The dissolution of sphalerite in ferric sulfate media

The dissolution of sphalerite, (Zn,Fe)S, in ferric sulfate media was investigated using closely sized fractions of crushed sphalerite crystals. Linear kinetics were observed, and the rate increased in proportion to the surface area, as the average particle size of the sphalerite decreased. The predominant reaction products are ZnSO4, FeSO4, and elemental sulfur. The leaching rate increases with increasing temperature, and the apparent activation energy is 44 kJ/mol. The relatively high apparent activation energy suggests that the rate is chemically controlled, a conclusion supported by the insensitivity of the rate of the rotation speed that was observed in complementary rotating disk experiments. The rate increases as the 0.3 to 0.4 power of the Fe(SO4)1.5 concentration, and is nearly independent of the pulp density, in the presence of a stoichiometric excess of ferric sulfate. In 0.3 M Fe(SO4)1.5 media, the rate increases with increasing acid concentrations >0.1 M H2SO4, but is insensitive to more dilute acid concentrations. In the absence of ferric ions, the rate increases rapidly with increasing H2SO4 concentrations, and relatively rapid rates are observed in solutions containing >0.5 M H2SO4. The rate decreases with increasing initial concentrations of ZnSO4, MgSO4, or FeSO4 in the ferric sulfate leaching solution, and this emphasizes the importance of maintaining the dissolved iron in a fully oxidized state in a commercial leaching operation.