Dr Silvia Black with a 15kg ingot of gold produced by
the traditional cyanide process

Research undertaken in Perth is contributing to the global search for a safer alternative to the use of cyanide in the gold mining industry.

Dr Silvia Black has been researching the use of copper, ammonia and thiosulfate (used in silver smelting and leatherworks) as an alternative method to the potentially dangerous cyanide process to leach gold from the ore.

The search has been spurred on by incidents such as the infamous Baia Mare spill in Romania in 2000, where thousands of fish were killed in three countries when cyanide leaked from a tailings dam into several rivers including the Danube.

“Cyanide has been used in mining for more than 100 years and when you look at its toxicity, it’s generally been handled very well by the industry,” Dr Black says.

“But there’s increasing pressure around the world, especially in the US, to try to move away from cyanide and we’re following a similar path.

“The thiosulfate-copper-ammonia process is the most suitable alternative in terms of being environmentally friendly, economically viable and giving you pretty good recovery rates of gold.”

The thiosulfate process has been around for some time but because the chemical reaction between the three reactants is a complex one, it has not been adopted by industry.

“My project was looking at the specific chemistry of copper and ammonia and thiosulfate to try to unravel some of the complexity involved with this process,” Dr Black explains.

Part of the challenge is that thiosulfate reacts with gold and brings it into solution, but thiosulfate and ammonia also react with the copper. Because of these reactions, copper changes state from the copper (II) oxidation stage – which is needed to recover the gold – to the less desirable copper (I) stage which affects how much gold can be recovered.

“The work involved trying to identify what compounds were there, so you have a mixture of six or seven compounds in solution so you try to identify any new ones – which I did – and determine the relative stability of each other,” she explains.

Dr Black researched the use of thiosulfate as part of her PhD thesis undertaken while working with the Parker Centre, a Cooperative Research Centre for the mining industry, based in Perth.

Her thesis, The Thermodynamic Chemistry of the Aqueous Copper-Ammonia-Thiosulfate System, was showcased at a presentation of early career researchers at the annual conference of the Cooperative Research Centres Association held recently in Sydney.

Dr Black now works as a researcher at the Chemistry Centre in Perth, with her work still focusing on cyanide chemistry, including environmental sampling involving cyanide.

She says it might be 10 years or so before the thiosulfate process is fully understood and adopted by the gold mining industry, but the new method looks promising.

“As someone said to me at the conference: ‘When this thiosulfate process starts being used by the mining industry, you could call it ‘green’ gold because it’s so much more environmentally friendly’,” she says.