Thermodynamic properties of CaF2+SiO2+CaO melts

The thermodynamic properties of CaF2 + SiO2 + CaO melts were studied in the composition ranges (mol %) 0-100 CaF2, 0-56 SiO2, and 3-58 CaO at temperatures between 1452 and 1823 K by Knudsen effusion measurements in combination with mass spectrometry of vapor species. The major vapor species identified are CaF2, SiF4, SiO and MO(2) (M = Nb, Ta, Mo). The MO(2) molecules result from melt reduction by the effusion-cell material. The measured activities of CaO, SiO2, and CaF2 are independent of measurement procedure and effusion-hole area. The experimental data set contains more than 1200 activity values for all components and various melt temperatures and compositions. In modeling the thermodynamic properties of CaF2 + SiO2 + CaO melts, we use an associated solution approximation. The parameters characterizing association reactions between SiO2 and CaO, SiO2 polymerization, and interactions between associated solution components agree with those measured in the constituent binary systems. The thermodynamic data are used to calculate phase equilibria in the system studied.