Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~[)SB563
BACKGROUMD OF THE INVENTION
The present invention relates to improved electrodes
particularly adapted for use as anodes in electrochemical pro-
cess involving the electrolysis of brines.
A variety of materials have been tested and used as
chlorine anodes in electrolytic cells~ In the past, the material
most commonly used for this purpose has been graphite. However,
the problems associated with the use of graphite anodes are
several. The chlorine overvoltage of graphite is relatively
high, in comparison for example with the noble metals. Further-
more, in the corrosive media of an electrochemical cell graphite
wears readily, resulting in substantial loss of graphite and
the ultimate expense of replacement as well as continued main-
tenance problems resulting from the need for frequent adjustment
of spacing between the anode and cathode as the graphite wears
away. The use of noble metals and noble metal oxides as anode
materials provides substantial advantages over the use of
graphlte. The elec~rical conductivity of the noble metals
is substantially higher and the chlorine overvoltage substan-
tially lower than that of graphite. In addition, the dimen-
sional stability of tha noble metals and noble metal oxides
represents a substantial improvement over graphite. However,
the use of noble metals as a ma~or material of construction
in anodes results~in an economic disadvantage due to the ex-
cessively high cost of such materials.
In a~tempts to avoid the use of the expensive noble
metals various other anode materials have been proposed for
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use as coatings over valve metal substrates. In U.S. patent
3,627,669, it is disclosed that mixtures of tin dioxide and
oxides of antimony can be formed as adherent coatings on a
valve metal substrate to form an anode useful in electro-
chemical processes. In the electrolytic production of chlorine,
anodes of this type provide the advantage of economy in the
elimination of the use of expensive noble metals or noble
metal oxides. In addition the tin oxide coa~ing provides an
effective protection or the substrate. However, the tin
oxide compositions, although useful as an anode material,
exhibit a chlorine overvoltage that is substantially higher
than that of the noble metals or noble metal oxides. Thus,
despite the elimination of expensive noble metals, the cost of
chlorine production, in processes using such anodes, is rela-
tively high.
Considerable effort has been expended in recent years in
attempts to develop improved anode materials and structures
utilizing the advantages of noble metals or noble metal oxides.
A great amount of effort has been directed to the development
of anodes having a high operative surface area of noble metal
or noble metal oxide in comparison with the total quantity
of the material employed. This may be done, for example, by
employing the noble metal as a thin film or coating over an
electrically conductive substrate. However, when it is at-
tempted to minimize the aforementioned economic disadvantage
of the noble metals by applying them in the form of very thin
films over a metal substrate, it has heen found that such
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very thin films are often porous. The result is an exposure
of the substrate to the anode environment, through the pores
in the outer layer. In addition, in normal use in an electro-
lytic cell, a small amount of wear, spalling or flaking off
of portions of the noble metal or noble metal oxide is likely
to occur, resulting in urther exposure of the substrate.
Many materials, otherwise suitable for use as a substrate
are susceptible to chemical attack and rapid deterioration
upon exposure to the anode environment~ In an attempt to
assure minimum deterioration of the substrate under such
circumstances, anode manufacturers commonly utilize a valve
metal such as titanium as the substrate material. Upon ex-
posure to the anodic environment, ti~anium, as well as other
valve metals, will form a surface layer of oxide which serves
to protect the substrate from further chemical attack. The
oxide thus formed, however, is not catalytically active and
as a result the operative surface area of the anode is decreased.
Accordingly, it is an object of the present invention to
provide improved electrodes for use as anodes in electrolytic
processes. It is a further object to provide such anodes
having an operative surface of noble metal or noble metal
oxide and having improved efficiency and maintenance charac-
teristics.
ST~TEMENT OF INVENTION
This invention provides a novel electrode, especially
suited for use as an anode in chlor-alkali cells; the novel
electrode comprising a valve metal substrate having a pro-
tective coating of conductive tin oxide on the surface
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thereof and an outer, thin layer of a noble metal or noble
metal oxide. Electrodes of this type exhibit a high degree
of durability in addition ~o the relatively low overvoltage
characteristics of a noble metal or noble metal oxide, making
them well-suited for use as anodes in the electrolytic pro-
duction of chorine.
Amvng the advantages of such construction is the pro-
tection afforded the metal substrate by the coating of con
ductive tin oxide. The preferred substrate materials of the
anodes of the invention are the valve metals, such as titanium
tantalum, niobium or zirconium, especially titanium. However,
where suitably thick intermediate layers of tin oxide are
employed, other more conductive metals may be considered for
use as substrates. The tin oxide coating, which may range
in coating weight from about 0.1 grams per square meter to
100 grams per squar~ meter or more, depending on the degree
of protectlon desired, prevents contact of the substrate and
the electrolyte, thus preventing or minimizing corrosion or
surface oxidation and the attendant deterioration or passi-
vation of the substrate. At the same time, the outer layer
provides the advantageous catalytic properties of the noble
metals or noble metal oxides. In addition, the protective
layer of conductive tin oxide permits the use of a relatively
thin layer of the noble metal or noble metal oxide and a
consequent savings resulting from a minimal use of the pre-
cious metal. Typically, the layer of noble metal or noble
metal oxide will have a coating weight in the range of about
0.1 grams per square meter to about 20 grams per square meter
or higher and preferably about 3 to 10 grams per square meter
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in thickness. The disadvantage of pores or pinholes in the
noble metal layer common in extremely thin layers is obviated
by the presence of ~he intermediate layer of conductive tin
oxide. Pores or pinholes in the noble metal layer, or wear-
ing away of that outer layer over long periods of use result
in the gradual exposure of ~he tin oxide layer. The inter-
mediate layer of tin oxide will continue to provide a cata-
lytically active sur~ace in those exposed areas. The cata-
lytic characteristics o~ tin oxide, although not as high as
the noble metals or noble metal oxides, is quite substantially
higher than the valve metal oxide. Thus, the overall deter-
ioration of the catalytic properties of the anode is more
gradual and maintenance problems are accordingly lesse~ed.
In additionr it has been found that the intermediate
layer of tin oxide provides an increase in surface area of
the anode with a consequent improvement in overvoltage.
It has further been found that the adhesion of the noble
metal or noble metal oxide to the substrate is increased by
the presence of the intermediate layer of tin oxide and the
problem of spalling of the surface layer is thereby reduced.
DE5CRIPTION OF THE :PREF RRED EMBODIMENTS
The valve metal substrate which forms the inner or base
component of the electrode is an electroconductive metal having
sufficient mechanical strength to serve as a support for the
coating and having a high degree of chemical resistivity, es-
pecially to the anodic environment of electrolytic cells.
Typical valve metals include, for example, Ti, Ta, Nb, Zr, and
5~3
alloys thereof. The valve metals are well known for their
tendency to form an inert oxide film upon exposure to an
anodic environment. The preferred valve metal, based on
cost and availability as well as electrical and chemical
properties is titanium. The conductivity of the substrate
may be improved, if desired, by providing a central core of
a highly conductive metal such as copper. In such an arrange-
mentr the core must be electrically connected to and completely
protected by the valve metal substrate.
Tin oxide can be readily formed as an adherent coatiny
on a valve metal substrate, in a manner described hereinafter,
to provide a protective, electrically conductive layer which
is especially resistant to chemical attack in anodic environ
ments. Pure tin oxide however has a relatively high electrical
resistivity in comparison to metals and exhibits undesireble
change in electrical resistivity as a function of temper-
ature. It is well known that the electrical stability of tin
oxide coatings may be substantially improved and the electrical
resistivity lowered through the introduction of a minor prop-
ortion of a suitable inorganic material (commonly referred
to as a "dopant"). A variety of materials, especially various
metal oxides and other metal compounds and mixtures thereof,
have been disclosed in the prior art as suitable dopants
for stabilizing and lowering the electrical resistivity of
tin oxide compositions. Among the materials shown in the
prior art to be useful as dopants in conductive tin oxide
compositions and which may be employed in the tin oxide coat-
ing compositions of the anodes of this invention are included,
for example, fluorine compounds, especially the metal salts
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of fluorine, such as sodium fluoride, potassium fluoride,
lithium fluoride, berylium fluoride, aluminum fluoride,
lead fluoride, chromium fluoride, calcium fluoride, and
other metal fluorides; hydrazine, phenylhydrazine; phos-
phorus compounds such as phosphorus chloride, phosphorus
oxychloride, ammoni~ phosphate, organic phosphorus esters
such as tricresyl phosphate; as well as compounds of tel-
luriumr tungsten, antimony, molybdenum, arsenic, and others
and mixtures thereof. The conductive tin oxide coatings of
this invention comprise tin oxide, preferably containing a
minor amount of a suitable dopant. The preferred dopant is
an antimony compound which may be added to the tin oxide
coating composition either as an oxide or as a compound such
as SbC13 which may form the oxide when heated in an oxidizing
atmosphere. Although the exact form o the antimony in the
final coatinq is not certain, it is assumed to be present as
Sb2O3 and data and proportions in this specification and the
appended claims are based on that assumption. The pre~erred
compositions of this invention comprise mixtures of tin oxide
and a minor amount of antimony oxide, the latter being present
preferably in an amount of between about 0.1 and 20 weight per-
cent (calculated on the basis of total weight of SnO2 and
Sb203 ) .
Conductive tin oxide coatings may be adherently formed
on the surface of the valve metal substrate by various
methods known in the art. Typically such coatings may be
formed by first chemically cleaning the substrate, for
example, by degreasing and etching the surface in a suitable
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acid, e.g., oxalic acid, then applying a solution of appro-
priate thermally decomposable salts, drying and heating in
an oxidizing atmosphere. The salts that may be employed
include, in general, any thermally decomposable inoxganic
or organic salt or ester of tin and dopant, e.g., antimony,
including for example their chlorides, alkoxides, alkoxy
halides, resinates, amines and the like. Typical salts in-
clude for example, stannic chloride, dibutyltin dichloride,
tin tetraethoxide, antimony trichloride, antimony penta-
chloride and the like. Suitable solvents include for ex-
ample, ethyl alcohol, propyl alcohol, butyl alcohol, pentyl
alcohol, amyl alcohol, toluene, benzene and other organic
solvents as well as water.
The solution of thermally decomposable salts, containing
for example, a salt of tin and a salt of antimony, or other
dopant, in the desired proportions, may be applied to the
cleaned surface of the valve metal substrate by painting,
brushing, dipping~ rolling, spraying or other method. The
coating is then dried by heating for example at about 100
to 200 C for several minutes to evaporate the solvent, and
then heating at a higher temperature, e.g., 250 to 800 C
in oxidizing atmosphere to convert the tin and antimony com-
pounds to their respective oxides. The procedure may be re-
peated as many times as necessary to achieve a desired coat-
ing weight or thickness. The final coating weight of this
conductive tin oxide coating may vary considerably, but is
preferably in the range of about 3 to about 30 grams per
square meter.
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Optionally, a small amount, such as up to 3 percent by
weight of a chlorine discharge catalyst such as at least one
of the difluorides of manganese, iron~ cobalt or nickel may
by included in the tin oxide coating to lower the overpoten-
tial required for chlorine gas liberation in a chlor-alkali
cell. The chorine discharge catalyst may be added to the tin
oxide coating by suspending a fine particulate preformed sinter
of tin dioxide and the catalyst in the solution of thermally
decomposable salts. Such chlorine discharge catalysts in the
tin oxide coating is not essential to the anodes of this inven-
tion but may be employed if desired in a known manner such as
disclosed in U.S. patent 3,627,669.
The outer coating of the anode comprises a noble metal
or noble metal oxide such as platinum, iridium, rhodium, pal-
ladium ruthenium or somium or mixtures or alloys of these
metals or the oxides or mixtures of the oxides of these metals.
An outer coating of a noble metal may be applied by known
methods such as electroplating, chemical deposition from a
platinum coating solution, spraying, or other methods.
Preferably, the outer coating of the anode comprises a
noble metal oxide. Noble metal oxide coating may be applied
by first depositing the noble metal in the metallic state and
then oxidizing the noble metal coating, for example, by gal-
vanic oxidation or chemical oxidation by means of an oxidant
such as an oxidizing salt melt, or by heating to an elevated
temperature, e.g., 300 C to 600 C or higher in an oxidizing
atmosphere such as air oxygen, at atmospheric or superatmos-
pheric pressures to convert the noble metal coating to a
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coating of the corresponding noble metal oxide. Other suit-
able methods include, for example, electrophoretic deposition
of the noble metal oxide; or application of a dispersion of
the noble metal oxide in a carrier, such as alcohol, by
spraying, brushing, rolling, dipping, painting, or other
method on to the tin oxide surface followed by heating at an
elevated temperature to evaporate the carrier and sinter the
oxide coating. A preferred method for the formation of the
noble metal oxide coating involves coating the conductive
tin oxide surface with a solution of a noble metal compound,
evaporating the solvent and converting the coating of noblP
metal compound to the oxide by chemical or electrochemical
reaction. For example, the conductive tin oxide surface may
be coated with a solution of a thermally decomposable salt
of a noble metal, such as a solution of a noble metal halide
in an alcohol, evaporation of the solvent, followed by heat-
ing at an elevated temperature such as between about 300 C
and 800 C in an oxidizing atmosphere such as air or oxygen
for a period of time sufficient to convert the noble metal
halide to a noble metal oxide. The procedure for formation
of a noble metal or noble metal oxide coating may be repeated
as often as necessary to achieve the desired thickness. The
foregoing and other methods for the preparation of coatings
of noble metals and noble metal oxides are well known in the
art and may be found for example in U.S~ patent 3,711,385.
The following specific examples will serve to further
illustrate this invention. In the examples and elsewhere in
this specification and claims~ all temperatures are in degrees
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Celsius and all parts and percentages are by weight unless
otherwise indicated.
EXAMPLE I
IA. Pre~ration of conductive tin oxide coating
A strip of titanium plate was prepared by immersion in
hot oxalic acid for several hours to etch the surface, then
washed and dried. The titanium was then coated with a compo-
sition of tin oxide doped with antimony oxide, following the
procedure of Example 4 of U.S. patent 3,627,669, in the fol-
lowing manner:
Tin dioxide was prepared by dissolving metallic tin (84
parts) in concentrated nitric acid and heating until tin di-
oxide was precipitated. Antimony trioxide (18 parts) was
boiled in concentrated nitric acid until evolution of nitrogen
oxides ceased, then thoroughly mixed with the precipitated tin
oxide. The mixture was further treated with hot nitric acid,
then washed free of acid and air dried at about 200 C. About
3 percent by weight of manganese difluoride was added and mixed
with the dried mixed oxides. The mixture was then compressed
into pellets, heated in air at about 800 C for 24 hours~ then
crushed and reduced to a particle size of less than 60 microns.
The crushed mixed oxide composition was again pelletized and
heated as before and then crushed and ball-milled to a par-
ticle size of less than 5 mcirons.
An antimony trichloride-alkoxy-tin solution was prepared
by boiling at reflux conditions for 24 hours a mixture of 15
parts of stannic chloride and S5 parts of n-amyl alcohol then
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dissolving therein 2.13 parts of antimony trichoride.
A suspension of 0.17 parts of the mixed oxide compo-
sition in 3.6 parts of the antimony trichloride-alkoxy-tin
solution was prepared and painted on to the cleaned titanium
surface and the coating was oven-dried at 150 C. Two addi-
tional coats of the same composition were similarly applied
and dried after which the coated strip was heated in air at
450 C for about 15 minutes to convert the coating substan-
tially to oxides of tin and antimony with manganese fluoride.
The coating operation, including the final heating at 450 C
was repeated three times to increase the thickness of the
coating.
The theoretical composition of the conductive coating
thus prepared, was 85.6 percent SnO2; 13.7 percent antimony
oxides ~calculated as Sb~03); and 0.7 percent MnF2. The
coating weight of the finished coating was 21.2 grams per
square meter.
IB. Preparation of RuO~ Coating
The conductive tin oxide coated titanium was further
coated in the following manner:
A solution of 1 gram of ruthenium trichloride in 0.4
cubic centimeters-of 36% hydrochloric acid and 6.2 cubic
centimeters of butyl alcohol was brushed several times on to
the tin oxide surface and then allowed to dry in air at room
temperature. After drying, the samples were heated in air at
560 C for 25 minutes to decompose the RuC13 and form Ru02.
An additional coating of RuC13 was similarily applied, dried
and thermally treated, to yield a final coating of Ru02
~8S~
having a coating weight of about 6.0 grams of ruthenium per
square meter.
In the foxegoing Example, a minor proportion of a
chlorine discharge agent, manganese difluoride was incorporated
in ~he conductive tin oxide coating. An anode may also be
prepared in accordance with this invention, following the
procedure of Example I except that no chlorine discharge
agent is added.
EXAMPLE II - Chlorine Cell Test
The anode, prepared as describèd in Example IB, was in-
stalled and-tested as an anode in a chlorine cell having a
steel cathode separated from the anode by a cationic membrane.
The anode compartment was supplied with preheated brine having
a composition of about 310 g/l NaCl and pH of about 4.5. The
anolyte was maintained at about 95 C. The test was conducted
at a constant current density of 310 ma!cm2 (2.0 ASI). The
anode exhibited a potential of 1.19 volts (v~ a saturated
calomel electrode) which potential remained stable during an
extended test period.
For purposes of comparison, a commercially available
anode composed of a titanium substrate leaving a coating of
ruthenium oxide directly on the surface thereof was installed
and tested under identical conditlons. The anode exhibited
a potential of 1.26 volts (v. a saturated calomel electrode).
Thus, it will be seen that an improvement in overvoltage is
achieved in anodes, such as the anode of Example IB, where
the outer coating of noble metal oxide is deposited on the
surface of a layer of conductive tin oxide rather than directly
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on the surface of the valve metal substrate.
EXAMPLE I I I
An anode prepared in accordance with Example IB, that
is, an anode consisting of a titanium substxate, an outer
coating of ruthenium oxide, and an intermediate layer of
conductive tin oxide, was tested in comparison with an anode
prepared in accordance with Example IA, that is t an anode
consisting of a titanium substrate and a coating of conductive
tin oxide. The anodes were installed and tested under iden-
tical conditions in a chlorine cell having a steel cathode,
separated from the anode by a cationic membrane. The anode
compartment was supplied with preheated brine having a con-
centration of abou~ 310 grams of NaCl per liter and a pH of
~ about 4.5. The anolyte was maintained at about 95 C and
the test was conducted at a constant current density of 310
ma/cm2 (2.0 ASI). The anode of Example IB exhibited an initial
potential of about 1.20 ~olts (v. a saturated calomel elec-
trode), the potential remaining essentially constant over a
127 hour test period. Under identical test conditions, the
anode of Example IA exhibited an initial potential of about
1.52 volts (v. a saturated calomel electrode), the potential
rising to 1.76 volts over the 128 hour test period.
EXAMPLE IV
A. A sample of titanium mesh was coated with a layer
of conductive tin oxide following the procedure of Example
IA.
B. A sample of titanium mesh coated with conductive
tin oxide as described in Example IVA was further coated with
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an outer layer of ruthenium dioxide following the procedure
of Example IB.
The mesh anodes, prepared as described in A and B above,
were installed and tested as anodes in chlorine cells wherein
the electrode gap between the anode and a steel cathode was
1/8 inch, and the anode and cathode were separated by a cat-
ionic membrane. The cells were operated with anolyte con-
centrations ranging from ~50 to 310 grams NaCl/liter at a pH
of 4.5, and temperatures ranging from 8a C to 90 C. rrhe
tests were conducted at a constant current density of 310
ma/cm2 (2.0 ASI). The anode of Example IVB exhibited an
initial potential of about 1.32 v and remained substantially
stable over a 60 day period of operation whereas the anode
of Example IVA exhibited an intial potential of about 1.50
volts, and the potential rose gradually to about 1.90 on
the 50th day of operation, then rose very rapidly on the 52nd
day and achleved complete passivation on the 55th day.
EXAMPLE V
Anode plates (5" x 6") prepared in accordance with the
procedures of Examples IA and IB, were installed and tested
in a chlorate cell which employs two anode plates surrounded
by a mild steel cathode shell. The gap between the anode and
cathode was 1/8 inch. The cell was operated at a current
density of 4.0 ASI and maintained at about 70 C. The
electrolyte compos$tion ranged from 400 to 550 grams of
NaClO3 and 120 to 150 grams NaCl and 1.0 to 1.5 grams sodium
dichromate per liter and a pH of about 6.7.
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The anode of Example IA, having an outer coating of
conductive tin oxide, exhibited an initial potential of
4.0 volts. The potential rose gradually to 5.4 volts during
the first 40 hours of operation and the anode failed com-
pletely in less than two days of operation~ Under identical
conditions the anode of Example IB exhibited a lower initial
potential (3.50 volts) and excellent stability, rising to
about 4.05 volts over an operating time of 91 days.
The foregoing s~ecification is intended to illustrate
~ the invention with certain preferred embodiments, but it is
understood that the details disclosed herein can be modified
without departing from the spirit and scope of the invention.
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