Some Phenomena Taking Place at the Electrodes in Aluminium Cells with Inert Anodes

Inert anodes may replace carbon anodes in the future, but there are still many obstacles to overcome. The present paper deals with three different phenomena related to inert anodes. With anodes based on nickel ferrite, consumption of the anode itself by an "autocannibal" reaction will take place at low alumina concentration. The risk is higher at high bath acidity. On the other hand, a highly acidic bath has lower nickel solubility, which may be necessary for avoiding excessive anode wear and contamination of the metal during normal operation. The bath at the cathode is more acidic than the bulk, which may lead to formation of solid cryolite at wetted cathodes. This can only be avoided by using a very high superheat. The entropic heat effects in a cell with inert anodes turn out to be very similar to the conditions prevailing in a Hall-Héroult cell. There is a relatively strong cooling of the anode, a moderate cooling of the bath, and a moderate heating of the cathode. The effects can be regarded as beneficial, as they counteract dissolution of the anode and crystallisation at the cathode.