Enhanced separation of praseodymium and neodymium by kinetic “push and pull” system of [A336][NO₃]-DTPA in a column extractor

A new approach was suggested in present work for improving the separation between Pr(III) and Nd(III) by a so-called kinetic “push and pull” system consisting of [A336][NO₃] and DTPA in a column extractor. It is revealed that, when organic extractant [A336][NO₃] is continuously pumped into the column extractor in the form of dispersed oil droplets and at the same time DTPA was injected into the aqueous feed solution when the extraction was just started, the separation factor of Pr(III) to Nd(III), β_Pr/Nd, increased obviously with the time, and could even achieve 21.7. Such an amazing increase in β_Pr/Nd value might be due to the extraction rate of Pr(III) by [A336][NO₃] oil droplets being faster than that of Nd(III), while the complexing rate of Nd(III) with DTPA in the aqueous solutions being faster than that of Pr(III). The opposite order of the two rates for Pr(III) and Nd(III) result in their kinetic “push and pull” separation. In contrast, the β_Pr/Nd value in traditional thermodynamic separation reported in previous literatures is only around 5 or even less, even though using the same extractant [A336][NO₃] and DTPA but by previously adding DTPA into the aqueous feed solutions for pre-complexing of Pr(III) and Nd(III). Various effects from the pH and addition amount of DTPA aqueous solutions, LiNO₃ concentrations in initial aqueous feed solutions, the initial concentration ratios of Pr(III) to Nd(III) ions, the initial pH of aqueous feed solutions, and the concentrations of [A336][NO₃] in organic phases, on the kinetic separation of Pr(III) and Nd(III) are discussed. The present work highlights a promising approach for separation of rare earths or other targets with extreme similarity in physicochemical properties.