Thermodynamic Investigation on Ionic Structure and Electrical Conductivity of Molten NaF-AlF3 Salts

Abstract

The behavior of the ions in the molten NaF-AlF3 salts plays a significant role in aluminum reduction process. Many modelling works have been carried out to investigate the physical properties of the molten salts and the mass transfer behavior in the aluminum reduction cells. The Temkin model was widely used in the models above, in which the molten salts were assumed to be consisted by F−, AlF4−, AlF52−, AlF63−, and Al2F7−. However, beside those in the Temkin model, some additional fluoro-aluminate complexes were reported as well, such as Al2F93−, Al2F104− and Al3F145−. In this paper, the influence of the cryolite ratios (CR) and the temperature on the ionic structure of the molten salts was investigated by thermodynamic calculation based on the Temkin model combining with the Al2F93−, Al2F104− and Al3F145−. The ionic structure of the molten salts with the various CR was discussed by analyzing fractions of all the anions. Additionally, the calculation of electrical conductivity of molten NaF-AlF3 salts based on thermodynamic model involving eight anions was performed to illustrate the effect of fluoro-aluminate complexes on the electrical conductivity, which is compared and agrees well with the experimental results reported in the previous work.