Thermodynamic Investigation on Ionic Structure and Electrical Conductivity of Molten NaF-AlF₃Salts

Abstract

The behavior of the ions in the molten NaF-AlF 3 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 − , AlF 4 − , AlF 5 2− , AlF 6 3− , and Al 2 F 7 − . However, beside those in the Temkin model, some additional fluoroaluminate complexes were reported as well, such as Al 2 F 9 3− , Al 2 F 10 4− and Al 3 F 14 5− . 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 Al 2 F 9 3− , Al 2 F 10 4− and Al 3 F 14 5− . 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-AlF 3 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.

Review—Primary Production of Aluminium with Oxygen Evolving Anodes

“…Meanwhile, the concentration of complex ions such as AlF , and Al 3 F 14 5− complex ions are also present in the cryolite system, and they influence the conductivity to some extent. 80 The electrical conductivity of the NaF-AlF 3 decreases with a decrease in CR and the melt's operating temperature (in Fig. 8).…”

Anode Process on Platinum in Low Temperature KF-AlF3-Al2O3 Based Melts: An Update

“…Proposed mechanism of the anodic process on platinum.-According to spectral data, 10 . Also, according to thermochemical estimates, [43][44][45][46] the energy of chemical bonds of atoms in the Al 2 O 2 F 4 2ion (per oxygen atom) is lower than in the Al 2 OF 6 2ion. Consequently, it can be assumed that the first of these ions (Al 2 O 2 F 4 2-) is more active during anodic (electrochemical) oxidation, while the first oxygen atom cleaves off from it during oxidation easier than the second.…”