Microwave acid baking of red mud for extraction of titanium and scandium values

Abstract

The recovery of valuable metals present in the red mud is investigated through the microwave acid baking–water leaching process. The titanium and scandium dissolution during the leaching stage were optimized using the statistical design of experiments by varying microwave power, exposure time, and acid ratio. Microwave power is the governing factor for heating the mixture during the baking process. The acid ratio and exposure time are influential parameters in the investigated range for metal dissolution. The microwave irradiation synergizes the baking process by generating submicron cracks in the iron(III) sulfate and titanium oxysulfate particles. The SEM-EDS analysis of the baked products revealed that the aluminum(III) sulfate particles have fine morphology, leading to higher aluminum dissolution irrespective of the baking conditions. The linear correlation between the scandium and titanium dissolution suggests that the scandium values are majorly associated with the anatase phase. The microcracks generation in the iron and titanium sulfate particles improves the penetration of water molecules and enhances the scandium dissolution. The optimal baking conditions (600 W, 6 min, 1 mL/g) yielded titanium dissolution of 73.3% and scandium dissolution of 88.4%. The leach residue contains unreacted hematite, anatase, and quartz phases majorly. The material balance indicates that acid baking of 1 kg red mud containing 0.094 kg titanium and 60 g t−1 scandium yielded 0.27 kg residue containing 0.12 kg iron, 0.026 kg titanium, and <10 g t−1 scandium at optimal conditions. The cost analysis of the process depicts the expenditure of ~0.27 $ including 0.11 $ for raw material yielded products possessing a value of ~0.37 $ per kg of red mud.