Preparation of lanthanum and cerium metals by hydrometallurgy

Article abstract of DOI:10.1016/S0925-8388(02)01167-2

Lanthanum and cerium metals were prepared by electrolytic synthesis of amalgams in aqueous solutions followed by the thermal decomposition. The amalgamation yields were almost quantitative. The preparation yield of lanthanide metals, however, deteriorated during thermal decomposition due to a density difference between lanthanide and mercury. The gaseous impurities in the prepared metals were found to be comparable with those in commercially available ones.

Full text of DOI:10.1016/S0925-8388(02)01167-2

Journal of Alloys and Compounds 352 (2003) 246–249
L
www.elsevier.com/locate/jallcom
Preparation of lanthanum and cerium metals by hydrometallurgy
a
b
b
a ,
*
K. Hasegawa , Y. Sano , A. Aoshima , Y. Shiokawa
a
Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan
b
Tokai Works, Japan Nuclear Cycle Development Institute, Tokai-mura 319-1194, Japan
Received 26 August 2002; accepted 12 September 2002
Abstract
Lanthanum and cerium metals were prepared by electrolytic synthesis of amalgams in aqueous solutions followed by the thermal
decomposition. The amalgamation yields were almost quantitative. The preparation yield of lanthanide metals, however, deteriorated
during thermal decomposition due to a density difference between lanthanide and mercury. The gaseous impurities in the prepared metals
were found to be comparable with those in commercially available ones.

2002 Elsevier Science B.V. All rights reserved.
Keywords: Metal preparation; Lanthanum metal; Cerium metal; Amalgam; Hydrometallurgy
1
. Introduction
trolytic amalgamation in aqueous solutions followed by the
thermal decomposition.
Metal ions belonging to groups 6–16 are amalgamated
in aqueous solutions [1], whereas electropositive metals
such as lanthanide and actinide have usually been de-
posited in molten salts [2,3] because the deposition po-
tentials are more negative than the cathodic limit of the
aqueous potential window. It has been shown, however,
that uranium and neptunium metals have been quantitative-
ly prepared by the controlled current electrolysis in aque-
ous solutions using a mercury electrode followed by
thermal decomposition of the amalgam. Moreover, the
purity of uranium metals was found to be more than
2. Experimental details
The amalgamation cell of 300 ml capacity consisted of
cathode and anode compartments divided by a cation-
exchange membrane of the H type. The surface area and
volume of the mercury cathode were 32 cm and 15 ml,
respectively, and a platinum anode was immersed in 1 M
sulfuric acid solution.
1
2
2
The stock solutions of La(III), Ce(III) and U(VI) were
prepared by dissolving the oxides in hydrochloric acid
solutions. Prior to the amalgamation, the hexavalent
uranium was electrochemically reduced to the trivalent
state, which was confirmed by the spectrophotometry. The
amalgamations of La(III), Ce(III) and U(III) were carried
out at the mercury cathode in acetate buffer solutions of
pH 3.4–3.7 containing 1.0 M acetic acid and 0.4 M sodium
acetate by the controlled current electrolysis at the density
9
9.99% and the contents of oxygen and nitrogen were
below 10 and 5 ppm, respectively [4,5]. The result shows
actinide ions are amalgamated outside aqueous potential
window and the quality of electropositive metals prepared
by hydrometallurgy seems to be comparable with that by
the conventional pyrometallurgy.
3
1
The standard potentials E (M /M) of La, Ce, U and
o
Np are 22.37, 22.34, 21.66 and 21.79 V, respectively.
Therefore, it is interesting whether more electropositive
lanthanide ions can be reduced to metallic states in
aqueous solutions. In the present work, preparation of
lanthanum and cerium metals is attempted by the elec-
2
2
of 220 mA cm . The pH of the catholyte was kept almost
constant during the electrolysis by supplying hydrogen
ions through an ion-exchange membrane from the anode
compartment, although the vigorous evolution of hydrogen
gas took place.
*
Corresponding author.
The resulting amalgam was rinsed by water and ethyl
alcohol and then transferred to a magnesia crucible,
E-mail address: shiokawa@imr.tohoku.ac.jp (Y. Shiokawa).
0
925-8388/02/$ – see front matter
 2002 Elsevier Science B.V. All rights reserved.
doi:10.1016/S0925-8388(02)01167-2

K. Hasegawa et al. / Journal of Alloys and Compounds 352 (2003) 246–249
247
degassed at 1000 8C in a vacuum. The amalgam was
2
7
heated in a vacuum below 10 Torr at temperatures above
the melting points of lanthanide metals after most of the
mercury was evaporated below 250 8C.
The mercury, nitrogen, oxygen and carbon in the
prepared metals were analyzed, because these four ele-
ments were apt to contaminate the products except the
impurities in lanthanide oxides used as the source materi-
als.
3
. Results and discussion
It is necessary to mention the potential applied to the
working mercury electrode. The amalgamation is accom-
panied by reduction of hydrogen ions and the current–
voltage characteristics depends on the pH as illustrated in
Fig. 1. At the controlled current electrolysis of 150
Fig. 2. Amalgamation behaviors of La(III), Ce(III) and U(III) at pH 3.6
2
2
by the controlled current electrolysis of 220 mA cm
.
2
2
mA cm , for example, the potentials are controlled at
1.32 V in the solution of pH 1.0 and at 21.66 V for pH
.0, respectively. Then, some electropositive ions are
amalgamated even in the aqueous solution by adjusting of
the pH and current density.
intermetallic compounds between Ce and Hg decompose
into cerium and gaseous mercury up to 555 8C according to
the phase diagram [6], cerium metals with less mercury
contents can be prepared at the decomposition of 1250 8C.
The mercury removal from the amalgams seems to be
more effective at the higher temperature than for the longer
heating.
The preparation of lanthanide metals is summarized in
Table 2 and amalgamations of the lanthanides are almost
quantitative except run La-5, which shows that La and Ce
ions, more electronegative than actinide ions, can be
reduced in the aqueous solution. On the other hand, the
preparation yields of lanthanum and cerium metals were
usually smaller than 100% as tabulated in the seventh
column. Since the amalgamation is almost quantitative, the
degradation in the preparation yields results from the
thermal decomposition. In this process, the lanthanide
amalgam decomposes into lanthanide and mercury [6] and
the former with lower density is apt to occupy the upper
part in the crucible. During the thermal decomposition,
lanthanide metals would be scattered from the crucible
2
3
Fig. 2 shows the typical reduction behavior of La(III),
Ce(III) and U(III) at pH 3.6 by the controlled current
2
2
electrolysis at 220 mA cm . The amalgamation of La(III)
and Ce(III) is found to be more sluggish than that of U(III)
and the reduction of La(III) seems to be more rapid than
that of Ce(III).
The resultant amalgams were decomposed in the mag-
2
6
nesia crucible below 1310 Torr at 900–1250 8C. The
mercury contents in the decomposed products were ana-
lyzed as tabulated in Table 1, where previously reported
results [5] are also included. The ratio of mercury in the
prepared metal is as high as 30% below the decomposition
temperatures of 1000 8C and the contents of mercury are
found to be dependent on the temperature. Although the
Table 1
The relationship between the thermal decomposition and contents of Hg
in the product metals
Decomposition of the amalgam
Contents
of Hg
Temp (8C)
Time (h)
La-1
La-2
La-3
La-4
Ce-1
Ce-2
Ce-3
Ce-4
Ce-5
Ce-6
1000
1050
1250
1250
900
2
2
1
1
1
3
4
1
1
1
17%
8%
27 ppm
17 ppm
38%
34%
14%
8%
61 ppm
31 ppm
950
1000
1150
1250
1250
Fig. 1. Current–potential curves for hydrogen evolution in the acetate
buffer solutions.

2
48
K. Hasegawa et al. / Journal of Alloys and Compounds 352 (2003) 246–249
Table 2
The amalgamation and preparation yields of lanthanum and cerium metals
Run
[Metal] 2
(mol dm
[Acetate]
(mol dm )
pH
Amalgamation
yield (%)
Prepared
metal (g)
Yield of metal
preparation (%)
3
23
)
La-1
La-2
La-3
La-4
La-5
Ce-1
Ce-2
Ce-3
Ce-4
Ce-5
0.109
0.109
0.107
0.109
0.109
0.100
0.100
0.100
0.107
0.100
1.47
1.49
1.65
1.49
1.45
1.47
1.47
1.40
1.49
1.29
3.6–2.9
3.7–3.1
3.6–3.2
3.7–3.1
3.7–3.4
3.6–3.0
3.7–3.0
3.6–3.1
3.7–3.6
3.6–2.8
99.9
98.3
99.4
98.1
91.8
99.2
99.2
99.7
99.5
99.8
1.074
0.767
0.728
0.597
0.545
1.400
1.216
1.143
1.097
0.892
70.9
50.6
49.1
39.4
36.0
99.5
86.4
81.2
73.2
63.6
accompanied by the intense evaporation of mercury oc-
cupied at the bottom. Actually, fine depositions of metals
were observed at the inside of the crucible. Such disper-
sions were never observed in the preparation of uranium
metals because of the higher density than Hg [4]. How-
ever, the careful control of the temperature in the de-
composition process achieves the high preparation yield of
tionally produced by electrolysis in fused salts or metal-
lothermic reduction [2], it is found that the metals are also
prepared electrochemically in the aqueous solution fol-
lowed by thermal decomposition. Moreover, the procedure
would be useful for the small scale preparation and would
be especially for the preparation composed of some
isotope, such as Sm-154 metal which is required for
neutron scattering experiments to elucidate the magnetic
properties of the intermetallic compounds. Thus, the metal
preparation of not only actinide but also lanthanide is
found to be practicable by hydrometallurgy.
9
9.5% as is run Ce-1 in Table 2. The prepared cerium
metals were shaped into a dull yellowy button of metal, a
platy or rod-like metal. The photograph of a 0.876 g
sample of cerium metal is shown in Fig. 3.
Since the amount of mercury in the prepared metal can
be reduced at high temperatures as shown in Table 1, the
impurities of nitrogen, oxygen and carbon in the prepared
metals were analyzed as tabulated in Table 3. Nitrogen and
oxygen contents in the cerium metals by hydrometallurgy
are smaller than or almost equal to those in the commercial
ones. It should be emphasized that the gaseous impurities
in the lanthanide metals prepared by hydrometallurgy are
comparable with those by pyrometallurgy. Gaseous im-
Acknowledgements
We are indebted to Dr T. Ogawa and A. Itoh, Research
Group on Actinide Science, Department of Materials
Research, Japan Atomic Energy Research Institute, for
analysis of nitrogen, oxygen and carbon in the prepared
metals. This work is supported by JSPS Research Fellow-
ship for Young Scientists and also by a Grant-in-Aid for
Scientific Research by the Ministry of Education, Science
and Culture of Japan.
1
2
purities of the cerium metal, however, were 10 –10 times
of that in uranium metal. It must be due to a difference in
affinity of the metals for nitrogen and oxygen.
Although lanthanum and cerium metal have been tradi-
Table 3
Analysis of gaseous impurities in lanthanum and cerium metals
Sample
N (wt. ppm)
O (wt. ppm)
C (wt. ppm)
1
3
La-1
La-2
La-3
Ce-1
Ce-2
Ce-3
Ce-4
Ce-5
cm-Ce-1
cm-Ce-2
cm-Ce-3
cm-Ce-4
cm-Ce-5
cm-Ce-6
3.0310
1.1310
1.5310
3.1310
3.8310
4.7310
1.9310
3.4310
6.0310
7.8310
3.5310
7.0310
6.9310
7.0310
1.1310
1.5310
1.6310
3.6310
4.8310
5.4310
5.5310
1.8310
3.6310
4.6310
5.2310
8.9310
9.2310
9.9310
1
3
2
1.3310
1.7310
1.0310
1.2310
8.0310
2.5310
1.8310
1
3
2
1
2
2
1
2
2
1
2
1
1
2
2
1
3
2
1
2
1
2
1
2
1
2
1
2
1
2
La-N, Ce-N: prepared by electrolysis of aqueous solutions; cm-Ce-N:
Fig. 3. A 0.876-g sample of cerium metal decomposed at 1250 8C.
commercially available cerium metals of ¯99.9 wt%.

K. Hasegawa et al. / Journal of Alloys and Compounds 352 (2003) 246–249
249
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