Recovery of Pd from spent fuel: 4. Precipitation of Pd from nitric acid solutions with hydrazine and CO

Article abstract of DOI:10.1007/s11137-005-0105-7

Precipitation of palladium with hydrazine and CO from nitric acid solutions simulating process solutions formed in regeneration of spent nuclear fuel was studied. The influence of various factors on the degree of palladium recovery was studied. From 1.0-4

Full text of DOI:10.1007/s11137-005-0105-7

Radiochemistry, Vol. 47, No. 4, 2005, pp. 378 381. Translated from Radiokhimiya, Vol. 47, No. 4, 2005, pp. 347 350.
Original Russian Text Copyright 2005 by Gevirts, Zelentsova, Kozlov, Kolobov, and Pokhitonov.
Recovery of Pd from Spent Fuel: 4. Precipitation of Pd
from Nitric Acid Solutions with Hydrazine and CO
V. B. Gevirts , L. A. Zelentsova, A. E. Kozlov, E. A. Kolobov, and Yu. A. Pokhitonov
Khlopin Radium Institute, Research and Production Association, St. Petersburg, Russia
Received June 25, 2004
Abstract Precipitation of palladium with hydrazine and CO from nitric acid solutions simulating process
solutions formed in regeneration of spent nuclear fuel was studied. The influence of various factors on the
degree of palladium recovery was studied. From 1.0 4.0 M HNO solutions, 50 90 and 70 98% of Pd is
3
precipitated with hydrazine and CO, respectively. Palladium recovery from simulated and real solutions
formed in processing of spent nuclear fuel was studied. The decontamination factor of palladium with respect
2
3
to most of elements including radionuclides was 10 10 .
Recovery of palladium from nitric acid solutions
with sorbents and extracting agents of different classes
was extensively studied. Recovery of palladium by
precipitation of its difficultly soluble salts and pal-
ladium metal was studied to a lesser extent.
Pd metal with carbon monoxide from 3 4 M HNO3
solutions. The content of concomitant radionuclides
in the product was low.
EXPERIMENTAL
Precipitation procedures require simple equipment
and yield high-purity palladium. However, this meth-
od was not applied to recover Pd from high-level
waste (HLW). Wu et al. [1], developed a procedure
for precipitating metallic Pd, Rh, and Ru in the course
of denitration of HLW with formic acid. The purity of
the recovered metals is high [2].
To prepared the stock palladium solution (9 g l 1
of Pd in 4 M HNO ), Pd metal was dissolved in 9 M
3
HNO and the resulting solution was filtered. The
3
working solutions were prepared by dilution of an
aliquot of the stock solution.
Palladium was precipitated by slow addition of
a 300 g l 1 hydrazine solution to a nitric acid solution
of palladium. The mixture was stirred for 30 min and
the palladium precipitate was filtered off.
Asano et al. [3] used trichlorostannate to precipi-
tate platinum metal complexes from HLW. It was
shown that Ru, Rh, and Pd react with SnCl to form
3
4
3
Precipitation with CO was performed by either
bubbling CO through nitric acid solution of Pd or
by passing of CO over the stirred solution. For this
purpose, a palladium solution was charged to a flask
fixed on a magnetic stirrer. The flask was closed with
a ground-glass adapter and purged with CO for 30
40 min. The precipitate was filtered off, washed with
[
(
RuCl(SnCl ) ] , [RhCl (SnCl ) ] , and [PdCl
3
5
3
3 3
2
SnCl ) ] complexes which are precipitated from
the solution with CsCl and Et NHCl. The degree of
recovery of the platinum metal was close to 100%.
3
3
3
However, the industrial use of chlorine-containing
reagents and chloride solutions requires equipment
made from special corrosion-resistant materials for
waste processing. In addition, precipitation of palladi-
um metal is preferable, since voluminous precipitates
of difficultly soluble palladium compounds are dif-
ficultly filterable and partially occlude the mother
liquor.
0.05 M HNO and distilled water, dried at 150 C, and
3
weighed. In some experiments the weighed precipitate
was dissolved in concentrated HNO . The Pd content
3
in the solution was determined spectrophotometrically
with thiourea or -nitroso- -naphthol. In the latter
case, the colored Pd complex was extracted with tolu-
ene, and the optical density of the toluene solution
at 370 nm was measured on an SF-16 spectrophotom-
eter. The difference between the palladium concen-
trations determined by gravimetric and spectrophoto-
metric procedures did not exceed 2 7%.
A search for compounds precipitating Pd from
1
4 M HNO solutions without their dilution is an
3
urgent problem. In this study we examined precipita-
tion of palladium metal with hydrazine and CO. We
determined conditions for quantitative precipitation of
The impurity content was determined on an ISPJ-
MS atomic emission spectrometer. The sensitivity of
Deceased.
1
066-3622/05/4704-0378 2005 Pleiades Publishing, Inc.

RECOVERY OF Pd FROM SPENT FUEL: 4.
379
1
the determination was 1 g l , and the determination
Table 1. Degree of Pd precipitation with hydrazine from
nitric acid solutions
error was no more than 10%. The content of -emit-
ting radionuclides and 2 Pu was determined by the
conventional procedures.
39
Composition of initial solution
, M C , mg l 1
Degree of
Pd precipita-
tion, %
T,
C
CHNO3, M
C
N H
Pd
RESULTS AND DISCUSSION
2 4
0
1
2
3.8
2.5
2.5
2.5
2
3
3
.9
.7
.6
0.67
0.67
0.67
0.67
1.32
0.98
0.48
0.06
0.60
0.60
95
95
95
95
80
80
80
80
100
100
20
20
20
20
20
20
20
20
24
40
89.7
89.0
81.4
77.1
75.0
70.0
61.2
46.2
78.0
66.0
Analysis of the Pd behavior in different steps of
spent nuclear fuel processing shows that the major
fraction of Pd passes into the solution during UO2
dissolution in HNO . Palladium is concentrated in
3
the raffinate of the first extraction cycle after extrac-
tion separation of U and Pu [4 7].
.5
.0
.0
First, we studied Pd precipitation with hydrazine. It
is known that the reaction of palladium with hydrazine
yields a white precipitate of [Pd(N H ) ](NO )
2
4 2
3 2
xH O, which decomposes on heating to form Pd
2
metal. At elevated temperatures, hydrazine is partially
2
4 HNO because of high solubility Pd in HNO .
3 3
oxidized to HN which reacts with Pd to form dif-
3
ficultly soluble palladium azide [8] by the equation
The mechanism of Pd dissolution in HNO is fairly
3
complex and involves HNO being in equilibrium
with HNO in solutions. Therefore, we looked for
2
2
HN3 + Pd2+ = Pd(N3)2 + 2H+.
3
compounds that affect the equilibrium concentration
of HNO in HNO .
Palladium azide is gradually converted under the
solution layer into palladium metal.
2
3
Introduction of H O , KMnO , N H , sulfamic
2
2
4
2 4
We studied the influence of various factors on the
degree of Pd precipitation from model solutions. The
results are summarized in Table 1.
acid, and other compounds allowed relatively com-
plete precipitation of palladium with CO from solu-
tions of high acidity.
From 50 to 90% of Pd was precipitated from 0.9
We suggest that these compounds decompose
3
.0 M HNO at room temperature.
HNO , thus shifting the equilibrium of Pd dissolution.
3
2
When hydrazine is added in portions to a hot Pd
We also suggest that formation of an oxide film on
the palladium surfcae decreases its solubility in
solution, palladium metal is formed without precipita-
tion of a voluminous palladium hydrazinate. As deter-
mined by X-ray diffraction analysis, the precipitate
contains 90 95% palladium metal with an oxide im-
purity. Although palladium is efficiently recovered
by this method, the presence of excess hydrazine and
its oxidation products prevents the use of this method
in industrial processing of HLW.
HNO . An X-ray diffraction analysis of the Pd pre-
3
cipitate showed the presence of palladium oxide; its
content increased with increasing HNO concentra-
3
tion. The content of palladium oxide in Pd precipi-
tated from 4 M HNO was as high as 25 30%.
3
The dependence of the degree of palladium precipi-
tation with CO on the concentrations of HNO and
3
Gaseous compounds (H , CO, CH , C H , etc.)
2
4
2
4
additives are shown in Figs. 1 and 2.
that selectively precipitate Pd from hydrochloric acid
solutions are more preferable. However, Pd can be
precipitated with these compounds only from solu-
Carbon monoxide can be introduced in the reaction
vessel not only in the gaseous form but also in the
form of the copper complex CuCl CO 2H O. Nitric
tions with the HNO concentration lower than 0.5 M.
2
3
acid oxidizes copper in this complex to the bivalent
state and the complex decomposes with CO liberation.
In the subsequent experiments, we used CO as the
precipitating agent. At low concentration of HNO3,
CO reduces palladium to the metal by the reaction
Thus, CO can be used to precipitate Pd from solu-
tions with HNO concentration of up to 4 M. Prior to
3
Pd(NO3)2 + CO + H O = Pd + CO2 + 2HNO3.
the precipitation, compounds decomposing HNO2
2
should be added to the solution. Among them, H O2
2
However, CO does not precipitate palladium from
and sulfamic acid are the most efficient.
RADIOCHEMISTRY Vol. 47 No. 4 2005

380
GEVIRTS et al.
compositions close to those of solutions formed in
processing irradiated nuclear fuel.
The conditions of Pd precipitation with hydrazine
were similar to those described above. Prior to pre-
cipitation with CO, hydrogen peroxide was added to
the solution. The Pd precipitates were dissolved in
concentrated HNO . The contents of Pd and impurity
3
metals in the solutions were determined.
The results are summarized in Table 2.
As seen from Table 2, Pd precipitated with both
hydrazine and CO with H O addition is efficiently
separated from all macropimurities without apparent
selectivity.
Fig. 1. Degree of Pd precipitation with CO as a function of
2
2
HNO concentration (1) without additives and with addition
3
of (2) N H , (3) KMnO , (4) sulfamic acid, (5) H O , and
2
4
4
2 2
1
(
0
6) urea. Initial concentration, g g Pd: N H 0.8, KMnO
2 4 4
.5, H O 1.0, and sulfamic acid 1.0.
2 2
To determine the decontamination factor of Pd
with respect to fission products and Am, we added an
1
44
154
155
aliquot of a solution containing
and
Ce,
Eu,
Eu,
241
Am to a nitric acid solution of the composition
similar to that of the nitric acid solution used in the
previous experiments. The results are presented in
Table 3.
As seen from Table 3, the decontamination factors
1
37
144
154, 155
of Pd with respect to
Cs,
Ce,
Eu, and
241
3
Am are higher than 1 10 . At the same time, the
decontamination factor with respect to
106
Ru and
Fig. 2. Degree of Pd precipitation with CO from 3 M HNO
125
3
Sb dose not exceed 10.
as a function of the amount of (1) KMnO , (2) H O ,
4
2 2
(
3) N H , and (4) sulfamic acid added to the solution.
Thus, the decontamination factors of Pd with
respect to radionuclides of REE and alkali metals
2
4
The results of these experiments have been patented
in the Russian Federation [9].
were similar to those for their macrocomponents. The
106
decontamination factors with respect to
Ru and
125
We studied the efficiency of Pd precipitation from
nitric acid solutions of the chemical and radiochemical
Sb were low.
Palladium can be precipitated with carbon monox-
ide from concentrates formed in processing of radioac-
tive solutions by different methods (extraction, sorp-
tion, etc.). Most of radionuclides are additionally re-
moved during precipitation.
Table 2. Distribution of macroimpurities between the solu-
tion and Pd precipitated with hydrazine and CO. Palladium
1
concentration in the initial solution 100 mg g ; HNO3
concentration 2.8 M
The distribution of radionuclides between the solu-
tion and Pd precipitated from the desorbate formed in
sorption recovery of palladium from true HLW [10] is
presented in Table 4 as an example. The solution con-
tained 690 mg l 1 Pd, 3 M HNO and 0.1 M DTPA
3
(the specific activity per 1 mg of Pd is presented in
Concentra-
tion in ini-
Decontamination factor
Macro-
impuri-
ties
tial solu- Pd precipitated Pd precipitated
1
tion, g l
with N H
with CO (+H O )
2
4
2 2
3
2.3 103
Cr
Sr
0.21
0.18
0.36
2.03
0.12
0.27
0.20
0.47
1.13
1.3 10
3
4
Table 4).
2.4 10
6.0 10
3
4.8 103
Ba
Ce
Pb
Mn
Ni
Cs
La
3.0 10
As seen from Table 4, most of radionuclides are
removed in the course of palladium precipitation
with CO.
3
3
2.4 10
5.5 10
3
4.6 103
0.4 10
3
4
3.6 10
1.8 103
3
Thus, palladium is precipitated with CO from solu-
tions with a nitric acid concentration higher than 1 M
only after preliminary introduction of special reagents
0.9 10
8 10
3
3
3.5 103
2.8 10
1.7 10
2.3 103
(
H O , KMnO , sulfamic acid, etc.) in the solution.
2
2
4
RADIOCHEMISTRY Vol. 47 No. 4 2005

RECOVERY OF Pd FROM SPENT FUEL: 4.
381
1
Table 3. Composition of the model solution and decontamination factors of Pd. Pd and HNO concentrations 100 mg l
3
and 2.8 M, respectively. Total activity 28.4 MBq l 1
Initial solution
fraction of total activity, %
Decontamination factor
radionuclide
Pd precipitated with hydrazine
Pd precipitated with CO
1
06
Ru
Sb
Cs
2.5
3.9
4.2
2.5
3.9
4.2
37.0
9.2
3
1
20
7
>1.2 10
>1 10
>1.6 10
1.2 10
0.6 10
>1 10
1
1
1
25
34
37
>5 102
3
3
3
Cs
>2 10
1
1
1
44
54
55
>8 103
4
Ce
Eu
Eu
Am
2.5 103
4
3
4
>2 10
2
41
>1 103
3
Table 4. Distribution of radionuclides between the solution and Pd precipitated with CO
Activity, Bq mg 1 Pd
Decontamination factor
Radionuclide
Pd backwash after sorption precipitated Pd precipitated Pd
after sorption and precipitation of Pd
2
1
1
1
1
1
41
06
25
55
54
5
0.3 102
6.0 104
2.9 106
Am
Ru
Sb
Eu
Eu
1.9 10
4.19 10
1.29 10
6.3 10
5
4
5
5
4
4
4
1
2
1.4 10
3.0 10
9.8 10
0.7 102
1.8 102
1.9 105
1
4
6
<1.5 102
>4.0 103
>2.3 106
1.26 10
1.82 10
4.5 10
0.2 10
6.0 10
5.8 10
37, 134
2
2
4
Cs
0.33 10
5.5 10
4.0 10
144Ce
0.2 102
2.0 103
4.2 105
The decontamination factor of Pd with respect to most
elements including radionuclides in the course of Pd
precipitation from model and true solution is 10 10 .
Tech. Rep. Ser., Vienna: IAEA, 1989, no. 308.
5. Shmidt, V.S. and Shorokhov, N.A., At. Energ., 1988,
2
3
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7
. Pokhitonov, Yu.A. and Romanovskii, V.N., Radio-
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RADIOCHEMISTRY Vol. 47 No. 4 2005