Research On The Efficient Separation Of Lithium Isotopes With Crown Ethers Based On Ion Pair Mechanism

The lithium isotopes（⁶Li,⁷Li）are sparking much interests for its important applications in the development of nuclear energy.Crown ether molecules have unique cavity structures and size effect,so the separation factor of ⁶Li/⁷Li for which is comparable to that of lithium amalgam method,which is the only one that put into industrialization.Crown ethers are the most studied and reported lithium isotope separation systems.However,the interaction between crown ether and lithium（Li）is relatively weak,which leads to the low distribution coefficients of lithium(D_Li)in crown ether chemical exchange systems(D_Li=10^-3～10^-5).Moreover,the value of D_Li is greatly affected by the type of anion of the lithium salt and obeys the Hofmeister sequence.For the Li salt of chloride or sulfate,the most abundant Li salts in nature,the low distribution coefficient of lithium is still a major problem.Herein,the crown ethers,benzo-15-crown-5（B15C5）or benzo-12-crown-4（B12C4）,are used as the extractants and[FeCl₄]^-is introduced as the counter anion in the Li Cl aqueous solution for lithium isotope extractive separation.The extraction efficiency of Li⁺and the separation performance of ⁶Li/⁷Li for the crown ethers/Li Cl-FeCl₃-Al Cl₃-HCl_（aq）extraction system were studied systematically.（1）The extraction behavior of Li⁺with B15C5 or B12C4 in different Li-containing solutions,Li Cl-HCl_（aq）,Li Cl-FeCl₃-HCl_（aq）,Li Cl-Al Cl₃-HCl_（aq）and Li Cl-FeCl₃-Al Cl₃-HCl_（aq）,was investigated.The effects of initial hydrochloric acid concentration,Fe/Li molar ratio,Al³⁺concentration in aqueous phase and crown ethers concentration in organic phase on the distribution coefficients of Li⁺in B15C5-1,2-dichloroethane/Li Cl-FeCl₃-Al Cl₃-HCl_（aq）system were highlightly explored.The results show that FeCl₃ and Al Cl₃ have the synergistic effect on the enhancement of the extraction of Li⁺from acid system.The optimal operation conditions are as follows:the initial concentration of hydrochloric acid is 0.4 mol/L;the molar ratio of Fe/Li is 2;the concentration of Al³⁺in aqueous phase is 2 mol/L and the concentration of crown ether in organic phase is 0.3 mol/L.The D_Liof around 30 was obtained at 298 K under the optimal conditions.This result is much better than that for the crown ether/Li Cl_（aq）system reported in the literatures.The single crystal structures of the crown ether-lithiumcomplexes were determined to be[Li（B15C5）（H₂O）][FeCl₄]or[Li（B12C4）₂][FeCl₄]by X-ray single crystal diffraction（XRD）,indicating that the formation of ion pairs promotes the efficient phase transfer of Li⁺.（2）The effects of crown ether concentration,temperature,organic solvent and contact time on the lithium isotopes separation were investigated.The separation factor of lithium isotope was predicted theoretically based on density functional theory（DFT）and Urey model.The separation mechanism of ⁶Li/⁷Li was revealed from the microscopic scales.The thermodynamic functions of the ion exchange system for the B15C5-1,2-dichloroethane/Li Cl-FeCl₃-Al Cl₃-HCl_（aq）were calculated.The results show that the solvent style of organic phase and temperature are the key factors affecting the separation of⁶Li/⁷Li.The maximum single-stage separation factors of ⁶Li/⁷Li for B15C5 and B12C4 were 1.038 and 1.049 at 273 K,respectively.The theoretical calculation results show that the coordination and electrostatic environment of Li⁺play important roles in the separation of ⁶Li/⁷Li.Weakening the coordination and electrostatic interactions associated with Li⁺can facilitate the separation of⁶Li/⁷Li.In addition,thermodynamic studies show that low temperature is beneficial for the separation of ⁶Li/⁷Li.（3）The results of the stripping experiment show that the ultrapure water can strip lithium and iron effectively from the Li/Fe-loaded B15C5-Li ion-pair complex.B15C5 can be reused and recycled for at least ten times after being regenerated.It was found that the structure of B15C5 did not change after stripping through the UV-Vis analysis,indicating that the organic phase is stable.After ten-stage extraction-stripping processes,the extraction efficiency of lithium maintained at 83%.Compared with the literatures,the crown ether ion pair systems by introducing[FeCl₄]^-as the counter anion reported in this work exhibited more efficient transfer of Li⁺from extremely hydrophilic chlorides to the organic phase,leading higher distribution coefficients of Li⁺.The separation factors of⁶Li/⁷Li were also improved.For the chlorides system,D_Li increased by 10³～10⁵orders of magnitude and the single-stage separation factor of ⁶Li/⁷Li for B15C5increased from 1.002 to 1.032.In addition,the method has the advantages of simple system and easy construction of ion pairs,which is of great value for the industrial application of crown ethers for lithium isotope separation.