Crystallization is the process by which a chemical is separated from solution as a high-purity, definitively shaped solid.

A crystal may be defined as a solid composed of atoms arranged in an orderly, repetitive array. The inter-atomic distances in a crystal of any definite material are constant and characteristic of that material. Crystals are, in short, high-purity products with consistent shape and size, good appearance, high bulk density, and good handling characteristics.

Because the pattern or arrangement of the atoms is repeated in all directions, there are definite limitations on the shapes which crystals may assume. For each chemical compound, there are unique physical properties differentiating that material from others, so the formation of a crystalline material from its solution, or mother liquor, is accompanied by unique growth and nucleation characteristics.

While crystallization is a unit operation embracing well known concepts of heat and mass transfer, it is nevertheless strongly influenced by the individual characteristics of each material handled. Therefore, each crystallization plant requires many unique features based upon well-established general principles. Each application must be evaluated on an individual basis to achieve optimum results.

The mechanical design of the crystallizer has a significant influence on the nucleation rate due to contact nucleation (that which is caused by contact of the crystals with each other and with the pump impeller or propeller when suspended in a supersaturated solution). This phenomenon yields varying rates of nucleation in scale up and differences in the nucleation rates when the same equipment is used with different materials.

It is important when performing pilot plant tests that designs are used in which both the small- and large-scale characteristics are well known, and that the testing is performed by highly experienced personnel.

Swenson Crystallizers

Swenson has an unusually broad background in the crystallization field and can supply a wide range of crystallizers and evaporators to meet the requirements for almost any application.

The Swenson draft tube baffle (DTB) crystallizer is the most successful of the crystallizers designed to make the large, uniform crystals required for fertilizer and similar applications where superior filtration, centrifugation, washing, drying, and storage characteristics are required. This design is the result of years of research and development as well as proven plant scale operation in hundreds of installations.