Extracting metals from low-grade ores is usually a tricky business, but some bacteria have no problems with it. The metal industry is using these microbes to make their mines both green and profitable. In 1947 US microbiologists discovered that micro-organisms could be used to extract copper from copper ore. The leaching of copper by micro-organisms Bacteria such as Thiobacillus ferro-oxidans and Thiobacillus thio-oxidans they obtain energy by oxidising Fe2+ ions and S2- ions. The S2- ions are present in insoluble minerals of copper, zinc and lead. The oxidation of the S2- ions liberates the copper and this valuable metal can be collected.
The mining industries are starting to use these methods to extract copper from lower grade materials. The tailings from conventional mining are piled up in an area where the ground has been made impermeable. Then it is sprayed with an acidic leaching solution containing T. ferro-oxidans and T. thio-oxidans. The bacteria used, thrive in an acidic environment. They only require a supply of Fe2+ ions or S2- ions, oxygen and carbon dioxide. The result of this process is that the copper ore separates out into its different elements. The solution contains Fe2+, Cu2+, Fe3+ and SO42- ions.
Copper ions can be selectively removed from a bacterial leaching solution by a process called LIGAND EXCHANGE SOLVENT EXTRACTION. A ligand is a compound with a lone pair of electrons that binds to metal ions. A complex is formed in which the central mental ions are surrounded by a number of ligands. A good ligand compound for copper is mixed with an organic solvent, which is immiscible in water such as kerosene. When this solution is mixed with water a reaction takes place in which the copper attaches to the ligand compound. The following reaction takes place: Cu2+(aq) + 2LH(organic) CuL2(organic) + 2H+(aq) Where L represents the ligand. The effect of this reaction is transferring the copper from a low concentration in the solution to a high concentration in the organic solvent.
The remaining leaching solution is recharged by catalysing other oxidation reactions. The copper is then extracted as sheets by a process called ‘Electro-winning’. The process involves passing an electric current through the copper solution. The copper metal collects on the negative electrode. The bacterial leaching of gold Between 15% and 30% of the worlds gold supply occurs as refractory minerals. This means that the gold is encapsulated in a mineral matrix. Well-known examples of these gold-containing minerals are arsenopyrite (FeAsS), iron pyrites (FeS2) and chalcopyrite (FeCuS2).
The usual why of getting the gold out of the refractory minerals is a process called cyanidation. In 1984, a group of students at kings college London where investigating refractory minerals in there last year. They hit upon a problem, at a mine they could only extract 10% of the gold in the refractory minerals. This was because the roasting technique was not allowed in the national park area where the mine was. So the students investigated alternatives to roasting. The students used acidic solutions of thiourea and different oxidants to try to get the gold in the concentrate into soluble compounds. They showed that this could be achieved but not to an extent that could be economic.
But at the Queen elizabeth college people were already investigating metal-microbe interaction. The two groups soon teamed up. The researchers treated the refractory sulphide concentrate with the thermophilic bacterium Sulpholobus acidocalderius. These bacteria catalyse the oxidation of the encapsulating sulphide minerals by dioxygen under aqueaous conditions at 700c. Cynidation of the resulting extract led to a jump from10% to 100% in gold extraction. We can take a closer look at the reaction-taking place. Taking arsenopyrite (FeAsS) as the example, the overall oxidation can be represented by the following equation. 2feAsS + 7O2 + 4H+ + 2H2O 2Fe3+ + 2H3AsO4 + 2HSO4
Bacterial oxidation occurs in two stages. The first stage involves the interface of the cell surface and the arsenopyrite. The bacteria catalyses the formation of soluble compounds of iron (ii), arsenic (iii) and sulphur (vi). FeAsS Fe(ii) + As(iii) + S(vi) The second stage is the oxidation of iron(ii) and arsenic(iii) in separate reactions. Fe(ii) Fe(iii) As(iii) As(v) These reactions liberate the Gold from the refractory minerals. The advantages and disadvantages of bacterial leaching Bacterial leaching has clear environmental and economic benefits but still mining companies are still reluctant use it. I will discuss the advantages and disadvantages of bacterial leaching to hopefully discover why mining companies are not taking to this clean and environmentally friendly process.
Arguably the biggest benefits of bacterial leaching is to the environment. Production of copper metal is a very polluting process. Biological extraction avoids this. Also Biohydro-metallugry may provide a method of underground mining without the environmental damage associated with conventional techniques. This is changing old tailing sites from polluting nuisances into valuble sources of raw materials.
Biohydro-metallugry also has other economic benefits for example-smelting copper by traditional methods has cost between$130-$200 per tonne. But by biohydro-metallugry it has cut the cost down to about $70 per tonne. But still biohydro-metallugry is only a last resort when mining copper. The problem lies in the slowness of the process. According to Keith Debus from Montana State University, ” Biological metal recovery may take decades to remove all copper from a mining site. Where both techniques have been evaluated, biological approaches have often been found to be cheaper, but delay in cash flow from slower production has hindered adoption”.
Bacterial leaching is only used as a secondary mining option for copper but is a primary mining process for gold. This is because copper can be mined and smelted very quickly and relatively cheaply with mining companies getting a high percentage of copper out of the ore. So there is very little call for other techniques. But in gold mining, trying to get gold out of refractory minerals by cynidation does not always produce a high percentage of gold out of the ore. But this graph shows what happens when bacterial leaching is used.
So this gold mining technique is very useful and cost effective so most mining companies take up the technique. Development stages for new mining techniques For a new mining technique to operate commercially it must go through a number of stages. The first stage would be identifying a need for the new technique because if one isn’t needed there is no point. The second would be identifying how the process worked and seeing if it was applicable to an industrial scale. The third would be trying to see if the new process would have greater benefits than the old process. The forth and final one would be to see if the new process was economically profitable at an industrial scale.