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Investigations on the Oxidation of Iron-chromium and Iron-vanadium Molten Alloys
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Process Science.
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

With the progress of high alloy steelmaking processes, it is essential to minimize the loss of valuable metals, like chromium and vanadium during the decarburization process, from both economic as well as environmental view points. One unique technique to realize this aim, used in the present work, is the decarburization of high alloy steel grades using oxygen with CO2 in order to reduce the partial pressure of oxygen. In the present work, the investigation on the oxidation of iron-chromium and iron-vanadium molten alloys under CO2-O2 mixtures was carried out and presented in this dissertation.

For oxidation study on Fe-Cr molten alloy with CO2-O2 mixtures, on the basis of thermodynamic analysis, energy balance calculation and modeling results, experimental validation in laboratory was carried out, and later on, the oxidation kinetics of Fe-Cr and Fe-Cr-C melts under controlled partial pressure of oxygen was investigated. Thermodynamics calculation and energy balance estimation demonstrated that, it is possible to use CO2 or CO2-O2 mixtures as decarburizers during EAF process and high initial carbon contents in the steel can be adopted at the beginning in order to reduce the cost.

A generic model has been developed to describe the overall process kinetics prevailing in metallurgical reactors containing liquid metal and gas bubbles. This model is general and can be extended further to consider any gas liquid reactions in any chemical engineering reactor, and especially the metallurgical ones, like AOD. In the present dissertation, the model is applied in predicting the evolution of Cr and C contents in a Fe-C-Cr melt during injection of different O2-CO2 mixtures. The related simulation results illustrated that CO2 is efficient in Cr retention.

In order to verify the modeling results, 1kg induction furnace experiments were carried out in the present laboratory. The results indicated that the predictions of the model are in good agreement with the experimental results. Meanwhile, the experimental results indicated that the Cr-losses can be significantly lowered by replacing the oxygen with CO2 in the injected gas, specifically for Fe-Cr-C melts with carbon levels higher than about 0.8 mass%. Subsequently, the oxidation kinetics of Fe-Cr and Fe-Cr-C melts was investigated under different CO2-O2 mixtures. It is indicated that, the oxidation rate is controlled by the chemical reaction at the initial stage and the reaction rate can be expressed as  at the Cr range of 11-21 mass% in the Fe-Cr melt.

For oxidation study on Fe-V liquid alloy, the investigation of the oxidation kinetics was carried out under CO2-O2 mixtures, which is followed by the study on thermodynamic properties of vanadium containing slags. During oxidation of Fe-V melt, in the case of alloys with vanadium contents exceeding 10 mass%, there exists an incubation period before the chemical reactions prevail the process. In addition, the ‘incubation time’ increased with the increase of temperature and the vanadium content, whereas it decreased with the increase of oxygen partial pressure in the oxidant gas.

High-temperature mass spectrometric method was used to determine the activity of the vanadium oxide in CaO-MgO-Al2O3-SiO2-V2O3 slags, whereas, the oxidation states of vanadium in the CaO-MgO-Al2O3-SiO2-VOxslag system was studied by XANES method. The results indicated that, higher basicities stabilize higher vanadium oxidation state, whereas, higher temperature stabilizes lower vanadium oxidation state.

The present work, which was carried out within the ECO-STEELMAKING project funded by MISTRA via Jernkontoret is expected to lead to implementation of some modifications in high alloy steel production based on fundamental concepts towards more environment-friendly steel processing.

Place, publisher, year, edition, pages
Stockholm: KTH , 2010. , viii, 58 p.
Series
Theses in Risk and Safety from the Division of Philosophy at the Royal Institute of Technology, ISSN 1654-627X
Keyword [en]
Oxidation, High-alloy steelmaking, Retention, CO2-O2, Fe-Cr melt, Fe-Cr-C melt, Fe-V melt, Vanadium containing slags, Kinetics, Thermodynamics properties.
National Category
Metallurgy and Metallic Materials Materials Engineering
Identifiers
URN: urn:nbn:se:kth:diva-12955ISBN: 978-91-7415-638-6 (print)OAI: oai:DiVA.org:kth-12955DiVA: diva2:319866
Public defence
2010-05-31, Salongen, KTHB, Osquars Backe 31, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC20100628

Available from: 2010-05-21 Created: 2010-05-20 Last updated: 2014-06-16Bibliographically approved
List of papers
1. Modelling of Physico-Chemical Phenomena between Gas inside a Bubble and Liquid Metal during Injection of Oxidant Gas
Open this publication in new window or tab >>Modelling of Physico-Chemical Phenomena between Gas inside a Bubble and Liquid Metal during Injection of Oxidant Gas
2010 (English)In: INT J CHEM REACT ENG, ISSN 1542-6580, Vol. 8Article in journal (Refereed) Published
Abstract [en]

Gas liquid reactors are extensively used in many metallurgical processes involving the refining of liquid metals. In these processes, reactions leading to the oxidation of various solutes in liquid metal often compete with each other, which ultimately determine the liquid metal composition. In the present paper, a model has been proposed to simulate the evolution of solute contents in a metallic melt considering mass transfer of solutes in the melt in the vicinity of the bubble, equilibrium at the gas-metal interface and gas composition evolution in the bubble during its ascent through the melt. The composition of solutes at the metal-gas interface in principle can be altered by changing the injected gas composition.

The model was applied to the case of oxygen injection through a lance into liquid steel-containing C and Cr, aiming sufficient decarburization without much oxidation of Cr to the slag. The Cr loss to the slag by oxidation is generally much more than that expected based on equilibrium thermodynamics applied to the bulk metal and gas. The actual Cr loss, as shown by the present model, is determined by the composition of solutes at the metal-gas interface rather than in the bulk. The effect of change of the partial pressure of oxygen in the bubble by replacing oxygen by carbon dioxide in the injected gas and the corresponding evolution of C and Cr contents in the melt was simulated. Some preliminary experiments were conducted to validate the model predictions. The frame work of the model is generic and can be extended to many gas-liquid metal reactors in liquid metal processing.

Keyword
gas injection, gas-liquid equilibrium, O-2-CO2 mixture, bubble, mass transfer, oxidation, DECARBURIZATION REFINING PROCESS, STAINLESS-STEEL, MATHEMATICAL-MODEL
National Category
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-12983 (URN)000274473700005 ()2-s2.0-77249120825 (Scopus ID)
Note
QC 20110131Available from: 2010-05-21 Created: 2010-05-21 Last updated: 2011-01-31Bibliographically approved
2. Modeling of Reactions between Gas Bubble and Molten Metal Bath-Experimental Validation in the Case of Decarburization of Fe-Cr-C melts
Open this publication in new window or tab >>Modeling of Reactions between Gas Bubble and Molten Metal Bath-Experimental Validation in the Case of Decarburization of Fe-Cr-C melts
2009 (English)In: High Temperature Materials and Processes, ISSN 0334-6455, E-ISSN 2191-0324, Vol. 28, no 6, 407-419 p.Article in journal (Refereed) Published
Abstract [en]

A theoretical generic model describing the mass transfer phenomena between rising gas bubbles and a metal bath has earlier been developed by the present authors, to predict the composition change in the melt as consequence of blowing different oxidant gases. In order to verify the model predictions, a series of experiments involving reactions between Fe-Cr-C melts and different O(2)-CO(2) gas mixtures were carried out. The results showed that the decarburization deviates significantly from thermodynamic paths predicted on the basis of bulk compositions and that the model was able to make reasonably reliable predictions of the changes of chromium and carbon contents in the melt as a function of time. According to the model, the compositions at the vicinity of injection point as well as at the gas-melt interface in the bubble are likely to be far from that in the bulk. The results of the present set of experiments showed, with CO(2) injection, the utilization of the available oxygen for decarburization was higher as compared to O(2) injection in the case of melts containing higher carbon levels (>1mass%). Reverse is the case in low carbon melts. The results also indicate relatively less Cr-losses from the metal bath when CO(2) is used as the oxidant. As the model predictions are found to be reasonably reliable, the model predictions are extended to predict the impact of the variation of different process parameters.

Keyword
Gas-liquid reactions, Bubbles, Simulation, Mass transfer, Decarburization, Partial pressure of oxygen, Fe-Cr-C melt
National Category
Natural Sciences
Identifiers
urn:nbn:se:kth:diva-46956 (URN)10.1515/HTMP.2009.28.6.407 (DOI)000279749700008 ()
Note
QC 20111107Available from: 2011-11-07 Created: 2011-11-07 Last updated: 2017-12-08Bibliographically approved
3. Investigation of the oxidation kinetics of Fe-Cr and Fe-Cr-C melts undercontrolled oxygen partial pressures
Open this publication in new window or tab >>Investigation of the oxidation kinetics of Fe-Cr and Fe-Cr-C melts undercontrolled oxygen partial pressures
2012 (English)In: Metallurgical and materials transactions. B, process metallurgy and materials processing science, ISSN 1073-5615, E-ISSN 1543-1916, Vol. 43, no 6, 1476-1487 p.Article in journal (Other academic) Published
Abstract [en]

In the current work, oxidation kinetics of Fe-Cr and Fe-Cr-C melts by gas mixtures containing CO2 was investigated by Thermogravimetric Analysis (TGA). The experiments were conducted keeping the melt in alumina crucibles, allowing the alloy melt to get oxidized by an oxidant gas. The oxidation rate was followed by the weight changes as a function of time. The oxidation experiments were conducted using various mixtures of O-2 and CO2 with = 10(-2) to 10(4) Pa. In order to understand the mechanism of oxidation, the wetting properties between the alumina container and the alloys used in the thermogravimetric analysis (TGA) experiments and the change of the alloy drop shape during the course of the oxidation were investigated by X-ray radiography.The experiments demonstrated that the oxidation rate of Fe-Cr melt increased slightly with temperature under the current experimental conditions, but it is strongly related to the Cr-content of the alloy as well as the oxygen partial pressure in the oxidant gas mixture, both of which caused an increase in the rate. For the Fe-Cr-C system, the oxidation rate has a negative relationship with carbon content, viz. with increasing carbon, the oxidation rate of the alloy melt slightly decreased. The chemical reaction was found to be the rate determining step during the initial stages, whereas as the reaction progressed, the diffusion of oxygen ions through slag phase to the slag-melt interface was found to have a strong impact on the oxidation rate. The overall impact of different factors on the chemical reaction rate for the oxidation process derived from the current experimental results can be expressed by the relationship: A model for describing the kinetics of oxidation of Fe-Cr and Fe-Cr-C alloys under pure CO2 was developed. Simulation of the oxidation kinetics using this model showed good agreement with the experimental results.

National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-13769 (URN)10.1007/s11663-012-9703-x (DOI)000312342900025 ()2-s2.0-84865097674 (Scopus ID)
Note

Updated from "Submitted" to "Published". QC 20140127

Available from: 2010-06-28 Created: 2010-06-28 Last updated: 2017-12-12Bibliographically approved
4. Oxidation of Fe-V Melts Under CO2-O-2 Gas Mixtures
Open this publication in new window or tab >>Oxidation of Fe-V Melts Under CO2-O-2 Gas Mixtures
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, 1042-1051 p.Article in journal (Refereed) Published
Abstract [en]

The oxidation mechanism of liquid Fe-V alloys with V content from 5 to 20 mass pct under different oxygen partial pressures using CO2-O-2 mixtures with CO2 varying from 80 pct to 100 pct was investigated by thermogravimetric analysis between 1823 K and 1923 K (1550 degrees C and 1650 degrees C). The products after oxidation were identified by scanning electron microscopy energy-dispersive spectrograph and X-ray diffraction. The results indicate that the oxidation process can be divided into the following steps: an apparent incubation period, followed by a chemical reaction step with a transition step before the reaction, and diffusion as the last stage. At the initial stage, a period of slow mass increase was observed that could be attributed to possible oxygen dissolution in the liquid iron-vanadium coupled with the vaporization of V2O. The length of this period increased with increasing temperature as well as vanadium content in the melt and decreased with increasing oxygen partial pressure of the oxidant gas. This analysis was followed by a region of chemical oxidation. The oxidation rate increased with the increase of the O-2 ratio in the CO2-O-2 gas mixtures. During the final stage, the oxidation seemed to proceed with the diffusion of oxygen through the product layer to the reaction front. The Arrhenius activation energies for chemical reaction and diffusion were calculated, and kinetic equations for various steps were setup to describe the experimental results. The transition from one reaction mechanism to the next was described mathematically as mixed-control equations. Thus, uniform kinetic equations have been setup that could simulate the experimental results with good precision.

Keyword
liquid-iron, vanadium, equilibrium, spinels, oxygen, slags
National Category
Metallurgy and Metallic Materials Materials Engineering
Identifiers
urn:nbn:se:kth:diva-27945 (URN)10.1007/s11663-010-9391-3 (DOI)000284333700014 ()
Note
QC 20110104Available from: 2011-01-04 Created: 2011-01-03 Last updated: 2017-12-11Bibliographically approved
5. High-temperature mass spectrometric study of the vaporization processes of V2O3 and vanadium-containing slags
Open this publication in new window or tab >>High-temperature mass spectrometric study of the vaporization processes of V2O3 and vanadium-containing slags
Show others...
2010 (English)In: Rapid Communications in Mass Spectrometry, ISSN 0951-4198, E-ISSN 1097-0231, Vol. 24, no 16, 2420-2430 p.Article in journal (Refereed) Published
Abstract [en]

A Knudsen effusion mass spectrometric method was used to study the vaporization processes and thermodynamic properties of pure V2O3 and 14 samples of vanadium-containing slags in the CaO-MgO-Al2O3-SiO2 system in the temperature range 1875-2625 K. The system was calibrated using gold in the liquid state as the standard. Vaporization was carried out from double tungsten effusion cells. First it was shown that, in vapor over V2O3 and the vanadium-containing slags in the temperature range 1875-2100 K, the following vapor species were present: VO2, VO, O, WO3 and WO2, with the latter two species being formed as a result of interaction with the tungsten crucibles. The temperature dependencies of the partial pressures of these vapor species were obtained over V2O3 and the slags. The ion current comparison method was used for the determination of the V2O3 activities in slags as a function of temperature with solid V2O3 as a reference state. The V2O3 activity coefficients in the slags under investigation indicated positive deviations from ideality at 1900 K and a tendency to ideal behavior at 2100K. It was shown that the V2O3 activity as a function of the slag basicity decreased at 1900 K and 2000 K and was practically constant in the slag melts at 2100K. The results are expected to be valuable in the optimization of slag composition in high-alloy steelmaking processes as well as for their environmental implications. Copyright (C) 2010 John Wiley & Sons, Ltd.

National Category
Biochemistry and Molecular Biology
Identifiers
urn:nbn:se:kth:diva-26866 (URN)10.1002/rcm.4657 (DOI)000280687200014 ()2-s2.0-77955359931 (Scopus ID)
Note
QC 20101201Available from: 2010-12-01 Created: 2010-11-29 Last updated: 2017-12-12Bibliographically approved
6. Determination of vanadium oxidation states in CaO-MgO-Al2O3-SiO2-VOxsystem by K edge XANES method
Open this publication in new window or tab >>Determination of vanadium oxidation states in CaO-MgO-Al2O3-SiO2-VOxsystem by K edge XANES method
(English)In: Metallurgical and materials transactions. B, process metallurgy and materials processing science, ISSN 1073-5615, E-ISSN 1543-1916Article in journal (Other academic) Submitted
National Category
Metallurgy and Metallic Materials Materials Engineering
Identifiers
urn:nbn:se:kth:diva-13772 (URN)
Note
QS 20120328Available from: 2010-06-28 Created: 2010-06-28 Last updated: 2017-12-12Bibliographically approved

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