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See Inside December 2011

Microbe Miners

Bacteria extract metals and clean up the mess afterward

MINING HASN'T CHANGED MUCH SINCE THE Bronze Age: to extract valuable metal from an ore, apply heat and a chemical agent such as charcoal. But this technique requires a lot of energy, which means that it is too expensive for ores with lower metal concentrations.

Miners are increasingly turning to bacteria that can extract metals from such low-grade ores, cheaply and at ambient temperatures. Using the bacteria, a mining firm can extract up to 85 percent of a metal from ores with a metal concentration of less than 1 percent by simply seeding a waste heap with microbes and irrigating it with diluted acid. Inside the heap Acidithiobacillus or Leptospirillum bacteria oxidize iron and sulfur for energy. As they eat, they generate reactive ferric iron and sulfuric acid, which degrade rocky materials and free the valued metal.

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