Crystallization

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.