Chemical and electrochemical basis of bioleaching processes

The bioleaching of sulfide minerals involves electrochemical and chemical reactions of the mineral with the leach liquor and the extra-cellular polysaccharide layers on the microorganisms. The microorganisms derive energy by oxidising the sulfur moiety and ferrous iron, which can be interpreted using electrochemistry and chemiosmotic theory. Recently, significant advances have been made in understanding the mechanism by which the bioleaching of sulfide minerals occurs. Kinetic models based on the proposed mechanism are being used successfully to predict the performance of continuous bioleach reactors. The measurement of oxygen and carbon dioxide consumption rates together with the measurement of redox potentials has led to this further elucidation of the mechanism of bioleaching of sulfide minerals and enabled the kinetics of the sub-processes involved to be determined separately. It has been shown that bioleaching involves at least three important sub-processes, viz., attack of the sulfide mineral, microbial oxidation of ferrous iron and some sulfur moiety. The overall process occurs via one of two pathways depending on the nature of the sulfide mineral, a pathway via thiosulfate resulting in sulfate being formed or a polythionate pathway resulting in the formation of elemental sulfur. For the case of pyrite, the primary attack of the sulfide mineral is a chemical ferric leach producing ferrous iron. The role of the bacteria is to re-oxidise the ferrous iron back to the ferric form and maintain a high redox potential as well as oxidising the elemental sulfur that is formed in some cases. The first two sub-processes of chemical ferric reaction with the mineral and bacterial oxidation of the ferrous iron are linked by the redox potential. The sub-processes are in equilibrium when the rate of iron turnover between the mineral and the bacteria is balanced. Rate equations based on redox potential or ferric/ferrous-iron ratio have been used to describe the kinetics of these sub-processes. The kinetics have been described as a function 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 is presented and discussed. The model is able to predict which bacterial species will predominate at a particular redox potential in the presence of a particular 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. Research on the chemistry and electrochemistry of the ferric leaching of sulfide minerals and an electrochemical mechanism for ferrous iron oxidation based on chemiosmotic theory will be presented and reviewed.     

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.     

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 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.     

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.     

A Microscopic study of the transformation of sphalerite particles during the roasting of zinc concentrate

The formation of zinc ferrite (ZnFe₂O₄) during the roasting of iron-bearing zinc concentrates requires substantial additional processing to recover the zinc from this compound by leaching and to eliminate the iron from the leachate. The phase changes that occur in the particles of a typical industrial zinc sulfide concentrate during roasting in a fluidized bed at 1223 K were investigated by the use of light microscopy, electron microprobe analysis, and SEM with EDS. The processes which the iron undergoes during its eventual transformation into ferrite have been clarified by examination of the phases and the morphology of partially roasted marmatitic sphalerite particles (Zn, Fe)S, and by reference to the known phase equilibria involved in the Zn-Fe-S-0 system. The oxidation of ironbearing sphalerite occurs in three stages. The first involves the selective diffusion of most of the iron to the particle surface resulting in the formation of an iron oxide shell enclosing a largely unreacted zinc sulfide kernel. In the second stage, this kernel is oxidized to form a solid solution of zinc oxide and iron oxide. The iron is initially present in the ferrous state but, with the disappearance of the sulfide kernel, is oxidized to ferric iron. In the final stage, this dissolved iron oxide and the iron oxide shell react with the surrounding zinc oxide to form the refractory spinel zinc ferrite.     

硫化矿细菌浸出机理及协同作用研究现状

细菌浸出作为一种经济、环保的处理技术,广泛应用于金属硫化物的金属提取和预处理过程中。为进一步理解细菌在硫化物浸出过程中的作用,详细阐述了细菌在浸出过程中的作用机理及其在矿物表面的作用机制,系统描述了金属硫化物溶解的硫代硫酸盐与多硫化合物途径。同时,还重点介绍了常见浸矿细菌的生存环境,并对不同种类浸矿细菌之间的协同作用进行了分析。     

A kinetic study on the pressure leaching of sphalerite

The dissolution of sphalerite (ZnS) in sulfuric acid solution under oxygen pressure was investigated. Effects of temperature, percent solids, agitation, sample size, oxygen partial pressure and foreign ions were evaluated. The effect of hydrogen pretreatment on sphalerite leaching rate was also examined. Leaching of sphalerite at 90°C and 150 psi oxygen pressure was found to occur at a constant rate. This rate was determined from the experimental data observed under the different leaching conditions mentioned above. The constant leaching rate was attributed to the chemical reaction occurring on the surface of the flat-plate type sphalerite sample. The rate-controlling step of the reaction was determined to be the oxidation of hydrogen sulfide to elemental sulfur. Oxidation of hydrogen sulfide was studied through the addition of iron and through the observation of the change in iron concentration during leaching. The oxidation was concluded to be by reaction with ferric ion rather than by direct oxygen oxidation. Leaching tests run with samples pretreated with hydrogen do not show any increase in the rate of zinc extraction. M. T. HEPWORTH, formerly with University of Denver.     

锑金精矿矿浆电解阳极区选择性氧化浸出研究

研究了锑金精矿矿浆电解过程中阳极区硫化矿物选择性氧化分解热力学基础。结果表明,黄铁矿标准氧化分解电位较辉锑矿低,从热力学角度看,黄铁矿会被优先氧化分解;当溶液中加入Cl~-以后,辉锑矿和黄铁矿氧化分解电位都呈现下降趋势,其中辉锑矿下降更为迅速;在研究条件下,当Cl~-浓度大于2.05mol/L时,辉锑矿的分解电位小于黄铁矿,在热力学上使得辉锑矿优先选择性氧化分解成为可能。矿浆电解法处理锑金精矿实验中,锑的浸出率可超过98%,而铁的浸出率在8%以下,可实现锑的选择性浸出。     

Towards a Novel Bioleaching Mechanism

Abstract

Research on the chemistry of pyrite bioleaching demonstrated-contradictory to the text book theory on direct or indirect leaching mechanisms-that only the indirect one is functioning. Cells of Thiobacillus ferrooxidans and Leptospirillum ferrooxidans primarily attach to the surface of pyrite particles by electrostatic interactions caused by iron(III) ion containing extracellular polymeric compounds. If sufficient free iron(III) ions are available (0.2 g/l) pyrite degradation starts. The first degradation products are thiosulfate and iron(II) ions. Thiosulfate will be rapidly oxidized by iron(lll) ions to tetrathionate. Tetrathionate adsorbs to the pyrite surface and is hydrolized to sulfane-monosulfonic acid and sulfate plus one proton. From the sulfane-monosulfonic acid several polythionales and elemental sulfur may arise. Consequently, the function of the leaching bacteria is “only” the maintenance of a high redox potential by keeping the iron(III) ions mainly in the oxidized state to optimize the indirect attack on the metal sulfide.     

微生物槽浸中铁的沉淀研究

以永平铜矿石为研究对象 ,通过摇瓶条件试验、有菌与无菌对比实验 ,研究了微生物槽浸中铁的沉淀。研究结果表明 :在菌液培养过程中 ,介质 pH值和二价铁浓度越低 ,铁沉淀量就越少 ,但会影响菌种的发育 ,在pH1 6～ 1 8、二价铁浓度为 4 g/L条件下 ,能减少铁的沉淀量和保证细菌较好的发育。在槽浸过程中 ,细菌产生的代谢物会促进铁沉淀的产生 ,影响细菌的浸矿效果。     

辉钼矿生物浸出研究进展

简要回顾了国内外钼矿生物浸出的发展历程,总结了钼矿生物浸出率低的原因。对钼矿生物浸出中的关键问题,即辉钼矿的可浸性、钼矿浸出的菌种、生物浸出的作用机理、钼离子对菌种生长的影响和沉淀对浸出的抑制作用作了探讨。此外,提出了浸矿菌种基因改良、多级生物反应器浸出和浸出体系溶液电位调控等辉钼矿生物高效浸出方法。     

黄铁矿促进黄铜矿微生物浸出影响因素

采用摇瓶实验,以氧化亚铁硫杆菌(Acidithiobacillus ferrooxidans,At.f)浸出黄铁矿-黄铜矿,重点研究了基础培养基、矿物配比和粒度组成等因素的影响.黄铁矿能促进黄铜矿的微生物浸出,以采用无Fe 9K培养基效果较好,它对应铜浸出率是9K培养基的1.68倍;采用宽粒级矿物时铜浸出效果较好,且铜浸出率与黄铁矿和黄铜矿的质量比有关,当质量比为2:2时铜浸出率最高可达45.58%;黄铁矿含量大小是影响铜浸出率高低的实质,当质量比小于等于5:2时以At.f菌的氧化作用为主,当质量比为10:2时以硫化矿间的原电池效应为主.浸渣的X射线衍射分析表明,采用无Fe 9K培养基时浸渣中生成的钝化物黄钾铁矾较少,故黄铁矿可以很好地替代9K培养基中的FeSO₄,并能与黄铜矿形成原电池效应,从而促进铜的浸出.      

The bioleaching of different sulfide concentrates using thermophilic bacteria

The bioleaching of different mineral sulfide concentrates with thermophilic bacteria (genusSulfolobus @#@) was studied. Since the use of this type of bacteria in leaching systems involves stirring and the control of temperature, the influence of the type of stirring and the pulp density on dissolution rates was studied in order to ascertain the optimum conditions for metal recovery. At low pulp densities, the dissolution kinetic was favored by pneumatic stirring, but for higher pulp densities, orbital stirring produced the best results. A comparative study of three differential concentrates, one mixed concentrate, and one global concentrate was made. Copper and iron extraction is directly influenced by bacterial activity, while zinc dissolution is basically due to an indirect mechanism that is activated in the presence of copper ions. Galvanic interactions between the different sulfides favors the selective bioleaching of some phases (sphalerite and chalcopyrite) and leads to high metal recovery rates. However, the formation of galvanic couples depends on the type of concentrate.     

A model of the dump leaching process that incorporates oxygen balance,heat balance,and air convection

A one dimensional, nonsteady-state model of the copper waste dump leaching process has been developed which incorporates both chemistry and physics. The model is based upon three equations relating oxygen balance, heat balance, and air convection. It assumes that the dump is composed of an aggregate of rock particles containing nonsulfide copper minerals and the sulfides, chalcopyrite and pyrite. Leaching occurs through chemical and diffusion controlled processes in which pyrite and chalcopyrite are oxidized by ferric ions in the lixiviant. Oxygen, the primary oxidant, is transported into the dump by means of air convection and oxidizes ferrous ion through bacterial catalysis. The heat generated by the oxidation of the sulfides promotes air convection. The model was used to simulate the leaching of copper from a small test dump, and excellent agreement with field measurements was obtained. The model predicts that the most important variables affecting copper recovery from the test dump are dump height, pyrite concentration, copper grade, and lixiviant application rate.     

砷化钴矿的生物浸出

为了从亚砷酸盐矿中回收钴,开发了二段工艺。在第一段中,二价铁离子溶液被氧化成三价铁离子溶液。该反应受到细菌的催化。在第二个反应器中,用三价铁离子溶液浸出矿石。该工艺主要为两个化学反应钴溶解;砷与铁以臭葱石形式沉淀。与常用的细菌浸出比较,这样的二段工艺的益处是两个主要反应能够分别被优化。 本文阐明了该工艺的化学原理、微生物的性质(适应范围)和试验的结果。     

Visualisation of pyrite leaching by selected thermophilic archaea: Nature of microorganism–ore interactions during bioleaching

In this study, pyrite (FeS₂) was leached by Acidianus brierleyi, Metallosphaera sedula and Sulfolobus metallicus during a 60 day experimental period. Leaching occurred over a redox potential range of 800 to 860 mV (S.H.E.) and in the presence of increasing Fe³⁺ levels. A modified ferrozine assay was developed to detect the increase of iron in solution as bioleaching of the ore progressed. For the first time, the interactions of these extreme thermophiles with the metal sulfide ore particles were extensively documented using SEM and TEM. As the pyrite degraded, there appeared to be a progression of deposited structures forming, ranging from sub-micron precipitates and disc-shaped structures on the ore's surface, which ultimately were similar for all leaching cultures. Furthermore, the residues resulting from the leaching of pyrite by M. sedula, the most active thermophile, were characterised using SEM/EDX, and appeared to be dominated by iron sulfate precipitates. The nature of the deposits formed, together with our other results, indicate that A. brierleyi, M. sedula and S. metallicus acted through the ‘contact’ and ‘non-contact’ sub-mechanisms of the indirect bioleaching mechanism for the dissolution of pyrite. The role of the bioleaching microorganisms is thus to maintain sufficient levels of Fe³⁺ and acid during pyrite leaching, for maximal mineral dissolution.     

矿石粒度和矿浆浓度对原生硫化铜矿细菌浸出的影响

通过摇瓶试验研究矿石粒度和矿浆浓度对原生硫化铜矿石中细菌浸出的影响,并初步探讨了浸矿过程中细菌的氧化活性及其对浸矿的影响。研究结果表明:在原生硫化铜矿石细菌浸出过程中,有利于铜浸出的矿石粒度和矿浆浓度分别是5 mm和20%~25%;溶液中三价铁含量过高或产生的铁沉淀都会直接影响细菌的氧化活性和浸矿效果。     

Bacterial leaching of low-grade ZnS concentrate using indigenous mesophilic and thermophilic strains

In this study low-grade sphalerite has been treated by the bioleaching process using native cultures of Acidithiobacillus ferrooxidans and Sulfobacillus thermosulfido-oxidans in order to determine the ability of these bacteria to the leaching of zinc. The effects of bacterial strain, pH, temperature, pulp density, iron precipitation, and initial concentration of ferric iron on the zinc leaching were evaluated. The bioleaching experiments were carried out in shake flasks at pH 1.5, 180 rpm, 33 °C and 60 °C for mesophilic and thermophilic bacteria, respectively. Compared with the use of laboratory reference strains or control conditions, zinc recovery from the respective concentrate was greater when native isolates were employed. The experimental data show that the selection of the suitable pH and temperature during the bioleaching processes would be important. The results indicate that the increase in pulp density generates a decrease in the dissolved zinc concentration. The maximum zinc extraction reached was 87% using native thermophile S. thermosulfido-oxidans culture after 30 days.     

Leaching kinetics and stoichiometry of pyrite oxidation from a pyrite–marcasite concentrate in acid ferric sulfate media

A pyrite concentrate with minor marcasite was oxidized in an acidic ferric sulfate medium at temperatures from 45 to 75 °C and at constant potentials corresponding to Fe(III) to Fe(II) ratios from 10 to 300. Potassium permanganate (KMnO₄) was found to be both a suitable oxidant for controlling the solution potential and a convenient and reliable indicator of leaching progress.The stoichiometry of pyrite oxidation was found to be essentially independent of temperature and only slightly dependent on solution potential over the range of conditions studied. Each unit of sulfide sulfur oxidized yielded 64 ± 2% sulfate, the rest elemental sulfur as discrete particles approximately 2 μm in diameter.The pyrite oxidation rate was very sensitive to the temperature, giving a large activation energy (83 kJ/mol), and was proportional to the Fe(III)/Fe(II) concentration ratio to the power of 0.57. The shrinking sphere model fitted very well the changing grain topology. A single mathematical expression combines the thermal, chemical, and topological functions to predict the pyrite conversion as a function of the known temperature, ferric concentration, ferrous concentration, particle size, and time. The model predictions are excellent over the range of conditions tested.