CATHODIC PROCESS AND CYCLIC REDOX REACTIONS IN ALUMINUM ELECTROLYSIS CELLS

The cathode processes in aluminium electrolysis cells are discussed, w ith detailed descriptions of the chemical reactions and transport proc esses leading to loss of current efficiency with respect to aluminium. The cathode current consuming reactions can be described by (i) the a luminium formation reaction, and (ii) reduction reactions forming so-c alled dissolved metal species (reduced entities). The rate determining steps for the aluminium forming process are mass transport of AlF3 to the metal surface, and mass transport of NaF away from the metal surf ace. In commercial cells there is continuous feed of impurity species to the electrolyte, depressing the concentration of dissolved metal sp ecies to very low equilibrium values in the bulk phase of the electrol yte. However, the equilibrium values of reduced entities in the electr olyte at the metal surface are much higher than in the bulk phase. Thi s 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 pl ane within the cathode boundary layer, and (ii) mass transport of impu rity species from the electrolyte bulk phase to the reaction plane in the cathode boundary layer. This means that there is negligible transp ort of dissolved metal species through the bulk of the electrolyte pha se during normal operation of commercial cells.