Relaxation of the lead-deficient sulfide surface layer on oxidized galena

The surface oxidation of galena in air and in aqueous media has been investigated using electron spectroscopy and linear potential sweep voltammetry to elucidate the incongruent oxidation indicated by previous X-ray photoelectron spectroscopic studies. It has been confirmed that the initial oxidation reaction involves the removal of lead atoms from the sulfide lattice to generate a metal-deficient surface layer. It has been shown that, when the mineral is removed from the oxidizing environment, the composition of this layer relaxes slowly back towards that of the bulk mineral as a result of lead atoms diffusing from the bulk to the surface to fill metal vacancies in the sulfide lattice. The relaxation accounts for the absence of a shifted component in sulfur electron spectra from galena exposed to air or basic solutions in equilibrium with air. Correlation of the potential dependence of self-induced flotation and the extent of surface sulfur excess determined electrochemically indicates that a metal-deficiency equivalent to about half a monolayer of excess sulfur is necessary for significant flotation to be effected.