Thermal behavior of goethite during transformation to hematite

Under non-isothermal conditions, thermal transformation kinetics of goethite was assessed from the shape of a derivative thermogravimetric (DTG) curve and then verified by the correlation between the maximum decomposition degrees and thermal kinetic mechanisms. Kinetic analysis showed that this process was governed by the three-dimensional diffusion. Microstructural changes during the process were characterized by use of infrared spectra (IR), transmission electron microscopy (TEM) and differential thermal analysis (DTA). Combining non-isothermal kinetic results and microstructural changes, thermal behavior of this transformation was described as followed. In the beginning, surface autodiffusion played a dominant role. With increasing temperatures, micropores were produced due to three-dimensional diffusion of hydrogen or hydroxyl groups in bulk goethite crystals and then merged into the slits because of the high water pressure inside. The morphology of hematite particles with regular texture was changed due to the recrystalyzation role, and finally spherical particles were formed.