Phase Equilibrium in the Aqueous Ternary System KH2PO4+ KCl +

May 19, 2015 - In this study, the solubility and density of a ternary system (KH2PO4 + KCl + H2O) at (288.15 and 303.15) K were determined, using isot...
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Phase Equilibrium in the Aqueous Ternary System KH2PO4+ KCl +H2O at (288.15 and 303.15) K Jun Long, Jian-hua Tang,* Yu-kun You, Lian-mei Guo, and Ke Chen Department of Chemical Engineering and Technology, Sichuan University, Chengdu, Sichuan 610065, People’s Republic of China ABSTRACT: In this study, the solubility and density of a ternary system (KH2PO4 + KCl + H2O) at (288.15 and 303.15) K were determined, using isothermal solution saturation and wet residues. The equilibrium solids were detected by the Schreinemaker’s method of wet residues and verified by X-ray diffraction (XRD). According to the experimental data, the diagram of density versus composition and the phase diagram were plotted. The crystallization regions of KH2PO4 and KCl were determined, and the crystallization region of KH2PO4 was much larger than that of KCl. The density of the saturated solution was calculated, and the model precision was tested. All results can offer fundamental basis for crystallization and separation processes in chemical industry.

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INTRODUCTION

METHODOLOGY Materials. Potassium dihydrogen phosphate (KH2PO4, ≥ 0.990 mass fraction) was purchased from Tianjin Bodi Chemical Holding Co. Ltd., China. Potassium chloride (KCl, ≥ 0.990 mass fraction) used in the work was purchased from Chengdu Kelong Chemical Reagent Co. Ltd., China. Doubly deionized water (electrical conductivity ≤ 1 × 10−4 S·m−1) was used in the work. Apparatus. A SHZ-88 type constant temperature water bath oscillator with a precision of 0.3 K was employed for phase equilibrium measurement. The oscillator was made in Jintan Medical Instrument Corporation, Jiangsu, China. The Philips X Pert Pro MPD X-ray diffraction (XRD) analyzer was employed for XRD characterizations. Experimental Methods. The method of isothermal solution saturation13−15 was employed to determine the solubility of the ternary system. The famous Schreinemaker’s method of moist residues16,17 was employed to analyze the composition of solid phase in this experiment indirectly. Because the traditional Schreinemaker’s method may have errors in some cases, the solid phase was also tested by XRD to verify the crystalloid composition. In this process, it turned out that the equilibrium time was 10 h. The oscillation was stopped and the system was allowed to stand for 2 h to make sure that all the suspended crystals settled. After equilibrium, the liquid phase and wet residues were transferred to a 250 mL volumetric flask, respectively. More details of the experimental method and the procedure of the preparation, collection, and transfer of samples were described in our previous papers.15

Potassium dihydrogen phosphate, an important chemical material used as the efficient compound fertilizer, provides both potassium and phosphorus. In agriculture, KH2PO4 does not contain chloride and is used for a variety of soils and crops. Moreover, potassium dihydrogen phosphate is utilized for flavoring additives, the in preparation of buffer solution and microbiological culture media, and also in the industry of medicine.1 Potassium chloride, as a basic raw material, is widely used in the manufacturing of various potassium salt or alkali. Potassium dihydrogen phosphate can be manufactured by reaction of potassium chloride with phosphoric acid or phosphate.2,3 However, the potassium dihydrogen phosphate solution produced often contains many impurities, like chloride ions.4,5 The crystallization process is an important step in producing KH2PO4.6 The purity,7 crystal shape,8,9 and growth rate10−12 of KH2PO4 crystal were obviously affected. Therefore, to get pure KH2PO4, the concentration of the Cl− and P2O5 must be controlled. From the phase equilibrium, the appropriate concentration of the Cl− and P2O5 in the solution can be attained for the crystallization process. The phase equilibrium of the KH2PO4 + KCl + H2O system is very important in crystallization and separation processes. However, the complete phase equilibrium data and phase diagram of KH2PO4 + KCl + H2O system at (288.15 and 303.15) K have not been reported yet. In this work, we determined phase equilibrium data of the ternary system at (288.15 and 303.15) K and we plotted the phase diagram and diagram of density versus composition. These can help to crystallize and separate the mixed solution. Meanwhile, it can provide fundamental data support for chemical industry development. © XXXX American Chemical Society

Received: February 13, 2015 Accepted: May 7, 2015

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DOI: 10.1021/acs.jced.5b00141 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Table 1. Mass Fraction Solubility of the Ternary KH2PO4 + KCl + H2O System at Temperature = (303.15 and 288.15) K and Pressure = 0.1 MPaa composition of liquid phase, 100w

composition of wet residue phase, 100w

no.

100w1b

100w2

100w1

100w2

1, S 2 3 4 5, C 6, C 7, C 8 9 10 11 12 13 14 15 16 17 18 19, R

27.20 26.39 25.80 25.55 25.35 25.35 25.35 22.61 20.40 18.70 15.96 15.00 12.28 9.55 7.51 5.24 3.45 2.04 0.00

0.00 1.19 2.59 3.37 3.66 3.66 3.66 4.53 5.45 6.12 7.29 7.82 9.38 11.37 12.94 15.45 17.22 18.45 21.55

NDd 65.10 61.20 59.75 44.58 20.55 13.06 6.16 5.80 5.02 4.49 3.93 3.24 2.44 2.13 1.42 1.11 0.71 ND

ND 0.66 1.37 1.91 32.92 57.10 65.84 73.92 72.96 74.97 74.48 75.53 76.23 77.81 76.10 76.67 75.90 74.48 ND

1, E 2 3 4 5 6 7 8 9 10 11 12, N 13, N 14, N 15 16, F

0.00 2.21 4.12 6.27 8.31 10.30 11.95 14.05 17.50 20.63 21.72 23.41 23.41 23.41 24.06 24.84

16.45 14.28 12.19 10.60 9.09 8.03 6.90 6.19 4.70 3.82 3.47 3.11 3.11 3.11 1.78 0.00

ND 0.82 1.44 2.07 2.63 2.76 3.20 3.64 4.26 5.08 5.52 15.74 42.63 69.34 62.95 ND

ND 69.67 70.01 71.10 71.44 75.48 75.32 76.07 76.75 76.33 75.74 60.32 47.09 11.37 1.01 ND

densities, ρ/(g·cm−3) equibrium solid phase 303.15 K KCl KCl KCl KCl KCl + KH2PO4 KCl + KH2PO4 KCl + KH2PO4 KH2PO4 KH2PO4 KH2PO4 KH2PO4 KH2PO4 KH2PO4 KH2PO4 KH2PO4 KH2PO4 KH2PO4 KH2PO4 KH2PO4 288.15 K KH2PO4 KH2PO4 KH2PO4 KH2PO4 KH2PO4 KH2PO4 KH2PO4 KH2PO4 KH2PO4 KH2PO4 KH2PO4 KCl + KH2PO4 KCl + KH2PO4 KCl + KH2PO4 KCl KCl

exp value

calcd value

relative errorc

1.1831 1.1887 1.1959 1.1995 1.2028 1.2026 1.1994 1.1862 1.1761 1.1689 1.1603 1.1569 1.1514 1.1478 1.1468 1.1474 1.1486 1.1509 1.1588

1.1831 1.1870 1.1942 1.1989 1.1999 1.1999 1.1999 1.1864 1.1775 1.1704 1.1598 1.1570 1.1498 1.1460 1.1439 1.1476 1.1489 1.1486 1.1588

0.0000 −0.0015 −0.0014 −0.0005 −0.0025 −0.0023 0.0004 0.0002 0.0012 0.0013 −0.0005 0.0001 −0.0014 −0.0016 −0.0026 0.0002 0.0003 −0.0020 0.0000

1.1223 1.1205 1.1180 1.1196 1.1221 1.1262 1.1311 1.1382 1.1530 1.1686 1.1766 1.1819 1.1847 1.1839 1.1774 1.1687

1.1223 1.1208 1.1177 1.1203 1.1228 1.1286 1.1312 1.1406 1.1535 1.1692 1.1744 1.1839 1.1839 1.1839 1.1777 1.1687

0.0000 0.0003 −0.0003 0.0006 0.0006 0.0021 0.0001 0.0021 0.0004 0.0005 −0.0019 0.0017 −0.0007 0.0000 0.0003 0.0000

a Standard uncertainties u(T) = 0.3 K, u(p) = 0.05 kPa, u(w2) = 0.01, u(w1) = 0.02, u(ρ) = 0.01 g·mL−1. bw1, mass fraction of KCl; w2, mass fraction of KH2PO4. cRelative error = (calcd value − exp value)/calcd value. dND, not determined. S, C, R, E, N, and F have the same meaning as described in Figure 2.

Analysis. The P2O5 concentration was analyzed by the quinoline phosphomolybdate gravimetric method,18 and the average relative deviation of the determination was less than ± 0.2 %. The chloride was measured by Volhard method,19,20 and the average relative deviation of the determination was less than ± 0.5 %. The density was calculated by a 5 mL gravity bottle, and the absolute uncertainties in the density measurements was estimated to be within 0.01 g·mL−1. Each experimental result was achieved from the average value of three parallel determinations. The equilibrium solid phase was verified by XRD characterizations.

1, the liquid phase diagram of the ternary system is presented in Figure 1, the ternary phase diagram is shown in Figure 2, and the diagram of density versus composition is given in Figure 3. In Figure 1, the experimental data is compared with literature data21 and it is found that the experimental data is in good agreement with the literature values, which demonstrates that experimental methods and devices are feasible in this study. As indicated in Figure 2, A, B, and W represent solid KH2PO4, solid KCl, and H2O, respectively. Point C (N), an invariant point at 303.15 K (288.15 K), reflects the cosaturated solution of KH2PO4 and KCl. Points R (E) and S (F) show the solubility of different single salts. R (E) represents the solubility of KH2PO4 in water at 303.15 K (288.15 K). S (F) represents the single salt mass fraction (100w) of KCl that saturated in water 303.15 K (288.15 K). The saturated liquid line RCS consists of two sections. The curve between points R and C

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RESULTS AND DISCUSSION The phase equilibrium experimental data is shown in Table 1. The ion concentration values in this equilibrium system are measured in mass fraction. According to the data listed in Table B

DOI: 10.1021/acs.jced.5b00141 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Figure 1. Equilibrium liquid phase diagram of the ternary system KH2PO4 + KCl + H2O at 288.15 K: ○, literary data at 288.2 K;21 ■, experiment data at 288.15 K; A, pure solid of KH2PO4; B, pure solid of KCl; W, water; E, solubility of KH2PO4 in water; F, solubility of KCl in water; N, cosaturated point of KH2PO4 + KCl.

Figure 3. Density vs composition: ▲, 303.15 K; ■, 288.15 K. S, C, R, E, N, and F have the same meaning as described in Figure 2.

As shown in Figure 2, alongside the curve RC, we connect the composition points of wet residue phase with liquid phase and then extend it to Y axis; the point of intersection is approximately the solid phase component for the KH2PO4. The same method is applied to determine the equilibrium solid phase component of CS, and the result turns out to be KCl. With the help of XRD, the solid phases of A and B are certified to be KH2PO4 and KCl, respectively. The system belongs to a simple eutectic type and neither double salt nor solid solution is formed at the investigation temperature. As shown in Figure 4, the equilibrium solid phase of the invariant point N is detected by XRD and verified to be coexistence of KH2PO4 and KCl. Figure 2 shows that the area of WRS and WEF represents unsaturated region at (303.15 and 288.15) K, respectively; ARC and AEN represents crystallization region of KH2PO4, whereas CSB and NFB stands for crystallization region of KCl. Area ACB and ANB represents the mixed crystallization region of KH2PO4 + KCl. This diagram further illustrates that the

Figure 2. Equilibrium phase diagram of the ternary system KH2PO4 + KCl + H2O at (303.15 and 288.15) K: ▲, 288.15 K; ●, 303.15 K; A, pure solid of KH2PO4; B, pure solid of KCl; W, water; R (E), solubility of KH2PO4 in water; S (F), solubility of KCl in water; C (N), cosaturated point of KH2PO4 + KCl.

indicates that KCl has been saturated in the water, whereas KH2PO4 has been precipitated by salting out. Additionally, the curve between points C and S presents the saturation process of KH2PO4, whereas KCl has been precipitated by salting out. The saturated liquid line ENF is similar. Figure 1 indicates that the solubility of KH 2 PO 4 decrease sharply with the concentration of KCl increasing in the solution. That is, KCl has a strong salting-out effect on KH2PO4. The solubility of KCl decline slightly with an increase of the concentration of KH2PO4 and KH2PO4 has a weak salting-out effect on KCl.

Figure 4. X-ray diffraction pattern of the invariant point N. C

DOI: 10.1021/acs.jced.5b00141 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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temperature can influence the equilibrium of the ternary system. Increasing the temperature from (288.15 to 303.15) K, the unsaturated region becomes larger apparently, and the invariant point moves right from point N to C, which illustrates that salting out effect of potassium chloride to potassium dihydrogen phosphate does not change significantly with temperature (from 288.15 K to 303.15 K). Both at (288.15 and 303.15) K, it is obvious that the crystalline region of KH2PO4 is much larger than that of KCl. Figure 3 indicates the relationship between the mass fraction of KH2PO4 and the density. With an increase of the concentration of KH2PO4, the density has the tendency to increase. But at the cosaturated point, which is represented by C (N), the density reaches its maximum value and then decreases afterward. At the point T (T′), the density reaches a minimum value and increases afterward. According to the following empirical equation of density in electrolyte solutions developed in the previous study,22 we calculated the density of solution

ln

ds = d0

REFERENCES

(1) Fan, L.; Zhao, J. G. The current status and prospect of potassium dihydrogen phosphate in China. Phosphate Compd. Fert. 2006, 21 (3), 34−37. (2) Yang, L.; Zhang, Z. Y.; Chen, Z. Y. Summary of processes for production of potassium dihydrogen phosphate. Phosphate Compd. Fert. 2004, 19 (1), 54−56. (3) Yang, W. H.; Li, X. Z.; Hu, Z. P. Summary for preparation of potassium dihydrogen phosphate from wet-process phosphoric acid. Inorg. Chem.: Indian J. 2014, 46 (4), 7−9. (4) Elierer, R. Production of KH2PO4 from KCl and H3PO4 in an Organic Liquid Medium. Ind. Eng. Chem. Process Des. Dev. 1978, 17 (4), 460−468. (5) Mazunin, S. A.; Chechulin, N. S.; Frolova, S. A.; Kistanova, N. S. Technology of obtaining of potassium dihydrophosphate in the system with salting-out. Russ. J. Appl. Chem. 2010, 88 (3), 553−561. (6) Dang, Y. G.; Fei, D. J.; Hu, X. Y. Effects of Impurities on Crystallization Process of Potassium Dihydrogen Phosphate. J. Chem. Eng. Chin. Univ. 2008, 22 (6), 911−914. (7) Zaitseva, N.; Atherton, J.; Rozsa, R. Design and Benefits of Continuous Filtration in Rapid Growth of Large KDP and DKDP Crystals. J. Cryst. Growth 1999, 197, 911−920. (8) Sangwal, K. Effects of Impurities on Crystal Growth Processes. Prog. Cryst. Growth Charact. 1996, 32, 3−43. (9) Li, G. H.; Xue, L. P.; Su, G. B.; Zhuang, X. X.; Li, Z. D.; He, Y. P. Study on the growth and characterization of KDP-type crystals. J. Cryst. Growth 2005, 274, 555−562. (10) Guzman, L. A.; Kubota, N. Growth Rate Hysteresis of a Potassium Dihydrogen Phosphate (KDP) Crystal in the Presence of Traces of Impurity. J. Cryst. Growth 2005, 275, e237−e242. (11) Seif, S.; Chang, J. M.; Bhat, K.; Penn, B.; Lal, R. B. Investigation of Temperature Dependence and Impurity Content on the Growth Rate of KDP Crystals. Cryst. Growth Des. 2003, 1, 359−362. (12) Kubota, N.; Yokota, M.; Doki, N.; Guzman, L. A.; Sasaki, S. A Mathematical Model for Crystal Growth Rate Hysteresis Induced by Impurity. Cryst. Growth Des. 2001, 3, 397−402. (13) Wang, P.; Li, J.; Luo, J. H.; Jin, Y.; Yang, Z. P. Solid−Liquid Phase Equilibrium for the Ternary System K2SO4 + KH2PO4 + H2O at (298.15 and 333.15) K. J. Chem. Eng. Data 2012, 57 (3), 836−839. (14) Shu, Y. G.; Lu, B. L.; Wang, X. R. Phase Diagram Analyses of Inorganic Chemical Production: Basic Theory (in Chinese); Chemical Industry Press: Beijing, 1985. (15) Liu, M.; Tang, J. H.; Cui, C.; Li, C.; Chen, X. Z. Phase Equilibrium in the Aqueous Ternary System NaH2PO4 + CO (NH2)2 + H2O at 298.15 K. J. Chem. Eng. Data 2012, 58 (1), 132−135. (16) Schott, H. A Mathematical Extrapolation for the Method of Wet Residues. J. Chem. Eng. Data 1961, 6, 324. (17) Xie, P.; Li, J.; Liang, C. Research on the Phase Equilibrium in the Production of Urea Phosphate. Phosphate Compd. Fert. 2006, 21 (15−16), 31. (18) ESO/TC 47.ISO 3706-1976. Phosphoric Acid for Industrial Use (Including Foodstuffs)-Determination of Total Phosphorus (V) Oxide Content-Quinoline Phosphomolybdate Gravimetric Method; ISO Information Handing Services: Switzerland, 1976. (19) Sheen, H. T.; Kahler, H. L. Effect of ions on Mohr method for chloride determination. J. Ind. Eng. Chem. Anal. Ed. 1938, 10 (11), 628−629. (20) Zhu, T.; Li, C. M. Determination of the content of chloride ion in fertilizer material by Volhard method. Chem. Fert. Ind. 2002, 1, 23− 26. (21) Zhao, F.; Lu, Y.; Wang, K.; Luo, G. Solubility of KH2PO4 in KCl, H3PO4, and Their Mixture Solutions. J. Chem. Eng. Data 2014, 59, 439−443. (22) Lin, L. J.; Fang, C. H.; Fang, Y.; Qin, X. F. A new model forprediction density of electrolyte solutions. J. Salt Lake Res. 2006, 14 (2), 56−61.

∑ Ai ·wi

where d0 = 0.995646 g·cm−3 and d0 = 0.999127 g·cm−3, the density of deionized water at (303.15 and 288.15) K; 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 KH2PO4 and KCl for calculation are 0.007044 (0.007067) and 0.006342 (0.006312) at 303.15 (288.15) K, respectively. The experimental data is compared with calculated data to determine the relative error. All of the data are listed in Table 1. Test of the model precision uses average relative error criterion and the average relative error of the empirical equation is 0.09 %, which demonstrates that the calculation method is feasible.

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CONCLUSIONS The phase equilibrium of the ternary system KCl + KH2PO4 + H2O at (288.15 and 303.15) K was investigated.The data of solubility and density of this system were obtained. According to the solubility data measured, the phase diagram was plotted, the solid phase which was in equilibrium with the saturated solution was detected, and crystalline regions of both solid phases were determined. There is only one invariant point (E or N) in each phase diagram and the crystallization fields of KH2PO4 are both larger than that of KCl at (288.15 and 303.15) K. KCl has a strong salting-out effect on KH2PO4. Increasing the temperature from (288.15 to 303.15) K, salting out effect of potassium chloride on potassium dihydrogen phosphate does not change significantly. The empirical equation has a high precision to calculate the density of the saturated solution. All results can offer fundamental data support; these data, conducive to designing and optimizing the processes of separation and crystallization, could be applied in forthcoming researches.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. D

DOI: 10.1021/acs.jced.5b00141 J. Chem. Eng. Data XXXX, XXX, XXX−XXX