Effects of Varying Anode-cathode Distance in an Aluminium Electrolysis Cell with a Sloping Tib2, Composite Cathode

The effects of anode-cathode distance (ACD) on the cell potential and electrical bath resistivity were studied in a laboratory scale-cell that utilized a sloping TiB{2} carbon composite cathode. The bath resistivity of the electrolyte increased with decreasing ACD due to an increasing gaseous phase fraction in the electrolyte. The effective bath resistivity was found to increase rapidly after ACD approached 10mm and was proportional to the inverse of ACD within the ACD range of 2 to 10mm. The effective bath resistivity was found to be dependent on the bath composition and its temperature. Addition of NaCl to the bath, lowered the effective bath resistivity by increasing the electrical conductivity of the bath and at low ACD it also lowered the bubble contribution to resistivity by altering the hydrodynamic properties of the system. Addition of AlF3 to lower the cryolite ratio (i.e. the molar ratio of NaF to AlF3) gave the opposite result (i.e. a higher bath resistivity) to the NaCl addition. These results indicate that reduction of the anode-cathode distance and optimization of process conditions to reduce the effect of bubbles, will allow enormous savings in the energy requirements of the Hall - Heroult process.