Bioleaching reaction
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Cu<sub>2</sub>S + 4Fe<sup>3+</sup> → 2Cu<sup>2+</sup>+4Fe<sup>2+</sup>+S | Cu<sub>2</sub>S + 4Fe<sup>3+</sup> → 2Cu<sup>2+</sup>+4Fe<sup>2+</sup>+S | ||
| - | Fe<sup>3+</sup> is used as oxidant in the leaching of copper from chalcocite (Cu<sub>2</sub>S) and as shown above the ferrous iron is oxidised by the bacteria: | + | Fe<sup>3+</sup> is used as oxidant in the leaching of copper from chalcocite (Cu<sub>2</sub>S) and as shown above the ferrous iron and sulphur is oxidised by the bacteria: |
4Fe<sup>2+</sup> + O<sub>2</sub> + 4H<sup>+</sup> → 4Fe<sup>3+</sup> + 2H<sub>2</sub>0 | 4Fe<sup>2+</sup> + O<sub>2</sub> + 4H<sup>+</sup> → 4Fe<sup>3+</sup> + 2H<sub>2</sub>0 | ||
| + | |||
| + | S + 1.5O<sub>2</sub> + H<sub>2</sub>0 → H<sub>2</sub>SO<sub>4</sub> | ||
The overall reaction then becomes: | The overall reaction then becomes: | ||
Revision as of 14:33, 12 September 2007
Microorganisms are catalyzing the production and recycling of some leaching reagents. These biologically produced or recycled leaching reagents may then attack minerals so that metals are leached in abiotic reactions.
Bioleaching microbes cause mineralytic effects by:
- The formation of organic or inorganic acids (protons)
- Oxidation and reduction reactions. For example, Fe3+ is one important leaching reagent which is regerated by the oxidation of Fe2+
- Excretion of complexing agents
The type of sulfide mineral will affect by what mechanism the oxidation will proceed.
- Acid-soluble metal sulfides (such as chalcopyrite, sphalerite and galena) are leached by both of the leaching chemicals Fe3+ and H+ via the polysulfide pathway.
- Acid-nonsoluble metal sulfides (such as pyrite and molybdenite) are leached by the leaching chemical Fe3+ alone via the thiosulfate pathway.
This difference in mechanisms explains why sulfur oxidizers are able to leach some minerals but not others.
Examples
Pyrite is the most common of the sulfide minerals. In bioleaching of pyrite there is an initial chemical leaching where Fe3+ oxidises the mineral:
4FeS2 + 4Fe2(SO4)3 → 12Fe(SO4) + 8S
The ferrous sulphate and elemental sulphur formed is then oxidised with the aid of microbes according to the following reactions:
12Fe(SO4) + 3O2 + 6H2SO4 → 6Fe2(SO4)3 + 6H20
8S + 12O2 + 8H20 → 8H2SO4
The overall summary reaction of pyrite oxidation is as follows:
4FeS2 + 15O2 + 2H20 → 2Fe2(SO4)3 + 2H2SO4
Pyrite + Oxygen + Water → Ferric sulfate + Sulfuric acid
2UO2 + 4Fe3+ → 2UO22+ + 4Fe2+
Fe3+ is used as oxidant in the leaching of uranium from uraninite (UO2)
Ferric iron is then regenerated by the bacteria to complete the cycle:
4Fe2+ + O2 + 4H+ → 4Fe3+ + 2H20
Giving the overall reaction:
2UO2 + O2 + 4H+ → 2UO22+ + 2H20
Cu2S + 4Fe3+ → 2Cu2++4Fe2++S
Fe3+ is used as oxidant in the leaching of copper from chalcocite (Cu2S) and as shown above the ferrous iron and sulphur is oxidised by the bacteria:
4Fe2+ + O2 + 4H+ → 4Fe3+ + 2H20
S + 1.5O2 + H20 → H2SO4
The overall reaction then becomes:
Cu2S + O2 + 4H+ → 2Cu2+ + 2H20 + S
Contact, non-contact and cooperative leaching
Bioleaching microbes interact with the metal containing material by direct contact or by affecting the water-solution holding the metal containing material.
- Non-contact leaching: Free microbes produce the leaching chemicals Fe3+ and H+.
- Contact Leaching: Attached microbes produce the leaching chemicals Fe3+ and H+.
- Cooperative leaching
