Effects of anode-cathode distance on the cell potential and electrical bath resistivity in an aluminium electrolysis cell with a sloping TiB₂ composite cathode

Abstract

A study of the effects of anode-cathode distance (ACD) on the cell potential and electrical bath resistivity in a laboratory scale cell that utilizes a sloping TiB₂ composite cathode shows an increasing bath resistivity as the ACD decreases, due to a simultaneously increasing gaseous phase fraction in the electrolyte. The effective bath resistivity was found to increase rapidly after ACD=10 mm and was proportional to the inverse of ACD within this low ACD range studied (2–10 mm). Addition of NaCl to the electrolyte (5.5 wt%) lowered the electrical resistivity by increasing the conductivity of the electrolyte, and at low ACD it also lowered the bubble contribution to the bath resistivity, most likely due to changes in the hydrodynamic properties of the system. Lowering the cryolite ratio (i.e. molar ratio of NaF to AlF₃) resulted in a higher electrical resistivity in the electrolyte by decreasing the conductivity of the melt, as well as by changing the hydrodynamic properties of the melt, leading to an increased bubble contribution to bath resistivity at low ACD. An increase in temperature resulted in a reduced bubble contribution at low ACD in a similar manner as the NaCl addition. From these results, it is clear that for a commericial process, reduction of the ACD and optimization of process conditions to reduce the effect of bubbles should allow significant savings in the energy requirements of the Hall-Heroult process.