Future Research and Developments on Reuse and Recycling of Steelmaking By-Products
- increasingly stringent legislation on waste/residues disposal;
- high content of metal oxides in residues, which can be used to (partially) replace costly virgin raw material;
- chemical and physical properties of steel residues and by-products that make them useful in other sectors.
2. Current Industrial Utilization of Residues Derived from the Steel Sector
- Basic materials in spent refractories (around 50% of granular materials recycled) can be used in EAF or converted as a substitute of the lime (usually containing calcium oxide and/or calcium hydroxide) and dololime (mainly containing calcium and magnesium oxides). The possible partial substitution of fired lime and calcined dolomite with adequate quantities of granulated materials obtained from spent refractories (refer to Table 2, Fines 1, Fines 2 and Fines 3) with the aim of obtaining EAF slags with characteristics suitable for the process has been theoretically evaluated within the project activities.
- Aluminous materials contained in spent refractories (around 40% of high alumina fluxes restored in the process) can be used as substitute of alumina fluxes for slag conditioning/forming.
- Spent refractories, containing about 5% of steel scrap, after separation, can be directly reused in the steelmaking process.
- Only 5% of the collected refractory materials are disposed in landfill.
- 39 million tonnes for forestry
- 88 million tonnes for secondary residues from wood industry
- 90 million tonnes for biowaste
- 16 million tonnes for post-consumer wood.
3. Information Gathering
- internal slag recycling and valorisation outside the steel production cycle;
- extraction of valuable material from waste and wastewater;
- internal and external recycling of Fe-bearing by-products different from slag
- internal and external recycling of by-product with other beneficial and valuable contents, such as metals, coal and lime.
- elimination of harmful elements;
- minimisation of waste generation and landfill;
- process integration solutions for by-products management;
- modelling and simulation.
4. Research and Development Projections
- hydrogen-rich auxiliary reduction gases (CnHm) injected via the tuyere;
- charging scrap via the burden;
- biogenic/alternate carbon sources that can partly replace fossil carbon resources.
4.1.1. BF/BOF Route Slag
- Internal utilisation:
- Recovery of Fe metal (e.g., Fe from BOF slag as scrap substitute, such as recycling into the BOF as iron carrier and cooling material);
- recovery of oxides (e.g., FeO/Fe2O3 from BOF slag as iron ore substitute in sinter plant or BF).
- External utilisation:
- P recovery/V/Fe and a slag product for the cement industry;
- changing BOF slag properties to be like granulated BF slag;
- road construction;
- extraction of valuable material (liming/fertilizer);
- improvement of the quality to better meet utilization requirements of slag for specific applications;
- investigation of processes for crystallization to minimize the leaching of certain elements to reduce the amount of slag to be landfilled;
- industrial processes for producing material for the building industry.
- industrial processes for heat recovering from slag.
4.1.2. EAF Route Slag
- Internal slag utilisation
- Recovery of Fe and other metals
- Fe recovery without carbon-based reduction or with biogenic carbon;
- Recovery of Fe for internal utilization (e.g., recycling in the EAF);
- Recovery of non-Fe metal (e.g., hot slag reduction for Ni/Cr/Fe/Mn (stainless slag) or P recovery/V/Fe/Mn (C steel) and for the cement industry);
- Recovery of oxides (e.g., substitution of lime with LF slag to EAF, LF slag internal recovery for refractory applications);
- Heat recovery from EAF and LF slag.
- External slag utilisation
- treatment (reduction, modification, granulation) to create hydraulic properties for Portland cement and to reduce CO2 emissions in cement industry;
- research on slag during transitions to H2 ironmaking;
- construction materials for road construction, earthworks, rail and hydraulic engineering (e.g., improving properties by decreasing leaching of specific elements, volume stability, etc.);
- improving the slag quality to better meet requirements for its recycling;
- investigation of slag use as liming/fertilizer material on long-term effects on soil and plants.
- Research on slag from all processes without carbon-based reduction:
- slag from a DRI–EAF route, considering slag conditioning and treatment as well as the utilization of low-grade ore and recycling material as feedstock;
- slag from DR/SAF/BOF route, considering also slag conditioning and treatment as well as the utilization of low-grade ore and recycling materials as feedstock, to produce a material similar to granulated BF slag;
- Research to improve the environmental compatibility of new EAF slags for different applications.
4.2. Dust and Sudge
4.2.1. BF/BOF Route Sludge and Dust
- Internal utilization
- External utilization
4.2.2. EAF Route Sludge and Dust
- Pyrometallurgical processes
- optimizing the separation of iron from other (high volatile) oxides using dedicated furnaces operating in reducing conditions;
- using briquettes and/or pellets, including EAF dust and other fine steel by-products;
- self-reducing briquettes using carbon from alternative sources (e.g., biochar). Using innovative technologies for facilitating the reduction process of reducible oxides (e.g., FeO);
- optimization of the synergy between procedures allows obtaining hot metal and/or metallic alloys to be reused internally, inert slag to be reused externally, ZnO-enriched dust to be treated for valuable metal recovery;
- innovative microwave heating technologies allowing the efficiency increase of the pyrometallurgical process;
- the use of devoted reducing furnaces for pyrometallurgical processes operating in parallel with EAF as a valid option to be implemented for serving single steel mills.
- Hydrometallurgical processes
- different leaching solutions can be proposed, i.e., based on ammonium chloride ready for industrial implementation;
- an integrated pyro/hydro metallurgical process to obtain the pre-selection of the different fractions, thus on the yield of obtained product (metallic zinc having high purity as main objective);
- recovering residues from the different stages of the hydro process, containing Pb, Ni, Cd, Cu, Al, Si as oxides or as different species according to the different leaching methods (e.g., sulphates);
- considering possible new options facilitating the leaching process (e.g., the ultrasound-assisted sulphuric leaching that allows maintaining the process efficiency as dissolution of franklinite at lower acid concentration);
- implementing hydrometallurgical plants fitted for serving a single steel mill.
4.2.3. Decarbonisation: Sludge and Dust
- partial replacement of fossil coal and coke by pre-treated biomass and plastic wastes;
- partial replacement of iron oxides by suitable scrap as secondary raw material;
- injection of hydrogen-rich reduction gases or other carbon sources in the BF.
- recovering the contained iron after application of agglomeration (palletisation, briquetting) for recycling these fine-grained residues into the DR plant or the EAF.
- recycling of DR plant dust/sludge to the same plant through briquetting technology , involving tests of new organic or inorganic binder systems to optimize the size of briquettes, the thermal stability and reducibility as well as the selection of a continuous agglomeration device, such as a roller press.
- The implementation of the DR/EAF route would result in the newly upcoming DR- dust and in an increased amount of produced EAF or SAF dust. Nevertheless, this EAF dust will contain low zinc, which can be recycled back to the DR plant or the EAF.
4.3. Mill Scale
- Internal utilization
- External utilization
- developing internal utilization routes for mill scale and other waste iron oxides from carbon-based to hydrogen-based integrated steelworks involving processing of these residues in the DR plant (shaft furnace) or in the electric furnace for DRI melting (EAF or SAF);
- developing advanced material preparation processes for internal utilization of mill scale in DR/EAF plants;
- defining limits for metallurgical reuse in hydrogen-based steel production due to presence of detrimental components;
- developing advanced processing routes for the mill scale to meet the new carbon free metallurgical technologies.
- Open-loop recycling (for internal recycling):
- developing/refining processes to select spent materials for a proper selection of the adequate mixtures to be used in different applications (e.g., grinding/sieving and sorting of the spent material);
- evaluating through laboratory tests the behaviour of mixtures, such as slag formers and fractions from spent refractories. They include dissolution kinetics with the support of theoretical models;
- developing/refining theoretical models for calculating kinetics and thermodynamics aspects of the mixtures, including spent refractories. These models should aim at selecting proper mixtures ensuring slag formation in due time and with adequate fluidity;
- possible external reuse of selected fractions of spent refractories, such as cement and ceramics.
- Closed-loop recycling (suitable for internal and external recycling)
- refining/optimising of automatic systems for identifying different classes of spent materials for a more efficient and objective sorting (e.g., methods based on cameras/laser systems, such as LIBS);
- developing/refining intelligent software systems to be coupled with the optical devices. In particular, application of artificial intelligence (AI), machine learning (ML) as well as Big Data and edge computing are expected;
- optimizing methods for purification of materials coming from spent refractory to achieve a purity level comparable to the correspondent virgin material.
- demonstrative projects involving the use of refractories from recycled refractories, showing adequate performance in real operating conditions, to overcome the mistrust still present in stakeholders and refractory producers.
- development of refractory materials with reduced carbon or carbon-free content;
- identifying adequate materials to be used as substitutes, such as graphite, pet-coke, tar-pitch, petroleum pitch;
- laboratory- and pilot-scale studies on needed refining of the new materials’ chemical composition, focusing also on the possible impact on steel quality;
- laboratory- and pilot-scale studies to identify possible refinements of chemical composition to be used in process conditions involving use of hydrogen;
- developing new refractory materials and new process routes also requires adaptation of the processing methods for recycling refractory materials;
4.5. Secondary Raw Materials
- using real plastic material waste streams;
- integrating biomass treatment and upgrading with EAF processes (e.g., use of waste heat from the EAF process for biomass processes);
- charging and EAF operation when materials with high volatile matter are used (to ensure the efficient use of the alternative carbon as an energy source);
- increasing the amount of polymers blended with fossil coke for injection carbon.
4.6. Modelling/Simulation for By-Products
Institutional Review Board Statement
Informed Consent Statement
Data Availability Statement
Conflicts of Interest
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|BF slag (GB/AB slag)||Cement/concrete, road, others||17.9 M tonnes (2021)|
|BOF slag||Cement/concrete, road, hydraulic engineering, fertilizer, metallurgical use, others||12.5 M tonnes (2021)|
|EAF C slag||Landfill replacement, landfill building material, aggregate|
|EAF S slag||Landfill replacement, landfill building material, metal extraction, aggregates (e.g., unbound mixtures)|
|LF slag||Acid mine drainage prevention, treatment, remediation; soil stabilization and road base reclamation; sludge solidification and stabilization; hazardous waste stabilization; flowable fill and excavatable backfill.|
|Sinter dust||Internal recycling as sinter raw material||167.7 M tonnes (2018)|
|BF dust (coarse)||Internal: mixed and granulated in sinter raw material, pelletized/ briquetted in BF burden or injected to BF via tuyere|
|BF sludge (fine)||Internal: dezincing pre-treatment by hydro cyclone, afterwards: mixed and granulated in sinter raw material, briquetted in BF|
External: dezincing (Shaft furnace—Oxycup, DK Recycling—Waelz process); sent to the landfill
|BOF dust (coarse)||Internal: used in the sinter plant, BF and BOF|
|BOF dust/sludge (fine)||External: dezincing (Shaft furnace process); sent to the landfill|
|EAF C dust||External: zinc recovery through the pyrometallurgical Waelz process (rotary kiln).|
|EAF S dust||External: processed to recover Cr and Ni in the form of ferroalloys.|
|Mill scale||Internal: recovering metal to be reused in the steel production|
External: cement sector, cement clinker manufacturing, used in counterweights, ferroalloy production, production of friction agents, production of refractories, welding electrodes, iron salts and iron oxides.
|0.3–1.3 M tonnes/year|
|Refractories||As slag conditioners in EAF and BOF; as a substitute of the lime and dololime in EAF and BOF; disposed in landfill.||Consumption:|
8–10 kg/t (BF/BOF)
5–7 kg/t (EAF)
|Secondary raw materials||Biomass: replacing fossil fuel and carbon|
Plastic: as alternative carbon source in steelmaking
|Fines 1||50||46.1||0.8||2.2||0.9||Granulated dolomia based (5/50 mm) from exhaust refractories|
|Fines 2||88.1||1.6||7.1||1.1||1||Granulated magnesia based (5/50 mm) from exhaust refractories|
|Fines 3||86.3||1.7||8.7||1.4||1||Granulated magnesia based (5/50 mm) from exhaust refractories|
|Ref-1||0.8||99||0.3||1||0.2||Fired lime 5/50 mm|
|Ref-2||27||72||0.4||1||0.2||Calcined dolomite 5/50 mm|
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Colla, V.; Branca, T.A.; Pietruck, R.; Wölfelschneider, S.; Morillon, A.; Algermissen, D.; Rosendahl, S.; Granbom, H.; Martini, U.; Snaet, D. Future Research and Developments on Reuse and Recycling of Steelmaking By-Products. Metals 2023, 13, 676. https://doi.org/10.3390/met13040676
Colla V, Branca TA, Pietruck R, Wölfelschneider S, Morillon A, Algermissen D, Rosendahl S, Granbom H, Martini U, Snaet D. Future Research and Developments on Reuse and Recycling of Steelmaking By-Products. Metals. 2023; 13(4):676. https://doi.org/10.3390/met13040676Chicago/Turabian Style
Colla, Valentina, Teresa Annunziata Branca, Roland Pietruck, Simon Wölfelschneider, Agnieszka Morillon, David Algermissen, Sara Rosendahl, Hanna Granbom, Umberto Martini, and Delphine Snaet. 2023. "Future Research and Developments on Reuse and Recycling of Steelmaking By-Products" Metals 13, no. 4: 676. https://doi.org/10.3390/met13040676