BIOLEACHING - METAL SOLUBILIZATION BY MICROORGANISMS

Bioleaching is a simple and effective technology for metal extraction from low-grade ores and mineral concentrates. Metal recovery from sulf ide minerals is based on the activity of chemolithotrophic bacteria, m ainly Thiobacillus ferrooxidans and T. thiooxidans, which convert inso luble metal sulfides into soluble metal sulfates. Non-sulfide ores and minerals can be treated by heterotrophic bacteria and by fungi. In th ese cases metal extraction is due to the production of organic acids a nd chelating and complexing compounds excreted into the environment. A t present bioleaching is used essentially for the recovery of copper, uranium and gold, and the main techniques employed are heap, dump and in situ leaching. Tank leaching is practised for the treatment of refr actory gold ores. Bioleaching has also some potential for metal recove ry and detoxification of industrial waste products, sewage sludge and soil contaminated with heavy metals.