Green Extraction Of Lithium Based On Ionic Liquid System And Its Thermodynamic Study

Lithium and its compounds are the key materials to support the industrial and national defense system,the global lithium resources mainly exist in salt lakes.As an important method,solvent extraction has the advantages of low cost,good selectivity and being beneficial to industrial production,but the pollution of traditional extraction systems to the environment can not be ignored.As a new green solvent,ionic liquid has extremely low vapor pressure,good recyclability and solubility,which shows its great application prospect in the extraction direction.Therefore,in this study,ionic liquid was used to replace the extractant and diluent of traditional extraction systems to construct a green and efficient binary ionic liquid extraction system for lithium extraction.In this paper,three kinds of ionic liquids tetrabutylphosphonium bis（2,4,4-trimethylpentyl）phosphinate[P₄₄₄₄][BTMPP],tetrabutylammonium bis（2-ethylhexyl）phosphate[N₄₄₄₄][DEHP]and tetrabutylammonium 2-ethylhexyl hydrogen-2-ethylhexylphosphonate[N₄₄₄₄][EHEHP]were synthesized by two-step method.The products were characterized by NMR and FT-IR,and it was proved that the product was the target ionic liquid.The above three kinds of functionalized ionic liquids with excellent selectivity for Li⁺were used as extractant respectively,at the same time,ionic liquid[C₅mim][NTf₂]with strong hydrophobicity was selected as diluent to construct[P₄₄₄₄][BTMPP]-[C₅mim][NTf₂],[N₄₄₄₄][DEHP]-[C₅mim][NTf₂]and[N₄₄₄₄][EHEHP]-[C₅mim][NTf₂].The effects of extractant molality(0.005～0.03mol·kg^-1),temperature（293.15～318.15 K）and ionic strength(0.3～1.8 mol·kg^-1)in different systems on lithium extraction capacity were investigated through conditional experiments.Under the optimum conditions(m=0.02 mol·kg^-1,T=293.15 K,I=0.3mol·kg^-1),the extraction rates of[P₄₄₄₄][BTMPP]-[C₅mim][NTf₂],[N₄₄₄₄][DEHP]-[C₅mim][NTf₂]and[N₄₄₄₄][EHEHP]-[C₅mim][NTf₂]were 83.68%,86.54%and88.22%,respectively.Furthermore,the changes of functional groups of three extractants before and after loading Li⁺were characterized by FT-IR,and the extraction mechanism was analyzed.Finally,the extraction data of the three systems were analyzed and calculated in detail.Based on the Pitzer electrolyte solution theory,the chemical statistical mechanical analysis of m{Li⁺},m{Na⁺}and m{Cl^-}in the process of lithium extraction were carried out,and the standard extraction equilibrium constants at different temperatures were obtained.Other thermodynamic parameters,such as Gibbs free energy change,enthalpy change and entropy change were calculated by polynomial fitting method.It was revealed that the extraction reactions of the three systems were both spontaneous exothermic reactions.This study laid a solid foundation for the industrial production of lithium extraction from ionic liquid systems in the future.