A comparison of the kinetics and mechanism of acid leaching of sphalerite containing low and high concentrations of iron

Abstract

The leaching behaviour of three Fe-containing sphalerite minerals (0.45, 11.40, and 12.90 wt.% Fe) was investigated at pH 1 in O 2-purged HClO 4 from 25 to 85 °C. For all three sphalerite samples, the leach data closely fitted a rate expression which incorporates both a shrinking sphere term and rate control via diffusion through a reacted surface layer. The leach rate increased with increasing Fe content of the sphalerite. At 85 °C after 168 h of leaching, 0.89 of the Zn had been leached from the 12.90 wt.% Fe-containing sphalerite while, over the same period of time, 0.49 of the Zn had leached from the 0.45 wt.% Fe-containing sphalerite. The activation energies for leaching decreased with increasing Fe content of the sphalerite (i.e. for Zn leaching 63±6, 50±7 and 39±2 kJ mol −1 for the 0.45, 11.40 and 12.90 wt.% Fe-containing sphalerite samples, respectively). The activation energy of leaching of Fe and Zn was within experimental error for the 11.40 and 12.90 wt.% sphalerite samples. The solution Fe content was not measured reliably for the 0.45 wt.% Fe-containing sphalerite. The leaching mechanisms have been interpreted as a function of surface speciation, which was analysed using time of flight secondary ion mass spectrometry (ToF-SIMS), X-ray photoelectron spectroscopy (XPS), powder X-ray diffraction (XRD) and scanning electron microscopy (SEM) for samples leached at 85 °C. The decrease in Zn dissolution rate on leaching is attributed to the formation and growth of a polysulfide surface layer, which forms during the initial rapid leach period. The formation of elemental sulfur during the subsequent slow leach period does not further affect the leach rates. The concentrations of polysulfide and elemental sulfur that form increase with increasing Fe concentration as expected from the higher leach rates. The formation of polysulfide and elemental sulfur on the ZnS surface consumes H + (via conversion to H 2O) thus decreasing solution acidity.