Extraction performance and mechanism of TBP in the separation of Fe3+ from wet-processing phosphoric acid

Fe3+ is an impurity of the raw phosphoric acid from wet-process route and the waste phosphoric acid from the etching process of electronic industry. The effects of temperature, the Cl− content and TBP dosage on the extraction performance of synthesized extractants to Fe3+ in raw phosphoric acid from HCl wet-process route were investigated systematically. The results show that the extractant containing 12.5 vol% TBP and 87.5 vol% kerosene effectively extracted Fe3+ at 293.15 K from raw phosphoric acid containing 15.8 wt% Cl−. The extraction efficiency of 100% was realized by counter-current three-stage extraction. The extract phase was characterized by FT-IR and UV–vis. The results show that Fe3+ was extracted by TBP with the form of HFeCl4. The interaction between TBP and HFeCl4 was simulated by DFT (density functional theory). The lowest energy structure, electrostatic potential distribution, energy change, bond level analysis, electron transfer, frontline molecular orbital and other information of the possible complex were simulated systematically. The results of the characterization of the extraction phase and DFT simulation were used to clarify the extraction mechanism and the structure of the complex. The simulation results show that the lone-pair electrons of the PO double bond in TBP has a strong complexation interaction with the empty orbital of HFeCl4 and a strong bond FeO{TBP} is formed in the extraction process. The bond length of formed FeO{TBP} is 2.05A, the LBO bond level of FeO{TBP} is 0.076 (0.075), the Mayer bond level of FeO{TBP} is about 0.16, the ρb values of FeO{TBP} bond critical points range from 0.252 to 0.271 e−/bohr3. A minimum electrostatic potential of −50.59 kcal/mol is existed in TBP at the vicinity of O atom in PO double bond. Two maximum electrostatic potential of 69.43 kcal/mol and 69.92 kcal/mol are distributed symmetrically on the opposite sides of the Fe vicinity of HFeCl4. Each HFeCl4 molecule combines two TBP molecules on the opposite sides, and the binding sites are near the O atom of the PO double bond of TBP and near the Fe atom of HFeCl4. The absolute value of the binding energy is greater than 165 kJ/mol. The complexation reaction of TBP and HFeCl4 occurs through the surface electrostatic potential attraction and molecular orbital complementation forms. The results are helpful to understand the extraction mechanism of TBP to extract Fe3+ from phosphoric acid.