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IN-SITU CHELATION AND EXTRACTION OFHEAVY METAL OXIDES USING MODIFIEDSUPERCRITICAL CARBON DIOXIDE

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					BARC Newsletter                                                                                  Issue No. 249



IN-SITU CHELATION AND EXTRACTION OF
HEAVY METAL OXIDES USING MODIFIED
SUPERCRITICAL CARBON DIOXIDE
Tessy Vincent and P.K. Wattal
Nuclear Recycle Group
Bhabha Atomic Research Centre
and
Mamata Mukhopadhay
Department of Chemical Engineering, IIT Bombay

                                                  Abstract
      Conventional processes used in nuclear facilities, engaged in the extraction of heavy metals,
      generate considerable quantity of radioactive liquid wastes that requires further treatment. Use of
      SFE significantly reduces the waste volumes. It, additionally, avoids the dissolution step prior to
      extraction. A method of extracting metals directly from metal oxides by exposing the oxide to a SC
      solvent containing a chelating agent is described. Cerium oxide has been considered as a candidate
      material to carry out feasibility studies along with TTA as the chelating agent for extraction.
      Solubility of Ce (TTA)3-chelate has been investigated in the pressure range of 150-350 bar at 400C.
      A linear relationship between the solubility and SC CO2 density is observed. TG-DTA and IR spectra
      of the chelate and TTA are discussed. Feasibility of in-situ conversion of cerium oxide to Ce (TTA)3-
      chelate in SC CO2 with TTA and water followed by its recovery by SC CO2 extraction has been
      experimentally demonstrated. Both static as well as dynamic extraction studies were performed at
      350 bar and 500 C in a stainless steel extractor of 500 ml capacity without the use of nitric acid. The
      samples collected in n-dodecane were analysed for Ce by ICP-AES. ESR as well as IR spectra of the
      chelate collected during dynamic extraction are discussed. X-ray powder pattern of both Cerium
      oxide fed to the extractor and the residue after the extraction have been brought out. This study
      illustrates the potential of SC CO2 modified with organic ligands to process heavy metals directly
      from irradiated spent fuel.

Introduction                                                products. Spent fuel reprocessing flow diagram
                                                            is given in Fig. 1.


U
         ranium and plutonium are recovered
         from irradiated nuclear fuel through the           The Purex process generates considerable
         widely practiced Purex process [1].                volumes of several kinds of nuclear wastes
Fuel cladding encasing the fuel is removed either           which require different treatment steps before
chemically or mechanically and the fuel is                  the liquid wastes could be released to the
dissolved in nitric acid. Pu and U are co-                  environment safely. The use of SFE in place of
extracted using TBP/n-dodecane leaving the                  Purex would significantly         reduce the
fission products in the raffinate stream. The               reprocessing cost by      improving the waste
loaded organic is contacted with the reductant to           management and process simplification [2].
separate Pu from U. Finally U is stripped into              The unique ability to fine tune the solvent
dilute nitric acid solution from TBP. Additional            properties by adjusting its temperature and
extraction strip cycles are performed with the              pressure and the excellent transport properties
separated uranium and plutonium streams in                  makes SCF a favourable solvent for extraction.
order to complete the purification from fission             Since CO2 is inert, odourless, tasteless,


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BARC Newsletter                                                                            Issue No. 249




                                  Fig. 1 Block diagram of purex process


environment–friendly, inexpensive and is easily         Ce is also high compared to other lanthanides in
available, it has been regarded as a safe and           the spent nuclear fuel, Ce was selected as
viable SCF solvent. Further, radiation stability of     candidate metal.
SC CO2 is high. Since solubility of metal oxides
in SC CO2 is poor, it needs to be associated with       Experimental
organic ligands in order to enhance the capacity
of SC CO2 for metal extraction. It has been             Solubility studies
reported that β-diketones, can be used as the           SFE depends on number of factors including i)
chelating agents for the extraction of f-block          stability and solubility of ligand, ii) solubility of
elements using SCF[3].                                  the metal–chelate iii) water and pH iv)
                                                        temperature and pressure v) chemical form of
The objective of the work is to explore the
                                                        the metal species and vi) matrix. Since the
potential of SC CO2 for the extraction of
                                                        solubility of the metal–chelate provides the limit
valuable heavy metal ions from the spent fuel
                                                        of extractability, solubility studies were
without dissolution step For the present study
                                                        performed prior to extraction studies.
TTA which is a solid at atmospheric conditions
has been selected as the chelating agent. Use of        For experimental measurements of the neat
TTA leads to better safety control, absence of          solubility in SC CO2, TTA (99.0% purity,
phosphorous aerosols and lesser corrosion as            Aldrich make) and Ce (NO3)3.6H20 (99.9%
compared to HNO3/TBPsystem. Since the                   purity, M/s. Indian Rare Earths) were used. Ce
chemistry of Pu and Ce is similar and yield of          (TTA)3-chelate was prepared by dissolving TTA


Founder’s Day Special Issue                                                                               58
BARC Newsletter                                                                       Issue No. 249


in acetone followed by addition of required
quantity of cerium nitrate. Upon evaporation, the
solution was made into solid Ce (TTA)3-chelate.
The chelate was subjected to DTA/TG in
nitrogen atmosphere at a heating rate of 20C/min
from room temperature to 2000C and is given in
Fig. 2. Thermographs show that the chelate starts
melting at 480C and then volatilizing. The
chelate was found to be stable till 1180C without
any decomposition as seen from DSC curve
shown in Fig. 3.                                            Fig. 4: TG-DTA of chelating agent TTA




                                                          Fig. 5 : IR Spectra of prepared chelate
Fig. 2: TG-DTA of prepared Ce-TTA chelate




                                                                 Fig. 6 : IR Spectra of TTA


  Fig. 3: DSC Analysis of prepared Ce- TTA          The spectra indicate the presence of free TTA
          chelate                                   and water in the prepared chelate.

IR spectra of TTA as well as prepared               A schematic diagram of the experimental setup
chelate are represented in Fig. 5 and Fig. 6        is given in Fig. 7. The internal capacity of the
                                                    stainless steel extractor was 500 ml. Before
respectively.
                                                    starting the studies, each part of the system was


Founder’s Day Special Issue                                                                         59
BARC Newsletter                                                                                                         Issue No. 249


                                                     Table 1: Solubility of Ce (TTA)3 - chelate in
                                                              supercritical CO2 at 40oC
                                                                          Pressure                  Solubility of
                                                                           (bar)                    Ce (TTA)3 - chelate
                                                                                                    (ppm)
                                                                               180                             57
                                                                               250                             69
                                                                               350                             79

                                                     Solubility data has been plotted against sc-CO2
                                                     density. A linear relationship is observed in
                                                     Fig. 8.

     Fig. 7: Schematic of experimental set up                             85



                                                                          80
cleaned with acetone and lines were flushed with
liquid CO2. About 40g of the prepared chelate           Solubility(ppm)   75

was charged into the extractor and the remaining
                                                                          70
portion was packed with glass beads. Stainless
steel filters were used to prevent any carryover                          65

of the solute. Set point of the pressure was kept
                                                                          60
at 180bar. After closing the metering valve
completely, the chelate in the cell was                                   55
pressurized to the liquid CO2 pressure. When the                            780   800   820   840   860   880   900   920   940
                                                                                                                            3
                                                                                          Density of Supercritical CO2(kg/m )
                                                                                                                                  960   980


pressure indicated was 58.5 bar, corresponding
to the CO2 cylinder pressure, air driven pump           Fig. 8: Chelate solubility v/s SC CO2 density
was put on to reach the pressure to the set value.
The pressure was maintained within ± 20bar
through an on/off control system.            Skin    Extraction Studies
temperature was maintained around 44-45oC and
was monitored with temperature indicator. The        The same experimental set up given in Fig. 7
whole system was kept under static conditions        was used. Bottom portion of the extractor was
for 2 hrs. In order to prevent plugging, the         packed with glass beads. CeO2, TTA and
restrictor was warmed electrically. Then the         distilled water were made into paste and added
metering valve was opened slowly to release          to the extraction vessel. Based on the solubility
1/3rd volume of the extractor. The chelate was       studies, pressure and temperature conditions for
collected in dodecane. Solubility studies were       the extraction studies were chosen as 350 bar
also conducted for 250 & 350 bar at 400C in the      and 50oC. The entire system was kept in static
same manner. After that the whole system was         condition for 45 minutes to reach equilibrium at
depressurized from 350bar to atom-spheric            350bar and 500C. Dynamic extraction was
condition.                                           achieved by slowly opening the metering valve
                                                     and continued for 1 ½ hours by maintaining the
The samples collected in n- doecane were             same temperature and pressure values with
analysed for Ce by ICP-AES. The solubility data      average CO2 flow rate of 6.35lpm. Flow rate was
is given in Table 1.                                 controlled manually to achieve smooth flow of


Founder’s Day Special Issue                                                                                                                   60
BARC Newsletter                                                                      Issue No. 249


the chelate. The chelate was collected in
dodecane through a restrictor. After that the
whole system was depressurized from 350 bar to
atmospheric conditions. The collected chelate in
dodecane was analysed for cerium by ICP-AES
and results are given in Table 2.
Table 2: Recovery of cerium from cerium
         oxide (350 bar, 500C)
 CeO2       TTA      Water            %
 (g)        (g)      added (ml)       extraction
    1         10           10              1.25

%extraction =
                                                      Fig. 10: ESR spectra of Cerium in Ce-TTA
      concn. of metal in SC phase                              chelate collected in methanol taken at
concn. of metal in feed before extraction                      liquid nitrogen temperature

Results and Discussion
From the results of the solubility studies, it is
noted that Ce (TTA)3-chelate is quite soluble in
SC CO2. Since the percentage extraction
obtained is low and the solubility observed is
quite high, it is inferred that the kinetics of
chelation is the rate controlling step rather than
the mass transfer. IR spectra of the chelate
collected during dynamic extraction is
represented by Fig. 9. It confirms the presence
of water molecules which can be replaced by
neutral ligands to enhance the solubility. To
confirm the valency state of Ce, ESR
spectrometry was done. It can be seen from
                                                     Fig. 11: X-ray powder pattern of CeO2 residue
Fig. 10 that peak exists in the ESR spectra taken
at low temperature.




                                                     Fig. 12: X-ray powder pattern of Ce O2 fed to
        Fig. 9: IR spectra of collected chelate              the extractor



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BARC Newsletter                                                                     Issue No. 249


This ensures that the Ce remains in +3 state in      References
the supercritical phase. The material left in the    1.    Glasstone S. and Sesonske A., Nuclear
extractor was washed with dodecane and the X-              Reactor Engineering, CBS Publishers,
ray powder pattern of the residue is represented           New Delhi (1986).
by Fig. 11. It matches with that of CeO2 fed to      2.    Shimda T. et al., Super Green 1st
the extractor as shown in Fig. 12. Since there is          International Symposium on Supercritical
no variation in the line intensities, it indicates         Fluid Technology for Energy and
that unconverted CeO2 has not undergone any                Environment Applications (2002).
structural changes even after 2 hours of             3.    Wai, C.M., Anl. Chem., 66 (13), 1971
extraction process.                                        (1994).

Conclusion                                           Nomenclature
1. A mixed ligand approach can be tried to
                                                     Ce         Cerium
   increase the percentage extraction since the
                                                     DSC        Differential Scanning Calorimeter
   chelate collected during dynamic extraction
                                                     ESR        Electron Spin Resonance
   contains water molecules.
                                                     HNO3       Nitric acid
2. As the kinetics is controlling, parametric
                                                     IR         Infra Red
   studies have been planned to investigate the
                                                     Pu         Plutonium
   effects of the temperature, pressure,
                                                     Purex      Plutonium uranium extraction
   residence time for chelation and relative
                                                     SC CO2     Supercritical Carbon dioxide
   ratios of CeO2-TTA-H2O.
                                                     SCF        Supercritical Fluid
3. It can be concluded from this study that SC       SFE        Supercritical Fluid Extraction
   CO2 modified with TTA and water has the           TBP        Tributyl Phosphate
   capacity for recovering Ce directly from          TG-DTA     Thermo-Gravimetry &
   oxides.                                                      Differential Thermal Analysis
                                                     TTA        Thenoyl Trifluoro Acetone
                                                     U          Uranium




    This paper has been adjudged as the Best Presentation paper in Indian
    Chemical Engineering Congress-2003 (CHEMCON-2003) at Regional
    Research Laboratory, Bhubaneswar, Orissa, during December
    19-22, 2003



Founder’s Day Special Issue                                                                         62
BARC Newsletter                                                                                    Issue No. 249




About the authors ...

                    Ms Tessy Vincent is from 35th batch of BARC Training School and is presently working in
                    Back-End Technology Development Division of Nuclear Recycle Group, BARC. She did her
                    post graduation in Chemical Engineering from Bharathiar University, Coimbatore. She is
                    currently working on application oriented process and equipment development for fuel
                    reprocessing plants. Additionally, she is perusing her Ph.D. programme in supercritical fluid
                    extraction with specific reference to nuclear fuel reprocessing and waste management.


                        Prof. Mamata Mukhopadhyay is a professor in Chemical Engineering Department of
                        Indian Institute of Technology, Bombay. She holds a Ph.D. from Ohio State University,
                        U.S.A. and has been a Visiting Professor to a number of Universities in U.S.A. in the area of
                        Supercritical Fluid Technology. She is a recipient of many awards including prestigious
                        Suman Sharma Design award by Institute of Engineers (India); Dr P.K. Patwardhan award
                        for Technology Development & Transfer, etc. She has authored a number of books notably
                        on Natural Extracts using Supercritical Carbon Dioxide, Phase Equilibrium in Solid
                        Supercritical Fluid Systems, etc.




Founder’s Day Special Issue                                                                                      63

				
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