Why do impurities usually collect at one end of a rod during zone refining?

In zone refining, impurities collect at one end of a rod because of the way atoms or molecules behave at the liquid-solid interface during the freezing process.1

When a molten zone moves through a solid, the impurity atoms generally prefer the liquid phase over the solid phase. As the material freezes at the rear of the moving zone, the solid crystal that forms excludes these impurities, which are then rejected back into the liquid. Consequently, the molten zone accumulates an increasing concentration of impurities as it travels along the ingot.12

This process is governed by the distribution coefficient, which measures the ratio of the impurity concentration in the forming solid to that in the liquid. Because the freezing point of a substance is typically depressed by the presence of impurities, the liquid zone remains enriched with these substances until it reaches the end of the rod, where the final, highly impure portion solidifies. Once the process is complete, this contaminated end can simply be cut off to leave behind a purified material.12