Impact of aluminium pad and operation parameters on graphitised and graphitic cathodes expansion and bath penetration

Cathode sodium expansion and bath penetration are important properties to consider in the Hall–Héroult process. Rapoport test is the standard method for sodium expansion determination. However, there is no standard methodology for bath diffusion measurement. In this work, expansion measurements are achieved on laboratory scale alumina reduction cells for graphitic and graphitised cathode block grades. Cathode carrots were then extracted to measure bath penetration. Results revealed that the presence of an aluminium pad at the beginning of the experiment has reduced the sodium expansion and bath penetration by about 50%. Sodium concentration in the bath has increased the expansion rate and bath penetration depth, but not the expansion level, which is driven by the current density. This demonstrates that the startup conditions are crucial to the lifespan of the electrolysis cell.