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Recycling of Blast Furnace Sludge to the Blast Furnace via Cold-Bonded Briquettes: Evaluation of Feasibility and Influence on Operation
Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Minerals and Metallurgical Engineering.
Swerim AB.
Swerim AB.
SSAB Merox .
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2019 (English)In: ISIJ International, ISSN 0915-1559, E-ISSN 1347-5460Article in journal (Refereed) Epub ahead of print
Abstract [en]

Ore-based steelmaking generates various residues including dusts, sludges, scales and slags. Recycling of these residues within the process or via other applications is essential for sustainable production of steel. In blast furnace (BF) ironmaking, the gas-cleaning equipment generally recovers the particles in the off-gas as dust and sludge. Traditionally, the dry dust is recycled via the sinter or, in the case of pellet-based BF operation, via cold-bonded briquettes and injection. As the BF sludge mainly consists of iron and carbon, this residue is of interest to recycle together with the BF dust. However, depending on how the BF is operated, these two residues are more or less the major outlet of zinc from the furnace. Thus, to limit the recycled load of zinc, both materials cannot be recycled without dezincing the sludge prior to recycling. Dezincing and recycling of the low-zinc fraction of BF sludge via sinter have been reported whereas recycling via cold-bonded briquettes has not been performed. In the present study, cold-bonded briquettes containing the low-zinc fraction of dezinced BF sludge were charged as basket samples to the LKAB Experimental Blast Furnace (EBF). The excavated basket samples from the quenched EBF suggested that additions of up to 20 wt.% of upgraded BF sludge was feasible in terms of reducibility and strength. Based on these results, BF sludge were added to cold-bonded briquettes and charged in industrial-scale trials. The trials indicated that the annual generation of BF sludge, after dezincing, could be recycled to the BF.

Place, publisher, year, edition, pages
Iron and Steel Institute of Japan , 2019.
Keywords [en]
blast furnace sludge, recycling, cold-bonded briquettes, pilot-plant scale blast furnace trials, industrial-scale blast furnace trials
National Category
Metallurgy and Metallic Materials
Research subject
Process Metallurgy
Identifiers
URN: urn:nbn:se:ltu:diva-75198DOI: 10.2355/isijinternational.ISIJINT-2019-080OAI: oai:DiVA.org:ltu-75198DiVA, id: diva2:1334573
Funder
Swedish Energy Agency, JK21069Available from: 2019-07-03 Created: 2019-07-03 Last updated: 2019-08-22
In thesis
1. Recycling of Blast Furnace Sludge within the Integrated Steel Plant: Potential for Complete Recycling and Influence on Operation
Open this publication in new window or tab >>Recycling of Blast Furnace Sludge within the Integrated Steel Plant: Potential for Complete Recycling and Influence on Operation
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Ore-based steelmaking generates various residues including dust, sludges, scales and slags. Internal and external recycling has allowed for 68-90 % of the dust, sludges and scales to be recycled. However, several residues are landfilled despite containing elements valuable as raw material in the production of steel. One such residue is the blast furnace (BF) sludge which has a chemical composition dominated by iron and carbon. In 2008, the annual worldwide landfilling of BF sludge was estimated to 8 million metric tons in dry weight. Furthermore, as the iron production via the BF route has increased significantly since 2008, the landfilling of BF sludge could be even higher as of today. Thus, the potential to reclaim valuable iron and carbon while improving the raw material efficiency is substantial.

Traditionally, in-plant recycling of residues generated in the integrated steel plant is conducted via the sinter or, in the case of pellet-based BFs, via cold-bonded briquettes and injection in the BF tuyeres. The challenges in recycling BF sludge via these routes are the fine particle size distribution, the high water content and the zinc content. Of these challenges, the latter is the main concern as too high zinc loads in the BF lead to increased reductant rates, reduced lining life of carbon-based bricks and scaffold formation, which may disturb the process. The challenge regarding zinc has previously been addressed by pretreating the sludge, generating a low-zinc and high-zinc fraction where the former has been recycled to the BF via the sinter or cold-bonded pellets. Although pretreatment and recycling of the low-zinc fraction have been achieved in industrial scale, the reported sludges are generally coarse in size and high in zinc. Furthermore, recycling of pretreated BF sludge to the BF utilizing cold-bonded briquettes has not been reported and the internal recycling of the high-zinc fraction has not been considered.

In the present thesis, newly produced BF sludge with a fine particle size distribution and low zinc content was characterized finding that a majority of the zinc was present in weak acid soluble phases and that the finest fraction of the sludge carried most of the zinc. Based on these findings, the BF sludge was pretreated using sulfuric acid leaching, hydrocycloning and tornado treatment, respectively. Sulfuric acid leaching was the most effective method in selectively separating zinc from the iron, carbon and solids. However, both hydrocycloning and tornado treatment were successful in generating a fraction low in zinc.

The low-zinc fraction of the tornado-treated BF sludge was incorporated in cold-bonded briquettes and tested for strength, swelling and intrinsic reducibility. Furthermore, the briquettes were charged as basket samples in the LKAB Experimental Blast Furnace (EBF) in order to study the behavior in actual BF conditions. The results suggested that the low-zinc fraction of the BF sludge could be added to the briquettes without negatively affecting the performance of the briquettes in the BF. The results were confirmed in industrial-scale trials where non-treated BF sludge was added to cold-bonded briquettes in an amount that would facilitate complete recycling of the low-zinc fraction. Charging these briquettes to the BF did not induce any negative effects on the process or the hot metal (HM) quality.

The high-zinc fraction of the tornado-treated BF sludge was added in self-reducing cold-bonded agglomerates and studied in technical-scale smelting reduction experiments aiming at recycling to the HM desulfurization plant. The experiments suggested that melt-in problems could be expected when using either briquettes or pellets. Nonetheless, industrial-scale trials were performed aiming to study the feasibility of recycling cold-bonded briquettes to both the HM desulfurization plant and basic oxygen furnace (BOF). These trials suggested that a substantial amount could be recycled without affecting the final quality of the steel. However, additional experiments were identified to be required in order to enable 100 % recycling of the high-zinc fraction of the tornado-treated BF sludge.

Based on the results from the experimental work, a holistic concept to completely recycle the BF sludge within the integrated steel plant was suggested.

Place, publisher, year, edition, pages
Luleå University of Technology, 2019. p. 104
Series
Doctoral thesis / Luleå University of Technology 1 jan 1997 → …, ISSN 1402-1544
National Category
Metallurgy and Metallic Materials
Research subject
Process Metallurgy
Identifiers
urn:nbn:se:ltu:diva-75651 (URN)978-91-7790-419-9 (ISBN)978-91-7790-420-5 (ISBN)
Public defence
2019-10-18, 09:00 (English)
Opponent
Supervisors
Funder
Swedish Energy Agency, JK21069
Available from: 2019-08-22 Created: 2019-08-22 Last updated: 2019-09-13Bibliographically approved

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Andersson, AntonAhmed, HeshamSundqvist Ökvist, Lena

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