Production of steel

Ramesh Singh , in Applied Welding Engineering (Third Edition), 2020

Melting

The EAF process has developed as an efficient melting apparatus. The modern designs focus on increased capacity of the furnace. Melting is accomplished by supply of energy to the furnace interior. This energy can be electrical or chemical. Electrical energy is supplied via the graphite electrodes and is usually the largest contributor in melting operations. Initially, an intermediate voltage tap is selected until the electrodes dig into the scrap. Usually, light scrap is placed on top of the charge to accelerate bore-in. About 15% of the scrap is melted during the initial bore-in period. After a few minutes, the electrodes penetrate the scrap deep enough to allow for a high voltage tap that would develop long arc without fear of radiation damage to the roof. The long arc maximizes the heat of the arc, as it transfers the power to the scrap and a liquid pool of metal in the hearth of the furnace.

The initiation of arc creates an erratic and unstable arc, and wide swings in current are observed accompanied by rapid movement of the electrodes. As the temperature in the furnace rises, the arc stabilizes. As the molten pool is formed, the arc becomes quite stable and the average power input increases.

Chemical energy is supplied via several sources including oxy-fuel burners and oxygen lances. Oxy-fuel burners burn natural gas using oxygen or a blend of oxygen and air. Heat is transferred to the scrap by flame radiation and convection by the hot products of combustion. Heat is transferred within the scrap by conduction. Large pieces of scrap take longer to melt into the bath than smaller pieces. In some operations, oxygen is injected via a consumable pipe lance to "cut" the scrap. The oxygen reacts with the hot scrap and burns iron to produce intense heat for cutting the scrap. Once a molten pool of steel is generated, oxygen can be lanced directly into the molten bath of steel. The oxygen reacts with several components in the bath including aluminum, silicon, manganese, phosphorus, carbon, and iron. All of these reactions are exothermic and supply additional energy to aid in the melting of the scrap. The metallic oxides that are formed end up in the slag. The reaction of oxygen with carbon in the bath produces carbon monoxide, which either burns in the furnace if there is sufficient oxygen, and/or is exhausted through the direct evacuation system where it is burned and conveyed to the pollution control system.

Once enough scrap has been melted to accommodate the second charge, the charging process is repeated. Once the final scrap charge is melted, the furnace sidewalls are exposed to intense radiation from the arc. As a result, the voltage is reduced. Alternatively, creation of a foamy slag allows the arc to be buried. This protects the furnace shell. In addition, a greater amount of energy is retained in the slag and is transferred to the bath resulting in greater energy efficiency.

Once the final scrap charge is fully melted, flat bath conditions are reached. At this point, bath temperature is taken and a sample is collected. The analysis of the bath chemistry allows the melter to determine the amount of oxygen to be blown during refining. At this point, the melter can also start to arrange for the bulk tap alloy additions to be made. These quantities are finalized after the refining period.

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Production of Steel

Ramesh Singh , in Applied Welding Engineering (Second Edition), 2016

Melting

The EAF process has developed as an efficient melting apparatus. The modern designs focus on increased capacity of the furnace. Melting is accomplished by supply of energy to the furnace interior. This energy can be electrical or chemical. Electrical energy is supplied via the graphite electrodes and is usually the largest contributor in melting operations. Initially, an intermediate-voltage tap is selected until the electrodes dig into the scrap. Usually, light scrap is placed on top of the charge to accelerate bore-in. About 15% of the scrap is melted during the initial bore-in period. After a few minutes, the electrodes penetrate the scrap deep enough to allow for a high-voltage tap that would develop long arc without fear of radiation damage to the roof. The long arc maximizes the heat of the arc as it transfers the power to the scrap and a liquid pool of metal in the hearth of the furnace.

If the initiation of arc creates an erratic and unstable arc, wide swings in current are observed accompanied by rapid movement of the electrodes. As the temperature in the furnace rises, the arc stabilizes. As the molten pool is formed, the arc becomes quite stable, and the average power input increases.

Chemical energy is supplied via several sources, including oxy-fuel burners and oxygen lances. Oxy-fuel burners burn natural gas using oxygen or a blend of oxygen and air. Heat is transferred to the scrap by flame radiation and convection by the hot products of combustion. Heat is transferred within the scrap by conduction. Large pieces of scrap take longer to melt into the bath than smaller pieces. In some operations, oxygen is injected via a consumable pipe lance to "cut" the scrap. The oxygen reacts with the hot scrap and burns iron to produce intense heat for cutting the scrap. After a molten pool of steel is generated, oxygen can be lanced directly into the molten bath of steel. The oxygen reacts with several components in the bath, including aluminum, silicon, manganese, phosphorus, carbon, and iron. All of these reactions are exothermic and supply additional energy to aid in the melting of the scrap. The metallic oxides that are formed end up in the slag. The reaction of oxygen with carbon in the bath produces carbon monoxide (CO), which either burns in the furnace if there is sufficient oxygen or is exhausted through the direct evacuation system, where it is burned and conveyed to the pollution control system.

When enough scrap has been melted to accommodate the second charge, the charging process is repeated. After the final scrap charge is melted, the furnace sidewalls are exposed to intense radiation from the arc. As a result, the voltage is reduced. Alternatively, creation of a foamy slag allows the arc to be buried; this protects the furnace shell. In addition, a greater amount of energy is retained in the slag and is transferred to the bath, resulting in greater energy efficiency.

When the final scrap charge is fully melted, flat bath conditions are reached. At this point, the bath temperature is taken, and a sample is collected. The analysis of the bath chemistry allows the melter to determine the amount of oxygen to be blown during refining. At this point, the melter can also start to arrange for the bulk tap alloy additions to be made. These quantities are finalized after the refining period.

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Waste Materials in Construction

Jinying Yan , ... Ivars Neretnieks , in Studies in Environmental Science, 1997

2.1 Experiments

2.1.1 Material

The steel slag used in this study is a scrap metal based steel slag produced from the electric arc furnace process. The main chemical composition of the slag is shown in Table 1. The samples of slag were oven dried at 105 ± 1 °C to constant weight. In order to minimize the effects of diffusion in the particles, the slag was finely ground to particle sizes less than 0.160 mm.

TABLE 1. Major elements in the steel slag

Element Ca Mg Na K Al Fe Mn Ti P S Si
(mmol/g) 5.51 1.86 0.02 0.01 0.81 4.33 0.72 0.06 0.15 0.04 2.05
(% by weight) 22.2 4.50 0.04 0.05 2.20 24.3 3.90 0.28 0.46 1300 * 5.80
*
The unit is ppm.

2.1.2 pH titration procedure and analysis of leachate

Long-term batch pH titration experiments were carried out for the steel slag by using the automatic titrator (Metrohm 719S Titrino). The slag sample was mixed with pure water in a liquid to solid ratio 5:1 (40 g ground slag and 200 ml of water) in a plastic bottle. HNO3 solution (usually 1 M) was used as titrant. The acid was automatically added to keep a constant pH value (a given pH ± 0.02). The experiments were performed at different pH levels, and were run from 4000 to 6000 hours. The acid neutralizing capacities (ANC) of the steel slag were determined for different titration times and for various pH levels.

The leachates were analyzed after one week. The samples of leachate were taken from the batch experiments. The main cations, calcium (Ca2+), magnesium (Mg2+), sodium (Na+) and potassium (K+) were determined by using the DIONEX DX-300 Series Ion Chromatography System with suppressed conductivity detection.

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Production of Steel

Ramesh Singh , in Applied Welding Engineering, 2012

Publisher Summary

Steel making is an ancient process and numerous developments have taken place in the technology over the many years of its use. The two processes that are now commonly used for the production of steel are the Basic Oxygen Furnace (BOF) and the Electric Arc Furnace (EAF); they use variety of charge materials and technologies. The EAF process uses virtually 100% old steel to make new steel. EAFs make up approximately 60% of today's steel making in the US. This chapter illustrates the operational details of EAF and BOF processes. The electric arc furnace operating cycle is called the tap-to-tap cycle. The EAF has developed as an efficient melting apparatus, with designs focusing on increased capacity. In modern EAF operations, especially those operating with a 'hot heel' of molten steel and slag retained from the prior heat, oxygen is often blown into the bath. This technique allows for the simultaneous operation of melting and refining in the furnace. The Basic Oxygen Furnace process uses 25 to 35 percent old steel to make new steel. The iron-iron carbide phase diagram is a good basis for understanding the effect of temperature and components on the properties of steel.

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Ferrous metal production and ferrous slags

George C. Wang , in The Utilization of Slag in Civil Infrastructure Construction, 2016

2.1 Introduction

Ferrous, which is different from ferric in chemistry, means iron related, especially iron with a valance of 2 (Fe++) that exists as oxidation state (FeO). Ferrous metals, in general, refer to iron and steel materials. Steel is the major ferrous metal, an alloy of iron and carbon, and is widely used in construction and other applications. Modern steelmaking is an integrated process consisting of blast furnace (BF) ironmaking and basic oxygen furnace (BOF) or electric arc furnace (EAF) steelmaking. Molten steel from BOF or EAF process can undergo a secondary refining process in a ladle furnace, or be sent directly to the continuous caster. During ironmaking, when smelting ore, iron scrap, coke, and flux, BF slag is formed and subsequently discharged. Steel slag is formed and discharged when smelting iron, steel scrap, and flux during the steelmaking process. After molten slag is discharged, being air-cooled, or treated under different cooling regimes and processed, slag products for various applications can be produced. Fig. 2.1 is a flow chart to present the iron- and steelmaking processes and the types of slag generated from each stage.

Fig. 2.1. The integrated iron- and steelmaking process and ferrous slag generation.

 Adapted from Yildirim, I. Z., & Prezzi, M. (2011). Chemical, mineralogical, and morphological properties of steel slag. Advances in Civil Engineering. http://dx.doi.org/10.1155/2011/463638.

As indicated in Fig. 2.1, the raw materials used in the BF-BOF system include iron ore, coal, fluxes (mainly limestone and dolomite), recycled steel scraps, and alloys. On average, this process uses approximately 1400   kg (3086   lb) of iron ore, 800   kg (1764   lb) of coal, 300   kg (661   lb) of flux, and 120   kg (265   lb) of recycled steel to produce 1000   kg (2205   lb) of crude steel. The EAF process uses primarily recycled steel scraps and electricity. On average, the recycled steel-EAF process uses 800   kg (1764   lb) of recycled steel, 16   kg (35   lb) of coal, and 64   kg (141   lb) of flux to produce 1000   kg (2205   lb) of crude steel (World Steel Association, 2014). To make final steel products, other materials may be used, which include manganese, silicon, nickel, chromium zinc, tin, and tungsten.

Unlike manufactured engineering materials, which are produced under quality control required by technical specifications and supplied on demand, ferrous slag is produced as a coproduct simultaneously with iron- and steelmaking and is generated daily. A considerable amount of ferrous slag is produced each year in the world. As the amount of slag tapped from the furnaces is not normally routinely measured and not all of the ferrous slag formed is tapped during a heat, the ferrous slag output levels are normally broadly estimated based on the typical slag to metal production ratios, which in turn are related to the chemistry of the raw materials to the furnaces. For typical high iron ore grades (60–66% iron), a BF normally produces approximately 0.25–0.30 tonne (0.28–0.33 ton) of slag per tonne (ton) of crude iron produced. For lower grade ores, the slag output will be higher, in some cases as much as 1.0–1.2 tonne (1.1–1.32 ton) slag per tonne (ton) of crude iron (USGS, 2013).

Steel furnaces typically produce approximately 0.2 tonne (0.22 ton) of slag per tonne (ton) of crude steel. However, up to 50% of this slag is entrained metal, most of which is recovered during slag processing and returned to the furnaces. The amount of marketable steel slag after processing entrained metal is usually between 10% and 15% of the crude steel output (USGS, 2014).

On the other hand, steel will continue to be the most commonly used structural and functional materials in terms of quantity in the future. From 2012 to 2013, the world crude steel production increased by 4.0%. The output of crude steel had increased from approximately 200 million tonnes (220 million tons) in 1950 to 1600 million tonnes (1760 million tons) in 2013 (World Steel Association, 2014). Based on the slag to metal production ratios and the ferrous metal production, the estimated ferrous slag generated in the world in 2013 was approximately 600 million tonnes (660 million tons), which includes 408 million tonnes (449 million tons) of BF slag and 193 million tonnes (212 million tons) of steel slag. Iron BF slag and steel slag make up the largest portion of the slag family, which includes nonferrous slag and nonmetallurgical slag.

In terms of the properties, solidified ferrous slag is a nonmetallic and energy-containing by-product and possesses some chemical, physical, and mechanical properties that match or are similar to those of some natural or manufactured engineering materials. In addition, due to some special characteristics of ferrous slag, there exists the possibility of altering or modifying physical and chemical properties of the conventional engineering materials to produce special construction materials that can be utilized for special applications. With increasing concern about sustainable development, greenhouse emissions, and emphasis on materials reduction, reuse, and recycling, it is critical that the full potential of the use of ferrous slag is developed to reduce potential environmental impacts and for natural resources sustainability, with financial return, rather than disposal costs. Processed BF, BOF, and EAF slags are being considered to be conventional or nontraditional construction materials. The availability and properties of ferrous slag open avenues for potential engineering utilizations, and also bring scientific, technical, and managerial challenges to people in developing various optimal applications.

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Ironmaking

Yongxiang Yang , ... Lauri Holappa , in Treatise on Process Metallurgy: Industrial Processes, 2014

1.1.1.4.1 Blast Furnace Process for Integrated Steelmaking

Steel is produced by using two types of raw materials: hot metal or pig iron, and steel scrap, and through two types of processes: basic oxygen furnaces (BOF) or basic oxygen steelmaking and electric arc furnaces (EAF). In BOF steelmaking, about 75% of iron comes from the hot metal produced by blast furnace process, the remaining 25% of the raw materials is steel scrap. EAF process uses 100% steel scrap in general, however, in some plants significant share of direct reduced iron (DRI) is used. Further, solid pig iron is used as "pure" raw material and carbon source in EAFs. The blast furnace–BOF route produces almost 66% total crude steel, and EAF route accounts for about 31%, while the blast furnace–open hearth process, which dominated steelmaking in the first half of the 1900s, had only a share of about 3% left [7].

The blast furnace process has been for the past two centuries the dominating ironmaking route to provide the raw materials for steelmaking industry. Nowadays, over 93% of the total iron production from ores is taking place via BF route. An alternative route for iron production is direct reduction, which is described more thoroughly in a separate chapter. Figure 1.1.1 illustrates the position of the blast furnace as well as direct reduction in the overall steelmaking process [8].

Figure 1.1.1. Illustration of ironmaking and steelmaking flowsheet [8].

Blast furnace uses iron ore as the iron-bearing raw materials, and coke and pulverized coal as reducing agents and heat source, lime, or limestone as the fluxing agents. The main objective of blast furnace ironmaking is to produce hot metal with consistent quality for BOF steelmaking process. Typically the specification of steel works requires a hot metal with 0.3–0.7% Si, 0.2–0.4% Mn, and 0.06–0.13% P, and a temperature as high as possible (1480–1520   °C at the tapping). A modern large blast furnace has a hearth diameter of 14–15   m, and a height of 35   m with an internal volume of about 4500   m3. One such large blast furnace can produce 10,000   tons of hot metal per day (THM/day). Figure 1.1.2 illustrates the modern ironmaking blast furnaces.

Figure 1.1.2. Blast furnace No. 6 and 7 of Tata Steel in IJmuiden, The Netherlands.

 Courtesy: Tata Steel Europe, IJmuiden (photographer: Vincent Bloothoofd).

Since blast furnace process consumes a large amount of metallurgical coke, other ironmaking processes are being developed and are emerging as eventual future alternative ironmaking processes: smelting reduction and direct reduction where metallurgical coke can be replaced by pulverized coal or other gaseous reducing agents. The examples of commercial processes are MIDREX [9] as direct reduction process and COREX [10] as smelting reduction process.

Annual hot metal production by blast furnace process, DRI, as well as crude steel from 1980 to 2011 is shown in Figure 1.1.3.

Figure 1.1.3. World iron and steel production: 1980–2011.

 Data taken from World Steel Association [11].

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Life cycle analysis of strengthening concrete beams with FRP

Sebastian George Maxineasa , Nicolae Taranu , in Eco-Efficient Repair and Rehabilitation of Concrete Infrastructures, 2018

24.2.2 Environmental impact of traditional building materials

The impact of the construction industry is highly affected by the impact of the specific products (i.e., materials and structures) used in this sector. The environmental performances of the built environment can be greatly influenced by the preoperation and postoperation phases of a structure—the considered structural solutions adopted in the design stage, and the building materials consumed. Therefore, we will try to clarify what the environmental implications resulted from the manufacturing stage of traditional building materials.

24.2.2.1 Concrete

Concrete is the most utilized product in the construction sector; the total quantity of concrete used is two times larger than the amount of other materials used in this sector. 25 billion tons of concrete are produced each year, which equals to 3.8 tons processed for every person. In the last 60 years, the levels of concrete consumption have multiplied 10-fold, an alarming situation given that concrete is the second most consumed resource, the first one being water (EPC, 2009; Gursel et al., 2014; Marinkovic et al., 2014).

It is estimated that, for the production of the amount of concrete used worldwide, between 8 and 12 billion tons of natural aggregates are consumed annually (Shafigh et al., 2014), while the manufacturing stage processes around 80   L of water per ton of concrete (MPA, 2015). From the concrete mix, cement is the component material with the most important influence on the overall environmental impact. In the last 60 years, the amount of cement produced and consumed worldwide has increased from 0.5 to ~3 billion tons (Estrada et al., 2012; Gursel et al., 2014; Habert, 2014). It is also estimated that, considering the current consumption trends, the quantity of cement used at the global level will reach between 3.69 and 4.40 billion tons (WBCSD, 2009). Probably the most important fact related to the environmental impact of cement, and therefore of concrete, is that 1   kg of CO2 is released as a result of the production of 1   kg of cement (Estrada et al., 2012; Habert, 2014). Taking into account the significant amount of concrete consumed each year and its overall environmental impact, concrete represents an important component in achieving sustainability (its environmental dimension, at least) in the construction sector.

24.2.2.2 Steel

The processes specific to steel manufacturing are responsible for approximately 9% of the total amounts of global CO2 emissions (Moynihan and Alwood, 2012). It is estimated that in the last 40 years, the amount of steel produced globally has increased about 2.5 times, from 595 million tons in 1970 to approximately 1600 million tons in 2013 (WSA, 2013a, b). The impact of this material is very important in assessing the environmental footprint of the built environment, due to the fact that about 50% of the total volume of steel produced worldwide is consumed in the construction sector (Wang et al., 2007; Moynihan and Alwood, 2012).

The environmental performances of steel are highly influenced by the producing technique. Two methods are generally used: the basic oxygen furnace (BOF) process and the electric arc furnace (EAF) process (Estrada et al., 2012; Strezov et al., 2013). Taking into account that the EAF process uses electricity and the BOF method coal and natural gas, and also considering that in the first manufacturing method, larger amount of recycled material is utilized, it can be concluded that the steel produced by using the EAF method has a lower environmental impact. Compared with other traditional construction materials, steel has an important environmental advantage in that it can be 100% recycled numerous times without any quality loss (Estrada et al., 2012).

It must be mentioned that, in recent decades, the steel industry has made significant progress with respect to its environmental impact. For example, in the United States of America since 1975, greenhouse gas emissions resulted from the manufacturing process have decreased by approximately 45%; the carbon footprint from steel is also smaller by 47%, compared with the one from 1990 (AISC, 2016). Even if the environmental footprint has decreased, the steel industry must take supplementary actions in order to produce a material with a nearly zero impact over the Earth's ecosystem.

24.2.2.3 Masonry

The level of environmental burdens characteristic to these elements is highly influenced by the type of material used in the manufacturing stage of masonry units. Fired clay bricks have a level of embedded energy higher than the one corresponding to concrete masonry units (or concrete bricks). Being a fired material, during the production stages of clay bricks, significant amounts of energy are consumed, which has a direct effect over the carbon footprint of the manufactured products (Volz and Stonver, 2010a; Estrada et al., 2012). In order to produce fired clay bricks, temperatures have to reach and be maintained between 900°C and 1050°C. Another important fact that influences the impact of clay units is represented by the time consumed during the heating and cooling processes, that can last up to 150   hours (Bingel and Bown, 2010; Volz and Stonver 2010a; Lourenco şi Vasconcelos, 2015).

As in the case of concrete, the environmental performances of masonry concrete blocks are highly influenced by the amount of cement used in the manufacturing stage (Bingel and Bown, 2010; Volz and Stonver, 2010a). In order to improve the environmental performances of these type of bricks, the masonry industry has decided to use complementary materials, like fly ash. This type of material has been used to replace a volume of cement in the case of concrete masonry units, or to produce a new type of brick, the fly ash brick, resulting in a type of unit with a lower environmental footprint than fired clay bricks concrete bricks, due to the amount of energy consumed in the manufacturing processes. The embodied energy of fly ash bricks is approximately 2 times lower than the one of classic concrete masonry blocks (Bingel and Bown, 2010; Volz and Stonver, 2010b; Estrada et al., 2012).

24.2.2.4 Timber

It is well known that forests are Earth's most proficient tool for enhancing air quality by sequestering an important volume of carbon dioxide (CO2) and replacing it with oxygen (O2). A well-managed wood plantation has the ability of sequestering approximately 670   g of CO2, releasing at the same time up to 490   g of O2, which makes timber a carbon negative material (DeStefano, 2009; Estrada et al., 2012). Forests also provide a natural habitat for a significant part of the wildlife, having probably one of the most important roles in the global efforts of achieving the environmental dimension of sustainability. Another benefit of wood, with respect to the Earth's ecosystem, is that this material is biodegradable and can also be recycled as bio fuel (Estrada et al., 2012; Hafner et al., 2014).

One of the most important environmental disadvantages of wood as a building material is the vulnerability to insect attacks and decay (Estrada et al., 2012). This means timber elements must be protected in order to ensure a long-term durability by using preservatives, which usually are chemical solutions with a negative influence over the natural environment. Even if the use of timber products made by processing tress from sustainable wood plantations (known as certified forests) is encouraged, there is still another major problem: illegal deforestation. Being a widely used material in the built environment, the consumption rates of timber in this sector have an important effect over the overall state of the Earth's ecosystem.

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Carbon dioxide sequestration on steel slag

Liwu Mo , in Carbon Dioxide Sequestration in Cementitious Construction Materials, 2018

8.2.1 Type, composition, and basic properties of steel slag

From the process of crude iron production to the following refining of crude iron into steel, various types of slags are produced at different stages of the steel manufacturing with different furnace processes. BFS is produced during the manufacturing of crude steel in blast furnace. BOFS and electric arc furnace slag (EAFS) are produced in basic oxygen furnace and electric arc furnace respectively, which are the most widely used processes for the steel manufacturing. In the electric arc furnace process, the electric arc furnace oxidizing slag is firstly produced when the iron scrap is melted and refined into steel, and then the electric arc furnace reducing slag is produced in the ladle refining furnace. Further refining of the stainless steel in the ladle furnace generates argon oxygen decarburization slag and desulfurization slag.

Generally, the main chemical compositions of steel slag include CaO, FeO (Fe2O3), MgO, SiO2, Al2O3, Na2O, K2O, and small amounts of heavy metals, for example, Pb, Cr, V, Mn, etc. However, the quantities of different chemical compositions vary with the type of steel slag. Fig. 8.1 shows the normalized CaO(MgO)-SiO2(Na2O, K2O)-Al2O3(Fe2O3) composition diagram of various types of steel slags drawn by Pan et al. (2016) on the basis of a series of data from literature. Apparently the steel slag has a wide range of chemical compositions depending on the raw materials and furnace process for the steel refining. For BFS, the compositions are primarily silica and alumina from the original iron ore, with some calcium and magnesium oxides from the added flux (mainly lime), while for the BOFS and EAFS more quantity of FeO and little quantity of Al2O3 are contained. The chemical components of the steel slag exist in different mineral forms and exhibit various physico-chemical properties. As a main component, Ca presents in many mineral forms, for instance, free-CaO, Portlandite (hydration products of free-CaO), various calcium silicates (e.g., larnite (Ca2SiO4) γ−C2S, β−C2S, C3S), complicated Ca-containing silicates (e.g., bredigite/merwinite(Ca14Mg2(SiO4)8), gehlenite(Ca2Al(Al, Si)2O7), etc.), and other solid solutions containing Mg, Fe, Al, etc. Free-CaO is very hydration and carbonation reactive; the hydration of free-CaO (when the contents are high) in steel slag may cause excessive volume expansion and hence induce unsoundness. Small party of calcium silicate presents in the forms of β−C2S, C3S, C4AF, and C2F, exhibiting cementitious performance when mixed with water. Therefore, steel slag can act as raw material for cement clinker production and as a potential mineral admixture for concrete at a proper replacement level (Wang and Yan, 2010; Li et al., 2011; Wang et al., 2011). Most part of the calcium silicate, in particular, in the form of the solid solution of Ca, Mg, Fe, and O, presents very low hydraulic property. Nonetheless, under the concentrated CO2 environment, most of the Ca containing phases in steel slag are carbonation reactive. Iron mainly exists in the mineral forms of wustite (FeO), hematite (Fe2O3), magnetite (Fe3O4), magnesioferrite (MgFe2O4), etc. Fe influences the grindability of steel slag. With the increasing Fe content in the slag, the hardness of steel slag increases and thus decreases its grindability. This makes the grinding of steel slag very energy intensive and inefficient. For example, in comparison to the reductive EAFS, the oxidizing EAFS normally contains 24–38   wt% of iron oxide, and exhibits higher hardness and grinding resistance.

Figure 8.1. Normalized CaO(MgO)-SiO2(Na2O, K2O)-Al2O3(Fe2O3) composition diagram of various types of steel slags drawn by Pan et al. (2016) on the basis of data from literature. AODS, argon oxygen decarburization slag; BFS, blast furnace slag; EAFOS, electric arc furnace oxidizing slag; EAFRS, electric arc furnace reducing slag; LFS, ladle furnace slag; PS, phosphorus slag.

The Mg content presents in two main forms of periclase and Mg containing solid solution (i.e., wustite in the formula of Fe(Mn, Mg, Ca, O)) (Wang et al., 2010). The presence of periclase contained in steel slag could also hydrate to form Mg(OH)2, giving rise to volume expansion and may cause unsoundness when the periclase content is excessive. These Mg phases exhibit relatively low hydration reactivity (Wang and Yan, 2010; Wang et al., 2011). However, magnesium can also be carbonated and therefore has an impact on the capacity of CO2 sequestration.

Heavy metals in terms of Ba, Cr, V, Mn, As, Cd, Hg, etc., are contained in the steel slag, which may leach out from the steel slag and hence induce potential pollution of the environment or detrimental effects on human health (Proctor et al., 2000; Das et al., 2007). Therefore the potential leaching of heavy metals is also an obstacle for its application.

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Nonmetallurgical slags

George C. Wang , in The Utilization of Slag in Civil Infrastructure Construction, 2016

4.1 Introduction

In a broad sense, the definition of slag can be extended to the fused agglomerated materials that are generated during the production of a base element to separate it from the impurities in the raw materials, for example, element phosphorus and phosphorus slag; or from coal combustion and incinerating process, for example, boiler slag from power generation and incinerator slag from incinerating municipal solid waste (MSW). These by-products or residues undergo high-temperature processes in a molten condition and subsequently are cooled under ambient air conditions or by water quenching, becoming solid and hard materials. In this broad sense, slag has been extended to the agglomerated materials from nonmetallurgical processes. In this chapter the focus is placed on the formation of phosphorus slag, boiler slag, and municipal solid waste incinerator (MSWI) slag, and their basic properties.

Phosphorus slag is formed during the elemental phosphorus production process. When phosphate rock, phosphorus concentrate, and fluxes are smelted in an electric arc furnace under temperatures of 1400–1500   °C (2552–2732   °F), molten slag is formed and tapped out from the furnace and goes through an air-cooling or water quenching process. The elemental phosphorus is separated from the impurities during the slagging process.

Similar to nonferrous metals production, the ore (ie, phosphate rock) is normally beneficiated before the smelting because the ores normally contain low phosphorus content. For example, for the phosphate rock produced in Florida, the content of P2O5 ranges approximately from 28% to 30% (Zhang, 2015).

In phosphorus production, large quantities of raw materials have to be handled and processed. To produce one tonne of elemental phosphorus, approximately 12.7 tonnes of solid materials are required. This typically includes 9.5 tonnes of phosphate rock, 1.5 tonnes of coke, and 1.7 tonnes of silica rock. To produce 1 tonne of elemental phosphorus, approximately 8–10 tonnes of phosphorus slag is produced (Barber, 1975; Corbridge, 1995; Zhou, Shu, Hu, & Wang, 2010).

Worldwide phosphate rock mine production in 2013 was 224 million tonnes (246 million tons) increased by 3.2% from 217 million tonnes (239 million tons) in 2012 (USGS, 2014). In the United States, phosphate rock mining is the fifth largest mining industry in terms of quantity of material mined. In 2013 the total production of phosphate rock in the US was estimated at 32 million tonnes (35 million tons). Most of the phosphate production goes to the making of fertilizers (USGS, 2014). In 2013, China was the largest producer of sedimentary phosphorite, which accounted for approximately 43% of the worldwide output of phosphate rock mining, and the United States came in second at 15% of the worldwide production (USGS, 2014).

In the phosphate industry, approximately 5% of the phosphate rock is processed via the electric arc furnace process to make elemental phosphorus ( Corbridge, 1995; Goldwhite, 1981; Zhang, 2015). The remainder is processed using the "wet method" to produce phosphoric acid and other phosphorus compounds. During the wet process, phosphogypsum, a by-product of this process, is produced. In 2013, approximately 1.5 million tonnes of element phosphorus is produced worldwide. The total annual phosphorus slag generated from the elemental phosphorus production therefore can be estimated as approximately 12–15 million tonnes (13.2–16.5 million tons).

Boiler slag is the agglomerated residues formed in coal-burning facilities; coal combustion furnaces in thermal power stations, for instance.

Boiler slag is formed at the base of the furnaces, mainly slag-tap and cyclone types, at temperatures of 1500–1700   °C (2732–3092   °F). The molten slag is collected and quenched with water-forming boiler slag. When the molten slag comes in contact with the quenching water, it fractures, crystallizes, and forms pellet-shaped particles. Boiler slag is a vitreous, grained material made up of hard, black, and angular particles with a smooth and glassy appearance (ECOBA, 2015).

Boiler slag is one of the coal combustion products (CCPs), along with fly ash, bottom ash, and flue gas desulfurization (FGD) gypsum. Coal, as a source of energy, has played a very significant role in generating electricity in (approximately) the past 150   years. In 2013, approximately 67% of the electricity generated in the United States was from fossil fuel (ie, coal, natural gas, and petroleum) with 39% attributed from coal. Worldwide, electricity generated by burning coal is approximately 41%. In some countries this number is higher. For example, in South Africa, it is 93%, Poland 87%, China 79%, Australia 78%, Kazakhstan 75%, India 68%, Israel 58%, and Czech Republic 51%. According to the International Energy Agency (IEA, 2015), in 2010, 7200 million tonnes (7920 million tons) of coal was consumed in the world, which increased 60% in 10   years from 2000.

Huge amounts of CCPs are produced each year. According to the American Coal Ash Association (ACAA), combustion of coal in the United States alone generated approximately 120 million tonnes (132 million tons) of CCPs in 2010, which included approximately 62 million tonnes (68.2 million tons) of fly ash, 17 million tonnes (18.7 million tons) of bottom ash, 30 million tonnes (33 million tons) of FGD materials, and 2 million tonnes (2.2 million tons) of boiler slag. Although boiler slag accounted for approximately 15% of the total CCPs generated, the utilization rate of boiler slag is higher than other CCPs; that is, fly ash, bottom ash, and FGD gypsum. In the United States, the utilization rate of boiler slag was 66.16% in 2013, while the utilization rates for fly ash, bottom ash, and FGD gypsum were 43.67%, 39.02%, and 48.85%, respectively. In Europe, the utilization rates of CCPs are generally higher than those of the United States. The utilization rate in Europe for boiler slag was 100% in 2007. For fly ash and bottom ash, the utilization rates were 49% and 51%, respectively (ACAA, 2015). Although the total production of boiler slag accounts for a relatively small portion of the total CCPs, the successful applications of boiler slag can be a reference to the utilization of other CCPs, especially fly ash and bottom ash, as the chemical and mineral composition of boiler slag, bottom ash, and fly ash are typically very similar, consistent, and uniform, if the coal used is from the same source, and they all are defined as nonhazardous materials.

Incinerator slag, most often is generated from the process of incinerating MSW. Incinerating is one of the three most frequently selected methods for MSW treatment along with landfilling and composing. Incineration is the combustion of MSW in a controlled manner to destroy it or transform it into less hazardous, less bulky, or more controllable constituents. Incineration may also be used to treat a wider range of waste streams, including commercial, clinical, and certain types of industrial waste (Cheremisinoff, 2003).

Solid waste management practices still differ widely throughout the world. MSW incineration is becoming increasingly important for waste management. In some countries, for example, countries in Europe, the European regulations prohibit storing of untreated waste in landfills (Müller & Rübner, 2006). In the United States and some other industrialized countries, the disposal of MSW is one of the more serious and controversial urban issues facing local governments. In the United States, approximately 84% of the MSW goes to landfills, while in Japan and most European countries over 70% of their MSWs are incinerated (Cheremisinoff, 2003).

It should be noted that boiler slag and MSWI slag are two different materials. Boiler slag, like other CCPs, results from burning coal under controlled conditions. The US Environmental Protection Agency (EPA) has determined that CCPs are nonhazardous and have excluded CCPs from their list of hazardous wastes. Incinerator slag is obtained as a result of burning MSW or combinations of MSW and other wastes. The chemical and mineral composition of incinerator slag vary because of the wide variety of MSW materials burned (Ramme & Tharaniyil, 2013).

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Characteristics of steel slags and their use in cement and concrete—A review

Yi Jiang , ... Shu-Yuan Pan , in Resources, Conservation and Recycling, 2018

3.2 Physico-chemical characteristics

Using steel scrap instead of melted iron as feed material, the EAF process is actually a steel scrap recycling process and the chemical composition of EAF slag can vary over a wider range than BOF slag. EAF-C slag shares many things in common with BOF slag such as the primary oxides (Table 3), mineral phases and physical appearance (e.g. color and morphology). However, EAF-S slag from stainless steel production contains a lower FeO content but a higher Cr content (Shi, 2004; Yildirim and Prezzi, 2011). The mineral phases identified for EAF slag include merwinite (3CaO·MgO·2SiO2), wustite (solid solutions of FeO), olivine, C2S and C3S (Adegoloye et al., 2016; Muhmood et al., 2009; Santamaría et al., 2016; Yildirim and Prezzi, 2011; Piatak et al., 2015).

Table 3. Chemical compositions of EAF slags (wt.%) used from the literature.

References Sources SiO2 Al2O3 Fe/FeO/Fe2O3 CaO MgO MnO/Mn2O3 P2O5 f- CaO Cr2O3 Others LOI Treatment
Muhmood et al. (2009) India 23.3 6.1 24.1 30.8 12 1.5 0.6 0.4 – 0.9(TiO2) – Air cooled and water sprayed
Muhmood et al. (2009) India 29.0 5.9 1.2 38.8 21.4 1.4 0.5 0.1 – 0.7(TiO2) – Water quenched
Roslan et al. (2016) Malaysia 26.4 4.84 43.4 16.9 1.86 2.66 – – – 0.15(Na2O) – Air cooled
Li et al. (2013) China 24.9 4.89 1.23 54.0 7.26 – 0.05 – – 0.15(SO3) – –
Yu et al. (2016) Australia 19.8 20 14.5 37.8 4.3 2.6 – – – 0.5(TiO2)/0.2(Na2O) /0.2(K2O) 0.1 Air cooled and weathered
Hekal et al. (2013) Egypt 13.1 5.51 36.8 33 5.03 4.18 0.7(P2O3) – – 0.14(SO3)/0.6(TiO2) – –
Mombelli et al. (2016) Europe 15-20 10-15 30-50 15-25 2-5 – – – 2-5 0.1-0.2(V2O5)/0.05-0.1(Ba) – –
Mombelli et al. (2016) Europe 5-25 1-3 30-50 15-25 1-3 – – – 5-30 1-2(V2O5) – –
Mombelli et al. (2016) Europe 10-40 5-15 5-30 20-50 5-15 – – – 0.5-5 0.05-0.4(V2O5)/0.1-0.5(Ba) – –
Sheen et al. (2015a) Taiwan 38.6 2.43 – 31.5 12.8 – – – – – – Air cooled and weathered

Remark: – means not detected or clarified, LOI = Loss on ignition.

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