DIAMEX Solvent Behavior under Continuous Degradation and

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Chapter 21

DIAMEX Solvent Behavior under Continuous Degradation and Regeneration Operations B. Camès,* I. Bisel, P. Baron, C. Hill, D. Rudloff, and B. Saucerotte CEA, Nuclear Energy Division, Radiochemistry & Processes Department, SCPS, LPCP, F-30207 Bagnols sur Cèze, France *[email protected]

Industrial implementation of partitioning processes based on solvent extraction requires solvent recycling with constant separation and physicochemical properties. In nuclear applications, both radiolysis and acidic hydrolysis lead to degradation products which need to be removed from solvent before its recycling. The long term evolution of DIAMEX process solvent (0.65M DMDOHEMA–HTP) under continuous degradation by acidic hydrolysis and γ-radiolysis was studied in the laboratory-scale MARCEL γ–irradiation facility, with and without alkaline treatment process (AT). With AT, analyses of organic phase showed the accumulation of only one degradation product, MDOHEMA, probably responsible for the molybdenum accumulation observed. Distribution coefficients of Zr, Pd, Fe and Nd, surface tension, refraction index, settling time, viscosity, and density were constant during the tests. Furthermore, no catalytic effect of fission and corrosion products was observed. These studies were consolidated by comparative α- and γ-radiolysis batch tests which showed that α-radiolysis and γ-radiolysis led to the same degradation products.

Introduction As decreed by the French law of 30th December 1991, a 15-year research program was carried out by the CEA. Its objective was the investigation of separation processes for subsequent transmutation of long-lived radionuclides, © 2010 American Chemical Society Wai and Mincher; Nuclear Energy and the Environment ACS Symposium Series; American Chemical Society: Washington, DC, 2010.

in order to significantly reduce the radiotoxicity of the final waste produced by the nuclear industry. The main targets were the minor actinides (MA) – i.e. neptunium, americium and curium – to be recovered quantitatively and selectively from PUREX (Plutonium - Uranium Refining by EXtraction) raffinate, the product of nuclear spent fuel reprocessing. The reference strategy for separating MA from spent fuel is based on an adaptation of the PUREX process for the separation of Np, and the development of new liquid-liquid partitioning processes, called DIAMEX (DIAMide Extraction) and SANEX (Separation of ActiNides by Extraction), for the other minor actinides (Am and Cm).

Aims of the Studies In the DIAMEX process, the use of a malonamide diluted in an aliphatic diluent permits the co-extraction of the Actinides (III) and the Lanthanides (III) from the waste. Different studies were performed to demonstrate the technical feasibility of this process: some focused on solvent stability, as the organic phase is degraded due to hydrolysis by aqueous nitric phases and radiolysis by fission products. The resulting degradation products modify the extraction and/or the hydrodynamic performances of the process. It is therefore necessary to remove these degradation compounds from the spent solvent by a specific treatment before recycling, and in parallel to have a fresh solvent supply. The extraction, separation, and hydrodynamic properties can thus be maintained. To anticipate and scale-up the DIAMEX solvent behavior under continuous operations, two kinds of studies were realized: -

-

Batch studies to determine the parameters influencing the degradation kinetics of the extractant molecule, in particular the effects of γ and α radiation and those of acidic hydrolysis, and to establish the efficiency of solvent treatment. Long term tests to study the behavior over time of the solvent (hydrolysis effect, combined hydrolysis and radiolysis effects, impact of the solvent clean-up), the kinetics of solvent consumption and of degradation product accumulation, and the impact of these on process performances.

Experiments Batch Studies The Diamex Solvent A DIAMEX solvent is an organic phase consisting of a malonamide diluted to 0.65 M in Hydrogenated TetraPropene (HTP). The reference molecule is N,N′-DiMethyl-N,N′-DiOctyl-Hexyl-Ethoxy-Malonamide (DMDOHEMA). This malonamide molecule’s formula was optimized to minimize long-alkyl-chain degradation products (1). 256 Wai and Mincher; Nuclear Energy and the Environment ACS Symposium Series; American Chemical Society: Washington, DC, 2010.

Malonamide degradation under hydrolysis and radiolysis has been studied in detail using GC-FTIR and GC-MS techniques (2). The main degradation products identified in the organic solution after radiolysis or hydrolysis in presence of nitric acid aqueous phase were an amidic-acid (1), a monoamide (3), diamides (DMOHEMA (4) with a loss of octyl group and MDOHEMA (5) with a loss of methyl group), carboxylic acids and amines such as MethylOctyl Amine (2) (MOA) (Figure 1).

Degradation Conditions of DMDOHEMA The diluted organic phase (0.65M or 1M) was pre-equilibrated with HNO3 media (2 to 4M) to obtain different concentrations of HNO3 in the organic phase, and then exposed to γ (60Co or 137Cs source) or α (244Cm aqueous solution) radiation or to hydrolysis at 45°C in presence of HNO3 aqueous phase. Next, different analyses of the solvent were performed: -

-

Gas chromatography (GC) coupled with a flame ionization detector (FID) to quantify compounds with a molecular weight higher than 170 g/mol, such as amidic-acid, monoamide, MDOHEMA or DMOHEMA and the remaining DMDOHEMA. Potentiometric titration in nonaqueous medium to determine the concentration of amide and/or amine compounds, such as methyl octyl amine, and acidic species such as carboxylic acids.

Figure 1. Simplified scheme for radiolytic or hydrolytic degradation of DMDOHEMA 257 Wai and Mincher; Nuclear Energy and the Environment ACS Symposium Series; American Chemical Society: Washington, DC, 2010.

Determination of the Kinetic of the Malonamide Degradation The solvent composed of DMDOHEMA 0.65M in HTP, pre-equilibrated with 3M HNO3 media, was degraded either at 4 kGy·h-1 by α (244Cm) or γ (60Co) radiations, or by hydrolysis at 45°C in presence of 3M HNO3 aqueous phase. The monitoring of the remaining DMDOHEMA by GC-FID in all the degraded solutions allowed an estimation of the malonamide degradation kinetic. The results summarized in Figure 2 show that under these conditions, DMDOHEMA disappearance is linear with time under hydrolysis and α and γ radiolysis, according to an apparent zero order kinetics. From the slope of the curves, the different apparent kinetic constants (k1) are calculated: -

For hydrolysis at 45°C during 700 h, k1h = - 0.636 mmol·L-1·h-1. For radiolysis at 4 kGy·h-1 with a cumulated dose of 0.7 MGy (175h), ○ ○

k1γ = - 1.666 mmol·L-1·h-1 for gamma radiation k1α = - 0.438 mmol·L-1·h-1 for alpha radiation.

These results thus show that the impact of alpha radiation is 4 times slower than that of gamma radiation.

Figure 2. Evolution of the remaining DMDOHEMA after degradation, with duration. [DMDOHEMA]0=0.65M in HTP pre-equilibrated with 3M HNO3. 258 Wai and Mincher; Nuclear Energy and the Environment ACS Symposium Series; American Chemical Society: Washington, DC, 2010.

To complete these results, the influence of DMDOHEMA concentration, initial acidity of the organic phase and dose rate were studied. The different degradation conditions are summarized in Table 1, the GC-FID analysis of the irradiated organic phases revealed that the variation of: -

The remaining DMDOHEMA concentration is linear with time, but different slope values (k1γ) are obtained. The plotting of these k1γ values increases linearly with dose rate, but different slope values (k2γ) are obtained. The plotting of these k2γ values increases linearly with acidity of the organic phase and leads to one slope value of 0.316.10-3 KGy-1 (Figure 3).

Table 1. Summary of the different irradiation conditions of the DMDOHEMA solution in HTP and of the apparent kinetic constants (k1γ and k2γ) obtained. γ source: 137Cs or * 60Co [DMDOHEMA]0 (M)

(M)

(M)

2

0.25

Time (h)

Dose rate (KGy/h)

k1γ (mmol/ L/h)

461

1.0

-0.374

461

1.9

-0.610

1.3

-0.545

k2γ (mmol/ L/KGy) -0.335

74 234 0.65

1

3

330

0.44

4

0.60

3

0.73

4 (3)

1.0

-0.405

479 461

1.9

-0.639

175

4.1*

-1.666

461

0.9

-0.404

461

1.9

-0.594

461

1.0

-0.595

461

1.8

-0.883

188

4.0*

-2.293

259 Wai and Mincher; Nuclear Energy and the Environment ACS Symposium Series; American Chemical Society: Washington, DC, 2010.

-0.449

-0.510 -0.573

Figure 3. Evolution of the apparent kinetic constants (k2γ) with initial organic phase acidity for the different irradiated DMDOHEMA solutions From these results, it seems that the kinetic of DMDOHEMA degradation is a function of 3 factors: initial nitric acid concentration in the organic phase, dose rate, and time, according to the following relationship:

: dose rate (KGy/h), t: radiolysis time (h), k3γ: consumption kinetic constant = 0.316.10-3 KGy-1 and constant = 0.265.10-3 KGy-1

Degradation Product Accumulation For the different degraded organic phases, GC-FID analyses also permitted quantification of the degradation products, such as monoamide (MOHOBA), amidic-acid (MOCHOBA) and diamide (MDOHEMA) only formed by radiolysis. Potentiometric titration revealed that there were more amide and/or amine compounds and acidic species than those quantified by GC-FID. Except for the methyloctyl amide (MOA) characterized by its pKa value (9.8), the other products were not able to be identified. For these studies, they are called “light amide and/or amine” and “light acidic” compounds, because they have a molecular weight lower than the species quantified by GC-FID. 260 Wai and Mincher; Nuclear Energy and the Environment ACS Symposium Series; American Chemical Society: Washington, DC, 2010.

Figure 4 shows an example of quantification for all degradation products formed in the case of radiolysis at a dose rate of 4 KGy/h (60Co).

Figure 4. Evolution of the degradation products with radiolysis duration. (60Co irradiation, dose rate = 4 KGy/h, [DMDOHEMA]0=0.65M,

= 0.44M

Removal of the Degradation Products These degradation products must all be removed from the solvent if its performance is to be maintained. Different treatments were therefore tested, including washing with HNO3 (0.1M), which is efficient towards MOA and carboxylic acids bearing less than 6 carbon atoms. The remaining products were monoamide, amidic-acid, MDOHEMA only in the case of radiolysis, and the light amides/amines or acid compounds. Table 2 summarizes the accumulation kinetic constants of these degradation products, estimated in the linear parts of the curves. An additional alkaline treatment with 0.3M sodium hydroxide permitted the removal of: -

Amidic-acid and light acids, in the case of hydrolysis. Amidic-acid and half of light acids, in the case of radiolysis.


Table 2. Apparent kinetic constant values for degradation product accumulation, after HNO3 0.1M washing of degraded DIAMEX solvent (Pre-equilibrated solvent with HNO3 3M and radiolysis at 4 KGy/h or hydrolysis at 45°C) k1γ (mmol.L1.h-1)

Monoamide

Amidic acid

MDOHEMA

“Light*” Amides/ amines

“Light*” acids

Radiolysis

0.2

0.3

0.2

1.4

0.4

Hydrolysis

0.4

0.3

0

0.5

0.2

Impact of the Remaining Degradation Products After solvent clean-up, the impacts of the remaining degradation products on the process performances were assessed. The measurement of the phase Disengagement Time Ratio (DTR) permits the evaluation of the degraded solvent’s hydrodynamic properties.

Figure 5 shows that the DTR values: -

Do not completely depend on the diamide concentration. Depend on the presence of some of the remaining degradation products.

The impact of the remaining degradation products after 0.1M HNO3 washing is low in the case of hydrolysed solvent and high for radiolysed solvent. A 0.5M Na2CO3 treatment permitted the restoration of hydrolysed organic phase hydrodynamic performances, but was not sufficient for radiolytically degraded solvent. Measurement of the different metal (americium or europium) distribution ratios between organic phases and a 3M HNO3 aqueous phase showed (Figure 6) that the degradation products formed by radiolysis do not greatly affect the DEu or DAm values, which decrease with decreasing DMDOHEMA concentration. On the other hand, the degradation products formed by hydrolysis affect the DEu or DAm values. The removal of some of these compounds by acido-basic treatments did not enable the restoration of the solvent’s extracting properties.


Figure 5. Evolution of the phase Disengagement Time Ratio (DTR) with DMDOHEMA concentration for non degraded or degraded solvent after acido-basic treatment

Continuous Operations Methodology The long term behavior of the DIAMEX solvent was studied in the MARCEL gamma irradiation facility. On this process platform, all is automated and monitored to ensure the facility’s safety: batteries of mixer settlers, centrifugal contactors (CC type 1, with a mixing chamber volume and a rotation speed lower than the CC type 2), pumps to feed in the aqueous solutions and the organic phase, scales to monitor the aqueous phase and level controllers for system safety. All the liquid-liquid contactors are installed under laboratory hoods, except the irradiated reactor (with a 137Cs source), located in the irradiator extension. The chemical composition, the physico-chemical properties (refraction index, surface tension, DTR, density and viscosity) and extraction/separation performances (distribution ratios of Fe, Nd, Pd, Zr and Mo) were monitored.


Figure 6. Distribution ratios of Am or Eu as a function of the DMDOHEMA concentration in the case of undegraded or hydrolysed or radiolysed (60C) solvent. [DMDOHEMA]0=0.65M,

= 0.44M

DIAMEX Flow-Sheet Scenarios Different flow-sheets were tested on the process platform under penalizing chemical conditions. The solvent (DMDOHEMA 0.65M in HTP) was: -

-

spiked with 3M nitric acid and cations, neodymium only, or in addition to iron, molybdenum, zirconium, and palladium, submitted to hydrolysis at 40°C with a residence time of 2.5 hours, alone or coupled with gamma radiolysis at a dose rate of 1.3 kGy/h with a residence time of 6.25 hours, after scrubbing and stripping (0.1M HNO3), the degraded solvent was either cleaned up with an alkaline treatment (AT) or not.

The main conditions are summarized in Table 3.


Table 3. Different DIAMEX flow-sheet scenarios tested on the Marcel process platform Cations

Degradation

Alkaline Treatment

Test duration

Nd

Hydrolysis

None

222 h

Nd

Hydrolysis

NaOH 0.3M

262 h

Nd

Hydrolysis + Radiolysis

None

334 h

Nd


NaOH 0.3M

298 h

Nd


NaOH 0.3M

765 h

Nd, Fe, Mo, Zr, Pd


NaOH 0.3M

446 h

The longest DIAMEX test with hydrolysis, radiolysis, Nd and alkaline treatment lasted 765 hours and corresponded to 43 solvent cycles, that is to say about 270 days of industrial operations for UOX2 spent fuels. The accumulated dose was 358 KGy, and the accumulated hydrolysis duration 104 h. The monitoring of the DMDOHEMA concentration by GC-FID permitted an adjustment of the solvent to its nominal composition. From the addition of DMDOHEMA and HTP, it was possible to estimate the apparent rate constant of solvent consumption (kγ). For example, in the case of the DIAMEX long test (Figure 7), the k values were: -

kγDMDOHEMA = 4.4 mmol/L/h hydrolysis (or 1.3 mmol/L/kGy) kγHTP = 69.4 mmol/L/h hydrolysis (or 20.2 mmol/L/kGy)

The more important consumption of HTP with regard to the DMDOHEMA (15 times more) is probably due to heating in the Centrifugal Contactors (CC) used for alkaline treatment, which increase the evaporation of HTP within these devices. Furthermore, the comparison of this test with the others (Table 4) showed that: -

the HTP evaporation is higher for the CC type 2 than for the CC type 1, the degradation conditions were such that the radiolysis impact was 3 times higher than the hydrolysis effect.


Table 4. Estimation of the apparent rate constants of solvent consumption (kγ) for the different DIAMEX flowsheets tested on the Marcel platform kγDMDOHEMA

kγHTP

Flowsheets

(mmol/L/h hydrolysis)

CC type 1 or 2

Nd+ Hydrolysis

1.4

7.3

Nd+ Hydrolysis+AT

2.4

20.9

Nd+ Hydrolysis + Radiolysis

3.9

19.1

Nd+ Hydrolysis + Radiolysis

7.9

37.6

CC type 1

Nd+ Hydrolysis + Radiolysis+AT

4.4

69.4

CC type 2

Nd, Fe, Mo, Zr, Pd + Hydrolysis + Radiolysis+AT

5.7

76.3

CC type 2

AT apparatus

CC type 1

Figure 7. Variation of the extractant concentration and make-up volumes during the DIAMEX test: 765 h – Nd+ H+R+AT For all these tests, the accumulation rate constants of the degradation products were estimated in the linear part of the curves (Table 5).


Table 5. Apparent kinetic constant values of degradation product accumulation for the different DIAMEX flowsheets kγamidicFlowsheets

kγmonoamide

acid

kγMDOHEMA

kγ“light” acids

(mmol/L/h hydrolysis) Nd+Hydrolysis

0.2

0.5

0

0

Nd+Hydrolysis+AT

0

0

0

0

Nd+Hydrolysis+Radiolysis

0.1

0.6

0.4

0.7

Nd+Hydrolysis+Radiolysis+AT

0

0

0.3

0

Nd, Fe, Mo, Zr, Pd +Hydrolysis+Radiolysis+AT

0

0

0.3

0

The comparison of all these kγ values allows the following assumptions: -

-

the amidic-acid is a primary degradation product formed by radiolysis or hydrolysis, MDOHEMA is a primary degradation product only formed by radiolysis, the monoamide is a secondary degradation product formed by degradation of amidic-acid, as it is not present in the solvent when the amidic-acid is removed by alkaline treatment, and the “light” acid compounds are primary degradation products only formed by radiolysis.

Moreover, the “light” amide/amine compounds detected in batch studies were not formed in continuous operations. Therefore, the use of alkaline treatment for the DIAMEX process allows only MDOHEMA to accumulate.

Impact of the Degradation Products on the Physico-Chemical Properties and Performances of the Solvent To study the impact of the degradation products on the process performances, different physico-chemical and distribution ratio measurements were monitored. For all the flow-sheets tested, there is no impact of the remaining degradation products on the distribution ratios of Fe, Nd, Pd, and Zr, or on the refraction index and the surface tension. These species can however modify the DTR values, the density, the viscosity or the distribution ratio of Mo. Table 6, which summarizes all the results, shows that for flowsheets with: -

-

neodymium only, the removal of some of the degradation products by alkaline treatment enabled the DTR values, the density and the viscosity to be stable, radiolysis degradation, the remaining degradation products led to molybdenum accumulation in the organic phase. In the case of the test 267 Wai and Mincher; Nuclear Energy and the Environment ACS Symposium Series; American Chemical Society: Washington, DC, 2010.

with all the cations including molybdenum, this phenomenon increased the density and the viscosity. The Mo accumulation in the organic phase was probably due to one compound, MDOHEMA, because it was the only common degradation product accumulating in the solvent. Furthermore, the variation of Mo accumulation is linear with MDOHEMA concentration (Figure 8).

Table 6. Impact of the Diamex solvent degradation products on process performances Flowsheets

DTR

Density

Viscosity

DMo

Nd+Hydrolysis

+31%

+0.4%

+4%

None

Nd+Hydrolysis+AT

None

None

None

None

Nd+Hydrolysis+Radiolysis

+22%

+1%

+22%

+338%

Nd+Hydrolysis+Radiolysis+AT

None

None

None

+272%

Nd, Fe, Mo, Zr, Pd +Hydrolysis+Radiolysis+AT

None

+1%

+25%

+0.2%/h hydrolysis

Figure 8. Variation of molybdenum in the solvent with MDOHEMA concentration for flowsheets with radiolysis 268 Wai and Mincher; Nuclear Energy and the Environment ACS Symposium Series; American Chemical Society: Washington, DC, 2010.

Figure 8 also shows that for the test with neodymium and hydrolysis and radiolysis degradation without alkaline treatment, the remaining “light” acid compounds probably contributed to this phenomenon.

Conclusion The MARCEL gamma-irradiation facility enabled the long term evolution of DIAMEX process solvent to be studied. The longest test, with hydrolysis and radiolysis degradation and alkaline treatment, represented at least 270 days of industrial operations for UOX2 spent fuels and in fact probably more, as γradiation is 3 times more penalizing than α-radiation. The batch studies showed that the DMDOHEMA consumption seems to depend only on nitric acid concentration in the organic phase, dose rate and time. Furthermore, these studies allowed an optimization of the acido-basic treatment efficiency for some of the degradation products (methyloctyl amine, amidic-acid and a part of the “light” species). The remaining degradation products are MDOHEMA, residual “Light” acid compounds, the monoamide and “Light” amide or amine compounds. The continuous degradation/regeneration operations permitted the determination of the solvent consumption kinetic and the identification of the degradation processes. Methyloctyl amine, amidic-acid and MDOHEMA are primary degradation products formed by hydrolysis and/or by radiolysis. “Light” acid species, amide/amine compounds and monoamide are secondary degradation products, formed by radiolysis only or by hydrolysis. Therefore, the use of an alkaline treatment permitted the removal of all the products formed by hydrolysis and/or radiolysis except one, MDOHEMA. This compound has no impact on the physico-chemical properties of the solvent apart from the accumulation of molybdenum in the solvent, which increases its density (+1%) and viscosity (+25%). However, the MDOHEMA accumulation had no impact on DIAMEX process performances. Additional studies will be realized to confirm the impact of this degradation product and to determine the concentration from which it will become problematic for the process.

References 1. 2. 3.

Charbonnel, M. C.; Berthon, L. Rapport Scientifique 1997; CEA Report CEA-R-5801; Direction du Cycle du Combustible, 1998; pp 114−119. Berthon, L.; Camès, B. Rapport Scientifique 1999; CEA Report CEA-R5892; Direction du Cycle du Combustible, 2000; pp 206−211. Berthon, L.; Charbonnel, M. C. In Ion Exchange and Solvent Extraction: A Series of Advances; Moyer, B. A, Ed.; CRC Press Taylor & Francis Group: Boca Raton, FL, 2010; Volume 19, pp 464−470.


DIAMEX Solvent Behavior under Continuous Degradation and

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