Sol-Gel formation and kinetic analysis of the in-situ/self-seeding transformation of bayerite [Al(OH)₃] to α-alumina

The sol-gel formation of Bayerite, Al(OH)₃, precipitated from AlCl₃·6H₂O (13.6 g/l) and NH₄OH (4N) at a basic pH 9 was studied. The resulting Bayerite was in the form of prisms ∼3 μm in length. The transformation of this Bayerite to α-alumina was [Al(OH)₃] → γ-Al₂O₃ → δ-Al₂O₃ → θ-Al₂O₃ → α-Al₂O₃. The slowest of these transitions, the one which controls the transformation by nucleation and growth kinetics, is the θ-Al₂O₃ to α-Al₂O₃. This rate-controlling transformation was studied by two different thermal treatments. One was isothermal and the other had two steps, the first of which created "in situ" nuclei, to enhance the transformation rate. The latter treatment reduced the time for the transformation at 1100°C from >80 h to only ∼13 h and reduced the activation energy from 419.02 kJ mol^-1 to 317.52 kJ mol^-1. It produced a finer crystal size of α-alumina.