The purpose of a crystallization plant is to produce, as far as possible, crystals of the required shape, size distribution, purity, and yield.
PRINCIPLES UNDERLYING THE DESIGN AND OPERATION OF CRYSTALLIZERS
The
purpose of a crystallization plant is to produce, as far as possible, crystals
of the required shape, size distribution, purity, and yield. This is achieved
by maintaining a degree of supersaturation at which nucleation and crystal
growth proceed at appropriate rates. Control of the number of nuclei formed
controls the size of the crystals deposited from a given quantity of solution.
Alter-natively, crystal number and size can be controlled by adding the correct
amount of artificial nuclei or seeds to a system in which little or no natural
nucleation is taking place.
In
the majority of cases, the mode of operation is determined by the relation
between the solubility of the solute and the temperature, examples of which are
shown in Figure 9.4. This determines how supersaturation is to be achieved.
Other factors of importance are the thermal stability of the solute, the
impurities that may be present, and the degree of hydration required. If the
solubility of the solute increases greatly with temperature, supersaturation
and deposition of a large proportion of the solute are brought about by cooling
a hot concentrated solution. Sodium nitrate provides an example. Sodium
chloride and calcium acetate, on the other hand, exemplify materials with a
small or negative temperature coefficient of solubility. Here, supersaturation
can best be achieved by evaporating a part of the solvent. In some cases, both
evaporation and cooling are employed. The mother liquors following evapo-rative
crystallization can be cooled to yield a further crop of crystals provided
there is a suitable change in solubility and impurities present do not prohibit
the process. In other crystallizers, flash cooling is used. A hot solution is
passed into a vacuum chamber in which both evaporation and cooling take place.
Supersaturation
can also be induced by the addition of a third substance that reduces the
solubility of a solute in a solvent. These precipitation processes, which are
important in the processing of thermolabile materials, are controlled by the
temperature of mixing, the agitation, and the rate at which the third substance
is added. Water-insoluble materials dissolved in water-miscible organic
solvents can be precipitated by adding water. Alternatively, the aqueous
solubility of many materials can be reduced by the change of pH or by the
addition of a common ion. Proteins can be salted out of solution by the
addition of ammonium chloride and adjustment of pH. Finally, precipitation of a
crys-talline solid may be the result of a chemical reaction.
A
crystallizer should produce crystals of uniform particle size. This facilitates
removal of the mother liquor and washing. If large quantities of the liquor are
occluded in the mass of crystals, drying will yield an impure product. In
addition, crystals of even size are less likely to cake on storage.
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