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Fate of Mercury in Cement Kilns
Paper #1203
C.L. Senior, A.F. Sarofim
Reaction Engineering International, Salt Lake City, UT
E. Eddings
Chemical and Fuels Engineering, University of Utah, Salt Lake City, UT
ABSTRACT
Mercury is one of a number of pollutants (like dioxins) that persist in the environment
and bioaccumulate in the food chain. Because of its toxicity and the potential for
bioaccumulation, mercury emissions to the environment are the subject of environmental
regulation. U.S. EPA estimates that 87% of the man-made emissions of mercury come
from point sources of combustion. There are currently emission limits on mercury from
certain categories of combustion sources, including cement kilns and incinerators burning
hazardous waste. Cement kilns that do not burn hazardous waste are not subject to these
emission standards. However, EPA is currently reviewing the need for emission
standards for mercury and other pollutants from cement kilns. In this paper, we review
the chemistry of mercury in the cement-making process and develop a simple model for
understanding the distribution of mercury among various streams in different types of
cement kilns.
INTRODUCTION
EPA, in its Mercury Study Report to Congress1 in 1997, put the amount of mercury
released into the atmosphere from human activities between 50 and 75 percent of the total
yearly release from all sources. According to this estimate, 158 tons of mercury was
emitted annually from all human activities in the U.S., most of this (87%) from
combustion point sources. Hazardous waste combustors, including Portland Cement
facilities that burn wastes, contributed 7.1 tons per year or 4.4% of the total.
Manufacturing sources contributed 10% to this estimate. The Portland Cement
manufacturing industry (excluding facilities burning hazardous waste) was estimated to
contribute 4.8 tons per year, or 3.1% of the total.
Only a fraction of Portland Cement kilns in the U.S. burn hazardous waste. In a survey
of cement plants in 20002, only 8 plants were identified as burning waste as a primary
fuel (Table 1). The capacity of these plants corresponds to about 5% of the total
manufacturing capacity in the U.S. However, a larger number of plants identified waste
as an alternate fuel, corresponding to about 50% of the total manufacturing capacity.
There are currently emission limits on mercury from certain categories of combustion
sources, including facilities burning hazardous waste. Table 2 summarizes the current
mercury emission standards for existing hazardous waste combustors in the U.S. and
Europe, compared with those for municipal waste combustors in the U.S. The EPA uses
a Maximum Achievable Control Technology (MACT) standard to specify control
technologies for specific pollutants. For mercury emissions from hazardous waste
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combustors, the MACT standard is feed rate control, based on input feed rates to the
combustor.
Table 1. Portland Cement Plants in the U.S. Using Waste as Fuel (Reference 2).
Plants using waste as a primary fuel
Capacity
Process Plants tons/day 1000's tons/yr
Dry 1 2,130 680
Wet 7 11,654 3,889
Total 8 13,784 4,569
Plants using waste as an alternate fuel
Capacity
Process Plants tons/day 1000's tons/yr
Dry 7 13,498 4,295
Dry(Preheater) 8 13,444 4,334
Dry(Precalciner) 19 54,941 17,694
Wet 17 31,139 9,951
Total 51 113,022 36,274
Table 2. Mercury emission standards for existing sources in µg per dry standard cubic meter
in µg/dscm at 7% O2 (Reference 3).
Hazardous Hazardous Hazardous Hazardous Large
Waste Waste Cement Waste Waste Municipal
Incinerators Kilns Lightweight Combustors Waste
Aggregate (Europe) Combustors
Kilns
130 120 47 130 80
Cement kilns that do not burn hazardous waste are not subject to these emission
standards. However, EPA is currently reviewing the need for emission standards for
mercury and other pollutants from cement kilns.
MERCURY BEHAVIOR IN COMBUSTION SYSTEMS
Coal is a common fuel used in cement kilns. Mercury is present in coal in low
concentrations, on the order of 0.1 µg/g. Mercury concentrations in petcoke and tires,
which are often burned in kilns, are lower.
Mercury is also present in certain wastes burned in cement kilns, in various
concentrations. In the high temperature combustion, all the mercury in the fuel is
vaporized as elemental mercury. Coal and liquid wastes are injected through the primary
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kiln burner and any mercury contained in them is exposed to flame temperatures, which
will convert all the mercury to the gaseous, elemental form.
Mercury is also found in the raw materials that enter the kiln. The range of mercury in
limestone, the largest component of the raw material, is reported to be 0.005 to 0.45 ug/g,
but measurements are scanty.4
In high temperature combustion systems, mercury exits the flame region in the elemental
gaseous form (Hg0). Subsequently, mercury can be oxidized homogeneously, oxidized
heterogeneously or adsorbed by ash or activated carbon. Gas-phase thermodynamic
equilibrium calculations suggest that HgCl2 is the dominant oxidized species in flue gas
at temperatures below 900-1100 F. Equilibrium predictions of mercury speciation do not
agree with measured mercury speciation at the inlet to particulate control devices in coal-
fired power plants5 nor with the speciation of mercury in medical waste incinerators
utilizing a water quench after the combustion chamber, which suggests that mercury
species are not in equilibrium as the flue gas cools.
At moderate temperatures in a flue gas environment, mercury is thought to react with
chlorine atoms, which are formed by the interaction of HCl with free radicals. Kinetic
calculations have been carried out on simulated combustion flue gas containing
chlorinated compounds.5 These calculations showed that equilibrium was not achieved
for chlorinated compounds in a rapidly cooling gas with cooling rates typical of the
convection section of a coal-fired power plant waste-to-energy plant. Thus, it seems
reasonable to conclude that the oxidation of mercury via chlorinated compounds does not
reach equilibrium under conditions of rapid quenching.
In incinerators that use a water quench, cooling is even more rapid than in combustion
systems that generate steam. There is indirect evidence of the inability of wet scrubbers
on incinerators with water quench to capture mercury,6 presumably because in those
systems, the mercury is still predominantly in the elemental form at the scrubber.
Elemental mercury, as discussed below, is not removed by wet scrubbers, unlike oxidized
forms of mercury. Experiments carried out on the speciation of mercury in a reactor that
mimicked the flue gas composition and residence time of a typical incinerator6
demonstrated that even at high (3000 ppmv) levels of HCl in the flue gas, mercury was
not oxidized if the gas was quenched too rapidly.
Mercury can be also oxidized heterogeneously or adsorbed by fly ash or activated carbon.
Ash plays a role in both the adsorption of mercury and the oxidation of elemental
mercury in flue gas at temperatures characteristic of particulate control device. Unburned
carbon in ash has been suspected of adsorbing mercury in coal-fired power plants.
In a dry process kiln, the gas exits the kiln at much high temperatures such that mercury
will exit the kiln in the gaseous form. But in the preheater tower, mercury may be
adsorbed on the raw meal, enriching both the kiln feed and the CKD. As with wet
process kilns, concentrations of mercury in the kiln itself should be higher than predicted
from the mercury content of the input streams.
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Detailed measurements of mercury within Portand cement kilns have not been made.
However, the behavior of thallium has been studied. Thallium is reported by Sprung7 to
have the highest volatility of the trace metals that were assayed in cement kilns; the other
metals were Zn, Pb, As, Ni, Cr, Cd. Figure 1 shows the concentrations of thallium and
other metals measured in solids collected from various stages in a preheater kiln. Volatile
metals (e.g.,
Figure 1. Distribution of selected metals in stages of preheater thallium) condense
7
kiln. at lower
temperatures on the
10000
Stage: I II III IV
solids than other
9000 metals. Since the
solids are being
Hourly quantity in feed material, g/h
8000
7000 Pb preheated before
introduction into the
6000
kiln, the condensed
5000
metal will be
4000 recycled back into
3000 K2O x10 -3 the kiln. The
2000
Thallium concentration of
1000
Cl x10 -3 volatile metals will
build up in kiln over
0
0 500 1000 1500 2000 2500 3000
time.
Temperature, F
The example of
thallium suggests
that mercury will have similar behavior; the boiling point of mercury is considerably
lower than that of thallium.
In a wet process kiln, the gases cool to 400oF or less at the kiln exit. Mercury may be
oxidized and/or it may condense on the raw material. In the latter case, the mercury will
move along the kiln in the solid bed and then vaporize in mid-kiln after the solids dry and
begin calcining. This will result in the gaseous mercury concentration within the kiln to
build up to high levels, setting up a recycle loop for mercury within the kiln. As
discussed below, some of the mercury will adsorb on the cement kiln dust (CKD) and be
removed in the particulate control device and some will leave the kiln in gaseous form.
When CKD is reinjected into the kiln, as is commonly done, levels of mercury in the kiln
itself will be considerably higher than one would predict from looking at the content of
mercury in the fuel and raw materials.
MERCURY EMISSIONS FROM PORTLAND CEMENT
MANUFACTURE
Sources of mercury in cement kiln feed streams
Coal is a primary fuel for many cement kilns, whether or not they burn hazardous waste.
Coal contains very small amounts of mercury; typical values of mercury in bituminous
coals of economic importance in the U.S. are 0.05 to 0.25 µg/g coal (dry basis). When
translated into concentration in the flue gas, this corresponds to 4 to 20 µg/DSCM at 7%
O2.
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Figure 2 shows the cumulative distribution of mercury in solid fuels. These data come
from EPA’s Information Collection Request (ICR) and represent multiple fuel samples
from every coal-fired power plant in the US taken during the fourth quarter of 1999.8
The figure demonstrates the range of mercury concentrations in solids fuels.
Figure 2. Distribution of mercury concentrations in solid fuels from ICR, Part 2 data
for fourth quarter, 1999.8
100%
Petcoke
80%
Lignite
Tires
%Less than value
60% Bituminous
Subbituminous
40%
20%
0%
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5
Hg content, ug/g
Cement kilns in the United States have been co-firing hazardous waste with other fuels
for 30 years. Most kilns that burn hazardous waste are wet process kilns. The reason for
this has more to do with economics than process, however. Wet process kilns require
more energy per ton of clinker than dry process kilns. Since fuel costs are an important
part of the cost of cement production, burning a “fuel” that generates income helps wet
process kilns reduce operating costs. Liquid hazardous wastes are most commonly fired
in cement kilns through the primary burner. Liquid wastes include the following:
• Residues from industrial or commercial painting operations;
• Metal-cleaning fluids and lubricants;
• Electronic industry solvents;
• Solvents from automotive aftermarket operations.
Some of these wastes contain trace metals, including mercury. Many of them also
contain significant amounts of chlorine, which can affect the chemistry of mercury in the
flue gas.
The kiln feed materials also contain measurable amounts of mercury as illustrated in
Table 3 using data from EPA’s database of emissions from hazardous waste combustors
(HWCs)9 as part of the process of setting maximum achievable control technology
(MACT) standards for HWCs. This database contains emissions data from 13 different
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wet process cement kilns that burn hazardous waste. In some cases, metals and/or
organics were spiked into the feed for testing purposes. In Table 3, a subset of the data is
shown for samples taken without spiking of metals in the feed and presumably (from the
low percentage of mercury in the raw material) without recycle of CKD into the raw
material. All the mercury concentrations have been converted to the equivalent of
µg/DSCM at 7% O2 in the flue gas. The amount of mercury entering the kiln in the raw
material appears to be on the same order as that in the coal, for these particular kilns. The
Table 3. Mercury in feed streams to select wet process kilns expressed as µg/DSCM
at 7% O2 in the flue gas (Reference 9).
Source
ID Haz. Raw
Number Waste Mat’l Coal Total
203 17.6 6.2 6.6 30.4
302 17.1 2.1 19.2
302A 17.1 2.1 19.2
319 18.9 0.8 0.8 20.5
319 5.9 0.7 0.3 7.9
322 71.4 1.2 72.5
323 3.1 3.6 6.7
323 152.9 5.4 158.2
323 24.0 3.1 27.0
323 112.8 3.4 114.5
404 86.9 8.0 7.5 98.4
404 27.8 8.8 5.9 42.5
473 456.8 456.8
hazardous waste typically has more mercury than either the coal or the raw material and
there is a very wide range of mercury in the hazardous waste.
Emissions of mercury from cement kilns
In order to get an idea of the range of mercury emissions from cement kilns, we will look
at two sets of data, one for kilns burning hazardous waste and one for kilns that do not
burn hazardous waste.
The Portland Cement Association compiled a database of cement kiln emissions,10
including emissions of mercury for kilns that do not burn hazardous wastes. The mercury
measurements are from 35 different sampling reports with a total of 50 measurements.
Various types of kilns, fuels, and particulate control devices are represented in the
database. Figure 3 shows a frequency distribution of the measured mercury emission.
Mercury concentration is reported in µg per dry standard cubic meter (DSCM) at 7% O2.
Statistical analysis of the data did not reveal significant differences between the
emissions as a function of type of process, but the mean mercury emission was higher for
kilns with fabric filters as compared to those with electrostatic precipitators. One group
of measurements falls into the expected range for coal-fired boilers; this might not be
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surprising since many of these kilns burn coal as the primary fuel. What is surprising,
however, is the number of measurements above 50 µg/DSCM.
Figure 3. Distribution of stack concentrations of mercury (in µg/DSCM at 7%
O2) for selected kilns that do not burn hazardous waste (Reference 10).
Range of Range
Coal-fired boilers of MWCs
Number of Tests in Range
20
18
16
14
12
10
8
6
4
2
0
0.01- 0.1-1 1-10 10-20 20-50 50- 100- >500
0.1 100 500
Mercury, ug/DSCM
Figure 4. Distribution of stack concentrations of mercury
The EPA database of
(in µg/DSCM at 7% O2) for selected kilns that burn
emissions from hazardous
hazardous waste (Reference 9).
waste combustors
(HWCs)10 previously
Range of Range mentioned contains
Coal-fired boilers of MWCs emissions data from 13
different wet process
Number of Tests in Range
18 cement kilns that burn
16 hazardous waste. In some
14
cases, metals and/or
12
10 organics were spiked into
8 the feed for testing
6 purposes. Figure 4
4 summarizes the distribution
2 of mercury emissions based
0 on 60 different samples.
0.01- 0.1-1 1-10 10- 20- 50- 100- >500 The mercury emission for
0.1 20 50 100 500 each sample represents an
Mercury, ug/DSCM average of three
measurements.
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Table 4 compares the two datasets. The non-hazardous waste dataset represents all types
of kilns and the hazardous waste dataset, only wet process kilns. However, there was no
statistically significant difference noted in the former dataset among various kiln types.
The dataset of hazardous-waste burning kilns has a mean and median that are about twice
that of the dataset from the kilns that don’t burn hazardous waste. Since the compositions
of wastes vary, no absolute comparison can be made. However, we can get an idea of the
range of mercury emissions from cement kilns, whether or not they fire hazardous
wastes.
There is an inherent recycle of a volatile
Table 4. Mercury emissions from metal such as mercury within the kiln,
cement kilns burning hazardous waste9 both due to recycle of mercury-containing
and not burning hazardous waste10 (in cement kiln dust and to large temperature
µg/DSCM at 7% O2). gradients (in wet process kilns), it may
take a long time for mercury to reach
steady state in a cement kiln. This makes
Kiln type Wet All it difficult to make accurate mass balance
HW non-HW measurements of mercury, particularly
Median 17.4 8.3 when mercury is only spiked in the feed
for short periods of time or where mercury
Mean 66.4 28.0
concentrations are changing rapidly in the
Standard deviation 201.2 62.7 feed stream. To illustrate the difficulties
Minimum 0.2 0.0 in closing the mercury mass balance, we
Maximum 1237.7 385.6 compare the mercury in the feed streams
to the mercury in the stack for selected
samples from the EPA HWC database.9
We have excluded any measurements in which mercury was spiked into the feed, with
the understanding that it can take a long time to come to steady state when metals are
spiked into the feed. The data on mercury in the raw material appear to include both
CKD and raw meal in some cases, which suggests that CKD was being recycled for those
cases. We have adjusted the concentration of mercury in the raw material in those cases
either to reflect the mercury level in the absence of recycle (if those data are available for
that kiln) or the mercury level in typical raw material. Figure 5 shows the ratio of
mercury in the stack emission to mercury in the feed. There was generally a significant
amount of mercury in the stack, relative to the feed concentration. In many cases, the
amount of mercury in the stack was greater than the amount of mercury in the feed. As
discussed above, this illustrates the inherent difficulty of obtaining a steady state
measurement of mercury in cement kilns.
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Figure 5. Ratio of mercury in stack emission to mercury in feed stream for selected wet
process kilns with and without recycle of cement kiln dust (Reference 9).
8
No Recycle
7
Recycle
6
Number of Tests in Range
5
4
3
2
1
0
0-0.1 0.1-0.5 0.5-1 1-2 2-5 5-10
Hg in stack emission/Hg in Feed
The distribution of mercury in a dry process, preheater kiln is illustrated in Figure 6.
These data are taken from a large study of trace metals in many different kilns.11 In this
particular kiln, mercury was also spiked into the feed, but the amount of the spike has
been removed from the mass balance because of the uncertainty of the time needed to
Figure 6. Mercury flow rates in a dry process, preheater kiln; all flows in lb/hr
(Reference 11).
Main Stack: 0.0046
Feed:
0.0399
Bypass Stack: 0.0021 Cooler Stack: 0.0003
Preheater
Bypass Dust: 0.0009
Kiln Cooler Clinker:
0.0101
Liquid Waste:
Inputs: 0.0419 lb/hr
Tires: 0.0003
Outputs: 0.0179 lb/hr 0.00007
Coal:
Mass Balance Closure: 0.0016
43%
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reach steady state. The mass balance closure is 43%.
MODELING MERCURY IN CEMENT KILNS
Models have been developed12 that can be used to predict the distribution of trace metals
including mercury in cement kilns. In this work, a model previously developed by
Reaction Engineering International was used to examine the effect of recycle on mercury
emissions from cement kilns.
The model for the long, wet kilns is illustrated in Figure 7. Table 5 gives the assumptions
used in the model. As shown previously, recycle of kiln dust back into the kiln has an
impact on mercury emissions. Figure 8 illustrates the predicted stack emissions with and
without recycle of cement kiln dust. Stack emissions are higher with recycle, which is
the behavior that has been observed (Figure 5).
Figure 7. Long, wet kiln process model for mercury.
Main Stack
Coo
Vent
M18 A M9
M15
C
M6 M5 M13
M19 Long Kiln
M14
M10 M11 M12
M1 M2 M3 M4
Raw Meal Clinker
M21
Kiln fuel
Table 5. Assumptions in long kiln process model.
Dust entrainment in kiln, g CKD/g
clinker 8.0%
Hg vaporization/recycle in kiln 90.0%
Dust entrainment in clinker cooler 10.0%
Collection efficiency, main ESP 95.0%
Collection efficiency, cooler ESP 30.0%
CKD recycle 0.0%
Hg concentration in fuel, mg/kg 0.10
Hg concentration in limestone, mg/kg 0.10
Fuel usage, kg fuel/kg clinker 0.20
Limestone usage, kg/kg clinker 1.52
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The model for the long, wet kilns is illustrated in Figure 8. Table 6 gives the assumptions
used in the model. As shown previously, recycle of kiln dust back into the kiln has an
impact on mercury emissions.
In precalciner kilns, the preheater/precalciner can be bypassed. A portion of the flue gas
is often recycled through a separate stack. The bypass stream does not pass through the
preheater, where mercury can condense on the raw material and be recycled into the kiln.
Figure 9 illustrates the predicted emissions from the main stack and bypass stacks. The
percentage of flue gas that exits through the bypass stack has an influence on stack
concentrations of mercury. Bypass shifts mercury emissions from the main stack to the
bypass stack.
Figure 8. Precalciner kiln process model for mercury.
Bypass Stack
M7 M17
B
M16
Main Stack Preheater/Precalciner
Cooler
Vent
M18 A M9 M8
M20 M15
C
M6 M5 M13
M19 Kiln
M14
M10 M11 M12
M1 M2 M3 M4
Inputs Clinker
M22 M21
Precalciner fuel Kiln fuel
Table 6. Assumptions in precalciner process model.
Dust entrainment in kiln, g CKD/g
clinker 14.0%
Hg vaporization/recycle in kiln 70.0%
Dust entrainment in clinker cooler 10.0%
Kiln bypass 30.0%
Carryover from preheater to APCD 60.0%
Collection efficiency, main ESP 30.0%
Collection efficiency, bypass ESP 30.0%
Collection efficiency, cooler ESP 30.0%
Hg concentration in fuel, mg/kg 20.0%
Hg concentration in limestone, mg/kg 0.26
Fuel usage, kg fuel/kg clinker 0.11
Fraction of fuel burned in kiln 1
Limestone usage, kg/kg clinker 1.52
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Figure 9. Effect of bypass ratio on the emissions of mercury from main and bypass
stack on precalciner kiln.
0.30
Main Stack
0.25 Emissions
0.20
Bypass Stack
Hg/Hg Input
0.15 Emissions
0.10
0.05 Bypass ESP Dust
0.00
0% 5% 10% 15% 20% 25% 30% 35%
Bypass
CONCLUSIONS
There are currently emission limits on mercury from certain categories of combustion
sources, including cement kilns that burn hazardous waste. Cement kilns that do not burn
hazardous waste are not subject to these emission standards. However, EPA is currently
reviewing the need for emission standards for mercury and other pollutants from cement
kilns.
Mercury enters a cement kiln in the coal, in the raw material (kiln feed) and in hazardous
wastes (if they are burned). Mercury leaves the kiln in the clinker, CKD or via stack
emissions. The distribution of mercury within a cement kiln is difficult to measure
quantitatively because of the difficulty in reaching a steady state. There is an inherent
recycle of mercury between the hot and cold end of a kiln, and also from the reinjection
of mercury-containing CKD into the kiln. Models can be used to predict the distribution
of mercury in a cement kiln. In the future, such models might be used in conjunction
with control technology to minimize emissions of mercury from cement kilns.
REFERENCES
1 Keating, M.H., et al. Mercury Study Report to Congress, Volume I: Executive
Summary, EPA-452/R-97-003, December 1997.
2. Portland Cement Association, U.S. and Canadian Portland Cement Industry: Plant
Information Summary, Data as of December 31, 2000 (Portland Cement Association,
2001).
3. U.S. Environmental Protection Agency, Hazardous Waste Combustion Frequently
Asked Questions, http://www.epa.gov/hwcmact/faqs.html (March 26, 2002).
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4. Johansen, V.C, Hawkins, G. J., Mercury Speciation in Cement Kilns: A Literature
Review, PCA R&D Serial 2567 (Portland Cement Association, 2003).
5. Senior, C.L., Sarofim, A.F., Zeng, T., Helble, J.J., Mamani-Paco, R., “Gas-phase
transformations of mercury in coal-fired power plants,” Fuel Process. Technol.,
2000, 63, 197-213.
6. Gaspar, J.A., Widmer, N.C., Cole, J.A., Seeker, W.R., “Study of Mercury Speciation
in a Simulated Municipal Waste Incinerator Flue Gas,” paper presented at the
International Conference on Icineration and Thermal Treatment Technologies,
Oakland, California, May 12-16, 1997.
7. Sprung, S., Technological Problems in Pyroprocessing Cement Clinker: Cause and
Solution. (Beton-Verlag, 1985).
8. EPA Air Toxics Website - Utility Toxics HAP Study,
http://www.epa.gov/ttn/atw/combust/utiltox/utoxpg.html#DA4
9. U.S. Environmental Protection Agency, Hazardous Waste Combustion: NODA
Documents, http://www.epa.gov/epaoswer/hazwaste/combust/comwsite/cmb-
noda.htm, December 19, 2002.
10. Richards, J. “Compilation of Cement Industry Air Emissions Data for 1989 to 1996”,
SP125, Portland Cement Association, Skokie, Illinois. .
11. Portland Cement Association, “An Analysis of Selected Trace Metals in Cement
Kiln Dust,” SP109T, Skokie, Illinois, 1992.
12. Owens, W.D., Sarofim, A.F., Pershing, D.W. “The use of recycle for enhanced
volatile metal capture,” Fuel Process. Technol., 1994, 39, 337-356.
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