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

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 CaF ₂ additions on depolymerisation of the aluminosllicate network. To simulate commercial mould fluxes, synthetic slags with compositions of xCaF ₂-5Na₂O-6Al₂O₃-(89 - x)CaO.SiO ₂ 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 CaF ₂ additions.