SLAG—IRON AND STEEL

In the making of crude or pig iron in a blast furnace, iron oxide ores are stripped of oxygen and other impurities by means of high-temperature reactions with carbon reducing agents and fluxes. Most of the impurities and fluxing agents combine to form a liquid silicate melt called iron or blast furnace slag, which floats above the liquid crude iron and which is tapped (removed) from the blast furnace separately from the iron. The crude iron is then transferred to a steel furnace, where the iron’s residual carbon content of about 4% is reduced, generally to below 0.5%, and other impurities are removed; this process involves lime and silicate fluxes and the formation of steel slag. Steel furnaces, particularly electric arc furnaces (EAF), also may be fed with scrap iron and steel, but again the impurities are removed by fluxing agents that form a slag. Apart from the original furnace feedstock impurities, slags (especially steel slags) also may contain significant amounts of entrained free metal. The physical attributes of slags when solidified—glassy, metallic, or stony; hard and compact or vesicular—depend mainly on how the material was cooled. The cooling method also largely determines how a particular slag may be used. Slags have been used for construction purposes, especially for road surfaces, since Roman times, but with the advent of the industrial revolution, iron and steel production rose dramatically, and the volume of slag produced soon far outpaced consumption of slag. The result was a growth of unattractive slag piles on industrial land. By the mid-19th century, new uses for slags had been found, particularly as an aggregate in hydraulic cement concrete and, for some slags, as a cementitious material in its own right. Consumption remained modest, however, until the 20th century, when slag was found to be an excellent aggregate for asphaltic concrete (asphalt) road paving. This and other new uses, together with a rapidly increased use of hydraulic cement concrete worldwide, led to the consumption of most existing slag piles and current slag consumption roughly keeps pace with new slag production. Ferrous slags are now properly recognized as useful coproducts of the iron and steel industry, not waste products. Notwithstanding their utility, most slags have very low unit values compared to pig iron and steel products. Iron and steel companies, accordingly, consider the slag they produce to be a nuisance and contract with outside slag processing companies to get rid of it; the slag processing company may also be responsible for cooling the slag. Although the financial arrangements vary, typically the processing company receives the cooled slag for free, crushes it to various marketable sizes, uses screens and magnetic separators to recover entrained metal from the slag (this metal to be returned to the furnace for a low charge), sells the slag on the open market, and pays a small percentage of the net slag sales revenues or profits to the iron or steel company. Blast furnace slags are of three main types. The first, aircooled blast furnace slag, is formed by allowing the molten slag to cool relatively slowly under ambient conditions; final cooling