Called in to cure a sick solvent extraction (SX) circuit, researchers discovered a yeast infection which, when treated, restored the circuit to health.

Their work could also help other mineral processing plants control similar problems with their SX circuits.

Solvent extraction is used to selectively extract metals such as uranium, copper, nickel, cobalt and vanadium from their ores. SX is simple, fairly cheap to operate and can produce a high grade of product.

In SX, a solution of a metal leached from an ore is mixed with an organic solvent containing specialist extractant chemicals. The aqueous and organic phases form an emulsion, just as vinegar and olive oil do when mixed to make a salad dressing. In this emulsion, the extractant chemicals in the organic solvent bind to the metal ions and pull the metal into the organic phase.

The mixture is left to settle so that the organic and aqueous phases separate. The organic phase containing the concentrated, purified metal is removed. The metal ions are then transferred from the solvent back into an aqueous solution from which they can be recovered in high purity.

"But there is a problem with SX," said Dr David Ralph, leader of a Parker Centre team which studies interactions between microbes and minerals. And its name is crud, he said.

In most SX circuits, instead of obtaining a sharp boundary between the two separated phases, crud or goop forms at the interface. Crud is a mixture of unseparated solvent and aqueous phases, stabilised by solids originating from the extracts of sand and crushed rock.

Dr Ralph said crud sometimes slows down separating the two phases and reduces the amount of solvent that can be recovered. "And when large amounts of crud form, it can bring the whole process to a halt while it is removed. So crud problems can be expensive problems."

About 18 months ago, a SX circuit at a minesite started behaving badly. Dr Ralph said a curious metallurgist examined its crud with a microscope and saw little filaments like string through it. So she sent him some samples. "We had a look under the microscope and it was alive with yeast - as single cells and long filaments," he said.

Although its main business is research, the Parker Centre runs a sideline - troubleshooting for the sometimes baffling short-term upsets in processing plant operations.

So the doctor was asked to make a plant call. He noted the symptoms of the sick SX circuit - increased losses of the expensive extractant chemicals, foam produced in some stages and excessive amounts of crud, teeming with yeast.

"There is no precedent for treating a problem like that and hydrometallurgists do not take kindly to things being added to their circuits," said Dr Ralph. "But after consultation, a remedy was prescribed." The circuit was treated with a biocide, a chemical that kills microbes. "And the symptoms faded away with normal operation resuming relatively rapidly."

"They'd had a yeast infection in their SX circuit. Since it's an open system and there's no way to sterilise it, all they can do is go into a management mode. A management strategy has been developed for their circuit that has prevented these plant excursions reoccurring."

Since then, the team has found yeast cells inside crud samples from other SX circuits they've examined. "It seems that circuits are colonised by microbes in the same way our skin surface is colonised. We think that unless you manage the circuit carefully, then depending on what you're producing and the particular conditions, you can get runaway infections which exacerbate crud problems." Dr Ralph believes SX operators need to be aware of this problem and that there are strategies to manage it, which can be tailored to a specific circuit.

"The fact that microbes can live on a diet of hydrocarbons such as those making up the organic solvent has been known for some time," he said. "So we should not have been surprised when we found them closely associated with SX problems. The real surprise was that cells would flourish in the extreme, often acidic, conditions at the hydrocarbon-aqueous interface of an SX circuit."

The researchers are now trying to find out more about these yeast. Although the infection problem can be fixed now, their work might suggest new ways to control the yeast which could become immune to biocides with time, like bacteria develop antibiotic resistance.

For further information, contact
Dr David Ralph, tel: (08) 9360 2887, e-mail: dralph@chem.murdoch.edu.au

By Ros Dilworth, Communications Officer, the Parker Centre.
First published in the Minerals Gazette.