The initial products of the anodic oxidation of galena in acidic solution and the influence of mineral stoichiometry

Abstract

The oxidation products retained at the surface of galena samples from Broken Hill (lead-rich) and Missouri (sulphur-rich) following mild anodic treatment in acetate solution of pH 4.6 have been determined by X-ray photoelectron spectroscopy (XPS) using excitation both by Al K α X-rays and by synchrotron radiation (SR). Elemental sulphur was found to be the major oxidation product on both galenas. For short anodization times, S 0 could only be detected by the more surface sensitive SR-XPS. Electrochemical studies showed that the quantity of sulphur on a Missouri galena surface did not decrease significantly when the galena was held for 40 min at open circuit. This contrasts with previous studies on Broken Hill galena in which the sulphur coverage was found to decrease to a low value in this time period, a behaviour that was assigned to diffusion of lead atoms from the bulk. The different behaviour of the two galenas can be explained by the different stoichiometry of the Broken Hill and Missouri minerals. According to SR-XPS measurements, the intensity of the S 0 component in S 2p spectra for galena surfaces held for 40 min at open circuit following anodization at 0.3 V for 20 s was less than for the equivalent spectra carried out within 7 min of anodization. The S 0 intensity was negligible for the Broken Hill mineral in agreement with electrochemical studies. A small but significant decrease in the S 2p intensity was also observed for Missouri galena and this was assigned to accumulation of the sulphur product into clusters. S 2p components at the binding energies expected for lead polysulphide were also observed in the spectra for the Missouri mineral. Conventional XPS revealed a significant variability in the S 0 2p intensity following short anodization times and this could also arise from sulphur clustering.