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A blast furnace is a type of Furnace for Smelting metal Ore , usually iron ore. The Combustion material and Ore are supplied from the top while an Air flow is supplied from the bottom of the chamber, so that the Chemical Reaction takes place throughout the ore, not only at the surface. This type of furnace is typically used for smelting Iron Ore to produce Pig Iron , the raw material for Wrought and Cast Iron . The blast furnace is to be distinguished from the Bloomery in that the object of the blast furnace is to produce molten metal that can be tapped from the furnace, whereas the intention in the Bloomery is to avoid it melting so that carbon does not become dissolved in the iron. Bloomeries were also articially blown using Bellows , but the term 'blast furnace' is normally reserved for furnaces were iron (or other metal) was melted. HISTORY The Ancient World The oldest known blast furnaces were built in Han China in the 1st Century BC . However, Cast Iron artifacts found in China have been dated as early as the 5th Century BC , so it is possible that the history of the blast furnace in China is older than presently known. These early furnaces had Clay walls and used Phosphorus -containing minerals as a Flux . {Link without Title} In Europe, the iron was made in Bloomeries by the Greeks , Celt s, Romans , and Carthaginians in the ancient period; several examples have been found in France; and materials found in Tunisia suggest their use there as well as in Antioch during the Hellenistic Period . Though little is known of its use during the Dark Ages , the process probably continued in use. Medieval Europe
Early modern blast furnaces: origin and spread
Coke blast furnaces
PROCESS The blast furnace relied on the fact that the unwanted Silicon and other impurities were lighter than the molten iron, pig iron, that was its main product. The furnace was built in the form of a tall Chimney -like structure lined with Refractory brick. Coke, Limestone and iron ore (iron oxide) are poured in the top. Air is blown in through tuyeres near to the base. This "blast" allows combustion of the fuel there. This reduces the oxide to the metal, which being heavier sank to the bottom of the furnace. The exact nature of the reaction is: Fe 2 O 3 + 3 C O → 2Fe + 3CO2 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 typically runs at about 1500°C, which is enough to also decompose Limestone ( Calcium Carbonate ) into Calcium Oxide and additional carbon dioxide: CaCO3 → CaO + CO2 The calcium oxide reacts with various acidic impurities in the iron (notably Silica ), forming a slag containing Calcium Silicate , Ca Si O 3 which floats on the iron. The pig iron produced by the blast furnace is not useful for most purposes due to its high carbon content, around 4-5%, making it very Brittle . Some pig iron is used to make cast iron goods, often being remelted in a foundry cupola. For other purposes further processing is needed to reduce the carbon content to enable iron to be used for tools or as a construction material. There have been various processes for this. The earliest process was conducted in the Finery Forge . In the late 18th century, this began to be displaced by 'potting and stamping', but the most successful new process of the Industrial Revolution period was Puddling . This is now done by forcing a jet of high-pressure oxygen into a special rotating container containing the pig iron. Some of the carbon is oxidised into Carbon Monoxide , CO, and carbon dioxide, CO2. This also oxidizes impurities in the pig iron. The container is rotated and the processed pig iron can be separated from the oxidised impurities. Before the mid 19th century, pig iron from the blast furnace was made into Wrought Iron , which is commercially pure iron. At that period, if steel was needed, particularly pure varieties of iron were heated with charcoal in a cementation furnace to produce blister steel (with about 1-2% carbon). This might be further purified using the Crucible Technique , but steel was too expensive to use on a large scale. However with the introduction of the Bessemer Process in the late 1850s and then other processes, the production of steel was dramatically increased. By the early 20th century most iron was being converted to steel before use. The blast furnace remains an important part of modern iron production. Modern furnaces include Cowper Stove s to Pre-heat the blast air to high temperatures in order to avoid cooling (and thus having to re-heat) the mix, and use fairly complex systems to extract the heat from the hot carbon dioxide when it escapes from the top of the furnace, further improving efficiency. The largest blast furnaces produce around 60,000 tonnes of iron per week, enough for about four cars per minute. This is a great increase from the 18th century, when charcoal blast furnaces averaged 400 tons per year. FOOTNOTES
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