Limits for the Current Efficiency in Hall-Héroult Cells

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