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, XSiO2= 0.635 and T= 1723 K, XSiO2= 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 XCaO= 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.