Bioleaching of fluoride-bearing secondary copper sulphides: Column experiments with Acidithiobacillus ferrooxidans

Abstract

Bioleaching is a mature technology, which is widely employed commercially in the leaching of low-grade secondary copper sulphide ores. In this work, the bioleaching potential of a low-grade (marginal) ore with a significant content of fluoride was assessed. Small columns were inoculated with a strain of At. ferrooxidans, at 30°C, and resulted in copper extractions above 89%, in 255days. The dissolution of fluorite from the gangue minerals accounted for up to 2.5g/L fluoride in solution, which affected bioleaching shortly after column inoculation. However, both the released of aluminium from the ore and ferric iron production by the bacteria reduced fluoride toxicity. A fluoride-toxicity parameter (η) was proposed to represent the mass ratio between total fluoride, total aluminium and total ferric iron concentrations in the system. Low values of the η parameter resulted from high ferric iron and aluminium concentrations and implied in large bacterial population in the systems. Thus, the presence of fluoride-bearing minerals in the ore may be an important issue, but the content of both cations should be also considered. Despite the absence of ferric iron and a high fluoride concentration prior to first solution bleeding, the η parameter was lower than 0.3 in the column containing an external aluminium source. In this condition, as the aluminium content surpassed that of fluoride the bacterial population reached 107cells/mL and enabled copper bioleaching.