| It was well known that there were many technical puzzles on how to enhance the strength of alumina in the domestic production of alumina, and a lot of correlative researches were focus on the factors of technologic conditions which affected on strength of alumina. However, studies on crystallization of gibbsite and transformation of crystal structure in seeded agglomeration process of sodium aluminate were not often seen in current references. It was not doubt that studies on technologic conditions had direct relations with actual industrial production, but industrialization production of alumina with steady characters would be difficult to implement in fact, except on the condition that the influencing factors affected onstrength of alumina essentially had been searched out.Based on actual requirement of production with high strength in industry production of alumina, theoretical study on the crystallization process and crystalmicrocosmic combination of gibbsite had been carried through in this paper. Specially, this work was one part of the series research work named as "the relationship between crystal combination of gibbsite and the strength of alumina".First of all, the experimental studies on the microstructure of gibbsite in the process of nucleation, radial and agglomeration growth of gibbsite were implemented. The microstructure, quality and transformation of crystal structure of alumina that calcinated by gibbsite had been analyzed. In succession, the relationship between the crystal qualities and the transformation of crystal structure of gibbsite (alumina) were discussed. Therefore, aiming at the question about "the relationship between the transformation of crystal structure and the strength of gibbsite (alumina)", it was a good decision to introduce the method of theoretical calculation, and apply to the experiment investigation combinated with theoretical means in this paper.Then, the theoretic studies on the properties, such as crystal lattice parameters, electronic structure and optics properties of Al(OH)3 crystal (gibbsite,bayerite,nordstrandite) were calculated using CASTEP program module, enthalpies of formation and Gibbs free energy of formation of Al2O3·3H2O, Al(OH)6-3, Al(OH)4(H2O)2- and Al(OH)4- were also calculated using Guassian98 program. The influences of pressure on the properties of gibbsite were predicted by theoretical calculation method at the first time. The calculation results were compared with correlation literature data and experimental facts, and it can be proved that the theoretic study methods are feasible and reliable.The theoretic studies aimed at the favorite growth unit Al6(OH)18(H2O)6 of glbbsite and the combination of growth units for the first time. The calculation results showed that figure of the favorite growth unit Al6(OH)18(H2O)6 was most likely to be hexagon flat and the geometry structure parameters of Al6(OH)18(H2O)6 were confirmed. Number of Al6(OH)18(H2O)6 which were needed in nucleation process was calculated. The strength of "face-face" combination mode was most fastness among various double combination modes of the favorite growth unit Al6(OH)18(H2O)6. Intermolecular force among growth units "face" type was larger than "side" type among various [Al6(OH)18(H2O)6]n (n=3, 4, 5, 6, 7, 8) combination systems. Accordingly, the relationship between the combination of growth units Al6(OH)18(H2O)6 of gibbsite and the linkage strength of growth units were confirmed.The microcosmic mechanism of nucleation of glbbsite which grow from the favorite growth unit Al6(OH)18(H2O)6 was studied, and it was considered that large numbers of favorite growth units were combined with "side"-"side" and "face"-"face" combination modes. Space lattice of Al6(OH)18(H2O)6 structure unit was restricted by space lattice of gibbsite, and finally, the effectual crystal nucleus would be appeared by dehydration and condensation processes. The microcosmic mechanism of the crystallization of gibbsite was found in theory.Total energy, electronics structure, population and optics properties of various surface slab models of gibbsite were calculated using CASTEP program, and the stability, bond strength, and active sites of the surfaces were studied. The calculation results indicated that from the point of view of energy, the state of gibbsite with more (001), (101) and (100) revealable facets were steadier than the system with more (010) revealable facets. While the frontier electrons of (001) slab of gibbsite were quite active, and there was some active site on this (001) slab. Al-0 bond strength of (001) slab of gibbsite was relative weaker than that of (011) and (110) slab, so that Al-0 bond strength of gibbsite surface might be enhanced if the revealable facet proportions of (011) and (110) slab were increased and that of (001) slab was decreased. Thereby, the law of relationship between the revealable characteristic and the chemical bond force of crystal facets of gibbsite was gained.The scanning electron micrographs and abrasion index of alumina products were analyzed, it could be found that there were some relationship between the strength of gibbsite grains and their micro structure. If the proportion to revealable of (110) face were reduced, the strength of products would be weak. It has been proved that the relativities between surface properties of crystal facies calculated by quantum chemistry method and strength, production technics of gibbsite or alumina products were existented in fact.In the end, the theoretic analyzed and calculation on crystal growth and habit were performed for the first time. The mechanism and combination mode models of radial and agglomeration growth of gibbsite were discussed, The equilibrium morphology of gibbsite and various surface slab models were calculated and the revealable facet were all forecasted commendably. These results were consistent with experimental facts, so that it was indicated that equilibrium morphology method is feasible and believable to study crystal growth morphology. The calculation results of crystal growth habit indicated that the growth morphology of 3×4×2 super cell of gibbsite would be quasi-hexagonal prism or thick quasi-hexagonal plate, (001), (100) and (010) slab were be inclined to be wedge-shaped plate, and (011) slab were likely to be thin flake shape. Hence, the morphology diversity of practical gibbsite crystal grain was explained by theoretic habit calculation method.Based on these series studies, some useful results were acquired. The experimental studies on the microstructure of gibbsite in the process of nucleation, radial and agglomeration growth of gibbsite were implemented, and the relationship between the crystal qualities and the transformation of crystal structure of gibbsite (alumina) were discussed. The relationship between the combination of growth units Al6(OH)18(H2O)6 of gibbsite and the linkage strength of growth units were confirmed, and the microcosmic mechanism of the crystallization of gibbsite was found in theory. The morphology diversity of practical gibbsite crystal grain was explained by theoretic habit calculation method. The law of relationship between the revealable characteristic and the chemical bond force of crystal facets of gibbsite was gained. It has been proved that the relativities between surface properties of crystal faces which calculated by quantum chemistry method and strength, production technics of gibbsite or alumina products were existented in fact.
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