Crystallization zone refining, principles

Purification by zone refining relies on the same principles of crystal formation as the solvent and melt methods of crystallization. Zone refining was developed as crystallization from the melt with the restriction that only a portion of the material is molten... 

To circumvent the various limitations of the basic technique, several continuous zone refining methods have been developed in which feed enters at one point in the sample while the product and waste leave at other points (Figure 4). The effect of countercurrent movement of solid and liquid phases is achieved by the movement of molten zones. In addition to the horizontal continuous refiner, the zone void vertical refiner and zone transport refiner are other modifications of this class. Cross-flow zone refiners and rotating drum multistage crystallizers based on the above principle are mainly used in growing single crystals rather than in purification of materials. 

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