A non-aqueous phase extraction system using tributyl phosphate for H3PO4 separation from wet-process superphosphoric acid: Extraction equilibrium and mechanism

Conventional wet-process phosphoric acid (WPA) extraction route encounters unsatisfactory extraction efficiency, phosphorus yield, and raffinate utilization. Herein, a new extraction route for H3PO4 separation from wet-process superphosphoric acid (WSPA) is proposed to improve these dilemmas. We focus on the equilibrium of H3PO4 extraction by tributyl phosphate (TBP) from WSPA and the extraction mechanism of TBP under high H3PO4 loading conditions. Several critical factors affecting the extraction equilibrium were investigated to optimize the extraction process, including the initial phase ratio (R0), the volume fraction of TBP in extradant (φTBP), temperature (T), and the crosscurrent extraction stages. The results show that the single-stage extraction rate of H3PO4 reaches 70% at R0=6, φTBP=80% and T=80oC with separation factors βP/Fe, βP/Al, βP/Mg, and βP/Ca of 12.48, 21.66, 47.57, and 8.89, respectively. In addition, Fourier transform infrared spectroscopy and Raman spectroscopy enlighten the extraction mechanism at high loading conditions. The characteristic peak positions of P=O, P=O⋯H2O, and P=O⋯H3PO4 in the infrared spectra are determined to be centered at 1,283, 1,267, and 1,233 cm−1, respectively. The semi-quantitative analysis implies that the self-polymerization behavior of the extraction complex TBP·H3PO4 and the mutual attraction of reverse micelles (RMs) through their polar cores is the trigger for the formation of a third phase. Furthermore, the red shift of P-(OH)3 asymmetrical stretching vibration in the Raman spectrum indicates the formation of hydrogen bonds among H3PO4 molecules in the organic phase, which corroborates the formation of RMs. Conclusions can be obtained that H3PO4 enters the organic phase under high loading capacity by reversed micellar extraction. The feasibility of this extraction process is further tested by scrubbing, stripping, and cycling performance experiments. The results are promising for the design of a new efficient route for separating H3PO4 from WPA.