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Blast furnace Process engineering and chemistry

Views: 4     Author: Site Editor     Publish Time: 2023-03-30      Origin: Site

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 is the provision of combustion air above atmospheric pressure.In a blast furnace, fuel (coke), ore, and flux (limestone) are supplied continuously from the top of the furnace, while hot air (sometimes enriched with oxygen) 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 Blast furnace

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 (burned 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.

The main chemical reaction producing the molten iron is:

Fe2O3 + 3CO → 2Fe + 3CO2

This reaction might be divided into multiple steps, with the first being that preheated air blown into the furnace reacts with the carbon in the form of coke to produce carbon monoxide and heat:

2 C(s) + O2(g) → 2 CO(g)

The hot carbon monoxide is the reducing agent for the iron ore and reacts with the iron oxide to produce molten iron and Carbon dioxide.Depending on the temperature in the different parts of the furnace (warmest at the bottom) the iron is reduced in several steps.At the top,where the temperature usually is in the range between 200 °C and 700 °C, the iron oxide is partially reduced to iron(II,III) oxide, Fe3O4.

3 Fe2O3(s) + CO(g) → 2 Fe3O4(s) + CO2(g)

The temperatures 850 °C, further down in the furnace, the iron(II,III) is reduced further to iron(II) oxide:

Fe3O4(s) + CO(g) → 3 FeO(s) + CO2(g)

Hot carbon dioxide, unreacted carbon monoxide, and nitrogen from the air pass up through the furnace as fresh feed material travels down into the reaction zone. As the material travels downward, the counter-current gases both preheat the feed charge and decompose the limestone to calcium oxide and carbon dioxide:

CaCO3(s) → CaO(s) + CO2(g)

The calcium oxide formed by decomposition reacts with various acidic impurities in the iron (notably silica), to form a fayalitic slag which is essentially calcium silicate, CaSiO 3:

SiO2 + CaO → CaSiO3

As the iron(II) oxide moves down to the area with higher temperatures, ranging up to 1200 °C degrees, it is reduced further to iron metal:

FeO(s) + CO(g) → Fe(s) + CO2(g)

The carbon dioxide formed in this process is re-reduced to carbon monoxide by the coke:

C(s) + CO2(g) → 2 CO(g)

The temperature-dependent equilibrium controlling the gas atmosphere in the furnace is called the Boudouard reaction:

2CO ⇌ CO2 + C.

"Pig iron" produced in blast furnaces has a relatively high carbon content of around 4-5%, and often contains too much sulfur, making it very brittle and of limited immediate commercial use.Some pig iron is used to make cast iron. Most of the pig iron produced in blast furnaces is further processed to reduce carbon and sulfur content and produce various grades of steel used in construction materials, cars, ships and machinery.Desulfurization usually occurs during the transport of molten steel to the steel mill.This is done by adding calcium oxide, which reacts with the iron sulfide contained in the pig iron to form calcium sulfide (known as lime desulfurization).In a further process step, so-called basic oxygen steelmaking, carbon is oxidized by blowing oxygen onto liquid pig iron to form crude steel.Although the efficiency of blast furnaces has continued to evolve, the chemical processes within blast furnaces have remained the same. One of the biggest disadvantages of blast furnaces is the unavoidable CO2 production as iron is reduced from iron oxide by carbon, and as of 2016 there are no economical alternatives steelmaking is one of the largest CO2 emitting industries in the world (see greenhouse gases) are investigating several alternatives such as plastic waste, biomass or hydrogen as a reducing agent, which can significantly reduce carbon emissions.

A European initiative called ULCOS (Ultra Low CO2 Steelmaking) are addressing the challenges posed by blast furnace greenhouse gas emissions.Several new process routes have been proposed and intensively studied to reduce specific emissions (CO2 per tonne of steel) by at least 50%.Some rely on carbon dioxide capture and further storage (CCS), while others choose to decarbonize steel production by switching to hydrogen, electricity and biomass.Recently, a technology that incorporates CCS into the blast furnace process itself and is called top gas recovery blast furnace is being developed and is being scaled up to commercial scale blast furnaces.


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