FINE CHEMICAL and pharmaceutical companies often employ reactive crystallization or precipitation to make intermediates and finished products. It is perhaps best to think of precipitation as embodying fast crystallization in which the particles can be either amorphous, crystalline or a combination of the two. The reactions can be very fast compared to the mass transfer rates to the crystals and their growth rates, often leading to high local supersaturations and extensive nucleation. In some cases, due to high supersaturation, the crystals or precipitates are small, in the submicron to several micron range. Such small particles usually are undesirable because they can present problems in downstream operations such as filtration, washing and drying. Agglomeration of some of the individual crystallites frequently occurs - but doesn't remove all small particles.
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