This document provides an introduction to the process of crystallization. It explains that crystallization involves arranging atoms or molecules into rigid crystals from solutions or melts. Crystallization is widely used for separation and purification in industry. The key steps of crystallization are achieving supersaturation of a solution, nucleation of seed crystals, and crystal growth until saturation is reached. Common methods to supersaturate solutions include changing temperature, evaporation, or adding anti-solvents. The objectives of crystallization are typically to achieve high yields, narrow crystal size distributions, maximum purity, and specific morphologies in an economic process.

1. This project has received funding from the European Union’s Horizon 2020
research and innovation programme under grant agreement No 869993.
Introduction to
crystallization

2. Crystallization
• In the crystallization process,
atoms, ions or molecules
are arranged into rigid
crystals from gas, melt, or
most commonly, from
solution.
• Crystallization is a widely
used separation and
purification technique in
industry.
• Crystallization occurs also in
nature, e.g. in formation of
ice crystals or salt. Crystallized salt along the Dead sea coast. (Pixabay)

3. Various crystals

4. Steps of the crystallization process
• Crystallization requires a
supersaturated solution. There are
different methods to achieve the
supersaturation.
• In the supersaturated solution, the
nucleation may take place. Some
seed crystals are usually needed to
initiate the nucleation process. Nuclei
are like little baby crystals.
• When stable nuclei are formed, they
begin to grow into crystals.

5. Supersaturation
• Most solids are more suluble at
higher temperatures. When you have
a saturated solution and then cool it
down, you will get a supersaturated
solution.
• If the solubility does not vary with
temperature, the concentration can
be increased by evaporation.
• Other methods for supersaturation
are anti-solvent addition and
precipitation.
The solubility of sucrose increases strongly
with temperature.

6. Nucleation
• The process in which nuclei start to form in
supersaturated solution, is called nucleation.
• If the solution is supersaturated enough,
nucleation might start spontaneously.
(Primary, homogeneous nucleation)
• Sometimes nuclei are generated by crystals
existing in the supersaturated solution.
(Secondary nucleation)
• Nuclei may also occur on the surface of the
vessel or on some impurities in the system.
(Primary, heterogeneous nucleation)

7. Crystal growth
• When stable nuclei have formed, they begin
to grow into crystals.
• At the same time, the concentration of the
solution reduces, because the particles of
the substance are attached onto the
crystals.
• The crystal growth may go on until the
saturation point of the solution is reached.
Then the solution is not supersaturated any
more. In practice, the crystal growth ends
before the saturation point is reached.

8. Filtration, washing and drying
• At the end of the crystal growth, crystals must
be separated from the mother liquid.
• After filtration, crystals are usually washed.
• Crystals should be insoluble in the wash
liquid and the wash liquid should be miscible
in the original solvent and also reasonably
volatile for rapid drying.
• The main objective of these final steps is to
minimize crystal changes.
• Sometimes the mother liquid is the desired
product of the crystallization process.

9. Objectives
• The crystallization process can be
modified with various techiques.
• Typical objectives are:
• high yield
• narrow crystal size distribution
• maximum crystal purity
• specific crystal morphology
• economic and trouble-free process
• The choices between different
alternatives are usually compromises.

10. This project has received funding from the European Union’s Horizon 2020
research and innovation programme under grant agreement No 869993.
References
Beckmann, W (ed.) 2013. Crystallization: Basic Concepts and Industrial Applications. Weinheim:
Wiley‐VCH Verlag GmbH & Co. KGaA. pp. 1-5.
Hipple, J. 2017. Chemical Engineering for Non‐Chemical Engineers. Hoboken, New Jersey: John
Wiley & Sons, Inc. pp. 209-216.
Mullin, J. W. 2001. Crystallization. Oxford: Elsevier Science & Technology. pp. 181, 216, 244.
Videos:
• What is crystallization? https://youtu.be/xzCF_IHWRXg
• Grow crystals from sugar: https://youtu.be/Vp7cWycX35w
• Beauty of crystallization: https://youtu.be/lo0cp2uhxb0