Synergistic strategies for defluoridation from RECl3–AlCl3–F complex solution: pH regulation and hydrolysis coprecipitation of aluminum by Ca2Al(OH)6Cl·2H2O

Abstract

Excessive fluoride and Al–F complex- ions severely impede the efficient extraction of rare earth elements (REEs) from rare earth chloride (RECl₃) solution. Herein, hydrated calcium chloroaluminate (Ca₂Al(OH)₆Cl·2H₂O) was prepared via a facile neutralization precipitation method and applied as a defluoridant to remove fluoride and aluminum to address the challenges in RECl₃ solution. Under the optimized conditions (initial pH = 2.0, 15 g L⁻¹ dosage, and 2 hours), the F concentration was reduced to 15.3 mg L⁻¹, achieving a defluoridation yield of 96.4% and a dealumination yield of 85.4% by using Ca₂Al(OH)₆Cl·2H₂O. X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS) analyses reveal that the defluoridation mechanism primarily involves in situ hydrolysis coprecipitation of Al–F complex ions. Based on the study of reaction kinetics, the defluoridation rate equation was −d[F]/dt = 7.389 exp(−2.96/RT)C_F0^0.916C_CA^0.972. The activation energy of the hydrolysis coprecipitation defluoridation was 2.96 kJ mol⁻¹, indicating a relatively rapid reaction rate. This work provides an effective strategy for the simultaneous removal of fluoride and aluminum, eliminating their hazards in REE extraction and facilitating the recovery of aluminum resources.