VACUUM CARBOTHERMIC REDUCTION OF ALUMINA

The current industrial production of aluminum from alumina is based on the electrochemical Hall-Heroult process, which has the drawbacks of high-greenhouse gas emissions, reaching up to 0.70kg CO2-equiv/kg Al, and large energy consumption, about 0.055GJ/kg Al. An alternative process is the carbothermic reduction of alumina. Thermodynamic equilibrium calculations and experiments by induction furnace heating indicated that this reaction could be achieved under atmospheric pressure only above 2200 degrees C. Lower required reaction temperatures can be achieved by alumina reduction under vacuum. This was experimentally demonstrated under simulated concentrated solar illumination and by induction furnace heating. By decreasing the CO partial pressure from 3.5mbar to 0.2 mbar, the temperature required for almost complete reactant consumption could be decreased from 1800 degrees C to 1550 degrees C. Deposits condensed on the relatively cold reactor walls contained up to 71wt% of Al. Almost pure aluminum was observed as Al drops, while a gray powder contained 60-80% Al and a yellow-orange powder contained only Al4C3, Al-oxycarbides and Al2O3.