Competitive extraction of lanthanides by solvent extraction using Cyanex 272: Analysis, classification and mechanism

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In comparison to bis-2-ethylhexylphosphoric acid (HDEPA) and 2-ethylhexyl phenylphosphonic acid (HEHΦP), bis(2,4,4-trimethylpentyl)phosphinic acid (Cyanex 272) did not extract the lanthanide elements from very acidic solutions (H3O+ > 0.5 M)15. However, it exhibits excellent physical phase separation properties at low acid concentrations compared to the two extractants. Although not widely used for lanthanide elements separation, the present work sought not to preclude the possibility. The novelty of our work is that the proposed models show that it is possible to predict Cyanex 272 concentrations/pH values for the sequential extraction/separation of individual or quartets of lanthanide elements, in a solution that contains all fourteen elements. The lanthanide elements could be separated into two broad groups, light (La-Gd) and heavy (Tb-Lu), and each group could be further separated into two subgroups – the tetrad26. The extraction order for the lanthanides was La < Ce < Pr < Nd (light sub-group 1) < Sm < Eu < Gd (light sub-group 2) < Tb < Dy < Ho < Er (heavy sub-group 1) < Tm < Yb < Lu (heavy sub-group 2). The lanthanide series could be fractionated using Cyanex 272 as ligand by starting at high acidity and progressively increasing pH. The proposed models (which are backed with experimental data) reveal the existence of a characteristic diagonal relationship for elements in the series. The diagonal relationship is an interesting pattern that could be very important fundamental information in developing separation processes for the lanthanide elements. The developed models could be predictive tools for the sequential extraction/separation of individual or quartets of lanthanides in a solution that contained all the elements, within 95% confidence level. Although organophorus acids used in liquid-liquid extraction may have consistent mechanism, the results also show that specific stoichiometries could result based on the Cyanex 272-aqueous phase equilibrium pH combination. One of the challenges our model addressed is the feasibility to quantitatively extract/separate the lanthanide elements from leach/soluble waste solutions.

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