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

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 (alpha-Al2O3) phases, although the exact role of Na is unclear. The influence of Na+ ions on the phase transformation of gamma-Al2O3 to alpha-alumina was investigated by varying the concentration of NaOH in aqueous gamma-Al2O3 suspensions. The phase transformation behavior of gamma-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 gamma-alumina suspensions increased, the amount of alpha-Al2O3 decreased in samples that were heat treated at 1200 degrees 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 alpha-Al2O3. Schematic representation of the alumina nanoparticles before and after addition of NaOH showing the effect of Na+ on phases and morphological changes. Samples were heat treated up to 1200 degrees C for 2 h. image