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Experimental Studies on the Oxidation States of Chromium Oxides in Slag Systems
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Process Science.
2010 (English)In: Metallurgical and materials transactions. B, process metallurgy and materials processing science, ISSN 1073-5615, E-ISSN 1543-1916, Vol. 41, no 5, 946-954 p.Article in journal (Refereed) Published
Abstract [en]

In view of the importance of the thermodynamic behavior of chromium in the slag phase as well as the serious discrepancies in the earlier reports on the valence state of chromium in slag melts, the oxidation state of chromium oxides in CaO-SiO2-CrOx and CaO-MgO-(FeO-) Al2O3-SiO2-CrOx were investigated experimentally in the present study using two different experimental techniques. The gas-slag equilibrium technique was adopted to study the CaO-SiO2-CrOx system between 1823 K (1550 °C) and 1923 K (1650 °C) and equilibrated with mixtures of CO-CO2-Ar gases corresponding to three different oxygen partial pressures (between 10-4 and 10-5 Pa). After equilibrating, the samples were quenched and subjected to analysis using the X-ray absorption near edge structure method to determine the distribution ratio of Cr2+/Cr3+ in the slags. The second technique examined the applicability of the high-temperature mass spectrometric method combined with the Knuden effusion cell for quantifying the valence states of Cr in the multicomponent system CaO-MgO-(FeO-) Al2O3-SiO2-CrOx up to a maximum temperature of 2000 K (1727 °C). The results showed that the Knudsen cell-mass spectrometric method could be used successfully to estimate the valence ratio for Cr in silicate melts. According to the present study, the Cr2+/Cr3+ ratio increased with increasing temperature and a decreasing slag basicity as well as the oxygen partial pressure prevailing in the system. A mathematical correlation of XCrO/XCrO1.5 as a function of temperature, oxygen partial pressure, and basicity was developed in the present work based on the present results as well as on those assessed from earlier data.

Place, publisher, year, edition, pages
2010. Vol. 41, no 5, 946-954 p.
Keyword [en]
strongly reducing conditions, silicate melts, 1500-degrees-c, xanes
National Category
Materials Engineering
URN: urn:nbn:se:kth:diva-24105DOI: 10.1007/s11663-010-9383-3ISI: 000284333700004ScopusID: 2-s2.0-84860219095OAI: diva2:343460
Tidigare titel: Experimental studies on the oxidation states of chromium oxides in slag phase QC 20100813. Uppdaterad från submitted till published(20110104).Available from: 2010-08-13 Created: 2010-08-13 Last updated: 2011-01-04Bibliographically approved
In thesis
1. Experimental and modelling studies of the thermophysical and thermochemical properties of some slag systems
Open this publication in new window or tab >>Experimental and modelling studies of the thermophysical and thermochemical properties of some slag systems
2009 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In order to optimize the metallurgical processes it is necessary to improve the understandings of the industrial slag properties, which include thermophysical and thermochemical properties. In the present study Mass Triangle Model was applied to predict these properties. According to the model formula, a program was developed in Visual C++ environment to calculate different properties in limited solubility ternary system. Successful applications of this model in predicting viscosity, surface tension, and density have been demonstrated in the case of a number of ternary slags in liquid state, as for example, RExOy-CaF2-SiO2 (RE refer to rare earth metal), MnO-TiO2-SiO2, CaO-Al2O3-SiO2, BaO-FeO-Fe2O3, CaO-MnO-SiO2 etc. In addition, the method has also been extended to predict electrical conductivities and sulphide capacities, which is first attempt to compute properties. Furthermore, an extension of this model has also been performed in the present study to obtain the corresponding properties in a homogeneous ternary system. Good agreement between model calculation and literature values had demonstrated that mass triangle model offers a powerful and efficient tool for estimations of various properties of molten system only based on limited experimental information.

Knowledge of the thermodynamic properties of chromium oxides-containing slags is of great importance to chromium retention in stainless steel production, in view of chromium impacts on economic costs and environmental protection. Thus, the oxidation states of chromium in slags as well as sulphide capacity of CrOx-containing slag have been studied. In the studies of oxidation state of chromium in slags, gas/slag equilibrium technique was used for CaO-SiO2-CrOx system and the ratio of Cr2+/Cr3+ in multicomponent slags was measured by X-ray absorption near edge spectra (XANES). High-temperature mass spectrometry method was also used to obtain the distribution of chromium oxides in CaO-MgO(-FeO)-Al2O3-SiO2-CrOx system. It is to be noted that the present work is probably the first to employ Knudsen cell-mass spectrometry as an effective way to estimate chromium valences through vapor species. Utilizing the data obtained in present study as well as those reported in literature, a mathematical correlation was established for estimating the ratio of Cr2+/Cr3+ as a function of temperature, partial pressure of oxygen and slag basicity. The comparison between experimental valence ratio values and estimated ones presents a satisfactory agreement.

The sulphide capacities of CaO-SiO2-CrOx pseudo-ternary slags were measured using the gas-slag equilibration technique in the temperature range 1823–1923 K under two different oxygen partial pressures 9.80×10-3 and 9.88×10-4 Pa. The results showed that log10Cs varied linearly with the reciprocal temperature, and the slope was higher than the corresponding value reported in the case of the binary CaO-SiO2 of corresponding composition. By using the equation developed as part of this project relating Cr2+/Cr3+ with basicity, oxygen partial pressure and temperature, it was possible to understand the effect of CrO on the sulphide capacities; viz. the sulphide capacity shows a decreasing trend as Cr2+ replaces Ca2+ in the slag. With further increase of Cr2+ content, there are indications of the occurrence of a minimum point beyond which the sulphide capacities show a slight increasing trend. The latter is attributed to the increasing extent of the polymerization reaction releasing oxygen ions for sulphide reactions in the metasilicate region. The behavior of CrO in the slag was found to be analogous to FeO.

Estimation of liquidus and solidus temperature of slag was also carried out in the present study. The results indicated that it is possible to get a reasonable idea of the solidus temperature from X-ray radiography while DSC would indicate the liquidus temperature of slag

Place, publisher, year, edition, pages
Stockholm: KTH, 2009. viii, 50 p.
National Category
Materials Engineering
urn:nbn:se:kth:diva-11232 (URN)978-91-7415-440-5 (ISBN)
Public defence
2009-10-30, Salongen, KTHB, Osquars Backe 31, stockholm, 10:00 (English)

QC 20100813

Available from: 2009-10-08 Created: 2009-10-07 Last updated: 2013-11-12Bibliographically approved

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