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Due to the high temperatures present in the HIsarna® process (∼1773 K), the two reaction mechanisms thermal decomposition and gas reduction occur simultaneously during the pre-reduction of the injected iron ore fines. The objective of this study is to investigate the impact of H2-enrichment in HIsarna reducing gas atmospheres on the pre-reduction of iron ores. Therefore, several iron ores, named OreA, OreB and OreC, with increasing amounts of goethite, were reduced by gas mixtures of H2-CO-CO2-Ar in a thermogravimetric analyser (TGA). A first approach studied the gas reduction as isolated process in isothermal conditions, whereas the second, non-isothermal approach, investigated thermal decomposition and gas reduction occurring simultaneously. In isothermal conditions the increase of H2 in the atmosphere leads to an increase in conversion to wustite, and also to an increase in reaction speed. At 7.5 % H2 in the atmosphere, the reaction saturates after 502–794 s, at 22.5 % H2 after 375–583 s and at 37.5 % H2 after 206–297 s, depending on the temperature. The reaction is controlled by the diffusion of the reducing gas through the developing product layer and the calculated activation energy is 77.60, 69.12 and 79.91 kJ/mol for H2-contents of 7.5 %, 22.5 % and 37.5 %, respectively. In non-isothermal conditions, the influence of ore mineralogy is more significant compared to the H2 increase in the atmosphere. An increase in total conversion is observed with an increase in goethite content in the ores.
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Due to the high temperatures present in the HIsarna® process (∼1773 K), the two reaction mechanisms thermal decomposition and gas reduction occur simultaneously during the pre-reduction of the injected iron ore fines. The objective of this study is to investigate the impact of H2-enrichment in HIsarna reducing gas atmospheres on the pre-reduction of iron ores. Therefore, several iron ores, named OreA, OreB and OreC, with increasing amounts of goethite, were reduced by gas mixtures of H2-CO-CO2-Ar in a thermogravimetric analyser (TGA). A first approach studied the gas reduction as isolated process in isothermal conditions, whereas the second, non-isothermal approach, investigated thermal decomposition and gas reduction occurring simultaneously. In isothermal conditions the increase of H2 in the atmosphere leads to an increase in conversion to wustite, and also to an increase in reaction speed. At 7.5 % H2 in the atmosphere, the reaction saturates after 502–794 s, at 22.5 % H2 after 375–583 s and at 37.5 % H2 after 206–297 s, depending on the temperature. The reaction is controlled by the diffusion of the reducing gas through the developing product layer and the calculated activation energy is 77.60, 69.12 and 79.91 kJ/mol for H2-contents of 7.5 %, 22.5 % and 37.5 %, respectively. In non-isothermal conditions, the influence of ore mineralogy is more significant compared to the H2 increase in the atmosphere. An increase in total conversion is observed with an increase in goethite content in the ores.
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.
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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.
