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Access to remote and marginal gold deposits not suited to conventional mining could be made economically viable thanks to an in situ leaching technique adapted from a process commonly used to extract uranium.
Paul Roberts, who led the research at the Australian Commonwealth Scientific and Research Organisation (CSIRO), headquartered in Victoria, says early laboratory tests have successfully recovered gold from near-surface oxide ores.
‘In situ leaching involves drilling holes in a mineralised structure, pumping liquids that dissolve the target mineral down the holes, and then recovering the pregnant liquid for processing,’ he explains.
Cyanide is used as the lixiviant to extract uranium from low-grade ore by converting it to water-soluble aurocyanide complex ions. However, cyanide is not effective for dissolving gold, which is normally found in the presence of other elements, such as iron sulphide, which break down the lixiviants. There are also environmental concerns regarding the toxicity of cyanide, and its use is banned in a number of countries.
But without artificial permeability enhancement, gold extraction rates are too low to be economically viable, so Roberts and his team looked into alternative chemicals for in situ gold leaching.
‘The challenge to find a replacement for cyanide led to the use of sodium thiosulphate and ferric ethylenediaminetetraacetic acid (EDTA),’ says Roberts. ‘Focusing our research on oxide gold deposits delivered gold recoveries of between 80-95% in bottle roll tests.
These chemicals were selected because they are effective in anaerobic conditions. This is important because the rate of dissolution of gold in cyanide solution is directly proportional to the amount of oxygen present.
‘We will never extract all of the gold, but if we can extract 50% in the deposit, it will be economically viable,’ says Roberts.
He explains that the leaching process takes up to two weeks as the fluids must be run through the ground many times before gold recoveries are up to an economic level.
Field studies involving the injection of these new lixiviants are likely to start in 2011, once permits and environmental approvals have been obtained.
The team are now looking at ways to enhance the permeability of the rocks using mechanical rock breakage via hydraulic fracture. Roberts says a number of issues require further detailed study. These include the effects of permeability enhancement methods on gold recovery and the costs involved in storing and transporting the liquids.
He is keen to point out the environmental benefits of this technique, as well as the economic. ‘The chemicals we are using will have a limited affect on the environment. The sodium thiosulphate and EDTA system has nothing like the toxicity of cyanide, but research still needs to be done [to determine the exact toxicity]. He adds, ‘The other important point about this technique is that it could be a replacement for open pit mining and so, looking at this in a holistic way, the environmental benefits are obvious.’
John Monhemius, Professor of Mineral and Environmental Engineering at Imperial College London, UK, says that the work could be useful, but questions its economical viability. ‘Gold is usually found as minute, micron, or even sub-micron, sized particles dispersed throughout the host rock, in concentrations rarely exceeding a few grammes of gold per tonne of rock. In principle, some gold ores may be amenable to in-situ leaching, but the number of factors that would have to be favourable to make this process economic means that, if it is ever used commercially, it is likely only to find niche applications for small, near-surface, ore bodies.’
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