Depolarized gas anodes for electrowinning of aluminium from cryolite-alumina melts in a laboratory cell

Abstract

Consumable carbon anodes are used in the electrowinning of aluminium by the Hall-Heroult process and in other proposed processes for electrowinning in molten salts. Emissions of CO2 may be eliminated by introducing an inert oxygen evolving anode, which however will require a higher anode potential. By introducing natural gas or hydrogen to the anode the CO2 emissions can be reduced and the anode potential can be lowered. Laboratory experiments were carried out in a modified Hall-Heroult electrolyte with excess AlF3 at 850 °C. Anodes of platinum, tin oxide and graphite were tested during electrolysis at constant current, with the supply of argon, methane and hydrogen through or at the anodes. Laboratory studies showed that by introducing both hydrogen and methane separately through a porous SnO2 anode in molten Na3AlF6-AlF3-Al2O3 (4.5 wt.%) at 850 °C the anode potential was found to be lowered by several hundred millivolts for a limited time during electrolysis.