What is slag fuming and how does it recover zinc from processing residues?

Slag fuming is a metallurgical process used to recover valuable metals, particularly zinc, from residues left behind by other extraction methods, such as the leaching of zinc concentrates with sulfuric acid.1

In this process, the residue—which typically contains lead, silver, gold, and a remaining 5 to 10 percent of the original zinc content—is melted to form a slag. Powdered coal or coke is then blown through this molten slag along with air. This creates a reducing environment that facilitates two primary reactions:1

• Zinc Recovery: The zinc in the slag is reduced to its metallic form and subsequently vaporized, allowing it to be captured and recovered.1
• Lead and Precious Metal Recovery: The lead in the residue is also converted to its metallic form. As it forms, this molten lead acts as a collector, dissolving the silver and gold present in the residue.1

The resulting lead bullion, now enriched with precious metals, is periodically collected and sent for further refining.1