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-SiO$_{2}$, MgO-SiO$_{2}$, FeO-SiO$_{2}$, CaO-Al$_{2}$O$_{3}$ and CaO-SiO$_{2}$-Al$_{2}$O$_{3}$ have been brought into equilibrium, at temperatures between 1350 and 1650 degrees C, with gas phases made by mixing H$_{2}$, CO$_{2}$ and SO$_{2}$ at room temperature. It has been shown that, when p$_{\text{O}_{2}}$ 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, $\frac{1}{2}$S$_{2}$ + (O)$_{\text{melt}}$ = $\frac{1}{2}$O$_{2}$ + (S)$_{\text{melt}}$. When p$_{\text{O}_{2}}$ is greater than about 10$^{-3}$ atm, the sulphur is held as sulphate and the corresponding equilibrium is $\frac{1}{2}$S$_{2}$ + $\frac{3}{2}$O$_{2}$ + (O)$_{\text{melt}}$ = (SO$_{4}$)$_{\text{melt}}$. 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 O$^{2-}$ + $\vdots $Si-O-Si$\vdots $ = 2($\vdots $Si-O$^{-}$). From the manner in which both the sulphide and sulphate equilibria depend upon the proportions of metal oxide present, it appears that only the oxygen atoms which are unattached to silicon are of importance so far as these equilibria are concerned. The potential capacity of a melt to hold sulphur as sulphide is expressed as its sulphide capacity C$_{\text{S}}$ = (wt.% S) (p$_{\text{O}_{2}}$)$^{\frac{1}{2}}$/(p$_{\text{S}_{2}}$)$^{\frac{1}{2}}$. 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 CaSO$_{4}$ and approximately constant in CaO-SiO$_{2}$ mixtures. The sulphide equilibrium results have been used to improve the free energy of formation curve for the formation of CaO-SiO$_{2}$ melts, and to construct a similar curve for CaO-Al$_{2}$O$_{3}$ melts. The temperature coefficient of the sulphide capacity for CaO-SiO$_{2}$ melts has been combined with other data and used to extend the heat of formation curve for CaO-SiO$_{2}$ melts. The sulphur capacities, and presumably the lime activities, of the ternary melts CaO-SiO$_{2}$-Al$_{2}$O$_{3}$, have been shown to be a maximum at compositions for which the molar proportions of SiO$_{2}$ and Al$_{2}$O$_{3}$ are equal; this behaviour would be expected because the energy of interaction of SiO$_{2}$ and Al$_{2}$O$_{3}$ is much greater than that of either CaO and SiO$_{2}$ or CaO and Al$_{2}$O$_{3}$. It follows the pattern established for FeO activity coefficients in CaO-SiO$_{2}$ and CaO-P$_{2}$O$_{5}$ melts, these rising to a maximum at CaO:SiO$_{2}$ and CaO:P$_{2}$O$_{5}$ molar ratios of 2:1 and 3:1 respectively.