Biooxidation of gold-bearing sulfide ore and subsequent biological treatment of cyanidation residues

The percolation biooxidation parameters of ore from the Bakyrchik deposit were studied. An investigation of the technological parameters (such as the concentration of leaching agents, irrigation intensity, and pauses at various stages of the leaching) revealed the optimal mode for precious metal extraction. The stages of the ore processing were biooxidation, gold extraction by cyanidation or thiosulfate leaching, and biological destruction of cyanide. The gold and silver recovery rates by cyanidation were 64.0 and 57.3%, respectively. The gold and silver recovery rates by thiosulfate leaching were 64.0 and 57.3%, respectively. Gold and silver recovery rates from unoxidized ore (control experiment) by cyanidation were 20.9 and 26.8%, respectively. Thiosulfate leaching of unoxidized ore allowed the extraction of 38.8 and 24.2% of the gold and silver, respectively. Cyanidation residues were treated with bacteria of the genus Alcaligenes in order to destruct cyanide.     

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.     

Process simulation and gate-to-gate life cycle assessment of hydrometallurgical refractory gold concentrate processing

Purpose

Currently, almost all cyanide-free gold leaching processes are still in the development stage. Proactively investigating their environmental impacts prior to commercialization is of utmost importance. In this study, a detailed refractory gold concentrate process simulation with mass and energy balance was built for state-of-the-art technology with (i) pressure oxidation followed by cyanidation and, compared to alternative cyanide-free technology, with (ii) pressure oxidation followed by halogen leaching. Subsequently, the simulated mass balance was used as life cycle inventory data in order to evaluate the environmental impacts of the predominant cyanidation process and a cyanide-free alternative.

Methods

The environmental indicators for each scenario are based on the mass balance produced with HSC Sim steady-state simulation. The simulated mass balances were evaluated to identify the challenges in used technologies. The HSC Sim software is compatible with the GaBi LCA software, where LCI data from HSC-Sim is directly exported to. The simulation produces a consistent life cycle inventory (LCI). In GaBi LCA software, the environmental indicators of global warming potential (GWP), acidification potential (AP), terrestrial eutrophication potential (EP), and water depletion (Water) are estimated.

Results and discussion

The life cycle assessment revealed that the GWP for cyanidation was 10.1 t CO₂-e/kg Au, whereas the halogen process indicated a slightly higher GWP of 12.6 t CO₂-e/kg Au. The difference is partially explained by the fact that the footprint is calculated against produced units of Au; total recovery by the halogen leaching route for gold was only 87.3%, whereas the cyanidation route could extract as much as 98.5% of gold. The addition of a second gold recovery unit to extract gold also from the washing water in the halogen process increased gold recovery up to 98.5%, decreasing the GWP of the halogen process to 11.5 t CO₂-e/kg Au. However, both evaluated halogen processing scenarios indicated a slightly higher global warming potential when compared to the dominating cyanidation technology.

Conclusions

The estimated environmental impacts predict that the development-stage cyanide-free process still has some challenges compared to cyanidation; as in the investigated scenarios, the environmental impacts were generally higher for halogen leaching. Further process improvements, for example in the form of decreased moisture in the feed for halide leaching, and the adaptation of in situ gold recovery practices in chloride leaching may give the cyanide-free processing options a competitive edge.

     

Biohydrometallurgical technology of a complex copper concentrate process

Leaching of sulfide-oxidized copper concentrate of the Udokan deposit ore with a copper content of 37.4% was studied. In the course of treatment in a sulfuric acid solution with pH 1.2, a copper leaching rate was 6.9 g/kg h for 22 h, which allowed extraction of 40.6% of copper. As a result of subsequent chemical leaching at 80 degrees C during 7 h with a solution of sulphate ferric iron obtained after bio-oxidation by an association of microorganisms, the rate of copper recovery was 52.7 g/kg h. The total copper recovery was 94.5% (over 29 h). Regeneration of the Fe3+ ions was carried out by an association of moderately thermophilic microorganisms, including bacteria of genus Sulfobacillus and archaea of genus Ferroplasma acidiphilum, at 1.0 g/l h at 40 degrees C in the presence of 3% solids obtained by chemical leaching of copper concentrate. A technological scheme of a complex copper concentrate process with the use of bacterial-chemical leaching is proposed.     

Biohydrometallurgical technology of copper recovery from a complex copper concentrate

Leaching of sulfide-oxidized copper concentrate of the Udokan deposit ore with a copper content of 37.4% was studied. In the course of treatment in a sulfuric acid solution with pH 1.2, a copper leaching rate was 6.9 g/kg h for 22 hours, which allowed extraction of 40.6% of copper. At subsequent chemical leaching at 80°C during 7 hours with a solution of ferric sulfate obtained after biooxidation by an association of micro-organisms, the rate of copper recovery was 52.7 g/kg h. The total copper recovery was 94.5% (over 29 hours). Regeneration of the Fe³⁺ ions was carried out by an association of moderately thermophilic microorganisms, including bacteria of genus Sulfobacillus and archaea Ferroplasma acidiphilum, at 1.0 g/L h at 40°C in the presence of 3% solids obtained by chemical leaching of copper concentrate. A flowsheet scheme of a complex copper concentrate process with the use of bacterial-chemical leaching is proposed.     

Life cycle energy use and CO<Subscript>2</Subscript> emissions of small-scale gold mining and refining processes in the Philippines

Purpose

Gold is one of the most significant metals in the world, with use in various sectors including the electronic, health, and fashion industries. The Philippines has the world’s third largest known Au deposits and is ranked 20th in global gold production. Of the country’s annual production, about 80% is from the small-scale gold mining (SSGM) sector. This work estimates the first location-specific life cycle energy use and CO₂ emissions of SSGM establishments in the Philippines.

Methods

Process-based LCA was used with functional unit of 100 g Au and observed data from 2010 to 2011 for mining, comminution, recovery, and refining. Four gold production paths were observed in the provinces of Benguet and Camarines Norte, namely, amalgamation, cyanidation with carbon-in-leach (CIL), cyanidation with leaching with zinc, and combination of amalgamation and cyanidation with CIL.

Results and discussion

It was estimated that 3–18 g of Au was extracted for every ton of ore within 57–159 man-hours from mining to refining. Energy use estimates ranged from 3501 to 67,325 MJ/100 g Au, while CO₂ emission estimates ranged from 398 to 5340 kg CO₂/100 g Au. The combination of amalgamation and cyanidation with CIL processes was the least energy and carbon intensive, while cyanidation with CIL process was the most intensive. Electricity use accounted for 95–100% of total emissions, except in cyanidation with CIL where kerosene accounts for 77% of the total. Since SSGMs contributed 80% of the 40 tons of Au produced in the Philippines in 2014, the SSGM energy use was estimated to be between 1120 and 21,544 TJ and the CO₂ emissions to be between 129 and 1726 ktons CO₂. Energy estimates are most sensitive to refining process yield and electrical equipment efficiency.

Conclusions

The estimated life cycle emissions rate for SSGM in the Philippines is lower than available estimates of large-scale mining. Notwithstanding, given the sector’s reliance on fossil fuels for its energy needs and the Philippines’ pledge to reduce its CO₂ footprint by 70% in 2030, every effort to mitigate energy use and CO₂ emission counts. Three main recommendations toward energy consumption and CO₂ emissions reduction in SSGMs are proposed: (1) policy to promote technologies that are energy-efficient and processes that maximize gold process yield, (2) effective Minahang Bayan (SSGM mining zone mandated by law) implementation to ensure use of higher-grade ores, and (3) adoption of renewable energy in Minahang Bayans to promote energy independence and mitigate CO₂ emissions.

     

The effects of Fe(II) and Fe(III) concentration and initial pH on microbial leaching of low-grade sphalerite ore in a column reactor

In this study the effects of initial concentration of Fe(II) and Fe(III) ions as well as initial pH on the bioleaching of a low-grade sphalerite ore in a leaching column over a period of 120 days with and without bacteria were investigated. Four different modifications of medium were used as column feed solutions to investigate the effects of initial concentration of Fe(II) and Fe(III) ions on zinc extraction. The experiments were carried out using a bench-scale, column leaching reactor, which was inoculated with mesophilic iron oxidizing bacteria, Acidithiobacillus ferrooxidans, initially isolated from the Sarcheshmeh chalcopyrite concentrate (Kerman, Iran). The effluent solutions were periodically analyzed for Zn, total Fe, Fe(II) and Fe(III) concentrations as well as pH values. Bacterial population was measured in the solution (free cells). Maximum zinc recovery in the column was achieved about 76% using medium free of initial ferrous ion and 11.4 g/L of ferric ion (medium 2) at pH 1.5. The extent of leaching of sphalerite ore with bacteria was significantly higher than that without bacteria (control) in the presence of ferrous ions. Fe(III) had a strong influence in zinc extraction, and did not adversely affect the growth of the bacteria population.     

Effect of Activated Carbon Addition on the Conventional and Electrochemical Bioleaching of Chalcopyrite Concentrates

The effect of activated carbon addition on the rate and efficiency of copper mobilization from Sarcheshmeh chalcopyrite concentrate was studied in the presence and absence of a mixed culture of moderately thermophilic microorganisms. Conventional leaching at a 10% (w/v) pulp density in 500-ml Erlenmeyer flasks on a rotary shaker at 150 rpm, and electrochemical bioleaching in a stirred bioreactor at an ORP (oxidation-reduction potential) range of 400 to 430 mV measured against a Ag/AgCl reference electrode. The bioreactor contained ore concentrate at a pulp density of 20%, which was stirred at 600 rpm. All experiments were conducted in the presence and absence of 3 g/L activated carbon, at initial pH 1.5, temperature 50°C, in Norris's nutrient medium with an addition of 0.02% (w/v) yeast extract. The results showed that the addition of activated carbon increased the rate and yield of copper extraction from the concentrate especially in the presence of bacteria. Final recovery after 20 days was 52% and 44% in the shake flask experiments with and without carbon addition, respectively. Enhanced rates of copper mobilization were achieved in the electrochemical bioleaching experiments in which copper was leached selectively relative to iron. Final copper recovery after 10 days was 85% and 77% in the presence and absence of activated carbon, respectively. The positive effect of activated carbon on copper extraction could be related to the galvanic interaction between the inert carbon as cathode and chalcopyrite as anode. The bacterial elimination of sulfur produced on the sulfide minerals during chemical leaching is assumed to intensify the galvanic interaction. It seems that maintaining the ORP at a low potential and efficient mixing improves the bacterial and chemical subsystems in the electro-bioreactor that accelerates the rate of copper mobilization from the concentrate.     

The Sociedad Minera Pudahuel bacterial thin-layer leaching process at Lo Aguirre

The thin-layer leaching process originally conceived and developed for leaching oxide ores has been successfully adapted to bacterial leaching of mixed and secondary sulphide ores. The process is currently being applied at the Socicdad Minera Pudahuel Lo Aguirre Plant. About 3000 ton of ore per day are being processed to produce 14000 ton of high-grade copper cathodes per year, in a closed circuit integrated with SX-EW. Changes in the soluble copper grade of the ore from about 1.8C4 to (I.6% have occurred during the last years, which have been compensated by an equivalent increase in the insoluble copper grade. In addition, ore from satellite ore bodies has resulted in acid consumption variations ranging from 611 to 120 kg H₂SO₄ per ton of ore. The main sulphide mineralogical species are chatcocite and bornite, with small amounts of chalcopyrite and covellite. An intensive research program in columns and large-scale heaps has been carried out to define the operating conditions which assure adequate bacterial growth and bacterial activity towards the sulphides. Agglomerated ores with 1.7 2.5% Cur and (I.3-0.6%: Cus, with the insoluble copper mainly present as chalcocite bornite, were leached at a flow rate of 0.2 1 min¹ mu² with a SX-Raffinate solution containing (in g^-1) 5–10 H₂SO₄, 2–4 Fe_T, 1–3 Fe⁺³, 0.5 Cu, as well as impurities resulting from a closed circuit operation. Copper recoveries of 75 85% Cu_T were obtained after 180–250 days of total leaching time, depending on the copper grade, the mineralogical composition, and the acid consumption of the ore. Important bacterial activity was detected. About 10 ³−10⁵ bacteria ml⁻¹ were measured in effluent solutions. Iron oxidation rates of 7–100μg Fe²⁺ h⁻¹ g⁻¹, measured from respirometric tests on agglomerated ore, suggest that an adsorbed biomass of about 10⁷-10⁸ bacteria g⁻¹ must also be present. Further applications of the bacterial thin-layer leaching process to Cerro Colorado and Qucbrada Blanca ores in North Chile are being studied.     

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.     

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).     

Mineralogical controls on the bacterial oxidation of refractory Barberton gold ores

Abstract: The effect of mineralogical characteristics of gold ore minerals on the nature of sulphide oxidation during a bacterial leaching process was investigated. Three different ore types from the South African goldmines were used, i.e. an arsenopyritic-pyritic ore (Sheba goldmine), a pyritic ore (Agnes goldmine) and a loellingitic-arsenopyritic ore (New Consort goldmine). Detailed mineralogical characterization of each ore was performed. Thereafter, polished sections of the sulphides were suspended in a bacterial leach pulp in an air-stirred vessel for various periods of time. The effect of bacterial oxidation on the sulphides was monitored. Different types of gold-bearing arsenopyrite exist, each type having its own characteristic behaviour during the bacterial oxidation process. The rate of oxidation is controlled by the amount of defects in the crystal structure, and the amount of defects is again controlled by the composition of the arsenopyrite crystal. The distribution of refractory gold in the sulphide minerals can be correlated with the presence of compositional zones and structural deviations. These same mineralogical features also control the sites and rates of bacterial oxidation. Thus. refractory gold occurs at sites which are preferentially leached by the bacteria. The rate of gold liberation from sulphides is therefore being enhanced during the early stages of bacterial oxidation. Defects in a crystal structure influence the rate of bio-oxidation, and can be related directly to the crystal structure of the sulphide mineral, the crystallographic orientation of the exposed surfaces, and differences in chemical compositional and mechanical deviations in the crytals. A combination of all of these mineralogical factors influences the bacterial oxidation process. To optimize and to understand the leaching of an individual ore it is important to establish its controlling factors.     

A mathematical model for the bacterial oxidation of a sulfide ore concentrate

The effect of dilution rate and feed solids concentration on the bacterial leaching of a pyrite/arsenopyrite ore concentrate was studied. A mathematical model was developed for the process based on the steady-state data collected over the range of dilution rates (20 to 110 h) and feed solids concentrations (6 to 18% w/v) studied. A modified Monod model with inhibition by arsenic was used to model bacterial ferrous ion oxidation rates. The model assumes that (i) pyrite and arsenopyrite leaching occurs solely by the action of ferric iron produced from the bacterial oxidation of ferrous iron and (ii) bacterial growth rates are proportional to ferrous ion oxidation rate. The equilibrium among the various ionic species present in the leach solution that are likely to have a significant effect on the bioleach process were included in the model. (c) 1994 John Wiley & Sons, Inc.     

Bacterial leaching of uranium ore from Figueira—PR, Brazil, at laboratory and pilot scale

Abstract: A bacterial leaching program was carried out in order to evaluate the potential of applying this process to leach uranium from the ore of Figueira-PR, Brazil. The experiments were carried out in shake flasks, column percolation (laboratory and scmipilot scale)and in heap leaching. In shake flasks and in column percolation experiments at laboratory scale, bacterial activity on Ihe ore was confirmed: approximately 60% of uranium was leached, against around 30% in sterilized controls. Colunm percolation experiments at semipilot scale and heap leaching (850 tons of ore) showed uranium extractions of approximately 50%. In both experiments, a complementary sulfuric acid attack, after the bacterial leaching phase, was neccssaw to reach this level of uranium extraction.     

Effect of temperature on the rate of oxidation of pyrrhotite-rich sulfide ore flotation concentrate and the structure of the acidophilic chemolithotrophic microbial community

Oxidation of flotation concentrate of a pyrrhotite-rich sulfide ore by acidophilic chemolithoautotrophic microbial communities at 35, 40, and 45°C was investigated. According to the physicochemical parameters of the liquid phase of the pulp, as well as the results of analysis of the solid residue after biooxidation and cyanidation, the community developed at 40°C exhibited the highest rate of oxidation. The degree of gold recovery at 35, 40, and 45°C was 89.34, 94.59, and 83.25%, respectively. At 40°C, the highest number of microbial cells (6.01 × 10⁹ cells/mL) was observed. While temperature had very little effect on the species composition of microbial communities (except for the absence of Leptospirillum ferriphilum at 35°C), the shares of individual species in the communities varied with temperature. Relatively high numbers of Sulfobacillus thermosulfidooxidans, the organism oxidizing iron and elemental sulfur at higher rates than other acidophilic chemolithotrophic species, were observed at 40°C.     

Thermoacidophilic microbial community oxidizing the gold-bearing flotation concentrate of a pyrite-arsenopyrite ore

An aboriginal community of thermophilic acidophilic chemolithotrophic microorganisms (ACM) was isolated from a sample of pyrite gold-bearing flotation concentrate at 45–47°C and pH 1.8–2.0. Compared to an experimental thermoacidophilic microbial consortium formed in the course of cultivation in parallel bioreactors, it had lower rates of iron leaching and oxidation, while its rate of sulfur oxidation was higher. A new thermophilic acidophilic microbial community was obtained by mutual enrichment with the microorganisms from the experimental and aboriginal communities during the oxidation of sulfide ore flotation concentrate at 47°C. The dominant bacteria of this new ACM community were Acidithiobacillus caldus (the most active sulfur oxidize) and Sulfobacillus thermotolerans (active oxidizer of both iron and sulfur), while iron-oxidizing archaea of the family Ferroplasmaceae and heterotrophic bacteria Alicyclobacillus tolerans were the minor components. The new ACM community showed promise for leaching/oxidation of sulfides from flotation concentrate at high pulp density (S : L = 1 : 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.     

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.     

Oxidation of sulfur-containing substrates by aboriginal and experimentally designed microbial communities

Aboriginal and experimental (constructed of pure microbial cultures) communities of acidophilic chemolithotrophs have been studied. The oxidation of elemental sulfur, sodium thiosulfate, and potassium tetrathionate as sole sources of energy has been monitored. The oxidation rate of the experimental community is higher as compared to the aboriginal community isolated from a flotation concentrate of pyrrhotine-containing pyrite-arsenopyrite gold-arsenic sulfide ore. The degree of oxidation of the mentioned S substrates amounts to 17.91, 68.30, and 93.94% for the experimental microbial community and to 10.71, 56.03, and 79.50% for the aboriginal community, respectively. The degree of oxidation of sulfur sulfide forms in the ore flotation concentrate is 59.15% by the aboriginal microbial community and 49.40% by the experimental microbial community. Despite a higher rate of oxidation of S substrates as a sole source of energy by the experimental microbial community, the aboriginal community oxidizes S substrates at a higher rate in the flotation concentrate of pyrrhotine-containing pyrite-arsenopyrite gold-arsenic sulfide ore, from which it was isolated. Bacterial-chemical oxidation of the flotation concentrate by the aboriginal microbial community allows for the extraction of an additional 32.3% of gold from sulfide minerals, which is by 5.7% larger compared to the yield obtained by the experimental microbial community.     

Continuous microbial leaching of a pyritic concentrate by Leptospirillum-like bacteria

Summary Continuous leaching of a pyritic flotation concentrate by mixed cultures of acidophilic bacteria was studied in a laboratory scale airlift reactor. Enrichment cultures adapted to the flotation concentrate contained Thiobacillus ferrooxidans and Thiobacillus thiooxidans. During the late stationary growth phase of these thiobacilli growth of Leptospirillum-like bacteria was observed, too. In discontinuous cultivation no significant influence of Leptospirillum-like bacteria on leaching rates could be detected. During continuous leaching at pH 1.5 Leptospirillum-like bacteria displaced Thiobacillus ferrooxidans. The iron leaching rate achieved by Leptospirillum-rich cultures was found to be up to 3.9 times higher than that by Leptospirillum-free cultures.