Green Extraction And Purification Of Lithium Salt In Brine Water

Ionic liquid is green solvent and as extraction medium can avoid environmental pollution of conventional hydrometallurgy due to volatilization of organic solvent. The modified two-steps method using non-solvent and temperature programmed technologies was developed for synthesis of five ionic liquids 1-alkyl-3-methylimidazolium hexafluorophosphate abbreviated as [CxMIm][PF6], where X=4, 6, 8, 10 and 12. After the crude products were diluted by acetone, it was decolored with activated carbon and almost colorless ionic liquids were obtained. There was no obvious absorption peak in the wavelength range of 400~800 nm.The physical and chemical properties of the ionic liquid such as thermal stability and solubility were investigated in detail. We determined the solubility of ionic liquids in water by ultraviolet absorption spectrum method. Polarity of ionic liquid was determined by pyrene fluorescence probe, and found polarity decreasing with the increasing of alkyl chain length. Besides, the physical properties containing thermal stability, density, viscosity and surface tension were determined. The results showed the properties were decided by both cations and anions of ionic liquids and exhibited how melting point, viscosity, density and surface tension were affected by changes in alkyl chain length. In the series of ionic liquids studied here, the thermal stability, polarity, water content and surface tension of ionic liquids were decreased with the increasing in alkyl chain length. All these ionic liquids exhibited excellent thermal stability of 260 oC; density was of 1.1~1.4g.cm-3; surface tension lower than water and higher than hexane. Therefore, the excellent properties containing high stability, strong hydrophobicity and low surface tension of ionic liquids proved its advantage in extraction.A model for study on extraction lithium from salt lake brine was designed, in which the assayed ionic liquid (IL), tributyl phosphate (TBP) and FeCl3were used as extraction medium, extracting agent and synergic reagent, respectively. The effects of ionic liquid propertied and extraction conditions on the lithium extraction were investigated. The results indicated that the viscosity, polarity and hydrophobicity were the main factors which influenced the extraction efficiency. With increasing number of carbon atom of alkyl group in the ionic liquid the extraction efficiency of lithium will obviously increase. However, the ionic liquid was solid and resulted in appearing a third phase when number of alkyl group exceeded eight. Thus, 1-octyl-3-methylimidazolium hexafluorophosphate ionic liquid was finally selected as extraction medium. Optimal extraction conditions of the system included TBP/IL of 9/1(v/v), 0.03 mol/L HCl, O/A of 1.5:1 and Fe/Li of 1.3:1. Under optimal conditions, single extraction and back-extraction efficiencies are 87% and 90%, respectively. The interference study indicated the determination of lithium was free from the interference of almost all positive and negative ions such as K+, Ca2+, Mg2+ and Na+ found in brine water. Daisy extraction of salt lake brine indicated that total recovery of 97% was obtained by combination of three step extractions with two step back-extraction, and Mg/Li ratio was lower to about 2.2. The reuse of ionic liquid-TPB organic phase was up to 10 times. Study on the mechanism revealed Li+ combines with TBP and FeCl3 to form a low polar LiFeCI4·2TBP and was extracted into organic phase because, and addition of HCl in the organic phase will result that Li+ was again back-extracted into aqueous phase due to strong polarization force of H+. As LiFeCl4·2TBP is more easy to small polar ionic liquid than non-polar solvent paraffin, the system with ionic liquid is of higher extraction efficiency and the capacity of lithium.Moreover, the above system was applied to the preconcentration of ultra trace lithium in hair. Proposed procedure allowed the rapid lithium determination at 2.5μg.L-1 level in hair samples with satisfactory results. The detection limit, calculated using three times the standard error of estimate of the calibration graph, was 2.5 ng of lithium per liter hair sample. The enrichment factor of lithium was more than 100 times. Lithium recoveries between 89 and 92% for samples proof the accuracy of the proposed method.