DOI: 10.1021/cg901164r
Nucleation and Morphology of Monosodium Aluminate Hydrate from Concentrated Sodium Aluminate Solutions
2010, Vol. 10 1605–1610
Shaotao Cao,†,‡,§ Yifei Zhang,*,†,‡ and Yi Zhang†,‡ †
Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China, ‡National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology, Beijing 100190, China, and §Graduate University of Chinese Academy of Sciences, Beijing 100049, China Received September 22, 2009; Revised Manuscript Received January 30, 2010
ABSTRACT: The induction time for monosodium aluminate hydrate (MAH) crystallization from supersaturated solution at 333 to 373 K is systematically investigated by experiment and the primary nucleation is also identified according to the classical nucleation theory. The truncated quadrangular orthopyramid and the twin/composite truncated pyramid crystals of MAH except for the tabular crystals and octagonal platelets obtained in the research are presented for the first time; furthermore, the supersaturation and temperature of the nucleation are experimentally studied. The interfacial free energies γ of truncated pyramid and octagonal platelet MAH crystals in the supersaturated sodium aluminate solution are conducted, and then the octagonal platelet MAH crystal is considered more stable in the solution than the truncated pyramid crystals. The twodimensional (2D) mediated growth mechanism for MAH crystallization is suggested based on the regression of experimental data of nucleation.
1. Introduction Sodium aluminate is an important industrial inorganic chemical used as a water purifying agent, an additive in newsprint mills, and a pH adjustment chemical on many occasions.1-3 It is also a convenient source of alumina to prepare zeolites and other catalysts,2,3 and an indispensable intermediate to produce alumina from bauxite in the hydrochemical process.3,4 Sodium aluminate generally includes Na2O 3 Al2O3, 3Na2O 3 Al2O3 3 6H2O, 4Na2O 3 Al2O3 3 12H2O, 6Na2O 3 Al2O3 3 12H2O,1,2,5 and monosodium aluminate hydrate (MAH, Na2O 3 Al2O3 3 2.5H2O), where MAH can be conveniently and economically prepared from sodium aluminate solution in the industrial production process of alumina.3,4 Unlike the precipitation of Al(OH)3 from the supersaturated sodium aluminate solution of relatively low alkaline concentration, which has been extensively investigated,6-9 the crystallization of MAH from supersaturated sodium aluminate solutions of high alkaline concentration,1,2 especially the nucleation and morphology of MAH, has been scarcely studied so far. The formation of nuclei from supersaturated solution often significantly influences the subsequent crystallization process and the properties of the final crystals; thus, nucleation generally attracts attention in many respects.10-12 Nucleation can be considerably affected by various operating parameters such as initial supersaturation, temperature, pH, agitation speed, and the presence of additives or impurities.13,14 Theoretic and experimental investigations on nucleation are generally based on the induction time tind, which actually refers to the time elapsing since the creation of a supersaturated solution until a new solid phase is detected in the solution.13,15
The induction time tind is generally considered as the sum of the relaxation time (tr) required for the system to achieve a quasi-steady-state distribution of molecular clusters, the nucleation time (tn) elapsing from the creation of the quasisteady-state distribution until the formation of critical nuclei, and the growth time (tg) needed for the nuclei to grow to a detectable size.16 And the induction time is often measured visually.13,15-17 Moreover, the regression of the induction time and the supersaturation based on the model proposed by Kashchiev et al. is generally used to validate the growth mechanisms of crystals, especially for inorganic compounds.18-21 For example, the model has been successfully applied to the intensively studied precipitation of CaCO3.13,15,21-23 The crystal morphology influences not only the quality of the product, but also the downstream processing, for example, rate of crystal dissolution, agitation, washing, milling or grinding, and handling and storage processes.24 Investigations on the morphologies of crystals from supersaturated solution are also important for understanding the growth mechanism and the physicochemical properties of crystals, and exploiting their applications.25-28 As few information of MAH crystallization is presented in the references, the nucleation and morphology of MAH from supersaturated aluminate solution is experimentally and theoretically investigated in this research. The interfacial free energy of MAH in solution is also determined according to classic nucleation theory, and the growth mechanism for MAH crystallization is deduced using the model proposed by Kashchiev et al. 2. Experimental Section
*Corresponding author. Address: Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, No. 1, Beiertiao, Zhongguancun, Beijing 100190, P. R. China. Tel: (þ8610)62655828. Fax: (þ8610)62655828. E-mail:
[email protected]. ac.cn.
2.1. Materials. The qualified supersaturated sodium aluminate solutions were prepared in a nickel vessel, by heating the mixture with a proper proportion of sodium hydrate and alumina trihydrate of analytical grade, and water highly purified by a Millpore Milli-Q system, to fully react and form a clear solution, followed by further
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instant filtration of the solution with quadruple qualitative filter paper. In order to ensure the same thermal history of the prepared clear supersaturated solution for each run of crystallization in the experiments, all the supersaturated solutions were reheated to 6 K above and spontaneously dropped to the preset experimental temperature of the crystallization in this research.29,30 2.2. Experimental Equipment and Procedures. The 350 mL jacket crystallizer is made of stainless steel with a lining of Teflon and stirred with a Teflon impeller driven by a multivariable speed motor under autocontrolled agitation, where the heating oil circulates from a thermostat bath to precisely control the crystallization temperature within (0.5 C. The crystallizer is sealed with plexiglass to prevent the evaporation of water from the solution and the entrance of dust to influence the experimental accuracy and facilitate the observation. The induction times (tind) of MAH crystallization with the agitation speed of 300 rpm were recorded from the solution reaching the preset experimental temperature until the emergence of visible particles by the naked eye. Meanwhile, the concentration variation of the clear filtrate of the suspension samples during nucleation was also measured by ICP analysis to validate the induction time in this research.15-17,31 Each experiment was conducted three to five times, and then their average values of the induction time were obtained. The crystallization of all experiments proceeded for a further 6-9 h after the elapse of the induction time. The suspension samples were filtrated, and the filter cake was washed with absolute alcohol, and then dried in the oven at 353 K for 12 h to get the sample of crystal product. 2.3. Analytical Methods. The chemical components of solutions were analyzed by an Optima 5300DV ICP-AES, and the morphologies of crystals were identified by field-emission scanning electron microscopy (FESEM, JSM-6700F, JEOL, Japan). The X-ray diffraction patterns of crystals were determined on the powder diffractometer (XRD, X’Pert MPD Pro, PanAnalytical, Netherlands) with Cu KR (λ = 0.15408 nm) radiation at room temperature.
3. Results and Discussion 3.1. Induction Time of MAH Primary Nucleation. On the basis of the classical nucleation theory,10,16,17 if the induction time tind of crystallization is supposed to be equal to the nucleation time for critical nuclei formation from supersaturated solution,13 which is inversely proportional to the nucleation rate, the linear relationship between tind and supersaturation S of the solution can be obtained as log tind ¼ K þ Rðlog SÞ -2
ð1Þ
where K is a dimensionless empirical constant and the supersaturation S of the solution is defined as CA ð2Þ S ¼ CA where CA (g L-1 Al2O3) is the actual concentration of Al2O3 in the solution, and CA* is the equilibrium concentration as obtained from the references,5,32 under the experimental conditions where the initial Na2O concentrations of all the experiments in this research are 490 g L-1. For homogeneous nucleation and the particles with arbitrary shape, R in eq 1 depends on a number of variables according to the classical nucleation theory,13,16,31 as given by 4NA fs3 γ3 ν2mol ð3Þ R ¼ ð2:3RÞ3 27fν2 T 3 where fs and fv are the surface and volume shape factors, respectively, which can be calculated by the geometrical parameters of the MAH crystal, γ is the interfacial free energy of the crystal in the actual supersaturated solution
Figure 1. Dependence of log tind on (log S)-2 for MAH primary nucleation at 333, 353, and 373 K.
(J m-2), νmol is the molecular volume of per mole solute (m3 mol-1) as 8.93 10-5 m3 mol-1 for MAH calculated from the density and molecular weight of MAH crystals (data from ICSD reference code 01-083-0315), NA is the Avogadro number (mol-1), and R is the ideal gas constant (J mol-1 K-1). The linear dependence of log tind on (log S)-2 is regressed and shown in Figure 1. The induction time decreases with the increases of supersaturation and temperature. Furthermore, the curves in Figure 1 for any fixed temperature basically consist of two straight-line sections with different slopes, where the section having a larger slope represents homogeneous nucleation at high supersaturation, while the other section having a small slope represents heterogeneous nucleation at low supersaturation.13,16 Similar results of many other crystallization systems have been observed.13,16,31,33 The reason for the occurrence of two distinct regions of nucleation related to homogeneous and heterogeneous nucleation is that the homogeneous nucleation rate is dominative in the solution with high supersaturation, but the heterogeneous nucleation rate is advantaged in the solution with low supersaturation. 3.2. Morphology of MAH Crystals. The various morphologies of MAH prepared in this research, as shown in Figure 2, include the truncated quadrangular orthopyramid, the twin/composite truncated pyramid, the octagonal platelet and the twin/composite octagonal platelet, while only the tabular crystals or octagonal platelets have been reported in the literature.2 The formation of the MAH crystals with different morphologies basically depends on the supersaturation of the solution and the temperature of nucleation. Figure 3 shows their relationship, where the MAH crystals of truncated quadrangular orthopyramid preferentially form in the solution of high supersaturation at low temperature, while the octagonal platelets from the solution of low supersaturation at high temperature, and the twin or composite appearance does in the solution of moderate supersaturation. It is well-known that the changes of crystal habit may be indicative of a phase transition;14 hence, the XRD patterns of both the MAH crystals of pyramid and platelet prepared in the experiments were analyzed. Apart from intensity differences of the XRD patterns shown in Figure 4, which can be attributed to preferred orientation of the samples, all the diffraction peaks coinstantaneously appear at their same
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Figure 2. Morphologies of MAH crystals formed at different conditions of crystallization: (a) S0 = 7, (b) S0 = 3, (c) S0 = 2, at 333 K, (d) S0 = 4.92, (e) S0 = 2.73, (f) S0 = 2.19, at 353 K; (g) S0 = 2.61, (h) S0 = 2.24, (i) S0 = 1.49, at 373 K.
Figure 3. The schematic division of different MAH morphologies crystallized from supersaturated aluminate solutions.
angular positions, by which the tetragonal structure of MAH is indexed with the ICSD reference code 01-083-0315. Therefore, it is validated that no phase transition of pure MAH occurs during the crystallizations in this research and those crystals with different morphologies prepared have the same crystal phase. Each face of a faceted crystal often presents different growth rates during crystallization, where the face with relatively high growth rate enlarges relatively slowly during crystallization or even disappears in the ultimate crystal, while those with a slow growth rate are well developed.16,34 Various growth rates of different faces of a crystal result from the electrostatic, geometric, and stereochemical features of the crystals in the supersaturated solution,35 and
Figure 4. Comparison of the XRD patterns of the MAH crystals with the truncated pyramid and platelet appearances, and the reference pattern from ICSD.
considerably decide the morphology of the final crystals. Hulliger proposed that the morphology of pure crystal was essentially kinetically controlled and determined by stability/ instability criteria in the crystal-solvent interface.36 Therefore, the morphology of a crystal during crystallization considerably varied with the supersaturation of the solution, temperature, and other operating factors.14,16,37 In the MAH crystallization, the growth rates along the lateral faces of the MAH crystal but in the Æ001æ direction greatly increase with the increase of crystallization temperature and the decrease of supersaturation, and the distinction
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Figure 5. MAH morphology crystallized from the solution with S0 of 2.24 at 333 K: (a) crystals emerged 0.5 h after being observed by naked eyes; (b) crystals after growth further for 9 h. Table 1. Corresponding Expressions of F(S) and f(S) for the Different Growth Mechanisms of the Mononuclear and Polynuclear Modela growth mechanism
F(S, t)
normal growth spiral growth 2D-nucleation mediated growth diffusion-controlled growth
tind ln½S ðS -1Þ ln½S1=n ðS -1Þ2ðn -1Þ=n tind ln½Sðn þ 2Þ=3n ðS -1Þ½2ðn -1Þ=3n tind ln½S1=n ðS -1Þðn -1Þ=n tind
a
1=n
ðn -1Þ=n
f(S)
ν
1 ln2 S 1 ln2 S 1 ln S
1 1 1
1 ln2 S
1 2
From refs 13, 15, and 21.
between the growth rates of the lateral faces and the Æ001æ direction enlarges in the solution of low supersaturation at high temperature, and then the platelet MAH crystal is about to form. The flake MAH crystals easily aggregate to diminish the surface energy, which induces the formation of tangled particles and the intergrowth phenomenon, as shown in Figure 2b,e. Similar results of crystallizations from various systems are presented in references. Che et al reported that high temperature led to more anisotropic growth, but lower temperature led to isotropic growth in the preparation of mesoporous silica.38 Chichakli found that the crystallization of petroleum waxes tended to produce plates at slow cooling rate or low supersaturation level.39 Zhang presented crystal with more complex morphology from the solution of low supersaturation.40 Viswanath and Ravishankar et al. recently presented the nucleation of two-dimensional (2D) islands and layer-by-layer growth mechanism to achieve the plate-shaped morphology at low driving forces, which could predicate accurately the formation of plate-shaped structures from the solution with low supersaturation.41-44 On the other hand, when the driving force for interface motion is large, all interfaces of the formed nucleus can move normal to themselves, leading to the formation of nearly equiaxed structures. 3.3. Interfacial Energy for MAH Crystallization. The interfacial energy (γ) of MAH crystal in supersaturated sodium aluminate solutions is calculated by eq 3 according to the experimental data of homogeneous nucleation shown in Figure 1. The statistic geometric parameters b/a and β of the truncated crystals shown in Figure 3 are 0.6 and 60, respectively, and the h/a of the octagonal platelets is 0.1. Therefore, the fs3/fv2 in eq 3 for the truncated crystals is conducted as 441, and that for the octagonal platelets is 1018. As a result, the interfacial free energies (γ) of MAH crystals in the supersaturated sodium aluminate solutions are determined to be 7.8, 7.6, and 4.1 mJ m-2 at 333, 353, and 373 K, respectively. The higher interfacial free energy γ of a crystal in solution means more difficult homogeneous formation of nuclei from
supersaturated solution, and more instability of the crystal formed in the solution.16 The interfacial free energy γ of truncated pyramid MAH crystals emerged by homogeneous nucleation at 333 to 353 K is 7.8 to 7.6 mJ m-2, while that of octagonal platelet crystals formed at 373 is 4.1 mJ m-2, which implies that the octagonal platelet MAH crystal is more stable compared to the truncated pyramid. It is also proved by the transition of MAH morphology from truncated pyramid to octagonal platelet after aging of 9 h at a relatively high temperature of 373 K, as shown in Figure 5. Using an online imaging technique, De Anda et al. also found that the transition of crystal morphology to a thermodynamically stable state is promoted of the crystallization at high temperature.24 Furthermore, it is considered that the crystal of thermodynamically unstable phases initially forms, followed by recrystallization of a thermodynamically stable phase according to Ostwald’s rule of stages.16,45 By the way, the interfacial energies of MAH crystals in aluminate solutions are much smaller than the interfacial energies 45 mJ m-2 of Al(OH)3 in aluminate solutions at 333 K measured by Rossiter et al,8 or 33 mJ m-2 at 338K by Li et al,7 indicating that the crystallization of MAH is much faster than the precipitation of Al(OH)3 from supersaturated sodium aluminate solution.4 4. Identification of the Crystal Growth Mechanism The mechanisms for crystal growth of MAH can be differentiated by the mononuclear and polynuclear model proposed by Kashchiev et al., including no growth, normal growth, spiral growth, 2D-mediated growth, and volume diffusioncontrolled growth mechanism.13 The induction time in this model, regardless of the number of nuclei appearing and growing in the supersaturated solution, is expressed as 1 δ þ ð4Þ tind ¼ JV ðan JGn -1 Þ1=n where J is the nucleation rate (# m-3 s-1), V is the volume of the supersaturated solution (m3), δ is the volume fraction of the new phase formed, an is a shape factor, G is the growth rate (m s-1), and the power n = mv þ 1, where the dimensionalities of growth m are 1, 2, 3 for the particles of needles, disks or plates, and spheres or cubes, respectively, v is equal to 0.5 for the volume diffusion-controlled growth mechanism, and 1 for all the normal, spiral, and 2D-mediated growth mechanism.13,15 Equation 4 involves two main approaches that are responsible for the breakdown of the metastable equilibrium in the supersaturated solution, where the first item of the multinomial equation expresses the mononuclear approach corresponding to no growth mechanism, that is, the metastable equilibrium of the system is broken down by the appearance of the initially emerging nucleus, and the second item expresses the polynuclear approach corresponding to the other mechanisms; that is, the metastable equilibrium is broken down by the formation of a statistically large number of nuclei and their growth to a detectable size. Upon the rearrangement of eq 4, the linear relationships of functions F(S, t) and f(S) for four growth mechanisms are given as listed in Table 1, and those experimental values of F(S, t) and f(S) based on the regression of data in Figure 1, as well as their simulated straight-lines are shown in Figure 6. All four growth mechanisms have been included because the mononuclear approach alone is only applicable to the system with a rather
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Figure 6. Dependence of F(S) on (ln S)-2 or (ln S)-1 for primary nucleation of MAH at 333 K (the expressions of F(S) are listed in Table 1).
small volume (,10 mm3).18 The fitting correlation of function F(S, t) to f(S) gives the information of the most possible growth mechanism. From Figure 6, the 2D-mediated growth mechanism is suitable for the crystallization of the truncated pyramid crystals of MAH in supersaturated aluminate solution at 333 K, and a three-dimensional growth (m = 3) is assumed for the crystal of truncated pyramid, and then the exponent power n in eq 4 of 4 is also obtained. The same growth mechanism is also found to be applicable for the crystallization of MAH at both 353 and 373 K, based on the regression of the experimental data. 5. Conclusions The induction time for MAH crystallization from supersaturated sodium aluminate solution at 333 to 373 K is systemically correlated to the supersaturation of the solution, and the primary nucleation of homogeneous nucleation at high supersaturation and the heterogeneous nucleation at low supersaturation is experimentally identified according to the classical nucleation theory. Except for the tabular crystals and octagonal platelets of MAH crystallized from the supersaturated aluminate solution in this research, the truncated quadrangular orthopyramid and the twin/composite truncated pyramid of MAH are presented for the first time. And the MAH crystals of truncated quadrangular orthopyramid preferentially forms in the solution with high supersaturation at low temperature, while the octagonal platelet does in the solution with low supersaturation at high temperature, and the twin or composite
MAH crystals does in the solution of moderate supersaturation. The interfacial free energy γ of the truncated pyramid MAH crystals in the supersaturated sodium aluminate solution at 333 to 353 K is measured as 7.8 to 7.6 mJ m-2, while that of octagonal platelet crystals at 373 K is 4.1 mJ m-2, which implies that the octagonal platelet MAH crystal is more stable than the truncated pyramid crystal. Furthermore, the interfacial free energies of MAH crystals in supersaturated sodium aluminate solution are much smaller than that of Al(OH)3 crystals, indicating the much faster crystallization rate of MAH. The 2D-mediated growth mechanism of MAH crystallization has been identified by the regression of experimental data of the nucleation according to the model proposed by Kashchiev et al. Acknowledgment. We acknowledge the National Basic Research Program of China (973 program, No. 2007CB613501), the Knowledge Innovation Project of Chinese Academy of Sciences (KGCX2-YW-321-2), and the National Key Technologies R&D Program (No. 2006BAC02A05) for funding this work.
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