Crystallization: What It Consists, Method of Separation, Types and Examples

In the process of crystallization A solid is formed with atoms or molecules in organized structures, which are called crystalline networks. Crystals and crystalline networks can be formed through the precipitation of a solution, by fusion and, in some cases, by direct deposition of a gas.

The structure and nature of this crystalline network will depend on the conditions under which the process occurs, including the time elapsed to reach this new state. Crystallization as a separation process is extremely useful, since it makes it possible to ensure that only the desired compound is obtained.

In addition, this process guarantees that the passage of other species will not be allowed given the ordered nature of the crystal, making this method an excellent alternative for the purification of solutions. Many times in chemistry and chemical engineering it is necessary to use a process of separation of mixtures.

This need is generated either to increase the purity of the mixture or to obtain a specific component thereof, and for this reason there are various methods that can be used depending on the phases in which this combination of substances is found.

What is the crystallization?

Crystallization requires two steps that must occur before there can be crystalline network formation: first, there must be enough accumulation of atoms or molecules at the microscopic level for the so-called nucleation to begin to occur.

This stage of crystallization can only occur in supercooled fluids (that is, cooled below their freezing point without making them solid) or supersaturated solutions.

After beginning the nucleation in the system, nuclei can be formed sufficiently stable and large enough to begin the second stage of crystallization: crystalline growth.

Nucleation

In this first step the arrays of the particles that will form the crystals are determined and the effects of the environmental factors on the crystals formed are observed; for example, the time it takes for the first crystal to appear, called nucleation time.

There are two stages of nucleation: primary and secondary nucleation. In the first, new nuclei are formed when there are no other crystals in the middle, or when the other existing crystals have no effect on the formation of these.

The primary nucleation can be homogeneous, in which there is no influence by solids present in the medium; or it can be heterogeneous, where the solid particles of external substances cause an increase in the nucleation rate that normally would not occur.

In secondary nucleation new crystals are formed by the influence of other existing crystals; This can occur due to shear forces that cause segments of existing crystals to become new crystals that also grow at their own rate.

This type of nucleation benefits in systems of high energy or flow, where the fluid involved generates collisions between crystals.

Crystal growth

It is the process in which the crystal increases its size by the aggregation of more molecules or ions to the interstitial positions of its crystalline network.

Unlike fluids, crystals only grow uniformly when molecules or ions enter these positions, although their shape will depend on the nature of the compound in question. Any irregular arrangement to this structure is called a crystal defect.

The growth of a crystal depends on a series of factors, among which are the surface tension of the solution, pressure, temperature, relative speed of the crystals in the solution and Reynolds number, among others.

The simplest way to ensure that a crystal grows to larger sizes and that it is of high purity is through a controlled and slow cooling, which prevents the crystals from forming in a short time and that foreign substances are trapped inside. they.

In addition, it is important to note that small crystals are much more difficult to manipulate, store and move, and it costs more to filter them from a solution than the larger ones. In the vast majority of cases, the largest crystals will be the most desired, for these and more reasons.

As a separation method

The need to purify solutions is common in chemistry and chemical engineering, since it may be necessary to obtain a product that is homogeneously mixed with another or other dissolved substances. This is why equipment and methods have been developed to carry out crystallization as an industrial separation process.

There are different levels of crystallization, depending on the requirements, and can be carried out on a small or large scale. Because of this, it can be divided into two general classifications:

Recrystallization

It is called recrystallization to the technique that is used to purify chemicals on a smaller scale, usually in a laboratory.

This is done with a solution of the desired compound together with its impurities in a suitable solvent, thereby seeking to precipitate some of the two species in the form of crystals and then be removed.

There are several ways to recrystallize the solutions, among which is the recrystallization with a solvent, with several solvents or with hot filtration.

-A single solvent

When a single solvent is used, a solution of compound"A", impurity"B"and the minimum required amount of solvent (at high temperature) is prepared to form a saturated solution.

The solution is then cooled, causing the solubility of both compounds to fall, and the compound"A"or impurity"B"to be recrystallized. What is ideally desired is that the crystals are of pure"A"compound. It may be necessary to add a core to begin this process, which may even be a fragment of glass.

-Various solvents

In the recrystallization of several solvents, two or more solvents are used and the same process is carried out as with a solvent. This process has the advantage that the compound or impurity will precipitate while the second solvent is added, since they are not soluble in it. In this recrystallization method it is not necessary to heat the mixture.

-Filtering hot

Finally, recrystallization with hot filtration is used when there is insoluble matter"C", which is removed with a high temperature filter after doing the same recrystallization procedure of a single solvent.

In the industrial field

In the industrial field, it is desired to carry out a process called fractional crystallization, which is a method that refines the substances according to their solubility differences. These processes resemble those of recrystallization, but use different technologies to handle larger quantities of product.

Two methods are applied, which will be better explained in the following statement: crystallization by cooling and crystallization by evaporation.

Being a large scale this process generates waste, but these are usually recirculated by the system to ensure absolute purity of the final product.

Types of crystallization

There are two types of large-scale crystallization, as stated above: by cooling and by evaporation. Hybrid systems have also been created, where both phenomena occur simultaneously.

Crystallization by cooling

In this method the solution is cooled to decrease the solubility of the desired compound, causing it to start precipitating at the desired rate.

In chemical (or process) engineering, crystallizers are used in the form of tanks with mixers, which circulate coolant fluids in compartments surrounding the mixture so that both substances do not come into contact while heat transfer from refrigerant to solution occurs.

To remove the crystals, scrapers are used, which push the solid fragments into a pit.

Crystallization by evaporation

This is the other option to achieve the precipitation of the solute crystals, making use of a solvent evaporation process (at a constant temperature, unlike the previous method), in order to make the solute concentration surpass the solubility level.

The most common models are the so-called forced circulation models, which maintain the liquor of crystals in a homogeneous suspension through the tank, controlling their flow and speed, and usually generate crystals of larger average size than those formed in the crystallization by cooling.

Examples

Crystallization is a process frequently used in industry, and several examples can be cited:

- In the extraction of salt from seawater.

- In the production of sugar.

- In the formation of sodium sulphate (Na ₂ SW ₄ ).

- In the pharmaceutical industry.

- In the making of chocolate, ice cream, butter and margarine, in addition to many other foods.

References

5. Wikipedia. (s.f.) Retrieved from en.wikipedia.org
6. Anne Marie Helmenstine, P. (s.f.). ThoughtCo. Retrieved from thoughtco.com
7. Boulder, C. (s.f.). University of Colorado at Boulder. Retrieved from orgchemboulder.com
8. Britannica, E. (s.f.). Encyclopedia Britannica. Retrieved from britannica.com
9. Chemist, Y. M. (s.f.). Your Mother Was a Chemist. Retrieved from kitchenscience.sci-toys.com