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Polymorphic Transformation of Aluminum Hydroxide Precipitated from Reactive NaAl(OH)4-NaHCO3 Solution Yan Li, Yifei Zhang,* Fangfang Chen, Chao Yang,* and Yi Zhang Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China ABSTRACT: For the first time, the polymorphism and nucleation mechanism of the precipitates from concentrated sodium aluminate solution reacted with sodium bicarbonate solution were investigated. The interface tensions of gibbsite and bayerite in the solution were measured as 45.9 and 58.3 mJ/m2, respectively. Temperature and supersaturation significantly affected the polymorph as well as the polymorphic transformation of the initial precipitates in the reactive solution. Gibbsite solely nucleated at 70 °C, while bayerite solely nucleated at 50 °C regardless of supersaturation; however, the high supersaturation benefited the nucleation of bayerite and inversely the nucleation of gibbsite at 60 °C. The sequence of polymorphic transformation in the reactive solution was proposed as Al-containing cluster f boehmite f bayerite f gibbsite. And the transformation rate from bayerite to gibbsite in the mother solution increased with temperature and caustic concentration.
’ INTRODUCTION The control of polymorphism is extremely important since polymorphs often differ in their physicochemical properties such as crystal habit, morphology, density, solubility, and chemical reactivity, which lead to differences in the functionality and performance of the polymorphs.1 Aluminum trihydroxide (Al(OH)3) is known to exhibit four polymorphs, viz., gibbsite, bayerite, doyleite, and nordstrandite, and their various physical and chemical characteristics contribute to their wide technical and commercial usages.2-5 The polymorphic sequence of aluminum hydroxide precipitated from acidic Al3þ solutions or dilute alkaline aluminate solution has been extensively studied.6-15 A gel formed initially from the base hydrolysis of acidic Al3þ aqueous solutions, and then the gel transformed according to the following sequence:12 gel f pseudo-spinel f boehmite f bayerite f gibbsite A similar polymorphic sequence from dilute potassium aluminate solution (10-4 M < Al < 16 10-4 M; OH/Al = 6.5) neutralized by dilute HNO3 solution was presented.10,11 amorphous f boehmite f bayerite f gibbsite In alumina industry, aluminum hydroxide is usually produced by the neutralization of concentrated sodium aluminate solution (NaAl(OH)4; 4 M < NaOH < 6 M; 2 M < Al(OH)3 < 4 M) with CO2 (the sinter process) or by the seeded hydrolysis of the aluminate solution (the Bayer process). Although the polymorphic sequence of aluminum hydroxide precipitated from acidic Al3þ solution and dilute aluminate solution has been observed, polymorphism from concentrated aluminate solution is rarely investigated. Only a few literature reports investigated how the conditions of r 2011 American Chemical Society
aluminate solution influenced the crystal phase of aluminum hydroxide. Li et al.16 found that NaOH concentration in aluminate solution significantly affected the phase of aluminum hydroxide precipitated at 22 ( 1 °C, viz., bayerite precipitated at lower NaOH concentration (NaOH e 1.00 M), gibbsite at a higher NaOH concentration (NaOH > 2.50 M), and a mixture of the two phases at the intermediate concentration between 1 and 2.5 M. Gerson et al.17 obtained a mixture of gibbsite and bayerite from pure sodium aluminate solution at 65 °C when the solution was made from aluminum metal or gibbsite and heated up to 160 °C, but obtained gibbsite alone when the aluminate solution was made from gibbsite at 100 °C or the solution was seeded with either gibbsite or bayerite. They did not observe the transformation from bayerite to gibbsite in the solution where both bayerite and gibbsite crystallized initially. An easily controllable process using sodium bicarbonate (NaHCO3) aqueous solution instead of CO2 to neutralize the aluminate solution was proposed for alumina production in the sinter process.18 The novel process can be controlled precisely, which provides an effective alternative to investigate the polymorphic transformation of aluminum hydroxide precipitated from a reactive solution. Therefore, the induction time of the reactive NaAl(OH)4-NaHCO3 system was first measured in this research, and then the nucleation mechanism and the interfacial tension of precipitates in solution were determined according to the classical nucleation theory. The polymorphism as well as the polymorphic transformation of aluminum hydroxide at various temperatures and supersaturations was subsequently investigated. Received: October 22, 2010 Revised: January 27, 2011 Published: March 09, 2011 1208
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Figure 1. Plots of conductivity versus time at S = 3.36 and S = 3.51.
’ EXPERIMENTAL SECTION Reagents. Sodium hydroxide (AR), gibbsite (AR), and sodium bicarbonate (AR) were purchased from Beijing Chemicals Factory, China. Sodium bicarbonate aqueous solution was prepared by the dissolution of sodium bicarbonate in ultrapure water produced by Milli-Q from Millipore, USA. Sodium aluminate solution was made by dissolving gibbsite into hot alkaline solution at the boiling point, and the liquor after complete dissolution was vacuum filtered twice through a 0.22 μm membrane filter and then stored in a caustic-resistant polyethylene vessel. According to the alumina industry terminology, the total caustic concentration C in the aluminate solution was expressed as g of Na2Ok/L and the concentration of aluminum hydroxide A was expressed as g of Al2O3/L. The molar ratio (MR = 1.645C/A) of the aluminate solution was kept at 1.5 or 2.0 for all the experiments in this research. Determination of Solubility. As the required solubility data for the NaAl(OH)4-NaHCO3-H2O system are lacking in literature, the solubility data at 50, 60, and 70 °C were measured. The solution with known concentrations of sodium hydroxide and sodium bicarbonate and an excess of aluminum hydroxide crystals was vigorously mixed at a constant temperature for a period of time that was long enough for the system to reach equilibrium. During that period, about 10 mL sample of clear solution was filtered periodically from the slurry and analyzed for Al concentration by inductively coupled plasma-atomic emission spectroscopy (ICP-AES). The equilibrium was achieved when the Al concentrations of two consecutive samples were identical, and then the equilibrium concentration of aluminum hydroxide Aeq was obtained. Procedures. The nucleation experiments were performed in a 400 mL cylindrical stainless steel crystallizer with a mechanical stirrer, a conductivity electrode, and an inlet for sodium bicarbonate solution. The nucleation temperature was controlled by immersing the crystallizer in a water bath thermostatically controlled within (0.5 °C. Solution conductivity was measured using an in situ conductivity analyzer Mettler M300 fitted with an inpro7108-VP/CPVC 3.1B sensor. A certain volume of aluminate solution was poured into the crystallizer and stirred at 400 rpm. When the solution was heated to the desired temperature, a required quantity of NaHCO3 solution was added rapidly into the crystallizer and mixed with aluminate solution. When the aluminate and bicarbonate solutions were mixed completely, the conductivity of the reactive solution was monitored continuously. The conductivity of the solution was constant before nucleation, and it started to increase when nuclei formed and grew into detectable crystals, which was much different from the decrease of conductivity during nucleation
in other systems.19 The induction time was then determined based on the time elapsed from the complete mixing until the initial precipitates were identified by the increase of the conductivity. The final induction time was the average of duplicate measurements. When the induction period was measured, the slurry was filtered and the dried initial precipitates were analyzed by scanning electronic microscopy (SEM) and X-ray powder diffractometry (XRD) for crystals morphology and form. As the initial precipitates appeared in trace amounts, adequate amounts of solids for X-ray analysis were gathered by repeating the same experiment. Thirty-three nucleation experiments were performed to investigate the influence of temperature, supersaturation, and MR of the aluminate solution on the induction time and the polymorph of the initial precipitates. Experiments to investigate the polymorphic transformation during aging were also carried out. The slurry after the induction period was kept suspending under the same temperature and agitation speed for several hours; meantime, 10-30 mL of slurry was sampled at regular intervals and the dried products were then examined by XRD and SEM. The percentage of a polymorph in the product was determined by direct linear analysis, as suggested by Klug and Alexander.20
’ RESULTS AND DISCUSSION Nucleation of Aluminum Hydroxide. The induction time (tind) was determined by the intersection of the two horizontal and upward straight lines regressed from the plot of conductivity versus time since the complete mixing of the aluminate and bicarbonate solutions. The representative plots at supersaturations (S = A/Aeq) of 3.36 and 3.51 at 60 °C, presented in Figure 1, show induction times of about 39 and 20 min, respectively. Moreover, the conductivity increases faster at higher supersaturation which implies that more crystalline surface is generated at the higher supersaturation. Detailed experimental conditions and the corresponding induction time results are listed in Table 1. Clearly, the induction time decreases with supersaturation at constant temperature and decreases with temperature at constant supersaturation. This behavior agrees with classical results on crystallization. According to the classical nucleation theory,21 the primary nucleation mechanisms (homogeneous or heterogeneous) can be distinguished from the variation of induction time with supersaturation, and the interfacial tension (γ) of precipitate in solution can be estimated by plots of ln tind versus 1/(ln S)2. The plots of ln 1209
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Table 1. Induction Time and Polymorph of Initial Precipitates at Various Temperatures and Supersaturations expt T, °C MR Na2Ok/Na2Oc
S
tind (min) initial crystalline form
1
50
1.5
13.02
4.23
235
Ba
2 3
50 50
1.5 1.5
9.76 7.81
4.38 4.54
147 90
B B
4
50
1.5
6.51
4.71
54
B
5
50
1.5
5.58
4.88
28
B
6
50
1.5
4.88
5.07
6
B
7
50
2.0
2.47
5.71
16
B
8
50
2.0
2.50
5.70
24
B
9
50
2.0
2.78
5.27
50
B
10 11
50 50
2.0 2.0
2.79 3.12
5.22 4.88
58 91
B B
12
60
1.5
13.13
3.36
220
Gb
13
60
1.5
9.85
3.48
175
G
14
60
1.5
7.88
3.60
118
G
15
60
1.5
6.57
3.73
70
G
16
60
1.5
5.63
3.88
42
B
17
60
1.5
4.92
4.01
23
B
18 19
60 60
2.0 2.0
2.22 2.47
4.99 4.55
5 25
B (B þ G)c
20
60
2.0
2.78
4.16
55
21
60
2.0
3.17
3.82
140
G
22
60
2.0
3.70
3.50
173
G
23
70
1.5
12.24
2.86
227
G
24
70
1.5
9.79
2.94
220
G
25
70
1.5
8.16
3.02
132
G
26 27
70 70
1.5 1.5
7.00 6.12
3.12 3.21
67 38
G G
28
70
1.5
5.44
3.31
26
G
29
70
2.0
2.48
4.05
15
G
30
70
2.0
2.76
3.74
35
G
31
70
2.0
3.10
3.46
60
G
32
70
2.0
3.55
3.20
190
G
33
70
2.0
4.14
2.96
442
G
Figure 2. Dependence of induction time on supersaturation, nucleation temperature, and MR.
GþB
B denotes polymorph bayerite obtained. b G denotes polymorph gibbsite obtained. c B þ G denotes the mixture of bayerite and gibbsite obtained and bayerite is predominant.
Figure 3. Dependence of induction time on temperature at constant supersaturation.
a
tind versus 1/(ln S)2 at 50, 60, and 70 °C and MRs of 1.5 and 2 based on the data in Table 1 are illustrated in Figure 2. The linearity of all the plots in Figure 2 implies either homogeneous or heterogeneous nucleation alone under the investigated experimental conditions. The slopes of the regressed straight lines of MR = 1.5 at 50, 60, and 70 °C are 25.5, 16.0, and 11.9, respectively, while that of MR = 2.0 are 23.5, 17.1, and 10.0, respectively. The comparatively high values of the slopes suggest that the predominant mechanism is primary homogeneous nucleation,22 which is consistent with the result of Li et al.19 but different from that of Rossiter et al.23 who found a shift from homogeneous to heterogeneous nucleation. The dependence of the interfacial tension on temperature can be visualized by plotting ln tind versus 1/T3 at a constant supersaturation. The regressed straight lines at the supersaturation of 2.51 for gibbsite and 4.36 for bayerite, as shown in Figure 3, suggest that the interfacial tensions of both gibbsite and
Table 2. The Calculated Interfacial Tensions of Aluminium Hydroxide Crystals in the Reactive Solution interfacial tensions (mJ/m2) T (°C)
MR = 1.5
MR = 2.0
average
50
59.0
57.5
58.3
60
50.6
51.7
70
47.2
44.5
polymorph B B/G/GþB
45.9
G
bayerite in the solution are independent of temperature under the investigated conditions. The interfacial tensions of aluminum hydroxide crystals in the reactive solution were calculated21 from the slopes of these plots in Figure 2 and are shown in Table 2. The interfacial tensions varied from 44.5 to 59.0 mJ/m2, generally decreasing with temperature. The interfacial tension for gibbsite, 45.9 mJ/m2, is comparable to 45 ( 6 mJ/m2 reported by Rossiter et al.23 for gibbsite nucleation from the pure Bayer liquor but is larger than 33 ( 3 mJ/m2 reported by Li et al.19 for colloidal gibbsite from pure aluminate solution. The interfacial tension for bayerite, 1210
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Figure 4. The XRD patterns of three typical aluminum hydroxide precipitates.
58.3 mJ/m2, is less than 67 ( 20 mJ/m2 reported by Van Straten and de Bruyn11 for bayerite nucleation under a very dilute caustic concentration of 10-3 M NaOH, compared to 1-5 M NaOH in the present research. The calculated interfacial tension of the metastable bayerite is unexpectedly higher than that of the stable gibbsite, which implies the more difficult nucleation of the metastable phase. Actually, similar results of a metastable polymorph with comparatively high interfacial tension were also found for salmeterol xinafoate nucleation by Tong et al.24 and for famotidine nucleation by Lu et al.25 The MR of aluminate solution influences insignificantly the resistance for nuclei formation in this system since the almost equivalent interfacial tensions at MR = 2.0 and 1.5 in Table 2. Polymorphs of the Initial Precipitates. The crystalline initial precipitates from the reactive NaAl(OH)4-NaHCO3 solution were different from the gelatinous or amorphous precipitates from the acidic Al3þ solution or dilute aluminate solution. According to the reference diffraction patterns, gibbsite shows a very strong peak at 18.3° and 20.3° (2θ) while bayerite shows strong peaks at 18.8° and 40.6° (2θ). The XRD patterns of three representative initial precipitates obtained in Expts 2, 19, and 21, presented in Figure 4, are attributed to gibbsite, bayerite, and their mixture, respectively. The crystal form can also be identified easily by the morphology of the crystals shown in Figure 5. The sample of Expt. 21 exhibits the typical hexagonal prismatic morphology of gibbsite, while that of Expt. 2 exhibits cone-like morphology of bayerite, and Expt. 19 shows both hexagonal and cone-like morphology indicating the presence of a mixture of gibbsite and bayerite.2 The polymorphs of other initial precipitates detected by XRD and SEM are summarized in Table 1. Regardless of supersaturation, the initial precipitates at 50 °C were only bayerite while that were only gibbsite at 70 °C. The polymorph precipitated at 60 °C varied with the initial supersaturation of the solution, viz., gibbsite formed at low supersaturation, bayerite at high supersaturation, and a mixture of gibbsite and bayerite at the intermediate supersaturation. Gibbsite precipitation alone at 70 °C was also observed by Fogg et al.26 using in situ XRD technique. The polymorphism of aluminum hydroxide in this research was very different from that precipitated from the aluminate solution neutralized by CO2 where the bayerite precipitated only
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at 30 °C and the mixtures of gibbsite and bayerite precipitated at 50 and 70 °C.27 This difference was caused by the high supersaturation and the effect of aging during the carbonation process. The polymorphic behavior in this research is also different from that nucleated from the unseeded aluminate solution where gibbsite spontaneously nucleated even at the lower temperature of 22 °C.16 Precipitation Sequence during the Primary Nucleation Stage. In our nucleation experiment at a quite low supersaturation of 3.65 at 50 °C, a trace amount of transitional boehmite was detected. Actually, detection of the transitional phase at high temperature would require the measurement of times in the millisecond range which was difficult to execute. But, the transitional phase might disappear more slowly at a lower temperature. Therefore, to identify the presence of transitional boehmite in the reactive NaAl(OH)4-NaHCO3 solution, an experiment was performed at 35 °C. Figure 6 shows the XRD patterns and SEM images of the solid phases formed at 35 °C and at two different nucleation times tn which was recorded from the end of the induction period. The presence of the transitional boehmite phase during nucleation was confirmed in Figure 6. The thin plate-like particles shown in Figure 6a were established as boehmite (AlOOH).1,14,15 The plate-shaped particles dissolved very quickly within a few minutes, and the somatoids grew in size and thickness resulting in the formation of cone-like particles (Figure 6b). Furthermore, the XRD pattern of the precipitates isolated immediately after nucleation (tn = 0 min) has the typical peak of boehmite at 11° (2θ); however, the peak disappears at tn = 20 min, which suggests that the initially formed boehmite has transformed into more stable bayerite. The transitional phase of boehmite between the Al cluster and the final Al(OH)3 was also reported in the literature.28 Therefore, although boehmite was not identified from all the experiments at 50-70 °C in Table 1, the polymorphic sequence of the precipitate emerged during the nucleation stage in the reactive solution can be summarized as Al-containing cluster f boehmite f bayerite f gibbsite The amorphous or gelatinous aluminum hydroxide usually emerging in the precipitation sequence in acidic Al3þ solution or the dilute aluminate solution was not detected in this proposed sequence, and this result was consistent with that of AddaiMensah.29 The above proposed precipitation sequence is consistent with the decreasing thermodynamic stability of the solid forms, boehmite < bayerite < gibbsite, which confirms the well-known “Ostwald’s Rule of Stages”, a principle based on nonequilibrium thermodynamics stating that the thermodynamically least-stable phase would form prior to more stable polymorphs.21 On the other hand, the precipitation kinetics of different polymorphs, such as the relative rates of nucleation and growth of the stable and metastable forms, can influence the sequence significantly. The number-based polymorph fraction RA, aA = (JA)/((JA þ JB)) was used often as a simple way to evaluate the nucleation sequence of two polymorphs,30-33 where the nucleation rate J is expressed as21 ! βγ3 ν2m J ¼ B exp ν2 k3 T 3 ðln SÞ2 where B is the pre-exponential factor which depends on fluid dynamics in the system, β is the geometric shape factor (= 16π/3 1211
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Figure 5. SEM photographs of three typical initial aluminum hydroxide precipitates.
Figure 6. The XRD patterns and SEM images of the precipitates obtained at 35 °C and S = 4.36, (a) tn = 0 min, (b) tn = 20 min.
Table 3. Nucleation Rates of Boehmite, Bayerite, and Gibbsite From the NaAl(OH)4-NaHCO3-H2O Reactive System at 50 °C form boehmite bayerite gibbsite
γ, mJ/m2 25 58.3 45.9
vm 1029, m3 3.31 5.12 5.37
for spheres), γ is the interfacial tension, ν is the number of ions into which a solute molecule dissociates (ν = 2 for Al(OH)3), k is the Boltzmann constant (1.38 10-23J/K), T is the nucleation temperature and S = A/Aeq is the supersaturation, νm = Mw/FcN is the molecular volume of the crystalline phase, where Mw is the molar mass, N is Avogadro’s number, 6.02 1023 mol-1, Fc is the crystal density, 2420 kg/m3 for gibbsite, 2530 kg/m3 for bayerite, and 3010 kg/m3 for boehmite. The nucleation rates for various polymorphs are often different because the corresponding parameters γ, vm, and S are different. The calculated nucleation rates of boehmite, bayerite, and gibbsite from the reactive system at 50 °C are enumerated in Table 3. As the pre-exponent factors B in Table 3 are basically equivalent for boehmite, bayerite, and gibbsite under identical dynamic conditions,33 the nucleation rate of boehmite is always several orders of magnitude higher than that of bayerite and gibbsite, which is consistent with the prior nucleation of boehmite observed in this research. Therefore, the preferential nucleation of boehmite is not only thermodynamically but also kinetically favored. However, the higher nucleation rate of gibbsite compared with bayerite suggests
exp(-(βγ3ν2m)/(ν2k3T3(ln S)2))
S 1.33 1.60 2.00
J
4.8 10
-5
4.8 10-5B
3.2 10
-48
3.2 10-48B
1.2 10
-12
1.2 10-12B
that gibbsite should nucleate prior to bayerite, which is opposite to the observed precipitation sequence. Actually, both gibbsite and bayerite have the same sheet-like structural unit which consists of a layer of aluminum ions sandwiched between two layers of hexagonally packed hydroxyl ions. However, gibbsite has the lattice order of AB-BA type while bayerite has the lattice order of AB-AB type, and the intralayer hydrogen bonds in gibbsite are stronger than in bayerite. The structure of boehmite consists of double layers of octahedral oxygen in cubic close packing partially filled with Al cations. The Al-O-Al and Al-HO-Al bonds between adjacent layers parallel to the {1 1 1} planes of boehmite in caustic solution will cleave rapidly, and the cleavage continues until the oxygen framework is not cubic closest packed; therefore, a well crystalline bayerite phase but not gibbsite forms subsequently.34,35 Transformation of the Polymorphs during Aging. The polymorphic transformation of aluminum hydroxide during aging in its precipitating solution was investigated under various initial supersaturations and temperatures while the agitation rate was kept at 400 rpm. The change of the molar percentage (XB) of bayerite in precipitates with aging time is shown in Figure 7. 1212
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from concentrated aluminate solution reacted with sodium bicarbonate, Al-containing cluster f boehmite f bayerite f gibbsite was proposed and discussed thermodynamically and kinetically. The transformation from bayerite to gibbsite occurred only in the caustic solution and the transformation rate increased with temperature and caustic concentration.
’ AUTHOR INFORMATION Corresponding Author
*Tel: (þ8610)82544826. Fax: (þ8610)82544826. Email: (Y.Z.)
[email protected]. Email: (C.Y.)
[email protected]. cn.
Figure 7. Variation of the polymorphic fraction of bayerite with aging time.
The crystals initially precipitated under various temperatures and supersaturations in Figure 7 were all pure bayerite (XB = 100%), which was consistent with the above results. The temperature significantly influenced the polymorphic transformation from bayerite to gibbsite, which occurred earlier and faster at 60 °C than at 50 °C; for instance, at the initial supersaturation of 4.91, the transformation occurred after aging 1 h at 60 °C but occurred after aging 5 h at 50 °C. Furthermore, the influence of initial supersaturation on transformation was significant at 50 °C but not at 60 °C. For example, at 50 °C, the transformation at the initial supersaturation of 4.91 and 5.46 occurred after aging 5 and 2 h, respectively; however, at 60 °C, the transformation at the initial supersaturation of 4.91 and 3.84 both started after aging 1 h. When the bayerite crystals were separated immediately from the caustic mother liquor after nucleation, they were definitely stable and not converted to gibbsite in this research. The transformation process of the metastable bayerite occurring only in the caustic solution was considered as “solution-mediated” transformation; that is, the bayerite crystals dissolved and gibbsite nucleated and subsequently grew from the solution. The solution-mediated transformation rate was quite low owing to the small difference in the solubilities between bayerite and gibbsite in the caustic solution. The transformation rate was accelerated considerably by increasing the temperature, which indicated that the growth of stable gibbsite was a kinetically controlled process.
’ CONCLUSIONS The nucleation mechanism of aluminum hydroxide from the reactive NaAl(OH)4-NaHCO3 solution was determined as primary homogeneous nucleation according to the regression of measured induction times by the classical nucleation theory. The interfacial tensions of gibbsite and bayerite in the solution were estimated as 45.9 and 58.3 mJ/m2, respectively, which were independent of MR of the aluminate solution and of temperature under the investigated conditions. The polymorph of the initial precipitates was affected significantly by temperature. Gibbsite nucleated at 70 °C while bayerite nucleated at 50 °C regardless of supersaturation; however, for the nucleation at 60 °C, the high supersaturation benefited the formation of bayerite, and inversely the formation of gibbsite. The precipitation sequence of aluminum hydroxide
’ ACKNOWLEDGMENT The authors acknowledge the National Basic Research Program of China (973 Project 2007CB613501), the Knowledge Innovation Project of Chinese Academy of Sciences (KGCX2YW-321-2), and the National Science Fund for Distinguished Young Scholars (21025627) for funding this work. ’ REFERENCES (1) Blagden, N.; Davey, R. J. Cryst. Growth Des. 2003, 3, 873. (2) Misra, C. Industrial Alumina Chemicals; American Chemical Society: Washington, DC, 1984. (3) Demichelis, R.; Civalleri, B.; Noel, Y.; Meyer, A.; Dovesi, R. Chem. Phys. Lett. 2008, 465, 220. (4) Cesteros, Y.; Salagre, P.; Medina, F.; Sueiras, J. E. Chem. Mater. 2001, 13, 2595. (5) Lefevre, G.; Pichot, V.; Fedoroff, M. Chem. Mater. 2003, 15, 2584. (6) Fu, G.; Nazar, L. F.; Bain, A. D. Chem. Mater. 1991, 3, 602. (7) Cesteros, Y.; Salagre, P.; Medina, F.; Sueiras, J. E. Chem. Mater. 1999, 11, 123. (8) Wang, S. L.; Wang, M. K.; Tzou, Y. M. Colloid. Surf. A 2003, 231, 143. (9) Rousseaux, J. M.; Weisbecker, P.; Muhr, H.; Plasari, E. Ind. Eng. Chem. Res. 2002, 41, 6059. (10) Van Straten, H. A.; Holtkamp, B. T. W.; De Bruyn, P. L. J. Colloid Interface Sci. 1984, 98, 342. (11) Van Straten, H. A.; De Bruyn, P. L. J. Colloid Interface Sci. 1984, 102, 260. (12) Bradley, S. M.; Hanna, J. V. J. Am. Chem. Soc. 1994, 116, 7771. (13) Dash, B.; Tripathy, B. C.; Bhattacharya, I. N.; Das, S. C.; Mishra, C. R.; Mishra., B. K. Hydrometallurgy 2009, 95, 297. (14) Cai, W. Q.; Yu, J. G.; Mann, S. Microporous Mesoporous Mater. 2009, 122, 42. (15) Cai, W. Q.; Yu, J. G.; Gu, S. H.; Jaroniec, M. Cryst. Growth Des. 2010, 10, 3977. (16) Li, H.; Addai-Mensah, J.; Thomas, J. C.; Gerson, A. R. J. Cryst. Growth 2005, 279, 508. (17) Gerson, A. R.; Counter, J. A.; Cookson, D. J. J. Cryst. Growth 1996, 160, 346. (18) Li, Y.; Zhang, Y. F.; Yang, C.; Chen, L. B.; Zhang, Y. Chem. Eng. Sci. 2010, 65, 4906. (19) Li, J.; Prestidge, C. A.; Addai-Mensah, J. J. Colloid Interface Sci. 2000, 224, 317. (20) Klug, H. P., Alexander, L. E. X-Ray Diffraction Procedures, 2nd ed.; Wiley: New York, 1974. (21) Mullin, J. W. Crystallization; Butterworth-Heinemann: London, 2001. (22) S€ohnel, O.; Mullin, J. W. J. Colloid Interface Sci. 1988, 123, 43. 1213
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