Bio-reduction of sulfide minerals to recover invisible gold

Sulfide minerals, like pyrite and arsenopyrite, are of economical interest due to the presence of invisible gold locked inside these minerals. As fine grinding is often not sufficient to liberate the gold from these minerals, additional destruction techniques, based on chemical and biological oxidation processes, are required to access the gold via cyanidation. These techniques have proven to be successful to reach satisfactory gold recoveries, but operation costs are high and challenging waste streams (sulfuric acid) are produced. As an alternative to the oxidation methods in this thesis the bio-reduction of sulfide minerals was proposed and investigated. Bio-reduction, the use of hydrogen to convert mineral-sulfur to hydrogen sulfide, has as major advantage that the hydrogen sulfide can be recovered from the solution (to produce bio-sulfur) leaving a waste stream without diluted sulfuric acid. Furthermore, electrical energy will be saved. Theoretically, this more environmental friendly alternative should work, but no bio-reduction reaction was observed in practice. It appeared that sulfur/sulfate reducing bacteria were not able to use sulfur when fixed in the crystal lattice of pyrite and arsenopyriteat the selected conditions (pH 5, 35ºC). In order to make the mineral-sulfur bio-available for these bacteria the sulfur should first enter the solution. As alternative the combination between partial bio-oxidation and bio-reduction was therefore investigated. Partial bio-oxidation at pH 2 results in the formation of elemental sulfur, which can serve as a substrate at pH 5 for the sulfur/sulfate reducing bacteria. This combined method was found to be successful at 35°C, as the gold leachability of the used concentrate was increased from 6% to 39%. Optimization of this process is needed to reach gold recoveries >90% in 1 or 2 ox/red cycles, but when successful a new method (called the Paroxsul process) with a lower environmental impact, less costs, and application to a large number of minerals, is ready to be introduced to the precious metal industry.