Depending on the operational conditions inside a direct reduction shaft furnace, e.g., ingoing gas temperature, feeding rate of material, and gas composition, the outgoing material will differ. This study investigates how the heating rate affects the reduction during pure hydrogen reduction of commercial iron ore pellets. As expected, the reduction rate increased with increasing heating rate. The heating rate also significantly affected the microstructure evolution inside the pellet. Inside the hydrogen direct reduced pellets, the iron had two appearances: (1) porous iron containing small and numerous intragranular pores, or (2) dense iron with larger but fewer intragranular pores. The pellet reduced with the slowest heating rate consisted of only porous iron, while the faster heating rates comprised porous and dense iron. The amount of dense iron gradually increased with increasing heating rate and was found to start forming at a temperature of around 668°C. The solid iron aggravated the mass transfer through the product layer and decreased the total reaction rate. This led to an expanded spread of the reaction zone as the heating rate increased. Through this work, it was also shown that insignificant reduction took place below a temperature of 450°C. Lastly, the microstructure that evolved during the non-isothermal reduction vastly differs from the microstructure formed during isothermal reduction. Consequently, an effective diffusivity and thermal conductivity that varies with time and temperature must be considered when optimizing the shaft furnace reactor.

As the iron and steel industry now strives for a carbon neutral industry, hydrogenbased direct reduction shaft furnace technology has become an alternative to theheavily fossil-depending blast furnace route. Research questions related to thefuture full-scale production have, therefore, become more interesting. Dependingon the operational conditions, the H2 concentration and temperature will varyacross the length of the reactor. This work studies the effect of nitrogen in ahydrogen-reducing gas during the reduction of commercial iron ore pellets usingthermogravimetric analysis. The reducing gas consisted of either pure hydrogenor a mixture of 90–70 vol% hydrogen and 10–30 vol% nitrogen at 773, 873, 973,1073, and 1173 K. It is found that the reduction rate decreased with decreasingtemperature and increasing nitrogen content. The effect of nitrogen on thereduction rate is more profound than expected from the decreased hydrogenpartial pressure alone. To aid the discussion, partially reduced pellets are studiedusing optical and scanning electron microscopy. It is found that the microstructure is strongly dependent on the temperature but independent of thenitrogen content.

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.

The development of new materials and their production processes along with the environmental constraints demand new data of high quality, especially thermodynamic and physical property data. As slags play a crucial role in metallurgical processes and recycling, the need of better understanding of the reaction mechanisms between slag and metal is also increasingly felt. High precision data and better understanding of the reaction mechanism require efficient collaboration between the researchers in the laboratory and in the industries. Unfortunately, in some cases, the reported data are not obtained in well-controlled experimental conditions. Without the knowledge of the experimental conditions, the use of the data in industrial practice would possibly lead to unintended results. In other cases, the measurements themselves, even their principles, are questionable. This review article addresses how to make the laboratory investigation more relevant to the industrial reality. Some existing uncertainties in the laboratory studies are also discussed. To help a sensible discussion, some specially designed experiments are conducted to support the argument. The review is focused on slag properties (namely, sulfide capacity, phosphate capacity, apparent viscosity, and apparent interfacial tension) and studies of interfacial slag phenomena.

Ni-based superalloy, which has excellent high-temperature strength and corrosion resistance, is mainly used in aviation materials, high-performance internal combustion engines, and turbines for thermal and nuclear power generation. For this reason, refining the impurities in Ni-based superalloys is a very important technical task. Nevertheless, the original technology for the melting and refining of Ni-based superalloys is still insufficient. Therefore, in this study, the effect of the CaO-Al2O3-MgO-TiO2 slag on the removal efficiency of an impurity element sulfur in Incoloy® 825 superalloy, one of the representative Ni-based superalloys, was investigated. The desulfurization behavior according to the change of TiO2 content and CaO/Al2O3 (=C/A, basicity) ratio as experimental variables was observed at 1773 K (1500 °C). Although the TiO2 content in the slag increases to 15 mass pct, the mass transfer coefficient of sulfur in molten alloy showed a constant value. Alternatively, under the condition of C/A > 1.0 of slag, the mass transfer coefficient of sulfur showed a constant value, whereas under the condition of C/A < 1.0, the mass transfer coefficient of sulfur greatly decreased as CaO decreased. Hence, in the desulfurization of Incoloy® 825 superalloy using the CaO-Al2O3-MgO-TiO2 slag, the TiO2 content in the slag does not have a considerable effect on the desulfurization rate and desulfurization mechanism (metal phase mass transfer controlled regime), but the basicity of the slag has a significant effect on desulfurization mechanism. When the slag basicity decreases below the critical level, i.e., C/A < 1.0, which is corresponding to sulfur distribution ratio, Ls < 200, it was confirmed that the desulfurization mechanism shifts from the metal phase mass transfer-controlled regime to the slag phase mass transfer-controlled regime due to the variation in the physicochemical properties of the slag such as viscosity and sulfide capacity. In addition, the different desulfurization rates between steel and Ni alloy melts were discussed by employing the diffusivity of sulfur in both systems.

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.

The paper presents a study on the decarburization of pig iron droplets in synthetic BOF slag. The effects of droplet size and slag composition were studied. The results show evidently that the decarburization is very fast in general. One gram of pig iron is mostly decarburized within one minute. The reaction is shown to be faster when many small droplets were employed instead of one big droplet with equal total mass. This is explained by the bigger interfacial area between the liquid slag and the pig iron of the many small particles. It is also interesting to see that the decarburizing reaction in the slag having lower dynamic viscosity and higher FeO activity is slower than in the slag with higher viscosity and lower FeO activity. The slower reaction could be explained by the longer incubation time in this slag.

The structure of foaming synthetic BOF-converter slags was studied by freezing the foam and using ocular examination. The foams were generated by CO gas formed due to the reaction between FeO in the slag and carbon in the hot metal. The character of the foams varied a lot with slag composition. Slag with lower viscosity resulted in foams with small gas bubbles, while slag having high viscosity resulted in very big bubbles.

A semi-empirical model was developed to predict the apparent velocity of particles falling through foams. Different foams were generated from liquids with different viscosities and surface tensions. Particles with different sizes and densities were dropped into the foam and average velocities were calculated. Based on the experimental work, the semi-empirical model was derived from an energy balance between buoyancy, drag and the energy needed for the particles to deform the bubbles in their path.