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A blast furnace is a metallurgical furnace used to smelt and produce industrial metals,usually pig iron but also other metals such as lead or copper.Blast means that the combustion air is "forced" or provided at a pressure above atmospheric.In a blast furnace,fuel (coke),ore and flux (limestone) are supplied continuously from the top of the furnace,while hot air (sometimes oxygen-enriched) is passed through a series of tubes called tuyeres,so that as the material falls downward,chemical reactions occur throughout the furnace reaction.The final products are usually molten metal and slag phases exiting from the bottom,and waste gases (flue gases) exiting the furnace top.The flux of the downward flow of ore and contact with the upward flow of hot CO-rich combustion gases is a countercurrent exchange and chemical reaction process.In contrast,air furnaces (such as reverberatory furnaces) are naturally aspirated,usually by convection of hot gases in the flue.Under this broad definition,iron-making steelworks,tin converting plants,and lead smelting plants would be classified as blast furnaces. However,the term is usually limited to those used to smelt iron ore to produce pig iron (an intermediate material used to produce commodity steel),and shaft furnaces used in conjunction with sinter plants in base metal smelting.Blast furnaces are estimated to be responsible for more than 4% of global greenhouse gas emissions between 1900 and 2015, but are difficult to decarbonise.
Process Engineering and Chemistry
The blast furnace operates on the principle of chemical reduction,where carbon monoxide converts iron oxide into elemental iron.Blast furnaces differ from roughing and reverberatory furnaces in that in blast furnaces the flue gases come into direct contact with the ore and iron, allowing carbon monoxide to diffuse into the ore and reduce iron oxide.Blast furnaces operate as a countercurrent exchange process,while tempering furnaces do not.Another difference is that steelmaking plants operate in batch mode,while blast furnaces operate continuously for long periods of time.Continuous operation is also preferred because blast furnaces are difficult to start and stop.In addition,the carbon in pig iron makes the melting point lower than that of steel or pure iron; iron,on the contrary, does not melt in the factory.Silica must be removed from pig iron.
It reacts with calcium oxide (burnt limestone) and forms silicates,which float to the surface of the molten pig iron as "slag".Historically,the best quality iron was produced using charcoal to prevent sulfur contamination.The downward moving column of ore,flux,coke or charcoal and reaction products must have sufficient porosity for the passage of flue gas.To ensure this permeability,the particle size of the coke or charcoal is very important.Therefore, the coke must be strong enough not to be crushed by the weight of the material above it.In addition to the physical strength of the particles, the coke must also be low in sulfur, phosphorus and ash.
Blast Furnace Metallurgy
A blast furnace,a vertical shaft furnace, produces liquid metal by reacting a stream of air introduced under pressure into the bottom of the furnace with a mixture of metal ore,coke and flux fed to the top.Blast furnaces are used to produce pig iron from iron ore,which is then processed into steel,and they are also used to process lead,copper,and other metals.Rapid combustion is maintained by air flow under pressure.Blast furnaces produce pig iron from iron ore by the reduction of carbon (provided in the form of coke) at high temperatures in the presence of a flux such as limestone.An ironmaking blast furnace consists of several areas: a crucible-shaped hearth at the bottom; an intermediate area called the hearth between the hearth and the chimney;a vertical shaft (chimney) extending from the hearth to the top of the furnace; and the roof,which contains the The mechanism for charging the furnace.Furnace charge or charge of ferrous material (such as iron ore pellets and sinter), coke and flux (such as limestone) descends through the shaft, where it is preheated and reacts with rising reducing gases to produce molten iron and accumulates in the furnace of slag.Air preheated to 900 to 1,250 °C (1,650 and 2,300 °F) and injected fuel (such as oil or natural gas) are blown into the furnace near the top of the furnace through a number of tuyeres (nozzles) located around the furnace perimeter; On large furnaces, the number of these nozzles may vary from 12 to 40.Preheated air is again supplied from a large diameter duct that surrounds the furnace.The preheated air reacts violently with the preheated coke,causing reducing gases (carbon monoxide) rising through the furnace and very high temperatures around 1,650 °C (3,000 °F) to produce molten iron and slag.
The bosh is the hottest part of the furnace partly because of its proximity to the reaction between air and coke.The molten iron collects in the hearth, which has a taphole for the discharge of the molten iron and a slag hole at a higher position for removing the mixture of impurities and flux.The hearth and bosh are thick-walled,lined with carbon refractory bricks,and the hearth is lined with high-quality clay bricks to protect the furnace shell.In order to prevent these refractories from burning out, plates,sidings or sprayers for circulating cooling water are built into them. During continuous operation, the chimney is always filled with alternating layers of coke,ore and limestone.The coke is ignited at the bottom and burns rapidly by forced air from the tuyeres.The iron oxide in the ore is chemically reduced to molten iron by the carbon and carbon monoxide in the coke.
The slag formed consists of limestone flux, ash from the coke,and impurities in the ore reacting with the flux; it floats in molten iron in a molten state.Hot gases rise from the combustion zone, heat the fresh material in the chimney, and exit through ducts near the roof. A blast furnace may have the following auxiliary facilities: a bunker to prepare the charge before the bin is lifted to the roof by a hopper car or belt conveyor system; a top charging system consisting of a set of vertical double bells (cones) or rotary chutes ,to prevent the release of furnace gas during charging; a furnace that uses furnace exhaust gases to preheat the air delivered to the tuyeres; and a foundry consisting of troughs that distribute molten iron and slag to appropriate ladles for transfer to the steelmaking furnace and slag recovery area.
In Europe, blast furnaces gradually evolved over centuries from small smelters operated by the Romans, in which charcoal was used to reduce ore to semi-solid iron nuggets containing relatively small amounts of carbon and slag.The block of iron is then hammered with a hammer to remove the slag, resulting in wrought iron.The increased height of the furnace,combined with the mechanical bellows used to introduce more air into it,allowed for the higher temperatures required to produce the high carbon iron known as cast or pig iron.This method of production was used in Central Europe in the mid-14th century and arrived in England around 1500.Charcoal was the only furnace fuel until the 17th century,when the depletion of the forests that supplied charcoal in England led to experiments with coke made from coal.By the mid-18th century, coke was widely used in blast furnaces, and the principle of heating the air before entering the blast furnace was introduced in the early 19th century. Modern blast furnaces range in size from 20 to 110 meters (70 to 360 ft), with hearth diameters from 6 to 15 meters (20 to 50 ft),and can produce from 1,000 to about 15,000 tons of pig iron per day.Steelmaking accounts for about 8% of global greenhouse gas emissions - the most of any industry - and these emissions contribute to global warming. Some blast furnaces have been equipped with carbon capture technology to reduce CO2 emissions. However, some steelmakers plan to use a hydrogen-based process that does not require a blast furnace and only emits water as a by-product.
Reverberatory Furnace Metallurgy
Reverberatory furnace, in copper, tin, and nickel production,a furnace used in smelting or refining in which the fuel is not in direct contact with the ore but is heated by a flame blown through another chamber.In steelmaking, this now largely obsolete process is known as the open-hearth process.The heat passes through the hearth where the ore is placed, and then reverberates back.The roof is vaulted,with the highest point above the firebox.It slopes down to a flue bridge, which deflects the flames and thus reverberates.The hearth is dense and impermeable so that heavy matte or molten impure metal cannot penetrate and pass through the hearth,and the walls are made of a material that resists chemical attack by the slag.The process is continuous in the reverberatory furnace: the concentrate is charged through the opening at the top; the slag rising to the top overflows continuously at one end; matte is extracted at intervals from the deepest part of the pool and transported to the converter for further refining.Although its basic structure remains the same, many technical innovations have increased the furnace's production capacity.The roof is made of refractory bricks instead of the normal bricks previously used,which allows for higher temperatures and thus faster refining.Reflective melting has recently given way to continuous melting and the use of newer processes such as electric or flash furnaces.