Iron oxide, coke and limestone are added to the top of the blast furnace. The limestone absorbs acid impurities in the iron ore. Waste gases can also get out at the top and their heat is transferred to the clean air that comes in at the same point.
The blast of hot air coming from the bottom of the furnace burns the carbon in the coke and this produces carbon monoxide which reacts with the iron oxide in the ore. Iron metal is produced and it is so hot in the furnace, this is a liquid which runs to the bottom of the furnace and can be collected.