Mining with microbes: a labor of bug.

Mining with microbes: A labor of bug

Back in the days when forty-niners were miners, the gold bug drove folks mad for the motherlode. Today's Forty-Niners play football in San Francisco and dream of getting rich doing TV commercials. But the gold bug lives on in a new, improved form. Indeed, modern gold bugs do the mining themselves.

The bugs are bacteria, mostly strains of the naturally occurring Thiobacillus ferrooxidans. Science-savvy prospectors report increasing success using these single-celled slaves to retrieve gold, copper and other valuable minerals from otherwise unprofitable mines.

T. ferrooxidans has long thrived on mineral-rich rock. The bacteria oxidize sulfide-bound minerals and proliferate in the acidic solutions created when rainwater reacts with ore. Mining companies long viewed these microbes as pests because their metabolic munching unleashes sulfur-entrapped toxins such as cyanide and arsenic. But the process also releases quantities of valuable metals whose concentrations are too low to extract economically by traditional methods.

In recent years, scientists have taken advantage of these reactions by "seeding" heaps of ore with bacteria that release bound-up copper. Mining companies dispose of the toxic runoff but keep the copper. Now the technique has begun to show promise as a means of recovering gold, cobalt and uranium.

After studying 28 strains of T. ferrooxidans collected from mines around the world, researchers at the Idaho National Engineering Laboratory in Idaho Falls report they have isolated one variant that leaches out about 10 percent of the available cobalt from low-grade ore. Their experiments so far have been small in scale, within 3-foot-long tubes. But the method shows promise as an economical way to extract the metals, says Karl Noah, a senior engineer on the project. Both known cobalt deposits in the United States today remain unexploited because conventional methods of recovery have proved too expensive.

Cobalt is a critical ingredient in some metals that retain their strength at high temperatures, such as the alloys used in aircraft engine parts. If scale-up experiments funded by the U.S. Bureau of Mines confirm current projections, cobalt mining may become a matter of "sprinkling the bacteria on piles of ore that are 20 feet high and as long as you want," Noah says. After hosing down these heaps with acidified water, scientists can recover the cobalt-rich runoff and extract the mineral using traditional electrochemical methods.

A similar technique shows promise for mining refractory gold ores, in which gold is bound to iron and sulfur, says William Reid, president of the Denver-based U.S. Gold Corp. The company's newly constructed mill in Tonkin Springs, Colo., pretreats these ores with a strain of T. ferrooxidans to liberate the gold, which is then recovered by conventional methods. As easily extracted gold-oxide ores become depleted in the next few years, "bioleaching" will become increasingly commonplace among gold-mining operations, Reid and others predict.