Electrocapillarity in the aluminum reduction cell

Abstract

The effect of the current density on the interfacial tension between aluminum and cryolite containing melts was measured based on the sessile drop method and an X-ray radiographic technique. The experiments were carried out under constant current densities in graphite crucibles with BN lining. When the aluminum drop was the cathode, the interfacial tension was almost independent of the current density. During electrolysis, the interfacial tension increased with decreasing NaF/AlF₃ ratio in a similar manner to that observed when no electrolysis was performed. The interfacial tension between aluminum and an electrolyte containing between 5 to 10 wt pct A1F₃, 5 wt pct CaF₂, and 5 wt pct A1₂O₃ is 690 ± 60 mN/m for cathodic current densities between 0.1 and 0.6 A/cm₂. Interruption of electrolysis caused an instantaneous decrease in the interfacial tension followed by a slow increase with time. This sudden drop together with a decrease in interfacial tension with reversal of cell polarity indicate that the metallic side of the interface has an excess positive charge. The interface was enriched with NaF during electrolysis as indicated by the slow recovery of the interfacial tension after current interruption.