Continuous synthesis of barium sulfate nanoparticles in a new high

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Continuous synthesis of barium sulfate nanoparticles in a new high-speed spinning disk reactor Javid Jahanshahi-Anboohi, and Asghar Molaei Dehkordi Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b02738 • Publication Date (Web): 09 Aug 2019 Downloaded from pubs.acs.org on August 13, 2019

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Industrial & Engineering Chemistry Research

Continuous synthesis of barium sulfate nanoparticles in a new high-speed spinning disk reactor Javid Jahanshahi-Anboohi and Asghar Molaei Dehkordi*

Department of Chemical and Petroleum Engineering, Sharif University of Technology, P.O. Box 11155–9465, Tehran, Iran

KEYWORDS: Nanoparticles; Barium sulfate; High-speed spinning disk reactor; Reactive precipitation; Supersaturation; Micromixing

* Corresponding author. Tel.: +98-21-6616 5412; Fax: +98-21-6602 2853; E-mail address: [email protected] (Asghar Molaei Dehkordi).

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ABSTRACT A new high-speed spinning disk reactor (HSSDR) was proposed and tested successfully. In this regard, barium sulfate (BaSO4 ) nanoparticles were synthesized using reactive crystallization processes. In this reactor, the rotational disk speed was varied from 5000 to 15000 rpm. The effects of various design and operating parameters such as the rotational disk speed, feed entrance radius, volumetric flow rate of feed solutions, supersaturation, and free ion ratio were investigated in detail. The mean particle size (MPS) and specifications of the synthesized barium sulfate were investigated using scanning electron microscopy (SEM) combined with energy-dispersive X-ray spectroscopy (EDXS) and powder X-ray diffraction (XRD). Nanoparticles with smaller MPS and narrower particle size distribution (PSD) were synthesized due to an increased micromixing obtained in the HSSDR. Moreover, BaSO4 nanoparticles with the MPS of 16.4 nm and PSD of 4 to 26 nm were synthesized successfully. This clearly shows the proposed reactor is a promising contacting device in process intensification.

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1. Introduction One of the chemical processes widely used in the chemical industry is reactive crystallization that can be performed particularly via contacting two liquid phases. The quality of the product in this process depends on the uniformity of mixing of reactants before the reaction takes place. Therefore, improvement of the reactors used in this process with the goal of achieving higher levels of micromixing quality and decreasing residence time in micro-scale has received much attention in recent decades. Nanoparticles have unique properties and many applications in various industries and, thus, extensive efforts have been paid to synthesize nanoparticles with desired specifications such as smaller mean particle size (MPS) and narrower particle size distribution (PSD).1 The synthesis of nanoparticles has attracted extensive attention because of desired specifications such as large surface-to-volume ratio and high energy of each particle. The most important applications of nanoparticles are in the pharmaceutical,2 corrosion,3 composites,4 ceramic and tile,5 environmental,6 electronics7 and food8 industries. Over the past years, numerous contacting devices such as microchannels,9 confined impinging jet reactors,10 rotating packed bed reactors,11 and spinning disk reactors (SDRs)1215

have been proposed and tested for the synthesis of nanoparticles using reactive

crystallization processes. In this regard, SDRs, consisting of one or two disks, and the introduction of one or more liquid jets on the disk surface, exert shear and centrifugal forces on the fluid, resulting in the formation of a thin and unstable fluid film on the disk surface. This provides unique feature and clearly shows the importance of using this type of contacting device because it also provides high heat-and mass-transfer rates and short mixing time in the system.1,16 Considering these unique features, various nanoparticles and chemicals

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have been synthesized using SDRs such as copper (Cu),17 diesel,13 silver (Ag),18 silver iodide (AgI),19 zinc oxide (ZnO),20 barium sulfate (BaSO4 ),15,21 titanium dioxide (TiO2 ),22 etc. Barium sulfate nanoparticles have found wide applications such as bone cement,23 biomedical devices,24 catalyst carriers,25 adsorbent materials26 and engineering plastics27 owing to their unique optical properties, excellent stability and inertness, and high specific gravity.28,29 In addition, the synthesis of BaSO4 nanoparticles has been used by numerous investigators to evaluate the capability of new contacting devices. In this regard, synthesis of BaSO4 nanoparticles via reactive precipitation processes has been widely used as a chemical process to evaluate the performance of various contacting devices including SDRs. For example, Cafiero et al.30 used an SDR with a diameter of 50 cm, rotational speed of 900 rpm, feed inlet radius of 50 mm, free ion ratio of 1, and a supersaturation of 2000 to synthesize BaSO4 nanoparticles. They reported the synthesis of BaSO4 nanoparticles with an MPS of ≈ 700 nm and a wide PSD of 500 to 1000 nm. In addition, they compared this device in terms of energy consumption with T-mixers, and reported a 1000-fold decrease in energy consumption. They also claimed that by using rotating disks, the mixing time could be reduced to be less than the induction time of BaSO4 synthesis. Molaei Dehkordi and Vafaeimanesh21 used an SDR with a diameter of 20 cm, rotational speed of 1500 rpm, feed inlet radius of 10 mm, free ion ratio of 5, and a supersaturation of 2000 to synthesis BaSO4 particles, and also reported the synthesis of BaSO4 nanoparticles with an MPS of 38 nm and a wide PSD within range of 20 to 130 nm. In addition, they examined the effects of various operating and design parameters such as the free ion ratio, supersaturation, rotational disk speed, and the disk diameter on the MPS. According to the obtained results and reported in their work, smaller particles could be synthesized by increasing the rotational disk speed. They stated that this can be attributed to the reduction of the mixing time compared to the

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induction time for the rotational speeds of more than 1000 rpm. Jacobsen and Hinrichsen31 used an SDR with a radius of 20 cm, rotational speed of 5000 rpm, feed inlet radius of 10 mm, free ion ratio of 2.8, and a supersaturation of 990 to synthesize BaSO4 nanoparticles. They reported the synthesis of BaSO4 nanoparticles with an average size of ≈ 27 nm. Bagheri Farahani et al.15 designed a new double spinning disks reactor (DSDR) consisting of two coaxial rotating disks placed horizontally in a cylindrical chamber. In this reactor, the reactive precipitation process was performed continuously by contacting two or more feeds in the space between the two rotating disks. They used two rotating disks with a diameter of 20 cm, with the same rotational speed of 4750 rpm in opposite direction (clockwise and counterclockwise) for the two disks. The radius of Na2 SO4 outflow from the lower disk was 70 mm and 16 nozzles were also provided on this disk and BaCl2 solution was introduced to the center of the disk and the distance between the two disks was set to 0.1 mm. The free ion ratio and supersaturation were set to 0.42 and 14000, respectively, to synthesize BaSO4 nanoparticles with an MPS of 23.4 nm and a narrow PSD from 10 to 45 nm. They reported that by using two disks, more uniform particles could be synthesized and also by using the quick dilution of the solution, agglomeration could be prevented. In addition, it was found that SDRs increases the local supersaturation in the reaction zone and the nucleation and the molecular-to-growth rates significantly increase with the local supersaturation; as well as the number of nuclei increases and the PSD of synthesized nanoparticles becomes narrower. The results of MPS reported by other investigators regarding the synthesis of barium sulfate nanoparticles in SDRs are summarized in Table 1. The main objective of the present work was to propose and test a new high-speed spinning disk reactor (HSSDR) in order to improve the MPS and narrow PSD of BaSO4 nanoparticles compared to those obtained in conventional SDRs. In this regard, to evaluate the performance

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capability of the proposed HSSDR, the influences of various design and operating parameters on the MPS and PSD of synthesized BaSO4 nanoparticles were carefully investigated.

2. Theory In the reactive or precipitation crystallization processes, supersaturation plays an important role as a thermodynamic driving force for primary and secondary nucleation and growth phenomena. When a nucleus is formed inside the solution, it has the ability to grow until it becomes a crystal. In addition to the nucleation and growth processes, other phenomena also affect MPS and PSD. Among the most important phenomena and operating conditions, agglomeration, breakage, attrition, and longer residence time play important roles. More generally, secondary phenomena are caused by the collision of the nuclei with each other.32,33 However, large values of supersaturation increase the nucleation rate and produce a large number of particles with a high surface-to-volume ratio. This increases the collision rate between the particles and, hence, increases the agglomeration phenomenon. Because the collision of nanoparticles does not have much energy; the breakage and attrition phenomena can be neglected as observed and reported by Heyer et al.34. They observed that, the nucleation and agglomeration are the main mechanisms to be considered, whereas crystal growth does not play a significant role.35 Therefore, in order to synthesize fine particles particularly nanoparticles, the agglomeration phenomenon should be also prevented in addition to providing conditions for homogeneous nucleation. In this work, this phenomenon can be prevented using rapid dilution of the liquid solution. The reactive crystallization process studied in the present work was carried out according to the following chemical reaction:

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BaCl2 ∙ 2H2 O + Na2 SO4 ⇄ BaSO4 ↓ +2NaCl + 2H2 O

(1)

In this process, by the reaction of Ba2+ ions within the solution of barium chloride and SO2− 4 ions within the sodium sulfate solution, soluble barium sulfate can be obtained. If the amount of these ions in the solution is larger than the solubility of this low-soluble salt (BaSO4 ), the excess amount of this salt appears in the form of white solid particles and the solution becomes cloudy. The value of solution supersaturation (𝑆BaSO4 ) based on activity is an indicator that shows the degree of solution deviation from the thermodynamic equilibrium conditions. The supersaturation of the barium sulfate solution can be defined as follows:36,37

𝑆BaSO4 = 𝛾 (

𝐶Ba2+ ∙ 𝐶SO2− 4 𝐾sp

0.5

(2)

)

where 𝑆BaSO4 is the solution supersaturation, C is the molar concentration of chemical species (M), 𝐾sp is the solubility product of barium sulfate solution with a value of 9.82 × 10−11 mol2 ⁄L2 at 25℃, 𝛾 is the mean activity coefficient, and Ba2+ and SO2− 4 are barium and sulfate ions, respectively.38 The supersaturation can be evaluated using the semi-empirical formulae provided by Bromley and Leroy,38 and Öncül et al.37 at a temperature of 25℃. In fact, these formulae are an advanced and variant of the Debye-Hookel law, which is acceptable for an amount of ion strength of up to 6 M. It should be added that, an ion strength of 4.7 M was used in the present work because the maximum concentrations of BaCl2 and Na2 SO4 that could be solved in the double distilled water were 1.4 M and 3.3 M, respectively. These concentrations provide us the maximum ionic strength of 4.7 M. Thus, the maximum concentrations of BaCl2 and Na2 SO4 used in the present work were 1.4 M and 3.3 M, respectively. The mean activity coefficient of the barium sulfate salt can be expressed as

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log 𝛾BaSO4 =

−𝐴γBaSO4 |𝑍Ba2+ ∙ 𝑍SO2− |√𝐼 4 1 + √𝐼

+

|𝑍Ba2+ ∙ 𝑍SO2− | 4 |𝑍Ba2+ | + |𝑍SO2− | 4

∙[

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𝐹SO2− 𝐹Ba2+ 4 + ] |𝑍Ba2+ | |𝑍SO2− |

(3)

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where I is the ionic strength of the solution and Z is the ion charge number (absolute charge value). 𝐴γBaSO4 is the Debye-Hookel constant, equals to 0.511 kg 0.5⁄mol0.5 . 𝑍Ba2+ and 𝑍SO2− 4 are, respectively, the ion charge number of cations and anions. 𝐹Ba2+ and 𝐹SO2− can be 4 evaluated using eqs (4), (5), and (1). 2 2 𝐹Ba2+ = (𝐵̇BaSO4 ∙ 𝑍̅BaSO4 ∙ 𝐶SO2− ) + (𝐵̇BaCl2 ∙ 𝑍̅BaCl2 ∙ 𝐶cl− ) 4

(4)

2 ̅ SO 2 ∙ 𝐶Na+ ) 𝐹SO2− = (𝐵̇BaSO4 ∙ 𝑍̅BaSO4 ∙ 𝐶Ba2+ ) + (𝐵̇Na2SO4 ∙ 𝑍Na 2 4 4

(5)

̅ The arithmetic mean of ion charge numbers (𝑍BaSO , 𝑍̅BaCl2 , 𝑍̅Na2 SO4 ) and the variables 4 𝐵̇BaSO4 , 𝐵̇BaCl2 , and 𝐵̇Na2 SO4 can be evaluated as follows:38 |𝑍Ba2+ | + |𝑍SO2− | 4

𝑍̅BaSO4 = 𝑍̅BaCl2 =

|𝑍Ba2+ | + |𝑍cl− | 2

𝑍̅Na2 SO4 = 𝐵̇BaSO4 =

𝐵̇BaCl2 =

(6)

2

(7)

|𝑍Na2+ | + |𝑍SO2− | 4

(8)

2

(0.06 + 0.6𝐵BaSO4 ) ∙ |𝑍Ba2+ ∙ 𝑍SO2− | 4 1.5 (1 + ∙ 𝐼) |𝑍Ba2+ ∙ 𝑍SO2− | 4

2

(0.06 + 0.6𝐵BaCl2 ) ∙ |𝑍Ba2+ ∙ 𝑍cl− |

𝐵̇Na2SO4 =

2 1.5 (1 + ∙ 𝐼) |𝑍Ba2+ ∙ 𝑍cl− |

+ 𝐵BaSO4

+ 𝐵BaCl2

(0.06 + 0.6𝐵Na2SO4 ) ∙ |𝑍Na+ ∙ 𝑍SO2− | 4 2

1.5 (1 + ∙ 𝐼) |𝑍Na+ ∙ 𝑍SO2− | 4

+ 𝐵Na2 SO4

8


(9)

(10)

(11)

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where the constants 𝐵BaSO4 , 𝐵BaCl2 , and 𝐵Na2 SO4 defined in eqs (9) to (11) can be determined as follows: 𝐵BaSO4 = 𝐵Ba2+ + 𝐵SO2− + (𝛿Ba2+ ∙ 𝛿SO2− ) 4 4

(12)

𝐵BaCl2 = 𝐵Ba2+ + 𝐵cl− + (𝛿Ba2+ ∙ 𝛿cl− )

(13)

𝐵Na2SO4 = 𝐵Na+ + 𝐵SO2− + (𝛿Na+ ∙ 𝛿SO2− ) 4 4

(14)

The individual ion values associated with eqs (12) to (14) are presented in Table 2 and, also, eq (15) can used to determine the ionic strength of the solution.

𝐼=

2 (𝐶Ba2+ ∙ 𝑍Ba2+ 2 ) + (𝐶SO2− ∙ 𝑍SO2− ) + (𝐶Na+ ∙ 𝑍Na+ 2 ) + (𝐶cl− ∙ 𝑍cl− 2 ) 4 4

2

(15)

Therefore, 𝛾BaSO4 value can be obtained using eq (3) and, then, by substituting its value in eq (2), 𝑆BaSO4 can be determined. Particles obtained from the process of reactive precipitation are divided into two groups of primary particles synthesized from the nucleation and growth stages, and secondary particles resulting from the agglomeration and breakage of the primary particles. To achieve a highquality product in the field of nanotechnology (i.e., small MPS and narrow PSD), it is highly desirable to prevent the formation of secondary particles as much as possible. Meanwhile, the rate of nucleation and growth phenomena that are called "primary phenomena" depends on the level of supersaturation of the solution; whereas the rate of the secondary phenomena depends on the collision frequency of the particles and the mechanical stress exerted on them. Therefore, increasing the value of supersaturation as the driving force for the reactive precipitation process, a region with a high mechanical stress during the reaction and reducing the collision frequency of particles by diluting the products quickly are the main strategies to improve the specifications of the product in the process of reactive precipitation. On the other

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hand, the reaction between the reactants in the process of reactive precipitation is almost instantaneous; however, the formation of a solid phase, which consists of nucleation, growth, aggregation, and breakage phenomena; despite the high rate, is not instantaneous. The specifications of the particles produced in this process are strongly affected by the relative rate of these phenomena in comparison with the mixing process. The induction time (𝑡i ) and the mixing time (𝑡m ) on the micro-scale are two main indices in order to compare the rate of these phenomena. Note that 𝑡i is defined as the sum of the time required for the reaction, nucleation, and particle growth to a detectable size; while 𝑡m at the micro-scale is defined as the time required for the effective mixing of the reactants at the molecular level. If the value of mixing time on a micro-scale is far smaller than t i , before significant improvement in the nucleation, the solution becomes completely uniform. Under these conditions, not only all the particles experience the same conditions in the solution but also the highest possible level of supersaturation dominates throughout the solution. These two points make it possible to synthesize particles of the smallest size and the narrowest PSD.39,40 The induction time of the precipitation of barium sulfate can be evaluated as follows:39,40

log(𝑡i ) =

15.5 − 4.2 (log(𝑆BaSO4 ))2

(16)

The value of 𝑡i is mainly affected by the nucleation phenomenon. Because of the greater influence of nucleation rate compared to the growth rate, an increase in the solution supersaturation results in the reduction of MPS and narrows PSD.15 Using eq (16) for the applied range of supersaturation (1137–4403) in the present work, the calculated induction time varies from 0.93 to 2.88 ms. The residence time (𝑡r ) of the liquid solution on the disk can be also evaluated by30

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1⁄ 3

3 12 𝜋 2 𝜈 𝑡r = ( 2 2 ) 4 𝜔 𝑄

4⁄ 3

(𝑟2

4⁄

(17)

− 𝑟1 3 )

where 𝜈, 𝜔, 𝑄, 𝑟2 , 𝑟1 , and 𝑡r , respectively, are the kinematic viscosity of the solution, the angular velocity of the disk, the volumetric flow rate of feed solutions, the disk radius, the entrance radius of the feed on the disk, and the residence time of the liquid solution on the disk. Considering this equation, as the disk rotates faster, 𝑡r becomes smaller. Moreover, Moore41 examined the mixing time (𝑡m ) using eq (18) for barium sulfate.30

𝑡m =

0.3785 𝜈 𝑡r ((𝑟2 𝜔2 )2 − (𝑟1 𝜔1 )2 + (𝑢22 − 𝑢12 ))

(18)

where the subscripts 1 and 2 are, respectively, the feed location on the disk surface and the outer edge of the disk where the solution is located, and 𝑢 is the average velocity of the liquid solution on the disk. It should be noted that whenever the mixing time is less than the induction time, the homogeneous nucleation dominates and, hence, for 𝑡m < 𝑡i , the total time of the reactive crystallization process is not affected by the mixing intensity, and only depends on the reaction, nucleation, and crystal growth phenomena.21 The specific energy dispersed (𝜖) on a rotating disk can be evaluated as follows:41

𝜖=

(𝑢2 + 𝑢1 ) 2 2 (𝜔 (𝑟2 − 𝑟12 ) + (𝑢22 − 𝑢12 )) ) 4(𝑟2 − 𝑟1

(19)

As can be noticed from this equation, the specific energy dissipated on a rotating disk increases with an increase in the rotational disk speed. This clearly shows that the proposed reactor needs more power input requirement than that of conventional SDRs.

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3. Experimental Section 3.1. HSSDR design and fabrication To investigate the effects of operating and design parameters such as the rotational disk speed, the feed entrance radius, the volumetric flow rate of feed solutions, supersaturation, and the free ion ratio on the size of the barium sulfate nanoparticles, a newly-designed SDR was fabricated. To fabricate this HSSDR, due to the high rotational speed of the disk (up to 15000 rpm), and in order to prevent possible accidents and increase the security and safety levels, the reactor was made of aluminum 2618-T6. A schematic diagram of the experimental setup used in the present work is shown in Figure 1. A rotating disk (11) made of aluminum was horizontally placed inside the reactor. The transparent fixed disk (12) made of polycarbonate were placed at a distance of 5 cm (15) far from the surface of the rotating disk. The precipitation process mostly takes place on the disk surface and in order to collect the liquid drops exiting from the disk surface, the upper fixed disk was designed and placed 5 cm far apart of the lower disk, and the holes on them were plugged by pins (14), as well as the upper cover was connected to the reactor body by an Allen screw (16). For better sealing and preventing accumulation, facilitating the complete exit of the product solution from the reaction chamber and reducing its residence time, the bottom section of the reactor was made of an polyethylene incomplete cone (10) as well as two outlet valves (1/2" nickel plated brass ball valve - full port 600 Psi (WOG), designed and manufactured by DuraChoice Company) (6) was used as the outlet port of the product solution (5) at the lowest points of the reaction chamber. The rotating disk was driven counter-clockwise by a variable speed three-phase electric motor (Forza Spindle Engine, designed and manufactured in Turkey, Model FS1-A16-1, with three-phase input, 220 V, Power 0.25 kW, 200 Hz, 15000 RPM) (9). A frequency inverter (LS Starvert iC5, Model SV004iC5-1F, 0.37kW 230V 1 ph to 3 ph AC Inverter

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Drive, C3 EMC, Speed Control Range: 0–400 Hz) (7 and 8) was used to convert single-phase electricity into three-phase electricity and as a disk speed controller. In this experimental rig, Na2 SO4 (2) and BaCl2 (1) solutions were introduced to the reaction chamber using two peristaltic pumps (Heidolph Pump Drives 5101) (3) from the feed tanks (1 and 2), which have been already calibrated. In addition, Na2 SO4 and BaCl2 solutions were introduced to the disk surface via two nozzles. To facilitate quick dilution and prevent agglomeration of synthesized particles, by using two peristaltic pumps (Heidolph Pump Drives 5106) (3), double distilled water from two water tanks (4) was introduced to the reactor through two nozzles (17). To prevent possible vibration, the entire HSSDR setup was fitted with 3 steel bases (18) on a steel plate (19) with 3 rubber bases (stabilizers) (20). Considering the design and fabrication described in this section, the cost of the HSSDR was lower than that of DSDR proposed and tested in our previous work and, also, the design of HSSDR is so simpler than that of DSDR.15 Note that considering the large distance between the two disks (5 cm far apart), the transparent disk does not play an important role in the overall processes and was designed to prevent from exiting the liquid drops and collecting them and to better view of the inside of the reactor. A Lutron Laser Tachometer was used to measure the rotational disk speed, that each input frequency in the frequency inverter corresponds a rotational speed. Figure 2 shows the rotational disk speed vs. the input frequency to the electric motor that could be correlated as follows: 𝜔 = −0.1304𝑓 2 + 106.57𝑓 − 4622.5

(20)

3.2. Chemicals In the present work, double-distilled water with a specific conductivity of 1.26 µS/cm was used for the preparation of the feed solutions, washing the laboratory equipments, and also

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the dilution of the solution exiting from the reactor. The chemicals used in this work were of analytical grade. The feed solutions were prepared by dissolving specified amounts of sodium sulfate (Na2 SO4 , MERCK product, 1.06649.1000) and barium chloride dihydrate (BaCl2 ∙ 2H2 O, MERCK product, 1.01717.1000) in double-distilled water. 3.3. Analysis The MPS and PSD of the synthesized barium sulfate for each experimental run were measured by scanning electron microscopy (SEM; MIRA3 FEG-SEM, Tescan Company, Czech Republic). The SEM images taken were analyzed using ImageJ software (NIH, USA). Four images were taken and analyzed for each sample. X-ray diffraction spectroscopy (XRD; STOE STADI, MP, Germany, anode material: Cu Kα, 𝜆=1.5405 °A) as a rapid analytical technique was used to identify the specifications of synthesized particles that confirmed to be barium sulfate. Energy-dispersive X-ray spectroscopy (EDXS) was also used. 3.4. Experimental procedure To carry out an experimental run, feed solutions of barium chloride and sodium sulfate were prepared according to the desired concentrations and stored in the feed tanks. The feed flow rates and the rotational disk speed were adjusted using the peristaltic pumps and frequency inverter, respectively. Before turning on the feed pumps, the peristaltic pump connected to the distilled water tank was turned on with a constant flow rate of 400 mL⁄min. This was conducted in order to dilute the product as fast as possible and to get further reduction of secondary phenomena. In the next step, the electric motors were turned on and after the speed of the disk reached the desired value, the feed pumps were turned on for 60 s. At the same time, the solution exiting from the disk surface was diluted inside the chamber and introduced to a large tank. During the experimental runs, the temperature of the solution

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exiting from the disk was measured and found to be almost constant (i.e., 25 to 27°C). Finally, a 100 mL sample was taken and sonicated for 10 to 20 min and after that one or two drops of the solution were taken by a dropper or pipette for SEM analysis. The SEM images of samples were taken and analyzed using the ImageJ software and finally the MPS and PSD of the synthesized BaSO4 particles were evaluated carefully.

4. Results and Discussion The design and operating conditions used in this work are presented in Table 3. In the present work, the effects of various operating and design parameters on the MPS and PSD of barium sulfate synthesized in HSSDR were investigated in detail. In each experimental run, only one parameter was varied and its effects on the MPS and PSD were investigated. These parameters include the rotational disk speed, feed entrance radius, the volumetric feed flow rate, supersaturation, and the free ion ratio. Moreover, XRD and EDXS analyses were performed to ensure that barium sulfate nanoparticles were synthesized. To make sure about the experimental result, most of the experimental runs were repeated twice to confirm the repeatability of the experimental results. On the basis of obtained experimental results, the standard deviation of the experimental results was smaller than 4%. In what follows, the obtained results are presented and discussed. For the better and more accurate comparison of MPS and PSD, the operating conditions of all the experimental runs performed are summarized in Table 4. 4.1. Analysis of the synthesized 𝐵𝑎𝑆𝑂4 nanoparticles XRD analysis was performed to ensure that BaSO4 nanoparticles were synthesized and to determine its crystalline structure. Figure 3 shows the XRD patterns of the obtained products (Experiment E-30). All the peaks can be readily indexed to a pure orthorhombic crystalline

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phase of BaSO4 , which are consistent with the corresponding data reported in the literature (Barite JCPDS-ICDD No. 00-001-1229, space group: Pmna, Space group number: 53).42 Moreover, as can be observed from Figure 3, the peak broadening clearly shows that the synthesized crystals are in the nanosize range. To evaluate MPS, seven peaks having high intensities were identified. The diffraction peaks at 20.40° (011), 22.84° (111), 25.88° (210), 28.78° (211), 31.59° (112), 32.90° (020), and 43.04° (122) are the characteristic peaks of orthorhombic BaSO4 crystals. In addition, the size of the BaSO4 crystallites can be evaluated from Scherrer eq (21)43 (In our system, 𝐾 = 0.89, 𝜆 = 0.15405 nm). The XRD patterns clearly show that the size of the BaSO4 crystallite is 10.8 nm.

𝐷=

𝐾𝜆 𝛽 𝑐𝑜𝑠 𝜃

(21)

To verify the XRD results, energy dispersive X-ray spectroscopy (EDXS) analysis was also performed to determine the composition of elements on the surface of BaSO4 . The results of the surface analysis are presented in Figure 4. As may be observed from this figure, synthesized BaSO4 consists of barium, sulfur, and oxygen elements. The percentage composition of surface elements is presented in Table 5. It can be observed that the trend is consistent with the findings of Bala et al.44 and Wang et al.45. 4.2. Effect of rotational disk speed The most important parameter studied in this work was the effect of rotational disk speed on MPS and PSD. In this regard, a number of experimental runs were carried out, which their results are summarized in Table 4 and analyzed. In these experiments, all the parameters except the rotational disk speed were kept constant such that; the volumetric feed flow rates

16


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were set to 150 mL/min, the concentration of each feed solution was 0.75 M, and the free ion ratio was R = 1. Using eq (2), the supersaturation value was evaluated to be 3551, the radius of the rotating disk was 10 cm and the entrance radius of both the feeds was 10 mm. The rotational disk speed was varied from 5000 to 15000 rpm. The results obtained from these experiments are shown in Figure 5. This figure shows that as the rotational disk speed rises from 5000 to 15000 rpm, the MPS of the synthesized barium sulfate decreases from 83.1 to 47.6 nm. It is quite clear that the variation of MPS of nanoparticles for the disk speeds larger than 11000 rpm is negligible, which is an indication of slight influence for high speeds, however, a small reduction in the MPS of synthesized nanoparticles is of importance. The reason for the reduction in the size of the synthesized particles is that increasing the rotational disk speed improves the micromixing efficiency. In addition, eq (16) shows that at a constant supersaturation, the induction time (𝑡i ) is constant, while the mixing time (𝑡m ) decreases with an increase in the rotational disk speed (eq (18)). Therefore, under the conditions of high local supersaturation and high levels of uniform concentration, in addition to the successful synthesis of smaller particles, a narrower PSD can be achieved. Figure 6 shows the SEM images of the synthesized barium sulfate for the disk speeds of 5000 and 15000 rpm. It is quite clear that with an increase in the disk speed, the smaller MPS and more uniform PSD can be obtained and the agglomeration of the synthesized particles is reduced as explained in Section 2. To evaluate the particle size using the method described in Section 3.3, an attempt was made to remove the agglomerated particles from the images and then measure the MPS and the PSD. The results of these experimental results are consistent with those reported by Molaei Dehkordi and Vafaeimanesh,21 Cafiero et al.,30 and Bagheri Farahani et al.15. It should be also added that in this work, with a much higher rotational disk speed than those reported in the above-mentioned works nanoscale particles obtained become smaller and more

17



uniform and as a result, the rotational disk speed has a great influence on the size of the synthesized particles. 4.3. Effect of feed entrance radius The second parameter examined and analyzed was the effect of the feed entrance radius on the MPS of synthesized barium sulfate. Note that on the top of the disk, there were inlet nozzles located at radii 2, 10, 30, 50, 70, and 90 mm, and in each radius given, there was a pair of symmetrical apertures connecting the nozzles of the feed. In this regard, twelve experimental runs were carried out, which their results are presented in Table 4. The rotational disk speed was set to 9000 and 15000 rpm, while the other parameters were identical to those mentioned in section 4.2 and the obtained results are presented in Figure 7. These experimental results indicate that the variation of the feed entrance radius of BaCl2 and Na2 SO4 solutions from 10 to 70 mm reduces the MPS of BaSO4 for both the rotational disk speeds. In this study, the rate of nucleation and growth is very high and the reaction between sulfate and barium ions takes place immediately as explained in Section 2. Therefore, the precipitation process of BaSO4 is complete shortly with the contact of both the feed solutions. The reason for the observed trend is as follows: as the entrance radius increases, due to an increase in the linear velocity of the flow field near the disk surface, the turbulence intensity of the flow field and, consequently, the rate of mixing at the microscale increase. Hence, if the feed solutions are introduced at larger radii, the solution on the disk will become more uniform with a faster rate consequently smaller particles can be synthesized with a narrower PSD. Note that the entrance radius of 2 mm represents the status of the impinging jets. In this case, the mixing intensity obtained from the collision of the jets is greater than that obtained from the disk rotation for small radii. Therefore, entering the feed solutions at near zero radius results in the synthesis of smaller particles compared to those synthesized for entering

18


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the feed near the disk center (about 10 mm radius). In addition, it can be observed from Figure 7 that with an increase in the rotational disk speed, the effects of the feed location can be enhanced for larger radii, while this increase in the rotational disk speed does not affect the size of the synthesized particles for the feed inlet at a radius of 2 mm. Moreover, with an increase in the rotational disk speed from 9000 to 15000 rpm the discrepancy between the MPS of synthesized nanoparticles increases significantly for larger feed entrance radii (i.e., re > 2 mm) while the influence of the rotational disk speed on the MPS in the case of re = 2 mm is negligible as shown in Figure 7. This can be attributed to the influence of the rotational disk speed at larger feed entrance radius. However, at a radius near the radius of the disk (feed entrance radius of 90 mm), the trend is completely changed and as shown in Figure 7, the particle size increases with a jump due to the very low mixing on the disk surface. Figure 8 shows the SEM images of Experimental runs E-11 and E-15 where Figure 8a shows the fully agglomerated particles obtained in E-11, whereas Figure 8b clearly demonstrates more uniform particles obtained in E-15. Therefore, because the greater energy required for the larger disks, an appropriate radius for the feed inlet can be found. The effects of the feed entrance radius were also studied by Jacobsen and Hinrichsen31 and Bagheri Farahani et al.15 that they reported the same trend. It was found that as the feed entrance radius increases beyond a specific value, the MPS of synthesized nanoparticles increases in addition to the results reported in the above-mentioned studies. 4.4. Effect of feed flow rate The third parameter studied in this work was the effect of the volumetric feed flow rate on the MPS of the synthesized barium sulfate particles. In this regard, three experiments were carried out with various volumetric feed flow rates while other parameters were kept

19



constant. In all the experiments conducted throughout this work, the feed ratio was 1:1, which in this case, the total volumetric feed flow rates were 150, 300, and 450 mL/min, while the other operating parameters were set to the best result obtained in Section 4.3 (i.e., in terms of MPS) corresponding to the test number E-15. The obtained experimental results show that the MPS varies slightly from 43.5 to 41.1 nm (see Table 4) that is negligible; however in the next sections, the total volumetric feed flow rate is 450 mL/min. Increasing the volumetric feed flow rate increases the radial velocity of the solution on the disk and as a result, increases the micromixing process. Under the operating conditions of these experiments, the radial velocity magnitude is much smaller than the tangential velocity at the feed entrance radius (𝑟e = 70 mm). Hence, the influence of increasing the volumetric feed flow rate on the net linear velocity is negligible and, the MPS does not change significantly. The obtained results of these experimental runs are consistent with those reported in the literature.15,31 4.5. Effect of supersaturation Note that the maximum concentrations of barium chloride and sodium sulfate solutions used in the present work were 1.4 M and 3.3 M, respectively. In this section, five experiments were performed that according to eq (2), the value of 𝑆BaSO4 varies with the concentrations of the of barium chloride and sodium sulfate solutions. Other parameters were to the same as those applied for the test No. E-18. Figure 9 shows the obtained results of these experimental runs. As can be observed from this figure, when the supersaturation varies from 3067 to 4403 or when the feed concentrations change from 0.5 to 1.4 mol/L, the MPS of BaSO4 synthesized decreases from 47.7 to 31.7 nm. Using eqs (2) to (15), the value of 𝑆BaSO4 for each concentration of feed solutions can be obtained. This decrease in the MPS can be attributed to the homogeneous nucleation wherein with an increases in the local supersaturation, the nucleation rate increase sharply, and this behavior is the dominant

20


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mechanism.46 The experimental results obtained in the present work and those reported in the literature15,47 are consistent. Moreover, the slope of Figure 9b is larger than that of Figure 9a; hence, it can be concluded that concentration has a greater influence on the MPS reduction than that of the supersaturation. In addition, by solving eqs (2) to (15), the results shown in Figure 10 can be obtained. As may be observed from this figure, when R is smaller or larger than 1.0, the value of supersaturation becomes smaller and at R=1, the largest supersaturation value can be obtained. For R>1 (see Figure 10a) with an increase in R value, the supersaturation decreases dramatically; however, no significant changes can be observed for R1, the slope is less than that for R1, and this has a negligible influence on the size of nanoparticles. When R is larger or smaller than one, the MPS of synthesized nanoparticles becomes smaller as presented in section 4.6. Therefore, in the last step, experiments with a maximum concentration of barium chloride and sodium sulfate were carried out with two values of R=5 and 0.2 such that there were three modes as follows: (1) The maximum concentration of barium chloride, (2) The maximum concentration of sodium sulfate, and (3) The maximum possible concentration of both the feed solutions as mentioned in section 4.5. The obtained results of these three experiments are summarized in Table 4 (Experiments E-28, E-29, and E-30). As can be observed from these results, the MPS and PSD of synthesized nanoparticles are, respectively, smaller and narrower in the third case (E-30). Figure 13 shows the SEM images taken for the results of Experiment E-30. It can be

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concluded that under high micromixing intensity conditions, the feed concentration and the free ion ratio have considerable influence on the MPS of synthesized barium sulfate.

5. Conclusion In this work, barium sulfate nanoparticles were successfully synthesized using a new HSSDR by improving the micromixing efficiency. The influences of various operating and design conditions on the MPS and PSD of BaSO4 nanoparticles were thoroughly investigated. These parameters include the rotational disk speed, feed entrance radius, volumetric feed flow rate, supersaturation, and the free ion ratio. Analysis of SEM images taken indicates that increasing the rotational disk speed up to 15000 rpm, increasing the feed entrance radius (the appropriate radius was 70 mm), increasing the feed concentrations or supersaturation (𝐶BaCl2 =1.4 M, 𝐶Na2 SO4 =3.3 M, and 𝑆BaSO4 =3362), and increasing free ion ratio except R=1 (the feed concentrations with a non-stoichiometric ratio, R≠1), result in the synthesis of BaSO4 nanoparticles with smaller MPS and narrower PSD. In addition, it was found that the influences of volumetric feed flow rate on the MPS and PSD of the synthesized particles are not remarkable. It was also found that the high speeds of the disk decrease the size of the nanoparticles. Moreover, the analysis conducted in this work confirms that by using the new HSSDR, a high level of micromixing efficiency that increases the local supersaturation on the disk can be achieved. Furthermore, the smallest BaSO4 nanoparticles with an MPS of 16.4 nm and a PSD of 4–26 nm were synthesized successfully. Finally, the results of this work show that the synthesis of nanoparticles using the new proposed HSSDR has the following advantages: (1) The synthesis of smaller particles compared to those obtained using conventional SDRs.

23



(2) The synthesis of nanoparticles with the desired MPS and narrower PSD. (3) The cost of manufacturing of HSSDR is lower than that of DSDR proposed in our previous work.15 (4) The design of HSSDR is so simpler than that of DSDR.

Acknowledgment The present authors would like to acknowledge the financial support provided by Sharif University of Technology (Tehran, Iran).

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Nomenclature 𝐴γBaSO4 = Debye-Hookel constant [kg 0.5⁄mol0.5 ] 𝐵 = individual ion values [kg/mol] 𝐵̇ = variables ion values [kg/mol] 𝐶 = concentration [mol/m3 ] 𝐶Ba2+ , 𝐶cl− = concentration of BaCl2 in feed solution [mol/m3 ] 𝐶Na+ , 𝐶SO2− = concentration of Na2 SO4 in feed solution [mol/m3 ] 4 𝐷 = diffusion coefficient [m2 /s] 𝐹 = function [−] 𝑓 = frequency [Hz] 𝐼 = ionic strength of solution [mol/kg] 𝐾sp = solubility product [mol2 /m6 ] 𝑄 = volumetric flow rate [m3 /s] 𝑅 = free ion ratio [−] 𝑟 = disk radius [𝑚] 𝑟e = feed entrance radius [m] 𝑆BaSO4 = activity-based supersaturation [−] 𝑡i = induction time [s] 𝑡m = mixing time [s] 𝑡r = residence time [s] 𝑢 = average velocity of liquid solution on the disk [m/s] 𝑍 = ion charge number of cation or anion [−] 𝑍̅ = arithmetic mean of ion charge number [−]

Greek symbols 𝛾BaSO4 = mean activity coefficient [−] 𝜃 = Bragg’s diffraction angle [degree] 𝜖 = specific dispersed power [W/kg]

25



𝜈 = kinematic viscosity [m2 /s] 𝜆 = X-ray wavelength [nm] 𝜔 = rotational disk speed [rad/s]

Subscripts/Superscripts 1 = inner side of the disk 2 = outer side of the disk Ba2+ = barium ion BaCl2 = barium chloride solution BaSO4 = barium sulfate solution Cl− = chloride ion Na+ = sodium ion NaCl = Sodium chloride solution Na2 SO4 = sodium sulfate solution SO2− 4 = sulfate ion

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(47) Kucher, M.; Babic, D.; Kind, M. Precipitation of barium sulfate: experimental investigation about the influence of supersaturation and free lattice ion ratio on particle formation. Chem. Eng. Process. 2006, 45, 900–907.

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Table 1. The appropriate operating conditions, design parameters, and specifications of the barium sulfate particles synthesized in SDRs Author

Cafiero et al.

Year

30

Molaei Dehkordi and Vafaeimanesh21 Jacobsen and Hinrichsen31 Bagheri Farahani et al.15

Rotational

Total

Initial

Feed

Free

Disk

MPS

speed

volumetric

supersaturation

inlet

ion

diameter

(nm)

(rpm)

feed flow rate

radius

ratio

(cm)

(mL/min)

(mm)

2002

1000

~160

2000

50

1

50

~700

2009

1500

160

2000

10

5

15

~38

2012

5000

360

990

10

2.8

20

27

2017

4750

400

14000

70

0.42

20

23

33



Table 2. Individual ion values in a 𝐁𝐚𝐒𝐎𝟒 solution at 25 °C 38 Ion

|𝑍ion |

𝐵ion (kg⁄mol)

𝛿ion

Ba2+

2

0.0022

+0.098

SO2− 4

2

0.0000

−0.400

Na+

1

0.0000

+0.028

1

0.0643

−0.067

Cl

−

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Table 3. Operating Conditions The internal and external temperature of the reactor

𝟐𝟓 − 𝟑𝟎 ℃

Barium chloride-to-sodium sulfate flow ratio

𝟏: 𝟏

Rotational disk speed (𝜔)

𝟓𝟎𝟎𝟎 − 𝟏𝟓𝟎𝟎𝟎 𝐫𝐩𝐦

Disk radius (𝑟)

𝟏𝟎 𝐜𝐦

Free ion ratio (𝑅 = [Ba2+ ]⁄[SO2− 4 ])

𝟎. 𝟐 − 𝟓

Supersaturation (𝑆BaSO4 )

𝟏𝟏𝟑𝟕 − 𝟒𝟒𝟎𝟑

The total volumetric feed flow rate (Q)

𝟏𝟓𝟎 − 𝟒𝟓𝟎 𝐦𝐋⁄𝐦𝐢𝐧

Kinematic viscosity21 (𝜈) (~ constant)

𝟏. 𝟎𝟐𝟐 × 𝟏𝟎−𝟔 𝐦𝟐 ⁄𝐬

Diffusion coefficient21 (𝐷) (~ constant)

𝟗. 𝟒𝟐 × 𝟏𝟎−𝟗 𝐦𝟐 ⁄𝐬

Feed nozzle diameter

𝟓 𝐦𝐦

The distance between two feed nozzles

𝟒 − 𝟏𝟒𝟎 𝐦𝐦

Distance between the outlet of feed nozzle and the disk

𝟓 𝐦𝐦

35



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Table 4. The operating conditions and the obtained results for the experimental runs performed Experiment No.

Rotational disk speed (𝐫𝐩𝐦)

Feed entrance radius (𝐦𝐦)

Total 𝐁𝐚𝐂𝐥𝟐 feed flow concentration rate (𝐦𝐨𝐥⁄𝐋) (𝐦𝐋⁄𝐦𝐢𝐧)

𝐍𝐚𝟐 𝐒𝐎𝟒 feed concentration (𝐦𝐨𝐥⁄𝐋)

E-01 E-02 E-03 E-04 E-05 E-06 E-07 E-08 E-09 E-10 E-11 E-12 E-13 E-14 E-15 E-16 E-17 E-18 E-19 E-20 E-21 E-22 E-23 E-24 E-25 E-26 E-27 E-28 E-29 E-30

5000 7000 9000 11000 13000 15000 9000 9000 9000 9000 9000 15000 15000 15000 15000 15000 15000 15000 15000 15000 15000 15000 15000 15000 15000 15000 15000 15000 15000 15000

10 10 10 10 10 10 2 30 50 70 90 2 30 50 70 90 70 70 70 70 70 70 70 70 70 70 70 70 70 70

300 300 300 300 300 300 300 300 300 300 300 300 300 300 300 300 150 450 450 450 450 450 450 450 450 450 450 450 450 450

0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.50 1.00 1.20 1.40 0.18 0.11 0.10 0.25 0.85 0.28 3.30 3.30

0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.50 1.00 1.20 1.40 0.18 0.33 0.48 0.15 0.17 1.40 0.66 1.40

36


free ion ratio (#) 𝑹 [𝐁𝐚𝟐+ ] = [𝐒𝐎𝟐− 𝟒 ] 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 3.00 5.00 0.60 0.20 5.00 0.20 0.42

Supersaturation (𝑺𝐁𝐚𝐒𝐎𝟒 ), (#)

MPS (PSD) (𝐧𝐦)

3551 3551 3551 3551 3551 3551 3551 3551 3551 3551 3551 3551 3551 3551 3551 3551 3551 3551 3067 3924 4178 4403 2000 2000 2000 2000 2000 1137 3422 3362

83.1 (30-190) 67.8 (24-168) 56.3 (24-160) 50.9 (22-154) 48.7 (20-130) 47.6 (18-92) 46.3 (20-116) 55.2 (26-140) 53.4 (26-150) 49.3 (22-92) 73.1 (48-184) 44.7 (18-88) 45.1 (18-88) 44.2 (24-78) 42.4 (20-62) 66.9 (30-80) 43.5 (20-64) 41.1 (18-60) 51.0 (26-80) 42.3 (20-58) 39.6 (28-68) 35.0 (20-50) 46.1 (26-66) 36.8 (18-52) 29.9 (16-44) 30.9 (16-42) 25.0 (12-36) 28.6 (10-40) 21.9 (6-34) 16.4 (4-26)

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Table 5. Elemental analysis of the surface of 𝐁𝐚𝐒𝐎𝟒 powder obtained in the experiment No. E-30 Element

Atomic

Weight

(%)

(%)

Oxygen

56.46

19.03

Sulfur

20.30

13.71

Barium

23.24

67.26

Total

100

100

37



Figure 1. Schematic layout of the rig: (1) 𝑩𝒂𝑪𝒍𝟐 solution tank, (2) 𝑵𝒂𝟐 𝑺𝑶𝟒 solution tank, (3) peristaltic pumps, (4) distilled water tank, (5) product, (6) outlet valves, (7) frequency inverter, (8) single phase po wer, (9) variable speed three -phase electric motor, (10) polyethylene cone, (11) rotating disk, (12) polycarbonate f ixed disk, (13) upper cover, (14) pin, (15) distance between the disks (5 cm), (16) Allen screw, (17) feed nozzles, (18) steel base, (19) steel plate, (20) stabilizers.

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Page 39 of 51

17000 15000 13000 Rotational speed, ω (rpm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


11000 9000 7000

y = -0.1304 x2 + 106.57 x - 4622.5

5000

R² = 0.9992

3000 80

100 120 140 160 180 200 220 240 260 280 300 Frequency, f (Hz)

Figure 2. Rotational disk speed vs. the input frequency to the electric motor.

39



Figure 3. XRD pattern. (a) E-30, (b) Barite JCPDS-ICDD No. 00-001-1229.

40


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Figure 4. EDXS image of the 𝐵𝑎𝑆𝑂4 powder by experiment No. E-30.

41



90 85 80 75 MPS (nm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 42 of 51

70 65

60 55 50 45 40 4000

6000

8000

10000

12000

14000

16000

Rotational disk speed, ω (rpm) Figure 5. Effect of the rotational disk speed on the MPS of the synthesized barium sulfate. Operating conditions: 𝑄=300 mL/min, 𝐶𝐵𝑎𝐶𝑙2 =𝐶𝑁𝑎2 𝑆𝑂4 =0.75 mol/L, 𝑟𝑒 =10 mm, 𝑆𝐵𝑎𝑆𝑂4 =3551, and R=1.

42


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Figure 6. SEM micrographs of the barium sulfate particles synthesized at different rotational disk speeds. Operating conditions: 𝑄=300 mL/min, 𝐶𝐵𝑎𝐶𝑙2 =𝐶𝑁𝑎2 𝑆𝑂4 =0.75 mol/L, 𝑟𝑒 =10 mm, 𝑆𝐵𝑎𝑆𝑂4 =3551, and R=1. (a) 𝜔=5000 rpm (E-01), and (b) 𝜔=15000 rpm (E-06).

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75 ω = 9000 rpm ω = 15000 rpm

70 65 MPS (nm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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60 55 50 45 40 0

10

20

30

40

50

60

70

80

90

100

Feed entrance radius, re (mm) Figure 7. Effect of the feed entrance radius on the MPS of the synthesized barium sulfate. Operating conditions: 𝑄=300 mL/min, 𝐶𝐵𝑎𝐶𝑙2 =𝐶𝑁𝑎2 𝑆𝑂4 =0.75 mol/L, 𝑆𝐵𝑎𝑆𝑂4 =3551, and R=1.

44


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Figure 8. SEM micrographs of the barium sulfate particles synthesized at different entrance radius. Operating conditions: 𝑄=300 mL/min, 𝐶𝐵𝑎𝐶𝑙2 =𝐶𝑁𝑎2 𝑆𝑂4 =0.75 mol/L, 𝑆𝐵𝑎𝑆𝑂4 =3551, and R=1. (a) 𝜔=9000 rpm and 𝑟𝑒 =90 mm (E-11), and (b) 𝜔=15000 rpm and 𝑟𝑒 =70 mm (E-15).

45



50

MPS (nm)

45

40 MPS = -0.011S + 81.377 35

R² = 0.9693 30 3000

3200

3400

3600

3800

4000

4200

4400

4600

Supersaturation, SBaSO4 (#) 50

45

MPS (nm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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40 MPS = -16.36C + 55.029 35 R² = 0.9674 30 0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

1.2

1.3

1.4

1.5

Barium Chloride or Sodium Sulfate Concentration, CBaCl2 (M) Figure 9. Effect of (a) supersaturation or (b) 𝐵𝑎𝐶𝑙2 concentration on the MPS of the synthesized barium sulfate. Operating conditions: 𝑄=450 mL/min, 𝐶𝐵𝑎𝐶𝑙2 =𝐶𝑁𝑎2 𝑆𝑂4 , 𝑟𝑒 =70 mm, 𝜔=1500 rpm, and R=1.

46


Page 47 of 51

4500

(a)

Supersaturation (#)

R=1

4000

R=2

3500

R=3 R=4

3000

R=5

2500 2000 1500

1000 500 0 0

0.25

0.5

0.75

1

1.25

1.5

Barium Chloride Concentration, CBaCl2 (M) 4500

(b)

R=1

Supersaturation (#)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4000

R=0.8

3500

R=0.6 R=0.4

3000

R=0.2

2500 2000 1500 1000 500 0 0

0.25

0.5

0.75

1

1.25

1.5

Barium Chloride Concentration, CBaCl2 (M) Figure 10. Effect of ion concentration on the supersaturation: (a) 1 ≤ 𝑅 ≤ 5, and (b) 0.2 ≤ 𝑅 ≤ 1.

47



50 45 40

MPS (nm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 48 of 51

35 30 25 20 0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

5.5

Free ion ratio, R (#) Figure 11. Effect of Free ion ratio (R) on the MPS of the synthesized barium sulfate. Operating conditions: 𝑄=450 mL/min, 𝑟𝑒 =70 mm, 𝑆𝐵𝑎𝑆𝑂4 =2000, and 𝜔=1500 rpm.

48


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Figure 12. SEM micrographs of the barium sulfate particles synthesized at different R. Operating conditions: 𝑄=450 mL/min, 𝑟𝑒 =70 mm, 𝑆𝐵𝑎𝑆𝑂4 =2000, and 𝜔=1500 rpm. (a) R=1 (E-23), (b) R=5 (E-25), and (c) R=0.2 (E-27).

49



Figure 13. SEM micrographs of the barium sulfate particles synthesized for the final experimental run (E30). Operating conditions: 𝑄=450 mL/min, 𝐶𝐵𝑎𝐶𝑙2 =1.4 mol/L, 𝐶𝑁𝑎2𝑆𝑂4 =3.3 mol/L, 𝑟𝑒 =70 mm, 𝑆𝐵𝑎𝑆𝑂4 =3362, R=0.42, and ω=15000 rpm. MPS=16.4 nm & PSD=4 to 26 nm.

50


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Graphical Abstract

51


Continuous synthesis of barium sulfate nanoparticles in a new high

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