The effect of ore mineralogy on the speciation of arsenic in bacterial oxidation of refractory arsenical gold ores.

Abstract

The bacterial leaching of refractory arsenic-bearing gold ores from different deposits were investigated to determine the effects of mineralogy on the fate of arsenic during processing and to assess the effectiveness of biological pre-treatment on gold extraction. Experiments were conducted using a mixed culture of iron and sulphur oxidising bacteria dominated by Thiobacillus ferrooxidans . The speciation of arsenic during bioleaching was studied using polarography. Leaching of a concentrate which was predominantly arsenopyrite resulted in 80% of the leached arsenic reporting in solution as arsenic(Ill) at the end of the bacterial oxidation stage. Study of the oxidation of concentrates from different ore deposits with various ratios of pyrite to arsenopyrite showed that the amount of pyrite was an important factor in determining the dominating arsenic oxidation state in the leach solutions. Iron(Ill) alone was found not to be able to oxidise arsenic(m), instead pyrite was needed as a catalyst. It was found that adding pyrite and iron(m) to arsenopyrite concentrates promotes the formation of arsenic(V) leaving less than 1% of the extracted arsenic as arsenic(m). Furthermore a strong dependence of pyrite leaching and arsenic(Ill) oxidation was demonstrated. These results demonstrate the importance of ore composition in terms of pyrite to arsenopyrite ratio in the production of environmentally acceptable biooxidation residues containing arsenic(V).