Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
210460
WO 94124321 - PCT/US93103605
This invention relates to components of aluminium
production cells made of composite materials comprising
ordered aluminide compounds of nickel, iron and/or titanium,
for use in particular as anodes and cathodes and cell
linings in aluminium production cells containing a fluoride-
based molten electrolyte containing dissolved alumina and
cerium species.
The invention is more particularly concerned with the
production of components of aluminium production cells made
of composite materials comprising ordered aluminide
compounds of nickel, iron and/or titanium, by the
micropyretic reaction of a mixture of reactive powders,
which reaction mixture when ignited undergoes a micropyretic
reaction to produce a net-shaped reaction product, it being
understood that the reaction product may be used directly as
an anode or cathode, or as substrate carrying an outer
protective coating, or as a cell component.
US Patent N° 4,614,569 describes anodes for aluminium-
production coated with a protective coating of cerium
oxyfluoride, formed in-situ in the cell or pre-applied, this
coating being maintained by the addition of cerium to the
molten cryolite electrolyte. The inclusion of cerium in the
substrate was proposed to promote formation of the cerium
oxyfluoride coating and enhance its properties, but so far
no practical way was found to effectively implement this.
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US Patent N° 4,948,676 describes a ceramic/metal
composite material for use as an anode for aluminium
production particularly when coated with a protective cerium
oxyfluoride based coating, comprising mixed oxides of cerium '
and one or more of aluminium, nickel, iron and copper in the
form of a skeleton of interconnected ceramic oxide grains
interwoven with a metallic network of an alloy or an
intermetallic compound of cerium and one or more of
aluminium, nickel, iron and copper. The production methods
included reactive sintering, reactive hot-pressing and
reactive plasma spraying of a metal powder mix optionally
including some oxides. The described process conditions led
to a complex porous microstructure which through dissolution
and redeposition of cerium provided a self-healing effect
'_5 when the anode is first used. However, difficulties were
encountered in controlling the porosity of this
microstructure.
US Patent N° 4, 909, 842 discloses the production of
dense, finely grained composite materials with ceramic and
metallic phases by self-propagating high temperature
synthesis (SHS) with the application of mechanical pressure
during or immediately after the SHS reaction. The ceramic
phase may be carbides or borides of titanium, zirconium,
hafnium, tantalum or niobium, silicon carbide or boron
carbide. The intermetallic phase may be aluminides of
nickel, titanium or copper, titanium nickelides, titanium
ferrites or cobalt titanides, and the metallic phase may
include aluminium, copper, nickel, iron or cobalt. The final
product, which has ceramic grains in~an intermetallic and/or
metallic matrix, has a density of at least about 95~ of the
theoretical density obtained by the application of pressure.
Interconnected porosity is not obtained, nor does the '
process control porosity. Because the pressure is applied
uniaxially, it is not possible to produce a net-shaped '
article, i.e. whose final shape and dimensions may be
largely or even completely achieved in the manufacturing
process, and for which no or only minor post manufacturing
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processing such as grinding are required. hlso. :.he required
application of pressure prevents high production rates.
Moreover, materials produced by the described method but
without the application of pressure are weak and have a
_ porosity of about 45 to 48%, which makes them unsuitable as
electrodes for aluminium production.
PCT patent application w0/13977 describes the
production of ceramic or ceramic-metal electrodes for
electrochemical processes, in particular for aluminium
.0 production, by combustion synthesis of particulate or
fibrous reactants with particulate or fibrous fillers and
binders. The reactants included aluminium usually with
__tanium and boron; the binders included copper and
aluminium; .he fillers included various oxides, ~itrides,
_.. borides, carbides and silicides. The described composites
included copper/aluminium oxide-titanium diboride etc. This
method has prospects of improved process control leading to
a better microstructure, but the compositions are still in
need of improvement.
~0 PCT patent application WO/92/22582 describes an
improvement of the just mentioned production method with
specific fillers. The described reactants included an
aluminium nickel mixture, and the binder could be a metal '
mixture including aluminium, nickel and up to 5 weight%
25 copper. Among the many combinations covered is a combination
of 85-90 weight% nickel-aluminium-copper with 10-15 weight%
of cerium oxide. However, such combinations are very
reactive and the described method does not provide details
as to how to control the microstructure.
30 Co-pending Canadian Patent Application 2,131,287 discloses the
production of a protective refractory coating on
carbonaceous and other substrates by applying to the
substrate a micropyretic reaction layer from a slurry
containing particulate reactants in a colloidal carrier, and
~5 initiating a micropyretic reaction. This application is
WO 94/24321 PCTIUS93l03605
specially concerned with the production of refractory
borides coatings suitable for cathodic applications.
So far, attempts to produce an electrode suitable as
anode for aluminium production and based on intermetallic
compounds of aluminium with nickel, iron and/or titanium
have not been successful. Additionally, no combination of
such intermetallics with a ceramic has been achieved which
maintains the property of a ceramic to resist oxidation at
the same time achieving good conductivity at high
temperatures. Moreover, attempts to incorporate cerium in an
anode substrate to be coated with cerium oxyfluoride have
not been successful.
Summary of the Invention
The invention provides a method of manufacturing
i5 components of aluminium production cells made of composite
materials comprising ordered aluminide compounds of nickel,
iron and/or titanium, for use in particular as anodes and
cathodes and cell linings in aluminium production cells
containing a fluoride-based molten electrolyte containing
dissolved alumina and cerium species, by micropyretic
reaction of a reaction mixture comprising reactants which
react to produce the aluminide-based composite material,
which reaction mixture when ignited undergoes a micropyretic
reaction.
According to the invention, the reaction mixture is
mixed with a cerium-based colloidal carrier, dried and
compacted into a reaction body bonded by the cerium-based
colloid, and the colloid-bonded reaction body is ignited to
initiate the micropyretic reaction. The use of a cerium-
based colloidal carrier - usually colloidal ceria or cerium
acetate, usually in an aqueous medium - has been found to
assist bonding of the reaction mixture to form the reaction
body, and contributes to moderating the micropyretic
reaction as well as.considerably improving the properties of
the reaction product. Comparable reaction mixtures without
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the cerium-based colloidal carrier are difficult to bond,
react poorly and do not produce a satisfactory product.
Moreover, the cerium-based colloid improves the
reaction product, in particular when it is to be used as an
anode for aluminium production coated with a protective
cerium oxyfluoride coating. When such anode is initially
immersed in a cerium-containing fluoride-based electrclyte,
the colloid-originating cerium in the anode promotes initial
cerium oxyfluoride formation and improves the impermeability
of the cerium oxyfluoride coating by its dissolution and re-
deposition, which provides a self-healing effect. These
effects are enhanced when the composite material of the
anode also contains copper oxide. The colloid-originating
cerium in the composite material also improves its
i5 performance when used as cathode or cell lining in an
aluminium production cell with a cerium-containing fluoride-
based electrolyte.
The cerium-based colloidal carrier may comprise
colloidal ceria, colloidal cerium acetate or mixtures
thereof. These cerium-based colloids may also include some
colloidal silica, alumina, yttria, thoria, zirconia,
magnesia, lithia or monoaluminium phosphate, and hydroxides,
acetates and formates thereof as well as oxides and
hydroxides of other metals, cationic species and mixtures
thereof. Some particulate ceria can be included in the
colloidal ceria.
The cerium-based colloid may be derived from colloid
precursors and reagents which are solutions of at least one
salt such as chlorides, sulfates, nitrates, chlorates,
perchlorates or metal organic compounds such as alkoxides,
formates, acetates The aforementioned solutions of metal
organic compounds, principally metal alkoxides, may be of
the general formula M (OR) Z where M is a metal or complex
cation, R is an alkyl chain and z is a number usually from 1
to 12.
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The dry colloid content of the cerium-based colloidal
carrier usually corresponds to 10 - 30 weighty of the
colloidal carrier, preferably 10 - 20 weight , but may
account for up to 40 weighty or even 50 weighty of the
colloidal carrier, there being preferably from 10 to 20 ml
of the colloidal carrier per 100 grams of the powder
mixture.
The colloid-originating cerium usually amounts to 0.2
to 10~ by weight of the composite material.
The reaction mixture usually comprises particulate
metals from the group of aluminium, nickel, iron, titanium,
copper, chromium, manganese, vanadium, molybdenum,
zirconium, niobium and cerium, and/or compounds of these
metals, and mixtures thereof.
A typical reaction mixture comprises 50 to 100 parts by
weight of particulate nickel, iron and/or titanium and 2 to
50 parts by weight of particulate aluminium. There may also
be a further 1 to 30 parts by weight of particulate
additives selected from copper, chromium, manganese,
vanadium, molybdenum, zirconium, niobium and cerium and
compounds thereof, as well as compounds of aluminium,
nickel, iron and titanium.
One preferred reaction mixture comprises 50 to 100
parts by weight of particulate nickel, 2 to 50 parts by
weight of particulate aluminium and 1 to 25 parts by weight
of particulate copper. Another comprises 50 to 90 parts by
weight of particulate nickel, 5 to 30 parts by weight of
particulate aluminium, 5 to 25 parts by weight of
particulate copper and 0 to 15 parts by weight of additives
selected from chromium, manganese, vanadium, molybdenum,
zirconium, niobium and cerium and compounds thereof, as well
as compounds of aluminium, nickel, iron, titanium and a
copper.
For some applications, especially for anodes, the
reaction mixture includes one or more oxides of at least one
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metal from the group of aluminium, nickel, copper, chromium,
manganese and cerium.
For cathodic applications, the reaction mixture may
comprise at least one boride of at least one metal from the
S group titanium, chromium, vanadium, molybdenum, zirconium,
niobium and cerium, or precursors that react to form said
borides.
The micropyretic reaction (also called self-propagating
high temperature synthesis) can be initiated by applying
local heat to one or more points of the reaction body by a
convenient heat source such as an electric arc, electric
spark, flame, welding electrode, microwaves or laser, in
which case the reaction propagates through the reaction body
along a reaction front which may be self-propagating or
assisted by a heat source. Reaction may also be initiated by
heating the entire body to initiate reaction throughout the
body in a thermal explosion mode. In either case, the
reaction proceeds without supplying further heat as in a
furnace. The reaction atmosphere is not critical, and
reaction can take place in ambient conditions without the
application of pressure.
A coating may be applied to the component produced by
micropyretic reaction, the composition of this coating
depending on the intended use. Such coatings may in general
contain the same components as the additives listed above.
A preferred coating for aluminium-production anodes is
cerium oxyfluoride according to US Patent No 4,614,569,
formed in-situ in the cell or pre-applied. The cerium
oxyfluoride may optionally contain additives such as
compounds of tantalum, niobium, yttrium, tantalum,
praesodymium and other rare earth elements, this coating
( being maintained by the addition of cerium and possibly
other elements to .the molten cryolite electrolyte. When a
cerium oxyfluoride coating is to be applied in-situ, the
?5 anode substrate preferably includes cerium or cerium oxide
as an additive in the composite material, in addition to the
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cer=um _rom the colloidal carrier. Production o: such a
coating _..-situ leads to dense and homogeweous cerium
cxyfluoride. The presence of copper oxide _.. the anode
sur~ace is believed to enhance tie cerium cxy~luoride
=gating.
A cathode according to the invention can also be coated
wit': a protective refractory coating, typically containing
an aluminium-wettable Refractory Hard Metal compound such as
the borides and carbides of metals of Group IVB (titanium,
zirconium, hafnium) and Group VB (vanadium, niobium,
tantalum). Boride-containing coatings are preferred.
Such a protective coating may be formed b:J applying to
=::~.e cathode a micropyretic reaction layer °=om a slurry
containing particulate reactants in a colloidal carrier, and
:5 initiating a micropyretic reaction as described in
co-pending Canadian Patent Application 2,131,287. Such a
micropyretic slurry comprises particulate micropyretic reactants
in combination with optional particulate or fibrous non-
reactant fillers or moderators in a carrier of colloidal
materials or other fluids such as water or other aqueous
solutions, organic carriers such as acetone, urethanes, etc.,
or inorganic carriers such as colloidal metal oxides.
When the cathode is coated with a refractory coating
e5 forming a cathodic surface in contact with the cathodically-
produced aluminium, it can be used as a drained cathode. the
refractory coating forming the cathodic surface on which the
aluminium is deposited cathodically, and the component being
arranged usually upright or at a slope for the aluminium to
?0 drain from the cathodic surface.
Advantageously, before use, the operative surface of
the cell component is conditioned by impregnating it with
colloidal ceria or ,cerium acetate or other colloids such as
colloidal silica, alumina, yttria, thoria, zirconia,
?5 magnesia or lithia followed by drying the colloid-
impregnated electrode, these impregnation/drying steps being
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q
repeated preferably until the electrode surface is saturated
with the colloid. Impregnation of the component should be
followed by a heat treatment and is preferably also preceded
by a heat treatment. For anodes used in molten salt
electrolysis, coated or not with cerium oxyfluoride, this
impregnation preferably takes place with colloidal ceria or
cerium acetate.
The invention also pertains to a cell component of an
aluminium production cell, made of a composite material
comprising at least one ordered aluminide compound of at
least one of nickel, iron and titanium. The cell component
is produced by micropyretic reaction of a dried reaction
mixture comprising compacted particulate reactants which
react to produce the composite material, bonded by a cerium-
based colloidal carrier. Cerium from the colloid is
dispersed in the aluminide compound forming the cell
component. Usually, the colloid-originating cerium amounts
to 0.2 to 10~ by weight of the composite material.
A preferred composite material making up the cell
component comprises nickel aluminide in solid solution with
copper, and possibly also in solid solution with other
metals and oxides. Another composite material comprises a
major amount of Ni3A1 and minor amounts of NiAl, nickel, a
ternary nickel-aluminium-copper intermetallic compound and
Ce02.
Other composite materials comprise at least one
intermetallic compound from the group AlNi, AlNi3, Al3Fe,
AlFe3, AlTi and AlTi3 as well as ternary intermetallic
compounds derived therefrom, and solid solutions and
mixtures of at least one of said intermetallic compounds
with at least one of the metals aluminium, nickel, iron,
titanium, copper, chromium, manganese, vanadium, molybdenum,
zirconium, niobium and cerium and oxides of said metals.
Another composite material comprises an intimate
mixture of at least one intermetallic compound of nickel-
aluminium, at least one intermetallic compound of nickel-
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aluminium-copper, copper oxide, and a solid solution of at
least two of the metals nickel, aluminium and copper.
The component produced by the micropyretic reaction may
comprise an intimate mixture of at least one intermetallic
compound of nickel-aluminium such as Ni3A1 and Al3Ni, at
least one intermetallic compound of nickel-aluminium-copper
such as Al~3NilgCug, copper oxide, and a solid solution of
two or three metals nickel, aluminium and copper. It is
believed that this material and materials like it contain
non-stoichiometric conductive oxides wherein lattice
vacancies are occupied by the metals or intermetallics,
providing an outstanding conductivity while retaining the
property of ceramic oxides to resist oxidation.
The aforementioned nickel aluminide based materials and
nickel aluminide composites and solid solutions have been
found to perform particularly well as dimensionally stable
anodes for aluminium production.
As explained above, the cell component is
advantageously impregnated with colloidal ceria, cerium
acetate, silica, alumina, yttria, thoria, zirconia, magnesia
or lithia.
Another aspect of the invention is a precursor of a
component of an aluminium production cell which is ignitable
to produce by micropyretic reaction a cell component made of
a composite material comprising at least one ordered
aluminide compound of at least one of nickel, iron and
titanium. This precursor is a body formed of a dried
reaction mixture, as explained above, comprising compacted
particulate reactants which react to produce the composite
material, mixed with and bonded by a cerium-based colloidal
carrier. The properties of this precursor are substantially
enhanced by the cerium-based colloid. .
_ Yet another aspect of the invention is a reaction
mixture for producing a component of an aluminium production
cell, which component is made of a composite material
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comprising at least one ordered aluminide compound of at
least one of nickel, iron and titanium, by micropyretic
reaction of the reaction mixture after drying and
compacting. The reaction mixture comprises particulate
reactants, as set out above, which react to produce the
composite material, mixed with a cerium-based colloidal
carrier in an amount of at least 5m1 of colloid per 100
grams of the reaction mixture.
The invention will be further described in the
following examples.
E x a:r
A powder mixture was prepared from nickel powder, -100
mesh, aluminium powder, -325 mesh, and copper powder, -200
mesh. First the nickel and aluminium powders were mixed in a
ratio Ni:Al 87:13 wt~. Then this mixture was mixed with
copper powder in a ratio Ni/Al:Cu 90:10 wt~ in 12m1 of
colloidal cerium acetate per 100 grams of the powder
mixture.
After 10 minutes mixing, which was sufficient to
produce a good mixture, the mixture was compacted into
samples by applying a pressure of about 170 MPa for 2-3
minutes, and allowed to dry in air for at least 3 hours.
When the sample was almost dry, an exothermic reaction
between the powders and cerium acetate occurred. To keep the
samples cool and avoid cracking, cool air was blown on the
samples by an air gun.
After the samples had dried completely, a small hole
was drilled in the bottom of each sample to threadably
receive a nickel-based superalloy rod to provide for
electrical connection to the sample.
The samples were then combusted in a furnace at 900°C
to initiate a micropyretic reaction which swept through the
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sample, and afterwards allowed to cool slowly to avoid
cracking.
Example 1 was repeated varying the proportion of Ni:Al,
in the ratios 75:25; 86.6:13.4; 90:10; 92:8; 94:6 and 96:4. '
The weight ratio of Ni/Al:Cu was kept constant at 90:10.
Colloidal cerium acetate was added to the different series
of samples in amounts of 12m1, 24m1 and 36m1 per 100 grams
of powder mixture. Compacting was carried out at approx. 170
MPa for 4 minutes. After drying, the samples were combusted
in a furnace at 950°C. All samples underwent a micropyretic
reaction.
A sample prepared as in Example 1 was conditioned for
use as an aluminium electrowinning anode by heating in air
at 1000°C for 4 hours to oxidize its surface. After cooling,
the sample was dipped in colloidal cerium acetate until no
more is absorbed. The sample was then heated in an oven to
dry it. After cooling the sample was again dipped in
colloidal cerium acetate and dried. The dipping and drying
steps were repeated until no more cerium acetate was
absorbed.
A cylindrical piece of 25 mm diameter and 40 mm height
was prepared using the micropyretic technique of Example 2,
with the composition Ni:Al 86.6:13.4, mixed with colloidal
cerium acetate in an amount of 24m1/100 grams of the powder
mixture. The material was then submitted to a heat treatment
in air at 1000°C for 10 hours. The weight uptake due to
oxidation was about 6$. The oxidized material was
impregnated by dipping into a colloidal solution of cerium
acetate for 10 minutes and drying at 250°C . This operation
was repeated twice. The sample was then tested as an anode
in a small electrolytic cell containing molten cryolite at
1000°C with 5~ alumina and 1.5~ cerium fluoride, at a
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~3
current density of 0.3 A/cm2 for 4 hours. The cell voltage
remained stable at 4V during the test. The test anode was
then cross-sectioned and no significant corrosion was
observed.
FxamB~
The same pretreatment and test procedures were applied
to a second sample with the composition Ni:Al 90:10 mixed
with colloidal cerium acetate in an amount of 24ml~per
100 grams cf the powder mixture. The test results were
similar to the previous material.
The same pretreatment and test procedures were applied
to a.third sample with the composition Ni:Ai 90:10 but mixed
with colloidal cerium acetate in an amount of 35m1 per
100 grams of the powder mixture. The weight uptake after the
heat treatment was more important (about 20~ greater) but
the material did not show any crack or fissure. The
electrolytic test gave results similar to the previous
examples with a somewhat higher cell voltage of 5 Volts.
Example 7
The previous examples were repeated varying the size of
the particulate nickel (1 to 10 micrometer diameter), copper
(1 to 100 micrometer diameter) and aluminium (1 to 100
micrometer diameter). Best results in terms of lowest
porosity and electrochemical performance were obtained with
nickel 3 micrometer diameter, copper 10 micrometer diameter
and aluminium 44 micrometer diameter (-325 mesh).
The previous examples were repeated replacing the
colloidal cerium acetate with colloidal ceria optionally
containing some ~ceria powder. Excellent results were
obtained.
