Abstract
Hydrous alumina gels and amorphous alumina were aged in water at 70°C for a long period, and the gels in part were hydrothermally treated at temperatures from 100° to 180°C for 5h. The gels were prepared by adding ammonia water to an aqueous solution of aluminium chloride. Amorphous alumina was obtained by calcining aluminium chloride at 600°C for 10h. The water-to-sample ratio and stirring were adopted as principal variables in aging experiments at 70°C. Aged products were examined by X-ray diffraction, differential thermal analysis (DTA) and thermogravimetry (TG). Pseudo-boehmite was formed rapidly at very early stages of aging, which was followed by the formation of alumina trihydrate sensitively detected by DTA for the dehydration. The development of DTA peaks for the dehydration of aged products corresponded to the degree of crystallinity and the amount of alumina hydrates detected by X-ray diffraction. The thermal dehydration of trihydrates as aging products at 70°C proceeded directly to alumina without the intermediate formation of crystalline boehmite or pseudo-boehmite. Thermal analysis techniques, especially DTA, were found to be powerful to studying the formation and crystallization of alumina hydrates from hydrous alumina gels and amorphous alumina in water.
