Importance of secondary nucleation

Provided that primary nucleation is reliably avoided by working within the metastable zone, the granulometry of the crystallized masses produced depends on the control of secondary nucleation. Secondary nucleation is therefore the most important control factor for the granulometry of crystallization processes. 

For a produced quantity of 10 g, 10 crystals with a mass of 1 g each are produced from 10 crystal nuclei, while 10 000 crystal nuclei will lead to 10 000 crystals with a mass of only 1 mg each. As the supersaturation level cannot be increased above the metastable zone - otherwise the number of nuclei would increase spontaneously - it takes approximately 20 times longer (see time arrow) to incorporate the production quantity oflOginlOcrystalsoflg each than to create 10 000 cry stals of only 1 mg each. 

In the case of the uncontrollable spontaneous nucleation with its tremendously higher nuclei production instead of secondary nucleation, the distribution of the production capacity to all of these nuclei will consequently lead to extremely fine crystals, difficult to separate from the impure mother liquor that is, the targets for granulometry and purity will be entirely missed. 

On this basis, it is possible to deduce the following two principles for the design and layout of crystallizers  

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Another important development which altered our view of crystallization processes was the realization of the importance of secondary nucleation due to contact between crystals and the impeller and vessel. Secondary nucleation of this type has been shown (2-6) to often have a dominant role in determining crystallizer performance. Our understanding of crystal growth, nucleation, fluid mechanics and mixing have all greatly improved. A number of review (2r 101 have appeared in recent years which describe the advances in these and... 

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