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 (L_P and L_V) 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 L_P and L_V were in the ranges 8-26 and 501-1994, respectively. The L_V values increased with decreased reduction degrees. The values for L_P increased with decreasing reduction degrees until a 97% degree of reduction. Further lowered reduction degrees correlated with decreased L_P 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 p_O2 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 p_O2=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.

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 (L_P och L_V) samt den autogena slaggens fasförhållande. L_P och LV studeras genom att smälta H-DRI med reduktionsgrader mellan 91 och 99% i stängda system vid 1873 K. De erhållna L_P och L_V låg i intervallen 8-26 respektive 501-1994. L_V ökade med minskad reduktionsgrad. L_P ökade med minskad reduktionsgrad ner till en reduktionsgrad på 97%. Ytterligare sänkning av reduktionsgraden korrelerade med en sänkning av L_P. 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 p_O2=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 p_O2=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.

To meet the future environmental challenges, hydrogen direct reduced iron (H-DRI) is expected to constitute the principal material for virgin steel production. For an efficient value chain, knowledge of the melting mechanism and dephosphorization mechanism of H-DRI is needed. The in situ melting behavior, the melting mechanism, and the dephosphorization mechanism during heating of H-DRI are investigated experimentally at 1773 and 1873 K. It is found that the melting rate of H-DRI varies with the reduction degree (91–99.5%), increasing with decreasing reduction degree. An autogenous slag forms during heating and flows through the pores of the H-DRI, thus increasing its effective thermal conductivity. The fraction of filled pores varies with reduction degree explaining the difference in melting rate. At this stage, the dissolution of apatite is initiated and completed upon melting of the metal phase. A gradual reversion of phosphorus from the autogenous slag to the liquid metal is observed after complete melting. The rate of reversion is discussed based on the properties of the H-DRI, for example, reduction degree and carbon addition.

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.

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.

Experiments were carried out to determine the activities of Sr, Al and Zr in liquid silicon at temperatures between 1723K and 1873K (1450°C and 1600°C). Different oxide pellets, made of a SrSiO₃-Sr₂SiO₄ mixture, an Al₂O₃-Al₆Si₂O₁₃ mixture and a ZrSiO₄-SiO₂ mixture (for the Sr-Si system, Al-Si system and Zr-Si system respectively), were equilibrated with liquid silicon in a closed system. The activities of the metals in the silicon were calculated based on the contents of the element dissolved in the silicon. The temperature had negligible effect on the activity coefficients for the Sr and Al in the liquid silicon at the temperature range studied, while the activity coefficient of Zr in liquid silicon increased with temperature.

Inclusions in crude steels are linked to employed feedstock material and secondary steelmaking should be tailored correspondingly for optimized praxis. Thus, when implementing hydrogen direct reduced iron (H-DRI) as feedstock for electric arc furnace (EAF) operation, knowledge of the number, size, type of inclusions, and mechanism of formation are needed to support future process development. Crude steel samples were collected from a pilot-scale EAF, employing 100% H-DRI, to study the inclusions originating from the novel feedstock. A sampler containing aluminum was used, and the effect of deoxidation is discussed. Laboratory experiments were conducted to understand the mechanisms of formation and growth of inclusions. It was clear that the inclusions formed because oxide particles merged and grew when the pores coalesced, and the iron grains sintered during the melting of H-DRI. The composition varied between the different inclusions and a total of three inclusion types were found. The size of the inclusions varied mainly between 5 and 70 µm, but the largest inclusion had a diameter of approximately 300 µm. The smallest inclusions (5-10 µm) were solid and contained mainly magnesiowüstite with a high MgO content (Type I-3). Type I-2 was larger (3-60 µm), and contained liquid, spinel, and no or small amounts of magnesiowüstite phase. Type I-1 inclusion varied greatly in size (13-300 µm) and contained all three phases with relatively high MgO content. H-DRI was heated for different durations and quenched prior to melting to study the formation mechanism. The link between feedstock material and inclusions in crude steel is discussed.