The Stable Phase Equilibria of the Ternary Systems Na2SO4 + Rb2SO4

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Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX

The Stable Phase Equilibria of the Ternary Systems Na2SO4 + Rb2SO4 (Cs2SO4) + H2O at 298.2 K Linxin Wu,† Ying Zeng,*,†,‡ Yu Chen,† Xudong Yu,†,‡ Peijun Chen,† Peng Huang,† and Jiu Sun† †

College of Materials and Chemistry & Chemical Engineering, Chengdu University of Technology, Chengdu 610059, P. R. China Collaborative Innovation Center of Panxi Strategic Mineral Resources Multi-Purpose Utilization, Chengdu University of Technology, Chengdu 610059, P. R. China

J. Chem. Eng. Data Downloaded from pubs.acs.org by UNIV DE BARCELONA on 01/09/19. For personal use only.

‡

ABSTRACT: To understand the crystallographic forms of salts in sulfate systems containing sodium and rubidium (cesium), the stable phase equilibria of ternary systems Na2SO4 + Rb2SO4 + H2O and Na2SO4 + Cs2SO4 + H2O at 298.2 K were studied with the isothermal dissolution method. The composition and crystallographic forms of the equilibrated solid phase was verified by Schreinemaker’s wet residue method combined with X-ray diffraction. Experimental results show that there is no double salt or solid solution formed. The stable phase diagram of Na2SO4 + Rb2SO4 + H2O at 298.2 K is characterized by one invariant point (H), two univariant curves, and two crystallographic forms (corresponding to Na2SO4·10H2O and Rb2SO4). The ternary system Na2SO4 + Cs2SO4 + H2O is of the hydrate type II, there are two invariant points (E1, E2), three univariant curves, and three crystallographic forms (corresponding to salts Na2SO4·10H2O, Na2SO4 and Cs2SO4). Affected by the unique structure of Cs+, both salt Na2SO4·10H2O and Na2SO4 have been found in Na2SO4 + Cs2SO4 + H2O system at 298.2 K. Furthermore, the density and refractive index of the equilibrated solution possess the analogous variation tendency and regularly change with the change of concentration.

1. INTRODUCTION Rubidium (Rb) and cesium (Cs), precious alkali metals, are widely used in aeronautics, electronics, and defense industries. The current demand for cesium (rubidium) and their compounds in the international market has grown due to their necessity in application and difficulty in production.1 Recently, the process of reasonably extracting Rb and Cs is receiving more and more attention in academia and industry. The salt-lake brines, one of the most important liquid mineral resources, are abundant with desirable components as well as rubidium and cesium. Phase equilibrium studies provide the requisite theoretical and practical importance for the comprehensive utilization of salt lake brines.2 Plenty of studies about phase equilibrium data and phase diagrams have been performed to utilize salt lake brines or underground brines.3,4 A series of ternary, quaternary, and quinary aqueous systems containing rubidium have been investigated in our previous work at (298−348) K. Results show that the double salt RbCl·MgCl2·6H2O was formed in chloride systems containing rubidium and magnesium, and the continuous solid solution [(K, Rb)Cl] was found in chloride systems containing potassium and rubidium.5−11 In the saturated solution of borate systems containing rubidium, the crystallographic form for rubidium borate is RbB5O6(OH)4·2H2O.12−16 So far, previous phase equilibria studies of cesium containing systems mainly focused on the chloride type brine systems17−19 or the mixed electrolyte systems,20,21 and rarely dealt with the sulfate system. To effectively utilize the brine resource, the phase diagram of sulfate systems containing © XXXX American Chemical Society

rubidium or cesium should be investigated to obtain more information about the crystallographic forms and phase behavior. Ternary systems Na2SO4 + Rb2SO4 + H2O and Na2SO4 + Cs2SO4 + H2O are two basic subsystems for sulfate-type old brine, but the solubilities and thermodynamic data about these two systems have not been reported yet. Therefore, the phase equilibria of these two ternary systems Na2SO4 + Rb2SO4 + H2O and Na2SO4 + Cs2SO4 + H2O were investigated by the isothermal dissolution method at 298.2 K, as well as the solubility, density (ρ), and refractive index (nD).

2. EXPERIMENTAL SECTION 2.1. Reagents and Instruments. Deionized water (κ < 1.0 × 10−4 S·m−1) was used to prepare artificial solution and chemical analysis. The main chemicals consist of sodium Table 1. Reagents Description Table chemical

CASRN

purity

sodium sulfate (Na2SO4) rubidium sulfate (Rb2SO4) cesium sulfate (Cs2SO4)

7757-82-6

≥99.9%

source

7488-54-2

≥99.9%

10294-54-9

≥99.9%

Chengdu Kelong Chemical Reagent Plant, China Shanghai Dingli Chemical Co., Ltd., China Shanghai Dingli Chemical Co., Ltd., China

Received: August 6, 2018 Accepted: December 21, 2018

A

DOI: 10.1021/acs.jced.8b00693 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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Table 2. Instruments Description Table instruments

type

precision

thermostatic water bath oscillator analytical balance Abbe refractometer atomic absorption spectrometer X-ray diffractometer

HZS-HA BSA124S WYA iCE-3300 DX-2700

0.5 K 0.0001 g 0.0001 0.50%

source Jintan Guosheng Experimental Instrument Manufactory, China Mettler Toledo Instruments Co., Shanghai INESA Physics optical Instrument Co., Thermo fisher scientific Instrument Corp., Dandong Fangyuan Instrument Co.,

Ltd. Ltd. America Ltd., China

Table 3. Solubility, Density, and Refractive Index of the Invariant Points in the Binary Subsystems at 298.2 K and Pressure p = 0.1 MPaa system

solubility, 100w

refractive index/nD

density/ρ(g·cm−3)

equilibrium solid phase

data source

Na2SO4−H2O

21.80 21.9424 33.65 33.7025 64.63 64.5025

1.3628 -b 1.3649 1.365226 1.4095 1.409126

1.1830 -b 1.3631 1.347026 2.0162 2.007026

Na2SO4·10H2O Na2SO4·10H2O Rb2SO4 Rb2SO4 Cs2SO4 Cs2SO4

this work reference this work reference this work reference

Rb2SO4−H2O Cs2SO4−H2O

a Standard uncertainties u are u(T) = 0.20 K; u(p) = 0.05; u(ρ) = 0.012 kg·m−3; u(nD) = 0.001; ur(w(Na2SO4)) = 0.0050; ur(w(Rb2SO4)) = 0.0050; ur(w(Cs2SO4)) = 0.0050; b-, not detected.

Table 4. Determined Values of Solubilities, Refractive Indices (nD), and Densities (ρ) of the Ternary System Na2SO4 + Rb2SO4 + H2O at 298.2 K and Pressure p = 0.1 MPaa composition of liquid phase, w(M) × 102

composition of wet solid phase, w(M) × 102

no.

density/ρ(g·cm−3)

refractive index /nD

w(Na2SO4)

w(Rb2SO4)

w(Na2SO4)

w(Rb2SO4)

equibrium solid phaseb

1,C 2 3 4 5 6 7 8 9 10 11 12, H 13 14 15 16 17 18 19 20 21 22 23,D

1.1830 1.2084 1.2294 1.2516 1.2818 1.3075 1.3357 1.3738 1.3856 1.4388 1.4809 1.5401 1.5213 1.5142 1.4930 1.4783 1.4658 1.4477 1.4142 1.3984 1.3829 1.3705 1.3631

1.3628 1.3637 1.3660 1.3684 1.3711 1.3740 1.3753 1.3780 1.3805 1.3844 1.3872 1.3894 1.3875 1.3861 1.3842 1.3820 1.3800 1.3778 1.3735 1.3704 1.3685 1.3670 1.3649

21.80 21.18 21.31 21.65 21.98 22.44 22.67 22.61 22.38 22.73 23.17 23.10 21.88 20.56 18.42 16.11 13.75 11.87 6.68 4.75 2.99 1.60 0.00

0.00 1.85 3.50 5.69 8.51 10.86 13.20 15.67 17.46 21.41 23.35 25.65 25.89 26.12 26.32 26.98 27.96 28.37 30.11 30.84 32.13 32.58 33.65

41.83 40.33 42.90 38.42 42.05 43.31 40.15 41.95 40.86 40.22 36.34 2.84 1.20 0.19 0.14 0.11 0.06 0.34 0.35 0.85 0.28 -

0.22 0.37 0.42 1.37 0.51 1.55 1.27 1.05 3.13 4.46 19.29 87.56 93.42 96.36 96.01 95.92 94.35 92.89 93.10 91.42 92.87 -

S10 S10 S10 S10 S10 S10 S10 S10 S10 S10 S10 S10 + Rb2SO4 Rb2SO4 Rb2SO4 Rb2SO4 Rb2SO4 Rb2SO4 Rb2SO4 Rb2SO4 Rb2SO4 Rb2SO4 Rb2SO4 Rb2SO4

Standard uncertainties u are u(T) = 0.20 K; u(p) = 0.05; u(ρ) = 0.012 kg·m−3; u(nD) = 0.001; ur(w(Na2SO4)) = 0.0050; ur(w(Rb2SO4)) = 0.0050. S10: Na2SO4·10H2O.

a

b

sulfate (Na2SO4), rubidium sulfate (Rb2SO4), and cesium sulfate (Cs2SO4). The main instruments in this work consist of thermostatic water bath oscillator, analytical balance, Abbe refractometer, atomic absorption spectrometer, and X-ray diffractometer. The key information for chemical reagents and instruments involved in this work was listed in Table 1 and Table 2, respectively. 2.2. Experimental Method. The isothermal dissolution method was adopted to investigate the stable phase equilibria of the ternary systems Na2SO4 + Rb2SO4 + H2O and Na2SO4

+ Cs2SO4 + H2O at 298.2 K. The moderate saturated solution of a single salt was added into 100 mL rigid plastics bottles, then the second salt was gradient added to form a series artificial test solution. The bottles were securely sealed and immersed in a thermostatic water bath oscillator with 120 rpm speed at 298.2 K ± 0.5 K. Once the equilibrium reached, the vibrating was stopped and the bottles were kept standing for at least 24 h. The solid−liquid phase completely separated. The separated solid phases were identified using Schreinemaker’s wet residue method22 and X-ray diffraction. The density (ρ) B



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was tested by the specific gravity bottle method with a correction for air buoyancy,6 and its precision is ±0.0002 g· cm−3. The refractive index (nD) was tested by a WYA Abbe refractometer, with a precision of ±0.0001. 2.3. Analytical Method.23 The concentration of Rb+ and Cs+ was measured by a sodium tetraphenylborate−cetyltrimethylammonium bromide titration (precision, ±0.5%); The concentration of Na+ is obtained by atomic absorption spectrometer (precision, ±0.6%), and verified using the ionic equilibrium calculation method. The SO42− concentration was determined by the gravimetric method (precision, ± 0.5%).

3. RESULTS AND DISCUSSIONS The binary systems (Na2SO4−H2O, Rb2SO4−H2O and Cs2SO4−H2O) are the subsystems of Na2SO4 + Rb2SO4 + Figure 3. Density vs composition diagram of the ternary system Na2SO4 + Rb2SO4 + H2O at 298.2 K.

Figure 1. Stable phase diagram of the ternary system Na2SO4 + Rb2SO4 + H2O at 298.2 K: ●, liquid point; ◆, wet solid point; , isothermal dissolution curve; ---, wet residue curve.

Figure 4. Refractive index vs composition diagram of the ternary system Na2SO4 + Rb2SO4 + H2O at 298.2 K.

which means that the data reliability has verified to a certain degree and the experimental procedures are rational. The Ternary System Na2SO4 + Rb2SO4 + H2O. The compositions and the physico-chemical properties of the ternary system Na2SO4 + Rb2SO4 + H2O at 298.2 K were presented in Table 4. The compositions of Na2SO4 and Rb2SO4 are represented as mass fraction w(M). C and D are the invariant points of the binary subsystems Na2SO4−H2O and Rb2SO4−H2O, respectively. H represents the invariant point of this ternary system. On the basis of these experimental data in Table 4, the stable phase diagram containing the dissolution curves and crystallographic zones was drawn and shown in Figure 1. Figure 2 is the XRD pattern of the cosaturated salts corresponding to the invariant point H. Figure 3 and Figure 4 are the diagrams of physicochemical properties (density and refractive index) vs composition, respectively. As shown in Figure 1, the ternary system Na2SO4 + Rb2SO4 + H2O is of a simple type at 298.2 K, without double salt or solid solution formed, while only two single salts formed. Its stable phase diagram contains one invariant point (H), two univariant curves (DH, CH), and two crystallographic zones. Figure 2 is the XRD pattern of the solid phase at invariant

Figure 2. X-ray diffraction patterns of the solid phases corresponding to the invariant point H.

H2O and Na2SO4 + Cs2SO4 + H2O. The solubility, density, and refractive index data of the binary subsystems in this work were compared with the data in previous studies,24−26 as listed in Table 3. The results show that the experimental data obtained in this work matches well with the literature value, C



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Table 5. Determined Values of Solubilities, Refractive Indices (nD), and Densities (ρ) of the Ternary System Na2SO4 + Cs2SO4 + H2O at 298.2 K and p = 0.1 MPaa composition of liquid phase, w(M)×102

composition of wet residue, w(M)×102

no.

density/ρ (g·cm−3)

refractive index /nD

w(Na2SO4)

w(Cs2SO4)

w(Na2SO4)

w(Cs2SO4)

equibrium solid phaseb

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

1.1830 1.2212 1.2392 1.2458 1.2751 1.3072 1.3532 1.4037 1.4497 1.5178 1.6242 1.6658 1.7270 1.7804 1.8345 1.8391 1.8813 1.8922 1.9725 2.0128 2.0482 2.0423 2.0168 2.0153 2.0140 2.0124 2.0156 2.0162

1.3628 1.3630 1.3635 1.3650 1.3672 1.3700 1.3731 1.3770 1.3805 1.3894 1.3960 1.3977 1.3995 1.4024 1.4046 1.4051 1.4063 1.4090 1.4137 1.4160 1.4169 1.4160 1.4146 1.4140 1.4130 1.4124 1.4112 1.4095

21.80 20.09 19.93 20.02 20.09 19.77 19.69 19.57 18.98 19.50 19.43 19.12 18.95 18.48 17.60 17.03 16.15 15.43 14.03 13.15 12.87 11.23 7.70 6.05 4.47 3.17 1.58 0.00

0.00 2.26 3.19 4.83 7.29 9.70 13.18 16.62 19.57 21.43 23.04 24.13 25.90 28.83 32.82 37.08 40.43 42.25 47.46 48.75 51.30 52.91 55.04 55.47 56.68 57.51 60.07 64.63

43.23 41.12 43.19 42.10 43.09 42.78 39.97 42.80 42.05 81.40 76.45 73.14 88.69 83.46 76.43 79.56 84.4 76.19 78.29 19.72 1.06 0.26 0.21 0.23 0.15 0.07 -

0.21 0.35 0.23 0.57 0.75 1.03 1.29 1.44 1.36 4.79 7.62 8.05 3.25 6.01 10.69 10.34 7.15 14.12 12.59 61.71 98.29 93.38 90.19 90.36 87.02 91.59 -

S10 S10 S10 S10 S10 S10 S10 S10 S10 S10 S10 + S S S S S S S S S S S + Cs2SO4 Cs2SO4 Cs2SO4 Cs2SO4 Cs2SO4 Cs2SO4 Cs2SO4 Cs2SO4

Standard uncertainties u are u(T) = 0.20 K; u(p) = 0.05; u(ρ) = 0.012 kg·m−3; u(nD) = 0.001; ur(w(Na2SO4)) = 0.0050; ur(w(Cs2SO4)) = 0.0050. S10: Na2SO4·10H2O; S: Na2SO4.

a

b

of w(Rb2SO4), and reaching the maximum value at the invariant point H. The Ternary System Na2SO4 + Cs2SO4 + H2O. The determined data were tabulated in Table 5, including solubility, density, and refractive index of the ternary system Na2SO4 + Cs2SO4 + H2O at 298.2 K, in which w(M) was the mass fraction of Na2SO4 or Cs2SO4. On the basis of the data in Table 5, the stable phase diagram was portrayed and shown in Figure 5. The XRD patterns of the solid phases at points E1 and E2 were shown in Figure 6 and Figure 7. The Diagrams of Physicochemical Properties (Density and Refractive Index) vs Composition Were Shown in Figure 8 and Figure 9. As shown in Figure 5, the ternary system Na2SO4 + Cs2SO4 + H2O is of the hydrate type II, with the dissolution curves of Na2SO4·10H2O and Na2SO4 at 298.2 K. The stable phase diagram comprises two invariant points (E1, E2), three univariant curves (AE1, E1E2 and E2B), and three crystallographic zones. The univariant curves, AE1, E1E2, and E2B, are the solubility isotherms of Na2SO4·10H2O, Na2SO4, and Cs2SO4, respectively. The three crystallization fields correspond with salt Na2SO4·10H2O, Na2SO4, and Cs2SO4. Figure 6 and Figure 7 are the XRD pattern diagrams of the solid phases corresponding to the invariant points E1 and E2. Comparing the X-ray diffraction patterns with standard databases, invariant point E1 is saturated with salts Na2SO4· 10H2O (PDF no. 75-1077) and Na2SO4 (PDF no. 74-1738),

point H. Compared with the standard card, the X-ray diffraction pattern matches well with Na2SO4·10H2O (PDF no. 75-1077) and Rb2SO4 (PDF no. 70-1017), which means that the salt Na2SO4·10H2O and Rb2SO4 coexist at invariant point H. Furthermore, the equilibrium solution corresponding at the invariant point H is w(Na2SO4) = 23.10%, w(Rb2SO4) = 25.65%, and w(H2O) = 51.25%. The univariant curves (CH, DH) are the dissolution curve of Na2SO4·10H2O and Rb2SO4, cosaturated with the salt Na 2 SO 4 ·10H 2 O and Rb 2 SO 4 , respectively. Notably, w(Na2SO4) increases slightly on the univariant curve CH, demonstrating that salt Rb2SO4 has a slight salting-in effect on salt Na2SO4. The two crystallographic zones correspond with Na2SO4·10H2O and Rb2SO4. Salt Na2SO4·10H2O has a smaller crystallization field than Rb2SO4. Figure 3 is the diagram of density vs composition, the ordinate of which is the density and the abscissa is the mass fraction of Rb2SO4. On the univariant curve CH, the density increases regularly with the increase of w(Rb2SO4) and reaches the maximum value at the invariant point H, whereas on the univariant curve DH, the density decreases as the Rb2SO4 content increases. Figure 4, the diagram of refractive index vs composition, is similar to Figure 3, which shows that the refractive index has the analogous variation tendency with density, increasing first and then decreasing with the increases D



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Figure 5. Stable phase diagram of the ternary system Na2SO4 + Cs2SO4 + H2O at 298.2 K: dissolution curve; ---, wet residue curve.

▲,

liquid point; ◆, wet solid point; , isothermal

Figure 6. X-ray diffraction patterns of the solid phases corresponding to the invariant point E1. Figure 8. Density vs composition diagram of the ternary system Na2SO4 + Cs2SO4 + H2O at 298.2 K.

and invariant point E2 is saturated with salts Na2SO4 (PDF no. 74-1738) and Cs2SO4 (PDF no. 70-0228). The composition of the equilibrated solution at point E1 is w(Na2SO4) = 19.43%, w(Cs2SO4) = 23.04%, w(H2O) = 57.53%, and those at point E2 is w(Na2SO4) = 12.87%, w(Cs2SO4) = 51.30% and w(H2O) = 35.83%. According to the phase rule, invariant point E1, outside the triangle formed by its cosaturated salts Na2SO4, Na2SO4· 10H2O, and H2O, is of an incommensurate invariant point; whereas, invariant point E2, sited in the triangle formed by its cosaturated salts Na2SO4, Cs2SO4, and H2O, is of a commensurate invariant point.

Figure 7. X-ray diffraction patterns of the solid phases corresponding to the invariant point E2. E



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invariant point, it should be the evaporative dry point during the evaporation process, and it also should be the extreme point for the physicochemical properties.

4. CONCLUSION In this paper, the densities, refractive indices, and solubilities of the ternary systems Na2SO4+ Rb2SO4 (Cs2SO4) + H2O were obtained at 298.2 K. Results show that there is no double salt or solid solution formed. The stable phase diagram of Na2SO4 + Rb2SO4 + H2O at 298.2 K is composed of one invariant points, two univariant curves, and two crystallographic zones corresponding to two simple salts Rb2SO4 and Na2SO4· 10H2O, respectively. The ternary system Na2SO4 + Cs2SO4 + H2O is of the hydrate type II, its stable phase diagram contains two invariant points, three univariant curves, and three crystallographic zones corresponding to three simple salts Na2SO4·10H2O, Na2SO4, and Cs2SO4. Caused by the unique structure of Cs+, there are two types of crystallization of sodium sulfate, namely Na2SO4·10H2O and Na2SO4 in the system with sodium and cesium sulfate at 298.2 K. The density and refractive index of the solution have an analogous variation trend, regularly changing with the change of concentration.

Figure 9. Refractive index vs composition diagram of the ternary system Na2SO4 + Cs2SO4 + H2O at 298.2 K.

In regard to the crystallization form of sodium sulfate, different systems may have different results at 298 K. In Cd2+, Na+, K+//SO42−−H2O system,27 there is only decahydrate sodium sulfate (Na2SO4·10H2O) formed, whereas in the system Na+, K+//Br−, SO42−−H2O28 and Na+, Mg2+//Cl−, SO42−, NO3−−H2O,29 there are decahydrate sodium sulfate (Na2SO4·10H2O) and anhydrous sodium sulfate (Na2SO4) formed simultaneously. In this work, results show that salt Na2SO4 has two crystallization forms (Na2SO4·10H2O and Na2SO4) in the system Na2SO4 + Cs2SO4 + H2O, whereas it has only one crystallization form (Na2SO4·10H2O) in the system Na2SO4 + Rb2SO4 + H2O. This difference may be caused by the difference of the atomic dimension, electronegativity, electron concentration, and the hydration capacity between Rb+ and Cs+. Gong’s30 and Marcus’31 calculation results show that the total binding energy of Cs+ with water is lesser than Rb+, thus Cs+ has stronger water binding ability than Rb+, which effects the binding force between Na+ and H2O molecule or between Na2SO4 and H2O molecules. Affected by the unique structure of Cs+, the decahydrate sodium sulfate dehydrated (Na2SO4·10H2O) and the anhydrous sodium sulfate (Na2SO4) dissolved out with the increase of w(Cs2SO4) in the system Na2SO4 + Cs2SO4 + H2O at 298.2 K. Conversely, only Na2SO4·10H2O formed in the system Na2SO4 + Rb2SO4 + H2O. Furthermore, the phase equilibrium of Na2SO4 + Cs2SO4 + H2O at 298.15 K was calculated by Filippov using Pitzer’s model,32 and the same results as this paper were obtained, with Na2SO4 appearing as well as Na2SO4·10H2O at 298 K, and a slight difference between the calculated solubility value and the experimental value, as shown in Figure 5. The diagrams of physicochemical properties (density and refractive index) vs composition were shown in Figure 8 and Figure 9. The solubility of Cs2SO4 is greater than that of Na2SO4, thus, the content of Cs2SO4 becomes the main effect on the changes of density and refractive index in the equilibrated solution. The two figures show that both density and refractive index of the equilibrated solution rise first then descend with the increase of w(Cs2SO4), and reach the maximum value at the invariant point E2. This variation phenomenon is in accordance with the phase rules about the invariant point properties; that is, Point E2 is a commensurate

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], [email protected]. ORCID

Linxin Wu: 0000-0001-7964-1799 Xudong Yu: 0000-0002-3848-9484 Peijun Chen: 0000-0001-6067-6502 Funding

This project was supported by the National Natural Science Foundation of China (U1607121, 41473059, U1507111). Notes

The authors declare no competing financial interest.

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The Stable Phase Equilibria of the Ternary Systems Na2SO4 + Rb2SO4

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