The Study On Extraction And Separation Of Rare Earth Elements Yttrium And Gadolinium With Pyridinium-based Ionic Liquids

Yttrium is an important rare earth element,which is widely used in aerospace,high temperature superconductors,alloys,fluorescent materials,etc.It is difficult for extraction and separation of yttrium due to the chemical properties of yttrium are similar with the adjacent holmium and erbium elements.As the largest number of unpaired electrons in rare earth elements,gadolinium has unique properties,such as strong magnetic moment,strong attraction to thermal neutron and so on,and is widely used in neutron absorber,magnetic resonance imaging,magnetic memory,the nuclear reaction masking agent,etc,these applications require high-purity gadolinium with purity greater than 99.995%.At present,organic solvent extraction is often used to separate and purify rare earth yttrium and gadolinium,but organic solvents such as toluene and kerosene are volatile and toxic,and the separation coefficient of yttrium with holmium and erbium is low(?1.3),and the impurity aluminum in rare earth gadolinium solution is difficult to remove.In recent years,ionic liquids have been used as a new medium for the separation and purification of rare earth yttrium and gadolinium,but there are still some problems,for example,low extraction efficiency of conventional ionic liquids,and ionic liquid with high viscosity usually needs to be dissolved in organic solvents such as toluene and kerosene,which is contrary to the concept of green chemistry.Therefore,it is of great significance to develop an efficient,clean and highly selective ionic liquid extraction system for rare earth.In this paper,three kinds of pyridinium-based ionic liquids with carboxylic acid functional groups and three kinds of conventional ionic liquids with low viscosity were designed and synthesized,and a new method for selective separation of yttrium from holmium and erbium,and efficient removal of aluminum from rare earth gadolinium were established based on these ionic liquids.The main research contents are as follows:1.According to the designability of ionic liquids,three kinds of pyridinium-based ionic liquids([(CH₂)_nCOOHpyr][NTf₂],n=3,5,7)were designed and synthesized by introducing coordination carboxylic acid functional groups into pyridine cation,and three kinds of conventional ionic liquids[C₄mim][NTf₂]?[C₄mim][PF₆]?[C₄pyr][NTf₂]with low viscosity were designed and synthesized.Moreover,the structures of ionic liquids were characterized by ¹H NMR,¹³C NMR and FTIR,the viscosity density,thermal stability and water content of ionic liquids were determined.The results show that the ionic liquids can meet the experimental requirements.2.The pyridinium-based ionic liquids([(CH₂)_nCOOHpyr][NTf₂],n=3,5,7)were dissolved in the conventional ionic liquid[C₄mim][NTf₂]to construct a new ionic liquid extraction system.A series of experiments were carried out to investigate the effects of extraction time,p H value of aqueous phase,extractant concentration,temperature and alkyl chain length of ionic liquid on the extraction of yttrium.In addition,the selective separation of yttrium from holmium and erbium was investigated.It could be found that both the separation coefficients of Ho/Y and Er/Y were greater than 2,which were higher than other ionic liquid extractants.A possible cation exchange extraction mechanism was proposed by slope analysis and FTIR spectroscopy,and the extraction efficiency of the[(CH₂)₇COOHpyr][NTf₂]-[C₄mim][NTf₂]system for Y still maintained about 98%after 8 cycles.These results indicate that the ionic liquid system is a potential new extractant for separation of yttrium from holmium and erbium.3.The pyridinium-based ionic liquids([(CH₂)_nCOOHpyr][NTf₂],n=3,5,7)were dissolved in ionic liquid diluents[C₄mim][NTf₂]?[C₄mim][PF₆]?[C₄pyr][NTf₂]to form an ionic liquid phase for efficient removal of aluminum from gadolinium solution.The main factors affecting the extraction separation of gadolinium and aluminum ions were investigated,such as the types of diluent and extractant,p H value of aqueous phase,etc.The high efficiency of removing aluminum from gadolinium solution was achieved by adjusting the p H value of water phase.Among them,the separation factor of aluminum and gadolinium was up to 6.4×10³ in[(CH₂)₅COOHpyr][NTf₂]-[C₄mim][NTf₂]and[(CH₂)₇COOHpyr][NTf₂]-[C₄mim][NTf₂],which is significantly higher than the results reported in the literature.The extraction mechanism of gadolinium in[(CH₂)₇COOHpyr][NTf₂]-[C₄mim][NTf₂]system was proposed as cation exchange mechanism by slope analysis and infrared spectroscopy.In addition,the extraction efficiency of the[(CH₂)₇COOHpyr][NTf₂]-[C₄mim][NTf₂]system for Gd was still higher than 95.5%after 8 cycles.This study provides a new and promising method for the efficient removal of aluminum from gadolinium.