This review focuses on the reduction of iron oxides using hydrogen as a reducing agent. Due to increasing requirements from environmental issues, a change of process concepts in the iron and steel industry is necessary within the next few years. Currently, crude steel production is mainly based on fossil fuels, and emitting of the climate‐relevant gas carbon dioxide is integral. One opportunity to avoid or reduce greenhouse gas emissions is substituting hydrogen for carbon as an energy source and reducing agent. Hydrogen, produced via renewable energies, allows carbon‐free reduction and avoids forming harmful greenhouse gases during the reduction process. The thermodynamic and kinetic behaviors of reduction with hydrogen are summarized and discussed in this review. The effects of influencing parameters, such as temperature, type of iron oxide, grain size, etc. are shown and compared with the reduction behavior of iron oxides with carbon monoxide. Different methods to describe the kinetics of the reduction progress and the role of the apparent activation energy are shown and proofed regarding their plausibility.

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To optimize existing iron ore reduction processes or to develop new ones, it is necessary to know the reduction kinetics of the iron ore of interest under the relevant operating conditions. In this work the reduction kinetics of hematite fine iron ore was studied for industrial-scale processes using the fluidized bed technology. Especially designed batch tests were performed in a laboratory-scale fluidized bed reactor fluidized with H 2 , H 2 O, CO, CO 2 , N 2 at atmospheric and elevated pressures to simulate the relevant process conditions. To obtain the reduction rates and the degree of reduction, the concentrations of H 2 O, CO, and CO 2 in the outlet gas were analyzed by FT-IR spectroscopy.Preliminary reduction tests showed a strong effect of the sample weight on the reduction rates, especially in the early stages of reduction. The optimum sample weight was determined by partly replacing the hematite with silica sand. Additionally, the silica sand provided a constant and stable flow pattern throughout the reduction tests. The effects of temperature, gas composition, particle size and pressure on the rates of reduction were tested and discussed.Rate analysis showed the existence of two phases with different rates during the reduction tests. Additional investigations (microscope analysis, SEM) demonstrated that in the first phase the rates were controlled by mass transport in the gas phase and in the second phase by the reduction process within the small grains of the iron ore particles.KEY WORDS: iron ore reduction; high temperature fluidized bed; reduction kinetics; elevated pressure; H 2 -CO gas mixture.

A new model for the simulation of the converter steelmaking process was developed after the detailed study of existing thermodynamic and kinetic models and approaches. This model consists of the reaction model and models for charge materials melting and dissolution. The reaction model is based on the coupled reaction model, which includes the combination of both thermodynamics and kinetics of the involved phases. The models of charge material melting and dissolution include the definition of the driving force for the melting and dissolution, and the mass transfer coefficients in the metal and slag phases.

The reduction kinetics of hematite iron ore fines to metallic iron by hydrogen using a laboratory fluidized bed reactor were investigated in a temperature range between 873 K and 1073 K, by measuring the weight change of the sample portion during reduction. The fluidization conditions were checked regarding plausibility within the Grace diagram and the measured pressure drop across the material during experiments. The apparent activation energy of the reduction was determined against the degree of reduction and varied along an estimated twopeak curve between 11 and 55 kJ mol À1 . Conventional kinetic analysis for the reduction of FeO to metallic iron, using typical models to describe gas-solid reactions, does not show results with high accuracy. Multistep kinetic analysis, using the Johnson-Mehl-Avrami model, shows that the initial stage of reduction from Fe 2 O 3 to Fe 3 O 4 , and partly to FeO, is controlled by diffusion and chemical reaction, depending on the temperature. Further reduction can be described by a combination of nucleation and chemical reaction, whereby the influence of nucleation increases with an increasing reduction temperature. The results of the kinetical analysis were linked to the shape of the curve from apparent activation energy against the degree of reduction.

High temperature confocal scanning laser microscopy (HT‐CSLM) is used to study the dissolution behavior of Al2O3 inclusions in various slag compositions in the system CaO‐Al2O3‐SiO2‐MgO. This method enables the in situ observation of the dissolution at steelmaking temperatures. The change of the diameter of the spherical inclusion is measured by image analysis of pictures obtained from the HT‐CSLM. Subsequently, dissolution rates and normalized dissolution curves are determined, and the governing dissolution mechanism is identified by the use of a modified approach of the diffusion equation introduced by Feichtinger et al. and compared with the dissolution of SiO2 previously reported by the same authors. Finally, effective binary diffusion coefficients are calculated. Slag viscosity is shown to essentially affect the dissolution behavior, changing the normalized dissolution pattern from rather S‐shaped (high slag viscosity) to a parabolic form (low slag viscosity).

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The metallurgical properties and the microstructure of coke after gasification reaction with pure H2O and pure CO2 were investigated in this study. Moreover, the first-principles calculation was conducted to study the reaction process of the carbon with pure H2O and pure CO2. The results show that the CRI (coke reaction index) increases sharply and the CSR (coke strength after reaction) decreases sharply, when the cokes are gasified with H2O as compared to CO2. The scanning electronic microscopy images and the coke panoramagrams show that H2O more easily leads to the generation of large pores (>500 μm) and destroys the coke structure than CO2. The X-ray diffraction results indicate that the arrangement of carbon atoms of coke becomes regular and the ordered degree of coke increases after reaction with CO2 and H2O; however, after being gasified with H2O, the cokes have a higher ordered degree than with CO2. The results of the first-principles calculation show that the H2O molecule is more likely to react with carbon as compared to the CO2 molecule due to the lower energy barriers of H2O adsorption and H2 formation. The M2 → FS reaction process is the controlled step of the C-H2O reaction process, as well as in the C-CO2 reaction system.