Valorization of acid mine drainage treatment sludge as remediation component to control acid generation from mine wastes,part 2: Field experimentation

The possibility of using acid mine drainage (AMD) treatment sludge as a cover component to control AMD generation from mine wastes was investigated through laboratory characterization and kinetic column testing (companion paper). The results showed that mixtures of sludge and waste rock, and sludge and tailings, may be integrated in an AMD prevention and control strategy at Doyon mine site (northwestern Quebec, Canada). In order to further investigate these scenarios in realistic climatic conditions, instrumented field test cells were installed on site to evaluate the performance of the mixtures to control AMD generation from tailings and waste rock under natural field conditions. The main findings from two seasons of monitoring are presented in the paper. The waste rock-sludge mixture placed over waste rock was able to reduce the generation of AMD from the waste rock, therefore confirming lab results, and was able to produce a neutral effluent with low concentrations of dissolved metals. The tailings-sludge mixture placed over tailings, with an evaporation protection layer, maintained a high volumetric water content and reduced sulphide oxidation from the tailings as exhibited by a neutral effluent. Monitoring of the field cells will continue to provide valuable information on the possible sludge valorization options.     

Electrical and Seismic Tomography Used to Image the Structure of a Tailings Pond at the Abandoned Kettara Mine, Morocco

The Kettara site (Morocco) is an abandoned pyrrhotite ore mine in a semi-arid environment. The site contains more than 3 million tons of mine waste that were deposited on the surface without concern for environmental consequences. Tailings were stockpiled in a pond, in a dyke, and in piles over an area of approximately 16?ha and have generated acid mine drainage (AMD) for more than 29?years. Geophysical methods have been used at the Kettara mine site to determine the nature of the geological substrate of the tailings pond, the internal structure of the mine wastes, and to investigate the pollution zones associated with sulphide waste dumps. Electrical resistivity tomography (ERT) and seismic refraction data were acquired, processed, and interpreted; the results from ERT and seismic refraction were complementary. A topographical survey of the tailings disposal area was also undertaken to estimate the volume of wastes and quantify the AMD process. Two-dimensional inverse models were used to investigate the geophysical data and indicated alteration zones at depth. It was determined that the material could be classified into three categories: tailings, with low resistivity (5?C15??? m) and low velocity (500?C1,800?m/s); altered, black shales, with intermediate resistivity (20?C60??? m) and velocity (2,000?C3,500?m/s), and; materials with high resistivity and velocity (>60????m and >4,000?m/s, respectively), including unaltered shales associated with quartzite seams. The low-resistivity zone generates AMD, which migrates downward through fractures and micro-fractures. The substrate is composed of broken and altered shale, which facilitates AMD infiltration.     

Lime Treatment of Mine Drainage at the Sarcheshmeh Porphyry Copper Mine,Iran

Laboratory and field treatment tests were performed to evaluate the effectiveness of lime treatment for mitigation of environmental effects of acid mine drainage (AMD) at the Sarcheshmeh porphyry copper mine. AMD associated with the rock waste dumps is contaminated with Al (>36,215 μg/L), Cd (>105 μg/L), Co (>522 μg/L), Cu (>53,250 μg/L), Mn (>42,365 μg/L), Ni (>629 μg/L), and Zn (>12,470 μg/L). The concentrations of other metals (Fe, Mo, Pb, and Se) are low or below detection limits (As, Cr, and Sb). Due to the very high Al and Mn content and the low concentration of Fe, a two-stage lime treatment method was chosen for the laboratory tests. In the first stage, the AMD was treated at four pH set points: 7.5, 8.9, 9, and 10. In the second stage, after removing the sludge at pH 9, treatment was continued at pH 10 and 11. The results indicated that a two-stage treatment method was not necessary because elements such as Al, Cu, Co, and Zn were easily treated at pH 7.5, while complete removal of Cd, Mn, and Ni only required a pH of 10. Increasing pH during the treatment process only caused a slight increase in Al. Field treatment tests support the laboratory results. Lime treatment of highly contaminated AMD from dump 11, using simple low density sludge pilot scale equipment, show that contaminant metals are treatable using this method. The mean treatment efficiency for contaminant metals was 99.4% for Al, % for Cd, 99.6% for Co, 99.7% for Cu, 98.5% for Mn, 99.7% for Ni, 99% for U, and 99.5% for Zn. The optimum pH for AMD treatment by lime was in the range of 9–10. The produced sludge in the treatment process was highly enriched in the contaminant metals, especially Cu (>7.34%), Al (>4.76%), Mn (>2.94%), and Zn (>1.25%). A correlation coefficient matrix indicates that the distribution pattern of the contaminant metals between soluble and precipitated phases is consistent with the hydrochemical behavior of the metals during the lime treatment process.     

Acid Mine Drainage at the Abandoned Kettara Mine (Morocco): 2. Mine Waste Geochemical Behavior

Weathering and humidity cell tests were used to predict the potential for acid mine drainage (AMD) and to estimate the mineral reaction rates and depletion of fine and coarse tailings from the abandoned Kettara mine, Morocco. The geochemistry of the fine and coarse mine wastes was similar and, as expected by static tests, the wastes produced significant amounts of AMD. The sulfate production rate of both fine and coarse tailings was very high (2,000–8,000 and 2,400–560 mg SO₄/kg/week, respectively) during the first weeks of kinetics tests. After 9 weeks, sulfate release became low, ranging between 600 and 78 mg SO₄/kg/week for fine tailings and 500–120 mg SO₄/kg/week for coarse tailings. Effluent water samples had low pH (2.9–4.2) and elevated concentrations of acidity, sulfate, iron, copper, and zinc. Most or all of the dissolved K, Na, Al, Mg, and Si in the AMD result from the acidic dissolution of silicates (chlorite, talc, muscovite, and albite). Fine tailings produce much higher concentrations of acidity and sulfate than coarse tailings. However, due to greater transport of oxygen and water within the coarse waste, coarse tailings could be of greater environmental significance than fine tailings. The coarse waste continued to release acid after 378 days of leaching, whereas the fine tailings naturally passivates. These laboratory results agree with field observations; the upper profile of the coarse waste rock dam is highly oxidized (75 cm) whereas oxidation in the fine tailings does not extend more than 5–15 cm beneath the surface. A comparison between weathering and humidity cell tests indicated that the general trend of dissolution of metals was essentially similar for both methods. However, sulfate depletion rates were higher for the weathering cell tests. These tests indicate that the Kettara tailings piles and dam will continue to release acid for a long time unless remedial action is taken.     

Valorisation of acid mine drainage treatment sludge as remediation component to control acid generation from mine wastes,part 1: Material characterization and laboratory kinetic testing

In operating mines, acid mine drainage (AMD) is often treated using lime treatment. This process generates a significant amount of sludge that contains metal hydroxide precipitates, gypsum, and unreacted lime. The sludge may have interesting geotechnical and geochemical properties to be used as a part of covers (oxygen barriers) to prevent AMD generation from waste rocks and tailings. The main results of a project aiming to evaluate the use of sludge from the Doyon mine site (Canada) as a material in mine site rehabilitation are presented. The first part of the project involved detailed characterization of sludge, waste rock, and tailings samples. Then, laboratory column leaching tests were performed to evaluate the performance of the mixtures to control AMD produced by tailings and waste rocks. It was found that a sludge–waste rock mixture placed over waste rock reduces the metal loads in the column effluent, which remained acidic, as well as a mixture of sludge and tailings deposited over tailings can reduce metal content in effluents from tailings.     

Prediction of Acid Mine Drainage: Importance of Mineralogy and the Test Protocols for Static and Kinetic Tests

Static tests, which compare the acid-generating potential and acid-neutralizing potential for a given mine waste (tailings or waste rocks), are characterized by a wide uncertainty zone in which it is impossible to accurately predict the acid-generating potential (AGP). Then, to better assess long-term AGP, kinetic tests are usually performed to provide more information about the reaction rates of the acid-generating and acid-neutralizing minerals. The present work compares the classic Sobek static test with three mineralogical static tests to assess the importance of sample mineralogy in acid mine drainage (AMD) prediction. We also investigated how experimental procedures related to static tests can influence prediction results. We used three synthetic tailings samples made by mixing well-characterized pure minerals in calibrated proportions. Although basically different in their principles and procedures, the modified Sobek and mineralogical static tests gave similar results. These AGP predictions were then validated by the use of a kinetic test. The kinetic test protocol was also modified in this study and the results obtained correlated well with the static test results, in contrast to the standard kinetic test protocol. The present work highlights the limitations of static and kinetic test procedures, and provides recommendations for a better use of these tests for more reliable AMD prediction.     

Reclamation of Abandoned Coal Mine Waste in Korea using Lime Cake By-Products

Abstract.  There are hundreds of abandoned coal mines in Korea's steep mountain valleys. Enormous amounts of coal waste from these mines were dumped on the slopes, contaminating streams with sediment and acid mine drainage. A limestone slurry by-product (lime cake), which is produced during the manufacture of soda ash, was investigated for its potential use in reclaiming the coal waste. The lime cake is fine grained, has low hydraulic conductivities (10^-8 to 10^-9 cm sec^-1), high pH, high electrical conductivity, and trace amounts of heavy metals. A field experiment was conducted; each plot was 20 x 5 m in size on a 56% slope. Treatments included a control (waste only), lime (CaCO₃), and lime cake. The lime requirement (LR) of the coal waste to pH 7.0 was determined; treatments consisted of adding 25, 50, and 100% of the LR. The lime cake and lime were applied either as a layer between the coal waste and topsoil or mixed into the topsoil and waste. Each plot was hydroseeded with grasses, and planted with trees. In each plot, soils, surface runoff, and subsurface water were collected and analyzed, and plant cover was measured. Lime cake treatments increased the pH of the coal waste from 3.5 to 6, and neutralized the pH of the runoff and leachate of the coal waste from 4.3 to 6.7. Moreover, the surface cover of seeded species was significantly increased; sufficient acidity in the coal waste was neutralized in the 25% LR plots to allow seed germination.     

Laboratory Evaluation of the Use of Alkaline Phosphate Wastes for the Control of Acidic Mine Drainage

Pyrrhotite tailings at the abandoned Kettara mine site in Morocco are producing acid mine drainage (AMD). We investigated the use of alkaline phosphate waste (APW) rock from a nearby operating open-pit phosphate mine to control the AMD. The neutralizing potential of the APW, using the Paktunc method, was estimated between 500 and 680 kg CaCO₃/t. In laboratory column tests, the addition of 15 wt% APW to the coarse Kettara tailings produced leachates with significantly lower acidities and metal concentrations than unamended controls. The high calcium concentration in the flushed solutions indicates that calcite was responsible for the neutralization. Dolomite dissolution seems to be negligible and fluorapatite was stable under the testing conditions. It was also observed that when the treated solution comes in contact with unweathered Kettara coarse tailings, the pH becomes acidic, although the metal concentrations remain low.     

Engineering Closure of an Open Pit Gold Operation in a Semi-Arid Climate

Along with material characteristics and geometry, the climate in which a mine is located can have a dramatic effect on the appropriate options for rehabilitation. The paper outlines the setting, mining, milling and waste disposal at Kidston Gold Mine's open pit operations in the semi-arid climate of North Queensland, Australia, before focusing on the engineering aspects of the rehabilitation of Kidston. The mine took a holistic and proactive approach to rehabilitation, and was prepared to demonstrate a number of innovative approaches, which are described in the paper. Engineering issues that had to be addressed included the geotechnical stability and deformation of waste rock dumps, including a 240 m high in-pit dump: the construction and performance monitoring of a “store and release” cover over potentially acid forming mineralised waste rock; erosion from the side slopes of the waste rock dumps; the in-pit co-disposal of waste rock and thickened tailings; the geotechnical stability of the tailings dam wall; the potential for erosion of bare tailings; the water balance of the tailings dam; direct revegetation of the tailings; and the pit hydrology. The rehabilitation of the mine represents an important benchmark in mine site rehabilitation best practice, from which lessons applicable worldwide can be shared.     

Soil Infiltration,Permeability, and Rock Fracturing Assessment to Establish Water Flow Patterns in a Mine with Acid Mine Drainage

The flow of acid mine drainage (AMD) and neutral water were assessed inside an underground mine using a volumetric capacity technique, while water infiltration through the ground was assessed by performing three double ring constant load infiltrometer tests. Measured infiltration velocities were slow to moderate, between 0.036 and 2.07 cm/h. Total rainwater infiltration for the sub-basin was 0.031 hm³/yr. Tests performed on representative rock cores of the stratigraphical column of the area indicated that the permeabilities of the altered schist was 4.73E?10 cm/s, the tuff was 1.47E?09, and the graphitic phyllite was 2.47E?06. A structural analysis was performed inside the mine to assess fractures and faults using the mine plans for three different levels. Three major discontinuity paths were identified. Since the ground and rock permeabilities were low, the large water volumes inside the mine were attributed to these fractures. The results of this study are being used to understand flow patterns in the mine and to optimally locate a water treatment system to control the AMD.     

Prediction of Minerals Producing Acid Mine Drainage Using a Computer-Assisted Thermodynamic Chemical Equilibrium Model

Methods such as optical microscopy, electron microscopy, and X-ray diffraction are sometimes used to identify minerals involved in the production of acid mine drainage (AMD). A simpler method to identify the minerals without losing accuracy would decrease costs. This paper presents an overview of the important oxidation reactions of sulphide minerals and related chemical components produced by these oxidation reactions. A methodology for predicting the minerals producing AMD using MINTEQ is also discussed. This method can be used in conjunction with analytical techniques to characterize AMD for a specific site. While it does not replace analytical tests, it can decrease the number and frequency of these expensive tests. The model has been validated with data from the Wolverine coal mine in northeastern BC, Canada.     

Assessment of Trace Elements in Soils and Mine Water Surrounding a Closed Manganese Mine (Anti Atlas,Morocco)

Five mine water samples, 23 topsoil samples, and four mine waste (tailings) samples were collected to assess the effects of a closed Moroccan Mn mine. Based on the pH, electrical conductivity, and concentrations of sulphate, Cu, Zn, As, Cd, Pb, and Mn, mining has not adversely influenced mine water quality. Soil samples were analyzed for 23 chemical elements and the results were interpreted by univariate and multivariate statistical techniques. Based on an enrichment factor (EF) calculation, only Cd, As, V, and Mn were selected for further study. Geochemical background (GB) and geoaccumulation index (I_geo) were determined for these elements to differentiate between geogenic and anthropogenic enrichment. The GB values showed that the Tiwiyyine soils contained a high geogenic content of Cd, As, V, and Mn that reflected the geochemistry of the parental rocks in this mineralized region. The I_geo calculation revealed that these soils were moderately influenced by anthropogenic activity, which had increased the concentrations of those elements. Finally, geochemical maps revealed that mining was likely responsible for the anthropogenic soil pollution.     

Effect of loess for preventing contamination of acid mine drainage from coal waste

Acid mine drainage (AMD) that releases highly acidic, sulfate and metals-rich drainage is a serious environmental problem in coal mining areas in China. In order to study the effect of using loess for preventing AMD and controlling heavy metals contamination from coal waste, the column leaching tests were conducted. The results come from experiment data analyses show that the loess can effectively immobilize cadmium, copper, iron, lead and zinc in AMD from coal waste, increase pH value, and decrease Eh, EC, and SO ₄ ^2? concentrations of AMD from coal waste. The oxidation of sulfide in coal waste is prevented by addition of the loess, which favors the generation and adsorption of the alkalinity, the decrease of the population of Thiobacillus ferrooxidans, the heavy metals immobilization by precipitation of sulfide and carbonate through biological sulfate reduction inside the column, and the halt of the oxidation process of sulfide through iron coating on the surface of sulfide in coal waste. The loess can effectively prevent AMD and heavy metals contamination from coal waste in in-situ treatment systems.     

Numerical modelling of transfer processes in a waste rock pile undergoing the temporal evolution of its heterogeneous material properties

Waste rock piles producing acid mine drainage (AMD) are complex and heterogeneous unsaturated systems in which multiple coupled processes are involved: multiphase flow of fluids (liquid and gas), heat transfer and mass transport. Numerical modelling is required to consider together these processes. Such modelling was used to evaluate the effects of closing options for a waste rock pile, including resloping and the placement of a cover, on AMD production from the South Dump of the Doyon Mine (Quebec, Canada). The thermal data recorded within the pile during 15?years show a steady reduction of temperature over the last 10?years. The objective of this study was to numerically reproduce the temporal evolution of thermal conditions to answer two key questions: (1) Can the evolution of material properties over time explain observed changes in thermal conditions? (2) If waste rock properties have evolved, is it effective to cover the pile to mitigate AMD production? Modelling showed that reducing the permeability of materials in the model can reproduce observed temperature changes over time, which cannot be achieved by reducing the reactivity of materials. Results also show that the evolution of hydraulic properties of the waste rock pile can have a direct effect on the global oxidation rate because of their control over the supply of atmospheric air and oxygen in the pile. When the production of AMD is already in decline, simulations also show that a simple remodelling of the surface may be sufficient to significantly decrease the oxidation rate and provide immediate environmental gains, without a cover. However, there may be other benefits related to cover placement that have not been evaluated by our study, especially in relation to the management of water runoff and leachate.     

Production of a Poly-ferric Sulphate Chemical Coagulant by Selective Precipitation of Iron from Acidic Coal Mine Drainage

The aim of this work was to produce a ferric sulphate rich solution from acidic coal mine drainage that could be used as coagulant. Precipitating the iron at pH 3.8, followed by dissolution in sulphuric acid, produced a coagulant consisting of 12.4% iron and 1.3% aluminium. Water treatment tests proved that this coagulant was as efficient as the coagulant chemicals conventionally used in water treatment plants. The process can be easily incorporated into conventional AMD treatment plants, thereby reducing sludge waste issues and producing a valuable chemical reagent.     

矿山排土场稳定性主动调控机制研究

矿山排土工艺正朝高阶段排土、高强度排土和设备大型化方向发展。伴随而来的是排土平台或其地基失稳剧增,如降低堆置高度则容量减少,如加固或清除地基软土层,巨额的工程费用又难以承受。分析了矿山排土场拦碴坝、坡脚挡墙等被动的过程调控措施的实用性,评价了排(截)水沟的有效性;基于对环境的影响后果,从排土场边坡与基底的相互作用机理出发,采用允许变形和部分破坏的设计原则。基于空间效应,构架了合理使用排土空间,调整排土场形成过程的时空关系的主动调控机制:1对凸形排土场应控制排弃强度,确保土场沉降正常,以废石流量为安全调控关键指标;2对软弱地基,首要是控制第一层排土段高,采用堆载预压原理提高基底承载力,作为上覆土场基础,规避大面积清除软基层;3当场址内梁、谷交错且地基软弱,对称均衡纵向推进的排土,可规避侧向挤压作用触发的牵引式滑坡危害。同时,利用分区间隔点排法和基于界面疏导原理,构筑的泄流基底确保降雨不诱发排土场滑坡及突变成泥石流。经过现场试验论证,利用排土工艺的主动调控方法符合矿山需求,可兼顾安全性和经济性。     

Use of permeable reactive barriers to treat acid mine effluents

Acid mine drainage (AMD) is one of the most serious environmental problems facing the Canadian Mineral Industry. The AMD results from oxidation of sulphide minerals (e.g. pyrite or pyrrhotite) contained in mine waste or mine tailings. It is characterised by acid effluents rich in heavy metals, which are released into the environment. A new acid remediation technology is presented in this article by which alkaline metallurgical residues (red mud) from the aluminum extraction industry are used to construct permeable reactive barriers (PRB) to treat acid mine effluents. This article describes column tests performed to simulate a PRB constructed using Bauxsol?, a chemically and physically treated bauxite refinery residue, as a reactive material. The results from these experiments show that these PRB are efficient in neutralising pH and removing metals from acidic mine effluents.     

Predicting Geochemical Behaviour of Waste Rock with Low Acid Generating Potential Using Laboratory Kinetic Tests

Techniques developed for acid mine drainage (AMD) prediction might not be suitable for contaminated neutral drainage (CND) generating sites. The Tio mine waste is known to generate Ni contaminated neutral drainage in some of the piles, but humidity cell tests fail to generate the Ni concentrations observed in the field. Weathering cell tests (small-scale humidity cell tests) were performed on fresh and weathered (produced 25 years ago) waste rock samples from the Tio mine containing various levels of hemo-ilmenite ore, and results were compared to humidity cell results on similar samples. The main constituents of the waste rock are the hemo-ilmenite ore and the plagioclase gangue; these minerals were purified from the waste rocks and the purified fractions were also submitted to weathering cell tests. The fresh waste rock samples were also submitted to sorption cells (modified weathering cells), which showed that the waste rocks have an important Ni sorption potential and that the sorbed phases are stable under weathering cell conditions. Even though the Ni concentrations obtained from the laboratory tests remain significantly lower than those obtained in field conditions (from field test pads and from waste rock piles), the results from the present study give important insight into the geochemical processes implicated in CND generation.     

Kinetic Testing and Sorption Studies by Modified Weathering Cells to Characterize the Potential to Generate Contaminated Neutral Drainage

Techniques developed for acid mine drainage (AMD) prediction might not be suitable for contaminated neutral drainage (CND) generating sites. The Tio mine waste is known to generate Ni contaminated neutral drainage in some of the piles, but humidity cell tests fail to generate the Ni concentrations observed in the field. Weathering cell tests (small-scale humidity cell tests) were performed on fresh and weathered (produced 25 years ago) waste rock samples from the Tio mine containing various levels of hemo-ilmenite ore, and results were compared to humidity cell results on similar samples. The main constituents of the waste rock are the hemo-ilmenite ore and the plagioclase gangue; these minerals were purified from the waste rocks and the purified fractions were also submitted to weathering cell tests. The fresh waste rock samples were also submitted to sorption cells (modified weathering cells), which showed that the waste rocks have an important Ni sorption potential and that the sorbed phases are stable under weathering cell conditions. Even though the Ni concentrations obtained from the laboratory tests remain significantly lower than those obtained in field conditions (from field test pads and from waste rock piles), the results from the present study give important insight into the geochemical processes implicated in CND generation.     

Use of Steel Slag Leach Beds for the Treatment of Acid Mine Drainage

Steel slag from the Waylite steel-making plant in Bethlehem, Pennsylvania was leached with acidic mine drainage (AMD) of a known quality using an established laboratory procedure. Leaching continued for 60 cycles and leachates were collected after each cycle. Results indicated that the slag was very effective at neutralizing acidity. The AMD/slag leachates contained higher average concentrations of Ba, V, Mn, Cr, As, Ag, and Se and lower average concentrations of Sb, Fe, Zn, Be, Cd, Tl, Ni, Al, Cu, and Pb than the untreated AMD. Based on these tests, slag leach beds were constructed at the abandoned McCarty mine site in Preston County, West Virginia. The leach beds were constructed as slag check dams below limestone-lined settling basins. Acid water was captured in limestone channels and directed into basins to leach through the slag dams and discharge into a tributary of Beaver Creek. Since installation in October 2000, the system has been consistently producing net alkaline, pH 9 water. The treated water is still net alkaline and has a neutral pH after it encounters several other acidic seeps downstream.