Abstract
The review on the hydroxides and oxides of aluminium is given from the viewpoint of their kinds and structures. Crystalline aluminium hydroxide is composed of five kinds of alumina hydrates due to hydrargillite (and/or gibbsite), bayerite and nordstrandite classified as alumina trihydrate and due to boehmite and diaspore classified as alumina monohydrate. Gelatinous aluminium hydroxide consists of amorphous aluminium hydroxide, pseudoboehmite and intermediate boehmite, indicating the particular increase in the intensity of X-ray diffraction peak at the plane (200) as the orthorhombic structure similar to boehmite. The aluminium hydroxides prepared by various procedures are thermally decomposed to anhydrous aluminas. As it has been reported by many researchers that the thermal transformations of aluminium hydroxides yield anumber of aluminas, such as χ, γ, η, δ, δ1, κ, κ', θ, E, ε, ρ and λ, being a transition stage in a process leading ultimately to α-alumina, the thermal decomposition processes are discussed on the basis of the results obtained by the present author. Further the formation of anhydrous aluminas by the thermal decompositions of aluminium salts of inorganic and organic acids such as hydrates of chloride, nitrate and sulphate of aluminium, ammonium alum, formate and acetate of basic aluminium and lactate, citrate and tartrate of aluminium is compared with the thermal transformation processes of aluminium hydroxides to α-alumina. In addition, the preparations of nordstrandite and ρ-alumina are investigated in comparison with the previous results of the author.
