crystallisation

Crystallisation is a process by which a solid with a highly ordered internal structure forms from a solution, melt, or vapour. The resulting product is a crystal or a family of crystals with a defined lattice. Crystallisation is widely used for purification, isolation, and the production of crystalline materials.

In solution crystallisation, a dissolved substance becomes supersaturated and forms solid nuclei that grow into crystals. This can be achieved by cooling, evaporating solvent, or adding an anti-solvent. Nucleation can be primary (occurring spontaneously) or secondary (on existing crystals or impurities). Growth depends on temperature, concentration, solvent, and impurities, and the rate of cooling or evaporation strongly influences crystal size and quality.

Crystallisation from a melt occurs when a substance is cooled below its melting point, allowing molecules to arrange into a crystalline lattice as the solid forms. Deposition from vapour or gas involves condensation of material into a solid crystal.

Key stages are nucleation and crystal growth. Supersaturation drives crystallisation, while impurities, agitation, and the surface area of solids influence rates and crystal morphology. Use of seed crystals, controlled cooling, and careful solvent choice can help control crystal size, habit, and purity.

Applications include purification of chemicals, production of salts and sugars, and formulation of pharmaceutical ingredients where crystal form can affect solubility and stability. Crystallisation also enables characterization of materials through crystal size distribution, morphology, and phase identification.

Characterisation methods include microscopy, X-ray diffraction, and thermal analysis. Considerations such as polymorphism, hydration state, and solubility influence process design, scale-up, and downstream processing.