Concentration gradients of individual anion species in the cathode boundary layer of aluminium reduction cells

Abstract

It is well known that the system NaF-AlF3, which constitutes the “backbone” of the electrolyte used in primary aluminium manufacture, forms Na+, F-, and a number of fluoro-aluminate anion complexes in the molten state. Since mainly the Na+ ion carries electric current, the aluminium-containing complexes must diffuse towards the cathode, resulting in concentration gradients in the cathode boundary layer. Starting from a structural model for the melt containing five anion species, it was possible to calculate the concentration gradients of the individual ions using the Stefan-Maxwell equation for diffusion in a multi-component system. Generally, AlF4- and Al2F7- were transported towards the cathode, while F- and AlF63- moved away from the cathode. For NaF/AlF3 molar ratios higher than 2.0, AlF52- moved towards the cathode, while it diffused away from the cathode in more acid melts.