Efficient utilization of red mud waste via stepwise leaching to obtain α-hematite and mesoporous γ-alumina

Abstract

Utilizing the red mud sustainably is now a challenging issue. Red mud due to its wide production, presence of some radioactive elements, high alkalinity, and salinity, has a dramatic potential to contaminate soil and groundwater. Notwithstanding its drawbacks, Red mud consists several elements, including Ca, Al, Ti, Si, and Fe, in various mineral forms. In this study, stepwise leaching was applied as a proper method to separate and purify the main valuable elements using available and affordable HCl. The pre-leaching step under optimized conditions using HCl (0.2 M) at room temperature for 2 h removed 89% of the calcium content from red mud. To selectively remove the solid silica, the residue was treated with concentrated HCl (3.0 M, L/S of 20 mL/g) at 95 °C, resulting in the dissolution of iron and aluminum content with up to 90% efficiency. After precipitation of the Fe3+ and Al3+, they were characterized using FT-IR, BET, EDS, XRD, SEM and TEM monographs, confirming the formation of nanosized hematite (α-Fe2O3) and mesoporous gamma alumina (γ-Al2O3). Consequently, inexpensive red mud was converted into highly valuable nano-sized metal oxides using simple, sustainable techniques and cheap reagents. Moreover, this technique generates the lowest amounts of waste during the leaching process and all reagents can be recycled for further uses, making this method a sustainable utilization.