FLUIDIZED DRUM GRANULATION - STUDIES OF AGGLOMERATE FORMATION

Agglomerate formation in fluidized drum granulation was studied experi mentally using a 0.3 m diameter laboratory fluidized drum granulator. Glass ballotini of various sizes were coated with molten paraffin wax. The effect of particle size (1.5 to 8 mm), liquid spray rate (17 to 4 3 g/min) and drum speed (20 to 40 rpm) on the fraction of agglomerates formed was measured. The formation of agglomerates was controlled by collisions between wetted granules in the falling particle curtain in or near the spray zone. There is a sharp transition between high fract ions of agglomerates (non-inertial regime) and almost no agglomerates (coating regime) as particle size increases. This transition point inc reases strongly with increasing liquid spray rate but is only a slight function of drum speed. These results agree, qualitatively and quanti tatively, with the granulation regime analysis proposed by Ennis, Tard os and Pfeffer, Powder Technol., 65 (1991) 257 which is based on the b alance between inertial energy and viscous dissipation during a collis ion between two wetted granules. Analysis of the data shows the charac teristic collision velocity is low, of the order of 0.02 m s(-1), and this is consistent with agglomerates forming in the falling particle c urtain near the spray zone. A better understanding of particle-particl e and particle-binder interactions in the spray zone is needed to exte nd the theoretical analysis and develop design and scale-up rules.