limestone is used to remove impurities in the furnace and the main impurity of the blast furnace is silica (sand and rock) which is silicon dioxide. read more
Chemistry of the blast furnace . Limestone is calcium carbonate (CaCO 3) and it is added to the blast furnace to remove the impurities in the iron ore. Calcium carbonate is decomposed by heat in the furnace to give calcium oxide and carbon dioxide. This is called thermal decomposition (see examples of other carbonates). read more
The limestone decomposes to quicklime and carbon dioxide, and the quicklime removes acidic impurities (eg SiO2). read more