The bacteria then convert the tailings into a solution containing, amongst other things, Cu2+ ions, which can then be drained off from the other ions that are present. The rest of the leaching solution has Thiobacillus ferro-oxidans added to it to catalyse the oxidation of Fe2+ ions into Fe3+ ions, this solution can then be reused, and so the process starts again. In the mean time the solution containing the Cu2+ ions are added to a ligand, this ligand has lone pairs of electrons which are able to bind to metal ions,
Cu2+ (aq) + 2LH (organic) CuL2 (organic) + 2H+ (aq) 2
L represents the ligand, a suitable compound for this is kerosene as it is immiscible with water, the resulting reaction liberates the copper ions from the water. When the copper is in this compound it is then electrolysed, and the copper metal collects at the negative electrode.
The extraction of Gold
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Flotation separates the refractory minerals from unwanted ores. These refractory minerals form a sulphide concentrate that is then roasted to liberate the gold. The gold is extracted by treating the mixture with sodium cyanide, this process is called cyanidation. The problems that result from roasting and cyanidation mean that only a fraction of the gold is extracted from the sample. 3
Bacteria at work.
If the refractory sulphide concentrate is treated with a thermophilic bacterium, Sulpholobus acidocalderius, it helps catalyse the oxidation of sulphide ions and then this resulting mixture, when treated with sodium cyanide, produces a total extraction of the gold. More research lead to the discovery of a mixture of different thermophilic bacterium that worked over a range of temperatures, PHs, and arsenic conditions making it ideal for gold extraction in other countries and increasing the extraction process even more. The bio-oxidation process works in two stages, taking arsenopyrite as an example, the bacteria catalyses the formation of iron(II), arsenic(III) and sulphur(VI):
FeAsS Fe(II) + As(III) + S(VI)
During the second stage two of the products are oxidised:
Fe(II) Fe(III)
As(III) As(V)
All the products from the oxidation of arsenopyrite are soluble in water, so after the process has taken place the water is treated with hydrated calcium hydroxide, this neutralises sulphuric acid and helps form a layer of iron(III) arsenates and other products on the surface of the water.
Advantages and Disadvantages.
Copper extraction.
The new process of extracting copper from its ores is much cheaper than the traditional method, mining copper in the traditional way cost around $130 and $200 per tonne, but the introduction of bacteria lowered this cost to less than $70. There are environmental factors that are caused by traditional mining, the main pollutant being the emissions of SO2, due to restrictions in amounts of SO2 that are allowed to be produced bacterial extraction of copper seems to be the way to go, but at the moment it is only being used as a last resort, when mining operations have no other choice. This is due to the fact that the bacteria is slow working it takes months, years to produce profitable amounts of copper, so the process of biohydrometallurgy is used a secondary process. The biological methods are economically feasible for low grade ores, but for there to be a profitable result the process would need to speeded up and this advance hasn’t been made, mining operations are reluctant to try a process with such few commercial successes. The traditional method is preferred as more copper can be extracted from the ores.
Gold extraction.
The bacterial process for extracting gold is preferred to the traditional method, the extraction of gold using bacteria is much higher. Bio-oxidation is cost effective, efficient, safe, and environmentally friendly. Compared to roasting, bio-oxidation reduces capital costs by 12-20%, operating costs by 10% and construction time by 25%. The yield of gold is also increased, the process doesn’t take months and the process is extremely profitable. By using the bacterial process primarily it eliminates the emissions produced by roasting, large amounts of SO2, As2 and O3, and also the costs of disposing of arsenic. The operation is simple and a skilled workforce isn’t required, the plants are also healthier and safer. Only 20% of all the refractory gold treatment plants use bio-oxidation, for this to increase the governments would need to provide funding. Political powers would also need to approve and regulate new processes, checking it is safe and not very environmentally damaging. 5
Sources:
1 Article one – by John Merson, Mining with microbes
2 Article two – extracting copper ions from leaching solutions
3 Article three – by Jack Barrett and Martin Hughes, A Golden Opportunity
4 ,
5 ,
articles from the engineering and mining journal from 1999.
