Phase Equilibria in the Aqueous Ternary System (NH4)2SO4 +

Feb 22, 2018 - Phase Equilibria in the Aqueous Ternary System (NH4)2SO4 + Na2SO4 + H2O at T = (303.15 and 313.15) K and p = 0.1 MPa ... Phone: +86-22-...
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Phase Equilibria in the Aqueous Ternary System (NH4)2SO4 + Na2SO4 + H2O at T = (303.15 and 313.15) K and p = 0.1 MPa Dongchan Li,*,†,‡ Rong Fan,† Xiaofu Guo,†,‡ Sennan Yang,† and Ziyi Zhang† †

School of Marine Science and Engineering, Hebei University of Technology, Tianjin 300130, P. R. China Engineering Research Center of Seawater Utilization Technology, Ministry of Education, Tianjin 300130, P. R. China

‡

ABSTRACT: The solubility for the aqueous ternary system (NH4)2SO4 + Na2SO4 + H2O at T = (303.15 and 313.15) K and p = 0.1 MPa was investigated experimentally with the method of isothermal dissolution equilibrium. In the phase diagram of the ternary system at 303.15 K, there are eight crystallization regions, four univariant curves, and three invariant points corresponding to (Na2SO4·10H2O + Na2SO4), (Na2SO4 + Na2SO4·(NH4)2SO4·4H2O), and (Na2SO4·(NH4)2SO4·4H2O + (NH4)2SO4). However, in the phase diagram of the same ternary system at 313.15 K, the crystallization region of Na2SO4·10H2O is disappeared, and there are three single salt crystallization regions corresponding to Na2SO4, Na2SO4·(NH4)2SO4·4H2O, and (NH4)2SO4. A comparison of the phase diagrams for this ternary system at (273.15, 288.15, 298.15, 303.15, 313.15, and 333.15) K shows that the area of Na2SO4·(NH4)2SO4·4H2O and (NH4)2SO4 is increased obviously, whereas the area of Na2SO4 (or Na2SO4·10H2O) is decreased significantly and no solid solution was found. The experimental phase equilibria and phase diagrams can provide a fundamental basis for salt recovery in industrial wastewaters.

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INTRODUCTION

guidance for the salt crystallization. It has been widely used in many industries with its cost-effective, energy-saving, and green process.3 It also has become a potential method for extracting and recycling salts from industrial wastewaters. To obtain the pure salts of (NH4)2SO4 and Na2SO4 from the above-mentioned industrial wastewaters, the solubility data of (NH4)2SO4 and Na2SO4 is essential and necessary. There have been a number of literature reports on the system containing (NH4)2SO4 and Na2SO4, such as the systems (NH4Cl + (NH4)2SO4 + H2O),4 (Na2SO4 + MgSO4 + (NH4)2SO4 + H2O),5,6 and ((NH4)2SO4 + Na2SO4 + NH4Cl + NaCl + H2O),7 and its subsystem ((NH4)2SO4 + Na2SO4 + H2O).8,14 However, many of the thermodynamic properties relevant to the systems are still not sufficient. In this study, the phase equilibria for the system of ((NH4)2SO4 + Na2SO4 + H2O) were determined at (303.15 and 313.15) K for the factory requirement. The research will provide guidance for designing the production of Na2SO4 and (NH4)2SO4.

As important inorganic chemicals, Na2SO4 and (NH4)2SO4 are used widely in industry and agriculture. The salt mixtures (NH4)2SO4 and Na2SO4 are found in a large number of industrial processes, especially in flue gas desulfurization. It has developed rapidly in recent years with the production and consumption of coal as the cheapest fuel in the foreseeable future. Nevertheless, the desulfurization operation generates a lot of wastewater rich in (NH4)2SO4 and Na2SO4 which mainly come from the regeneration process of the flue gas desulfurization agent.1 However, what should be noticed is that these industrial wastewaters are neither recycled nor discharged. At present, the treatment is usually to evaporate them to dryness and transport to stack somewhere. As a result, these large numbers of mixed salts with impurities not only occupy large area lands but also lead to secondary pollution with rainfall infiltration because of their high water solubility and thus are identified as hazardous wastes.2 Hence, separating and recovering of these salts from such industrial wastewaters become an urgent task for the enterprise. As we know, crystallization is a usually used method for separating and extracting pure salts from the mixed solutions, while the phase diagram is the crucial basis and gives the whole © XXXX American Chemical Society

Received: September 25, 2017 Accepted: February 12, 2018

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DOI: 10.1021/acs.jced.7b00847 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Table 1. Chemical Samples Used in This Study chemical name

CAS no.

source

purification method

mass fraction purity

analysis method

(NH4)2SO4 Na2SO4

7783-20-2 7757-82-6

Sinopharm Chemical Reagent Co. Ltd. Sinopharm Chemical Reagent Co. Ltd.

recrystallization recrystallization

0.995 0.995

gravimetric method for SO42− gravimetric method for SO42−

Co., Ltd., China) with a temperature range from (273.15 to 372.15) K and a precision of 0.01 K was used in the equilibrium experiments. The application of an X-ray diffractometer (D8 Focus, Bruker AXS, Germany) was used to identify solid phase minerals. An inductively coupled plasma optical emission spectrometer (ICP-OES, Prodigy, Leman Co., USA) was employed in determining the concentrations of Na+ in solution. The chemicals used were recrystallized, and their purities were analyzed before use; the chemical sample descriptions were listed in Table 1. Doubly deionized water (DDW) with a conductivity less than 1.0 × 10−4 S·m−1 and pH 6.60 at 298.15 K was used for the (solid + liquid) phase equilibrium experiments and chemical analysis. Experimental Method. The solid−liquid phase equilibrium was studied with the isothermal dissolution equilibrium method.9−11 First, according to relevant phase equilibrium compositions, the appropriate salts (Na2SO4, (NH4)2SO4) and doubly deionized water were placed and sealed in the equilibrium bottles, which must ensure the solids were not dissolved entirely and always existed in the whole equilibrium process. Then, the sealed bottles were fixed in the magnetic stirring thermostatic bath (HXC-500-6A) in which the temperatures were set at either (303.15 ± 0.01) or (313.15 ± 0.01) K with 150 rpm stirring speed. These artificial synthesized complexes were stirred for about (3−5) days with the electric magnetic stirrer. It was the time that the equilibrium had been reached when the compositions of the liquid phase gave the identical analysis results and became constant state. Before sampling, the equilibrium bottles were allowed to settle and separate for 5 h to clarify an

Figure 1. Comparison of experimental solubilities (100w) of (NH4)2SO4 with literature data: red ○, refs 13 and 14; blue ▲, this work.

Table 2. Compositions of Solubilities of (NH4)2SO4 and Na2SO4 at T = (303.15 and 313.15) K and p = 0.1 MPa T (K) (NH4)2SO4 Na2SO4 a

303.15 313.15 303.15 313.15

compositions of solubilities 100w (%) 43.82a 44.75a 29.28a 32.52a

43.798 44.798 29.088 32.488

43.9113 44.8113 29.013 32.613

44.774 32.4415

Solubility data in this work.

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EXPERIMENTAL SECTION Reagents and Apparatus. A magnetic stirring thermostatic bath (HXC-500-6A, Beijing Fortune Joy Science Technology

Table 3. Solubility of the Ternary System ((NH4)2SO4 + Na2SO4 + H2O) at 303.15 K and Pressure p = 0.1 MPaa composition of liquid phase 100wb (%)

a b

composition of wet solid phase 100w (%)

no.

(NH4)2SO4

Na2SO4

H2O

(NH4)2SO4

Na2SO4

solid phase

1, A 2 3 4, E1 5 6 7, E2 8 9 10 11 12 13 14 15 16 17 18, E3 19 20 21, B

0.00 2.32 4.91 6.49 9.94 12.90 14.64 16.49 18.20 20.12 22.12 25.64 27.65 30.15 31.53 33.84 35.58 38.27 39.62 41.48 43.82

29.28 29.91 30.38 30.76 28.58 25.78 25.35 24.00 21.84 20.08 18.72 16.14 15.08 13.28 12.63 11.23 10.74 8.36 6.47 3.17 0.00

70.72 67.77 64.71 62.75 61.48 61.32 60.01 59.51 59.96 59.80 59.16 58.22 57.27 56.57 55.84 54.93 53.68 53.37 53.91 55.35 56.18

c 0.87 2.09 1.42 3.49 4.57 14.84 29.00 30.11 31.47 32.23 33.19 34.01 34.82 36.51 36.80 36.90 57.44 79.63 79.28 c

c 38.31 38.14 49.37 73.95 73.08 68.43 34.49 34.01 33.57 33.10 31.75 31.85 30.72 31.90 30.60 31.21 19.80 2.19 1.12 c

Na2SO4·10H2O Na2SO4·10H2O Na2SO4·10H2O Na2SO4·10H2O + Na2SO4 Na2SO4 Na2SO4 Na2SO4 + Na2SO4·(NH4)2SO4·4H2O Na2SO4·(NH4)2SO4·4H2O Na2SO4·(NH4)2SO4·4H2O Na2SO4·(NH4)2SO4·4H2O Na2SO4·(NH4)2SO4·4H2O Na2SO4·(NH4)2SO4·4H2O Na2SO4·(NH4)2SO4·4H2O Na2SO4·(NH4)2SO4·4H2O Na2SO4·(NH4)2SO4·4H2O Na2SO4·(NH4)2SO4·4H2O Na2SO4·(NH4)2SO4·4H2O Na2SO4·(NH4)2SO4·4H2O + (NH4)2SO4 (NH4)2SO4 (NH4)2SO4 (NH4)2SO4

Standard uncertainties u are u(T) = 0.01 K, u(p) = 0.005 MPa. u(w) for (NH4)2SO4 and Na2SO4 are 0.003 and 0.003 in mass fraction, respectively. w = mass fraction. cNot detected. B

DOI: 10.1021/acs.jced.7b00847 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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aqueous solution in the thermostat with the temperature at either (303.15 ± 0.01) or (313.15 ± 0.01) K. Then, the liquid samples and wet residues of the equilibrium solutions were taken out from the bottles, respectively. The liquid samples were taken with the syringes equipped with a filter and then weighed and diluted to 250 mL volumetric flask with distilled water for the chemical analysis. The corresponding wet residues of the samples were separated with vacuum filtration and then weighed and diluted for the chemical analysis. The solids were identified by Schreinemakers’ method combined with X-ray powder diffraction analysis. Analytical Methods. The compositions of NH4+ in the liquid phases and wet residues were determined by a method of acid−base titration with the precision in triplicate ±0.003 in mass fraction. It was titrated by the standard NaOH solution and phenolphthalein as the indicator with the presence of formaldehyde solution neutralized before use.12 The concentration of SO42− was obtained by the gravimetric method with 5% (w/v)

Figure 2. Equilibrium phase diagram of the ternary system ((NH4)2SO4 + Na2SO4 + H2O) at 303.15 K: black ●, experimental points; , isothermal curve; red ●, wet residue points; - - -, Schreinemakers lines.

Figure 3. X-ray diffraction pattern of the eutectic point E1 (Na2SO4·10H2O + Na2SO4).

Figure 4. X-ray diffraction pattern of the eutectic point E2 (Na2SO4+ Na2SO4·(NH4)2SO4·4H2O). C

DOI: 10.1021/acs.jced.7b00847 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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BaCl2 as the precipitator with the uncertainty in triplicate ±0.003 in mass fraction. The Na+ concentration was determined in triplicate with ICP-OES with an uncertainty within ±0.005.

The Ternary System (NH4)2SO4 + Na2SO4 + H2O at 303.15 K. The compositions in the liquid phases and wet residues of the ternary system (NH4)2SO4 + Na2SO4 + H2O at 303.15 K were determined and presented in Table 3. The corresponding phase diagram for the ternary system at 303.15 K was shown in Figure 2. In Figure 2, presented as a triangular coordinate, the vertex corresponded to pure water, the points (A and B) on the sides represented the components of binary systems, and the points inside the equilateral triangle characterized the compositions of ternary mixtures. As shown in Figure 2, there are eight crystallization fields corresponding to AHE1 (Na2SO4·10H2O), E1HC (Na2SO4·10H2O + Na2SO4), E1CE2 (Na2SO4), E2CG

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RESULTS AND DISCUSSION The solubilities (100w) of (NH4)2SO4 in water at different temperatures have been shown in Figure 1. It was observed that the solubility of (NH4)2SO4 increases with increasing temperature. The solubilities of (NH4)2SO4 and Na2SO4 at T = (303.15 and 313.15) K were obtained and shown in Table 2. As shown in Figure 1 and Table 2, our experiment data keep consistent with the literature reported data, which supports that the experiment method and data quality in this work are reliable.

Figure 5. X-ray diffraction pattern of the eutectic point E3 (Na2SO4·(NH4)2SO4·4H2O + (NH4)2SO4).

Table 4. Solubility of the Ternary System ((NH4)2SO4 + Na2SO4 + H2O) at 313.15 K and Pressure p = 0.1 MPaa composition of liquid phase 100wb (%)

a b

composition of wet solid phase 100w (%)

no.

(NH4)2SO4

Na2SO4

H2 O

(NH4)2SO4

Na2SO4

solid phase

1, C 2 3 4 5 6 7 8 9, F1 10 11 12 13 14 15 16, F2 17 18 19 20 21, D

0.00 4.41 7.38 10.55 12.37 14.66 15.89 17.01 19.58 23.41 25.84 28.89 32.84 34.73 35.71 38.41 39.00 39.96 42.37 43.66 44.75

32.52 31.88 30.56 28.69 27.76 26.30 25.69 24.96 23.08 20.44 18.82 16.84 14.28 12.88 12.00 10.23 9.44 7.57 4.49 2.28 0.00

67.48 63.71 62.06 60.76 59.87 59.04 58.42 58.03 57.34 56.15 55.34 54.27 52.88 52.39 52.29 51.36 51.56 52.47 53.14 54.06 55.25

c 0.15 2.51 3.30 3.98 4.89 5.40 5.66 15.39 32.27 32.97 34.75 36.19 37.02 37.57 52.41 80.67 79.74 79.88 81.28 c

c 77.25 77.18 77.25 76.24 75.86 73.96 75.30 65.90 32.38 31.23 31.73 31.68 31.38 31.16 23.33 2.87 2.51 1.52 0.87 c

Na2SO4 Na2SO4 Na2SO4 Na2SO4 Na2SO4 Na2SO4 Na2SO4 Na2SO4 Na2SO4 + Na2SO4·(NH4)2SO4·4H2O Na2SO4·(NH4)2SO4·4H2O Na2SO4·(NH4)2SO4·4H2O Na2SO4·(NH4)2SO4·4H2O Na2SO4·(NH4)2SO4·4H2O Na2SO4·(NH4)2SO4·4H2O Na2SO4·(NH4)2SO4·4H2O Na2SO4·(NH4)2SO4·4H2O + (NH4)2SO4 (NH4)2SO4 (NH4)2SO4 (NH4)2SO4 (NH4)2SO4 (NH4)2SO4

Standard uncertainties u are u(T) = 0.01 K, u(p) = 0.005 MPa. u(w) for (NH4)2SO4 and Na2SO4 are 0.003 and 0.003 in mass fraction, respectively. w = mass fraction. cNot detected. D

DOI: 10.1021/acs.jced.7b00847 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Point E1: w((NH4)2 SO4 ) = 6.49%,

w(Na 2SO4 ) = 30.76%,

w(H 2O) = 62.75%

Point E2: w((NH4)2 SO4 ) = 14.64%,

w(Na 2SO4 ) = 25.35%,

w(H 2O) = 60.01%

Point E3: w((NH4)2 SO4 ) = 38.27%,

w(Na 2SO4 ) = 8.36%,

w(H 2O) = 53.37%

Figures 3, 4, and 5 show the X-ray diffraction patterns of invariant points E1, E2, and E3, respectively. The abscissa ordinate is the 2θ from 5 to 85°; the vertical ordinate is the intensity. The XRD pattern of the invariant point E1 shown in Figure 3 was well matched to the standard diffraction pattern of Na2SO4·10H2O and Na2SO4 with powder diffraction file (PDF) numbers “11-0647” and “37-1465”, respectively. It shows that salts Na2SO4·10H2O and Na2SO4 coexist at the invariant point E1. Accordingly, as shown in Figures 4 and 5, salts Na2SO4· (NH4)2SO4·4H2O, Na2SO4 and Na2SO4·(NH4)2SO4·4H2O, (NH4)2SO4 coexist at the invariant points E2 and E3, respectively. The Ternary System (NH4)2SO4 + Na2SO4 + H2O at 313.15 K. The compositions in the liquid phases and wet residues of the ternary system (NH4)2SO4 + Na2SO4 + H2O at 313.15 K were determined and presented in Table 4 and shown in Figure 6. As shown in Figure 6, there are three univariant isothermal dissolution curves of CF1, F1F2, and F2D, which are saturated with a single salt (Na2SO4, Na2SO4·(NH4)2SO4·4H2O, or (NH4)2SO4), and six crystallization fields corresponding to CHF1 (Na2SO4), F1HG (Na2SO4 + Na2SO4·(NH4)2SO4· 4H2O), F2GT (Na2SO4·(NH4)2SO4·4H2O + (NH4)2SO4), F2TD ((NH4)2SO4), and an all-solid region HGT, respectively. The single salt’s crystallization area decreases in the order of Na2SO4, Na2SO4·(NH4)2SO4·4H2O, (NH4)2SO4 at 313.15 K.

Figure 6. Equilibrium phase diagram of the ternary system ((NH4)2SO4 + Na2SO4 + H2O) at 313.15 K: ●, experimental points; , isothermal curve; red ●, wet residue points; - - -, Schreinemakers lines.

(Na2SO4 + Na2SO4·(NH4)2SO4·4H2O), E2GE3 (Na2SO4· (NH4)2SO4·4H2O), E3GD (Na2SO4·(NH4)2SO4·4H2O + (NH4)2SO4), E3DB ((NH4)2SO4), and an all-solid region CGD. The area of the single salt crystallization region of Na2SO4·(NH4)2SO4·4H2O is largest, whereas the Mir crystallization area is smallest. These results indicate that double salt Na2SO4·(NH4)2SO4·4H2O is relatively easy to saturate and crystallize than the other three salts from solution at 303.15 K. The salt’s crystallization area decreases in the order of Na2SO4· (NH4)2SO4·4H2O, (NH4)2SO4, Na2SO4, Na2SO4·10H2O. Na2SO4·(NH4)2SO4·4H2O is an incongruent double salt in this ternary system, and no solid solution was found at 303.15 K. In the phase diagram, there are four univariant solubility curves of AE1, E1E2, E2E3, and E3B indicating solution cosaturated with Na2SO4·10H2O, Na2SO4, Na2SO4·(NH4)2SO4·4H2O, and (NH4)2SO4, respectively. There are three invariant points corresponding to E1 (Na2SO4·10H2O + Na2SO4), E2 (Na2SO4· (NH4)2SO4·4H2O + Na2SO4), and E3 ((NH4)2SO4 + Na2SO4· (NH4)2SO4·4H2O) in this ternary system, and the mass fraction compositions of the equilibrated solution are listed as follows:

Figure 7. X-ray diffraction pattern of the eutectic point E2 (Na2SO4+ Na2SO4·(NH4)2SO4·4H2O). E

DOI: 10.1021/acs.jced.7b00847 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Figure 8. X-ray diffraction pattern of the eutectic point E3 (Na2SO4·(NH4)2SO4·4H2O + (NH4)2SO4).

In the phase diagram, there are two invariant points corresponding to F2 (Na2SO4 + Na2SO4·(NH4)2SO4·4H2O) and F2 (Na2SO4·(NH4)2SO4·4H2O + (NH4)2SO4), and the mass fraction compositions of the equilibrated solution are listed as follows: Point F1: w((NH4)2 SO4 ) = 19.58%,

w(Na 2SO4 ) = 23.08%,

w(H 2O) = 57.34%

Point F2: w((NH4)2 SO4 ) = 38.41%,

w(Na 2SO4 ) = 10.23%,

w(H 2O) = 51.36% Figure 9. Comparison of the phase diagram of the ternary system ((NH4)2SO4 + Na2SO4 + H2O) at (273.15, 288.15, 298.15, 303.15, 313.15, and 333.15) K: navy ●, isothermal dissolution curve at 273.15 K; pink △, isothermal dissolution curve at 288.15 K;14 magenta ■, isothermal dissolution curve at 298.15 K;14 red ○, isothermal dissolution curve at 303.15 K (this work); blue ■, isothermal dissolution curve at 313.15 K (this work); purple ◊, isothermal dissolution curve at 333.15 K.14

Figures 7 and 8 show the X-ray diffraction patterns of invariant points F1 and F2, respectively. The XRD pattern of the invariant point F1 shown in Figure 7 was well matched to the standard diffraction pattern of Na2SO4 and Na2SO4· (NH4)2SO4·4H2O with powder diffraction file (PDF) numbers “37-1465” and “15-0283”, respectively. It shows that salts Na2SO4·10H2O and Na2SO4 coexist at the invariant point E1. Accordingly, as shown in Figure 8, salts Na2SO4·(NH4)2SO4· 4H2O and (NH4)2SO4 coexist at the invariant point F2. The comparison of the solubility data for the ternary system (NH4)2SO4 + Na2SO4 + H2O at (273.15, 288.15, 298.15, and 333.115) K14 and (303.15 and 313.15) K (this study) is shown in Figure 9. The solid phase numbers and existing minerals are different. The area of the crystallization region of Na2SO4· (NH4)2SO4·4H2O and (NH4)2SO4 is increased obviously as the temperature increases from (273.15 to 333.15) K. Sodium sulfate has two stable solid phases Na2SO4·10H2O and Na2SO4 in this system, and experimental research shows that the transition temperature of solid phase Na2SO4·10H2O and Na2SO4 is 305.5 K. The solubility of Na2SO4 dramatically increases with increasing temperature at T = (273.15 to 313.15) K and gradually decreases with increasing temperature at T > 313.15 K. It indicates that Na2SO4 holds the potential for both cooling crystallization at low temperatures and evaporation crystallization at high temperatures. Moreover, it is worthwhile to note that the crystallization region of double salt Na2SO4·(NH4)2SO4·4H2O has a significant

difference with temperature. Na2SO4·(NH4)2SO4·4H2O is an incongruent double salt when the temperatures are in the lower range (273.15−288.15 K), while it is a congruent double salt when the temperatures are in a relatively higher range (298.15− 313.15 K) and it disappeared at 333.15 K. This behavior of transformation and crystallization for double salt Na2SO4·(NH4)2SO4· 4H2O at lower temperatures from (273.15 to 298.15) K is important for separating and purifying the ammonium sulfate.

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CONCLUSION Phase equilibria of the ternary system ((NH4)2SO4 + Na2SO4 + H2O) at (303.15 and 313.15) K were determined with the isothermal dissolution equilibrium method. According to the experimental data, the phase diagrams were plotted. With the increase of temperatures from (273.15 to 333.15) K, the area of the crystallization regions is very different; especially the crystallization area of double salt Na2SO4·(NH4)2SO4·4H2O increased obviously. Na2SO4·(NH4)2SO4·4H2O is an incongruent double salt at lower temperature, while it transformed into a congruent F

DOI: 10.1021/acs.jced.7b00847 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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(14) Silcock, H. L. Solubilities of Inorganic and Organic Compounds: Ternary and multicomponent systems of inorganic substances; Pergamon: New York, 1979. (15) Block, J.; Waters, O. B. Calcium sulfate-sodium sulfate-sodium chloride-water system at 25°C to 100°C. J. Chem. Eng. Data 1968, 13, 336−344.

double salt, and this behavior of transformation and crystallization is important for separating and purifying the ammonium sulfate. The phase diagrams are of great use for surmounting the intractable problem of salt recovery in industrial wastewater treatment.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +86-22-60202241. ORCID

Dongchan Li: 0000-0002-5333-3840 Funding

Financial support from the Program of the Natural Science Foundation of Tianjin (15JCQNJC06100, 17JCYBJC19500), the Natural Science Foundation of Hebei Province (B2017202198), and the National Natural Science Foundation of China (21406048, U1507109) is greatly acknowledged. Notes

The authors declare no competing financial interest.

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DOI: 10.1021/acs.jced.7b00847 J. Chem. Eng. Data XXXX, XXX, XXX−XXX