STRUCTURES OF OXYFLUOROALUMINATES IN MOLTEN CRYOLITE-ALUMINA MIXTURESINVESTIGATED BY DFT-BASED CALCULATIONS

Aluminum is produced throughout the world using the Heroult-Hall proce ss, which consists in the electrolysis of alumina dissolved in cryolit e melts. This dissolution of alumina gives rise to the formation of ox yfluoroaluminates, the structures of which vary with the composition o f 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 alum ina in molten cryolite has led us to undertake structural, energetic a nd vibrational studies of these species using the methods of computati onal chemistry. Conformational and vibrational analyses of the ionic c omplexes were performed with the DGauss software on Gray C98 and C916 machines located at Gray Research, Eagan, MN, USA. A systematic study was carried out on the complexes [AlOFx](1-z) (1 less than or equal to x less than or equal to 5), [Al2OFx](4-x) (4 less than or equal to x less than or equal to 9), [Al2O2Fx](2-x) (2 less than or equal to x le ss than or equal to 7), which are those most commonly suggested for in terpreted experimental measurements. The Al4OF10 and Al4OF82+ complexe s were also investigated because of their possible occurrence in the e lectrorefining process of aluminum. Some of the studied complexes are much more stable than the others and so are the complexes most probabl y involved in the electrowinning process of aluminum. These complexes are AlOF2-, Al2OF62- and Al2O2F42-. Their infrared spectra have also b een calculated. Concerning the two complexes Al4OF10 and Al4OF82+, we find that oxygen is three-coordinated and not four-coordinated. The la st atom of aluminum is stabilized by two fluoride bridges. This comput ational approach has enabled us to eliminate the majority of the struc tures suggested in the literature for oxyfluoroaluminate ions. Current ly we are examining in our laboratory the effects of cations on the an ionic complexes.