Surface Reactions in Aqueous Metal Sulfide Systems: 5. The Complexation of Sulfide Ions at the ZnS-H2O and Pbs-H2O Interfaces

Abstract

The reactions between hydrous ZnS or PbS surfaces and sulfide ions have been studied. The measurements were performed as potentiometric titrations in 0.1 M NaNO3 or 0.1 M NaClO4 media at 25°C. Besides determining -log[H+] using a glass electrode, log[S2-] was measured with an Ag/Ag2S selective electrode. Synthetically prepared ZnS (predominantly wurtzite) and PbS, as well as the minerals sphalerite and galena, were studied. When adding sulfide ions to acidic or slightly neutral suspensions, bulk precipitation of metal sulfides occurred. The solubility products for the metal sulfides formed were determined and found to be log K(Me2+ + HS- ⇌ MeS(s) + H+) = 10.6 for ZnS and 15.6 for PbS (synthetic preparations). Sulfide additions to neutral and alkaline suspensions resulted in adsorption of H2S, HS-, and S2-. In the PbS-system the best fit was found with a model with the surface complexes ≡SPbSH2, ≡SPbSH-, and ≡SPbS2- without any charge dependent surface complexation constants. In the ZnS-system two models were found to fit experimental data almost equally well. One with the three surface complexes ≡SZnSH2, ≡SZnSH-, and ≡SZnS2- and no charge dependence or one with two surface complexes (≡SZnSH2 and ≡SZnS2-) with charge dependent equilibrium constants. Experimental data were evaluated using the computer program FITEQL. The results explain part of the mechanism behind depression of sulfide minerals in the flotation process.