Crystallization (from the Greek κρύσταλλος, originally ice, later rock crystal, crystal) is the process of forming crystals from gases, solutions, melts or glasses. Crystallization is also the formation of crystals with a given structure from crystals of a different structure (polymorphic transformations) or the process of transition from a liquid state to a solid crystalline state.
Crystallization is the cause of the formation of minerals and ice, tooth enamel and bones of living organisms. The simultaneous growth of a large number of small crystals (mass crystallization) is used in metallurgy and other industries. In the chemical industry, crystallization is used to obtain substances in pure form.
The crystallization process begins only after the liquid has cooled to a certain temperature.
During crystallization, the temperature does not change.
The crystallization temperature is equal to the melting temperature.
When crystals are formed, a phase transition occurs, that is, a transition of a substance from one thermodynamic phase to another. The formation of crystals from gases, solutions, melts or glasses is a first-order phase transition, and crystallization during polymorphic transformations can be a second-order phase transition.
Crystallization begins when a certain limiting condition is reached, such as supercooling of the liquid or supersaturation of the vapor, when a multitude of small crystals, the centers of crystallization, appear almost instantly. The crystals grow by adding atoms or molecules from the liquid or vapor. The growth of the crystal faces occurs layer by layer; the edges of the unfinished atomic layers (steps) move along the face during growth. The dependence of the growth rate on the crystallization conditions leads to a variety of crystal growth forms (polyhedral, plate-like, needle-like, skeletal, dendritic and other forms, pencil structures, etc.). Various defects inevitably arise during crystallization.
The number of crystallization centers and the growth rate are significantly affected by the degree of supercooling.
The degree of supercooling is the level of cooling of liquid metal below the temperature of its transition to the crystalline (solid) modification. Supercooling is necessary to compensate for the energy of the latent heat of crystallization. Primary crystallization is the formation of crystals in metals (alloys and liquids) during the transition from a liquid to a solid state.
How does crystallization occur?
When the liquid cools, the movement of the particles slows down. They start to attract to each other and form ordered structures – crystals.
Mass crystallization
The simultaneous growth of many crystals, widely used in industry, is called mass crystallization. The properties of ingots and castings during crystallization of metallurgical melts depend to a great extent on the number of crystallization centers and the conditions of their growth. During solidification of metal castings, crystallization centers first appear on the cooled walls of the mold, into which the molten metal is poured. Of the randomly oriented crystals, those that grow perpendicular to the wall survive. They form a columnar zone near the wall. Convection currents in the melt can break off the branches of dendrites, supplying secondary crystallization centers to the melt. Mass crystallization in solutions begins on heterogeneous crystallization centers or on specially introduced seed crystals. Collisions of these crystals with each other and with the walls of the vessel in a stirred solution give rise to secondary crystallization centers. Ultrasonic crushing of growing crystals or additives of surfactants are used to create additional crystallization centers. Mass crystallization is also used to purify substances from impurities.
Application of crystallization
In nature, crystallization leads to the formation of minerals, ice, and plays an important role in many biological processes. Crystallization also occurs during some chemical reactions, in the process of electrolysis. It is the basis of many technological processes: in metallurgy, in the production of materials for electronics, optics. By crystallization, massive single crystals and thin films are obtained. Crystallization is widely used in the chemical, food, and medical industries: in the technology of purification of substances, in the production of salt, sugar, and medicines.
Everyday examples
- Growing sugar crystals from sugar syrup.
- Forming ice crystals when water freezes.
- Obtaining salt from seawater by evaporation.
Nature examples
- Formation of minerals in igneous rocks.
- Formation of stalactites and stalagmites in caves.
- Crystallization of salt in salt lakes.
Examples of crystallization
- Crystallization of water: When water freezes in the freezer, ice crystals are formed.
- Sugar crystals: Cooking sugar syrup and cooling it can lead to the formation of sugar crystals. This process is used to create lollipops and crystallized sugar (candied).
- Snowflake Formation: In cold clouds, water vapor crystallizes to form snowflakes. Each snowflake has a unique shape due to the complex crystallization process.
- Salt Crystallization: When salt water evaporates, such as from seawater, the salt crystallizes and settles to the bottom. This process is often used to extract salt from salt lakes and pools.
- Honey Crystals: When honey is stored at certain temperatures, the glucose in it can begin to crystallize, turning liquid honey into a solid or semi-solid product. This is a natural process that does not affect the quality of the honey.
Observation of this phase transition in everyday life and nature
- Formation of frost on the surface of objects at low temperatures.
- Formation of snowflakes during snowfall.
- Crystallization of minerals in the earth’s crust during the formation of rocks.
Use of crystallization in everyday life and technology
- Obtaining salt from seawater by evaporation.
- Purification of sugar in industries by crystallization.
- Production of medicinal products, when crystallization is used to purify and extract the desired substances.