Solid–Liquid Phase Equilibrium in the Ternary System MgSO4 +

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Solid−Liquid Phase Equilibrium in the Ternary System MgSO4 + MgCl2 + H2O at 263.15 K Yuan Zhong,†,‡ Hongjun Yang,†,‡ Huaiyou Wang,†,‡ Haiwen Ge,†,‡ and Min Wang*,†,‡ †

Key Laboratory of Comprehensive and Highly Efficient Utilization of Salt Lake Resources, Qinghai Institute of Salt Lakes, Chinese Academy of Sciences, Xining 810008, PR China ‡ Key Laboratory of Salt Lake Resources Chemistry of Qinghai Province, Xining 810008, PR China ABSTRACT: The stable solid−liquid phase equilibrium in the ternary system MgSO4 + MgCl2 + H2O at 263.15 K was investigated with the isothermal dissolution method, and the phase diagram was plotted. Two isothermal invariant points cosaturated with MgSO4·7H2O + ice and MgCl2· 6H2O + MgSO4·7H2O, three stable crystallization fields corresponding to MgCl2·6H2O (Bis), MgSO4·7H2O (Eps), and ice, and three isothermal univariant curves were determined in the ternary system. Compared to the phase equilibrium at 273.15 K, the ice crystallization region and ice−salt invariant point are found at 263.15 K. Neither solid solution nor double salts are found in the system.

1. INTRODUCTION In China, magnesium sulfate subtype saline lakes are mainly spreading in Qinghai and Xinjiang, such as the Da Qaidam saline lake.1 The winter around saline lake areas is bitter cold and lasts very long. According to a report,2 the local average air temperature in Qinghai ranges from 255.15 to 266.15 K in January. To better guide solar pond production in winter, it is meaningful to study the phase equilibrium of water−salt systems below zero. However, the research in this area is rare. The ternary system Mg/SO42−,Cl−−H2O is one of the most important subsystems concerning magnesium sulfate subtype saline lakes, which has been investigated over a wide temperature range from 273.15 to 431.15 K.4 Bousmina3 established the diagram Mg/SO42−,Cl−−H2O at 288.15 K and gave evidence for the existence of MgSO4·6H2O. Due to the contradiction of a previous study, Li et al.5 reinvestigated the stable solubility of the ternary system at 323.15 and 348.15 K and recognized that MgSO4·nH2O (n = 5, 4) were metastable phases and MgSO4·nH2O (n = 6, 1) were stable phases at 323.15 K. Gao et al.6 studied the metastable phase equilibrium of the system at 308.15 K and discussed the differences between the metastable crystallization regions (MgSO4·7H2O, MgSO4·6H2O, MgSO4·5H2O, MgSO4·4H2O, and MgCl2· 6H2O) and the stable crystallization regions (MgSO4·7H2O, MgSO4·6H2O, MgSO4·4H2O, and MgCl2·6H2O). More attention is usually paid to guiding solar pond production in summer, and the solid−liquid phase equilibrium of the ternary system below 273.15 K is more complicated than that above 273.15 K, so most of the phase equilibrium studies on the ternary system have been performed at temperatures above 298.15 K, besides a few studies at 288.15 and 273.15 K.7 However, the local environmental temperature in winter is © XXXX American Chemical Society

always below 273.15 K. Moreover, ice may be found as a solid phase if the ternary system below 273.15 K. To better use the cold energy and guide the salt production in winter, it is necessary to study the phase equilibrium of the ternary system at 263.15 K. This is also useful to provide the basic data for obtaining thermodynamic model parameters. In this present work, we focused on the phase equilibrium of this system MgSO4 + MgCl2 + H2O at 263.15 K with the isothermal method.

2. EXPERIMENTAL SECTION 2.1. Reagents and Instruments. Magnesium chloride hexahydrade (MgCl2·6H2O, purity in mass fraction >0.99, Tianjin Guangfu Technological Development Co., Ltd.) and magnesium sulfate heptahydrate (MgSO4·7H2O, purity in mass fraction >0.995, Tianjin Guangfu Technological Development Co., Ltd.) were recrystallized two times, each time with 50% of the salt recovery to limit the impurity ions. The impurity ion contents of the recrystallized reagents (MgCl2·6H2O and MgSO4·7H2O) were detected by the ICP-OES method (ICAP 6500 DUO, Thermo Scientific), and the final mass fraction purities were more than 0.999 by the subtraction method, as shown in Table 1. Doubly deionized water with conductivity less than 1.2 × 10−4 S·m−1 was used to prepare samples for phase equilibrium experiments and chemical analysis. Ethylenediamine tetraacetic acid disodium salt (AR grade, Sinopharm Chemical Reagent Co., Ltd.) and mercuryReceived: October 16, 2017 Accepted: February 25, 2018

A

DOI: 10.1021/acs.jced.7b00894 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

Table 1. Description of the Recrystallized Reagents (MgCl2·6H2O and MgSO4·7H2O) chemical compound

source

initial mass fraction purity

purification method

final mass fraction purity

analysis method

MgCl2·6H2O MgSO4·7H2O

Guangfu Guangfu

>0.990 >0.995

recrystallization recrystallization

>0.999 >0.999

ICP-OES ICP-OES

Table 2. Solubility of the Ternary System MgSO4 + MgCl2 + H2O at 263.15 K and 0.1 MPaa composition of liquid phase, 100wb

a

composition of solid phase, 100wb

no.

MgCl2

MgSO4

H2O

1 2 3 4 5 6 7 8 9 10 11 12 13

34.11 33.56 32.01 31.05 29.48 25.94 20.80 18.59 12.72 6.96 8.43 10.12 11.39

0 0.86 0.74 0.89 1.04 1.13 2.10 2.78 5.43 9.90 6.56 2.80 0

65.89 65.58 67.25 68.06 69.48 72.93 77.10 78.63 81.85 83.14 85.01 87.08 88.61

MgCl2

MgSO4

H2O

11.28 13.33 10.79 9.63 8.71 8.03 5.00

31.97 28.27 31.38 31.20 29.26 28.99 31.73

56.75 58.40 57.83 59.17 62.03 62.98 63.27

1.32 2.46

1.21 0.61

97.47 96.93

solid phasec Bis Bis+Eps Eps Eps Eps Eps Eps Eps Eps Eps+ice ice ice ice

Standard uncertainties u are u(T)= 0.1 K, u(P) = 1 kPa, and ur(w) = 0.003. bw = mass fraction. cBis, MgCl2·6H2O; Eps, MgSO4·7H2O.

concentration of Mg2+ was determined by EDTA complexometric titration at pH 9.5−10 using Eriochrome Black-T as the indicator. The concentration of Cl− was measured through Hg(NO3)2 complexometric titration at pH 3−3.5 with phenylazoformic acid a-phenylhydrazide and bromophenol blue as the mixture indicator. The concentration of SO42− was evaluated by an mass balance and the ICP-OES method. The solid phase type was identified by Schreinemaker’s method and X-ray diffraction.

(II) nitrate (AR grade, Shanghai Zhongqin Chemical Reagent Co., Ltd.) were used for chemical analysis. The solid−liquid phase equilibrium at 263.15 K was investigated in a constant temperature bath controlled by the refrigeration instrument (DHJF-4010, Zhengzhou Greatwall Scientific Industrial and Trade Co., Ltd.) with cooling and heating function to control the bath temperature. The bath temperature ranged from 263.24 to 262.97 K measured by a digital display thermal resistance thermometer (JW-1-AF, Hebei Huage Electronic Instruments Plant), and the temperature of the solution ranged from 263.2 to 263.1 K (±0.05 K). A balance (ME204, Mettler Toledo) was used for weighing, with an error of ±0.1 mg. An X-ray diffractormeter (PANalytical X’pert Pro, Holland) was used for the solid phase identification. 2.2. Experimental Method. The solution was prepared in a 250 mL three-mouth flask placed in the thermostatic bath at 263.15 K. Using a stirring paddle to stir the solution for equilibrium. The connection between the stirring paddle and the middle flask mouth was filled with silicone oil to prevent the evaporation of water. Experiments showed that the equilibriumof this system was attained in 48 h at 263.15 K, which was shorter than the equilibrium time for MgSO4·H2O at 323.15 and 348.15 K,5 so 48 h was selected as the equilibrium time. When equilibrium was reached, the liquid was sampled (quickly with an injector connected to a filterable rubber tube to avoid adsorbing solid), weighed, diluted, and analyzed. After the liquid was sampled, the flask was transferred into a freezer to separate the solid and liquid, keeping the process at 263.15 K. After the wet solid was sampled quickly, the remaining mixture in the flash was filtrated by a vacuum pump as quickly as possible. The wet solid was also weighed, diluted, and analyzed. The separated solid was identified by X-ray diffraction. 2.3. Analysis Method. The compositions of the liquid and solid (wet) phase samples were analyzed by mass titration.8 The parallel samples were within a relative error ≤0.2%. The

3. RESULTS AND DISCUSSION The solubility data of the ternary system Mg/SO42−,Cl−−H2O at 263.15 K are presented in Table 2 with the corresponding phase diagram plotted in Figure 1. As shown in Figure 1, three stable crystallization regions correspond to MgCl2·6H2O, MgSO4·7H2O, and ice and two isothermal invariant points concerning E1 and E2 are cosaturated with Eps+ice and Bis

Figure 1. Stable equilibrium phase diagram of the ternary system MgSO4 + MgCl2 + H2O at 263.15 K: ○, ice; ◓, ice+Eps; □, Eps; ◧, Eps+Bis; ●,▼, ■, ◆, wet solid phases. B

DOI: 10.1021/acs.jced.7b00894 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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solubility data of the binary system at 263.15 K are presented in Table 3. The relative errors between Mg2+ and Cl− are 0.36% in the liquid phase and 0.26% in the solid phase, respectively. The average content of MgCl2·6H2O in the solid is 95.90%, and the rest is regarded as mother liquor. Combined with X-ray diffraction, as shown in Figure 2, the solid phase type is also identified as MgCl2·6H2O. This is different from the reported solid phase type MgCl2·8H2O in the binary system MgCl2 + H2O at 263.15 K.9 The solubility data of MgCl2·6H2O in the binary system changed a little from 348.15 K (39.41%)5 to 263.15 K (34.11%). In the ternary system, as shown in Figure 3, the crystallization area of MgCl2·6H2O is always the smallest and has small changes from 348.155 to 263.15 K. This indicates the temperature has little influence on the solubility of MgCl2· 6H2O. Compared to the solubility of MgCl2·6H2O in the binary system with the ternary system Mg/SO42−,Cl−−H2O, MgSO4·nH2O has little impact on the solubility of MgCl2· 6H2O. On the contrary, MgCl2·6H2O has a salting-out effect on MgSO4·nH2O. The ice phase in the system at 263.15 K was observed clearly and identified by Schreinemaker’s method. The concentrations of the ions in the wet solids were determined by ICP-OES. According to Figure 1, the ice phase is confirmed. Different from the phase equilibrium of the system at 273.15 K, as shown in Figure 3, the ice crystallization region and ice−salt invariant point (MgSO4·7H2O + ice) are found at 263.15 K. If the concentration of the solution is very low, the ice or Eps+ice crystallizes from the system at 263.15 K. Taking advantage of the ice formation, the solution can be concentrated and water can be recycled by the natural freezing at the saline lake areas in winter. From 273.15 to 263.15 K, two isothermal univariant curves corresponding to MgSO4·7H2O and MgCl2·6H2O are very close. This indicates the solubilities of MgSO4·7H2O and MgCl2·6H2O have small changes from 273.15 to 263.15 K, respectively. Because of the ice−salt invariant point (MgSO4· 7H2O + ice), part of the MgSO4·7H2O crystallization area at 273.15 K is instead a cocrystallization area (MgSO4·7H2O + ice) at 26.15 K.

Table 3. Solubility of the Binary System MgCl2 + H2O at 263.15 K and 0.1 MPaa composition of liquid phase, 100wb

composition of solid phase, 100wb

Mg2+

Cl−

relative error, %

Mg2+

Cl−

relative error, %

8.69

25.44

0.36

11.45

33.49

0.26

a

Standard uncertainties u are u(T) = 0.1 K, u(P) = 1 kPa, and ur(w) = 0.003. bw = mass fraction.

Figure 2. X-ray diffraction pattern for (a) MgCl2·6H2O and (b) MgSO4·7H2O.

+Eps, respectively. The crystallization area of MgCl2·6H2O is the smallest, whereas the areas of ice and MgSO4·7H2O are relatively large. That is, the solubility of MgCl2·6H2O is the highest in the ternary system. In the binary system MgCl2 + H2O at 263.15 K, MgCl2· 6H2O is the equilibrium solid phase identified by the described analysis method and X-ray diffraction. When the equilibrium time was extended to 7 days, the result was the same. The

Figure 3. Stable equilibrium phase diagram of the ternary system MgSO4 + MgCl2 + H2O at 26.15, 273.15,7 323.15,5 and 348.15 K:5 ○, at 263.15 K; △, at 273.15 K; □, at 323.15 K; ◇, at 348.15 K. C

DOI: 10.1021/acs.jced.7b00894 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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Table 4. Solubility of the Isothermal Invariant Points at 263.15 K and 0.1 MPaa composition of liquid phase, 100wb No. E2

E1

MgCl2 33.59 33.53 33.55 33.56 6.94 6.98 6.96

MgSO4 0.84 0.85 0.88 0.86 9.89 9.92 9.90

composition of solid phase, 100wb H2O d

65.57 65.62e 65.57e 65.58h 83.17f 83.10g 83.14h

MgCl2

MgSO4

H2O

solid phasec

36.50 14.57 15.76

8.74 29.59 28.68

54.76 55.84 55.56

Bis+Eps

1.23 1.17

8.12 29.02

90.65 69.81

Eps+ice

Standard uncertainties u are u(T) = 0.1 K, u(P) = 1 kPa, and ur(w) = 0.003. bw = mass fraction. cBis, MgCl2·6H2O; Eps, MgSO4·7H2O. dFrom the MgCl2-rich side. eFrom the MgSO4-rich side. fFrom the ice-rich side. gFrom the MgSO4-rich side. hThe mean value.

a

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The MgSO4·7H2O phase at 263.15 K was identified by Schreinemaker’s method and X-ray diffraction, as shown in Figures 1 and 2. From 348.15 to 263.15 K, the MgSO4·nH2O crystallization field in the ternary system decreases largely, so the temperature has a greater impact on the solubility of MgSO4·nH2O than MgCl2·6H2O. MgSO4·nH2O can be obtained by decreasing the system temperature. In addition, the combined water number of MgSO4 is increasing as the equilibrium temperature decreases. MgSO4·H2O is the stable solid phase at 348.15 K, MgSO4·6H2O and MgSO4·H2O are the stable solid phases at 323.15 K, and MgSO4·7H2O is the stable solid phase at 263.15 K. The equilibrium time for MgSO4· 7H2O at 263.15 K was found to be much shorter than that for MgSO4·H2O at 348.15 K. The solubility data of the isothermal invariant points corresponding to E1 and E2 are presented in Table 4. E1 and E2 were sampled from two sides in the phase diagram, respectively. The solid type of invariant points E1 and E2 was identified by Schreinemaker’s method and X-ray diffraction. According to Table 4, the composition of each isothermal invariant point among different samples is equivalent. In the Figure 3, the invariant points (MgSO4·7H2O + MgCl2·6H2O) from 348.15 to 263.15 K were found to be close.

ACKNOWLEDGMENTS The authors thank Professor P. S. Song for his advice and support on the project. Thanks also professor B. Sun for her support on the analysis method.

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5. CONCLUSIONS The table phase equilibrium of the ternary system MgSO4 + MgCl2 + H2O at 263.15 K was investigated with the isothermal dissolution method. Three stable crystallization regions are Bis, Eps, and ice, respectively. Two isothermal invariant points were determined, which are cosaturated with Bis+Eps and Eps+ice, repectively. The ice region and ice−MgSO4·7H2O cosaturated points are defined in the ternary system. Neither solid solution nor double salts are found in the system.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*M. Wang. E-mail: [email protected] Tel.:+8613519750944. ORCID

Yuan Zhong: 0000-0002-3887-6118 Funding

National Natural Science Foundation of China (grant No. U1407129). Financial support from the Youth Foundation of Qinghai institute of salt lakes, Chinese academy of Sciences (grant No. Y560321148). Notes

The authors declare no competing financial interest. D

DOI: 10.1021/acs.jced.7b00894 J. Chem. Eng. Data XXXX, XXX, XXX−XXX