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 degrees 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: 1/S dC/dt = (2.0 +/- 0.2) a(Fe3+)/a(H+)(0.7) where 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 (M-0.7 h(-1)) and a(Fe3+) and a(H+) 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 Fe-2(SO4)(3)) at 75 degrees C under various pH and Eh regimes. The only condition under which this rate model was found not to hold was at simultaneously low a(Fe3+) and high a(H+), that is at pH 1 and a(Fe3+) < 5 x 10(-5) M, where the concentration of dissolved O-2 may be leach rate determining. (c) 2010 Elsevier Ltd. All rights reserved.