Solid–Liquid Equilibrium in the Aqueous System Containing the

Oct 5, 2015 - The stable phase equilibrium and phase diagram of the quaternary system K+, Rb+, Mg2+ // borate·H2O at 348 K were determined by isother...
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Solid−Liquid Equilibrium in the Aqueous System Containing the Borates of Potassium, Rubidium, and Magnesium at 348 K Hao Shi,†,‡ Shan Feng,†,‡ Jiangle Zhang,† Yujuan Zhang,† Xudong Yu,†,‡ and Ying Zeng*,†,‡ †

College of Materials 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 stable phase equilibrium and phase diagram of the quaternary system K+, Rb+, Mg2+ // borate·H2O at 348 K were determined by isothermal dissolution method. The densities, refractive indices, and solubility values of the equilibrated solution in the quaternary system were measured. The crystalloid forms of the solid phase of invariant point were identified by X-ray diffraction method. The stable phase diagram, water content diagram, and the diagrams of the densities/refractive indices versus composition were plotted based on the measured data. Results show that this quaternary system at 348 K without double salt or solid solution formed belongs to a simple eutectic type. The invariant point E saturated with three salts corresponding to potassium tetraborate tetrahydrate (K2B4O5(OH)4· 2H2O), rubidium pentaborate tetrahydrate (RbB5O6(OH)4·2H2O), and hungchaoite (MgB 4 O 5 (OH) 4 ·7H 2 O). The crystallization field of RbB5O6(OH)4·2H2O is the maximum, meaning that it has the smallest solubility among the coexisting salts. On the solubility curves AE (cosaturated with salts MgB4O5(OH)4·7H2O and RbB5O6(OH)4·2H2O) and BE (cosaturated with salts K2B4O5(OH)4·2H2O and RbB5O6(OH)4·2H2O), the water content decreases, the densities and refractive indices increase with the increase of J(K2B4O7); on the curve CE (cosaturated with salts MgB4O5(OH)4·7H2O and K2B4O5(OH)4·2H2O), the water content and the densities increase, the refractive indices decrease with the increase of J(K2B4O7).

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stable phase equilibrium of the system K+, Rb+, Mg2+ // borate· H2O in aqueous solution at 348 K has not been reported. The quaternary system K+, Rb+, Mg2+ // borate·H2O consists of three ternary subsystems. In our previous research, the phase equilibria of its ternary subsystems Rb+, Mg2+ // borate·H2O10 and K+, Mg2+ // borate·H2O11 at 348 K have been studied. Results show that these two ternary systems at 348 K are all of simple type; no double salt or solid solution formed. This work is a continuation of our previous research. Consequently, the solubilities and physicochemical properties (densities and refractive indices) of the quaternary system are presented here in detail.

INTRODUCTION

With the rapid expansion development of national economy and the rising of new industry, solid mineral resources are difficult to fulfill the national deficiency security and economy globalizing; the shortage of mineral resources is increasing intensifying. Due to the exploitation of the salt lake and underground brine resource, the economic value of the liquid mineral resource is displayed to make up the deficiency of solid mineral resources. However, the development of liquid mineral resources more focused on the common salts, such as KCl, NaCl, H3BO3, etc. Until now, some valuable components such as lithium and rubidium are still not used. The thermodynamic investigation of the phase diagrams of salt−water system plays an important role in exploiting brine resources.1 The relationship of above-mentioned liquid mineral resources can be described as the complex system Li + K + Mg + Rb + Cl + B + H2O. To understand the thermodynamics behavior of the system mentioned above, some phase equilibria have been performed, such as the ternary system K+ // BO2−, OH−·H2O at 263 K,2 Li+, K+ // borate·H2O3 and Rb+, Cl− // borate·H2O4 at 348 K, and K+, Rb+ (Mg2+) // Cl−·H2O at 298 K,5 323 K6 and 348 K;7 the quaternary system Li+, K+, Rb+ // borate·H2O8 and Li+, K+, Mg2+ // borate·H2O9 at 348 K. Commonly, the crystalloid form of the borates is affected by the coexist ions. To date, the © XXXX American Chemical Society

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EXPERIMENTAL SECTION Reagents and Apparatus. The doubly deionized water (κ ≤ 1.0 × 10−4 S·m−1, pH ≈ 6.60) was required in the preparation of artificial solutions and analytical operations. The chemicals used in this work were of analytical purity grade and tabulated in Table 1. The apparatus used for phase equilibrium was a THZ-82 type digital display constant temperature water bath kettle, made by Jintan Guosheng Experimental Instrument Manufactory. The Received: June 2, 2015 Accepted: September 29, 2015

A

DOI: 10.1021/acs.jced.5b00455 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Table 1. Solubility and Analytical Experimental Reagents initial purity

chemical name potassium tetraborate (K2B4O7·5H2O)

0.990

magnesium oxide (MgO)

0.998

boric acid (H3BO3)

0.990

rubidium carbonate (Rb2CO3)

0.998

lithium tetraborate (Li2B4O7)

0.990

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

purified method

final purity

source

0.990

0.990

Sinopharm Chemical Reagent Co., Ltd. Sinopharm Chemical Reagent Co., Ltd. Sinopharm Chemical Reagent Co., Ltd. Jiangxi Dongpeng New Materials Co., Ltd. Chengdu Kelong Chemical Reagent Plant synthesized in laboratory12

0.990

synthesized in laboratory13

0.990

recrystallization

recrystallization

0.998 0.998 0.998 0.995 0.990

analytical method15 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− alkalimetry in the presence of mannitol for B4O72−

Table 2. Experimental Values of Densities, Refractive Indices, and Solubility of the Equilibrium Solution in the Quaternary System K+, Rb+, Mg2+ // Borate·H2O at 348 K and Pressure p = 0.1 MPaa Jänecke index of dry salt J(K2B4O7) + J(Rb2B4O7) + J(MgB4O7) = 100

composition of solution, w(B) × 100 no.

density (g·cm−3)

refractive index

w(K2B4O7)

w(Rb2B4O7)

w(MgB4O7)

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

1.1424 1.1492 1.1691 1.2067 1.2316 1.2584 1.2969 1.3342 1.3882 1.4018 1.4233 1.4380 1.4486 1.4888 1.5538 1.5899 1.5989 1.5981 1.5893 1.5881 1.5878 1.5870 1.5860 1.5899 1.5634 1.5651 1.5679 1.5744 1.5811 1.5899

1.3404 1.3415 1.3429 1.3441 1.3450 1.3520 1.3560 1.3644 1.3702 1.3725 1.3807 1.3821 1.3840 1.3890 1.3975 1.3995 1.4080 1.4066 1.4046 1.4040 1.4021 1.4014 1.3999 1.3995 1.4128 1.4103 1.4069 1.4029 1.3998 1.3995

0.00 0.34 0.83 2.54 5.06 7.07 8.93 12.37 15.31 16.75 24.77 25.64 26.26 28.79 31.95 35.29 36.47 36.44 36.36 36.26 35.66 35.49 35.47 35.29 42.28 42.16 37.27 36.73 35.48 35.29

3.79 3.78 3.68 3.66 3.65 3.65 3.65 3.64 3.63 3.62 3.60 3.57 3.56 3.44 3.43 3.22 7.53 7.26 6.95 6.18 4.88 4.02 3.25 3.22 0.00 0.15 0.62 1.96 3.07 3.22

3.75 3.75 3.74 3.65 3.61 3.55 3.45 3.37 3.31 3.19 3.12 3.05 2.97 2.89 2.79 2.77 0.00 0.22 0.47 0.94 1.88 2.51 2.72 2.77 8.11 6.67 4.94 3.58 2.86 2.77

w(H2O) J(K2B4O7) 92.46 92.13 91.75 90.15 87.68 85.73 83.97 80.62 77.75 76.44 68.51 67.74 67.21 64.88 61.83 58.72 56.00 56.08 56.22 56.62 57.58 57.98 58.56 58.72 49.61 51.02 57.17 57.73 58.59 58.72

0.00 4.29 9.97 25.64 40.92 49.45 55.72 63.91 68.93 71.31 78.88 79.73 80.38 82.24 84.01 85.66 87.13 86.92 86.68 86.53 85.73 85.25 85.82 85.66 80.04 82.77 84.44 85.84 85.71 85.66

J(Rb2B4O7)

J(MgB4O7)

J(H2O)

equilibrated solid phase

35.75 34.15 31.63 26.45 21.12 18.27 16.30 13.46 11.70 11.03 8.20 7.94 7.80 7.03 6.45 5.59 12.87 12.39 11.86 10.55 8.40 6.91 5.63 5.59 0.00 0.21 1.01 3.28 5.31 5.59

64.25 61.55 58.40 47.91 37.96 32.28 27.99 22.64 19.38 17.66 12.92 12.33 11.82 10.73 9.54 8.74 0.00 0.68 1.46 2.92 5.88 7.84 8.56 8.74 19.96 17.02 14.55 10.88 8.98 8.74

15785 15067 14275 11790 9185 7767 6787 5395 4535 4216 2826 2729 2665 2401 2106 1846 1733 1733 1736 1750 1793 1804 1835 1846 1217 1297 1678 1748 1833 1846

RB + MB RB + MB RB + MB RB + MB RB + MB RB + MB RB + MB RB + MB RB + MB RB + MB RB + MB RB + MB RB + MB RB + MB RB + MB KB + RB + MB KB + RB KB + RB KB + RB KB + RB KB + RB KB + RB KB + RB KB + RB + MB KB + MB KB + MB KB + MB KB + MB KB + MB KB + RB + MB

Standard uncertainties u are u(T) = 0.50 K; ur(p) = 0.05; ur(ρ) = 2.0 × 10−3; ur(n) = 1.0 × 10−4; ur(K2B4O7) = 0.0050; ur(Rb2B4O7) = 0.0050; ur(MgB4O7) = 0.0050; w(B), mass fraction of B; J(B), the Jänecke index values of B; KB, K2B4O7·4H2O; MB, MgB4O7·9H2O; RB, RbB5O6(OH)4· 2H2O.

a

uncertainty of 0.002 g·cm−3. A WYA type Abbe refractometer, conducted in a thermostat at (348 ± 0.5) K, was used for measuring the refractive index of the equilibrated solution with a stand uncertainty of 0.0001. The rubidium ion was determined by ICP−OES (type 5300 V, PerkinElmer Instrument Corp. of America). The solid phase of the invariant point

range of temperature is (298 to 348) K and the stand uncertainty was 0.5 K. AL104 type standard analytical balance of 110 g capacity and 0.0002 g resolution, made by Mettler Toledo Instruments Co., Ltd., was employed for the mass determination. The density of the equilibrated solution was measured by using the gravity bottle method14 with a stand B

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was identified by DX-2700 type X-ray diffraction analyzer with Cu Kα radiation. Experimental Methods. In the solubility experiments, the isothermal dissolution method was carried out. The system points for the quaternary system were obtained by adding the third salt gradually to the solution from the invariant point of ternary system at 348 K. A series of artificial samples were transferred into tightly sealed bottles and then placed in the THZ-82 type thermostat water bath at 348 ± 0.5 K and a constant oscillation frequency (120 rpm) to achieve a balance. 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 K. Meanwhile, the densities and refractive indices of the liquid phase were determined by the previously mentioned apparatus. The composition of solution was measured by chemical or instrumental analysis; the densities and the refractive indices of equilibrated liquid phase were determined by the abovementioned instrument. The equilibrium solid samples were dried at 348 K and identified by the powder X-ray diffraction method. Analytical Methods. The samples of the liquid phase were analyzed for three times in parallel, and the average value of three measurements was considered as the final value of the analysis. The Mg2+ concentration was determined by titration with EDTA stand solution (stand uncertainty: 0.5%).15 The composition of borate was measured by alkalimetry in the presence of mannitol (stand uncertainty: 0.5%).15 When the K+ and Rb+ coexist, the total amount of K+ and Rb+ was analyzed by sodium tetraphenylborate (STPB)−hexadecyl trimethylammonium bromide (CTAB) back-titration with a stand uncertainty of 0.5%; the composition of Rb+ was analyzed by ICP-OES (stand uncertainty: 0.50%),16 and then the concentration of the K+ was calculated by the subtraction method.

Figure 1. Phase diagram of the quaternary system K+, Rb+, Mg2+ // borate·H2O at 348 K.

invariant point E is w(K2B4O7) = 35.29%, w(Rb2B4O7) = 3.22%, w(MgB4O7) = 2.77%, w(H2O) = 58.72%. The component of the cosaturated salt was confirmed with an Xray diffraction method and demonstrated in Figure 2.

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RESULTS AND DISCUSSION The solubility data, the values of density and refractive index, and the equilibration solid phase of the quaternary system are listed in Table 2. The mass fraction w(B) and Jänecke index J(B) were used to represent the ion concentration values of the stable equilibrium solutions, where B represents MgB4O7, Li2B4O7, K2B4O7, or H2O. The data of Jänecke index ought to comply with the formulas listed below: [M] =

w(MgB4O7 ) 179.54

J(K 2B4 O7 ) = J(H 2O) =

+

w(K 2B4 O7 ) w(Rb2 B4 O7 ) + 233.43 326.17

w(K 2B4 O7 ) × 100 233.43 × [M]

w(H 2O) × 100 18.02 × [M]

Figure 2. X-ray diffraction pattern for the cosaturated salts corresponding to the invariant point E (cosaturated salts K2B4O5(OH)4·2H2O, RbB5O6(OH)4·2H2O, and MgB4O5(OH)4· 7H2O).

Comparing the sample spectrum of solid phase of point E at the larger rectangle with the standard cards found that K2B4O5(OH)4·2H2O (a = 11.785 × 10−10 m, b = 12.917 × 10−10 m, c = 6.865 × 10−10 m), MgB4O5(OH)4·7H2O (a = 8.811 × 10−10 m, b = 10.644 × 10−10 m, c = 7.888 × 10−10 m), and RbB5O6(OH)4·2H2O (a = 11.307 × 10−10 m, b = 10.964 × 10−10 m, c = 9.341 × 10−10 m) coexist at the invariant point E. The stable phase diagram (Figure 1) consists of one invariant point, three univariant curves, and three crystallization fields. The three crystallization fields correspond to three single salts, K2B4O5(OH)4·2H2O, MgB4O5(OH)4·7H2O, and RbB5O6(OH)4·2H2O. The size of crystalline area is in the order RbB 5 O 6 (OH) 4 ·2H 2 O > MgB 4 O 5 (OH) 4 ·7H 2 O > K2B4O5(OH)4·2H2O, which shows that the solubility of RbB5O6(OH)4·2H2O is the lowest in the system, it means the salt RbB5O6(OH)4·2H2O can be more easily separated from the solution than the other coexisting salts at 348 K.

(1)

(2)

(3)

According to the experimental data in Table 2, the stable phase diagram of the quaternary system K+, Rb+, Mg2+ // borate·H2O at 348 K was plotted in Figure 1. In Figure 1, the three apexes of the triangle denote the invariant points of three ternary subsystems, viz., (A) Rb+, Mg2+ // borate·H2O; (B) K+, Rb+ // borate·H2O; (C) K+, Mg2+ // borate·H2O. The invariant point of the quaternary system labeled as E is cosaturated with three salts and one liquid phase. The mass fraction of the solution at equilibrium corresponding to C

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Commonly, the crystallization form of salts was affected with coexisting ions and temperature. In the chloride solution, the solid solution [(K, Rb)Cl] and double salts KCl·MgCl2·6H2O and RbCl·MgCl2·6H2O can be formed in the system containing potassium, magnesium, and rubidium.17 While when the anion is boron, like the system in this article, the solubility behavior is complex because boron appears in various forms such as B5O8−, B4O72−, et al.18 In this case, even the coexist cation ions were the same as above-mentioned system, because of the unique structure of anion ions; there are only three single salts formed: without the solid solution and a double salt. The three univariant curves, namely, curves AE, BE, and CE, are cosaturated with two salts and an equilibrated solution, respectively. The cosaturated salts for the three univariant curves are listed below: AE, saturated with MgB 4 O 5 (OH) 4 ·7H 2 O and RbB5O6(OH)4·2H2O BE, saturated with K 2 B 4 O 5 (OH) 4 ·2H 2 O and RbB5O6(OH)4·2H2O CE, saturated with MgB 4 O 5 (OH) 4 ·7H 2 O and K2B4O5(OH)4·2H2O From the solubility experiments, it can be seen that the solubility of K2B4O7 is the greatest among the coexisting salts at 348 K, thus the physicochemical properties are mainly affected by the K2B4O7 content in the equilibrium solution. On account of this, the water content diagram (Figure 3), the diagrams of

Figure 4. Densities vs composition diagram of the quaternary system Li+, K+, Mg2+ // borate·H2O at 348 K.

Figure 5. Refractive indices vs composition diagram of the quaternary system Li+, K+, Mg2+ // borate·H2O at 348 K.

+

salt or solid solution formed belongs to a simple eutectic type. The stable phase diagram comprises one invariant point, three univariant curves, and three crystallization fields. The crystallographic form of borates of potassium, rubidium, and magnesium at 348 K in the quaternary system are K2B4O5(OH)4·2H2O, RbB5O6(OH)4·2H2O, and MgB4O5(OH)4·7H2O, respectively. The crystallization field of RbB5O6(OH)4·2H2O is the maximum, meaning that it has the smallest solubility among the coexisting salts and is more easily separated from the solution than the other coexisting salts at 348 K.

+

Figure 3. Water content diagram of the quaternary system Li , K , Mg2+ // borate·H2O at 348 K.

density versus composition (Figure 4) and refractive index versus composition (Figure 5) were constructed with J(K2B4O7) as abscissa. On the univariant curves AE and BE, the water content decrease, and the densities and refractive indices increase with the increase of J(K2B4O7); on the curve CE, the water content and the densities increase, and the refractive indices decrease with the increase of J(K2B4O7).

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CONCLUSIONS The compositions of the quaternary system K+, Rb+, Mg2+ // borate·H2O at 348 K were determined by isothermal dissolution method, the values of densities and refractive indices were measured by gravity bottle method and Abbe refractometer. The quaternary system at 348 K without double

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: 86-28-84079016. Fax: 86-2884079074. Notes

The authors declare no competing financial interest. D

DOI: 10.1021/acs.jced.5b00455 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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ACKNOWLEDGMENTS This research supported by the National Natural Science Foundation (41173071, 41473059), the National High Technology Research and Development Program of China (2012AA061704), 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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