Efficient extraction and separation of heavy rare earths from chloride medium with N, N′-di(2-ethylhexyl)aminomethyl phosphonic acid mono-2-ethylhexyl ester

Abstract

The separation of heavy rare earths (HREs) at a higher efficiency using solvent extraction has been one of the most difficult tasks in hydrometallurgy. Herein, a novel extractant N, N′-di(2-ethylhexyl)aminomethyl phosphonic acid mono-2-ethylhexyl ester (HEDEAP, abbreviated as HA) was synthesized and used for the extraction and separation of HREs from chloride medium. The pKa and purity of HA was determined to be 7.82 and 93.8% by potentiometric titration. The separation factors (SF) between adjacent HREs, i.e. SFEr/Ho, SFTm/Er, SFYb/Tm, and SFLu/Yb were determined to be 3.05, 3.30, 3.53 and 1.52, respectively, which are higher than those of the analogues of HA and some typical extractants such as P204 and P507 under the same conditions. The extraction of REs by HA follows cation exchange mechanism and the extracted complex of trivalent REs was determined to be REH2ClA4 by slope analysis. Thermodynamic parameters (ΔHo, ΔGo and ΔSo) of Lu3+ were calculated as 4.02 kJ mol−1, −7.21 kJ mol−1 and 37.7 J mol−1 K−1 at 298 K, respectively, indicating that the extraction of Lu3+ is an entropy-driven endothermic process. The loading capacities of 30% (V/V) HA toward Y, Yb and Lu were about 10.3 g/L (Y2O3), 21.6 g/L (Yb2O3) and 25.4 g/L (Lu2O3), respectively. Among inorganic acids such as H2SO4, HNO3 and HCl, H2SO4 was the most effective stripping acid, and 98% Yb3+ can be stripped from the loaded organic phase by just one stage of stripping with 0.25 mol/L H2SO4. Therefore, HA would be a potential extractant for the separation of HREs.