Abstract
AbstractCrystallization of Al(OH)3 that occurs during the decomposition of caustic soda solutions is an important part of Bayer process for alumina production. Several phenomena, which influence the physicochemical characteristics of precipitated Al(OH)3, occur simultaneously during this process. They are nucleation, agglomeration, and crystal growth of Al(OH)3. In this article, we have investigated the mechanism of Al(OH)3 crystal growth from pure caustic soda solutions and in the presence of oxalic acid.The results have shown that the growth of Al(OH)3 crystals from caustic soda solutions follow the B + S model (birth and spread). New Al(OH)3 particles, formed during the decomposition process of pure caustic soda solutions, are characterized by regular hexagonal shape. The nuclei have the same geometry as the contact face. However, microstructural investigations of Al(OH)3 samples, obtained by crystallization from caustic soda solutions in the presence of oxalic acid, have shown the presence of nuclei of irregular shape in addition to regular ones. So, the presence of oxalic acid in the caustic soda solutions leads to a change in crystal habit.Besides, the results obtained by kinetic investigation confirmed the mentioned mechanism of Al(OH)3 crystal growth.
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