U.S. patent application number 13/514858 was filed with the patent office on 2012-10-04 for metal electrowinning anode and electrowinning method.
Invention is credited to Masatsugu Morimitsu.
Application Number | 20120247971 13/514858 |
Document ID | / |
Family ID | 44145455 |
Filed Date | 2012-10-04 |
United States Patent
Application |
20120247971 |
Kind Code |
A1 |
Morimitsu; Masatsugu |
October 4, 2012 |
METAL ELECTROWINNING ANODE AND ELECTROWINNING METHOD
Abstract
The present invention has a problem aiming to provide an
electrowinning system capable of suppressing accumulation of a side
reaction product on an anode and a rise of an electrolysis voltage
caused thereby, and also to provide an electrowinning method using
the system. To solve this problem, the electrowinning system of the
present invention applies predetermined electrolysis current
between an anode and a cathode placed in an electrolyte, thereby
depositing a desired metal on the cathode, in which the electrolyte
is a sulfuric acid-based or chloride-based solution containing ions
of the metal, and the anode has a catalytic layer, containing
amorphous iridium oxide or amorphous ruthenium oxide, formed on a
conductive substrate.
Inventors: |
Morimitsu; Masatsugu;
(Kyoto, JP) |
Family ID: |
44145455 |
Appl. No.: |
13/514858 |
Filed: |
November 22, 2010 |
PCT Filed: |
November 22, 2010 |
PCT NO: |
PCT/JP2010/070809 |
371 Date: |
June 8, 2012 |
Current U.S.
Class: |
205/560 ;
204/290.13; 204/290.14 |
Current CPC
Class: |
C25C 7/02 20130101; C25C
1/08 20130101; C25C 1/16 20130101 |
Class at
Publication: |
205/560 ;
204/290.14; 204/290.13 |
International
Class: |
C25C 7/02 20060101
C25C007/02; C25C 1/00 20060101 C25C001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 8, 2009 |
JP |
2009-278607 |
Claims
1-7. (canceled)
8. An electrowinning anode for use in electrowinning of any one of
nickel, zinc or cobalt, comprising a conductive substrate and a
catalytic layer formed on the conductive substrate, wherein the
catalytic layer contains amorphous iridium oxide, so that a side
reaction on the anode is suppressible.
9. The electrowinning anode according to claim 8, being used in
nickel electrowinning.
10. The electrowinning anode according to claim 8, wherein the
catalytic layer contains amorphous iridium oxide and amorphous
tantalum oxide.
11. An electrowinning anode for use in electrowinning of any one of
nickel, zinc or cobalt, comprising a conductive substrate and a
catalytic layer formed on the conductive substrate, wherein the
catalytic layer contains amorphous ruthenium oxide, so that a side
reaction on the anode is suppressible.
12. The electrowinning anode according to claim 11, being used in
nickel electrowinning.
13. The electrowinning anode according to claim 11, wherein the
amorphous ruthenium oxide in the catalytic layer is a composite
oxide of amorphous ruthenium oxide and amorphous titanium
oxide.
14. The electrowinning anode according to claim 8, wherein the side
reaction is a reaction in which a manganese compound is
generated.
15. The electrowinning anode according to claim 8, further
comprising an intermediate layer made of tantalum or a tantalum
alloy, wherein the intermediate layer is formed between the
conductive substrate and the catalytic layer.
16. An electrowinning method using an anode which is an
electrowinning anode of claim 8.
17. (canceled)
18. (canceled)
19. The electrowinning anode according to claim 11, wherein the
side reaction is a reaction in which a manganese compound is
generated.
20. The electrowinning anode according to claim 11, further
comprising an intermediate layer made of tantalum or a tantalum
alloy, wherein the intermediate layer is formed between the
conductive substrate and the catalytic layer.
21. An electrowinning method using an anode which is an
electrowinning anode of claim 11.
Description
[0001] This nonprovisional application is a national Stage of
International Application No. PCT/JP2010/070809, which was filed on
Nov. 22, 2010, and which claims priority to Japanese Patent
Application No. 2009-278607, which was filed in Japan on Dec. 8,
2009, and which is herein incorporated by reference.
TECHNICAL FIELD
[0002] The present invention relates to an electrowinning anode for
electroextracting a desired metal from an electrolyte and an
electrowinning method for electroextracting a desired metal from an
electrolyte.
BACKGROUND ART
[0003] Generally, in metal electrowinning, an anode and a cathode
are dipped into a solution (hereinafter, an electrolyte) containing
ions of a metal to be recovered and are brought into conduction,
thereby depositing the metal on the cathode. An anode often used is
made of lead or a lead alloy. Examples of the electrolyte used
include various electrolytes, such as sulfuric acid-based and
chloride-based electrolytes.
[0004] For example, a sulfuric acid-based electrolyte that contains
zinc ions extracted from a zinc ore is used in zinc electrowinning.
In such zinc electrowinning, zinc is deposited on the cathode, and
the main reaction on the anode is oxygen evolution. In addition,
oxidation of divalent manganese cations mingled into the
electrolyte during the zinc ion extraction process occurs on the
anode, and manganese compounds, such as manganese oxyhydroxide
(MnOOH) and manganese dioxide (MnO.sub.2), are deposited on the
anode. Such manganese compounds have extremely low catalytic
properties with respect to oxygen evolution and also have low
conductivity, and therefore they increase the oxygen evolution
potential, resulting in an increase in electrolysis voltage.
Moreover, in the case where a lead electrode or a lead alloy
electrode is used as the anode, divalent cobalt cations might be
added to the electrolyte for the purpose of enhancing the
durability of the electrode. In such a case, a side reaction occurs
on the anode where divalent cobalt cations are oxidized, generating
trivalent cobalt cations, which react with the electrode to
generate a compound. In the case where electrolysis lasts for a
long period of time, such a compound precipitates in the
electrolyte as sludge. Thereafter, the precipitate is partially
dissolved into the electrolyte, so that lead and cobalt ions elute
and are mingled into zinc generated on the cathode, resulting in
reduced purity of the zinc metal.
[0005] Similarly, a sulfuric acid-based electrolyte is also used in
cobalt and nickel electrowinning. In such a case also, cobalt or
nickel is deposited on the cathode, and oxygen evolution occurs on
the anode as a main reaction, so that divalent manganese cations,
which are present in the electrolyte as impurities, cause a side
reaction as mentioned above, resulting in an increased electrolysis
voltage. Moreover, in cobalt electrowinning, divalent cobalt
cations, which are present in the electrolyte, are reduced on the
cathode, so that a cobalt metal is deposited, and oxidation of
divalent cobalt cations, along with oxygen evolution, occurs on the
anode, resulting in cobalt oxyhydroxide (CoOOH). Cobalt
oxyhydroxide has extremely low catalytic properties with respect to
oxygen evolution and also has low conductivity, and therefore it
raises the electrolysis voltage as in the case of manganese
compounds.
[0006] Furthermore, in cobalt and nickel electrowinning, a
chloride-based electrolyte might be used. For example, cobalt
electrowinning uses a hydrochloric acid electrolyte which contains
divalent cobalt cations extracted from a cobalt-containing ore. In
the case where such an electrolyte is used, cobalt is deposited on
the cathode, and chlorine evolution occurs on the anode as a main
reaction. However, oxidation of divalent cobalt cations contained
in the electrolyte occurs on the anode as a side reaction, so that
cobalt oxyhydroxide is deposited on the anode, causing a raised
electrolysis voltage as in the case mentioned above. Moreover, in
the case of nickel electrowinning using a chloride-based
electrolyte, if divalent manganese cations are present in the
electrolyte, manganese oxyhydroxide or manganese dioxide is
deposited on the anode, causing a raised ed electrolysis voltage,
as described earlier.
[0007] As described above, in zinc, cobalt, and nickel
electrowinning, the main reaction on the anode is oxygen evolution
or chlorine evolution, and in the case where oxygen evolution
occurs as the main reaction, an anode made of lead or a lead alloy
is often used. Lead or lead alloy electrodes are advantageous in
that they are low-cost, but they have a high overvoltage for oxygen
evolution, hence a high electrolysis voltage, and have an issue in
that lead ions elute into the electrolyte, as described above, so
that the purity of a metal deposited on the cathode is reduced.
Moreover, they have low catalytic properties with respect to
chlorine evolution, and are not preferable in that overvoltage is
higher than in the case of oxide evolution. Accordingly, in recent
years, insoluble electrodes which have their conductive substrates,
such as titanium, coated with a catalytic layer containing noble
metal or noble metal oxide, have been increasingly used as anodes
for overcoming such issues. For example, Patent Document 1
discloses a conventional copper electrowinning method which uses an
insoluble electrode covered with an active coating containing
iridium oxide.
PRIOR ART DOCUMENT
Patent Document
[0008] Patent Document 1: Japanese Laid-Open Patent Publication No.
2007-162050
DISCLOSURE OF THE INVENTION
Problems to be Solved by the Invention
[0009] Electrowinning using the anode described in Patent Document
1 does not have an issue due to the aforementioned elution of lead
ions. In addition, the issue with a high electrolysis voltage can
be solved to a certain degree. However, even the use of such an
anode cannot solve various issues caused by an insulating side
reaction product being deposited on the anode due to a side
reaction.
[0010] These issues will be described below taking zinc
electrowinning and nickel electrowinning as examples. Specifically,
in zinc electrowinning or nickel electrowinning in which a sulfuric
acid-based electrolyte is used, when the conventional insoluble
electrode is used as an anode, manganese ions are oxidized on the
anode to change from the divalent to trivalent cations before
oxygen evolution, as is expected, and the trivalent manganese
cations react with water and are changed to insoluble manganese
oxyhydroxide, which is deposited/accumulated on the anode. The
manganese oxyhydroxide might be further oxidized and
deposited/accumulated as manganese dioxide.
[0011] Unlike the catalytic layer of the insoluble electrode, such
accumulated manganese compounds do not have high catalytic
properties with respect to oxygen evolution. Moreover, manganese
oxyhydroxide has extremely low conductivity and provides electrical
insulation, and therefore it inhibits electron exchange that should
occur at the interface between the electrolyte and the anode.
Accordingly, accumulation of the manganese compounds on the anode
results in inhibited catalytic properties of the insoluble
electrode, a raised oxygen evolution potential, and an increased
electrolysis voltage. That is, conventional electrowinning using
the insoluble electrode described in Patent Document 1, has an
issue in that:
[0012] (1) the electrolysis voltage can be reduced at the early
stage of electrolysis, but the electrolysis voltage rises as
manganese compounds are accumulated, resulting in a large amount of
power being consumed at the time of electrolysis.
[0013] In addition to this, conventional electrowinning has the
following issues due to accumulation of manganese compounds on the
anode:
[0014] (2) non-uniform current distribution on the anode causes
nonuniform metal deposition on the cathode;
[0015] (3) dendritically grown metal on the cathode reaches the
anode, causing short circuit between the anode and the cathode;
and
[0016] (4) to avoid the short-circuit issue, it is necessary to
lengthen an inter-electrode distance between the anode and the
cathode, resulting in an increased electrolysis voltage due to
ohmic loss in the electrolyte.
[0017] Furthermore, when a task for removing manganese compounds
from the anode is performed in order to avoid the above issues, the
following issues are brought about:
[0018] (5) electrolysis needs to be suspended during the task and,
therefore, cannot be performed without interruption; and
[0019] (6) the catalytic layer is damaged when removing the
manganese compounds, reducing the durability of the insoluble
electrode.
[0020] On the other hand, also in conventional cobalt
electrowinning in which a chloride-based electrolyte is used,
insulating cobalt oxyhydroxide is continuously
deposited/accumulated on the insoluble electrode, as is expected,
so that the same issues with the manganese compounds as specified
in (1) to (6) above are brought about.
[0021] Furthermore, conventional cobalt electrowinning also has the
following issues:
[0022] (7) divalent cobalt cations, which are expected to be
reduced on the cathode, are consumed around the anode to generate
cobalt oxyhydroxide; and
[0023] (8) cobalt oxyhydroxide grows to the outside of an anode bag
provided around the anode, so that chlorine evolution occurs
outside the anode bag, releasing chlorine, which is harmful to the
human body, in the environment where electrowinning is
performed.
[0024] The present invention has been made under the above
circumstances, and a problem thereof is to provide an
electrowinning anode and an electrowinning method capable of
decreasing an electrolysis voltage compared to conventional lead or
lead alloy electrodes and insoluble electrodes which act as anodes
and suppressing deposition and accumulation of a side reaction
product on the anode and a rise of the electrolysis voltage caused
thereby.
Solution to the Problems
[0025] The present inventor carried out various studies to solve
the above problem, and arrived at the present invention based on
findings that an electrode having a catalytic layer, containing
amorphous iridium oxide, formed on a conductive substrate is used
as an anode for electrowinning using a sulfuric acid-based
electrolyte, and an electrode having a catalytic layer, containing
amorphous iridium oxide or amorphous ruthenium oxide, formed on a
conductive substrate is used as an anode for electrowinning using a
chloride-based electrolyte.
[0026] Specifically, a first electrowinning anode according to the
present invention to solve the problem is for use in electrowinning
of any one of nickel, zinc or cobalt, and includes a conductive
substrate and a catalytic layer formed on the conductive substrate,
in which the catalytic layer contains amorphous iridium oxide, so
that a side reaction on the anode is suppressible.
[0027] Here, preferable conductive substrates are valve metals,
such as titanium, tantalum, zirconium and niobium, valve
metal-based alloys, such as titanium-tantalum, titanium-niobium,
titanium-palladium and titanium-tantalum-niobium, and conductive
diamonds (e.g., boron-doped diamonds). In addition, they can take
various shapes, including plate-like, meshed, rod-like, sheet-like,
tubular, linear, porous plate-like shapes, and shapes of
three-dimensional porous materials composed of bonded spherical
metal particles. In addition to the foregoing, conductive
substrates to be used may be metals other than valve metals, such
as iron and nickel, or conductive ceramics, having surfaces coated
with the aforementioned valve metals, alloys, or conductive
diamonds.
[0028] A substantial reactive surface area (effective surface area)
of the amorphous iridium oxide in the catalytic layer is larger
than that of crystalline iridium oxide. Accordingly, by
incorporating amorphous iridium oxide in the catalytic layer, it is
rendered possible to achieve an anode having high catalytic
activity for oxygen evolution, i.e., the anode has a low oxygen
evolution potential and is capable of promoting oxygen evolution.
Moreover, oxygen evolution accompanies proton (H.sup.+) generation
and, therefore, as oxygen evolution is promoted, the electrolyte on
the anode surface has a higher proton concentration.
[0029] When a sulfuric acid-based electrolyte is used, the main
reaction on the anode is oxygen evolution. This oxygen evolution is
promoted by a catalytic layer containing amorphous iridium oxide,
so that the proton concentration on the anode surface naturally
increases. On the other hand, when a chloride-based electrolyte is
used, the main reaction on the anode is normally chlorine
evolution, which does not accompany proton generation, but
amorphous iridium oxide has extremely high catalytic activity for
oxygen evolution and, therefore, oxygen evolution occurs
simultaneously with chlorine evolution, which is the main reaction.
Accordingly, either in the case where a chloride-based electrolyte
is used or in the case where a sulfuric acid-based electrolyte is
used, the proton concentration on the anode surface is high.
[0030] On the other hand, in the case where divalent manganese
cations are present in the electrolyte, the divalent manganese
cations are oxidized on the anode, as mentioned earlier, resulting
in manganese oxyhydroxide, which becomes manganese dioxide as the
oxidization further progresses. However, in the case where oxygen
evolution is promoted, the oxygen evolution potential is low, which
suppresses generation of manganese dioxide. Moreover, both
generation of manganese oxyhydroxide and generation of manganese
dioxide accompany proton generation, and particularly the reaction
where manganese oxyhydroxide and protons are generated from
trivalent manganese cations is promoted when the pH of the
electrolyte in which the reaction occurs is high (the proton
concentration is low) and is suppressed when the pH is low (the
proton concentration is high).
[0031] Here, in the case where electrowinning is carried out with
constant electrolysis current, the electrolysis current is consumed
by oxygen evolution and manganese compound generation (side
reaction), which simultaneously progress on the anode. Moreover,
oxygen evolution is promoted by using an anode with a catalytic
layer, containing amorphous iridium oxide, formed thereon,
therefore, in the first electrowinning anode according to the
present invention, the proportion of electrolysis current to be
consumed by oxygen evolution is high, and the proton concentration
on the anode surface is high as well. Consequently, manganese
compound generation, which is a side reaction, is suppressed, so
that deposition/accumulation of a manganese compound (side reaction
product) on the anode is suppressed.
[0032] As described above, firstly, in the first electrowinning
anode according to the present invention, a catalytic layer, which
contains amorphous iridium oxide having high catalytic activity for
oxygen evolution, is formed so that the oxygen evolution potential
is reduced, making it possible to decrease the electrolysis
voltage. Secondly, it is possible to keep a manganese compound from
being generated and accumulated on the anode in a side reaction as
a result of promotion of oxygen evolution, so that the electrolysis
voltage can be prevented from being increased when electrowinning
continues over a long period of time.
[0033] Furthermore, the catalytic layer of the first electrowinning
anode preferably further contains amorphous tantalum oxide.
[0034] In the case of a catalytic layer containing amorphous
iridium oxide and amorphous tantalum oxide, the amorphous tantalum
oxide promotes amorphization of the iridium oxide, thereby further
reducing the oxygen evolution potential, and the tantalum oxide
functions as a binder to enhance compactibility of the catalytic
layer, resulting in improved durability. Note that metallic
elements in the catalytic layer are preferably 45 to 99 at. %,
particularly preferably 50 to 95 at. %, of iridium oxide in terms
of metal and preferably 55 to 1 at. %, particularly preferably 50
to 5 at. %, of tantalum oxide in terms of metal.
[0035] Furthermore, a second electrowinning anode according to the
present invention to solve the problem is for use in electrowinning
of any one of nickel, zinc or cobalt, and includes a conductive
substrate and a catalytic layer formed on the conductive substrate,
in which the catalytic layer contains amorphous ruthenium oxide, so
that a side reaction on the anode is suppressible.
[0036] In the second electrowinning anode also, the conductive
substrate of the first electrowinning anode can be used.
[0037] The amorphous ruthenium oxide in the catalytic layer has
higher catalytic activity for chlorine evolution than crystalline
ruthenium oxide. Accordingly, by incorporating amorphous ruthenium
oxide in the catalytic layer, it is rendered possible to achieve an
anode having a low chlorine evolution potential and capable of
promoting chlorine evolution.
[0038] Here, in the case where electrowinning is carried out using
a chloride-based electrolyte with constant electrolysis current,
the electrolysis current is consumed by chlorine evolution and
generation of a side reaction product, which simultaneously
progress on the anode. Moreover, chlorine evolution is promoted by
using an anode with a catalytic layer, containing amorphous
ruthenium oxide, formed thereon, therefore, in the second
electrowinning anode according to the present invention, the
proportion of electrolysis current to be consumed by chlorine
evolution is high, and side reaction product generation is
relatively suppressed. For example, in cobalt electrowinning,
cobalt oxyhydroxide, which is a side reaction product, is kept from
being generated and accumulated on the anode, and in nickel
electrowinning, manganese oxyhydroxide and manganese dioxide, which
are side reaction products, are kept from being generated and
accumulated on the anode.
[0039] As described above, firstly, in the second electrowinning
anode according to the present invention, a catalytic layer, which
contains amorphous ruthenium oxide having high catalytic activity
for chlorine evolution, is formed so that the chlorine evolution
potential is reduced, making it possible to decrease the
electrolysis voltage. Secondly, it is possible to keep a side
reaction product from being generated and accumulated on the anode
as a result of promotion of chlorine evolution, so that the
electrolysis voltage can be prevented from being increased when
electrowinning continues over a long period of time. Note that
electrowinning can be carried out in a sulfuric acid-based
electrolyte using an anode with a catalytic layer, containing
amorphous ruthenium oxide, formed, but for a sulfuric acid-based
electrolyte, it is preferable, from the viewpoint of durability, to
use an anode with a catalytic layer, containing amorphous iridium
oxide, formed.
[0040] Furthermore, the amorphous ruthenium oxide in the catalytic
layer of the second electrowinning anode is preferably a composite
oxide of amorphous ruthenium oxide and titanium oxide.
[0041] By using a composite oxide of amorphous ruthenium oxide and
titanium oxide, amorphization of ruthenium oxide can be promoted,
thereby further reducing the chlorine evolution potential, and
ruthenium and titanium are in a solid solution as the composite
oxide, which functions as a binder to keep the catalytic layer from
wearing, peeling, flaking and cracking, resulting in improved
durability. Note that metallic elements in the catalytic layer are
preferably 10 to 90 at. %, particularly preferably 25 to 35 at. %,
of ruthenium oxide in terms of metal and preferably 90 to 10 at. %,
particularly preferably 75 to 65 at. %, of titanium oxide in terms
of metal.
[0042] Furthermore, in the first and second electrowinning anodes,
an intermediate layer made of tantalum or a tantalum alloy can be
formed between the conductive substrate and the catalytic
layer.
[0043] Forming the intermediate layer makes it possible to prevent
the conductive substrate from being oxidized and corroded by an
acidic electrolyte penetrating through the catalytic layer,
resulting in improved durability of the anode. The intermediate
layer is beneficial particularly in the case where electrowinning
is carried out over a long period of time using electrolysis
current with its density raised to about 0.1 A/cm.sup.2. Note that
sputtering, ion plating, CVD, electroplating, etc., can be used as
methods for forming the intermediate layer.
[0044] The first and second electrowinning systems have been
described for zinc, nickel, or cobalt electrowinning, but in the
case where deposition/accumulation of a side reaction product as
mentioned above causes an issue, they can be expected to exert
action/achieve effect on reduction of the electrolysis voltage and
suppression of side reaction product generation/accumulation for
electrowinning of metals other than zinc, nickel, and cobalt as
well.
[0045] Furthermore, in an electrowinning method according to the
present invention to solve the problem, any one of nickel, zinc or
cobalt is recovered using any of the aforementioned electrowinning
anodes.
Effect of the Invention
[0046] The present invention can provide an electrowinning anode
capable of suppressing accumulation of a side reaction product on
an anode and a rise of an electrolysis voltage caused thereby, and
the invention can also provide an electrowinning method using the
electrowinning anode.
[0047] Since the electrowinning anode and the electrowinning method
according to the present invention can suppress accumulation of a
side reaction product on an anode, not only the aforementioned
issue (1) but also the aforementioned issues (2) to (8) can be
solved as well.
BRIEF DESCRIPTION OF THE DRAWINGS
[0048] FIG. 1 is a graph for electrolysis voltage where zinc
electrowinning is carried out using electrowinning systems
according to Example 1, Comparative Example 1, and Comparative
Example 2.
MODE FOR CARRYING OUT THE INVENTION
[0049] While the present invention will be described in detail
below by way of Examples and Comparative Examples, the present
invention is not limited to the following examples.
[0050] [Zinc Electrowinning Using Sulfuric Acid-Based
Electrolyte]
EXAMPLE 1
[0051] In Example 1, zinc electrowinning was carried out using an
electrowinning system which included a sulfuric acid-based
electrolyte containing zinc ions, an electrowinning anode
(hereinafter, simply referred to as an "anode") having a catalytic
layer, containing amorphous iridium oxide, formed on a conductive
substrate, and a cathode placed in the electrolyte along with the
anode.
[0052] In anode production, initially, a commercially available
titanium plate (5 cm long, 1 cm wide, 1 mm thick) was dipped and
etched in a 10% oxalic acid solution at 90.degree. C. for 60
minutes, and then washed with water and dried. A coating solution
was prepared by adding iridium chloride acid hexahydrate
(H.sub.2IrCl.sub.6.6H.sub.2O) and tantalum chloride (TaCl.sub.5) to
a butanol (n-C.sub.4H.sub.9OH) solution containing 6 vol. %
concentrated hydrochloric acid, such that the molar ratio of
iridium to tantalum was 80:20 and a total amount of iridium and
tantalum was 70 mg/mL in terms of metal. The coating solution was
applied to the dried titanium plate and then dried at 120.degree.
C. for 10 minutes before thermal decomposition for 20 minutes in an
electric furnace maintained at 360.degree. C. The coating, drying
and thermal decomposition was repeated five times in total, thereby
producing an anode having a catalytic layer formed on the titanium
plate, which acted as a conductive substrate.
[0053] The anode was structurally analyzed by X-ray diffraction,
resulting in an X-ray diffraction pattern with no diffraction peak
profile corresponding to either IrO.sub.2 or Ta.sub.2O.sub.5. That
is, the catalytic layer of the anode was formed of amorphous
iridium oxide and amorphous tantalum oxide.
[0054] A zinc plate (2cm.times.2cm) was used as the cathode, and
the electrolyte used was a sulfuric acid-based electrolyte obtained
by dissolving 0.8 mol/L ZnSO.sub.4 in distilled water and adjusting
the pH to -1.1 with sulfuric acid. Moreover, the anode was embedded
in a polytetrafluoroethylene holder, so that the electrode area to
be contacted with the electrolyte was regulated to 1 cm.sup.2.
[0055] The anode and the cathode were placed in the electrolyte so
as to oppose each other at a predetermined inter-electrode
distance. Then, an inter-terminal voltage (electrolysis voltage)
between the anode and the cathode was measured while carrying out
zinc electrowinning in which electrolysis current with current
densities of 10 mA/cm.sup.2, 50 mA/cm.sup.2, and 100 mA/cm.sup.2
based on the electrode area of the anode was applied between the
anode and the cathode.
COMPARATIVE EXAMPLE 1
[0056] In Comparative Example 1, zinc electrowinning was carried
out using the same electrowinning system as in Example 1 under the
same conditions as in Example 1 except that the thermal
decomposition temperature at which to form the catalytic layer was
changed from 360.degree. C. to 470.degree. C.
[0057] The anode according to Comparative Example 1 was
structurally analyzed by X-ray diffraction, the result being that a
sharp diffraction peak profile corresponding to IrO.sub.2 was
recognized but any diffraction peak profile corresponding to
Ta.sub.2O.sub.5 was not recognized. That is, the catalytic layer of
the anode was made of crystalline iridium oxide and amorphous
tantalum oxide.
COMPARATIVE EXAMPLE 2
[0058] In Comparative Example 2, zinc electrowinning was carried
out using the same electrowinning system as in Example 1 under the
same conditions as in Example 1 except that a commercially
available Pb--Sb (5% Sb) alloy electrode was used as the anode.
[0059] Where the electrowinning systems according to Example 1,
Comparative Example 1, and Comparative Example 2 were used in zinc
electrowinning with the electrolyte temperature at 30.degree. C.,
the electrolysis voltage was as shown in FIG. 1 for each system. In
addition, the electrolysis voltage at three minutes after the start
of electrolysis was shown in Table 1.
TABLE-US-00001 TABLE 1 Difference (Improvement) Electrolytic
Electrolysis Voltage In Electrolysis Voltage Current Comp. Comp.
Comp. Ex. Comp. Ex. Density Ex. 1 Ex. 1 Ex. 2 1 - Ex. 1 2 - Ex. 1
10 mA/cm.sup.2 2.37 V 2.49 V 2.91 V 0.12 V 0.54 V 50 mA/cm.sup.2
2.49 V 2.67 V 3.07 V 0.18 V 0.58 V 100 mA/cm.sup.2 2.62 V 2.82 V
3.20 V 0.20 V 0.58 V
[0060] As shown in Table 1, the electrolysis voltage was 0.12V to
0.20V lower in Example 1, where the catalytic layer containing
amorphous iridium oxide was used, than in Comparative Example 1,
where the catalytic layer containing crystalline iridium oxide was
used. Moreover, the electrolysis voltage was even 0.54V to 0.58V
lower in Example 1 than in Comparative Example 2, where the
commercially available Pb--Sb alloy electrode was used as the
anode. That is, the electrowinning system according to Example 1 of
the present invention succeeded in significantly reducing power
consumption for electrolysis.
[0061] Furthermore, similar experiments were conducted where the
electrolyte temperature was changed to 40.degree. C., 50.degree.
C., and 60.degree. C. The results are shown in Table 2 (40.degree.
C.), Table 3 (50.degree. C.), and Table 4 (60.degree. C.).
TABLE-US-00002 TABLE 2 Difference (Improvement) Electrolytic
Electrolysis Voltage In Electrolysis Voltage Current Comp. Comp.
Comp. Ex. Comp. Ex. Density Ex. 1 Ex. 1 Ex. 2 1 - Ex. 1 2 - Ex. 1
10 mA/cm.sup.2 2.34 V 2.46 V 2.86 V 0.12 V 0.52 V 50 mA/cm.sup.2
2.46 V 2.62 V 3.01 V 0.16 V 0.55 V 100 mA/cm.sup.2 2.57 V 2.76 V
3.12 V 0.19 V 0.55 V
TABLE-US-00003 TABLE 3 Difference (Improvement) Electrolytic
Electrolysis Voltage In Electrolysis Voltage Current Comp. Comp.
Comp. Ex. Comp. Ex. Density Ex. 1 Ex. 1 Ex. 2 1 - Ex. 1 2 - Ex. 1
10 mA/cm.sup.2 2.32 V 2.43 V 2.82 V 0.11 V 0.50 V 50 mA/cm.sup.2
2.43 V 2.58 V 2.96 V 0.15 V 0.53 V 100 mA/cm.sup.2 2.53 V 2.71 V
3.06 V 0.18 V 0.53 V
TABLE-US-00004 TABLE 4 Difference (Improvement) Electrolytic
Electrolysis Voltage In Electrolysis Voltage Current Comp. Comp.
Comp. Ex. Comp. Ex. Density Ex. 1 Ex. 1 Ex. 2 1 - Ex. 1 2 - Ex. 1
10 mA/cm.sup.2 2.29 V 2.41 V 2.77 V 0.12 V 0.48 V 50 mA/cm.sup.2
2.40 V 2.55 V 2.91 V 0.15 V 0.51 V 100 mA/cm.sup.2 2.49 V 2.66 V
3.03 V 0.17 V 0.54 V
[0062] As shown in Tables 2 to 4, the electrolysis voltage was 0.11
V to 0.19V lower in Example 1, where the catalytic layer containing
amorphous iridium oxide was used, than in Comparative Example 1,
where the catalytic layer, containing crystalline iridium oxide,
was used. Moreover, the electrolysis voltage was even 0.48V to
0.55V lower in Example 1 than in Comparative Example 2, where the
commercially available Pb--Sb alloy electrode was used as the
anode. That is, the electrowinning system according to Example 1 of
the present invention succeeded in significantly reducing power
consumption for electrolysis as well for the case where the
electrolyte temperature was in the range from 40.degree. C. to
60.degree. C.
[0063] [Cobalt Electrowinning Using Sulfuric Acid-Based
Electrolyte]
EXAMPLE 2
[0064] In Example 2, cobalt electrowinning was carried out using an
electrowinning system according to the present invention, which
included a sulfuric acid-based electrolyte containing cobalt ions,
an anode having a catalytic layer, containing amorphous iridium
oxide, formed on a conductive substrate, and a cathode placed in
the electrolyte along with the anode.
[0065] The present example used the same anode as in Example 1,
i.e., an anode which had a catalytic layer, containing amorphous
iridium oxide, formed on a titanium plate and whose electrode area
for acting on electrolysis was regulated to 1 cm.sup.2. Moreover, a
cobalt plate (2cm.times.2cm) was used as the cathode, and the
electrolyte used was a sulfuric acid-based electrolyte obtained by
dissolving 0.3 mol/L CoSO.sub.4 in distilled water and adjusting
the pH to 2.9 with sulfuric acid.
COMPARATIVE EXAMPLE 3
[0066] Comparative Example 3 used the same anode as in Comparative
Example 1, i.e., an anode which had a catalytic layer, containing
crystalline iridium oxide, formed on a titanium plate and whose
electrode area for acting on electrolysis was regulated to 1
cm.sup.2. Other than that, the same electrowinning system as in
Example 2 was used for cobalt electrowinning.
COMPARATIVE EXAMPLE 4
[0067] In Comparative Example 4, the same electrowinning system as
in Example 2 was used for cobalt electrowinning, except that a
commercially available Pb--Sb alloy electrode (5% Sb) was used as
the anode.
[0068] Where the electrowinning systems according to Example 2,
Comparative Example 3, and Comparative Example 4 were used in
cobalt electrowinning with electrolysis current having a current
density of 10 mA/cm.sup.2 based on the electrode area of the anode
and the electrolyte temperature at 40.degree. C., the electrolysis
voltage at three minutes after the start of electrolysis was as
shown in the following table.
TABLE-US-00005 TABLE 5 Difference (Improvement) Electrolytic
Electrolysis Voltage In Electrolysis Voltage Current Comp. Comp.
Comp. Ex. Comp. Ex. Density Ex. 2 Ex. 3 Ex. 4 3 - Ex. 2 4 - Ex. 2
10 mA/cm.sup.2 1.91 V 2.01 V 2.08 V 0.10 V 0.17 V
[0069] As shown in Table 5, the electrolysis voltage was 0.10V
lower in Example 2, where the catalytic layer containing amorphous
iridium oxide was used, than in Comparative Example 3, where the
catalytic layer containing crystalline iridium oxide was used, and
also 0.17V lower in Example 2 than in Comparative Example 4, where
the commercially available Pb--Sb alloy electrode was used as the
anode. That is, also in cobalt electrowinning using a sulfuric
acid-based electrolyte, the electrowinning system according to
Example 2 of the present invention succeeded in significantly
reducing power consumption for electrolysis.
[0070] [Nickel Electrowinning Using Chloride-Based Electrolyte]
EXAMPLE 3
[0071] In Example 3, nickel electrowinning was carried out using an
electrowinning system, which included a chloride-based electrolyte
containing nickel ions, an anode having a catalytic layer,
containing amorphous iridium oxide, formed on a conductive
substrate, and a cathode placed in the electrolyte along with the
anode.
[0072] The present example used the same anode as in Example 1,
i.e., an anode which had a catalytic layer, containing amorphous
iridium oxide, formed on a titanium plate and whose electrode area
for acting on electrolysis was regulated to 1 cm.sup.2. Moreover, a
nickel plate (2 cm.times.2cm) was used as the cathode, and the
electrolyte used was a chloride-based electrolyte obtained by
dissolving 0.08 mol/L NiCl.sub.2 in a 0.5 mol/L HCl aqueous
solution.
COMPARATIVE EXAMPLE 5
[0073] Comparative Example 5 used the same anode as in Comparative
Example 1, i.e., an anode which had a catalytic layer, containing
crystalline iridium oxide, formed on a titanium plate and whose
electrode area for acting on electrolysis was regulated to 1
cm.sup.2. Other than that, the same electrowinning system as in
Example 3 was used for nickel electrowinning.
COMPARATIVE EXAMPLE 6
[0074] In Comparative Example 6, the same electrowinning system as
in Example 3 was used for nickel electrowinning, except that a
commercially available Pb--Sb alloy electrode (5% Sb) was used as
the anode.
[0075] Where the electrowinning systems according to Example 3,
Comparative Example 5, and Comparative Example 6 were used in
nickel electrowinning with electrolysis current having a current
density of 10 mA/cm.sup.2 based on the electrode area of the anode
and the electrolyte temperature at 40.degree. C., the electrolysis
voltage at three minutes after the start of electrolysis was as
shown in the following table.
TABLE-US-00006 TABLE 6 Difference (Improvement) Electrolytic
Electrolysis Voltage In Electrolysis Voltage Current Comp. Comp.
Comp. Ex. Comp. Ex. Density Ex. 3 Ex. 5 Ex. 6 5 - Ex. 3 6 - Ex. 3
10 mA/cm.sup.2 1.76 V 1.82 V 2.75 V 0.06 V 0.99 V
[0076] As shown in Table 6, the electrolysis voltage was 0.06V
lower in Example 3, where the catalytic layer containing amorphous
iridium oxide was used, than in Comparative Example 5, where the
catalytic layer containing crystalline iridium oxide was used, and
also even 0.99V lower in Example 3 than in Comparative Example 6,
where the commercially available Pb--Sb alloy electrode was used as
the anode. That is, also in nickel electrowinning using a
chloride-based electrolyte, the electrowinning system according to
Example 3 of the present invention succeeded in significantly
reducing power consumption for electrolysis.
[0077] [Cobalt Electrowinning Using Chloride-Based Electrolyte]
EXAMPLE 4
[0078] In Example 4, cobalt electrowinning was carried out using an
electrowinning system, which included a chloride-based electrolyte
containing cobalt ions, an anode having a catalytic layer,
containing amorphous ruthenium oxide, formed on a conductive
substrate, and a cathode placed in the electrolyte along with the
anode.
[0079] In anode production, initially, a commercially available
titanium plate (5 cm long, 1 cm wide, 1 mm thick) was dipped and
etched in a 10% oxalic acid solution at 90.degree. C. for 60
minutes, and then washed with water and dried. A coating solution
was prepared by adding ruthenium chloride trihydrate
(RuCl.sub.3.3H.sub.3O) and titanium n-butoxide
(Ti(C.sub.4H.sub.9O).sub.4) to a butanol (n-C.sub.4H.sub.9OH)
solution, such that the molar ratio of ruthenium to titanium was
30:70 and a total amount of ruthenium and titanium was 70 mg/mL in
terms of metal. The coating solution was applied to the dried
titanium plate and then dried at 120.degree. C. for 10 minutes
before thermal decomposition for 20 minutes in an electric furnace
maintained at 340.degree. C. The coating, drying and thermal
decomposition was repeated five times in total, thereby producing
an anode having a catalytic layer formed on the titanium plate,
which acted as a conductive substrate.
[0080] The anode was structurally analyzed by X-ray diffraction,
resulting in an X-ray diffraction pattern with no diffraction peak
profile corresponding to RuO.sub.2, but a weak diffraction line in
a broadened pattern corresponding to a RuO.sub.2--TiO.sub.2 solid
solution was recognized. That is, the catalytic layer of the anode
was made of a composite oxide of amorphous ruthenium oxide and
titanium oxide.
[0081] A platinum plate (2 cm.times.2cm) was used as the cathode,
and the electrolyte used was a chloride-based electrolyte obtained
by dissolving 0.9 mol/L CoCl.sub.2 in distilled water and adjusting
the pH to 1.6 with hydrochloric acid. Moreover, the anode was
embedded in a polytetrafluoroethylene holder, so that the electrode
area for acting on electrolysis was regulated to 1 cm.sup.2.
[0082] The anode and the cathode were placed in the electrolyte so
as to oppose each other at a predetermined inter-electrode
distance. Then, cobalt electrowinning was carried out for 40
minutes with electrolysis current having a current density of 10
mA/cm.sup.2 based on the electrode area of the anode and the
electrolyte temperature at 60.degree. C.
COMPARATIVE EXAMPLE 7
[0083] In Comparative Example 7, cobalt electrowinning was carried
out using the same electrowinning system as in Example 4 except
that the thermal decomposition temperature at which to form the
catalytic layer was changed from 340.degree. C. to 450.degree.
C.
[0084] The anode according to Comparative Example 7 was
structurally analyzed by X-ray diffraction, resulting in an X-ray
diffraction pattern in which a sharp diffraction peak profile
corresponding to a solid solution (composite oxide) of crystalline
RuO.sub.2 and TiO.sub.2 was recognized. That is, the catalytic
layer of the anode contained crystalline ruthenium oxide but did
not contain amorphous ruthenium oxide.
[0085] On the basis of the weight of the anodes before and after
electrolysis for cobalt electrowinning with the electrowinning
systems according to Example 4 and Comparative Example 7, the
weight of cobalt oxyhydroxide deposited on the anodes was
calculated, and the percentage of the quantity of electrolytic
electricity (hereinafter, simply referred to as current efficiency)
consumed for generating cobalt oxyhydroxide was calculated. The
results are shown in the following table.
TABLE-US-00007 TABLE 7 Electrolytic Current Efficiency Current
Density Ex. 4 Comp. Ex. 7 10 mA/cm.sup.2 8.3% 24%
[0086] As shown in Table 7, in Example 4, where the catalytic layer
containing amorphous ruthenium oxide was used, the current
efficiency was about 1/3 of that in Comparative Example 7, where
the catalytic layer containing crystalline ruthenium oxide was
used. Specifically, in Example 4, most of the electrolysis current
(90% or more) was consumed for chlorine evolution, the main
reaction, resulting in a considerable reduction of power required
for electrowinning of the same amount of cobalt. Moreover, in
Example 4, cobalt oxyhydroxide was kept from being accumulated on
the anode, thereby suppressing a rise of the electrolysis voltage
while electrowinning was continued for a long period of time.
[0087] [Other (Variants)]
[0088] While the electrowinning anode according to the present
invention has been described above with respect to preferred
embodiments, the present invention is not limited to these
configurations, and numerous types of variants are conceivable.
[0089] For example, amorphous tantalum oxide can be omitted from
the catalytic layers of the electrowinning anodes according to
Examples 1 to 3, so long as amorphous iridium oxide is at least
included. However, from the viewpoint of reducing oxygen evolution
potential or increasing durability, the catalytic layers preferably
include both amorphous iridium oxide and amorphous tantalum
oxide.
[0090] Similarly, titanium oxide can be omitted from the catalytic
layer of the electrowinning anode according to Example 4, so long
as amorphous ruthenium oxide is at least included.
[0091] Furthermore, the catalytic layers of the electrowinning
anodes according to Examples 1 to 3 may be formed on protective
layers, containing crystalline iridium oxide, formed on the
conductive substrates. In the case where a metal such as titanium
or tantalum is used as the conductive substrates, the protective
layers containing crystalline iridium oxide adhere well to the
conductive substrates, and also to the catalytic layers, so that
the catalytic layers can be more stably formed on the conductive
substrates, resulting in enhanced durability. Particularly suitable
for such a protective layer is a mixed oxide layer made of
crystalline iridium oxide and amorphous tantalum oxide.
[0092] Similarly, in order to achieve enhanced durability, the
catalytic layer of the electrowinning anode according to Example 4
may be formed on a protective layer, containing crystalline
ruthenium oxide, formed on the conductive substrate. Particularly
suitable for such a protective layer is a composite oxide layer
made of crystalline ruthenium oxide and titanium oxide.
[0093] Furthermore, in the case where the current density of the
electrolysis current is increased to 0.1 A/cm.sup.2 or more, an
intermediate layer made of tantalum or a tantalum alloy is
preferably formed between the conductive substrate and the
catalytic layer. Forming the intermediate layer makes it possible
to prevent the conductive substrate from being oxidized and
corroded by an acidic electrolyte penetrating through the catalytic
layer, resulting in improved durability of the anode. The
intermediate layer can be formed by methods such as sputtering, ion
plating, CVD, electroplating, etc.
[0094] Furthermore, zinc, cobalt, and nickel electrowon in the
examples are illustrative, and the electrowinning anode and the
electrowinning method according to the present invention can
electrowin noble metals, rare metals, and other metals.
* * * * *