Preparation and Application of Polyaluminum Ferric Sulfate from Red Mud: Behaviors of Leaching, Polymerizing, and Coagulation

Red mud is a solid waste containing valuable components, such as aluminum and iron. This paper aims to recover aluminum and iron from red mud by acid leaching and prepare polyaluminum ferric sulfate (PAFS) to apply for the turbidity reduction treatment of coal slurry water. The behaviors of leaching, polymerizing, and coagulation were analyzed by leaching thermodynamics and advanced micro-detection methods. More than 90% of aluminum and 60% of iron in red mud were dissolved into the leaching solution by using 50% sulfuric acid (v/v) with 7 mL/g at 100 °C for 2 h, where the crystal lattice of cancrinite was significantly destroyed to promote the dissolution of aluminum. The low polymerization (Al + Fe)_a, medium polymerization (Al + Fe)_b, and high polymerization (Al + Fe)_c could be generated in PAFS by adjusting the basicity of the leaching solution with 0.7-0.9. The removal efficiency of turbidity of wastewater could reach more than 95% by using PAFS at 25 mg/L in the pH range of 6.0-7.0. The turbidity reduction mechanism included not only the electric neutralization of (Al + Fe)_a but also the adsorption of (Al + Fe)_b and the entrapment effect of (Al + Fe)_c. This current study contributes to the future development of red mud based on flocculants containing aluminum and iron for wastewater treatment.