Slag is aby-product or co-product ofsmelting (pyrometallurgical)ores and recycled metals depending on the type of material being produced.[1] Slag is mainly a mixture of metaloxides andsilicon dioxide. Broadly, it can be classified asferrous (co-products of processing iron and steel),ferroalloy (a by-product of ferroalloy production) ornon-ferrous/base metals (by-products of recovering non-ferrous materials likecopper,nickel,zinc andphosphorus).[2] Within these general categories, slags can be further categorized by their precursor and processing conditions. Examples includeblast furnace slags, air-cooled blast furnace slag, granulated blast furnace slag,basic oxygen furnace slag, andelectric arc furnace (EAF) slag. Slag generated from the EAF process can contain toxic metals, which can be hazardous to human and environmental health.[3]
Due to the large demand for ferrous, ferralloy, and non-ferrous materials, slag production has increased throughout the years despite recycling (most notably in the iron andsteelmaking industries) andupcycling efforts. TheWorld Steel Association (WSA) estimates that 600 kg of co-materials (co-products and by-products; about 90wt% is slags) are generated pertonne of steel produced.[5]
Slag is usually a mixture of metaloxides andsilicon dioxide. However, slags can contain metalsulfides and elemental metals. It is important to note, the oxide form may or may not be present once the molten slag solidifies and forms amorphous and crystalline components.
The major components of these slags include the oxides ofcalcium,magnesium,silicon, iron, and aluminium, with lesser amounts ofmanganese,phosphorus, and others depending on the specifics of the raw materials used. Furthermore, slag can be classified based on the abundance of iron among other major components.[1]
Slag forms during the production of metals in a liquid state. Its low density (2.4) causes it to float above the molten metal (density ofsteel at 20 °C: 7.85). The metal separates easily from it because slag is anionic compound, notmiscible with the molten metal[6]
It is a co-product from the production ofpig iron in ablast furnace, where it corresponds to the sterilegangue of theiron ore combined with the ashes of thecoke.[7] The amount of slag produced directly correlates with the richness of the iron ore used.[8] For a modern blast furnace operating with iron-rich ores, a proportion of 180 to 350 kilograms (400 to 770 lb) of slag per 1 tonne (1.1 tons) of pig iron is typical. Extreme values are possible: 100 kilograms per tonne (220 lb/long ton) for a blast furnace usingcharcoal, or 1,300 kilograms per tonne (2,900 lb/long ton) for poor ores and cheap fuel.[7]
For the steelmaker, blast furnace slag enables control of the pig iron composition (notably by removingsulfur, an undesirable element, as well asalkalis, which disrupt furnace operation)[9]
Experienced steelmakers can estimate the approximate composition and properties of molten slag. Often, a simple "hook test" suffices, where an iron hook is dipped into the molten slag. If the slag adheres in small droplets to the hook (short slag): it is fluid and basic, with a basicity indexi, defined by the weight ratioCaO /SiO
2 greater than 1. If the slag flows off the hook in long threads (long slag): it is viscous and acidic, with a ratioi = CaO /SiO
2 < 1.[10]
However, while a basic slag removes acidic sulfur (SO
2 orH
2S depending on the system's redox conditions),alkalis are only removed from the blast furnace with an acidic slag. Thus, the slag composition faces an additional compromise: the dilemma faced by the blast furnace operator is sometimes resolved by accepting a relatively high sulfur content in the pig iron […], or by replacing, at constant basicity, the lime (CaO) in the slag withmagnesia (MgO), a condition more favorable for alkali removal and refractory wear control.[7]
However, from a thermal perspective, slag is a sterile material to melt, even if itsenthalpy of fusion, around 1,800 megajoules per tonne (510 kWh/long ton) of slag, accounts for only 3.5% of the blast furnace's energy balance, its value, though non-negligible, is far less significant than that of pig iron. Poor iron ores, likeminette ore, which increase coke consumption in the blast furnace, have been abandoned because theamount of material to heat is greater. Indeed, even for a blast furnace using iron-rich ores, the slag volume equals that of the produced pig iron (due to density differences),[11] the sale price of granulated slag contributes less than 5% to the pig iron production cost.[12]
| Typical compositions of pig iron slag (in % by weight) | |||||||||
|---|---|---|---|---|---|---|---|---|---|
| Slag type | FeO /Fe 2O 3 | MnO | SiO 2 | Al 2O 3 | CaO | MgO | P 2O 5 | TiO 2 | S |
| Blast furnace[13] (hematite pig iron) | 0.16–0.2 | < 1 | 34–36 | 10–12 | 38–41 | 7–10 | 1 | 1–1.5 | |
| Cupola furnace (melting furnace).[14] | 0.5–2.5 | 1–2 | 25–30 | 5–15 | 45–55 | 1–2 | |||
![Molten slag flowing into a pelletizing plant [fr]](/mt/?noimg=&dark=on&url=https%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aBouletage_proces.jpg)
In asteel mill, slag comes fromconverters, where it is highly oxidized, from ladle metallurgy, or fromelectric arc furnaces. For one ton of steel produced, approximately 150 to 200 kilograms (330 to 440 lb) of steelmaking slag is generated, regardless of the process (blast furnace–converter or scrap melting).[15]
Converter slag (or black slag) is produced by theoxidation of undesirable elements (such assilicon,sulfur, andphosphorus). However, the oxidation of certain metals (likeiron andmanganese) is unavoidable due to the process's nature (injection ofO
2 to oxidizecarbides in pig iron).[16]
| Typical compositions of primary metallurgy slag (in % by weight, at the end ofrefining)[17] | ||||||||
|---|---|---|---|---|---|---|---|---|
| Slag type | FeO / Fe2O3 | MnO | SiO2 | Al2O3 | CaO | MgO | P2O5 | S |
| Original Bessemer | 15 | 7 | 75 | 3 | 0 | |||
| Original Thomas | 17 | 9 | 15 | 37 | 10 | 12 | 0.5 | |
| Improved Thomas | 10 | 3 | 7 | 2 | 52 | 5 | 20 | 0.3 |
| OLP-type converter | 12 | 4 | 7 | 57 | 20 | |||
| LD-type converter | 20 | 7 | 13 | 48 | 2 | |||
| Electric arc furnace[18] | 32 | 5 | 15 | 5 | 34 | 9 | ||
| Electric arc furnace with OLP treatment | 20–30 | 7 | 50 | 1–2.5 | ||||
The role of secondary metallurgy slag (or white slag[16]) is as varied as it is complex. It gathers impurities and undesirable chemical elements by absorbing dissolvedoxide inclusions in the metal, typically fromdeoxidation. For this, managing its composition to make it reactive is essential. For example, a high lime and fluoride content promotes the capture of acidic alumina inclusions. However, this slag must also protectrefractory bricks… the adjustment ofsteelmaking slag is thus a compromise.[16]
Moreover, certain slag oxides, likeFeO, can oxidize alloy additions such asferrotitanium,aluminum, orferroboron… In this case, these alloying elements are consumed before reaching the liquid metal: their oxidation is thus wasteful. Excessive slag quantities or poorly controlledoxidation of the slag are prohibitive in this case.[16]
Inladle metallurgy or secondary metallurgy, tools for slag treatment typically include a "rake" to "skim" the slag floating on the liquid steel.Hoppers allow the addition of products to form oramend the slag.[16]
Steelmaking slag is generally lime-alumina forcarbon steels intended for flat products and lime-silica for carbon steels intended for long products. Forstainless steels, their high chromium content makes them unsuitable for use as fill, but their internal recycling within the steel mill is economically viable.[16]
| Typical compositions of secondary metallurgy slag (in % by weight, at the end ofrefining) | ||||||||
|---|---|---|---|---|---|---|---|---|
| Slag type | FeO /Fe 2O 3 | MnO | SiO 2 | Al 2O 3 | CaO | MgO | P 2O 5 | S |
| Aluminum-deoxidized steel (desulfurizing slag) | 0 | 0 | 10 | 35 | 45 | 10 | 0.1 | |
The termslag is used for the crust that forms on the weld pool when using aflux (electrode coating,powder or granules). It protects the pool from atmosphericoxygen and thermally insulates it. It also contributes to the chemical composition of theweld pool, adding or removing elements (e.g., removingsulfur).
Inshielded metal arc welding, the coating, when melting, creates the slag.
Electrodes are distinguished by their coating: basic (rich inlime), which is difficult to use but ensures excellent mechanical strength, or acidic (rich insilica), which is easier to use.
| Oxide | Basicity |
|---|---|
| Na 2O | 1.15 |
| CaO | 1.0 |
| MgO | 0.78 |
| CaF 2 | 0.67 |
| TiO 2 | 0.61 |
| Al 2O 3 | 0.61 |
| MnO | 0.59 |
| Cr 2O 3 | 0.55 |
| FeO | 0.51 |
| Fe 2O 3 | 0.48 |
| SiO 2 | 0.48 |
When molten, slag is a solution of oxides. The most common areFeO,Fe
2O
3,SiO
2,Al
2O
3,CaO, andMgO. Somesulfides may also be present, but the presence of lime and alumina reduces their solubility.[20]
Themolecular geometry of molten slag can be categorized into three oxide groups: acidic, basic, and neutral. The most common acidic oxides aresilica andalumina.[a] When molten, these oxidespolymerize, forming long complexes. Acidic slags are thus highlyviscous[b] and do not readily assimilate acidic oxides present in the molten metal.[20]
Basic oxides, such as lime (CaO) or magnesia (MgO), dissolve in an acidic slag asionic compounds. They break the molecular chains of acidic oxides into smaller units, making the slag less viscous and facilitating the assimilation of other acidic oxides. Up to a certain limit, adding basic oxides to an acidic slag or acidic oxides to a basic slag lowers themelting point.[20]
Neutral oxides (slightly acidic), such aswustite (FeO) orCu
2O, react minimally with oxide chains.[20]
In general,electrical conductivityvaries greatly with temperature,[clarify] and increases with basicity (i.e., with slag fluidity, promotingion diffusion in the molten medium) and the content of copper and iron oxides.Surface tension, however, depends little on temperature and increases with acidity, i.e., with slagviscosity.[20]
In nature, iron, copper, lead,nickel, and other metals are found in impure states calledores, oftenoxidized and mixed in withsilicates of other metals. During smelting, when the ore is exposed to high temperatures, these impurities are separated from the molten metal and can be removed. Slag is the collection of compounds that are removed. In many smelting processes, oxides are introduced to control the slag chemistry, assisting in the removal of impurities and protecting the furnacerefractory lining from excessive wear. In this case, the slag is termedsynthetic. A good example is steelmaking slag:quicklime (CaO) andmagnesite (MgCO3) are introduced for refractory protection, neutralizing thealumina andsilica separated from the metal, and assisting in the removal of sulfur and phosphorus from the steel.[citation needed]
As a co-product ofsteelmaking, slag is typically produced either through theblast furnace –oxygen converter route or theelectric arc furnace – ladle furnace route.[21] To flux the silica produced during steelmaking,limestone and/ordolomite are added, as well as other types of slag conditioners such ascalcium aluminate orfluorspar.
There are three types of slag:ferrous,ferroalloy,non-ferrous slags, which are produced through different smelting processes.
Ferrous slags are produced in different stages of the iron and steelmaking processes resulting in varying physiochemical properties. Additionally, the rate of cooling of the slag material affects its degree ofcrystallinity further diversifying its range of properties. For example, slow cooled blast furnace slags (or air-cooled slags) tend to have more crystalline phases than quenched blast furnace slags (ground granulated blast furnace slags) making it denser and better suited as an aggregate. It may also have higher freecalcium oxide and magnesium oxide content, which are often converted to its hydrated forms if excessive volume expansions are not desired. On the other hand, water quenched blast furnace slags have greateramorphous phases giving it latent hydraulic properties (as discovered by Emil Langen in 1862) similar toPortland cement.[22]
During the process of smelting iron, ferrous slag is created, but dominated by calcium and silicon compositions. Through this process, ferrous slag can be broken down into blast furnace slag (produced from iron oxides of molten iron), then steel slag (forms when steel scrap and molten iron combined). The major phases of ferrous slag contain calcium-richolivine-group silicates andmelilite-group silicates.
Slag fromsteel mills in ferrous smelting is designed to minimize iron loss, which gives out the significant amount of iron, following by oxides ofcalcium,silicon,magnesium, and aluminium. As the slag is cooled down by water, several chemical reactions from a temperature of around 2,600 °F (1,430 °C) (such asoxidization) take place within the slag.[1]
Based on a case study at the Hopewell National Historical Site inBerks andChester counties,Pennsylvania, US, ferrous slag usually contains lower concentration of various types oftrace elements thannon-ferrous slag. However, some of them, such asarsenic (As), iron, andmanganese, can accumulate ingroundwater andsurface water to levels that can exceed environmental guidelines.[1]
Non-ferrous slag is produced from non-ferrous metals of natural ores. Non-ferrous slag can be characterized into copper, lead, andzinc slags due to the ores' compositions, and they have more potential to impact the environment negatively than ferrous slag. The smelting of copper, lead andbauxite in non-ferrous smelting, for instance, is designed to remove the iron and silica that often occurs with those ores, and separates them as iron-silicate-based slags.[1]
Copper slag, the waste product of smelting copper ores, was studied in an abandoned Penn Mine in California, US. For six to eight months per year, this region is flooded and becomes a reservoir fordrinking water andirrigation. Samples collected from the reservoir showed the higher concentration ofcadmium (Cd) and lead (Pb) that exceeded regulatory guidelines.[1]
Slags can serve other purposes, such as assisting in thetemperature control of the smelting, and minimizing any re-oxidation of the final liquid metal product before the molten metal is removed from the furnace and used to make solid metal. In some smelting processes, such asilmenite smelting to producetitanium dioxide, the slag can be the valuable product.[23]
During theBronze Age of theMediterranean area there were a vast number of differential metallurgical processes in use. A slag by-product of such workings was a colorful, glassy material found on the surfaces of slag from ancient copper foundries. It was primarily blue or green and was formerly chipped away and melted down to make glassware products and jewelry. It was also ground into powder to add to glazes for use in ceramics. Some of the earliest such uses for the by-products of slag have been found in ancientEgypt.[24]
Historically, the re-smelting of iron ore slag was common practice, as improved smelting techniques permitted greater iron yields—in some cases exceeding that which was originally achieved. During the early 20th century, iron ore slag was also ground to a powder and used to makeagate glass, also known as slag glass.
Use of slags in theconstruction industry dates back to the 1800s, whenblast furnace slags were used to buildroads and railroad ballast. During this time, it was also used as an aggregate and had begun being integrated into thecement industry as ageopolymer.[25]
Today, groundgranulated blast furnace slags are used in combination withPortland cement to create "slag cement". Granulated blast furnace slags react withportlandite (Ca(OH)2), which is formed during cement hydration, via thepozzolanic reaction to produce cementitious properties that primarily contribute to the later strength gain of concrete. This leads to concrete with reduced permeability and better durability. Careful consideration of the slag type used is required, as the high calcium oxide and magnesium oxide content can lead to excessive volume expansion and cracking in concrete.[26]
These hydraulic properties have also been used for soil stabilization in roads andrailroad constructions.[27]
Granulated blast furnace slag is used in the manufacture of high-performance concretes, especially those used in the construction of bridges and coastal features, where its low permeability and greater resistance to chlorides and sulfates can help to reduce corrosive action and deterioration of the structure.[28][user-generated source?]
Slag can also be used to create fibers used as an insulation material calledslag wool.
Slag is also used as aggregate inasphalt concrete forpaving roads. A 2022 study in Finland found that road surfaces containingferrochrome slag release a highly abrasive dust that has caused car parts to wear at significantly greater than normal rates.[29]
Dissolution of slags generate alkalinity that can be used to precipitate out metals, sulfates, and excess nutrients (nitrogen and phosphorus) in wastewater treatment. Similarly, ferrous slags have been used as soil conditioners to re-balancesoil pH andfertilizers as sources of calcium and magnesium.[30]
Because of the slowly released phosphate content inphosphorus-containing slag, and because of itsliming effect, it is valued as fertilizer in gardens and farms in steel making areas. However, the most important application is construction.[31]
Slags have one of the highest carbonation potential among the industrial alkaline waste due their high calcium oxide and magnesium oxide content, inspiring further studies to test its feasibility inCO2 capture and storage (CCS) methods (e.g., direct aqueous sequestration, dry gas-solid carbonation among others).[32][33] Across these CCS methods, slags can be transformed intoprecipitated calcium carbonates to be used in the plastic, and concrete industries andleached for metals to be used in the electronic industries.[34]
However, high physical and chemical variability across different types of slags results in performance and yield inconsistencies.[35] Moreover,stoichiometric-based calculation of the carbonation potential can lead to overestimation that can further obfuscate the material's true potential.[36] To this end, some have proposed performing a series of experiments testing the reactivity of a specific slag material (i.e.,dissolution) or using thetopological constraint theory (TCT) to account for its complex chemical network.[37]
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Slags are transported along with slag tailings to "slag dumps", where they are exposed to weathering, with the possibility ofleaching of toxic elements and hyperalkaline runoffs into the soil and water, endangering the local ecological communities. Leaching concerns are typically around non-ferrous or base metal slags, which tend to have higher concentrations of toxic elements. However, ferrous and ferroalloy slags may also have them, which raises concerns about highly weathered slag dumps and upcycled materials.[38][39]
Dissolution of slags can produce highlyalkalinegroundwater withpH values above 12.[40] Thecalcium silicates (CaSiO4) in slags react with water to producecalcium hydroxide ions that leads to a higher concentration ofhydroxide (OH-) inground water. Thisalkalinity promotes the mineralization of dissolved CO2 (from the atmosphere) to producecalcite (CaCO3), which can accumulate to as thick as 20 cm. This can also lead to the dissolution of other metals in slag, such as iron (Fe),manganese (Mn),nickel (Ni), andmolybdenum (Mo), which become insoluble in water and mobile asparticulate matter. The most effective method todetoxify alkaline ground water discharge isair sparging.[40]
Fine slags and slag dusts generated frommilling slags to be recycled into the smelting process orupcycled in a different industry (e.g. construction) can be carried by the wind, affecting a larger ecosystem. It can be ingested and inhaled, posing a directhealth risk to the communities near theplants, mines, disposal sites, etc.[38][39]
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