Solubility and Metastable Zone Width of Sodium Chromate Tetrahydrate

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Solubility and Metastable Zone Width of Sodium Chromate Tetrahydrate † ‡ † ‡ † ‡ † † † † Liping Wang, , Jiaoyu Peng, , Lili Li, , Haitao Feng, Yaping Dong,*, Wu Li, Jian Liang, † and Zhulin Zheng † Qinghai Institute of Salt Lakes, Chinese Academy of Sciences, 810008, Xining, China ‡ University of Chinese Academy of Sciences, 100039, Beijing, China

ABSTRACT: The solubility of sodium chromate tetrahydrate has been investigated in the temperature range from 290.15 K to 325.15 K using the conventional polythermal method by means of the laser technique. The metastable zone width (MZW) and the apparent nucleation order of sodium chromate tetrahydrate were calculated. The eﬀects of cooling rate, initial composition, stirring rate, and seed particles on the MZW were also studied. The results show that the MZW of sodium chromate tetrahydrate ﬁrst increases and then narrows with increasing initial composition. The MZW becomes wider by reducing the cooling rate and the mass of seed particles, whereas the MZW will be reduced by enhancing the rotation frequency of stirring and the diameter of seed particles.

■ INTRODUCTION ■ EXPERIMENTAL SECTION · Sodium chromate, which is a basic compound in chromate Materials and Apparatus. Na2CrO4 4H2O, provided from chemical industry, can be used in pigments, metal finishing, Tianjin Paisen Technology Corporation, was recrystallized textile, leather tanning, and wood preservative fields.1 It also from an aqueous solution with the mass fraction purity of more plays a crucial part in the production of sodium dichromate as than 99.5 %. Water (resistivity: 18.25 MΩ·cm) was deionized alkali metal chromate.2 Generally, roasted chromite is leached by a water purification system (UPT-II-20T, Chengdu by water, acidulated, and crystallized.3 The techniques for Ultrapure Technology Co., Ltd.). The experimental setup for producing sodium dichromate from sodium chromate include measuring MZW is shown schematically in Figure 1. The − sulfuric acidification,4 carbonization acidification,5 7 sodium temperature and turbidity control was accomplished by a − bisulfate,8,9 and the electrosynthesis method.10 12 CrystalSCAN with four parallel reactors (E1061, United The electrosynthesis method, as a green technology, has Kingdom He., Ltd.) containing systems for temperature control competitive potential and enormous advantages such as a lack and computer processing as well as a crystallizer assisted with a of pollution, high product purity, defined byproducts, and high yields compared with other traditional techniques.12 The impurity of sodium chromate negatively influences the electrosynthesis process, including blocking of ion-exchange membrane channels, decline in current efficiency, and life expectancy of ion-exchange membrane, poor quality of product, and so forth. Hence, obtaining high-purity sodium chromate has become a first-class issue needed to be solved. Investigation of the metastable zone properties of sodium chromate has been considered as a criterion for the design of a crystallization 13 process. Figure 1. Apparatus for solubility and crystallization measurements: 1, In this paper, the metastable zone widths (MZWs) of sodium low constant temperature bath; 2, temperature control system; 3, chromate tetrahydrate in the temperature range from 290.15 K computer processing system; 4, crystallizer; 5, turbidity sensor; 6, to 325.15 K were determined. Moreover, the effects of cooling overhead stirring; 7, temperature sensor; 8, seed injector. rate, initial composition, stirring rate, and mass and size of seed particles on the MZW of sodium chromate tetrahydrate were Received: July 3, 2013 discussed. The findings can provide an experimental basis for Accepted: October 8, 2013 the crystallization and purification of sodium chromate. Published: October 18, 2013

© 2013 American Chemical Society 3165 dx.doi.org/10.1021/je4006272 | J. Chem. Eng. Data 2013, 58, 3165−3169 Journal of Chemical & Engineering Data Article programmable thermostatic bath (SF-01-T, Ningbo Haishu Seif Experimental Instrument Factory, China). The crystallizer was a 100 mL glass vessel with an internal overhead stirrer, temperature sensor, and turbidity sensor, as can be seen in Figure 1. The accuracy of the temperature sensor was 0.1 K. The XRD analysis (X′Pert PRO, 2006 PANalytical) and simultaneous thermal analysis (STA449F3, Netzsch Instru- ments Manufacturing Co., Ltd.) were used for confirming the identity of the solid phase containing hydrates crystallized from the sodium chromate solutions. Metastable Zone Width Measurements. The MZW of the sodium chromate tetrahydrate was determined in the temperature range from 290.15 K to 325.15 K by the conventional polythermal method.14 A 100 mL well-sealed − glass vessel with a Na2CrO4 H2O mixture (80 g) was placed into the CrystalSCAN and then heated to 5 K above the saturation temperature, remaining for 10 min. Then the solution was cooled with a settled cooling rate until a remarkable increase in turbidity was observed. The temperature Figure 2. XRD pattern of sodium chromate tetrahydrate: (a) pure; (b) fi crystallized from solution. at this point was de ned as the nucleation temperature, Tnuc. Finally, the mixture was kept 5 K below the nucleation ff temperature for 10 min and then was heated at the same from solution. The X-ray powder di raction patterns of pure constant rate until dissolution of the solute in which the sodium chromate tetrahydrate and the product crystallized turbidity remained constant. The corresponding temperature from solution are identical, which reveals the crystallized 15 product is sodium chromate tetrahydrate. was recorded as Tdis. The steps described above were repeated at five cooling/heating rates of 15 K·h−1,25K·h−1,35 Simultaneous Thermal Analysis. Simultaneous thermal K·h−1,45K·h−1, and 55 K·h−1 with a constant stirring rate of analysis was used to determine the crystal water of sodium chromate. The measurements were carried out using nitrogen 450 rpm. fl · −1 In the dissolution process, it requires sufficient time for the purge gas with a ow rate of 70 mL min . A portion of 10 mg to 15 mg of the sample in an aluminum sample pan was heated solid dissolved to solution completely. For fast heating rates, · −1 the dissolution time will increase the dissolution temperature from 300 K to 1025 K at a heating rate of 300 K h . The TG greatly, so the dissolution temperature measured at the point of (a) and DTG (b) curves are shown in Figure 3. It can be disappearance is greater than the actual saturation temperature. Accordingly, as the heating rate slows down, the dissolution temperature is closer to the saturation temperature.16,17 Therefore, the saturation temperature, recorded as Tsat, of the sodium chromate tetrahydrate can be obtained by the extrapolation of the measured dissoluiton temperature Tdis to a virtual heating rate “zero”.15 The difference between the saturation temperature and the temperature at the point of Δ nucleation is considered as the MZW( Tmax), which could be Δ − calculated by the formula of Tmax = Tsat Tnuc. The estimated uncertainties were summarized in Table 1.

Table 1. Uncertainties of Measurements Estimated for These Studies

property estimated uncertainty solubility ± 0.30 to 0.39 ± Figure 3. TG/DTG curves of sodium chromate crystals: (a) thermal saturation temperature 0.06 K ff · ± gravity curve; (b) di erential thermal gravity curve. 100w (Na2CrO4 4H2O) 0.21 to 0.26 Δ ± Tmax 0.06 K observed that the crystallized product starts to lose crystal water at room temperature and completes at 354.7 K. The unique ■ RESULTS AND DISCUSSION sharp valley from the DTG curve indicates that the process of XRD Analysis. Powder X-ray diffraction (XRD) is useful for losing crystal water is continuous. The residue mass is 8.9425 confirming the identity of a solid material and its phase purity. mg (69.76 % of initial simple mass). The crystal water of The XRD analysis was performed using a tube voltage and crystallized product is 3.90 by calculation which is close to 4. current of 40 kV and 30 mA, respectively. The scanning The result of simultaneous thermal analysis further proves that position 2θ is from 5.0014° to 69.9754°. Figure 2a shows the the crystallized product is sodium chromate tetrahydrate. pure sodium chromate tetrahydrate pattern obtained from the Solubility of Sodium Chromate Tetrahydrate. The · PDF card (reference code: 00-022-1366). Figure 2b shows the solubility of Na2CrO4 4H2O in aqueous solution was XRD spectrum of sodium chromate tetrahydrate crystallized determined in the temperature range from 290.15 K to

3166 dx.doi.org/10.1021/je4006272 | J. Chem. Eng. Data 2013, 58, 3165−3169 Journal of Chemical & Engineering Data Article

325.15 K. The experimental solubility data are shown in Table 2. They are very similar to those reported previously.18

Table 2. Solubility of Sodium Chromate Tetrahydrate at Diﬀerent Saturation Temperatures · ζ · Tsat/K 100w (Na2CrO4 4H2O) 100 (Na2CrO4 4H2O) 290.58 61.29 158.33 301.09 66.00 194.12 310.34 68.49 217.35 321.07 71.37 249.28 324.21 73.28 274.25

Eﬀect of the Stirring Rate on MZW. Solutions of · constant mass fraction (100w Na2CrO4 4H2O = 72.90) of sodium chromate tetrahydrate were prepared, and the MZW was determined at diﬀerent stirring rates with variable cooling rates (55 K·h−1;45K·h−1;35K·h−1;25K·h−1;15K·h−1). The Figure 5. Changes of MZW of sodium chromate tetrahydrate in results are plotted in Figure 4. It can be observed that the − dependences of initial composition. Cooling rate (K·h 1): ■, 55; ●, 45; ▲, 35; ▼, 25; ◆, 15.

The MZW first increases and then decreases with increasing initial composition. The basic reason is that at low initial composition the solution viscosity increases with increasing initial composition, which hinders the solute diffusion and heat transfer, thus causing a decrease of nucleation and an increase of the metastable zone width. However, at high initial composition, the highly increasing saturation temperature becomes the dominant factor affecting the MZW greatly. When the saturation temperature increases, the diffusion and transfer rate of solute increase, improving the collision probabilities. Consequently, the formation of crystals is facilitated, and the MZW narrows. Calculation of Apparent Nucleation Order (m). For Figure 4. MZW of sodium chromate tetrahydrate with different crystal nucleation theory, the equation of MZW and cooling stirring rates. Stirring rate (rpm): ■, 250; ●, 300; ▲, 350; ▼, 400; ◆, rate can be calculated by means of the following formula at a 450. constant stirring rate:19

⎛ ⎞ log k 1 −∗m ⎜⎟dc n 1 stirring rate has a certain impact on the values of MZW, log Δ=Tmax log −+log β m ⎝ dT ⎠ mm especially when the rate is greater than 350 rpm. The MZW of sodium chromate tetrahydrate decreases with the increase of Δ β − Tmax is the value of MZW, is the cooling rate, and kn is a stirring rate. At the cooling rate of 55 K·h 1, the values of * Δ constant related to the nucleation rate. dc /dT is the slope of Tmax are reduced to more than a half when the stirring rate the solubility in dependence of a temperature change. m means increases from 250 rpm to 450 rpm. The possible reason is that Δ ff the apparent nucleation order. The dependence of log Tmax the collision chances of crystal nucleation and the di usion of from log β of sodium chromate tetrahydrate is summarized in solute have an obvious increase by increasing the stirring rate, Table 3 and plotted in Figure 6. resulting in the appearance of an earlier crystal nucleation. For different mass fractions of sodium chromate tetrahydrate, Consequently, the MZW of sodium chromate tetrahydrate the slope of the line is almost identical. To obtain a more actual tends to narrow with the increase of the stirring rate. 14 Effect of the Cooling Rate and Initial Composition on result, m can be calculated by the following formula. MZW. The MZW of sodium chromate tetrahydrate with different initial compositions in the settled cooling rate (55 K· Table 3. Nucleation Equation of Sodium Chromate h−1;45K·h−1;35K·h−1;25K·h−1;15K·h−1) was investigated. Tetrahydrate in Pure Water The experimental data were analyzed and are shown in Figure · 2 5. 100w (Na2CrO4 4H2O) nucleation equation R Δ − β The MZW tends to narrow with the decrease of the cooling 61.29 log Tmax = 0.3173 + 0.5834 log 0.9899 Δ − β rate in a settled initial composition. Since the supersaturated 66.00 log Tmax = 0.2807 + 0.5955 log 0.9898 Δ − β solution has enough time for nucleation at slow cooling rates, 68.49 log Tmax = 0.23722 + 0.5779 log 0.9962 Δ − β the crystallization temperature will be higher, and the MZW 71.37 log Tmax = 0.39022 + 0.5752 log 0.9838 Δ − β will become narrower. 73.28 log Tmax = 0.57321 + 0.5803 log 0.9974

3167 dx.doi.org/10.1021/je4006272 | J. Chem. Eng. Data 2013, 58, 3165−3169 Journal of Chemical & Engineering Data Article

Table 4. MZW of Sodium Chromate Tetrahydrate Added α with Diﬀerent Sizes of Seed Particles Δ mesh no. diameter Tmax 20 to 30 600 to 850 3.84 60 to 80 180 to 250 3.13 100 to 120 125 to 150 2.99 180 to 200 75 to 80 2.44 300 to 320 45 to 48 2.05 α μ Δ · Diameter in m; Tmax in K; 100w (Na2CrO4 4H2O) = 72.32; heat/ · −1 cooling rate = 45 K h ; stirring rate = 450 rpm; mseed = 20 mg.

Δ β Figure 6. Relationship between log Tmax and log .Initial · ■ ● ▲ composition, 100w (Na2CrO4 4H2O): , 61.29; , 66.00; , 68.49; ▼, 71.37; ◆, 73.28.

∑∑p −∑·∑ 1 j=1 [/]i xyi i i xij Ni yi = p 22 m ∑∑−∑j=1 [()/]i xxNi i ij β Δ where xi = log i, yi = log( Tmax)i, P is the total number of straight lines, and Nj is the number of measurements carried out for each line. The apparent nucleation order of sodium chromate tetrahydrate is about 1.72. ff E ect of Seed Particles on MZW. Crystal nucleation is a Figure 7. MZW of sodium chromate tetrahydrate added with different complex process, since nuclei can be generated by many · masses of seed particles: initial composition, 100w (Na2CrO4 4H2O) = different mechanisms. Most nucleation classification schemes 71.40; heat/cooling rate = 45 K·h−1; stirring rate = 450 rpm; seed distinguish between primary nucleationin the absence of mesh: 20 to 30. crystalsand secondary nucleationinthepresenceof crystals.14,20 Most primary nucleation is almost certainly heterogeneous rather than homogeneous in industrial crystal- addition, the smaller seeds at the same mass have larger lization; besides secondary nucleation has a profound influence quantities, which increase the collision probabilities, generating on virtually all industrial crystallization processes.21,22 The more new nuclei due to the breakage and attrition. As a result, presence of seed particles enhances the secondary nucleation the secondary nucleation is faster, and the MZW becomes rate, which reduces the adsorption of impurities and improves smaller. the purity of products. Moreover, since seeded nucleation The MZW reduces from 5.60 K to 2.16 K (Figure 7), while occurs more readily at a lower degree of supersaturation with the mass of seed particles increases from 0 mg to 200 mg. The the addition of seed particles, the crystallization process can be increased number of the seed crystals could improve the easily controlled in the metastable zone. collision chance of crystals with each other or with container, The seed crystals were crystallized from 200 mL of mother resulting in the reduction of the induction time and making the solution. To obtain crystals of different sizes after sucking MZW of sodium chromate tetrahydrate narrow. filtration, the material was sieved by means of different standard sieves (mesh number: 20, 30, 60, 80, 100, 120, 180, 200, 300, ■ CONCLUSIONS 320). A portion of 50 mL of identical solution was crystallized/ The solubility and MZW of sodium chromate tetrahydrate have dissolved in the settled cooling/heat and stirring rate three been determined exactly by a laser technique. The apparent times. The dissolution temperatures were determined, and then nucleation order of sodium chromate tetrahydrate is calculated. seed particles were added below 2 K of the average dissolution It is found that the cooling rate, initial composition, stirring temperature in the cooling process. rate, and presence of seed particles have a significant effect on The effects of mass and size of seed particles on MZW of the MZW. In industrial crystallization, the optimum cooling sodium chromate tetrahydrate are shown in Table 4 and Figure rate, initial composition, stirring rate, and presence of seed 7, respectively. particles are of great importance. The MZW data studied may Table 4 reveals that the MZW reduces with the increase of give important information for the optimum design of the diameter of seed particles. A higher constant cooling rate industrial crystallizers. (45 K·h−1) makes the solution being more supersaturated. The supersaturation cannot be consumed by seed crystal growth ■ AUTHOR INFORMATION completely; thus it causes the second nucleation. With higher Corresponding Author surface energy and greater area the smaller seeds attract more *E-mail: [email protected]. Phone: 86-971-6302023. Fax: 86- nuclei to coagulate, causing an earlier secondary nucleation. In 971-6310402.

3168 dx.doi.org/10.1021/je4006272 | J. Chem. Eng. Data 2013, 58, 3165−3169 Journal of Chemical & Engineering Data Article

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3169 dx.doi.org/10.1021/je4006272 | J. Chem. Eng. Data 2013, 58, 3165−3169