Recovery of Iron Resources from Wet-Process Phosphoric Acid and the Preparation of High Purity Ferrous Oxalate: Solvent Extraction and Photochemical Reaction

Oxalic acid was used as a stripping agent to recover the associated metal resource Fe from wet-process phosphoric acid by solvent extraction, and ferrous oxalate was prepared by the photochemical reaction of Fe(III)–oxalate complexes. This paper focuses on the stripping of Fe(III), the preparation of FeC2O4·2H2O, and the macroscopic kinetics of the photochemical reaction of Fe(III)(C2O4)33–. The stripping rate of Fe(III) reached 83.01% using oxalic acid. pH was decisive for the transformation of Fe(II)(C2O4)22– into FeC2O4·2H2O. Ferrous oxalate began to precipitate when pH < 3.13 and almost all iron precipitated when pH < 1.5. The high O2 content in the atmospheric gas of the photochemical reaction significantly decreased the yield of FeC2O4·2H2O. FeC2O4·2H2O had different morphologies and crystal structures at different temperatures. The total impurity content of the ferrous oxalate was less than 120 ppm, and the quality met the battery-grade standard. The photochemical reaction rate was proportional to the ultraviolet intensity, independent of the Fe(III) concentration, oxalate ligand concentration, and reaction temperature. On the basis of the macroscopic kinetic results, the photochemical reaction mechanism and kinetic equation of Fe(III)(C2O4)33– in the system were obtained. The transformation of substances during the photochemical reaction was analyzed by liquid in situ infrared spectrometry. This study confirmed the feasibility of using oxalic acid to recover iron from wet-processed phosphoric acid and prepare high purity ferrous oxalate, realizing high-value utilization of iron resources.