British Columbia Mine Reclamation Symposium

Organic cover materials for tailings : do they meet the requirements of an effective long term cover? Elliott, Linda C. M.; Liu, Liangxue; Stogran, S. Wade 1996

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Proceedings of the 20th Annual British Columbia Mine Reclamation Symposium  in Kamloops, BC, 1996. The Technical and Research Committee on Reclamation ORGANIC COVER MATERIALS FOR TAILINGS: DO THEY MEET THE REQUIREMENTS OF AN EFFECTIVE LONG TERM COVER? Linda C.M. Elliott, Liangxue Liu, Ph.D. and S. Wade Stogran Lakefield Research Limited Postal Bag 4300 185 Concession Street Lakefield, Ontario KOL 2HO A literature review conducted in 1992 revealed that an organic layer on sulphide tailings could be beneficial in the suppression of tailings oxidation and acidic mine drainage, in the following five ways: 1) creation of a physical oxygen barrier, 2) maintenance of an oxygen-consuming barrier, 3) chemical inhibition, 4) chemical amelioration, and 5) reduction of water infiltration due to compaction through decomposition. Organic cover materials were also believed to be potentially beneficial in the provision of a top cover for the introduction and maintenance of a vegetative cover. The natural cycling of vegetative growth and decay would assist in the replenishment of the organic carbon content of the cover layer. Three different organic materials (peat, lime stabilized sewage sludge (LSSS) and municipal solid waste compost) were evaluated in a combination of bench and pilot scale laboratory test programs. A fourth non- organic material, desulphurized tailings, was also tested to provide comparative data. The organic cover materials tested demonstrate that there are significant differences in the ability of each material to provide a beneficial tailings cover. The results to date from the one year pilot scale test cells show that, of all the materials tested, the LSSS performed best at meeting the objectives of a good tailings cover. The data to date show no evidence of the dissolution of metals caused by the migration of organic acids into the underlying tailings, or the effect of reductive dissolution. The following paper summarizes the results of bench scale tests and a one year pilot scale test program designed to evaluate the effectiveness of organic cover materials at reducing acid generation. INTRODUCTION Large quantities of organic material are now stockpiled, or may be available in the near future, from urban and industrial sources. The cities in Ontario, alone, are capable of producing approximately 680,000 tonnes of municipal solid waste (MSW) compost annually and create comparable amounts of sewage sludge, which is currently landfilled (Pierce, 1992). Peat from bogs in the Canadian Shield region, although not a waste material, represents a vast renewable source of organic matter. Peat bogs are often found near base metal and precious metal mines. 196 Proceedings of the 20th Annual British Columbia Mine Reclamation Symposium  in Kamloops, BC, 1996. The Technical and Research Committee on Reclamation These materials may provide effective and affordable solutions to the reclamation of acidic mine tailings. A literature review of the physical and chemical characteristics of MSW compost and other organic materials (Pierce, 1992) revealed that an organic layer on sulfide tailings could be beneficial in the suppression of tailings oxidation and acidic mine drainage, in the following five ways: 1. Physical oxygen barrier - The organic cover layer may be saturated with water over at least part of its depth.   This saturation would provide the limiting factor for the rate of oxygen diffusion into the tailings and this rate would be approximately the same as the low diffusivity of oxygen in water. 2. Oxygen-consuming barrier - The continued decomposition of organic material may create a large biological oxygen demand which would act as a sink for atmospheric oxygen and dissolved oxygen in infiltrating water. 3. Chemical inhibition - Compounds and decomposition products in the organic material that leach into the tailings may inhibit the growth and metabolism of sulphate-producing (acidifying) bacteria. 4. Chemical amelioration - Organic compounds in the organic material may cause the reductive dissolution of iron oxides (either directly or indirectly by providing metabolic substrates for bacteria), the reduction of sulphate, and the prevention of indirect ferrous sulphide oxidation and acid generation. 5. Reduced water infiltration - The decomposition and resultant compaction of an organic cover layer may result in the decrease of the hydraulic conductivity of the cover. This would result in a subsequent decrease in infiltration, thus decreasing tailings ground water flow. Sulfate and iron reduction rates may be limited in a tailings system by a lack of organic substrates for bacterial metabolism. Consequently, an organic cover layer on the tailings may provide an important source of carbon compounds for bacteria as decomposition and leaching proceeds. The biological oxygen demand of an aerobic, actively-decomposing organic layer also constitutes a strong sink for atmospheric oxygen, preventing it from moving down the oxygen concentration gradient toward the AMD oxidation processes in the tailings. Oxidation of an organic cover layer material will eventually decline as the remaining material becomes more humified and more resistant to further decomposition (Pierce, 1992). Therefore, the biological oxygen demand of the organic cover layer will eventually reach a lower level based on a lower input rate of natural carbon compounds and other nutrients into the cover layer. The resistance that the cover layer will still be able to offer to the downward diffusion of atmospheric oxygen will then be determined by its physical properties, especially its depth and gas-filled porosity. Gas-filled porosity, in turn, will be mainly 197 Proceedings of the 20th Annual British Columbia Mine Reclamation Symposium  in Kamloops, BC, 1996. The Technical and Research Committee on Reclamation determined by the structural composition of the material and the degree that the pore spaces are filled with water (i.e. water content). STUDY PURPOSE The purpose of this program was to investigate the effectiveness of various organic materials on eliminating or reducing acid generation and the movement of acidity and heavy metals from tailings ponds. The program involved a study of four different cover materials. The materials studied included: peat, municipal solid waste compost, lime-stabilized sewage sludge (LSSS), and desulphurized tailings (DST). The organic materials investigated were selected for their physical and chemical characteristics that would decrease the rates of acid generation processes in sulfide tailings or reverse the acid generation processes. These would result in the precipitation of sulfides. It was believed that these materials could also decrease the movement of water through the tailings pond (Pierce 1992, Pierce et al. 1994). The organic materials studied were also potentially available in large quantities at low cost. Desulphurized tailings was used as a contrasting, non-organic cover layer that may prove to be cost-effective. A control cell was used to evaluate the differences between the tailings with and without a cover. Background information from previous studies illustrated that it was important to evaluate the process of organic leachate interaction with tailings. This interaction was assessed by examining the processes in situ during the pilot experiments. The research project had the following objectives: 1) To experimentally compare the effectiveness of organic and inorganic cover layers in reducing acid generation and the mobilization of trace metals from partially oxidized acid generating tailings. 2) To evaluate the extent and rates of upward salt migration in a range of cover materials under worst case conditions, and assess the need and effectiveness of a capillary barrier. 3) To monitor the organic and inorganic chemistry of three organic and one inorganic tailings cover systems in an effort to understand and quantify the aerobic and anaerobic organic degradation rates and processes and their interaction with acid generating tailings. 4) To evaluate the physical characteristics of three organic and one inorganic cover in an effort to determine their ability to remain saturated enough to minimize oxygen transport. 198 Proceedings of the 20th Annual British Columbia Mine Reclamation Symposium  in Kamloops, BC, 1996. The Technical and Research Committee on Reclamation PROGRAM DESIGN The covers evaluation test program included three main components of study: 1. Characterization of Tailings and Cover Materials 2. Salt Migration Column Bench Scale Test 3. Pilot Scale Cover Test Program The majority of this paper will focus on the results of the Pilot Scale Cover Test Program. A brief review of the background preparation and a listing of the characterization work and parallel bench scale tests conducted during the covers evaluation program follow. Receipt and Preparation of the Tailings and Cover Materials Thirty drums of oxidized tailings from a nickel sulphide mine in northern Ontario were received at Lakefield in the fall of 1994. To break up and homogenize the tailings, fully autogeneous grinding (FAG) was conducted using a Nordberg FAG mill. The tailings were pumped into the pilot cells and allowed to settle. Settling was aided through the use of a buried vacuum system at the base of the pilot cells. Sixty three drums of fresh mill cyclone overflow was received at Lakefield Research Limited for desulphurization by flotation, prior to placement as a cover. Approximately 2.5 tonnes of Lime Stabilized Sewage Sludge (LSSS) was received and placed outside in a sheltered location adjacent to the cover cell laboratory and covered over with black polyethylene until ready for placement. Municipal solid waste mature compost was received at Lakefield Research Limited, from the Corcan Central Composting Facility, at Millhaven, Ontario. The peat was obtained from a local source near Bridgenorth, Ontario. Physical and Chemical Characterization and Bench Scale Tests Conducted Due to the large amounts of data collected, only a list of those tests conducted initially to provide an understanding of the individual material characteristics and behaviours is given below. The tests included: • multi-element scans by Inductively Couple Plasma      •    whole    rock    analysis     by    X-Ray - Emission Spectroscopy Fluorescence • particle   size   distribution   using   sieve   series,          •    in-situ    hydraulic    conductivity    tests cyclosizer analyses and hygrométrie analyses, (infiltration test) 199 Proceedings of the 20th Annual British Columbia Mine Reclamation Symposium  in Kamloops, BC, 1996. The Technical and Research Committee on Reclamation • bulk density determinations • moisture content determinations, • specific gravity determinations • porosity calculations • mineralogical examination of polished sections • drainage curve tests. • humidity, temperature and potential evaporation • oxygen consumption and decomposition of the lab area rates • evaporative flux tests • salt migration column tests PILOT SCALE COVER TESTS - PROGRAM DESIGN Pilot scale models were designed to simulate a section taken out of a tailings pond. The section is shown in Figure 1, and contains several interactive components of the tailings cover system. Atmospheric/weather effects, tailings cover interaction and water flows through the system are some of the system components which are replicated in the models. The pilot model is, in effect, a microsystem of a tailings pond area. Due to the ability to control some of the external factors (such as rainfall), to measure and correlate others to the field (such as evaporation and temperature) and to monitor the water balance of the system, the complex field system is somewhat simplified allowing for more direct interpretations of cause-effect relationships. Pilot Cell Design The pilot scale models (pilot cells) were constructed of 1.25 cm thick sheets of PVC plastic welded together and supported within a frame of angle iron. The pilot cells are 2.5 m long, 1.5 m high and 0.6 m wide (Figure 2). One sheet of clear Plexiglas™ forms one long side of the cells, (2.5 m by 1.5 m) to permit visual observations of the layered systems. The back, sides and bottom are constructed of opaque PVC. One end of each pilot cell was covered with a filter fabric to allow the cover and tailings layers to drain freely. This was welded into place using PVC strips and a PVC welder. Seven sets of sampling ports and monitoring sensor ports were installed verticailly in the Plexiglas™ side of the cells, to allow for the collection of profiles of pore water and sensor data from the cover and tailings. Two sets of sampling and monitoring ports were installed in the tailings layer and five sets of ports were installed in the cover layer(s). The ports were installed at 0.15 intervals up the Plexiglas™. The ports were installed 1.75 m from the cell end covered with the filter fabric to reduce boundary effects on the data collected. 200 Proceedings of the 20th Annual British Columbia Mine Reclamation Symposium  in Kamloops, BC, 1996. The Technical and Research Committee on Reclamation   Figure 1 :   Pilot Scale Model of a Tailings-Cover System Figure 2: Pilot Cell Configuration A dual probe TDR Moisture Probe system was installed in the cells along with temperature probes, Eh probes, and solution samplers. PVC collection trays were installed at the drainage end of the cells to collect runoff water from the surface, at the cover-tailings interface and from the base of the tailings layer. Monitoring Program Sampling and monitoring of the pilot cells was conducted immediately after loading, one week after cell saturation, and then 2 weeks, 1 month, 3 months, 9 monte and 12 months after cell saturation. Samples were analyzed for: pH, sulphate, total sulphur, total iron, ferric iron, ferrous iron, total phosphorous, nitrates, nickel, copper, and lead. Additional analyses were conducted for phenols, PAH's and pathogens. Full 24 element metal scans were conducted on the pore water every 6 months and similar scans were conducted on the tailings discharge, or leachate, collected from the base of the cells. Simulated rainwater was added to the cells at a pH of 4.2. The pH was adjusted using a 60/40 mixture of sulphunc (H2SO4): nitric (HNO3) acid. This rain was used for the initial saturation and rainfall application on the cells Rainfall was applied once weekly at a rate calculated to approximate the average annual precipitation rate at Sudbury, Ontario. The laboratory par, evaporation rate and the field pan evaporatic rate for the Sudbury Airport were used to calculate a ratio of lab:field conditions and this was used to generate the weekly rainfall rate. 201  Proceedings of the 20th Annual British Columbia Mine Reclamation Symposium  in Kamloops, BC, 1996. The Technical and Research Committee on Reclamation In-situ monitoring was conducted for Eh and temperature one week after cell saturation, and then 2 weeks, 1 month, 3 months, 6 months, 9 months and 12 months after cell saturation. The moisture content using TDR was monitored before and after the weekly rainfall events. This method was unable to accurately measure moisture contents greater than 40% by volume in the tailings or cover materials being tested. Fortunately, the design of the cells permitted destructive testing without compromising the integrity of the test. Destructive coring and moisture content determinations were, therefore, conducted at several stages during the program. To provide information useful to water balance determinaitions, the rainfall application, pan evaporation, free drainage from the cells and the vacuum drain water volumes were monitored. Laboratory relative humidity and temperature were also recorded to supplement the laboratory climatic information. In-Situ pore gas sampling was added to the program after 6 months of operation. The gas samples were extracted using a syringe, through a stainless steel tube fitted with a septum. Oxygen (O2) concentration analysis was performed using an oxygen concentration apparatus designed by Ron Nicholson of the University of Waterloo. RESULTS OF THE PILOT SCALE COVER TESTS The oxidized tailings received at Lakefield exhibited the typical trends of ARD, i.e. a low pH and high dissolved metals content. The following results summarize the physical and chemical differences exhibited by the tailings under the different cover materials. The data plotted in Figures 3 to 10 are the results of tests conducted on the pore water extracted from approximately 0.25 m below the tailings-cover interface. Moisture Content and Degree of Saturation Moisture content data from the destructive coring conducted on the cells is plotted in Figure 3. The equivalent degree of saturation for each material is plotted in Figure 4. The data plotted in both Figures 3 and 4 are from a single sampling event conducted near the end of the one year monitoring program. The results from previous samplings were very similar in value to those plotted. Figure 3 shows that fairly large fluctuations in moisture contents occurred with depth in the peat, while fairly consistent moisture contents over depth were noted in the control, desulphurized tailings (DST), the LSSS and the compost. 202 Proceedings of the 20th Annual British Columbia Mine Reclamation Symposium  in Kamloops, BC, 1996. The Technical and Research Committee on Reclamation The degree of saturation plotted in Figure 4, however, shows that only the LSSS and the DST were able to maintain >90% saturation throughout their depth.   Figure 3:     Volumetric Moisture Content versus Depth in the Covers and Tailings Figure 4:     Degree of Saturation versus Depth in the Covers and Tailings Pore Water pH The migration of metals as relatively stable, soluble, organo-metal complexes will largely be controlled by the solution pH. Under conditions where the pH of the solution is imposed by factors other than the presence of the organic acids, metal dissolution through complexation will be greater at pH above 5 than at pH below 4. However, metal dissolution due to low pH will generally increase progressively with decreasing pH (the classic ARD mechanism). Consequently, the combined effects of ARD and organic acids on metal dissolution and migration are likely to be veiy complex. Figure 5 illustrates the changes which occurred over the one year test program to the pH in the oxidized tailings pore water under the various cover materials. The results show that the pH in the tailings pore water under all but one of the covers remained essentially constant around 3.5 to 4. The pH in tailings beneath the lime stabilized sewage sludge (LSSS) showed a marked increase from 3.5 to 6.1 after a period of twelve months. 203 Proceedings of the 20th Annual British Columbia Mine Reclamation Symposium  in Kamloops, BC, 1996. The Technical and Research Committee on Reclamation Pore Water Sulphate Concentrations In typical ARD, sulphide minerals oxidize and release both soluble metals and sulphate to the receiving environment. Sulphate concentrations in the oxidized tailings under the various cover materials are plotted in Figure 6. The data show that the sulphate concentrations under the LSSS cover decreased both quickly and substantially from initial concentration levels. The compost cover appears to have had no effect on sulphate production, while the desulphurized tailings cover resulted in the generation of much higher sulphate production than the control cell.  Dissolved Organic Carbon A potential problem associated with the use of organic materials as covers is the effect of decomposition products, particularly organic acids, on the solubility of tailings minerals and on the mobility of dissolved metals (Pierce, 1992). In a short term experiment using MSW compost as a cover layer on oxidized tailings, an increase in the concentrations of trace metals in pore water was observed in some bench scale models tested. This trace metal increase observed may have been due to dissolution or chelation by organics in methods similar to the acid generation process in sulfides; or, the dissolution of previously oxidized and precipitated metals as they are reduced to a sulfide form through soluble phases during the reduction process (reductive dissolution). An example of reductive dissolution would be the transformation of ferric iron through ferrous iron to iron sulfide. To evaluate the effect of the interaction of organic 204 Proceedings of the 20th Annual British Columbia Mine Reclamation Symposium  in Kamloops, BC, 1996. The Technical and Research Committee on Reclamation leachates with the oxidized tailings, the dissolved organic carbon (DOC) content of the pore waters was measured. The change in DOC over time is plotted in Figure 7. It was noted that the DOC content in the underlying tailings remained effectively unchanged under both the compost and peat, while a steady increase in DOC concentrations was noted under the LSSS cover. This indicates that organic rich leachates are migrating downwards from the LSSS into the underlying tailings. Dissolved Iron Concentrations Although analyses for several metals were conducted during the program, in general the concentration versus time trends were similar. Therefore, only the dissolved iron concentrations are plotted in Figure 8. By comparing the data shown in Figure 8 with the DOC concentrations plotted in Figure 7 it is apparent that metals concentrations did not increase with the increase in organic leachate interaction with oxidized tailings under the LSSS. In fact the results show that the increase in DOC is mirrored by an approximately equally large decrease in dissolved iron concentrations in the underlying tailings pore water. Results for the remaining cells are similar to those shown by the sulphate concentrations, with substantial increases occurring under the DST cover, a minor change under the compost, and a slight decrease, after an initial increase in concentrations noted under the peat cover.   Figure 7: Pore Water DOC in the Oxidized Tailings     Figure 8: Dissolved Fe in the Oxidized Tailings Under Various Cover Materials Under Various Cover Materials 205 Proceedings of the 20th Annual British Columbia Mine Reclamation Symposium  in Kamloops, BC, 1996. The Technical and Research Committee on Reclamation Visible evidence of this interaction between the leachates from the LSSS and the underlying oxidized tailings was noted in the pilot cell. After a period of approximately 6 months a thin black band started to form at the interface. This band grew to approximately 0.6 metres thickness by the end of one year. The band has been interpreted to be evidence of the reduction of sulphides and precipitation of metals. Oxygen Gas Concentrations Direct measurements of oxygen gas concentrations versus depth in the cover tailings system are plotted in Figure 9. The plot shows the marked decrease in O2 in the control cell, which was to be expected by the consumption of oxygen in the oxidation of sulphides. Similarly strong decreases in O2 concentrations were evident in the compost and LSSS cover materials. The desulphurized tailings and peat contained relatively high oxygen concentrations throughout their entire depth. Cell Loading Calculations Measurements of the volumes of runoff, interface discharge and basal leachate and their respective water quality analyses were used to calculate the whole cell loading for sulphate, nickel and iron. These are shown relative to the control which is represented by 100% loading in Figure 10. Figure 10 indicates that the DST and LSSS covers resulted in a lower release of metals and salts to the environment than the other covers. O2 Concentration (%) 5 10 15 20 25  Figure 9:   Oxygen Gas Concentrations with Depth       Figure 10:   SO4, Fe and Ni Loading Relative to the Control Cell 206 Proceedings of the 20th Annual British Columbia Mine Reclamation Symposium  in Kamloops, BC, 1996. The Technical and Research Committee on Reclamation DISCUSSION Fairly large fluctuations in moisture content occurred with depth in the peat and control cells. The compost, although able to maintain a constant moisture content, only maintained a 60-70% degree of saturation. High degrees of saturation (>90%) and consistent moisture contents over depth were noted in both the DST and the LSSS. This ability to maintain consistent moisture content levels during cyclic rain and dry events and maintain >90% saturation levels will provide a benefit to the underlying tailings by limiting the rate of oxygen infiltration to approximately the rate of diffusion through water. This reduction in O2 infiltration will serve to decrease the rate of tailings oxidation and increase the life of the cover. The pH in tailings beneath the lime stabilized sewage sludge (LSSS) showed a marked increase (from 3.5 to 6.1) after a period of twelve months. This may be largely due to the leaching of alkalinity from the LSSS downwards into the underlying tailings as the LSSS had an initial pH of 12. The positive effects of the interaction of the leachates from the LSSS with the underlying tailings is also evident in the decreases in the sulphate and iron concentrations noted. In addition, visible evidence of sulphide reduction at the interface has been noted. The increasing immobilization of metals by complexation to organic compounds and the formation of metal sulfide precipitates are mediated primarily by the soluble humic acids and by the sulfate-reducing bacteria (SRB) that are found in anoxic environments. These anaerobic microbes require organic compounds, particularly simple organic acids, for their metabolism and thereby consume acidity. This consumption of organic acids results in an increase in pH which would further enhance the SRB activity. SRB and other bacteria also cause an increase in pH through an acid consuming process, which result in the formation of methane or hydrogen gas. Increasing pH and certain organic compounds also suppress the acid-generating process by inhibiting the growth and activity of the autotrophic iron bacteria Thiobacillus ferrooxidans that thrive at the low pH range of 1.5 to 3.5 (Pierce 1992). Through this interactive system a reversal of the ARD process is seen to be occurring beneath the LSSS cover. The observed decreases in oxygen concentrations over depth in the LSSS and compost materials are believed to be due to the combination of the high moisture contents in these covers and the active consumption of oxygen in the ongoing biological decomposition of the organic cover. The minor decrease 207 Proceedings of the 20th Annual British Columbia Mine Reclamation Symposium  in Kamloops, BC, 1996. The Technical and Research Committee on Reclamation in O2 concentration seen in the peat, which has a lower degree of saturation and is an older largely decomposed (aged) material, supports this conclusion. The DST, however, which has >90% saturation and contains sufficient sulphides to actively consume incoming oxygen, is not acting as effectively as might have been assumed. The desulphurized tailings cover allowed the generation of much higher sulphate and dissolved iron production in the underlying tailings than the control cell. This may be due to the continuous formation of cracks at the surface which extend to considerable depths in the DST cover. These cracks create direct pathways for oxygen migration into the cover and underlying tailings from numerous directions. Further work is on-going. Destructive sampling of the interfacial area and at several depths through the covers and tailings has been conducted and mineralogical examinations, hydraulic conductivity tests and chemical analyses are being conducted to identify and quantify the changes which have occurred both to the covers and the underlying tailings. Tests are also planned to examine methods of reducing cracking of the DST tailings, and the effects of mixing the LSSS with DST. Incubation tests are being conducted to evaluate the active oxygen consuming life of the LSSS. CONCLUSIONS Of the covers tested, the DST and LSSS appear to offer the greatest potential for reducing metal loading in water migrating from oxidized tailings to the environment. The reasons for this effect are different for each cover. The LSSS is actively changing the underlying tailings environment by reversing the ARD processes with an increase in pH, decrease in dissolved metals concentrations and formation of a reducing environmental at the tailings - cover interface. The DST has a low hydraulic conductivity that results in the slow release of high concentration pore waters to the environment, however, the oxidation process is continuing in the underlying tailings. The effects of reducing cracking and of mixtures of the two materials is under investigation. 208 Proceedings of the 20th Annual British Columbia Mine Reclamation Symposium  in Kamloops, BC, 1996. The Technical and Research Committee on Reclamation ACKNOWLEDGMENTS The authors would like to thank Mike Sudbury, Mark Wiseman, Joe Fyfe, and Glen Hall of Falconbridge Limited for their support of this program. We would also like to thank the technical experts from outside of Lakefield Research who provided input at various stages in the program. The experts consulted included: Luc St. Arnaud and Keith Shikitani of Noranda Technology Centre, Glen Pierce, S. Lee Barbour of the University of Saskatchewan, Ron Nicholson of the University of Waterloo, Gene Shelp, Ward Chesworth, and Graeme Spiers of the University of Guelph, Bill Hook of Gabel Corporation, Randy Knapp of Senes Consultants, Terry Logan of Ohio State University. REFERENCES Pierce, W.G. 1992. Reclamation of Sulphide Tailings Using Municipal Solid Waste Compost: Literature Review and Recommendations. Report prepared for Falconbridge Ltd. by the Centre in Mining and Mineral Exploration Research, Laurentian University, Sudbury Pierce, W.G., Belzile, N., Wiseman, M.E. and Winterhalder, K. 1994. Composted Organic Wastes as Anaerobic Reducing Covers for Long Term Abandonment of Acid-Generating Tailings. Proceedings of the International Land Reclamation and Mine Drainage Conference and Third International Conference on the Abatement of Acidic Drainage, United States Department of the Interior, Bureau of Mines Special Publication SP 06B-94. Volume 2 of 4: Mine Drainage, pp. 148-157. 209


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