Stability of metal/electrolyte interface in Hall-Héroult cells: Effect of the steady velocity

Abstract

An instability of the metal-bath interface in Hall-Héroult cells is described. The instability is hydrodynamic rather than magnetohydrodynamic in nature. Above a critical value of the velocity of the fluids (which are assumed to have the same steady velocity), the interface becomes unstable and waves grow; this result is calculated without any specific electromagnetic mechanism. The critical velocity depends on the cell’s flow pattern; for example, a one loop pattern will have a critical velocity different than that of a two loop pattern. The critical velocities calculated are near the velocities measured for aluminum in commercial cells. Also, the form of the waves which are generated is a set of traveling waves whose pattern depends on the pattern of the steady velocity. The variety of observed motions is reminiscent of the variety seen in interface topographical animations generated from measured anode currents. In this analysis, increasing the anode-to-cathode distance decreases the cell’s stability, as indicated by the value of the steady velocity at which flow would become unstable. Two possible technical explanations are developed for this difference from conventional wisdom. Additional study in this area is recommended. Finally, the application of this analysis to an operating cell is illustrated, showing the potential of the method in assessing the stability effects of various operating or design parameters.