Surface and bulk activation of a siliceous bauxite during attrition milling

Physicochemical changes in a siliceous bauxite ore during attrition milling, and corresponding changes in reactivity are described in this paper. Atrition milling of bauxite has been found to have high milling efficiency. BET Specific surface area has sharply increased in the initial stages, but flattened eventually. Decrease in the enthalpy and temperature of transformation (gibbsite to boehmite) with increase in milling time is a consequence of mechanically induced reactivity. The surface charge, as measured by zeta potential, altered as the milling time is increased; such changes in the surface, surface activation, are also a manifestation of reactivity enhancement with milling. The increase in surface reactivity has been explained in terms of generation of more proton active species (≡ AlOH) at the surface. Analysis of X-ray diffraction patterns of bauxite milled for different duration shows progressive breakdown of gibbsite structure to an amorphous state - high energy state - with milling. Reactivity enhancement ensuing attrition milling leads to near complete digestion of bauxite (alumina) in Bayer (caustic soda) solution even at ambient pressure. Alumina recovery correlates well with specific surface area and degree of amorphisation; however, amorphisation (mechanical activation) is more relevant than specific surface area for multiphase systems like that of bauxite used in this study.