Microscopic surface processes observed during the oxidative dissolution of sphalerite

Sphalerite cleavage surface dissolution in acidic (HCl) and oxygen-saturated solutions was investigated by liquid-cell Atomic Force Microscopy (AFM). The sphalerite surface was cleaved in the laboratory and then mounted in the liquid cell at 298 K with a fast continuous renewal of the interacting solution. AFM data indicate that unreacted (110) surfaces are characterised by flat surface terraces delimited by step edges aligned along [110] crystallographic directions. AFM imaging allowed us to investigate removal of matter only at pH = 0 (HCl). Under these conditions, etch pits develop that are delimited by 1-3-nm-high step edges. However, surface terraces are covered by nanometric protrusions, while the step edges are microrough. Ex-situ solution chemistry measurements performed in flow-through-reactor indicates strong undersaturation with respect to both zinc sulphide and zinc sulphate. The reactivity of the dissolving (110) surface decreases significantly during the 24 hours of run time. Such a decrease suggests a change in the mechanism governing the overall dissolution process. We interpret nanometric protrusions as due to oxidative reactions at the interface that result in a reorganisation of the surface at the nanometric scale. The mechanism limiting the rate of sphalerite dissolution would be the process of protrusion formation and dissolution. A similar phenomenon was observed in an AFM study of the galena surface. Finally, we propose that the process of protrusion formation could be general in the oxidative dissolution of metal sulphides.