What Is a Crystallizer?
Crystallization is the process of forming solid crystals from a liquid or gas. It is a long-used technique, notably in salt production and the pharmaceutical industry.
The solubility of a solute in a solvent depends on temperature. At saturation, the solute is maximally soluble. Cooling, evaporation, solvent changes, and reactions can decrease solubility, causing some solutes to precipitate as crystals.
Uses of Crystallizers
Crystallizers produce different crystal shapes, with two common types:
- Crystal Oslo Type: Separates solvent evaporation and crystal growth, using external heat in the evaporation part to precipitate crystals. Its advantage is less crystal fracturing in the growth section due to no circulation by the pump. However, it requires larger equipment due to the need for lower solution concentration. Example: Production of Sodium cyanide used in plating.
- Inverted Cone Type: Utilizes an inverted cone to separate solvents, exploiting the slower sedimentation rate of larger crystals. This method produces crystals with varied grain sizes. Example: Production of Alum used as a food additive.
Principles of Crystallizers
Crystallization involves the formation and growth of crystal nuclei:
- Generation of Crystal Nuclei: Under varying pressure and temperature, the solid phase may be more energetically stable than liquid or gas phases. This stability leads to the generation of nano-sized crystal nuclei. Primary nucleation occurs spontaneously from the solution, while secondary nucleation happens when a nucleus detaches from a larger crystal due to an applied force. Primary nucleation is common in laboratory settings, whereas secondary nucleation is prevalent in industrial processes.
- Crystal Growth: Occurs when atoms and molecules attach to a crystal nucleus’s surface. The crystal surface has steps where atoms and molecules incorporate into kinks, leading to crystal layer thickening and growth. This process is complex and not yet fully understood.