Recovery of Th(IV) from leaching solutions of rare earth residues using a synergistic solvent extraction system consisting of Cyanex 572 and n-octyl diphenyl phosphate (ODP)

Abstract

The synergistic effect of mixed Cyanex 572 and n-octyl diphenyl phosphate for Th(IV) extraction from nitric acid solution has been investigated in this paper. The effect of acidity together with mole fraction of extractant on the synergistic effect was studied. Based on the obtained results, the maximal synergistic enhancement coefficient reaches 12.40 at a ratio of 1:2 of Cyanex 572 and n-octyl diphenyl phosphate. Under the experimental conditions, the phase separation performance, stripping property and selectivity of Th and REE in the mixed extraction system are better than those from individual Cyanex 572 or n-octyl diphenyl phosphate system. The synergistic extraction mechanism for Th(IV) in nitric acid medium is proposed as Thaq4++HA2org+2.5Borg+3NO3−aq↔KexThNO33HA2B2.5org+Haq+. The mixed extraction system is successfully used for the separation of Th(IV) and REE(III) from the industrial feed solution leached from ion-absorbed type rare earth mineral waste residue. Lower stripping acid, stronger extraction ability, better phase separation performance and selectivity provide the synergistic extraction system with good application prospects.