How does a moving heated ring create a molten zone in a solid rod?

In the context of industrial zone melting, a moving heated ring creates a molten zone by maintaining a temperature several degrees above the melting point of the solid material.2

As this ring travels slowly along a rod—typically at a rate of a few centimeters per hour—it creates a localized liquid region. The process functions as follows:12

• Melting: As the ring moves, solid material is continuously melted at the front interface of the zone.2
• Crystallization: Simultaneously, as the ring passes, the material at the rear interface of the zone cools and crystallizes back into a solid.2
• Impurity Redistribution: Because impurities are generally more soluble in the liquid phase than the solid phase, they are rejected by the freezing solid and concentrated within the moving molten zone.12

By moving the ring along the length of the rod, the impurities are effectively swept toward one end, where they can be removed. This process can be repeated or performed with multiple rings to achieve ultrahigh purity.12