Interfacial processes and the performance of cathode linings in aluminum smelters

Measurements of the rate at which aluminum carbide dissolves in aluminum smelting electrolytes show that the dependence on electrolyte acidity (or excess aluminum fluoride concentration) follows a similar trend to that for the carbide saturation solubility, indicating mass transfer control. However, since the electrolyte supply at the aluminum/cathode carbon interface is limited, preferential corrosive wear will occur in areas where there is maximum accumulation of sludge and subsequent back feeding. The rate of the carbide corrosion can be reduced by adding Ti(IV) in relatively small concentrations (about 100 ppm); this also causes wetting of the carbon by the electrolyte. With further increases in the Ti(TV) concentrations—typically to about 350 ppm—apparent wetting with the metal also occurs, but under such conditions an electronically conducting electrolyte/carbide layer exists between the carbon and the wetted metal. Because of the higher viscosity of the titanium-rich metal phase when metal wetting occurs, the thickness of the film increases significantly. This mitigates titanium diboride formation on the carbon surface when the electrolyte contains both titanium oxide and boron oxide.