From Wikipedia,
the free encyclopedia
Crystallization is a
solid-liquid separation technique,
or the process of formation of
solid
crystals from a
homogeneous
solution.
For crystallization to occur
the solution at hand ought to be
supersaturated. Put simply,
the solution should contain more
solute molecules than it would
under ordinary conditions. This
can be achieved by various methods
-- solvent evaporation, cooling,
chemical reaction, 'drowning'
being the most common ones used in
industrial practice. The picture
of the snow crystallization is an
example of crystallization.
To make things clear we can use
a simple example. We take a bowl
of water to which we add sugar
crystals. We keep adding sugar to
it until we reach a stage when no
more crystals can be dissolved.
This solution so obtained is a
saturated one. One can dissolve
further crystals into this
saturated solution, by heating it:
solubility of solutes generally
increases with temperature. When
the temperature of the solution is
allowed to attain equilibrium with
the surroundings, the solubility
of the solute decreases (because
the temperature of the solution
has decreased) and the 'excess'
sugar so added crystallizes out.
This process illustrates the
simplest of supersaturation
techniques. 'Drowning' is the
addition of a nonsolvent in the
solution that decreases the
solubility of the solid.
Alternatively, chemical reactions
can also be used to decrease the
solubility of the solid in the
solvent, thus working towards
supersaturation.
Crystallization can be divided
into stages - primary nucleation
is the first. Simply defined, it's
the growth of a new crystal, which
in turn causes secondary
nucleation - the final stage (if
removal of the crystals is not an
issue). Secondary nucleation
requires existing crystals to
perpetuate crystal growth. In our
sugar example, we had obtained
such nuclei when the 'excess'
sugar had just about crystallized
out assisting further crystal
formation. Secondary nucleation is
the main stage in crystallization
for this is what causes the 'mass
production' of crystals.
