Phase Diagram for the Na2SO4–NaOH–H2O System and Na2SO4

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Phase Diagram for the Na2SO4−NaOH−H2O System and Na2SO4 Solubility in Sodium Aluminate Solution with Caustic Ratios of 12 and 15 at 80 °C Aijuan Ding,†,‡ Yuejiao Wang,† Shuhua Ma,*,† Shili Zheng,† and Fen Guo‡ †

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 ‡ School of Chemical Engineering, Beijing University of Chemical Technology, Beijing 100029, People’s Republic of China

ABSTRACT: The phase diagram for the Na2SO4−NaOH−H2O system and solubility data for Na2SO4 in sodium aluminate solution with Na2O to Al2O3 ratios (caustic ratios) of 12 and 15 were studied in the full alkali concentration region at 80 °C. The contents of Al and Na in the solution were analyzed by means of inductively coupled atomic plasma emission spectrometry (ICPOES). The sulfur content was determined using ion chromatography (IC) and the ethylenediaminetetraacetic acid (EDTA) volumetric method. The phases of the equilibrium solids were identified via X-ray diffraction (XRD) as well as Schreinemaker’s method. The results showed that the Na2SO4 solubility declines as the concentration of NaOH in the ternary system increases. At a NaOH concentration exceeding 32.96 % (mass percent), the Na2SO4 solubility is lower than 1.13 %. For this system, the corresponding equilibrium solid phase was identified as anhydrous sodium sulfate (Na2SO4) and sodium hydroxide (NaOH). The Na2SO4 solubility in the quaternary system approximates the values obtained in the Na2SO4−NaOH−H2O system. At NaOH concentrations higher than 30.68 % and 30.76 % the Na2SO4 solubility values in the sodium aluminate solution with caustic ratios of 12 and 15 are lower than 1.03 % and 1.05 %, respectively.

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INTRODUCTION The alumina industry is an important basic raw material industry that has developed steadily worldwide in recent years, with production increasing at an average annual rate of 3.11 %. In China, the total yield of alumina was 3407.8 million tons (Mt) in 2011, accounting for 40 % of the total world output, with a year-on-year growth rate of 17.70 %.1 Swelling domestic demand for alumina increased the ratio of imported bauxite to more than 50 % in 2011.2 Thus, the search for alternatives to bauxite resources is becoming increasingly urgent. China is abundant in high-alumina fly ash, especially in Inner Mongolia, Shanxi Province. The mass content of alumina in the fly ash in these regions is in excess of 30 % and even 50 % in certain instances. Thus, fly ash can be considered as a precious alumina containing resource.3,4 At present, almost all of the high-alumina coal ash has been piled away due to the lack of economic and flexible utilization technology. This piling not only occupies a vast area of land but also contaminates the environment.5 This is particularly harmful for the northwest © XXXX American Chemical Society

region of China, which experiences predominantly windy weather conditions. Moreover, stockpiling fails to make good use of the valuable components contained in the fly ash, such as alumina and silica. Thus, unearthing means of actively exploiting such nontraditional resources is of great significance to the alumina industry. Alkaline processing, the Bayer process in particular, is the primary method of alumina production, which has been widely used in the alumina industry for more than one hundred years and plays an important role in aluminum production. However, the Bayer process cannot be applied to fly ash because of the low mass ratio of alumina to silica (A/S).6,7 As is well-known, the content of alumina in fly ash is high, but the content of silica is also very high. The A/S of high alumina fly ash is generally in the range of 0.80 to 1.00. Received: November 29, 2012 Accepted: March 1, 2013

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used for the experiments at 80 °C. Polyethylene bottles, each of 200 cm3 capacity, were used for storing the experimental samples and placed in the thermostat. A vacuum drying oven (D2X-6020B, Shanghai Fuma Laboratory Equipment Co., Ltd.) was used to dry the solid obtained when equilibrium was attained. Inductively coupled atomic plasma emission spectrometry (ICP-OES, PE Optima 5300DV, Perkin-Elmer) was used to analyze the chemical composition of the liquid and solid. The equilibrium solid phase was identified via X-ray diffraction (XRD, Philips PW226/30 with Cu Kα radation, 40 kV and 100 mA). The Na2SO4 content was determined via ion chromatography (IC, Dionex, DX-500) coupled with the ethylenediaminetetraacetic acid (EDTA) volumetric method. All chemicals used in this work, including sodium hydroxide, anhydrous sodium sulfate, and aluminum hydroxide, were of analytical grade and were manufactured by Sinopharm Chemical Reagent Company. High-purity Milli-Q water, with a resistivity of above 18.2 MΩ·cm at ambient temperature, was used for all experiments. Procedure. For the Na2SO4−NaOH−H2O ternary system, a range of sodium hydroxide solutions were prepared by dissolving various amounts of sodium hydroxide in high purity water; these solutions were placed in a constant temperature water bath for quick dissolution of the sodium hydroxide. When the solutions became clear, anhydrous sodium sulfate was added to the aqueous alkali while stirring. Stirring was discontinued when no further dissolution of the salts occurred. The solutions were then transferred into polyethylene bottles, and the bottles were subsequently sealed and placed in thermostatted air shaking baths with the temperature maintained at 80 °C. The shaking speed of the air baths was set at 190 rpm to improve the mass transfer process and accelerate the system equilibration. To guarantee the accuracy of the solubility data, excess anhydrous sodium sulfate was added to the prepared solutions. Stirring was stopped at the point of analysis, and the samples were allowed to stand in the baths long enough for sedimentation of the suspended precipitates. The Na and Al contents of the clear liquid and the solid phase were determined via ICP-OES and titration analysis, respectively. The SO42− content of the liquid and solid phases was obtained by IC with dilution to obtain concentrations within the calibration range. As a comparative analysis, the EDTA volumetric method was also used to determine the concentration of SO42− in selected samples. The detailed procedure is as follows: aliquots of the liquid samples were withdrawn using a sampling gun. For each sample, ca. 1 mL of solution was withdrawn and placed into a weighing bottle. The solution was then quickly weighed. Subsequently, the solution was transferred to a volumetric flask and diluted to a suitable concentration using high-purity water for further analysis. The clear liquid phase was removed from the bottle, leaving behind the solid phase with a small amount of the liquid phase, namely, the wet solid phase. The chemical composition of the wet solid was analyzed in a manner similar to the aforementioned process. This procedure was repeated over the full alkali concentration. Schreinemaker’s method was used to identify the equilibrium solid phase using the compositions of the liquid phase and the wet solid phase. The solid phases were also identified using X-ray diffraction (XRD). The solids obtained were washed with ethanol several times prior to analysis and then dried in a vacuum oven to constant weight. To prevent decomposition of the solid phase, the drying temperature was set at 40 °C. The results of this analysis were

Accordingly, a highly efficient and environmental friendly alumina extraction process has been developed by the Institute of Process Engineering (IPE), Chinese Academy of Sciences (CAS). This new process involves the use of highly concentrated NaOH solution to extract alumina from the fly ash, with an alumina extraction rate of above 85 %. Unfortunately, fly ash contains a number of impurities, some of which are highly deleterious to alumina production. As a primary impurity, sulfur is often present in fly ash in the form of calcium sulfate, which is transformed to sodium sulfate in the process of extracting alumina from coal ash. This has negative effects on alumina production to varying degrees. For example, the impurity in the form of SO42‑ can increase the viscosity and enhance the stability of the solution, while the presence of Na2SO4 retards seed decomposition. In the seed decomposition process, sulfur accumulation increases the alkali consumption, decreases the precipitation ratio, and especially enhances the concentration of soluble iron, which results in contamination of the alumina product and serious equipment corrosion. Furthermore, the production operations are negatively impacted by sodium sulfate crystallization when it accumulates to a certain extent. Additionally, serious scaling occurs in the mother liquor evaporator and digestion reactor, with a consequent sharp reduction of the heat transfer coefficient.8−10 Based on these considerations, removal of sulfur compounds from the process is necessary to restrict the sodium sulfate content within a harmless limit. The solubility of sodium sulfate in sodium aluminate solution is the theoretical basis for removing the sulfur component. Thus, evaluation of the solubility of sodium sulfate in sodium aluminate solution is of particular importance. Due to the complexity of the quaternary system, such studies often start from empirical evaluation of the phase diagram of the ternary system. Certain solubility data for sodium sulfate in these solutions at 25 °C, 35 °C, 50 °C, 90 °C, 140 °C, and so forth, are currently available.11−13 However, corresponding data at 80 °C, which is the temperature at which the solid−liquid separation is performed in the new process developed by IPE, CAS for extracting alumina from fly ash, has not yet been published. Furthermore, only the solubility data for Na2SO4 in sodium aluminate solution with a caustic ratio of 3.6 is currently available in the literature published.14 Few studies have focused on the sodium sulfate solubility data for the system mentioned above at higher caustic ratios. In this regard, the phase diagram for the Na2SO4−NaOH−H2O system is evaluated herein over a wide alkali concentration using X-ray diffraction coupled with Schreinemaker’s method.15 Based on Schreinmaker’s method, the equilibrium solid phase can be determined via extrapolation. At equilibrium, the line connecting the saturated liquid phase with the corresponding wet solid phase also represents the intermediate composition for various compositions of solid and liquid phases. Thus, by joining the points for the saturated liquid with the corresponding wet solid, on the extrapolated line there would be a point representing the composition of the equilibrium solid. The lines connecting different pairs of liquid and solid phases have a common intersection if the equilibrium solids are the same. The solubility of Na2SO4 in sodium aluminate solutions with caustic ratios of 12 and 15 at are also evaluated at 80 °C herein.

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EXPERIMENTAL SECTION Apparatus and Reagents. A specially designed HZ-9612K thermostatted shaking air bath with a precision of 0.2 °C was B

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consistent with those obtained by Schreinemaker’s method. All of the samples were analyzed at three-week intervals. Equilibrium was considered to be attained when the liquid phase concentrations no longer changed. For the quaternary system having a caustic ratio of 12, a range of aluminate solutions with different alkali concentrations were prepared by adding various amounts of Al(OH)3 to the sodium hydroxide solutions. The process used for preparing the sodium hydroxide solutions was the same as that used for the ternary system. The amount of Al(OH)3 utilized was selected to guarantee that the caustic ratio of each solution was 12. The same process was used to prepare the solutions with caustic ratios of 15. Excess anhydrous sodium sulfate was then added to the sodium aluminate solutions. The subsequent operations were the same as those used for the ternary system. For both systems, the analysis of duplicate solutions demonstrated that the results were reproducible.

Figure 1. Phase diagram for Na2SO4−NaOH−H2O system at 80 °C. A: the solubility of Na2SO4 in pure water at 80 °C; B: the solubility of NaOH in pure water at 80 °C; C: Na2SO4; D: NaOH; E: three-phase point; S: saturated solution; Combination of symbols (such as C+S) means that the items coexist. AE and EB indicate the compositions of saturated ternary solutions that are in equilibrium with the solids Na2SO4 (C) and NaOH (D), respectively. Thick solid lines are tielines between coexisting phases. The dashed lines connect the compositions of saturated solution with the corresponding wet solid.

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RESULTS AND DISCUSSION Phase Diagram for the Na2SO4−NaOH−H2O System. The equilibrium composition data for the Na2SO4−NaOH− H2O system at 80 °C is summarized in Table 1, and the corresponding phase equilibrium diagram is shown in Figure 1. Table 1. Equilibrium Data for the Na2SO4−NaOH−H2O System at 80 °C and Pressure p = 0.1 MPaa,b composition of alkaline solution

composition of wet solid phase

sample no.

100 w1

100 w2

100 w1

100 w2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

0.00 2.90 6.52 8.90 11.60 17.19 21.74 27.93 32.96 38.66 44.64 50.59 57.31 63.33 68.69 72.97 74.32 75.80

25.02 20.30 15.20 12.57 9.85 6.04 3.67 1.63 1.13 0.79 0.69 0.60 0.41 0.27 0.23 0.06 0.05 0.00

2.61

45.62

4.92

55.17

19.93

27.70

36.15

30.96

48.03 54.92 64.35

24.04 19.55 11.01

Figure 1 shows the solubility of Na2SO4 in pure water at 80 °C, with a value of 25.02 %. Point B on the abscissa shows the solubility of NaOH in pure water at 80 °C, with a value of 75.80 %. Point C represents pure solid Na2SO4. Point D represents pure solid NaOH. Point E is the three-phase point where solid anhydrous sodium sulfate and sodium hydroxide coexist at saturation. S represents the solution saturated with Na2SO4. The lines AE and EB indicate the compositions of solutions with different concentrations of NaOH and Na2SO4 that are in equilibrium with solids Na2SO4 (C) and NaOH (D). The area below the saturated liquid line AB represents the unsaturated sodium sulfate and alkali solution region and above the saturated line represents the saturated sodium sulfate and alkali solution and Na2SO4 (C). The crystalline phase of Na2SO4 (anhydrous sodium sulfate) for this system at 80 °C is the same as that present at 50 °C in the wide alkali concentration region.13 Thus, it can be concluded that the equilibrium solid Na2SO4 (anhydrous sodium sulfate) can exist stably in the temperature region spanning 50 °C to 80 °C. Figure 2 shows the XRD pattern of the equilibrium solids for the ternary system, identified as anhydrous sodium sulfate. The dashed lines connecting the compositions of saturated solution with the corresponding wet solid in Figure 1 show that the equilibrium solids for the samples in the full alkali concentration region are all the same. Solubility Data for Na2SO4 in Sodium Aluminate Solutions. The solubility data for Na2SO4 in sodium aluminate solutions with caustic ratios of 12 and 15 at 80 °C are summarized in Table 2, and the corresponding solubility data curves are shown in Figure 3. As can be seen from Figure 3, the data for the sodium aluminate solutions containing sulfate, with caustic ratios of 12 and 15, are very similar to each other and are very close to the curve obtained for the ternary system. The Na2SO4 solubility decreases monotonically with increasing alkali concentration. For the system with a caustic ratio of 12, there is an obvious decrease in the solubility of Na2SO4 with increasing NaOH

equilibrium crystalline phases C C C C C C C C C C C C C C C C C+D D

a w1, mass fraction of NaOH; w2, mass fraction of Na2SO4; C, anhydrous sodium sulfate solid phase; D, sodium hydroxide solid phase. bStandard uncertainties u are u(T) = 0.02 K, u(w) = 0.0003.

The results demonstrate that the Na2SO4 solubility decreases monotonically with an increase in the NaOH concentration. In the low alkali region (where the mass percent of NaOH is less than 32.96 %) the Na2SO4 solubility decreases much more sharply than in the high alkali region. Where the mass percent of NaOH is less than 32.96 %, the solubility of Na2SO4 becomes lower than 1.13 %. For this system, the equilibrium solid phase is identified as anhydrous sodium sulfate and sodium hydroxide, the phase identification analysis of which is shown graphically in Figure 1. Point A on the ordinate of C

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Figure 2. XRD patterns of the equilibrium solids obtained from the ternary system; the equilibrium solid of the system is anhydrous sodium sulfate.

Figure 3. Solubility data for Na2SO4 in sodium solution and in the sodium aluminate solutions with caustic ratios of 12 and 15 at 80 °C.

Table 2. Solubility Data for Na2SO4 in Sodium Aluminate Solutions at 80 °C and Pressure p = 0.1 MPaa,b composition of liquid phase for solutions with caustic ratios of 12

composition of liquid phase for solutions with caustic ratios of 15

sample no.

100 w1

100 w2

100 w1

100 w2

1 2 3 4 5 6 7 8 9 10 11 12 13

5.21 7.58 9.97 12.06 15.22 19.79 23.35 25.96 28.45 30.68 32.59 37.27 40.59

20.54 17.25 14.02 11.77 8.29 4.87 2.98 1.92 1.33 1.03 0.78 0.51 0.44

3.42 6.56 8.96 11.25 13.85 17.14 20.13 23.69 27.05 30.76 33.78 37.44 41.82

22.73 18.56 15.22 12.73 10.08 6.87 4.86 3.14 1.66 1.05 0.62 0.52 0.42

Figure 4. Solubility data for Na2SO4 in the sodium aluminate solutions with caustic ratios of 12 and 15 at 80 °C compared with the solubility data of Na2SO4 in the sodium aluminate solutions with caustic ratios of 3.6 at 75 °C.

a

w1, mass fraction of NaOH; w2, mass fraction of Na2SO4. bStandard uncertainties u are u(T) = 0.02 K, u(w) = 0.0003.

concentration in the region where the concentration of NaOH is less than 30.76 %. As the alkali concentration increases to values higher than 30.76 %, the solubility of Na2SO4 becomes lower than 1.05 %. For the same system with a caustic ratio of 15, the Na 2 SO 4 solubility is below 1.03% when the concentration of NaOH reaches 30.68 %. Compared with the solubility data for Na2SO4 in the sodium aluminate solutions with a caustic ratio of 3.6 at 75 °C, the solubility of Na2SO4 obtained at caustic ratios of 12 and 15 are a little lower, but the values are overall very similar, as illustrated in Figure 4. Thus, the addition of Al(OH)3 has no evident impact on the Na2SO4 solubility in the sodium hydroxide solution at the same temperature.

system at 80 °C demonstrated that increasing the NaOH concentration monotonically decreases the solubility of Na2SO4. In the region where the concentration of NaOH is less than 30.76 % the Na2SO4 solubility decreases much more sharply than in the high alkali region. The solubility of Na2SO4 becomes lower than 1.05 % when the concentration of NaOH is higher than 30.76 %. The solubility curves of Na2SO4 in the sodium aluminate solutions with caustic ratios of 12 and 15 exhibited trends similar to the ternary system. As the concentration of NaOH exceeds 30.68 % and 30.76 % for the respective systems, the corresponding sodium sulfate solubility becomes lower than 1.03 % and 1.05 %. The equilibrium solid produced in the ternary systems is anhydrous sodium sulfate. The addition of Al(OH)3 causes a slight decrease of the Na2SO4 solubility in the sodium hydroxide solution. This study provides a significant database for the study of removal of sulfate impurities, with potential applicability to the process of alumina extraction from fly ash.

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CONCLUSIONS The phase diagram for the Na2SO4−NaOH−H2O system and the solubility of Na2SO4 in sodium aluminate solutions with caustic ratios of 12 and 15 at 80 °C were investigated in this study. The phase diagram for the Na2SO4−NaOH−H2O D

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +86-10-82544856. Fax: +86-10-82544856. Funding

The authors gratefully acknowledge the National Science and Technology Support Program of China under Grant No. 2012BAF03B01, the National High Technology Research and Development Program of China (863 program) under Grant No. 2011AA060701. Notes

The authors declare no competing financial interest.

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