The mining industry has recently started using a new technique to extract minerals such as gold and copper from mines using bioleaching technology. Traditionally, the extraction would involve many expensive stages such as roasting in a smelting process.
The process
There are two bacteria involved in the process, [Thiobacillus ferro-oxidans]? and Thiobacillus thio-oxidans. In stage 1, bacteria catalyse (accelerate without being used up) the break-up of the mineral arsenopyrite (FeAsS) by oxidising (when the ions lose electrons) the sulfur and metal (in this case arsenic) ions to higher oxidation states whilst reducing dioxygen H2 and Fe3+. This allows the soluble products to dissolve in the solution.
FeAsS(s) -> Fe2+(aq) + As3+(aq) + S6+(aq)
This process occurs actually at the cell membrane of the bacteria. The electrons pass in to their cells and are used in biochemical processes to produce energy for the bacteria to reduce oxygen molecules to water.
In stage 2, bacteria then oxidise Fe2+ in to Fe3+ (whilst reducing O2).
Fe2+ -> Fe3+
They then oxidise the metal to a higher positive oxidation state. With the electrons gained from that they reduce Fe3+ to Fe2+ to continue the cycle.
M3+ -> M5+
The gold is now separated from the ore, disolved in solution.
The process for copper is very similar. The mineral chalcopyrite (CuFeS2) follows the two stages of being dissolved and then further oxidised, with copper2+ ions being left.
Details of metal extraction from mixture
Copper
Copper (Cu2+) ions are removed from the solution by 'ligand exchange solvent extraction' which leaves other ions in the solution. The copper is removed by bonding to a ligand, which is a large molecule consisting of a number smaller molecules each possessing a lone pair. The ligand is dissolved in an organic? solvent such as kerosene, 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 polar? water molecules and dissolves in the kerosene, which is then easily separated from the solution. Because the initial reaction is reversible?, and therefore not a [displacement reaction]?, it is determined by pH. By adding concentrated acid it 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 purity. This is where an electric current is passed through the resulting the 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 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 as it accepts electrons, iron is the reducing agent as it loses them.
Gold
Left in the original solution there may be traces of precious metals such as gold. Treating the mixture with [sodium cyanide]? with free oxygen present dissolves the gold. The gold is removed from the solution by adsorbing (taking it up on the surface) to charcoal?.
Advantages/disadvantages of bacterial leaching compared to traditional mining processes
Advantages of bioleaching
Disadvantages
Choice of extraction methods
For copper, at the moment, it is more economical to smelt the ores as they generally have quite a high concentration of the metal. This means that the profit from more copper being produced faster, is more than the lower cost and higher yield efficiency of bacteria make up for in terms of profit. Whereas because the concentration of gold in ore is so low the cost to smelt it, despite the fast rate equaling more gold, would be so high compares to the slower rate but cheaper cost of bacterial leaching.
Development stages that a new mining process must pass through before it can operate commercially
First research has to be done proving that this method is economically viable. When additional funding is gained, the research is continued to try and further improve the efficiency of the process, varying factors such as temperature, pressure, pH, (and in bacterial oxidation) strain of bacteria. Then the experiments start to scale the process up to the real size, using more realistic conditions and amounts. This enables researchers to anticipate problems and further refine the process. Finally a plant is built using this technique.
Further Reading