The copper ions are extracted from the leaching solution by a process called ‘ligand exchange solvent extraction.’ A ligand is a compound with a lone pair of electrons that binds to metal ions. The central metal ion is surrounded by a number of ligands. A compound that is a suitable ligand for copper ions is dissolved in an organic solvent that will is immiscible with water. This is then shaken with the water containing the copper ions. The following reaction occurs:
Cu2+(aq)+2LH(organic) = CuL2(organic)+2H+(aq) (L= Ligand.)
This removes the copper ions from the water and places them in a solution where they are in higher concentration.
The copper ions are removed from the solution through an ‘electro-winning’ process, in which an electric current is passed through the solution, and the metal collects at the negative electrode. The metal can then be removed in sheets.
A similar process can be used for mining gold. Extracting gold from the ground is difficult. Up to 30% of the world’s gold occurs as refractory minerals – microscopic particles of gold trapped within other minerals. Treating the refractory sulphides with the bacterium ‘sulpholobus acidocalderius’. The bacteria catalyse the oxidation of the sulphide minerals encasing the gold. The extract is then cyanised, and the gold recovery can be up to 100%.
The traditional method for extracting copper is to smelt the ores at high temperatures. The ore is first ground into a fine powder. The grains of copper mineral are separated out by froth flotation. The slurry of copper mineral and water produced is then pumped into the smelter. In the smelter, the concentrate is passed down the heated reaction shaft with oxygen-enriched air. The sulphides and the iron oxides are converted to a molten slag. The bottom layer is copper sulphide, which is run off into another furnace where it is blown with air to produce copper.
The traditional method of extracting copper by smelting uses a lot of energy, and is highly polluting – for every tonne of copper produced, two tonnes of sulphur dioxide are pumped into the air, which causes environmental damage. Bacterial leaching of copper is much cheaper, and cleaner, but is also much slower than obtaining copper by smelting.
The traditional method for extracting gold is to use froth flotation to separate the refractory minerals from any non-metallic minerals present. This produces a sulphide concentrate, which is then roasted to liberate the gold. The resulting mixture is then treated with aerated sodium cyanide to extract the gold. This final process is called cyanidation.
The traditional method of extracting gold used a lot of energy, and produces a lot of pollution in the form of sulphur dioxide and arsenic (III) oxide, which causes environmental damage. Roasting the gold does not always liberate all the metal, so it yields only a fraction of contained gold. Bacterial leaching yields at least 92% of the contained gold, is much cheaper, and produces no gasses.
Bacterial leaching is usually used for secondary extraction of copper because the traditional methods produce the copper faster, and produce a higher yield than bacterial leaching, which is more useful for extracting the remaining copper from the tailings and low-grade ore. It is, however, more suitable for the primary extraction of gold, as it produces a much higher yield of gold than traditional methods, and causes little pollution.
