A new process uses bacteria to break down the toxic effluent produced by gold extraction from sulfide ores.

Cyanide is used to leach gold from gold-bearing ores. However, the gold in sulfide gold ores, often found in underground deposits, is locked away from the leaching cyanide within the sulfide mineral matrix. One solution to this problem uses bacteria to oxidise the sulfides. This breaks up the matrix, letting the cyanide in to dissolve the gold.

While slower than the traditional ore roasting solution, the bacteria don't release environmentally unfriendly sulfur dioxide gas, as roasting does. A major drawback of biological (bio-) oxidation is that the subsequent cyanide leach produces thiocyanate as a waste product - and it's pure poison to the bacteria.

The effluent water cannot then be reused in the bio-oxidation stage and goes to waste. Recycling would be useful in arid Australian goldfields where quality water is a precious resource. Further, in the tropics where, due to too much water, containment can be an environmental issue, decontamination of the water may be required.

So, Parker Centre scientists teamed with two CSIRO microbiologists and biotechnology company BacTech Ltd to develop a bacterial process to get rid of the thiocyanate.

Dr Martin Houchin (from the Parker Centre) says the team went looking for bacteria in the environment in which they wanted them to operate - in the tailings dam of a gold mine using bio-oxidation. "Anything that's living in the tailings dam is growing in the presence of thiocyanate and so will be adapted to survive in that environment," he says. "It may even use the thiocyanate to exist."

Bacteria able to live by breaking down thiocyanate were selected by feeding them a diet with thiocyanate as the only energy source. In this way, several new species of native Aussie bacteria were discovered.

The ability of these thiocyanate chompers to act like an army of mini wastewater treatment plants was then tested in the lab in a specially built bio-reactor. The bacteria were grown on spiked balls inside a rotating cage sitting half immersed in a tank through which the effluent flowed. As the cage rotated, the bacteria alternatively fed on the thiocyanate in the effluent and "breathed" in oxygen from the air.

"We could treat high concentrations of thiocyanate in the bio-reactor and get it down to basically undetectable levels," says Dr Houchin. "Eventually it's degraded to ammonia and carbon dioxide and sulfate which are all benign."

The bio-reactor. The bacteria inside the rotating cages broke down high concentrations of thiocyanate to clean up test waste streams flowing through the bio-reactor.

The final proof was showing that the bacteria used to oxidise the sulfide ore could still do their job in the water treated with the thiocyanate-degrading bacteria, whereas their oxidising activity flatlined in the original effluent. If the detoxifying process was used on a gold mine, it would purify the water, allowing recycling and at the same time cleaning up the environment.

The process could be valuable in the future when sulfide gold ores become more common as the surface deposits are mined out.

The potential use of bio-reactors for effluent treatment could widen. For example, Parker Centre researchers have worked on the bacterial destruction of cyanide in gold plants which are being decommissioned.

By Ros Dilworth, Communications Officer, the Parker Centre.
First published in the Exploring CRC Research: Highlights of the Mining & Energy Sector CRCs in Australia booklet.