United States
Environmental Protection
Agency
Industrial Environmental Research
Laboratory
Cincinnati OH 45268
Research and Development
EPA-600/S2-83-093 Dec. 1983
SERA Project Summary
Control of Air Pollution
Emissions from Molybdenum
Roasting
N. Masarky, K. Schwitzgebel, C. D. Wolbach, R. D. Delleney, T. P. Nelson, R.
L Glover, and J. M. Burke
Molybdenum, a relatively rare ele-
ment, occurs principally as molybdenite
(MoS2) and wulfenite (PbMoO4). Molyb-
denite is the commercial source of
molybdenum. In 1979, three primary
molybdenum mines accounted for
about 68% of the domestic production
with the balance obtained as a principal
byproduct chiefly from 16 porphyry
copper mines. This latter form is pre-
sently our sole source of rhenium,
which is recovered in the processing of
molybdenite.
While a minor metal in terms of com-
mercial volume, molybdenum is widely
used in the form of molybdenum
oxide as an alloying agent in ferrous
metals, its principal application, and is
essential for tool steels. Molybdenum
compounds are also used in chemicals,
catalysts, pigments, lubricants, and
electronics. In 1979, the value of U.S.
exports of molybdenum ore, concen-
trates, and products was about three
quarter billion dollars.
Molybdenum oxide is derived from a
concentrate of molybdenite via a
thermal process known as roasting. In
practice, the concentrate is processed
in a vertical multiple hearth type
furnace to produce the oxide. The roast-
ing results in the generation of particu-
late and weak sulfur dioxide emissions.
This program was undertaken to: (1)
determine the capabilities of a unique
fabric filtration system using Teflon®*
coated bags in a hot, corrosive atmo-
sphere for particulate and trace element
control and (2) explore and evaluate the
"Mention of tradenames or commercial products
does not constitute endorsement or recommendation
for use by the U.S. Environmental Protection Agency.
feasibility of a variety of weak SO2 con-
trol systems for application to the molyb-
denum roaster and potentially to other
smelter weak SO2 off-gases.
This Project Summary was developed
by EPA's Industrial Environmental
Research Laboratory, Cincinnati. OH.
to announce key findings of the research
project that is fully documented in
three separate reports (see Project
Report ordering information at back.
Introduction
This project was undertaken jointly by
Molycorp and the lERL-Ci because of
mutual interest in addressing the problems
of weak SO2 stream control. At the time
the tests were conducted, Molycorp was
concerned with the problem of controlling
sulfur dioxide emissions and eliminating
a visible plume in order to meet state
requirements. The IERL objectives were
similar. The first task was to characterize
the pollution control capabilities of the
Teflon® coated fabric filter for removal of
particulate and trace metals in the flue
gas prior to atmospheric discharge or
treatment in a sulfur dioxide control
system. The second task was to determine
the technical feasibility of applying a flue
gas wet scrubbing system for weak
stream (about 1%) sulfur dioxide control.
The joint study was subdivided into three
tasks: (1) characterization of emissions
and particulate control, (2) a study of
alternatives for control of weak sulfur
dioxide emissions, and (3) a pilot-plant
scale test study of one of the approaches
identified in the second task. The
magnesium oxide system was selected
for this study. The results of these tasks
achieved our objectives and aided Moly-
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corp in developing a plan to modernize
their smelter.
At the time these tests were conducted,
Molycorp, Inc. operated two multi-level
hearth roasters at its facility in Washington,
Pennsylvania. The plant processed 10
million pounds of molybdenum per year.
In the roasting process, molybdenum
disulfide concentrate was oxidized by air
to molybdenum trioxide and sulfur
dioxide. Particulate control was achieved
by a baghouse, followed by a spray cooler
and a packed bed scrubber. The scrubber
water was recirculated with blow-down.
A diagram showing the process flow, flue
gas handling and baghouse is given in
Figure 1. The baghouse employs Teflon®
coated fabric filter bags, a unique appli-
cation in the nonferrous industry.
Emissions Characterization and
Particulate Control
The sampling and analysis effort
described in this report was conducted to
characterize the particulate emissions
from the molybdenum roaster perform-
ance of the associated particulate control
devices consisting of a high-temperature
baghouse and a spray scrubber-packed
bed clean-up facility. The characterization
was accomplished by chemical analyses
of all streams, flow-rate measurements,
grain-loading determinations, and particle-
size distribution measurements under
different operating conditions. Spark
Source Mass Spectrometry (SSMS) was
used to semiquantitatively analyze the
samples. From these results, 15 elements
were selected for quantitative determina-
tion. The selection was based on concen-
tration level, volatility, and toxicity. The
elements investigated more fully were:
Arsenic Bismuth Nickel
Lead Antimony Copper
Molybdenum Cadmium Manganese
Mercury Silver Iron
Selenium Zinc Rhenium
The quantitative analytical results were
combined with the total mass flow in
each individual stream to derive an
elemental flow rate. These data were
used to establish material balances
around the roaster and the baghouse.
The major findings were:
• Semiquantitative survey analyses by
SSMS for 73 elements indicate that
Twin Buttes and Questa concen-
trates contain low concentrations
(ppm range) of most metals. Excep-
tions are: copper, lead, zinc, iron,
and manganese. The Twin Buttes
concentrate is high in copper con-
centrate (24,000 ppm versus 1000
ppm in Questa). The Questa concen-
Contains 4 ft.
of Packin
g*
Scrubber
Blowdown Liquid Fed-
to a Thickener
Conveyor J
' Lead Lined Quench Vessel
"oints
Ambient Air Blended to Maintain Baghouse
Exit Temperature at 400-450°F
Figure 1. Dust collection system at Molycorp.
trate shows a high lead concentration
(4300 ppm versus 930 ppm in Twin
Buttes concentrate).
• Mercury and selenium are volatilized
in the roasting process, pass
through the baghouse essentially
uncontrolled, and are partially
removed in the quench scrubber.
For mercury, a feed rate of 0.104
gm/hr (2.3 x 10~4 Ib/hr) was
measured with 0.0014 gm/hr (0.3 x
10~4 Ib/hr) reporting to the product;
therefore 0.1026 gm/hr was volati-
lized in the roaster of which 90% was
removed in the quench scrubber.
For selenium, a feed rate of 68.1
gm/hr (0.15 Ib/hr) was measured,
with 1.4 gm/hr (0.003 Ib/hr) report-
ing to the product; therefore 66.7
gm/hr was volatilized in the roaster
of which 50% was removed in the
quench scrubber.
• Pollutant content of the stack gas
was, in gm/hr (in Ib/hr): molybdenum
— 4.5 (0.01), selenium — 3.6
(0.008); lead — 6.8 (0.015); iron —
3.2 (0.007); organics — 6.8 (0.015).
• The baghouse was effective for par-
ticulate control. The average inlet
loading was 10.3 gm/Nm3 (4.51
grains/DSCF) and 0.1 Mg/hr (223
Ibs/hr); the average outlet loading
was 0.091 gm/Nm3 (0.040 grains/
DSCF) and 1.8 kg/hr (4.04 Ibs/hr).
This indicates a control efficiency of
99.1% on a concentration basis and
98.2% on a mass basis. One would
expect an efficiency of 99% or better
for an installation of this type. For
zero air inleakagefi.e., dilution) the
concentration efficiency should
equal the mass efficiency. It is not
clear if the difference here is real or
whether the lower mass efficiency
value is attributable to: (1) flow
measurement error as the location
of the measurement points was less
than ideal owing to the limitations
imposed by the equipment configura-
tion or (2) interference by acid mist
formed by condensation at the outlet
test point.
The source of the plume had been a
controversial point. Some observers
held that it was caused by the
presence of organic material intro-
duced by flotation agents while
others felt that the cause was
sulfuric acid mist formed in the
interaction of the roaster gas with
the quench scrubber. It was found
that sulfate particles and sulfuric
acid mist, not organics, were princi-
pally responsible for the visible
plume problem. The high oxygen,
high 862 concentration of the
roaster off-gases favors the forma-
tion of S03 as the flue gas cools. The
SOa reacts with solids to form sul-
fates and with water vapor to form
sulfuric acid mist. This increases the
particulate loading as the gas tem-
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perature decreases. Acid dew points
were 309°F at the roaster outlet,
and 269°F at the baghouse outlet.
Paniculate loading determinations
showed a concentration at the roaster
exit of 10.3 gm/Nm3 (4.51 grains/scf).
Analysis of the filter for sulfate indicated
a sulfate contribution of 0.4%. An
average particulate loading of 0.091
gm/Nm3 (0.040 grains/scf) was measured
at the baghouse exit. Sulfate contribution
accounted for 17% of the particulate
matter. Water droplets and acid mist
were present in the quench-scrubber
exit gases. Water had to be evaporated in
order to determine a particulate loading.
The EPA method 5 was used. Contributions
of sulfuric acid mist accounted for 35%,
50%, 74% and 91 % at sampling tempera-
tures of 380°F, 365°F, 300°F and 250°F,
respectively. These results point to
sulfuric acid mist as the cause of the
plume formation. This mist was completely
removed in a bench-scale wet electrostatic
precipitator.
Alternatives for Control of
Weak Sulfur Dioxide Emissions
Processes in the primary nonferrous
metals industry produce off-gases con-
taining significant quantities of SO2.
Typically, these streams are classified as
"strong" or "weak" depending on their
SO2 concentration. Strong gas streams
have SOa concentrations greater than 3.5
to 4.0 volume percent while weak gas
streams contain between 0.5 and 3.5
volume percent S02.
The technology for sulfur dioxide
control in emissions from smelters
containing weak concentrations, i.e.,
from about 0.5 to 3.0 percent S02, is a
slowly developing area. This survey of
alternative control technologies was
undertaken to provide information to aid
in selection of system for pilot-scale
testing. The technical feasibility for a
number of scrubbing systems has been
established through application and full-
scale operation at certain smelters. It
should be noted that, with the exception
of cold sea water absorption, each full-
scale scrubber system is unique in the
sense that it has been applied at only one
smelter site. Some twelve systems were
examined for suitability for application to
nonferrous smelters and molybdenum
roasting in particular.
• CIBA-GEIGY nitrosyl-sulfuric acid
process
• CIBA-GEIGY SO2 sorption-stream
stripping process
• Limestone FGD
• Dual Alkali
• Magnesium Oxide
• Wellman-Lord
• Sulf-X
• Endako
• Chiyoda Thoroughbred (2)
• U.S. Bureau of Mines Citrate
• Sodium Carbonate Throwaway
• Dowa Basic aluminum sulfate
The magnesium oxide system was
selected for pilot-scale testing because of
the state-of-development, available
information for test design purposes,
compatibility with existing pilot plant
equipment, and the need to develop
applicability data to support analysis of
economic feasibility of this system.
Pilot-Scale Test Results for
Magnesium Oxide Scrubbing
The magnesium oxide (MgO) system
was selected for pilot-scale testing to
generate engineering design data for an
MgO system which, in turn, could be used
as a basis for exploring economic
feasibility. Specifically, the tests were
designed to quantify the SO2 removal
which could be attained by the MgO
system, and to develop data on the MgO
system which could be used to design an
absorber for treating similar gas streams
in other smelter applications.
A schematic of the pilot unit is shown in
Figure 2. As shown, the pilot unit
consisted of an absorber, reaction tank,
and solids separator. For the purpose of
these brief tests, the solids produced
were disposed of by ponding rather than
being dried and regenerated.
The absorber was a 76 cm (30 inch)
diameter tower containing two beds of
2.5 cm (1 inch) Tellerette® packing. The
upper bed depth was 76 cm (30 inches)
and the lower bed was 51 cm (20 inches)
deep. The piping and valve arrangement
was designed to distribute slurry through
both beds or the lower bed only. This
permitted scrubbing tests at packing
depths of 51 cm (20 inches) and 127 cm (50
inches). Full cone spray nozzles were
used for liquid distribution and were
arranged so that the edge of the spray
contacted the tower wall at the level of
the packing.
Gas flow rates through the absorber
ranged from 2800 to 4000 NmVhr (1700
to 2600 scfm) while slurry flow rates
ranged from 0.4 to 2.4 I/sec(6 to38gpm).
The SO2 concentration of the gas was
controlled by a damper arrangement on
the inlet gas stream so that inlet gas
strength and flow rate could be maintained
at a constant value through dilution
without interfering with plant operations.
During most tests, the pH of the absorber
feed was maintained between 7.5 and
8.0. Slurry exiting the absorber was
gravity fed to a 5680 (1500 gallon)
reaction tank. The pH of the slurry leaving
the absorber ranged from 4.5 to 6.5, and
Mg (OH)2 was added to the reaction tank
to raise the pH back to 7.5-8.0.
The scrubbing slurry contained 4 to 8
weight percent Mg SO3 • 6H^O solids. A
bleed stream of slurry was removed from
the reaction tank and concentrated to
approximately 35 weight percent solids in
Solids
•Gas
Makeup
Water
Figure 2.
Schematic of the magnesium oxide scrubber.
3
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a clarif ier. Overflow from the clarif ier was
either returned to the reaction tank or
used to prepare fresh Mg(OH)2 in the
slurry tank.
A well-defined test program was
conducted in which the process param-
eters of interest were varied to observe
their effect on SO2 removal. The data
collected during these tests were then
correlated in the form of a design
equation. As this program was considered
preliminary effort, long-term testing to
demonstrate reliability and to acquire
operability data was considered beyond
the scope of this effort and was not
attempted.
The statistically designed test plan
consisted of a matrix of short-term tests
which included all possible combinations
of three liquid-to-gas ratios, three gas
flow rates, and two packing depths (18
tests). These tests were supplemented
with a limited number of tests in which
the pH of the scrubbing slurry and the S02
concentration of the off-gas were varied.
In addition, a longer-term, or steady-state
test was scheduled. This test was
included to provide data which would be
used to complete material balance
calculations for the system.
The test results indicate that the MgO
scrubbing system is applicable for
treating gas streams containing up to
about one percent SO2. Removal effici-
encies of over 90 percent of S02 were
possible at gas velocities and absorber
pressure drops similar to those used in
design of utility MgO FGD systems. In the
evaluation of the results of the tests
conducted at Molycorp, a correlation was
developed which can be used to predict
SO2 removal in the pilot plant for a given
set of absorber operating parameters.
This correlation was developed based on
absorption theory, using the two-film
mass transfer model. Regression analysis
was used to develop an empirical value
for KgA, resulting in the design equation:
R = 1-exp[(4.36 x 1CT3) (L/AC)0899
(pH)286 (W)0284 (SO2)"0621 (P) (D)].
vhere
R = fractional efficiency of S02 removal
L = absorbent flow rate, liters per second
S02 = SOz concentration of inlet gas, ppm
W = MgSO3 solids in the absorber feed,
% by weight
Ac = absorber cross sectional area,
square meters
D = packing depth; centimeters
P = operating pressure, atmospheres
pH = pH of the absorber feed slurry
It is interesting to note that the flue gas
flow rate does not appear. This is due to
an offsetting effect associated with a
change in gas flow rate into the absorber.
By increasing gas rate, more SO2 enters
the absorber and thus tends to lower
removal. However, increasing the gas
flow rate increases the value of Kga and
thus offsets the effect of additional S02
entering the absorber. The reader is
cautioned that this is only true because
the exponent of (G/AC) is 1.0 for this test
program, and should be re-established for
other tests conditions.
The correlation between actual and
predicted S02 removal efficiency is quite
good with a correlation coefficient of 0.98.
The absorber design equation can be
used to predict the performance of a
particular absorber design configuration.
However, this equation has been proved
valid only for specific conditions and
ranges of absorber operating parameters
explored in this program. In using the
equation, the most obvious restriction is
that it is only applicable for use with an
MgO scrubbing system processing gas in
an absorber packed with 2.5 cm (1 inch)
Tellerette® packing. This design equation
is not applicable to other absorber
configurations (e.g., a spray tower) or other
types or sizes of packing material. The
other principal restriction is that the
scrubber operating parameters selected
for an absorber design must be within the
range in which the parameters were
tested during the program. Specific
ranges for these parametersare presented
in Table 1. The applicable range for each
parameter represents the range over
which the parameters were varied during
the test program.
Another significant result of the test
program was determination of the
absorber pressure drop as a function of
various absorber operating parameters.
Of the parameters which were examined
during the test program, only packing
depth and gas velocity were found to have
a significant impact on the pressure drop
in the absorber (Figure 3). Based on theo-
retical considerations and on measure-
ments made on the pilot absorber, pressure
drop was found to be a linear function of
packing depth. That is, a doubling in the
packing depth results in a doubling of the
pressure drop in the absorber.
The relationship between gas velocities
and pressure drop is somewhat more
complex. Pressure drop appears to
increase exponentially with increasing
gas velocity. This is an important consid-
eration in designing an absorber since
the gas velocity would be selected based
on a trade-off between absorber cross
section and pressure drop. Relatively low
gas velocities result in a larger absorber
with a lower pressure drop. The trade-off
which must be evaluated is an economic
one; the capital costs for a larger absorber
versus the operating costs of overcoming
a high pressure drop.
The major conclusions of the pilot-
scale tests are:
1 )The MgO scrubbing system which was
tested at Molycorp's Washington,
Pennsylvania plant was capable of
removing over 90 percent of the S02
from an off-gas stream containing up
to 9000 ppm SO2. The use of a packed
absorber in conjunction with the MgO
sorbent was the major factor which
contributed to the good system per-
formance. This is due to the high
(relative to a spray tower) liquid
residence time and overall area for
mass transfer which exist in a packed
absorber. The relatively long liquid
residence time in the absorber helped
promote dissolution of MgSOa solids
in the absorber which effectively
increased the liquid phase alkalinity of
the scrubbing slurry.
2) An equationwasdevelopedtocorrelate
the results of the pilot-scale tests. The
correlation of experimental data was
excellent and the equation can be
used to design an MgO absorber using
identical packing material treating a
gas stream which is similar in com-
position to the one at Molycorp. In
using the design equation, the level of
absorber operating parameters should
be in the range of parameters examined
during the pilot-scale tests.
3) For most operating conditions, plugging
of the absorber bed was not a problem.
However, at very high SO2 removal
efficiencies (95 percent) MgSOa solids
did begin to accumulate in the absor-
ber. In order to prevent such accumula-
tion, the S02 concentration at the
outlet of the packed bed should be
maintained above 500 ppm. If addi-
tional S02 removal is required, a clear
Table 1. Range of Absorber Operating Parameters Applicable to Design Equation
Operating parameter Applicable range
Gas Velocity (m/sec)
pH
Liquid Velocity (I/see-in )
Weight Percent Solids
SOz Concentration (ppm)
1.8 to 2.6
6.0 to 8.0
1.0 to 5 4
4 to 10
2000 to 9000
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72.0-
70.0-
t
-*e
I
8.0'
8
6.0 —
4.0'
2.0-
7.5
\
2.0
\
2.5
\
3.0
Figure 3.
Gas Velocity (m/sec)
Absorber pressure drop as a function of gas velocity
SOa removal efficiencies and lower
absorber pressure drops than those
reported by utility applications. The
principal reason for the superior
performance is the absorber design
used, a packed absorber in the pilot
tests versus a venturi type absorber in
the utility systems. In general, the
packed absorber appears well suited
to treating off-gas streams which
contain high concentrations of SO2
(up to 9000 ppm).
liquor spray installed above the
packing should permit absorber oper-
ation without plugging.
4) Process control of the pilot-scale MgO
system was not difficult. Measurement
of reaction tank pH provided an
indicator which could be used to
accurately control the Mg(OH)2 feed
rate to the system. Also, changes in
gas flow rate (over the range tested)
did not require any corresponding
changes in other absorber operating
parameters to maintain a constant
SOa removal efficiency.
5) Because the pilot unit was not
operated as a closed-loop system,
there were certain issues which were
not addressed during the test program.
Of these, it appears that buildup of
MgSO4 (aq) will have the largest
impact on test results. This is due to
the fact that as MgS04 (aq) concentra-
tion builds up, MgSOj (aq) solubility
decreases, thus decreasing available
liquid phase alkalinity in the scrubbing
slurry and possibly decreasing the SO2
removal efficiency. Theoretically, a
decrease in alkalinity can be offset by
a corresponding increase in liquid
flow rate to the absorber, but tests
should be conducted to confirm this
fact prior to using the design equation.
6) A comparison of the pilot plant test
results to those reported for utility
applications of the MgO system
indicates that the system tested at
Molycorp was generally superior. The
Molycorp pilot tests resulted in higher
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N. Masarky is with Molycorp, Inc., Washington, PA 15301; K. Schwitzgebel, C. D.
Wolbach, R. D. Delleney, T. P. Nelson. R. L Glover, and J. M. Burke are with
Radian Corp., Austin, TX 78759; the EPA Project Officer J. O. Burckle (also the
author of this Project Summary) is with the Industrial Environmental Research
Laboratory, Cincinnati, OH 45268.
The complete report consists of three volumes, entitled "Control of Air Pollution
Emissions from Molybdenum Roasting,"(Set Order No. PB 83-264 184; Cost:
$33.00, subject to change)
"Volume I. Emissions Characterization and Paniculate Control," (Order No.
PB 83-264 192; Cost: $11.50, subject to change)
"Volume II. Alternatives for Control of Weak Sulfur Dioxide Emissions,"
(Order No. PB 83-264 200; Cost: $13.00, subject to change)
" Volume III. Pilot Scale Test Results for Magnesium Oxide Scrubbing," (Order
No. PB 83-264 218; Cost: $14.50, subject to change)
National Technical Information Service
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Springfield. VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Industrial Environmental Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
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