Chemical, crystallographic, and electromagnetic variability in natural chalcopyrite and implications for leaching

Leaching studies of chalcopyrite (CuFeS₂) notoriously reach divergent conclusions about kinetics, mechanism, and passivation behavior during leaching, even for the same lixiviant-oxidant systems. One seldom-explored reason for this is likely natural variability in the chemical, crystallographic, and electromagnetic properties of chalcopyrite samples tested. Chalcopyrite can accommodate up to almost 1% of Zn, Ag, Co, Se, and other galvanically active trace elements that catalyze or inhibit leaching depending on impurity siting and concentration. Chalcopyrite can be slightly over-saturated in Cu, leading to faster dissolution and possibly to enhanced passivation. Annealed chalcopyrite from metamorphosed or recrystallized deposits tends to have high crystallinity, but defects and lattice strain are common in other environments and also enhance leach kinetics. Though most chalcopyrites are n-type semiconductors, measured room-temperature resistivities vary over 4 orders of magnitude and an estimated 3–5% are weak to moderate p-type semiconductors. While being p-type semiconductors seems to lead to faster leaching, the effects of resistivity are ambiguous. A review of the literature shows that the analytical techniques employed in current hydrometallurgical investigations (powder XRD, SEM-EDS) are not capable of measuring most of these relevant characteristics. Thus, natural variability in chalcopyrite samples is largely unknown, and this likely accounts for some of the disparities in the results of leaching studies. While a comprehensive characterization is beyond the scope and equipment of most laboratories, analyzing samples with a broader range of common techniques (EPMA, LA-ICP-MS, resistivity and Hall effect measurements) would go most of the way toward resolving the uncertainties in chalcopyrite leaching studies.