Screening Of Deep Eutectic Solvent With Low Viscosity And Strong Affinitv For Lithium And Its Application In Lithium Extraction From Seawater

Lithium shortage is expected to be fully resolved by the efficient recovery of lithium from seawater.Unfortunately,the extremely low concentration of lithium-ion(Li+)and the existence of many foreign metal-ions that are far more abundant leading to no major advancement in the effective utilization of lithium resources in seawater.Liquid-liquid extraction(LLE)is a well-known and extensively used technique for the concentration and purification of metal ions.The conventional extraction system mainly introduces organic solvents with high volatility,toxicity and flammability,which can easily cause environmental pollution and safety accidents.Deep eutectic solvent(DES)emerged,offering to design chemical processes without harmful chemicals used or produced.Therefore,the application potential of hydrophobic deep eutectic solvent(HDES)in lithium extraction from seawater was discussed.In order to achieve the ultimate goal of extracting lithium from seawater,the HDES with low viscosity and high Li+affinity was selected by viscosity prediction and molecular simulation.Then,the lithium was extracted in a simple dilute solution of pure lithium chloride before real seawater.This will facilitate the design of green LLE systems for lithium recovery from low-grade resources like seawater and concentrated seawater,and further promote the application development of DES.Viscosity of solvents strongly affects the mass transfer process of LLE.For the numerous designable solvents,it is economically and technically unreasonable to determine their viscosity solely by experimental measurement.Therefore,a model based on transition state theory for calculating viscosity of mixtures was proposed.Group contribution method was used to determine the model parameters and the viscosity database of DES containing 2229 experimental data was used for subsequent validation.With just temperature and composition as inputs,a mathematical model was obtained which can predict the viscosity of DES with high accuracy.The key of the lithium extraction process is to find highly hydrophobic extractants with strong affinity for Li+ and little attraction to other metal ions.Therefore,this work established a set of DES screening methods using the molecular simulation software Materials Studio.First,the Blends module was used to calculate the compatibility data between DES and water in order to filter HDES with high hydrophobicity.Then,the interaction energy data of HDES with LiCl and NaCl were calculated by DMol3 module to screen potential HDES for selective extraction of LiCl.Finally,the established viscosity prediction model was used to calculate the viscosity data of HDES with affinity for LiCl at 298 K,and the HDES with low viscosity and high Li+affinity was finally screened.Since the composition of seawater is complex,this work first evaluated the ability of the screened HDES to recover Li+from extremely dilute lithium chloride solutions in order to establish an efficient extraction system for lithium.The extraction experiment confirmed that HDES composed of methyltrioctyl ammonium chloride(Ns881Cl)and decanoic acid(DecA)was an effective extractant for Li+extraction.Furthermore,HDES was used as the primary extraction solvent,and bis(2-ethylhexyl)phosphate acid(DEHPA)was used as the secondary solvent to create a synergistic extraction system.In optimum conditions,it was feasible to extract lithium with an efficiency of 80%.Ten cycles of organic phase regeneration showed that the extraction system had good stability for Li+extraction.The selective extraction performance of Li+from high concentration of Na+,K+,Ca2+and Mg2+by the N8881Cl/2DecA-DEHPA(DEHPA/HDES)system was further investigated.Aiming at the problem that DEHPA/HDES system had a stronger affinity for multivalent metal ions,a two-step LLE strategy was proposed.Firstly,DEHPA/KERO(DEHPA dissolves in kerosene)was used to remove highly interfering bivalent metal ions,and then DEHPA/HDES was used to selectively extract Li+.After optimizing the extraction conditions,the extraction efficiency of Li+was 66.44%,and the selectivity coefficient of lithium and sodium was 11.57.Finally,the proposed two-step LLE strategy was applied to the simulate seawater and real seawater,the extractant can still be recycled more than ten times.