Study on the Solubility of the Aqueous Quaternary System Li2SO4 +

Article pubs.acs.org/jced

Study on the Solubility of the Aqueous Quaternary System Li2SO4 + Na2SO4 + K2SO4 + H2O at 273.15 K Ying Zeng,*,†,‡ Xiaofeng Lin,§ and Xudong Yu† †

College of Materials and Chemistry & Chemical Engineering, Chengdu University of Technology, Chengdu 610059, China Mineral Resources Chemistry Key Laboratory of Sichuan Higher Education Institutions, Chengdu 610059, China § Panzhihua Steel Corporation, Panzhihua 617067, China ‡

ABSTRACT: The aqueous quaternary system Li2SO4 + Na2SO4 + K2SO4 + H2O is one of the most important subsystems of the sulfate-type salt lake brine located in the Qinghai-Tibet Plateau. The phase equilibria of the aqueous quaternary system Li2SO4 + Na2SO4 + K2SO4 + H2O were investigated at 273.15 K using an isothermal evaporation method. A sodium and lithium sulfate double salt (3Na2SO4·Li2SO4·12H2O) and a potassium and lithium sulfate double salt (KLiSO4) are formed in this quaternary system at 273.15 K. There are three invariant points, seven univariant curves, and five crystallization fields in the phase diagram of this system. The five crystallization fields correspond to sodium sulfate decahydrate (Na2SO4·10H2O), potassium sulfate (K2SO4), lithium sulfate monohydrate (Li2SO4·H2O), and two double salts KLiSO4 and 3Na2SO4·Li2SO4·12H2O. The double-salt glaserite (3K2SO4·Na2SO4) is not formed in this system at research temperature. The three invariant points have different attributes, which invariants E1 and E2 are of incommensurate points, whereas invariant E3 is of a commensurate point. The water content and the density of the equilibrated solution change regularly with the mass fraction composition of potassium sulfate.

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INTRODUCTION With the depletion of solid mineral resource, more and more research focuses on the exploitation of liquid resource. Salt-lake brine, bounded with more than 50 kinds of elements, is a very important liquid mineral resource.1 The Qinghai-Tibet Plateau, located in the west of China, is the main distribution area of salt lakes in the world. Because of the influence of geologic structure, topoclimate, ionic migration, and the characteristic of hydrochemistry, the modern salt lakes are usually provided with zonal aggregation.2 Salt lakes in the Qaidam Basin are of a magnesium-sulfated-chlorine-based subtype, whereas salt lakes in Tibet are of a carbonate-sulfate-based subtype.3 On the one hand, the water salt phase equilibrium is the basis to explore the regular transform and precipitations of natural salts in salt lakes and also gives the information about the evaporation, concentration, and crystallization behavior of salts. On the other hand, the Qinghai-Tibet Plateau is of the typical plateau continental climate, with the characteristics of drying, less rain, windiness, and long sunshine hours. The dry, windy, and sunny climate has enabled the development of solar pond techniques to exploit salt-lake brine resources. The metastable equilibrium solubilities of salts are the basic data for the design of the solar ponds process, and thus investigation on the metastable equilibrium is essential for comprehensive and utilization of the salt-lake brine. There is a series of salt lakes located in Tibet, the most influential including Lake Zabuye, Lake Bangkogco, Lake Margog, Lake Zacang, and so on. Among them, Lake Zabuye and Lake Bangkogco belong to the carbonate subtype, whereas © 2012 American Chemical Society

Lake Margog and Lake Zacang are of magnesium-sulfated subtype. Different type brine has different mineral equilibria. By now, the metastable and stable phase equilibrium aimed at different types of salt lakes have been reported, including our previous works focused on the component characteristics of the Zabuye Salt Lake,4−6 but much research still needs to be done before the solar pond technique can be successfully used in the Zabuye Salt Lake. The salt-water quaternary system Li2SO4 + Na2SO4 + K2SO4 + H2O is one of the most important subsystems of the sulfatetype salt-lake brine. In this article, the salt formation and precipitation characteristics of these sulfates at 273.15 K are discussed in accordance with the basic solubility data of the saltwater system Li2SO4 + Na2SO4 + K2SO4 + H2O.

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EXPERIMENTAL SECTION

Reagents and Apparatus. The analytical purity grade chemicals lithium sulfate (Li2SO4, 99.0 % (w/w)), sodium sulfate (Na2SO4, 99.0 % (w/w)), and potassium sulfate (K2SO4, 99.0 % (w/w)) used in this work were gained from the Chengdu Chemical Reagent Plant. A doubly deionized water, with an electrical conductivity less than 1 × 10−4 S·m−1 and pH 6.6, was obtained from a Millipore water system in our laboratory. The experimental solutions for each experiment Received: August 5, 2012 Accepted: October 24, 2012 Published: November 2, 2012 3672

dx.doi.org/10.1021/je300874c | J. Chem. Eng. Data 2012, 57, 3672−3676

Journal of Chemical & Engineering Data

Article

Table 1. Experimental Solubility and Density Values of Equilibrated Solution in the Quaternary System Li2SO4 + Na2SO4 + K2SO4 + H2O at 273.15 Ka composition of solution, w(B) × 102

Jänecke index n(Li2SO4) + n(Na2SO4) + n(K2SO4) = 100 mol

no.

density (g·cm−3)

w(Li2SO4)

w(Na2SO4)

w(K2SO4)

n(Li2SO4)

n(Na2SO4)

n(K2SO4)

n(H2O)

equilibrated solid phase

1A 2 3 4 5B 6 7 8E1 9 10 11E2 12C 13 14E3 15 16 17E 18 19 20 21D

1.1209 1.1774 1.1946 1.2028 1.1524 1.1964 1.1896 1.1924 1.1925 1.2031 1.2552 1.2651 1.2689 1.2839 1.2582 1.2624 1.2280 1.2457 1.2391 1.2486 1.2668

0.00 5.83 7.29 11.81 11.22 13.59 13.32 14.17 14.96 18.35 22.60 25.95 26.85 28.35 27.14 27.99 12.39 14.24 15.48 18.63 23.37

5.14 6.08 6.36 5.52 0.00 2.01 2.25 3.22 3.05 3.10 3.48 0.00 1.69 1.65 0.45 0.28 6.91 5.90 3.00 4.75 2.78

7.56 8.65 8.85 8.75 6.82 8.48 7.24 6.45 5.45 4.50 1.78 1.43 1.48 1.41 1.38 1.40 0.00 1.16 0.72 1.45 0.00

0.00 36.46 40.93 54.65 72.27 66.29 67.82 68.34 72.05 77.79 87.45 96.66 92.35 92.95 95.73 96.24 70.02 73.04 84.89 80.35 91.63

45.44 29.40 27.66 19.77 0.00 7.60 8.88 12.01 11.36 10.17 8.85 0.00 4.46 4.15 1.21 0.73 29.97 23.23 12.62 15.72 8.36

54.56 34.14 31.41 25.58 27.73 26.11 23.30 19.65 16.59 12.04 3.70 3.34 3.19 2.9 3.06 3.03 0.01 3.73 2.49 3.93 0.01

6089 3022 2649 2079 3203 2248 2386 2229 2237 1904 1655 1638 1457 1361 1517 1464 2761 2444 2683 1963 1753

NS+KS NS+KS NS+KS NS+KS KL+KS KL+KS KL+KS KL+KS+NS KL+NS KL+NS KL+NS+NL KL+LS KL+LS KL+LS+NL KL+NL KL+NL NL+NS NL+NS NL+NS NL+NS NL+LS

w(B), mass fraction of B; n(B), the Jänecke index values of B; NS, Na2SO4·10H2O; KS, K2SO4; LS, Li2SO4·H2O; KL, KLiSO4; NL, 3Na2SO4·Li2SO4·12H2O. a

liquid solutions were analyzed by chemical methods, and the solid phases were assayed by X-ray diffraction method. In the mean time, the density of the liquid solution was determined using a specific gravity bottle method with correction of air floating force and used to calculate the mass fraction of liquid components. The sodium and lithium ion in solution were measured using an inductively coupled plasma optical emission spectrometer (ICP-OES method) and with a precision of 0.06 mass %. The potassium ion of solution was analyzed by a sodium tetraphenyborate (STPB)−hexadecyl trimethyl ammonium bromide (CTAB) back-titration method and with a precision of 0.5 mass %.8 The sulfate ion concentration was determined by titration with a standard solution of EDTA in the presence of an excess Ba−Mg mixture solution8 and with a precision better than 1.0 mass %.

were produced by required amounts of the reagents dissolved in enough deionized water. The isothermal evaporation experiments were performed in a thermostatic evaporator (SHH-250 type, Chongqing INBORN Instrument Corporation, China). The temperature-controlling precision of the evaporator was ± 0.1 K. The powder X-ray diffraction (XRD) measurements used to analyze the crystalloid form of solid phases were recorded on a DX-2700 diffractometer with monochromatized Cu Kα radiation. The ion concentration of lithium and sodium in solution was determined by inductively coupled plasma optical emission spectrometer (ICP-OES, type 5300 V, PerkinElmer Instrument of America). Experimental Method. The isothermal evaporation method was employed.7 The isothermal evaporation experiments were carried out in a thermostatic evaporator (SHH-250 type). A series of initial evaporating solutions should be prepared prior to use. The required reagents were completely dissolved in 1000 mL of deionized water to produce the solutions. The amounts of the reagents depend on the salt’s solubility in aqueous solution and the composition of the invariant point in the ternary subsystem at test temperature. The prepared solutions were loaded into clean opened polyethylene containers (24 cm long, 14 cm wide, and 7 cm high) and then placed in the thermostatic evaporator; after being held at 273.15 K for 2 days, the lids of the containers ready for isothermal evaporation were opened. The evaporating temperature was strictly controlled at [(273.15 ± 0.1) K] and measured by a J thermocouple with an operating range of (258.15 to 338.15) K, whose system precision is of ± 0.1 K. The measuring point depended on the changes of solid phase in the process of evaporation.7 The determination at each measuring point includes liquid solution and solid phases. The

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RESULTS AND DISCUSSION The solubilities and densities data of the equilibrated solution in the quaternary system Li2SO4 + Na2SO4 + K2SO4 + H2O at 273.15 K are listed in Table 1. On the basis of the data of Jänecke index n(B), the isothermal evaporation phase diagram of this quaternary system at 273.15 K is constructed, as shown in Figure 1. The partially enlarged diagram of Figure 1 is shown in Figure 2. As shown in Figures 1 and 2, there are two kinds of double salt formed in this system at 273.15 K, which are a sodium and lithium sulfate double salt (3Na2SO4·Li2SO4·12H2O) and a potassium and lithium sulfate double salt (KLiSO4). The isothermal evaporation phase diagram consists of five crystallization fields, seven univariant curves, and three invariant points, which are labeled as E1, E2, and E3, respectively. The coexisting phases corresponding to the univariant curve are two 3673



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3Na2SO4·Li2SO4·12H2O + KLiSO4 + Na2SO4·10H2O and with w(Li2SO4) = 26.60 %, w(Na2SO4) = 3.48 %, and w(K2SO4) = 1.78 %; and E3, saturated with salts Li2SO4·H2O + KLiSO4 + 3Na2SO4·Li2SO4·12H2O and with w(Li2SO4) = 26.35 %, w(Na2SO4) = 1.05 %, and w(K2SO4) = 1.61 %. Figures 3 and 4 give the X-ray diffraction patterns of invariant points E2 and E3. At these two invariant points, the double salts KLiSO4 and 3Na2SO4·Li2SO4·12H2O exist together. Obviously, the univariant curve E2E3 is cosaturated with the double salt KLiSO4 and 3Na2SO4·Li2SO4·12H2O. The five crystallization fields correspond to three single salts, decahydrate sodium sulfate (Na2SO4·10H2O), anhydrous potassium sulfate (K2SO4), and monohydrate lithium sulfate (Li 2 SO 4 ·H 2 O), and two double salts, KLiSO 4 and 3Na2SO4·Li2SO4·12H2O. Salt Li2SO4·H2O has the smallest crystallization field, whereas salt Na2SO4·10H2O has the largest crystallization field, which shows that sodium sulfate has smaller solubility in water than lithium sulfate under the coexisting ions and can be easier to separate from solution using evaporation method in this system at research temperature. Lithium, sodium, and potassium belong to the alkali metal family; their sulfates have the same or similar crystal structure, and it is easy to form double salts between each other. Double salt formation can change salt-dissolving capacity. As in this system, the solubilities of the double salts KLiSO4 and 3Na2SO4·Li2SO4·12H2O are all smaller than the single salt lithium sulfate, that is to say, the formation of the double salt makes the lithium concentrating not only in mother liquids but also in double salts during the process of sodium and potassium extracting. In this quaternary system at 273.15 K, the double salt glaserite (3K2SO4·Na2SO4) is also not found. The double salts formed between the alkali metals lithium, potassium, and sodium in this quaternary system are the same as those in the quaternary system Li2B4O7 + Na2B4O7 + K2B4O7 + Li2SO4 + Na2SO4 + K2SO4 + H2O,8 which shows that the introduction of the borate ion in solution has no influence on the double salts of lithium, potassium, and sodium sulfate formed.

Figure 1. Metastable phase diagram of the quaternary system Li2SO4 + Na2SO4 + K2SO4 + H2O at 273.15 K.

Figure 2. Partial enlarged diagram of Figure 1.

solid phases and one saturated liquid phase, whereas those corresponding to the invariant points are three solid phases and one saturated liquid phase. Among the three invariant points, points E1 and E2 are of incommensurate invariant points, whereas point E3 is of a commensurate invariant point. The saturated salts and the composition of the equilibrated solution corresponding to the three invariant points are listed as follows: E1, saturated with salts K2SO4 + Na2SO4·10H2O + KLiSO4 and with w(Li2SO4) = 14.17 %, w(Na2SO4) = 3.22 %, and w(K2SO4) = 6.45 %; E2, saturated with salts

Figure 3. X-ray diffraction pattern of the invariant point E2 (KLiSO4 + Na2SO4·10H2O + 3Na2SO4·Li2SO4·12H2O). 3674



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Figure 4. X-ray diffraction pattern of the invariant point E3 (KLiSO4 + Li2SO4·H2O + 3Na2SO4·Li2SO4·12H2O).

Figure 5 is the water content diagram of the system, with the ordinate in accordance with the Jänecke index of water (mol

Figure 6. Density−composition diagram of the equilibrated solution in the quaternary system Li2SO4 + Na2SO4 + K2SO4 + H2O at 273.15 K.

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Figure 5. Water-content diagram of the equilibrated solution in the quaternary system Li2SO4 + Na2SO4 + K2SO4 + H2O at 273.15 K.

CONCLUSIONS The isothermal evaporation method was employed to study the phase diagram of the aqueous quaternary system Li2SO4 + Na2SO4 + K2SO4 + H2O at 273.15 K. The phase diagram of this quaternary system is composed of five salt-crystallization fields and three invariant points. The five salt-crystallization fields include three single salts and two double salts. It is easier to form a double salt among the potassium, lithium, and sodium sulfate. In this article, there are two kinds of double salts formed: one is the potassium and lithium sulfate double salt KLiSO4, and the other is the sodium and lithium sulfate double salt. Glaserite (3K2SO4·Na2SO4) is not formed in this quaternary system at 273.15 K. Salt Li2SO4·H2O has the smallest crystallization field, whereas salt Na2SO4·10H2O has the largest crystallization field. Relatively, sodium sulfate is easier to separate from solution using the evaporation method than lithium sulfate in this system at research temperature. Among the three invariant points in the diagram, points E1 and E2 are of incommensurate invariant points, whereas point E3 is of a commensurate invariant point. At points E3, the density of the solution reaches the maximum value, whereas the water content reaches the minimum value.

H2O/100 mol dry salts) and abscissa in accordance with the mass fraction of potassium sulfate. The water content changes regularly with a composition of potassium sulfate. At the univariant curves cosaturated with salt lithium sulfate, the water content changes scarcely with the slow evaporation rate caused by the higher content, whereas at the univariant curves cosaturated with salt sodium sulfate or potassium sulfate the water content sharply declines with the faster evaporation rate and reaches the smallest value at the invariant point E3. Figure 6 is the density versus composition diagram of the quaternary system at 273.15 K and shows that as the composition of potassium sulfate changes , the density of the solution regularly changes. At the univariant curves cosaturated with salt lithium sulfate, the density changes little, whereas at the univariant curves cosaturated with salt sodium sulfate or potassium sulfate, the density sharp increases with the quick concentrating of lithium sulfate and reaches a maximum value at the invariant point E3. 3675



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Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: 86-28-84079016. Fax: 8628-84079074. Funding

Financial support for this work was provided by the National High Technology Research and Development Program of China (2012AA061704), National Nature Science Foundation of China (no. 40673050, no. 41173071), the Project of China Geological Survey (1212011085523), and the Research Fund for the Doctoral Program of Higher Education from the Ministry of Education of China (20115122110001). Notes

The authors declare no competing financial interest.

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REFERENCES

(1) Zheng, M. P.; Liu, W. G.; Xiang, J. Qinghai-Xizhang Plateau Saline Lakes; Beijing Science and Technology Press: Beijing, China, 1989. (2) Zheng, X. Y.; Zhang, M. G.; Xu, T.; Li, B. X. Salt Lakes of China; Science Press: Beijing, China, 2002. (3) Song, P. S. The Exploitation and Utilization of Salt Lake Resource. J. Salt Lake Research 1993, 1, 68−80. (4) Zeng, Y.; Cao, F. J.; Li, L. G..; Yu, X. D.; Lin, X. F. Metastable Phase Equilibrium in the Aqueous Quaternary System (Li2SO4 + Na2SO4 + Li2B4O7 + Na2B4O7 + H2O) at 273.15 K. J. Chem. Eng. Data 2011, 56, 2569−2573. (5) Yang, H. X.; Zeng, Y. Metastable Phase Equilibrium in the Aqueous Quaternary System LiCl + Li2SO4 + Li2B4O7 +H2O at 273.15 K. J. Chem. Eng. Data 2011, 56, 53−57. (6) Wang, R. L.; Zeng, Y. Solid-Liquid Metastable Equilibria in the Quaternary System (NaCl + Na2CO3 + Na2SO4 + H2O) at 273.15 K. Chem. Res. Chin. Univ. 2010, 26, 114−117. (7) 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. (8) Institute of Qinghai Salt-Lake of Chinese Academy of Sciences. Analytical Methods of Brines and Salts, 2nd ed.; Chinese Science Press: Beijing, 1988.

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Study on the Solubility of the Aqueous Quaternary System Li2SO4 +

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