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How does smelting separate molten metal from waste rock?

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Diagram of a zinc-lead blast furnace system showing ore and coke entering through charge hoppers, with labeled furnace, air preheater, blower, taphole for slag and lead bullion, and a connected lead-splash condenser using rotors to separate and cool lead and zinc, with pathways for molten metals and off-gas. — metallurgy A zinc-lead blast furnace and lead-splash condenser. This technology is utilized mainly for producing zinc while managing both lead and zinc from raw materials, a technique that was highly influential in 20th-century smelting.

In smelting, the separation of molten metal from waste rock—collectively known as gangue—is achieved through the use of a flux.

During the heating process, a flux (such as limestone, silica, or dolomite) is added to the furnace. The flux reacts with the impurities in the ore to form a substance called slag. Because slag is composed of mixed oxides and other by-products, it typically has a lower density than the molten metal and floats on its surface. This physical separation allows the slag to be drawn off, effectively removing the waste material and protecting the underlying molten metal from oxidation.123