Copper can be extracted from its ores by using bacteria.
Firstly the low-grade copper ore and also the tailings are stacked up where the ground has been made impermeable.
The ground is sprayed with the acidic leaching solution (Thiobacillus Ferro oxidans and T. Thio-oxidans). This solution is used because the bacteria used, thrive in acidic conditions and they also don't need any organic material on which to feed. The bacteria require Fe ions, or S ions, oxygen and carbon dioxide. The bacterium may also need bacterial nutrients containing nitrogen and phosphorus. The result is that the bacteria transfer the insoluble sulphide minerals into a solution of Cu, Fe, Fe and SO4 ions.
Due to the piles sitting on the impermeable base layer, it's easy to drain off the solution carrying the copper ions.
Secondly, the copper ions are removed using another solvent. The left over leaching solution flows into an open pond, where Thiobacillus Ferro oxidans catalyses oxidation of the remaining Fe ions to Fe ions. This process of oxidation recharges the leaching solution, which is pumped back to the top of the pile for the cycle to begin again.
The copper is separated from its ore because the leaching solution containing bacteria react with the ores and a form of ionisation takes place and electrons are knocked of the ore and the copper is liberated. (Article 1 Mining with Microbes by John Merson)
Gold could also be extracted in a similar way.
Firstly, the refractory sulphide concentrate was treated with the thermophillic bacterium sulpholobus acidocalderius. These bacteria then catalysed the oxidation of the encapsulating sulphide minerals by dioxygen under fixed aqueous conditions at 70 degrees. Cyanidation of the final and resulting extract led to a breakthrough, in which the gold recovered increased from 10% to 100%.
The advantage of using bacterial leaching rather than traditional extraction methods is that it cost much less than the traditional method. Mining copper by the traditional method can cost between $130 and $200 per tonne. The introduction of biohydrometallurgy rapidly reduces the cost to around $70 per tonne. The result is that one tonne of mined copper typically results in two tonnes of sulphur dioxide being pumped into the atmosphere whereas biological extraction avoids this.
Another advantage is that this biological process of mining has no environmental problems associated with it. This is because the whole process is biological apart from in the copper extraction where Electro winning is used where the ions are separated.
The disadvantages of using bacterial leaching rather than traditional extraction methods are that it is slow, requiring decades rather than years, and methods for breaking ores into small enough particles for efficient extraction are not yet available. Already, biological methods are economically feasible for low-grade ores that elude conventional methods, but further technological innovations may make them competitive in more situations.
Also the mining industry have not yet taken on this new process because there is a general lack of familiarity with the technology - there are few trained biohydrometallurgists and few commercial success. The industry is also concerned with the reliability of this biological process because of the length of time it occurs in. (Article 3 A Golden Opportunity by Jack Barrett and Martin Hughes)
Bacterial leaching is used as a secondary process in the extraction of copper because the process is too slow and uneconomic because less money is made. Also when using the traditional method when extracting copper, more copper can be obtained from the ore.
The bacterial process of extracting gold is used as a primary process (secondary process is the traditional method) because the research and technology has been developed fast. More economic because more gold is produced so more money to be made. There is also a low cost of running the plants. Also the whole process is more feasible and economically competitive with the conventional process.
For a new process for bacterial leaching to become commercial the government and political powers must approve and also regulate a new process and may need to provide funding.
A Political power may need to hold an enquiry into the effects of this new technology to check if it is mandatory and safe. Thus granting permission for the new technology to go ahead.
References
- Article 1 Mining with microbes
- Article 2 Extracting Copper Ions from Leaching Solutions
- Article 3 A Golden Opportunity