Mechanisms of mixing of granular materials in drum mixers under rolling regime

Experimental investigations on mixing of non-ideal powders (granular tetraacetylendiamine (TAED)) are described. The evolution of mixing in rotating batch cylinders, in rolling regime has been addressed. Characterization and quantification of the local mixture composition have been obtained through an efficient solidification technique, coupled with computerized image analysis. Starting from a completely segregated configuration, the fort-nation of a temporary, poorly mixed core at low rotation speed has been observed. Investigation of intermediate configurations during the mixing process allows to identify some unexpected granular mixing mechanism. The observed core has been explained in terms of transient axial convective fluxes superimposed on diffusive motion. Small differences of dynamic angle of repose between the two granular materials have been suggested to drive the axial convection, similarly to the mechanism reported in the literature to explain axial segregation phenomena. The differences in repose angle result from surface and shape irregularities typical of actual (i.e. non-ideal) granules. Convective fluxes due to the friction of powder with the end plates are also identified at the extremities of the mixer. Short-circuiting zones are created that hinder both axial diffusion and convection from the center of the vessel. Eventually, we suggest a mixing mechanism of non-ideal granular material where convection plays a major role. (C) 2004 Elsevier Ltd. All rights reserved.