Preparation and Phase Changes of Amorphous Alumina

Amorphous alumina was prepared (i) from aluminium nitrate solution at pH 6.0 and 9.0 and (ii) by coprecipitationfrom aluminium nitrate solution containing 5% magnesium and chromium nitrate at pH 9. Phase changes taking place in them during calcination were compared with those of aluminium hydroxide by DTA and X-ray diffraction techniques.DTA of the prepared samples generally showed a sharp endothermic peak at 325°C, corresponding to the dehydration of the possible traces of fine gibbsite and an exothermic peak at 345° C which could be considered as characteristic of amorphous phase. Aluminium hydroxide, however, showed three endothermic peaks at 240°, 325° and 540°C corresponding to the coarse gibbsite to boehmite conversion, gibbsite dehydration and boehmite dehydration respectively.The X-ray diffraction study of the prepared samples showed mostly amorphous patterns. When calcined to 750°, 950° and 1450°C respectively the gamma, delta and alpha phases were formed.