Progress in bioleaching: part B: applications of microbial processes by the minerals industries

This review presents developments and applications in bioleaching and mineral biooxidation since publication of a previous mini review in 2003 (Olson et al. Appl Microbiol Biotechnol 63:249–257, 2003). There have been discoveries of newly identified acidophilic microorganisms that have unique characteristics for effective bioleaching of sulfidic ores and concentrates. Progress has been made in understanding and developing bioleaching of copper from primary copper sulfide minerals, chalcopyrite, covellite, and enargite. These developments point to low oxidation–reduction potential in concert with thermophilic bacteria and archaea as a potential key to the leaching of these minerals. On the commercial front, heap bioleaching of nickel has been commissioned, and the mineral biooxidation pretreatment of sulfidic-refractory gold concentrates is increasingly used on a global scale to enhance precious metal recovery. New and larger stirred-tank reactors have been constructed since the 2003 review article. One biooxidation–heap process for pretreatment of sulfidic-refractory gold ores was also commercialized. A novel reductive approach to bioleaching nickel laterite minerals has been proposed.     

Bacterial Oxidation of Refractory Sulfide Ores for Gold Recovery

Abstract

The microbiological leaching of refractory sulfide ores (pyrite, arsenopyrite) for recovery of gold is reviewed in this article. The underlying physiological, biochemical, and genetic fundamentals of the bacteria involved (Thiobacillus and Sulfolobus spp.) are complex and have yet to be elucidated in depth. The chemistry of acid and biological leaching of pyrite and arsenopyrite minerals is also complex, and many of the individual reactions are not known in detail. Bacterial leaching is discussed in relation to chemical speciation at acid pH values. Attempts to develop models for a better understanding of bioleaching processes are summarized. The importance of pH, redox potential, temperature, sulfur balance, and toxic metals is evaluated for optimizing conditions for bacterial activity. Gold is finely disseminated in refractory sulfide ores, thereby decreasing Au recoveries upon conventional cyanidation for gold dissolution. In the bioleaching process, bacteria remove the sulfide minerals by oxidative dissolution and thus expose Au to extraction with cyanide solution. Stirred tank reactors appear most suited for this biological leaching process. The overall oxidation of the sulfides is an important variable for gold recovery. Pilot- and commercial-scale bioleaching processes for gold-containing pyrite and arsenopyrite ores are reviewed. This application of mineral biotechnology competes favorably with pressure leaching and roasting processes, both of which are problematic and energy-intensive alternatives for pretreatment of auriferous pyrite/arsenopyrite ores.     

Industrial practice and the biology of leaching of metals from ores The 1997 Pan Labs Lecture

Biomining processes have been used successfully on a commercial scale for the recovery of metals, the most important of which are copper, uranium and gold. These processes are based on the activity of chemoautolithotrophic bacteria which are able to use either iron or sulfur as their energy source and which grow in highly acid conditions. In general, low-rate dump and heap leaching processes are used for copper recovery while the biooxidation of difficult-to-treat gold-bearing arsenopyrite ores is carried out commercially in highly aerated stirred tank reactors. Because of the high levels of bacterial activity required, limitations in the growth rate of the microorganisms which were not apparent in low-rate processes have become an important factor. A key to the commercialization of the gold-bearing arsenopyrite biooxidation process was the development of a rapidly-growing, arsenic-resistant bacterial consortium. The empirical technique of mutation and selection in a continuous-flow system was used to improve the ability of the bacteria to decompose the ore. This approach resulted in a dramatic initial enhancement in growth rate but a plateau in improvement of performance has been reached. Further advances will require a more direct approach based on an understanding of the underlying physiological mechanisms and an application of the tools of molecular biology. Considerable advances have been made in our understanding of the molecular biology of Thiobacillus ferrooxidans. However much less is known about the other biomining bacteria. Recent studies using 16S rRNA analysis techniques have indicated that T. ferrooxidans may play a smaller role in continuous flow stirred tank biomining processes than was previously thought. Received 20 November 1997/ Accepted in revised form 2 March 1998     

Recent advances in microbial mining

Microbial mining of copper sulphide ores, has been practiced on an industrial scale since the late 1950s. Since then, advances in microbial mining and the role of microorganisms involved in solubilization of metals have assumed commerical importance. The fact that bioleaching processes save energy, have a minimum pollution potential and are able to yield value-added by-products make these processes invaluable. The metal extraction processes using microorganisms, which are currently in active use, concern copper and uranium bioleaching. Biobeneficiation is also applied at an industrial scale for recovery of gold from arsenopyrites. The developments in these processes during the last 15 years, with particular reference to developing nations, are reviewed. Information gathered on molecular genetics of these microorganisms should lead to a better understanding and control of microbial leaching processes. Areas still needing research to sustain economic expansion of microbial mining techniques are indicated.The author is with the Agharkar Research Institute, Agarkar Road, Pune 411 004, India     

Bioleaching of copper from chalcopyrite ore by fungi

Microorganisms have been geologically active in mineral formation, mineral diagenesis and sedimentation via direct action of their enzymes or indirectly through chemical action of their metabolic products. This property of microorganisms is being harnessed during the recent years for extraction of metals from their ores, especially from low-grade ores. In the present study bioleaching of copper from its low-grade chalcopyrite ore using 26 isolates of acidophilic fungi is reported. Most of these fungal strains belonged to the genera Aspergillus, Penicillium and Rhizopus. The leaching experiments were conducted in Czepek Dox minimal medium containing 1% (100 mesh) ore with shaking at room temperature for 20 days. Out of these, 4 isolates exhibited significant bioleaching activities. Maximum leaching of copper (78 mg/L) was observed with Aspergillus flavus (DSF-8) and Aspergillus niger (DOF-1). Nutritional and environmental conditions for optimum bioleaching were standardized. Present study indicates the usefulness of acidophilic fungi in bioleaching of copper from its low-grade ores.     

Selective leaching of zinc from copper-zinc concentrate

Biooxidation of copper-zinc concentrate with the use of consortia of mesophilic and moderately thermophilic acidophilic chemolithotrophic microorganisms was studied. Pyrrhotite, sphalerite, and chalcopyrite were the main sulfide minerals of the concentrate. The possibility in principal of complete selective leaching of zinc from sulfide concentrate coupled with minimal recovery of copper (less than 20%) was demonstrated. Selective leaching of zinc could be caused by galvanic interactions between minerals of the concentrate during the biooxidation. The results can be used as the basis for the development of the technologies for production of grade copper concentrate not containing zinc from sulfide copper-zinc concentrate obtained from refractory ores.     

Acdidophilic thermophilic archaebacteria: Potential application for metals recovery

Abstract The acidophilic thermophilic archaebacteria Sulfolobus and Acidianus have the potential for applid use in the recovery of metal values from ores through the process of baterial leaching. These microbes readily adapt to the conditions of low pH and high concentrations of metals required for bacterial leaching. In addition, these archaebacteria can exist at high temperatures which can occur during the oxidation of metal sulfides in bioleaching reactors. The acidophilic of copper and molybdenum from chalcopyrite and molybdenite minerals, respectively. The microbes can also enhance the recovery of gold by oxidation of pyrite which occludes gold preventing recovery by standard metallurgical procedures. The ability of this group of microbes to facilitate metals recovery is yet to be developed on a commercial scale.     

Bacterial oxidation of propane

Abstract Much recent work in the field of biohydrometallurgy has been directed to the study of bio-oxidation of gold ores by acidophilic iron and sulfur oxidizing microorganisms. This work has been done worldwide and has resulted in several pilot plant and commercial scale operations for gold ore bio-oxidation. Bioleaching of gold by metabolic products of microorganisms has received less attention, but also offers opportunities for industrial application, especially if future regulations restrict the use of cyanide. This paper reviews recent progress in the use of microorganisms tooxidize the sulfidic matrix in refractory gold ores (bio-oxidation) and to solubilize elemental gold (bioleaching).     

Two-stage process of bacterial-chemical oxidation of refractory pyrite-arsenopyrite gold-bearing concentrate

A technology for tank biooxidation of refractory gold-bearing concentrate under variable temperature conditions has been improved: the temperature of the first of two stages was changed from 30°C to 34–36°C. Gold in this concentrate is mainly associated with sulfide minerals: arsenopyrite and pyrite, which underlies a low gold recovery (16.68%) as a result of cyanidation. To resolve the problem, an association of mesophilic acidophilic chemolithotrophic microorganisms and moderately thermophilic bacteria of the Sulfobacillus genus were used for the concentrate oxidation. The composition of the used microbial association was studied; it was shown that it depends upon temperature: at 42°C, the population of the mesophilic thiobacteria decreased, whereas that of thermophilic sulfobacilli enhanced as compared to 36°C. The accepted scheme of the process ensures a high extent of gold recovery (94.6%) within a short space of time for biooxidation (96 h).     

Thermophilic microorganisms in biomining

Biomining is an applied biotechnology for mineral processing and metal extraction from ores and concentrates. This alternative technology for recovering metals involves the hydrometallurgical processes known as bioleaching and biooxidation where the metal is directly solubilized or released from the matrix for further solubilization, respectively. Several commercial applications of biomining can be found around the world to recover mainly copper and gold but also other metals; most of them are operating at temperatures below 40–50 °C using mesophilic and moderate thermophilic microorganisms. Although biomining offers an economically viable and cleaner option, its share of the world´s production of metals has not grown as much as it was expected, mainly considering that due to environmental restrictions in many countries smelting and roasting technologies are being eliminated. The slow rate of biomining processes is for sure the main reason of their poor implementation. In this scenario the use of thermophiles could be advantageous because higher operational temperature would increase the rate of the process and in addition it would eliminate the energy input for cooling the system (bioleaching reactions are exothermic causing a serious temperature increase in bioreactors and inside heaps that adversely affects most of the mesophilic microorganisms) and it would decrease the passivation of mineral surfaces. In the last few years many thermophilic bacteria and archaea have been isolated, characterized, and even used for extracting metals. This paper reviews the current status of biomining using thermophiles, describes the main characteristics of thermophilic biominers and discusses the future for this biotechnology.     

Continuous Bioleaching of Chalcopyritic Concentrate at High Pulp Density

The main objective of the present study was to investigate the continuous bioleaching of chalcopyrite concentrate at a high pulp density by moderate thermophilic microorganisms. Using a flotation concentrate containing 46% chalcopyrite and 23% pyrite, bioleaching tests were carried out at a high pulp density (15%) and temperature of 47°C using a setup consisting of three continuous stirred tank bioreactors in series. A two-level full factorial design of experiments was used to assess the effects of residence time, particle size and acidity of the leaching solution on the copper recovery. From the results of these tests, we concluded that under the best process conditions (d₈₀ = 30 μm, T = 47°C, and acidity of 130 kg/ton) more than 54% of copper was extracted from the concentrate after 7 days. Also, the concentration of copper in the final solution was higher than 20 g/L.     

Reduction of arsenic content in a complex galena concentrate by <Emphasis Type="Italic">Acidithiobacillus ferrooxidans</Emphasis>

Background  

Bioleaching is a process that has been used in the past in mineral pretreatment of refractory sulfides, mainly in the gold, copper and uranium benefit. This technology has been proved to be cheaper, more efficient and environmentally friendly than roasting and high pressure moisture heating processes. So far the most studied microorganism in bioleaching is Acidithiobacillus ferrooxidans. There are a few studies about the benefit of metals of low value through bioleaching. From all of these, there are almost no studies dealing with complex minerals containing arsenopyrite (FeAsS). Reduction and/or elimination of arsenic in these ores increase their value and allows the exploitation of a vast variety of minerals that today are being underexploited.     

Leaching of pyrite-arsenopyrite concentrate in bioreactors during continuous cultivation of a thermoacidophilic microbial community

A community of thermoacidophilic chemolithotrophic microorganisms was shown to exhibit enhanced efficiency of leaching and biooxidation of the gold-bearing pyrite-arsenopyrite flotation concentrate in continuous mode of cultivation. Under the optimal values of growth parameters, the degree of oxidation of sulfide arsenic, iron, sulfur, and antimony in the line of three laboratory reactors (D = 0.004 h^?1) was 99.55, 98.87, 99.65, and 97.08%, respectively, while gold recovery from the solid biooxidation residue was 97.4%.     

Biooxidation of Gold-, Silver,and Antimony-Bearing Highly Refractory Polymetallic Sulfide Concentrates,and its Comparison with Abiotic Pretreatment Techniques

Effectiveness of different pure and mixed cultures of three moderately thermophilic, extremely acidophilic bacterial strains (Acidimicrobium ferrooxidans ICP, Sulfobacillus sibiricus N1, Acidithiobacillus caldus KU) were investigated for biooxidation of highly refractory polymetallic gold ore concentrates. Despite of its complex mineralogy and the presence of a mixture of potentially inhibitory metals and metalloids, the concentrate was readily dissolved in defined mixed cultures including both iron and sulfur oxidizers, releasing as much as 80% of soluble Fe and 61% of soluble As. Factors to affect microbial mineral dissolution efficiencies (i.e. microbial As(III) oxidation ability, formation of secondary mineral precipitation (e.g. jarosite, elemental sulfur, scorodite, anglesite), and microbial population dynamics during biooxidation) were studied, based on which roles of individual microbes and their synergistic interactions during biooxidation were discussed. Applying the biooxidation pretreatment using the most efficient mixed cultures containing all three strains significantly improved the recovery of both Au (from 1.1% to 86%) and Ag (from 3.2% to 87%). Finally, this study provides one of the very few available comparisons of the effectiveness of different pretreatment techniques for refractory gold ore concentrates: Compared with other abiotic pretreatment approaches (roasting, pressure oxidation, and alkali dissolution), biooxidation was shown to be one of the most effective options in terms of the recovery of Au and Ag.     

Genomics, metagenomics and proteomics in biomining microorganisms

The use of acidophilic, chemolithotrophic microorganisms capable of oxidizing iron and sulfur in industrial processes to recover metals from minerals containing copper, gold and uranium is a well established biotechnology with distinctive advantages over traditional mining. A consortium of different microorganisms participates in the oxidative reactions resulting in the extraction of dissolved metal values from ores. Considerable effort has been spent in the last years to understand the biochemistry of iron and sulfur compounds oxidation, bacteria-mineral interactions (chemotaxis, quorum sensing, adhesion, biofilm formation) and several adaptive responses allowing the microorganisms to survive in a bioleaching environment. All of these are considered key phenomena for understanding the process of biomining. The use of genomics, metagenomics and high throughput proteomics to study the global regulatory responses that the biomining community uses to adapt to their changing environment is just beginning to emerge in the last years. These powerful approaches are reviewed here since they offer the possibility of exciting new findings that will allow analyzing the community as a microbial system, determining the extent to which each of the individual participants contributes to the process, how they evolve in time to keep the conglomerate healthy and therefore efficient during the entire process of bioleaching.     

Manganese biomining: A review

Biomining comprises of processing and extraction of metal from their ores and concentrates using microbial techniques. Currently this is used by the mining industry to extract copper, uranium and gold from low grade ores but not for low grade manganese ore in industrial scale. The study of microbial genomes, metabolites and regulatory pathways provide novel insights to the metabolism of bioleaching microorganisms and their synergistic action during bioleaching operations. This will promote understanding of the universal regulatory responses that the biomining microbial community uses to adapt to their changing environment leading to high metal recovery. Possibility exists of findings ways to imitate the entire process during industrial manganese biomining endeavor. This paper reviews the current status of manganese biomining research operations around the world, identifies factors that drive the selection of biomining as a processing technology, describes challenges in exploiting these innovations, and concludes with a discussion of Mn biomining’s future.     

Characteristics and adaptability of iron- and sulfur-oxidizing microorganisms used for the recovery of metals from minerals and their concentrates

Microorganisms are used in large-scale heap or tank aeration processes for the commercial extraction of a variety of metals from their ores or concentrates. These include copper, cobalt, gold and, in the past, uranium. The metal solubilization processes are considered to be largely chemical with the microorganisms providing the chemicals and the space (exopolysaccharide layer) where the mineral dissolution reactions occur. Temperatures at which these processes are carried out can vary from ambient to 80°C and the types of organisms present depends to a large extent on the process temperature used. Irrespective of the operation temperature, biomining microbes have several characteristics in common. One shared characteristic is their ability to produce the ferric iron and sulfuric acid required to degrade the mineral and facilitate metal recovery. Other characteristics are their ability to grow autotrophically, their acid-tolerance and their inherent metal resistance or ability to acquire metal resistance. Although the microorganisms that drive the process have the above properties in common, biomining microbes usually occur in consortia in which cross-feeding may occur such that a combination of microbes including some with heterotrophic tendencies may contribute to the efficiency of the process. The remarkable adaptability of these organisms is assisted by several of the processes being continuous-flow systems that enable the continual selection of microorganisms that are more efficient at mineral degradation. Adaptability is also assisted by the processes being open and non-sterile thereby permitting new organisms to enter. This openness allows for the possibility of new genes that improve cell fitness to be selected from the horizontal gene pool. Characteristics that biomining microorganisms have in common and examples of their remarkable adaptability are described.     

Copper-resistant microorganisms and their role in the environment

Naturally occurring, copper-resistant microorganisms are common in the environment. This review discusses the mechanism of copper resistance, which is not yet clearly understood, though the copper resistance conferred by a plasmid pPT23D has been elucidated at the molecular level. The different applications of copper-resistant microorganisms are described, including commerical bloleaching of copper ores. The construction of novel bioleaching strains through recombinant DNA technology may be possible. Biosorbents, the non-living microbial blomass, have been successfully used for metal recovery operations and are also reviewed.     

Bioleaching of chalcopyrite concentrate by a moderately thermophilic culture in a stirred tank reactor

A mixed culture of moderately thermophilic microorganisms was enriched from acid mine drainage samples collected from several chalcopyrite mines in China. Such mixed culture can be used to effectively extract copper from chalcopyrite. Furthermore, after being adapted to gradually increased concentration of chalcopyrite concentrate, the tolerance of the mixed culture to chalcopyrite concentrate was brought up to 80 g/L. The effects of several leaching parameters on copper recovery in stirred tank reactor also had been investigated. The results of the investigation show that it was possible to achieve a copper extraction rate of 75% in 44 days at a pulp density of 8%. The leaching rate of chalcopyrite concentrate tended to increase with dissolved total iron concentration. At low pH ranges, more microscopic counts of microorganisms were found in the solution. Furthermore, the analysis of leached residues indicates that the passivation of chalcopyrite concentrate was mainly due to a mass of jarosite and PbSO(4) on the mineral surface, other than the elemental sulphur layer. The bacterial community composition was analyzed by using Amplified Ribosomal DNA Restriction Analysis. Two moderately thermophilic bacteria species were identified as Leptospirillum ferriphilum and Acidithiobacillus caldus with abundance of 67% and 33% in the bio-pulp, respectively.     

Non‐homogeneous biofilm modeling applied to bioleaching processes

A two‐dimensional non‐homogeneous biofilm model is proposed for the first time to study chemical and biochemical reactions at the microorganism scale applied to biological metal leaching from mineral ores. The spatial and temporal relation between these reactions, microorganism growth and the morphological changes of the biofilm caused by solid inorganic precipitate formation were studied using this model. The model considers diffusion limitations due to accumulation of inorganic particles over the mineral substratum, and allows the study of the effect of discrete phases on chemical and microbiological mineral solubilization. The particle‐based modeling strategy allowed representation of contact reactions between the microorganisms and the insoluble precipitates, such as those required for sulfur attack and solubilization. Time‐dependent simulations of chemical chalcopyrite leaching showed that chalcopyrite passivation occurs only when an impervious solid layer is formed on the mineral surface. This mineral layer hinders the diffusion of one kinetically determinant mineral‐attacking chemical species through a nearly irreversible chemical mechanism. Simulations with iron and sulfur oxidizing microorganisms revealed that chemolithoautotrophic biofilms are able to delay passivation onset by formation of corrosion pits and increase of the solid layer porosity through sulfur dissolution. The model results also show that the observed flat morphology of bioleaching biofilms is favored preferentially at low iron concentrations due to preferential growth at the biofilm edge on the surface of sulfur‐forming minerals. Flat biofilms can also be advantageous for chalcopyrite bioleaching because they tend to favor sulfur dissolution over iron oxidation. The adopted modeling strategy is of great interest for the numerical representation of heterogeneous biofilm systems including abiotic solid particles. Biotechnol. Bioeng. 2010;106: 660–676. © 2010 Wiley Periodicals, Inc.