Coupled thermal, hydraulic and geochemical evolution of pyritic tailings in unsaturated column experiments

The evolution of pore-water and the composition of solid phases in the vadose zone of pyritic tailings was studied by means of unsaturated column experiments. Several columns of water-saturated mine tailings were dried during 125 days under controlled laboratory conditions. The columns were dismantled at four successive drying stages and the evolution of pore-water, mineralogy, water content and temperature was characterized.Sulfide and aluminosilicate minerals present in the waste dissolved, releasing sulfate and other solutes (mainly Fe, Zn, Cu, Al, Mg and Ca) to the pore-water. Evaporation caused a crust of efflorescent, water-soluble sulfates to develop over the complete top surface of the columns and into the pores of the underlying waste material. This crust, which has also been identified in the field, changed the hydraulic properties of the tailings and produced a decrease in the evaporation rate of the columns. Moreover, these water-soluble precipitates (mainly rozenite, szomolnokite, halotrichite, hexahydrite, mirabilite and gypsum) acted as temporary sinks for Cd, Pb, Co and Ni, which could be released to the surface run-off or the groundwaters during rainfall events under field conditions.Pore-water evolution was determined not only by geochemical processes (dissolution of sulfides and aluminosilicates, precipitation of secondary phases) but also by thermal and hydraulic processes. Progressive dilution was observed in the lower part of the columns. Dilution was caused by the thermally driven vapor flux from the top of the column to its colder bottom and subsequent condensation therein. This process, which may also occur in tailings under sub-arid climate, played a key role on the evolution of pore-water with increasing drying.     

Geochemical and mineralogical aspects of sulfide mine tailings

Tailings generated during processing of sulfide ores represent a substantial risk to water resources. The oxidation of sulfide minerals within tailings deposits can generate low-quality water containing elevated concentrations of SO₄, Fe, and associated metal(loid)s. Acid generated during the oxidation of pyrite [FeS₂], pyrrhotite [Fe_(1−x)S] and other sulfide minerals is neutralized to varying degrees by the dissolution of carbonate, (oxy)hydroxide, and silicate minerals. The extent of acid neutralization and, therefore, pore-water pH is a principal control on the mobility of sulfide-oxidation products within tailings deposits. Metals including Fe(III), Cu, Zn, and Ni often occur at high concentrations and exhibit greater mobility at low pH characteristic of acid mine drainage (AMD). In contrast, (hydr)oxyanion-forming elements including As, Sb, Se, and Mo commonly exhibit greater mobility at circumneutral pH associated with neutral mine drainage (NMD). These differences in mobility largely result from the pH-dependence of mineral precipitation–dissolution and sorption–desorption reactions. Cemented layers of secondary (oxy)hydroxide and (hydroxy)sulfate minerals, referred to as hardpans, may promote attenuation of sulfide-mineral oxidation products within and below the oxidation zone. Hardpans may also limit oxygen ingress and pore-water migration within sulfide tailings deposits. Reduction–oxidation (redox) processes are another important control on metal(loid) mobility within sulfide tailings deposits. Reductive dissolution or transformation of secondary (oxy)hydroxide phases can enhance Fe, Mn, and As mobility within sulfide tailings. Production of H₂S via microbial sulfate reduction may promote attenuation of sulfide-oxidation products, including Fe, Zn, Ni, and Tl, via metal-sulfide precipitation. Understanding the dynamics of these interrelated geochemical and mineralogical processes is critical for anticipating and managing water quality associated with sulfide mine tailings.     

Reactive transport modelling of mine tailings columns with capillarity-induced high water saturation for preventing sulfide oxidation

A series of laboratory column tests on reactive mine tailings was numerically simulated to study the effect of high water saturation on preventing sulfide mineral oxidation and acid mine drainage (AMD). The approach, also known as an elevated water table (EWT), is a promising alternative to full water covers for the management and closure of sulfidic tailings impoundments and for the long term control of acid mine drainage. The instrumented columns contained reactive tailings from the Louvicourt mine, Quebec, and were overlain by a protective sand cover. Over a 13–19 month period, the columns were exposed to atmospheric O₂ and flushed approximately every month with demineralized water. A free draining control column with no sand cover was also used. During each cycle, water table elevations were controlled by fixing the pressure at the column base and drainage water was collected and analyzed for pH and Eh, major ions, and dissolved metals (Fe, Zn, Cu, Pb, and Mg). The columns were simulated using the multi-component reactive transport model MIN3P which solves the coupled nonlinear equations for transient water flow, O₂ diffusion, advective–dispersive transport and kinetic geochemical reactions. Physical properties and mineralogical compositions for the material layers were obtained from independent laboratory data. The simulated and observed data showed that as the water table elevation increased, the effluent pH became more neutral and SO₄ and dissolved metal concentrations decreased by factors on the order of 10²–10³. It is concluded that water table depths less than or equal to one-half of the air entry value (AEV) can keep mine tailings sufficiently saturated over the long term, thus reducing sulfide oxidation and AMD production.     

Hydrogeochemistry and microbiology of mine drainage: An update

The extraction of mineral resources requires access through underground workings, or open pit operations, or through drillholes for solution mining. Additionally, mineral processing can generate large quantities of waste, including mill tailings, waste rock and refinery wastes, heap leach pads, and slag. Thus, through mining and mineral processing activities, large surface areas of sulfide minerals can be exposed to oxygen, water, and microbes, resulting in accelerated oxidation of sulfide and other minerals and the potential for the generation of low-quality drainage. The oxidation of sulfide minerals in mine wastes is accelerated by microbial catalysis of the oxidation of aqueous ferrous iron and sulfide. These reactions, particularly when combined with evaporation, can lead to extremely acidic drainage and very high concentrations of dissolved constituents. Although acid mine drainage is the most prevalent and damaging environmental concern associated with mining activities, generation of saline, basic and neutral drainage containing elevated concentrations of dissolved metals, non-metals, and metalloids has recently been recognized as a potential environmental concern. Acid neutralization reactions through the dissolution of carbonate, hydroxide, and silicate minerals and formation of secondary aluminum and ferric hydroxide phases can moderate the effects of acid generation and enhance the formation of secondary hydrated iron and aluminum minerals which may lessen the concentration of dissolved metals. Numerical models provide powerful tools for assessing impacts of these reactions on water quality.     

湘西金矿尾矿-水相互作用:2.动力学模拟

从硫化物的氧化、脉石矿物的溶解、氧的扩散、水的流动和溶质质量迁移等过程的耦合作用建立了尾矿-水相互作用的动力学模型。对湖南湘西金矿尾矿库的数学模拟表明：早期尾矿-水的相互作用可以引起酸水的产生和重金属的释放及对环境的污染，影响较大的主要是前30年，随着时间增加，尾矿孔隙水逐渐中性化，污染元素含量显著降低。脉石矿物的溶解和有机物反应可降低尾矿中氧的扩散和产生酸的中和作用。尾矿库上部孔隙水中各组分含量明显高于尾矿库下部，并在约6m深处存在突变带。尾矿库的水文分带是导致地球化学分带的主要原因。     

Mobilization and attenuation of heavy metals within a nickel mine tailings impoundment near Sudbury, Ontario, Canada

 The oxidation and the subsequent dissolution of sulfide minerals within the Copper Cliff tailings area have led to the release of heavy metals such as Fe, Ni, and Co to the tailings pore water. Dissolved concentrations in excess of 10 g/l Fe and 2.2 g/l Ni have been detected within the shallow pore water of the tailings, with increasing depth these concentrations decrease to or near analytical detection limits. Geochemical modelling of the pore-water chemistry suggests that pH-buffering reactions are occurring within the shallow oxidized zones, and that secondary phases are precipitating at or near the underlying hardpan and transition zones. Mineralogical study of the tailings confirmed the presence of goethite, jarosite, gypsum, native sulfur, and a vermiculite-type clay mineral. Goethite, jarosite, and native sulfur form alteration rims and pseudo-morphs of the sulfide minerals. Interstitial cements, composed of goethite, jarosite, and gypsum, locally bind the tailings particles, forming hardpan layers. Microprobe analyses of the goethite indicate that it contains up to 0.6 weight % Ni, suggesting that the goethite is a repository for Ni. Other sinks detected for heavy metals include jarosite and a vemiculite-type clay mineral which locally contains up to 1.6 weight % Ni. To estimate the mass and distribution of heavy metals associated with the secondary phases within the shallow tailings, a series of chemical extractions was completed. The experimental design permitted four fractions of the tailings to be evaluated independently. These four fractions consisted of a water-soluble, an acid-leachable, and a reducible fraction, as well as the whole-rock total. Twenty-five percent of the total mass of heavy metals was removed in the acid-leaching experiments, and 100% of the same components were removed in the reduction experiments. The data suggest that precipitation/coprecipitation reactions are providing an effective sink for most of the heavy metals released by sulfide mineral oxidation. In light of these results, potential decommissioning strategies should be evaluated with the recognition that changing the geochemical conditions may alter the stability of the secondary phases within the shallow tailings. Received: 9 April 1997 · Accepted: 21 July 1997     

Integrated geophysical and geochemical study on AMD generation at the Haveri Au–Cu mine tailings,SW Finland

The Haveri tailings area contains 1.5 Mt of sulfide-bearing waste from the Au–Cu mine that operated during 1942–1961. Geophysical and geochemical methods were used to evaluate and characterize the generation of acid mine drainage (AMD). Correlations were examined among the electrical resistivity tomography (ERT) data, the total sulfide content and concentrations of sulfide-bound metals (Cu, Co, Fe, Mn, Ni, Pb and Zn) of tailings samples, and the resistivity and geochemistry of surface water. The resulting geophysical–geochemical model defines an area in the vadose tailings, where a low resistivity anomaly (<10 Ohm m) is correlated with the highest sulfide content, extensive sulfide oxidation and low pH (average 3.1). The physical and geochemical conditions, resulting from the oxidation of the sulfide minerals, suggest that the low resistivity anomaly is associated with acidic and metal-rich porewater (i.e., AMD). The lower resistivity values in the saturated zone of the central impoundment suggest the formation of a plume of AMD. The natural subsoil layer (silt and clay) and the bedrock surface below the tailings area were well mapped from the ERT data. The detected fracture zones of the bedrock that could work as leakage pathways for AMD were consistent with previous geological studies. The integrated methodology of the study offers a promising approach to fast and reliable monitoring of areas of potential AMD generation and its subsurface movement over large areas (ca. 9 ha). This methodology could be helpful in planning drill core sampling locations for geochemical and mineralogical analysis, groundwater sampling, and choosing and monitoring remedial programs.     

Mineral/water interactions in tailings from a tungsten mine, Mount Pleasant, New Brunswick

The pore-water geochemistry and mineralogy of tailings derived from a granitic tungsten deposit were characterized by collecting pore-water samples at discrete depth intervals throughout the tailings for the analysis of major and minor element concentrations. Mineralogical samples from the oxidation zone were analyzed by X-ray diffraction, scanning electron microscopy combined with energy dispersive X-ray spectroscopy (SEM/EDS), electron microprobe (EMP) combined with wavelength dispersive X-ray spectroscopy (WDS), and transmission electron microscopy (TEM). The oxidation of sulfide minerals in the near-surface tailings leads to a decrease in pore-water pH and elevated SO₄, As, and metal concentrations. The unusual mineralogy of this deposit, compared with that of commonly studied base-metal and gold deposits, results in several unique geochemical characteristics. The dissolution of fluorite releases F into the pore water; the F forms strong complexes with Al and enhances the dissolution of aluminosilicate minerals within the oxidation zone. As a result, high Al concentrations (up to 151.7 mg/L) are detected in the near-neutral pore water in the oxidation zone. The combined dissolution of aluminosilicates and carbonate minerals maintains the pH near 10 in the pore water at depth. Elevated concentrations of W (up to 7.1 mg/L) are detected in the pore water throughout the tailings, likely as a result of the dissolution of wolframite. Consistent with geochemical model calculations, results from SEM/EDS, EMP/WDS and TEM/EDS analyses indicate that secondary minerals, which occur as orange-brown coatings on grains of primary-minerals, are Fe oxyhydroxides. Examples of these secondary minerals display a fibrous habit at high resolution in the TEM. One of these minerals, which contains substantial amounts of Al, As, and Si as impurities, was identified by selected-area electron diffraction (SAED) analyses to be goethite. Another mineral contains relatively high amounts of Si, Pb, Bi, and As, and SAED analyses suggest that the mineral is two-line ferrihydrite.     

Acid neutralization mechanisms and metal release in mine tailings: a laboratory column experiment

Mining and milling of base metal ore deposits can result in the release of metals to the environment. When sulfide minerals contained in mine tailings are exposed to oxygen and water, they oxidize and dissolve. Two principal antagonistic geochemical processes affect the migration of dissolved metals in tailings impoundments: sulfide oxidation and acid neutralization. This study focuses on acid neutralization reactions occurring in the saturated zone of tailings impoundments. To simulate conditions prevailing in many tailings impoundments, 0.1 mol/L sulfuric acid was passed continuously through columns containing fresh, unoxidized tailings, collected at Kidd Creek metallurgical site. The results of this column experiment represent a detailed temporal observation of pH, Eh, and metal concentrations. The results are consistent with previous field observations, which suggest that a series of mineral dissolution-precipitation reactions control pH and metal mobility. Typically, the series consists of carbonate minerals, Al and Fe(III) hydroxides, and aluminosilicates. In the case of Kidd Creek tailings, the dissolution series consists of ankerite-dolomite, siderite, gibbsite, and aluminosilicates. In the column experiment, three distinct pH plateaus were observed: 5.7, 4.0, and 1.3. The releases of trace elements such as Cd, Co, Cr, Cu, Li, Ni, Pb, V, and Zn were observed to be related to the pH buffering zones. High concentrations of Zn, Ni, and Co were observed at the first pH plateau (pH 5.7), whereas Cd, Cr, Pb, As, V, and Al were released as the pH of the pore water decreased to 4.0 or less.     

Kinetic tests comparison and interpretation for prediction of the Joutel tailings acid generation potential

Five sulfide mine tailings coming from the Joutel mine tailing ponds (Quebec, Canada) were tested by the humidity cell test (30 to 52 cycles duration) and the column test (11 to 12 cycles duration). The objectives of this study were twofold. First, there was the determination of the tailings acid generation potential for site reclamation. Second, there was the kinetic test comparison for understanding the tailings geochemical behavior under different test conditions. The samples used had a wide diversity in terms of acid-generation potential, particle size distribution, and parameters influencing reaction rates. Leachates produced remained at a near neutral pH for the duration of the tests. Evolution of the main elements involved in the dissolution processes demonstrated neutralization by carbonates as a response to the acid generated by sulfide oxidation. Depletion rates given by sulfates are higher for the humidity cell tests when compared to those obtained for the column tests. This is consistent with most studies to date, the humidity cell test being considered as more severe. However, by taking the ratio between cumulative elements coming from neutralization and the ones coming from oxidation, similar curves (named herein oxidation –neutralization curves) for all tests were obtained. These results show that overall geochemical behavior of the tailings is similar at near neutral pH for both types of tests. With this interpretation method, the acid-generation potential of the Joutel tailings were tested and compared to the static test results to constrain their uncertainty zone with regard to the studied tailings. The tailings geochemical behavior (carbonate dissolution response to sulfide oxidation) at near neutral pH condition appears slightly dependent of test conditions under certain hypothesis.     

Chemistry of acid production in black coal mine washery wastes

Column leaching tests on black coal mine washery wastes were performed, to determine the chemistry of acid generation. Coal mine coarse rejects and tailings were subjected to wet and dry cycle dissolution and subsequently column leached. The rates of iron sulphide oxidation and carbonate mineral dissolution were determined based on the drainage chemistry. The kinetic data from column leach experiments are used to predict the time required to deplete the acid producing and acid consuming minerals in the mine wastes. The acid production in the mine rejects was found to depend upon iron chemistry, carbonate chemistry, diffusion of oxygen, and permeability. The chemistry of the drainage from two different coal mines is compared.     

Mineralogical,geochemical, and microbial investigation of a sulfide-rich tailings deposit characterized by neutral drainage

Mineralogical, geochemical and microbial characterization of tailings solids from the Greens Creek Mine, Juneau, Alaska, was performed to evaluate mechanisms controlling aqueous geochemistry of near-neutral pH pore water and drainage. Core samples of the tailings were collected from five boreholes ranging from 7 to 26 m in depth. The majority of the 51 samples (77%) were collected from the vadose zone, which can extend >18 m below the tailings surface. Mineralogical investigation indicates that the occurrence of sulfide minerals follows the general order: pyrite [FeS₂] >> sphalerite [(Zn,Fe)S] > galena [PbS], tetrahedrite [(Fe,Zn,Cu,Ag)₁₂Sb₄S₁₃] > arsenopyrite [FeAsS] and chalcopyrite [CuFeS₂]. Pyrite constitutes <20 to >35 wt.% of the tailings mineral assemblage, whereas dolomite [CaMg(CO₃)₂] and calcite [CaCO₃] are present at ?30 and 3 wt.%, respectively. The solid-phase geochemistry generally reflects the mineral assemblage. The presence of additional trace elements, including Cd, Cr, Co, Mo, Ni, Se and Tl, is attributed to substitution into sulfide phases. Results of acid–base accounting (ABA) underestimated both acid-generating potential (AP) and neutralization potential (NP). Recalculation of AP and NP based on solid-phase geochemistry and quantitative mineralogy yielded more representative results. Neutrophilic S-oxidizing bacteria (nSOB) and SO₄-reducing bacteria (SRB) are present with populations up to 10⁷ and 10⁵ cells g⁻¹, respectively. Acidophilic S-oxidizing bacteria (aSOB) and iron-reducing bacteria (IRB) were generally less abundant. Primary influences on aqueous geochemistry are sulfide oxidation and carbonate dissolution at the tailings surface, gypsum precipitation–dissolution reactions, as well as Fe reduction below the zone of sulfide oxidation. Pore-water pH values generally ranged from 6.5 to 7.5 near the tailings surface, and from approximately 7–8 below the oxidation zone. Elevated concentrations of dissolved SO₄, S₂O₃, Fe, Zn, As, Sb and Tl persisted under these conditions.     

尾矿中硫化物风化氧化模拟实验研究

为防治矿山尾矿造成环境污染，对方铅矿，闪锌矿，磁黄铁矿、黄铜矿，黄铁矿进行了风化氧化实验研究，结果显示，硫化物的氧化速率顺序为：方铅矿＞闪锌矿＞磁黄铁矿＞黄铜矿＞黄铁矿，侵蚀液pH值越低，硫化物氧化速率越大，有机物存在对硫化物氧化起缓冲和抑制作用。     

金川镍矿尾矿砂中Ni和Cu赋存状态研究

金川镍矿浮选尾矿数量巨大,含有相当可观的有价金属,其中有价金属的回收受到越来越多的关注。金川老尾矿库尾矿砂中Ni和Cu赋存状态复杂,水溶性矿物态、可交换离子态、碳酸盐态、结晶度较差的Fe氧化物态、结晶度较好的Fe氧化物态、硫化物态和残渣态中都含有数量不等的有价金属Ni和Cu;尾矿砂风化作用释放的金属阳离子大多数在发生氧化的硫化物位置原位发生水解沉淀形成次生矿物,有价金属Ni和Cu在尾矿库中没有发生明显的富集。金川镍矿尾矿砂中Ni和Cu适于用化学酸溶浸出的方法进行二次回收。     

Influence of climate, mineralogy and mineral processing on the weathering behaviour within two, low-sulfide, high-carbonate, gold mine tailings in the Eastern Desert of Egypt

Despite its importance within environmental management strategies, little concern is shown to sulfide oxidation and/or hardpan formation at neutral pH where dry condition prevails. Two gold mine tailings in Egypt, El Sid and Barramiya, were studied for their geochemical/mineralogical properties, and climate influence on hardpan formation. The tailings are characterised by homogeneous silt-sized sediments (>42%), have high carbonate, predominantly as calcite for El Sid and dolomite-ankerite for Barramiya, and low-sulfide contents, chiefly as pyrite, galena and sphalerite for El Sid, and arsenopyrite–pyrite for Barramiya. El Sid is characterised by high average concentrations of Pb (2,758 mg/Kg) and Zn (2,314 mg/Kg), its lower part dominated by mafics, overlaid by granitoids. Barramiya has higher As (average 2,836 mg/Kg) content and represents a mixture of mica-schists/mafics-ultramafics. During field investigations, no hardpans were identified, only bassanite and gypsum were found at the surface of El Sid tailings, forming thin layers and desiccation crack fillings. Column experiments showed a thin crust consisting of gypsum, halite and sodium sulfate formed at the top of the column of El Sid tailings after 2 weeks, this was not recognized in the column from Barramiya. The homogenous thickened tailings deposition in both areas did not favour hardpan formation, since the critical amounts of reacting sulfides were never achieved in individual lamina, due to missing mineral/grain size fractionation. The high-temperature/low-water availability, characteristic for desert climate regions did not allow significant sulfides oxidation. Therefore, both tailings will suffer from continuous erosion and spreading out of contaminants to the environment for a prolonged period of time by sporadic flash floods.     

Element discharge from pyritic mine tailings at limited oxygen availability in column experiments

Sulfide mineral oxidation in mine tailings deposits poses a long term threat to surrounding ground water and surface waters. Soil or water cover remediation aims at reducing the rate of sulfide mineral oxidation by decreasing the O₂ ingress rate. In this study, the authors addressed the rate of sulfide oxidation and pH buffering in ∼33 months long, well-controlled laboratory studies of water saturated columns of sulfidic mine tailings from the Kristineberg site in Sweden at reduced O₂ availability. The element discharge rates slowly declined towards a quasi-steady state over hundreds of days. Non-reactive tracer tests showed an anomalously large dispersion, indicating strong flow heterogeneity, possibly including preferential flow and/or stagnant water zones. Congruent dissolution of pyrite and sphalerite by injected oxidants (dissolved O₂ and Fe(III)) adequately explained the discharge rate of Fe, S and Zn at quasi-steady state. Arsenic, Pb and Cu were partly retained in the tailings. Base cation discharge rates, and thus pH buffering, were apparently controlled by the rate of acidity production, with actual pH levels, available mineral surface area, and water residence times being of less importance.     

尾矿料的动力特性试验研究

通过对某铜矿的尾矿料进行动三轴和共振柱试验,研究了尾矿材料动力变形特性,提出了简单实用的孔隙水压力模型,给出了能更加准确地预测尾矿材料的动孔隙水压力的公式,并将其与Seed提出的预测公式进行了比较。在不同密度尾矿料的动三轴试验基础上,分析了相对密度对液化特性的影响,得出了相对密度小于70 %时抗液化强度随相对密度的增加而明显增加的结论。在不同围压下进行动三轴试验,结果表明：在相同的液化振次条件下,围压越高,动剪应力比越低。由共振柱试验可知,尾矿料的阻尼比随着动剪应变幅的增大而增大,而动剪模量随动剪应变幅的增大而减小,动剪模量和阻尼比与动剪应变幅的关系受围压影响不太敏感。     

砂柱排水真空阻滞效应的试验和动力学解释

含低渗透盖层的强透水层在排水过程中可能产生包气带真空并阻滞排水。这种真空阻滞效应具有什么动力学特征,是岩土渗流领域有待解决的新课题。以细砂作为粗砂的盖层,进行了双层结构砂柱的排水试验,观测到了包气带真空度先快速增大,然后缓慢减小的过程,排水速率显著低于无细砂覆盖的情况。细砂盖层厚度越大,真空阻滞效应越强烈。基于水-气渗流理论提出了解释试验现象的水流方程和气流方程,并得到了排水早期和后期真空度变化的近似解析公式,说明真空度峰值随盖层厚度呈非线性增加趋势。对试验结果进行参数分析,发现盖层透气性在排水后期明显大于早期,反映了盖层含水率对透气性的影响。     

矿山尾矿矿物学研究进展

矿山尾矿由于产生大量酸性排水和释放重金属，污染地下水和地表水，破坏生态环境而成为人们关注的焦点。尾矿引起的环境问题是地表条件下，水一气一矿物复杂反应的综合结果。近年来尾矿矿物学的研究已经揭示出尾矿中矿物分解和形成的规律，水一气一矿物反应的机制和影响因素，酸性排水和重金属迁移规律，为尾矿环境危险性评价和尾矿污染防治提供了基础资料和新的思路。