Thermodynamic properties of the intermediate phases of { xCaF 2 + ySiO 2 + (1 − x − y)CaO}

Abstract

A Knudsen-effusion method with mass-spectrometric analysis of evaporation products has been used to study the gaseous phase over {xCaF2 + ySiO2 + (1 − x − y)CaO} and to determine thermodynamic properties of intermediate phases. The temperature range studied was 1370 K to 1849 K. Compositions varied in the range x = 0 to 1, y = 0 to 0.67, and (1 − x − y) = 0.03 to 0.77. Double effusion cells made from niobium, tantalum, and molybdenum of high-purity grades were used. In mass spectra of the saturated vapour the ions SiF+3, SiF+2, SiF+, CaF+2, CaF+, Ca+, SiO+, MeO+, and MeO+2 (where Me is Nb, Ta, or Mo) were detected. A method of mass-spectral decoding was developed. Experiments on isothermal evaporation of slags as well as determination of the vapour pressures of components as functions of temperature were used to calculate partial pressures of components. All the regions of the heterogeneous equilibria were studied, with the exception of those where intermediate phases did not exist. Molar Gibbs energies of formation of the intermediate phases CaO·SiO2, 3CaO·2SiO2, 2CaO·SiO2, and 3CaO·SiO2 were calculated with the help of partial characteristics of CaO and SiO2 determined simultaneously for a number of compositions corresponding to different phase fields of {xCaF2 + ySiO2 + (1 − x − y)CaO} as well as found by integration of Gibbs-Duhem equations. Compounds 2CaO·SiO2 and CaO·SiO2 were found to be the most stable in the system. Molar Gibbs energies of formation of the intermediate phases obtained in present work were compared with literature values.