Stability of Sodium Sulfate Dicarbonate - American Chemical Society

Crystallization of sodium sulfate dicarbonate (∼2Na2CO3‚Na2SO4) is known to be a primary contributor to the fouling of heat-transfer equipment in ...
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Stability of Sodium Sulfate Dicarbonate (∼2Na2CO3‚Na2SO4) Crystals Obtained from Evaporation of Aqueous Solutions of Na2CO3 and Na2SO4 Cosmas Bayuadri,† Christopher L. Verrill,‡ and Ronald W. Rousseau*,† School of Chemical & Biomolecular Engineering and Institute of Paper Science and Technology, Georgia Institute of Technology, Atlanta, Georgia 30332

Crystallization of sodium sulfate dicarbonate (∼2Na2CO3‚Na2SO4) is known to be a primary contributor to the fouling of heat-transfer equipment in spent-liquor concentrators used in the pulp and paper industry. Therefore, understanding the conditions leading to the formation and in-process stability of this double salt and the related burkeite salt is crucial to the elimination or reduction of industrial problems. In this work, double salts were generated in a batch crystallizer at close-to-industrial process conditions. X-ray diffraction, calorimetry, and microscopic observation were used to investigate the stability of the salts to in-process aging, isolation and storage, and exposure to high temperature. The results verify that sodium sulfate dicarbonate exists as a unique phase in this system and that it remains stable at process conditions. Introduction Two double-salt species formed during the evaporation of a solution of sodium carbonate and sodium sulfate are identified as sodium sulfate dicarbonate (∼2Na2CO3‚Na2SO4) and burkeite (∼2Na2SO4‚Na2CO3).1-3 Thermonatrite (Na2CO3‚H2O) and anhydrous sodium carbonate (Na2CO3) can also crystallize from this system depending on the solution composition and temperature. The concentration of spent pulping liquors in pulp and paper industry evaporators is one major industrial process that suffers from costly deposition of these salt scales Shi and Rousseau1 obtained sodium sulfate dicarbonate phase crystals from the evaporative crystallization of a salt solution when the mole fraction x ) CO32-/(CO32- + SO42-) of the liquid phase was in the range 0.833 < x < 0.889. Burkeite crystals were found predominantly at lower mole fractions. The crystal composition of sodium sulfate dicarbonate varies slightly with the solution composition, as does that of burkeite. This behavior raised questions as to whether sodium sulfate dicarbonate is an intermediate (metastable) species that can transform to a different species. Hypothetically, if sodium sulfate dicarbonate is not stable, a change in chemical composition or temperature in the system might lead to degeneration of the crystal phase. Hydrothermal and crystal-phase stability have been investigated by researchers working with varied chemical systems. Ryoo and Jun4 investigated the hydrothermal stability of a mesoporous molecular sieve by observing changes in intensity and line shape of X-ray diffractograms while varying time and temperature. Forbes et al.5 assessed the dehydration of the potassium and sodium salts of p-aminosalicylic acid with a thermogravimetric method that differentiates between a weight loss due to dehydration and one due to decomposition. The thermal stability of the hydrates was considered to have a relationship with the strength of metal ion-to-water dipole bonding that is a function of the distance between the atoms. Johnson et al.6 observed the phase stability of cadmium arsenate salt crystals stored in the presence of atmospheric water by * To whom correspondence should be addressed. E-mail: rwr@ chbe.gatech.edu. † School of Chemical & Biomolecular Engineering. ‡ Institute of Paper Science and Technology.

measuring the unit cell and the change in structure using a quantitative X-ray diffraction method. The purpose of our research is to advance the limited information available about double-salt structures and their properties in an aqueous Na2CO3-Na2SO4 system. Prior work by Euhus suggested that sodium sulfate dicarbonate, isolated from mother liquor, changes to other phases upon prolonged exposure to ambient air.7 The present investigation addressed speculative concerns that sodium sulfate dicarbonate is an intermediate or metastable species that transforms into other double salt during equilibration with its mother liquor. Double-salt crystals were produced in a batch evaporator/ crystallizer from aqueous solutions with different Na2CO3/Na2SO4 molar ratios. As in earlier work,1 the experiments were conducted at 115 °C. This temperature was selected because evaporation equipment typically operates in the range 110125 °C when the total solids content of the spent pulping liquor exceeds the solubility limit of sodium compounds. The crystals were isolated and analyzed by powder X-ray diffraction (XRD), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and microscopic visual observation with polarized light. In the context of this work, stability is defined as the ability of the crystal species to maintain its unique phase properties during a change in composition or temperature or an extended period of time in contact with the mother liquor. If the crystals are not stable under these conditions, then either a degeneration of the crystal or a change in crystallographic properties is expected to occur. Materials and Methods Anhydrous Na2CO3 and anhydrous Na2SO4 of at least 99% purity were dissolved in deionized water to create salt solutions with 30% dry weight initial concentration, which is close to the solubility limits of the particular salts.8,9 Because calcium has been found to influence the nucleation characteristics of salts formed from such solutions,10 a calcium content of less than 300 mg/kg in the reagents was noted. The initial molar ratios of Na2CO3 to Na2SO4 in the solution were (A) 1:2, (B) 6:1, and (C) 6:1 with ethylenediamine tetraacetic acid (EDTA). On the basis of previous studies,1-3 burkeite crystals were expected to form from solution A, and sodium sulfate dicar-

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Figure 3. XRD analysis of burkeite crystals from evaporation of an aqueous solution containing a 1:2 molar ratio of Na2CO3 and Na2SO4 before and after heat exposure at 200 °C showing insignificant change in structure. Figure 1. Schematic diagram of bench-scale batch crystallizer.

Figure 4. Two sequential DSC scans of a single sample of burkeite crystals.

Figure 2. Photomicrograph showing possible trona formation due to prolonged exposure to ambient air; arrow indicates trona-like structures between sodium sulfate dicarbonate crystal agglomerates.

bonate crystals were expected from B and C. Addition of EDTA creates a complex with Ca2+ so that burkeite or sodium sulfate dicarbonate can crystallize freely without inhibition.3,10 An excess of EDTA relative to calcium in the amount of 1000 mg/ kg of solution was added to solution C. The solutions were continuously mixed in a beaker for 30 min to ensure that all solids were dissolved. Batch evaporative crystallizations were carried out in a 1-L stirred pressure vessel. The schematic of the bench-scale batch crystallizer can be seen in Figure 1. Crystals formed in the process were detected in situ by a Lasentec focused beam reflectance measurement (FBRM) system. Approximately 800 mL of clear solution was added to the vessel for each run. The mixer speed was 250 rpm. The reactor was sealed, and the mixture was heated to 115 °C before the vapor valve was opened. Evaporations were carried out at a constant rate (5 g/min) at 115 °C to reach nucleation, which was defined as corresponding to more than 2000 particle counts per second in the FBRM-measured size range of 9-100 µm. Evaporation was then stopped, and the sample was collected through a preheated,

insulated 2-µm in-line filter. The mother liquor was collected into a known volume of dilution water, and the crystals retained on the filter were optimally rinsed sequentially using a 1:1 mixture of ethylene glycol and water at 45-50 °C, followed by ethylene glycol at room temperature, and finally ethanol. Crystal sample contamination was encountered when an abbreviated wash procedure was used, as discussed below. The crystals were then removed from the filter chamber and stored in an evacuated desiccator. The early work in which this procedure was developed suggested that any exposure of saturated solutions rich in Na2CO3 to temperatures below 109 °C (e.g., a cold filter housing) must be avoided to prevent Na2CO3‚H2O formation during crystal sampling and isolation. Powder X-ray Diffraction (XRD). The stability of crystal samples was investigated by following the change in intensity and specific peak location on diffractograms as a function of time and temperature exposure. The crystals were ground and analyzed using a Philips PW1710 automatic powder diffractometer with a Cu anode and PANalytical X′Pert HighScore Plus analysis software. Differential Scanning Calorimetry (DSC). Thermal properties were evaluated with a Pyris 1 differential scanning calorimeter (DSC) from Perkin-Elmer. The DSC technique

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Figure 5. XRD analysis of a mixture of sodium sulfate dicarbonate and thermonatrite crystals obtained from evaporation of an aqueous solution of Na2CO3 and Na2SO4 (6:1 mole ratio, without EDTA) after samples had been aged 0, 3, and 6 h.

Figure 6. DSC scan of a crystal mixture sample harvested after 6 h of aging showing an apparent overlap of two individual peaks in the range 78-140 °C due to evaporation of ethanol and dehydration of Na2CO3‚ H2O.

measures the amount of energy (heat) absorbed or released by a sample as it is undergoing a programmed temperature change in an inert atmosphere. Samples of exactly 10-mg of crystals were inserted into an aluminum chamber and heated under a nitrogen purge at a rate of 20 °C/min, so that the sample temperature increased from 50 to 300 °C. The samples were then held for 1 min at 300 °C before being cooled back to 50 °C. The energy absorbed was measured and plotted versus sample temperature. When a substance is evaporating, the temperature will not increase until all of the substance has evaporated, and the programmed controller increases the heat input as it attempts to increase the temperature. This extra heat flow during evaporation shows as a peak on the DSC plot. The area under the curve can be correlated with the latent heat of evaporation. An endothermic peak on a DSC plot indicates melting, evaporation, dehydration, or a change in the crystal phase when the peak occurs during heating; on the other hand, an exothermic peak (during cooling) indicates solidification or a change in the crystal phase. Thermogravimetric Analysis (TGA). Analyses by this method were performed on a Perkin-Elmer Pyris 1 TGA

Figure 7. TGA analysis of the crystal mixture analyzed in Figure 6.

Figure 8. XRD analysis of a crystal mixture of sodium sulfate dicarbonate and thermonatrite harvested after 6 h of aging before and after exposure to hot air at 200 °C.

thermogravimetric analyzer. The results were used to supplement the DSC data by verifying whether any moisture or other substance evaporated during heating. The technique involves monitoring the weight loss of the sample in nitrogen as a function of temperature using an auto stepwise method. As for the DSC technique, 10-mg samples were heated incrementally from 50 to 300 °C with automatic internal weight measurements. Polarized Light Microscopy (PLM). Polarized light micrographs were obtained using a Canon CCD camera coupled to a microscope with crossed polarizers, a red compensator, and 4-40× objective lenses. The microscope lenses provide very little depth of field; therefore, the images are essentially twodimensional. Structural changes can lead to visual changes in the shape, color, or optical properties of the crystals. Kim et al.11 observed changes in the optical properties of dehydrated crystals that might be indicative of thermonatrite dehydration. Thermal Exposure. Crystal samples were exposed to high temperature (200 °C) in an air-drying oven for 1 h and then reanalyzed to observe whether any changes in their structural, thermal, or morphological properties took place.

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Figure 9. Photomicrographs of the sodium sulfate dicarbonate-thermonatrite mixture (a) before and (b) after exposure to 200 °C.

Figure 10. XRD analysis of sodium sulfate dicarbonate crystals obtained from evaporation of an aqueous solution of Na2CO3 and Na2SO4 (6:1 molar ratio, with EDTA) sampled after being aged 0 and 24 h.

Results and Discussion Stability of Crystal Samples on Storage. The sodium sulfate dicarbonate crystal samples produced during this work were kept in a desiccator to avoid contact with moisture and air, as there is a possibility of sample degradation after exposure to moist ambient air and/or CO2. According to Datta et al.,12 the normal conditions of the atmosphere encourage the formation of trona (Na2CO3‚NaHCO3‚2H2O) in wet salt mixtures containing thermonatrite. Johnson et al.6 also mentioned the possible effect of

dissolved carbon dioxide on the stability of precipitated phases of different inorganic salts. Figure 2 shows a photomicrograph of a sample that had been exposed to ambient air for about 1 week. On the basis of similar observations by Datta et al.,12 it is thought that the thin monoclinic rodlike crystal is trona. We speculate that moisture from the air partially dissolved sodium sulfate dicarbonate, which recrystallized as a mixture of thermonatrite and burkeite. Further exposure of this mixture to atmospheric CO2 produced trona. In contrast to the above observations, there was no apparent change in crystal samples that were stored for 1 month or longer under vacuum; i.e., the X-ray diffraction patterns remain the same. Thermal Stability of Burkeite Crystals. Crystals obtained from a solution with a 1:2 Na2SO4-to-Na2CO3 molar ratio were examined by XRD. The crystals were then heated to 200 °C and reexamined. The powder XRD results for burkeite crystals before and after heating are shown in Figure 3 over the range of 2θ from 10° to 40°. The characteristic burkeite pattern was observed before and after heating, suggesting that no transformation occurred. The peak positions and relative intensities matched the standard burkeite XRD patterns obtained by the JCPDS-ICDD and U.S. National Bureau of Standards (PDF 241134 and 85-1731-85-1733). The DSC analysis shown in Figure 4 supports the conclusion that there was no transformation of burkeite during heating. The first scan (shown as the solid curve) shows that residual ethanol from the washing step was evaporated starting at around 78.4 °C. Judging from the area under the peak, the amount of ethanol

Figure 11. PLM observations of sodium sulfate dicarbonate crystals from evaporation of salt solutions with EDTA; typical agglomerates (left) and rodlike elements of clusters (right).

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Figure 12. XRD analysis of sodium sulfate dicarbonate crystals obtained from evaporation of an aqueous solution of Na2CO3 and Na2SO4 (6:1 mole ratio, without EDTA) after samples had been aged 12, 18, and 34 h.

evaporated can be estimated as 1.99 wt %. The second scan (shown as the dashed curve) follows the path of the first scan except for the portion caused by the evaporation of ethanol. This suggests that there were no volatile impurities remaining in the sample after the first stage of heating. Furthermore, the similarity of the two scans indicates that no structural change in the crystalline sample occurred, confirming that burkeite is stable and is not degenerated under this thermal exposure. These results for the well-characterized burkeite salt provide an important reference for the following discussion of the relatively unknown sodium sulfate dicarbonate salt. Crystal Mixtures of Sodium Sulfate Dicarbonate and Thermonatrite. On the basis of prior work, evaporation from of a solution containing a mole ratio of Na2CO3 to Na2SO4 in the range between 5:1 and 7:1 is expected to produce crystals of sodium sulfate dicarbonate.1 During the batch evaporation process in the experiments of the present work, the evaporation was stopped after crystallization was detected (i.e., when the FBRM particle counts were greater than 2000 per second). The slurry of crystals and mother liquor were maintained in the vessel at constant temperature and pressure, and crystal samples were obtained after 0, 3, and 6 h and rinsed only with ethanol before being analyzed by XRD. The results are presented in Figure 5. Peaks corresponding to thermonatrite (Na2CO3‚H2O) are apparent at 2θ values of 16.7°, 16.9°, 21.6°, 32.2°, 33.6°, 33.8°, 34.5°, 36.2°, 36.6°, and 38°, and characteristic peaks of sodium sulfate dicarbonate appear at 20°, 23.2°, 23.5°, 24.04°, 24.5°, 27.2°, and 36°. The increase in intensity with time of the characteristic XRD peaks for thermonatrite appears to suggest that more thermonatrite formed in the crystal-liquor slurry during aging. Chemical analysis of the crystals at each of the recovery times was performed by Coulometric titration for CO32- content and capillary ion electrophoresis for SO42-. These results show an apparent increase in carbonate content with aging time. To confirm the presence of thermonatrite, DSC tests were performed on 10 mg of the crystals that were harvested after being aged for 6 h. The results are plotted in Figure 6. On the first scan, a significant endotherm was observed at around 78 °C, which is close to the temperature at which ethanol evolution was noted in Figure 4. However, the size of the peak and the change in slope as the peak rises makes it clear that the

Figure 13. DSC scan of sodium sulfate dicarbonate from evaporation of a salt solution with EDTA showing features of residual ethanol and ethylene glycol (crystal washing solvent) release.

Figure 14. XRD analysis of sodium sulfate dicarbonate obtained from evaporation of an aqueous solution of Na2CO3 and Na2SO4 (6:1 mole ratio) with EDTA before and after exposure to 200 °C.

endotherm results from the evolution of both ethanol and water. The first shoulder likely comes from the evaporation of residual ethanol on the crystal surfaces, whereas the latter main peak is associated with the dehydration of sodium carbonate monohydrate (thermonatrite). A similar endotherm was not observed during the second heating loop, which confirmed that the crystal sample was fully dehydrated after the first loop. The TGA data shown in Figure 7 were obtained using a sample of the material that had been aged for 6 h. It shows that the total sample weight decreased by about 10.2 wt %. If all of the water mass evolved came from thermonatrite, the maximum thermonatrite content of this sample was 61 wt %. Heating the mixed-salt sample to 200 °C revealed a significant deformation of the structure when reanalyzed by XRD. Figure 8 shows that the specific peaks of thermonatrite were deformed and the intensity was significantly decreased. This result confirms that the thermonatrite content converted to anhydrous sodium carbonate during heating. Thermonatrite peaks at 16.7° and 16.9° disappeared, and the characteristic anhydrous sodium carbonate peak appeared at 30.29°. Sodium sulfate dicarbonate peaks can be seen at 2θ values of 23.2°, 23.5°, 24.04°, 24.5°, 27.2°, 33.2°, 34°, 34.8°, and 36°.

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Figure 15. Photomicrographs of sodium sulfate dicarbonate crystals (left) before and(right) after being heated to 200 °C.

Dehydration of thermonatrite (Na2CO3‚H2O) also impacted the optical properties of the crystal mixture. Figure 9 shows photomicrographs (a) before and (b) after heating. Thermonatrite crystals appear as bright orthorhombic-pyramidal shapes and fragments. Thermonatrite crystals experienced a darkening effect when dehydrated. Kim et al.11 also observed that dehydration reduced the transparency of crystal samples. Note that the nearly spherical objects with dark cores in Figure 9 are typical of sodium sulfate dicarbonate agglomerates observed in earlier studies.2,14 These results appear to suggest that a mixed-crystal phase can result from evaporative crystallization of aqueous solutions of Na2CO3 and Na2SO4 in the mole ratio expected to produce sodium sulfate dicarbonate when EDTA is not used to sequester free divalent metal ions. However, we later determined that ethanol alone was not a suitable solvent for crystal washing. Upon the mixing of ethanol with the trapped mother liquor in the crystal sample, the solubility of thermonatrite is reduced and can precipitate in the crystal mass during washing. Shi and Rousseau1 and Oosterhof and co-workers15 suggested the use of ethylene glycol to displace the mother liquor. In our later experiments, we used a sequence of hot ethylene glycol/water, room-temperature ethylene glycol, and ethanol to prevent formation of a second phase during crystal sample preparation. When improperly harvested from solution, a mixed-crystal phase results that is not chemically or thermally stable because of the tendency of thermonatrite to dehydrate upon heat exposure. However, results from analyses (XRD, DSC, TGA, optical microscopy) of burkeite and mixed thermonatritesodium sulfate dicarbonate crystals suggest that the specific burkeite and sodium sulfate dicarbonate fractions remain stable during exposure to elevated temperatures. Sodium Sulfate Dicarbonate Crystals. Earlier work2,3 showed that the introduction of EDTA into solutions of Na2CO3 and Na2SO4 apparently prevents the formation of thermonatrite. It was hypothesized that the EDTA sequestered calcium ions and kept them from interfering with the nucleation and growth of crystalline species. To evaluate this effect in the present work, solutions of Na2CO3 and Na2SO4 in a mole ratio of 6:1 both with and without 1000 mg of EDTA per kilogram of solution were evaporated by the procedure described above. In the first experiment, with the addition of EDTA, two samples were taken 0 and 24 h after nucleation was detected by the FBRM. The XRD results in Figure 10 confirm that a single phase of sodium sulfate dicarbonate was obtained. Apparent peaks at 23.2°, 23.5°, 24.04°, 24.5°, 27.2°, 33.2°, 34°, and 34.8° are the characteristic features of this salt as reported by Shi and

Rousseau.1 No loss of integrity of the sodium sulfate dicarbonate structure occurred after it had been aged for 24 h. It can be concluded that sodium sulfate dicarbonate is hydrothermally stable at 115 °C. Polarized light microscopy shows the structure of sodium sulfate dicarbonate formed from evaporation of salt solution with EDTA (Figure 11). The left-hand image is typical of the nearly spherical agglomerates formed under most conditions in these experiments. The higher-magnification image to the right suggests that the agglomerates are composed of short, rodlike segments. The second experiment, without the addition of EDTA, was done using the same parameters, and crystal samples were obtained and rinsed sequentially after 12, 18, and 34 h of aging in the solution. The XRD results in Figure 12 confirm that sodium sulfate dicarbonate crystals maintained the same phase for up to 34 h while in contact with mother liquor at the industrial process temperature of 115 °C. Peaks corresponding to sodium sulfate dicarbonate are apparent at 2θ values of 12.6°, 23.2°, 23.5°, 24.04°, 24.5°, 27.2°, 33.2°, 34°, 34.8°, and 36°. A DSC result for sodium sulfate dicarbonate crystals sample is presented in Figure 13. The small, broad peak between 78 and 108 °C is likely a result of evaporation of the residual ethanol used in crystal washing. The larger peak between 133 and 193 °C likely results from evaporation of a residual mixture of water and ethylene glycol (bp 197.3 °C at 1 atm) that was used to displace mother liquor from the crystal sample. The second heating loop shows no peaks, confirming that the crystal sample is stable. A sample of sodium sulfate dicarbonate was exposed to 200 °C for 1 h. As shown in Figure 14, XRD analysis revealed no changes in the crystal after exposure to the elevated temperature. The PLM observation in Figure 15 reveals two apparent habits of sodium sulfate dicarbonate crystals, but with no significant changes in the optical properties of either form upon heating. These results confirm that sodium sulfate dicarbonate is a stable phase and does not readily transform into other Na2CO3-Na2SO4 salts upon heating. Conclusions The experiments conducted in this work verify earlier findings that sodium sulfate dicarbonate is a unique phase that can be crystallized from aqueous solutions of Na2CO3 and Na2SO4. Sodium sulfate dicarbonate crystals harvested from mother liquor will form trona upon contact with water vapor and carbon dioxide in ambient air. Therefore, crystal samples obtained from this system must be protected from air contact to avoid trona formation between sampling and analysis.

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Sodium sulfate dicarbonate is the predominant species crystallized from the evaporation of an aqueous solution containing a 6:1 molar ratio of Na2CO3 and Na2SO4. Care must be taken to avoid formation of thermonatrite during isolation of these crystals. The thermonatrite component of the resulting mixture is not thermally stable and releases a water of hydration when heated above 100 °C. Sodium sulfate dicarbonate appears to be chemically stable at 115 °C. Sodium sulfate dicarbonate formed and was stable in salt solutions with a high fraction of Na2CO3, yet anhydrous Na2CO3 was not detected. This unique double salt does not transform or degenerate to other species when kept in contact with its mother liquor for prolonged times or when exposed to air at a temperature of 200 °C. It can be concluded that sodium sulfate dicarbonate is not a metastable species. The basic crystal structure of sodium sulfate dicarbonate has not yet been reported, and our work is limited to the ability to repeatedly synthesize this double salt and examine its stability. A detailed structural model of sodium sulfate dicarbonate is needed to explain its stability using, e.g., Rietveld analysis to examine distortions or dislocations in the crystal structure during heating, aging, and other environmental changes. Acknowledgment The authors thank Dr. Xin-Sheng Chai from the Institute of Paper Science and Technology at Georgia Tech for insightful discussions and performing the thermal analysis. Partial funding for the work was provided by a consortium of Institute of Paper Science and Technology member companies. Some of the equipment used in the study was provided by the State of Georgia Traditional Industries Program. Literature Cited (1) Shi, B.; Rousseau, R. W. Structure of burkeite and a new crystalline species obtained from solutions of sodium carbonate and sodium sulfate. J. Phys. Chem. B 2003, 107, 6932-6937. (2) Frederick, W. J., Jr.; Shi, B.; Euhus, D. D.; Rousseau, R. W. Crystallization and control of sodium salt scales in black liquor concentrators. TAPPI J. 2004, 3 (6), 7-13. (3) Shi, B.; Rousseau, R. W.; Frederick, W. J., Jr. Nucleation of Burkeite from Aqueous Solutions and Black Liquor. In Proceedings of the 2001 International Chemical RecoVery Conference, PAPTAC: Montreal, 2001; pp 177-181.

(4) Ryoo, R.; Jun, S. Improvement of hydrothermal stability of MCM41 using salt effects during the crystallization process. J. Phys. Chem B 1997, 101, 317-310. (5) Forbes, R. T.; York, P.; Fawcett, V.; Shields L. Physicochemical properties of salts of p-aminosalicylic acid. I. Correlation of crystal structure and hydrate stability. Pharm. Res. 1992, 9 (11), 1428-1435. (6) Johnson, C. D.; Skakle, J. M. S.; Johnston, M. G.; Feldman, J.; Macphee, D. E. Hydrothermal synthesis, crystal structure and aqueous stability of two cadmium arsenate phases, CdNH4(HAsO4)OH and Cd5H2(AsO4)4‚4H2O. J. Mater. Chem. 2003, 13, 1429-1432. (7) Euhus, D. D. Nucleation in Bulk Solutions and Crystal Growth on Heat-Transfer Surfaces during Evaporative Crystallization of Salts Composed of Na2CO3 and Na2SO4. Ph.D. Dissertation, Georgia Institute of Technology, Atlanta, GA, 2003. (8) Seidell, A.; Linke, W. Solubility of Inorganic and Metal Organic Compounds, 4th ed.; Van Nostrand: Princeton, NJ, 1965; Vol II, pp 915, 938-939, 1122. (9) Green, S.; Frattali, F. The system sodium carbonate-sodium sulfatesodium hydroxide-water at 100 °C. J. Am. Chem. Soc. 1964, 68, 17891794. (10) Shi, B.; Frederick, W. J., Jr; Rousseau, R. W. Effects of Calcium and Other Ionic Impurities on the Primary Nucleation of Burkeite. Ind. Eng. Chem. Res. 2003, 42, 2861-2869. (11) Kim, Y.-s.; Paskow, H. C.; Rousseau, R. W. Propagation of SolidState Transformations by Dehydration and Stabilization of Pseudopolymorphic Crystals of Sodium Naproxen. Cryst. Growth Des. 2005, 5 (4), 1623-1632. (12) Datta, S.; Thibault, Y.; Fyfe, W. S.; Powell, M. A.; Hart, B. R.; Martin, R. R.; Tripthy, S. Occurrence of trona in alkaline soils of the IndoGangetic Plains of Uttar Pradesh (U.P.), India. Episodes 2002, 25 (4), 236239. (13) Shi, B.; Rousseau R. W. Crystal properties and nucleation kinetics from aqueous solutions of Na2CO3 and Na2SO4. Ind. Eng. Chem. Res. 2001, 40, 1541-1547. (14) Shi, B.; Frederick, W. J., Jr.; Rousseau, R. W. Nucleation, growth, and composition of crystals obtained from solutions of Na2CO3 and Na2SO4. Ind. Eng. Chem. Res. 2003, 42, 6343-6347. (15) Oosterhof, H.; Witkamp, G. J.; van Rosmalen, G. M. Evaporative crystallization of anhydrous sodium carbonate at atmospheric conditions. AIChE J. 2001, 47 (10), 2220-2225.

ReceiVed for reView April 27, 2006 ReVised manuscript receiVed August 4, 2006 Accepted August 15, 2006 IE060540Q