Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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ULTRASTABLE ~T(,~DES FOR ALUMINUM PRODUCTION CELL
Related A~nlic~tion
This application is a continuation-in-part of the US designation of
International
application PCT/US96/15176, filed September 23, 1996.
Field o~the Invention
This invention relates to cell components, particularly anodes, for use in the
S electrowinning of aluminum by the electrolysis of alumina in a molten
fluoride electrolyte,
in particular cryolite.
The invention is more particularly concerned with the production of cell
components, particularly anodes, of aluminum production cells made of
composite
materials by the micropyretic reaction of a mixture of reactive powders, which
reaction
mixture when ignited undergoes a micropyretic reaction to produce a reaction
product.
ac ground Art
US Patent 4,614,569 (Duruz et al) describes anodes for aluminum electrowinning
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.
US Patent 4,948,676 (Darracq et al) describes a ceramic/metal composite
material
for use as an anode for aluminum electrowinning particularly when coated with
a protective
cerium oxyfluoride based coating, comprising mixed oxides of cerium and one or
more of
aluminum, nickel, iron and copper in the form of a skeleton of interconnected
ceramic
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oxide grains interwoven with a metallic network of an alloy or an
intermetallic compound
of cerium and one or more of aluminum, nickel, iron and copper.
US Patent 4,909,842 (Dunmead et al) 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.
US Patent S,217,583 (Sekhar et al) describes the production of ceramic or
ceramic-
metal electrodes for electrochemical processes, in particular for aluminum
electrowinning,
by micropyretic reaction of particulate or fibrous reactants with particulate
or fibrous fillers
and binders. The reactants included aluminum usually with titanium and boron;
the binders
included copper and aluminum; the fillers included various oxides, nitrides,
borides,
carbides and silicides. The described composites included copper/aluminum
oxide-titanium
diboride etc.
US Patent 5 , 316, 718 (Sekhar et al) describes an improvement of US Patent
5,217,583 with specific fillers. The described reactants included an aluminum
nickel
mixture, and the binder could be a metal mixture including aluminum, nickel
and up to 5
weight % copper.
US Patents 4,374,050 (Ray) and 4,374,761 (Ray) disclose anodes for aluminum
electrowinning composed of a family of metal compounds including oxides. It is
stated that
the anodes could be formed by oxidizing a metal alloy substrate of suitable
composition.
However, it has been found that oxidized alloys do not produce a stable,
protective oxide
film but corrode during electrolysis with spalling off of the oxide. US patent
4,620,905
(Tarcy et al) also discloses oxidized alloy anodes.
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US Patents 4,4S4,O1S (Ray/Rapp) and 4,678,760 (Ray) disclose aluminum
production anodes made of a composite material which is an interwoven network
of a
ceramic and a metal formed by displacement reaction. These ceramic metal
composites
have not been successful.
S US Patents S,069,771, 4,960,494 and 4,9Sb,068 (all Nyguen et al) disclose
aluminum production anodes with an oxidized copper-nickel surface on an alloy
substrate
with a protective barrier layer. However, full protection of the alloy
substrate was difficult
to achieve.
US Patent 5,284,562 (Beck et al) discloses alloy anodes made by sintering
powders
of copper nickel and iron. However, these sintered alloy anodes cannot resist
electrochemical attack.
Published international application W094/24321 (Sekhar et al), discloses
aluminum
production anodes comprising ordered aluminide compounds of nickel, iron and
titanium
produced by micropyretic reaction with a cerium based colloidal carrier.
1S A significant improvement was described in US Patent S,S 10,008, and in
International Application W096/12833 (Sekhar et al). Prior to this, a11
attempts to produce
an electrode suitable as anode for aluminum production and based on metals
such as nickel,
aluminum, iron and copper or other metals had proven to be unsuccessful in
particular due
to the problem of poor adherence due partly to thermal mismatch between the
metals and
the oxide formed prior to or during electrolysis.
This teaching provided an anode for aluminum production where the problem of
poor adherence due partly to thermal mismatch between a metal substrate and an
oxide
coating formed from the metal components of the substrate was resolved, the
metal
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electrode being covered with an oxide layer which remained stable during
electrolysis and
protected the substrate from corrosion by the electrolyte.
Such an anode for the production aluminum by the electrolysis of alumina in a
molten fluoride electrolyte comprises a porous micropyretic reaction product
derived from
particulate nickel, aluminum and iron, or particulate nickel, aluminum, iron
and copper,
optionally with small quantities of doping elements such as chromium,
manganese,
titanium, molybdenum, cobalt, zirconium, niobium, cerium, oxygen, boron and
nitrogen
included in a quantity of up to 5 wt% in total.
The porous micropyretic reaction product contained metallic and/or
intermetallic
phases, and a composite oxide surface formed in-situ from the metallic and
intermetallic
phases contained in the porous micropyretic reaction product, by anodically
polarizing the
micropyretic reaction product in a molten fluoride electrolyte containing
dissolved alumina.
The in-situ formed composite oxide surface comprised an iron-rich relatively
dense outer
portion, and an aluminate-rich relatively porous inner portion.
Comparative anodes of similar compositions (i.e. similar to those of the
anodes of
U.S. Patent 5,510,008 and WO 96/12833, Sekhar et al), but prepared from alloys
not
having a porous structure obtained by micropyretic reaction, show poor
performance. This
is believed to be a result of the mismatch in thermal expansion between the
oxide layer and
the metallic substrate with the alloy anodes. The differences in thermal
expansion
coefficients allow cracks to form in the oxide layer, or the complete removal
of the oxide
layer from the alloy, which induced corrosion of the anode by penetration of
the bath
materials, leading to short useful lifetimes.
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In contrast, the porous anodes according to U.S. Patent 5,510,008 and WO
96/12833 (Sekhar et al) accommodate the thermal expansion, leaving the dense
protective
oxide layer intact. Bath materials such as cryolite which may penetrate the
porous metal
during formation of the oxide layer become sealed off from the electrolyte,
and from the
active outer surface of the anode where electrolysis takes place, and did not
lead to
corrosion but remain inert inside the electrochemically inactive inner part of
the anode.
These in-situ oxidized anodes represent a considerable improvement over
earlier
proposals. However, the composite oxide layer of these in-situ oxidized anodes
of U.S.
Patent 5,510,008 and W096/12833 (Sekhar et al) may grow to a thickness that
reduces
process efficiency, which limits the useful lifetime of the anodes. Attempts
to remove this
limitation of the anodes by including the additives disclosed in U.S. Patent
5,510,008 and
W096/12833 (Sekhar et al) were not successful, in that such additives were
found either
not to have an effect of limiting the growth rate of the thickness of the
oxide layer, or a
thickness limiting effect was achieved but to an inadequate amount and/or this
effect would
be offset by problems of contamination of the product aluminum.
~umm~rv of the Invention
The invention is based on the discovery that the performance of the anodes of
U. S.
Patent 5,510,008 and W096/12833 is unexpectably improved by certain additive
elements.
The invention relates to a cell component, preferably an anode, for the
electrowinning of aluminum by the electrolysis of alumina dissolved in a
molten fluoride
electrolyte, comprising a porous micropyretic reaction product of particulate
nickel,
aluminum, iron and optionally, copper, and of at least one additive element in
an effective
amount usually up to 8 wt % of the total, the porous micropyretic reaction
product
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containing metallic and intermetallic phases which preferably form a composite
oxide
surface layer, more preferably comprising an iron-rich relatively dense outer
portion and
an aluminate-rich relatively porous inner portion, wherein said layer is
formed when the
porous micropyretic reaction product is anodically polarized in a molten
fluoride electrolyte
containing dissolved alumina, or is oxidised by being subjected to contact
with oxygen at
high temperatures. After electrolysis or oxidation, the product comprises a
porous core
and the composite oxide surface. Thus, the product can be characterized as a
"graded"
material.
According to the invention, the overall performance of the prior art
electrodes of
U.S. Patent 5,510,008 and W096/12833 (Sekhar et al) is greatly enhanced by
using, as an
additive element, at least one element from the group consisting of silicon,
tin, zinc,
vanadium, indium, hafnium, tungsten, elements from the lanthanide series
starting from
praesodymium and misch metal. The combustion behavior of the reactant mixture
is
improved considerably as will be explained in greater detail below.
IS With these additive elements, it has been discovered that the composite
oxide
surface layer formed in-situ when the cell components are used as anodes grows
to a
thickness which is much less than the thickness obtained with the best
formulations
disclosed in the prior art. This is due to the much smaller thickness growth
rate observed
for the cell components of the present invention. As a result, anodes
according to the
invention can operate at a lower overvoltage, and for a considerably longer
useful life.
The composition of the micropyretic reaction product is important to the
formation
of a dense composite oxide surface preferably comprising an iron-rich
relatively dense
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outer portion which is associated with an aluminate-rich relatively porous
inner portion by
diffusion of the metals/oxides during the in-situ production of the oxide
surface.
The micropyretic reaction product is preferably produced from the particulate
nickel, aluminum, iron, copper and the additive element in the amounts 50-90
wt % nickel,
3-20 wt % aluminum, 5-20 wt % iron, 0-15 wt % copper and 0.5-5 wt % of said at
least one
additive element from the group consisting of silicon, tin, zinc, vanadium,
indium,
hafnium, tungsten, elements from the lanthanide series starting from
praesodymium and
misch metal and optionally other additives. More preferably still, the
micropyretic reaction
product is produced from 60-80 wt % nickel, 3-10 wt % aluminum, S-20 wt % iron
and 5-15
wt% copper, plus 0.5-5 wt% of the selected additive element(s).
In micropyretic synthesis, it is known that the ignition temperature T; and
the
combustion temperature T~ are important processing parameters. See, for
example,
J'rocessing of Composite Materials by the Micropyretic Synthesis Method, M. Fu
and J.A.
Sekhar, Key Eng. Mat., Trans. Tech Publications, vol. 108-110, pp. 19-44
(1995)). It has
bee noted when using the compositions of the present invention that the
combustion
temperature decreased with the presence of iron, copper and zinc, which do not
contribute
to the energy developed during the reaction. Contrarily the combustion
temperature
increased with the presence of nickel, aluminum, silicon or tin, because these
elements
participate in the micropyretic reaction.
Preferred embodiments of the invention include silicon, tin or zinc as
additive
element in an amount of 0. S to 3 wt % of the total .
Preferred elements from the lanthanide series are praesodymium, neodymium and
ytterbium as well as misch metal which is a mixture of cerium, lanthanum,
neodymium and
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other rare-earth metals. These elements are also preferably included as
additive element in
an amount of 0.5 to 3 wt % of the total.
The micropyretic reactions product was tested in the absence of the additive
elements for effect of the aluminum content. In the preferred aluminum content
range of 3-
10 wt% , the resulting composites have good adherence with cerium oxyfluoride
coatings
when such coatings are used for protection, and the lowest corrosion rate.
Below 3
aluminum, the composites still have low corrosion, but surface spalling is
found after
testing. With increasing aluminum content above 10 wt%, corrosion increases
gradually,
and above about 20 wt % aluminum the composites have low porosity due to the
increase of
combustion temperature. It is expected that these effects would continue even
with the
inclusion of the additive elements of the present invention.
The micropyretic reaction products were also tested in the absence of the
additive
elements for effect of iron content. Below 5 wt % iron or no iron, the samples
have higher
corrosion and a nonconducting layer is found after testing. Above 20 wt% iron,
results in
1 S surface spalling after oxidation, 15 wt % being a preferred upper limit.
It is expected that
these effects would continue even with the inclusion of the additive elements.
The micropyretic reaction products were further tested in the absence of the
additive
elements for effect of copper content. Below 5 wt % copper down to 0 wt %
copper results
in anodes with higher corrosion rate but which are nevertheless acceptable,
and more than
15 wt% , in particular more than 20 wt% copper, results in surface spalling
after oxidation.
In the cell components tested as anodes, it was found that the composite oxide
layer is
depleted in copper, whereas the unoxidized portion of the micropyretic
reaction product
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adjacent to the aluminate rich inner portion of the oxide surface is rich in
copper. It is
expected that these effects would continue even with addition of the additive
elements.
It is preferred to use very reactive iron and copper, by selecting a small
particle size
of 44 micrometers or less for these components.
S The particulate nickel may advantageously have a larger particle size than
the
particulate aluminum, iron and copper. Large particle size nickel, for example
up to about
l50 micrometers, is preferred. Fine nickel particles, smaller than 10
micrometers, tend to
lead to very fine NiAI, Ni3Al or NiOx particles which may increase corrosion
when the
finished product is used as anode. Using large nickel particles enhances the
formation of
aluminates such as NiAlO, NiAlFeO or FeAlO phases on the surface, which
inhibits
corrosion and also promotes a porous structure. However, good results have
also been
obtained with nickel particles in the range 10 to 20 micrometers, these small
nickel
particles leading to a finer and more homogenous porous microstructure.
It is recommended to use aluminum particles in the size range 5 to 20
micrometers.
Very large aluminum particles (100 mesh) tend to react incompletely. Very fine
aluminum
particles, below 5 micrometers, tend to have a strong oxidation before the
micropyretic
reaction, which may result in corrosion when the finished product is used as
an anode.
The powder mixture may be compacted preferably by uniaxial pressing usually at
about 200-250 Mpa, or cold isostatic pressing (CIP), and the micropyretic
reaction may be
ignited in air or an inert atmosphere such as argon. The thermal explosion
mode of
micropyretic synthesis is preferred, at about 1000 ~ C . Excellent results
have been obtained
with combustion in air. The powder mixture is preferably compacted dry, such
as by ball
milling. Alternatively, liquid binders may be used for compaction.
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The micropyretic reaction (also called self propagating high temperature
synthesis
or combustion 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 to initiate a reaction which propagates
through the
reaction body along a reaction front which may be self propagating or assisted
by a heat
source, as in a furnace. Reaction may also be initiated by heating the entire
body to initiate
reaction throughout the body in a thermal explosion mode. The reaction
atmosphere is not
critical, and reaction can take place in ambient conditions without the
application of
pressure.
The micropyretic reaction product has a porous structure comprising at least
two
metallic and/or intermetallic phases. Generally, the micropyretic reaction
product
comprises at least one intermetallic compound from the group consisting of
nickel-iron,
nickel-aluminum, nickel-copper, aluminum-iron, nickel-aluminum-copper and
nickel-
aluminum-iron-copper containing intermetallic compounds.
The porosity and microstructure of the micropyretic reaction product are
important
for the in-situ formation of the preferred surface oxide layer since the pores
accommodate
for thermal expansion, leaving the outer oxide layer intact during
electrolysis.
Pre-electrolysis, the porous micropyretic reaction product may comprise nickel
aluminide (Ni3A1), in solid solution with copper, and possibly also in solid
solution with
other metals and oxides, including silicon, tin, zinc and compounds thereof
(including
oxides), or of the other additive elements, and mixtures. Post-electrolysis,
the core of the
preferred cell component/anode material comprises a major amount of Ni and
Ni3Al and
minor amounts of NiCu and NiFe in the substrate and a major amount of Ni0 and
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amount of NiFe204, Zn0 and NiZnFe204 (nickel zinc ferrite) in the mixed oxide
surface
layer. It is believed that the surface of such materials contains non-
stoichiometric
conductive oxides wherein lattice vacancies are occupied by the metals,
providing an
outstanding conductivity while retaining the property of ceramic oxides to
resist oxidation.
During electrolysis, the aluminum is depleted from the core of the cell
component/anode material, with the Ni3Al being replaced by Ni3Fe. The aluminum
migrates to the surface. Most of the copper is also present in the core as is
the iron,
because both copper and iron are highly soluble in nickel. It has been
observed that Ni3A1
and Ni3Fe are both considerably superior to NiA1 and NiFe, respectively, in
terms of
corrosion resistance and oxidation resistance. Both pre- and post-
electrolysis, the preferred
cell components of the present invention have a predominance of Ni3A1 and
Ni3Fe versus
NiAI and NiFe.
The micropyretic reaction product can also be produced from a mixture
containing,
in addition to said at least one additive element from the group consisting of
silicon, tin,
zinc, vanadium, indium, hafnium, tungsten, elements from the lanthanide series
starting
from praesodymium, and misch metal (preferably in an amount of OS . to 3 wt %
of the
total), an optional additional additive element from the group consisting of
chromium,
manganese, titanium, molybdenum, cobalt, zirconium, niobium, tantalum,
yttrium, cerium,
lanthanum, oxygen, boron and nitrogen. Although these additional additive
elements are
not as effective as silicon, tin, zinc, vanadium, indium, hafnium, tungsten,
elements from
the lanthanide series starting from praesodymium, and misch metal in reducing
the
thickness of the oxide layer, they may be included as additional "dopants"in
small
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quantities with favorable effects. The total of the main and the additional
additive elements
preferably should not exceed 7 wt % of the total.
The composite oxide surface usually comprises an iron-rich relatively dense
outer
portion, and an aluminate-rich relatively porous inner portion which integrate
into the
porous structure of the substrate. Analysis of some of the specimens has shown
that there
is present between the iron-rich outer portion and the aluminate-rich inner
portion, an
aluminum-depleted intermediate portion comprising predominantly oxides of
nickel and
iron.
The outermost iron-rich oxide layer, when present, is a homogenous, dense
layer
usually comprising oxides of aluminum, iron and nickel with predominant
quantities of
iron, usually mainly nickel ferrite and nickel-zinc ferrite (NiZnFe204) doped
with
aluminum (when zinc is the additive element).
Nickel-zinc ferrite has been observed to have excellent properties as an anode
coating material for aluminum production, even being superior to nickel
ferrite. In one
advantageous embodiment, the composite oxide surface comprises nickel oxide,
nickel
ferrite, zinc oxide and nickel-zinc ferrite.
The aluminum-depleted intermediate oxide layer, when present, usually
comprises
oxides of nickel and iron, with nickel highly predominant, for example iron-
doped nickel
oxide which provides good electrical conductivity of the anode and contributes
to good
resistance during electrolysis.
The innermost aluminate-rich oxide part, which is usually present, is slightly
more
porous that the two preceding oxide layers and is essentially an oxide of
aluminum, iron
and nickel, with aluminum highly predominant. This aluminate-rich part may be
a
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homogenous phase of aluminum oxide with iron and nickel in solid solution, and
usually
comprises mainly iron nickel aluminate.
Minor amounts of oxides of the principal additive elements or additional
additive
elements may also be present in the intermediate layers.
The porous metal substrate, close to the oxide layer, often comprises nickel
in
solution with copper and iron and also includes small quantities of aluminum.
The
substrate is usually largely depleted in aluminum as the aluminum is used to
create the
preferred aluminate-rich part on it. Preferably) the substrate is also
depleted in iron. The
metallic and intermetallic core deeper inside the substrate is also preferably
depleted of
aluminum as a result of internal oxidation in the open pores of the material
and diffusion of
the oxidized aluminum to the surface. The metallic and intermetallic core
(deep down in
the sample), can have a similar composition to the metallic core nearer the
oxide surface.
Interconnecting pores in the metal substrate may be filled with cryolite by
penetration during formation of the oxide layer, but the penetrated material
becomes sealed
i5 off from the electrolyte by the dense oxide coating and does not lead to
corrosion inside the
anode.
The invention also provides a method of manufacturing a cell component,
preferably an anode, for the production of aluminum by the electrolysis of
alumina in a
molten fluoride electrolyte, comprising reacting a micropyretic reaction
mixture of
particulate nickel, aluminum, iron and optionally copper, and at least one
additive element
selected from the group consisting of silicon, tin, zinc, vanadium, indium,
hafnium,
tungsten, elements from the lanthanide series starting from praesodymium, and
misch metal
in an amount up to 8 wt% of the total reactants, to produce a porous
micropyretic reaction
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product containing metallic and intermetallic phases, and preferably
anodically polarizing
the micropyretic reaction product in a molten fluoride electrolyte containing
dissolved
alumina, or subjecting it to contact with oxidizing gas at high temperatures,
to produce,
from the metallic and intermetallic phases contained in the porous
micropyretic reaction
product, an in-situ or ex-situ formed composite oxide surface usually
comprising an iron-
rich relatively dense outer portion and an aluminate-rich relatively porous
inner portion.
Another aspect of the invention is a method of electrowinning aluminum by the
electrolysis of alumina in a molten fluoride electrolyte. The electrowinning
method
comprises providing a starter anode, which is a porous micropyretic reaction
product
comprising metallic and intermetallic phases produced by reacting a
micropyretic reaction
mixture of particulate nickel, aluminum, iron and optionally copper, and at
least one
additive element from the group comprising silicon, tin, zinc, vanadium,
indium, hafnium,
tungsten, elements from the lanthanide series starting from praesodymiurn, and
misch metal
in an amount up to 8 wt% of the total reactants, and anodically polarizing it
in a molten
fluoride electrolyte containing dissolved alumina, or subjecting it to contact
with oxidizing
gas at high temperatures, to produce a composite oxide surface usually
comprising an iron-
rich relatively dense outer portion and an aluniinate-rich relatively porous
inner portion.
Electrolysis is then continued, using the same electrolyte (in which the in-
situ oxide
layer was formed) or a different molten fluoride electrolyte containing
dissolved alumina,
to produce aluminum using the in-situ oxidized starter anode. For example, the
composite
oxide surface would be formed in a cerium-free molten fluoride electrolyte
containing
alumina, then cerium would be added to deposit a cerium oxyfluoride based
protective
coating upon the composite oxide layer.
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In principle, the preferred final stage of production (formation of the
composite
oxide layer on the anode surface) will be performed in-situ in the aluminum
production cell
during production of aluminum. However, for special applications, it is
possible to form
the in-situ oxide layer in a special electrolytic cell and then transfer the
composite oxide
coated cell anode to a production cell.
For uses as cell components other than anodes, it is possible to pre-form a
composite oxide surface by anodic polarization or by oxidation in an oxidizing
gas such as
air prior to use of the component.
Yet another aspect of the present invention is a precursor of a cell component
of an
aluminum production cell which is ignitable to produce by micropyretic
reaction, a cell
component made of a composite material, said precursor comprising particulate
nickel,
aluminum, iron and, optionally copper, and at least one additive element
selected from the
group consisting of silicon, tin, zinc, vanadium, indium, hafnium, tungsten,
elements from
the lanthanide series starting from praesodymium, and misch metal, said
additive element
I5 being present in an amount of up to 8 weight percent of the total
precursor, possibly with
other additives as explained above.
A coating may be applied to the preferred in-situ formed oxide layer; a
preferred
coating being in-situ formed cerium oxyfluoride according to US Patent No.
4,614,569
(Duruz et al). The cerium oxyfluoride may optionally contain additives such as
compounds
of tantalum, niobium, yttrium, praesodymium and other rare earth elements;
this coating
being maintained by the addition of cerium and possibly other elements to the
molten
cryolite-based electrolyte. Production of such a protective coating in-situ
leads to dense
and homogeneous cerium oxyfluoride.
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Brief Description of Drawing
Figure 1 shows X-ray diffraction spectra, at the Two-Theta position, of two
metallic/intermetallic micropyretic reaction substrates, one without and the
other with zinc
as additive element; and
S Figure 2 shows corresponding X-ray diffraction spectra of the
metallic/intermetallic
micropyretic oxide layers formed on the substrates of Figure 1.
Detailed Desr,~jp,~~on
The invention will be further described and will be compared to the prior art
in the
following Examples.
Exam lp e1 (Co~parativel
A powder mixture was prepared from 73 wt % (68 atomic % ) nickel, 100 mesh
( < l49 micrometer), 6 wt% (12 atomic % ) aluminum, 325 mesh ( < 42
micrometer), 11
wt % ( 11 atomic % ) iron, 10 micrometers particle size, and 10 wt % (9 atomic
% ) copper,
5-10 micrometers particle size. After mixing, the dry mixture (i.e. without
any liquid
fiber), was uniaxially pressed at a pressure of 176 Mpa for a holding time of
3 minutes.
The pressed samples were then ignited in a furnace at 900~C or 1050~C to
initiate a
micropyretic reaction in air.
A11 reacted specimens were inhomogeneous and semiporous. Analysis of the
specimens showed the following composition in atomic % : 59. 8 % nickel, 18.6
aluminum, 11.2 % iron and 10. 5 % copper at the surface and 62. 8 % nickel, 13
.9 %
aluminum, 12.3 % iron, and 11.0 % copper in the core. The intermetallic
compound NiAl3
was present.
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Some specimens were then subjected to an oxidizing treatment in air at 1000 ~C
for
several hours, typically 5 hours. Other specimens were not subjected to this
oxidizing
treatment and it has been found that, this oxidizing treatment is not
essential .
The specimens were then used as anodes in a cryolite-based electrolyte
containing 7
wt% alumina and 1 wt% cerium fluoride at 980~C. A typical test for a specimen
with an
anode surface area of 22.4 cm2 ran for a first period of 48 hours at a current
density of 0.3
A/cm2, followed by a second period of 54 hours at a current density of 0.5
A/cm2. During
the first period, the cell voltage was from 2.9 to 2.5 Volts, and during the
second period
the cell voltage was from 3.3 to 4.4 Volts. At the end of the test, the anode
specimens
were removed. The specimens showed no signs of dimensional change, and the
metallic
substrate of dense appearance Was covered by a coarse, dense, uniform and well
adhering
layer of cerium oxyfluoride.
After the electrolytic test, the specimens were examined by scanning electron
microscope and energy dispersive spectroscopy (SEM/EDS).
The cerium oxyfluoride coating appeared homogeneous and very dense, with no
apparent porosity. On the surface of the specimen, below the cerium
oxyfluoride coating,
there was an in-situ formed composite oxide layer, total thickness about 300
micrometers,
made up of three different oxide layers, as described above.
The outermost oxide layer was a homogeneous, dense, oxide-only layer devoid of
fluoride. This oxide layer comprised oxides of nickel, aluminum and iron with
predominant quantities of iron. The quantities of metals present in atomic %
were 32
nickel, 21 % aluminum, 45 % iron and 2 % copper. It is believed that this
phase comprises
nickel ferrite doped with aluminum.
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The intermediate oxide layer was composed of large grains which
interpenetrated
with the outermost layer. Analysis showed no detectable fluoride, and the
intermediate
oxide layer comprised oxides of nickel and iron, with nickel highly
predominant. The
quantities of metals present in atomic % were 83 % nickel, 3 % aluminum, 13 %
iron and
1 % copper. It is believed that this phase is iron-doped nickel oxide which
would explain
the good electrical conductivity of the cell component/anode and its
resistance to
dissolution during electrolysis.
The oxide layer below the intermediate layer was slightly more porous that the
top
two oxide layers. Analysis identified it is an oxide of nickel, aluminum and
iron with
aluminum highly predominant. A small quantity of fluoride was detected in the
pores.
The quantities of metals present in atomic % were 22. 6 % nickel, 53.87 %
aluminum,
21.54 % iron and 1.99 % copper. It is believed that this phase may be a
homogeneous
phase of aluminum oxide with iron and nickel in solid solution, forming an
aluminate-rich
layer such as an iron nickel aluminate.
The porous metal substrate in contact with the oxide layer is comprised of
nickel
with small quantities of copper, iron and aluminum. It is largely depleted in
aluminum, as
the aluminum is used to create the aluminate layer on top of it. The
composition of the
porous substrate in atomic % was 77. 8 % nickel, 5 . 3 % aluminum, 3 . S %
iron and 13 . 5
copper.
The metallic core deeper inside the substrate is also depleted of aluminum as
a
result of internal oxidation in the open pores of the material and diffusion
of the oxidized
aluminum. Here, the composition in atomic % was 77 .2 % nickel, 1. 8 %
aluminum, 9.7 %
iron and 11.3 % copper.
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A11 interconnecting pores in the metal substrate were filled with cryolite,
and in
some cryolite-filled pores, a second phase identified as aluminum fluoride is
seen, probably
resulting from phase separation during the cooling of the cryolite within the
sample. No
other metallic fluorides were detected in the metallic core.
The above procedure was repeated varying the proportions in the starting
mixture,
as shown in Table I. The resulting specimens were subjected to electrolytic
testing as
described above.
Table 1
Ni/wt % Al/wt % Fe/wt % Cu/wt
76.1 4.9 10 10
71.4 3.6 15 10
62 8 20 10
79 10 11 0
66.4 3.6 15 15
64 6 15 15
71 8 11 10
Exa ple 2 lComparative)
The procedure of Example 1 was repeated varying the proportions in the
starting
mixture and with zirconium, chromium, titanium, yttrium or niobium as an extra
component in a total amount up to 5 wt % of the total reactants . The particle
size of the
chromium was 325 mesh ( < 425 micrometer) . The composition was nickel 73 wt %
,
aluminum 6 wt % , iron 6 wt % , copper 10 wt % and chromium or other additive
up to 5
wt% . Results comparable to those for the samples of Example 1 were obtained.
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l~
The procedure of the preceding examples was repeated, but all compositions
included silicon, tin or zinc as additives in an amount up to 5 % of the total
reaction
mixture, to the base Ni-Al-Cu-Fe in the weight percentages shown in Table 2
below:
Table 2
72Ni - 6A1 - lOCu - llFe - 1Si
70Ni - 6A1 - lOCu - llFe - 3Si
68Ni - 6A1 - lOCu - llFe - SSi
72Ni - 6A1 - lOCu - llFe - 1Zn
70Ni - 6A1 - lOCu - llFe - 3Zn
68Ni - 6A1 - IOCu - llFe - SZn
72Ni - 6A1 - lOCu - llFe - 1Sn
70Ni - 6A1 - lOCu - llFe - 3Sn
68Ni - 6A1 - IOCu - llFe - SSn
It was found that these samples exhibited superior oxidation resistance
compared to
the samples of Comparative Examples 1 and 2, in particular the selected
additives resulted
in formation of a thinner oxide layer, i.e. an oxide layer which grew at a
much slower rate
during oxidation than the oxide layers for the samples of Comparative Examples
1 and 2,
as is apparent from the oxidation test data reported below.
Electrochemical and Gas Oxidation Testing
Compared to the (other) prior art, samples prepared according to comparative
Examples 1 and 2, i.e. according to U.S. Patent 5,510,008 and W096/12833
(Sekhar et
CA 02269727 1999-04-23
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al), have shown outstanding properties as dimensionally stable anodes or anode
substrates
for aluminum production in the Hall-Heroult process, i.e. electrolysis of
alumina dissolved
in molten cryolite at temperatures of about 950 ~ . The anodes also show
outstanding long
term resistance to oxidation when subjected to oxidation tests in air or
oxygen at 950~C.
However, it has been observed that in long term electrolysis test, the samples
"over-passivate" (as described below) hence reducing their useful life and
compromising
their viability. It has been postulated that the oxidizing conditions under
anodic
polarization in molten cryolite at 950~C are more stringent than when the
samples are
heated in air/oxygen at the same temperature, and that the oxidizing
conditions prevailing
in a Hall Heroult cell at 950~C could be "simulated" by exposing the samples
to oxygen in
an oven at temperature about 200~C higher, i.e. at about 1150~C.
Tests, reported below, were therefore carried out to assess the resistance of
the
specimens to oxidation in air or oxygen at 1000~C and 1150~C. It was found
that the
samples according to Comparative Examples 1 and 2 (generally according to the
teaching
of U.S. Patent 5,510,008 and W096/12833, Sekhar et al), formed a relatively
thick oxide
layer under these conditions. It seems probable that if these specimens are
used as anodes
in aluminum production, their useful life would be limited by the formation of
such a thick
oxide layer. Additionally, as the thickness rises beyond a certain threshold
{"over-
passivation"), mechanical stresses between the oxide and the anode body are
great enough
to cause the oxide layer to lose its integrity, causing part of the oxide
layer to dissolve in
the electrolyte. Furthermore, such growth can increase the overall resistance
(reduced
conductivity) and the overvoltage and lower the cell efficiency.
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Surprisingly, it was found that with addition of the silicon, tin or zinc
additives of
this invention, the oxide surface layer was much thinner, i.e. it grew at a
much slower rate
than that for comparative examples 1 and 2. Moreover, the added metals, in
particular
zinc, are well tolerated in small quantities as impurities in the product
aluminum.
S The oxidation testing was carried out as follows: Samples prepared according
to
the preceding Examples were subjected to oxidation testing in a tube furnace
at l000~C or
1150 ~ C in air under static conditions or with a high airflow rate (0.2 1
/min), for a time of
about 24 hours.
After removal from the furnace, the thickness of the oxide film formed was
measured by an average of three readings from three different locations of
each sample, in
accordance with ASTM G54-90 procedures. The results of each series of
measurements
are reported in Table 3 below.
Table 3
Composition (wt % ) ~ Oxide Layer
Thickness
(
Ni(73)Al(6)Cu(10)Fe{11) 2.50
Ni(72)Al(6)Cu(10)Fe(11)Zn(1) 0.54
Ni(70}Al(6)Cu(10)Fe(11}Zn(3) 0.33
Ni(72)Al(6)Cu(10)Fe(11)Sn(1) 0.39
Ni(70)Al(6)Cu(10)Fe(11)Sn(3) 0.34
Ni(72)Al(6)Cu(10)Fe(11)Si(I) 0.35
Ni(70)Al(6)Cu(10)Fe(11)Si(3) 0.38
The zinc, silicon and tin additives led to a reduced oxide layer thickness due
to the
lowest growth rate compared to the sample without. Importantly, zinc is
relatively better
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tolerated as an impurity in aluminum, than the additives of the prior art such
as titanium
and chromium. Comparatively, zinc is also better tolerated as an impurity than
silicon and
tin.
A11 of the samples with silicon, tin and zinc additives has a reduced oxide
layer
thickness compared to the samples without additives. The no-additives samples
had a
considerably thicker oxide layer thickness (about 5 to 10 times greater). It
is believed that
the lower growth rates of the samples with the additives of the present
invention would
allow aluminum production cells including such components as anodes, to be
operated at
much greater current densities than the traditional current densities of 0.3
to 0.5 Aicm2. It
is believed that current densities of up to 8 A/cm2 would be possible.
X-ra~Diffraction Analysis
The phase of the metallic/intermetallic substrate and the mixed oxide layer of
samples produced according to Examples 1 and 3 were determined by X-ray
diffraction.
Figures 1 and 2 show the Two-Theta X-ray diffraction spectra of two samples,
one with a
composition Ni(73)Al(6)Cu(10)Fe(11) wt%, the other with a composition
Ni(73)Al(6)Cu(10)Fe(I1)Zn(3) parts by weight, before and after oxidation. As
is well
known, Theta is half the angle between the diffracted X-ray beam and the
original X-ray
beam direction. In a typical diffractometer, a moving X-ray detector records
the 2 Theta
angles at which the X-ray beams is diffracted, giving a characteristic
diffraction pattern as
seen in Figs. 1 and 2. For more detailed information on X-ray diffraction, a
textbook such
as The Science and Engineering of Materials, D.R. Askeland, PWS Publishing
Company
(1994), may be consulted.
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The primary phases of the metallic/intermetallic substrate are Ni and Ni3Al,
which
indicates that the major reactions during the synthesis process are between Ni
and AI
particles. Cu and Fe have high solubility in Ni and Ni3Al, and exist in solid
solution in
these phases or form minor amounts of copper and iron compounds such as NiCu
and NiFe
(not detected by the X-ray diffraction).
The mixed oxide layer is the part of the anode which contacted the cryolite
alumina
electrolyte during electrolytic testing. For the non-zinc specimen, this layer
is primarily
made up of Ni0 and NiFe204, which have low solubility in cryolite.
The addition of zinc does not change the phases of the substrate as shown in
Figure
1. Zn has a high solubility in the Ni and the Ni3A1 and exists in solid
solution or as zinc
compounds in small amounts that are not detected by the X-ray diffraction.
As is apparent from Figure 2, the mixed oxide layer formed with the zinc-
containing specimen is significantly different, with formation of complex
mixed oxides.
The oxides include NiO, NiFe204, NiZnFe204 and ZnO. It is believed that the
complex
oxides density the mixed oxide layer and enhance the oxidation resistance,
especially
NiZnFe204.
Thus, it is apparent that there have been provided, in accordance with the
invention,
cell components which fully satisfy the objects and advantages set forth
above. While the
invention has been described in conjunction with specific embodiments thereof,
it is evident
that many alternatives, modifications, and variations will be apparent to
those skilled in the
art in light of the foregoing description. Accordingly, it is intended to
embrace all such
alternatives, modifications and variations which fall within the spirit and
broad scope of the
appended claims.
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