The use of microorganisms e.g. bacteria to extract minerals particularly metals from low-grade ore. This is commonly known as microbial leaching. This is usually practiced in the extraction of copper and uranium. This is usually carried out by microorganisms known to be chemolithotrophic as they obtain their energy from the oxidation of inorganic substances. Many of which are also autotrophic that is they capture carbon for the synthesis of cellular components using atmospheric carbon dioxide as their carbon source. The leaching bacteria live in environments that would be quite unfriendly to other organisms e.g. the concentration of sulfuric acid and of soluble metals is often very high. Some thermophilic species require temperatures above 50oc and a few strains have been found at temperatures near the boiling point of water (Butler , 2003).
Butler (2003) continues to states microbial leaching is commonly used in the extraction of copper from the copper ores approximately 25% of all copper mined worldwide is now obtained by this processes because copper sulfate formed during the oxidation of copper sulfide ores is very water-soluble.
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The acidophilic archaea Sulfolobus metallicus and Metallosphaera sedula tolerate up to 4% of copper and have been misused for mineral mining. Between 40 and 60% copper extraction was achieved in primary reactors and more than 90% extraction in secondary reactors with overall residence times of about 6 days.
Agate (1996) states that some microorganisms use the oxidation of the ferrous ion (Fe2+) to the ferric ion (Fe3+) as their source of energy. In this reaction only a small amount of energy is obtained, hence large amounts of (Fe2+) have to be oxidized. Furthermore, (Fe3+) forms the insoluble Fe(OH)3 precipitate in water. Many Fe2+ oxidizing microorganisms also oxidize sulfur and are therefore obligate acidophiles that further acidify the environment by the production of H2SO4. This is due in part to the fact that at neutral pH Fe2+ is rapidly oxidized chemically in contact with the air. In these conditions there is not enough Fe2+ to allow significant growth. At low pH, however, Fe2+ is much more stable. The most considered Fe2+ oxidizing bacterium is Acidithiobacillus ferrooxidans, an acidophililic chemolithotroph.
Even though the microbiological oxidation of Fe2+ is essentially for acidic pH's in mines which contributes to a serious ecological problem, this process can also be usefully exploited when well managed. The sulfur containing ore pyrite (FeS2) is at the start of the microbial mining process. Pyrite is an insoluble cristalline structure that is rich in coal- and mineral ores. It is produced by the following reaction: S + FeS → FeS2 (Agate, 1996).
Agate (1996) continues by stating that the pyrite is usually protected from contact with oxygen therefore unavailable for microorganisms. Upon exploitation of the mine however pyrite is brought into contact with air exposing it to oxygen and microorganisms hence oxidation start. This oxidation be dependent on a combination of chemically and microbiologically catalyzed processes and can be influenced by two electron acceptors: O2 and Fe3+ ions. The latter will only be present in significant amounts in acidic conditions of pH less than 2.5. First a slow chemical process with O2 as electron acceptor will initiate the oxidation of pyrite: FeS2 + 7/2 O2 + H2O → Fe2+ + 2 SO42- + 2 H+
This reaction acidifies the environment and stable Fe2+ will be designed. In such an environment Acidithiobacillus ferrooxidans will be able to grow promptly. Upon more acidification Ferroplasma will also develop and further acidify. As a result of the microbial activity energy is produced: Fe2+ → Fe3+.This Fe3+ that remains soluble at low pH reacts naturally with the pyrite: FeS2 + 14 Fe3+ + 8 H2O → 15 Fe2+ + 2 SO42- + 16 H+. Fe2+ can again be used by the microorganisms and hence a cascade reaction will be initiated (Agate ,1996).
In the case copper extraction which is available in two forms chalcocite and the covellite, Acidithiobacillus ferrooxidans is able to oxidize the Cu+ in chalcocite (Cu2S) to Cu2+, thus removing some of the copper in the soluble form, Cu2+, and forming the mineral covellite (CuS). This oxidation of Cu+ to Cu2+ is an energy yielding reaction such as the oxidation of Fe2+ to Fe3+. Covellite can then be oxidized, giving out sulfate and soluble Cu2+ as products (Helingerová et.al. 2010).
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Helingerová et.al. ( 2010) continues by stating that a second mechanism, and probably significant in mining operations, involves chemical oxidation of the copper ore with ferric (Fe3+) ions formed by the microbial oxidation of ferrous ions derived from the oxidation of pyrite. Three possible reactions for the oxidation of copper ore are:
Cu2S + 1/2 O2 + 2 H+ → CuS + Cu2+ + H2O
CuS + 2 O2 → Cu2+ + SO42-
CuS + 8 Fe3+ + 4 H2O → Cu2+ + 8 Fe2+ + SO42- + 8 H+
The copper metal is then recuperated by using Fe0 from steel cans: Fe0 + Cu2+ → Cu0 + Fe2+.
According to Kalin et.al. (2004) in the same way as in copper, Acidithiobacillus ferrooxidans can also oxidize U4+ to U6+ with O2 as electron acceptor. Though the uranium leaching process relies more on the chemical oxidation of uranium by Fe3+, with At. ferrooxidans contributing mainly through the reoxidation of Fe2+ to Fe3+ as described above.
UO2 + Fe(SO4)3 → UO2SO4 + 2 FeSO4
Akcil and Mudder (2003) states that microbial mining can also be used in the extraction of gold as is frequently found in nature associated with minerals containing arsenic and pyrite. The process take place as the At. ferrooxidans and relatives are able to attack and solubilize the arsenopyrite minerals, and in the process, releasing the trapped gold (Au): 2 FeAsS[Au] + 7 O2 + 2 H2O + H2SO4 → Fe(SO4)3 + 2 H3AsO4 + [Au].
The use of microorganisms to leach minerals from mine tailings has improved recovery rates and reduced operating costs. Moreover, it permits extraction from low grade ores - an important consideration in the face of the depletion of high grade ores (Helingerová et.al. 2010).