The use of bacterial leaching became a reality for the mining industry in the 1950’s initially in copper mining and later in gold mining. This report looks at the process involved and discusses the advantages and disadvantages of this method of extracting metals from low grade ore. EXTRACTING COPPER In 1752, at the Rio Tinto mines, it was first noted that blue-green streams of liquid running from the excavated rock left a brown film on the iron implements. This film was pure copper. In 1947 it was discovered that this was caused by micro-organisms (Merson,J. 1992).
Thiobacillus ferro-oxidans and Thiobacillus thio-oxidans thrive by oxidising Fe2+ ions and S2- ions. By spraying the low-grade ore left by traditional mining methods with an acidic leaching solution containing these bacteria results in the conversion of the insoluble sulphide materials into a solution containing CU2+, Fe2+,, Fe3+ and SO42- ions S2- ions are present in copper and the oxidisation of these by bacteria releases the metal ions into a solution. Extraction of copper by this method is known as bacterial leaching and produces a solution of copper sulphate which is diluted and impure (Chemical Storylines).
Solvent extraction techniques are used to recover the copper ions from the solution by a process of ligand exchange solvent extraction. (Article 2) The ligand, dissolved in an organic solvent such as kerosene is immiscible in water. Cu2+(aq) + 2LH (organic) = CuL2(organic) +2H+(aq) (L represents the ligand) The copper is removed from a low concentration in water to a high concentration in the solution. Mixing the organic solution with a small volume of concentrated acid reverses the process and pushes the Cu2+ ions back into the aqueous solution increasing their concentration. The concentrated copper solution is then sent to through electro winning cells with cathodes for recovery as copper plates (Copper mining & processing).
The remaining leaching solution is allowed to flow into an open pond where it is recharged by the oxidation of the Fe2+ ions to Fe3+ ions by Thiobacillus ferro-oxidans. The recharged leaching solution can then be pumped to the top of the pile and reused again. EXTRACTING GOLD Microscopic particles of gold embedded in a mineral matrix make up between 15-30% of the world’s gold reserves. Obtaining gold from the ore can be conducted in heaps similar to those used for the bacterial leaching of copper.
The ore is stacked and a leach solution applied to the top. This is allowed to percolate through the heap. As it migrates through the ore it leaches the gold and holds it in a solution. By the use of froth flotation the refractory gold containing minerals which include arsenopyrite (FeAsS), iron pyrites (FeS2) and chalcopyrite (FeCuS2) are separated from the oxide ores and non-metallic elements. There are 2 stages of bacterial oxidation. Firstly there is a reaction between the surface of the bacterial cell and arsenopyrite and catalyses the formation of soluble compounds of iron (II), arsenic (III) and sulphur (VI). Secondly there are reactions which oxidises the iron (II) and arsenic (III).
In order to release the gold the sulphide concentrate is roasted and the resulting mixture then treated with sodium cyanide. This process is called cyanidation (Barrett & Hughes, 1997). The leach area is lined with several layers of impermeable liners to prevent the possibility of the cyanide escaping into the ground and poisoning the surrounding area (Gold Ore Processing Plant). The gold is now in a solution as sodium aurocyanide and is precipitated as slime by thread like turnings of zinc. The ‘slimes’ are treated with sulphuric acid (H2SO4), or with lead (Pb) to recover the gold in a pure form (The British Encyclopaedia).
Advantages and Disadvantages of Bacterial Leaching Bacterial leaching is far less polluting than traditional methods of smelting ore to obtain gold and copper and as more and more regulations come into force with regards to emissions this is a strong consideration for the mining industries. Obtaining copper and gold by smelting is very energy intensive whereas the opposite can be said for bacterial leaching.
This has many environmental benefits Smelting copper by traditional methods cost the industry between $130 and $200 per kilo, but the cost has been cut in half by the introduction of biohydrometallurgy. It also means that low grade ores can be fully utilised by the mining industry and this becomes increasingly important as sources of high grade ore become exhausted. However, despite the environmental advantages, bacterial leaching is a much slower method and in some case, such as the extraction of gold at the Clogau St David’s mine in North Wales, liberating the gold in a way that met the regulations of the National Park proved uneconomic.
Bacterial leaching is still the second preferred method for copper extraction because of the slowness of the biological process and the fact that it cannot be carried out on a large scale means that there is a slow rate of return on the business capital. However, the opposite is the case for gold mining where bacterial leaching is the primary extraction process. This is because almost all the gold is recovered and therefore proves to be more cost effective. Also the recovery of other base metals during the gold extraction is high and contributes to the profitability of the mine.
Development of a New Mining Process Before a new mining process can be put into practice commercially it has to be tested fully to ensure that it will benefit the industry on a commercial basis and not be damaging to the environment. A newly discovered method would have to be piloted over a period of time and the results analysed to ensure its commercial viability. Following this the process would be patented and then the processed commissioned. Mines and researchers work hand in hand to ensure continued progress in developing methods for extraction of metals.
References:
Article 2 (1997) IN GCE Chemistry (Salter) examination paper Barrett, J. & Hughes, M. (1997) A Golden Opportunity. IN Chemistry in Britain. The Royal Society of Chemistry Chemical Storylines 2nd Ed.(2000) Heinemann.