Phase Equilibria for the KHSO4–H2SO4–H2O and KHSO4–CrO3

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Phase Equilibria for the KHSO4−H2SO4−H2O and KHSO4−CrO3−H2SO4−H2O Systems at 313.15 K Wenwen Cui,†,‡,§ Ping Li,*,†,‡ Shili Zheng,†,‡ Hailin Zhang,†,‡,§ Chuang Liu,†,‡ Yongan Chen,†,‡ and Yi Zhang†,‡ †

National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology, Institute of Process Engineering, Chinese Academy of Sciences, Beijing, 100190, People’s Republic of China ‡ Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing, 100190, People’s Republic of China § University of Chinese Academy of Sciences, No. 19A Yuquan Road, Beijing, 100049, People’s Republic of China ABSTRACT: Phase equilibria data for the KHSO4−H2SO4− H2O ternary system and the KHSO4−CrO3−H2SO4−H2O quaternary system at 313.15 K were measured, and the phase diagrams were constructed. Furthermore, analyses and discussions were made on the crystallization zones in the phase diagrams. All results can offer fundamental basis for preparation and purification of chromium trioxide by reacting potassium dichromate with sulfuric acid.

1. INTRODUCTION Chromium trioxide (CrO3) finds wide applications in electroplating, wood preservation, medicine, printing, and tanning due to its excellent performance in reducibility, solubility, and corrosion.1,2 At present, the industrial production of CrO3 generally employs the frit reaction between Na2Cr2O7 and concentrated H2SO4 at 468.15 to 478.15 K, and the supertoxic and smelly chromyl gas is discharged to worsen work circumstances.5 To eliminate the chromyl gas, CrO3 is made by reacting Na2Cr2O7 solution with H2SO4 at 338.15 to 393.15 K.3,4 However, serious environmental pollution results from the Cr(VI)-containing Na2SO4 as byproduct which is difficult to achieve comprehensive utilization. In recent years, various methods for preparing CrO3 have been developed with special consideration for environmental protection. Yuan et al.6 developed a two-stage melting method of preparing CrO3 to reduce emissions of the chromyl gas. Qi et al.7,8 electrolyzed Na2Cr2O7 to prepare CrO3 and NaOH as byproduct which was easily crystallized to obtain the base chemicals (caustic soda). Chen9 and Chen et al.10 reported a mild chemical method of decomposing K2Cr2O7 with HNO3 to prepare CrO3. Fernandez et al.11 prepared CrO3 by milling Cr(VI)-containing hazardous wastes, extracting Na2CrO4 from leaching solution of the milled Cr(VI)-containing hazardous wastes, and then precipitating as CrO3, through gasification. A new green process for CrO3 preparation has been developed by the Institute of Process Engineering, Chinese Academy of Sciences.12 With the design objective of eliminating the toxic chromyl gas, CrO3 was prepared by reacting H2SO4 © XXXX American Chemical Society

with K2Cr2O7 solution at 313.15 to 353.15 K, and KHSO4 as byproduct could be effectively utilized as fertilizers. The process consists of three steps: CrO3 crystal nucleus formation at 353.15 K and growth with the decrease of temperature from 353.15 to 313.15 K, separation of CrO3 crystal and KHSO4 solutions, and evaporative crystallization of KHSO4. The main reaction is as follows: K 2Cr2O7 (s) + H 2SO4 (aq) → 2CrO3(s) + 2KHSO4 (s) + H 2O

In acidic aqueous solution, two CrO3 molecules tend to associate with each other and combine with one water molecule, which becomes H2Cr2O7. Therefore, the KHSO4−CrO3− H2SO4−H2O system can be regarded as a reciprocal system which comprises double positive ions, K+ and H+, and double anions, Cr2O72−, and SO42−. The phase equilibria for the KHSO4−CrO3−H2SO4−H2O system provides the theoretical basis to guide the new process. Some studies have been performed on the solubility for the subsystems of the reciprocal system. The solubility data for the CrO3−H2SO4−H2O system at 298.15 K13 and the K2O− CrO3−H2O system at 283.15, 303.15, and 333.15 K14 have been determined earlier. The solubility data for the system K2Cr2O7−CrO3−H2O at 298.15 and 363.15 K has been reported by Cao et al.15 Some relevant systems, such as Received: July 14, 2015 Accepted: December 2, 2015

A

DOI: 10.1021/acs.jced.5b00594 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Na2Cr2O7−NaNO3−H2O16 and K2Cr2O7−KNO3−H2O,17 have also been investigated. To the best of our knowledge, the phase equilibria for the KHSO4−H2SO4−H2O ternary system and the KHSO4−CrO3−H2SO4−H2O quaternary system have not been reported so far. This study thus aimed at investigating the solubilities of the KHSO4−H2SO4−H2O and the KHSO4−CrO3−H2SO4−H2O systems at 313.15 K, and the phase diagrams were also plotted and applied to analyze the new process.

2.2. Experimental Method. The phase diagram was determined by the method of isothermal equilibrium. Various concentrated supersaturated solutions of the KHSO4−H2SO4− H2O ternary system were prepared in which the mass of KHSO4 was a little bit more than that in a saturated solution. CrO3 was gradually added into the supersaturated ternary solutions to obtain KHSO4−CrO3−H2SO4−H2O quaternary systems. Besides, the K2Cr2O7−CrO3−H2O ternary subsystem as foreshadows was also prepared by gradually adding CrO3 into the supersaturated solution of K2Cr2O7−H2O system. The mixtures were then put into sealed polytetrafluoroethylene (PTFE) bottles in the thermostat vibrator of which the temperature was controlled at the 313.15 K with 160 rpm to accelerate the equilibrium. The samples were agitated for 2 days and then needed stay still for 2 days to enable all the suspended crystals to subside. A certain volume of liquid from each bottle was sampled for analysis at 4 days’ intervals until identical results of twice analyses were obtained, which was assumed that equilibrium had been reached. After equilibrium, 1 mL of liquid was sampled for analysis, and the densities of the equilibrium solutions were calculated by weighing the 1 mL sample on a precision balance. Finally, solid phase was also sampled and dried at room temperature, grinded into powder, and analyzed by X-ray diffraction (XRD). 2.3. Analytical Method. The concentration of K+ was determined by inductively coupled plasma-optical emission spectrometry (ICP-OES, PE Optima 6300DV, PerkinElmer), and the concentration of SO42− was determined by ion chromatograph (Dionex DX-500). The concentrations of Cr2O72− and H+ were titrated by using N-phenylanthranilic acid as the indicator and the standard solution of NaOH, respectively. The solid phase was analyzed by an X-ray diffraction (XRD, APD-10X, Philips, Netherlands) with scanning range from 5° to 90°. All of the solubility data are the average values of three measurements with a standard deviation of less than 0.02%.

2. EXPERIMENTAL SECTION 2.1. Apparatus and Reagents. The reagents used in this work, including chromium trioxide (CrO3), potassium bisulfate (KHSO4), and sulfuric acid (H2SO4), are of analytical reagent grade, and the description for the reagents is listed in Table 1. Table 1. Suppliers and Mass Fraction Purity of the Reagentsa reagent

source

KHSO4

Sinopharm Chemical Reagent Co. Ltd., Beijing, China Sinopharm Chemical Reagent Co. Ltd., Beijing, China Beijing chemical plant, Beijing, China

CrO3 H2SO4 a

mass fraction purity

analysis method

>0.999

ICP-OES

>0.999

ICP-OES

>0.985

titration by NaOH

All chemical reagents were used without further purification.

All of reagents were used without further purification. The deionized water (high-purity Milli-Q water, with a resistivity of above 18.2 MΩ cm at ambient temperature) used in this work, was produced through a laboratory water purification system. An HZ-9212 s-type thermostatic vibrator was employed for the sample preparation at desired temperature with a precision of 0.1 K.

Table 2. Solubility Data of KHSO4−H2SO4−H2O System at 313.15 K and (0.1 ± 0.01) MPaa composition of the liquid phase wb/wt %

E

ρb/(g·cm−3)

H2SO4

KHSO4

H2O

composition of the solid phase

1.391 1.359 1.376 1.368 1.324 1.319 1.308 1.336 1.377 1.408 1.593 1.645 1.688 1.711 1.759 1.817 1.801 1.832

0.00 0.32 4.90 9.68 15.66 23.10 30.88 37.23 44.45 46.56 52.33 55.82 55.61 54.99 54.08 57.08 60.94 61.51

47.22 45.60 43.68 37.72 27.91 18.48 12.73 8.40 5.48 5.50 6.11 11.57 22.82 30.74 35.41 31.62 30.16 38.49

52.78 54.08 51.42 52.60 56.43 58.42 56.39 54.37 50.07 47.94 41.56 32.61 21.57 14.27 10.51 11.30 8.90 0.00

KHSO4 KHSO4 KHSO4 KHSO4 KHSO4 KHSO4 KHSO4 KHSO4 KHSO4 KHSO4 KHSO4 KHSO4 KHSO4 KHSO4 KHSO4 + KHSO4·H2SO4 KHSO4·H2SO4 KHSO4·H2SO4 KHSO4·H2SO4

a Standard uncertainty of the temperature is u(T) = 0.1 K, and relative standard uncertainties ur are ur(ρ) = 0.001, ur(w) = 0.02%; bThe density is calculated by weighing 1 mL solution sample on a precision balance (the equation is ρ = m/v), and w(H2SO4) and w(KHSO4) are calculated by measured values of the concentration of H+ and K+.

B

DOI: 10.1021/acs.jced.5b00594 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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3. RESULTS AND DISCUSSION 3.1. The Solubility Data of KHSO4−H2SO4−H2O Ternary System at 313.15 K. The solubility data of the KHSO4−H2SO4−H2O system at 313.15 K is presented in Table 2 and the compounds concentration values in the equilibrium solution are expressed in mass fraction. According to the solubility data, the phase diagram of the system is plotted in Figure 1. As shown in Figure 1, the phase diagram consists of

was an invariant point of the two solid phases KHSO4 and KHSO4·H2SO4. In the crystallization region of KHSO4, the solubility of KHSO4 decreased first and increased latterly with the further increase of H2SO4 concentration. Specifically, the solubility of KHSO4 decreased to 5.48 wt % when the concentration of H2SO4 increased to 44.45 wt %. The results indicate that KHSO4 could be effectively separated by adjusting the concentration of H2SO4. 3.2. The Solubility Data of KHSO4−CrO3−H2SO4−H2O Quaternary System at 313.15 K. In order to draw the phase diagram conveniently, this work takes HSO4−, the common anion of H2SO4 and KHSO4, as a kind of anion in the reciprocal system; and the content of ions in liquor is expressed by the Janecke index (J value), which is calculated as follows: n(B) × 100 n(K ) + n(H+) n(B) = × 100 n(1/2Cr2O7 2 −) + n(HSO4 −)

J(B) =

+

(1)

J(H 2O) =

n(H 2O) n(K+) + n(H+) + n(1/2Cr2O7 2 −) + n(HSO4 −) (2)

× 100 Figure 1. Phase diagram of the KHSO4−H2SO4−H2O system at 313.15 K. ○, experimental points; E, invariant point of KHSO4 and KHSO4·H2SO4.

where J(B) is the Janecke index value, B can be the ions of K+, H+, 1/2Cr2O72−, HSO4−; n(K+), n(H+), n(1/2Cr2O72−), n(HSO4−), and n(H2O) are the molar concentration of K+, H+, 1/2Cr2O72−, HSO4−, and H2O, respectively. The solubility data of the quaternary system at 313.15 K is presented in Table 3, and the phase diagrams are plotted in

two crystallization regions corresponding to the large area of KHSO4 and the relative small area of KHSO4·H2SO4. Point E

Table 3. Solubility Data of the KHSO4−CrO3−H2SO4−H2O Quaternary System at 313.15 K and (0.1 ± 0.01) MPaa,b composition of liquid phase w/wt % −3

ρ /(g·cm )

H

1.354

0.00 1.02 0.770 0.00 0.00 0.080 0.130 0.162 0.200 0.235 0.251 0.252 0.177 0.315 0.367 0.352 0.352 0.323 0.370 0.361 0.407 0.472

c

A B C D E1

P E2

E3

1.592 1.127 1.362 1.401 1.373 1.381 1.345 1.356 1.347 1.306 1.454 1.352 1.367 1.363 1.372 1.533 1.372 1.409 1.423 1.462

+

composition of liquid phase Jc/(mol/100 mol dry salts)

K

Cr2O72−

HSO4−

H+

K+

1/2 Cr2O72−

HSO4−

H2O

11.54 0.00 0.00 7.16 14.36 12.39 9.84 10.87 7.73 8.06 6.89 5.12 4.82 4.90 4.59 4.28 3.78 3.05 3.23 2.79 2.85 3.02

0.00 0.00 83.12 19.84 1.59 3.13 3.59 3.40 4.14 3.99 4.57 8.77 32.50 7.43 6.73 8.12 8.18 43.34 20.04 21.81 20.66 17.54

28.69 97.96 0.00 0.00 34.28 37.59 37.24 34.69 36.63 33.12 32.65 26.49 0.00 31.95 35.02 34.30 38.95 0.00 22.53 33.02 33.92 35.68

0.00 100 100 0.00 0.00 20.19 34.07 36.81 50.30 53.18 58.67 65.78 58.90 71.50 75.68 76.21 78.39 80.52 81.74 83.43 84.77 85.93

100 0.00 0.00 100 100 79.81 65.94 63.19 49.70 46.82 41.33 34.22 41.10 28.50 24.32 23.79 21.61 19.48 18.26 16.57 15.23 14.08

0.00 0.00 100 100 1.03 1.84 2.12 2.15 2.48 2.63 3.04 6.92 100 4.96 4.14 5.05 4.51 100 16.65 12.92 12.03 9.94

100 100 0.00 0.00 98.97 98.16 97.88 97.85 97.52 97.37 96.96 93.08 0.00 95.04 95.86 94.95 95.50 0.00 83.35 87.08 87.97 90.06

612

+

composition of solid phase

93 1766 381.30 327.93 352.79 350.52 362.62 383.01 399.07 487.15 923 391.07 343.85 352.67 311.37 590 408.66 283.64 266.82 251.11

KH H C KC KH+KC KH + KC KH+KC KH+KC KH+KC KH+KC KH+KC KH+KC+KCO KH+KCO KH+KCO KH+KCO KH+KCO KH+KCO KCO+C KCO+C KCO+C KCO+C KCO+C

KH−KHSO4, H−H2SO4, C−H2Cr2O7(CrO3), KC−K2Cr2O7, KCO−K2Cr3O10. bStandard uncertainty of the temperature is u(T) = 0.1 K; relative standard uncertainties ur are ur(ρ) = 0.001 and ur(w) = 0.02%. cThe density is calculated by weighing 1 mL solution sample on a precision balance (the equation is ρ = m/v), and J(x) (x = H+, K+, 1/2Cr2O72−, HSO4−, H2O) is calculated by measured values of the concentration of H+, K+, Cr6+, and SO42− by eqs 1 and 2. a

C

DOI: 10.1021/acs.jced.5b00594 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Figure 2. The XRD patterns of the equilibrium solid phase are showed in Figure 3a−d. The phase diagrams in the Figure 2 consist of a dry salt diagram (a) and a water-content diagram (b). Diagrams (a) and (b) have the same abscissa, and diagram (b) reflects the content of water in the quaternary system. The dry salt diagram shows that the KHSO4−CrO3−H2SO4−H2O system has four crystallization zones, including the KHSO4 crystallization zone (AE1PBA), K2Cr2O7 crystallization zone (E1PE2DE1), K2Cr3O10 crystallization zone (E2PBE3E2), and H2Cr2O7 (CrO3) crystallization zone (BCE3B). Among the four crystallization zones, the crystallization zone of K2Cr2O7 is the largest, and the crystallization zone of CrO3 is the smallest. The crystallization zone of KHSO4 shows a trend of expanding first and shrinking latterly, which agrees with the results of the KHSO4−H2SO4−H2O system. The corners A, C, and D represent the pure crystals of KHSO4, H2Cr2O7 (CrO3), and K2Cr2O7, respectively. The corner B represents the hypothetical pure H2SO4 solid, which does not exist theoretically. Point P is the three-salt isothermal invariant point; points E1, E2, and E3 represent the equilibrium of two phases at the two extremes of the corresponding side. In the water-content phase diagram, the points and curves are consistent with those in the dry-salt phase diagram rigidly. Figure 3 displays the X-ray diffraction spectra of solid phases, including KHSO4, K2Cr2O7, K2Cr3O10, and H2Cr2O7 (CrO3), which agree with the crystallization zones in the dry salt phase diagram. Chen et al.10 proved that K2Cr4O13 existed as one of the solid equilibrium phases in the K2Cr2O7−CrO3−KNO3− HNO3−H2O system. Unlike the HNO3 addition, the H2SO4 addition probably inhibited the formation of K2Cr4O13 phase due to stronger cohesion between SO42− and K+. By comparing with the solid phase change of the quaternary system, it could

Figure 2. Phase diagrams of KHSO4−CrO3−H2SO4−H2O system at 313.15 K: (a) the dry salt phase diagram; (b) the water-content phase diagram. ■, experimental points; solid line, cosaturation composition line of two salts.

Figure 3. XRD patterns of equilibrium solid phases of KHSO4−CrO3−H2SO4−H2O system at 313.15 K: (a) XRD patterns of KHSO4 and K2Cr2O7; (b) XRD patterns of KHSO4 and K2Cr3O10; (c) XRD patterns of K2Cr3O10 and CrO3; (d) XRD patterns of KHSO4, K2Cr2O7, and K2Cr3O10. D

DOI: 10.1021/acs.jced.5b00594 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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(11) Fernandez; Francisco G. Process and plant for converting hazardous waste containing chromium VI into non-hazardous waste. U.S. Patent, No. 8471089, 2013. (12) Zhang, Y.; Li, Z. H.; Qi, T. Green chemistry of chromate cleaner production. Chin. J. Chem. 1999, 17, 258−266. (13) Howard, S. Solubilities of inorganic and organic compounds, Vol.2, 3rd.; Pergamon Press: New York. 1979. (14) Mellor, J. W. A comprehensive treatise on inorganic and theoretical chemistry; Longmans: London, 1953; Vol. XI. (15) Cao, D. W.; Zhang, Y. J.; Qi, T. et. Phase diagram for the system K2Cr2O7 + CrO3 + H2O at (25 and 90) °C. J. Chem. Eng. Data 2007, 52, 766−768. (16) Korin, E.; Soifer, L. Phase diagram for the system Na2Cr2O7 + NaNO3 + H2O in the temperature range 20 to 40 °C. J. Chem. Eng. Data 1996, 41, 885−887. (17) Korin, E.; Soifer, L. Phase diagram for the system K2Cr2O7 + KNO3 + H2O in the temperature range 20 to 40 °C. J. Chem. Eng. Data 1997, 42, 508−510.

be found that high H2SO4 concentration facilitate the formation of the CrO3. To prepare CrO3, the crystallization zone of CrO3 should exist alone. The dry salt phase diagram (Figure 2a) confirmed that the crystallization zones of KHSO4 and H2Cr2O7 (CrO3) at the diagonal vertexes did not connect to each other at 313.15 K. As a result, separation of CrO3 and KHSO4 could be easily realized. However, the CrO3 phase existed in the high H2SO4 concentration region, while the KHSO4 phase existed in the any H2SO4 concentration region, indicating that the CrO3 and partial KHSO4 would be crystallized at the same time and further recrystallization of CrO3 was required. All of the analysis about the KHSO4−CrO3−H2SO4−H2O system provides the theoretical basis for the design of the possible process to prepare CrO3 by reacting K2Cr2O7 with H2SO4.

4. CONCLUSIONS Phase equilibria for the KHSO4−H2SO4−H2O and the KHSO4− CrO3−H2SO4−H2O systems at 313.15 K was investigated. The phase diagrams show that there exist KHSO4, K2Cr2O7, K2Cr3O10, and CrO3 phases, the CrO3 formation will be facilitated by high H2SO4 concentration, and the sole CrO3 phase can be obtained in the quaternary system at 313.15 K. These findings provide a significant database for the CrO3 preparation by reacting H2SO4 with K2Cr2O7 solution.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: lipinggnipilmailto:@ipe.ac.cn; tel.: +86-10-82544856; fax: +86-10-82544856. Funding

Financial support was provided by the National Nature Science Foundation of China (No. 51574212, 51204154, U1403195). Notes

The authors declare no competing financial interest.

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REFERENCES

(1) Ding, Y.; Ji, Z. Production and application of chromium compounds; Chemical Industry Publishing House: Beijing, China, 2002; pp 151− 168. (2) Li, P.; Zheng, S. L.; Qing, P. H.; et al. The vanadate adsorption on a mesoporous boehmite and its cleaner production application of chromate. Green Chem. 2014, 16, 4214−4222. (3) Ji, Z. The technologies for the production of chromium trioxide. Inorg. Chem. Ind. 2002, 34, 15−17. (4) Danvers, A. S.; Michael, A. M. Process for the production of chromium trioxide. U.S. Patent, No. 4291000, 1981. (5) Michael, A. M.; Danvers, A. S. Michael, A. M. Production of chromium trioxide. U.S. Patent, No. 4431625, 1984. (6) Yuan, D. J.; Hu, G. M.; Gu, L. Z. A two-stage method for preparation of chromium trioxide. Chinese Patent, No. 201110144577.6. (7) Qi, T.; Li, C. W.; Yu, Z. H. et al. A electrochemical synthesis method for preparation of chromium trioxide with sodium dichromate. Chinese patent, No. 200810101864.7. (8) Qi, T.; Li, C. W.; Yu, Z. H. et al. A electrochemical synthesis method for preparation of chromium trioxide with potassium dichromate. Chinese Patent, No. 200810101864.7. (9) Chen, H. F. Fundamental study on the preparation of high-purity chromium trioxide and potassium nitrate in the new cleaner chromate production process. Doctoral Dissertation: Institute of Process Engineering, Chinese Academy of Sciences, Beijing, 2004. (10) Chen, H. F.; Li, Z. H.; Zhang, Y. Solubility in the K2Cr2O7 + CrO3 + KNO3 + HNO3 + H2O system. J. Chem. Eng. Data 2004, 49, 143−147. E

DOI: 10.1021/acs.jced.5b00594 J. Chem. Eng. Data XXXX, XXX, XXX−XXX