How does zone melting move impurities through a solid ingot?

Zone melting moves impurities through a solid ingot by leveraging the difference in impurity concentration between the liquid and solid phases of a material.1

The process involves creating a narrow molten zone within a solid rod or ingot and moving this zone slowly along its length. As the zone travels, the material melts at the front interface and recrystallizes at the rear interface. Because impurities typically prefer the liquid phase over the solid phase, they are rejected by the forming crystal and accumulate within the molten zone.12

As the zone moves, it effectively "sweeps" these impurities forward, concentrating them in the liquid as it progresses. By the time the zone reaches the end of the ingot, the impurities are deposited in the final region to freeze, which can then be removed. Passing multiple molten zones through the ingot in the same direction—a technique known as zone refining—repeatedly carries fractions of the impurities toward the end of the charge, resulting in a highly purified remainder.12