Current efficiency in the Hall–He´roult process for aluminium electrolysis: experimental and modelling studies

Results are presented from a laboratory study of the influence of electrolyte composition, temperature, cathodic current density and interpolar distance on the current efficiency with respect to aluminium (CE). The current efficiency was determined from the weight gain of metal, in a laboratory cell designed to attain good and reproducible convective conditions, and with a flat cathode surface which ensures uniform cathodic current distribution. The cell is believed to more closely represent conditions in industrial cells than traditional small-scale cells, and is a good basis for an experimental study of the influence of isolated variable parameters on the current efficiency with respect to aluminium. The results show a nonlinear decrease of CE with increasing electrolyte temperature, a close to linear decrease of CE with increasing NaF/AlF3 ratio in the electrolyte, a slight increase of CE with increasing electrolyte CaF2 concentration, and no influence of electrolyte Al2O3 concentration on CE. A current efficiency model, based on previous work and theory of electrochemistry and mass transport, shows good agreement with the obtained results.