Supercritical Carbon Dioxide - ACS Publications - American Chemical

Chapter 2

Extraction of Uranium and Lanthanides from Their Oxides with a High-Pressure Mixture of T B P - H N O H O-CO 3

2

2

1

2

3

YouichiEnokida ,IchiroYamamoto ,and ChienM.Wai Downloaded by MONASH UNIV on May 4, 2015 | http://pubs.acs.org Publication Date: August 31, 2003 | doi: 10.1021/bk-2003-0860.ch002

1

2

Research Center for Nuclear Materials Recycle and Department of Nuclear Engineering, Nagoya University, Nagoya 4648603, Japan Department of Chemistry, University of Idaho, Moscow, ID 83844-2343

3

The solubility of tri-n-butylphosphate (TBP) complex with nitric acid in supercritical CO (SF-CO ) is very high and (HNO ) (H O) TBP is completely miscible in SF-CO if the mixture is pressurized more than 14 MPa at 323 K . A mixture of 0.5 mol•dm UO (NO ) (TBP) and SF-CO is also miscible if the mixture is pressurized more than 13MPaat 323 K . Uranium was extractable from the solid matrices of UO with the high pressure solution of TBP-HNO -H O-CO at 323 Κ and 13 MPa. Molecular ratios of HNO to TBP in the high pressure mixture affected extraction rates and the higher ratio gave the higher extraction rates. Currently an application of supercritical fluid extraction to the nuclear fuel reprocessing is under investigation in Japan. Another promising field of application is treatment of uranium wastes. Decontamination factor of UO or U O was determined to be higher than 3x10 for the decontamination using the simulated uranium waste. The solid waste after the supercritical fluid leaching treatment was dry and contained low concentrations of TBP and HNO . 2

3 1.8

2

2

0.6

2

-3

2

3

2

2

2

2

3

2

2

3

2

3

8

2

3

10

© 2003 American Chemical Society

In Supercritical Carbon Dioxide; Gopalan, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.

Downloaded by MONASH UNIV on May 4, 2015 | http://pubs.acs.org Publication Date: August 31, 2003 | doi: 10.1021/bk-2003-0860.ch002

11 Uranium is a widely and dispersedly distributed element on the earth. Apart from its long-standing though small-scale use for coloring glass and ceramics, its only significant use is as a nuclear fuel (I). The nuclear fuel has a distinct feature that a spent fuel can be recycled if it is reprocessed properly (2-4). Therefore separation of uranium from the other elements is very important in the processes related to the nuclear fuel cycle. For this reason, separation of uranium as well as plutonium is indispensable. The conventional reprocessing process is known as the P U R E X process, which involves a solvent extraction using tri-n-butylphosphate (TBP). The P U R E X process is used commercially without almost any exception, because we can expect economical recovery of the products with a high purity. However, recently several alternative processes have been investigated in pursuit of a more economical process. The economical decline of the P U R E X process was brought about mainly by increasing cost of waste management. A n innovative uranium separation method is also expected for treatment of uranium wastes which arise from nuclear fuel fabrication. In these fields, avoidance of generating the secondary contaminated wastes is important point of view to develop new processes. Lanthanide elements are used for miscellaneous purposes (I) and recently they are used as additives in fluorescence light tubes to improve their spectra. Additionally, recycling of fluorescence light tubes has just started because mercury is contained in the tubes. A new method of recovering lanthanides is required to improve economical aspect of the recycle. Extraction of uranium and lanthanides from solid matrices is required in the industrial processes previously mentioned. Conventionally, leaching with strong acids such as nitric acid is widely used in these processes, but the treatment and recycling of waste acids are costly and their potential impact on the environment is a major concern. Supercritical fluid extraction (SFE) techniques have been reported for recovering uranium and lanthanides from their oxides and from environmental samples using fluorinated β-diketones and TBP as extractants (5-7). The reasons for developing SFE technologies are mostly due to the changing environmental regulations and increasing costs for disposal of conventional liquid solvents. Supercritical fluids exhibit gas-like mass transfer rates and yet have liquid-like solvating capability. The high diffusivity and low viscosity of supercritical fluids enable them to penetrate and transport solutes from porous solid matrices. From this point of view, SFE is an ideal method to extract uranium and lanthanides from solid wastes. Carbon dioxide (C0 ) is most frequently used in SFE because of its moderate critical pressure (P ) and temperature (T % inertness, low cost, and availability in pure form. Figure 1 illustrates moderate values of P and T compared with those of water. This new extraction technology appears promising for effective processing with marked reduction in waste generation because no aqueous solutions and organic solvents are involved and phase-separation can be easily achieved by depressurization. Recently, the authors found a C 0 soluble TBP complex with nitric acid was very effective for dissolution of U 0 and U Og and extracted as U 0 ( N 0 ) ( T B P ) in supercritical C0 (SF-C0 )(S-1I). In this chapter, applicability of SFE of uranium and lanthanides using T B P complex with nitric acid in S F - C 0 is explained and discussed. 2

c

c

c

c

2

2

2

3

2

2

2

3

2

2



12

Introduction of Acid into Supercritical C 0

2

Extraction of metal compounds by S F - C 0 is feasible when they are complexed with organic ligands (7). For example, nitrates of uranium and lanthanides are easily complexable with T B P and solubilities of the complex are very high in supercritical C 0 . This method is also applicable to hexavalent uranium extraction from solid U 0 using T T A and T B P in S F - C 0 (6). Since U 0 is the most common uranium oxide found in nuclear fuels, uranium extraction from U 0 is indispensable in the nuclear fuel reprocessing and uranium waste treatment. From solid U 0 , however, uranium extraction is not possible with this method because T T A nor T B P form complexes with U 0 directly. If we introduce acid homogeneously in S F - C 0 , we can expect U 0 dissolution and conversion into U 0 , which can be complexed with T B P by the charge neutralization as T B P U 0 ( N 0 ) . For example, concentrated nitric acid is widely used to dissolve U 0 in aqueous solution forming U 0 ( N 0 ) . Nitric acid is one of the three major acids of the modern chemical industry and has been known as a corrosive solvent for metals since alchemical times in the thirteenth century. The concentrated nitric acid is a mixture of H N 0 and H 0 , and the phase diagram H N 0 - H 0 shows the presence of two hydrates, H N 0 * H 0 and Η Ν 0 · 3 Η 0 (i). Solubility of nitric acid in S F - C 0 is low due to the weak solute-solvent interaction. However, if nitric acid is complexed with an organic ligand, it may become quite soluble in S F - C 0 . We have demonstrated this technique using a TBP complex of nitric acid (12). 2

2

3

2

2

2

2

2

2

2

2 +

2

2

2

3

2

2

2

3

2

3

3

3

2

2

3

2

2

2


2

13 In our experimental works, the certain volume of an anhydrous T B P (Koso Chemical, Japan) was contacted with different volumes of 60% H N 0 (Wako Pure Chemicals, Japan) by shaking vigorously in a flask for 30 min followed by centrifugation to prepare an organic solution of TBP complex with nitric acid. Note that the prepared organic solution was a mixture of TBP, H N 0 and H 0 and formed a single phase. The concentration of H 0 in the organic phase was measured by Karl-Fischer titration (Aquacounter AQ-7, Hiranuma, Japan). The concentration of H N 0 in the organic phase was measured with a titrator (COM-450, Haranuma, Japan) with 0.1 m o l f (M) NaOH solution after adding a large excess amount of deionized water. 3

3

2

2

3


1

Acidities of the T B P complex with nitric acid and equilibrated aqueous nitric acid are shown in Figure 2 for different phase ratios. In the P U R E X process, acidity of 3 to 6 M is applied to dissolve spent fuels in aqueous solution. Figure 4 suggests the organic solution equilibrated with aqueous nitric acid of more than 6 M is easy to prepare, and this organic solution may dissolve U 0 . Without S F - C 0 , we have tried to dissolve U 0 powder with two kinds of TBP complexes with nitric acid under the normal pressure. The two composition can be described as TBP(HN0 ) . (H 0)o. and TBP(HNO )i. (H O) . . Although, with the former, U 0 was not dissolved, with the latter well dissolved. When we pressurized TBP(HN0 ) . (H 0)o. with S F - C 0 , an aqueous phase appeared, and U 0 was expected to be dissolved in the aqueous phase. The appearance of the aqueous phase can be understood as an antisolvent effect with SFC 0 . For TBP(HN0 )i. (H 0)o.6, however, no visible droplet was observed when the organic solution was mixed with S F - C 0 . Figures 3 illustrate molecular ratios observed in the organic solution. These results suggest that there may be two different chemical structures for the organic solutions. In this chapter, we describe the T B P complex with nitric acid, T B P ( H N 0 ) ( H 0 ) a s T B P - H N O H 0 . Similarly, T B P - H N 0 - H 0 - C 0 denotes a high pressure mixture of T B P - H N 0 - H 0 and S F - C 0 . 2

2

2

3

0

7

2

7

3

8

2

0

6

2

3

0

7

2

7

2

2

2

3

8

2

2

3

x

2

y

r

2

3

3

2

2

2

2

Solubility of TBP Complex with Nitric Acid We studied the solubility of T B P - H N 0 - H 0 in S F - C 0 carefully. Figure 4 illustrates an experimental apparatus used for solubility measurements. This apparatus was prepared to operate up to 25 MPa in pressure and up to 373.15 Κ in temperature. Pressurized C 0 (99.9% purity, Nippon Sanso) was introduced from a cylinder to the experimental apparatus via a syringe pump (Model 260D, ISCO) with a controller (Series D, ISCO). The temperature of the pressurized C 0 was kept at 268.15 Κ through a heat exchanger placed in a cryogenic bath (CCA-1000, Eyela) before feeding to the pump. The equilibrium pressure in the cell was measured with a pressure gauge (NPG-350L, Nihonseimitsu). The internal volume of the cell is 60 cm . The temperature of the fluid in the cell was maintained constant with circulating water from a water bath (UA-10S, Eyela) within ±0.03 K . A magnetic stirrer (RCN3, Eyela) was used for mixing the fluid in the cell. 3

2

2

2

2

3


14

100

-•-Aqueous acidity -•-Organic acidity


Y

0.1

1

Initial NO volume ratio

•

to

Figure 2. Acidities of TBP complex with nitric acid and equilibrated aqueous phase.

IHN0 J /fH 0) 3 A

2

|HN0 yiH OJ

A

3

2

A

Figure 3. Molecular ratio of(a)HN0 to TBP(left) and (b)HN0 to H 0 (right) in the TBP complex with nitric acid as a function of molecular ratio of nitric acid to water in equilibrated aqueous nitric acid. 3

3

2



15

Figure 4. A schematic diagram of the experimental apparatus for solubility measurement 1; CO2 cylinder, 2; syringe pump, 3; pump controller, 4; thermostated water bath, 5; view cell, 6; magnetic stirrer, 7; liquid trap, 8; gas reservoir, 9; gas volume meter.

10

s

10*

ίο*

3

10"

10"

2

Mole fraction of T B P ( H N O

) 3 t.8

10

1

0

( H O) 2

0.6

Figure 5. Experimental data and correlated curves for transition pressures from two-phase to single-phase for a mixture ofSF-C0 and TBP(HN0 )i.g(Fifi)o.6 system 2

3


16 For each run of the experiments, the gas in the whole line of the apparatus was replaced with C 0 at the atmospheric pressure following a charge of a known amount of T B P - H N 0 - H 0 in the cell. The pressurized C 0 was charged into the cell with the syringe pump and well mixed with the organic solution. In order to maintain the isothermal conditions, liquid C 0 through the pump was heated through the heat exchanger placed in the water bath. The charging of CO? was continued until a single phase was formed in the view cell with pressure increase of the fluid in the cell. The solubilities of T B P - H N 0 - H 0 were determined with the bubble point method. The phase transitions resulting from pressure variations were performed and observed by direct visualization. Near the point of a single-phase formation for binary mixtures, an increment of charging pressure was set to a small value (less than 0.1 MPa), and kept the fluid in a half hour to confirm whether the fluid mixture was a single-phase equilibrium or not. A transition pressure was determined as the pressure at which the two-phase to single-phase transition took place. After it was confirmed a single-phase equilibrium, the pressure in the cell was lowered by opening the exit valve very slightly and the appearance of the two-phase mixture was checked. The total amount of the charged C 0 gas was measured with a gas volume meter (DC-1C, Shinagawa) at a constant temperature. 2

3

2

2

2


3

2

2

We found that this organic complex is homogeneously miscible with S F - C 0 with an arbitrary fraction when the mixture is pressurized more than 14 MPa at 323.15 K . This finding implies that we can introduce nitric acid in S F - C 0 i n order to dissolve metals including uranium and lanthanides. Figure 5 illustrates experimental data obtained for the transition pressure from two phases to a single phase at different temperatures. In this figure, the upper part is the single-phase region and the lower part is the two-phase region. There exists the maximum point of the locus of the transition pressure, with increasing the concentration of T B P complex near 0.03 mole fraction of T B P complex under isothermal condition. The similar phase behavior has been reported for TBF and C 0 system (8). However, the maximum transition pressure reported for the mixture of T B P and C 0 was 11 MPa at 323.15 K , which was lower than the value we obtained for the mixture of S F - C 0 and ΤΒΡ(ΗΝ0 ) (Η 0)ο.6. The critical opalescence was observed for this system; this means that there exists a pseudocritical point of this mixture where the system goes through a smooth change from two phase coexistence to a one phase state, without a true phase transition. The correlated curves plotted in Figure 5 were obtained by using the same approach described in the literature (14) and the detail is given in Ref. (16) 2

2

2

2

2

3

1

Supercritical Carbon Dioxide - ACS Publications - American Chemical

Recommend Documents