British Columbia Mine Reclamation Symposium

Selenium release and removal from the Red Dog Mine operation Brienne, Stephane H. R.; Falutsu, Maria; Weakley, J. O.; Kulas, Jim E.; Kuit, Walter J.; Geist, D. J.; Gustafson, Jennifer A.; Wood, Scott; Baker, Leslie L.; Rosenzweig, R. Frank; Ramamoorthy, Srividhya; Crawford, Don L.; Prisbrey, Keith A.; Moller, Gregory; George, John B. 2000-06-23

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Proceedings of the 24th Annual British Columbia Mine Reclamation Symposium  in Williams Lake, BC, 2000. The Technical and Research Committee on Reclamation SELENIUM RELEASE AND BLEMOVAL FROM THE RED DOG MINE OPERATION S. H. Brienne  Cominco Research, Trail B.C., Canada M. Falutsu, J. O. Weakley and J. E. Kulas Cominco Alaska Inc., Kotzebue, AK, USA W. J. Kuit  Cominco Ltd., Vancouver B.C., Canada D. J. Geist, J. A. Gustafson, S. A. Wood, L. L, Baker, R. F. Rosenzweig, S. Ramamoorthy, D. L. Crawford, K. A. Prisbrey, G. Moller and J. George University of Idaho, Idaho, USA ABSTRACT Recent changes in the Red Dog water discharge permit reduced the allowable selenium discharge concentration to 6.0 ug/L. When these changes were introduced, the selenium concentration in the treated effluent was 9 ug/L. Over the last two years, this concentration has decreased to 4.5 ug/L. The release and subsequent selenium reactions in the mill circuit were investigated using mill and concentrator surveys. Designed flotation experiments were also carried out to determine the factors responsible for selenium release. The fate of selenium in the tailings impoundment was investigated using thermodynamic tailings pond simulations. Laboratory tests were conducted to determine the long term release rates. A water and sediment sampling campaign from different areas in the tailings impoundment was conducted. INTRODUCTION The Red Dog Mine, situated in Northwest Alaska (Figure 1) is the world's largest producer of zinc concentrates. After the recent production rate increase (1999), the production of zinc concentrate will be in excess of 1.1 Mt/y. The mine is operated by Cominco Alaska Incorporated and the deposit is owned by the NANA Regional Corporation. The impact of Red Dog Mine on the surrounding aquatic systems is minimized by collecting all impacted site water (tailings water from the concentrator, site run off and mine drainage) and storing it in a tailings impoundment. The tailings impoundment is located at the South Fork of the Red Dog Creek. The water reclaimed from the tailings impoundment is treated in one of two water treatment plants (WTP) where metals/metalloids are removed by precipitation, clarification and sand filtration. The treated water is either recycled for use in the concentrator or discharged during the spring and summer months to the Middle Fork Red Dog Creek. This creek flows into Ikalukrok Creek, which in turn flows into the Wulik River and then into the Chukchi Sea. - 194- Proceedings of the 24th Annual British Columbia Mine Reclamation Symposium  in Williams Lake, BC, 2000. The Technical and Research Committee on Reclamation  Figure 1. Location of the Red Dog Mine in Alaska. Metals concentrations in the receiving waters have become more stable without the seasonal extreme spikes observed in the pre-mining period. This has resulted in an improved environment for aquatic life. Studies by the Alaska Department of Fish and Game indicate an increase in the utilization of Red Dog Creek by Chum Salmon and Arctic Greyling among other species [Weber-Scannell, 1999]. The discharged water is subject to various federal and state permit regulations. The discharge permit, renewed and effective in August 1998, required a significant reduction in the concentration limits for many metals and other analytes (Table 1). Modifications to the effluent water treatment plant, WTP#2, allowed the Red Dog operation to meet the new limits for all elements except for selenium whose permitted value is now 6.0 ug/L (Interim Minimum Level). This limit for selenium is based on the LLS. EPA freshwater chronic criterion of 5 ug/L, established in 1987. Selenium is an essential nutrient but it is toxic/teratogenic at high concentrations [Frankenberger and Engberg, 1998; Bodek et al., 1988]. A collaborative project was set up between Cominco Ltd. and the University of Idaho, with US EPA approval, to study selenium release and the fate of selenium at the Red Dog operation. Parts of this review have already been given [Edwards, 1998]. This report reviews the current findings of the study with an emphasis on the selenium release in the concentrator circuit, distribution and final fate of selenium in the tailings impoundment. - 195- Proceedings of the 24th Annual British Columbia Mine Reclamation Symposium  in Williams Lake, BC, 2000. The Technical and Research Committee on Reclamation DISCHARGE CONCENTRATIONS Over the last few years, the selenium concentrations in the treated discharge water have decreased. This is shown in Table 1 along with NPDES-imposed limits for different cations. The selenium concentration in treated discharge water was 4.5 ug/L at the end of the 1999 discharge season. The average selenium concentration in the treated effluent was 3.0 ug/L (August, 1999) compared to 8.7 ug/L (August, 1998) and 12.0 ug/L (August, 1997).  The total treated effluent discharged in 1999 was 5.8 x 109 L (1.53 x 109 gal) with an average volumetric flow of 28,000 L/min (7,300 gpm). The average selenium concentration over the 1999 discharge season was 3.6 ug/L. The average daily selenium discharged to the Red Dog river was 0.15 kg/d in 1999 and 0.5 kg/d in 1998 [Edwards, 1998]. The lower selenium discharge in 1999 is a result of a decrease in the selenium concentrations in the treated effluent. The selenium concentrations in the effluent comply with the discharge limits, however the underlying reasons behind the observed decreases in selenium concentrations are not understood. Decreases in the selenium concentrations in the ore, changes in the mill chemistry or changes in the chemistry of the tailings impoundment may be responsible for the observed decrease in selenium concentrations of the treated discharged effluent. The mill chemistry and tailings impoundment will be investigated in this report. SELENIUM INPUTS The only significant source of selenium in the Red Dog water circuit is the ore [Edwards, 1998], The selenium is widely distributed in the Red Dog region. No direct relationship between a specific ore type and high selenium concentrations exists; however, the selenium concentration increases with the organic content of the ore [Millholland, 1997]. Selenium is concentrated in the organic component in Texas lignite [Clark et al, 1980] and shales [Yudovich, 1984]. The median selenium concentrations in the Red Dog ore and regional rocks are 5.0 mg/kg and 9.9 mg/kg, respectively [Millholland, 1997]. The selenium input to the Red Dog mill has been estimated at 50 kg/d, though only a fraction of the contained selenium is solubalized in the flotation circuit [Edwards, 1998]. The recent observed decrease in the selenium concentrations in the treated discharge water may be due to a decrease in the selenium concentrations in the ore. The selenium concentrations present in previously treated Red Dog ores are currently being investigated to address this question. - 196- Proceedings of the 24th Annual British Columbia Mine Reclamation Symposium  in Williams Lake, BC, 2000. The Technical and Research Committee on Reclamation MILL CHEMISTRY The selenium variation in the Red Dog milling and flotation circuit (Figure 2) was surveyed in 1998. The same circuit was sampled in July 31, 1999 during a period of stable plant operation. The results of the most recent plant survey are given in Table 2.  - 197-Proceedings of the 24th Annual British Columbia Mine Reclamation Symposium  in Williams Lake, BC, 2000. The Technical and Research Committee on Reclamation Selenium release increased with addition of reagents in the milling and flotation circuit [Edwards, 1998]. In particular, there was an increase in the selenium concentration after cyanide addition [Moller, 2000]. The selenium was present in the slurry as predominantly selenocyante (SeCN-). The selenocyante is formed by cyanide attack on selenide in the sulphide minerals in an analogous reaction to cyanide attack on sulphide to form thiocyanate (SCN-) [Cotton and Wilkinson, 1980]. This concentration rapidly decreased in the lead rougher tails as the selenocyanate was oxidised. The solution survey shows speciation data for different points in the flotation circuit. The following comments can be made about the results. • Selenocyanate: Selenocyanate was present in all samples except the process water and flotation feed.  Selenocyanate also was present in the lead rougher tails, zinc scavenger tails and final tails.    The concentration of s;elenocyanate increased to 93 ug/L after cyanide addition.  This concentration rapidly decreased, suggesting that an effective mechanism for selenium removal exists in the flotation circuit. The selenocyante was likely oxidised. • Selenate:  Selenate was present in all samples at concentrations from 5-11 ug/L.  Selenate is the final oxidised form of selenium and is likely to be the only selenium species present in the tailings impoundment water. • Selenite:  Selenite was detected at low levels (1.1 ug/L to 1.4 ug/L) in the lead rougher tails and final tails.  This was not observed in previous surveys.  The presence of selenite in the process water suggests that complete oxidation did not occur during the flotation step. The concentrations of selenium in the tailings and reclaim water obtained during 1999 circuit surveys were 4-6 ug/L and 5 ug/L, respectively. In the 1998 survey, the total selenium concentrations in the tailings and reclaim water were 20 ug/L and 9.5 ug/L, respectively. These results may point to a decrease in the selenium liberated in the flotation circuit, an increase in the selenium removal efficiency or a decrease in the selenium concentrations in the ore. Flow through experiments, using continuous stirred tank reactors, were carried out on Red Dog ore under flotation conditions but failed to simulate concentrator chemistry. The aim of the experiments was to determine the ore selenium leaching rate with residence times close to that of the concentrator circuit. The selenium concentrations in the slurries were close to the detection limit (< 2 ug/L) rather than the 100 ug/L selenium observed in plant surveys and in batch flotation tests. These results suggest that high selenium concentrations may be achieved instantaneously in the plant because of reagent addition, and that an efficient mechanism for selenium removal may also exist in the concentrator circuit. TAILINGS CHEMISTRY The observed decrease in the selenium concentrations in the treated discharge may also be due to processes occuring in the tailings impoundment. For example, the water chemistry may change over time inducing selenium precipitation. Selenium removal via bioreaction [Nelson, 1996; Masscheleyn, 1993], photolysis [Sanuki, 1999; Moller, 1999] and adsorption [Overman, 1999; Kuan, 1998; Manceau, 1997] are possible mechanisms that have been suggested in the literature. - 198. - Proceedings of the 24th Annual British Columbia Mine Reclamation Symposium  in Williams Lake, BC, 2000. The Technical and Research Committee on Reclamation Changes in water chemistry in the system were tested by taking reclaim water and different ratios of tails in static tests. The tests simulate the processes occurring as tailings are deposited in the tailings impoundment. The tests were conducted for 7 months with samples taken in the initial time period and then every month after this. The selenium concentrations as well as other metal ions were monitored over the time period. The results are presented in Figure 3.  Figure 3. Selenium variation with time for static tests carried out with reclaim water (r) in the presence of tailings (t). The percentages are for the proportion of tailings to reclaim water. The following comments may be made about the process: • The control experiment shows that the selenium concentration of reclaim water in the absence of tailings remains approximately constant over time. • The selenium concentrations for reclaim water in the presence of tailings shows a marked decrease after 60 days. This suggests that other species, such as iron oxides are responsible for removing selenium. • Selenium was initially present in the reclaim water as selenate.  In the presence of tailings, some selenocyante was present in the water. After one week, no selenocyanate was detected in the water, suggesting conversion to the selenate occurred. • The selenium concentration was correlated with iron concentrations in the slurry water. The onset of selenium removal coincided with iron concentrations of >50 mg/L from an initial value of <0.1 mg/L. After 4 months, the iron concentrations had decreased to 6 mg/L and the pH had decreased from 6.3 to 3.6.  This result suggests that the selenium precipitates with iron from solution.  The adsorption of selenate on iron hydroxides is known [Zhang, 1990; Yamaguchi, 1999]. The Red Dog tailings impoundment water is supersaturated in both gypsum (CaSO4.2H2O) and hematite (Fe2O3) [Moller, 2000]. This result suggests that both gypsum and hematite may be removed from the impoundment water by precipitation. Selenium may also be removed from the impoundment water at the same time. Geochemical modelling results suggest 1 g of precipitated gypsum will remove 380 ng of selenium (as CaSeO4.2H2O). The modeling result suggests precipitating gypsum to reach the equilibrium concentration would decrease the selenium concentration in the tailings impoundment by 1 ug/L. - 199- Proceedings of the 24th Annual British Columbia Mine Reclamation Symposium  in Williams Lake, BC, 2000. The Technical and Research Committee on Reclamation Selenium may also be removed from tailings water by freezing. For example, selenium concentrations in the ice obtained after freezing Red Dog tailings impoundment water were >70% lower compared to the original solution. This may be another possible mechanism of reducing selenium concentrations in the tailings impoundment water, if the selenium could be converted into an insoluble form before the spring melt occurs. TAILINGS SEDIMENT SAMPLING Microbial action is another possible method of immobilising selenium in tailings sediments. The Red Dog tailings impoundement (Dam Stage VI) has a capacity of approximately 27 x 109 L (7.3 x 109 gal). The tailings have been deposited in varying locations over the period that the mine has been operating, to ensure even coverage. The effect of microbial action in immobilising selenium in the Red Dog tailings impoundment was studied by collecting core samples of tailings at different locations. The sampling took place in October 1999. The following locations were sampled: • Site A (mature tailings):   Sediments were fine-grained and poorly consolidated.   Sediments were black in colour and slimy in texture. Cores were homogeneous throughout with respect to appearance and texture. The entire gravity coring device consistently penetrated below the sediment-water interface. Samples obtained at this site were heterogeneous. • Site B (freshly deposited tailings). Sediments consisted of coarse recently deposited tailings. Sediments were dark grey in colour and granular texture with no well-defined stratigraphie boundaries evident. • Site C (uncontaminated area).   Sediments consisted of compact tundra soil and presented colours ranging from straw to dark brown. Poorly decomposed organic matter was abundant and consisted of plant root and stems. This; material was spongy in texture. • Site D (intermediate between C and B). Sediments revealed a 5-10 cm layer of fine grained tailings atop material identical to that described for site C.   The bottom <20 cm contained abundant plant matter. A number of core samples were obtained at each location, for example A3 and A4, to determine the variability in the sampling. The selenium concentrations in pore water as a function of depth for the different core samples are given in Figure 4.  Figure 4: Total dissolved selenium with depth from ICP analysis.- 200 -Proceedings of the 24th Annual British Columbia Mine Reclamation Symposium  in Williams Lake, BC, 2000. The Technical and Research Committee on Reclamation The following results were collated from the sampling campaign and subsequent analysis. • pH and Eh measurements.  Redox measurements indicate the A core samples (old tailings) are highly reducing. The C and D core samples are acidic (pH 5.0 to 6.5) compared to the A and B cores (pH 8.5 to 9.5).   Under reducing conditions, selenium may be present as selenuim(0) or as selenide. • Metal ion concentrations. The cut off between tundra and tailings deposition is very clearly seen in the variation in metals concentrations with depth in the cores. In particular, the iron depth profile clearly indicates where tailings deposition occurred over the underlying tundra. • Selenium concentrations. Very little variation in the selenium concentration in the pore water as a function of depth was evident (Figure 4). •  Bacterial activity. Most probable number (MPN) analyses indicate 102 to 105cells/g (wet weight of sediment) cultivable sulphate reducing bacteria (SRB) are present in the sediment. There are no obvious patterns in the SRB distribution by either depth or by site. This bacterial density is lower than for sediments found in other environments [LaForce, 1998; Harrington, 1998]. Regions of active sulphate reduction were found between 1 and 10 cm, though at levels lower than for other mining-impacted freshwater sediments. This suggests that sulphate reduction to form sulphide occurs in the Red Dog sediments. Recent results also suggest that selenium-reducing bacteria may be present. The presence of selenium reducing bacteria may be another mechanism of reducing selenium concentrations in the tailings impoundment. SUMMARY AND CONCLUSIONS A decrease in the selenium concentration has been observed in the treated tailings discharge water, tailings water and reclaim water over the last few years. The selenium concentrations in the discharge water are now with the compliance level of 6 ug/L. A detailed understanding of the reasons behind this underlying trend is required. The collaborative work carried out by the Cominco /US EPA and the University of Idaho support the following conclusions: • Selenium appears to be released after cyanide addition prior to the lead rougher flotation. The concentrations  of selenium (as   selenocyanate) are higher than 90 ug/L.     These concentrations rapidly decrease in the subsequent flotation processes. The final tails contain selenium as selenocyanate as well as selenate. Flotation tests also show similar results. •  A decrease in the selenium concentrations was observed for tailings simulation tests where reclaim water was added to tailings. The decrease in the selenium concentrations coincided with iron precipitation. • Geochemical  modeling  indicates  that  the  Red  Dog  tailings   impoundment   water  is supersaturated with gypsum and hematite.   Selenium may possibly precipitate as a calcium selenate dihydrate with gypsum. • Sulphate and selenium reducing bacteria are present in the Red Dog tailings impoundment. These bacteria may possibly immobilize selenium by conversion of selenate to selenide. Progress has been made in understanding the possible factors responsible for the observed decrease in the selenium concentrations in the treated tailings impoundment discharge water. If these causes can be identified, treatment strategies can be targeted. Selenium removal is difficult to accomplish on a commercial scale [Chamberlin 1996]. Implementing of different treatment technologies (iron co-precipitation, hydroxide precipitation, sulphide precipitation, adsorption, oxidation, reduction or through the use of proprietory reagents) is even more complex in the Red Dog case [Mudder and Botz, 1997]. -201 - Proceedings of the 24th Annual British Columbia Mine Reclamation Symposium  in Williams Lake, BC, 2000. The Technical and Research Committee on Reclamation Future work is targeted at getting a better understanding of selenium release in the flotation process, collate selenium inputs into the mine and to simulate the tailings impoundment. Lab-scale experiments to verify some of the proposed selenium release and immobilization mechanisms should also be carried out. REFERENCES Bodek, I., Lyman, W. J., Reehl, W. F., Rosenblatt, D. H., 1988. Environmental Inorganic Chemistry, Properties, Processes and Estimation Methods. SETAC Special Publication Series, Pergamon Press. Chamberlin, P. D., 1996. Selenium removal from Waste Waters - an update. Randol Gold Forum'96, 119-127. Clark, P. J., Zingaro, R. A., Irgolic, K. J., McGinley, A. N., 1980. Arsenic and Selenium in Texas Lignit. International Journal of Environmental and Analytical Chemistry, 7, 295-314. Cotton, F. A., and Wilkinson, G., 1980. Advanced Inorganic Chemistry. Fourth Edition, John Wiley and Sons, New York. Edwards, M. Kulas, J. E., Weakley, J. O., Kuit, W. J., Bloom, N. S., and Wallschlager, D., 1999. Aquatic Selenium at Cominco's Red Dog Mine: Sources, Speciation, Distribution and Control. Tailings and Mine Waste, 1999, 535-542. Frankenberger, W. T. Jr., Engberg, R. A., 1998. Environmental Chemistry of Selenium. New York: Marcel Dekker, Inc. Kuan, W. H., Lo, S. L, Wang M. K., Lin, C. F., 1998. Removal of Se(IV) and Se(VI) from Water by Aluminium-Oxide Coated Sand. Water Resources, 32(3), 915-923. Harrington, I. M., LaForce, M. I., Rember, W. C., Fendorf, S. E., Rosenzweig, R. F., 1998. Phase Associations and Mobilisation of Iron and Trace Elements in Coeur d'Alêne Lake, Idaho. Environmental Science and Technology, 32, 650-656. LaForce, M. J., Fendorf, S. E., Li, G. C., Schneider, G. M., Rosenzweig, R. F., 1998. Heavy Metals in the Environment. A Laboratory Evaluation of Trace Element Mobility from Flooding and Nutrient Loading of Coeur d'Alêne River Sediments. Journal of Environmental Quality, 27, 318-328. Masscheleyn, P., H., Patrick, W. H. Jr., 1993. Biogeochemical Processes Affecting Selenium Cycling in Wetlands. Environmental Toxicology and Chemistry, 12, 2235-2243. Manceau, A., Gallup, D. L., 1997. Removal of Selenocyanate in Water by Precipitation: Characterisation of Copper-Selenium Precipitates by X-Ray Diffraction, Infrared and X-Ray Absorption Spectroscopy. Environmental Science and Technology, 31(4), 968-976. Millholland, M., 1997. Study of Selenium in Red Dog Ores, Host Rocks and Regional Rocks of the Noatak District. Report to Cominco Alaska by Millholland and Associates, April. -202- Proceedings of the 24th Annual British Columbia Mine Reclamation Symposium  in Williams Lake, BC, 2000. The Technical and Research Committee on Reclamation Moller, G., 1999. Molecular Studies of Selenium Chemodynamics in Natural and Engineered Abiotic and Biotic Processes: Application to Red Dog Discharge Control. Progress Report November 15. Moller, G., 2000. Molecular Studies of Selenium Chemodynamics in Natural and Engineered Abiotic and Biotic Processes: Application to Red Dog Discharge Control. Progress Report March 22. Mudder, T., Botz, M., 1997. Evaluation of Selenium Control Technologies for the Red Dog Mine. Report to Cominco Alaska,, Nelson, D. C., Casey, W. H., Sison, J. D., Mack, E. E., Ahmad, A., Pollack, J. S., 1996. Selenium Uptake by Sulphur-Accumulating Bacteria. Geochimica et Cosmochimica Acta, 60(18)3531-3539. Overman, S. D.., 1999. Process for Removing Selenium from Refinery Process Water and Waste Water Streams. United States Patent 5,993,667, November 30. Sanuki, S., Kojima, T., Arai, K., Nagaoka, S., Majima, H., 1999. Photocatalytic Reduction of Selenate and Selenite Solutions using TiOa Powders. Metallurgical and Materials Transactions B., 3OB, 15-20. Weber-Scannell, P., Andersen, S., 1999. Aquatic Taxa Monitoring Study at the Red Dog Mine. Alaska Department of Fish and Game, Habitat and Restoration Division, February. Yamaguchi, N. U., Okazaki, M., Hashitani, T., 1999. Volume Changes due to Sulphate, Selenate and Dihydrogen phosphate Adsorption on Amorphous Iron(III) Hydroxide in an Aqueous Suspension. Journal of Colloid and Interfacial Science, 209, 386-391. Yudovich, Y. A., Ketris, M. P., 1984. Selenium in Pay Khoy Black Shales. Geokhimiya, 11,1767-1174. Zhang, P., Sparks, D. L., 1990. Kinetics of Selenate and Selenite Adsorption/Desorption at the Goethite/Water Interface. Environmental Science and Technology, 24, 1848-1856. - 203 - 

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