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Recovery of lithium from salt-lake brines using solvent extraction with TBP as extractant and FeCl3 as co-extraction agent
Hydrometallurgy 191 (2020) 105244
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Recovery of lithium from salt-lake brines using solvent extraction with TBP as extractant and FeCl3 as co-extraction agent Zhiyong Zhou, Jiahui Fan, Xueting Liu, Yafei Hu, Xiaoyu Wei, Yulei Hu, Wei Wang, Zhongqi Ren
T
⁎
College of Chemical Engineering, Beijing University of Chemical Technology, Beijing 100029, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Liquid-liquid extraction Utilization of magnesium compound Extraction efficiency Separation factor
A novel extraction process to recover lithium ions from simulated salt lake brines with high Mg/Li ratio was developed. Tributyl phosphate (TBP), FeCl3 and diethyl succinate were used as extractant, co-extraction agent and diluent, respectively. Effects of extraction conditions containing the volume concentration of TBP, Fe/Li molar ratio and O/A phase ratio on extraction performances were studied. The highest one-stage extraction efficiency of lithium ions and separation factor were about 65% and 350, respectively. In addition, the third phase did not appear over all the volume concentrations of TBP. HCl + MgCl2 and HCl + NaCl were used as washing and stripping agents, respectively. Effects of washing and stripping conditions on washing and stripping performances were investigated too. Mg(OH)2 and MgCO3 were used as regeneration agents to regenerate the organic phase. The extraction efficiency of lithium ions remained at about 53% during reusing the organic phase ten times, demonstrating that the proposed extraction system had good stability and reusability.
1. Introduction Lithium is a silver-white metal and has a very high electrochemical potential (3.045 V). Lithium is widely used in many industrial fields, such as glass, ceramics, grease and lithium-ion battery (Meshram et al., 2014; Monmaturapoj et al., 2013; Palacios-Bereche et al., 2012; Song et al., 2013; Sullivan and Gaines, 2012; Xu et al., 2016). Among them, lithium-ion battery is the most potential application in now and future. With the increasing depletion of fossil resources, electric vehicles have attracted more and more attention. It is estimated that the market share of hybrid electric vehicle will reach up to 20% by 2020 (Swain, 2016a). Therefore, a large amount of lithium resource is needed for producing lithium-ion batteries in near future. According to statistics (Swain, 2016b; Yan et al., 2012), the content of lithium in the earth's crust is about 0.007% and lithium mainly exists in the ore, seawater and brines. Due to the gradual depletion of lithium ore resource and high energy consumption for recovery of lithium ions from seawater with very low concentration, the extraction of lithium ions from brines has attracted increasing attention (Diallo et al., 2015; Luong et al., 2013; Rezza et al., 2010). Most of the salt lake brines in China have the characteristic of high Mg/Li ratio (Xing et al., 2016). Since the magnesium and lithium ions have similar chemical properties, the extraction of lithium ions from salt lake brines with high Mg/Li ratio is very difficult (Song et al.,
⁎
2017). The commonly used recovery methods include precipitation method, adsorption method, membrane method and liquid-liquid extraction method (Swain, 2016b; Chen et al., 2017; Liu et al., 2015; Nie et al., 2016; Paranthaman et al., 2017; Qing et al., 2018). Since the liquid-liquid extraction technology has the advantages of simple process, easy control of operating conditions, low cost and very high extraction efficiency and selectivity, it is considered as the most potentially industrial method for recovery of lithium ions from salt lake brines in China. The typical extraction systems include extractant, diluent and co-extraction agent. The extractant mainly includes β-diketone (Cui et al., 2019; Zhang et al., 2018; Zhang et al., 2019), crown ether (Torrejos et al., 2016; Xu et al., 2017) and organophosphorus agents. The organophosphorus extractants are widely studied in labs and even applied in industrial production in Qinghai province. Tributyl phosphate (TBP) is the most widely used extractant due to the high extraction selectivity. Diluent is mainly used to reduce the viscosity and density of TBP, and improve the mass transfer effect to promote the extraction performance. The co-extraction agent plays a key role in the extraction system. The addition of co-extraction agent can significantly increase both the extraction efficiency and selectivity of lithium ions. FeCl3 is the most commonly used co-extraction agent in lithium ion extraction process (Shi et al., 2016; Zhou et al., 2011). In recent years, fluorine-containing anions like PF6−, BF4− and NTf2− have been developed as co-extraction agents to instead of FeCl3 and excellent
Corresponding author. E-mail address: [email protected] (Z. Ren).
https://doi.org/10.1016/j.hydromet.2019.105244 Received 23 September 2019; Received in revised form 14 December 2019; Accepted 22 December 2019 Available online 24 December 2019 0304-386X/ © 2019 Published by Elsevier B.V.
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98%), KCl (purity > 99%), Mg(OH)2 (purity > 99%) and MgCO3 (40% MgO) were purchased from Sinopharm Group Co., Ltd. (Beijing, China). HCl (36–38 wt%) was purchased from Beijing Chemical Plants (Beijing, China). TBP (purity > 99%) and Diethyl succinate (purity > 99%) were purchased from Shanghai Macklin Biochemical Co., Ltd. (Shanghai, China). All chemicals were used as received without any further purification.
extraction performances have been obtained (Gao et al., 2015; Shi et al., 2014; Shi et al., 2015; Gao et al., 2016). However, these fluorinecontaining co-extraction agents are easy to decompose during the whole liquid-liquid extraction process to generate HF, which is extremely harmful to both equipment and operator. In our previous studies (Zhou et al., 2011; Zhou et al., 2012a; Zhou et al., 2012b), two extraction systems, TBP-FeCl3-Kerosene and TBPFeCl3-Methyl isobutyl ketone (MIBK) were used for selective extraction of lithium ions from simulated salt lake brines with high Mg/Li ratio. The addition of FeCl3 could significantly increase the extraction efficiency of lithium ions. It is worth noting that the concentration of Cl− in the aqueous solution needs to be strictly controlled to prevent the dropping of Fe3+ from the organic phase to the aqueous phase during the whole extraction process. The extractive reactions are showed as follows (Zhou et al., 2012a; Shi et al., 2018):
FeCl3 + Cl− ↔ FeCl−4
2.2. Analysis The atomic absorption spectrometer (AA-6880, Shimadzu, Japan) was used to determine the concentration of metal ions. The aqueous samples obtained before and after extraction were first diluted by ultrapure water and then measured by using the atomic absorption spectrometer. The concentration of Li+ could be directly obtained. However, since the concentrations of the other metal ions in the raffinate were still very high after extraction, large error may be obtained during the direct measurement by the atomic absorption spectrometer. Thus, the concentrations of the other metal ions in the aqueous solution after extraction were determined by mass conservation. That is, the other ions were stripped from the organic phase obtained after extraction with HCl solution, and the concentration of each ion in the organic phase could be obtained by directly measuring the HCl solution using the atomic absorption spectrometer. Then the concentration of each ion in the raffinate could be obtained by mass conservation.
(1)
Li+ + FeCl−4 + n TBP ↔ LiFeCl 4⋅n TBP
(2)
−
It can be seen that FeCl4 is the real co-extraction anion for the extraction process. Since the formation reaction of FeCl4− is a reversible process, if the concentration of Cl− in aqueous solution is low, Eq. (1) will move to the left, resulting in dropping Fe3+ from the organic phase to the aqueous phase, which greatly affects the extraction performance. The solubility of the organic complex formed by Eq. (2) in the organic phase is much higher than that in brine, which is beneficial to the extraction of Li+ into the organic phase. However, the polarity of the diluent also has great influence on the extraction performance (Zhou et al., 2011; Zhou et al., 2012a). Since the formed organic complex has very high polarity, a non-polar diluent like kerosene could not dissolve this complex. When the volume concentration of TBP in the organic phase is lower than 30%, a third phase will appear, which has a bad influence on the extraction process. Although MIBK as a polar solvent can increase the extraction efficiency and avoid the formation of the third phase, it is too volatile and the low flash point (15.6 °C) is not suitable for industrial production. At the same time, the solubility of MIBK in the aqueous phase is large. The dissolution problem in actual industrial production can't be ignored. Therefore, a new diluent with high flash point, low volatilization, low solubility in water, and low toxicity is needed to be developed. In addition, HCl solution is usually used as typical washing and stripping agent and NaOH solution is commonly used as the regeneration agent. However, HCl solution with high concentration is harmful to both the equipment and the health of human body. Moreover, both the loss of Fe3+ and yield of Li+ are hard to control. For the regeneration process, Fe3+ is easy to form precipitate when the organic phase mixes with NaOH solution. Therefore, the new and suitable washing, stripping and regeneration agents are needed to be developed too. In this work, TBP and FeCl3 were used as extractant and co-extraction agent, respectively. Diethyl succinate with high polarity and flash point and low solubility in water was selected as a new diluent to instead of kerosene and MIBK. MgCl2 as a chlorine source was innovatively used as an addition salt for washing agent to prevent the loss of Fe3+. Mg(OH)2 and MgCO3 were selected as the regeneration agents to regenerate the organic phase. Various extraction conditions, such as the volume concentration of TBP, the molar ratio of Fe3+ to Li+ and the O/A phase ratio on extraction performances were studied. Moreover, the effects of washing and stripping conditions on washing and stripping performances were investigated too. Finally, the regeneration and recycling performances of the organic phase were also studied.
where Ci and Cr are the initial and residual concentrations of metal ions in the aqueous phase, respectively, Vaq and Vorg are the volumes of the aqueous and organic phases, DLi is the distribution coefficient of Li+ and DM represents the distribution coefficient of other metal ions (M = Mg2+, K+ or Na+). The washing or stripping efficiency (E') was calculated according to the following equation.
2. Experimental
E ′ (%) =
2.1. Reagents
where Co represents the ion concentration in the organic phase before washing or stripping, Ca represents the ion concentration in the aqueous phase after washing or stripping.
2.3. Methods All experiments were carried out at ambient temperature and the pH of the brine was not adjusted additionally. The simulated salt lake brine consisted of 0.35 g·L−1 Li+, 96.00 g·L−1 Mg2+, 1.84 g·L−1 Na+ and 0.62 g·L−1 K+. Then a certain amount of FeCl3·6H2O was added to the simulated salt lake brine. The organic phase was obtained by mixing TBP and diethyl succinate at a certain volume ratio. The prepared organic phase was mixed with simulated brine to extract metal ions. The mixture was stirred for 10 min in a water bath shaker and then centrifuged for 1 min to completely separate the two phases. After extraction, the organic phase was transferred to subsequent washing, stripping and regeneration operations. The extraction efficiency (E), distribution coefficient (D) and separation factor (β) were calculated according to the following equations.
E (%) =
LiCl (purity > 99%), NaCl (purity > 99%), MgCl2·6H2O (purity > 2
Ci ‐Cr × 100 Ci
(3)
D=
V aq Ci ‐Cr × Cr V org
(4)
β=
DLi DM
(5)
V aq Ca × × 100 Co V org
(6)
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Fig. 1. Flow chart of the whole process.
3.1.2. Effect of TBP volume concentration on extraction performance During the extraction process, the volume ratio of TBP to diethyl succinate greatly influences the extraction efficiency. Effects of volume concentration of TBP on extraction performances are shown in Figs. 2 and 3. Fig. 2 shows that the extraction efficiency of Li+ first increases and then decreases with increasing the volume concentration of TBP from 10% to 90%. In addition, the extraction efficiency of Li+ is much higher than that of the other three metal ions. This is mainly because as the volume concentration of TBP increases at the beginning, the chance of contact between TBP and Li+ gradually increases, leading to the increase of the extraction efficiency of Li+. However, when the volume concentration of TBP is larger than 50%, the influences of high viscosity and density of TBP on extraction efficiency occupy a dominant position, which weakens the mass transfer effect between the two phases, resulting in slightly decreasing the extraction efficiency. As shown in Fig. 3, both the separation factors of Li+ to Mg2+ (βLi+/Mg2+) and Li+ to K+ (βLi+/K+) increase first and then decrease with the increase of volume concentration of TBP, while the separation factor of Li+ to Na+ (βLi+/Na+) increases slowly with increasing the volume concentration of TBP. Since the concentration of Mg2+ in the brine is extremely high, the separation of Li+ and Mg2+ is very important for the extraction process. Therefore, 30% was selected as the suitable volume concentration of TBP for further study.
2.4. Flow chart The whole process included extraction, washing, stripping and regeneration processes. The flow chart of the whole process is shown in Fig. 1.
3. Results and discussions 3.1. Effect of extraction conditions 3.1.1. Selection of diluent The organic phase for extracting lithium ions from salt lake brine by liquid-liquid extraction is usually composed of extractant, diluent and co-extraction agent. FeCl3 is a commonly used co-extraction agent because of its good co-extraction effect, low cost and easy availability. TBP as an organic phosphorus compound has the advantages of easy to purchase, good stability and high extraction efficiency and selectivity for Li+. Thus, TBP is considered as the preferential extractant for extraction of lithium ions. However, the viscosity and density of TBP are relatively high, which is not conducive to the separation and mass transfer of the two phases during the extraction process. Therefore, in order to obtain high extraction efficiency and eliminate the formation of the third phase, a polar diluent is needed. The good diluent should have the properties of large polarity, high flash point, small dissolution loss and low toxicity. The comparison of physicochemical properties and extraction effects of different commonly used polar diluents is listed in Table 1. The volume ratio of TBP to diluent, n(Fe3+)/n(Li+) and O/A phase ratio were 1.0, 1.2 and 1.0, respectively. By comprehensive comparison, diethyl succinate was selected as a suitable diluent for subsequent investigation.
3.1.3. Effect of molar ratio of Fe3+ to Li+ on extraction performance The solvent extraction of Li+ from salt lake brines is a co-extraction process. Therefore, the addition of FeCl3 used as co-extraction agent has a significant effect on the extraction performance. The effect of addition amount of FeCl3 on extraction efficiency and separation factor was studied and the results are shown in Figs. 4 and 5. It can be seen from Fig. 4 that the extraction efficiency of Li+ is much higher than that of the other three metal ions and increases slowly with the increase of the
Table 1 Comparison of physicochemical properties and extraction effects of different diluents. Diluent
ELi+/%
βLi+/Mg2+
Solubility in water (25 °C)
Flash point (°C)
Toxicity
Diisopropyl malonate P-methylpropiophenone Geranyl acetate 1-Bromododecane Diethyl succinate
69.78 73.06 66.49 67.45 67.65
186.24 253.17 205.85 136.20 236.05
Slightly soluble Insoluble 1 mg/mL Insoluble Insoluble
88 96 104 110 90
Irritating Skin irritation Slightly harmful to water Causing anesthesia Non-toxic
3
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Fig. 2. Effect of TBP volume concentration on extraction efficiencies of metal ions (n(Fe3+)/n(Li+) = 1.2, O/A = 1:1).
Fig. 5. Effect of molar ratio of Fe3+ to Li+ on separation factor (30% TBP/70% diethyl succinate (v/v), O/A = 1:1).
molar ratio of Fe3+ to Li+. Meanwhile, the extraction efficiencies of Mg2+, K+ and Na+ also slightly increase with increasing the molar ratio of Fe3+ to Li+. Fig. 5 shows the variation of βLi+/Mg2+, βLi+/K+ and βLi+/Na+ with increasing the molar ratio of Fe3+ to Li+. These three separation factors follow the sequence βLi+/Mg2+ > βLi+/K+ > βLi+/Na+. Both βLi+/K+ and βLi+/Na+ maintain nearly unchanged with increasing the molar ratio of Fe3+ to Li+. However, βLi+/Mg2+ decreases gradually with the increase of the molar ratio of Fe3+ to Li+. When the molar ratio of Fe3+ to Li+ is < 1.3, βLi+/Mg2+ decreases slightly. Then the separation factor of Li+ to Mg2+ decreases dramatically with increasing the molar ratio of Fe3+ to Li+ from 1.3 to 2.0. This is mainly because excessive addition of FeCl3 would increase the chances of contact between TBP and Mg2+, 2+ resulting in a decrease of βLi+/ . Considering both the extraction efMg ficiency of Li+ and the separation factor of Li+ to Mg2+, 1.3 was selected as the suitable molar ratio of Fe3+ to Li+. 3.1.4. Effect of O/A phase ratio on extraction performance During the extraction process, the volume ratio of the organic phase to the aqueous phase also has a significant influence on the extraction performance. The volume ratio of these two phases is usually expressed as R(O/A). Figs. 6 and 7 show the variations of extraction efficiency and separation factor vs various O/A phase ratios. As shown in Fig. 6, the extraction efficiency of Li+ first increases and then maintains nearly unchanged with the increase of O/A phase ratio. The extraction efficiencies of Mg2+, K+ and Na+ are not significantly influenced by changing O/A phase ratio. It can be seen from Fig. 7 that both βLi+/Mg2+ and βLi+/K+ first increases, then maintains constant and finally decreases with the increase of O/A phase ratio. The change of βLi+/Na+ is not very obvious with increasing O/A phase ratio. These results are mainly due to the fact that when the O/A phase ratio is small, the TBP content for extracting is also small, which greatly influences the extraction performance of Li+. When the O/A phase ratio increases from 1.0 to 1.8, the effect of O/A phase ratio on the extraction performance becomes weak. At this time, the molar ratio of Fe3+ to Li+ plays a leading role in the extraction process. However, the excessive increase of O/A phase ratio is not conducive to the mass transfer between the two phases. Considering both the extraction efficiency and the separation factor, 1.0 was selected as the suitable O/A phase ratio.
Fig. 3. Effect of TBP volume concentration on separation factor (n(Fe3+)/n (Li+) = 1.2, O/A = 1:1).
3.2. Washing experiments
Fig. 4. Effect of molar ratio of Fe3+ to Li+ on extraction efficiencies of metal ions (30% TBP/70% diethyl succinate (v/v), O/A = 1:1).
After extraction process, the organic phase contained a large 4
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Fig. 6. Effect of O/A phase ratio on extraction efficiencies of metal ions (n (Fe3+)/n(Li+) = 1.3, 30% TBP/70% diethyl succinate (v/v)).
Fig. 8. Effect of O/A phase ratio on washing efficiency of different metals ions.
3.2.1. Effect of O/A phase ratio on washing performance In the washing process, increasing the O/A phase ratio is an efficient way to improve washing efficiency and prevent the transfer of Fe3+ from the organic phase to the aqueous phase. When investigating the effect of O/A phase ratio on washing performance, the total Cl− concentration in the washing solution was fixed as 7.2 mol/L, and the molar ratio of H+ to all the impurity metal ions (Mg2+, Na+ and K+) was fixed as 1:1. The washing results are shown in Fig. 8. Both the washing efficiencies of Li+ and Na+ gradually decrease with the increase of O/A phase ratio. However, the washing efficiency of Mg2+ first increases and then maintains nearly unchanged with increasing O/ A phase ratio. The washing efficiency of K+ was basically not affected by O/A phase ratio. Considering washing impurity metal ions (Mg2+, Na+ and K+) while avoiding the loss of Li+ as much as possible during the washing process, 20:1 was selected as suitable O/A phase ratio for the washing process. 3.2.2. Effect of HCl concentration on washing performance In order to further decrease the washing efficiency of Li+, effect of + H concentration on washing performance was investigated. The washing efficiencies of metal ions at different H+ concentrations are shown in Fig. 9 (the total Cl− concentration was fixed as 7.2 mol/L). It
Fig. 7. Effect of O/A phase ratio on separation factor (n(Fe3+)/n(Li+) = 1.3, 30% TBP/70% diethyl succinate (v/v)).
amount of Li+ and other metal ions. If the organic phase is directly stripped, the purity of the product obtained in the stripping solution will be not high enough. Therefore, the organic phase is usually washed with a suitable washing agent before the stripping process, thereby removing the impurity ions doped in the organic phase, particularly Mg2+, while retaining Li+ in the organic phase as much as possible and increasing the purity of the product. According to the extraction mechanism reported in our previous studies (Wang et al., 2019; Zhou et al., 2019), the binding energy of TBP to metal ions followed the sequence H+ > Li+ > Na+ > Mg2+ ≈ K+. Therefore, if the binding energy of TBP to a cation is higher than that to Mg2+, this cation could be selected as the washing agent. In our previous work (Zhou et al., 2012a; Wang et al., 2019), both HCl + LiCl and LiCl+NaCl were used as the washing agents. In this study, the combination of HCl and MgCl2 was innovatively used to wash the extracted organic phase. The addition of HCl was mainly used to wash Na+, K+ and Mg2+ from the organic phase to the aqueous phase. The addition of MgCl2 was to provide enough Cl− in the solution to maintain the stable presence of FeCl4− in the organic phase.
Fig. 9. Effect of HCl concentration on washing efficiency of different metals ions (O/A = 20). 5
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can be seen that all the washing efficiencies of these four metal ions increase with increasing the concentration of H+ in the washing solution from 0.5929 mol/L to 0.7623 mol/L. In addition, both the washing efficiencies of Mg2+ and K+ are maintained above 80%, which is much higher than that of Na+ and Li+. When the H+ concentration is higher than 0.6564 mol/L, both the washing efficiencies of Mg2+ and K+ are larger than 90%. Under this condition, the washing efficiency of Li+ is about 10%. Considering all the washing efficiencies of these four metal ions, 0.6564 mol/L was selected as suitable H+ concentration in the washing solution. In summary, the suitable O/A phase ratio, H+ concentration and total Cl− concentration for the washing process were 20:1, 0.6564 mol/L and 7.2 mol/L, respectively. Under this condition, the washing efficiencies of Li+, Mg2+, Na+ and K+ were 11.44%, 90.78%, 48.15% and 90.27%. 3.3. Stripping experiments In order to recover the extracted Li+, the organic phase needs to be stripped. According to the sequence of binding energy of TBP to metal ions and cation exchange mechanism, only H+ could be used to exchange Li+ in the organic phase. Therefore, HCl was selected as the stripping agent. In order to prevent the loss of Fe3+, at least 6 mol/L HCl solution is often added to provide a sufficient amount of Cl−. However, such high concentration of HCl solution causes not only the waste of acid, but also the corrosion of the equipment and environmental pollution. Therefore, a combination of HCl and NaCl was used as the stripping agent.
Fig. 11. Effect of HCl concentration on stripping efficiency of Li+ (O/A = 7).
stripping solution is higher than 0.07 mol/L, the stripping efficiency of Li+ reaches up to about 99% after two-stage cross-current stripping processes. Therefore, considering both the stripping efficiency of Li+ and the amount of H+ used, 0.07 mol/L was selected as suitable H+ concentration in the stripping solution. In summary, the suitable total Cl− concentration, O/A phase ratio and H+ concentration were 5 mol/ L, 7:1 and 0.07 mol/L for the stripping process and the stripping efficiency of Li+ was 99.14% after two-stage cross-current stripping processes.
3.3.1. Effect of O/A phase ratio on stripping performance The effect of O/A phase ratio on the stripping efficiencies of Li+ and 3+ Fe was investigated. In the experiment, the equivalent concentration of HCl was fixed as 0.1 mol/L. The total Cl− concentration in the stripping solution was fixed as 5 mol/L and the results are shown in Fig. 10. It can be seen that increasing O/A phase ratio can significantly reduce the loss of Fe3+. When the O/A phase ratio is 7, the stripping efficiency of Fe3+ is lower than 0.1%. Under this condition, the stripping efficiency of Li+ is still higher than 90%. Therefore, 7:1 was selected as suitable O/A phase ratio for the stripping process.
3.4. Regeneration process Since the binding energy of TBP to H+ is higher than that to Li+, the organic phase obtained after stripping process is hard to be reused directly in next extraction process. Therefore, it is necessary to regenerate the organic phase to recover the extraction capacity of Li+. NaOH is currently the most widely used regeneration agent. The cation of the complex in the organic phase becomes Na+ after the reaction between NaOH and H+. The binding energy of TBP to Na+ is lower than that to Li+, which is conductive to the extraction of Li+. However, since the alkalinity of NaOH is too strong, Fe3+ is easy to form various precipitates during the regeneration process. Therefore, two kinds of magnesium compounds, Mg(OH)2 and MgCO3, were used to instead of NaOH as regeneration agents in this work. In the regeneration process, 3.6 mol/L MgCl2 solution was added to provide sufficient Cl− and the O/A phase ratio was fixed as 7:1. Since both the regeneration agents are solid substances, the addition of MgCl2 solution is also beneficial to improve the phase separation. The MgCl2 solution mixed with different concentrations of regeneration agents (Mg(OH)2 or MgCO3) was used to regenerate the organic phase and the mixed system was shaken to make the solid regeneration agent fully contact with the organic phase. After regeneration, the mixture of two phases was allowed to stand for a while and then divided into two phases. The effects of the addition amounts of Mg(OH)2 and MgCO3 on regeneration performance were investigated and the results are shown in Fig. 12. As shown in Fig. 12, when Mg(OH)2 was used as the regeneration agent, the extraction efficiency after regeneration process first increases and then decreases with increasing the addition concentration of Mg(OH)2. When the addition concentration of Mg(OH)2 is > 1.466 g/L, the color of the organic phase changed from pale yellow to dark yellow, indicating that ferric precipitation may be formed, which is not conductive to the next extraction process. In order to prevent excessive alkalinity, the suitable addition concentration of Mg (OH)2 was determined as 1.466 g/L, and the extraction efficiency of the regenerated organic phase was 53.21%. As for MgCO3 in Fig. 12, when
3.3.2. Effect of HCl concentration on stripping performance In order to decrease the concentration of HCl solution, a multi-stage cross-current stripping experiment was carried out. The total Cl− concentration was still fixed as 5 mol/L. The effect of HCl concentration on stripping efficiency of Li+ was studied and the results are shown in Fig. 11. It can be seen that the stripping efficiency of Li+ increases with increasing H+ concentration. When the H+ concentration in the
Fig. 10. Effect of O/A phase ratio on stripping efficiencies of Li+ and Fe3+. 6
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was proposed. A series of condition optimizations were carried out for the extraction, washing, stripping and regeneration processes. The optimal extraction conditions were as follows: volume concentration of TBP = 30%, Fe3+/Li+ molar ratio = 1.3 and O/A phase ratio = 1:1. Under these extraction conditions, the extraction efficiency of lithium ion reached up to 65.53% and βLi+/Mg2+, βLi+/Na+ and βLi+/K+ were 347.38, 29.10 and 202.02, respectively. A combination of HCl and MgCl2 was used as the washing agent. The washing efficiencies of Li+, Mg2+, Na+ and K+ were 11.44%, 90.78%, 48.15% and 90.27% under the washing conditions with O/A phase ratio as 20, the total Cl− concentration as 7.2 mol/L, and the H+ concentration as 0.6564 mol/L. Most of Mg2+ and K+ could be washed down and Li+ was retained in the organic phase as much as possible. The combination of HCl and NaCl was used as the stripping agent and the extracted Li+ could be nearly completely stripped after two-stage cross-current stripping processes. Finally, the organic phase after stripping was regenerated by Mg (OH)2 and MgCO3. After ten cycles of reusing the organic phase, no significant decrease in the extraction efficiency of lithium ions was observed, demonstrating that these two magnesium compounds showed good regeneration abilities. This work proposed a new thought to use a series of magnesium compounds as the chloride source and regeneration agents without compromising the extraction performance. This proposed extraction method makes the whole process more reasonable and environmentally friendly.
Fig. 12. Effect of addition concentration of regeneration agents on regeneration efficiency.
Declaration of Competing Interest The authors declare no conflict of 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 Big Science Project from BUCT (XK180301). The authors gratefully acknowledge these grants. References Chen, P., Tang, S.Y., Yue, H.R., Liu, C.J., Li, C., Liang, B., 2017. Lithium enrichment of high mg/Li ratio brine by precipitation of magnesium via combined CO2 mineralization and solvent extraction. Ind. Eng. Chem. Res. 56, 5668–5678. Cui, L., Jiang, K., Wang, J.F., Dong, K., Zhang, X.P., Cheng, F.Q., 2019. Role of ionic liquids in the efficient transfer of lithium by cyanex 923 in solvent extraction system. AICHE J. 65, e16606. Diallo, M.S., Kotte, M.R., Cho, M., 2015. Mining critical metals and elements from seawater: opportunities and challenges. Environ. Sci. Technol. 49, 9390–9399. Gao, D.L., Yu, X.P., Guo, Y.F., Wang, S.Q., Liu, M.M., Deng, T.L., Chen, Y.W., Belzile, N., 2015. Extraction of lithium from salt lake brine with trisobutyl phosphate in ionic liquid and kerosene. Chem. Res. Chin. Univ. 31, 621–626. Gao, D.L., Guo, Y.F., Yu, X.P., Wang, S.Q., Deng, T.L., 2016. Extracting lithium from the high concentration ratio of magnesium and lithium brine using imidazoliumbased ionic liquids with varying alkyl chain lengths. J. Chem. Eng. Jpn 49, 104–110. Liu, X.H., Chen, X.Y., He, L.H., Zhao, Z.W., 2015. Study on extraction of lithium from salt lake brine by membrane electrolysis. Desalination 376, 35–40. Luong, V.T., Kang, D.J., An, J.W., Kim, M.J., Tran, T., 2013. Factors affecting the extraction of lithium from lepidolite. Hydrometall. 134, 54–61. Meshram, P., Pandey, B.D., Mankhand, T.R., 2014. Extraction of lithium from primary and secondary sources by pre-treatment, leaching and separation: a comprehensive review. Hydrometall. 150, 192–208. Monmaturapoj, N., Lawita, P., Thepsuwan, W., 2013. Characterisation and properties of lithium disilicate glass ceramics in the SiO2-Li2O-K2O-Al2O3 system for dental applications. Adv. Mater. Sci. Eng. 2013, 763838. Nie, X.Y., Sun, S.Y., Sun, Z., Song, X., Yu, J.G., 2016. Ion-fractionation of lithium ions from magnesium ions by electrodialysis using monovalent selective ion-exchange membranes. Desalination 403, 128–135. Palacios-Bereche, R., Gonzales, R., Nebra, S.A., 2012. Exergy calculation of lithium bromide–water solution and its application in the exergetic evaluation of absorption refrigeration systems LiBr-H2O. Int. J. Energy Res. 36, 166–181. Paranthaman, P., Li, L., Luo, J.Q., Hoke, T., Ucar, H., Moyer, B.A., Harrison, S., 2017. Recovery of lithium from geothermal brine with lithium–aluminum layered double hydroxide chloride sorbents. Environ. Sci. Technol. 51, 13481–13486. Qing, H., Williams, N.J., Hyun, O.J., Lynch, V.M., Kuk, K.S., Moyer, B.A., Sessler, J.L., 2018. Selective solid-liquid and liquid-liquid extraction of lithium chloride using
Fig. 13. Reusability of the organic phase with Mg(OH)2 and MgCO3 as regeneration agents.
MgCO3 was used as the regeneration agent, the extraction efficiency of the organic phase after regeneration also increases first and then decreases with the increase of the addition concentration of MgCO3. When the addition concentration of MgCO3 was 2.5 g/L, the extraction efficiency of the regenerated organic phase reached up to 54.85%. 3.5. Study on the reusability of the organic phase The reusability of the organic phase is a key factor in determining whether the extraction system is stable. Therefore, when Mg(OH)2 and MgCO3 were used as regeneration agents separately, the organic phase was reused ten times for the whole separation process including extraction, washing, stripping and regeneration, and the results are shown in Fig. 13. It can be seen that both extraction efficiencies of Li+ maintain at about 53% during ten cycles, indicating that both Mg(OH)2 and MgCO3 have excellent regeneration abilities for the extraction system with FeCl3 as co-extraction agent. 4. Conclusions In this work, a new extraction system consisting of TBP as extractant, FeCl3 as co-extraction agent and diethyl succinate as diluent 7
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Recovery of lithium ions from salt lake brine with a high magnesium/lithium ratio using heteropolyacid ionic liquid. ACS Sustain. Chem. Eng. 7, 3062–3072. Xing, L.X., Song, J.F., Li, Z.S., Liu, J.D., Huang, T., Dou, P.J., Chen, Y., Li, X.M., He, T., 2016. Solvent stable nanoporous poly (ethylene-co-vinyl alcohol) barrier membranes for liquid-liquid extraction of lithium from a salt lake brine. J. Membr. Sci. 520, 596–606. Xu, X., Chen, Y.M., Wan, P.Y., Gasem, K., Wang, K.Y., He, T., Adidharma, H., Fan, M.H., 2016. Extraction of lithium with functionalized lithium ion-sieves. Prog. Mater. Sci. 84, 276–313. Xu, X.C., Li, Y., Yang, D.Y., Zheng, X.D., Wang, Y.Y., Pan, J.M., Zhang, T., Xu, J.C., Qiu, F.X., Yan, Y.S., Li, C.X., 2017. A facile strategy toward ion-imprinted hierarchical mesoporous material via dual-template method for simultaneous selective extraction of lithium and rubidium. J. Clean. Prod. 171, 264–274. Yan, Q.X., Li, X.H., Yin, Z.L., Wang, Z.X., Guo, H.J., Peng, W.J., Hu, Q.Y., 2012. A novel process for extracting lithium from lepidolite. Hydrometall. 121-124, 54–59. Zhang, L.C., Li, L.J., Shi, D., Peng, X.W., Song, F.G., Nie, F., Han, W.S., 2018. Recovery of lithium from alkaline brine by solvent extraction with β-diketone. Hydrometall. 175, 35–42. Zhang, L.C., Shi, D., Li, L.J., Peng, X.W., Song, F.G., Rui, H.M., 2019. Solvent extraction of lithium from ammoniacal solution using thenoyltrifluoroacetone and neutral ligands. J. Mol. Liq. 274, 746–751. Zhou, Z.Y., Qin, W., Fei, W.Y., 2011. Extraction equilibria of lithium with tributyl phosphate in three diluents. J. Chem. Eng. Data 56, 3518–3522. Zhou, Z.Y., Qin, W., Liang, S.K., Tan, Y.Z., Fei, W.Y., 2012a. Recovery of lithium using tributyl phosphate in methyl isobutyl ketone and FeCl3. Ind. Eng. Chem. Res. 51, 12926–12932. Zhou, Z.Y., Qin, W., Liu, Y., Fei, W.Y., 2012b. Extraction equilibria of lithium with tributyl phosphate in kerosene and FeCl3. J. Chem. Eng. Data 57, 82–86. Zhou, Z.Y., Liu, H.T., Fan, J.H., Liu, X.T., Hu, Y.F., Hu, Y.L., Wang, Y., Ren, Z.Q., 2019. Selective extraction of lithium ion from aqueous solution with sodium phosphomolybdate as a coextraction agent. ACS Sustain. Chem. Eng. 7, 8885–8892.
strapped calix[4]pyrroles. Angew. Chem.-Int. Edit. 57, 11924–11928. Rezza, I., Salinas, E., Calvente, V., Benuzzi, D., Tosetti, M.I.S.D., 2010. Extraction of lithium from spodumene by bioleaching. Lett. Appl. Microbiol. 25, 172–176. Shi, C.L., Duan, D.P., Jia, Y.Z., Jing, Y., 2014. A highly efficient solvent system containing ionic liquid in tributyl phosphate for lithium ion extraction. J. Mol. Liq. 200, 191–195. Shi, C.L., Jia, Y., Zhang, C., Liu, H., Jing, Y., 2015. Extraction of lithium from salt lake brine using room temperature ionic liquid in tributyl phosphate. Fusion Eng. Des. 90, 1–6. Shi, C.L., Jing, Y., Jia, Y.Z., 2016. Solvent extraction of lithium ions by tri-n-butyl phosphate using a room temperature ionic liquid. J. Mol. Liq. 215, 640–646. Shi, D., Zhang, L.C., Peng, X.W., Li, L.J., Song, F.G., Nie, F., Ji, L.M., Zhang, Y.Z., 2018. Extraction of lithium from salt lake brine containing boron using multistage centrifuge extractors. Desalination 441, 44–51. Song, Z.H., Liang, Y.M., Fan, M.J., Zhou, F., Liu, W.M., 2013. Lithium-based ionic liquids as novel lubricant additives for multiply alkylated cyclopentanes (MACs). Friction 1, 222–231. Song, J.F., Long, D.N., Li, X.M., He, T., 2017. Lithium extraction from chinese salt-lake brines: opportunities, challenges, and future outlook. Environ. Sci.-Wat. Res. 3, 593–597. Sullivan, J.L., Gaines, L., 2012. Status of life cycle inventories for batteries. Energ. Convers. Manage. 58, 134–148. Swain, B., 2016a. Recovery and recycling of lithium: a review. Sep. Purif. Technol. 172, 388–403. Swain, B., 2016b. Separation and purification of lithium by solvent extraction and supported liquid membrane, analysis of their mechanism: a review. J. Chem. Technol. Biotechnol. 91, 2549–2562. Torrejos, R.E.C., Nisola, G.M., Song, H.S., Han, J.W., Chung, W.J., 2016. Liquid-liquid extraction of lithium using lipophilic dibenzo-14-crown-4 ether carboxylic acid in hydrophobic room temperature ionic liquid. Hydrometall. 164, 362–371. Wang, Y., Liu, H.T., Fan, J.H., Liu, X.T., Hu, Y.F., Hu, Y.L., Zhou, Z.Y., Ren, Z.Q., 2019.
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Recovery of lithium from salt-lake brines using solvent extraction with TBP as extractant and FeCl3 as co-extraction agent Zhiyong Zhou, Jiahui Fan, Xueting Liu, Yafei Hu, Xiaoyu Wei, Yulei Hu, Wei Wang, Zhongqi Ren
T
⁎
College of Chemical Engineering, Beijing University of Chemical Technology, Beijing 100029, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Liquid-liquid extraction Utilization of magnesium compound Extraction efficiency Separation factor
A novel extraction process to recover lithium ions from simulated salt lake brines with high Mg/Li ratio was developed. Tributyl phosphate (TBP), FeCl3 and diethyl succinate were used as extractant, co-extraction agent and diluent, respectively. Effects of extraction conditions containing the volume concentration of TBP, Fe/Li molar ratio and O/A phase ratio on extraction performances were studied. The highest one-stage extraction efficiency of lithium ions and separation factor were about 65% and 350, respectively. In addition, the third phase did not appear over all the volume concentrations of TBP. HCl + MgCl2 and HCl + NaCl were used as washing and stripping agents, respectively. Effects of washing and stripping conditions on washing and stripping performances were investigated too. Mg(OH)2 and MgCO3 were used as regeneration agents to regenerate the organic phase. The extraction efficiency of lithium ions remained at about 53% during reusing the organic phase ten times, demonstrating that the proposed extraction system had good stability and reusability.
1. Introduction Lithium is a silver-white metal and has a very high electrochemical potential (3.045 V). Lithium is widely used in many industrial fields, such as glass, ceramics, grease and lithium-ion battery (Meshram et al., 2014; Monmaturapoj et al., 2013; Palacios-Bereche et al., 2012; Song et al., 2013; Sullivan and Gaines, 2012; Xu et al., 2016). Among them, lithium-ion battery is the most potential application in now and future. With the increasing depletion of fossil resources, electric vehicles have attracted more and more attention. It is estimated that the market share of hybrid electric vehicle will reach up to 20% by 2020 (Swain, 2016a). Therefore, a large amount of lithium resource is needed for producing lithium-ion batteries in near future. According to statistics (Swain, 2016b; Yan et al., 2012), the content of lithium in the earth's crust is about 0.007% and lithium mainly exists in the ore, seawater and brines. Due to the gradual depletion of lithium ore resource and high energy consumption for recovery of lithium ions from seawater with very low concentration, the extraction of lithium ions from brines has attracted increasing attention (Diallo et al., 2015; Luong et al., 2013; Rezza et al., 2010). Most of the salt lake brines in China have the characteristic of high Mg/Li ratio (Xing et al., 2016). Since the magnesium and lithium ions have similar chemical properties, the extraction of lithium ions from salt lake brines with high Mg/Li ratio is very difficult (Song et al.,
⁎
2017). The commonly used recovery methods include precipitation method, adsorption method, membrane method and liquid-liquid extraction method (Swain, 2016b; Chen et al., 2017; Liu et al., 2015; Nie et al., 2016; Paranthaman et al., 2017; Qing et al., 2018). Since the liquid-liquid extraction technology has the advantages of simple process, easy control of operating conditions, low cost and very high extraction efficiency and selectivity, it is considered as the most potentially industrial method for recovery of lithium ions from salt lake brines in China. The typical extraction systems include extractant, diluent and co-extraction agent. The extractant mainly includes β-diketone (Cui et al., 2019; Zhang et al., 2018; Zhang et al., 2019), crown ether (Torrejos et al., 2016; Xu et al., 2017) and organophosphorus agents. The organophosphorus extractants are widely studied in labs and even applied in industrial production in Qinghai province. Tributyl phosphate (TBP) is the most widely used extractant due to the high extraction selectivity. Diluent is mainly used to reduce the viscosity and density of TBP, and improve the mass transfer effect to promote the extraction performance. The co-extraction agent plays a key role in the extraction system. The addition of co-extraction agent can significantly increase both the extraction efficiency and selectivity of lithium ions. FeCl3 is the most commonly used co-extraction agent in lithium ion extraction process (Shi et al., 2016; Zhou et al., 2011). In recent years, fluorine-containing anions like PF6−, BF4− and NTf2− have been developed as co-extraction agents to instead of FeCl3 and excellent
Corresponding author. E-mail address: [email protected] (Z. Ren).
https://doi.org/10.1016/j.hydromet.2019.105244 Received 23 September 2019; Received in revised form 14 December 2019; Accepted 22 December 2019 Available online 24 December 2019 0304-386X/ © 2019 Published by Elsevier B.V.
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98%), KCl (purity > 99%), Mg(OH)2 (purity > 99%) and MgCO3 (40% MgO) were purchased from Sinopharm Group Co., Ltd. (Beijing, China). HCl (36–38 wt%) was purchased from Beijing Chemical Plants (Beijing, China). TBP (purity > 99%) and Diethyl succinate (purity > 99%) were purchased from Shanghai Macklin Biochemical Co., Ltd. (Shanghai, China). All chemicals were used as received without any further purification.
extraction performances have been obtained (Gao et al., 2015; Shi et al., 2014; Shi et al., 2015; Gao et al., 2016). However, these fluorinecontaining co-extraction agents are easy to decompose during the whole liquid-liquid extraction process to generate HF, which is extremely harmful to both equipment and operator. In our previous studies (Zhou et al., 2011; Zhou et al., 2012a; Zhou et al., 2012b), two extraction systems, TBP-FeCl3-Kerosene and TBPFeCl3-Methyl isobutyl ketone (MIBK) were used for selective extraction of lithium ions from simulated salt lake brines with high Mg/Li ratio. The addition of FeCl3 could significantly increase the extraction efficiency of lithium ions. It is worth noting that the concentration of Cl− in the aqueous solution needs to be strictly controlled to prevent the dropping of Fe3+ from the organic phase to the aqueous phase during the whole extraction process. The extractive reactions are showed as follows (Zhou et al., 2012a; Shi et al., 2018):
FeCl3 + Cl− ↔ FeCl−4
2.2. Analysis The atomic absorption spectrometer (AA-6880, Shimadzu, Japan) was used to determine the concentration of metal ions. The aqueous samples obtained before and after extraction were first diluted by ultrapure water and then measured by using the atomic absorption spectrometer. The concentration of Li+ could be directly obtained. However, since the concentrations of the other metal ions in the raffinate were still very high after extraction, large error may be obtained during the direct measurement by the atomic absorption spectrometer. Thus, the concentrations of the other metal ions in the aqueous solution after extraction were determined by mass conservation. That is, the other ions were stripped from the organic phase obtained after extraction with HCl solution, and the concentration of each ion in the organic phase could be obtained by directly measuring the HCl solution using the atomic absorption spectrometer. Then the concentration of each ion in the raffinate could be obtained by mass conservation.
(1)
Li+ + FeCl−4 + n TBP ↔ LiFeCl 4⋅n TBP
(2)
−
It can be seen that FeCl4 is the real co-extraction anion for the extraction process. Since the formation reaction of FeCl4− is a reversible process, if the concentration of Cl− in aqueous solution is low, Eq. (1) will move to the left, resulting in dropping Fe3+ from the organic phase to the aqueous phase, which greatly affects the extraction performance. The solubility of the organic complex formed by Eq. (2) in the organic phase is much higher than that in brine, which is beneficial to the extraction of Li+ into the organic phase. However, the polarity of the diluent also has great influence on the extraction performance (Zhou et al., 2011; Zhou et al., 2012a). Since the formed organic complex has very high polarity, a non-polar diluent like kerosene could not dissolve this complex. When the volume concentration of TBP in the organic phase is lower than 30%, a third phase will appear, which has a bad influence on the extraction process. Although MIBK as a polar solvent can increase the extraction efficiency and avoid the formation of the third phase, it is too volatile and the low flash point (15.6 °C) is not suitable for industrial production. At the same time, the solubility of MIBK in the aqueous phase is large. The dissolution problem in actual industrial production can't be ignored. Therefore, a new diluent with high flash point, low volatilization, low solubility in water, and low toxicity is needed to be developed. In addition, HCl solution is usually used as typical washing and stripping agent and NaOH solution is commonly used as the regeneration agent. However, HCl solution with high concentration is harmful to both the equipment and the health of human body. Moreover, both the loss of Fe3+ and yield of Li+ are hard to control. For the regeneration process, Fe3+ is easy to form precipitate when the organic phase mixes with NaOH solution. Therefore, the new and suitable washing, stripping and regeneration agents are needed to be developed too. In this work, TBP and FeCl3 were used as extractant and co-extraction agent, respectively. Diethyl succinate with high polarity and flash point and low solubility in water was selected as a new diluent to instead of kerosene and MIBK. MgCl2 as a chlorine source was innovatively used as an addition salt for washing agent to prevent the loss of Fe3+. Mg(OH)2 and MgCO3 were selected as the regeneration agents to regenerate the organic phase. Various extraction conditions, such as the volume concentration of TBP, the molar ratio of Fe3+ to Li+ and the O/A phase ratio on extraction performances were studied. Moreover, the effects of washing and stripping conditions on washing and stripping performances were investigated too. Finally, the regeneration and recycling performances of the organic phase were also studied.
where Ci and Cr are the initial and residual concentrations of metal ions in the aqueous phase, respectively, Vaq and Vorg are the volumes of the aqueous and organic phases, DLi is the distribution coefficient of Li+ and DM represents the distribution coefficient of other metal ions (M = Mg2+, K+ or Na+). The washing or stripping efficiency (E') was calculated according to the following equation.
2. Experimental
E ′ (%) =
2.1. Reagents
where Co represents the ion concentration in the organic phase before washing or stripping, Ca represents the ion concentration in the aqueous phase after washing or stripping.
2.3. Methods All experiments were carried out at ambient temperature and the pH of the brine was not adjusted additionally. The simulated salt lake brine consisted of 0.35 g·L−1 Li+, 96.00 g·L−1 Mg2+, 1.84 g·L−1 Na+ and 0.62 g·L−1 K+. Then a certain amount of FeCl3·6H2O was added to the simulated salt lake brine. The organic phase was obtained by mixing TBP and diethyl succinate at a certain volume ratio. The prepared organic phase was mixed with simulated brine to extract metal ions. The mixture was stirred for 10 min in a water bath shaker and then centrifuged for 1 min to completely separate the two phases. After extraction, the organic phase was transferred to subsequent washing, stripping and regeneration operations. The extraction efficiency (E), distribution coefficient (D) and separation factor (β) were calculated according to the following equations.
E (%) =
LiCl (purity > 99%), NaCl (purity > 99%), MgCl2·6H2O (purity > 2
Ci ‐Cr × 100 Ci
(3)
D=
V aq Ci ‐Cr × Cr V org
(4)
β=
DLi DM
(5)
V aq Ca × × 100 Co V org
(6)
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Fig. 1. Flow chart of the whole process.
3.1.2. Effect of TBP volume concentration on extraction performance During the extraction process, the volume ratio of TBP to diethyl succinate greatly influences the extraction efficiency. Effects of volume concentration of TBP on extraction performances are shown in Figs. 2 and 3. Fig. 2 shows that the extraction efficiency of Li+ first increases and then decreases with increasing the volume concentration of TBP from 10% to 90%. In addition, the extraction efficiency of Li+ is much higher than that of the other three metal ions. This is mainly because as the volume concentration of TBP increases at the beginning, the chance of contact between TBP and Li+ gradually increases, leading to the increase of the extraction efficiency of Li+. However, when the volume concentration of TBP is larger than 50%, the influences of high viscosity and density of TBP on extraction efficiency occupy a dominant position, which weakens the mass transfer effect between the two phases, resulting in slightly decreasing the extraction efficiency. As shown in Fig. 3, both the separation factors of Li+ to Mg2+ (βLi+/Mg2+) and Li+ to K+ (βLi+/K+) increase first and then decrease with the increase of volume concentration of TBP, while the separation factor of Li+ to Na+ (βLi+/Na+) increases slowly with increasing the volume concentration of TBP. Since the concentration of Mg2+ in the brine is extremely high, the separation of Li+ and Mg2+ is very important for the extraction process. Therefore, 30% was selected as the suitable volume concentration of TBP for further study.
2.4. Flow chart The whole process included extraction, washing, stripping and regeneration processes. The flow chart of the whole process is shown in Fig. 1.
3. Results and discussions 3.1. Effect of extraction conditions 3.1.1. Selection of diluent The organic phase for extracting lithium ions from salt lake brine by liquid-liquid extraction is usually composed of extractant, diluent and co-extraction agent. FeCl3 is a commonly used co-extraction agent because of its good co-extraction effect, low cost and easy availability. TBP as an organic phosphorus compound has the advantages of easy to purchase, good stability and high extraction efficiency and selectivity for Li+. Thus, TBP is considered as the preferential extractant for extraction of lithium ions. However, the viscosity and density of TBP are relatively high, which is not conducive to the separation and mass transfer of the two phases during the extraction process. Therefore, in order to obtain high extraction efficiency and eliminate the formation of the third phase, a polar diluent is needed. The good diluent should have the properties of large polarity, high flash point, small dissolution loss and low toxicity. The comparison of physicochemical properties and extraction effects of different commonly used polar diluents is listed in Table 1. The volume ratio of TBP to diluent, n(Fe3+)/n(Li+) and O/A phase ratio were 1.0, 1.2 and 1.0, respectively. By comprehensive comparison, diethyl succinate was selected as a suitable diluent for subsequent investigation.
3.1.3. Effect of molar ratio of Fe3+ to Li+ on extraction performance The solvent extraction of Li+ from salt lake brines is a co-extraction process. Therefore, the addition of FeCl3 used as co-extraction agent has a significant effect on the extraction performance. The effect of addition amount of FeCl3 on extraction efficiency and separation factor was studied and the results are shown in Figs. 4 and 5. It can be seen from Fig. 4 that the extraction efficiency of Li+ is much higher than that of the other three metal ions and increases slowly with the increase of the
Table 1 Comparison of physicochemical properties and extraction effects of different diluents. Diluent
ELi+/%
βLi+/Mg2+
Solubility in water (25 °C)
Flash point (°C)
Toxicity
Diisopropyl malonate P-methylpropiophenone Geranyl acetate 1-Bromododecane Diethyl succinate
69.78 73.06 66.49 67.45 67.65
186.24 253.17 205.85 136.20 236.05
Slightly soluble Insoluble 1 mg/mL Insoluble Insoluble
88 96 104 110 90
Irritating Skin irritation Slightly harmful to water Causing anesthesia Non-toxic
3
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Fig. 2. Effect of TBP volume concentration on extraction efficiencies of metal ions (n(Fe3+)/n(Li+) = 1.2, O/A = 1:1).
Fig. 5. Effect of molar ratio of Fe3+ to Li+ on separation factor (30% TBP/70% diethyl succinate (v/v), O/A = 1:1).
molar ratio of Fe3+ to Li+. Meanwhile, the extraction efficiencies of Mg2+, K+ and Na+ also slightly increase with increasing the molar ratio of Fe3+ to Li+. Fig. 5 shows the variation of βLi+/Mg2+, βLi+/K+ and βLi+/Na+ with increasing the molar ratio of Fe3+ to Li+. These three separation factors follow the sequence βLi+/Mg2+ > βLi+/K+ > βLi+/Na+. Both βLi+/K+ and βLi+/Na+ maintain nearly unchanged with increasing the molar ratio of Fe3+ to Li+. However, βLi+/Mg2+ decreases gradually with the increase of the molar ratio of Fe3+ to Li+. When the molar ratio of Fe3+ to Li+ is < 1.3, βLi+/Mg2+ decreases slightly. Then the separation factor of Li+ to Mg2+ decreases dramatically with increasing the molar ratio of Fe3+ to Li+ from 1.3 to 2.0. This is mainly because excessive addition of FeCl3 would increase the chances of contact between TBP and Mg2+, 2+ resulting in a decrease of βLi+/ . Considering both the extraction efMg ficiency of Li+ and the separation factor of Li+ to Mg2+, 1.3 was selected as the suitable molar ratio of Fe3+ to Li+. 3.1.4. Effect of O/A phase ratio on extraction performance During the extraction process, the volume ratio of the organic phase to the aqueous phase also has a significant influence on the extraction performance. The volume ratio of these two phases is usually expressed as R(O/A). Figs. 6 and 7 show the variations of extraction efficiency and separation factor vs various O/A phase ratios. As shown in Fig. 6, the extraction efficiency of Li+ first increases and then maintains nearly unchanged with the increase of O/A phase ratio. The extraction efficiencies of Mg2+, K+ and Na+ are not significantly influenced by changing O/A phase ratio. It can be seen from Fig. 7 that both βLi+/Mg2+ and βLi+/K+ first increases, then maintains constant and finally decreases with the increase of O/A phase ratio. The change of βLi+/Na+ is not very obvious with increasing O/A phase ratio. These results are mainly due to the fact that when the O/A phase ratio is small, the TBP content for extracting is also small, which greatly influences the extraction performance of Li+. When the O/A phase ratio increases from 1.0 to 1.8, the effect of O/A phase ratio on the extraction performance becomes weak. At this time, the molar ratio of Fe3+ to Li+ plays a leading role in the extraction process. However, the excessive increase of O/A phase ratio is not conducive to the mass transfer between the two phases. Considering both the extraction efficiency and the separation factor, 1.0 was selected as the suitable O/A phase ratio.
Fig. 3. Effect of TBP volume concentration on separation factor (n(Fe3+)/n (Li+) = 1.2, O/A = 1:1).
3.2. Washing experiments
Fig. 4. Effect of molar ratio of Fe3+ to Li+ on extraction efficiencies of metal ions (30% TBP/70% diethyl succinate (v/v), O/A = 1:1).
After extraction process, the organic phase contained a large 4
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Fig. 6. Effect of O/A phase ratio on extraction efficiencies of metal ions (n (Fe3+)/n(Li+) = 1.3, 30% TBP/70% diethyl succinate (v/v)).
Fig. 8. Effect of O/A phase ratio on washing efficiency of different metals ions.
3.2.1. Effect of O/A phase ratio on washing performance In the washing process, increasing the O/A phase ratio is an efficient way to improve washing efficiency and prevent the transfer of Fe3+ from the organic phase to the aqueous phase. When investigating the effect of O/A phase ratio on washing performance, the total Cl− concentration in the washing solution was fixed as 7.2 mol/L, and the molar ratio of H+ to all the impurity metal ions (Mg2+, Na+ and K+) was fixed as 1:1. The washing results are shown in Fig. 8. Both the washing efficiencies of Li+ and Na+ gradually decrease with the increase of O/A phase ratio. However, the washing efficiency of Mg2+ first increases and then maintains nearly unchanged with increasing O/ A phase ratio. The washing efficiency of K+ was basically not affected by O/A phase ratio. Considering washing impurity metal ions (Mg2+, Na+ and K+) while avoiding the loss of Li+ as much as possible during the washing process, 20:1 was selected as suitable O/A phase ratio for the washing process. 3.2.2. Effect of HCl concentration on washing performance In order to further decrease the washing efficiency of Li+, effect of + H concentration on washing performance was investigated. The washing efficiencies of metal ions at different H+ concentrations are shown in Fig. 9 (the total Cl− concentration was fixed as 7.2 mol/L). It
Fig. 7. Effect of O/A phase ratio on separation factor (n(Fe3+)/n(Li+) = 1.3, 30% TBP/70% diethyl succinate (v/v)).
amount of Li+ and other metal ions. If the organic phase is directly stripped, the purity of the product obtained in the stripping solution will be not high enough. Therefore, the organic phase is usually washed with a suitable washing agent before the stripping process, thereby removing the impurity ions doped in the organic phase, particularly Mg2+, while retaining Li+ in the organic phase as much as possible and increasing the purity of the product. According to the extraction mechanism reported in our previous studies (Wang et al., 2019; Zhou et al., 2019), the binding energy of TBP to metal ions followed the sequence H+ > Li+ > Na+ > Mg2+ ≈ K+. Therefore, if the binding energy of TBP to a cation is higher than that to Mg2+, this cation could be selected as the washing agent. In our previous work (Zhou et al., 2012a; Wang et al., 2019), both HCl + LiCl and LiCl+NaCl were used as the washing agents. In this study, the combination of HCl and MgCl2 was innovatively used to wash the extracted organic phase. The addition of HCl was mainly used to wash Na+, K+ and Mg2+ from the organic phase to the aqueous phase. The addition of MgCl2 was to provide enough Cl− in the solution to maintain the stable presence of FeCl4− in the organic phase.
Fig. 9. Effect of HCl concentration on washing efficiency of different metals ions (O/A = 20). 5
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can be seen that all the washing efficiencies of these four metal ions increase with increasing the concentration of H+ in the washing solution from 0.5929 mol/L to 0.7623 mol/L. In addition, both the washing efficiencies of Mg2+ and K+ are maintained above 80%, which is much higher than that of Na+ and Li+. When the H+ concentration is higher than 0.6564 mol/L, both the washing efficiencies of Mg2+ and K+ are larger than 90%. Under this condition, the washing efficiency of Li+ is about 10%. Considering all the washing efficiencies of these four metal ions, 0.6564 mol/L was selected as suitable H+ concentration in the washing solution. In summary, the suitable O/A phase ratio, H+ concentration and total Cl− concentration for the washing process were 20:1, 0.6564 mol/L and 7.2 mol/L, respectively. Under this condition, the washing efficiencies of Li+, Mg2+, Na+ and K+ were 11.44%, 90.78%, 48.15% and 90.27%. 3.3. Stripping experiments In order to recover the extracted Li+, the organic phase needs to be stripped. According to the sequence of binding energy of TBP to metal ions and cation exchange mechanism, only H+ could be used to exchange Li+ in the organic phase. Therefore, HCl was selected as the stripping agent. In order to prevent the loss of Fe3+, at least 6 mol/L HCl solution is often added to provide a sufficient amount of Cl−. However, such high concentration of HCl solution causes not only the waste of acid, but also the corrosion of the equipment and environmental pollution. Therefore, a combination of HCl and NaCl was used as the stripping agent.
Fig. 11. Effect of HCl concentration on stripping efficiency of Li+ (O/A = 7).
stripping solution is higher than 0.07 mol/L, the stripping efficiency of Li+ reaches up to about 99% after two-stage cross-current stripping processes. Therefore, considering both the stripping efficiency of Li+ and the amount of H+ used, 0.07 mol/L was selected as suitable H+ concentration in the stripping solution. In summary, the suitable total Cl− concentration, O/A phase ratio and H+ concentration were 5 mol/ L, 7:1 and 0.07 mol/L for the stripping process and the stripping efficiency of Li+ was 99.14% after two-stage cross-current stripping processes.
3.3.1. Effect of O/A phase ratio on stripping performance The effect of O/A phase ratio on the stripping efficiencies of Li+ and 3+ Fe was investigated. In the experiment, the equivalent concentration of HCl was fixed as 0.1 mol/L. The total Cl− concentration in the stripping solution was fixed as 5 mol/L and the results are shown in Fig. 10. It can be seen that increasing O/A phase ratio can significantly reduce the loss of Fe3+. When the O/A phase ratio is 7, the stripping efficiency of Fe3+ is lower than 0.1%. Under this condition, the stripping efficiency of Li+ is still higher than 90%. Therefore, 7:1 was selected as suitable O/A phase ratio for the stripping process.
3.4. Regeneration process Since the binding energy of TBP to H+ is higher than that to Li+, the organic phase obtained after stripping process is hard to be reused directly in next extraction process. Therefore, it is necessary to regenerate the organic phase to recover the extraction capacity of Li+. NaOH is currently the most widely used regeneration agent. The cation of the complex in the organic phase becomes Na+ after the reaction between NaOH and H+. The binding energy of TBP to Na+ is lower than that to Li+, which is conductive to the extraction of Li+. However, since the alkalinity of NaOH is too strong, Fe3+ is easy to form various precipitates during the regeneration process. Therefore, two kinds of magnesium compounds, Mg(OH)2 and MgCO3, were used to instead of NaOH as regeneration agents in this work. In the regeneration process, 3.6 mol/L MgCl2 solution was added to provide sufficient Cl− and the O/A phase ratio was fixed as 7:1. Since both the regeneration agents are solid substances, the addition of MgCl2 solution is also beneficial to improve the phase separation. The MgCl2 solution mixed with different concentrations of regeneration agents (Mg(OH)2 or MgCO3) was used to regenerate the organic phase and the mixed system was shaken to make the solid regeneration agent fully contact with the organic phase. After regeneration, the mixture of two phases was allowed to stand for a while and then divided into two phases. The effects of the addition amounts of Mg(OH)2 and MgCO3 on regeneration performance were investigated and the results are shown in Fig. 12. As shown in Fig. 12, when Mg(OH)2 was used as the regeneration agent, the extraction efficiency after regeneration process first increases and then decreases with increasing the addition concentration of Mg(OH)2. When the addition concentration of Mg(OH)2 is > 1.466 g/L, the color of the organic phase changed from pale yellow to dark yellow, indicating that ferric precipitation may be formed, which is not conductive to the next extraction process. In order to prevent excessive alkalinity, the suitable addition concentration of Mg (OH)2 was determined as 1.466 g/L, and the extraction efficiency of the regenerated organic phase was 53.21%. As for MgCO3 in Fig. 12, when
3.3.2. Effect of HCl concentration on stripping performance In order to decrease the concentration of HCl solution, a multi-stage cross-current stripping experiment was carried out. The total Cl− concentration was still fixed as 5 mol/L. The effect of HCl concentration on stripping efficiency of Li+ was studied and the results are shown in Fig. 11. It can be seen that the stripping efficiency of Li+ increases with increasing H+ concentration. When the H+ concentration in the
Fig. 10. Effect of O/A phase ratio on stripping efficiencies of Li+ and Fe3+. 6
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was proposed. A series of condition optimizations were carried out for the extraction, washing, stripping and regeneration processes. The optimal extraction conditions were as follows: volume concentration of TBP = 30%, Fe3+/Li+ molar ratio = 1.3 and O/A phase ratio = 1:1. Under these extraction conditions, the extraction efficiency of lithium ion reached up to 65.53% and βLi+/Mg2+, βLi+/Na+ and βLi+/K+ were 347.38, 29.10 and 202.02, respectively. A combination of HCl and MgCl2 was used as the washing agent. The washing efficiencies of Li+, Mg2+, Na+ and K+ were 11.44%, 90.78%, 48.15% and 90.27% under the washing conditions with O/A phase ratio as 20, the total Cl− concentration as 7.2 mol/L, and the H+ concentration as 0.6564 mol/L. Most of Mg2+ and K+ could be washed down and Li+ was retained in the organic phase as much as possible. The combination of HCl and NaCl was used as the stripping agent and the extracted Li+ could be nearly completely stripped after two-stage cross-current stripping processes. Finally, the organic phase after stripping was regenerated by Mg (OH)2 and MgCO3. After ten cycles of reusing the organic phase, no significant decrease in the extraction efficiency of lithium ions was observed, demonstrating that these two magnesium compounds showed good regeneration abilities. This work proposed a new thought to use a series of magnesium compounds as the chloride source and regeneration agents without compromising the extraction performance. This proposed extraction method makes the whole process more reasonable and environmentally friendly.
Fig. 12. Effect of addition concentration of regeneration agents on regeneration efficiency.
Declaration of Competing Interest The authors declare no conflict of 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 Big Science Project from BUCT (XK180301). The authors gratefully acknowledge these grants. References Chen, P., Tang, S.Y., Yue, H.R., Liu, C.J., Li, C., Liang, B., 2017. Lithium enrichment of high mg/Li ratio brine by precipitation of magnesium via combined CO2 mineralization and solvent extraction. Ind. Eng. Chem. Res. 56, 5668–5678. Cui, L., Jiang, K., Wang, J.F., Dong, K., Zhang, X.P., Cheng, F.Q., 2019. Role of ionic liquids in the efficient transfer of lithium by cyanex 923 in solvent extraction system. AICHE J. 65, e16606. Diallo, M.S., Kotte, M.R., Cho, M., 2015. Mining critical metals and elements from seawater: opportunities and challenges. Environ. Sci. Technol. 49, 9390–9399. Gao, D.L., Yu, X.P., Guo, Y.F., Wang, S.Q., Liu, M.M., Deng, T.L., Chen, Y.W., Belzile, N., 2015. Extraction of lithium from salt lake brine with trisobutyl phosphate in ionic liquid and kerosene. Chem. Res. Chin. Univ. 31, 621–626. Gao, D.L., Guo, Y.F., Yu, X.P., Wang, S.Q., Deng, T.L., 2016. Extracting lithium from the high concentration ratio of magnesium and lithium brine using imidazoliumbased ionic liquids with varying alkyl chain lengths. J. Chem. Eng. Jpn 49, 104–110. Liu, X.H., Chen, X.Y., He, L.H., Zhao, Z.W., 2015. Study on extraction of lithium from salt lake brine by membrane electrolysis. Desalination 376, 35–40. Luong, V.T., Kang, D.J., An, J.W., Kim, M.J., Tran, T., 2013. Factors affecting the extraction of lithium from lepidolite. Hydrometall. 134, 54–61. Meshram, P., Pandey, B.D., Mankhand, T.R., 2014. Extraction of lithium from primary and secondary sources by pre-treatment, leaching and separation: a comprehensive review. Hydrometall. 150, 192–208. Monmaturapoj, N., Lawita, P., Thepsuwan, W., 2013. Characterisation and properties of lithium disilicate glass ceramics in the SiO2-Li2O-K2O-Al2O3 system for dental applications. Adv. Mater. Sci. Eng. 2013, 763838. Nie, X.Y., Sun, S.Y., Sun, Z., Song, X., Yu, J.G., 2016. Ion-fractionation of lithium ions from magnesium ions by electrodialysis using monovalent selective ion-exchange membranes. Desalination 403, 128–135. Palacios-Bereche, R., Gonzales, R., Nebra, S.A., 2012. Exergy calculation of lithium bromide–water solution and its application in the exergetic evaluation of absorption refrigeration systems LiBr-H2O. Int. J. Energy Res. 36, 166–181. Paranthaman, P., Li, L., Luo, J.Q., Hoke, T., Ucar, H., Moyer, B.A., Harrison, S., 2017. Recovery of lithium from geothermal brine with lithium–aluminum layered double hydroxide chloride sorbents. Environ. Sci. Technol. 51, 13481–13486. Qing, H., Williams, N.J., Hyun, O.J., Lynch, V.M., Kuk, K.S., Moyer, B.A., Sessler, J.L., 2018. Selective solid-liquid and liquid-liquid extraction of lithium chloride using
Fig. 13. Reusability of the organic phase with Mg(OH)2 and MgCO3 as regeneration agents.
MgCO3 was used as the regeneration agent, the extraction efficiency of the organic phase after regeneration also increases first and then decreases with the increase of the addition concentration of MgCO3. When the addition concentration of MgCO3 was 2.5 g/L, the extraction efficiency of the regenerated organic phase reached up to 54.85%. 3.5. Study on the reusability of the organic phase The reusability of the organic phase is a key factor in determining whether the extraction system is stable. Therefore, when Mg(OH)2 and MgCO3 were used as regeneration agents separately, the organic phase was reused ten times for the whole separation process including extraction, washing, stripping and regeneration, and the results are shown in Fig. 13. It can be seen that both extraction efficiencies of Li+ maintain at about 53% during ten cycles, indicating that both Mg(OH)2 and MgCO3 have excellent regeneration abilities for the extraction system with FeCl3 as co-extraction agent. 4. Conclusions In this work, a new extraction system consisting of TBP as extractant, FeCl3 as co-extraction agent and diethyl succinate as diluent 7
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