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

Abstract

The electronic polarisability of F αF 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 αF 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 αF than CaF2, which would be a sign of Al–F and/or Si–F bonds being produced. From the value of αF, 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.