An XPS study of the oxidation of pyrite and pyrites in coal

Abstract

X-ray Photoelectron Spectroscopy (XPS) was used to study mineral, synthetic and coal-associated pyrites, oxidized for various time intervals at low temperatures with humid air or oxygen. This was done to find out if XPS could detect, monitor and clarify pyrite surface-oxidative changes that influence surface-dependent coal-cleaning methods such as froth flotation, and could provide a means of directly analysing coal sulphur, by determining if oxidizing conditions existed which would effectively eliminate the surface pyrite whose XPS peak may occur at the same energy as the organic sulphur peak of coal. The conditions of study were as follows: a mineral and two coals containing pyrite were exposed to air at 24 ± 3 ° C and 33 ± 8% relative humidity up to 600 h; two mineral pyrites were exposed to oxygen at 100% relative humidity and 35 ° C for up to 200 h; and the two mineral and a synthetic pyrite were exposed to oxygen at 100% relative humidity and 55 ° C for up to 300 h and at 72°C for 25 h. The results indicated that the XPS S2p pyrite peak at ≈169 eV and the surface-oxidation-product(s) peak(s) at ≈163 eV could be detected and followed with XPS, although no conclusions could be made about the oxidation mechanism. The pyrite XPS peak became small compared to that of its oxidation products when the synthetic and mineral pyrites were exposed to 55 ° C oxygen at 100% relative humidity for 300 h. These conditions may prove useful in trying to determine directly the organic sulphur in coal.