Study On The Structural Characteristics And Decomposition Mechanism Of Supersaturated Sodium Aluminate Solution

The crystallization of Gibbsite (Al (OH)3) from supersaturated sodium aluminate solution is an important part of commercial alumina production. However, the mechanism by which tetrahedral coordinated Al(OH)4"(aq) in solution becomes octahedral coordinated in Al(OH)3(cry.) is still not well understood . Because supersaturated sodium aluminate solution is apart far away from its equilibrium state, it presents some special properties different from general one just in equilibrium. For example, the structure of solution changes with time; there is non-linear action during its decomposition; very slow decomposition rate though it is a highly supersaturated, and so on. The aim of the work is to probe the structural characteristics and decomposition mechanism of supersaturated sodium aluminate solution ir order to provide fundament for the research on new methods to enhance or improve the crystallization of Al (OH)3 from supersaturated sodium aluminate solution.Clarifying the species and structures of ions in aluminate solution is the first step to reveal the decomposition mechanism of supersaturated sodium aluminate solution. In our research, the ion species in supersaturated aluminate solutions with various concentration and caustic ratios had been detected by Infrared adsorption spectra, Raman Scattering Spectra, 27A1-NMR and solution x-ray diffraction, while the electronic structure, total energies, stability energy from medium as well as theoretical infrared spectra of a series of possible aluminum containing anionic species had all been studied by ab initio Quantum chemical methods. Based on theoretical and experimental results, the actual aluminate ion species and the possible transformation among species were researched intensively; the reason that the structure of sodium aluminate solution changes with concentration was investigated. It was found for the first time that the potential function of solvent medium changes with the solution concentration resulting in the structure and stability of aluminates varying with the concentration. The main Al-contained species in intermediate concentrated solutions is S4 symmetric [A1(OH)4(H2O)4]' , there is not [A1(OH)5]2' or [A1(OH)6]3' in supersaturated sodium aluminate solutions. With the amount of Al (III) increasing, [A1(OH)4(H2O)4]" reacts with [A1(OH)3(H2O)], three kinds of dimeric aluminate such as [A12O(OH)6]2- , [A12(OH)7]- , [Al2(OH)g(H2O)J2~ were formed.; tri-polymerized and multi-polymerized aluminates were not detected in stable and metastable solution.Making a systematic exposition of the structure and component of growth units for Al(OH)3(cry.) precipitating from supersaturated sodium aluminate solution is another key step in revealing the decomposition mechanism of supersaturated sodium aluminate solution. By inspecting the decomposition process of supersaturated aluminate solution with in-situ Raman spectrum measurement, it was found for the first time that , the structure of sodium aluminate solution was reconstructed and six coordinated poly-anions connected by double A1-(OH)2-A1 linkage were formed during the reduction period of decomposition; when there was seed crystal of Al(OH)3(cry.) in solution, the reconstruction of anions took place on the liquid-solid interface. In order to research the structure of growth unit, the crystallization habitof Gibbsite under different physical-chemical condition were inspected systemically by SEM observation,, then the morphology of Gibbsite were analyzed from the point of view of Ion Coordination Polyhedral Theory. Based on the results of experiments and computer simulation of the polymerization of possible growth units, the structure and component of growth units of A1(OH)3 crystals precipitating from supersaturated caustic aluminate solution were put forward and illustrated for the first time, which indicated that Al-(OH)6 is the configuration unit of growth units; [A1(OH)4(H2O)2]" is the minimum growth unit; while [A16(OH)18(H2O)6] is the favorable growth unit, the nucleation of Al(OH)3(cry.) resul...