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How does slag remove iron and other impurities from copper matte?

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Molten copper pours from a large industrial furnace into rectangular molds at a copper smelting facility, with glowing metal and industrial equipment visible. — copper processing Molten copper being made into plates.

In copper metallurgy, slag does not actively "remove" impurities on its own; rather, it acts as a medium that collects oxidized impurities, allowing them to be separated from the molten copper matte.

The process relies on the addition of a flux, typically silica, to the furnace. When air or oxygen is blown into the molten matte, iron and other impurities are oxidized. The iron oxide then reacts with the silica flux to form a light, fluid layer of slag that floats on top of the heavier matte.12

Because the slag is less dense than the matte, it forms a distinct layer that can be physically removed—a process known as skimming—thereby separating the oxidized impurities from the copper-rich material. In the subsequent converting stage, this process is repeated to further purify the matte into "blister" copper, with the slag being periodically skimmed off to ensure the removal of remaining iron and other oxidized elements.1