Ionic Liquids Based Selective Separation Of Rare Earth Metal From Transitional Metals

From the viewpoint of the sustainable development and green economy,one of the effective measure to solve the supply risk of rare earth metals in the future is the highly efficient separation of rare earth metal from the waste materials and the paragenetic mineral.Therefore,it is significance to design and develop environmentally friendly liquid-liquid separation technologies.Due to their unique physicochemical properties such as low vapor pressure and melting point,non-flammability and tunable structures,ionic liquids have been widely applied in organic synthesis,catalysis reaction,electrochemistry and preparation of the advanced functionalized materials.Especially,ionic liquids materials have play important role in the liquid-liquid extraction of metals to replace the volatile,flammable and toxic organic solvents.However,based on the concept of green chemistry,there are still some serious problems in the ionic liquids based extraction and separation of metal ions,such as the use of extra extractants,the use organic solvents as the media of ionic liquids,the low selective separation efficiency,the loss of ionic liquids in water phase from an exchange mechanism in extraction process,the efficient stripping of metal ions and the recycling of ionic liquids.According to the characteristic of tunable structure of ionic liquids,it is one of the important strateges to design and develop sustainable functionalized-ionic liquids system to solve these problems.As part of the project supported by the National Natural Science Foundation of China(No.21273062),a series of carboxyl-functionalized imidazolium ionic liquids were designed and synthesized,and their acid-base properties were investigated quantitatively in this work.New strategies for the selective separation of rare earth(neodymium,samarium and scandium)from the transition metals(iron,cobalt)and the lanthanide metal ions have been proposed.In addition,an aqueous two-phase system composed of a hydrophilic ionic liquid and an inorganic salt was constructed and used to study the separation of neodymium from cobalt and nickel.The following main contents are included in the thesis:1.Four imidazolium bromides with one carboxylic acid substitute group in their alkyl chain([{(CH2)nCOOH}mim]Br,n=1,3,5,7)were designed and synthesized,and their dissociation constants(pKa)at different ionic strengths were determined in aqueous and aqueous organic solvents at 0.1 mole fraction(x)of ethanol,glycol,iso-propanol or dimethyl sulfoxide by potentiometric titrations at 298.2 K.The standard thermodynamic dissociation constants(pKaT)of the ionic liquids in these solvents were calculated from the extended Debye-Hiickel equation.It was found that ionic strength of the media and the addition of organic solvent to water have obvious effect on the pKa and pKaT values.In addition,the effect of solvent nature on pKaT values was interpreted from solvation of the dissociation components and their Gibbs energy of transfer from water to aqueous organic solutions.The determination of the acid-base dissociation constants of the carboxyl-functionalized ionic liquids has laid the foundation for the understanding of their role in the extraction and separation processes of metal ions.2.Three hydrophobic ionic liquids 1-alkylcarboxylic acid-3-methylimidazoliumbis(trifluoro methylsulfonyl)imide,[(CH2)nCOOHmim][Tf2N](n=3,5,7),were designed,synthesized and directly utilized to selectively separate neodymium(?)from Fe(?)and samarium(?)from Co(?)in aqueous solutions.The factors affecting the solvent extraction process such as phase volume ratio,contact time,pH value of aqueous phase,alkyl chain length of the ionic liquids and temperature of the system were examined systematically.It was found that the maximum extraction efficiency of the investigated metal ions was as high as 99.0%under optimal conditions,and Sm(?)and Nd(?)could be selectively separated from Co(?)and Fe(?)with separation factors of 104-105 by simply modulation of the aqueous phase pH value.After extraction,the metal ions could be effectively stripped from ionic liquid phase by using dilute aqueous HCl,HNO3 or oxalic acid,and the ionic liquids could be recovered and reused in the next extraction process.It is worthy to mention that for the ionic liquids based solvent extraction systems reported here,the experimental condition was mild,no organic solvents and other extraction agents were necessary,and no ionic liquid components were lost in the extraction process by ionic exchange.Therefore,the carboxyl-functionalized ionic liquids developed here can provide a highly efficient,simple and green separation method for the selective recovery of rare earth metals from the waste NdFeB and SmCo permanent magnets3.A selective separation strategy for Sc(?)from yttrium(?)and lanthanides(?)in aqueous solution was developed by using ionic liquids 1-alkylcarboxylic acid-3-methylimidazolium bis(trifluoromethanesulfonyl)imide([(CH2)nCOOHmim][Tf2N])(n=3,5,7)as extractants and 1-alkyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide([Cnmim][Tf2N])(n=4,6,8,10)ionic liquids as diluents.The extraction parameters such as phase volume ratio,equilibrium time,pH value of the aqueous phase,alkyl chain length of the extractants and diluents,and system temperature were investigated systematically.It was shown that the extraction efficiency of Sc(?)was as high as 99.5%at pH 4.2.Sc(?)could be extracted selectively from yttrium and lanthanides by simple tuning of pH value of the aqueous phase,and the separation factor was 103.After the extraction,stripping of Sc(?)from the ionic liquids was attempted by using dilute aqueous HNO3.In addition,an ion exchange mechanism of the proton of extractants and the cation of the diluents was proposed from a series of control experiments.Since both the proton of the extractants and the cation of diluents participated in the extraction process,amount of the ionic liquids transferred to water phase can be greatly reduced.The combination of the functionalized hydrophobic ionic liquids and conventional ionic liquids provide efficient and environmentally friendly advantages for the selective separation of scandium from yttrium and lanthanides4.A hydrophilic ionic liquid tetrabutylphosphonate nitrate([P4444][NO3])was prepared and used for the formation of an ATPS with inorganic salt NaNO3 or NH4NO3.The bimodal curves of ATPSs at 298.1 K were determined for the design of the extraction process.The extraction parameters such as contact time,aqueous phase pH,phase-forming amount of NaNO3 and ionic liquid were investigated in detail.It was shown that under optimal conditions,the extraction efficiency of neodymium(?)is as high as 99.7%,and neodymium(?)could be selectively separated from cobalt and nickel with a separation factor of 1626 After extraction,neodymium(?)can be stripped from the ionic liquid-rich phase by using dilute aqueous sodium oxalate,and the ionic liquids can be quantitatively recovered and reused in the next extraction process.Since[P4444][NO3]worked as one of the components of the ATPS and the extractant for the neodymium,no organic diluent,extra etractant and fluorinated ILs were used in the separation process Thus,the strategy described here shows potential in selective and green separation of neodymium from cobalt and nickel by using simple ionic liquid aqueous two-phase system.