Crystallization

Definition:

It is the (natural or artificial) process of formation of solid crystals precipitating from a solution, melt or more rarely deposited directly from a gas.

Crystallization is also a chemical solid-liquid separation technique in which mass transfer of a solute from the liquid solution to a pure solid crystalline phase occurs.

Process of crystallization:
There are two major events in the process of crystallization

a. Nucleation
b. Crystal growth

Nucleation:
A step where the solute molecules dispersed in the solvent start together into clusters on the nanometer scale. These become stable under the current operating conditions.
However, when the slucters are not stable they redissolve. Therefore, the clusters need to reach a critical size in order to become stable nuclei.
It is at the stage of nucleation that the atoms arrange in a defined and periodic manner that defines the crystal growth.

Crystal growth:
It is the subsequent growth of the nuclei that succeed in achieving the critical cluster size.
Supersaturation:
Nucleation and crystal growth continue to occur simultaneously while the supersaturation exists.

Supersaturation is the driving force of the crystallization hence the rate of nucleation and growth is driven by the existing supersaturation in the solution.

Once the supersaturation is exhausted, the solid-liquid system reaches equilibrium and the crystallization is complete.

Polymorphism:
Many compounds have the ability to crystallize with different crystal structures a phenomenon called polymorphism.

Each polymorph is in fact a different thermodynamic solid state crystal polymorphs of the same compound exhibit different physical properties such as dissolution rate, shape and melting point etc.

So, polymorphism is of major importance in industrial manufacture of crystalline product.

Artificial method for crystallization:
For crystallization to occur from a solution, it must be supersaturated. This can be achieved by solution cooling, addition of a second solvent to reduce the solubility of the solute (techniques known as antisolvent or drawn out), chemical reaction or change in pH being the most common methods used in industrial practice.

Other methods such as solvent evaporation can also be used.

Applications:
Crystal production such as powdered slat for food industry, silicon crystal wafer production and production of sucrose from sugar beet, where the sucrose is crystallized out from an aquous solution.

Purification:
Crystallization separates out a product from a liquid (feedstream) often in extremely pure form by cooling the feedstream or adding precipitants which lower the solubility of the desired product so that it forms crystals.

Well formed crystals are expected to be pure because each molecule or ion must fit perfectly into the crystal as it leaves the solution.

Apparatus for crystallization:
Tank crystallizers:
Saturated solutions are allowed to cool in open tanks. After a period of time the mother liquid is drained and the crystals removed. In this method, nucleation and size of crystal are difficult to control. Labor costs are high.

Scrapped surface crystallizers:
One type of scraped surface crystallizer consists of Swensen-Walker crystallizer consisting an open of an open trough 0.6 meter wide with a semicircular bottom having a cooling jacket outside.

A slow speed spiral agitator rotates and suspends the growing crystals on turning the blades pass close to the walls and break off any deposits or crystals on the cooled wall.