Is fractional freezing a separation technique for mixtures?

1 Answer
Jun 13, 2014

Yes. Just as distillation is a way of separating components of a solution that have different vapor pressures, fractional freezing (sometimes called zone refining) can be used to separate components that have different melting points.

When a liquid mixture is cooled, the component with the highest melting point will freeze first. The liquid phase will then become more concentrated in the lower-melting component and the solid component more concentrated in higher-melting component.

If the liquid and solid phases are separated, and each is subjected to additional freezing and melting cycles, it is possible to produce highly purified materials.

The process of zone refining consists of passing a solid object (e.g., a rod) past a heated zone in an enclosed system. The heated zone produces a liquified layer that passes gradually through the material. The liquid becomes concentrated in the low-melting impurity and is eventually removed when the melted zone reaches the end of the rod. This process of melting and freezing is repeated until the desired level of purity is attained.