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Abstract

A method of leaching sulphidic material that contains metals and arsenic under oxidising conditions and with the aid of microorganisms is characterised by mixing the material with an acid aqueous solution to form a pulp, and in that the material is leached in a first leaching stage at a pH of below 2 and at a temperature that ranges from room temperature to about 55.degree. C. in the presence of an active quantity of microorganisms of the mesophilic and/or moderately thermophilic type, wherewith the major part of the arsenic content of the material and possibly also part of its metal content is/are leached-out and the arsenic leached from said material is oxidised successively to a pentavalent state. The remaining leachable metal content of the material is leached-out in a following leaching stage in the presence of an active quantity of an extremely thermophilic micro-organism and at a temperature within the active range of said micro-organism, said temperature being higher than the temperature in the first stage. The pH is suitably raised in the second stage to a level at which the pentavalent arsenic present in solution is caused to transform to a solid state and precipitate-out as a metal-containing arsenate which is separate from the leaching solution together with unleached material in the pulp after termination of the leaching process.

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