4.7 : Crystal Growth: Principles of Crystallization

Crystallization is a phase transformation process in which crystals are precipitated from a supersaturated solution or formed from other sources. During crystallization, atoms or molecules arrange themselves into a well-defined, rigid crystal lattice to minimize energy.

Initiating crystallization involves manipulating the concentration of the solute and the temperature of the solution. Since crystal growth occurs when the ratio of concentration and solubility of the solute in the solvent – the supersaturation ratio, S – exceeds one, the solution must be concentrated, and the temperature must be low enough. As crystallization actively occurs, increasing the solute concentration through solvent evaporation or decreasing the solubility of the solute by reducing the temperature increases the value of S, aiding the process. Another important factor in crystal growth is the cooling rate, which plays an essential role in the size and morphology of the crystals. Slow cooling leads to larger crystals, whereas rapid cooling results in small crystals.

The physical process of crystallization involves two steps: nucleation and crystal growth. During nucleation, the solvent initiates random accumulation of the solute molecules, forming clusters that ultimately form the first crystal called the nucleus or the seed. If crystal nuclei are formed from a solution that contains no pre-existing crystals, they are referred to as primary nucleation. Primary nucleation occurs through either homogeneous or heterogeneous mechanisms. During heterogeneous nucleation, the crystals form on surfactants like dust particles. On the other hand, homogeneous nucleation takes place in a clear solution without any heterogeneous particulates. In secondary nucleation, growth occurs from parent crystals like those introduced through seeding. During the second phase of crystallization -- crystal growth -- more and more solute molecules join the rigid lattice structure of the seed, eventually increasing the crystal size to macroscopic scales. Once the crystals have formed and the solution has cooled to room temperature, the solution is filtered using vacuum filtration, and the separated crystals are dried and stored.

Crystallization is often used as a separation and purification step in the pharmaceutical and chemical industries. It is also frequently employed in the food industry to manufacture sugar, salt, artificial sweeteners, and other crystalline seasonings.