/ 24 January 1997

They’re out of sight, but they can make you rich

AMONG all the miniscule creatures that live around us, invisible to the human eye, is a strange, rod-shaped bacterium called Thiobacillus ferrooxidans. It does not cause disease, cannot pollute the environment and is happiest in an extremely acidic environment where it can “eat” the minerals in rock.

It is these creatures (and their relatives) that added colour to the Johannesburg urban landscape many years ago by turning its most notable landmarks – the mine dumps — from grey to yellow.

A conference of the South African Society of Biochemistry and Molecular Biology heard this week that these tiny bacteria are worth millions to Gencor (and South Africa) as the mining company has developed a process using them to release gold. Gencor currently markets the process and uses it for joint projects with other companies.

The process works something like this: mines produce “refractory gold”, gold that is scattered in particles through sulphide minerals. The minerals around the gold do not dissolve easily, and so getting the gold out by conventional means is a costly process.

But the minerals that lock in the gold are minerals the bacteria can use to “feed” their energy needs. So Gencor scientists grow the bacteria in vast colonies, adapt them to function efficiently in the sulphide environment and release them on the ore.

After about four days of the bacteria’s work, known as “oxidation”, the gold is liberated enough for the mining company to extract about 97% of it.

Gencor’s Dr David Dew, who gave a paper this week at the conference in Grahamstown, describes the bacterium as shaped like an unshelled peanut, with a surface coating that it uses to attach to the mineral it is going to oxidise.

He says the bacterium is small in real terms and can only be seen under a microscope with a magnification of 700. “But it’s big for a bacteria.”

He says Gencor has designed five plants that use the process around the world, and two more will come into operation in the next three years.

The technology is unique to Gencor, which means the company can charge a licence fee for the companies that want to use it for their refractory gold. But Gencor is keen on benefiting from the technology by forming commercial partnerships using it as their stake in the deal, says Dew.

Gencor researchers are also looking for ways to use this type of mineral-eating bacteria for nickel ores, in a similar process.

Besides saving money, the process is more environmentally friendly than conventional technology. There are no by-products (like sulphur dioxide in one process) and it uses less energy than high-temperature processes.