TABLE OF CONTENTS Crystals are solids composed of atoms arranged in an orderly repetitive array. Thus, the interatomic distances in a crystal of any definite compound are constant and are characteristic of that material. Crystallization is an important unit operation because the process of crystallization is capable of producing very high purity products from solutions containing significant amounts of impurities with very low energy input compared to other unit operations such as distillation. The crystals produced have a good appearance with high bulk density and good handling characteristics, so drying requirements are minimized due to the low moisture content of the cake off of a centrifuge or filter.
Crystals can be stored for long periods of time compared to agglomeration or compaction, which have much shorter storage lives before breaking down.
Dissolved or molten substances are recoverable in solid form by precipitation upon cooling or upon removal of the solvent or by addition of precipitating agents or by reaction between solutions or a solution and a gas. For convenience a distinction is made between two kinds of processes:
In solution crystallization, the crystals are separated away from a solvent, often water. In the case of inorganic solids particularly, the operating temperature is far below their melting points.
In melt crystallization, two or more substances of comparable melting points are separated by some degree of cooling. The degree of completeness of such separations depends on the phase equilibrium relations. When the crystals must be refined to remove occluded substances, the recovered material may...
