Kinetics of thermal transformation of synthetic al-maghemites into al-hematites

Abstract

Maghemite (γ-Fe2O3) and hematite (α-Fe2O3) are important iron oxides in Brazilian soils derived from basalt. Maghemite can transform into hematite when exposed to high temperatures. However, isomorphic substitution (e.g., Al3+) may largely influence this process. We analyzed the kinetics of thermal transformation of Al-maghemites into Al-hematites and some of its mineralogical aspects. Synthetic substituted maghemites with different degrees of Al-substitution (0.0, 1.0, 2.0, 2.9, 3.8, 5.6, 6.7, 10.0, 12.0, and 17.1 mol% Al) were subjected to a temperature of 500±10 °C for 0, 5, 10, 16, 64, 128, 192, 360, 720, 2160, 3600, 5040, and 6480 min. After thermal treatment, samples were characterized by X ray diffraction (XRD), differential thermal analysis (DTA), specific surface area (SSA) measurement, and total chemical analysis. XRD patterns were analyzed by Rietveld refinement, and maghemite and hematite contents were calculated using Rietveld refinement and the method proposed by Sidhu. Isomorphic substitution of Fe by Al increased the critical temperature of transformation and the time necessary for maghemite to hematite transformation. Rietveld refinement data showed a better fit than the data adjusted by the Sidhu method. Increasing isomorphic substitution also decreased lattice parameters and mean crystallite dimension (MCD) values in maghemite; but only c-dimension and MCD decreased with increasing Al-substitution in hematite. For maghemite, the SSA increased with isomorphic substitution, rising up to 5.9 mol% Al; for hematite, SSA increased linearly. SSA decreased with heating time, regardless of isomorphic substitution.