Agglomeration by coalescence

The type of movement required for mixing particulate matter also produces ideal conditions for agglomeration by coalescence. Therefore, unwanted agglomeration is often observed in powder mixers, especially if the particle size of the solids is small and/or a small amount of moisture is present. Considerable problems can arise if components of the bulk mass have different particle sizes because, in that case, the smaller fractions may selectively agglomerate, thus making it impossible to obtain an ideal mixture. Such selective agglomeration is of particular concern in the pharmaceutical industry where an extremely small amount of a finely divided active substance must often be mixed uniformly and reliably with a relatively large amount of inert filler material. 

Often, the process is characterized by conversion of a liquid feed (solution, suspension, or melt) into a granular product whereby the liquid is sprayed on or into a fluidized layer of primary particles. Sometimes the liquid is only a binder and, therefore, is introduced in small amounts together with or without finely divided solids. While in the first case growth occurs chiefly by layering, random agglomeration by coalescence prevails in the second. 

The most common equipment for large scale agglomeration by coalescence and growth in tumbling particle beds, pelletizing drums, and discs (see Figure 83)... 

Kapur and Fuerstenau (K6) have presented a discrete size model for the growth of the agglomerates by the random coalescence mechanism, which invariably predominates in the nuclei and transition growth regions. The basic postulates of their model are that the granules are well mixed and the collision frequency and the probability of coalescence are independent of size. The concentration of the pellets is more or less fixed by the packing... 

Sastry, K. V. S., Similarity size distribution of agglomerates during their growth by coalescence in granulation or green pelletization, Int. J. Miner. Process. 2, 187 (1975). 

In continuous systems, recycled undersize material or agglomerates formed by coalescence of the fresh feed act as nuclei. However, since the fine feed particles can form stronger bonds with the nuclei particles than with each other, the majority of the feed material would be expected to simply layer onto the established agglomerates rather than to coalesce with other fine particles. 

Agglomerates formed by coalescence and by layering can usually be distinguished by their appearance. Layered granules tend to a smooth spherical shape and examination of their interior cross sections reveals a layered onion skin construction. Coalesced granules tend to a more irregular spheroidal shape and often exhibit a rough raspberry -type surface. 

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