The CSIRO Division of Minerals has unearthed savings of up to half a million dollars a year for the average-sized Australian gold mine, achievable through improved ore processing.
The research was jointly funded by WMC Resources Ltd and the Australian Minerals Industry Research Association (AMIRA), and focussed on Western Australia's Goldfields, source of 80 per cent of Australia's $4.4 billion gold exports. Most of the potential savings would come from neutralising the effects of the highly saline underground water used on the Goldfields in the gold leaching process.
The research team leader, Dr John Rumball from CSIRO Minerals' Waterford laboratory in Perth, says the high salt levels create a pH buffering effect. "It is exactly the opposite to what's needed for efficient and safe use of cyanide - the key ingredient for leaching the gold from the tonnes of useless dirt."
To keep cyanide in solution and avoid the formation of toxic hydrogen cyanide gas, the pH of leach solutions is kept high, achieved by adding lime. WA Goldfields' producers use up to 6 kg of lime for every tonne of ore treated, while plants elsewhere using fresh water have a typical usage rate of just 0.5 kg of lime per tonne of ore.
"The cost of additional lime for a typical Goldfields' miner is $1 million to $2 million per year," according to Dr Rumball.
"After exploring the cyanide - lime - pH relationship, we have come up with a formula which delivers maximum gold extraction, maximum worker safety, and minimum cost inputs. Already one gold plant, taking advantage of the results, has cut the extra lime costs by as much as one third," Dr Rumball said.
The CSIRO team also identified large potential savings from what is discarded to tailings dumps.
Dr Rumball estimates that in every tonne of waste there is about 0.2 gms of pure gold. "For some mines this represents almost $10 million per year," he said. Tests showed 20 per cent of the tailings gold - 'free gold' - is readily retrieved with minor alterations to the refining process.
"The problem is that in the leach tanks thin films of salts are coating the gold particles and in some cases are thick enough to prevent the cyanide from dissolving them into solution. The 'free gold' can be unlocked with a simple combination of extra chemicals and a physical roughing up of the particles' surfaces," Dr Rumball said.
More information: Dr John Rumball (09) 334 8070 (bh) or (09) 385 4993 (ah)