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.

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.

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Magnetite-based iron ore usually shows a high sticking tendency and a poor reducibility in the fluidized bed because of its dense structure. To enhance the fluidization and reduction behaviors of magnetite-based iron ore during hydrogen-induced fluidized bed reduction, the effect of a prior oxidation treatment is investigated. The results show that the untreated magnetite-based iron ore cannot be fluidized successfully in the tested temperature range between 600 °C and 800 °C. At 600 °C reduction temperature, the de-fluidization can be avoided by a prior oxidation treatment. At higher reduction temperatures, the fluidization behavior can be further improved by an addition of 0.5 wt pct MgO. Magnesiowüstite (FexMg1−xO) is formed, which decreases the contact chance of the sticky surface between particles. Regarding to the reduction rate, a prior partial oxidation is more beneficial compared to deep oxidation. The kinetic analysis shows that MgO could promote the initial reaction. The reaction rate limiting step is no longer diffusion but chemical reaction for prior partly oxidized samples. A prior partial oxidation combined with an addition of MgO is considered to be a promising pretreatment method for a successful processing of magnetite-based iron ore.

The reduction behavior of raw and prior-oxidized magnetite iron ore ultra-fines with hydrogen was investigated. Reduction tests were conducted with a thermogravimetric analyzer in a temperature range from 873 K to 1098 K at 1.1 bar absolute, using hydrogen as reducing gas. The experimental results show that a prior oxidation of the magnetite has a positive effect on the reduction behavior because of changing morphology. The apparent activation energies show a turnaround to negative values, depending on the prior oxidation and degree of reduction. A multi-step kinetic analysis based on the model developed by Johnson–Mehl–Avrami was used to reveal the limiting mechanism during reduction. At 873 K and 948 K, the reduction at the initial stage is controlled by nucleation and chemical reaction and in the final stage by nucleation only, for both raw and pre-oxidized magnetites. At higher temperatures, 1023 K and 1098 K, the reduction of raw magnetite is mainly controlled by diffusion. This changes for pre-oxidized magnetite to a mixed controlled mechanism at the initial stage. Processing magnetite iron ore ultra-fines with a hydrogen-based direct reduction technology, lower reduction temperatures and a prior oxidation are recommended, whereby a high degree of oxidation is not necessary.

Replacing carbon by hydrogen is a huge step towards reducing CO2 emissions in the iron- and steel-making industry. The reduction of iron oxides using hydrogen plasma smelting reduction as an alternative to conventional steel-making routes has been studied at Montanuniversitaet Leoben, Austria. The aim of this work was to study the slag formation during the reduction process and the reduction behaviour of iron oxides. Furthermore the reduction behaviour of iron ore during continuous feeding was assessed. Mixtures of iron ore and calcined lime with a basicity of 0, 0.8, 1.6, 2.3, and 2.9 were melted and reduced by hydrogen. The off-gas composition was measured during the operations to calculate the process parameters. The reduction parameters, namely the degree of reduction, degree of hydrogen utilisation, produced iron, and slag, are presented. The results of the batch-charged experiments showed that at the beginning of the reduction process, the degree of hydrogen utilisation was high, and then, it decreased over the operation time. In contrast, during the continuous-feeding experiment, the degree of hydrogen utilisation could be kept approximately constant. The highest degrees of reduction and hydrogen utilisation were obtained upon the application of a slag with a basicity of 2.3. The experiment showed that upon the continuous feeding of iron ore, the best conditions for the reduction process using hydrogen could be applied.

One possible route for ironmaking process is using a hydrogen‐gas‐based fluidized bed to produce direct reduced iron (DRI), which allows to use the natural magnetite directly in particle scale. The magnetite particles are oxidized during the preheating stage before being charged into the reduction unit. The exothermic effect, the crystal transformation, and the structural evolution during the oxidation of magnetite are introduced. The oxidation of magnetite is summarized in both thermodynamics and kinetics aspects. Furthermore, the influence of the oxidation of magnetite on its subsequent reduction behavior is examined.

This review focuses on the usability of iron ore ultra-fines for hydrogen-based direct reduction. Such technology is driven by the need to lower CO2 emissions and energy consumption for the iron and steel industry. In addition, low operational and capital expenditures and a high oxide yield because of the direct use of ultra-fines can be highlighted. The classification of powders for a fluidized bed are reviewed. Fluid dynamics, such as minimum fluidization velocity, entrainment velocity and fluidized state diagrams are summarized and discussed regarding the processing of iron ore ultra-fines in a fluidized bed. The influence of the reduction process, especially the agglomeration phenomenon sticking, is evaluated. Thus, the sticking determining factors and the solutions to avoid sticking are reviewed and discussed. The essential theoretical considerations and process-relevant issues are provided for the usability of iron ore ultra-fines for hydrogen-based fluidized bed direct reduction.