Article pubs.acs.org/jced
Solubility Data for the Quaternary KOH−K3VO4−K2CrO4−H2O System at (40 and 80) °C Man Feng,†,‡ Hao Du,† Shili Zheng,† Shaona Wang,*,† 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, People’s Republic of China ‡ University of Chinese Academy of Sciences, No. 19A Yuquan Road, Beijing, 100049, People’s Republic of China ABSTRACT: The separation of K3VO4 and K2CrO4 plays an important role in the vanadium cleaner production process using concentrated alkaline solution as reaction media. Equilibrium solubility data for the KOH−K3VO4−K2CrO4− H2O system at (40 and 80) °C therefore were measured and compared with ternary KOH−K3VO4−H2O and KOH−K2CrO4−H2O subsystems, respectively. Based on the data, a strategy for effective separation of K3VO4 and K2CrO4 from the KOH−K3VO4−K2CrO4−H2O system has been proposed.
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INTRODUCTION Vanadium and its compounds are extensively used as color modifying additives for pigments, as electrolytes for electrochemical storage devices, and as catalysts for the chemical and polymer synthesis.1,2 It has also drawn much research attention in recent years as its complexes are known to exhibit antitumor activity, which makes it a great prospect for therapeutic usage.3,4 However, about 87 % of the vanadium is consumed as an alloying element in producing high-grade steel and iron for construction and aerospace applications.2 Hence, the global vanadium market is closely related to the growth and development of the steel and iron making industries.5 At present, over 95 % of vanadium is produced in China, Russia, and South Africa.6 The most important source of vanadium is vanadium slag produced during steel making using vanadium titano-magnetite ore, accounting for 58 % of the world’s total vanadium production.5 The vanadium-bearing materials are typically treated via several processes including salt roasting, leaching, solvent extraction, and ion exchange,7−10 among which the roasting of vanadium slag or other vanadium bearing resources with NaCl or/and Na2CO3 as additives is the most representative route.11 However, this process suffers from its low vanadium extraction efficiency, high energy consumption, and severe environmental problems such as the emission of hazardous ammonia−nitrogen wastewater and toxic gases (SO2 and Cl2). These disadvantages of the traditional process trigger the development of more efficient and environmental friendly vanadium extraction process. Recently, targeting at recovery of vanadium associates with chromium from the vanadium slag simultaneously, a new liquid oxidation process was developed by the Institute of Process Engineering, Chinese Academy of Sciences.12 This new process © 2013 American Chemical Society
treats vanadium slag by concentrated KOH solution. In comparison with the traditional salt roasting process, which typically operates at 800 °C, the temperature of the liquid oxidation reaction can be significantly reduced to 200 °C, and high extraction efficiency of vanadium and chromium can be achieved simultaneously. Followed by dissolution with water and filtering separation of the reaction slurry, KOH, K3VO4, and K2CrO4 are dissolved into the solution and separated with the tailing. To separate K3VO4 and K2CrO4 from the KOH solution effectively, it is necessary to study the equilibrium solubility of the quaternary KOH−K3VO4−K2CrO4−H2O system. The equilibrium data for the ternary KOH−K3VO4− H 2 O and KOH−K 2 CrO 4 −H 2 O subsystems have been measured and reported previously.13,14 However, due to the lack of relevant data of quaternary KOH−K3VO4−K2CrO4− H2O system at (40 and 80) °C in literature, the design of an effective separation method with respect to K3VO4 and K2CrO4 from the KOH−K3VO4−K2CrO4−H2O quaternary system is yet to be established. Therefore, in this work, the examination of K3VO4 and K2CrO4 solubility in the KOH−K3VO4− K2CrO4−H2O quaternary system has been performed systematically, and the salting-out effect between K3VO4 and K2CrO4 has been discussed, providing fundamental support for the design of a K3VO4 and K2CrO4 separation strategy.
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EXPERIMENTAL SECTION Apparatus and Reagents. Polypropylene bottles with good heat resistance and chemical stability (capacity of 250 Received: January 11, 2013 Accepted: March 14, 2013 Published: March 21, 2013 1029
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Table 1. Solubility Data for the KOH−K3VO4−K2CrO4−H2O System at 40 °C composition of alkaline solution (g·L−1) sample no. 1 2 3 4 5 6 7 8 9 10 11 12 13 14
ρ/g·cm
−3
1.4948 1.7864 1.7334 1.5186 1.6146 1.5153 1.5125 1.5197 1.5207 1.4422 1.5729 1.5211 1.5929 1.5796
composition of alkaline solution (wt %)
KOH
K3VO4
K2CrO4
KOH
K3VO4
K2CrO4
20.27 61.81 149.81 155.09 240.87 265.07 275.89 323.55 437.81 480.50 631.77 626.73 676.39 711.16
707.71 791.93 675.53 549.98 499.94 436.02 424.56 363.15 265.12 189.33 132.97 122.78 121.50 128.19
10.38 9.15 1.66 1.53 1.71 1.47 2.07 1.76 1.12 0.98 0.58 0.57 0.54 0.36
1.36 3.46 8.64 10.21 14.92 17.49 18.24 21.29 28.79 33.32 40.17 41.20 42.46 45.02
47.34 44.33 38.97 36.22 30.96 28.77 28.07 23.90 17.43 13.13 8.45 8.07 7.63 8.12
0.69 0.51 0.10 0.10 0.11 0.10 0.14 0.12 0.07 0.07 0.04 0.04 0.03 0.02
Table 2. Solubility Data for the KOH−K3VO4−K2CrO4−H2O System at 80 °C composition of alkaline solution (g·L−1) sample no. 1 2 3 4 5 6 7 8 9 10 11 12 13 14
ρ/g·cm
−3
1.6282 1.6285 1.6187 1.6097 1.6164 1.6233 1.6984 1.6423 1.697 1.6395 1.6299 1.659 1.6363 1.6525
composition of alkaline solution (wt %)
KOH
K3VO4
K2CrO4
KOH
K3VO4
K2CrO4
30.10 57.63 126.95 179.93 226.24 276.14 334.82 356.27 411.19 449.15 503.25 567.93 653.95 641.61
782.51 731.48 680.08 646.87 610.02 565.44 559.83 515.31 510.85 437.61 387.91 373.58 304.65 311.65
12.32 11.10 7.72 4.70 2.64 2.22 1.60 1.56 1.54 1.33 0.93 0.72 0.43 0.50
1.85 3.54 7.84 11.18 14.00 17.01 19.71 21.69 24.23 27.40 30.88 34.23 39.97 38.83
48.06 44.92 42.01 40.19 37.74 34.83 32.96 31.38 30.10 26.69 23.80 22.52 18.62 18.86
0.76 0.68 0.48 0.29 0.16 0.14 0.09 0.10 0.09 0.08 0.06 0.04 0.03 0.03
mL) were used for preparing the samples. These bottles were placed in a HZ-9212S type thermostatic shaking water bath with a precision of 0.1 °C and a HZ-9612K type thermostatic shaking air bath with a precision of 0.2 °C to reach equilibrium at (40 and 80) °C, respectively. The concentrations of sodium, vanadium, and chromium were determined using ICP-OES (PE Optima 5300DV, Perkin-Elmer). The chemicals used in this study were of analytical grade. Potassium hydroxide (KOH) and potassium chromate (K2CrO4) were purchased from Sinopharm Chemical Reagent Company, and potassium vanadate (K3VO4·3H2O) was provided by the Chengde Iron & Steel Company. The purities of potassium vanadate and potassium chromate are above 99.5 %, and potassium hydroxide is above 85 %. High-purity Milli-Q water, with a resistivity of above 18.2 MΩ·cm at ambient temperature, was used for preparing the solutions. Experimental Procedure. The supersaturated solutions were prepared via two methods. One approach was based on the principle of crystallization of supersaturated K2CrO4 and K3VO4 in the KOH solutions. First, alkaline solutions with predetermined KOH concentrations were prepared in beakers positioned in a constant-temperature water bath at (60 and 95) °C, higher than the equilibrium temperatures in order to reach supersaturation, and the temperature fluctuations were controlled to be less than 0.1 °C. Then K2CrO4 and K3VO4
were added into the alkaline solutions while stirring with stirring speed controlled to be 250 rpm. The stirring was stopped when the K2CrO4 and K3VO4 salts could not dissolve any more as suggested by slurry being clearly formed. These slurries, 200 mL each, were then transferred into polyethylene bottles and sealed with polytetrafluoroethylene tape to prevent evaporation during equilibrating. The samples were then placed in the thermostatic shaking bath maintained at temperatures of (40 and 80) °C with fluctuation less than (0.1 and 0.2) °C, and constant shaking with a speed of 180 rpm was maintained to accelerate the equilibration and homogeneity of the slurries. The second approach was based on the dissolution of K2CrO4 and K3VO4 in KOH solutions, and the experimental procedure was as follows. First, KOH solutions with predetermined concentrations were prepared as described in the above section, apart from the difference that the solutions were prepared at (40 and 80) °C, respectively. Then small amounts of K2CrO4 and K3VO4 salts were added into the KOH solutions every day at constant temperature (40 and 80) °C until the solution reached saturation. Sampling of the liquid phase and examining of the solid phase were performed every day before further addition of K2CrO4 and K3VO4 salts to the solutions. The equilibrium state was considered to be achieved when the composition of liquid phase did not change with either time or further addition of salts, and vanadium-bearing 1030
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can be seen from Figure 1 that the solubility of K3VO4 and K2CrO4 decrease monotonically with the increase of KOH concentration within the whole alkali range examined. The solubility of K3VO4 is substantially higher than that of K2CrO4 in the equivalent system. For example, the concentrations of K3VO4 are 205 and 254 times higher than that of K2CrO4 when the KOH concentration is 28 wt % at (40 and 80) °C, respectively. Besides, with the increase of KOH concentration, the solubility of K3VO4 shows significant decrease until the KOH concentration reaches 40 wt % at (40 and 80) °C. Consequently, it is possible to separate K3VO4 through evaporative crystallization in this alkali region. Further, when the KOH concentration is above 40 wt %, though the solubility of K3VO4 shows no significant independence with the alkali concentration, the impact of the temperature is obvious. Therefore, cooling crystallization separation of K3VO4 would be more effective at a high alkali concentration range (KOH concentration of more than 40 wt %). The solubility of K2CrO4 in the quaternary system exhibits quite a different trend in comparison with that of K3VO4, as suggested by the significant decrease of solubility initially and negligible variation thereafter. For example, when the KOH concentration increases from (1.36 to 8.64) wt % at 40 °C or (1.85 to 19.71) wt % at 80 °C, the K2CrO4 solubility decreases from (0.69 to 0.10) wt % or (0.76 to 0.09) wt %; when the KOH concentration increases from (8.64 to 45.02) wt % at 40 °C or (19.71 to 38.83) wt % at 80 °C, the K2CrO4 solubility shows no obvious decrease which is less than 0.07 wt %. On the other hand, the K2CrO4 solubility is also weakly dependent on the temperature. In view of the above analysis, it is possible to separate K2CrO4 from the quaternary systems at low alkali concentration range by evaporation crystallization. Similar solubility behavior as well as separation strategy have also been reported by Cui14 in her study of the KOH− K2CrO4− H2O ternary system. However, an effective separation method cannot be designed with the detailed information regarding the salting-out effect between K2CrO4 and K3VO4, which will be discussed in the following section in detail. Comparison of the KOH−K3VO4−H2O System and KOH−K3VO4−K2CrO4−H2O System. To evaluate the salting out effect of K2CrO4 to K3VO4 in KOH solutions, the equilibrium data for the ternary system KOH−K3VO4−H2O were measured, and the results are presented in Table 3. For the convenience of comparison, the corresponding K3VO4 solubility curves of both ternary and quaternary systems are shown in Figure 2. From the data in Table 3, it is clear that when K2CrO4 is added into the ternary KOH solution at 40 °C, there is a decrease of K3VO4 concentration in the whole alkali region. The decrease of K3VO4 solubility changes from 12 wt % to almost zero when the KOH concentration increases from (1.36 to 28.79) wt %. However, with further increase of the KOH concentration to 45.02 wt %, the decrease of K3VO4 solubility becomes appreciable again. At 40 °C, the average difference of K3VO4 concentration with or without the addition of K2CrO4 is about 7 wt %, and the maximum difference is less than 12 wt %, which occurs at the KOH concentration of 3.46 wt %, suggesting that the existence of K2CrO4 shows no significant salting-out effect on K3VO4 at 40 °C. At 80 °C, the salting-out effect of K2CrO4 on K3VO4 is more significant as suggested by larger K3VO4 solubility difference with or without the addition of K2CrO4 in comparison with what has been
and chromium-bearing phase were presented in the solid phases simultaneously. The solution composition of our samples prepared via the above-mentioned two different approaches were compared constantly, and equilibrium was assumed to realize when the solution compositions agree with each other. Once the system was in equilibrium state, the shaking was stopped, and the samples were kept in the baths for 10 more days in order for the suspended precipitates to settle. Each equilibrium liquid phase sample of 1 mL was taken using a sampling gun and transferred into a volumetric flask, followed by dilution with high purity water for further analysis. To prevent the saturation liquid from crystallization as the temperature changed during sampling, the sample tubes were first heated to (40 and 80) °C, respectively. The concentrations of sodium, vanadium, and chromium were determined using ICP-OES. To guarantee the accuracy of the analysis, each equilibrium system was sampled and analyzed at least three times, and the results hereafter were the average of multiple measurements with standard deviation of less than 3 %.
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RESULTS AND DISCUSSION KOH−K3VO4−K2CrO4−H2O System. The equilibrium data for the quaternary KOH−K3VO4−K2CrO4−H2O system at (40 and 80) °C are summarized in Tables 1 and 2 and Figure 1. It
Figure 1. Solubility diagrams of the KOH−K3VO4−K2CrO4−H2O system at (40 and 80) °C: (I) at 40 °C; (II) at 80 °C. 1031
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Table 3. Solubility Data of the KOH−K3VO4−H2O System at (40 and 80) °C
From Figure 2, it is obvious that, regardless of adding K2CrO4 or not, the average difference in K3VO4 concentration in relative low alkalinity region (that is the region where the KOH concentration below 28.79 wt %, equivalent to 437.81 g·L−1) is about 6 wt %, while the difference reduces to 3 wt % when the KOH concentration is above 28.79 wt % at 40 °C. In contrast, at 80 °C, the difference in K3VO4 is monotonically dependent on the amount of K2CrO4 in the solution, as suggested by the 0.44 wt % K3VO4 solubility decrease when the K2CrO4 concentration is 0.03 wt % (40 wt % KOH solutions) and 18 wt % difference when the K2CrO4 concentration increases to 0.29 wt % (11 wt % KOH solutions). Consequently, the higher solubility of K2CrO4 at 80 °C plays a more significant role in changing the K3VO4 solubility in comparison with that of 40 °C. From discussion above, it can be concluded that due to the generally weak salting-out effect, evaporative crystallization at low alkali concentration range, and cooling crystallization at the high alkali region are both effective methods to separate K3VO4 from this quaternary solution. Comparison of the KOH−K2CrO4−H2O System and the KOH−K3VO4−K2CrO4−H2O System. Equilibrium data for the ternary system KOH−K2CrO4−H2O are presented in Table 4, and Figure 3 shows corresponding K2CrO4 solubility
composition of alkaline solution (wt %) T = 40 °C
T = 80 °C
sample no.
KOH
K3VO4
KOH
K3VO4
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
3.41 6.60 8.80 11.86 13.35 15.84 18.13 22.37 28.00 33.93 39.45 42.87 47.55 50.72 53.30
56.87 51.64 47.34 41.85 38.39 34.68 30.31 24.56 18.28 14.75 12.13 10.73 8.73 8.19 8.18
47.25 43.34 40.29 37.39 32.52 29.78 25.42 22.25 17.62 13.00 10.94
18.15 18.15 19.06 20.03 23.79 25.99 31.49 36.54 44.61 53.23 58.43
Table 4. Solubility Data of the KOH−K2CrO4−H2O System at (40 and 80) °C composition of alkaline solution (g·L−1) T = 40 °C
T = 80 °C
sample no.
KOH
K2CrO4
KOH
K2CrO4
1 2 3 4 5 6 7 8 9 10 11 12 13 14
22.38 39.97 71.94 114.57 119.9 133.22 193.17 227.81 269.78 319.74 331.73 639.47 783.36 879.28
494.07 425.53 397.25 311.71 306.71 270.26 177.4 140.91 104.46 54.28 43.09 2.84 0.93 0.3
27.98 115.11 122.57 245.13 273.11 399.67 439.64 479.61 511.58 799.34 879.28
565.36 375.8 364.76 188.33 153.86 59.69 40.22 23.54 11.45 2.35 0.9
curves of both ternary and quaternary systems. From Figure 3, it is observed that there is a substantial decrease of K2CrO4 solubility in the low alkali region when K3VO4 is saturated in the solution. For example, K2CrO4 solubility decreases from (494.07 to 10.38) g·L−1 at 40 °C and from (565.36 to 12.32) g·L−1 at 80 °C with the addition of K3VO4 when the KOH concentration around 22.38 g·L−1. However, the difference in K2CrO4 solubility becomes negligible in both the ternary and quaternary systems when the KOH concentration is above 631.77 g·L−1 at 40 °C and 641.61 g·L−1 at 80 °C, respectively. The maximum K2CrO4 solubility differences between the quaternary system and the ternary KOH−K2CrO4−H2O system is 483 g·L−1 at 40 °C and 553 g·L−1 at 80 °C, suggesting that temperature is a significant factor in manipulating K2CrO4 solubility. The temperature effect can be explained due to the fact that the salting effect of K3VO4 to K2CrO4 is more significant while the solubility of K3VO4 is higher at elevated temperature. When the solubility of K3VO4
Figure 2. Solubility diagrams of the KOH−K3VO4−H2O system and KOH−K3VO4−K2CrO4−H2O at (40 and 80) °C: (I) at 40 °C; (II) at 80 °C.
observed at 40 °C. When the concentration of KOH is 10 wt %, the K3VO4 solubility in the KOH−K3VO4−K2CrO4−H2O quaternary system is 18 wt % lower than that of the ternary KOH−K3VO4−H2O system at 80 °C. With a further increase of KOH concentration, the difference in K3VO4 solubility between the ternary and quaternary system reduces and eventually becomes negligible when the KOH concentration is above 34.23 wt %. 1032
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Funding
The authors gratefully acknowledge the financial support from the Major State Basic Research Development Program of China (973 program) under Grant No. 2013CB632605, Key Program of the Chinese Academy of Sciences under Grant No. KGCX2EW-215, and National Natural Science Foundation of China under Grant Nos. 51274179 and 51090382. Notes
The authors declare no competing financial interest.
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Figure 3. Solubility diagrams of the KOH−K2CrO4−H2O system and KOH−K3VO4−K2CrO4−H2O system at (40 and 80) °C: (I) at 40 °C; (II) at 80 °C.
decreases from (707.71 to 128.19) g·L−1 at 40 °C and from (782.51 to 311.65) g·L−1 at 80 °C, the K2CrO4 solubility change decreases from (483.69 to 0.5) g·L−1 and from (553.04 to 4.5) g·L−1, respectively, further suggesting that the existence of K3VO4 shows a significant salting-out effect on K2CrO4 especially in the low alkali region. Therefore it can be concluded that K3VO4 should be first separated from the quaternary solution followed by evaporative crystallization to obtain high-purity K2CrO4.
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CONCLUSIONS Solubility data for the KOH−K3VO4−K2CrO4−H2O system at (40 and 80) °C were investigated in this study. From the data obtained, it can be concluded that the solubility of both K2CrO4 and K3VO4 decreases as the KOH concentration increases. K2CrO4 solubility decreases obviously with the introduction of K3VO4, while the K3VO4 solubility shows no apparent change with the introduction of K2CrO4. From the analysis of solubility data, it is proposed that, to effectively separate K3VO4 and K2CrO4 from the KOH−K3VO4− K2CrO4−H2O quaternary system, evaporative/cooling crystallization of K3VO4 first followed by evaporative crystallization to separate K2CrO4 is preferred.
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dx.doi.org/10.1021/je400051n | J. Chem. Eng. Data 2013, 58, 1029−1033
