Study On Solvent Extraction Of Lithium Ions From Salt Lake Brine With Ionic Liquids Containing Complex Anions With Stereo Symmetric Structure As Co-extraction Agent

As a promisingly strategic metal,lithium is widely used in various fields,especially in the battery industry,in which the demand for lithium is increasing year by year.There are abundant lithium resources in the salt lake brines in China.The extraction of lithium from salt lake brine gradually becomes the main way to obtain lithium resources.However,the salt lake brines in China have the characteristics of high Mg/Li ratio,resulting in difficult extraction of lithium.Therefore,it is of great significance to choose a suitable and effective method to extract lithium ions from salt lake brines.Solvent extraction is one of common methods for extracting lithium ions.However,fluorine-containing or iron-containing co-extractant agents have poor stability and reusability problems in the extraction process.Thus,in order to solve these problems,non-fluorine and non-iron ionic liquids containing complex anions with stereo symmetric structure were designed and synthesized as co-extraction agents for extraction of lithium ions.In this paper,two hydrophobic ionic liquids were designed and synthesized as co-extraction agents based on the idea that the co-extraction agents require complex anions with stereo symmetric structure.The 1-butyl-3-methylimidazolium phosphotungstate([Bmim]3PW12O40)was prepareed by the reaction between phosphotungstic acid(H3PW12O40)and 1-butyl-3-methylimidazolium bromide(BmimBr).Then the prepared[Bmim]3PW12O40 was characterized by Fourier transform infrared spectra(FT-IR),nuclear magnetic resonance(NMR)and thermogravimetric analysis(TGA).Then,the lithium ions were selectively extracted using an organic extraction system consisting of tributyl phosphate(TBP)as extractant,[Bmim]3PW12O4O as co-extraction agent and dimethyl phthalate(DMP)as diluent,and the experimental conditions of extraction,washing and stripping processes were optimized.Under the optimal extraction conditions,the separation factor followed the sequence ?Li/Mg2+>?Li+/K+>?Li+/Na+,and the extraction efficiency of lithium ions reached up to 99.23%after five-stage cross-flow extraction processes.Besides,the washing efficiency of Mg2+ and washing loss of Li+ were 100%and 0%when the organic phase was washed with washing agent composed of NaCl and LiCl solution.The stripping efficiency of Li+ was 100%when the organic phase was stripped by HCl or Na2CO3 solution,and the single stage extraction efficiency of Li+ maintained at about 60%during 10 cyclical extraction processes.Finally,FT-IR,NMR and simulated calculation with Materials Studio software were used to reveal the mechanism of the extraction process as cation exchange.The metal ions were extracted into the organic phase by ion-pair extraction between complex anion in co-extraction agent and organic cation formed by coordination between metal ions and P=0 group in TBP.In addition,the coordination ability of different metal ions by TBP followed the sequence Li+>Na+>K+?Mg2+.In order to increase the single stage extraction efficiency and separation factor,1-butyl-3-methylimidazolium tetraphenylborate([Bmim]Bph4)was synthesized by reaction between sodium tetraphenylborate(NaBph4)and BmimBr.The prepared[Bmim]Bph4 was characterized by NMR.Then,the lithium ion was extracted using an organic extraction system consisting of TBP as extractant,[Bmim]Bph4 as co-extraction agent and bromochloromethane(CH2BrCl)as diluent,and the experimental conditions of extraction,washing and stripping processes were investigated.The separation factor followed the sequence ?Li+/Mg2+>?Li+/K+>>?Li+/Na+,and the extraction efficiency of lithium ion reached up to 99.47%after four-stage cross-flow extraction processes.The washing efficiency of Mg2+ was 100%and the washing loss of Li+ was 0%when the organic phase was washed with NaCl and LiCl solution.The stripping efficiency of Li+ was up to 100%when the organic phase was stripped by Na2CO3 solution,and the single stage extraction efficiency of Li+ maintained at about 90%during 10 cyclical extraction processes.Finally,the mechanism of the extraction process was explored by FT-IR,NMR and simulated calculation with Materials Studio software.Cation exchange was demonstrated as the extraction mechanism,and the coordination ability of different metal ions by TBP followed the sequence Li+>>Na+>K+? Mg2+.