Study On The Application Of Ionic Liquid Systems For Extraction Of Lithium From Salt Lake Brine

Solvent extraction is considered one of the most powerful techniques for the separation of lithium from the brine sources of a high Mg/Li ratio.However,there are some problems in the conventional extraction systems,such as the pollution of volatile solvent,severe equipment corrosion and emulsification.As a new type of green solvent,ionic liquids exhibit several properties that make them attractive as a potential solvent for the separation processes,including high thermal stability,nonflammability,negligible volatility,etc.In this paper,several ionic liquids extraction systems have been used to extract lithium ions form the salt lake brine of a high Mg/Li ratio.The extraction efficiencies and distribution ratios of lithium ions were measured as a function of various extraction parameters,including aqueous acidity,extractant concentration,ionic liquids concentration and phase ratio.The extracted species were determined using a slope analysis method.The main functional groups of the extractant and ionic liquids were characterized by infrared spectrum and ultraviolet spectrum.Applied B3 LYP method of density functional theory at the 6-31G?d,p?level with Gaussian 09 program,the binding energies,bond length and ionization energies of the ionic liquids were calculated,which help to learn about the extraction mechanism at the atomic level.The major findings of this study are as follows:1.The optimal extraction conditions of [C_nmim][PF₆]-TBP-C₂H₄Cl₂?n = 4,6,8?system were obtained,including the ratio of TBP/IL/C₂H₄Cl₂ at 8/1/1?v/v?,O/A at 2:1 and extraction temperature at 20 oC.Under the optimal conditions,the highest single extraction efficiency of lithium was 86.48%.The UV and IR spectrum showed that the P=O?Li coordination bond was formed and a cation exchange mode was occurred in [C_nmim][PF₆]-TBP-C₂H₄Cl₂?n = 4,6,8?systems.The stoichiometry of the extracted complex was determined using a slope method.It was found that only one molecule of TBP was included in it.The thermodynamic parameters were determined by changing the temperature of the ionic liquids systems.The reaction of lithium extraction was spontaneous and exothermic in nature??Ho < 0,?Go < 0?.2.The viscosity and solubility of ionic liquids [C_nmim][NTf₂]?n = 4,6,8?were lower than that of [C_nmim][PF₆].The single extraction efficiency of 90.73% was obtained with 10%[C₄mim][NTf₂]-80%TBP-10%C₂H₄Cl₂ system at phase ratio of 2/1.After a three-stage simulation of countercurrent extraction,the extraction efficiency of lithium reached 99.15%.At the same time,the separation factor?? = DLi/DMg?was calculated and it was 921.33,which indicated the ionic liquid systems can effectively separate Li and Mg.The cations of ionic liquids in aqueous phase showed one ultraviolet absorption peak??max = 211 nm?.It was found that the extraction of lithium ions into ionic liquid systems involved a cation-exchange mode.According to the slope analysis,the ratio of Li⁺ to TBP in the extracted complex was 1:1.And the extraction complex was identified as [Li·TBP]+.The thermodynamics constants of the extraction process showed that the extraction efficiencies of lithium ions decreased with increasing temperature.The extraction equilibrium was a spontaneous and exothermic reaction which was suited for low temperature.3.Box-Behnken response surface analysis methodology was used to optimize the extraction process of Li⁺ from brine in [C₄mim][NTf₂]-TBP-C₂H₄Cl₂ system.On the basis of single factor,the volume concentration of TBP,volume concentration of ionic liquid and phase ratio?O/A?were selected as independent variable.At the same time,we selected the extraction efficiency of Li⁺ as response value and a quadratic model about the extraction efficiency associated with the above factors was derived.Under the optimal condition?ionic liquid of 11.46%,TBP of 85% and phase ratio of 2.77?,there was no significant difference between the experimental value of lithium extraction efficiency of 95.61% and the predicted value of 97.81%.The relative error was calculated and it was 2.24%.So the response model determined by RSM was reasonable and can be used to predict the extraction efficiencies of lithium ions.4.Two functionalized ionic liquids?FILs?,namely [N4444][DEHP] and [N8888][DEHP],were synthesized and fully characterized by 1H and 13 C NMR spectroscopy.The high extraction efficiencies of Li⁺ were obtained for both FILs,although the value for [N4444][DEHP] would be more higher than that of [N8888][DEHP].Linear regression analysis of the extraction data resulted in a slope of about 1 for both of the FILs,suggesting that one molecule of FIL formed a complex with a single lithium ion during the extraction process?i.e.,a 1:1 complex?.The IR spectrum showed that the lithium ion was coordinated to the oxygen atom of the P=O group in [DEHP]-and the P=O?Li coordination bond was formed.The extraction mechanism of FILs for Li⁺ was indicated to be an ion association mechanism.Thermodynamics study indicated that the extraction equilibrium was a spontaneous and exothermic reaction.5.Applied B3 LYP method of density functional theory at the 6-31G?d,p?level with Gaussian 09 software,the ionic liquids [C₄mim][PF₆],[C₄mim][NTf₂] and [N4444][DEHP] were calculated and geometrically optimized.Several parameters of the ionic liquids were obtained,such as cation-anion interaction energies,bond length,local ionization energy and electron density.All the calculation results showd that the oxygen atom from [DEHP]-anion was easier to attract the lithium ion electrostatically,thus leads to the high efficiency of extraction.However,the ionic liquids [C₄mim][PF₆] and [C₄mim][NTf₂] could not react with Li⁺.