Synthesis of Barium Sulfate Nanoparticles Using a Spinning Disk

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Ind. Eng. Chem. Res. 2009, 48, 7574–7580

Synthesis of Barium Sulfate Nanoparticles Using a Spinning Disk Reactor: Effects of Supersaturation, Disk Rotation Speed, Free Ion Ratio, and Disk Diameter Asghar Molaei Dehkordi* and Alireza Vafaeimanesh Department of Chemical and Petroleum Engineering, Sharif UniVersity of Technology, P.O. Box 11155-9465, Tehran, Iran

The aim of this research was to synthesize barium sulfate nanoparticles using a spinning disk reactor. Barium sulfate was produced by continuously pumping two aqueous solutions of BaCl2 and Na2SO4, respectively, into the chamber of spinning disk reactor, where a liquid-liquid reaction took place to form BaSO4. The influences of various operating and design parameters such as the initial supersaturation, disk rotation speed, free ion ratio, and the disk diameter on the size of barium sulfate nanoparticles were carefully investigated. By varying the supersaturation and disk rotation speed, a broad range of particle size ranging from micrometer sizes down to nanoparticles smaller than 100 nm can be produced. Using high disk rotation speed and high initial supersaturation, crystals ∼38 nm in size were produced. It was found that the variation of free ion ratio has a significant influence on the obtained particle size. Moreover, at a constant supersaturation and disk rotation speed, precipitation experiments with the excess amount of barium or sulfate ions lead to smaller mean particle size compared to those prepared under stoichiometric conditions. 1. Introduction Barium sulfate, so-called Barite, used in the paint and paper industries as well as in oil production, can be extracted from the ground or synthesized by a precipitation method. In addition, it has been largely used as a model system to investigate the effects of various process conditions on the precipitation reactions and to develop precipitation models. Nanoparticles, one of advanced materials, have tremendous potential and applications in many industries, such as electronics,1,2 opticals,3,4 chemicals,5,6 ceramics,7,8 metallurgy,1 pulp and paper,9 environment,10 pharmaceutics,11-13 and so forth. In the past decade, significant international research efforts have been directed toward the synthesis, characterization, and properties of nanoparticles.14 More recently, the concerns of mass production of nanoparticles have encouraged research activities in the development of a synthesis method with lowcost and high-volume production. On the other hand, many methods for preparing nanoparticles were developed and reported in the open literature. These methods could be classified as physical vapor deposition,15 chemical vapor deposition,15,16 reactive precipitation,17,18 sol-gel,19 microemulsion,20 sonochemical processing,21 supercritical chemical processing,22 etc. Among these methods, reactive precipitation is of high industrial interest because of its convenience in processing, low cost, and massive production. The precipitation process is fast, operable at ambient temperature, and involved nucleation, growth, and agglomeration phenomena. In general, the nucleation and growth processes take place concurrently to secondary phenomena, such as agglomeration, attrition, and breakage. It is therefore possible to identify primary particles, which are formed by crystal nucleation and growth, and secondary particles, which derive mainly from agglomeration phenomena.23 Supersaturation is known to play the main role in controlling the mechanism and the kinetics of nucleation and growth processes. In particular, heterogeneous nucleation can take place at the low level of supersaturation. In this case, the generation * To whom correspondence should be addressed. E-mail: amolaeid@ sharif.edu. Tel.: +98-21- 66165412. Fax: +98-21- 66022853.

of nuclei is catalyzed by foreign particles, typically dust. On the contrary, very high levels of supersaturation are required for homogeneous nucleation, since in this case the critical nucleus can be only generated by the collisions of a high number of solute clusters randomly moving in solution. Depending on the level of supersaturation, nucleation phenomena are generally very fast in precipitation processes. Since the driving force of the nucleation process is local supersaturation, the intensity of mixing plays a fundamental role in determining the precipitation mechanism and, hence, particle properties and crystal size distribution. Thus, very high levels of supersaturation and intense mixing are required to ensure that homogeneous nucleation is the dominant nucleation mechanism.24 Conventional precipitation processes are often carried out in stirred tank or column reactors, therefore, the control of product quality is difficult and the morphology and particle size distribution changes during the production time. The high-gravity (Higee) technique proposed by Ramshaw25 is an example of process intensification and is defined to minimize the equipment scale, to save space, resources, and energy, and thus to make the chemical industry cleaner and safer. During the past two decades, Higee systems have been extensively applied in chemical processes because of the great enhancement in the mass-transfer rate in many unit operations such as distillation, absorption, stripping, extraction, and adsorption, and it has recently been adopted in the field of precipitation.26 There are two types of devices, that is, the high-gravity rotating packed-bed reactor (RPBR) and the spinning disk reactor (SDR). Chen et al.14 have successfully produced submicrometer particles and even nanoparticles such as CaCO3, SrCO3, and Al(OH)3 by the RPBR without adding surfactants to the reacting solution. Cafiero et al.27 have prepared BaSO4 particles with a mean size of around 0.7 µm by using a SDR in which the uniform distribution of supersaturation arisen from high mixing efficiency was essential for producing particles with a narrow size distribution. Note that Cafiero et al.27 have only examined the effects of disk rotation speed and the supersaturation on the crystal size of BaSO4. The main objectives of the present investigation were (1) to synthesis barium sulfate nanoparticles in an SDR and (2) to

10.1021/ie801799v CCC: $40.75  2009 American Chemical Society Published on Web 07/22/2009

Ind. Eng. Chem. Res., Vol. 48, No. 16, 2009

examine the effects of various operating and design parameters, including the disk rotation speed, initial supersaturation, free ion ratio, and the disk diameter on the particle size distribution of barium sulfate.

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The mixing time, tm, of the reactant solutions on the SDR could be evaluated on the basis of the hydrodynamic properties of the aqueous solution and of the specific power, , given by32 tm ) 2(νL /ε) sin-1(0.05 ln Sc)

(4)

2. Theory Supersaturation is the thermodynamic driving force for the nucleation and growth phenomena. Because of the relatively high ionic strength in supersaturated solutions (up to 0.4 mol/L in the present work), ion activities instead of ion concentrations are used for the calculation of the supersaturation. To evaluate the activity-based supersaturation, the method described by Vicum et al.28 is used. In the present work, the precipitation of barium sulfate according to the following reaction is studied: BaCl2 · 2H2O + Na2SO4 a BaSO4 V +2NaCl + 2H2O (1) The activity-based supersaturation, Sa, is normally defined by Sa )



aBa2+aSO42Ksp



) γ(

CBa2+CSO42Ksp

Sc ) νL /D

(3)

where Sa is the supersaturation ratio defined earlier. Note that the concentration of barium sulfate used for the experiments in the present work is higher than that used by Carosso and Pelizzetti31 who proposed eq 3. Nevertheless, it has been supposed that eq 3 is valid for the present experimental data. Moreover, Cafiero et al.27 used the same equation for the calculation of the induction time. Using eq 3, the corresponding induction times for all the experiments could be calculated.

(5)

where D denotes the molecular diffusion coefficient. In addition, the specific power, , can be evaluated by27 )

( )

1 [(r2N2 + u2)o - (r2N2 + u2)i] 2tres

(6)

where tres, r, N, and u are, respectively, the residence time of the liquid solution on the disk, radial distance from the center of the disk, angular velocity of the disk, and the average velocity of the liquid solution on the disk; the subscripts “o” and “i” represent the outer and inner radius of the disk, respectively. Moreover, the average velocity of the liquid solution on the disk is given by27

(2)

with the value of the solubility product Ksp ) 9.82 × 10-11 mol2/L2 at 25 °C taken from Kucher et al.29 The mean activity coefficient γ( can be calculated as a function of ionic strength by using the semiempirical method proposed by Bromley30 as an advanced and multicomponent version of the DebeyHu¨ckel limiting law, which is valid for ionic strengths up to 6 mol/L. To check whether the operating conditions used in the present investigation are such that the homogeneous nucleation predominates or not, the micromixing for all experimental runs was examined. Micromixing is defined as a very effective mixing of the two reagents at the molecular level, which can be slow or fast. The homogeneous nucleation predominates when the mixing time, tm, is shorter than the induction time, tind. The latter is defined as the overall time required for the reaction, nucleation, and outgrowth of the generated crystals to the detectable size. For the tm < tind, the overall rate of the reactive crystallization process is not affected by the mixing intensity. Note that for the reactive crystallization of the slightly soluble salts, the reaction step is a very fast process and the time that elapses between the reaction and nucleation is negligible. Thus, the tind can only be attributed to the nucleation and outgrowth processes. Because there were some difficulties to detect the appearance of the first precipitated particles in the liquid film flowing over the surface of the disk, therefore, to estimate the tind for the precipitation of barium sulfate on the SDR, the following expression can be used.31 log(tind) ) 15.5 log-2 Sa - 4.2

where νL is the kinematic viscosity of the solvent. Sc is the Schmidt number given by

u)

[ ] FLQL2N2

1/3

(7)

12π2µLr

where FL, QL, µL, and r are the liquid density, volumetric flow rate of the liquid solution on the disk, liquid viscosity, and the radial distance from the center of the disk, respectively. Furthermore, the residence time of the liquid solution on the disk can be evaluated by27

[

tres ) 3/4(12π2)1/3

µL(ro2 - ri2) FLN2QL2

]

1/3

(8)

3. Experimental Section 3.1. Chemicals. All chemicals used in the present study were of analytical grade. Stock solutions were prepared by dissolving barium chloride dehydrate (Merck product, 1.01719.1000) and sodium sulfate anhydrous (Merck product, 1.06645.2500) in deionized and distilled water with conductivity < 1 µS/cm. 3.2. Methods of Analysis. Morphology and the mean particle size of BaSO4 particles were analyzed by scanning electron microscopy (SEM; Leo, 1455 VP). The particle size distribution was determined using Clemex Image Analysis. The SEM micrographs of barium sulfate produced were analyzed by this software. Three micrographs were taken for each data point and in each micrograph, about 50 particles were measured and the particle size distribution was determined by the Clemex analyzer. Powder samples obtained were also analyzed with an X-ray diffractometer (XRD; Philips, PW3710) using Cu KR (λ ) 1.54°A) radiation to determine their crystal structures. 3.3. Experimental Apparatus. A schematic diagram of the experimental apparatus used in the present study is shown in Figure 1, which consisted of a liquid feeding system, a spinning disk reactor, and a product collection vessel. The liquid feeding system contains two feed tanks (1 and 2), from which liquid reactants are pumped into the reactor chamber through rotameters (4) and liquid distributors (5). The two liquid distributors, which are straight tubes 10 cm in length with a 3 mm hole at the end, are set up parallel to each other and 5 mm apart, and perpendicular to the spinning disk (7) at a distance of 10 mm.

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Ind. Eng. Chem. Res., Vol. 48, No. 16, 2009 Table 1. The Operating Conditions spinning disk speed (N) temperature within the reactor barium chloride to sodium sulfate flow ratio flow rate of liquid solutions disk diameter free ion ratio (R ) Ba2+/SO4-2) supersaturation (Sa) kinematic viscosity (νL)27 diffusion coefficient (D)27

Figure 1. Schematic diagram of the spinning disk reactor: (1,2) storage tanks; (3) feed pumps; (4) rotameters; (5) feed distributors; (6) reaction chamber; (7) spinning disk; (8) cooling water jacket; (9) variable-speed electric motor; (10) product vessels.

The main part of spinning disk reactor (6) is a stainless steel disk (7) with 15 and 20 cm in diameter, driven by a variablespeed electric motor (9). The electric motor was connected to the bottom of the disk via a central shaft. The precipitated slurry solution was collected in product collection vessels (10) as shown in Figure 1. The temperature of the slurry solution exiting from the spinning disk was kept constant by cooling water introduced into the jacket (8). 3.4. Experimental Procedures. In each experimental run the desired amount of analytical grade chemicals was dissolved in dionized water to prepare feedstocks with given concentrations. The reactant solutions were fed over the surface of the disk through open air. The solution flow rates were regulated using the rotameters. The liquid was accelerated due to centrifugal force, causing it to spread over the disk surface and forming a thin liquid film where the BaSO4 precipitation reaction took place. The slurry solution containing BaSO4 particles left the disk and collected in the product vessels, where the product was diluted with deionized distilled water. Experiments were carried out at 25 °C and the reactant solution flow rates were set to be 1.33 × 10-6 m3/s. In each experimental run, 2 mL samples of the slurry solution were taken at fixed time intervals from the disk housing. After the withdrawal of samples, they were quickly introduced into deionized distilled water. This procedure avoids the agglomeration and settling of the particles, and the dilution of samples reduces the growth rate of crystals. Selected samples were then analyzed by SEM, and for SEM observations, the suspensions were diluted and filtered with a 0.2 µm pore filter paper or for even smaller particles, a 0.1 µm pore filter paper was used. X-ray diffraction analysis of the solid products was performed. The suspension was filtered on a blue Whatman filter paper using a vacuum pump and dried in an oven at 100 °C for 1 day. The dried precipitate was then bombarded by Cu KR rays and was confirmed to be anhydrous BaSO4. For each data point, the experimental run was repeated at least two times, and thus each data point was determined on the basis of the mean value of at least two measurements with a standard deviation of 2-4%.

500-1500 rpm 25 °C 1:1 0.080 dm3 min-1 0.15 and 0.20 m 0.1 - 5 400-2000 1.022 × 10-6 m2 s-1 9.42 × 10-9 m2 s-1

speed, supersaturation, disk diameter, and the free ion ratio. In what follows the influences of these important operating and design parameters on the particle size distribution of BaSO4 are presented and discussed. 4.1. Effect of Supersaturation on the BaSO4 Particle Size. To investigate the effect of supersaturation on the BaSO4 particle size, other operating variables were fixed such that the flow rates of BaCl2 and Na2SO4 solutions were set at 0.080 L/min (1.33 × 10-6 m3/s), the disk rotation speed was set at 1000 rpm, the free ion ratio (i.e., R ) [Ba2+]/[SO42-]) was kept at R ) 5, and the disk diameter was 15 cm. The number mean particle sizes obtained are presented in Figure 2 while the initial supersaturation, Sa, was varied from 400 to 2000. As may be observed from this figure, despite increasing supersaturation from 400 to 800, the final particle size remains nearly constant. Further increase in the supersaturation leads to a decrease in the mean particle size. In the range of Sa e 800 heterogeneous nucleation seems to be the dominant mechanism, which is indicated by the constant mean particle size. This is because that with increasing supersaturation the heterogeneous nucleation rate increases and indeed more heterogeneous nuclei are activated leading to more particles being formed. On the other hand, with increasing supersaturation a larger amount of mass is brought into the system and deposited upon this increased number of nuclei. In addition, agglomeration is usually significant at low mixing intensity. The result of these contrary effects is a nearly constant final particle size, which is independent of supersaturation. If the initial supersaturation is further increased (i.e., Sa > 800), the critical supersaturation at which homogeneous nucleation becomes important is crossed. This results in a sudden strong increase in the nucleation rate and, thus, in the number of particles being formed, leading to continuously smaller particle sizes with increasing supersaturation.33 The typical SEM micrographs of these experiments are shown in

4. Results and Discussion The operating and design conditions used in the present investigation for the determination of particle size distribution of BaSO4 are summarized in Table 1. In the present investigation, the effects of important design and operating parameters on the particle size distribution of BaSO4 produced in the SDR were investigated. These parameters were the disk rotation

Figure 2. Effect of supersaturation on the number mean size of BaSO4 particles. Operating conditions: QL ) 0.080 L/min, N ) 1000 rpm, R ) 5, and disk diameter ) 15 cm.

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Figure 3. SEM micrographs of BaSO4 particles produced at different supersaturations. Operating conditions: QL ) 0.080 L/min, N ) 1000 rpm, and R ) 5. (a) Sa ) 400, (b) Sa ) 600, (c) Sa ) 800, (d) Sa ) 1000, (e) Sa ) 1500, and (f) Sa ) 2000.

Figure 3a-f. As shown in these panels, the BaSO4 particles produced are very small, and few aggregates are observed. 4.2. Effect of Disk Rotation Speed on the BaSO4 Particle Size. The effect of the rotation speed of the spinning disk on the BaSO4 particle size was examined. In these experiments the other operating variables were kept constant such that the solution flow rates were set at 0.080 L/min (1.33 × 10-6 m3/s), the supersaturation was 2000, the free ion ratio was kept at R ) 5, and the disk diameter was 15 cm. The results of these experiments are shown in Figure 4. This figure shows that when the rotation speed increases from 500 to 1000 rpm, the number mean size of the BaSO4 particle decreases from 5.62 µm to 116 nm. This is because that increasing the disk rotation speed improves the mixing efficiency, which promotes micromixing. Thus, a uniform and high supersaturation is achieved and the smaller crystals with less aggregation are produced. Moreover, the effects of disk rotation speed on the mixing time, tm, are shown in Figure 5. Equation 3 shows that at a constant

supersaturation the induction time, tind, is constant, whereas with increasing the disk rotation speed the mixing time decreases. As shown in Figure 5, in the range of N g 900, the mixing time is shorter than the induction time, hence the homogeneous nucleation seems to be the dominant mechanism. Cafiero et al.27 have also investigated the effect of disk rotation speed on the BaSO4 particle size. They have reported that when the disk rotation speed increases from 200 to 1000 rpm, the average crystal size of BaSO4 evaluated by transmission electron micrographs (TEM) decreases from 3.0 to 0.7 µm. 4.3. Effect of Free Ion Ratio on the BaSO4 Particle Size. In these experiments, the supersaturation and the disk rotation speed were kept constant and the influence of the free ion ratio on the BaSO4 particle size was investigated. In these experiments, the other operating variables were fixed such that the flow rates of the reactant solutions and supersaturation were set at 0.080 L/min (1.33 × 10-6 m3/s) and 2000, respectively. In addition, the spinning disk speed and the disk diameter were

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Figure 4. Effect of disk rotation speed on the number mean size of BaSO4 particles. Operating conditions: QL ) 0.080 L/min, Sa ) 2000, R ) 5, and disk diameter ) 15 and 20 cm.

Figure 5. Effect of disk rotation speed on the mixing and induction times. Operating conditions: QL ) 0.080 L/min, Sa ) 2000, R ) 5, and disk diameter ) 15 and 20 cm.

Figure 7. SEM micrographs of BaSO4 particles produced at different free ion ratios. Operating conditions: QL ) 0.080 L/min, N ) 1000 rpm, Sa ) 2000, and disk diameter ) 15 cm. (a) R ) 0.1, (b) R ) 1, and (c) R ) 5.

Figure 6. Effect of free ion ratio on the number mean size of BaSO4 particles. Operating conditions: QL ) 0.080 L/min, Sa ) 2000, N ) 1000 rpm, and disk diameter ) 15 cm.

1000 rpm and 15 cm, respectively. The results of these experiments are presented in Figure 6. As may be observed from this figure, the largest particle diameter was found to be around 2.46 µm at R ) 1, whereas for the free ion ratios larger than 1 (i.e., R > 1), the mean particle size of BaSO4 decreases as

expected. As may be noticed from this figure, the mean particle size reaches ∼133 nm at R ) 5. In the range of R < 1, the mean particle size slowly decreases such that at R ) 0.1, the number mean size of BaSO4 particles is still larger than that for R > 1. The SEM micrographs of these experiments are shown in Figure 7a-c. Figures 6 and 7 show that particle aggregation can also be affected by the initial free ion ratio (R * 1). This behavior can be explained by the fact that the specific adsorption of barium ions as well as sulfate ions on the barium sulfate particles creates charges on the surface of the particles, resulting in higher repulsive particle-particle interactions and, hence, aggregation can be avoided. As shown in Figure 6, the particle size around 133 nm was produced at R ) 5. By decreasing the free ion ratio and reaching a value of R ) 1, the particle size varies almost 1 order of magnitude. 4.4. Effect of Disk Diameter on BaSO4 Particle Size. As far as we searched, the effect of disk diameter on the particle size

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Figure 8. XRD analysis of BaSO4 nanoparticles produced by the SDR. Operating conditions: QL ) 0.080 L/min, N ) 1000 rpm, Sa ) 2000, R ) 5, and disk diameter ) 15 cm.

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Figure 10. Cumulative particle size distribution of BaSO4 particles on the number basis. Operating conditions: QL ) 0.080 L/min, N ) 1500 rpm, Sa ) 2000, R ) 5, and disk diameter ) 20 cm.

shown in Figure 9 indicates that the morphology of BaSO4 is spherical. The number mean particle size evaluated using SEM micrographs was ∼38 nm. The cumulative size distribution is also shown in Figure 10. As can be observed from this figure, the particle size of the BaSO4 produced in the SDR was between 20 and 130 nm, and that the percentage of BaSO4 particles smaller than 100 nm was greater than 96%, which meets the requirement of the nanoscale crystal. 5. Conclusions

Figure 9. SEM micrograph of BaSO4 nanoparticles produced by the SDR.

of BaSO4 has not been investigated. In the present work, two stainless steel disks with different diameters (i.e., 15 and 20 cm) were used to examine the effect of this parameter on the particle size of BaSO4. A number of experiments were carried out with the disk diameters of 15 and 20 cm under the same operating conditions. In these experiments, the operating variables were kept constant such that the flow rates of reactant solutions and the supersaturation were set at 0.080 L/min (1.33 × 10-6 m3/s) and 2000, respectively, whereas the free ion ratio was R ) 5. To examine the effect of disk diameter at different disk rotation speeds, two series of results are shown in Figure 4. As shown in Figure 4, the number mean sizes of BaSO4 particles produced using the larger disk (i.e., 20 cm) are smaller than those produced using the disk diameter of 15 cm. Note that this may be attributed to the more rigorous agitation, which breaks down the agglomerate beyond 15 cm. The effect of disk rotation speed on the mixing time of 20-cm disk diameter is shown in Figure 5. As shown in this figure, in the range of N g 750, the mixing time is shorter than the induction time and, hence, the homogeneous nucleation seems to be the dominant mechanism. Therefore, crystals with smaller particle size are produced. Furthermore, the effect of disk diameter on the particle size produced at low disk speeds is more significant. 4.5. Analysis of the Produced BaSO4 Nanoparticles. The produced BaSO4 with the smallest size was analyzed by XRD and SEM. Figure 8 shows the results of XRD analysis, which was confirmed to be barium sulfate. The SEM micrograph

In this study, barium sulfate nanoparticles were successfully synthesized using a spinning disk reactor. The produced barium sulfate particles were ∼38 nm in diameter. The influences of various operating and design variables on the particle size of BaSO4 were carefully investigated. These parameters include the initial supersaturation, rotation speed of the spinning disk, free ion ratio of Ba2+ to SO42-, and the disk diameter. It was found that up to the supersaturation of Sa ) 800, the heterogeneous nucleation seems to be the dominant nucleation mechanism and, the obtained number mean size was around 3.3 µm, with no dependence on the supersaturation. Moreover, further increase of the supersaturation leads to the homogeneous nucleation, which results in continuously smaller particle size up to far below 133 nm. The critical supersaturation where the change in nucleation mechanism has a significant influence on the particles size was found to be Sa ) 800. By changing the free ion ratio larger than R ) 1 (i.e., barium ion is excess) the aggregation of produced particles in all size ranges can be influenced. Through the adsorption of the potential determining barium ions, repulsive electrostatic particle-particle interactions are created on the particle surface and aggregation can be avoided. This study also shows that the spinning disk reactor is a powerful tool for producing nanoparticles. Acknowledgment The authors gratefully acknowledge Sharif University of Technology for providing financial support. Nomenclature a ) activity, mol m-3 C ) concentration, mol m-3 D ) diffusion coefficient, m2 s-1 I ) ionic strength, mol m-3

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Ksp ) solubility product, mol2 m-6 N ) disk rotation speed, s-1 QL ) volumetric flow rate of BaCl2 and Na2SO4 solutions, m3 s-1 r ) radial distance from the center of the disk, m R ) initial free ion ratio Sa ) activity-based supersaturation Sc ) Schmidt number tind ) induction time, s tm ) mixing time, s tres ) residence time, s u ) average velocity of the liquid solution on the disk, m s-1 Greek Symbols γ( ) mean activity coefficient ε ) specific dispersed power, W kg-1 µL ) viscosity of the solvent, kg m-1 s-1 νL ) kinematic viscosity, m2 s-1 FL ) solution density, kg m-3 Subscripts/Superscripts Ba2+) barium ion i ) inner side of the disk o ) outer side of the disk SO42- ) sulfate ion

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ReceiVed for reView November 24, 2008 ReVised manuscript receiVed July 7, 2009 Accepted July 8, 2009 IE801799V