Phase equilibria of MgO-Al2O3-TiO2 system at 1600°C in air: Emphasis on pseudobrookite and spinel solid solution phases

The recycling of titanium from titania-bearing slag is an important part of the circular economy; however, the yield of titania is restricted due to the lack of fundamental thermodynamic data. In the present work, the equilibrium phase relations of MgO-Al2O3-TiO2 system in air at 1600 degrees C have been determined using a high-temperature equilibration-quenching technique followed by X-ray photoelectron spectroscopy, scanning electron microscopy-energy-dispersive X-ray spectrometry, and X-ray diffraction. Ti4+ was confirmed as the stable valence of titanium, and the rutile and spinel solid solution phases were detected to be coexisting with pseudobrookite solid solution phase, respectively. The formation principles of pseudobrookite and spinel solid solutions were confirmed in view of the corresponding crystal structure and the composition substitution relationship of the oxides at 1600 degrees C. However, great discrepancies were found from the comparison of the present experimental results with the thermodynamic calculations and the literature results, and the present equilibrium phase relations are therefore significant for updating the present titania related thermodynamic database for a superior prediction of the thermodynamic behaviors of relevant process.