Stable (solid + liquid) phase equilibrium for the ternary systems (K2SO4 + KH2PO4 + H2O), (K2SO4 + KCl + H2O) at T = 313.15 K

J. Chem. Thermodynamics 90 (2015) 15–23

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Stable (solid + liquid) phase equilibrium for the ternary systems (K2SO4 + KH2PO4 + H2O), (K2SO4 + KCl + H2O) at T = 313.15 K Wei Shen, Yongsheng Ren ⇑, Ting Wang, Cui Hai School of Chemistry and Chemical Engineering, Ningxia University, Yinchuan 750021, PR China

a r t i c l e

i n f o

Article history: Received 16 February 2015 Received in revised form 4 June 2015 Accepted 8 June 2015 Available online 16 June 2015 Keywords: KH2PO4 (Solid + liquid) equilibrium Phase diagram Physico-chemical properties

a b s t r a c t The stable (solid + liquid) phase equilibria and physico-chemical properties of the solution in the ternary systems (K2SO4 + KH2PO4 + H2O) and (K2SO4 + KCl + H2O) at T = 313.15 K were determined by using an isothermal solution saturation method. According to the experimental data, the phase diagrams and the diagrams of physico-chemical properties versus composition were plotted. It was found that there were one invariant point, two univariant curves and three fields of crystallization in the ternary systems (K2SO4 + KH2PO4 + H2O) and (K2SO4 + KCl + H2O) at T = 313.15 K. On the basis of the phase diagrams obtained, we analyzed each invariant point. When K2SO4 and KH2PO4 were analyzed as (7.71, 20.99) wt%, respectively, the ternary system (K2SO4 + KH2PO4 + H2O) reached saturation at T = 313.15 K. When K2SO4 and KCl were analyzed as (1.15, 27.08) wt%, respectively, the ternary system (K2SO4 + KCl + H2O) reached saturation at T = 313.15 K. The solubility of K2SO4 is the lowest among the three salts, and the crystalline fields of K2SO4 in the two ternary systems are larger than that of KH2PO4 and KCl, which shows that K2SO4 can be easily separated from the mixed aqueous solution. Thus, knowledge of (solid + liquid) equilibrium, especially the crystalline fields at T = 313.15 K, would be extremely valuable to design and optimize solvent extraction process of KH2PO4 and other crystallization processes involving the two ternary systems in the industrial production. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction KH2PO4 is a widespread useful fine chemistry product used in industrial and agricultural fields. It has aroused considerable interest with the advantage of wide frequency conversion, high efficiency, high damage threshold against high power laser [1]. In addition, it also can be used as a commercially valuable intermediate for producing the widest variety of derivatives, such as other potassium salts, penicillin and sodium glutamate [2,3]. Potassium Sulfate is widely used in agriculture. It has been reported evidence describing the significant effect of K2SO4 fertilizer on increased yield in calcareous-alkaline soils [4]. Pure KH2PO4 can be obtained by several production processes, such as neutralization technology, direct chemical conversion method, crystallization method, ion exchange method and solvent extraction method [5,6]. Among them solvent extraction method was considered to be very promising. Solvent extraction method ⇑ Corresponding author at: School of Chemistry and Chemical Engineering, Ningxia University, No. 539, Helanshan West Road, Xixia district, Yinchuan, Ningxia Hui Autonomous Region 750021, PR China. Tel.: +86 0951 2062004; fax: +86 0951 2062860. E-mail addresses: [email protected], [email protected] (Y. Ren). http://dx.doi.org/10.1016/j.jct.2015.06.011 0021-9614/Ó 2015 Elsevier Ltd. All rights reserved.

has the advantages of low energy consumption and high product purity. Purified wet-process phosphoric acid and KCl as raw materials were used to product KH2PO4 with the method of solvent extraction [7]. A simple process of solvent extraction was shown in figure 1. Crystallization process is a crucial step in producing KH2PO4 with the method of extraction technology. Because Cl and SO2 4 [8] will accumulate in crystallization mother liquor and could not remove after a long series of removing impurity process, which results in lower quality of KH2PO4. In order to prepare high quality of KH2PO4 product, the key point is to separate KCl and K2SO4 from the mixture solution. In this way, we can not only obtain refined KH2PO4, but also retrieve a small quantity of byproducts K2SO4 and KCl in the crystallization process. In previous publications, a literature survey shows that Bel’teschev, F.V. [9] was the earliest to report (K2SO4 + KH2PO4 + H2O) phase equilibrium data at T = (273.15, 293.15, 303.15) K. Furthermore, Pan Wang [10] had investigated the ternary system of (K2SO4 + KH2PO4 + H2O) at T = (298.15 and 333.15) K by the method of isothermal solution saturation and moist residues. (K2SO4 + KH2PO4 + H2O) phase diagram in the region of heterogeneous supercritical fluids has been reported in the literature [11]. The phase equilibrium data for the ternary system

16

W. Shen et al. / J. Chem. Thermodynamics 90 (2015) 15–23

H2O

KCl

(Zhengzhou Ying Yuhua Instrument Co. Ltd., China), which was used to keep constant temperature in the experiment. And all the temperature was controlled with a mercury thermometer (u(T) = 0.05 K).

H3PO4

Solution

2.2. Experimental methods Filtration

Recycling mother liquor

Residues

Organic phase

Extraction Recycling organic phase

Stripping agent

Crystallization Stripping

Products KH2PO4 FIGURE 1. A schematic diagram of solvent extraction for KH2PO4 production.

(K2SO4 + KCl + H2O) at T = (273.15, 298.15, 323.15, 348.15) K have been reported in the literature [12]. So far, the completely phase equilibrium data for the systems (K2SO4 + KH2PO4 + H2O) and (K2SO4 + KCl + H2O) at T = 313.15 K have not been reported in the literature yet. The phase diagram of (solid + liquid) system is of theoretical and practical importance to the crystallization process [13]. Thus, (solid + liquid) equilibrium data of (K2SO4 + KH2PO4 + H2O) and (K2SO4 + KCl + H2O) at T = 313.15 K are basically required for designing and optimizing the solvent extraction process of KH2PO4. At the same time, it also can be applied to other crystallization processes involving the two ternary systems in the mixed solution at T = 313.15 K. Therefore, it is necessary to investigate and supply more phase equilibrium data for the systems (K2SO4 + KH2PO4 + H2O) and (K2SO4 + KCl + H2O) at T = 313.15 K.

2. Experimental section 2.1. Materials and instruments The chemicals used were of analytical grade, where KCl, KH2PO4 and K2SO4 were produced from the Tianjin Kermel Chemical reagent Co. Ltd., China. All materials were employed with recrystallization. The sources and CAS numbers were listed in table 1. Doubly deionized water (DDW) with conductivity less than 104 S  m1 was used for the (solid + liquid) phase equilibrium experiments and chemical analysis. A constant temperature bath oscillator (SHZ-C, Shanghai Langgan Laboratory Equipment Co. Ltd., China) with a temperature range from (293.15 to 373.15) K was employed for equilibrating samples. The (solid + liquid) equilibrium experiments were carried out in an XMTD-4000 type super constant-temperature water bath

The solubility was determined employing the method of isothermal solution saturation [14] and Schreinemaker’s wet residue method [15] in this study. At a certain temperature, when pure K2SO4 was put into the saturated solution of pure KH2PO4, the solubility of KH2PO4 decreased. With further addition of K2SO4, the KH2PO4 solute kept decreasing to a stop point. The solution is known as co-saturation solution, in which K2SO4 or KH2PO4 cannot be dissolved anymore. This point is named as the invariant point in the phase diagram. Although K2SO4 existed in the solution, only KH2PO4 was saturated in the water before the solution reached co-saturation. Therefore, we get a series of points that represent the solubility of K2SO4 and KH2PO4 in saturated solutions. In this way, the solubility of ternary system (K2SO4 + KCl + H2O) can also be determined. As we all know, the composition of wet residues indicates that a little mother liquor adheres to the crystals. The composition of the solid phase is usually not determined directly, because it is quite difficult to separate the crystals from mother liquor completely. Schreinemaker’s method [16] of wet residues was used to analyze the composition of the solid phases and ascertain the crystallization fields. Extrapolation is made by Schreinemaker’s method of wet residues. In a phase diagram, the compound point must be on the tie line joining the composition of the pure solid and the saturated liquid in equilibrium. Some have a common intersection point that is the composition of the pure solid phase. In this way, the crystallization fields were ascertained. The appropriate quantity of KH2PO4 (KCl), K2SO4 and DDW were mixed together in a conical flask (250 mL) for the solubility experiments, and the conical flask was sealed and placed in the constant temperature bath oscillator with temperature controlled at around 313.15 K. (Solid + liquid) mixtures in the conical flask were vibrated to accelerate the establishment of equilibrium states. To ensure that sampling is performed at equilibrium, several samples of the system at T = 313.15 K were analyzed for Cl, H2PO 4 and SO2 after shaking (3, 4, and 5) h separately. The experimental 4 2 results showed that the concentrations of Cl, H2PO 4 and SO4 in the solution remain unchanged after 3 h. But for caution’s sake, 4 h was selected to ensure solubility equilibrium. After stirring 4 h, the samples were kept in the stopped oscillator for 24 h to allow remaining solids to settle. And the temperature was kept at around 313.15 K. The actual temperature of the oscillator was monitored by a mercury thermometer (u = 0.05 K). When a run achieved equilibrium, the saturated solution was transferred to a 100 mL beaker at around T = 313.15 K, and it was weighed accurately before sampling. A certain mass of saturated solution was removed into a volumetric flask and diluted with deionized water for quantitative analysis. In this way, the composition of saturated solution was measured by analytical methods. The remainder of liquid phase was used to measure the relative physico-chemical

TABLE 1 The purities and suppliers of chemicals.

a

Chemical

Mass fraction purity

CAS No.

Source

Analytical methoda

KCl KH2PO4 K2SO4

P99.5% P99.5% P99.5%

7447-40-7 7778-77-0 7778-80-5

Tianjin Kermel Chemical reagent Co. Ltd., China Tianjin Kermel Chemical reagent Co. Ltd., China Tianjin Kermel Chemical reagent Co. Ltd., China

Volhard method Quinolinephosphomolybdate gravimetric method Barium sulfate turbidimetry

Standard uncertainties u are u(KCl) = 0.005 (mass fraction), u(KH2PO4) = 0.004 (mass fraction), u(K2SO4) = 0.002 (mass fraction).

W. Shen et al. / J. Chem. Thermodynamics 90 (2015) 15–23

properties (q, nD, pH) individually according to the analytical method. Finally, the solid phase was separated from the solution and evaluated. The wet solids were weighed accurately, and appropriate quantity of it was taken out into 100 mL beaker for quantitative analyzed. Afterwards these were dissolved by deionisd water, transferred the solution to a 100 mL volumetric flask, and diluted to the mark with deionized water. In this way, the composition of the moist solid samples and the equilibrium liquid phase were determined.

17

3, our experimental data keep consistent with the tendency comparing with the literature data. But several data cannot keep consistent with the tendency, especially the density value of KCl–H2O at T = 308.15 K in Ref. [34] and refractive index data of K2SO4–H2O at T = 318.15 K in reference [33]. Maybe, it can be explained by different conditions, such as the precision of apparatus. Although the density value of KCl–H2O is 1.2364 g  cm3 at T = 308.15 K in reference [34], it is 1.2020 g  cm3 at T = 323.15 K in reference [24]. Except these literature data, we can see that our density value of KCl–H2O at T = 313.15 K keep consistent with the tendency comparing with other data.

2.3. Analysis 3.1. The ternary system (K2SO4 + KH2PO4 + H2O) The composition of the moist solid samples and the equilibrium liquid phase were determined by the method of chemical analysis. The H2PO-4 concentration was determined by the quinoline phosphomolybdate gravimetric method [17–19] with a standard uncertainty of 0.004 in mass fraction. The sulfate ion concentration was obtained by means of barium sulfate turbidimetry [20,21] with a standard uncertainty of 0.002 in mass fraction. The Cl concentration was determined by Volhard method [22,23] with a standard uncertainty of 0.005 in mass fraction. The details of the above analytical methods can be found in the literature. The liquid-phase physico-chemical properties (q, nD, pH) were measured. The densities (q) were measured with a specific weighing bottle, and standard uncertainty is 0.005 g  cm3. The specific gravity bottle method with correction of the air floating force was used [24]. The pH values were measured with a PB-10 pH meter supplied by the Sartorius Scientific Instruments, and standard uncertainty is 0.1. Before it was used, the pH meter was calibrated with standard buffer solutions that were prepared by the mixing agents of both mixing potassium dihydrogen phosphate (pH = 6.86) and potassium hydrogen phthalate (pH = 4.01). The refractive index (nD) value was determined by an Abbe refraction (WAY-2S), and standard uncertainty is 0.002 [25]. All the measurements were maintained in a constant temperature bath monitored by a mercury thermometer (u = 0.05 K). 3. Results and discussion Many researchers have previously reported binary systems data (KH2PO4–H2O, K2SO4–H2O and KCl–H2O) at T = 313.15 K. Several solubility data for the binary systems were collected in table 2. And there were several density and refractive index data for binary systems in literature, which were listed in table 3. Saturated solution is a solution in equilibrium with another phase. Data in tables 2 and 3 show solubility and physico-chemical properties value of the saturated solution. In order to ensure the quality of our data, our experimental data were also listed in table 2, which is a good support for experimental methods and the quality of the data measured in this work. Data in table 3 show physico-chemical properties value of the saturated solution at different temperatures. As we can see in table

TABLE 2 Solubility (S) in pure water (wt%) at T = 313.15 K and P = 88.35 kPa. This worka

Literature

S/ (wt%)

Standard uncertainty/(mass fraction)

S/(wt%)

KH2PO4

25.56

0.004

KCl K2SO4

28.79 12.88

0.005 0.002

27.15 [26], 25.10 [27], 25.00 [28] 28.728 [29], 28.01 [27] 12.86 [30], 12.90 [31], 13.0 [32]

Salt

a

Standard uncertainties u are u(T) = 0.05 K, u(P) = 0.5 kPa.

The phase equilibrium experimental results of solubility and the relevant physico-chemical properties measured for the ternary system (K2SO4 + KH2PO4 + H2O) at T = 313.15 K were shown in table 4. All solution has achieved saturated, data in table 4 show the solubility and physico-chemical properties value of saturated solutions for the ternary system (K2SO4 + KH2PO4 + H2O) at T = 313.15 K. Saturated solution compositions are expressed in terms of weight percentage. The corresponding phase diagram for the ternary system was shown in figure 2. As shown in figure 2, points A, U and W represent the pure KH2PO4, H2O and K2SO4, respectively. Points E and F represent the saturation points of binary systems KH2PO4–H2O and K2SO4– H2O at T = 313.15 K, where the mass fractions of salts are (25.56 and 12.88) wt%, respectively. The phase diagram consists of one invariant point, two univariant curves and three crystallized regions. Point C is the invariant point of two co-saturated salts, corresponding to the coexistence of solids K2SO4 and KH2PO4 with the saturated solution. The composition of the corresponding equilibrated solution at invariant point C is w(KH2PO4) = 20.99 wt%, w(K2SO4) = 7.71 wt%. Along the solubility curve EC, connecting the points of the equilibrium liquid phase and wet solid phase and then extended, the point of intersection of these tie-lines represent the solid phase component for KH2PO4. The curve EC indicates that K2SO4 has been saturated in the water, where KH2PO4 has been precipitated. Similarly, along the solubility curve CF, connecting the points of the equilibrium liquid phase and wet solid phase and then extended, the point of intersection of these tie-lines represent the solid phase component for K2SO4. Thus, the curve CF indicates KH2PO4 has been saturated in the water, where K2SO4 has been precipitated. According to experimental results and Schreinemaker’s method, we can conclude that there are there crystallization fields and one unsaturated solution field. The three crystallization regions correspond to KH2PO4 (I), the mixed of (K2SO4 + KH2PO4) (II), K2SO4 (III). Field EUFC (IV) corresponds to unsaturated solution. Bel’teschev, F.V. [9] had reported phase equilibrium data at T = (273.15, 293.15, 303.15) K for the ternary system (K2SO4 + KH2PO4 + H2O) by the method of visual polythermic. Furthermore, and Pan Wang [10] had investigated this ternary system at T = (298.15 and 333.15) K by the method of isothermal solution saturation and moist residues. The quality of the measured data may be investigated by comparing it with literature values. The available solubility data in literature were plotted for temperatures between (293.15 and 333.15) K in figure 3(a). From the figure 3(a), we can see that there is significant consistency between our solubility data at T = 313.15 K and the data from reference [9] at (293.15, 303.15) K for the ternary system (K2SO4 + KH2PO4 + H2O), even if the solubilities measured in this work are systematically higher when compared with the values published by Pan Wang et al. Thus, it is possible to analyze the data reported in reference [9] with other solubility data for the ternary system. It is obvious to observe that invariant points, binary

18

W. Shen et al. / J. Chem. Thermodynamics 90 (2015) 15–23

TABLE 3 A comparison of density and refractive index data for saturated solutions of the binary systems between this work and literature at different temperatures. No.

T (K)

1 2 3 4 5 6 7 8 9 a

KCl–H2O

278.15 288.15 298.15 308.15 313.15a 318.15 323.15 333.15 348.15

K2SO4–H2O 3

KH2PO4–H2O 3

nD

q/(g  cm )

nD

q/(g  cm )

1.3646[34]

1.3683

1.1678[34] 1.1798[29] 1.2364[34] 1.1956

1.33885[33] 1.34065[33] 1.34240[33] 1.34392[33] 1.3473 1.34560[33]

1.3696[24]

1.2020[24]

1.3717[36]

1.3120[36]

q/(g  cm3)

nD

1.088[35]

1.0802 [10]

1.0999

1.3600

1.1847

1.110[35] 1.0587[10] 1.120[35]

1.1210 [10]

This work: Standard uncertainties u are u(nD) = 0.002, u(q) = 0.005 g  cm3.

TABLE 4 (Solid + liquid) equilibrium and physic-chemical properties of saturated solutions for the ternary system (K2SO4 + KH2PO4 + H2O) at T = 313.15 K and P = 88.35 kPa.a No.

1,E 2 3 4,C 5,C 6,C 7 8 9 10 11 12,F a b c d

Composition of liquid phase, 100w

Composition of wet residue, 100w

Physic-chemical properties of liquid phase

100w1b

100w1

100w2

pH

nD

q/g  cm3

ND 59.50 75.96 28.61 23.01 26.77 7.44 6.34 4.05 3.33 1.78 ND

4.14 3.90 3.20 4.22 4.21 4.22 3.87 3.93 3.95 3.45 4.20 6.65

1.3600 1.3632 1.3640 1.3642 1.3637 1.3638 1.3596 1.3575 1.3550 1.3530 1.3495 1.3473

1.1847 1.2041 1.2028 1.2137 1.2136 1.2132 1.1959 1.1564 1.1484 1.1348 1.1272 1.0999

0.00 2.36 4.86 7.63 7.71 7.79 8.60 9.38 10.12 10.49 11.28 12.88

100w2b

c

25.56 24.50 21.63 20.86 20.99 20.48 16.93 13.72 9.63 7.12 3.89 0.00

ND 1.70 1.17 46.21 39.81 46.55 55.31 54.67 51.85 58.84 54.64 ND

Equilibrium solid phase

P P P S+P S+P S+P S S S S S S

d

Standard uncertainties u are u(T) = 0.05 K, u(P) = 0.5 kPa, u(w1) = 0.002 (mass fraction), u(w2) = 0.004 (mass fraction), u(q) = 0.005 g  cm3, u(nD) = 0.002, u(pH) = 0.1. w1, mass fraction of K2SO4, w2, mass fraction of KH2PO4. ND, not determined. S, K2SO4; P, KH2PO4.

100

A

35 30

80

w(KH2PO4)

25 60

100w2 40

I

E

10

C III

5

IV 0

15

II

20

U0

20

F

20

40

60

80

W

100

100w1

0 0

2

4

6

8

10

12

14

w(K2SO4) FIGURE 2. Phase diagram for the ternary system (K2SO4 + KH2PO4 + H2O) at T = 313.15 K and P = 88.35 kPa d equilibrium liquid phase composition; h, moist solid phase composition; 100w1: K2SO4 wt%; 100w2: KH2PO4 wt%; U, H2O; A, pure solid of KH2PO4; W, pure solid of K2SO4; C, invariant point of KH2PO4 and K2SO4; E, solubility of KH2PO4 in water at T = 313.15 K; F, solubility of K2SO4 in water at T = 313.15 K.

endpoints and most solubilities data reported in reference [10] are systematically lower when compared with the values of Bel’teschev, F.V. et al. In figure 3(b), the solubility data of K2SO4–H2O measured in this work keep consistent with the values in reference [12,35]. And

FIGURE 3a. SLE data for the ternary system (K2SO4 + KH2PO4 + H2O) T = 293.15 K, s T = 303.15 K. reference [9]; at T = 298.15 K, at T = 333.15 K, reference .[10]; Exp at T = 313.15 K, this work.

solubilities data of K2SO4–H2O in reference [10] are systematically lower when compared with the values in reference [12,35]. We suspect that maybe it was caused by different apparatuses, different shaking time and different purity of raw materials. It seems that it is necessary to put a long time for the ternary system to achieve (solid + liquid) equilibrium. Maybe chemically pure

W. Shen et al. / J. Chem. Thermodynamics 90 (2015) 15–23

18 16

w(K2SO4)

14 12 10 8 6 270

280

290

300

310

320

330

340

350

T/K FIGURE 3b. SLE temperature for K2SO4–H2O reference [12]; j Exp.

reference [10];

reference [35];

K2SO4 and KH2PO4 should be recrystallized to increase the solubility of K2SO4. Thus, it may be normal for our data to be higher than the data reported in reference [10]. A comparison of the solubility values for the ternary system (K2SO4 + KH2PO4 + H2O) at T = (293.15 and 313.15) K was shown in figure 4. In figure 4, a similar tendency is obtained between the literature data and experimental data over the saturated solution for the ternary system (K2SO4 + KH2PO4 + H2O) at T = (293.15 and 313.15) K. From the figure 4, we can see that (1) the solubility of K2SO4 always decreases with KH2PO4 concentration increasing. The dissolution equilibrium of KH2PO4 and K2SO4 can be shown in equations (1) and (2).

K2 SO4 ðsÞ !2Kþ ðaqÞ þ SO2 4 ðaqÞ

ð1Þ

KH2 PO4 ðsÞ !Kþ ðaqÞ þ H2 PO4 ðaqÞ

ð2Þ

When solid KH2PO4 was added into the saturated K2SO4 solution and dissolved, the concentration of K+ increases. Because of the common ion effect, the equilibrium in equation (1) shifts to the left. That is to say, (1) KH2PO4 has an obviously inhibitive effect

100

A

80

60 100w2 40 C 20 M 0 U0

W 20

40 100w1

60

80

100

FIGURE 4. Phase diagram for the ternary system (K2SO4 + KH2PO4 + H2O) at T = (293.15 and 313.15) K. s, literary data at T = 293.15 K [9]; d, experiment data in this study at T = 313.15 K; 100w1: K2SO4 wt%; 100w2: KH2PO4 wt%; U, H2O; A, pure solid of KH2PO4; W, pure solid of K2SO4; C, invariant point of KH2PO4 and K2SO4 at T = 313.15 K; M, invariant point of KH2PO4 and K2SO4 at T = 293.15 K.

19

on the dissolution of K2SO4. (2) the invariant point moves upward from point M to C with temperature increasing from (293.15 to 313.15) K. (3) the fields of crystallization of K2SO4 and KH2PO4 increase a little with increasing temperature in the ternary system. Because the solubility of K2SO4 and KH2PO4 increase with increasing temperatures from (293.15 to 313.15) K. (4) the field of crystallization of K2SO4 in the ternary system is larger than that of KH2PO4 at T = (293.15 and 313.15) K. Mainly because K2SO4 has lower solubility, which shows that it is easy to saturate and crystallize from solution. Experimental results of physico-chemical properties (q, nD, pH) for the ternary system (K2SO4 + KH2PO4 + H2O) at T = 313.15 K were tabulated in table 4. Figures 5–7 show the relationship between the physico-chemical properties and the weight percentage values of KH2PO4 in the saturated solution for the ternary system. As we all know, the number of independent variables for saturated solution (in equilibrium with solid phase) is 3, such as pressure, temperature, and concentration of one of the salt. Since pressure and temperature are constant, we can easily plot density, refractive index and pH as a function of weight percentage of KH2PO4 for saturated solutions. Thus, our experimental solutions are saturated for the ternary system. It can be found that physico-chemical properties of this ternary system changed regularly with the content of KH2PO4. In this system, KH2PO4 has higher solubility and common ion effect on K2SO4. Therefore, the concentration of KH2PO4 is the main factor affecting the physico-chemical properties values of the solution. According to the following empirical equation about the density calculation of the mixed solution and the literature, we can find that the density has a positively correlated relationship with the composition [37,38]:

lnðd=d0 Þ ¼

X ðAi  W i Þ

ð3Þ

where d and d0 refer to the density values of the solution and pure water at T = 313.15 K, respectively; Ai is the constant for the ith component in the solution and can be obtained from the densities of the two boundary values in the ternary system with the mass fraction at T = 313.15 K; Wi is the salt of i in the solution in mass fraction. The d0 values of the pure water at T = 313.15 K is 0.99224 g  cm3. Values of the Ai constants of KH2PO4 and K2SO4 at T = 313.15 K are 0.0069358 and 0.007998, respectively. The density values of the solution were calculated by equation (3). All of the calculated results have a maximum relative error < 0.017, which is a good support for the quality of the density data measured in this work. Density data for (K2SO4 + H2O) can be found in previous studies [35]. A comparison of density data for (K2SO4 + H2O) saturated solution was shown in figure 5(a). As we can see from figure 5(a), there is a significant consistency between our density data at T = 313.15 K and the literature data reported in reference [35], even if the densities measured in reference [35] are higher when compared with the values published by Pan Wang et al. As we know from empirical equation (3), density has a positively relationship with the composition. According to figure 3(a), it is obvious to see that the solubility data measured in this work are systematically higher when compared with the values reported in reference [10]. Thus, we concluded that density data in this work may not keep consistent with the density values reported in reference [10]. Figures 5(b) shows the relationship between density and the weight percentage of KH2PO4 in the saturated solution. As shown in figure 5(b), the density increases with the increase of KH2PO4 concentration and then produces a decreasing trend beyond the invariant point C. The maximum value of the density of the

20

W. Shen et al. / J. Chem. Thermodynamics 90 (2015) 15–23

1.20

1.370

refractive index

density/(g.cm-3)

C

1.365

1.15

1.10

E

1.360

1.355

1.05

1.350

1.00 260

F 280

300

320

340

360

1.345

380

0

5

10

T/K

15

20

25

30

100w (KH2PO4)

FIGURE 5a. Temperature vs density for K2SO4–H2O saturated solutions [35]; reference [10]; j Exp.

reference FIGURE 6. Refractive index vs w(KH2PO4) in the saturated solutions of ternary system (K2SO4 + KH2PO4 + H2O) at T = 313.15 K and 88.35 kPa.

C

1.22

8.0 7.5

1.20 1.18

F

6.5 6.0

1.16

pH

density/(g.cm-3 )

7.0

E

1.14

5.5 5.0 C

4.5

1.12

E

4.0 3.5

1.10 F 0

3.0

5

10

15

20

25

30

2.5

100w(KH2 PO4 )

solution is 1.2137 g  cm3, where the KH2PO4 is 20.86 wt% at the invariant point C. According to the following empirical equation about the refractive index calculation of the mixed solution, the refractive index has a positively correlated relationship with the composition [39]:

X ðBi  W i Þ

5

10

15

20

25

30

100w (KH2PO4)

FIGURE 5b. Density vs w(KH2PO4) in the saturated solutions of ternary system (K2SO4 + KH2PO4 + H2O) at T = 313.15 K and 88.35 kPa.

lnðn=n0 Þ ¼

0

FIGURE 7. pH vs w(KH2PO4) in the saturated solutions of ternary system (K2SO4 + KH2PO4 + H2O) at T = 313.15 K and 88.35 kPa.

ion concentration increases with the dissociation of KH2PO4 in the system, which causes the system to become acidic. Thus, the pH values of this ternary system tended to decrease slowly with the increase of KH2PO4 concentration. When it reached at invariant point C, the pH value is 4.22.

ð4Þ

where n and n0 refer to the refractive index of the solution or the pure water at T = 313.15 K, respectively; Bi represents the constants of each possible component i in the system, and it can be obtained from the saturated solubility of the binary system at T = 313.15 K. Wi is the salt of i in the solution in mass fraction. Figures 6 shows the relationship between refractive index and the weight percentage of KH2PO4 in the saturated solution. As shown in figure 6, refractive index increases rapidly with an increase of the concentration of KH2PO4 and reaches a maximum value at the invariant point C. And then, it decreases slowly. The maximum value of the refractive index of the solution is 1.3642, where KH2PO4 concentration is 20.86 wt% at point C. Figures 7 shows the relationship between pH and the weight percentage of KH2PO4 in the saturated solution. According to figure 7, the pH values range from 6.65 to 3.90, mainly due to hydrogen

3.2. The ternary system (K2SO4 + KCl + H2O) The phase equilibrium experimental results of solubility and the relevant physico-chemical properties measured for the ternary system (K2SO4 + KCl + H2O) at T = 313.15 K were listed in table 5. All solution has achieved saturated. Data in table 5 show the solubility and physico-chemical properties value of saturated solutions for the ternary system (K2SO4 + KCl + H2O) at T = 313.15 K. The concentration values of the equilibrated solution and the wet residues were both expressed in weight percentage values. Composition points of the liquid phase and solid phase measured in the (K2SO4 + KCl + H2O) system at T = 313.15 K were connected into lines in figure 8. As can be seen from figure 8, points D, U and W represent the pure KCl, H2O and K2SO4, respectively. The phase diagram consists

21

W. Shen et al. / J. Chem. Thermodynamics 90 (2015) 15–23 TABLE 5 (Solid + liquid) equilibrium and physic-chemical properties of saturated solutions for the ternary system (K2SO4 + KCl + H2O) at T = 313.15 K and P = 88.35 kPa.a No.

1,G 2,S 3,S 4 5 6 7 8 9 10 11 12 13 14,H a b c d

Composition of liquid phase, 100w

Composition of wet residue, 100w

Physic-chemical properties of liquid phase

100w1b

100w1

100w3

pH

nD

q/g  cm3

c

ND 52.89 46.16 12.97 9.73 10.93 8.47 5.97 6.01 3.87 2.60 1.82 0.83 ND

5.22 4.82 4.67 5.74 5.71 5.47 5.95 6.37 6.64 6.65 6.77 6.79 6.81 6.65

1.3683 1.3706 1.3705 1.3702 1.3656 1.3637 1.3623 1.3574 1.3561 1.3555 1.3535 1.3501 1.3483 1.3473

1.1956 1.1946 1.1944 1.1828 1.1661 1.1583 1.1492 1.1273 1.1197 1.1139 1.1095 1.0999 1.0959 1.0999

100w3b

0.00 1.15 1.37 3.75 5.77 6.76 7.72 8.54 9.53 9.61 10.21 11.32 12.43 12.88

28.79 27.08 27.94 25.71 23.28 21.19 18.95 16.10 14.62 13.34 9.50 7.00 2.83 0.00

ND 6.73 9.51 40.71 39.58 33.61 46.46 54.64 57.37 64.26 60.09 60.56 51.44 ND

Equilibrium solid phase

Cl Cl + Sd Cl + S S S S S S S S S S S S

Standard uncertainties are u(T) = 0.05 K, u(P) = 0.5 kPa, u(w1) = 0.002 (mass fraction), u(w3) = 0.005 (mass fraction), u(q) = 0.005 g  cm3, u(nD) = 0.002, u(pH) = 0.1. w1, mass fraction of K2SO4 w3, mass fraction of KCl. ND, not determined. S, K2SO4; Cl, KCl.

D 100

100

I

D

90 80

80

70 60

60

II 100w3

100w3

50 40

40

30

G

S

20

III

20

10

IV 0

U0

S Z

W H

20

40

60

80

100

0 U0

20

40 100w1

60

80

W 100

100w1 FIGURE 8. Phase diagram for the ternary system K2SO4 + KCl + H2O at 313.15 K and P = 88.35 kPa d equilibrium liquid phase composition; h, moist solid phase composition; 100w1: K2SO4 wt%; 100w3: KCl wt%; U, H2O; D, pure solid of KCl; W, pure solid of K2SO4; S, invariant point of KCl and K2SO4; G, solubility of KCl in water at T = 313.15 K; H, solubility of K2SO4 in water at T = 313.15 K.

of one invariant point, two univariant curves and three crystallized regions. The point S is invariant point at T = 313.15 K, which shows the two pure solids KCl and K2SO4 saturated with equilibrium solution. The composition of the corresponding equilibrated solution at invariant point S is w(KCl) = 27.08 wt%, w(K2SO4) = 1.15 wt%. GS and SH are two univariant curves. As shown in figure 8, some lines can be extended to the point D, which represents the solid phase component for KCl. Other lines can also be extended to the point W, which represents the solid phase component for K2SO4. When these lines can be extended to the line DW, it indicates that the solid phase components consist of K2SO4 and KCl. The phase diagram consists of three crystallization fields: field DGS (I) corresponding to the salt KCl with saturated solution, field DSW (II) corresponding to the coexistence of the K2SO4 and KCl with saturated solution, field SHW (III) corresponding to the salt K2SO4 with saturated solution. Field GUHS (IV) corresponds to unsaturated solution. The phase equilibrium data for the ternary system (K2SO4 + KCl + H2O) at T = 303.15 K have been reported in the

FIGURE 9. Phase diagram for the ternary system (K2SO4 + KCl + H2O) at T = (303.15 and 313.15) K , literary data at T = 303.15 K [40]; d, experiment data at T = 313.15 K; 100w1: K2SO4 wt%; 100w3: KCl wt%; U, H2O; D, pure solid of KCl; W, pure solid of K2SO4; S, invariant point of KCl and K2SO4 at T = 313.15 K; Z, invariant point of KCl and K2SO4 at 303.15 K.

literature [40]. Figure 9 shows a comparison of the equilibrium phase diagrams for the ternary system (K2SO4 + KCl + H2O) at T = (303.15 and 313.15) K. Obviously, there are one invariant point, two univariant curves and three crystallization regions both in the phase diagram for this two temperatures. It can be seen from figure 9 that (1) the solubility of KCl decreases with the addition of K2SO4. Thus, an effect of the common ion effect existence was also observed in the (K2SO4 + KCl + H2O) system. (2) The field of crystallization of K2SO4 in the ternary system is larger than that of KCl at T = (303.15 and 313.15) K, and the field of crystallization of KCl is small. Field DSW (DZW) corresponding to the coexistence of the K2SO4 and KCl with saturated solution is bigger than other fields. (3) When the temperature increases from (303.15 to 313.15) K, the solubility of K2SO4 and KCl increases. (4) The crystalline regions of K2SO4 and KCl increase a little, and the mixed crystalline region of the K2SO4 and KCl decreases a little when the temperature increase from (303.15 to 313.15) K. According to Figs. 4 and 9, we can see that (1) The solubility of KH2PO4 in water changes greatly with temperature increasing from (293.15 to 313.15) K, which is from 17.80 wt% [9] to 25.56 wt%. (2)

W. Shen et al. / J. Chem. Thermodynamics 90 (2015) 15–23

G

1.20

K

1.18 density/(g.cm-3 )

the solubility of KCl in water changes a little with temperature increasing from (303.15 to 313.15) K, which is from 26.96 wt% [12] to 28.78 wt%. (3) The solubility of K2SO4 in water changes a little with temperature increasing from (303.15 to 313.15) K, which is from 11.50 wt% [12] to 12.88 wt%. (4) At T = 313.15 K, the solubility of salts in water is in the order of KCl > KH2PO4 > K2SO4. (5) In figure 4, the scope of areas of crystallization of salts is in the order of K2SO4 > KH2PO4. (6) In figure 9, the scope of areas of crystallization of salts is in the order of K2SO4 > KCl. According to the analysis, it indicates that K2SO4 can achieve better separation from their mixed solution. On the basis of data collected in table 5, relationships between the physico-chemical properties of the saturated solutions and the weight percentage values of KCl were shown in figures 10–12. According to the Gibbs phase rule for a ternary mixture, the number of independent variables for saturated solution (in equilibrium with solid phase) is 3. In this experiment, pressure and temperature are constant, only concentration of one of the salt is variable. Our experimental solutions are saturated for the ternary system. Thus, we can easily plot density, refractive index and pH as a function of weight percentage of KCl for saturated solutions. An effect of the common ion effect existence was observed in the K2SO4 + KCl + H2O system, and KCl has common ion effect on K2SO4. Therefore, the concentration of KCl is the main factor affecting the solution the physico-chemical properties values. As shown in figures 10–12, the physico-chemical properties values of the equilibrated solution are positively correlated with the concentration of KCl and changed regularly with the changing of the liquid phase concentration of KCl. Figure 10(a) is the density vs composition diagram of saturated solutions for the ternary system. Densities of saturated solutions increase with increasing concentration of KCl and reach the maximum at point G. The maximum value of the density for the system is 1.1956 g  cm3, where KCl concentration is 28.79 wt% at point G. Figure 10(b) is a comparison for the density data in the ternary system (K2SO4 + KCl + H2O) between 303.15 K and 313.15 K. According to figure 10(b), we can see that there is a significant consistency between our density data at 313.15 K and the literature data reported in reference [40]. The density data of the equilibrium solution change gradually with the increase of KCl concentration, and a singular value appears at the invariant point G and K, respectively. Thus, we concluded that density data in this work keep consistent with the density values reported in reference [40]. Figure 11 is the refractive index vs composition diagram of saturated solutions for the ternary system. Refractive index

1.16 1.14 1.12 1.10 1.08 0

5

10

15

20

25

30

100w(KCl) FIGURE 10b. Density data vs w(KCl) in the saturated solutions of the ternary system (K2SO4 + KCl + H2O) at T = (303.15 and 313.15) K at T = 313.15 K, this study; at T = 303.15 K, reference [40].

1.38

S 1.37

refractive index

22

G 1.36

1.35 H

1.34

0

5

10

15

20

25

30

100w(KCl) FIGURE 11. Refractive index vs w(KCl) in the saturated solutions of the ternary system (K2SO4 + KCl + H2O) at T = 313.15 K and 88.35 kPa.

7.0 H

1.20

S

6.5

G

1.18 1.16

pH

density/(g.cm-3)

6.0

5.5

1.14

G

1.12

5.0

H

S

1.10 4.5

1.08

0

5

10

15 20 100w(KCl)

25

30

FIGURE 10a. Density vs w(KCl) in the saturated solutions of the ternary system (K2SO4 + KCl + H2O) at T = 313.15 K and 88.35 kPa.

0

5

10

15

20

25

30

100w(KCl) FIGURE 12. pH vs w(KCl) in the saturated solutions of the ternary system (K2SO4 + KCl + H2O) at T = 313.15 K and 88.35 kPa.

W. Shen et al. / J. Chem. Thermodynamics 90 (2015) 15–23

increases with an increase of the concentration of KCl and reaches a maximum value at the invariant point S. And then, it decreased. The maximum value of the refractive index for the system is 1.3706, where KCl concentration is 27.08 wt% at point S. Figure 12 is the pH vs composition diagram of saturated solutions for the ternary system. From the figure 12, the pH values range from 4.82 to 6.81, mainly due to the KCl and K2SO4 in this system being strong acid and strong base salts. Hydrogen ion concentration is almost unchanged with the dissociation of these salts. We can find that the pH values of solution are decreasing with the increase of the KCl concentration and reached the minimum at the invariant point S. When KCl concentration is 27.94 wt% at point S, the minimum pH value of the solution is 4.67. 4. Conclusion The (solid + liquid) equilibrium experiments of the ternary systems (K2SO4 + KH2PO4 + H2O) and (K2SO4 + KCl + H2O) were investigated at T = 313.15 K. Solubilities and the physico-chemical properties (q, nD, pH) of the solution were obtained experimentally. On the basis of the experimental data, phase diagrams and the diagrams of physico-chemical properties vs composition for the two ternary systems were plotted. It indicates that there were all in one invariant point, two uninvariant curves, and three crystallization regions in the two ternary phase diagrams. The composition of the corresponding equilibrated solution is w(KH2PO4) = 20.99 wt%, w(K2SO4) = 7.71 wt% at invariant point C, which shows that the two pure solids KH2PO4 and K2SO4 saturated with equilibrium solution in the ternary system (K2SO4 + KH2PO4 + H2O) at T = 313.15 K. The composition of the corresponding equilibrated solution is w(KCl) = 27.08 wt%, w(K2SO4) = 1.15 wt% at invariant point S, which shows that the two pure solids KCl and K2SO4 saturated with equilibrium solution in the ternary system (K2SO4 + KCl + H2O) at T = 313.15 K. The crystalline field of K2SO4 is larger than that of KH2PO4 and KCl in the two ternary systems. Therefore, we can conclude that K2SO4 is easy to saturate and crystallize from the mixed solution. All results obtained in this experiment can be used for the solvent extraction process of KH2PO4 in the industrial production and further theoretical studies. Acknowledgements This research was financially supported by the ‘‘Western Light’’ talent cultivation program of Chinese Academy of Sciences (CAS), 2013.

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JCT 15-100