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A Study of Parameters that Influence the Kinetics of the AOD Decarburisation Process
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.ORCID iD: 0000-0003-2916-3666
2020 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis focuses on the AOD process, which is an important metallurgical reactor in stainless steel production. To be more specific, the thesis is limited to study the decarburisation step, which is the first of three process steps in the AOD converter. The main research questions is to increase the knowledge of reasons for random differences in decarburisation rates during the process. In the first part of the study, physical modeling is used to study the mixing in AOD converters. Parameters that were studied included, bath heights, gas flow rates and chemical reactions. The results showed that the mixing time decreased with an increased gas flow rate or an increased bath height. In addition, the influence of the top slag on the fluid flow and mixing time was studied. The results showed that the flow field was influenced by the slag phase and that it is of importance to account for the solid slag fractions to simulate the fluid flow and mixing time to resemble AOD converters. However, the results from this first part of the thesis illustrates that mixing is not the rate-limiting step for decarburisation in AOD converters. Instead, the focus was shifted to study if the slag was the cause for random differences in the decarburistaion rate. Slag samples were collected from an industrial AOD reactor. These slags are quite unique since they contain mainly solids and a small liquid fraction. Thus, petrography was used to study the samples and a new methodology was developed to characterize the slag samples. Methods for characterising the top slag samples from the AOD process were established, including combinations of different techniques. The common slag phases in decarburisation slag were identified. The results showed good agreement with samples analysed with SEM and EDS compared to calulations made in Thermo-Calc. Overall, it was shown that the slag characteristics changes during the decarburization period and that these changes can be determined using the new methodology. In the last part of the thesis, the commercial AOD process control model TimeAOD2 was used in combination with Thermo-Calc calculations to study how the process could be improved so that the slag composition became most beneficial for improving the kinetics of the decarburisation part of the AOD converter process. The results show that it is possible to predict the slag composition and especially the amount of liquid slag in the sample. This in turn, makes it possible to better estimate the optimal lime addition depending on the silicon content in steel and the amount of carry-over slag from the electric arc furnace.  Furthermore, it is shown that to large lime additions will lead to an increased heating time while not improving the decarburization rate.

Abstract [sv]

Denna avhandling fokuserar på AOD processen, vilken är en viktig metallurgisk reactor vid tillverkning av rostfritt stål. Mer specifikt, så är avhandlingen fokuserad på avkolningsteget, vilket är det första av tre processteg i AOD konvertern. Den huvudsakliga forskningsfrågan är att öka kunskapen gällande de slumpmässiga variationerna i avkolningshastighet i processen. I första delen av studien så används fysikalisk modellering för att studera omblandning i AOD konvertrar. Parametrar som studerades innefattade badhöjd, gasflöde och kemiska reaktioner. Resultaten visade att omblandingstiden minskade med ett ökat gasflöde och en ökad badhöjd. Dessutom så studerades inverkan av toppslaggen på stålets rörelse. Resultaten visade att stålflödet påverkades av slaggen och att det är av betydelse att även ta hänsyn till den fasta slaggfraktionen för att simulera stålflödet i AOD konvertrar. Däremot så visade resultaten från denna första del av avhandlingen att omblandningen inte är det hastighetsbestämmande steget vid avkolning i AOD konvertrar. Därför så skiftades fokus till att studera om slaggen var orsaken till slumpmässiga variationer i kolfärskningshastighet. Slaggprover togs från en industriell AOD konverter. Dessa slagger är unika eftersom de innehåller i huvudsak fasta partiklar och en liten flytande fraktion. Petrografi användes för att studera proverna och en ny metodologi utvecklades för att karakterisera slaggproverna. Detta innefattade användning av metoder för att karakterisera slaggproverna från AOD processen, inkluderande en kombination av olika tekniker. De mest vanliga slaggfaserna i slagger från avkolningssteget identifierades. Resultaten visade en god överensstämmelse mellan prover studerade med SEM och EDS jämfört med beräkningar gjorda med användande av Thermo-Calc. Sammanfattningsvis så visade resultaten att slaggkarakteristiken förändrades under avkolningsperioden och att dessa förändringar kunde bestämmas med den nyutvecklade metodologin. I den sista delen av avhandlingen så användes den kommerciella processkontrollmodellen TimeAOD2 i kombination med Thermo-Calc beräkningar för att studera hur processen skulle kunna förbättras så att slaggsammansättningen kunde vara mer fördelaktig för att förbättra kinetiken under avkolningssteget i AOD processen. Resultaten visade att det är möjligt att beräkna slaggsammansättningen och mer specifikt mängden flytande slagg som finns. Detta i sin tur gör det möjligt att bättre uppskatta den optimala kalktillsatsen vid olika kiselhalter i stål och vid olika mängder restslagg från ljusbågsugnen. Slutligen visade resultaten att an för stor kalktillsats kan leda till en ökad värmningstid utan att förbättra avkolningshastigheten.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2020. , p. 58
Series
TRITA-ITM-AVL ; 2020:26
Keywords [en]
AOD, decarburization, steel, slags, composition, petrography, physical modeling, industrial plant trials
Keywords [sv]
AOD, avkolning, stål, slagg, sammansättning, petrografi, fysikalisk modellering, industriella försök
National Category
Metallurgy and Metallic Materials
Research subject
Materials Science and Engineering; Metallurgical process science
Identifiers
URN: urn:nbn:se:kth:diva-273348ISBN: 978-91-7873-511-2 (print)OAI: oai:DiVA.org:kth-273348DiVA, id: diva2:1430303
Public defence
2020-06-05, https://kth-se.zoom.us/webinar/register/WN_TpALHOTpSde-B8jj2BMo6w, http://Vid fysisk närvaro eller Du som saknar dator/ datorvana kan kontakta service@itm.kth.se (English), Stockholm, 10:00 (English)
Opponent
Supervisors
Available from: 2020-05-14 Created: 2020-05-14 Last updated: 2020-06-04Bibliographically approved
List of papers
1. Mixing Time in a Side-Blown Converter
Open this publication in new window or tab >>Mixing Time in a Side-Blown Converter
2010 (English)In: ISIJ International, ISSN 0915-1559, E-ISSN 1347-5460, Vol. 50, no 5, p. 663-667Article in journal (Refereed) Published
Abstract [en]

The mixing time in a side-blown converter was studied using physical modelling. Water was used to simulate steel and a KCI tracer was added during the experiments. Thereafter, the mixing time was determined experimentally by measuring the electrical conductivity in the water bath. Experiments were done for two bath diameters of 200 mm and 300 mm, respectively. Furthermore, for gas flow rates between 30 cm(3)/s and 800 cm(3)/s as well as bath heights ranging from 106 to 314 mm. The mixing times were also calculated based on an expression involving the Strouhal and Reynolds numbers. The experimentally determined mixing times were found to be within +/- 20% of the theoretical values, which is considered to be good in physical modelling. Overall, the mixing time was found to be influenced by the gas flow rate and the vessel diameter, but not by the bath height.

Keywords
physical modeling, mixing time, converter, side-blown, conductivity measurements
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-27582 (URN)10.2355/isijinternational.50.663 (DOI)000277996600004 ()2-s2.0-77954764310 (Scopus ID)
Note

QC 20101214

Available from: 2010-12-14 Created: 2010-12-13 Last updated: 2020-05-14Bibliographically approved
2. A physical modelling study to determine the influence of slag on the fluid flow in the AOD converter process
Open this publication in new window or tab >>A physical modelling study to determine the influence of slag on the fluid flow in the AOD converter process
Show others...
2018 (English)In: Ironmaking & steelmaking, ISSN 0301-9233, E-ISSN 1743-2812, Vol. 45, no 10, p. 944-950Article in journal (Refereed) Published
Abstract [en]

A 1:4.6 scale physical model of a production argon oxygen decarburisation (AOD) converter was used to study the influence of top slag on the AOD process. Specifically, the gas penetration depth, fluid flow and slag behaviour under different nozzle diameters, nozzle numbers and gas flow rates were studied. The results show that the relative gas penetration depth generally increases linearly with an increased gas flow rate and a decreased nozzle size. Furthermore, the slag thickness increases linearly with an increased gas flow rate. In addition, the open-eye size was found to increase exponentially with an increased gas flow rate. Overall, three kinds of fluid flow patterns were found in the experiments: (i) a counter-clockwise rotation, (ii) a clockwise rotation and (iii) a double circulation with the plume in the middle of the converter. A counter-clockwise rotation was most common for the current experimental conditions.

Place, publisher, year, edition, pages
Taylor & Francis, 2018
Keywords
Physical modelling, AOD, slag, side-gas injection, penetration depth
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-240764 (URN)10.1080/03019233.2017.1415012 (DOI)000453823900014 ()2-s2.0-85039167810 (Scopus ID)
Note

QC 20190107

Available from: 2019-01-07 Created: 2019-01-07 Last updated: 2020-05-14Bibliographically approved
3. Physical Modeling of Side-Blown Converters
Open this publication in new window or tab >>Physical Modeling of Side-Blown Converters
2006 (English)In: Proc. InternationalConference on Advanced Fluid/Solid Science and Technologyin Experimental Mechanics, Sapporo, Japan, Sept. 11-14, 2006, 2006Conference paper, Published paper (Other academic)
Abstract [en]

In order to increase the knowledge of the metallurgical process in an AOD converter, physical modeling has been done. The work has been focused on the chemical reactions, measuring the pH level in a NaOH solution with injection of CO2 gas. Also, some trials have been made to investigate the effect of the slag phase upon the flow field using PIV measurement. The effect of different ratios of solid/liquid slag on the flow field and slag entrapment has been studied. It was found that the mass transfer product of CO2 in water, kwA, increase with an increased gas flow rate and bath height. Furthermore, the effect of slag layer on the flow field was also found necessary to consider in order to more realistically resemble the actual system in an industrial AOD. Here, it was found necessary to take the solid fraction of the slag into account to describe the viscosity of an AOD slag.

National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-273053 (URN)
Conference
International Conference on Advanced Fluid/Solid Science and Technology in Experimental Mechanics Sept. 11-14, 2006
Note

QC 20200511

Available from: 2020-05-06 Created: 2020-05-06 Last updated: 2020-05-14Bibliographically approved
4. Using an AOD simulator workbench to support process control development
Open this publication in new window or tab >>Using an AOD simulator workbench to support process control development
2011 (English)In: METEC InSteelCon 2011, Düsseldorf, Germany, 27 June - 1 July, 2011, 2011Conference paper, Published paper (Other academic)
Abstract [en]

The AOD (Argon Oxygen Decarburization) process is simulated with a model developed within the RFCS project OPCONSTAINLESS. The focus on the project has been on the influence of slag on the decarburization process. Here, a slag property is usually a parameter that is not used for process control, often due to the complexity of the slag system. The workbench, TimeAOD2, has been coupled with the Thermo-Calc software [1], giving the ratios of different slag phases and amount of liquid slag throughout the operation. The workbench gives the process manager an opportunity to see the outcome of different process layouts, control paradigms and raw material choices under different production conditions. A number of scenarios are outlined and different solutions evaluated. The simulation data is for some cases compared to operational data from AOD plants in Sweden.

National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-273055 (URN)
Conference
METEC InSteelCon 2011
Note

QC 20200511

Available from: 2020-05-06 Created: 2020-05-06 Last updated: 2020-05-14Bibliographically approved
5. Methods to Determine Characteristics ofAOD-Converter Decarburization-Slags
Open this publication in new window or tab >>Methods to Determine Characteristics ofAOD-Converter Decarburization-Slags
Show others...
2020 (English)In: Metals, ISSN 2075-4701, Vol. 10, no 3, article id 308Article in journal (Refereed) Published
Abstract [en]

Argon Oxygen Decarburization (AOD) converter slags are known to consist of both liquid and solid phases, but limited information on the slag characteristics has been published in the open literature. Therefore, a new methodology to study the characteristics of slag samples taken from the AOD converter process during production was developed based on petrography. The results show that the preparations of the slag samples using the borax method are suitable to use when determining the chemical composition of AOD slag samples using the X-ray fluorescence (XRF) method. The results also showed that both the light optical microscopy (LOM) method and a method combining scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS) can be used to characterize the slag samples and that the correlation between the methods was found to be good. This means that it is possible to use the faster LOM method instead of the more complicated SEM-EDS method to characterize AOD slag samples. Finally, the results show that the difference between calculated values based on stoichiometry and measured data for Ca and Cr in AOD slags are 11.7 mass% and 11.3 mass%, respectively. This is considered to be a good agreement for industrial samples.

Place, publisher, year, edition, pages
MDPI, 2020
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-273054 (URN)10.3390/met10030308 (DOI)
Note

QC 20200511

Available from: 2020-05-06 Created: 2020-05-06 Last updated: 2020-05-14Bibliographically approved

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