(3c) High-Shear Granulation: An Investigation into the Kinetics of Granule Consolidation

High-shear granulation, the production of agglomerates called granules by agitation of wet powders in heavy mixers, is widely used in many industries. Despite its wide application, the key mechanisms of the granulation process, i.e., wetting and nucleation, consolidation and growth, and breakage and attrition, are not fully understood. This lack of knowledge makes the design of granulated products a laborious process.

The aim of this work is to elucidate the kinetics of consolidation and layered growth in high-shear mixers. In order to achieve this, a novel consolidation-only granulator (COG) has been developed to isolate growth and consolidation behaviour of granules. The obtained kinetics are then compared to an existing kinetic model in the literature, based on surface tension-driven growth. Although this model has been shown to predict static growth behaviour correctly, it has not been validated for dynamic situations.

For the investigation of dynamic growth kinetics, both the newly developed COG and a high-shear mixer with a flat blade impeller were used. Prenucleated granules were consolidated in the granulators, extracted and weighed. In this way, the growth of the granules could be monitored over time. For the COG, nuclei were produced with drop nucleation on static powder beds, whereas nuclei for the high-shear mixer were produced from paste in order to increase their survivability. True and envelope densities of the granules were determined using helium and powder pycnometry, respectively. The structure of a select number of granules was also investigated with X-ray tomography.

Four different powder-binder systems were investigated: lactose-silicone oil, lactose-polyethylene glycol (PEG), glass beads-silicone oil and glass beads-PEG. The binder viscosity was expected to have a significant effect on the growth rate. Therefore, binder viscosities were varied to investigate this dependency by using different grades of silicone oil and PEG.

Our work shows that the COG is a new, reliable tool for generating layered growth and consolidation. Furthermore, the kinetics obtained from the studies with the COG reveal that growth behaviour is linear with the square root of time, as predicted by the surface tension-driven growth model found in literature. This finding shows that consolidation behaviour has predictable kinetics, which makes it possible to model consolidation and layered growth.

Additionally, we present the comparison between kinetics in the COG and the high-shear mixer. Differences in kinetics, granule structure and density are discussed in terms of number of impacts, shear and granule preparation method.

The ultimate objective of this study is the proposal of a kernel for consolidation and growth for population balance modelling. In this way, the physical relevance and reliability of such models can be improved.