Study On Extraction And Separation Of Lithium Isotope By Crown Ether-ionic Liquids System

With the reduction of non-renewable energy reserves and the gradual deterioration of the ecological environment,it is urgent to develop green,clean and environmentally friendly energy.As an efficient,clean and green energy,nuclear energy is an important part of China's energy system.Nuclear energy is produced by fusion and fission.Fusion is a safer,more efficient and greener way to provide than fission.There are two stable isotopes of lithium in nature,⁶Li and ⁷Li,with natural abundances of 7.53%and 92.47%,respectively.⁶Li and ⁷Li play an important role in the nuclear reaction process.⁶Li is extremely susceptible to neutron collisions to produce tritium,which is an important raw material in nuclear fusion reactions.The neutron absorption cross section of ⁷Li is small and it is often used as a core coolant,speed reducer and p H regulator for nuclear reactions.Therefore,the research on the separation and extraction of lithium isotopes is of great significance to the development of clean energy and the implementation of strategic security in China.At present,a more effective method for the separation of lithium isotopes is to use crown ether as an extractant to separate lithium isotopes.The molecular structure of crown ether has a great influence on the separation of lithium isotopes.We selected three types of crown ethers with different pore diameters as extraction agents and room temperature ionic liquids with better hydrophobicity as co-extractants and extraction solvent to systematically explore the influence of the diameter of crown ether cavity on the effect of lithium isotope separation.Based on the experimental results,crown ether with good separation effect was selected and used as an extractant in the multistage countercurrent extraction experiments.At the same time,the extraction exchange mechanism and thermodynamic parameters in the process of lithium isotope separation were also investigated.The specific work is as follows:?1?Study on the extraction and separation of lithium isotopes by crown ether with different pore diameters-ionic liquid system:the effects of volume ratio of ionic liquid in organic phase,ionic liquid cationic carbon chain length,crown ether concentration,lithium salt anion and temperature on the separation of lithium isotopes were systematically investigated.Through experimental investigation,it was found that the order of effect of crown ether with different cavity sizes on lithium isotope separation factorwas:DB15C5?Dibenzo-15-crown-5?>DB18C6?Dibenzo-18-crown-6?>DB14C4?Dibenzo-14-crown-4?.When dibenzo-15-crown-5 with a diameter slightly larger than the diameter of lithium ion was selected as the extractant,the system had a large lithium isotope separation factor with a maximum separation factor of 1.031.?2?Study on the separation of lithium isotopes by multistage countercurrent extraction with crown ether-ionic liquid system:a multistage countercurrent extraction experiment was performed by using dibenzo-15-crown-5 with better lithiumisotopeseparationeffectastheextractant,1-Ethyl-3-methyl-imidazolium-bis?trifluoromethysulfonyl?-imide?[EMIm][NTf₂]?as extraction solvent and anisole as diluent.The results of the multistage countercurrent extraction extraction experiment showed that the concentration of lithium ions in the organic phase gradually increases with the increase of the number of extraction stages.The abundance of ⁶Li in the organic phase also showed an increasing trend.After ten stage countercurrent extraction experiments,the extraction rate of lithium ions can reach 37.4%and the abundance of ⁶Li in the organic phase can reach 7.77%.?3?The mechanism of extraction and separation of lithium isotopes by crown ether-ionic liquid system was investigated.The complex of crown ether and lithium ion is formed by ion-dipole interaction.When the diameter of the crown ether pore cavity is similar to that of the lithium ion,it is easier to complexate with lithium ion to form crown ether-lithium ion complex.At this time,the extraction rate of lithium ions was higher,but the separation effect of lithium isotope was not better.When the diameter of the crown ether pore cavity is too large,the ion-dipole effect of the crown ether donor atom and lithium ions will be reduced,which is not conducive to the separation of lithium isotopes.When the crown pore diameter is slightly larger than the lithium ion diameter,it is beneficial for the separation of lithium isotopes.In the process of extracting metal ions by crown ether-ionic liquid system,the extraction mechanism of crown ether-ionic liquid system is mostly cation exchange.In DB15C5-ionic liquid system and DB14C14-ionic liquid system,the exchange ratio between ionic liquid cations and lithium ions is close to 1:1.Through the study of the extraction ratio of the system,we found that the extraction ratio of the DB15C5-ionic liquid system and the DB18C6-ionic liquid system is close to 1,which means that a molecule of crown ether can form a complex with a molecule of lithium ion.The extraction ratio of DB14C4 system is slightly less than 1.The thermodynamic results of the system indicated that the separation of lithium isotope in the crown ether-ionic liquid system is a spontaneous exothermic process and the reduction of temperature is beneficial for the separation of lithium isotope.