Note: Descriptions are shown in the official language in which they were submitted.
C~216d468
Treated Carbon or Carbon-Based Cathodic
Components of Aluminium Production Cells
Field of the Invention
This invention relates to carbon or carbon-based cathodic cell components of
electrolytic cells for the production of aluminium in particular by the
electrolysis of
alumina in a sodium-containing molten halide electrolyte such as cryolite.
Background Art
Aluminium is produced conventionally by the Hall-Heroult process, by the
electrolysis of alumina dissolved in cryolite-based molten electrolytes at
temperatures
up to around 950°C. A Hall-Heroult reduction cell typically has a steel
shell provided
with an insulating lining of refractory material, which in turn has a lining
of carbon
which contacts the molten constituents. Conductor bars connected to the
negative
pole of a direct current source are embedded in the carbon cathode substrate
forming
the cell bottom floor. The cathode substrate is usually an anthracite based
carbon
lining made of prebaked cathode blocks, joined with a ramming mixture of
anthracite,
coke, and coal tar.
In Hall-Heroult cells, a molten aluminium pool acts as the cathode. The carbon
lining or cathode material has a useful life of three to eight years, or even
less under
adverse conditions. The deterioration of the cathode bottom is due to erosion
and
penetration of electrolyte and liquid aluminium as well as intercalation of
sodium,
which causes swelling and deformation of the cathode carbon blocks and ramming
mix. In addition, the penetration of sodium species and other ingredients of
cryolite
or air leads to the formation of toxic compounds including cyanides.
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The problems associated with penetration of sodium
into the carbon cathode have been extensively studied and
discussed in the literature.
Several papers in Light Metals 1992 published by the
The Minerals, Metals and Materials Society discuss these
problems. A paper "Sodium, Its Influence on Cathode Life in
Theory and Practice" by Mittag et al, page 789, emphasises the
advantages of using graphitic carbon over anthracite. Reasons
for the superiority of graphitic carbon were also set out in
a paper "Change of the Physical Properties and the Structure
in Carbon Materials under Electrolysis Test" by Ozaki et al,
page 759. Another paper "Sodium and Bath Penetration into TiB2
Carbon Cathodes During Laboratory Aluminium Electrolysis" by
Xue et al, page 773, presented results showing that the
velocity of sodium penetration increased with increasing TiB2
content. Another paper "Laboratory Testing of the Expansion
Under Pressure due to Sodium Intercalation in Carbon Cathode
Materials for Aluminium Smelters" by Peyneau et al, page 801,
also discusses these problems and describes methods of
measuring the carbon expansion due to intercalation.
There have been several attempts to avoid or reduce
the problems associated with the intercalation of sodium in
carbon cathodes in aluminum production.
Some proposals have been made to dispense with carbon
and instead use a cell bottom made entirely of alumina or a
similar refractory material, with a cathode current supply
arrangement employing composite current feeders using metals
and refractory hard materials. See for example, EP-B-0 145
412, March 16, 1988; EP-A-0 215 555, March 25, 1987; EP-B-0 145
411, March 16, 1988; and EP-A-0 215 590, March 25, 1987. So
far, commercialisation of these promising designs has been
hindered due to the high cost of the refractory hard materials
and difficulties in producing large pieces of such materials.
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Other proposals have been made to re-design the cell bottom making use of
alumina or similar refractory materials in such a way as to minimize the
amount of
carbon used for the cathode - see US Patent 5,071,533. Using these designs
will
reduce the problems associated with carbon, but the carbon is still subject to
attack
by sodium during cell start up.
There have been numerous proposals to improve the carbon materials by
combining them with TiBz or other refractory hard materials, see e.g. US
Patent No.
4,466,996. But, as pointed out in the above-mentioned paper of Xue et al.,
with
such composite materials, the penetration increases with increasing TiB2
content.
WO/93/20027 proposes applying a protective coating of refractory material to
a carbon cathode by applying a micropyretic reaction layer from a slurry
containing
particulate reactants in a colloidal carrier, and initiating a micropyretic
reaction. To
assist rapid wetting of the cathode by molten aluminium, it was proposed to
expose
the coated cathode to a flux of molten aluminium containing a fluoride, a
chloride or
a borate of lithium and/or sodium. This improves the wetting of the cathode by
molten aluminium, but does not address the problem of sodium attack on the
carbon,
which is liable to be increased due to the presence of TiB2.
No adequate solution has yet been proposed to substantially reduce or
eliminate the problems associated with sodium penetration in carbon cathodes,
namely swelling especially during cell start-up, displacement of the carbon
blocks
leading to inefficiency, reduced lifetime of the cell, the production of large
quantities
of toxic products that must be disposed of when the cell has to be overhauled,
and
the impossibility to use low density carbon.
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Summary of the Invention
A primary object of the present invention is to improve the resistance of
carbon
cathodes of aluminium production cells or, more generally, of carbon-
containing
catholic components of such cells, to the penetration therein of molten
electrolyte
components and in particular to intercalation by sodium, thereby improving the
resistance of the components to degradation during use.
The invention applies to cathodes or other catholic cell components made of
carbon or other carbon-based microporous materials which have an open porosity
which extends to the surfaces of the component which, in use, are exposed to
the
conditions in the cell.
The term carbon cathode is meant to include both pre-formed carbon blocks
ready to be assembled into a cathode in the bottom of an aluminium production
cell,
as well as installed cathodes forming the cell bottom and the carbon side
walls
extending up from the bottom and which are also cathodically polarized and
therefore
subject to attack by sodium from the molten cell content. Other carbon
catholic
components include weirs and baffles secured on the cell bottom.
The invention provides a method of treating carbon-based catholic components
of electrolytic cells for the production of aluminium in particular by the
electrolysis of
alumina in a sodium-containing molten halide electrolyte such as cryolite, in
order to
improve their resistance to attack in the aggressive environment in the cells,
in
particular their resistance to intercalation by sodium.
The method according to the invention comprises impregnating and/or coating
the catholic cell component with colloidal alumina, ceria, cerium acetate,
silica, lithia,
yttria, thoria, zirconia, magnesia or monoaluminium phosphate and drying the
colloid
impregnated component.
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Colloidal alumina is preferred, and mixtures of colloidal alumina with the
other colloids
can also be used.
The method also includes optionally coating the surface of the component, or
including in the surface of the component, an aluminium-wettable refractory
material,
such as titanium diboride. In this case, the material of the component under
the
aluminium-wettable refractory material must be impregnated with the colloid,
in order
to provide an effective barrier to penetration of sodium species.
Thus, when the component is coated with colloid, the colloid coating may
optionally contain aluminium-wettable refractory components such as titanium
diboride provided the component is impregnated with colloid in order to
provide a
barrier to sodium penetration. But the colloid coating may be devoid of
aluminium-
wettable refractory components particularly in the case where the component is
coated with, for example, "thick" colloidal alumina, in which case the coating
already
provides a barrier to sodium penetration at the surface and the colloid need
not
penetrate so deeply into the carbon or carbon-based material.
Such impregnation and/or coating the carbon or carbon-based component, in
particular with colloidal alumina, has been found to improve the resistance of
the
carbon to damage by sodium impregnation due to the fact that the colloids are
stabilized by sodium or other monovalent ions. This stabilization, which
occurs during
use of the component in the cathodic environment of the aluminium production
cell,
makes the diffusion of fresh sodium difficult. Such stabilization is
particularly
effective when the sodium attack occurs through micropores in the carbon or
carbon-
based material. Therefore, to optimize the protective effect, it is preferred
to
impregnate the microporous carbon or carbon-based material with the colloid.
In addition, the colloid impregnation and/or coating prevents or inhibits
cryolite
penetration due to the fact that sodium impregnation in the surface generally
makes
the
CA~1 b446~
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carbon or carbon-based material more wettable by cryolite. By limiting sodium
penetration to the colloid surface, this enhances wettability of the surface
by cryolite,
which assists in keeping the cryolite at the surface. Hence, the enhanced
resistance
to sodium penetration unexpectedly is associated with an enhanced protection
against
damage by cryolite penetration.
This surprising synergistic effect leads to several further advantages. For
example, as a consequence of the inhibition of sodium and cryolite penetration
into
the bulk of the carbon or carbon-based material, the formation of toxic
components
is greatly reduced.
Furthermore, the colloid impregnated in the carbon or carbon-containing
surface, or coated on the surface, improves the resistance of the carbon or
carbon-
based material to abrasion by sludge that deposits on the cathode surface and
may
move with the cathodic pool of aluminium and thereby wear the surface.
Also, by protecting the carbonaceous cell components from attack by NaF or
other aggressive ingredients of the electrolyte, the cell efficiency is
improved.
Because NaF in the electrolyte no longer reacts with the carbon cell bottom
and walls,
the cell functions with a defined bath ratio without a need to replenish the
electrolyte
with NaF.
Impregnation and/or coating of the component is preferably followed by a heat
treatment and may also be enhanced by preceding it with a heat treatment, for
example at about 1000°C. Sometimes, a single impregnation suffices, but
usually
the impregnation and drying steps are repeated until the component is
saturated with
the colloid. Generally, impregnation will take place when the viscosity of the
colloid
is low, and the number of impregnations needed to saturate the material can be
determined by measuring the weight gain. Coating will take place when the the
colloid is thicker, i.e. paste-like. Impregnation with a
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_,_
low-viscosity colloid an be followed by coating with a pasty colloid.
The component is conventionally impregnated by dipping it into the colloid,
which can take place in ambient conditions, but the impregnation may be
assisted by
the application of a pressure differential, by applying pressure or a vacuum.
Coating
can be by dipping or other application techniques such as brushing.
The 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
and
mixtures thereof. 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.
The colloid usually has a dry colloid content corresponding to up to 50
weight% of the colloid plus liquid carrier, preferably from 10 to 20 weight%.
The
liquid carrier is usually water but could be non-aqueous.
The carbon or carbon-based microporous material making up the cathode or
cathodic component usually has an open porosity usually from 5% to 40%, often
from about 15% to about 30%. Such microporous materials are in particular
liable
to be attacked by the corrosive cell contents at the high operating
temperatures.
Impregnation of the pores with a selected colloid greatly increases the
materials'
resistance to corrosion, as set out above.
It is advantageous for the carbon or other carbon-based microporous material
making up to the cathode or the cathodic component to be impregnated with
alumina
or with colloidal monoaluminium phosphate which will be converted to alumina.
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Especially when the electrolyte in the aluminium
production cell contains cerium, for instance cryolite
containing cerium which maintains a protective cerium
oxyfluoride coating on the anode, the carbon-based cathode
component may be impregnated and/or coated with a cerium-based
colloid, typically comprising at least one of colloidal ceria
and colloidal cerium acetate. This cerium-based colloidal
carrier may further comprise colloidal alumina or other
colloids such as yttria, silica, thoria, zirconia, magnesia,
lithia and/or monoaluminium phosphate. Colloid cerium
impregnated in the microporous carbon or carbon-based material
improves its performance when used as cathode or cell lining,
while the cerium-based colloid is compatible with a
cerium-containing fluoride-based electrolyte.
One advantageous impregnating agent greatly
improving the material's resistance to penetration by sodium
from the molten content of the cell, is colloidal lithia. The
liquid carrier of the colloid, preferably colloidal alumina
and/or colloidal lithia, is a solution containing at least one
compound of lithium, sodium and potassium, preferably a lithium
compound. Impregnation of carbon cathodes with colloidal
lithia and/or with a colloid in a solution of a lithium, sodium
or potassium salt, followed by heat treatment greatly improves
the cathodes resistance to sodium impregnation, as taught in
copending application SN 2,155,205.
A colloid impregnated cathode or cathodic
component according to the invention can also be coated with
a protective 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) ,
usually applied after impregnation of the carbon or
carbon-based material with the colloid.
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Such a protective coating may be formed by applying
to the treated carbon cathode a micropyretic reaction layer
from a slurry containing particulate reactants in a colloidal
carrier, and initiating a micropyretic reaction as described
in WO/93/20027. Such micropyretic slurry comprises particulate
micropyretic reactants in combination with optional particulate
of 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.
Such coatings may give an additional protection against sodium
attack.
Protective coatings can also be formed from a
colloidal slurry of particulate non-reactants, such as
pre-formed TiB2, as described in WO/93/20026.
Such protective coatings applied directly to a carbon
or carbon-based material in a colloidal carrier have good
adherence to the substrate and good wettability by molten
aluminium. However, as discussed in the Background Art
section, the presence of aluminium-wettable refractory material
such as titanium diboride enhances the penetration of sodium
and inhibits the potential beneficial effect of the colloid as
a barrier to sodium penetration. For this reason, components
coated with aluminium-wettable refractory materials must be
impregnated with the colloid in order to inhibit sodium
penetration in accordance with the invention.
When the impregnated carbon or carbon-based cathode
or catholic component is coated with a refractory coating
forming a catholic surface in contact with the
cathodically-produced aluminium, it can be used as a drained
cathode. The refractory coating forms the catholic surface on
which the aluminium is deposited cathodically usually with the
component arranged upright or at a slope for the aluminium to
drain from the catholic surface.
a
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It is advantageous for cathodes or cell bottoms of
low density carbon to be impregnated with a colloid according
to the invention. Low density carbon embraces various types
of relatively inexpensive forms of carbon which are relatively
porous and very conductive, but hitherto could not be used
successfully in the environment of aluminium production cells
on account of the fact that they were subject to excessive
corrosion or oxidation. Now it is possible, by impregnating
these low density carbons with a colloid according to the
invention, to make use of them in these cells instead of the
more expensive high density anthracite and graphite, taking
advantage of their excellent conductivity and low cost.
The cathode or cathodic components may, for instance,
be made of petroleum coke, metallurgical coke, anthracite,
graphite, amorphous carbon, fullerene such as fullerene C6o or
C,o or of a related family, low density carbon or mixtures
thereof . Most usually, the component will be made of the usual
grades of carbon used as cathodes in conventional Hall-Heroult
cells.
The material making up the component may also be a
carbon-based composite material comprising carbon and at least
one further component selected from refractory oxycompounds,
in particular alumina, and possibly also refractory hard metal
borides, carbides and silicides, in particular titanium
diboride, it being understood that any aluminium-wettable
refractory material will be adjacent to the surface in which
case the underlying carbon or carbon-based material will be
impregnated with the colloid. Examples of such composite
materials are described in copending application
PCT/US93/05459(MOL0512).
The component of the invention may be a carbon
cathode or a carbon cell bottom or lining advantageously
Y
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impregnated with dried colloidal alumina and coated with a protective coating
comprising a Refractory Hard Metal boride.
Alternatively the component may be a carbon cathode or a carbon cell bottom
or lining impregnated and coated with dried colloidal alumina.
A further aspect of the invention is an electrolytic cell for the production
of
aluminium, in particular by the electrolysis of alumina in a sodium-containing
molten
halide electrolyte such as cryolite, comprising a cathodic component made of
carbon
or a carbon-based material, wherein the component is impregnated and/or coated
with
colloidal alumina, ceria, cerium acetate, silica, lithia, yttria, thoria,
zirconia, magnesia
or monoaluminium phosphate, as set out above.
The invention also concerns a method of producing aluminium by the
electrolysis of alumina dissolved in molten cryolite in a cell having a
colloid
impregnated and/or coated carbon cathode as set out above; an electrolytic
cell for
producing aluminium by the electrolysis of alumina dissolved in molten
cryolite
provided with such a colloid impregnated and/or coated carbon; a method of
conditioning carbon cathodes for use in such cells; as well as a method of
reconditioning these electrolytic cells. The electrolyte may be cryolite or
modified
forms of cryolite in particular containing LiF, and may be at the usual
operating
temperature of about 950°C, or lower temperatures.
Detailed Description
The invention will be further described in the following examples.
Example 1
Samples of cathode-grade carbon were impregnated with colloidal alumina by
dipping them in NyacoIT"' colloidal alumina containing 20 wt% alumina for 5
minutes,
removing
CAZ1ba468
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them and air drying in an oven for 1 hour at 200°C. This produced a
weight uptake
of approximately 1 .7%. The dipping process was repeated, but there was no
further
weight uptake, indicating that the sample was saturated with alumina.
These impregnated samples and corresponding non-impregnated samples were
then subjected to a sodium penetration test. This test consisted of
cathodically
polarizing the samples in an approximately 33/67 wt% sodium fluoride/sodium
chloride electrolyte at about 710°C and at a current density of 0.15
A/cm2 or
0.1 A/cm2 for variable test periods, usually between 5 and 10 hours. These
test
conditions simulate the effects of sodium penetration in commercial working
conditions over much longer periods.
The impregnated samples showed a higher resistance to sodium penetration
than the non-impregnated samples which showed signs of substantial degradation
after only about 3 hours.
Several of the impregnated samples were sectioned and submitted to analyses
to determine the extent of alumina penetration. Alumina was detected uniformly
through the sample to a depth of 1 Omm, corresponding to the center of the
sample.
The samples had a random distribution of narrow pores from the sample surface
to
a depth of 1 mm. Impregnation to the center of the sample took place through
an
interconnected inner pore system, in the carbon.
Example 2
Serval of the colloid-impregnated samples of Example 1 were further coated
with a TiBz coating as follows.
A slurry was prepared from a dispersion of 10g TiB2, 99.5% pure, -325 mesh
( < 42 micrometer, in 25m1 of colloidal alumina containing about 20 weight% of
solid
alumina. Coatings with a thickness of 150 ~ 50 to 500 ~ 50 micrometer
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were applied to the faces of carbon blocks. Each layer of slurry was allowed
to dry
for several minutes before applying the next, followed by a final drying by
baking in
an oven at 100-150°C for 30 minutes to 1 hour.
The above procedure was repeated varying the amount of TiB2 in the slurry
from 5 to 15g and varying the amount of colloidal alumina from 10m1 to 40m1.
Coatings were applied as before. Drying in air took 10 to 60 minutes depending
on
the dilution of the slurry and the thickness of the coatings. In all cases, an
adherent
layer of TiB2 was obtained.
The colloid-impregnated TiB2-coated samples showed an even higher resistance
to sodium penetration than the colloid-impregnated uncoated samples, when
submitted to the same sodium penetration test. These coated samples
additionally
exhibited improved wettability by molten aluminium. Compared to non-
impregnated
samples coated in the same way, the impregnated and coated samples showed a
better resistance to sodium penetration.
Example 3
40m1 10% HCI in aqueous solution was added to 50g of a petroleum coke
based particulate mixture and stirred for a sufficient time to wet the
petroleum coke
particles, followed by drying at 200°C for approximately 2 hours to dry
the petroleum
coke completely. The particulate mixture was made of 84 wt% petroleum coke ( 1-
200 micrometerl, 15wt% A1203 (3 micrometer) and 1 wt% B203 ( 1 micrometer.
80 ml of colloidal alumina (AL-20 grade, 20% solid alumina) was added to the
dried acidified petroleum coke based mixture and stirred well. The resulting
slurry of
petroleum coke, particulate alumina, colloid alumina and HCI mixture was then
dried
at 200°C in an air furnace for approximately 2 to 3 hours to produce a
paste.
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The resulting paste was pressed at 57 mPa into cylinder form. In the pressing
process, some liquid was squeezed out. The cylinders were then held at
200°C in
an air furnace until dried. The resulting material was a microporous
carbon/alumina
composite.
A specimen produced this way was impregnated with colloidal cerium acetate
by dipping the dried cylinder in the colloid, then drying it again at
200°C.
Compared to non-impregnated cylinders, impregnated cylinders prepared this
way were found to have enhanced resistance to sodium penetration when used as
cathodes in a laboratory scale aluminium production cell.
Example 4
The above Examples can be repeated including in the liquid carrier of the
colloid
at least one compound of lithium, aluminium, cerium, calcium, sodium and/or
potassium, preferably a soluble compound.
The lithium compound may be lithium acetate, lithium carbonate, lithium
fluoride, lithium chloride, lithium oxalate, lithium nitride, lithium nitrate,
lithium
formate and lithium aryl, lithium tetraborate and mixtures thereof.
The aluminium compound is preferably a soluble compound, but some insoluble
compounds can also be used. Soluble compounds include aluminium nitrate,
carbonate, halides and borate. Insoluble aluminium carbide can also be used.
Preferably, there is at least one of these lithium compounds together with at
least one of these aluminium compounds. These compounds react together and,
when the component is made of carbon, with the carbon to form aluminium
oxycarbide and/or aluminium carbide AI4C which act as an oxidation-resistant
and
electrically-conductive binder for the carbon and contribute to the great
oxidation
resistance of the material and make it wettable by molten
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cryolite. Altogether, the addition of these lithium and aluminium compounds
greatly
increases the stability of the material in the environment of an aluminium
production
cell.
For instance, a solution can be prepared by thoroughly mixing 5g of
A1 N03.9H20(98%) and 5g of LiN03(99%) in 50m1 of water, and this carrier
solution
then mixed with colloidal alumina to provide a solid alumina colloid content
of about
to 20 weight% of the total. Cathode grades of carbon impregnated with this
reagent-containing colloidal alumina followed by heat treatment at about
1000°C
10 show improved stability and greater resistance to penetration by sodium.
