View source for Slag

{{Short description|By-product of smelting ores and used metals}}
{{about|the metalworking by-product|other meanings|slag (disambiguation)}}
[[File:Caletones.jpg|thumb|upright=1.15|Molten slag is carried outside and poured into a dump. [[Caletones copper smelter]] in [[El Teniente|El Teniente mine]], Chile.]]
'''Slag''' is a [[by-product]] or co-product of [[smelting]] ([[Pyrometallurgy|pyrometallurgical]]) [[ore]]s and recycled metals depending on the type of material being produced.<ref name=":0">{{Cite journal|last1=Piatak|first1=Nadine M.|last2=Parsons|first2=Michael B.|last3=Seal|first3=Robert R.|date=2015|title=Characteristics and environmental aspects of slag: A review|journal=Applied Geochemistry|volume=57|pages=236–266|doi=10.1016/j.apgeochem.2014.04.009|bibcode=2015ApGC...57..236P |issn=0883-2927}}</ref> Slag is mainly a mixture of metal [[oxide]]s and [[silicon dioxide]]. Broadly, it can be classified as [[ferrous]] (co-products of processing iron and steel), [[ferroalloy]] (a by-product of ferroalloy production) or [[Non-ferrous metal|non-ferrous]]/[[base metal]]s (by-products of recovering non-ferrous materials like [[copper]], [[nickel]], [[zinc]] and [[phosphorus]]).<ref>{{Cite book|last1=Stroup-Gardiner|first1=Mary|last2=Wattenberg-Komas|first2=Tanya|date=2013-06-24|title=Recycled Materials and Byproducts in Highway Applications—Summary Report |volume=1|doi=10.17226/22552|isbn=978-0-309-22368-3}}</ref> Within these general categories, slags can be further categorized by their precursor and processing conditions.  Examples include [[blast furnace]] slags, air-cooled blast furnace slag, granulated blast furnace slag, [[basic oxygen furnace]] slag, and [[electric arc furnace]] (EAF) slag. Slag generated from the EAF process can contain toxic metals, which can be hazardous to human and environmental health.<ref name="National Academies Press 2023 p. ">{{cite book | title=Health Risk Considerations for the Use of Unencapsulated Steel Slag | publisher=National Academies Press | publication-place=Washington, D.C. | date=17 November 2023 | isbn=978-0-309-70011-5 | doi=10.17226/26881 | page=| pmid=38190460 | bibcode=2023nap..book26881N }}</ref>

[[File:IronAndSteelProduction.jpg|thumb|upright=1.15|Global production of iron and steel, 1942–2018, according to [[United States Geological Survey|USGS]]<ref>{{Cite web|title=Iron and Steel Statistics and Information|url=https://www.usgs.gov/centers/nmic/iron-and-steel-statistics-and-information?qt-science_support_page_related_con=0#qt-science_support_page_related_con|access-date=2021-11-27|website=www.usgs.gov}}</ref> ]]
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 and [[steelmaking]] industries) and [[upcycling]] efforts. The [[World Steel Association]] (WSA) estimates that 600 kg of co-materials (co-products and by-products; about 90 [[Mass fraction (chemistry)|wt%]] is slags) are generated per [[tonne]] of steel produced.<ref>{{Cite web|website=worldsteel |title=Steel industry co-products position paper|url=http://www.worldsteel.org/publications/position-papers/co-product-position-paper.html|access-date=2021-11-27|language=en}}</ref>

==Composition==

Slag is usually a mixture of metal [[oxide]]s and [[silicon dioxide]]. However, slags can contain metal [[sulfide]]s 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 of [[calcium]], [[magnesium]], [[silicon]], iron, and aluminium, with lesser amounts of [[manganese]], [[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.<ref name=":0" />

== Production ==
[[File:Schéma_procédés_fabrication_acier_1870-1970.svg|thumb|Diagram of the principles of traditional steel production processes in the mid-20th century: 1: [[Blast furnace]]; 2: Gas cleaner; 3: [[Regenerative heat exchanger|Cowper stoves]]; 4: [[Blowing engine]]; 5: Mixer; 6: [[Open-hearth furnace]]; 7: [[Bessemer process|Bessemer]] / Thomas converter; 8: [[Electric arc furnace]]; A: [[Limestone]] (flux); B: [[Iron ore]]; C: [[Coke (fuel)|Coke]]; D: Hot air blast; E: Slag; F: [[Pig iron]]; G: [[Compressed air]]; H: [[Steel]]; I: [[Scrap|Scrap metal]]]]
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 of [[steel]] at 20 °C: 7.85). The metal separates easily from it because slag is an [[Ion|ionic compound]], not [[Miscibility|miscible]] with the molten metal<ref>{{Cite book |last1=Krundwell |first1=Frank K. |url=https://books.google.com/books?id=yx5L85iuOicC&q=converter |title=Extractive Metallurgy of Nickel, Cobalt and Platinum Group Metals |last2=Moats |first2=Michael S. |last3=Ramachandran |first3=Venkoba |last4=Robinson |first4=Timothy G. |last5=Davenport |first5=William G. |publisher=[[Elsevier]] |year=2011 |isbn=978-0-08-096809-4 |page=67 |language=en}}</ref>

=== Blast furnace slag ===
It is a co-product from the production of [[pig iron]] in a [[blast furnace]], where it corresponds to the sterile [[gangue]] of the [[iron ore]] combined with the ashes of the [[Coke (fuel)|coke]].<ref name="Ingo">{{Cite book |last1=Schmidt |first1=Karl-Heinz |title=Kohle, Erdöl, Erdgas: Chemie und Technik |last2=Romey |first2=Ingo |last3=Mensch |first3=Fritz |publisher=Vogel Verlag |year=1981 |isbn=3-8023-0684-8 |location=Wurzburg |language=de |trans-title=Coal, Oil, Natural Gas: Chemistry and Technology}}</ref> The amount of slag produced directly correlates with the richness of the iron ore used. For a modern blast furnace operating with iron-rich ores, a proportion of {{convert|180|to|350|kg}} of slag per {{convert|1|tonne}} of pig iron is typical. Extreme values are possible: {{convert|100|kg/t}} for a blast furnace using [[charcoal]], or {{convert|1300|kg/t}} for poor ores and cheap fuel.<ref name="Ingo" />

For the steelmaker, blast furnace slag enables control of the pig iron composition (notably by removing [[sulfur]], an undesirable element, as well as [[Alkali metal|alkalis]], which disrupt furnace operation)<ref>{{Cite book |last=Wagner |first=Donald B. |url=http://donwagner.dk/Fate/Fate_Figs/Fate_Figs.html |title=The traditional Chinese iron industry and its modern fate |publisher=[[Routledge]] |year=1997 |isbn=0-7007-0951-7 |language=en}}</ref>

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 index ''i'', defined by the weight ratio {{nobr|[[lime (material)|CaO]] / [[silicon dioxide|{{chem|SiO|2}}]]}} greater than 1. If the slag flows off the hook in long threads (long slag): it is viscous and acidic, with a ratio {{nobr|''i'' {{=}} CaO / {{chem|SiO|2}} < 1}}.<ref>{{Cite book |last=Verein Deutscher Eisenhüttenleute |title=Gemeinfassliche Darstellung des Eisenhüttenwesens |publisher=Stahleisen mbH |year=1970 |edition=17 |location=Dusseldorf |pages=83–84 |language=de |trans-title=Accessible Presentation of Ironworks}}</ref>

However, while a basic slag removes acidic sulfur ({{chem|SO|2}} or {{chem|H|2|S}} depending on the system's redox conditions), [[Alkali metal|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 with [[Magnesium oxide|magnesia]] (MgO), a condition more favorable for alkali removal and refractory wear control.<ref name="Ingo" />

However, from a thermal perspective, slag is a sterile material to melt, even if its [[enthalpy of fusion]], around {{convert|1800|MJ/t}} 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, like [[Minette (ore)|minette ore]], which increase coke consumption in the blast furnace, have been abandoned because the [[Amount of substance|amount 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),<ref name="VanDerStel">{{Cite web |last=van der Stel |first=Jan |date=19–20 September 2012 |title=Improvements in Blast Furnace Ironmaking operations |url=ftp://ftp.cordis.europa.eu/pub/coal-steel-rtd/docs/20120919-20-09-vanderstel_en.pdf |publisher=Tata Steel RD&T |language=en }}</ref> the sale price of granulated slag contributes less than 5% to the pig iron production cost.<ref name="Yearbook2013">{{Cite web |date=January 2013 |title=2011 Minerals yearbook; Slag-iron and steel |url=http://minerals.usgs.gov/minerals/pubs/commodity/iron_&_steel_slag/myb1-2011-fesla.pdf |publisher=U.S. Department of the Interior U.S. Geological Survey |language=en }}</ref>
{| class="wikitable center"
! colspan="10" |Typical compositions of pig iron slag (in % by weight)
|-
!Slag type
![[Iron(II) oxide|FeO]] / [[Iron(III) oxide|{{chem|Fe|2|O|3}}]]
![[Manganese(II) oxide|MnO]]
![[Silicon dioxide|{{chem|SiO|2}}]]
![[Alumina|{{chem|Al|2|O|3}}]]
![[Calcium oxide|CaO]]
![[Magnesium oxide|MgO]]
![[Phosphoric acid|{{chem|P|2|O|5}}]]
![[Titanium dioxide|{{chem|TiO|2}}]]
![[Sulfur|S]]
|-
! width="40%" scope="row" |Blast furnace<ref>{{Cite book |last=Taube |first=Karl |title=Stahlerzeugung kompakt: Grundlagen der Eisen- und Stahlmetallurgie |publisher=Vieweg Technik |year=1998 |isbn=3-528-03863-2 |location=Braunschweig/Wiesbaden |pages=158–159 |language=de |trans-title=Compact Steel Production: Fundamentals of Iron and Steel Metallurgy}}</ref> (hematite pig iron)
|0.16–0.2
|< 1
|34–36
|10–12
|38–41
|7–10
|
|1
|1–1.5
|-
! scope="row" |[[Cupola furnace]] (melting furnace).<ref>{{Cite book |last=Hasse |first=Stephan |title=Giesserei Lexikon (lexique de la fonderie) |publisher=Fachverlag Schiele & Schoen |year=2000 |isbn=3-7949-0655-1 |page=1097 |language=de |trans-title=Foundry Lexicon}}</ref>
|0.5–2.5
|1–2
|25–30
|5–15
|45–55
|1–2
|
|
|
|}
<gallery mode="packed">
File:Bouletage_proces.jpg|Molten slag flowing into a {{ILL|pelletizing plant|fr|Installation de bouletage}}
File:HKM116331.jpg|Granulated (or vitrified) blast furnace slag
File:Granulated_slag_—_Гранульований_шлак_1.jpg|Sample of granulated slag
File:Slag_-_geograph.org.uk_-_297686.jpg|Block of crystallized slag used as fill
</gallery>

=== Steelmaking slag ===

==== Primary metallurgy slag (or black slag) ====
In a [[steel mill]], slag comes from [[Converter (metallurgy)|converters]], where it is highly oxidized, from ladle metallurgy, or from [[Electric arc furnace|electric arc furnaces]]. For one ton of steel produced, approximately {{convert|150|to|200|kg}} of steelmaking slag is generated, regardless of the process (blast furnace–converter or scrap melting).<ref>{{Cite web |date=February 2010 |title=Steel industry by-products |url=http://www.worldsteel.org/pictures/programfiles/Fact%20sheet_By-products.pdf |language=en}}</ref>

Converter slag (or black slag) is produced by the [[Redox|oxidation]] of undesirable elements (such as [[silicon]], [[sulfur]], and [[phosphorus]]). However, the oxidation of certain metals (like [[iron]] and [[manganese]]) is unavoidable due to the process's nature (injection of {{chem|O|2}} to oxidize [[Cementite|carbides]] in pig iron).<ref name="Zulhan">{{Cite web |last=Zulhan |first=Zulfiadi |date=November 2013 |title=Iron and steelmaking slags: are they hazardous waste? |url=https://www.researchgate.net/publication/260980474 |publisher=Bandung Institute of Technology |language=en}}</ref>
{| class="wikitable center"
! colspan="9" |Typical compositions of primary metallurgy slag (in % by weight, at the end of [[Refining (metallurgy)|refining]])<ref>{{Cite journal |last1=Richardson |first1=F.D. |last2=Jeffes |first2=J.H.E. |year=1989 |title=Sydney Thomas invention and its later impact |url=https://books.google.com/books?id=NKCpAVR011MC&pg=PA84 |journal=MASCA Research Papers in Science and Archaeology |language=en |volume=History of technology: the role of metals |issue=6 |isbn=978-0-924171-95-6 }}</ref>
|-
!Slag type
!FeO / Fe<sub>2</sub>O<sub>3</sub>
!MnO
!SiO<sub>2</sub>
!Al<sub>2</sub>O<sub>3</sub>
!CaO
!MgO
!P<sub>2</sub>O<sub>5</sub>
!S
|-
! width="40%" scope="row" |[[Bessemer process|Original Bessemer]]
|15
|7
|75
|3
|
|
|0
|
|-
! scope="row" |[[Thomas process|Original Thomas]]
|17
|9
|15
|
|37
|10
|12
|0.5
|-
! scope="row" |Improved Thomas
|10
|3
|7
|2
|52
|5
|20
|0.3
|-
! scope="row" |[[Converter (metallurgy)|OLP-type converter]]
|12
|4
|7
|
|57
|
|20
|
|-
! scope="row" |LD-type converter
|20
|7
|13
|
|48
|
|2
|
|-
! scope="row" |[[Electric arc furnace]]<ref>{{Cite book |last=Pretorius |first=Eugene |url=https://www.academia.edu/35667652 |title=Fundamentals of EAF and ladle slags and ladle refining principles |date=April 2002 |page=14 |language=en |doi=10.1179/030192302225003495}}</ref>
|32
|5
|15
|5
|34
|9
|
|
|-
! scope="row" |Electric arc furnace with OLP treatment
|20–30
|
|7
|
|50
|
|1–2.5
|
|}
<gallery>
File:Slag_runoff_Republic_Steel.jpg|Molten slag flowing into a channel, exiting a [[Martin-Siemens process|Martin-Siemens converter]] in 1941
File:2015.028.033_Tapping_open_hearth_furnaces_and_carbonizing_molten_metal_with_charcoal_or_coke.tif|Molten slag flowing into a slag pot, collecting overflow from the metal ladle behind, exiting a Martin-Siemens converter
</gallery>

==== Secondary metallurgy slag (or white slag) ====
The role of secondary metallurgy slag (or white slag<ref name="Zulhan" />) is as varied as it is complex. It gathers impurities and undesirable chemical elements by absorbing dissolved [[oxide]] inclusions in the metal, typically from ''deoxidation''. 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 protect [[Refractory|refractory bricks]]… the adjustment of [[steelmaking]] slag is thus a compromise.<ref name="Zulhan" />

Moreover, certain slag oxides, like [[Iron(II) oxide|FeO]], can oxidize alloy additions such as [[ferrotitanium]], [[aluminum]], or [[ferroboron]]… In this case, these alloying elements are consumed before reaching the liquid metal: their oxidation is thus wasteful. Excessive slag quantities or poorly controlled [[Redox|oxidation]] of the slag are prohibitive in this case.<ref name="Zulhan" />

In [[Steelmaking#Ladle metallurgy|ladle metallurgy]] or secondary metallurgy, tools for slag treatment typically include a "rake" to "skim" the slag floating on the liquid steel. [[wiktionary:hopper|Hoppers]] allow the addition of products to form or [[Soil amendment|amend]] the slag.<ref name="Zulhan" />

Steelmaking slag is generally lime-[[alumina]] for [[Carbon steel|carbon steels]] intended for flat products and lime-[[Silicon dioxide|silica]] for carbon steels intended for long products. For [[Stainless steel|stainless steels]], their high chromium content makes them unsuitable for use as fill, but their internal recycling within the steel mill is economically viable.<ref name="Zulhan" />
{| class="wikitable center"
! colspan="9" |Typical compositions of secondary metallurgy slag (in % by weight, at the end of [[Refining (metallurgy)|refining]])
|-
!Slag type
!FeO / {{chem|Fe|2|O|3}}
!MnO
!{{chem|SiO|2}}
!{{chem|Al|2|O|3}}
!CaO
!MgO
!{{chem|P|2|O|5}}
!S
|-
! width="35%" scope="row" |Aluminum-deoxidized steel
(desulfurizing slag)
|0
|0
|10
|35
|45
|10
|
|0.1
|}

=== Weld slag ===
The term '''slag''' is used for the crust that forms on the weld pool when using a ''flux'' ([[Shielded metal arc welding|electrode coating]], [[Submerged arc welding|powder or granules]]). It protects the pool from atmospheric [[oxygen]] and thermally insulates it. It also contributes to the chemical composition of the [[weld pool]], adding or removing elements (e.g., removing [[sulfur]]).

In [[shielded metal arc welding]], the coating, when melting, creates the slag.

Electrodes are distinguished by their coating: basic (rich in [[Lime (material)|lime]]), which is difficult to use but ensures excellent mechanical strength, or acidic (rich in [[Silicon dioxide|silica]]), which is easier to use.

== Physicochemistry ==
{| class="wikitable right"
|+Optical basicity (Λ) of slag oxides<ref>{{harvsp|Pretorius|2002|p=8}}</ref>
! Oxide
! Basicity
|-
|[[Sodium oxide|{{chem|Na|2|O}}]]
|1.15
|-
|[[Calcium oxide|CaO]]
|1.0
|-
|[[Magnesium oxide|MgO]]
|0.78
|-
|[[Calcium fluoride|{{chem|CaF|2}}]]
|0.67
|-
|[[Titanium dioxide|{{chem|TiO|2}}]]
|0.61
|-
|[[Alumina|{{chem|Al|2|O|3}}]]
|0.61
|-
|[[Manganese(II) oxide|MnO]]
|0.59
|-
|[[Chromium(III) oxide|{{chem|Cr|2|O|3}}]]
|0.55
|-
|[[Iron(II) oxide|FeO]]
|0.51
|-
|[[Iron(III) oxide|{{chem|Fe|2|O|3}}]]
|0.48
|-
|[[Silicon dioxide|{{chem|SiO|2}}]]
|0.48
|}
When molten, slag is a solution of oxides. The most common are [[Iron(II) oxide|FeO]], [[Iron(III) oxide|{{chem|Fe|2|O|3}}]], [[Silicon dioxide|{{chem|SiO|2}}]], [[Alumina|{{chem|Al|2|O|3}}]], [[Calcium oxide|CaO]], and [[Magnesium oxide|MgO]]. Some [[Sulfide mineral|sulfides]] may also be present, but the presence of lime and alumina reduces their solubility.<ref name="ExtractiveCopper">{{Cite book |last1=Davenport |first1=William G. I. |url=https://www.academia.edu/9598830 |title=Extractive Metallurgy of Copper |last2=King |first2=Matthew J. |last3=Schlesinger |first3=Marc E. |last4=Biswas |first4=A. K. |publisher=[[Elsevier]] |year=2002 |isbn=0-08-044029-0 |edition=4 |location=Oxford/New York/Tokyo |pages=59–63 |language=en}}</ref>

The [[molecular geometry]] of molten slag can be categorized into three oxide groups: acidic, basic, and neutral. The most common acidic oxides are [[silica]] and [[alumina]].{{efn|text=Other rarely dominant acidic oxides include {{chem|Cr|2|O|3|link=chromium(III) oxide}} and {{chem|ZrO|2|link=zirconium dioxide}}. Rarer acidic oxides are {{chem|As|2|O|3|link=arsenic trioxide}}, {{chem|Bi|2|O|3}}, and {{chem|Sb|2|O|3}}. Among basic oxides, besides lime (CaO) and magnesia (MgO), are [[iron(II) oxide|FeO]], {{chem|Fe|2|O|3|link=iron(III) oxide}}, [[lead(II) oxide|PbO]], {{chem|Cu|2|O|link=copper(I) oxide}}, {{chem|Na|2|O|link=sodium oxide}}, {{chem|K|2|O|link=potassium oxide}}, and {{chem|CaF|2|link=calcium fluoride}}, the last three often considered [[flux (metallurgy)|fluxes]].}} When molten, these oxides [[Polymerization|polymerize]], forming long complexes. Acidic slags are thus highly [[Viscosity|viscous]]{{efn|text=Acidic slags are viscous, much like [[lava|volcanic lavas]] that are [[felsic rock|felsic]], acidic, and silica-rich (causing [[Plinian eruption|Plinian]] or [[Peléan eruption|Peléan]] volcanic eruptions and highly destructive [[pyroclastic flow|pyroclastic explosions]] from sudden gas release followed by the fallout of devastating [[pyroclastic surge|pyroclastic surges]]).}} and do not readily assimilate acidic oxides present in the molten metal.<ref name="ExtractiveCopper" />

Basic oxides, such as lime (CaO) or magnesia (MgO), dissolve in an acidic slag as [[Ion|ionic 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 the [[melting point]].<ref name="ExtractiveCopper" />

Neutral oxides (slightly acidic), such as [[wustite]] (FeO) or [[Copper(I) oxide|{{chem|Cu|2|O}}]], react minimally with oxide chains.<ref name="ExtractiveCopper" />

In general, [[electrical conductivity]] {{clarify span|varies greatly with temperature,|Does it decrease or increase? See [[Electrical conductivity#Expression]]|date=August 2022}} and increases with basicity (i.e., with slag fluidity, promoting [[Diffusion|ion 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 slag [[viscosity]].<ref name="ExtractiveCopper" />

==Ore smelting==
[[File:The_Manufacture_of_Iron_--_Carting_Away_the_Scoriae.jpg|right|thumb|''The Manufacture of Iron – Carting Away the Scoriæ'' (slag), an 1873 [[wood engraving]]]]
In nature, iron, copper, lead, [[nickel]], and other metals are found in impure states called [[ores]], often [[Redox|oxidized]] and mixed in with [[silicate]]s 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 furnace [[refractory]] lining from excessive wear. In this case, the slag is termed ''synthetic''. A good example is steelmaking slag: [[quicklime]] (CaO) and [[magnesite]] (MgCO<sub>3</sub>) are introduced for refractory protection, neutralizing the [[alumina]] and [[silica]] separated from the metal, and assisting in the removal of sulfur and phosphorus from the steel.{{Citation needed|date=June 2021}}

As a co-product of [[steelmaking]], slag is typically produced either through the [[blast furnace]] – [[Basic oxygen steelmaking|oxygen converter]] route or the [[electric arc furnace]] – ladle furnace route.<ref>{{Cite book|last=Fruehan|first=Richard|title=The Making, Shaping, and Treating of Steel, Steelmaking and Refining Volume |edition=11th |publisher=The AISE Steel Foundation|year=1998|isbn=0-930767-02-0|location=Pittsburgh, Pennsylvania, USA|page=10}}</ref> To flux the silica produced during steelmaking, [[limestone]] and/or [[Dolomite (mineral)|dolomite]] are added, as well as other types of slag conditioners such as [[Calcium aluminates|calcium aluminate]] or [[Fluorite|fluorspar]].

== Classifications ==
[[File:Slag_runoff_Republic_Steel.jpg|left|thumb|upright|Slag run-off from one of the [[open hearth furnace]]s of a steel mill, [[Republic Steel]], Youngstown, Ohio, November 1941. Slag is drawn off the furnace just before the molten steel is poured into ladles for [[ingot]]ting.]]
There are three types of slag: [[ferrous]], [[ferroalloy]], [[non-ferrous]] slags, which are produced through different smelting processes.

=== Ferrous slag ===
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 of [[crystallinity]] 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 slag]]s) making it denser and better suited as an aggregate. It may also have higher free [[calcium 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 greater [[Amorphous solid|amorphous]] phases giving it latent hydraulic properties (as discovered by Emil Langen in 1862) similar to [[Portland cement]].<ref>{{Citation|last=Cwirzen|first=Andrzej|title=10 – Properties of SCC with industrial by-products as aggregates|date=2020-01-01|url=https://www.sciencedirect.com/science/article/pii/B9780128173695000106|work=Self-Compacting Concrete: Materials, Properties and Applications|pages=249–281|editor-last=Siddique|editor-first=Rafat|series=Woodhead Publishing Series in Civil and Structural Engineering|publisher=Woodhead Publishing|language=en|isbn=978-0-12-817369-5|access-date=2021-11-26}}</ref>

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-rich [[olivine]]-group silicates and [[melilite]]-group silicates.

Slag from [[steel mill]]s in ferrous smelting is designed to minimize iron loss, which gives out the significant amount of iron, following by oxides of [[calcium]], [[silicon]], [[magnesium]], and aluminium. As the slag is cooled down by water, several chemical reactions from a temperature of around {{convert|2600|F|abbr=on}} (such as [[Redox|oxidization]]) take place within the slag.<ref name=":0" />[[File:Slag2.jpg|thumb|upright|A path through a slag heap in [[Clarkdale, Arizona|Clarkdale]], Arizona, showing the striations from the rusting corrugated sheets retaining it]]

Based on a case study at the Hopewell National Historical Site in [[Berks County, Pennsylvania|Berks]] and [[Chester County, Pennsylvania|Chester]] counties, [[Pennsylvania]], US, ferrous slag usually contains lower concentration of various types of [[trace element]]s than [[non-ferrous slag]]. However, some of them, such as [[arsenic]] (As), iron, and [[manganese]], can accumulate in [[groundwater]] and [[surface water]] to levels that can exceed environmental guidelines.<ref name=":0" />

=== Non-ferrous slag ===
Non-ferrous slag is produced from non-ferrous metals of natural ores. Non-ferrous slag can be characterized into copper, lead, and [[zinc]] slags due to the ores' compositions, and they have more potential to impact the environment negatively than ferrous slag. The smelting of copper, lead and [[bauxite]] 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.<ref name=":0" /> 

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 for [[drinking water]] and [[irrigation]]. Samples collected from the reservoir showed the higher concentration of [[cadmium]] (Cd) and lead (Pb) that exceeded regulatory guidelines.<ref name=":0" />

== Applications ==
Slags can serve other purposes, such as assisting in the [[temperature 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 as [[ilmenite]] smelting to produce [[titanium dioxide]], the slag can be the valuable product.<ref>{{cite journal|last=Pistorius|first=P. C.|date=2007|title=Ilmenite smelting: the basics|url=http://www.saimm.co.za/Conferences/HMC2007/075-84_Pistorius.pdf|journal=The 6th International Heavy Minerals Conference 'Back to Basics'|pages=75–84}}</ref>
[[File:Slag from iron ore melting.jpg|thumb|Early slag from Denmark, c.&nbsp;200–500&nbsp;[[Common Era|CE]]|left]]

===Ancient uses===
During the [[Bronze Age]] of the [[Mediterranean]] 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 ancient [[Egypt]].<ref>{{Cite web|title=The chemical composition of glass in Ancient Egypt by Mikey Brass (1999)|url=http://www.antiquityofman.com/AE_glass.html|access-date=2009-06-18}}</ref>

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 make [[agate glass]], also known as slag glass.

===Modern uses===

==== Construction ====
Use of slags in the [[construction industry]] dates back to the 1800s, when [[Blast furnace slag|blast furnace slags]] were used to build [[roads]] and railroad ballast. During this time, it was also used as an aggregate and had begun being integrated into the [[cement industry]] as a [[Geopolymer cement|geopolymer]].<ref>{{Citation|last1=Netinger Grubeša|first1=Ivanka|title=4 – Application of blast furnace slag in civil engineering: Worldwide studies|date=2016-01-01|url=https://www.sciencedirect.com/science/article/pii/B978008100368800004X|work=Characteristics and Uses of Steel Slag in Building Construction|pages=51–66|editor-last=Netinger Grubeša|editor-first=Ivanka|publisher=Woodhead|language=en|isbn=978-0-08-100368-8|access-date=2021-11-27|last2=Barišić|first2=Ivana|last3=Fucic|first3=Aleksandra|last4=Bansode|first4=Samitinjay S.|editor2-last=Barišić|editor2-first=Ivana|editor3-last=Fucic|editor3-first=Aleksandra|editor4-last=Bansode|editor4-first=Samitinjay S.}}</ref>

Today, ground [[Ground granulated blast-furnace slag|granulated blast furnace slags]] are used in combination with [[Portland cement]] to create "[[slag cement]]". Granulated blast furnace slags react with [[portlandite]] ({{chem2|Ca(OH)2}}), which is formed during cement hydration, via the [[pozzolanic 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.<ref>{{Cite journal|last1=Ortega-López|first1=Vanesa|last2=Manso|first2=Juan M.|last3=Cuesta|first3=Isidoro I.|last4=González|first4=Javier J.|date=2014-10-15|title=The long-term accelerated expansion of various ladle-furnace basic slags and their soil-stabilization applications|url=https://www.sciencedirect.com/science/article/pii/S0950061814007387|journal=Construction and Building Materials|language=en|volume=68|pages=455–464|doi=10.1016/j.conbuildmat.2014.07.023|issn=0950-0618|url-access=subscription}}</ref>

These hydraulic properties have also been used for soil stabilization in roads and [[Railroad construction|railroad constructions]].<ref>{{cite book|last1=Grubeša|first1=Ivanka Netinger|chapter=Chapter 7: Diverse Applications of Slags in the Construction Industry|date=2021-08-04|url=https://pubs.rsc.org/en/content/chapter/bk9781788018876-00194/978-1-78801-887-6|title=Metallurgical Slags|pages=194–233|language=en|access-date=2021-11-27|last2=Barišić|first2=Ivana|series=Chemistry in the Environment|doi=10.1039/9781839164576-00194|isbn=978-1-78801-887-6|s2cid=238965391}}</ref>

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.<ref>{{Cite web|title=High Performance Cement for High Strength and Extreme Durability by Konstantin Sobolev|url=http://www.geocities.com/ResearchTriangle/Forum/1657/Cement/high_performance_cement.html|archive-url=https://web.archive.org/web/20090803185555/http://geocities.com/ResearchTriangle/Forum/1657/Cement/high_performance_cement.html|archive-date=2009-08-03|access-date=2009-06-18}}</ref>{{ugc|reason=Geocities was a personal web hosting service|date=March 2024}}

Slag can also be used to create fibers used as an insulation material called ''[[slag wool]]''.

Slag is also used as aggregate in [[asphalt concrete]] for [[Road surface|paving roads]]. A 2022 study in Finland found that road surfaces containing [[ferrochrome slag]] release a highly abrasive dust that has caused car parts to wear at significantly greater than normal rates.<ref>{{cite web |date=20 September 2022 |title=Autojen jakohihnojen rikkoutumisen taustalla ferrokromikuonan eli OKTO-murskeen aiheuttama kuluminen |url=https://www.gtk.fi/ajankohtaista/autojen-jakohihnojen-rikkoutumisen-taustalla-ferrokromikuonan-eli-okto-murskeen-aiheuttama-kuluminen/ |access-date=20 September 2022 |website= |publisher=Geological Survey of Finland |language=fi}}</ref>

==== Wastewater treatment and agriculture ====
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-balance [[soil pH]] and [[fertilizer]]s as sources of calcium and magnesium.<ref>{{Citation|last1=Gomes|first1=Helena I.|title=Chapter 8: Environmental Applications of Slag|date=2021-08-04|url=https://pubs.rsc.org/en/content/chapter/bk9781788018876-00234/978-1-78801-887-6|work=Metallurgical Slags|pages=234–267|language=en|access-date=2021-11-27|last2=Mayes|first2=William M.|last3=Ferrari|first3=Rebecca|series=Chemistry in the Environment|doi=10.1039/9781839164576-00234|isbn=978-1-78801-887-6|s2cid=238967817|url-access=subscription}}</ref>  

Because of the slowly released phosphate content in [[phosphorus]]-containing slag, and because of its [[Liming (soil)|liming]] effect, it is valued as fertilizer in gardens and farms in steel making areas. However, the most important application is construction.<ref>{{Cite journal|last1=O'Connor|first1=James|last2=Nguyen|first2=Thi Bang Tuyen|last3=Honeyands|first3=Tom|last4=Monaghan|first4=Brian|last5=O'Dea|first5=Damien|last6=Rinklebe|first6=Jörg|last7=Vinu|first7=Ajayan|last8=Hoang|first8=Son A.|last9=Singh|first9=Gurwinder|last10=Kirkham|first10=M. B.|last11=Bolan|first11=Nanthi|date=2021|title=Production, characterisation, utilisation, and beneficial soil application of steel slag: A review|journal=[[Journal of Hazardous Materials]]|volume=419|article-number=126478|doi=10.1016/j.jhazmat.2021.126478|pmid=34323725|bibcode=2021JHzM..41926478O |issn=0304-3894}}</ref>

==== Emerging applications ====
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 in [[Carbon capture and storage|{{chem2|CO2}} capture and storage]] (CCS) methods (e.g., direct aqueous sequestration, dry gas-solid carbonation among others).<ref>{{Cite journal|last=Doucet|first=Frédéric J.|date=2010-02-01|title=Effective CO2-specific sequestration capacity of steel slags and variability in their leaching behaviour in view of industrial mineral carbonation|url=https://www.sciencedirect.com/science/article/pii/S0892687509002313|journal=[[Minerals Engineering]]|series=Special issue: Sustainability, Resource Conservation & Recycling|language=en|volume=23|issue=3|pages=262–269|doi=10.1016/j.mineng.2009.09.006|issn=0892-6875|url-access=subscription}}</ref><ref>{{Cite journal|last1=Romanov|first1=Vyacheslav|last2=Soong|first2=Yee|last3=Carney|first3=Casey|last4=Rush|first4=Gilbert E.|last5=Nielsen|first5=Benjamin|last6=O'Connor|first6=William|date=2015|title=Mineralization of Carbon Dioxide: A Literature Review|url=https://onlinelibrary.wiley.com/doi/abs/10.1002/cben.201500002|journal=ChemBioEng Reviews|language=en|volume=2|issue=4|pages=231–256|doi=10.1002/cben.201500002|osti=1187926|issn=2196-9744|url-access=subscription}}</ref> Across these CCS methods, slags can be transformed into [[precipitated calcium carbonate]]s to be used in the plastic, and concrete industries and [[Leaching (chemistry)|leached]] for metals to be used in the electronic industries.<ref>{{Cite journal|last1=Ragipani|first1=Raghavendra|last2=Bhattacharya|first2=Sankar|last3=Suresh|first3=Akkihebbal K.|date=2021|title=A review on steel slag valorisation via mineral carbonation|url=http://xlink.rsc.org/?DOI=D1RE00035G|journal=Reaction Chemistry & Engineering|language=en|volume=6|issue=7|pages=1152–1178|doi=10.1039/D1RE00035G|s2cid=236390725|issn=2058-9883|url-access=subscription}}</ref>

However, high physical and chemical variability across different types of slags results in performance and yield inconsistencies.<ref>{{Cite journal|last1=Brand|first1=Alexander S.|last2=Fanijo|first2=Ebenezer O.|date=2020-11-19|title=A Review of the Influence of Steel Furnace Slag Type on the Properties of Cementitious Composites|journal=[[Applied Sciences]]|language=en|volume=10|issue=22|page=8210|doi=10.3390/app10228210|issn=2076-3417|doi-access=free|hdl=10919/100961|hdl-access=free}}</ref> Moreover, [[stoichiometric]]-based calculation of the carbonation potential can lead to overestimation that can further obfuscate the material's true potential.<ref>{{Cite journal|date=1956|title=Some Effects of Carbon Dioxide on Mortars and Concrete|journal=ACI Journal Proceedings|volume=53|issue=9|doi=10.14359/11515|issn=0002-8061}}</ref> To this end, some have proposed performing a series of experiments testing the reactivity of a specific slag material (i.e., [[Solvation|dissolution]]) or using the [[Rigidity theory (physics)|topological constraint theory]] (TCT) to account for its complex chemical network.<ref>{{Cite journal|last1=La Plante|first1=Erika Callagon|last2=Mehdipour|first2=Iman|last3=Shortt|first3=Ian|last4=Yang|first4=Kai|last5=Simonetti|first5=Dante|last6=Bauchy|first6=Mathieu|last7=Sant|first7=Gaurav N.|date=2021-08-16|title=Controls on CO2 Mineralization Using Natural and Industrial Alkaline Solids under Ambient Conditions|journal=ACS Sustainable Chemistry & Engineering|volume=9|issue=32|pages=10727–10739|doi=10.1021/acssuschemeng.1c00838|bibcode=2021ASCE....910727L |osti=1858885 |s2cid=238670674}}</ref>

==Health and environmental effect==
{{See also|Particulates#Controlling technologies and measures}}
{{Expand section|small=no|date=January 2024}}<!-- new sources on talk page can be used to expand the article -->

[[File:Indiana-Harbor-scrap.jpg|left|thumb|227x227px|Pile of steelmaking slag at the [[Cleveland-Cliffs]] Indiana Harbor steelmaking facility]]
Slags are transported along with slag tailings to "slag dumps", where they are exposed to weathering, with the possibility of [[Leaching (chemistry)|leaching]] 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.<ref name=":2" /><ref name=":3" />

Dissolution of slags can produce highly [[alkaline]] [[groundwater]] with [[pH]] values above 12.<ref name=":1">{{Cite journal|last1=Roadcap|first1=George S.|last2=Kelly|first2=Walton R.|last3=Bethke|first3=Craig M.|date=2005|title=Geochemistry of Extremely Alkaline (pH > 12) Ground Water in Slag-Fill Aquifers|journal=[[Ground Water]]|volume=43|issue=6|pages=806–816|doi=10.1111/j.1745-6584.2005.00060.x|pmid=16324002|bibcode=2005GrWat..43..806R |s2cid=12325820 |issn=0017-467X}}</ref> The [[calcium silicate]]s (CaSiO<sub>4</sub>) in slags react with water to produce [[calcium hydroxide]] ions that leads to a higher concentration of [[hydroxide]] (OH-) in [[ground water]]. This [[alkalinity]] promotes the mineralization of dissolved {{CO2}} (from the atmosphere) to produce [[calcite]] (CaCO<sub>3</sub>), which can accumulate to as thick as 20&nbsp;cm. This can also lead to the dissolution of other metals in slag, such as iron (Fe), [[manganese]] (Mn), [[nickel]] (Ni), and [[molybdenum]] (Mo), which become insoluble in water and mobile as [[Particulates|particulate matter]]. The most effective method to [[Detoxification|detoxify]] alkaline ground water discharge is [[air sparging]].<ref name=":1" />

Fine slags and slag dusts generated from [[Milling (machining)|milling]] slags to be recycled into the smelting process or [[upcycling|upcycled]] 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 direct [[health risk]] to the communities near the [[Chemical plant|plants]], mines, disposal sites, etc.<ref name=":2">{{Citation|last1=Ettler|first1=Vojtěch|title=Chapter 6: Environmental Impact of Slag Particulates|date=2021-08-04|url=https://pubs.rsc.org/en/content/chapter/bk9781788018876-00174/978-1-78801-887-6|work=Metallurgical Slags|pages=174–193|language=en|access-date=2021-11-27|last2=Kierczak|first2=Jakub|series=Chemistry in the Environment|doi=10.1039/9781839164576-00174|isbn=978-1-78801-887-6|s2cid=238952198|url-access=subscription}}</ref><ref name=":3">{{Citation|last1=Ettler|first1=Vojtěch|title=Chapter 5: Slag Leaching Properties and Release of Contaminants|date=2021-08-04|url=https://pubs.rsc.org/en/content/chapter/bk9781788018876-00151/978-1-78801-887-6|work=Metallurgical Slags|pages=151–173|language=en|access-date=2021-11-27|last2=Vítková|first2=Martina|series=Chemistry in the Environment|doi=10.1039/9781839164576-00151|isbn=978-1-78801-887-6|s2cid=238945892|url-access=subscription}}</ref>

==See also==
* [[Calcium cycle]]
* [[Circular economy]]
* [[Clinker (waste)]]
* [[Dross]]
* [[Fly ash]]
* [[Ground granulated blast furnace slag]]
* [[Heavy metals]]
* [[Mill scale]]
* [[Pozzolan]]
* [[Slag (welding)]]
* [[Spoil tip]]
* [[Tailings]]

==Notes==
{{notelist}}

==References==
<references />

==Further reading==

* {{Cite journal|last=Dimitrova|first=S.V.|date=1996|title=Metal sorption on blast-furnace slag|journal=Water Research|volume=30|issue=1|pages=228–232|doi=10.1016/0043-1354(95)00104-S|bibcode=1996WatRe..30..228D }}
* {{Cite journal|last=Roy|first=D.M.|author-link= Della Roy |date=1982|title=Hydration, structure, and properties of blast furnace slag cements, mortars, and concrete|journal=ACI Journal Proceedings|volume=79|issue=6}}
* {{Cite journal|last=Fredericci|first=C.|author2=Zanotto, E.D.|author3=Ziemath, E.C.|date=2000|title=Crystallization mechanism and properties of a blast furnace slag glass|journal=Journal of Non-Crystalline Solids|volume=273|issue=1–3|pages=64–75|bibcode=2000JNCS..273...64F|doi=10.1016/S0022-3093(00)00145-9}}

==External links==
{{commons category|Slag}}
* [https://www.chc.com.tw/en/source.html Types of Slag]  
* [https://www.epa.gov/smm/electric-arc-furnace-eaf-slag Electric Arc Furnace (EAF) Slag], US EPA  
* [[usgs.gov]]: [https://pubs.usgs.gov/periodicals/mcs2025/mcs2025.pdf#page=98 iron and steel slag] (Mineral Commodity Summaries 2025)

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