What role does freezing play in zone-melting purification?

In zone-melting purification, freezing is the fundamental mechanism that enables the separation of impurities from a substance. The process relies on the fact that a freezing crystal typically differs in composition from the liquid from which it forms.1

As a molten zone travels slowly through a solid ingot, liquid is continuously created at the front of the zone, while solid material crystallizes at the rear. Because the freezing point of a substance is often depressed by the presence of impurities, these impurities prefer the liquid phase over the solid phase. Consequently, as the material freezes at the rear interface, the impurities are rejected into the remaining liquid.12

This process effectively concentrates impurities within the molten zone, which carries them toward the end of the ingot as it moves. By repeating this freezing process, the bulk of the material is left behind in a highly purified solid state, while the impurities are sequestered at the end of the rod, where they can be removed.12