Fundamental studies of the contribution of galvanic interaction to acid-bacterial leaching of mixed metal sulfides

A systematic study has been made towards understanding the role of galvanic interactions in the leaching of sulfide minerals both in the absence and presence of bacteria. When two sulfide minerals are in contact with each other in an acid-aqueous solution, the mineral lower in the electromotive series dissolves rapidly while the one higher in the series is galvanically protected. To ascertain the magnitude of galvanic interaction when chalcopyrite (CuFeS 2) and pyrite (FeS 2) are in contact, potentiodynamic polarization measurements were carried out in 1 M H 2SO 4 and in Bryner and Anderson medium. The individual rest potentials of CuFeS 2 and FeS 2 were found to be 0.52 V and 0.63 V [vs. Standard Hydrogen Electrode (SHE)] respectively. The mixed potential of the CuFeS 2: FeS 2 couple was found to be 0.56 V and 0.28 V (vs. SHE) in 1 M H 2SO 4 and in Bryner and Anderson medium respectively. The corrosion current (5 μA/cm 2) was calculated from the polarization curves. These results were found to be in agreement with the actual leaching rates of copper from contacting large mineral specimens composing the CuFeS 2/FeS 2 system. Both powdered samples as well as large mineral specimens of CuFeS 2, FeS 2 and ZnS were used to observe the effect of galvanic interactions on the process of leaching. Chalcopyrite and pyrite powders, as pulp mixtures, were leached under a variety of experimental conditions to optimize their ratio (total pulp density) and size fraction for efficient metal extraction by way of maximum contact with each other. A CuFeS 2:FeS 2 ratio of 5:5 (g/g) and size fraction of −200 mesh was found to be the most preferable when leaching was carried out in the presence of T. ferrooxidans. The effect of temperature and thermophilic bacteria on the rate of leaching was also studied. The presence of pyrite in the optimized quantities considerably enhanced the rate of copper dissolution which was increased, by a factor of 2, to 15. Under these experimental conditions, E h increased from 338 mV to 580 mV while pH dropped from 2.30 to 1.56. In order to have a definite control over surface area of the ores and a constant contact between two sulfide ores, large crystalline-mineral samples of CuFeS 2, FeS 2 and ZnS were leached singly as well as in intimate contact (coupled galvanically). This also made it possible to observe the surfaces of the specimens as the leaching proceeded in the presence of galvanic interaction. Coupled CuFeS 2/FeS 2, ZnS/FeS 2 and Cu