The biological approach is to treat the refractory sulphide concentrate with the thermophilic bacterium Suipholobus acidocalderius. These bacteria catalyse the oxidation of the encapsulating sulphide minerals by dioxygen under aqueous conditions. When tested at 70°C, cyanidation of the resulting extract led to increase in gold recovery from 10% to 100%.
Chemical equations
Taking the refractory mineral arsenopyrite (idealised formula FeAsS) as the example, the overall oxidation process is represented by the following equation.
2FeAsS+ 7O2 + 4H+ + 2H2O → 2Fe3+ +2H3AsO4 + 2HSO4-
Bacterial oxidation occurs in two stages:
Stage 1
Stage 2
Leaching copper
Explanation
Performed to heaps of waste ground ore irrigated in closed circuit with an acidic leaching solution; it contains Kerosene, T ferro-oxidans and T thio-oxidans (These bacteria thrive in an acidic environment, and require only a supply of Fe2+ ions, or S2- ions, O2 and CO2). The overall result is that the bacteria convert the insoluble sulphide minerals into a solution containing Cu2+, Fe2+ Fe3+ & and SO42- ions. The solution is then pumped to the recovery section.
Because the piles sit on an impermeable base layer, it is easy to drain off the solution carrying the copper ions. Copper ions, Cu2+, can be selectively removed from bacterial leaching solutions by the process of ligand exchange solvent extraction. Other metal ions, such as Fe2+ and Fe3+ are left behind in the aqueous solution.
The remaining leaching solution flows into an open pond, where T ferro-oxidans catalyses oxidation of the remaining Fe2+ ions to Fe3+ ions. This recharges the leaching solution, which is pumped back to the top of the pile for the cycle to begin again.
Chemical equations (Ligand Exchange Solvent extraction)
The Organic Solvent used
Forms a complex with a Cu2+ ion to form a six-membered chelate ring containing the Cu2+ ion and five of the atoms of the ligand molecule. Two molecules of the ligand react with each copper ion to form a planar complex. The complex has no overall charge and dissolves readily in the kerosene but not in water.
Advantages of leaching
The main advantages are its simplicity, mild operation conditions, low capital costs, low energy input, and in its friendliness towards the environment. Humans can achieve the same result only by smelting ores at high temperatures, a far more polluting and energy-intensive approach.
Also, regulatory agencies were beginning to restrict emissions of sulphur dioxide, adding to the cost of metal production. Smelting copper ore by traditional methods had cost between $130 and $200 per kilo. The introduction of biohydrometallurgy cut the cost to less than $70 per kilo. Now 30% of the copper produced in the US is extracted in this way.
Disadvantages of leaching
In Copper mining, using heaps of waste material as a source has environmental and time consuming advantages, however they present severe limitations: the piled material is very heterogeneous and practically no close process control can be exerted. Moreover, the rates of O2 and CO2 transfer occurring are limited and the leaching rates are low, so extended periods of operation are required in order to achieve the desired conversions.
Why bacterial leaching is a secondary mining process for copper, but a primary extraction method for gold.
In bacterial leaching for copper, the extraction process is too slow to provide enough capital per annum to be profitable. This means it is impractical for companies to use. The higher tax rates on traditional methods still do not reduce profits made by a company sufficiently to make it a less cost-effective extraction process.
However, in gold mining, the increase in gold extraction by leaching is so significant, that the slower processing still outweighs income made from other methods.
How is a mining process developed to industrial use?
- Mining process is thought up by individual or workgroup
- Tests are carried out in a lab to check how well process works in a reaction vessel
- a medium-sized laboratory plant is built for further testing
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A large-sized Pilot plant is built for further testing in various mines
- A full-scale plant is built for industrial use
References
- http://www.epa.gov/grtlakes/arcs/EPA-600-991-001/EPA-600-991-001.html
- Section 2.5 – “Conclusion”
- http://www.ejbiotechnology.info/content/vol3/issue3/full/4/bip/
- “Mining with Microbes” John Merson
- “Extracting copper ions from leaching solutions”
- “A Golden Opportunity” Jack Barrett & Martin Hughes
28/04/2007 Bacterial Leaching in the mining industry Page
Matthew Stone