Compositional and structural alteration of pyrrhotite surfaces in solution: XPS and XRD studies

Ground pyrrhotite (Fe_1-xS) surfaces oxidised by exposure to (i) air, (ii) water and (iii) de-oxygenated perchloric acid solution (0.05-1M) were examined using X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). In air or water, the surfaces form amorphous layers containing carbonate species; sulfate species; iron(III) oxide/hydroxides; and an iron-deficient sulfide species with an S(2p) doublet shifted 1.0-1.8 eV to higher binding energy (BE). After acid reaction, the surface partly restructures to a crystalline, defective tetragonal Fe₂S₃ product in which linear chains of S_n atoms have a S-S distance similar to elemental sulfur (S₈) but the S(2p) BE is still 0.2-0.6 eV less than S₈. Initially, the acid-reacted surface may be partly hydrophobic, giving flotation separation, but, as oxidation proceeds, hydrophilic iron hydroxides deposit on the surface depressing flotation. The chemical forms of Fe and S in the surface layers are discussed in detail with changes in the proportion of the oxidised and iron-deficient sulfide products.