Crystallization

Types of Crystallization - Definition, Process and Examples

Crystallization is a natural process which happens when the materials solidify from a liquid, or as they precipitate out of a liquid or gas. This process can be carried out by causing a physical change, like a change in temperature, or a chemical change like acidity. The crystallization process is carried out on the basis of the size and shapes of the molecules involved, and their chemical properties. Crystals can be made out of 1 species of atom, different species of ions, or even huge molecules like proteins. Some big molecules have a difficult time going through the crystallization process, as their internal chemistry is not symmetrical or interacts with itself to avoid crystallization.

The unit cell is known as the smallest unit of crystal. It is the base form of atoms or molecules upon which more units can be attached. Think of this as a children’s building block, to which other blocks can be joined. Crystallization goes on as if the blocks are getting attached in all the directions. Some of the materials form different shaped crystals, which results in great variation in shape, size, and color of various crystals.

Types of Crystallization

Crystallization process can be differentiated by the manner in which the supersaturation is made. The types of crystallization are;

         Evaporative crystallization
         Cooling crystallization

Evaporative Crystallization

In the process of evaporative crystallization, the crystallization is extracted from the evaporation of the solvent. This process created a vapor and a suspension of the main liquid. The main liquid will still contain the equilibrium concentration of the product. The residual amount of the product can be harvested by recycling the main liquid. The recycling of the main liquid can be obstructed by the impurities. At some point in time, the concentration of impurities will get so high that they can influence the crystallization or the purity of the product. In that case, the main liquid stream cannot be recycled anymore and the remaining liquid has to be discharged by a bleed or purge stream.

Cooling Crystallization

Crystallization is pretty when the solubility of the product increases extensively with increasing temperature. In such cases, cooling crystallization is generally more energy friendly than evaporative crystallization. In a cooling crystallization process, the product is cooled in a heat exchanger, which can be located inside the crystallizer or an external loop. The wall of the crystallizer can be used as an internal heat exchanger, but the heat exchanger can also be incorporated in the crystallizer in the form of cooling tubes or plates. Crystallization can be carried on when the liquid is cooled to a temperature below the equilibrium solubility. The lowest temperature in the system is at the bottom of the heat exchanger which will lead to encrustation. Basically the measures to prevent this unwanted phenomenon is to lower the temperature difference between the coolant and the crystallizing solution, to increase the liquid velocity along the bottom of the heat exchanger to even out the temperature difference over the length of the heat exchanger or to use a scraper to keep the bottom of the heat exchanger free of solids. Another method of cooling that does not require a heat exchanger is flash cooling methods which involve evaporation of the solvent or direct cooling by insertion of cold gas or coolant.

Melt crystallization can be called as a special form of cooling crystallization process. The important difference with cooling crystallization from solution is the absence of solvents, which shows that most melt crystallization processes are operated close to the melting point of the original product. The product for a melt crystallization process is an impure melt. Cooling this melt below the equilibrium temperature will basically result in the formation of a solid phase that is purer than the product, while the impurities would prefer to be there in the impure liquid.

Crystallization Process

Nucleation

The initial step in the crystallization process is Nucleation. The 1st atom in the mass to form a crystal structure becomes a center, and more atoms gather around the nucleus. As this process takes place, more unit cells assemble around the nucleus, a small seed crystal is formed. In crystallization the process of nucleation is very important as the nucleus of a crystal will determine the structure of the whole crystal. Imperfections in the nucleus and the seed crystal can lead to extreme rearrangements as the crystal continues to form. Nucleation occurs in a supercooled liquid or a supersaturated solvent.

A supercooled liquid is a liquid which is on the verge of becoming a solid. An initial nucleus has to be made in order to convert the liquid into solid. Around this nucleus, the process of crystallization will continue. In a cooling liquid, the nucleus will form when atoms or molecules have no more kinetic energy to bounce off of each other. Instead, they start to interact with each other and form stable crystal structures. Pure elements basically form a crystal structure, while bigger molecules may be difficult to crystallize at normal temperature and pressures.
In a supersaturated solution, the solvent which carries the desired crystal is at max level of storing. As the temperature cools down, or the acidity changes, the solubility of the atoms or molecules in the solutions change, and the solvent can carry less of them. As such, they fall out of the solution, dashing against each other. This also causes nucleation and subsequent crystallization.

Crystal Growth

When the molecules and atoms surround the nucleus, they fall out from the symmetry which has already been set up, adding to the seed crystal. This process can occur on very quickly, or on a very slow pace, depending upon the conditions water can be crystallized into ice in a couple of minutes, while it takes a lot of time to form a “typical” geological crystal like quarts and diamonds. The simple formation set up around the nucleus determines the completion of the crystal structure. This difference information is responsible for the difference in crystals, from the uniqueness of a snowflake to the clarity of a diamond.

There are very less amount of geometric shapes that crystals can take. These are determined by the bonds and interactions of the molecules used. The different shapes are made due to the different bond angles of atoms, based on the original nucleus. An impurity in the solution or material will cause a diversion from the typical pattern. As we have seen in snowflakes, even a small impurity in the nucleus can lead to a completely new and unique design.

Uses of Crystallization

Crystallization is generally used in laboratories. It can be used to purify substances and can be merged with advanced imaging methods to understand the nature of the substances crystallized. In laboratory crystallization, a substance can be mixed into an appropriate solvent. Heat and changes in acidity can help the material to dissolve completely. When these conditions are altered, the materials in the solution precipitate out at different rates. If the conditions are used properly, pure crystals of desired substances can be made.

Crystallography is an advanced imaging technique. In this technique, high-energy beams or x-rays and particles can be shot through the crystal structure of a pure substance. While this does not form a visible image, the rays and particles are diffracted in particular patterns. These patterns can be seen by a special developing paper or electronic detectors. The patterns can be analyzed by mathematics and computers, and a structure of the crystal can be formed. The diffraction patterns are made when particles or beams are redirected by dense electron-clouds in the crystal structure. These dense areas are the atoms and bonds there in the crystal, formed in the process of crystallization. Using this particular method, scientists can recognize almost any substance on the basis of its crystal form.

Crystallization Examples

Human time-scale

Crystals take a lot of time to form, or they can form quickly. Scientists were able to study crystallization because there are a lot of events in nature wherein crystallization is fast. As we discussed earlier, ice and snowflakes are a great example of crystallization of water. Another example is the crystallization of honey. When the bees eject honey into the honeycomb, it is in liquid form. Over a period of time, the sugar molecules in the honey start to form crystals, through the process of crystallization described earlier. When you look inside an old bottle of honey, you will notice there are small crystals of sugar formed in the liquid.

Geological time scale

Even though the process is similar, the time it requires to form things like quarts, ruby, and granite is very much. These crystals are made under extremely high pressures within the crust and magma of the Earth. While crystallization is the same, it takes a very long time for the conditions to unite in the right way to crystallize. Laboratories also grow crystals seed which can be used for the production of a larger number of crystals at once.