Cu²+(aq) + 2LH(organic) CuL2(organic) + 2H+(aq)
(Where L represents the Ligand)
This basically removes the copper ions from the water where they are in low concentration, and transfers them to the solvent where they are in high concentration. To further increase the concentration of the Cu2+ ions, this reaction can be reversed, i.e. mixing the copper/solvent complex with a small amount of acid. This reversible reaction depends on the concentration of H+ ions. If it is wrong, the reaction will not take place.
(Ref 1)
Extraction of Gold
Bacterial Oxidation is the process used to extract gold. As gold is found within minerals and in very small amounts, the mineral reduces the yield of gold. So, bacterial oxidation is used to remove the mineral matrix by oxidation. Once the gold is in residue, it can be recovered by cyanidation. (Ref 2)
The bacteria used are Thermophiles. Being only 2 micrometers long, they can only be seen under a microscope:
(Ref 3)
Unlike most living things, Thermophiles (as in the diagram to the left) do not rely on organic matter to survive. They thrive on inorganic matter and are harmless to living things. Their diet consists of pyrite, arsenopyrite and other metal sulphides such as chalcocite and chalcopyrite. This is good for the extraction of gold as it is found in these mineral sulphides. The bacteria thrive in acidic conditions and thus the oxidation process takes place at around a pH of 0.5 – 1.5 and a temperature of any where between 30 and 55 degrees Celsius. The process however has been known to work at temperature as extreme as –22 and 55 degrees Celsius. (Ref 4)
The process
Using arsenopyrite (FeAsS) as the sulphide, bacterial oxidation occurs in two stages. The first is the interaction between the bacteria and the mineral sulphide. The bacteria catalyse the formation of soluble compounds of iron (II), arsenic (III) and sulphur (VI).
FeAsS Fe(II) + As(III) + S(VI)
The second stage has two different reactions where iron and arsenic are oxidised. There are no gasses produced, as all the products are soluble.
Fe(II) Fe(III) As(III) As(V)
The overall reaction looks like this:
2FeAsS + 7O2 + 4H+ + 2H2O 2Fe3+ + 2H3AsO4 + 2HSO4-
The remaining waste water is treated with either crushed limestone or hydrated calcium hydroxide to neutralise the sulphuric acid. This leaves a gelatinous mixture of iron arsenates and iron oxohydroxide along with gypsum. Iron arsenate dihydrate is a stable compound that is formed; this could, if in contact with river water or rain, dissolve arsenic into the water system. However the concentration of arsenic is still well below the US limit of 2 ppm.
Pros and Cons Of Bacterial Leaching
In Copper Extraction
Copper is conventionally extracted (Ref 5) using the blast furnace. This is very polluting, expensive and hard to maintain due to its energy intensive approach. As emission limits were being established by environmental agencies, bacterial leaching becomes desirable due to its very low polluting effect. It can turn piles of tailings from previous mining into valuable sources of copper. It is possible to mine like this without actually mining huge networks of tunnels, as the leaching solution can be pumped underground. It is a good alternative where traditional methods are not profitable and it can also extract from low-grade ores. However it is slower than the blast furnace and in today’s fast paced world, this method is placed second to traditional ones.
In Gold Extraction
In the old days of the gold (Ref 6) rush, pure gold was almost impossible to extract due to its occurrence in mineral matrices. Roasting was used to melt the compounds but this converted any sulphur or cyanide into undesirable substances. Old techniques gave very small percentage yields of just 10% where as bacterial oxidation gives yields as high as 100%. There is no need for extreme conditions as the bacteria can work at 30 to 55 degrees Celsius and 0.5 to 1.5 pH. The bacteria are also robust and adaptable and can be used again and again reducing costs of maintaining the process. Although the set-up cost of the oxidising plants may be high, this soon repays for its self and is kinder to the environment. This is why bacterial oxidation is preferred to roasting.
Summary
For a new mining process to become commercialised it needs to be assessed for cost effectiveness, yield of metal, how polluting it is, the initial cost and how lucrative the process will be. Also the speed of the extraction is a major factor and it may be developed further to increase the speed. All these have to be considered and not all are industrial factors. A lot factors that go unnoticed are financial and even political.
References
