Slag, Metallurgical

a melt (after solidifying, a rocklike or glasslike substance) usually covering the surface of molten metal in metallurgical processes, as the smelting of raw material, the processing of molten intermediates, and the refining of metals. Slags are alloys of oxides of various compositions. The major components of slags are the acidic oxide SiO₂ and the basic oxides CaO, FeO, and MgO, as well as the neutral Al₂O₃ and (less often) ZnO. Slags are classified as acidic or basic depending on the predominant oxides. They play an important role in the physicochemical processes of metallurgical production: they remove undesirable impurities in metals and protect metals from the harmful effects of the gaseous medium in the furnace (that is, from oxidation and gas saturation). Slags are formed from waste rock of ores (or ore materials), fluxes, combustion ash, oxidation products of processed materials, and the lining of smelting units. In steel smelting, slags are sometimes prepared in special furnaces (synthetic slags) and then used to process steel upon its removal from the furnace or converter to the ladle. Previously prepared slags are used in the electroslag remelting of metals.

In ferrous metallurgy, slags are usually by-products. They are secondary raw materials for the production of building materials (such as slag sitalls) and lime and phosphorus fertilizers; they are also used as a recycle in metallurgy. Blast-furnace slag is used to prepare granulated slag (used in the production of cement and slag blocks), pumice (a lightweight aggregate in concrete), gravel, rock castings, and slag wool. Steel-smelting slag is used in the production of gravel for road construction; in sintering blast furnaces and cupola furnaces it is used to extract the manganese, iron, and calcium oxide present in the slag. Ferroalloy slags are processed into ground limestone, gravel, and slag sand for construction as well as a powder used in foundry production; a portion of such slags is used in the production of alloys. Special slags used in the production of ferroalloys are produced by the integrated processing of ores and pig iron. Oxidation is an efficient method of transferring vanadium, manganese, niobium, and other metals from naturally alloyed pig iron into slag; the slag is then used for the production of such metals or their alloys.

In nonferrous metallurgy, a distinction is made between reduction slags and waste slags. Reduction slags contain a higher content of valuable metals and are formed as a by-product in metal production or refining. For example, converter slags are a byproduct of nickel and copper production, and anode slime is a byproduct of copper refining. Reduction slag may be the major product of a production process in which an extracted metal is concentrated, for example, titanium and vanadium slag. Slag yields are extremely high in nonferrous metallurgy. Up to 100–120 tons of slag is produced per ton of metal in the processing of poor ores. Thus, even a small amount of the nonferrous metal in the slag leads to significant losses. Metals may be extracted from reduction slags by recycling the slags to one of the primary production stages or by subjecting them to special processing, such as fuming, electrosmelting with the addition of a reducing agent and a sulfidizer, and flotation. Metal oxides not suitable for extraction in metallurgical processing are concentrated in waste slags, as are various admixtures and small amounts of valuable metals, the extraction of which is not economically feasible at the present level of technology. Waste slags are used partially for the production of slag wool and other building materials.

The complete and integrated use of slags ensures a waste-free production process and less pollution of the environment.