Steelmaking is very aptly touted as the art of making the right slag. But, once the steel is produced, the leftover slag has to be taken care of.
A slag is a mixture of oxides, mainly of silicon, aluminum, calcium, magnesium, iron, among traces of others. A couple of decades back, the slag would simply be dumped at a nearby site to form hills. This is not the case anymore as slag from steel industry now finds a variety of applications
Primarily, two slag types are formed in the steel industry-
- The blast furnace slag formed in the blast furnace along with hot metal, contains the impurities separated out from the iron ore;
- The steel slag formed in an LD converter or an Electric Arc Furnace during refining of the hot metal with oxygen to form steel
The role of the flux, which always is lime (CaO) with or without dolomite (MgO), is to fix itself with these impure oxides and form the diffusible slag. In addition to these slags, small quantities of ladle slag may be produced during alloy steelmaking.
For every tonne of hot metal tapped from a blast furnace, 350-500 kgs of slag is produced. On the other hand, in steelmaking, 150-200 kgs of slag is generated per tonne of liquid steel. However, the compositions of the two slags are markedly different, affecting their behavior, and therefore, applicability a great deal.
The BF Slag
All of the iron making slag generated today can be utilized. Based on application, the slag is treated in one of the following two ways after exiting the BF.
Quenching: The slag is poured in a stream of water which cools it immediately. The water is then filtered out to obtain slag granules. Here, unlike air cooling, the slag doesn’t solidify to form crystals. This slag is ground to required size to obtain ‘ground granulated blast furnace slag’ (GGBS). GGBS is used in manufacturing of Portland Slag Cement (PSC).
The slag is dumped in the open, allowing it to cool naturally. It is used in the manufacture of glass. In developed countries, both the types of BF slags find widespread use in the construction industry for making concrete, as aggregates, asphalt production, road construction and the like.
A brief comparison of the slag compositions earlier listed would instantly point out the same constituents, only in varying proportions. However, this variation narrows the versatility of the steel slag. The main culprits are the excessive free & un-reacted oxides of calcium and magnesium present in the slag which react with any moisture in the air or with water and swell, thus leading to volumetric instability and ruling out any possibility of consumption in construction activities. This excessive lime is a metallurgical requirement for producing steel. The cement industry does not prefer it either as the silica content is halved along with the additional presence of FeO, which makes it difficult to grind.
Steel slags can neutralize acidic soils and be used for water treatment. Many steel plants recycle the steel slag in blast furnaces and in sintering units for its lime content, compensating partly for the requirement of lime.