How do repeated molten-zone passes improve the purity of an ingot?

Repeated molten-zone passes improve the purity of an ingot through a process of continuous redistribution. In zone refining, each molten zone travels through the solid charge in a single direction, acting as a carrier for impurities.1

As the zone moves, it melts the material at its leading edge and allows it to recrystallize at its trailing edge. Because impurities typically prefer the liquid phase over the solid phase, they are effectively "swept" toward the end of the ingot with each successive pass.12

By repeating this process, the concentration of impurities at the beginning of the ingot is progressively reduced. With each additional pass, the material becomes increasingly pure until it reaches a theoretical limit known as the ultimate distribution, where the impurity concentration is reduced to extremely low levels. This method is highly effective because it avoids the need for complex separation or recombination steps, relying instead on the inherent differences in composition between the liquid and solid phases during the freezing process.1