Wear of carbon cathodes in cryolite-alumina melts

Abstract

Wear of carbon cathode blocks is one of the main factors limiting the lifetime of aluminium electrolysis cells. A laboratory test cell set-up has been designed to investigate the wear mechanism. A homogeneous graphite material was exposed to three different bath compositions (CR = 2.25, 1.80 and 1.50) with variable cathode rotation speed, cathodic current density and time of electrolysis. The wear was strongly influenced by bath acidity, i.e higher wear was observed at CR = 1.8, probably due to higher carbide solubility in the bath. The wear also increased with current density at CR = 1.8, indicating an electrochemical wear mechanism. The cathode samples have been investigated after the tests using optical and scanning electron microscopy. Aluminium carbide was only found at the surface area of the cathode that was protected by a sintered alumina lining, while on the areas directly exposed to the electrolyte no carbide was detected.