ARTICLE pubs.acs.org/IECR
Thermodynamic Preparation Window of Alpha Calcium Sulfate Hemihydrate from Calcium Sulfate Dihydrate in Non-Electrolyte GlycerolWater Solution under Mild Conditions Baohong Guan,*,† Guangming Jiang,† Hailu Fu,† Li Yang,† and Zhongbiao Wu†,‡ † ‡
Department of Environmental Engineering, Zhejiang University, Hangzhou 310058, China Zhejiang Provincial Engineering Research Center of Industrial Boiler & Furnace Flue Gas Pollution Control, Hangzhou 310058, China
bS Supporting Information ABSTRACT: Through vapor pressure measurements, water activities of the glycerolwater solution were first examined under temperatures of 3085 °C and glycerol concentrations of 050 mol %. The water activity is the only determinant of phasetransition point among CaSO4 3 2H2O (DH), α-CaSO4 3 0.5H2O (α-HH), and CaSO4 (AH) in glycerolwater solution. Accordingly, a phase-transition diagram was constructed as a function of temperature (0140 °C) and glycerol concentration (050 mol %), which provided a thermodynamic preparation window of α-HH from DH. Controlling glycerol concentration and temperature within the window, direct transition of DH to α-HH can be regulated. However, the window is apt to shrink as the operating time is shortened, indicating that its available profile is directed by transition kinetics of DH to α-HH besides transition thermodynamics. Overall, the glycerolwater solution could act as a novel category of nonelectrolyte medium for α-HH preparation from DH.
1. INTRODUCTION Calcium sulfate hemihydrate (HH), including α- (α-HH) and β-form (β-HH), is a widely applied class of cementitious materials. α-HH is superior to β-HH due to its better workability and higher strength and has gained more acceptance in modern building materials, molding, special binder systems, dental materials, and some other innovative applications. α-HH is prepared from the conversion of calcium sulfate dihydrate (DH). Current commercial processes, e.g., the SICOWA-ProMineral autoclaving process,1 are energy-intensive procedures to heat DH in an autoclave at elevated temperature and pressure. Since 1960, the salt solution method to cure DH in boiling or near-boiling aqueous solutions of electrolytes (inorganic acids or their salts) under atmospheric pressure has been favorably developed.27 Dominant transition mechanism is a dissolution of DH, followed by the crystallization of α-HH.8 Electrolytes could lower the transition temperature through decreasing the water activity6 and exert profound effects on the crystallization kinetics and properties of α-HH. Our previous work revealed that α-HH could also be prepared in a nonelectrolyte aqueous solution composed of only methanol and water.9 In fact, aqueous solutions of alcohols have been widely adopted in synthesizing calcium-based minerals with nanoscales, particular morphologies, and novel functions.1013 The properties of these solutions, such as dielectric constant, solvent polarity, density, and viscosity, distinguish much from those of the electrolyte aqueous solutions. Preparation of α-HH from DH in such solutions, therefore, deserves being further studied and is also essential to describe the crystallization chemistry. In comparison to the salt solution method, a foreign ion-free crystallization surrounding in this procedure is quite attractive, which could help achieve a higher purity of α-HH. r 2011 American Chemical Society
This is due to the fact that undesired minerals (for example, substituted phases of α-HH or double salts) and hazardous impurities (for example, chloride ion) are inclined to be involved in products when some salts are used as the crystal growing media, such as NaCl, KCl, and MgCl2.1416 As a novel method, much work is needed to assess whether this procedure is more effective and viable. From an industrial point of view, methanol with its high toxicity and volatility appears disadvantageous for application as the preparation medium for α-HH. Glycerol, as an additive without toxicity and volatility, has been proved able to provide a stabilizing effect on the α-HH in the aqueous solution under mild conditions.17 Therefore, glycerolwater solution was attempted here as a medium to transform DH into α-HH. This work aims at acquiring a thermodynamic preparation window of α-HH from DH in glycerolwater solution. This was accomplished by determining the phase-transition diagram of calcium sulfate in the glycerolwater solution. Based on this a new method, which we called the water activity method, was then introduced. Moreover, the dehydration processes of DH in this thermodynamic preparation window were further investigated.
2. EXPERIMENTAL SECTION 2.1. Materials. Reagent grade DH (Silian Chemical Co., Ltd., Shanghai, China) was adopted as the raw material. The composition is presented in Table 1. Received: May 15, 2011 Accepted: October 21, 2011 Revised: October 15, 2011 Published: October 21, 2011 13561
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Table 1. Chemical Composition of the Raw Material CaSO4 3 2H2O wt %
99.40
Cl
NH4+ CO32 MgO, Na2O HCl insoluble
0.005 0.003
0.040
0.200
0.005
The glycerolwater solution as the medium for the DH transition to α-HH was prepared by deionized water and glycerol (reagent grade, Sinopharm Chemical Reagent Co., Ltd., Shanghai, China) with a purity of 99.5 wt %. 2.2. Methods. Precise experimental determination of the phase-transition diagram seems fussy through classical solubility equilibrium method,18,19 since the equilibrium time for solubility measurement differs under various solution conditions, such as solution composition, solute concentration, and temperature.20 Another less tedious method of determining the phase-transition point, which we called the water activity method, was then introduced from Richadl’s work.21 In aqueous solution, calcium sulfate occurs in three forms: DH, α-HH, and calcium sulfate anhydrite (AH). There are two phase equilibria among them: CaSO4 3 2H2 OðsÞðDHÞ S CaSO4 3 0:5H2 OðsÞðα-HHÞ þ 1:5H2 OðlÞ ð1Þ CaSO4 3 2H2 OðsÞðDHÞ S CaSO4 ðsÞðAHÞ þ 2H2 OðlÞ
ð2Þ
Taking the reaction 1 for an example, its Gibbs energy change (ΔG) is as follows: ! KSP, DH 1:5 ΔG ¼ RT 3 ln KDH-α-HH RT 3 ln aw ¼ RT 3 ln KSP, α-HH RT 3 ln a1:5 w
ð3Þ
where KDH‑α‑HH denotes the thermodynamic equilibrium constant of the reaction 1, Ksp is the thermodynamic solubility product, and aw is the water activity. At the phase-transition point, where the equilibrium between DH and α-HH reaches ΔG = 0 and the eq 3 could then be arranged as follows: aw ¼
KSP, DH KSP, α-HH
!2=3 ð4Þ
Since KSP of DH, α-HH, and AH are reported only temperature dependent, the water activity is suggested to be the only determinant of phase-transition point. Equations KSP,DH/α‑HH/AH = f(T) have been given respectively in Li’s work.6 With the aid of eq 4, the needed water activity to reach the phase equilibrium could be calculated. It then makes it possible to get the phasetransition point where the water activity of the glycerolwater solution equals the calculated one. The water activity of a solution is defined as the ratio of the vapor pressure of water over the solution to that over the pure water at the same temperature: pH O ð5Þ aw ¼ o 2 p H2 O Equation 5 is valid in this work as the operating pressure p is moderate (the Poynting factor equals 1) and the solute (glycerol, Po = 7.9 103 KPa at 85 °C) is nonvolatile. Generally, calcium
Figure 1. Apparatus for vapor pressure determination. (1) U-shaped tube; (2) solution sample; (3) equilibrium cell (50 mL); (4) thermostatted water bath; (5) thermometer; (6) heater; (78) valve; (9) thermostat; (10) ZP-B8 vacuum detector; (11) Pyrex glass pipe; (12) vacuum pump.
sulfate phases are sparingly soluble. Therefore, vapor pressures of the solution saturated with CaSO4 could be substituted, during experimental measurements, by those of the CaSO4-free solutions. Figure 1 shows the experimental apparatus for vapor pressure measurement based on a static method. The pipe was constructed of Pyrex glass, and the thermometer and heater were made of stainless steel. The U-shaped tube and equilibrium cell (50 mL) containing solution sample were immerged into a water bath with approximate volume of 20 dm3, which was kept at an expected temperature with a deviation of (0.5 °C using a thermostat. The actual temperature was monitored by a thermometer. The pressure of the system was tracked by ZP-B8 vacuum detector (Duozhu Co. Ltd., Nanjing, China) with an accuracy of 1%. Valve 7 was connected with the vacuum pump and used to control the gas removing from the system, whereas valve 8 was connected with the atmosphere and used to control the gas suction into the system. Prior to the solution sample addition, the equipment was checked for air tightness. This was performed by opening valve 7 but closing 8 until the system pressure dropped to 0.02 Kpa. The air tightness was acceptable if the vacuum could keep steady for 5 min. Once air tightness was guaranteed, 40 mL of glycerolwater solution was injected into the U-shaped tube and equilibrium cell at the same level with the help of a special plastic injector, and then preheated to the expected temperature. Before the vapor pressure determination, air removing from the system was conducted in the case of closing valve 8 and opening valve 7 until the sample in equilibrium cell boiled, to make sure (1) the system and solution sample were degassed thoroughly; (2) the pipe between the U-shaped tube and equilibrium cell was filled only with the vapor of the solution sample. Vapor equilibrium was reached as the solution level in the two arms of U-shaped tube was equal through adjusting the valves 7 and 8. The saturated vapor pressure was then read from the vacuum detector. This was performed in triplet to get a good reproducibility of the vapor pressure. To identify the composition, the solid samples obtained during the transition process of DH to α-HH were subjected to a powder X-ray diffraction analyzer (XRD, D/Max-2550 pc, Rigaku Inc., Japan) and a Fourier transform infrared analyzer (IR, IRAffinity-1, SHIMADZU, Japan). The XRD analysis was performed with Cu-Kα radiation at a scanning rate of 8° min1 in the 2θ range from 5° to 85°. For IR analysis, thin films of ground sample and KBr were prepared and the spectra were collected at 13562
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Table 2. Vapor Pressures and Water Activities of the GlycerolWater Solution at Various Glycerol Concentrations and Temperatures vapor pressure/Kpa temperature/°C
glycerol concentration/mol %
this work
22
reference
water activity deviation
this work
reference23
deviation
25
7.0
0.954
0.947
+0.74%
25
14.3
0.935
0.912
+2.46%
25 40
0.0 0.0
3.26 7.46
3.17 7.37
+2.76% +1.21%
60
0.0
20.32
19.92
+1.97%
80
0.0
48.21
47.34
+1.80%
Figure 2. Dependence of water activity of the glycerolwater solution on the glycerol concentration (mol %) under various temperatures (3085 °C).
4 cm1 resolution over the frequency of 500 to 4000 cm1. The differential scanning calorimetry (DSC) analysis (NETZSCH STA 409 Luxx, Selb/Bavaria, Germany) was used to distinguish α-HH from β-HH. For DSC measurement, 20 mg of dry sample was sealed in an Al2O3 crucible with a lid and scanned at a rate of 10 °C min1 under N2 gas atmosphere.
3. RESULTS AND DISCUSSION 3.1. Influence of Glycerol Concentration and Temperature on the Water Activity. To evaluate the validity of the water
activity measurement, comparisons were conducted between our work and the references on the vapor pressures of pure water at temperatures of 25, 40, 60, and 80 °C, as well as the water activities of glycerolwater solutions under concentrations of 7.0 and 14.3 mol % at 25 °C. The results are presented in Table 2. The small deviations of 1.212.76% between the measured vapor pressures of pure water and those calculated from IAPWS formulation22 confirm the reliability of the experimental apparatus and operation. The water activities measured experimentally are very close to those reported in the reference23 with deviations of 0.742.46%, further approving the accuracy of the data. The water activities of the glycerolwater solution were then measured under glycerol concentrations of 050 mol % and temperatures of 3085 °C. Parts of the results as a function of glycerol concentration and temperature are presented in Figures 2 and 3, respectively.
Figure 3. Dependence of water activity of the glycerolwater solution on the temperature under various glycerol concentrations (1150 mol %).
The diagonal straight line in Figure 2 corresponds to the ideal behavior according to the Raoult’s law, from which the glycerol water solution is implied to deviate negatively from ideality at nearly all investigated temperatures.24 It may be a consequence of the intense interaction between the water and glycerol molecules, which then restricts the free movement of the water molecules. The deviation increases as the glycerol concentration increases. The water activity exhibits a monotonic decline with increasing glycerol concentration. It drops to 0.39 when the concentration reaches 50 mol % at 75 °C. In electrolyte types of solution, the transition of DH to α-HH is provoked by the electrolytes due to their negative effect on the water activity.6 Glycerol similarly shows the same function with electrolytes, indicating that a novel medium, i.e., the glycerolwater solution, could be expected for α-HH preparation from DH. The temperature is also found in Figure 3 to exert a negative effect on the water activity, mainly owing to the greater and greater intense interactions that occur between glycerol and water molecules as the temperature rises. However, in all cases, the maximum drop in the water activity is only 10% as the temperature increases from 30 to 85 °C, demonstrating the weak capacity of the temperature on lowering the water activity. 3.2. Phase-Transition Diagram of Calcium Sulfate in Glycerol Water Solution. At the phase-transition point of DH and α-HH, water activity of the glycerolwater solution should equal that calculated from eq 4 at corresponding temperature. Consequently, it is possible to get the phase-transition point from 13563
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Figure 4. Graphical illustration of the DHα-HH phase-transition point determination at 50 and 85 °C through the water activity method.
Table 3. Phase-Transition Points of DHα-HH and DHAH in GlycerolWater Solution DHα-HH
Figure 5. Phase-transition diagram of CaSO4 in glycerolwater solution. The stability order is DH > AH > α-HH in region I, AH > DH > α-HH in region II, and AH > α-HH > DH in region III.
DHAH
glycerol
temperature/
glycerol
temperature/
concentration/mol %
°C
concentration/mol %
°C
48
30
17
30
44
40
9
35
38
50
2
40
33 28
60 65
0
43.5
21
75
12
85
0
100
intersection of the curve aw = aCalculated water activity and the curve aw = f(cGlycerol concentration). As an example, the calculated water activities for the phase equilibrium of DH and α-HH at 50 and 85 °C are 0.54 and 0.81, respectively. Figure 4 indicates that the intersection, i.e., the phase-transition point, corresponds to 38 mol % glycerol at 50 °C and 12 mol % glycerol at 85 °C. Accordingly, the phase-transition points of DHα-HH and DHAH were determined in a larger range of temperatures (30100 °C) and glycerol concentrations (050 mol %). The transition temperatures and corresponding glycerol concentrations of the phase-transition points are presented in Table 3. The phase-transition diagram of calcium sulfate in glycerol water solution was then outlined in Figure 5. It describes three regions (regions I, II, and III) with different stability orders of DH, α-HH, and AH. Region I borders region II by the DHAH phase-transition line, while region II borders region III by the DHα-HH phase-transition line. The stability order is DH > AH> α-HH in region I, AH > DH> α-HH in region II, and AH > α-HH> DH in region III. DH is the stable phase in region I, while AH is the stable phase in regions II and III. α-HH occurs only as a metastable phase in glycerolwater solution, just as in the electrolyte aqueous solution.6 The dehydration routes of DH differ in three regions due to the change in their stability order. The dehydration product must be the thermodynamic stable phase provided that the operating time is long enough. Therefore in theory, when DH is put into
Figure 6. XRD patterns of the solid samples obtained during the transition process from calcium sulfate dihydrate (DH) to calcium sulfate hemihydrate (HH) in glycerolwater solution with 33 mol % glycerol at 80 °C.
the glycerolwater solution under the conditions in region I, the final phase remains DH, whereas in region II DH will dehydrate directly to AH. In region III, DH will first transform to α-HH followed by further transition to AH as described by Ostwald’s rule of stages4 or transform directly to AH25 depending on the solution conditions. Therefore, α-HH is only expected to be prepared from DH in region III before its further transition to AH. Region III provides a thermodynamic preparation window of α-HH from DH in glycerolwater solution. The window is confined by the boiling point line and the DHα-HH phasetransition line. If the final product is desired to be α-HH, the glycerol concentration and temperature should be strictly controlled in this window. Increasing glycerol concentration appears to lower the phase-transition temperature. It would fall below 40 °C once the added glycerol reaches 50 mol %. 3.3. Validation of the Calcium Sulfate Dihydrate Transition to α-Hemihydrate in the Thermodynamic Preparation Window. A typical point of glycerol concentration 33 mol % and temperature 80 °C was chosen from the thermodynamic preparation window to confirm the feasibility of the DH transition to 13564
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Figure 7. Infrared spectra of the solid samples obtained during the transition process from calcium sulfate dihydrate (DH) to calcium sulfate hemihydrate (HH) in glycerolwater solution with 33 mol % glycerol at 80 °C.
α-HH. Figure 6 shows the XRD patterns of raw material DH (PDF-ICDD 033-0311) and solid samples obtained at an interval of 6 h during transition process. In the first 12 h, the solid phase remains unchanged and no new phase occurs. But 18 h later, the diffraction peaks of DH at 2θ = 11.62°, 23.39°, and 29.11° shrink seriously, while those of HH (PDF-ICDD 041-0244) at 2θ = 14.70°, 25.62°, and 29.71° show relatively high intensity. During the next 6 h, intensities of the diffraction peaks for HH increase at the expense of those for DH. After 36 h, the XRD pattern exhibits only the diffraction peaks for HH, indicating that DH has completely transformed into HH. This process was further followed by IR analysis (Figure 7).26 The IR spectrum of the 0 h sample shows all absorption peaks characteristic for DH. For SO42 groups, the ν1 vibration (symmetric stretching) is centered at 1005 cm1, ν3 vibration (asymmetric stretching) occurs at 1145 and 1117 cm1, while ν4 vibration (asymmetric bending) lies at 603 and 670 cm1. The librational vibration of H2O groups appears at 3405 and 3551 cm1. HH is evidenced in the 18 h sample since its characteristic peaks emerge at 3610 cm1 (librational vibration of H2O groups), 1008 (ν1 SO42 stretching), 1096, 1115, and 1154 (ν3 SO42 stretching), 601 and 660 cm1 (ν4 SO42 stretching). Thereafter, characteristic peaks of HH go more intense, while those of DH shrink gradually. Until 36 h, the last remained peak of DH at 3405 cm1 disappears, which indicates that DH has completely transformed into HH. This result is quite consistent with the XRD analysis. It should be noted that the ν3 (SO4)2 vibrations of the DH and HH exhibit some distinct adsorption bands from those of the pure ones, which exhibit only one band for DH at 1120 cm1 and three bands for HH at 1080, 1110, and 1195 cm1.27 The impurities involved in the material, such as sulfate, carbonate, or oxide of Mg and Na, should take this responsibility. Due to the extremely small amount, their effects on the water activity, which is the only determinant factor of the transition thermodynamics, could be ignored. The DSC graph of the 48 h sample (Figure 8) displays an exothermal peak at 193 °C, further confirming that the HH was α-form. As demonstrated, DH could directly transform to α-HH in glycerolwater solution under mild conditions as expected by the thermodynamic preparation window. The glycerolwater
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Figure 8. DSC graph of the solid sample obtained after 48 h of the calcium sulfate dihydrate (DH) transition in glycerolwater solution with 33 mol % glycerol at 80 °C.
Figure 9. Validation of the thermodynamic preparation window of αHH from DH in the glycerolwater solution: (O) the transition occurs within 12 h; (k) the transition occurs between 12 and 24 h; (9) the transition does not occur within 24 h.
solution may act as a novel category of medium available for the α-HH preparation from DH. To validate the profile of the preparation window, a series of DH dehydration tests was conducted under several solution conditions chosen from the window. The transition is regarded as a failure if no α-HH is detected in XRD or IR pattern of the product. The results are presented in Figure 9. When 12 h is selected as the operating time, the actual DHα-HH phasetransition line (dash line) shifts to the top right of that thermodynamically determined through the water activity method, resulting in a shrinkage of preparation window by around 50%. However, this line (dot line) goes closer when the operating time increases to 24 h, resulting in a shrinkage of preparation window by about 25%. This reflects a fact that the scope of the operable preparation window is directed by not only the thermodynamics but also the kinetics of the DH transition to α-HH. Assuming that the operating time is prolonged or the transition kinetics is promoted, the available preparation window would be much closer to the theoretical one given by the phase-transition diagram in Figure 5. Overall, the thermodynamic preparation window of α-HH is deemed satisfactory and the water activity method for phase-transition diagram determination is credible. 13565
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Industrial & Engineering Chemistry Research One can thus expect to regulate the transition of DH to HH with the aid of this thermodynamic preparation window. 3.4. Occurrence of Calcium Sulfate Anhydrite during the Preparation Process of α-Hemihydrate. In all experiments, direct transition of DH to AH is not observed. AH is only found to coexist with α-HH under the conditions in the top right corner of the preparation window (see the Supporting Information) where the temperature and the glycerol concentration are relatively high (Figure 9). Furthermore, AH only emerges as the operating time is over 48 h. The phenomena may be explained by the dissolutionrecrystallization mechanism that accompanies the dehydration or transformation process.8,28 The nucleation of AH, the thermodynamically stable calcium sulfate phase in region III, is usually kinetically retarded in aqueous solution, which then favors the precipitation of the metastable phase α-HH.29 As the temperature and glycerol concentration increase, the nucleation rate of AH is promoted, otherwise the nucleation rate of α-HH is retarded,30,31 leading to the occurrence of AH in advance. Therefore, to acquire high purity of α-HH, the preparation conditions should be controlled not only in thermodynamic preparation window but also within a suitable operating time. Further work is still needed to determine the relation between operating time and solution conditions in the window.
4. CONCLUSIONS The water activity method has been successfully adopted to construct the phase-transition diagram of calcium sulfate in glycerolwater solution. The good agreement between the diagram and experimental results implies that this method could qualify for the phase-transition diagram determination. The water activities of the glycerolwater solution were measured as a function of glycerol concentration (050 mol %) and temperature (3085 °C). It is reduced by increasing glycerol concentration and temperature, which accounts for the feasibility of DH transition to α-HH in the glycerolwater solution. The phase-transition diagram provides a thermodynamic preparation window of α-HH in glycerolwater solution. Through controlling the glycerol concentration and temperature within the window, direct transition of DH to α-HH was regulated. However, the window was apt to shrink as the operating time decreased, indicating that the available profile of the window was also directed by the transition kinetics of DH to α-HH besides the transition thermodynamics. α-HH acts only as an intermediate product during the dehydration process of DH, followed by further transition to AH. The nucleation of AH proceeds earlier under a high glycerol concentration and temperature. To obtain pure α-HH, the operating time should be controlled within a suitable value. ’ ASSOCIATED CONTENT
bS
Supporting Information. Figures I and II. This information is available free of charge via the Internet at http:// pubs.acs.org/.
’ AUTHOR INFORMATION Corresponding Author
*Tel.: +86-0571-88982026. Fax: +86-0571-88273687. E-mail:
[email protected].
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’ ACKNOWLEDGMENT We gratefully appreciate the Project 21176219 supported by NSFC, the Project 2006AA03Z385 supported by the National High-Tech R&D Program of China and Changjiang Scholar Incentive Program (Ministry of Education, China, 2009). ’ REFERENCES (1) Engert, H. J.; Koslowski, T. The New Gypsum Binder Alpha 2000 Production Technology and Products. ZKG Int. 1998, 51, 229. (2) Dahlgren, S. E. Fertilizer Materials, Calcium Sulfate Transitions in Superphosphate. J. Agric. Food Chem. 1960, 8, 411. (3) Powell, D. A. Calcium Sulfate Hemihydrate Prepared in Sodium Chloride Solution. Aust. J. Chem. 1962, 15, 868. (4) Z€urz, A.; Odler, I.; Thiemann, F.; Berghoefer, K. Autoclavefree Formation of α-Hemihydrate Gypsum. J. Am. Ceram. Soc. 1991, 74, 1117. (5) Nijhof, E.; Witkamp, G. J.; Mulder, K. H. Direct Formation of Alpha-calcium Sulfate Hemihydrate under Atmospheric Pressure. Eur. Patent 0642467, 1995. (6) Li, Z. B.; Demopoulos, G. P. Model-based Construction of Calcium Sulfate Phase-transition Diagrams in the HCl-CaCl2-H2O System between 0 and 100 °C. Ind. Eng. Chem. Res. 2006, 45, 4517. (7) Guan, B. H.; Shen, Z. X.; Wu, Z. B.; Yang, L. C.; Ma, X. F. Effect of pH on the Preparation of α-Calcium Sulfate Hemihydrate from FGD Gypsum with the Hydrothermal Method. J. Am. Ceram. Soc. 2008, 91, 3835. (8) Nancollas, G. H.; Reddy, M. M.; Tsai, F. Calcium Sulfate Dihydrate Crystal Growth in Aqueous Solution at Elevated Temperatures. J. Cryst. Growth 1973, 20, 125. (9) Guan, B. H.; Jiang, G. M.; Wu, Z. B.; Mao, J. W.; Kong, B. Preparation of α-Calcium Sulfate Hemihydrate from Calcium Sulfate Dihydrate in Methanol-Water Solution under Mild Conditions. J. Am. Ceram. Soc. 2011, 10, 3261. (10) Li, Q.; Ding, Y.; Li, F. Q.; Xie, B.; Qian, Y. T. Solvothermal Growth of Vaterite in the Presence of Ethylene Glycol, 1,2-Propanediol and Glycerin. J. Cryst. Growth 2002, 236, 357. (11) Dickinson, S. R.; McGrath, K. M. Switching between Kinetic and Thermodynamic Control: Calcium Carbonate Growth in the Presence of a Simple Alcohol. J. Mater. Chem. 2003, 13, 928. (12) Zhang, L.; Yue, L. H.; Wang, F.; Wang, Q. Divisive Effect of Alcohol-Water Solvents on Growth Morphology of Calcium Carbonate Crystals. J. Phys. Chem. B 2008, 112, 10668. (13) Flaten, E. M.; Seiersten, M.; Andreassen, J. P. Polymorphism and Morphology of Calcium Carbonate Precipitated in Mixed Solvents of Ethylene Glycol and Water. J. Cryst. Growth 2009, 311, 3533. (14) Block, J.; Waters, O. B. The CaSO4-Na2SO4-NaCl-H2O System at 25 o to 100 °C. J. Chem. Eng. Data 1968, 13, 336. (15) Guan, B. H.; Ma, X. F.; Wu, Z. B.; Yang, L. C.; Shen, Z. X. Crystallization Routes and Metastability of α-Calcium Sulfate Hemihydrate in Potassium Chloride Solutions under Atmospheric Pressure. J. Chem. Eng. Data 2009, 54, 719. (16) Yang, L. C.; Guan, B. H; Wu, Z. B. Solubility and Phase Transitions of Calcium Sulfate in KCl Solutions between 85 and 100 °C. Ind. Eng. Chem. Res. 2009, 48, 7773. (17) Kirk, D. W.; Tang, S. T. Process for the Production of Alpha Hemihydrate Calcium Sulfate from Flue Gas Sludge. U.S. Patent 00562892A, 1996. (18) Sullivan, J. M.; Kohler, J. J.; Grinstead, J. H. Solubility of αCalcium Sulfate Hemihydrate in 40, 50, and 55% P2O5 Phosphate Acid Solution at 80, 90, 100 and 110 °C. J. Chem. Eng. Data 1988, 33, 367. (19) Ling, Y. B.; Demopoulos, G. P. Solubility of Calcium Sulfate Hydrates in (0 to 3.5) mol 3 kg1 Sulfuric Acid Solutions at 100 °C. J. Chem. Eng. Data 2004, 49, 1263. (20) Li, Z. B.; Demopoulos, G. P. Solubility of CaSO4 Phases in Aqueous HCl-CaCl2 Solutions from 283 to 353 K. J. Chem. Eng. Data 2005, 50, 1971. (21) Richardl, L. G. Stability of Calcium Sulfate Hydrates in Wetprocess Phosphoric Acid. Ind. Eng. Chem. Proc. Des. Dev. 1967, 6, 371. 13566
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