ARTICLE IN PRESS
Journal of Crystal Growth 312 (2010) 2345–2350
Contents lists available at ScienceDirect
Journal of Crystal Growth
journal homepage: www.elsevier.com/locate/jcrysgro
Crystallization kinetics of Li2CO3 from LiHCO3 solutions
Wen-tao Yi a,n, Chun-yan Yan a, Pei-hua Ma b
a
b
Department of Chemistry and Chemical Engineering, Zaozhuang University, Zaozhuang 277160, China
Qinghai Institute of Salt Lakes, Chinese Academy of Sciences, Xining 810008, China
a r t i c l e in fo
abstract
Article history:
Received 8 October 2009
Received in revised form
25 April 2010
Accepted 1 May 2010
Communicated by M. Uwaha
Available online 6 May 2010
The crystallization kinetics of Li2CO3 from LiHCO3 solutions was studied by a non-evaporation volumeconstant batch reactor. The factors influencing the crystallization process such as initial concentration,
temperature and stirring speed were investigated and discussed. In the experiment, CO2 was stripped
from a slightly acidic solution of LiHCO3 by stirring resulting in critical supersaturation and
precipitation of Li2CO3. The reduction of the Li + concentration was recorded as a function of time by
determination with flame atomic absorption spectrometry (FAAS), and the pH values were also
measured.
The results showed that the crystallization rate increased and the induction period shortened with
increase in the initial concentration and temperature. In addition, high stirring speed could significantly
promote this process.
Thermodynamic parameters at critical conditions such as the surface free energy and critical radius
of nucleus were estimated based on the crystallization theory and experimental results. The results
showed that the crystallization kinetics of this process coincided with the second-order reaction rate
equation, and the apparent activation energy of the process was obtained. The analysis of the crystal
growth mechanism showed that the growth of Li2CO3 from LiHCO3 solutions was mainly controlled by
diffusion.
& 2010 Elsevier B.V. All rights reserved.
Keywords:
A1.Diffusion
A1.Nucleation
A2.Growth from solution
B1.Lithium compound
1. Introduction
High-purity Li2CO3 can be used for pharmaceutical applications,
production of electronic grade crystals (e.g. LiNbO3 and LiTaO3
crystals), preparation of battery-grade lithium metal, etc. [1]. In
recent years, with the development of battery and single crystal
industries in the world, the demand for it is dramatically growing,
whereas its supply is in some degree lacking. Consequently,
promoting production of high-purity Li2CO3 has become extremely
necessary nowadays. The carbonation–decomposition method is a
promising way to produce high-purity Li2CO3 for its simple
operation, low cost, high efficiency, and low pollution. For this
method, the crude Li2CO3 should be first converted to watersoluble LiHCO3 by carbonation with CO2–H2O solutions; impurities
in the crude Li2CO3 are either solubilized or precipitated out, and
the dissolved impurities are separated from LiHCO3 solutions by
suitable purifying methods, such as ion exchange or solvent
extraction; then, high-purity Li2CO3 is precipitated out by heating
the purified LiHCO3 solution. Clearly, the last step, i.e. crystallization of Li2CO3 from LiHCO3 solutions, is one of the most
important links in the production chain.
The crystallizations of the sparingly soluble alkaline earth
carbonates, such as the different CaCO3 polymorphs calcite,
aragonite and vaterite, witherite (BaCO3), and strontianite (SrCO3),
have been studied already [2–10]. The results suggest that the rates
of nucleation and the crystal growth are both strongly dependent on
supersaturation. The crystal growth rate increased with increase in
supersaturation on the whole. The growth rate is mostly transportcontrolled at high supersaturation, and integration-controlled
growth occurs only at low supersaturation approximately.
However, there are sparse data of the crystallization of alkali
carbonates, and especially, the crystallization kinetics of the
sparingly soluble Li2CO3 from LiHCO3 solutions has never been
reported until now. The present study provides experimental data of
nucleation and crystal growth of Li2CO3 in pure and supersaturated
LiHCO3 solutions. This method is used here and is convenient for
studying inorganic processes that occur in aqueous systems where
the major components are Li + and HCO3 ions. The study on the
crystallization kinetics and mechanism of Li2CO3 from LiHCO3
solutions will provide theoretical basis for crystallizer design and
the optimization of the process operation.
2. Materials and methods
n
Corresponding author. Tel./Fax.: +86 632 3786872.
E-mail address: [email protected] (W.-t. Yi).
0022-0248/$ - see front matter & 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jcrysgro.2010.05.002
Metastable LiHCO3 solutions were prepared by dissolution of
known quantities of Li2CO3 in CO2–H2O mixtures, whereby the
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final Li + concentrations of the resulting solutions range from 4.02
to 9.93 g L 1. The solution pH was measured using a digital pH
211 meter with an uncertainty of 70.01 unit. Due to the
dissolved free CO2, the pH values of the solutions were about
5.8070.01 at the initial stage. The crystallization of Li2CO3 was
carried out under different temperatures which were controlled
by a super thermostat bath, and the measuring temperature can
be thermostatically controlled to 70.1 K. The reduction of Li +
concentration was determined by flame atomic absorption
spectrometry (FAAS) with an uncertainty of 73%.
In a glass flask, 400 mL of each LiHCO3 solution was stirred
continuously by a magnetic stirrer. The dissolved CO2 was
stripped from slightly acidic solutions to achieve the critical
supersaturation of Li2CO3, and then the Li2CO3 was precipitated
out. As concentration is a key index to reflect the crystallization
process, the resulting reduction of the Li + concentration was
determined by FAAS (GBC-932 plus AAS from Australia) as
a function of time. Furthermore, the pH value was determined
as a function of time, because the alterations of CO2 concentrations are correlated with alterations of pH values.
3. Results and discussion
3.1. The analysis of the crystallization process
The Li2CO3 precipitation described above is a very elaborate
process, and the possible mechanism can be expressed by the
equations below:
þ
LiHCO3 "HCO
3 þ Li
ð1Þ
þ
HCO3 "CO2
3 þH
ð2Þ
H þ þ HCO
3 "CO2 m þ H2 O
ð3Þ
2Li þ þCO2
3 -Li2 CO3 saturated solution
ð4Þ
Li2 CO3 saturated solution-Li2 CO3 supersaturated solution
ð5Þ
Li2 CO3 supersaturated solution-Li2 CO3 molecular clusters
ð6Þ
Li2 CO3 molecular clusters-Li2 CO3 crystal nuclei
ð7Þ
Li2 CO3 crystal nuclei-Li2 CO3 crystal grain
ð8Þ
and the total equation can be expressed by
2LiHCO3 -Li2 CO3 k þ H2 Oþ CO2 m
ð9Þ
According to the analysis above, it can be found that the
formation of CO23 is crucial for the crystallization process, which
was also found by Dreybrodt et al. [11] and Liu [12] when they
investigated the precipitation kinetics of CaCO3.
3.2. The calculation of the supersaturation of Li2CO3
The supersaturation S of Li2CO3 is defined as
S¼
a2 Li þ UaCO2
3
ð10Þ
KSP
where KSP is the thermodynamic solubility product of Li2CO3 at
different temperatures, and it is calculated by ion entropy
correspondence principle, which is used to calculate the thermodynamic solubility product of the insoluble or slightly soluble
compounds [13]; the results for Li2CO3 from 298 to 363 K were
listed in Table 1. a, the activities of the ionic species in the
solutions, are calculated by the professional program of Visual
MINTEQ, which can be used to calculate the activities of ions
based on their concentrations [14], and some calculation results
of the activities of lithium and carbonate ions under different
LiHCO3 concentrations and temperatures are listed in Table 2.
The supersaturation is the impetus for a crystallization
process, while the temperature and the concentration are the
main factors influencing the supersaturation. The supersaturations of Li2CO3 at different temperatures and Li + concentrations
were calculated, and the results are shown in Fig. 1.
It can be seen from Fig. 1 that the supersaturations of Li2CO3
increased with increase in temperature and Li + concentration.
3.3. Effects of the parameters
3.3.1. Effect of initial concentration
Concentration is one of the most important factors influencing
the crystallization process. The change of ion concentration with
time can reflect the crystallization kinetics in a large manner. The
crystallization results of LiHCO3 solutions with different initial
concentrations under 353 K are shown in Fig. 2. And it can be seen
from the figure that the lower the initial concentration, the longer
the induction period of the system. In the prior period of the
reaction, the ion concentration decreases sharply, and it can be
considered that this is related with the nucleation; while in the
later period, the ion concentration decreases slowly till
equilibrium, and it can be considered that this is related with
the crystal grain growth.
3.3.2. Effect of temperature
Temperature is another most important factor influencing the
crystallization process. The experimental results under different
temperatures from 323 to 363 K are shown in Fig. 3. The results
showed that the crystallization rate of Li2CO3 from LiHCO3
solutions increased with increase in temperature. And the
reasons can be interpreted as follows: on the one hand, the
solubility of Li2CO3 decreased with increasing temperature which
in turn increased its supersaturation; on the other hand, the
elevation of temperature accelerated the diffusion of the ions to
the surface and the lattice of the crystals which in turn
accelerated the crystal growth. However, the elevation of
temperature will increase the energy consumption, and 353 K
was selected for the rest of the experiments.
3.3.3. Effect of stirring
Mechanical effects such as stirring, vibration, etc. often affect
the crystallization evidently. As to a supersaturated solution, the
rate of the nuclei formation can be accelerated by even a slight
vibration. Effect of stirring speed on Li2CO3 crystallization was
investigated in this part, and the results are shown in Fig. 4. It is
obvious that with the increase of stirring speed, the crystallization
rate of Li2CO3 increases. The reasons can be interpreted as
follows: on the one hand, the stirring accelerated the ion
Table 1
KSP of Li2CO3 at different temperatures calculated by using ion entropy correspondence principle.
Temp. (K)
KSP
298
1.17 10 3
323
4.99 10 4
333
3.34 10 4
343
2.17 10 4
353
1.36 10 4
363
8.37 10 5
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2.59
4.18
5.97
8.06
11.3
4.06
5.13
6.20
7.16
8.41
4.12
5.18
6.29
7.24
8.47
0.711
1.43
3.89
5.19
7.53
0.852
1.85
4.65
5.48
7.84
4.09
5.14
6.25
7.20
8.44
0.979
2.93
5.36
7.84
10.9
(10 3 mol L 1)
(10 1 mol L 1)
aLi þ
(10 3 mol L 1)
aCO3 2-
(10 1 mol L 1)
aLi þ
(10 1 mol L 1)
(10 1 mol L 1)
4.15
5.21
6.33
7.28
8.50
0.264
0.517
1.46
3.74
5.12
Fig. 2. Effect of LiHCO3 initial concentration on Li2CO3 crystallization from LiHCO3
solutions: 353 K, medium-speed stirring.
4.25
5.33
6.46
7.40
8.60
aLi þ
(10 1 mol L 1)
aCO3 2-
aLi þ
(10 3 mol L 1)
aCO3 2-
aLi þ
(10 3 mol L 1)
aCO3 2-
353 K
343 K
333 K
298 K
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Fig. 1. Relationship between Li2CO3 supersaturation and temperature, and LiHCO3
concentration.
diffusion, which induced the aggregation of the ions; on
the other hand, the stirring was beneficial for CO2 release from
the system, and the supersaturation of Li2CO3 increased
accordingly, hence the acceleration of Li2CO3 crystallization.
3.4. Crystallization kinetics
3.4.1. Correlation between induction period and supersaturation
According to the thermodynamic theory of new-phase formation, i.e. the Nielsen theory, the dependence of the measured
induction period tind on the solution supersaturation S is given by
[15–17]
log tind ¼
B
2
log S
þA
ð11Þ
thereinto,
4.02
5.06
6.13
6.98
8.03
cLiHCO3 (g L 1)
Table 2
Some calculation results of the activities of lithium and carbonate ions under different LiHCO3 concentrations and temperatures.
(10 3 mol L 1)
363 K
aCO3 2-
W.-t. Yi et al. / Journal of Crystal Growth 312 (2010) 2345–2350
B¼
b g 3 u2
ð2:303kB TÞ3 n2
ð12Þ
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where kB is the Boltzmann constant (1.38 10 23 J K 1); T is
absolute temperature (K); g is interfacial tension or surface energy
of the solid in contact with the solution (J m 2); u is the molar
volume of Li2CO3 (u¼ 5.824 10 29 m3 here); v is the mole
number of ions per mole solute (as for Li2CO3, v ¼3 here); S is
supersaturation; b is a structural factor (b ¼16p/3 for the
spherical nucleus); and A is a constant.
Under given conditions, supersaturations of Li2CO3 were
calculated, and the induction periods were recorded precisely.
Fig. 5 shows the induction period as a function of supersaturation.
The experimental data demonstrate that log tind versus (log S) 2
function is linear with correlation coefficient of 0.9993, and the
slope of 1.2797 was obtained. So, we can say the Nielsen theory
can describe the relationship of the two physical properties
properly.
Fig. 3. Effect of reaction temperature on Li2CO3 crystallization from LiHCO3
solutions: Li + initial concentration [Li + ]0 ¼8.23 g L 1, medium-speed stirring.
3.4.2. Surface energy and critical thermodynamic parameters of
Li2CO3
According to Eqs. (11) and (12) and the slope in Fig. 5, the
surface energy of Li2CO3 was calculated to be 65.92 mJ m 2.
With the surface energy, several thermodynamic parameters
of Li2CO3 under critical conditions can be predicted [16,17].
Before the formation of nuclei, the incipient crystals should be
firstly formed. However, only when the radii of the incipient
crystals are bigger than that of the critical nuclei, the nuclei will
be stable, otherwise, they will be dissolved and disappeared. As
soon as enough stable nuclei have been formed in the supersaturated solution, they begin to grow into crystals of visible size.
The critical nucleus radius r of Li2CO3 can be calculated by
r ¼ 2gu=f
ð13Þ
where g is the surface energy of Li2CO3; u is the molar volume of
Li2CO3 (u¼5.824 10 29 m3 here); f is the affinity (J), and the
relationship of it between supersaturation S can be represented
by
f ¼ kB T ln S
ð14Þ
So, here is
r ¼ 2gu=ðkB T ln SÞ
Fig. 4. Effect of stirring speed on Li2CO3 crystallization from LiHCO3 solutions: Li +
initial concentration [Li + ]0 ¼8.23 g L 1, 353 K.
ð15Þ
According to Eq. (15), the critical nucleus radius of Li2CO3 was
calculated to be 4.308 10 10 m.
A critical nucleus contains several particles generally. The
numbers of the particles in a nucleus nn can be obtained by
3
n ¼ 2bg3 n2 =f
ð16Þ
Substitute Eq. (14) into Eq. (16), Eq. (17) can be found.
n ¼ 2bg3 n2 =ðkB T ln SÞ3
ð17Þ
The result shows that there are 5 particles in a critical nucleus.
3.4.3. Macro-kinetics of the process
The results mentioned above showed in the time interval of
60 min except induction period (denoted by tind-60 min), the ion
concentration decreased sharply, and it could be considered that
this period was mainly related with the nucleation, so we called it
the nucleation period; while from 80 to 120 min (denoted by
80–120 min), the ion concentration decreased slowly which was
mainly related with the grain growth, and we called it the crystal
grain growth period. The time interval of 60–80 min can be
assumed as the transition adjusting period from nucleation to
grain growth.
The crystallization of Li2CO3 follows a second rate equation
expressed in the form of
Fig. 5. Plot of log tind as a function of (log S)
2
according to Eq. (11).
d½Li þ ¼ k½Li þ 2
dt
ð18Þ
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where k (L mol 1 min 1) is the rate constant of the crystallization
process and [Li + ] is the Li + concentration (g L 1) measured at
time t. Analysis of Li2CO3 crystal growth data is facilitated by the
integrated form
1
1
¼ kt þ C
½Li þ ½Li þ 0
ð19Þ
where [Li + ]0 is the initial concentration of lithium ions (g L 1) and
C is a constant related with the crystallization process.
Fig. 6. Plot of 1/[Li + ] 1/[Li + ]0 versus time during the nucleation period under
different temperatures (the data of [Li + ], [Li + ]0 and time are from Fig. 3).
Fig. 7. Plot of 1/[Li + ] 1/[Li + ]0 versus time during the grain growth period under
different temperatures (the data of [Li + ], [Li + ]0 and time are from Fig. 3).
2349
The linear plots of 1/[Li + ] 1/[Li + ]0 as a function of time
presented in Figs. 6 and 7 confirm that Eq. (19) is valid to interpret
the experimental results.
Table 3 shows the rate constants k in different time intervals
calculated via Eq. (19) from Figs. 6 and 7 with the corresponding
correlation coefficient f.
According to Arrhenius equation
ln k ¼ Ea
þb
RT
ð20Þ
where k is the apparent rate constant (L mol 1 min 1); Ea is the
apparent activation energy (kJ mol 1); R is the gas constant
(8.314 J mol 1 K 1); T is the absolute temperature (K); and b is a
constant.
Plot ln k versus 1/T is shown in Fig. 8; two lines were obtained
with correlation coefficients of 0.9997 and 0.9973. Ea of the two
periods were found to be 19.78 kJ mol 1 in the nucleation period
and 6.988 kJ mol 1 in the crystal grain growth period. It is
obvious that Ea of the nucleation period is bigger than that of the
crystal grain growth period.
3.4.4. The growth mechanism of Li2CO3 crystals
The crystal growth can be controlled by diffusion, surface
mono-nuclear growth or surface polynuclear growth. A simple
method to judge the crystal growth mechanism is to evaluate the
activation energy of the reaction. Generally, as far as a process
controlled by diffusion is concerned, its activation energy is less
than 20 kJ mol 1, while as for a process controlled by chemical
reaction, its activation energy is higher than 40 kJ mol 1 [18–20].
From the activation energy we obtained above, which are less
Fig. 8. Arrhenius diagram for determining the activation energy (the data k are
calculated from Figs. 6 and 7).
Table 3
Kinetic parameters fitted under different temperatures.
Parameters
k(tind 60 min) (L mol 1 min 1)
f(tind 60 min)
k(80–120 min) (L mol 1 min 1)
f(80–120 min)
Ea(tind 60 min) (kJ mol 1)
Ea(80–120 min) (kJ mol 1)
T (K)
323
333
343
353
363
0.00202
0.9976
0.00168
0.9992
0.00256
0.9987
0.00179
0.9986
0.00316
0.9981
0.00197
0.9967
19.78
6.988
0.00380
0.9980
0.00208
0.9981
0.00457
0.9979
0.00223
0.9970
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than 20 kJ mol 1, it can be concluded that diffusion mechanism is
dominant for crystallization of Li2CO3 from LiHCO3 solutions.
4. Conclusions
The analysis of the crystallization mechanism showed that the
formation and diffusion of CO23 were very important for the
crystallization process. Factors influencing the crystallization
process were investigated and discussed, and the critical thermodynamic parameters of Li2CO3 crystallization were calculated
based on the discussion for the relationship between induction
time and supersaturation. The results showed that the crystallization kinetics coincided with the second-order reaction rate
equation, and the apparent activation energy of the process was
obtained. The analysis of the crystal growth mechanism showed
that diffusion mechanism was dominant for crystallization of
Li2CO3 from LiHCO3 solutions. This study will provide theoretical
basis for the crystallizer design and optimization of the process
operation.
Acknowledgements
The authors would like to thank the Chinese Academy of
Sciences and the Ministry of Science and Technology of China for
their financial support. The authors are also indebted to those
who helped in one way or other to complete this work.
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