Preparation of Rho Alumina

Abstract

The crystalline aluminium hydroxides of hydrargillite-I (synonym of gibbsite) and II (H-I, II) and bayerite-I and II (B-I, II), in which I and II consist of the coarse and fine particles, respectively, were heated at 200-400°C under reduced pressure. The starting materials were checked up by electron microscopy and thermal analysis (TG and DTA), and then the resulting products were examined by x-ray diffraction study. Consequently, it was found that upon heating under reduced pressures at 0.667-6.67 Pa, amorphous alumina was formed from H-I and II bearing no relation to the heating rate at 1.25 or 5°C min-1 , and similarly amorphous alumina was also formed from B-I and II at the rapid heating rate of 5°C min-1 ; while very poor crystalline alumina, which is regarded as the material corresponding to rho alumina (ρ-alumina) was obtained at the slow heating rate of 1.25°C min-1 from B-II, and a mixture of boehmite and ρ-alumina was obtained from B-I at the same heating rate. Accordingly it is seen that a ρ-alumina is prepared from the dehydration of bayerite which consist of fine particles by heating at 200-400°C at the slow rate under reduced pressure. Crystalline aluminium hydroxides are composed of five kinds of alumina hydrates : hydrargillite (and/or gibbsite), bay-erite and nordstrandite, classified as three aluminium trihydrox-ides ; and boehmite and diaspore, classified as two aluminium oxyhydroxides. Hydrargillite and boemite occur naturally as the primary composition of the mineral bauxite, but they can also be produced artificially. The terms hydrargillite and gibbsite are interchangeable, the former used in Europe and the latter generally in the rest of the world. The preparation of aluminas is tipically carried out by thermal decomposition of aluminium hysroxides. Similarly, alumi-nas are also prepared by thermal decomposition of the inorganic and organic salts of aluminium. Until quite recently, the view was generally accepted that aluminium hydroxides, except diaspore, were thermally transformed to stable α-alumina through boehmite and γ-alumina. It has been reported by quite a few observers that thermal transformation of the various aluminium hydroxides yields a number of aluminas, such as χ, γ, η, δ, δ 1 , κ, κ' , θ, ε, ρ and λ and amorphous, which are the transition stages in a process leading ultimately to α-alumina 1-4). In paticular, Stumpf et al 5). have suggested the following stages through which thermal transformation passes : hydrargil-lite proceeds through the stages via boehmite, χ-, γ-, κ-, θ-and α-aluminas, bayerite via boehmite, η-, δ-, θ-and α-aluminas, boehmite via γ-, δ-, θ-and α-aluminas, and diaspore goes to α-alumina. On the one hand, Sato has also proposed the following sequences for the thermal transformation of crystalline aluminium hydroxides : hydrargillite and bayerite are classified as types I and II, respectively, according to their dehydration behaviors ; hydrargillite-I dehydrates to a mixture of boehmite and hydrargillite and then of boehmite and λ-alumina, and to λ-alumina, while hydrargillite-II dehydrates directly to λ-alu-mina ; for hydrargillite-I and II, λ-alumina passes to κ-, θ-and α-aluminas ; bayerite-I dehydrates to a mixture of boehmite and bayerite and then of boehmite and η-alumina, and to η-alumina, while bayerite-II dehydrates directly to η-alumina ; for bayerite-I and II, η-alumina passes to θ-and α-aluminas ; nordstrandite proceeds to the stages via η-, θ-and α-aluminas, boehmite via γ-, δ-, θ-and α-aluminas, and diaspore goes to α-alumina. Accordingly many aluminas have been introduced as the transition state to α-alumina. Moreover, the presence of ρ-alumina has been indicated in the thermal decomposition of aluminium hydroxide under reduced pressure. Rho alumina (ρ-alumina) is primarily a vacuum produced phase. It is substantially amorphous, since the x-ray pattern shows only a few diffuse bands, the most pronounced of which has a spacing of 1.40Å. The diffraction peak at 1.40Å, which arises from the spinel diffraction index 440, corresponds to half of the distance between oxygen and oxygen atoms in the packing structure of oxygen and aluminium ions and is common to a number of close-packed structures of oxygen atoms. For this reason, it was assumed that ρ-alumina had only a foundamental structure and not developed in atomic arrangement regularity. Accordingly it is considered that the formation of ρ-alumina depends on preparation conditions. Tertian et al 6) suggested that when hydrargillite is heated under reduced pressure at 1.33Pa, ρ-alumina was formed at 200-400°C and went to α-alumina via η and θ-aluminas. On heating under atomospheric conditions, hydrargillite formed ρ-＊