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Homogeneous precipitation of aluminum hydrous oxide and its application for obtaining composite materials


Homogeneous precipitation of aluminum hydrous oxide and its application for obtaining composite materials

Kudryavtsev Pavel

Incoming article date: 20.06.2018

The paper presents the results of studies of the process of hydrated alumina homogeneous precipitation. The variants of this process are considered. As the most convenient embodiment of this process, precipitation of hydrated alumina from its salts using auxiliary substances. As such auxiliary substances, it is proposed to use urea. Urea hydrolysis proceeds slowly even with heating, which allows a homogeneous precipitation process under the most mild conditions. It is shown that the process of hydrolytic precipitation in the aluminum salt-urea system proceeds according to the dissociative mechanism SN1. For SN1, a two-step mechanism is characteristic, in which the first step is slow monomolecular hydrolytic dissociation, followed by a rapid coordination reaction with the second reagent. To determine the nature of the interaction of the aluminum salt with alkaline agents, experiments were conducted on potentiometric titration of the aluminum salt with solutions of ammonia and sodium hydroxide in the presence of various amounts of ammonium chloride and urea. When titrating with sodium hydroxide solution, the process proceeds to the formation of sodium aluminate, titration with a solution of ammonia produces a precipitate of hydrated alumina. Addition of ammonium salts shifts the equilibrium of the hydrolysis reaction towards the formation of aluminum hydroxide. The presence of urea virtually does not affect the course of the deposition process. To determine the uniformity of the formed sediment, the laser radiation scattering method. For these purposes, a special installation has been created that allows the capillary to be scanned through a focused laser beam. The homogeneity of the hydrated alumina precipitate formed was shown. Studies have been carried out on the use of homogeneously precipitated aluminum hydroxide as a binder in lightweight fibrous refractory materials. The process of precipitation of aluminum hydroxide particles proceeds primarily on the filler fibers, covering them with a thin film. The resulting film acts as a gluing agent. After drying and calcination, the gel deposited by the thin film becomes a solid that binds the fibers into a single monolith. The resulting materials have a developed specific surface area and porosity.

Keywords: homogeneous precipitation, hydrated alumina, composite materials, urea