Zinc Distillation

Zinc distillation is a metallurgical process that recovers pure metallic zinc by heating roasted zinc ore inside a sealed clay retort until the zinc turns to vapor, then condensing that vapor back into liquid metal in a separate vessel, a method required because zinc boils at a lower temperature than the reaction that frees it from its ore needs to reach. The earliest known industrial-scale version of this process was developed in India, where the mines at Zawar in Rajasthan produced zinc by this method from around the twelfth century, using sealed cylindrical clay retorts packed with roasted ore, dolomite and organic matter, set vertically in a furnace and heated to roughly 1,100 degrees Celsius so that the zinc vapor condensed downward into a collection vessel at the base of the retort, a downward-distillation design that predates any comparable European process by centuries. Zawar's furnaces are estimated to have produced on the order of 200 kilograms of zinc a day during the process's most active centuries, between about 1400 and 1800, and a similar zinc-distillation industry developed independently in China from around the sixteenth century.