Gold mining is one of Western Australia's largest mineral industries, with an annual production of over 192 tonnes worth around $3.2 billion. Almost all mines use a cyanide leaching process to extract the gold from the ore. In this process the finely milled ore is mixed with a sodium cyanide solution that dissolves the gold. The reaction between the gold and the cyanide ions is described by Elsner's equation:
4Au + 8CN- + O2 + 2H2O = 4Au(CN)2- + 4OH-
Activated carbon is then used to recover the gold from this solution. The leaching takes place in large tanks holding many thousands of cubic metres of solution and generally takes several days.
Naturally all miners want this process to be as efficient as possible and to maximise the gold extracted from the ore. Across Australia over $100 million worth of cyanide is used each year in this process. Hence improving the efficiency is of value not only in terms of the extra gold recovered but also in the reduction of costs. In addition, if the leaching process can be accelerated then a plant can increase its throughput, further reducing the cost of treating each tonne of ore.