The conventional flotation can't provide fair results when applied to very low grade ores or to "refractory" ores. This class includes intimate and nonuniform mineral associations, with partialy oxidized minerals and high secondary mineral content and also high soluble salt content, preactivated minerals because of the excessive permeability of the deposit and of the intense circulation of waters with heavy metal ions etc. Bacterial oxidation as a means to the recovery of metals from sulphidic deposits has been used specially gold recovery. The achieved results by microorganisms leaching by low grade cholcopyrite ores are folowing: gold recovery up to 85% and silver recovery up to 80%..
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