Chelyabinsk State University, Chelyabinsk, Russia:

A. V. Tolchev, Head of the Department of Chemical Technology and Computational Chemistry, Doctor of Chemiсal Sciences, Professor, e-mail: avtolchev@csu.ru
A. P. Tronov, Senior Lecturer at the Department of Chemical Technology and Computational Chemistry, e-mail: arteq_tron@mail.ru

Using such analysis techniques as X-ray analysis, electron scanning microscopy and isothermal and non-isothermal analysis, the authors examined the structure of particles in synthetic aluminium hydroxide with different degrees of dispersion, as well as the composition and structure of the products of their thermolysis in order to identify factors contributing to the formation of a γ-modification of aluminium oxohydroxide in the process of thermal decomposition of γ-Al(OH)₃. The study focused on synthetic aluminium hydroxide produced by the Urals Aluminium Smelter (gibbsite). The material was screened into two size fractions: coarse material — with the particles bigger than 200 μm, and fine material — with the particles <20 μm. The initial particles of aluminium hydroxide studied with the help of electron scanning microscopy can be characterized with a block structure, and they consist of smaller crystals, the sizes and the shapes of which may differ depending on the initial particle size distribution. Electron microscopy and X-ray studies showed that the coarse particles of aluminium hydroxide consist of imperfect little plate-like crystals with the form factor of 30–50. The fine particles of γ-Аl(ОН)₃ are mainly comprised of formations that contain a few or dozens of little prismatic crystals or, which is more rare, individual little prismatic crystals. Through experiments it was established that thermolysis of γ-Аl(ОН)₃ with the particles bigger than 200 μm resulted in a greater amount of aluminium oxohydroxide with the structure of boehmite (approximately by 25%) formed compared with thermolysis of fine particles with the size <20 μm. It is demonstrated that the main differentiating characteristics of the coarse particles of aluminium hydroxide include imperfections and the platelike shape of the crystals which build the particles of the coarse-disperse γ-Аl(ОН)₃. The authors suggest that they are the above differentiating characteristics that cause microstresses in the crystals, which increase the reactivity of aluminium hydroxide and, under otherwise equal conditions, help the formation of the γ-AlOOH phase.

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