| The crystallization of Al(OH)3 from seeded sodium aluminate liquors plays a vital role in alumina production. To promote decomposition of sodium aluminate liquors and get high quality Al(OH)3 are the eager goals of many alumina producers. In present study, seed activation and organic additives were applied to the enhancement of Al(OH)3 crystallization, and the mechanisms were extremely emphasized. The results will impregnate some new ideas to the subsequent investigations for the enhancement of Al(OH)3 crystallization.A series analysis techniques, laser particle size analyzer, IR, 27Al NMR, 27Al MAS NMR, XRD, SEM and quantum chemical calculation were assembled to investigate the crystallization process of seeded sodium aluminate liquors. The effect of activated seed on Al(OH)3 crystallization were studied. The intensification effect of 15-crown-5-ether was proved to be related to its caged structure. The effects of steric configuration and electronic structure of functional group in the additive molecules was investigated systematically. The effect of oily additives on Al(OH)3 agglomeration was evaluated and their action mechanism was also probed. A model of critical agglomeration time was established to interpret periodic agglomeration of Al(OH)3. The control of Al(OH)3 morphology by additive was proved to be feasible both from experimental results and quantum calculation. Probe molecules were initially applied to investigate the Al(OH)3 crystallization mechanism. Finally, general rules for additive design are proposed on the basis of present investigations. The main inclusions were drawn as follows:1. Industrial grade Al(OH)3 was activated by boiling in the distilled water. Either in agglomeration experiment or whole Al(OH)3 crystallization try, activated seed can accelerate the precipitation rate of Al(OH)3 significantly. The precipitation ratio was found to be promoted for about 5.0% in the agglomeration process, and for about 4.0% in the whole process of Al(OH)3 crystallization. It is proposed that the poisonous organic impurities desorb from the active site of seed surface during the actvation process, which leads to the increase of seed activity.2. The effect of 15-crown-5-ether on Al(OH)3 crystallization was investigated. The stability of sodium aluminate liquors decreases with the addition of 15-crown-5-ether and the Al(OH)3 crystallization is intensified. The peak width of 27Al NMR of sodium aluminate liquor broadens as the 15-crown-5-ether is introduced. The enhancement mechanism is visualized as follow: the presence of 15-crown-5-ether in the sodium aluminate liquor favors the combination of aluminate ions and the formation of network, and the crystallization is enhanced subsequently.3. The influence of steric configuration and electronic structure of functional group in the additive molecules on the enhancement were evaluated systematically. The enhancement of 1-butanol, 1,4-dioxane, tetrahydrofurane are strongerly correlated with the net charge of oxygen atom in the molecules. High concentration of ethylene glycol inhibits Al(OH)3 precipitation slightly, while ethylene glycol monoethylether acetate, which has more steric hindrance effect, can enhance Al(OH)3 crystallization.4. A series of oily additives on Al(OH)3 agglomeration were studied. Low concentrations of oleic acid and moderate concentrations of tall oil can greatly enhance the agglomeration of Al(OH)3 . High agglomeration degree Al(OH)3 can be achieved without the decrease of crystallization rate when tall oil and 1-Dodecanol are mixed and introduced at a certain proportion. The concept of critical agglomeration time was proposed. Taking the fractal characteristic of Al(OH)3 crystallization kinetics into account, an integral model critical agglomeration time is established as follow:5. Using addtives to control Al(OH)3 morphology was invented and verified. The SEM observation of seeded crystallization product shows that the (001) surface area of gibbsite under the influence of oleic acid is larger than that of the blank. The homogeneous crystallization results suggest that oleic acid, 1-octadecanol and stearic acid can significantly influence the gibbsite morphology. Result of theoretical calculation indicates that the (001) surfaces appear stubbornly when oleic acid molecules are adsorbed by a reasonable mode.6. The techniques of 27AlMAS NMR and probe molecules were applied to unveil the mechanism of Al(OH)3 crystallization. Some alcohol compounds were verified to inhibit crystallization by disturbing the structure of sodium aluminate solution. The configuration inversion of tetra-coordinated aluminate is completed in the bulk solutions. The crystallization process is susceptible to some organics, for the attachment energy of growth unit on lattice is relatively small. The crystallization of Al(OH)3 on seed surface would be severely inhibited by the polyols, which characterized with a matching configuration of functional group to crystal lattice and an electronic structure of charge donor.7. General rules for additive design are proposed on the basis of present investigations. The additive molecule design for crystallization enhancement should emphasize on the decrease of stability of sodium aluminate solutions and the improvement of the nature of interface of solid-solution. While the ones for agglomeration enhancement should pay more attention to the soluble polymers and oily organics.
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