Article pubs.acs.org/jced
Experimental Study on Solid and Liquid Equilibrium of the System MgCl2−MgBr2−MgSO4−H2O at 323.15 K Lingzong Meng,*,†,‡ Dan Li,† Yafei Guo,‡ Tianlong Deng,‡ and Jingjing Ming† †
School of Chemistry and Chemical Engineering, Linyi University, Linyi 276000, P. R. China Tianjin Key Laboratory of Marine Resources and Chemistry, Tianjin University of Science and Technology, Tianjin 300457, P. R. China
‡
ABSTRACT: The solubility and the refractive index data of the system MgCl2−MgBr2− MgSO4−6H2O was found in the system.H2O at 323.15 K were determined using the isothermal dissolution method in this study. The dry-salt phase diagram, water-phase diagrams, and refractive index versus composition of the solution diagrams were plotted. The system belongs to the solid solution type. The dry-salt diagram consists of three univariant solubility curves and four crystallization zones (MgSO4·6H2O, Hex; MgSO4· 4H2O, Tet; MgSO4·H2O, Kie; Mg(Cl,Br)2·6H2O, Mcb). Solid solution Mg(Cl,Br)2·6H2O was found in the system. J(H2O) increases with the increase of J(MgCl2), while the refractive indices of the equilibrium solution decrease with the increase of J(MgCl2). The calculated refractive index data agree well with the experimental results.
1. INTRODUCTION Recently, bromine has been widely used in the technology fields and pharmaceuticals fields. Bromine-containing brine is widely distributed in the Qinghai-Tibet Plateau and Laizhou Bay of China.1,2 These brines belong mostly to the complex system (Li−Na−K−Mg−Cl−Br−SO4−borate−H2O). Moreover, bromide can be strongly enriched in hydrothermal water compared with seawater.3 The solubilities and the phase diagrams both are the theoretical foundation for the exploitation of the brine resources. The solubility data (phase diagrams) of the brines can be used to predict the path of mineral crystallization and describe the geochemical behavior of brine. Therefore, investigation of the solubility data of brominecontaining systems at high temperature is important for the comprehensive and effective exploitation of bromine from the brines. The quaternary system MgCl2−MgBr2−MgSO4−H2O is an important subsystem for the complex multiple-system. Although the solubilities of some systems containing bromine have been reported,4−6 there are few systems containing both chloride and bromide reported in the literature, especially for quaternary systems.7 There are several studies on the solubility of the subsystem MgCl2−MgBr2−H2O in the literatures at T = (288.15 and 333.15) K .8−10 The solid solution Mg(Cl,Br)2· 6H2O, was found to form in the system and the composition of Mg(Cl,Br)2·6H2O is continuous from MgCl2 to MgBr2. From literature about the system MgBr2−MgSO4−H2O,11,12 it is found that the phase diagram of the MgBr2−MgSO4−H2O system is similar to that of the system MgCl2−MgSO4− H2O.12,13 The subsystems of the quaternary system MgCl2− MgBr2−MgSO4−H2O have been reported over a wide temperature range, but the quaternary system has not been © XXXX American Chemical Society
reported in the literature to date. In this study, the solubilities and the refractive indices of the quaternary system at 323.15 K were determined.
2. EXPERIMENTAL SECTION Apparatus and Reagents. The phase equilibrium was done in a THZ-82 type thermostatic shaker. The temperature in the thermostatic bath was always maintained at (323.15 ± 0.1) K. An X-ray diffractometer (X′pert PRO, Spectris. Pte. Ltd., The Netherlands) was used to identify the solid phase minerals. The chemicals used in this work were analytical purity grade. Bischofite (MgCl2·6H2O, 0.99 in mass fraction), epsomite (MgSO4·7H2O, 0.99), and kieserite (MgSO4·H2O, 0.99) were supplied from Tianjin Kermel Chemical Reagent Ltd., and magnesiun bromide hexahydrate (MgBr2·6H2O, 0.98) was supplied from the Shanghai Xinbao Fine Chemical Plant. The purity of the minerals was determined using a titrimetric analysis method. All the chemicals were recrystallized before use except kieserite. The water used in experiments such as chemical analysis was double distilled water (DDW) with conductivity ≤1.2·10−4 S·m−1 and pH ≈ 6.60 at 298.15 K. Experimental Steps. The isothermal dissolution method was used in this study. The solubilities of the quaternary system were estimated, then a series of artificial synthesized brines with calculated salts and DDW were loaded into clean polyethylene bottles, which were placed in the thermostatic rotary shaker, with the temperature at (323.15 ± 0.1) K and rotation speed at Received: August 31, 2013 Accepted: November 5, 2013
A
dx.doi.org/10.1021/je400785d | J. Chem. Eng. Data XXXX, XXX, XXX−XXX
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Table 1. Experimental Solubility Data of the System MgCl2−MgBr2−MgSO4−H2O at 323.15 K composition of liquid phase, 100 wib no
a
1,A 2 3 4 5 6 7,A′ 8,B 9 10 11 12 13 14 15 16,B′ 17,C 18 19 20 21 22 23 24 25 26 27,C′
Jänecke index Ji/(g/100g S)
MgCl2
MgSO4
MgBr2
MgCl2
MgSO4
MgBr2
H2O
equilibrium solid phasec
0.00 0.16 6.05 10.92 15.77 17.99 21.10 0.00 5.42 10.36 13.47 16.66 20.66 24.28 25.67 26.90 0.00 8.53 12.13 18.14 18.72 18.91 23.29 26.46 30.51 36.00 36.60
10.82 10.42 11.07 10.94 11.22 11.08 10.70 3.74 4.21 4.62 5.43 5.8 5.86 6.22 6.43 6.54 0.93 1.08 1.24 1.78 1.58 1.81 1.91 1.71 1.63 1.01 0.95
27.75 28.00 19.95 14.52 7.40 4.44 0.00 43.46 34.81 26.08 19.99 13.93 8.31 3.28 1.47 0.00 51.79 37.12 30.82 21.68 21.28 20.78 14.99 11.55 6.09 0.69 0.00
0.00 0.41 16.31 30.02 45.84 53.69 66.35 0.00 12.20 25.23 34.64 45.78 59.31 71.88 76.47 80.44 0.00 18.26 27.44 43.61 45.02 45.56 57.95 66.61 79.80 95.5 97.47
28.05 27.02 29.87 30.06 32.63 33.06 33.65 7.92 9.48 11.26 13.96 15.94 16.83 18.41 19.15 19.56 1.76 2.32 2.82 4.28 3.80 4.36 4.74 4.30 4.27 2.67 2.53
71.95 72.57 53.81 39.92 21.53 13.26 0.00 92.08 78.32 63.52 51.4 38.28 23.86 9.71 4.38 0.00 98.24 79.42 69.74 52.11 51.18 50.08 37.31 29.09 15.93 1.83 0.00
159.27 159.21 169.74 174.91 190.80 198.42 214.47 111.86 124.97 143.53 157.12 174.78 187.05 196.08 197.87 199.04 89.68 113.94 126.27 140.38 140.55 140.96 148.86 153.98 161.51 165.32 166.31
Hex + Tet Hex + Tet Hex + Tet Hex + Tet Hex + Tet Hex + Tet Hex + Tet Tet + Kie Tet + Kie Tet + Kie Tet + Kie Tet + Kie Tet + Kie Tet + Kie Tet + Kie Tet + Kie Kie + Mb Kie + Mcb Kie + Mcb Kie + Mcb Kie + Mcb Kie + Mcb Kie + Mcb Kie + Mcb Kie + Mcb Kie + Mcb Kie + Bis
a The data for points A, B, and C are from ref 11, and the data for points A′, B′,C′ are from ref 13. bwi, mass fraction; cHex, MgSO4·6H2O; Tet, MgSO4·4H2O; Kie, MgSO4·H2O; Mb, MgBr2·6H2O; Mcb, Mg(Cl,Br)2·6H2O; Bis, MgCl2·6H2O.
bromide ions total concentrations were measured by titration with a standard solution of Hg(NO3)2 in the presence of mixed indicator of diphenylcarbazone and bromophenol blue. The Br− ion was converted into Br2 by potassium permanganate in the solution, then the Cl− ion concentration was determined by titration with Ag(NO3) using the indicator ferriammonium sulfate. The Br− concentration was evaluated by the subtraction method.15 The precision of the ion concentrations was less than ±0.003 in mass fraction. The SO42− concentration was obtained by subtraction via charge balance. A WZS-1 type abbe refractometer was used for the liquid phase refractive index (nD) measurement at (323.15 ± 0.1) K, with an uncertainty of ± 0.0001.
120 rpm to quicken the equilibrium of those brines. The shaker would stop for 1 h and the refractive indices of the clarified solutions in the bottles were measured periodically. If the difference between two refractive index measurements was within ± 0.0002, then the equilibrium state was achieved. Otherwise, the solution was rotated continually until the equilibrium state was achieved. Generally, it took approximately 60 days to reach the equilibrium state. A sample of approximately 3.0 cm3 was taken, weighed accurately and diluted in a 250.0 cm3 volumetric flask with DDW twice at different times when the equilibrium state achieved. The concentration of the sample in the volumetric flask was measured with titrimetric analysis. If the difference between the concentration of the two samples taken from the same polyethylene bottle was within ± 0.3 % in mass fraction, then the solubility of this equilibrium point, which was the average of the experimental data, was obtained. At the same time some of the wet residue in the bottle was separated from the solution, and one portion of the wet residue was used for titrimetric analysis, the other wet residue was dried with filter paper, ground into powder, and analyzed by X-ray diffraction. Then the minerals were identified by Schreinemaker’s wet residue method12 (the liquid phase point, the wet residue point, and the solid phase mineral point are in line) and X-ray diffraction. Analytical Methods. All the ion concentrations except SO42− of the solution and the wet residues were determined with titrimetric analysis methods. The concentration of magnesium ion was measured with an EDTA standard solution using the indicator Eriochrome Black-T.14 The chloride and
3. RESULTS AND DISCUSSION The experimental data on the solubilities of the system MgCl2− MgBr2−MgSO4−H2O at 323.15 K are listed in Table 1. wi in Table 1 is the concentration (mass fraction) for the mineral i. Ji, whose unit is g/100g S (dry salt), is the Jänecke index for the mineral i. Ji can be calculated by wi using the equations as follows, J(MgCl2) =
w(MgCl2) w(MgCl2) + w(MgBr2) + w(MgSO4 )
100 (1)
B
dx.doi.org/10.1021/je400785d | J. Chem. Eng. Data XXXX, XXX, XXX−XXX
Journal of Chemical & Engineering Data J(MgBr2) =
w(MgBr2) w(MgCl2) + w(MgBr2) + w(MgSO4 )
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In Table 1 and Figure 1, the points A, B, C and A′, B′, C′ are the invariant points of the ternary systems MgBr2−MgSO4− H2O and MgCl2−MgSO4−H2O. The dry−salt phase diagram has four crystallization zones: hexahydrite (MgSO4·6H2O, Hex), tetrahydrite (MgSO4·4H2O, Tet), kieserite (MgSO4· H2O, Kie), and solid solution (Mg(Cl,Br)2·6H2O, Mcb). The crystallization area of Hex, Tet, Kie, and Mcb decreases in sequence. These results indicate that magnesium sulfate hydrate is easy to saturate and crystallize from solution and that Mg(Cl,Br)2·6H2O has a high solubility. The concentration of magnesium chloride decreases with the increase of the magnesium bromide concentration, which shows that magnesium bromide has a strong salt-out effect on magnesium chloride. It was also found that the composition of Mg(Cl,Br)2· 6H2O is continuous from MgCl2 to MgBr2, which is similar to that in the literature.8−10 The three univariant solubility curves AA′, BB′, and CC′ are saturated with Hex + Tet, Tet + Kie, and Kie + Mcb, respectively. From the aforementioned aspects, it can be concluded that the system belongs to the solid solution type, and it is very difficult to get pure magnesium chloride or magnesium bromide from the brines. Figure 2 shows that the Jänecke index of J(H2O) changes regularly with the increase of J(MgCl2) or J(MgBr2). J(H2O) increases with the increase of J(MgCl2) on the univariant curve AA′, BB′, and CC′. In contrast, J(H2O) decreases with the increase of J(MgBr2). The maximum point is A′ (Hex +Tet) and the minimum point is C (Mb + Kie). The main reason is that the solubility of magnesium bromide is higher than that of magnesium chloride. The refractive index data (n) of the liquid phase were presented in Table 2. It is worthwhile to note that the no. column (first column) in Table 2 corresponds to the no. column in Table 1. Figure 3 was drawn on the basis of the refractive index results and J(MgCl2) or J(MgBr2). The refractive index decreases with the J(MgCl2) increasing, while the opposite is true for J(MgBr2). The maximum and minimum points in Figure 3 are just contrary to that in Figure 2. There was positive correlation between the refractive index and the solubilities, while there was a negative correlation between the Jänecke index of J(H2O) and the solubilities.
100 (2)
J(MgSO4 ) =
w(MgSO4 ) w(MgCl2) + w(MgBr2) + w(MgSO4 )
100 (3)
J(H 2O) =
w(H 2O) 100 w(MgCl2) + w(MgBr2) + w(MgSO4 ) (4)
The Jänecke index of MgCl2, MgSO4 and MgBr2 were used as X axis, Y axis, and Z axis, respectively. Then the dry−salt phase diagram was plotted, as shown in Figure 1. Figure 2, which was plotted using the Jänecke index of MgCl2 (MgBr2) and H2O, was the water−phase diagram.
Figure 1. Dry-salt phase diagram of the system MgCl2−MgBr2− MgSO4−H2O at 323.15 K: △, solubility data in this work; −, experimental isotherm curve; Hex, MgSO4·6H2O; Tet, MgSO4·4H2O; Kie, MgSO4·H2O; Mcb, Mg(Cl,Br)2·6H2O.
4. EMPIRICAL EQUATIONS FOR REFRACTIVE INDEX On the basis of the following empirical equation of the refractive index in electrolyte solutions developed in the
Figure 2. Water-phase diagrams of the system MgCl2−MgBr2−MgSO4−H2O at 323.15 K: phase isotherm curve. C
△,
solubility data in this work; , experimental water-
dx.doi.org/10.1021/je400785d | J. Chem. Eng. Data XXXX, XXX, XXX−XXX
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323.15 K. The Bi for MgBr2 and MgSO4 were given in the literature,11 and the Bi for MgCl2 was fitted as 0.001948. According to the coefficients and the solubility data of the system in this work, the refractive indices of the points in Table 1 were calculated. The comparison of the calculated and experimental values is presented in Table 2, and the maximal relative error in Table 2 is 0.37%. These results indicate that the coefficients Bi obtained in this study are reliable and can be used to calculate the refractive index of the solution in more complicated systems. Using the coefficients Bi and the refractive index data, the solubilities can also be roughly calculated. However, it is impossible to get the accurate concentration in a more complicated system only using the equation. Therefore, the results here only give the relationship between the refractive index and the concentrations.
Table 2. Calculated and Experimental Refractive Index Data of the System MgCl2−MgBr2−MgSO4−H2O at 323.15 K refractive index, n no.
a
experimental value
calculated value
relative error/%b
1.4167 1.4164 1.4124 1.4108 1.4102 1.4099 1.4085 1.4421 1.4325 1.4249 1.4212 1.4178 1.4160 1.4150 1.4138 1.4122 1.4648 1.4507 1.4457 1.4401 1.4392 1.4387 1.4354 1.4339 1.4339 1.4335 1.4340
1.4185 1.4188 1.4160 1.4155 1.4115 1.4099 1.4065 1.4438 1.4378 1.4302 1.4250 1.4193 1.4163 1.4143 1.4140 1.4139 1.4594 1.4459 1.4402 1.4347 1.4349 1.4346 1.4323 1.4320 1.4292 1.4294 1.4292
0.13 0.17 0.26 0.34 0.10 0.00 −0.14 0.12 0.37 0.37 0.27 0.10 0.02 −0.05 0.02 0.12 −0.37 −0.33 −0.38 −0.37 −0.30 −0.28 −0.22 −0.13 −0.33 −0.28 −0.33
1,A 2 3 4 5 6 7,A′ 8,B 9 10 11 12 13 14 15 16,B′ 17,C 18 19 20 21 22 23 24 25 26 27,C′
5. CONCLUSION The solubilities and the refractive indices of the MgCl2− MgBr2−MgSO4−H2O system at 323.15 K were obtained with the isothermal dissolution method. The dry-salt diagram, water diagrams and refractive index diagrams were plotted. The drysalt diagram consists of four crystallization zones (Hex, Tet, Kie, and Mcb) and three univariant solubility curves saturated with Hex + Tet, Tet + Kie, and Kie + Mcb, but no invariant points were found in the diagram. Solid solution Mg(Cl,Br)2· 6H2O was found in the system. The system belongs to the solid solution type. J(H2O) increases with the increase of J(MgCl2) and decreases with the increase of J(MgBr2), while the refractive indices of the equilibrium solution decrease with the increase of J(MgCl2) and increase with the increase of J(MgBr2). With the use of the equation from the literature, the coefficient for MgCl2 in the equation is obtained and the calculated refractive index data agree well with the experimental results.
a
The data for points A, B, C are from ref 11. bRelative error = (calculated value − experimental value)/experimental value.
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previous study,16,17 the refractive index of the solution was calculated. ln(n/n0) =
∑ Bi wi
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Tel./Fax: +86-5398766600.
(5)
n and n0 refer to the refractive index of the solution and water at 323.15 K, wi is the concentration of the salt i in the solution in mass fraction, which is the same as that in Table 1. Bi represents the coefficient for the salt i in the system, and it was calculated from the experimental data of the ternary system at
Funding
This study was supported by the NNSFC (Grants 21276194 and 21306136), the Key Pillar Program of Tianjin Municipal Science and Technology (Grant 11ZCKGX02800), the
Figure 3. The refractive index versus composition diagrams of the system MgCl2−MgBr2−MgSO4−H2O at 323.15 K: △, experimental data in this work; −, experimental curve. D
dx.doi.org/10.1021/je400785d | J. Chem. Eng. Data XXXX, XXX, XXX−XXX
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Specialised Research Fund for the Doctoral Program of Chinese Higher Education (Grant 20101208110003) and the Tianjin Key Laboratory of Marine Resources and Chemistry (Grant 201101). Notes
The authors declare no competing financial interest.
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REFERENCES
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