Bioleaching of Covellite from Low Grade Copper Sulphide Ore and Tails

The economic importance of copper production in Chile has prompted the study of new strategies for the optimal management of mineral resources, including processing of low grade ore and even processing tails (processed ore). Currently, heap bioleaching is the most convenient technology to treat low grade copper sulphide ores bearing chalcopyrite, chalcocite and covellite. Covellite is a secondary copper sulphide occurring in copper sulphide deposits and it is also formed by acidic dissolution of chalcocite. In leaching processes, the main factors influencing the dissolution of covellite are the electrochemical potential (Eh) and temperature.This work aimed at assessing the effect of different concentrations of total iron (3.0 and 6.0 g/L) and temperatures (30 and 50°C) in the bioleaching of covellite contained in a low grade copper ore and the same ore after a previous process of bioleaching at ambient temperature. Initial composition of low-grade ore was chalcocite (0.58%), chalcopyrite (0.56%), covellite (0.30%) and pyrite (2.73%), while the processed ore contained chalcopyrite (0.47%), covellite (0.28%), chalcocite (0.10%) and pyrite (3.32%). Covellite was mostly dissolved (>90%) in both unprocessed and processed ore, independently of the temperature and total iron concentrations. However a higher total copper recovery was achieved in tests performed at 50°C due to higher dissolution of primary ores, especially in unprocessed ore. The microbial activity was confirmed by Most Probable Number (MPN) analyses of iron and sulfur oxidizing microorganisms. The microbial population was dominated by Acidithiobacillus spp. in tests operated at 30°C, while in tests operated at 50° Sulfobacillus spp. were the most abundant organisms.