Biohydrometallurgy is an application in mining.

Biohydrometallurgy

"Biohydrometallurgy is an application in mining. In its natural state a metal such as copper is found combined with other elements in the mineral chalcopyrite. A bacterium is then used to form the compound copper sulphate (CuSO4), which in turn can be treated chemically to obtain pure copper. This microbiological mining process is used only with low-grade ores and currently accounts for 10% of copper production world-wide."

Copper can be extracted from its ores by using bacteria. This is done by, firstly the low-grade copper ore and also the tailings (left from an earlier traditional mine) are stacked up where the ground has been made impermeable. The ground is then sprayed with the acidic leaching solution (T. 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 eventual 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. The next stage is that the copper ions are removed using another solvent. The left over leaching solution flows into an open pond, where T. 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.

Gold could also be extracted in a similar way but more research was needed for the process to be successful. 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 C. Cyanidation of the final and resulting extract led to ...

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More research was carried out, as funding became available. A student called Ali Nobar introduced a mixed culture of moderately thermophillic bacteria from certain samples. This culture became effective over a wide range of conditions such as temperatures, pH, water salinity and arsenic concentration. Research found out That the bacteria worked best at 46 degrees c (if the temperature is to high the bacteria cell will became denatured due to a higher temperature) and in solutions with a pH of between 0.5 and 1.5. These greatly enhanced the percentage of gold extraction

A graph to show the enhancement of gold recovery using bacteria

Key

Cyanidation alone

Bacterial oxidation followed by cyanidation

This new process has many advantages and disadvantages. One of the main advantages of this process 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 main disadvantages of biohydrometallurgy 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.

Bacterial leaching is used as a secondary process in the extraction of copper because the process is to 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.

Summary

This type of bacterial chemistry in order to obtain metal from its ore is a new technology but the industry is still yet to recognise it. Its main advantage is it cheap and mostly environmentally friendly compared to traditional methods. This type of process will lead us into the future.