Reaction and transformation behaviors of ferric oxides and titanium oxide in diasporic bauxite during an alkaline hydrothermal process

Ferric and titanium oxides are the main impurities in low-grade bauxite that significantly affect alumina extraction in the Bayer process. This study aims to investigate the reaction and transformation behavior of ferric and titanium oxides in diasporic bauxite during a mild alkaline hydrothermal process to minimize the inhibitory effects of Fe and Ti impurities on bauxite digestion. Analytical techniques such as X-ray diffraction, X-ray fluorescence, inductively coupled plasma, scanning electron microscopy with energy dispersive X-ray spectroscopy, mineral liberation analysis, and Mössbauer spectroscopy were used to analyze the occurrence states of impurities in the raw bauxite and residue, explain their reaction mechanisms, and understand their transformation under hydrothermal conditions. Starch was used as a reducing agent to transform hematite into magnetite and facilitate the subsequent reaction of magnetite with anatase to minimize adverse effects. Thermodynamic calculations and results from experimental investigations showed that under optimized operating conditions of 180 °C, reaction time of 90 min, alkali concentration of 60 g/L, and a small quantity of starch, significant transformations of iron and titanium occurred. These include transformation of (i) hematite to magnetite (54.8 %) and (ii) anatase to Ti-containing stable phases such as ilmenite (19.7 %), ferrous pseudo brookite (12.7 %), ulvite (12.9 %) iron titanate, and perovskites. After the pretreatment with bauxite, alumina digestion can exceed 90.0 %.