Slag handling is not heavily documented, mostly because it is not a process requiring a high degree of technology. After cooling to ambient temperature, slag is processed similar to bulk aggregate (sizing, separation, and transport). Other than storage requirements to achieve stabilization, limited literature exists detailing the physical handling of slag from the furnace to end-use. Steel producers have not documented the process in detail, as the most common business approach is to subcontract slag handling and processing to a specialist handling company.
2.2.1. Physical Handling. Physical handling of slag is carried out similar to
gravel aggregate. One slag processor (MultiServ) lists the handling steps on their
website.6 In their business model the contractor takes possession of the slag as it is tapped from the furnace (under furnace removal) into transport pots. The contractor transports the slag to a pour pit or pad where it is dumped and allowed to cool naturally or by spray quenching. The cooled slag is dug out of the pit with a front loader and brought to the processing station.
2.2.2. Slag Processing. Slag processing typically consists of crushing, screening,
and magnetic separation.6,9 Magnetic separation may be followed by screening. The oversized material may be crushed and sent back through the process again. Magnetic recovered material is sold back to the steel mill for furnace feed. Van Oss3 estimates that up to 50% of the slag volume is recovered as magnetic (entrained metal) for return to the furnace. The non-magnetic material is graded by size, and stockpiled for sale.
2.2.3. Slag Stabilization. The end use destination will dictate storage time in the
stockpile. This is because slag must go through an ageing period to reach stabilization. During ageing, compounds in the slag (initially free lime) will react with water and carbon dioxide from the air to form hydroxides and carbonates. Formation of these compounds results in a volume change or “swelling” of the slag, and converts these compounds to a more stable form. Stabilization in a stockpile prevents end use in-situ swelling or leaching. Details on the swelling mechanism are discussed in subsequent sections.
Slag swelling is a concern when the slag is used as construction aggregate. Crawford and Burns document the case of an office building on the Canadian side of the
international bridge at Sault Ste. Marie, Michigan that experienced wall buckling and floor heave due to swelling of steel slag foundation fill.23 This was open hearth slag that generated 9% vertical lift on the building floor slab upon in-situ swelling. To prevent in- situ swelling, many state governments now require steelmaking slag to be aged before use. For example, the Missouri Department of Transportation requires slag used in asphaltic concrete to be aged for at least three months after crushing and screening7, while the Pennsylvania Department of Transportation requires slag to be tested for expansion after six months stockpile curing.16
An additional problem that may occur with the use of raw slag is leaching of alkaline materials into the surface water. While testing on slag shows the leachate
reporting to the surface water is non-hazardous18, it may be perceived as a problem by the public. For example, slag used in the Cleveland airport runway produced a “milky white, sulfuric runoff”.8 While analysis showed that free lime leaching into the surface water precipitated calcium carbonate (milky white), which is stable and non-hazardous, the perception led to a project delay and negative publicity for the use of slag in construction.
2.3. USES
The National Slag Association (NSA) employs promotional and research efforts to identify and develop innovative applications for steel slag’s unique chemical and/or physical properties. The key uses the NSA have identified are as a source of iron and flux materials into blast furnace operations, high quality mineral aggregate, Portland cement, unconfined construction applications, soil conditioning, and environmental pH
neutralization of abandoned mines and contaminated sites.10,11 In 2003, the U.S.
Geological survey listed the breakdown of use as shown in Figure 2.1, which supports these same categories.3 High quality mineral aggregate (asphaltic concrete, road bases, and surfaces) account for more than 63% of the steel slag sold. The next largest use is unconfined construction (fill), which accounts for approximately 12-13%, followed by clinker feed at 5.4%. The “other” category accounts for railroad ballast, roofing, mineral wool, and soil conditioning. Unspecified use is not detailed but probably accounts for recycling into iron making and environmental neutralization.
Figure 2.1 Sales of Steel Slag in the United States in 2003, by use3
2.3.1. Recycling Into Blast Furnace. Steelmaking slag finds use in recycling
into the blast furnace, both for recovery of iron content and for fluxing properties.9,11 The amount that can be recycled is limited by the chemical composition, specifically
phosphorus and sulfur content.170 As the lime and magnesia are in calcined form, a reduction in energy consumption may be realized compared to conventional flux charge.9
2.3.2. High Quality Mineral Aggregate. The physical characteristics of steel
slag provide road surfaces with high stability, rutting resistance, skid resistance, and long life.12 Steel slag qualifies as a premier aggregate for Hot Mix Asphalt (HMA),
“Superpave”, and Stone Matrix Asphalt (SMA) applications. Steel slag used in asphaltic concrete is less susceptible to swelling, as the slag particles are coated with a water resistant barrier (asphalt). Steel slag in asphaltic concrete mixes results in 1.5 to 3 times greater stability, 20-80% higher resilient modulus (@ 29°C), and lower resilient
Poisson’s ratio.13 The acceptance of steel slag for these applications has led to the generation of ASTM standard D5106-03 Standard Specification for Steel Slag Aggregates for Bituminous Paving Mixtures.1
Asphaltic Concrete 17.0% Road Base & Surface 46.4% Unspecified 17.6% Fill 11.1% Other 2.5% Clinker 5.4%
2.3.3. Portland Cement. Steel slag is similar enough in chemical composition
and character to Portland cement raw materials that it can be used as a cost effective feed additive.10 The beneficial features of steel slag result from exposure to high furnace temperatures thus eliminating heat of decarbonation, reducing CO2 and volatile
emissions, and reducing total raw material moisture contents when used in clinker feedstock. Production increases with moderate additions (8-11%) of steel slag can be almost proportionate to the amount utilized.10 An improved slag for clinker use can be generated by first oxidizing the FeO in the slag to Fe2O3, then quenching with water.170
2.3.4. Unconfined Construction. Steel slag finds excellent application as an
aggregate in unconfined construction resulting in very high stability. Unpaved shoulders, bases, or parking lots, erosion control, and railroad ballast all find improved life and stability compared to standard mineral aggregates.10,11 As the slag may expand with time, confined applications such as bases under pavements and structures are generally avoided unless the slag is stabilized prior to use.
2.3.5. Soil Conditioning. Steel slag utilization for agricultural purposes was first
introduced in North America in 1916 by United States Steel.9,10 Used first as a lime and phosphate amendment, other trace minerals such as Fe, Mn, Mg, Zn, and Mo have been shown to provide beneficial crop growth. Traditionally, use has been for corn and soybean crops, but sugar cane and rice crops are now being treated.
2.3.6. Environmental pH Neutralization. The free lime and magnesia content
of steel slag has led to the investigation of steel slag as a pH neutralizer for acid
containing wastes. Specifically steel slag has proven beneficial use in acid mine drainage where it serves as a fill material, bulk filter, and pH neutralizer. Research by the National Mine Reclamation Center in West Virginia on several Appalachian coal mines and
Canadian pyrite mines, show that additions of steel slag is a more effective acid discharge neutralizer than lime or limestone, as it provides pH control during a longer period of time, and has no hazardous leachate by products.14, 15