Note: Descriptions are shown in the official language in which they were submitted.
8~
.
STATE OF T~IE ART
Dimensionally stable electrodes for anodic and
cathodic reactions in electrolysis cells have recently become
of general use in the electrochemical industry replacing the
consumable electrodes of carbon, graphite and lead alloys.
They are particularly useful in flowing mercury cathode cells
and in diaphragm cells for the production of chlorine and
caustic, in metal electrowinning cells wherein pure metal is
recovered from aqueous chloride or sulfate solution as well
as for the cathodic protection of ships' hulls and other metal
structures,
Dimensionally stable electrodes generally comprise a
valve metal base such as Ti, Ta, Zr, Hf, Nb and W, which
under anodic polarization develop a corrosion-resistant but
non-electrically conductive oxide layer or "barrier layer",
coated over at least a portion of their outer surface with
an electrically conductive and electrocatalytic layer of
platinum group metal oxides or platinum group metals (see U,S,
Patents No, 3,711,385, No, 3,632,498 and No, 3,846,273,
Electroconductive and electrocatalytic coatings made of or
containing platinum group metals or platinum group metal
oxides are, however, expensive and are eventually subjected
. ~ to consumption or deactivation in certain electrolyti;c
processes and, therefore, reactivation or recoating is
necessary to reactivate exhausted electrodes.
Furthermore, electrodes of this type are not
operable in a number of electrolytic processes. For example,
in molten salt electrolytes, the valve metal support is
--1-- '
bm. .
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~.' -,. ' ~ ,' :-: ,',. ,' . ` ':
1~8(~i~i4
rapidly dissolved, since thë thin,,~rotection oxide layer is
either not formed at all or is rapidly destroyed by the molten
electrolyte with the consequent dissolution of the valve metal
base and loss of the catalytic noble metal coating. Moreover,
in several aqueous electrolytes, such as fluoride solutions
or in sea-water, the breakdown voltage of the protective
oxide layer on the exposed valve metal base is too low and
the valve metal base is often corroded under anodic polariza-
tion.
Recently, other types of electrodes have been
suggested to replace the rapidly consumed carbon anodes and
carbon cathodes used up to now in severely corrosive applica-
tions such as the electrolysis of molten metal salts,
typically for the electrolysis of molten fluoride baths such
as those used to produce aluminum from molten cryolite. In
this particular electrolytic process which is of great econo-
mic importance, carbon anodes are consumed at a rate of
approximately 450 to S00 kg of carbon per ton of aluminum
produced and expensive constant adjustment apparatus is
needed to maintain a small and uniform gap between the corro-
ding anode surfaces and the liquid aluminum cathode. It is
estimated that over 6 million tons of carbon anodes are consu-
med in one year by aluminum producers. The carbon anodes are
burned away according to the reaction:
Al~0, + 3/2 C -~ 2Al + 3/2 CO2
but the actual consumption rate is much higher due to fragi-
lization and breaking away of carbon particles and to inter-
mittent sparking which takes place across anodic gas films
which often form over areas of the anode surface since carbon
bm,
,
.: .. . ..
- ~8~1S~
- is poorly wetted by the molten salts electrolytes, or to short
circuiting caused by "bridges" of conductive particles coming
from the corroding carbon anodes and from dispersed particles
of the depositing metal.
British Patent No. 1,295,117 discloses anodes for
molten cryolite baths consisting of a sintered ceramic oxide
material consisting substantially of SnO2 with minor amounts
of other metal oxides, namely, oxides of Fe, Sb, Cr, Nb, Zn,
W, Zr, Ta in concentration of up to 20~. While electrically
conducting sintered SnO 2 with minor additions of other metal
oxides, such as oxides of Sb, Bi, Cu, U, Zn, Ta, As, etc.~,
has been used for a long time as a durable eLectrode material
in alternating current glass smelting furnaces (see U~S. Patents
No. 2,490,825, No. 2,490,826, No. 3,287,284 and No. 3,502,597),
it shows considerable wear and corrosion when used as an anode
material in the electrolysis of molten salts. We have found
wear rates of up to 0.5 grams per hour per cm from samples
of the compositions described in the patents mentioned above
when operated in fused cryolite electrolyte at 3000 A/ml,
The high wear rate of sintered SnO2 electrodes is thought to
be due to several factors: a) chemical attack by the halogens,
in fact SnIV gives complexes of high coordination numbers with
halogen ions; b) reduction of SnO2 by aluminum dispersed in
the electrolyte; and c) mechanical erosion by anodic gas
evolution and salt precipitation within the pores of the
material.
Japanese Patent Application No. 112589 (Publication
No. 62,114 of 1975) discloses electrodes having a conductive
support of titanium, nickel or copper or an alloy thereof,
--3--
bm.
- - ., . ~ : .
lV8(~
carbon graphite or other conductive material coated with a
layer consisting substantially of spinel and/or perovskite
type metal oxides and alternatively electrodes obtained by
sintering mixtures of said oxides. Spinel oxides and pero-
vskite oxides belong to a family of metal oxides which
typically show good electronic conductivity and have been
proposed previously as suitable electroconductive and electro-
catalytic anodic coating materials for dimensionally stable
valve metal anodes ( see U,S, Patents No. 3,711,382 and No.
3,711t297; Belgian Patent No. 780,303).
Coatings of particulate spinels and/or perovskites
have been found, however, to be mechanically weak as the
bonding between the particulate ceramic coating and the metal
or carbon substrate is inherently weak, because the crystal
structure of the spinels and of the perovskites are not
isomorphous with the oxides of the metal support and various
binding agents such as oxides, carbides, nitrides and borides
have been tried with little or no improvement. In molten salt
electrolytes, the substrate material is rapidly attacked due
to the ineviatable pores through the spinel oxide coating and
the coating is quickly spalled off the corroding substrate,
Furthermore, spinels and perovskites are not chemically or
electrochemically stable in molten halide salt electrolytes
and show an appreciable wear rate due to halide ion attack
and to the reducing action of dispersed metal.
In the electrolytic production of metals from molten
halide salts, the mentioned anodes of the prior art have been
found to have another disadvantage, The appreciable
dissolution of the ceramic oxide material brings metal cations
into the solution which deposit on the cathode together with
4--
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. ,
. ,, . : ,
. : : ::- ' ., ~ ' , ,: .
.: . -
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lO~V~
the metal which is being produced and the impurity content
~in the recovered metal is so high that the metal can no
longer be used for applications re~uiring electrolytic grade
purity. In such cases, the economic advantages of the
electrolytic process which are due, to a large extent, to
the high purity attainable compared to the smelting processes
are partially or entirely lost.
An electrode material to be used successfully in
severely corrosive conditions such as in the electrolysis of
molten halide salts and particularly of molten fluoride
salts, should primarily be chemically and electrochemically
stable at the operating conditions, It should also be
catalytic with respect to the anodic evolution of oxygen
and/or halides, so that the anode overpotential is lowest
for high overall efficiency of the electrolysis process,
The electrode should also have thermal stability at operating
temperatures of, i,e,, about 200 to 1100C, good electrical
conductivity and be sufficiently resistant to accidental
contact with the molten metal cathode, Excluding coated
metal electrodes, since hardly any metal substrate could
resist the extremely corrosive conditions found in molten
fluoride salt electrolysis, we have systematically tested
the performances of a very large number of sintered substantially
ceramic electrodes of different compositions.
THE INVENTION
It has now been found that highly efficient, insoluble
electrodes are prepared by sintering yttrium oxide and at least
one electroconductive agent into a self-sustaining body and
- - ~mO ............................... ..
: , . .:
.. ' ~ , r
l~B`~S~
providing on at least the surface thereof at least one
electrocatalyst.
The sintered yttrium oxidè electrodes of the invention
are particularly useful in electrowinning processes used in
the production of various metals such as aluminu~, magnesium,
sodium, potassium, calcium, lithium and other metals from
molten salts. Yttrium oxide and at least one electro-
conductive agent, when used as an anode in direct current
electrolysis of molten salt electrolytes, has been found to
be unusually stable as an inert, dimensionally stable anode
of sufficient electrical conductivity, and when provided on
the surface thereof with oxide electrocatalysts such as
C0904, Ni30~, MnO2, Rh203, IrO2, Ru02, Ag20, etc., has high
electrocatalytic activity, particularly for chlorine evolution.
The term "sintered yttrium oxide" is meant to describe
a self-sustaining, essentially rigid body consisting
principally of yttrium oxide, and at least one electroconduc-
tive agent produced by any of the known methods used in the
ceramic industry such as by the application of temperature
and pressure to a powdered mixture of yttrium oxide and other
materials to shape the mixture to the desired size and shape,
or by casting the material in molds, by extrusion ! or by the
use of bonding agents and so forth, and then sintering the
shaped body at high temperature into a self-sustaining elec-
trode.
The electrical conductivity of the sintered ceramic
electrodes are improved by adding to the composition 0.1 to
20% by weight of at least one electroconductive agent
selected from the group consisting of (A) doping oxides,
bm,
l~VlS~ 1
typically of metals having a valence which is lower or higher
than the valence of the metals of the oxides constituting the
matrix, such as the alkaline earth metal oxides of Ca, Mg,
Sr and Ba and metals such as Zn, Cd, In2, Tl2, As2, Sb~, Bi2
and Sn; (B) oxides showing electroconductivity due to
intrinsic Redox systems such as spinel oxides, perovskites
oxides etc.; (C) oxides showing electroconductivity due to
metal to metal bonds such as CrO2, MnO2, TiO, Ti203 etc.; .
borides, silicides, carbides and sulfides of the valve
metals such as Ti, Zr, Hf, V, Nb, Ta, Cr, Mo and W or the metals
Y, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Pd and Ag or alloys thereof
or mixtures of (A) and/or (B) and/or (C).
By admixing with the powder of the matrix material, a
minor amount, typically from 0.5 to about 30%, of powders of
a suitable electrocatalytic material and by sintering the
mixture into a self-sustaining body, it shows, when used as
an electrode, satisfactory electroconductive and electro-
catalytic properties which retains its chemical stability
even though the admixed catalyst would not be resistant per
Se to the conditions of the electrolysis.
The catalyst may be a metal or an inorganic oxy-
compound. The preferred admixed catalyst powders are the
powdered metals Ru, Rh, Pd, Ir, Pt, Fe, Co, Ni, Cu and Ag,
especially the platinum group metals; powdered oxy-compounds
of Mn, Fe, Co, Ni, Ru, Rh, Pd, Ir, Pt, Ag, As, Sb and Bi and
especially oxy-compounds of the platinum group metals.
Specifically preferred are ~MnO2, Co904, Rh203, IrO2,
Ru02, Ag20, Ag202, Ag209, As209, Sb~03, Bi209, CoMn204, NiMn~04,
CoRh~04 and NiCo~04 and mixtures of said powdered metals and
cxy-compounds.
.
bm,
.. . . .
`` ~08Vl~
It has been found to be especially advantageous to
add to the yttrium oxide a material such as stannous oxide,
zirconium oxide or the like and that also by adding a small
amount of at least one metal belonging to the group
comprising yttrium, chromium, molybdenum, zirconium,
tantalumt tungsten, cobalt, nickel, palladium and silver,
both the mechanical properties and the electrical conductivity
of thè sintered yttrium oxide electrodes are improved without
appreciably decreasing their chemical and electrochemical
corrosion resistance.
These additives are added in powder form and mixed
with the powdered yttrium oxide in percentages which may range
from 40 to 1~ calculated in terms of weight of the metal
content. Optionally, yet other organic and/or inorganic
compounds may be added to the powder mixture to improve on the
bonding of the particles during both the moulding and
sintering processes,
Anodes containing a major portion of Y~0~ have a high
melting point welL above the temperature of the molten salt
electrolytes being used and they undergo no phase change under
working conditions of the electrolysis. Moreover, the thermal
elongation co-efficient is not far different from that of the
halide salts used in the molten salts bath, which helps
preserve the proper electrode spacing between the anode and the
cathode and avoids expansions and contractions which might
break the salt crust on the top of the molten salt electrolyte
in the normal aluminum electrowinning process.
The conductivity of the sintered yttrium oxide
electrodes of the invention is comparable with that of
--8-- -
bmO
,
:
~o~3Q~54
graphite. The matrix has acceptable work-ability in the
forming and sintering operation and in use forms a thin layer
of oxyhalides on its surface under anodic conditions. The
yttrium oxide free formation energy is more negative than the
oxide free formation energy of the corresponding halide-phase
fused salt electrolyte, so that these sintered yttrium oxide
anodes have a high degree of chemical stability,
The sintered yttrium oxide electrodes of the
invention may also be used as bipolar electrodes. According
to this latter embodiment, the sintered yttrium oxide
electrodes may be conveniently produced in the form of a
slab or plate whereby one of the two major surfaces of the
electrode is provided with a layer containing the anodic
electrocatalyst such as the oxides COgO4~ Ni304, MnO2, Rh203,
IrO2, Ru02, Ag20 etc. and the other major surface is provided
with a layer containing suitable cathodic materials such as
carbides, borides, nitrides, sulfides, carbonitrides etc.
of metals, particularly of the valve metals and most preferably
of yttrium, titan:ium and zirconium.
The self-sustaining sintered body consisting of à
major portion of yttrium oxide may be prepared by grinding
the materials together, or separately, preferably to a grain
size between 50 and 500 microns, to provide a powder mixture
which contains a range of grain sizes to obtain a better
degree of compaction. According to one of the preferred
methods, the mixture of powders is mixed with water or with
an organic binding agent to obtain a plastic mass having
suitable flowing properties for the particular forming
process used. The material may be molded in known manner
bm.
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- ~08~3~5~ ~
either by ramming or pressing the mixture in a mold or by
slip-casting in a plaster of Paris mold or the material may
be extruded through a die into various shapes,
The molded electrodes are then subjected to a drying
process and heated at a temperature at which the desired bonding
can take place, usually between 800 to 1800C for a period of
between 1 to 30 hours, normally followed by slow cooling to
room temperature, The heat treatment is preferably carried
out in an inert atmosphere or one that is slightly reducing,
for example in H2 + N2 (80%), when the powdered mixture is
composed essentially of yttrium oxide with a minor portion
of other metal oxides or metals.
When the powdered mixture contains also metallic
powders, it is preferable to carry out the heat treatment in
an oxidizing atmosphere, at least for a portion o the heat
treatment cycle to promote the oxidation of metallic particles
in the outside layer of the electrodes, The metallic
particles remaining inside the body of the sintered material
improve the electrical conductivity properties of the electrode.
The forming process may be followed by the sintering
process at a high temperature as mentioned above or the forming
process and the sintering process may be simultaneous, that is,
pressure and temperature may be applied simultaneously to the
powder mixture, for example by means of electrically-heated
molds. Lead-in connectors may be fused into the ceramic
electrodes during the molding and sintering process or
attached to the electrodes after sintering or molding. Other
methods of shaping, compressing and sintering the yttrium oxide
powder mixture may of course be used.
--10-- . `
bm.
~s~
The electrocatalyst, usually applied to the electrode
surface due to costs, should have a high stability, a low anodic
overpotential for the wanted anodic reaction, and a high anodic
overpotential for non-wanted reactions~ In the case of chlorine
evolution, oxides of cobalt, nickel, iridium, rhodium, ruthenium
or mixed oxides thereof such as Ru02 - TiO2 etc, can be used,
and in the case of fluoride containing electrolytes wherein
oxygen evolution is the wanted anodic reaction, oxides of
silver and manganese are preferable, Other oxides for use as
electrocatalysts may be oxides of platinum, palladium and lead,
Poisons for the suppression of unwanted anodic
reaction may be used, such as, for example, to suppress oxygen
evolution from chloride electrolytes. ~oisons which present
a high oxygen overpotential should be used, suitable materials
are the oxides of arsenic, antimony and bismuth, These
oxides which are used in small percentages may be applied
together with the electrocatalyst oxides in percentage of 1
to 10% of the electrocatalyst calculated in terms of the
respective metals weight,
The application of the electrocatalyst, and
optionally of the poisoning agent may be effected by any of
known coating methods. Preferably the electrocatalvst, and
optionally the poisoning agent, are applied to the sintered
yttrium oxide electrodes as a solution of decomposable salts
Qf the metals. The sintered yttrium oxide body is impregna-
` ted with the solution containing the appropriate metal salts
and dried. Hence the electrode is heated in air or in other-
wise oxygen containing atmosphere to convert the salts into
the wanted oxides.
- bm.
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-: ,,
. . ~
~o~
Usually the porosity of the sintered yttrium oxide
body and the method used to impregnate the-surface layers of
the sintered body with the metal salts should provide for
the penetration of the solution down to a depth of at least
1 to 5 millimeters, preferably 3 mm, inward from the surface
of the electrode so that after the heat treatment the
electrocatalysts are present in the pores of the sintered
yttrium oxide body down to a certain depth inward from the
surface of the electrodes
Alternatively, by appropriate powder mixing techniques,
preformed electrocatalyst oxides and optionally preformed
poisoning oxides, may be ground into ~owder form and added to the
powder mixture during the moulding of the electrodes in such
a way that the external layers of the moulded electrodes are
enriched with powders of the electrocatalyst oxides, and
optionally of the eoisoning oxidesr during the formin~-process
whereby after sintering the surface of the electrodes is
already provided with the electrocatalyst.
The sintered yttrium oxide electrodes of the invention
may be used as bipolar electrodes. According to this
embodiment of the invention, the yttrium oxide electrodes may
be provided over one surface with the anodic electrocatalyst,
and optionally with the poisoning agent for the unwanted
anodic reaction by one of the methods disclosed above while '~
the other surface may be provided with a coating of suitable
cathodic material. For example, the surface of the bipolar
electrode which will function as a cathode during the process
of electrolysis may be provided with a layer of metal carbides,
borides, nitrides~ sulfides and/or carbonitrides of yttrium,
-12-
bm.
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tantalum, titanium, zirconium etc,
One preferred method to apply a layer is by plasma-jet
technique whereby powders of the selected materials are sprayed
and adhere to the surface of the sintered yttrium oxide body
with a flame under controlled atmosphere. Alternatively,
the selected powdered material may be added during the forming
process to the yttrium oxide powder mixture and thence be
sintered together whereby the cathodic surface of the bipolar
electrode is provided with a layer of the selected cathodic
material.
The sintered yttrium oxide activated with suitable
electrocatalyst can be used as a non-consumable electrode in
the electrolysis of molten salts and for other processes in
which an electric current is passed through an electrolyte
for the purpose of decomposing the electrolyte, for carrying
out oxidat.ion and reductions of organic and inorganic
compounds or to impress a cathodic potential to a metallic
structure which has to be protected from corrosion, as well
as for primary and secondary batteries containing molten
salts such as aluminum halides - alkali metal halides,
The electrodes of the invention may be polarized as anodes or
as cathodes or may be utilized as bipolar electrodes, whereby
one face or end of the electrode acts as anode and the
opposite face or end of the electrode acts as cathode with
respect to the electrolyte contacting each face of the
electrode respectively, as is known in the art of
electrolysis.
The electrolysis cell of the invention comprises
a cell provided with at least one set of a spaced anode and
cathode and a means for impressing an electrolysis current
-13-
bm,
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on the said cell, the said anode being a dimensionally stable
three component electrode as discussed above, The cell is
preferably used for electrolysis of molten metal salts such
as aluminum chloride.
In the following examples there are described several
preferred embodiments to illustrate the invention, However,
it should be understood that the invention is not intended
to be limited to the specific embodiments. The percentages
of the electrode components are calculated in percent by
weight as free metal based on the total metal content of the
composition.
EXAMPLE 1
About 250 g of a mixture of the matrix material and
additive materials indicated in Table I were ground in a
; mixer for 20 minutes and the powder mixtures were poured
into cylindrical plastic molds and pre-compressed manually
with a steel cylinder press, Each mold was placed in an
isostatic pressure chamber and the pressure was raised to
about 1500 Kg/cm2 in 5 minutes and then reduced to zero in a
few seconds. The samples were then taken out of the plastic
molds and polished, The pressed samples were put into an
electrically heated furnace and heated from room temperature
to 1200C under a nitrogen atmosphere over a period of 24
hours, held at the maximum temperature for 2 to 5 hours and
then cooled to 300C over the following 24 hours, The sin-
tered samples were then taken out of the furnace and after
cooling to room temperature, they were weighed and their
apparent density and electrical conductivity at 25C and at
1000C were measured, The results are reported in Table I
-14-
bm,
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TABLE I
_ - ~ , - , ........
Sample Ccmposition :Sintering time Apparent Electrical conductivitv
. No. ~ by weight at max. temp, 1l density j at 1000C at 250C .
,, _ ,_.5hr,s) q/cm3 Q-~ cm-' Q- cm-_ ___ _
A Y203 75 5 5,2 ,
Ti203 25 0.001 _
_ . ._ _ , . . _ .
Y203 65
1 Ti203 15 5 5.2 0.2 . ,
Rh203 20 . _ .
... _ .. _ _ .. _ .
ZnO2 30
Y203 30
2 WO 10 5 5.5 0.4 _ ''
~g20 10 ,
Ru02 10 ..
Y20350 _
. - .`'
YOF 15
3 Y 15 2 5,9 11 2.5
IrO2 10 .
. Ag20 10
_............... ... _
. . Y~O, 60 . .
. . NiCo~0420 .
4 CdO 5 2 5.7 7 0.2
CaO 1 .
Rh~09 19 .
. .
-15-
hn.
.
,
:, : ,.:, - :
.~8~31S4
The data in Table I shows that the electrical
conductivity of the sintered ceramic electrodes at high
temperatures of 1000C is 5 to 10 times higher than the
electrical conductivity at 25C, The addition of oxides
having conductivity equivalent to metals to the substantially
non-conductive ceramic oxides of the matrix increases the
conductivity of the electrodes by a magnitude of 10 2 as can
be seen from electrodes A and No. 1. The addition of a metal
stable to molten salts such as yttrium or molybdenum, etc. to
the ceramic electrodes of the invention increases the
electrical conduc~ivity of the electrodes by 2 to 5 times.
EXAMPLE 2
- ,.
The conditions of operation of an electrolytic cell
for the production of aluminum metal from a molten cryolite
bath were simulated in a laboratory test cell. In a heated
crucible of graphite, a layer of liquid aluminum was
, provided on the bottom and a melt consisting of cryolite
(80 to 85%), alumina (5 to 10%) and AlF3 (from 1 to 5~) was
' poured on top thereof. The sample electrodes with a working
surface area of 3 cm2 prepared according to the procedure
described in Example 1 and to which at Pt wire was brazed to
provide an easy means for electrical connection were dipped
into the salt melt and held at a distance of about 1 cm from
the liquid aluminum layer, The crucible was maintained at a
temperature ranging from 950 to 1050C and the current density
was 0.5 A/cm~ and the cell was operated for 2000 hours. The
experimental data obtained is shown in Table II. The sample
-16-
bm,
,:, . . .- : ::,: ~
1~8VlS~
number indicates that the electrode tested corresponds to the
sample described in Table I with the same number.
TABLE II
Sample I Aluminum ' Weight loss of .
No. produced anodes
(g/h) (gr/cm2)
__ , ............... .. _ . _ j . _ _ _ . .
1 0.48 0.1
2 . 0.50 0.02
The test sample electrodes operated successfully
as anodes in the cryolite melt and the observed wear rates
appear to be quite acceptable for the electrolytic production : ^
of aluminum from molten cryolite. Both tested electrodes
showed a low wear rate during 2000 hours of operation, In
general, the wear rate of the electrodes containing thermal
. stabilizers such as oxy-compounds of metals of group III of
the Periodic Table is about 10 times less than the electrodes
without thermal stabilizers,
-17-
bm.
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.. : , , ,, : , .
, , : : . .
1~8~15~
EXAMPLE 3
Electrode No. 1 described in Table 1 was used as an
anode for the electrolysis of a molten aluminum chloride
electrolyte in the test cell described in Example 2, The
electrolysis conditions were the following:
Electrolyte : AlClg from 31 to 35% b.w,t,
NaCl from 31 to 35% b,w,t;
BaCO3 from 31 to 35% b,w,t,
Temperature
of Electrolyte : from 690 to 720C
. .
Anodic current
density : 2000 Amp/m
Cathode : Molten Aluminum
Interelectrodic gap : 1 cm,
The tested electrode operated successfully and the
weight loss after 2000 hours of operation was negligible,
:,
-18-
bm.
. - . . . ~, . .
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: ' '. .'. ' ''
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EXAMPLE 4
Sintered anodes comprised of 70% by weight of YzO9
and 30% by weight of ZnO2 measuring 10 x 10 x 10 mm were
impregnated with an aqueous solution of the chloride salt of
the metals of Table III and were then heated in air at 300 to
650C. The process was repeated until the amount of metal
catalyst was 10 g/m2 calculated as metal, The electrodes
were then used as anodes in the electrolysis of a 5:1 mixture r
of AlCl3-NaCl at a current density of 1000 A/m2 at 750C.
The initial anode potential and the potential and the wear
rate after 100 hours were determined. The results are in
Table III,
TABLE III
.
Anode Potential ¦ Wear rate g/m2
V(RCGE) Iafter
Electrocatalyst initial after 100 hrs 100 hrs.
Rh2O, 0.2 0,2 none detectable
IrO2 0,0 0,0 n
Co 34 __ - O, O O . O ll
. _
The electrodes showed excellent characteristics
in the electrolysis of molten aluminum chloride.
--19-- ,
bm.
1~8015~i
EXAMPLE 5
Sintered anode coupons 10 x 10 x 10 mm consisting
.of 70% by weight of Y209 and 30% by weight of ZrO2 were
impregnated with the metals of Table IV with the process of
Example 4, The anodes were used for the electrolysis of
molten metal carbonate-fluoride salt and the wear rate anode
potentials were determined as in Example 4. The results are
reported in Table IV.
TABLE IV
_ . _ . ..
Anode Potential Wear rate
. VC~CÇE) _ g/m
Electrocatalyst initial after 100 hrs after 100 hrs
_ ._ ...
2 x 1 0.O 0.O nil
IrO2 0.1 0.2 ..
MnO 2 0,2 0.2 ..
., _ . _ . - ' :
The anodes operated successfully for the electro-
lysis of molten carbonate-fluoride salts in which oxygen was
evolved at the anode.
-20-
bm.
-
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.
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. . .
EXAMPLE 6
Electrode sample No. 4 of Example 1 was used
alternatively as anode and as cathode in the electrolysis of
synthetic sea-water in a test cell in which the electrolyte
was pumped through the electrodic gap of 3 mm at a speed of
3 cm/sec. The current density was maintained at 1500 A/mZ
and the spent electrolyte contained 0.8 to 2.4 of sodium
. hypochlorate with a faraday efficiency of more than 88~.
The weight loss of the electrode after 200 hours at operation -
was negligible.
EXAMPLE 7
Electrode sample No. 3 of Example 1 was used as an
anode in the electrolysis of an aqueous acidic cupric sulfate
solution in a cell with a titanium cathode blank~ The
electrolyte contained 150 to 200 gpl of sulfuric acid and
40 gpl of cupric sulfate as metallic copper and the anode
current density was 300 A/cm2, The electrolyte temperature
was 60 to 80C and an average of 6 mm of copper was deposited
on the flat cathode at a faraday efficiency ranging from 92
to 98%. The quality of the metal deposit was good and free
of dendrites and the anode overvoltage was very low, ranging
from 1.81 to 1.95 V(NHE).
bm.
. ~ . : : . .
... , ,. . .: . :
, ~ : . : .::: : ~
1~30~S~ -
EXAMPLE 8
One block of sintered Y203: SnOz :Y m~tal in the
ratios of 7:2:1 by weight of free metals; one block of
sintered Y203:ZrO~Zr metal in the ratios of 6:3:1 by weight
of free metals; and one block of sintered YzO3:Pd metal in . r
the ratios of 9:1 by weight of free metals were activated by
impregnating the sintered samples with an aqueous solution
of CO Cl3 followed by drying and heating in air at 300 to
650C to convert the chloride into C0304. The cycle was
repeated to obtain a final coating of the electrodes of
15 g/m2 of Co304 of anode surface. The activated anodes were
used for the electrolysis of molten AlCl3 + NaCl electrolyte
and the anode potential and the wear rates are reported in
Table V.
., .
TABLE V
Anode Potential Wear Rates _
Sample. I i ~ V_ S.C!G E-) _ g/m2
_ _
y~o9-Sno2- Y 0.1 0.1 Nil
Y20~-ZrO2- Zr 0.1 0.1 Nil
Y~O~-Pd 0.0 0.0 0.5
~m.
.
1~8~5~ . ....
EXAMPLE 9: r
.
One block of sintered Y~03 ~ ZrO2 and Pd metal in
the weight ratio of 7:2.5:0.5 of free metals was impregnated
over one of its larger surfaces with 15 g/m2 of surface of
Co904 by the method of Example 8, The opposite larger surface
was coated with a layer 1 millimeter thick of zirconium
di-boride applied by flame spraying under nitrogen atmosphere,
The block was placed between two counter electrodes of gra-
phite in electrical conducting relationship and spaced from
the same. The interelectrodic spaces were filled with molten
AlCl3 + NaCl and the graphite counter electrode facing the
zirconium di-boride coated surface of the sintered bipolar
electrode was connected to the positive pole of the direct
current supply and the graphite counter electrode facing the
Co904 activated surface of the sintered bipolar electrode
was connected to the negative pole of the D,C. supply. The
sintered electrode operated as a bipolar electrode and molten
aluminum metal flowed down the zirconium di-boride coated
surface and was recovered at the bottom of the same while
chlorine was evolved on the Co 3 0 4 activated surface of the
electrode. The electrolysis process was conducted satisfac-
torily for a period of 28 hours when the test cell fabricated
mainly with graphite failed, The bipolar electrode after
this period of operation did not show signs of deterioration
and no wear was detected.
Other electrocatalysts which may be used in the
electrolysis of molten halide salts for halide ion discharge
are RuO2, and oxides such as ~5~09~ Sn209 and si2oa may be
-23-
bm.
: ::.. - : :: : : - ~
added in percentages up to 10~ by weight of free metal based
upon the total metal content to rise the oxygen overpotential
without affecting the halide ion discharge potential.
For anodes to be used in molten fluoride elect-
rolytes where oxygen is evolved, the catalyst may be those
listed in Example 5 or Rh~03, PbO2 and IrOz.TiO2,
The components of the anodes given in the examples
are calculated in percent by weight of free metal based upon
the total metal content of the anode composition.
The electrolyte may contain other salts than those
used in the Examples such as alkali metal chloride or
fluoride as well as the salt of the metal undergoing electro-
lysis. The metal halides are effective to reduce the melting
point of the salt undergoing electrolysis thus permitting use
of lower temperatures while maintaining the salt bath in molten
or melted state.
The above examples include fused or molten metal
salt electrolysis, primarily the electrolysis of molten
aluminum chloride or fluoride salts. In a similar manner,
the molten chlorides of other metals such as alkali metal or
alkaline earth metals may be electrolyzed using the designated
anodes, accoridng to otherwise standard practice, In addition,
other molten salts, such as the molten nitrates, may by
electrolyzed in the same way. A molten alumina-cryolite
electrolyte or the like alkali metal aluminum fluoride may be
electrolyzed to produce molten aluminum.
These electrodes may be used in place of graphite
anodes in standard aluminum electrowinning cells with either
aluminum ore feed into a cryolite bath or with aluminum
-24-
bm.
- ~08~3~S4
chloride feed into a predominately aluminum chloride bath.
The use of these sintered yttrium oxide anodes for
the recovery of the desired metals from fused salts of the
metals to be won results in reduced power consumption per
unit weight of metal produced and in purer recovered metals.
The electrodes are dimensionally stable in service and there-
fore do not require frequent interventions to restore the
optimum distance from the cathode surface as it is necessary
with the consumable anodes of the prior art,
The sintered yttrium oxide anodes of our invention
may also be used in aqueous or non-aqueous solutions of elec-
trolytes for the recovery of one or more constituents of the
electrolytes,
Various modifications of the electrodes and
processes of the invention may be made without departing from
the spirit or scope of our invention and it is to be under-
stood that the invention is intended to be limited only as
defined in the appended claims.
-25-
- bm.
.. . .
" ~. . ..
