Alumina Vacuum Carbothermal Reduction And Carbon Thermal Reduction - Chloride - Disproportionation Reaction

Presently aluminum is produced industrially via the electrolytic Hall-Heroult process. When the electrolytic process has been improving, there has been much research of other methods continuously. During the past more than 100 years, numerous attempts have been made to produce aluminum using carbothermal reduction of alumina. However, it has not been industrialized practically, due to the high temperatures required, and the difficulty in separating aluminum from the residues. The disproportionation of aluminum subchloride to aluminum in vacuum is founded on the basis that AlCl(g) is generated by the system of Al2O3-C-AlCl3, and then disproportionate into aluminum and AlCl3(g) at lower temperatures. The process solved the problem of separating aluminum from the residues and, moreover, the required temperature is lower greatly than that of the carbothermal reduction. However, its industrial application needs to further thorough research.The disproportionation of aluminum subchloride to aluminum in vacuum is founded on the carbothermal reduction-chlorination-disproportionation of the Al2O3-C-AlCl3 system. The mechanism of the process is the theoretical basis for its industrial application. There are still different opinions on the mechanism of the carbothermal reduction of the solid Al2O3-C system, by solid-solid reaction or gas reaction. In addition, there is a lack of unified interpretation on the formation of aluminum carbide and oxycarbide. Therefore, this paper firstly studied the carbothermal reduction of alumina in vacuum, and then on this basis, studied the carbothermal reduction-chlorination-disproportionation of alumina in vacuum, and finally studied several influence factors of the disproportionation of aluminum subchloride to aluminum in vacuum.The thermodynamics of the reaction systems were investigated. The experiments were conducted with alumina, graphite and aluminum chloride as the raw materials, with the three-zone vacuum furnace designed by ourselves as the main equipment and with X-ray diffraction and scanning electron microscope as the main analysis methods.The research on the carbothermal reduction of alumina demonstrated the following conclusions:(1) The carbothermal reduction of alumina at a temperature ranged from 1643K to 1843K conformed to the gas phase reaction mechanism, comprising two stages:firstly alumina reacted carbon to generate Al2O, Al and CO gas by gas-solid reactions, and then the gaseous Al2O and Al reacted with CO to form condensates at lower temperatures. (2) The equilibrium temperature of the generation of Al2O was lower than that of the generation of Al by the carbothermal reduction of alumina. However, the Gibbs free energy value of the generation of Al2O was decreasing less sharply than that of the formation of Al with increasing temperature so that the curves of the Gibbs free energy values of the two reactions verses temperature intersected. Above the intersection temperature, Al was the main product, and below that, Al2O was the main product. The intersection temperature was decreasing with the system pressure. (3) The condensates were comprised of Al4O4C, Al2O3, Al4C3 and C when the carbothermal reduction of alumina formed Al2O and CO, and were comprised of Al, Al4O4C, Al2O3, Al4C3 and C when the carbothermal reduction of alumina formed Al and CO. The composition of condensates depended on the gas composition and temperature, which could be deduced by the equilibrium curves.The research on the carbothermal reduction-chlorination-disproportionation of alumina demonstrated the following conclusions:(1) The formation of AlCl by the reactions of Al2O3-C-AlCl3 system was comprised of two steps:firstly alumina reacted carbon to generate gaseous Al and Al2O, and then the gaseous Al2O and Al reacted with AlCl3 to form AlCl. (2) The formation of AlCl by the reactions of the gaseous Al2O, Al with AlCl3 accompanied by the side reactions that the gaseous Al2O, Al reacted with CO, resulting in the formation of Al4O4C, Al2O3, AI4C3 and C. (3) The disproportionation of AlCl to aluminum and gaseous AlCl3 at lower temperatures accompanied by the side reactions that CO disproportionated, and aluminum reacted with CO, resulting in the existence of C, Al4C3 and Al2O3 in the aluminum product.The research on the chlorination-disproportionation of the condensates formed by the carbothermal reduction of alumina demonstrated that the condensates composing of Al4O4C, Al2O3, Al4C3 and C could react with gaseous AlCl3 to generate AlCl, and AlCl disproportionated to generate aluminum at lower temperatures. The extent of the side reactions was greatly reduced, and there existed only the disproportionation of CO, thereby existing only carbon in aluminum.The research on the influence factors of the disproportionation of aluminum subchloride to aluminum in vacuum demonstrated the following conclusions:(1) With increasing the temperature of carbothermal reduction-chlorination, alumina reacted more completely, and the extent of side reactions increased. The highest aluminum yield was obtained when the reaction temperature was 1743K. (2) With decreasing the AlCl disproportionation temperature, the extent of side reactions decreased, and the C, Al4C3 and Al2O3 content in aluminum decreased. (3) With decreasing the interface between aluminum and CO, the extent of side reactions decreased, and the C, Al4C3 and Al2O3 content in aluminum decreased. (4) When the reaction temperature was 1793K, the extent of side reactions decreased and the aluminum yield increased on the condition that the carbothermal reduction of alumina occurred first, and then the chlorination-disproportionation occurred with gaseous AlCl3 entering. The three-zone vacuum furnace had been designed on the basis of the existing one for the carbothermal reduction-chloridation-disproportionation of alumina. (1) The independent condenser was designed, in which AlCl disproportionated to aluminum and gaseous AlCl3. Aluminum condensed in the condenser pans and AlCl3 went out through the hole of the condenser cover, and collectively condensed on the furnace cover with cooling water. The problem of condensates spreading on the furnace wall was solved. (2) The aluminum chloride sublimation equipment was provided with an automatic temperature control system. The problem of control temperature of aluminum chloride sublimation in the existing furnace was solved. (3) The heat insulation sleeve consisted of two graphite layers with a carbon layer between them, reducing the furnace volume; therefore the furnace had the advantage of convenient operation and lower power consumption.The experiments on the carbothermal reduction-chlorination-disproportionation of alumina to aluminum were conducted in the three-zone vacuum furnace. The aluminum yield was low owing to failing to continuously convey AlCl3 gas. The C, Al4C3 and Al2O3 content in aluminum products was high due to the side reaction of aluminum and CO.