Bioleaching is a simple and effective technology for metal extraction from low-grade ores and mineral concentrates. Metal recovery from sulfide minerals is based on the activity of chemolithotrophic bacteria, mainly Thiobacillus ferrooxidans and T. thiooxidans, which convert insoluble metal sulfides into soluble metal sulfates. Non-sulfide ores and minerals can be treated by heterotrophic bacteria and by fungi. In these cases metal extraction is due to the production of organic acids and chelating and complexing compounds excreted into the environment. At present bioleaching is used essentially for the recovery of copper, uranium and gold, and the main techniques employed are heap, dump and in situ leaching. Tank leaching is practised for the treatment of refractory gold ores. Bioleaching has also some potential for metal recovery and detoxification of industrial waste products, sewage sludge and soil contaminated with heavy metals.

Bioleaching: metal solubilization by microorganisms

The acid-insoluble metal sulfides FeS2, MoS2, and WS2 are chemically attacked by iron(III) hexahydrate ions, generating thiosulfate, which is oxidized to sulfuric acid. Other metal sulfides are attacked by iron(III) ions and by protons, resulting in the formation of elemental sulfur via intermediary polysulfides. Sulfur is biooxidized to sulfuric acid. This explains leaching of metal sulfides by Thiobacillus thiooxidans.

Bacterial leaching of metal sulfides proceeds by two indirect mechanisms via thiosulfate or via polysulfides and sulfur

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

Aqueous Pyrite Oxidation and the Consequent Formation of Secondary Iron Minerals

Leaching bacteria such as Thiobacillus ferrooxidans attach to pyrite or sulfur by means of extracellular polymeric substances (EPS) (lipopolysaccharides). The primary attachment to pyrite at pH 2 is mediated by exopolymer-complexed iron(III) ions in an electrochemical interaction with the negatively charged pyrite surface. EPS from sulfur cells possess increased hydrophobic properties and do not attach to pyrite, indicating adaptability to the substrate or substratum.

Importance of Extracellular Polymeric Substances from Thiobacillus ferrooxidans for Bioleaching

Metallurgical applications of bacterial leaching and related microbiological phenomena

The effects of carbon dioxide-enriched air on the rate of zinc extraction during the microbiological leaching of a, high-grade zinc sulfide concentrate by Thiobacillns ferrooxidans have been studied. Under normal air-aeration conditions, the leach rate is limited initially by the availability of solid substrate surface area per unit volume of leach liquor, then by availability of carbon dioxide. If carbon dioxide-enriched air is supplied, along with excess substrate, the zinc extraction rate increases as the carbon dioxide content increases until some other, as yet unknown, factor becomes limiting.

The effect of carbon dioxide and particle surface area on the microbiological leaching of a zinc sulfide concentrate

The microbiological extraction of zinc from a high-grade zinc sulfide concentrate has been investigated, using a pure strain of Thiobacillus ferrooxidans. Conditions such as temperature, pH, pulp density, nutrient, concentration, and specific surface of solids have been studied in terms of their effects on zinc extraction rate and in some instances on final zinc concentration in solution. Where appropriate, optimum conditions for leaching have been specified.

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Microbiological leaching of a zinc sulfide concentrate

The microbiological oxidation of ferrous iron in batch and continuous systems has been investigated in relation to uranium extraction from a low-grade ore by Thiobacillus ferrooxidans. The influence of the parameters, agitation, and aeration on oxygen saturation concentration, rate of oxygen mass transfer, and rate of ferrous iron oxidation was demonstrated. The kinetic values, Vmax and K were determined using an adapted Monod equation for different dilution rates and initial concentrations of ferrous iron. The power requirements for initial leaching conditions were also calculated. Uranium extraction as high as 68% has been realized during nine days of treatment. Regrinding the leach residue and its subsequent leaching yielded 87% uranium solubilization.

Ferrous iron oxidation and uranium extraction by Thiobacillus ferrooxidans

Thiobacillus ferrooxidans, grown on either ferrous sulfate, lead sulfide concentrate, or chalcopyrite concentrate demonstrated oxygen uptake and CO2 fixation in the presence of ferrous sulfate, cha...

Arpad E. Torma

Papers

Metallurgical applications of bacterial leaching and related microbiological phenomena

The effect of carbon dioxide and particle surface area on the microbiological leaching of a zinc sulfide concentrate

Microbiological leaching of a zinc sulfide concentrate

Ferrous iron oxidation and uranium extraction by Thiobacillus ferrooxidans

Oxidation of metal sulfides by Thiobacillus ferrooxidans grown on different substrates.