EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

The steadily increasing amount of ultra-fine iron ores, especially magnetite, due to the intensive beneficiation of low-grade ore deposits in combination with the industrial de-carbonization towards a hydrogen-based economy, require new solutions for the iron and steel industry. Taking the endothermic behavior of the hydrogen-based reduction of iron ore into account, a pre-heating step of the input material is a straightforward approach providing sufficient energy to the process. In case of magnetite, a pre-heating step under oxidizing atmosphere would facilitate the heating due to the exothermic character of the oxidation to Fe2O3. Therefore, the effect of a pre-heating step under oxidizing atmosphere on the reduction behavior of magnetite-based iron ore ultra-fines was investigated. The reduction tests were conducted with a thermogravimetric analyzer with hydrogen as reducing gas in a temperature range of 873 to 1098 K at ambient pressure. The experimental results show that the oxidizing pre-heating step has a significant effect on the reduction performance of magnetite-based ore using hydrogen as reducing gas. A multi-step kinetic analysis, based on the model developed by Johnson-Mehl-Avrami, was used to identify the limiting mechanism during the reduction as well as to evaluate the influence of the pre-oxidation. The kinetic analysis reveals that at temperatures of 873 and 948 K for both, raw and oxidized magnetite, the reduction proceeds at the initial stage correspondingly to nucleation and reaction and for the final stage nucleation only. At 1023 and 1098 K, the reduction of the raw magnetite is mainly controlled by diffusion, which has changed for oxidized magnetite towards a mixture of all three mechanisms at the initial stage but still diffusion for the final stage. The results are confirmed by the morphology of the reduced samples.