The crystallization It is the chemical process of transformation of a gas, liquid or a solution, in a network of bonds that results in a crystalline network. For instance: frost formation, silicon purification, aspirin manufacturing.
This process can be used to separate solid components from a solid-liquid mixture, which is why it is used many times in the purification of a solid. The method consists of transferring the component from the liquid phase to the solid phase in the form of precipitated crystals.
Thus, crystallization can be used to separate components of some homogeneous mixture, for example, water with salt (NaCl). The separation can be carried out through several types of crystallization, including the selective alteration of the temperature or pressure of the mixture, as well as the addition of other substances.
The shape, size and quality of the crystals obtained will depend on the conditions and the time during which their formation is allowed. If the crystals are very small, they can become covered in impurities, and if they are very large, the impurities can be trapped within the crystal lattice.
The crystals they are solid formations that present a well-defined diffraction pattern. They are common in nature and are classified according to their constitution in:
- Solid crystals. They are solid crystalline formations. They have an orderly structure, unlike glass and amorphous compounds (without regular or orderly shape), which have disordered structures.
- Luminous crystals. They are formations with structures whose degree of ordering is intermediate between the crystalline and the amorphous. They are used to make screens.
- Ionic crystals. They are made up of cations and anions, held together by electrostatic force.
- Covalent crystals. They are made up of atoms linked by covalent bonds. They are very tough.
- Molecular crystals. They are made up of molecules held together by Van der Waals forces or hydrogen bonds, not by chemical bonds. They have relatively low melting points.
- Metallic crystals. They are made up of atoms of the same metal and almost all of them are good conductors of electricity.
Examples of crystallization
- Frost formation. On particularly cold days, ambient water vapor can crystallize on cold surfaces such as glass or certain metals, much like snow forms. This is known as frost, but they are water crystals with a very regular and well-formed constitution.
- Freezing water. Although ice as such is not a crystal, during the first stages of freezing of water it is possible to see the formation of dendrites and other submerged structures in the container that are very similar in appearance to crystals.
- Salt water evaporation. This procedure is very common both in obtaining salt crystals and in desalinating water. When the liquid boils, it turns into a gaseous state and the salts dissolved in it remain, and they rejoin their molecules in the form of saline crystals at the bottom.
- The manufacture of aspirin. Acetylsalicylic acid, the active compound of the popular remedy, is obtained in an esterification reaction that crystallizes in the presence of ethanoic anhydride and sulfuric acid.
- The winterization of oils. This process is useful to obtain oils of greater clarity and lower density. It is made from the rapid and sustained cooling of the oil to cause the crystallization of stearins, saturated glycerides, waxes and other unwanted substances. Once these have formed solid crystals, the oil is filtered and centrifuged before they can recover their liquidity and they are extracted from the mixture.
- The crystallization of sugar. Sucrose and other sweeteners (whose commercial presentation is in crystals to be dissolved in beverages) have undergone a crystallization process from the sweet syrup from which they are obtained. The mixture is then centrifuged to separate the crystals from the honey. Brown or brown sugar (not white) is just sugar in its first stage of crystallization (unrefined).
- Covalent crystals of carbon. Subjected to enormous underground pressures and slow processes of metamorphosis, carbon can become any of its three allotropes: carbon, graphite or diamond. This last case is precisely an example of a crystal, whose atoms are so closely united that they have a recognized hardness and very low melting point.
- Backward sublimation. Certain solids that, when exposed to heat, become a gaseous state (sublimation) can later recover their physical form as crystals when exposed to a decrease in temperature, in what is called reverse sublimation. In the process the impurities of the solid will have been lost and there will be pure crystals in their place. This process is useful for purifying iodine or sulfur, for example.
- Silicon purification. Although silicon does not sublimate, it is possible to purify it by melting it and then selectively cooling it, to cleave the soluble impurities from the high-purity silicon single crystals that are then used in the semiconductor industry.
- Benzoic acid crystallization. This crystallization process occurs from a solution of benzoic acid in acetone, with the simple addition of water. The interaction between the two solvents creates a new mixture and the benzoic acid crystallizes at the bottom of the container.
- Marine limestone formations. Like those of mollusks, corals and bivalves, which through the action of certain proteins can not only precipitate, but also mold the creation of calcite or quartz crystals on the rock in which their colony will form.
- The formation of molecular crystals. In substances such as sulfur dioxide (SO2), Van der Waals forces and hydrogen bonds lead to the formation of molecular crystals, which are usually brittle and meltable below 100 ° C.
- Silver crystals for films. Obtaining silver crystals is useful for certain implements of the filmic or early photographic industry (not digital), since they are sensitive to light and allow the rearrangement of the substance according to the light impression through the lens. They are obtained from chemical compounds such as silver bromide, chloride or iodide.
- Calcium Oxalate Crystals. These crystals are formed by the deposition of salts and calcium in the kidneys, where they oxidize and form small dark stones that must then be painfully expelled along with the urine. It is a common kidney ailment known as kidney stones, or also “stone” or “grit” in the kidneys.
- Uric acid crystallization. This is the phenomenon of the disease known as drop, in which uric acid crystals form in the joints, causing pain and decreased movement. It can be a consequence of excess purine intake or kidney failure of varying degrees.