Separation System And Separation Law Of Lithium Isotope By Electromigration Method

The separation of lithium isotopes holds significant importance for the development of nuclear industry.There are two stable isotopes of lithium in nature:~6Li and ~7Li,with relative abundances of about 7.5%and 92.5%,respectively.~6Li is an important fuel for nuclear fusion reactors,while ~7Li can be used as a coolant,heat transfer medium,and thorium molten salt medium in fusion reactor cores.However,the only industrialized method for lithium isotope separation is the lithium-mercury amalgamation method.The large-scale use of toxic mercury may lead to potential environmental pollution issues if leaked.Therefore,our research group proposed a green method for lithium isotope separation called the"aqueous solution-organic solution-aqueous solution"electromigration method.Based on the previous research of our group,this paper first improved the migration device for lithium ions to increase system stability and lithium ion transport flux.The effect of the ionic liquid ratio in the organic phase was systematically investigated using the optimized ionic liquid ratio suitable for the migration process.Furthermore,based on the experimental results obtained above,we explored multi-stage migration and clarified the influence of factors such as migration level on lithium isotope separation efficiency during the multi-stage migration process.The specific conclusions are as follows:(1)Based on the"aqueous solution-organic solution-aqueous solution"system constructed in previous studies,we optimized and improved the experimental device.The transport rate of lithium ions in the system was increased by 18 times,and the stability of the separation effect was also enhanced.Using the improved device,we investigated the effect of crown ether concentration and voltage on the transport rate of lithium ions in the organic phase.Increasing the crown ether concentration in the organic phase promotes the migration of lithium ions from the anolyte to the organic phase but has a migration limit.The introduction of crown ether facilitates the migration of lithium ions from the organic phase to the catholyte,but excessive crown ether concentration reduces the migration ratio.When an external voltage of 16 V is applied,the most suitable crown ether concentration for a lithium salt concentration of 1 mol/L is 0.2 mol/L.Applying voltage at both ends of the system promotes the migration of lithium ions from the anolyte to the organic phase and catholyte.As the voltage increases,the concentration of lithium ions in the organic phase continues to increase,while the concentration of lithium ions in the catholyte shows an upward trend followed by a downward trend.(2)This study investigated the effect of different ionic liquid ratios in the organic phase under different voltages on lithium ion transport and lithium isotope separation efficiency during the migration process in the"aqueous solution–“organic solvent-ionic liquid”-aqueous solution"system.Results showed that with an increasing ionic liquid ratio in the organic phase,the migration ratio of lithium ions from the lithium salt solution(anolyte)to the organic phase and from the organic phase to the ammonium chloride solution(catholyte)showed an upward trend followed by a downward trend at0 V and 4 V.At 16 V,the migration ratio of lithium ions from the lithium salt solution into the organic phase continued to increase,and the increase became more significant,while the migration ratio of lithium ions from the organic phase into the catholyte showed an upward trend followed by a downward trend.The enrichment effect of ~6Li in the organic phase continued to decrease with an increasing ionic liquid ratio,and applying 16 V voltage at both ends of the system weakened the enrichment effect of ~6Li in the organic phase.As the ionic liquid ratio increased,the enrichment effect of lithium isotopes in the catholyte showed a similar changing pattern as that in the organic phase.The study discussed the influence mechanism and regularity of different ionic liquid ratios on lithium ion migration and lithium isotope separation.Within the voltage range of 0-16 V,the ionic liquid ratio of 20%-40%in the organic phase was found to balance the concentration of lithium ions and the lithium isotope separation effect.(3)After preliminary experimental research,it was discovered that the separation effect of lithium isotopes mainly comes from the water-organic interface.Based on this,a multi-stage migration trough with different migration levels was designed,taking into account the need for long-term stability of the liquid-liquid interface in multi-stage separation systems.The appropriate proportion of ionic liquids in the organic phase was determined.After examination,it was found that the separation effects of the multi-stage interface can effectively overlap,achieving the effect of multi-stage separation in a single separation operation.Based on this,the study further explored the effects of the type and concentration of lithium salt solution in the system,the concentration of crown ether in the organic phase,the applied voltage,migration time,and level on lithium ion migration and lithium isotope separation in multi-stage migration.Finally,the cumulative separation coefficient of the 3-stage system can be stabilized at 1.073.The study found that by changing the lithium ion concentration in the lithium salt solution,the diffusion driving force of lithium ions in the migration process can be altered,thereby changing the ability of each stage to enrich lithium isotopes.In the absence of an external electric field,by changing the migration time of lithium ions,it was found that ~7Li would migrate out of the organic phase first in each liquid phase,resulting in the water phase being relatively enriched with ~7Li compared to the organic phase,weakening the ability of the entire system to enrich ~6Li.By extending the migration time of lithium ions,the ability of the ammonium salt solution to enrich ~7Li compared to the organic phase can be weakened,thereby enhancing the ability of the entire system to enrich ~6Li.