Mass transfer at the metal/bath interface in Hall-Heroult electrolysis

Mass transfer at the metal/bath interface (the cathode) during aluminum reduction from cryolitic melts was studied under reproducible convective conditions.; Two reference electrodes were developed and tested so that cathodic overpotential could be measured. The preferred, "wetted molybdenum hook" design was reproducible within 5 mV and stable for an 8 hour testing period. It consisted of a well-defined length of aluminum-wetted molybdenum wire in cryolitic bath. The hook design allowed the immersion, in the molten aluminum, of a seal between the molybdenum lead and a protective sheath. In the "densified bath inverted" design, aluminum floated above a cryolitic mixture containing 30% barium fluoride.; A thin film of liquid aluminum on a rotating molybdenum cylinder acted as the cathode. Concentration overpotential was measured as a function of rotation rate, current density, and bath composition and converted to mass transfer coefficients using data for the EMF of an aluminum electrode and four reactant concentration models that accounted in different ways for complexing of aluminum fluoride anions with alumina. 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. The effective diffusivities reflect the concentration model employed and an averaging across the mass transfer boundary layer of diffusion, transference, and physical property changes.; A multicomponent transport model was used to describe mass transfer in Hall-Heroult electrolytes. The limitations of the treatment were discussed in terms of the ionic constitution of the melts. The relationship to the effective diffusivity to the transference number and the diffusion coefficient was addressed for alumina-free electrolytes of CR {dollar}>{dollar} 3 by modeling the system as two binary molten salts with a common ion. Average diffusion coefficients were estimated to be about 10% higher than effective diffusivities. Transport coefficients for binary interactions, the reciprocals of which are similar to "friction coefficients", were calculated.