V. N. Belan

Phenomenology of corundum crystal formation in supercritical water fluid

This paper describes a solid phase transformation of aluminium hydroxide into oxyhydroxide (boehmite), and then into well formed fine crystals of corundum (α-Al 2 O 3 ) in the medium of a supercritical water fluid at a temperature of 450 °C and a pressure of 30 MPa. Using electron microscopy the development and processing of corundum crystal formation from incipient crystals to crystal growth and their perfecting is traced.

Hydrargillite → Boehmite Transformation

The hydrargillite → boehmite transformation has been studied at 200 and 250°C in water and water vapor. Samples have been characterized by X-ray diffraction, differential thermal analysis, thermogravimetry, IR spectroscopy, and transmission and scanning electron microscopy before and after autoclaving for various lengths of time. The hydrargillite → boehmite transformation is shown to be a solid-state process, and its steps are identified.

Scientific basis of technology of fine-crystalline quartz and corundum

Abstract Authors have investigated the mechanism and the kinetics of fine-crystalline quartz and corundum synthesis in supercritical water. The thermovaporous treatment of raw materials has been carried out in laboratory (v=20 ml) and technical (v = 2m3 and v=4m3) autoclaves at temperatures between 390 and 410°C and vapor pressures of water from 20 to 27 MPa in the presence of miacroadditives of activators. The samples of products after thermovaporous treatment have been studied by X-ray diffraction, optical and electron microscopy, mass-spectroscopy, ESR-, IR-spectroscopy, thermal analysis and pycnometry. It was shown that microadditives of activator into silica or alumina might not only accelerate the attainment of the best hydroxylation, but also initiate the transformation of precursors in an ordered way. The control of the transformation mechanism by introducing activators into the starting materials has allowed us to obtain fine-crystalline quartz and corundum with desired properties: various habitues and size of crystals. On the basis of these data, technology of fine-crystalline quartz and corundum has been developed.

Aluminum hydroxide transformations during thermal and vapor heat treatments

We have studied the mechanisms underlying the formation of corundum crystals from hydrargillite and boehmite during thermal and vapor heat treatments and have identified the major stages in the formation of corundum structure, thought of as the evolution of the system toward the most thermodynamically stable state. The results indicate that isothermal autoclaving at temperatures from 390 to 450°C gives rise to structural changes in alumina, which involve the following stages: the formation of microcrystalline boehmite from coarser hydrargillite crystals, accompanied by disintegration of agglomerates, boehmite flocculation and dehydration, conversion of the dehydrated boehmite to alpha-alumina within each floccule, and formation of corundum single crystals. Unflocculated boehmite crystals are absorbed by the growing crystal faces. If presynthesized boehmite is used as the precursor, the process also involves flocculation, dehydration, conversion to alpha-alumina, and formation of corundum single crystals. The structural changes induced in hydrargillite and boehmite crystals by annealing in air and vacuum at temperatures from 1400 to 1500°C have been analyzed. The formation of alpha-alumina from boehmite below <1200°C is accompanied by no changes in crystal habit, whereas annealing in the range 1400–1500°C gives rise to sintering of fine alumina crystals and changes in crystal habit. The degree of alignment of the crystals is shown to influence their structural transformations.

The thermodynamic properties and role of water contained in dispersed oxides in precursor-boehmite conversion, based on the example of aluminum hydroxide and oxide under hydrothermal conditions in different environments

The hydrothermal conversion of precursors (MDGA hydrargillite/γ-Al2O3) to boehmite is studied. It is found that the specific surface area of the precursors increases and the peak of the bending vibrations of molecular water is shifted from 1640 to 1690 cm−1 for hydrargillite at the early stages of hydrothermal treatment. The heats of vaporization of water from a two-phase system (original precursor/resulting boehmite) are found to be 27.5 and 6.5 kJ/mol H2O, which is less than the tabulated value. Based on the literature data, conclusions are drawn with regard to the state of water in the structure of the treated material in the form of single molecules, dimers, and trimers: the boehmite structure is not perfect, and the heat effects of the dehydration of boehmite in forming γ-Al2O3 have values of 13.5 and 21.4 kJ/mol AlOOH. It is found that further hydrothermal treatment leads to the formation of boehmite crystals with perfect structure and, regardless of the composition of the reaction medium used for the treatment, the heat effects of the dehydration of boehmite are 26.9–27.2 kJ/mol AlOOH, which is close to the tabulated value.

Preparation of Silicon Dioxide with a Fixed Content of Fluorine from Fluorosilicic Acid

The process of obtaining silicon dioxide upon the interaction of fluorosilicic acid with an aqueous solution of ammonia is studied. The fluorine-containing compounds in synthesized dioxide are identified by 19F NMR spectroscopy. It is shown that the two-stage method of precipitation followed by washing, allows producing silicon dioxide with a fluorine content of units of ppm.

Increasing Crystallization Rate in Growing Single Crystals of Quartz

A new method was proposed to increase the crystallization rate in growing single crystals of quartz. In this method, the crystallization is accelerated by using surfactants (polyethyleneamine, tetramethylammonium base, and polyethyleneimine) added to a soda-alkaline solution of standard concentration. Within studied ranges of pressures and concentrations of components of the solution, polyethyleneimine at a concentration of 0.002–0.05 wt % is the most efficient crystallization accelerator, which can increase the quartz crystal growth rate on the average by a factor of 2.

New techniques for processing wastepaper

The paper presents processing techniques of waste newsprint paper. Wastepaper was soaked in a soda or a hydrochloric solution and heated in a reactor with limited air access or in an autoclave. In the first stage, at 300°C, we obtained brittle carbonated coal with particle sizes of 10–50 µm, which can be used after milling as an additive in poor coals in the production of a coal-water slurry fuel. At the second stage, at 500–600°C, we obtained an amorphous carbon containing of up to 91.2% of C, which is widely used in the tire, rubber, and paint and varnish industries.

The study of hydrargillite and γ-alumina conversion process in boehmite in different hydrothermal media

The transition of hydrargillite and γ-alumina in boehmite was investigated under hydrothermal conditions at a temperature of 200°C. The methods of X-ray fluorescnce (XRF) analysis, infrared spectroscopy (IR spectroscopy) DTA, TGA, DSC, scanning electron microscopy (SEM), and thermal desorption were used to study the samples subjected to autoclave treatment in different media. It was shown that, at the hydrothermal treatment of hydrargillite and γ-alumina at the initial stages of treatment, regardless of the conditions of the process and the reaction medium, the formed boehmite is characterized by a high specific surface and low value of the heat effect of the formation and decomposition, which is determined by the weak ordering of its structure.

Effect of pre-heat treatment and cobalt doping of hydrargillite on the kinetics of the hydrargillite-corundum transformation in supercritical water fluid

We have studied the kinetics of corundum formation in supercritical water fluid from aluminum hydroxide (hydrargillite) and from the products of its thermal decomposition at 300°C over a period of 5 h. In all cases, the reaction involves the formation of boehmite, AlOOH, as an intermediate. The use of the product of the thermal decomposition of hydrargillite as a precursor considerably increases the reaction rate and reduces the induction period in comparison with the parent hydrargillite. Cobalt doping further increases the reaction rate and reduces the induction period. When the product of the thermal decomposition of hydrargillite is used as a precursor, the temperature dependence of the reaction rate constant shows Arrhenius behavior, whereas that for synthesis from uncalcined aluminum hydroxide has a more complex character.