Structural effects of neutral organophosphorus extractants on solvent extraction of rare-earth elements from aqueous and non-aqueous nitrate solutions

Abstract

A series of neutral organophosphorus extractants, comprising phosphine oxides, phosphinates, phosphonates and phosphates with linear and branched alkyl chains have been synthesized. Together with some similar commercial extractants, the performance of these new compounds in the extraction of Nd(III) and Dy(III) from both aqueous nitrate and non-aqueous ethylene glycol nitrate solutions was investigated. It was observed that: (1) the order of extraction efficiency of the rare-earth elements is the same as the basicity (electronegativity) of the extractant molecules: phosphine oxide > phosphinate > phosphonate > phosphate; (2) the same type of extractants with longer alkyl chains show higher metal extraction efficiencies, and branched chains hamper the extraction; (3) extraction from ethylene glycol solutions give higher separation factors for all types of neutral organophosphorus extractants than the corresponding aqueous solutions; and finally (4) phosphinates and phosphonates are more promising extractants than phosphine oxides and phosphates for separation of Nd(III) and Dy(III) by solvent extraction from ethylene glycol solutions. These results are useful for the development of more efficient and more selective new extractants and separation processes.