The catalytic effect of silver on acidic ferric-sulfate leaching of chalcopyrite: A microscopic cyclic reaction

Copper extraction from chalcopyrite is challenging, because acid dissolution is slow, occurring incongruently via a complex three-step reaction mechanism. Silver has been known to catalyse copper extraction from chalcopyrite since the 1970's; yet the mechanism remains controversial. Microcharacterisation of experimental products obtained under optimal leaching conditions (50–150 μm chalcopyrite grains in ferric/ferrous-sulfate solution with a redox potential around 500 mV vs. Ag/AgCl, approximately 1ppm Ag; Ag 6.4 × 10^-6 mol/L; 70 °C; 4 days) highlights the heterogeneity of the reaction: µm-thick layers of a porous copper-sulfide with variable composition formed both in cracks within, and on the surface of the chalcopyrite grains. There is no evidence for formation of Ag-rich phases (Ag₂S_(s), Ag⁰_(s)). The fundamental three-step reaction mechanism remains the same with or without added silver; silver merely accelerates the initial dissolution step.An integrated model for the catalytic effect of silver is proposed that incorporates recent advances in the reactivity of sulfide minerals. The initial reaction follows a ‘Fluid-Induced Solid State Diffusion Mechanism’, where diffusion of Fe in the chalcopyrite lattice is driven towards the surface by its rapid removal into solution, resulting in a Fe-deficient surface layer. The large Ag⁺ ion, relative to Cu⁺/Fe³⁺, diffuses into this Fe-deficient surface layer and accelerates chalcopyrite dissolution in the subsequent step, whereby chalcopyrite is replaced by copper sulfides via an interface coupled dissolution reprecipitation reaction as a consequence of the sulfide-rich micro-environment at the mineral surface. Effective Ag⁺ recycling is key to the catalytic effect of silver, and occurs as a result of the strong affinity of Ag⁺ for bisulfide ligands accumulating at the surface of dissolving chalcopyrite.