Why does crystallization occur

Crystals are grown in many shapes, which are dependent upon downstream processing or final product requirements. Crystal shapes can include cubic, tetragonal, orthorhombic, hexagonal, monoclinic, triclinic, and trigonal.

In order for crystallization to take place a solution must be "supersaturated". Supersaturation refers to a state in which the liquid solvent contains more dissolved solids solute than can ordinarily be accomodated at that temperature.

So how do you grow crystals? Let's consider an example that is fairly easy to envision. Take a pot of boiling water and add table salt while stirring to make a water-salt solution.

Continue adding salt until no more salt will dissolve in the solution this is a saturated solution. Now add one final teaspoon of salt. Discover a detailed explanation of crystallization, its uses in industry, and the various methods and technology available for performing it. Crystallization is defined as a process by which a chemical is converted from a liquid solution into a solid crystalline state. The widespread use of crystallization within industry is in part due to the fact that crystallization acts as both a separation and purification step; almost all chemical processes utilize at least one crystallization step either as key separation mechanism or final product engineering.

Development of crystallization processes represents a complex and challenging issue, requiring simultaneous control of various product properties, including purity, crystal size and shape, and molecular level solid structure. The control of the nucleation phase is difficult but is the key to process control; crystallization chemists usually aim to achieve goals of high purity and high yield by solely using controlled cooling crystallization techniques.

Depending on the conditions used, either nucleation or crystal growth may be predominant over the other, leading to crystals with different shapes and sizes. Therefore, controlling polymorphism is of significant interest in chemical manufacture. A common example of the importance of crystal size can be found with ice-cream. Small ice crystals, formed through rapid cooling, improve the texture and taste of the ice-cream compared with larger ice crystals.

Traditionally, crystal formation has been achieved by reducing the solubility of the solute in a saturated solution in a variety of ways. Figure 2 shows that the given material is highly soluble in Solvent A, meaning more material can be crystallized from a given volume of solvent. Conversely, the given material has a low solubility in Solvent C across all temperatures, potentially making it a good anti-solvent for this material.

As cooling continues, at a certain temperature, crystal nucleation will begin. By carefully controlling the level of supersaturation of a solution, scientists can control the crystallization process. As can be seen from the above schematic, at low levels of supersaturation, crystals grow more quickly than they nucleate resulting in large crystal size distribution. At high supersaturation levels, nucleation dominates crystal growth, providing smaller crystals.

This makes understanding and controlling supersaturation vitally important when creating crystals of a desired size and distribution. Crystallization is one of the most widely used technologies in chemical industry, and process robustness governs process productivity and economics.

Materials Science and Engineering , 65 1 , 7— Toward a Molecular Understanding of Crystal Agglomeration. Crystal breakage in a mixed suspension crystallizer. Kristall Und Technik , 8 8 , — Organic crystallization processes. Powder Technology , 2 , — Strategy for control of crystallization of polymorphs.

CrystEngComm , 11 6 , Solubility and Metastable Zone Width Crystallization. Supersaturation and Crystallization. Seeding Studies For Crystallization. Oiling Out in Crystallization. Particle Engineering and Wet Milling.

Using AntiSolvent For Crystallization. Kinetics of Crystallization in Supersaturation. The Impact of Mixing on Crystallization. Chemical Reaction Kinetics Studies. Polymorphism Chemistry. Protein Crystallization. Lactose Crystallization. See details. Request Info Get a Quote. Overview Publications Related Products. See less. Overview Applications Publications Related Products. Overview Related Products. White Papers.

By quickly identifying unnecessary hold times and determining how cooling rate influences crystal growth and nucleation, the cycle time for an interme Understand Crystallization with In Situ Microscopy. Dynamic mechanisms key to understanding crystallization processes can now be observed with in situ microscopy.

A white paper explains how leading chem Effective Crystallization Process Development. The quality of a crystallization process greatly influences the quality of the final product. Our new white paper introduces you to the fundamentals o This white paper discusses strategies to optimize crystal size distribution during process development and manufacturing. Improve Industrial Crystallization. Industrial crystallization is an important separation and purification step in the chemical industry.

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Designing Continuous Crystallization Platforms. This webinar describes an investigation into the characterization, design and operation of continuous crystallization. Robust crystallization process Figure 3. The oily acetanilide droplets appear more colored than the solution, indicating a higher quantity of dissolved methyl red impurity. If an oiled out liquid eventually solidifies, it often forms an impure glass-like non-crystalline solid.

The reasons for oiling out are several, and it can happen while dissolving the solid and during crystallization. It may be that the melting point of the solid is naturally low. It may also be that a solid is so impure that its melting point is dramatically lowered as impurities lower the melting point.

If either of these methods fail, recover the crude solid by rotary evaporation and attempt another crystallization. If the failed attempt used a mixed solvent, try a single solvent if possible.

Or choose another solvent with similar solubility properties, but with a lower boiling point. For example, if ethanol were used as the solvent the first time, repeat the crystallization using methanol.

Methanol has similar solubility properties as ethanol, but its lower boiling point may allow for the solid to come out of solution above its melting point. Lisa Nichols Butte Community College. Complete text is available online. Crystallization is Too Quick Rapid crystallization is discouraged because impurities tend to become incorporated into the crystal, defeating the purpose of this purification technique.

Although more compound will dissolve in the mother liquor, the compound will stay soluble longer once set aside to cool.

For example, in the crystallization of trans -cinnamic acid with a mixed solvent of methanol and water, use of the minimum amount of hot solvent to dissolve the solid Figure 3. To remedy this problem, the solid was placed back on the steam bath, additional methanol soluble solvent was added, brought to a boil to dissolve the solid, and then cooled.