Chalcopyrite leaching: The rate controlling factors

The processes that determine the rate of chalcopyrite leaching are central to understanding how chalcopyrite (CuFeS2) behaves under the environmentally adverse conditions of acid rock drainage. To this end the effect of the acid anion on chalcopyrite leach rates using a variety of acidic media (H2SO4, HClO4, HCl and H2SO4 with 0.25 M NaCl) under carefully controlled solution conditions (pH 1 and 2, Eh 750 mV (SHE) and 75 °C) has been examined. These conditions have been chosen to enable sufficient leach rates for accurate experimental determination and to compare to the previous mechanistic analysis carried out by Harmer et al. (2006).Extensive surface analysis of leach residues demonstrated that variations in the surface speciation could not be responsible for the observed variations in leach rate. The rate of Cu release, however, was found to be first order with respect to Fe3+ activity and inversely proportional with respect to H+ activity to the power of 0.7: 1SdCdt=(2.0±0.2)aFe3+aH+0.7where S is the relative surface area, C is concentration of Cu in the solution (M), t is the time (h), 2.0 is the rate constant (M0.7 h−1) and aFe3+aFe3+ and aH+aH+ are Fe3+ and H+ activities, respectively (M). The rate model was further validated against additional leaches carried out in H2SO4 media with the initial addition of Fe3+ (8 mM as Fe2(SO4)3) at 75 °C under various pH and Eh regimes. The only condition under which this rate model was found not to hold was at simultaneously low aFe3+aFe3+ and high aH+aH+, that is at pH 1 and aFe3+<5×10-5M, where the concentration of dissolved O2 may be leach rate determining.