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Slag is a by-product or co-product of smelting (Pyrometallurgy) and recycled metals depending on the type of material being produced. Slag is mainly a mixture of metal 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/ (by-products of recovering non-ferrous materials like copper, nickel, zinc and phosphorus). (2013). 9780309223683 ISBN 9780309223683 Within these general categories, slags can be further categorized by their precursor and processing conditions (e.g., blast furnace slags, air-cooled blast furnace slag, granulated blast furnace slag, basic oxygen furnace slag, and electric arc furnace slag). Slag generated from the EAF process can contain toxic metals, which can be hazardous to human and environmental health. (2023). 9780309700115, National Academies Press. ISBN 9780309700115
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.
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.
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, which disrupt furnace operation)
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), Alkali metal 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 (MgO), a condition more favorable for alkali removal and refractory wear control.
However, from a thermal perspective, slag is a sterile material to melt, even if its enthalpy of fusion, around 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, which increase coke consumption in the blast furnace, have been abandoned because the 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), the sale price of granulated slag contributes less than 5% to the pig iron production cost.
Converter slag (or black slag) is produced by the Redox 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 Cementite in pig iron).
Moreover, certain slag oxides, like 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 of the slag are prohibitive in this case.
In ladle metallurgy or secondary metallurgy, tools for slag treatment typically include a "rake" to "skim" the slag floating on the liquid steel. allow the addition of products to form or Soil amendment the slag.
Steelmaking slag is generally lime-alumina for Carbon steel intended for flat products and lime-Silicon dioxide for carbon steels intended for long products. For Stainless steel, their high chromium content makes them unsuitable for use as fill, but their internal recycling within the steel mill is economically viable.
In shielded metal arc welding, the coating, when melting, creates the slag.
Electrodes are distinguished by their coating: basic (rich in lime), which is difficult to use but ensures excellent mechanical strength, or acidic (rich in Silicon dioxide), which is easier to use.
| +Optical basicity (Λ) of slag oxides ! Oxide ! Basicity | |
| Sodium oxide | 1.15 |
| Calcium oxide | 1.0 |
| Magnesium oxide | 0.78 |
| Calcium fluoride | 0.67 |
| Titanium dioxide | 0.61 |
| Alumina | 0.61 |
| MnO | 0.59 |
| 0.55 | |
| FeO | 0.51 |
| 0.48 | |
| Silicon dioxide | 0.48 |
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 Polymerization, forming long complexes. Acidic slags are thus highly Viscosity and do not readily assimilate acidic oxides present in the molten metal.
Basic oxides, such as lime (CaO) or magnesia (MgO), dissolve in an acidic slag as Ion. 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.
Neutral oxides (slightly acidic), such as wustite (FeO) or , react minimally with oxide chains.
In general, electrical conductivity and increases with basicity (i.e., with slag fluidity, promoting 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.
As a co-product of steelmaking, slag is typically produced either through the blast furnace – oxygen converter route or the electric arc furnace – ladle furnace route. 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 Fluorite.
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 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 Redox) take place within the slag., Arizona, showing the striations from the rusting corrugated sheets retaining it]]
Based on a case study at the Hopewell National Historical Site in Berks and Chester counties, Pennsylvania, US, ferrous slag usually contains lower concentration of various types of 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.
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.
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.
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 Road surface. 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.
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.
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., Solvation) or using the topological constraint theory (TCT) to account for its complex chemical network.
Dissolution of slags can produce highly alkaline groundwater with pH values above 12. The (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 Particulates. The most effective method to Detoxification alkaline ground water discharge is air sparging.
Fine slags and slag dusts generated from milling slags to be recycled into the smelting process or upcycling 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, mines, disposal sites, etc.
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