Follow
The four main mechanisms of granule formation initially described by Ennis include:
1. Wetting and Nucleation
This is the first and an important stage of granule formation. It involves the initial wetting of the powder bed and the nucleation of existing grains by the granulating liquid. This step is influenced by spray rate or liquid distribution as well as feed structure characteristics compared to mechanical mixing. It is worth noting that the nucleation process, i.e., the initial aggregation of primary particles around larger wetting drops, is strongly associated with the wetting phase.
2. Coalescence or increase in force
During the coalescence or force growth phase, partially wet primary particles and larger nuclei coalesce to form grains composed of several particles. The more general term cohesion refers to the successful collision of two granules to form a new, larger granule.
3. Consolidation
As the particles grow in size, they are brought together by compressive forces due to bed movement. The extent of consolidation depends on the movement of the granulating tool and the resistance of the grains to deformation. This phase of granule formation controls the internal granule porosity and therefore the final properties of the granule, such as granule strength, hardness or dissolution.
4. Attrition or breakage
At this stage, the formed grains break up into fragments that bond with other grains to form a layer of material over the surviving grains.
The above processes may occur simultaneously in all processes of wet granulation. However, certain processes may dominate a particular manufacturing process depending on the type of equipment used.
