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Minimizing Chromium Leaching from Low-Alloy Electric Arc Furnace (EAF) Slag by Adjusting the Basicity and Cooling Rate to Control Brownmillerite Formation
Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Minerals and Metallurgical Engineering.
AB Indesko, Västerås, Sweden.
Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Minerals and Metallurgical Engineering.
Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Minerals and Metallurgical Engineering.ORCID iD: 0000-0002-9588-0180
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2020 (English)In: Applied Sciences, E-ISSN 2076-3417, Vol. 10, no 1, p. 35-50, article id 35Article in journal (Refereed) Published
Abstract [en]

Brownmillerite is connected to chromium leaching when present in steel slags. To prevent chromium leaching, brownmillerite in slag should be prevented. Two methods for decreasing brownmillerite content in low-alloy electric arc furnace (EAF) slag were investigated: decreasing the basicity and increasing the cooling rate. The methods were tried on both laboratory scale and in full-scale production. In the laboratory scale experiments, chromium leaching decreased as the basicity decreased until brownmillerite was no longer present, slower cooling resulted in increased chromium leaching, and faster cooling decreased chromium leaching. In full-scale production, basicity modified single batches, with a basicity below 2.2, generally leached less chromium than slag batches with higher basicity, thus verifying the correlation between basicity and chromium leaching seen in laboratory scale experiments. The cooling process in the full-scale experiments was achieved either by letting the slag cool by itself in the air or by water spraying. The water-sprayed slag, which cooled faster, had less chromium leaching than the air-cooled slag. The full-scale production experiments confirmed that both decreasing basicity below 2.2 and increasing the rate of cooling could be used to decrease chromium leaching.

Place, publisher, year, edition, pages
MDPI, 2020. Vol. 10, no 1, p. 35-50, article id 35
Keywords [en]
EAF slag, chromium, leaching, brownmillerite, laboratory scale, full scale
National Category
Metallurgy and Metallic Materials
Research subject
Process Metallurgy
Identifiers
URN: urn:nbn:se:ltu:diva-77396DOI: 10.3390/app10010035ISI: 000509398900035Scopus ID: 2-s2.0-85078897464OAI: oai:DiVA.org:ltu-77396DiVA, id: diva2:1385406
Note

Validerad;2020;Nivå 2;2020-01-31 (johcin)

Available from: 2020-01-14 Created: 2020-01-14 Last updated: 2024-03-27Bibliographically approved
In thesis
1. Minimisation of Chromium Leaching from Low-Alloy Electric Arc Furnace Slag by Mineral Modifications
Open this publication in new window or tab >>Minimisation of Chromium Leaching from Low-Alloy Electric Arc Furnace Slag by Mineral Modifications
2020 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Iron and steel are produced continuously as a vital part of modern life. The largest by-product of metal production is slag. Slag is an essential part of the production of steel since, in the furnace, the molten slag removes impurities from the metal. When the slag which consists of oxides, solidifies minerals form. Slag can then be used in different applications: fertiliser, water purification, cement, concreate and as building material. By using slag, natural resources can be saved. In the electric arc furnace, EAF, scrap is melted to produce steel. The scrap contains chromium which partly distributes to the slag. Unfortunately, excessive leaching of chromium restricts slag usage. Chromium leaching occurs when chromium-containing minerals in slag dissolve. In low-alloy electric arc furnace slag there are three chromium containing minerals: spinel (Mg,Fe)Cr2O4, magnesiowüstite (Mg,Mn,Fe)O and brownmillerite Ca2(Al,Fe)2O5. Of these minerals the spinel has already been determined to be stable. The aim of this thesis is to minimise the chromium leaching of low-alloy EAF slag by modifying the minerals and/or mineralogy in the slag so that the chromium-containing minerals do not dissolve. In addition, it was discovered that ageing of low-alloy electric arc furnace slag may increase the chromium leaching. Autoclave treatment of remelted slag resulted in chromium leaching at the same magnitude as before remelting. Autoclave treatment was performed before the leaching test to determine the chromium leaching of aged slag.

Instead of indirectly examining the chromium leaching from magnesiowüstite and brownmillerite by comparing chromium leaching from slag samples, the dissolution properties of the minerals were investigated individually. Dissolution of different compositions of magnesiowüstite and brownmillerite were studied at pH 7 and pH 10 for 40 hours. The conclusion was that increased iron content decreased the dissolution rate of both magnesiowüstite and brownmillerite. Magnesiowüstite (Mg,Fe)O did not dissolve at pH 10 when it contained 60 wt% FeO and did neither dissolve at pH 7 or pH 10 at higher FeO content. According to thermodynamic calculations, the FeO content in the magnesiowüstite can be affected and this was investigated. Laboratory and full-scale experiments showed that the composition of magnesiowüstite in slag could not be correlated to chromium leaching. The conclusion was that magnesiowüstite was not the main mineral leaching chromium.

Brownmillerite composition is difficult to control in slag and had a significant impact on the dissolution rate; therefore, brownmillerite in slag should be avoided to minimise leaching of chromium. From thermodynamic calculations two options to avoid brownmillerite formation were identified: decreasing the basicity (CaO/SiO2 ratio) or increasing the cooling rate. Both methods were tested in laboratory scale using low-alloy electric arc furnace slag. When the basicity was decreased by SiO2 addition, the chromium leaching of the slag decreased. The chromium leaching was correlated to the brownmillerite content, the chromium leaching decreased with the decreasing basicity until brownmillerite was no longer be detected by XRD analysis. After that the chromium leaching did not decrease significantly. This indicates a correlation between basicity, brownmillerite and chromium leaching. When cooling of slag was investigated the slower cooling rates increased the chromium leaching and fast cooling rates decreased the chromium leaching. The results were verified by full-scale experiments. Si-sand addition was used to decrease the basicity to 2.2; according to thermodynamic calculations this would prevent brownmillerite formation. The targeted basicity of 2.2 was difficult to achieve, as the basicity varied between batches since the scrap composition and other parameters changed. Batches from other experiments were included for more data. It was seen that the chromium leaching of batches below basicity 2.2 did not reach the same magnitude as the batches above basicity 2.2. The full-scale trials with different cooling methods confirmed that the faster-cooled water-sprayed slag had lower chromium leaching than slag that had cooled by its own.

Basicity between 2.0 and 2.2 minimises chromium leaching but decreasing the basicity to specific values in full-scale is challenging. A decrease in basicity is the most efficient method of decreasing chromium leaching, but faster cooling also decreases the chromium leaching.

Place, publisher, year, edition, pages
Luleå University of Technology, 2020
Series
Doctoral thesis / Luleå University of Technology 1 jan 1997 → …, ISSN 1402-1544
Keywords
slag, leaching, minerals
National Category
Metallurgy and Metallic Materials
Research subject
Process Metallurgy
Identifiers
urn:nbn:se:ltu:diva-78073 (URN)978-91-7790-556-1 (ISBN)978-91-7790-557-8 (ISBN)
Public defence
2020-05-12, C305, Luleå, 10:00 (English)
Opponent
Supervisors
Available from: 2020-03-16 Created: 2020-03-16 Last updated: 2020-05-15Bibliographically approved

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Strandkvist, IdaAndersson, AntonOlofsson, JennyLennartsson, AndreasSamuelsson, CaisaEngström, Fredrik

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