of water treatment used to eliminate acid mine drainage from abandoned
underground mines can be grouped into two types, active and passive. The following
techniques are BAT for treating acid effluents:
   • active treatments:
           o   addition of limestone (calcium carbonate), hydrated lime or quicklime
           o   addition of caustic soda for ARD with a high manganese content
   •   passive treatment:
           o constructed wetlands
           o   open limestone channels/anoxic limestone drains
           o   diversion wells.
Active treatment requires constant maintenance and involves neutralizing acid-polluted water
with lime, sodium hydroxide (caustic soda), sodium carbonate (soda ash) or ammonia. This
treatment reduces acidity and significantly decreases iron and other metals, but is expensive
to construct and operate. The laws require treatment as long as mine drainage is produced,
which can be several decades.
Passive treatment involves the construction of a treatment system that is typically designed to
last 20 – 40 years and requires much less maintenance than active treatment. This technology
involves the use of wetlands, ponds, and anoxic limestone drains. Passive treatment systems
are relatively inexpensive to construct and many are very successful
Also, curtain grouting, relief wells and compartmentalized barriers are several of the
techniques suggested for controlling ARD discharges.
By a combination of compaction and sealing of the underlying strata, ARD generation is
minimized and uncontrolled seepage into the ground is avoided. A number of factors dictate
the level of sophistication of the treatment system that is necessary to ensure that effluent
standards will be met.
    These factors include:
   • the chemical characteristics of the Acid Mine Drainage,
   • the quantity to be treated,
   • climate,
   • terrain,
   • sludge characteristics, and
   • projected life of the plant.
The chemicals used for Acid Mine Drainage treatment include limestone, hydrated lime,
soda ash, caustic soda and ammonia.
Cleaning up Acid Mine Drainage from abandoned coal mines is very difficult and expensive.
The least costly and most effective method of controlling ARD is to prevent its initial
formation. Optimal strategies are site-specific and a function of geology, topography,
hydrology, mining method and cost effectiveness.


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Accidental bursts or collapses
On the 30th of August the drainage culvert underneath the waste (“tailings”) dump at the Sasa
lead-zinc mine in FYROM collapsed. There was a spill of tailings into the river which had
extremely serious consequences. Bursts or collapses of tailings dams at several other
operations (e.g. in Aznalcollar and Baia Mare) have brought public attention to the
management of tailings ponds and tailings dams. Such collapses of tailings and waste-rock
heaps can cause severe environmental damage.
The dimensions of either type of tailings management facility can be enormous. Dams can be
tens of metres high, heaps even more than 100 m and several kilometres long possibly
containing hundreds of millions of cubic metres of tailings or waste rock.
At the other extreme are ponds the size of a swimming pool or heaps smaller than a
townhouse. Tailings dams are built to retain slurried tailings. In some cases, material
extracted from the tailings themselves is used for their construction. Tailings dams have
many features in common with water retention dams. Actually, in many cases they are built
as water retaining dams, particularly where there is a need for the storage of water over the
tailings.
Heaps are used to pile up more or less dry tailings or waste-rock.
The collapse of any type of TMF can have short-term and long-term effects. Typical short-
term consequences include:
     • flooding
     • blanketing/suffocating
     • crushing and destruction
     • cut-off of infrastructure
     • poisoning.
Potential long-term effects include:
     • metal accumulation in plants and animals
     • contamination of soil
     • loss of animal life.
Guidelines for the design, construction and closure of safe TMFs are available in many
publications. If the recommendations given in these guidelines were to be closely followed,
the risk of a collapse would be greatly reduced. However, major incidents continue to occur
at an average of more than one a year (worldwide).
An investigation of 221 tailings dam incidents has identified the main causes for the reported
cases of dam failures. The main causes were found to be lack of control of the water balance,
lack of control of construction and a general lack of understanding of the features that control
safe operations. It was found that only in very few cases did unpredictable events, such as
unexpected climatic conditions or earthquakes, cause the bursts.
Chapter 4 of the BREF document on the Management of Tailings and Waste-Rock in Mining
Activities contains generic as well as more specific guidelines on studies and plans that
should be developed in the design of an TMF WRMF (conceptual, preliminary and detailed
design stages) and then maintained throughout the sites operation and closure:
     • site selection documentation
     • environmental impact assessment
     • risk assessment
     • emergency preparedness plan
     • deposition plan
     • water balance and management plan, and
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    • decommissioning and closure plan.
The plan contents listed above only represent the minimum requirements. In practice, on a
case-by-case basis there may be additional aspects which need to be included.

Site closure and after-care
When an operation comes to an end, the site needs to be prepared for subsequent use.
Usually, these plans are part of the permitting of the site from the planning stage onwards
and should, therefore, have undergone regular updating, depending on changes in the
operation and in negotiations with the permitters and other stakeholders. In some cases, the
aim is to leave as little a footprint as possible, whereas in other cases, a complete change of
landscape may be aimed for. The concept of ‘design for closure’ implies that the closure of
the site is already taken into account in the feasibility study of a new mine site and is then
continuously monitored and updated during the life cycle of the mine. In any case, negative
environmental impacts need to be kept to a minimum.
Some sites can be handed over to the subsequent user after a relatively simple reclamation,
e.g. after reshaping, covering and re-vegetation. In other cases, after-care will need to be
undertaken for long periods of time, sometimes even in perpetuity.
It is impossible to restore a site to its original condition. However, the operator, the
authorities and the stakeholders involved have to agree on the successive use. It will usually
be the operators responsibility to prepare the site for this. In order to receive a permit for the
closure, the characteristics of the impounded material should be well determined (e.g.
amounts, quality/ consistency, possible impacts). Avoiding future ARD is a main concern for
the closure design for tailings with a net ARD potential.
Locally, pollution may still occur from drainage from abandoned mines and by mobilization
of mine tailings. The uptake of heavy metals by plants and the consequences for human
health are also to be noted.

BAT for primary lead smelting
The lead smelting processes to consider are: -
   • For mixed lead and zinc concentrates after sintering - the Imperial smelting furnace
       incorporating a splash condenser and New Jersey distillation column for zinc and
       cadmium purification and separation. Sintering stages should have good gas
       collection.
   • For lead concentrates and some secondary raw materials - the blast furnace and the
       electric furnace after sintering or roasting or smelting of the concentrates. The direct
       smelting processes that use the Kaldo, ISA Smelt/Ausmelt, QSL or Kivcet processes.
   • For mixed copper and lead concentrates - the electric furnace after roasting the
       concentrate in a fluidised bed roaster.




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Table 23: Overview of primary lead smelters
The following techniques, when used with appropriated collection and abatement techniques,
are considered to be BAT for the production of lead.
Good gas collection and abatement systems and energy recovery applied to these processes
offer advantages in energy efficiency, cost, throughput and ease of retrofitting.
Gases from the sintering, roasting and direct smelting processes should be treated to remove
dust and volatile metals, to recover heat or energy and the sulphur dioxide recovered or
converted to sulphuric acid depending on local markets for sulphur dioxide.




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Table 24: Primary lead smelters considered as BAT
The abatement system to consider for primary smelting processes is dust removal and the
removal of other metals followed by the recovery of sulphur dioxide. This is usually
achieved by conversion to sulphuric acid in a double contact process with four or more
passes, sometimes a caesium-doped catalyst is used.
Conversion of part of the SO2 into liquid SO2 can be practised, with the balance being
converted into sulphuric acid. The use of a single contact plant or WSA process is a
technique to consider for weak sulphur dioxide gas streams. The gases are cooled (with
heat/energy recovery) and cleaned before conversion. A combination of coolers and hot
electrostatic precipitators or a combination of scrubbers (radial or jet) and wet EPs are used.
Mercury recovery systems are employed using the techniques discussed in section 2.8.of the
BREF document on Non Ferrous Metals Industries.
Steel belt, up-draught or fully enclosed down-draft sintering processes are techniques to be
considered. Steel belt sintering has several advantages for certain metal groups and can
minimise gas volumes, reduce fugitive emissions and recover heat.

Slag treatment
Slag fuming and slag reduction stages as well as techniques discussed in section 2.8 of the
Non Ferrous Metals Industries BREF document that are appropriate to the process, are BAT
techniques to consider. The specific feed materials will influence the final process choice.

BAT for lead refining
During refining temperature control of the kettles and fume collection and abatement systems
should be considered. Refining kettles are not considered suitable for melting scrap lead that
is contaminated with organic materials.



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The refining stages that are considered to be BAT are any of the following techniques that
are listed as applied techniques; the combination of refining processes will depend on the
metals contained in the lead bullion.
    • Copper removal and separation as sulphide dross.
    • Arsenic, antimony and tin are removed by oxidation with a mixture of sodium nitrate
         and caustic soda, followed by mechanical skimming to remove the oxide dross.
         Air/oxygen can also be used.
    • De-silvering by the Parkes process and zinc removal by vacuum distillation.
    • Bismuth removal by treatment with a mixture of calcium and magnesium in the
         Kroll-Betterton process.
The processes would be used with efficient primary and if necessary, secondary fume
collection systems. Temperature control of the refining kettles is particularly important to
prevent lead fume and indirect heating is more effective in achieving this.

BAT for fume/gas collection and abatement
The techniques discussed in section 2.7 and 2.8 of the BREF document for Non Ferrous
Metals Industries on the removal of SO2, VOCs, dioxins and dust are techniques to consider
for the various process stages involved in the production of the metals in this group.
The use of secondary hoods is also a technique to consider. The design of the hooding system
needs to take account of access for charging and other furnace operations and the way the
source of process gases change during the process cycle. This can be achieved by the use of a
system of intelligent control to target fume emissions automatically as they occur during the
cycle without the high-energy penalty of continuous operation.
The fume collection systems used can exploit furnace-sealing systems and be designed to
maintain a suitable furnace depression that avoids leaks and fugitive emissions. Systems that
maintain furnace sealing or hood deployment can be used. Examples are through hood
additions of material, additions via tuyeres or lances and the use of robust rotary valves on
feed systems. An intelligent fume collection system capable of targeting the fume extraction
to the source and duration of any fume will consume less energy.
Best Available Techniques for gas and fume treatment systems are those that use cooling and
heat recovery if practical before a fabric filter except when carried out as part of the
production of sulphuric acid and this is covered below. Fabric filters that use modern high
performance materials in a well-constructed and maintained structure are applicable. They
feature bag burst detection systems and on-line cleaning methods.
The sulphur recovery systems and the associated dust and metal recovery stages are those
described in section 2.8 of this document, the production of sulphuric acid is most applicable
technique unless a local market exists for sulphur dioxide. The gas cleaning stage that is used
prior to the sulphuric acid plant will contain a combination of dry EPs, wet scrubbers,
mercury removal and wet EPs.
Slag granulation systems need a venturi scrubber or wet EP because of the high steam
loading.
The ISF process gases also need to use wet scrubbing so that the gases are cooled prior to use
as a fuel.
The abatement systems that are considered to be BAT for the components likely to found in
the off gases are summarised in the following table.



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Table 25: Summary of abatement options for components in the off-gas
There may be variations in the raw materials that influences the range of components or the
physical state of some components such as the size and physical properties of the dust
produced, these should be assessed locally
The use of hoods for tapping and casting is also a technique to consider. Tapping fumes will
consist mainly of oxides of the metals that are involved in the smelting process. The design
of the hooding system needs to take account of access for charging and other furnace
operations and the way the source of process gases change during the process cycle. An
example of a coincident hood for charging and tapping is shown below.

EXAMPLE 5.04 COLLECTION OF FUME
Description: - Co-incident charging and tapping zone for a rotary furnace.




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Figure 14: Co-incident fume collection system
Furnace lining wear may mean that door end tapping holes may not allow all of the
metal to be tapped.
Main environmental benefits: - Easier fume collection from a single point.
Operational data: - Non available.
Cross media effects: - Positive effect - good collection efficiency with reduced power
   consumption.
Economics: - Low cost of modification. Several examples are operating viably.
Applicability: - All rotary furnaces.
Example plants: - France, UK, Germany
There are several site-specific issues that will apply and some of these are discussed
earlier in this chapter. The process technologies discussed in this chapter, combined
with suitable abatement are capable of meeting the demands of stringent environmental
protection.




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Table 26: Chemical treatment methods for gaseous components

Emissions to air associated with the use of BAT
Emissions to air consist of the captured/abated emissions from the various sources, plus the
fugitive or uncaptured emissions from these sources. Evidence indicates that fugitive
emissions can be the largest contributor to the total emissions, however, modern, well
operated systems can bring about efficient removal of pollutants.
For all processes the total emissions to air are based on the emissions from:
    • The materials handling and storage, drying, pelletising, sintering, roasting and
        smelting stages.
    • Slag fuming and Waelz kiln processes.
    • Chemical refining, thermal refining and electro-winning stages.
Fugitive emissions can be highly significant and can be predicted from the process gas
capture efficiency and by environmental monitoring.
The following tables summarise the techniques and the associated emissions.




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Table 27: Emissions to air from primary smelting, roasting and sintering associated with the use of BAT
in the lead and zinc sector




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Table 28: Emissions to air from materials pre-treatment, secondary smelting, thermal refining, melting,
slag fuming and Waelz kiln operation
The metal content of the dust varies widely between processes. In addition, for similar
furnaces there are significant variations due to the use of varying raw materials. It is
therefore not accurate to detail specific achievable concentrations for all metals emitted to air
in this document.
Some metals have toxic compounds that may be emitted from the processes and so need to
be reduced to meet specific local, regional or long-range air quality standards. It is
considered that low concentrations of heavy metals are associated with the use of high
performance, modern abatement systems such as a membrane fabric filter -provided the
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operating temperature is correct and the characteristics of the gas and dust are taken into
account in the design. The issue is site specific but the following table gives some indication
of the effects on the content of metals in dust that will be encountered locally.




Table 29: Metal content of some dusts from various lead and zinc production processes



BAT for process control
The principles of Best Available Techniques include the concepts of how a process is
designed, operated, controlled, manned and maintained. These factors allow good
performance to be achieved in terms of emission prevention and minimisation, process
efficiency and cost savings.
Good process control is used to achieve these gains and also to maintain safe conditions.
Some of the furnaces and processes are capable of improvement by the adoption of many of
these techniques. Particular attention is needed for the temperature measurement and control
for furnaces and kettles used for melting the metals in this group so that fume formation is
prevented or minimised. The processes and the techniques for furnace control and melting
temperature control are suitable for use with new and existing installations.




Table 30: Typical furnace applications



BAT for wastewater treatment
This is a site-specific issue; existing treatment systems are reported to be to a high standard.
All wastewater should be treated to remove dissolved metals and solids. In some cases a two
stage precipitation process is used with a hydroxide stage followed by a sulphide stage to
ensure the removal of lead and cadmium. Absorbed acid gases (e.g. sulphur dioxide, HCl)
and should be reused or neutralised if necessary. The techniques listed in Table 31 are the
techniques to consider. In a number of installations cooling water and treated wastewater
including rainwater is reused or recycled within the processes.


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Table 31: Overview of wastewater streams and their treatment methods
For primary and secondary production of the metals in this group, the total emissions
to water are based on:
    • The slag treatment or granulating system.
    • The waste gas treatment system.
    • The leaching and chemical purification system.
    • The electro-winning process.
    • The wastewater treatment system:
    • Surface drainage.
The following table gives associated emissions to water after effluent treatment. The data
given may not be transposable to all installations.




Table 32: Summary of associated emissions to water for some processes

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BAT for process residues
The processes that were discussed earlier as available techniques are all techniques to
consider in the determination of BAT. The specific feed materials will influence the final
process choice.
The use or recycling of slags, slimes and filter dusts is considered to be part of the processes.
The iron precipitation method used (Goethite or Jarosite) depends on local conditions and the
composition of the concentrate. The effective washing and precipitation of the leachable
metalsas sulphides before disposal should be considered. The solubility of the residue should
be monitored using a standard leachate test. Disposal should meet the requirements set out in
the directive on landfill.
The production processes in this sector have been developed by the industry to maximise the
reuse of the majority of process residues from the production units or to produce residues in
form that enables them to be used in other non-ferrous metal production processes. An
overview of the potential end uses for residues is given earlier in this chapter and some
specimen quantities are also given for specific installations.
The quantity of residues produced is strongly dependent on the raw materials in particular the
iron content of primary materials, the content of other non-ferrous metals in primary and
secondary materials and the presence of other contaminants such as organic materials. The
emissions to land are therefore very site and material specific and depend on the factors
discussed earlier. It is therefore not possible to produce a realistic, typical table of quantities
that are associated with the use of BAT without detailing the raw material specification. The
principles of BAT include waste prevention and minimisation and the re-use of residues
whenever practical. The production of arsine and stibine from the action of water or water
vapour on some residues should be taken into account.
The industry is particularly effective in these practices the use and treatment options for
some residues from the production of lead and zinc is given in Table 21.
The processes that were presented in Table 21 as available techniques are all techniques to
consider in the determination of BAT. The specific feed materials will influence the final
process choice. Furthermore, to achieve effective waste minimisation and recycling the
following can be considered:
    • Waste minimisation audits can be conducted periodically according to a programme.
    • The active participation of staff can be encouraged in these initiatives.
    • Active monitoring of materials throughput, and appropriate mass balances should be
        available. Monitoring should include water, power, and heat.
    • There should be a good understanding of the costs associated with waste production
        within the process. This can be achieved by using accounting practices that ensure
        that waste disposal and other significant environmental costs are attributed to the
        processes involved and are not treated simply as a site overhead.


Costs associated with the techniques
Cost data has been compiled for a variety of process variations and abatement systems. The
cost data is very site specific and depends on a number of factors but the ranges given may
enable some comparisons to be made. The data is provided in an appendix to this note so that
costs for processes and abatement systems over the whole of the non-ferrous metal industry
can be compared.


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                                   REFERENCES
•   Integrated Pollution Prevention and Control (IPPC), Reference Document on Best
    Available Techniques for Management of Tailings and Waste-Rock in Mining
    Activities (2004) European Commission
•   Integrated Pollution Prevention and Control (IPPC), Reference Document on Best
    Available Techniques in the Non Ferrous Metals Industries (2001) European
    Commission
•   Lead Review Of Trends In 2006, (2007) International Lead and Zinc Study Group
    Press Release, http://www.ilzsg.org/
•   Lead and Zinc Smelting, (1998) Pollution Prevention and Abatement Handbook,
    World Bank Group
•   Lead and Zinc, Mining Industry Of The Future (2002) Energy and Environmental
    Profile of the U.S. Mining Industry, U.S. Department of Energy (D.O.E.), Office of
    Energy Efficiency and Renewable Energy
•   Extraction and Mining Industry in Southeastern Europe, (2007) SeeNews – Research
    & Profiles, http://www.seenews.com/news/latestnews/seenewsresearch_profiles-
    extractionandminingindustryinsoutheasterneurope-143903/
•   Thornton I., Rautiu R., and Brush S. (2001) “Lead The facts” On-Line Fact Book,
    Imperial College Consultants Ltd (London, UK) Lead Development Association
    International




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            FERROSILICON




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    The purpose of this report, in the framework of the PREWARC project, PL 517574, is to present
feasible preventive and remedial technologies that can be adopted by the mining and metallurgical
industries during production as well as during waste management during ferrosilicon production.


1. General information
Silicon (Si) is a light chemical element having metallic and nonmetallic characteristics. In the
form of silicates it constitutes more than 25% of the Earth’s crust. Silica (SiO) is a silicate
consisting entirely of silicon and oxygen. Silica, in the form of quartz or quartzite contains
more than 98% SiO2 and is the main raw material for the production of ferrosilicon.
Ferrosilicon is an alloy of iron and silicon containing calcium, aluminum, carbon, sulphur
and phosphorous as impurities; industry standards are set for common impurities, including
iron and aluminium.
Silicon metal and ferrosilicon are referred to according to the silicon contained in the
material and the maximum amount of trace impurities present. There are two standard grades
of ferrosilicon; respectively containing about 50% and 75% silicon by weight.
Ferrosilicon is used:
       • as a deoxidiser in the production of steel
       • as a reducing and alloying element for iron, steel and ferroalloys
       • in the making of electrical grade steel
       • in the production of anti-corrosive and acid resistant steel
       • in the manufacturing of power rectifiers
       • in the manufacturing of welding electrodes
       • as graphitising agent in the production of cast iron
       • as a heavy-medium ore separation material
On average, 5.4 kg of ferrosilicon are used per tonne of stainless steel produced, and 17.0 kg
are used per tonne of cast iron produced.
World production of ferrosilicon has been approximately 4 million metric tons (gross weight)
per year over the past five years. The leading countries for ferrosilicon production, in
descending order of production, were China, Russia, Norway, and Ukraine.
On a silicon content basis, Western world consumption of ferrosilicon averages about 1.7
million metric tons per year, and silicon metal averages about 968,000 metric tons per year.
Demand for ferrosilicon as an alloying agent in carbon, stainless and alloy steels is at an all
time high and continuing to rise, with growth rates of over 6% in world steel output.
Generally ferroalloy production increases in accordance with steel production. Because of its
abundance in Earth’s crust, silica reserves around the world are more than adequate to sustain
silicon production levels indefinitely. (American Geological Institute 2007)

2. Raw materials
Although raw materials to produce ferro-silicon and silicon-metal are available everywhere
in the world, not all sources allow the production, under economic and quality conditions, of
all the ranges of silicon alloys. In order to achieve good process results, the selection of the
raw material is due to some quality requirements. The thermal strength of the quartzite for
example is of special importance, because it is connected to the gas permeability of the
charge where too much fine sized material may prevent gas flow. The carbon quality is
important for the environmental performance of the process, because the coal and coke
contains normally sulphur and some other unwanted elements. If for instance carbon contains

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mercury or other vaporous elements, they will evaporate in the process and will be
transferred as part of the off-gas into the environment.
The raw materials that are commonly used for the production of ferro-silicon, silicon metal
and silico-calcium are listed in the following table.




Table 1: Raw material for the production of ferro-silicon, Si-metal and silico-calcium

3. Production of ferro-silicon

Ferro-silicon is commonly produced in low-shaft three phase submerged electric arc
furnaces. The electric arc furnaces can be of the open or semi-closed type. Norway, the
biggest FeSi producer in the EU, uses primarily furnaces that are semi-open. In low-income
countries FeSi is manufactured in open furnaces. The open furnaces are commonly built up
with moveable curtains or gates around the hoods for furnace maintenance and eventually
manual feeding. The furnace hood is the upper part of the furnace. The hood has several
tasks. First of all it collects the process off-gas and shields the equipment from the process
heat. Secondly, the hood is the site for electrode arrangements, raw material charging and
cooling arrangement for the furnace. The furnace normally rotates e.g. once a week, in order
to connect the reaction areas around each electrode tip. This homogenises the molten metal in
the furnace and saves 5-to10% of electric energy. The rotation gives rise to some difficulties
in obtaining good capture efficiency of the fugitive emissions at the tap-hole as the location
of the tap-hole will rotate with the furnace. A typical electric arc furnace for the production
of ferro-silicon is shown in the following figure.




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Figure 1: Electric arc furnace for the production of ferro-silicon


Raw material is normally supplied to or closed to the plant by truck and train. Several
ferrosilicon and silicon plants are located near the sea or a river where boats are mainly used
for transportation of raw materials and products. The loading and unloading of raw material
is done with the use of crane grips, front-end loaders or dumper trucks.
First, the feed silica is washed, sized, and crushed. The silica is then mixed with a reducing
agent, and either coal, coke, or charcoal. Wood chips are added for porosity. The mixture is
fed into the furnace and through feeding tubes into the smelting zone around the electrodes.
In small furnaces the raw material can also be fed by using stocking cars. For the production
of FeSi and CaSi Søderberg electrodes are used. The electrodes normally supply about 5–
10% of the total requirement of reducing agent.
The chemical reaction is as follows (Sjardin 2003): Fe2O3 + 2 SiO2 + 7C -> 2 FeSi + 7 CO
 The process off-gas containing silica fume is cleaned in a bag house using fabric filters. The
 liquid metal is tapped off continuously or at regular intervals. The metal is cast from the ladle
 after the tapping is finished. Transportable tapping vessels can be brought to the tapping
 position by suitable vehicles or by overhead cranes. The metal can also be directly tapped to
 the casting area without using transport vessels The silicon alloy is then cast into moulds and
 crushed by using jaw-, rotary or roll crushers or granulated in water.
 The furnace is tapped periodically and the molten ferrosilicon or silicon metal is drawn out
 and cast into ingots. The ingots are allowed to cool, then are crushed to produce the final
 product.

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A process flow diagram for the production of ferro-silicon, silicon metal and calcium silicon
is presented in the next figure.




Figure 2: Process flow diagram for a modern production of ferro-silicon and silicon metal
To obtain a higher purity metal a further refining step is needed. The refining takes place by
oxidising the impurities in a ladle. Injection of oxygen gas or air is done through immersed
lances, porous plugs in the ladle bottom or injectors. Correcting slag can also be added to
improve the refining process. The refining stage is covered with a fume collection system
e.g. a fume collection hood that is connected with a bag house.

4. Materials handling and storage
The main raw materials used in the production of non-ferrous metals are ores and
concentrates, secondary raw materials, fuels (oil, gases and solid fuel) and process gases
(such as oxygen, chlorine and inert gases). Other materials such as fluxes, additives and
process chemicals (e.g. for abatement systems) are also used. This variety of materials
possesses many handling and storage problems and the specific technique used depends on
the physical and chemical properties of the material. The study identified that the following
techniques are used.
The main environmental impact by storage and handling of these materials are fugitive dust
emissions and contamination of surface water and soil caused by wash out from rainwater.

Best Available Techniques (BAT) for materials, storage and handling
The techniques that are used depend to a large extent on the type of material that is being
used. For example large, heavy items are treated by a completely different range of
techniques to fine, dusty material. These issues are specific to individual sites and materials.

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There are however, several techniques that are considered to be BAT in preventing emissions
from material storage and handling processes. These techniques are:
   • The use of liquid storage systems that are contained in impervious bunds that have a
      capacity capable of containing at least the volume of the largest storage tank within
      the bund. Various guidelines exist within each Member State and they should be
      followed as appropriate. Storage areas should be designed so that leaks from the upper
      portions of tanks and from delivery systems are intercepted and contained in the bund.
      Tank contents should be displayed and associated alarms used.
     • The use of planned deliveries and automatic control systems to prevent over filling
         of storage tanks.
     • Sulphuric acid and other reactive materials should also be stored in double walled
         tanks or tanks placed in chemically resistant bunds of the same capacity. The use of
         leak detection systems and alarms is sensible. If there is a risk of ground water
         contamination the storage area should be impermeable and resistant to the material
         stored.
     • Delivery points should be contained within the bund to collect spilled of material.
         Back venting of displaced gases to the delivery vehicle should be practised to
         reduce emissions of VOCs. Use of automatic resealing of delivery connections to
         prevent spillage should be considered.
     • Incompatible materials (e.g. oxidising and organic materials) should be segregated
         and inert gases used for storage tanks or areas if needed.
     • The use of oil and solid interceptors if necessary for the drainage from open storage
         areas. The storage of material that can release oil on concreted areas that have curbs
         or other containment devices. The use of effluent treatment methods for chemical
         species that are stored.
     • Transfer conveyors and pipelines placed in safe, open areas above ground so that
         leaks can be detected quickly and damage from vehicles and other equipment can
         be prevented. If buried pipelines are used their course can be documented and
         marked and safe excavation systems adopted.
     • The use of well designed, robust pressure vessels for gases (including LPG’s) with
         pressure monitoring of the tanks and delivery pipe-work to prevent rupture and
         leakage. Gas monitors should be used in confined areas and close to storage tanks.
     • Where required, sealed delivery, storage and reclamation systems can be used for
         dusty materials and silos can be used for day storage. Completely closed buildings
         can be used for the storage of dusty materials and may not require special filter
         devices.
     • Sealing agents (such as molasses and PVA) can be used where appropriate and
         compatible to reduce the tendency for material to form dust.
     • Where required enclosed conveyors with well designed, robust extraction and
         filtration equipment can be used on delivery points, silos, pneumatic transfer
         systems and conveyor transfer points to prevent the emission of dust.
     • Non-dusty, non-soluble material can be stored on sealed surfaces with drainage and
         drain collection.
     • Swarf, turnings and other oily material should be stored under cover to prevent
         washing away by rainwater.



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     •   Rationalised transport systems can be used to minimise the generation and transport
         of dust within a site. Rainwater that washes dust away should be collected and
         treated before discharge.
     •    The use of wheel and body washes or other cleaning systems to clean vehicles used
         to deliver or handle dusty material. Local conditions will influence the method e.g.
         ice formation. Planned campaigns for road sweeping can be used.
     •     Inventory control and inspection systems can be adopted to prevent spillages and
         identify leaks.
     •      Material sampling and assay systems can be incorporated into the materials
         handling and storage system to identify raw material quality and plan the
         processing method. These systems should be designed and operated to same high
         standards as the handling and storage systems.
     •       Storage areas for reductants such as coal, coke or woodchips need to be surveyed to
         detect fires, caused by self-ignition.
     •       The use of good design and construction practices and adequate maintenance.

The following table summarises the techniques on the basis of type and characteristics of the
material.




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Table 2: Summary of handling and storage techniques
To prevent contamination raw materials are preferably stored on hard surfaces where indoor
and outdoor storage may be used, depending on the potentially dusty nature and the chemical
properties of the materials. To keep the materials clean the storage area can also be divided in
different storage-bays. Dry fine-grained materials should be stored and handled inside where
closed silos, bins and hoppers are used to prevent fugitive emissions to the environment as
well as to the workspace. Excessive dusting can also be prevented by water spraying of dry
fine materials.
Closed conveyors and transfer systems are used for handling of dusty fine materials, where
extraction and filtration equipment is used for dusting delivery points. The dust leaden air
from the silos, closed conveyors and charging systems are cleaned by using bag filters, which
may be monitored by measuring the pressure drop to control the cleaning mechanism.


5. Processing techniques
Ores, concentrates and secondary raw materials are sometimes in a form that cannot be used
directly in the main process. Drying/thawing may be needed for control or safety reasons.

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The material size may need to be increased or decreased to promote reactions or reduce
oxidation.
Reducing agents such as coal or coke and fluxes or other slag forming materials may need to
be added to control the metallurgical process. Coatings may need to be removed to avoid
process abatement problems and improve melting rates. All of these techniques are used to
produce a more controllable and reliable feed for the main process and are also used in
precious metal recovery to assay the raw material so that toll recovery charges can be
calculated.
During such beneficiation processes dust is emitted.

BAT techniques for ore preparation and pre-treatment
This section presents a number of techniques for the prevention or reduction of emissions
and residues as well as techniques reducing the overall energy consumption. They are all
commercially available. Examples are given in order to demonstrate techniques, which
illustrate a high environmental performance. The techniques that are given as examples
depend on information provided by the industry, European Member States and the valuation
of the European IPPC Bureau.
The pre-processing and transfer operations often deal with materials that are dry or are likely
to produce process emissions to any of the environmental media. More detailed design of the
process equipment used at this stage is therefore needed and the processes need to be
monitored and controlled effectively. The nature of the material (e.g. dust forming,
pyrophoric) needs to be taken into account and the potential emissions of VOCs and dioxins
in thermal processes needs to be assessed. Extraction and abatement systems in particular
need to be carefully designed, constructed and maintained. The review of applied techniques
in this section includes the issues that will be encountered in the various process options. The
techniques listed for raw materials handling should also be referred to. The following items
however are considered to be the most important.
•      A shaft furnace is preferably used for coke drying were the use of recovered energy or
    the CO rich off-gas from the smelting furnace as a secondary fuel is suitable. Bag filters
    are used to clean the off-gas were the associated dust emission level is 5 mg/Nm3.
•   Use of pre-treatment and transfer processes with well designed robust extraction and abatement
    equipment to prevent the emission of dust and other material. The design of this equipment
    should take account of the nature of the emissions, the maximum rate of emissions and all of the
    potential sources.
•   Use of enclosed conveying systems for dusty materials. These systems should be provided with
    extraction and abatement equipment where dust emissions are possible.
•   Processes that “flow” directly into the following process if possible to minimise handling and
    conserve heat energy.
•   Use of wet grinding, blending and pelletising systems if other techniques for the control of dust
    are not possible or appropriate.
•   Thermal cleaning and pyrolysis systems (e.g. swarf drying and de-coating) that use robust after-
    burning equipment to destroy combustion products e.g. VOCs and dioxins. The gases should be
    held at a temperature greater than 850 °C (1100 °C if there is more than 1% halogenated organic
    material), in the presence of at least 6% oxygen for a minimum of 2 seconds. Lower residence
    times may as well result in the complete destroying of VOCs and dioxins but this should be
    demonstrated on a local level. Gases should be cooled rapidly through the temperature window of
    dioxin reformation.
•   To reduce the impact of VOC’s, washing processes to remove oil or other contaminants should
    use benign solvents. Efficient solvent and vapour recovery systems should be used.

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•   Steel belt, up-draught or fully enclosed down-draft sintering processes are techniques to be
    considered. Steel belt sintering has several advantages for certain metal groups and can minimise
    gas volumes, reduce fugitive emissions and recover heat. These are discussed later. Off gas
    extraction systems should prevent fugitive emissions.
•   The use of rotary kilns with wet ash quenching for the processes involving the volume reduction
    of material such as photographic film. Smaller installations may use a moving grate furnace. In
    both cases the combustion gases should be cleaned to remove dust and acid gases if they are
    present.
•   If required to minimise the generation of smoke and fumes and to improve the melting rates,
    separation processes should be designed to produce clean materials that are suitable for recovery
    processes.
•   Collection and treatment of liquid effluents before discharge from the process to remove non-
    ferrous metals and other components.
•   The use of good design and construction practices and adequate maintenance.




Table 3: Summary of pre-treatment methods
Some plants use crushers or agglomeration equipment to obtain the desired size of charging
material. Bag filters clean the suction air of crushers and agglomeration equipment. Wet
grinding, filtering and pelletising systems are as well suitable to prevent dusting. In this case
the water is recycled.
The collected dust is recycled to the charging system, which may need an additional
agglomeration step.



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6 Pre-reduction and pre-heating
The electricity consumption of the smelting furnace can be decreased by pre-heating the feed
materials Pre-heating for instance as it is used in the production of FeCr increases at the same
time the productivity of the smelting furnace. However, the technology of pre-reducing ore
and concentrates is fully implemented only in two plants worldwide. As reported, there are
still some problems operating this technology. Pre-reduction is therefore not yet
recommended as a general BAT in this sector.


7. Smelting process
In the production of ferro-alloys the most important stage is the reduction of metal oxides
and alloying with the iron present in the process. Depending on the reducing agent, different
types of smelting systems (such as the electric arc furnace, the blast furnace or a reaction
crucible) are used. Electric arc furnaces are normally operated submerged as a closed, semi-
closed or open type. The concept of the different smelting systems is influenced by the
desired flexibility in the production, the range of raw material, the possibilities of energy
recovery and the environmental performance. The different techniques considered for the
recovery of energy, are very much dependent on the used smelting system but also on local
conditions such as local energy prices, periods of production and the presence of potential
customers.

BAT for smelting
The different furnaces used for the ferro-alloy production are listed in the following table that
summarises the advantages and disadvantages of the various systems.




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Table 4: Summary of advantages and disadvantages of the used smelting systems in the ferro-alloy industry.
Taking account of the above advantages and disadvantages the smelting systems to
consider are:
     • Open furnace for special applications and small capacities connected with a bag filter
         
      •    Semi-closed furnace connected with a bag filter
      •     Closed furnace systems in different applications cleaned by a wet scrubber or dry cleaning
           system
      •       Blast furnace if the waste energy will be recovered
      •        Reaction crucibles with an appropriate hooding system connected with a bag filter
      •         Reaction crucibles in a closed chamber connected with a bag filter
      •      Multiple heard furnace for molybdenite roasting with an dust removal and an acid recovery
The open furnace for producing bulk ferro-alloys is not a technique to be considered in the
determination of BAT. The main reasons are the higher electrical energy consumption due to
the higher off-gas volume to be cleaned in the filter-house. This higher off-gas volume
induces, even with a high standard bag house, a larger amount of fine dust emitted to the
environment. In addition the energy used to operate an open furnace cannot be recovered.
According to the different ferro-alloys produced and the environmental impact of the processes,
which are influenced by the smelting system, the smelting furnaces presented in the next table are
considered to be BAT for this sector.




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Table 5: Smelting furnaces considered as BAT for the production of ferro-alloys

As can be seen from the table the recommended furnace for the production of ferro-silicon is the
semi-closed electric arc furnace. Due to operational problems, however, FeSi and Si-metal cannot yet
be produced in a closed furnace.
The considered furnaces are in general all applicable to new and existing plants. However the long
furnace life and the very high investment cost to build a new or replace an existing furnace should be
taken into account. Therefore the best available techniques for smelting furnaces is strongly
applicable only for new plants and a substantially change or replacement of a furnace.
This is especially the case for replacing an open furnace by a closed furnace, because main parts of
the abatement technique need to be changed as well.
The open furnace itself has not a significantly higher electrical or coke consumption, but huge
amounts of cold ambient air are sucked into the furnace to burn the CO which is present in the off-
gas. This consequently results in a very large volumetric flow of waste gas, which does not allow the
recovery of its energy content because the temperature level is low and the flow rate large to build
technically and economically efficient heat exchangers. The CO generated by the smelting process in
this case is transformed into CO2 and heat without using its energy content that is lost. Due to this the
open furnace has not been considered as BAT, but can be tolerated if local conditions, for instance
local prices of energy, periods of production and the absence of possible customers didn’t allow the
recovery of energy from a semi-closed furnace under economic viable conditions.
For existing open furnaces retrofitting with an appropriate hood in order to change the open furnace
into a semi-closed furnace is suitable and possible. By applying a nearly close hooding it is possible
to limit the infiltration of air, but at the same time supply enough air to combust the CO generated in
the furnace. Defining the off-gas temperature, which is about 300 – 400 ºC for an open furnace and
about 600 - 800 ºC for a semi-closed furnace, can be used to make the distinction between open and
semi-closed furnaces. The volumetric flow rate, which can be up to 100000 Nm3/t of metal for an
open furnace and up to 50000 Nm3/t of metal for the semiclosed furnace can be used as an indication.
Due to the increased off-gas temperature in a semiclosed furnace, the installation of an appropriate
energy recovery system should than also be taken into consideration, because the major advantage of
a semi-closed furnace is the possibilityto recover a significant part of the process heat. The energy
recovery can be done by producing steam in a waste-heat boiler and transformation into electrical
energy.


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For a semi-closed furnace with a nearly closed hood it should as well be noticed that the capital
requirement for a collection and abatement system is proportional to the volumetric flow of gas, so
that minimisation of gas volume is emphasised. This will also affect the environmental impact
concerning the total amount of dust emitted. Assuming a bag filter with the same filter efficiency is
used, the mass stream of dust emitted to the atmosphere will be reduced in the same way as the
volumetric flow of gas will be reduced.


8. Post furnace operations
Using a pneumatic or hydraulic drill normally opens the tap hole of the smelting furnace.
Oxygen lancing is also used, either as the only method or as a back up or complement to
drilling. A tapping gun helps to remove blockages, but slugs containing lead and zinc should
only be used if an appropriate hood is installed to remove tapping fumes. This is necessary
because the lead and particularly the zinc, will to a large extent vaporise in the tap hole, and
create zinc and lead fumes that otherwise would pollute the working area and subsequently
participate in the ventilation air. The tap hole is closed using a mud gun.
The most frequently used technique of tapping is the cascade tapping. In this case the metal
and slag is tapped together in the same vessel. The lower density slag float at the top and
eventually overflows through the spout to the next ladle.
Slag granulation and water spraying of slag in a pit or teeming station will contribute to
reduce emissions of fumes and dust. The used water needs a treatment in a settler to remove
particles before using it again as quenching water.
The generation of very fine powder (dust) that is collected in the bag filter used for de-
dusting the furnace off gases may create problems in handling, storage and transport of
powders.

EXAMPLE 9.07 DENSIFICATION OF SILICA POWDER AND OTHER DUST
COLLECTED IN BAG FILTERS FROM FERRO-ALLOY SMELTING FURNACES
Description: - To handle silica fume (micro silica) and other ferro-alloy filter dust a
densification process involving a micro-pelletisation step has been reported. The
process that forms a powder into small spheres about 0.5 - 1 mm in diameter.
Main environmental benefits: -Higher bulk density reduces the environmental impact of
transportation. This means less air pollution and less noise problems from truck traffic.
Operational data: - The bulk density of raw silica dust is less than 0.2 t/Nm3. The process
of micro pelletisation increases the bulk density to 0.5 – 0.6 t/Nm3. These reduce the
transport costs by about 65% and the environmental impact of transportation.
Cross media effects: - Less truck traffic
Economics:- Not available
Applicability: - To new and existing plants where silica fume, SiMn-powder, FeCr-
powder, manganese and ferro oxides need to be handled.


9. Process control
Process operation and control has developed recently in this sector and is applied to a variety
of processes. Full process design is approached with care using professional engineers who
have experience and knowledge of the process and of the environmental impact and
requirements.


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BAT for process control
The principles of Best Available Techniques include the concepts of how a process is
designed, operated, controlled, manned and maintained. These factors allow good
performance to be achieved in terms of emission prevention and minimisation, process
efficiency and cost savings.
Good process control is used to achieve these gains and also to maintain safe conditions. The
following techniques are used: -
       • Sampling and analysis of raw materials is commonly used to control plant
          conditions. Good mixing of different feed materials should be achieved to get
          optimum conversion efficiency and reduce emissions and rejects.
       • Feed weighing and metering systems are used extensively. Loss in weight silos,
          belt weighers and scale weighers are use extensively for this purpose.
       • Microprocessors are used to control material feed-rate, critical process and
          combustion conditions and gas additions. Several parameters are measured to allow
          processes to be controlled, alarms are provided for critical parameters: -
           o   On-line monitoring of temperature, furnace pressure (or depression) and gas volume
               or flow is used to prevent the production of metal and metal oxide fume by
               overheating.
           o   Gas components (O2, SO2, CO) are monitored. Process gases are collected using
               sealed or semi-sealed furnace systems. Interactive, variable speed fans are used to
               ensure that optimum gas collection rates are maintained and can minimise energy
               costs. Solvent vapours are collected and recovered as far as possible. Further removal
               of solvent vapours is practised to prevent the emission of VOC and odours.
           o   On-line monitoring of vibration is used to detect blockages and possible equipment
               failure.
           o   On-line monitoring of the current and voltage of electrolytic processes.
           o   On-line monitoring of emissions to control critical process parameters. Process
               control using relevant methods so that it is possible to maintain operating conditions
               at the optimum level and to provide alarms for conditions that are outside the
               acceptable operating range.
     •    Slag, metal and matte are analysed on the basis of samples taken at intervals. On-
          line analysis of these streams is an emerging technique.
      • Operators, engineers and others should be continuously trained and assessed in the
          use of operating instructions, the use of the modern control techniques described
          and the significance of and the actions to be taken when alarms are given. There is
          growing use of dedicated maintenance staff forming part of the operator teams who
          supplement the dedicated maintenance teams.
      • Environmental management and quality systems are used. Hazard and operability
          studies are carried out at the design stages for all process changes. For some
          processes special regulations such as the Seveso or Waste Incineration Directives
          may have to be taken into account.
Computerised control systems are also used for instance in the production of FeSi and Si-
metal in order to follow and control the generation of silica-fume.




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10. Emissions to air
Dust emissions
According to the raw material that is needed and the unit operations used, e. g. crushing,
drying, sintering, smelting tapping and product handling the most important source of
environmental input are dust and fume emissions. The following figure shows the potential
emission points for dust and fume emissions from a ferroalloy producing plant.




Figure 3: Ferroalloy production flow diagram showing potential points of air emissions
Unloading and storage of raw material can generate dust when the material falls from one
conveyor to another. Dust can also be produced if the conveyor is running too fast (i.e. more
than 3.5 m/s). If a front–end loader is used, dusting is seen during the transport distance.
The dust that is produced by the smelting process is collected by hoods or in case of a closed
furnace by the furnace sealing directly and transferred to an abatement plant and de-dusted (e
g. by a fabric filter or a wet scrubber). Scrubbing is used for closed furnaces.
Tapping off-gas consists of dust and fumes from oxygen lancing, dust from drilling, fumes
from vaporised slugs if a tapping gun is used and fumes from all exposed metal and slag
surfaces.
These fumes that arise from tapping will mainly be oxides of the metals involved in the
smelting process.
The following tables present the available emission data for the emission of dust by
producing ferrosilicon and silicon metal:




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Table 6: Dust emissions by producing ferro-silicon based on one tonne of produced alloys

Other emissions to air
The most important pollutants from the production of ferro-alloys beside dust are
      • SO2,
      • NOx,
      • CO-gas CO2,
      • HF,
      • poly cyclic aromatic hydrocarbon (PAH),
      • volatile organic compounds (VOCs) and
      • heavy metals (trace metals).
The formation of dioxins in the combustion zone and in the cooling part of the off-gas
treatment system (de-novo synthesis) may be possible. The emissions can escape the process
either as stack emissions or as fugitive emissions depending on the age of the plant and the
used technology. Stack emissions are normally monitored continuously or periodically and
reported by on-site staff or off-site consultants to the competent authorities. Data provided


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has confirmed that the significance of fugitive emissions in many processes is very high and
that fugitive emissions can be much greater than those that are captured and abated.
The sulphur content in metallurgical coke varies between 0.4 and 1.0%. 60 – 85% of the
sulphur remains in the slag and about 5% escapes the furnace as SO2. The production of
silicon alloys requires different reducing agents like coal, coke, petrol-coke and charcoal.
This material contains different amounts of sulphur, typical variations are between 0.5-and
3%. In the silicon alloy production, which is almost slag free, nearly all sulphur escapes the
furnace as SO2 or as bounded sulphur to the micro-silica. By using a reducing agent or a
mixture of different carbon sources, which contains in total a high sulphur content of about 2
- 3%, higher SO2 emissions may occur.
In the carbo-thermic process only the fixed carbon content is used as a reductant, that means
the content what is left when volatile matters ashes and moisture are deducted. The volatile
matter consists mainly of hydrocarbons, do not take part in the reaction but leaves the
furnace together with the CO when the furnace is closed or burns near the surface in a semi-
closed or open furnace. In both cases the energy content in the volatile matters is utilised.
Heavy metals are carried into the process as trace elements in the raw material. The metals
with boiling points below the process temperature will escape as gases in form of metal
vapour, which partly condenses and oxidises to form part of the dust from the smelting
furnace. Even after tapping and especially during refining the temperature of the molten
metal and slag are high enough to allow vaporisation of components both from the metal and
from the slag. The fumes arising from this evaporation evolves the whole time, from start of
tapping until casting is finished. Even after the ladle is emptied, some fumes may evolve
from the metal scull. During tapping most of the fumes are collected and cleaned, trough the
tapping fume collection. The table below presents some figures of recently measured
emissions to air by producing bulk ferro-alloys.




Table 7: Emissions to air (after abatement) by producing bulk ferro-alloys


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Emissions of noise and vibrations
The heavy machinery such as crushers and large fans used in the ferro-alloy production can
give rise to emissions of noise and vibration. Also the mechanical scull releasing from the
ladles may be a source of noise.

Process Gas Collection Techniques
This section deals with process gases. The techniques involved follow the hierarchy of
prevention, minimisation and collection of fume. Dust, fume and gases are collected by using
sealed furnace systems, by total or partial enclosure or by hooding. Sealed furnaces can be
charged from sealed lance or burner systems, through hollow electrodes, through hoods or
tuyeres or by docking systems that seal onto the furnace during charging. Hoods are designed
to be as close as possible to the source emission while leaving room for process operations.
Movable hoods are used in some applications and some processes use hoods to collect
primary and secondary fume.
Gas collection requires the movement of significant volumes of air. This can consume vast
amounts of electrical power and modern systems focus the design on capture systems to
increase the rate of capture and minimise the volume of air that is moved. The design of the
collection or hood system is very important as this factor can maintain capture efficiency
without excessive power consumption in the remainder of the system. Sealed systems such as
sealed furnaces can allow a very high capture efficiency to be attained.
Ducts and fans are used to convey the collected gases to abatement or treatment processes.
The effectiveness of collection depends on the efficiency of the hoods, the integrity of the
ducts and on the use of a good pressure/flow control system. Variable speed fans are used to
provide extraction rates that are suitable for changing conditions such as gas volume, with
minimum energy consumption. The systems can also be designed to take account of the
characteristics of the plant that it is associated with, e.g. the abatement plant or sulphuric acid
plant. Good design and maintenance of the systems is practised.
Collector systems and extraction rates are designed on the basis of good information about
the characteristics of the material to be collected (size, concentration etc), the shape of the
dust cloud at the extremes of operation and the effects of volume, temperature and pressure
changes on the system.
Correct measurement or estimation of the gas volume, temperature and pressure are made to
ensure that sufficient rates of extraction are maintained during peak gas flows. Some of the
characteristics of the gas and dust are also critical to good design to avoid problems of
abrasion, deposition, corrosion or condensation and these are measured. Another significant
factor is the provision of access to furnace filling or tapping areas while maintaining good
rates of collection, operator experience is used at the design stage to provide this.


BAT for process gas collection
The techniques to consider are based on the application of the principles of good practice
recorded above. Good practice relies on the professional design and maintenance of the
collection systems as well as on-line monitoring of emissions in the clean gas duct. The
following examples are used to illustrate good practice, it is not an exhaustive list and other
examples may also be applicable.
      • The use of sealed furnaces can contain gases and prevent fugitive emissions.
          Examples are sealed smelting furnaces, sealed electric arc furnaces and the sealed

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          point feeder cell for primary aluminium production. Furnace sealing still relies on
          sufficient gas extraction rates to prevent pressurisation of the furnace.
      • The use of sealed charging systems for the furnaces to prevent fugitive emissions
          during furnace opening. Examples are the use of charging skips that seal against a
          furnace feed door and the use of through-hood charging systems. These techniques
          may be applicable to some new and existing processes particularly for non-
          continuous processes.
      • An important established practise to achieve good extraction is the use of automatic
          controls for dampers so that it is possible to target the extraction effort to the source
          of fume without using too much energy. The controls enable the extraction point to
          be changed automatically during different stages of the process. For example,
          charging and tapping of furnaces do not usually occur at the same time and so the
          charging and tapping points can be designed to be close together so that only one
          extraction point is needed. The extraction point is also designed to allow good
          access to the furnace and give a good rate of extraction.
      • The hooding is constructed robustly and is maintained adequately. An example of
          this is an adaptation of a short rotary furnace. The feed door and tapping holes are
          at the same end of the furnace and the fume collection hood allows full access for a
          slag ladle and feed conveyor, it is also robust enough to withstand minor impacts
          during use. This principle is easily applied to a short rotary furnace but the principle
          of targeting the extraction effort to a changing source of fume is also be achieved
          by automatically controlling dampers to extract the main source of fume during the
          operating cycle e.g. charging, tapping etc. The short rotary furnace and the TBRC
          may also be totally enclosed.
These techniques may be applicable to all new and existing processes particularly for
noncontinuous processes. If sealed furnaces are not available for example when retrofitting
an existing open furnace, maximum sealing to contain furnace gases can be used.




Figure 4: Fourth hole fume collection
An example of this is the use of a “fourth hole” in the roof of an electric arc furnace to
extract the process gases as efficiently as possible and is shown in the above figure.
Maintenance of the collector hood, the ducts, the filter system and the fan is vital to ensure
that collection or extraction rates remain at the designed level. Physical damage from
collision or abrasion, deposition in ductwork and deposition on fan blades are some of the
problems that can be encountered. Regular inspection and preventative maintenance is used
to ensure this. This technique is applicable to all new and existing processes.

BAT for sulphur dioxide removal
The Best Available Techniques for the removal of sulphur dioxide removal depends on the
degree of fixation of sulphur in a matte or slag to prevent the formation of sulphur dioxide


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and on the strength of the gas that is produced. For very low strength gases a wet or semi-dry
scrubber, producing gypsum for sale if possible, is considered to be BAT.
For higher strength gases the recovery of sulphur dioxide using cold water absorption
followed by a sulphuric acid plant for the remaining gas and the stripping and production of
liquid sulphur dioxide from the absorbed liquor is considered to be BAT where local markets
exist for the material. The use of a double contact sulphuric acid plant with a minimum of
four passes is considered to be BAT. The principle of maximising the inlet gas concentration
is also considered as BAT so that the subsequent removal process can operate at maximum
efficiency.
The following factors are considered to be BAT for a sulphuric acid plant using smelter
offgases.
       • A double contact, double absorption plant with a minimum of 4 passes can be used
           in a new installation. A caesium doped catalyst can be used to improve conversion.
           It may be possible to improve existing catalysts during maintenance periods by
           incorporating caesium doped catalysts when catalyst additions are made. This can
           be particularly effective when used in the final passes where the sulphur dioxide
           content is lower but to be fully effective must be accompanied by improvements in
           other areas.
       • Gases are diluted before the contact stages to optimise the oxygen content and give
           a sulphur dioxide content to ~ 14% or slightly above to suit the thermal limits of the
           catalyst carrier material. Caesium oxide doping is required for such high inlet
           concentrations as it allows a lower first pass inlet temperature.
       • For low, varying sulphur dioxide concentrations (1.5 to 4%) a single absorption
           plant such as the WSA process, could be used for existing plants. The use of a
           caesium oxide doped catalyst in the final pass can be used to achieve optimum
           performance and can be incorporated during routine catalyst changes or during
           maintenance. To be fully effective this should be accompanied by improvements in
           other areas such as gas cleaning to protect the catalyst from poisoning. Conversion
           to double contact is complex and expensive but the use of a single contact plant
           with tail gas de-sulphurisation if necessary, to produce gypsum for sale can allow
           energy savings and lower waste generation.
       • Fluorides and chlorides should be removed to prevent damage to downstream plant
           structure.

Fugitive emissions
Fugitive emissions are generally considered to be gases emanating from sources that cannot
be easily localized, for example, leaks from various types of industrial equipment including
valves, flanges, and compressors. The EPA defines fugitive emissions as “emissions that (1)
escape capture by process equipment exhaust hoods; (2) are emitted during material transfer;
(3) are emitted from buildings, housing, material processing or handling equipment; and (4)
are emitted directly from process equipment.” Gases and fume that escape from the processes
are released into the working area and then escape into the surrounding environment. They
therefore affect operator health and safety and contribute to the environmental impact of the
process.
Fugitive emissions are very important but are hard to measure and quantify. Methods of
estimating ventilation volumes or deposition rates can be used to estimate them. One reliable
method has been used over a number of years at one site. The results show that the

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magnitude of fugitive emissions can be much more significant than collected and abated
emissions. The lower the controlled emissions, the more significant the fugitive emissions.
Fugitive emissions can be more than two to three times the quantity of controlled emissions.

BAT for fugitive emissions
It is possible to reduce environmental impact of fugitive emissions by following the
hierarchy of gas collection techniques from material storage and handling, reactors or
furnaces and from material transfer points. Potential fugitive emissions must be considered at
all stages of process design and development. The hierarchy of gas collection from all of the
processes stages is: -
       • Process optimisation and minimisation of emissions such as thermal or mechanical
           pretreatment of secondary material to minimise organic contamination of the feed.
       • The use of sealed furnaces or other process units to prevent fugitive emissions,
           allow heat recovery and allow the collection of process gases for other use (e.g. CO
           as a fuel and SO2 as sulphuric acid) or to be abated.
       • The use of semi-sealed furnaces where sealed furnaces are not available.
       • The minimisation of material transfers between processes is particularly important.
       • Where such transfers are unavoidable, the use of launders in preference to ladles for
           molten materials.
       • In some cases, restricting techniques to those that avoid molten material transfers
           would prevent the recovery of some secondary materials that would otherwise enter
           the waste stream. In these cases the use of secondary or tertiary fume collection is
           appropriate.
       • Hooding and ductwork design to capture fume arising from hot metal, matte or slag
           transfers and tapping.
       • Furnace or reactor enclosures may be required to prevent release of fume losses
           into the atmosphere.
       • Where primary extraction and enclosure are likely to be ineffective, then the
           furnace can be fully closed and ventilation air drawn off by extraction fans to a
           suitable treatment and discharge system.
       • Roofline collection of fume is very energy consuming and should be a last resort.
       • Environmental samples can be taken to measure the impact of fugitive emissions.
           In this case samples of air or dust are collected at a series of points determined by
           an atmospheric modelling exercise. Correlation with atmospheric conditions is
           needed to estimate releases.
       • Fugitive releases from a building such as a furnace room can be measured by taking
           samples from the building ventilators. The flow of gases from the ventilators can be
           estimated by measuring the temperature difference between the flow from the
           ventilators and the ambient air

As reported above, fugitive emissions can be highly significant, therefore if fugitive
emissions cannot be prevented or minimised to an acceptable level, secondary fume
collection systems can be used as illustrated by the example below.
      • Some furnaces can be equipped with secondary hoods in order to prevent fugitive
          emissions during charging or tapping as described above. The fan suction is
          provided directly at the source of fume to optimise the reduction of fugitive
          emissions. Alternatively, the air could be extracted at the roof ventilator, but a large

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         volume of air would have to be handled which might not be cleaned effectively in a
         fabric filter. Other disadvantages are high-energy consumption, high investment,
         more waste (used filter media). Secondary fume collection systems are designed for
         specific cases. Energy use can be minimised by automatically controlling the point
         of extraction using dampers and fan controls so that the systems are deployed when
         and where they are needed, for example during charging or during “roll out” of a
         converter.


Air Abatement and Recovery
Collected gases are transferred to abatement plant where contaminants are removed and
some components recovered. Dust and acid gases are commonly removed and valuable or
toxic metal components are recovered for use in other processes. The design of the abatement
process is critical, factors such as efficiency, suitability of the method and the input and
output loading of the material to be collected are used.

BAT for Air Abatement and Recovery
This section presents a number of techniques for the prevention or reduction of emissions
and residues as well as techniques reducing the overall energy consumption. They are all
commercially available. Examples are given in order to demonstrate techniques, which
illustrate a high environmental performance. The Techniques that are given as examples
depend on information provided by the industry, European Member States and the valuation
of the European IPPC Bureau.

General principles
The choice and design of a suitable abatement technique is particularly important. Several
techniques exist and although some may seem to offer a very high performance, problems
may be encountered unless the characteristics such as the loading and nature of the gases,
dust and other components are fully considered. For example the fabric filter using modern
materials is considered to offer better environmental performance than other techniques for
dust removal; however it cannot be considered to be universally applicable due to problems
of stickiness and abrasion with some types of dust. These issues are specific to individual
sites and materials and the operator should take these factors into account in a professional
design brief.
The volume, pressure, temperature and moisture content of the gas are important parameters
and have a major influence on the techniques or combination of techniques used. In
particular the dew point will be affected by all of these parameters and their variations
throughout a production cycle must be taken into account.
The characterisation of the nature of the dust or fume is very important to identify any
unusual properties (hygroscopic, pyrophoric, sticky, abrasive etc). The particle size and
shape, wet ability and density of the material are also factors to optimise the choice of
technique. The dust concentration and its’ variability should also be taken into account
producing a reliable, robust design.
Many operators have identified that performance may deteriorate with time as equipment
wears and maintenance is needed. Modern systems should be used to continuously monitor
performance by direct measurement of the gases emitted (for example dust, CO, SO2).


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Alternatively critical control parameters can be monitored. Alarm systems should be
incorporated in these systems. The following table presents an overview of dust abatement
techniques:




Table 7: Overview of dust abatement techniques




Table 8: Measured performance of dust removal systems when using various dust abatement
techniques with suitable dusts

The measured levels are quoted as ranges. They will vary with time depending on the
condition of the equipment, its maintenance and the process control of the abatement plant.
The operation of the source process will also influence dust removal performance, as there
are likely to be variations in temperature, gas volume and even the characteristics of the dust
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throughout a process or batch. The achievable emissions are therefore only a basis from
which actual plant performance can be judged and the achievable and associated emissions
discussed in the metal specific chapters take account of the suitability of the dusts
encountered and the cost/benefits of the particular application of the technique. Process
dynamics and other site specific issues need to be taken into account at a local level.
According to the techniques to consider that are presented for fume/gas collection and
abatement, BAT for this sector is considered as follows.
      • Bag filter or wet scrubbers like cascade or venturi scrubbers are suitable for de-
         dusting furnace off gases. A residual particulate matter concentration of less than 5
         mg/Nm3 for a bag filter and less than 10 mg/Nm3 for a wet scrubber is the
         associated level.
      • Dust emissions well below the associated levels may be achieved for instance with
         membrane bag filters if local air quality standards or the presence of harmful metal
         compounds requires this.
      • Some metals have toxic compounds that may be emitted from the processes and so
         need to be reduced. For metal compounds such as nickel, vanadium, chrome,
         manganese etc. as part of the total dust, emissions much lower than the associated
         dust emissions of 5 mg/Nm3 for a bag filter and 10 mg/Nm3 for a wet scrubber are
         achievable. For nickel compounds emissions less than 1 mg/Nm3 is the associated
         level.
      • By recovering ferro-alloys from steel mill residues, dust and volatile metals notably
         mercury and to a lesser extents cadmium and lead should be reduced. Using a two-
         stage bag house with injection of activated carbon or lignite coke can do this.
         Alternatively a 3-step venturi scrubber combined with a wet electrostatic
         precipitator and a selenium filter can also be used.
      • For harmful toxic vaporised metals like mercury, cadmium and lead as part of the
         off-gas, the associated emission level is below 0.2 mg/Nm3.
      • Appropriate hooding systems connected with a bag filter are preferably used for
         collecting and cleaning of tapping and casting fumes. Proper design and good
         maintenance can ensure a high capture efficiency.
      • The sulphur-dioxide content in the molybdenite roasting off-gas should be removed
         and preferably converted to sulphuric acid. The associated conversion efficiency for
         a single contact plant is 98-99%. For new plants 99.3% conversion is achievable.
         The following table summarises the captured emission associated with the use of
         best available technique and the techniques that can be used to reach these levels.




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Table 9: Emission levels to air associated with the use of BAT


BAT for removal of mercury
Mercury removal is necessary when using some raw materials that contain the metal.
Specific instances are referred to in the metal specific chapters and in these cases the
following techniques are considered to be BAT.
• The Boliden/Norzink process with the recovery of the scrubbing solution and production of
mercury metal.
• Bolchem process with the filtering off the mercury sulphide to allow the acid to be returned
to the absorption stage.
• Outokumpu process.
• Sodium thiocyanate process.
• Activated Carbon Filter. An adsorption filter using activated carbon is used to remove mercury
   vapour from the gas stream as well as dioxins.


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For processes where mercury removal from the gases is not practicable the two processes to
reduce the mercury content in sulphuric acid produced during the production of non-ferrous
metals are considered to be BAT.
• Superlig Ion Exchange process.
• Potassium iodide process.
The emissions associated with the above processes are related to any residual mercury that
will be present in the acid that is produced; the product specification is normally < 0.1 ppm
(mg/l) and is equivalent to ~ 0.02 mg/Nm3 in the cleaned gas.

11. Liquid effluents/Wastewater
9.2.2.3 Emission to water
The production of non-ferrous metals by pyrometallurgical and hydrometallurgical methods
is associated with the generation of different liquid effluents. The possible wastewater
streams are:
•   Surface run-off and drainage water
•   Waste water from wet scrubbers
•   Waste water from slag and metal granulation
•   Cooling water
The main sources of the most important effluent streams can be classified as shown in the
following table
.




Figure 5: Effluent Classification

The above wastewater streams can be contaminated by metal compounds from the
production processes and may have a high environmental impact. Even at low concentrations
some metals like mercury and cadmium are very toxic. This can be illustrated by the fact that
mercury and cadmium head the list of priority hazardous substances drawn up by the North
Sea Conference of 1984, which calls for a 50% reduction of emissions into the North Sea.
The toxic effect of some metal compounds is also due to the fact that under the correct
chemical conditions metals can easily enter natural watercourses as soluble species and be
quickly and irreversibly assimilated into the food chain
The composition of the liquid effluents from pyrometallurgical as well as from
hydrometallurgical methods depends very much of the metal being produced, the production
process and the raw material that is used. However, the liquid effluents from a non-ferrous
metal production plant normally contain heavy metals, e.g. copper, lead, zinc, tin, nickel,
cadmium, chromium, arsenic, molybdenum and mercury, and suspended solids.
For the production of ferro-alloys the emissions to water are also very dependent on the
process; for instance the abatement system and the type of wastewater treatment used.
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BAT for effluent treatment and water reuse
There exist a variety of different water collection and wastewater treatment systems in the
ferro-alloy industry. Some plants use a central wastewater treatment plant in which water
from different production processes as well as surface run-off water will be cleaned together.
Other facilities are using a separate treatment system for rainwater and special treatment
processes for the different process wastewater streams. The main water pollutants are
suspended solids and metal compounds. The wastewater is treated in order to remove
dissolved metals and solids and is recycled or reused as much as possible in the process.
The contaminated water is normally led to a thickener or a settling pond to settle out the
suspended solids. Precipitation steps are often used to remove metal compounds from the
water.
In special cases for instance by cleaning scrubbing water from a molybdenite-roasting
furnace ion exchangers are used to remove metal compounds such as selenium and rhenium
from the scrubbing water. The particles mostly consist of very fine particles, it may therefore
be necessary to add flocculent to assist settling in thickeners. After the treatment in a
thickener or a settling pond the suspended solids are usually below 20 mg/litre, which allows
reuse in scrubbers as cooling water or as process water for other purposes.
Where necessary, wastewater should be treated to remove dissolved metals and solids. In a
number of installations cooling water and treated wastewater including rainwater is reused or
recycled within the processes.
A water treatment is needed in the processes with wet scrubbers and granulation processes,
because suspended solids should be removed before the water is recirculated. To reach
acceptable values of harmful components, it may in some cases be necessary to polish the
bleed that has to be taken from the scrubbing water cycle. This may take place by using sand
filters, carbon filters or by adding suitable chemicals to precipitate harmful compounds.
The most important factors to decide, which in a specific case would be the best solution in
order to minimise the amount of wastewater and the concentration of the pollutants are:
•   The process where the wastewater is generated,
•   The amount of water,
•   The pollutants and their concentrations,
The most common pollutants are metals and their compounds and initial treatment focuses on
precipitation of the metals as hydroxides or sulphides using one or more stages followed by
the removal of the precipitate by sedimentation or filtration. The technique will vary
depending on the combination of pollutants but the following table summarises the methods
described earlier.




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Table 10: Overview of wastewater streams
The best available techniques are a combination of the different treatment methods and can
only be chosen on a site-by-site basis by taking into account the site-specific factors. The
following table presents the advantages and disadvantages of the most common treatment
techniques.




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Table 11: a) Summary of advantages and disadvantages of common wastewater treatment techniques




Table 12: b) Summary of advantages and disadvantages of common wastewater treatment techniques

According to the techniques to consider that are presented for water treatment, BAT
for this sector is considered as follows:
      • Closed water cycles are suitable for wet scrubbers, cooling systems and granulation
          processes.
      • The bleed form closed water cycles need to be treated to remove particulate matter
          and metal compounds from the water.

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      •  Treated waste waster should be recycled and reused as much as possible
      •  Scrubbing liquids should also be treated and analysed before discharge.
      •   Containment of plant drainage systems where possible and treatment of effluents
        according to their content with analysis before discharge.
      • In case of using the wet cleaning system in the alloy recovery process the bleed
        from the scrubber can be cleaned by a
           -   cyanide removal,
           -   reduction of Cr6+ to Cr3+, precipitation of metal hydroxides at high pH together with
               oxidation of cyanides
           -   precipitation of fluoride and cleaning the water from particles in a sand filter.


12. Solid waste/process residues
The production of non-ferrous metals from primary and secondary raw material is related to
the generation of a wide variety of by-products, intermediate products and residues. The most
important process specific residues are
       • filter dusts,
       • sludge from wet scrubbers,
       • slag form the smelting process,
       • used furnace linings and packaging material like drums or big-bags.
These residues are partly sold as by-products, recycled to the process or in cases of wastes
without economic utility transported to a deposit or a landfill The content and value of the
elements contained in the residue influences its potential for reuse e.g. anode slime is a viable
raw material for the recovery of precious metals. Any designation of a residue as waste for
disposal takes this into account. Also some filter dust such as silica fume that arises from the
smelting process of ferro-silicon and silicon metal is today recommended as a by-product.
According to the current EU legislation, many of these residues are regarded as wastes.
However the non-ferrous metal industry has for many decades used many residues as raw
materials for other processes and an extensive network of metallurgical operators has been
established for many years to increase the recovery of metals and reduce the quantities of
waste for landfill. It has been reported that some legislative measures to control waste
movements are inhibiting the recycling of residues from metallurgical operations. It is also
well known that the metal producing industries obtain one of the highest recycling rates in all
industrial sections. This helps to reduce Cross Media Issues to a minimum. Nevertheless the
problem of residues from production facilities and the designation of some of these materials
will also play an important role in future permits and the techniques tend to concentrate on
this aspect.
The most important factor to reduce the environmental impact of discharging residues as
waste is process-integrated measures that result in the generation of fewer residues. If the
amount of process residues is minimised by using primary measures, the extended amount
should be recycled or reused as much as possible. The specific feed materials will influence
the final process choice. To achieve effective waste minimisation and recycling the following
can be considered:
       • Waste minimisation audits can be conducted periodically according to a
          programme.
       • The active participation of staff can be encouraged in these initiatives.


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      • Active monitoring of materials throughput, and appropriate mass balances should
        be available. Monitoring should include water, power, and heat.
      • There should be a good understanding of the costs associated with waste production
        within the process. This can be achieved by using accounting practices that ensure
        that waste disposal and other significant environmental costs are attributed to the
        processes involved and are not treated simply as a site overhead.

The production of ferrosilicon is accompanied by the formation of considerable amounts of
dusts and sludges resulting, respectively, from the dry and wet scrubbing of gaseous wastes
from the production process. Originally, the baghouse dust (microsilica) was considered of
little or no value. However, microsilica is today recommended as a by-product and used as an
additive in a number of different products. Existing data suggests that this material does not
exhibit any characteristics of hazardous waste. Such dust collected by the off gas cleaning
can be agglomerated and sent back to the smelter or supplied as a raw material for further
metal recovery in other facilities. Silica fume (micro silica) with a silicon content of at least
75 percent, is collected in the bag filter by smelting silicon-metal or ferro-silicon and is sold
as a valuable by-product to the construction industry in order to produce high-strength
concrete.
The amount of slag and filter dust or sludge generated per tonne of produced ferro-silicon
and other ferroalloys as well as the possibilities of valorisation are shown in the next tables.




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Table 13:Generation, recycling, reuse and discharge of ferro-alloy slag




Table 14: Generation, recycling, reuse and discharge of dust and sludge from the air abatement system

BAT for solid waste treatment
In general all process steps should be analysed in order to minimise the generation of process
residues and the exhaust the possibilities of recycling and reuse. The following ways of

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recycling and reuse of slag and filter dust and sludge are considered as Best Available
Techniques for ferrosilicon:




Table 15: Recycling and reuse of filter dust, sludge and slag from the production of ferro-silicon


13. Energy consumption and recovery
The production of ferro-alloys is a high energy consuming process, because high
temperatures are needed for the reduction of metal oxides and smelting. Factors affecting the
energy consumption are among other things the quality of raw material and their pre-
treatment before smelting, the utilisation of reaction energies and the heat content of the
processes.
The energy used in the process can be supplied as electrical energy or fossil fuel in form of
coal, coke charcoal or sometimes natural gas. The supplied energy either in a blast furnace or
in an electric arc furnace is transformed into chemical energy formed by the reduction
process as well as off gas energy (CO rich gas) and heat.
The off-gas energy is mainly represented as process heat in case of a semi-closed furnace or
by the content of CO, CH4 and H2 when a closed furnace is used. The process-gases are
produced in the smelting process if carbon is used as a reducing agent. The CO can be
utilised as a secondary fuel and transferred by means of pipelines within the plant area like
any other fuel gas. It can be used by direct burning for instance in the sinter-furnace and for
drying or preheating the furnace charge, as well as for energy recovery in form of hot water,
steam and/or electricity. If a semi-closed submerged electric arc furnace is used for the
production of FeCr, FeSi, siliconmetal, SiMn or FeMn, the CO gas from the smelting process
burns in air thus creating a hot off- gas. Therefore the semi-closed furnaces are sometimes
equipped with a waste heat boiler as an integrated energy recovery system. The waste heat
boiler generates superheated steam that can be sold to neighbouring mills or used for
electricity production in a backpressure turbine. The next tables give an overview about the
different possibilities of energy recovery and the use of the recovered energy.




Table 16: Energy reuse by producing bulk ferro-alloys

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Article 3(d) of the IPPC Directive requires that energy is used efficiently and this note
includes comment on energy use and its place in the assessment of BAT under each of the
metal production chapters. Energy use in the non-ferrous metal industry is covered by a
series of reports prepared by the Centre for the Analysis and Dissemination of Demonstrates
Energy Technologies (CADDET). These reports have been used extensively in comparing
techniques.
Raw materials and energy consumption depend on the quality of the ores used. The energy
sources going into the production process consist of electrical energy and latent chemical
energy in the carbonaceous material. One kg of carbon has a potential gross energy content
on conversion to CO2 of approximately 8.8 kWh, or approximately 7.7 kWh/kg coke. If
these figures are used, the gross consumption of energy for the production of bulk ferro-
alloys can be calculated as shown in the next tables. The emitted gross amount of CO2 will
be directly proportional to the amount of coke consumed in the process.
Production of silicon metal and silicon alloys is also extremely power intensive, requiring a
power input, for some operations, of up to 14,000-kilowatt hours per ton of silicon content.
The consumption of energy and raw material for the production of ferro-silicon and silicon
metal is presented in terms of specific input factors as an example in the following table,
because due to plant and product specific reasons also other raw material combinations are
common. The amount of electrical energy that is given in the table is due for a commonly
used open or semi-closed submerged electric arc furnace without energy recovery.




Table 17: Consumption of raw material and energy by producing ferro-silicon

Recent development work has resulted in a new electrode type utilising a combination of the
Søderberg-technology and a graphite core, to allow the system for the production of silicon
metal. The aim is to reduce the iron impurities caused by the electrode casing.

BAT for energy consumption and recovery
Energy and heat recovery is practised extensively during the production and casting of
nonferrous metals. Pyrometallurgical processes are highly heat intensive and the process
gases contain a lot of heat energy. As a consequence recuperative burners, heat exchangers

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and boilers are used to recover this heat. Steam or electricity can be generated for use on or
off site and process or fuel gases can be pre-heated. The technique used to recover heat varies
from site to site. It is governed by a number of factors such as the potential uses for heat and
energy on or near the site, the scale of operation and the potential for gases or their
constituents to foul or coat heat exchangers.
The following examples are typical and constitute techniques to consider for use in the
processes to produce non-ferrous metals. The techniques described can be incorporated into
many existing processes:
       • The hot gases produced during the smelting or roasting of sulphide ores are almost
           always passed through steam raising boilers. The steam produced can be used to
           produce electricity and/or for heating requirements. An example of this in where a
           copper smelter produces 25% of its electrical requirements (10.5 MVA) from the
           steam produced by the waste heat boiler of a flash furnace. In addition to electricity
           generation, steam is used as process steam, in the concentrate dryer and residual
           waste heat is used to pre-heat the combustion air.
       • Other pyrometallurgical processes are also strongly exothermic, particularly when
           oxygen enrichment of combustion air is used. Many processes use the excess heat
           that is produced during the smelting or conversion stages to melt secondary
           materials without the use of additional fuel. For example the heat given off in the
           Pierce-Smith converter is used to melt anode scrap. In this case the scrap material is
           used for process cooling and the additions are carefully controlled, this avoids the
           need for cooling the converter by other means at various times of the cycle. Many
           other converters can use scrap additions for cooling and those that are not able are
           subject to process developments to allow it. The use of oxygen enriched air or
           oxygen in the burners reduces energy consumption by allowing autogenic smelting
           or the complete combustion of carbonaceous material. Waste gas volumes are
           significantly reduced allowing smaller fans etc to be used.
       • Furnace lining material can also influence the energy balance of a melting
           operation. In this case Low Mass refractories are reported to have a beneficial effect
           by reducing the thermal conductivity and storage in an installation. This factor must
           be balanced with the durability of the furnace lining and metal infiltration into the
           lining and may not be applicable in all cases.
       • Separate drying of concentrates at low temperatures reduces the energy
           requirements. This is due to the energy required to super heat the steam within a
           smelter and the significant increase in the overall gas volume, which increases fan
           size. The production of sulphuric acid from the sulphur dioxide emitted from
           roasting and smelting stages is an exothermic process and involves a number of gas
           cooling stages. The heat generated in the gases during conversion and the heat
           contained in the acid produced can be used to generate steam and /or hot water.
       • Heat is recovered by using the hot gases from melting stages to pre-heat the furnace
           charge. In a similar way the fuel gas and combustion air can be pre-heated or a
           recuperative burner used in the furnace. Thermal efficiency is improved in these
           cases. For example, nearly all cathode/copper scrap melting shaft furnaces are
           natural gas fired, the design offers an thermal efficiency (fuel utilisation) of 58% to
           60%, depending on diameter and height of the furnace. Gas consumption is
           approximately 330 kWh/tonne of metal. The efficiency of a shaft furnace is high,
           principally because of charge preheating within the furnace. There can be sufficient

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          residual heat in the off-gas to be recovered and re-used to heat combustion air and
          gas. The heat recovery arrangement requires the diversion of the furnace stack
          gases through a suitably sized heat exchanger, transfer fan and ductwork. The heat
          recovered is approximately 4% to 6% of the furnace fuel consumption.
       • Cooling prior to a bag filter installation is an important technique as it provides
          temperature protection for the filter and allows a wider choice of fabric. It is
          sometimes possible to recover heat at this stage. For example in a typical
          arrangement used by a shaft furnace to melt metal, gases from the top of the furnace
          are ducted to the first of two heat exchangers that produces preheated furnace
          combustion air. The temperature of the gases after this heat exchanger can be
          between 200 and 450 ºC. The second heat exchanger reduces the gas temperature to
          130 ºC before the bag filter. The heat exchangers are normally followed by a
          cyclone, which removes larger particles and acts as a spark arrester.
       • Carbon monoxide produced in an electric or blast furnace is collected and burnt as
          a fuel for several different processes or to produce steam or other energy.
          Significant quantities of the gas can be produced and examples exist where a major
          proportion of the energy used by an installation is produced from the CO collected
          from an electric arc furnace installation. In other cases the CO formed in an electric
          furnace burns in the furnace and provides part of the heat required for the melting
          process.
       • The re-circulation of contaminated waste gas back through an oxy-fuel burner has
          resulted in significant energy savings. The burner recovers the waste heat in the gas,
          uses the energy content of the contaminants and removes them [tm 116, Alfed
          1998]. Such a process can also reduce nitrogen oxides.
       • The use of the heat content of process gases or steam to raise the temperature of
          leaching liquors is practised frequently. In some cases a portion of the gas flow can
          be diverted to a scrubber to recover heat into the water, which is then used for
          leaching purposes. The cooled gas is then returned to the main flow for further
          abatement.
       • During the smelting of electronic scrap or battery scrap in metallurgical vessels the
          heat content of the plastic content is used to melt the metal content and other
          additional scrap and slag forming components.
       • The advantage of preheating the combustion air used in burners is well
          documented. If an air preheat of 400 °C is used there is an increase in flame
          temperature of 200 °C, while if the preheat is 500 °C the flame temperature
          increases by 300 °C. This increase in flame temperature results in a higher melting
          efficiency and a reduction in energy consumption. The alternative to preheating the
          combustion air is to preheat the material charged to the furnace. Theory shows that
          8% energy savings can be obtained for every 100 °C preheat and in practice it is
          claimed that preheating to 400 °C leads to 25% energy savings while a preheat of
          500 °C leads to a 30% energy savings. Pre-heating is practised in a variety
          processes for example the pre-heating of the furnace charge using the hot furnace
          off-gases during the production of ferro-chrome.
Heat and energy recovery is therefore an important factor in this industry and reflects the
high proportion of costs that energy represents. Many techniques for energy recovery are
relatively easy to retrofit [tm 118, ETSU 1996] but there are occasionally some problems of
deposition of metal compounds in heat exchangers. Good design is based on a sound

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knowledge of the compounds released and their behaviour at various temperatures. Heat
exchanger cleaning mechanisms are also used to maintain thermal efficiency.
Whilst these savings are examples of individual components of installations they are
critically dependant upon the site and process specific conditions including economics.
The following table shows that the difference in process energy consumption between
production alternatives is not very big. Indeed, the “conventional” process routes may have
an advantage if a considerable part of the recoverable energy can be sold externally. Most
often plants do not have external energy customers. Choosing a process route that can utilise
recovered heat, either for added process steps that increase efficiency and output, or for
electricity generation, will then be advisable options. An important point of the closed
furnace process that uses pelletising/sintering and pre-heating is to minimise the use of fossil
carbon per tonne of produced alloy, which will also minimise the specific CO2 emission.
However, the pelletising/sintering will only reduce the impact of greenhouse gases if an
alternative, less energy efficient process would lead to a deficiency of CO gas.




Table 18: Comparison of electrical and fuel energy consumption
Ore quality is also an important factor for energy consumption. Of primary importance is the
content of metal oxide and the non ferrous metal/iron ratio, which should both be as high as
possible. Secondarily the content of gangue minerals should be as low as possible in the ore
or the ore mix (this will partly be a consequence of a high amount of metal oxide), and of a
composition to minimise use of slag additives. This will lower the slag amount, and thus the
proportion of the electric power necessary to melt slag. Concerning the energy usage, the
disadvantage of the smelting furnaces used without energy recovery is the high amount of
energy lost as CO in the off gas and as waste heat. For instance by producing FeSi and
silicon metal only about 32% of the energy consumed is chemical energy in the product, that
means about 68% of the energy is lost as heat in the furnace off-gas.
Energy can be recovered from the cooling cycles as hot water and from the off gas as heat
which can be transferred into high pressure steam and subsequently into electrical energy or
by using the CO content directly as a secondary fuel.
There are some direct plant improvements that can be done to reduce the energy
consumption, such as running the process with a high metal yield, improving the furnace
design to achieve lower energy loss. In addition to the direct plant improvements about 15 –
20% of the electric energy consumed by the electric arc furnace can be recovered as
electricity by an energy recovery system. This percentage is considerably higher for a system

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that produces electricity and uses the thermal energy of the furnace cooling and the off-gas
volume. This will be as well the case if the CO-gas is utilised directly as a secondary fuel in
order to replace fossil fuels.
Beside the production of electricity the CO gas can also be transferred by means of pipelines
in the plant area and used as a secondary fuel for many purposes. The best utilities are
achieved in direct burning replacing fossil fuels, e.g. heavy oil or coal.
CO gas can as well be used as a fuel in the steel belt sintering furnace in order to reduce the
primary energy consumption of the furnace. The CO rich gas can as well be cleaned and then
supplied as a synthetic gas to a neighbouring chemical plant, in which the gas serves as a raw
material.
In a semi-closed furnace the CO-gas from the smelting furnace burns in the suction air thus
creating a hot off gas of about 400 – 800 .C with can also reach peaks up to 1200 .C. The
furnaces can be equipped with an integrated energy recovery system, which contains the
following components:
•   Exhaust hood with furnace ducting
•   Waste-heat boiler
•   Feed-water system
•   Heat distribution system or steam turbine with generator and condenser The CO rich gas can as
    well be cleaned and then supplied as a synthetic gas to a neighbouring chemical plant, in which
    the gas serves as a raw material.

EXAMPLE 9.09 ENERGY RECOVERY FOR A SEMI-CLOSED ELECTRIC ARC
FURNACE
Description: - The energy form hot off-gas of the furnace can be recovered in a waste heat
boiler, which produces superheated steam. Relatively conventional water pipe boilers with
super heater, economiser and condenser sections are used, combined with an efficient
cleaning system to keep the heating surfaces clean in the heavily dust polluted flue gas.
The furnace top hood is highly exposed to the internal furnace heat, and is conventionally
cooled with a water piping system covered by a ceramic lining. About 25% of the furnace
heat emissions are lost to the top hood cooling water. For energy recovery the top hood may
be cooled by unshielded high-pressure water piping, producing steam to the recovery boiler
system. Such hood exists and contributes substantially to the energy recovery.
The steam can be used in a back pressure turbine in order to produce electricity or be sold to
a neighbouring mill. The recovery system can be designed also to produce hot water, which
can be used by a local heating system.




Figure 6: Energy recovery from a semi-closed furnace

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Main environmental benefits: -The recovery of energy from the hot off gas reduces the
overall energy consumption of the process, which consequently minimise the impact of
global warming by emitting CO2 from burning fossil fuel. The off-gas energy presented a
large available, partly unexploited energy source that can provide new electricity without
pollution and additional CO2 emission.
Operational data: The off-gas energy can be used to produce electric power, heat energy or
both. If the waste heat is utilised as electric power the recovery is up to 28 - 33% of the
energy consumption. Alternately, the steam can be drained at mean pressure and be used for
district heating, and the recovery will increase to approximately 80 - 90%. But then only 20%
of the waste heat is recovered as electric power. The demand of district heating often varies
trough the year and the most efficient solution is co-generation of electric power and heat
energy to supply heat energy only when needed.
Cross media effects: - The recovered energy replaces in most cases fossil fuel like oil or
coal and reduces therefore at the same time the emission of SO2. The energy recovery
produces no pollution, as the flue gas composition is not changed by the recovery. The
emission of hot cooling air and water from the plant is reduced. The energy recovery creates
no visual changes of the landscape FeSi production with an electricity consumption of 60
MW uses a semi-closed furnace with about 750 .C off-gas temperature. The waste heat boiler
consists of 3 sections and each section has 4 economisers, 2 evaporators and 2 super heaters.
The gas exits the boiler at approximately 170 .C. The produces superheated steam is fed to a
multistage turbine. The generator produces 17 MW of electric power equals to 90 GWh/a,
which corresponds to 28% of the flue gas Energy and 16.5% of the electric power
consumption in the furnace. The investment costs for the recovery plant has been in 1987
about 11.7 M € (20 Years annuity, 11.5% interest, electricity cost 0.02 €/kWh) FeSi
production with an electricity consumption of 60 MW uses a semi-closed furnace with about
750 ºC off-gas temperature. The waste heat boiler consists of 3 sections and each section has
4 economisers, 2 evaporators and 2 super heaters. The gas exits the boiler at approximately
170 ºC. The produces superheated steam is fed to a multistage turbine. The generator
produces 17 MW of electric power equals to 90 GWh/a, which corresponds to 28% of the
flue gas Energy and 16.5% of the electric power consumption in the furnace. The investment
costs for the recovery plant has been in 1987 about 11.7 M € (20 Years annuity, 11.5%
interest, electricity cost 0.02 €/kWh)
Applicability: - The technology is in general applicable to both new and existing plants.
Since this energy source normally presents existing installation, one of the obvious demands
towards the energy recovery is that it is applicable to existing plants. For the production of
FeSi and Si-metal is has been reported that a smelting furnace, which slowly rotates may
contribute to the reduction of the overall energy consumption by about 10% and increase the
metal yield.
The above possibilities of energy recovery are presently in operation in various systems in
the ferro-alloy industry and performed satisfactory for many years. However it should be
noticed that an appropriate energy recovery system means a high capital investment. Taking
local conditions, such as local energy prices, periods of production and the absence of
potential customers into account, the returns of investments may in several cases not be high
enough to justify such investments from an economic point of view.
According to the considered techniques and routes of utilisation the CO gas or to recover the
heat energy from a smelting process, BAT for energy recovery in this sector is considered as
follows:


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Table 19: BAT for energy recovery by producing ferro-alloys

For the production of FeSi and Si-metal it has been reported that a smelting furnace, which
slowly rotates may contribute to the reduction of the overall energy consumption by about
10% and increase the metal yield.
Concerning the energy usage, the disadvantage of the smelting furnaces used without energy
recovery is the high amount of energy lost as CO in the off gas and as waste heat. For
instance by producing FeSi and silicon metal only about 32% of the energy consumed is
chemical energy in the product, that means about 68% of the energy is lost as heat in the
furnace off-gas (Bref) Energy can be recovered from the cooling cycles as hot water and
from the off gas as heat which can be transferred into high pressure steam and subsequently
into electrical energy or by using the CO content directly as a secondary fuel.
The above mentioned best available techniques for energy recovery are techniques that are
applicable to new plants and in case of a substantial change of an existing plant. This
includes also the case where a furnace needs to be replaced.
For existing plants retrofitting of a smelting furnace with an appropriate energy recovery
system is possible especially when an open furnace will be changed into a semi-closed
furnace. The energy content can then be recovered by producing steam in a waste-heat boiler
where the furnace hood can advantageously be integrated in the recovery system and used as
superheater.
The produced steam may be used in the process, in neighbouring mills but most often for the
generation of electrical energy will be economically the best solution. By building a closed
furnace or replacing of an existing furnace by a closed one a cleaning and recovery system
for the CO-gas is unavoidable. The CO, that otherwise needs to be flared off can be used as
high quality secondary fuel for a variety of purposes or as raw material or fuel in
neighbouring mills. Flaring of CO-gas is only acceptable in the case where customers inside
or outside the plant are temporarily not available. The recovered CO gas can as well be used
for the production of electrical energy.
The recovery of process energy reduces the consumption of natural energy resources and
consequently contributes to minimise the CO2 emissions and the effect of global warming if
the total impact of the process, and the saved energy elsewhere are included into the global
energy and CO2 balance. Energy recovery is therefore a desirable option and will in future

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be more and more important, but it is suitable only if local conditions (e.g. local prices of
energy, the presence of external energy customers, and periods of production) justify the
investment. As already mentioned in the part of BAT for smelting furnaces the recovery of
energy is strongly related to the used furnace type (semi-closed or closed furnace). Energy
recovery should therefore also be seen in the context and the requirements of changing
existing furnaces.



                                       REFERENCES
2007 American Geological Institute as recovered from
http://www.agiweb.org/geotimes/nov03/resources.html

Sjardin, Milo CO2 EMISSION FACTORS FOR NON-ENERGY USE IN THE NON-FERROUS
METAL, FERROALLOYS AND INORGANICS INDUSTRY, Copernicus Institute Department of
Science, Technology and Society University of Utrecht The Netherlands, June 2003




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              FERRONICKEL




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The purpose of this report, in the framework of the PREWARC project, PL 517574, is to present
feasible preventive and remedial technologies that can be adopted by the mining and metallurgical
industries during production as well as during waste management during ferronickel production.



1. General information
     Ferro-nickel (FeNi) is the major alloying agent in the production of stainless steel. There are two
types of nickel deposits, which are of economic interest: magmatic sulfide deposits where the
principal ore mineral is pentlandite [(Ni,Fe)9S8], or laterites where the principal ore minerals are
nickeliferous limonite [(Fe,Ni)O(OH)] and garnierite (hydrous nickel silicate). Nickel laterites and
other nickel oxide ores constitute the world’s largest known reserves of this metal. The oxidic ores
are used to make nickel mattes or ferronickel and the sulphidic ores are usually used to produce
nickel metal (Sjardin, 2003).
     Laterite ore is characterised by a relatively low nickel content (1.2 – 3%) and a high moisture
content together with chemically bound water in the form of hydroxide. The mined ore, being of a
porous nature, can hold a large content of free moisture, commonly 25 to 30% H2O, although it can
contain even 40% or more. In addition to this, the chemically bound water, which is not completely
driven off until a temperature of 700 to 800°C is reached, can amount to up to 15% based on the dry
ore weight. Because of the quantity of water present, and because water requires so much energy to
evaporate and heat up, it is clear that some pretreatment of the ore (at least drying and calcining) is
required before smelting.
     The chemical composition of laterite usually varies considerably, and this adds to the difficulty in
processing this material. Normal electric furnaces, operating with a layer (or partial layer) of feed
material on top of the melt, are reportedly difficult to operate with an SiO2/MgO ratio greater than
2.0, or an iron content of more than 20% by mass, as these conditions can cause operational
instability, mainly due to a tendency to slag foaming. These compositional problems can be
overcome, to some extent, by blending different ores.
     Major sources of nickel ore are found in Russia, Canada and Australia. In European Union, nickel
ore is mined in Greece, and on a smaller scale in Spain and Finland. Nickel smelting operations take
place in Greece, Austria and Finland. High purity nickel and nickel chemicals are produced in special
refineries. Important nickel refineries operate in the UK, France, Finland and Norway (ENIA, 2007).
     Japan is the largest producer of ferronickel in the world with almost half of the world production.
The EU produces only about 11% of the total world production (Sjardin, 2003).




                               Figure 1: World production of ferronickel


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    The major ferronickel producers are shown in the following figure: (Hurbe 2005)




                                   Figure 2: Ferronickel producers

     Nickel is extracted through both underground and surface mining.
     Underground mining uses a sub-level caving method. Access to the deposit is carried out with
horizontal calcareous galleries and spiral ramps. The main phases are the drilling, the charging the
drills with explosives and their firing, the collection of the produced ore and the support of the
galleries. The produced ore is then transported to the surface.
     Surface mining combines open and closed pits. The height of the benches varies between 12 and
15m, with the width depending on whether they are active benches or close to decommissioning. To
begin, they are around 25m wide, decreasing to around 12m before decommissioning. Stripping is
performed by use of explosives while cutting with bulldozers and other mechanical equipment.
     Extracted ore is stored temporarily in piles before transporting for crushing and separating. The
magnetic portion of the ore (concentrate) is led to the stacker system and on to the homogenization
yard. The homogenized ore is loaded and transported to the smelting plant, where it is weighed and
fed to the kilns for further processing.
     The primary extraction processes can be defined as the processes that receive nickel concentrate,
or prepared ore, to produce final metal products, ferronickel and nickel oxide, as well as intermediate
products such as matte and liquor. Nickel primary extraction is comprised of two main methods:
            • Pyrometallurgical methods
            • Hydrometallurgical methods
     Over 90% of the world's nickel sulphide concentrates are treated by pyrometallurgical processes,
e.g. reverberatory furnace or flash smelting, to form nickel-containing mattes. In several modern
operations the roasting step has been eliminated, and the nickel sulphide concentrate is treated
directly in the smelter.
     In the pyrometallurgical process, sulphur is generally added to the oxide ore during smelting,
usually as gypsum or elemental sulphur, and an iron-nickel matte is produced. The smelting process
that does not include adding sulphur produces a ferronickel alloy, containing less than 50% nickel,
which can be used directly in steel production.
     Hydrometallurgical techniques involve leaching with ammonia or sulphuric acid, after which the
nickel is selectively precipitated. Alloys, such as stainless steels, are produced by melting primary


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metals and scrap in large arc furnaces and adjusting the carbon content and concentration of alloying
metals to the desired levels (Sjardin, 2003).
   The following figure shows the main processing routes for nickel production (E. Norgate, et al.
2007).




                      Figure 3: The main processing routes for nickel production



2. Production of ferronickel from primary raw material
    The production of ferro-nickel from primary raw material is carried out by the rotary kiln-electric
furnace process. The first step is drying. The next process steps are crushing and homogenisation
where the different ores are mixed with coal and pelletised dust, which is recycled from the main
process. The hot pre-reduced calcine can be introduced directly to the smelting furnace or by
insulated containers. The containers may be used for two reasons: first to conserve the heat and
second to add coke or coal required for complete reduction before they are discharged into the electric
furnace, where melting and final reduction occurs. The reductant is either coke or coal and in the
electric arc furnace (EAF) Søderberg electrodes are used.

Phase 1: Drying in the Rotary Dryer
    As mentioned before, the raw material carries a significant amount of water; therefore the first
step of the process is a drying operation. Drying normally takes place in a directly fired rotary dryer
where the moisture content can be reduced from about 45 to 15%. Further drying below 15% should
be avoided in order to keep the dust generation in the subsequent calcining and smelting process as
low as possible.
    Energy may be recovered from the process by using the thermal calorific value of the off-gases
from the furnace to assist in the energy requirements for drying or calcining of the laterite feed. The
CO-rich off-gas may also be used for pre-reduction purposes.




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Phase 2: Crushing.
    The ore is crushed to an acceptable particle size for calcining. Due to the physical properties of
the ores, a large fraction of fines is generated during the pre-treatment stages.

Phase 3: Homogenisation:
    The next process step is homogenisation where the different ores are mixed with coal and
pelletised dust, which is recycled from the main process. The dry feed mix is then fed to a rotary kiln.

Phase 4: Calcination
    The rotary kiln is used to de-hydrate the ore by calcination and to pre-reduce the nickel and iron
oxide. The process takes place at about 900 – 1000 ºC. The calcining and pre-reducing process results
in a furnace feed which contains about 40% of the nickel as a metal and the iron content in form of
iron (II)-oxides.




Figure 4: Rotary kiln
   Rotary kilns use the same arrangement as a rotary furnace but operate without melting the charge.
Apart from the production of calcine for the ferronickel process, these kilns are used for a variety of
fuming and calcining processes.

Phase 5: Smelting
     The hot pre-reduced calcine can be introduced directly to the smelting furnace or by insulated
containers. The containers may be used for two reasons: first to conserve the heat and second to add
coke or coal required for complete reduction before they are discharged into the electric furnace,
where or melting and final reduction occurs
     Ferro-nickel smelting today only takes place in electric arc furnaces. In the electric furnace the
reductive smelting operation occurs by the combined action of carbon electrodes and added solid
carbonaceous reductant. The slag melting temperature in the ferro-nickel smelting process is strongly
dependent on the FeO-content. The operation mode of the furnace therefore changes if the slag
melting temperature is above or below the melting temperature of the metal. If the melting
temperature of the slag is higher than the melting point of the metal the furnace can easily be operated
with a covered bath. In this case the electrode tips are not immersed in the slag and the final reduction
of the nickel and iron oxides takes mostly place in the hot charge, which covers the slag layer. If the
melting temperature of the slag is below the melting temperature of the metal the furnace is more
difficult to operate. In order to reach the melting temperature of the metal, the electrodes should
penetrate deep in to the slag layer. The highest bath temperature will then be around the electrode tips
where smelting takes place in the slag-metals interface. These operating conditions result in a high
generation rate of CO-gases, which requires an open bath surface around the electrodes.


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    To reduce a high content of nickel oxides commonly the burden contains an excess proportion of
carbon. These also increase the amount of iron that will be reduced and the final carbon content of the
crude ferro-nickel. To reduce the iron and carbon content a further refining step is necessary. To
avoid further refining several process improvements had been made. For instance in the “Ugine
Ferro-nickel Process” no reductant is added. The electric furnace produces a molten ore, which is
reduced to ferro-nickel by using ferro-silicon in a further ladle furnace. In the “Falcondo Ferro-nickel
Process” a shaft furnace is used instead of a rotary kiln. In the shaft furnace a briquetted ore is
reduced with a reducing gas (low sulphur naphta). The subsequent electric furnace is then only used
to melt the metal and to separate it from the slag. (ANTAM 2005)




Figure 5: A flow sheet of a ferro-nickel production.

Phase 6: Refining
     Ferro-nickel produced by the conventional process needs further refining. Besides the reduction
of iron and carbon the impurities like sulphur, silicon and phosphorus should be removed. For ferro-
nickel refining a variety of equipment are available e.g. shaking reaction ladle, induction furnace,
electric arc furnace and oxygen blown converters. The purified ferro-nickel is cast into ingots or
granulated under water.
     The dust containing off-gas from the rotary kiln, the electric arc smelting furnace and the refining
step is treated by an appropriate abatement system. The dust content can be pelletised and recycled to
the raw material blending station and can be used profitably as a feedstock for the production of
ferronickel.



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Figure 6: Rotary kiln-electric arc furnace process for the ferro-nickel production




3. Raw materials and energy
     The manufacturing of ferro-alloys is in general an energy consuming process, because the
smelting takes place at high temperatures. The ferro-alloy production is therefore related to a
relatively high consumption of raw materials such as ore, concentrates and fluxes as well as
reductants and fuels like coke or coal and electrical energy.
     Typically, raw materials and energy consumption depend on the quality of the ores used. The
impact of declining ore grade on Gross Energy Requirement (GER) comes about largely because of
the additional energy that must be consumed in the mining and mineral processing stages to move and
treat the additional gangue material. Once a concentrate or mineral product of a specified grade has
been produced, emissions from downstream processing (e.g. smelting and refining) are not
significantly affected by the original ore grade (Norgate, et al 2007)
     To avoid false differences between process alternatives it is important to present only the gross
energy consumption. The energy sources going into the production process consist of electrical
energy and latent chemical energy in the carbonaceous material. One kg of carbon has a potential
gross energy content on conversion to CO2 of approximately 8.8 kWh, or approximately 7.7-kWh/kg
coke. If these figures are used, the gross consumption of energy for the production of bulk ferro-
alloys can be calculated. The emitted gross amount of CO2 will be directly proportional to the
amount of coke consumed in the process.
     Water is used in the production of ferro-nickel both as process water and cooling water. Process
water is used for scrubbing and slag granulation.
     The available information about the consumption of raw material and energy for the production
of various special ferro-alloys are presented in the following table in terms of specific input factors
based on a tonne of produced product:




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Table 1: Raw material and energy consumption for the production of ferro-alloys

4. Materials handling and storage
    The main raw materials used in the production of non-ferrous metals are ores and concentrates,
secondary raw materials, fuels (oil, gases and solid fuel) and process gases (such as oxygen, chlorine
and inert gases). Other materials such as fluxes, additives and process chemicals (e.g. for abatement
systems) are also used. This variety of materials possesses many handling and storage problems and
the specific technique used depends on the physical and chemical properties of the material. The main
environmental impact by storage and handling of these materials are fugitive dust emissions and
contamination of surface water and soil caused by wash out from rainwater.

Best Available Techniques (BAT) for materials, storage and
handling
    The techniques that are used depend to a large extent on the type of material that is being used.
For example large, heavy items are treated by a completely different range of techniques to fine,
dusty material. These issues are specific to individual sites and materials. There are however, several
techniques that are considered to be BAT in preventing emissions from material storage and handling
processes. These techniques are:
          • The use of liquid storage systems that are contained in impervious bunds that
            have a capacity capable of containing at least the volume of the largest storage
            tank within the bund. Various guidelines exist within each Member State and
            they should be followed as appropriate. Storage areas should be designed so that
            leaks from the upper portions of tanks and from delivery systems are intercepted
            and contained in the bund. Tank contents should be displayed and associated
            alarms used.
          • The use of planned deliveries and automatic control systems to prevent over
            filling of storage tanks.

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•   Sulphuric acid and other reactive materials should also be stored in double
    walled tanks or tanks placed in chemically resistant bunds of the same capacity.
    The use of leak detection systems and alarms is sensible. If there is a risk of
    ground water contamination the storage area should be impermeable and
    resistant to the material stored.
•   Delivery points should be contained within the bund to collect spilled of
    material. Back venting of displaced gases to the delivery vehicle should be
    practised to reduce emissions of VOCs. Use of automatic resealing of delivery
    connections to prevent spillage should be considered.
•   Incompatible materials (e.g. oxidising and organic materials) should be
    segregated and inert gases used for storage tanks or areas if needed.
•    The use of oil and solid interceptors if necessary for the drainage from open
    storage areas. The storage of material that can release oil on concreted areas that
    have curbs or other containment devices. The use of effluent treatment methods
    for chemical species that are stored.
•     Transfer conveyors and pipelines placed in safe, open areas above ground so
    that leaks can be detected quickly and damage from vehicles and other
    equipment can be prevented. If buried pipelines are used their course can be
    documented and marked and safe excavation systems adopted.
•      The use of well designed, robust pressure vessels for gases (including LPG’s)
    with pressure monitoring of the tanks and delivery pipe-work to prevent rupture
    and leakage. Gas monitors should be used in confined areas and close to storage
    tanks.
•      Where required, sealed delivery, storage and reclamation systems can be used
    for dusty materials and silos can be used for day storage. Completely closed
    buildings can be used for the storage of dusty materials and may not require
    special filter devices.
•       Sealing agents (such as molasses and PVA) can be used where appropriate and
    compatible to reduce the tendency for material to form dust.
•           Where required enclosed conveyors with well designed, robust extraction and
    filtration equipment can be used on delivery points, silos, pneumatic transfer
    systems and conveyor transfer points to prevent the emission of dust.
•            Non-dusty, non-soluble material can be stored on sealed surfaces with drainage
    and drain collection.
•             Swarf, turnings and other oily material should be stored under cover to prevent
    washing away by rainwater.
•        Rationalised transport systems can be used to minimise the generation and
    transport of dust within a site. Rainwater that washes dust away should be
    collected and treated before discharge.
•         The use of wheel and body washes or other cleaning systems to clean vehicles
    used to deliver or handle dusty material. Local conditions will influence the
    method e.g. ice formation. Planned campaigns for road sweeping can be used.
•          Inventory control and inspection systems can be adopted to prevent spillages
    and identify leaks.

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          • Material sampling and assay systems can be incorporated into the materials
            handling and storage system to identify raw material quality and plan the
            processing method. These systems should be designed and operated to same
            high standards as the handling and storage systems.
          • Storage areas for reductants such as coal, coke or woodchips need to be
            surveyed to detect fires, caused by self-ignition.
          • The use of good design and construction practices and adequate maintenance.

   The following table summarises the techniques on the basis of type and characteristics of the
material.




Table 2: Summary of handling and storage techniques

    To prevent contamination raw materials are preferably stored on hard surfaces where indoor and
outdoor storage may be used, depending on the potentially dusty nature and the chemical properties
of the materials. To keep the materials clean the storage area can also be divided in different storage-
bays. Dry fine-grained materials should be stored and handled inside where closed silos, bins and

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hoppers are used to prevent fugitive emissions to the environment as well as to the workspace.
Excessive dusting can also be prevented by water spraying of dry fine materials.
    Closed conveyors and transfer systems are used for handling of dusty fine materials, where
extraction and filtration equipment is used for dusting delivery points. The dust leaden air from the
silos, closed conveyors and charging systems are cleaned by using bag filters, which may be
monitored by measuring the pressure drop to control the cleaning mechanism.

5. Ore processing and preparation
    Ores, concentrates and secondary raw materials are sometimes in a form that cannot be used
directly in the main process. Drying/thawing may be needed for control or safety reasons. The
material size may need to be increased or decreased to promote reactions or reduce oxidation.
    Reducing agents such as coal or coke and fluxes or other slag forming materials may need to be
added to control the metallurgical process. Coatings may need to be removed to avoid process
abatement problems and improve melting rates. All of these techniques are used to produce a more
controllable and reliable feed for the main process and are also used in precious metal recovery to
assay the raw material so that toll recovery charges can be calculated. During such beneficiation
processes dust is emitted.
    It is advantageous to pre-reduce the laterite prior to its introduction into the furnace. In such a
case, the laterite ore is dried first, followed by dry milling, and calcining at 700 to 900°C in a
fluidized bed. The calcined laterite is then pre-reduced in a fluidized reduction reactor, using a solid
carbonaceous or gaseous reductant, at 800 to 850°C, prior to feeding to the furnace bath. As the DC-
arc furnace can smelt fine materials, a fluid bed reactor can be linked to the furnace.

BAT techniques for ore preparation and pre-treatment
     The pre-processing and transfer operations often deal with materials that are dry or are likely to
produce process emissions to any of the environmental media. More detailed design of the process
equipment used at this stage is therefore needed and the processes need to be monitored and
controlled effectively. The nature of the material (e.g. dust forming, pyrophoric) needs to be taken
into account and the potential emissions of VOCs and dioxins in thermal processes needs to be
assessed. Extraction and abatement systems in particular need to be carefully designed, constructed
and maintained. The review of applied techniques in this section includes the issues that will be
encountered in the various process options. The techniques listed for raw materials handling should
also be referred to. The following items however are considered to be the most important.
     A shaft furnace is preferably used for coke drying were the use of recovered energy or the CO
rich off-gas from the smelting furnace as a secondary fuel is suitable. Bag filters are used to clean the
off-gas where the associated dust emission level is 5 mg/Nm3.
      Use of pre-treatment and transfer processes with well designed robust extraction and abatement
equipment to prevent the emission of dust and other material. The design of this equipment should
take account of the nature of the emissions, the maximum rate of emissions and all of the potential
sources.
      Use of enclosed conveying systems for dusty materials. These systems should be provided with
extraction and abatement equipment where dust emissions are possible.
      Processes that “flow” directly into the following process if possible to minimise handling and
conserve heat energy.
      Use of wet grinding, blending and pelletising systems if other techniques for the control of dust
are not possible or appropriate.
      Thermal cleaning and pyrolysis systems (e.g. swarf drying and de-coating) that use robust after-
burning equipment to destroy combustion products e.g. VOCs and dioxins. The gases should be held
at a temperature greater than 850 °C (1100 °C if there is more than 1% halogenated organic material),

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in the presence of at least 6% oxygen for a minimum of 2 seconds. Lower residence times may as
well result in the complete destroying of VOCs and dioxins but this should be demonstrated on a
local level. Gases should be cooled rapidly through the temperature window of dioxin reformation.
    To reduce the impact of VOC’s, washing processes to remove oil or other contaminants should
use benign solvents. Efficient solvent and vapour recovery systems should be used.
    Steel belt, up-draught or fully enclosed down-draft sintering processes are techniques to be
considered. Steel belt sintering has several advantages for certain metal groups and can minimise gas
volumes, reduce fugitive emissions and recover heat. These are discussed later. Off gas extraction
systems should prevent fugitive emissions.
    The use of rotary kilns with wet ash quenching for the processes involving the volume reduction
of material such as photographic film. Smaller installations may use a moving grate furnace. In both
cases the combustion gases should be cleaned to remove dust and acid gases if they are present.
    If required to minimise the generation of smoke and fumes and to improve the melting rates,
separation processes should be designed to produce clean materials that are suitable for recovery
processes.
    Collection and treatment of liquid effluents before discharge from the process to remove non-
ferrous metals and other components.
    The use of good design and construction practices and adequate maintenance.
    According to the Environmental Protection Agency’s (EPA) Standards For Emissions Of Air
Toxics From Ferronickel Production, controlled particulate emissions from the ferronickel ore
processing emission sources would be limited to a concentration limit of 0.030 gr/dscf. or69 mg/dscm
(
  EPA)
    The following table presents a summary of pre-treatment methods.




Table 3: Summary of pre-treatment methods


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6 Pre-reduction and pre-heating
    The electricity consumption of the smelting furnace can be decreased by pre-heating the feed
materials Pre-heating for instance as it is used in the production of FeCr increases at the same time
the productivity of the smelting furnace. However, the technology of pre-reducing ore and
concentrates is fully implemented only in two plants worldwide. As reported, there are still some
problems operating this technology. Pre-reduction is therefore not yet recommended as a general
BAT in this sector.

7. Smelting process
    In the production of ferro-alloys the most important stage is the reduction of metal oxides and
alloying with the iron present in the process. Depending on the reducing agent, different types of
smelting systems (such as the electric arc furnace, the blast furnace or a reaction crucible) are used.
Electric arc furnaces are normally operated submerged as a closed, semi-closed or open type. The
concept of the different smelting systems is influenced by the desired flexibility in the production, the
range of raw material, the possibilities of energy recovery and the environmental performance. The
different techniques considered for the recovery of energy are very much dependent on the used
smelting system, but also on local conditions such as local energy prices, periods of production and
the presence of potential customers.

BAT for smelting
   The different furnaces used for the ferro-alloy production are listed in the following table that
summarises the advantages and disadvantages of the various systems.




Table 4: Summary of advantages and disadvantages of the used smelting systems in the ferro-alloy industry.


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    Taking account of the above advantages and disadvantages the smelting systems to consider are:
          •    Open furnace for special applications and small capacities connected with a
              bag filter
          •    Semi-closed furnace connected with a bag filter
          •    Closed furnace systems in different applications cleaned by a wet scrubber or
              dry cleaning system
          •    Blast furnace if the waste energy will be recovered
          •    Reaction crucibles with an appropriate hooding system connected with a bag
              filter
          •    Reaction crucibles in a closed chamber connected with a bag filter
          •    Multiple heard furnace for molybdenite roasting with an dust removal and an
              acid recovery
    The open furnace for producing bulk ferro-alloys is not a technique to be considered in the
determination of BAT. The main reasons are the higher electrical energy consumption due to the
higher off-gas volume to be cleaned in the filter-house. This higher off-gas volume induces, even
with a high standard bag house, a larger amount of fine dust emitted to the environment. In addition
the energy used to operate an open furnace can not be recovered.
    According to the different ferro-alloys produced and the environmental impact of the processes,
which are influenced by the smelting system, the smelting furnaces presented in the next table are
considered to be BAT for this sector.




•
•




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•
Table 5: Smelting furnaces considered as BAT for the production of ferro-alloys

•
     The considered furnaces are in general all applicable to new and existing plants. However the
long furnace life and the very high investment cost to build a new or replace an existing furnace
should be taken into account. Therefore the best available techniques for smelting furnaces is strongly
applicable only for new plants and a substantially change or replacement of a furnace.
     This is especially the case for replacing an open furnace by a closed furnace, because main parts
of the abatement technique need to be changed as well.
     The open furnace itself has not a significantly higher electrical or coke consumption, but huge
amounts of cold ambient air are sucked into the furnace to burn the CO which is present in the off-
gas. This consequently results in a very large volumetric flow of waste gas, which does not allow the
recovery of its energy content because the temperature level is low and the flow rate large to build
technically and economically efficient heat exchangers. The CO generated by the smelting process in
this case is transformed into CO2 and heat without using its energy content that is lost. Due to this the
open furnace has not been considered as BAT, but can be tolerated if local conditions, for instance
local prices of energy, periods of production and the absence of possible customers didn’t allow the
recovery of energy from a semi-closed furnace under economic viable conditions.
     For existing open furnaces retrofitting with an appropriate hood in order to change the open
furnace into a semi-closed furnace is suitable and possible. By applying a nearly close hooding it is
possible to limit the infiltration of air, but at the same time supply enough air to combust the CO
generated in the furnace. Defining the off-gas temperature, which is about 300 – 400 ºC for an open
furnace and about 600 - 800 ºC for a semi-closed furnace, can be used to make the distinction
between open and semi-closed furnaces. The volumetric flow rate, which can be up to 100000 Nm3/t
of metal for an open furnace and up to 50000 Nm3/t of metal for the semiclosed furnace can be used
as an indication. Due to the increased off-gas temperature in a semiclosed furnace, the installation of
an appropriate energy recovery system should than also be taken into consideration, because the
major advantage of a semi-closed furnace is the possibility to recover a significant part of the process
heat. The energy recovery can be done by producing steam in a waste-heat boiler and transformation
into electrical energy.


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    For a semi-closed furnace with a nearly closed hood it should as well be noticed that the capital
requirement for a collection and abatement system is proportional to the volumetric flow of gas, so
that minimisation of gas volume is emphasised. This will also affect the environmental impact
concerning the total amount of dust emitted. Assuming a bag filter with the same filter efficiency is
used, the mass stream of dust emitted to the atmosphere will be reduced in the same way as the
volumetric flow of gas will be reduced.

8. Post furnace operations
    Using a pneumatic or hydraulic drill normally opens the tap hole of the smelting furnace.
    Oxygen lancing is also used, either as the only method or as a back up or complement to drilling.
A tapping gun helps to remove blockages, but slugs containing lead and zinc should only be used if
an appropriate hood is installed to remove tapping fumes. This is necessary because the lead and
particularly the zinc, will to a large extent vaporise in the tap hole, and create zinc and lead fumes that
otherwise would pollute the working area and subsequently participate in the ventilation air. The tap
hole is closed using a mud gun.
    The most frequently used technique of tapping is the cascade tapping. In this case the metal and
slag is tapped together in the same vessel. The lower density slag float at the top and eventually
overflows through the spout to the next ladle.
    Slag granulation and water spraying of slag in a pit or teeming station will contribute to reduce
emissions of fumes and dust. The used water needs a treatment in a settler to remove particles before
using it again as quenching water.
    The generation of very fine powder (dust) that is collected in the bag filter used for de-dusting the
furnace off gases may create problems in handling, storage and transport of powders.

9. Process control
    Process operation and control has developed recently in this sector and is applied to a variety of
processes. Full process design is approached with care using professional engineers who have
experience and knowledge of the process and of the environmental impact and requirements.

BAT for process control
     The principles of Best Available Techniques include the concepts of how a process is designed,
operated, controlled, manned and maintained. These factors allow good performance to be achieved
in terms of emission prevention and minimisation, process efficiency and cost savings.
     Good process control is used to achieve these gains and also to maintain safe conditions. The
following techniques are used: -
          • Sampling and analysis of raw materials is commonly used to control plant
            conditions. Good mixing of different feed materials should be achieved to get
            optimum conversion efficiency and reduce emissions and rejects.
          • Feed weighing and metering systems are used extensively. Loss in weight silos,
            belt weighers and scale weighers are use extensively for this purpose.
          • Microprocessors are used to control material feed-rate, critical process and
            combustion conditions and gas additions. Several parameters are measured to
            allow processes to be controlled, alarms are provided for critical parameters: -
    o   On-line monitoring of temperature, furnace pressure (or depression) and gas volume or flow
        is used to prevent the production of metal and metal oxide fume by overheating.
    o   Gas components (O2, SO2, CO) are monitored. Process gases are collected using sealed or
        semi-sealed furnace systems. Interactive, variable speed fans are used to ensure that optimum

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        gas collection rates are maintained and can minimise energy costs. Solvent vapours are
        collected and recovered as far as possible. Further removal of solvent vapours is practised to
        prevent the emission of VOC and odours.
    o   On-line monitoring of vibration is used to detect blockages and possible equipment failure.
    o   On-line monitoring of the current and voltage of electrolytic processes.
    o   On-line monitoring of emissions to control critical process parameters. Process control using
        relevant methods so that it is possible to maintain operating conditions at the optimum level
        and to provide alarms for conditions that are outside the acceptable operating range.
          • Slag, metal and matte are analysed on the basis of samples taken at intervals.
            On-line analysis of these streams is an emerging technique.
          • Operators, engineers and others should be continuously trained and assessed in
            the use of operating instructions, the use of the modern control techniques
            described and the significance of and the actions to be taken when alarms are
            given. There is growing use of dedicated maintenance staff forming part of the
            operator teams who supplement the dedicated maintenance teams.
          • Environmental management and quality systems are used. Hazard and
            operability studies are carried out at the design stages for all process changes.
            For some processes special regulations such as the Seveso or Waste Incineration
            Directives may have to be taken into account.

10. Emissions to air
Dust emissions
    According to the raw material that is needed and the unit operations used, e.g. crushing, drying,
sintering, smelting, tapping and product handling, the most important source of environmental input
is dust and fume emissions. The following figure shows the potential emission points for dust and
fume emissions from a ferroalloy producing plant.




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Figure 6: Ferroalloy production flow diagram showing potential points of air emissions


    Unloading and storage of raw material can generate dust when the material falls from one
conveyor to another. Dust can also be produced if the conveyor is running too fast (i.e. more than 3.5
m/s). If a front–end loader is used, dusting is seen during the transport distance.
    The dust that is produced by the smelting process is collected by hoods or, in case of a closed
furnace, by the furnace sealing directly and transferred to an abatement plant and de-dusted (e g. by a
fabric filter or a wet scrubber). Scrubbing is used for closed furnaces.
    Tapping off-gas consists of dust and fumes from oxygen lancing, dust from drilling, fumes from
vaporised slugs if a tapping gun is used and fumes from all exposed metal and slag surfaces. These
fumes that arise from tapping will mainly be oxides of the metals involved in the smelting process.
    The following tables present the available emission data for the emission of dust by producing
various ferro-alloys.




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Table 6: Dust emissions to air (after abatement) by producing ferro-alloys


    The proposed EPA standard would require that the emissions of particulate matter (PM) (as a
surrogate for metallic HAP) from each air pollution control device serving new and existing
ferronickel calciners and electric arc melt furnaces shall not exceed 34 milligrams per dry standard
cubic meter (mg/dscm) (0.015 grains per dry standard cubic foot (gr/dscf)). (EPA)

Other emissions to air
    The most important pollutants from the production of ferro-alloys beside dust are
           •   SO2,
           •   NOx,
           •   CO-gas CO2,
           •   HF,
           •   poly cyclic aromatic hydrocarbon (PAH),
           •   volatile organic compounds (VOCs) and
           •   heavy metals (trace metals).
     The formation of dioxins in the combustion zone and in the cooling part of the off-gas treatment
system (de-novo synthesis) may be possible. The emissions can escape the process either as stack
emissions or as fugitive emissions depending on the age of the plant and the used technology. Stack
emissions are normally monitored continuously or periodically and reported by on-site staff or off-
site consultants to the competent authorities.
     In the carbo-thermic process only the fixed carbon content is used as a reductant, which means
the content what is left when volatile matters ashes and moisture are deducted. The volatile matter,
which consists mainly of hydrocarbons, does not take part in the reaction but leaves the furnace
together with the CO when the furnace is closed or burns near the surface in a semi-closed or open
furnace. In both cases the energy content in the volatile matters is utilised. The sulphur content in
metallurgical coke varies between 0.4 and 1.0%. 60 – 85% of the sulphur remains in the slag and
about 5% escapes the furnace as SO2.
     Heavy metals are carried into the process as trace elements in the raw material. The metals with
boiling points below the process temperature will escape as gases in form of metal vapour, which
partly condenses and oxidises to form part of the dust from the smelting furnace. Even after tapping
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and especially during refining, the temperature of the molten metal and slag are high enough to allow
vaporisation of components both from the metal and from the slag. The fumes arising from this
evaporation evolves the whole time, from start of tapping until casting is finished. Even after the ladle
is emptied, some fumes may evolve from the metal scull. During tapping most of the fumes are
collected and cleaned, trough the tapping fume collection. Depending of the type of ore that is used,
mercury may be emitted to air. Therefore control of mercury-input in the furnace and control of
subsequent mercury-output of the processes is advisable, if such raw materials are used. In this case
the raw material needs a pre-treatment to remove the mercury otherwise the mercury has to be
removed from the furnace off-gas by using a mercury-removal step.
    The table below presents some figures of recently measured emissions to air by producing bulk
ferro-alloys.




Table 7: Emissions to air from Ferro-nickel

Emissions of noise and vibrations
     The heavy machinery such as crushers and large fans used in the ferro-alloy production can give
rise to emissions of noise and vibration. Also the mechanical scull releasing from the ladles may be a
source of noise.




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                                               (
                                                   MIGA)

Process Gas Collection Techniques
    This section deals with process gases. The techniques involved follow the hierarchy of
prevention, minimisation and collection of fume. Dust, fume and gases are collected by using sealed
furnace systems, by total or partial enclosure or by hooding. Sealed furnaces can be charged from
sealed lance or burner systems, through hollow electrodes, through hoods or tuyeres or by docking
systems that seal onto the furnace during charging. Hoods are designed to be as close as possible to
the source emission while leaving room for process operations. Movable hoods are used in some
applications and some processes use hoods to collect primary and secondary fume.
    Gas collection requires the movement of significant volumes of air. This can consume vast
amounts of electrical power and modern systems focus the design on capture systems to increase the
rate of capture and minimise the volume of air that is moved. The design of the collection or hood
system is very important as this factor can maintain capture efficiency without excessive power
consumption in the remainder of the system. Sealed systems such as sealed furnaces can allow a very
high capture efficiency to be attained.
    Ducts and fans are used to convey the collected gases to abatement or treatment processes. The
effectiveness of collection depends on the efficiency of the hoods, the integrity of the ducts and on the
use of a good pressure/flow control system. Variable speed fans are used to provide extraction rates
that are suitable for changing conditions such as gas volume, with minimum energy consumption.
The systems can also be designed to take account of the characteristics of the plant that it is
associated with, e.g. the abatement plant or sulphuric acid plant. Good design and maintenance of the
systems is practised.
    Collector systems and extraction rates are designed on the basis of good information about the
characteristics of the material to be collected (size, concentration etc), the shape of the dust cloud at
the extremes of operation and the effects of volume, temperature and pressure changes on the system.
    Correct measurement or estimation of the gas volume, temperature and pressure are made to
ensure that sufficient rates of extraction are maintained during peak gas flows. Some of the
characteristics of the gas and dust are also critical to good design to avoid problems of abrasion,
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deposition, corrosion or condensation and these are measured. Another significant factor is the
provision of access to furnace filling or tapping areas while maintaining good rates of collection,
operator experience is used at the design stage to provide this.

BAT for process gas collection
    The techniques to consider are based on the application of the principles of good practice
recorded above. Good practice relies on the professional design and maintenance of the collection
systems as well as on-line monitoring of emissions in the clean gas duct. The following examples are
used to illustrate good practice, it is not an exhaustive list and other examples may also be applicable.
          • The use of sealed furnaces can contain gases and prevent fugitive emissions.
            Examples are sealed smelting furnaces, sealed electric arc furnaces and the
            sealed point feeder cell for primary aluminium production. Furnace sealing still
            relies on sufficient gas extraction rates to prevent pressurisation of the furnace.
          • The use of sealed charging systems for the furnaces to prevent fugitive
            emissions during furnace opening. Examples are the use of charging skips that
            seal against a furnace feed door and the use of through-hood charging systems.
            These techniques may be applicable to some new and existing processes
            particularly for non-continuous processes.
          • An important established practise to achieve good extraction is the use of
            automatic controls for dampers so that it is possible to target the extraction
            effort to the source of fume without using too much energy. The controls enable
            the extraction point to be changed automatically during different stages of the
            process. For example, charging and tapping of furnaces do not usually occur at
            the same time and so the charging and tapping points can be designed to be
            close together so that only one extraction point is needed. The extraction point is
            also designed to allow good access to the furnace and give a good rate of
            extraction.
          • The hooding is constructed robustly and is maintained adequately. An example
            of this is an adaptation of a short rotary furnace. The feed door and tapping
            holes are at the same end of the furnace and the fume collection hood allows full
            access for a slag ladle and feed conveyor, it is also robust enough to withstand
            minor impacts during use. This principle is easily applied to a short rotary
            furnace but the principle of targeting the extraction effort to a changing source
            of fume is also be achieved by automatically controlling dampers to extract the
            main source of fume during the operating cycle e.g. charging, tapping etc. The
            short rotary furnace and the TBRC may also be totally enclosed.
    These techniques may be applicable to all new and existing processes particularly for
noncontinuous processes. If sealed furnaces are not available for example when retrofitting an
existing open furnace, maximum sealing to contain furnace gases can be used.




Figure 7: Fourth hole fume collection

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     An example of this is the use of a “fourth hole” in the roof of an electric arc furnace to extract the
process gases as efficiently as possible and is shown in the above figure.
     Maintenance of the collector hood, the ducts, the filter system and the fan is vital to ensure that
collection or extraction rates remain at the designed level. Physical damage from collision or
abrasion, deposition in ductwork and deposition on fan blades are some of the problems that can be
encountered. Regular inspection and preventative maintenance is used to ensure this. This technique
is applicable to all new and existing processes.

BAT for sulphur dioxide removal
    The Best Available Techniques for the removal of sulphur dioxide removal depends on the
degree of fixation of sulphur in a matte or slag to prevent the formation of sulphur dioxide and on the
strength of the gas that is produced. For very low strength gases a wet or semi-dry scrubber,
producing gypsum for sale if possible, is considered to be BAT.
    For higher strength gases the recovery of sulphur dioxide using cold water absorption followed
by a sulphuric acid plant for the remaining gas and the stripping and production of liquid sulphur
dioxide from the absorbed liquor is considered to be BAT where local markets exist for the material.
The use of a double contact sulphuric acid plant with a minimum of four passes is considered to be
BAT. The principle of maximising the inlet gas concentration is also considered as BAT so that the
subsequent removal process can operate at maximum efficiency.
    The following factors are considered to be BAT for a sulphuric acid plant using smelter off gases.
          • A double contact, double absorption plant with a minimum of 4 passes can be
            used in a new installation. A caesium doped catalyst can be used to improve
            conversion. It may be possible to improve existing catalysts during maintenance
            periods by incorporating caesium-doped catalysts when catalyst additions are
            made. This can be particularly effective when used in the final passes where the
            sulphur dioxide content is lower but to be fully effective must be accompanied
            by improvements in other areas.
          • Gases are diluted before the contact stages to optimise the oxygen content and
            give a sulphur dioxide content to ~ 14% or slightly above to suit the thermal
            limits of the catalyst carrier material. Caesium oxide doping is required for such
            high inlet concentrations as it allows a lower first pass inlet temperature.
          • For low, varying sulphur dioxide concentrations (1.5 to 4%) a single absorption
            plant such as the WSA process, could be used for existing plants. The use of a
            caesium oxide doped catalyst in the final pass can be used to achieve optimum
            performance and can be incorporated during routine catalyst changes or during
            maintenance. To be fully effective this should be accompanied by
            improvements in other areas such as gas cleaning to protect the catalyst from
            poisoning. Conversion to double contact is complex and expensive but the use
            of a single contact plant with tail gas de-sulphurisation if necessary, to produce
            gypsum for sale can allow energy savings and lower waste generation.
          • Fluorides and chlorides should be removed to prevent damage to downstream
            plant structure.




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Fugitive emissions
    Fugitive emissions are generally considered to be gases emanating from sources that cannot be
easily localized, for example, leaks from various types of industrial equipment including valves,
flanges, and compressors. The EPA defines fugitive emissions as “emissions that (1) escape capture
by process equipment exhaust hoods; (2) are emitted during material transfer; (3) are emitted from
buildings, housing, material processing or handling equipment; and (4) are emitted directly from
process equipment.” Gases and fume that escape from the processes are released into the working
area and then escape into the surrounding environment. They therefore affect operator health and
safety and contribute to the environmental impact of the process.
    Fugitive emissions are very important but are hard to measure and quantify. Methods of
estimating ventilation volumes or deposition rates can be used to estimate them. One reliable method
has been used over a number of years at one site. The results show that the magnitude of fugitive
emissions can be much more significant than collected and abated emissions. The lower the
controlled emissions, the more significant the fugitive emissions. Fugitive emissions can be more
than two to three times the quantity of controlled emissions.

BAT for fugitive emissions
    It is possible to reduce environmental impact of fugitive emissions by following the hierarchy of
gas collection techniques from material storage and handling, reactors or furnaces and from material
transfer points. Potential fugitive emissions must be considered at all stages of process design and
development. The hierarchy of gas collection from all of the processes stages is: -
          •   Process optimisation and minimisation of emissions such as thermal or
              mechanical pretreatment of secondary material to minimise organic
              contamination of the feed.
    The use of sealed furnaces or other process units to prevent fugitive emissions allow heat
recovery and allow the collection of process gases for other use (e.g. CO as a fuel and SO2 as
sulphuric acid) or to be abated.
          •   The use of semi-sealed furnaces where sealed furnaces are not available.
          •    The minimisation of material transfers between processes is particularly
              important.
          •     Where such transfers are unavoidable, the use of launders in preference to ladles
              for molten materials.
          •     In some cases, restricting techniques to those that avoid molten material
              transfers would prevent the recovery of some secondary materials that would
              otherwise enter the waste stream. In these cases the use of secondary or tertiary
              fume collection is appropriate.
          •     Hooding and ductwork design to capture fume arising from hot metal, matte or
              slag transfers and tapping.
          •     Furnace or reactor enclosures may be required to prevent release of fume losses
              into the atmosphere.
          •   Where primary extraction and enclosure are likely to be ineffective, then the
              furnace can be fully closed and ventilation air drawn off by extraction fans to a
              suitable treatment and discharge system.
          •   Roofline collection of fume is very energy consuming and should be a last
              resort.


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          • Environmental samples can be taken to measure the impact of fugitive
            emissions. In this case samples of air or dust are collected at a series of points
            determined by an atmospheric modelling exercise. Correlation with atmospheric
            conditions is needed to estimate releases.
          • Fugitive releases from a building such as a furnace room can be measured by
            taking samples from the building ventilators. The flow of gases from the
            ventilators can be estimated by measuring the temperature difference between
            the flow from the ventilators and the ambient air
          • Some furnaces can be equipped with secondary hoods in order to prevent
            fugitive emissions during charging or tapping as described above. The fan
            suction is provided directly at the source of fume to optimise the reduction of
            fugitive emissions. Alternatively, the air could be extracted at the roof
            ventilator, but a large volume of air would have to be handled which might not
            be cleaned effectively in a fabric filter. Other disadvantages are high-energy
            consumption, high investment, more waste (used filter media). Secondary fume
            collection systems are designed for specific cases. Energy use can be minimised
            by automatically controlling the point of extraction using dampers and fan
            controls so that the systems are deployed when and where they are needed, for
            example during charging or during “roll out” of a converter.

Air Abatement and Recovery
    Collected gases are transferred to abatement plant where contaminants are removed and some
components recovered. Dust and acid gases are commonly removed and valuable or toxic metal
components are recovered for use in other processes. The design of the abatement process is critical,
factors such as efficiency, suitability of the method and the input and output loading of the material to
be collected are used.

BAT for Air Abatement and Recovery
    This section presents a number of techniques for the prevention or reduction of emissions and
residues as well as techniques reducing the overall energy consumption. They are all commercially
available. Examples are given in order to demonstrate techniques, which illustrate a high
environmental performance. The techniques that are given as examples depend on information
provided by the industry, European Member States and the valuation of the European IPPC Bureau.

General principles
    The choice and design of a suitable abatement technique is particularly important. Several
techniques exist and although some may seem to offer a very high performance, problems may be
encountered unless the characteristics such as the loading and nature of the gases, dust and other
components are fully considered. For example the fabric filter using modern materials is considered
to offer better environmental performance than other techniques for dust removal; however it cannot
be considered to be universally applicable due to problems of stickiness and abrasion with some types
of dust. These issues are specific to individual sites and materials and the operator should take these
factors into account in a professional design brief.
    The volume, pressure, temperature and moisture content of the gas are important parameters and
have a major influence on the techniques or combination of techniques used. In particular the dew
point will be affected by all of these parameters and their variations throughout a production cycle
must be taken into account.


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     The characterisation of the nature of the dust or fume is very important to identify any unusual
properties (hygroscopic, pyrophoric, sticky, abrasive etc). The particle size and shape, wet ability and
density of the material are also factors to optimise the choice of technique. The dust concentration
and its’ variability should also be taken into account producing a reliable, robust design.
     Many operators have identified that performance may deteriorate with time as equipment wears
and maintenance is needed. Modern systems should be used to continuously monitor performance by
direct measurement of the gases emitted (for example dust, CO, SO2).
     Alternatively critical control parameters can be monitored. Alarm systems should be incorporated
in these systems.
     Bag filter and wet scrubbers are normally used for de-dusting the process off-gases. There exist a
number of different bag filter designs using different kinds of filter materials, which in principal all
achieve low emission values that means dust emissions below 5 mg/Nm3.
     The use of the membrane filtration techniques (surface filtration) results additionally in an
increasing bag life, high temperature limit (up to 260 .C) and relatively low maintenance costs
combined with dust emissions in the range of 1 - 5 mg/Nm3. There are different suppliers in Europe
who are able to provide bag filter with membrane filter bags. The membrane filter bags consist of an
ultra-fine expanded PTFE membrane laminated to a backing material. The particles in the off-gas
stream are captured on the bag surface. Rather than forming a cake on the inside or penetrating into
the bag fabric, particles are repelled from the membrane thus forming a smaller cake. This technique
is applicable for all new and existing plants and may also be used for rehabilitation of existing fabric
filters.
     Bag house filters are in many cases in the ferro-alloy and metallurgical industry pressure filters
with fans on the dirty fume/gas side. Recent developments led to a closed suction filter with fans on
the clean-gas side. This combines the advantages of gentle bag cleaning that means longer bag life,
low operating and maintenance costs and due to the closed filter a defined gas volume].
     Wet scrubbers are techniques to consider by operating closed furnaces where the CO-rich off gas
need to be washed and de-dusted at very high temperatures. Modern wet scrubbers achieve dust
emissions below 10 mg/Nm3, with coarser dust, even achieved dust concentrations of 4 mg/Nm3 by
using a cascade scrubber to clean the off-gas from a sinter furnace have been reported.
     The use of hoods for tapping and casting is also a technique to consider. Tapping fume will
consists of fumes from oxygen lancing, dust from drilling, fumes from the vaporised slugs if a
tapping gun is used and fumes from all exposed metal and slag surfaces. These fumes will consist
mainly of oxides of the metals that are involved in the smelting process. The design of the hooding
system needs to take account of access for charging and other furnace operations and the way the
source of process gases change during the process cycle.




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Table 8: Overview of dust abatement techniques




Table 9: Measured performance of dust removal systems when using various dust abatement
techniques with suitable dusts
     The measured levels are quoted as ranges. They will vary with time depending on the condition
of the equipment, its maintenance and the process control of the abatement plant. The operation of the
source process will also influence dust removal performance, as there are likely to be variations in
temperature, gas volume and even the characteristics of the dust throughout a process or batch. The
achievable emissions are therefore only a basis from which actual plant performance can be judged
and the achievable and associated emissions discussed in the metal specific chapters take account of
the suitability of the dusts encountered and the cost/benefits of the particular application of the
technique. Process dynamics and other site specific issues need to be taken into account at a local
level.

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  According to the techniques to consider that are presented for fume/gas collection and abatement,
BAT for this sector is considered as follows.
         •   Bag filter or wet scrubbers like cascade or venturi scrubbers are suitable for de-
             dusting furnace off gases. A residual particulate matter concentration of less
             than 5 mg/Nm3 for a bag filter and less than 10 mg/Nm3 for a wet scrubber is
             the associated level.
         •   Dust emissions well below the associated levels may be achieved for instance
             with membrane bag filters if local air quality standards or the presence of
             harmful metal compounds requires this.
         •   Some metals have toxic compounds that may be emitted from the processes and
             so need to be reduced. For metal compounds such as nickel, vanadium, chrome,
             manganese etc. as part of the total dust, emissions much lower than the
             associated dust emissions of 5 mg/Nm3 for a bag filter and 10 mg/Nm3 for a
             wet scrubber are achievable. For nickel compounds emissions less than 1
             mg/Nm3 is the associated level.
         •   By recovering ferro-alloys from steel mill residues, dust and volatile metals
             notably mercury and to a lesser extents cadmium and lead should be reduced.
             Using a two-stage bag house with injection of activated carbon or lignite coke
             can do this. Alternatively a 3-step venturi scrubber combined with a wet
             electrostatic precipitator and a selenium filter can also be used.
         •   For harmful toxic vaporised metals like mercury, cadmium and lead as part of
             the off-gas, the associated emission level is below 0.2 mg/Nm3.
         •   Appropriate hooding systems connected with a bag filter are preferably used for
             collecting and cleaning of tapping and casting fumes. Proper design and good
             maintenance can ensure a high capture efficiency.
         •   The sulphur-dioxide content in the molybdenite roasting off-gas should be
             removed and preferably converted to sulphuric acid. The associated conversion
             efficiency for a single contact plant is 98-99%. For new plants 99.3%
             conversion is achievable. The following table summarises the captured emission
             associated with the use of best available technique and the techniques that can
             be used to reach these levels.




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Table 10: Emission levels to air associated with the use of BAT

BAT for removal of mercury
    Mercury removal is necessary when using some raw materials that contain the metal. Specific
instances are referred to in the metal specific chapters and in these cases the following techniques are
considered to be BAT.
           •   The Boliden/Norzink process with the recovery of the scrubbing solution and
               production of mercury metal.
           •   Bolchem process with the filtering off the mercury sulphide to allow the acid to
               be returned to the absorption stage.
           •   Outokumpu process.
           •   Sodium thiocyanate process.
           •   Activated Carbon Filter. An adsorption filter using activated carbon is used to
               remove mercury vapour from the gas stream as well as dioxins.

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    For processes where mercury removal from the gases is not practicable the two processes to
reduce the mercury content in sulphuric acid produced during the production of non-ferrous metals
are considered to be BAT.
           •    Superlig Ion Exchange process.
           •    Potassium iodide process.
    The emissions associated with the above processes are related to any residual mercury that will be
present in the acid that is produced, the product specification is normally < 0.1 ppm (mg/l) and is
equivalent to ~ 0.02 mg/Nm3 in the cleaned gas.

11. Liquid effluents
    The production of non-ferrous metals by pyrometallurgical and hydrometallurgical methods is
associated with the generation of different liquid effluents. The possible wastewater streams are:
           •    Surface run-off and drainage water
           •    Waste water from wet scrubbers
           •    Waste water from slag and metal granulation
           •    Cooling water
     The main sources of the most important effluent streams can be classified as shown in the
following table




Figure 8: Effluent Classification

    The above wastewater streams can be contaminated by suspended solids and metal compounds
from the production processes and may have a high environmental impact. Wet electrostatic
precipitators (ESPs) are often used for gas treatment and a resulting wastewater could have high
metal concentrations. Process bleed streams may contain antimony, arsenic or mercury. Even at low
concentrations some metals like mercury and cadmium are very toxic. This can be illustrated by the
fact that mercury and cadmium head the list of priority hazardous substances drawn up by the North
Sea Conference of 1984, which calls for a 50% reduction of emissions into the North Sea. The toxic
effect of some metal compounds is also due to the fact that under the correct chemical conditions
metals can easily enter natural watercourses as soluble species and be quickly and irreversibly
assimilated into the food chain
    The composition of the liquid effluents from pyrometallurgical as well as from
hydrometallurgical methods depends very much of the metal being produced, the production process
and the raw material that is used. However, the liquid effluents from a non-ferrous metal production
plant normally contain heavy metals, e.g. copper, lead, zinc, tin, nickel, cadmium, chromium, arsenic,
molybdenum and mercury, and suspended solids. For the production of ferro-alloys the emissions to
water are also very dependent on the process; for instance the abatement system and the type of
wastewater treatment used.

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BAT for effluent treatment and water reuse
    There exist a variety of different water collection and wastewater treatment systems in the ferro-
alloy industry. Some plants use a central wastewater treatment plant in which water from different
production processes as well as surface run-off water will be cleaned together. The contaminated
water is led to a thickener or a settling pond to settle out the suspended solids. Precipitation steps are
often used to remove metal compounds from the water.
    Other facilities are using a separate treatment system for rainwater and special treatment
processes for the different process wastewater streams. The wastewater is treated in order to remove
dissolved metals and solids and is recycled or reused as much as possible in the process.
    In special cases for instance by cleaning scrubbing water from a molybdenite-roasting furnace ion
exchangers are used to remove metal compounds such as selenium and rhenium from the scrubbing
water. The particles mostly consist of very fine particles, it may therefore be necessary to add
flocculent to assist settling in thickeners. After the treatment in a thickener or a settling pond the
suspended solids are usually below 20 mg/litre, which allows reuse in scrubbers as cooling water or
as process water for other purposes.
    Where necessary, wastewater should be treated to remove dissolved metals and solids. In a
number of installations cooling water and treated wastewater including rainwater is reused or
recycled within the processes.
    A water treatment is needed in the processes with wet scrubbers and granulation processes;
because suspended solids should be removed before the water is recirculated. To reach acceptable
values of harmful components, it may in some cases be necessary to polish the bleed that has to be
taken from the scrubbing water cycle. This may take place by using sand filters, carbon filters or by
adding suitable chemicals to precipitate harmful compounds.
    The following guidelines present emission levels normally acceptable to the World Bank Group:




                                              (MIGA)

    The most important factors to decide, which in a specific case would be the best solution in order
to minimise the amount of wastewater and the concentration of the pollutants are:
          •   The process where the wastewater is generated,
          •   The amount of water,
          •   The pollutants and their concentrations,
    The most common pollutants are metals and their compounds and initial treatment focuses on
precipitation of the metals as hydroxides or sulphides using one or more stages followed by the
removal of the precipitate by sedimentation or filtration. The technique will vary depending on the
combination of pollutants but the following table summarises the methods described earlier.

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Table 11: Overview of wastewater streams



    The best available techniques are a combination of the different treatment methods and can only
be chosen on a site-by-site basis by taking into account the site-specific factors. The following table
presents the advantages and disadvantages of the most common treatment techniques.




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Table 12: Summary of advantages and disadvantages of common wastewater treatment techniques




Table 13: Summary of advantages and disadvantages of common wastewater treatment techniques




12. Solid wastes, residues and by-products
    The production of ferro-alloys is related to the generation of a number of by-products, residues
and wastes, which are also listed in the European Waste Catalogue (Council Decision 94/3/EEC). The
most important process specific residues are:

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          •   filter dusts,
          •   sludge from wet scrubbers,
          •   slag form the smelting and refining process,
          •   used furnace linings and
          •   packaging material like drums or big-bags.
    According to the current EU legislation, many of these residues are regarded as wastes. However
the non-ferrous metal industry has for many decades used many residues as raw materials for other
processes and an extensive network of metallurgical operators has been established for many years to
increase the recovery of metals and reduce the quantities of waste for landfill. It has been reported
that some legislative measures to control waste movements are inhibiting the recycling of residues
from metallurgical operations. It is also well known that the metal producing industries obtain one of
the highest recycling rates in all industrial sections. This helps to reduce Cross Media Issues to a
minimum. Nevertheless the problem of residues from production facilities and the designation of
some of these materials will also play an important role in future permits and the techniques tend to
concentrate on this aspect.

BAT for solid waste treatment
    The most important factor to reduce the environmental impact of discharging residues as waste is
process-integrated measures that result in the generation of fewer residues. If the amount of process
residues is minimised by using primary measures, the extended amount should be recycled or reused
as much as possible. The specific feed materials will influence the final process choice. To achieve
effective waste minimisation and recycling the following can be considered:
          • Waste minimisation audits can be conducted periodically according to a
            programme.
          • The active participation of staff can be encouraged in these initiatives.
          • Active monitoring of materials throughput, and appropriate mass balances
            should be available. Monitoring should include water, power, and heat.
          • There should be a good understanding of the costs associated with waste
            production within the process. This can be achieved by using accounting
            practices that ensure that waste disposal and other significant environmental
            costs are attributed to the processes involved and are not treated simply as a site
            overhead.
    The amount of slag and filter dust or sludge generated per tonne of produced ferroalloy and their
possibilities of valorisation are shown in the next tables.




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Table 14: Generation, recycling, reuse and discharge of ferro-alloy slag




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Table 15: Generation, recycling, reuse and discharge of dust and sludge from the air abatement system

    In general all process steps should be analysed in order to minimise the generation of process
residues and the exhaust the possibilities of recycling and reuse. The following ways of recycling and
reuse of slag and filter dust and sludge are considered as Best Available Techniques for ferroalloys:




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Table 16: Recycle and reuse of the collected filter dust and sludge from the production of ferroalloys

13. Energy consumption and recovery
     The production of ferro-alloys is a high energy consuming process, because high temperatures are
needed for the reduction of metal oxides and smelting. Factors affecting the energy consumption are
among other things the quality of raw material and their pre-treatment before smelting, the utilisation
of reaction energies and the heat content of the processes. The energy used in the process can be
supplied as electrical energy or fossil fuel in form of coal, coke charcoal or sometimes natural gas.
The supplied energy either in a blast furnace or in an electric arc furnace is transformed into chemical
energy formed by the reduction process as well as off gas energy (CO rich gas) and heat.
     The off-gas energy is mainly represented as process heat in case of a semi-closed furnace or by
the content of CO, CH4 and H2 when a closed furnace is used. The process-gases are produced in the
smelting process if carbon is used as a reducing agent. The CO can be utilised as a secondary fuel and
transferred by means of pipelines within the plant area like any other fuel gas. It can be used by direct
burning for instance in the sinter-furnace and for drying or preheating the furnace charge as well as
for energy recovery in form of hot water, steam and/or electricity.

BAT for energy consumption and recovery
    Article 3(d) of the IPPC Directive requires that energy is used efficiently and this note includes
comment on energy use and its place in the assessment of BAT under each of the metal production
chapters. Energy use in the non-ferrous metal industry is covered by a series of reports prepared by
the Centre for the Analysis and Dissemination of Demonstrates Energy Technologies (CADDET).
These reports have been used extensively in comparing techniques.



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     Raw materials and energy consumption depend on the quality of the ores used. The energy
sources going into the production process consist of electrical energy and latent chemical energy in
the carbonaceous material. One kg of carbon has a potential gross energy content on conversion to
CO2 of approximately 8.8 kWh, or approximately 7.7 kWh/kg coke. If these figures are used, the
gross consumption of energy for the production of bulk ferro-alloys can be calculated as shown in the
next tables. The emitted gross amount of CO2 will be directly proportional to the amount of coke
consumed in the process.
     Recent development work has resulted in a new electrode type utilising a combination of the
Søderberg-technology and a graphite core, to allow the system for the production of silicon metal.
The aim is to reduce the iron impurities caused by the electrode casing.
     Energy and heat recovery is practised extensively during the production and casting of nonferrous
metals. Pyrometallurgical processes are highly heat intensive and the process gases contain a lot of
heat energy. As a consequence recuperative burners, heat exchangers and boilers are used to recover
this heat. Steam or electricity can be generated for use on or off site and process or fuel gases can be
pre-heated. The technique used to recover heat varies from site to site. It is governed by a number of
factors such as the potential uses for heat and energy on or near the site, the scale of operation and the
potential for gases or their constituents to foul or coat heat exchangers.
     The following examples are typical and constitute techniques to consider for use in the processes
to produce non-ferrous metals. The techniques described can be incorporated into many existing
processes:
          • The hot gases produced during the smelting or roasting of sulphide ores are
            almost always passed through steam raising boilers. The steam produced can be
            used to produce electricity and/or for heating requirements. An example of this
            in where a copper smelter produces 25% of its electrical requirements (10.5
            MVA) from the steam produced by the waste heat boiler of a flash furnace. In
            addition to electricity generation, steam is used as process steam, in the
            concentrate dryer and residual waste heat is used to pre-heat the combustion air.
          • Other pyrometallurgical processes are also strongly exothermic, particularly
            when oxygen enrichment of combustion air is used. Many processes use the
            excess heat that is produced during the smelting or conversion stages to melt
            secondary materials without the use of additional fuel. For example the heat
            given off in the Pierce-Smith converter is used to melt anode scrap. In this case
            the scrap material is used for process cooling and the additions are carefully
            controlled, this avoids the need for cooling the converter by other means at
            various times of the cycle. Many other converters can use scrap additions for
            cooling and those that are not able are subject to process developments to allow
            it. The use of oxygen enriched air or oxygen in the burners reduces energy
            consumption by allowing autogenic smelting or the complete combustion of
            carbonaceous material. Waste gas volumes are significantly reduced allowing
            smaller fans etc to be used.
          • Furnace lining material can also influence the energy balance of a melting
            operation. In this case Low Mass refractories are reported to have a beneficial
            effect by reducing the thermal conductivity and storage in an installation. This
            factor must be balanced with the durability of the furnace lining and metal
            infiltration into the lining and may not be applicable in all cases.
          • Separate drying of concentrates at low temperatures reduces the energy
            requirements. This is due to the energy required to super heat the steam within a
            smelter and the significant increase in the overall gas volume, which increases
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    fan size. The production of sulphuric acid from the sulphur dioxide emitted
    from roasting and smelting stages is an exothermic process and involves a
    number of gas cooling stages. The heat generated in the gases during conversion
    and the heat contained in the acid produced can be used to generate steam and
    /or hot water.
•   Heat is recovered by using the hot gases from melting stages to pre-heat the
    furnace charge. In a similar way the fuel gas and combustion air can be pre-
    heated or a recuperative burner used in the furnace. Thermal efficiency is
    improved in these cases. For example, nearly all cathode/copper scrap melting
    shaft furnaces are natural gas fired, the design offers an thermal efficiency (fuel
    utilisation) of 58% to 60%, depending on diameter and height of the furnace.
    Gas consumption is approximately 330 kWh/tonne of metal. The efficiency of a
    shaft furnace is high, principally because of charge preheating within the
    furnace. There can be sufficient residual heat in the off-gas to be recovered and
    re-used to heat combustion air and gas. The heat recovery arrangement requires
    the diversion of the furnace stack gases through a suitably sized heat exchanger,
    transfer fan and ductwork. The heat recovered is approximately 4% to 6% of the
    furnace fuel consumption.
•   Cooling prior to a bag filter installation is an important technique as it provides
    temperature protection for the filter and allows a wider choice of fabric. It is
    sometimes possible to recover heat at this stage. For example in a typical
    arrangement used by a shaft furnace to melt metal, gases from the top of the
    furnace are ducted to the first of two heat exchangers that produces preheated
    furnace combustion air. The temperature of the gases after this heat exchanger
    can be between 200 and 450 ºC. The second heat exchanger reduces the gas
    temperature to 130 ºC before the bag filter. The heat exchangers are normally
    followed by a cyclone, which removes larger particles and acts as a spark
    arrester.
•   Carbon monoxide produced in an electric or blast furnace is collected and burnt
    as a fuel for several different processes or to produce steam or other energy.
    Significant quantities of the gas can be produced and examples exist where a
    major proportion of the energy used by an installation is produced from the CO
    collected from an electric arc furnace installation. In other cases the CO formed
    in an electric furnace burns in the furnace and provides part of the heat required
    for the melting process.
•   The re-circulation of contaminated waste gas back through an oxy-fuel burner
    has resulted in significant energy savings. The burner recovers the waste heat in
    the gas, uses the energy content of the contaminants and removes them [tm 116,
    Alfed 1998]. Such a process can also reduce nitrogen oxides.
•   The use of the heat content of process gases or steam to raise the temperature of
    leaching liquors is practised frequently. In some cases a portion of the gas flow
    can be diverted to a scrubber to recover heat into the water, which is then used
    for leaching purposes. The cooled gas is then returned to the main flow for
    further abatement.



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          • During the smelting of electronic scrap or battery scrap in metallurgical vessels
            the heat content of the plastic content is used to melt the metal content and other
            additional scrap and slag forming components.
          • The advantage of preheating the combustion air used in burners is well
            documented. If an air preheat of 400 °C is used there is an increase in flame
            temperature of 200 °C, while if the preheat is 500 °C the flame temperature
            increases by 300 °C. This increase in flame temperature results in a higher
            melting efficiency and a reduction in energy consumption. The alternative to
            preheating the combustion air is to preheat the material charged to the furnace.
            Theory shows that 8% energy savings can be obtained for every 100 °C preheat
            and in practice it is claimed that preheating to 400 °C leads to 25% energy
            savings while a preheat of 500 °C leads to a 30% energy savings. Pre-heating is
            practised in a variety processes for example the pre-heating of the furnace
            charge using the hot furnace off-gases during the production of ferro-chrome.
     Heat and energy recovery is therefore an important factor in this industry and reflects the high
proportion of costs that energy represents. Many techniques for energy recovery are relatively easy to
retrofit but there are occasionally some problems of deposition of metal compounds in heat
exchangers. Good design is based on a sound knowledge of the compounds released and their
behaviour at various temperatures. Heat exchanger cleaning mechanisms are also used to maintain
thermal efficiency.
     Whilst these savings are examples of individual components of installations they are critically
dependant upon the site and process specific conditions including economics.
     The following table shows that the difference in process energy consumption between production
alternatives is not very big. Indeed, the “conventional” process routes may have an advantage if a
considerable part of the recoverable energy can be sold externally. Most often plants do not have
external energy customers. Choosing a process route that can utilise recovered heat, either for added
process steps that increase efficiency and output, or for electricity generation, will then be advisable
options. An important point of the closed furnace process that uses pelletising/sintering and pre-
heating is to minimise the use of fossil carbon per tonne of produced alloy, which will also minimise
the specific CO2 emission. However, the pelletising/sintering will only reduce the impact of
greenhouse gases if an alternative, less energy efficient process would lead to a deficiency of CO gas.




Table 17: Comparison of electrical and fuel energy consumption

    Ore quality is also an important factor for energy consumption. Of primary importance is the
content of metal oxide and the non ferrous metal/iron ratio, which should both be as high as possible.

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Secondarily the content of gangue minerals should be as low as possible in the ore or the ore mix (this
will partly be a consequence of a high amount of metal oxide), and of a composition to minimise use
of slag additives. This will lower the slag amount, and thus the proportion of the electric power
necessary to melt slag. Concerning the energy usage, the disadvantage of the smelting furnaces used
without energy recovery is the high amount of energy lost as CO in the off gas and as waste heat. For
instance by producing FeSi and silicon metal only about 32% of the energy consumed is chemical
energy in the product, that means about 68% of the energy is lost as heat in the furnace off-gas.
     Energy can be recovered from the cooling cycles as hot water and from the off gas as heat which
can be transferred into high pressure steam and subsequently into electrical energy or by using the CO
content directly as a secondary fuel.
     There are some direct plant improvements that can be done to reduce the energy consumption,
such as running the process with a high metal yield, improving the furnace design to achieve lower
energy loss. In addition to the direct plant improvements about 15 – 20% of the electric energy
consumed by the electric arc furnace can be recovered as electricity by an energy recovery system.
This percentage is considerably higher for a system that produces electricity and uses the thermal
energy of the furnace cooling and the off-gas volume. This will be as well the case if the CO-gas is
utilised directly as a secondary fuel in order to replace fossil fuels.
     Beside the production of electricity the CO gas can also be transferred by means of pipelines in
the plant area and used as a secondary fuel for many purposes. The best utilities are achieved in direct
burning replacing fossil fuels, e.g. heavy oil or coal.
     CO gas can as well be used as a fuel in the steel belt sintering furnace in order to reduce the
primary energy consumption of the furnace. The CO rich gas can as well be cleaned and then
supplied as a synthetic gas to a neighbouring chemical plant, in which the gas serves as a raw
material.
     In a semi-closed furnace the CO-gas from the smelting furnace burns in the suction air thus
creating a hot off gas of about 400 – 800 .C with can also reach peaks up to 1200 .C. The furnaces
can be equipped with an integrated energy recovery system, which contains the following
components:
          •   Exhaust hood with furnace ducting
          •   Waste-heat boiler
          •   Feed-water system
          •   Heat distribution system or steam turbine with generator and condenser The CO
              rich gas can as well be cleaned and then supplied as a synthetic gas to a
              neighbouring chemical plant, in which the gas serves as a raw material.
•
EXAMPLE: ENERGY RECOVERY FOR A SEMI-CLOSED ELECTRIC ARC
FURNACE
     Description: - The energy form hot off-gas of the furnace can be recovered in a waste heat boiler,
which produces superheated steam. Relatively conventional water pipe boilers with super heater,
economiser and condenser sections are used, combined with an efficient cleaning system to keep the
heating surfaces clean in the heavily dust polluted flue gas.
     The furnace top hood is highly exposed to the internal furnace heat, and is conventionally cooled
with a water piping system covered by a ceramic lining. About 25% of the furnace heat emissions are
lost to the top hood cooling water. For energy recovery the top hood may be cooled by unshielded
high-pressure water piping, producing steam to the recovery boiler system. Such hood exists and
contributes substantially to the energy recovery.



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    The steam can be used in a back pressure turbine in order to produce electricity or be sold to a
neighbouring mill. The recovery system can be designed also to produce hot water, which can be used
by a local heating system.




Figure 8: Energy recovery from a semi-closed furnace
     Main environmental benefits: -The recovery of energy from the hot off gas reduces the overall
energy consumption of the process, which consequently minimise the impact of global warming by
emitting CO2 from burning fossil fuel. The off-gas energy presented a large available, partly
unexploited energy source that can provide new electricity without pollution and additional CO2
emission.
     Operational data: The off-gas energy can be used to produce electric power, heat energy or both.
If the waste heat is utilised as electric power the recovery is up to 28 - 33% of the energy
consumption. Alternately, the steam can be drained at mean pressure and be used for district heating,
and the recovery will increase to approximately 80 - 90%. But then only 20% of the waste heat is
recovered as electric power. The demand of district heating often varies trough the year and the most
efficient solution is co-generation of electric power and heat energy to supply heat energy only when
needed.
     Cross media effects: - The recovered energy replaces in most cases fossil fuel like oil or coal and
reduces therefore at the same time the emission of SO2. The energy recovery produces no pollution,
as the flue gas composition is not changed by the recovery. The emission of hot cooling air and water
from the plant is reduced. The energy recovery creates no visual changes of the landscape FeSi
production with an electricity consumption of 60 MW uses a semi-closed furnace with about 750 .C
off-gas temperature. The waste heat boiler consists of 3 sections and each section has 4 economisers,
2 evaporators and 2 super heaters. The gas exits the boiler at approximately 170 .C. The produces
superheated steam is fed to a multistage turbine. The generator produces 17 MW of electric power
equals to 90 GWh/a, which corresponds to 28% of the flue gas Energy and 16.5% of the electric
power consumption in the furnace. The investment costs for the recovery plant has been in 1987
about 11.7 M € (20 Years annuity, 11.5% interest, electricity cost 0.02 €/kWh) FeSi production with
an electricity consumption of 60 MW uses a semi-closed furnace with about 750 ºC off-gas
temperature. The waste heat boiler consists of 3 sections and each section has 4 economisers, 2
evaporators and 2 super heaters. The gas exits the boiler at approximately 170 ºC. The produces
superheated steam is fed to a multistage turbine. The generator produces 17 MW of electric power
equals to 90 GWh/a, which corresponds to 28% of the flue gas Energy and 16.5% of the electric
power consumption in the furnace. The investment costs for the recovery plant has been in 1987
about 11.7 M € (20 Years annuity, 11.5% interest, electricity cost 0.02 €/kWh)
     Applicability: - The technology is in general applicable to both new and existing plants. Since
this energy source normally presents existing installation, one of the obvious demands towards the
energy recovery is that it is applicable to existing plants. For the production of FeSi and Si-metal is


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has been reported that a smelting furnace, which slowly rotates may contribute to the reduction of the
overall energy consumption by about 10% and increase the metal yield.
    The above possibilities of energy recovery are presently in operation in various systems in the
ferro-alloy industry and performed satisfactory for many years. However it should be noticed that an
appropriate energy recovery system means a high capital investment. Taking local conditions, such as
local energy prices, periods of production and the absence of potential customers into account, the
returns of investments may in several cases not be high enough to justify such investments from an
economic point of view.
    According to the considered techniques and routes of utilisation the CO gas or to recover the heat
energy from a smelting process, BAT for energy recovery in this sector is considered as follows:
    Concerning the energy usage, the disadvantage of the smelting furnaces used without energy
recovery is the high amount of energy lost as CO in the off gas and as waste heat. For instance by
producing FeSi and silicon metal only about 32% of the energy consumed is chemical energy in the
product, that means about 68% of the energy is lost as heat in the furnace off-gas (Bref) Energy can
be recovered from the cooling cycles as hot water and from the off gas as heat which can be
transferred into high pressure steam and subsequently into electrical energy or by using the CO
content directly as a secondary fuel.
    According to the considered techniques and routes of utilisation the CO gas or to recover the heat
energy from a smelting process, BAT for energy recovery in this sector is considered as follows:




Table 15: BAT for energy recovery by producing ferro-alloys

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    The above mentioned best available techniques for energy recovery are techniques that are
applicable to new plants and in case of a substantial change of an existing plant. This includes also
the case where a furnace needs to be replaced.
    For existing plants retrofitting of a smelting furnace with an appropriate energy recovery system
is possible especially when an open furnace will be changed into a semi-closed furnace. The energy
content can then be recovered by producing steam in a waste-heat boiler where the furnace hood can
advantageously be integrated in the recovery system and used as superheater. The produced steam
may be used in the process, in neighbouring mills but most often for the generation of electrical
energy will be economically the best solution.
     By building a closed furnace or replacing of an existing furnace by a closed one a cleaning and
recovery system for the CO-gas is unavoidable. The CO, that otherwise needs to be flared off can be
used as high quality secondary fuel for a variety of purposes or as raw material or fuel in
neighbouring mills. Flaring of CO-gas is only acceptable in the case where customers inside or
outside the plant are temporarily not available. The recovered CO gas can as well be used for the
production of electrical energy.
    The recovery of process energy reduces the consumption of natural energy resources and
consequently contributes to minimise the CO2 emissions and the effect of global warming if the total
impact of the process, and the saved energy elsewhere are included into the global energy and CO2
balance. Energy recovery is therefore a desirable option and will in future be more and more
important, but it is suitable only if local conditions (e.g. local prices of energy, the presence of
external energy customers, and periods of production) justify the investment. As already mentioned in
the part of BAT for smelting furnaces the recovery of energy is strongly related to the used furnace
type (semi-closed or closed furnace). Energy recovery should therefore also be seen in the context
and the requirements of changing existing furnaces.




                                       REFERENCES
          • EPA, Proposed Air Toxics Rule For Ferroalloys Production, Fact Sheet, Air
            Quality Strategies and Standards Division (OAQPS), recovered from
            http://www.epa.gov/ttn/oarpg/t3/fact_sheets/ferrofs.pdf
          • Sjardin, Milo CO2 Emission Factors For Non-Energy Use In The Non-Ferrous
            Metal, Ferroalloys And Inorganics Industry, Copernicus Institute Department of
            Science, Technology and Society University of Utrecht The Netherlands, June
            2003
          •  Integrated Pollution Prevention and Control (IPPC) Reference Document on
            Best Available Techniques in the Non Ferrous Metals Industries December
            2001
          • European Nickel Industry Association 2007 as recovered from
            http://www.enia.org/index.cfm/ci_id/12916/la_id/1.htm
          • Hurbe, Jean-Yves Ferronickel: A Specialty Product For The Stainless Industry
            China Nickel 2005 10 September 2005
          • E. Norgate, S. Jahanshahiand and Rankin, W.J. Assessing the environmental impact of
            metal production process Journal Volume 15, Issues 8-9, 2007, Pages 838-848.
          • ANTAM Nickel Strategic Business Unit, Pomalaa Nickel Mine & Ferronickel
            Smelting Plant, Macquarie IMEC Site Visit Friday, January 28 2005

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•   Multilateral Investment Guarantee Agency (MIGA) Environmental Guidelines
    for Nickel Smelting and Refining




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