How does slag remove impurities during tin smelting?

During the smelting of tin, slag acts as a crucial medium for separating unwanted materials from the molten metal. The process involves adding limestone to the furnace feed to serve as a flux and a slag-producing agent.1

As the furnace reaches high temperatures, the flux reacts with impurities—such as iron oxides and other elements like silicon, sulfur, phosphorus, and aluminum—to form a molten slag. Because this slag is less dense than the molten tin, it floats on the surface of the metal pool. This layer serves a dual purpose: it captures and holds most of the unwanted impurities, and it protects the molten tin from oxidation by the atmosphere. Once the smelting process is complete, the impure tin is tapped off, while the slag is removed and often granulated for further treatment to recover any remaining tin.12