Synthesis of aluminium hydrates by a precipitation method and their use in coatings for ceramic membranes

Aluminum hydrates were synthesized using AlCl3·6H2O as the starting material by the precipitation method. The phases of the obtained powder were amorphous, boehmite, bayerite and nordtsrandite depending on the pH of the solutions. The weight loss of the powders by dehydration was increased with the lowering pH condition of solution. Aluminum hydrates transformed to α-Al2O3 via γ-Al2O3, δ-Al2O3 and θ-Al2O3, and particle sizes were grown by increment of the heating temperature. The crystallite size and specific surface area were rapidly changed over 1100°C which is the formation temperature of the α-Al2O3 phase. The γ-Al2O3 powder was coated on an intermediate layer of ceramic membrane by the slip casting method, and an unsupported membrane was also prepared for comparison. The supported microfiltration layer showed a porous structure with small grains, but the unsupported layer revealed interconnected larger grains. Grain growth was more dominant in the unsupported layer than in the supported one due to interaction between the support and the supported layer. An ultrafiltration membrane with homogeneous microstructure was formed using boehmite particulate sol, and abnormal grain growth in unsupported membrane was also observed similar to microfiltration layer.