Cadmium electroextraction from alkali oxide melts

Article abstract of DOI:10.1007/s10789-005-0172-3

A method is proposed for cadmium electroextraction from alkali oxide melts, suitable for CdO-containing waste processing. A bench-top electrolyzer design is described, and the process parameters are optimized.

Full text of DOI:10.1007/s10789-005-0172-3

Inorganic Materials, Vol. 41, No. 6, 2005, pp. 576–579. Translated from Neorganicheskie Materialy, Vol. 41, No. 6, 2005, pp. 667–670.
Original Russian Text Copyright © 2005 by Barbin, Kazantsev, Vatolin.
Cadmium Electroextraction from Alkali Oxide Melts1
N. M. Barbin*, G. F. Kazantsev**, and N. A. Vatolin**
* Institute of High-Temperature Electrochemistry, Ural Division, Russian Academy of Sciences,
ul. S. Kovalevskoi 20, Yekaterinburg, 620219 Russia
** Institute of Metallurgy, Ural Division, Russian Academy of Sciences,
ul. Amundsena 101, Yekaterinburg, 620016 Russia
e-mail: a.lystov@ihte.uran.ru
Received September 1, 2004
Abstract—A method is proposed for cadmium electroextraction from alkali oxide melts, suitable for CdO-con-
taining waste processing. A bench-top electrolyzer design is described, and the process parameters are opti-
mized.
1
30−50 A/m2 for 24 h. The current yield is 70–90%. If
INTRODUCTION
the electrolyte is stirred, the current density can be
The processing of Cd-containing waste and battery
raised to 100–200 A/m2. The energy consumption is
scrap yields cadmium oxide. Metallic cadmium is used
1400–1600 kW h/t. In rotating-cathode electrolyzers,
in various technological applications: nickel–cadmium
the current density can be raised to 250–300 A/m2.
and silver–cadmium batteries, special alloys, various
Cadmium sponge can be obtained by electroextrac-
tion from a chloride solution with a Cd concentration of
15–30 g/l, after passing the solution through ion-
exchange and contact columns. In electrolyzers with a
graphite anode and aluminum cathode, cadmium is
deposited at current densities from 100 to 800 A/m2.
The current yield in terms of cadmium exceeds 90%,
and the Cd purity is 98.0–98.5%. The energy consump-
tion is 2–4 kW h/kg [6, 7].
In the course of electrolysis, about 10% of the cad-
mium converts to scrap and waste, which are the again
dissolved. Cadmium cathodes are remelted in steel ket-
tles under a layer of molten sodium hydroxide, which
reacts with Zn and Pb to form sodium zincate and
sodium plumbite. The melt temperature is 400–500°C,
and the cadmium loss is 2–4% [8].
After remelting, cadmium contains 0.002–0.005 wt %
Pb, 0.001–0.005 wt % Zn, and 0.002–0.003 wt % Cu.
To achieve higher purity, the cast metal must be refined
further by electrolysis or distillation [3].
Waste processing is finding ever wider industrial
application. This is of particular importance in the pro-
duction of expensive and rare metals, in particular cad-
mium.
coatings, and others [1]. Cadmium is also applied in the
synthesis of chalcogenides and II–VI compound semi-
conductors. In the nuclear industry, cadmium is used as
a material for control rods [2].
There are no deposits containing only cadmium or
its compounds. Cadmium is always present in complex
and zinc ores. For this reason, cadmium production
always develops in combination with the technologies
of zinc and lead. Most cadmium plants are integrated
with zinc and lead works [3].
The basic raw materials for cadmium production are
by-products of zinc and lead metallurgy: copper–cad-
mium cakes, zinc blast-smelting dust, and others. Cad-
mium can be extracted from these materials using
either pyrometallurgical (fractional distillation) or
hydrometallurgical processes or combined approaches.
The hydrometallurgical approach is used most
widely and involves the following steps: oxidation of
cadmium, leaching, solution purification and deposi-
tion of cadmium sponge, oxidation of the sponge,
sponge dissolution and solution purification, electroex-
traction, and remelting of the cathodic cadmium [3, 4].
The aqueous electrolyte for electroextraction con-
tains Cd 100–250, H2SO4 ≤ 12, Zn ≤ 80, Cu 0.001, Fe
0.02–0.03, and Mn ≤ 10 g/l [5].
Cadmium electroextraction is conducted in electro-
lyzers with insoluble Pb anodes and Al cathodes. The
electrolyte temperature is maintained in the range
25−40°C. The process is run at a current density of
From Cd-containing waste and battery scrap result-
ing from the production of nickel–cadmium batteries,
cadmium can be selectively extracted by cyclic leach-
ing. The process involves several steps:
washing of plates with the aim of removing the elec-
trolyte, containing KOH;
calcination at 250–300°C in order to oxidize the
metallic cadmium and decompose cadmium and nickel
salts;
1
Presented in part at the XII Conference High-Purity Substances
and Materials: Preparation, Analysis, and Application, Nizhni
Novgorod, 2004.
0020-1685/05/4106-0576 © 2005 Pleiades Publishing, Inc.

CADMIUM ELECTROEXTRACTION FROM ALKALI OXIDE MELTS
577
leaching of the waste with an ammoniacal carbonate
(+)
(–)
solution, yielding an aqueous solution containing cad-
mium, nickel, and cobalt in the form of ammine com-
plexes and also a remainder containing the residual Fe;
9
5
8
treatment of the aqueous solution in a water-insolu-
ble organic solution containing hydroxy oxime, which
selectively extracts nickel compounds, with subsequent
nickel isolation in the form of nickel sulfate by adding
an aqueous solution of sulfuric acid;
+
+
6
4
1
cobalt precipitation in carbonate form by removing
ammonia from the solution;
11
cadmium precipitation in carbonate form by remov-
ing ammonia from the solution and isolation of cad-
mium carbonate.
In the leaching step, Ni and Cd pass into solution,
whereas Fe remains in the parent material. Under opti-
mal conditions, the nickel yield exceeds 95%, and the
cadmium yield is above 99%. This process ensures
high-purity of the resulting cadmium carbonate. Subse-
quent calcination converts cadmium carbonate to cad-
mium oxide.
7
2
10
3
Metallic cadmium can be obtained from CdO via cad-
mium electroextraction from alkali oxide melts [10, 11].
As reported by Rolla and Salani [12], the CdO
decomposition voltage in the electrolysis of NaOH +
5 wt % CdO at 480°C is 0.89 V.
Chizhikov [3] proposed the use of molten NaOH for
cadmium electrorefining.
Melting a mixture of CdO and NaOH at 450–500°C
leads to the formation of sodium cadmiate [3, 13] by the
reaction
Schematic of the bench-top electrolyzer: (1) BeO crucible,
(2) melt, (3) cathode, (4) anode, (5, 6) current leads,
(7, 8) BeO tubes, (9) asbestos cement clips, (10) molten
cadmium, (11) cadmium oxide.
not corrode up to 600°C but has a high electrical con-
ductivity. Ni anodes do not dissolve in alkaline melts.
During electrolysis, the anodic process was oxygen
release. The cathode was made of Fe or Ni because
these metals are poorly soluble in molten cadmium.
CdO + 2NaOH = Na2CdO2 + H2O.
The drawbacks to cadmium electroextraction from
aqueous solutions are the low current density and low
current yield. This led us to focus on cadmium electro-
extraction from alkali oxide melts. The purpose of this
work was to optimize this process in terms of the cur-
rent yield and cadmium purity, which would enhance
the efficiency of cadmium waste processing.
As the high-temperature solvent, we used a mixture
of reagent-grade NaOH or KOH (80 wt %) and sodium,
potassium, or calcium carbonate (20 wt %). The molten
mixture was held for 2 h at 500–550°C to remove most
of the moisture. The rest of the water was removed from
the melt by electrolysis at a current of 1.2A and voltage
of 1.17 V (current density no higher than 0.01 A/cm2).
After water removal, Cd metal (10–15 g) and CdO (5 g)
were introduced into the cell. Subsequently, CdO was
added at regular intervals. The dc current through the
cell was 2.5 to 25 A. The experimental conditions were
varied so that the anode current density was 0.25 to
1.911 A/cm2 and the cathode current density was 0.204
to 1.531 A/cm2. The cathode current density was
always lower because the area of the liquid cathode was
larger than the anode area (the anode diameter was
smaller than the inner diameter of the crucible in order
not to prevent oxygen release).
EXPERIMENTAL
A bench-top electrolyzer (figure) was mounted in a
temperature-controlled Silit-heater furnace. The melt
(2) was contained in a BeO crucible (1). The cathode
(3) and anode (4) were circular in shape. The current
leads (5, 6) were protected by BeO tubes (7, 8). To
maintain a fixed separation between the electrodes, the
current leads were secured between asbestos cement
plates (9). In our experiments, we used a liquid cathode.
To this end, the lower electrode (3) was immersed in
molten cadmium (10). Cadmium oxide was placed on
the anode (11). The melt was stirred by the released gas,
which accelerated CdO dissolution.
In our experiments, we varied the separation
The anode was made of N-0 nickel. In alkaline between the electrodes from 2 to 5 cm and the melt tem-
melts, nickel is covered with a thin oxide film and does perature from 450 to 600°C.
INORGANIC MATERIALS Vol. 41 No. 6 2005

578
BARBIN et al.
Table 1. Process parameters and current yield
Current density, A/cm2
Run
no.
Electrode
separation, cm
wt % CdO
in the melt
Current
yield, %
Melt*
t, °C
anode
cathode
1
2
NaOH–Na2CO3
NaOH–Na2CO3
NaOH–Na2CO3
KOH–Na2CO3
NaOH–Na2CO3
KOH–Na2CO3
NaOH–Na2CO3
NaOH–Na2CO3
NaOH–Na2CO3
NaOH–Na2CO3
NaOH–Na2CO3
NaOH–CaCO3
NaOH–Na2CO3
0.254
0.382
0.637
0.625
0.625
0.625
0.625
0.764
0.937
0.955
0.937
1.273
1.911
0.204
0.306
0.510
0.510
0.510
0.510
0.510
0.612
0.765
0.765
0.765
1.020
1.531
4
500
500
500
450
550
450
450
500
600
500
600
500
500
6.0
3.0
3.0
3.0
6.0
6.0
6.0
3.0
4.0
3.0
6.0
8.0
10.0
54.1
74.2
98.85
36.0
78.34
20.0
35.1
89.0
71.8
82.7
71.94
42.0
55.52
4
3
4
4
3
5
4.5
5
6
7
2
8
4
9
5
10
11
12
13
4
3.5
4
4
* 80 wt % NaOH or KOH and 20 wt % Na CO or CaCO .
2
3
3
Table 2. Impurity concentration in cadmium
Weight percent
Run no.
Zn
Pb
Fe
Cu
Na
Ni
3
5
8
2.3 × 10–4
2.5 × 10–4
1.8 × 10–4
3 × 10–4
3 × 10–4
9 × 10–4
1 × 10–3
1.5 × 10–3
1 × 10–3
2.8 × 10–3
1.5 × 10–3
3.5 × 10–3
1 × 10–3
1 × 10–3
1 × 10–3
1 × 10–3
1 × 10–3
1 × 10–3
After each experiment, the melt was poured into a deficiency. At CdO contents above 6 wt %, the melt
graphite mold. The solidified metal was separated from contains a thick, viscous CdO precipitate.
the salt and weighed in order to determine the current
yield. The Cd metal was analyzed for impurities on a
PE-404 spectrophotometer.
Reducing the melt temperature to below 450°C
increases the melt viscosity, and the current yield drops
to 20%. Above 600°C, the melt vaporizes rapidly, cor-
roding the unprotected parts of the electrolyzer.
To separately assess the effects of several process
parameters (melt composition, cathode and anode cur-
rent densities, electrode separation, CdO concentration,
and melt temperature), on the current yield, we used the
simplex design approach [14]. Acceptable results (cur-
rent yield of 72–78%) can only be achieved if no more
than two process parameters are close or equal to their
limiting (best) values (runs 5, 9, 11). At three and four
limiting parameters, the current yield drops to 36–55
and 20%, respectively (runs 6, 7).
RESULTS AND DISCUSSION
Table 1 summarizes the process parameters and the
results on the preparation of cadmium by the electroly-
sis of Cd-containing melts. The best results were
obtained at a cathode current density from 0.510 to
0.765 A/cm2, anode current density from 0.637 to
0.955 A/cm2, melt temperature of 500°C, electrode
separation of 4 cm, and CdO content of 3 wt %.
The optimal electrode separation is 3.5–4.5 cm. It
ensures the maximum current yield and a low, Ohmic
melt resistance.
At current densities of 0.5–0.6 A/cm2 (5000–
6000 A/m2), the voltage across the electrolyzer was no
higher than 3.0–3.5 V. The energy consumption was
13–15 kW h/kg Cd, which is lower in comparison with
Reducing the CdO content of the melt to below
3 wt % impairs the current yield because of the Cd2+ the electrolysis of aqueous solutions. The productivity
INORGANIC MATERIALS Vol. 41 No. 6 2005

CADMIUM ELECTROEXTRACTION FROM ALKALI OXIDE MELTS
579
5. Prikladnaya elektrokhimiya (Applied Electrochemistry),
per unit area of the electrolyzer described here is tens of
times higher than that of electrolyzers employing aque-
ous solutions.
The chemical composition of the cadmium prepared
in some of our experiments is given in Table 2. The
metal is close in the concentrations of major impurities
to KdO cadmium (RF Standard GOST 1467-77) [15].
Rotinyan, A.L., Ed., Leningrad: Khimiya, 1974.
6. Schmidt, W., Kadmium Gewinnung Puhz-Zink GmbH in
Datten, Erzmetall., 1979, vol. 32, no. 12, pp. 522–524.
7. Vladulescu, C., Valea, T., and Cornea, A., Rom. Patent
81200 C.P.P., 1978.
8. Gudima, N.V. and Shein, Ya.P., Kratkii spravochnik po
metallurgii tsvetnykh metallov (Concise Handbook of
Nonferrous Metallurgy), Moscow: Metallurgiya, 1975.
9. Sittig, M., Resource Recovery and Recycling Handbook
of Industrial Wastes, Park Ridge: Noyes Data, 1975.
Translated under the title Izvlechenie metallov i neor-
ganicheskikh soedinenii iz otkhodov: Spravochnik, Mos-
cow: Metallurgiya, 1985.
CONCLUSIONS
A method is proposed for cadmium electroextrac-
tion via electrolysis in alkali carbonate melts contain-
ing cadmium oxide, and the process parameters are
optimized.
This method enables the preparation of pure cad-
mium in a single electrolysis cycle.
10. Kazantsev, G.F., Barbin, N.M., Sofinskii, A.V, et al., RF
Patent 2123544.
11. Barbin, N.M. and Kazantsev, G.F., Preparation of Pure
Cadmium by Electroextraction from Alkaline Melts, XII
Konferentsiya po vysokochistym veshchestvam i materi-
alam (XII Conf. on High-Purity Substances and Materi-
als), Nizhni Novgorod, 2004, p. 96.
ACKNOWLEDGMENTS
This work was supported by the Russian Foundation
for Basic Research (Ural), project no. 04-03-96113.
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ione delle miscele fuse di soda ed ossido di zinco e di
soda ed ossido di cadmio, J. Chim. It., 1922, vol. 52,
pp. 286–313.
13. Lidin, R.A., Molochko, V.A., and Andreeva, L.L.,
Khimicheskie svoistva neorganicheskikh veshchestv
(Chemical Properties of Inorganic Substances), Mos-
cow: Khimiya, 1996.
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rabot po planirovaniyu promyshlennogo eksperimenta
(Applied Research on the Design of Industrial Experi-
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INORGANIC MATERIALS Vol. 41 No. 6 2005