Aluminium Carbide and Carbon Dust in Aluminium Electrolysis Cells-A Conceptual Model for Loss in Current Efficiency

A qualitative mass balance for carbon dust and aluminium carbide in aluminium electrolysis cells is suggested. The structure model by Odegard et al. for dissolved aluminium carbide in the bath was verified using formal activity data as well as an ideal Temkin activitymodel. The conditions for transport across the metal-bath interface (cathode) of carbon stemming from cathode wear were modelled. The main part of the current work consists of the derivation of a conceptual model for loss in current efficiency (CE) caused by aluminium carbide formation by a reaction between carbon dust and dissolved metal close to the cathode. The reaction entails a steeper concentration gradient of dissolved metal and thereby increased loss in CE. The concentrations of dissolved metal, carbon particles, and aluminium carbide close to the cathode were calculated. The loss in CE was computed for different values of the reaction rate constant and the amount of carbon dust. It appeared that the loss in CE is probably limited by the kinetics of the chemical reaction since diffusion control at small carbon particles would produce an unrealistically high loss in CE.