Chemical state of fluorine in fluoroaluminosilicate slags in glassy and molten states from perspective of electronic polarisability

Abstract

The electronic polarisability of F α _F has been determined for fluoroaluminosilicate slags from density and refractive index data measured to investigate the chemical state of F in the slags and the effect of CaF₂ additions on depolymerisation of the aluminosilicate network. To simulate commercial mould fluxes, synthetic slags with compositions of xCaF₂–5Na₂O–6Al₂O₃–(89–x)CaO.SiO₂ were prepared as samples, where x ranged between 5 and 12·5 mol.-%. Densities and refractive indices of the slags were measured in glassy and molten states; for the melts, the sessile drop method and ellipsometry were applied to density and refractive index measurements respectively. Values of α _F determined for the slags are on the whole in good agreement with those for CaF₂, irrespective of temperature, which suggests that F ions in the slags are in ionic bonding of Ca–F with a possibility of a Na–F bond. However, only a glassy slag with a lower concentration of CaF₂ exhibits a slightly smaller α _F than CaF₂, which would be a sign of Al–F and/or Si–F bonds being produced. From the value of α _F, F ions are likely to terminate silicate anions by forming Ca–F bonds in the slags and depolymerisation of the network by this termination would be responsible for lowering the viscosity of molten slags with CaF₂ additions.