The reactions taking place in the lower temperature range (500-1500K) in the blast furnace are:
3Fe₂O₃ + CO → 2Fe₃O₄ + CO₂
Fe₃O₄ + 4CO → 3Fe + 4CO₂
Fe₂O₃ + CO → 2FeO + CO₂
Limestone decomposes in the hot furnace, forming calcium oxide (CaO) which removes silicate impurity of ore as slag.
CaCO₃ → CO₂ + CaO
CaO + SiO₂ (Impurity) → CaSiO₃ (Slag)