Industrial uses of slag (the use and re-use of iron and steelmaking slags)

Abstract

Traditionally, iron ore has been reduced with coke in a blast furnace and the hot metal product of the blast furnace containing carbon, manganese, silicon, sulphur and phosphorous, was subsequently refined in a steelmaking furnace. During ironmaking as well as during steelmaking, significant amounts of slag are produced. Two decades ago, more than 13 million tonne (mt) of blast furnace slag and 4 mt of steelmaking slag per annum were produced in the USA alone. It is therefore not surprising that many attempts have been made to re-use iron and steelmaking slags. However, these slags are not merely metallurgical waste products but, on the contrary, their compositions and physical properties are judiciously designed to optimise the operations in the respective metallurgical reactors. The variation in composition and properties of blast furnace slags are largely determined by the gangue in the ore and ash in the coke and much progress has been made in the re-use of such slag. With respect to steelmaking slag, emphasis has in recent years been placed on hot metal treatment before refining and ladle treatment after decarburisation. The bulk removal of silicon, phosphorous and sulphur from the hot metal product of the blast furnace by specially designed slags prior to refining in a basic oxygen furnace, has reduced tap-to-tap time, lowered cost and produced steel of much higher quality. The use of synthetic slags in ladle refining techniques subsequent to decarburisation in the basic oxygen furnace has resulted in reduced hydrogen and nitrogen contents and reduced solute impurities. Moreover, inclusion shape and composition control, again by the use of specially designed slags has led to the achievement of much improved mechanical properties of the steel. In these revised process routes, slags of different compositions are produced in the variety of unit processes and hence the re-use of steelmaking slags has become much more complex. Clearly, a thorough understanding of the design and use of these different slags is a prerequisite to the development of re-use strategies. Japanese steelmakers have spearheaded attempts to reduce slag volumes in the individual unit processes and to strive towards 'slagless steelmaking' by recycling slags internally. Steelmaking slag volumes have been reduced on commercial scale from about 140 kg t⁻¹ steel to 60 kg t⁻¹ steel. Moreover, model predictions have shown that in principle, all slag can be recycled and the phosphorous extracted to produce fertiliser. This development is of particular significance since the supply of low-phosphorous iron ores is diminishing at an ever increasing rate.