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

Abstract 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‐Al 2 O 3 ) phases, although the exact role of Na is unclear. The influence of Na + ions on the phase transformation of γ‐Al 2 O 3 to α‐alumina was investigated by varying the concentration of NaOH in aqueous γ‐Al 2 O 3 suspensions. The phase transformation behavior of γ‐Al 2 O 3 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‐Al 2 O 3 decreased in samples that were heat treated at 1200°C for 2 h. XPS analysis indicated that washing transition alumina powders with NH 4 OH solutions had decreased the surface concentration of Na + and facilitated the phase transformation to α‐Al 2 O 3 .