A multi-spectroscopic investigation of sulphur speciation in silicate glasses and slags

Sulphur K-edge x-ray absorption near-edge structure (XANES), sulphur Kα and Kβ high resolution x-ray emission spectroscopies (XES), electron paramagnetic resonance (EPR) and optical absorption spectroscopies have been used to study the speciation of sulphur in a range of soda-lime-silica glasses and silicate slags. Several inorganic standards with known sulphur oxidation states and structural environments have also been analysed. Results confirm that the average oxidation state of sulphur in glasses decreases, as expected, in the order (colourless>light olive>dark olive>light amber>dark amber). This behaviour is consistent with decreasing S6+/ΣS ratio, which has been quantified by linear combination fitting of XES S Kα spectra, and with analysed sulphur contents which exhibit a characteristic relationship with oxygen partial pressure, pO2. A combination of S6+, S5+, S4+ and more reduced sulphur species has been detected in olive and amber glasses. The S4+ and S5+ species are most evident in olive-coloured glasses produced under moderately reducing conditions that coincide with minimum sulphur solubilities. The reduced form of sulphur, present in all reduced glasses, is interpreted as being present as S2− on the basis of XANES, XES, EPR and optical measurements. An alternative interpretation of the data is that there is a continuum of less strongly reduced species, primarily S+ and S2+; this interpretation has less merit. In this paper we show that the established relationship that describes sulphur redox only in terms of S6+ and S2−, and which states that only these two species co-exist over a narrow, moderately reducing range of pO2, does not fully describe the behaviour of S in the industrial, non-equilibrated glasses studied. Hence this relationship requires slight modification for non-equilibrated systems to explain the existence of intermediate sulphur oxidation states such as S4+ and S5+, particularly within the intermediate pO2 range that corresponds with sulphur solubility minima.