Michel Leroy

Structures of oxyfluoroaluminates in molten cryolite-alumina mixtures investigated by DFT-based calculations

Abstract Aluminum is produced throughout the world using the Heroult-Hall process, which consists in the electrolysis of alumina dissolved in cryolite melts. This dissolution of alumina gives rise to the formation of oxyfluoroaluminates, the structures of which vary with the composition of the electrolyte mixture. Up to now there has been no real consensus as to the structural entities of these fluoroaluminate ions. The lack of knowledge of the ionic species arising from the dissolution of alumina in molten cryolite has led us to undertake structural, energetic and vibrational studies of these species using the methods of computational chemistry. Conformational and vibrational analyses of the ionic complexes were performed with the DGauss software on Cray C98 and C916 machines located at Cray Research, Eagan, MN, USA. A systematic study was carried out on the complexes [ AlOF x ] 1− x (1 ≤ x ≤ 5), [ Al 2 OF x ] 4− x (4 ≤ x ≤ 9), [Al 2 O 2 F x ] 2−x (2 ≤ x ≤ 7), which are those most commonly suggested for interpreted experimental measurements. The Al 4 OF 10 and Al 4 OF 8 2+ complexes were also investigated because of their possible occurrence in the electrorefining process of aluminum. Some of the studied complexes are much more stable than the others and so are the complexes most probably involved in the electrowinning process of aluminum. These complexes are AlOF 2 − , Al 2 OF 6 2− and Al 2 O 2 F 4 2− . Their infrared spectra have also been calculated. Concerning the two complexes Al 4 OF 10 and Al 4 OF 8 2+ , we find that oxygen is three-coordinated and not fourcoordinated. The last atom of aluminum is stabilized by two fluoride bridges. This computational approach has enabled us to eliminate the majority of the structures suggested in the literature for oxyfluoroaluminate ions. Currently we are examining in our laboratory the effects of cations on the anionic complexes.