Fabriction of Hydrophobic Ionic Liquid Extraction System and Its Applications in Separation of Lithium Form Salt Lake Brines
Author:Wang Xing Quan
Supervisor:jia yong zhong
As an important strategic resource,lithium and its compounds are widely used in modern cutting-edge science and technology,military industry,national defense construction and the development of the national economy.With the continuous development of high and new technology,lithium batteries as a clean energy source can not only reduce environmental pollution,but also have high safety performance;especially the electric equipment powered by lithium batteries is increasingly welcomed and valued by human beings.Qinghai Province is rich in lithium resources of salt lakes,but it is located in areas where the ecological environment of the Qinghai-Tibet Plateau is relatively weak.Therefore,it is of great significance to research and develop high-efficiency and green salt lake lithium extraction technology.In this paper,an oxygen-containing solvent was selected as the extractant of lithium,and the hydrophobic ionic liquid was selected as the synergist and solvent.Five ionic liquid extraction systems were constructed,and the single-stage Extraction efficiency of lithium in the salt lake brine was selected and the selectivity was good.[BPy][NTf2]-TBP-C2H4Cl2 and[N4,4,4,1][NTf2]-TBP-C2H4Cl2 two extraction systems were studied in depth.The mass transfer kinetics model of lithium extracted from pyridine ionic liquid system was judged by constant interface cell method.The thermodynamics of two extraction systems were studied,and UV spectroscopy,infrared spectroscopy,density functional theory and cavity theory were used.The mechanism of the extraction reaction process was discussed,and the extraction mechanism of the ionic liquid system was further explained from the microscopic level.The main contents and conclusions are as follows:1.The oxygen-containing solvent was selected as the extractant of lithium,and the ionic liquids of pyridine,imidazole,piperidine,quaternary amine and pyrrole cation and bis（trifluoromethanesulfonyl）imide salt as the common anion were selected as the co-extractant and solvent.The extractant type and ionic liquid structure and the extraction effect of diluent on Li+,Na+,K+,Mg2+and B in brine,it is found that this ionic liquid extraction system has a good extraction and separation effect on lithium in salt lake brine.Different ionic liquid cation structures affect the extraction efficiency of lithium.2.The constant interface method was used to study the extraction reaction control mode.It was found that with the increase of the stirrer rotation speed and the boundary area“A”,the extraction efficiency of Li+also increased,which increased linearly.From this,it is judged that the extraction process is diffusion control.The activation energy of the[BPy][NTf2]-TBP-C2H4Cl2 extraction Li+is determined by the Arrhenius equation.The activation energyΔE=8.08KJ·mol-1<20 kJ·mol-1 is also confirmed by the Arrhenius equation.The extraction process of the extraction system is controlled by diffusion.3.By investigating the relationship between TBP volume fraction and lithium extraction efficiency in[BPy]-[NTf2]-TBP-C2H4Cl2 and TBP-[N4,4,4,1][NTf2]-C2H4Cl2extraction systems,the slope method was used.The linear slopes of the two extraction systems obtained by linear fitting were 0.9037 and 1.1426,respectively,which were close to 1,and the resulting extract structure was expressed as[Li·TBP][NTF2].According to the extraction reaction equation,the Gibbs free energy of the extraction reactions of the two extraction systems were calculated as-1986.997 J·mol-1 and-830.722 J·mol-1 respectively,ΔG0<0.It indicated that the extraction of Li+by the ionic liquid system is spontaneously in the positive direction,which is consistent with the experimental results.4.By infrared characterization of the organic phase before and after extraction of TBP-[BPy][NTf2]-C2H4Cl2 and TBP-[N4,4,4,1][NTf2]-C2H4Cl2 extraction systems,it was found that the P=O characteristic peak of the intermediate extractant TBP is shifted to the low frequency direction,while the infrared characteristic peak of the ionic liquids[BPy][NTf2]and[N4,4,4,1][NTf2]and the diluent 1,2-dichloroethane T is hardly changed.It is concluded that only TBP in the extraction system is coordinated with Li+in the brine.5.Using the Gaussian 09 package,the two ionic liquid structures[BPy][NTf2]and[N4,4,4,1][NTf2]were carried out under the theoretical level of B3LYP/6-31G（d,p）.Optimization,the binding energy between the anion and cation of the ionic liquid,the local ionization energy,the surface electrostatic potential,the electron density difference and other parameters were obtained.The calculation results show that the two ionic liquids are similar in nature and cannot directly react with Li+.However,during the experiment,as the extraction efficiency of Li+:[BPy]NTf2]>[N4,4,4,1][NTf2],because[N4,4,4,1]+has 3 longer alkyl chains.Its hydrophobicity is higher than[BPy]+,which makes it difficult to exchange cations,resulting in lower extraction efficiency of[N4,4,4,1][NTf2]system.6.Comparing the ionic structure of five ionic liquids,it is found that the order of hydrophobicity of ionic liquid cations is JA>PD>BL>MZ>BD in the case of the same anion structure,and the extraction order of lithium in five ionic liquid systems in the experiment is BD>MZ>BL>PD>JA,which indicated a negative correlation existed between the hydrophobicity of the ionic liquid cation and the extraction rate,and the stronger the hydrophobicity,the smaller the extraction rate.The reason is that the mechanism of lithium extraction by ionic liquid extraction system is that cation exchange between ionic liquid cation and lithium ion in aqueous phase,the stronger the hydrophobicity of the cation,the more difficult it is to enter aqueous phase for exchanging with lithium,and thus the extraction rate of lithium by JA is lower than other.7.According to the theoretical analysis of the cavity,it is found that the addition of ionic liquid increases the radius（R）of the Li+ligand.The larger the volume of the extracted molecule,the larger the extraction energyΔE value.Li+is extracted into the organic phase to form an extract[Li·TBP][NTf2]which is larger than LiCl in the brine,and the extract needs to break the hydrogen bond to enter the aqueous phase,so the formed extract[Li·TBP][NTf2]is more likely to stay in an organic phase with lower potential energy,and in order to make the organic phase and the aqueous phase in an electrically neutral state,the ionic liquid cation enters the aqueous phase and binds to Cl-,and a cation exchange process occurs,thereby Li+extraction efficiency is improved.