Influence of sodium hydroxide on phase transformation of gamma alumina to alpha alumina

Official URL: https://dx.doi.org/10.1111/jace.19630

Corundum and transition alumina powders are commonly produced through the Bayer process—that is, by digestion of bauxite in a sodium hydroxide solution, followed by gibbsite precipitation, which contains significant amounts of Na+ ions. During the de-hydroxylation of gibbsite, Na+ is associated with retardation of the structural phase transformations between transition alumina and corundum (α-Al2O3) phases, although the exact role of Na is unclear. The influence of Na+ ions on the phase transformation of γ-Al2O3 to α-alumina was investigated by varying the concentration of NaOH in aqueous γ-Al2O3 suspensions. The phase transformation behavior of γ-Al2O3 was monitored through thermal analysis, X-ray diffraction, scanning electron microscope, X-ray photoelectron spectrometer (XPS), and transmission electron microscope. As the Na concentration in γ-alumina suspensions increased, the amount of α-Al2O3 decreased in samples that were heat treated at 1200°C for 2 h. XPS analysis indicated that washing transition alumina powders with NH4OH solutions had decreased the surface concentration of Na+ and facilitated the phase transformation to α-Al2O3.