Fe Separation as Ferric Phosphate from Spent Pickling Liquors Through Anti-synergistic Solvent Extraction

An efficient separation method is crucial for recovering low-value iron from the spent pickling solutions as valuable materials. In this study, selective separation of Fe(III) from the spent pickling effluent was achieved through anti-synergistic solvent extraction, where Cyanex923 and TBP were used as the extractants. The extraction of both Fe(III) and Zn(II) was depressed owing to the molecular interaction between TBP and Cyanex923, but this anti-synergistic role is more evident for zinc. The separation coefficient of Fe(III)/Zn(II) can be improved from 125 to 525 when 0.2 mol/L TBP was mixed with 0.1 mol/L Cyanex923. The structures of the organic phase before and after extraction were disclosed by UV–vis, FT-IR, and SAXS spectroscopies. The addition of TBP can boost Fe(III) stripping because the molecular interaction impairs the combining capacity between Fe(III) and Cyanex923. Moreover, the loaded Fe(III) was recovered as the ferric phosphate precipitates. Therefore, the interaction between Cyanex923 and TBP enhances the separation of Fe(III)/Zn(II) and promotes Fe(III) stripping, thus realizing its efficient recovery and utilization. These results will contribute to the efficient treatment and recycling of spent pickling liquors.