Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
34Z 7 ~
15 1! STATE OF T.;_ ART
Dimensionally stable electrodes ~or anodic and
cathodic reactions in electrolysis cells have recently beco.me-
;.
jo~ general use in the electrochemical industry replacing the
~iconsumable electrodes Or carbon, graphite and lead alioys.
I,,They are particularly usePul in flo~ling mercury cathode cells~
¦and in diaphragm cells for the production Or chlorine and
caustic, in metal electrowinning cells wherein pure metal is i
,recovered from chloride or sulfate aqueous solution as well
.as in the cathodic protection of ships' hulls and other metal
,struceures
~ . .
- !
-, " ~! . .
,.
....
.
~' :
" :
3~2`7
s
Dimensionally stable electrodes generally co~prise a
valve metal base, such as Ti, Ta, Zr, Hf, Nb and W, which
( under anodic polarization develope a corrosion-resistant but
, non-electrically conductive oxide layer or "barrier layer",
coated over at least a portion of the surface with an electri~
cally conductive and electrocatalytic layer containingplatinum
group metal oxides or platinu.~ group ~etals (see U.S.3,nl,38r,
3,763,498 and 3,846,273) and sometimes oxides o~ also valve
metals. Molybdenum, vanadium, aluminum and yttrium are also
metals wh~ch, within certain environments, show distinct valve
-'metal characteristics, i.e. the formation of a filming layer
,Or oxides substantialIy protecting the metal from further
oxidation or corrosion. (e.g. anodic treatment of Al).
i Electroconductive and electrocatalytic coatings made'
'Or or containing platinum group metals or platinum group meta~
,oxldes are, however, expensive and are eventually subjected
to consumption or deactivation in certain electrolytic pro-
Ilcesses and, therefore, reactivation or recoating is necessary
j to reactivate exhausted electrodes.
Furthermore, electrodes o~ this type are not operablé
in a number of electrolytic p.ocesses. For example, in molten
salt electrolytes, the valve ~etal support is rapidly dis-
- solved, since the thin protective oxide layer is either not
'
formed at all or is rap~dly destroyed by the electrolyte with
the consequent dissolution of the valve metal base and loss
'~of tbe catalytic noble metal coating. Moreover, in several
aqueous electrolytes, such as bro~ide solutions or in sea-
C ~ water, the breakdo~n voltage o~ thc protective oxide layer on
,', i
,' ~
r
~.~3 3~7 -
the exposed valve metal base is too low and the valve metal
base is often corroded under anodic polarization.
Recently, other types of electrodes have been sug-
~gested to replace the rapidly consumed anodes and carbon
. ~ cathodes in severely corrosi~e applications~ such as the
electrolysis Or molten salts, typically for the electrolysis
Or molten fluoride baths such as those used to producealuminu~
from molten cryolite. In this particular electrolytic proces$
hich is of great economic importance, carbon anodes are
~consumed at a rate of approximately 500 kg Or carbon per ton ,
of aluminum produced and expensive constant adjustment appara-
! tus is used to maintain a small and uniform gap between the
- corroding znode surface and the liquid aluminum cathode. It
"is estimated that over 6 million tons Or carbon anodes are
consumed~in one year by aluminum producers. The carbon anodes
are burned away according to the reaction: ¦
~ A123 + 3/2 C ~ 2A1 ~ 3/2 C02
iibut the actual consumption rate is much higher due to fragi-
I lization and breaking away Or carbon particles and to inter-
mittent sparking which takes place across anodic gas films
which orten form over areas of the anode surface since carbon
is poorly wetted by the molten salt electrolytes, or to shcrtt
circuiting caused by "bridges" of conducti~e particles coming~
from the corroding carbon anodes and from dlspersed particles
Or the depositing metal.
British Patent No. 1,295,117 discloses anodes for
; molten cryolite baths cons~sting of a sintered ceramic oxide
.. ' . I
,- , . I
1 1
i3~Z 7
material consisting substantially of sno~ with minor amounts
of other metal oxides, namely, oxides of Fe, Sb, Cr, Nb, Zn,
W, Zr, Ta in concentrations of up to 20%.
While electrically conducting sintered SnO2 with
minor additions of other métal 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 Nos. 2,490,~25; 2,490,826;
3,287,284 and 3,502,597), it shows considerable wear and
corrison when used as anode material in the electrolysis of
molten salts.
~ e have found wear rates of up to 0.5 grams per hour
per cm2 from samples of the compositions described in the
patents mentioned above when operated in fused cryolite
electrolytes at 3000 A/m2. 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 197~) discloses electrodes having a conductive
support of tltanium, nickel or copper or an alloy thereof,
carbon, graphite ox other conductive material coated with a
layer consisting of substantially of spinel and/or perovskite
type metal oxides and alternatively electro~es produced from
sintered mixtures of said oxides. Spinel oxides and perovs-
- ~ dm: -
~ite oxldes belong to a family of metal oxides which typically
show ~ood electronic conductivity and have been proposed pre-
viously as suitable electroconductive and electrocatalytic
anodic coating materials for dimensionally stable valve mot21
anodes (see U.S. Patents Nos. ~,711,382 and 3,711,297;
- Belgian Patent No. 780,303).
Coatings o~ particulate spinels and~or perovskites
have been ~ound, however, to be m~chanically weak as the
bonding bet~een the particula~e ceramic coating and the metal
or carbon substrate is inherently ~reak, because the crystal
structure Or the spinels and of the perovskites are not iso-
morphous with the oxides Or the metal support and various
' bindlng agents such as oxides, carbides, nitrides and borides
have been tried wlth little or no improvement. In molten
salt el-ctrolytes~ the substrate material is rapidly attacked
due to the inevitable pores through the spinel oxide coating
and thecoating is ~uickly ~palled offt~hecorroding substra~e.
~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 o~ dispersed metal.
In the electrolytic production of metals ~rom molten
, halide salts, the mentloned anodes o~ the prior art have beenj
,~ound to have another disadvantage. The appreciable dissolu-l
, tlon of the ceramic oxide material brings metal cations into ,
the solution which deposit on the cathode together with the
metal which is being produced and the impurity content in the~
( recovered metal is so high that the metal can no longer be
, ~ ! i
-5- 1
~ ; !
F
15 ~.3~Z 7 t
.. ' ' .
used for applications reouiring 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, co.~pared to the smelting processes are parti~lly;
or entirely lost.
An electrode material to be used successfully in
severely corrosive conditions such as in the electrolysis Or
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
'~ ef~iciency Or the electrolysis process. The electrode should
; also have thermal stability at operating temperatures of i.e.,
about 2~0 to 1100C, good electrical conductivity and be
sufficienct~/ resistant to accidental contact with the molten
metal cathode. Excluding coated metal electrodes, since
i hardly any metal substrate could resist the extremely corro-
i! sive conditions ~ound in molten fluoride salts electrolysisj
20 we have systematically tested the performances of a very large
number of sintered substantially ceramic electrodes of dif-
ferent compositions.
Patent No. 3,636,856 describes electrodes made of
t~tanium carbide impregnated graphite for electrolysis of
25 manganese sul~ate solutions to produce manganese dioxide and !
U.S. Patents ~o. 3,028,324; No. 3,215,615; No. 3,314,876 and
- No. 3,330,756 relate to aluminwD electrolysis cells using
~ralve metal borides and valve metal carbides as current
,
~ --6- i
!
; . .. , . .. ,. ,.. . L
... . , ...... , ... ~
1~3~2`~
collectors. Patent No. 3,459,515 relates to an alurninum elect-
rolytic cell with a current collector consisting of titanium
carbide-titanium boride and/or zirconium boride and up to 30~ of
aluminum. Patent No. 3,977,959 describes an electrode of
tantalum, tantalum boride, tantalum carbide and a metal of the
iron group.
OBJECTS OF THE INVENTION
It is an object of the invention to provide novel im-
proved electrodes consisting essentially of silicon carbide-valve
metal boride-carbon and to novel bipolar electrodes.
It is another object of the invention to provide a
novel electrolysis cell equipped with silicon carbide-valve metal
boride-carbon anodes.
It is an additional object of the invention to provide
a novel electrochemical process using the electrodes of the
invention.
In accordance with the present invention there is pro-
vided a sintered anode consisting essentially of 40 to 90% by
weight of at lease one boride of a metal selected from the group
~0 consisting of titanium, tantalum zirconium, aluminum, hafnium,
niobium, tungsten, yttrium, molybdenum and vanadium, 5 to 40~ by
weight of silicon carbide and 5 to 40% by weight of carbon.
These and other objects and advantages of the invention will be-
come obvious from the following detailed description in conjunc-
tion with the accompanying drawings, in which:
Figure 1 is a graph of the chlorine potential of the
electrodes of Example 1 with reference to a silver electrode,
and
j k/J o
Figure 2 is a graph of the chlorine potential with
the electrodes of Example 2.
THE INVENTION
l'he novel sintered electrodes of the invention
consist essentially of 40 to 50~ by weight of at least one
valve metal boride, 50 to 40~ by weight of silicon carbide and
5 to 40% by weight of carbon.
The said electrodes are useful in electrochemical pro-
cesses such as the electrolysis of aqueous halide solutions,
-7a-
' jk/
;J~ 7
for electrowinning of metals from aqueous sulfate or halide
solutions and for other processes in which an electric cur-
rent is passed through an electrolyte for the purpose of
decomposing the electrolyte, for carrying out oxidations and
reduction 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. The electrodes of the invention may be polarized
as anodes or as cathodes or may be utilized as bipolar elec-
trodes~ whereby one face or end of the electrode acts as anodeand 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 term "sintered electrode" is used to describe the
mixture of the specified silicon carbide-valve metal boride-
graphite in a self-sustaining essentially rigid body produced
by any of the known methods used in the ceramic industry such as
by application of pressure and temperature to a powder mixture,
by casting of the material in molds, by extrusion or by bonding
agents, etc. The words "bonded electrodes", "cast electrodes"
or 'Isintered electrodes'l, even when used separately are
essentially synonymous and the component materials may be in
the crystalline and/or amorphous state. Valve metal is
intended to include titanium, tantalum, hafnium, zirconium,
aluminum, niobium and tungsten and alloys thereof particularly
suited for anodic polarization and molybdenum, vanadium and
yttrium particularly suited for cathodic polarization.
dm~ 8 -
~;
~3~
Electrodes made of valve metal borides such as zir-
conium boride or titanium boride tend to dissolve when used
as an anode in a molten salt bath like aluminum chloride and
have a rather high overpotential for chlorine. Valve metal
carbides when used in such molten salt baths tend to disintegrate
and carbon or graphite alone has a poor life.
In contrast thereto, the electrodes of the invention
have good electronic and electrical conductivity, a chlorine
overpotential lower than that of graphite and mixtures of
1~ ~alve metal boride-silicon carbide electrodes, good corrosion
resistance and good wettability by the molten salt electro-
lyte in which it comes in contact. Moreover, the electrodes
can be operated as anodes at high current density such as
5,000 to 10,000 amperes or more per square meter.
When the electrodes are obtained by sintering, the
particles of the component powders may have a grain size which
can vary between 50 to 500 microns and normally the powder
mixture is arranged to contain a certain range of grain sizes
to obtain a better degree of compaction. The electrodes may be
prepared by the conventional methods used in the ceramic
industry. In 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 either by ramming or pressing the mixture in
a mold or by slip-casting in a plaster of ~aris mold or the
material may be ex-truded through a die in various shapes.
dm:ij
`7
, .
The molded electrodes are then subjected to a dryin~ -
process and heated at a te~perature at ~hich the uesired
bonding can take place, for a period of be.~;een 1 to 30 hours,
normally follo-led by slow cooling to room temperature. The
heat treatment is preferably carried out in an inert atmos- ~
phere or one that is slightly reducing, for example in H2 ~ N2
(80%).
The forming process may be followed by the sintering
process at a high temperature as mentioned above or the form-
- 10 ing process and the sintering process may be-simultaneous,
that is, pressure and temperature may be appliedsimultaneously
to the powder mixture, for example by means of electrically
heated molds. Lead-in connectors may be fused into the
~electrodes during the molding and sintering process or attached
to the electrodes after sintering or molding.
` A metal netting or core or flexible core material may
be provided inside the body of the sintered electrodes to
jimprove the current distribution and to provide for easier
electrical connection of the electrode to the electric supply;
system and to reinforce the sintered body.
The process of the invention may be used effectively
-for the electrolysis of many electrolytes. The electrodes
may be used as anodes and/or cathodes in electrocnemical pro-
cess such as the electrolysis of ai~ueous chloride solutions-
~or production of chlorine, caustic, hydrogen, hypochlorite,
chlorate and perchlorate; the electro~inning of metals from
aqueous sulfate or chloride solutions for production of co2P~r`,
C zinc, nickel, cobalt and other metals; and for th- electrolySis
., . I
-10- 1
!
.
j t
... . ... . . ...... ..... . ... ..
.
~\
~3~2 7
.
of bromides, sulridcs, sulfuric acid, hydrochloric acid and
hydrofluoric acid.
Generally, the process Or the invention is useful
; where an electric current ispassed through an electrolyte to
decompose the electrolyte, for effecting oxidation and re- t
duction Or organic and inorganic compounds or to impress a
; cathodic potential on a metallic structure to pro~ect it from~
corrosion as well as in primary and secondary batteries.
~` When the process of the invention uses bipolar elect-
rodes, the composltion Or the cathode portion Or the elect-
rodes must be such that it will be resistant to the particular
., ,
cathodic conditions. -,
Therefore, the cathode portion Or the bipolar elect-
~rode ma~ contain other materials which improve the character-
~'istics Or the electrodes of the invention such as the carbi~s,
l,borides, silicides, nitrides, sulfides and/or carbonitrides
'lof metals, particularly the valve metals, molybdenum, vanadiu~and yttrium. Yttrium, titanium or zirconium borides are pre-
l~ferred materials for the cathodic side Or bipolar electrodes.
' By appropriate powder mixing techniques the composi-
~tion Or the bipolar electrodes of the invention may be varied
across the cross-section of the electrode. That is the super
ical layers Or the cathodic surface Or the bipolar elect-
~ rode may be enriched with yttrium, titanium or zlrconium
I,bor~de during the molding process and before sintering is
icompleted.
( ' The electrolysis cell Or the in~ention comprises a
Icell prov~ded with at least one set of a spaced anode and
!1 ¦
. .
1~ .
..
34~`7
.
. '
cathode and a means ~or impressing an electrolysis current onthe said cell, the said anode being a dimensionally stable,
three component electrode as discussed above. TAe cell is
preferably used for electrolysis of molten metal salts such
as aluminum chloride. f
The ~ollowing examples describe 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.
EXAMPLE 1
About 250 g of the 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-compresseid
manually-with a steel cylinder press. Each mold was placed
~n an ~sostatic pressure chamber and the pressure W2S raised
to about 1500 Kg/cm2 in 5 minutes and then reduced to zero in
ja few seconds. The samples were then taken out of the plasti
,molds and polished. ~ne pressed samples ~rere put into an
~electrically heated furnace and heated from room temperature ¦
to 1500C under a nitrogen atmosphere over a period of 24 hours~
held at the maximum temperature for 2 to 5 hours and then
cooled to 20 C over the follo~ing 24 hours. The sintered
samples were then taken out of the furnace and after cooling
to room temperature, they were weighed.
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
-12-
' I
crucible of graphite, a layer of liouid aluminum -.~as Drovide~
on the bottom and a melt consl~tin~ Or 56% by ~Jeight of hlC13,
lg~5~ b~ weight of NaCl and 24 5~ by wei~ht of .~Cl was p~ured
on top thereof. The sample electrodes prepared according to ¦
the procedure describ~d above and to which a Pt ~ire ~Yas
brazed to provide an easy means for electrical conn~ction w~2re
cipped into the salt melt and held at a distance ol about 1 cm
~rom the liquid aluminum layer. The crucible was maintained
at a temperature ranging from 700C and the current density
0 ~Y25 5KA/m2 and the cell was operated for 8 hours. The
experimental data obtained is shown in Figure 1.
TABLE I
.
,;~lectrode No, j Co~position , Dimensions
,. , . ,
1 ~ , Graphite l 20 x 20 x 30 mm
2 i ZrB2(80%) + SiC(20%) 20 x 20 x 30 mm
i: 3 ZrB2(72%) -t SiC(~8%)
+ C(10%) 1 ~60 x 10 mm
1~ 4 ZrB2(56%) + SiC(14/o)
!` -t C(30%) ~ ~60 x 10 mm `,
.. ~
j.
.
.
'.'' I
: t
.
-13- !
!~ .
~i3~2`7
The above results show that the chlorine potential
for graphite is 1.5 to 1.7 volts higher than the electrodes of
the invention. Moreover, the chlorine potential for electrodes
3 and 4 of the invention is less than that of electrode No. 2
which does not contain any free carbon. No corr~sion was noted
during the 8 hours operation. Moreover, as can be seen from
the curves of electrodes 3 and 4, the chlorine potential is
slightly lower as the carbon content increases.
-14-
jk/~
lil3~.~`7
The chlorine potential for the electrodes No. 1 to 4
of Example 1 was determined with reference to a silver electrode
at 2.5 XA/m and the results reported in Figure 2
show no change in the chlorine potential after 8 hours.
Various modifications of the electrodes, cells and
electrochemicai processes of the invention may be made without
departing from the spirit or scope thereof and it is to be
understood tnat the invention is to be limited only as defined
in the appended claims.
-15-
jk/~
