Mechanochemical phenomena of gibbsite, bayerite and boehmite by grinding

Three hydrated aluminas, gibbsite, bayerite and boehmite, were ground in air for 0.25–20 h by a planetary ball mill. The effect of grinding on their texture, structure and thermal behaviors has been examined by means of XRD, TG, DTA, SEM, IR, particle size distribution and nitrogen gas adsorption. It was found that in the initial stages of grinding (within the first 1 h), the specific surface area, S, had increased in the gibbsite, decreased in the bayerite and had a maximum in boehmite, and then proceeded to achieve their respective equilibrium values. These behaviors were correlated with changes in the median size, d₅₀, as a measure of the aggregate size of the particles. Prolonged grinding times, led to a decrease in the intensity of X-ray diffraction lines and an increase in the half-maximum line breadth. After 4 or 8 h grinding all the hydrated aluminas had been changed to the amorphous phase without mechanochemical dehydration. Moreover, the DTA endotherms attributable to the dehydration of the hydrated aluminas gradually diminished on grinding, and a set of broad endotherms appeared at about 150 or 200 °C for the amorphous phases. Whereas the transformation sequences of the unground hydrated aluminas to α-Al₂O₃ showed gibbsite → χ → κ → α, bayerite → η → θ → α and boehmite → γ → δ → θ → α, all the amorphous phases showed the amorphous hydrated alumina → amorphous alumina → η → α and the temperature of α-transformation had been lowered by more than 200°C.