Electrochemical measurement of effective diffusivity in Hall‐Heroult electrolyte

Forced‐convection mass transfer at the metal/bath interface during aluminum reduction from cryolitic melts was studied under reproducible convective conditions. A film of molten aluminum on a rotating molybdenum cylinder was the cathode. Concentration overpotential measured as a function of rotation rate, current density, and bath composition was converted to concentration differences between the bulk and the metal surface. Chosen as the basis for calculation of a mass transfer coefficient was the concentration of aluminum fluoride given by:\documentclass{article}\pagestyle{empty}\begin{document}$$ C_{{\rm AlF}_{\rm 3} } = \frac{{2\rho (100 - Al_2 {\rm O}_3 \%- {\rm CaF}_2 \%- IMP\%)}}{{100M_{{\rm AlF}_3} (CR + 2)}} $$\end{document}where the amounts of indicated compounds are in weight percent, IMP designates impurities, M is molecular weight, ρ is the density of the melt, and CR is the cryolite ratio, the ratio of moles NaF to moles AlF 3 . Agreement with a correlation for mass transfer to a rotating cylinder allowed the calculation of effective diffusivities for aluminum fluoride species, in alumina‐saturated melts, of: 11.1 ± 1.1 × 10 −5 cm 2 /s at 1.8 CR ; 11.4 ± 1.7 × 10 −5 cm 2 /s at 2.3 CR ; 5.4 ± 0.8 × 10 −5 cm 2 /s at 3.0 CR ; and 4.4 ± 0.9 × 10 −5 cm 2 /s at 4.0 CR .