Thermodynamic calculation of phase equilibria in the CaF2–SiO2–CaO system

Abstract

Thermodynamic properties of intermediate phases and melts of the system CaF₂–SiO₂–CaO, found experimentally, have been applied to computation of phase equilibria. Four intermediate phases CaO·SiO2, 3CaO·SiO₂, 3CaO·2SiO₂, 2CaO·SiO₂ and two deep eutectics at T= 1717 K, X_SiO2= 0.635 and T= 1723 K, X_SiO2= 0.445 situated on either side of CaO·SiO₂ exist in the binary system CaO–SiO₂. The CaF₂–CaO binary system has a simple eutectic diagram with the eutectic point at T = 1631 K and X_CaO= 0.198. Isotherms of the CaF₂–SiO₂-CaO phase diagrams were calculated at 1473, 1573, 1700, 1773 and 1823 K. Four pseudobinary joins in the CaF₂–SiO₂–CaO phase diagram were also computed. A large region of liquid immiscibility is predicted to exist in the system and confirmed experimentally. One liquid phase is composed of almost pure CaF₂ with additions of several mol% CaO and SiO₂, and the other contains 56–70 mol% CaF₂. There are five invariant points in the CaF₂–SiO₂–CaO system, four of them being ternary eutectics and one being a ternary peritectic. The primary phase fields of crystallization of all the phases are elongated toward the CaF₂ apex; those of the end-member components are the widest, and those of the compounds 3CaO·SiO₂ and 3CaO·2SiO₂ are the narrowest. Experiments were carried out to confirm the reliability of the locations of the computed phase equilibria boundaries.