Study on the dissolution of aluminium carbide formed on the graphite cathode in aluminium electrolysis

Abstract The behaviors of the ions in the molten NaF-AlF3-Al4C3 salts play significant roles in mechanism of cathode wear in aluminium reduction process. Models were proposed based on the fitting the experimental solubility of Al4C3 in molten salts, while ionic composition of the molten NaF-AlF3-Al4C3 salts are still controversial, and the spectroscopic investigation is rarely performed. In this paper, the influence of CR on the ionic structure of the molten NaF-AlF3-Al4C3 salts at 1300 K was investigated by thermodynamic calculation based on the models containing NaF, NaAlF4, Na2AlF5, Na3AlF6, and different combinations of the carbon-containing solutes, such as Na3Al3CF8, Na4Al4CF12, NaAl3CF6, Na6Al2CF8, and so on. On the other hand, the Raman spectra of carbon-containing solutes were obtained by the quantum chemical calculations. A scheme was performed to calculate the high temperature Raman spectra of the molten NaF-AlF3-Al4C3 salts based on the thermodynamic and quantum chemical calculations, in which the characteristic peaks of Na3Al3CF8 were determined at 445 cm−1 and 715 cm−1. Furthermore, the high temperature in situ Raman spectra were obtained to clarify the ionic composition of the molten NaF-AlF3-Al4C3 salts, in which the existence of Na3Al3CF8 in the molten NaF-AlF3-Al4C3 salts has been confirmed.