Theoretical study of vacuum carbothermic reduction of alumina chlorination disproportionation
|School||Kunming University of Science and Technology|
|Course||Physical Chemistry of Metallurgy|
|Keywords||carbothermal reduction and chloride Disproportionation reaction reactionmechanism ab initio molecular dynamics density functional theory|
Aluminum production by carbothermo-chlorination-disproportionation reduction of alumina under vacuum was a new method, which proposed by National Engineering Laboratory of Vacuum Metallurgy, Kunming University of Science and Technology, focus on the disproportionation reaction of AlCl, and has achieved initial progress. At present, the reaction mechanism of carbon thermal reduction process, the chlorination process and the disproportionation process need to be solved to improve the yield and the purity of the aluminum. To solve these problems needs to research on kinetics of the reactions of carbothermo-chlorination-disproportionation reduction of alumina under vacuum and high temperature and low pressure, but the further insights into this type of reaction kinetics by experimental is often difficult. Therefore, this paper focus on three outstanding problems encountered in the reaction of carbothermo-chlorination-disproportionation reduction of alumina, based on the experimental studies results, by using quantum chemistry, ab initio molecular dynamics method to study on the interfacial reaction mechanisms and simulate the process of multiphase reactions of the carbothermo-chlorination-disproportionation reduction of alumina under high-temperature and low-pressure, with the corresponding experimental results, to obtain reaction mechanism of the carbothermo-chlorination-disproportionation reduction of alumina under high-temperature and low-pressure and provide systematic theoretical basis for the experimental study of aluminum carbothermic reduction disproportionation reaction.Firstly, the Gibbs free energies with temperature changes under different pressures (100KPa,100Pa,60Pa,30Pa) of the main reactions which may occur during the process of aluminum carbothermic reduction disproportionation reaction have been acquired by thermodynamics calculations, to determine reactions which can occur under1760K,60Pa. On this basis, by using the methods of quantum chemistry and ab initio molecular dynamics simulation to study on the carbothermal reduction, chlorination, disproportionation process respectively.The interaction of Al2O3、Al4CO4、Al2CO and Al4C3with carbon and CO in the process of carbothermal reduction have been calculated by using density functional theory, and ab initio molecular dynamics simulations method has been used to simulate the reactions which may occur between Al2O3、Al4CO4、Al2CO and Al4C3in the process of carbothermal reduction. And Al2O(g) and CO(g), the products of carbothermal reduction, which may occur reactions in the condensation area, have been studied on preliminarily. The results showed that during the carbon thermal reduction process, the appear order of gaseous products, which produced by the reaction of Al2O3and carbon, should be CO→Al2O→Al. The reaction products of the carbon thermal reduction of Al2O3were affected by the quantity of carbon. Under1760K,60Pa, CO could reacted only with Al4C3, the product of carbon thermal reduction, and it seemed CO could not reacted apparently with Al2O3、Al2CO、Al4CO4. And the order of the strength of interaction between CO of the species in Al-C-O system is Al2O3<Al4CO4<Al2CO<Al4C3, the order of the strength of interaction between carbon of the species in Al-C-O system is Al2O3> Al4CO4> Al2CO, contrary to both the order. Therefore, it can be considered that solid-solid reaction is the form of Al2O3carbothermal reduction reaction under the experimental conditions, the reducing agent is carbon, instead of CO. The calculations results of the reactions of Al2O (g) which produced during the process of carbothermal reduction of Al2O3show that the reaction which occur first in the condenser area is the reaction of Al2O(g) form Al and Al2O3, secondly, Al2O reacted with CO to generate Al4CO4and carbon, then Al2O reacted with carbon generated Al4C3and Al4CO4and Al4C3reacted with CO generated Al4CO4, the occurrence of these reactions are not conducive to the subsequent chlorination reaction.The interaction of Al4CO4、Al2CO and Al4C3with AlCl、AlCl2and AlCl3in the process of chlorination have been calculated by using density functional theory, and ab initio molecular dynamics simulations method has been used to simulate the reactions which may occur between Al2O3、Al4CO4、Al2COand Al4C3in the process of chlorination. The results showed that AlCl3, AlCl2could occur dissociation reaction on Al4C3(001) surface, but there is no obvious chemical reaction for AlC13, AlC12on Al4CO4(001), Al2CO(001) surface. AlCl is not easy to adsorb on Al4C3(001) surface, while on Al4CO4(001)、Al2CO(001) surface, AlCl tends to exist in the adsorption state. The ab initio molecular dynamics simulation calculations for the related reactions of Al2O3、Al4CO4、Al2CO and Al4C3with AlCl3at temperature of1760K, pressure of60Pa showed that under the experimental conditions(1760K,60Pa), when Al4CO4in the presence of carbon, Al4C3in the presence of oxygen tend to react easier with AlCl3to produce AlCl than there is no oxygen(or carbon). At the conditions of1760K,60Pa, the order of the strength of the dissociation reaction of AlCl3on the surface is:Al2O3(with carbon)<Al4CO4(with carbon)<Al2CO<Al4C3<Al4C3(with oxygen), it means that Al4C3in the presence of oxygen (coexistence with Al2O3or Al4CO4) is more susceptible to react with AlCl3and generated AlCl.The density functional theory has been used to calculate the stable structures and search the transition state of the clusters generated process for oligomers [AlCl]n(n=1～10), to discuss on the oligomers growth mechanism by the total energy and the activation energy.In additional, the disproportionation reaction mechanism of AlCl on Al(110) and Al (100) surfaces have been studied by using pseudopotential plane-wave method which based on density functional theory. Three possible reaction mechanism of AlCl disproportionation on Al(110) and Al (100) surfaces have been studied, the structures of the reactants and products on Al(110) and Al(100) surfaces have been optimized and further calculation have been carried out to acquire the transition state structure and reaction energies, as well as desorption energies and adsorption energies for these reactions. The results indicate that the ground state geometric configurations of [AlCl]n(n=1～10) clusters are formed by Aln frameworks connected to n Cl atom, and have well geometrical symmetry. Activation energy obtained from the transition state calculation indicates the activation energy of reverse process is always bigger than that of the forward process. It means that AlCl has the trend of formation [AlCl]n. The research results can help to understand the mechanism of produce Aluminum by carbothermic reduction-disproportionation reaction. The results of surface disproportionation reaction mechanism of AlCl on aluminum surfaces indicate: The reaction2AlCl(*)→Al(*)+AlCl2(*)is the rate determining step of the surface reaction of3AlCl(g)→2Al(*)+AlCl3(g) in mechanism B and C. when the reaction3AlCl(g)→2Al(*)+AlCl3(g) proceed forward, the reaction will mainly follow mechanism C, next, to follow mechanism B, and finally, to follow mechanism A. When reaction3AlCl(g)→2Al(*)+AlCl3(g) proceed reversely, the reaction will mainly follow mechanism A, next, to follow mechanism B, and finally, to follow mechanism C. The reaction3AlCl(g)→2Al(*)+AlCl3(g) which react on Al(110) surface is exothermic. In words, when the temperature rises, the reaction3AlCl(g)→2Al(*)+AlCl3(g) will proceed reversely, while when the temperature drops, the reaction3AlCl(g)→2Al(*)+AlCl3(g) will proceed forward, the result is in accordance with the experiment phenomenon that AlCl is stable in high temperature and disproportionation into Al and AlCl3in low temperature.