Phase Equilibria in the Ternary Systems K2SO4–K2B4O7–H2O and

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Phase Equilibria in the Ternary Systems K2SO4−K2B4O7−H2O and Na2SO4−Na2B4O7−H2O at 348 K Rui-Zhi Cui,†,‡,§ Shi-Hua Sang,*,†,‡,§ Kai-Jie Zhang,†,§ and Ting Li†,§ †

College of Materials and Chemistry & Chemical Engineering, Chengdu University of Technology, Chengdu 610059, P. R. China State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation (Chengdu University of Technology), Chengdu 610059, P. R. China § Mineral Resources Chemistry Key Laboratory of Sichuan Higher Education Institutions, Chengdu 610059, P. R. China ‡

ABSTRACT: Phase equilibria in the ternary systems K2SO4− K2B4O7−H2O and Na2SO4−Na2B4O7−H2O at 348 K were studied by the isothermal solution saturation method. The solubilities and densities of the solution in these ternary systems were determined experimentally. Using the experimental data, the phase diagrams of the two ternary systems were obtained. The ternary systems K2SO4−K2B4O7−H2O and Na2SO4−Na2B4O7−H2O at 348 K are a type of simple common saturation and without complex salt and solid solution. In the phase diagrams and both the two ternary systems at 348 K, all have one invariant point, two univariant curves, and two regions of crystallization. The two crystallization regions of the ternary system K2SO4−K2B4O7−H2O at 348 K correspond to potassium borate tetrahydrate (K2B4O7·4H2O) and potassium sulfate (K2SO4). The two single-salt crystallization regions are sodium borate pentahydrate (Na2B4O7·5H2O) and sodium sulfate (Na2SO4) in the ternary system Na2SO4−Na2B4O7−H2O at 348 K.

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298.15 K have been studied.8,9 The metastable phase equilibria of the quaternary systems K2CO3−K2B4O7−KCl−H2O at 298 K and quinary system Na+, K+ //CO32−, B4O72−−H2O at 273 K have been discussed.10,11 Solubility calculations of ternary systems LiCl−Li2B4O7−H2O and Li2B4O7−MgCl2−H2O at 298 K have been researched.12,13 The ternary systems K2SO4−K2B4O7−H2O and Na2SO4− Na2B4O7−H2O are two important subsystems of complex system of the underground brine in the Western Sichuan Basin. Phase equilibria in the ternary system K2SO4−K2B4O7−H2O at 288 K have been published,14 but no report has been found to describe the phase equilibria of the two systems at 348 K. In this paper, the phase equilibrium of these two ternary systems were studied on the basis of our former research. The solubilities and densities of the equilibrated solutions were determined at 348 K. This can provide basic solubility data for boron resource development.

INTRODUCTION The underground gasfield brines, widely distributed in the Sichuan Basin, contain Na, K, Cl, B, I, Br, and S, which are all useful elements, and exceed the grade of extraction individually and comprehensive utilization.1 It has been reported that the gasfield brines discovered in the west of Sichuan Basin contain high concentrations of potassium and borate. The salinity of this brine is up to 377 g·L−1. The boron content appears to have an abnormally high value, up to 4994.36 mg·L−1, which is far higher than other world’s boron brine.2 These brines are rare liquid mineral resources in the world, so they have fine exploitation prospects. As is known to all, the phase equilibria and phase diagrams are important foundations for the exploitation of underground brine resources. Our research group has carried out a series of works about the underground brine previously in Western Sichuan Basin, such as quaternary systems Na2CO3 + K2B4O7 + K2CO3 + Na2B4O7 + H2O at 288 K, Na2SO4−Na2B4O7−Na2CO3−H2O at 298 K, Na+, K+// Br−,SO42−−H2O at 323 K, and Na2B4O7−NaBr−Na2SO4−H2O at 348 K.3−6 Boron is one of the most important elements in the Earth's crust, and a boron mine is a source of major chemical raw materials. Boron resources of China are rich, but not easy to process.7 Effective development and utilization of underground brine will help to make up for the shortage of solid boron mines. So far, many studies have focused on phase equilibria of systems containing boron. The phase equilibria of the quaternary system Na2B4O7−Na2CO3−NaCl−H2O at 273 K and the quinary system Li+, Na+, K+//Cl−, B4O72−−H2O at © 2012 American Chemical Society

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EXPERIMENTS Reagents and Instruments. All of the chemicals used in this work were of analytical purity grade and obtained from Chengdu Kelong Chemical Reagent Manufactory, China. They were K2B4O7·4H2O, K2SO4, Na2B4O7·10H2O, and Na2SO4. Deionized water (conductivity less than 1·10−5 S·m−1, pH = 6.6) was used to prepare the experimental solutions. Received: June 15, 2012 Accepted: November 13, 2012 Published: November 20, 2012 3498

dx.doi.org/10.1021/je3006387 | J. Chem. Eng. Data 2012, 57, 3498−3501

Journal of Chemical & Engineering Data

Article

A standard analytical balance of a 110 g capacity and 0.0001 g resolution (AL104, supplied by the Mettler Toledo Instruments Co., Ltd.) was employed for the determination of the solution density. An SHA-GW type oil bath thermostatted vibrator (Jintan Guowang Instrument Plant of China) with an uncertainty of 0.1 K was used for the equilibria measurement.

Table 1. Solubilities and Densities of Solution in the Ternary System K2SO4−K2B4O7−H2O at T = 348 K and p = 0.1 MPaa composition of solution 100·w(b)

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EXPERIMENTAL METHOD The phase equilibria of the ternary systems were studied at 348 K using the isothermal solution method. The system points for the ternary system were compounded by adding the second component gradually on the basis of the binary subsystem salt saturation points. The prepared salts were dissolved into 50 mL of deionized water to form the artificial synthesized brines and poured into a sealed glass bottle. Then, the bottles were placed in the thermostatted vibrator (SHA-GW). The temperature was controlled to (348 ± 0.1) K. The solutions were taken out periodically to analyze by chemical methods. The sign of the equilibrium is based on the unchanged concentration of the solution. This process took about 15 days. After equilibrium, the liquid and solid phases were separated and taken out. The liquid phases were analyzed quantitatively by chemical methods, and the solid phases were analyzed by X-ray diffraction to ascertain the crystalloid form. The densities of the equilibrated solution were measured using a density bottle method with an uncertainty of 0.004 g·cm−3. Analytical Methods. The borate ion concentration (B4O72−) was measured by using basic titration (0.05 mol·L−1 NaOH) with the existence of mannitol with a phenolphthalein solution as the indicator (uncertainty of 0.3 mass %). The potassium ion concentration (K+) was determined by a sodium tetraphenylborate (STPB)−hexadecyl trimethyl ammonium bromide (CTAB) titration (uncertainty of 0.5 mass %). The sulfate ion concentration (SO42−) was analyzed by titration with a standard solution of ethylenediaminetetraacetic acid (EDTA) in the presence of an excess Ba−Mg mixture solution (uncertainty of 1 mass %). The sodium ion concentration (Na+) was evaluated on an ion balance and assisted by ICPOES (uncertainty of 0.5 mass %). Each analysis was repeated twice to estimate the reproducibility.

density

no.

w(K2B4O7)

w(K2SO4)

solid phase

ρ/g·cm−3

1(D1) 2 3 4 5 6 7(E1) 8(C1) 9 10 11 12 13 14

0.00 8.17 12.92 18.13 26.27 29.93 30.74 35.77 32.79 31.34 30.66 29.91 29.86 29.59

17.10 14.90 12.35 10.00 7.45 5.07 4.43 0.00 2.41 4.42 5.18 5.42 5.63 6.07

K2SO4 K2SO4 K2SO4 K2SO4 K2SO4 K2SO4 K2SO4 + K2B4O7·4H2O K2B4O7·4H2O K2B4O7·4H2O K2B4O7·4H2O K2B4O7·4H2O K2B4O7·4H2O K2B4O7·4H2O K2B4O7·4H2O

1.272 1.314 1.345 1.389 1.457 1.493 1.497 1.494 1.510 1.492 1.499 1.496 1.506 1.493

a

Note: w(b) is the mass fraction of b. Standard uncertainties u are u(T) = 0.1 K, u(ρ) = 0.004 g·cm−3, u(w) = 0.005.

Figure 1. Equilibrium phase diagram of the ternary system K2SO4− K2B4O7−H2O at T = 348 K and p = 0.1 MPa.

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(A1B1F1) is the crystallization area of K2SO4, K2B4O7·4H2O and K2B4O7. The salt K2SO4 has the smaller solubility, which means that it can be easily separated from solution, whereas the salt K2B4O7·4H2O has the larger solubility. Two univariant curves are D1E1 and C1E1. Point E1 is the invariant point for the system K2SO4−K2B4O7−H2O at 348 K. It saturate with salts K2SO4 + K2B4O7·4H2O and the mass fraction compositions of the corresponding liquid phase is w(K2SO4) = 4.43 %, w(K2B4O7) = 30.74 %. In this system, the solubility of the salt K2B4O7 is greater than that of the salt K2SO4, Therefore, the concentration of K2B4O7 is the main factor affecting the solution density. Figure 2 is the density vs composition diagram of the ternary system K2SO4− K2B4O7−H2O at 348 K. According to Table 1 and Figure 2, the density increases with an increase of the concentration of K2B4O7 and decreases with an increase of the concentration of K2SO4. At the invariant point E1, the density reaches a maximum value equal with 1.511 g·cm−3. Comparison with the Phase Diagrams at (288 and 348) K for the Ternary System K2SO4−K2B4O7−H2O. The phase diagrams of the ternary system K2SO4−K2B4O7−H2O have been studied at 288 K.14 Compared with the two phase

RESULTS AND DISCUSSION K2SO4−K2B4O7−H2O System. The experimental results of phase equilibria in the ternary system K2SO4−K2B4O7−H2O at 348 K are listed in Table 1. w(b) is the mass fraction of b, and it expresses the b salt in grams per 100 g of the equilibriated solution, that is, g/(100 g of solution). ρ is the density, and it expresses the equilibriated solution in grams per cubic centimeter, that is, g·cm−3. Figure 1 is the phase diagram of ternary system K2SO4− K2B4O7−H2O at 348 K that was plotted on the basis of the experimental data. It is seen from Table 1 and Figure 1 that this ternary system is a type of simple common saturation and without complex salt and solid solution at the investigated temperature. There is one invariant point, two univariant curves, and two regions of crystallization in the phase diagram. The two crystallization fields correspond to potassium borate tetrahydrate (K2B4O7·4H2O) and potassium sulfate (K2SO4). The crystallization area of potassium sulfate (A1D1E1) is the larger, and the crystallization area of potassium borate tetrahydrate (B1C1D1) is the smaller. The area (A1B1E1) is the crystallization area of K2SO4 and K2B4O7·4H2O, and area 3499

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Figure 3. Equilibrium phase diagram of the ternary system Na2SO4− Na2B4O7−H2O at T = 348 K and p = 0.1 MPa.

Figure 2. Density value vs composition of the ternary system K2SO4− K2B4O7−H2O at T = 348 K and p = 0.1 MPa.

which there was one invariant point, two univariant curves, and two crystallization fields (Na2B4O7·5H2O and Na2SO4). Figure 3 shows that the crystallizing zone of sodium sulfate (A2D2E2) is larger than the crystallizing zone of borate pentahydrate (B2C2D2). The area (A2B2E2) is the crystallization area of Na2SO4 and Na2B4O7·5H2O, and area (A2B2F2) is the crystallization area of Na2SO4, Na2B4O7·5H2O, and Na2B4O7. The salt Na2SO4 has the smaller solubility, whereas the salt Na2B4O7·5H2O has the larger solubility. These results indicate that salt Na2B4O7·5H2O is more easily saturated and crystallized from the solution. The two univariant curves saturated with one salt in the system corresponding to curves D2E2 and C2E2. The one invariant point is E2, where E2 was saturated with salts Na2SO4 and Na2B4O7·5H2O. The mass fraction composition of liquid phase of invariant point E2 is w(Na2SO4) = 25.68 %, w(Na2B4O7) = 9.61 %. In this system, the solubility of the salt Na2SO4 is greater than that of the salt Na2B4O7. Therefore, the concentration of Na2SO4 is the main factor affecting the solution density. Figure 4 is the density vs composition diagram of the ternary system

diagrams at different temperatures, the result shows that the shape of them is similar. They all have one invariant point, two univariant curves, and two regions of crystallization. Two crystallizations of this ternary system correspond to K2SO4 and K2B4O7·4H2O. The salts in this system have the same crystallization forms, but it is easy to observe that the crystallization fields have changed. The solubility of salt K2B4O7·4H2O in water is 11.52 % at 288 K, and it is 35.77 % at 348 K. Obviously, the solubility at 348 K is much greater at 288 K, so the crystallization field of the salt K2B4O7·4H2O is smaller at 348 K than at 288 K, whereas the crystallization field of K2SO4 is larger at 348 K than 288 K. Na2SO4−Na2B4O7−H2O System. The experimental data on the solubilities and densities of the ternary system Na2SO4− Na2B4O7−H2O at 348 K are presented in Table 2. According to the experimental data in Table 2, the phase diagram of the system was shown in Figure 3 at 348 K. It is seen from Table 2 and Figure 3 that this ternary system is of simple eutectic type; no double salt or solid solution formed at 348 K. The equilibrium phase diagram was constructed in Table 2. Solubilities and Densities of Solution in the Ternary System Na2SO4−Na2B4O7−H2O at T = 348 K and p = 0.1 MPaa composition of solution 100·w(b)

density

no.

w(Na2B4O7)

w(Na2SO4)

solid phase

ρ/g·cm−3

1(D2) 2 3 4 5 6 7(E2) 8(C2) 9 10 11 12 13 14

0.00 2.05 3.80 8.65 9.32 9.44 9.61 21.72 17.70 16.17 13.34 11.11 10.39 9.43

30.30 29.74 28.86 26.10 25.57 25.90 25.68 0.00 7.14 9.22 12.45 20.15 23.52 26.02

Na2SO4 Na2SO4 Na2SO4 Na2SO4 Na2SO4 Na2SO4 Na2SO4 + Na2B4O7·5H2O Na2B4O7·5H2O Na2B4O7·5H2O Na2B4O7·5H2O Na2B4O7·5H2O Na2B4O7·5H2O Na2B4O7·5H2O Na2B4O7·5H2O

1.457 1.475 1.497 1.510 1.516 1.518 1.525 1.356 1.383 1.409 1.438 1.483 1.492 1.516

Figure 4. Density value vs composition of the ternary system Na2SO4−Na2B4O7−H2O at T = 348 K and p = 0.1 MPa.

Na2SO4−Na2B4O7−H2O at 348 K. According to Table 2 and Figure 4, the density of the system changed with the content change of Na2SO4. The density value first increases to a maximum in a w (Na2SO4) range of (0 up to 25.68) % and, then, decreases in a w (Na2SO4) range of (25.68 up to 30.30) %. The system has the largest density equal with 1.525 g·cm−3 at the invariant point E2.

a

Note: w(b) is the mass fraction of b. Standard uncertainties u are u(T) = 0.1 K, u(ρ) = 0.004 g·cm−3, u(w) = 0.005. 3500

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(12) Yang, G. M.; Yao, Y.; Zhang, A. Y.; Song, P. S. Isopiestic studies on thermodynamic properties for LiCl-Li2B4O7-H2O system at 298.15 K. J. Salt Lake Res. 2004, 12 (9), 31−37 ; in Chinese. (13) Zhang, A. Y.; Yao, Y.; Yang, G. M.; Song, P. S. Isopiestic studies of thermodynamic properties and representation with ion-interaction model for Li2B4O7-MgCl2(B)-H2O system at 298.15 K. Acta. Chim. Sin. 2004, 62 (12), 1089−1094 ; in Chinese. (14) Sang, S. H.; Zhang, X. Solubility Investigations in the systems Li 2SO4 + Li 2B4O7 + H2O and K2SO4 + K 2B4O7 + H2O at 288 K. J. Chem. Eng. Data 2010, 2, 808−812.

CONCLUSIONS Phase equilibrium in the ternary systems K2SO4−K2B4O7−H2O and Na2SO4−Na2B4O7−H2O were investigated at 348 K using an isothermal solution saturation method. According to the experimental data, the phase diagrams and density diagrams of the two systems were plotted. The results show that these two ternary systems come from the simple system, without any solid solution or double salts formed, and both them have one invariant point, two univariant curves, and two regions of crystallization.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: 13032845233. E-mail: [email protected], [email protected]. Funding

The project was supported by Open Fund (PLC201204) of State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation (Chengdu University of Technology), the National Nature Science Foundation of China (No. 40973047), and the Youth Science Foundation of Sichuan Province in China (08ZQ026-017). Notes

The authors declare no competing financial interest.

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dx.doi.org/10.1021/je3006387 | J. Chem. Eng. Data 2012, 57, 3498−3501