Key Takeaways & Executive Findings
- •• Finer magnetite particles generally reduce faster and more completely, but gangue composition can override this size effect. • MgO gangue forms magnesio-wustite solid solutions that create dense microstructures, hindering hydrogen diffusion and limiting reduction. • CaO gangue promotes calcium ferrite formation, leading to porous microstructures and enhanced reducibility. • Gangue composition is a dominant factor in reduction kinetics, phase formation, and product morphology, crucial for optimizing hydrogen-based ironmaking.
Abstract
The steel industry’s transition to hydrogen-based ironmaking necessitates a deeper understanding of magnetite ore reduction, a crucial yet underexplored pathway for decarbonization. This study systematically investigates the combined effects of particle size and gangue composition on hydrogen-based reduction behavior of four industrial magnetite ore concentrates with varying CaO and MgO contents. Thermogravimetric analysis at 973 K, interrupted reduction experiments, and post-reduction characterization steps are used to evaluate reduction extent and phase transformations across different particle size fractions and bulk ores. The finer fractions generally exhibit faster and more complete reduction. However, this trend is overridden by gangue effects in certain ores. Magnetite ores with MgO as gangue tend to form magnesio-wustite solid solution (Mg,Fe)O during reduction, resulting in dense microstructures that impede hydrogen diffusion and limit reduction progress. In contrast, magnetite ores with CaO as gangue facilitate the formation of intermediate calcium ferrites, which promote porous morphology and enhanced reducibility. Notably, even the finer particles of ore containing MgO show a lower reduction degree than the coarser particles of the ore containing CaO as gangue. This highlights the dominant role of gangue composition in governing reduction kinetics, intermediate phase formation and final product morphology. These findings contribute to the growing knowledge necessary to enable fossil-free ironmaking by emphasizing the importance of considering both granulometric characteristics and heterogeneity when evaluating magnetite ores for hydrogen-based reduction.
1. Introduction
The global steel industry faces the dual challenge of meeting rising steel demand while significantly cutting greenhouse gas emissions. Accounting for approximately 7%–9% of global CO2 emissions [1–2], the steel industry relies heavily on the carbon-intensive blast furnace (BF) route for ironmaking, which contributed to 71% of global steel production in 2023 [3]. The industry is actively exploring alternative, low-carbon ironmaking technologies to mitigate these environmental impacts while sustaining economic viability and production levels. Hydrogen-based iron ore reduction has emerged as a promising approach, replacing carbon-based fossil fuels with hydrogen (H2) gas to lower emissions.
This transition is being driven by ambitious carbon neutrality goals, successful early-stage pilot projects, and small-scale implementations [4–5]. Landmark efforts like HYBRIT’s (Hydrogen Breakthrough Ironmaking Technology) production of the world’s first hydrogen-reduced sponge iron in 2021 [6], Stegra’s planned construction of a 5Mt/year green steel facility by 2030 [7], SALCOS’s (Salzgitter Low CO2 Steelmaking) initiatives to cut emissions by 95% by the end of 2033 [8], and Midrex’s development of MIDREX H2 process for using 100% hydrogen as a reductant [9] reflect growing momentum. ArcelorMittal is also advancing multiple hydrogen-based projects across Europe [10]. These initiatives reflect a global push to develop scalable and sustainable technologies for iron and steelmaking.
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Pritesh Garg, Hesham Ahmed, Charlotte Andersson, Jan-Olov Wikström, TK Sandeep Kumar, Daniel Marjavaara, Susanne Rostmark (2025). Influence of particle size and inherent gangue on hydrogen-based reduction of magnetite iron ores. Int. Journal of Minerals, Metallurgy and Materials (矿物冶金与材料学报). https://doi.org/10.1007/s12613-025-3232-5
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Frequently Asked Questions
What is the main objective of this study?
The study systematically investigates the combined effects of particle size and gangue composition on the hydrogen-based reduction behavior of magnetite iron ores, aiming to understand how these factors influence reduction kinetics and final product morphology.
How does gangue composition affect the reduction of magnetite?
MgO gangue forms magnesio-wustite solid solutions that create dense microstructures, hindering hydrogen diffusion and limiting reduction. In contrast, CaO gangue promotes the formation of calcium ferrites, leading to porous microstructures and enhanced reducibility.
What experimental methods were used in this research?
The study employed thermogravimetric analysis at 973 K, interrupted reduction experiments, and post-reduction characterization to evaluate reduction extent and phase transformations across different particle size fractions and bulk ores.
Why is hydrogen-based reduction of magnetite important?
Hydrogen-based reduction of magnetite is a promising pathway for decarbonizing the steel industry, as it replaces carbon-intensive processes with hydrogen, significantly reducing CO2 emissions.
What are the key findings regarding particle size?
Finer fractions generally exhibit faster and more complete reduction, but this trend can be overridden by gangue effects. For instance, finer particles of MgO-containing ore show lower reduction degree than coarser particles of CaO-containing ore.
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