Physicochemical properties of Na<sub>2</sub>SO<sub>4</sub>-CaSO<sub>4</sub> melts

The aim of the work was to study the properties of melts of the Na2SO4–CaSO4 system, which are best suited for practical application in metallurgy, for example, for improving the operation of industrial furnaces. In addition, information on the properties of this system is of interest for applied chemistry of molten salts, for example, for cleaning gases, metal and ceramic surfaces of structural elements. The properties of melts of binary sulfate systems, necessary for selecting application parameters, have not been studied sufficiently. The most important for practice are density, surface tension and dynamic viscosity. The work uses modern experimental research methods: maximum pressure in an argon bubble blown into the melt through a capillary to determine the density and surface tension, and the vibration method – for dynamic viscosity. The temperature and concentration ranges of measurements were 1050–1200°C and 0–60 mol. %. For the entire studied range of melt compositions, linear dependences of density and surface tension on temperature were obtained. Dynamic viscosity obeys an exponential dependence. As a result of statistical processing of the experimental data, general equations of polytherms of density, surface tension and viscosity were derived. Analysis of the experimental results showed that the values of density, surface tension and viscosity have deviations from additivity, which reflects a change in the structure of sodium sulfate – calcium sulfate melts with an increase in the concentration of CaSO4. Comparison of the obtained experimental data with those already known for binary carbonate melts indicated changes in the structure of the melts, in particular, the formation in the volume, in addition to the Na+ and Ca2+ cations and the SO42- anion, of complex anions [NaSO4]- predominantly on the surface and [Ca(SO4)]n- predominantly in the volume.