Journal of Nuclear and Radiochemical Sciences, Vol. 6, No. 2, pp. 127-130, 2005

ArticlesSupercritical Fluid Extraction of Silicone Oil from Uranate Microspheres Prepared by
Sol-gel Process
R. Kumar, R. Venkatakrishnan, N. Sivaraman, T. G. Srinivasan, and P. R. Vasudeva Rao*
Fuel Chemistry Division, Chemistry GroupIndira Gandhi Centre for Atomic Research, Kalpakkam - 603 102, India
Received: December 14, 2004; In Final Form: May 3, 2005
Supercritical fluid extraction of silicone oil from urania microspheres prepared through sol-gel route was investigated. The influence of pressure, temperature, and flow rate on the extraction efficiency was studied. Experimental
conditions were optimised for the complete removal of silicone oil from urania microspheres.

1. Introduction
Supercritical fluids provide faster, cleaner, and efficient
extractions. The low viscosity, high density, low surface tension,and better diffusive properties help supercritical fluids to easily
penetrate through the pores of a sample matrix and extract thecomponents of interest efficiently.1−3 Supercritical fluid extraction (SFE) results in little or no waste. Carbon dioxide as supercritical fluid is non-toxic and inexpensive, making it an attractive
candidate as a solvent for extraction. The extraction power ofsupercritical carbon dioxide (SC-CO2) depends on its density
which can be altered by varying the pressure/temperature.
Conventionally used techniques for the removal/recovery ofmaterials from different waste matrices e.g. from nuclear waste,
generate relatively large amount of secondary liquid waste withproportional consumption of solvents. But the supercriticalfluid extraction technique enables the same task with minimum
or no waste since the fluid escapes as a gas after extraction, thus
reducing the burden of secondary waste generation considerably. This is obviously an attractive feature especially in nuclear
industry where the main concern is accumulation of radioactive
waste. In addition, the SFE technique obviates in many cases
the cumbersome pre-treatment of waste matrix. Application of
SFE technique for radioactive materials using modified SC-CO2
demonstrated the feasibility of efficient extraction.4−10 The SFEof uranium from tissue paper waste was also demonstrated fromour laboratory.11−12 This paper presents an important application
related to the removal of silicone oil from urania microspheresproduced by sol-gel process wherein silicone oil is employed as
the gelation medium.
Sol-gel process for the production of nuclear material offers
the advantages such as amenability for remotisation and elimination of powder handling leading to less man rem problems.
Hence, it is being considered for the development of future fuelcycles by international initiatives such as INPRO by IAEA.13−14
Internal gelation process is one of the important sol-gel routes
for the preparation of microspheres of nuclear fuel materials inwhich in-situ homogeneous release of ammonia by decomposition of hexamethylenetetramine (HMTA) is used for the conversion of solution droplets into hydrous gel particles.15 In thisprocess, silicone oil is used as the gelation medium. After thecompletion of gelation, the silicone oil is removed from themicro-spheres by washing with carbon tetrachloride. This leads
to a generation of large amount of radioactive liquid waste. To
obviate this disadvantage, extraction of silicone oil using SC*Corresponding author. E-mail: vasu@igcar.ernet.in. FAX: +914114-280065.

CO2 was investigated.
The extraction of silicone oil was initially studied using SCCO2 as a function of its pressure, temperature, and flow rate.
Subsequently, silicone oil present in urania microspheres was
extracted. The influence of various SFE parameters was investigated and the results are discussed.
2. Experimental
Preparation of microspheres. The urania gel preparation
was carried out in an assembly described elsewhere.15 The broth
was prepared by mixing desired quantities of acid-deficienturanyl nitrate and HMTA-urea mixture pre-cooled to –2˚C andcharging into the feed tank. The broth was fed into the gelation
column in the form of droplets. The droplets slowly traveled
down into the wider limb of the column with the hot silicone oil
flowing upwards. As the droplets reached the bottom of thecolumn, the oil flow carried them up to the narrow limb. The
gelled droplets were transferred to the oil-separator assembly forseparating the silicone oil.
Removal of silicone oil. The gelled spheres were separated
into two equal lots. The first lot of the spheres was washed by
the conventional method, i.e. initial wash with carbon tetrachloride to remove oil and with 5% NH4OH solution to remove
ammonium nitrate and residual gelation agents. During eachbatch of ammonia wash, 10 minute period was given for eachrinse with 500 mL of 5% NH4OH, the electrical conductanceof the wash solution was measured using conductivity meter.
The washing was continued until the conductivity of the wash
solution reduces to the background values of 5% NH4OH solution. The second lot of the spheres was subjected to SFE using
SC-CO2.
SFE system. The SFE system (M/S Jasco, Japan) used in
the present studies consisted of a solvent delivery system, aconstant temperature oven (± 0.1˚C) and a back pressure regulator. Extraction vessels (EV) of 10 mL and 50 mL capacity
were employed for the studies. Carbon dioxide used in the
SFE was of supercritical grade (99.99%).
Silicone oil collection and assay. The extracted silicone oil
was collected either in an empty flask or in a flask containing
small quantity of n-heptane. It was assayed by weight and alsousing IR spectroscopic technique (with BOMEM FTIR MB
100 spectrophotometer) when present in smaller quantities(ppm levels). After the extraction, the spheres were shaken
with known volume of n-heptane, and an aliquot was used forthe IR analysis. Calibration graph for the IR determination
was prepared for the range 5 to 75 ppm with a detection limitof 1 ppm.

© 2005 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 11/07/2005

Kumar128 J. Nucl. Radiochem. Sci., Vol. 6, No. 2, 2005

3. Results and Discussion
Extraction of silicone oil. The results on the extraction ofsilicone oil (5 g) with no matrix present in the 10 mL extraction
vessel for various operating pressures using SC-CO2 are shownin Figure 1. A SC-CO2 flow rate of 5 mL/min was employed.
Complete extraction was observed in about 20 min when thepressure was about 300 bar. It was about 90% at 250 bar for asimilar extraction period. The extraction kinetics was, however,found to be slow at a lower pressure of 100 bar, and about 45%
extraction only for a period of 90 min. Increase in pressure
leads to higher CO2 density resulting in better solubilisation ofsilicone oil, thus enhancing the extraction kinetics. Accordingly
higher pressures are required for faster and complete extractionof silicone oil.
When the same experiment was carried out using 50 mL
extraction vessel under similar conditions for the same quantity of oil, extraction was found to be slower and about 100
min extraction period was required for complete removal ofsilicone oil at a pressure of 250 bar and a flow rate of 5
mL/min with a 50 mL extraction vessel (Figure 2). However,
the trend in extraction behavior with pressure variation
remains the same, as in the case of 10 mL extraction vessel.
Extraction profiles of 5 g of silicone oil both in 10 and 50
mL capacity extraction vessels were compared. The 10 min
extraction with the flow rate of 5 mL/min in 10 mL vessel (fivechanges of fluid) caused complete extraction whereas the 10
min extraction with the same flow rate in 50 mL extraction
vessel (one change of fluid) effected only about 20% extraction.
Even though in both the cases, the total amount of CO 2 wasmaintained same i.e. 50 mL in 10 min, it had 5 incremental

100

80
250 bar, 5 mL/min
250 bar, 2 mL/min
200 bar, 2 mL/min
150 bar, 2 mL/min
100 bar, 2 mL/min

60

40

20

0
0

100

200

300

400

500

600

700

800

Extraction period/ minFigure 2. Extraction of silicone oil using SFE from a larger capacity
vessel. Experimental conditions: SC-CO2 P: 100 – 250 bar, T: 308 K,
flow rate: 2 – 5 mL/min, EV: 50 mL.

100

silicone oil extraction (cumulative %)100% extraction (cumulative)

extractions in 10 mL EV but only 1 extraction in 50 mL EV in
the given 10 min. This observation agrees well with the established fact in the conventional solvent extraction technique thatthe higher incremental extractions always lead to better extraction efficiency. Hence it suggests that the minimum capacity
extraction vessel for the given amount of solute is always
preferred for the more efficient and economical supercriticalfluid extraction technique.
Better extraction behaviour was observed at lower temperatures i.e. 308 K compared to higher temperature i.e. 323 K(Figure 3). For example, complete extraction of about 1.3 g ofsilicone oil was observed at 308 K for an extraction period of
140 min. However, only 60% extraction was observed whenthe extraction temperature was kept at 323 K for the same
period. This behaviour is explained by the fact that the densityof SC-CO 2 is high only at lower temperatures and density
decreases with raise in temperature. Hence the solubility ofsilicone oil in SC-CO2 decreases with raise in temperature.
The extraction of silicone oil was also studied as a functionof SC-CO2 flow rate (Figure 4). Better extraction behaviour
was observed at higher flow rates. For example, at 100 bar
and 308 K, extraction of about 70% and 45% were observed

silicone oil extraction (cumulative %)SFE of silicone oil. The silicone oil and the same present inuranate micro spheres were subjected to SFE in 10 and 50 mL
extraction vessels. The extraction was studied as a function ofpressure in the range of 100 – 300 bar and in the temperature
range of 308 – 323 K. The influence of silicone oil content in
the microsphere, extraction vessel capacity and SC-CO2 flowrate was investigated.
Preparation of simulated samples of silicone oil in urania
microspheres. A batch of samples were prepared by equilibrating urania microspheres of about 5 – 10 g with 1 g of silicone oil in n-hexane followed by evaporation of the volatilesolvent. Another batch of samples were prepared by equilibrating about 10 g of urania microspheres with different
amounts of silicone oil in n-hexane followed by evaporation of
the volatile solvent, n-hexane.

300 bar
250 bar
100 bar

80

60

40

20

308 K
323 K
80

60

40

20

0

0
0

20

40
60
Extraction period/ min

80

100

Figure 1. Extraction of silicone oil (no matrix) using SFE. Experimental
conditions: SC-CO2 P: 100 – 300 bar, flow rate: 5 mL/min, T: 308 K,
EV: 10 mL, silicone oil content: 5 g.

0

50

100
150
200
Extraction period/ min

250

Figure 3. Influence of temperature on the extraction of silicone oil.
Experimental conditions: SC-CO2 P: 150 bar, flow rate: 2 mL/min,
EV: 10 mL, silicone oil taken: 1.29 g.

J. Nucl. Radiochem. Sci., Vol. 6, No. 2, 2005 129

Supercritical Fluid Extraction of Silicone Oil100

silicone oil extraction (cumulative %)silicone oil extraction (cumulative %)

100
80

60

40

2 mL/min
5 mL/min
20

0

80

60

250 bar
300 barSilicone oil extracted = 2 g from 12 g of spheres.

40

20

0
100
150
200
Extraction period/ min

250

300

Figure 4. Influence of SC-CO2 flow rate on the extraction efficiencyof silicone oil. Experimental conditions: SC-CO 2 P: 100 bar, T: 308
K, EV: 10 mL, silicone oil employed: 3.5 g.

for a flow rate of 5 and 2 mL/min, respectively. Thus higher
operating SC-CO2 flow rate is essential for faster removal ofsilicone oil. It is also illustrated (Figure 2) with a 50 mL
capacity extraction vessel.
Extraction of silicone oil from spheres. Initially the gelled
spheres were directly subjected to SFE extraction in the pressure range of 73 – 300 bar at 308 K. It was observed that thespheres in the extraction vessel crumbled during extraction.
Initially this was assumed to be due to the extraction of theresidual gelation agents, HMTA-urea along with the silicone
oil from the soft wet gels. Subsequently, the gelled spheres
were washed with 5% NH 4 OH solution and the complete
removal of gelating agents was ensured by measuring the electrical conductance of the wash solution. It was observed againthat the spheres got crumbled during the extraction despite
ammonia wash. Thus it is evident from these studies that the
wet gels are not suitable for extraction under the SFE conditions. The microspheres prepared are intended to be used forthe fabrication of sphere pac fuel pins for the fast reactors. The
crack or crumbling of spheres is undesirable as it will create
small particles or powders and thereby we lose one of the main
advantage of the sol-gel process i.e. elimination of powder
handling. Thus it is essential to maintain the integrity of spheres
during extraction.
Therefore the spheres were washed with NH4OH and thendried in an oven at 373 K for 12 hours and the dried spheres
were then subjected to SFE. The sphere integrity was now
observed to be intact under these experimental conditions. The
results of the experiment on the extraction of silicone oil frommicrospheres are shown in Figure 5. About 2 g of oil was
extracted completely in about 50 min at a pressure of 300 bar
from about 12 g of spheres. Silicone oil content after extraction was found to be as low as 7 ppm in a typical run where 12
g of sphere was loaded in 10 mL extraction vessel with 5
mL/min CO 2 pumping. The typical extraction period wasabout 60 min. About 200 g of spheres were subsequently
processed in three batches using 50 mL capacity vessel at a
SC-CO2 flow rate of 5 mL/min. Complete extraction of silicone oil from microspheres was ensured by IR spectroscopic
analysis (< 7 ppm). The extracted quantity of silicone oil fromthe microsphere was found to be about 16%.
The results on the influence of silicone oil content in the
microsphere (simulated samples) on the extraction efficiency
are presented in Figures 6 and 7. A small variation in the rateof extraction was observed. For example, silicone oil from
10% oil loaded sample was extracted at a relatively faster rate

0

20

40
60
Extraction period/ min80

Figure 5. Extraction of silicone oil from urania microspheres.
Experimental conditions: SC-CO2 flow rate: 5 mL/min, EV: 10 mL.

100% extraction (cumulative)

50

80

60
10 g sphere containing 1 g of oil (10% oil load)
8.12 g sphere containing 1 g of oil (12.3% oil load)
5.05 g sphere containing 1 g of oil (19.8% oil load)

40

Filled with dummy spheres wherever required andtotal weight used for SFE : 10 g

20

0
0

10

20

30

40

50

Extraction period/ minFigure 6. Influence of silicone oil content in urania microsphere.
Experimental conditions: SC-CO2 P: 250 bar, flow rate: 5 mL/min, T:
308 K, EV: 10 mL.

100% extraction (cumulative)

080

20% oil load
11% oil load
15% oil load

60

40

20

0
0

10

20
30
40
Extraction period/ min50

Figure 7. Extraction of silicone oil from urania microspheres. SC-CO2
P: 300 bar, flow rate: 3 mL/min, T: 308 K, EV: 10 mL, 10 g of spheres
employed.

130 J. Nucl. Radiochem. Sci., Vol. 6, No. 2, 2005
compared to one corresponding to 12.3 and 19.8% oil loaded
samples (Figure 6). However the difference in the rates ofextraction were not significant. Data in Figure 7 also indicatethat silicone oil from microsphere samples with lesser oil content
could be extracted at a faster rates compared to samples withhigher oil content.
These spheres were subsequently subjected to heat treatments
along with the spheres washed by conventional procedures i.e.
these spheres were initially heated to 573 K for 4 hours, reduced
under Ar + 8% H2 at 1073 K for 4 hours and sintered at 1473 K
for 4 hours.
The spheres obtained from SFE and the one preparedthrough conventional washing procedure were compared under
microscope and found to be crack free. The microscopic
image of the sintered microspheres, which underwent SFE, isshown in Figure 8. The density of the microspheres wasmeasured using helium pycnometer. The average density ofthe sintered spheres which were subjected to SFE extraction
was found to be 10.87 g/mL (99% TD) which is similar to theone obtained for the spheres which underwent CCl4 wash.
The SEM pictures under the same magnification of thesintered microspheres, prepared by washing under SFE andconventional methods are shown in Figures 9 and 10, respectively. The microstructure of both the spheres is flaky innature. As seen in the figure the individual grain and pore sizeof spheres washed by SFE method appears to be an order ofmagnitude higher than that washed under conventional
washing procedure. This may be due to extraction of certain
residual gelating reagents by supercritical CO2 after passing
through the pores of matrix which might increase the pore size.

Kumar

Figure 10. SEM of sintered microspheres wherein silicone oil has
been removed by washing under conventional method.

4. Conclusion
Silicone oil is extracted completely from uranate microspheres at 300 bar and 308 K. The microspheres after the
extraction were found to maintain the integrity and similar tothe conventionally washed spheres. They were also suitable
for subsequent steps in the preparation of UO2 spheres. The
supercritical fluid extraction is thus an excellent alternate
method with negligible generation of liquid waste for theremoval of silicone oil from microspheres produced throughsol-gel route.
Acknowledgement. Authors acknowledge Dr. K. S.
Viswanathan, Chemistry Group, IGCAR for his help in analysing
silicone oil using IR spectroscopy technique.
References

Figure 8. Microscopic image of urania microspheres after removal ofsilicone oil through SFE technique followed with heat treatment.

Figure 9. SEM of sintered microspheres subjected to SFE.

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