Steel slag, a by-product of steel making, is produced during the separation of the molten steel from impurities in steel-making furnaces. The slag occurs as a molten liquid melt and is a complex solution of silicates and oxides that solidifies upon cooling.

Virtually all steel is now made in integrated steel plants using a version of the basic oxygen process or in specialty steel plants (mini-mills) using an electric arc furnace process. The open hearth furnace process is no longer used.

In the basic oxygen process, hot liquid blast furnace metal, scrap, and fluxes, which consist of lime (CaO) and dolomitic lime (CaO.MgO or “dolime”), are charged to a converter (furnace). A lance is lowered into the converter and high-pressure oxygen is injected. The oxygen combines with and removes the impurities in the charge. These impurities consist of carbon as gaseous carbon monoxide, and silicon, manganese, phosphorus and some iron as liquid oxides, which combine with lime and dolime to form the steel slag. At the end of the refining operation, the liquid steel is tapped (poured) into a ladle while the steel slag is retained in the vessel and subsequently tapped into a separate slag pot.

The steel slag produced during the primary stage of steel production is referred to as furnace slag or tap slag. This is the major source of steel slag aggregate. After being tapped from the furnace, the molten steel is transferred in a ladle for further refining to remove additional impurities still contained within the steel. This operation is called ladle refining because it is completed within the transfer ladle. During ladle refining, additional steel slags are generated by again adding fluxes to the ladle to melt. These slags are combined with any carryover of furnace slag and assist in absorbing deoxidation products (inclusions), heat insulation, and protection of ladle refractories. The steel slags produced at this stage of steel making are generally referred to as raker and ladle slags.

Steel Slag Uses

Steel slag is very similar to blast furnace slag, yet it is also very distinct. Both types of slags are produced as limestone is chemically converted in high temperature processes. The blast furnace slag is made in a blast furnace, and consequently steel slags are produced in steel making processes (e.g. basic oxygen furnace or in an electric arc furnace). Steel slag product names are identical to blast furnace slag products and the steel slags are also PennDOT certified.

Since steel is produced at much lower temperatures, it typically has a much different chemistry than its blast furnace counterpart. There is usually much more residual iron in steel slags, which account for its darker color. Steel slags also have a higher magnesium concentration, so care must be taken when incorporating them into the cement/concrete processes. However, they can be used just about anywhere that blast furnace slags can be used.

Environmental Benefits to Recycle Steelmaking Slag

An independent nationally renowned chemical laboratory and risk assessment team has conducted a human and ecological risk assessment of both BFS and SFS. The risk assessment scientists analyzed samples from a representative cross-section of the slag industry in accordance with the EPA’s risk assessment guidelines. The results of this study reinforced that BFS and SFS conforms to the EPA’s stringent requirements and does not pose a threat to human or plant life. As in most applications, when materials are used in environmentally sensitive areas they should be tested to assess environmental impact. It should be noted that BFS and SFS have a long history of environmentally safe application, and that the use of slag has very positive environmental benefits. The use of slag in cement manufacturing significantly decreases carbon dioxide emissions, and reduces the energy needed to calcine limestone. The use of slag as aggregate reduces the need for virgin materials and the energy use and emissions produced during the mining, processing and transportation of those materials.

Specifically slag generated from EAFs, BOFs, and BFs during the iron/steel making process -- has many important and environmentally safe uses. Not only does slag offer a superior material for many construction, industrial, agricultural, and residential applications, but the use of slag promotes the conservation of natural resources. As a result, the market for slag has remained strong and, as further applications are promoted, is expected to grow. The existence of this market and the broad variety of potential uses offered by steelmaking slag demonstrate that it is clearly a safe, useful and valuable product and not a "solid waste."

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In addition to slag recovery, the liquid furnace slag and ladle slags are generally processed to recover the ferrous metals. This metals recovery operation (using magnetic separator on conveyor and/or crane electromagnet) is important to the steelmaker as the metals can then be reused within the steel plant as blast furnace feed material for the production of iron.

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Steel slag aggregates generally exhibit a propensity to expand. This is because of the presence of free lime and magnesium oxides that have not reacted with the silicate structures and that can hydrate and expand in humid environments. This potentially expansive nature (volume changes of up to 10 percent or more attributable to the hydration of calcium and magnesium oxides) could cause difficulties with products containing steel slag, and is one reason why steel slag aggregates are not suitable for use in Portland cement concrete or as compacted fill beneath concrete slabs.

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