Synthesis, Characterization, Neutron Activation, and Application of

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Synthesis, Characterization, Neutron Activation, and Application of Scandium Oxide Microsphere in Radioactive Particle Tracking Experiments Jayashree Biswal,† Sunil Goswami,† Harish Jagat Pant,*,† Yashwant Ramdas Bamankar,‡ Tadipatri Venkovarao Vittalrao,‡ Rajesh Kumar Upadhay,§ and Ashutosh Dash† †

Isotope Production and Applications Division and ‡Fuel Chemistry Division, Bhabha Atomic Research Centre, Mumbai 400085, India § Department of Chemical Engineering, Indian Institute of Technology, Guwahati, Assam 781039, India ABSTRACT: This paper describes a novel approach for synthesis of scandium oxide (Sc2O3) microspheres based on sol−gel technique of internal gelation followed by neutron activation in a nuclear reactor to produce 46Sc for use as radiotracers in radioactive particle tracking (RPT) experiments. The synthesis of scandium oxide microspheres of regular and spherical shapes essentially consists of formation of concentrated colloidal sol of the Sc(OH)3 and transformation of this sol into a semi rigid gel followed by heating. The synthesized microspheres were characterized by X-ray diffraction, scanning electron microscopy, thermogravimetry analysis/differential thermal analysis, and surface area analyses. The neutron-activated microspheres were successfully used in RPT experiments to investigate hydrodynamics of solids in a pilot-scale liquid−solid fluidized bed.

1. INTRODUCTION Radioactive particle tracking (RPT) technique has emerged as a promising and versatile radiotracer technique to investigate dynamics of a specific phase in laboratory as well as pilot-scale multiphase flow systems. In RPT technique, the motion of a single particle labeled with gamma-emitting radionuclide in a flow system is tracked using an array of NaI(Tl) scintillation detectors strategically positioned around the system of interest.1−14 In view of the perceived need to obtain various hydrodynamic parameters, the data recorded by different detectors are processed in conjunction with calibration data using suitable software. Over the years, the RPT technique has undergone rapid technological evolution to provide a wealth of crucial information on flow characteristics of fluids/solids, evaluation, and improvement of design of multiphase flow systems. This has been possible primarily because of availability of multichannel nuclear data acquisition systems and developments in preparation procedures of radioactive particles.15−19 While RPT lies at the interface between many disciplines, its dependence on radioactive tracer particle is arguably the strongest. The radioactive particle should not only be hydrodynamically similar to that of the phase whose motion is being followed, but also ideally should have similar physical properties (size, shape, density, and buoyancy) of the tracked phase. The radionuclide used for radiolabeling of the particle should have a fairly long half-life (few days), high specific activity, emit highly penetrating γ radiation, and have chemistry amenable for incorporation on to the particle. With expanding areas of applications and growing interest in the use of RPT, acquiring local production capability of radioactive tracer particle is an interesting proposition deemed worthy of consideration. Conspicuous harnessing of the RPT in conjunction with radioactive tracer particle development would not only unveil bountiful applications, but also seem © XXXX American Chemical Society

poised to broaden the palette of RPT. In pursuit of a pragmatic approach to undertake preparation of radioactive tracer particle for RPT experiments, radioactive decay characteristics of various radionuclide were scrupulously scrutinized. There appears to be enticing interest to consider the use of 46Sc, a radionuclide with half-life of 83.8 days, which emits two γ rays [0.889 MeV (99.98%) and 1.120 MeV (99.98%)]. The highly penetrating gamma radiations are sufficient for transmission through thick metal walls of the investigated system and offer the scope for in situ detection. Moreover, cost-effective production of 46Sc with adequate activity can be carried out in medium flux research reactors by neutron activation of natural Sc (100% 45Sc) target owing to reasonable thermal neutron capture cross-section (σ = 27 barn) of 45Sc. In the quest for an effective approach for the preparation of 46 Sc particle amenable for RPT experiments, the prospect of making Sc2O3 microspheres followed by activation in a nuclear reactor seems to be an intuitive pathway and motivated us to pursue further. Among the various viable chemical methods used for the preparation of microspheres, the sol−gel method, which involves the gelation of a droplet of sol into a gel microsphere and subsequent conversation into dry microsphere by heat treatment, appears to be a favorable pathway. This strategy is technically simple, reproducible, requires small amounts of precursors, and offers the scope for particle size control. In pursuit of a viable method within the realm of sol− gel method, the prospect of using internal gelation method was deemed worthy of consideration owing to adaptability at small laboratory scale.20−26 This theme was simplified in this work Received: June 22, 2015 Revised: October 15, 2015 Accepted: December 15, 2015

A

DOI: 10.1021/acs.iecr.5b02261 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research and directed toward the development of a viable method for the preparation of Sc2O3 microspheres amenable for RPT experiments. While the outlook of internal gelation technique seems to be quite promising, technical realization of the technique requires cognitive introspection of process, conscious development of process equipment, and adherence to microsphere specifications. These challenges were considered and addressed scrupulously. The present paper describes a systematic approach for the synthesis and characterization of Sc2O3 microspheres using internal gelation method followed by neutron activation in a nuclear reactor to produce 46Sc. One of the prepared radioactive microspheres (46Sc2O3) was successfully used for tracking solid phase in a laboratory-scale liquid solid fluidized column, and results are briefly discussed in the present paper.

2. EXPERIMENTAL SECTION 2.1. Synthesis of Sc2O3 Microspheres. Synthesis of Sc2O3 microspheres essentially consists of two steps. The first step entails conversion of Sc2O3 powder in to Sc(NO3)3, and the second step involves preparation of Sc2O3 microspheres by sol−gel process based on internal gelation technique. In this method, 2 mol dm−3 scandium nitrate solution was prepared by dissolving scandium oxide powder in excess concentrated nitric acid at 80 °C. The pH of the metal ion solution was brought to 3 by the addition of ammonium hydroxide. A chilled feed solution containing 1.5 mol dm−3 scandium nitrate, 1.5 mol dm−3 hexamethylenetetramine (HMTA), and 1.5 mol dm−3 urea was dispersed as droplets through a capillary into a gelation column containing hot silicone oil at 90 °C to obtain gel spheres. Upon extraction, the gel microspheres were degreased with carbon tetrachloride to remove the oil and then rinsed with 0.5 mol dm−3 ammonium hydroxide to remove ammonium nitrate, unreacted HMTA, and urea. The microspheres were dried at 100 °C, and one set of gel spheres was calcined at 300 °C and another set at 900 °C to obtain Sc2O3 microspheres. Henceforth, the dried untreated microspheres and the microspheres heat treated at 300 °C and at 900 °C are represented as UT, HT300, and HT900, respectively. A flowchart of the process is depicted in Figure 1. The reagent and chemicals such as nitric acid, Sc2O3, carbon tetrachloride (CCl4), urea, HMTA, etc. used in synthesis of Sc2O3 were of analytical grade and procured from E. Merck (Germany) and BDH (England). Solutions were prepared from doubly demineralized water obtained by passing distilled water through a Millipore Milli-Q water purification system. All other GR/AR grade chemicals were procured from local manufacturers in India. 2.2. Characterization of Microspheres. After synthesis, the microspheres were characterized for surface topography and morphology, surface area, structural information, and changes in physical and chemical properties of the microspheres as a function of increasing temperature. Particle morphologies were observed by high-resolution scanning electron microscopy (SEM) (Phillips XL 30ESEM, operating voltage 30 kV). Particles were spread on the surface of a silicon plate and were coated with gold thin film for better conductivity. Specific surface area of the Sc2O3 powder, SBET, was measured by the Brunauer−Emmett−Teller method on Quatachrome Autosorb1C surface analyzer via liquid nitrogen chemisorption at 77 K. Thermogravimetry analysis (TGA) and differential thermal analysis (DTA) curves were recorded on Ntezsch thermal analyzer, Germany, Model-STA 409 at a heating rate of 10 °C/

Figure 1. Flowchart of synthesis of scandium oxide microspheres.

min under a flow of N2 using pure alumina as reference. The plot of mass loss as a function of temperature was recorded. Xray diffraction (XRD) was performed on a Philips PW1800 Xray diffractometer operating at 40 kV/50 mA using nickelfiltered Cu Kα radiation and at a scanning speed of 0.5° in 2θ/ min. 2.3. Neutron Activation of Microspheres. Out of the synthesized and calcined Sc2O3 microspheres, nine particles of different sizes were selected and appropriately packed in a coldpressure-weld type cylindrical 1S aluminum can (diameter, 22 mm; length, 44 mm) and irradiated in the DHRUVA reactor at Trombay, Mumbai, India at a neutron flux of 3 × 1013 n cm−2 s−1 for 7 days. At the end of the irradiation, the irradiated microspheres were cooled for 24 h and subsequently transported to the laboratory in lead shielded flask. The irradiation container containing neutron irradiated Sc2O3 microspheres was taken to a lead shielded facility where the irradiation container was opened with the help of an opening unit, and irradiated microspheres were retrieved. 2.4. Gamma Ray Spectrum. To identify presence of radionuclide other than 46Sc, gamma-ray spectrum of the irradiated microspheres was measured using a high-resolution gamma ray spectrometry system consisting of a coaxial HPGe detector (ORTEC, Oakridge, TN, USA) with 10% relative efficiency and integrated to a 8K multichannel analyzer (MCA). The energy resolution of the detector was 0.5 keV for 1332.46 keV gamma energy of 60Co. Energy and efficiency calibration of the HPGe detector was carried out using 152Eu and 133Ba reference sources. The neutron-activated microspheres were counted for a period of 300 s, and the spectra were analyzed for B


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addition to this, the polymer coating also improves mechanical strength of the microspheres. Different researchers have used different polymeric material for coating.11,15 In the present study, the microspheres were coated with a composite of polypropylene (PP) and submicron sized iron particles. A known quantity of iron particles was mixed with the PP matrix with xylene as the solvent. The coating of microspheres was carried out by dipping in the PP-iron particles composites solution for 2 min and draining the excess of solution. The coated microspheres were then placed onto a wire tray where the excess of the uncoated PP-iron particles composites drained out and air-dried. PP-iron particles composites coated microspheres were washed with lukewarm water and dried under infrared lamp at 60 °C for 1 h. 2.7. Measurement of Density of the Microsphere. The density of the PP-coated microsphere was measured by carrying out settling experiments. In this experiment, a 2 m long, 0.076 m diameter column filled with glycerol (density, 1.26 g cm−3) was used. The microsphere was dropped in the column with zero initial velocity, and the time taken by the particle to reach the bottom of the column was noted. The experiment was repeated five times to get better accuracy. Because the microsphere has very small size (∼0.9 mm) and very low settling velocity due to high viscosity of the fluid, it always has Reynolds number less than 1. The density of the particle was calculated by using the following equations:

the photo peaks. The measured gamma ray spectrum of the neutron-irradiated microspheres is shown in Figure 12. 2.5. Radioactivity Measurement. It was not possible to measure the radioactivity of 46Sc in neutron activated microspheres using above-mentioned HPGe detector based MCA because of relatively higher gamma ray counts produced by the irradiated microspheres. The error in measurement of radioactivity due to “dead time” is higher at higher count rates. Therefore, the activity measurements were performed in a special type of ionization chamber (CENTRONICS IG 12A ionization chamber) often referred to as radionuclide calibrator. The equipment consists of a well-type ionization chamber filled with argon at 2 MPa and coupled to a digital electronic system, which allows direct readings in units of the activity for the radioisotope analyzed. The calibration factor for different radioactive sources is preprogrammed in the system. The activities of microspheres having sizes of 0.83 mm, 0.86 mm, and 0.91 mm were found to be 18.6−21.6 MBq, 20.8−27.7 MBq, and 56.273.6 MBq, respectively (Table 1). Table 1. Measured Activity of Irradiated Sc2O3 Microsphere microsphere code

size of microsphere (μm)

HT300-MS1 HT300-MS2 HT300-MS3 HT300-MS4 HT300-MS5 HT300-MS6 HT300-MS7 HT300-MS8 HT300-MS9

∼830

∼860

∼910

activity MBq (μCi) 18.6 16.5 21.6 23.3 20.8 27.7 73.6 56.2 60.6

(504) (446) (586) (632) (562) (749) (1990) (1520) (1640)

VS =

Vs =

(ρp − ρl )gd p2 18μ

L Δt

(1)

(2)

where Vs is the settling velocity of microsphere, ρp the density of microsphere, ρl the density of liquid or glycerol (1.26 g cm−3), μ the viscosity of glycerol (1.412 Pa.s), dp the diameter of particle (0.9 mm), L the height of liquid level in the column (1.8 m), and Δt the time taken by the particle to reach the

2.6. Polymer Coating. To prevent leaching of 46Sc, the irradiated microspheres need to be coated with polymeric material. The polymer coating is applied not only for preventing leaching of 46Sc, but also for matching the density and size of the microsphere for an intended application. In

Figure 2. Schematic diagram of solid−liquid fluidized bed and experimental setup for RPT study. C


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Figure 3. Camera-ready picture of Sc2O3 microspheres obtained at different temperatures.

3. RESULTS 3.1. Synthesis Process. In view of the perceived need to produce structurally strong and stable Sc2O3 microspheres by the internal gelation process, optimum broth formulations and gel-forming temperatures were necessary and therefore were pursued diligently. Preliminary experiments were carried out to assess the gelation parameters for scandium. Scandium nitrate solution mixed with optimal proportions of HMTA and urea resulted in the creation of a hard wax when heated as a bulk mixture in a beaker, and formation of soft, gelled spheres after falling through a column of hot silicone oil. Following the above-described methodology, several batches of Sc2O3 microspheres were synthesized. The microspheres obtained were found to be of 0.7−1.2 mm diameter sizes, mostly spherical in morphology, and contained very little broken microspheres. After washing and drying at room temperature, the gel microspheres were calcined at two different temperatures, that is, 300 °C and at 900 °C. The heat treatment is necessary to drive off organic impurities and to get structurally strong, stable, desired sized dioxide microsphere. The heat treated (HT) Sc2O3 microspheres were structurally strong and stable as compared to their untreated (UT) counterparts. Figure 3 depicts the cameraready picture of Sc2O3 microspheres obtained at different temperatures. Spherical microspheres with a smooth external surface were observed in each case. The UT microspheres showed higher mean diameter as compared to HT microspheres. The internal gelation sol−gel process involves the reaction of scandium nitrate with ammonia to produce a spherical gel, and the ammonia is produced internally by the decomposition of HMTA.20 Cooling of the reaction mixture to 0 °C is necessary to prevent early gelation owing to the combination of urea with the nitrate salt to stabilize the solution. Thus,

bottom of the column (6920 s). The average settling velocity of the microsphere was 0.26 mm/s, and the corresponding density was estimated to be 2.08 g cm−3. 2.8. Application of Radioactive Microspheres. The synthesized and neutron activated 46Sc2O3 microspheres can be used for tracking solid or liquid phase in RPT experiments conducted in multiphase flow systems. In RPT experiments, a single radioactive particle is used to extract information about the flow dynamics of a specified phase. In the present work, a single radioactive microsphere (density, 2.5 g cm−3) having activity about 18.5 MBq (0.5 mCi) was used to investigate flow patterns of solids in a liquid−solid fluidized bed column as shown in Figure 2. The experimental setup consists of a perspex column having inner diameter of 0.1 m and height of 1.5 m. A perforated perspex plate having holes of 0.001 m diameter and fitted at the bottom of the column was used as distributor for liquid. Glass beads (diameter, 0.0012 m; density, 2.5 g cm−3) and water were used as solid and liquid phase, respectively. The water from a tank was pumped into the column through the distributor that fluidizes the glass beads in the column. The water flows out of the column at the top and is recirculated back to the tank. The liquid flow rate was regulated with the help of a rotameter in such a way that the glass beads do not over flow from the column. A series of experiments were conducted, and in each experiment, mass of the bed was kept constant to 2 kg. The density of the microsphere was matched with the density of the glass beads during the preparation and polymer coating. The density matched (2.5 g cm−3) 46Sc2O3 microsphere was introduced into the column during the experiments, and its movement within the fluidized bed was tracked by eight NaI(Tl) detectors (2 in × 2 in, Bicorn USA make) mounted strategically at four axial locations around the column. The gamma-rays emitted by the radioactive microsphere were recorded by the detectors connected to a multiinput data acquisition system (MIDAS). At a particular operating condition, the data were continuously recorded at an interval of 20 ms for about 8 h. Subsequently, the recorded data were transferred to a computer for further processing.

Sc(NO3)3 + 2(NH 2)2 CO ⇋ Sc(NO3)3 2(NH 2)2 CO

(3)

After the broth is stabilized, the temperature is increased by introduction of the broth into the silicone oil at 90−110 °C; the urea decomplexes, and the above reaction is driven to the left D


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Figure 4. SEM images of the microspheres (UT, HT300, and HT900).

Figure 5. Magnified surface images of the microspheres (UT, HT300, and HT900).

Figure 6. (A) SEM image of polymer coated microsphere. (B) Magnified surface images of the polymer coated microsphere.

Sc(NO3)3 + 3H 2O ⇌ Sc(OH)3 + 3HNO3

consumed as fast as it is produced. Once the ammonium is produced, the final product is precipitated as a solid. Ammonium hydroxide generated reacts with hydrogen ion to neutralize and form the metal ion polymer (Sc(OH)3)n. Hence, a rigid gel structure of hydrated scandium oxide is formed. Since the reaction sequence transpires within the spherical solution droplet suspended in the silicone oil, the final product is a solid sphere. The spheres form rapidly, but the reaction needs time to complete so that the spheres are “aged” in the hot oil. 3.2. SEM, TGA/DTA, and XRD Analyses. Microstructures of the synthesized Sc2O3 spheres were studied by SEM analysis. The SEM images of the microspheres, that is, UT, HT300, and HT900, are shown in Figure 4, panels A, B, and C, respectively. The corresponding surface structures of the particles at higher magnification are shown in Figure 5, panels A1, B1, and C1, respectively. The SEM images revealed that microspheres were spherical, had narrow distribution, and were porous in nature. The two surface features that noticed in the SEM images were sphericity and porosity of the microspheres. The microspheres retain sphericity even after heating up to 900 °C temperature.

(4)

When the droplets come in contact with the hot silicon oil, the complexes dissociate as they are unstable at higher temperature. The metal salt loses its nitrate group and undergoes hydrolysis. The hydrogen ion produced by the hydrolysis reaction and then undergoes protonation reaction with the HMTA.21 Thus, (CH 2)6 N4 + H+ ⇌ (CH 2)6 N4 · H+

(Fast)

(5)

The protonated HMTA undergoes decomposition to produce ammonium (NH4+) and formaldehyde (CH2O).22 Thus, (CH 2)6 N4 ·H+ + 3H+ + 6H 2O ⇌ 4NH4 + + 6CH 2O (6)

Up to this stage, all species are still soluble in the matrix. Urea present in the sol plays another important role. Urea and formaldehyde readily react together to produce monomethylol urea.21 As a result, the reaction as shown in eq 6 is driven completely to the right direction as the formaldehyde product is E


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Industrial & Engineering Chemistry Research The magnified surface revealed that the surface was not smooth, which can be attributed to the pores formed on the surface. The particle diameters obtained from SEM image were found to be about 1210 μm, 830−910 μm, and 640−750 μm for UT, HT300, and HT900 microspheres, respectively. Hence, it was observed that there was reduction in the particle size on heat treatment at elevated temperatures. The SEM image of polymer coated microsphere is shown in Figure 6. The magnified surface revealed that the surface was relatively smoother than the uncoated microsphere, which indicated porosity decreases due to polymer coating on the surface. The TGA and DTA Sc2O3 microspheres (UT, HT300, and HT900) were conducted to evaluate the mass changes and identify any possible reaction during the heating process. The results of TGA are shown in Figure 7. TGA curve in Figure 7, panel A

Figure 8. DTA plot of Sc2O3 microspheres (A) untreated and (B) calcined at 300 °C.

Figure 7. TGA plot of Sc2O3 microspheres (A) before heat treated, (B) calcined at 300 °C, and (C) calcined at 900 °C.

shows that there is a continuous weight loss up to a temperature of 500 °C, which is attributed to the evaporation of molecular water, hydroxide groups, and also decomposition of the gel. A small weight loss is observed up to 150 °C in Figure 7, curve B, which may be due to evaporation of molecular water, whereas there is no appreciable weight loss observed in Figure 8, curve C. Subsequently, the weight becomes stable that represents the formation of Sc2O3. The results of DTA are shown in Figure 8. In the DTA curve, the plot in Figure 8, panel B does not exhibit any exothermic effect, whereas the plot in Figure 8, panel A exhibits an exothermic effect at 456 °C that could be attributed to the decomposition of organic compound. In an attempt to obtain detailed information on the thermal behavior of the microsphere obtained after heating to 900 °C, TGA/DTA of microspheres (HT900) were carried out separately, and the results are shown in Figure 9. In DTA curve, endothermic peak at 483−555 °C is due to decomposition of organic compound, and that at 602 °C is due to phase change. There is no appreciable weight change in the entire region. The surface area analyses of the UT, HT300, and HT900 microspheres were carried out to extract information about the effect of heat treatment, and the results are presented in Table 2. It was observed that the surface area of untreated microspheres, that is, UT, is more than that of the heat treated microspheres, that is, HT300 and HT900. Also, with the

Figure 9. TGA/DTA curve of Sc2O3 microparticles heated at 900 °C.

Table 2. Surface Area Analysis microsphere batch

Sc2O3 microspheres

surface area (m2/g)

UT HT300 HT900

untreated heated at 300 °C heated at 900 °C

176 ± 13 136 ± 10 74 ± 7

increase in calcination temperature, the surface area decreases, indicating shrinking of the pores. The XRD analysis was carried out to characterize the crystal structure and identify the phases present in the microspheres. The XRD patterns of the Sc2O3 microspheres before heat treatment and after calcinations at 300 °C are shown in Figure 10. It was observed that untreated microspheres are amorphous in nature, whereas the heat-treated microspheres are crystalline in nature. The X-ray diffractogram of scandium oxide calcined at 300 °C showed prominent peaks at 2θ values of 22.29°, 31.51°, 52.65°, and 62.73° representing the 211, 222, 440, and 622 Brags reflections. The XRD pattern of Sc2O3 microsphere calcined at 900 °C is shown in Figure 11, which reveals (2 2 2) diffraction peak as the prominent peak indicating that the crystal structure is in pure cubic phase. F


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shows two peaks at 0.889 MeV and 1.12 MeV, which are the photopeaks of Scandium-46 (46Sc) radioisotope. This indicates that there is no other radioisotope present in the microspheres. The activities of the neutron activated microspheres ranged from 18.6−73.6 MBq (Table 1). These activities were sufficient for different RPT experiments. Therefore, the synthesized microspheres contained sufficient amount of target material (45Sc) to produce required activity of 46Sc when irradiated at a neutron flux of 3 × 1013 n cm−2 s−1 for 7 days. 3.4. Density Adjustment. The average settling velocity of the microsphere was 0.26 mm/s, and the corresponding density was estimated to be 2.08 g cm−3. Since in the present study a scandium oxide microsphere was used for tracking glass particles having density 2.5 g cm−3, the density of the prepared microsphere was also adjusted to about 2.5 g cm−3 by coating the microsphere with a mixture of polypropylene and submicron iron particles as discussed earlier. 3.5. RPT Experiments. The utility of the radioactive microspheres was successfully demonstrated by conducting the RPT experiments in a liquid−solid fluidized bed column as shown in Figure 2. The experiments were performed at three different superficial velocities of liquid, that is, 0.03, 0.04, and 0.05 m/s, which correspond to three, four, and five times of minimum fluidization velocity (Umf) of solids, respectively. The position of the tracer particle with respect to time and space were reconstructed using a Monte Carlo based reconstruction algorithm.13 From the reconstructed particle positions of the particle, mean velocities and axial root−mean−square velocities (rms velocities, Vz) of solids phase were calculated. Figure 13

Figure 10. XRD of Sc2O3 microspheres (A) before heat treatment and (B) calcined at 300 °C.

Figure 11. XRD of Sc2O3 microspheres calcined at 900 °C.

3.3. Gamma Ray Spectrum and Radioactivity Measurements. The measured gamma ray spectrum of the neutronirradiated microspheres is shown in Figure 12. The spectrum

Figure 13. Radial variation of mean axial velocity of solids for different liquid velocities (r, distance from center; R, radius of the column).

shows the radial variation of mean axial velocity of solids for different liquid velocities (UL) at half of the height of the column, where R is the radius of the column and r represents the radial position at which data are plotted. Results show that for all the liquid velocities, solids are moving upward at the center of the column and downward near the wall of the column. Further, it indicated that with increase in liquid velocities, the velocities of solids increase due to higher momentum provided by the liquid to the solids. Figure 14 shows the radial variation of the axial rms velocities of solids for different liquid velocities. Results indicate that with increase in liquid velocity, the axial rms velocities of solid increase. This is primarily because the bed becomes relatively

Figure 12. Gamma spectrum of neutron activated scandium oxide microsphere. G


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or organic metal compounds such as metal alkoxides are used. The sol essentially consists of a suspension of the precursors, which undergo hydrolysis and condensation polymerization reactions to produce gels. The chemical reactions that take place during the sol−gel process are hydrolysis, polycondensation, and gelation. The method used in this work is a ureabased internal gelation method where ammonia is produced internally by the decomposition of HMTA. 20−26 The combination of the scandium nitrate, urea, and HMTA is commonly referred to as the “broth”. Chilling of the broth is necessary to stabilize the solution and prevent early gelation. Decomposition of organic precursors (HMTA and urea) under heat produces ammonia, which increases the pH and ensures the gelation. Once the ammonium is produced, the final product is precipitated as a solid. The role of urea was to catalyze the decomposition of HMTA into NH3 and to bring porosity to the microsphere. As the reaction sequence transpires within the spherical solution droplet suspended in the silicone oil, this leads to the formation of a solid microsphere. In light of the perceived need for the reaction to be complete, the spheres are “aged” in the hot oil. For RPT studies, the radioactive particle has to be representative of the solids being tracked. Therefore, the physical parameters such as density, size, and shape of the particle have to be identical to that of the solid particles. In addition to visual inspection, the SEM images of the microspheres showed that the particles were spherical. To match the density and size of the radioactive microsphere to that of the glass particles for RPT studies in a solid−liquid fluidized bed, additional coatings of PP mixed with submicron size iron particles were applied. In addition to this, the particle was also coated with a polymeric paint of red color and subsequently air-dried. The microsphere painted with red color helps to identify and isolate the radioactive tracer particle from bulk of the solids after completing the experiments. The polymeric coating also helps to make particle hydrophobic and thus prevents any change in density of the microsphere during RPT experiments in liquid−solid flow systems. Such a strategy seemed sagacious as it not only maintains the density to about 2.5 g cm−3, but also renders the radioactive microspheres to resist wear-off during the experiments. The utility of liquid−solid fluidized bed in various industries such as chemical, hydrometallurgy, food technology, biochemical processing, and water treatment for conducting different types of chemical reactions, crystallization, ion exchange, and adsorption needs hardly to be reiterated.27−31 While fluidized beds are an important asset to many industrial processes and offer several advantages including high heat and mass transfer rates, lower pressure drops, uniform temperature distribution, an understanding the hydrodynamic behavior of solids represents the key determinant for successful designing and scaling-up of industrial liquid−solid fluidized systems. In this context, use of noninvasive measurement techniques such as RPT seemed attractive as they provide insight into the flow behavior and general hydrodynamic characteristics of multiphase flow and opaque systems.

Figure 14. Radial variation of axial rms velocity of solids for different liquid velocities.

more violent at higher velocities. Further, it was observed that the rms velocities of solids are higher at the wall and lower at the center of the column for all the liquid velocities. This indicates that the solids near the wall are relatively more dynamic as compared to the center of the column, which could be mainly due to the downward motion of the solids at walls. The results of the present study are in good agreement with the results reported in literature.27−30 These results of the present study indicate that the radioactive 46Sc2O3 microsphere is able to track the motion of solids (glass beads) accurately and reproduce the basic flow phenomena.

4. DISCUSSION The RPT technique has emerged as a powerful noninvasive technique for flow visualization of solid and liquid phases in multiphase flow systems in chemical engineering research. One of the prime requirements of the technique is to have a suitable particle for representing and tracking movement of a specific phase. Therefore, there appear to be enticing interest in synthesis and preparation of suitable microspheres that can be activated and labeled with suitable radioisotopes and subsequently used in radioactive particle tracking experiments. The present work successfully addresses several issues such as synthesis of Sc2O3 microspheres of uniform sizes, characterization of the synthesized microspheres, neutron activation of the microspheres, measurement of gamma-ray spectrum and radioactivity produced in microspheres, coating of the microspheres with a thin layer of PP submicron iron particles composites, and demonstration of application of the radioactive 46 Sc2O3 microspheres for tracking flow dynamics of solids in a laboratory-scale liquid−solid fluidized bed column using particle tracking experiments. The present approach to synthesize Sc2O3 microspheres followed by neutron activation rather than using radioactive 46Sc for preparation of microspheres precludes radioactive contamination during the handling of radioactive solution. This approach facilitates the fine control on the activity of the microspheres by judicious optimization of neutron irradiation parameters. Among the available methods to prepare Sc2O3 microspheres, sol−gel process is the obvious choice owing to its simplicity, reliability, reproducibility, and proven effectiveness. The sol−gel process is a wet-chemical synthesis technique for preparing gels/solids from solution. As precursors, commonly inorganic metal salts

5. CONCLUSIONS The process for synthesis of Sc2O3 microspheres having particle size ranging from 0.5−1.2 mm was successfully developed and optimized exploiting the internal gelation process (IGP). The synthesized and calcined microspheres were found to be mechanically and structurally strong, have lower tendency for H


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surface erosion, and can be used repeatedly. The successful use of a mixture of polypropylene and submicron iron particles or polymer paint to coat neutron activated 46Sc2O3 microspheres for adjusting the density of the microspheres to about 2.5 g cm−3 amenable for a RPT study has been amply demonstrated. The utility of the radioactive 46Sc2O3 microspheres for tracking solids (glass beads) in a RPT study has been prolifically demonstrated. The developed process for synthesis of 46Sc2O3 microspheres was found to be facile, robust, efficient, costeffective, easily up-scalable, uses a simple apparatus, and would serve a good steadfast method for ensuring routine availability of radioactive microspheres for RPT experiments. The encouraging results, as documented in this communication, can provide a platform for the preparation of solid radioactive tracers using other radionuclide for RPT.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +91-22-25505151. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Research at the Bhabha Atomic Research Centre is part of the ongoing activities of the Department of Atomic Energy, India and is fully supported by government funding. The authors wish to express their sincere gratitude to Dr. K. L. Ramakumar, Director, Radiochemistry and Isotope Group, for the keen interest and providing necessary support during this study. The authors would like to thank Dr. K. T. Pillai of Fuel Chemistry Division, Bhabha Atomic Research Centre (BARC), Mumbai for the help and support in conducting the experiments for preparation of the microspheres. The authors also acknowledge the help extended by Dr. J. G. Shah of Process Development Division, BARC, Mumbai and D. B. Kulkarni of Radiological Physics and Advisory Division, BARC, Mumbai for measurement of SEM images and radioactivity measurements of the microspheres, respectively.

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REFERENCES

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