EXTRACTION
Iron is usually extracted from its oxides or siderite. When extracting iron from siderite, the ore is first roasted in air to convert it to iron (II) oxide which is an unstable oxide of iron. The siderite (carbonate) is first decomposed by heat to form Iron (II) oxide and Carbon (IV) Oxide.
Iron II Oxide is then oxidised by the oxygen in the atmosphere to form Iron III Oxide. (haematite)
Iron III Oxide is also mined as the ore. The iron III oxide obtained by either method is ground into a powder then mixed with limestone and coke then fed into a furnace from the top. The mixture is heated by blasts of hot air at temperatures of between 800-1000 degrees celcious from the bottom of the furnace.
The blast furnace is about 30m high. It is made of steel and the inner side is lined with bricks made from magnesium oxide. The fire bricks conserve heat energy in order to maintain the optimum temperature needed for the reduction of iron III oxide.
In the blast furnace three important reactions take place.
At the bottom of the furnace, coke (carbon) is oxidised to carbon (IV) oxide. The reaction raises temperature of the furnace to 1600 degrees celcious since it is exothermic.
In the middle part of the furnace, carbon IV oxide is reduced by coke to Carbon II Oxide and the temperature drops to about 1000 degrees celcious since the reaction is endothermic.
At the upper of the furnace where the temperature has fallen to about 700 degrees celcious iron (III) oxide is reduced to iron metal.
Both Carbon and Carbon (II) Oxide act as reducing agents, but Carbon II Oxide is the main reducing agent because of the large surface that is in contact with the Iron (III) Oxide. The Carbon IV Oxide produced is recycled. The limestone fed into the furnace alongside coke and iron III Oxide is decomposed by heat in the furnace to form Calcium Oxide and Carbon (IV) Oxide.
Calcium Oxide being basic reacts with acidic and amphoteric oxide in the furnace to form slag.
The iron obtained from the blast furnace is 90-95% pure and is called "pig iron".