Agglomerate Formation And Growth Agglomeration mechanisms

Early work reviewed by Capes (1980) and Sherrington and Oliver (1981) has established that the following mechanisms for agglomerate formation and growth apply to many granulating systems:

1. Nucleation of primary particles by random coalescence

2. Coalescence between colliding agglomerates

3. Layering of primary particles or fines from degradation of established agglomerates

Nucleation of primary particles is a mechanism common to all agglomeration processes. Beyond or parallel to the nucleation stage, granule growth may proceed by coalescence between agglomerates when the starting material has a wide size distribution, or by the layering mechanism when the size distribution is narrow. Balling of agglomerates may be seen in mixer-granulators where larger agglomerates roll and slide over inclined surfaces. Kristensen (1988a) claimed that ball growth is essentially uncontrolled coalescence which proceeds in granulation regimes where shear effects are absent or low.


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fig. 8 Formal presentation of mechanisms for size changes in an agglomeration process.

Adapted from Sastry and Fuerstenau (1977).

fig. 8 Formal presentation of mechanisms for size changes in an agglomeration process.

Adapted from Sastry and Fuerstenau (1977).

Figure 8 shows the mechanisms of size enlargement and size reduction which are likely to control most agglomeration processes. Agglomerate growth proceeds in a balance between size enlargement and size reduction, which means a balance between binding and disruptive forces. What happens in a particular process depends on the very complex interactions between factors related to the granulator and its mode of action, the process conditions, and the properties of the powder to be agglomerated. Clearly, the strength of the moist agglomerates and their ability to resist the disruptive forces resulting from agitation of the mass are important to the overall growth process.

In wet granulation of pharmaceutical products, the starting materials are usually fine powders with wide particle size distributions which produce strong agglomerates that grow primarily by nucleation and coalescence. A uniform liquid distribution is a prerequisite for controlled growth. Good wetting properties of the binder liquid are, therefore, essential. Furthermore, it might be advantageous to add the binder solution slowly, by atomi-zation, in order to avoid local over-wetting which gives rise to the presence of larger lumps in the final product, especially when the powders are soluble in the liquid.

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