DEHYDROXYLATION SEQUENCES OF GIBBSITE AND BOEHMITE - STUDY OF DIFFERENCES BETWEEN SOAK AND FLASH CALCINATION AND OF PARTICLE-SIZE EFFECTS

The thermal transformation sequences of boehmite (gamma-AlOOH) and two grades of gibbsite [gamma-Al(OH)(3)] upon soak and flash calcination are reported. The techniques used were X-ray diffraction (XRD), differ ential thermal analysis (DTA), Fourier-transform IR (FTIR) and Al-27 m agic-angle spinning(MAS) NMR spectroscopies. Boehmite undergoes the de hydroxylation sequence boehmite, gamma, delta, theta, alpha-Al2O3 unde r both soak and hash calcination. The dehydroxylation sequence of gibb site, however, depends on the calcination method and the particle size of the feed material. Soak calcination of a: fine gibbsite (ca. 0.5 m u m) gave the dehydroxylation sequence gibbsite, chi, kappa, alpha-Al2 O3; with flash calcination the sequence gibbsite, chi, gamma, delta, t heta, alpha-Al2O3 was observed. Soak calcination of coarse gibbsite (c a. 14 mu m) gave both the dehydroxylation pathways (a): gibbsite, boeh mite, gamma, delta, theta, alpha-Al2O3 and (b): gibbsite, chi, kappa, alpha-Al2O3, and pathway (a) was predominant. Flash-calcined coarse gi bbsite experiences a crossover between these routes (chi-gamma) withou t formation of kappa-Al2O3. Flash calcines of gibbsite undergo this ch i-gamma phase change at ca. 800 degrees C.