Preparation of Barium Sulfate Nanoparticles in an Interdigital

Article pubs.acs.org/IECR

Preparation of Barium Sulfate Nanoparticles in an Interdigital Channel Configuration Micromixer SIMM-V2 Hui Wu,† Chongqing Wang,† Changfeng Zeng,‡ and Lixiong Zhang*,† †

State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry and Chemical Engineering and ‡College of Mechanic and Power Engineering, Nanjing University of Technology, Nanjing 210009, PR China S Supporting Information *

ABSTRACT: BaSO4 nanoparticles were synthesized by precipitation of Na2SO4 and BaCl2 at their concentrations close to their saturation concentrations in a commercially available micromixer, SIMM-V2. The particle size of BaSO4 was dependent on the flow rate at the saturation concentrations, exhibiting a Z-type change with increasing the flow rate. The average particle size of BaSO4 particles could be adjusted by decreasing the Na2SO4 concentration. Decreasing the molar ratio R to less than 1 resulted in agglomerated round BaSO4 particles, while dispersible rod-like ones were produced when R = 1.2−1.8. The wastewater separated from each synthesis solution could be reused to prepare the BaCl2 solution, resulting in a slight increase in the mean particle size and broadened particle size distribution after eight cycles. The optimized preparation process could produce 2 kg/h of BaSO4 nanoparticles with a mean particle size of 28 nm with a narrow particle size distribution. Kucher et al.13 reported that the mean diameter of BaSO4 decreased first, and then remained constant with increasing the flow rate; whereas McCarthy et al.12 found that the mean diameter of BaSO4 slightly decreased or increased with increasing the flow rate at high or low reactant concentrations in Y-micromixers. On the other hand, Schwarzer et al.7,8 observed a decreasing trend, but Kockmann et al.10 revealed an upward parabola trend of the particle size with an increase in the flow rate in the T-micromixers. Furthermore, the use of a two phase system resulted in complexity of the product separation.17 To increase the production capacities of the microreactors, a microporous tube-in-tube microchannel reactor (MTMCR) exhibiting the combined effects of many T-type microchannels was developed for large scale production of BaSO4 nanoparticle at a total flow rate of up to 9 L/min,18 producing nanoparticles with an average size of 37 nm and PSD ranging from 15 to 60 nm. However, a low Na 2 SO 4 concentration of 0.1 mol/L was employed. Nevertheless, the microreactors used in the above research are mainly based on simple convective mixing principle.19,20 In the present work, we used a commercially available micromixer, SIMM-V2, for the preparation of BaSO4 nanoparticles. SIMM-V2 has a mixing element of 2 × 16 interdigital microchannels with a size of 45 μm (width) × 200 μm (depth) for each of the 32 channels and operates on a more complex mixing principle than the T- or Y-type micromixer. As stated by Hessel, et al.,21 interdigital feeds, termed in analogy with the respectively arranged electrode structures, provide multiple outlet ports with alternately arranged fluids (type A−B−A−B, etc.). The typical architecture of interdigital feeds comprises a large reservoir from which many equal substream channels

1. INTRODUCTION Barium sulfate, also known as barite, is a relatively simple inorganic material. Generally, it exhibits some superior properties such as high specific gravity, opaqueness to X-rays, inertness, and whiteness, and thus it is widely used as fillers and additives in polymers and paints, catalyst carriers and reflector material of optical devices.1,2 Nanosized BaSO4 particles, particularly those with narrow particle size distributions, also show interesting optical characteristics and unique flow behavior, and thus can be used in pigment, printing ink, medicine, and high quality paint.3 BaSO4 can be simply prepared by precipitation of Ba2+ with SO42‑ in batch or semibatch tank reactors.4,5 However, it is difficult to obtain uniform BaSO4 nanoparticles because of the difficulty in controling the mixing of the reactants in these reactors. To overcome this limitation, microreactors are employed in the preparation of BaSO4 nanoparticles because of their high mixing efficiency on the microscale.6 Generally, Tor Y-type micromixers are mainly used as the mixing units for the preparation of nanosized BaSO4, by either directly introducing the Ba2+ and SO42‑ solutions into the microreactors6−13 or forming a gas−liquid two phase segmented flow14 or water-in-oil microemulsions.15,16 Table 1 lists the channel sizes of the T- or Y-type microreactors, the preparation conditions, including the reactant concentrations, the initial Ba2+/SO42‑ molar ratio (R), the flow rates, and the mean particle sizes and particle size distributions (PSDs) of the resultant BaSO4 particles produced in these microreactors reported in literature. The channel sizes of the microreactors are mostly over 400 μm, and the Ba2+ and SO42‑ solution concentrations are not greater than 0.5 mol/L, although the usage of high reactant concentrations is practically more appreciable for increasing the production capacities of the microreactors. In addition, the conclusions drawn from these researches on the effects of the flow rate and the R on the particle size are not consistent. For example, Gradl et al.9 and © 2013 American Chemical Society

Received: Revised: Accepted: Published: 5313

October 16, 2012 March 16, 2013 March 21, 2013 March 21, 2013 dx.doi.org/10.1021/ie302825b | Ind. Eng. Chem. Res. 2013, 52, 5313−5320

Industrial & Engineering Chemistry Research

Article

Table 1. The Preparation Conditions and Particle Size of Barium Sulfate Produced in Microreactors Reported in Literature and This Work

a

type of microreactor

channel size (μm)

Ba2+/SO42− molar ratio

Ba2+ solution concentration (mol/L)

total flow rate (mL/min)

particle size (PSD) (nm)

ref

Ta Ta Tb Tc Y Y MTMCRd SIMM-V2

1000 600 × 300 830 × 1000 500 500 500 20 45 × 200

3 1.5 5 1 1 5 1 1.2

0.7 0.5 0.5 0.01 0.1 ≤0.5 0.35 1.8

180 24 6.5 0.8 30 600 9000 50

35(20−90) 91 (50−200) 300 (260−700) ∼15 200 (100−500) 40(14−105) 37 (15−60) 28 (10−60)

8 10 14 15 12 13 18 this work

Liquid−liquid direct reaction system. bTwo-phase system. cMicroemulsion. dMicroporous tube-in-tube microchannel reactor.

branch ending in nozzle-type outlets, for each fluid. The equidistribution to these channels is given by applying a pressure barrier; that is, the hydraulic diameter of the substream channel is much smaller than that of the reservoir. Consequently, the micromixer, with an inner volume of 8 μL, can create fast mixing and result in the mixing time of several to about 50 ms (obtained by dividing the inner volume of the mixer by the total liquid flow rate) at the total liquid flow rates up to 2.5 L/h.22,23 SIMM-V2 has been mainly used for production of various chemical products, including biodiesel,24 tetraethyl thiuram disulfide,25 imidazolium-based ionic liquids (ILs),26 nitrated alkyl-pyrazole, and brominated electron-rich heteroaromatic substrates,27 etc. However, its application in the preparation of inorganic particles is limited to Au nanoparticles.28,29 The purpose of this paper is to examine the effect of channel size, the mixing process of the micromixer, the flow rate, the reactant concentration, and the initial ratio R on the particle size and PSD of the resulting BaSO4 nanoparticles, and find a suitable operating condition at high reactant concentrations (close to the saturation concentration) and low initial ratio R. The contradictory conclusions on the effects of the flow rate and the R reported in literature are expected to be clarified. Furthermore, the effect of recycled water on BaSO4 preparation is also examined, which is very important for commercial production.

Figure 1. Schematic presentation of the process to produce nanoBaSO4 (a). SIMM-V2 device: individual parts of the SIMM-V2 device, details of the microchannels of the mixing element with interdigital channel configuration, and photograph of the interdigital flow passes slit to create multilamellae (b).31 Comparison diagram of deionized water with the collected sample solution produced at both 1.5 mol/L Na2SO4, and 1.8 mol/L BaCl2 flow rates of 25 mL/min before separation (c).

2. EXPERIMENTAL SECTION The preparation of nanosized BaSO4 was carried out by simply pumping the sodium sulfate (AR, the Chinese Chemical Reagent Co.) and barium chloride (AR, Xilong Chemical Co.) aqueous solutions into the SIMM-V2 micromixer (Institut für Mikrotechnik Mainz GmbH) using two high performance liquid chromatograph (HPLC) pumps (P3000, Beijing ChuangXinTongHeng Science and Technology Co., Ltd.) at room temperature, respectively (Figure 1). Precipitation of BaSO4 occurred immediately as soon as the Ba2+ contacted with SO42− in the micromixer. The outlet of the micromixer was connected with a Φ1/16 in. stainless steel capillary with a length of 1 cm. The produced suspension was collected with a beaker, and the BaSO4 nanoparticles were separated from the suspension every 15 min after precipitation by centrifuging in a centrifuge tube with a diameter of 15 mm (∼10 mL) at 10000 rpm for 15 min. The separated water after centrifuging was collected and used to prepare the BaCl2 aqueous solution to examine the effect of Na+ and Cl− on the size of BaSO4. The residual Ba2+ content was measured by using the method reported by Meeks et al.30 Briefly, about 10 drops of diethylamine were added to 20 mL of

the separated water to adjust the pH of the water to approximately 11.5, and methylthymol blue was added as an indicator. The separated water was then titrated with a standard EDTA solution. The color of the Ba2+-methylthymol blue complex at this pH is deep blue, while the uncomplexed form of the indicator gives the solution a light pink appearance. Therefore, when barium is titrated with EDTA, the color of the solution at the end-point changes from blue to pink to quite sharp. The residual Ba2+ content was finally calculated according to the consumed volume of standard EDTA solution. X-ray diffraction (XRD) measurements of powder samples were carried out on a BRUKER Axs D8 ADVANCE diffractometer equipped with Cu Kα at 40 kV and 40 mA. Fourier transform infrared (FTIR) spectra were obtained using a NICOLET infrared spectrophotometer, and the specimens were prepared using the KBr pellet technique. The morphology of BaSO4 precipitates was observed by transmission electron microscopy (TEM, JEM2010-UHR), and their sizes were measured from TEM pictures using the Adobe Photoshop software. For each sample, three pictures were taken at different regions. We measured about 100 particles on each picture, and the three values obtained on these three pictures were 5314

dx.doi.org/10.1021/ie302825b | Ind. Eng. Chem. Res. 2013, 52, 5313−5320


Article

(eq 1) and using the peaks with high intensities labeled in Figure 2a.33

normalized to obtain the mean PSD value that we used in this paper. The experimental error for each sample, calculated by normalizing the absolute value of the difference between the measured value and the mean value, is no more than ±2 nm. Since the prepared BaSO4 particles are in ellipsoid shapes with different length-diameter ratio or rod morphology, we calculated the particle size of nano-BaSO4 according to the length of the spheroid or rod particles. Nevertheless, no agglomeration of these stored particles was observed as the samples could be redispersed well in water.

D=

0.94λ β cos θ

(1)

where D, λ, β, and θ are the average diameter, the X-ray wavelength, half width of the peak (full width at halfmaximum), and the Bragg’s diffraction angle, respectively. Figure 2b displays the FTIR spectrum of BaSO4 particles. Absorption bands could be found at wave numbers of 1179− 1083 cm−1, assigned to S−O stretching of inorganic sulfate, and 1190−1070 cm−1 and the shoulder at 981 cm−1 assigned to the symmetrical vibration of SO42−. The peaks at 608 and 637 cm−1 are due to the out-of-plane bending vibration of SO42−.33,34 Figure 2 panels c and d show the TEM photos of the product at different magnifications, indicating spheroid morphology of the particles with a mean length of ca. 28 nm, which is in good agreement with the XRD analysis, and a mean width of 21 nm and the largest size of less than 55 nm. There exist small pores with a width of ca. 5 nm on these particles. The electron diffraction pattern (inset in Figure 2d) reveals the single crystal structure of the BaSO4. These results indicate that BaSO4 nanoparticles were synthesized in the SIMM-V2 micromixer. 3.1. Effect of the Flow Rate. The effect of the flow rate was examined at a Na2SO4 concentration of 1.5 mol/L, an R of 1.2 and the flow rate of each solution of 5, 15, 25, and 50 mL/ min. The corresponding Reynolds numbers (Re) are 98, 295, 491, and 983, respectively. Figure 3 shows TEM photos and the corresponding mean particle size and PSD of the resulting

3. RESULTS AND DISCUSSION We first conducted the experiment with a Na2SO4 concentration of 1.5 mol/L (the saturation concentrations of BaCl2 and Na2SO4 at 20 °C are 1.72 mol/L and 1.37 mol/L, respectively32) and an R of 1.2 at equal flow rates of the Ba2+ and SO42‑ solution of 25 mL/min. The collected liquid suspension showed a milk-white color. After dilution with water, it exhibited light blue color and an obvious Tyndall effect (Figure 1b), indicating the formation of nanoparticles. The system could be smoothly operated for the set time of 3 h, without clogging. The XRD and FTIR analyses were conducted to verify if pure BaSO4 was produced. Figure 2a shows the XRD

Figure 2. XRD pattern (a), FTIR spectrum (b), and TEM pictures (c,d) with different magnifications and electron diffraction pattern (inset) of the BaSO 4 nanoparticles prepared with a Na 2 SO 4 concentration of 1.5 mol/L at R = 1.2 and a total flow rate of 50 mL/min.

pattern of the separated powders. Diffraction peaks could be observed at 20.4°, 22.8°, 25.8°, 26.8°, 28.7°, 31.5°, 32.8°, and 42.9°, corresponding to (101), (111), (021), (210), (121), (211), (002), and (212) faces of orthorhombic barite crystals (JCPDS card no.: 24-1035), indicating the formation of pure BaSO4. The half width of the three main XRD peaks at 25.8°, 26.8°, and 28.7° were compared with those of the conventionally made microsized BaSO4 particles (Figure S1 in the Supporting Information). The peak broadening was clearly observed, suggesting that the particles produced in this work are nanosized. The mean particle size of the particles is calculated to be about 28 nm by the Debye−Scherrer equation

Figure 3. TEM pictures (a to c) of the BaSO4 nanoparticles prepared with an equal flow rate of 5 mL/min (a), 15 mL/min (b), 50 mL/min (c), and the mean particle size (d) and particle size distribution (e) at different Re numbers. 5315



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BaSO4 particles produced at different Re numbers. With increasing flow rate, the morphology of BaSO4 particles changes from mostly nanorods to a more uniform round or spheroid shape. Generally, the mean particle sizes are about 37 nm at flow rates of 5 and 15 mL/min and decrease to 28 nm at 25 and 50 mL/min, exhibiting a Z-type change with Re and slightly broader PSD at a lower Re. This Z-type tendency is different from those observed by other groups in the preparation of BaSO4 nanoparticles using T, Y, or MTMCR microreactors (Supporting Information, Figure S2),6−8,10,11,14,18 which can be attributed to the synergistic effect of both mixing intensity and supersaturation, as the increase in the supersaturation and enhanced mixing intensity favor the precipitation of nanosized particles with a narrower size distribution.35 3.2. Effect of the Reactant Concentration. The effect of the reactant concentration was examined at an R of 1.2 and both the BaCl2 and Na2SO4 solution flow rates of 25 mL/min, with change of the Na2SO4 concentration from 0.1 to 1.5 mol/ L. The TEM photos of the resulting products (Figure 4a−d)

particles. Figure 4 panels e and f show the mean particle size and PSDs of the products prepared at different SO42− concentrations. It demonstrates that the increase in the reactant concentration produces smaller particles. As the Na2SO4 concentration increases from 0.1 to 0.6 mol/L, the mean particle size decreases from 46 to 32 nm, and the PSD changes from 20−98 to 15−77 nm. A further increase in the SO42− concentration to 1.0 mol/L slightly decreases the particle size to 29 nm with a change of PSD to 14−58 nm. With further increasing the Na2SO4 concentration from 1.0 to 1.5 mol/L, the particle size does not change much (28 nm and PSD of 10−60 nm). This general trend toward smaller particles at higher reactant concentrations is similar to previous investigations performed by other groups with lower reactant concentrations in T, Y or MTMCR microreactors.11,13,18 This is due to the high supersaturation level at a high reactant concentration which promotes the nucleation rate rather than the growth rate,11 resulting in small particles. A further increase in the Na 2SO4 concentration from 1.0 to 1.5 mol/L cannot significantly reduce the particle size, because of the increasing role of the activity at higher supersaturations, as analyzed by Pieper et al.11 It should be pointed out that the mean particle size and the PSD of the BaSO4 nanoparticles produced in SIMM-V2 at a lower reactant concentration are smaller and narrower than those produced in T or Y microreactors at close reaction conditions, respectively. A further increase of the reactant concentrations results in even smaller particles with narrower PSD in SIMM-V2. This can be explained by the efficient mixing and the narrow channel size of SIMM-V2 and the high reactant concentration used in this work, as will be discussed in section 3.5. 3.3. Effect of the Initial Ratio R. The effect of R was examined at a Na2SO4 concentration of 1.0 mol/L, both the BaCl2 and Na2SO4 solution flow rates of 25 mL/min and the R ranging from 0.5 to 1.8 by changing the BaCl2 concentration from 0.2 to 1.8 mol/L. Figure 5 shows TEM photos of the resulting products and their mean particle sizes and PSDs. It is clear that the increase in R has a strong effect on the morphology. When R = 0.2 and 0.5, most of the particles are round or square with serious agglomeration. At R = 1, most of the particles are round or spheroid with some nanorods with also agglomeration. At R = 1.2−1.8, most of the round or spheroid particles change to nanorod-shaped particles with better dispersibility. On the other hand, the effect of the R on the particle size at R ≤ 1 is different from that at R > 1. When R ≤ 1, the average particle sizes calculated by neglecting the agglomeration are about 41−34 nm with bimodal PSDs at R = 0.2, 0.5, and 1. This may result from weak adsorption of excessive sulfate ions at R < 1 on these particles, producing less charged particles with low stability in water.13 When R > 1, the PSD of the BaSO4 particles is in the range of 14−78, 14−58, and 15−52 nm with the mean particle size of 29, 29, and 28 nm at R = 1.2, 1.5, and 1.8, respectively, suggesting no obvious influence of R on the particle sizes. This is different from the trend that the increase in R results in a decrease in the mean particle size in T- and Y-type micromixers, which is explained by enhancement of the positive charges on BaSO4 particle surface because of adsorption of Ba2+ with the increase in the R (i.e., increase in the Ba2+ concentration) and subsequent preference to produce smaller particles because of the repulsive particle−particle interaction.7,13,36 In our case, the reactant concentrations we used at R > 1 are over 1.0 mol/L, much higher than those used in T-type microreactors (up to 0.5 mol/

Figure 4. TEM pictures (a to d), and the influence of reactant concentration on the mean particle size (e) and particle size distributions (f) of the BaSO4 nanoparticles prepared at an R = 1.2, total flow rate of 50 mL/min, and Na2SO4 concentration of 0.1 mol/L (a), 0.3 mol/L (b), 0.6 mol/L (c), and 1.0 mol/L (d).

show that the reactant concentration, that is, the initial supersaturation, has a certain effect on the particle morphology. When the Na2SO4 concentration is 0.1 mol/L, the rodlike particles are predominantly formed. Increasing Na 2 SO 4 concentration to 0.3 mol/L results in an increase in the number of the spheroid and round particles and some rodlike ones. Further increase of Na2SO4 concentration to 0.6 mol/L and 1.5 mol/L leads to more uniform disperse spheroid 5316



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SIMM-V2. The resulting products are pure BaSO4, as determined by XRD and FTIR (Supporting Information, Figure S3). Figure 6 shows the TEM images and PSDs of

Figure 6. TEM pictures (a−c) and particle size distributions (d) of the BaSO4 nanoparticles prepared with 1.5 mol/L Na2SO4 and 1.8 mol/L BaCl2 in the first cycle (a), the third cycle (b), and the eighth cycle (c).

the resulting products after 1, 3, and 8 cycles. It is apparent that BaSO4 nanoparticles produced from the separated water exhibit also spheroid and rodlike morphology, similar to those prepared from the fresh solutions (Figure 2c,d). But more rodlike particles are observed than spheroid ones in the products produced from the separated water. Their mean particle sizes are about 38 nm with PSDs in the range of 19−90 nm, slightly larger and broader than those prepared from the fresh solutions under the same conditions. This can be attributed to the existence of Na+ and Cl− in the recycled solutions, which promotes higher crystal growth rates as revealed by He et al. using a BaCl2−Na2SO4−NaCl−H2O system.37 In our work, the Na+ and Cl− concentrations in the reused water are ∼1.8 and ∼1.5 mol/L in the first cycle, ∼2.9 and ∼2.6 mol/L in the third cycle, ∼3.3 and ∼3.0 mol/L in the eighth cycle, respectively. Afterward, they hardly change, thus allowing unlimited recycling of the separated water.

Figure 5. TEM pictures (a−f), the influence of R on the mean particle size (g) and particle size distributions (h) of the BaSO4 nanoparticles prepared with a Na2SO4 concentration of 1.0 mol/L, a total flow rate of 50 mL/min at R = 0.2 (a), R = 0.5 (b), 1.0 (c), 1.2 (d), 1.5 (e), 1.8 (f).

4. DISCUSSION The above results reveal that the particle sizes and the PSDs of the BaSO4 nanoparticles produced at both low and high reactant concentrations in SIMM-V2 are smaller and narrower than most of those produced in T or Y microreactors. This can be partially ascribed to the fast diffusion in SIMM-V2, as the average time for molecules to diffuse from a region of higher concentration to one of lower concentration over the distance x(t) follows eq 2,38 while that in a T- or Y-type micromixer can be expressed as eq 3.39

L),6−13 suggesting much higher supersaturation. Under such a circumstance, the effect of the reactant concentration on the particles may overwhelm that of the R. Hence, the increase in R does not have an obvious effect on the mean particle size and PSD. 3.4. Effect of Wastewater Recycling. Recycle of the separated water after each synthesis is quite important for commercial mass production because the wastewater has to be treated to meet the discharge standard. The separated water contains mainly unreacted Ba2+, Na+, and Cl−. The effect of the Na+ and Cl− on production and the particle size of BaSO4 had not been clear. We started the experiment using the BaCl2 solution concentration of 1.8 mol/L, R = 1.2 and a flow rate of each solution of 25 mL/min. In the next run, we prepared the BaCl2 solution with the same concentration by adding a certain amount of BaCl2 to the separated water. The experiments could be run up to eight recycles smoothly without clogging in

x2 2n2D

(2)

x 2 = 2Dt

(3)

t=

where D is the diffusion coefficient and n is the number of parallel fluid substreams. 5317



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SIMM-V2 is designed to form a multilamellaed flow40 by splitting the flow into 32 parallel channels with a 45 μm microchannel width. Thus, the t is decreased by a factor of n2 = 322 = 1024 times than that in a T- or Y-type micromixer. Faster mixing leads to a higher local supersaturation during the reaction, which in turn favors nucleation over growth, thus producing smaller particles with a narrow PSD. On the other hand, the narrow channel width of SIMM-V2 may also account for the small particle sizes of BaSO 4 nanoparticles, as the use of narrower channels increases the nucleation rate.12 This can be ascribed to the increased mixing intensity in narrow channels, which is described as the specific power input, ε, by Trippa et al. (eq 4).41 ε = 28.4ν

Q2 dh 6

(4)

where, Q is the fluid volumetric flow rate, m /s, ν is the kinematic viscosity of the BaSO4 suspension, m2/s, and dh is the channel hydraulic diameter, m. Hence, the ε value in SIMM-V2 is about 6 orders of magnitude higher than that in a Y-type micromixer with a channel width of 0.5 mm at the same flow rates, suggesting higher nucleation rates in SIMM-V2. McCarthy et al.12 also found that the nucleation rate of BaSO4 in the 0.5 mm channel size Y-type micromixer was 5 orders of magnitude higher than that in the one with 2.0 mm channel width. The mean particle size of BaSO4 particles they obtained in the 0.5 mm channel size Y-type micromixer was about 160 nm with a broad PSD of 100−500 nm. In our case, the largest BaSO4 particles produced in SIMM-V2 are smaller than 100 nm in size with a much narrower PSD. Furthermore, the reactant concentration may play an important role in determining the particle sizes of BaSO4 nanoparticles, as the nucleation rate is sensitive to supersaturation.12 The BaCl2 and Na2SO4 concentrations we used at R > 1 are over 1.0 mol/L, much higher than those used in T- or Y-type microreactors (up to 0.5 mol/L).6−13 The supersaturation S in the high reactant concentration system should be quite high, as can be calculated by eq 5.42 3

S=

aBa,free2+aSO4 ,free2− K sp

= γ±

Figure 7. Effect of supersaturation on the particle nucleation rate and nucleation/growth rate ratio in the SIMM-V2 and Y-type micromixer with 0.5 mm channel width.12

SIMM-V2 increase sharply as increases from 1576 to 8598, corresponding to the BaCl2 and Na2SO4 concentrations of 0.1 and 0.12 mol/L to 1.5 and 1.8 mol/L, respectively, indicating that the nucleation rate is much higher than the crystal growth rate. In the Y-type micromixer, the J value increases slightly with an increase in S from 707 to 4483, corresponding to the equal BaCl2 and Na2SO4 concentrations of 0.1 to 1.0 mol/L.12 However, the J/G value slightly decreases, suggesting that the effect of S on the crystal growth rate is stronger than that on the nucleation rate in the Y-type micromixer. As the increased supersaturation results in a sudden strong increase in the nucleation rate, the number of particles being formed is consequently increased, leading to smaller particle sizes.13 Therefore, the BaSO4 particles produced in SIMM-V2 under our experimental conditions should be quite small. On the other hand, the J values in SIMM-V2 are mostly smaller than those in the Y-type micromixer. This is possibly due to the much smaller Re number in SIMM-V2. In section 3.1, we simply attributed the effect of the flow rate on the particle size to the increased mixing intensity with increasing the flow rate. The above discussion indicates that the effect can, more specifically, be ascribed to the synergistic effect of both mixing intensity and supersaturation. It is known that increasing mixing intensity and high supersaturation result in a high nucleation rate and formation of increased number of nuclei.47 At low flow rates, the corresponding specific power inputs are not significantly different, as from eq 3. Under such a condition, the high supersaturation leads to formation of a large number of nuclei, which mainly turn to deposit and agglomerate at low flow rates.48 Consequently, large particles whose sizes do not significantly change with slightly increasing flow rate are produced. At medium flow rates, a large number of nuclei are also formed, which are quickly brought out with the main stream, avoiding deposition and agglomeration. Therefore, the particle size decreases with increasing flow rate at this stage. At high flow rates, both high supersaturation and quite intense mixing ensure that homogeneous nucleation is dominant.47 Under such a circumstance, the overall rate of the reaction crystallization process is not affected,41 thus producing steady number of nuclei. Therefore, the number of nuclei formed reach a plateau, which does not change any more with increasing flow rate, leading to formation of particles

c Ba,free2+cSO4 ,free2− K sp

(5)

where Ksp is the solubility product of barium and sulfate ions in the equilibrium state (∼1.1 × 10−10 kmol2/m6 at 25 °C).43 The mean activity coefficient γ ± can be calculated as a function of ionic strength by using the semiempirical method proposed by Bromley.44 The supersaturation determines the nucleation rate, J, and the crystal growth rate, G, following the expression defined in eq 6 and eq 7.45 ⎡ A ⎤ J = B exp⎢ − 2 ⎥ ⎣ ln S ⎦

(6)

G = kg(S − 1)2

(7)

where B = 1 × 10 number of crystals/(m /s) and A = 2686 for S > 1000, while B = 1.46 × 10−12 number of crystals /(m3/ s) and A = 67.3 for 1 < S < 1000; kg = 4 × 10−11 m/s.46 We correlated the S value with J and J/G in SIMM-V2 and compared them with those in a Y-type micromixer with 0.5 mm channel width (Figure 7). Obviously, both the J and J/G in 36

3

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without changing particle sizes. This synergistic effect can also explain why the particles in SIMM-V2 are smaller than those produced in the Y-type micromixer,12 although the J values in SIMM-V2 are mostly smaller than those in the Y-type micromixer. From the above discussion, we can conclude that the flow rate, the channel size and configuration, the high reactant concentration, and the Ba2+/SO42‑ molar ratio can exert a synergistic effect on the particle size and its distribution. However, the reactant concentration should have a stronger influence than the flow rate, evidenced by a limited change of particle size with a change in the flow rate at a high reactant concentration and relatively sharp change of the particle size with the reactant concentration at a fixed flow rate. The R value exhibits the least effect, as the mutual effect of the activity-based supersaturation and the mixing intensity is overwhelming, resulting in no change of the particle size at R > 1.2. Nevertheless, their synergistic effect can only lead to a decrease in the particle size to certain extent. To further decrease the particle size, micromixers with smaller channel size and more complex mixing principle than SIMM-V2 and suitable for high flow rates have to be applied, as the increase in the reactant concentration is limited to the saturation concentration.

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ASSOCIATED CONTENT

* Supporting Information S

Figures S1, S2, and Table 1 as mentioned in the text. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by Natural Science Key Project of the Jiangsu Higher Education Institutions (12KJA530002) and the Priority Academic Program Development of Jiangsu Higher Education Institutions.

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REFERENCES

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5. CONCLUSIONS BaSO4 nanoparticles were produced by precipitation of BaCl2 with Na2SO4 in an interdigital channel configuration SIMM-V2 micromixer at reactant concentrations close to their saturation concentrations, low Ba2+/SO42− molar ratio, and room temperature without any stabilizer. The particle size of BaSO4 was dependent on the flow rate at the saturation concentrations, exhibiting a constant value first, a decrease afterward, and a constant value again with increasing the flow rate. The mean particle size of the product was found to decrease with an increase in the concentrations of BaCl2 and Na2SO4 solutions, ascribed to higher degree of supersaturation at higher reactant concentrations. The BaSO4 particles were mostly round with agglomeration at R ≤ 1 and are of spheroid or nanorods morphology at R > 1 without agglomeration. The particle size was less dependent on the value of R at saturation concentrations, possibly resulting from the overwhelming effect of the activity based supersaturation and the mixing intensity. Thus, the particle size and its distribution are the result of the synergistic effect of the flow rate, the channel size and configuration, the high reactant concentration, and the Ba2+/ SO42‑ molar ratio. The wastewater separated from each synthesis could be reused to prepare the BaCl2 solution for preparation of BaSO4 nanoparticles. The Na+ and Cl− ions in the wastewater had little effect on the morphology and crystallinity of BaSO4 particles, but resulted in a slight increase in the particle size and the PSD. The prepared BaSO4 exhibits smaller particle sizes and narrower particle size distributions than most of the products prepared in T- or Y-type micromixers, ascribed to the narrow channel size and complex mixing principle of the SIMM-V2 micromixer as well as the high supersaturation resulting from the reactant concentration. Under suitable conditions, BaSO4 nanoparticles with a mean particle size of 28 nm and a PSD ranging from 15 to 52 nm could be produced at a production capacity of 2 kg/h. 5319



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Preparation of Barium Sulfate Nanoparticles in an Interdigital

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