The Mechanism and Kinetics of Bioleaching Sulphide Minerals

Abstract

The measurement of oxygen and carbon dioxide consumption rates together with the measurement of redox potentials, has led to the further elucidation of the mechanism of bioleaching of sulphide minerals and enabled the kinetics of the sub-processes involved to be determined separately. This has shown that the primary attack of the sulphide mineral is a chemical ferric leach with the role of the bacteria to re-oxidise the ferrous iron formed back to the ferric form and maintain a high redox potential as well as oxidising the elemental sulphur which is formed in some cases. The kinetics of bacterial ferrous oxidation by Thiobacillus ferrooxidans and Leptospirillum ferrooxidans have been determined over a range of expected operating conditions. Also the chemical ferric leach kinetics of pyrite have been measured under conditions similar to those in bioleach systems. The kinetics have been described as functions of the ferric/ferrous-iron ratio or redox potential which enables the interactions of the two sub-processes to be linked at a particular redox potential through the rate of ferrous iron turn-over. The use of these models in predicting bioleach behaviour for pyrite presented and discussed. The model is able to predict which bacterial species will predominate at a particular redox potential in the presence of a mineral, and which mineral will be preferentially leached. The leach rate and steady state redox potential can be predicted from the bacterial to mineral ratio. The implications of this model on bioleach reactor design and operation are discussed.     

A Visual Insight into the Oxidation of Sulfide Minerals During Bioleaching and Chemical Leaching of a Complex Ore

A scanning electron microscope (SEM) study was performed to provide a visual insight into the oxidation patterns of sulfide minerals during chemical and bacterial leaching of a complex ore for 3 days. The mineral grains were studied under SEM before and after bacterial and chemical leaching with or without the addition of ferrous iron to generate ferric iron in situ by bacteria or chemical oxidant (MnO₂). Both mesophilic and moderately thermophilic cultures of bacteria were used in bioleaching tests. A limited oxidation of sphalerite and pyrite, similar to those in acid leaching (control), was observed to occur when no ferrous iron was added. However, the initial addition of ferrous iron into bioleaching media was shown to significantly improve the oxidation of sphalerite and pyrite. Galena was readily oxidized in the presence or absence of bacteria. Sphalerite was oxidized more extensively/selectively than chalcopyrite and pyrite, consistent with their respective nobility/electrochemical activity. Provided that chemical/biological oxidation of sphalerite was intensive, a sulfur-rich layer appeared to form on mineral surface. But, no such layer on pyrite surfaces was discernable. Supplementary bioleaching data were also provided to support SEM observations and to further elucidate the bioleaching characteristics of these sulfide phases. It can be inferred from this study that the oxidation of sulfides proceeds most discernibly via “indirect mechanism” and the generation of ferric iron by bacteria in sufficient quantity is essential for the effective oxidation of sulfide minerals.     

A review of rate equations proposed for microbial ferrous-iron oxidation with a view to application to heap bioleaching

In view of the fact that the microbial oxidation of ferrous iron to the ferric form is an essential sub-process in the bioleaching of sulphide minerals, the development of a comprehensive rate equation for this sub-process is critical. Such a rate equation is necessary for the design and modelling of both tank and heap bioleach systems.Most of the rate equations presented in the literature define the specific microbial growth rate using a Monod-type form for ferrous substrate limitation, with further terms added to account for ferric product inhibition, ferrous substrate limitation and inhibition. A few of the published rate equations describe the specific substrate utilization rate in terms of a modified Michaelis–Menten equation and include the maximum yield constant and cell maintenance via the Pirt equation. Other rate equations are based on chemiosmotic theory or an analogy with an electrochemical cell.In the present paper a selection of rate equations are compared against each other by calibrating them against the same set of data and comparing the fits. It was found that none fits the data particularly well and that some of the underlying assumptions need to be questioned. In particular, it appears that ferric inhibition is perhaps not as significant a factor than previously assumed and that rate control by the availability of ferrous is more significant.Some rate equations include terms to account for the effects of temperature, pH, biomass concentration, ionic strength as well as inhibition due to arsenic. In general these effects have been studied in isolation and in ranges not too far off the optimum. Few rate equations combine more than 2 effects and there is no clarity on how a comprehensive model to account for all effects should be constructed.Rate equations have been applied to tank bioleach systems, which usually operate under controlled conditions near the optimum. Heap bioleach systems, on the other hand, often operate far from optimum conditions with respect to temperature, pH, solution conditions, etc., at the same time. The kinetics of such sub-optimal systems are still poorly understood. Future studies should be directed towards the development of a comprehensive rate equation useful for describing the kinetics of heap bioleaching over a wide range of conditions.     

硫化矿细菌氧化浸出机理

硫化矿的细菌浸出技术已在黄金工业和提铜工业中获得广泛应用,浸出机理的研究也已有很大进展,然而,至今仍未提出一个普遍接受的过程机理认识。从化学、电化学和生物化学3 个方面综述了近年来对这一过程机理研究的主要成果,综合论述了硫氧化的2 种不同宏观化学历程,矿物与浸出液间、矿物与细菌间的电化学反应,细菌中参与反应的基本结构单元及电子转移途径,以期获得广泛关注和深入研究,为强化浸出和提高过程效率奠定理论基础。     

Mobilisation of metals in mineral tailings at the abandoned São Domingos copper mine (Portugal) by indigenous acidophilic bacteria

The composition of microbial populations of tailings waste at the abandoned São Domingos copper mine was investigated, and the abilities of pure and mixed cultures of indigenous microorganisms to leach metals from the tailings was examined. The indigenous acidophiles comprised Gram-negative sulfur-oxidising bacteria and Gram-positive bacteria that oxidised ferrous iron as well as sulfur. A mixed culture, obtained from the tailings by enrichment, was shown to leach virtually all the acid-extractable iron from the tailings when augmented with inorganic nutrients. In contrast, pure cultures of two bacteria from the tailings (a strain of Acidithiobacillus thiooxidans and an iron/sulfur-oxidising firmicute) were both ineffective at enhancing metal mobilisation from the mineral waste, in cultures amended with either inorganic or organic (yeast extract) nutrients. Mixed populations of isolates from the São Domingos tailings were more effective at leaching the residual sulfide minerals in the tailings, but were again less effective than the mixed enrichment culture. Changes in microbial populations during bioleaching of mineral tailings were determined using terminal restriction fragment length polymorphism (T-RFLP) analysis, and by plating of samples on selective solid media. Both techniques confirmed that At. thiooxidans was present throughout culture incubation but, although the other bacteria identified were iron/sulfur-oxidising firmicutes, the exact identity of these strains varied with the technique used. The significance of these data in terms of the potential for re-processing the metal rich tailings, either to recover metals or to limit the time frame of acid mine drainage genesis, is discussed.     

Raman characterization of secondary minerals formed during chalcopyrite leaching with Acidithiobacillus ferrooxidans

Chalcopyrite passivation greatly reduces the yields from leaching and bioleaching but the problem has not been successfully resolved. Passivation involves the formation of a layer of secondary minerals on chalcopyrite surface, which becomes a diffusion barrier to fluxes of reactants and products. This study aims to identify secondary minerals formed during chalcopyrite passivation in the presence of iron- and sulfur-oxidizing bacteria (Acidithiobacillus ferrooxidans) in mineral salts solution. The minerals were characterized with X-ray diffraction, Fourier transform-infrared spectroscopy, and Raman spectroscopy. Potassium jarosite was the initial product covering chalcopyrite grains, followed by the formation of ammonio-jarosite. Covellite and elemental sulfur were also detected in the passivation layer. The results suggest that passivation may be reduced by controlling jarosite precipitation and prior acclimatization of bacteria to oxidize CuS and elemental S in the presence of ferrous and ferric iron.     

Silicate mineral dissolution in the presence of acidophilic microorganisms: Implications for heap bioleaching

Silicate minerals are found with sulfide minerals and therefore, can be present during heap bioleaching for metal extraction. The weathering of silicate minerals by chemical and biological means is variable depending on the conditions and microorganisms tested. In low pH metal rich environments their dissolution can influence the solution chemistry by increasing pH, releasing toxic trace elements, and thickening of the leach liquor. The amenity of five silicate minerals to chemical and biological dissolution was tested in the presence of either ‘Ferroplasma acidarmanus’ Fer1 or Acidithiobacillus ferrooxidans with olivine and hornblende being the most and least amenable, respectively. A number of the silicates caused the pH of the leach liquor to increase including augite, biotite, hornblende, and olivine. For the silicate mineral olivine, the factors affecting magnesium dissolution included addition of microorganisms and Fe²⁺. XRD analysis identified secondary minerals in several of the experiments including jarosite from augite and hornblende when the medium contained Fe²⁺. Despite acidophiles preferentially attaching to sulfide minerals, the increase in iron coupled with very low Fe²⁺ concentrations present at the end of leaching during dissolution of biotite, olivine, hornblende, and microcline suggested that these minerals supported growth. Weathering of the tested silicates would affect heap bioleaching by increasing the pH with olivine, fluoride release from biotite, and production of jarosite during augite and hornblende dissolution that may have caused passivation. These data have increased knowledge of silicate weathering under bioleaching conditions and provided insights into the effects on solution chemistry during heap bioleaching.     

The Indirect Mechanism of Bacterial Leaching

Abstract

Mathematical models of the chemical leaching of a sulphide mineral and the bacterial oxidation of ferrous ions are combined in a mathematical model of bacterial leaching. It is shown that the models of the chemical leaching of sphalerite and the bacterial oxidation of ferrous ions are in excellent agreement with the experimental results. The indirect mechanism of bacterial leaching, which is a combination of these two sub-processes, is able to account for the shape of the reaction curve obtained from bacterial leaching experiments. It is also shown that even at very low concentrations of iron in solution the indirect mechanism may be the dominant pathway in bacterial leaching or sulphidic minerals.     

The preferential oxidation of orthorhombic sulfur during batch culture

The formation of sulfur is predicted by the current understanding of the mechanisms involved in mineral sulfide oxidation and observed in studies of the leaching products that accumulate on the surface of the mineral. Sulfur oxidising bacteria can exploit this energy source and can remove a potentially ‘rate-limiting’ diffusion barrier. In this study on the activity of sulfur oxidising bacteria cultured on mixed solid sulfur allotropes, it was observed that a heterogeneous culture preferentially oxidised the orthorhombic allotrope and no significant growth on the polymeric allotrope could be demonstrated.     

紫金山铜矿生物堆浸过程酸铁平衡实践与优化方向

紫金山铜矿低品位矿石采用生物堆浸—萃取—电积工艺产出阴极铜。矿石中主要铜矿物为蓝辉铜矿及铜蓝,同时含有较高含量的黄铁矿,耗酸脉石含量低。铜矿物浸出过程中,伴随着黄铁矿的氧化产酸产铁,造成堆浸系统溶液中酸铁浓度的不断累积,影响到浸出、萃取及环保处理工序,需要通过不断地中和来降低酸铁浓度。介绍了紫金山铜矿生物堆浸的技术特点,对生物堆浸过程中高酸高铁和低酸低铁两种工艺实践中酸铁平衡实践进行总结;结合紫金山铜矿矿石矿物学信息,进行酸平衡计算,确定了堆浸过程中黄铁矿氧化过程对酸铁平衡的影响;分析工艺条件对酸铁平衡的影响,并提出未来解决酸铁过剩的工艺优化方向。     

The role of iron and its compounds in processes of enrichment of sulfide ores of non-ferrous and noble metals

On the basis of the analysis of the literature data and the data of our investigations, the role of iron and products of its oxidation in sulfide pulps is shown. The technological factors are noted, such as the pH value of the medium, concentration of modifiers, mineral composition, and points and the order of batching flotation reagents; which affect the redox processes with the participation of iron and the characteristics of separation of sulfide minerals during flotation.     

锌的生物浸出技术现状及研究进展

锌是现代工业所必需的有色金属,属于很重要的战略资源,其在世界所有金属产量中排名第四,仅次于铁、铝和铜。随着低品位难处理锌资源的种类和产量的不断增加,以及湿法冶金技术的不断发展,锌的生物浸出技术得到了研究人员的广泛关注,并展示出了良好的潜在应用前景。本文首先较为详细的介绍了含锌资源的矿物特征,并对其生物可浸性进行了分析。其次,对目前锌的生物浸出体系,所用浸矿菌种,浸出过程所涉及的电化学、热力学、动力学以及浸出机理进行了归纳总结;接着,对锌的生物浸出技术现状和工艺新进展进行了阐述。最后,展望了锌的生物浸出工艺的发展趋势及后续的研究热点。研究表明高效浸锌菌种的选育驯化、与之相匹配的工艺及装备研发,是锌的生物浸出当今研究热点及未来发展方向。      

加压氧化浸出工艺的机理研究

加压浸出法具有流程短、砷浸出率高、浸出时间短及无SO2等有毒物质产生的优点,是预处理含硫、砷金矿石或金精矿的有效手段.在酸性介质中,硫化物、铁化合物与砷化物发生高温氧化的主要反应包括3种形式:硫化物全部被氧化成硫或硫酸盐,反应过程中产生的Fe2+被氧化成Fe3+,砷被氧化成砷酸盐.随着易处理矿石资源日益减少,加压浸出法...     

Copper recovery from chalcopyrite concentrates by the BRISA process

The technical viability of the BRISA process (Biolixiviación Rápida Indirecta con Separación de Acciones: Fast Indirect Bioleaching with Actions Separation) for the copper recovery from chalcopyrite concentrates has been proved. Two copper concentrates (with a copper content of 8.9 and 9.9 wt.%) with chalcopyrite as the dominant copper mineral have been leached with ferric sulphate at 12 g/L of ferric iron and pH 1.25 in agitated reactors using silver as a catalyst. Effects of temperature, amount of catalyst and catalyst addition time have been investigated. Small amounts of catalyst (from 0.5 to 2 mg Ag/g concentrate) were required to achieve high copper extractions (>95%) from concentrates at 70 °C and 8–10 h leaching. Liquors generated in the chemical leaching were biooxidized for ferrous iron oxidation and ferric regeneration with a mixed culture of ferrooxidant bacteria. No inhibition effect inherent in the liquor composition was detected. The silver added as a catalyst remained in the solid residue, and it was never detected in solution. The recovery of silver may be achieved by leaching the leach residue in an acid-brine medium with 200 g/L of NaCl and either hydrochloric or sulphuric acid, provided that elemental sulphur has been previously removed by steam hot filtration. The effect of variables such as temperature, NaCl concentration, type of acid and acidity–pulp density relationship on the silver extraction from an elemental sulphur-free residue has been examined. It is possible to obtain total recovery of the silver added as a catalyst plus 75% of the silver originally present in concentrate B (44 mg/kg) by leaching a leach residue with a 200 g/L NaCl–0.5 M H₂SO₄ medium at 90 °C and 10 wt.% of pulp density in two stages of 2 h each. The incorporation of silver catalysis to the BRISA process allows a technology based on bioleaching capable of processing chalcopyrite concentrates with rapid kinetics.     

Kinetics of Dissolution and Equilibrium in Solution of Complex Ores

The development of the kinetic expression for the dissolution of a nickeliferous sulfide by ferric sulfate and of pyrite by oxygen and ferric sulfate on the basis of its oxidation mechanism is discussed. The rate of dissolution of complex ores is determined not only by the kinetics of the heterogenous reactions (solid-liquid reactions) but also by the homogenous reactions taking place simultaneously in the leach liquor. The liquid phase in a leaching system contains a large number of species which are simultaneously reacting with one another such as in acid-base and complexation reactions at near equilibrium or at equilibrium. The heterogenous mineral solid-leaching solution reactions are limited by the kinetics of dissolution and often do not reach equilibrium. During the leaching process, the concentrations of chemical species in the liquid phase adjust rapidly to the changes in the liquid phase analytical concentrations. Chemical equilibrium is maintained in the homogenous phase although the mineral-leach solution reactions are far from equilibrium. If the reactions of the solid with the leach solution is considered to take place in very small increments, the pertubations to the analytical concentrations within the liquid phase can adjust quickly and thus remain at equilibrium as the leaching reaction proceeds. Following the changes in the liquid phase concentrations during the leaching step can be useful in optimizing the dissolution process and specifying the influent conditions to achieve optimum conditions. It may also be possible to specify the conditions under which selective leaching can be attained. The partial equilibrium model is capable of characterizing the minerals dissolution reactions and the associated changes in solution species concentrations. The modeling of the dissolution of chalcopyrite by ferric sulfate, ilmenite by hydrochloric acid, uranium dioxide-pyrite by ferric sulfate-sulfuric acid, and uraninite by ammonmium carbonate-hydrogen peroxide systems are discussed.     

The Dissolution of Sphalerite in Ferric Chloride Solutions

The kinetics of dissolution of both sintered sphalerite disks and untreated sphalerite particles in ferric chloride-hydrochloric acid solutions have been investigated. Over the temperature interval 25 to 100°C, the dissolution occurred according to a linear rate law and with an associated apparent activation energy of about 10 kcal/mole. Most of the oxidized sulfide ion reported as elemental sulfur in the leach residues. The leaching rate was independent of the disk rotation speed and this fact, together with various hydrodynamic calculations, indicated that the reaction was chemically controlled. The dissolution rate increased as the 0.36 power of the ferric chloride concentration and it also increased substantially in the presence of dissolved CuCl₂. The accumulation of the ferrous chloride reaction product severely retarded the leaching reaction, but the presence of dissolved zinc chloride only slightly impeded it. The leaching rate was relatively insensitive to low levels of HC1 (>1 M), but increased dramatically at higher acid concentrations because of direct acid attack of the ZnS.     

铁粉预还原—硫化亚铁脱除高酸性硫酸铁溶液中的砷

焙烧氰化法提金尾渣经酸浸后产出大量酸性硫酸铁溶液,因其中含有较高的砷而限制了其高值化利用。采用铁粉预还原—硫化亚铁脱砷对溶液中的砷进行脱除研究。结果表明,溶液中砷的存在形式及分布与溶液体系电位密切相关,铁粉可以有效降低溶液电位,经铁粉预还原后硫酸铁溶液中的砷可用硫化亚铁有效脱除。当铁粉添加量为溶液中铁含量的0.6倍,溶液加入36.6g/L的FeS,搅拌30min,可使溶液中砷含量由0.253g/L降低至4.79mg/L。空气对脱砷过程有不利影响。     

固定化细胞技术在生物浸矿中应用的可行性分析

根据生物冶金技术中金属硫化矿细菌氧化机理,分析了浸矿微生物固定化的必要性、优势和方法以及具体操作步骤等,提出了利用微生物菌种是生物浸矿的发展方向,具有巨大潜力.     

基于PHREEQC模拟嗜酸氧化亚铁硫杆菌浸铀研究

为模拟微生物浸铀过程中各组分的动态变化,通过将微生物浸铀动力学过程的3个反应和4个方程嵌入PHREEQC地球化学数据库拟合试验结果。结果表明:建立的基于PHREEQC软件的微生物浸铀动力学数据库能够模拟微生物浸铀的化学反应动力学过程,模拟结果分析发现,铀的浸出既受溶液中铁离子浓度的直接影响,又受微生物数量和亚铁离子浓度及氧化的间接影响。     

The Effect of Sulfur Concentration in Liquid Iron on Mineral Layer Formation During Coke Dissolution

The effects of sulfur concentration in liquid iron on mineral layer development between coke and iron as coke dissolves in a 2 mass pct carbon-iron liquid have been investigated at 1773 K (1500 °C). The initial sulfur in iron concentrations used ranged from 0.006 to 0.049 mass pct. Key findings include that the two-stage dissolution behavior exhibited in the carbon transfer from coke to iron, as reported in a previous study by the authors, at low initial sulfur in iron contents, was also apparent at the higher values used in this study. This two-stage behavior was attributed to a change in the mineral layer density as a result of changes in mineral morphology at the interface. In addition to confirming the two-stage behavior of the carbon-transfer kinetics at the higher sulfur concentration in iron levels, after a period of time, a solid calcium sulfide layer formed on the mineral layer. The sulfide layer formed after approximately 40 minutes, and the proportion of sulfide in the mineral layer increased with increased experimental time and initial sulfur concentration in iron. It was usually found at the iron side of the mineral layer and was associated with calcium-enriched calcium aluminates. Thermodynamic analysis of this layer confirmed that the sulfide is stabilized as the mineral layer is enriched by calcium.