INFLUENCE OF GRAIN AND CRYSTALLITE SIZE ON THE GIBBSITE TO BOEHMITE THERMAL TRANSFORMATION

Thermal decomposition processes of three different samples of hydrated alumina: Bayer precipitated size fractioned, Bayer precipitated ground and fine precipitated, were studied. These were investigated with special regard to evolution of boehmite. The original samples contained 75-85 wt% of gibbsite, while remaining material with gibbsite-like Al2O3-H2O ratio did not show long range order crystallinity. Decomposition reactions were observed by thermal analysis and reaction products were investigated by powder X-ray diffraction. Grain sizes were determined by laser diffraction and morphological changes of grains were observed by scanning electron microscopy. Boehmite formation is influenced mainly by grain and crystallite sizes of starting materials, while degree of crystallinity is of less importance. Transformation of gibbsite to boehmite was most pronounced in the case of coarse grains, nevertheless in fine particles boehmite evolution seemed retarded.