Mixing kinetics of granular materials in drums operated in rolling and cataracting regime

Most of the earlier studies on mixing have been focused on drums operated in or close to rolling regime, considered the most convenient for metallurgical, cement and mining applications in rotary kilns. However, many other industrial applications deal with powder mixing in rotating drums or other tumblers such as in pharmaceutical, detergent or food industry. In these cases, effectiveness of mixing may be given by other regimes. Here we compare mixing efficiency and kinetics of two different regimes, i.e. rolling and cataracting. The attention has been specifically focused on both the internal composition patterns and the mixing kinetics, aiming at optimising the operation time and the final homogeneity of the mixtures. The internal structure of the bed, after mixing, has been investigated through a solidification technique. Images of the transverse plane of the mixture at the ends of the drum provide information on the mixture composition there, during mixing. Both information from the interior and the ends have been used to point out differences in the mixing patterns and kinetics of the two regimes considered. It is known since a long time in the industry that the evolution of the mixing process strongly depends on the history whom the bulk has been subjected to before mixing begins or during the early stages of the process. This work aims at providing some mechanistic and quantitative explanation of this knowledge and shows that not a single regime, but a proper combination of the two regimes allows to achieve a better mixing quality more rapidly.