Limits for the Current Efficiency in Hall-Heroult Cells

A chemical model for the current efficiency (CE) in aluminium cells was used for estimating the maximum attainable CE in industrial cells. The model assumes that the loss in CE is governed by the mass transfer of dissolved metal from the metal surface (cathode) to the bulk of the bath. The mass transfer-dependent effect of the NAF/AlF3 ratio at the cathode was included. The model implies that when the CE is plotted as a function of the mass transfer coefficient at the cathode, there will be a maximum in CE. For a high-productivity cell with a contemporary bath composition, the maximum appears to be about 96.5%. Some other factors that lower the current efficiency are briefly discussed (too low anode-cathode distance, formation of a layer of solid matter at the cathode (metal-bath interface), polyvalent impurity elements, cathode wear, carbon particles, metal droplets, and small gas bubbles in the bath). A solid layer of cryolite and/or alumina is not necessarily detrimental to the CE, at least if the surface fraction is moderate.