An environmental-friendly method for recovery of soluble sodium and harmless utilization of red mud: Solidification, separation, and mechanism

Red mud is a hazardous waste produced from Bayer process, and its high alkalinity caused by soluble sodium (Na) poses a major threat to environment. In this study, an environmental-friendly method was proposed for recovery of soluble sodium (Na) and harmless utilization of red mud. First, soluble sodium was selectively solidified into Na-rich phases (anorthite and melilite). Subsequently, high-purity Na-rich phases were efficiently recovered from red mud under super gravity. Finally, solidification mechanism of sodium in anorthite and melilite was confirmed by characterizing high-purity Na-rich crystals. The leaching rates of Na+ were only 0.01% and 0.04% in anorthite and melilite, respectively, and high alkalinity was minimized. In addition, the anorthite was found to possess a stronger solidification capacity of Na as compared with melilite based on the reaction mechanism of Na+ + Si4+= Ca2+ + Al3+. This provides a theoretical basis for alkalinity minimization and harmless utilization of red mud.