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Extraction kinetics of lithium from salt lake brine by N,N-bis(2-ethylhexyl) acetamide using Lewis Cell
Accepted Manuscript Extraction kinetics of lithium from salt lake brine by N,Nbis(2-ethylhexyl) acetamide using Lewis Cell
Hui-Fang Li, Li-Juan Li, Xiao-Wu Peng, Lian-Min Ji, Wu Li PII: DOI: Reference:
S0304-386X(18)30025-2 doi:10.1016/j.hydromet.2018.03.022 HYDROM 4786
To appear in:
Hydrometallurgy
Received date: Revised date: Accepted date:
9 January 2018 21 March 2018 31 March 2018
Please cite this article as: Hui-Fang Li, Li-Juan Li, Xiao-Wu Peng, Lian-Min Ji, Wu Li , Extraction kinetics of lithium from salt lake brine by N,N-bis(2-ethylhexyl) acetamide using Lewis Cell. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Hydrom(2017), doi:10.1016/ j.hydromet.2018.03.022
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Extraction Kinetics of Lithium from Salt Lake Brine by N,N-bis(2-ethylhexyl) Acetamide Using Lewis Cell Hui-fang LI1,2,3,4 , Li-juan LI1*, Xiao-wu PENG 1,3, Lian-min JI1 , Wu LI1,2* 1 Key Laboratory of Comprehensive and Highly Efficient Utilization of Salt Lake Resources, Qinghai Institute of Salt Lakes, Chinese Academy of Sciences, Xining, 810008, China; 2 Key Laboratory of Salt Lake Resources Chemistry of Qinghai Province, Xining, 810008, China; 3 University of Chinese Academy of Sciences, Beijing, 10049, China
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4 Qinghai University, Xining, 810010, China
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Abstract: Lithium, as the green energy, its application and exploitation attract much attention. The lithium extraction kinetics with N,N-bis(2-ethylhexyl) acetamide(N523) in sulfonated kerosene has been investigated experimentally using Lewis Cell. The effects of diffusion resistance area, interfacial area, temperature and the concentration of reagents were studied. The results showed that the diffusion resistance mainly existed in aqueous phase, the
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extraction process was a diffusion-controlled kinetics process with an interfacial reaction and
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Keywords: Lithium; Extraction kinetics; Mechanism;
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the extraction kinetic equation was obtained.
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1 Introduction The growing demand of lithium as energy storage material has increased research interest on it applications such as glass ceramics, medicine, aircraft alloys, catalyst and controlled nuclear fusion fuel. Due to the appearance of electric vehicle, the requirement of
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lithium product has increased urgently in recent years. Although lithium resource is abundant in salt lake brine of Qinghai and Tibetan in China,
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the low concentration of lithium ion and the presence of other cations such as magnesium, calcium, boric and sodium ions lead to the recovery of lithium a difficult process(Wang et al.,
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2018; Zhao et al., 2017). The art of extracting lithium from salt lake brine is not so easy, but some attention has given to this problem. The extraction methods of electrodialysis(Ball and
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Boateng, 1987; Iizuka et al., 2013), adsorption(Lemaire et al., 2014), nanofiltration(Somrani et al., 2013; Tang et al., 2010; Sun et al., 2015), precipitation, supported liquid
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membrane(Song et al., 2018; He et al., 2004; Song et al., 2014; Song et al., 2017)and solvent extraction(Ji et al., 2016; Shi et al., 2017) were mentioned in literatures. Among those
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methods solvent extraction is considered to be the most promising method to achieve industrial for the advantages of high selectivity, high recovery, high capacity and simple
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equipment. Furthermore, solvent extraction can also obtain a satisfied separation of lithium ions and the other cations.
A typical solvent extraction system for lithium recovery is composed of neutral
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organophosphate extractant tributyl phosphate (TBP) and co-extractant FeCl3 (Zhang et al., 2012; Zhou et al., 2013; Zhou et al., 2012). The selective extraction of TBP is better than other neutral organophosphate extractants(Shi et al., 2013; Zhu and Pu, 2000), but the aqueous phase must be under strong acidity in the process of extraction, which has severe corrosion to the extraction equipment. In addition, the strong acidity may cause serious pollution to environment. Currently, a novel extraction system in which N,N-bis(2-ethylhexyl) acetamide was extractant(N523) and FeCl3 was co-extractant, has successfully applied to extract lithium with a higher lithium extraction efficiency(Li et al., 2016; Shi et al., 2013). If the new extraction system would be applied to the industrial production in future, the extraction mechanism should be given in detail. The kinetics studies were essential to clarify
ACCEPTED MANUSCRIPT the extraction mechanism and mass transfer to give guidance to optimize of extraction equipment. Hence, extraction kinetics is of paramount importance to all industrial applications and largely determined by the interfacial chemistry, which is particularly significant in the design of any type of contactor to a given system. The chemical reaction kinetics can also determine the size and types of extractors to be used.
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Our group has studied the mass transfer and kinetics of lithium with N523 using rising single drop method(Li et al., 2016). The effect of different parameters and the extraction
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mechanism were investigated. However, the single drop method cannot avoid some disadvantages(Sun and Gao, 1993; Sun et al., 1986), such as end effect, the instability of drop
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rising and the change of drop size. By contrast, due to the stability and the simplicity, Lewis Cell was widely used to study the extraction kinetic(El-Hefny, 2007; Yang et al., 2013; Sun
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and Xue, 1996). Furthermore, Lewis Cell can make the process of extraction no less real. In this paper, the extraction kinetics of lithium from salt lake brine by N523 was studied
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using Lewis Cell. The effects of different parameters on the extraction rate were investigated and the mechanism was proposed to explain the results obtained.
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2 Experimental 2.1 Materials and apparatus
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The brine was obtained from Qinghai Xitai, China. The experiments were performed at the acidity of 0.05 mol/L, and boric acid could precipitate out under this condition. The existence of boric acid in brine would affect the extraction of lithium. Therefore, boric acid
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needed to be removed before extracting lithium from brine. After getting rid of the boric acid, the brine including lithium was obtained, and the molar concentrations of major ions in the solution were shown in Table 1. The extractant N523, was prepared by Shanghai Institute of Organic Chemistry of the Chinese Academy of Sciences. The structure of N523 was shown in literature[18]. Sulfonated kerosene as diluent was bought from the local market. The organic phase was made up of 50% (v/v) N523 in kerosene in this work. All other chemicals were of analytical reagent grade. The lithium stock solution were prepared by dissolving ferric chloride in brine with the molar ratio of Fe/Li=1.5. The concentration of lithium ion in the organic phase was determined from material balance. All other reagents were of analytical grade.
ACCEPTED MANUSCRIPT Table 1 Major component in brine Ions Concentration (mol/L)
Li+
Mg2+
Na+
K+
Ca2+
Cl-
SO42-
B
0.383
4.16
0.23
0.066
<0.001
8.47
0.26
0.13
The Lewis Cell device was manufactured by China Institute of Atom Science Research. The construction used in the experiments was shown as Fig. 1. The volume of the Lewis cell
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was 30 cm3 and its interfacial area was 9.85 cm2 . The Lewis cell was made of stainless steel container having a double-bladed paddle, which can make interfacial area enter the laminar
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flow regime in the limited speed. The paddles turn in opposite direction in experiments and the turning speed was 150 r/min. This device was used in the study of the extraction kinetic of
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rare earth metal by many scientific organizations. The each of paddles was controlled by the
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experimental results could be replicated.
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single electrical machinery respectively. The experiments were conducted in duplicate and the
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Fig.1 The extraction device
2.2 Data treatment
The experimental data was treated according to the method used by Danesi. Assuming
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that the mass transfer involves only lithium ions in the process of the experiment, the following equation could be written:
Li ( a )
kao koa
Li ( o )
(1)
Where “a”, “o”, “k ao ”, and “k oa ” represent the aqueous phase, organic phase, forward mass transfer constant and backward mass transfer constant, respectively. Considering that the forward rate is much larger than the backward rate in this reaction, namely, k ao>> k oa , the average mass transfer rate through the interfacial area cell can be calculated using the following equation:
dm Vdc[ Li ]( o ) R dt Adt
(2)
ACCEPTED MANUSCRIPT Where “R” refers to the extraction rate; “V “is the volume of the aqueous phase in the interfacial area cell; “A” is the interfacial area through which mass transfer occurs; and “t “is the contact time of the two phases. The extraction rate of lithium is proportional to [Li+]x , [FeCl4 -]y, and [N523]z , as shown below: R=k[Li+]x [FeCl4 -]y[N523]z
(3)
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Where “k” is the forward extraction rate constant; and “x”, “y” and “z” are reaction orders
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with respect to Li+, FeCl4 - and N523, respectively.
For a particular set of Li+, FeCl4 - and N523 concentrations, the effect of the temperature
apparent activation energy (Ea ) can be calculated:
Ea l nA RT
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l nk
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on the extraction rate can be treated by the Arrhenius equation, from which the value of the
(4)
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Where “Ea” is the apparent activation energy; “R” is the universal gas constant (8.314 J.mol-1.K-1 ); “T “is the absolute temperature (K); and “A “is the pre-exponential factor. 3 Results and discussion
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Experimental result on the extraction of lithium in the N523-sulfonated kerosene system at equilibrium has shown that 1 mol Li+ was coordinated with 1 mol FeCl4 - and 1 mol N523 to form the extracted complex. The extraction reaction is represented by the following:
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𝐿𝑖 + + 𝐹𝑒𝐶𝑙4− + 𝑁523 → LiFeCl4 ∙ N523
(5)
3.1 Determination of the diffusion resistance area
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In the kinetics experiment, the criterion generally used to determine the diffusion resistance area was the effect of the stirring of two phases on the concentration of lithium. The concentration of the lithium in the aqueous phase with the increasing extraction time is investigated under the following conditions: the aqueous phase was stirred only, the organic phase was stirred only and the two phases were stirred. The result was plotted in Fig.2.
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Fig.2 Effect of stirring on the extraction of lithium(conditions: [Li+]=0.288 mol/L;
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[Fe3+]=0.432 mol/L; [H+]=0.05 mol/L; [N523]=0.5 mol/L; A=9.85 cm2 ; and T=298.15 K) From Fig.2, the decrease of lithium ion concentration in the aqueous phase was similar
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when stirring two phases or only stir aqueous phase. However, the lithium ion concentration declined very slowly compared with the two other cases when stirring organic phase only.
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This phenomenon indicated that the diffusion resistance was mainly in the aqueous phase. Therefore, the extraction reaction could only happen in the aqueous phase or interface and
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could not happen in the organic phase.
3.2 Identification of extraction reaction zone
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An important criterion to identify whether the chemical reaction that controlled the extraction rate in kinetic regime occurred in the bulk phase or at the interfacial area was the
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relationship between the initial rate of extraction variety and the specific interfacial area(A). If it is the first case, the extraction rate is independent of the interfacial area, in contrast, the extraction rate increases with the increase of interfacial area. The effect of the specific
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interfacial area was studied used different areas in the range of 4.51-9.85 cm2 . The effect of the interfacial area on the extraction of lithium was shown in Fig. 3.
Fig. 3 Effect of the interfacial area on the extraction rate of lithium (conditions: [Li+]=0.288
ACCEPTED MANUSCRIPT mol/L; [Fe3+]=0.432 mol/L; [H+]=0.05 mol/L; [N523]=0.5 mol/L; and T=298.15 K) Obviously, the plot of the extraction rate R against the interfacial area A showed that extraction rate increased with the increase of interfacial area, giving a straight line that passes through the origin. Hence, this phenomenon indicated that the extraction rate was dependent on the variation in the interfacial area, and the rate controlling reaction took place at the
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interfacial area rather than in the bulk phase. This result agrees with literature(Li, 2016 ). 3.3 Effect of temperature on the extraction kinetics
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The activation energy of extraction process can enable to distinguish between a diffusion-controlled and a kinetic regime. According to the reports by Danesi and Chiarizia, if
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the rate is controlled by chemical reaction, Ea is more than 40 kJ/mol, but if the extraction rate is controlled by diffusion process, Ea is less than 20 kJ/mol, and the value of Ea is between 20
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kJ/mol and 40 kJ/mol is expected for a mixed controlled regime. The effect of temperature on the extraction kinetics was analyzed in the temperature range of 298.15-338.15 K(In Fig. 4),
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The apparent activation energy of the reaction was 16.16 kJ/mol according to the Arrhenius equation and the plot, which suggested a possible diffusion controlled regime in the
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temperature range of 298.15-338.15 K.
Fig. 4 Effect of temperature on the extraction rate(conditions: [Li+]=0.288 mol/L; [Fe3+]=0.432 mol/L; [H+]=0.05 mol/L; [N523]=0.5 mol/L; and A=9.85 cm2 )
3.4 Extraction kinetics equation The influence of Li+ , FeCl4 - and N523 concentration on the extraction rate were shown in Fig. 5. In Fig. 5, the relationship between the extraction rate and concentration of lithium ion remained linear with increasing [Li+]. The slope (1.0835) of logR vs. log[Li +] was close to 1, hence, it deduced that the reaction order with respect to [Li+] was 1. In Fig. 5, the linear correlation between logR vs. log[FeCl4 -] was close to 1 and the linear correlation between
ACCEPTED MANUSCRIPT logR vs. log[N523] was close to 1 respectively. The extraction rate constant could be
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calculated according to the intercept of Fig. 5, namely, k=2.63 × 10 −7.
Fig. 5 Effect of concentration of reactant on the extraction rate(conditions: [Li+]=0.288 mol/L;
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[Fe3+]=0.432 mol/L; [H+]=0.05 mol/L; [N523]=0.5 mol/L; A=9.85 cm2 ; and T=298.15 K)
experimental rate equation could be obtained:
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Based on the above discussion, the index was rounded off to the closest integer, and the
R = 2.63 × 10 −7 [𝐿𝑖 +][𝐹𝑒𝐶𝑙4−][𝑁523]
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3.5 Extraction mechanism
(6)
According to the previous studies, the stoichiometry of the extracted lithium ion species
(1)
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was LiFeCl4 ·N523. The mechanism of lithium extraction by N523 can be assumed as follows: Li+ in the aqueous phase was combined with the FeCl4 - , forming the extracted
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complex, which diffused from bulk to the boundary layer. Step (1) including the two processes: The Li+ ions run away from the atmosphere of H2 O molecules, diffusing from the aqueous phase to the interface firstly, then the ions Li+
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coordinated with the FeCl4 - forming the extracted compound LiFeCl4 . This phenomenon can happen at the condition that chloride ion concentration is more than 7mol/L. The salt lake brine contains saturated magnesium chloride and the other ions. Hence, the formation of compound LiFeCl4 can realize. (2) The organic phase provided N523 as the extractant, which diffused from bulk to boundary layer. (3) The extractant N523 and the extracted complex LiFeCl4 meet at the interface and chemically reacted to form LiFeCl4 ·N523. The extracted species LiFeCl4 coordinated with N523 to form LiFeCl4 ·N523, at the same time, the H2 O around Li+ molecules were replaced completely. Finally, the products
ACCEPTED MANUSCRIPT diffused from the interface to the organic phase. Chemical reaction at the interfacial zone would be considered: k1
k2
LiFeCl4( a )
LiFeCl4(i ) N 523( i ) LiFeCl4 N 523(i )
k5 k5
LiFeCl4( i )
k2 k3 k3
k4 k4
(7) (8)
N 523( i )
(9)
LiFeCl4 N 523( i )
LiFeCl4 N 523( o )
(10) (11)
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N 523( o )
LiFeCl4( a )
k1
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Li ( a ) FeCl4 ( a )
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The subscript of “i” represents the interfacial zone. The reaction in Eq.(10) is assumed to be the rate-controlling step, the following rate equation is deduced:
R k4 [ LiFeCl4 ](i ) [ N 523]( i )
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(12)
According to steady-state approximation (QSSA):
k2 [ LiFeCl4 ]( a ) k2 k4 [ N 523]( i )
k3[ N 523] o(
)
(14)
k3 k4 [ N 523]( i )
k1 k2[ L i ]a( )[ F e C4 l]a
(
[k3 5N2 o3 ](
)
(k2 k 4 [N 5 2 3i ( ]) k) (1k LiFeCl 2 [
a4
k(] ) )k 3 N
k1 k2 k3 k[ 4 L ]ia ( [ ) F e C4]al ([
)
5N2 o3 ] (
(k2 k4 [ N 5 2 3i ( ]) k) (1 k 2 L[ i F e aC4 l(] ) ) (k3
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R
(13)
(k2 k4 [ N 523]( i ) )(k1 k2 [ LiFeCl4 ]( a ) )
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R k4
k1k2 [ Li ]( a ) [ FeCl4 ]( a )
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[ N 523]( i )
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[ LiFeCl4 ]( i )
4
)
[
i
)
5 2 3( ])
(15)
(16)
k[N ) i 5 2 (3)]
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Due to k2 k4 [ N 523](i ) , k1 k2 [ LiFeCl4 ]( a ) , k3 k4 [ N 523]( i ) , the extraction rate can be written as:
R
Where k
k1k2k3k4 [ Li ]( a ) [ FeCl4 ]( a ) [ N 523]( o ) k1k2k3
(17)
k1k2k3k4 , so k1k2k3
R k[ Li ]( a ) [ FeCl4 ]( a ) [ N 523](o )
（18）
Eq.(18) deduced from the rate-determining step (10) was consistent with the rate
ACCEPTED MANUSCRIPT equation obtained from the experimental results. 4 Conclusions The extraction kinetics of lithium ion from salt lake brine by N523 using Lewis Cell has been performed, it was found: (1) Studies on the effects of diffusion resistance area, interfacial area, temperature and the concentration of reagent on the extraction kinetics showed that the diffusion resistance
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was mainly in the aqueous phase, the chemical reactions occured at the interface rather than in the bulk phase, and the process of extraction was controlled by diffusion.
(2) Based on the experimental data correlated as a function of the concentration of Li+,
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constant interfacial area cell could be expressed as follows:
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FeCl4 - and extractant N523, the extraction rate equation of lithium ion from salt lake brine by
R = 2.63 × 10 −7 [𝐿𝑖 +][𝐹𝑒𝐶𝑙4−][𝑁523]
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Acknowledgments
This work was financially supported by Qaidam Salt Lake Chemical Joint Scientific
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Research Funds (U1407203) and Qinghai University project (2015-QGY-6). The authors are grateful for Shanghai Institute of organic Chemistry for the experimental instruction.
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References
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ACCEPTED MANUSCRIPT Highlights:
Lithium extraction kinetics with N523 was investigated using a Lewis Cell.
The diffusion resistance mainly existed in aqueous phase.
The extraction kinetics was diffusion-controlled.
The
extraction
rate
equation
could
be
expressed
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R = 2.63 × 10−7 [𝐿𝑖 + ][𝐹𝑒𝐶𝑙 − 4 ][𝑁523].
by
Hui-Fang Li, Li-Juan Li, Xiao-Wu Peng, Lian-Min Ji, Wu Li PII: DOI: Reference:
S0304-386X(18)30025-2 doi:10.1016/j.hydromet.2018.03.022 HYDROM 4786
To appear in:
Hydrometallurgy
Received date: Revised date: Accepted date:
9 January 2018 21 March 2018 31 March 2018
Please cite this article as: Hui-Fang Li, Li-Juan Li, Xiao-Wu Peng, Lian-Min Ji, Wu Li , Extraction kinetics of lithium from salt lake brine by N,N-bis(2-ethylhexyl) acetamide using Lewis Cell. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Hydrom(2017), doi:10.1016/ j.hydromet.2018.03.022
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Extraction Kinetics of Lithium from Salt Lake Brine by N,N-bis(2-ethylhexyl) Acetamide Using Lewis Cell Hui-fang LI1,2,3,4 , Li-juan LI1*, Xiao-wu PENG 1,3, Lian-min JI1 , Wu LI1,2* 1 Key Laboratory of Comprehensive and Highly Efficient Utilization of Salt Lake Resources, Qinghai Institute of Salt Lakes, Chinese Academy of Sciences, Xining, 810008, China; 2 Key Laboratory of Salt Lake Resources Chemistry of Qinghai Province, Xining, 810008, China; 3 University of Chinese Academy of Sciences, Beijing, 10049, China
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4 Qinghai University, Xining, 810010, China
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Abstract: Lithium, as the green energy, its application and exploitation attract much attention. The lithium extraction kinetics with N,N-bis(2-ethylhexyl) acetamide(N523) in sulfonated kerosene has been investigated experimentally using Lewis Cell. The effects of diffusion resistance area, interfacial area, temperature and the concentration of reagents were studied. The results showed that the diffusion resistance mainly existed in aqueous phase, the
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extraction process was a diffusion-controlled kinetics process with an interfacial reaction and
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Keywords: Lithium; Extraction kinetics; Mechanism;
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the extraction kinetic equation was obtained.
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1 Introduction The growing demand of lithium as energy storage material has increased research interest on it applications such as glass ceramics, medicine, aircraft alloys, catalyst and controlled nuclear fusion fuel. Due to the appearance of electric vehicle, the requirement of
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lithium product has increased urgently in recent years. Although lithium resource is abundant in salt lake brine of Qinghai and Tibetan in China,
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the low concentration of lithium ion and the presence of other cations such as magnesium, calcium, boric and sodium ions lead to the recovery of lithium a difficult process(Wang et al.,
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2018; Zhao et al., 2017). The art of extracting lithium from salt lake brine is not so easy, but some attention has given to this problem. The extraction methods of electrodialysis(Ball and
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Boateng, 1987; Iizuka et al., 2013), adsorption(Lemaire et al., 2014), nanofiltration(Somrani et al., 2013; Tang et al., 2010; Sun et al., 2015), precipitation, supported liquid
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membrane(Song et al., 2018; He et al., 2004; Song et al., 2014; Song et al., 2017)and solvent extraction(Ji et al., 2016; Shi et al., 2017) were mentioned in literatures. Among those
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methods solvent extraction is considered to be the most promising method to achieve industrial for the advantages of high selectivity, high recovery, high capacity and simple
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equipment. Furthermore, solvent extraction can also obtain a satisfied separation of lithium ions and the other cations.
A typical solvent extraction system for lithium recovery is composed of neutral
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organophosphate extractant tributyl phosphate (TBP) and co-extractant FeCl3 (Zhang et al., 2012; Zhou et al., 2013; Zhou et al., 2012). The selective extraction of TBP is better than other neutral organophosphate extractants(Shi et al., 2013; Zhu and Pu, 2000), but the aqueous phase must be under strong acidity in the process of extraction, which has severe corrosion to the extraction equipment. In addition, the strong acidity may cause serious pollution to environment. Currently, a novel extraction system in which N,N-bis(2-ethylhexyl) acetamide was extractant(N523) and FeCl3 was co-extractant, has successfully applied to extract lithium with a higher lithium extraction efficiency(Li et al., 2016; Shi et al., 2013). If the new extraction system would be applied to the industrial production in future, the extraction mechanism should be given in detail. The kinetics studies were essential to clarify
ACCEPTED MANUSCRIPT the extraction mechanism and mass transfer to give guidance to optimize of extraction equipment. Hence, extraction kinetics is of paramount importance to all industrial applications and largely determined by the interfacial chemistry, which is particularly significant in the design of any type of contactor to a given system. The chemical reaction kinetics can also determine the size and types of extractors to be used.
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Our group has studied the mass transfer and kinetics of lithium with N523 using rising single drop method(Li et al., 2016). The effect of different parameters and the extraction
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mechanism were investigated. However, the single drop method cannot avoid some disadvantages(Sun and Gao, 1993; Sun et al., 1986), such as end effect, the instability of drop
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rising and the change of drop size. By contrast, due to the stability and the simplicity, Lewis Cell was widely used to study the extraction kinetic(El-Hefny, 2007; Yang et al., 2013; Sun
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and Xue, 1996). Furthermore, Lewis Cell can make the process of extraction no less real. In this paper, the extraction kinetics of lithium from salt lake brine by N523 was studied
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using Lewis Cell. The effects of different parameters on the extraction rate were investigated and the mechanism was proposed to explain the results obtained.
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2 Experimental 2.1 Materials and apparatus
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The brine was obtained from Qinghai Xitai, China. The experiments were performed at the acidity of 0.05 mol/L, and boric acid could precipitate out under this condition. The existence of boric acid in brine would affect the extraction of lithium. Therefore, boric acid
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needed to be removed before extracting lithium from brine. After getting rid of the boric acid, the brine including lithium was obtained, and the molar concentrations of major ions in the solution were shown in Table 1. The extractant N523, was prepared by Shanghai Institute of Organic Chemistry of the Chinese Academy of Sciences. The structure of N523 was shown in literature[18]. Sulfonated kerosene as diluent was bought from the local market. The organic phase was made up of 50% (v/v) N523 in kerosene in this work. All other chemicals were of analytical reagent grade. The lithium stock solution were prepared by dissolving ferric chloride in brine with the molar ratio of Fe/Li=1.5. The concentration of lithium ion in the organic phase was determined from material balance. All other reagents were of analytical grade.
ACCEPTED MANUSCRIPT Table 1 Major component in brine Ions Concentration (mol/L)
Li+
Mg2+
Na+
K+
Ca2+
Cl-
SO42-
B
0.383
4.16
0.23
0.066
<0.001
8.47
0.26
0.13
The Lewis Cell device was manufactured by China Institute of Atom Science Research. The construction used in the experiments was shown as Fig. 1. The volume of the Lewis cell
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was 30 cm3 and its interfacial area was 9.85 cm2 . The Lewis cell was made of stainless steel container having a double-bladed paddle, which can make interfacial area enter the laminar
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flow regime in the limited speed. The paddles turn in opposite direction in experiments and the turning speed was 150 r/min. This device was used in the study of the extraction kinetic of
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rare earth metal by many scientific organizations. The each of paddles was controlled by the
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experimental results could be replicated.
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single electrical machinery respectively. The experiments were conducted in duplicate and the
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Fig.1 The extraction device
2.2 Data treatment
The experimental data was treated according to the method used by Danesi. Assuming
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that the mass transfer involves only lithium ions in the process of the experiment, the following equation could be written:
Li ( a )
kao koa
Li ( o )
(1)
Where “a”, “o”, “k ao ”, and “k oa ” represent the aqueous phase, organic phase, forward mass transfer constant and backward mass transfer constant, respectively. Considering that the forward rate is much larger than the backward rate in this reaction, namely, k ao>> k oa , the average mass transfer rate through the interfacial area cell can be calculated using the following equation:
dm Vdc[ Li ]( o ) R dt Adt
(2)
ACCEPTED MANUSCRIPT Where “R” refers to the extraction rate; “V “is the volume of the aqueous phase in the interfacial area cell; “A” is the interfacial area through which mass transfer occurs; and “t “is the contact time of the two phases. The extraction rate of lithium is proportional to [Li+]x , [FeCl4 -]y, and [N523]z , as shown below: R=k[Li+]x [FeCl4 -]y[N523]z
(3)
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Where “k” is the forward extraction rate constant; and “x”, “y” and “z” are reaction orders
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with respect to Li+, FeCl4 - and N523, respectively.
For a particular set of Li+, FeCl4 - and N523 concentrations, the effect of the temperature
apparent activation energy (Ea ) can be calculated:
Ea l nA RT
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l nk
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on the extraction rate can be treated by the Arrhenius equation, from which the value of the
(4)
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Where “Ea” is the apparent activation energy; “R” is the universal gas constant (8.314 J.mol-1.K-1 ); “T “is the absolute temperature (K); and “A “is the pre-exponential factor. 3 Results and discussion
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Experimental result on the extraction of lithium in the N523-sulfonated kerosene system at equilibrium has shown that 1 mol Li+ was coordinated with 1 mol FeCl4 - and 1 mol N523 to form the extracted complex. The extraction reaction is represented by the following:
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𝐿𝑖 + + 𝐹𝑒𝐶𝑙4− + 𝑁523 → LiFeCl4 ∙ N523
(5)
3.1 Determination of the diffusion resistance area
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In the kinetics experiment, the criterion generally used to determine the diffusion resistance area was the effect of the stirring of two phases on the concentration of lithium. The concentration of the lithium in the aqueous phase with the increasing extraction time is investigated under the following conditions: the aqueous phase was stirred only, the organic phase was stirred only and the two phases were stirred. The result was plotted in Fig.2.
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Fig.2 Effect of stirring on the extraction of lithium(conditions: [Li+]=0.288 mol/L;
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[Fe3+]=0.432 mol/L; [H+]=0.05 mol/L; [N523]=0.5 mol/L; A=9.85 cm2 ; and T=298.15 K) From Fig.2, the decrease of lithium ion concentration in the aqueous phase was similar
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when stirring two phases or only stir aqueous phase. However, the lithium ion concentration declined very slowly compared with the two other cases when stirring organic phase only.
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This phenomenon indicated that the diffusion resistance was mainly in the aqueous phase. Therefore, the extraction reaction could only happen in the aqueous phase or interface and
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could not happen in the organic phase.
3.2 Identification of extraction reaction zone
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An important criterion to identify whether the chemical reaction that controlled the extraction rate in kinetic regime occurred in the bulk phase or at the interfacial area was the
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relationship between the initial rate of extraction variety and the specific interfacial area(A). If it is the first case, the extraction rate is independent of the interfacial area, in contrast, the extraction rate increases with the increase of interfacial area. The effect of the specific
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interfacial area was studied used different areas in the range of 4.51-9.85 cm2 . The effect of the interfacial area on the extraction of lithium was shown in Fig. 3.
Fig. 3 Effect of the interfacial area on the extraction rate of lithium (conditions: [Li+]=0.288
ACCEPTED MANUSCRIPT mol/L; [Fe3+]=0.432 mol/L; [H+]=0.05 mol/L; [N523]=0.5 mol/L; and T=298.15 K) Obviously, the plot of the extraction rate R against the interfacial area A showed that extraction rate increased with the increase of interfacial area, giving a straight line that passes through the origin. Hence, this phenomenon indicated that the extraction rate was dependent on the variation in the interfacial area, and the rate controlling reaction took place at the
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interfacial area rather than in the bulk phase. This result agrees with literature(Li, 2016 ). 3.3 Effect of temperature on the extraction kinetics
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The activation energy of extraction process can enable to distinguish between a diffusion-controlled and a kinetic regime. According to the reports by Danesi and Chiarizia, if
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the rate is controlled by chemical reaction, Ea is more than 40 kJ/mol, but if the extraction rate is controlled by diffusion process, Ea is less than 20 kJ/mol, and the value of Ea is between 20
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kJ/mol and 40 kJ/mol is expected for a mixed controlled regime. The effect of temperature on the extraction kinetics was analyzed in the temperature range of 298.15-338.15 K(In Fig. 4),
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The apparent activation energy of the reaction was 16.16 kJ/mol according to the Arrhenius equation and the plot, which suggested a possible diffusion controlled regime in the
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temperature range of 298.15-338.15 K.
Fig. 4 Effect of temperature on the extraction rate(conditions: [Li+]=0.288 mol/L; [Fe3+]=0.432 mol/L; [H+]=0.05 mol/L; [N523]=0.5 mol/L; and A=9.85 cm2 )
3.4 Extraction kinetics equation The influence of Li+ , FeCl4 - and N523 concentration on the extraction rate were shown in Fig. 5. In Fig. 5, the relationship between the extraction rate and concentration of lithium ion remained linear with increasing [Li+]. The slope (1.0835) of logR vs. log[Li +] was close to 1, hence, it deduced that the reaction order with respect to [Li+] was 1. In Fig. 5, the linear correlation between logR vs. log[FeCl4 -] was close to 1 and the linear correlation between
ACCEPTED MANUSCRIPT logR vs. log[N523] was close to 1 respectively. The extraction rate constant could be
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calculated according to the intercept of Fig. 5, namely, k=2.63 × 10 −7.
Fig. 5 Effect of concentration of reactant on the extraction rate(conditions: [Li+]=0.288 mol/L;
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[Fe3+]=0.432 mol/L; [H+]=0.05 mol/L; [N523]=0.5 mol/L; A=9.85 cm2 ; and T=298.15 K)
experimental rate equation could be obtained:
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Based on the above discussion, the index was rounded off to the closest integer, and the
R = 2.63 × 10 −7 [𝐿𝑖 +][𝐹𝑒𝐶𝑙4−][𝑁523]
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3.5 Extraction mechanism
(6)
According to the previous studies, the stoichiometry of the extracted lithium ion species
(1)
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was LiFeCl4 ·N523. The mechanism of lithium extraction by N523 can be assumed as follows: Li+ in the aqueous phase was combined with the FeCl4 - , forming the extracted
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complex, which diffused from bulk to the boundary layer. Step (1) including the two processes: The Li+ ions run away from the atmosphere of H2 O molecules, diffusing from the aqueous phase to the interface firstly, then the ions Li+
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coordinated with the FeCl4 - forming the extracted compound LiFeCl4 . This phenomenon can happen at the condition that chloride ion concentration is more than 7mol/L. The salt lake brine contains saturated magnesium chloride and the other ions. Hence, the formation of compound LiFeCl4 can realize. (2) The organic phase provided N523 as the extractant, which diffused from bulk to boundary layer. (3) The extractant N523 and the extracted complex LiFeCl4 meet at the interface and chemically reacted to form LiFeCl4 ·N523. The extracted species LiFeCl4 coordinated with N523 to form LiFeCl4 ·N523, at the same time, the H2 O around Li+ molecules were replaced completely. Finally, the products
ACCEPTED MANUSCRIPT diffused from the interface to the organic phase. Chemical reaction at the interfacial zone would be considered: k1
k2
LiFeCl4( a )
LiFeCl4(i ) N 523( i ) LiFeCl4 N 523(i )
k5 k5
LiFeCl4( i )
k2 k3 k3
k4 k4
(7) (8)
N 523( i )
(9)
LiFeCl4 N 523( i )
LiFeCl4 N 523( o )
(10) (11)
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N 523( o )
LiFeCl4( a )
k1
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Li ( a ) FeCl4 ( a )
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The subscript of “i” represents the interfacial zone. The reaction in Eq.(10) is assumed to be the rate-controlling step, the following rate equation is deduced:
R k4 [ LiFeCl4 ](i ) [ N 523]( i )
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(12)
According to steady-state approximation (QSSA):
k2 [ LiFeCl4 ]( a ) k2 k4 [ N 523]( i )
k3[ N 523] o(
)
(14)
k3 k4 [ N 523]( i )
k1 k2[ L i ]a( )[ F e C4 l]a
(
[k3 5N2 o3 ](
)
(k2 k 4 [N 5 2 3i ( ]) k) (1k LiFeCl 2 [
a4
k(] ) )k 3 N
k1 k2 k3 k[ 4 L ]ia ( [ ) F e C4]al ([
)
5N2 o3 ] (
(k2 k4 [ N 5 2 3i ( ]) k) (1 k 2 L[ i F e aC4 l(] ) ) (k3
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R
(13)
(k2 k4 [ N 523]( i ) )(k1 k2 [ LiFeCl4 ]( a ) )
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R k4
k1k2 [ Li ]( a ) [ FeCl4 ]( a )
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[ N 523]( i )
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[ LiFeCl4 ]( i )
4
)
[
i
)
5 2 3( ])
(15)
(16)
k[N ) i 5 2 (3)]
4
Due to k2 k4 [ N 523](i ) , k1 k2 [ LiFeCl4 ]( a ) , k3 k4 [ N 523]( i ) , the extraction rate can be written as:
R
Where k
k1k2k3k4 [ Li ]( a ) [ FeCl4 ]( a ) [ N 523]( o ) k1k2k3
(17)
k1k2k3k4 , so k1k2k3
R k[ Li ]( a ) [ FeCl4 ]( a ) [ N 523](o )
（18）
Eq.(18) deduced from the rate-determining step (10) was consistent with the rate
ACCEPTED MANUSCRIPT equation obtained from the experimental results. 4 Conclusions The extraction kinetics of lithium ion from salt lake brine by N523 using Lewis Cell has been performed, it was found: (1) Studies on the effects of diffusion resistance area, interfacial area, temperature and the concentration of reagent on the extraction kinetics showed that the diffusion resistance
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was mainly in the aqueous phase, the chemical reactions occured at the interface rather than in the bulk phase, and the process of extraction was controlled by diffusion.
(2) Based on the experimental data correlated as a function of the concentration of Li+,
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constant interfacial area cell could be expressed as follows:
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FeCl4 - and extractant N523, the extraction rate equation of lithium ion from salt lake brine by
R = 2.63 × 10 −7 [𝐿𝑖 +][𝐹𝑒𝐶𝑙4−][𝑁523]
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Acknowledgments
This work was financially supported by Qaidam Salt Lake Chemical Joint Scientific
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Research Funds (U1407203) and Qinghai University project (2015-QGY-6). The authors are grateful for Shanghai Institute of organic Chemistry for the experimental instruction.
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References
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ACCEPTED MANUSCRIPT Highlights:
Lithium extraction kinetics with N523 was investigated using a Lewis Cell.
The diffusion resistance mainly existed in aqueous phase.
The extraction kinetics was diffusion-controlled.
The
extraction
rate
equation
could
be
expressed
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R = 2.63 × 10−7 [𝐿𝑖 + ][𝐹𝑒𝐶𝑙 − 4 ][𝑁523].
by