Metastable Equilibrium of the Salt Lake Brine System Na+ + K+ +

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Metastable Equilibrium of the Salt Lake Brine System Na+ + K+ + CO32- + SO42- + B4O72- + H2O at 273.15 K Ying Zeng,* Shan Feng, and Zhi-Yuan Zheng Department of Geochemistry, College of Materials and Chemistry and Chemical Engineering, Chengdu University of Technology, Chengdu, 610059, P. R. China

The metastable phase equilibrium of the quinary system Na+ + K+ + CO32- + SO42- + B4O72- + H2O was studied at 273.15 K using an isothermal evaporation method. The solubilities and densities of the equilibrated solution were measured. On the basis of the experimental data, the stereo metastable phase diagram and the projected phase diagram saturated with salt Na2B4O7 of the quinary system at 273.15 K were constructed. The projected phase diagram of this system consists of six invariant points, 12 univariant curves, and seven crystallization fields corresponding to the single salts Na2SO4 · 10H2O, K2SO4, K2B4O7 · 4H2O, Na2CO3 · 10H2O, and K2CO3 · 3/2H2O and double salts 2Na2SO4 · Na2CO3 and K2CO3 · Na2CO3 · 12H2O. The double salts 2Na2SO4 · Na2CO3 and K2CO3 · Na2CO3 · 12H2O are formed in the quinary system and also exist in the other quinary or quaternary systems containing the Na2CO3-Na2SO4-H2O and K2CO3-Na2CO3-H2O ternary subsystems at 273.15 K. The forms of sodium and potassium borate are Na2B4O5(OH)4 · 8H2O and K2B4O5(OH)4 · 2H2O, respectively. In contrast to the phase diagrams of its subsystems, the crystallization area of K2CO3 · 3/2H2O increases clearly in the quinary system.

Introduction Salt-water system phase diagrams remain a topic of current interest, largely due to the potential for the comprehensive utilization of the saline brines. Salt lakes are widely distributed in the area of the Qinghai-Xizang (Tibet) Plateau and a series of salt lakes in Tibet that are famous for their abundance of sodium, potassium, and lithium, as well as chloride, borate, sulfate, and carbonate, especially the Zabuye Salt Lake. The main components of the brines belong to the complex system (Li+ + Na+ + K+ + Cl- + CO32- + SO42- + borate + H2O).1 Because of the rainless and windy climate in the saline lake country, natural evaporating technologies are widely used to extract salts from the saline brines. Earlier research has proved that, in the process of evaporation, the equilibrium relationships among the salts in the brines are always metastable.2,3 Thus, investigations on metastable equilibria are more important than the stable phase equilibria for salts exploited from the saline brines. A series of work has been done on metastable phase equilibria of saline brines; however, most effort has focused on temperatures above 288 K, including our earlier work aimed at the Zabuye Salt Lake.4,5 The metastable phase equilibrium at T ) 273.15 K is scarcely reported in the literature. The Zabuye Salt Lake region is arid, with high daily evaporation, and its average temperature is about 273 K.1 Thus, studies on the phase equilibria at 273 K will be more close to reality and will be of great use in exploiting the brine. Compared to phase studies at higher temperatures, studies at 273.15 K are more difficult because of the lower evaporating velocity, stronger viscosity of solution, and longer time to reach the equilibrium. * Corresponding author. Tel.: +86-28-84079016. Fax: +86-28-84079074. E-mail: [email protected].

The experimental phase diagram studies at T < 273 K are limited by the freezing point of water at atmospheric pressure. Some researchers6 have tried to calculate the phase diagram for saline brine systems at lower temperatures by using Pitzer’s equation, but it was not possible due to a lack of low-temperature Pitzer single salt parameters and mixture ion interaction parameters. Pitzer and his coworkers7,8 have established the temperature-dependent equations for the virial coefficients and ion interaction parameters for the simple ternary system NaCl + Na2SO4 (MgCl2) + H2O. These equations work well at temperatures between (298 and 373) K with concentrations up to the saturated solution for NaCl + Na2SO4 and NaCl + MgCl2. However, the temperature-dependent equations are not used at lower temperatures, and the Pitzer parameters obtained from the solubility were not set up in the absence of any solubility literature data at temperatures lower than 273.15 K. The quinary system Na+ + K+ + CO32- + SO42- + B4O72+ H2O is one of the basic and characteristic subsystems of the composition of the Zabuye Lake saline brine. We have undertaken studies on this quinary system at 273.15 K, with the hope that the metastable phase diagrams of this system at lower temperature can be obtained and also with the hope that the determined solubility data will help in calculating the lower temperature Pitzer parameters of carbonate and borate.

Experimental Section Reagents and Instruments. The chemicals used were of analytical grade purity and were obtained from the Chengdu Chemical Reagent Plant. They are potassium carbonate (K2CO3, 99.0 %), potassium sulfate (K2SO4, 99.0 %), potassium borate (K2B4O7, 99.5 %), sodium carbonate (Na2CO3, 99.0 %), sodium sulfate (Na2SO4, 99.0 %), and sodium borate (Na2B4O7, 99.5 %). Before being used to make the experimental solution, the potassium and sodium carbonate were pretreated to remove the

10.1021/je100791a  2010 American Chemical Society Published on Web 11/29/2010

Journal of Chemical & Engineering Data, Vol. 55, No. 12, 2010 5835

small amount of bicarbonate in them. The commercial potassium carbonate or sodium carbonate was put into a beaker, heated to 470 K, kept the temperature for up to 20 min to remove the bicarbonate, and then cooled to room temperature for use. Doubly deionized water (electrical conductivity less than 1 · 10-4 S · m-1, pH ) 6.6) was used to produce the experimental solutions. A SHH-250 type thermostatic evaporator made by the Chongqing INBORN Instrument Corporation, China, was used for the metastable phase equilibrium experiment. The temperature controlling precision is ( 0.1 K. A Siemens D500 X-ray diffraction analyzer with Ni-filtered Cu KR radiation was used for the analysis of solid phases. The operating conditions are 35 kV and 25 mA. An atomic absorption spectrophotometer (type WYD-YII) was employed for the determination of the sodium ion concentration in solution. Experimental Methods. The isothermal evaporation method was used for the metastable phase equilibria experiments.9 The experiments were done at (273.15 ( 0.1) K, and a thermostatic evaporator was used for evaporation. Because of the low evaporation temperature, the time for reaching equilibrium was comparatively long. The measuring point depended on the changes of the solid phase in the process of evaporation. When enough new solid phase appeared in the complex, the liquid and solid phases were separated by a rapid filtration method.4 The composition of solution was determined by an analytical method. The wet crystals in the solid phase were separated from each other according to crystal shape as much as possible, dried at 273.15 K, pestled into a powder, and then analyzed by X-ray diffraction. The densities of solution were also determined in this study and used for the mass fraction calculation of components. The densities were determined by the specific gravity bottle method with correction for the floating force of air (precision: 0.0001 g · cm-3).9 Analytical Methods. The composition of potassium ion was measured by titration with a sodium tetraphenylborate-hexadecyl trimethyl ammonium bromide aqueous solution.10 The average relative deviation of the determination was less than 0.5 %. The sulfate ion concentration was determined by titration with a standard solution of ethylenediaminetetraacetic acid (EDTA) in the presence of an excess Ba-Mg mixture solution, with a precision of less than 0.5 %.10 The borate ion concentration was determined by a neutralization titration in the presence of propanetriol, with a precision of less than 0.3 %.10 The carbonate ion concentration was determined by acid-base neutralization titration, with a precision of less than 0.5 %.10 The sodium ion concentration was determined by atomic absorption spectrophotometry, with a precision of less than 0.06 %.10

Results and Discussion The solubilities and densities of the equilibrated solution in the quinary system Na+ + K+ + CO32- + SO42- + B4O72- + H2O at 273.15 K are tabulated in Table 1. In Table 1, w(B) is the mass fraction of B, and J(B) is the Ja¨necke index values of B, with J(K22+) + J(SO42-) + J(CO32-) ) 100. On the basis of the experimental data, the stereo metastable phase diagram and the projected phase diagram saturated with salt Na2B4O7 were constructed, as shown in Figures 1 and 2, respectively. In Figure 1, the six apexes of the regular three-prism denote six pure salts that are Na2SO4, K2SO4, Na2CO3, K2CO3,

Na2B4O7, and K2B4O7, respectively. The nine touchlines express the nine ternary subsystems, and points 1 to 11 correspond to the invariant points for these ternary subsystems. Among them, the ternary system K2CO3 + Na2CO3 + H2O and Na2SO4 + Na2CO3 + H2O are of a complex type, where the double salts K2CO3 · Na2CO3 · 12H2O and 2Na2SO4 · Na2CO3 are formed, respectively. The upper regular triangle denotes the simple quaternary system K2CO3 + K2SO4 + K2B4O7 + H2O, with the invariant point N. The down regular triangle denotes the quaternary system Na2CO3 + Na2SO4 + Na2B4O7 + H2O, with the invariant points E1 and E2. The double salt 2Na2SO4 · Na2CO3 is found in this subsystem. The three squares denote the three reciprocal quaternary subsystems Na2CO3 + K2CO3 + Na2SO4 + K2SO4 + H2O, Na2SO4 + K2SO4 + Na2B4O7 + K2B4O7 + H2O, and Na2CO3 + K2CO3 + Na2B4O7 + K2B4O7 + H2O, respectively. The double salts K2CO3 · Na2CO3 · 12H2O and 2Na2SO4 · Na2CO3 are simultaneously found in the quaternary system Na2CO3 + K2CO3 + Na2SO4 + K2SO4 + H2O. Figure 2, the projected diagram saturated with salt Na2B4O7, clearly shows that the metastable phase diagram of the quinary system Na+ + K+ + CO32- + SO42- + B4O72- + H2O at 273.15 K consists of seven crystallization fields, 12 univariant curves, and six invariant points. The seven crystallization fields correspond to sodium sulfate decahydrate (Na2SO4 · 10H2O), potassium sulfate (K2SO4), potassium borate tetrahydrate (K2B4O7 · 4H2O), sodium carbonate decahydrate (Na2CO3 · 10H2O), potassium carbonate hydrate (K2CO3 · 3/2H2O), sodium carbonate-sulfate double salt (2Na2SO4 · Na2CO3), and a potassium-sodium carbonate double salt (K2CO3 · Na2CO3 · 12H2O). Borates can form different polyanions in aqueous solution. The various species of boron in aqueous solution depend on the pH value, the total concentration of boron and salts, and the kinds of coexistent salts.11 B4O72- is just a traditional stoichiometric expression for various boric species in solution. In this metastable system at the studied temperature, the equilibrium solid phase of both sodium borate and potassium borate exists as a tetraborate, with the crystallization form Na2B4O5(OH)4 · 8H2O and K2B4O5(OH)4 · 2H2O, respectively. The crystallization form of sodium carbonate existing in this system is Na2CO3 · 10H2O, and neither Na2CO3 · 7H2O nor Na2CO3 · H2O is found. There are two kinds of double salts formed in this quinary system; those are a sodium carbonate-sulfate double salt (2Na2SO4 · Na2CO3) and a potassium-sodium carbonate double salt (K2CO3 · Na2CO3 · 12H2O). It is worthy to point out that the potassium-sodium sulfate double salt (3K2SO4 · Na2SO4) is formed in the quinary system Li+ + Na+ + K+ + SO42- + B4O72- + H2O at 288.15 K4 but exists in neither this system nor the quinary system Li+ + Na+ + K+ + SO42- + B4O72- + H2O at 273.15 K.12 This result shows that the temperature affects the interaction of sodium and potassium sulfate. The six invariant points labeled as K1, K2, K3, K4, K5, and K6 are cosaturated with four salts. The saturated salts and the mass fraction composition for the invariant points in this system are listed below. K1, saturated with salts Na2B4O7 · 10H2O + Na2SO4 · 10H2O + K2SO4 + K2CO3 · 3/2H2O, with w(Na+) 2.73 %, w(K+) 0.72 %, w(SO42-) 4.94 %, w(CO32-) 1.00 %, w(B4O72-) 0.06 %. K2, saturated with salts Na2B4O7 · 10H2O + K2B4O7 · 4H2O + K2SO4 + K2CO3 · 3/2H2O, with w(Na+) 10.73 %, w(K+) 4.70 %, w(SO42-) 20.36 %, w(CO32-) 2.96 %, w(B4O72-) 4.97 %.

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Table 1. Solubilities and Densities of Solutions in the Quinary System Na+ + K+ + CO32- + SO42- + B4O72- + H2O at 273.15 K composition of equilibrium solution, w(B) · 10-2

density

Ja¨necke index of dry salt, mol/100 mol dry salt J(K22+) + J(CO32+) + J(SO42+) ) 100

no.

g · cm-3

w(Na+)

w(K+)

w(SO42-)

w(CO32-)

w(B4O72-)

w(H2O)

J(Na22+)

J(K22+)

J(SO42-)

J(CO32-)

J(H2O)

equilibrium solid phasea

1, K2 2 3 4 5 6 7 8 9 10 11 12, L1 13, L2 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31, K1 32 33 34 35 36 37 38 39 40 41 42 43, K4 44 45 46 47 48 49, F1 50, K6 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78

1.1006 1.1104 1.1052 1.1326 1.1348 1.1610 1.1555 1.1477 1.1477 1.1438 1.1326 1.1286 1.1329 1.1013 1.0992 1.0979 1.0925 1.0949 1.0815 1.0847 1.1116 1.1298 1.1440 1.1395 1.1654 1.1138 1.1025 1.0730 1.0742 1.0722 1.0691 1.1033 1.0961 1.0825 1.0714 1.0753 1.1038 1.1077 1.1129 1.1237 1.1102 1.1181 1.1134 1.1262 1.1449 1.1459 1.1687 1.1857 1.2561 1.1201 1.0532 1.137 1.1591 1.1678 1.1542 1.1305 1.1183 1.1034 1.0927 1.0882 1.1286 1.2153 1.1288 1.1268 1.1217 1.1192 1.1165 1.1210 1.1163 1.0927 1.0467 1.0549 1.0595 1.0716 1.0729 1.0876 1.1095 1.1083

10.73 10.90 10.88 10.95 10.47 10.71 9.54 10.36 4.43 2.31 0.46 0.17 2.13 6.32 8.10 11.42 10.55 10.83 10.63 10.50 12.21 10.82 11.27 11.01 10.63 5.85 4.15 2.31 2.32 2.44 2.73 11.63 4.87 3.43 2.46 3.09 13.78 13.10 13.34 13.8 12.14 12.37 3.87 1.93 3.58 4.20 8.10 11.75 16.15 12.42 13.85 13.53 13.16 13.92 12.05 11.32 11.54 3.78 4.26 7.35 13.07 13 7.82 4.25 4.90 4.79 4.71 8.65 4.59 2.56 1.62 1.79 1.87 2.14 2.23 2.45 1.44 1.49

4.70 4.89 5.53 5.43 5.86 6.78 8.7 6.72 8.17 7.95 7.08 5.33 3.91 3.85 3.67 3.59 3.13 3.03 2.79 2.92 2.63 3.90 4.05 3.90 4.89 2.38 1.42 0.59 0.58 0.61 0.72 1.78 1.15 0.96 0.72 0.73 4.90 4.96 5.25 5.87 1.87 1.99 0.99 5.51 6.90 5.49 3.08 2.36 2.19 2.13 2.39 2.38 2.67 2.87 1.91 1.24 0.21 0.88 0.47 0.35 2.27 3.68 1.97 1.52 1.39 1.21 1.04 9.39 8.51 7.70 0.67 0.72 0.86 1.02 1.11 1.54 1.19 2.32

20.36 20.67 20.7 20.55 19.94 20.46 20.08 20.2 12.21 8.29 5.23 3.26 8.79 16.87 21.36 26.2 23.83 24.28 23.53 23.38 26.4 24.84 25.61 24.75 24.97 12.58 8.19 4.1 4.05 4.31 4.94 19.08 8.06 6.98 4.24 5.06 23.41 21.58 25.28 27.27 18.32 18.48 0.45 0.57 0.44 0.43 0.67 0.59 0.00 18.81 20.3 19.38 18.48 19.05 13.21 9.92 8.21 0.50 0.53 0.79 19.08 17.98 10.44 4.08 4.11 2.95 1.29 16.81 8.33 4.03 0.54 0.52 0.66 0.78 0.81 0.97 0.62 0.57

2.96 3.14 3.38 3.5 3.57 4.22 4.33 4.48 2.63 1.72 0.52 0.00 0.00 0.35 0.62 0.92 0.92 0.93 0.96 0.97 1.06 1.20 1.47 1.55 1.67 2.11 1.18 0.88 0.91 0.93 1.00 3.53 1.80 1.63 1.09 1.40 5.93 5.73 4.66 4.61 5.33 5.60 5.48 6.04 9.64 9.01 12.09 16.34 14.88 5.63 6.67 6.87 7.19 7.95 6.65 6.22 6.58 5.25 4.60 6.69 6.40 8.00 4.77 3.83 4.60 5.09 6.00 7.46 6.91 6.36 1.95 2.22 2.36 2.75 2.92 3.43 2.27 3.20

4.97 4.94 5.46 5.49 5.50 5.58 5.80 4.04 4.69 5.77 5.83 5.92 0.79 0.81 0.89 0.91 0.94 0.92 0.88 0.93 0.97 1.00 0.87 0.86 0.90 0.73 0.54 0.08 0.07 0.06 0.06 1.13 1.05 0.79 0.06 0.06 3.06 2.73 2.52 2.21 1.29 1.34 0.13 0.93 1.12 1.08 1.11 1.11 0.95 1.15 1.40 1.24 1.23 1.28 5.89 8.52 9.07 0.12 2.53 6.90 1.23 1.40 1.09 0.85 0.76 0.76 0.37 1.33 1.07 1.00 0.87 0.91 0.86 0.88 0.86 0.89 0.35 0.46

56.28 55.46 54.05 54.08 54.66 52.25 51.55 54.20 67.87 73.96 80.88 85.32 84.38 71.80 65.36 56.96 60.63 60.01 61.21 61.30 56.73 58.24 56.73 57.93 56.94 76.35 84.52 92.04 92.07 91.65 90.55 62.85 83.07 86.21 91.43 89.66 48.92 51.90 48.95 46.24 61.05 60.22 89.08 85.02 78.32 79.79 74.95 67.85 65.83 59.86 55.39 56.60 57.27 54.93 60.29 62.78 64.39 89.47 87.61 77.92 57.95 55.94 73.91 85.47 84.24 85.20 86.59 56.36 70.59 78.35 94.35 93.84 93.39 92.43 92.07 90.72 94.13 91.96

72.51 71.73 68.99 69.60 66.49 62.86 52.79 60.67 34.92 23.15 6.50 3.61 32.68 59.50 62.91 74.27 75.52 76.62 77.84 76.82 81.33 71.55 71.39 71.74 65.91 64.65 73.24 77.33 77.84 77.76 76.72 90.16 82.26 66.47 74.73 78.66 73.87 74.19 71.02 68.78 86.92 86.37 77.39 23.67 30.68 40.57 71.01 82.74 127.17 85.15 85.23 84.79 82.55 82.29 95.98 110.40 126.78 79.02 104.98 128.63 84.94 76.84 79.62 73.43 77.58 79.44 80.77 44.79 32.08 22.56 75.39 75.34 71.03 69.40 67.96 61.21 52.57 36.39

18.73 18.98 20.68 20.35 21.95 23.47 28.39 23.21 37.99 46.99 58.95 66.80 35.38 21.38 16.81 13.77 13.21 12.64 12.05 12.60 10.33 15.21 15.13 14.99 17.88 16.62 14.78 11.65 11.48 11.46 11.93 8.14 11.46 10.97 12.90 10.95 15.49 16.56 16.48 17.25 7.90 8.19 11.68 39.88 34.87 31.28 15.92 9.80 10.17 8.61 8.67 8.80 9.88 10.01 8.97 7.13 1.36 10.85 6.82 3.61 8.70 12.83 11.83 14.31 12.97 11.83 10.51 28.68 35.08 40.04 18.39 17.87 19.26 19.51 19.95 22.69 25.62 33.41

65.93 65.18 62.89 62.59 60.67 57.54 53.24 56.68 46.12 39.80 35.42 33.20 64.62 76.12 79.51 81.64 81.74 82.31 82.56 81.96 84.26 78.71 77.73 77.27 74.18 71.37 69.32 65.77 65.11 65.81 66.52 70.88 65.23 64.78 61.72 61.70 60.14 58.56 64.49 65.12 62.85 61.83 4.31 3.35 1.81 1.99 2.81 1.99 0.00 61.79 59.86 58.20 55.55 53.96 50.41 46.37 43.21 5.01 6.35 6.67 59.41 50.92 50.94 38.80 31.20 23.42 10.64 41.71 27.89 17.01 12.04 10.49 12.01 12.12 11.83 11.61 10.85 6.67

15.34 15.84 16.43 17.06 17.38 18.99 18.37 20.11 15.89 13.21 5.63 0.00 0.00 2.50 3.68 4.59 5.05 5.04 5.39 5.44 5.41 6.08 7.14 7.74 7.94 12.01 15.90 22.59 23.41 22.72 21.55 20.98 23.31 24.25 25.39 27.35 24.37 24.88 19.03 17.63 29.26 29.98 84.01 56.77 63.33 66.73 81.26 88.21 89.83 29.59 31.47 33.01 34.58 36.03 40.62 46.50 55.43 84.14 86.83 89.72 31.89 36.25 37.24 46.89 55.83 64.75 78.85 29.62 37.03 42.95 69.57 71.64 68.72 68.37 68.22 65.70 63.53 59.92

972.0 932.7 875.8 878.5 887.0 783.7 729.0 811.1 1367 1894 2919 4634 3308 1727 1297 946.7 1109 1085 1145 1146 965.6 984.2 918.3 964.6 902.2 2156 38128 7874 7895 7464 6503 1245 3586 4269 7098 5833 670.2 751.1 666.0 589.0 1117 1074 4552 2665 1715 1970 1679 1221 1325 1049 871.1 906.5 918.1 829.9 1227 1565 1808 4780 5518 3485 962.4 845.0 1923 3774 3409 3611 3795 745.8 1261 1764 11221 10094 9065 7660 7170 5792 8782 5739

nb+kb+ks+kc nb+kb+ks nb+kb+ks nb+kb+ks nb+kb+ks nb+kb+ks nb+kb+ks nb+kb+ks nb+kb+ks nb+kb+ks nb+kb+ks nb+kb+ks nb+ks+ns nb+ks+ns nb+ks+ns nb+ks+ns nb+ks+ns nb+ks+ns nb+ks+ns nb+ks+ns nb+ks+ns nb+ks+ns nb+ks+ns nb+ks+ns nb+ks+ns nb+ks+ns nb+ks+ns nb+ks+ns nb+kc+ks nb+kc+ks nb+ns+kc+ks nb+ns+kc nb+ns+kc nb+ns+kc nb+ns+kc nb+ns+kc nb+kb+kc nb+kb+kc nb+kb+kc nb+kb+kc nb+kb+kc nb+kb+kc nb+kc+nc+nsc nb+nc+knc nb+nc+knc nb+nc+knc nb+nc+knc nb+nc+knc nb+nc+nkc nb+ns+nsc+kc nb+ns+nsc nb+ns+nsc nb+ns+nsc nb+ns+nsc nb+ns+nsc nb+ns+nsc nb+ns+nsc nb+nc+nsc nb+nc+nsc nb+nc+nsc nb+kc+nsc nb+kc+nsc nb+kc+nsc nb+kc+nsc nb+kc+nsc nb+kc+nsc nb+kc+nsc nb+kc+knc nb+kc+knc nb+kc+knc nb+kc+nc nb+kc+nc nb+kc+nc nb+kc+nc nb+kc+nc nb+kc+nc nb+kc+nc nb+kc+nc

Journal of Chemical & Engineering Data, Vol. 55, No. 12, 2010 5837 Table 1 Continued composition of equilibrium solution, w(B) · 10-2

density no. 79 80 81, 82 83 84 85, 86 87 88 89 90 91, 92, 93, 94, 95, 96, 97, 98, 99, a

K3

K5

F2 F3 M1 M4 M2 M3 N E1 E2

Ja¨necke index of dry salt, mol/100 mol dry salt J(K22+) + J(CO32+) + J(SO42+) ) 100

g · cm-3

w(Na+)

w(K+)

w(SO42-)

w(CO32-)

w(B4O72-)

w(H2O)

J(Na22+)

J(K22+)

J(SO42-)

J(CO32-)

J(H2O)

equilibrium solid phasea

1.0939 1.0872 1.0766 1.0732 1.0803 1.0970 1.1552 1.1552 1.1552 1.2117 1.3251 1.3749 1.4289 1.5578 1.1529 1.3518 1.3086 1.1561 1.4968 1.1069 1.0857

1.96 1.54 1.26 10.84 10.92 11.72 11.82 9.43 6.78 5.02 3.10 2.15 1.87 1.11 5.29 2.94 5.69 2.19 0.00 9.27 7.87

3.17 3.82 4.75 2.64 2.75 3.96 6.32 10.58 15.67 17.21 17.43 18.78 21.05 28.53 4.16 25.90 4.13 16.15 27.10 0.00 0.00

0.96 0.89 0.92 19.55 19.71 21.02 20.78 20.51 20.82 18.45 7.77 2.84 0.00 0.00 5.77 1.56 5.24 0.00 0.93 5.37 1.75

4.10 4.07 4.36 3.38 3.46 4.59 6.50 6.74 6.90 7.42 8.56 13.8 18.31 23.10 3.04 21.42 3.60 14.88 18.53 5.02 5.38

0.75 0.83 0.92 1.43 1.50 1.54 2.00 2.22 2.48 2.16 1.78 0.81 0.73 0.60 0.00 0.00 0.00 0.95 2.06 9.60 9.80

89.06 88.85 87.79 62.16 61.66 57.17 52.58 50.52 47.35 49.74 61.36 61.62 58.04 46.66 81.74 48.18 81.34 65.83 51.38 70.74 75.20

35.81 26.55 19.14 80.20 79.60 73.59 63.32 44.41 27.66 20.34 15.07 9.34 7.07 3.21 70.08 9.06 73.83 10.46 0.00 144.35 158.57

34.15 38.82 42.54 11.52 11.82 14.66 19.97 29.38 37.71 41.13 49.98 48.12 46.93 48.72 32.50 47.08 31.61 45.50 52.17 0.00 0.00

8.40 7.38 6.69 69.31 68.84 63.24 53.34 46.28 40.71 35.82 18.10 5.91 0.00 0.00 36.63 2.30 32.58 0.00 1.45 40.07 16.90

57.45 53.80 50.76 19.17 19.34 22.10 26.70 24.33 21.59 23.05 31.91 45.97 53.07 51.28 30.87 50.62 35.81 54.50 46.37 59.93 83.10

4159 3915 3407 1175 1149 917.3 719.8 608.0 493.7 515.1 762.5 684.2 560.7 345.3 2767 379.5 2697 803.7 428.6 2815 3872

nb+kc+nc nb+kc+nc nb+kc+knc+nc nb+kb+knc nb+kb+knc nb+kb+knc nb+kb+kc+nkc nb+kb+knc nb+kb+knc nb+kb+knc nb+kb+knc nb+kb+knc nb+kb+knc kb+kc+knc ns+nsc+ks ks+knc+kc nc+nsc+ks nc+knc+nb kc+ks+kb nb+ns+nsc nb+nc+nsc

nc: Na2CO3 · 10H2O; ns: Na2SO4 · 10H2O; kc: K2CO3 · 3/2H2O; ks: K2SO4; nsc: 2Na2SO4 · Na2CO3; knc: K2CO3 · Na2CO3 · 12H2O; kb: K2B4O7 · 4H2O.

Figure 2. Projected diagram of quinary system Na+ + K+ + CO32- + SO42- + B4O72- + H2O at 273.15 K (saturated with Na2B4O7 · 10H2O).

Figure 1. Stereo phase diagram of quinary system Na+ + K+ + CO32- + SO42- + B4O72- + H2O at 273.15 K.

K3, saturated with salts Na2B4O7 · 10H2O + Na2CO3 · 10H2O + K2CO3 · 3/2H2O + K2CO3 · Na2CO3 · 12H2O, with w(Na+) 1.26 %, w(K+) 4.57 %, w(SO42-) 0.92 %, w(CO32-) 4.36 %, w(B4O72-) 0.92 %. K4, saturated with salts Na2B4O7 · 10H2O + K2CO3 · 3/2H2O + 2Na2SO4 · Na2CO3 + Na2CO3 · 10H2O, with w(Na+) 3.87 %, w(K+) 0.99 %, w(SO42-) 0.45 %, w(CO32-) 5.48 %, w(B4O72-) 0.13 %. K5, saturated with salts Na2B4O7 · 10H2O + K2CO3 · 3/2H2O + K2CO3 · Na2CO3 · 12H2O + K2B4O7 · 4H2O, with w(Na+) 11.82 %, w(K+) 6.32 %, w(SO42-) 20.78 %, w(CO32-) 6.50 %, w(B4O72-) 2.00 %. K6, saturated with salts Na2B4O7 · 10H2O + K2CO3 · 3/2H2O + 2Na2SO4 · Na2CO3 + Na2SO4 · 10H2O, with w(Na+) 12.42 %, w(K+) 2.13 %, w(SO42-) 18.81 %, w(CO32-) 5.63 %, w(B4O72-) 1.15 %.

In the equilibrated solution corresponding to invariant point K1, the order of the ion concentration is SO42- > Na+ > CO32> K+ > B4O72-, with the largest concentration of SO42- present only in the salt K2SO4 among the equilibrated solid phases, so the invariant point K1 is an incongruent invariant point. Similarly K2, K3, K5, and K6 are incongruent invariant points. However, in the equilibrated solution corresponding to invariant point K4, the order of the ion concentration is CO32- > Na+ > K+ > SO42> B4O72-, with the largest concentration of CO32- present in the salts K2CO3 · 3/2H2O, 2Na2SO4 · Na2CO3, and Na2CO3 · 10H2O among the equilibrated solid phases, so invariant point K4 is a congruent invariant point. At the congruent invariant point K4, four salts simultaneously precipitated, whereas at the invariant points K1, K2, K3, K5, and K6, dissolving conversion occurred, with one salt dissolving and the other three salts precipitating. On the basis of the measured density data of the equilibrated solution tabulated in Table 1, we concluded that, at the univariant curves, the density of the solution smoothly changes with the J(CO32-). Comparisons between this system and the

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ternary or quaternary subsystems containing potassium carbonate show that, in this quinary system, the contribution of salt potassium carbonate to the density has been reduced, and the crystallization field of salt K2CO3 · 3/2H2O enlarges clearly.13

Conclusions The projected phase diagram saturated with the salt Na2B4O7 of this system at 273.15 K consists of six invariant points, 12 univariant curves, and seven crystallization fields corresponding to the single salts Na2SO4 · 10H2O, K2SO4, Na2CO3 · 10H2O, K2CO3 · 3/2H2O, K2B4O7 · 4H2O, and double salts 2Na2SO4 · Na2CO3 and K2CO3 · Na2CO3 · 12H2O. The crystallization forms of sodium and potassium borate are Na2B4O5(OH)4 · 8H2O and K2B4O5(OH)4 · 2H2O, respectively. In contrast to the subsystems containing potassium carbonate, the crystallization area of K2CO3 · 3/2H2O increases clearly in the quinary system.

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(5) Zeng, Y.; Shao, M. Liquid-Solid Metastable Equilibria in the Quinary System Li+ + K+ + Cl- + CO32- + B4O72- + H2O at T ) 288 K. J. Chem. Eng. Data 2006, 51, 219–222. (6) Qi, W.; Zheng, M. P. Simulation with Pitzer model for lake brine evolution of Zabuye Salt Lake, Tibetan Plateau. Acta Geol. Sin. 2007, 81, 1734–1741. (7) de Limma, M. C. P.; Pitzer, K. S. Thermodynamics of saturated electrolyte mixtures of NaCl with Na2SO4 and MgCl2. J. Solution Chem. 1983, 12, 187–199. (8) Phutela, R. C.; Pitzer, K. S. Thermodynamics of electrolyte mixtures: Enthalpy and the effect of temperature on the activity coefficient. J. Solution Chem. 1986, 15, 649–662. (9) Zeng, Y.; Sang, S. H.; Ni, L. Q. Liquid-solid equilibrium for quaternary system Na2SO4 + K2SO4 + Na2B4O7 + K2B4O7 + H2O at 288 K. J. Chem. Eng. Data 2005, 50, 928–931. (10) Institute of Qinghai Salt-Lake of Chinese Academy of Sciences. Analytical Methods of Brines and Salts, 2nd ed.; Chinese Science Press: Beijing, 1984; pp 75-80. (11) Farmer, J. B. Metal borates. In AdVances in Inorganic Chemistry and Radiochemistry; Emeleus, H. J., Sharpe, A. G., Eds.; Academic Press: New York, 1982; Vol. 25. (12) Zeng, Y.; Lin, X. F. Solubility and density measurements of concentrated Li2B4O7 + Na2B4O7 + K2B4O7 + Li2SO4 + Na2SO4 + K2SO4 + H2O Solution at 273.15 K. J. Chem. Eng. Data 2009, 54, 2054–2059. (13) Zheng, Z. Y. Studies on the Metastable Phase Equilibrium in the Quinary System K+, Na+//CO32-, SO42-, B4O72--H2O at 273 K; College of Materials and Chemistry & Chemical Engineering, Chengdu University of Technology: Chengdu, China, 2008. Received for review July 30, 2010. Accepted November 12, 2010. Financial support for this work was provided by the National Nature Science Foundation of China (No. 40673050), the Doctoral Fund of the Ministry of Education of China (20070616008), the Scholarship Leaders Training Fund from Sichuan Province (2008-140), and the Program for New Century Excellent Talents in University (NCET-08-0900).

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