Why does the beginning of an ingot become purer after several zone-refining passes?

The beginning of an ingot becomes purer after several zone-refining passes because the process exploits the difference in impurity concentration between the liquid and solid phases of a material.1

When a molten zone moves through the ingot, impurities that prefer the liquid phase over the solid phase are rejected by the freezing crystal at the trailing edge of the zone. As the zone travels in one direction, it effectively "sweeps" these impurities toward the end of the charge.1

With each successive pass, the molten zone picks up impurities at its melting interface and leaves behind a solid that is purer than the liquid. Because the zone continuously carries a fraction of the impurities forward, the concentration of impurities at the beginning of the ingot decreases with every pass, eventually reaching a state known as the ultimate distribution. This method is highly effective for achieving extreme levels of purity, as it avoids the need for complex separation or chemical reagents.1