PHASE-EQUILIBRIA IN THE CAF2-AL2O3-CAO SYSTEM

Computations of phase equilibria in the CaF2-Al2O3-CaO system have been carried out on the basis of experimentally found thermodynamic properties of all intermediate phases and melts. Coordinates of the phase equilibrium boundaries were determined by solving a system of equations expressing equality of chemical potentials of the components in coexisting phases. The nature and quantity of the coexisting phases were established by a search for the Gibbs energy minimum of the system. All the phases of the CaF2-Al2O3-CaO system were taken into consideration. Calculated phase diagrams of the CaO-CaF2, CaO-Al2O3 and CaF2-Al2O3 binary subsystems are in good agreement with the data available in the literature. Isotherms of the CaF2-Al2O3-CaO system were calculated at 1600, 1650, 1723 and 1773 K. A wide region of liquid separation into two phases is observed in the system. One phase is composed of practically pure CaF2 with additions of several mol% of CaO and Al2O3, and the other consists of 50 to 65 mol% of CaF2 only. Eleven invariant points of the CaF2-Al2O3-CaO system include seven ternary eutectics, two ternary peritectics and two points of four-phase monotectic transition. The primary fields of crystallization of all the phases are alongated toward the CaF2 apex, the CaO field being the widest and the 3CaO.Al2O3 field the narrowest. Seven junctions of the CaF2-Al2O3-CaO phase diagram were represented. Computed saturation lines of CaF2-Al2O3-CaO melt with CaO, Al2O3, CaO.6Al2O3 and CaO.2Al2O3, and also the positions of a number of characteristic points, agree well with the experimental data available. The present calculations reveal a number of details and peculiarities of the constitution of the CaF2-Al2O3-CaO phase diagram.