Li ratio by the crystallization-precipitation method

Journal Pre-proof Extraction of lithium from brines with high Mg/Li ratio by the crystallization-precipitation method

Xianrong Lai, Pan Xiong, Hui Zhong PII:

S0304-386X(18)30782-5

DOI:

https://doi.org/10.1016/j.hydromet.2020.105252

Reference:

HYDROM 105252

To appear in:

Hydrometallurgy

Received date:

19 October 2018

Revised date:

7 December 2019

Accepted date:

5 January 2020

Please cite this article as: X. Lai, P. Xiong and H. Zhong, Extraction of lithium from brines with high Mg/Li ratio by the crystallization-precipitation method, Hydrometallurgy(2019), https://doi.org/10.1016/j.hydromet.2020.105252

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© 2019 Published by Elsevier.

Pre-proof Extraction of lithium fromJournal brines with high Mg/Li ratio by the crystallization-precipitation method Xianrong Laia, Pan Xionga, Hui Zhonga, * College of Materials and Chemistry & Chemical Engineering, Chengdu University of Technology, Chengdu 610059, Sich uan, PR China

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* Corresponding author. E-mail address: [email protected] (H. Zhong)

Journal Pre-proof ABS TRACT A new crystallization-precipitation method is proposed to extract lithium from salt-lake brine with high M g/Li mass ratio (M g/Li>40). A closed loop path for carnallite crystallization is established and a high magnesium removal efficiency is achieved via M gHPO 4(s) precipitation through thermodynamic analysis and experiments. In the first stage, KCl is added into the brine to remove about 50% of magnesium by forming carnallite. When the amount of KCl is 55.9% of stoichiometric requirement and further evaporation at 17.6%, the removal efficiency of total magnesium is 53.1% and the loss of lithium is 5.4%. In the second stage, residual magnesium in the brine is further removed by forming M gHPO 4(s) . With the amount of Na2HPO 4 at stoichiometric requirement, reaction temperature 40℃, reaction time 30 mins and aging time 3 h, the removal efficiency of residual magnesium is 99.2% and the recovery of residual lithium is 98.5%. To sum up, the total removal efficiency of magnesium is 99.6% and the recovery of lithium is 93.2%. In addition, KCl and Na2HPO 4 used in this method can be recycled, which will significantly reduce the cost.

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Keywords: Extraction of lithium; High Mg/Li mass ratio; Brine; Carnallite; Magnesium hydrogen phosphate

1. Introducti on

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The demand for lithiu m products in the international market continues to grow at an average annual rate of over 10% (Financial Bu zz, 2018). Currently, recovery of lithiu m fro m saline lake brine containing more than 60% of the global lithium reserves has become the main way to produce lithiu m salts (Xu et al., 2016). However, most of the salt lakes have the characteristic of a high mass ratio of magnesium to lithiu m (Mg/Li>40) (Wang, 2018), which makes it d ifficu lt to extract lithium from brine. Therefore, a series of methods have been developed for separating lithiu m fro m magnesium in brines with high mass ratio of Mg/Li, such as solvent extraction (Xiang et al.,2017; Song et al., 2017), adsorption (Chitrakar et al., 2000; Zandevakili et al., 2014; Wang et al., 2017), membrane method (Yang et al., 2011; Ji et al., 2017), electrochemical method (Kim et al., 2015; Zhao et al., 2017) and precipitation (An et al., 2012; Tran et al., 2016; Liu et al., 2018). Ho wever, these methods have a few disadvantages, such as high cost, low y ield o f lithiu m or incomp lete separation of magnesiu m, therefore, few can be applied co mmercially. Th us, a fresh crystallizat ion-precipitation method is proposed to extract lithiu m fro m brine, which is especially suitable for brine with magnesiu m content closing to saturation. Firstly, potassium ch loride (KCl) is added into the brine to remove half of magnesium by the crystallization of carnallite. Then, residual magnesiu m in the brine is fu rther removed v ia the precipitation o f magnesium ions by Na2 HPO4 . Carnallite is a natural mineral formed by KCl (26.8wt.%) and MgCl2 (34.2wt.%). In light of the characteristics of high concentration of Mg 2+ and ultra-low content of K+ in the brine investigated, a high removal of magnesium can be ach ieved by adding KCl into the brine to form carnallite. Zhang et al. (2016) carried out a three-step evaporation crystallization with saturated KCl solution. After the third stage of crystallization, the mass ratio of Mg/Li decreased fro m 10.1 to 0.39 and t he total recovery of lithiu m was 77.9%. Ho wever, three stages of crystallization have the follo wing disadvantages. Firstly, the use of saturated solution of KCl increases the energy consumption for evaporation. Secondly, after the third step of evaporation, the concentration of Li+ reaches as high as 43.1 g/L in brine, which leads to a considerable entrain ment loss of Li+ (22.1wt.%). In order to resolve those problems, solid KCl is used as the feeding material in our study to avoid the introduction of water. When the content of Li+ is doubled after the second step of evaporation, the process of evaporation crystallization is stopped, which g reatly reduces the loss of lithium entrainment (5.4wt.%). For the phosphate precipitation method, soluble phosphates have b een applied to precip itate Mg 2+ fro m solutions containing lithiu m. Xiao et al. (2015) and He et al. (2017) utilized (NH4 )3 PO4 to precipitate Mg 2+ fro m LiCl solution with an init ial Mg/Li mass ratio of 0.065 and 4.1, respectively. In this study, we adopt a real b rine as the raw material (Mg/ Li>20, p H=6.7). In o rder to preserve the lake ecosystem, Na 2 HPO4 is used as the precipitant, because the pH value of the reaction is close to that of the brine. Therefore, in order to develop an efficient, reliable and lo w-cost crystallization-precip itation method for recovering lithiu m, this study focuses on reducing the energy consumption of carnallite crystallization and the amount of phosphate us ed in the process of precipitating magnesium.

Journal Pre-proof

2. Experimental 2.1. Materials

The brine was sampled fro m the West Taijinar salt-lake in Qinghai Province of Ch ina, which was obtained after KCl and NaCl extracted fro m the original b rine by natural evaporation and crystallization. The brine before and after treat ment was analyzed b y the atomic absorption spectrometry (AAS) and chemical t itrations. The concentrations of components are listed in Tab le 1. All chemicals used in the study were of analytical grade; deionized water was used for dissolving and cleaning. 2.2. Experimental procedure The flowsheet of this experiment is shown in Fig. 1. The main process consists of two parts, i.e., removing half of magnesium fro m b rine by the crystallization of carnallite and removal of the remaining magnesium in the brine via the precip itation of MgHPO4 . Table 1 Concentrations of components in the West Taijinar salt -lake brine before and after treatment. Li+

Na+

Raw Brine

25.1

0.20

85.4

After extraction of KCl and NaCl

103

2.50

2.00

K+

B2 O3

Cl-

7.30

1.30

213

126

0.40

0.20

316

41.1

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Mg2+

Mg/Li ratio

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Ion/oxide (g L-1 )

Fig. 1. Flowsheet for the crystallization-precipitation method.

2.2.1. Removal of magnesium by the crystallization of carnallite The crystallization of carnallite was carried out in t wo steps. In the first step, 1 L raw brine (L0 , Density=1.2858 g/ mL, p H=6.7) was added into a beaker and heated to a specified temperature. Then, a certain amount of solid KCl was added into the brine with the magnetic stirring until KCl was dissolved and reacted completely. After that, the brine was cooled to 25℃ naturally and held for 4 h. The cooled slurry was filtered to obtain filtrate L1 and filter cake S1 . In the second step, some solid KCl was added into filtrate L1 followed by the magnetic stirring until a certain amount of water was evaporated by heating at a specified temperature. Then, L1 was cooled to 25℃ naturally and held for 4 h. This cooled slurry was filtered to obtain filtrate L2 and filter cake S2 . The stoichiometric amount of KCl was calcu lated according to the total amoun t of magnesium in the brine on the basis of Eq. (a). Evaporation efficiency (β), removal efficiency of Mg 2+ (RMg ), recovery efficiency of Li+ (RLi ) and loss of Li+ (γ) were calculated according to Eq. (b-e). KCl + MgCl2 + 6H2 O = KCl·MgCl2 ·6H2 O

(a)

β = (W 1 -W 2 )/W 1 ×100

(b)

RMg = (M 1 -M 2 )/M 1 ×100

(c)

RLi = (N1 -N2 )/N1 ×100

Journal Pre-proof (d)

γ= 100-RLi

(e)

The terms β (%) is the evaporation efficiency, W 1 (g) is the amount of brine and KCl added into the brine before evaporation, W 2 (g) is the remain ing amount of the brine after evaporation, RMg (%) is the removal efficiency of Mg 2+, M 1 (g) is the amount of Mg 2+ before a specified treatment, M 2 (g) is the amount of Mg 2+ after a specified treat ment, RLi (%) is the removal efficiency of Li+, N1 (g) is the amount of Li+ before a specified treatment, N2 (g) is the amount of Li+ after a specified treat ment, γ (%) is the recovery of Li+. The carnallite flotation method mentioned in Fig.1 is an industrial method to recover KCl fro m potassium-rich brine. The brief description of this process is as follows: carnallite is collected f ro m the raw brine by evaporation and crystallizat ion naturally and then decomposed by adding water. Typically, Octadecyl amine hydrochloride is used as a collector and flotation oil is used as a foaming agent to capture potassium in the deco mposing solution . The yield of KCl per passing is about 40wt.%~50wt.% and the purity of KCl is over 90wt.% (Wang et al., 2016). The remaining solution is discharged to the saline pool for further evaporating and crystallizing. In this paper, the carnallite obtained by the crystallization method will be sent to a nearby potash fertilizer p lant to recover KCl by the carnallite flotation method.

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2.2.2. Removal of magnesium by the precipitation method of MgHPO 4 Precip itation react ion of Mg HPO4 is depicted as Eq. (f). This experiment was carried out in a three-necked flask with a mechanical stirrer. In a typical react ion process, Na2 HPO4 solution was added to the raw b rine at a certain flow rate. The react ion temperature was maintained by a thermostat water bath and the pH of the solution was adjusted to 6.5 with sodium hydro xide solution or hydrochloric acid solution and detected by a pH meter. After feeding, the solution continued to be stirred for a period of time. When the reaction was comp leted, the slurry was filt ered by vacuum filtration. The filter cake S3 was washed with deionized water and dried at 110℃ fo r 4 h. The filtrate L3 was rich in lithiu m and suitable for the production of lithiu m salts in the downstream process. The removal efficiency of Mg 2+, the removal efficiency of Li+ and the recovery of Li+ are calcu lated according to Eq. (c-e). Next, MgHPO4(s) obtained by the precipitation method was recycled via the melamine co mplex method which was inspired by the synthesis of melamine phosphate salt as a non-halogen flame retardant (Cichy et al., 2014; Nowak et al., 2016). After MgHPO4(s) dissolved in hydrochloric acid solution (Eq.(g)), melamine was added to the solution to form melamine phosphate precipitate (Eq.(h)) wh ich was then neutralized by sodium carbonate solution or sodium hydro xide solution to produce Na 2 HPO4 and melamine (Eq. (i, j)). The by-product of this method was MgCl2 ·6H2 O.

2.3. Analytical methods

(f) (g) (h) (i) (j)

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Mg 2+ + HPO4 2- = MgHPO4 ↓ MgHPO4 + HCl = MgCl2 + H3 PO4 C3 H6 N6 + H3 PO4 = C3 H6 N6 ∙ H3 PO4 ↓ C3 H6 N6 ∙ H3 PO4 + 2NaOH = C3 H6 N6 + Na2 HPO4 + 2H2 O C3 H6 N6 ∙ H3 PO4 + Na2 CO3 = C3 H6 N6 + Na2 HPO4 + CO2 + H2 O

The concentrations of Mg 2+, Li+, Na+ , and K+ in the brine samples were determined by ato mic absorption spectrometry (AAS, AA800, Perkin - Elmer). The chloride and boron were analyzed by silver nitrate t itration and mannitol conversion acid base titration, respectively. The total phosphorus content was determined by the quinoline phosphomolybdate gravimetric method. The phase structures of samples were characterized by X-ray diffraction analysis (XRD, DX-2700, Fangyuan) using Cu-Kα radiation. The morphology of samples was characterized by a scanning electron microscope (SEM, ProX, Phenom). 3. Results and discussion 3.1 Removal of magnesium by carnallite crystallization 3.1.1 Phase diagram analysis The crystallization route of carnallite is predicted by the phase diagram of the KCl-Mg Cl2 -H2 O system at 25℃ (Fig.2), because other minor co mponents in the brine have litt le influence on the format ion of carnallite. Fig .2 can be d ivided into six reg ions: the liquid phase region (KEF0), the KCl crystallization reg ion (AEK), the KCl and carnallite crystallization region (A EC), the carnallite crystallization reg ion (CEF), the carnallite and bischofite crystallization reg ion (CDF) and the solid phase region (ABD). The system point G outside the CEF region represents the initial co mposition of Mg 2+ and K+. In order to separate MgCl2 in the form of carnallite, addit ional KCl is required to move point G into the CEF region, and the new system point H must be on line A G. To get as much carnallite as possible, point H should be on the intersection point between line AG and line CE. Content of KCl at point H (8.30%) is obtained through dissolving additional KCl in the brine by heating, and the brine is cooled at 25℃ to form carnallite. Carnallite S1 and filtrate L1 are obtained by filtering. After that, system point of filtrate L1 moves to system po int E. Because point E is on the saturated liquidus, extra KCl is needed to get the system point back to the CEF region. The new

Pre-proof system point I must be on line A E which is outsideJournal the CEF area because the concentration of Mg Cl 2 decreases in the filtrate L1 . In order to increase the concentration of MgCl2 , L1 needs to be evaporated and the new system point J must be on the extension line o f WI. And to make full use of KCl, the system point J should be on the intersection point between line C G and the extension line o f WI. Accordingly, point J can be obtained by adding KCl and evaporating water. After carnallite was formed by cooling crystallization, carnallite S2 and filtrate L2 are obtained by filtration. The system point of filtrate L2 will move to G’. Therefore, the path of the above two steps is G→H→E→I→J→G’, wh ich is a closed loop path. The composition of key points is listed in Table 2. Nevertheless, the amount of KCl added in the second step should be determined through experiments to ensure a low lithiu m loss rate, because Fig.2 cannot show the entrainment loss of lithiu m ions. System points I and J given in Fig.2 are under the most suitable conditions. Table 2 Solubility of KCl and MgCl2 in the KCl-MgCl2 -H2 O system and key points of carnallite crystallization at 25℃ (Deng et al., 2013). Composition of liquid phase (%)

F

H2 O

26.5

0.00

73.5

KCl

20.3

5.00

74.7

KCl

14.9

10.0

75.1

KCl

10.5

15.0

74.5

KCl

6.70

20.0

73.3

KCl

4.10

25.0

3.40

26.9

1.10 0.10 0.10

H

8.30

I

8.20 10.4 0.00

C

26.8

KCl+Car

30.0

68.9

Car

35.0

64.9

Car

35.6

64.3

Car+Bis

35.7

64.3

Bis

31.0

68.9

Car.

28.4

63.3

25.5

66.3

32.3

57.3

46.8

53.2

34.2

39.0

Car.

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KCl

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E

MgCl2

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Car. -Carnallite, KCl·MgCl2·6H2O; Bis. - Bischofite, MgCl2 ·6H2O.

Fig.2. Phase diagram of KCl-MgCl2 -H2 O system at 25℃ and the evaporation-crystallization path (C-carnallite; D-bischofite).

Journal Pre-proof 3.1.2 Synthesis conditions of carnallite The amount of KCl added to the brine was determined at 36.8% of the stoichio metric amount in the first step of crystallizat ion according to the system point H discussed above. As the solubility of KCl varies with temperature, the co mposition of solid and mother liquor obtained at different heating temperatures was investigated. The results are shown in Table 3 and Fig. 3. Table 3 Composition of liquid phases and solid phases after the first step of crystallization at different temperature. Temp. (℃)

Composition of solid phase (wt .%)

Mg

Li+

Na+

K+

Cl-

Vol. (L)

Mg2+

Li+

Na+

K+

Cl-

Wt. (g)

93.58 90.75 89.51 89.33 89.62

2.73 2.79 2.80 2.79 2.80

2.19 2.24 2.23 2.24 2.24

13.64 15.74 19.17 21.97 22.48

301.64 296.65 296.19 298.14 299.55

0.903 0.877 0.873 0.874 0.869

7.38 8.05 8.37 8.51 8.54

0.015 0.018 0.020 0.021 0.022

0.009 0.012 0.014 0.015 0.016

19.40 16.44 15.08 14.43 14.30

39.22 38.52 38.20 38.05 38.02

251.11 287.33 293.85 289.47 290.16

Solid phase (XRD)

Evaporation (g)

Car.+ KCl

7.78 8.92 11.6 16.0 20.5

Car.+ KCl Car.+ KCl Car. Car.

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35 45 55 65 75

Composition of liquid phase (g L-1 ) 2+

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Fig.3. Effect of dissolution temperature on the removal efficiency of Mg and Li.

Fig.4. XRD patterns of carnallite obtained via the first step of crystallization at: (a) 75℃; (b)65℃; (c)55℃; (d)45℃; (e)35℃.

As shown in Fig.3, the removal of magnesium rises with the increasing temperature and tends to be stable at about 24.0%, indicating that KCl can be co mpletely dissolved when the temperat ure is over 55℃. This can be proved fro m Fig.4 that when the temperature is below 55℃, the phase of KCl is apparent. Meanwhile, the evaporation of water increases rapidly and reaches 1.5% finally and the removal of lithiu m increases slightly and stabilizes at around 2.5%. Therefore, the most suitable d issolution temperature is chosen at 55℃.

Journal Pre-proof In the second step of crystallization, the evaporation temperature was chosen at 80℃ accord ing to the reference data (Zhang et al., 2016). The results of different additions of KCl on the evaporation efficiency are shown in Tab le 4 and Fig.5. In order to make full use of KCl added, the final system point of filtrate L2 was selected near the initial co mposition of the brine. As shown in Fig.4, the removal efficiency of magnesium rises with the increasing amount of KCl and evaporated water. At the same t ime, the lithium loss increases with the addition of KCl and this trend is significant when the amount of KCl is more than 19.0%. The XRD diagram of carnallite obtained by the second step of crystallizat ion is shown in Fig.6. At KCl addition below 15.9%, carnallit e is the only phase. However, b ischofite appears at KCl addit ions higher than 15.9% wh ich may be due to local supersaturation and slightly excessive evaporation. Therefore, to reduce the loss of lith iu m and energy consumption, the most suitable amount of KCl added in the second step is chosen at 19.1%. Table 4 Composition of liquid phases and solid phases after the second step of crystallization. Composition of solid phase (wt .%)

Mg

Li+

Na+

K+

Cl-

Vol. (L)

Mg2+

Li+

Na+

K+

Cl-

Wt. (g)

100.83 102.67 102.94 104.91 106.78

4.39 4.74 5.14 5.25 5.44

3.52 3.80 4.18 4.27 4.43

4.46 5.55 5.93 8.12 10.94

325.93 334.53 337.63 346.74 355.97

0.551 0.506 0.467 0.430 0.395

8.71 8.76 8.81 8.76 8.77

0.009 0.015 0.021 0.049 0.072

0.008 0.012 0.012 0.032 0.049

13.60 13.42 13.14 13.30 13.28

37.78 37.77 37.74 37.95 38.09

258.95 298.74 340.41 374.93 408.60

Solid phase (XRD) Car. Car. Car. Car.+ Bis. Car.+ Bis.

Evaporation (g)

208.2 233.2 245.6 259.7 277.9

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40 50 60 70 80

Composition of liquid phase (g L-1 ) 2+

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KCl added (g)

Fig.5. Effect of KCl dosage on the removal efficiency of Mg and Li.

Fig.6. XRD patterns of carnallite obtained via the second step of crystallization with KCl at: (a)25.4%; (b)22.2%; (c)19.1%; (d)15.9%; (e)12.7%.

To sum up, at the most suitable conditions, the Journal total amount Pre-proof of KCl added to the brine was 55.9% (36.8%+19.1%=55.9%) of the stoichiometric amount, the evaporation was 17.6% ((11.6+245.6)/(1285.8+175.8)×100%=17.6%) which was 60.7% of the result (29.0%) reported in the literature (Zhang et al., 2016) in the case of similar magnesium content . The total remo val efficiency of magnesium accounted for 53.1% (24.0%+29.1%=53.1%) of the total amount of magnesium in the brine, and the total loss of lithiu m was 5.40% (2.36%+3.04%=5.40%). After two steps of crystallization, the concentrations of components in filtrate L2 is shown in Table 5. Table 5 Concentrations of components in filtrate L2 . Ion/oxide (g L-1 )

Mg2+

Li+

Na+

K+

B2 O3

Cl-

Mg/Li ratio

Filtrate L2

102.9

5.1

4.1

5.9

0.4

337.6

20.18

3.2 Removal of magnesium by MgHPO4 precipitation

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3.2.1 Thermodynamic analysis of MgHPO4 precipitation In order to locate the most suitable conditions for Mg2+ removal by the precipitation of MgHPO4(s) fro m the brine, phosphate species in the solution at different pH should be studied at first. Because phosphoric acid is a tribasic acid, there are mu ltistage ionization in the solution and four species can be produced, namely H3 PO4 , H2 PO4 -, HPO4 2-and PO4 3-. The distribution of phosphate anions at pH 0-14 was calcu lated by the fo llo wing equations (Eq.(k-p)). And the relat ionship between fraction of each species (α) and pH values is shown in Fig. 7.

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[P]t =[H3 PO4 ] +[H2 PO4 -] +[HPO4 2-] +[PO4 3-] (k) φ=10pK1+pK2+pK3-3pH+10pK2+pK3-2pH+10pK3-pH+1 (l) α(PO4 3-) =1/φ (m) α(HPO4 2-) =10pK3-pH /φ (n) α(H2 PO4 -) =10pK2+pK3-2pH/φ (o) α(H3 PO4 ) =10pK1+pK2+pK3-3pH/φ (p) The terms [P]t (mol/ L) denotes the total concentration of phosphorus, φ signifies the coefficient of total concentration of phosphorus, α designates fraction of H3 PO4 , H2 PO4 -, HPO4 2-and PO4 3-.

Fig.7. Relationship between fraction of each species (α) and pH.

As shown in Fig.7, a phosphate anion is dominant in a certain range of pH, which is conducive to the format ion of corresponding salt. The predominant areas are H3 PO4 at pH=0-2, H2 PO4 at pH=2-7, HPO4 2- at pH=7-12 and PO4 3- at 12-14. 2+ + + + Meanwhile, the main cat ions in the brine are Mg , Li , Na , K . The in fluence of Na +, K+ is not considered here, as the phosphate of Na+, K+ are soluble salts. Table 6 shows the reaction of transformation of various insoluble phosphate salts of Mg 2+, Li+ such as MgHPO4(s), Mg 3 (PO4 )2(s), Li3 PO4(s) and Mg(OH)2(s). Table 6 T ransforming reactions of precipitates and determination of reaction direction at the standard state. No.

Reactions

1

MgHPO4(s) + 2PO43- → Mg3 (PO4) 2(s) +3HPO42-

2

Mg3 (PO4 )2(s) + 6OH- → 3Mg(OH) 2(s) + 2PO4 3-

K 10

7.88

(K10 )

10 7.84

△G0 = -RT lnK

Reaction direction

＜0

Forward

＜0

Forward

Mg3 (PO4 )2(s) + 6Li+ → 2Li3 PO4(s) + 3Mg2+

3

Journal Pre-proof 10

＞0

-7.4

Reverse

K represents equilibrium constant, R=8.314J/(mol·K), T=273.15K.

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According to Table 6, the spontaneous transformat ion of MgHPO4(s) to Mg 3 (PO4 )2(s) and Mg3 (PO4 )2(s) to Mg(OH)2(s) is thermodynamically feasible. On the basis of the distribution of phosphate anions in Fig.7 and data from Table 6, the sequence of precipitate fo rmation is MgHPO4(s), Mg 3 (PO4 )2(s), Mg(OH)2(s) with the increase of pH. In addit ion, Mg 3 (PO4 )2(s) cannot be converted to Li3 PO4(s) spontaneously in the standard state because of △G0 ＞0. On ly when Mg 3 (PO4 )2(s) is transformed to Mg(OH)2(s) and free PO4 3- is produced, or when Mg 3 (PO4 )2(s) is formed and excess PO4 3- exists in the solution can Li3 PO4(s) be produced. Therefore, in the process of MgHPO4(s) precipitation, the mo lar amount of precipitant (Na 2 HPO4 ) should be equal to that of Mg 2+ in order to avoid Li3 PO4(s). According to the thermodynamic princip le, △G0 is the change of free energy of reactants and products in the standard state and it can only judge the change direction of reaction under this specific condition. For the actual state, Eq.(q) is used as the criterion. △GT = -RT ln Kc + RT ln Jc (q) When Jc < Kc, △GT < 0, the reaction proceeds to the right. When Jc = Kc, △GT = 0, the reaction reaches equilibrium. When Jc > Kc, △GT > 0, the reaction proceeds to the left. Jc is the activity ratio o f product to reactant in the actual system. Because of the lack of th ermodynamic data, it is impossible to calculate the strength of each ion. At the same time, the concentration o f each ion in the system is large, so the activity is replaced by the concentration. For reaction (1) in Table 6, in order to obtain more MgHPO4(s), the reaction should proceed to the left, as follows: J10 = [HPO4 2-]3 /[PO4 3-]2 > K10 Under given [P]t and pH, concentrations of HPO4 2- and PO4 3- were calculated according to the fraction of phosphate ions in Fig.7. And the relationship between J 10 and pH are shown in Fig.8.

Fig.8. Relationship between J10 and pH at [P] t=4.0mol.

As can be seen from Fig.8, J10 decreases rapidly with the increase of pH. Line A B represents the equilibriu m line of react ion (1). When pH＜8.25, J10 is greater than K10 , which is beneficial to the formation of MgHPO4(s). Therefore, the format ion condition for MgHPO4(s) is pH < 8.25. In the process of MgHPO4(s) precipitation, 14 species (H+, OH-, PO4 3-, HPO4 2-, H2 PO4-, H3 PO4 , Li+, LiOH, LiHPO4 , Mg 2+, + + MgOH , MgPO4 , MgHPO4(aq), MgH2 PO4 ) may correspondingly exist in solution according to the literature report (He et al., 2017). Equilibriu m reactions for the 14 species and mass balance equations of the system are shown in Table 7. According to the equations in Table 7, the magnesium soluble species distribution, the total magnesium, lithiu m and phosphate concentrations at given pH values in the solution were calculated by Microsoft Excel 2016. The results are shown in Fig.9. Table 7 Equilibrium reactions, constants and mass balance equations for the Li+-Mg2+-HPO42--H2O system at 298K.a Eq no. 1 2 3 4

Equilibrium reactions H2 O=H+ +OHH3 PO4 = H+ + H2 PO4H2 PO4 - = H+ + HPO4 2HPO4 2- = H+ + PO4 3-

Equilibrium constant -14.0 -2.04(K1 ) -7.20(K2 ) -12.36(K3 )

Equation 10 -14 =[H+][OH-] 10 -2.04 =[H+][ H2PO4- ]/[ H3 PO4 ] 10 -7.2 =[H+][ HPO4 2-]/[ H2 PO4-] 10 -12.36=[H+][ PO4 3-]/[ HPO4 2-]

5 6 7 8 9 10 11 12 13 14

Mg2+ + OH- = MgOH+ Mg2+ + PO4 3-=MgPO4 Mg2+ + HPO4 2- =MgHPO4(aq) Mg2+ + H2 PO4 - =MgH2PO4+ Li+ + OH- = LiOH Li+ + HPO4 2- = LiHPO4MgHPO4(s) = Mg2+ + HPO42Mg3 (PO4 )2(s) = 3Mg2++2PO43Mg(OH) 2(s) = Mg2+ + 2OHLi3 PO4(s) = 3Li+ + PO43-

Journal Pre-proof 2.57 4.80 2.91 0.45 0.36 -0.72 -5.50 -24.38 -10.74 -8.49

10 2.57=[MgOH+]/{[Mg2+][OH-]} 10 4.8 =[ MgPO4- ]/{[Mg2+][ PO4 3-]} 10 2.91=[ MgHPO4]/{[Mg2+][ HPO4 2-]} 10 0.45=[ MgH2PO4+]/{[Mg2+][ H2 PO4- ]} 10 0.36=[ LiOH]/{[Li+][ OH- ]} 10 -0.72 =[LiHPO4- ]/{[Li+][ HPO42-]} 10 -5.5 =[Mg2+][ HPO42 -] 10 -24.38=[Mg2+] 3[ PO43 -] 2 10 -10.74=[Mg2+][ OH-] 2 10 -8.49 =[Li+] 3[ PO43 -]

T he balance equations of the systems. 15 [P] T = [ H3 PO4 ] + [ H2 PO4-] + [ HPO42-] + [ PO4 3-] + [MgPO4-] + [MgHPO4(aq)] + [MgH2 PO4 +] + [LiHPO4-] 16 [Mg] T = [Mg2+] + [MgOH+] + [MgPO4 -] + [MgHPO4(aq)] + [MgH2 PO4+] 17 [Li] T = [Li+] + [LiOH] + [LiHPO4 -] 18 [Mg] Ti-[Mg] T=[P]Ti-[P] T

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K1 , K2 and K3 are the first, second and third ionization constant of H3PO4 ; [P] T designates the total concentration of phosphorus; [Mg] T designates the total concentration of magnesium; [Li] T designates the total concentration of lithium. a T hermodynamic data were obtained from [Kotrlý et al., 1985; Speight et al., 2004].

Fig.9. Magnesium soluble species distribution (a) and Log C-pH diagram of the Li+-Mg2+-HPO4 2--H2 O system (b). Conditions: [Li] Ti=0.7 mol L-1 , [Mg] Ti=4.0 mol L-1 , [P] Ti=4.0 mol L-1 . -

As can be seen from Fig.9 (a), the concentration of MgPO4 and MgOH+ is close to 0, and the concentration of Mg HPO4(aq) keeps rising, mainly related to the increase of MgHPO4(s) production with the increase of p H. Content of MgH2 PO4 + decreases with the increase of pH and the content of Mg 2+ increases first and then decreases. As shown in Fig.9 (b), the total lithium

Journalindicating Pre-proof concentration in the solution remains basically unchanged, that Li+ is difficult to precip itate in this pH range. The curves of total magnesium and total phosphorus are similar in shape and decrease rapidly with the increase of pH value, indicating that large extent of precip itation occurs. But when pH＞6.5, the curves of total magnesium and total phosphorus no longer coincide, and the curves of total magnesium tend to be flat, while the curves of tot al phosphorus continue to decline, indicating that a small amount of phosphate participates in the precipitation reaction. Considering that the total magnesium tends to be constant, the decrease of total phosphorus content is caused by the formation of trace Li3 PO4(s). Therefore, in order to av oid Li3 PO4(s), it is appropriate to adjust the pH of MgHPO4(s) reaction to about 6.5. 3.2.2. Effect of feeding methods on magnesium precipitation Due to the low solubility of MgHPO4(s) in water, addit ion of too much water will lead to a reduction in removal efficiency of magnesiu m. Therefo re, two feed ing methods were investigated, which were adding solid Na 2 HPO4 or saturated solution of Na2 HPO4 . The experiment was carried out under the conditions that the initial concentration of Mg 2+ was 102.9 g/ L, the init ial pH value of brine was 6.7, the amount of Na 2 HPO4 was 1.0 time of the stoichiometric amount, reaction temperature was 25℃, reaction time was 30 mins and aging time was 1 h. The results are shown in Table 8. Table 8 Effect of feeding methods on magnesium precipitation. Solid Na2 HPO4

Saturated solution of Na 2 HPO4

3.96 2.97 93.8 5.90 1.30 6.60

0.32 2.92 99.5 6.80 0.10 6.50

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Concentration of Mg in filtrate (g L-1 ) Concentration of Li in filtrate (g L-1 ) Removal efficiency of Mg (%) Loss of Li (%) Final mass ratio of Mg/Li Final pH value in filtrate

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Table 8 shows that a lower removal efficiency of magnesium is obtained by adding solid Na 2 HPO4 . That is because the solid Na2 HPO4 does not dissolve completely in a short time, a s mall part of wh ich is encapsulated by MgHPO 4(s) produced by the reaction, so that the precipitation reaction is incomplete. Due to the small d ifference in lithiu m loss by the two feeding methods, adding saturated solution of Na 2 HPO4 is adopted as the feeding method. In addition, the pH value of the solution changes lit tle before and after the reaction, which is in accordance with the results of thermodynamic analysis above. Therefore, this magnesiu m ext raction process does not lead to significant changes on the pH value of the salt-lake brine which is beneficial to protect the salt-lake ecosystem.

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3.2.3. Separation results of magnesium precipitation In order to further study the operating conditions, the dosage of precipitant, react ion temperature, reaction time and aging t ime were considered. The results are shown in Table 9. Table 9 Effects of reaction temperature, dosage of Na 2HPO4, reaction time and aging time on the separation of Mg and Li. No.

1

2

Temperature (℃)

25

40

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

60

80

90

40

40

40

40

40

40

40

40

40

40

40

40

40

100

100

100

100

100

~100

~100

~100

~100

100

100

100

100

100

100

100

100

100

Reaction time (min)

30

Aging time (h)

1

30

30

30

30

30

30

30

30

10

20

40

50

60

30

30

30

30

1

1

1

1

1

1

1

1

1

1

1

1

1

2

3

4

C Mg in filtrate (g L-1 )

5

0.32

0.78

0.91

1.41

3.18

0.44

0.31

0.25

0.19

4.48

1.71

0.63

0.51

0.41

0.51

0.51

0.51

0.51

C Li in filtrate (g L-1 )

2.92

3.06

3.12

3.17

3.20

3.02

3.00

3.02

2.99

3.15

3.09

3.05

3.02

3.01

3.07

3.10

3.10

3.10

R Mg (%)

99.5

99.0

98.5

97.8

95.1

99.3

99.5

99.6

99.7

93.1

97.3

99.0

99.2

99.3

99.2

99.2

99.2

99.2

Loss of Li (%)

6.8

3.9

1.5

0.4

0.3

4

4.1

4.1

4.2

2.2

3.1

4.0

4.1

4.0

2.6

1.5

1.4

1.4

Mg/Li ratio

0.11

0.25

0.29

0.44

0.99

0.15

0.1

0.08

0.06

1.42

0.55

0.21

0.17

0.15

0.17

0.16

0.16

0.16

Dosage of Na2 HPO4 (%)

As shown in Table 9, with the reaction temperature rising fro m 25℃ to 90℃, the removal efficiency of magnesium decreases slightly fro m 99.5% to 95.1%, indicating that the solubility of Mg HPO4(s) increases at a higher temperature. Meanwhile, the removal o f lithiu m declines fro m 6.8% to 0.3% and the mass ratio of Mg/Li increases from 0.11 to 0.99. That is exp lained by the fact that the higher temperature is conducive to the format ion of larger particles of MgHPO 4(s) (He et al., 2017), which is beneficial to reduce the loss of lithiu m entrain ment. Therefore, considering the recovery efficiency of lithiu m and energy consumption, the most suitable reaction temperature is chosen at 40℃. Obviously, there is little effect on raising the remo val efficiency of magnesium by increas ing the amount of Na 2 HPO4. Thus, it is better to choose the amount of Na 2 HPO4 at 1.0 time of the stoichiometric requirement. Table 9 also shows that the removal efficiency of magnesium increases from 93.1% to 99.3% with prolonging the reaction time fro m 10 mins to 60 mins. When the reaction time is 30 mins, the removal efficiency of magnesium

Journal Pre-proof reaches a relatively great value of 99.1% which increases slightly in the next 30 mins, indicating that the reaction is complete in 30 mins. Meanwhile, the change of lithiu m loss is not significant in the same conditions, but the mass ratio of Mg/Li drops from 1.42 to 0.15. In order to make the react ion comp leted, the most suitable reaction time is chosen at 30 mins. When the aging time is 3 h, the removal efficiency of magnesium is 99.2%, and the loss of lithiu m reaches the minimu m value of 1.5% and almost no longer changes in the next 2 h. Hence, it is appropriate to choose the aging time for 3 h. The concentrations of components in filtrate L3 after precipitation of MgHPO4(s) under the most suitable conditions is shown in Table 10. The material balance for magnesiu m and lithiu m through the crystallization -precipitation process is listed in Table 11. It can be seen that the material balance of Mg and Li in the whole process is basically equal. The slight difference may be due to the experimental operation error and analysis error. Therefore, the data obtained by this method are reliable. Table 10 Concentrations of components in filtrate L 3 . Mg2+

Li+

Na+

K+

B2 O3

Cl-

P 2 O5

Mg/Li ratio

Filtrate L 3

0.44

2.7

104.8

3.1

0.2

183.5

1.3

0.16

Table 11 Material balance of Mg and Li by the process of crystallization-precipitation. Composition of liquid phase (g L-1 ) Li 2.50 2.80 5.14 2.70

Brine Crystallization I Crystallization II Precipitation of MgHPO4(s)

Vol. (L) 1.000 0.873 0.467 0.867

Composition of solid phase (wt.%)

Li+ 0.020 0.021 0.007

Mg2+ 8.37 8.81 9.77

Wt.(g) 293.85 340.41 488.54

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102.80×1.000-8.37%×293.85-8.81%×340.41-9.79%×488.54-0.44×0.867=0.103g, Relative error = 0.103÷102.80×100 = 0.10% 2.50×1.000-0.020%×293.85-0.021%×340.41-0.008%×488.54-2.70×0.867= -0.005g, Relative error = -0.005÷2.50×100 = -0.20%

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Material balance of Mg Material balance of Li

Mg2+ 102.8 89.5 102.9 0.44

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Ion/oxide (g L-1 )

dC A = kC AmCBn dt dC  dCA = kC An ' 2 A  C dtdC = kC t A A 1  dt 21   kdt 

A

0 C A  kt dCCA A,0 2

C A ,0

rCA

dt

 1.07C A

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3.2.4 Kinetics of MgHPO4 precipitation According to the experimental results, the precipitation of MgHPO4 is basically co mpleted in 1 h. Therefore, the kinetics of the precipitation react ion in 1 h is studied. The reaction rate equation can be exp ressed as Eq. (r). As the stochiometric coefficients of Mg 2+ and HPO4 2- are equal to 1 and the in itial concentration of CA is equal to that of CB, the concentration of the two reactants is equal at any time. Thus , Eq. (r) can be expressed as Eq. (s). Generally, the differential method, half-life method and trial meth od are used to determine the reaction order. Since the rate equation of a similar chemical reaction conforms to the second-order rate equation, the trial method can be used, assuming that the order o f the rate equation is 2 as Eq.(t ). Then linear regression analysis is carried out to determine the rate constant k and R2 (correlation coefficient) used to determine whether the equation accords with the second-order rate equation. Therefore, Eq.(t) is integrated into Eq.(u) and Eq.(v). (r)

(s) (t) (u) (v) (w)

The terms CA (mo l/ L) and CB (mo l/ L) denote the concentrations of Mg 2+ and HPO4 2- respectively, k represents the rate constant, m and n are the order of reaction, n’ is the sum of m and n, CA,0 (mol/ L) is the initial concentration of Mg2+, r (mol/(L·min)) is the reaction rate. Fig.10 is drawn with t ime as an independent variable and 1/ CA -1/ CA,0 as a function, where CA can be obtained by experiments. Kinetic fitting equation of MgHPO4 precip itation is y=1.07 x-5.08, and the value of R2 is 0.99 wh ich shows good lin ear correlation. Therefore, the precip itation reaction of MgHPO4 is a second-order reaction with a rate constant k of 1.07 L/(mo l· min). The rate equation (Eq.(w)) of this reaction is obtained by introducing k into Eq.(t).

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Fig.10. Kinetic Fitting Curve of MgHPO4 precipitation reaction.

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3.2.5 Identification of MgHPO4 The solid obtained by MgHPO4 precip itation method under the most suitable conditions were analy zed and tested. From the XRD result shows in Fig.11, it can be seen that the diffraction peaks o f the product agree well with that of MgHPO4 ·3H2 O (PDF card: 70-2345). Moreover, Fig.12 indicates that MgHPO4 ·3H2 O obtained is a bulky grain with good crystallinity, thus showing good filtering performance for lithium.

Fig.11. XRD pattern of the precipitate obtained via MgHPO4 precipitation method.

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Fig.12. SEM image of MgHPO4(s) obtained at stoichiometric amount of Na2 HPO4 , reaction temperature 40 ℃, reaction time 30 mins and aging time 3 h.

4. Conclusions

Acknowledgements

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In order to recover lithiu m fro m brine with high Mg/Li mass ratio, a combined method of carnallite crystallization and MgHPO4 precipitation is proposed. In the first stage, about half of magnesium content in the raw brine is removed by formi ng carnallite. When the amount of solid KCl added to the brine is 55.9% of the stoichio metric amount and the evaporation efficiency at 17.6%, the removal efficiency of magnesium accounts for 53.1% of the total magnesium in the brine and the loss of lithium corresponds to 5.40%. In the second stage, residual magnesium is removed by forming MgHPO4(s). With the amount of Na 2 HPO4 at stoichiometric requirement, reaction temperature 40℃, reaction time 30 mins and aging time 3 h, the removal efficiency of residual magnesium is 99.2% and the recovery of residual lith iu m is 98.5 %. After two stages of treatment, the total remo val efficiency of magnesium is 99.6% (55.9%+99.2%×(1-0.559)=99.6%) and the total recovery of lithium is 93.2% (98.5%× (1-0.054)=93.2%) and the mass ratio of Mg/ Li is dropped from 41.1 to 0.16 wh ich is low enough for the production of lithium carbonate in industry. Therefore, the crystallization-precip itation method can be used for extracting lithiu m fro m brine with high Mg/Li ratio effectively. Meanwhile, MgCl2 ·6H2 O is recovered as a by-product. In addition, KCl and Na2 HPO4 used in this process can be recycled, which will significantly reduce the cost.

This work was supported by the research scholarship from Chengdu University of Technology. References

An, J.W., Kang, D.J., Tran, K.T., Kim, M.J., Lim, T., Tran, T., 2012. Recovery of lithium from Uyuni salar brine. Hydrometallurgy 117-118, 64–70. Chitrakar, R., Kanoh, H., Miyai, Y., Ooi, K., 2000. A new type of manganese oxide (MnO 2·0.5H2O) derived from Li1.6 Mn 1.6O4 and its lithium ion-sieve properties. Chem. Mater. 12, 3151-3157. Cichy, B., Kuzdzal, E., 2014. Obtaining monodisperse melamine phosphate grains by a continuous reaction crystallization proce ss. Ind. Eng. Chem. Res. 53: 6593-6599. Deng, T.L., Zhou, H., Chen, X., 2013. Salt-water system phase diagrams and application, first ed. Chemical Industry Press, Beijing, pp. 300-301. FinancialBuzz.com, 2018. Lithium demand projected to grow with electric vehicle market. https://www.autoblog.com/press-releases/lithium-demand-projected-to-grow-with-electric-vehicle-market_11661/ (accessed 10 April 2018). He, L., Xu, W., Song, Y., Liu, X., Zhao, Z., 2017. Selective removal of magnesium from a lithium -concentrated anolyte by magnesium ammonium phosphate precipitation. Sep. Purif. Technol. 187, 214-220. Ji, Z., Chen, Q., Yuan, J., Liu, J., Zhao, Y., Feng, W., 2017. Preliminary study on recovering lithium from high Mg2+/Li+ ratio brines by electrodialysis. Sep. Purif. Technol 172, 168-177. Kim, S., Lee, J., Kang, J.S., Jo, K., Kim, S., Sung, Y.-E., Yoon, J., 2015. Lithium recovery from brine using a λ-MnO2 /activated carbon hybrid supercapacitor system. Chemosphere 125, 50-56. Kotrlý, S., Sucha, L., 1985. Handbook of Chemical Equilibria in Analytical Chemistry, Ellis Horwood, Chichester. Lassin, A., Andre, L., Lach, A., Thadee, A.-L., Cezac, P., Serin, J.-P., 2018. Solution properties and salt -solution equilibria in the

Journal Pre-proof

Jo ur

na

lP

re

-p

ro of

H-Li-Na-K-Ca-Mg-Cl-H2O system at 25 degrees C: A new thermodynamic model based on Pitzer’s equations. CALPHAD 61: 126-139. Liu, X., Zhong, M., Chen, X., Zhao, Z., 2018. Separating lithium and magnesium in brine by aluminum-based materials. Hydrometallurgy 176, 73-77. Nowak, M., Cichy, B., Kuzdzal, E., 2016. Kinetic analysis of the process of melamine pyrophosphate synthesis. J. Therm Anal Calorim 124: 329-339. Speight, J.G., Lange, N.A., 2004. Lange’s Handbook of Chemistry, 16th ed., McGraw-Hill Professional, Maidenhead. Song, J., Huang, T., Qiu, H., Li, X., He, T., 2017. Recovery of lithium from salt lake brine of high Mg/Li ratio using Na[FeCl4 ∗2TBP] as extractant: T hermodynamics, kinetics and processes. Hydrometallurgy 173, 63-70. Tran, K.T., Han, K.S., Kim, S.J., Kim, M.J., Tran, T., 2016. Recovery of magnesium from Uyuni salar brine as hydrated magnesi um carbonate. Hydrometallurgy 160, 106-114. Wang, G., Li, J.G., 2016. Salt chemical engineering and technology, first ed. Tsinghua University Press, Beijing, pp. 99-100. Wang, S., Li, P., Zhang, X., Zheng, S., Zhang, Y., 2017. Selective adsorption of lithium from high Mg-containing brines using HxTiO3 ion sieve. Hydrometallurgy 174, 21-28. Wang, H.Y., Zhong, Y., Du B.Q., Zhao, Y.J., Wang, M., 2018. Recovery of both magnesium and lithium from high Mg/Li ratio brines using a novel process. Hydrometallurgy 175, 102-108. Xiang, W., Liang, S., Zhou, Z., Qin, W., Fei, W., 2017. Lithium recovery from salt lake brine by counter-current extraction using tributyl phosphate/FeCl3 in methyl isobutyl ketone. Hydrometallurgy 171, 27-32. Xiao, C., Xiao, L., Gao, C., Zeng, L., 2015. Thermodynamic study on removal of magnesium from lithium chloride solutions using phosphate precipitation method. Sep. Purif. Technol 156, 582-587. Xu, X., Chen, Y., Wan, P., Gasem, K., Wang, K., He, T., Adidharma, H., Fan, M., 2016. Extraction of lithium with functionaliz ed lithium ion-sieves. Prog. Mater. Sci. 84, 276-313. Yang, G., Shi, H., Liu, W., Xing, W., Xu, N., 2011. Investigation of Mg2+/Li+ separation by nanofiltration. Chinese J. Chem. Eng. 19(4), 586-591. Zandevakili, S., Ranibar, M., Ehteshamzadeh, M., 2014. Recovery of lithium from Urmia Lake by a nanostructure MnO 2 ion sieve. Hydrometallurgy 149, 148-152. Zhao, M.-Y., Ji, Z.-Y., Zhang, Y.-G., Guo, Z.-Y., Zhao, Y.-Y., Liu, J., Yuan, J.-S., 2017. Study on lithium extraction from brines based on LiMn 2 O4 /Li1-x Mn2O4 by electrochemical method. Electrochim. Acta 252, 350-361. Zhang, L.-F., Zhang, D.-Y., Ouyang, H.-Y., Wang, Q.-Y., Ning, S.-M., Wan, H.-Q., 2016. Technique for separating magnesium and lithium from salt lake brine with high Mg/Li ratio. Ming and Metallurgical engineering 36(4): 83-87.

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Highlights  A closed-loop path for carnallite crystallization is proposed through adding KCl.  99% of Mg2+ in brine can be removed by the crystallization-precipitation method.

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 KCl and Na2HPO4 used in the process can be recycled.