Influence of electrolyte impurities on current efficiency in aluminium electrolysis cells

Abstract

The influence of electrolyte impurity species on current efficiency with respect to aluminium (CE) was studied in a specially designed laboratory cell at 980∘C, with a graphite anode and a cathodic current density of 0.85Acm−2. The electrowinning was performed in a base melt of Na3AlF6 with a NaF/AlF3 molar ratio of 2.5 and with 4–6wt% Al2O3 and 5wt% CaF2. Impurity species, probably present in only one valence state in the electrolyte, Mg, Ba and B, had no measurable effect on CE for low impurity concentrations. Sn, added to the electrolyte as SnO2, also did not affect current efficiency, probably due to its low solubility. The results show a linear decrease in CE with increasing electrolyte concentration of the polyvalent impurity species from the elements, Fe, P, V, Si, Zn, Ti and Ga. The decrease was found to be within the range 0.1 to 0.7% in C E per0.01wt% of impurity cations present in the electrolyte, with phosphorus ions as the most detrimental. The effects of the individual impurity species on CE appear to be roughly additive for electrolytes containing more than one impurity species. The results obtained cannot be explained by a simple codeposition mechanism or a single reduction to a soluble species of a lower valency. The most likely mechanism explaining the observed decrease in CE for a large number of impurity species is cyclic redox reactions in the cathode and anode/CO2 interfacial boundary layers. Such a mechanism may also be the dominant one in commercial cells, since the impurity levels are of the same size as in the laboratory cell.