Biohydrometallurgy
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Biohydrometallurgy is a technique in the world of metallurgy that utilizes biological agents (bacteria) to recover metals. It is a subset or specialized form of hydrometallurgy, which refers to the use of aqueous solutions for metal extraction through a series of chemical reactions.[1][2] Bioleaching is closely related to biohydrometallurgy. It focuses on extraction or liberation of metals from their ores through the use of living organisms.[3]
Relative to traditional forms of metallurgy, biohydrometallurgy or bioleaching is slow but low cost.[4] These techniques are mainly applied to recovery of copper and gold from low-grade ores. These techniques have been proposed to the extraction of uranium, nickel, and other metals.[5]
Direct vs indirect leaching
Biohydrometallurgy can be productively implemented in two ways: direct and indirect leaching. Direct leaching entails physical contact between the ore and the microbe. The sulfide ore serves as an electron donor, which supplies energy to the organisms when coupled to the reduction of oxygen. Many sulfide ores are susceptible to direct leaching: covelite (CuS), chalcocite (Cu2S), galena (PbS), molybdenite (MoS2), and more. The energy producing conversion can be represented:
- MS + 2 O2 → MSO4
"Indirect leaching" requires no physical contact between the organism and the sulfide mineral. Here the bacterial produce Fe3+ (ferric) ions, which can be viewed as the lixiviant. Ferric ions attack the sulfide (usually) ore. The general equation for the solubilization is:
- MS + Fe2(SO4)3 + O2 → 2 FeSO4 +MSO4 + S
The bacterial then promote the reoxidation of ferrous by air and the oxidation of sulfur-containing products to sulfuric acid.
The sulfate salts are metal aquo complexes, not anhydrous as depicted. Similar reactions apply to the proposed leaching of nickel ions from pentlandite ores and uranium from UO2-containing ores.[3] The net reaction is:[6]
- FeS2 + O2 + 2 H2O → FeSO4 + 2 H2SO4
Applications
Biohydrometallurgy was first used more than 300 years ago to recover copper.[4][5][7][8] Early work on copper bioleaching was carried out at the mines of Chuquicamata and Lo Aguirre in Chile.[9]
Pyrite
Bioleaching from pyritic ores (pyrite, marcasite, arsenopyrite) utilize iron- and sulfur-oxidizing bacteria, including Acidithiobacillus ferrooxidans (formerly known as Thiobacillus ferrooxidans) and Acidithiobacillus thiooxidans (formerly known as Thiobacillus thiooxidans). There is no interest in obtaining iron salts from this kind of treatment. Rather, traces of precious metals such as gold may be liberated in the process since tiny particles of gold are often associated with pyrite.[10]
One method of bacterial leaching, also known as "Indirect leaching,Fe3+ ions are used to oxidize the pyrite. This step is entirely independent of microbes. The role of the bacteria is oxidation of the liberated ferrous ions. The bacteria require nutrients such as ammonium and phosphate.[3] The net reaction is:[6]
- FeS2 + O2 + 2 H2O → FeSO4 + 2 H2SO4
In terms of mechanism, an early step entails oxidation of pyrite to thiosulfate by ferric ion (Fe3+), which in turn is reduced to give ferrous ion (Fe2+). The ferrous ion is then oxidized by bacteria using oxygen:
- 4 FeSO4 + O2 + 2 H2SO4 → 2Fe2(SO4)3 + 2 H2O
Thiosulfate is also oxidized by bacteria to give sulfate:
- S2O2−3 + 2 O2 + H2O → 2HSO−4
Although depicted as anhydrous, ferrous and ferric sulfates exist in aqueoue solutions as aquo complexes.
Copper ores
The process for copper is very similar. The main copper mineral chalcopyrite (CuFeS2) is not leached very efficiently, which is why the dominant technology remains flotation. The leaching of CuFeS2 proceeds according the route indicated for indirect leaching above.[6]
Other ores
Bioleaching of non-sulfidic ores such as pitchblende also uses ferric iron as an oxidant (e.g., UO2 + 2 Fe3+ ==> UO22+ + 2 Fe2+). In this case, the purpose of the bacterial step is the regeneration of Fe3+. Sulfidic iron ores can be added to speed up the process and provide a source of iron. Bioleaching of non-sulfidic ores by layering of waste sulfides and elemental sulfur, colonized by Acidithiobacillus spp., has been demonstrated, which provides a strategy for accelerated leaching of materials that do not contain sulfide minerals.[11]
Related speculative concepts
With fungi
Several species of fungi can be used for bioleaching. Fungi can be grown on many different substrates, such as electronic scrap, catalytic converters, and fly ash from municipal waste incineration. Experiments have shown that two fungal strains (Aspergillus niger, Penicillium simplicissimum) were able to mobilize Cu and Sn by 65%, and Al, Ni, Pb, and Zn by more than 95%. Aspergillus niger can produce some organic acids such as citric acid. This form of leaching does not rely on microbial oxidation of metal but rather uses microbial metabolism as source of acids that directly dissolve the metal.[12]
In space
Microorganisms could be employed to mine extraterrestrially.[13][14]
Environmental impact
The process is more environmentally friendly than traditional extraction methods.[15] For the company this can translate into profit, since the necessary limiting of sulfur dioxide emissions during smelting is expensive. Less landscape damage occurs, since the bacteria involved grow naturally, and the mine and surrounding area can be left relatively untouched. As the bacteria breed in the conditions of the mine, they are easily cultivated and recycled.[16]
Sulfuric acid is produced in the process of pyrite ores, potentially forming "Yellow Boy" pollution.[17] The Finnish Talvivaara project proved to be environmentally and economically disastrous.[18][19]
Further reading
- Kundu et al. 2014 "Biochemical Engineering Parameters for Hydrometallurgical Processes: Steps towards a Deeper Understanding"
- T. A. Fowler and F. K. Crundwell – "Leaching of zinc sulfide with Thiobacillus ferrooxidans"
- Brandl H. (2001) "Microbial leaching of metals". In: Rehm H. J. (ed.) Biotechnology, Vol. 10. Wiley-VCH, Weinheim, pp. 191–224
- Watling, H. R. (2006). "The bioleaching of sulphide minerals with emphasis on copper sulphides — A review". Hydrometallurgy. 84 (1–2): 81. Bibcode:2006HydMe..84...81W. doi:10.1016/j.hydromet.2006.05.001.
- Olson, G. J.; Brierley, J. A.; Brierley, C. L. (2003). "Bioleaching review part B". Applied Microbiology and Biotechnology. 63 (3): 249–57. doi:10.1007/s00253-003-1404-6. PMID 14566430. S2CID 24078490.
- Rohwerder, T.; Gehrke, T.; Kinzler, K.; Sand, W. (2003). "Bioleaching review part A". Applied Microbiology and Biotechnology. 63 (3): 239–248. doi:10.1007/s00253-003-1448-7. PMID 14566432. S2CID 25547087.
