The present thesis investigates physical, mechanical and melting properties of hydrogen direct reduced iron (H-DRI) and how these relate to both reduction conditions and steelmaking practice. A combination of industrial pilot trials and controlled laboratory work is used in this endeavor. Carbonaceous reduction and resulting direct reduced iron are studied as references. Properties of H-DRI are of interest as hydrogen-based reduction is a proposed alternative to the blast furnace process, which represented 7-8% of global CO2 emissions in 2025.  As a first step, DRI reduced by natural gas or hydrogen in an industrial pilot shaft was investigated in terms of carbon and cementite contents, apparent porosity, fractures, tumbling strength and compression strength. It was found that tumbling and compression strength decreased with an increasing carbon content. The carbon-free H-DRI sample produced 90% less fines after the longest tumbling test and had nearly twice the average compression strength in comparison to the NG-DRI sample having the highest carbon content of 3 wt%. Cementite was found as concentrated near the pellets’ surfaces for the carbon-containing samples.  To gain deeper understanding, iron ore pellets were fully reduced in lab with hydrogen gas under varying temperature and time conditions. All reduction reached a maximum temperature in the range 700 to 1000 °C. The applied heating rates were between 1.5 and 10 °C/min. Holding times at the final reduction temperature were between 0 and 360 min. Volume change, apparent porosity, surface fractures, tumbling strength, compression strength, and microstructures at the pellet cross-sections were studied. The maximum reduction temperature had the most significant impact on the samples’ physical and mechanical properties. Samples reduced for a total time of five hours to 1000 °C in comparison to 700 °C had 9% lower fines generation after a 4-hour tumbling test, 3% lower apparent porosity and 13% fewer pellets with surface fractures. Increasing heating rate and holding time also had positive effect on apparent porosity, surface fractures, tumbling and compression strength. Further investigation is needed to understand the influence of conditions in the industrial reactor such as water content and pressure. Isothermal reduction experiments at 700 or 900 °C were conducted over an experimental time between 1 and 5 hours. CO+H2 gas mixture supplied into 

the bottom and exited through the top of the iron ore pellet bed with a gas flow rate of either 2 or 4 nL/min. Carbon and oxygen content analysis as well as cementite quantification over the pellets’ cross-sections was made for pellets placed along the bed’s vertical axis. Metallization, total carbon content and cementite content was greatest at the bottom and lowest at the top of the pellet bed for all experiments. Cementite content varied further within individual pellets, mostly found as concentrated near the pellets’ surfaces.  Melting of H-DRI and reference NG-DRI was investigated in a pilot-scale Electric Arc Furnace (EAF). Selected results from a large testing plan are included in this thesis work with focus on the melting behavior of highly metallized H-DRI (99%), low-metallized H-DRI (95%) and reference NG-DRI. Extensive sampling of slag composition, steel composition, temperature, ingoing and outgoing material weights was made. Slag foaming performance and identification of agglomerates of unmelted material termed ferrobergs were noted by visual observation. Maintaining an FeO content in the top slag above ~30 wt% seemed to have a greater influence on melting behavior than did slag basicity, foaming index or type of DRI. This may be because higher FeO content lowers the slag viscosity, which in turn enhances convective heat transfer within the bulk slag. Deeper understanding of melting behavior of H-DRI in EAF-type slags was sought through lab experimental work. A single H-DRI pellet was submerged in molten slags having varying FeO, Al2O3 and MgO. The slag and submerged pellet were quenched after melting times of 5, 10 or 15 s. The cross-section was studied and the surrounding slag was crushed and magnetically separated to see amount of melting. After 15 s melting time, between 50 and 80% apparent melting was achieved. The latter was achieved by submersion in the slag having theoretically highest thermal conductivity, i.e lowest FeO content and highest Al2O3 content. This stands in contrast to the findings in pilot-scale trials, showing that convection seems to be the dominant heat transfer mechanism in industrial-like conditions. 

I denna avhandling undersöks fysiska, mekaniska och smältegenskaper hos vätgasreducerade järnmalmspellets (H-DRI, Hydrogen Direct Reduced Iron) och hur dessa relaterar till både förhållanden vid reduktion och ståltillverkningsprocessen. Industriella pilotförsök och labbskaliga experiment kombinerades. Reduktion med CO-innehållande gas och resulterande naturgasreducerade järnmalmspellets (NG-DRI, Natural Gas Direct Reduced Iron) studerades som referenser. Egenskaperna av H-DRI är av stort intresse då vätgasreduktion är ett alternativ till masugnsprocessen som stod för 7-8% av globala CO2 utsläpp under 2025.  Som ett första steg studeras NG-DRI och H-DRI från ett industriellt pilotschakt med avseende på kol och cementithalt, uppskattad porositet, sprickor, tumligsstyrka, och tryckhållfasthet. Resultaten visade på att tumlingsstyrka och tryckhållfashet minskade med ökande kolhalt. Det kolfria H-DRI provet producerade 90% mindre finkornigt material och hade nästan dubbelt så högt tryckhållfasthet i jämförelse med NG-DRI provet med den högsta kolhalten på 3 wt%. Cementit identifierades i högsta koncentrationer nära ytan på pellets för de kolinnehållande proverna.   För att öka förståelsen för H-DRI utfördes fullständig reduktion med vätgas i labbskala med varierande temperatur och tidsförhållanden. En temperaturökning mellan 1.5 and 10 °C/min applicerades för att nå maximitemperaturer mellan 700 och 1000 °C. Vid maximitemperaturen hölls provet mellan 0 och 360 min. Volymförändring, uppskattad porositet, ytsprickor, tumlingsstyrka, tryckhållfasthet och mikrostruktur vid tvärsnittet på pellets undersöktes efter reduktionen. Maximitemperatur hade mest signifikant påverkan på provets fysiska och mekaniska egenskaper. Prov som reducerats totalt 5 timmar till 1000 °C respektive 700 °C producerade 9% lägre finkornigt material efter 4 timmars tumlingstester, hade 3% lägre uppskattad porositet och 13% färre pellets med synliga ytsprickor. Snabbare temperaturökning samt längre hålltid vid maximitemperaturen hade också en positiv påverkan på fysiska och mekaniska egenskaper. Ytterligare studier behövs för att förstå förhållanden i den industriella reaktorn, till exempel påverkan av vattenhalt i reduktionsgasen och tryck på pelletsbädden.  För att förstå hur kol och cementit bildas vid reduktion gjordes isoterma reduktionsexperiment med CO+H2 gasblandning vid 700 eller 900 °C, en tid mellan 1 och 5 timmar och ett flöde på 2 eller 4 nL/min. Analys av kol och 

syreinnehåll samt kvantifiering av cementit vid pelletsens tvärsnitt gjordes för pellets placerade längs bäddens vertikala axel. Metalliseringsgrad, totalt kolinnehåll och cementitmängd var högst vid botten och lägst vid toppen av pelletsbädden för alla experiment. Cementitmängd varierade även inom individuella pellets, där högst koncentration återfanns nära ytan på pellets.  Smältning av H-DRI och NG-DRI undersöktes i en pilotskalig ljusbågsugn. Utvalda resultat från en större testplan inkluderas i avhandlingen med fokus på smältningsförloppet för högmetalliserad H-DRI (99%), lågmetalliserad H-DRI (95%), och referens NG-DRI. Utförlig provtagning utfördes på slagg- och stålsammansättning, temperatur, samt vikter på ingående och utgående material. Hur bra slaggen skummade samt förekomsten av osmält material, s. k ferroberg, bedömdes visuellt. Att bibehålla en FeO halt i slaggen över 30 wt% verkade ha större positiv påverkan på smältningen än slaggens övriga egenskaper eller typen av DRI som smältes. Detta kan bero på att en stigande FeO halt i slaggen bidrar till lägre viskositet vilket förbättrar konvektiva värmeöverföringen i slaggen. Smältning undersöktes ytterligare genom labbexperiment där en H-DRI pellet sänktes ner i flytande smält slagg vars FeO, MgO och Al2O3 halter varierades. Smältningen avstannades efter 5, 10 eller 15 s genom att slaggen och den nedsänkta pelleten snabbt kyldes ner och stelnade. Provets tvärsnitt samt krossad och magnetseparerad slagg användes för att kvantifiera smältningen. Efter 15 s uppnåddes en smältning mellan 50 och 80%. Den sistnämnda uppnåddes för slaggen med teoretiskt högst värmeledning, i.e lägst FeO halt och högst Al2O3 halt. Detta stämmer inte överens med resultaten från pilotskaliga försöken, vilket visar på att konvektion verkar vara den dominerande värmeöverföringsmekanism i industriella förhållanden.   

Reducing the greenhouse gas emissions from steel production can be done through direct reduction inside a shaft furnace using hydrogen gas as a reductant, generating water as an off gas. The temperature varies along the height of the shaft furnace, and studying the non-isothermal reduction is therefore necessary. In this work, industrial hematite pellets were non-isothermally reduced in a vertical tube furnace. Different gas mixtures containing water and hydrogen were used for reduction. The reduction gas used contained water vapor contents of 5%, 10%, and 20%, respectively, and the remaining gas was hydrogen. The experimental setup was carefully designed for the reductions to be carried out under well-controlled experimental conditions. It was clear that the water present in the reduction gas significantly decreased the reduction rate, especially at the lower temperatures. Moreover, the onset temperature of reduction was increased to around 525°C when water was present, compared to 450°C when pure hydrogen was used. Water contents above 5% lead to a low-rate stage at reduction degrees between 0.11 to 0.15. The low-rate stage ended when the wüstite phase became stable, changing the mechanism of reduction, which altered the chemical reaction rate. The reduction rate was less affected by water when the heating rate increased, since an increasing heating rate led to the reduction occurring at a higher temperature. Finally, the present study showed that the kinetics of non-isothermal reduction, using different water vapor contents, are very different from isothermal reduction.

Hydrogen direct reduced iron ore pellets (H-DRI) are an increasingly relevant raw material for electric-arc furnace steelmaking as steelmakers strives to reduce CO2 emissions. Understanding the melting behavior of carbon-free H-DRI pellets in slag is needed to facilitate efficient industrial melting operations. The present study focuses on the role of slag during heating and subsequent melting of H-DRI pellets. Experiments where single H-DRI pellets are submerged in EAF slags with different FeO, Al2O3, and MgO contents have been done and evaluated. Results show that, within the experimental range, fastest melting progression is achieved with low FeO, high Al2O3 content slags. At the most favorable conditions, 80% apparent melting is reached after 15 s, in contrast to 50% under least favorable conditions. This is likely due to differences in thermal conductivity of the slag. In pilot-scale EAF trials where carbon-free H-DRI pellets is melted by continuous operation, melting performance improved with increasing FeO content. This can be explained by a decreasing viscosity which facilitated convective heat transfer. Therefore, the combination of experimental and pilot-scale results shows that convection is the dominating heat transfer mechanism in the industrial case why it should be the focus of future research and development efforts.

Direct Reduction processes for iron ore employing natural gas or hydrogen are becoming more important. Correspondingly, properties of outgoing products are of importance. Degradation of Direct Reduced Iron (DRI) feedstock greatly influences storage, handling, risk of reoxidation, and following melting operations. The focus of this work has been on the physical properties of DRI, and their effect on mechanical properties. Four types of industrially produced DRI having varying metallization, carbon content, structure and apparent porosity have been studied. Three of the selected DRI types have been reduced using natural gas, and one DRI type has been reduced using pure hydrogen. Cold compression strength and tumbling tests are performed. Qualitative analysis of samples before and after mechanical testing is made using SEM and LOM. Results show that total carbon content has a significant influence on overall mechanical strength of DRI. Degradation of samples in tumbling tests is least significant for the hydrogen-reduced – and hence carbon-free – DRI type. The DRI type containing the most carbon (3 wt%) exhibits the most fines generation and lowest compression strength. Furthermore, apparent porosity and fractures formed during reduction have a small though not as significant impact as carbon content on mechanical properties.

Direct reduction, with reducing gases containing CO and H2, is becoming an increasingly important process for reduction of iron ore to iron. There is a need for understanding reduction and carburization mechanisms for CO + H2 gases in shaft-like conditions. The experimental setup includes a column of pellets, where ingoing gas of 40 pct CO and 60 pct H2 and known temperature enters at the bottom and exits at the top. Experiments are carried out at 600 °C, 700 °C, 800 °C, and 900 °C for 1, 2.5, or 5 hours with gas flow rate of 4 or 6 nL/min. After reduction, pellets are removed, taking note of vertical position, and analyzed for reduction degree, total carbon, and cementite content. Results show that there is a gradient in reduction and carburization over the column height, which decreases with increasing time. Comparison of thermodynamic calculations with experimental results shows that kinetic factors have a strong influence on reduction and carburization. Consumption of gas by reduction or carburization reactions limits gas suitability at the local reaction sites. It could therefore be of interest to design the shaft process so that reduction and carburization take place in two steps.

The contents of hydrogen and nitrogen in the liquid steel after melting hydrogen direct reduced iron (H-DRI) inside an EAF just before tapping were investigated in this work. In addition, the contents of hydrogen and nitrogen inside H-DRI were also studied to understand their effects on hydrogen and nitrogen pickup and removal in EAF practice. The contents of hydrogen and nitrogen in the H-DRI were much greater than the solubility limit of pure iron at relevant process temperatures. Hence, a significant amount of the total contents come from adsorption on the pellet pore surface. Furthermore, the contents depend on the surface area and the type of cooling gas. The highest initial hydrogen (65-87 ppm) and nitrogen (100-120 ppm) contents were found in the laboratory-reduced H-DRI. A sudden and drastic decrease of the contents were noted when subjecting the pellet to temperatures of 1400°C, 1500°C, and 1550°C. The significant decreaseof hydrogen and nitrogen content in the pellet was due to the a) increased desorption rate, b) decreased surface area for adsorption and increased diffusion rate of the gas phase, when the temperature was increased. However, applying a longer heating time did not further decrease the contents and the crude steel sample contained a final hydrogen and nitrogen content of 6-22 ppm and 20-30ppm, respectively. Since the solubility limit of hydrogen and nitrogen significantly increase when the steel melts, the adhered gases can dissolve into the liquid metal. Furthermore, due to the small atom size of the hydrogen and fast diffusion rate, a significant amount of the hydrogen that adhered to the pellet pore surface dissolved into the liquid metal. In the case of nitrogen, the initial content (100-120 ppm) dropped below 20 ppm during heating at a temperature of 1400°C. Due to the slower dissolution rate of nitrogen, most of the adhered nitrogen is desorbed from the surface during heating. This preliminary work sheds light on the need for optimizing the refining process when H-DRI is used as input material.

In recent years, several new projects have been initiated aiming to utilize hydrogen as reducing gas for direct reduction of iron ore. One important aspect in developing this new process are the physical and mechanical properties of the outgoing hydrogen direct reduced iron (H-DRI). Product quality is essential in order to maximize yields and avoid safety hazards associated with for example fines generation or uncontrolled oxidation. As such, an investigation with focus on the impact of reduction conditions on physical and mechanical properties of the outgoing H-DRI is conducted. Non-isothermal reduction experiments are carried out using pure hydrogen gas to final temperatures of 700 °C, 800 °C, 900 °C or 1000 °C. In addition, heating rates between 1.5 and 10 °C/min as well as holding times at the final temperature of up to 6 hours are applied. The lab-produced H-DRI is investigated to determine degree of reduction, microstructural characteristics, volume change, apparent porosity, frequency of surface fractures, compression strength and tumbling strength. Results show that an increasing final reduction temperature leads to lower apparent porosity, higher tumbling strength and slightly higher compression strength. Increasing heating rate seems to have a similar effect as increasing final reduction temperature, with the exception of the fastest heating rate where significant fracture formation took place. Holding time causes slight coalescence of grains and decrease in apparent porosity, which in general gives higher tumbling strength. Comparison to an obtained pilot reduced H-DRI sample reveals considerable differences, including more than double the tumbling and compression strengths. As such, further study should be made into the remaining shaft reduction parameters.  

A 10 MT pilot-scale Electric Arc Furnace (EAF) was employed to study melting of hydrogen direct reduced iron (H-DRI) and natural gas direct reduced iron (NG-DRI). Two H-DRI types with 99% and 95% metallization, respectively, and 0 wt% carbon were tested. The NG-DRI had a metallization of 93% and carbon content of 2 wt%. A substantial trial program was planned and executed. Selected results are presented in the present paper. During a trial, H-DRI/NG-DRI is continuously fed and melted by electrical power input through the three electrodes. Slag formers, carbon and oxygen are continuously added to match the H-DRI/NG-DRI feeding rate and desired slag composition. Considerable sampling is made of ingoing raw materials, formed steel, and formed slag in terms of composition and weighed amounts. Temperature is measured frequently. Slag foaming and identification of ferrobergs is noted as per visual inspections. An FeO content above 30 wt% was found to significantly lower the likelihood for formation of a ferroberg. Good slag foaming had a secondary positive effect on lowering likelihood for ferroberg formation. The type of H-DRI/NG-DRI, including the metallization and carbon content, did not influence ferroberg formation. There was an effect on total consumption of carbon and oxygen needed to maintain desired slag composition and foaming slag conditions. Increasing FeO content relates to decreasing slag viscosity, which would enhance convective heat transfer in the slag. This could explain the seen impact of FeO content on melting of H-DRI and NG-DRI. Further study in full-scale is needed to verify the results.