Phase Equilibrium in the Aqueous Ternary System KH2PO4

Jun 30, 2017 - Phase equilibrium in the system KH2PO4–NaH2PO4–H2O at 288.15 and 318.15 K was determined using methods of isothermal solution ...
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Phase Equilibrium in the Aqueous Ternary System KH2PO4− NaH2PO4−H2O at 288.15 and 318.15 K Yu-kun You, Jian-hua Tang,* Sheng-qiang Lin, and Si Shen Department of Chemical Engineering and Technology, Sichuan University, Chengdu, Sichuan 610065, People’s Republic of China ABSTRACT: Phase equilibrium in the system KH2PO4−NaH2PO4− H2O at 288.15 and 318.15 K was determined using methods of isothermal solution saturation and moist residues. The equilibrium phase solid was also verified by X-ray diffraction (XRD). Furthermore, the crystallization areas in the phase diagram were analyzed and discussed. In the phase diagram of the two temperatures, there is one invariant point, two single saturated liquid curves, and two crystallization fields corresponding to the single salts NaH2PO4·2H2O, KH2PO4 at 288.15K and NaH2PO4·H2O, KH2PO4 at 318.15K. The density of saturated solutions for the systems studied at 288.15 and 318.15 K was measured and calculated to test the precision of the chosen model. This research filled the blank of phase diagram in this ternary system and can be applied to the production of potassium dihydrogen phosphate process to optimize the crystallization and separation process of potassium dihydrogen phosphate.

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INTRODUCTION Potassium dihydrogen phosphate (KDP) is an important industrial material and compound fertilizer that is widely used in industrial and agricultural fields.1 Many processes for the production of KH2PO4 have been reported, such as the neutralization technology, direct chemical conversion method, crystallization method, ion exchange method, and extraction technology.2 The technology of producing KDP through neutral reaction of potassium carbonate with phosphoric acid is simple and easy to put into production.3 But the basic raw materials are insufficient and expensive for big production of KDP through the neutral reaction. KDP can be also manufactured by the reaction of potassium chloride with phosphoric acid, which is much cheaper in raw materials.4 But the problems such as environmental pollution and pipeline corrosion have appeared when the process produces hydrogen chloride. So an environmentally friendly route is that phosphoric acid reacts with sodium carbonate to form sodium dihydrogen phosphate, then sodium dihydrogen phosphate reacts with potassium chloride to form potassium dihydrogen phosphate and sodium chloride. 5,6 The key step is the double decomposition reaction of producing KDP from KCl and NaH2PO4. After the reaction, a crucial step is to separate KDP from the KH2PO4−NaH2PO4−KCl−NaCl−H2O system.The research for this system provides the basic thermodynamic data for the crystallization of KDP. Solubility data in the system KH2PO4 + NaH2PO4 + H2O at 303.15 K has been studied in our lab.7 Also, a small amount of solubility data of this system was investigated about 50 years ago.8 However, the data provided is insufficient to solve the practical separation above, and an extensive study at other temperatures needs to be done. The ternary system KH2PO4−NaH2PO4−H2O in stable © XXXX American Chemical Society

equilibrium at 288.15 and 318.15 K are chosen as the further research direction. In this study, the solubility of equilibrium state and physicochemical property data of the ternary system are determined and the phase diagram and the diagram of densities versus composition are plotted, which could help to fill in the blanks of data of this research aspect.

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METHODOLOGY Apparatus and Materials. The purity of all reagents is shown in Table 1. An HZS-88 type constant temperature water bath oscillator with the standard uncertainty of 0.3 K is employed for phase equilibrium measurement and made in Donglian Electronic & Technology Development Co. Ltd., Beijing, China. The Philips X Pert Pro MPD X-ray diffraction (XRD) analyzer is employed for XRD characterizations. Experimental Method. The method of isothermal solution saturation9,10 is employed to determine the solubility of the ternary system. The famous Schreinemaker’s method of moist residues11−13 is applied to analyze the equilibrium solid phase component in the experiments indirectly, and the solid phase is also tested by XRD to verify the crystalloid composition. In a pre-experiment, the liquid phase of the samples is analyzed at every 2 h and it is shown that the phase equilibrium is reached in 10 h. According to a certain proportion and making sure that one of the salts is excessive, the experimental components are added into a series of conical flasks (250 mL) gradually, and the sealed flask is placed into the oscillator. The Received: January 8, 2017 Accepted: June 20, 2017

A

DOI: 10.1021/acs.jced.7b00020 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Table 1. Provenance and Purity of the Material Useda

a

reagent

source

mass fraction purity

CASRN

KH2PO4 NaH2PO4 H2O

Tianjin Bodi Chemicals Chengdu Kelong Chemicals Chengdu Kelong Chemicals

≥0.995 ≥0.995 electrical conductivity ≤10−4 S·m−1

7778-77-0 7558-80-7 7732-18-5

The sample purities were stated by the suppliers and no purification was applied to the chemicals.

5 and 6. The ion concentration values in this equilibrium system are measured in mass fraction, and ρ is the density for the equilibrated solution, which unit is g·cm−3. In Figures 1−, A, B, C, D, and W denote solid KH2PO4, NaH 2PO 4, NaH2 PO4·2H2O, NaH2 PO4·H2O, and H2O, respectively. Point F, an invariant point, reflects the cosaturated solution of KH2PO4 and NaH2PO4·2H2O at 288.15 K. N represents the solubility of KH2PO4, and E represents the solubility of NaH2PO4 in water at 288.15 K. The saturated liquid line EFN consists of two branches. Branch EF corresponds to the saturated NaH2PO4 solution and visualizes changes of the NaH2PO4 concentration with increasing the KH2PO4 concentration. Branch FN corresponds to the saturated KH2PO4 solution and indicates changes of the KH2PO4 concentration with increasing the NaH2PO4 concentration in the equilibrium solution. As indicated in Figures 1 and 2, along the curve EF, we connect the composition points of wet residue phase with liquid phase and then extend, the intersection of these straight lines is approximately the equilibrium solid phase for NaH2PO4·2H2O. The same method is utilized to analyze the equilibrium solid phase component of FN, and the intersection is KH2PO4. Similarly, the equilibrium solid phases of A and D at 318.15 K are KH2PO4 and NaH2PO4·H2O, respectively. As indicated in Figures 5 and 6, the equilibrium solid phase of F and T are analyzed by XRD. In Figure 5, all the main peaks are consistent with the KH2PO4 and NaH2PO4·2H2O standard data, and the equilibrium solid phase of the invariant point T is verified to be coexistence of KH2PO4 and NaH2PO4·2H2O. In Figure 6, all the main peaks are consistent with the KH2PO4 and NaH2PO4·H2O standard data, and the equilibrium solid phase of the invariant point T is verified to be coexistence of KH2PO4 and NaH2PO4·H2O. The solid phases of A, C, and D are certified to be KH2PO4, NaH2PO4·2H2O, and NaH2PO4· H2O, respectively. Consequently, the ternary system pertains to a simple eutectic type and does not form complex salt and solid solution at the investigated temperature. Figure 1 shows that WEFN denotes unsaturated area at 288.15 K. AFN denotes crystallization region of KH2PO4, whereas CFE denotes crystalline region of NaH2PO4·2H2O. Zone AFC denotes the mixed crystalline region of KH2PO4 + NaH2PO4·2H2O. Figures 3 and 4 indicate the relationship between the mass fraction of NaH2PO4 in the solution and the density at 288.15 and 318.15 K. With an increase of the concentration of NaH2PO4, the density has the tendency to increase, and then the density declines slightly afterward. At the cosaturated point F (T), the density reaches a maximum value. A comparison between the phase equilibrium for NaH2PO4− KH2PO4−H2O at 288.15, 303.15, and 318.15 K is presented in Figure 7. The diagram further reveals that the temperature can affect the phase equilibrium. With the temperature increasing from 288.15 to 318.15 K, it is discovered that (1) the crystalline region of KH2PO4 is much larger than crystallization region of NaH2PO4·2H2O (NaH2PO4·H2O) at both temperatures, and

oscillator vibrates continuously at the two specific temperatures: 288.15 and 318.15 K (the standard uncertainty of 0.3 K). After equilibration, the oscillation is stopped, and the system is allowed to stand for 2 h to make sure that all the suspended crystals settle. The wet residues and liquid phase are transferred to a 250 mL volumetric flask, respectively. Meanwhile, some other liquid phases are used to measure density individually. Finally, these samples are quantitatively analyzed by chemical methods. More details of the experimental method and the procedure of the transfer, collection, and preparation of samples are described in our previous papers.7,14,15 Analysis. The P2O5 concentration is determined by the quinoline phosphomolybdate gravimetric method,16 and the average deviation of the determination is less than 0.01. The K2O concentration is determined by means of the sodium tetraphenylborate gravimetric method,17 and the mean relative deviation of the determination is less than 0.01. The concentration of sodium dihydrogen phosphate is determined by subtraction method. The density is measured with Ostwald− Sprenge type pycnometers with a bulb volume of 1 mL, and the double deionized water (electrical conductivity ≤10−4 S·m−1) is used for pycnometer calibration; the absolute uncertainties in the density measurements are estimated to be within 0.001. Each experimental result is achieved from the average value of three parallel measurements. The equilibrium solid phase is verified by X-ray diffraction (XRD).

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RESULTS AND DISCUSSION In Table 2, the experimental data are compared with literature data to determine the relative error,18,19 and it is discovered Table 2. Solubility for NaH2PO4 or KH2PO4 in Pure Water at 288.15 and 318.15 K salt

experimental data, 100wa

KH2PO4 NaH2PO4

16.86 43.60

KH2PO4 NaH2PO4

26.85 59.92

literature data, 100w

relative errorb

16.8719 43.4218

0.0006 −0.0041

26.9019 59.7118

0.0019 −0.0035

288.15 K

318.15 K

w, mass fraction. bRelative error = (literature data − experimental data)/ literature data.

a

that the experimental results fit well with the literature results, which demonstrates that the experimental devices and methods are feasible. The phase equilibrium experimental data is summarized in Tables 3. On the basis of the data listed in Table 3, the phase diagram at 288.15 and 318.15 K are plotted in Figures 1 and 2, respectively. The data of density versus composition at 288.15 and 318.15 K are drawn in Figures 3 and 4, respectively. The Xray diffraction pattern of the invariant point is given in Figures B

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Table 3. Mass Fraction Solubility of the Ternary KH2PO4 + NaH2PO4 + H2O System at Temperature T = 288.15 and 318.15 K and Pressure p = 0.1 MPaa composition of liquid phase, 100w no

100w1

1E 2 3 4 5F 6 7 8 9 10 11 12 13 14 15 16N 1H 2 3 4 5 6 7 8T 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24K

b

composition of wet residue phase, 100w

100w2

100w1

100w2

0 3.11 5.59 8.19 8.67 8.73 8.77 9.26 9.95 10.33 11.25 11.93 13.06 13.97 15.23 16.86

43.60 43.24 41.87 41.03 40.68 40.01 38.67 35.07 31.89 29.29 25.63 20.85 16.34 11.91 6.26 0

NDd 1.35 3.05 4.8 16.07 53.68 56.55 58.73 59.84 61.57 63.26 64.82 67.35 70.41 72.5 ND

ND 61.33 59.77 56.56 50.14 21.07 18.47 15.9 14.2 12.51 10.55 8.2 6.12 4.1 1.98 ND

0 2.04 4.42 6.06 7.37 8.06 9.80 10.00 10.81 11.36 12.11 12.69 13.52 14.71 15.17 15.63 16.4 17.86 19.29 20.75 22.06 23.12 24.24 26.85

59.92 59.43 59.01 58.75 57.43 56.71 56.06 56.13 52.04 50.53 48.57 45.56 42.86 38.67 37.64 36.07 33.85 28.42 24.19 20.2 15.68 10.69 6.51 0

ND 1.46 3.42 5.03 6.17 7.08 8.59 11.95 22.75 21.33 23.17 28.69 31.56 38.64 41.41 44.73 48.84 53.35 56.24 60.17 64.59 68.78 70.12 ND

ND 66.63 64.68 63.08 61.84 60.4 58.89 57.01 45.18 45.09 42.45 37.41 34.26 28.18 26.03 24.01 20.57 15.85 13.17 10.33 7.15 4.54 2.81 ND

densities ρ/(g·cm−3) equibrium solid phase 288.15 K NaH2PO4·2H2O NaH2PO4·2H2O NaH2PO4·2H2O NaH2PO4·2H2O NaH2PO4·2H2O + KH2PO4 KH2PO4 KH2PO4 KH2PO4 KH2PO4 KH2PO4 KH2PO4 KH2PO4 KH2PO4 KH2PO4 KH2PO4 KH2PO4 318.15 K NaH2PO4·H2O NaH2PO4·H2O NaH2PO4·H2O NaH2PO4·H2O NaH2PO4·H2O NaH2PO4·H2O NaH2PO4·H2O NaH2PO4·H2O + KH2PO4 NaH2PO4·H2O KH2PO4 KH2PO4 KH2PO4 KH2PO4 KH2PO4 KH2PO4 KH2PO4 KH2PO4 KH2PO4 KH2PO4 KH2PO4 KH2PO4 KH2PO4 KH2PO4 KH2PO4

exp. value

calcd. value

relative errorc

1.3884 1.4222 1.4282 1.4405 1.4530 1.4345 1.4287 1.3940 1.3678 1.3439 1.3141 1.2732 1.2375 1.2034 1.1647 1.1264

1.3884 1.4156 1.4260 1.4434 1.4445 1.4379 1.4238 1.3905 1.3642 1.3413 1.3133 1.2729 1.2402 1.2073 1.1673 1.1264

0.0000 −0.0047 −0.0016 0.0020 −0.0059 0.0024 −0.0035 −0.0025 −0.0027 −0.0019 −0.0006 −0.0002 0.0022 0.0032 0.0022 0.0000

1.5733 1.6056 1.6204 1.6276 1.6355 1.6271 1.6377 1.6426 1.6005 1.5932 1.5747 1.5481 1.5273 1.4850 1.4772 1.4668 1.4493 1.4118 1.3724 1.3455 1.3177 1.2725 1.2450 1.2046

1.5733 1.6057 1.6135 1.6296 1.6286 1.6277 1.6403 1.6436 1.6019 1.5897 1.5744 1.5447 1.5220 1.4864 1.4795 1.4666 1.4498 1.4051 1.3742 1.3468 1.3130 1.2732 1.2428 1.2046

0.0000 0.0001 −0.0043 0.0012 −0.0042 0.0004 0.0016 0.0006 0.0009 −0.0022 −0.0002 −0.0022 −0.0034 0.0010 0.0016 −0.0001 0.0003 −0.0048 0.0013 0.0010 −0.0036 0.0006 −0.0018 0.0000

a Standard uncertainties u(T) = 0.3 K, ur(p) = 0.05, ur(w1) = 0.01, ur(w2) = 0.01, and ur(ρ) = 0.001. bw1, mass fraction of KH2PO4; w2, mass fraction of NaH2PO4. cRelative error = (calcd. value − exp. value)/calcd. value. dND, not determined. E, F, N, H, T, and K have the same meaning as described in Figures 1 and 2.

the unsaturated area expands apparently. (2) The invariant point shifts upward from F to T, which illustrates that the salting out effect of NaH2PO4 to KH2PO4 increases more significantly. (3) With the temperature rising, the crystallization region of NaH2PO4·2H2O at 288.15 and 303.15 K transforms into NaH2PO4·H2O at 318.15 K. On the basis of the following empirical equation of density in electrolyte solutions developed in the previous study, the density of the solution is calculated.20 The experimental data are compared with the calculated data to determine the relative error. All of the data mentioned above are listed in Table 3.

ln

d = d0

∑ Ai . wi

where d0 = 0.999099 g·cm−3 and d0 = 0.990208 g·cm−3, the density of the pure water at 288.15 and 318.15 K;21 Ai is the constant of each possible component i in the system. wi is the salt of i in the solution in mass fraction. Constants Ai of NaH 2 PO 4 and KH 2 PO 4 for calculation are 0.007547 (0.007727) and 0.007113 (0.007299) at 288.15 (318.15) K, respectively. C

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Figure 3. Density versus composition at 288.15 K. E, N, and F have the same meaning as described in Figure 1.

Figure 1. Equilibrium phase diagram of the ternary system KH2PO4 + NaH2PO4 + H2O at 288.15 K. ■, Equilibrium liquid phase composition; ●, moist solid phase composition; A, pure solid of KH2PO4; B, pure solid of NaH2PO4; W, water; C, pure solid of NaH2PO4·2H2O; E, solubility of NaH2PO4 in water; N, solubility of KH2PO4 in water; F, cosaturated point of KH2PO4 and NaH2PO4· 2H2O.

Figure 4. Density versus composition at 318.15 K. K, T and H have the same meaning as described in Figures 2.

Figure 2. Equilibrium phase diagram of the ternary system KH2PO4 + NaH2PO4 + H2O at 318.15 K. ■, Equilibrium liquid phase composition; ●, moist solid phase composition; A, pure solid of KH2PO4; B, pure solid of NaH2PO4; W, water; D, pure solid of NaH2PO4·H2O; H, solubility of NaH2PO4 in water; K, solubility of KH2PO4 in water; T, cosaturated point of KH2PO4 and NaH2PO4· H2O.

The experimental result is contrasted with the calculated result to determine the relative error. The related data are in Table 2. Test of the model precision uses the average relative error criterion, and the average relative error of the empirical equation is 0.19%, which demonstrates that the calculation method is feasible.

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CONCLUSIONS The phase equilibrium of KH2PO4−NaH2PO4−H2O at 288.15 and 318.15 K was investigated. The solubility and density were obtained. On the basis of the solubility data, the phase diagram

Figure 5. X-ray diffraction pattern of the invariant point F.

D

DOI: 10.1021/acs.jced.7b00020 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].. ORCID

Jian-hua Tang: 0000-0003-0018-4084 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work described in this paper was fully supported by a grant from the National Natural Science Foundation of China (No. 21476151).

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REFERENCES

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Figure 6. X-ray diffraction pattern of the invariant point T.

Figure 7. Solubility isotherms of the ternary system NaH2PO4 + KH2PO4 + H2O at 288.15, 303.15, and 318.15 K. ■, 288.15 K; △, 303.15 K;7 ●, 318.15 K; A, B, C, D, E, F, N, H, T, K, and W have the same meaning as described in Figures 1 and 2

was plotted; the solid phase that is in equilibrium with the solution was analyzed, and the crystallization areas were determined. The crystalline region of KH2PO4 was much larger than that of NaH2PO4·H2O (NaH2PO4·2H2O) at the investigation temperature (at 288.15 and 318.15 K). There were in all two crystallization regions, one invariant point, and two univariant curves in the phase diagrams. The empirical equation had a high precision to calculate the density of the saturated solution. With the temperature increasing from 288.15 to 318.15 K, the crystallization region of NaH2PO4· 2H2O transformed into NaH2PO4·H2O. NaH2PO4 had a strong salting-out effect on KH2PO4, and the salting-out effect was stronger at higher temperatures. All results can offer fundamental data support for the separation processes of KH2PO4 in the industrial production and further theoretical researches. E

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(18) Eysseltová, J. Sodium dihydrogenphosphate. Alkali Metal Orthophosphates 1988, 31, 43−45. (19) Eysseltová, J. Potassium dihydrogenphosphate. Alkali Metal Orthophosphates 1988, 31, 213−219. (20) Lin, L. J.; Fang, C. H.; Fang, Y.; Qin, X. F. A new model for prediction density of electrolyte solutions. J. Salt Lake Res. 2006, 14, 56−61. (21) Yang, W.; He, R. X. Physical and Chemical Testing II; Science Press: Beijing, China, 2014.

F

DOI: 10.1021/acs.jced.7b00020 J. Chem. Eng. Data XXXX, XXX, XXX−XXX