Extraction of metal: Iron
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Explanation:
Extraction of Iron
Extraction of iron from its ore is the third and the penultimate process in the Metallurgy. The extraction of metals and its isolation occurs over a few major steps:
Concentration of Ore
Extraction of metal from concentrated Ore
Purification of the metal
It’s a long process which begins with Concentration through calcination roasting. Concentration removes the water and other volatile impurities such as sulphur and carbonates. This concentrated ore is mixed with limestone (CaCO3) and Coke and fed into the blast furnace from the top. It is in the blast furnace that extraction of iron occurs. The extraction of iron from its ore is a long and subdued process, that helps in separating the useful components from the waste materials such as slag.
The purpose of a Blast Furnace is to reduce the concentrated ore chemically to its liquid metal state. A blast furnace is a gigantic, steel stack lined with refractory brick where the concentrated iron ore, coke, and limestone are dumped from the top, and a blast of hot air is blown into the bottom. All the three ingredients are crushed into small round pieces and mixed and put on a hopper which controls the input.
Hot air is blown from the bottom and coke it burned to yield temperatures up to about 2200K. Burning coke provides the majority of the heat required for this process. At such high temperatures, Coke reacts with the oxygen in the hot air to form Carbon Monoxide (CO). The CO and heat now move upwards and meet the raw material running down from the top. The temperature in the upper parts of the Blast Furnace is considerably lower than the 2200K at the bottom. In this part, Haematite (Fe2O3) and Magnetite (Fe3O4) are reduced to Ferrous Oxide (FeO).
Reactions in the Blast furnace at 500 – 800 K, In the upper parts with lower temperatures,
3Fe2O3+CO→2Fe3O4+CO2
Fe3O4+4CO→3Fe+4CO2
Fe2O3+CO→2FeO+CO2
At 900 – 1500 K, In the lower sections of the furnace,
C+CO2→2CO
3FeO+CO2→Fe+CO2
The limestone also decomposes to CaO which removes the silicate impurity of the ore in the form of Slag. It can be easily separated out of molten iron. The iron manufactured in Blast Furnaces contain about 3 – 4 % of Carbon and smaller quantities of many other impurities such as sulphur, Silicon, etc. This is called Pig Iron. It is a hard but brittle metal and the impurities severely hamper its strength. Carbon seems to play a significant role in influencing the brittleness and hardness balance in iron. To further reduce the carbon content of pig iron, it is melted again with scraps of iron and coke and subjected to the blast of hot air. This kind of iron is called Cast Iron and has a slightly lower carbon content 2 – 3 %. This is even harder than pig iron.
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