The effect of Al3+ concentration on the properties of alumina powders obtained by reaction between aluminium sulphate and urea in boiling aqueous solution

Homogeneous precipitation of alumina precursors was carried out in boiling aqueous solutions that contained excess urea and 0.010–0.200 M Al3+. The obtained precursor precipitates were dried for 2 h at 100 °C to yield precursor powders and the precursor powders were calcined for 2 h at 1000 °C to yield alumina powders. Precursor powders and alumina powders were investigated by X-ray diffraction (XRD), electron microscopy (EM), particle size distribution (PSD) analysis and nitrogen adsorption-desorption techniques for characterisation. Inspection of the XRD data showed that the precursor powders were amorphous in nature and the final product was δ-Al2O3. Investigation of the EM and PSD data of the powders showed that they contained unequally sized spheres and agglomerates. It was determined that, as the Al3+ concentration in the boiling aqueous solution increased, the number of 0.3–1.0 μm sized independent particles decreased and the sizes of agglomerates grew between 1.00–100 μm while their volume percentages increased from 50 to 80%. It was also observed that the specific surface areas of the alumina powders decreased from 98 m2 g−1 to 54 m2 g−1 and their specific micropore-mesopore volumes decreased from 0.31 cm3 g−1 to 0.14 cm3 g−1 as the Al3+ concentration in the boiling aqueous solution increased. It was observed that it was possible to prepare alumina powders that have different particle size distributions and different porosities by changing the Al3+ concentration while keeping the [urea]/[Al3+] ratio constant at a value of 5.4 in the boiling aqueous solution.