Producing hollow granules from hydrophobic powders in high-shear mixer granulators

The formation of hollow granules from hydrophobic powders in a high-shear mixer granulator has been investigated by changing the binder/powder mass ratio and studying its effects on granule size and structure. In this study, a mixer granulator was filled with 100 g of hydrophobic fumed silica and then varying quantities of 5% Hydroxy Propyl Cellulose solution was slowly sprayed into granulator. A range of liquid to solid mass ratios between from 0.5:1 to 15:1 was used. Granules were then dried at 60 °C in a fan forced oven. This paper compares the particle size distributions, scanning electron microscopy (SEM) images and X-ray tomography (XRT) images of hollow granules as a function of the liquid to solid mass ratio. The granule mean size increased and the fraction of un-granulated (fine) particles decreased as the liquid to solid mass ratio increased. Simultaneously, the morphology and structure of the hollow granules changed from a spherical to a deformed structure which indicates the importance of choosing the optimal liquid to solid mass ratio. The optimal liquid to solid mass ratio for Aerosil R202 powder in this study was found to be between 3:1 and 6:1. The final granule shape and size distribution are dependent on the liquid to solid ratio if the liquid marble nucleation process starts with a preformed droplet template.