Numerical analysis of ionic mass transfer in the electrolytic bath of an aluminium reduction cell

In the Hall-Héroult process, the electrolytic bath is a molten solution of cryolite and alumina. Like all other molten salts, it ends up in different moving ions driven by physical mechanisms such as convection, diffusion and migration. The motion of these ions and their concentration distribution are important because they determine many functional macroscopic parameters of the electrolytic cell like current density distribution, heat generation, back reactions, current efficiency, and mass-transfer controlled reactions at the electrodes. In this study, a numerical model for the fluxes of most important ions in a NaF-AlF₃-Al₂O₃ system has been proposed. The reactions in the bath and the resulted ions have been added to the reactions that take place at the cathode and anode, and a finite element model has been presented for the electrolyte portion of the aluminium reduction cell. The transient motion of the different ions under the migration and diffusion mechanisms have been modelled based on the classical mass transfer equations. The results illustrate the significant role of the migration in the early stages of electrochemical process. This mechanism is also the dominating effect in the motion of nonelectroactive species. For larger time scales, because of the depletion of the consumed species and accumulation of the produced species near the electrodes, the mass transfer is dominated by the diffusion.