Cathodic process and cyclic redox reactions in aluminium electrolysis cells

Abstract

The cathode processes in aluminium electrolysis cells are discussed, with detailed descriptions of the chemical reactions and transport processes leading to loss of current efficiency with respect to aluminium. The cathode current consuming reactions can be described by (i) the aluminium formation reaction, and (ii) reduction reactions forming so-called dissolved metal species (reduced entities). The rate determining steps for the aluminium forming process are mass transport of AIF₃ to the metal surface, and mass transport of NaF away from the metal surface. In commercial cells there is continuous feed of impurity species to the electrolyte, depressing the concentration of dissolved metal species to very low equilibrium values in the bulk phase of the electrolyte. However, the equilibrium values of reduced entities in the electrolyte at the metal surface are much higher than in the bulk phase. This means that polyvalent impurity species are involved in cyclic redox reactions in the electrode and gas boundary layers. The most important rate-determining steps related to these cyclic processes are (i) mass transport of reduced entities from the metal surface to a reaction plane within the cathode boundary layer, and (ii) mass transport of impurity species from the electrolyte bulk phase to the reaction plane in the cathode boundary layer. This means that there is negligible transport of dissolved metal species through the bulk of the electrolyte phase during normal operation of commercial cells.