Steelmaking slag is a liquid product generated during the production of steel. It can be categorized as carbon steel slag, or stainless steel slag according to the type of steel produced, or as primary steelmaking slag such as basic oxygen furnace slag, electrical furnace (electric arc furnace, and induction furnace) slag, secondary steelmaking slag, or continuous casting slag according to the steelmaking process.
Steelmaking slag is defined as the solid material resulting from the interaction of flux and impurities during the smelting and refining of steels. The American Society for Testing Materials (ASTM) defines steelmaking slag as ‘a non-metallic product, consisting essentially of calcium silicates and ferrites combined with fused oxides of iron, aluminum, manganese, calcium and magnesium, that is developed simultaneously with steel in basic oxygen, electric arc, or open hearth furnaces’.
Steelmaking slag is a by-product of steelmaking. It is produced during the separation of the liquid steel from impurities in steelmaking furnaces. The slag occurs during steelmaking in liquid state. It is a complex solution of silicates and oxides that solidifies during cooling. The chemical as well as phase composition of steelmaking slag is very variable due to the different facilities in which they originate and due to variability of produced steel grades. The portion of crystalline phases and glass phase also depends on cooling speed of liquid slag. The production, processing, and the main uses of steelmaking slag are shown in Fig 1.
Fig 1 Production processing and uses of steelmaking slag
The utilization of the steelmaking slag is closely related to its chemical and physical characteristics. Compared to blast furnace slags, the steelmaking slags normally contain much higher amounts of iron and manganese, are lower in silica and can be higher in lime producing a much higher lime-silica ratio, and normally quite low sulphur contents. This lime precipitates out during cooling and exists as free CaO, which then forms Ca(OH)2 in the presence of water. Since free CaO expands, steam aging is normally performed at a steam aging plant, to saturate free CaO to Ca(OH)2 in advance and thereby prevent expansion.
Since steelmaking slag when it is formed, it is in a liquid or red-hot state at a temperature range of 1,300 deg C to 1,700 deg C, the slag is to be immediately subjected to the cooling process upon removal from the furnace. Normally, this is done in a cooling yard by air cooling and moderate water sprinkling. This method requires a considerable amount of time to cool the hot slag down to a workable temperature and requires the allocation of a spacious yard. Hence, a number of more efficient cooling processes have been developed and put into practical use.
The other cooling processes include (i) the air granulation process, whereby a high pressure gas is blown onto the liquid slag to solidify and granulate the slag while it is being cooled, (ii) ISC (instantaneous slag chill) process, whereby the hot slag is first poured into a steel box for accelerated cooling/solidification and then subjected to water sprinkling and immersion cooling, and (iii) the rapid cooling process, whereby the liquid slag is poured into a special drum and cooled rapidly by sprinkling water over the slag. In addition, one more process is under development. In this process, the liquid slag is poured between twin-rolls and cooled very rapidly. There is the possibility that this process can allow for effective control of slag solidification and provide for the recovery of a large portion of the heat loss in the cooling process.
Steelmaking slag contains around 10 % to 40 % of metal iron (excluding iron oxides), which is derived from the refining process or from some processing vessel (e.g., the converter, hot metal / liquid steel ladle, or tundish). It is advantageous to separate this iron from the slag for the purpose of using the iron recovered as a substitute for scrap iron and / or making effective use of the slag for applications where iron is an impurity. Hence, after cooling, the steelmaking slag is crushed and the iron is recovered by magnetic separation. In general, with the aim of increasing the metal iron recovery ratio and improving the quality of metal iron recovered, the above crushing and magnetic separation process is repeated several times, as shown in Fig 2.
Fig 2 Flowsheet of processing of steelmaking slag
For commercial use of the produced steelmaking slag, it is necessary to process it to the grain size specified by the customer. Hence, the slag is adjusted to the customer-specified grain size through a process of crushing and classification. In the real practice, as shown Fig 2, an efficient process using crushers, magnetic separators, and screens is designed in which these equipments are arranged in such a way so as to allow for simultaneous recovery of metal (scrap) and adjustment of the slag grain size. Steelmaking slag is shown in Fig 3.
Fig 3 Steelmaking slag
Since the flux added to refine the steel forms a component of the steelmaking slag. It is particular significance that CaO and MgO contained in the flux remain partly inactive and reside in the slag or crystallize while the slag is solidified and cooled. These inactive materials, referred to as free-CaO and free-MgO, react with water, and, in the process, their volume nearly doubles through hydration. As a measure to prevent this, the steelmaking slag is subjected to the aging treatment process, whereby the hydration reactions are completed by the time the slag is dispatched. The hydration reactions are CaO + H2O = Ca (OH)2 and MgO + H2O = Mg (OH)2.
There are two representative methods of aging treatment. The first is the normal aging treatment, whereby the hydration reaction is allowed to take place by natural rainfall on the cooling yard, and accelerated aging treatment, whereby the hydration reaction is induced to complete in a shorter period of time. The accelerated aging treatment processes which have been developed and put into practical use include ‘steam aging treatment’ which uses high-temperature steam, ‘hot water aging treatment’ which immerses the slag in hot water, and ‘high pressure aging treatment’ which causes the slag to react with steam in a vessel under high pressure (e.g., 0.6 – 1 MPa). In recent years, steam aging treatment or high pressure aging treatment is often applied to steelmaking slag before its dispatch to ensure moderate, uniform expansion of the slag, especially when the slag is to be used (for example, for road base course material) where expansion can become a major issue.
Physical and chemical characteristics of steel slag
Physically, the steelmaking slags are much heavier, harder, denser and less vesicular in nature, and have unusually high resistance to polishing and wear. Processed steelmaking slag is strong, hard, durable, dense and roughly cubical particles which make it especially suitable for use in road construction. Not all steelmaking slag is expansive, but water quenching is the first step in weathering of steelmaking slag which is essential to provide a stable, non-expansive construction material. In the cooling process, some of the lime in the slag can be ‘hard burnt’. The lime has a hard outer shell or surface which masks the unsound soft inner core of unhydrated lime. This lime is to be saturated by water for the particle to be stable
The density of steelmaking slag lies between 3.3 to 3.6 tons/cum. The bulk density of the steelmaking slag is in the range of 1.6 tons/cum to 1.9 tons/cum. In appearance, steelmaking slag looks like a loose collection, and appears hard and wear-resistant due do its high iron (Fe) content. The grindability index of steel slag is 0.7 in contrast with the value of 0.96 and 1 for blast furnace slag and standard sand respectively.
The cooling rate of steel making slag is sufficiently low so that crystalline compounds are generally formed. The relative proportions of these compounds depend on the steelmaking practice and the steel slag cooling rate. The steelmaking slag mainly consists of SiO2, CaO, Fe2O3, FeO, Al2O3, MgO, MnO, and P2O5. The chemical components of steelmaking slag varies with the furnace type, steel grades and pretreatment method. The steelmaking slag is a high basicity material, since some of the lime which is charged during the steelmaking remains in an undissolved form. The chemical ranges of BOF (basic oxygen furnace) and EAF (electric arc furnace) slag are given in Tab 1.
|Tab 1 Composition range of steelmaking slag|
|Sl. No.||Oxide name||Formula||Unit||Value|
Like blast furnace slag, steelmaking slag oxides are combined when cooled to form various forms of minerals common to both lime and silica based materials. Normally, steelmaking slags are mainly composed of calcium silicates, calcium alumino-ferrites, and fused oxides of calcium, iron, magnesium, and manganese. The compositions vary with type of furnace, composition of furnace charges, grades of steel produced, and with individual furnace operating practices. Materials added to the melt just before the end of a heat may not be completely incorporated in the slag. Hence, some free oxides, including CaO, can be found in some slags.
The common mineral phases present in steel slags include merwinite (3CaO·MgO·2SiO2), olivine (2MgO·2FeO·SiO2), di-calcium silicate (2CaO·SiO2) both beta-C2S , and alpha-C2S, tri calcium silicate C3S (3CaO·SiO2), tetra calcium aluminoferrite C4AF (4CaO·Al2O3·FeO3), di-calcium-ferrite C2F (2CaO·Fe2O3), the RO phase (a solid solution of CaO-FeO-MnO-MgO), akermanite, gehlenite, cristobalite, CaO (free lime), periclase, sulphides, and others. Since BOF and EAF slags both have high iron oxide contents, solid solutions of FeO (wustite) are typically observed as one of the main mineral phases. Ladle slag has a lower FeO content, and polymorphs of C2S are therefore frequently observed as the main phase.
Due to the presence of unstable phases in its mineralogy, steelmaking slags can show volumetric instability, caused mainly by the presence of free CaO. In the presence of water, free lime hydrates and forms portlandite [Ca(OH)2]. Portlandite has a lower density than CaO and, hence, hydration of free CaO results in volume increase.
Tab 2 links the characteristics of the steelmaking slag with its applications.
|Tab 2 Characteristics and applications of the steelmaking slag|
|1||Hard, wear-resistant, adhesive, rough||Aggregates for road and hydraulic construction|
|2||Porous, alkaline||Waste water treatment|
|3||FeOx, Fe components||Iron reclamation|
|4||CaO,MgO,FeO,MgO,MnO components||Fluxing agent|
|5||Cementitious components (C3S, C2S and C4AF)||Cement and concrete production|
|6||CaO, MgO components||CO2 capture and flue gas desulphurization|
|7||FeO, CaO, SiO2 components||Raw material for cement clinker|
|8||Fertilizer components (CaO, SiO2, MgO, FeO)||Fertilizer and soil improvement|
Mechanical properties – Processed steelmaking slag has favourable mechanical properties for aggregate use. It has good abrasion resistance, good soundness characteristics, and high bearing strength. Some typical mechanical properties of steelmaking slag are given in Tab 3.
|Tab 3 Typical mechanical properties of steelmaking slag|
|1||Los Angeles abrasion||%||20-25|
|2||Sodium sulphate soundness loss||%||Less than 12|
|3||Angle of internal friction||Degrees||40-50|
|4||Hardness (measured by Moh’s scale of mineral hardness)*||6-7|
|5||California Bearing Ratio (CBR), top size 19 mm**||%||upto 300|
|*Hardness of dolomite measured on same scale is 3 to 4|
|**Typical CBR value for crushed limestone is 100 %|
Thermal properties – Due to the high heat capacity, steel making slag aggregates have been observed to retain heat considerably longer than conventional natural aggregates. The heat retention characteristics of steelmaking slag aggregates can be advantageous in hot mix asphalt repair work.
Issues connected with steelmaking slag
Expansion of steelmaking slag – Lime (CaO) and other minerals are used as the flux for steel refining. Since the melting point of pure lime is as high as 2,572 deg C, it does not melt during the steelmaking in BOF or EAF (at roughly 1,300 deg C to 1,700 deg C). For this reason, despite slag composition design to lower the melting point by having it form compounds with other elements, some of it may remain in the slag unreacted or as precipitate during slag cooling. Such unreacted and not melted CaO is called free CaO. When free CaO comes in contact with water, it hydrolyzes, and its volume duplicates. When free CaO is there in the slag used for road base course, it undergoes hydrolysis and expands. This can lead to bulging or cracking of the pavement. Since steelmaking slag contains free CaO by some percent, it is necessary to immediately carry out after producing, the aging treatment so that the hydration reaction of CaO is completed before slag is dispatched to the users.
Alkalinity – Unlike lime, steel making slag does not absorb CO2 from the air and convert back to relatively insoluble limestone. This is an important property. Because of this property steelmaking slag, when left outside exposed to the atmosphere for years, achieves high levels of alkalinity upon dissolution.
Because of the CaO content, water leaching from steelmaking slag after the hydrolysis reaction shows a pH of 10 to 12.5, which is the same alkalinity as that of road bed material of recycled concrete or cement-stabilization soil. As slag is a coarse glass, it maintains high permeability (around 0.045 cm/sec) regardless of how much water has passed through it. The permeability of the slag is reduced when it is compacted or grounded into smaller particles. The neutralization potential of the steelmaking slag is in the range of 45 % to 78 %
In case, when the soil is acidic, then even when alkali contents are leached from slag, they get absorbed in soil and neutralized as far as the slag use is adequate, and thus there no environmental issue exist, However, when slag is used for civil engineering purposes in very large amounts locally, the slag can constitute a significant alkali source such as in the case of using recycled waste concrete. However, considering such risk, and to put in efforts to comply with environmental regulations, the normal measures which are taken are (i) the suitability of slag use is judged where necessary through field investigation (on topography, alkali adsorption capability, and water permeability of the soil, underground water vein at the site and surrounding areas, etc. (ii) development of pH simulation technology to evaluate the risk of alkali leaching out and diffusion by considering the soil quality and topographic conditions, and (iii) the development of methods for suppressing alkali leaching in ground and marine uses.
Heavy metal content – The steelmaking slag can contain elements like sulphur, selenium, carbon, cadmium, lead, copper, and mercury. Many of the residuals are encased within a glassy matrix.
Application of steelmaking slag
Typical applications of steelmaking slag include sealing aggregate (skid resistant), asphalt aggregate, base, sub-base, construction fills, subsoil drains, grit blasting and waste water treatment.
Reclamation of steel scrap – Steelmaking slag contains around 10 % steel scrap, which can be reclaimed through crushing, sorting, magnetic separation and screening process and can be recycled.
Utilization in the sintering process – Steelmaking slag with CaO content normally above 50 % can be used as flux in the sintering process, partially replacing the lime and limestone. The slag addition can improve the quality, reduce fuel consumption due to the heat liberation of Fe and FeO oxidation reaction, and decrease the cost of sinter ore.
Phosphorus removal – Phosphorus from agricultural wastewater is one of the major pollutants in natural water which cause the algae growth and eutrophication of lakes. Hence, phosphorus is one of the major nutrients which are needed to be removed from domestic wastewater before being discharged into water bodies. In order to remove phosphorus from wastewater, various methods or approaches have been attempted, including biological, chemical process (precipitation, metal salt addition) and physical (electro-dialysis, reverse osmosis). Among all the methods, the chemical precipitation is expensive and increasing sludge volume by upto 40 %. The biological phosphorus removal needs a lot more volume or space (anaerobic unit) than the other processes.
The P removal is along with the development of constructed wetlands. This method is a low cost technology for pollutions treatment, which slag performs an important role in absorbing impurities especially phosphorus. By comparing most of industrial by-products, steelmaking slag is a low-cost and abundant material, which its combination with small secondary treatment systems (such as constructed wetlands) is preferred in compared to the others methods. The steelmaking slag contains various metal oxides such as iron oxides and alumina which can be effective in phosphorus reduction from domestic, agricultural, and municipal effluents. Steelmaking slag is a light weight porous medium with numerous sites for adsorption. Steelmaking slags have different physico-chemical property due to various feedstock ores, fluxes and manufacturing process. These differences cause a board range of phosphorus adsorption capacity by using slag, ranging from 76.4 mg/kg to 8390 mg/kg.
Utilization for hot metal dephosphorization – Nippon Steel Corporation has developed the MURC (Multi-Refining Converter) process, in which de-phosphorization and de-carbonization are conducted in the same converter. In this process, around 50 % of de-carbonization slag stays in the converter after the de-carbonization process and hot recycled as de-phosphorization and de-siliconization slag of the next charge, resulting in increased de-phosphorization efficiency and decreased CaO consumption. Sumitomo Metal Industries, Ltd. has developed a duplex process, called SRP (Simple Refining Process), in which de-phosphorization and de-carbonization tasks are assigned to be carried out by two converters. In this process, de-carbonization slag and converter slag are recycled and applied to dephosphorization.
Utilization for road and hydraulic construction – Steelmaking slag, due to its high strength and durability, can be processed to aggregates of high quality comparable with those of natural aggregates. The high bulk density, the high level of strength and abrasion as well as the rough texture qualify steelmaking slag as a construction material for hydraulic engineering purposes.
In Japan artificial reefs for sea wood/coral breeding (marine block) have been constructed using carbonated steel slag. The artificial reefs show a high stability in seawater due to the fact that it consists of CaCO3, like shells and coral, and they act as great breeding habitats for seaweeds and coral.
Also, based on high level of strength, high binder adhesion as well as high frictional and abrasion resistance, steel slag can be used as an aggregate not only in surface layers of the pavement but also in unbound bases and sub-bases, especially in asphaltic surface layers. Around 60 % of slag is used for road engineering in Japan and European countries, and even 98 % of that is utilized as aggregates of cement and bituminous pavement in UK.
The influence of the utilization of steel slag as a coarse aggregate on the properties of hot mix asphalt has been shown in a study. The results of the study have shown that steelmaking slag when used as a coarse aggregate has improved the mechanical properties of asphalt mixtures. Further, volume resistivity values have demonstrated that the electrical conductivity of the steelmaking slag asphalt mixtures has been better than that of limestone asphalt mixtures. It has been observed that asphalt concrete mixes containing 30 % of the steelmaking slag has the highest skid number compared to the competing materials.
Another study has evaluated the effectiveness of steel slag as a substitute for virgin aggregates on mechanical properties of cold mix recycling asphalt pavement. The results has shown that the use of steelmaking slag can enhance Marshall stability, resilient modulus, tensile strength, resistance to moisture damage and resistance to permanent deformation of CIR (Cold In Place Recycling) mixes.
Volume instability and heavy metal leaching are two considerable unsafe factors for steelmaking slag using as aggregates in road and hydraulic engineering. In contact with water, free CaO and MgO in steel slag react to hydroxides. Depending on the rate of free lime and/or free MgO this reaction causes a volume increase of the slag mostly combined with a disintegration of the slag pieces and a loss of strength. Hence, the volume stability is a key criterion for using steelmaking slags as a construction material.
The immersion expansion ratio is limited to 2 % for road construction according to the specifications of relative standards. In Germany, experience has found that steel slags with a free lime content upto 7 % can be used in unbound layers and upto 4 % in asphaltic layers. The heavy metal leaching is mostly related to the stainless steel slag since it contains a higher amount of chromium (Cr) and nickel (Ni) than the ordinary steelmaking slag. In Germany steel slag processed to aggregates for road construction and hydraulic structures have to be analyzed by leaching tests twice a year. The concentration of Cr total is limited to 3 mg/litre.
Utilization as railroad ballast – The specification for steelmaking slag for use as railroad ballast has the requirements which are identical to those for other aggregate, except imposing of a minimum unit weight.
Utilization for production of cement and concrete – In cement and concrete industries, slag can be used either as an aggregate or binder in stabilized base courses. In order to conserve natural resources and reduce environmental impact, slags can be used as an aggregate. To lessen the need for cement which is expensive in cost, slag is replaced and used as a binder.
Addition of slag to cement presents some important technical advantages over ordinary Portland cements. These benefits can be enumerated as development of mechanical strengths, low solubility of the hydrates and porosity, lower heat of hydration, excellent durability and stronger aggregate–matrix interface. However, there are some disadvantages such as high shrinkage, formation of micro-cracks and rapid setting.
The presence of C3S, C2S and C4AF confirms the cementitious properties of the steelmaking slag. It is normally agreed that the cementitious properties of steelmaking slag increases with its basicity. Hence, the steelmaking slag ground into fine powder can be used as cement additives and concrete admixtures.
A study has used the mixture of fly ash, steel slag powder, and cement clinker to prepare composite cement and it has been observed that a certain amount of steelmaking slag admixture in cement can reduce the porosity, improve pore distribution and increase the consistency of cement.
It has been found that concrete mixed with steelmaking slag has a compressive strength of 100 MPa and an excellent anti-chloride ion penetration performance. It has been observed that the ground EAF slag addition in concrete has performed an excellent water-reducing effect. However, it is noticed that the beta-C2S and C3S formed in BOF slag have lesser activity compared with those in cement clinker due to their large crystal size and the amount of cementitious minerals in BOF slag is much lower relative to that in cement clinker, sometimes beta-C2S, which is having no cementitious activity, is a predominant mineral of BOF slag. All these mentioned factors lead to a low cementitious activity of BOF slag. Hence the blended cements with BOF slags normally present low strengths, especially for early strengths. An experiment studying the effect of accelerators on the early strength of steelmaking slag cementitious materials has found that a combination of inorganic and organic early strength agent can improve the early strength but no influence on the 28 days compressive strength.
Since the steelmaking slag is hard and has a low grindability index, it is necessary that the crushing and grinding is carried out in highly efficient and energy-saving crushing and grinding equipment.
Steelmaking slag can also be used an aggregate for high-strength and refractory concrete. The compressive strength of concrete using steelmaking slag as fine aggregates is 1.1 to 1.3 times of the common concrete. A high-strength (higher than 70 MPa) concrete utilizing EAF slag as aggregates has been produced. A study has shown that when the steelmaking slag is heated upto a temperature of 1000 deg C prior to its use for refractory concrete, the final product shows mechanical properties which are comparable to concrete with conventional refractory aggregate. Another study has shown that that the steelmaking slag concrete shows similar fire resistance to the river aggregate mixture upto 400 deg C, and much improved fire resistance at high-temperature ranges.
Utilization for other construction materials – Experiments carried out for the application of the steelmaking slag for preparing glass ceramics have shown that steelmaking slag is a potential material for ceramic production. An appropriate addition of steelmaking slag can reduce the firing temperature needed of clay bricks. Steelmaking slag can also be used to produce coloured pavement bricks and tiles.
Utilization for materials of waste water treatment – As the steelmaking slag has good sportive characteristics and is low cost, it is widely used in wastewater and water treatment, and it is an alternative of using granular activated carbon. Steelmaking slag has porous structure and large surface area. In addition, it is easy to separate from water due to its high density. Hence, the application of steelmaking slag in industrial waste water treatment has received high attention in recent years.
For the adsorptions of dye, heavy metals and organics, the uptake capacity of slag is dependent on the pH solution. The hydration of slag composition in the aqueous solutions provides a high pH. With the high pH condition, the slag surface is negatively charged and adsorption metal ions especially cations are preferred. However, heavy metal absorption by using steelmaking slag takes place with a condition of either high temperature or low pH under certain conditions.
In a study which has been carried out to investigate the treatment of mercury-containing sea water with steelmaking slag, the high adsorption capacity of steelmaking slag for mercury has been observed. In another study, the steelmaking slag has been used as a low-cost adsorbent for arsenic in aqueous system. The study has shown 95 % to 100 % removal efficiency near initial pH of 2. The removal mechanism included the co-precipitation and adsorption of CaCO3.
The removal mechanism of copper using steelmaking slag has been investigated and the results show that the major mechanisms are adsorption and precipitation. In addition, steelmaking slag as a separated adsorbent can be used to remove aqueous ammonium nitrogen, phosphorous, and phenol. The combined use of steelmaking slag and hydrogen peroxide (H2O2) can decompose organic pollutions due to the ferrous ion produced from FeO in steelmaking slag reacting with hydrogen peroxide to form Fenton’s reagent which has strong oxidation. Fenton’s reagent is a solution of hydrogen peroxide with ferrous iron (typically iron sulphate, FeSO4) as a catalyst that is used to oxidize contaminants or waste waters. Steelmaking slag can also be used as raw material for coagulant preparation.
Application in CO2 capture and flue gas desulphurization – Carbon dioxide is one of the primary green house gases, which gives great contribution to the climate change. Hence, carbon capture and storage (CCS) has been the focus for the carbon dioxide CO2 reduction. Among current popular carbon dioxide sequestration routes, the mineral carbon dioxide sequestration is regarded as a potential important technology due to its benefits such as environmentally friendly, permanent trapping of carbon dioxide in form of carbonate, and without the need of post-storage surveillance for CO2 leakage. In mineral carbonation, carbon dioxide gas is stored by promoting magnesium or calcium oxides in silicate minerals to react to store carbon dioxide in carbonates forms using steelmaking slag slurry with mild conditions of temperature and carbon dioxide pressure.
Studies have been carried out to investigate the technological condition of carbon dioxide sequestration with steelmaking slag slurry, including reaction time, liquid-to-solid ratio, temperature, carbon dioxide pressure, and initial pH. It has been shown that that the maximum carbon dioxide capture capacity can reach to 211 kg of carbon dioxide / ton steelmaking slag with consideration of the contribution of Mg(HCO3)2 in capturing carbon dioxide and the precipitate obtained under optimized carbonation condition was rich in CaCO3 with composition percentage reaching upto 96 %. Although carbon dioxide capture with steelmaking slag in laboratory has been successful, there is no industrialization process till now.
The method of flue gas de-sulphurization includes wet process, dry process and semi-dry process, among which wet limestone / lime method is most widely used. Steelmaking slag can be used for de-sulphurization due to its high CaO, especially the free CaO content
Application in agriculture – Steelmaking slag contains fertilizer components CaO, SiO2, and MgO. In addition to these three components, it also contains components such as FeO, MnO, and P2O5, so it is being used for a broad range of agricultural purposes. Its alkaline property counteracts soil acidity. In several countries, BOF slag is used to produce siliceous fertilizer, phosphorus fertilizer and micronutrient fertilizer.