Scientific basis of technology of fine crystalline quartz and corundum.

M.N. Danchevskaya, Yu.D. Ivakin, S.N. Torbin, G.P. Panasyuk, V.N Belan, I.L Voroshilov.

3-rd International Conference on Solvothermal Reactions ICSTR - 3 Bordeaux, July 19-23, 1999, p. 25

ABSTRACT. The developed technology of fine crystalline quartz and corundum is based on the previous investigation of the behavior of solids during heating in water vapor atmosphere. In particular processes of solid-phase transformation of silica and alumina in supercritical water (T= 380-450 C; P H2O =15,0- 30,0 MPa) were studied. Authors made an emphasis on investigation of mechanism and kinetics of quartz synthesis (alfa-SiO2) and corundum (alfa-Al2O3) because these products are widely used in industry. The thermovaporous treatment of raw materials has been carried out in laboratory (v=20 ml) and technical (v=4 m3) autoclaves at temperatures between 390 and 410 C and vapor pressures of water (18,0-30,0) MPa in the presence of activating microadditives. 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, the necessary condition for the formation of the most thermodynamically stable phases (alfa-SiO2 and alfa-Al2O3) during thermovaporous treatment of silica and alumina is the formation of an intermediate phase with a certain degree of hydroxylation. The kinetics of transformation of intermediate solid phases (cristobalite-SiO2.nH2O and boehmite-AlOOH ) respectively to quartz and corundum, satisfied the Avrami - Erofeev equation. The activation energy of transformation depends on the activator type. Microadditives of activator into silica or alumina may 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 on solid-phase transformations of silica and alumina in supercritical water new economic and ecologically pure technology of fine crystalline quartz and corundum have been developed. This research was made with financial support of RFBR pr. 98-03-32645a.

Laboratory of Catalysis and Gas Electrochemistry