Slag

thumb|upright=1.15|Molten slag is carried outside and poured into a dump. [[Caletones copper smelter in El Teniente mine, Chile.]]

Slag is a by-product or co-product of smelting (pyrometallurgical) ores and recycled metals depending on the type of material being produced. Slag is mainly a mixture of metal oxides 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/base metals (by-products of recovering non-ferrous materials like copper, nickel, zinc and phosphorus). 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.

thumb|upright=1.15|Global production of iron and steel, 1942–2018, according to [[United States Geological Survey|USGS ]]

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 wt% is slags) are generated per tonne of steel produced.

Composition

Slag is usually a mixture of metal oxides and silicon dioxide. However, slags can contain metal sulfides and elemental metals. 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.

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. 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 of slag per of pig iron is typical. Extreme values are possible: for a blast furnace using charcoal, or for poor ores and cheap fuel.

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 greater than 1. If the slag flows off the hook in long threads (long slag): it is viscous and acidic, with a ratio .

However, while a basic slag removes acidic sulfur ( or 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 with magnesia (MgO), a condition more favorable for alkali removal and refractory wear control. the sale price of granulated slag contributes less than 5% to the pig iron production cost.

{| class="wikitable center"

! colspan="10" |Typical compositions of pig iron slag (in % by weight)

!Slag type

!FeO / Iron(III) oxide|

!MnO

!Silicon dioxide|

!Alumina|

!CaO

!MgO

!Phosphoric acid|

!Titanium dioxide|

! width="40%" scope="row" |Blast furnace (hematite pig iron)

|0.16–0.2

|< 1

|34–36

|10–12

|38–41

|7–10

|1–1.5

! scope="row" |Cupola furnace (melting furnace).

|0.5–2.5

|1–2

|25–30

|5–15

|45–55

|1–2

File:Bouletage_proces.jpg|Molten slag flowing into a

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 converters, where it is highly oxidized, from ladle metallurgy, or from electric arc furnaces. For one ton of steel produced, approximately of steelmaking slag is generated, regardless of the process (blast furnace–converter or scrap melting).

Converter slag (or black slag) is produced by the 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 to oxidize carbides in pig iron).

{| class="wikitable center"

! colspan="9" |Typical compositions of primary metallurgy slag (in % by weight, at the end of refining)

!Slag type

!FeO / Fe2O3

!MnO

!SiO2

!Al2O3

!CaO

!MgO

!P2O5

! width="40%" scope="row" |Original Bessemer

|15

|75

! scope="row" |Original Thomas

|17

|15

|37

|10

|12

|0.5

! scope="row" |Improved Thomas

|10

|52

|20

|0.3

! scope="row" |OLP-type converter

|12

|57

|20

! scope="row" |LD-type converter

|20

|13

|48

! scope="row" |Electric arc furnace

|32

|15

|34

! scope="row" |Electric arc furnace with OLP treatment

|20–30

|50

|1–2.5

File:Slag_runoff_Republic_Steel.jpg|Molten slag flowing into a channel, exiting a 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

! Oxide

! Basicity

|Sodium oxide|

|1.15

|CaO

|1.0

|MgO

|0.78

|Calcium fluoride|

|0.67

|Titanium dioxide|

|0.61

|Alumina|

|0.61

|MnO

|0.59

|Chromium(III) oxide|

|0.55

|FeO

|0.51

|Iron(III) oxide|

|0.48

|Silicon dioxide|

|0.48

When molten, slag is a solution of oxides. The most common are FeO, Iron(III) oxide|, Silicon dioxide|, Alumina|, CaO, and MgO. Some sulfides may also be present, but the presence of lime and alumina reduces their solubility.

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. When molten, these oxides polymerize, forming long complexes. Acidic slags are thus highly viscous and do not readily assimilate acidic oxides present in the molten metal. To flux the silica produced during steelmaking, limestone and/or dolomite are added, as well as other types of slag conditioners such as calcium aluminate or fluorspar.

Classifications

left|thumb|upright|Slag run-off from one of the [[open hearth furnaces 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 ingotting.]]

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 slags) 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 phases giving it latent hydraulic properties (as discovered by Emil Langen in 1862) similar to Portland cement.

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 mills 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 (such as oxidization) take place within the slag.

thumb|Early slag from Denmark, c. 200–500 [[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.

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 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.

Today, ground granulated blast furnace slags are used in combination with Portland cement to create "slag cement". Granulated blast furnace slags react with portlandite (), 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.

These hydraulic properties have also been used for soil stabilization in roads and railroad constructions.

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.

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 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.

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 fertilizers as sources of calcium and magnesium.

Because of the slowly released phosphate content in phosphorus-containing slag, and because of its liming effect, it is valued as fertilizer in gardens and farms in steel making areas. However, the most important application is construction.

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 capture and storage (CCS) methods (e.g., direct aqueous sequestration, dry gas-solid carbonation among others). Across these CCS methods, slags can be transformed into precipitated calcium carbonates to be used in the plastic, and concrete industries and leached for metals to be used in the electronic industries.

However, high physical and chemical variability across different types of slags results in performance and yield inconsistencies. Moreover, stoichiometric-based calculation of the carbonation potential can lead to overestimation that can further obfuscate the material's true potential. To this end, some have proposed performing a series of experiments testing the reactivity of a specific slag material (i.e., dissolution) or using the topological constraint theory (TCT) to account for its complex chemical network.

Health and environmental effect

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 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. The calcium silicates (CaSiO4) 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 (from the atmosphere) to produce calcite (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), and molybdenum (Mo), which become insoluble in water and mobile as particulate matter. The most effective method to detoxify alkaline ground water discharge is air sparging.