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Study of Phase Equilibrium of NaBr + KBr + [H.sub.2]O and NaBr + Mg[Br.sub.2] + [H.sub.2]O at 313.15 K.

1. Introduction

Phase equilibrium in salt-water systems and phase diagram are the foundation of inorganic chemical production and salt mineral resources exploitation [1-4]. To extract relevant products from the potassium, magnesium, and bromine salt mine, it is essential to investigate the phase equilibrium of NaBr + KBr + [H.sub.2]O and NaBr + Mg[Br.sub.2] + [H.sub.2]O. By now, a number of studies on the Br-bearing phase equilibria have been done, such as quaternary systems KCl-KBr-[K.sub.2]S[O.sub.4]-[H.sub.2]O at 323 K, 348 K, and 373 K [5-7], NaBr-Sr[Br.sub.2]-Mg[Br.sub.2]-[H.sub.2]O and KBr-Sr[Br.sub.2]-Mg[Br.sub.2]-[H.sub.2]O at 323 K [8], and quinary system [Na.sup.+], [K.sup.+]//[Cl.sup.-], [Br.sup.-], and S[O.sub.4]2-[H.sub.2]O at 373 K [9]. The two ternary systems NaBr-KBr-[H.sub.2]O and NaBr-Mg[Br.sub.2]-[H.sub.2]O also have been reported at 323 K and 348 K [10-12]. However, the data provided is far from enough, so an extensive study at other temperatures needs to be done. The phase equilibrium of NaBr + KBr + [H.sub.2]O and NaBr + Mg[Br.sub.2] + [H.sub.2]O at 313.15 K has not been reported yet. This paper is conducive to fill the blank of data. In this study, the solubility and density of the ternary systems were obtained. The equilibrium solid phases were analyzed, and the crystallization regions were determined. All results can offer fundamental data support for salt mineral resources exploitation and further theoretical studies.

2. Methodology

2.1. Materials and Apparatus. The sources and purity of the chemicals are listed in Table 1. Doubly deionized water (electrical conductivity [greater than or equal to] 1 x [10.sup.-4] S x [m.sup.- 1]) is used in the work. A HZS-H thermostatic water bath shaker is employed to carry out the experiments.

2.2. Experimental Methods. The method of isothermal solution saturation [13-15] was employed to determine the solubility of the ternary systems. The famous Schreinermarks method of moist residues [15-17] was applied to determine the equilibrium solid phase in the experiments.

Based on a fixed ratio and ensuring that one of the components is excessive, the experimental components are added to a series of conical flasks (125 mL) gradually, and the sealed flasks are placed into the oscillator. The oscillator vibrates continuously at 313.15 K (the standard uncertainty of 0.3 K). In a pre-experiment, the liquid phase of the samples is analyzed every 2 days, and it is shown that the phase equilibrium is reached in 10 days. After equilibrium, the oscillation is stopped and the system is allowed to stand for 4 days to make sure that all the suspended crystals settle. The wet residues and liquid phase are transferred to two volumetric flasks, respectively. Simultaneously, some other liquid phases are used to determine density individually. Finally, these samples are quantitatively analyzed by chemical methods.

More details of the experimental method and the procedure are presented in the previous papers [12-14].

2.3. Analysis. The concentration of potassium ion was analyzed by a sodium tetraphenylborate (STPB) hexadecyl trimethyl ammonium bromide (CTAB) titration [18-20] (uncertainty of 0.0058); the concentration of magnesium ion was measured with an EDTA standard solution using the indicator Eriochrome Black-T [21] (uncertainty of 0.0072); the concentration of bromine ion was determined by Mohr's method using a silver nitrate standard solution [21] (uncertainty of 0.0037); and the concentration of sodium was evaluated according to the ion charge balance. The density is measured using a pycnometer (uncertainty of 0.002). Each experimental result is achieved from the average value of three parallel measurements.

3. Results and Discussion

To compare with literature data [22, 23], the experimental data on the solubility for NaBr, KBr, or Mg[Br.sub.2] in pure water at 313.15 K are in good agreement with the literature values, which demonstrates that the experimental devices and methods are feasible.

3.1. Solid-Liquid Phase Equilibrium for NaBr + KBr + [H.sub.2]O. The experimental data were listed in Table 2. The ion concentration values were expressed in mass fraction in the equilibrium solution. The solution densities were given in grams per cubic centimeter. According to the experimental results, the phase diagram was plotted in Figure 1 and the relationship of the solution densities was plotted in Figure 2. In the ternary system NaBr + KBr + [H.sub.2]O at 313.15 K, it contains one invariant point, two univariant curves, and two crystallization regions.

As indicated in Figure 1, A, B, C, and W denote solid NaBr, solid KBr, solid NaBr x 2[H.sub.2]O, and [H.sub.2]O, respectively; point S, an invariant point, reflects the cosaturated solution of KBr and NaBr x 2[H.sub.2]O at 313.15 K, with w (NaBr) = 0.4612 and w (KBr) = 0.0820; P and H denote the solubility of KBr and NaBr in water at 313.15 K, respectively. Two univariant solubility curves of this ternary system are PS and HS. Curve PS corresponds to the saturated KBr solution and visualizes changes of the KBr concentration with increasing the NaBr concentration. Curve SH corresponds to the saturated NaBr solution and indicates changes of the NaBr concentration with the KBr concentration increasing in the equilibrating solution. The KBr concentration decreases sharply with increasing the NaBr concentration, which illustrates that NaBr has a strong salting-out effect on KBr.

As indicated in Figure 1, along the curve PS, we connect the composition points of wet residue phase with liquid phase and then extend the intersection of these straight lines which is approximately the equilibrium solid phase for KBr. The same method is utilized to analyze the equilibrium solid phase of SH, and the intersection is NaBr x 2[H.sub.2]O. WPSH denotes unsaturated region at 313.15 K. BPS denotes crystallization region of KBr, while SHC denotes crystallization region of NaBr x 2[H.sub.2]O. Zone BSC represents the mixed crystalline region of KBr + NaBr x 2[H.sub.2]O. It is obvious that the crystalline region of NaBr-2[H.sub.2]O is much smaller than that of KBr.

The phase diagrams of the ternary system NaBr + KBr + [H.sub.2]O at 323 and 348 K have been reported [10]. Apparently, the three phase diagrams have very similar shapes, each of them having an invariant point, two univariant curves, and two crystallization regions. The equilibrium solid phases in the ternary system NaBr + KBr + [H.sub.2]O are potassium bromide (KBr) and sodium bromide dihydrate (NaBr x 2[H.sub.2]O) at 313 K and 323 K, and those are potassium bromide (KBr) and sodium bromide (NaBr) at 348 K.

Figure 2 indicates the relationship between the mass fraction of NaBr and the density in the solution. With increasing the NaBr concentration, the density first increases and then the density declines afterwards. At the invariant point S, the density reaches a maximum value.

3.2. Solid-Liquid Phase Equilibrium for NaBr + Mg[Br.sub.2] + [H.sub.2]O. The phase equilibrium experimental data is shown in Table 3, and the ternary phase diagram is drawn in Figure 3.

As indicated in Figure 3, A, M, D, C, and W denote solid NaBr, solid Mg[Br.sub.2] x 6[H.sub.2]O, solid Mg[Br.sub.2], solid NaBr x 2[H.sub.2]O, and [H.sub.2]O, respectively; point Q, an invariant point, reflects the cosaturated solution of Mg[Br.sub.2] x 6[H.sub.2]O and NaBr x 2[H.sub.2]O at 313.15 K, with w (NaBr) = 0.0418 and w (Mg[Br.sub.2]) = 0.4781; N and H represent the solubility of Mg[Br.sub.2] and NaBr in water at 313.15 K, respectively. Two univariant solubility curves of this ternary system are PS and HS. Curve NQ corresponds to the saturated Mg[Br.sub.2] solution and visualizes changes of the Mg[Br.sub.2] concentration with increasing the NaBr concentration. Curve QH corresponds to the saturated NaBr solution and indicates changes of the NaBr concentration with increasing the Mg[Br.sub.2] concentration. The solubility of NaBr decreases sharply with increasing the Mg[Br.sub.2] concentration.

The polarization of ions has a certain effect on the dissolution of ionic crystals. The results show that the ionic dipole intensity in the solution depends on the electric field strength. In this study, the electrolyte concentration increased with the higher solubility of Mg[Br.sub.2] added to the solution; also, the polarity of the solution increases, and the dielectric coefficient of the dielectric medium is reduced, while the ionic electric field strength increases, making it easy to bound more water to its surrounding, so that the reduction of water in the dissolution of other substances leads to enhanced salting out. In this system, it illustrates that Mg[Br.sub.2] has a strong salting-out effect on NaBr.

In Figure 3, the same method used in Figure 1 is utilized to analyze the equilibrium solid phase of the system NaBr + Mg[Br.sub.2] + [H.sub.2]O. Consequently, curve NQ corresponding equilibrium solid phase is Mg[Br.sub.2] x 6[H.sub.2]O and curve HQ corresponding equilibrium solid phase is NaBr x 2[H.sub.2]O. WNQH denotes unsaturated region at 313.15 K. NQM denotes crystallization region of Mg[Br.sub.2] x 6[H.sub.2]O, while HQC denotes crystallization region of NaBr x 2[H.sub.2]O. Zone MQC denotes the mixed crystalline region of Mg[Br.sub.2] x 6[H.sub.2]O + NaBr x 2[H.sub.2]O. It is obvious that crystallization region of Mg[Br.sub.2] x 6[H.sub.2]O is much smaller than that of NaBr x 2[H.sub.2]O.

The phase diagram of the ternary system NaBr + Mg[Br.sub.2] + [H.sub.2]O has been studied at 323 K and 348 K [11,12]. Compared with the three phase diagrams at different temperatures, the result shows that the solubility of Mg[Br.sub.2] x 6[H.sub.2]O is highest at three temperatures. But the numbers of invariant points, crystallization fields, and univariant curves are different. The quaternary systems at 313 K and 348 K are all simple cosaturation type without complex salt and solid solution. They all include one invariant point, two univariant curves, and two crystallization regions (Mg[Br.sub.2] x 6[H.sub.2]O and NaBr x 2[H.sub.2]O at 313 K, Mg[Br.sub.2]-6[H.sub.2]O and NaBr at 348 K). The phase diagram at 323 K includes two invariant points, three univariant curves, and three crystallization regions, where the solids are NaBr x 2[H.sub.2]O, NaBr, and Mg[Br.sub.2] x 6[H.sub.2]O, respectively.

Figure 4 indicates the relationship between the mass fraction of Mg[Br.sub.2] and the density in the solution. With an increase of the Mg[Br.sub.2] concentration, the density first increases and then, the density declines afterwards. At the invariant point Q, the density reaches a maximum value.

4. Conclusions

The phase equilibria in the NaBr + KBr + [H.sub.2]O and NaBr + Mg[Br.sub.2] + [H.sub.2]O ternary systems at 313.15 K were investigated. The solubility and density data of the ternary systems were obtained. The diagrams of density versus composition and the ternary phase diagrams were plotted. The equilibrium solid phases were analyzed and the crystalline regions were determined. In ternary system NaBr + KBr + [H.sub.2]O, the crystalline region of KBr is much larger than that of NaBr x 2[H.sub.2]O and NaBr has a strong salting-out effect on KBr. In ternary system NaBr + Mg[Br.sub.2] + [H.sub.2]O, the crystalline region of NaBr x 2[H.sub.2]O is much larger than that of Mg[Br.sub.2] x 6[H.sub.2]O and Mg[Br.sub.2] has a strong salting-out effect on NaBr. There are in all two crystalline regions, one invariant point, and two univariant curves in the ternary phase diagrams. All results can offer fundamental data support for optimizing the processes and further theoretical studies.

https://doi.org/ 10.1155/2017/2319635

Conflicts of Interest

The authors declare that there are no financial conflicts of interest.

References

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Qing Chen, Jiping She, and Yang Xiao

College of Energy, Chengdu University of Technology, Chengdu, Sichuan 610059,

China

Correspondence should be addressed to Jiping She; 437290779@qq.com

Received 28 March 2017; Revised 21 May 2017; Accepted 5 June 2017; Published 16 July 2017

Academic Editor: Christophe Coquelet

Caption: FIGURE 1: Equilibrium phase diagram of the ternary system NaBr + KBr + [H.sub.2]O at 313.15 K. *, equilibrium liquid phase composition; *, moist solid phase composition; A, pure solid of NaBr; B, pure solid of KBr; C, pure solid of NaBr-2H2 O; W, water; H, solubility of NaBr in water; P, solubility of KBr in water; S, cosaturated point of NaBr x 2[H.sub.2]O and KBr.

Caption: FIGURE 2: Density versus 100w (NaBr) in the ternary system (NaBr + KBr + [H.sub.2]O). H, S, and P have the same meaning as described in Figure 1.

Caption: FIGURE 3: Equilibrium phase diagram of the ternary system NaBr + Mg[Br.sub.2] + [H.sub.2]O at 313.15 K. *, equilibrium liquid phase composition; *, moist solid phase composition; A, pure solid of NaBr; D, pure solid of Mg[Br.sub.2]; C, pure solid of NaBr x 2[H.sub.2]O; M, pure solid of Mg[Br.sub.2] x 6[H.sub.2]O; W, water; H, solubility of NaBr in water; N, solubility of Mg[Br.sub.2] in water; Q, cosaturated point of Mg[Br.sub.2] x 6[H.sub.2]O and NaBr x 2[H.sub.2]O.

Caption: FIGURE 4: Density versus 100w (Mg[Br.sub.2]) in the ternary system (NaBr + Mg[Br.sub.2] + [H.sub.2]O). N, Q, and H have the same meaning as described in Figure 3.
TABLE 1: Purities and suppliers of chemicals.

Chemical             Mass fraction purity      Source

NaBr                [greater than or equal     Tianjin Bodi Chemical
                          to] 99.0%            Holding Co. Ltd., China
KBr                 [greater than or equal     Tianjin Bodi Chemical
                          to] 99.0%            Holding Co. Ltd., China
Mg[Br.sub.2] x      [greater than or equal     Tianjin Bodi Chemical
  6[H.sub.2]O             to] 99.0%            Holding Co. Ltd., China

TABLE 2: Mass Fraction Solubility of the ternary NaBr + KBr +
[H.sub.2]O system at temperature = 313.15 K and pressure = 0.1 MPa
(a).

Number           Composition of liquid           Composition of wet
                      phase, 100w              residue phase, 100 w
         100[w.sub.1] (b)   100[w.sub.2]   100[w.sub.1]   100[w.sub.2]

1,P             0              43.51           NDc             ND
2              3.56            40.23           2.45          59.74
3              7.23            36.77           5.43          53.13
4             10.88            33.61           6.84          58.31
5             14.88            30.33           9.60          55.30
6             18.75            27.21          10.57          59.18
7             21.68            24.98          12.22          57.81
8             25.23            22.18          14.66          54.92
9             29.28            19.08          15.63          56.89
10            33.05            16.62          15.95          59.95
11            37.45            13.87          18.97          56.55
12            41.23            11.48          18.92          59.52
13, S         46.12             8.20          49.09          20.61
14            47.31             4.97          59.53           2.84
15            49.56             2.36          54.95           1.91
16, H         51.43             0.00            ND             ND

Number        Densities of         Equilibrium solid phase
              liquid phase
         [rho]/(g x [cm.sup.-3])

1,P              1.4208                      KBr
2                1.4247                      KBr
3                1.4286                      KBr
4                1.4351                      KBr
5                1.4446                      KBr
6                1.4546                      KBr
7                1.4636                      KBr
8                1.4738                      KBr
9                1.4863                      KBr
10               1.5034                      KBr
11               1.5254                      KBr
12               1.5439                      KBr
13, S            1.5658            NaBr x 2[H.sub.2]O + KBr
14               1.5462               NaBr x 2[H.sub.2]O
15               1.5355               NaBr x 2[H.sub.2]O
16, H            1.5296               NaBr x 2[H.sub.2]O

(a) Standard uncertainties u(T) = 0.3 K, [u.sub.r](p) = 0.05,
[u.sub.r]([K.sup.+]) = 0.0058, [u.sub.r]([Br.sup.-]) = 0.0037, and
[u.sub.r]([rho]) = 0.002. (b) [w.sub.1], mass fraction of NaBr;
[w.sub.2], mass fraction of KBr. (c) ND, not determined. H, S, and P
have the same meaning as described in Figure 2.

TABLE 3: Mass Fraction Solubility of the ternary NaBr + Mg[Br.sub.2] +
[H.sub.2]O system at temperature = 313.15 K and pressure = 0.1
MPa (a).

              Composition of liquid        Composition of wet residue
Number             phase, 100w                   phase, 100 w
         100[w.sub.1] (b)   100[w.sub.2]   100[w.sub.1]   100[w.sub.2]

1, H          51.43             0.00           NDc             ND
2             45.58             4.98          59.07           2.67
3             40.05            10.49          60.65           4.21
4             34.65            15.45          57.84           6.50
5             30.38            20.43          59.93           6.71
6             23.82            26.87          56.46           9.52
7             17.41            32.79          55.09          11.08
8             10.80            38.47          53.04          12.91
9              6.76            43.23          50.73          15.14
10, Q          4.18            47.81           7.78          51.15

11             2.72            48.95           2.01          52.78
12             1.32            50.28           0.98          54.15
13, N          0.00            51.62            ND             ND

Number   Densities of liquid phase    Equilibrium solid phase
          [rho]/(g x [cm.sup.-3])

1, H              1.5296                 NaBr x 2[H.sub.2]O
2                 1.5391                 NaBr x 2[H.sub.2]O
3                 1.5496                 NaBr x 2[H.sub.2]O
4                 1.5663                 NaBr x 2[H.sub.2]O
5                 1.5843                 NaBr x 2[H.sub.2]O
6                 1.6049                 NaBr x 2[H.sub.2]O
7                 1.6242                 NaBr x 2[H.sub.2]O
8                 1.6488                 NaBr x 2[H.sub.2]O
9                 1.6723                 NaBr x 2[H.sub.2]O
10, Q             1.6846                NaBr x 2[H.sub.2]O +
                                     Mg[Br.sub.2] x 6[H.sub.2]O
11                1.6795             Mg[Br.sub.2] x 6[H.sub.2]O
12                1.6705             Mg[Br.sub.2] x 6[H.sub.2]O
13, N             1.6584             Mg[Br.sub.2] x 6[H.sub.2]O

(a) Standard uncertainties u(T) = 0.3 K, [u.sub.r](p) = 0.05,
[u.sub.r]([Mg.sup.2+]) = 0.0072, [u.sub.r]([Br.sup.-]) = 0.0037,
and [u.sub.r] ([rho][right arrow]) = 0.002. (b) [w.sub.1], mass
fraction of NaBr; [w.sub.2], mass fraction of Mg[Br.sub.2]. (c) ND,
not determined. N, Q, and H have the same meaning as described in
Figure 4.
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Title Annotation:Research Article
Author:Chen, Qing; She, Jiping; Xiao, Yang
Publication:Journal of Chemistry
Article Type:Technical report
Date:Jan 1, 2017
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