Thermodynamics Phase Equilibria of the Aqueous Ternary Systems

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Thermodynamics Phase Equilibria of the Aqueous Ternary Systems LiCl + KCl (MgCl2) + H2O at 348 K Pengtao Mu,† Qi Tan,† Xudong Yu,† Qiang Li,† and Ying Zeng*,†,‡ †

College of Material and Chemistry & Chemical Engineering, Chengdu University of Technology, Chengdu, 610059, P. R. China Mineral Resources Chemistry Key Laboratory of Sichuan Higher Education Institutions, Chengdu, 610059, P. R. China

‡

ABSTRACT: The thermodynamics phase equilibria of the aqueous ternary systems LiCl + MgCl2 + H2O and LiCl + KCl + H2O at 348 K were investigated using an isothermal evaporation method. The solubilities and physicochemical properties such as density and refractive index of the equilibrated solution were determined. The compositions of solid phases were confirmed using the Scherinemakers’ wet residue method. In accordance with the experimental data, the phase diagrams and physicochemical vs composition diagrams were constructed. The ternary system LiCl + MgCl2 + H2O is of a complex type with double salt LiCl·MgCl2·7H2O (lithium carnallite) formed at 348 K, and its phase diagram consists of two invariant points, three univariant curves, and three crystallization fields corresponding to lithium chloride monohydrate (LiCl·H2O), magnesium chloride hexahydrate (MgCl2·6H2O), and lithium carnallite (LiCl·MgCl2·7H2O). The ternary system LiCl + KCl + H2O is of a simple eutectic type at 348 K, and its phase diagram is composed of one invariant point, two univariant curves and two crystallization fields corresponding to potassium chloride (KCl) and lithium chloride monohydrate (LiCl·H2O). Lithium chloride has a salting out effect on magnesium chloride or potassium chloride. Comparisons between the metastable and stable phase diagrams at 348 K of these two ternary systems have been drawn. In the system LiCl + MgCl2 + H2O, the crystallization region of MgCl2·6H2O under the metastable equilibrium is smaller, while the crystallization region of the LiCl·MgCl2·7H2O is larger. In the system LiCl + KCl + H2O, the crystallization regions of LiCl·H2O and KCl under metastable equilibrium are both smaller. Comparisons between the metastable phase diagrams of system LiCl + KCl + H2O at (298, 323, and 348) K show that the crystallization region of LiCl·H2O increased with the rise of temperature, whereas the crystallization region of salt KCl is decreased.

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INTRODUCTION The phase equilibrium of aqueous system deals with the solutions which two or more phases coexist in the thermodynamic equilibrium. For metastable phase equilibrium, the system does not reach a certain equilibrium, and the phase characteristics of the system change slightly and continually along with the evaporation of water. The metastable phase equilibrium is more approach to the evaporation and precipitation process of bittern, thus, dependable knowledge of metastable phase equilibria plays an irreplaceable role in the brine resources development. Pingluo underground brine, located in the west of Sichuan province of China, is one of the most significant liquid mineral resources enriched with lithium, sodium, potassium, magnesium, rubidium, cesium, bromine, iodine, borate, etc.1 At the early stage of the brine’s development, large proportions of NaCl and KCl were separated from the brine by means of evaporation and precipitation methods; bromine and borate were separated by means of oxidation style method and extraction method, respectively. After that, the composition of the brine can be summarized as the quinary system Li+, K+, Mg2+, Rb+//Cl−−H2O1 which contains six ternary systems, that are ternary system LiCl + KCl + H2O, LiCl + MgCl2 + H2O, LiCl + RbCl + H2O, KCl + MgCl2 + H2O, KCl + RbCl + H2O, © XXXX American Chemical Society

and MgCl2 + RbCl + H2O. Up to now, some ternary subsystems among them have been investigated in our previous research, such as the metastable phase equilibria of ternary system LiCl + RbCl + H2O at 298 K2 and ternary systems KCl + MgCl2 + H2O and KCl + RbCl + H2O at 298 K,3 323 K,4 and 348 K.5 For the ternary systems LiCl + KCl + H2O and LiCl + MgCl2 + H2O, some research has been done, such as the metastable phase equilibria of the ternary system LiCl + KCl + H2O at 298 K6 and 323 K,7 the stable phase equilibria of system LiCl + KCl + H2O at 348 K,8 the stable phase equilibria of the ternary system LiCl + MgCl2 + H2O at 273 K,9 298 K,10 333 K, and 348 K.11 However, the metastable phase equilibria of these two ternary systems at 348 K have not been reported in the literature. The Pingluo underground brine belongs to the marine sedimentary deep brine, with distinguishing features of deep buried depth (over 4500 m), high pressure (about 97 MPa), and high temperature (about 393 K).1 When the brine reaches the ground, its temperature is still up to 373 K; by this stage, Received: July 27, 2014 Accepted: December 29, 2014

A

DOI: 10.1021/je500700d J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Table 1. Chemical Reagents Used in the Experiment chemical name

source

lithium chloride (LiCl) potassium chloride (KCl) magnesium chloride hexahydrate (MgCl2·6H2O) ethylenediaminetetraacetic acid (EDTA) cetyltrimethylammonium bromide (CTAB) sodium tetraphenylborate (STPB) silver nitrate (AgNO3)

Chengdu Kelong Chemical Reagent Plant Chengdu Kelong Chemical Reagent Plant Chengdu Kelong Chemical Reagent Plant Chengdu Kelong Chemical Reagent Plant Chengdu Kelong Chemical Reagent Plant Sinopharm Chemical Reagent Co., Ltd. Sinopharm Chemical Reagent Co., Ltd.

mass fraction purity 99.0 99.5 99.0 99.0 99.0 99.0 99.8

% % % % % % %

Table 2. Solubilities and Physicochemical Properties of the Ternary System LiCl + MgCl2 + H2O at T = 348 K and Pressure p = 0.1 MPaa composition of equilibrated solution, w(B)·102 no.

density(g·cm−3)

refractive index nD

w(MgCl2)

w(LiCl)

1, A 2 3 4 5 6, E1 7 8 9 10, E2 11 12 13 14 15 16, B

1.4437 1.4617 1.4998 1.5182 1.5281 1.5262 1.5240 1.5222 1.5217 1.5011 1.4806 1.4997 1.5031 1.5020 1.5061 1.5036

1.4454 1.4471 1.4498 1.4511 1.4511 1.4518 1.4460 1.4439 1.4441 1.4447 1.4400 1.4392 1.4388 1.4385 1.4388 1.4380

0.00 4.43 5.30 9.35 11.79 12.06 15.69 18.82 22.84 24.83 28.80 31.82 32.81 34.95 35.80 39.38

50.89 46.63 45.83 41.63 39.16 36.91 31.47 26.20 21.26 18.64 14.22 9.12 7.50 4.79 3.34 0.00

composition of wet solid phase, w(B)·102 w(MgCl2)

w(LiCl)

3.76 5.93 8.32 9.64

53.00 50.25 46.01 47.94

36.06 35.48 39.37 39.4 39.32

7.06 7.16 3.09 2.38 2.40

equilibrium solid phase LiCl·H2O LiCl·H2O LiCl·H2O LiCl·H2O LiCl·H2O LiCl·H2O+LiCl·MgCl2·7H2O LiCl·MgCl2·7H2O LiCl·MgCl2·7H2O LiCl·MgCl2·7H2O LiCl·MgCl2·7H2O + MgCl2·6H2O MgCl2·6H2O MgCl2·6H2O MgCl2·6H2O MgCl2·6H2O MgCl2·6H2O MgCl2·6H2O

Note: Standard uncertainties u are u(T) = 0.10 K; ur(p) = 0.05; u(ρ) = 2.0·10−4 g·cm−3; u(n) = 1.0·10−4 nD; u(LiCl) = 0.0050; u(MgCl2) = 0.0050; w(B) = mass fraction of B.

a

salt NaCl first separates from the brine by evaporation using thermal energy from itself. At the later stage of evaporation process, the temperature of brine is keeping around 348 K. To make sure the interaction effect of coexisted salts at 348 K, research focusing on the metastable phase equilibria of the ternary systems LiCl + KCl + H2O and LiCl + MgCl2 + H2O at 348 K has been done. Comparisons of the phase diagrams of these two systems at different temperature have also been made.

diffractometer (DX-2700 type), made by Dandong Fangyuan Instrument Co., Ltd., was used to analyze the crystal form of the solid phase. Experimental Method. The metastable phase equilibrium was studied using an isothermal evaporation method. The experiment was divided into two parts: evaporation and measurement, and these two processes were cycled until the solution drying by evaporation. First, an appropriate amount of salt was dissolved into the deionized water in an opened polyethylene container to form synthesized solution. Second, the polyethylene container was placed in a thermostatic evaporator (type SHH-250 thermostatic evaporator instrument) to evaporate with the temperature maintained at (348 ± 0.1 K) and observe the evaporation process. Once a certain amount salt precipitated from the solution, the solid phase and the liquid phase were quickly separated from each other. For the liquid phase, 5 mL sample was taken out and transferred to a 50 mL volumetric flask, diluted with deionized water to volume, and mixed to determine the chemical composition of liquid phase. Meanwhile, the density and refractive index of the solution were determined experimentally. For the wet solid phase, about 3 g of wet sample was weighed accurately, dissolved in a 50 mL volumetric flask, and used to analyze the composition of wet solid. According to the composition of wet solid and its corresponding liquid phase, by means of Schreinemaker’s

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EXPERIMENTAL SECTION Reagents. The chemical reagents used in the experiment were of analytical purity grade, and detailed information is listed in Table 1. The deionized water, with an electrical conductivity less than 1·10−4 S·m−1 and pH ≈ 6.60, was used in the preparation of saline solution and chemical analysis. Apparatus. A thermostatic evaporator (SHH-250 type), with a temperature range from (258 to 373) K (precision ± 0.1 K), was made by Chongqing Inborn Instrument Corp., China. An inductively coupled plasma optical emission spectrometer (ICP-OES: 5300 V type) was made by PerkinElmer Instrument Corp., America. For an Abbe refractometer (2WAJ type), used together with a water bath device to keep the temperature constant, the precision is ± 0.0001. A standard analytical balance (AL104 type), made by Mettler Toledo Instruments Co., Ltd. and with 110 g capacity and ± 0.0002 g resolution, was used to determine the density of solution. X-ray B

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method, the crystallization form of salt can be determined. The result was verified by the method of X-ray diffraction. Analytical Methods. The composition of liquid and solid phases was obtained using instrumental analysis and chemical analysis.12,13 The analytical methods for ions are listed below. Li+: ICP-OES method (precision: less than 0.50 %). Mg2+: ethylenediaminetetraacetic acid (EDTA) titration using K−B as indicator (precision: ± 0.50 %). K+: sodium tetraphenylborate (STPB)−hexadecyl trimethylammonium bromide (CTAB) back-titration using titan yellow as indicator (precision: ± 0.50 %). Cl−: silver nitrate titration using potassium chromate as indicator (precision: ± 0.30 %).

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RESULTS AND DISCUSSION Ternary System LiCl + MgCl2 + H2O. The experimental data for solubility and physicochemical property values (density and refractive index) of the ternary system LiCl + MgCl2 + H2O at 348 K are listed in Table 2. Figure 1 is the metastable

Figure 2. X-ray diffraction photograph of the invariant point E1 (LiCl· H2O + LiCl·MgCl2·7H2O).

Figure 3. X-ray diffraction photograph of the invariant point E2 (MgCl2·6H2O + LiCl·MgCl2·7H2O).

Figure 1. Metastable phase diagram of the ternary system LiCl + MgCl2 + H2O at 348 K.

MgCl2·7H2O. On the univariant curves, the solubility of MgCl2 is decreased with the increase of LiCl, which indicates that the salt LiCl has salting-out effect on salt MgCl2 at 348 K. Both lithium chloride and magnesium chloride have various hydrated forms, that are LiCl·nH2O (n = 1, 2, 3, 5) and MgCl2· mH2O (m = 2, 4, 6, 8, 12).14 The experimental results show that the existence forms of lithium chloride and magnesium chloride in this system are LiCl·H2O and MgCl2·6H2O at 348 K, respectively. Figure 4 shows the metastable and stable phase diagrams11 of system LiCl + MgCl2 + H2O at 348 K, with dots for metastable and triangles for stable. It is obviously shown that the crystallization forms of salts are the same, whereas the crystallization zones have changed slightly. Under the metastable equilibrium, the crystallization region of MgCl2· 6H2O is smaller than that under stable equilibrium, while the crystallization region of the double salt LiCl·MgCl2·7H2O is larger than that under stable equilibrium, and the crystallization region of LiCl·H2O has no significant change. Figures 5 and 6 are the diagram of the density versus composition and the diagram of the refractive index versus composition, respectively. It is shown that both the density and refractive index values change regularly with the increase of LiCl. In Figure 5, at the commensurate invariant point E1, the

phase diagram which constructed based on the data listed in Table 2. As shown in Figure 1, the ternary system LiCl + MgCl2 + H2O at 348 K is of a complex type with double salt LiCl· MgCl2·7H2O (lithium carnallite) formed. Its phase diagram is composed of two invariant points (E1 and E2), three univariant curves (AE1, E1E2, and BE2), and three crystallization fields corresponding to LiCl·H2O (aAE1a), MgCl2·6H2O (bBE2b), and LiCl·MgCl2·7H2O (cE1E2c). Symbols “a”, “b”, and “c” are the composition points of salts LiCl·H2O, MgCl2·6H2O, and LiCl·MgCl2·7H2O. Figures 2 and 3 showed the XRD photographs of the invariant points E1 and E2, respectively. The univariant curve for the ternary system corresponds to one salt and an equilibrated solution; the cosaturated salt for the three univariant curves are LiCl·H2O for AE1, LiCl·MgCl2· 7H2O for E1E2, and MgCl2·6H2O for BE2, respectively. Invariant point E1 is of a commensurate invariant point, while invariant point E2 is of an incommensurate invariant point. The composition of the liquid phase corresponding to the invariant point E1 is w(MgCl2) = 12.06 %, w(LiCl) = 36.91 %, and the corresponding solid phases are LiCl·H2O and LiCl·MgCl2· 7H2O. The composition of the liquid phase corresponding to the invariant point E2 is w(MgCl2) = 24.83 %, w(LiCl) = 18.64 %, and the concomitant solid phases are MgCl·6H2O and LiCl· C

DOI: 10.1021/je500700d J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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System LiCl + KCl + H2O. The experimental data of the solubility and physicochemical property (density and refractive index) of the ternary system at 348 K are shown in Table 3. From Table 3, the metastable phase diagram was constructed, as shown in Figure 7. Obviously, the ternary system LiCl + KCl + H2O at 348 K is of a simple eutectic type, without double salt or solid solution formed. The metastable phase diagram consists of one invariant point (E), two univariant curves (AE: solubility curve of KCl, BE: solubility curve of LiCl), and two crystallization fields corresponding to single salts potassium chloride (aAEa) and lithium chloride monohydrate (bBEb). Symbols “a” and “b” are the composition points of salts KCl and LiCl·H2O, respectively. The composition of the liquid phase corresponding to point E is w(KCl) = 6.16 %, w(LiCl) = 42.30 %, and the relevant cosaturated salts are LiCl·H2O and KCl. The XRD photograph of the invariant point E was demonstrated in Figure 8. As showed in Figure 7, on the univariant curves, the solubility of salt KCl decrease quickly with the accretion of salt LiCl, which indicates that salt LiCl has salting-out effect on salt KCl. It is attributed to the strong polarization ability of the lithium ion. Lithium ions and hydrones react to form hydrated lithium ions and cause a high solubility of LiCl. When the salt KCl is added into the aqueous solution, intense common ion effect prevents the dissolution of salt KCl. On the basis of the experimental data of physicochemical properties in Table 3, the density vs composition diagram and refractive index vs composition diagram were plotted in Figure 9 and Figure 10, respectively. In Figure 9, the density changes regularly with the percentage composition of LiCl. On the univariant curve AE, the density rises after an initial decline with the increase of LiCl content. The decrease is attributed to the strong salting-out effect of LiCl. In Figure 10, the refractive index increases gradually with the increase of the content of LiCl. Comparisons between the metastable phase diagrams of system LiCl + KCl + H2O at 298 K,6 323 K7 and 348 K are shown in Figure 11. It is showed that the crystallization region of LiCl·H2O increases with the rise of temperature, whereas the crystallization region of the KCl is decreased. Figure 12 includes the metastable and stable phase diagrams8 of system LiCl + KCl + H2O at 348 K. It is shown that the crystallization forms of salts have no changes; however, the crystallization zones have changed slightly. The crystallization regions of LiCl· H2O and KCl under metastable equilibrium are both smaller than that under stable equilibrium.

Figure 4. Metastable and stable phase diagram11 of the ternary system LiCl + MgCl2 + H2O at 348 K. (●, metastable; ▲, stable; E1, E2, invariant points of metastable; F1, F2, invariant points of stable).

Figure 5. Density vs composition diagram of the ternary system LiCl + MgCl2 + H2O at 348 K.

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CONCLUSIONS The metastable phase equilibria of the ternary systems LiCl + MgCl2 + H2O and LiCl + KCl + H2O at 348 K were investigated by the isothermal evaporation method. Based on the experimental data, the metastable phase diagrams and the physicochemical properties such as density and refractive index vs composition diagrams of the system were constructed. The ternary system LiCl + MgCl2 + H2O at 348 K is of a complex type with double salt LiCl·MgCl2·7H2O formed. Its metastable phase diagram consists of two invariant points, three univariant curves, and three crystallization fields. The crystallization forms of lithium chloride and magnesium chloride at the temperature of 348 K are LiCl·H2O and MgCl2·6H2O, respectively. Salt LiCl has a salting-out effect on salt MgCl2. The comparison between the stable and metastable

Figure 6. Refractive index vs composition diagram of the ternary system LiCl + MgCl2 + H2O at 348 K.

liquid phase has the largest density. In Figure 6, the refractive index values reach the extrema at invariant points E1 and E2, respectively. D

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Table 3. Solubilities and Physicochemical Properties of the Ternary System LiCl + KCl + H2O at T = 348 K and Pressure p = 0.1 MPaa composition of equilibrated solution, w(B)·102 no.

density(g·cm−3)

refractive index nD

w(KCl)

w(LiCl)

1, A 2 3 4 5 6 7 8 9 10 11 12, E 13 14 15 16, B

1.3120 1.3079 1.3076 1.2843 1.2811 1.2789 1.3069 1.3247 1.3218 1.3733 1.4500 1.5023 1.4974 1.4729 1.4634 1.4393

1.3713 1.3768 1.3773 1.3796 1.3812 1.3857 1.3924 1.3960 1.3981 1.4212 1.4318 1.4478 1.4488 1.4468 1.4498 1.4461

32.29 28.69 25.24 21.56 18.99 14.84 9.63 8.61 7.24 6.45 6.68 6.16 5.13 3.75 3.17 0.00

0.00 1.72 4.01 8.38 10.76 15.00 21.42 23.84 25.83 32.32 40.49 42.30 46.33 47.75 50.15 50.89

composition of wet solid phase, w(B)·102 w(KCl)

w(LiCl)

75.51 70.02 56.20 70.88 69.35 52.76 40.55 49.70 54.78 52.83 3.96

0.62 0.99 6.08 5.12 6.52 11.82 15.78 13.67 15.23 19.95 54.16

equilibrium solid phase KCl KCl KCl KCl KCl KCl KCl KCl KCl KCl KCl KCl + LiCl·H2O LiCl·H2O LiCl·H2O LiCl·H2O LiCl·H2O

Note: Standard uncertainties u are u(T) = 0.10 K; ur(p) = 0.05; u(ρ) = 2.0·10−4 g·cm−3 ; u(n) = 1.0·10−4 nD; u(LiCl) = 0.0050; u(KCl) = 0.0050; w(B) = mass fraction of B. a

Figure 7. Metastable phase diagram of the ternary system LiCl + KCl + H2O at 348 K.

Figure 9. Density vs composition diagram of the ternary system LiCl + KCl + H2O at 348 K.

Figure 8. X-ray diffraction photograph of the invariant point E (KCl + LiCl·H2O).

Figure 10. Refractive index vs composition diagram of the ternary system LiCl + KCl + H2O at 348 K. E

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Funding

This project was supported by National High Technology Research and Development Program of China (2012AA061704), China National Nature Science Foundation (41173071, 41473059), the Sichuan Youth Science and Technology Innovation Research team funding (2013TD0005), the Project of the China Geological Survey (Grant 12120113087700), and Innovation Team of CDUT (KYTD201405). Notes

The authors declare no competing financial interest.

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Figure 11. Metastable phase diagram of the ternary system LiCl + KCl + H2O at 298 K,6 322 K,7 and 348 K. (●, 348 K; ⧫, 323 K; ■, 298 K; E, invariant point at 348 K; F, invariant point at 323 K; G, invariant point at 298 K).

Figure 12. Metastable and stable phase diagram8 of the ternary system LiCl + KCl + H2O at 348 K. (●, metastable; ▲, stable; E, invariant point of metastable; H, invariant point of stable).

phase diagrams of this system at 348 K shows that the crystallization region of salts has changed. The ternary system LiCl + KCl + H2O at 348 K is of a simple eutectic type. Its metastable phase diagram consists of one invariant point, two univariant curves and two crystallization fields. Salt LiCl also has a salting-out effect on salt KCl. Under the metastable equilibrium, the crystallization regions of LiCl· H2O and KCl are both smaller than that under stable equilibrium. Comparisons between the metastable phase diagrams of this ternary system at 298 K, 323 K, and 348 K show that the crystallization region of LiCl·H2O is increased and KCl is decreased with the rise of temperature.

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

DOI: 10.1021/je500700d J. Chem. Eng. Data XXXX, XXX, XXX−XXX