SLAG—IRON AND STEEL

As early as 350 B.C., the Greek philosopher-physician Aristotle prescribed iron slags for healing wounds. In the ensuing centuries, a number of applications for iron and steel slags were found. However, it was not until the early 20th century, when the modern processes of iron and steel production were developed, that the commercial, large-scale use of iron and steel slags became economical. The American Society of Testing and Materials (ASTM) (1999) defines blast furnace slag as “the non-metallic product consisting essentially of calcium silicates and other bases that is developed in a molten condition simultaneously with iron in a blast furnace. ” To the casual observer, one of the most striking features of an iron and steel plant is the site of a mountainous slag pile that presents a continuous display as ladle after ladle pours its incandescent load down the side in the endless accumulation of this “waste product. ” Almost a century ago, the term waste product aptly described slag. Slag was considered to be essential in the production of iron, but once it served its purpose in refining the metal, it was strictly a nuisance with little or no use. The use of slags became a common practice in Europe at the turn of the 19th century, where the incentive to make all possible use of industrial byproducts was strong and storage space for byproducts was lacking. Shortly after, many markets for slags opened in Europe, the United States, and elsewhere in the world. In the production of iron, the blast furnace (BF) is charged with iron ore, fluxing agents (usually limestone and/or dolomite), and coke as both fuel and reducing agent. The iron ore is a mixture of iron oxides, silica, and alumina. From this and the added fluxing agents (alkaline earth carbonates), molten slag and iron are formed. Oxygen in the preheated air that is blown into the furnace combines with the carbon of the coke to produce the needed heat and carbon monoxide. At the same time, the iron ore is reduced to iron, mainly through the reaction of the carbon monoxide with the iron oxide to yield carbon dioxide (CO2) and metallic iron. The fluxing agents dissociate into oxides and CO2. The oxides combine with silica and alumina to form slag. For air-cooled slag, the slag is transported into a cooling pit either directly or via iron ladles, depending on the distance between the pit and the furnace. The principal constituents of iron and steel slags are silica, alumina, calcia, and magnesia, which together make up 95% of the composition. Minor elements included are manganese, iron, and sulfur compounds, and trace amounts of several other elements are included. The physical characteristics, such as density, porosity, and particle size, are affected by the cooling rates of the slag and its chemical composition. Depending on the cooling method, three types of BF slag are produced—aircooled, expanded, and granulated. Fine particle slags also are pelletized. Allowing the molten slag to cool slowly in air in an open pit produces air-cooled slag. Air-cooled BF slag is defined in ASTM standard C-125 as “the material resulting from solidification of molten blast furnace slag under atmospheric conditions. Subsequent cooling may be accelerated by application of water to the solidified surface” (American Society for Testing and Materials, 1999). The solidified slag has a vesicular structure with closed pores. The rough vesicular texture of slag gives it a greater surface area than smoother aggregates of equal volume and provides an excellent bond with portland cement as well as high stability in asphalt mixtures. Expanded slag is formed through controlled rapid cooling of molten slag in water or in water combined with steam and compressed air. Steam and other gases enhance the porosity and vesicular nature of the slag, resulting in a lightweight aggregate suitable for use in concrete. Quenching the molten slag into glass granules by using high-pressure water jets produces granulated slag. Quenching prevents the crystallization of minerals constituting the slag composition, thus resulting in a granular, glassy aggregate. This slag is crushed, pulverized, and screened for use in various applications, particularly in cement production, owing to its pozzolanic (hydraulic cementitious) characteristics. Slags are also coproducts of steelmaking processes. Production of steel calls for the removal of excess silicon by mineralization and excess carbon (by oxidation) from pig or crude iron. Steel slag is a hard, dense material somewhat similar to air-cooled iron slag. It contains significant amounts of free steel or metal, giving it its high density and hardness, which make it particularly suitable as a road construction aggregate and raw material for portland cement manufacture. Slag is transported to processing plants, where it undergoes crushing, grinding, and screening operations to meet various use specifications. Processed slag is either shipped to its buyer for immediate use or stored to cure or to accumulate adequate stocks for large orders.