THE INITIAL PRODUCTS OF THE ANODIC-OXIDATION OF GALENA IN ACIDIC SOLUTION AND THE INFLUENCE OF MINERAL STOICHIOMETRY

The oxidation products retained at the surface of galena samples from Broken Hill (lead-rich) and Missouri (sulphur-rich) following mild ano dic treatment in acetate solution of pH 4.6 have been determined by X- ray photoelectron spectroscopy (XPS) using excitation both by Al K-alp ha X-rays and by synchrotron radiation (SR). Elemental sulphur was fou nd to be the major oxidation product on both galenas. For short anodiz ation times, SO could only be detected by the more surface sensitive S R-XPS. Electrochemical studies showed that the quantity of sulphur on a Missouri galena surface did not decrease significantly when the gale na was held for 40 min at open circuit. This contrasts with previous s tudies on Broken Hill galena in which the sulphur coverage was found t o decrease to a low value in this time period, a behaviour that was as signed to diffusion of lead atoms from the bulk. The different behavio ur of the two galenas can be explained by the different stoichiometry of the Broken Hill and Missouri minerals. According to SR-XPS measurem ents, the intensity of the S-o component in S 2p spectra for galena su rfaces 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-o intensity was negligible for the Broken H ill mineral in agreement with electrochemical studies. A small but sig nificant decrease in the S 2p intensity was also observed for Missouri galena and this was assigned to accumulation of the sulphur product i nto clusters. S 2p components at the binding energies expected for lea d polysulphide were also observed in the spectra for the Missouri mine ral. Conventional XPS revealed a significant variability in the S-o (2 )p intensity following short anodization times and this could also ari se from sulphur clustering. (C) 1998 Elsevier Science B.V.