Synergistic extraction of rare earths using acid–base coupling extractants of calix[4]arene carboxyl derivative and primary amine N1923

Synergistic effect in the extraction of rare earth (RE) metals by the acid–base coupling (ABC) extractants of calix[4]arene carboxyl derivative tBu[4]CH2COOH (H4A) and primary amine N1923 (RNH2) has been investigated. The extraction of RE was enhanced by the addition of sodium cations into the aqueous phase not only in the extraction system of tBu[4]CH2COOH alone but also in the mixture of tBu[4]CH2COOH and N1923. The separation factors (SFs) indicating the extraction selectivity of adjacent RE elements became higher in the mixture system. The synergistic extracted species and reaction mechanism were deduced for the mixture system by continuous variation and slope analysis methods. The plot of synergistic enhancement coefficient Rvs. the increasing atomic number showed a “double peak effect”. The equilibrium constants and thermodynamic parameters (ΔG, ΔH and ΔS) were calculated by theoretical formula. The stripping of RE can be achieved by changing the acidity of the aqueous solution.