Stable Phase Equilibrium and Phase Diagram of the Quinary System

Article pubs.acs.org/jced

Stable Phase Equilibrium and Phase Diagram of the Quinary System Li+, K+, Rb+, Mg2+//Borate‑H2O at T = 348.15 K Xudong Yu,†,‡ Ying Zeng,*,†,‡ Shanshan Guo,† and Yujuan Zhang† †

College of Materials and Chemistry and Chemical Engineering, Chengdu University of Technology, Chengdu 610059, People’s Republic of China ‡ Mineral Resources Chemistry Key Laboratory of Sichuan Higher Education Institutions, Chengdu 610059, People’s Republic of China ABSTRACT: The stable phase equilibrium of mixed aqueous electrolyte system Li+, K+, Rb+, Mg2+//borate-H2O has been studied using the isothermal dissolution method at T = 348.15 K. The space diagram, the planar projection diagram (saturated with Li2B4O7), water content diagram, and diagrams of the physicochemical properties (densities and refractive indices) have been constructed based on the Jänecke method. The crystallographic form of the solid phase at the quinary system invariant point was identified by the X-ray diffraction method. Results show that the quinary system belongs to a simple type without solid solution or double salt formed. The crystallographic form of borates formed in this work are Li 2 B 4 O 7 ·3H 2 O, K 2 B 4 O 5 (OH) 4 ·2H 2 O, RbB5O6(OH)4·2H2O, and MgB4O5(OH)4·7H2O. Under the condition of Li2B4O7 saturated, the crystallization area of salts decrease in the order of RbB5O6(OH)4·2H2O > MgB4O5(OH)4·7H2O > K2B4O5(OH)4·2H2O.

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INTRODUCTION Over the last year, phase separation technique have been widely used in many industrial processes such as precipitation, evaporation of water from brines, and extraction chemical products from liquid minerals (Searls Salt Lake, Great Salt Lake, Dead Sea, Zabuye Salt Lake, and so forth).1 It is wellknown that the thermodynamic investigations of the phase diagrams are the basis and guidance of utilization of liquid mineral resources and separation techniques of salts.2 Therefore, it should be no surprise that consideration attention was directed in the references to the solid−liquid phase equilibria of binary, ternary, quaternary, quinary, and multicomponentsystem.3−13 Pingluo underground brine with 1.77 × 1011 m3 resource reserves and high content of potassium, boron, lithium, and rubidium is famous for its huge reserves and excellent quality. The content of boron in Pingluo underground brine is up to 4.99 g·L−1, which is higher than other famous salt lake brines such as Zabuye Salt Lake (2.66 g·L−1), and Searles Salt Lake (3.11 g·L−1).14−16 Incomplete statistics show that the amount of boron resource calculated as H3BO3 in Pingluo underground brine is nearly 2.987 × 107 t, and the content of boron is 32.29 times that of the industrial grade for comprehensive utilization.17 Accordingly, the Pingluo brine has a great potential for extraction borates. To comprehensively utilize the brine borates resources, phase equilibria studies about the borate containing system are necessary. Ingri and Gao’s previous research found that in the aqueous borate solution, the solubility behavior of borate compounds is © XXXX American Chemical Society

rather complex because boron appears in many polymeric forms such as B(OH)3, BO2−, B(OH)4−, B4O5(OH)42−, B5O6(OH)4− et al.18−20 Therefore, aiming at understanding the thermodynamic behaviors of the borate containing system, some saltwater systems of the borate system have been done by our research group, such as Rb+//Cl−, borate-H2O at 348.15 K,21 Rb+, Mg2+//borate-H2O at 348.15 K,22 Rb+, Mg+//Cl−, borate-H2O at 348.15 K,23 Li+, Na+, K+//SO42−, borate-H2O at 273.15 K,24 and so forth. Results show that the crystalloid form of rubidium borate is RbB5O6(OH)4·2H2O at 348.15 K;21−23 the equilibrium solid phases of lithium borate correspond to LiBO2·8H2O at 273.15 K;24 the crystalloid form of potassium borate and hungchaoite correspond to K2B4O5(OH)4·2H2O at 273.15 K,24 and MgB4O5(OH)4·7H2O at 348.15 K.22,23 The main compositions of the Pingluo underground brine can be simplified as the complex six-component system Li+, K+, Rb+, Mg2+//Cl−, borate-H2O. The quinary system Li+, K+, Rb+, Mg2+//borate-H2O is a most important subsystem of the complex system. The quinary system contains four quaternary subsystems and the phase equilibria and phase diagram of the quaternary systems have been studied in our previous work.25−28 The chief results were as follows: the quaternary systems Li+, K+, Rb+//borate-H2O,25 Li+, Rb+, Mg2+//borateH2O,26 Li+, K+, Mg2+//borate-H2O,27 and K+, Rb+, Mg2+// borate-H2O28 at 348.15 K are all of simple type without double Received: October 20, 2015 Accepted: January 29, 2016

A

DOI: 10.1021/acs.jced.5b00888 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

Table 1. Solubility and Analytical Experimental Reagents final purity (w/w %)

initial purity (w/w %)

purified method

source

lithium tetraborate (Li2B4O7)

99.0

recrystallization

potassium tetraborate (K2B4O7· 5H2O) magnesium oxide (MgO)

99.0 99.8

boric acid (H3BO3)

99.0

rubidium carbonate (Rb2CO3)

99.8

hungchaoite (MgB4O5(OH)4· 7H2O) rubidium pentaborate (RbB5O6(OH)4·2H2O)

99.0

Chengdu Kelong Chemical Reagent Plant, China Sinopharm Chemical Reagent Co., Ltd., China Sinopharm Chemical Reagent Co., Ltd., China Sinopharm Chemical Reagent Co., Ltd., China Jiangxi Dongpeng new materials Co., Ltd., China Synthesized in laboratory34

chemical name

99.0

recrystallization

recrystallization

Synthesized in laboratory35

99.5 99.0 99.8 99.8 99.8 99.0 99.8

analytical method30 alkalimetry in the presence of mannitol for B4O72− alkalimetry in the presence of mannitol for B4O72− titration with EDTA stand solution for Mg2+ alkalimetry in the presence of mannitol acid−base titration for CO32− alkalimetry in the presence of mannitol for B4O72− alkalimetry in the presence of mannitol for B4O72−

Table 2. Experimental Data Corresponding to the Invariant Points of the Ternary and Quaternary Subsystems in the Quinary System Li+, K+, Rb+, Mg2+//borate-H2O at 348.15 K and pressure p = 0.1 MPaa Jänecke index of dry salt J(Li2B4O7)+ J(K2B4O7) + J(Rb2B4O7) + J(MgB4O7) = 100

composition of equilibrium solution w(b) × 102 no.

system

w(Li2B4O7)

w(K2B4O7)

w(Rb2B4O7)

w(MgB4O7)

w(H2O)

J(Li2B4O7)

J(K2B4O7)

J(Rb2B4O7)

JMgB4O7)

J(H2O)

a b c d e f A B C

LiKB25,27 KMgB27,28 LiMgB26,27 LiRbB25,26 KRbB25,28 RbMgB26,28 LiKMgB27 LiKRbB25 LiRbMgB26

6.33 0.00 5.26 3.79 0.00 0.00 6.44 4.63 6.68

36.71 42.28 0.00 0.00 36.47 0.00 40.19 32.35 0.00

0.00 0.00 0.00 8.03 7.53 3.79 0.00 5.16 3.33

0.00 8.11 0.12 0.00 0.00 3.75 7.31 0.00 3.23

56.96 49.61 94.62 88.18 56.00 92.46 46.06 57.86 86.76

19.23 0.00 97.90 47.65 0.00 0.00 15.17 15.06 58.35

80.77 80.04 0.00 0.00 87.13 0.00 68.61 76.24 0.00

0.00 0.00 0.00 52.35 12.87 35.75 0.00 8.70 15.08

0.00 19.96 2.10 0.00 0.00 64.25 16.22 0.00 26.57

1624 1217 16527 10405 1733 15785 1019 1767 7112

D

KRbMgB28

0.00

35.29

3.22

2.77

58.72

0.00

85.67

5.59

8.74

1847

E

LiKRbMgB

4.51

35.21

3.00

3.08

54.20

13.08

73.99

4.51

8.42

1476

equilibrated solid phase LiB + KB KB + MgB LiB + MgB LiB + RbB KB + RbB RbB + MgB LiB + KB + MgB LiB + KB + RbB LiB + RbB + MgB KB + RbB + MgB LiB + KB + RbB + MgB

a

Note: Standard uncertainties u are u(T) = 0.20 K, ur(p) = 0.05; ur(Li2B4O7) = 0.0050, ur(K2B4O7) = 0.0050, ur(Rb2B4O7) = 0.0050, ur(MgB4O7) = 0.0050; LiKB, Li+, K+//borate-H2O; KMgB, K+, Mg2+//borate-H2O; LiMgB, Li+, Mg2+//borate-H2O; LiRbB, Li+, Rb+//borate-H2O; KRbB, K+, Rb+/borate-H2O; RbMgB,: Rb+, Mg2+//borate-H2O; LiKMgB, Li+, K+, Mg2+//borate-H2O; LiKRbB, Li+, K+, Rb+//borate-H2O; LiRbMgB, Li+, Rb+, Mg2+//borate-H2O; KRbMgB, K+, Rb+, Mg2+//borate-H2O; LiKRbMgB, Li+, K+, Rb+, Mg2+//borate-H2O; LiB-Li2B4O7·3H2O, KB-K2B4O7·4H2O, RbB-RbB5O8·4H2O, MgB-MgB4O7·9H2O.

K) was used for the phase equilibrium experiments with which the temperature could be controlled to 348.15 ± 0.2 K. The standard analytical balance of 120 g capacity and 0.0001 g resolution (BSA124S, supplied by the Sartorius Scientific Instruments (Beijing) Co., Ltd., China) was applied to determine the weight of liquid samples. The density of the equilibrated solution was measured by using the gravity bottle method29 with a precision of 0.0001 g·cm−3. The Abbe refractometer (WYA, supplied by the Shanghai Precision & Scientific Instrument Co., Ltd., China), which was conducted in a thermostat that electronically controlled the set temperature at (348.15 ± 0.5) K with a precision of 0.0001, was used for refractive index measurements. A DX-2700 X-ray diffraction analyzer (supplied by the Dandong Fangyuan Instrument Co., Ltd., China) was used for the solid phase X-ray analysis. The rubidium and lithium ion were determined by ICP-OES (type 5300 V, PerkinElmer Instrument Corp. of America). Experimental Method. The phase equilibrium experiments were performed by means of an isothermal dissolution method. The component of the invariant points of the quaternary subsystems (saturated with three salts) was taken

salt or solid solution found, and in the quaternary system mentioned above boron appears with the form of B4O5(OH)42− and B5O6(OH)4−. The present paper is a continuation of a previously undertaken project. To the best of our knowledge, the phase equilibrium of the quinary system has not been reported. Consequently, the stable phase equilibrium of the quinary system at 348.15 K are presented here in detail, including the densities and refractive indices of this quinary system.

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EXPERIMENTAL SECTION Reagents and Apparatus. The chemicals used in this work were of analytical purity grade and tabulated in Table 1, and the purities reported in Table 1 were expressed in mass fraction. The chemicals were dried at 393.15 K for about 5−8 h. All solutions were prepared using the doubly deionized water, which was obtained using a Millipore water system with an electrical conductivity less than 1 × 10−4 S·m−1. The constant-temperature water bath (THZ-82 type, supplied by the Jintan Guosheng Experimental Instrument Manufactory, China, with the temperature range of 298∼373 B



Article

Table 3. Densitites, Refractive Indices, and Compositions of Solutions of the Quinary System Li+, K+, Rb+, Mg2+//borate-H2O at 348.15 K and Pressure p = 0.1 MPa (Saturated with Salt Li2B4O7)b Jänecke index of dry salt J(K2B4O7) + J(Rb2B4O7) + J(MgB4O7) = 100


density (g/cm−3)

refractive index

w(Li2B4O7)

w(K2B4O7)

w(Rb2B4O7)

w(MgB4O7)

w(H2O)

J(K2B4O7)

J(Rb2B4O7)

J(MgB4O7)

J(H2O)

1, A27 2 3 4 5 6 7 8, E

1.6077 1.6079 1.6082 1.6084 1.6090 1.6097 1.6116 1.6129

1.4230 1.4220 1.4209 1.4205 1.4202 1.4200 1.4197 1.4195

6.44 6.25 6.12 5.90 5.74 5.55 5.11 4.51

40.19 40.05 39.81 39.38 38.75 37.88 36.61 35.21

0.00 0.30 0.71 1.34 1.93 2.67 2.96 3.00

7.31 6.77 5.65 4.72 4.28 3.63 3.23 3.08

46.06 46.63 47.71 48.66 49.30 50.27 52.09 54.20

80.87 81.62 83.52 84.74 84.80 85.11 85.29 85.13

0.00 0.44 1.07 2.06 3.02 4.29 4.93 5.19

19.13 17.94 15.41 13.20 12.18 10.60 9.78 9.68

1201 1232 1297 1357 1398 1464 1573 1698

9, B25 10 11 12 13 14 15, E

1.5821 1.5852 1.5877 1.5934 1.5983 1.6081 1.6129

1.4154 1.4161 1.4168 1.4176 1.4181 1.4185 1.4195

4.63 4.61 4.60 4.57 4.56 4.55 4.51

32.35 32.45 33.45 33.82 34.29 34.62 35.21

5.16 4.85 4.17 3.45 3.14 3.05 3.00

0.00 0.59 1.17 2.06 2.39 2.97 3.08

57.86 57.50 56.61 56.10 55.62 54.81 54.20

89.75 88.45 88.13 86.79 86.49 85.14 85.13

10.25 9.46 7.86 6.34 5.67 5.37 5.19

0.00 2.09 4.01 6.87 7.84 9.49 9.68

2081 2031 1933 1866 1818 1747 1698

16, C26 17

1.2276

1.3515

6.68

0.00

3.33

3.23

86.76

0.00

36.20

63.80

17083

1.2335

1.3519

6.67

0.14

3.33

3.21

86.65

2.09

35.59

62.32

16771

18

1.2415

1.3523

6.65

0.31

3.34

3.20

86.50

4.52

34.84

60.64

16341

19

1.2433

1.3539

6.57

0.77

3.28

3.20

86.18

10.58

32.25

57.17

15348

20

1.2536

1.3561

6.40

1.38

3.26

3.18

85.78

17.59

29.73

52.68

14168

21

1.2621

1.3595

6.18

1.90

3.23

3.17

85.52

22.80

27.74

49.46

13302

22

1.2715

1.3676

6.10

2.56

3.22

3.15

84.97

28.57

25.72

45.71

12291

23

1.3209

1.3701

5.89

3.28

3.21

3.15

84.47

33.91

23.75

42.34

11319

24

1.3993

1.3784

5.56

5.00

3.20

3.14

83.10

43.96

20.14

35.90

9471

25

1.4495

1.3868

5.21

8.25

3.21

3.13

80.20

56.44

15.72

27.84

7112

26

1.5300

1.3965

4.93

11.73

3.16

3.12

77.06

64.99

12.53

22.48

5534

27

1.5634

1.4050

4.47

15.28

3.14

3.11

74.00

70.83

10.42

18.75

4446

28

1.5853

1.4121

4.68

18.84

3.13

3.12

70.23

74.95

8.91

16.14

3621

29

1.5964

1.4145

4.66

22.45

3.13

3.10

66.66

78.17

7.80

14.03

3008

30

1.6055

1.4171

4.64

26.04

3.09

3.11

63.12

80.63

6.85

12.52

2533

31

1.6087

1.4178

4.62

31.56

3.07

3.10

57.65

83.52

5.81

10.67

1977

32, E

1.6129

1.4195

4.51

35.21

3.00

3.08

54.20

85.13

5.19

9.68

1698

33, D28 34

1.5899

1.3995

0.00

35.29

3.22

2.77

58.72

85.67

5.59

8.74

1847

1.5932

1.4019

0.56

35.30

3.21

2.82

58.11

85.54

5.57

8.89

1825

35

1.5989

1.4055

1.16

35.28

3.18

2.84

57.54

85.53

5.52

8.95

1808

36

1.6012

1.4080

2.10

35.27

3.17

2.88

56.58

85.43

5.50

9.07

1776

37

1.6045

1.4100

3.25

35.25

3.15

2.90

55.45

85.41

5.46

9.13

1741

38

1.6078

1.4142

4.18

35.24

3.09

2.96

54.53

85.33

5.35

9.32

1711

39

1.6100

1.4176

4.41

35.23

3.08

3.00

54.28

85.23

5.33

9.44

1702

C

equilibrated solid phase LiB + KB + MgB LiB + KB + MgB LiB + KB + MgB LiB + KB + MgB LiB + KB + MgB LiB + KB + MgB LiB + KB + MgB LiB + KB + RbB + MgB LiB + KB + RbB LiB + KB + RbB LiB + KB + RbB LiB + KB + RbB LiB + KB + RbB LiB + KB + RbB LiB + KB + RbB + MgB LiB + RbB + MgB LiB + RbB + MgB LiB + RbB + MgB LiB + RbB + MgB LiB + RbB + MgB LiB + RbB + MgB LiB + RbB + MgB LiB + RbB + MgB LiB + RbB + MgB LiB + RbB + MgB LiB + RbB + MgB LiB + RbB + MgB LiB + RbB + MgB LiB + RbB + MgB LiB + RbB + MgB LiB + RbB + MgB LiB + KB + RbB + MgB KB + RbB + MgB KB + RbB + MgB KB + RbB + MgB KB + RbB + MgB KB + RbB + MgB KB + RbB + MgB KB + RbB + MgB



Article

Table 3. continued Jänecke index of dry salt J(K2B4O7) + J(Rb2B4O7) + J(MgB4O7) = 100


density (g/cm−3)

refractive index

w(Li2B4O7)

w(K2B4O7)

w(Rb2B4O7)

w(MgB4O7)

w(H2O)

J(K2B4O7)

J(Rb2B4O7)

J(MgB4O7)

J(H2O)

40, E

1.6129

1.4195

4.51

35.21

3.00

3.08

54.20

85.13

5.19

9.68

1698

b

equilibrated solid phase LiB + KB + RbB + MgB

Note: Standard uncertainties u are u(T) = 0.20 K; ur(p) = 0.05; ur(ρ) = 2.0 × 10−3 g·cm−3; ur(n) = 1.0 × 10−4.

Figure 1. Space diagram of the quinary system Li+, K+, Rb+, Mg2+//borate-H2O at 348.15 K.

(STPB)-hexadecyl trimethylammonium bromide (CTAB) back-titration with a standard uncertainty of ±0.5%; the concentration of Rb+ was analyzed by ICP-OES method with a standard uncertainty of ±0.5%, and then the composition of K + was calculated by the subtraction method; 32 the concentration of Li+ was measured by ICP-OES method with a standard uncertainty of ±0.5%.33

as the composition of initial samples. The desired samples were compounded by adding different quantities of the fourth salt to the initial samples. The ratio of the fourth salt was dependent upon the solubility of the salts at 348.15 K. All of the samples were transferred into waterproof and tightly sealed bottles and then placed in the THZ-82 type thermostatic water bath oscillator with the temperature at 348.15 ± 0.2 K and a constant oscillation frequency at 120 rpm to accelerate equilibration. Once the composition of the sample remained constant, it indicated that the equilibrium had been achieved. Then, the liquid and solid phases were separated by filtration at 348.15 K. Meanwhile, the corresponding density and refractive index were measured with the instrument mentioned above. The compositions of the liquid phases were measured by chemical or instrument analysis; the solid phases were identified by the X-ray diffraction method. Analytical Methods. Each sample was measured three times, and the average value of the three measurements was considered as the final value of the analysis. The concentration of borate was determined by neutralization titration in the presence of mannitol with a stand uncertainty of ±0.3%;30 the Mg2+ ion concentration was analyzed by EDTA complexometric titration at pH 9.0−10.0 (ammonia buffer) using Eriochrome black T indicator with a standard uncertainty of ±0.5%. The interference of the Li+ ion was eliminated by adding different amounts of mixed ethanol.31 The total amount of K+ and Rb+ was analyzed by sodium tetraphenylborate

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RESULTS AND DISCUSSION The solubility data corresponding to the invariant points in the ternary and quaternary subsystems of the quinary system Li+, K+, Rb+, Mg2+//borate-H2O at 348.15 K are tabulated in Table 2. The experimental values of solubilities, densities, refractive indices, and composition of equilibrated solid phases in the quinary system are presented in Table 3. In Tables 2 and 3, the concentration of each solution component is expressed in mass fraction w(B), and the Jänecke index (dry salt mole index) is expressed by J(B)(B can be Li2B4O7, K2B4O7, Rb2B4O7, MgB4O7, or H2O). B4O72− is just a traditional stoichiometric expression for various boric species in solution. Thus, in Tables 2 and 3 the composition of rubidium borate have been expressed as Rb2B4O7. The space phase diagram, dependent on the Jänecke index J(B) displayed in Tables 2 and 3, was plotted in Figure 1. In Figure 1, the four apexes of the regular tetrahedron denote four pure salts, viz., Li2B4O7, K2B4O7, Rb2B4O7, and MgB4O7. The six bimodal phase lines present the six ternary subsystems, viz., D



Article

Figure 2. X-ray diffraction pattern of the invariant point E (Li2B4O7·3H2O + K2B4O5(OH)4·2H2O + RbB5O6(OH)4·2H2O + MgB4O5(OH)4·7H2O).

Figure 3. Planar projection diagram of the quinary system Li+, K+, Rb+, Mg2+//borate-H2O at 348.15 K (saturated with salt Li2B4O7).

Li2B4O7·3H2O, K2B4O5(OH)4·2H2O, RbB5O6(OH)4·2H2O, and MgB4O5(OH)4·7H2O. Coexisting ions is one of the main influence factors of salt crystals form. In the previous research of quinary system Li+, K+, Rb+, Mg2+//Cl−-H2O at 323 K,12 the solid solution [(K, Rb)Cl] and double salts KCl·MgCl2·6H2O, and RbCl·MgCl2· 6H2O can be formed. Whereas even the coexist cation ions were the same as the above-mentioned system, when the anion is boron, because of the unique structure of anion ions, such as B5O8−, B4O72−, there are only four single salts formed without the solid solution and double salt found. The four isothermal dissolution univariant curves of the space diagram, namely, curves AE, BE, CE, and DE, are cosaturated with three salts and an equilibrated solution, respectively. The cosaturated salts for the four univariant curves are listed below.

Li+, K+//borate-H2O, K+, Mg2+//borate-H2O, Li+, Mg2+// borate-H2O, Li+, Rb+//borate-H2O, K+, Rb+//borate-H2O, Rb+, Mg2+//borate-H2O, and points a−f correspond to the invariant points for these six ternary subsystems. The four regular triangles represent the four quaternary systems Li+, K+, Mg2+//borate-H2O, Li+, K+, Rb+//borate-H2O, Li+, Rb+, Mg2+//Cl−-H2O, K+, Rb+, Mg2+//borate-H2O, and points A− D correspond to the invariant points for these four quaternary subsystems. All of these four quaternary subsystems belong to simple type without double salt or solid solution formed. The invariant point of the quinary system, labeled as E, is cosaturated with four salts and one liquid phase. The mass fraction of the solution at equilibrium corresponding to invariant point E is w(Li2B4O7) = 4.51%, w(K2B4O7) = 35.21%, w(Rb2B4O7) = 3.00%, w(MgB4O7) = 3.08%, w(H2O) = 54.20%. The component of the cosaturated salts was confirmed with an X-ray diffraction method and demonstrated in Figure 2. As shown in Figure 2, the crystallization forms for borates of lithium, potassium, rubidium, and magnesium are, respectively, E



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univariant curve AE, the Jänecke index of water increased with the increase of J(K2B4O7). As shown in Figures 5 and 6, with

AE: Li 2B4 O7 ·3H 2O + K 2B4 O5(OH)4 ·2H 2O + MgB4 O5(OH)4 · 7H 2O

(1)

BE: Li 2B4 O7 ·3H 2O + K 2B4 O5(OH)4 ·2H 2O + RbB5O6 (OH)4 · 2H 2O

(2)

CE: Li 2B4 O7 · 3H 2O + MgB4 O5(OH)4 · 7H 2O + RbB5O6 (OH)4 · 2H 2O

(3)

DE: K 2B4 O5(OH)4 ·2H 2O + RbB5O6 (OH)4 ·2H 2O + MgB4 O5(OH)4 · 7H 2O

(4)

With the data of Jänecke index J(B) in Table 3, the planar projection diagram (saturated with salt Li2B4O7) of the quinary system at 348.15 K was constructed in Figure 3. Figure 3 consists of three invariant points, three univariant curves, and three crystallization zones corresponding to salts K2B4O5(OH)4·2H2O, RbB5O6(OH)4·2H2O, and MgB4O5(OH)4·7H2O. Results show that under the condition of Li2B4O7 saturated, the scope of areas of crystallization of salts is such that RbB5O6(OH)4·2H2O > MgB4O5(OH)4·7H2O > K2B4O5(OH)4·2H2O, which means that the crystallization path is RbB 5 O 6 (OH) 4 ·2H 2 O → MgB 4 O 5 (OH) 4 ·7H 2 O → K2B4O5(OH)4·2H2O. In the quinary system Li+, K+, Rb+, Mg2+//borate-H2O, the solubility of K2B4O7·4H2O is greater than those of Li2B4O7· 3H2O, RbB5O8·4H2O, and MgB4O7·9H2O; thus, in solution the water content and physicochemical properties are mainly affected by the content of K2B4O7. Accordingly, with the Jänecke index of water or the values of densities and refractive indices as ordinate, and the Jänecke index of K2B4O7 as abscissa, the water content diagram, densities versus composition diagrams, and refractive indices versus composition diagrams were plotted in Figures 4 to 6. Figure 4 shows that at the univariant curves BE and CE, the Jänecke index of water decreased with the increase of J(K2B4O7), while at the

Figure 5. Density versus composition diagram of the quinary system Li+, K+, Rb+, Mg2+//borate-H2O at 348.15 K (saturated with salt Li2B4O7).

Figure 6. Refractive index versus composition diagram of the quinary system Li+, K+, Rb+, Mg2+//borate-H2O at 348.15 K (saturated with salt Li2B4O7).

the increase of J(K2B4O7) the densities of the univariant curves AE and CE increased, the refractive indices of the univariant curve AE decreased, and that of the curve CE increased, while at the univariant curve BE, the values of density and refractive index increased with the decrease of J(K2B4O7)

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CONCLUSIONS Solid-liquid phase equilibrium in the aqueous quinary system Li+, K+, Rb+, Mg2+//borate-H2O at T = 348.15 K was investigated by using the isothermal dissolution method. Results show that the quinary system is of a simple type and

Figure 4. Water content diagram of the quinary system Li+, K+, Rb+, Mg2+//borate-H2O at 348.15 K (saturated with salt Li2B4O7). F



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the space diagram consists of one invariant point, four univariant curves, and four crystallization zones. The solid phases in equilibrium with the saturated solutions are found as Li2B4O7·3H2O, K2B4O5(OH)4·2H2O, RbB5O6(OH)4·2H2O, and MgB4O5(OH)4·7H2O. Under the condition of Li2B4O7 saturated, the crystallization path is RbB5O6(OH)4·2H2O → MgB4O5(OH)4·7H2O → K2B4O5(OH)4·2H2O. The Jänecke index of water decreased with the increase of J(K2B4O7) at the univariant curves BE and CE, while J(H2O) increased with the increase of J(K2B4O7) at the univariant curve AE. The densities of the univariant curves AE and CE increased with the increase of J(K2B4O7). The values of density and refractive index increased with the decrease of J(K2B4O7) at the univariant curve BE.

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AUTHOR INFORMATION

Corresponding Author

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Supported by the National High Technology Research and Development Program of China (2012AA061704), the National Natural Science Foundation of China (41173071, 41473059), the Sichuan youth science and technology innovation research team funding scheme (2013TD0005) and Innovation Team of Chengdu University of Technology (KYTD201405).

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Stable Phase Equilibrium and Phase Diagram of the Quinary System

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