Philipp Leerhoff

During high-temperature in-flight flash reduction of iron ores, goethitic particles undergo rapid structural changes. This study examines the influence of goethite on pre-reduction performance in a drop-tube furnace, focusing on the hematite to magnetite reduction step. Three iron ores (OreA, OreB, and OreC) with distinct mineralogical and physical characteristics were reduced in N ₂ and atmospheres containing 95 % CO ₂ with 5 % CO or H ₂ at temperatures between 1658 and 1760 K. Compared to ores containing hematite as the sole iron-bearing phase, goethite-containing ores exhibit enhanced cracking and fragmentation during thermal decomposition, leading to changes in particle size and density. A comparison of OreB (lower goethite) and OreC (higher goethite) shows that the importance of particle residence time increases with goethite content. Higher pre-reduction was achieved at 80 ms for OreB and at 430 ms for OreC during thermal decomposition. Despite the use of reducing gases, reduction beyond magnetite was not observed due to large particle sizes (> 100 µm) and residence-time limitations of the drop-tube furnace. Further reduction (Fe ₃ O ₄ → FeO) was assessed using thermogravimetric analysis. Across both experimental set-ups, thermal decomposition has a higher impact on the total reduction than the gas reduction. EBSD analysis identified two reaction mechanisms, internal pore diffusion and surface product layer diffusion. Balanced mineralogical properties (as in OreB) were found to be critical for effective early stage in-flight flash reduction.

Suspension smelting technology is emerging as a promising alternative in ironmaking to conventional hydrogen-based direct reduction (H-DRI) for low-carbon ironmaking. Unlike the slower solid-state reduction reactions in DRI processes, suspension smelting uses rapid in-flight reduction of fine iron ore particles. Within seconds, the ore fines undergo several process steps like heating up, phase transformation, reduction and partially melting, due to the high temperatures. This review explores hydrogen adaptation in suspension smelting technologies such as the flash ironmaking technology (FIT) and the HIsarna ^® process. The key reaction mechanisms, thermal decomposition and gas reduction with H ₂ /CO, are examined through thermodynamic and kinetic analyses. A comparison of reduction kinetics between classical DRI processes and suspension smelting is provided, highlighting the significantly faster reaction rates of suspension smelting. Finally, the influence of the water–gas shift reaction and its potential negative impact on the reduction behaviour is analysed.

In the pre-reduction cyclone of the HIsarna process, both thermal decomposition and gas reduction of the injected iron ores occur simultaneously at gas temperatures of 1723–1773 K. In this study, the kinetics of the thermal decomposition of three iron ores (namely OreA, OreB and OreC) for HIsarna ironmaking were analysed as an isolated process with a symmetrical thermogravimetric analyser (TGA) under an inert atmosphere. Using various methods, the chemical and mineralogical composition, particle size distribution, morphology and phase distribution of the ores were analysed. The ores differ in their mineralogy and morphology, where OreA only contains hematite as iron-bearing phase and OreB and OreC include goethite and hematite. To obtain the kinetic parameters in non-isothermal conditions, the Coats–Redfern Integral Method was applied for heating rates of 1, 2 and 5 K/min and a maximum temperature of 1773 K. The TGA results indicate that goethite and hematite decomposition occur as a two-stage process in an inert atmosphere of Ar. The proposed reaction mechanism for the first stage of goethite decomposition is chemical reaction with an activation energy ranging from 46.55 to 60.38 kJ/mol for OreB and from 69.90 to 134.47 kJ/mol for OreC. The proposed reaction mechanism for the second stage of goethite decomposition is diffusion, showing an activation energy ranging between 24.43 and 44.76 kJ/mol for OreB and between 3.32 and 23.29 kJ/mol for OreC. In terms of hematite decomposition, only the first stage was analysed. The proposed reaction mechanism is chemical reaction control. OreA shows an activation energy of 545.47 to 670.50 kJ/mol, OreB one of 587.68 to 831.54 kJ/mol and OreC one of 424.31 to 592.32 kJ/mol.