The behaviour of sulphur in silicate and aluminate melts

Abstract As part of a general programme of research on the nature of inorganic melts, a study has been made of the manner in which sulphur is held in liquid silicates and aluminates. Mixtures of CaO-SiO2, MgO-SiO2, FeO-SiO2, CaO-Al2O3 and CaO-SiO2-Al2O3 have been brought into equilibrium, at temperatures between 1350 and 1650°C, with gas phases made by mixing H2, CO2 and SO2 at room temperature. It has been shown that, when pO2 is less than about 10–5 atm, a sulphur atom can only enter the melt by displacing a suitable oxygen atom. The sulphur is then held in the slag entirely as sulphide, its concentration being controlled by the general equilibrium, ½S2 + (O)melt = ½O2 + (S)melt. When pO2 is greater than about 10-3 atm, the sulphur is held as sulphate and the corresponding equilibrium is ½S2 + 3/2O2 + (O)melt = (SO4)melt∙ In the binary silicate melts there must be three kinds of oxygen atoms present: those bonded to two silicon atoms, those bonded to one silicon and those unattached to silicon. Their proportions are presumably connected by an equilibrium of the type O2– + ⋮ Si-O-Si ⋮ = 2(⋮ Si-O¯). From the manner in which both the sulphide and sulphate equilibria depend upon the proportions of metal oxide present, it appears that only the oxygen atoms which are unattached to silicon are of importance so far as these equilibria are concerned. The potential capacity of a melt to hold sulphur as sulphide is expressed as its sulphide capacity Cs = (wt. % S) (pO2)½/(ps2)½. For binary silicate and aluminate melts there is a close parallelism between the value of this capacity and the activity of the basic metal oxide. The activity coefficients for CaS have been found to be the same as for CaSO4 and approximately constant in CaO-SiO2 mixtures. The sulphide equilibrium results have been used to improve the free energy of formation curve for the formation of CaO-SiO2 melts, and to construct a similar curve for CaO-Al2O3 melts. The temperature coefficient of the sulphide capacity for CaO-SiO2 melts has been combined with other data and used to extend the heat of formation curve for CaO-SiO2 melts. The sulphur capacities, and presumably the lime activities, of the ternary melts CaO-SiO2 -Al2O3, have been shown to be a maximum at compositions for which the molar proportions of SiO2 and Al2O3are equal; this behaviour would be expected because the energy of interaction of SiO2 and Al2O3 is much greater than that of either CaO and SiO2 or CaO and Al2O3. It follows the pattern established for FeO activity coefficients in CaO-SiO2 and CaO-P2O5 melts, these rising to a maximum at CaO : SiO2 and CaO; P2O5 molar ratios of 2:1 and 3:1 respectively.