Separations for the Nuclear Fuel Cycle in the 21st Century - American

Chapter 8

Downloaded by NANYANG TECHNOLOGICAL UNIV on October 14, 2015 | http://pubs.acs.org Publication Date: June 9, 2006 | doi: 10.1021/bk-2006-0933.ch008

Comparison of Uranyl Third-Phase Formation in 30% TBP-Nitric Acid in Dodecane and HPT Using UV-Visible Spectroscopy 1

2

2

3

3

J . Plaue , S. Goeury , J . Petchsaiprasert , M . Draye , J . Foos , and K . Czerwinski 2,3

1

Defense Nuclear Facilities Safety Board, 625 Indiana Avenue, NW, Suite 700, Washington, DC 20004 Department of Chemistry, University of Nevada at Las Vegas, 4505 Maryland Parkway, Las Vegas, NV 89154-4003 Conservatoire National des Arts et Métiers, 11 Rue Pierre et Marie Curie, 75005 Paris, France 2

3

Third phase formation behavior was studied in the uranyl / 30% tri-n-butyl phosphate/high molar nitric acid system. U s i n g metal ion titration coupled with U V visible spectroscopy, a comparison was made between the diluents dodecane and hydrogenated polypropylene tetramer ( H P T ) . This work presents the first quantitative comparison o f dodecane with H P T , which is commonly used

in the

French

reprocessing

program.

As

anticipated, higher metal loading prior to third phase formation was observed in the H P T system due to the branched configuration o f the diluent. Spectral evidence o f differences in speciation between each phase, as well as a change in the single organic phase speciation prior to phase splitting is presented.

© 2006 American Chemical Society

In Separations for the Nuclear Fuel Cycle in the 21st Century; Lumetta, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2006.

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Introduction There exists a lack of quantitative information regarding third phase formation in solvent extraction systems. Historically, the primary methods relied upon determination of the phase split boundary though visual identification. This process has been shown to be error prone and has resulted in inconsistencies across the literature (/). This chapter attempts to quantitatively dissect third phase formation using ultraviolet-visible spectroscopy. While general spectroscopic studies of the third phase have been performed previously, this work utilized multiple spectroscopic titrations to elucidate changes in the speciation of uranium as the third phase is formed. In addition, a comparison is made between the diluents w-dodecane and hydrogenated polypropylene tetramer (HPT). Branched hydrocarbons, such as HPT, have been reported to have higher resistance to third phase formation, and are commonly used in the French reprocessing industry. Little published data is available on the behavior or composition of HPT, however it is commonly assumed to be a branched dodecane. While the range of acid conditions studied is limited and under non-typical conditions, they are important to understanding the system chemistry and can be applied to off-normal plant scenarios. Third phase formation is the phenomenon in which the organic phase of a normal two-phase solvent extraction systems splits into heavy and light phases, thereby creating a three-phase system. Historically, the study of third phase formation in solvent extraction systems has exclusively focused on defining the boundaries where it may occur. Typically, results are reported as limiting organic concentrations (LOC), which is the maximum concentration of metal found in the organic phase prior to a visual observation of phase splitting. Although the effects of several variables (nitric acid and extractant concentrations, temperature, diluent and, extractant types, and ionic strength) have been studied on the formation boundaries (2-5), very little progress has been made on the formation mechanism or characterization of third phase species. A complete understanding of the third phase



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phenomenon is important from a process safety perspective. Serious upsets, such as criticality or runaway reactions, can result from third phase formation. Uranyl third phase formation has been the focus of renewed studies (6-7). Unique spectra were identified for the third phase, while the normal organic phase spectra closely resembled that of the light organic phase. These results suggested an abrupt change in the uranyl speciation rather than the progressive buildup of insoluble organic complexes. However, these UV-Visible spectroscopy results contradicted several other reports tending to show third phase formation as the result from an aggregation-type process (8-9). The primary evidence for this conclusion is based upon small angle neutron scattering measurements indicating the presence of reverse micelle which interact more strongly with increasing organic metal concentration. These techniques were initially developed for other systems involving other extractants (10-11), but the results in the TBP systems have lead to various models for prediction of third phase formation (12-13). This chapter does not attempt to propose a unique model, but rather add to the small body of work on this topic, especially with respect to the diluent HPT.

Methods Stock solutions of uranyl nitrate were prepared at constant nitric acid concentration by dissolution of known quantities of U 0 (N0 ) *6H 0 (Merck) in nitric acid solutions. Organic extraction phases were prepared using 30 vol. % tri-w-butyl phosphate (Aldrich 97 %) with respective diluents; w-dodecane (Prolabo) and hydrogenated polypropylene tetramer (Novasep SAS, France). The conditions examined are given in Table 1. All work was performed at room temperature of approximately 22 °C. Equal volumes ( 4 - 7 mL) of organic and aqueous phases were contacted and vigorously mixed for a period of 10 minutes using a wrist action shaker. After centrifuging, samples of each phase were drawn off and 2

3

2

2



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spectra taken using a Cary 5E UV-visible spectrometer with precision quartz cuvettes (1 cm path length) measured against a deionized water reference cell. Four sets of spectra were collected for each sample and the average used in analysis performed with Microsoft Excel™ software. Samples were returned, and a small volume of aqueous uranyl ion was titrated into the system at constant acid molarity. Phase volumes were determined by mass using density measurements. Using this methodology, spectral evidence of third phase behavior was collected before and after phase separation was observed. Extinction coefficients for aqueous phase uranyl nitrate were determined using laboratory prepared samples. Acid concentration in each phase was determined by titration with a Metrohm titration apparatus. The organic phase was washed twice with water to remove the acid, and oxalate was added to the aqueous phase to bind the uranium and prevent complexation with hydroxide during the titration. Material balance calculations enabled determination of total organic phase metal content, from which extinction coefficients for the light and heavy organic phases were determined.

Table 1. Experimental conditions

Diluent Dodecane HPT

[HN0 ] M 11, 11.5, 12, 13, 14,14.2 12,13,14 3

2+

fU0 J M 0-0.65 0.1-0.75 2

aa

Results and discussion

Organic acid phase concentration

The titration data showed a distinct change in organic phase acid concentration upon third phase formation. In the presence of a single organic phase, the acid concentration in the organic



123

phase tends to increase with increasing aqueous acid concentration in the absence of uranium (Table 2). For comparison purposes, the experimental distribution values are compared to model predictions using the Argonne Model for Universal Extraction (AMUSE) (14). When uranium is present in the system, a decrease in the single phase organic acid concentration with increasing uranium organic phase concentration is observed (Table 3). The data presented in Table 3 provide averages of the measured nitric acid concentration under set phase conditions. Additional measurements taken as the system was loaded with uranium indicate trends. In the single organic phase, the organic nitric acid concentration is observed to decrease on approach of the phase split. Upon phase splitting, there is a pronounced decrease in organic nitric acid concentration in the light phase. Further measurements taken as uranium is added to the system show a decrease in the light organic phase acid concentration. However, the heavy organic phase experiences a dramatic increase in nitric acid concentration. The total acid concentration in the heavy phase is generally higher than any other organic phase conditions, particularly at higher total acid concentrations. This behavior indicates a minimum nitric acid concentration is required for the formation of the heavy organic phase under a set of given initial conditions. These observations are seen for both the dodecane and HPT systems studied. For both the dodecane and HPT organic phases, the nitric acid distribution increases with increasing total nitric acid concentration. This trend differs with the nitric acid distribution in the absence of uranium (Table 2), where the distribution decreases and reaches a minimum with increasing total nitric acid concentration. For the single and light phase there is a similar decrease in acid distribution with increasing uranium concentration at a given initial nitric acid concentration. However, as shown in Figure 1, differences between the nitric acid distributions for the organic diluents are evident. For the dodecane, the heavy organic phase has a higher nitric acid distribution compared to the HPT systems, indicating a lower acid concentration in the HPT


124 Table 2. Nitric acid concentrations in moI/L for single organic phase in dodecane with no uranium

[HNO ] 8 9 10 11 12 13 14 14.2


}

a(lim

[HN0 ] 1.09±0.08 1.12±0.07 1.20±0.02 1.20±0.01 1.30±0.01 1.39±0.01 1.51±0.03 1.55±0.03 3

ore

AMUSE 0.170 0.163 0.157 0.155 0.153 0.150 0.146 0.145

[HNOsUt/fHNOsJao 0.157 0.142 0.136 0.122 0.121 0.120 0.121 0.120

0.25 O Dodecane

0.20 -

•

HPT

0.15 -

0.10

0.05

10

11

12

13

14

15

[HNOjk, M

Figure 1. Nitric acid distribution between the heavy organic phase and aqueous phase for dodecane and HPT diluents as a function of total nitric acid concentration.



1.31±0.15

14 2.93±0.18

0.0200±0.0017

0.0791±0.0068

0.193±0.008

1.93±0.10

1.44±0.11

2.42±0.14

1.14±0.04

13

]

0.343±0.018

2 +

0.381±0.021

2

1.23±0.11

11

[U0

phase

1.07±0.15

Heavy

0.65±0.17 0.84±0.08

phase

11.5 12

Single [HNO3]

3

[HN0 ]

[HNO3]

Aqueous

concentration

]

0.026±0.024

0.762±0.167

0.797±0.157

1.88±0.21E-3

5.06±0.81E-3

0.0177±0.0016

0.0968±0.009

2 +

0.0502±0.0048

2

0.474±0.211

[U0

phase

0.384±0.048

[HNO3]

Light

presence of uranium. The uranium concentration is 110% of the L O C for the nitric acid

Table 3. Average organic phase nitric acid concentration in mol/L for dodecane diluent in the


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third phase. As apparent in the comparison of the uranium concentration described below, the acid behavior is opposite of the uranium behavior, where the formation of the HPT third phase requires a higher uranium concentration.


Organic uranium phase concentration

The uranium concentration in the organic phase was observed to increase with increasing total uranium concentration, regardless if the organic is composed of a single phase or two phases. For the two phase organic condition, there is a noticeable difference in the uranium concentration and increase due to total uranium present (Figure 2). For the single phase system, the expected increase in organic uranium concentration is consistent with the expected distribution behavior. This trend is continued with the heavy organic phase upon formation of the second phase. The light phase exhibits only the slightest organic uranium concentration increase. These observations are consistent with all the conditions examined, including both HPT and dodecane diluents. As previously stated, the organic uranium behavior differs from the observed nitric acid organic phase behavior for the heavy and light organic phases. This appears to indicate a radical difference in the extracted uranium species in the heavy and light phase. However, if the ratio of organic acid to uranium is evaluated, similarities between the light and heavy organic phase are recognizable (Figure 3). The ratio decreases with increasing total uranium for both the light and heavy organic phases. Furthermore, the ratio values are similar with the largest differences at outset of the phase splitting. For all systems examined, the acid to uranium ratio of the two organic phases converge under the highest total uranium initial concentrations. This indicates there are similarities between the extracted organic uranium species in each phase. At the initial onset of third phase it appears the heavy phase has a relatively larger acid concentration. This can be attributed to the extraction of nitric acid by either uncomplexed TBP or


127 0.50


0.40 %

0.30

O

0.20

• ± *

Single phase Light phase Heavy phase

0.10

..A, * . . . A .

if

0.00 0.05

0.15

0.1

2+

[UO2

0.25

0.2

0.3

0.35

] aq initial M

Figure 2. Organic uranium concentration for the 12 MHNO /30 % TBP in dodecane system 3

12.00 11.00

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Separations for the Nuclear Fuel Cycle in the 21st Century - American

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