Fractional freezing

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Fractional freezing is a process used in process engineering and chemistry to separate two liquids with different melting points. It can be done by partial melting of a solid, for example in zone refining of silicon or metals, or by partial crystallization of a liquid, for example freeze distillation, also called "normal freezing" or "progressive freezing". Partial crystallization can also be achieved by adding a diluent solvent to the mixture, and cooling and concentrating the mixture by evaporating the solvent, a process called "solution crystallization".^[1] Fractional freezing is generally used to produce ultra-pure solids, or to concentrate heat-sensitive liquids.

1 Purification of solids
2 Concentration of liquids
3 References
4 See also

[edit] Purification of solids

When a pure solid is desired, two possible situations can occur. If the contaminant is soluble in the desired solid, a multiple stage fractional freezing is required, analogous to multistage distillation. If, however, a eutectic system forms (analogous to an azeotrope in distillation), a very pure solid can be recovered, as long as the liquid is not at its eutectic composition (in which case a mixed solid forms, which can be hard to separate) or above its eutectic composition (in which case the undesired solid forms).

[edit] Concentration of liquids

When the requirement is to concentrate a liquid phase, fractional freezing can be useful due to its simplicity, for example when it is used to increase the alcohol concentration in fermented alcoholic beverages, a process sometimes called freeze distillation. Examples are applejack, made from hard cider, and ice beer. Freeze distillation of alcoholic beverages is illegal in many countries, as it concentrates poisonous compounds, for example fusel alcohols, in the original fermented beverage to unhealthy levels.

Fractional freezing is also used in the production of fruit juice concentrates and other heat-sensitive liquids, as it does not involve heating the liquid (as happens during evaporation).