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- ------- 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- ------- 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 ------- 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 ------- 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 ------- 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 5285 Port Royal Road 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 BULK RATE U.S. POSTAGE PAID Cincinnati, Ohio Permit No. G35 Official Business Penalty for Private Use $300 U.S. GOVERNMENT PRINTING OFFICE: 1984-759-102/807 ------- |