Solubility and Metastable Zone Width of Sodium TetraborateDecahydrate in a Solution Containing Lithium ChlorideJiaoyu Peng,† Zhen Nie,§ Lili Li,†,‡ Liping Wang,†,‡ Yaping Dong,*,† and Wu Li†

†Qinghai Institute of Salt Lakes, Chinese Academy of Sciences, 810008, Xining, China‡Graduate University of Chinese Academy of Sciences, 100039, Beijing, China§Chinese Academy of Geological Sciences, 100037, Beijing, China

ABSTRACT: The solubility and the metastable zone width of sodiumtetraborate decahydrate in solutions containing lithium chloride have beenmeasured at temperature a range of (20 to 35) °C. It is found that the lithiumimpurity has a salt-in effect on the solubility of sodium tetraboratedecahydrate. The metastable zone width broadens with an increase of theimpurity concentrations. This increase can be attributed to the absorptionmechanism on the crystal surface. The apparent nucleation order of sodiumtetraborate decahydrate with and without impurity was calculated by amodified regression method. The results show that the addition of lithiumimpurity has no great effect on the apparent nucleation order of sodiumtetraborate decahydrate.

■ INTRODUCTION

On Qinghai-Tibet Plateau, China, most of the salt lakes are richin boron and lithium mineral resources.1 A typical example ofthe boron-containing lake is the Zabuye Salt Lake. This salt lakebelongs to a carbonate-type2−5 of salt lake, and the brine of thissalt lake contains a great quantity of lithium, boron, andpotassium resources. During the late period of evaporation, theborate crystallizes out from the concentrated brine in the formof high-grade sodium tetraborate decahydrate, which isfavorable for the industrial production of refined sodiumtetraborate decahydrate or boric acid. Because of the mainconstituents of boron-containing salt lake, the influence ofcoexisting elements in brine on the crystallization path of boraxcannot be neglected and until now still remained unclear.Knowledge of the meatastable zone width is important for

designing a crystallization process. The metastable zone widthmay affect crystallization in many aspects such as nucleation,6

crystal growth,6,7 morphology,8 and quality of the product.However, the limit of the metastable zone in contrast to thesaturation limit is thermodynamically not defined. It dependson a number of parameters such as temperature,9 coolingrates,10 impurities,11−13 and solution dynamics, etc. Theinfluence of impurities on the metastable zone width iscomplicated and not predictable. Impurities can change theequilibrium solubility or the solution structure by absorption onnuclei or heteronuclei14 and also by the complex formation inthe solution.15 In this paper, the influence of temperature,cooling rates, and lithium impurity on the metastable zonewidth of sodium tetraborate decahydrate has been investigatedby laser technique.

■ EXPERIMENT SECTION

Materials and Apparatus. The chemical reagentsemployed in the experiment are listed in Table 1. The sodiumtetraborate decahydrate was recrystallized from aqueoussolutions. The anhydrous lithium chloride, provided fromJ&K Scientific Co., Ltd., was used without further purification.The water used (resistivity, 18.25 MΩ·cm) was deionized froma water purification system (UPT-II-20T, Chengdu UltrapureTechnology Co., Ltd.) before experiments. Figure 1 shows theexperimental apparatus for measuring the solubility and thesupersolubility of sodium tetraborate decahydrate.16

Solubility Measurements. Solubility measurements by thepolythermal method were performed in the temperature rangeof (20 to 35) °C. First, lithium chloride solution and solidsodium tetraborate decahydrate were weighed and placed in an80 mL well-sealed triple glass vessel. The suspensions werethen stirred at 200 rpm and heated until dissolution of all thesolid phase. The corresponding temperature of dissolution wasrecorded as Tdis. These steps were repeated at five heating ratesof (15, 25, 35, 45 and 55) °C·h−1. Last, the saturationtemperature Tsat of borax can be obtained by the extrapolationof Tdis to a virtual heating rate “zero”.17 The content of sodiumtetraborate decahydrate in the solution was determined bytitration, and the Li+ was measured by atomic absorptionspectrometry. Each experiment was conducted in duplicate.

Metastable Zone Width Measurements. The metastablezone width of sodium tetraborate decahydrate in the solution

Received: January 18, 2013Accepted: March 22, 2013

Article

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containing lithium chloride was measured. The programmedmethod was based on the conventional polythermal techniqueand involved the following process. (i) An amount ofNa2B4O7−LiCl−H2O mixture was prepared. The mixture wasthen heated above saturation temperature, filtered using amembrane filter (Millipore 0.22 μm pore size). The filtrate (80mL) was placed into a well-sealed triple-jacketed glass vessel.(ii) The solution was cooled at five cooling rates of (15, 25, 35,45 and 55) °C·h−1. The temperature at the point of nucleationwas recorded as Tnuc. The difference between the saturationtemperature and the temperature at the point of nucleation isconsidered as the metastable zone width ΔTmax, which can begiven as

Δ = −T T Tmax sat nuc

Table 2 shows a summary of the estimated uncertainties.

■ RESULTS AND DISCUSSIONXRD Analysis. The XRD analysis was performed using a

tube voltage and current of 40 kV and 30 mA. The scanningposition 2θ is from 5.0014° to 69.9754°. Figure 2 scans a and bshow the XRD patterns of the pure sodium tetraboratedecahydrate (reference code:18 01-075-1078) and with addedlithium impurity. As can be seen from Figure 2, the X-raypowder diffraction patterns of both pure and with lithiumimpurity are identical, illustrating that the mentioned impurityhas not entered into the structure of sodium tetraboratedecahydrate crystal. The crystallographic parameters of sodiumtetraborate decahydrate added lithium impurity are: C2/c, Z =4, a = 11.875, b = 10.578, c = 12.282 Å, β = 106.365°, α = 90°,and γ = 90°, which is in close agreement with the reportedvalues.18,19

FTIR Spectral Analysis. The FTIR spectrum of sodiumtetraborate decahydrate was also recorded in the range of (400to 4000) cm−1 at room temperature. Figure 3 curves a and b

show the spectra of sodium tetraborate decahydrate both in thepure state and with added lithium impurity, respectively. InFigure 3a, the broad band positioned between 3200.00 cm−1

and 3579.72 cm−1 corresponds to the O−H stretchingvibration. The presence of H2O in borax is revealed by thefrequency at 1646.92 cm−1. The peaks at 948.68 cm−1 and1419.38 cm−1 can be assigned to the asymmetric and symmetricstretching vibrations of B(3)-O. The presence of B−O−H in-plane bending corresponds to the peak at 1154.37 cm−1.

Table 1. Chemical Reagents Employed

chemical name source initial mass fraction purity puritification method final mass fraction purity analysis method

sodium tetraborate decahydrate Tianjin Yongdaa > 0.995 recrystallization 0.9995 titrationanhydrous lithium chloride J&K Scientific Co., Ltd. > 0.99 noneultrapure grade water UPT-II-20Tb 18.25 MΩ·cm (resistivity) purification system

aTianjin Yongda Chemical reagent Co., Ltd. China. bChengdu Ultrapure Technology Co., Ltd. China.

Figure 1. Apparatus for solubility and crystallization measurements: 1,laser generator; 2, lectromagnetic stirrer; 3, jacketed glass vessel; 4,precise thermometer; 5, photoelectric converter; 6, programmablethermostatic bath.

Table 2. Uncertainties of Measurements Estimated for ThisResearch

property estimated uncertainty

solubility ± (0.01 to 0.06) g of 100 g H2Osaturation temperature ± 0.06 °Cω (LiCl) ± (0.004 to 0.04) %metastable zone width (ΔTmax) ± 0.06 °C

Figure 2. XRD pattern of sodium tetraborate decahydrate: (a) pure;(b) with lithium impurity.

Figure 3. FTIR spectrum of sodium tetraborate decahydrate: (a) pure;(b) with lithium impurity.

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Asymmetric and symmetric stretching vibrations of B(4)-O areobserved at peaks of 1073.10 cm−1 and 997.41 cm−1. The bandat 533.79 cm−1 is assigned to the characteristic vibration peak ofpolyborate anions. The peak at 448.91 cm−1 can be attributedto the bending of B(4)−O. In Figure 3b, the spectrum iscompletely identical with the pure sodium tetraboratedecahydrate. There is not any peak corresponding to thelithium impurity. As a result, it can be concluded that the crystalstructure of sodium tetraborate decahydrate has not changedwhen adding lithium impurity in the solution.Effect of Impurity on Solubility of Sodium Tetrabo-

rate Decahydrate. The effect of lithium impurity on thesolubility of sodium tetraborate decahydrate was investigated.The obtained experimental solubility data and the uncertaintiesare listed in Table 3 and demonstrated graphically in Figure 4.

As can be seen in Figure 4, the presence of a lithium impurityhas led to an increase in the solubility of sodium tetraboratedecahydrate. The solubility of sodium tetraborate decahydrateincreases linearly with increasing concentrations of impurities.The observations made can be explained in terms of the salteffect of the addition of electrolytes, which is described in detailin our previous paper.16

Effect of Impurity on Metastable Zone Width ofSodium Tetraborate Decahydrate. Saturated solutions withvariable concentrations of impurity were prepared and differentcooling rates were applied to determine the metastable zonewidth of sodium tetraborate decahydrate. The correspondingmetastable zone width of sodium tetraborate decahydrate isgiven in Figure 5. It is found that the metastable zone width ofsodium tetraborate decahydrate increases with increasing

cooling rates and decreases with the increase in saturationtemperature. In the presence of lithium impurity, themetastable zone width broadens with increasing concentrationsof lithium chloride. There is little impact on the metastablezone width when the concentration of lithium chloride is below0.30 %. This indicates that the changes of metastable zonewidth can be ignored at low lithium chloride concentration.The impurities affect the width of the metastable zone by

different mechanisms.11−14 A possible mechanism of theinfluence of impurities on metastable zone width can beexplained by the adsorption of impurities on nuclei or crystalgrowth surface. Generally, a delay in nucleation and a growthreduction can lead to an increase in metastable zone width.Contrary to this, enhanced nucleation with a moderate growthreduction can reduce the metastable zone width. Impurities caneither enhance the nucleation rate due to a reduced interfacialtension or suppress it by occupying active growth sites onnuclei or foreign particles. In terms of the adsorption model,the influence of the lithium impurity on metastable zone widthcan be interpreted as a progressively increasing surface coverageon the nuclei or on the crystal surface.On the other hand, the solubility changes of sodium

tetraborate decahydrate in the presence of lithium impuritycan also affect the meatastable zone width.14 As studied above,the solubility of sodium tetraborate decahydrate in the presenceof a lithium impurity is greater than the equilibrium solubility.This means that the saturation temperature of sodiumtetraborate decahydrate has been changed and becomes lowercompared to that of pure solution. From Figure 5a to Figure5d, the metastable zone width increases with the decrease ofsaturation temperatures. Therefore, the increase in thesolubility of sodium tetraborate decahydrate tends to enhancethe metastable zone width depending on the concentrations ofimpurity.

Calculation of Apparent Nucleation Order m. Accord-ing to the classical theory of nucleation,20−22 the nucleationrate, in the number of crystals, can be expressed as

= = ΔJNt

k cdd

mn max (1)

where J is the nucleation rate, kn is the nucleation rate constantand Δcmaxm is the maximum allowable supersaturation. Theexponent “m” is the apparent order of nucleation.When the supersaturation is created by cooling, the

nucleation rate is also expressed as a function of cooling rate,assuming that the nucleation rate equals the supersaturationrate (within a limited period during which it is possible toneglect the growth of just formed crystals):21

β=*

−JcT

dd

( )(2)

Table 3. Solubility of Sodium Tetraborate Decahydrate in the Lithium Chloride Solution at Different Saturation Temperaturea

s

ω(LiCl) Tsat = 20.74 Tsat = 25.63 Tsat = 30.63 Tsat = 34.63

none 4.97 ± 0.01 6.21 ± 0.03 7.78 ± 0.04 9.59 ± 0.050.12 ± 0.004 5.07 ± 0.03 6.34 ± 0.04 7.99 ± 0.04 9.66 ± 0.050.30 ± 0.01 5.16 ± 0.04 6.43 ± 0.04 8.14 ± 0.05 9.79 ± 0.060.60 ± 0.03 5.29 ± 0.03 6.56 ± 0.04 8.36 ± 0.05 9.98 ± 0.061.18 ± 0.04 5.51 ± 0.03 6.86 ± 0.05 8.61 ± 0.06 10.39 ± 0.06

aω in %; s in g of 100 g water; Tsat in °C.

Figure 4. Effect of lithium impurity on the solubility of sodiumtetraborate decahydrate at different saturation temperature: ▼, 34.63°C; ▲, 30.63 °C; ●, 25.63 °C; ■, 20.74 °C.

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where dc*/dT is the slope of the equilibrium solubility line andcan be obtained from a given saturation temperature; β is thecooling rate. The relationship between the maximum allowablesupersaturation, Δcmax and the maximum allowable under-cooling, ΔTmax can be expressed by

Δ =*

Δ⎛⎝⎜

⎞⎠⎟c

cT

Tddmax max

(3)

Combining eq 1, 2, and 3 yields

βΔ = − *− − −

⎛⎝⎜

⎞⎠⎟T

mm

cT

km m

log1

logdd

log 1log( )max

n

(4)

Equation 4 can be given by the equation of straight line,

= +Y A Bx (5)

where x = log(−β) and Y = log(ΔTmax). The slope of thestraight line given by eq 5 is the inverse of the apparent order mof a given system. The relationships between nucleation ratesand metastable zone width and the apparent order of sodiumtetraborate decahydrate in the presence of lithium impurity arelisted in Table 4.

Since there are some deviations because of the measurementerrors, the apparent orders mi of sodium tetraboratedecahydrate with different saturation concentrations are notexactly identical. A modified linear regression method has beenused to correct the apparent orders by the following formula.20

=∑ ∑ − ∑ ·∑

∑ ∑ − ∑=

=m

x y x N y

x x N1 [ / ]

[ ( ) / ]jp

i i i i i j i i

jp

i i i i j

1

12 2

(6)

where xi = log(−βi), yi = log(ΔTmax)i, p is the total number ofstraight lines, and Nj is the number of measurements carriedout for each line. As can be observed from Table 4, thenucleation order m of sodium tetraborate decahydrate in a puresystem is about 3.40, which is in good agreement with theliterature value 3.3.20 We have also calculated the nucleationorder m in the impure system. It is found that there is only alittle influence of the lithium impurity on the nucleation orderm of sodium tetraborate decahydrate.

■ CONCLUSIONThe influence of lithium impurity on the solubility andmetastable zone width of sodium tetraborate decahydrate hasbeen investigated by the polythermal method. The presence of

Figure 5. Effect of lithium impurity on the metastable zone width of sodium tetraborate decahydrate at different saturation temperature. Massfractions of LiCl (%): ■, 0.00; ○, 0.12; ◇, 0.30; Δ, 0.60; □, 1.18. Saturation temperature: (a) 20.74 °C; (b) 25.63 °C; (c) 30.63 °C; (d) 34.63 °C.

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a lithium impurity has led to an increase value in the solubilityof sodium tetraborate decahydrate in all concentrations. Thiscould be attributed to the salt effect of the addition of lithiumchloride electrolyte in the solution. The metastable zone widthbroadens with impurity concentrations. There is a negligibleeffect on the metastable zone width at low lithium impurityconcentrations. The possible reason may be due to theabsorption mechanism on the nuclei or crystal surface. As faras the apparent nucleation order of sodium tetraboratedecahydrate in the absence of impurity is concerned, theaddition of lithium impurity has no great influence on theapparent nucleation order of sodium tetraborate decahydrate.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: dyp811@126.com. Tel.: 86-971-6302023. Fax: 86-971- 6310402.FundingThis work is financially supported in part by the NationalNatural Science Foundation of China (No. 41273032 and No.41073050).NotesThe authors declare no competing financial interest.

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(6) Mullin, J. W.; Amatavivadhan, A.; Chakrabprty, M. Crystal HabitModification Studies with Ammonium and Potassium DihydrogenPhosphate. J. Appl. Chem. 1970, 20, 153−158.(7) Ginde, R. M.; Myerson, A. S. Effect of Impurities on ClusterGrowth and Nucleation. J. Cryst. Growth 1993, 126, 216−222.(8) Dash, S. R.; Rohani, S. Effect of Magnesium and Sulfate Ions onKCl Crystallization in a Continuous Cooling MSMPR Crystallization.Chem. Eng. Commun. 1993, 125, 211−226.(9) Sayan, P.; Ulrich, J. Effect of Various Impurities on theMetastable Zone Width of Boric Acid. Cryst. Res. Technol. 2001, 36,411−417.(10) Ma, Y.; Zhu, J. W.; Ren, H. R.; Chen, K. Effects of Impurity Ionson Solubility and Metastable Zone Width of Phosphoric Acid. Cryst.Res. Technol. 2009, 44, 1313−1318.(11) Rauls, M.; Bartosch, K.; Kind, M.; Kuch, St.; Lacmann, R.;Mersmann, A. The Influence of Impurities on CrystallizationKineticsA Case Study on Ammonium Sulfate. J. Cryst. Growth2000, 213, 116−128.(12) Dhanaraja, P. V.; Bhagavannarayana, G.; Rajesh, N. P. Effect ofAmino Acid Additives on Crystal Growth Parameters and Propertiesof Ammonium Dihydrogen Orthophosphate Crystals. Mater. Chem.Phys. 2008, 112, 490−495.(13) Ginde, R. M.; Myerson, A. S. Effect of Impurities on ClusterGrowth and Nucleation. J. Cryst. Growth 1993, 126, 216−222.(14) Titiz-Sargut, S.; Ulrich, J. Influence of Additives on the Width ofthe Metastable Zone. Cryst. Growth Des. 2002, 2, 371−374.(15) Rajesh, N. P.; Meera, K.; Srinivasan, K.; Raghavan, P. S.;Ramasamy, P. Effect of EDTA on the Metastable Zone Width of ADP.J. Cryst. Growth 2000, 213, 389−394.(16) Peng, J. Y.; Dong, Y. P.; Nie, Z.; Kong, F. Z.; Meng, Q. F.; Li, W.Solubility and Metastable Zone Width Measurement of BoraxDecahydrate in Potassium Chloride Solution. J. Chem. Eng. Data2012, 57, 890−895.(17) Barrett, P.; Glennon, B. Characterizing the Metastable ZoneWidth and Solubility Curve using Lasentec FBRM and PVM. Trans.IChemE, Part A 2002, 80, 799−805.(18) Levy, H. A.; Lisensky, G. C. Crystal Structures of SodiumSulfate Decahydrate (Glauber’s Salt) and Sodium TetraborateDecahydrate (Borax): Redetermination by Neutron Niffraction. ActaCrystallogr. 1978, B34, 3502−3510.

Table 4. Calculation of Apparent Order m of Sodium Tetraborate Decahydrate in the Solution Containing Lithium Chloridea

ω(LiCl) ω(Na2B4O7·10H2O) relationships between β and ΔTmax mi m (modified)

0.00 4.74 log ΔTmax = 0.6492 + 0.2705 log(−β) 3.72 3.405.85 log ΔTmax = 0.5521 + 0.3045 log(−β) 3.287.14 log ΔTmax = 0.5086 + 0.2984 log(−β) 3.358.75 log ΔTmax = 0.5085 + 0.2707 log(−β) 3.95

0.12 ± 0.004 4.82 log ΔTmax = 0.6486 + 0.2656 log(−β) 3.76 3.685.95 log ΔTmax = 0.6002 + 0.2744 log(−β) 3.647.39 log ΔTmax = 0.5184 + 0.2923 log(−β) 3.428.80 log ΔTmax = 0.5476 + 0.2402 log(−β) 4.16

0.30 ± 0.01 4.90 log ΔTmax = 0.6114 + 0.2921 log(−β) 3.42 3.706.03 log ΔTmax = 0.5831 + 0.2849 log(−β) 3.517.51 log ΔTmax = 0.5413 + 0.2821 log(−β) 3.538.85 log ΔTmax = 0.5603 + 0.2340 log(−β) 4.27

0.60 ± 0.03 5.00 log ΔTmax = 0.6392 + 0.2761 log(−β) 3.62 3.796.12 log ΔTmax = 0.6240 + 0.2613 log(−β) 3.827.67 log ΔTmax = 0.5462 + 0.2839 log(−β) 3.528.95 log ΔTmax = 0.5630 + 0.2344 log(−β) 4.27

1.18 ± 0.04 5.16 log ΔTmax = 0.6503 + 0.2692 log(−β) 3.71 3.916.34 log ΔTmax = 0.6565 + 0.2474 log(−β) 4.047.84 log ΔTmax = 0.5650 + 0.2781 log(−β) 3.599.30 log ΔTmax = 0.5838 + 0.2285 log(−β) 4.38

aω in %; β in °C·h−1; ΔTmax in °C.

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(19) Gainsford, G. J.; Kemmitt, T.; Higham, C. Redetermination ofThe Borax Structure from Laboratory X-ray Data at 145 K. ActaCrystallogr. 2008, E64, i24−i25.(20) Nyvlt, J.; Sohnel, O.; Matuchova, M.; Broul, M. The Kinetics ofIndustrial Crystallization; Elsevier: Amsterdam, The Netherlands, 1985.(21) Mullin, J. W. Crystallization, 2nd ed.; Butterworth: London,1972.(22) Jaroslay, Nyv́lt. Kinetics of nucleation in solutions. J. Cryst.Growth 1968, 3−4, 377−383.

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