In-situ synchrotron diffraction study of the hydrothermal transformation of goethite to hematite in sodium aluminate solutions

The hydrothermal crystallization of hematite (α-Fe₂O₃) from goethite (α-FeOOH), in sodium aluminate solutions has been studied for the first time using in-situ synchrotron time-resolved energy dispersive diffraction. This technique has allowed the transformation to be followed in real time and under the high temperature and pressure conditions (200-250 °C, 42 bar) required to allow (or induce) crystallization. To promote crystallization from goethite, hematite seed was added to accelerate the otherwise slow kinetics of crystallization. The reaction proceeds via goethite dissolution and subsequent hematite growth and the kinetic data was fitted using Avrami-Erofe'ev expression and Sharp-Hancock method. The rate of transformation of goethite to hematite increases at higher caustic concentrations, hematite surface area, and temperature. Crystallization is inhibited by the presence of anatase (TiO₂).