COMPUTATIONAL AND ANALYTICAL-CHEMISTRY - METHODOLOGY TO STUDY CHEMICAL-REACTIONS BETWEEN SODIUM, CALCIUM, AND ALUMINUM FLUORIDES IN MOLTEN CRYOLITE

Reaction constants and composition profiles in molten cryolite have be en theoretically investigated. Hartree-Fock and density functional cal culations were applied to determine the exact nature (structure and en ergetic) of complexes existing in molten cryolite. The aim of this wor k was thus the understanding of chemical processes occurring in the el ectrowinning of aluminum and was a demonstration of how computational chemistry (based on density functional theory) can help us to determin e structures and reaction energies in particularly complex medium such as cryolite. An analytical study, based on mass balance and equilibri um constants has been undertaken. This was performed on molecular Liqu id entities taking into account the four-, five- and sixfold coordinat ed aluminum complexes of the AlF3-3NaF melt system. Moreover, the effe ct of calcium has been studied by substituting two sodium atoms with o ne calcium atom, thus leading to the CaNaAlF6 system. Two conformers ( instead of three for Na) were obtained for this system They can be des cribed as representing the four- and fivefold coordinated aluminum com plexes in molten cryolite. The structurizing effect of calcium was cle arly illustrated by the resulting optimized structures, showing that c alcium stabilizes the TV and V coordinations of aluminum. By computing reaction constants, we have obtained composition profiles that are pr esented with those based on experimental data. Comparisons point out t hat computational chemistry techniques match with experimental results , especially in the case of pure cryolitic melts. For the presence of the fivefold coordinated aluminum complex in cryolite, and the predomi nance of the fourfold coordinated complex with calcium, it is clear th at these computational techniques show us correct trends in predicting the main species in molten media. (C) 1997 John Wiley & Sons, Inc.