In metallurgy of iron, when limestone is added to the blast furnace, the calcium ends up in:
slag
CaCO3Δ−→CaO+CO2
The coke further reacts with this carbon dioxide to form carbon monoxide:
CO2+C⟶2CO
Reduction:
Fe2O3+3C⟶2Fe+2CO
The CaO from the added limestome (calcium carbonate) reacts with silica forming slag:
CaO+SiO2⟶CaSiO3