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Article: Crystallization


Crystallization Process and Factors Affecting Crystallization

  • Crystallization is the process by which solid forms, organized into a crystal structure.
  • Crystals can form through precipitation from a solution, freezing, or deposition from a gas.
  • Factors such as temperature, air pressure, and time of fluid evaporation affect crystal attributes.
  • Crystallization occurs in two steps: nucleation and crystal growth.
  • Loose particles form layers on the crystal's surface and lodge into inconsistencies like pores and cracks.
  • The ease of crystallization depends on atomic forces (mineral substances), intermolecular forces (organic and biochemical substances), or intramolecular forces (biochemical substances).
  • Minerals and organic molecules generally crystallize easily without visible defects.
  • Larger biochemical particles, like proteins, are often difficult to crystallize.
  • Crystallization is influenced by temperature, supersaturation, and other operating conditions.
  • Polymorphism is the ability of compounds to crystallize in different crystal structures.

Crystallization in Nature and Methods of Crystal Formation

  • Natural processes involving crystallization include mineral crystal formation, stalactite/stalagmite formation, snowflake formation, and honey crystallization.
  • Snowflakes exhibit different geometries due to subtle differences in crystal growth conditions.
  • Nearly all types of honey crystallize.
  • Crystallization processes occur on geological and human time scales.
  • Crystallization plays a role in the formation of gemstones.
  • Crystal formation can involve cations and anions (salts) or uncharged species.
  • Methods of crystal formation include cooling, evaporation, antisolvent or drown-out techniques, solvent layering, sublimation, and changing the cation or anion.
  • Supersaturated solutions do not always guarantee crystal formation and may require a seed crystal or scratching the glass to induce nucleation.
  • Laboratory techniques for crystal formation include dissolution, filtration, washing, and recrystallization.
  • Biological molecules often use microbatch crystallization under oil and vapor diffusion methods.

Typical Equipment for Crystallization

  • Tank crystallizers are an old method still used in specialized cases.
  • Saturated solutions are allowed to cool in open tanks, and the mother liquor is drained and crystals are removed.
  • Nucleation and crystal size control are challenging in tank crystallization.
  • Tank crystallization is associated with high labor costs.
  • Other industrial equipment for crystallization exists but needs further expansion.

Crystallization Dynamics, Nucleation, and Crystal Growth

  • Crystals form at lower temperatures through supercooling.
  • The release of heat during crystallization increases the entropy of the universe.
  • Heating a pure crystal causes it to become a liquid at a specific temperature.
  • Melting occurs because the gain in entropy overcomes the loss in enthalpy.
  • Crystalline form is regained when the molten crystal is cooled.
  • Nucleation is the initiation of a phase change from liquid to solid.
  • Primary nucleation occurs when no other crystals are present.
  • Homogeneous nucleation is not influenced by solids, while heterogeneous nucleation is.
  • Secondary nucleation is the formation of nuclei influenced by existing crystals.
  • Fluid-shear and contact nucleation are common methods of secondary nucleation.
  • Once a nucleus forms, it acts as a convergence point for solute molecules.
  • Growth occurs in successive layers, resembling the rings of an onion.
  • The growth rate is influenced by physical factors such as surface tension and temperature.
  • Supersaturation value, crystal surface area, retention time, and flow pattern are important factors to control.
  • Well-designed crystallizers optimize these values for efficient crystal growth.

Size Distribution and Factors Affecting Crystallization

  • The appearance and size range of crystals are crucial in crystallization.
  • Large crystals may require further processing.
  • Size distribution affects the quality and usability of the crystalline product.
  • Controlling size distribution is important for industrial applications.
  • The design and operation of crystallizers impact the size distribution of crystals.
  • Crystallization is governed by thermodynamic and kinetic factors.
  • Impurity level, mixing regime, vessel design, and cooling profile impact crystal size, number, and shape.
  • Supersaturation is a fundamental factor in crystallization.
  • The difference between actual and theoretical solute concentration is called supersaturation.
  • Factors such as temperature and pressure conditions influence solid formation.

Crystallization Data Sources

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