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 3200K would be required to obtain from Al2Si2O5(OH)(4)+9C an Al-Si alloy with 39wt.% Si. At 10(-4)bar and 1800K, the predicted Al-Si alloy would contain 2.4wt.% Si. From mullite, the reaction of Al6Si2O13+13C at 10(-4)bar and either 1800K or 2200K should produce an Al-Si alloy with 0.65 or 24wt.% 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.