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Mechanisms of Bacterial Leaching in Metal Recovery

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Abstract

Bacterial leaching of minerals is a simple, effective and environmental by benign technology in the treatment of sulphidic ores. This method has been successfully applied for the recovery of copper, gold and uranium in commercial scale for the past 25 years. Efficiency and cost-effectiveness of the bacterial leaching process depend mainly on the activity of bacteria and mineralogical and chemical composition of the ores. Bacterial leaching is based on the activity of mesophilic iron- and/or sulphur-oxidizing bacteria, notably Acidithiobacillus ferrooxidans, Acidithiobacillus thiooxidans and Leptospirillum ferrooxidans. These bacteria oxidize metal compounds to water soluble metal sulphates by a series of biological and chemical oxidation reactions occurring in leaching medium. After the isolation of above bacteria from acidic mine drainage waters, two oxidation mechanisms (direct and indirect bacterial leaching) have been discussed as related to oxidation/leaching of sulphidic ores in leaching systems. Fully understanding the bacterial leaching mechanisms of sulphidic ores improves the design and operation of bacterial leaching plants. In this article, the importance of various leaching mechanisms employed for metal recovery and their application aspects are critically reviewed with emphasis on copper, lead, zinc and nickel minerals.

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... Chemolithotrophic acidophiles with the capability of oxidizing iron and reduced sulphur compounds as well as surviving in an environment of high metal concentrations and acidity (Brierley 1978, Brierley 1982, Johnson 2001, Akcil and Ciftci 2006) play a vital role in the bioleaching of sulphide minerals. These microorganisms can grow autotrophically and/or mixotropically by obtaining their cellular carbon from atmospheric carbon dioxide and/or at least a proportion of it from organic compounds. ...
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... Chemolithotrophic acidophiles with the capability of oxidizing iron and reduced sulphur compounds as well as surviving in an environment of high metal concentrations and acidity (Brierley 1978, Brierley 1982, Johnson 2001, Akcil and Ciftci 2006) play a vital role in the bioleaching of sulphide minerals. These microorganisms can grow autotrophically and/or mixotropically by obtaining their cellular carbon from atmospheric carbon dioxide and/or at least a proportion of it from organic compounds. ...
... Bioleaching of sulphide minerals is inherently a complex process occurring through a set of oxidative chemical and biological reactions whereby mineral sulphide is oxidised and electrons are transferred from iron and/or sulphur moiety of the mineral to oxygen. The mode of contribution of bacteria to the oxidation of sulphides has been the controversy and until recently the direct and indirect mechanisms have been extensively discussed in the literature (Bosecker 1997, Fowler and Crundwell 1998, Suzuki 2001, Sand et al. 2001, Tributsch 2001, Crundwell 2003, Akcil and Ciftci 2006. Direct mechanism assumes that the oxidation of a sulphide mineral occurs directly by the enzymatic action of bacteria i.e. no intermediate products form (eq.3). ...
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... high metal concentrations and acidity (Brierley 1978, Brierley 1982, Johnson 2001, Akcil and Ciftci 2006) play a vital role in the bioleaching of sulphide minerals. These microorganisms can grow autotrophically and/or mixotropically by obtaining their cellular carbon from atmospheric carbon dioxide and/or at least a proportion of it from organic compounds. ...
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This study evaluates different bioleaching treatments of a molybdenite concentrate using mesophilic and thermophilic bacterial cultures. Further studies on the chemical leaching and the electrochemical behavior of the MoS2 concentrate were carried out. Bioleaching tests showed a progressive removal of chalcopyrite from the molybdenite concentrate with an increase in temperature. Chemical leaching tests support the idea of an indirect attack of the concentrate. Electrochemical tests indicate that chalcopyrite dissolution is favored when molybdenite is present. Therefore, this type of bioleaching treatment could be applied to purify molybdenite flotation concentrates by selectively dissolving chalcopyrite.
Article
It has been shown (a) that bacterial leaching of metal sulfides apparently requires the attachment of leach bacteria to metal sulfides, (b) that exopolymerbound iron compounds are responsible for or at least considerably increase the rate of the biological attack over the chemical rate, (c) that the primary attacking agent in leaching environments is the ferric iron hexahydrate ion, (c) that thiosulfate is the first intermediate sulfur compound, giving rise to a variety of other compounds including polythionate-containing periplasmic granula, and (d) that we have no idea about the actual concentrations of protons, ferrous/ferric and/or other cations, and sulfur compounds in the reaction space between the bacterium and the sulfide surface.
Chapter
The present article illustrates the increased interest which is manifested in the microorganisms, Thiobacillus ferrooxidans, involved in the biohydrometallurgical extraction processes. The wide varieties of problems currently studied are very important in order to gain a better understanding about the factors which are governing the growth of microorganisms, and as a consequence, the metal dissolution phenomena. In several mining sites, the microbiological leaching techniques are currently practiced at industrial-scale, especially for recovery of copper and uranium from low-grade materials. However, an accurate assessment of further potential possibilities for the application of microorganisms in leaching metal sulfides requires a more fundamental knowledge about the interactions of the physical and chemical factors with the growth of T. ferrooxidans in pure and mixed cultures including heterotrophic and thermophilic cohabitants. Altogether, the future industrial exploitation of these microbiological leaching techniques are very attractive in many countries of the world.
Article
The dissolution of metal sulfides is controlled by their solubility product and thus, the [H+] concentration of the solution, and further enhanced by several chemical mechanisms which lead to a disruption of sulfide chemical bonds. They include extraction of electrons and bond breaking by [Fe3+], extraction of sulfur by polysulfide and iron complexes forming reactants [Y+] and electrochemical dissolution by polarization of the sulfide [high Fe3+ concentration]. All these mechanisms have been exploited by sulfide and iron-oxidizing bacteria. Basically, the bacterial action is a catalytic one during which [H+], [Fe3+] and [Y+] are breaking chemical bonds and are recycled by the bacterial metabolism. While the cyclic bacterial oxidative action via [H+] and [Fe3+] can be called indirect, bacteria had difficulties harvesting chemical energy from an abundant sulfide such as FeS2, the electron exchange properties of which are governed by coordination chemical mechanisms (extraction of electrons does not lead to a disruption of chemical bonds but to an increase of the oxidation state of interfacial iron). Here, bacteria have evolved alternative strategies which require an extracellular polymeric layer for appropriately conditioned contact with the sulfide. Thiobacillus ferrooxidans cycles [Y+] across such a layer to disrupt FeS2 and Leptospirillum ferrooxidans accumulates [Fe3+] in it to depolarize FeS2 to a potential where electrochemical oxidation to sulfate occurs. Corrosion pits and high resolution electron microscopy leave no doubt that these mechanisms are strictly localized and depend on specific conditions which bacteria create. Nevertheless, they cannot be called ‘direct’ because the definition would require an enzymatic interaction between the bacterial membrane and the cell. Therefore, the term ‘contact’ leaching is proposed for this situation. In practice, multiple patterns of bacterial leaching coexist, including indirect leaching, contact leaching and a recently discovered cooperative (symbiotic) leaching where ‘contact’ leaching bacteria are feeding so wastefully that soluble and particulate sulfide species are supplied to bacteria in the surrounding electrolyte.
Article
The kinetics and rate limiting sub-processes in the bacterial oxidation of zinc sulphide with Thiobacillus ferrooxidans were examined. The oxidation rate of synthetic ZnS in the presence and absence of bacteria at equal ferrous and ferric iron concentration and pH, was measured in parallel batch experiments. Bacteria oxidize Fe2+ produced in the chemical oxidation of ZnS with Fe3+. In the fermenter without bacteria hydrogen peroxide was added to regenerate ferric iron and maintain the Fe2+ concentration equal to that in the fermenter with bacteria. No significant differences in the oxidation rates of zinc sulphide were found between the slurry with and without bacteria at equal Fe2+ and Fe3+ concentrations. Consequently, an indirect mechanism (i.e. chemical oxidation of ZnS with Fe3+ to Zn2+, S0 and Fe2+, and bacterial oxidation of S0 to SO42− and Fe2+ to Fe3+) determines the oxidation rate of ZnS in the bacterial oxidation process. From the experiments it was shown that T. ferrooxidans prefers S0 as a substrate and the regeneration of Fe3+ can even become terminated. From the experimental data several suggestions for improved process design and experimental research are given.
Article
Batch biooxidation data were obtained for a gold-bearing arsenopyrite-pyrite concentrate at 12% solids, pH of 1.6 and temperature of 40°C. The specific rates of biooxidation of arsenopyrite and pyrite were found to be very similar at 0,15 d−1 or as mass rates, for arsenopyrite 2,1 kg m−3 d−1 and pyrite 6,5 kg m−3 d−1. However, it was found that the biooxidation of arsenopyrite preceded that of pyrite and was almost complete before the oxidation of pyrite commenced.Data were also obtained for four narrow size fractions prepared from the bulk concentrate. Sequential oxidation of arsenopyrite and pyrite was observed for all size fractions. The oxidation rate obtained from the linear portion of the curve was found to have a linear dependence on surface area concentration suggesting that area-based rates are more appropriate for characterizing biooxidation kinetics.The logistic equation was found to be a good fit to all the data obtained. It was found that gold liberation was linearly dependent upon the extent of arsenic oxidation. However, sulphide oxidation appeared to be preferential in the gold-rich regions, leading to a non-linear dependence of gold liberation on extent of sulphide oxidation.
Article
The aim of the present investigation was to study the biooxidation of a refractory gold-bearing pyrrhotite, in order to increase the gold recovery during the subsequent conventional cyanidation.Bacterial cultures utilised in the biological test consisted predominantly of Thiobacillus genus. Tests were conducted at laboratory scale. The gold content of the ore sample, coming from Bolivia, was of 10 g t−1 Au.After 24 h leaching time by direct cyanidation, low gold recovery was obtained (<20% Au), with a high reagent consumption. On the other hand, a high gold recovery was achieved for the biooxidated samples: after 24 h cyanidation gold dissolution reached about 91% Au.Experimental results have shown the technical feasibility of the biooxidative pretreatment prior to conventional leaching and a complete circuit of treatment, on laboratory scale, has been developed considering also the subsequent gold recovery by carbon adsorption/desorption and electrowinning.A gold extraction yield of about 86% was determined in the whole process for gold extraction from pyrrhotite (biooxidation, solid–liquid separation, cyanidation, adsorption, desorption, electrowinning).
Article
Bacterial leaching of zinc from chat (chert) pile rock and copper from tailings pond sediment was studied in shake flask experiments. Thiobacillus ferrooxidans at 26°C leached 38% of zinc from pulverized rock in 15 days and extracted copper completely from native sediment in 24 h. Experiments omitting single medium constituents for the bacterial leaching of both zinc and copper indicated that the primary mechanism of bacterial leaching is very likely by the indirect route, in which the bacteria catalyze the ferric-ferrous redox cycle. The small particle size of the native tailings pond sediment (particles of ∼ 5 μm in size contributed almost all of the surface area) is a probable reason for the relatively short amount of time required for the copper leaching. Because of the rapidity of the extraction of copper with T. ferrooxidans, larger scale tests were carried out in a bioreactor with mechanical agitation and aeration. Complete extraction of copper was obtained in 12 h. A separate fluidization experiment showed that the tailings pond sediment could be kept suspended with a low fluid bulk velocity (0.3 cm/s). Two possible routes to scale up the bacterial leaching of copper from tailings pond sediment are discussed.
Article
A laboratory-scale method for treating a bulk concentrate (CuFeS2–PbS–ZnS) for metals recovery was developed utilising a combination of thermal process (roasting) and pressure leaching as an alternative to conventional pyrometallurgical processing. Pyrometallurgy is becoming less acceptable from environmental standpoints for the treatment of bulk concentrates. Additionally, high capital costs make modern facilities cost prohibitive. The leaching agents employed, namely, sulphuric acid (lixivant) and ferric sulphate, are selective for metal sulphides, this coupled with the fact that they create fewer environmental problems and are economical makes this new process highly favourable. In the laboratory evaluation of this process, the metal values in the flotation concentrates were selectively recovered by combining roasting and pressure leaching. The experimental parameters studied included roasting temperature, and pressure leaching pulp density, temperature, and retention time. Laboratory results indicate that roasting followed by pressure leaching is an efficient and cost effective method of treating base metal sulphide concentrates.
Article
In the present work the applicability of bioleaching using a mixed culture of mesophilic microorganisms (Thiobacillus ferrooxidans, Thiobacillus thiooxidans and Leptospirilum ferrooxidans) on a bulk concentrate of a Spanish complex sulphide ore was studied. The bulk concentrate mainly consisted of by chalcopyrite, sphalerite and pyrite. Effects of nutrient medium, stirring, pulp density, temperature and the addition of CO2 (1% v/v) to the air flow were also studied. The highest leaching rates and recoveries were obtained with mechanically stirred reactors at 5% pulp density and 9K medium. However, by using 9K medium higher jarosite precipitation was observed. Results showed that the optimum temperature for copper bioleaching was 30°C, whereas zinc dissolution increased with a rise in the temperature.
Article
The bioleaching of pyrite has been found to occur via an indirect mechansim. Ferric iron leaches the pyrite, and is reduced to ferrous iron. Bacteria such as Thiobacillus ferrooxidans oxidise the ferrous iron to ferric iron, thus maintaining a high redox potential. The effect of the redox potential on the ferric leach rate was investigated by developing an experimental technique where dynamic redox potential measurements were used to study the kinetics of the sub process.The ferric leach rate of pyrite was found to be of the order of 3 × 10−7 moles pyrite per mole pyrite per second, which is of the same order of magnitude as rates reported for the bioleaching of pyrite over similar ranges of redox potential. The rate decreased as the redox potential decreased, in what appeared to be a Butler-Volmer-like manner. This, along with the observation that there was no significant effect of the total iron concentration, suggested the likelihood of an electrochemical mechanism being operative, with charge transfer at the pyrite surface being rate limiting.
Article
Pyrite is one of the sulfide minerals most refractory to leaching and bioleaching because its dissolution kinetics are relatively slow. Bioleaching of coal pyrite does not appear to differ considerably from that of pyrite of igneous origin and is probably influenced by the semiconductor properties of the mineral. Several mesophilic, moderately thermophilic and extremely thermophilic microorganisms exhibit the ability to enhance pyrite oxidation and solubilization. However, the mesophiles seem to be the most suitable for a commercial process. On the bench scale, very encouraging results have been achieved with batchwise operation of conventional stirred tank or air-lift (Pachuca-type) bioreactors; however, these bioreactors have the disadvantage that they are unsuitable for processing pulps containing solids concentrations above ~20%. The enhancement of pyrite bioleaching rate, the development of more effective bioreactor types and downstream processing are the major topics that warrant further research. To supplement laboratory data adequately, a 50 kg h −1 pilot plant is being built at the Porto Torres (Sardinia) factory of EniChem-Anic, in the framework of a project supported by the Commission of European Communities, in which research institutions from Germany, Holland, Italy and the UK are taking part. The operation of this plant is expected to provide information on the technical and economic feasibility of a commercial coal biodepyritization process.
Article
In the history of Turkey the first use of cyanide for gold recovery has been at the Ovacik Gold Mine. During one-year test period, this mine has successfully been mining and processing after a complicated and extensive environmental impact procedure. In Turkey about 2500 ton of sodium cyanide are used with about 240 ton of sodium cyanide being used at this mine annually. During the test period, it has been shown that an effluent quality (CNWAD) between 0.06 ppm (min) and 1 ppm (max) was achievable after cyanide destruction with the Inco Process. It was also found that treated effluent values (CNWAD) of process water (decant) were between 0.04 ppm (min) and 0.59 ppm (max). This paper presents a review of the cyanidation and cyanide destruction processes at the Ovacik Gold Mine.
Article
Biooxidation of a fine-grained, complex zinc and gold-containing sulphide ore has been performed in a series of experiments at bench scale with 201 leaching volume in a series of three continuously stirred reactors. A mixed culture of moderate thermophilic bacteria was used for bioleaching at 45°C and a mixed culture of extreme thermophilic archea were used for bioleaching at 65°C.The leaching yields for zinc were in the range 80–87% with the moderate thermophilic bacteria and 96–98% with the extremely thermophilic microorganisms. It was found that, to obtain a high zinc recovery with a low degree of pyrite oxidation, a fine particle size was essential. Changes in retention time did not influence zinc solubilisation to any greater extent. Due to a high limestone content in the ore, the bioleaching was acid consuming. The acid consumption was strongly dependent on the throughput of ore in the leaching system. Recoveries of gold and silver, of ∼90% and 60–80%, respectively, after cyanidation of the bioleaching residue were obtained, irrespective of experimental conditions.
Article
The combination of roasting and pressure leaching is an alternative process that offers advantages over conventional processes because of the shorter leaching time and higher metal recovery. The copper and iron sulphide minerals examined in this study were chalcopyrite (CuFeS2) and pyrite (FeS2). The best results obtained were with a pre-treatment by roasting followed by acid pressure leaching in an autoclave system. The extraction of copper achieved was over 85%. Copper dissolution in this system is affected by particle size, leaching time and oxygen pressure. This paper presents the preliminary research on acid leaching of pyritic copper ore in an autoclave system under laboratory conditions.
Article
The effects of substrate type in the growth medium, mixing time of lignite into the growth medium and the biodesulfurization time on sulfur removal were studied. Biodesulfurization experiments were carried out with Mengen lignite under optimum growth conditions of Rhodococcus rhodochrous. The highest reduction of organic sulfur forms was 27.1% when sodium acetate was the substrate. Sulfate sulfur could be totally reduced when lignite was added to the culture medium 24 h after incubation. Compared with sodium acetate, glycerol yielded higher sulfate sulfur reduction rates when lignite was added at the time of incubation. The highest organic sulfur removal rates were found when sodium acetate was the substrate.
Article
The kinetics of leaching of sphalerite by the thermophilic Acidianus brierleyi were studied in a batch stirred reactor. Experiments were done at 65°C and pH 2.0 on the adsorption of A. brierleyi onto sphalerite and the bioleaching of sphalerite particles. The distribution of A. brierleyi cells between the mineral and solution was attained within the first 30 min of exposure to sphalerite, and the equilibrium distribution data were correlated with the Langmuir isotherm. The addition of 0.3 and 1.4 kg/m3 ferric iron to the A. brierleyi culture resulted in a significant decline in the leaching rates, probably because of the formation of iron precipitates such as jarosite. Rate data collected in iron-free leach solutions were analyzed to determine microbial kinetic and stoichiometric parameters for the growth of A. brierleyi on sphalerite. These growth parameters demonstrated that the rate of bioleaching with the thermophilic A. brierleyi is about seven times that with the common leaching mesophile, Thiobacillus ferrooxidans.
Article
Microorganisms are important in metal recovery from ores, particularly sulfide ores. Copper, zinc, gold, etc. can be recovered from sulfide ores by microbial leaching. Mineral solubilization is achieved both by ‘direct (contact) leaching’ by bacteria and by ‘indirect leaching’ by ferric iron (Fe3+) that is regenerated from ferrous iron (Fe2+) by bacterial oxidation. Thiobacillus ferrooxidans is the most studied organism in microbial leaching, but other iron- or sulfide/sulfur-oxidizing bacteria as well as archaea are potential microbial agents for metal leaching at high temperature or low pH environment. Oxidation of iron or sulfur can be selectively controlled leading to solubilization of desired metals leaving undesired metals (e.g., Fe) behind. Microbial contribution is obvious even in electrochemistry of galvanic interactions between minerals.
Article
The oxidation by Ferrobacillus ferrooxidans of untreated pyrite (FeS(2)) as well as HCl-pretreated pyrite (from which most of the acid-soluble iron species were removed) was studied manometrically. Oxygen uptake was linear during bacterial oxidation of untreated pyrite, whereas with HCl-pretreated pyrite both a decrease in oxygen uptake at 2 hr and nonlinear oxygen consumption were observed. Ferric sulfate added to HCl-pretreated pyrite restored approximately two-thirds of the decrease in total bacterial oxygen uptake and caused oxygen uptake to revert to nearly linear kinetics. Ferric sulfate also oxidized pyrite in the absence of bacteria and O(2); recovery of ferric and ferrous ions was in excellent agreement with the reaction Fe(2)(SO(4))(3) + FeS(2) = 3FeSO(4) + 2S, but the elemental sulfur produced was negligible. Neither H(2)S nor S(2)O(3) (2-) was a product of the reaction. It is probable that two mechanisms of bacterial pyrite oxidation operate concurrently: the direct contact mechanism which requires physical contact between bacteria and pyrite particles for biological pyrite oxidation, and the indirect contact mechanism according to which the bacteria oxidize ferrous ions to the ferric state, thereby regenerating the ferric ions required for chemical oxidation of pyrite.