In general, crystallinity is the most important underlying property of a solid dictating many of its physicochemical properties including stability.
Crystallization
INTRODUCTION
In
general, crystallinity is the most important underlying property of a solid
dictating many of its physicochemical properties including stability. As a unit
operation, the term crystallization describes the production of a solid,
single-component, crystalline phase from a multicomponent fluid phase (Mullin,
1993). It may be applied to the production of crystalline solids from vapors,
melts, or solutions. Crystallization from solution is most important. To
complete the preparation of a pure, dry solid, it is also necessary to separate
the solid from the fluid phase. This is usually carried out by centrifugation
or filtration and by drying. The importance of crystallization lies primarily
in the purification achieved during the process and in the physical properties
of the product. A crystalline powder is easily handled, is stable, and often
possesses good flow properties and an attractive appearance.
Crystallization
from a vapor, which occurs naturally, for example, in the formation of hoar
frost, is employed in sublimation processes and for the con-densation of water
vapor during freeze-drying. Equipments may be regarded as specialized
condensers in which the principal problems are the removal of the latent heat
of crystallization and the discharge of the solid condensate. Con-densers are
commonly mounted in parallel so that one can be shut down and emptied manually,
by conveyor or by melting and draining, without inter-rupting sublimation. This
process is not further considered.
In
the pharmaceutical industry, crystallization is usually performed on a small
scale from solutions, often in jacketed or agitated vessels. The conditions of
crystallization, necessary for suitable purity, yield, and crystal form, are
usually established by experiment. Nevertheless, a study of the principal
factors that control crystallization is important. In this study, much
information is derived from the behavior of carefully prepared melts. These
reveal more clearly than solutions the two stages of crystallization:
nucleation and crystal growth.
Nucleation
describes the formation of small nuclei around which crystals grow. Without the
formation of nuclei, crystal growth cannot occur.
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