Limestone is a key ingredient in blast furnace ironmaking along with iron ore and coke. While the role of iron ore is to provide the metallic input and coke the process heat and reducing gases while also playing a structural role within the furnace, limestone is there to react with impurities introduced by the other two ingredients to form a slag which can be removed from the furnace without contaminating the iron.
The heat inside the blast furnace (up to 1,800oC) converts limestone into calcium oxide and CO2 gas. Calcium oxide readily reacts with impurities like silica, sulphur, alumina and magnesia to form a slag. This percolates down through the furnace to settle on top of the liquid iron where it can be tapped off.
The limestone used for ironmaking may be pure calcium limestone or dolomite (containing some magnesia), or a blend of the two. It is crushed and screened to approximately 10-35mm and charged to the furnace at a rate of approximately 250kg/tonne of liquid iron. Depending on its composition blast furnace slag can be used in road construction and cement production. And for more detail click and see wikipedia entry.