How does distillation refining remove lead and cadmium from blast-furnace zinc?

Distillation refining removes lead and cadmium from blast-furnace zinc through a two-stage fractional distillation process that exploits the different boiling points of these metals.12

• Removing Lead: In the first fractionating column, the impure zinc is vaporized. Because lead has a higher boiling point (1,744 °C) than zinc (907 °C), it remains in the liquid phase and is separated from the zinc vapor.12
• Removing Cadmium: The distilled zinc vapor is condensed and fed into a second column. Here, the process targets cadmium, which has a lower boiling point (765 °C) than zinc. The cadmium is distilled off as a vapor, leaving behind high-purity zinc, which is then collected from the bottom of the column.12

This operation is conducted in columns equipped with superposed trays made of silicon carbide refractory material, which facilitate the necessary countercurrent flow of liquid metal and metal vapor.1