Recovery of Lithium Ions from Salt Lake Brine with a High Magnesium

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Recovery of lithium ions from salt lake brine with a high Mg/Li ratio using heteropoly acid ionic liquid Yong Wang, Haotian Liu, Jiahui Fan, Xueting Liu, Yafei Hu, Yulei Hu, Zhiyong Zhou, and Zhongqi Ren ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04694 • Publication Date (Web): 26 Dec 2018 Downloaded from http://pubs.acs.org on January 6, 2019

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ACS Sustainable Chemistry & Engineering

Recovery of lithium ions from salt lake brine with a high Mg/Li ratio using heteropoly acid ionic liquid Yong Wang, Haotian Liu, Jiahui Fan, Xueting Liu, Yafei Hu, Yulei Hu, Zhiyong Zhou*, and Zhongqi Ren* College of Chemical Engineering, Beijing University of Chemical Technology, Beijing 100029, China

ABSTRACT:

A

fluoride-free

ionic

liquid

1-butyl-3-methylimidazolium

phosphotungstate ([Bmim]3PW12O40) was synthesized and first used as co-extraction reagent for recovery of lithium ions from salt lake brine with a high Mg/Li ratio. Fourier transform infrared spectra (FT-IR), nuclear magnetic resonance (NMR) and thermogravimetric analysis (TGA) were used for characterizations of the prepared ionic liquid. Tributyl phosphate (TBP), [Bmim]3PW12O40, and dimethyl phthalate (DMP) were used as extractant, co-extraction reagent and diluent, respectively. Effects of molar ratio of [Bmim]3PW12O40 to Li+, O/A phase ratio, volume fraction of TBP and carbon chain length of ionic liquid on extraction efficiency and separation factor of Li+ were investigated. The overall extraction efficiency of Li+ was 99.23% after five stages cross-current extraction experiments under optimal conditions. The washing and stripping experiments of the organic phase were conducted and the parameters were optimized. The reusability of the organic phase was investigated. A total of 10 regeneration cycles of extraction processes showed that the organic phase was highly stable for selective extraction of Li+. Finally, the extraction mechanism using the proposed extraction system was also explored. The results showed that the highly

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selective extraction of lithium ion was realized by cation exchange and coordination between TBP and Li+. KEYWORDS: Solvent extraction, Ionic liquid, Co-extraction reagent, Cross-current extraction, Cation exchange INTRODUCTION Lithium, as a new type of energy metal, has become a new hot spot in the global economic development. Compared to the other alkali metals, lithium has the highest redox potential and specific heat capacity. In addition, the chemical property of metallic lithium is very active. Due to these excellent characteristics, lithium is currently used in a wide range of applications, such as Li-ion battery, alloy manufacturing, ceramics, glass, rubber, pharmaceuticals, aerospace, and so on.1-5 At present, about 70% of the world's lithium resources exist in salt lake brines. Therefore, separating lithium from salt lake brines has attracted more and more attention.6 Various traditional separation methods have been used for recovery of lithium from salt lake brines, such as precipitation method, ion exchange, selective semipermeable membrane, and evaporative crystallization.7-10 Since Li and Mg are in a diagonal direction in the periodic table of elements, they have similar physical/chemical properties. That is, the large amount of magnesium chloride in salt lake brine is not conducive to the extraction of lithium by traditional methods. Therefore, solvent extraction with high extraction efficiency and selectivity is considered as an effective method for extracting lithium from salt lake brine with high Mg/Li ratio, which also shows a high industrial application prospect.


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At present, β-diketones, crown ethers and organic phosphine are the main extractants for extracting lithium ions from salt lake brines by liquid-liquid extraction. β-diketones show a good extraction performance for recovering lithium. However, the disadvantages of this kind of extractants are obvious, such as high cost, high dissolution loss and difficult to recycle, which limits the industrial application. Torrejos et al.11 explored a DB14C4-C18-COOH/CYPHOSIL 109 system for selective extraction of Li+. The results showed that the extraction behavior of Li+ using this extraction system was efficient and stable. However, the synthesis of crown ether compounds was complicated and expensive, which is not conducive to industrial application. Various acidic phosphorus-containing extractants are used for extraction of lithium, such as bis (2ethylhexyl) phosphoric acid (P204 or D2EHPA), 2-ethylhexylphosphonic acid mono2-ethylhexyl ester (P507), phosphonic acid mono (2- ethylhexyl) ester (H2MEHP), and so on. However, this kind of extractants show higher extraction efficiencies of divalent metal ions than that of monovalent metal ions.12 Therefore, this type of extractants could not be used for extraction of lithium ions from salt lake brines with a high Mg/Li ratio.13 Neutral phosphorus-containing extractants including tributyl phosphate (TBP), triisobutyl phosphate (TIBP), trioctyl phosphate (TOP), and trioctylphosphine oxide (TOPO) are considered as a kind of more suitable extractants for recovery of lithium from salt lake brines with high Mg/Li ratio. TBP is the most commonly used extractant among them. In addition, the addition of co-extraction reagent to the extraction system can greatly increase the extraction efficiency and selectivity of Li+. FeCl3 is a typical co-extraction reagent for TBP-kerosene extraction system. However, a third phase is



easy to form during the extraction process with TBP-FeCl3-kerosene, which is not conducive to the continuous production. Moreover, very high concentration of HCl solution is used as stripping reagent, which shows a badly threat to human body and operating apparatus. Based on the above problems, Zhou et al.14-17 used polar diluents like ketones and alcohols to replace non-polar diluent kerosene. The results showed that no appearance of the third phase could be observed during the whole extraction process with different molar ratios of TBP to polar diluent. Besides, the concentration of HCl solution used as stripping reagent could be greatly decreased by optimizing process parameters. However, some inherent problems with FeCl3 as co-extraction reagent can’t easily be solved. In order to realize the continuous processing, the extraction system should be maintained in the organic phase. For extraction of lithium ions from aqueous solution, one molecule of neutral FeCl3 used as co-extraction reagent should first from one complex anion of with one anion of chloride ion. Based on charge conservation, the complex of LiFeCl4·nTBP is formed in the organic phase. Since the reaction between FeCl3 and Cl- to form FeCl4- is a reversible reaction, it is necessary to ensure a certain concentration of chloride ions in all the aqueous solutions used in the whole recovery process, such as original aqueous solution containing lithium ions, washing reagent, stripping reagent and regenerant, which extremely limits the application fields and the selection of these reagents. In addition, ferric ions can be easily precipitated with the addition of NaOH in the regeneration process of the organic phase. Thus, it is necessary to explore an efficient co-extraction reagent to replace FeCl3 for recovery of lithium ions from salt lake brines with high Mg/Li ratio by solvent


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extraction. Ionic liquids (ILs) as one kind of green and clean ionic solvent have many excellent characteristics, such as good solubility, high stability, nonflammability, negligible vapor pressure and adjustable acidity.18-20 Hence, ionic liquids have exhibited attractive applications in chemical reaction, catalysis, biodiesel production, gas absorption, extraction of metal ions and the other separation fields. The traditional extraction and separation industries usually use volatile organic solvents, some of which show obvious disadvantages like low efficiency, high cost and pollution. The use of ionic liquid can greatly improve extraction efficiency and reduce environmental pollution.

21,22

In previous studies, ILs were used as extractants or solvents for the

extraction of multivalent metal ions. However, rare reports about the extraction of Li+ using ILs could be found. Shi et al.23,24 investigated the extraction efficiency of Li+ with TBP and several ionic liquids (PF6- and NTf2- as anions). Gao and coworkers25 investigated the extraction performance of Li+ with triisobutyl phosphate (TIBP) and several ionic liqiuid (PF6- and NTf2- as anions). The anions of these ionic liquids used as co-extraction reagents have good hydrophobicity and stereosymmetric structure which is similar to FeCl4-. The extraction system showed high extraction efficiency for Li+. However, the fluorine containing anions in these ionic liquids extremely influence their “greenness”.26 The presence of fluorine atoms may cause serious concerns if the hydrolysis and thermal stabilities of the anion are poor (e.g. for PF6- and NTf2-). The addition of hydrochloric acid solution in the washing and stripping processes would cause the hydrolysis of the anions of these ionic liquids, resulting in the formation of



HF which would greatly threaten the human body. In addition, the extraction efficiency of lithium ion decreases with continuous hydrolysis of the ionic liquids. In order to avoid the liberation of toxic and corrosive HF into the environment, additional effort is needed. Therefore, it is necessary to explore a fluorine-free and relatively hydrolysisstable ionic liquid with stereosymmetric anions as co-extraction reagent for recovery of Li+. Heteropolyacid as an environmentally friendly catalyst has many advantages, such as hydrolysis-stable structure, high catalytic activity, high redox property, thermal stability, and etc. Therefore, it shows broad application prospects.27,28 At present, heteropolyacids are mainly used in the preparation of catalysts and desulfurization process. No previous reports about the application of heteropolyacids in the extraction of Li+ could be found. Since heteropolyacid usually has stereosymmetric anions, it has a great application prospect in extraction of lithium ions from salt lake brines. However, most heteropolyacids like sodium phosphotungstate (Na3PW12O40) have poorly direct solubility either in aqueous solution or in the organic phase, leading to low extraction efficiency of Li+. However, the heteropolyacids ionic liquid has good direct solubility in the organic phase. Thus, the synthesis of ionic liquids using phosphotungstic acid was conducted to improve the solubility. Heteropolyacid ionic liquid is usually prepared by combining organic cations with heteropolyacid anions.29,30 In this study, ionic liquids with phosphotungstate anion ([Bmim]3PW12O40, [Hmim]3PW12O40 and [Omim]3PW12O40) were synthesized and used as co-extraction reagents.31 This class of ionic liquid was usually used as catalysts for organic reaction31-


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33

and desulfurization.34-36 However, no reports about the application of this class of IL

in extraction of metal ions especially lithium ion can be found. In addition, this class of ionic liquid not only inherits the advantages of ionic liquid but also shows very high hydrolysis and thermal stabilities,37,38 which avoids the treatment of hydrolyzed coextraction reagent and second pollution of salt lake brines. Therefore, the expansion and development of the application field of this class of ionic liquid is worthwhile. The prepared ionic liquid was dissolved in the organic phase, which consisted of tributyl phosphate (TBP) as extractant and dimethyl phthalate (DMP) as diluent. The prepared [Bmim]3PW12O40 was characterized by Fourier transform infrared spectra (FT-IR), nuclear magnetic resonance (NMR) and thermogravimetric analysis (TGA). Effects of different extraction parameters on extraction performances of Li+ were studied. Saturated extraction and multi-stage countercurrent extraction experiments were conducted. Then washing, stripping and organic phase recovery experiments were also carried out. Finally, the extraction mechanism with TBP-DMP-[Bmim]3PW12O40 extraction system was investigated. EXPERIMENTAL SECTION Reagents and Apparatus. Lithium chloride anhydrous (> 99% purity) was purchased from Beijing HWRK Chem Co., Ltd (Beijing, China). Magnesium chloride hexahydrate, sodium chloride, potassium chloride, sodium hydroxide and hydrochloric acid (> 98% purity) were purchased from Beijing Chemical Plants (Beijing, China). Phosphotungstic acid (> 99% purity) was purchased from Shanghai Macklin Biochemical Co., Ltd (Shanghai, China). Tributyl phosphate (TBP) (> 99% purity) and



dimethyl phthalate (DMP) (> 98% purity) were purchased from Shanghai Macklin Biochemical Co., Ltd (Shanghai, China). 1-Bromobutane (> 98% purity) and 1methylimidazole (> 99% purity) were purchased from Aladdin Industrial Co., Ltd (Shanghai, China). The composition of simulated salt lake brine are as follows: Li+ (0.35 g·L−1), Mg2+ (96 g·L−1), Na+ (1.84 g·L−1) and K+ (0.62 g·L−1). Thermostatic temperature water bath oscillator (THZ-82AB, Jiangsu Jintan Ronghua Instrument Manufacturing Co., Ltd.) was applied for the liquid-liquid extraction process. High speed centrifuge (SF-TDL-5A, Shanghai Fichal Analysis Instrument Co., Ltd.) was used for rapid separation of the aqueous and organic phases after extraction. Ultra-pure water machine (TY0476, Pall filter Beijing Co., Ltd.) was used for the preparation of the simulated salt lake brine and dilution of the aqueous solution after extraction. The other instruments used in this study are as follows: vacuum drying oven (DZF-6020, Shanghai New Miao Medical Equipment Manufacturing Co., Ltd.), rotary evaporator (EV322, Beijing Lab Tech Instrument Limited by Share Ltd.), thermostatically heated magnetic stirrer (DF-101S, Beijing Kewei Yongxing Instrument Co., Ltd.), water-circulation multifunction vacuum pump (SHB-III, Shanghai Zhen Jie Experimental Equipment Co., Ltd.). Synthesis of Ionic Liquids. [Bmim]3PW12O40 was prepared by two steps method reported previously.33,35,39 Figure S1 shows the structures of precursors and ionic liquid. The synthesis procedures of [Hmim]3PW12O40 and [Omim]3PW12O40 were similar to that of [Bmim]3PW12O40 except for replacing 1-bromobutane by 1-bromohexane and 1-bromooctane.


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Characterization. The Fourier transform infrared spectra (FT-IR) of [Bmim]Br, H3PW12O40, [Bmim]3PW12O40 and the organic phase before and after extraction were performed on a Nicolet 8700 attenuated total refection Fourier transform infrared spectrometer (Fisher Scientific, USA). 1H NMR spectra of [Bmim]Br, H3PW12O40, [Bmim]3PW12O40, TBP and the organic phase before and after extraction and 31P NMR spectra of the organic phase before and after extraction were recorded on an Inova-400 spectrometer (Bruker AV400, Germany) with DMSO as the solvent. The thermogravimetric analysis of IL was carried out on a TG209C thermal analyzer (Netzsch, Germany). Analysis. The atomic absorption spectrometer (AA-6880, Shimadzu, Japan) was used to determine the concentration of metal ions (Li+, Mg2+, Na+ and K+) in aqueous solution before and after extraction. Liquid-Liquid Extraction Experiment. All the extraction experiments were carried out at room temperature. Ultra-pure water was used for preparation of the simulated brine and dilution of aqueous solution. The certain volumes of the organic phase (TBP + DMP), ionic liquid and simulated brine solution were first mixed. Then the mixture was shaken on a water bath oscillator for 10 min and then centrifuged for 3 min for complete separation of the organic and aqueous phases. After that the aqueous sample was taken out for the determination of the concentration of Li+ by atomic absorption spectrometer. The concentration of Li+ in the organic phase was calculated by conservation of mass. However, since the concentration difference of Na+, K+ and Mg2+ in aqueous solution before and after the extraction is very small, resulting in very



high calculation error, the extraction parameters of Na+, K+ and Mg2+ can’t be calculated from the concentrations of aqueous solution before and after extraction. Therefore, it is necessary to strip the organic phase using a HCl solution to determine the ionic concentration in the hydrochloric acid solution after stripping. Then the obtained values would be converted to the ionic concentration of the organic phase based on the volume ratio. The extraction efficiency (E), distribution coefficient (D) and separation factor (β) are calculated according to the following equations: 𝐸(%) = 𝐷=

𝑐i

𝐶i

― 𝑐f

𝑐f 𝛽=

― 𝐶f

𝐶i

× 100

(1)

× 𝑉(A/O)

𝐷Li 𝐷M

(2)

(3)

where Ci and Cf are the initial and final concentrations of metal ions in the aqueous phase, respectively, V(A/O) represents the volume ratio of the aqueous phase to the organic phase, DLi represents the distribution coefficient of lithium ion, DM represents the distribution coefficients of other metal ions (K+, Na+ and Mg2+). The washing and stripping efficiencies (𝐸′) are calculated according to the following equation:

𝐸′(%) =

𝐶a 𝐶o

× 𝑉(A O) × 100

(4)

where Co is the concentration of metal ion in the organic phase before the washing or stripping process, Ca is the final concentration of metal ion in the washing or stripping


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solution. RESULTS AND DISCUSSION Characterizations

of

[Bmim]3PW12O40,

[Hmim]3PW12O40

and

[Omim]3PW12O40 FT-IR. The FT-IR spectra of [Bmim]Br, H3PW12O40 and [Bmim]3PW12O40 are shown in Figure S2. Since H3PW12O40 has four typical Keggin structures, the characteristic peaks at 1080.3 cm-1, 983.4 cm-1, 888.7 cm-1 and 805.4 cm-1 correspond to the stretching vibrational absorptions of P-O, W=O, W-Ob1-W and W-Ob2-W, respectively (Ob1 and Ob2 represent the bridge oxygen in the Keggin structure). Meanwhile, four characteristic peaks of Keggin structures at 1079.9 cm-1, 978.2 cm-1, 896.1 cm-1 and 807.3 cm-1 could also be found in the FT-IR spectrum of [Bmim]3PW12O40, indicating that the Keggin structures of H3PW12O40 still remain in the structure of prepared ionic liquid. The characteristic peaks at 3144.4 cm-1, 1571.3 cm-1 and 2960.5 cm-1 in the FT-IR spectrum of [Bmim]Br correspond to the stretching vibrations of C-H and -C=N- on the imidazole ring and the stretching vibration of C-H on the substituent, respectively. These characteristic peaks could also be found in the FT-IR spectrum of [Bmim]3PW12O40, which confirms the existence of [Bmim]+ in the prepared ionic liquid. Both the results demonstrate the successful synthesis of the ionic liquid [Bmim]3PW12O40. NMR. The nuclear magnetic resonance spectra of [Bmim]Br, H3PW12O40 and [Bmim]3PW12O40 are shown in Figure S3. It can be seen that all the characteristic peaks of [Bmim]Br appear in the NMR spectrum of the synthesized ionic liquid



[Bmim]3PW12O40. However, no characteristic peak of H3PW12O40 can be found in the NMR spectrum of the synthesized ionic liquid [Bmim]3PW12O40, demonstrating the successful synthesis of ionic liquid, which agrees with the results of FT-IR characterization. The results of 1H NMR also indicate that the prepared ionic liquid has high purity. Besides, the characteristic peak at 3.338 ppm is an absorption peak of water in the prepared ionic liquid. TG-DSC. The TG-DSC curves of the prepared ionic liquid [Bmim]3PW12O40 are shown in Figure S4. It can be seen that the prepared ionic liquid shows almost no weight loss when the temperature is lower than 400 °C, indicating that the prepared ionic liquid has good thermal stability. Two decomposition points at around 500 °C and 600 °C can be seen in the DSC curve of prepared ionic liquid. The decomposition of [Bmim]+ happens at the temperature ranging from 400 °C to 500 °C. Then [PW12O40]3- begins to decompose at the temperature around 500 °C. Generally speaking, extraction process usually operate at room temperature. Therefore, the prepared ionic liquid can maintain its structure during the extraction process. Characterizations of [Hmim]3PW12O40 and [Omim]3PW12O40 are shown in the Supporting Information (Figures S5-S10). Effect of Extraction Conditions Effect of Length of Carbon Chain on Ionic Liquid. The length of carbon chain will influence the physical and chemical properties of ionic liquid, which may also affect the extraction performance of metal ions with ionic liquid as co-extraction reagent. Therefore, three kinds of ionic liquids with different lengths of carbon chains were


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synthesized, which were used as co-extraction reagents for the extraction experiments performed under the determined optimum conditions. Figure 1 shows that the length of carbon chain on ionic liquid indeed has a significant effect on extraction efficiency of Li+. The extraction efficiency of Li+ decreases with the increase of the length of carbon chain. The extraction process is probably a cation exchange process. The increase of the length of carbon chain on IL increases both the hydrophobicity and viscosity of IL.23,25 High hydrophobicity of IL is beneficial to maintaining the cation of IL in the organic phase, which is not conductive to the cation exchange process. In addition, the increase of viscosity of IL shows a contrary influence on transfer of Li+ from aqueous solution to the organic phase, which also is not conductive to the cation exchange process. Therefore, [Bmim]3PW12O40 was selected as the co-extraction reagent. Effect of Molar Ratio of [Bmim]3PW12O40 to Li+. Ionic liquid acted as coextraction reagent plays a key role in the extraction process. Effect of molar ratio of [Bmim]3PW12O40 to Li+ on extraction efficiencies of metal ions was investigated. Figure 2a shows that the extraction efficiency of Li+ increases with the increase of the molar ratio of [Bmim]3PW12O40 to Li+ from 0.4 to 1.2. However, the extraction efficiency of Li+ maintains constant when the molar ratio of [Bmim]3PW12O40 to Li+ is higher than 1.2. It is because that the solubility of ionic liquid in the organic phase is limited and no more [Bmim]3PW12O40 can be dissolved in the organic phase when the molar ratio of [Bmim]3PW12O40 to Li+ is higher than 1.2. However, the extraction efficiency of Li+ is much higher than that of other metal ions. All the extraction efficiencies of Na+, K+ and Mg2+ are lower than 10% and almost maintain constant with



increasing of molar ratio of [Bmim]3PW12O40 to Li+. As shown in Figure 2b, the separation factors of Li/Mg and Li/K first increase and then maintain constant with the increase of the molar ratio of [Bmim]3PW12O40 to Li+. However, the separation factor of Li/Na keeps unchanged with increasing the molar ratio of [Bmim]3PW12O40 to Li+. The separation factor follows the sequence Li/Mg > Li/K > Li/Na. Therefore, 1.2 was selected as the optimal molar ratio of [Bmim]3PW12O40 to Li+. Effect of O/A Phase Ratio. The volume ratio of the organic phase to the aqueous phase has a great influence on the extraction performance. Effect of O/A phase ratio on extraction efficiencies of metal ions was investigated. Figure 3a shows that the extraction efficiency of Li+ increases gradually with the increase of O/A phase ratio. The increase of O/A phase ratio enhances the dispersibility of IL in the organic phase and increases the relative quantity of TBP, which increases extraction efficiency of Li+. Meanwhile, the extraction efficiencies of Na+, K+ and Mg2+ are much lower than that of Li+. Figure 3b shows that the separation factors of Li/Mg and Li/K first increase and then slightly decrease with the increase of O/A phase ratio. In addition, the separation factor of Li/Na remains nearly unchanged with increasing O/A phase ratio. The separation factor follows the same sequence Li/Mg > Li/K > Li/Na. Therefore, 1 was selected as the optimal O/A phase ratio. Effect of Volume Fraction of TBP. The volume fraction of TBP may have a great influence on the extraction performance. Since the viscosity of TBP is high and the density of TBP is close to that of water, both of which are not conducive to the extraction process, a diluent is needed to improve the physical properties of TBP. In


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this work, DMP was selected as a diluent for TBP. Figure 4a shows that the extraction efficiency of Li+ first increases and then maintain constant with the increase of TBP volume fraction. When the volume fraction of TBP increases from 30% to 80%, the chance of contact between Li+ and TBP increases, leading to the increase of extraction efficiency of Li+. However, when the volume fraction of TBP continues to increase, both the viscosity and density of the organic phase increase, which is not favor for the phase separation, resulting in unchanging the extraction efficiency of Li+. The extraction efficiencies of Na+, K+ and Mg2+ are much lower than that of Li+ and increase gradually with the increase of the volume fraction of TBP. As shown in Figure 4b, the separation factor of Li/Mg first increases slightly and then decreases sharply with the increase of the volume fraction of TBP. However, the separation factor of Li/K first increases sharply and then decreases slightly with the increase of volume fraction of TBP. Besides, the separation factor of Li/Na remains basically unchanged. The higher the volume fraction of TBP is, the relative quantity of free TBP molecules is, leading to the increase of extraction efficiency of Li+. However, the extraction of Li+ approaches to saturation with the increase of volume fraction of TBP. The increase of the number of residual free TBP molecules can also increase the extraction efficiencies of other metal ions at different levels. Therefore, all the separation factors show a decrease trend with the increase of volume fraction of TBP. Since the concentration of Mg2+ is much higher than that of Na+ and K+, the increase of separation factor of Li/Mg as much as possible should be top priority. However, the decrease of volume fraction of TBP leads to sharp decrease of the extraction efficiency of Li+, which greatly



influences the final yield of LiCl product. Therefore, comprehensive consideration of the extraction efficiency and separation factor, 60% TBP was selected as the optimal volume fraction of TBP. In addition, the comparison of extraction performances with different ionic liquids as co-extraction reagents are listed in Table 1. Although the maximum one-stage extraction efficiencies of lithium ions with [C4mim][NTf2] (92.37%) and [C4mim][PF6] (90.93%) as co-extraction reagents and TBP as extractant are higher than that with [Bmim]3PW12O40 as co-extraction reagent (69.18%), the extraction phase ratios (O/A) with [C4mim][NTf2] and [C4mim][PF6] as co-extraction reagents are also larger than that with [Bmim]3PW12O40 as co-extraction reagent. Moreover, although the maximum one-stage extraction efficiency of lithium ion obtained in this work (69.18%) is lower than that obtained with [C4mim][NTf2] and TIBP as co-extraction reagent and extractant (83.71%), the separation factor of Li+ to Mg2+ obtained in this work (283.06) is much higher than that obtained with [C4mim][NTf2] and TIBP as co-extraction reagent and extractant (71.24). More importantly, the initial Mg/Li molar ratio of the simulated salt lake brine prepared in this work is much higher than that used in all the previous studies, indicating that [Bmim]3PW12O40 used as co-extraction reagent shows very high extraction and selective ability for lithium ions. Saturated Extraction. In order to explore the maximum extraction capacity of the extraction system used in this study, the saturated extraction experiments of the organic phase was conducted. In the saturated extraction experiments, the same organic phase continuously extracted a new aqueous solution at each extraction stage. A total


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of seven extraction times were performed and the results are shown in Figure 5. Since the extraction capacity of a fixed organic phase is limited, the extraction efficiency of Li+ gradually decreases with increasing extraction times. In other words, after seven extraction times, the loading of the organic phase reached saturation. Multistage Cross-Current Extraction. In order to explore the maximum extraction stages of Li+, a multistage cross-current extraction experiments under optimal conditions were conducted. Figure 6 shows that nearly no lithium ions can be found in the aqueous phase after five-stage cross-current extraction experiments. Moreover, the overall extraction efficiency of Li+ reaches up to 99.23% after five-stage cross-current extraction experiments. The results demonstrate that the extraction system used in this work shows high separation efficient for lithium ions. Washing Experiments. After the extraction process, a large amount of Li+ and small amounts of K+, Na+ and Mg2+ existed in the organic phase. To obtain pure Li2CO3 in the final production process, washing experiments are necessary to be conducted for removing the impurity ions in the organic phase. Since MgCO3 is easier to be formed than Li2CO3 under the same conditions, Mg2+ is the main impurity ion which must be removed during washing process. Besides, the loss of Li+ should be reduced as much as possible. According to ion exchange theory, the binding ability of cations to TBP follows the sequence H+ > Li+ ≫ Na+ > K+ ≈ Mg2+. Therefore, in order to elute Mg2+, NaCl aqueous solution was first used as a washing reagent for washing experiments. Effect of NaCl concentration on washing efficiency of metal ions in the organic phase was studied. Figure 7 shows that the washing efficiencies of K+ and Mg2+ increase with



the increase of the concentration of NaCl. When the concentration of NaCl is 1.235 mol·L-1, K+ and Mg2+ can be eluted completely from the organic phase. However, about 40% Li+ could be eluted from the organic phase too, which greatly affects the yield of lithium ions. To reduce washing loss of Li+, 0.926 mol·L-1 was selected as the optimal concentration of NaCl. Then effect of O/A phase ratio on washing efficiency of metal ions in the organic phase was investigated. As shown in Figure 8, the mass transfer driving force between the two phases decreases with increasing O/A phase ratio, resulting in the slightly decrease of the washing efficiencies of K+ and Li+. However, the washing efficiency of Mg2+ first increases and then maintains constant with increasing O/A phase ratio. In order to reduce the washing loss of Li+, the O/A phase ratio was selected as 8. In order to further reduce the washing loss of Li+, LiCl was added to NaCl aqueous solution to prevent the lithium ions from the organic phase into the aqueous phase. Effect of LiCl concentration on washing efficiency of metal ions in the organic phase was studied. Figure 9 shows that K+ and Mg2+ could be eluted completely when the concentration of LiCl is higher than 0.252 mol·L-1. Moreover, the washing efficiency of Li+ decreases gradually with the increase of the concentration of HCl. The washing efficiency of Li+ is negative when the concentration of HCl is higher than 0.252 mol·L-1. This is mainly because small amounts of Li+ in the washing reagent would be extracted into the organic phase in the washing process. In short, the washing efficiencies of K+ and Mg2+ are approximately 100% with no loss of Li+ under the optimal conditions (O/A phase ratio is 8, the concentrations of NaCl and LiCl are 0.926 mol·L-1 and 0.252


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mol·L-1, respectively). Stripping Experiments. After washing process, the concentrations of K+ and Mg2+ in the organic phase are basically zero. Li+ and Na+ in the organic phase need to be stripped for subsequent production of Li2CO3. Since only the binding ability of TBP to H+ is higher than that to Li+, HCl aqueous solution can be used as a stripping reagent for Li+. Effects of the concentration of HCl solution and O/A phase ratio on stripping efficiency of Li+ were investigated. The molar ratio of [H+]/[M+] (M = Na or Li) was fixed as 1 and the O/A phase ratio was selected as 1, 2, 4, 6, 8 or 10. Figure 10 shows that the stripping efficiency of Li+ first increases and then slightly decreases with increasing O/A phase ratio. This is mainly because the contact chance between H+ in the aqueous phase and Li+ in the organic phase is relatively small when O/A phase ratio is higher than 6, which is not conducive to the transfer of Li+ between the two phases. Thus, 6 was selected as the optimal O/A phase ratio for stripping process. Then effect of the concentration of HCl solution on stripping efficiency of Li+ was studied. Figure 11 shows that the stripping efficiency of Li+ first increases and then maintains constant with increasing the HCl concentration. The stripping efficiency of Li+ reaches up to 100% when the HCl concentration is 0.309 mol·L-1. Hence, 0.309 mol·L-1 was selected as the HCl concentration for stripping process. Reusability of The Organic Phase. The stability of the organic phase plays a critical role in solvent extraction process. Therefore, the reusability of the organic phase under the optimal conditions was investigated. After extraction, washing and stripping processes, the organic phase contains large amounts of H+, which is not conducive to



the extraction of Li+. Therefore, sodium hydroxide solution was used to neutralize hydrogen ions for regeneration and reuse of the organic phase. A total of ten regeneration cycles of the whole extraction process were conducted. Figure 12 shows that all the extraction efficiencies of Li+ during the ten regeneration cycles of the whole extraction process are approximately 60%, indicating that the extraction system used in this study is very stable and suitable for high selective extraction of Li+. The stability of the organic phase is mainly due to the use of ionic liquid [Bmim]3PW12O40 as the coextraction reagent. Since [PW12O40]3- is a fairly stable anion, which is resistant to most acids and alkalis, it would not be decomposed by the influence of the acid and alkali solutions in stripping and regeneration processes. Therefore, the ionic liquid [Bmim]3PW12O40 is a suitable and promising co-extraction reagent to replace fluorinecontaining ionic liquids for solvent extraction of Li+ from salt lake brine with high Mg/Li ratio. Extraction Mechanism. The extraction system of TBP/[Bmim]3PW12O40 showed high extraction efficiency and separation factor for Li+ in simulated brine. Therefore, it is of great significance to study the extraction mechanism of metal ions. Firstly, 1H NMR characterizations of various organic solvents were performed. Figure 13 shows that the 1H NMR of IL shows eight different characteristic peaks of H in the cation of [Bmim]3PW12O40 (except for the absorption peak of water at 3.338 ppm). These eight characteristic peaks can be observed in the organic phase before extraction too. However, no characteristic peaks of H in the cation of [Bmim]3PW12O40 can be found in the organic phase after extraction and only the characteristic peaks of TBP


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appear, indicating that the extraction process took place by cation exchange. That is, the cation of the ionic liquid exchanged with metal ions. Therefore, metal ions were extracted into the organic phase and the cation of ionic liquid was exchanged to the aqueous phase. The FT-IR characterizations of the organic phase before and after extraction were performed. As shown in Figure 14, the characteristic peaks at 2961.2 and 2874.8 cm-1 corresponding to asymmetric and symmetric stretching vibrations of -CH3, the characteristic peaks at 1464.2 and 1382.4 cm-1 corresponding to asymmetric and symmetric angular vibrations of -CH3, and the characteristic peak at 1028 cm-1 corresponding to asymmetric stretching vibration of P-O-C in the FT-IR spectrum of the organic phase before extraction nearly remain unchanged after extraction. However, the characteristic peak at 1282.45 cm-1 corresponding to stretching vibration of P=O on TBP molecule before extraction shifts to 1259.31 cm-1 after extraction. Since the electronegativity of O atom is higher than that of P atom, the electron cloud is largely biased toward O atom. There is a lone pair electron on O atom, which has strong affinity to metal ions. The interaction between O atom and metal ions makes the electron cloud shift, resulting in the elongation of the P=O bond and the decrease of vibration frequency. The large shift of characteristic peak corresponding to stretching vibration of P=O demonstrates that TBP has strong affinity to metal ions in simulated salt lake brines. To further investigate the extraction mechanism, several extraction experiments with LiCl, NaCl, KCl and MgCl2 solutions separately were carried out and the binding



ability of TBP in the organic phase to four metal ions was studied by

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31P

NMR. As

shown in Figure 15, it can be seen that the position of characteristic peak of P atom in the organic phase before extraction shifts after extraction. The shift value of the position of characteristic peak of P atom in the organic phase after extraction of various metal ions solutions follows the sequence Li+ > Na+ > K+ ≈ Mg2+. Since the electronegativity of O atom in P=O group is very high, the density of electron cloud around P proton is also relatively high. The degree of shielding was enhanced after extraction process. Therefore, the resonance signal of P proton was toward the high field, resulting in the decrease of the value of chemical shift. The larger the chemical shift is, the stronger the binding ability is. Therefore, the results obtained in Figure 15 indicate that the binding ability of TBP to metal ions follows the sequence Li+ > Na+ > K+ ≈ Mg2+. Moreover, the results also demonstrate that the extraction process of metal ions from the aqueous solution to the organic phase is realized by the coordination between P=O group on TBP molecule and metal ion. CONCLUSIONS In this work, a new type of ionic liquid [Bmim]3PW12O40 was synthesized and used as co-extraction reagent for selective extraction of lithium ions with TBP and DMP as extractant and diluent, respectively. The results of various characterizations confirmed the successful synthesis of ionic liquid. Effects of several operation factors on extraction efficiency and separation factor of Li+ were studied. The optimal extraction conditions were as follows: n(ionic liquid)/n(Li+) = 1.2, 60% TBP/40% DMP (v/v), O/A = 1:1 and unadjusted pH. The extraction efficiencies followed the sequence


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Li+ > Na+ > K+ ≈ Mg2+ under the optimal conditions. Moreover, the separation factor followed the sequence β (Li/Mg) > β (Li/K) ≫ β (Li/Na). The loading of the organic phase reached saturation after seven extraction times with fresh aqueous solution. The results of multi-stages cross-current extraction experiments showed that the overall extraction efficiency of Li+ was higher than 99% after five stages cross-flow extraction processes. The mixture of 0.926 mol·L-1 NaCl and 0.252 mol·L-1 LiCl solutions was selected as the washing reagent with the optimal O/A phase ratio of 8. Besides, 0.309 mol·L-1 hydrochloric acid solution was selected as stripping reagent with O/A phase ratio of 6. Since the prepared [Bmim]3PW12O40 was structurally stable and resistant to acids and alkalis, this extraction system was very stable during 10 regeneration cycles of extraction experiments. The study of extraction mechanism indicated that the binding ability of TBP to metal ions follows the sequence Li+ > Na+ > K+ ≈ Mg2+, which agrees with the extraction efficiency results. Cation exchange and coordination between P=O group on TBP molecule and metal ion were the inner mechanism of the high selective extraction process for Li+. The prepared [Bmim]3PW12O40 showed great potential for replacing traditional organic solvents and fluorine-containing ionic liquids in liquidliquid extraction process of lithium ions from salt lake brine with a high Mg/Li ratio. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI : Description of synthesis process of [Bmim]3PW12O40, the structures of



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precursors and [Bmim]3PW12O40, FT-IR spectra, 1H NMR spectra and TGDSC curves of [Bmim]3PW12O40, [Hmim]3PW12O40 and [Omim]3PW12O40 (PDF) AUTHOR INFORMATION Corresponding Authors *Tel.: +86-10-64433872. E-mail: [email protected]. *Tel.: +86-10-64197101. E-mail: [email protected]. ORCID Zhongqi Ren: 0000-0002-2571-5702 Zhiyong Zhou: 0000-0001-6436-1399 Notes The authors declare no competing financial interest. Acknowledgements This work was supported by the National Natural Science Foundation of China (21576010, 21606009, U1607107 and U1862113), Beijing Natural Science Foundation (2172043) and Fundamental Research Funds for the Central Universities (BUCTRC201515). The authors gratefully acknowledge these grants. REFERENCE (1) Shi, C. L.; Jing, Y.; Xiao, J.; Wang, X. Q.; Yao, Y.; Jia, Y. Z. Solvent extraction of lithium from aqueous solution using non-fluorinated functionalized ionic liquids as extraction

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DOI

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List of Tables Table 1. Comparison of extraction performances with different ionic liquids as coextraction reagents



Table 1. Comparison of extraction performances with different ionic liquids as co-extraction reagents Initial Mg/Li Maximum oneExtractant Ionic liquid O/A βLi/Mg Reference molar ratio stage ELi / % TBP [C4mim][NTf2] 13.05 92.37 2 403.33 [23] TBP [C4mim][PF6] 12.58 90.93 2 [24] TIBP [C2mim][NTf2] 9.54 83.71 1 71.24 [25] TBP [Bmim]3PW12O40 78.33 69.18 1 283.06 This work


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Figure Captions Figure 1. Effect of the length of carbon chain on ionic liquid on extraction efficiency of lithium ion (n(ionic liquid)/n(Li+) = 1.2, 60% TBP/40% DMP (v/v), O/A = 1:1, unadjusted pH). Figure 2. Effects of molar ratio of [Bmim]3PW12O40 to Li+ on extraction efficiencies (a) and separation factors (b) of metal ions (60% TBP/40% DMP (v/v), O/A = 1:1, unadjusted pH). Figure 3. Effect of O/A phase ratio on the extraction efficiencies (a) and separation factors (b) of metal ions (n([Bmim]3PW12O40)/n(Li+) = 1.2, 60% TBP/40% DMP (v/v), unadjusted pH). Figure 4. Effect of the volume fraction of TBP on the extraction efficiencies (a) and separation factors (b) of metal ions (n([Bmim]3PW12O40)/n(Li+) = 1.2, O/A = 1:1, unadjusted pH). Figure

5.

Saturated

extraction

experiments

of

the

organic

phase

(n([Bmim]3PW12O40)/n(Li+) = 1.2, 60% TBP/40% DMP (v/v), O/A = 1:1, unadjusted pH). Figure 6. Multi-stage cross-current extraction experiments of the organic phase (n(ionic liquid)/n(Li+) = 1.2, 60% TBP/40% DMP (v/v), O/A = 1:1, unadjusted pH). Figure 7. Effect of NaCl concentration on washing efficiency of metal ions in the organic phase (O/A = 4). Figure 8. Effect of O/A phase ratio on washing efficiency of metal ions in the organic phase (0.926 mol·L-1 NaCl solution).



Figure 9. Effect of LiCl concentration on washing efficiency of metal ions in the organic phase (O/A = 8, 0.926 mol·L-1 NaCl solution). Figure 10. Effect of O/A phase ratio on stripping efficiency of Li+ in the organic phase (0.0643 mol·L-1 HCl solution). Figure 11. Effect of HCl concentration on stripping efficiency of Li+ in the organic phase (O/A = 6). Figure 12. Reusability of the organic phase. Figure 13. 1H NMR of different samples (400MHz, DMSO) (n(ionic liquid)/n(Li+) = 1.2, 60% TBP/40% DMP (v/v), O/A = 1:1, unadjusted pH). Figure 14. FT-IR spectra of the organic phase before (a) and after (b) extraction. Figure 15.

31P

NMR of different samples (400MHz, DMSO) (a) The organic phase

before extraction; the organic phase after extraction of MgCl2 solution (b), KCl solution (c), NaCl solution (d) and LiCl solution (e).


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60

+ Li

40

E/%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


20

0

[Bmim]3PW12O40

[Hmim]3PW12O40 [Omim]3PW12O40

Figure 1. Effect of the length of carbon chain on ionic liquid on extraction efficiency of lithium ion (n(ionic liquid)/n(Li+) = 1.2, 60% TBP/40% DMP (v/v), O/A = 1:1, unadjusted pH).



75

E/%

60

45

Li+ Mg2+ K+ Na+

30

15

0

0.4

0.6

0.8

1.0

1.2

1.4

+

n([bmim]3[PW12O40]/n(Li )

(a)

300

β (Li+/Na+) β (Li+/K+)

250

+ β (Li+/Mg2 ) 200

β

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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150

100

50

0

0.4

0.6

0.8

1.0

1.2

1.4

+

n([bmim]3[PW12O40])/n(Li )

(b) Figure 2. Effects of molar ratio of [Bmim]3PW12O40 to Li+ on extraction efficiencies (a) and separation factors (b) of metal ions (60% TBP/40% DMP (v/v), O/A = 1:1, unadjusted pH).


Page 37 of 50

100

Li+ Mg2+

80

K+ Na+

E/%

60

40

20

0

0.5

1.0

1.5

2.0

2.5

3.0

2.0

2.5

3.0

V(O/A)

(a)

350

300

β (Li+/Na+) β (Li+/K+) β (Li+/Mg2+)

250

200

β

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


150

100

50

0

0.5

1.0

1.5

V(O/A)

(b) Figure 3. Effect of O/A phase ratio on the extraction efficiencies (a) and separation factors (b) of metal ions (n([Bmim]3PW12O40)/n(Li+) = 1.2, 60% TBP/40% DMP (v/v), unadjusted pH).



75

Li+ Mg2+ K+

60

Na+

E/%

45

30

15

0

30

40

50

60

70

80

90

100

Volume fraction of TBP (%, V:V)

(a)

300

250

200

β

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 50

β (Li+/Na+) β (Li+/K+) β (Li+/Mg2+)

150

100

50

0

30

40

50

60

70

80

90

100

Volume fraction of TBP (%, V:V)

(b) Figure 4. Effect of the volume fraction of TBP on the extraction efficiencies (a) and separation factors (b) of metal ions (n([Bmim]3PW12O40)/n(Li+) = 1.2, O/A = 1:1, unadjusted pH).


Page 39 of 50

Li+

50

40

E/%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


30

20

10

0

1

2

3

4

5

6

7

Extraction times

Figure

5.

Saturated

extraction

experiments

of

the

organic

phase

(n([Bmim]3PW12O40)/n(Li+) = 1.2, 60% TBP/40% DMP (v/v), O/A = 1:1, unadjusted pH).



400

Li

+

300

-1 Li+/mgL

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 40 of 50

200

100

0

0

1

2

3

4

5

Nunber of stages

Figure 6. Multi-stage cross-current extraction experiments of the organic phase (n(ionic liquid)/n(Li+) = 1.2, 60% TBP/40% DMP (v/v), O/A = 1:1, unadjusted pH).


Page 41 of 50

100

Li+ K+

80

Mg2+

E/%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60

40

20

0 0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

NaCl/mol·L-1

Figure 7. Effect of NaCl concentration on washing efficiency of metal ions in the organic phase (O/A = 4).



100

80

E/%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 42 of 50

Li+ K+

60

Mg2+

40

20

0

0

2

4

6

8

10

V(O/A)

Figure 8. Effect of O/A phase ratio on washing efficiency of metal ions in the organic phase (0.926 mol·L-1 NaCl solution).


Page 43 of 50

100

80

Li+ K+

60

E/%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Mg2+

40

20

0

-20 0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

LiCl/mol·L-1

Figure 9. Effect of LiCl concentration on washing efficiency of metal ions in the organic phase (O/A = 8, 0.926 mol·L-1 NaCl solution).



100

+ Li

80

E/%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 44 of 50

60

40

20

0

0

2

4

V(O/A)

6

8

10

Figure 10. Effect of O/A phase ratio on stripping efficiency of Li+ in the organic phase (0.0643 mol·L-1 HCl solution).


Page 45 of 50

100

+ Li

80

E/%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60

40

20

0 0.20

0.25

0.30

0.35

0.40

0.45

0.50

0.55

0.60

0.65

HCl/mol·L-1

Figure 11. Effect of HCl concentration on stripping efficiency of Li+ in the organic phase (O/A = 6).



+ Li 60

40

E/%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 46 of 50

20

0

1

2

3

4

5

6

7

8

Regeneration times

Figure 12. Reusability of the organic phase.


9

10

Page 47 of 50 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 13. 1H NMR of different samples (400MHz, DMSO) (n(ionic liquid)/n(Li+) = 1.2, 60% TBP/40% DMP (v/v), O/A = 1:1, unadjusted pH).



3600

3000

2400

1800

Wavenumber / cm-1

1200

1028.5

1282.45

1733.2

2961.2

2874.8

1464.2 1382.7

(a)

1029.4

1259.31

2961.0

1733.5

2874.5

1463.8

1382.4

(b)

Transmittance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 48 of 50

600

Figure 14. FT-IR spectra of the organic phase before (a) and after (b) extraction.


Page 49 of 50 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Li Na K Mg The organic phase before extrtaction

-0.650

(a) -0.668

(b) -0.671

(c) -0.678

(d) -0.746

(e) 0.0

-0.5

-1.0

-1.5

-2.0

f1 / ppm

Figure 15.

31P

NMR of different samples (400MHz, DMSO) (a) The organic phase

before extraction; The organic phase after extraction of MgCl2 solution (b), KCl solution (c), NaCl solution (d) and LiCl solution (e).



For Table of Contents Use Only

A fluoride-free and relatively hydrolysis-stable ionic liquid [Bmim]3PW12O40 was synthesized and first used as co-extraction reagent for recovery of Li+ from salt lake brine.


Page 50 of 50

Recovery of Lithium Ions from Salt Lake Brine with a High Magnesium

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