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 for Oxidation Acceleration and Passivation of Sulfides in Waste Rock Piles to Reduce Contaminated Neutral Drainage Generation Potential

Contaminated neutral drainage (CND), generated when metals are leached and enough neutralizing minerals are present to keep a near-neutral pH, is a growing environmental concern related to mine waste management. There is a need to find ways to reduce CND generation from mine waste to avoid perpetual effluent treatment. Nickel concentrations at the Lac Tio mine effluent have been sporadically higher than provincial environmental standards. In this project, three different treatment solutions were applied to Lac Tio mine waste rock samples in 70 kg waste rock columns in an attempt to reduce nickel CND generation potential by forcing sulfide oxidation and/or passivation. Following the applications, kinetic tests were used to determine any reduction in sulfide oxidation and nickel leaching. Hydrogen peroxide and sodium silicate were the main products tested, alone and in combination with a sodium bicarbonate buffer. Buffered hydrogen peroxide showed the best potential to reduce CND generation, with nickel release reduced from 1.4 × 10–3 mg/kg/day (untreated) to 4.0 × 10–5 mg/kg/day (treated) and near-neutral pH values. This passivation treatment also reduced the sulfur release rate by more than 50%. Optical and scanning electron microscopy observations supported the laboratory results. To conclude, this research project proposes a new way to reduce the CND generation potential of low sulfide waste rock.     

Humidity cell tests for the prediction of acid rock drainage

This paper presents a study of various geochemical humidity-style weathering tests that were carried out on waste mine rock from Avoca, County Wicklow, Ireland. The aim of this paper is to present data that demonstrate some of the geochemical controls on weathering rates together with release rates from laboratory testwork. These data are used to determine the applicability of various interpretations of humidity cell data for prediction of acid rock drainage. Furthermore, within this context the paper offers opinion on common questions related to the use of such tests: should humidity cells be aerated? How long should the test be run for? Is pre-treatment of the samples required? Is inoculation of the samples with iron and sulfur oxidising microbes required? And should these tests really be considered to be accelerated weathering tests?     

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.     

Combined extraction of metals and production of Mg(OH)2 for CO2 sequestration from nickel mine ore and overburden

Carbon storage by mineralisation (CSM) is an option for CO₂ emissions mitigation that has been under intensive study at Åbo Akademi (ÅA), Finland. Magnesium from silicate rocks is extracted with recoverable ammonium salts at 250–500 °C. The extracted elements are separated by selective precipitation and Mg is subsequently carbonated in a pressurised fluidised bed. This work studies the applicability of the process for co-extraction of Ni, Cu, Fe and Mg from nickel ore and nickel mine waste rock (overburden). The rock samples were collected from the Hitura Nickel mine, Finland. The results show that it is possible to co-extract the listed elements and integrate CSM (by the ÅA process) with steelmaking and Ni production industries.     

Acid Mine Drainage at the Abandoned Kettara Mine (Morocco): 1. Environmental Characterization

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 have been deposited on the surface without concern for environmental issues. Tailings were stockpiled in a dyke and pond and in piles, over an area of about 16 ha, and have generated acid mine drainage (AMD) for more than 24 years. The mine waste and secondary precipitates from this mine were characterized using geochemical and mineralogical techniques. The Kettara wastes contain 1.6–14.5 wt% sulfur, mainly sulfide minerals (e.g., pyrrhotite, pyrite, chalcopyrite, galena, and sphalerite). The main gangue minerals were goethite, quartz, chlorite-serpentine, talc, muscovite, and albite. Carbonates occur at very low quantities (less than 1 wt%). The most abundant heavy metals were Cu, Zn, Cr, Pb, Co, As, Cd, and Ni. Acid–base accounting static test results showed that all the samples have low values of acid-neutralizing potential (NP) (0–9 kg CaCO3/t). The mine waste has high acid-producing potential (AP) (51–453 kg CaCO3/t). Abundant secondary mineralogy is present, consisting mainly of halotrichite, goethite, jarosite-hydroanion, hydroniumjarosite, starkeyite, gypsum, alunite, copiapite, butterite, and coquimbite. Hardpans, which can prevent water infiltration to fresh tailings beneath and thereby lessen the rate of sulfide reactivity, were observed during sampling of the fine tailings. Mineralogical analysis indicated that the cementitious phase of the hardpan is mainly goethite. The alteration observed in the tailings pond does not extend more than 5–15 cm.     

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.     

金矿石粉碎特性对半自磨-球磨功耗的影响研究

为了定量评价矿石性质对碎磨工艺的影响,对云南某金矿深部原生矿体坑采样和岩芯样进行了落重试验和磨矿功指数测定,将试验结果与数据库数据比较,可以看出,坑采样中等偏硬,岩芯样属较硬矿石。根据SMC试验和邦德球磨功指数试验结果,坑采样和岩芯样的粉碎特性参数A、b、ta以及邦德球磨功指数并没有太大的差异,但落重指数DWi的差异将会导致在前端粗碎机排矿口尺寸条件下坑采样的产能将高于岩芯样。     

花岗岩矿矿床地质特征及开采技术条件分析

研究了河南省南召县田家庄饰面用花岗岩矿的矿床地质特征及开采技术条件,分析了矿区地层、构造、岩浆岩、覆盖层及风化层地质特征,研究了矿体特征、矿石质量及其特征、断裂及节理和裂隙分布发育特征、矿石类型和品级、矿体围岩及夹石、矿床成因及找矿标志等矿床地质特征。通过水文地质、工程地质、环境地质调查工作,基本查明了矿区的水文地质、工程地质和环境地质条件,区内岩石类型简单,矿体及围岩主要为黑云母二长花岗岩,岩体呈块状结构,地质构造简单,不会形成岩体滑动,矿体与围岩工程地质条件稳定,不易发生矿山工程地质问题,工程地质条件为第二类简单型。矿石和废石不易分解出有害组分,但在矿山开采时会对原有地形地貌造成破坏,存在一定的不良地质灾害隐患。矿区地表附近无污染源,无放射性异常,环境地质条件中等。研究可作为矿山建设的依据。     

A critical review of acid rock drainage prediction methods and practices

Failure to accurately predict acid rock drainage (ARD) leads to long-term impacts on ecosystems and human health, in addition to substantial financial consequences and reputational damage to operators. Currently, a range of chemical static and kinetic tests are used to evaluate the acid producing nature of materials, from which risk assessments are prepared and waste classification schemes designed. However, these well-established tests and practices have inherent limitations, for example: (i) best-practice sampling is not pursued; (ii) risk assessments rely on limited static and kinetic test data, thus compromising the accuracy of resulting ARD block models; (iii) static tests are completed off-site and do not reflect actual field measurements; (iv) kinetic test data do not become available until later stages of mine development; (v) waste classification schemes generally categorise materials as only three types (i.e., PAF, NAF and UC) with other drainage forms (e.g., neutral metalliferous or saline) not considered; and (vi) conventional testing fails to consider that reactivity of waste is controlled by parameters other than chemistry (e.g., microbiology, type and occurrence of minerals, texture and hardness). Thus, accurate prediction is challenging because of the multifaceted processes leading to ARD. Hence, risk assessments need to consider mineralogical, textural and geometallurgical rock properties in addition to predictive geochemical test data. Instead, a new architecture of integrative, staged ARD testing should be pursued. Better ARD prediction must start with improving the definition of geoenvironmental models and waste units. Then, a range of low-cost and rapid tests for the screening of samples should be conducted on site prior to the performance of established tests and advanced analyses using state-of-the-art laboratories. Such an approach to ARD prediction would support more accurate and cost-effective waste management during operation, and ultimately less costly mine closure outcomes.     

回填废石层下放矿损贫预测及颗粒流数值分析

露天转地下开采,回填废石层在放矿过程中对矿石的损失和贫化有一定的影响。回填废石层下放矿损贫的主要影响因素有回填层厚度、废石的粒径和非均匀度3个方面,对不同试验者的实验数据重新处理,总结出了回填废石层下放矿贫化率和损失率的数学模型,并结合司家营铁矿Ⅲ采场的露天转地下工程进行了回填废石粒径与非均匀度的设计。通过颗粒流数值正交试验方法,分别从绝对因素和相对因素回归建立了贫化率λ、回填层厚度h、回填层粒径d、回填层粒径非均匀度K4个因素和损失率之间的关系,为类似条件下的回填层合理设计提供参考。     

淮南煤矸石山周边土壤中蚯蚓对重金属的富集特征

为研究蚯蚓对煤矸石周边土壤中重金属的富集作用,从不同堆积年限煤矸石山周边土壤采集蚯蚓,分析了蚯蚓中Zn,Pb,Cd,Ni,Cr,V,Cu与土壤中相应重金属浓度关系。结果表明：随煤矸石堆积时间的增加,蚯蚓体内Pb,Cd,Cu含量增加,Ni,Cr,V含量先增加再降低,而Zn含量变化规律不明显;蚯蚓体内Zn和Cd含量可较准确反映煤矸石周边土壤中重金属浓度,而蚯蚓体内Ni,Cr,V含量可在距离上显示与煤矸石山远近关系;蚯蚓只对Zn和Cd产生富集效应（富集系数大于1）,且对Cd富集作用最大。蚯蚓可作为煤矸石山周边土壤Cd和Zn的指示生物。     

硫化镍矿废石和尾矿中重金属排放特征与防治对策

硫化镍矿由于镍品位低、提取工艺复杂,形成大量的废石和尾矿。废石目前普遍在废石场堆存,尾矿粒度较细,一般排入尾矿库。废石和尾矿中含有的重金属元素,在堆放过程中由于淋滤作用,不仅可能污染地表水,而且可以通过水力联系发生污染转移。选取我国典型硫化镍矿采选企业有代表性的废石和尾矿样品进行重金属组分分析和浸出毒性实验分析,目标重金属包括五种毒性较高的重金属镉、铬、铅、砷、汞及矿石主金属镍、钴、铜,确定每种重金属的污染负荷率,分析得出废石和尾矿中重点防控的重金属为砷、镉、铬,主金属镍和铜也应重点防控。针对现有企业废石和尾矿处置过程中存在的问题,提出有针对性的防治措施,可为镍采选企业废石和尾矿的重金属污染监管提供依据。     

Laboratory Simulation of Groundwater Along Uranium-Mining-Affected Flow Paths Near the Grand Canyon,Arizona, USA

Mining of volumetrically small, but relatively enriched (average 0.6% U₃O₈) breccia pipe uranium (BPU) deposits near the Grand Canyon, Arizona, USA has the potential to affect groundwater and springs in the area. Such deposits also contain base metal sulfides that can oxidize to generate acid mine drainage and release trace metals. In this study, sequential batch experiments were conducted to simulate the geochemistry of local shallow groundwater that contacts BPU ore and then moves downgradient through sedimentary strata. The experiments simulated shallow groundwater in a carbonate aquifer followed by contact with BPU ore. The experiments subsequently simulated contact with sedimentary rocks and changing oxygen availability. Concentrations of several contaminants of potential concern became substantially elevated in the waters exposed to BPU ore, including As, Co, Ni, U, and Zn, and to a lesser extent, Mo. Of these, Co, Mo, Ni, and U were minimally attenuated by downgradient processes, whereas Zn was partially attenuated. Sb and Tl concentrations were more moderately elevated but also generally minimally attenuated. Although the mixture of elements is particular to these BPU ore deposits, sulfide oxidation in the ore and carbonate buffering of pH by sedimentary rocks generates patterns of water chemistry common in acid mine drainage settings. Ultimately, downgradient concentrations of elements sourced from BPU ore will also be strongly influenced by non-geochemical factors such as the quantities of water contacting BPU materials, heterogeneity of materials along flow paths, and mixing with waters that have not contacted BPU materials.

     

Mineralogical and Geochemical Characterization of Mine Tailings and Pb, Zn, and Cd Mobility in a Carbonate Setting (Northern Tunisia)

Millions of tonnes of Pb–Zn ore flotation tailings and waste rock have been discharged at sites in northern Tunisia without concern for environmental issues. The tailings are dominantly fine grained (<125 μm), with high porosity and permeability. The tailings were characterized to assess base metal (Pb, Zn, and Cd) mobility. The relatively low percentage of iron sulphide and the dominance of carbonates in the matrices of the tailings indicated that only neutral mine drainage is likely. Batch sequential testing showed that the calcium and sulphate, which are the major ionic species in solution, are derived mainly from the dissolution of gypsum and not from neutralization of acidity generated by pyrite oxidation. Yet, despite the carbonate setting, the resultant neutral to slightly alkaline pH, and prolonged weathering, the studied flotation tailings maintain their capacity to release contaminants, notably Zn and Cd, into the environment. The amount of Zn that dissolves (2,400 μg L^?1, on average), though significant, is below the background concentrations in the Mejerda River and the environmental norms established for surface waters. Pb concentrations come close to the standards, but only Cd (18 μg L^?1, on average) sometimes exceeds current river water concentrations and environmental standards.     

Prevention of acid drainage from gold mining in the western United States and impacts on water quality

Water quality regulations recently adopted by states in the western USA have increasingly stringent limitations on allowable changes in the quality of surface water and groundwater. The “nondegradation of water” provisions in state regulations require accurate predictions and control of the quantity and quality of acid mine water and strictly limit the entry of acid water into natural water systems. Long-term water quality impacts from mine waste rock, spent ore from heap leaching, tailings and open pits must be considered in design, operation and reclamation of proposed or expanded mining operations. Acid-base testing, humidity cells, column testing and shake flask tests have been used with mixed success to predict the extent of acid water production. The types and forms of sulfide minerals present, bacterial catalysis of the sulfide oxidation reaction and configuration of the reclaimed facilities are all important elements in accurately predicting acid mine drainage. A critical factor in prediction of acid mine impacts is a pathway and fate analysis which includes geochemical reactions with aquifer materials and dilution and dispersion of parameters in the leachate plume. Of particular concern is the production and transport of arsenic, metals and residual cyanide from mined areas. Evaluation of three major operating gold mines in the northwestern United States shows the relationship between production of acidic water, movement of this water in aquifers and impacts on groundwater and surface water. Column testing showed reduction in concentrations of most metals by 50 to over 90 percent during travel through aquifers. Clays and silt zones were very effective in adsorbing metals. Operational control of water/rock reactions and reclamation design can significantly reduce or eliminate acid drainage. Soil cover, revegetation and slope are the major components that limit long-term acid drainage and metal contamination of surface water and groundwater. Compliance with water quality limits can be achieved only by design and operation of mining facilities to minimize the formation of acidic waters.     

Assessment of Phosphate Limestone Wastes as a Component of a Store-and-Release Cover in a Semiarid Climate

In a semiarid climate where the annual precipitation is low and the evaporation rate is high, contaminated drainage production from mine tailings can be controlled by reducing water infiltration. Store-and-release covers that use capillary barrier effects can prevent water percolation by storage and evaporation (or evapotranspiration) during wet and dry climatic periods. In Morocco, sedimentary phosphate mines are located close to contaminated sites, which includes the abandoned Kettara mine. This mine site generates highly contaminated acid rock drainage (ARD) with negative impacts on its surrounding area. In order to validate if phosphate mine wastes can be used as cover material to reclaim the Kettara site, instrumented test columns were exposed to field conditions and tested for a period of one and a half years. Under natural conditions, more than 94 % of the total net infiltration (246.5 mm) was released to the atmosphere by evaporation. Preliminary tests showed that the studied scenarios can limit deep water infiltration even during extreme simulated rainfall (155 mm/d) and could be used to efficiently control contaminated drainage in a semiarid climate.     

Determining the weathering characteristics of a waste dump with field tests

Prediction of the relative rates of acid generation and neutralization is required to determine if and when acidic drainage will occur for mining waste. Results of laboratory kinetic tests are routinely used to predict the long-term weathering rates of a waste dump. More specifically, an estimate of the ‘time to acidity’ (lag time) is needed to predict if exposed waste rock will become acidic before underwater disposal. A meaningful prediction requires consideration of the differences between the conditions in the laboratory and an actual waste dump. Field tests, which are less commonly conducted, provide site-specific weathering conditions but return results that are difficult to interpret. The results from field tests were compared to those obtained from laboratory kinetic tests. The field rates of weathering were determined to be an order of magnitude greater than the adjusted laboratory results, which were corrected for both surface area and temperature. The difference between the field and laboratory weathering rates is believed to be caused by inadequate hydrogeological assumptions and deficiencies in the experimental protocols. Insights were gained into building and operating kinetic field tests, interpreting results and understanding the limitations and advantages of field test data for prediction purposes. Field test data were found to be very useful for predicting depletion rates of neutralization potential and for validating sulphide oxidation rates obtained from laboratory tests.     

Evaluation of Laboratory Kinetic Test Methods for Measuring Rates of Weathering

A study was conducted to compare laboratory kinetic test methods for predicting acid rock drainage rates of weathering. Five laboratory kinetic test protocols (standard humidity cells, non-aerated cells, tall cells, shaken cells and NP depletion columns) were evaluated by comparing sulfate release and NP depletion rates, and predicted time to acidity (defined as pH 6). Our tests indicate that the standard humidity cell creates an unnatural oxidizing environment due to its extreme wetting and drying cycles, and therefore produces erratic results. The non-aerated test cells likely create an oxidizing environment that more closely represents the natural conditions, producing more consistent results and therefore a better estimate of the sulfide oxidation rate. The practice of shaking cells to promote efficient rinsing of weathering products disturbs the oxidizing environment and may retard the oxidation rate. Accelerating NP depletion by the high addition of acidic water creates an unnatural leaching environment, producing results that are not consistent with those obtained from other testing protocols having more natural leaching environments.