Blast furnaces,construction and working.
Blast Furnace:
A blast furnace is a type of furnace used for smelting metal ore, usually iron ore. The combustion material and ore are supplied from the top while air flow is supplied from the bottom of the chamber, so that the chemical reaction takes place, not only at the surface throughout the ore. This type of fumace is typically used for smelting iron to produce pig iron, the raw material for wrought and cast iron.
Blast furnace is named so because very high temperature developed inside the furnace by means of forcing a blast of heated air. Its height is about 30 m and interior diameter is of 8 m.
Working process:
In this furnace the unwanted silicon and other impurities are lighter than the molten iron (pig iron) which is its main product. The furnace is built in the form of a tall, chimney like structure lined with refractory bricks. Coke, limestone and iron ore (iron oxide) are poured in at the top. Air is blown in through tuyers near the base. This “blast” allows combustion of the fuel. This reduces the oxide in the metal, which is being heavier sinks into the bottom of the furnace. The nature of reaction takes place inside the furnace is:
Fe203 + 3C0 => 2Fe +3C02
More precisely, the compressed air blown into the furnace reacts with. the carbon in the fuel to produce carbon monoxide, which then mixes with the iron oxide, reacting chemically to produce. iron and carbon dioxide, which leaks out of the furnace at the top, The temperature in the furnace is typically about 1500°C, which is also enough to decompose limestone (calcium carbonate) into calcium oxide and additional carbon dioxide:
CaC03 => Ca0+C02
The calcium oxide reacts with various acidic impurities in the iron (notably silica), forming a slag containing calcium silicate,CaSiO3 which floats on the iron.
The pig iron produced by the blast furnace is not very useful due to its high carbon content (around 4 -5 %) which makes it very brittle. It is used to make cast iron goods, often being remelted in a foundry eupola. The blast furnace remains an important part of modern production. Modern fumaces include Céwper stoves to pre-heat the blast air to high temperatures in order to avoid cooling (and the having re-heat) the mix they use fairly complex systems to extract the heat from the hot carbon dioxide when it escapes from the top of the furnace, further irnproving efficiency. The largest blast furnaces produce around 60,000 tonnes of the iron per week.
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