Vacuum Carbothermic Production of Aluminum and Al-Si Alloys From Kaolin Clay: A Thermodynamic Study

Examination of the thermodynamic constraints for the carbothermic reduction of iron-free kaolinite, Al2Si2O5(OH)4, or of its calcination product mullite, Al6Si2O13, either at atmospheric pressure or under vacuum of 10−3 to 10−5 bar, indicates the conditions required at equilibrium to produce either elementary Al or Al-Si alloys. At atmospheric pressure, a very high temperature of 3200 K would be required to obtain from Al2Si2O5(OH)4 + 9C an Al-Si alloy with 39 wt.% Si. At 10−4 bar and 1800 K, the predicted Al-Si alloy would contain 2.4 wt.% Si. From mullite, the reaction of Al6Si2O13 + 13C at 10−4 bar and either 1800 K or 2200 K should produce an Al-Si alloy with 0.65 or 24 wt.% Si. The CO produced by the carbothermic reactions may be by water-gas shift converted to syngas, and further either to methanol or by a Fischer–Tropsch reaction to liquid fuels or chemical intermediates. Concentrated solar energy may be used to supply the required process heat of these high-temperature reactions.