- Email: [email protected]
Transport of thorium from nitric acid solution by non-dispersive solvent extraction using a hollow fibre contactor
Desalination 232 (2008) 272–280
Transport of thorium from nitric acid solution by non-dispersive solvent extraction using a hollow fibre contactor C.B. Patila, P.K. Mohapatrab, V.K. Manchandab* a
PREFRE Lab, Nuclear Recycles Group, BARC, Tarapur, Maharashtra-401502, India b Radiochemistry Division, BARC, Trombay, Mumbai-400085, India Tel. +91 (22) 25593688; Fax +91 (22) 25505151; email: [email protected]
Received 22 May 2007; accepted revised 19 November 2007
Abstract Membrane based non-dispersive solvent extraction (NDSX) of Th(IV) from aqueous nitric acid medium was carried out using di-n-hexyl octanamide (DHOA) in normal paraffin hydrocarbon (NPH) using a commercial hollow fiber module containing microporous hydrophobic polypropylene capillaries. The NDSX operation was carried out with pumping various concentrations of nitric acid (1–6 M) containing Th(IV) through the tube side and organic extractant (usually 1.1 M DHOA in NPH) through the shell side of the hollow fibre capillaries at aqueous and organic phase flow rates of 3.5 mL/s and 4.5 mL/s, respectively. Extraction studies were performed under different hydrodynamic conditions and the overall mass transfer was evaluated under counter-current flow condition. The percentage NDSX of Th(IV) increased with the increase in the extractant concentration (from 0.1 M DHOA to 1.1 M DHOA) as well as with nitric acid concentration (from 1 M to 6 M). Stripping studies were carried out using both distilled water as well as oxalic acid as the strippant. The possibility of the separation of U from Th was also evaluated. Keywords: Hollow fibre; Non-dispersive solvent extraction; Thorium(IV); Di-n-hexyl octanamide
1. Introduction There are various stages in the front as well as the back end of the nuclear fuel cycle which require efficient separations. The separation meth*Corresponding author.
ods used are: solvent extraction (SX), ion exchange (IX), precipitation, etc. Out of these, solvent extraction technique has a wider acceptance due to its rapidity, ease of operation and large throughputs. However, the major disadvantages of solvent extraction are: dependence on phase
Presented at the Symposium on Emerging Trends in Separation Science and Technology — SESTEC 2006 Bhabha Atomic Research Centre (BARC), Trombay, Mumbai, India, 29 September – 1 October 2006 0011-9164/08/$– See front matter © 2007 Elsevier B.V. All rights reserved doi:10.1016/j.desal.2007.11.055
C.B. Patil et al. / Desalination 232 (2008) 272–280
disengagement time, third phase formation, solvent entrainment, etc. These can be alleviated by using state-of-the-art technologies such as the nondispersive solvent extraction involving hollow fibre membranes (NDSX) [1]. Distinct features of micro-porous hollow fibre membranes are: large surface to volume ratio, faster mass transfer rates, continuous flow, no flooding and no loading limitations, the possibility to realize extreme phase ratios, and independence of phase densities and phase ratios [2–6]. Prasad and Sirkar initiated the work on membrane-based extraction systems using both single and double module hollow fibre contactors [7]. Non-dispersive solvent extraction (NDSX) techniques have been extensively deployed in separation science applications, such as metal recovery from leach solutions, recovery of precious and strategic metals and treatment of large volumes of the effluents including toxic and hazardous wastes generated by chemical industries. Ortiz et al. have extensively studied the non-dispersive extraction of Cr(VI) with Aliquat 336 [8,9]. Kathios et al. [10] demonstrated the utility of the membrane-based extraction modules for the extraction of Nd(III) which was used as a surrogate of trivalent actinides present in the high level waste. The extractants tested by them were CMPO and DHDECMP in di-iso-propyl benzene while the strippant used was 0.01 M HNO3. Geist et al. [11–13] used the hollow fibre membrane technique for lab-scale separation of actinides from lanthanides using BTP (2,6-bis(5,6-dipropyl1,2,4-triazin-3-yl)pyridine) and substituted di-thio phosphinic acid as the extractants. Uranium is used as the fuel in nuclear reactors. In view of the fast depleting uranium reserves and availability of vast thorium reserves, India has designed future reactors based on the conversion of Th-232 to the fissile U-233. Separation of U-233 from irradiated Th-232 is traditionally done using tri-n-butyl phosphate (TBP) which is the work horse of the nuclear industry. However, with the increasing concern for the environment, ‘green
273
solvents’ which can be easily incinerated have drawn the attention of separation/synthetic scientists. Amongst these ‘green solvents’, monoamides, such as di-n-hexyl octanamide (DHOA) and din-hexyl decanamide (DHDA) are suggested as viable alternatives to TBP. They have been found to be particularly promising in view of their improved re-extraction behaviour, complete incinerability and the innocuous nature of their radiolytic degradation products. Membrane-based non-dispersive solvent extraction technique has additional advantages of a) lower ligand inventory, b) lower volume of secondary wastes and c) lower volume of the inflammable diluent. We have earlier reported the extraction of U(VI) employing NDSX technique using hollow fiber contactors where DHOA in normal paraffin hydrocarbon (NPH) was used as the solvent [14]. The aim of this work is to evaluate the extraction behaviour of DHOA towards Th(IV) in nitric acid medium which is the major metal ion present in the THOREX feeds. The effect of various chemical and hydrodynamic parameters in optimizing the performance of the hollow fiber module for the efficient NDSX of U(VI) over Th (IV) from nitric acid solution has also been studied. The possibility of the separation of low quantities of U(VI) from macro concentrations of Th was also evaluated. 2. Experimental 2.1. Materials All the chemicals used were of AR grade. Th(IV) stock solution was prepared by dissolving thorium nitrate in 4 M nitric acid and standardizing by titration. DHOA was synthesized by the reported method and the purity was checked by conventional methods such as 1H NMR, elemental analysis and non-aqueous titration [15]. 2.2. The NDSX set-up Liqui-Cel hollow fiber module (1.25″×9″) containing about 3600 polypropylene micro porous
274
C.B. Patil et al. / Desalination 232 (2008) 272–280
lumens (40% porosity; surface area: 0.5 m2) supported by epoxy resin in a polysulfone housing was procured from M/s Alting, France. Table 1 shows the details of the module used in the present work. A schematic view of the membrane-based extraction process based on the hollow fiber contactor in the recirculation mode (using peristaltic pumps) is shown in Fig. 1. 2.3. Solvent extraction studies Table 1 Characteristics of the Liqui-Cel hollow fibre module (1.25″×9″) used in the present work
In the solvent extraction studies, solutions of desired concentrations of DHOA prepared in NPH (normal parraffinic hydrocarbon) were employed after pre-equilibration at the respective acidities. Equal volumes of the organic and aqueous phases (usually containing 0.5–1.0 g/L of U or Th in 4.0 M nitric acid) were equilibrated in a rotary thermostated water bath for 1 h at 25.0±0.2°C. The two phases were then centrifuged and assayed by taking suitable aliquots from both phases. The distribution ratio (DM) is defined as the ratio of concentration of metal ion in the organic phase to that in the aqueous phase. 2.4. NDSX studies
Characteristics
Values
Fiber Internal diameter, µm Wall thickness, µm Number of fibers Nominal porosity, % Pore size, µm Effective mass transfer length, mm Effective mass transfer area, m2 Surface area to volume ratio, m2/m3
X-40/polypropylene 240 30 3000 40 0.03 200 0.5 5000 approx.
The aqueous and organic phases were circulated through the hollow fiber module in co-current as well as counter-current modes. The aqueous and organic solutions were agitated continuously in the reservoir. Since the fibers were hydrophobic, the pressure on the aqueous side was maintained higher (0.2–0.3 bar) than the pressure on the organic phase side by the use of a throttle on the aqueous outlet. The higher pressure on the aqueous side prevented emulsion formation by
Organic phase in Lean feed 1.1 M DHOA in NPH
Feed (Th(IV) in HNO3) Th(IV) loaded organic phase Stacked lumens Module body
Fig. 1. Extraction mechanism at the pore mouth of the fiber in the hollow fiber contactor.
C.B. Patil et al. / Desalination 232 (2008) 272–280
intrusion of the organic phase in the aqueous phase. The organic phase used in the present experiments was 1.1 M DHOA in NPH, which flowed through the shell side of the fibers while the feed containing about 10 g/L of Th as well as U at varying concentrations of HNO3 flowed through the lumen side. The volume of the organic phase was 200 cm3, while the aqueous feed volume was usually 300 cm3. The feed and organic solutions were re-circulated by means of calibrated peristaltic pumps.
2.5. Analysis of metal ions Th-estimation at mg/mL level was performed by EDTA-complexometric titration following a method described in the literature [16]. The samples were titrated using buffered solutions maintained at pH 4. Estimation of uranium was carried out spectrophotometrically using thiocyanate as the chromogenic agent (λmax = 420 nm; ε = 1000 M–1cm–1). Higher amounts of U were estimated by Davies–Gray potentiometric titrations [17].
3. Results and discussion 3.1. Extraction equilibria The extraction of Th(IV) and U(VI) in DHOA is well established [15]. The solvated complexes predominate in the nitric acid media ranging from 1 to 5 M HNO3. The extraction equilibria can be represented as follows: m+ ⎯⎯→ M aq. + mNO3− aq. + nDHOA org. ←⎯⎯ kex
M(NO3 ) m ⋅ nDHOA org.
(1)
where Mm+ is UO22+ or Th4+. The extraction equilibrium can be represented as:
275
[ M(NO3 )m ⋅ nDHOA ]org. m n ⎡⎣ M m + ⎤⎦ ⎡⎣ NO3− ⎤⎦ [ DHOA ]org.
(2)
DM = [ M(NO3 ) m ⋅ nDHOA ]org. / ⎡⎣ M m + ⎤⎦
(3)
kex =
Upon rearranging and taking logarithm: log DM = log kex + m log ⎡⎣ NO3− ⎤⎦ + n log [ DHOA ]
(4)
The value of m is 2 for UO22+ and 4 for Th4+, while the number of DHOA molecules in the extracted complex was determined by solvent extraction studies at varying concentrations of DHOA. Fig. 2 shows the results of the DHOA variation experiments suggesting that the extracted species were Th(NO3)4· 2DHOA and UO2(NO3)2· 2DHOA, for Th(IV) and U(VI), respectively. Similar species are expected for the NDSX studies as well. 3.2. NDSX studies During our earlier NDSX studies of U(VI) from nitric acid medium using di-n-hexyl octanamide (DHOA) in NPH as the carrier, a systematic study was carried out for flow rate optimization. The organic and aqueous phase flow rates were optimized at 4.5 mL/s and 3.5 mL/s [3]. Breakthrough of the organic phase into the feed stream was reported earlier at higher organic phase flow rates. Similarly, counter-current NDSX studies resulted in better mass transfer rates as compared to the co-current studies. Therefore, for the present work, the flow rates were maintained as 4.5 mL/s and 3.5 mL/s for the organic and aqueous phases, respectively, and all studies were carried out in the counter-current mode. In the acidity variation experiments, increasing the aqueous phase acidity helped in enhancing the Th uptake from the feed which is reflected in the decrease of Th concentrations in the feed
276
C.B. Patil et al. / Desalination 232 (2008) 272–280
Fig. 2. Dependence of the distribution ratios of Th(IV) and U(VI) on DHOA concentration. Nitric acid concentration = 4.0 M; metal ion concentrations were in the range 0.5–1 g/L; equilibration time 1 h; temperature 25°C.
(Table 2). An increase in Th extraction was observed with increasing the aqueous phase acidity (Fig. 3). Only about 20% Th transport (feed 10 g/L) was observed in 50 min which is in sharp con-
trast to >95% U transport (feed 12 g/L U) achieved in comparable time of operation at a feed acidity 4 M HNO3 (Fig. 3). Enhancing the aqueous phase acidity to 6 M increased the % Th transport to
Table 2 Mass transfer rates from a feed containing about 10 g/L thorium in varying concentrations of nitric acid. Organic phase: 1.1 M DHOA in NPH
Time (min) 0 5 10 15 20 30 50 120 a
Th concentration (g/L) in feed at varying feed acidities 1.0 Ma
2.0 M
3.0 M
4.0 M
6.0 M
9.98 —a —a —a —a —a —a —
9.87 9.89 9.805 9.783 9.701 9.507 9.401 —
9.62 9.512 9.438 9.378 9.265 8.993 8.66 —
9.520 9.433 9.396 9.061 8.863 8.090 7.630 —
9.75 7.75 7.34 7.20 6.80 6.34 5.93 3.99
Insignificant extraction was observed in the entire time scale of operation
C.B. Patil et al. / Desalination 232 (2008) 272–280
277
Fig. 3. Effect of the aqueous phase acidity on the rate of Th/U NDSX. Feed 10 g/L Th and 12 g/L U; organic phase 1.1 M DHOA; aqueous phase flow rate 3.5 mL/s; organic phase flow rate 4.5 mL/s.
>38% in 50 min which increased to about 60% in 2 h of operation. This is in contrast to the behaviour with U extraction which saturated beyond 4 M acidity. The relatively slow mass transfer rates in the case of Th(IV) as compared to U(VI) are probably due to slow metal uptake in the case of the former at the feed–membrane interface. In the reagent concentration variation experiments, a significant enhancement in percent Th extraction was observed which was due to the direct correlation between the Th extraction with DHOA concentration [15]. It appears that 1.1 M DHOA can be used as the organic phase due to the significantly higher extraction rates (Fig. 4). However, the extraction rates of Th were remarkably lower as compared to those with U suggesting the possibility of separation of U from the bulk Th. In another set of studies involving the loading effect of Th(IV) using 1.1 M DHOA, it was observed that only less than 0.2% extraction of Th(IV) was possible when 200 g/L Th in 4 M
HNO3 was used as the feed solution. Under identical conditions, >99.9% extraction of U is possible for a feed containing 1 g/L U in 4 M HNO3. This appears quite promising as the NDSX method can be used for preferential extraction of uranium over thorium which may find application for the selective extraction of U-233 from irradiated Th232. Fig. 5 gives a comparative performance of NDSX of U (1 g/L) and Th (200 g/L). It is interesting to note that the organic phase extracted about 0.4 g/L Th while it extracted >99.9% (1 g/L) U at a contact time of 20 min. When the contact time was increased to 50 min, Th concentration increased to ~1.4 g/L Th while the U concentration remained constant. 3.3. Stripping studies Stripping studies were carried out using different strippants viz. deionized water and oxalic acid. Ideally, distilled water cannot be used as the strippant as it can cause the hydrolysis of Th(IV).
278
C.B. Patil et al. / Desalination 232 (2008) 272–280
Fig.4. Effect of the carrier concentration on the rate of thorium NDSX. Feed 10 g/L Th in 4 M HNO3; aqueous phase flow rate 3.5 mL/s; organic phase flow rate 4.5 mL/s.
Fig. 5. Comparative NDSX data of U (1 g/L) and Th (200 g/L) through the hollow fibre module. Organic phase 1.1 M DHOA; aqueous phase flow rate 3.5 mL/s; organic phase flow rate 4.5 mL/s.
C.B. Patil et al. / Desalination 232 (2008) 272–280
However, there is always some amount of acid transported into the organic phase due to the high basicity of DHOA (KH = 0.17) as per the following equation: + DHOA org. + H aq. + NO3− aq.
(5)
KH ⎯⎯→ DHOA ⋅ HNO3 org. ←⎯⎯
which can decrease the aqueous phase pH of the strip solution to prevent any significant hydrolysis. However, as a precautionary measure, the stripping of uranyl ions was carried out earlier using dilute nitric acid (pH 2) which was found to be an effective strippant [16]. Table 3 shows the stripping data of Th(IV) when the metal ion
279
loaded (1.46 g/L) 1.1 M DHOA in NPH was used. The kinetics of stripping was slower with DW and about 52% stripping was achieved in 60 min. On the other hand, >96% stripping was observed in the same contact time when 0.5 M oxalic acid was used as the strippant. We have earlier reported that the percentage stripping of U(VI) decreased with increasing the carrier concentration [16]. Similar studies were carried out for Th(IV) and the data are presented in Table 4. As can be seen from the table, 30 min was sufficient for quantitative stripping of Th when 0.3 M DHOA was used as the extractant. Interestingly, though the NDSX of uranium was efficient with DHOA in NPH, it was partly offset by the relatively inefficient stripping. On the other
Table 3 Stripping data for Th loaded organic phases by NDSX using deionized water and 0.5 M oxalic acid solution. Organic phase: Th loaded (1.46 g/L) 1.1 M DHOA in NPH
Time (min)
Deionized water
15 30 45 60 75
0.5 M oxalic acid
Th (g/L)
% strip
Th (g/L)
% strip
0.819 0.760 0.708 0.698 —
43.9 47.9 51.5 52.2 —
0.936 0.658 0.411 0.208 0.056
35.9 54.9 71.9 85.8 96.2
Table 4 Stripping data for Th loaded organic phases by NDSX. Aqueous phase: 0.5 M oxalic acid solution. Organic phase: varying concentrations of DHOA in NPH
a
Time (min)
0.3 M DHOAa
0.5 M DHOAb
0.8 M DHOAc
1.1 M DHOAd
Th (g/L)
% strip
Th (g/L)
% strip
Th (g/L)
% strip
Th (g/L)
% strip
15 30 45 60 75
0.021 nil — — —
75.9 100 — — —
0.108 0.05 0.024 nil —
48.6 76.2 88.6 100 —
0.185 0.128 0.069 0.048 nil
33.7 54.3 75.5 83.0 100
0.936 0.658 0.411 0.208 0.056
35.9 54.9 71.9 85.8 96.2
Total initial Th in organic phase was 0.087 g/L Total initial Th in organic phase was 0.21 g/L
b
c
Total initial Th in organic phase was 0.28 g/L Total initial Th in organic phase was 1.46 g/L
d
280
C.B. Patil et al. / Desalination 232 (2008) 272–280
hand, the NDSX of thorium was relatively inefficient while its stripping rate was quite satisfactory. For successful separation of low quantities of U from a large concentration of Th, a lower concentration of DHOA can be used. This can suppress Th extraction in the first stage itself leading to a better separation factor.
4. Conclusions NDSX of Th was carried out using DHOA in NPH as the organic phase. Though increasing extractant concentration as well as increasing nitric acid concentration resulted in improved the extraction of Th(IV), it was much less efficiently extracted as compared to U(VI) as was reported earlier. The stripping behaviour was remarkably better as compared to that observed for U(VI). Relative inefficient extraction of Th(IV) as compared to U(VI) can be used to separate U-233 from the bulk Th-232 and the former can be used as a fissile material in the Th-based AHWR reactors.
Acknowledgements One of the authors (C.B.P.) wishes to thank Dr. P.B. Gurba, Mr. P. Janardan and Mr. R.D. Changrani for their keen interest and constant encouragement.
References [1] P.K. Mohapatra and V.K. Manchanda, Ind. J. Chem., 42A (2003) 2985. [2] P.R. Alexander and R.W. Callahan; J. Membr. Sci.,
35 (1962) 546–549. [3] C.H. Yun, R. Prasad, A.H. Guha and K.K. Sirkar, Ind. Eng. Chem. Res., 32 (1993) 1186–1195. [4] F.J. Alguacil, M. Alonso, A.M. Sastre and A. Kumar; Recent Res. Devel. Chem. Eng., 5 (2003) 145–159. [5] L. Dahuron and E.L. Cussler; AIChE J., 341 (1988) 30–36. [6] M.E. Campderrós, A. Acosta and J. Marchese, Talanta, 47 (1998) 19–24. [7] R. Prasad and K.K. Sirkar, J. Membr. Sci., 50 (1990) 153–175. [8] I. Ortiz, B. Galan and A. Irabien, Ind. Eng. Chem. Res., 35 (1996) 1369. [9] A. Urtiaga, M.J. Abellan, J.A. Irabien and I. Ortiz, J. Membr. Sci., 257 (2005) 161. [10] D.J. Kathios, G.D. Jarvinen, S.L. Yarbro and B.F Smith, J. Membr. Sci., 97 (1994) 251. [11] A. Geist, M. Weigl and K. Gompper, Radiochim. Acta, 93 (2005) 197. [12] A. Geist, M. Weigl U. Muellich and K. Gompper, Membr. Technol., 5 (2003) 5–7. [13] A. Geist, M. Weigl and K. Gompper, Sep. Sci. Tech., 37 (2002) 3369. [14] C.B. Patil, P.K. Mohapatra, R.R. Singh, P.B. Gurba, P. Janardan, R.D. Changrani and V.K. Manchanda, Radiochim. Acta, 94 (2006) 331–334. [15] K.K. Gupta, V.K. Manchanda, M.S. Subramanian and R.K. Singh; Solv. Extr. Ion Exch., 18 (2000) 273. [16] G.H. Jeffery, J. Bassett, J. Mendham and R.C. Denney, Vogel’s Textbook of Quantitative Chemical Analysis, Addison Wesley Longman, Singapore, 1996. [17] W. Davies and W. Gray, A rapid and specific titrimetric method for the precise determination of uranium using iron(II) sulphate as reductant, Talanta, 11 (1964) 1203. [18] C.B. Patil, P.K. Mohapatra, P.B. Gurba, P. Janardan, R.D. Changrani and V.K. Manchanda, Proc. Indian Nuclear Society Annual Conference (INSAC-2005).
Transport of thorium from nitric acid solution by non-dispersive solvent extraction using a hollow fibre contactor C.B. Patila, P.K. Mohapatrab, V.K. Manchandab* a
PREFRE Lab, Nuclear Recycles Group, BARC, Tarapur, Maharashtra-401502, India b Radiochemistry Division, BARC, Trombay, Mumbai-400085, India Tel. +91 (22) 25593688; Fax +91 (22) 25505151; email: [email protected]
Received 22 May 2007; accepted revised 19 November 2007
Abstract Membrane based non-dispersive solvent extraction (NDSX) of Th(IV) from aqueous nitric acid medium was carried out using di-n-hexyl octanamide (DHOA) in normal paraffin hydrocarbon (NPH) using a commercial hollow fiber module containing microporous hydrophobic polypropylene capillaries. The NDSX operation was carried out with pumping various concentrations of nitric acid (1–6 M) containing Th(IV) through the tube side and organic extractant (usually 1.1 M DHOA in NPH) through the shell side of the hollow fibre capillaries at aqueous and organic phase flow rates of 3.5 mL/s and 4.5 mL/s, respectively. Extraction studies were performed under different hydrodynamic conditions and the overall mass transfer was evaluated under counter-current flow condition. The percentage NDSX of Th(IV) increased with the increase in the extractant concentration (from 0.1 M DHOA to 1.1 M DHOA) as well as with nitric acid concentration (from 1 M to 6 M). Stripping studies were carried out using both distilled water as well as oxalic acid as the strippant. The possibility of the separation of U from Th was also evaluated. Keywords: Hollow fibre; Non-dispersive solvent extraction; Thorium(IV); Di-n-hexyl octanamide
1. Introduction There are various stages in the front as well as the back end of the nuclear fuel cycle which require efficient separations. The separation meth*Corresponding author.
ods used are: solvent extraction (SX), ion exchange (IX), precipitation, etc. Out of these, solvent extraction technique has a wider acceptance due to its rapidity, ease of operation and large throughputs. However, the major disadvantages of solvent extraction are: dependence on phase
Presented at the Symposium on Emerging Trends in Separation Science and Technology — SESTEC 2006 Bhabha Atomic Research Centre (BARC), Trombay, Mumbai, India, 29 September – 1 October 2006 0011-9164/08/$– See front matter © 2007 Elsevier B.V. All rights reserved doi:10.1016/j.desal.2007.11.055
C.B. Patil et al. / Desalination 232 (2008) 272–280
disengagement time, third phase formation, solvent entrainment, etc. These can be alleviated by using state-of-the-art technologies such as the nondispersive solvent extraction involving hollow fibre membranes (NDSX) [1]. Distinct features of micro-porous hollow fibre membranes are: large surface to volume ratio, faster mass transfer rates, continuous flow, no flooding and no loading limitations, the possibility to realize extreme phase ratios, and independence of phase densities and phase ratios [2–6]. Prasad and Sirkar initiated the work on membrane-based extraction systems using both single and double module hollow fibre contactors [7]. Non-dispersive solvent extraction (NDSX) techniques have been extensively deployed in separation science applications, such as metal recovery from leach solutions, recovery of precious and strategic metals and treatment of large volumes of the effluents including toxic and hazardous wastes generated by chemical industries. Ortiz et al. have extensively studied the non-dispersive extraction of Cr(VI) with Aliquat 336 [8,9]. Kathios et al. [10] demonstrated the utility of the membrane-based extraction modules for the extraction of Nd(III) which was used as a surrogate of trivalent actinides present in the high level waste. The extractants tested by them were CMPO and DHDECMP in di-iso-propyl benzene while the strippant used was 0.01 M HNO3. Geist et al. [11–13] used the hollow fibre membrane technique for lab-scale separation of actinides from lanthanides using BTP (2,6-bis(5,6-dipropyl1,2,4-triazin-3-yl)pyridine) and substituted di-thio phosphinic acid as the extractants. Uranium is used as the fuel in nuclear reactors. In view of the fast depleting uranium reserves and availability of vast thorium reserves, India has designed future reactors based on the conversion of Th-232 to the fissile U-233. Separation of U-233 from irradiated Th-232 is traditionally done using tri-n-butyl phosphate (TBP) which is the work horse of the nuclear industry. However, with the increasing concern for the environment, ‘green
273
solvents’ which can be easily incinerated have drawn the attention of separation/synthetic scientists. Amongst these ‘green solvents’, monoamides, such as di-n-hexyl octanamide (DHOA) and din-hexyl decanamide (DHDA) are suggested as viable alternatives to TBP. They have been found to be particularly promising in view of their improved re-extraction behaviour, complete incinerability and the innocuous nature of their radiolytic degradation products. Membrane-based non-dispersive solvent extraction technique has additional advantages of a) lower ligand inventory, b) lower volume of secondary wastes and c) lower volume of the inflammable diluent. We have earlier reported the extraction of U(VI) employing NDSX technique using hollow fiber contactors where DHOA in normal paraffin hydrocarbon (NPH) was used as the solvent [14]. The aim of this work is to evaluate the extraction behaviour of DHOA towards Th(IV) in nitric acid medium which is the major metal ion present in the THOREX feeds. The effect of various chemical and hydrodynamic parameters in optimizing the performance of the hollow fiber module for the efficient NDSX of U(VI) over Th (IV) from nitric acid solution has also been studied. The possibility of the separation of low quantities of U(VI) from macro concentrations of Th was also evaluated. 2. Experimental 2.1. Materials All the chemicals used were of AR grade. Th(IV) stock solution was prepared by dissolving thorium nitrate in 4 M nitric acid and standardizing by titration. DHOA was synthesized by the reported method and the purity was checked by conventional methods such as 1H NMR, elemental analysis and non-aqueous titration [15]. 2.2. The NDSX set-up Liqui-Cel hollow fiber module (1.25″×9″) containing about 3600 polypropylene micro porous
274
C.B. Patil et al. / Desalination 232 (2008) 272–280
lumens (40% porosity; surface area: 0.5 m2) supported by epoxy resin in a polysulfone housing was procured from M/s Alting, France. Table 1 shows the details of the module used in the present work. A schematic view of the membrane-based extraction process based on the hollow fiber contactor in the recirculation mode (using peristaltic pumps) is shown in Fig. 1. 2.3. Solvent extraction studies Table 1 Characteristics of the Liqui-Cel hollow fibre module (1.25″×9″) used in the present work
In the solvent extraction studies, solutions of desired concentrations of DHOA prepared in NPH (normal parraffinic hydrocarbon) were employed after pre-equilibration at the respective acidities. Equal volumes of the organic and aqueous phases (usually containing 0.5–1.0 g/L of U or Th in 4.0 M nitric acid) were equilibrated in a rotary thermostated water bath for 1 h at 25.0±0.2°C. The two phases were then centrifuged and assayed by taking suitable aliquots from both phases. The distribution ratio (DM) is defined as the ratio of concentration of metal ion in the organic phase to that in the aqueous phase. 2.4. NDSX studies
Characteristics
Values
Fiber Internal diameter, µm Wall thickness, µm Number of fibers Nominal porosity, % Pore size, µm Effective mass transfer length, mm Effective mass transfer area, m2 Surface area to volume ratio, m2/m3
X-40/polypropylene 240 30 3000 40 0.03 200 0.5 5000 approx.
The aqueous and organic phases were circulated through the hollow fiber module in co-current as well as counter-current modes. The aqueous and organic solutions were agitated continuously in the reservoir. Since the fibers were hydrophobic, the pressure on the aqueous side was maintained higher (0.2–0.3 bar) than the pressure on the organic phase side by the use of a throttle on the aqueous outlet. The higher pressure on the aqueous side prevented emulsion formation by
Organic phase in Lean feed 1.1 M DHOA in NPH
Feed (Th(IV) in HNO3) Th(IV) loaded organic phase Stacked lumens Module body
Fig. 1. Extraction mechanism at the pore mouth of the fiber in the hollow fiber contactor.
C.B. Patil et al. / Desalination 232 (2008) 272–280
intrusion of the organic phase in the aqueous phase. The organic phase used in the present experiments was 1.1 M DHOA in NPH, which flowed through the shell side of the fibers while the feed containing about 10 g/L of Th as well as U at varying concentrations of HNO3 flowed through the lumen side. The volume of the organic phase was 200 cm3, while the aqueous feed volume was usually 300 cm3. The feed and organic solutions were re-circulated by means of calibrated peristaltic pumps.
2.5. Analysis of metal ions Th-estimation at mg/mL level was performed by EDTA-complexometric titration following a method described in the literature [16]. The samples were titrated using buffered solutions maintained at pH 4. Estimation of uranium was carried out spectrophotometrically using thiocyanate as the chromogenic agent (λmax = 420 nm; ε = 1000 M–1cm–1). Higher amounts of U were estimated by Davies–Gray potentiometric titrations [17].
3. Results and discussion 3.1. Extraction equilibria The extraction of Th(IV) and U(VI) in DHOA is well established [15]. The solvated complexes predominate in the nitric acid media ranging from 1 to 5 M HNO3. The extraction equilibria can be represented as follows: m+ ⎯⎯→ M aq. + mNO3− aq. + nDHOA org. ←⎯⎯ kex
M(NO3 ) m ⋅ nDHOA org.
(1)
where Mm+ is UO22+ or Th4+. The extraction equilibrium can be represented as:
275
[ M(NO3 )m ⋅ nDHOA ]org. m n ⎡⎣ M m + ⎤⎦ ⎡⎣ NO3− ⎤⎦ [ DHOA ]org.
(2)
DM = [ M(NO3 ) m ⋅ nDHOA ]org. / ⎡⎣ M m + ⎤⎦
(3)
kex =
Upon rearranging and taking logarithm: log DM = log kex + m log ⎡⎣ NO3− ⎤⎦ + n log [ DHOA ]
(4)
The value of m is 2 for UO22+ and 4 for Th4+, while the number of DHOA molecules in the extracted complex was determined by solvent extraction studies at varying concentrations of DHOA. Fig. 2 shows the results of the DHOA variation experiments suggesting that the extracted species were Th(NO3)4· 2DHOA and UO2(NO3)2· 2DHOA, for Th(IV) and U(VI), respectively. Similar species are expected for the NDSX studies as well. 3.2. NDSX studies During our earlier NDSX studies of U(VI) from nitric acid medium using di-n-hexyl octanamide (DHOA) in NPH as the carrier, a systematic study was carried out for flow rate optimization. The organic and aqueous phase flow rates were optimized at 4.5 mL/s and 3.5 mL/s [3]. Breakthrough of the organic phase into the feed stream was reported earlier at higher organic phase flow rates. Similarly, counter-current NDSX studies resulted in better mass transfer rates as compared to the co-current studies. Therefore, for the present work, the flow rates were maintained as 4.5 mL/s and 3.5 mL/s for the organic and aqueous phases, respectively, and all studies were carried out in the counter-current mode. In the acidity variation experiments, increasing the aqueous phase acidity helped in enhancing the Th uptake from the feed which is reflected in the decrease of Th concentrations in the feed
276
C.B. Patil et al. / Desalination 232 (2008) 272–280
Fig. 2. Dependence of the distribution ratios of Th(IV) and U(VI) on DHOA concentration. Nitric acid concentration = 4.0 M; metal ion concentrations were in the range 0.5–1 g/L; equilibration time 1 h; temperature 25°C.
(Table 2). An increase in Th extraction was observed with increasing the aqueous phase acidity (Fig. 3). Only about 20% Th transport (feed 10 g/L) was observed in 50 min which is in sharp con-
trast to >95% U transport (feed 12 g/L U) achieved in comparable time of operation at a feed acidity 4 M HNO3 (Fig. 3). Enhancing the aqueous phase acidity to 6 M increased the % Th transport to
Table 2 Mass transfer rates from a feed containing about 10 g/L thorium in varying concentrations of nitric acid. Organic phase: 1.1 M DHOA in NPH
Time (min) 0 5 10 15 20 30 50 120 a
Th concentration (g/L) in feed at varying feed acidities 1.0 Ma
2.0 M
3.0 M
4.0 M
6.0 M
9.98 —a —a —a —a —a —a —
9.87 9.89 9.805 9.783 9.701 9.507 9.401 —
9.62 9.512 9.438 9.378 9.265 8.993 8.66 —
9.520 9.433 9.396 9.061 8.863 8.090 7.630 —
9.75 7.75 7.34 7.20 6.80 6.34 5.93 3.99
Insignificant extraction was observed in the entire time scale of operation
C.B. Patil et al. / Desalination 232 (2008) 272–280
277
Fig. 3. Effect of the aqueous phase acidity on the rate of Th/U NDSX. Feed 10 g/L Th and 12 g/L U; organic phase 1.1 M DHOA; aqueous phase flow rate 3.5 mL/s; organic phase flow rate 4.5 mL/s.
>38% in 50 min which increased to about 60% in 2 h of operation. This is in contrast to the behaviour with U extraction which saturated beyond 4 M acidity. The relatively slow mass transfer rates in the case of Th(IV) as compared to U(VI) are probably due to slow metal uptake in the case of the former at the feed–membrane interface. In the reagent concentration variation experiments, a significant enhancement in percent Th extraction was observed which was due to the direct correlation between the Th extraction with DHOA concentration [15]. It appears that 1.1 M DHOA can be used as the organic phase due to the significantly higher extraction rates (Fig. 4). However, the extraction rates of Th were remarkably lower as compared to those with U suggesting the possibility of separation of U from the bulk Th. In another set of studies involving the loading effect of Th(IV) using 1.1 M DHOA, it was observed that only less than 0.2% extraction of Th(IV) was possible when 200 g/L Th in 4 M
HNO3 was used as the feed solution. Under identical conditions, >99.9% extraction of U is possible for a feed containing 1 g/L U in 4 M HNO3. This appears quite promising as the NDSX method can be used for preferential extraction of uranium over thorium which may find application for the selective extraction of U-233 from irradiated Th232. Fig. 5 gives a comparative performance of NDSX of U (1 g/L) and Th (200 g/L). It is interesting to note that the organic phase extracted about 0.4 g/L Th while it extracted >99.9% (1 g/L) U at a contact time of 20 min. When the contact time was increased to 50 min, Th concentration increased to ~1.4 g/L Th while the U concentration remained constant. 3.3. Stripping studies Stripping studies were carried out using different strippants viz. deionized water and oxalic acid. Ideally, distilled water cannot be used as the strippant as it can cause the hydrolysis of Th(IV).
278
C.B. Patil et al. / Desalination 232 (2008) 272–280
Fig.4. Effect of the carrier concentration on the rate of thorium NDSX. Feed 10 g/L Th in 4 M HNO3; aqueous phase flow rate 3.5 mL/s; organic phase flow rate 4.5 mL/s.
Fig. 5. Comparative NDSX data of U (1 g/L) and Th (200 g/L) through the hollow fibre module. Organic phase 1.1 M DHOA; aqueous phase flow rate 3.5 mL/s; organic phase flow rate 4.5 mL/s.
C.B. Patil et al. / Desalination 232 (2008) 272–280
However, there is always some amount of acid transported into the organic phase due to the high basicity of DHOA (KH = 0.17) as per the following equation: + DHOA org. + H aq. + NO3− aq.
(5)
KH ⎯⎯→ DHOA ⋅ HNO3 org. ←⎯⎯
which can decrease the aqueous phase pH of the strip solution to prevent any significant hydrolysis. However, as a precautionary measure, the stripping of uranyl ions was carried out earlier using dilute nitric acid (pH 2) which was found to be an effective strippant [16]. Table 3 shows the stripping data of Th(IV) when the metal ion
279
loaded (1.46 g/L) 1.1 M DHOA in NPH was used. The kinetics of stripping was slower with DW and about 52% stripping was achieved in 60 min. On the other hand, >96% stripping was observed in the same contact time when 0.5 M oxalic acid was used as the strippant. We have earlier reported that the percentage stripping of U(VI) decreased with increasing the carrier concentration [16]. Similar studies were carried out for Th(IV) and the data are presented in Table 4. As can be seen from the table, 30 min was sufficient for quantitative stripping of Th when 0.3 M DHOA was used as the extractant. Interestingly, though the NDSX of uranium was efficient with DHOA in NPH, it was partly offset by the relatively inefficient stripping. On the other
Table 3 Stripping data for Th loaded organic phases by NDSX using deionized water and 0.5 M oxalic acid solution. Organic phase: Th loaded (1.46 g/L) 1.1 M DHOA in NPH
Time (min)
Deionized water
15 30 45 60 75
0.5 M oxalic acid
Th (g/L)
% strip
Th (g/L)
% strip
0.819 0.760 0.708 0.698 —
43.9 47.9 51.5 52.2 —
0.936 0.658 0.411 0.208 0.056
35.9 54.9 71.9 85.8 96.2
Table 4 Stripping data for Th loaded organic phases by NDSX. Aqueous phase: 0.5 M oxalic acid solution. Organic phase: varying concentrations of DHOA in NPH
a
Time (min)
0.3 M DHOAa
0.5 M DHOAb
0.8 M DHOAc
1.1 M DHOAd
Th (g/L)
% strip
Th (g/L)
% strip
Th (g/L)
% strip
Th (g/L)
% strip
15 30 45 60 75
0.021 nil — — —
75.9 100 — — —
0.108 0.05 0.024 nil —
48.6 76.2 88.6 100 —
0.185 0.128 0.069 0.048 nil
33.7 54.3 75.5 83.0 100
0.936 0.658 0.411 0.208 0.056
35.9 54.9 71.9 85.8 96.2
Total initial Th in organic phase was 0.087 g/L Total initial Th in organic phase was 0.21 g/L
b
c
Total initial Th in organic phase was 0.28 g/L Total initial Th in organic phase was 1.46 g/L
d
280
C.B. Patil et al. / Desalination 232 (2008) 272–280
hand, the NDSX of thorium was relatively inefficient while its stripping rate was quite satisfactory. For successful separation of low quantities of U from a large concentration of Th, a lower concentration of DHOA can be used. This can suppress Th extraction in the first stage itself leading to a better separation factor.
4. Conclusions NDSX of Th was carried out using DHOA in NPH as the organic phase. Though increasing extractant concentration as well as increasing nitric acid concentration resulted in improved the extraction of Th(IV), it was much less efficiently extracted as compared to U(VI) as was reported earlier. The stripping behaviour was remarkably better as compared to that observed for U(VI). Relative inefficient extraction of Th(IV) as compared to U(VI) can be used to separate U-233 from the bulk Th-232 and the former can be used as a fissile material in the Th-based AHWR reactors.
Acknowledgements One of the authors (C.B.P.) wishes to thank Dr. P.B. Gurba, Mr. P. Janardan and Mr. R.D. Changrani for their keen interest and constant encouragement.
References [1] P.K. Mohapatra and V.K. Manchanda, Ind. J. Chem., 42A (2003) 2985. [2] P.R. Alexander and R.W. Callahan; J. Membr. Sci.,
35 (1962) 546–549. [3] C.H. Yun, R. Prasad, A.H. Guha and K.K. Sirkar, Ind. Eng. Chem. Res., 32 (1993) 1186–1195. [4] F.J. Alguacil, M. Alonso, A.M. Sastre and A. Kumar; Recent Res. Devel. Chem. Eng., 5 (2003) 145–159. [5] L. Dahuron and E.L. Cussler; AIChE J., 341 (1988) 30–36. [6] M.E. Campderrós, A. Acosta and J. Marchese, Talanta, 47 (1998) 19–24. [7] R. Prasad and K.K. Sirkar, J. Membr. Sci., 50 (1990) 153–175. [8] I. Ortiz, B. Galan and A. Irabien, Ind. Eng. Chem. Res., 35 (1996) 1369. [9] A. Urtiaga, M.J. Abellan, J.A. Irabien and I. Ortiz, J. Membr. Sci., 257 (2005) 161. [10] D.J. Kathios, G.D. Jarvinen, S.L. Yarbro and B.F Smith, J. Membr. Sci., 97 (1994) 251. [11] A. Geist, M. Weigl and K. Gompper, Radiochim. Acta, 93 (2005) 197. [12] A. Geist, M. Weigl U. Muellich and K. Gompper, Membr. Technol., 5 (2003) 5–7. [13] A. Geist, M. Weigl and K. Gompper, Sep. Sci. Tech., 37 (2002) 3369. [14] C.B. Patil, P.K. Mohapatra, R.R. Singh, P.B. Gurba, P. Janardan, R.D. Changrani and V.K. Manchanda, Radiochim. Acta, 94 (2006) 331–334. [15] K.K. Gupta, V.K. Manchanda, M.S. Subramanian and R.K. Singh; Solv. Extr. Ion Exch., 18 (2000) 273. [16] G.H. Jeffery, J. Bassett, J. Mendham and R.C. Denney, Vogel’s Textbook of Quantitative Chemical Analysis, Addison Wesley Longman, Singapore, 1996. [17] W. Davies and W. Gray, A rapid and specific titrimetric method for the precise determination of uranium using iron(II) sulphate as reductant, Talanta, 11 (1964) 1203. [18] C.B. Patil, P.K. Mohapatra, P.B. Gurba, P. Janardan, R.D. Changrani and V.K. Manchanda, Proc. Indian Nuclear Society Annual Conference (INSAC-2005).