Note: Descriptions are shown in the official language in which they were submitted.
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PCT!ls99lo0Q81
WO 99136593
SL'URRY FOR COATING NON-CAP.SON MSTAL-BASEA ANODES
FOR ALUMINIUM PRODUCTION CELLS
Field of the Invention
This invention relates to a slurry for coating
anodes for use in cells for the electrowinning of metals
from their oxides dissolved in molten salts, and to
methods for their fabrication and reconditioning, as well
as aluminium electrowinning cells containing coated
anodes and their use to produce aluminium.
Background Art
The production of metals by the electrolysis of
their oxides is usually carried out in very chemically
aggressive enviro=ents. Therefore, the materiaa.s used
for the manufacture of cornpoxients of productioza cells
must be resistant to attack by the environrnent of such
cell. Arnodes of cells for the productiori of metals by the
electrolysis of their oxides dissolved in molten salts
need to be resistant to attack by the electrolyte and by
the oxygen which is anodically produced' during
electrolysis.
Unfortunately, for the dissolution of the raw
material a highly aggressive electrolyte, such as a
fluoride-based electrolyte is required.
The surface of the anode must be electrochemically
active, substantially insoluble in the e].ectrolyte and
resistant to attacks by the nascent monoatomic oxygen and
by the subsequently formed molecular oxygen gas which are
anodically produced. Since monoatomic oxygen is far more
aggressive than biatomic molecular gaseous oxygen, the
constituents of the active suzface of the anor' should
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contain electro-catalytic materials for the reaction
which forms molecular oxygen from the monoatomic oxygen
to reduce monatomic oxygen attack.
The materials having the greatest resistance to
oxidation are metal oxides which are all to some extent
soluble in cryolite. Oxides are also poorly electrically
conductive, therefore, to avoid substantial ohmic losses
and high cell voltages, the use of oxides should be
minimal in the manufacture of anodes. Whenever possible,
a good conductive material should be utilised for the
anode core, whereas the surface of the anode is
preferably made of an oxide having a high
electrocatalytic activity.
In the field of aluminium production, it has been
described in US Patents 5,069,771, 4,960,494 and
4,956,068 (all Nyguen/Lazouni/Doan), and US Patent
5,510,008 (Sekhar/Liu/Duruz) that a metal core could be
protected by barrier layers and/or by oxidised metals but
these results have not as yet been commercially and
industrially applied.
Obiects of the Invention
An object of the invention is to provide a method
for coating an anode for metal electrowinning cells, in
particular aluminium electrowinning cells, which
substantially reduces the consumption of the active anode
surface that is attacked by nascent monoatomic oxygen by
enhancing the reaction of nascent oxygen to gaseous
molecular gaseous oxygen.
Another object of the invention is to provide a
slurry for coating anodes for metal electrowinning cells,
in particular aluminium electrowinning cells, which
provides a coating with high electrolytic activity, a
long life and which can be re-coated onto the anode as
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soon as such activity decreases or when the coating is
worn out.
A major object of the invention is to provide an
anode for metal electrowinning cells, in particular
aluminium electrowinning cells, which has no carbon so as
to eliminate carbon-generated pollution and reduce the
cell voltage and the high cost of cell operation.
Summary of the Invention
The present invention concerns a method of applying
a slurry onto a conductive, heat resistant anode
substrate to form an oxide coating on those parts of the
substrate which are exposed to oxidising or corrosive
cell environments.
The invention in particular relates to a method of
coating an electronically conductive and heat resistant
substrate of a non-carbon metal-based anode of a cell for
the electrowinning of metals from their oxides dissolved
in molten salt, to protect and make the surface of the
anode substrate active for the oxidation of the oxygen
ions present in the electrolyte. The method comprises
applying onto the substrate a slurry comprising at least
one oxide or a precursor thereof as a non-dispersed but
suspended particulate in a colloidal and/or inorganic
polymeric carrier, the slurry is then solidified and made
adherent to the substrate upon heat treatment to form an
adherent, protective, predominantly oxide-containing
coating.
An oxide may be present in the oxide-containing
coating as such, or in a multi-compound mixed oxide
and/or in a solid solution of oxides. The oxide may be in
the form of a simple, double and/or multiple oxide,
and/or in the form of a stoichiometric or non-
stoichiometric oxide.
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A typical application for this method is the coating
of anodes for the electrowinning of aluminium by the
electrolysis of alumina dissolved in a molten fluoride-
containing electrolyte, such as a cryolite-based
electrolyte or cryolite.
The colloidal and/or inorganic polymeric carrier may
be selected from alumina, ceria, lithia, magnesia,
silica, thoria, yttria, zirconia, tin oxide, zinc oxide
and mixtures thereof.
Advantageously, the colloidal and/or inorganic
polymeric carrier forms upon heat treatment the same
chemical compound as the non-dispersed particulate.
The oxides which may be used as a non-dispersed
particulate and/or as a carrier may be in the form of
spinels and/or perovskites or precursors thereof. Spinels
may be doped, non-stoichiometric and/or partially
substituted spinels, the doped spinels comprising dopants
selected from the group consisting of Ti4+, Zr4+, Sn4+,
Fe4+, Hf4+, Mn4+, Fe3+, Ni3+, C03+, Mn3+, A13+, Cr3+, Fe2+,
Ni2+, Co2+, jvig2+, Mri2+, Cu2+, Zn2+ and Li+.
The spinels may comprise a ferrite which can be
selected from cobalt, copper, chromium, manganese, nickel
and zinc ferrite, and mixtures and precursors thereof.
The ferrites may also be doped with at least one oxide
selected from chromium, titanium, tin, zinc and
zirconium. Nickel-ferrite is a preferred compound for an
electrochemically active coating for its high chemical
resistance and may be present as such or partially
substituted with Fe2+.
Alternatively, the spinels may also comprise a
chromite which can be selected from iron, cobalt, copper,
manganese, beryllium, calcium, strontium, barium,
yttrium, magnesium, nickel and zinc chromite, and
mixtures and precursors thereof.
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The slurry advantageously comprises one or more
electrocatalysts or a precursor thereof, however such a
constituent is not always necessary. When an
electrocatalyst is used, it may be advantageously
selected from iridium, palladium, platinum, rhodium,
ruthenium, silicon, tin, zinc, Mischmetal oxides and
metals of the Lanthanide series, and mixtures and
compounds thereof.
For the formation of the coating onto the substrate,
the oxide constituents of the slurry may react among
themselves. Alternatively the constituents of the slurry
may react with constituents of the electronically
conductive and heat resistant substrate. However, a
reaction is not always necessary for the formation of the
coating from the slurry.
The slurry may be applied onto the substrate by
conventional techniques such as brushing, spraying
dipping, electrodeposition or by using rollers.
The substrate can be chosen among metals, alloys,
intermetallics, cermets, and conductive ceramics. It may
for instance comprise at least one of chromium, cobalt,
hafnium, iron, molybdenum, nickel, copper, niobium,
platinum, silicon, tantalum, titanium, tungsten,
vanadium, yttrium and zirconium, and their combinations
and compounds.
The substrates may advantageously have a self-
healing effect, i.e. when exposed to electrolyte the
substrate passivates under the effect of the electrical
current and becomes substantially inert to the
electrolyte.
The adherence of the coating on the substrate may be
enhanced by applying onto the substrate a pre-coat before
applying the slurry. Several methods are known to obtain
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an oxide pre-coat on a metal substrate, e.g. heating in
air for prolonged periods at high temperatures (>1000 C).
However, a preferred pre-coat can be formed by
applying a metal oxide in a colloidal or polymeric
solution onto a clean metal substrate, drying and heat-
treating the pre-coat at 500 C. Oxides for the pre-coat
may be selected from Si02, A1203, Th02, Zr02, Sn02, Ti02
and Ce02. Preferably the colloid/polymer contains cerium
oxide having a crystallite size of about 5 to 10
nanometer and a NO3-/CeO2 mole ratio of approximately
0.25, which can be prepared by following the teachings of
US Patent 4,356,106 (Woodhead/Raw).
The pre-coat can be applied from a colloidal
dispersion having a concentration between 25 and 250 g/l.
Conventional techniques such as dipping, brushing or
spraying can be used prior to drying and/or heat-treating
the pre-coat.
The invention also relates to an anode coating
slurry for coating an electronically conductive and heat
resistant substrate of a non-carbon metal-based anode for
the electrowinning of metals from their oxides dissolved
in molten salts, to form an adherent, protective,
predominantly oxide-containing coating after heat
treatment and to make the surface of the anode active for
the oxidation of the oxygen ions present in the
electrolyte. The slurry comprises at least one oxide or
oxide precursor as a non-dispersed but suspended or
suspendable particulate in a colloidal and/or inorganic
polymeric carrier.
This method may also be applied for reconditioning a
non-carbon metal-based anode with a slurry as described
hereabove, the active coating of which anode has become
non-active or worn out. The method comprises clearing and
restoring the surface of the conductive substrate before
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applying the slurry onto the substrate as described
hereabove.
Another aspect of the invention is an anode of a
cell for the electrowinning of a metal, in particular of
an aluminium electrowinning cell, comprising an
electronically conductive substrate and a protective
electrochemically acti_ve coating obtained from a slurry
as described hereabove.
A further aspect of the invention is a cell for the
production of a metal by the electrolysis of its oxide
dissolved in a molten salt, in particular for the
electrowinning of aluminium or a lanthanide such as
neodymium, having at least one anode comprising an
electronically conductive substrate and a protective
electrochemically active coating obtained from a slurry
as described hereabove.
An aluminium electrowinning cell may advantageously
comprise at least one aluminium-wettable cathode. The
cell may be in a drained configuration by having at least
one drained cathode on which aluminium is produced and
from which aluminium continuously drains. The cell may be
of monopolar, multi-monopolar or bipolar configuration. A
bipolar cell may comprise the anodes as described above
as a terminal anode or as the anode part of a bipolar
electrode.
Preferably, the aluminium electrowinning cell
comprises means to improve the circulation of the
electrolyte between the anodes and facing cathodes and/or
means to facilitate dissolution of alumina in the
electrolyte. Such means can for instance be provided by
the geometry of the cell as described in co-pending
application WO 99/41429 (de Nora/Duruz) or by
periodically moving the anodes as described in co-pending
application WO 99/41430 (Duruz/Bello).
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The aluminium electrowinning cell may be operated
with the electrolyte at conventional temperatures, such
as 950 to 970 C, or at reduced temperatures as low as
750 C.
Yet another aspect of the invention is a method of
electrowinning aluminium in a cell comprising at least
one coated non-carbon metal-based anode as described
hereabove, the method comprising dissolving alumina in
the electrolyte and then electrolysing the dissolved
alumina to produce aluminium.
The slurry as described hereabove can be used for
coating a non-carbon metal-based anode for the production
of aluminium in a cell for the electrowinning of
aluminium by the electrolysis of alumina dissolved in a
fluoride-containing electrolyte, on which anode oxygen
ions in the electrolyte are oxidised and released as
biatomic molecular gaseous oxygen by the
electrochemically active anode slurry-obtained coating.
Detailed Description
The invention will be further described in the
following Examples:
Examnle 1
A polymeric slurry was prepared from: a non-
dispersable but suspendable particulate consisting of a
nickel-ferrite powder and a nickel aluminate (NiOAl2O3)
precursor material acting as a polymeric carrier and
binder for the nickel ferrite powder. The nickel-ferrite
powder was specially prepared; however, commercially-
available products could also have been used. The
precursor NiOA12O3 materials, solution and gel powder
reacted to form the spinel NiAl2O4 at < 1000 C.
When applied to a suitably prepared substrate such
as nickel, this slurry produced an oxide coating made
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from the pre-formed or the in-situ formed nickel ferrite
which adhered well onto the substrate and formed a
coherent coating when dried and heated. The slurry could
be applied by a simple technique such as brushing or
dipping to give a coating of pre-determined thickness.
Example 2
A carrier consisting of a nickel aluminate polymeric
solution containing a non-dispersed but suspended
particulate of nickel aluminate was made by heating 75 g
of Al(N03)3.9 H20 (0.2 moles Al) at 80 C to give a
concentrated solution which readily dissolved 12 g of
NiCO3 (0.1 moles). The viscous solution (50 ml) contained
200 g/l A1203 and 160 g/l NiO (total oxide, >350 g/1).
This nickel-rich polymeric concentrated anion
deficient solution was compatible with commercially-
available alumina sols e.g. NYACOLT"'.
A stoichiometrically accurate NiO.A1203 mixture was
prepared by adding 5 ml of the anion deficient solution
to 2.0 ml of a 150 g/l alumina sol; this mixture was
stable'to gelling and could be applied to smooth metal
and ceramic surfaces by a dip-coating technique. When
heated to 450-500 C, X-ray diffraction showed nickel-
aluminate had formed in the coating.
Other non-dispersable particulate than nickel
aluminate could be suspended in the anion-deficient
nickel aluminate precursor solution and applied as
coatings which when heat-treated would form nickel-
aluminate containing the added oxides.
Examiple 3
A colloidal solution containing a metal ferrite
precursor (as required for NiONiFe2O4) was prepared by
mixing 20.7 g Ni(N03)2.6 H20 (5.17 g NiO) with 18.4 g
Fe(NO3)3.9 H20 (4.8 g Fe203) and dissolving the salts in
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water to a volume of 30 ml. The solution was stable to
viscosity changes and to precipitation when aged for
several days at 20 C.
An organic solvent such as PRIMENET"' JMT (R3CNH2
molecular weight -350) is immiscible with water and
extracts nitric acid from acid and metal nitrate salt
solutions. An amount of 75 ml of the PRIMENE=M JMT (2.3 M)
diluted with an inert hydrocarbon solvent was mixed with
ml of the colloidal nickel-ferrite precursor solution.
10 Within a few minutes the spherical droplets of feed were
converted to a mixed oxide gel; they were filtered off,
washed with acetone and dried to a free-flowing powder.
When the gel was heated in air, nickel-ferrite formed at
< 800 C and the powder could be used as a non-dispersable
but suspended particulate in colloidal and/or inorganic
polymeric slurries as described in Example 1 or 2.
Commercially-available nickel-ferrite powder could also
have been used.
Examipl e 4
An amount of 5 g of NiC03 was dissolved in a
solution containing 35 g Fe(N03)3.9 H20 to give a mixture
(40 ml) having the composition required for the formation
of NiFe204. The solution was converted to gel particles
by solvent extracting the nitrate with PRIMENET"' JMT as
described in Example 3. The nickel-ferrite precursor gel
was calcined in air to give a non-dispersable but
suspended particulate in the form of a nickel-ferrite
powder, which could be hosted into nickel-aluminate
carrier for coating applications from colloidal and/or
polymeric slurries.
Example 5
An amount of 100 g of Cr(N03)3.9 H20 was heated to
dissolve the salt in its own water of crystallisation to
form a solution containing 19 g Cr203. The solution was
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heated to 120 C and 12.5 g of magnesium-hydroxy carbonate
containing the equivalent of 5.0 g MgO was added. Upon
stirring a solution was obtained in the form of an anion-
deficient polymer mixture with a density of approximately
1.5 g/cm3 suitable to act as a carrier. An amount of 50 g
of this carrier was evaporated to dryness to convert the
solution into a fine oxide powder. The oxides were then
calcined at 600 C into a magnesium chromite powder to
form a non-dispersable but suspended particulate.
After grinding to a fine powder, the magnesium
chromite particulate was suspended in the polymer carrier
to form a slurry suitable for coating treated metal
substrates.
Example 6
An amount of 150 g of Fe(N03)3.9 H20 was heated to
dissolve the salt in its own water of crystallisation to
form a solution containing 29 g Fe203. The solution was
heated to 120 C and 18.9 g of magnesium hydroxy-carbonate
dissolved in the hot solution to form 7.5 g MgO in form
of an inorganic polymer together with Fe203. An amount of
50 g of the polymer solution was evaporated to dryness
and then calcined at 600 C yielding approximately 13 g of
magnesium ferrite powder.
After calcination, the ferrite powder was ground in
a pestle and mortar and then suspended as a non-
dispersable particulate in the same inorganic polymer
acting as a carrier to give a slurry that was used to
coat a treated metal substrate.
Example 7
A cleaned surface of an InconelTM billet (typically
comprising 76 weight% nickel - 15.5 weight% chromium - 8
weight% iron) was pre-coated with a ceria colloid as
described in US Patent 4,356,106 (Woodhead/Raw), dried
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and heated in air at 500 C. The pre-coated billet was
then further coated with the polymeric slurry described
in Example 1 or 2, dried and heated in air at 500 C. The
ferrite coating was very adherent and successive layers
of the slurry could be applied to build up a coating of
ferrite/aluminate having a thickness above 100 micron.
A similar untreated InconelTM billet was coated with
a 10 micron thick layer using the polymeric slurry
described in Example 1 or 2 but without pre-coating the
billet with ceria colloid. After heat-treatment the
coating was cracked and easily broke away from the
substrate, which demonstrated the effect of the ceria
pre-coat.
