The dehydroxylation of aluminium hydroxides and the kinetics ofα-Al2O3 formation

Abstract

Studies were made of the dehydroxylation of several aluminium hydroxide modifications and the kinetics of a-A1203 formation. The investigated samples differed in both mineral composition and the level of alkali admixtures. It was found that the rate of formation and the quantity of a-A1203 depend mainly on the purity of the aluminium hydroxides, while the transition forms of alumina depend on the initial type of the aluminium hydroxide.Alumina is currently a valuable construction material in the various branches of industry. Its wide-ranging applications demand that alumina fulfils very high requirements as concerns chemical purity, hardness, crystallographic structure, degree of comminution and many other physieochemical features.Alumina is chiefly obtained by the dehydroxylation of aluminium hydroxides. The final aim is most often the manufacture of the most stable form of A1203 i.e. a-A1203, corundum. Corundum has exellent properties, such as high hardness, thermal resistance and high melting temperature. The final physicochemical properties of the produced ceramic material and the economy of the technological process of its manufacture depend on the rate of formation and the quantity of a-A1203.The dehydroxylation of aluminium hydroxides is a very complicated process. Numerous research works [1][2][3][4][5][6][7][8][9], have demonstrated this, though the conclusions are sometimes discrepant. The transformations of the structure of aluminium hydroxides during heating up to the formation of new crystalline products depend on many factors, such as the type and origin of the aluminium hydroxide, grain size, chemical purity, heating rate, calcination time, atmosphere, mineralizator additives and others. Irrespective of these factors, greater or smaller amounts of a-A1203 are always obtained if the calcination temperature is adequately high. The proposed schemes of dehydroxylation differ because of the various methods of manufacturing of John Wiley & Sons, Limited, Chichester Akaddmiai Kiad6, Budapest

Thermal decomposition of precipitated fine aluminium trihydroxide

“…The peak observed for both the materials at around 480–500°C is due to the dehydroxylation of boehmite phase. Therefore, the transition forms of various aluminas occur after this temperature only and there may not be any hydroxide phase above 500°C [1]. A similar behaviour is also observed for fine ATH particles of sizes 1 and 0.25 μm indicating that boehmite formation with ATH of low particle size is also a possibility particularly for precipitated particles.…”

“…The process of dehydration of gibbsite (aluminium hydroxide, γ‐Al(OH) 3 ) was studied extensively and is a very complicated process as reported in numerous research publications [1–17]. The dehydration sequence of γ‐Al(OH) 3 and the formation of α‐Al 2 O 3 is accomplished through various metastable processes (transition alumina).…”

“…Conversion of trihydroxide to boehmite is accomplished at around 500°K [15] under rapid heating conditions with coarse particle size. Beyond this temperature, a series of metastable phases such as chi, kappa aluminas are formed [1, 2, 10–14, 16]. Other forms of transition aluminas obtained are gamma, delta, theta, eta, rho etc.…”

Cryosol Synthesis of Nanocrystalline Alumina

“…27 Al NMR spectra for the reference solutions N1-N6 prepared by conventional hydrolysis correspond well to the previously reported data 11-13 (see Figure 5). The freeze-drying of the sols results in white, very friable, nonhygroscopic powders (F ∼ 0.05 g/cm 3 ). The as-prepared samples are amorphous to X-ray and electron diffraction.…”

Modelisation mathematique d un four rotatif de calcination d alumine

“…Pyzalski et Wojcik (1990) obtiennent par exemple une température pour la fin de transformation de l'alumine gamma en alumine alpha de 1350°C, ce qui se rapproche beaucoup de la température obtenue par le modèle.…”