How does a molten zone purify material as it travels through a solid rod?

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zone melting Figure 1: Schematic representation of (A) normal freezing, (B) zone refining

Zone melting purifies a material by leveraging the difference in impurity concentration between its liquid and solid phases. As a molten zone travels through a solid rod, it acts as a moving solvent that redistributes impurities based on their distribution coefficient.

The process works through the following mechanism:

Selective Solubility: Impurities typically prefer the liquid phase over the solid phase. As the molten zone moves, it melts the solid at its leading edge, taking impurities into the liquid.
Freezing Out: At the trailing edge of the zone, the material begins to freeze. Because the impurities are more soluble in the liquid, the newly forming solid crystal rejects them, leaving behind a purer solid than the liquid from which it crystallized.
Impurity Transport: By passing multiple molten zones through the rod in a single direction, impurities are progressively carried toward the end of the ingot, leaving the remainder of the material in a highly purified state.1

This technique is essential in the semiconductor industry for producing ultrapure materials like silicon and germanium.1