Separation Of Rare Earths And Its Mechanism By Three-Liquid-Phase Extraction

During traditional group separation of rare earths, by repeatedly adjusting the acidity of aqueous solution, the discrepancy in complexation of rare-earth ions with the complexing agent is controlled. The liquid-liquid-liquid three-phase system (TLPS) has a unique advantage on group separation of similar components. The group separation of components with similar properties can be achieved by the different hydrophilicity/ hydrophobicity of three different phases. Simultaneously, the feasibility of solving the difficulty in stripping of heavy rare-earth ions by TLPS was investigated. The main research contents and results are given as follows:The partition behaviors of rare-earth ions, La, Eu, Yb, in TLPS were investigated by controlling hydroplilic/hydrophobic properties of different rare-earth ions and liquid phases. The different complexing behaviors by introducing the complexing agent DTPA into aqueous phase were obtained.84% of Yb was extracted into organic phase.40% of Eu was partitioned into polymer-rich phase with the formation of Eu-DTPA. La was neither extracted into organic phase nor partitioned into PEG-rich phase.The three-liquid-phase group separation behaviors of fourteen lanthanides coexisted were investigated. By introducing a water-soluble complexing agent, the complexing behaviors of rare-earth ions La-Nd, Sm-Ho, Er-Lu with the complexing agent were adjusted, which resulted in the occurrence of different partition behaviors of these rare-earth ions into different liquid phases. By changing phase-forming conditions such as phase-forming salt concentration, polymer concentration and extractant concentration in organic phase, the different partition behaviors of all the lanthanides into different liquid phases were adjusted. All the lanthanides were just divided into (La, Ce, Pr, Nd), (Sm, Eu, Gd, Tb, Dy, Ho) and (Er, Tm, Yb, Lu) three rare-earth groups.During group separation of rare earths, the partitioning trend of associated non-rare earths was investigated. When a water-soluble complexing agent of phen was introduced, 85% of Fe and 50% of Si were partitioned into polymer-rich phase. The light and middle rare-earth ions were not extracted but remained in aqueous bottom phase. When two phen and EDTA were introduced simultaneously,50% of Al with together with Fe, Si was partitioned into polymer-rich phase. The rare-earth ions La, Eu, Yb remained in aqueous bottom phase. According to this idea, the occurrence of co-extraction of rare earths and associated non-rare earths during two-phase extraction can be avoided.The separation behaviors of rare earths and non-rare earths under the nonequilibrium state were investigated by a mixer-settler-mixer three chamber integrated extractor. The experimental results reveal that the extraction of rare-earth ions into top phase was enhanced with the increase of the agitation speed in three-phase mixer. Simultaneously, it was favorable for the partitioning of non-rare-earth ions into PEG-rich phase. With the pumping action of impeller in two-phase mixer, it is favorable for the transfer of non-rare-earth ions into PEG-rich phase. The increase in flow ratio of organic solvent to aqueous two-phase mixture resulted in the increase of the extraction percentages of rare-earth ions in top phase.The stripping of heavy rare earths in TLPS was investigated. According to experimental results, with the introduction of a water-soluble complexing agent DTPA into aqueous phase and existence of PEG-rich phase, three-phase partitioning behaviors of heavy rare earths was obtained. By adjusting phase-forming salt concentration, the heavy rare-earth ions in polymer-rich phase were easily stripped.Taking Eu as an example, the effect of phase-forming behaviors on partitioning was investigated. The correlation of partition coefficients of rare earths with mass fractions of hydrated PEG was established by considering volume ratios of polymer-rich phase to salt-rich phase, which was related to hydration behavior of PEG. The partition coefficients of rare-earth ion calculated by phase volume ratios fitted well with experimental values. Such an influence of water content reflects an effect of hydrogen-bonding interaction in polymer-rich phase on the partitioning of rare-earth ion.The mechanism for three-liquid-phase partitioning of rare earths was studied in details using a variety of characterization methods including MFS, FTIR, SAXS, IFT and NMR. MFS results indicated that the rare-earth ions can be partitioned into polymer-rich phase only by the formation of rare-earth complexes. FTIR and NMR data show that the reason for partitioning of the rare-earth ion in polymer-rich phase is the formation of an indirect hydrogen-bonding interaction with water as a bridge between rare-earth complexes and polymer molecules. SAXS, IFT and NMR results reveal that the hydrophilic/hydrophobic properties of rare-earth complexes did not match that of polymer-rich phase any more. And thus the mass fractions of rare earths in polymer-rich phase relatively low.