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Investigations of Stabilization of Cr in Spinel Phase in Chromium-Containing Slags
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Process Science.
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 [en]
Cr2O3 containing slags, gas-slag equilibrium, chromium partition, spinel
National Category
Metallurgy and Metallic Materials
Identifiers
URN: urn:nbn:se:kth:diva-50709ISBN: 978-91-7501-207-0 (print)OAI: oai:DiVA.org:kth-50709DiVA: diva2:462438
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
List of papers
1. Effect of basicity on chromium partition in CaO-MgO-SiO2-Cr 2O3 synthetic slag at 1873 K
Open this publication in new window or tab >>Effect of basicity on chromium partition in CaO-MgO-SiO2-Cr 2O3 synthetic slag at 1873 K
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.

Keyword
phase equilibrium, chromium partition, slag basicity, spinel, scanning electron microscope (SEM)
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-50908 (URN)10.1179/1743285513Y.0000000038 (DOI)2-s2.0-84901700588 (Scopus ID)
Note

QC 20150317. Updated from accepted to published.

Available from: 2011-12-08 Created: 2011-12-08 Last updated: 2017-12-08Bibliographically approved
2. Effect of the heat treatment on the chromium partition in CaO-MgO-SiO2-Cr2O3 synthetic slags
Open this publication in new window or tab >>Effect of the heat treatment on the chromium partition in CaO-MgO-SiO2-Cr2O3 synthetic slags
2013 (English)In: Metallurgical and materials transactions. B, process metallurgy and materials processing science, ISSN 1073-5615, E-ISSN 1543-1916, Vol. 44, no 6, 1586-1597 p.Article in journal (Refereed) Published
Abstract [en]

Mg-spinel phase is known to be important for control of Cr leaching from Cr-containing slags. The objective of the present study 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 in the range of 1673 K to 1873 K (1400 A degrees C to 1600 A degrees C) have been investigated experimentally and compared with the results from thermodynamic calculations. The slag compositions close to the industrial slag systems were chosen. The Cr2O3 and MgO contents in the slag were fixed to be 6 and 8 wt pct, respectively. The basicity (CaO/SiO2) of the slag was varied in the range of 1.0 to 2.0. The slags were synthesized at a pre-determined oxygen partial pressure (10(-4)) or air (2.13 x 10(4) Pa) at a temperature above the liquidus temperature. The samples were then soaked at targeted temperatures for 24 hours in controlled atmosphere in order to achieve the equilibrium state before quenching in water. Four different heat-treatment regimes (defined as Ia, Ib, II.a and II.b) in Section II-D) were used in the present experiments. The lower oxygen partial pressure was maintained by a suitable mixture of CO and CO2 gases. Phases present and their compositions in the quenched slags were studied using scanning electron microscopy coupled with energy-dispersive spectroscopy and X-ray diffraction techniques. The chromium content in the phases present was analyzed using wavelength-dispersive spectrometer. The experimental results obtained are compared with the calculation results from Factsage software. The size of spinel crystals increased drastically after slow-cooling from 1873 K (1600 A degrees C) followed by annealing at 1673 K (1400 A degrees C) for 24 hours (heating regimes II) compared to samples being quenched directly after soaking at 1873 K (1600 A degrees C) (heating regime I.a). It was found that the amount of foreign elements in the spinel phase, and other phases decreased after soaking at oxygen partial pressure of 10(-4) Pa resulting in phases with less defects and foreign oxide contents compared to those treated in air. The size of spinel crystals was found to be larger in samples with lower basicity.

Keyword
phase equilibrium, chromium partition, slag basicity, spinel, scanning electron microscope (SEM)
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-50928 (URN)10.1007/s11663-013-9939-0 (DOI)000327383100029 ()2-s2.0-84890559757 (Scopus ID)
Funder
Mistra - The Swedish Foundation for Strategic Environmental Research, MISTRA-88035
Note

Updated from "Manuscript" to "Articles" QC 20131105

Available from: 2011-12-08 Created: 2011-12-08 Last updated: 2017-12-08Bibliographically approved
3. Effect of oxygen partial pressure on the chromium partition in CaO-MgO-SiO2-Cr2o3 synthetic slag at 1673 k
Open this publication in new window or tab >>Effect of oxygen partial pressure on the chromium partition in CaO-MgO-SiO2-Cr2o3 synthetic slag at 1673 k
(English)Manuscript (preprint) (Other academic)
Keyword
Cr2O3 containing slags, gas-slag equilibrium, chromium partition, spinel
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-50929 (URN)
Available from: 2011-12-08 Created: 2011-12-08 Last updated: 2011-12-08Bibliographically approved

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