Phase Equilibrium of the Quaternary System Containing Lithium

Jul 10, 2014 - Qing Hai CITIC Guoan Science and Technology Development Co., Ltd, Golmud 816000, P. R. China. §. Mineral Resources Chemistry Key ...
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Phase Equilibrium of the Quaternary System Containing Lithium, Magnesium, Sulfate, and Borate in Aqueous Solution at 308 K Hongcan Li,†,‡ Shijun Ni,† and Ying Zeng†,§,* †

Chengdu University of Technology, Chengdu 610059, P. R. China Qing Hai CITIC Guoan Science and Technology Development Co., Ltd, Golmud 816000, P. R. China § Mineral Resources Chemistry Key Laboratory of Sichuan Higher Education Institutions, Chengdu 610059, P. R. China ‡

ABSTRACT: An isothermal dissolution method was employed to investigate the phase equilibrium at 308 K of the mixed aqueous system containing lithium, magnesium, sulfate, and borate. The solid phases of the invariant points were confirmed with the X-ray diffraction method. The stable phase diagram, the water content diagram, and the diagrams of the physicochemical properties versus the composition of the solution have been constructed. The system contained two invariant points (noted as E1 and E2), five univariant curves, and four crystallization fields corresponding to single salts epsomite (MgSO4·7H2O), hungchaoite (MgB4O7·9H2O), lithium sulfate monohydrate (Li2SO4· H2O), and lithium borate trihydrate (Li2B4O7·3H2O). Invariant points E1 and E2 are of commensurate invariant points. The crystallization fields decrease in the sequence MgB4O7· 9H2O, Li2B4O7·3H2O, Li2SO4·H2O, and MgSO4·7H2O, which was contrary with the solubility of the salts. Compared with the temperature at 298 K, the crystallization area of salts MgB4O7·9H2O and Li2SO4·H2O enlarged, whereas that of salts Li2B4O7·3H2O and MgSO4· 7H2O decreased at 308 K, which lead the change for property of invariant point, one invariant point change from incommensurate to commensurate. The solubility of salt MgSO4 increases with temperature strongly; thus, its crystallization field decreases obviously, whereas the solubility of salt Li2SO4 decreases with temperature and its crystallization field is enlarged.



INTRODUCTION A huge volume of natural salt lake and underground brine with high concentrations of lithium and borate are widely distributed in China, such as Zabuye salt lake, Chaidamu saline lake, Xitai Jinaier salt lake, Pingluoba underground brine, and Jianglin underground brine.1−3 According to the chemical composition of the brine, it can be divided into five types, which are chloride type, carbonate type, sulfate type, nitrate type, and borate type.4 For the comprehensive utilization of the brine resources, research on the phase equilibrium aimed different type brine is essential. Meanwhile, the solubility data and related equilibrium are necessary for developing appropriate process technologies to extract valuable elements and to investigate the geochemical origin of the brine. To date, based on the phase equilibrium and phase detachment technique, a series of chemical products have been extracted from the Great Salt Lake, Dead Sea, and Lop Nor.5 Xitai Jinaier salt lake, located in the Qaidam Basin with an area of 81 km2 and at a high altitude of 2678 m, is of magnesium sulfate type salt lake, which contains (188.410−192.189) g·L−1 chloride ion, (71.742−82.070) g·L−1 sodium, (21.887−28.780) g·L−1 sulfate, (22.940−30.176) g·L−1 magnesium, (7.650−9.523) g·L−1 potassium, (0.970−1.183) g·L−1 B2O3, and (0.261−0.270) g·L−1 lithium. Along with the exploiting process of the brine, sodium chloride and potassium chloride have been extracted as products, and the other ions are continuously enriched in the mother liquid to form an aqueous quinary system containing lithium, magnesium, chloride, sulfate, and borate. © 2014 American Chemical Society

According to the meteorological data, the temperature of the Xitai Lake in a year was between (298 and 313) K from June to October, with an average temperature of 308 K. Thus, the phase equilibrium, determined under the similar natural conditions, can objectively reveal the crystallization path and interactions among the salts. The reciprocal quaternary system Li+, Mg2+//SO42−, borate H2O is a most important subsystem of the complex quinary system mentioned above. Till now, the phase equilibrium for the ternary subsystem system Li2SO4 + MgSO4 + H2O had been widely studied over a wide temperature range,6−10 and the phase equilibrium for the quaternary system Li+, Mg2+//SO42−, borate−H2O has been studied by Song at 298 K,11 results show that no double salt is found and dehydration of the four original components does not occur in the system. Up to now, no report has been found in the literature about the phase equilibrium of this quaternary system at 308 K. To close these gaps, the phase equilibria of this quaternary system at 308 K have been studied in the present work.



EXPERIMENTAL SECTION The doubly deionized water (κ < 1.0 × 10−4 S·m−1) and chemicals with analytical grade purity were used to prepare solutions. All chemicals used were obtained from the Chengdu Received: March 31, 2014 Accepted: July 1, 2014 Published: July 10, 2014 2523

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Table 1. Experimental Values of Solubility and Physicochemical Properties Values of the Equilibrated Solution in the Reciprocal Quaternary System Li+, Mg2+//SO42−, borate− H2O at 308 K and Pressure p = 0.1 MPaa Jänecke index of dry salt composition of equilibrated solution, w(B)·10

2

J(Li22+)

+ J(Mg2+) = J(SO42−) + J(B4O72−) = 100

no.

density ρ/g·cm−3

refractive index

w(Li+)

w(Mg2+)

w(SO42−)

w(B4O72−)

w(H2O)

J(Li22+)

J(Mg2+)

J(SO42−)

J(B4O72−)

J(H2O)

equilibrated solid phase

1,A 2 3 4 5 6 7 8 9,E1 10,B 11,E1 12 13 14 15 16 17 18 19 20 21,E2 22 23 24 25,C 26,D 27 28 29 30 31 32 33,E2

1.0666 1.0762 1.0734 1.0756 1.0775 1.0872 1.1552 1.2094 1.2342 1.2753 1.2342 1.2447 1.2576 1.2664 1.2877 1.3015 1.3017 1.3219 1.3553 1.3845 1.4340 1.4051 1.3938 1.3951 1.3890 1.3436 1.3985 1.4002 1.4070 1.4241 1.4318 1.4332 1.4340

1.3485 1.3492 1.3495 1.3501 1.3509 1.3524 1.3675 1.3770 1.3820 1.3808 1.3820 1.3805 1.3827 1.3845 1.3875 1.3892 1.3908 1.3980 1.4048 1.4066 1.4083 1.4016 1.4000 1.3992 1.3994 1.4022 1.4020 1.4024 1.4040 1.4060 1.4078 1.4078 1.4083

0.34 0.34 0.36 0.39 0.44 0.57 1.73 2.37 2.84 2.68 2.84 2.35 2.25 2.29 2.12 2.21 2.19 1.76 1.29 1.09 1.15 1.23 1.42 1.53 1.66 0.00 0.12 0.24 0.51 0.86 1.11 1.16 1.15

0.05 0.04 0.04 0.04 0.04 0.04 0.04 0.04 0.03 0.00 0.03 0.27 0.62 0.76 0.96 1.10 1.87 3.36 4.48 4.62 4.75 4.94 4.95 4.85 4.86 6.15 5.91 5.67 5.20 5.20 4.74 4.86 4.75

0.00 0.23 0.42 0.90 1.60 2.62 11.04 15.95 19.40 20.04 19.40 16.91 17.63 18.44 18.05 19.29 22.20 24.95 26.21 25.33 26.09 27.77 29.36 29.82 29.80 23.81 23.59 23.42 23.45 25.93 25.82 26.57 26.09

4.00 3.74 3.67 3.11 2.55 2.39 1.71 0.96 0.66 0.28 0.66 0.74 0.72 0.72 0.71 0.69 0.69 1.11 1.03 1.10 1.36 0.76 0.39 0.22 0.00 1.23 1.38 1.41 1.45 1.38 1.34 1.38 1.36

95.61 95.65 95.51 95.56 95.37 94.38 85.48 80.68 77.07 77.00 77.07 79.73 78.78 77.79 78.16 76.71 73.05 68.82 66.99 67.86 66.65 65.30 63.88 63.58 63.68 68.81 69.00 69.26 69.39 66.63 66.99 66.03 66.65

92.25 93.70 94.03 94.47 95.06 96.15 98.70 99.05 99.40 100.0 99.40 93.84 86.40 84.07 79.45 77.86 67.22 47.84 33.52 29.23 29.77 30.36 33.43 35.58 37.42 0.00 3.43 6.90 14.66 22.45 29.08 29.47 29.77

7.75 6.30 5.97 5.53 4.94 3.85 1.30 0.95 0.60 0.00 0.60 6.16 13.60 15.93 20.55 22.14 32.78 52.16 66.48 70.77 70.23 69.64 66.57 64.42 62.58 100.0 96.57 93.10 85.34 77.55 70.92 70.53 70.23

0.00 9.04 15.62 31.88 50.36 63.93 91.26 96.41 97.94 99.14 97.94 97.37 97.54 97.64 97.63 97.84 98.11 97.32 97.63 97.38 96.88 98.34 99.19 99.55 100.0 96.90 96.51 96.41 96.32 96.81 96.89 96.89 96.88

100.0 90.96 84.38 68.12 49.64 36.07 8.74 3.59 2.06 0.86 2.06 2.63 2.46 2.36 2.37 2.16 1.89 2.68 2.37 2.62 3.12 1.66 0.81 0.45 0.00 3.10 3.49 3.59 3.68 3.19 3.11 3.11 3.12

19987 20306 19219 17833 15873 12264 3757 2597 2078 2213 2078 2453 2331 2200 2257 2082 1727 1441 1341 1402 1329 1242 1159 1139 1106 1509 1521 1534 1536 1340 1352 1292 1329

LiB + MB LiB + MB LiB + MB LiB + MB LiB + MB LiB + MB LiB + MB LiB + MB LiB + MB + LiS LiS + LiB LiB + MB + LiS LiS + MB LiS + MB LiS + MB LiS + MB LiS + MB LiS + MB LiS + MB LiS + MB LiS + MB LiS + MB + MS LiS + MS LiS + MS LiS + MS LiS + MS MS + MB MS + MB MS + MB MS + MB MS + MB MS + MB MS + MB MS + MB + LiS

Note: Standard uncertainties u are u(T) = 0.50 K, ur(p) = 0.05, ur(ρ) = 2.0·10−4 g·cm−3; ur(n) = 1.0·10−4, ur(Li+) = 0.0050, ur(Mg2+) = 0.0050, ur(SO24−) = 0.0060, ur(B4O72−) = 0.0030; LiB-Li2B4O7·3H2O, LiS-Li2SO4·H2O, MB-MgB4O7·9H2O, and MS-MgSO4·7H2O.

a

The WAY type Abbe refractometer, which was conducted in a thermostat that electronically controlled the set temperature at (308 ± 0.5) K, with a precision of 0.0001 was used for measuring the refractive index of solution at equilibrium. The composition of the liquid phase was determined by chemical or instrument analysis. The Mg2+ concentration was determined by ethylenediaminetetraacetic acid (EDTA) titration with a precision of ± 0.30 %.14 The concentration of borate ion was determined by neutralization titration in the presence of mannitol (precision: ± 0.30 %).15 The concentration of SO42− was analyzed by the gravimetric methods with the precision of ± 1.20 %.15 The composition of lithium ion was analyzed by ICP− OES method with a precision of ± 0.50 %.16 The composition of the solid phase was determined by X-ray diffractometer with Cu Kα radiation under the operating conditions of 40 kV and 30 mA.

Kelong Chemical Reagent Plant. They were lithium sulfate monohydrate (Li2SO4·H2O; 99.5 % (w/w)), magnesium sulfate heptahydrate (MgSO4·7H2O; 99.0 % (w/w)), and lithium teraborate anhydrous (Li2B4O7; 99.0 % (w/w)). Hungchaoite (MgB4O7·9H2O) was synthesized in our Lab with purities higher than 99.00 % (w/w).12 The isothermal dissolution method was employed in the experiment and the details were explained in a previous paper.13 The system points for the quaternary system were obtained by adding the third component gradually based on the ternary saturation points at 308 K. A series of artificial brine samples were put into the tightly sealed glass bottles, and the bottles were placed in the THZ-82 type thermostatic water bath oscillator with the temperature ((308 ± 0.5) K) and a constant oscillation frequency (120 rpm) to accelerate equilibration. The clarifying solutions were taken out periodically for chemical analysis, when the composition of the liquid sample remained constant, the equilibrium was reached. Experimental results show that the time of reached equilibria is more than 3 weeks with stirring. The gravity bottle method with a precision of ± 0.0002 g·cm−3 was applied to measure the density of solution at equilibrium.



RESULTS AND DISCUSSION The physicochemical property values (densities and refractive indices) and the composition of solution at equilibrium in the quaternary system were presented in Table 1. In Table 1, the ion concentration values were expressed in mass fraction w(B). J(B) 2524

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Figure 1. Phase diagram of the quaternary system Li+, Mg2+//SO42−, borate− H2O at 308 K.

correspond to single salts epsomite (MgSO4·7H2O), hungchaoite (MgB4O7·9H2O), lithium sulfate monohydrate (Li2SO4· H2O), and lithium borate trihydrate (Li2B4O7·3H2O), respectively. The five isothermal dissolution curves, namely curves AE1, BE1, CE2, DE2, and E1E2, are cosaturated with two salts and an equilibrated solution. The cosaturated salts for each univariant curves are listed as follows.

is the Jänecke index values of B, with J(Li22+) + J(Mg2+) = J(Cl22−) + J(B4O72−) = 100. The molar masses of the elements used in this calculation are adopted from an IUPAC Technical Report.17 The Jänecke index can be calculated according to the following correlations, w(Mg 2 +) =

⎡ w(SO4 2‐) w(B4O7 2 ‐) 24.305⎣⎢2 96.056 + 2 155.237 − 1 2

w(Li+) ⎤ ⎥g 6.941 ⎦

AE1: saturated with Li 2B4O7 ·3H 2O + MgB4O7 ·9H 2O

100g (solution)

BE1: saturated with Li 2B4O7 ·3H 2O + Li 2SO4 ·H 2O

w(H 2O) = 1 − w(SO4 2 ‐) − w(B4 O7 2 ‐) − w(Li+)

CE 2 : saturated with Li 2SO4 ·H 2O + MgSO4 ·7H 2O

− w(Mg 2 +)

DE 2 : saturated with MgSO4 ·7H 2O + MgB4 O7 ·9H 2O

w(SO4 2 ‐) w(B4 O7 2 ‐) Letting[M] = + 96.056 155.237 w(Mg 2 +) 1 w(Li+) = + 2 6.94 24.3050 J(Li 2 2 +) =

1 w(Li+) ·100 2 6.941[M]

J(SO4 2 +) =

w(SO4 2 ‐) ·100 96.056[M]

J(H 2O) =

E1E 2 : saturated with Li 2SO4 ·H 2O + MgB4 O7 ·9H 2O

As shown in Figures 3 and 4, the abscissa ordinate is the 2θ, the vertical ordinate is the intensity. The XRD pattern of the invariant E1 shown in Figure 3 was well matched to standard diffraction pattern of MgB4O7·9H2O, Li2SO4·H2O, and Li2B4O7· 3H2O with powder diffraction file(pdf) number“34−1288″, “15−0873″, “50−0564″, respectively. It shows that salts MgB4O7·9H2O, Li2SO4·H2O, and Li2B4O7·3H2O coexist at the invariant point E1. Similarly, as shown in Figrue 4, salts MgB4O7· 9H2O, Li2SO4·H2O, and MgSO4·7H2O coexist at the invariant point E2. The mass fraction composition of the equilibrated solution corresponding to invariant points E1 and E2 are listed as follow. At the invariant point E1, the mass fraction composition of the equilibrated solution is w(Li+) = 2.84 %, w(Mg2+) = 0.03 %, w(SO42−) = 19.40 %, w(B4O72−) = 0.66 % and w (H2O) = 77.07 %. At the invariant point E2, the mass fraction composition of the equilibrated solution is w(Li+) = 1.15 %, w(Mg2+) = 4.75 %, w(SO42−) = 26.09 %, w(B4O72−) = 1.36 % and w(H2O) = 66.65 %. Figure 1 shows that point E1 lies in the triangle which is formed by corresponding solid-phase salts MgB4O7·9H2O, Li2SO4·H2O, and Li2B4O7·3H2O, and point E2 lies in the triangle which is

w(H 2O) ·100 18.02[M]

The phase diagram, dependent on the Jänecke index displayed in Table 1, was constructed in Figure 1. Figure 2 is the partial enlargement diagram of Figure 1. Figures 1 and 2 presented as square coordinate, each vertex corresponded to pure components, and the points on the sides represented to the components of ternary systems, the points inside the square characterized the compositions of quaternary mixtures. The phase diagram consists of two invariant points, five isothermal dissolution curves and four crystallization fields. The four crystallization fields 2525

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Figure 2. Partial enlargement of Figure 1

Figure 3. X-ray diffraction pattern of the invariant point E1 (MgB4O7·9H2O + Li2SO4·H2O + Li2B4O7·3H2O).

formed by corresponding solid-phase salt MgB4O7·9H2O, Li2SO4·H2O, and MgSO4·7H2O, thus these two invariant points are all of commensurate invariant points. In the phase diagram, the crystallization field of salt MgSO4· 7H2O is the smallest because of its high solubility, whereas the crystallization area of salt hungchaoite (MgB4O7·9H2O) is the largest and almost occupies the entire phase region, which shows that hungchaoite is easier to saturate and crystallize than the other three salts from solution at 308 K. The salt’s crystallization fields decrease in the order of MgB4O7·9H2O, Li2B4O7·3H2O, Li2SO4·H2O, and MgSO4·7H2O. The reciprocal salt pair MgB4O7·9H2O and Li2SO4·H2O is stable, thus salts MgB4O7· 9H2O and Li2SO4·H2O can be obtained by cross - reaction

between Li2B4O7·3H2O, and MgSO4·7H2O in the inorganic chemical production. The stable phase diagram of the quaternary system Li+, Mg2+// SO42−, borate−H2O at 298 K has been investigated by Song.11 Figure 5 shows the stable phase diagrams of this system at (298 and 308) K. The results show that only the size of the crystallization zones of the salts is changed, with the crystallization zones of salts MgB4O7·9H2O and Li2SO4·H2O enlarging, Li2B4O7·3H2O and MgSO4·7H2O decreasing at 308 K, while the crystallization forms of the salts are not changed. In Figure 5, F1 and F2 are two invariant points of this system at 298 K. With the changes of the size of crystallization fields, the property of invariant point has also been changed. Although invariant point F1 at 298 K has the same cosaturated salts MgB4O7· 2526

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Figure 4. X-ray diffraction pattern of the invariant point E2 (MgB4O7·9H2O + Li2SO4·H2O + MgSO4·7H2O).

Figure 5. Projected phase diagram of the quaternary system Li+, Mg2+//SO42−, borate− H2O at 298 K11 and 308 K. ●, experimental point at 308 K; ▲, experimental point at 298 K; E1, E2, invariant points at 308 K; F1, F2, invariant points at 298 K.

9H2O, Li2SO4·H2O, and Li2B4O7·3H2O as invariant point E1 at 308 K, its lies out the triangle which is formed by these three salts, thus point F1 is of an incommensurate invariant point, whereas point E1 is of a commensurate invariant point. The area of the crystallization zone of the salts depends on the temperature, solubility of the coexisted salts, the salting-in or salting-out effect, etc. Commonly, the solubility of the inorganic salts increases with the increasing of temperature. While, lithium sulfate does not following the usual trend of solubility versus temperature, its solubility in water decrease with increasing

temperature, solubility decrease from 26.3 % (273 K) to 23.6 % (373 K).18 With the temperature rising from (298 to 308) K, the solubilities of the four components of the quaternary system increase or decrease no different degrees. These differences in the effects of temperature are reason for the change in width of the equilibrium crystallization fields. Thus, the solubility of MgSO4 increases with temperature more strongly than other three salts, the crystallization field is narrowed obviously. Similarly, the solubility of Li2SO4 decreases with temperature lead to broadening of the crystallization field of salt Li2SO4. 2527

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Figure 8. Diagram of refractive index versus composition for quaternary system Li+, Mg2+//SO42−, borate− H2O at 308 K.

Figure 6. Water-content diagram of quaternary system Li+, Mg2+//SO42−, borate− H2O at 308 K.

the physicochemical properties of the solution decrease with an increase of J(Mg2+).



The water content diagram of the system at 308 K is constructed in Figure 6. On the univariant curve BE1, DE2, and E1E2, with an increase of J(Mg2+), the Jänecke index of water changes slightly. While on the univariant curve AE1 and CE2, the Jänecke index of water obviously increases with an increase of J(Mg2+). To ensure the change rule of the density and refractive index in the solution, the diagrams of the physicochemical properties (density and refractive index) vs composition were plotted in Figures 7 and 8. On the univariant curve CE2 and E1E2, with an

CONCLUSION The physicochemical properties (density and refractive index) and the solubility of the quaternary system Li+, Mg2+//SO42−, borate− H2O at 308 K have been investigated by isothermal dissolution method. In the stable phase diagram of this system, the salt’s crystallization fields decrease in the order of MgB4O7· 9H2O, Li2B4O7·3H2O, Li2SO4·H2O, and MgSO4·7H2O. Thus, in the comprehensive utilization of the brine, MgB4O7·9H2O can be easier to separate from solution. The reciprocal salt pair MgB4O7· 9H2O and Li2SO4·H2O is stable, and salts MgB4O7·9H2O and Li2SO4·H2O can be obtained by cross-reaction between Li2B4O7· 3H2O and MgSO4·7H2O in the inorganic chemical production. Comparisons between the phase diagrams of this system at (298 and 308) K show that the crystallization zones of salts MgB4O7· 9H2O and Li2SO4·H2O are enlarged, whereas that of salts Li2B4O7·3H2O and MgSO4·7H2O are decreased at 308 K.



AUTHOR INFORMATION

Corresponding Author

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

The work was supported by Qing Hai CITIC Guoan Science and Technology Development Co., Ltd. Financial support from the National Nature Science Foundation of China (41173071), National High Technology Research and Development Program of China (2012AA061704), and the Sichuan Youth Science and Technology Innovation Research Team Foundation (2013TD0005), and Innovation Team of Chengdu University of Technology (KYTD201405).

Figure 7. Diagram of density versus composition for quaternary system Li+, Mg2+//SO42−, borate− H2O at 308 K.

Notes

The authors declare no competing financial interest.



increase of J(Mg2+), the values of the density and refractive index increase gradually until the invariant point E2 with the maximum value. While on the univariant curve AE1 and DE2, the values of

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