Bacterial catalytic processes for transformation of metals

Microorganisms actively participate in the transformation of metals and metalloids by various processes including adsorption, absorption, alkylation, oxidation and reduction reactions. Bacteria of the genera Thiobacillus and Sulfolobus have a chemolithotrophic mode of metabolism and catalyze various metal transformations. These bacteria are primarily involved in oxidation-reduction reactions of metals. Metal sulfides can be either directly oxidized by the microbes or oxidized by ferric iron, an end-product of microbial metabolism. These processes result in solubilization of iron, copper, molybdenum, uranium, and many other metals as well. Bacteria capable of these reactions are found, and they function in a variety of environments. Conditions which favor their growth are acidic pH (1.5 to 3.5) and temperatures which can be as high as 75 degrees C for Sulfolobus. The microbes have potential for mobilization of pollutants, or toxic elements, during resource extraction processes. In the absence of oxygen, iron, molybdenum and chromium may be reduced by microbes.     

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.     

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 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.     

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。     

氯离子对载金黄铁矿生物氧化的影响

研究了Cl-对载金黄铁矿生物氧化和生物氧化过程形成产物的影响。结果发现,0.01 mol/L Cl-对载金黄铁矿生物氧化具有显著的促进作用;而当Cl-浓度＞0.04 mol/L时,由于高浓度Cl-抑制了微生物的生长,进而抑制了载金黄铁矿的生物氧化。XRD分析结果表明,载金黄铁矿生物氧化过程中形成了单质硫,Cl-的加入有效消除了单质硫在矿物表面的累积。Cl-的促进作用主要是它可以消除阻碍载金黄铁矿生物氧化的单质硫在矿物表面的累积。     

Ecological Evaluation of Bacterial Leaching of Arsenous Gold Concentrates

Phase changes of arsenic in the process of biohydrometallurgical treatment of refractory arsenous gold concentrates, including bacterial leaching, sorption from cyanide solution using active carbons, and purification of sorption tailings by calcium hypochlorite were studied by physico-chemical methods.

It was found by X-ray crystal and X-ray spectrum analyses that in tailings from the process of purification of cyanide pulps, arsenic occurs mainly in the form of amorphous species of ferric sulphate arsenates, their composition depending on molar ratios of iron and arsenic. The presence of crystal phases of arsenic, mainly as scorodite, is observed. According to the results of sample surface study by X-ray photoelectronic spectroscopy, the base of amorphous species is presumed to be hydrated. Acid ferric arsenates and arsenic (III) composites were found in the cakes of bacterial leaching.

The washout of arsenic from the products of biohydrometallurgical treatment was studied. Experiments revealed that the migration of arsenic from the solid phase depends on molar ratios of iron and arsenic. It may be possible to reduce arsenic concentration in the liquid phase to 0.05 mg/1 by temperature treatment of arsenic products. The obtained results demonstrate the possibility to create an ecological acceptable technology for the treatment of refractory arsenous gold concentrates.     

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。     

pH对中度嗜热混合菌氧化载金黄铁矿生物氧化的影响

以载金黄铁矿为研究对象,研究pH值对中度嗜热混合菌氧化载金黄铁矿的影响.载金黄铁矿生物氧化过程中由于载金黄铁矿氧化溶解产酸使生物氧化体系不断下降,经过5 d搅拌氧化pH值已降至0.9左右,低于中度嗜热菌的最适生长pH值,抑制微生物的生长和载金黄铁矿生物氧化.控制生物氧化体系的pH值至1.2和1.6使铁离子浓度从13.08 g/L增大至15.75 g/L和13.58 g/L, 控制载金黄铁矿生物氧化体系pH值至1.2显著促进载金黄铁矿生物氧化.XRD分析结果表明:中度嗜热菌氧化载金黄铁矿过程中不生成黄钾铁矾和单质硫等产物.      

(Bio)chemistry of bacterial leaching—direct vs. indirect bioleaching

Bioleaching of metal sulfides is effected by bacteria, like Thiobacillus ferrooxidans, Leptospirillum ferrooxidans, Sulfolobus/Acidianus, etc., via the (re)generation of iron(III) ions and sulfuric acid.According to the new integral model for bioleaching presented here, metal sulfides are degraded by a chemical attack of iron(III) ions and/or protons on the crystal lattice. The primary iron(III) ions are supplied by the bacterial extracellular polymeric substances, where they are complexed to glucuronic acid residues. The mechanism and chemistry of the degradation is determined by the mineral structure.The disulfides pyrite (FeS₂), molybdenite (MoS₂), and tungstenite (WS₂) are degraded via the main intermediate thiosulfate. Exclusively iron(III) ions are the oxidizing agents for the dissolution. Thiosulfate is, consequently, degraded in a cyclic process to sulfate, with elemental sulfur being a side product. This explains, why only iron(II) ion-oxidizing bacteria are able to oxidize these metal sulfides.The metal sulfides galena (PbS), sphalerite (ZnS), chalcopyrite (CuFeS₂), hauerite (MnS₂), orpiment (As₂S₃), and realgar (As₄S₄) are degradable by iron(III) ion and proton attack. Consequently, the main intermediates are polysulfides and elemental sulfur (thiosulfate is only a by-product of further degradation steps). The dissolution proceeds via a H₂S*⁺-radical and polysulfides to elemental sulfur. Thus, these metal sulfides are degradable by all bacteria able to oxidize sulfur compounds (like T. thiooxidans, etc.). The kinetics of these processes are dependent on the concentration of the iron(III) ions and, in the latter case, on the solubility product of the metal sulfide.     

磨矿和浮选过程中黄铁矿电化学行为的研究进展

综述了黄铁矿在选矿过程中有关的电化学行为及工作机理,重点讨论了黄铁矿结构特性、溶液中氧化、金属离子作用和抑制剂对黄铁矿电化学行为的影响;此外,还讨论了磨矿过程中电偶相互作用、研磨介质形状、介质材料和研磨气氛对研磨中黄铁矿电化学行为的影响。其中黄铁矿晶体结构的不同对黄铁矿表面的氧化具有较大影响,从而间接的影响黄铁矿的可浮性,半导体性质对黄铁矿的导电率具有显著的影响;同时适度的氧化有利于黄铁矿的无捕收剂浮选,而强烈的还原电位或氧化电位会抑制黄铁矿的浮选;电位的增加,对铜活化黄铁矿有不利影响,主要原因是电位增加导致活化Cu+的浓度降低,同时黄铁矿表面被铁氧化物覆盖阻碍了铜离子的吸附。抑制剂的加入可以直接参与捕收剂与黄铁矿之间的氧化还原反应,从而抑制黄铁矿的浮选;同时磨矿介质及气氛条件的不同也会影响黄铁矿电化学行为。      

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...     

Studies on the effect of addition of silver ions on the direct oxidation of pyrite

The nature of the reaction between Ag⁺ and pyrite in 0.25 M H₂SO₄ solutions has been investigated in order to determine whether Ag⁺ can enhance the ferric sulfate leaching of this mineral. Analysis of reacted pyrite particles using scanning electron microscopy, X-ray photoelectron spectroscopy (XPS), and low-angle X-ray diffraction (XRD) indicates that elemental silver and elemental sulfur are the primary surface species formed by this interaction. Rest potential measurements of a pyrite electrode immersed in a solution containing 10⁻² M Ag⁺ are also consistent with what is expected for the deposition of metallic silver. Furthermore, the XRD data reveal that, at the most, only minor amounts of Ag₂S are being produced. The presence of Ag₂O has also been detected, but this is due to oxidation of silver after the experiment is complete and while the particles are being transferred for surface analysis. When 1 M ferric sulfate is contacted with pyrite which has been pretreated in a AgNO₃ solution, most of the silver immediately redissolves and does not redeposit while ferric ions are present. This indicates that the kinetics of the transfer reaction between Ag⁺ and pyrite is slower than the reaction between Fe³⁺ and pyrite and suggests that Ag⁺ does not likely enhance the ferric sulfate leaching.     

白云西矿某处含硫磁铁矿物的嵌布特性分析

文章对西矿某处含硫磁铁矿物进行了嵌布特性分析,结果表明,此含硫磁铁矿物含硫量达到2%以上,磁铁矿物中硫化铁主要包括黄铁矿与磁黄铁矿,并以黄铁矿为主;黄铁矿与磁铁矿嵌布关系复杂,是导致＂硫高难降＂的主要原因。     

氯化铁溶液浸出低冰镍提取镍铜的研究

利用三价铁离子(Fe3+)的氧化性,采用氯化铁溶液浸取低冰镍,提取其中的镍、铜元素。本研究考察了浸出液固比、浸出温度、浸出时间、氯化铁溶液浓度对镍和铜浸出率的影响。动力学研究表明:氯化铁溶液浸出低冰镍时,镍元素的浸出过程由化学反应控制,铜元素的浸出过程由混合反应控制,经计算镍的浸出活化能为70.26 kJ/mol、铜元素的浸出活化能为38.62 kJ/mol。低冰镍和浸出渣的物相分析结果表明,浸出反应发生时,低冰镍中的硫元素被氧化成单质硫。本研究避免了传统工艺中的含硫气体污染问题。      

载金硫化物焙烧-自浸出过程研究

针对传统氧化焙烧-氰化浸金工艺环境污染严重的现状,采用焙烧-自浸出工艺提取载金硫化物中的金.研究焙烧温度、焙烧时间和试样量对单质硫转化率和金浸出率的影响,通过X射线衍射分析、扫描电镜观察、能谱分析等手段分析焙烧过程中载金硫化物中硫的物相转变规律.载金硫化物中黄铁矿发生热分解反应生成单质硫和磁黄铁矿,随焙烧温度的升高和焙烧时间的延长,黄铁矿的特征衍射峰强度逐渐减小直到消失,磁黄铁矿的特征衍射峰逐渐生成并增强,原本致密状的黄铁矿颗粒变得疏松多孔.50 g试样在氮气流量1 L·min-1、焙烧温度800℃、焙烧时间60 min的条件下,单质硫的转化率达到42.53%,金浸出率达到88.70%,实现载金硫化物的高效非氰浸出.      

碱性环境中黄铁矿表面反应机理研究

采用SEM、XPS检测手段和前线轨道理论计算,对碱性环境下的黄铁矿表面反应机理进行研究。结果表明,黄铁矿中的S、Fe、Cu原子由晶格跃迁到表面,并氧化形成硫酸盐、氢氧化物、单质S或S8。随着处理时间的增加,黄铁矿表面氧化产物明显增多,在处理4 h后,黄铁矿表面已经完全被氧化。黄铁矿中Cu原子进入晶格后,改变了HOMO轨道中Fe、S原子电子得失能力（反应活性）,但对LUMO轨道的原子性质变化影响不大。在氧化过程中,Fe、S原子更易与氧作用,其次为Cu原子。这也可以解释黄铁矿表面先出现铁、硫化合物,之后才发现有铜的产物的现象。     

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.     

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.     

还原剂与脱硫剂对硫酸渣直接还原焙烧同步脱硫的影响

研究了还原剂云南煤和脱硫剂SH对硫酸渣在直接还原焙烧过程中提铁降硫效果的影响.采用X射线衍射与扫描电镜方法分析了云南煤与脱硫剂SH的作用机理.结果表明:在高温还原气氛下,硫酸渣中的黄铁矿生成具有挥发性的气态单质硫和气态羰基硫、金属铁和非磁性的陨硫铁;硫酸渣中的赤铁矿和磁铁矿则被还原为金属铁;云南煤对硫酸渣在焙烧过程中的脱硫效果比较明显,但无法达到要求的指标;添加脱硫剂SH可以进一步降低还原铁中的硫,其机理是脱硫剂与硫酸渣中的黄铁矿在直接还原焙烧过程中反应生成金属铁和没有磁性的硫化钙,通过磨矿-磁选的方法将硫化钙与金属铁分离,从而达到脱硫目标.