Frost crystallization on a shrub.

Crystallization is the (natural or artificial) process of formation of solid crystals from a uniform solution or melt, or more rarely directly from a gas. In Materials science, a crystal is a Solid in which the constituent Atoms Molecules or Ions are packed in a regularly ordered repeating In Chemistry, a solution is a Homogeneous Mixture composed of two or more substances Melting is a process that results in the phase change of a substance from a Solid to a Liquid. This page is about the physical properties of gas as a state of matter Crystallization is also a chemical solid-liquid separation technique, in which mass transfer of a solute from the liquid solution to a pure solid crystalline phase occurs. In Chemistry, a solution is a Homogeneous Mixture composed of two or more substances

Process

The crystallization process consists of two major events, nucleation and crystal growth. Nucleation is the onset of a Phase transition in a small region Crystal growth is a major stage of a crystallization process, after the Nucleation stage Nucleation is the step where the solute molecules dispersed in the solvent start to gather into clusters, on the nanometer scale (elevating solute concentration in a small region), that becomes stable under the current operating conditions. In Chemistry, a solution is a Homogeneous Mixture composed of two or more substances A solvent is a liquid or gas that dissolves a solid liquid or gaseous Solute, resulting in a Solution. A nanometre ( American spelling: nanometer, symbol nm) ( Greek: νάνος nanos dwarf; μετρώ metrό count) is a These stable clusters constitute the nuclei. However when the clusters are not stable, they redissolve. Therefore, the clusters need to reach a critical size in order to become stable nuclei. Such critical size is dictated by the operating conditions (temperature, supersaturation, etc. Temperature is a physical property of a system that underlies the common notions of hot and cold something that is hotter generally has the greater temperature The term supersaturation refers to a Solution that contains more of the dissolved material than could be dissolved by the Solvent under normal circumstances ). It is at the stage of nucleation that the atoms arrange in a defined and periodic manner that defines the crystal structure — note that "crystal structure" is a special term that refers to the relative arrangement of the atoms, not the macroscopic properties of the crystal (size and shape), although those are a result of the internal crystal structure. Periodicity is the quality of occurring at regular intervals or periods (in Time or Space) and can occur in different contexts A Clock marks In Mineralogy and Crystallography, a crystal structure is a unique arrangement of Atoms in a Crystal.

The crystal growth is the subsequent growth of the nuclei that succeed in achieving the critical cluster size. Nucleation and growth continue to occur simultaneously while the supersaturation exists. Supersaturation is the driving force of the crystallization, hence the rate of nucleation and growth is driven by the existing supersaturation in the solution. Depending upon the conditions, either nucleation or growth may be predominant over the other, and as a result, crystals with different sizes and shapes are obtained (control of crystal size and shape constitutes one of the main challenges in industrial manufacturing, such as for pharmaceuticals). Once the supersaturation is exhausted, the solid-liquid system reaches equilibrium and the crystallization is complete, unless the operating conditions are modified from equilibrium so as to supersaturate the solution again.

Many compounds have the ability to crystallize with different crystal structures, a phenomenon called polymorphism. Each polymorph is in fact a different thermodynamic solid state and crystal polymorphs of the same compound exhibit different physical properties, such as dissolution rate, shape (angles between facets and facet growth rates), melting point, etc. For this reason, polymorphism is of major importance in industrial manufacture of crystalline products.

Crystallization in nature

Snow flakes are a very well known example, where subtle differences in crystal growth conditions result in different geometries. Nature, in the broadest sense is equivalent to the natural world, physical universe, material world or material universe. "Snowfall" redirects here For other uses see Snow (disambiguation or Snowfall (disambiguation. The Koch snowflake (or Koch star) is a mathematical Curve and one of the earliest Fractal curves to have been described

There are many examples of natural process that involve crystallization.

Geological time scale process examples include:

• Natural (mineral) crystal formation (see also gemstone);
• Stalactite/stalagmite, rings formation. The geologic time scale is a chronologic schema (or idealized Model) relating Stratigraphy to time that is used by Geologists and other In Materials science, a crystal is a Solid in which the constituent Atoms Molecules or Ions are packed in a regularly ordered repeating A gemstone or gem, also called a precious or semi-precious stone, is a piece of attractive Mineral, which &mdash when cut and polished &mdash A stalactite ( Greek stalaktites, (Σταλακτίτης from the word for "drip" and meaning "that which drips" is a type of Speleothem A stalagmite (from the Greek stalagma ("Σταλαγμίτης" "drop" or "drip" is a

Usual time scale process examples include:

• Snow flakes formation (see also Koch snowflake);
• Honey crystallization (nearly all types of honey crystallize). "Snowfall" redirects here For other uses see Snow (disambiguation or Snowfall (disambiguation. The Koch snowflake (or Koch star) is a mathematical Curve and one of the earliest Fractal curves to have been described Honey is a sweet and Viscous fluid produced by Honey bees (and some other species and derived from the nectar of Flowers According to the

Artificial methods

For crystallization (see also recrystallization) to occur from a solution it must be supersaturated. Recrystallization (see also Crystallization) is a physical process that has meanings in Chemistry, Metallurgy and Geology. The term supersaturation refers to a Solution that contains more of the dissolved material than could be dissolved by the Solvent under normal circumstances This means that the solution has to contain more solute entities (molecules or ions) dissolved than it would contain under the equilibrium (saturated solution). In Chemistry, a solution is a Homogeneous Mixture composed of two or more substances An ion is an Atom or Molecule which has lost or gained one or more Valence electrons giving it a positive or negative electrical charge This can be achieved by various methods, with 1) solution cooling, 2) addition of a second solvent to reduce the solubility of the solute (technique known as anti-solvent or drown-out), 3) chemical reaction and 4) change in pH being the most common methods used in industrial practice. Other methods, such as solvent evaporation, can also be used.

Applications:

There are two major groups of applications for the artificial crystallization process: crystal production and purification.

Crystal production

From a material industry perspective:

• Macroscopic crystal production, for supply the demand of natural-like crystals with methods that "accelerate time-scale" for massive production and/or perfection:
  • ionic crystal production;
  • covalent crystal production. Materials Science or Materials Engineering is an interdisciplinary field involving the properties of matter and its applications to various areas of Science and Crystalline materials (mainly Metals and Alloys but also stoichiometric Salts and other materials are made up of solid regions of ordered matter (atoms placed An ionic bond (or electrovalent bond) is a type of Chemical bond that can often form between Metal and Non-metal Ions (or
• Tiny size crystals:
  • Powder, sand and smaller sizes: using methods for powder and controlled (nanotechnology fruits) forms. Sand is a naturally occurring Granular material composed of finely divided rock and Mineral particles Nanotechnology, sometimes shortened to nanotech, refers to a field of Applied science whose theme is the control of matter on an Atomic and Molecular
    • Mass-production: on chemical industry, like salt-powder production. The chemical industry comprises the companies that produce industrial chemicals
    • Sample production: small production of tiny crystals for material characterization. Characterization, when used in Materials science, refers to the use of external techniques to probe into the internal structure and properties of a material Controlled recrystallization is an important method to supply unusual crystals, that are needed to reveal the molecular structure and nuclear forces inside a typical molecule of a crystal. Recrystallization (see also Crystallization) is a physical process that has meanings in Chemistry, Metallurgy and Geology. Many techniques, like X-ray crystallography and NMR spectroscopy, are widely used in chemistry and biochemistry to determine the structures of an immense variety of molecules, including inorganic compounds and bio-macromolecules. X-ray crystallography is a method of determining the arrangement of Atoms within a Crystal, in which a beam of X-rays strikes a crystal and scatters Nuclear magnetic resonance spectroscopy, most commonly known as NMR spectroscopy, is the name given to a technique which exploits the magnetic properties of certain nuclei The term macromolecule by definition implies "large Molecule "
  • Thin film production. Thin-film deposition is any technique for depositing a Thin film of material onto a substrate or onto previously deposited layers

Massive production examples:

• "Powder salt for food" industry;
• Silicon crystal wafer production. Salt 's ability to preserve food was a foundation of Civilization. A wafer is a thin slice of Semiconductor material such as a Silicon crystal used in the fabrication of Integrated circuit and other microdevices
• Production of sucrose from sugar beet, where the sucrose is crystallized out from an aqueous solution. The beet or beetroot is a Flowering plant species ( Beta vulgaris) in the family Chenopodiaceae.

Purification

See also: Recrystallization

Well formed crystals are expected to be pure because each molecule or ion must fit perfectly into the lattice as it leaves the solution. Recrystallization (see also Crystallization) is a physical process that has meanings in Chemistry, Metallurgy and Geology. Impurities would normally not fit as well in the lattice, and thus remain in solution preferentially. Hence, molecular recognition is the principle of purification in crystallization. However, there are instances when impurities incorporate into the lattice, hence, decreasing the level of purity of the final crystal product. Also, in some cases, the solvent may incorporate into the lattice forming a solvate. In addition, the solvent may be 'trapped' (in liquid state) within the crystal formed, and this phenomenon is known as inclusion.

Thermodynamic view

The nature of a crystallization process is governed by both thermodynamic and kinetic factors, which can make it highly variable and difficult to control. Factors such as impurity level, mixing regime, vessel design, and cooling profile can have a major impact on the size, number, and shape of crystals produced.

Now put yourself in the place of a molecule within a pure and perfect crystal, being heated by an external source. At some sharply defined temperature, a bell rings, you must leave your neighbours, and the complicated architecture of the crystal collapses to that of a liquid. Temperature is a physical property of a system that underlies the common notions of hot and cold something that is hotter generally has the greater temperature Textbook thermodynamics says that melting occurs because the entropy, S, gain in your system by spatial randomization of the molecules has overcome the enthalpy, H, loss due to breaking the crystal packing forces:

This rule suffers no exceptions when the temperature is rising. In Thermodynamics (a branch of Physics) entropy, symbolized by S, is a measure of the unavailability of a system ’s Energy In Thermodynamics and molecular chemistry, the enthalpy (denoted as H, h, or rarely as χ) is a quotient or description of By the same token, on cooling the melt, at the very same temperature the bell should ring again, and molecules should click back into the very same crystalline form. The entropy decrease due to the ordering of molecules within the system is overcompensated by the thermal randomization of the surroundings, due to the release of the heat of fusion; the entropy of the universe increases.

But liquids that behave in this way on cooling are the exception rather than the rule; in spite of the second principle of thermodynamics, crystallization usually occurs at lower temperatures (supercooling). This can only mean that a crystal is more easily destroyed than it is formed. Similarly, it is usually much easier to dissolve a perfect crystal in a solvent than to grow again a good crystal from the resulting solution. The nucleation and growth of a crystal are under kinetic, rather than thermodynamic, control.

Equipment for Crystallization

1. Tank crystallizers. Tank crystallization is an old method still used in some specialized cases. Saturated solutions, in tank crystallization, are allowed to cool in open tanks. After a period of time the mother liquid is drained and the crystals removed. Nucleation and size of crystals are difficult to control. Typically, labor costs are very high.

2. Scraped surface crystallizers. One type of scraped surface crystallizer is the Swenson-Walker crystallizer, which consists of an open trough 0. 6m wide with a semicircular bottom having a cooling jacket outside. A slow-speed spiral agitator rotates and suspends the growing crystals on turning. Agitator is a term for a person that actively supports some Ideology or movement with speeches and especially actions The blades pass close to the wall and break off any deposits of crystals on the cooled wall. The product generally has a somewhat wide crystal-size distribution.

3. Double-pipe scraped surface crystallizer. Also called a votator, this type of crystallizer is used in crystallizing ice cream and plasticizing margarine. Ice cream or ice-cream (originally iced cream) is a frozen dessert made from Dairy products such as Milk and Cream, combined Margarine (ˈmɑrdʒərɨn /ˈmɑrdʒəriːn/ or /ˈmɑrgəriːn/ as a generic term can indicate any of a wide range of Butter substitutes Cooling water passes in the annular space. An internal agitator is fitted with spring-loaded scrapers that wipe the wall and provide good heat-transfer coefficients. Agitator is a term for a person that actively supports some Ideology or movement with speeches and especially actions

4. Circulating-liquid evaporator-crystallizer. Also called Oslo crystallizer. Here supersaturation is reached by evaporation. The term supersaturation refers to a Solution that contains more of the dissolved material than could be dissolved by the Solvent under normal circumstances The circulating liquid is drawn by the screw pump down inside the tube side of the condensing stream heater. The heated liquid then flows into the vapor space, where flash evaporation occurs, giving some supersaturation. The term supersaturation refers to a Solution that contains more of the dissolved material than could be dissolved by the Solvent under normal circumstances The vapor leaving is condensed. The supersaturated liquid flows down the downflow tube and then up through the bed of fluidized and agitated crystals, which are growing in size. The leaving saturated liquid then goes back as a recycle stream to the heater, where it is joined by the entering fluid. A heater is any object that emits Heat or causes another body to achieve a higher Temperature. The larger crystals settle out and slurry of crystals and mother liquid is withdrawn as a product.

5. Circulating-magma vacuum crystallizer. The magma or suspension of crystals is circulated out of the main body through a circulating pipe by a screw pump. A rotary screw compressor is a type of gas compressor which uses a rotary type positive displacement mechanism The magma flows though a heater, where its temperature is raised 2-6 K. The heated liquor then mixes with body slurry and boiling occurs at the liquid surface. Boiling (also called ebullition) a type of Phase transition, is the rapid vaporization of a Liquid, which typically occurs when a liquid This causes supersaturation in the swirling liquid near the surface, which deposits in the swirling suspended crystals until they leave again via the circulating pipe. The term supersaturation refers to a Solution that contains more of the dissolved material than could be dissolved by the Solvent under normal circumstances The vapors leave through the top. A steam-jet ejector provides vacuum. An injector, ejector, steam ejector or steam injector is a pump-like device that uses the Venturi effect of a converging-diverging This vacuum means "absence of matter" or "an empty area or space" for the cleaning appliance see Vacuum cleaner.

References

• Glynn P. D. and Reardon E. J. (1990) "Solid-solution aqueous-solution equilibria: thermodynamic theory and representation". Amer. J. Sci. 290, 164-201.
• Geankoplis, C. J. (2003) "Transport Processes and Separation Process Principles". 4th Ed. Prentice-Hall Inc.
• Stanley SJ. (2006) Tomographic imaging during reactive precipitation: mixing with chemical reaction, Chemical Engineering Science, 61 (23), pp 7850-7863

External links

• Industrial Crystallization

See also

Gallery

Solvent recrystallisation

The process

Hot-filtration, 1 solvent

2 solvent

2 solvent, with evaporation

X-ray crystals

slow evaporation 1 solvent	slow evaporation 2 solvent	slow gas diffusion 2 solvent	slow liquid diffusion
slow liquid diffusion - H Tube