XPS and SR-XPS techniques applied to sulphide mineral surfaces

X-ray photoelectron spectroscopy (XPS) is one of the major techniques in basic research of flotation-related surface studies of sulphide minerals. The advantages of the method, in general, are good surface sensitivity, rather straightforward elemental and chemical state analysis and reliable quantification of the data. The most serious disadvantage is the ex-situ nature of the technique. Surface sensitivity of the XPS technique can be considerably improved if an optimal excitation energy is chosen for a characteristic emission of each different element. This can be done by using a synchrotron radiation (SR) source, providing a continuous energy distribution over large energy region, instead of a fixed excitation energy source (either AlKalpha or MgKalpha radiation from sealed off X-ray tube) used in conventional XPS. In the case of identification of sulphur species, which often is the main task in surface analysis of sulphide surfaces, this leads to approximately an order of magnitude improvement in surface sensitivity and, hence, essentially better possibility to detect surface species of submonolayer coverage. Air oxidation of PbS and FeS2 has been studied with SR excitation. On PbS cleavage surface, oxidized sulphur (sulphate and/or polysulphide type) and lead species were detected after 10 min exposure time in air at room temperature, indicating congruent oxidation of PbS surface. The oxidation reactions proceeded relatively quickly during the first 4 h of contact with air. SR-XPS experiments with cleaved FeS2 also show the formation of sulphate as a primary oxidation product in air. However, the initial layer, formed immediately after cleavage, seemed to passivate the surface against further oxidation. (C) 1997 Elsevier Science B.V.