Glossary

Industrial slag is a byproduct of metal smelting, refining, and other metallurgical processes, consisting mainly of molten mixtures of metal oxides, silicates, and fluxes that form when impurities are separated from molten metals such as iron, steel, copper, or nickel. Once cooled and solidified, slag becomes a rock-like material that can be processed for use in various industrial, construction, and environmental applications.

In simple terms, slag is the waste layer that floats on top of molten metal during refining, acting as both a purifier and a protective blanket for the molten metal beneath. It’s intentionally formed by adding fluxes (like limestone, dolomite, or silica) that react with unwanted oxides and impurities in the ore or metal, creating a molten mixture that can be easily separated.

Formation and Chemistry

The chemistry of slag depends on the metal being produced and the flux materials used, but it generally includes:

- Metal oxides: such as FeO, MnO, CaO, MgO, and Al₂O₃

- Silicates: formed from reactions between silica (SiO₂) and metal oxides

- Minor constituents: including sulfides, phosphates, or residual metals

For example, in iron and steelmaking, slag forms through a series of reactions:

CaCO3 ​→ CaO + CO2​

CaO + SiO2 ​→ CaSiO3​

The calcium silicate (CaSiO₃) produced is a major component of blast furnace slag—a molten compound that floats on the molten iron and traps impurities like sulfur and phosphorus.

Types of Industrial Slag

1.Blast Furnace Slag: Produced during pig iron manufacture. It’s rich in calcium silicates and often granulated for use in cement and concrete.

2. Steelmaking Slag: Formed in basic oxygen furnaces (BOF) or electric arc furnaces (EAF); contains oxides of iron, calcium, and magnesium. Used in road construction and as a fluxing agent in secondary processes.

3. Non-Ferrous Slags: Produced from copper, nickel, lead, and zinc smelting. These often contain silicates and residual metals, requiring proper disposal or recycling.

Functions During Metal Production

- Refining: Slag removes impurities from the molten metal, including sulfur, phosphorus, and silica.

- Protection: It shields molten metal from oxidation by the air.

- Thermal Insulation: It helps maintain heat in the furnace and stabilize temperature distribution.

- Desulfurization: It absorbs sulfur from the molten bath through chemical reactions with calcium oxide and magnesium oxide.

Post-Processing and Uses

Rather than being discarded, modern industries treat slag as a valuable secondary resource. Once cooled and processed, it can be used for:

- Cement production: Granulated blast furnace slag (GBFS) is a key ingredient in Portland cement and supplementary cementitious materials (SCMs).

- Construction materials: Crushed slag is used in road base, rail ballast, and asphalt aggregate.

- Soil stabilization and reclamation: Slag’s alkaline nature can neutralize acidic soils or mine tailings.

- Metallurgical recycling: Some slags are reprocessed to recover valuable metals.