Iron is an element that has the chemical symbol, Fe. It is a metal that has been used since prehistory, by the Ancient Egyptians and Sumerians as early as 4000 B.C. These ancient peoples were able to get pure iron from meteorites that fell to the earth, and they used it in the tips of weapons and spears because of its strength and durability. Meteorites that have fallen from space are the only iron found in nature in its pure form.
Iron is found naturally in the form of ores that are oxides of the pure metal, such as hematite and magnetite. The most common of the modern methods used to extract iron from its ores is coke smelting in a blast furnace. The development of this technique is largely credited to Abraham Darby, who set up his Brass and Iron Works in Shopshire in 1709, which was important in the creation of the industrial era that followed.
The blast furnace is used in extracting iron and is a tall structure, usually about 30 meters high, which is made of iron or steel and lined with firebrick. It is narrow at the top, then increasing in diameter until it reaches the bottom, where it again narrows. Hot air is pumped in at the bottom through perforations known as truyeres, initiating some of the reactions for the smelting which results in pure molten iron production. The furnace is initially charged with pre-calculated quantities of coke, limestone (CaCO3) and the iron oxide to be reduced to pure iron.
The main reactions that take place in a blast furnace are fairly simple. Coke, which is a form of carbon, is produced by heating coal in the absence of air. It is inexpensive, and it reacts with oxygen from hot air that is blasted into the blast furnace to form carbon dioxide in a reaction which is very exothermic, or heat-producing.
C + O2 → CO2
The heat produced by this reaction is the main source of heat in the blast furnace that is used for iron production. At the bottom of the furnace, the temperature is very high, which causes more of the coke to react with carbon dioxide, producing carbon monoxide.
C + CO2 → 2CO
The carbon monoxide then becomes the “reducing agent” for the iron, as it donates electrons to the oxide (in this example hematite, with the formula Fe2O3, is being used) converting it to pure iron and more carbon dioxide.
Fe2O3 + 3CO → 2Fe + 3CO2
In the areas of the furnace where the temperature is high enough, the carbon from coke itself can also reduce the iron oxide, giving off more carbon monoxide. The pure iron then melts and goes to the bottom of the furnace.
The iron ore itself is not pure ore. It is also contaminated with a variety of rock, which will not necessarily melt at blast furnace temperatures and therefore could clog it up. For this reason, limestone, or calcium carbonate, is added to the initial furnace charge. The hot furnace reacts with the limestone to give calcium oxide.
CaCO3 → CaO + CO2
This reaction is endothermic, and it absorbs heat from the furnace, so in order to keep the furnace temperatures high, it is important to not introduce too much limestone into the charge mixture. The calcium oxide (CaO) produced by this reaction reacts with other oxides present in the rock, and the reaction products from this run down to the furnace bottom, forming a molten layer on top of the liquid iron known as slag. This is tapped off and used in making cement and roads.
The molten iron is tapped from the bottom of the furnace and then run into sand molds to harden. This product is known as pig iron, or cast iron. Usually, the pig iron is then converted to steel by removing any other impurities that are present and combining the molten iron with any other metals that are needed for the specific type of steel being produced.