INDUCTION MELTING FURNACE, ELECTRIC ARC FURNACE, STEEELMAKING

Document Sample
INDUCTION MELTING FURNACE, ELECTRIC ARC FURNACE, STEEELMAKING Powered By Docstoc
					                          COMPARISON BETWEEN EAF AND IMF

          (D. ERTAS – Project and Contracting Department - CVS MAKINA)

By far, the most important engineering and construction material in the world, steel
application figures prominently in many aspects of human life, from civil structures to
automotive manufacture, from paper clips to refrigerators and washing machines and from
aircrafts to the finest surgical instruments. All major industrial economies have a strong
domestic steel industry which shaped their economic growth in the initial stages of their
development.

Steel is produced either from basic raw materials namely iron ore, lime stone and coke,
using the blast furnace and basic oxygen furnace processes, or from recyclable steel scraps
via the electric arc furnace process or induction melting process. The molten steel is stored
and refined in ladles before being solidified and cast into slabs, blooms, billets and bars by
continuous casting machine and/or ingot casting. Semi-finished steel is re-rolled (formed and
finished) to produce finished steel items like plates, flats, bars, sections, tubes etc of varying
thickness and dimensions.

The High-Frequency induction furnace is widely used to produce tool steel (cast steel). The
melt consists of selected scrap of known carbon content. This is placed in a crucible, which is
surrounded by a water-cooled induction coil. A high frequency alternating current is passed
through the coil and this set up an alternating magnetic field within the melt which
generates intense heat and also has a stirring effect. Not only is it possible to obtain high
temperatures, but the degree of heat may be accurately controlled. The furnace is tilted to
pour out the molten metal. Induction furnace capacities range from less than one kilogram
to twenty tonnes capacity, and are used to melt iron and steel, copper, aluminium, and
precious metals. The one major drawback to induction furnace usage in a foundry is the lack
of refining capacity; charge materials must be clean of oxidation products and of a known
composition, and some alloying elements may be lost due to oxidation (and must be re-
added to the melt).

The electric-arc furnace consists of a large shallow bath with either an acid or basic lining,
carbon electrodes over the hearth which may be raised or lowered. Current is supplied to



                               CVS MAKINA İNŞ. SAN. ve TİC. LTD. ŞTİ.
    Dilovası Organize Sanayii Bölgesi 3. Kısım Muallimköy Cad. No:21 Gebze 41400 KOCAELİ / TURKEY
           Tel. +90 262 759 15 05 Fax. +90 262 759 18 80 www.cvs.com.tr    cvs@cvs.com.tr
these electrodes from special transformers. The hearth is charged with lime to remove the
impurities and form a slag. The metal which forms the melt is scrap steel of known
composition. When the furnace is charged the electrodes are lowered and the current is
switched on. The electrodes are then raised and an electric arc jumps across from the
electrodes to the metal and melting begins. The temperature near to electrode tip reaches
about 4100ºC, which is the heat to melt the scrap. Lime, fluorspar, carbon and Ferro-alloys
are added to de-oxidize the metal. The electric arc furnace can used to produce high grade
alloy steels, HSS, High Tensile Steel and Silver Steel. This is possible because of the greater
control over impurities and thus the steel making. It is increasingly used nowadays in the
production of common steel. Arc furnaces range in size from small units of approximately
one ton capacity used in foundries for producing cast iron products, up to about 400 ton
units used for secondary steelmaking (arc furnaces used in research laboratories and by
dentists may have a capacity of only a few dozen grams).

Following paper aims to give comparison between Electric Arc Furnace Technology with
Induction Melting Furnace Technology.




                               CVS MAKINA İNŞ. SAN. ve TİC. LTD. ŞTİ.
    Dilovası Organize Sanayii Bölgesi 3. Kısım Muallimköy Cad. No:21 Gebze 41400 KOCAELİ / TURKEY
           Tel. +90 262 759 15 05 Fax. +90 262 759 18 80 www.cvs.com.tr    cvs@cvs.com.tr
Induction Furnace

Induction heating process is a method in which the electrical conducting material is heated
through by eddy currents induced by a varying electromagnetic field. The principle of the
induction heating furnace is similar to that of a transformer.

Induction furnace has it's own limitation. The induction process used in foundries lacks
refining capacity. Charge materials must be clean of oxidation products and of a known
composition, and some alloying elements may be lost due to oxidation (and must be re-
added to the melt).




                                                              The   frequency     of   operation    of
                                                              induction furnace also vary. Usually
                                                              it depend on the material being
                                                              melted, the capacity of the furnace
                                                              and the melting speed required. A
                                                              high frequency furnace is usually
                                                              faster to melt a charge whereas
                                                              lower frequencies generate more
                                                              turbulence in the metal, reducing
the power that can be applied to the melt.

When the induction furnace operates it emits a hum or whine (due to magnetostriction), the
pitch of which can be used by operators to identify whether the furnace is operating
correctly, or at what power level.




                               CVS MAKINA İNŞ. SAN. ve TİC. LTD. ŞTİ.
    Dilovası Organize Sanayii Bölgesi 3. Kısım Muallimköy Cad. No:21 Gebze 41400 KOCAELİ / TURKEY
           Tel. +90 262 759 15 05 Fax. +90 262 759 18 80 www.cvs.com.tr    cvs@cvs.com.tr
     Following are the features of induction furnace:

         ·   Highest chemical durability.

         ·   High refractoriness.

         ·   Available in various sizes.

         ·   Comes in different capacities.




     Some parameters of induction furnaces are give in the following table:

Title          Unit   0.15Ton       0.3Ton      0.5Ton       1Ton            1.5Ton             2Ton           3Ton
Equipment
                Kw       100          160         250         500             750               1000           1500
rating power
Input
                 V       380          380         380      660        380   660        380   660        380   575-1250
Voltage
Melting rate Kg/hour     160          300         490        1120             1680              2300           3300
Equipment
             ton/hour     5            5           8             10               22               28           35
water rate
Equipment
             kwh/ton     850          800         750         700             650                650            650
power rate
Furnace
                 V       750          750        1500     1400-2500         1400-2500        1400-2500 2300-2500
rating power
Furnace
rating         Ton      0.15          0.3         0.5            1              1.5                2             3
capacity
match cover
               KVA    125-160       160-200    250-315      500-630         800-1000         1000-1250 1600-2000
Transformer
Rating
                'C      1250         1250        1250        1250             1250              1250           1250
Temperature
Capacitor      Kvar     4000         6000        8000    16000 32000 24000 48000 32000 64000                   48000




                                    CVS MAKINA İNŞ. SAN. ve TİC. LTD. ŞTİ.
         Dilovası Organize Sanayii Bölgesi 3. Kısım Muallimköy Cad. No:21 Gebze 41400 KOCAELİ / TURKEY
                 Tel. +90 262 759 15 05 Fax. +90 262 759 18 80 www.cvs.com.tr          cvs@cvs.com.tr
Electric Arc Furnace

Arc furnace provides a simple and effective way of melting various grades of scrap and then
going ahead to refine the metal to desired specification. It is also useful in making all kinds
of steels including tool steels and alloy steels. This provides a method of utilising low cost
scrap, which is available in abundance.The key advantage in EAF melting is that refining is
possible and you can also produce low carbon steels.

An electric arc furnace is mainly used for making steel and consist of devices like refractory-
lined vessel and electrodes. Electrodes are normally round in section and comes in segments
with threaded couplings, so that as the electrodes wear, new segments can be added. The
arc forms between the charged material and the electrode. The charge so formed is heated
both by current passing through the charge and by the radiant energy evolved by the arc.

Through automatic positioning system electrodes are raised and lowered. For positioning
electric winch hoists or hydraulic cylinders are used. The regulating system maintains an
approximately constant current and power input during the melting of the charge, even
though scrap may move under the electrodes while it melts. The mast arms holding the
electrodes are used to convey the current to the electrode holders. The transformer is
installed in a vault to protect it from the heat of the furnace.

The refractory lined vessel having a removable roof is separated from the electrical system.
The bottom of the furnace, is lined with refractory bricks and granular refractory material.
There is a tilting platform on which the furnace is built so that the liquid steel can be poured
into another vessel for transport in the steel making process. To prevent the liquid steel from
the contaminants like nitrogen and slag modern furnaces have a bottom tap-hole on the
spout. In some of the latest plant, scrap pre-heating is applied with different method to
decrease the electric consumption and to increase the productivity.

Operation
Scrap metal is delivered to a scrap bay, located next to the melt shop. The furnace is filled
with the scrap. After putting the scrap inside furnace the roof again cover the top of the
furnace where the melt down goes on.




                               CVS MAKINA İNŞ. SAN. ve TİC. LTD. ŞTİ.
    Dilovası Organize Sanayii Bölgesi 3. Kısım Muallimköy Cad. No:21 Gebze 41400 KOCAELİ / TURKEY
           Tel. +90 262 759 15 05 Fax. +90 262 759 18 80 www.cvs.com.tr    cvs@cvs.com.tr
The electrodes are lowered onto the scrap, an arc is struck and the electrodes are then
pushed into the layer of shred at the top of the furnace. Voltage selected for this level of
operation is usually small. Low voltages protect the roof and walls from excessive heat and
damage from the arcs.

After reaching the base of the furnace and the electrodes can be raised slightly, thereby
increasing the length of the arcs and increasing power to the melt. This helps in the
formation of molten pool even more rapidly. Modern furnaces are designed with some
additional features. In this oxygen is pushed into the scrap. sometimes chemical heat is
provided by wall-mounted oxy-fuel burners. Both processes accelerate scrap meltdown.

The formation of slag is an important part of steel making which floats on the surface of the
molten steel. Slag not only acts as thermal blanket but also reduce erosion of the refractory
lining. For a furnace with basic refractories causes the slag to foam, allowing greater thermal
efficiency, and better arc stability and electrical efficiency.

Once srap has been completely melted down, often another bucket of scrap is charged into
the furnace and melted down. After the second charge is completely melted, refining
operations take place to check and correct the steel chemistry and superheat the melt above
its freezing temperature in preparation for tapping. Once the temperature and chemistry are
correct, the steel is tapped out into a preheated ladle through tilting the furnace.

Alternating current furnaces have three moving graphite electrodes. Heating and melting of
metal is enabled by radiant energy of the arc burning between the electrodes and metal, and
the temperature in the arc zone reaches 4000 °С. The uniform burning of arc is regulated by
means of moving of current-carrying electrodes transversely to the surface of the melt. It is
possible to regulate the radiant energy by stretching and contracting the arc with two
moving electrodes.

Principles of control

Control of electrode movement is, in its turn, based on maintaining of constant level of the
burning electric arc external impedance. During the previous decade they have undergone a
drastic change due to the uprise of the new computing techniques generation in the industry
such as controllers and digital controlled electric drives.



                                CVS MAKINA İNŞ. SAN. ve TİC. LTD. ŞTİ.
     Dilovası Organize Sanayii Bölgesi 3. Kısım Muallimköy Cad. No:21 Gebze 41400 KOCAELİ / TURKEY
            Tel. +90 262 759 15 05 Fax. +90 262 759 18 80 www.cvs.com.tr    cvs@cvs.com.tr
The voltage of furnace power transformer may vary in steps via the tap changing device.
But, as a rule, the transformer taps change is not applied in small furnaces during the
technological process.

The problem of electrode failure at arc ignition in up-to-date electric drives is easily solved by
adjusting current limiting set points in the electrode movement controlled by hydraulic
cylinders, and the overcharge alarm is transmitted to the master controller to ensure the
appropriate system response.

With the help of modern automation equipment it becomes possible to integrate the control
system into a top-level network for organization of recording systems of different types,
control or document management systems.

The use of EAFs allows steel to be made from a 100% scrap metal feedstock, commonly
known as 'cold ferrous feed' to emphasise the fact that for an EAF, scrap is a regulated feed
material. The primary benefit of this is the large reduction in specific energy (energy per unit
weight) required to produce the steel.

EAFs can be rapidly started and stopped, allowing the steel mill to vary production according
to demand. Although steelmaking arc furnaces generally use scrap steel as their primary
feedstock, if hot metal from a blast furnace or direct-reduced iron is available economically,
these can also be used as furnace feed.

Applications
Electric Arc Furnace has following applications:

   ·   Electric arc furnace produces many grades of steel.

   ·   Concrete reinforcing bars common merchant-quality standard channels, bars, and
       flats.

   ·   Special bar quality grades used for the automotive and oil industry.

   ·   A typical steel making arc furnace is the source of steel for a mini-mill, which may
       make bars or strip product.




                               CVS MAKINA İNŞ. SAN. ve TİC. LTD. ŞTİ.
    Dilovası Organize Sanayii Bölgesi 3. Kısım Muallimköy Cad. No:21 Gebze 41400 KOCAELİ / TURKEY
           Tel. +90 262 759 15 05 Fax. +90 262 759 18 80 www.cvs.com.tr    cvs@cvs.com.tr
COMPARISON

When a foundry/meltshop is choosing between state-of-the-art Electric Arc Furnace or
medium frequency coreless induction melting for its operation, the discussion centers on key
factors such as energy costs, environmental regulations, charge materials, labor and
production levels. To perform a proper analysis of what system is best for an operation, all
factors must be quantified in dollars per ton of molten iron and then totaled to determine the
cost-effective melt solution.

This analysis reviews the following factors for both melt systems and assigns them a cost in
dollars per ton of iron:

* charge material/treatment cost;

* operational cost;

* labor cost;

* environmental cost;

* investment and breakeven analysis.



These factors then can be totaled to determine the melt system that is a more cost-effective
option for the specific production situation.



Charge Materials

A major difference between the compared melt processes is the ability to use differing
quality charge materials. Oxidation and reduction reactions take place within and above the
melt zone during electric steelmaking, which allows for the usage of highly oxidized and low
quality scrap material. Induction furnaces are more sensitive to low quality charge materials
and contaminants, resulting in premium scrap costs. A reductive atmosphere is not present
and therefore iron oxide will not be reduced. This increases iron loss through the slag.
Another charge material difference between the melt processes is the cost of alloys and
nonmetallic additions. An induction furnace operation uses a high-grade silicon carbide to




                                CVS MAKINA İNŞ. SAN. ve TİC. LTD. ŞTİ.
    Dilovası Organize Sanayii Bölgesi 3. Kısım Muallimköy Cad. No:21 Gebze 41400 KOCAELİ / TURKEY
           Tel. +90 262 759 15 05 Fax. +90 262 759 18 80 www.cvs.com.tr    cvs@cvs.com.tr
adjust iron chemistry. In addition, pure carbon in the form of graphite is used for
carburization. These additions are costs not required for the EAF.



Electricity

Because of higher electrical energy input with higher electrode current, higher electrical
efficiency with reactors and thanking to additional chemical energy input (nearly 35 % of
total energy) by oxy-fuel burners and C-injection, EAF operation requires lower specific
energy consumption around 380-420 kWh/ton whereas IMF has 680-720 kWh/ton
consumption.



Productivity

Thanking to faster equipment movements, EAF has advantageous for power-of time in one-
heat. Additionally, chemical energy input by supersonic oxygen and carbon injection, power-
on time of EAF is lower. In general, tap-to-tap time of EAF is around 50-60 minutes whereas,
one heat can take 120-150 minutes for IMF. The profitability of any industrial plant increases
and decreases directly with production rates.



Labor
EAF operation with state-of-the-art equipment has the following labor requirements: two EAF
operators, one charge crane operator, one foreman. This results in five workers per shift.

A medium frequency melt system's labor requirements are: two furnace operators, two
charger and crane operators, one supervisor. Five workers per shift are required for melting
within the outlined operation.



Refractory
The EAF is designed for a two-week melt campaign. After the two weeks, the anticipated
repairs would be the breast and tap hole, spout, well and melt zone. In addition, major




                               CVS MAKINA İNŞ. SAN. ve TİC. LTD. ŞTİ.
    Dilovası Organize Sanayii Bölgesi 3. Kısım Muallimköy Cad. No:21 Gebze 41400 KOCAELİ / TURKEY
           Tel. +90 262 759 15 05 Fax. +90 262 759 18 80 www.cvs.com.tr    cvs@cvs.com.tr
replacements and repairs for the spout, well, melt zone and brick lining above the melt zone
must be considered.

For the three medium frequency furnaces, the refractory and labor items to consider are
major furnace relining, general/small furnace repair and pouring spout repair.

The largest differences are in general refractory and small repairs because of the increased
level of materials and man-hours for a three-furnace system. The medium frequency system
does have the advantage of not using a holding furnace, which saves on refractory cost.


Waste Disposal

The wastes accumulating from both systems are slag and dust. The difference lies in the
quantity of waste. EAF generate around 8 % of their melt rate as slag and 20-30 lb of dust
per ton of iron melted. EAF slag from this analysis will have a commercial value for beneficial
reuse.

Medium frequency furnace wastes are less in volume, slag mass is 1% of the melt. This
operation does not produce slag for commercial reuse. In addition, in medium frequency
melting, the dust collected (1 lb/ton melted) in a baghouse requires treatment for leachable
heavy metals.

However, in addition to indirect suction from canopy hood in IMF case, direct suction from
furnace roof fume emission to atmosphere for EAF operation is less due to the chance to
exract the off-gasses from furnace directly



Maintenance
Maintenance costs for both operations are calculated assuming costs for equipment, spare
parts, in-house maintenance and outside contractors and based on reports by foundries.
Costs for the EAF operation are estimated at $6.40/ton of melted iron and $4.40/ton melted
iron for the medium frequency furnace operation.



Buildings and Other




                               CVS MAKINA İNŞ. SAN. ve TİC. LTD. ŞTİ.
    Dilovası Organize Sanayii Bölgesi 3. Kısım Muallimköy Cad. No:21 Gebze 41400 KOCAELİ / TURKEY
           Tel. +90 262 759 15 05 Fax. +90 262 759 18 80 www.cvs.com.tr    cvs@cvs.com.tr
This item includes building operational costs such as electricity and natural gas, office
operation, transportation and safety and is based on reports by foundries. This parameter is
equal for both operations.



Capital Investment

In this analysis, the capital investments for these two systems are based on past projects.
Based on this experience, medium frequency melt systems are installed at a 25% reduced
cost when compared to EAF systems.

For the electric melting system, the major cost factors are the furnaces, charging system,
buildings with power supply and environmental controls.




                               CVS MAKINA İNŞ. SAN. ve TİC. LTD. ŞTİ.
    Dilovası Organize Sanayii Bölgesi 3. Kısım Muallimköy Cad. No:21 Gebze 41400 KOCAELİ / TURKEY
           Tel. +90 262 759 15 05 Fax. +90 262 759 18 80 www.cvs.com.tr    cvs@cvs.com.tr
Which to Choose?

Since it gained prominence in the 1960s, electric melting has been an attractive option for
iron foundries/meltshops. The key is that when the selection of a system is being made, all
the proper data and factors must be considered to ensure the most efficient melt operation.

The temperature of the arc is much higher than the max temperature created by the
induction. A big difference will be the energy costs. Arc is electrically more efficient. Most
coreless induction furnaces are for non-ferrous melting and are much smaller than a small
arc furnace.

EAF practice needs experienced melters and operators who are in short supply. Also the cost
of graphite electrodes and refractories increase the operational cost. Energy cost for melting
compares with Induction furnace. The cost of EAF may be low compared to Induction
furnace, but the cost of electrical transformers,switch gear,cables will be expensive.
Additionally, additional pollution control equipments has to be installed to meet company’s
local laws.

Normally where the liquid metal demand is above 25 tons on a continuous basis, EAF is
installed.

The major drawback in Induction melting is the use of clean and segregated scrap, which is
expensive and the absence of any refining. Installation of the equipment is quick and a low
skilled operator can un the furnace. You are able to produce very low carbon (0.03%) grades
in induction furnace. Also changing grades of steel can be done on a heat to heat basis.

For hot metal requirements below 5 tph, the medium frequency coreless induction furnace
has proven to be the best choice from an operational and economic standpoint.

Dust generation and collection continue to be two serious problems with Induction Furnaces,
and no satisfactory solution appears imminent.

If the refractory and ladle practice is good, arc furnace metal should also be of better quality

Cast steel grades prone to form defects caused by nitrogen or hydrogen may pose a problem
when melted in induction furnaces.




                                 CVS MAKINA İNŞ. SAN. ve TİC. LTD. ŞTİ.
     Dilovası Organize Sanayii Bölgesi 3. Kısım Muallimköy Cad. No:21 Gebze 41400 KOCAELİ / TURKEY
              Tel. +90 262 759 15 05 Fax. +90 262 759 18 80 www.cvs.com.tr   cvs@cvs.com.tr
Both types of melting furnace operations benefit from the use of refining operations
(secondary metallurgy), particularly with respect to "clean steel" requirements.

Each melting operation presents its own set of unique requirements, and the optimum
practice must be developed on a case-by-case basis; rules-of-thumb are inadequate.

Due to the nature of Induction Furnace melting process, refining stage cannot be
accomplished. Consequently phosphorus reduction and slag removal cannot be succeeded.
To obtain proper quality, clean from hazardous elements and dirt free scrap should be used.
These kind of scrap is respectively expensive and difficult to find than the ordinary scrap.

Due to the shape of the Induction Furnace, scrap size is very limited. To prepare scrap blend
brings extra cost.

Electric consumption for EAF is very low. Even less than 400 kwh/ton can be quarantined.

Delivery time of the mechanical equipment is approximately 4 months, but electrical
equipment such as step-down transformer, furnace transformer etc. is 8-10 months. This will
be the same as Induction Furnace.

Productivity of EAF is two times more than the Induction Furnace in terms of ton/hour.

The scrap yield is 90% in EAF but as already mentioned on item 1, EAF has opportunity to
use cheaper scrap than the Induction Furnace




                               CVS MAKINA İNŞ. SAN. ve TİC. LTD. ŞTİ.
    Dilovası Organize Sanayii Bölgesi 3. Kısım Muallimköy Cad. No:21 Gebze 41400 KOCAELİ / TURKEY
           Tel. +90 262 759 15 05 Fax. +90 262 759 18 80 www.cvs.com.tr    cvs@cvs.com.tr
Following table gives the operational cost comparison:

CALCULATED                           UNIT EAF (UNIT EAF (PRICE IMF (UNIT IMF (PRICE
                        30 TON
FURNACE CAPACITY                     PRICE CONS.)   CONS.)     CONS.)    CONS.)

ELECTRICITY             KWH/TON 0,0967 380                   36,75          575             55,60

ELECTRODES              KG/TON       4,9      1,6            7,84           0               -

OXYGEN                  NM3/TON 0,08          30             2,40           0               -

NATURAL GAS             NM3/TON 0,1           2              0,20           0               -

LIME                    KG/TON       0,07     35             2,45           35              2,45

REFRACTORY              KG/TON       0,7      3              2,10           3               2,10

YIELD                   %            150      0,89           133,50         0,89            133,50

POWER-OFF TIME          min                   15             -              25              -

POWER-ON TIME           min                   35             -              55              -
TOTAL
OPERATIONAL COST        USD/TON               185,24                        193,65


Steel production using the electric arc furnace has increased significantly in recent years and
now accounts for a little over one third of total world output.




                               CVS MAKINA İNŞ. SAN. ve TİC. LTD. ŞTİ.
    Dilovası Organize Sanayii Bölgesi 3. Kısım Muallimköy Cad. No:21 Gebze 41400 KOCAELİ / TURKEY
           Tel. +90 262 759 15 05 Fax. +90 262 759 18 80 www.cvs.com.tr    cvs@cvs.com.tr

				
DOCUMENT INFO
Shared By:
Categories:
Stats:
views:638
posted:8/29/2010
language:English
pages:14