Extraction of lanthanide ions from aqueous solution by modified supercritical CO2: tri-n-butylphosphate+CO2 and bis-2-ethylhexyl phosphoric acid+CO2

High-pressure single-phase limit of binary tri-n-butylphosphate (TBP)+CO2 and bis-2-ethylhexyl phosphoric acid (B2EHPA)+CO2 was measured. By a variable-volume view-cell apparatus, the phase transition pressures of each system were measured at various phase-equilibrium conditions (313.15–333.15 K, 10–25 MPa, 3.7–18.9 mol% TBP and 1.0–45.0 mol% B2EHPA). Based on this data, supercritical fluid extraction (SFE) was carried out for lanthanide ions from acidic aqueous solution. In the first place, SFE was carried out for Nd3+ over a wide range of conditions (0–5 M HNO3, 313.15–333.15 K and 10–25 MPa) and optimum SFE process conditions are evaluated. In the second place, SFE was carried out at 313.15 K and 25 MPa for six types of lanthanide ions (La3+, Ce3+, Nd3+, Sm3+, Eu3+, and Yb3+) from 1 M HNO3 acidic solution and the characteristics of different metals on the SFE were compared.