Current Efficiency in Hall-Heroult Cells: The Role of Mass Transfer at the Cathode

The current efficiency (CE) in aluminium cells is governed by transport of dissolved metal (mainly sodium) across the boundary layer at the cathode. The transport takes place by ordinary mass transfer, but since solutions of alkali metals and their salts show electronic conductivity, the CE is also influenced by loss of electrons. The dependence of convection is not necessarily the same for the two loss mechanisms. A laboratory experiment was designed, where the mass transfer coefficient in the so-called Sterten-Solli laboratory cell for measuring CE was varied in a controlled manner by means of a mechanical stirrer. The effect of stirring on the mass transfer coefficient (k) was first surveyed by recording the limiting current density for potassium ferri- and ferrocyanide in an aqueous electrolyte as a function of the stirring rate, followed by measuring the CE in cryolitic melts at different stirring rates. It turned out that plots of the CE versus the mass transfer coefficient produced straight lines that extrapolated back to 99% CE at k = 0. This means that predictions of the CE can be made by using equations for ordinary mass transfer.