Crystallisation, one of the oldest of unit operations, is used to produce vast quantities
of materials, including sodium chloride, sodium and aluminium sulphates and sucrose
which all have production rates in excess of 108 tonne/year on a world basis. Many
organic liquids are purified by crystallisation rather than by distillation since, as shown
by MULLIN(1) in Table 15.1, enthalpies of crystallisation are generally much lower than
enthalpies of vaporisation and crystallisation may be carried out closer to ambient temperature
thereby reducing energy requirements. Against this, crystallisation is rarely the last
stage in a process and solvent separation, washing and drying stages are usually required.
Crystallisation is also a key operation in the freeze-concentration of fruit juices, the
desalination of sea water, the recovery of valuable materials such as metal salts from
electroplating processes, the production of materials for the electronic industries and in
biotechnological operations such as the processing of proteins.
Although crystals can be grown from the liquid phase—either a solution or a
melt—and also from the vapour phase, a degree of supersaturation, which depends on
the characteristics of the system, is essential in all cases for crystal formation or growth
to take place. Some solutes are readily deposited from a cooled solution whereas others
crystallise only after removal of solvent. The addition of a substance to a system in order to
alter equilibrium conditions is often used in precipitation processes where supersaturation
is sometimes achieved by chemical reaction between two or more substances and one of
the reaction products is precipitated.