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Synthesis and dissolution of slag minerals: A study of β-dicalcium silicate, pseudowollastonite and monticellite
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.
Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Minerals and Metallurgical Engineering.ORCID iD: 0000-0001-9297-8521
2015 (English)In: Canadian metallurgical quarterly, ISSN 0008-4433, E-ISSN 1879-1395, Vol. 54, no 4, p. 446-454Article in journal (Refereed) Published
Abstract [en]

Leaching of slag must be limited when slag is used in other areas than landfill. As slag is composed of different minerals, the properties of slag, inter alia the dissolution, depend on these minerals. If the leaching properties of each mineral are known, counter measurements can be taken to prevent the leaching of unwanted elements. In this study, the dissolution of three common slag minerals will be examined. The three minerals; boron-stabilised β-dicalcium silicate, pseudowollastonite and monticellite were synthesised. The dissolution was measured by setting pH to 4, 7 or 10 and adding 0.05 g mineral in the size range 20-38 μm, and the HNO3 consumption required 3 to maintain the pH level was recorded during 40 h. As expected, the dissolution increases for all minerals when pH decreases. The boron- stabilised β-dical cium silicate was the only mineral fully dissolved at all pH levels, while pseudowollastonite and monticellite only dissolved completely at pH 4.

Place, publisher, year, edition, pages
2015. Vol. 54, no 4, p. 446-454
National Category
Metallurgy and Metallic Materials
Research subject
Process Metallurgy
Identifiers
URN: urn:nbn:se:ltu:diva-9171DOI: 10.1179/1879139515Y.0000000022ISI: 000371430800009Scopus ID: 2-s2.0-84960841657Local ID: 7bca281c-f98d-4457-97f4-0a2b34fc3fb6OAI: oai:DiVA.org:ltu-9171DiVA, id: diva2:982109
Note
Validerad; 2016; Nivå 2; 20160329 (andbra)Available from: 2016-09-29 Created: 2016-09-29 Last updated: 2020-03-16Bibliographically approved
In thesis
1. A Dissolution Study of Common Ferrous Slag Minerals
Open this publication in new window or tab >>A Dissolution Study of Common Ferrous Slag Minerals
2016 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Slag is a vital part of metal production since it removes impurities from the metal. As slag is continuously produced, the options are either to dispose slag in landfill or acknowledge slag as a product. Slag can be used in many different applications, ranging from fertilizer to construction material; in some cases, the properties of slag make it a superior alternative to virgin stone materials. The properties and thereby the field of application is determined by the mineralogical composition of the slag. Slag is considered an environmentally friendly material as long as the leaching of certain elements stay below specified thresholds, for leaching of chromium the limit is at 0.5 mg/kg for slag to be considered inert material.

The most common leaching approach is to compare leaching analyses from slag samples to deduce which elements and/or phases contribute or prevent leaching of specific elements. With this method each slag need to be investigated separately and the result may only apply for that specific slag type. In this thesis the approach is different: individual minerals are synthesized and dissolved separately at various pH to accurately assess their dissolution capabilities. By studying the dissolution of individual minerals the leaching of any type of slag with known mineralogical composition can be anticipated. Slag leaching can then be tailored, for example, chromium leaching can be eliminated if all chromium containing phases are removed or not capable of dissolving.

In this thesis the dissolution of akermanite, β- and γ-dicalciumsilicate, merwinte, monticellite, pseudowollastonite and magnesiowüstite with varying FeO/MgO ratios are studied. Leaching tests of magnesiowüstite with 4 wt% Cr2O3 were also included. The dissolution of each mineral is calculated by the acid addition required to maintain a constant pH with 50 mg of mineral in 100 ml water.

As expected, the dissolution decreases as pH increases, with exception of the dicalcium silicates which dissolved completely at pH 4 to 10. The dissolution of the minerals is largely connected to the elemental composition. In the silica based minerals a high Ca ratio promotes dissolution while a high Si ratio impedes the dissolution rate. Both dissolution and chromium leaching of magnesiowüstite depends on the FeO content, with increasing FeO content the dissolution and leaching decreases, magnesiowüstite with at least 60 wt% FeO does not dissolve at pH 10. None of the magnesiowüstite compositions were close to the chromium leaching limit of inert material, 0.5 mg/kg, as the highest leaching sample, with 52 wt% FeO, leached 0.069 mg/kg.

Place, publisher, year, edition, pages
Luleå: Luleå University of Technology, 2016
Series
Licentiate thesis / Luleå University of Technology, ISSN 1402-1757
National Category
Metallurgy and Metallic Materials
Research subject
Process Metallurgy
Identifiers
urn:nbn:se:ltu:diva-60227 (URN)978-91-7583-747-5 (ISBN)978-91-7583-748-2 (ISBN)
Presentation
2016-12-16, F341, Luleå tekniska universitet, Luleå, 10:00 (English)
Available from: 2016-11-10 Created: 2016-11-09 Last updated: 2017-11-24Bibliographically approved
2. 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, IdaBjörkman, BoEngström, Fredrik

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