Bioleaching reaction

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Leaching mechanisms. Some of the best known bioleaching reactions include when metal is released from sulfide minerals by Fe3+ oxidation of the metal sulfide bond and is catalyzed by acidophilic iron oxidizing microorganisms that convert the resulting Fe2+ back to Fe3+.
Leaching mechanisms. Some of the best known bioleaching reactions include when metal is released from sulfide minerals by Fe3+ oxidation of the metal sulfide bond and is catalyzed by acidophilic iron oxidizing microorganisms that convert the resulting Fe2+ back to Fe3+.

Bioleaching microbes harvest energy for their metabolism by catalyzing the production and recycling of some leaching reagents. The biologically produced leaching reagents attack the minerals and leach the metals in abiotic reactions.

Bioleaching microbes cause mineralytic effects by:

  1. The formation of organic or inorganic acids (protons)
  2. Oxidation and reduction reactions. For example, Fe3+ is one important leaching reagent which is regenerated by the oxidation of Fe2+
  3. Excretion of complexing agents

Contents

Mineral sulfide oxidation pathways

Acid-soluble sulfides and acid stable sulfides are highly reduced compounds. The type of sulfide mineral will affect by which mechanism the oxidation will proceed:

  • Acid-soluble metal sulfides are leached by both of the leaching chemicals Fe3+ and H+ via the polysulfide pathway.
  • Acid-nonsoluble metal sulfides 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.

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

Acid-insoluble sulfides are attacked by ferric iron produced by iron-oxidising bacteria and archaea whether they are attached to the mineral surface or in the liquid phase, where ferric iron oxidises the mineral following transport by diffusion of mass action.

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 an 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 an oxidant in the leaching of copper from chalcocite (Cu2S) and as shown above the ferrous iron and sulphur are oxidised by the bacteria:

4Fe2+ + O2 + 4H+ → 4Fe3+ + 2H20

S + 1.5O2 + H20 → H2SO4

The overall reaction then becomes:

Cu2S + 2.5O2 + 4H+ → 2Cu2+ + H20 + H2SO4

See also

Questions

  • What is produced during leaching? (Dissolved metal will form but other products as well)
  • How will pH change during leaching?
  • Which are the leaching chemicals?
  • Which conditions make leaching possible?
  • Do leaching chemicals come to an end or can they be recycled?
  • What role do microbes play?
  • How can leaching be controlled?
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