In the blast furnace during iron extraction, limestone acts as a flux, reacting with impurities like silicon dioxide (sand) to form a molten slag that can be easily separated from the molten iron.
A flux is a substance added to a smelting process to react with and remove impurities from the molten metal.
In the blast furnace, limestone (calcium carbonate, CaCO3) decomposes at high temperatures into calcium oxide (CaO) and carbon dioxide (CO2). The calcium oxide (CaO) then reacts with the silicon dioxide (SiO2), which is a common impurity in iron ore, to form calcium silicate (CaSiO3), which is the molten slag.