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Effect of basicity on chromium partition in CaO-MgO-SiO2-Cr 2O3 synthetic slag at 1873 K
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Process Science.
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Process Science.
Divison of Extractive Metallurgy, Luleå University of Technology, Luleå, Sweden.
2014 (English)In: Transactions of the Institution of Mining and Metallurgy Section C - Mineral Processing and Extractive Metallurgy, ISSN 0371-9553, E-ISSN 1743-2855, Vol. 123, no 2, 116-122 p.Article in journal (Refereed) Published
Abstract [en]

The objective of the present work is to get an understanding of the phase relationships in the CaO-MgO-SiO2-Cr2O3 system with a view to control the precipitation of Cr-spinel in the slag phase. The equilibrium phases in CaO-MgO-SiO2-Cr2O3 slag system at 1873 K (1600°C) have been investigated experimentally and compared with the results from thermodynamic calculations. The Cr2O 3 and MgO contents in the slag were fixed at 6 and 8 wt-% respectively. The basicity (CaO/SiO2) of slag was varied in the range 1·0-2·0. A gas/slag equilibrium technique was adopted to synthesise the slag at a high temperature in air. The samples were heated to and soaked at 1873 K (1600°C) for 24 h in order to achieve the equilibrium state and subsequently quenched in water. The chromium distribution and phase compositions in the quenched slag were studied using scanning electron microscope wavelength dispersive spectroscopy and X-ray diffraction techniques. FactSage software was used for the phase equilibrium calculations. The experimental results obtained from the present work were compared with the calculation results from FactSage software. It was found that the spinel formation at 1873 K (1600°C) is favoured in the slag basicity range 1·0-1·4.

Place, publisher, year, edition, pages
2014. Vol. 123, no 2, 116-122 p.
Keyword [en]
phase equilibrium, chromium partition, slag basicity, spinel, scanning electron microscope (SEM)
National Category
Metallurgy and Metallic Materials
Identifiers
URN: urn:nbn:se:kth:diva-50908DOI: 10.1179/1743285513Y.0000000038Scopus ID: 2-s2.0-84901700588OAI: oai:DiVA.org:kth-50908DiVA: diva2:463058
Note

QC 20150317. Updated from accepted to published.

Available from: 2011-12-08 Created: 2011-12-08 Last updated: 2017-12-08Bibliographically approved
In thesis
1. Investigations of Stabilization of Cr in Spinel Phase in Chromium-Containing Slags
Open this publication in new window or tab >>Investigations of Stabilization of Cr in Spinel Phase in Chromium-Containing Slags
2011 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

The influence of basicity, heat treatment as well as different oxygen partial pressures on the phase relationships in the CaO-MgO-SiO2-Cr2O3 slags was studied with a view to control the precipitation of Cr-spinel in the slag phase. The equilibrium phases in CaO-MgO-SiO2-Cr2O3 slag system in the range on 1673-1873 K have been investigated under low oxygen partial pressure as well as in as air atmosphere. In low oxygen partial pressure experiments, a suitable mixture of CO and CO2 was used to control the oxygen partial pressure. The oxygen partial pressure was kept at 10-4 Pa. The Cr2O3 and MgO contents in the slag were fixed to be 6 and 8wt% respectively. The basicity (CaO/ SiO2) of the slag was varied in the range 1.0-2.0. Gas/slag equilibrium technique was adopted to synthesize the slag at a suitable temperature above the liquidus point. One heat treatment procedure is that the samples were heated to and soaked at 1873 K for 24h in order to achieve the equilibrium state and subsequently quenched in water. The other is that the samples were heated to and soaked at 1873 K for 24h, then slow cooled to 1673 K and soaked at this temperature for additional 24h in order to achieve the equilibrium state at lower temperature before quenching in water. The chromium distribution and phase compositions in the quenched slag were studied using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction techniques (XRD). FACTsage software was used for the phase equilibrium calculations. The experimental results obtained from the present work are compared with the calculation results from FACTsage software as well as with results from samples directly quenched after soaking at 1873K. It is found that the spinel formation at 1873 K in air atmosphere is favored in the slag basicity range of 1.0 to 1.6. The size of spinel crystals increased drastically after slow cooling followed by annealing compared to samples being quenched after soaking at 1873 K. The amount of foreign elements dissolved in the spinel phase, and matrix phases decreased after slow cooling followed by annealing at lower temperature, resulting in purer phases with less defects. It was found that the amount of foreign elements in the spinel phase, and other phases decreased after soaking at very low PO2. The size of the spinel crystals was found to be larger in samples with low basicity. Spinel phase precipitation has improved in the samples with higher basicities compared to the results obtained in air.

 

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2011. vi, 42 p.
Keyword
Cr2O3 containing slags, gas-slag equilibrium, chromium partition, spinel
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-50709 (URN)978-91-7501-207-0 (ISBN)
Presentation
2011-12-16, Sal B23, KTH, Brinellvägen 23, Stockholm, 10:00
Opponent
Supervisors
Note
QC 20111208Available from: 2011-12-08 Created: 2011-12-07 Last updated: 2011-12-08Bibliographically approved
2. Abatement of Chromium Emissions from Steelmaking Slags - Cr Stabilization by Phase Separation
Open this publication in new window or tab >>Abatement of Chromium Emissions from Steelmaking Slags - Cr Stabilization by Phase Separation
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Chromium is an important alloying element in stainless steel but also environmentally harmful element. A number of mineralogical phases present in the slag matrix can contain chromium and lead to chromium leaching. Chromium in slag if not stabilized, could oxidize to the cancerogenic hexavalent state, and leach out if exposed to acidic and oxygen rich environment. Other environmental concerns are slag dusting and chromium escape to the atmosphere. Despite the fact that there is a certain risk of Cr-emission from slags at operating conditions, still very little is known regarding the emission of the oxides of chromium during the slag tapping. Spinel phase is known to be important for controlling the leaching properties of chromium from the slag. The objective of the present study was to get an understanding of the phase relationships and chromium partition in the chromium-containing industrial slags and synthetic slags with a view to control the chromium stabilization in spinel phase. The impact of slag basicity, heat treatment, oxygen partial pressure and Al2O3 addition, on the phase relationships and chromium partition has been determined. The experimental results were compared with the phase equilibrium calculations. It was found that the oxygen partial pressure in the gas phase had a strong impact on chromium partition. The experimental results show that the impact of the slag basicity on chromium partition at lower oxygen partial pressures was negligible in contrast to that in air. The amount of spinel phase was found to increase with increased Al2O3 content. Slow cooling of slag and soaking at low oxygen partial pressure would improve the spinel phase precipitation. This treatment will also lead to less Cr dissolved in the unstable matrix phases. Chromium oxide was found to be emitted when chromium containing slags were exposed to oxidizing atmosphere. The results indicate that chromium oxide evaporation increases with increase in temperature and oxygen partial pressure, but decreases with slag basicity and sample thickness.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2013. xi, 53 p.
Keyword
spinel; slag basicity; oxygen partial pressure; chromium oxide vaporization, шпинель; шлак; парциальное давление кислорода, испарение оксидa хрома, 尖晶石;炉渣碱度;氧分压;铬氧化物汽化, spinell, slaggbasicitet, syrepartialtryck, förångning av kromoxid
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-133480 (URN)978-91-7501-666-5 (ISBN)
Public defence
2013-12-06, B3, Brinellvägen 23, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Projects
Steel Eco-Cycle
Funder
Mistra - The Swedish Foundation for Strategic Environmental ResearchKnut and Alice Wallenberg Foundation
Note

QC 20131114

Available from: 2013-11-14 Created: 2013-11-05 Last updated: 2013-11-14Bibliographically approved

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