Reduction kinetics of mechanically activated hematite concentrate with hydrogen gas using nonisothermal methods

Steam-iron process: Influence of steam on the kinetics of iron oxide reduction

The simultaneous effect of promoters and water on the reduction of an iron catalyst for ammonia synthesis

\n Low-temperature oxidation (LTO) of magnetite is an alteration process which occurs under normal atmospheric conditions, causing maghemitization. The use of magnetic properties as palaeoclimate proxies requires improved understanding of how humidity and temperature affect such processes. We exposed natural magnetite, with grain size ranging from <1 to ∼30 μm, to different humidity conditions at room temperature and 70 °C for 1 yr. Changes in room temperature setups were very minor, but in all 70 °C setups alteration was detected by magnetic and mineralogical properties. Lowering of the Verwey transition temperature (Tv) turned out to be the most sensitive indicator of LTO, and also lattice constants correlate well with the shift of Tv. Thermomagnetic curves and XRD-results indicate that LTO affects the entire volume of the particles rather than only surface layers. The sample exposed to high relative humidity (rH) >90  per cent at 70 °C showed the strongest degree of LTO with an increase of the oxidation degree by ∼3 per cent according to Tv, and it was the only setup where partial alteration to hematite was indicated by Mössbauer analysis. The sample with extremely dry conditions (rH of ∼5 per cent) at 70 °C, and the sample that was exposed to cycles of high and low humidity in 2-weeks alternation at 70 °C, both revealed a smaller degree of LTO. The smallest change of the high temperature setups was observed for the sample with intermediate rH of ∼13 per cent. The results suggest a non-linear sensitivity of magnetite alteration to humidity conditions, high humidity strongly favours alteration, but alteration is strongly reduced when extreme humidity alternates with dry conditions, suggesting an importance of seasonality in natural weathering.

Humidity related magnetite alteration in an experimental setup

The hydrogen reduction behavior of iron oxide composite pellets containing Ni, Fe, and Mn from 973 K to 1173 K was compared with iron oxide and Al2O3 containing reference composite pellets to determine the effect of metallic species on the kinetics of iron oxide reduction. The Mn and Ni containing pellets showed slightly faster initial reduction rates compared to the Fe and Al2O3 containing pellets. The effect of the metal phases was found to be more significant at lower temperatures when chemical reaction at the interface is a slower and more controlling factor. From the SEM of partially reduced pellets, a wide intermediate region between an O rich unreacted core and an Fe rich outer shell was observed. Although an initially short topochemical receding interface controlled region exists, the mixed control between the topochemical receding interface and pore diffusion was prevalent. For Fe2O3/Mn composite pellets, the thermodynamic stability of the MnO is higher and Mn can act as a reductant for iron oxide. Thus, the overall metallization of the Fe2O3/Mn composite pellets decreased compared to the other Fe2O3/metal composite pellets. From the temperature dependence of the iron-oxide/metal composite pellets, the apparent activation energy was calculated to be approximately between 15 to 20 kJ/mol, which is typical of a mixed control reduction mechanism of gas diffusion and interface reaction.

Effect of Metal Additions to the Reduction of Iron Oxide Composite Pellets with Hydrogen at Moderate Temperatures

Freshly formed metal accelerates the rate of reduction of ferric oxide in the presence of water vapour. This effect is explained on the basis of the spillover of hydrogen from the metal sites to the oxide phase through “portholes” of water.

M. V. S. Sastri

Papers

Hydrogen spillover effects in the reduction of iron oxide