Steel slag, a by-product of steelmaking, is produced by separating molten steel from impurities in steel furnaces. The slag changes as a molten liquid melt and is a complex solution of silicates and oxides, which solidifies upon cooling.
Today, virtually all steel is produced in integrated steel mills using basic oxygen process versions or in special steel mills (mini-mills) using an electric arc process. The open furnace process is no longer used. In the basic oxygen process, the flammable liquid blast furnace metal, scrap and fluxes that survive from lime (CaO) and dolomitic lime (CaO.MgO or “dolime”) are fed to a converter (kiln). A nozzle is started in the converters and high-pressure oxygen is injected. Oxygen combines impurities in the charge and removes them. These impurities remain from coal as gaseous carbon monoxide and silicon, manganese, phosphorus and parts of iron as liquid oxides, which combine with lime and lime to form steel. At the end of the refining operation, a liquid steel plug is poured into the ladle, while the steel slag is retained in the vessel and then poured into a separate slag vessel.