Synthesis of Uniformly Spherical Bayerite from a Sodium Aluminate

Article pubs.acs.org/IECR

Synthesis of Uniformly Spherical Bayerite from a Sodium Aluminate Solution Reacted with Sodium Bicarbonate Shaowei You,†,‡,§ Yan Li,†,§ Yifei Zhang,*,† Chao Yang,*,† and Yi Zhang† †

National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology, Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China ‡ University of Chinese Academy of Sciences, Beijing 100049, China S Supporting Information *

ABSTRACT: Bayerite was synthesized successfully by the precipitation reaction of a sodium aluminate solution with sodium bicarbonate in a mixed suspension−mixed product removal (MSMPR) crystallizer. The particle size distribution of the prepared bayerite was significantly influenced by the temperature, the stirring speed, and the molar ratio (MR) of Na2Oc/Na2Ok as well as the addition rate of aqueous NaHCO3 solution in the system. The uniformly spherical bayerite with narrow “missing-fines-type” particle size distribution was obtained at 50 °C, with the MR of 0.66, at the agitation speed of 300 rpm in the MSMPR crystallizer. The synthesis of bayerite from the reactive NaAl(OH)4−NaHCO3 system was without an aging process. The thermal decomposition of the prepared bayerite was analyzed by TGA/DSC and XRD.

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INTRODUCTION Bayerite and gibbsite are two technically important polymorphs of aluminum trihydroxide (Al(OH)3).1,2 Bayerite is used commercially to prepare aluminum oxide catalyst supports due to the incorporation of less sodium during its precipitation than with gibbsite.3 In addition, bayerite is usually used as an adsorbent for phosphonic acid and metal ions and as a precursor in the synthesis of boehmite (γ-AlOOH) and transition alumina (η- or θ-Al2O3).1,4−13 For those usages, bayerite particles with uniformly spherical morphology and narrow particle size distribution (PSD) are highly desired.1 Bayerite is rarely found in nature, and it is synthesized generally via (i) hydrolysis and/or hydration of aluminum, AlN, and/or alumina or (ii) neutralizations, either of inorganic aluminum salts with ammonia or of the alkaline aluminate solution with CO2 or acids.1,14−21 The bayerite particles synthesized by the above methods always presented uncontrolled size distribution and diverse shapes: cone, prism, wedge, rod, and hourglass (the global term for them is somatoid). Actually, uniformly spherical bayerite particles with narrow particle size distribution were difficult to prepare via the previous processes because they were all performed in batch or semibatch conditions, where the supersaturation varied significantly. A small deviation of the supersaturation would lead to an obviously different shape and size. The precise control of supersaturation to avoid spontaneous nucleation is crucial to the crystallization of a sparingly soluble substance with desired particle size and morphology. Compared with a batch or semibatch process, a continuous process can be precisely maintained at a supersaturation level, suitable for producing sparingly soluble substances with uniform shape and narrow size distribution.22,23 The reaction of the NaAl(OH)4− NaHCO3 system is quite easily manipulated, and uniform gibbsite has been prepared from the reaction using the MSMPR crystallization technique.24 Likewise, mesoporous alumina has been synthesized from the reaction system and, moreover, the © XXXX American Chemical Society

neutralization of sodium aluminate solution with aqueous NaHCO3 solutions can be integrated into the sintering process in the alumina industry.25 In this work, the uniform spherical bayerite synthesized by the process was investigated, and the thermal decomposition process of the prepared bayerite was identified as well.

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EXPERIMENTAL SECTION Materials. Aqueous sodium bicarbonate solution was made by the dissolution of sodium bicarbonate (AR, Beijing Chemical Works) in deionized water, and its concentration was expressed as grams of Na2Oc per liter. Sodium aluminate solution was made by dissolving a known mass of aluminum hydroxide (AR, Beijing Chemical Works) into the boiling sodium hydroxide (AR, Beijing Chemical Works) solution. After complete dissolution, the solution was filtered twice through a caustic-resistant 0.22 μm Teflon membrane and then stored in a Teflon bottle. According to alumina industry terminologies, the caustic and aluminum concentrations in the aluminate solution were expressed as grams of Na2Ok per liter and grams of Al2O3 per liter, respectively. The aluminate solution with concentrations of 150 g Na2Ok/L and 164.5 g Al2O3/L approximating that in the alumina industry was specifically studied in this work. Synthesis. The synthetic experiments were carried out in a continuous MSMPR crystallizer (500 mL, diameter = 90 mm) with a three-blade marine-type agitator.23 The liquors of NaAl(OH)4 and NaHCO3 were fed at fixed rates to maintain the mean residence time throughout the experiments. A total of 6−10 mean residence times was necessary for the crystallization system to attain a steady state. The agitation speed of 300 rpm Received: January 29, 2013 Revised: August 12, 2013 Accepted: August 12, 2013

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dx.doi.org/10.1021/ie401353t | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Industrial & Engineering Chemistry Research

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were performed using a Mettler Toledo TGA/DSC 1 STARe system. The sample was heated at a rate of 10 °C/min from 25 to 1200 °C; nitrogen was fed at 40 mL/min. The PSD of the dried bayerite crystals was measured by a Malvern Mastersizer Hydro 2000MU instrument with water as dispersion agent. The form of “in band” percentage, namely, the volume percentage of the particle sample within that band, was originally displayed by the Malvern particle sizer. To compare the results from various experiments, the frequency curve of PSD obtained by differentiating the cumulative undersize curve was used in this work. The volume mean size D4,3 and the span of distribution, which was used to reflect the width or the spread of PSD, are given as

ensured that all particles were homogeneously distributed throughout the crystallizer. The Al2O3 contents in the aluminate solution and the residual reactant were analyzed by inductively coupled plasma optical emission spectrometry (Optima 5300 DV, Perkin-Elmer, USA). The experimental conditions are listed in Table 1. Table 1. Experimental Conditions for the Synthesis of Bayerite parameter

value

temperature (°C) mean residence time (min) agitation speed (rpm) addition rate of NaHCO3 solution (mL/min) addition rate of NaAl(OH)4 solution (mL/min) molar ratio (MR) of Na2Oc/Na2Oka MR (αk) of Na2Ok /Al2O3

40−60 50−90 300−600 4−7 2 0.57−0.86 1.6−2.0

D4,3 =

span =

a

Where Na2Oc is the molar concentration of sodium bicarbonate and Na2Ok is the molar caustic concentration in aluminate solution.

∑ di3

(3)

d0.9 − d0.1 d0.5

(4)

where di is the mean diameter of size class i and d0.1, d0.5, and d0.9 are the sizes of particles below which 10, 50, and 90% (v/v) of the sample lies, respectively. The volume distribution is then transformed to the population density distribution according to the following relationship:10

Characterization. The magma density M T of the suspension is defined as the amount of the precipitated bayerite per unit of volume, as expressed in eqs 1 and 2, where [Al2O3]in and [Al2O3]out are the total Al2O3 concentration (g/ L) entering and leaving the crystallizer, respectively; [Al2O3]F is the Al2O3 concentration in the fed NaAl(OH)4 liquor; FAl2O3 and FNaHCO3 (mL/min) are the flow rates of the fed NaAl(OH)4 and NaHCO3 liquors, respectively.

n=

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wM T ρc νΔv ∑ w

(5)

RESULTS AND DISCUSSION Steady State of the Continuous MSMPR Precipitation. To identify the steady state of the continuous MSMPR precipitation in the reactive system, the population density distribution and the total population number of bayerite particles were measured at dwell times during precipitation, as shown in Figure 1. It was concluded that the steady state was attained after 630 min. At the start for the precipitation, as shown by the curve on 90 min in Figure 1a, a great number of fine particles formed, and the total population number reached the maximum in the system (Figure 1b). During precipitation in the MSMPR

magma density M T = 1.52 × ([Al 2O3]in − [Al 2O3]out ) (1)

[Al 2O3]in = [Al 2O3]F F Al 2O3/(F Al 2O3 + FNaHCO3)

∑ di4

(2)

The habit and morphology of the produced crystals were observed by the scanning electron microscope (SEM, JEOL JSM-6700F). The phase was identified by comparison of the powder diffraction patterns of the product examined by X-ray diffractometer (XRD, X’Pert Pro MPD, Panalytical Co.; 40 kV, 30 mA) using Cu Kα radiation. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) analysis

Figure 1. Population density distribution curves (a) and total population number (b) measured up to steady state for the continuous crystallization of bayerite. B



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crystallizer, particles enlarged until the population density distribution curves remained constant (Figure 1a). Meanwhile, the great number of fine particles was depleted due to agglomeration. The decrease for the depletion rate of total population number before the system reached steady state (Figure 1b) implied that significant agglomeration happened, and the breakup rate of particles increased. When the population density distribution as well as the total population number of bayerite particles was constant at steady state, both the agglomeration rate and the breakup rate of particles in the system should be basically constant. Therefore, uniformly spherical bayerite particles with narrow “missingfines-type” size distribution were obtained from the steady state MSMPR precipitation. Crystal Phase. The XRD patterns of the synthesized bayerite are shown in Figure 2. The products obtained under

Figure 3. SEM images of bayerite particles obtained at MR of 0.47 and various temperatures: (A) 40 °C; (B) 50 °C; (C) 60 °C (magnification: left = 50× and right = 10000×).

Figure 2. XRD patterns of the synthesized bayerite products.

the experimental conditions were confirmed as the bayerite phase; however, it contained some small, but insignificant, amount of gibbsite. The presence of gibbsite was identified according to the week gibbsite peak (002). The crystal phase of bayerite was formed initially, without transformation during the whole process of the precipitation between aluminate solution and bicarbonate solution in this research. It was quite different from the preparation of bayerite using CO2 to neutralize aluminate solution, where an amorphous precipitate was produced at the beginning of putting CO2 into the sodium aluminate, and the initial amorphous precipitate transformed finally to bayerite via pseudoboehmite after an aging process for quite a long time.15 Particle Morphology. The bayerite particles precipitated in the experiments at 40 °C (Figure 3A) were accompanied with a number of small particles, but those formed at 50 and 60 °C (Figure 3B,C) were all uniformly spherical. Moreover, as shown in the SEM images, the agglomerated bayerite particles of many quite small crystallites suggested that compared with growth, nucleation, and breakage during the bayerite precipitation, the agglomeration actually induced by particle collisions and cemented by the intergrowth of agglomerates would be responsible for the growth of the particle size. The growth of the originally agglomerated uniform bayerite particles can be called agglomeration-controlled growth,26,27 as shown in Figure 4. At the beginning of the precipitation, the formed bayerite

Figure 4. SEM images of bayerite particles sampled at dwell times during precipitation: (A) initial moment of MSMPR experiment; (B) 2τ; (C) 4τ; (D) 7τ.

crystals are small and irregularly tabular (Figure 4A), and the evolution process of the agglomeration for those small particles can be tracked by the SEM images shown in Figure 4B−D. Particle Size Distribution. Differently from the bimodal distributions generally presented in most other agglomerating systems,28−31 the bayerite particles obtained under appropriate conditions in this work consistently exhibit unimodal PSDs (Figure 5). It was found that synthesis parameters, such as temperature, MR, and stirring speed as well as addition rate of C



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Figure 5. PSDs of bayerite particles prepared at various conditions: (a) temperature; (b) MR; (c) stirring speed; (d) addition rate of aqueous NaHCO3 solution.

and more important with the progresion of precipitation. The size and number density of initially formed particles in the solution significantly influence the following agglomerationcontrolled growth process and then the final PSD. The high stirring speed for a reactive crystallization can increase, decrease, or even insignificantly affect the mean particle size because a high stirring rate can probably improve the rates of agglomeration and/or breakup.33 The effect of stirring speed at 300 and 600 rpm, which could make ensure solid−liquid mixing for bayerite precipitation, on the differential PSD of bayerite products is shown in Figure 5c. The narrow PSDs at two stirring speeds were obtained with quite similar shape and span. However, the significantly smaller particle size of the bayerite precipitated at 600 but not at 300 rpm shows that the breakup in the process, compared to the agglomeration and growth, is improved more significantly at high stirring speed. The PSD of bayerite particles was found to be very sensitive to the addition rate of aqueous NaHCO3 solution to aluminate solution as shown in Figure 5d. The faster was the addition rate of the aqueous NaHCO3 solution, the wider was the PSD curve of bayerite particles and the smaller was the mean particle size obtained in this research, especially when the addition rate was above 6 mL/min. This implies that the local supersaturation in bulk solution generated by the addition of NaHCO3 in this fast reaction system is significant for preparing bayerite particles with qualified PSD. The faster the addition of NaHCO3, the higher the local supersaturation in the solution, and more fine particles formed instantly in the place, but the particles are irregular.

aqueous NaHCO3 solution, significantly influenced the PSD of bayerite. The mean particle size increases and the PSD curve narrows significantly as the temperature is raised from 40 to 60 °C, as is shown in Figure 5a. This supports the agglomeration-controlled growth of bayerite precipitation in the system, where the wide PSD at low temperature is typically because of the high nucleation rate at low temperature, with more primary nuclei, and then more small agglomerates form. The agglomeration-controlled growth of bayerite particles can be postulated as the aggregation of primary particles with growing agglomerates, and the primary particle is adhered instantly on the surface of the growing agglomerate by the collision between them. The collision and the surface integration rates increase with temperature, and then the primary particle can incorporate more quickly into the growing agglomerate at higher temperature.26,27 Furthermore, the lower viscosity of the medium at higher temperature also results in a higher frequency of collisions and then the higher growth rate and the larger mean particle size. As is shown in Figure 5b, the volume-mean size of bayerite decreases approximately linearly from 131.69 to 80.59 μm and the span of distribution increases from 0.64 to 0.72 when the molar ratio of Na2Oc to Na2Ok increases from 0.57 to 0.86. The low caustic concentration at high MR in the solution, actually with low solubility of bayerite, means high supersaturation of the precipitation. More small particles are formed from the solution with high supersaturation because of the fast nucleation rate.32 This implies that the nucleation is quite significant in the bayerite precipitation process. However, the growth and breakup as well as the agglomeration become more D



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Figure 6. TGA (A) and DSC (B) curves of the synthesized bayerite.

The molar ratio (αk) of Na2O to Al2O3 in the sodium aluminate solution insignificantly influenced the PSD of bayerite; however, the lower the molar ratio (αk), the higher the yield of bayerite. The figure can be found in the Supporting Information. Thermal Analysis. Thermal analysis for the bayerite particles prepared from the reaction of aluminate and bicarbonate solution was carried out, and the TGA and DSC curves are shown in Figure 6. There are three endothermic peaks at about 210, 272, and 501 °C and an exothermic peak at about 966 °C in the DSC curve of the particles (Figure 6B), whereas the corresponding mass loss at endothermic temperatures is shown in the TGA curve (Figure 6A). The total mass loss of 34.64% is about the theoretical value of 34.62% for the transformation from Al(OH)3 to Al2O3. The XRD patterns of the calcined bayerite particles prepared in this research are shown in Figure 7, and results of phase analysis are listed in Table 2. Phase analysis reveals that the thermal decomposition sequence of the bayerite is bayerite → boehmite → γ-Al2O3 → δ-Al2O3 → θ-Al2O3 → α-Al2O3, which agrees with the literature.34−38 The dehydration of bayerite goes to boehmite and not to η-Al2O3 in this work. However, fine bayerite transforms as follows: bayerite → η-Al2O3 → θAl2O3 → α-Al2O3, according to the literature.2,36 The difference between the two transition paths is related to particle size. It has been presented that the dehydrating decomposition of bayerite, identical with that of gibbsite, is affected by particle size.35,39 Very fine particles might show less boehmite formation,36 but boehmite forms from large bayerite particles due to the intergranular hydrothermal conditions resulting from the buildup of steam pressure in the coarse particles.2,36 In addition, the original shape of bayerite particles is retained after transforming completely to α-Al2O3 during the calcination at 1300 °C, which suggests a new route to prepare spherical sandy alumina.

Figure 7. XRD patterns of bayerite samples calcined at various temperatures for 2 h.

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CONCLUSIONS The uniformly spherical bayerite was synthesized from reactive NaAl(OH)4−NaHCO3 systems in a continuous MSMPR crystallizer, and the prepared bayerite exhibited narrow “missing-fines-type” PSD with the volume-mean size ranging E



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(5) Luschtinetz, R.; Oliveira, A. F.; Frenzel, J.; Joswig, J.; Seifert, G.; Duarte, H. A. Adsorption of phosphonic and ethylphosphonic acid on aluminum oxide surfaces. Surf. Sci. 2008, 602, 1347−1359. (6) Seyssiecq, I.; Veesler, S.; Boistelle, R.; Lamérant, J. M. Agglomeration of gibbsite Al(OH)3 crystals in Bayer liquors. Influence of the process parameters. Chem. Eng. Sci. 1998, 53, 2177−2185. (7) Musić, S.; Dragčević, Đ.; Popović, S.; Vdović, N. Chemical and microstructural properties of Al-oxide phases obtained from AlCl3 solutions in alkaline medium. Mater. Chem. Phys. 1999, 59, 12−19. (8) Wang, J. Q.; Liu, J. L.; Liu, X. Y.; Qiao, M. H.; Pei, Y.; Fan, K. N. Hydrothermal transformation of bayerite to boehmite. Sci. Adv. Mater. 2009, 1, 77−85. (9) Weidmann, C.; Brezesinski, K.; Suchomski, C.; Tropp, K.; Grosser, N.; Haetge, J.; Smarsly, B. M.; Brezesinski, T. Morphologycontrolled synthesis of nanocrystalline η-Al2O3 thin films, powders, microbeads, and nanofibers with tunable pore sizes from preformed oligomeric oxo-hydroxo building blocks. Chem. Mater. 2012, 24, 486− 494. (10) Santos, P. S.; Coelho, A. C. V.; Santos, H. S.; Kiyohara, P. K. Hydrothermal synthesis of well-crystallised boehmite crystals of various shapes. Mater. Res. 2009, 12, 437−445. (11) Rajabi, L.; Derakhshan, A. A. Room temperature synthesis of boehmite and crystallization of nanoparticles: effect of concentration and ultrasound. Sci. Adv. Mater. 2010, 2, 163−172. (12) Britto, S.; Kamath, P. V. Structure of bayerite-based lithiumaluminum layered double hydroxides (LDHs): observation of monoclinic symmetry. Inorg. Chem. 2009, 48, 11646−11654. (13) Seo, C. W.; Jung, K. D.; Lee, K. Y.; Yoo, K. S. Influence of structure type of Al2O3 on dehydration of methanol for dimethyl ether synthesis. Ind. Eng. Chem. Res. 2008, 47, 6573−6578. (14) Aguilar-Santillán, J.; Balmori-Ramírez, H.; Bradt, R. C. Sol-gel formation and kinetic analysis of the in-situ/self-seeding transformation of bayerite [Al(OH)3] to α-alumina. J. Ceram. Process. Res. 2004, 5, 196−202. (15) Du, X. L.; Su, X. H.; Wang, Y. Q.; Li, J. G. Thermal decomposition of grinding activated bayerite. Mater. Res. Bull. 2009, 44, 660−665. (16) Koga, N.; Fukagawa, T.; Tanaka, H. Preparation and thermal decomposition of synthetic bayerite. J. Therm. Anal. Calorim. 2001, 64, 965−972. (17) Lee, Y. P.; Liu, Y. H.; Yeh, C. S. Formation of bayerite, gibbsite and boehmite particles by laser ablation. Phys. Chem. Chem. Phys. 1999, 1, 4681−4686. (18) Lefèvre, G.; Pichot, V.; Fédoroff, M. Controlling particle morphology during growth of bayerite in aluminate solutions. Chem. Mater. 2003, 15, 2584−2592. (19) Kocjan, A.; Dakskobler, A.; Kosmač, T. Evolution of aluminum hydroxides in diluted aqueous aluminum nitride powder auspensions. Cryst. Growth Des. 2012, 12, 1299−1307. (20) Lefevre, G.; Duc, M.; Lepeut, P.; Caplain, R.; Fedoroff, M. Hydration of γ-alumina in water and its effects on surface reactivity. Langmuir 2002, 18, 7530−7537. (21) Trawczynski, J. T. Effect of aluminum hydroxide precipitation conditions on the alumina surface acidity. Ind. Eng. Chem. Res. 1996, 35, 241−244. (22) Randolph, A. C.; Larson, M. A. Theory of Particulate Processes: Analysis and Technology of Continuous Crystallization, 2nd ed.; Academic Press: London, 1988. (23) Scherzberg, H.; Kahle, K.; Käs eberg, K. Continuous precipitation and reaction crystallization of inorganic substances in agitated crystallizers with an integrated clarification zone. Chem. Eng. Technol. 1999, 22, 412−417. (24) Li, Y.; Zhang, Y. F.; Yang, C.; Chen, L. B.; Zhang, Y. Crystallization of aluminium hydroxide from the reactive NaAl(OH)4−NaHCO3 solution: experiment and modeling. Chem. Eng. Sci. 2010, 65, 4906−4912. (25) Zhong, L.; Zhang, Y. F.; Zhang, Y. Cleaner synthesis of mesoporous alumina from sodium aluminate solution. Green Chem. 2011, 13, 2525−2530.

Table 2. Phases of the Bayerites Calcined at Various Temperatures temperature (°C)

calcined product phase

210 270 400 500 900 1000 1100 1200 1300

boehmite boehmite boehmite, γ-Al2O3 γ-Al2O3 δ-Al2O3 δ-Al2O3, θ-Al2O3 θ-Al2O3, α-Al2O3 θ-Al2O3, α-Al2O3 α-Al2O3

approximately from 80 to 130 μm. During precipitation, the synthesis parameters, such as temperature, stirring speed, MR, and the addition rate of aqueous NaHCO 3 solution, significantly influenced the PSD of bayerite products. The bayerite particles retained uniformly spherical shape during the calcination at 1300 °C to form α-Al2O3, which suggested a new route to prepare spherical sandy alumina. The thermal decomposition of bayerite to α-Al2O3 in this research was identified via the bayerite → boehmite → γ-Al2O3 → δ-Al2O3 → θ-Al2O3 → α-Al2O3 path. Moreover, the bayerite was prepared directly in the system without an aging process.

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ASSOCIATED CONTENT

* Supporting Information S

PSD of bayerite particles prepared at various molar ratios (αk) of Na2O to Al2O3 in the sodium aluminate solution (Figure S1). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*(Yifei Zhang) Tel.: +86 10 82544826. Fax: +86 10 82544826. E-mail: [email protected]. *(Chao Yang) Tel.: +86 10 62554558. Fax: +86 10 82544928. E-mail: [email protected]. Author Contributions §

Shaowei You and Yan Li contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the National Natural Science Foundation of China (21276258) and the National High Technology Research and Development Program of China (863 Program, 2011AA060701) for funding this work.

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REFERENCES

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Synthesis of Uniformly Spherical Bayerite from a Sodium Aluminate

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