How does the Parkes process separate silver and gold from lead bullion?

The Parkes process is a pyrometallurgical method used to extract silver and gold from lead bullion.12

The process functions through the following steps:

• Addition of Zinc: Small quantities of zinc are added to the molten lead bullion. Because gold and silver are more soluble in zinc than in lead, they react to form insoluble compounds.12
• Skimming: These zinc-gold-silver compounds are lighter than the surrounding lead. As the mixture cools to below 370 °C (700 °F)—but remains above the melting point of lead—the compounds rise to the surface to form a crust, which is then skimmed off.32
• Recovery: The skimmed crust is treated to recover the precious metals, often using cupellation to oxidize the remaining lead. Any residual zinc in the lead bullion is typically removed through vacuum retorting, where the zinc is vaporized and collected for reuse.12