Reduction mechanisms of goethite-rich iron ore fines under suspension smelting conditions

Abstract

The steel industry is one of the major contributors to anthropogenic CO₂-emissions, necessitating significant changes to achieve carbon neutrality in the future. Considerable efforts in recent years have focused on developing alternatives to the traditional blast furnace (BF) process, aiming for more sustainable ironmaking pathways. Within the framework of the ULCOS initiative in the early 2000s, the HIsarna® process was developed as a promising alternative ironmaking process. HIsarna produces liquid hot metal and slag, similar to the blast furnace, but enables rapid pre-reduction of fine iron ores in a suspension environment, often referred to as flash reduction due to the short reaction times and high temperatures, followed by a final smelting reduction step. Owing to its design, HIsarna offers several process advantages compared to the conventional BF. These are, for example, the elimination of material pre-treatment steps like ore agglomeration and coking, as well as the ability to use low-grade iron ores and secondary Fe-bearing materials.

This PhD project investigated the fundamental mechanisms of suspension smelting (flash ironmaking) in the pre-reduction stage of the HIsarna process, with a particular focus on the influence of ore mineralogy and H₂-enriched reducing atmospheres. The primary objective was to understand how these factors affect the pre-reduction kinetics and particle behaviour, providing relevant insights for optimizing the HIsarna process. To achieve this, a combination of thermogravimetric analysis (TGA) and high-temperature drop tube furnace (HTDF) experiments was carried out. TGA enables a detailed kinetic analysis under controlled non-isothermal or isothermal conditions and long residence times, while the HTDF simulates isothermal in-flight particle reduction at very short residence times (hundreds of milliseconds), which is much closer to the actual conditions in the upper part of the HIsarna furnace. Incorporation of both techniques provides a comprehensive understanding of the in-flight single particle reduction behaviour, and of molten particle films, which undergo longer reduction times.

The study begins with a comprehensive characterization of three potential iron ore fines for HIsarna ironmaking (OreA, OreB, OreC) using SEM-EDS, XRD, XRF for compositional analysis, Raman spectroscopy for Fe-phase distribution, light scattering for particle size distribution, He-pycnometer for true particle density, and BET analysis to assess specific surface area and porosity. Throughout the thesis, different size fractions were investigated. For OreA, the < 63 and (63-125) μm fractions were studied, for OreB < 63, (63-125) and (125-250) μm fractions, and for OreC the < 63 and (63-125) μm fractions. The primary mineralogical difference among the three types of iron ores is the goethite content, which is absent in OreA, intermediate in OreB, and high in OreC, influencing physical properties such as porosity, true particle density, specific surface area, and reactivity. Due to the lower iron content in goethite compared to hematite, OreB (58.80-61.60 total-Fe) and OreC (56.80-58.00 total-Fe) are considered low-grade ores, whereas OreA (60.10-64.00 total-Fe) can still be classified as a medium-grade ore. For all ores, SiO₂ and Al₂O₃ are the main gangue components. Additional mineralogical differences between the ores are the dolomite phase present in OreA and kaolinite phase in OreB.

Thermal decomposition of the ores in the TGA revealed that goethite decomposition occurs in a two-stage process. The initial stage is the chemically reaction-controlled decomposition of nearly pure goethite particles within the ore. The following second stage is the decomposition of goethite-hematite intertwined phases, which is diffusion controlled. Hematite thermal decomposition in the TGA is consistently chemical reaction controlled for all ores. Higher goethite content leads to higher reduction degrees during thermal decomposition, highlighting the mineralogical impact on the pre-reduction behaviour. Hence, at the end of the non-isothermal heating zone at 1773 K OreC reached the highest reduction degrees (11.91-13.81 %), followed by OreB (11.90-12.90 %), and OreA reached the lowest reduction degrees (9.48-10.75 %). While OreB and OreC were completely converted to magnetite during the heating period, OreA required additional holding times at 1773 K to fully convert to magnetite. HTDF experiments indicated that under in-flight conditions, thermal decomposition alone resulted in much lower pre-reduction due to extremely short particle residence times. Goethite-containing ores exhibited significant cracking, followed by fragmentation, altering particle size and density, which then influenced the extent of pre-reduction and its kinetics. OreC, with the highest goethite content, showed more extensive fragmentation, whereas OreB primarily expanded before fragmentation. Reducing gas enrichment (at low CO or H₂ levels) slightly enhanced pre-reduction under flash conditions, but its effect was more pronounced in the TGA experiments, where small amounts of wustite have formed. EBSD analysis confirmed that reduction under both rapid thermal decomposition and low reducing gas-enriched atmospheres proceeded via a mixed mechanism of product-layer and internal pore diffusion.

Further isothermal TGA studies of OreB with varying H₂-levels (7.5-37.5 %) demonstrated that increasing H₂ in the atmosphere significantly accelerates reduction rates, reducing the overall reduction time even though the total reduction extent is only moderately affected. For example, at a post-combustion ratio (PCR) of 60% and at a temperature of 1773 K, the reduction in 7.5 % H₂ took 502 s to achieve a PRD (pre-reduction degree) of 24.2 %. When the H₂-content in the atmosphere was increased to 22.5 %, the reaction took only 375 s to achieve a PRD of 25.0 %. By further decreasing the PCR to 40 % and, therefore, increasing the H₂-content to 37.5 %, the reaction only needed 206 s to reach a PRD of 27.1 %. SEM-EDS analysis indicated that the reduction was controlled by the diffusion of the reducing gas through a developing product layer. Within the same PCR, the activation energy decreased from 77.60 kJ/mol to 69.12 kJ/mol by increasing the H₂ content, but increased to 79.91 kJ/mol at a lower PCR. Non-isothermal experiments confirmed that mineralogy, especially the goethite content, enhances hematite to wustite reduction, while under isothermal conditions these effects were limited after initial magnetite formation. The water-gas shift reaction was observed at higher temperatures, impacting the extent of available H2 during in-flight reduction. In HTDF experiments with 7.5 % H₂, OreA exhibited higher pre-reduction at the particle surface, whereas OreB showed more bulk reduction, indicating surface-controlled interfacial reaction for OreA and a likely more mixed control of nucleation and surface control for OreB. Testing higher H₂ levels under flash conditions was limited by rapid water-vapour formation via the water-gas shift reaction, which caused excessive condensation in the quenching zone. This condensation prevented reliable sample collection after the experiments.

Overall, this thesis provides a comprehensive understanding of how ore mineralogy and H₂-enrichment influence reaction fundamentals under in-flight suspension smelting conditions. The findings highlight that under in-flight flash pre-reduction, goethite-containing ores perform better compared to ores containing only hematite due to favorable physical and morphological properties. H₂-enrichment offers a promising approach to accelerate reduction, potentially decreasing the amounts of coal injection needed in the HIsarna smelting stage. However, its application must consider issues arising from the water-gas shift reaction, which consumes H2 and generates large amounts of water-vapour. This work establishes critical mechanistic insights for optimizing the HIsarna ironmaking process, demonstrating that control of ore selection and gas composition can enhance pre-reduction efficiency and support the development of more sustainable iron production processes.