Synthesis of UO2F2 Nanoparticles in a Tubular Aerosol Reactor

Each experiment started with a clean mesh filter disk. ... Figure 3 (a) Photographic image of the aerosol reactor assembly and safety enclosure (horiz...
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2020

Ind. Eng. Chem. Res. 2007, 46, 2020-2033

Synthesis of UO2F2 Nanoparticles in a Tubular Aerosol Reactor: Reactor Design and Experimental Investigations Ruozhou Hou,* Tariq Mahmud, Nikolas Prodromidis, Kevin J. Roberts, and Richard A. Williams Institute of Particle Science and Engineering, School of Process, EnVironmental and Materials Engineering, UniVersity of Leeds, Leeds, LS2 9JT, United Kingdom

David T. Goddard and Terry Semeraz Nexia Solutions Ltd., Springfields, Salwick, Preston, Lancashire, PR4 0XJ, United Kingdom

The gas-phase synthesis of uranyl fluoride (UO2F2) particles via the hydrolysis of uranium hexafluoride (UF6) has been investigated in a purposely designed laboratory-scale aerosol reactor. An in situ particle sampling technique was developed for collecting particle samples from a chemically aggressive and safety critical environment. The effects of two key process variablessnamely, UF6 concentration and the molar ratio of steam to UF6son the particulate product properties were investigated. Image analysis of environmental scanning electron microscopy (ESEM) images of sampled particulates indicated an increase in the mean single particle size with residence time. The number and size of agglomerates also increased as a function of residence time, suggesting the presence of both surface reaction and coagulation growth mechanisms. A notable transition of the UO2F2 particles from a spherical morphology to a platelet morphology was observed when the UF6 concentration and the steam:UF6 ratio reached values of 1.3 × 10-3 mol/L and 10, respectively. Introduction Gas-phase synthesis has proven to be one of the most successful and efficient manufacturing routes for nanosized particles. The process has been routinely used over several decades for industrial-scale production of nanoparticulate commodities such as carbon black, fumed silica, pigmentary titania, and nuclear fuels.1 Gas-phase synthesis has also been extended to the synthesis of a variety of advanced speciality materials, including, for example, optical waveguides,2 high-temperature superconductors,3 and electronic substrates.4 These products have been shown to display highly desirable characteristics, such as superparamagnetic behavior,5 superplasticity,6 high fracture toughness, and ductility.7 Several studies concerning the evolution of particle size from gas-phase synthesis in aerosol reactors have been reported in the literature.1,8-10 It is generally agreed that the initial growth of aerosol particles starts with the formation of monomers, as a result of chemical reactions from gaseous reactants or precursors that grow via monomer addition into clusters of some minimum stable size. Further particle growth may occur through one of two different processes: the nucleation-driven formation process, in which particles grow via surface reaction and coagulation, or the coagulation-driven process, in which the particle growth is mainly governed by interparticle Brownian collisions. Various models have been proposed to account for the particle growth via different mechanisms, and reasonable agreement has been achieved between experimental observations and model predictions over specific stages during the history of the particle formation process.11-13 Although most aerosol dynamics models are capable of simulating the entire particle growth regime, ranging from monomers to nanosized products, satisfactory validation of the model predictions for the early stage of particle growth has never been obtained, because of the challenging * To whom correspondence should be addressed. Tel.: 0044 113 3432434. E-mail: [email protected].

experimental difficulties associated with removing and analyzing representative samples. In addition, very little is understood in regard to the evolution of particle characteristics, such as morphology, porosity, and chemical composition homogeneity, in relation to variations of process conditions. In the nuclear fuel industry, some manufacturers use gasphase synthesis to produce quality uranium dioxide (UO2) powders for nuclear fuels via the single-stage integrated dry route (IDR) process.14 In this process, enriched uranium hexafluoride (UF6) gas is first hydrolyzed with steam in an aerosol reactor section of the single vessel to produce nanosized uranyl fluoride (UO2F2) particles. This is followed by defluorination, in which the UO2F2 particles are reduced in a rotary kiln by a counter-current mixture of H2 and steam to produce UO2 powder: 200-500 °C

UF6 + 2H2O 98 UO2F2 + 4HF 600-800 °C

UO2F2 + H2 98 UO2 + 2HF

(1) (2)

Although UO2F2 is only an intermediate in an integrated process leading to the production of UO2 nuclear fuels, its particle size and morphology are thought to have a significant role in dictating the powder handling characteristics of the UO2 product, and the compaction and mechanical properties of the subsequently fabricated UO2 ceramic pellets. UO2F2 particles are not routinely sampled. However, the subsequent UO2 particles are known to have two morphological forms, labeled “spheres” and “platelets”, as shown in Figure 1. It is considered likely that the morphology is determined upon formation of the UO2F2 particles, rather than being a consequence of the defluorination stage, and, indeed, one of the objectives of the present work is to demonstrate that this is the case. Reported investigations into UF6 hydrolysis kinetics are scarce. Kessie15 investigated the hydrolysis rate of gaseous UF6 in a packed bed at temperature from 134 °C to 212 °C, using nitrogen gas as the carrier. It was shown, through experimental

10.1021/ie061289h CCC: $37.00 © 2007 American Chemical Society Published on Web 03/07/2007

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Figure 1. Scanning electron microscopy (SEM) photographs showing different forms of UO2 particle morphology observed in the commercial production line: (a) platelets and (b) spheres.

data fitting, that the overall reaction rate could be described by the following equation:

R)

KpUF6 pH2O 1 + k1pUF6 + k2 pHF

(3)

where R is the reaction rate (in units of mg U/(h cm2)), K is the rate constant, p is the partial pressure of the subscripted component at the surface (in units of mm Hg), and k1 and k2 are adsorption constants. Using x to denote the fraction of UF6 that is unreacted, then 0 pUF6(g) ) xpUF 6(g) 0 pH2O ) pH0 2O - 2pUF (1 - x) 6 0 pHF ) 4pUF (1 - x) 6

in which p0 is the partial pressure of the subscripted component at the inlet (in units of mm Hg). Klimov et al.,16 on the other hand, suggested that the hydrolysis of UF6 at atmospheric pressure in moist air with an excess of H2O (5-10 fold) was consecutive and proceeded in two stages: k1

UF6 + H2O 98 UOF4 + 2HF k2

UOF4 + H2O 98 UO2F2 + 2HF

(4) (5)

The first reaction was the limiting stage, having a rate constant of (4 ( 4) × 10-18 cm3/s. However, no further details were revealed. No reported work has been found in the public domain on UO2F2 particle size and morphology evolution under varying process conditions. Data accumulated thus far from the commercial production lines are also rather fragmented, because a collection of in situ process information is often hampered by the rapid chemical reaction progress and hostile environment in the system. This lack of understanding has largely limited the process development for better quality control and higher yield production of the UO2 powder. This paper reports some novel experimental work undertaken on the gas-phase synthesis of UO2F2 nanoparticles using a purpose-built laboratory-scale aerosol reactor. The general reactor design principles and implementation of the particle sampling facilities to enable collection of in situ particle growth

information are discussed. Preliminary data on the evolution of both UO2F2 crystal size and morphology is examined under varying UF6 concentrations and steam:UF6 ratios, to advance further our understanding of the gas-phase synthesis process and nanoparticulate growth mechanism in a closed aerosol system. There have been very few publications of work in such challenging experimental systems involving both chemically aggressive environments and radioactive materials. Design of Laboratory-Scale Aerosol Reactor Although there are many publications available in the literature on the use of aerosol reactors for various nanoparticle product synthesis,17,18 little work has been dedicated to the development of generally applicable scale-up/scale-down laws for different aerosol systems. Sadakata et al.17 identified the reactor temperature, pressure, initial precursor concentrations, together with the Reynolds number and the ratio of nozzle diameter to nozzle outlet velocity, as being the most influential factors affecting the aerosol reactor performance. Based on these criteria, for an aerosol reactor with no recirculating flow to have a high yield of monosized ultrafine particles, and for the particles to pass through the reactor without adhering to the walls, the following conditions had to be met:

td > t r > t p > t m

(6)

where td, tr, tp, and tm denote the diffusion time, residence time, particle production time, and mixing time, respectively. For a jet-mixing-type reactor, these characteristic times can be calculated from the following equations:17

rr2 td ) Dp

(7)

where Dp is the particle diffusion coefficient and rr is the reactor radius;

tr )

Vr Vtotal

(8)

where Vr is the reactor volume and Vtotal is the total gas volumetric feed rate;

tp )

Cp Ra

(9)

where Cp is the product particle concentration when the reaction

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Figure 2. Schematic diagram of the aerosol reactor (top) and multiple concentric nozzle (bottom).

is completed and Ra is the average reaction rate for the gas reactant converting to the product particles; and

()

tm ≈ 103

dj Vj

(10)

where dj is the inner diameter of the jet, and Vj is the jet gas velocity at the outlet. In our system, however, a direct scale-down design of the reactor from the commercial production conditions, following the recommended principles of Sadakata et al.,17 could not be applied, because this would result in a protocol reactor with an unrealistic size of only 2 mm in diameter. There were also several specific challenges to be addressed in the design process: (1) UF6 is both radioactive and chemotoxic, and the HF byproduct in the reaction is chemically aggressive, which can cause penetrative flesh burns to the human body. This required the design to give the reactor maximum flexibility and built-in redundancy, because after the reactor became contaminated during use, significant decontamination would need to be performed before any modifications could be made. (2) The hazardous nature of both the reactants and products also meant that the reaction had to occur in a closed vessel surrounded by a well-ventilated external enclosure, to minimize manual interaction and human exposure. (3) UO2F2 is both water-soluble and hygroscopic. Thus, all the gas streams, the reactor system, and the sampling devices had to be trace-heated, to prevent steam condensation and dissolution of the formed UO2F2 particles. (4) At elevated temperatures, the overall reaction rate in an aerosol system has a tendency to be controlled by the reactant mixing rate, because the intrinsic chemical reaction rate is almost instantaneous.18 This meant that the designed reactor length had to be long enough to give the gas streams sufficient residence time for complete mixing of the reactants, and, hence, completion of the reaction (tr > tm). (5) The reactor would preferably operate under laminar flow conditions. This allowed us to start with a simpler, better-defined system and increase the degree of complexity when we had a

better understanding of the system. Moreover, in the laminar flow regime, particles inside the reactor had a much longer radial diffusion time. This helped to eliminate the wall deposition (via td > tr), allowing all the particles produced to form in the growth processes and maintain a mass balance across all sections of the reactor. (6) Chemical compatibility of the construction materials with UF6/H2O/HF at elevated temperatures was required. Considerations of the above requirements led to the design of a tubular reactor, as shown schematically in Figure 2. At the maximum design (UF6 mass flow rate of 250 g/h, and a steam: UF6 molar ratio of 3), the reactor featured a residence time of tr ) 32.2 s, a mixing time of tm ) 10.5 s, and a diffusion time of td ) 31 208 s. The calculations were based on assumptions that the gas viscosity was taken to be 0.015 × 10-3 kg/(m s), and the primary particle size would be 0.02 µm, which gave particle diffusion coefficient of Dp ) 1.34 × 10-8 m2/s under the laminar flow conditions.9 The feed to the reactor was implemented via a multiple concentric nozzle, which allowed up to five different concentric reactants/shielding gases to be introduced simultaneously. The reactor body consisted of two identical Monel tube sections, each of which was 0.5 m long, with an inner diameter (ID) of 40.9 mm and an outer diameter (OD) of 48.26 mm. The two sections could be either bolted together or used individually. Along each section, five 6.35-mm-diameter stubs were welded to the wall. They were used for the insertion of probes, either for temperature measurement or particle sampling. The exit of the reactor was fitted with a product collection pot, which contained a 15 µm pore size Monel mesh filter disk. Each experiment started with a clean mesh filter disk. Particles produced inside the reactor were captured by the filter, either as agglomerates or through thermophoretic deposition, and, at the end of each run, after the reactor was cooled, they were collected from the pot for further analysis. The off-gases passed through three stages of scrubbing (two stages of self-indicating soda lime granules and one stage of sodium hydroxide (NaOH) solution) to neutralize the HF gas and any unreacted UF6 vapor. The pres-

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Figure 3. (a) Photographic image of the aerosol reactor assembly and safety enclosure (horizontal scale of 3.6 m), and (b) schematic flow diagram of the experimental setup.

sure drop across the filter was closely monitored during each experiment. A vacuum pump was incorporated at the end of the off-gas exit, to control reactor pressure between 0 mbar and 300 mbar. Reactor heating was implemented through the use of many electrical heating tapes, which were wrapped around the gas feed lines, reactor body, sampling devices, and the particle collection pot. Thermocouples were distributed throughout the entire reactor system, to control the reactor external wall temperature and monitor the internal gas temperature. Experimental Section All the experiments were performed on-site at Nexia Solutions, Ltd. (Springfields, Preston, UK). Figures 3a and b show a photographic image of the reactor assembly and a schematic diagram of the experimental setup, respectively. UF6 gas (with a purity of g99.5%) and steam, preheated to 125 and 200 °C, respectively, were fed to the multiple concentric nozzle. The UF6 heating temperature was determined through several active trial runs, which showed that, at 125 °C, the UF6 feeding rate could be steadily controlled and there was no sign of steam and HF condensations inside the reactor, whereas the steam temperature was set based on the plant process conditions. Feeding UF6 at a much lower temperature than that of steam

would also bring additional benefits to further boosting the reactor yield, as the subsequently induced thermophoresis could lead to enhanced mixing between UF6 and steam. UF6 vapor was generated by vaporizing liquid UF6 from a sample pot in a 2 kW recirculating air oven and then heating the vapor to the required temperature. The UF6 feed rate was controlled by a needle valve and metered using a loss-in-weight feed system. Steam was generated by injecting deionized water dispensed from a syringe pump (Razel A99FJZ, Fisher Scientific) into an electrically heated superheater. The syringe pump also controlled and measured the steam feeding rate. A pressure relief valve with a relief setting of 300 mbar was also installed in the steam feeding line to prevent back-feeding due to reactor pressure buildup. Nitrogen shielding gas, also preheated to 200 °C, was introduced between the UF6/steam reactant streams and along the reactor wall, to reduce the particle deposition on both the nozzle exit and the reactor wall. In the current investigations, the reactor wall heating temperature was set at 200 °C, which allowed the reactor internal temperature to be stabilized at ∼185 °C when steady state was reached. Figures 4a and b show the wall temperature profile and a representative reactor internal profile, respectively, in a typical experimental run. Particle Sampling. In situ sampling provided important information on the particle growth history and morphology changes. One requirement of sampling from reacting aerosol processes is that samples are taken rapidly from defined positions in the system, with a minimum disturbance to the flow field, so that both surface growth and agglomerate formation on the samples can be minimized. Thermophoretic sampling has been widely used as a sampling technique, especially in open flame aerosol synthesis systems.19,20 In this technique, a probe at room temperature is inserted rapidly into the flame and the ensuing temperature gradient causes particles to migrate and adhere to the cool surface of the probe. Unfortunately, such a technique could not be readily adopted in the current system, because the toxic and erosive nature of the materials involved forced the reactions to be conducted in a sealed environment. Moreover, introducing a cool surface in a steam and HF environment could have led to the dissolution and/or hydration of any deposited particles. A new in situ particle sampling system was designed for use in any closed aerosol reactor system. Figure 5 illustrates a schematic diagram of the system. It consisted of a standard ball valve (to open and close access to the reactor), a union cross (to allow nitrogen purging of the sample), and a compression fitting in which the metal ferrules were replaced by polytetrafluoroethylene (PTFE) (to enable the sliding movement of the sampling rod while the system remained sealed). For the current investigation, in situ particle sampling was performed by manual insertion of the preheated rod (trace-heated to 160 °C) into the reactor through the sample valves. An adhesive aluminum foil was attached to the tip of the sampling rod to capture the particle sample. This allowed the sample to be transferred for environmental scanning electron microscopy (ESEM) examination with minimum disturbance. The reactor was equipped with potentially six in situ sampling ports to capture growing particles from the reaction zone (see Figure 2), of which three were routinely used, with the others providing spare capacity. Particle deposition, as a function of sampling time, was investigated to ensure that excessive particle coverage did not mask any primary particles, and that the sampling time was short enough to minimize surface growth on the sampling stub. Environmental Scanning Electron Microscopy (ESEM) and X-ray Diffraction (XRD) Analysis. The captured particles

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Figure 4. Reactor temperature profiles in a typical experimental run: (a) reactor wall temperature profile and (b) internal temperature profile. (Time t ) 0 refers to the moment when UF6 vapor started to be introduced into the preheated reactor.)

Figure 5. In situ particle sampling assembly.

were examined using an FEI-Electroscan 2020 environmental scanning electron microscopy (ESEM) system operating at 20 kV. The ESEM system uses a gaseous secondary electron

detector with a LaB6 filament and has an optimum resolution of ∼5 nm. Samples typically were sputter-coated with gold at a current of 10 mA for 2 min, because this was determined to

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Figure 6. Typical XRD analysis result of the UO2F2 particle sample collected from the particle collection pot. Table 1. Experimental Conditions UF6 Feed Rate run 1 2 3 4

(mol/min) 10-3

1.42 × 1.42 × 10-3 5.11 × 10-3 5.11 × 10-3

(L/min)a 0.044 0.044 0.16 0.16

Steam (H2O) Feed Rate N2 feed rate

(L/min)b

0.5 + 3.4 0.5 + 2.9 0.5 + 2.8 0.5 + 1.4

(mol/min) 10-3

4.02 × 1.5 × 10-2 1.5 × 10-2 5.1 × 10-2

(L/min)a

UF6 concentration (mol/L)

steam (H2O):UF6 molar ratio

0.16 0.57 0.57 1.97

3.5 × 10-4 mol/L 3.5 × 10-4 mol/L 1.3 × 10-3 mol/L 1.3 × 10-3 mol/L

2.8 10.6 2.9 10.0

a If treated as ideal gases. b Data presented as the sum of two figures. The first figure represents the shielding gas feed rate between the UF and steam 6 streams, whereas the second figure represents the wall shielding gas feed rate.

improve the edge definition of the particles, which, in turn, made feature extraction using image analysis more reliable. Unlike conventional instruments, ESEM microscopes do not require samples to be coated with a conducting layer prior to imaging. By imaging samples before and after coating, it was possible to demonstrate conclusively that the coating procedure did not generate any artifacts in the images. For the analysis of particles, at least 10 fields of view at a magnification of 10000× were selected at random positions across each sample. Image processing was undertaken using Image Pro Plus software (Media Cybernetics, Inc., USA) on many of these images, using the segmentation function to extract the particles from the image background. Particles were then visually inspected and classed as either single particles or agglomerates. A classification was also made for each particle and agglomerate, in regard to its structure and whether it exhibited a spherical or platelet morphology. At least 40 particles were analyzed per sample using this procedure. Although this analysis was not expected to provide a rigorous evaluation of particle development, it was expected to show general trends in behavior. Some of the products captured on the Monel mesh filter at the reactor exit were also analyzed by XRD using a Philips X’pert MPD X-ray diffractometer to determine chemical structure. A typical spectrum (Figure 6) revealed that both anhydrous UO2F2 and a β-UO2F2‚1.5H2O hydrated phase were produced. Other spectra (not shown) also indicated the presence of the R-UO2F2‚1.5H2O phase alone. The UO2F2/HF/H2O system is complex, as described in the early work of Brooks et al.,21 with hydration, dehydration, and hydrofluorination of the uranyl fluoride all possible, not to mention the observation of polymorphism in at least one of the hydrates. With the current

reactor design, it was not feasible to collect XRD patterns of the material as formed in the reactor. However, the evidence of the ex situ data is consistent with the formation of the anhydrous uranyl fluoride in the reactor with subsequent conversion to hydrated phases upon exposure to the atmosphere, with the exact mechanism of hydration/dehydration likely to be determined by factors such as the relative humidity, the time of storage, and the depth of the powder bed that is produced. Results The experiments focused on the investigation of the effects of the two key process variablessUF6 concentration and the steam:UF6 molar ratioson the particulate product properties. Two levels of UF6 concentration (3.5 × 10-4 and 1.3 × 10-3 mol/L) and two levels of steam:UF6 molar ratio (2.8 and 10) were selected. The experimental conditions are listed in Table 1. Throughout the investigations, the flow rate of shielding gas (N2) between the UF6 and steam streams was not varied. The total feed rate of all the gas components was kept constant by adjustment of the wall shielding gas throughput. This meant that particle samples removed from the first, second, and third sampling ports had a residence time of 1.0, 4.9, and 14.6 s, respectively, regardless of the run conditions. Note that, in the current system, there was a gradual buildup of the filter cake inside the reactor. However, because the pressure increases inside the reactor had been controlled to