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A study of an autogenous slag for steel production with consideration of possible vanadium extraction
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Process. Swerim AB.ORCID iD: 0000-0001-9923-0145
2024 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The thesis discusses the use of the autogenous slag that forms from the residual oxides present in hydrogen reduced iron (H-DRI) pellets during melting. The studies are motivated by a better understanding of how H-DRI affect the steelmaking operations. A possible optimization of the steelmaking process is to recover the vanadium that is contained in the iron ore raw material. Therefore, understanding the role of vanadium during melting of H-DRI is given an extra focus. 

 

Taking advantage of the autogenous slag by utilizing its dephosphorization power to a maximum, or using it to extract vanadium, could make an important contribution to the process economics. To assist the developments in these directions, the phosphorus and vanadium partitions between slag and metal (LP and LV) as well as the phase relationship of the autogenous slag were investigated. The partitions were studied by melting H-DRI with reduction degrees between 91 and 99% in closed systems at 1873 K. The obtained LP and LV were in the ranges 8-26 and 501-1994, respectively. The LV values increased with decreased reduction degrees. The values for LP increased with decreasing reduction degrees until a 97% degree of reduction. Further lowered reduction degrees correlated with decreased LP values. The lowest phosphorus levels encountered in the iron (130 ppm) were obtained after melting of H-DRI with degrees of reduction between 94 and 98%. This indicates that the autogenous slag has a potential to make a significant contribution to the phosphorus refining. 

 

To find out about the phase relationship in the autogenous slag at 1873 K, small (5 g) samples of synthetic slag were equilibrated with 1 g iron in closed systems. Composition-wise, these slags corresponded to the autogenous slags from H-DRI with 98.4-99.7% reduction degrees. Preservation of the high temperature phase relationship required fast cooling; therefore, the samples were quenched in oil. This was also the reason for using small samples. Spinel, magnesiowüstite and liquid phase were identified as the stable phases at 1873 K. The spinel and magnesiowüstite phases were high in V, while the liquid contained almost no V. Increased FeO-contents (decreased degrees of reduction) correlated with a decreased amount of spinel, an increased amount of magnesiowüsite, as well as a decreased content of V in both phases.

 

To increase the understanding about the phases in the autogenous slag, a sub-system containing MgO and V2O3 was investigated under conditions relevant to H-DRI melting, namely temperatures between 1661-1873 K and pO2 values between 1.75×10-11 and 1.75×10-10 atm. The phase boundaries for the three stable phases MgO-halite, spinel and V2O3-corundum were established. The oxygen potential and the temperature had limited impacts on the phase boundaries for the spinel and V2O3-corundum phases, while the maximum solubility of V2O3 in MgO-halite was affected to a somewhat larger extent. As earlier research has shown that an acid slag could be suitable for V extraction, the pseudo-ternary phase diagram between Al2O3, SiO2 and V2O3 at 1873 K and pO2=3.4×10-11-3.4×10-9 atm was also investigated. 5 different phases were identified, namely mullite, Al2O3-corundum, V2O3-corundum, cristobalite, and a liquid phase. The most significant effect of the oxygen potential was on the invariant point representing double Al2O3 and V2O3 saturation of the liquid. The multivalent nature of vanadium is suggested as the reason for the slight impact of the oxygen potential on the phase diagrams.

 

To understand how the autogenous slag forms from the residual oxides, individual pellets with 90 and 99% reduction degrees were studied during heating to either 1773 or 1873 K. It was observed that the autogenous slag forms before iron melts. The slag likely forms as FeO melts and dissolves the other remaining oxides. Thereby, vanadium is transferred to the autogenous slag. Before iron melts, the movement of the autogenous slag is restricted to the pellet’s pore network. Thereafter, when iron melts, the slag starts to coalesce as well as to floatate.

 

As the autogenous slag may contain solid phases, the effect of the fraction of solid phase on the slags foamability was finally investigated. This was done by measuring the maximum foaming heights of slags containing Al2O3, CaO, FeO and SiO2, reminiscent in their compositions to the autogenous slag. The slag compositions were chosen so that the fraction of precipitated magnesiowüstite phase was the main variable. It was found that some amount of solid phase (1.6 vol%) increased the foaming height by approximately 7%, while ≥8.7 vol% more than halved the foaming height.

Abstract [sv]

I denna avhandling presenteras forskning om den autogena slagg som vid smältning av vätgasreducerade järnmalmspellets (H-DRI pellets) bildas från de kvarstående oxiderna. Studiens motiv är att öka förståelsen om hur användningen av H-DRI pellets påverkar ståltillverkningsprocessen. En möjlig optimering av ståltillverkningsprocessen är att ta vara på det vanadin som finns i järnmalmsråvaran. Därför ligger ett extra fokus på att förstå vanadinets roll vid smältning av H-DRI. 

 

Ett sätt att dra nytta av den autogena slaggen för att främja processekonomin är att maximalt utnyttja dess fosforreningsförmåga, eller att använda den för att extrahera vanadin. För att öka förståelsen kring hur detta skulle kunna gå till undersöks inledningsvis uppdelningarna av fosfor- och vanadin mellan slagg och metall (LP och LV) samt den autogena slaggens fasförhållande. LP och LV studeras genom att smälta H-DRI med reduktionsgrader mellan 91 och 99% i stängda system vid 1873 K. De erhållna LP och LV låg i intervallen 8-26 respektive 501-1994. LV ökade med minskad reduktionsgrad. LP ökade med minskad reduktionsgrad ner till en reduktionsgrad på 97%. Ytterligare sänkning av reduktionsgraden korrelerade med en sänkning av LP. De lägsta fosforhalterna i järnet (130 ppm) erhölls vid smältning av H-DRI med reduktiongrader mellan 94 och 97%. Detta tyder på att den autogena slaggen kan bidra signifikant till fosforreningen. 

 

För att undersöka fasförhållandet i den autogena slaggen vid 1873 K användes små (5 g) prover av syntetisk slagg, sammansättningsmässigt motsvarande autogen slagg från H-DRI med reduktionsgrader mellan 98.4 och 99.7%. Slaggerna smältes i stängda system med 1 g järn för att styra hur jämvikten ställde in sig. För att bevara högtemperatursfaserna krävdes snabb nedkylning. Av denna anledning användes små prov som släcktes i olja. Spinell, magnesiowüstit samt en flytande fas identifierades som stabila vid 1873 K. De två förstnämnda innehöll höga koncentrationer av vanadin, medan den sistnämnda knappt innehöll något vanadin. Ökad FeO-halt (minskad reduktionsgrad) korrelerade med minskad mängd spinellfas, ökad mängd magnesiowüstitfas samt en minskad halt av vanadin i båda dessa faser.

 

För att öka förståelsen kring faserna i den autogena slaggen undersöktes undersystemet MgO-V2O3 vid liknande förhållanden som vid H-DRI-smältning, nämligen 1661-1873 K och pO2=1.75×10-11-1.75×10-10 atm. Fasgränserna för de tre stabila faserna MgO-halit, spinell och V2O3-corundum fastställdes. Syrepotentialen och temperaturen hade en begränsad påverkan på fasgränserna för spinell och V2O3-corudum, medan maxlösligheten av V2O3 i MgO-halit påverkades i något större utsträckning. Eftersom tidigare forskning har visat att en sur slagg skulle kunna vara lämplig för vanadinextraktion undersöktes även det pseudo-ternära fasdiagrammet mellan Al2O3, SiO2 och V2O3 vid 1873 K och pO2=3.4×10-11-3.4×10-9 atm. 5 olika faser identifierades; mullit, Al2O3-corundum, V2O3-corundum, kristobalit samt en flytande fas. Punkten för samtidig Al2O3- och V2O3-mättnad i den flytande fasen påverkades mest anmärkningsvärt av syrepotentialen. Vanadinets förmåga att anta olika valenser framhålls i diskussionen som anledningen till att syrepotentialen har en viss påverkan på fasdiagrammen.

 

För att förstå hur den autogena slaggen bildas från de kvarvarande oxiderna studerades enskilda pellets med reduktionsgraderna 90 och 99% då de värmdes upp till 1773 K eller 1873 K. Det observerades att den autogena slaggen bildas innan järnet smälter. Slaggen bildas troligtvis av att FeO smälter och löser in vissa av de andra kvarvarande oxiderna. Därmed upptas vanadin i den autogena slaggen. Innan järnet smälter kan slaggen enbart röra sig i pelletens pornätverk. När järnet smälter kan slaggen gå samman till större enheter samt flyta upp till ytan. 

 

Eftersom det visat sig att den autogena slaggen kan innehålla fasta faser undersöktes slutligen hur mängden fast fas påverkar en slaggs förmåga att skumma. Detta gjordes genom att mäta den maximala skumhöjden för slagger innehållande Al2O3, CaO, FeO och SiO2, lika den autogena slaggen i sina sammansättningar. Slaggsammansättningarna valdes så att andelen magnesiowüstitfas utgjorde den huvudsakliga variabeln. Det observerades att lite fast fas (1.6 vol%) höjde den maximala skumhöjden med ca 7%, medan ≥8.7 vol% mer än halverade den maximala skumhöjden.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2024. , p. 65
Series
TRITA-ITM-AVL ; 2024:1
National Category
Metallurgy and Metallic Materials
Research subject
Materials Science and Engineering
Identifiers
URN: urn:nbn:se:kth:diva-343210ISBN: 978-91-8040-836-3 (print)OAI: oai:DiVA.org:kth-343210DiVA, id: diva2:1836313
Public defence
2024-03-04, F3, Lindstedtsvägen 26, Stockholm, 09:00 (English)
Opponent
Supervisors
Funder
Swedish Energy AgencyAvailable from: 2024-02-09 Created: 2024-02-08 Last updated: 2024-02-09Bibliographically approved
List of papers
1. Effect of Reduction Degree on Characteristics of Slag Formed by Melting Hydrogen-Reduced DRI and Partitions of P and V between Slag and Metal
Open this publication in new window or tab >>Effect of Reduction Degree on Characteristics of Slag Formed by Melting Hydrogen-Reduced DRI and Partitions of P and V between Slag and Metal
2021 (English)In: Steel Research International, ISSN 1611-3683, E-ISSN 1869-344X, Vol. 92, no 3, article id 2000432Article in journal (Refereed) Published
Abstract [en]

Self-fluxing hematite pellets are reduced by hydrogen to different degrees. The reduced pellets are melted in closed MgO crucibles at 1873 K to study the effect of reduction degree on the characteristics of slag formed. The results reveal that the phosphorus content in the metallic phase can be brought down to 130 ppm merely by the self-fluxing slag, even though the slag weighs only about 8% of the metal. It shows a great potential in reducing the amount of slag formers in the steelmaking process. The slag compositions obtained by melting the reduced pellets are used to prepare small synthetic slag samples for identifying the phases after melting. The use of the small samples is to ensure efficient quenching. Microscopic examination reveals that all the self-fluxing slags contain mainly three phases, namely, magnesiowüstite, spinel, and a liquid phase. Most of vanadium is found to be in the spinel and magnesiowüstite phases. The liquid phase only contains 1–2 wt% V2O3. Decreased FeO content of the slag increases the vanadium oxide contents in the spinel and magnesiowüstite phases. The fact that vanadium concentrates in the solid oxide phases provides essential information for sustainable extraction of vanadium from the steelmaking slag.

Place, publisher, year, edition, pages
Wiley-VCH Verlag, 2021
Keywords
hydrogen-reduced iron, phosphorus partition, self-fluxing slag, sustainable steelmaking, vanadium partition, Hematite, Hydrogen, Magnesia, Melting, Pelletizing, Steelmaking, Vanadium compounds, Hematite pellets, Metallic phase, Phosphorus contents, Reduction degree, Slag compositions, Steelmaking process, Synthetic slag, Vanadium oxides, Slags
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-290841 (URN)10.1002/srin.202000432 (DOI)000589827200001 ()2-s2.0-85096708879 (Scopus ID)
Note

QC 20210323

Available from: 2021-03-23 Created: 2021-03-23 Last updated: 2024-02-08Bibliographically approved
2. Phase relationship in the system MgO-V2O3 between 1661 and 1873 K at P-O2 = (3.55+/-0.2).10(-6) and (3.55+/-0.3) x 10(-5) Pa
Open this publication in new window or tab >>Phase relationship in the system MgO-V2O3 between 1661 and 1873 K at P-O2 = (3.55+/-0.2).10(-6) and (3.55+/-0.3) x 10(-5) Pa
2020 (English)In: Calphad, ISSN 0364-5916, E-ISSN 1873-2984, Vol. 70, article id 101777Article in journal (Refereed) Published
Abstract [en]

The composition ranges of the phases in the pseudo binary system MgO-V2O3 were studied between 1661 and 1873 K and at controlled oxygen partial pressures of (3.55 +/- 0.2) x 10(-6) and (3.55 +/- 0.3) x 10(-5) Pa. The phase relationship was determined by equilibrating MgO-V2O3 pellets in a CO-CO2 mixture followed by quenching and electron-probe microanalysis. To ensure sufficient quenching, a new setup was designed and developed, so that the equilibrated samples can be quenched in oil directly under the same atmosphere inside the experimental setup. Three different phases were found in the samples, namely MgO, MgO-V2O3 spinel and V2O3. The phase boundaries were determined with good reproducibility. The solubility of V2O3 in the MgO phase increased with temperature and was significantly higher than literature data. The spinel as well as the V2O3 composition range were found to change only a little with temperature in the investigated temperature range. Decreased oxygen potential led to a slight increase of the V2O3 content in the spinel phase and V2O3 phase. Furthermore, decreased oxygen potential resulted in a significant increase of the solubility of V2O3 in the MgO phase at the higher temperatures, especially at 1873 K.

Place, publisher, year, edition, pages
Elsevier, 2020
Keywords
V2O3, MgO, Spinel, Phase relationship, Oxygen potential
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-281471 (URN)10.1016/j.calphad.2020.101777 (DOI)000564496900002 ()2-s2.0-85084940956 (Scopus ID)
Note

QC 20201021

Available from: 2020-10-21 Created: 2020-10-21 Last updated: 2024-02-08Bibliographically approved
3. Reaction Mechanisms During Melting of H-DRI Focusing on Slag Formation and the Behavior of Vanadium
Open this publication in new window or tab >>Reaction Mechanisms During Melting of H-DRI Focusing on Slag Formation and the Behavior of Vanadium
2023 (English)In: Metallurgical and materials transactions. B, process metallurgy and materials processing science, ISSN 1073-5615, E-ISSN 1543-1916, Vol. 54, no 4, p. 2206-2215Article in journal (Refereed) Published
Abstract [en]

The reaction mechanisms during melting of hydrogen direct reduced iron pellets (H-DRI) with different degrees of reduction were studied experimentally at 1773 K to 1873 K at different times (60 to 600 seconds), focusing on the autogenous slag formation. It was found that an autogenous slag is formed inside the pellets prior to the melting of the metal phase. The formation of the autogenous slag started with the melting of FeO, initially located in the center of the iron grains. The liquid FeO flowed into the pore network of the pellet. While flowing, the liquid FeO dissolved parts of the residual oxides, forming an autogenous slag. The slag stayed in the pore network until the iron was molten. Upon melting of the iron, the slag coalesced into spherical droplets. The final state is reached upon the separation of the metal and slag phases by flotation, as a bulk slag was formed on the surface of the liquid iron. In addition, since the iron ore used in this study contains vanadium, the behavior of V was discussed separately based on the experimental observations to build a basis for future studies on V extraction.

Place, publisher, year, edition, pages
Springer Nature, 2023
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-338576 (URN)10.1007/s11663-023-02827-z (DOI)000999629500001 ()2-s2.0-85160601404 (Scopus ID)
Note

QC 20231107

Available from: 2023-11-07 Created: 2023-11-07 Last updated: 2024-02-08Bibliographically approved
4. Impact of Solid Particles and Liquid Droplets on Foams - Cold Model and High Temperature Experiments
Open this publication in new window or tab >>Impact of Solid Particles and Liquid Droplets on Foams - Cold Model and High Temperature Experiments
2022 (English)In: ISIJ International, ISSN 0915-1559, E-ISSN 1347-5460, Vol. 62, no 1, p. 104-111Article in journal (Refereed) Published
Abstract [en]

In order to obtain a realistic view of the foam in metallurgical slag, high temperature experiments where the foaming heights of FeO-CaO-SiO2-MgO slags containing precipitated MgO center dot FeO particles were measured. The foaming height slightly increased when small amounts of particles were present in the slag, but decreased to half height already when approximately 8 vol% particles were present in the liquid phase of the foam. To help the understanding, the foaming heights of silicone oil and food oil containing liquid insoluble droplets and non-reacting particles were also studied at room temperature. In these experiments, insoluble oil droplets were found to stabilize the foam, increasing the foaming height, while the addition of water droplets or solid particles had very little effect on foaming height. In line with the literature, it is believed that the interfacial energy between the droplets or particles and the bulk liquid as well as the interfacial energy between the droplets or particles and gas plays an important role. When the interfacial energy between the different phases becomes too high, the foaming height decreases, while when it's low enough, the foaming height increases.

Place, publisher, year, edition, pages
Iron and Steel Institute of Japan, 2022
Keywords
steelmaking, foaming slag, foaming height, particles, precipitation, droplets
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-311289 (URN)10.2355/isijinternational.ISIJINT-2021-226 (DOI)000778737700013 ()2-s2.0-85124654481 (Scopus ID)
Note

QC 20220422

Available from: 2022-04-22 Created: 2022-04-22 Last updated: 2024-02-08Bibliographically approved

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