Influence of X-Ray Penetration Depth on the XRD Characterization of Magnetite Hydrogen-Reduced in a Fluidized Bed Reactor

Hydrogen-based reduction of iron ores is a promising route toward low-carbon ironmaking, making reliable determination of the reduction degree essential. However, XRD analysis may overestimate conversion in coarse particles with radial phase gradients, as X-rays mainly probe the particle surface. This study investigates the effect of X-ray penetration depth on the XRD characterization of hydrogen-reduced magnetite particles (~100 μm radius). Two magnetite materials were reduced at 500–600 °C and analyzed before and after milling to ~20 μm. At short reaction times, both measurements gave comparable results, while at longer times non-milled particles showed significantly higher apparent Fe contents and reduction degrees. After 90 min at 600 °C, for example, the Ballast sample appeared fully reduced without milling, whereas milling revealed ~31 wt% residual magnetite and a reduction degree of ~75%. Thermogravimetric measurements confirmed the latter value. The results show that XRD can be strongly surface-weighted in coarse, heterogeneous particles, leading to overestimation of bulk conversion. Milling improves measurement representativeness, while particle size, X-ray energy and reaction pattern must be considered together.