Strength and structure of granules produced in continuous granulators

Abstract

The effect of the operating conditions of three continuous high shear granulators on the internal structure and strength of granules has been investigated and the possibility of seeded granulation has been explored. In a recently concluded programme of research on the scale-up of a high shear granulator, Cyclomix (manufactured by Hosokawa Micron B.V., The Netherlands), a novel method of granulation called seeded granulation was introduced, where each granule contained, at its core, a large particle from the upper tail end of the feed particle size distribution. Seeded granulation is particularly useful for process control of continuous granulators as there is the potential to control granulation by the flow rate of the seed particles. Hence, the performance of three different types of continuous granulators in terms of granule strength and structure has been evaluated here; these are Extrudomix, Modulomix (manufactured by Hosokawa Micron, UK and The Netherlands, respectively) and the Nica M6 Turbine continuous granulator (manufactured by GEA, UK). Calcium carbonate (Durcal 65) powder was granulated using an aqueous solution of polyethylene glycol (PEG) as binder in the same ratio as used previously in our batch granulation, to allow comparison between the continuous and batch processes. The crushing strength was characterised by quasi-static side crushing between two platens using a mechanical testing machine. The internal structure and morphology were evaluated by scanning electron microscopy and the extent of seeding quantified. Granules produced in all the three continuous granulators were significantly weaker than those of the batch granulator tested previously. Among the continuous granulators only the Modulomix granulator produced some seeded granules. It is considered that longer residence time is necessary to produce seeded granules.