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Why does molten slag float above molten iron in a blast furnace?

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How iron is turned into steel — iron processing Iron ore is one of the most abundant elements on Earth, and one of its primary uses is in the production of steel. When combined with carbon, iron changes character completely and becomes the alloy steel.

In a blast furnace, molten slag floats above molten iron primarily because of the difference in their densities. Slag, which is a by-product formed from flux (such as limestone), ash from coke, and impurities in the ore, is less dense than the molten pig iron produced during the smelting process.123

This physical separation is essential to the operation of the furnace. By floating on the surface of the molten metal, the slag layer serves a protective function, shielding the iron from oxidation by the atmosphere and helping to keep the metal clean. During the tapping process, the slag and iron accumulate in the hearth at the bottom of the furnace, where they can be removed—often through separate holes or the same taphole—to be processed further.123