Bioleaching reaction

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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.

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


UO2 + 2Fe3+ → UO22+ + 2Fe2+

Fe3+ is used as oxidant in the leaching of uranium from uraninite (UO2)

Again ferrous iron is oxidised 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)

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
Leaching mechanisms
Leaching mechanisms

See also

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