Thermodynamics of fluoride vaporisation from slags containing CaF ₂ at 1773 K

The changes in the composition of the CaO-SiO₂-CaF₂ and CaO-Al₂O₃-CaF₂ slags were measured after an equilibration at 1773 K to deduce the vaporisation loss of fluoride species. Furthermore, the volatilisation phenomena were discussed based on the thermodynamic fundamentals. In the CaO-SiO₂-CaF₂ system, the SiO₂ and CaF₂ contents simultaneously decreased at basicity B(≡(%CaO)/(%SiO₂)) ≤ 1.0, while the content of CaF₂ only decreased at basicity of the slags greater than about unity. On the other hand, the content of CaF₂ only decreased in the entire composition range in the CaO-Al₂O₃-CaF₂ system. The volatilisation behaviour of fluoride species from the slags could be explained from the thermodynamic assessment by taking the activity and the activity coefficient of CaF₂ into account. From the measured results and the thermodynamic calculations, it could be concluded that the major volatilisation species in the relatively acidic (B ≤ 0.8-1.0) silicate melts would be SiF₄ (g), while the CaF₂ (g) would be a dominant vaporisation species in the relatively basic (B ≥ 1.0) silicate and in the aluminate melts.