Wear and Sodium Expansion by Aluminum Electrolysis in Alumina Slurries

Abstract

The physical abrasion, electrochemical corrosion and sodium expansion of carbon cathode materials were studied in cryolitic melt-alumina slurries. The physical abrasion of graphite is about 5 times the physical abrasion of anthracitic carbon, this is similar to the room temperature result. The electrochemical corrosion is a much stronger wear process than the physical abrasion, but it is suppressed by excess alumina addition. Different carbon cathode materials have approximately the same resistance to the electrochemical corrosion. In the acidic, cryolitic melt-alumina slurry, a 25% to 65% increase in expansion was observed compared to the expansion in the corresponding slurry-free melt. The larger expansion is attributed to diffusion hindrance by the slurry resulting in an increased sodium concentration on the interface. The expansion increased radically in the basic cryolitic melt-alumina slurry compared to the expansion in the corresponding slurry-free melt: 180–560%. This large expansion increase is considered due to diffusion hindrance combined with precipitation of β-alumina on the interface. The present findings constitute a strong argument against basic start-up and muck-up of the cell bottom during this period.