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Gold Mine

 

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Overview
When you think of the gold rush days in the Old West, you probably picture the miner as a grizzled prospector with his mule and pickax. Gold mining today, however, is a scientific process that uses computers, geologic data, chemistry, microbiology, and sophisticated refining equipment to extract trace amounts of gold from rock blasted out from deep underground. If you look at a map of an underground mine, such as the one that the Homestake Mining Company operates in South Dakota, it looks like a very orderly ant farm, with rooms carved out of the solid rock for machine shops, laboratories, and other facilities. Data from geological core samples goes into a computer that makes a drawing of an area (like a connect-the-dot map in three dimensions) and tells the miners where to find gold-bearing rock. The miners then drill a series of precision holes into the rock face, pack in explosives, clear everyone out, and blast. After checking for gas leaks, workers reinforce the walls and ceiling to prevent cave-ins and then hoist the ore out of the mine through the vertical shafts. Gold, a pure element identified with the chemical symbol Au, exists in nature. (Fool's gold, a compound of iron and sulfur called a pyrite, looks a little like gold.) Tiny gold particles are encased in tons of rock, so the ore first goes to a mill where it is crushed very fine. The larger particles separate from the ground rock on a vibrating table that works on the same principle as panning, in which substances of different densities separate from each other. The smallest particles of gold then are dissolved (leached) out of the ground rock with a weak cyanide solution. This still doesn't get all the gold, and it leaves some very toxic wastewater behind, but miners have some valuable new helpers for both of these problems: bacteria. Some bacteria chew up the cyanide in wastewater. Others chemically alter stubborn rocks so that the cyanide treatment can be more effective. This is called bioleaching. A side benefit of bioleaching is the fast production (and consequent treatment) of acids that would otherwise leak slowly from the sludge into the environment.

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