Highly Efficient Separation of Magnesium and Lithium and High

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Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Highly Efficient Separation of Magnesium and Lithium and HighValued Utilization of Magnesium from Salt Lake Brine by a ReactionCoupled Separation Technology Xiaoyu Guo,† Shaofang Hu,† Chenxi Wang,† Haohong Duan,*,‡ and Xu Xiang*,† †

State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China Chemistry Research Laboratory, Department of Chemistry, University of Oxford, 12 Mansfield Road, Oxford OX1 3TA, U.K.

‡

S Supporting Information *

ABSTRACT: Lithium extraction from salt lake brines is one of the most important pathways for obtaining Li-related products, e.g., Li2CO3 and LiOH, and for further fabricating electric energy-storage products, e.g., lithium ion batteries. The high Mg/Li ratio and high Mg content are remarkable characteristics of the salt lakes in the Qaidam Basin in China, making the Mg/Li separation and Li extraction rather difficult. Herein, we proposed a reaction-coupled separation technology for Mg/Li separation from brine with a high Mg/Li ratio. The core idea of this technology is that the Mg2+ cations were reacted to form a solid via a nucleation−crystallization separation method. The solid product was MgAl-layered double hydroxide (MgAl-LDH), a widely used and high-valued product in the family of LDHs. The Li+ cations were left in the solution after Mg2+ cations were reacted with alkali solution, accompanied by foreign Al3+ cations. That is to say that the Mg2+ cations can be incorporated into the layers of MgAl-LDH while Li+ cations cannot. The findings indicated that Mg2+ cations were almost completely extracted into the solid phase to form the LDH. The Li+ cations remained in the solution having a weight loss less than 8%, which is an excellent level of Li extraction from the brine with a high Mg/Li ratio. The effects of reaction parameters, e.g., ionic strength, nucleation rotating speed, Mg/Al ratio, and crystallization temperature and time, on the separation performance and lithium loss were investigated. The optimal conditions were derived for lower lithium loss and more outstanding Mg/Li separation performance, which can be a useful guide for environmentally friendly and sustainable Li extraction from the brine.

1. INTRODUCTION Lithium, the first metallic element in the periodic table of elements, is one of the most important metal resources and has extensive applications in a variety of fields, such as nuclear, chemical, manufacturing, and metallurgical industries.1 The demand for lithium resources has experienced a tremendous increase in the last decades because of the rapid expansion of battery industries for developing electric vehicles, energy storage devices, and 3C electric products.2−4 There are two main pathways for mining lithium from natural resources. One is from Li-bearing ores, e.g., petalite, lepidolite, and spodumene. The other is from Li-containing salt lake brines. The latter accounts for 70% of the total recoverable lithium resources around the world.5−7 There are extremely abundant lithium deposits in salt lake brines in China, which are mostly distributed in the Qaidam Basin and the Qinghai Tibet Plateau.8 Most of the salt lake brines in the Qaidam Basin belong to the magnesium sulfate subtype classification, and the Mg/Li ratios vary from several tens to hundreds. The high Mg/ Li ratios are a major challenge for recovering lithium from salt lake brines owing to the very similar properties of Mg and Li elements (e.g., ionic radius, chemical reactivity).9−11 © XXXX American Chemical Society

The current methods of mining lithium from the salt lake brines of high Mg/Li ratios mainly include calcination, adsorption, extraction, and membrane methods. The calcination technology is relatively mature and has been applied in the West Taijinar salt lake brine.12 However, the high energy consumption and serious emission of acid mist limit its further scale production. The adsorption approaches have been widely studied in the past decades and applied for lithium recovery in the Qarhan Salt Lake.13−15 The main adsorbents include layered adsorbents, aluminum salt adsorbents, ionic sieve sorbents, and ion-exchange resins, which have a relatively high selectivity for lithium recovery.16−20 For instance, Heidari and co-workers reported that the adsorption of Li+ on Al(OH)3 is effective and 76.4% of lithium in the solution is adsorbed at an equilibrium condition.16 The adsorption on the lithium ionic sieves, e.g., Li−Mn−O series and LiFePO series, can reach as high as ∼90%.17,18 The main problems of the adsorption Received: Revised: Accepted: Published: A

March 15, 2018 April 24, 2018 April 25, 2018 April 25, 2018 DOI: 10.1021/acs.iecr.8b01147 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

Scheme 1. Structural Model of Hydrotalcite Lamellaa

method are the dissolution of adsorbents, the reduced adsorption capacity due to the blockage of the ion channels, and the lithium residue upon desorption.19,20 The extraction is meant to selectively recover lithium from the brines using organic extraction agents.21−24 For example, Zhou and colleagues developed an extraction system of tributyl phosphate (TBP)/methyl isobutyl ketone (MIBK)/FeCl3, in which TBP acts as an extraction agent, MIBK as a diluent and synergistic agent, and FeCl3 as a coextraction agent.21 The extraction has a characteristic of high selectivity to lithium ions. Nevertheless, the remaining issues are its large footprint, severe corrosion to the equipment by the extraction agents, and long equilibrium time. The membrane approach is an emerging technology for mining lithium from the brines, which mainly include electrodialysis, nanofiltration, and membrane distillation.25−28 Electrodialysis is a method in which the cations with different valences have distinct diffusing speed in the solution under an electric field and can be separated through a specific ionexchange membrane.26,27 The advantages of this method are relatively low energy cost and good cation selectivity based on the valences of cations. Nanofiltration (NF) refers to a pressure-driven membrane separation process. Nanofiltration membranes have better retention properties for divalent and high-valence ions and have good transmittance for univalent ions.28 However, the nanofiltration membranes cannot completely separate magnesium and lithium from the solution. Membrane distillation (MD) is a membrane technology that uses hydrophobic microporous membranes to separate ions. The previous studies showed that the direct contact membrane distillation is not suitable for the salt concentration above 7 mol/L. The osmotic membrane distillation is not suitable for the salt concentration above 10 mol/L.29 The lack of highefficiency membranes commercially available could be a barrier. The most serious problem of the current approaches for Li recovery could be the discarding of huge magnesium resources in the brine, which makes the utilization of brine resources unsustainable. Consequently, lithium recovery from brines having a high Mg/Li ratio remains a great challenge. Hydrotalcite is a layered double hydroxide (LDH) having alternatively arranged Mg2+ and Al3+ cations in the layers and charge-balancing carbonate anions in the interlayer, i.e., MgAlLDH.30−33 The versatile substitutions of cations in the layers and of anions in the interlayer create a huge family of LDHs, which display diverse applications in many fields.34−37 The chemical formula of hydrotalcite is Mg6Al2(OH)16CO3·xH2O. The Mg2+ and Al3+ cations can form stable layers of hydrotalcite, while Li+ cannot because of metal−oxygen octahedron (MO6) distortion angle criteria.38 The structural model of hydrotalcite lamella is depicted in Scheme 1. Inspired by the structural imprinting of hydrotalcite, we propose a reaction-coupled separation technology aiming at Mg/Li separation from brines with a high Mg/Li ratio. The Mg2+ cations can be selectively reacted to form a solid precipitate, i.e., hydrotalcite. The Li+ cations remain in the liquid phase because of the lack of structural imprinting. This technology enables simultaneously the Mg/Li separation and the efficient utilization of magnesium resources from the brines. In other words, the comprehensive utilization of magnesium and lithium in the brine can be achieved by this sustainable and pollutantfree technology.

a

Mg/Al = 3 in molar. The lamella layer consists of alternatively arranged Mg(OH)6 and Al(OH)6 octahedrons. The carbonate anions and water molecules are lying in the galleries of the interlayers.

2. EXPERIMENTAL SECTION 2.1. Materials. The AlCl3·6H2O (99.0%), NaOH (99.0%), and Na2CO3 (99.8%) were purchased from Beijing Chemical Corporation Ltd. The industrial grade AlCl3·6H2O (95.5%, Fe % ≤ 0.025%) was purchased from Suzhou Jia Ting Chemical Corporation Ltd. (China). The main chemical compositions of the brine after crystallization of KCl and NaCl from the East Taijinar salt lake (China) were listed in Table 1. 2.2. Methods. Solution A: AlCl3·6H2O was dissolved in the brine to form a clear salt solution. Solution B: NaOH and Na2CO3 were dissolved in deionized water to form the base solution. The volume of the base solution was the same as that of salt solution. The molar concentrations of the bases were related to the concentrations of metal ions in solution A as follows: [NaOH] = 1.6[Mg2+ + Al3+ ] and [CO32−] = 2.0[Al3+]. 2.2.1. Nucleation Rotating Speed. Solution A: The brine of 50 mL was diluted to 150 mL. The concentration of Mg2+ was 1.1722 mol/L in the diluted solution. AlCl3·6H2O with a molar ratio of Mg2+/Al3+ = 3.0 was dissolved in the brine. Solutions A and B were simultaneously poured into a homemade colloid mill, i.e., nucleation reactor, at a rotating speed of 2000, 3000, 4000, or 5000 rpm at room temperature for 3 min. The resulting slurry was transferred to a crystallization reactor and aged at 100 °C for 6 h. The slurry was filtered, washed thoroughly with deionized water, and dried at 80 °C for 6 h. 2.2.2. Ionic Strength. Solution A: The brine of 50 mL was diluted to 100, 125, 150, and 200 mL, and the concentration of Mg2+ was 1.7583, 1.4066, 1.1722, and 0.8791 mol/L in the diluted solution, respectively. AlCl3·6H2O with a molar ratio of Mg2+/Al3+ = 3.0 was dissolved in the brine. Solutions A and B were simultaneously poured into a colloid mill, i.e., nucleation reactor, at a rotating speed of 4000 rpm for 3 min. The resulting slurry was transferred to a crystallization reactor and aged at 100 °C for 6 h. The slurry was filtered, washed thoroughly with deionized water, and dried at 80 °C for 6 h. 2.2.3. Mg/Al Molar Ratio. Solution A: The brine of 50 mL was diluted to 150 mL. The concentration of Mg2+ was 1.1722 mol/L in the diluted solution. AlCl3·6H2O with Mg2+/Al3+ molar ratios of 2:1, 3:1, 4:1, and 5:1 was dissolved in the brine. B

DOI: 10.1021/acs.iecr.8b01147 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research Table 1. Chemical Composition of the East Taijinar Salt Lake Brine concentration (g/L)

Mg2+

Li+

B2O3

Na+

K+

SO42−

Cl−

85.47

6.75

15.77

10.42

7.69

29.58

251.60

radiation operating at 40 kV and 30 mA, λ = 0.15418 nm (Shimadzu XRD-6000 diffractometer). The ion concentrations were determined by inductively coupled plasma mass spectrometry (ICPS-7500, Shimazduo). A series of standard solutions of Mg2+, Li+, and Al3+ ions were prepared from 10 and 100 ppm. The standard curves were plotted. The filtrate samples were diluted to an appropriate concentration based on the standard curves before the measurements. A proper amount of MgAl-LDH solid was weighed and dissolved in dilute nitric acid to form a clear solution. The solution was tested for determining Mg2+ and Al3+ contents in the solid product. Transmission electron micrographs were taken using a transmission electron microscopy (TEM) microscope (HT7700, Hitachi). For TEM observations, the samples were ultrasonically dispersed in ethanol, and then a drop of the suspension was deposited onto a carbon-coated Cu grid followed by the evaporation of solvent in air. The particle sizes were examined by a laser particle size analyzer (Malvern Zetasizer Nano ZS). The 0.01 g samples were dispersed in 100 mL of ethanol for continuous ultrasonic 30 min and then tested. The secondary particle size value was defined as the mean particle size obtained by the laser particle size analyzer. The zeta potential was examined by a laser particle size analyzer (Malvern Zetasizer Nano ZS). The 0.1 g samples were dispersed in 100 mL of deionized water for continuous ultrasonic treatment for 30 min. The suspension was kept static for 12 h. The Cl− ions on the solid MgAl-LDH were measured by Thermo DIONEX ICS-5000 ion chromatography. A proper amount of MgAl-LDH was dissolved in dilute nitric acid to form a clear solution. The pH of the solution was adjusted to >2.0 using a solution of NaOH. The test conditions are as follows: column, IonPacAS11-HC analysis column, IonPacAG11-HC guard column; eluent, 30 mM KOH; suppressor, ASRS 300 4 mm; detector, conductivity detector; flow rate, 1.2 mL/min; injection volume, 25 μL; column temperature, 35 °C; standard curve, standard solution with a gradient of 1, 10, 20, 50, and 100 ppm. The solid-state nuclear magnetic resonance (NMR) spectra were measured by Bruker BioSpin GmbH NMR equipment. The samples were vacuum-dried and placed in a 4 mm diameter ZrO2 tube after grinding. The test conditions were as follows: resonant frequency, 78.172 MHz; cycle delay time, 0.5 s; excitation pulse width, 0.5 μs; scanning spectrum width, 400 ppm; number of scans, 7000−8000; rotating speed, 8 kHz; chemical shift reference, AlCl3·6H2O. The whiteness of the solid products was measured by the WSB-VI Smart Whiteness Tester. The whiteness value of the standard whiteboard is 83.3. First, the instrument was calibrated on the basis of the standard whiteboard. Then, the sample was put into the test port, and the counter showed the whiteness value. The lithium loss in percentage is defined as (1 − Mass(lithium in the filtrate after reaction)/Mass(lithium in the brine before reaction)) × 100.

Solutions A and B were simultaneously poured into a colloid mill, i.e., nucleation reactor, at a rotating speed of 4000 rpm for 3 min. The resulting slurry was transferred to a crystallization reactor and aged at 100 °C for 6 h. The slurry was filtered, washed thoroughly with deionized water, and dried at 80 °C for 6 h. 2.2.4. Crystallization Temperature. Solution A: The brine of 50 mL was diluted to 150 mL. The concentration of Mg2+ was 1.1722 mol/L in the diluted solution. AlCl3·6H2O with a molar ratio of Mg2+/Al3+=3.0 was dissolved in the brine. Solutions A and B were simultaneously poured to a colloid mill i.e. nucleation reactor at a rotating speed of 4000 rpm for 3 min. The resulting slurry was transferred to a crystallization reactor and aged at 40 °C, 60 °C, 80 and 100 °C, respectively, for 6h. The slurry was filtered, washed thoroughly with deionized water, and dried at 80 °C for 6 h. 2.2.5. Crystallization Time. Solution A: The brine of 50 mL was diluted to 150 mL. The concentration of Mg2+ was 1.1722 mol/L in the diluted solution. AlCl3·6H2O with a molar ratio of Mg2+/Al3+ = 3.0 was dissolved in the brine. Solutions A and B were simultaneously poured into a colloid mill, i.e., nucleation reactor, at a rotating speed of 4000 rpm for 3 min. The resulting slurry was transferred to a crystallization reactor and aged at 100 °C for 3, 6, 12, or 24 h. The slurry was filtered, washed thoroughly with deionized water, and dried at 80 °C for 6 h. 2.2.6. MgAl-LDH Prepared Using the Industrial Grade AlCl3·6H2O. Solution A: Industrial grade AlCl3·6H2O was dissolved in the brine according to a molar ratio of Mg2+/Al3+ = 3.0. Solutions A and B (NaOH and Na 2 CO 3 ) were simultaneously poured into a colloid mill, i.e., nucleation reactor, at a rotating speed of 4000 rpm for 3 min. The resulting slurry was transferred to a crystallization reactor and aged at 100 °C for 6 h. The slurry was filtered, washed thoroughly with deionized water, and dried at 80 °C for 6 h. 2.2.7. MgAl-LDH Prepared from the Solution Containing Mg2+ and Al3+ Ions. Solution A: MgCl2·6H2O was dissolved in 150 mL of deionized water to form a solution with Mg2+ concentration of 1.1722 mol/L. AlCl3·6H2O was dissolved in the solution of Mg2+ with a molar ratio of Mg2+/Al3+ = 3.0. Solutions A and B (NaOH and Na2CO3) were simultaneously poured into a colloid mill, i.e., nucleation reactor, at a rotating speed of 4000 rpm for 3 min. The resulting slurry was transferred to a crystallization reactor and aged at 100 °C for 6 h. The slurry was filtered, washed thoroughly with deionized water, and dried at 80 °C for 6 h. 2.2.8. MgAl-LDH Prepared by a Coprecipitation Method. Solution A: The brine of 50 mL was diluted to 150 mL. The concentration of Mg2+ is 1.1722 mol/L in the diluted solution. AlCl3·6H2O was dissolved in the brine with a molar ratio of Mg2+/Al3+ = 3.0. Solution B (NaOH and Na2CO3) was added dropwise to solution A under stirring at room temperature. The pH of the resulting slurry reached 10.0. The slurry was aged at 100 °C for 6 h. The slurry was filtered, washed thoroughly with deionized water, and dried at 80 °C for 6 h. 2.3. Analysis. Powder X-ray diffraction (XRD) patterns of all the samples were carried out using graphite-filtered Cu Kα C

DOI: 10.1021/acs.iecr.8b01147 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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tensification to the traditional coprecipitation method.39 The major characteristic of this method is the separation of nucleation and crystallization. A huge number of LDH nuclei are first formed in the nucleation stage. These nuclei are further aged in the crystallization stage. This enables no nucleation during the crystallization, and the crystallization just occurs on the pristine LDH nuclei. Consequently, this method results in relatively uniform particle size (∼100 nm) and narrow size distribution of LDH. Aiming at reducing lithium loss, we studied the factors of the nucleation rotating speed, the ionic strength of the pristine solution, and the Mg/Al ratio in the nucleation stage. The effects of crystallization temperature and the time on lithium loss were further investigated in the crystallization stage. 3.1. Factors in the Nucleation Stage. 3.1.1. Effect of Nucleation Rotating Speed. The XRD patterns of MgAl-LDH products are shown in Figure 1. The diffraction lines can be

The Mg/Li ratio is defined as the mass ratio of magnesium and lithium in the filtrate after solid−liquid separation. Scheme 2 depicts the process for the Mg/Li separation and the synthesis of MgAl-LDH from the brine. The salt solution Scheme 2. Flow Sheet for the Mg/Li Separation and the Synthesis of MgAl-LDH from the Brine after Crystallization of KCl and NaCla

Figure 1. XRD patterns of MgAl-LDH synthesized at varied nucleation rotating speeds: (a) 2000 rpm, (b) 3000 rpm, (c) 4000 rpm, and (d) 5000 rpm.

The aluminum salt was first dissolved in the brine to form a clear salt solution. The salt solution and alkali solution were simultaneously poured into a colloid mill, i.e., a nucleation reactor, for rapid nucleation within several minutes. The resulting slurry was transferred to a crystallization reactor for aging for a time under stirring. Subsequently, the suspension was filtered and washed for solid−liquid separation. After being dried, the solid MgAl-LDH product was collected. The lithium ions remained in the solution. a

indexed to the planes (003), (006), (009), (110), and (113) of the hydrotalcite phase (JCPDS No. 22-0700).40 The d-value of plane (003) is 7.83 Å (Figure 1c). According to the literature, the d(003) value of carbonate-intercalated MgAl-CO3-LDH is 7.84 Å (JCPDS No. 22-0700) and that of chlorine ionintercalated MgAl-Cl-LDH is 7.98 Å.41 The MgAl-LDH product has a d(003) value closer to that of the carbonateintercalated one. This is an indication that the MgAl-LDH products are charge-balanced by carbonate anions in the gallery of interlayers.42 No other phase was observed in the detection limit. It suggests that the nucleation rotating speed has little effect on the phase structure of the products. In addition, the MgAl-LDH product was studied by ion chromatography to measure the content of Cl− ions. The chlorine content was as low as 1.24 ppm, indicating that the MgAl-LDH is almost free of chlorine. The Mg/Al ratios of MgAl-LDH products prepared at varied nucleation rotating speeds was 3.20, 3.24, 3.16, and 3.24 (Table S1), which are close to the ratio in the feedstocks (Mg/Al = 3). This suggested that the nucleation rotating speed has little effect on the Mg/Al ratio of the products. The ICP results also showed that sodium content was lower than 0.05% and potassium content was below the detection limit in the MgAl-LDH solid products. Reducing the lithium loss is a critical factor for Mg/Li separation and Li extraction from salt lake brine. The lithium loss in weight percentage presents a reverse volcano-type curve with the increasing nucleation rotating speed from 2000 to

and alkali solution were simultaneously poured into a highspeed rotating colloid mill, i.e., nucleation reactor, to rapidly form LDH nuclei within a few minutes. A digital photo of the colloid mill is shown in Figure S1. The resultant slurry was transferred to a crystallization reactor, e.g., a flask or a tank, to be aged at a certain temperature and time. After aging, the suspension was separated by filtration. The solid product was MgAl-LDH, and the liquid phase was lithium-bearing solution.

3. RESULTS AND DISCUSSION The Mg/Li ratio equals 12.66 (in weight) in East Taijinar salt lake brine. In the case of Mg/Al = 3 (in molar), the nominal chemical formula of the MgAl-LDH is Mg6Al2(OH)16CO3· xH2O (x = 3−8). According to the molecular weight of Li2CO3 and MgAl-LDH, around 10 tons MgAl-LDH are produced, while 1 ton Li2CO3 is obtained from the brine. The LDH has wide applications and potentially huge market demand, e.g., additives in polymers, optoelectric materials, biomedical materials, catalyst support, etc.32,33 Consequently, the reaction-coupled separation technology enables simultaneous utilization of magnesium and lithium in the brine. The nucleation−crystallization separation method is process inD

DOI: 10.1021/acs.iecr.8b01147 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research 5000 rpm in the nucleation stage (Figure 2). The lithium loss reaches a minimum of 8.17% at a nucleation rotating speed of

The whiteness is an index for MgAl-LDH products. To consider the industrial production using this new technology, we prepared the MgAl-LDH using the industrial grade AlCl3· 6H2O (Fe% ≤ 0.025%) as aluminum feedstock. The whiteness value of the standard whiteboard is 83.3. The MgAl-LDH prepared using analytical grade AlCl3·6H2O and industrial grade AlCl3·6H2O has a whiteness value of 75.7 and 72.7, respectively. The whiteness of the latter is slightly lower than that of the former. The digital photos of the two products are shown in Figure S3. The impurity of Fe3+ in the industrial grade AlCl3·6H2O has a little effect on the whiteness of the LDH product. The zeta potential of MgAl-LDH was further measured to evaluate the variation of secondary particle size. It is found that the MgAl-LDH synthesized at the nucleation rotating speed of 4000 rpm has the highest zeta potential value of 15.9 mV (Figure 3). This is consistent with the result that the MgAl-

Figure 2. Effects of nucleation rotating speed on the lithium loss (squares), Mg/Li ratio (circles), and secondary particle size (triangles) of MgAl-LDH: 2000, 3000, 4000, and 5000 rpm. The curves were fitted using a B-spline function based on the data obtained at different nucleation rotating speeds.

4000 rpm and increases to 12.89% when the nucleation rotating speed is elevated to 5000 rpm. The Mg/Li ratio in the filtrate is remarkably reduced to 0.06 from the initial value of 12.66 in the brine at the nucleation rotating speed of 4000 rpm. This indicates that the Mg2+ cations almost completely enter the solid phase, i.e., MgAl-LDH and have negligible concentration in the liquid phase after reaction. The secondary particle size of MgAl-LDH measured by laser particle size analyzer shows a similar curve to that of lithium loss or Mg/Li ratio and has the minimum of 381 nm at the nucleation rotating speed of 3000− 4000 rpm. The secondary particle size is usually much larger than the primary particle size of LDH synthesized by a nucleation−crystallization separation method because of the agglomeration of sheetlike LDH nanoparticles. Therefore, the degree of agglomeration can be estimated by the secondary particle size to a certain extent.43 The smaller the secondary particle size, the less agglomeration of LDH nanoparticles. We measured the Li content in the MgAl-LDH product synthesized at the nucleation rotating speed of 4000 rpm by ICP. The lithium content on MgAl-LDH was measured to be 0.16% (in weight), which is consistent with the lithium loss in the liquid phase. This suggested that the lithium loss is possibly due to the adsorption on the solid LDH. A trace amount of Na+ ions (0.05%) was detected, possibly because the adsorption and the K+ ions were not detected in the solid MgAl-LDH. The chemical compositions of the brine after the precipitation of MgAl-LDH were listed in Table S2. To determine the effect of preparation methods on the lithium loss, we synthesized MgAl-LDH by a conventional coprecipitation method. The concentrations of Mg2+ and Al3+ ions are the same as those by a nucleation−crystallization separation method. The details are stated in Experimental Section. The MgAl-LDH products prepared using different methods showed the characteristic diffractions of hydrotalcitelike structure in the XRD patterns (Figure S2). The lithium loss reached as high as 19.85% when a coprecipitation method was applied to prepare MgAl-LDH from the brine. This verified that the nucleation−crystallization separation method is preferable to efficient Mg/Li separation and lower lithium loss by a process intensification mechanism.39

Figure 3. Zeta potentials of MgAl-LDH synthesized at varied nucleation rotating speeds.

LDH has the smallest secondary particle size under this synthesis condition. The higher zeta potential results in stronger repulsion interaction among LDH nanoparticles because of the positively charged layers, reducing the agglomeration.44 On the basis of the findings above, the lithium loss increases with the increasing secondary particle size or the decreasing zeta potential value of MgAl-LDH. 3.1.2. Effect of Ionic Strength. The ionic strength (I) is usually defined using the following formula: n

I=

1 ∑ cizi 2 2 i=1

(1)

where ci is the molar concentration of a specific component in the solution and zi is the charge number of a specific component (i). It reflects the comprehensive effect of ionic concentrations and charges on the reactions occurring in an aqueous solution. When a nonequilibrium crystallization method is applied, the larger the initial concentrations of nucleation ions, the smaller the LDH particle sizes synthesized. Generally, high ionic strength leads to the larger quantity of LDH nuclei formed in the nucleation stage, and thus smaller particle size of the product after crystallization.45 The brine after crystallization of KCl and NaCl possesses high ionic concentrations and further ionic strength. Consequently, it is critical to reveal the effect of ionic strengths on the Mg/Li separation from the brine. The XRD patterns of the solid products synthesized at varied ionic strengths show a single E

DOI: 10.1021/acs.iecr.8b01147 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research phase of hydrotalcite (Figure S4). No other phase is observed in all patterns. The lithium loss in the solution after reactions reduces with the decreasing ionic strengths (Figure 4), which

Figure 5. Zeta potentials of MgAl-LDH products synthesized at varied ionic strengths.

investigated the effect of Mg/Al ratio at a wider range of 2−5 in the feedstock. The XRD patterns indicated that only a single phase, i.e., hydrotalcite, exists in the products whatever the initial Mg/Al ratios (Figure S6). The Mg/Al ratios in LDH products are 2.04, 3.16, 3.90, and 4.78, as measured by ICP, basically consistent with their nominal ratio in the feedstock. When the Mg/Al ratio equals 2, a larger amount of aluminum salt was added to the brine, causing a higher total concentration of salt solution. As a result, the content of product after reaction is higher, and the filtration requires a longer time than those synthesized using higher Mg/Al ratios, which could lead to stronger inclusion of lithium ions within LDH particles. The lithium loss has a higher value of 14.79% (Figure 6). The

Figure 4. Effects of ionic strength on the lithium loss (squares), Mg/Li ratio (circles), and secondary particle size (triangles) of MgAl-LDH: 3.3946, 4.7586, 5.5419, and 6.8282 mol/kg. The curves were fitted using a B-spline function based on the data obtained at different ionic strengths.

reaches as low as 8.17% and 8.19% at the ionic strength of 4.7586 and 3.3946 mol/kg, respectively. The Mg/Li ratio in the filtrate slightly increases from 0.06 to 0.14 at the ionic strength of 4.7586 to 6.8282 mol/kg. However, the Mg/Li ratio after reaction is significantly lower than that in the brine before reaction (Mg/Li = 12.66), indicative of excellent Mg/Li separation capability. The secondary particle size is gradually increased with the elevating ionic strengths, having a smaller size of 375 nm at a low ionic strength of 3.3946 mol/kg and a larger size of 717 nm at a high ionic strength of 6.8282 mol/kg. The primary particle size is around the range of 30−40 nm, and the LDH particles gradually present an aggregate status at high ionic strengths (5.5419 and 6.8282 mol/kg) (Figure S5). The higher ionic strength in the feedstock solution enhances the collision possibility during the rapid nucleation in the colloid mill. A large amount of LDH nuclei are formed within a very short period of time. A too high quantity of LDH nuclei could lead to the more intensive agglomeration of the particles in the crystallization process because the small-sized and crystallization-deficient LDH nuclei possess large surface energy.46 It is found that the zeta potentials of MgAl-LDH products exhibit a decreasing trend from 18.8 to 4.82 mV with the increasing ionic strength (Figure 5). The larger zeta potential leads to less agglomeration because the positive surface charging of LDH causes stronger repulsion interaction among LDH particles. In addition, LDH particles form agglomeration more easily under a smaller zeta potential. This finding is consistent with the results of secondary particle size and lithium loss. As a result, lowering the ionic strength of the brine favors Mg/Li separation and suppresses Li loss. 3.1.3. Effect of Mg/Al Molar Ratios. The Mg/Al ratio cannot be too small because the substitution of Mg2+ ions by Al3+ ions in the layers of MgAl-LDH is not unlimited. Too large a portion of Al3+ ions can lead to the formation of Al-containing impurity phases, e.g., gibbsite and boehmite except the MgAlLDH phase.47 Usually, the MgAl-LDH is synthesized at an initial Mg/Al molar ratio of 2−4. Aiming for the sufficient utilization of magnesium resources in the salt lake brine, we

Figure 6. Effects of Mg/Al molar ratios on the lithium loss (squares), Mg/Li ratio (circles), and secondary particle size (triangles) of MgAlLDH: 2, 3, 4, and 5. The curves were fitted using a B-spline function based on the data obtained at different Mg/Al molar ratios.

lithium loss reaches a minimum of 8.17% at the Mg/Al ratio of 3 and increases to 12.26% and 16.03% when the Mg/Al ratio equals 4 and 5, respectively (Figure 6). Chitrakar and coworkers showed that different ratios of divalent cations to trivalent cations (M2+/M3+) leads to the differences in crystallinity of LDHs.48 The higher Mg/Al ratio results in lower crystallinity of LDH product. The imperfection and defects resulting from low crystallinity cause more inclusion of lithium ions in LDH particles. The secondary particle size increases with the increase of the Mg/Al ratio. It is found that the secondary particle size increases from 381 to 812 nm and further to 1026 nm when the Mg/Al ratio is elevated from 3:1 to 5:1. It is an indication that F

DOI: 10.1021/acs.iecr.8b01147 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

(Figure 8). When the aging time is increased to 12 h, the secondary particle size reaches down to 246 nm, and it shows

the agglomeration of LDH particles is more serious with the increasing Mg/Al ratios. The zeta potentials display a decreasing trend from 15.9 to 12.8 mV with the increase of Mg/Al ratios from 3 to 5 (Figure S7). The reduction in zeta potentials suggests the weakening of repulsion interaction among LDH particles, causing the agglomeration to be more possible. Consequently, the medium Mg/Al ratio (Mg/Al = 3) is favorable to abate the aggregates of LDH particles and reduce lithium loss. 3.2. Factors in the Crystallization Stage. 3.2.1. Effect of Crystallization Temperature. Tunig the crystallization temperature of inorganic nanoparticles has a marked effect on the size and the crystallinity of the products. When the crystallization temperature is increased, the crystallinity of the LDH particles can be improved and the intrinsically plateletlike shape can be developed well.49 Here, we studied the effect of crystallization temperature on the Mg/Li separation and lithium extraction. The secondary particle size of the LDH product gradually decreases from 977 to 381 nm when the crystallization temperature is elevated from 40 to 100 °C (Figure 7). The

Figure 8. Effects of crystallization time on the lithium loss (squares), Mg/Li ratio (circles), and secondary particle size (triangles) of MgAlLDH: 3, 6, 12, and 24 h. The curves were fitted using a B-spline function based on the data obtained at different crystallization times.

little change when the LDH was aged for 24 h. The Mg/Li ratio in the filtrate after reaction is reduced to below 0.10 when the aging time is extended to 6 h or above. The lithium loss is 8.17%, 8.22%, and 8.21% at the crystallization time of 6, 12, and 24 h, respectively. This suggests that the lithium loss is hardly affected by the crystallization time when LDH is aged for 6 h or longer. The zeta potentials of LDH particles increases from 14.2 to 15.9 mV when prolonging the crystallization time from 3 to 6 h and slowly rises to 17.2 mV at the crystallization time of 12 h (Figure S9). This trend is in accordance with the variation of the secondary particle size. The larger zeta potential indicates more robust electrostatic repulsion among LDH particles, preventing them from agglomerating. Based on the findings, the optimal crystallization time could be 6−12 h for less lithium loss and lower Mg/Li ratio in the liquid phase after reaction and separation. It is known that the brine is a salt solution containing multications, different from that usually used (containing only Mg2+ and Al3+ ions). For better comparison, we prepared the MgAl-LDH from the salt solution containing Mg2+ and Al3+ as a control. The LDH products prepared from different feedstocks were compared by X-ray diffraction (XRD) and NMR. The XRD patterns were similar for the two LDH products (Figure S10). The chemical shifts of Al in the solid LDH products were measured by 27Al NMR. Both shifts were 9.23 ppm (Figure S11). This confirmed that Al exists in a hexacoordinated octahedron in the two LDH products. The XRD and NMR results verified that the coexisting cations in the brine barely affected the structure of MgAl-LDH.

Figure 7. Effects of crystallization temperature on the lithium loss (squares), Mg/Li ratio (circles), and secondary particle size (triangles) of MgAl-LDH: 40, 60, 80, and 100 °C. The curves were fitted using a B-spline function based on the data obtained at different crystallization temperatures.

Mg/Li ratio in the filtrate after reaction shows a similar decreasing tendency with the secondary particle size from 977 to 381 nm. Lower crystallization temperature results in incomplete growth and crystallinity of LDH nanoparticles and causes more defects and structural distortion, which leads to more intensive agglomeration.49 One can find that the lithium loss is down to 8.17%, while the crystallization temperature rises to 100 °C (Figure 7). The zeta potentials were further measured and showed a rising trend from 8.74 to 15.9 mV with the elevating crystallization temperature (Figure S8). This result agrees with the variation of the secondary particle size, reflecting the degree of agglomeration of LDH particles. We did not increase the temperature above 100 °C because this reaction proceeds in an aqueous solution, and we also consider a balance between the lithium loss and energy consumption. 3.2.2. Effect of Crystallization Time. The effects of crystallization time on the growth of LDH particles have been investigated. Zhou and colleagues found that the extension of aging times could increase crystallinity of LDH and reduce the agglomeration among particles.44 In the present studies, we observed that the secondary particle size decreases from 398 to 381 nm while extending aging time from 3 to 6 h

4. CONCLUSIONS A reaction-coupled separation technology was established for lithium extraction from the salt lake brine in Qaidam (China) having a high Mg/Li ratio. The Mg2+ cations in the brine were reacted with the alkali solution together with the foreign Al3+ to form a MgAl-LDH solid based on a coprecipitate principle. A nucleation−crystallization separation method was utilized to divide the reaction into two individual stages. The factors in the nucleation and crystallization stages were investigated and correlated with the lithium loss and Mg/Li separation ability by G

DOI: 10.1021/acs.iecr.8b01147 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research this technology. The findings indicated that the nucleation rotating speed, ionic strength, initial Mg/Al ratio, and crystallization temperature and time have remarkable effects on the lithium loss. The appropriate nucleation rotating speed of 3000−4000 rpm results in less lithium loss because the LDH particles have larger zeta potentials and thus less severe agglomeration. To reduce the ionic strength of the feedstock solution favors suppressing lithium loss. For instance, the lithium loss is below 8.20% when the ionic strength is less than 4.7586 mol/kg. The agglomeration of LDH were reduced and the lithium loss was lowered at a medium Mg/Al ratio (Mg/Al = 3). The higher Mg/Al ratio could lead to lower crystallinity of LDH product, and the existing defects might cause more lithium loss by the inclusion of Li+ among LDH particles. To elevate the crystallization temperature (80−100 °C) and to prolong the crystallization time (6 h or above) favors reducing lithium loss and lowering Mg/Li ratio in the filtrate after reaction. The lithium loss reached as low as 8.17%, and the Mg/Li ratio in the filtrate is below 0.1% at the optimal reaction conditions, which are the state-of-the-art level for the Mg/Li separation from the brine with a high Mg/Li ratio. This technology enables most of the lithium ions (>90%) to remain in the solution and almost complete magnesium incorporation into the solid LDH, which achieves high-efficiency Mg/Li separation and simultaneously high-valued utilization of the abundant magnesium resource in the brine. Product engineering from the lithium-abundant solution after separation could be explored in future work to yield high-valued lithium products, for example, Li2CO3.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.8b01147. Mg/Al ratios in MgAl-LDH products, chemical compositions of the brine, digital photos of MgAl-LDH products, additional XRD patterns, TEM photographs, zeta potentials, and 27Al NMR spectra (PDF)

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NF = nanofiltration MD = membrane distillation I = ionic strength

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Haohong Duan: 0000-0002-9241-0984 Xu Xiang: 0000-0003-1089-6210 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant U1507202, U1707603), the Innovative Research Groups of National Natural Science Foundation of China (Grant 21521005) and the Key R&D Program of Qinghai Province (Grant 2017-GX-144).

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ABBREVIATIONS LDH = layered double hydroxide RCST = reaction-coupled separation technology rpm = revolutions per minute MO6 = metal−oxygen octahedron H

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DOI: 10.1021/acs.iecr.8b01147 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX