| This study was aimed to produce high-purity ammonium alum and further high-purity alumina to realize the utilization of crude product in a high value way with the metallurgical-grade as the raw material and using the method of ammonium alum crystallization where the impurity behavior was investigated and the crystallization process was adjusted and controlled on the technology platform of alumina cleaner production technology by sub-molten salt process. On the other hand, because sodium calcium hydrogen silicate (NaCaHSiO4) could be easily decomposed in the process of red mud treated by sob-molten salt, the decomposition reaction process of NaCaHSiO4 in sodium aluminate solution was investigated in order to provide a theoretical basis and practical guide for the industrial production operation of the process red mud treated by sob-molten salt. The main contents and results of this research were as follows.The induction time and metastable zone width (MSZW) of ammonium alum crystallization were measured and mechanisms about the nucleation and growth process were analyzed. According to the classical nucleation theory and the theoretical analysis of MSZW, it was concluded that the nucleation process of ammonium alum crystallization was continuous and was of three-dimensional nucleation mechanism. According to the surface entropy factor theory and the method of model determination, it was concluded that the growth process of ammonium alum was continuous growth. In addition, values of parameters, including the interfacial tension of ammonium alum crystals in solution, surface entropy factor, critical radius of nucleus, critical free energy, and the activation energy related with the diffusion of solute in solution, were calculated.High-purity of alumina with the purity of beyond 99.99% was produced with the metallurgical-grade as the raw material and two impurity removal processes of ammonium alum crystallization and washing. The process was simple and realized the utilization of crude product in a high value way. Results showed that crystallization parameters, including the initial concentration of sulfate aluminum, supersaturation, temperature and crystallization method, significantly affected the content of impurities in ammonium alum crystals. The impurities content could be greatly reduced by natural cooling crystallization of ammonium alum under the condition of no stirring.Effects of impurity ions, including Na+, K+, Mg2+ and Fe3+, on the kinetics of ammonium alum crystallization were investigated. Results showed that the presence of K.+ and Fe+ in solution shorten induction times and reduced MSZW, promoting the nucleation; the presence of Mg2+ in solution prolonged the induction times and increased MSZW, inhibiting the nucleation; the presence of Na+ in solution had no significant promoting or inhibiting effect on the nucleation. The kinetic parameters were calculated according to the theory on MSZW.High-purity ammonium alum was as the raw material to produce high-purity alumina with the purity of beyond 99.9% by the thermal decomposition of ammonium aluminum carbonate hydroxide, and the prepared ultrafine high-purity alumina has a size of less than 1μm with uniform particle size distribution. Effects of preparation conditions of ammonium aluminum carbonate hydroxide on the K condition of reaction product, calcination of ammonium aluminum carbonate hydroxide and impurity removal of alumina by washing were investigated.The decomposition reaction process of NaCaHSiO4 in sodium aluminate solution was investigated. When NaCaHSiO4 decomposed in sodium aluminate solution, NaCaHSiO4 transformed to Na8Al6Si6O24(OH)2(H2O)2 (sodalite) and Ca2.93Al1.97(Si0.64O2.56)(OH)944 (katoite), reducing the efficiency of alumina extraction in the process of red mud treated by sub-molten salt. |
PDF Full Text Request