Note: Descriptions are shown in the official language in which they were submitted.
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TITLE: CATHODE PROTECTION
Field of the Invention:
This invention relates to the protection of
refractory hard material cathodes used in aluminium smelting
cells and to aluminium smelting systems incorporating such
protected cathodes.
Background of the Invention:
In conventional designs for the Hall-Heroult cell,
the molten aluminium pool or pad formed during electrolysis
itself acts as part of the cathode system. The life span of
the carbon lining or cathode material may average three to
eight years, but may be shorter under adverse conditions. The
deterioration of the carbon lining materials is due to erosion
and penetration of electrolyte and liquid aluminium as well as
intercalation by metallic sodium, which causes swelling and
deformation of the carbon blocks and ramming mix. Penetration
of cryolite through the carbon body has caused heaving of the
cathode blocks. Aluminium penetration to the iron cathode bars
results in excessive iron content in the aluminium metal, or in
more serious cases, a tap-out.
Another serious drawback of the carbon cathode is its
non-wetting by aluminium, necessitating the maintenance of a
substantial height of pool or pad of metal in order to ensure
an effective molten aluminium contact over the cathode surface.
In conventional cell designs, a deep metal pad promotes the
accumulation of undissolved material (sludge or muck) which
forms insulating regions on the carbon cathode surface.
Another problem of maintaining such an aluminium pool is that
electromagnetic forces create movements and standing waves in
the molten aluminium. To avoid shorting between the metal and
the anode, the anode-to-cathode distance (ACD) must be kept at
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a safe 4 to 6 cm in most designs. For any given cell
installation there is a minimum ACD below which there is a
serious loss of current efficiency, due to shorting of the
metal (aluminium) pad to the anode, resulting from instability
of the metal pad, combined with increased back reaction under
highly stirred conditions. The electrical resistance of the
inter-electrode distance traversed by the current through the
electrolyte causes a voltage drop in the range of 1.4. to 2.7
volts, which represents from 30 to 60 percent of the voltage
drop in a cell, and is the largest single voltage drop in a
given cell.
To reduce the ACD, and associated voltage drop,
extensive research using Refractory Hard Materials (RHM), such
as titanium diboride (TiB2), as cathode materials has been
carried out since the 1950's. Because titanium diboride and
similar Refractory Hard Materials which are wetted by
aluminium, resist the corrosive environment of a reduction
cell, and are excellent electrical conductors, numerous cell
designs utilising Refractory Hard Materials have been proposed
in an attempt to save energy, in part by reducing anode-to-
cathode distance.
The use of titanium diboride current-conducting
elements in electrolytic cells for the production or refining
of aluminium is described in the following exemplary U.S.
patents: U.S. Pat. Nos. 2,915,442, 3,028,324, 3,215,615,
3,314,876, 3,330,756, 3,156,639, 3,274,093, and 3,400,061.
Despite the rather extensive effort expended in the past, as
indicated by these and other patents, and the potential
advantages of the use of titanium diboride as a current-
conducting element, such compositions have not been
commercially adopted on any significant scale by the aluminium
industry.
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Lack of acceptance of TiB2 or RHM current-conducting
elements of the prior art is related to their lack of stability
in service in electrolytic reduction cells. It has been
reported that such current-conducting elements fail after
relatively short periods in service. Such failure has been
associated with the penetration of the self-bonded RHM
structure by the electrolyte, and/or aluminium, thereby causing
critical weakening with consequent cracking and failure. It is
well known that liquid phases penetrating the grain boundaries
of solids can have undesirable effects. For example, RHM tiles
wherein oxygen impurities tend to segregate along grain
boundaries are susceptible to rapid attack by aluminium metal
and/or cryolite bath. Prior art techniques to combat TiB2 tile
disintegration in aluminium cells have been to use highly
refined TiB2 powder to make the tile, where commercially pure
TiB2 powder contains about 3000 ppm oxygen.
Moreover, fabrication further increases the cost of
such tiles substantially. However, no cell utilizing TiB2
tiles is known to have operated successfully for extended
periods without loss of adhesion of the tiles to the cathode,
or disintegration of the tiles. Other reasons proposed for
failure of RHM tiles and coatings have been the solubility of
the composition in molten aluminium or molten flux, or the lack
of mechanical strength and resistance to thermal shock.
Additionally, different types of TiB2 coating materials,
applied to carbon substrates, have failed due to differential
thermal expansion between the titanium diboride materials and
the carbon cathode block or chemical attack of the binder
materials. To our knowledge no prior RHM-containing materials
have been successfully operated as a commercially employed
cathode substrate because of thermal expansion mismatch,
bonding problems, chemical erosion, etc.
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Titanium diboride tiles of high purity and density
have been tested, but they generally exhibit poor thermal shock
resistance and are difficult to bond to carbon substrates
employed in conventional cells. Mechanisms of debonding are
believed to involve high stresses generated by the thermal
expansion mismatch between the titanium diboride and carbon, as
well as aluminium penetration along the interface between the
tiles and the adhesive holding the tiles in place, due to
wetting of the bottom surface of the tile by aluminium. In
addition to debonding, disintegration of even high purity tiles
may occur due to aluminium penetration of grain boundaries.
These problems, coupled with the high cost of the titanium
diboride tiles, have discouraged extensive commercial use of
titanium diboride elements in conventional electrolytic
aluminium smelting cells, and limited their use in new cell
design. To overcome the deficiencies of past attempts to
utilize Refractory Hard Materials as a surface element for
carbon cathode blocks, coating materials comprising Refractory
Hard Materials in a carbonaceous matrix have been suggested.
In U.S. Pat. Nos. 4,526,911, 4,466,996 and 4,544,469
by Boxall et al, formulations, application methods, and cells
employing TiB2/carbon cathode coating materials were disclosed.
This technology relates to spreading a mixture of Refractory
Hard Material and carbon solids with thermosetting carbonaceous
resin on the surface of a cathode block, followed by cure and
bake cycles. Improved cell operations and energy savings
result from the use of this cathode coating process in
conventionally designed commercial aluminium reduction cells.
Plant test data indicate that the energy savings attained and
the coating life are sufficient to make this technology a
commercially advantageous process.
Advantages of such composite coating formulations
over hot pressed RHM tiles include much lower cost, less
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sensitivity to thermal shock, thermal expansion compatibility
with the cathode block substrate, and less brittleness. In
addition, oxide impurities are not a problem and a good bond to
the carbon cathode block may be formed which is unaffected by
temperature fluctuations and cell shutdown and restart. Pilot
plant and operating cell short term data indicate that a
coating life of from four to six years or more may be
anticipated, depending upon coating thickness.
The aforesaid patents both teach that the baking
process should be carried out in an inert atmosphere, coke bed
or similar protective environment to prevent "excessive air
burn." In laboratory studies it is possible to bake the test
samples in a retort which maintains a high grade inert
atmosphere and excludes air/oxygen ingress, however this is not
practical for commercial use. Baking under a coke bed is
reported to give satisfactory protection for the TiB2/carbon
composite material.
Composite coatings have been tested in plants using
full scale aluminium reduction cells (U. S. Pat. No. 4,624,766;
Light Metals 1984, pp 573-588; A.V. Cooke et al., "Methods of
Producing TiB2/Carbon Composites for Aluminium Cell Cathodes",
Proceedings 17th Biennial Conference on Carbon, Lexington,
Kentucky (1985)). After curing, the coating is quite hard and
the coated blocks may be stored indefinitely until baking. For
baking, the coated blocks were placed in steel containers,
covered with a protective coke bed, and baked using existing
plant equipment such as homogenizing furnaces. Once baked, the
blocks could be handled without further precautions during cell
reline procedures. The integrity of the cured coating and
substrate bond remained excellent after baking. No changes in
cell start-up procedure were required for using the blocks
coated with composite TiB2 material. No difficulties were
encountered when the coated cathode cells were started-up using
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either a conventional coke resistror bake or hot metal start-up
procedure. Core samples from the test cells demonstrated areas
of good coating condition after 109 and 310 days of service in
the operating cell, but performance was non-uniform.
Extensive testing of TiB2/carbon composite materials
have been performed in both laboratory and plant tests. The
improved laboratory tests and more detailed cell autopsies have
shown a variability in material performance not observed in
previously reported tests. X-Ray Diffraction (XRD) analysis
was used to measure the trace impurities in the test samples.
It was discovered that the poor performance of a test material
had a direct correlation with the presence of oxidation
products of Ti and B such as Ti0 and/or TiB03, within the
structure of the material. A similar variation was detected in
the RHM coating applied to a carbon cathode.
Laboratory tests demonstrated that none of the
conventional methods (e. g. coke bed, inert gas, liquid metal,
boron oxide coating on anodes) for preventing/controlling
carbon oxidation was adequate to prevent the formation of TiB03
or similar oxidation products during the bake operation and/or
the cell start-up.
In addition to the above described problems
associated with RHM cathodes, the start-up phase of operation
of conventional cells can also result in oxidation damage
leading to reduced operational life, and the present invention
is not therefore limited to cells have RHM cathodes.
Brief Description of Invention and Objects:
It is the primary object of the present invention to
provide a method of protecting aluminium smelter cathodes
against deterioration in use, and more specifically to provide
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an improved start-up procedure by means of which the life of
aluminium smelter cell cathodes may be extended.
In its broadest form, the invention provides an
improved start-up procedure for aluminium smelting cells
characterized by the creation or establishment of conditions
which reduce the formation of oxides from external oxidant
sources in cathode materials during the start-up period of the
cell. This reduction in the formation of oxides will result in
cathode materials having superior longevity when compared with
Refractory Hard Materials and other cathode materials which
have not been similarly protected against the development of
oxide products.
In one currently preferred form of the invention, the
desired conditions are established in the smelting cell by the
formation of a barrier which is liquid or molten during the
start-up temperatures above about 400°C, which is in intimate
contact with the exposed surfaces of the cathode, which is
stable and effective at temperatures up to about 1000°C and
which is substantially impervious to oxygen throughout the
start-up period of the cell.
One of the major advantages of the use of a barrier
which is liquid or molten is that it allows outgassing from the
refractory material during the start up procedure while
preventing the return of such gases or other oxidants to the
cathode material. This would not be the case where say a
gaseous barrier is present since the outgasses and other
oxidants may readily mix with the barrier gas and will
therefore be free to react with the cathode material.
The barrier may be formed of two materials, one which
is effective up to one temperature and the other effective from
said one temperature to temperatures up to about 1000°C.
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In one form of the invention, this is achieved by the
use of boron oxide (B203), which melts at about 450°-470°C or
lower due to impurities, or some other suitable material which
is liquid or molten at temperatures above about 400°C, which is
substantially impervious to oxygen transport and which wets
carbon. This material provides_a barrier which substantially
prevents the Refractory Hard Materials (or other cathode
materials) of the cathode from being oxide contaminated. At
temperatures above about 650°-700°C at which the boron oxide
material is likely to be less effective, aluminium pellets or
the like which are added to the cell with the boron oxide and
form a molten aluminium barrier which functions during start-up
until the cell starts producing aluminium which functions as a
barrier for the remainder of the operating life of the cell.
Thus, by establishing a substantially oxygen impermeable
barrier which essentially prevents formation of oxides during
the start-up period, the cathode of the cell is protected
against subsequent damage of the type outlined above.
The boron oxide can be used directly or alternatively
can be formed in situ by controlled oxidation of TiB2
containing material such as the refractory hard material
coating or a commercially available product such as GraphiCoat.
In another aspect, the invention provides a method of
reducing the development of oxidation products in Refractory
Hard Material or other cathodes during the cell start-up
procedure, comprising the step of adding to the cell at least
one material which is liquid or molten at temperatures above
about 400°C and which is stable at temperatures up to about
1000°C, which covers the cathode of the cell and thereby forms a
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barrier to oxygen, and which does not materially affect the
operation of the cell.
In one preferred form, the method includes adding
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first material which is liquid or molten at temperatures above
about 400°C and which is substantially impervious to oxygen
transport, as well as a second material which is liquid or
molten at temperatures above about 600°C and which forms a
substantially impervious barrier to oxygen transport.
While a currently preferred first material is boron oxide
(B203), other materials which are liquid or molten at about
400°C and which form a carbon wetting film substantially
impervious to oxygen at temperatures above 400°C may be used.
For example, materials such as mixtures of chloride or
fluoride salts or liguid melts such as lead tin alloys may be
used, although they are currently considered to be less
practical than boron oxide. The boron oxide can be used
directly or alternatively can be formed in situ by controlled
oxidation of a TiB2 containing material such as the refractory
hard material coating or a commercially available product such
as Graphi-Coat*. While use of this alternative method may
result in an outer skin of oxide contaminated RHM, this skin
may be regarded as a sacrificial layer which an operator is
willing to lose in return for a protection system which is
less complex and costly to operate. The effectiveness of this
alternative protection method will be dependent on the
porosity of the ref ractory hard material with lower porosities
giving better results.
Clearly the most preferable second material, for
practical reasons, is aluminium metal since this is present in
* Trade-mark
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the cell in any event. However, other metals or compounds,
which are fluid at about 600°C and above, which completely
cover the carbon to create a substantially impervious barrier
to oxygen transport may be used.
In the post-start-up phase of operation of the cell, it
may be necessary or desirable to remove the viscous boron
oxide layer, or other viscous layer derived from the boron
oxide coating, which adhere to the surface of the cathode.
While this removal may be achieved in a number of ways, such
as flushing the cell with fresh metal to physically remove
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the layer, it is presently preferred to remove the layer
chemically by converting the boron oxide into a more innocuous
boron-containing phase such as by contacting the boron oxide
phase with a Ti-containing species, leading to the
precipitation of TiB2. For example, Ti-bearing additions such
as Ti02 may be added to the electrolyte or Ti-A1 alloys may be
added to the metal. Other transition metal species in the
fourth to sixth groups of the periodic system which are able to
form borides from the boron oxide layer may also be used with
acceptable results, such as Zr, Hf, V, Nb, Ta, Cr, Mo and W.
Description of Preferred Embodiments:
In the following description, the conditions under
which RHM material can be heated above 400°C without degrading
its consistency and service life in an aluminium cell will be
outlined in greater detail. Two types of TiB2/carbon composite
materials were evaluated in laboratory and plant exposure tests
to determine their uniformity and service life when used to
form an aluminium wetted cathode surface for the electrolyte
winning of aluminium from a molten cryolite based bath. The
cathode coating material was formulated, mixed, applied to the
cathode block top surface and cured as taught in U.S. Pat. No.
4,526,911 to Boxall et al. The cured coating blocks were then
baked under a fluid coke bed as described by Boxall et al. A
nitrogen purge was maintained through the metal box containing
the coated blocks and fluid coke to prevent any ingress of air
during the bake procedure. After cooling to less than 200°C,
the baked coated blocks were removed from the coke bed. Normal
cell construction procedures were used to construct a
conventional pre-bake cathode using the coated blocks.
The cathode tiles were moulded, cured and baked as
taught in U.S. Pat. No. 4,582,553 by Buchta. A fluid coke bed
with a nitrogen purge was used to protect the tiles from
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"excessive air burn". The tiles were attached to the top of
the cathode blocks in a conventionally rammed cathode using
UCAR C-34 cement as described by Buchta.
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A conventional resistor coke bed start-up procedure was
used to heat the coated lined cathode cell up to about
900°-950°C before fluxing with molten bath transferred from
other cells in the potline. The test cells were operated as
regular cells for approximately 6 weeks before the shut down
for autopsy. Most of the bath and metal were tapped from the
cell during the shutdown procedure. After cooling, the
remaining bath and metal were removed from the cathode surface
to expose the coated tiled surface. Visual inspection and
photographs of the cathode surface were used to evaluate the
condition of the exposed cathode coating tiles. Core samples
were taken for metallurgical and chemical analysis.
The seven day laboratory exposure test was performed in a
Hollingshead cell comprising an inconel pot, a graphite
crucible, a variable height graphite stirrer driven by a 60
r.p.m. geared motor and insulating lid of pyrocrete.
Test samples of T182/C composite were glued to the bottom
of the crucible with UCAR C-34* cement and were coated with
boron oxide paste. Samples were then buried in synthetic
cryolite (2kg) and about 2kg of aluminium metal granules were
placed on top. The temperature was raised at 40°/hr to 980°C
and the stirrer was immersed so that it mixed both metal and
bath. After seven days operation at 980°C, the graphite
crucible and contents were allowed to cool and then cross
sectioned to enable visual and chemical analysis of the test
samples. Test results confirmed that this long term dynamic
* Trade-mark
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exposure test can be used to screen RHM cathode materials,
glues, formulations and baking rates in the laboratory prior
to their use in industrial scale cells.
The following TiH2 composite failure mechanisms observed
in the industrial cells were reproduced in the test cell:
(a) delamination cracking of tiles and coatings
(b) complete debonding of tiles due to stresses set up
by sodium swellingf
(c) partial debonding of tiles due to chemical attack of
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the glue, and
(d) deformation of tiles.
Furthermore, the dynamic exposure testing of TiB2
composite materials also confirmed the following observations
made during cell autopsies and laboratory investigations.
~ glued joints between tiles and cathode block are
subject to chemical attack;
~ coating produced and baked under laboratory
conditions performs much better than that produced
and baked in the plant;
~ order of rank of laboratory performance is coated
anthracite block > coated MLI block > tiled
anthracite block > tiled graphite block;
~ structural integrity of the laboratory baked coatings
is better than the laboratory baked tiles and much
better than the plant baked coatings;
~ the bonding interface between coating and anthracite
block is at least as resistant to bath and sodium as
the coating itself.
A large variation in coating/tile quality was found
on the cathode surface of the autopsied test cells. There
appeared to be a random distribution of good, poor and missing
coating/tile areas over the cathode surface. The presence of
well bonded undeformed areas of coating/tile demonstrated that
the material could survive the aluminium cell environment
provided a more consistent material could be produced.
No correlation between the material test results and
the mixing, spreading, moulding and curing process parameters
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could be established to explain the variability observed in the
plant tests.
It was discovered that the condition of the exposed
coating/tile material was related to the presence of oxides of
titanium, including mixed oxides, in the material, the oxide
content being determined using known X-Ray Diffraction (XRD)
analysis.
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TABLE 1
TiB2/Carbon Composite Baking Tests
Test Protection Where Oxides of Titanium
Sample Systems Baked Relative XRD
Peak Height
Coatings
BN1 Coke bed Lab 10
BN1 B203 only Lab 6
BN1 B203 only Lab 5
BN1 A1 powder Lab 10
BN1 B203 + A1 Lab 1
BN1 Graphicoat Lab 6
BNl TiB2/C icing Lab 5
BN1 B203 Lab 7
BN1 Graphicoat Lab 5
BN1 TiB2/C icing Lab 7.5
BN1-2C Coke bed Plant-28/5/87 4
BN1-4C " " " " 10
BNl-6C " " " " 4
BN1-7C " " " " 10
BN1-8C " " " " 24
BN1-1C B203 + A1 Plant-4/8/87 1
BN1-3C " " " " 2
BN1-6C " " " " 2
Pitch Bonded Coke bed + Ar Lab 34
Pitch Bonded Coke bed + Ar Lab 34
BM1 Graphi-Coat + A1 Plant Test 2
BM1 TiB2/C icing + A1 Plant Test 2
Cast Tiles
BR7 Coke bed + Ar Lab 6
BR7 Coke bed " 8
BR7 B203 only " 5
BR7 B203 + A1 " 2
The preferred B203/A1 protection system was found to
provide the best results, although the use of a sacrificial
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layer or coating, such as Graphi-Coat or TiB2/C icing, in lieu
of the B203 component also produced acceptable results.
By preventing this low level oxidation of the TiB2,
the composite structure remains intact and a long service life
is maintained.
The appreciable oxidation of TiB2 evident during
unprotected start-up was not anticipated since data sheets for
TiB2 indicate a high resistance to air oxidation at
temperatures up to 1100°C (ICD Group Inc., New York, NY,
technical bulletin dated 10/79). Based on this data, the prior
art use of a coke bed to prevent air burn of the carbon matrix
and the carbon matrix itself was relied upon to provide
adequate oxidations protection for the TiB2.
The data in Table 1 show that the conventional
methods for protecting carbon from air burn are inadequate and
that an unexpected synergism was found when a combination of
B2O3 (or a suitable 'sacrificial' layer) plus Al was used to
protect the TiB2 material.
According to one practical embodiment, the B203/A1
protection system and cell start up procedure according to one
embodiment is as follows:
1. B203 powder is evenly distributed over the cured
composite surface of the cathode. About 30 kgs was
used in the 100 K ampere test cell. For difficult or
vertical surfaces a H3B03 powder added to water to
form a viscous paste is used.
2. Cover the B203 with aluminium foil to protect the
powder against disturbance during subsequent
operation. Overlapping strips of 1200mm wide heavy
duty foil has been found to be sufficient.
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3. Cover the foil with aluminium "pellets". The amount
should be calculated to provide at least 20mm of
molten metal over the highest part of the cathode.
About 4 tonnes of pellets was found sufficient for
the 100 K ampere test cell.
4. Baking is carried out by directing oil fired burners
between the anodes and the pellets, and heating at a
rate of about 50°C/hr. After the aluminium has
melted, the anodes can be lowered, current applied
and the baking process continued.
It will be evident from the above discussion that the
improved start-up procedure embodying the invention provides
the following advantages over the prior art practices:
1. Provides improved protection for materials from
oxidation damage at temperatures in excess of 400°C.
2. Provides low oxygen activity environment required to
prevent oxidation of RHM and RHM containing
composites when heated above 400°C.
3. Provides a quality control test for vendor supplied
RHM composite articles (XRD analysis procedure for
critical oxide impurities).
4. Improves reliability, uniformity and service life for
RHM type cathodes.
5. Enables the use of RHM cathode materials which were
previously unacceptable due to poor service life.
The above described start-up procedure leaves a
viscous boron oxide layer, or other layer derived from the
boron oxide coating, on the surface of the cathode. The
continued presence of the viscous boron oxide layer prevents a
sloping cathode cell from operating in its desired manner.
That is, the aluminium metal is restricted from draining to the
metal sump. Other operational difficulties may also occur, as
described elsewhere (E.N. KARNAUKIIOV et al, Soviet Journal of
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Non-Ferrous Metals Research, English version Vol. 6 No. 1 1978,
p. 16). Our own experience has shown that metal pooling may
occur on the cathode surface, leading to uneven anode burning
and/or short-circuiting, low current efficiency and general
cell instability. The transition from start-up conditions to
normal stable cell operation may therefore become problematic
unless the boron oxide layer can be effectively removed at the
end of the start-up phase. We have found that the
establishment of stable operating conditions can be
accomplished more effectively by accelerating the rate of
removal of the boron oxide. A number of methods have been
found successful for achieving this removal. For instance, by
flushing the cell with fresh metal the removal of the boron
oxide has been promoted. However, the transferring of large
volumes of molten metal into and out of the cell, whilst
effective, is inconvenient.
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hazardous and undesirable.
We have discovered that the removal of boron oxide can be
most conveniently facilitated by the chemical conversion in
situ to a separate and more innocuous boron-containing phase
that does not interfere with the draining of the cathode metal
to the sump. Hy contacting the B203 phase with a Ti-
containing species, chemical interaction between Ti and B is
achieved leading to the conversion of H203 to TiH2 and the
precipitation thereof. Importantly this chemical conversion
process provides for the removal of the potentially
problematic boron oxide viscous phase, which in turn allows
for a rapid transition to stable and efficient drained cathode
cell operation, as evidenced by normal bath temperatures and
the uninterrupted filling of the metal sump at a rate
consistent with the expected metal production rate.
Alternatively, it may be possible to use Ti in the form
of an alloy of aluminium (e. g. T1-A1) to provide close contact
between the H and Ti species, respectively. Ti-A1 alloys are
a preferred form of Ti addition since they are readily
available as master alloys in the aluminium foundry industry.
Furthermore, it is well known in aluminium foundry practice
(eg. AU 21393/83 issued to Alcan International Ltd., entitled
"Removal of Impurities from Molten Aluminium" published on May
24, 1994) that the removal of metal impurities from molten
aluminium can be achieved in a straightforward manner by
contacting molten aluminium with a boron-containing material,
thus leading to the generation of insoluble metal borides (eg.
(Ti, V) B2). The formation and deposition of TiH2 is
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therefore readily accomplished. However, the use of T1-A1
alloys for the removal of viscous boron-containing layers on
the cathode surface, by the chemical conversion to another
phase, has not been previously demonstrated.
While the use of T1 species is preferred for the above
reasons, any RHM species, such as the metals in the fourth to
sixth groups of the periodic system (Ti, Zr, Hf, V, Nb, Ta,
Cr, Mo and W), which can form borides from the boron oxide
layer may be used with acceptable results.
In one preferred form of the process, Ti-bearing
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additions, or other RHM boride forming species, such as those
mentioned above, may be made directly to the electrolyte.
Cryolite electrolytes are good solvents for oxide ores, so a
convenient form of the Ti-containing species is as Ti02,
although other additives may also be employed. The Ti-
containing species reacts with the B203 to form at least TiB2
precipitate, although other equally acceptable precipitates may
form.
In each of the above cases, an aluminium-RHM diboride
alloy phase is formed on the cathode surface, and this may
offer additional restorative and other benefits to the cathode
surface .
In laboratory tests it was observed that a 1.8758
addition to the bath of Ti02 effectively removed a 0.9758 layer
of B203 originally located at the interface between the
composite and the metal (ie. no B203 could be detected at the
interface by either visual or chemical microprobe methods).
The mass of Ti02 was chosen to be in excess of that needed for
stoichiometric conversion of TiB2 to ensure that all the B203
was removed. The mass ratio of Ti/B in TiB2 is 2.218:1 and the
mass ratio of Ti/B actually used was 3.71:1, which equates to a
Ti mass excess of 67~. Thus a Ti02/B203 mass ratio of
1.875/0.975 = 1.92 (ie. ~ 2) is effective for removing the B203
layer at the cathode surface.
The TiB2 precipitate is formed as randomly
distributed and irregularly shaped fine particles ranging in
size from less than 1 ~,m to about 10 Vim. These particles
sometimes aggregate as clusters consisting of from 3 or 4 to 30
or 40 particles. Because of the much higher density of TiB2
compared to Al (ie. 4.5g/cm3 vs 2.3g/cm3) the TiB2 has been
observed to form a sediment on the cathode surface and may
therefore provide restorative and other benefits for cathodes
containing RHM, such as TiB2 (eg. reduces solubility of the
CA 02010316 2000-O1-13
75626-3
- 17 -
RHM). Similar comments apply equally to the other RHM boride
forming species referred to above.
The above described post-start-up operations provide
the means for enhancing the removal of a major portion of the
boron oxide phase that is potentially disruptive to normal cell
operation. The enhanced rate of removal facilitates the smooth
transition from the start-up phase in which the boron oxide
layer performs a useful protective function-to cell operation.
