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.     

某难选金矿的选矿试验研究与工艺流程改造

甘肃某金矿金矿物主要为自然金,金属矿物主要为黄铁矿、毒砂、褐铁矿、辉锑矿。金矿物中24.88%的金以次显微或微细包裹体形式赋存于黄铁矿、毒砂等硫化矿物中,10.87%的金以微细包裹体形式赋存于褐铁矿、锑铁矿及石英等脉石矿物中。针对该微细粒浸染含砷含锑的金矿石,通过详细的条件试验及不同流程结构的试验研究,采用浮选+中矿再磨氰化浸出的选矿工艺流程,得金精矿金品位50.21g/t,金总回收率78.81%,实现了较高的金回收率。      

砷污染土壤治理工艺进展

砷是自然界常见元素,环境中的砷源自包括含砷金属矿石的开采、焙烧、冶炼、化工、炼焦、火电、造纸、皮革等生产过程中排放的含砷烟尘、废水、废气、废渣,在冶金工业生产过程中,约有30%左右的砷进入水、气和土壤中,使附近土地受到严重的砷污染。砷污染土壤给人类生态环境造成巨大危害,已经成为全球性的环境问题。人们对清除土壤砷方法进行了大量研究,当前砷污染土壤治理工艺主要有:固化/稳定化、焙烧挥发、沉淀、萃取、淋洗、生物法等。本文对砷污染土壤的处理工艺进行了简单介绍。     

Hydrogeochemistry of the Getchell Underground Mine––Part 1: Mine Water Chemistry

The Getchell underground operations in Northern Nevada intersect groundwater associated with marble and hornfel lithologies and a sulfide bearing ore hosted within a 30-km long shear zone system. The deposit is classified as Carlin-type gold mineralization. A distinct feature of the mineralization is the high proportion of arsenic sulfides present in the ore and associated altered wallrock. This results in an intense arsenic enrichment, with some zones containing as much as 30% arsenic, and 0.5–2% arsenic throughout the mineralized envelope. Most of the groundwater in the area is well buffered by the calcareous host rocks and show a macrochemistry of Ca-Na-HCO₃. Along the shear zone and in zones within the hornfel host rock, the waters are less alkaline and more saline, and have a chemistry of Na-Mg-Ca-SO₄-HCO₃. This latter water type occurs in sulfide-bearing zones. Arsenic speciation analysis and theoretical predictions demonstrate that higher arsenic concentrations are associated with reducing conditions, with higher Na/Ca ratios, and with low concentrations of Fe. In these waters, As occurs as arsenite, along with trace concentrations of mono-methyl arsonic acid and di-methyl arsinic acid. Natural attenuation of As appears to occur along groundwater flow paths due to co-precipitation and adsorption onto hydrous ferric oxide particles. However, elevated As is still a notable feature of groundwater quality throughout the Kelly Creek basin. This elevated As occurs in bedrock groundwater during underground mine development, rather than in near-surface alluvium groundwater. Due to this and the protracted history of mining, it is not possible to define a true background value for water quality in the area other than acknowledging that bedrock groundwater is mineralized and has little association with seasonal recharge and water quality in the alluvium cover.     

Low pH cyanidation of gold

Lime is used in CIP/CIL slurries to increase the pH, which partially stops the formation of aqueous hydrogen cyanide. Hydrogen cyanide gas can be evolved directly from the pulp surface or purged from the leach tank in oxygen or air that is sparged into the tanks to provide oxygen for the dissolution of gold.Savings in lime consumption can be achieved by leaching at a reduced pH, especially in aqueous solutions high in magnesium such as to be found in the Kalgoorlie Goldfields in Western Australia, but a greater percentage of cyanide is then present as aqueous hydrogen cyanide. Another proposed procedure to reduce lime consumption is closed tank leaching where a reduced pH or the natural pH of the ore is used and hydrogen cyanide loss is prevented by the closed vessel.The dissolution rate of pure gold as a function of pH for constant total cyanide concentration has been determined. Dissolution was directly proportional to the concentration of the cyanide ion present, with the contribution to dissolution from aqueous hydrogen cyanide too low to be measured.Ore leached in high magnesium, hyper-saline, process water showed increased gold dissolution at low pH compared to the pure gold. Reducing pH caused more cyanide to be consumed by the pulp but did not significantly increase the amount lost to the atmosphere.     

Impact of recycling cyanide and its reaction products on upstream unit operations

A typical operations outcome to dealing with the environmental and economic issues associated with cyanide in gold plant tailings streams involves the recycle of cyanide and its associated reaction products. This recycle stream may take the form of return dam water or tailings thickener overflow to the mill, or a cyanide recovery stream to the leach from a SART (sulphidization–acidification–recycling–thickening) or AVR (acidification–volatilization–reneutralization) circuit. The chemical composition and cyanide speciation of these solutions may be relatively complex, containing various metal cyanide species, thiocyanate, cyanate and other breakdown products.Several of these species are reported to have a deleterious effect on upstream unit operations. For example, while cyanide is in common use as a depressant for pyrite in mineral flotation, ferrocyanide and thiocyanate ions have also been shown to exhibit a depressant action in some cases. Primary flotation recovery of gold can thus be compromised and may be responsible for significant lost revenue, particularly from high-grade refractory gold operations. Bioleaching plants tend to separate cyanide circuits to an independent water balance to ensure that no cyanide or thiocyanate returns to inflict dire consequences on the bacterial population. Tolerable levels of these ions are exceedingly low, creating a risk factor and constraining water balance flexibility. Milling-in-cyanide has been periodically mooted as a beneficial circuit modification, particularly in operations that have tailings thickening, SART or dolomitic ore from which significant levels of free and complexed cyanides and thiocyanates are present. Potential consequences include unwanted side reactions with freshly activated mineral surfaces within mills operating at elevated temperatures and significant cyanide losses under these conditions.The evidence for the effects of recycled cyanides and associated species on upstream gold processing circuits have to date not been assessed in a systematic fashion. This paper presents a review of the available literature and analysis of associated data in these areas. Common arguments from operators justifying certain process plant configurations, modifications and operating setpoints around recycle streams are examined. Impacts of cyanide recycle on milling, flotation and bioleaching are quantified in the context of exemplary case studies. The various literature and case study flowsheets provide a useful analysis of the application of recycle streams to operating plant flowsheets, often requiring creative or inelegant solutions. In the current work, plant operating data is analyzed to quantify some of these effects; moreover, a simulation model has been developed to further quantify and explain some of these effects and provide insight into the decision-making process for plant designers and operators alike. Recycles without water treatment or other means may raise thiocyanate and cyanide levels above site-specific tolerances for operability.     

砷的地球化学习性及尾矿源砷污染防治

致癌和致畸有毒元素砷的污染普遍存在于金属硫化物矿山开采中。当物理化学条件发生变化时,不同形态和价态的砷可发生转化,并在土壤和水体中富集而造成砷污染,常见砷化物毒砂在O₂和H₂O的作用下生成臭葱石,并在酸性条件下水解成砷酸,从而被废水淋滤迁移。根据砷污染机理,论证了从矿山尾矿的源头控制和预防砷污染的重要性及可行性;在砷污染防治方面,新矿山尾矿的水下存储、酸性中和及表面封存技术,已存在污染的尾矿的渗透反应栅技术和生物修复技术均较为成熟、切实可行。     

硫精矿焙砂提金及铜锌综合回收试验研究

针对云南某矿山硫精矿焙砂中铜、锌、硫、砷含量高及铁氧化物包裹金等问题,开展了常规氰化浸出和酸浸—氰化浸出两种不同方案回收金的试验研究.结果表明,常规氰化浸出金的浸出率为84.52％,酸浸—氰化浸出的酸浸过程铜、锌浸出率分别为40.25％、38.79％,后续金的浸出率为85.82％.综合比较,酸浸—氰化浸出工艺更优,比常...     

Cyanide and removal options from effluents in gold mining and metallurgical processes

Cyanide has been widely used as an essential raw material in several industries including textile, plastics, paints, photography, electroplating, agriculture, food, medicine and mining/metallurgy. Because of its high affinity for gold and silver, cyanide is able to selectively leach these metals from ores. Cyanide and cyanide compounds in wastewater streams are regulated. Residues and wastewater streams containing cyanide compounds have to be treated to reduce the concentration of total cyanide and free cyanide below the regulated limits.Natural degradation reactions can render cyanide non-toxic, resulting in carbon dioxide and nitrogen compounds. These natural reactions have been utilised by the mining industry as the most common means of attenuating cyanide. However, the rate of natural degradation is largely dependent on environmental conditions and may not produce an effluent of desirable quality in all cases year round. Technologies that include chemical, biological, electrochemical and photochemical methods have been developed to remove cyanide and cyanide compounds to below the regulated limits in wastewaters. This paper discusses commercially available and emerging methods for removing cyanide from waste streams, particularly from tailings and tailings reclaim waters that are generated in the gold mining processes.     

Evaluation of artificial neural networks and kriging for the prediction of arsenic in Alaskan bedrock-derived stream sediments using gold concentration data

The detection of arsenic in sediments of placer gold mining areas is critical for planning future controls on migration and mitigation, or tapping uncontaminated groundwater resources for public water use. Arsenic (As) is often found to be collocated and correlated with gold in sediments. However, due to biogeochemical processes, arsenic can partition between the solid and the dissolved fractions in sediments and their interstitial waters. Such partitioning can mobilize arsenic into areas away from the co-located gold distribution in the sediments. In such cases, it is critical to detect the dispersed arsenic concentration. In this paper, neural network (NN) and kriging techniques were used to predict the presence of arsenic in the sediments of Circle City, Alaska using the gold concentration distribution within the sediments. The results obtained using kriging were more promising than those using NNs, albeit a statistically low correlation existed between the observed and the predicted arsenic concentrations. However, irrespective of the method used, the prediction of arsenic value without using gold concentration data was extremely poor.     

Processing of gold ores using Guinean bauxite waste

This paper describes the treatment of gold ores via the bauxite waste mud used as a pH modifier in the cyanidation of gold. The red mud is irrefutable in modifying the pH of gold ore sludges. So any gold contained in the red mud is concentrated by gravity separation and is recovered along with gold from the gold ore. Carbon tests are carried out on cyanide leach solutions to determine the level of deactivation resulted by organics in the bauxite waste mud.     

Improvement of cyanidation practice at Goldcorp Red Lake Mine

Goldcorp Red Lake Mine processes a mildly refractory high grade gold ore with 77.8 g/t Au and 1.9% arsenopyrite, 1.7% pyrite and 0.2% pyrrhotite at a grind of 77% −37 microns. A cyanidation study was conducted to determine the necessary retention time of the leach circuit and the optimal gold extraction and cyanide consumption. Results have shown that the gold leaching kinetics could be significantly improved by the addition 100–200 g/t lead nitrate directly in the grinding circuit, followed by a 6 h cyanidation. A 1 month trial indicated a throughput of 810 tpd can be sustained without detrimental effect on gold extraction. However, the mill is currently limited by the underground operation to a throughput ranging from 650 to 700 tpd for a 55 h retention time. In the event of future throughput increase over 810 tpd value, lead nitrate could become an essential part of the leaching efficiency. The experimental work indicated a gold extraction at 87.1–87.4% (tailings ∼9.8 g/t Au) which is similar to plant performance. The associated cyanide consumption was 0.7 kg/t. Gold dissolution can efficiently be conducted at an average cyanide concentration of 400 ppm. Previous control of cyanide concentration in the plant was 700 ppm in the first leach tank to 350 ppm NaCN at the end. Testwork realised in the mill allowed a 32% cyanide reduction predicted by the on-site cyanidation study. The new cyanide concentration is ranging between 500 ppm in the first leach tank and 300 ppm NaCN at the end of the leaching circuit. The cyanide consumption of the plant was reduced from 1.0 to 0.68 kg/t with reduction of effluent treatment costs. Grinding finer (91% −37 microns) slightly increased the gold extraction (tailings lower by 0.4 g/t) but lowers the leaching kinetics, increases cyanide consumption (35%) and the arsenic dissolution.     

A review of the selective leaching of gold from oxidised copper-gold ores with ammonia-cyanide and new insights for plant control and operation

The ammonia-cyanide leach system was first patented over 100 years ago and stands out as a unique method of selectively leaching up to 90% Au and <1% Cu from oxidised copper-gold ores using <10% NaCN used in conventional cyanidation processes. However, the system has proved to be difficult to control and predict performance with different ores due to a lack of understanding of the chemistry and mechanism and proper process control. Several laboratory studies have been carried out using various empirical concentrations of ammonia and cyanide with mixed success and only a few commercial operations have been successful.This paper reviews some of the recent applied and fundamental studies on the leaching of copper-gold ores with the ammonia-cyanide system and provides insights into the mechanism to give a better appreciation of the key parameters required for the optimum leaching of gold with minimum copper dissolution. Recommended leach compositions, Eh and pH are provided to enable process control measures to be adopted for a variety of ores. The selective recovery of gold from the leach solutions by cementation or adsorption onto activated carbon or ion-exchange resins is also discussed.     

广西某低品位金矿氰化浸出助浸剂实验研究

广西某低品位金矿石含金量为1.29 g/t,脉石矿物以石英为主,有色金属铜、铅、锌等及有害元素砷的含量极低。对该矿石进行氰化浸金实验研究,分别考查磨矿细度、溶液pH值、氰化物用量、搅拌转速、浸出时间对金浸出效果的影响;通过单矿物氰化助浸实验,确定多种助浸效果较好的助浸剂,并按同一比例混合,获得了三种新型助浸剂A、B、C;针对广西某低品位金矿石,进行氰化浸出助浸实验。结果表明,矿样细度-0.074 mm 93.27%,溶液pH值为10.5,氰化钾用量为4 kg/t,搅拌转速为1500 r/min,浸出时间为24 h的实验条件下,金的浸出率为92.58%;而氰化钾用量减少至3 kg/t,其余条件不变的情况下,加入新型助浸剂A浸出18 h后,金的浸出率可达93%。新型助浸剂的加入有效地提高了金的浸出率,同时将氰化物的损耗降低了25%,浸出时间缩短了6 h以上。     

两段石灰中和-洗涤-絮凝沉淀法脱除污酸中砷的研究

以高砷铜冶炼污酸废水为原料,采用两段石灰中和-洗涤-絮凝沉淀法处理,研究了该工艺除砷原理、影响因素、及效果,讨论了不同终了pH时溶液及渣中砷含量变化情况,洗涤对渣中砷含量变化的影响和絮凝剂PFSS对溶液深度除砷效果的影响。结果表明：一段中和后溶液中砷含量由13.69g/L降到12.90g/L,除砷率仅为5.77%,渣中砷含量为2.80%。用蒸馏水洗涤一段中和渣,液固比为10:1,洗涤5次后,渣中砷含量降至0.06%;用0.1mol/L的Na2CO3溶液洗涤一段中和渣,液固比为10:1,洗涤4次后,渣中砷含量降至0.05%。两者均可使中和渣由固体危废变为普通固废,减少危废排放量25%以上。当溶液pH中和到12.04时,砷的残留量仅为3.6mg/L,除砷率达99.97%,PFSS的滴加可使溶液中砷含量低于0.5mg/L,甚至低于0.01mg/L,出水砷含量满足排放标准。     

膜技术处理金矿氰化浸出溶液的应用

综述了处理金矿氰化浸出溶液的技术研究现状，分析现有技术的优缺点，介绍了电渗析法和液膜分离法在处理金矿氰化浸出溶液的应用情况，对该领域的发展现状及目前面临的主要问题作了简要的概述，并对未来的研究趋势进行了展望。     

甘肃某超大型金矿选矿试验研究

甘肃是西秦岭地区金矿床的主要集中区,新近又发现了一超大型金矿床。为了更好地利用该金矿资源,进行了详细地选矿试验研究。针对原矿中矿物种类复杂(金属矿物较少,非金属矿物含量较大)、嵌布粒度细小且不均匀的特性,通过全泥氰化法提金的工艺流程,获得了金浸出率85.13%的良好指标,因此推荐该方法作为矿床矿石的选矿工艺方法。     

柠檬酸助浸效果研究

研究了助浸剂柠檬酸的助浸效果及其作用机理。研究结果表明, 柠檬酸的分散、除杂及螯合作用有助于金的氰化浸出。氰化浸出时加入助浸剂柠檬酸可缩短浸出时间70%, 降低氰化物用量1/3。     

某含砷金精矿的焙烧氰化浸出工艺研究

采用化学物相分析法定量地研究了含砷金精矿及其焙砂中金的化学物相及其含量的变化，并对该精矿的焙烧及其氰化浸出过程进行了研究。介绍了焙烧温度和停留时间对该矿精的脱砷率和脱硫率以及浸出时间、氰化钠浓度、氧化钙浓度、液固比等对焙涛中氰化浸金率的影响。在最佳浸出条件下，其氰化浸金率可达到82％以上。     

A kinetic and electrochemical study of the ammonia cyanide process for leaching gold in solutions containing copper

One of the biggest challenges for the gold industry in the 21st century is the presence of copper in gold containing ore bodies. This is because copper consumes large quantities of cyanide. In addition, copper cyanide species are more stable than free cyanide, and hence are problematic in events of tailings spillage. One of the methods which has been suggested for treating copper containing ores is to leach with an ammonia cyanide solution. The effect of copper and ammonia addition on gold leaching kinetics was studied in the present paper. It will be shown that when the solutions do not contain copper, the addition of ammonia decreases the rate of gold leaching. When copper is added to solution, the leach rate does decrease due to the formation of the copper cyanide complexes. However it will be shown that under conditions of zero free cyanide, gold does leach readily via the Cu(CN)₃²⁻ complex. It was found that the addition of ammonia had little effect on the leaching of gold by Cu(CN)₃²⁻, but did increase the leaching kinetics when the major cyanide species present is Cu(CN)₂⁻. Under these conditions, leaching in the absence of ammonia is very slow. The effect of copper(II) addition was also studied, and it was found that in the absence of free cyanide the presence of copper(II) increases the leach rate, provided there is enough ammonia to stabilise it against reaction with the copper(I) cyanide complexes.