Phase Equilibria in the NaOH–Na2MoO4–Na2SO4–H2O System at

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Phase Equilibria in the NaOH−Na2MoO4−Na2SO4−H2O System at (313.15 and 403.15) K Yan Liu,†,‡ Yifei Zhang,*,† Fangfang Chen,† and Yi Zhang† †

National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology, Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China ‡ Graduate University of the Chinese Academy of Sciences, Beijing 100049, China ABSTRACT: Equilibria data for the NaOH−Na2MoO4−Na2SO4−H2O quaternary system at (313.15 and 403.15) K were measured with an isothermal method. According to the experimental results, dry-salt and water phase diagrams were constructed. It was found that there are two invariant points, five univariant curves, and four crystallization zones corresponding to Na2MoO4, Na2SO4, Na2MoO4·2H2O, and NaOH·H2O (NaOH at 403.15 K). In addition, the solubility of Na2MoO4 in NaOH solution was compared with that in NaOH solution saturated with Na2SO4 at both temperatures, while the solubility of Na2SO4 in NaOH solution saturated with Na2MoO4 at 313.15 K was compared with that at 403.15 K.

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INTRODUCTION Sodium molybdate is an important inorganic molybdate that is widely used in the corrosion inhibitor catalyst, fuel, pigment, pharmaceutical, water treatment, and oil chemical fields.1,2 In industry, sodium molybdate is mainly produced from molybdenum concentrate, ammonia leaching residues, and low-grade molybdenum ore. Usually, these materials are processed either by oxide roasting and subsequent alkali leaching or by oxygen-pressure NaOH leaching, and then the leach liquor is separated by evaporation crystallization or cooling crystallization.3 Because of the presence of sulfide in the materials, it is very important in the production of high-purity sodium molybdate that the sodium molybdate be effectively separated from the NaOH−Na2MoO4−Na2SO4−H2O system. Thus, studies of the phase diagram of the NaOH−Na2MoO4− Na2SO4−H2O quaternary system are necessary to get a separation method. The phase diagrams of ternary subsystems of the NaOH− Na2MoO4−Na2SO4−H2O quaternary system, such as the Na 2 MoO 4 −Na 2 SO 4 −H 2 O system 4−7 and the NaOH− Na2SO4−H2O system,8,9 have been previously studied. However, the phase diagram of the NaOH−Na2MoO4− Na2SO4−H2O system has not been reported to date. In this work, we studied the NaOH−Na2MoO4−Na2SO4−H2O system at (313.15 and 403.15) K in order to determine the boundaries of the crystallization fields for the salt components and to obtain a full understanding of the phase equilibria in the system.

respectively. The chemicals used in the experiments, including NaOH, Na2MoO4·2H2O, and Na2SO4, were all of analytical grade and manufactured by the Xilong Chemical Co., Ltd. Deionized water was used in all of experiments. Experimental Method. An isothermal method was used to study the solubility at the specified temperatures in this study. The experiment at 313.15 K was performed in sealed polyethylene bottles that were placed in a HZ-9612K-type thermostatic shaking incubator with a precision of 0.1 K, while the experiment at 403.15 K was performed in airtight stainless steel hydrothermal synthesis reactors with poly(tetrafluoroethylene) linings that were placed in a JXF-type thermostatic shaker with a precision of 0.1 K. Predetermined amounts of sodium hydroxide, sodium sulfate, and sodium molybdate dihydrate were mixed homogenously in a given amount of water and then added to the polyethylene bottles or airtight stainless steel hydrothermal synthesis reactors. The time found to be sufficient for the attainment of equilibrium in the quaternary system was about 4 days in our pre-experiments. The system was kept shaking for 6 days to make sure that equilibrium had been attained. The shaking was then discontinued, and the samples were left static at the original temperature for 24 h to ensure that all of the suspended crystals had settled. Samples of the solution and solid phases were then taken out and analyzed. The sampling pipet tips were preheated to a temperature slightly higher than the experimental temperature to prevent the possibility of crystallization. The sampling process lasted about 2 s. The 0.5 mL solution phase was weighed and then diluted into a 100 mL volumetric flask for further analysis. The remainder of the solution phase and the rest of the solid phase were filtered quickly. The residue was dried in an oven at 323.15 K for 3 h, pestled into power, and

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EXPERIMENTAL SECTION Apparatus and Reagents. An HZ-9612K-type thermostatic shaking incubator (Taicang Technology Equipment Factory) and a JXF-type thermostatic shaker (Shangdong Songling Chemical Equipment Co., Ltd.) were used for the equilibrium measurements at (313.15 and 403.15) K, © 2012 American Chemical Society

Received: June 19, 2012 Accepted: August 21, 2012 Published: August 28, 2012 2576

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Table 1. Solubility Data for the NaOH−Na2MoO4− Na2SO4−H2O System composition of the liquid phase/wt %a NaOH

a

Na2MoO4

Na2SO4

5.27 15.19 23.20 28.79 33.68 38.76 42.90 48.79 59.29 63.21 68.59 75.76 79.80 86.82 93.99 94.49 96.08

76.60 69.04 63.20 59.66 56.24 51.95 48.77 44.22 34.81 30.75 27.03 20.67 16.82 10.78 5.32 4.95 3.48

18.13 15.77 13.61 11.55 10.08 9.29 8.34 6.99 5.90 6.04 4.38 3.57 3.38 2.40 0.70 0.56 0.44

97.68 99.38

1.20 0.44

1.12 0.19

0 99.89 96.12 99.58

78.67 0 3.98 0.40

21.33 0.11 0 0

5.40 15.33 25.61 34.09 36.06

87.11 77.60 68.34 59.94 57.08

7.49 7.07 6.05 5.96 6.86

39.30 43.35 53.70 58.60 69.99 79.57 85.08 88.53 93.45 95.56 96.76 97.90 98.36 98.43 99.07 0 99.86 27.31 98.30

54.93 50.71 40.47 36.08 24.91 14.90 9.82 6.86 4.28 3.70 2.69 1.64 1.17 1.16 0.49 92.99 0 72.69 1.70

5.77 5.94 5.83 5.32 5.09 5.53 5.10 4.61 2.27 0.75 0.55 0.46 0.47 0.42 0.44 7.01 0.10 0 0

H2O

equilibrium solid phase

T = 313.15 K 152.54 Na2SO4 + Na2MoO4·2H2O 163.63 Na2SO4 + Na2MoO4·2H2O 175.48 Na2SO4 + Na2MoO4·2H2O 181.82 Na2SO4 + Na2MoO4·2H2O 193.39 Na2SO4 + Na2MoO4·2H2O 194.74 Na2SO4 + Na2MoO4·2H2O 192.99 Na2SO4 + Na2MoO4·2H2O 197.35 Na2SO4 + Na2MoO4·2H2O 208.95 Na2SO4 + Na2MoO4·2H2O 209.39 Na2SO4 + Na2MoO4·2H2O 211.41 Na2SO4 + Na2MoO4·2H2O 211.59 Na2SO4 + Na2MoO4·2H2O 213.38 Na2SO4 + Na2MoO4·2H2O 196.71 Na2SO4 + Na2MoO4·2H2O 178.40 Na2SO4 + Na2MoO4·2H2O 174.43 Na2SO4 + Na2MoO4·2H2O 161.07 Na2SO4 + Na2MoO4 + Na2MoO4·2H2O 144.11 Na2SO4 + Na2MoO4 84.64 Na2SO4 + Na2MoO4 + NaOH·H 2O 137.98 Na2SO4 + Na2MoO4·2H2O 87.06 Na2SO4 + NaOH·H2O 157.20 Na2MoO4 + Na2MoO4·2H2O 90.02 Na2MoO4 + NaOH·H2O T = 403.15 K 105.35 Na2SO4 + Na2MoO4·2H2O 107.88 Na2SO4 + Na2MoO4·2H2O 121.85 Na2SO4 + Na2MoO4·2H2O 123.72 Na2SO4 + Na2MoO4·2H2O 125.68 Na2SO4 + Na2MoO4·2H2O + Na2MoO4 130.33 Na2SO4 + Na2MoO4 131.38 Na2SO4 + Na2MoO4 137.84 Na2SO4 + Na2MoO4 144.51 Na2SO4 + Na2MoO4 141.45 Na2SO4 + Na2MoO4 123.91 Na2SO4 + Na2MoO4 112.49 Na2SO4 + Na2MoO4 97.01 Na2SO4 + Na2MoO4 70.56 Na2SO4 + Na2MoO4 63.97 Na2SO4 + Na2MoO4 54.94 Na2SO4 + Na2MoO4 54.48 Na2SO4 + Na2MoO4 48.49 Na2SO4 + Na2MoO4 45.99 Na2SO4 + Na2MoO4 44.61 Na2SO4 + Na2MoO4 + NaOH 96.11 Na2SO4 + Na2MoO4 35.32 Na2SO4 + NaOH 129.46 Na2MoO4 + Na2MoO4·2H2O 31.31 Na2MoO4 + NaOH

Figure 1. (a) Dry-salt phase diagram for the NaOH−Na2MoO4− Na2SO4−H2O system at 313.15 K. Points P1 and P2 are the invariant points, and points F1, F2, F3, and F4 represent the equilibrium compositions of the solid phases at the two extremes of the corresponding sides, respectively. A, Na2SO4 crystallization zone; B, Na 2 MoO 4 ·2H 2 O crystallization zone; C, Na 2 MoO 4 crystallization zone; D, NaOH·H2O crystallization zone. (b) Water phase diagram for the NaOH−Na2MoO4−Na2SO4−H2O system at 313.15 K.

in the samples was determined using inductively coupled plasma optical emission spectrometry (ICP-OES, 2400 type, PerkinElmer). The hydroxide ion concentration was measured by volumetric titration with a standard solution of hydrochloric acid in the presence of phenolphthalein solution as an indicator. The structures of the solid phases were identified by XRD using a Rigaku D/max-2400 X-ray diffractometer with Cu Kα radiation. Each reported experimental result is the mean value from three parallel determinations, with a relative standard deviation (RSD) of less than 2%.

Mass percent of dry salt.

analyzed by X-ray diffraction (XRD). Parallel experiments showed that the results were reproducible. Analytical Methods. The concentration of sodium sulfate in the samples was determined by ion chromatography (DX-500, Dionex). The concentration of sodium molybdate 2577

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expressed in mass percent of dry salt. According to the experimental data in Table 1, the dry-salt and water phase diagrams at (313.15 and 403.15) K are plotted in Figures 1 and 2, respectively. The dry-salt phase diagrams in Figures 1a and 2a show that the NaOH−Na2MoO4−Na2SO4−H2O quaternary system has two invariant points, five univariant curves, and four crystallization zones, which are Na2MoO4·2H2O, Na2MoO4, Na2SO4, and NaOH·H2O (NaOH at 403.15 K). Among the four zones, the Na2SO4 crystallization zone is far larger than the other three, and the NaOH·H2O (NaOH at 403.15 K) crystallization zone is the smallest. The NaOH·H2O crystallization zone at 313.15 K is smaller than NaOH crystallization zone at 403.15 K, and the Na2SO4 and Na2MoO4 crystallization zones at 313.15 K are smaller than those at 403.15 K; however, the Na2MoO4·2H2O crystallization zone at 313.15 K is larger than that at 403.15 K. Figures 1b and 2b are the relevant water phase diagrams of the system at (313.15 and 403.15) K, respectively. They show that the mass values of water change with the mass values of sodium hydroxide. Tables 2 and 3 contain data on the solubility of Na2MoO4 in aqueous NaOH solution and in aqueous NaOH solution Table 2. Comparison of the Solubility of Na2MoO4 in Aqueous NaOH Solution with That in Aqueous NaOH Solution Saturated with Na2SO4 at 313.15 K solubility without Na2SO4/wt %a

Figure 2. (a) Dry-salt phase diagram for the NaOH−Na2MoO4− Na2SO4−H2O system at 403.15 K. Points P1 and P2 are the invariant points, and points F1, F2, F3, and F4 represent the equilibrium compositions of the solid phases at the two extremes of the corresponding sides, respectively. A, Na2SO4 crystallization zone; B, Na2MoO4·2H2O crystallization zone; C, Na2MoO4 crystallization zone; D, NaOH crystallization zone. (b) Water phase diagram for the NaOH−Na2MoO4−Na2SO4−H2O system at 403.15 K.

a

solubility with Na2SO4/wt %a

NaOH

Na2MoO4

NaOH

Na2MoO4

Na2SO4

0 0.12 1.18 2.44 3.68 5.65 7.46 8.60 11.52 16.30 18.23 22.66 25.77 27.58 31.73 34.46 37.09 42.11 44.09 47.89 52.76

43.40 40.32 37.40 35.16 33.24 30.36 28.15 26.02 22.14 15.97 12.43 7.64 5.93 4.27 3.07 1.87 1.46 0.79 0.58 0.45 0.55

0 2.09 5.76 8.42 10.21 11.48 13.15 14.64 16.41 19.19 20.43 22.02 24.31 25.46 29.26 33.76 34.43 36.80 40.01 47.62 53.82

33.06 30.33 26.19 22.94 21.18 19.17 17.62 16.64 14.87 11.27 9.94 8.68 6.64 5.37 3.63 1.91 1.80 1.33 0.49 0.34 0.24

8.96 7.18 5.98 4.94 4.09 3.43 3.15 2.85 2.35 1.91 1.95 1.41 1.15 1.08 0.81 0.25 0.21 0.17 0.46 0.18 0.10

Mass percent of the total solution.

saturated with Na2SO4 at (313.15 and 403.15) K, respectively, and the corresponding isotherms are plotted in Figure 3. The concentration values are expressed in mass percent of the total solution. It is found that the concentration of Na2MoO4 decreases significantly with increasing NaOH concentration in both the NaOH−Na2MoO4−H2O and NaOH−Na2MoO4− Na2SO4−H2O systems at (313.15 and 403.15) K. For 313.15 K, the concentration of Na2MoO4 in aqueous NaOH solution saturated with Na2SO4 is a little lower than that in aqueous NaOH solution without Na2SO4 when the NaOH concentration less than 15 wt %, but when the NaOH concentration is

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RESULTS AND DISCUSSION The equilibrium data for the NaOH−Na2MoO4−Na2SO4− H2O quaternary system at (313.15 and 403.15) K are presented in Table 1, where the respective concentration values are 2578

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Table 3. Comparison of the Solubility of Na2MoO4 in Aqueous NaOH Solution with That in Aqueous NaOH Solution Saturated with Na2SO4 at 403.15 K solubility without Na2SO4/wt %a

a

solubility with Na2SO4/wt %a

NaOH

Na2MoO4

NaOH

Na2MoO4

Na2SO4

0.22 1.76 4.06 9.27 11.90 14.71 17.97 23.94 29.46 33.82 37.63 51.27 54.31 58.55

47.20 44.29 41.00 35.17 31.68 27.47 22.27 15.76 10.62 6.95 5.24 3.10 3.32 3.18

0 2.63 7.37 11.54 15.24 15.98 17.06 18.74 22.58 23.97 28.99 35.54 40.04 44.94 54.79 58.28

47.42 42.42 37.33 30.80 26.80 25.29 23.85 21.92 17.02 14.76 10.32 6.65 4.62 3.48 2.51 2.25

3.57 3.65 3.40 2.73 2.67 3.04 2.50 2.57 2.45 2.17 2.11 2.45 2.40 2.34 1.33 0.46

Figure 4. Solubility isotherms for Na2SO4 at (313.15 and 403.15) K in aqueous NaOH solution saturated with Na2MoO4.

concentration, the Na2SO4 solubility at 313.15 K is lower than that at 403.15 K.

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CONCLUSION Phase equilibria for the NaOH−Na2MoO4−Na2SO4−H2O quaternary system at (313.15 and 403.15) K have been studied. Phase diagrams for the system, solubility isotherms for Na2MoO4 in aqueous NaOH solution and in aqueous NaOH solution saturated with Na2SO4, and solubility isotherms for Na2SO4 in aqueous NaOH solution saturated with Na2MoO4 have been plotted. This study provides a theoretical basis for the separation of Na2MoO4 and Na2SO4 from NaOH solutions. Our solubility data will likely help to determine the crystallization conditions for the system’s components and the optimum schedule for their separation.

Mass percent of the total solution.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel./Fax: +86 10 82544826. Funding

The authors acknowledge the financial support by the National High Technology Research and Development Program of China (863 Program, 2011AA060701) and the National Natural Science Foundation of China (21101159).

Figure 3. Solubility isotherms for Na2MoO4 at (313.15 and 403.15) K: ■, in aqueous NaOH solution at 313.15 K; ●, in aqueous NaOH solution saturated with Na2SO4 at 313.15 K; ▲, in aqueous NaOH solution at 403.15 K;▼, in aqueous NaOH solution saturated with Na2SO4 at 403.15 K.

Notes

The authors declare no competing financial interest.

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REFERENCES

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greater than 15 wt %, the concentrations of Na2MoO4 in the NaOH−Na2MoO4−H2O and NaOH−Na2MoO4−Na2SO4− H2O solutions are equal. The results indicate that at 313.15 K it is beneficial to precipitate Na2MoO4·2H2O from NaOH− Na2MoO4−Na2SO4−H2O system coexisting with saturated Na2SO4 when the NaOH concentration is less than 15 wt %. In contrast, the concentrations of Na2MoO4 in the NaOH− Na2MoO4−H2O and NaOH−Na2MoO4−Na2SO4−H2O solutions are equal at all of the experimental NaOH concentrations at 403.15 K. That is, the salting out effect of Na2SO4 on Na2MoO4·2H2O can be ignored at this temperature. Figure 4 shows isotherms for the solubility of Na2SO4 in aqueous NaOH solution saturated with Na2MoO4 at (313.15 and 403.15) K. As shown in Figure 4, the concentration of Na2SO4 decreases strongly with increasing NaOH concentration in the NaOH−Na2MoO4−Na2SO4−H2O system at 313.15 K, but it does not change significantly at 403.15 K. At low NaOH concentration, the Na2SO4 solubility at 313.15 K is higher than that at 403.15 K, while at high NaOH 2579

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Sodium Hydroxide and Sodium Chloride. J. Am. Chem. Soc. 1935, 57, 1539−1546. (9) Green, S. J.; Frattali, F. J. The System Sodium Carbonate− Sodium Sulfate−Sodium Hydroxide−Water at 100 °C. J. Am. Chem. Soc. 1946, 68, 1789−1794.

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