Solubility of the System KOH–K2CrO4–Al2O3–H2O at 150° C in a

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Solubility of the System KOH−K2CrO4−Al2O3−H2O at 150 °C in a High Alkali Concentrated Region Chunhua Du,*,†,‡ Shili Zheng,‡ and Yi Zhang‡ †

College of Chemistry & Pharmacy of Qingdao Agricultural University, Qingdao, 266109, P. R. China Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100080, P. R. China

‡

ABSTRACT: The phase equilibria for the system KOH−K2CrO4−Al2O3− H2O and its subsystems at 150 °C in a high alkali concentrated region were investigated. In the concentrated alkali region of the system and its subsystem KOH−Al2O3−H2O, the equilibrium solid-phase K2O·Al2O3·2H2O was found. This is different from the system KOH−Al2O3−H2O at low temperature from which the equilibrium solid-phase K2O·Al2O3·3H2O was discovered. The salting-out effect of KOH and potassium aluminate to K2CrO4 is strong. These findings can provide a significant and precious database for the production of K2CrO4 and the removal of alumina from the concentrated alkali solution that is produced in the chromate cleaner production process.

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INTRODUCTION Chromates, such as alkali metal chromates and bichromates, find wide applications in many industries, including metal plating, leather tanning, pigment production, and erosion protection. They are typically manufactured from chromite (FeO·Cr2O3). In the traditional production process based on the oxidative roasting of chromium ore with alkali metal carbonate, limestone, and dolomite additives, large amounts of residues with highly dangerous Cr6+ are discharged. The cleaner production of chromate technology,1,2 which was based on the original core technology of liquid oxidation in concentrated KOH media, successfully achieves the zero emission of chromium-containing residues and thus has aroused widespread interest. The main reaction involved in the cleaner process is the oxidative reaction of chromite with air in a highly concentrated aqueous solution of KOH which is much more excessive than its stoichiometric amount. The main reaction can be expressed as follows:

process, this work mainly focused on solubility and chemical phase identification for the system KOH−K2CrO4−Al2O3− H2O at 150 °C. In fact, extensive work has been carried out on the solubility of the system K2O−Al2O3−H2O. In an earlier publication,3 we reviewed some important literature and found equilibrium solid-phase K2O·Al2O3·3H2O besides KOH·H2O at the concentrated alkali region of the ternary system at 40 °C. However, previous investigations mainly focused on (30, 40, 60, and 95) °C, so insufficient information has been reported on the system at high temperatures. The partial region phase equilibria of the system K2O−CrO3−H2O at (0, 30, and 60) °C are reported in solubility handbooks.4,5 To the authors’ knowledge, solubility data and chemical phases of the system KOH−K2CrO4−Al2O3−H2O and its subsystems KOH− K2CrO4−H2O and KOH−Al2O3−H2O at 150 °C are presently unavailable, although they are of vital importance for the cleaner process mentioned above.

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EXPERIMENTAL METHOD The solubility was determined by employing the method of isothermal solution saturation.6 Fresh and pure potassium aluminate hydrate crystal was prepared by the reaction of aluminum metal (purity ≥ 99.99 wt %) and analytical-grade potassium hydroxide (purity ≥ 85.0 wt %, containing 14.5 wt % H2O) according to the similar method given in our previous paper.3 The solutions of a preset composition, obtained through alternately dissolving a calculated amount of analytical-grade crystalline anhydrous

FeO·Cr2O3(s) + 4KOH + 7/4O2 (g) → 2K 2CrO4 + 1/2Fe2O3(s) + 2H 2O

Separation-related problems are of great significance for the cleaner process. Achieving effective crystallization separation of K2CrO4 is a key issue to reduce environmental pollution from the source. During the reaction, the associated aluminum in chromite is converted into a soluble state; thus the aqueous solution usually contains a certain amount of water-soluble alumina which impacts the conversion of chromium in chromite and the separation efficiency of K2CrO4. The results of tentative experiments indicated that the crystallization separation of K2CrO4 at the end temperature of 150 °C was of high efficiency. To give a theoretical basis for the separation © 2012 American Chemical Society

Received: April 25, 2012 Accepted: October 4, 2012 Published: October 15, 2012 2971

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potassium chromate (purity ≥ 99.0 wt %), aluminum hydroxide(purity ≥ 99.6 wt %) or the prepared potassium aluminate hydrate, and potassium hydroxide in a given amount of water, were put into a specially devised phase equilibrium apparatus whose main body is a solubility bomb with an inner liquid sampler and an inner detached solid sampler. The sealed bomb was then placed in a HZ-9613Y type thermostatted shaking bath with transmitting heat oil (YTD-320) which was maintained at 150 ± 0.2 °C. A comparison test demonstrated that the equilibrium always was reached in 48 h for nonaluminum system and in 360 h or less for the aluminumcontaining system. Under the experimental conditions, the viscosity of the saturated potassium aluminate solution was still low enough to easily obtain clear liquid through sedimentation within two days. Samples were taken after the phase equilibrium was achieved, and the bomb was allowed to stand for at least 24 hours to settle out the suspended solid. The corresponding equilibrium liquid and wet solids were analyzed. To eliminate the effects of the carbon dioxide in atmosphere, the operations of loading and transporting sample were performed in a nitrogen cabin. The content of potassium hydroxide was titrated by hydrochloric acid solution using phenolphthalein solution as the indicator. Barium chloride was added as a precipitator to deal with chromate for a chromate-containing system. All titrations were performed in triplicate, and the amount of sample was chosen for a required titrant volume of more than 5 mL. The mean value of the triplicate titration results was employed. Potassium, alumina, and chromium were determined by ICP (Optima 5300 DV) at the wavelengths of (766.490, 396.153, and 267.716) nm, respectively. The equilibrium solid samples were placed in a desiccator at room temperature, pestled into powder, and then analyzed by X-ray diffraction analyzer (X'Pert PRO MPD).

versus KOH concentration is sharp. That is, the salting-out effect of KOH on K2CrO4 is very strong. This finding suggested that the separation of K2CrO4 from the reactive system should be operated at the concentrated alkali region. Solids in the concentrated alkali region were identified by the X-ray diffraction (XRD) analytical software, X’pert Highscore 1.0, employing the database of PDF2-2003. Figure 1 displays

Figure 1. XRD pattern of the solid phase of the system KOH− K2CrO4−H2O at 150 °C.

the XRD spectra of the obtained solid. The d values of the eight most intense peaks at scattering angles (2θ) of 34.58°, 29.94°, 17.15°, 20.72°, 29.11°, 34.68°, 29.01°, and 23.28°, are 2.592, 2.984, 5.171, 4.286, 3.067, 2.591, 3.076, and 3.821, respectively, which are in relative agreement with the data of K2CrO4, indicating that in the concentrated alkali region, K2CrO4 exists as an equilibrium solid. Subsystem K2O−Al2O3−H2O. The equilibrium data in concentrated alkali region at 150 °C are collected in Table 2 and plotted in Figure 2. As shown in Figure 2, the solubility isotherm of Al 2 O 3 declines with the increase of the concentration of KOH.

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RESULTS AND DISCUSSION Subsystem KOH−K2CrO4−H2O. The equilibrium data in concentrated alkali region at 150 °C were summarized in Table 1, which shows that the solubility of K2CrO4 decreases with the increase of the KOH content. Because hexavalent chromium is very dangerous for human beings and the environment, reducing the amount of hexavalent chromium in recycled potassium hydroxide media is of significance. Based on this point, we consider that the decrease of chromate solubility

Table 2. Solubility of Potassium Aluminate Hydrate in the System K2O(w1a)−Al2O3(w2b)−H2O(w3c) at 150 °C composition of liquid phase (wt %)

Table 1. Solubility of K2CrO4 in the System KOH(w1)− K2CrO4(w2)−H2O(w3) at 150 °C mass fraction of solution (wt %) no.

100·w1a

100·w2b

100·w3

equilibrium solidc

1 2 3 4 5 6 7 8 10

51.92 52.49 53.42 56.50 60.50 65.26 68.40 72.50 75.74

1.14 1.09 1.01 0.85 0.69 0.57 0.50 0.42 0.36

46.94 46.42 45.57 42.65 38.81 34.17 31.10 27.08 23.90

K2CrO4 K2CrO4 K2CrO4 K2CrO4 K2CrO4 K2CrO4 K2CrO4 K2CrO4 K2CrO4

composition of wet solid (wt %)

no.

100·w1

100·w2

100·w1

100·w2

equilibrium solidd

1 2 3 4 5 6 7 8

40.02 41.92 46.12 51.33 52.76 56.94 60.06 63.15

18.24 13.92 8.17 3.90 2.93 1.76 0.95 0.52

40.12 41.55 45.13 49.09 50.46 54.05 55.79 57.82

21.97 24.13 14.80 12.28 10.82 9.15 10.52 10.95

K2O·Al2O3·2H2O K2O·Al2O3·2H2O K2O·Al2O3·2H2O K2O·Al2O3·2H2O K2O·Al2O3·2H2O K2O·Al2O3·2H2O K2O·Al2O3·2H2O K2O·Al2O3·2H2O

a Standard uncertainty (k = 1): ur(w1) = 0.0021. bStandard uncertainty (k = 1): ur(w2) = 0.0022. cw3 = 100 − w1 − w2. dIdentified by Schreinemaker’s method, the purity of any sample is higher than 98 % (counted as Al2O3).

The potassium aluminate hydrate solid obtained from concentrated alkali region was verified by Schreinemaker’s method6 and X-ray diffraction, as shown in Figures 2 and 3, respectively. It is clearly shown in Figure 2 that the different coupling lines cross at the neighborhood of the point (K2O·Al2O3·2H2O)

a Standard uncertainty (k = 1): ur(w1) = 0.0025. bStandard uncertainty (k = 1): ur(w2) = 0.0012. cIdentified by XRD, the purity of any sample is higher than 99 % (counted as CrO42−).

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Table 3. Solubility of K2CrO4 and Potassium Aluminate Hydrate in the System KOH(w1a)−K2CrO4(w2b)− Al2O3(w3c)−H2O(w4d) at 150 °C composition of liquid phase (wt %) no.

100·w1

100·w2

100·w3

1 2 3 4 5 6

48.8 52.17 54.88 61.56 64.29 67.61

0.49 0.44 0.38 0.33 0.31 0.29

17.48 12.62 7.44 3.95 2.89 2.00

equilibrium solide K2CrO4 K2CrO4 K2CrO4 K2CrO4 K2CrO4 K2CrO4

+ + + + + +

K2O·Al2O3·2H2O K2O·Al2O3·2H2O K2O·Al2O3·2H2O K2O·Al2O3·2H2O K2O·Al2O3·2H2O K2O·Al2O3·2H2O

a Standard uncertainty (k = 1): ur(w1) = 0.0025. bStandard uncertainty (k = 1): ur(w2) = 0.0012. cStandard uncertainty (k = 1): ur(w2) = 0.0022. dw4 = 100 − w1 − w2 − w3. eIdentified by XRD.

Figure 2. Phase diagram of the system K2O−Al2O3−H2O in the concentrated alkali region at 150 °C.

Figure 4. Solubility of K2CrO4 and potassium aluminate (counted as Al2O3) in the system KOH−K2CrO4−Al2O3−H2O at 150 °C: □, Al2O3; ●, K2CrO4. Figure 3. XRD pattern of the solid phase of the system K2O−Al2O3− H2O at 150 °C.

with the theoretical composition of 40.57 % of Al2O3 and 43.93 % of K2O, indicating that in the concentrated alkali region K2O·Al2O3·2H2O is an equilibrium solid before the potassium hydroxide in the mixed aqueous solution reaches saturation. Figure 3 displays the X-ray diffraction spectra of the obtained potassium aluminate hydrate whose relatively strong diffraction peaks differ from those of K2O·Al2O3·3H2O or K2O·Al2O3·H2O in the database of PDF-2 which has no diffraction data of K2O·Al2O3·2H2O. To give referenced data for the more complex system KOH−K2CrO4−Al2O3−H2O, relatively strong diffraction peaks of K2O·Al2O3·2H2O are labeled in Figure 3. System KOH−K2CrO4−Al2O3−H2O. The equilibrium data in concentrated alkali region at 150 °C are listed in Table 3 and plotted in Figure 4. As shown in Figure 4, the solubility isotherms of Al2O3 and K2CrO4 decline with the increase of the concentration of KOH. Comparing Table 1 with Table 3, the fact that potassium aluminate plays an evidently salting-out role to potassium chromate can be affirmed. This finding is of significance for the cleaner production of potassium chromate. The solids obtained from concentrated alkali region were verified by X-ray diffraction, as shown in Figure 5. Almost all relatively strong diffraction peaks of K2CrO4 in the database of PDF-2 are in relative agreement with those in Figure 5, indicating that, in the concentrated alkali region, K2CrO4 exists as an equilibrium solid phase. We identified the aluminate solid

Figure 5. XRD pattern of the solid phase of the system KOH− K2CrO4−Al2O3−H2O at 150 °C: peaks with a, ascribable to K2CrO4; peaks with various number, ascribable to K2O·Al2O3·2H2O.

phase by noting diffraction angles (2θ) and the corresponding d values of the most relatively strong diffraction peaks in Figure 5 and then comparing them one by one with the diffraction data of K2O·Al2O3·2H2O obtained by this work, which is displayed in Figure 3. The comparison indicates that all diffraction angles and d values were in relative agreement, so equilibrium solids contain K2O·Al2O3·2H2O. The main strong diffraction peaks 2973

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are all appointed ascriptions; thus equilibrium solids can be identified as K2CrO4 and K2O·Al2O3·2H2O.

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CONCLUSIONS The phase equilibria of the system KOH−K2CrO4−Al2O3− H2O and its subsystems at 150 °C in a high alkali concentrated region were investigated. The solubility isotherm of K2CrO4 decreases sharply with the increase of the concentration of KOH, and the separation of K2CrO4 from the reactive system should be performed at the concentrated alkali region. In the concentrated alkali region of the aluminum-containing system, K2O·Al2O3·2H2O exists as an equilibrium solid phase. This is different from the system KOH−Al2O3−H2O at low temperature from which the equilibrium solid-phase K2O·Al2O3·3H2O was discovered. Potassium aluminate plays an evident salting-out role to potassium chromate. These findings can provide a significant database for the production of K2CrO4 and the removal of alumina from the concentrated alkali solution that was produced in the chromate cleaner production process.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-532-88030522; fax: +86-532-88030364. E-mail address: [email protected]. Funding

The authors gratefully acknowledge the financial support from the National Science Foundation of China (No. 50234040, 50874099) and the Major Program Project of the Knowledge Innovation Program of the Chinese Academy of Sciences (No. KCCX1-SW-22). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors wish to thank Tao Qi, Huiquan Li, Hongbin Cao, and Hongbin Xu, who have offered valuable suggestions. REFERENCES

(1) Zhang, Y.; Li, Z.; Qi, T.; Zheng, S.; Li, H.; Xu, H. Green manufacturing process of chromium compounds. Environ. Prog. 2005, 24, 44−50. (2) Zhang, Y.; Li, Z.; Qi, T.; Wang, Z.; Zheng, S. Green Chemistry of Chromate Cleaner Production. Chin. J. Chem. 1999, 17, 258−266. (3) Du, C.; Zheng, S.; Zhang, Y. Phase Equilibria in the K2O-Al2O3H2O System at 40 °C. Fluid Phase Equilib. 2005, 238, 239−241. (4) Silcock, H. L. Solubilities of Inorganic and Organic Compounds, Vol. 3; Pernamon Press: New York, 1979. (5) Seidell, A. Solubilities of Inorganic and Metal Organic Compounds, Vol. 1; D. Van Nostrand Company Inc: New York, 1953. (6) Weissberg, A.; Rossiter, B. W. Physical methods of chemistry, Part V: Determination of thermodynamic and surface properties; WileyInterscience: New York, 1971.

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