Bacterial Leaching of Copper and Gold

This Process ocurs at the cell membrane of the bacteria. The electrons pass into the cells and are used in biochemical processes to produce energy for the bacteria in order to reduce the oxygen in water.

Stage 2

The bacteria then oxidise the Fe2+ to Fe3+ (whilst reducing the oxygen).

                                                   Fe2+ -> Fe3+

The bacteria then oxideses the metal to a higher oxidation state. With the electrons they gain from that (as the metals loose electrons from oxidation) they reduce Fe3+ to Fe2+ to continue the cycle. The gold is now seperated from the ore and its solution.

     

The process for  is very similar. The mineral  CuFeS2 (pictured) follows the two stages of being dissolved and then further oxidised, with Cu2+ ions being left.

Extraction from the Mixture

Copper (Cu2+) ions are removed from the solution by  exchange solvent extraction which leaves other ions in the solution.

Ligand - is an ,  or  that is  to one or more central atoms or ions, usually  generally through co-ordinate covalent bond. A lot of such ligands around a centre is termed a .

The copper is removed by bonding to a ligand, which is a large molecule consisting of a number of smaller groups each possessing a lone pair. The ligand is dissolved in an  solvent such as  and shaken with the solution producing this reaction:

Cu2+(aq) + 2LH(organic) -> CuL2(organic) + 2H+(aq) 

The ligand donates electrons to the copper, producing a complex - a central metal atom (copper) bonded to 2 molecules of the ligand. Because this complex has no charge, it is no longer attracted to  water molecules and dissolves in the kerosene, which is then easily separated from the solution. Because the initial reaction is , and therefore not a , it is determined by pH. Adding concentrated acid reverses the equation, and the copper ions go back into an aqueous solution.

Then the copper is passed through an electro-winning process to increase its purity: an electric  is passed through the resulting solution of copper ions. Because copper ions have a 2+ charge, they are attracted to the negative cathodes and collect there.

The copper can also be concentrated and separated by displacing the copper with Fe from scrap iron:

Cu2+(aq) + Fe(s) -> Cu(s) + Fe2+(aq) 

The electrons lost by the iron are taken up by the copper. Copper is the oxidising agent (it accepts electrons), and iron is the reducing agent (it loses electrons).

Traces of precious metals such as gold may be left in the original solution. Treating the mixture with Sodium Cyanide in the presence of free oxygen dissolves the gold. The gold is removed from the solution by adsorbing (taking it up on the surface) to .

The table below represents the advantages and disadvantages of Bacterial Leaching. From here we are able to see the true quality of this process.