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Crystallization is a solid-liquid separation technique, in which mass transfer of solute from liquid solution (parent phase) to a pure solid crystalline phase occurs (Figure 1). It is used to purify solid compounds. Cystallization is the oldest method for recovering solids from a solution.

Figure 1.Figure 1. Schematic diagrams showing the principles behind fractional crystallization in a magma
Figure 1. Schematic diagrams showing the principles behind fractional crystallization in a magma

Fractional crystallization is a step-wise separation technique, which relies upon liquid-solid phase transition and enables multi-component mixtures to split into narrow fractions.

Crystal

A crystal is a solid that has a short and long-range order with atoms or molecules in a fixed lattice arrangement. Internal structures of a crystal can be observed by X-ray diffraction (Figure 2) analysis.

Figure 2. Structure determination by X-ray crystallography
Figure 2. Structure determination by X-ray crystallography

Crystal Morphology

Crystal morphology is defined as the general appearance of crystals described by the Miller indices of the faces that show and give the crystals their characteristic shape. Industrial crystallization includes generation of crystals by nucleation, crystal growth, and recovery of crystals from residual mother liquor. It can be recovered from suspension or layer. Steps involved in crystallization are as follows.

  • Formation of a supersaturated solution
  • Formation of crystal nuclei and
  • Growth of the nuclei to the desired size

Super-saturation is a most important step for crystallization process. It can be controlled by regulating temperature during the process and composition of the parent solution. The type of equipment used in crystallization process depends primarily upon the solubility of solute being used. The most commonly used crystallizers are evaporative, falling-film, vacuum cooling, and batch crystallizer.

Application

More than 80% of the substances used in pharmaceuticals, chemicals, agrochemicals, food, and cosmetics are isolated or formulated in their solid form.

  • It is most commonly used for separation of organic materials ranging from isomer separations to chemical mixtures and from organic acids to monomers. Formation of crystals occurs by natural process (such as gemstones, stalactite, stalagmite, snow-flakes (figure 3), or artificial process (such as supersaturation of solution).
  • Microtube batch protein crystallization technique is used to study the structure of human immunodeficiency virus type II protease and human rennin.
  • Calcium phosphate crystals are used in several dental and surgical procedures.
  • Watson and Crick used crystallization to determine to structure of DNA.
  • It is used for studying biological membranes, cellular structure and organelles.
Figure 3. Small beautiful snowflake in the frozen grass
Figure 3. Small beautiful snowflake in the frozen grass