Quantum chemical prediction of the spectroscopic properties and ionic composition of the molten NaF-AlF3 salts

Abstract

The behavior of the ions in the molten NaF-AlF3 salts plays a significant role in aluminium reduction process. Many experimental and modelling works have been carried out to investigate the ionic composition of the molten cryolite, which are still controversial. In this study, a recent thermodynamic model was introduced to compare with the Temkin model, which was assumed to consist of F−, AlF4−, AlF52−, AlF63−, Al2F7−, Al2F93−, Al2F104−, and Al3F145−. The thermodynamic calculations were performed to investigate the influence of the cryolite ratios on the ionic composition of the molten NaF-AlF3 and calculate the fractions of all the possible anions in the molten salts. On the other hand, the Raman spectra of the fluoro-aluminate ionic clusters were obtained by the quantum chemical calculations considering the existence of the cations Na+ and the number of the ionic pairs in the structural models of the ionic clusters, which were used to parsing the high temperature in situ Raman spectrum of the molten cryolite. The ionic composition of the molten salts was discussed based on the thermodynamic calculations and the fitting results of the high temperature in situ Raman spectrum of the molten cryolite.