Going for gold: A non-toxic solution to gold leaching

Project leader Paul Breuer holds CSIRO’s ingot. Image: CSIRO

Researchers at the Commonwealth Scientific and Industrial Research Organisation (CSIRO) have extracted and produced gold using a non-toxic chemical process in an Australian first. Australian Mining speaks with project lead Paul Breuer to find out more.

Cyanide has long been a component of gold extraction. Used during gold leaching primarily for the extraction of fine gold, it is used in over 90 per cent of global gold production, but in modern times its harmful, toxic nature has seen regional bans in areas of the United States, Europe and South America.

For the past several years however, researchers at the Commonwealth Scientific and Industrial Research Organisation (CSIRO) have been working on a potentially less harmful alternative to cyanide leaching, and they may be on the verge of a new breakthrough.

While cyanide leaching has continued as the de facto gold leaching reagent in most countries, including Australia, CSIRO’s method may finally be coming to fruition in Australia after 20 years of research and roughly $10 million of investment.

The team has recently achieved Australia’s first ingot poured and produced using a non-toxic leaching process.

An extraction method involving the use of non-toxic thiosulphate (derived from oxygen and elemental sulphur) was successfully tested in Australia recently as part of CSIRO’s environmentally conscious Going for Gold initiative, which focuses on alternative extraction reagents that “don’t cost the Earth”.

CSIRO has been running the project in partnership with small gold miner Eco Minerals Research at a demonstration plant in Menzies, Western Australia, in order to show thiosulphate as a less toxic — but no less viable — alternative not just to cyanide leaching, but mercury leaching as well.

While largely usurped by cyanide in modern mining, mercury is still occasionally used for gold leaching by small-scale or artisanal miners.

From initial design, engineering, fabrication and setup of the facility, first pour at the project was delivered in 10 months, a feat Eco Minerals Research managing director Paul Hanna called “a fantastic achievement”.

Eco Minerals Research said the technology holds potential for use in major gold producing countries such as China, South Africa and the US, as well as the copper and silver industries too.

Fine gold (i.e. tiny fragments or molecules of gold that would be missed during gold recovery by gravity processes) can be recovered by having the thiosulphate-containing leach solution contact the ore in large tanks or vats. The gold is then collected from the leach solution via the application of ion exchange resins. The gold is recovered from the resin and further processed to produce gold doré (an impure gold bar).

CSIRO’s commemorative ingot, made from fine gold extracted using its non-toxic process. Image: CSIRO

 

The technology is not new in the strictest sense, but has taken a long time to develop to where it is now. The properties of thiosulphates for use in leaching were first discovered in the early 1970s and a number of patents were filed. However, the technology’s viability faced several hurdles at the time.

The CSIRO research team led by Breuer has seen commercial success with Toronto-based Barrick Gold at its Goldstrike mine in Nevada, where a specially tailored solution making use of the thiosulphate extraction has been used to maintain the mine’s gold production rates for nearly four years.

CSIRO gold processing team leader Paul Breuer says that the project posed many technical challenges for his team as well.

“The reason it’s taken so long to get to this point is the technical challenges in development of a process around the leaching of gold using thiosulphate,” he explains.

“One of the big ones was finding a method of recovering the gold thiosulphate from the solution,” he says.

“It doesn’t adsorb well onto activated carbon so we needed to find another means of recovering the gold using ion exchange resins. In 2005, CSIRO developed a process for recovering gold from these ion exchange resins that was quite efficient.

“We took that work and worked with Barrick Gold in terms of the development and demonstration of their tailored thiosulfate process and that knowledge and technology went into their process at Goldstrike.”

“Our collaboration started in about 2005 and went through a lot of research and then a demonstration plant that ran for about 16 months. After that the plant was retrofitted to use thiosulphate.”

While retrofitting worked well for Goldstrike, it is not suitable for all mine sites, it can be a complicated and costly process.

“Despite success at Goldstrike, their tailored leach system doesn’t work for other ores — we’ve been working away on other thiosulphate-based systems to try and get something that works more generically and is more applicable across a larger range of ores.”

In response to CSIRO’s success with the project, chief executive Larry Marshall said in a statement that the process could be a “game-changer” for small gold producers and producers with an interest in green commodity production. But the technology is not ready for widespread adoption yet and may take a long time to get to that stage.

Breuer stresses that despite the potential environmental upsides of using thiosulphates for gold leach extraction, it is not yet at a place where it could feasibly replace cyanide as an industry standard. Its current benefit is that it can pose as a viable alternative to cyanide in circumstances where cyanide leaching is ill suited to the project’s needs.

Additionally, it could be used at sites where cyanide leaching has already been banned, or may be banned at some point in the future.

“The technology is not at a point of replacing or substituting cyanide,” Breuer says. “The technology really has a market at the moment where cyanide can’t be used.

This article originally appeared in the October issue of Australian Mining.

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