British Columbia Mine Reclamation Symposia

The reclamation of heaps used for acid leaching Austin, J. 2009

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Proceedings of the 10th Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1986. The Technical and Research Committee on Reclamation  THE RECLAMATION OF HEAPS USED FOR ACID LEACHING J. Austin, B. Klein, B. Price, P. Kenny and G.Poling UNIVERSITY OF BRITISH COLUMBIA Department of Mining and Mineral Process Engineering Vancouver, B.C., Canada ABSTRACT Chemistry and physics of bacterially enhanced, acid leaching of waste dumps are reviewed from the prospective of subsequent reclamation practices.  Methods for treating acid mine waters and preventing their continuance are considered.  Since many questions and uncertainties still persist, research needs are emphasized. Proceedings of the 10th Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1986. The Technical and Research Committee on Reclamation  INTRODUCTION Several mines in British Columbia are examining the technical and economic potentials of acid-leaching their existing or yet-to-be-built waste dumps for additional copper recovery.  The Gibraltar Mines dump leach project has just recently become operational.  While substantial information has already been published on the establishment of bacterially enhanced leaching operations, information on how to terminate this leaching and reclaim these dumps is relatively sparse. This paper represents a class assignment by students in a course on "Treatment of Mineral Industry Effluents" in the Department of Mining and Mineral Process Engineering at the University of British Columbia. The object was to review relevant mine dump construction practices, dump chemistry and bio-chemistry with a view to establishing important links to subsequent reclamation procedures. While revegetation of leached dumps can be troublesome and costly the main concern is with the quality of ground waters or runoff waters from a previously leached dump.  Low pH and the oft-resulting elevated heavy metals concentrations can produce significant off-site impacts over the long term. In this paper, the chemistry and rate controlling reactions of acid mine water generation are first reviewed briefly, together with some operating constraints of dump construction and operation. Methods for predicting the changing balance of acid generating minerals with acid consuming minerals as the dump leaches are described.  Finally short and long term methods of reducing the rate of acid production are discussed. We hope that this review might lead to the collection and dissemination of existing operating data on dump leaching relevant to development of reasonable reclamation methods. Underlining the need for additional research to fill gaps in published information on acid-dump-leaching is a secondary objective. Proceedings of the 10th Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1986. The Technical and Research Committee on Reclamation  Chemistry and Physics of Acid-Dump Leaching Additional economic metal recovery can sometimes be achieved by leaching waste dumps containing sub-milling grades of metal values. Copper is the most common metal recovered by this technology (1). In- place dump leaching can be applied either to preexisting waste dumps not designed to be leached or to planned dumps placed on bases or layers constructed to be impermeable to the leach solutions. Acidic leach solutions are distributed over the surface of the dump, previously levelled and loosened (often by "ripping") to allow ready ingress of the acidic leach solutions.  Metal-laden solutions emanating from the toe of the dump are collected and treated by solvent extraction, ion exchange or cementation plants to recover the valuable metals.  Over a period of several months or even years, metal recovery might typically reach 60-70%. In general, for acid leaching to be economic, copper sulphide containing dumps must either be natural producers of acidic-mine-water or be very close to that condition.  To generate acidic mine water naturally, waste dumps must in general:  (1) contain acid-producing minerals (such as pyrite and pyrrhotite) in excess of acid-consuming minerals (such as carbonate rocks); (2) be sufficiently porous to allow oxygen infiltration; (3) contain an abundant and active population of microbes - i.e., "Thiobacillus ferroxidans" which use atmospheric carbon-dioxide as a carbon source for cell growth and, which oxidize ferrous iron and sulphides for growth energy.  The resulting high ferric ion concentration normally becomes the dominant and powerful oxidizing agent which together with the sulphuric acid also produced results in rapid leaching of the sulphide minerals. Figure 1 presents a highly simplified explanation of such a bacterially enhanced leaching reaction. While pyrite is shown to be the mineral attacked, copper sulphide minerals, although usually present in lesser amounts, are similarly solubilized. Proceedings of the 10th Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1986. The Technical and Research Committee on Reclamation   Figure 1 Simplified cyclic nature of bacterially enhanced acid mine water generation Although Thiobacillus ferroxidan bacteria are normally indigenous to sulphide dumps, to grow and multiply rapidly, they may require additional nutrients, such as nitrogen, phosphate, and the temperature and acidity must all be optimum.  High (> 50°C) and low (< 5°C) temperatures and pH's > ~ 4 will curtail their activity.  Certain chemicals called his biocides can also kill or render dormant these bacteria. The chemistry of the acid leaching reaction is relatively simple, and proceeds in stages.  Pyrite or pyrrhotite are almost always involved in the generation of acid mine waters.  As an approximation, one mole of pyrite can be oxidized by 3 1/2 moles of oxygen dissolved in water to produce one mole of dissolved ferrous sulphate plus one mole of sulphuric acid.  As early oxidation reactions progress and as the pH in the dump reduces to around 3 or 4 bacterial action picks up and produces rapidly escalating concentrations of ferric ion (Fe which then supercedes dissolved oxygen as the predominant oxidizing agent.  The rate of acid generation (and the rate of leaching of copper) then depends not so much on the chemistry of the reactions as the physics.  In other words, the rate of supply of oxidant (Fe the sulphide surfaces via diffusion mechanisms then controls the rate of leaching. Proceedings of the 10th Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1986. The Technical and Research Committee on Reclamation  Although chemically, one expects sulphide minerals such as chalcocite (Cu2S), covellite (CuS), bornite (Cu5FeS4) and pyrite to leach more readily than chalcopyrite this effect is seldom observed in practice.  Since the surface electrochemical reactions involved in the oxidation of all these sulphides are fast compared to the rate of supply of oxidant, all these minerals leach in practice at approximately the same rate once the initial "incubation period" is exceeded (2).  By "incubation period" we mean the time required for initial acidity to develop and for bacteria to fluorish. Figure 2 which is based on data taken from Table 1 attempts to illustrate how the rate of supply of oxidant controls pyrite (or chalcopyrite) leach rate (2).  This figure shows that oxidant reaches the sulphide surfaces via air convection, air diffusion or water diffusion.  Rates of diffusion were calculated using the constants given in Table 1.  Figure 2 indicates that with oxygen as oxidant, pyrite (and/or chalcopyrite) would be expected to leach at a rate of about 0.04% per month (line A - lower left corner).  Proceedings of the 10th Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1986. The Technical and Research Committee on Reclamation    Bacterial action typically generates Fe   concentrations of the order of 1000 ppm or higher which escalates the predicted leach rates into the order of 0.1-1.0% per month (Line B, Fig. 2).  These are typical of leach rates often achieved in acid leaching of copper waste dumps.  Figure 2 also indicates that oxygen can diffuse through air- filled spaces into a dump much faster than through water-filled pores. Table 1 shows that the much higher diffusion constant of oxygen in air and the nearly 3OX higher concentration of oxygen in air than in air- saturated water jointly account for this effect.  Figure 2 also indicates that to support leach rates approaching 1% per month at dump thicknesses in excess of approximately 10 ft, air convection becomes critically important.  The exothermic nature of the sulphide oxidation reactions creates heating within the zones of rapid oxidation and a chimney effect in the dump.  This effect sometimes results in higher leach rates in cold weather than in hot weather due to increased convection currents generated by the larger temperature differences. While an understanding of Figure 2 could be crucial in conducting a successful dump leaching operation, the same principles might obviously be important in attempting to halt generation of acid mine waters.  This will be discussed further in later sections. Proceedings of the 10th Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1986. The Technical and Research Committee on Reclamation  MINE RECLAMATION GUIDELINES AND REGULATIONS B.C. reclamation guidelines provide operators with a series of general requirements applicable to the reclamation of leached-waste dumps.  Grading dump faces to < 27° is recommended unless operators can show validity of doing otherwise.  Steps should also be taken to "ensure long-term water quality is preserved" and to ensure "all potential acid generating material shall be disposed of in a manner which prevents acid mine drainage" can also be found.  The government also requires monitoring procedures to ensure that potentially toxic materials are disposed of in a manner which prevents adverse effects. While general statements in guidelines and acts abound it is very clear that in large part onus is on the mining industry operators to develop reclamation technology to ensure high standards at reasonable costs. B.C. and Federal regulators have set effluent guidelines or standards to preserve water quality (15,16).  Table II shows examples of existing standards in comparison to ranges of contaminants typically present in acidic mine waters.  Proceedings of the 10th Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1986. The Technical and Research Committee on Reclamation  MONITORING OF WASTE DUMPS Chemical analyses of runoff waters or waters seeping from a waste dump provides an indirect measure of leaching reactions occurring perhaps deep within a dump.  Figure 3 shows how heavy metal concentrations and pH changed during one six hour period following a heavy rain on an acid-producing dump (3).  While heavy metal and dissolved SO4=  concentrations peaked towards the end of this six hour drainage event, total contaminant releases were actually highest near the peak flow rate.  This figure serves well to indicate that monitoring both flow rates and contaminant concentrations is crucial to understanding the severity of an acid mine water problem.  Proceedings of the 10th Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1986. The Technical and Research Committee on Reclamation  Basic questions about the progress of acid-base rock accounting as the dump leaches seem as yet unanswered.  Does dump leaching consume acid-producing components faster than acid-consuming rocks? If so the dump would be left in a state less able to generate acidic- mine waters naturally than prior to the leaching.  If the opposite is true, the acidic mine water problem might be worsened at least until the acid producing minerals are consummed.  Periodic sampling (drilling) and acid-base accounting should answer the above questions and indicate the potential problems that might continue after dump leaching is terminated. Acid-base accounting laboratory-test procedures differ in detail (4), (5), (6).  The potential of waste rock to produce acid or alkaline runoff or seepage waters can be indicated by making two determinations : (A) maximum potential acidity is usually equated chemically to determining total sulphur content of a small but representative sample and considering that all of this sulphur might be converted to sulphuric acid.  The acid production potential can be variously expressed as kg sulphuric acid per tonne of sample or as calcium carbonate equivalent to neutralize (1% sulphur = 31.25 kg calcium carbonate per tonne of sample).  Should significant sulphur, in the original waste rock sample exist in non-acid producing forms (such as inorganic sulfates or organic sulphur) this first determination will markedly overestimate acid-producing potential. (B) Measure of the neutralizers present in the waste rock is called either acid consuming potential (5) or neutralization potential (4).  In the B.C. Research procedure a 10 g ground-sample of the material is titrated with cold H2SO4 until a constant pH of 3.5 is attained (5).  This test can take many hours or perhaps days. In the Sturm Environment Services procedure, a sample is treated with an excess of HCl and heated to speed up reactions.  Excess HCl is back-titrated with a standard base to pH=7 (4).  This Proceedings of the 10th Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1986. The Technical and Research Committee on Reclamation  test takes only a few hours.  Results can again be expressed as acid consuming potential in kg H2SO4 per tonne or as CaCO3 equivalents per tonne.  Subtracting potential acidity from neutralization potential then indicates potential acidity.  In the sturm procedure, a negative value exceeding 5 tonnes CaCO3/1000 tonnes of rock is said to indicate a potential acid producer. When using the B.C. Research procedure, if the acid producing potential exceeds the acid-consuming potential then the waste rock might naturally go acid.  In the B.C. Research procedure, confirmation tests could then be conducted where an acidified sample is inoculated with Thiobacillus ferrooxidans to run a more realistic kinetic-type test (5).  If the pH attained is less than 3.5 a net acid production hazard is said to be confirmed. Arguments can be made for and against each of the above two methods of acid-base accounting.  The speed of the hot-acid titration procedure makes it much more attractive if samples are numerous. Ferguson (6) has recently evaluated the above procedures and found that the chemically-determined "static-type" tests were often accurate in predicting drainage quality.  Kinetic test methods are not commonly used due to their longer duration and higher cost. MITIGATION TECHNIQUES After shutting down a waste dump leach project, operators can expect to have to treat continuing acid mine drainage in the short term.  Longer term methods to terminate acid production are much more desirable since otherwise treatment might be expected to continue for dozens or even hundreds of years. Treatment most often involves collection and addition of alkaline reagents, such as lime, to neutralize acidity and to precipitate dissolved heavy metals.  Sometimes neutralization should be preceeded by an oxidation stage to make several metals more readily precipitate Proceedings of the 10th Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1986. The Technical and Research Committee on Reclamation  as hydroxides.  Local B.C. treatment plants have been well reported in our technical literature (7,8). Figure 4 shows highly simplified schematic flowsheets of such treatment plants.  Capital and operating costs can both be high and low-density precipitate sludge disposal problems can be difficult.  Proceedings of the 10th Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1986. The Technical and Research Committee on Reclamation  Passive water treatment systems sometime offer the prospect of continuous low cost treatment of AMD.  These systems attempt to utilize natural geochemical processes such as:  dilution, oxidation, neutralization, absorption and precipitation for heavy metal removal and acid neutralization.  Basic components of a passive treatment system include an ion exchange medium, a neutralization medium and an aeration device. Several studies have been made of the use of "WETLANDS" to treat AMD (9).  A peat bog filled with sphagrum moss seems to offer an effective organic ion exchange medium.  This moss has high tolerance of strongly acidic conditions and can remove four or five times as many cations as clays for example.  In addition sulphate-reducing bacteria thrive in the anerobic bottom of bog.  These bacterial appear capable of precipitating metal sulphides by microbiological processes (10).  Following bog-removal of most heavy metal ions, excess acidity can be removed by passing the effluent through or over limestone barriers or trenches.  Several artificially constructed wetland treatment sites are under evaluation in Pennsylvania and West Virginia (11).  One major practical problem is the large area requirement for a bog to handle high flow rates. Termination and Prevention of Acid Mine Drainage generally requires minimizing access of reactants such as air and water to areas undergoing biologically enhanced oxidation.  Sealing techniques might also be preceeded by the injection of biocides and neutralizers to attempt to "turn off" the acid-producing reactions quickly. Bacteriacides that have been tested with significant success are mainly anionic surfactants such as sodium lauryl sulphate (SLS). These reagents act at concentrations from 2-5 mg/l and are themselves biodegradable.  At low concentrations the surfactant is said to decrease the activity of pH sensitive enzymes within the bacterium. At high concentrations SLS can kill the bacteria by disagrregation of their cell walls (12). Proceedings of the 10th Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1986. The Technical and Research Committee on Reclamation  Field trials of SLS have been conducted at coal refuse sites in West Virginia (12) and at Equity Silver Mine in B.C. (13).  In West Virginia, application of SLS (as a 30% solution-diluted 175:1) amounted to -150 lbs SLS/acre.  Over a one to three month period, water quality improved dramatically as shown in Figure 5.  After four months, contaminant concentrations slowly rose to near previous levels.  Similar testwork at Equity Silver Mines produced negligible improvement in contaminant levels (13).  This failure was believed due to the rapid hydrolysis of SLS in the highly acidic Equity waste dump. Prior effective alkaline injection might have enabled the SLS to become biocidal before it degraded in the dump.  Proceedings of the 10th Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1986. The Technical and Research Committee on Reclamation  Alkaline Injection tests both in Pennsylvania and at Equity Silver have been largely unsuccessful.  In Pennsylvania a 4% lime slurry was injected using 15 wells 90 meters up-gradient from an AMD seepage area.  In one test nearly 120 tons of lime were injected over an eight month period (12). Minimal impact was observed due apparently to the deposition of the lime suspension in the immediate vicinity of each injection well.  Solubility of Ca(OH)2 was deemed too low for it to act effectively throughout the spoil pile.  Sodium carbonate was recommended for subsequent testing.  At Equity injection of an alkaline sludge into a waste rock dump caused an initial significant decrease in dissolved copper and zinc levels as shown in Figure 6.  After around 90 days however, re-dissolution of metal hydroxides in the injected sludge increased concentrations of these two heavy metals above those emanating previously.  These results did however show promise and probably deserve further testwork - particularly if alkaline injection can be combined with later sealing techniques. Minimizing access of air to a waste dump is generally regarded as the most effective long term measure for reducing AMD.  Sometimes water-flooding can achieve this goal (4).  More often, methods to seal the surface of the dump with impervious "coverings" must be sought. Membrane liners are usually too expensive and possibly ineffective in the long term. Testwork by Harries and Ritchie at the Rum Jungle mine site are particularly interesting (14).  These authors have conducted testwork on two test dumps over the last 10 years.  They examined attempts to limit the influx of air and water by recontouring and top covering the dump with graded clay and sands.  The top of one dump was resloped at a maximum 5° and covered with 22 cm of compacted clay plus 25 cm of sand and then another 15 cm of gravelly sand.  The sides were resloped at 1:3 horizontal and given a somewhat similar sealing-cover.  The tops were also revegetated to provide erosion control.  Holes were drilled to provide temperature probes and gas compositions within the dump.  Figure 7 shows the steady reduction in oxygen concentration at a 15 cm depth in one of the probe holes.  Figure 8 illustrates how Proceedings of the 10th Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1986. The Technical and Research Committee on Reclamation   Proceedings of the 10th Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1986. The Technical and Research Committee on Reclamation  temperatures also declined in response to this sealing technique. Harries and Ritchie conclude that their clay/soil cover was reasonably effective in reducing the oxidation rate within the dump over an extended period.  In more recent times they have found that air again has started to penetrate the cover and they expect that oxidation might again increase.  They have not communicated the cause of this deterioration in the clay/soil cover seal.  Their results do however lend considerable hope that surface sealing covers might prevent or at least control AMD from rehabilitated leached dumps!14).  Proceedings of the 10th Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1986. The Technical and Research Committee on Reclamation   Fig. 8 Temperature reduction as a function of time. (Harries & Ritchie 1985) (Ref. 14) Proceedings of the 10th Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1986. The Technical and Research Committee on Reclamation  CONCLUSIONS AND RECOMMENDATIONS Careful blending of waste dump materials to ensure acid mine water will not form naturally will generally be the most advisable operating procedure.  If this is impossible or if dumps are designed to be acid-leached then reasonable reclamation technology should be designed in to the mine plan. As shown in this review, no inexpensive and completely effective remedial measures are yet available once acid mine waters have formed.  Research into new methods is needed particularly in the western regions where the impact of waste rock dumps probably far exceeds the impacts of mill tailings.  Surface sealing techniques appear at present to have most merit for preventing restart of acid generation. ACKNOWLEDGEMENT The authors are indebted to Messrs.  J.H. Dick of B.C. Ministry of the Environment for promoting this project and K. Ferguson of Environment Canada and B. Marchant and R. Lawrence of Coastech Research Inc. for providing useful information.  Also thanks to Ms. Debbie Craswell for typing this report. Proceedings of the 10th Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1986. The Technical and Research Committee on Reclamation  References 1. Sheffer, H.W. and Evans, L.G., Copper leaching practices in the Western United States, USBM, Info. Circ 8341 (1968). 2. Cathles, L.M. Acid Mine Drainage, Earth and Mineral Sciences, 51, 37-41 (1982). 3. Harries, J.R. and Ritchie, A.I.M., Runoff fraction and pollution levels in runoff from waste rock dumps undergoing pyritic oxidation.  Water, Air & Soil Pollution, 19, 155-170 (1983). 4. Sturm Environmental Services, Evaluation of Acid Generation, Quinsam Coal Project, for Ministry of Environment of B.C., Victoria (1983). 5. Bruynesteyn, A., and Hackl, R.P., Evaluation of acid production potential of mining waste materials, Minerals and the Environment, Vol. 4, pp 5-8 (1984). 6. Ferguson, K.D., Static and kinetic methods to predict acid mine drainage, Int. Symp. Biohydrometallurgy, Vancouver August 22-24 (1985). 7. Patterson, R.J., Proc. 9th Annual Mine Reclamation Symposium, Kamloops (1985). 8. Kuit, W.J., Mine and tailing effluent treatment at the Kimberley, B.C. operations of Cominco Ltd., CIM Bulletin, 73, pp 105-112 (1980). 9. Burris, J.E. (éd.), Treatment of Mine Drainage by Wetlands, Pennsylvania State University, University Park, Pennsylvania (1984). 10. Wieder, R.K., Lang, G.E. and Whitehorse, A.E., The use of freshwater wetlands to treat acid mine drainage, Treatment of Mine Drainage by Wetlands, Pennsylvania State University (1984). 11. Kleinman, R.L.P. and Huntsmen, B.E., A pilot-scale demonstration of man-made methods for treatment of acid mine drainage, ibid, Pennsylvania State University (1984). 12. Kleinman, R.L.P. et al, Control of acid drainage from coal refuse using anionic surfactants, USBM - Report of Investigations, 8847 (1983). 13. Patterson, R.J., Environmental and reclamation practices at Equity Silver Mines Ltd., Proc. 9th Annual Mine Reclamation Symposium, Kamloops, (1985). Proceedings of the 10th Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1986. The Technical and Research Committee on Reclamation  14. Harries, J.R. and Ritchie, A.I.M., The impact of rehabilitation measure on the physiochemcial conditions within mine wastes undergoing pyritic oxidation, p. 341-351, Fundamental and Applied Biohydrometallurgy, Lawrence et al (éd.), Elsevier, New York (1986). 15. Fisheries and Environment Canada Report EPS l-WP-77-1, Metal mining liquid effluent regulations and guidelines, EPS April (1977). 16. Martin, R.T., Government guidelines for waste disposal, Tailings Seminar Vancouver (1981). 17. Kuit, W.J., Mine and tailings effluent treatment at the Kimberley, B.C. Operations of Cominco Ltd., CIM Bulletin, 73, pp. 105-112 (1980).

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