Domesticating the Gold Bugs, and the Copper Bugs Too
About a year ago, one of these columns reported research showing that at least some of Alaska's fine placer gold was---so to speak---bug sweat. Bacterial metabolism apparently has been responsible for generating a lot of gold dust. Now it turns out that bacteria may be put to work releasing the metal from gold-bearing ores.
To be fair, this isn't a new idea. University of Alaska researchers have been studying metal-extracting microbes for some time, and generations of miners have made use of bacterial action on ores without necessarily realizing that's what they were doing. Two thousand years ago, the Romans noticed that the runoff from the tailings pile of one of their copper mines in Spain was blue with copper salts. They found ways to recover the leached copper without worrying about how the metal entered solution. Forty years ago, someone finally figured that out, and blamed it on bacteria.
The tiny rod-shaped bacterium known as Thiobacillus ferrooxidans gets energy by oxidizing some inorganic materials such as sulfide-containing minerals. As the bacteria metabolize, they release acid and an oxidizing solution of ferric ions, which can wash metals right out of ore. The copper industry quickly and enthusiastically put this discovery to work.
Biological heap leaching is an inexpensive way to extract the metal from low-grade ores where copper is bound in a sulfide matrix. As the microbes chew up the ore, which has been treated with sulfuric acid to encourage them, the copper is released and concentrated in a solution that flows into a catch basin. The metal is extracted, and the acid solution is recycled. According to the journal Science, from which I gathered this information, fully 25 percent of the world's copper--worth about $1 billion annually--comes from such bioprocessing.
Though the busy bacteria may some day help extract copper from Alaskan ores, it's a sure bet they'll first see employment here as gold bugs. Elsewhere, T. ferrooxidans is pretreating gold-bearing ores to the satisfaction of mining companies ... to their considerable profit. Low-grade gold ore often contains the metal bound up with sulfides, and typically requires roasting or pressure oxidation to burn off the sulfides before the gold can be extracted with cyanide. Using bacteria does away with the need for the costly cooking treatments, and in at least one instance has improved the rate of gold recovery from 70 to 95 percent.
That instance was not in Alaska, nor even in the United States. Gencor, the company testing a process involving vats of bacterial brew, first employed it at their Fairview mine in South Africa. Pleased with the pilot test, Gencor managers intend to expand the biomining efforts to mines in Brazil, Ghana, and Australia.
Efforts to bring bioleaching technology to North American gold mines presently are led by Newmont Mining in Nevada. Newmont has obtained a patent for a simpler extractive process that sets bacteria to extracting gold from ores once thought so low-grade as to be worthless. The Newmont process puts the bacterial brew, including fertilizers to encourage the microbes, directly onto piles of crude ore; it's a genuine heap-leaching process. The Newmont people report it works, and it's about $2 a ton less expensive than conventional methods.
The next step the researchers foresee is to breed better bacteria. Right now, the hard-working microminers are wild-caught, naturally occurring bacteria, which (as one of the researchers put it) is about like using wild wheat for farming or an unselected strain of Penicillium for producing penicillin.
Some of those better bacteria will be genetically engineered, but others will stem from the same sort of selection processes that have improved farm animals over time. For example, researchers at the University of Illinois have found strains of Thiobaccilus that resist the toxic effects of heavy metals found with or used in processing gold, such as arsenic and mercury.
