Slag

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 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 using charcoal, or 1,300 kilograms per tonne (2,900 lb/long ton) 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 CaO / SiO2 greater than 1. If the slag flows off the hook in long threads (long slag): it is viscous and acidic, with a ratio i = CaO / SiO2 < 1. However, while a basic slag removes acidic sulfur (SO2 or H2S 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 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, 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.

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 150 to 200 kilograms (330 to 440 lb) 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 O2 to oxidize carbides in pig iron).

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