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 carbideAluminium carbide in aluminium electrolysis cellsAluminium electrolysis cells is suggested. The structure model by Ødegård et al. for dissolved aluminium carbideAluminium carbide in the bath was verified using formal activity data as well as an ideal Temkin activity model. The conditions for transport across the metal-bath interface (cathodeCathode) of carbon stemming from cathodeCathode wear were modelled. The main part of the current work consists of the derivation of a conceptual model for loss in current efficiencyCurrent efficiency (CE) caused by aluminium carbideAluminium carbide formationFormation by a reaction between carbon dust and dissolved metal close to the cathodeCathode. The reaction entails a steeper concentration gradient of dissolved metal and thereby increased loss in CE. The concentrations of dissolved metal, carbon particlesCarbon particles, and aluminium carbideAluminium carbide close to the cathodeCathode 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 diffusionDiffusion control at small carbon particlesCarbon particles would produce an unrealistically high loss in CE.