Solubility and Metastable Zone Width of Sodium Tetraborate

Apr 9, 2013 - tetraborate decahydrate in solutions containing lithium chloride have been ... impurity has a salt-in effect on the solubility of sodium...
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Solubility and Metastable Zone Width of Sodium Tetraborate Decahydrate in a Solution Containing Lithium Chloride Jiaoyu 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 sodium tetraborate decahydrate in solutions containing lithium chloride have been measured at temperature a range of (20 to 35) °C. It is found that the lithium impurity has a salt-in effect on the solubility of sodium tetraborate decahydrate. The metastable zone width broadens with an increase of the impurity concentrations. This increase can be attributed to the absorption mechanism on the crystal surface. The apparent nucleation order of sodium tetraborate decahydrate with and without impurity was calculated by a modified regression method. The results show that the addition of lithium impurity has no great effect on the apparent nucleation order of sodium tetraborate decahydrate.

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INTRODUCTION

EXPERIMENT SECTION Materials and Apparatus. The chemical reagents employed in the experiment are listed in Table 1. The sodium tetraborate decahydrate was recrystallized from aqueous solutions. The anhydrous lithium chloride, provided from J&K Scientific Co., Ltd., was used without further purification. The water used (resistivity, 18.25 MΩ·cm) was deionized from a water purification system (UPT-II-20T, Chengdu Ultrapure Technology Co., Ltd.) before experiments. Figure 1 shows the experimental apparatus for measuring the solubility and the supersolubility of sodium tetraborate decahydrate.16 Solubility Measurements. Solubility measurements by the polythermal method were performed in the temperature range of (20 to 35) °C. First, lithium chloride solution and solid sodium tetraborate decahydrate were weighed and placed in an 80 mL well-sealed triple glass vessel. The suspensions were then stirred at 200 rpm and heated until dissolution of all the solid phase. The corresponding temperature of dissolution was recorded as Tdis. These steps were repeated at five heating rates of (15, 25, 35, 45 and 55) °C·h−1. Last, the saturation temperature Tsat of borax can be obtained by the extrapolation of Tdis to a virtual heating rate “zero”.17 The content of sodium tetraborate decahydrate in the solution was determined by titration, and the Li+ was measured by atomic absorption spectrometry. Each experiment was conducted in duplicate. Metastable Zone Width Measurements. The metastable zone width of sodium tetraborate decahydrate in the solution

On Qinghai-Tibet Plateau, China, most of the salt lakes are rich in boron and lithium mineral resources.1 A typical example of the boron-containing lake is the Zabuye Salt Lake. This salt lake belongs to a carbonate-type2−5 of salt lake, and the brine of this salt lake contains a great quantity of lithium, boron, and potassium resources. During the late period of evaporation, the borate crystallizes out from the concentrated brine in the form of high-grade sodium tetraborate decahydrate, which is favorable for the industrial production of refined sodium tetraborate decahydrate or boric acid. Because of the main constituents of boron-containing salt lake, the influence of coexisting elements in brine on the crystallization path of borax cannot be neglected and until now still remained unclear. Knowledge of the meatastable zone width is important for designing a crystallization process. The metastable zone width may 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 the saturation limit is thermodynamically not defined. It depends on a number of parameters such as temperature,9 cooling rates,10 impurities,11−13 and solution dynamics, etc. The influence of impurities on the metastable zone width is complicated and not predictable. Impurities can change the equilibrium solubility or the solution structure by absorption on nuclei or heteronuclei14 and also by the complex formation in the solution.15 In this paper, the influence of temperature, cooling rates, and lithium impurity on the metastable zone width of sodium tetraborate decahydrate has been investigated by laser technique. © XXXX American Chemical Society

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

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Table 1. Chemical Reagents Employed

a

chemical name

source

initial mass fraction purity

puritification method

final mass fraction purity

analysis method

sodium tetraborate decahydrate anhydrous lithium chloride ultrapure grade water

Tianjin Yongdaa J&K Scientific Co., Ltd. UPT-II-20Tb

> 0.995 > 0.99 18.25 MΩ·cm (resistivity)

recrystallization none purification system

0.9995

titration

Tianjin 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, programmable thermostatic bath.

containing lithium chloride was measured. The programmed method was based on the conventional polythermal technique and involved the following process. (i) An amount of Na2B4O7−LiCl−H2O mixture was prepared. The mixture was then heated above saturation temperature, filtered using a membrane filter (Millipore 0.22 μm pore size). The filtrate (80 mL) 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 nucleation was recorded as Tnuc. The difference between the saturation temperature and the temperature at the point of nucleation is considered as the metastable zone width ΔTmax, which can be given as

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

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

ΔTmax = Tsat − Tnuc

Table 2 shows a summary of the estimated uncertainties. Table 2. Uncertainties of Measurements Estimated for This Research property solubility saturation temperature ω (LiCl) metastable zone width (ΔTmax)

estimated uncertainty ± ± ± ±

(0.01 to 0.06) g of 100 g H2O 0.06 °C (0.004 to 0.04) % 0.06 °C

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RESULTS AND DISCUSSION XRD Analysis. The XRD analysis was performed using a tube voltage and current of 40 kV and 30 mA. The scanning position 2θ is from 5.0014° to 69.9754°. Figure 2 scans a and b show the XRD patterns of the pure sodium tetraborate decahydrate (reference code:18 01-075-1078) and with added lithium impurity. As can be seen from Figure 2, the X-ray powder diffraction patterns of both pure and with lithium impurity are identical, illustrating that the mentioned impurity has not entered into the structure of sodium tetraborate decahydrate crystal. The crystallographic parameters of sodium tetraborate 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 reported values.18,19

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

show the spectra of sodium tetraborate decahydrate both in the pure state and with added lithium impurity, respectively. In Figure 3a, the broad band positioned between 3200.00 cm−1 and 3579.72 cm−1 corresponds to the O−H stretching vibration. The presence of H2O in borax is revealed by the frequency at 1646.92 cm−1. The peaks at 948.68 cm−1 and 1419.38 cm−1 can be assigned to the asymmetric and symmetric stretching vibrations of B(3)-O. The presence of B−O−H inplane bending corresponds to the peak at 1154.37 cm−1. B

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Table 3. Solubility of Sodium Tetraborate Decahydrate in the Lithium Chloride Solution at Different Saturation Temperaturea s ω(LiCl) none 0.12 ± 0.30 ± 0.60 ± 1.18 ± a

0.004 0.01 0.03 0.04

Tsat = 20.74 4.97 5.07 5.16 5.29 5.51

± ± ± ± ±

0.01 0.03 0.04 0.03 0.03

Tsat = 25.63 6.21 6.34 6.43 6.56 6.86

± ± ± ± ±

Tsat = 30.63

0.03 0.04 0.04 0.04 0.05

7.78 7.99 8.14 8.36 8.61

± ± ± ± ±

0.04 0.04 0.05 0.05 0.06

Tsat = 34.63 9.59 9.66 9.79 9.98 10.39

± ± ± ± ±

0.05 0.05 0.06 0.06 0.06

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

cooling rates and decreases with the increase in saturation temperature. In the presence of lithium impurity, the metastable zone width broadens with increasing concentrations of lithium chloride. There is little impact on the metastable zone width when the concentration of lithium chloride is below 0.30 %. This indicates that the changes of metastable zone width 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 the influence of impurities on metastable zone width can be explained by the adsorption of impurities on nuclei or crystal growth surface. Generally, a delay in nucleation and a growth reduction can lead to an increase in metastable zone width. Contrary to this, enhanced nucleation with a moderate growth reduction can reduce the metastable zone width. Impurities can either enhance the nucleation rate due to a reduced interfacial tension or suppress it by occupying active growth sites on nuclei or foreign particles. In terms of the adsorption model, the influence of the lithium impurity on metastable zone width can be interpreted as a progressively increasing surface coverage on the nuclei or on the crystal surface. On the other hand, the solubility changes of sodium tetraborate decahydrate in the presence of lithium impurity can also affect the meatastable zone width.14 As studied above, the solubility of sodium tetraborate decahydrate in the presence of a lithium impurity is greater than the equilibrium solubility. This means that the saturation temperature of sodium tetraborate decahydrate has been changed and becomes lower compared to that of pure solution. From Figure 5a to Figure 5d, the metastable zone width increases with the decrease of saturation temperatures. Therefore, the increase in the solubility of sodium tetraborate decahydrate tends to enhance the metastable zone width depending on the concentrations of impurity. Calculation of Apparent Nucleation Order m. According to the classical theory of nucleation,20−22 the nucleation rate, in the number of crystals, can be expressed as

Asymmetric and symmetric stretching vibrations of B(4)-O are observed at peaks of 1073.10 cm−1 and 997.41 cm−1. The band at 533.79 cm−1 is assigned to the characteristic vibration peak of polyborate anions. The peak at 448.91 cm−1 can be attributed to the bending of B(4)−O. In Figure 3b, the spectrum is completely identical with the pure sodium tetraborate decahydrate. There is not any peak corresponding to the lithium impurity. As a result, it can be concluded that the crystal structure of sodium tetraborate decahydrate has not changed when adding lithium impurity in the solution. Effect of Impurity on Solubility of Sodium Tetraborate Decahydrate. The effect of lithium impurity on the solubility of sodium tetraborate decahydrate was investigated. The obtained experimental solubility data and the uncertainties are listed in Table 3 and demonstrated graphically in Figure 4.

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

As can be seen in Figure 4, the presence of a lithium impurity has led to an increase in the solubility of sodium tetraborate decahydrate. The solubility of sodium tetraborate decahydrate increases linearly with increasing concentrations of impurities. The observations made can be explained in terms of the salt effect of the addition of electrolytes, which is described in detail in our previous paper.16 Effect of Impurity on Metastable Zone Width of Sodium Tetraborate Decahydrate. Saturated solutions with variable concentrations of impurity were prepared and different cooling rates were applied to determine the metastable zone width of sodium tetraborate decahydrate. The corresponding metastable zone width of sodium tetraborate decahydrate is given in Figure 5. It is found that the metastable zone width of sodium tetraborate decahydrate increases with increasing

J=

dN m = k nΔcmax dt

(1)

where J is the nucleation rate, kn is the nucleation rate constant and Δcmmax is the maximum allowable supersaturation. The exponent “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 supersaturation rate (within a limited period during which it is possible to neglect the growth of just formed crystals):21 J= C

dc * (−β) dT

(2)

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Figure 5. Effect of lithium impurity on the metastable zone width of sodium tetraborate decahydrate at different saturation temperature. Mass fractions 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.

Since there are some deviations because of the measurement errors, the apparent orders mi of sodium tetraborate decahydrate with different saturation concentrations are not exactly identical. A modified linear regression method has been used to correct the apparent orders by the following formula.20

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

⎛ dc * ⎞ =⎜ ⎟ΔTmax ⎝ dT ⎠

p

∑j = 1 [∑i xiyi − ∑i xi /Nj·∑i yi ] 1 = p m ∑j = 1 [∑i xi2 − (∑i xi)2 /Nj]

(3)

Combining eq 1, 2, and 3 yields log ΔTmax

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

⎛ dc * ⎞ log k n 1−m 1 = log⎜ − log( −β) ⎟− m m m ⎝ dT ⎠ (4)

Equation 4 can be given by the equation of straight line, Y = A + Bx

(6)

(5)

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where x = log(−β) and Y = log(ΔTmax). The slope of the straight line given by eq 5 is the inverse of the apparent order m of a given system. The relationships between nucleation rates and metastable zone width and the apparent order of sodium tetraborate decahydrate in the presence of lithium impurity are listed in Table 4.

CONCLUSION The influence of lithium impurity on the solubility and metastable zone width of sodium tetraborate decahydrate has been investigated by the polythermal method. The presence of D

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

ω(Na2B4O7·10H2O)

0.00

0.12 ± 0.004

0.30 ± 0.01

0.60 ± 0.03

1.18 ± 0.04

a

4.74 5.85 7.14 8.75 4.82 5.95 7.39 8.80 4.90 6.03 7.51 8.85 5.00 6.12 7.67 8.95 5.16 6.34 7.84 9.30

relationships between β and ΔTmax log log log log log log log log log log log log log log log log log log log log

ΔTmax ΔTmax ΔTmax ΔTmax ΔTmax ΔTmax ΔTmax ΔTmax ΔTmax ΔTmax ΔTmax ΔTmax ΔTmax ΔTmax ΔTmax ΔTmax ΔTmax ΔTmax ΔTmax ΔTmax

= = = = = = = = = = = = = = = = = = = =

0.6492 0.5521 0.5086 0.5085 0.6486 0.6002 0.5184 0.5476 0.6114 0.5831 0.5413 0.5603 0.6392 0.6240 0.5462 0.5630 0.6503 0.6565 0.5650 0.5838

+ + + + + + + + + + + + + + + + + + + +

0.2705 0.3045 0.2984 0.2707 0.2656 0.2744 0.2923 0.2402 0.2921 0.2849 0.2821 0.2340 0.2761 0.2613 0.2839 0.2344 0.2692 0.2474 0.2781 0.2285

log(−β) log(−β) log(−β) log(−β) log(−β) log(−β) log(−β) log(−β) log(−β) log(−β) log(−β) log(−β) log(−β) log(−β) log(−β) log(−β) log(−β) log(−β) log(−β) log(−β)

mi

m (modified)

3.72 3.28 3.35 3.95 3.76 3.64 3.42 4.16 3.42 3.51 3.53 4.27 3.62 3.82 3.52 4.27 3.71 4.04 3.59 4.38

3.40

3.68

3.70

3.79

3.91

ω in %; β in °C·h−1; ΔTmax in °C. (6) Mullin, J. W.; Amatavivadhan, A.; Chakrabprty, M. Crystal Habit Modification Studies with Ammonium and Potassium Dihydrogen Phosphate. J. Appl. Chem. 1970, 20, 153−158. (7) Ginde, R. M.; Myerson, A. S. Effect of Impurities on Cluster Growth and Nucleation. J. Cryst. Growth 1993, 126, 216−222. (8) Dash, S. R.; Rohani, S. Effect of Magnesium and Sulfate Ions on KCl Crystallization in a Continuous Cooling MSMPR Crystallization. Chem. Eng. Commun. 1993, 125, 211−226. (9) Sayan, P.; Ulrich, J. Effect of Various Impurities on the Metastable 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 Ions on 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 Crystallization KineticsA Case Study on Ammonium Sulfate. J. Cryst. Growth 2000, 213, 116−128. (12) Dhanaraja, P. V.; Bhagavannarayana, G.; Rajesh, N. P. Effect of Amino Acid Additives on Crystal Growth Parameters and Properties of Ammonium Dihydrogen Orthophosphate Crystals. Mater. Chem. Phys. 2008, 112, 490−495. (13) Ginde, R. M.; Myerson, A. S. Effect of Impurities on Cluster Growth and Nucleation. J. Cryst. Growth 1993, 126, 216−222. (14) Titiz-Sargut, S.; Ulrich, J. Influence of Additives on the Width of the 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 Borax Decahydrate in Potassium Chloride Solution. J. Chem. Eng. Data 2012, 57, 890−895. (17) Barrett, P.; Glennon, B. Characterizing the Metastable Zone Width 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 Sodium Sulfate Decahydrate (Glauber’s Salt) and Sodium Tetraborate Decahydrate (Borax): Redetermination by Neutron Niffraction. Acta Crystallogr. 1978, B34, 3502−3510.

a lithium impurity has led to an increase value in the solubility of sodium tetraborate decahydrate in all concentrations. This could be attributed to the salt effect of the addition of lithium chloride electrolyte in the solution. The metastable zone width broadens with impurity concentrations. There is a negligible effect on the metastable zone width at low lithium impurity concentrations. The possible reason may be due to the absorption mechanism on the nuclei or crystal surface. As far as the apparent nucleation order of sodium tetraborate decahydrate in the absence of impurity is concerned, the addition of lithium impurity has no great influence on the apparent nucleation order of sodium tetraborate decahydrate.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: 86-971-6302023. Fax: 86971- 6310402. Funding

This work is financially supported in part by the National Natural Science Foundation of China (No. 41273032 and No. 41073050). Notes

The authors declare no competing financial interest.

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REFERENCES

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

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