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Going for gold: Alternative processing methods

Editorial
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Gold is predicted to have an extremely rough year in 2014.

Some pundits are even forecasting a return to the lows the metal saw in the early 1980s.

If it doesn’t utilise technology as best it  can then many in the industry will be hard pressed to survive through this current trough of high cost extraction for relatively low returns.

One of the major areas in which operators can not only become more efficient, but also cut energy and production costs , is in the processing of the gold.

Importantly, unlike coal processing,  gold processing currently involves the use of environmentally hazardous cyanide, adding a raft of legal and regulatory burdens to operating.

In this climate the industry also now has to treat more complex ores as the easier to process gold ores are now all mined.

Many of these complex ores contain high levels of soluble copper, which consume the cyanide used to process the gold requiring larger amounts of the toxic chemical which increases the environmental risk.

However there are currently two innovative and greener methods currently being developed in Australia and the US which have the potential to change the way the industry processes gold.

Scientists at the CSIRO and the North Western University in Illinois are researching two different methods, thiosulfate and alpha-cyclodextrin (a substance commonly extracted from corn starch) respectively, to process and recover gold.

Thiosulfate and bromide are used to leach the gold, in a similar way to cyanide, whilst the alpha-cyclodextrin is used in a similar fashion to carbon to recover the gold.

Speaking to Paul Breuer at the CSIRO, he told Australian Mining that research is currently developing gold processes using thiosulfate as an alternative lixiviant to cyanide and have made some major breakthroughs.

“Gold only forms complexes with certain agents, and thiosulfate is one of the best lixiviants apart from cyanide,” he explained.

“While thiosulfate has been investigated as a potential alternative for decades, having first been looked at for gold processing back in the 1970s, it has challenges in terms of thiosulfate consumption and recovery of the gold thiosulfate complex. 

“The gold thiosulfate complex cannot be recovered using carbon, which however is advantageous in treating carbonaceous containing gold ores.”

The CSIRO has developed an elution process for removing gold from strong base ion exchange resin used to recover gold thiosulfates, with Breuer explaining that prior to the development there remained the challenge of effectively getting the gold thiosulfate off the resin.

“The breakthrough was the use of sulfite in the solution used to recover remove the gold thiosulfate from the resin, which has now opened up a new area of recovery potential applications of thiosulfate with ion exchange resin for gold recovery,” he said.

Thiosulfate was chosen for in-situ or in-place leaching of gold as it is more environmentally friendly than cyanide, however its stability and rate of gold recovery is typically not as good as cyanide.

But “when it is used with copper-ammonia as an oxidant for treating gravity concentrates, the gold leach rate can be higher than cyanide, though thiosulfate consumption in the process can also be high,” Breuer explained.

Breuer went on to tell Australian Mining that more than one Australian miner is already going down this track and investigating thiosulfate for in-situ treatment of gold ores.

However he stated that for the time being thiosulfate process development is fairly ‘niche focused’ as the CSIRO team is looking at its use for in-situ leaching and treatment of gold concentrates and that “in the short term it is not developing a complete replacement for cyanide, instead it is about building confidence and establishing the economics in the process in niche areas”.

Scientists at the North Western University in Illinois are using alpha-cyclodextrin, a substance commonly extracted from corn starch, to recover gold.

Zhichang Liu, at the University of Illinois, has discovered another green method of gold processing.

His team has uncovered an environmentally friendly replacement for activated carbon  - alpha-cyclodextrin, a material commonly composed of units derived from corn starch.

“We mixed nitric acid and hydrobromic acid, then we dissolved gold in the solution to extract it from the ore,” he told Australian Mining

By using this method Liu explained that the leach rate is much faster and operators don’t need to use strong acids and cyanide as “we can use bromide instead, and this solution is neutral, so we can add the alpha-cyclodextrin to this solution to aid in isolating gold”.

Fraser Stoddart, the chemistry professor leading Liu’s research team, said “alpha-cyclodextrin is the gold medal winner” for processing.

“We have replaced nasty reagents  with a cheap, biologically friendly material derived from cornstarch.”

Liu added that the process can also extract gold from consumer e-waste.

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