Influence of a Prior Oxidation on the Reduction Behavior of Magnetite Iron Ore Ultra-Fines Using Hydrogen

The reduction behavior of raw and prior-oxidized magnetite iron ore ultra-fines with hydrogen was investigated. Reduction tests were conducted with a thermogravimetric analyzer in a temperature range from 873 K to 1098 K at 1.1 bar absolute, using hydrogen as reducing gas. The experimental results show that a prior oxidation of the magnetite has a positive effect on the reduction behavior because of changing morphology. The apparent activation energies show a turnaround to negative values, depending on the prior oxidation and degree of reduction. A multi-step kinetic analysis based on the model developed by Johnson-Mehl-Avrami was used to reveal the limiting mechanism during reduction. At 873 K and 948 K, the reduction at the initial stage is controlled by nucleation and chemical reaction and in the final stage by nucleation only, for both raw and pre-oxidized magnetites. At higher temperatures, 1023 K and 1098 K, the reduction of raw magnetite is mainly controlled by diffusion. This changes for pre-oxidized magnetite to a mixed controlled mechanism at the initial stage. Processing magnetite iron ore ultra-fines with a hydrogen-based direct reduction technology, lower reduction temperatures and a prior oxidation are recommended, whereby a high degree of oxidation is not necessary.