SIDELEDGE FACING METAL IN ALUMINIUM ELECTROLYSIS CELLS: FREEZING AND MELTING IN THE PRESENCE OF A BATH FILM

摘要

Proper understanding of the sideledge in aluminium electrolysisrncells is important for making the right decisions in cell design andrnoperation. A conceptual model for the lower ledge, based on thernexistence of an alumina-saturated bath film stemming fromrnsludge, was published earlier. In the current study, the rates ofrnfreezing and melting were studied in a simplified model, withoutrntaking diffusion across the film into account. By considering thernheat- and mass balances during freezing or melting of sideledge, itrnturned out that the system will approach thermal balance as thernfilm creeps upwards along the ledge. As a result, melting andrnfreezing are rapid close to the bottom of the film, while ledge nearrnthe top of the metal changes only slowly depending on thernthickness of the film. Melting or freezing may also be initiated byrnvariation in the concentration of aluminium fluoride in the sludge.