U.S. patent number 3,855,084 [Application Number 05/380,325] was granted by the patent office on 1974-12-17 for method of producing a coated anode.
Invention is credited to Norman G. Feige, Jr..
United States Patent |
3,855,084 |
Feige, Jr. |
December 17, 1974 |
METHOD OF PRODUCING A COATED ANODE
Abstract
An electrode useful for electrowinning and other processes
including evolution of oxygen or chlorine or for plating is formed
of titanium particles, compacted cold, and coated and cemented with
a first layer of manganese dioxide thermally deposited from Mn
(NO.sub.3).sub.2 on the grains to form a coating of a combination
of manganese dioxide and titanium oxide having a rutile crystal
structure, and a further outer layer of manganese dioxide which is
electrodeposited, the two coatings not being limited to the surface
of the electrode but extending to all exposed surfaces of the
grains including those which are walls of channels between the
grains. The compression of the titanium powder is to a density
between 30 and 70 percent of the density of solid metal. Solid
metal such as metal mesh may be included in the titanium powder
prior to compacting, such as expanded titanium metal, to assist in
strengthening the form. In an alternate embodiment the metal
substrate is lead.
Inventors: |
Feige, Jr.; Norman G. (South
Salem, NY) |
Family
ID: |
23500754 |
Appl.
No.: |
05/380,325 |
Filed: |
July 18, 1973 |
Current U.S.
Class: |
205/199;
204/280 |
Current CPC
Class: |
C25B
11/054 (20210101); C25B 11/02 (20130101); C25B
11/091 (20210101); C25B 11/04 (20130101) |
Current International
Class: |
C25B
11/00 (20060101); C25B 11/02 (20060101); C25B
11/16 (20060101); C25B 11/04 (20060101); B01k
003/06 (); C23b 011/00 (); C01b 007/06 () |
Field of
Search: |
;204/38R,29R,29F,56R,96,83 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Edmundson; F. C.
Attorney, Agent or Firm: Wheeler, Morsell, House &
Fuller
Claims
I claim:
1. A method of manufacturing an electrode comprising the steps
of:
first, cold compacting metal particles chosen from the group
consisting of titanium and lead to form an electrode having an
extensively interconnected pore system between the particles;
second, depositing a first layer of manganese dioxide on the
exterior surface of the compact and within the pores between
particles by thermal decomposition of a solution of Mn
(NO.sub.3).sub.2 ;
and third, electrodepositing a layer of manganese dioxide on said
first layer.
2. The method of claim 1 in which the particles are titanium.
3. The method of claim 1 in which the particles are lead.
4. The method of claim 2 in which the particles comprise powder
predominantly in the size range of +100 mesh to -325 mesh.
5. The method of claim 1 in which the particles are compacted along
with at least one solid titanium metal reinforcement to increase
the mechanical strength of the electrode.
6. The method of claim 2 in which during the second step the
forming manganese dioxide coating interchanges atoms with the
titanium and titanium oxide normally present on the surface of the
particles to form a modified rutile crystal structure which is
electrically conductive.
7. The method of claim 1 in which the electrodeposited manganese
dioxide is approximately 100 microns thick.
8. The method of claim 1 wherein the density of the compacted
particles is between 30 and 70 percent of the density of the solid
metal of which the particles are composed.
9. A method of forming an electrode comprising the steps of:
first, forming the body of the electrode from particles of
titanium;
second, thermally decomposing manganeous nitrate in contact with
the electrode to form a layer of the intermediate product
manganeous-manganite;
third, causing ion exchange between the body of the electrode and
the coating to replace manganese ions with titanium ions;
fourth, causing the product to coat the surfaces of the particles
to form an adherent coating of a highly conductive film of
manganese-titanium oxide;
and fifth, thereafter electrodepositing a layer of manganese
dioxide on the surfaces of the coated particles.
10. The method of claim 9 in which the second step cements the
grains or particles together in the body of the electrode thereby
forming a mechanically sound electrode for use as an anode.
11. The method of claim 9 in which the powder is in the size range
of +100 mesh to -325 mesh.
12. The method of claim 9 in which the powder is compacted along
with at least one solid titanium metal reinforcement to increase
the mechanical strength of the electrode.
13. The method of claim 9 in which during step 2 the manganese
dioxide coating is reacted with the titanium oxide normally present
on the surface of the particles to form a modified rutile crystal
structure which is electrically conductive.
14. The method of claim 9 in which the electrodeposited manganese
dioxide is approximately 100 microns thick.
Description
BACKGROUND OF THE INVENTION
It has long been known that titanium metal has superior properties
for use as an electrode in cells, baths or solutions which would
corrode and be contaminated by metal from electrodes of other
compositions. Nevertheless there are difficulties in the use of a
titanium electrode as well. Solid titanium leads to a requirement
for high voltage, and gives poor current efficiency. Fox U.S. Pat.
Nos. 2,631,115 and 2,608,531 discuss some of the difficulties in
connection with specific systems, for instance the passive surface
coating on the titanium, and indicate that at least in the
production of oxides such as manganese dioxide for use in batteries
as a depolarizer, the use of a porous titanium anode made of chips
is of assistance. Under certain special conditions described by
Fox, the plating of MnO.sub.2 or in a battery, the use of a
specific form of titanium chips of about 35 mesh in one case and
the use of specific voltage relationships during electrolytic
formation of the surface of the electrode prior to use in the other
patent, improved results are observed using porous titanium anodes.
Fox discloses coating the surfaces of the porous titanium mass with
graphite, gold or iron, or with any good conductor which is inert
to the elctrolyte in which the electrode is to be used. Among other
things Fox discloses that his electrodes are useful for the
preparation of electrolytic manganese dioxide. His described
electrode does not function as an oxygen evolution anode, but
passivates under those conditions.
In industry it is most important to select a suitable anode that
does not contaminate the electrolyte or contaminate the cathode
deposit, that has a long life, and has a low oxygen overvoltage
during electrolysis. Platinum is an excellent known anode material
which satisfies the above mentioned characteristics.
Recently platinum and other precious metals have been applied to a
titanium substrate to retain their attractive electrical
characteristics and further reduce manufacturing cost. However such
anodes are expensive and are not suitable for some industrial uses.
Thus carbon and lead alloy electrodes have generally been used. The
carbon anode, however, has the disadvantage that it greatly
contaminates the electrolyte, wears fast, and has high electrical
resistance which results in an increase in cell voltage. It may
also be degraded to CO.sub.2 during oxygen evolution. The
disadvantage of the lead alloy anode is that PbO.sub.2 changes to
Pb.sub.3 O.sub.4 which is poorly conductive. O.sub.2 gets below
this layer and flakes off the film. These particles become trapped
in the deposited copper at the cathode, degrading it.
In order to overcome these disadvantages it has recently been
proposed to plate the surface of a titanium substrate with platinum
and to electrodeposit either lead dioxide or manganese dioxide on
the plated surface. Such anodes have the disadvantage of
comparatively high oxygen overvoltage. In addition both coatings
have high internal stress when electrolytically deposited and are
liable to be detached from the surface during commercial usage,
both contaminating the electrolyte and, in the case of lead, being
deposited on the cathode to reduce its value. Thus current density
with such anodes is very limited.
To improve the high oxygen overvoltage it has also been proposed to
compact and sinter titanium chips to increase the apparent surface
area. Such an anode has somewhat improved characteristics but does
not properly receive and retain the electrodeposited manganese
dioxide coating.
This invention is based on recognition that deposited manganese
dioxide is both insoluble and electrically conductive, and cannot
readily be deposited as a reduced product on the cathode.
Experiments have shown that my electrode has a low oxygen
overvoltage during electrolysis, economizes on electric power
necessary for electrolysis, reduces the loss of manganese from the
electrode to the bath to a very low figure, and is believed to be
the optimum electrode for electrolytic winning of copper, zinc and
nickel in sulfate electrolytes. It is also useful for evolving
oxygen in sulfate systems and chlorine in chloride systems.
SUMMARY OF THE INVENTION
The preferred electrode of my invention is made of titanium powder
cold compacted to the shape of an electrode, the compaction being
sufficient to produce a density in the powder between 30 and 70
percent of the density of the solid metal of which the powder is
composed. A layer of manganese dioxide is produced on the surface
of the grains throughout the mass of the electrode by thermal
decomposition of Mn (NO.sub.3).sub.2. That layer is modified by ion
exchange with the titanium dioxide surface layer and probably with
the titanium metal of the powder grains to contain titanium atoms
as well as manganese atoms. These do not significantly modify the
crystal structure which is essentially that of rutile. This mixed
structure is formed while the material of the coating is in the
intermediate manganeous-manganite form during the decomposition
from manganeous nitrate to manganese dioxide.
A final coating of manganese dioxide is electrodeposited over the
hybrid coating.
Preferably the structure is strengthened by including solid metal
in the electrode. A preferred form is an expanded metal lattice of
titanium although other shapes may be used. The preferred size
range of the titanium powder is +100 mesh to -325 mesh. The
preferred thickness of the outer coating of mananese dioxide is 100
microns. The density referred to is calculated on the weight of the
powder alone. The upper limit is the loss of connected pores
between the particles.
The preparation of an electrode in this manner produces a stronger
electrode without the necessity of sintering the powder because the
initial thermally deposited layer is extremely effective in
cementing the grains of the powder. It has been found that even
loose titanium powder may be cemented by this method to produce a
coherent shape. Powder which has been cold compacted to between 30
and 70 percent of metallic density and then coated as described
produces an extremely durable electrode suitable for use as the
anode in an electrowinning process. The anode evolves O.sub.2 in
sulfates and Cl.sub.2 in chlorides, and accordingly may also be
used for the evoluation of chlorine as well.
It has been observed that manganese dioxide coating of prior art
electrodes used in electrowinning is lost during periods of
shutdown, at which time it goes into solution and becomes a part of
the bath, degrading the electrode. The loss is largely in the form
of conversion to manganese cations and permanganate. With the anode
of my invention, the ions and permanganate are largely limited to
the pores or channels between the grains of titanium powder and a
very high percentage is redeposited as manganese dioxide upon
application of current for the electrowinning process, in the same
manner that the outer layer was originally deposited. Thus
degradation of the anode and pollution of the bath are both
avoided.
Finally the electrical and operating characteristics of my
electrode compare favorably with prior art electrodes.
The chief presently known use of my electrode is as an anode for
electrolysis. So used, an advantage of the anode is low oxygen
overvoltage during electrolysis, thus economizing the electrical
powder necessary.
A similar electrode may be made with lead particles, at lower cost.
As in the case of the titanium electrode a first coating of
MnO.sub.2 is thermally deposited and forms a hybrid with the lead
oxide surface layer normally present. A further layer of MnO.sub.2
is electrodeposited. A reinforcement of solid metal, which may be
titanium expanded metal mesh, may be used.
DRAWINGS
FIG. 1 is a perspective view of a rectangular electrode made
according to my invention with portions broken away to show
interior structure.
FIG. 2 is an enlarged cross-sectional view on line 2--2 of FIG.
1.
FIG. 3 is a graph of cell voltage against current density for an
anode according to my invention compared with a lead anode and a
solid titanium anode coated with MnO.sub.2.
FIG. 4 is an idealized cross-section of several grains showing the
two layers and the pores highly magnified.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Although the disclosure herein is detailed in order to enable those
skilled in the art to practice the invention, the embodiments
disclosed merely exemplify the invention which may take other
forms. The scope of the invention is defined in the claims.
FIG. 1 is a broken away perspective view showing a portion of an
electrode 10 formed according to my invention. An expanded metal
mesh 12 of titanium metal has been placed with -100 mesh titanium
powder in a mold and compacted in a press until the powder has a
density of 30 to 70 percent of theoretical density of the metal.
The precise shape is not a part of my invention and may conform to
specifications for a particular use, so the outline is not shown.
FIG. 2 is a cross-sectional view. As shown in FIG. 4, which is a
highly magnified and idealized cross-sectional view through several
particles of titanium, each grain 20 is coated with a first layer
22 and a second layer 24, leaving interconnected pores 26 between
the grains.
The first layer 22 is basically MnO.sub.2 which has been deposited
by thermal breakdown of Mn (NO.sub.3).sub.2 and which has exchanged
ions with the metal and surface layer of titanium oxide normally
present on the metal and not separately shown. Thus layer 22 has a
modified rutile character. It is strongly adherent and cements the
grains 20 and the mesh 12 together.
The second layer 24 is electrodeposited MnO.sub.2. It is not known
to interchange ions with the first layer and is believed therefore
to be identical with commercially electrodeposited manganese
dioxide.
The pores 26 are the spaces between the coated grains. As more
fully described elsewhere, they are highly interconnected. This not
only creates high available surface area (as opposed to surface in
a closed cavity not connected through pores 26 with the exterior of
the electrode 10) thus improving the apparent current density, it
also means that a substantial part of the first and second layers
22-24 are in the pores. During inactivity of the immersed electrode
MnO.sub.2 can break down to manganese cations and permanganate. In
my electrode these are largely trapped in pores 26. Upon
reapplication of current to the anode, MnO.sub.2 is re-formed on
the second layer with very little loss.
The method of my invention thus comprises the basic steps of
compacting metal particles to a density sufficient to make a
coherent final product but not so high as to destroy the high
interconnection between the pores, thermally breaking down Mn
(NO.sub.3).sub.2 at a temperature between about 120.degree. C. and
475.degree. C. to form a first layer on the particles or grains,
and depositing a second layer of MnO.sub.2 over the first layer, to
form an electrode. The metal may preferably be titanium but may
also be lead.
The density of the compacted titanium powder used in my electrode
has both a lower and upper limits for satisfactory performance. The
freely poured titanium powder of a typical sieve analysis as
follows:
TABLE I ______________________________________ Ti Powder Analysis
______________________________________ Typical Sieve Analysis
______________________________________ Size All powder less than
-100 mesh. -100, +150 mesh 25 percent -150, +200 mesh 21 percent
-200, +270 mesh 26 percent -270, +325 mesh 17 percent -325 mesh 11
percent Chemistry (%) Oxygen 0.30 max. Nitrogen 0.04 max. Carbon
0.04 max. Chlorine 0.20 max. Iron 0.50 max. Magnesium 0.30 max.
Total others 0.12 max. Titanium 98.50 min.
______________________________________
has a theoretical density of 25 percent, that is, its weight is 25
percent of the weight of an equal volume of solid titanium metal
having the same anlaysis. Using great care it is possible to
compact such powder to only 30 percent density in the shape of an
anode and cement the grains of the powder together by thermal
decomposition of Mn (NO.sub.3).sub.2 as described elsewhere in this
specification. Thus 30 percent density is believed to be a
practical lower limit of density. With greater compacting force the
density of the powder may be increased to about 70 percent while
retaining highly interconnected passageways between the grains of
metal of sufficient size for the application of the coatings of my
electrode. Above 70 percent theoretical density the pores lose
interconnection and surface area is lost to the extent that
following the process described in this specification does not
result in deposition of sufficient MnO.sub.2 either in the layer
produced by thermal decomposition or in the electrodeposited layer.
Above 70 percent density the oxygen overvoltage rises unduly as
shown in table 2 below:
TABLE NO. 2
__________________________________________________________________________
The Relationship Between Theoretical Density And Anode Performance
__________________________________________________________________________
Anode % Den. Thickness % MnO.sub.2 Back EMF Cell Voltage at 24
amps/ft.sup.2 at 120 amps/ft.sup.2
__________________________________________________________________________
72% 1.0cm 2% 1.15 volts 1.55 1.95 63% 1.1cm 3% 1.20 volts 1.54 1.90
49% 1.0cm 15% 1.06 volts 1.43 1.87 46% 1.6cm 6% 1.20 volts 1.48
1.84 38% 2.1cm 14% 1.11 volts 1.35 1.69 37% 1.2cm 17% 1.08 volts
1.30 1.71
__________________________________________________________________________
Another method of defining the upper limit is by blowing air
through the compacted mass. Substantial resistance to the passage
of air indicates that few passages are interconnected, showing that
compaction is too great. The upper limit may vary with particle
size and shape but includes only a degree of compaction which
leaves the pores highly interconnected.
In my electrode, the grains of titanium powder are cemented
together by the first coating of MnO.sub.2 which is produced by
thermal decomposition of Mn (NO.sub.3).sub.2 between 120.degree. C.
and 475.degree. C. During the thermal decomposition there is an
extensive reaction with the surface titanium oxide layer which is
normally present on titanium metal by ion exchange, during the
phase when the coating is black liquid manganeous-manganite. The
result is an adherent coating bonding the grains of titanium powder
with a highly conductive oxide film. Subsequently a further layer
of MnO.sub.2 is electrolytically deposited, resulting in an
electrode having extensively interconnected passages between the
grains, the surface of each grain both on the surface of the
electrode and in the passages being coated with a first layer of
modified rutile containing titanium, and then with MnO.sub.2.
Electrodeposited manganese dioxide is brittle, and has large
internal stresses. It is readily detached from a substrate when
deposited to an appropriate thickness for electrode use, making it
difficult to form an effective and long lived electrode from such
manganese dioxide alone. My invention permits the application of
heavy deposits in excess of 100 microns thick of manganese dioxide
within the pores between the grains of titanium powder, thus taking
advantage of the large internal stresses of the coating to improve
its adherence rather than to cause failure as in the case of a flat
sheet electrode.
Finally, by depositing much of the coating internally it is
protected both from mechanical dislodgement and from loss into the
solution when the electrode is inactive.
Although under anodic potential as applied during electrolysis,
manganese dioxide is insoluble, under an open circuit permanganate
is observed in solution. As the potential is again applied to the
anode the permanganate is redeposited on the anode as manganese
dioxide. By coating the porous substrate internally the dissolution
of manganese dioxide is reduced and the concentration of
permanganate is sufficiently high in the pores for redeposition to
reduce significantly the loss of manganese from the anode and the
pollution of the bath.
EXAMPLE 1
An example showing the comparison between my electrode and a
similar electrode made with solid titanium plate is as follows:
Two sheets of expanded titanium mesh, each 1.5 millimeters thick,
were placed in a mold 500 centimeters by 7.50 centimeters by 3
millimeters thick with 82 grams of titanium powder having the sieve
analysis and chemical analysis shown in Table 1. These were
subjected to a pressure of 5,400 kilograms, resulting in a
composite structure in which the powder component had a theoretical
density of 52 percent of the density of solid titanium. The
electrode was impregnated with an aqueous solution of Mn
(NO.sub.3).sub.2 and was baked at 176.degree. Centigrade. The
electrode developed a gray adherent coating. The electrode was then
placed in a bath of MnSO.sub.4 and H.sub.2 SO.sub.4 at 90.degree.
Centigrade and current was applied to electrolytically deposit a
coating of manganese dioxide following accepted techniques. It was
found that during this step current density could be varied from
0.1 ampere to 6.7 amperes per square foot, which was the observed
oxygen evolution value for the anode. A black layer of MnO.sub.2
developed over the first layer.
In the same manner a titanium plate 500 centimeters by 750
centimeters by 3 millimeters thick having a sandblasted surface was
coated with two layers as described above.
The two electrodes thus produced are compared in FIG. 3 along with
an electrode of solid lead. The bath was a copper plating bath
containing CuSO.sub.4 and H.sub.2 SO.sub.4 at 60.degree. Centigrade
and a gap of 1/2 inch. The anode made from titanium plate was less
durable and required higher cell voltage, despite the use of both
thermal and electrolytic deposition of MnO.sub.2 in accordance with
a portion of my invention. Line 34 is the curve for the porous
anode with two layers made according to my invention. Line 32 is
the curve for lead. Line 30 is the curve for the solid titanium
anode with two MnO.sub.2 layers.
A porous anode as described in this example was used for
electrolytic winning of copper, zinc and nickel in the respective
commercial sulfate-sulfuric acid electrolytes. The anode performed
satisfactorily and exhibited a substantial improvement in cell
voltage in the system as compared to lead anodes in each
electrolyte.
The porous electrode described has also been tested as an anode for
evolution of oxygen, and as an anode for evolution of chlorine,
both with good efficiencies and service life. The anode is useful
for chlorination of water, for instance.
EXAMPLE 2
The effect of changes in the density of the powder component of my
electrode may be seen in table 2 below. Anodes prepared according
to the first paragraph of Example 1, with the exception that the
compaction and amount of MnO.sub.2 varied, are compared.
TABLE NO. 2
__________________________________________________________________________
The Relationship Between Theoretical Density And Anode Performance
__________________________________________________________________________
Anode % Den. Thickness % MnO.sub.2 Back EMF Cell Voltage at 24
amps/ft.sup.2 at 120 amps/ft.sup.2
__________________________________________________________________________
72% 1.0cm 2% 1.15 volts 1.55 1.95 63% 1.1cm 3% 1.20 volts 1.54 1.90
49% 1.0cm 15% 1.06 volts 1.43 1.87 46% 1.6cm 6% 1.20 volts 1.48
1.84 38% 2.1cm 14% 1.11 volts 1.35 1.69 37% 1.2cm 17% 1.08 volts
1.30 1.71
__________________________________________________________________________
EXAMPLE 3
In a manner like that of Example 1, lead electrodes were prepared
using lead shot particles and titanium expanded metal mesh but
using lower pressure. In respective trials lead paraticles ranging
from no. 6 shot (0.110 in.) to no. 11 shot (0.065 in.) were used.
After the two layers of MnO.sub.2 are deposited as in Example 1,
the electrode was tested as an anode in a copper sulphate and
sulfuric acid conventional electrolyte. The graph of cell voltage
vs. amperes/sq. ft. for this electrode was intermediate between
such a graph for a solid lead anode and that for the anode of
Example 1 for the particle sizes tested.
It will be understood that the invention is not limited to the
examples described and that many modifications may be introduced
therein. The scope of the invention is intended to be limited only
by the scope of the appended claims.
* * * * *