The propensity of a divided solid to flow freely depends on:
- - particle form;
- - solid porosity.
The flow becomes more difficult as the form of the crystals becomes less spherical. The rate of non-sphericity is 1 for a sphere, reaching 5 for plates and 10, 50 or more for fibers or acicular forms. Angular particles flow poorly.
A divided solid with a loose structure, that is, with high porosity, flows more readily than a compacted solid (for example, by vibrations or repeated impacts) that may not even flow at all.
Impurities can be present in two ways, both of which are often unpredictable:
- - inclusions,
- - coating.
Impurity inclusions or even mother liquor inclusions are of greater concern when the crystal growth rate is higher.
Coating is due to the presence of mother liquor on the crystal surface, which has not been sufficiently drained by filtration. In addition, we should recall that tailor-made additives, made to modify the form of crystals, are located on the crystal surface. Any impurities present in the industrial solutions or in the atmosphere often have an unpredictable effect.
When, as is often the case, the impurity is more soluble in the mother liquor than the material that we wish to crystallize, this impurity becomes concentrated in the liquid, allowing for its elimination by a purge, the flow rate of which must be calculated judiciously.