Solubility, Metastable Zone Width, and Nucleation Kinetics of Boric

Article pubs.acs.org/jced

Solubility, Metastable Zone Width, and Nucleation Kinetics of Boric Acid in the NaCl−KCl−CaCl2−H2O System Jiaoyu Peng,†,‡ Naijin Dong,†,‡ Yaping Dong,*,† Zheng Pan,§ and Wu Li† †

Qinghai Institute of Salt Lakes, Chinese Academy of Sciences, 810008, Xining, China University of Chinese Academy of Sciences, 100039, Beijing, China § School of Chemical Engineering, Qinghai University, 810016, Xining, China ‡

ABSTRACT: The solubility and metastable zone width of boric acid in the simulated oilfield brine system of NaCl−KCl−CaCl2−H2O were studied at low temperature by the turbidity method. The nucleation kinetic constant A and the interfacial energy γ of boric acid were calculated with nucleation theory. Results showed that the solubility of boric acid in the simulated oilfield brine system was mainly determined by the coexisting salt of CaCl2 and decreased with increasing concentrations of CaCl2, even though the presence of NaCl and KCl has a salt-in effect on the solubility. However, the metastable zone width was affected by the three coexisting salts and it changes differently with the varied concentrations. The possible reason can be explained by the dependence of the interfacial energy γ on the slat concentration. On the basis of the above results, the crystallization behavior of boric acid during oilfield brine evaporation was investigated for the first time. It was deeply affected by the coexisting salts in brine.

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(5) halite (NaCl) + sylvite (KCl) + antarcticite (CaCl2· 6H2O)→ It is noticeable the boric acid coprecipitates with the halite, sylvite, and analogous carnallites during evaporation, which leads to the difficulty of boron extraction from deposited salts owing to the low grade of boron in the precipitated mineral salts. Possibly, the unique crystallization behavior of boric acid was affected by the coexisting components in the oilfield brine, especially the calcium ion having a great impact on the solubility and metastable zone with of boric acid.18−15 Therefore, it is necessary to study the effects of coexisting components on the crystallization behavior of boric acid, which may give important information for the boron extraction from oilfield brine. According to the results of the oilfield brine crystallization path, the main composition of oilfield brine between sylvite and boric acid is Na−K−Ca−Cl−H2O. In this study, the crystallization behavior including solubility, meatasble zone width (MZW), and nucleation kinetics of boric acid in the system of NaCl−KCl−CaCl2−H2O were investigated using turbidity technology.

INTRODUCTION Boron compounds can be used in different areas because they are lightweight and have high heat resistance, fire retardency, nonlinear optics, and antiwear properties.1−5 Boric acid is an important commercial boron compound and can be used as raw material for the chemical production of boron products. Generally, the extraction of boric acid includes acid precipitation from solid borate ores and solvent extraction from liquid boron resources such as seawater, oilfields, and salt lake brine. However, with the increasing demand of boron products and the difficult exploitation of low grade boron ores, great interest in recent years has been attracted to extract boron from salt lake brine, which has become an important means of boric acid production.6−8 Oilfield brine is a kind of underground brine, and it is a valuable resource to produce inorganic salts. Nanyi Mountain oilfield brine, located in the western Qaidam Basin Qinghai province of China, is abundant in potassium, boron, lithium, and iodine resources.9 The brine composition belongs to “chloride-calcium” type according to the B. A. Sulin’s classification.10 High in salinity, high in calcium ion, and low in magnesium and sulfate ions are its primary characteristics. The crystallization path of the oilfield brine at room temperature (295 ± 5) K was11,12 (1) halite (NaCl) → (2) halite (NaCl) + sylvite (KCl) → (3) halite (NaCl) + sylvite (KCl) + boric acid (H3BO3) → (4) halite (NaCl) + sylvite (KCl) + boric acid (H3BO3) + analogous carnallites (K(NH 4)Mg2 Cl6 ·12H 2O and K2(NH4)Mg3Cl9·18H2O) → © XXXX American Chemical Society

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EXPERIMENT SECTION Materials and Apparatus. Calcium chloride hexahydrate (CaCl2·6H2O), purchased from Tianjin Damao Chemical Reagent Co., Ltd., China, was recrystallized from aqueous solutions. Sodium chloride (NaCl, 99.95 % to 100.05 %) and potassium chloride (KCl, 99.95 % to 100.05 %) were purchased Received: July 12, 2015 Accepted: October 20, 2015

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DOI: 10.1021/acs.jced.5b00581 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

temperature Tdis. The above automated run was repeatedly at five different cooling and heating rates. The saturation temperature T0 of boric acid can be obtained from the extrapolation of the plot Tdis against the heating rate. The difference between the saturation temperature and the temperature point of nucleation was defined as the MZW(ΔTmax = T0 − Tlim). Every metastable zone width measurement was repeated at least two times to verify the reproducibility of experiments. Chemical Analysis.16 For the composition analysis of the NaCl−KCl−CaCl2−H3BO3−H2O system, the Ca2+ ion concentration was determined by EDTA complexometric titration with calconcarboxylic acid as indicator at pH 12. The K+ ion concentration was determined by quaternary ammonium backtitration with sodium tetraphenylboron as precipitant. The Cl− ion concentration was determined by mercurimetry. The boron content was determined by mannitol conversion acid−base titration. The ion charge balance method was used for the determination of Na+ ions. The accuracy of these analyses was about (0.1 to 0.3)%.

from Tianjin Yongda Chemical Reagent Co., Ltd., China. Boric acid was purchased from Sinopharm Group Chemical Reagent Co., Ltd., China. The water used (resistivity, 18.25 MΩ·cm) was deionized from a water purification system (UPT-II-20T, Chengdu Ultrapure Technology Co., Ltd.) prior to the experiments. Figure 1 shows the CrystalSCAN PolyBlock system (CrystalSCAN, HEL LIMITED) for measuring the solubility and metastable zone width of boric acid.

Figure 1. Schematic diagram of solubility and crystallization measurements: 1, low constant temperature bath; 2, CrystalSCAN PolyBlock; 3, computer processing system; 4, crystallizer; 5, temperature probes; 6, overhead stirring; 7, turbidity probes.

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RESULTS AND DISCUSSION Solid-State Characterization. It is necessary to identify the solid phase crystallized from the solution during boric acid MZW measurements. Figure 3 shows the XRD pattern of the

Solubility and MZW Determination. The experiments were carried out in a 100 mL crystallizer in which the temperature and turbidity probes were penetrated deeply into the solution. A stirrer speed of 300 rpm was used for all the probe monitored measurements. The automated cooling and heating cycles of boric acid mixture were performed at known rates of (12, 21, 30, 42, and 51) K·h−1. The whole automated control process was shown in Figure 2. First, an amount of 75 g of NaCl−KCl−CaCl2−H3BO3− H2O slurry was settled in a crystallizer and heated until the turbidity curve reached point A, which indicated that the crystals had dissolved, and the slurry became a clear solution. The temperature continued to rise up 5 K above the corresponding temperature of point A and was held constant for 10 min to ensure the nuclei had fully dissolved. Second, the solution was cooled with a constant cooling rate. When the turbidity curve reached point B, the corresponding temperature was recorded as the nucleation temperature Tlim. The clear solution became a slurry again. Last, the slurry was heated with a constant heating rate until the turbidity curve dropped to point C; the corresponding temperature was the dissolution

Figure 3. XRD pattern of the solid phase crystallized from a system of 12.11 % NaCl−5.10 % KCl−10.05 % CaCl2−1.61 % H3BO3−H2O: a, boric acid (PDF 01-073-2158); b, solid phase.

Figure 2. Temperature and turbidity cycles of boric acid during MZW measurements: dash, temperature; solid, turbidity. B



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great salt-out effect on the solubility of boric acid.13,16 These two opposite effects both influenced the solubility of boric acid. As can be seen from Figure 4, the solubility of boric acid in the studied systems was lower than that of boric acid in pure aqueous solution18 and decreased with the increasing concentration of CaCl2. This meant the solubility of boric acid in the studied systems was mainly affected by the coexisting CaCl2 salt, especially in systems with higher concentrations of CaCl2. As we know, when the oilfield brine was saturated with boric acid during evaporation, the concentration of boric acid was about 2.2 %. The coexisting salts of NaCl, KCl, and CaCl2 in brine were about 3 %, 4 %, and 22 %, respectively. Therefore, the saturation temperature of boric acid in this brine could be calculated from the solubility curve in the system of 2.74 % NaCl−4.11 % KCl−21.09 % CaCl2−H3BO3−H2O, and it was about 294 K. Note that the saturation temperature was within the room temperature (295 ± 5) K range; the boric acid would precipitate from the oilfield brine during the evaporation process. This may explain why boric acid coprecipitated with halite, sylvite, and analogous carnallites. MZW of Boric Acid in CaCl2−KCl−NaCl−H2O Solution. The measured MZW of boric acid in simulated oilfield brine is shown in Figure 5. It is noticeable that the MZW decreased rapidly with the rise in the solubility of boric acid due to higher solute concentration supplying more fresh material for nucleation. Figure 5 panels a and b show the MZW increased with increasing cooling rates, which was consistent with other work reported previously.19 It also can be seen from Figure 5 that the MZW changed with varied concentrations of coexisting salts in the solution. Previous studies showed that both coexisting salts of NaCl and KCl broaden the MZW of boric acid.13 In Figure 5, it was not difficult to find out the coexisting salt of CaCl2 narrows the MZW boric acid when the concentrations of NaCl and KCl remain constant (5.20 %, 5.10 %). The MZW of boric acid in the system of 12.14 % NaCl−5.10 % KCl−10.05 % CaCl2 was much wider than that in the system of 5.19 % NaCl−5.13 % KCl−13.19 % CaCl2, which suggests the MZW values in simulated oilfield brine are influenced by the above two opposite effects of coexisting salts in solution. Therefore, while

solid phase crystallized from 12.14 % NaCl-5.10 % KCl−10.05 % CaCl2−1.61 % H3BO3−H2O solution. From identity XRD patterns in Figure 3, it was known that the solid phase was boric acid. Furthermore, solids crystallized from other solutions studied in this paper were also identified as boric acid. Solubility of Boric Acid in CaCl2−KCl−NaCl−H2O Solution. According to evaporation experimental results,12 the mass concentrations of coexisting salts in the simulated oilfield brine system studied were (12−2.5) % NaCl, (6−4) % KCl, (10−22) % CaCl2. Owing to the low temperature climate (annual temperature about 278 K) of the oilfield brine in the Qaidam Basin area, the solubility of boric acid was investigated at temperature range from 265 K to 295 K. Figure 4 shows the solubility changes of boric acid with the varied concentrations of coexisting salts in solution.

Figure 4. Solubility of boric acid in NaCl−KCl−CaCl2−H2O solutions: ⧫, pure aqueous solution;18 ●, 12.14 % NaCl−5.10 % KCl−10.05 % CaCl2; ▲, 5.19 % NaCl−5.13 % KCl−13.19 % CaCl2;▼, 5.20 % NaCl−5.10 % KCl−17.17 % CaCl2; ■, 2.74 % NaCl−4.11 % KCl−21.09 % CaCl2.

In our previous studies, it was known that the coexisting NaCl salt had little effect on the solubility of boric acid,13,14 and the KCl salt had a salt-in effect,13,15 while the CaCl2 salt had a

Figure 5. MZW of boric acid in simulated oilfield brine solutions (NaCl−KCl−CaCl2−H2O): ■, 12.14 % NaCl−5.10 % KCl−10.05 % CaCl2; ●, 5.19 % NaCl−5.13 % KCl−13.19 % CaCl2; ▲, 5.20 % NaCl−5.10 % KCl−17.17 % CaCl2; ▼, 2.74 % NaCl−4.11 % KCl−21.09 % CaCl2. Cooling rates: a, 12 K·h−1; b, 30 K·h−1. C



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Table 1. Calculated A and γ of Boric Acid in NaCl−KCl−CaCl2−H2O System Based on Equation 1a m (salt concentrations)

s (solubility)

NaCl

KCl

CaCl2

H3BO3

T0

12.14

5.10

10.05

5.19

5.13

13.19

5.20

5.10

17.17

2.74

4.11

21.09

2.26 2.55 2.99 3.44 3.87 1.96 2.30 2.65 3.00 3.35 1.84 2.13 2.49 2.82 3.21 1.26 1.74 2.15 2.66 3.24

270.59 274.81 280.05 284.89 289.17 271.62 276.54 280.80 285.60 289.31 272.31 276.89 281.89 286.50 290.95 265.33 275.59 283.08 289.73 295.33

(T0/ΔTmax)2 ∼ lnR (T0/ΔTmax)2 (T0/ΔTmax)2 (T0/ΔTmax)2 (T0/ΔTmax)2 (T0/ΔTmax)2 (T0/ΔTmax)2 (T0/ΔTmax)2 (T0/ΔTmax)2 (T0/ΔTmax)2 (T0/ΔTmax)2 (T0/ΔTmax)2 (T0/ΔTmax)2 (T0/ΔTmax)2 (T0/ΔTmax)2 (T0/ΔTmax)2 (T0/ΔTmax)2 (T0/ΔTmax)2 (T0/ΔTmax)2 (T0/ΔTmax)2 (T0/ΔTmax)2

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

1846 2598 3552 3979 4579 2299 2974 3816 4621 5562 2213 2722 3807 4539 5729 1769 2990 3915 4996 5563

− − − − − − − − − − − − − − − − − − − −

284.19lnR 391.89lnR 562.02lnR 592.54lnR 673.81lnR 340.13lnR 469.78lnR 585.39lnR 678.87lnR 833.52lnR 328.06lnR 420.99lnR 600.29lnR 684.62lnR 904.22lnR 256.62lnR 454.94lnR 582.14lnR 745.70lnR 840.70lnR

A

γ

RC

3.18 3.52 2.49 3.57 3.75 4.10 2.58 3.02 3.89 3.32 4.03 2.94 2.51 3.24 2.34 4.90 3.31 3.65 3.40 3.01

3.95 3.56 3.18 3.14 3.03 3.72 3.36 3.14 3.01 2.82 3.77 3.49 3.12 3.00 2.75 4.06 3.39 3.15 2.93 2.83

0.9968 0.9887 0.9982 0.9979 0.9974 0.9957 0.9939 0.9962 0.9971 0.9830 0.9965 0.9996 0.9950 0.9974 0.9916 0.9928 0.9836 0.9949 0.9772 0.9757

Salt concentration in wt %; solubility in g of 100 g H2O; T0 in K; A in 1029 m−3.h−1; γ in mJ·m−2. Standard uncertainties u are u(m) = (0.04 to 0.86) %, u(s) = (0.01 to 0.09) g, u(T) = 0.06 K, u(ΔTmax) = 0.05 K.

a

Generally, the influence of coexisting salts on the MZW is closely related to the changes of nucleation kinetics in solution, especially the change of interfacial energy γ. The interfacial energy γ is the energy barrier of the formation of embryos into crystal nucleus in supersaturated solution. It indicates the difficulty degree of nucleating from supersaturated solution. Greater value of the interfacial energy γ means more difficulty of nucleation, which may lead to a wider MZW. Table 1 shows the calculated nucleation parameters A and γ according to eq 1. The corresponding adjust R-square (RC) values calculated by regression fit are also given in Table 1, and Figure 6 shows the dependence of interfacial energy γ on the coexisting salts based on the data in Table 1. As can be seen

the concentration of NaCl and KCl reached the lowest (∼3 %, ∼ 4 %) and the concentration of CaCl2 reached the highest (∼ 21 %), the MZW was much narrower than that of any other studied system. It is no wonder why boric acid began to coprecipitate with sylvite (KCl) during oilfield brine evaporation. Nucleation Kinetics of Boric Acid in CaCl2−KCl−NaCl− H2O Solution. According to the Classical Three-Dimensional Nucleation Theory Approach (3D CNT), the relationship between MZW(ΔTmax) and cooling rates R can be given by20 (T0/ΔTmax )2 = F − F1 ln R

(1)

where F = F1(X + ln T0) F1 =

1 ⎛ ΔHs ⎞ ⎜ ⎟ B ⎝ R GTlim ⎠

(2)

2

(3)

⎛ AR T ⎞ X = ln⎜ G lim ⎟ ⎝ f ΔHs ⎠

(4)

3 16π ⎛ γ Ω2/3 ⎞ ⎟ ⎜ B= 3 ⎝ kBTlim ⎠

(5)

ΔHs is the heat of dissolution and can be calculated from the solubility data using van’t Hoff equation; RG is the ideal gas constant, 8.3145 J·mol−1·K−1; f is a constant expressed in the number nuclei/m3 (f = 1/Ω); Ω is the molecular volume (m3), calculated by the density; kB is the Boltzmann constant (J·K−1); A is a kinetic constant (#·m−3·s−1) and γ is the interface energy. Equation 1 predicts the values of γ and A from the slope F1 and the intercept F, respectively. In this study, the values of the above parameters are ΔHs = 20870 J·mol−1, Ω = 7.15·10−29 m3, f = 1.40·1028 nuclei/m3 and kB = 1.38·10−23 J·K−1.

Figure 6. Changes of interfacial energy γ in simulated oilfield brine solutions (NaCl−KCl−CaCl2−H2O): ■, 12.14 % NaCl−5.10 % KCl− 10.05 % CaCl2; ●, 5.19 % NaCl−5.13 % KCl−13.19 % CaCl2; ▲, 5.20 % NaCl−5.10 % KCl−17.17 % CaCl2; ▼, 2.74 % NaCl−4.11 % KCl− 21.09 % CaCl2. D



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from Table 1 and Figure 6, the kinetic factor A remained fairly constant and was about 1029 m−3·h−1, which was in agreement with values (1026 m−3·s−1) reported by Kashchiev,21 while the value of interfacial energy γ decreased with the increasing saturation temperature. This is because high saturation temperature accelerates the ion movement and intensifies the ion collision frequency and facilitates the nucleation. Note that in Figure 6 the relationship between interfacial energy γ and coexisting salts was similar to that of the MZW (see Figure 5). It decreased with increasing concentrations of CaCl2 and with decreasing concentrations of NaCl and KCl salts. Analysis of Unique Crystallization Behavior of Boric Acid in Oilfield Brine. On the basis of the above results of solubility and MZW in simulated oilfield brine, the crystallization behavior of boric acid during evaporation was investigated for the first time. Figure 7 shows the evolution of

MZW of boric acid. Then, the boric acid (about 2.3 %) in brine saturated at the density point of 1.30 and coprecipitated jointly with sylvite and halite. The main composition of oilfield brine in this period was similar to the system of 2.74 % NaCl−4.11 % KCl−21.09 % CaCl2, and the saturation temperature of boric acid calculated from the simulated system was about 294.10 K, which suggests boric acid saturated at room temperature during this period. The above results show that the crystallization behavior of boric acid during evaporation was mainly influenced by the coexisting salts in brine, especially the high concentrated CaCl2 salt.

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CONCLUSION The solubility, MZW, and nucleation kinetics of boric acid in the simulated oilfield brine system of NaCl−KCl−CaCl2−H2O were studied for the purpose of investigating the unique crystallization behavior of boric acid during oilfield brine evaporation. It found that the bivalent cation Ca2+ had a great salt-out effect on the solubility and it determined the solubility of boric acid in the oilfield brine, even though the coexisting monovalent cations of Na+ and K+ had a salt-in effect on the solubility. Contrary to this, the MZW of boric acid was affected simultaneously by all the coexisting salts. It decreased with increasing concentrations of CaCl2 salt and increased with increasing concentrations of NaCl and KCl salts. Nucleation kinetics study showed that the interfacial energy γ in solution was affected by all of the three coexisting salts and changed differently with salt concentrations, which led to the change of the MZW in the studied system. The above results explain the crystallization behavior of boric acid during oilfield brine evaporation. It was deeply determined by the coexisting salts in brine, especially the concentrated CaCl2 salt during the late evaporation stage.

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Figure 7. Evolution of salt concentration in oilfield brine and in the precipitated salts during evaporation at room temperature: □, CaCl2 in brine; ○, NaCl in brine; △, KCl in brine; ◇, H3BO3 in brine; ■, H3BO3 in the precipitated salts.

AUTHOR INFORMATION

Corresponding Author

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

the major salts concentrations in the oilfield brine and the boric acid concentration both in brine and in the precipitated salts during evaporation.12 The evaporation experiments were carried out at room temperature (295 ± 5) K with the infrared lamp and the electric fan used as heat source and wind power, respectively. As can be seen from Figure 7, the following features may be noted: (1) When the brine density ρ value is about 1.25, the precipitated salt was halite (NaCl), the main composition of the oilfield brine was similar to the studied system of 12.14 % NaCl−5.10 % KCl−10.05 % CaCl2, the corresponding concentration of boric acid in brine was relatively low (about 1.60 %), and its saturation temperature calculated was about 273.15 K. During this period, the boric acid was enriched in the oilfield brine. (2) When the density ρ value increased to 1.27, the sylvite (KCl) salt began to precipitate from the oilfield brine, the concentration of boric acid in brine was about 2.0 %, and its saturation temperature was about (283 to 287) K calculated similarly from the system of 5.19 % NaCl−5.10 % KCl−(13.19 to 17.17) % CaCl2. Since the room temperature was (295 ± 5) K, the boric acid during this period continued to be enriched in the oilfield brine. (3) During the whole evaporation, the CaCl2 salt was concentrated in the oilfield brine, leading to the decrease in solubility and

This work is financially supported in part by Basic Research Project of Qinghai Province, China (Y341021053) and Leading Fund of Qinghai Institute of Salt Lakes (Y360161030). Notes

The authors declare no competing financial interest.

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Solubility, Metastable Zone Width, and Nucleation Kinetics of Boric

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