| Alpha alumina (α-Al2O3) is the main raw material in aluminum industry. In addition, it also can be used in many other fields such as electronics, petroleum, chemicals, ceramics, abrasives, and pharmaceutical fields. Alpha alumina is obtained mainiy by calcinating aluminum hydroxide. However, too much energy is consumed in this process. The most effective solution is to reduce the baking temperature and shorten the baking time of the process. This paper researched a new microwave calcination process.The main task of the process is to remove the absorption water and constitutional water. It is more effective for microwave to remove water from material. Therefore, in this paper, we will use microwave method to prepare alpha alumina from aluminum hydroxide. And the process of aluminum oxide crystal transformation was studied systematicly using the first principle studies. The thermal-dynamics by microwave calcination was performed through theoretical analysis. Accordingly, for comparison, the process was also researched in conventional method. The main contents and conclusions were as follows:1) The paper rstudied the effect of the reaction conditions on the yield and average crystal size of alpha alumina under the conventional and the microwave field. The results showed that the main factors are calcination temperature and holding time under conventional field. And the optitimal reaction conditions are the calcination temperature at1200℃and the holding time of2h. Under the microwave field, the main factors are calcination temperature, holding time and the mass of material. And the optimal reaction conditions are the calcination temperature at1000℃, the holding time of20min and the mass of material50g. So, it can be conclude that we will obtain alpha alumina with smaller average crystal size at lower temperature and shorter time under microwave field.2) The response surface methodology (RSM) is carried out to optimize the parameters of the experiment. The optimization results indicate that the optimum conventional calcination parameters are the calcination temperature at1206.81℃and the holding time of2.06h. In the optimum parameters, RSM forecast that the yield of alpha alumina is95.9251%and the average crystal size of alpha alumina is72.2455nm. The optimum microwave calcination parameters are the calcination temperature at1006.71℃, the holding time of18.04min, and the mass of material50.52g. In the optimum parameters, RSM forecast that the yield of alpha alumina is95.8578%and the average crystal size of alpha alumina is61.3243nm. The paper also verifies the reliability of the model used in the experiment.3) The XRD and FTIR analysis indicated that the reaction mechanism of the conventional field is A1(OH)3â†'AlO(OH)â†'χ-Al2O3,â†'γ+κ-Al2O31â†'θ+κ-Al2O3â†'α-Al2O3. However, the reaction mechanism of the microwave field is A1(OH)3â†'AlO(OH)â†'χ-Al2O3â†'γ+κ-Al2O3â†'α-Al2O3. Contrasting these two reaction courses, one finds that the AlO(OH) produces later in microwave field than in the conventional field and there is no θ-Al2O3in microwave field4) Crystal structure models of six crystals, which are A1(OH)3, AlO(OH), γ-Al2O3, κ-Al2O3,θ-Al2O3and α-Al2O3, were built respectively. Geometry optimizations were implemented by CASTEP program module using general gradient approximation (GGA) method, based on density functional theory (DFT). The total energy, bond structure, DOS, atomic and bond populations were also calculated. The calculation results indicate that the order of reactivity is γ-Al2O3>θ-Al2O3>κ-Al2O3>κ-Al2O3. It is consistent with the result and the conventional experiment. These three kinds of Al-O bond lengths of O-Al2O3are shorter than that of α-Al2O3. Thus, when the temperature rises sharply, there is too much energy in the system. It transforms γ-Al2O3into α-Al2O3directly. We speculate that it should be the main reason why the θ-Al2O3does not exist from the microwave processing. |
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