Abstract
Proper understanding of the sideledge in aluminium electrolysis cells is important for making the right decisions in cell design and operation. A conceptual model for the lower ledge, based on the existence of an alumina-saturated bath film stemming from sludge, was published earlier. In the current study, the rates of freezing and melting were studied in a simplified model, without taking diffusion across the film into account. By considering the heat- and mass balances during freezing or melting of sideledge, it turned out that the system will approach thermal balance as the film creeps upwards along the ledge. As a result, melting and freezing are rapid close to the bottom of the film, while ledge near the top of the metal changes only slowly depending on the thickness of the film. Melting or freezing may also be initiated by variation in the concentration of aluminium fluoride in the sludge.