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.
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.
Production
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 ionic compound, not miscible with the molten metal
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.
For the steelmaker, blast furnace slag enables control of the pig iron composition (notably by removing sulfur, an undesirable element, as well as alkalis, 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), 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.
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. Typical compositions of pig iron slag (in % by weight)Slag typeFeO / MnOCaOMgOSBlast furnace (hematite pig iron)0.16–0.2< 134–3610–1238–417–1011–1.5Cupola furnace (melting furnace).0.5–2.51–225–305–1545–551–2
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). Typical compositions of primary metallurgy slag (in % by weight, at the end of refining)Slag typeFeO / Fe2O3MnOSiO2Al2O3CaOMgOP2O5SOriginal Bessemer1577530Original Thomas179153710120.5Improved Thomas10372525200.3OLP-type converter12475720LD-type converter20713482Electric arc furnace325155349Electric arc furnace with OLP treatment20–307501–2.5
Secondary metallurgy slag (or white slag) The role of secondary metallurgy slag (or white slag) 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 bricks; the adjustment of steelmaking slag is thus a compromise.
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 oxidation 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. Hoppers allow the addition of products to form or amend the slag.
Steelmaking slag is generally lime-alumina for carbon steels intended for flat products and lime-silica for carbon steels intended for long products. For stainless steels, their high chromium content makes them unsuitable for use as fill, but their internal recycling within the steel mill is economically viable. Typical compositions of secondary metallurgy slag (in % by weight, at the end of refining)Slag typeFeO / MnOCaOMgOSAluminum-deoxidized steel (desulfurizing slag)00103545100.1
Weld slag The term slag is used for the crust that forms on the weld pool when using a flux (electrode coating, 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), which is difficult to use but ensures excellent mechanical strength, or acidic (rich in silica), which is easier to use.
Physicochemistry +Optical basicity (Λ) of slag oxides Oxide Basicity1.15CaO1.0MgO0.780.670.610.61MnO0.590.55FeO0.510.480.48 When molten, slag is a solution of oxides. The most common are FeO, , , , 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.
Basic oxides, such as lime (CaO) or magnesia (MgO), dissolve in an acidic slag as 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.
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 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.
Ore smelting
In nature, iron, copper, lead, nickel, and other metals are found in impure states called ores, often oxidized and mixed in with silicates 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 (MgCO3) 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.
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 fluorspar.
Classifications
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.