Development And Mechanistic Study Of Novel Hollow Fiber Membrane Systems For High-efficiency Separation Of Rare Earth Ions

The traditional rare earth separation process is plagued by numerous challenges,including complex stages,low efficiency,a large footprint,and slow mass transfer rates.At the same time,it involves significant consumption of organic solvents,acids,and bases,leading to increased costs and environmental pollution.Hollow fiber membrane technology,with its high mass transfer driving force and specific surface area,offers a potential solution to replace the traditional rare earth separation process.In this study,a dual-module hollow fiber membrane（D-HFMC）system was constructed for the separation of lanthanum-neodymium（La-Nd）as light rare earth elements and samarium-lutetium（Sm-Lu）as medium-heavy rare earth elements.The separation performance and mass transfer mechanism of the D-HFMC system for La-Nd and Sm-Lu were investigated.To address the low mass transfer separation efficiency of the D-HFMC system,the system was optimized by introducing Dean vortices and controlling the membrane’s specific surface area.This optimization led to the construction of a single-module braided spiral hollow fiber membrane（SBS-HFMC）system.The separation performance and strengthening mechanism of La-Nd and Sm-Lu in the SBS-HFMC system were studied.Moreover,during the separation process of medium-heavy rare earths Sm-Lu,the separation of yttrium（Y）and other heavy rare earth elements holmium-lutetium（Ho-Lu）proved to be particularly challenging.Therefore,an ionic liquid-coupled hollow fiber renewal supported liquid membrane（IL-HFRLM）system was proposed to address the separation of Y in heavy rare earth components,and various new carboxylic acid ILs were developed for this system.The separation properties and mass transfer mechanism of Y and Ho-Lu in the IL-HFRLM system were studied.The research results provide new ideas and theories for the application of hollow fiber membrane technology in rare earth separation.The main results of this study are as follows:（1）The D-HFMC system was constructed using simulated leach liquor of ion-type rare earth ore as the aqueous phase.The separation performance of the rare earth elements using extractants P507 and P204 in this system was investigated.Through experimental screening,the P507 system was determined to be the optimal system.Subsequently,the experimental conditions for separating rare earth elements using the P507 system were explored and optimized,including the acidity of the stripping phase,extractant concentration,and initial p H of the feed solution.Under the optimal conditions,the obtained mass percentages of La-Nd in the feed phase and Sm-Lu in the stripping phase were 98.6%and 91.9%,respectively,achieving the separation of the two rare earth groups.Furthermore,the mass transfer mechanism in the D-HFMC system was analyzed,and the results indicated that the transfer of rare earth ions in the D-HFMC system first involved the formation of complexes with the extractant driven by chemical driving force,followed by diffusion from the feed phase to the stripping phase driven by concentration gradient,ultimately achieving mass transfer from the feed phase to the stripping phase.（2）To further enhance the mass transfer separation efficiency of rare earth elements in the D-HFMC system,a new SBS-HFMC system was constructed to optimize D-HFMC system.The SBS-HFMC system effectively reduced spatial resistance and enhanced the mass transfer separation efficiency of rare earth elements by introducing Dean vortices and controlling the membrane’s specific surface area.The experimental conditions,including the acidity of the stripping phase,flow rate,rare earth ion concentration,and phase ratio,were optimized.The performance of the SBS-HFMC system in separating rare earth elements was evaluated based on the D-HFMC system,resulting in mass percentages of 98.9%for La-Nd in the feed phase and 95.9%for Sm-Lu in the stripping phase.In addition,the mass transfer enhancement mechanism in the SBS-HFMC system was analyzed,and the results demonstrated that the SBS-HFMC system,indicating that the spiral membrane structure and adjustable membrane specific surface area significantly improved the mass transfer and separation efficiency of rare earth elements,and the enhancement coefficient could reach above 5.（3）In the separation of medium-heavy rare earths Sm-Lu,the separation of Y and other heavy rare earth Ho-Lu presents a particular challenge.To achieve the separation of Y in the heavy rare earth component,an[N1888][CA12]-HFRLM system was developed,and its separation performance for Y in the simulated feed solution of heavy rare earth raw material was investigated.The mass transfer mechanism of rare earth in the[N1888][CA12]-HFRLM system was explored,and the results indicated that the membrane mass transfer coefficient（k_m）was the rate-controlling step.The diffusion coefficient of Y in the[N1888][CA12]-HFRLM system was calculated to be 8.06×10^-5 cm/s.By optimizing the experimental conditions such as the acidity of the stripping phase,phase volume ratio,Na Cl concentration,temperature,and[N1888][CA12]concentration,the[N1888][CA12]-HFRLM system successfully separated Y from other heavy rare earths,with the molar ratio of Y in the feed phase increasing from 70.9%to 98.4%.This demonstrated the feasibility of using ILs coupled with HFRLM for the separation of rare earth elements.（4）To further improve the separation efficiency of Y in the[N1888][CA12]-HFRLM system,six novel carboxylic acid ILs with superior separation performance for Y were designed and synthesized.Subsequently,an[N1888][C₉H₁₅O₂]-HFRLM system was constructed.The physical and chemical properties of the ILs were characterized through nuclear magnetic resonance,infrared spectroscopy,viscosity density,water content analysis,and thermogravimetric analysis,and the results showed that the synthesized ILs had superior physical and chemical properties.By optimizing the experimental conditions such as the initial p H of the feed solution,Na Cl concentration,extractant concentration,and stripping agent concentration,the[N1888][C₉H₁₅O₂]-HFRLM system achieved a significant improvement in the separation efficiency of Y,with the molar ratio of Y in the feed phase increasing from 70.9%to 99.2%.