Detailed introduction of Recrystallization

  • Recrystallization is a process in which crystals are dissolved or melted in a solvent and then recrystallized from a solution or melt. Recrystallization can purify impure materials or separate mixed materials from each other.

     

    Definition

    Recrystallization is a process in which a substance is dissolved in a solvent or melted, and then re-crystallized from a solution or melt. Recrystallization can purify impure materials or separate mixed materials from each other. The solid matter can be purified by recrystallization. Recrystallization of some metals or alloys can refine the crystal grains or change the crystal crystallization, thereby changing its properties.

     

    Principle

    The solubility of a solid mixture in a solvent is closely related to temperature. Generally, the temperature increases and the solubility increases. If it is dissolved in a hot solvent to reach saturation, the solution will become oversaturated and crystals will be precipitated due to the decrease in solubility during cooling. Because different substances often form different lattice structures, the chances of substances with the same lattice structure crystallizing together with substances with different lattice structures are very low; substances with the same lattice structure are more likely to crystallize together with similar radii. The solvent has different solubility to the purified substance and impurities, so that the purified substance can be separated from it. And let all or most of the impurities remain in the solution (if the solubility in the solvent is very small, it will be filtered to remove after the saturated solution), so as to achieve the purpose of purification. This method can also be used to separate optical isomers.

    To a hot saturated or supersaturated racemic solution, a seed crystal of a pure optically active isomer is added to create an asymmetric environment. When cooled to a certain temperature, a slight excess of the same isomer as the seed crystal will crystallize preferentially. After filtering out the crystals, add water and a hot saturated solution of racemate to the remaining mother liquor, and then cool to a certain temperature, then another slightly excess isomer will crystallize out. In theory, if the raw materials can form racemates of aggregates, then repeating the above process can convert all enantiomers into pure optical isomers. In the absence of pure enantiomer seed crystals, sometimes chiral compounds with similar structures, or even achiral compounds as seed crystals, can be successfully resolved.

     

    Method steps

    The recrystallization method generally includes the following steps:

    Choose a suitable solvent to make a hot saturated solution.

    Remove insoluble impurities. (Including bleaching)

    Cool to crystallize, filter with suction, and remove.

    Wash and dry to remove adhesion and solvent.

     

    Recrystallization refers to the crystallization of deposited minerals under the influence of temperature and pressure. For example, amorphous (colloid) opal becomes cryptocrystalline chalcedony after dehydration, and chalcedony recrystallizes to crystalline quartz. Therefore, recrystallization is a process in which sedimentary minerals change from amorphous to cryptocrystalline and crystalline, and their particles change from small to large. Recrystallization is the main action mode of chemical rock or biochemical rock diagenesis.

     

    Paraphrase

    Recrystallization is an important way of metamorphism, which has a significant impact on the formation of metamorphic rock structures. The intensity and speed of recrystallization are controlled by the composition and structure of the original rock, and are also related to various metamorphic factors. Roughly speaking, the simpler the composition of the original rock (siliceous rock, carbonate rock) and the finer the particles (clay structure, silty structure), the more conducive to the progress of recrystallization; on the contrary, the more complex the composition of the original rock, the greater the particle size. Coarse, the more unfavorable to the progress of recrystallization. The higher the temperature, the more fully the chemically active fluid content, the more conducive to the progress of recrystallization. Because the increase of heat energy can accelerate the dissolution of minerals in the interstitial solution, it can also increase the diffusion speed and distance of these components in the solution, which are conducive to the progress of recrystallization and the formation of coarser particles. In the recrystallization process, the components are first transferred from the surface of the mineral particles to the fluid phase solution dominated by the gap H20 and CO2, and then migrate to the surface of the growing particles through diffusion. The fluid phase plays an important solvent role in the whole process. Increasing stress is generally conducive to recrystallization.

     

    Causes and characteristics

    In the case of the same substance in the same volume and mass, the smaller the particle, the larger the area occupied, so the surface energy is also large, and the solubility is directly proportional to the surface energy, so the smaller the particle, the easier it is to be dissolved. The large particles are relatively stable; easy to grow. Therefore, as a result of recrystallization, the grain size of the rock becomes coarse and uniform.

    Recrystallization is an important way of metamorphism, which is of great significance to the formation of metamorphic rock structures. The size of mineral particles after recrystallization is related to the composition and structure of the original rock. Generally, under the same metamorphic conditions, rocks with simpler composition (such as carbonate rocks, siliceous rocks, etc.) have significant recrystallization, and the larger the rock particles , The more complex the composition, the more difficult it is to carry out recrystallization.

     

    Recrystallization of carbonate deposits

    There are two trends in the recrystallization of carbonate sediments: "progressive transformation and regenerating deformation" and "degeneration and regenerating deformation".

    Progressive recrystallization: After the sediment is buried, due to the continuous increase of temperature and pressure, various structural components (stucco, cement and particles) experience different degrees of recrystallization. There are the following situations.

    ①Coaxial metasomatic edge: like a crinoline fragment that replaces the surrounding matrix, forming metamorphic margins, and is still buried in the matrix; metamorphic margins contain matrix residues, which are slightly darker than crinoids, and can co-exist with bright crystal cements. The growth side of the shaft is different. ②Particle recrystallization: it can destroy the original structure and structure, and the crystal grains will increase; such as radial spherulites (formerly mud crystal structure), mollusk shell fragments with granular structure (formerly mud crystal structure, concentric structure), etc. It is the result of particle recrystallization. ③ Granular inlaid calcite plaques in limestone: The grains often contain matrix mineral inclusions, which are darker in color, curved on the boundary, and rarely see veneer bonding, often destroy the grain boundary, and do not have a generational structure. ④Sparkling limestone: the marine mortar is washed and leached by fresh water, and all aragonite and high-magnesium calcite are transformed and recrystallized into micro-sparkling calcite, forming micro-sparkling limestone. ⑤ Recrystallization of biological debris: The sensitivity to recrystallization is also different due to the difference in the mineral composition and internal structure that make up biological bones. The order of recrystallization from easy to difficult is: coral-green algae-mollusk-pelagic foraminifera. Beach-type foraminifera. Spindleworm. sea ​​lily. Red algae. ⑥Pseudo porphyritic structure: The tiny minerals "larger than the surface" in the micrite limestone are easy to dissolve after being stressed, and grow up to some minerals with larger crystal grains or crystalline bone flakes, forming a pseudo porphyritic structure.

    Degeneration and recrystallization: ① Micritic edges of oolitic grains, bone particles, etc.: As the cyanobacteria drill holes from the surface of the particles to the center, the holes are continuously filled with micritic calcite to form micritic edges. This effect Also known as micryization. ②Critical micrite: The particles are crushed (similar to the crushing effect in crushing metamorphism), which can transform sandy limestone into micrite limestone. ③The phenomenon of crushed mud crystallization of organisms (such as crinoline fragments).

     

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