On the formation of core-shell granules in batch high shear granulators at two scales

Abstract High shear granulation of powders with a wide size distribution can lead to the formation of core-shell particles consisting of a few larger particles in the core with a shell of fine particles. We demonstrate successful scale down of core-shell granule formation and, using a new model for interpreting observed cross-sections, show that above a critical speed the shell-thickness remains constant at about ~5% of the granule diameter with a crushing strength of about 5.5 MPa. Below this speed and at low fill-levels, shell-thickness increases and granule strength reduces. Under a low mixing intensity at the lowest fill-levels core-shell particles are not formed at all. Theoretical work on the strength of liquid bridges shows that small particles attached to each other would redistribute more easily than the same particles attached to a large particle. We propose therefore that a detachment-reattachment process is responsible for the formation of core-shell granules.