Learn about the process of nucleation. - primary/secondary nucleation - homogeneous/heterogeneous nucleation - nucleation rate

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

2. Crystallization process
• Crystallization process includes
two steps:
• nucleation
• crystal growth.
• In supersaturated solution,
constituent particles gather
together in clusters. When
they reach the critical cluster
size, they become stable
nuclei.
• This process is called
nucleation. Nuclei are like little
baby crystals. Picture: Kati Jordan CC0

3. Formation of nuclei
• The formation of nuclei is a difficult process
to envisage.
• Adequate amount of constituent particles
(atoms, ions or molecules) have to be
attached into a fixed lattice to form a
stable nucleus. For example, a stable ice
nucleus contains about 100 molecules.
• An unstable nuclei may either grow or
redissolve. The minimum size of a stable
nucleus is called its critical size.
• Nucleation requires a supersaturated
solution. It is not at equilibrium. The
solution crystallizes to achieve equilibrium.

4. Nucleation
• The region, where the solution is
only slightly supersaturated, is
called metastable zone. In
metastable zone nucleation is not
spontaneous. Some seeds are
needed.
• The rate of nucleation increases
with the level of supersaturation.
Above the metastable zone,
nucleation becomes spontaneous.
• The width of the metastable zone
is dependent on a number of
conditions, e.g. the rate of cooling.
The crystal growth occur in the
metastable zone. Picture: Kati Jordan CC0

5. Nucleation
• Nucleation can be divided into primary and
secondary nucleation.
• In secondary nucleation, the formation of
nuclei is induced by the existing crystals. It is
the main method in industrial crystallizers.
• In primary nucleation, no crystals are
involved. If the nucleation is caused by some
foreign particles (e.g. dust), process is called
heterogeneous nucleation.
• If nucleation occurs spontaneously, it is called
homogeneous nucleation. It is not possible in
industrial crystallizers because of the presence
of impurities and low levels of supersaturation.
Picture: Kati Jordan CC0

6. Nucleation rate
• Nucleation rate effects on crystal size distribution
and crystal habit.
• It depends on the level of supersaturation and the
seed surface area.
• The rate increases with increasing
supersaturation.
• The nucleation rate is usually given in units of the
number of nuclei formed / (m³ · s).
• It can also be determined as an induction time.
• Induction time is a time interval between the
creation of supersaturation and the appearance of
crystals. It can range from micro seconds to days.

7. This project has received funding from the European Union’s Horizon 2020
research and innovation programme under grant agreement No 869993.
References
• Beckmann & Beckmann, W. 2013. Crystallization: Basic Concepts and Industrial Applications.
Weinheim: Wiley‐VCH Verlag GmbH & Co. KGaA, pp. 17-25.
• McCabe, W. L., Smith, J. C. & Harriott, P. 2005. Unit operations of chemical engineering. 7th ed.
Boston: McGraw-Hill, pp. 936-945.
• Mullin, J. W. & W Mullin, J. 2001. Crystallization. Butterworth-Heinemann, pp. 181-215.
• Myerson, Myerson, A. S. & Myerson, A. 2002. Handbook of Industrial Crystallization.
Butterworth-Heinemann, pp. 43-53.
Videos:
• Dive deeper into the process of nucleation: https://youtu.be/Jbb6QDiE9dI
• Learn about the classical nucleation theory: https://youtu.be/cpPjN9x9NNE