Silver-catalyzed bioleaching of low-grade copper ores.: Part I: Shake flasks tests

A study of the effect of different variables (inoculation, pulp density, [Ag], nutrient medium, pH and [Fe3+]) on the silver-catalyzed bioleaching of a low-grade copper sulfide ore has been carried out in shake flasks. Chalcopyrite was the dominant copper mineral in the ore. Preliminary tests showed that addition of other ions (Sb, Bi, Co, Mn, Ni and Sn) did not enhance the copper dissolution rate. Conversely, an inoculation with mesophilic microorganisms and the addition of silver had a markedly catalytic effect on the extraction of copper. The kinetics of the silver-catalyzed chalcopyritic ore bioleaching was greatly affected by pulp density and silver concentration. Small amounts of silver (14.7 g Ag/kg Cu) dramatically accelerated the copper dissolution process while large amounts (294.12 g Ag/kg Cu) had an inhibitory effect. The copper dissolution rate was slightly affected in the range of pH between 1.2 and 2.5 but was significantly slower at pH 3.0. The effect of [Fe3+] in the presence of silver was studied both in abiotic and biotic conditions. High ferric iron concentrations in abiotic tests recovered similar copper amounts (∼ 95%) to those obtained without or with low [Fe3+] in the presence of bacteria. The leaching of copper from the low-grade copper ore can be very effectively enhanced with silver and mesophilic microorganisms. For that system, the onset of oxidizing conditions starts at an Eh value slightly higher than 650 mV. Above that critical value of potential the copper dissolution rate slows down. This also corresponds with the completion of the leaching process. As the potential rises past 650 mV, the copper extraction reaches a plateau.