Steelmaking is the process of producing steel from iron ore and/or scrap. In steelmaking, impurities such as nitrogen, silicon, phosphorus, sulfur and excess carbon (the most important impurity) are removed from the sourced iron, and alloying elements such as manganese, nickel, chromium, carbon and vanadium are added to produce different grades of steel.
Steelmaking has existed for millennia, but it was not commercialized on a massive scale until the mid-19th century. An ancient process of steelmaking was the crucible process. In the 1850s and 1860s, the Bessemer process and the Siemens-Martin process turned steelmaking into a heavy industry.
Today there are two major commercial processes for making steel, namely basic oxygen steelmaking, which has liquid pig-iron from the blast furnace and scrap steel as the main feed materials, and electric arc furnace (EAF) steelmaking, which uses scrap steel or direct reduced iron (DRI) as the main feed materials. Oxygen steelmaking is fueled predominantly by the exothermic nature of the reactions inside the vessel; in contrast, in EAF steelmaking, electrical energy is used to melt the solid scrap and/or DRI materials. In recent times, EAF steelmaking technology has evolved closer to oxygen steelmaking as more chemical energy is introduced into the process. [1]
Steelmaking is one of the most carbon emission intensive industries in the world. As of 2020 [update] , steelmaking is responsible for about 10% of greenhouse gas emissions. [2] To mitigate global warming, the industry will need to find significant reductions in emissions. [3]
Steelmaking has played a crucial role in the development of ancient, medieval, and modern technological societies. Early processes of steel making were made during the classical era in Ancient Egypt,[ citation needed ] Ancient China, India, and Rome.
Cast iron is a hard, brittle material that is difficult to work, whereas steel is malleable, relatively easily formed and a versatile material. For much of human history, steel has only been made in small quantities. Since the invention of the Bessemer process in 19th century Britain and subsequent technological developments in injection technology and process control, mass production of steel has become an integral part of the global economy and a key indicator of modern technological development. [4] The earliest means of producing steel was in a bloomery.
Early modern methods of producing steel were often labour-intensive and highly skilled arts. See:
An important aspect of the Industrial Revolution was the development of large-scale methods of producing forgeable metal (bar iron or steel). The puddling furnace was initially a means of producing wrought iron but was later applied to steel production.
The real revolution in modern steelmaking only began at the end of the 1850s when the Bessemer process became the first successful method of steelmaking in high quantity followed by the open-hearth furnace.
Modern steelmaking processes can be divided into three steps: primary, secondary and tertiary.
Primary steelmaking involves smelting iron into steel. Secondary steelmaking involves adding or removing other elements such as alloying agents and dissolved gases. Tertiary steelmaking involves casting into sheets, rolls or other forms. Multiple techniques are available for each step. [5]
Basic oxygen steelmaking is a method of primary steelmaking in which carbon-rich pig iron is melted and converted into steel. Blowing oxygen through molten pig iron converts some of the carbon in the iron into CO−
and CO
2, turning it into steel. Refractories—calcium oxide and magnesium oxide—line the smelting vessel to withstand the high temperature and corrosive nature of the molten metal and slag. The chemistry of the process is controlled to ensure that impurities such as silicon and phosphorus are removed from the metal.
The modern process was developed in 1948 by Robert Durrer, as a refinement of the Bessemer converter that replaced air with more efficient oxygen. It reduced the capital cost of the plants and smelting time, and increased labor productivity. Between 1920 and 2000, labour requirements in the industry decreased by a factor of 1000, to just 0.003 man-hours per tonne. in 2013, 70% of global steel output was produced using the basic oxygen furnace. [6] Furnaces can convert up to 350 tons of iron into steel in less than 40 minutes compared to 10–12 hours in an open hearth furnace. [7]
Electric arc furnace steelmaking is the manufacture of steel from scrap or direct reduced iron melted by electric arcs. In an electric arc furnace, a batch ("heat") of iron is loaded into the furnace, sometimes with a "hot heel" (molten steel from a previous heat). Gas burners may be used to assist with the melt. As in basic oxygen steelmaking, fluxes are also added to protect the lining of the vessel and help improve the removal of impurities. Electric arc furnace steelmaking typically uses furnaces of capacity around 100 tonnes that produce steel every 40 to 50 minutes. [7] This process allows larger alloy additions than the basic oxygen method. [8]
In HIsarna ironmaking process, iron ore is processed almost directly into liquid iron or hot metal. The process is based around a type of blast furnace called a cyclone converter furnace, which makes it possible to skip the process of manufacturing pig iron pellets that is necessary for the basic oxygen steelmaking process. Without the necessity of this preparatory step, the HIsarna process is more energy-efficient and has a lower carbon footprint than traditional steelmaking processes.[ citation needed ]
Steel can be produced from direct-reduced iron, which in turn can be produced from iron ore as it undergoes chemical reduction with hydrogen. Renewable hydrogen allows steelmaking without the use of fossil fuels. In 2021, a pilot plant in Sweden tested this process. Direct reduction occurs at 1,500 °F (820 °C). The iron is infused with carbon (from coal) in an electric arc furnace. Hydrogen produced by electrolysis requires approximately 2600 kWh per ton of steel. Costs are estimated to be 20-30% higher than conventional methods. [9] [10] [11] However, the cost of CO2-emissions add to the price of basic oxygen production, and a 2018 study of Science magazine estimates that the prices will break even when that price is €68 per tonne CO2, which is expected to be reached in the 2030s.
Secondary steelmaking is most commonly performed in ladles. Some of the operations performed in ladles include de-oxidation (or "killing"), vacuum degassing, alloy addition, inclusion removal, inclusion chemistry modification, de-sulphurisation, and homogenisation. It is now common to perform ladle metallurgical operations in gas-stirred ladles with electric arc heating in the lid of the furnace. Tight control of ladle metallurgy is associated with producing high grades of steel in which the tolerances in chemistry and consistency are narrow. [5]
As of 2021 [update] , steelmaking is estimated to be responsible for around 11% of the global emissions of carbon dioxide and around 7% of the global greenhouse gas emissions. [12] [13] Making 1 ton of steel emits about 1.8 tons of carbon dioxide. [14] The bulk of these emissions results from the industrial process in which coal is used as the source of carbon that removes oxygen from iron ore in the following chemical reaction, which occurs in a blast furnace: [15]
Fe2O3(s) + 3 CO(g) → 2 Fe(s) + 3 CO2(g)
Additional carbon dioxide emissions result from mining, refining and shipping the ore used, basic oxygen steelmaking, calcination, and the hot blast. Carbon capture and utilization or carbon capture and storage are proposed techniques to reduce the carbon dioxide emissions in the steel industry and reduction of iron ore using green hydrogen rather than carbon. [16] See below for further decarbonization strategies.
Coal and iron ore mining are very energy intensive, and result in numerous environmental damages, from pollution, to biodiversity loss, deforestation, and greenhouse gas emissions. Iron ore is shipped great distances to steel mills.
To make pure steel, iron and carbon are needed. On its own, iron is not very strong, but a low concentration of carbon - less than 1 percent, depending on the kind of steel - gives the steel its important properties. The carbon in steel is obtained from coal and the iron from iron ore. However, iron ore is a mixture of iron and oxygen, and other trace elements. To make steel, the iron needs to be separated from the oxygen and a tiny amount of carbon needs to be added. Both are accomplished by melting the iron ore at a very high temperature (1,700 degrees Celsius or over 3,000 degrees Fahrenheit) in the presence of oxygen (from the air) and a type of coal called coke. At those temperatures, the iron ore releases its oxygen, which is carried away by the carbon from the coke in the form of carbon dioxide.
Fe2O3(s) + 3 CO(g) → 2 Fe(s) + 3 CO2(g)
The reaction occurs due to the lower (favorable) energy state of carbon dioxide compared to iron oxide, and the high temperatures are needed to achieve the activation energy for this reaction. A small amount of carbon bonds with the iron, forming pig iron, which is an intermediary before steel, as it has carbon content that is too high - around 4%. [17]
To reduce the carbon content in pig iron and obtain the desired carbon content of steel, the pig iron is re-melted and oxygen is blown through in a process called basic oxygen steelmaking, which occurs in a ladle. In this step, the oxygen binds with the undesired carbon, carrying it away in the form of carbon dioxide gas, an additional source of emissions. After this step, the carbon content in the pig iron is lowered sufficiently and steel is obtained.
Further carbon dioxide emissions result from the use of limestone, which is melted at high temperatures in a reaction called calcination, which has the following chemical reaction:
CaCO3(s) → CaO(s) + CO2(g)
Carbon dioxide is an additional source of emissions in this reaction. Modern industry has introduced calcium oxide (CaO, quicklime) as a replacement. [18] It acts as a chemical flux, removing impurities (such as Sulfur or Phosphorus (e.g. apatite or fluorapatite) [19] ) in the form of slag and keeps emissions of CO2 low. For example, the calcium oxide can react to remove silicon oxide impurities:
SiO2 + CaO → CaSiO3
This use of limestone to provide a flux occurs both in the blast furnace (to obtain pig iron) and in the basic oxygen steel making (to obtain steel).
Further carbon dioxide emissions result from the hot blast, which is used to increase the heat of the blast furnace. The hot blast pumps hot air into the blast furnace where the iron ore is reduced to pig iron, helping to achieve the high activation energy. The hot blast temperature can be from 900 °C to 1300 °C (1600 °F to 2300 °F) depending on the stove design and condition. Oil, tar, natural gas, powdered coal and oxygen can also be injected into the furnace to combine with the coke to release additional energy and increase the percentage of reducing gases present, increasing productivity. If the air in the hot blast is heated by burning fossil fuels, which often is the case, this is an additional source of carbon dioxide emissions. [20]
There are several carbon abatement and decarbonization strategies in the steelmaking industry, depending on the basic manufacturing process used, of which blast furnace/basic oxygen furnace (BF/BOF) is currently the dominant process. Options fall in to three general categories: switching the energy source from fossil fuels to wind and solar, increasing the efficiency of processing, and innovative new technological processes. Most of the latter are still in speculative or experimental stages.
CO2 emissions vary according to energy sources. When sustainable energy such as wind or solar are used to power the process, in electric arc furnaces, or create hydrogen as a fuel, emissions can be reduced dramatically. European projects from HYBRIT, LKAB, Voestalpine, and ThyssenKrupp are pursuing this strategy. [21]
Top gas from the blast furnace is the gas that is normally exhausted into the air during steelmaking. This gas contains CO2 and is also rich in the reducing agents of H2 and CO. The top gas can be captured, the CO2 removed, and the reducing agents reinjected into the blast furnace.
One study claims this process can reduce BF CO2 emissions by 75%, [22] another study states that the emissions are reduced by 56.5% with the carbon capture and storage and reduced by 26.2% if only the recycling of the reducing agents is used. [23] To keep the carbon captured from entering the atmosphere, a method of storing it or using it would have to be found.
Another way to use the top gas would be in a top recovery turbine which then generates electricity, which could be used to reduce the energy intensity of the process, if electric arc smelting is used. [21] Carbon could also be captured from gasses in the coke oven. Currently, separating the CO2 from other gasses and components in the system, and the high cost of the equipment and infrastructure changes needed, have kept this strategy minimal, but the potential for emission reduction has been estimated to be up to 65% to 80%. [24] [21]
Scrap in steelmaking refers to steel that has either reached its end of life use or was generated during the manufacture of steel components. Steel is easy to separate and recycle due to its inherent magnetism and using scrap avoids the emissions of 1.5 tons of CO2 for every ton of scrap used. [25] Currently, steel recycling is high, with all the scrap being collected also being recycled in the steel industry.
In the blast furnace, the iron oxides are reduced by a combination of CO, H2, and carbon. Only around 10% of the iron oxides are reduced by H2. With H2 enrichment processing, the proportion of iron oxides reduced by H2 is increased, so that less carbon is consumed and less CO2 is emitted. [26] This process can reduce emissions by an estimated 20%.
Alternatively, hydrogen can be used in a shaft furnace to reduce the iron oxides. This allows solely relying on hydrogen (or natural gas) for the reduction, hence it allows near-zero emissions. This technology is employed in the HYBRID project in Sweden. However, this approach requires a substantial amount of renewables to produce the needed renewable hydrogen. For the European Union, it is estimated that the hydrogen demand for hydrogen-based steelmaking would require 180 GW of renewable capacity. [27]
The HIsarna ironmaking process was described above as a way of producing iron in a cyclone converter furnace without the pre-processing steps of choking/agglomeration, which reduces the CO2 emissions by around 20%. [28]
One speculative idea is and ongoing project by SuSteel to develop a hydrogen plasma technology that reduces the oxides with hydrogen, as opposed to with CO or carbon, and melts the iron at high operating temperatures. [21] This project is still at the developmental stage.
Another developing possible technology is iron ore electrolysis, where the reducing agent is simply electrons as opposed to H2, CO, or carbon. [21] One method for this is molten oxide electrolysis. Here, the cell consists of an inert anode, a liquid oxide electrolyte (CaO, MgO, etc.), and the molten steel. When heated, the iron ore is reduced to iron and oxygen. Boston Metal is at the semi-industrial stage for this process, with plans to reach commercialization by 2026. [29] Expanding a pilot plant in Woburn, Massachusetts, and building a production facility in Brazil, it was founded by MIT professors Donald Sadoway and Antoine Allanore. [30]
A research project which involved the steel company ArcelorMittal tested a different type of iron ore electrolysis process in a pilot project called Siderwin. [31] It operates on relatively low temperatures (around 110°C), while the Boston Metal process operates on high temperatures (~1.600°C). ArcelorMittal is currently investigating whether the company wants scale up the technology and build a larger plant, and expects an investment decision by 2025. [32]
In steelmaking, coal and coke are used for fuel and iron reduction. Biomass such as charcoal or wood pellets are a potential alternative fuel, but this does not actually reduce emissions, as the burning biomass still emits carbon, it merely provides a "carbon offset", where emissions are "traded" against the sequestration of the source biomass, "ofsetting" emissions by 5% to 28% of current CO2 values. [21]
Offsetting has a very low reputation globally, as cutting down the trees to create the pellets or charcoal does not sequester carbon, it interrupts the natural sequestration the tree was providing. Offsetting is not reduction.
Overall, there are a number of innovative methods to reduce CO2 emissions within the steelmaking industry. Some of these, such as top gas recovery and using hydrogen reduction in DRI/EAF are highly feasible with current infrastructure and technology levels. Others, such as hydrogen plasma and iron ore electrolysis are still in the research or semi-industrial stage. Despite these efforts emissions from steel making are not falling in 2023.[ citation needed ]
Smelting is a process of applying heat and a chemical reducing agent to an ore to extract a desired base metal product. It is a form of extractive metallurgy that is used to obtain many metals such as iron, copper, silver, tin, lead and zinc. Smelting uses heat and a chemical reducing agent to decompose the ore, driving off other elements as gases or slag and leaving the metal behind. The reducing agent is commonly a fossil fuel source of carbon, such as carbon monoxide from incomplete combustion of coke—or, in earlier times, of charcoal. The oxygen in the ore binds to carbon at high temperatures as the chemical potential energy of the bonds in carbon dioxide is lower than that of the bonds in the ore.
In chemistry and manufacturing, electrolysis is a technique that uses direct electric current (DC) to drive an otherwise non-spontaneous chemical reaction. Electrolysis is commercially important as a stage in the separation of elements from naturally occurring sources such as ores using an electrolytic cell. The voltage that is needed for electrolysis to occur is called the decomposition potential. The word "lysis" means to separate or break, so in terms, electrolysis would mean "breakdown via electricity".
Pig iron, also known as crude iron, is an intermediate good used by the iron industry in the production of steel. It is developed by smelting iron ore in a blast furnace. Pig iron has a high carbon content, typically 3.8–4.7%, along with silica and other constituents of dross, which makes it brittle and not useful directly as a material except for limited applications.
Slag is a by-product of smelting (pyrometallurgical) ores and recycled metals. Slag is mainly a mixture of metal oxides and silicon dioxide. Broadly, it can be classified as ferrous, ferroalloy or non-ferrous/base metals. Within these general categories, slags can be further categorized by their precursor and processing conditions.
A blast furnace is a type of metallurgical furnace used for smelting to produce industrial metals, generally pig iron, but also others such as lead or copper. Blast refers to the combustion air being supplied above atmospheric pressure.
Industrial processes are procedures involving chemical, physical, electrical, or mechanical steps to aid in the manufacturing of an item or items, usually carried out on a very large scale. Industrial processes are the key components of heavy industry.
Basic oxygen steelmaking, also known as Linz-Donawitz steelmaking or the oxygen converter process, is a method of primary steelmaking in which carbon-rich molten pig iron is made into steel. Blowing oxygen through molten pig iron lowers the carbon content of the alloy and changes it into low-carbon steel. The process is known as basic because fluxes of burnt lime or dolomite, which are chemical bases, are added to promote the removal of impurities and protect the lining of the converter.
A steel mill or steelworks is an industrial plant for the manufacture of steel. It may be an integrated steel works carrying out all steps of steelmaking from smelting iron ore to rolled product, but may also be a plant where steel semi-finished casting products are made from molten pig iron or from scrap.
An open-hearth furnace or open hearth furnace is any of several kinds of industrial furnace in which excess carbon and other impurities are burnt out of pig iron to produce steel. Because steel is difficult to manufacture owing to its high melting point, normal fuels and furnaces were insufficient for mass production of steel, and the open-hearth type of furnace was one of several technologies developed in the nineteenth century to overcome this difficulty. Compared with the Bessemer process, which it displaced, its main advantages were that it did not expose the steel to excessive nitrogen, was easier to control, and permitted the melting and refining of large amounts of scrap iron and steel.
New Zealand Steel Limited is the owner of the Glenbrook Steel Mill, a steel mill located 40 kilometres south of Auckland, in Glenbrook, New Zealand. The mill was constructed in 1968 and began producing steel products in 1969. Currently, the mill produces 650,000 tonnes of steel a year, which is either used domestically or exported. Over 90% of New Zealand's steel requirements are produced at Glenbrook, while the remaining volume is produced by Pacific Steel, a steel recycling facility in Ōtāhuhu, Auckland. The mill is served by the Mission Bush Branch railway line, which was formerly a branch line to Waiuku. Coal and lime trains arrive daily. Steel products are also transported daily. The mill employs 1,150 full-time staff and 200 semi-permanent contractors.
An electric arc furnace (EAF) is a furnace that heats material by means of an electric arc.
Direct reduced iron (DRI), also called sponge iron, is produced from the direct reduction of iron ore into iron by a reducing gas which either contains elemental carbon or hydrogen. When hydrogen is used as the reducing gas there are no greenhouse gases produced. Many ores are suitable for direct reduction.
Saldanha Steel was a South African steel company originally formed as a partnership between Iscor Limited and the Industrial Development Corporation (IDC). Saldanha Steel is now part of ArcelorMittal South Africa, which in turn is part of global steel company ArcelorMittal. The mill was shutdown and mothballed in 2020 resulting in the loss of 1500 jobs.
An Ellingham diagram is a graph showing the temperature dependence of the stability of compounds. This analysis is usually used to evaluate the ease of reduction of metal oxides and sulfides. These diagrams were first constructed by Harold Ellingham in 1944. In metallurgy, the Ellingham diagram is used to predict the equilibrium temperature between a metal, its oxide, and oxygen — and by extension, reactions of a metal with sulfur, nitrogen, and other non-metals. The diagrams are useful in predicting the conditions under which an ore will be reduced to its metal. The analysis is thermodynamic in nature and ignores reaction kinetics. Thus, processes that are predicted to be favourable by the Ellingham diagram can still be slow.
Electrometallurgy is a method in metallurgy that uses electrical energy to produce metals by electrolysis. It is usually the last stage in metal production and is therefore preceded by pyrometallurgical or hydrometallurgical operations. The electrolysis can be done on a molten metal oxide which is used for example to produce aluminium from aluminium oxide via the Hall-Hérault process. Electrolysis can be used as a final refining stage in pyrometallurgical metal production (electrorefining) and it is also used for reduction of a metal from an aqueous metal salt solution produced by hydrometallurgy (electrowinning).
Deoxidization is a method used in metallurgy to remove the oxygen content during steel manufacturing. In contrast, antioxidants are used for stabilization, such as in the storage of food. Deoxidation is important in the steelmaking process as oxygen is often detrimental to the quality of steel produced. Deoxidization is mainly achieved by adding a separate chemical species to neutralize the effects of oxygen or by directly removing the oxygen.
The HIsarna ironmaking process is a direct reduced iron process for iron making in which iron ore is processed almost directly into liquid iron (pig iron). The process combines two process units, the Cyclone Converter Furnace (CCF) for ore melting and pre-reduction and a Smelting Reduction Vessel (SRV) where the final reduction stage to liquid iron takes place. The process does not require the manufacturing of iron ore agglomerates such as pellets and sinter, nor the production of coke, which are necessary for the blast furnace process. Without these steps, the HIsarna process is more energy-efficient and has a lower carbon footprint than traditional ironmaking processes. In 2018 Tata Steel announced it has demonstrated that more than 50% CO2 emission reduction is possible with HIsarna technology, without the need for carbon capture technology.
The Corex Process is a smelting reduction process created by Primetals as a more environmentally friendly alternative to the blast furnace. Presently, the majority of steel production is through the blast furnace which has to rely on coking coal. That is coal which has been cooked in order to remove impurities so that it is superior to coal. The blast furnace requires a sinter plant in order to prepare the iron ore for reduction. Unlike the blast furnace, smelting reduction processes are typical smaller and use coal and oxygen directly to reduce iron ore into a usable product. Smelting reduction processes come in two basic varieties, two-stage or single-stage. In a single-stage system the iron ore is both reduced and melted in the same container. In a two-stage process, like Corex, the ore is reduced in one shaft and melted and purified in another. Plants using the Corex process have been put use in areas such as South Africa, India, and China. First COREX process was installed in 1988 at South Africa.
In 2022, the United States was the world’s third-largest producer of raw steel, and the sixth-largest producer of pig iron. The industry produced 29 million metric tons of pig iron and 88 million tons of steel. Most iron and steel in the United States is now made from iron and steel scrap, rather than iron ore. The United States is also a major importer of iron and steel, as well as iron and steel products.
In the iron and steel industry, direct reduction is a set of processes for obtaining iron from iron ore, by reducing iron oxides without melting the metal. The resulting product is pre-reduced iron ore.