High Energy Milling of Boehmite: Changes in Particulate Characteristics

Abstract

This work deals with the changes physicochemical characteristics of boehmite during high energy milling/ mechanical activation (MA). Reactivity of boehmite, typically low compared to its trihydrate polymorph (gibbsite), gets enhanced during MA as a result of physicochemical changes in the bulk and surface. Specific focus is on the changes in physicochemical characteristics. Boehmite used in the study has been prepared by thermal dehydroxylation of a synthetic gibbsite from an alumina refinery and has been characterised by a monomodal size distribution with median size (d50) of 110 m and high surface area (264 m2/g). A planetary mill has been used to activate the boehmite; extent of activation is varied by changing the milling time (0 to 240 min) while keeping the ball to powder ratio (30:1) and milling energy (400 rpm) constant. Particle size sharply reduced to few microns as a result of high impact forces prevailing in the mill and the size distribution transforms to a bimodal one in the early stages of milling. As milling progresses, the size reduction ceases; rather an increase in median size, a sign of particle aggregation, is observed. Particle aggregation has been confirmed by scanning electron microscopy. Ensuing geometric specific surface area changes reflected the changes in the size characteristics. BET specific surface area (SSABET), on the other hand, behaved differently; an anomalous decrease has been observed as the milling progressed. A decrease from 264 m2/g to about 65 m2/g has occurred during 240 min of milling. Detailed analysis of the adsorption isotherms, obtained from N2 adsorption studies (at liquid N2 temperature) carried out to understand the anomalous decrease, using BJH method has shown that the evolution of pore size distribution is responsible for the anomalous decrease in SSABET. In spite of the decrease in SSABET, reactivity in terms of alkali dissolution and thermal transformation has increased.