Note: Descriptions are shown in the official language in which they were submitted.
~L23378~L
-- 1 --
ALUMINUM WETTABLE MATERIALS
Technical Fie'd of the Invention
This invention relates to electrolytic cells for
the electrowinning of molten aluminum from alumina dis-
solved in molten cryolite contained within the cell. More
specifically this invention relates to components immersed
in molten aluminum within the cell, and where these
components are fabricated from a material substantially
non wettable by the molten aluminum, to methods for making
these components aluminum wettable.
Background of the Invention
--
Aluminum is commonly produced by electrowinning
aluminum from A12O3 (alumina) at about 900C to 1,000C.
Aluminum oxide being electrowon frequently is dissolved
in molten Na3AlF6 tcryolite) that generally contains
other additives helpful to the electrowinning process
such as CaF2, AlF3 and possibly LiF or MgF2.
In one popular configuration for these electrolytic
aluminum cells, anode and cathode are arranged in vertically
spaced configuration within the cell, the anode being
uppermost. Reduction of aluminum oxide to aluminum
occurs at the cathode which customarily is positioned at
the bottom or floor of the cell. Oxygen is disassociated
from A12O3, in most commerical cells combining with
carbonaceous material comprising the cell anode and being
emitted from the cell as CO and CO2.
Cryolite is an aggressive chemical necessitating
use of a cathode material substantially resistant to this
aggressive cryolite. One popular choice is the use of
molten aluminum as a cathode. While use of other cathodes
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~a~337~
such as bare graphite in contact with cryolite has been
contemplated, formation of undesirable by-products such
as aluminum carbide has discouraged use. In many commerical
cells, this cathode often covers substantially the entire
floor of the cell which typically can be 6 feet wide by
18 or more feet in length.
In utilizing aluminum for cathode purposes in a
cell, typically the cathode is included in an assembly of
a cathodic current feeder covered by a pool of aluminum
ranging in depth, depending upon the cell, from a few
inches to in excess of a foot, but generally about 6
inches. The aluminum pool functions effectively as a
cathode and also serves to protect current feeders made
from materials less than fully resistant to cell contentsO
These aluminum pool type cell cathode assemblies
contain conductive current collectors. Where these
conductive current collectors are utilized in certain
cell configurations, these collectors contribute to an
electrical current flow within the cell that is not per-
pendicular to the cell bottom. These nonperpendicular
electrical currents can interact with strong magnetic
fields established around cells by current flow through
busses and the like to contribute to strong electromagnetic
fluxes within the cell~
In cell employing a pool of aluminum covering the
cathode floor of the cell, the cryolite, containing A12O3
to be electrolyzed, floats atop this aluminum pool. The
cell anodes are immersed in this cryolite layer.
It is important that these anodes do not contact
the aluminum pool, for such contact would result in a
somewhat dysfunctional short circuit within the cell. The
electromagnetic flux within the cell arising from the
interaction of nonperpendicular electrical currents with
an electromagnetic field surrounding the cell contributes
to the formation of wave motion within the aluminum pool
~L2~3371EI~
contained in the cell, making prediction of the exact
depth of the aluminum pool somewhat imprecise. Therefore,
prediction of the minimum necessary spacing between the
anode and cathode current feeder and between the anode
and the interface between molten aluminum and molten
cryolite at any particular cell location is somewhat
imprecise. Consequentially, cell anodes are generally
positioned within the cryolite substantially above the
normal or expected level of the interface between molten
cryolite and molten aluminum within the cell. Usually,
a spacing of 1-1/2 to 2-1/2 inches is utilized.
The combination of a substantial aluminum pool
depth susceptible to wave motion and a positioning of
the anodes substantially above the cryolite-aluminum
normal interface position to forestall short circuits
caused, for example, by wave motion in the aluminum
establishes a substantial gap between the anode and cathode
in most conventional cells. Electrical power consumed in
operation of the cell is somewhat proportional to the
magnitude of this gap. Substantial reductions in the
anode-cathode spacing would result in considerable cost
savings via reduced cell electrical power consumption during
operation. Additionally, where the thickness of aluminum
in the pool could be reduced while reliably maintaining a
molten alumimum cover upon the cathodic current feeder,
considerable aluminum inventory savings would be realized.
One proposal to reduce spacing between anode and
cathode has been to employ so-called "drained cathodes"
in constructing aluminum electrolysis cells. In such
cells, no pool of aluminum is maintained upon a cathode
current feeder to function as a cathode; electrowon
aluminum drains from the cathode at the bottom of the
cell to be recovered from a collection area. In drained
cathode cells, without wave action attendant to a molten
aluminum pool, the anode and the cathode may be quite
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~.~337~
closely arranged, realizing significant electrical power
savings.
In these drained cathode cells, the cathode parti-
cularly where non-wettable by molten aluminum, is in
generally continuous contact with molten cryolite. This
aggressive material, in contact with a graphite or carbon
cathode, can contribute to material loss from the cathode
and can trigger formation of such undesirables as aluminum
carbide. Particularly carbon or graphite for use as a
drained cathode material of construction is therefore of
quite limited utility due to possible service life con-
straints and carbide contaminant formation.
Other longer lived materials are, in theory, available
for use ln a drained cathode. Generally, these materials
are both conductive and aluminum wettable refractory
materials such as TiB2. It has been found that unless
TiB2 and similar materials are in essentially pure form,
they too lose material or corrode at unacceptable rates
in the aggressive cell environment.
It is believed that the molten cryolite can con-
tribute to TiB2 corrosion by fluxing reaction products
of a reaction between impure TiB2 and aluminum, particularly
near grain boundaries of the mateiral. While it is known
that aluminum electrowinning cells utilizing essentially
pure TiB2 do not exhibit as substantial a corrosion suscept-
ibility as do those employing lower purity TiB2, cost and
availability factors seriously limit the use of TiB2
sufficiently pure to withstand an aggressive aluminum cell
environment.
In another proposal, a particular cathodic current
feeder configuration has been utilized to reduce signifi-
cantly non-perpendicular current flow within the cell,
thereby reducing wave motion. These solutions have not
proven wholly satisfactory however.
Conventionally, most cells employ construction
:1~337~i
materials that are either wettable by molten aluminum, are
relatively inert to the corrosive effects of cxyolite or
both. Where a substance is not readily wetted by molten
aluminum, even though immersed in molten aluminum the
substance may contact cryolite present at the interface
between the substance and the molten aluminum due to poor
wetting. Where the substance is significantly soluble
in cryolite, or corroded by cryolite, substantial material
losses to the substrate therefore can occurO
However, substrates substantially wettable by molten
aluminum tend, while immersed in the molten aluminum, to
be protected from the deleterious effects of contact with
molten cryolite. A sheathing effect by the molten alum-
inum protects the substance.
Aluminum wettable substances such as refractory
TiB2 have therefore been suggested for constructing
components of cells which are to be immersed in molten
aluminum. Conversely it has been found relatively less
acceptable to employ aluminum nonwettable materials,
particularly those such as alumina which are subject to
attack/dissolution by molten cryolite, for fabrication
of cell components. This reluctance may be greater where
dimensional stability of the component is relatively
important, for example in the fabrication of electrical
current feeders, weirs, sidewalls, and the like.
Were techni~ues available for rendering aluminum
nonwettable substrates wettable by molten aluminum,
these structures could then be utilized within aluminum
electrowinning cells, immersed in molten aluminum contained
in the cell to preclude attack/dissolution by molten
cryolite present in the cell. Where these normally non-
wettable substrates are relatively inexpensive, their
potential use in the electrolytic cell becomes quite
attractive.
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Disclosure of the Invention
.
The present invention provides a method for making
substrates used in, or components of an aluminum electro-
winning cell substantially wettable and thereby at leastpartially filled where porous by molten aluminum where
those components or substrates normally would not be
aluminum wetted in the environment of the cell. Used in
the electrowinning cell, these wettable components, when
immersed in molten aluminum contained in the cell are
stable in the cell environment even where the materials
from which the substrates were fabricated would otherwise
be subject to aggressive attack by materials such as
cryolite contained in the cell.
Substrates are made wettable by molten aluminum
by applying to the substrate a coating of wetting agent
and a solubility suppressor prior to or while the substrate
is immersed in molten aluminum, and molten aluminum is
; maintained near saturation with the wetting agent and
solubility suppressor by introducing the wetting agent
and the solubility suppressor into the molten aluminum
to maintain desired levels in the molten aluminum.
The coating of wetting agent and solubility sup
pressor applied to the substrate is preferably quite thin.
The coating need not be continuous.
The method preferably is utilized to make refractory
materials commonly non aluminum wettable amenable to
wetting by aluminum. Once aluminum wettable, these
refractory materials can be utilized for a variety of
purposes within an aluminum electrowinning cell including
weirs, current feeders, packing, baffles, structural
components and the like.
Thus, in accordance with the present teachings, an
improvement is provided in a method of operating an
electrolytic cell to produce aluminum by electrolyzing a
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~233~8~
molten cryolite electrolyte containing aluminum oxide by
means of direct current passing between the anode surfaces
immersed in the cryolite electrolyte and cathode surfaces
associated with a pool of molten aluminum underneath the
cryolite electrolyte. The improvement which is provided
comprises utilizing in the region of the cell below the
cryolite electrolyte, structural parts and/or functional
components formed from a normally aluminum non-wettable
refractory substrate which has a thin coating formed there-
on containing a metallic wetting agent selected from tit-
anium, hafnium or zirconium and as solubility suppressor,
boron, and providing the wetting agent and boron to the
molten aluminum pool in substantially saturating amounts.
In accordance with a further teaching, an improve-
ment is provided in an electrolytic process for electro-
winning aluminum metal from aluminum oxide in a solution
of molten cryolite electrolyte floating on a pool of molten
aluminum. The improvement which is provided comprises
providing therein in contact with the molten aluminum at
least one structural or functional component which is
formed from a normally aluminum non-wettable refractory
material having deposited on the surface thereof a thin
coating containing a metallic element wettable by aluminum
and chosen from the group consisting of titanium, zirconium
and hafnium and, as a solubility suppressor for the metallic
element, boron, and separately providing in the molten
aluminum pool nearly saturating proportions of boron and
metallic element.
By yet a further aspect, a molten aluminum-wettable
component is provided for use in an electrolytic cell for
winning aluminum from a molten cryolite bath containing
aluminum oxide, comprising an aluminum non-wettable sub-
strate of desired configuration fabricated from a refract-
ory compound chosen from the group consisting of alumina,
aluminum nitride, AlON, SiAlON, boron nitride, silicon
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-- 8 --
carbides, silicon nitrides, aluminum borides, alkaline
earth metal zirconates and aluminates and mixtures thereof,
which substrate has an adherent surface coating of titanium,
zirconium or hafnium borides between about 5 Angstroms and
100 microns in thickness.
The above and other features and advantages of the
invention will be more apparent from the description of
the preferred embodiment when considered in conjunction
with the accompanying drawings forming a part of the spec-
ification.
Description of the Drawings
Fig. 1 is a cross sectional representataion of an
aluminum electrowinning cell.
Best Embodiments of the Invention
Referring to the drawings, Fig. 1 shows in cross
section a representation of an aluminum electrowinningcell 10. The cell includes a base 14 and sidewalls 16,
18, generally of steel, surrounding the cell. The cell
includes a cathodic current feeder 20 and anodes 22, 24.
The base and sidewalls enclose the cathodic current
feeder 20 which in this best embodiment functions also
as a cell liner. Portions 26 of the liner define a floor
of the cell. Well known refractory materials and graphite
are suitable for fabricating this current feeder 20, as
are other suitable or conventional materials. A current
buss 28, embedded in the feeder 20 provides electrical
current for distribution within the cell 10. The buss
28 is connected to an external source of electrical
current (not shown).
The anodes 22, 24 are arranged in vertically spaced
relationship with the current feeder portions 26 defining
~;~337~31
g
the floor of the cell. The anodes 22, 24 are separated
from the cathodic current feeder by two pools 30, 32 of
molten material. One pool 30 comprises essentially molten
aluminum. This molten aluminum pool functions as a cathode
for electrowinning of aluminum within the cell. While
the pool consists essentially of molten aluminum, impurities
customarily assocated with aluminum produced electrolytically
may be present.
The remaining pool 32 is comprised of molten cyrolite,
Na3AlF6, containing dissolved Al2O3. A number of cryolite
formulations that include additives such as CaF2, LiF,
and AlF3 for enhancing electrolysis of the Al2O3 to
aluminum are possible and are contemplated as being utilized
within the scope of the invention. This cryolite layer,
being less dense than the molten aluminum, floats upon
the aluminum. An interface 36 separates the molten
aluminum 30 from the molten cryolite 32.
An insulating layer 39 is provided to resist heat
flow from the cell 10. While a variety of well-known
structures are available for making this insulating
strucutre, often the insulating layer 39 consists of
crystallized contents of the electrolytic cellO
The anodes 22, 24 are fabricated from any suitable
or conventional material and immersed in a cryolite phase
32 contained in the cell. Since oxygen is released in
some form at the anode, the anode material must be either
resistant to attack by oxygen or should be made of a
material that can be agreeably reacted with the evolving
oxygen, preferably producing a lower anode half cell
voltage by virtue of reactive depolarization. Typically,
carbon or graphite is utilized providing a depolarized
anode. The anodes 22, 24 should be arranged for vertical
movement within the cell so that a desired spacing can
be maintained between the anode and cathode notwithstanding
the anode being consumed by evolving oxygen.
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In this best embodiment a packed bed 41 of loose
elements 42 is positioned in the cell, in the molten
aluminum pool 30O These elements are formed of a substance
substantially non-wettable by alumium. The elements are
maintained in the molten aluminum at a level at or below
the interface 36 between the molten aluminum and molten
cryolite, the depth to which the elements are packed being
substantially uniform across the cell. In general, the
elements should be not further than 5 centimeters from the
interface, but should not extend substantially above the
interface, particularly where the elements 42 may be sub-
ject to aggressive attack by the cryolite.
The packed bed elements can be of any shape. It
is preferred that the shapes provide, when packed, inter-
stices through the packed bed whereby aluminum can fillgaps in the packing to maintain uniform electrical con-
ductivity through the packed pool of aluminum. Particularly,
packing in the formed of berl saddles, Raschig rings,
Intalox saddles, and equiaxed shapes such as cylinders and
spheres are much preferred; however randomly shaped packing,
blocks or bricks may be utilized.
The packing is fabricated from a material substan-
tially non-wettable by molten aluminum, preferably porous,
with alumina, A12O3, being much preferred. Since alumina
is soluble in the molten cryolite, and since aluminum is
being electrolyzed within the cell from alumina dissolved
in the cryolite layer 32, it is important that the alumina
packing be maintained reliably covered with alumina to
prevent consumption of the packing. This is conveniently
accomplished by maintaining the packing virtually continuously
beneath the interface; when the packing is non-wettable by
aluminum, a substantially aluminum thickness is required
to assure non-contact with cryolite. However should
portions of the aluminum non-wettable packing protrude from
the molten aluminum pool but be coated with molten aluminum,
~2~33~
the packing would thereby be protected. A coating of molten
aluminum shields the packing elements from aggressive
attack by the cryolite.
Shielding can be accomplished by making the normally
aluminum non-wettable packing wettable by molten aluminum
at operating temperatures within the cell. Wettability is
accomplished by providing the otherwise nonwettable packing
with a surface coating of a wetting agent and a solubility
suppressor for the wetting agent. This coating can include
any of a variety of elemental materials known for making
aluminum non-wettable materials wettable by aluminum. As
wetting agent Zr, Hf, Si, Mg, B, Cr, Nb, Ca and Ti are
suitable with Ti being substantially preferred in the
pxactice of the invention.
Elements substantially suppressing the solubility
of the wetting agents in molten aluminum are suitable for
use as solubility suppressors. Typically boron, carbon
and nitrogen are useful with boron being much preferred.
In this best embodiment the coating applied then is TiB2,
butthe practice detailed applies equally to other wetting
agents and solubility suppressors.
The surface coating can be applied to the packing
by a variety of methods. For example the packing can
be soaked in a slurry of titanium hydride and amorphous,
powdered boron in polyvinyl alcohol, and then fired at
800-15C¢C for 1 to 25 hours.
Alternatively titanium can be applied by electroless
metallization techniques in a fused salt bath. The tit-
anium coated packing is then packed in boron powder for
1 to 25 hours at 800 to 1200C. As an alternate to electro-
less metalliding, the titanium may be sputtered onto the
packing. In lieu of the sputtering of titanium onto thepacking,
boridization in boron powder may be eliminated by sputtering
TiB2 directly onto the packing.
TiB2 may also be applied directly to the packing
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by vapor depositionO Alternately a slurry of TiO2 and
B2O3 may be spray applied to the surface and reduced.
The packing can be soaked in aluminum containing
titanium and boron for 1 to 2 weeks to apply the cotaing.
Titanium may be present as Ti, TiO2 or TiB2 for example,
while boron may be present as B2O3,B for example. Where
the packing has been molded from a refractory material
such as alumina, titanium and boron compounds such as
TiO2 and B203 or TiB~ may be molded with the packing. Upon
heating, the boron and titanium will tend to migrate to
the surface of the packing to provide the desired coating.
Wetting of alumina or other suitable substrate can be
achieved using this procedure of soaking in aluminum con-
taining wetting agent and solubility suppressor outside
of the aluminum electrowinning cell, in which case the
coated wettable packing is transferred to the cell. Altern-
atively, the packing or substrate can be made wettable in-
situ by soaking in aluminum containing wetting agent and
solubility suppressor in the aluminum electrowinning cell.
The coating can be produced in-situ through the
aluminothermic reduction of titanium oxide and boron oxide
coatings on alumina or other substrates. The formation
of a surface coating of TiB2 combined with alumina results
through this in-situ reaction and wettability by aluminum
is achieved. If desired, this in-situ reaction coating
can be done by contact with molten aluminum in the electro-
winning cell.
An average coating thickness of between 5.0 angstroms
and 100 microns is preferable, with coatings in excess of
about 10 an~stroms being much preferred. The coating need
not be continuous; continuous coatings delivering only
marginally superior wettability over noncontinuous coatingsO
It should be noted that the inclusion of the wetting
agent and solubility suppressor is intended to produce a
surface effect only. Total inclusion of substances such
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as TiB2 generally will not exceed about 5% and usually
substantially below 1% by weight. Unless the substrate
being coated is electrically conductive, the coated sub-
strate remains relatively el~ctrically non-conductive.
It is believed that the TiB2 coating permits
virtually instantaneous wetting of the substrate. It is
further believed that the TiB2 coating functions to provide
a surfactant permitting molten aluminum to penetrate pores
of a coated structure. A partially aluminum filled porous
structure surface results, having advantageous physical
characteristics over a mere wetted surface. Ti and B dis-
solving from the surface coating penetrate the pores with
the molten aluminum, permitting in surfactant fashion the
passage of molten aluminum into pores otherwise inaccessible
to the molten aluminum by reason of surface tension. To
achieve this result, both the substrate surface and the
TiB2 coating should be porous, permitting infiltration
into substrate pores.
Titanium and boron present in the coating are, to-
gether marginally soluble in molten aluminum. Once immersed
in molten aluminum, the coating therefore tends to dissolve
into the molten aluminum unless the molten aluminum is near
or above saturation with titanium and boronO At operating
temperatures for an aluminum electrowinning cell, titanium
is soluble in molten aluminum to about 50 parts per million
(ppm) and boron to about 20 ppm. Therefore it is desirable
that molten aluminum present in the cell be maintained
saturated with titanium and boron by the addition of com-
pounds containing them. Typically, existing aluminum
electrowinning cells are equipped for introducing additives,
however suitable or conventional method for intrGducing
the Ti and B would suffice, including the introduction of
TiB2.
At the interface between molten aluminum and the
coated substrate, a quite elevated concentration of titanium
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exists. This concentration decreases exponentially with
distance into the substrate.
There is an affinity between molten cryolite and
titanium and boron that might lead to the conclusion that
tianium and boron present in the molten aluminum layer 30
within the cell 10 may lead to inclusion of titanium and
boron in the molten cryolite layer 32 thereby gradually
stripping the applied boron and titanium from the packing
42 particularly where cryolite is replaced from time to
time. However, while the cell 10 is under an electrical
potential such as during aluminum electrowinning, it has
been found that the cryolite does not tend to retain the
titanium and boron present in the cell, much of these
materials accumulating in excess of solubility as deposits
within the aluminum phase 30.
While in this preferred embodiment, packing has been
shown as the cell component being fabricated from a non-
aluminum wettable material, other components are suitable
candidates for fabrication using these wettability techniques.
For example, weirs for overflowing molten aluminum from the
cell, and current feeders may be fabricated using the
technique of the instant invention from aluminum nonwettable
materials. Other applications within the cell will become
apparent upon reflection.
A number of suitable or conventional materials sub-
stantially nonwettable by aluminum are available for use
in the instant invention. These materials, because of the
relatively elevated temperature they must withstand in an
aluminum electrolysis cell, are primarily refractory mat-
erials including alumina, aluminum nitride, AlON, SiAlON,
boron nitride, silicon nitride, aluminum borides such as
AlB12, silicon carbides, alkaline earth metal zirconates
and aluminates such as calcium zirconate, barium zirconate,
and magnesium aluminate, and mixtures of these materials.
Alumina is preferred.
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Boron, within the purview of this invention appears
to function to suppress the solubility of titanium in the
molten aluminum. Therefore it should be apparent that
sukstances such as the packing, coated solely with titanium,
and immersed in molten aluminum will be initially wetted
by the aluminum. However, in the absence of boron within
the system, this titanium coating will be relatively
readily removed from the substrate surface and solubilized
in the molten aluminum. In the presence of boron in the
molten aluminum, this titanium coating is relatively
rapidly transformed to a titanium and boron coating, while
suppressing Ti solubility in the aluminum, the coating
being stable while the aluminum within the cell remains
near saturation with titanium and boron.
By wettable, what is meant is a contact angle between
the coated substrate and molten aluminum of less than 90;
nonwettable being a contact angle in excess of 90.
Generally otherwise nonwettable substrates such as alumina,
coated according to the method of the instant invention
allow aluminum to spread over the substrate surface,
indicating a contact angle of about 30 or less. Vtilizing
the techniques of the instant invention, an alumina sub-
strate, normally subject to some aggressive attack by molten
cryolite even when immersed in an aluminum pool within an
aluminum electrowinning cell, can be coated and immersed
in molten aluminum within the cell with small concern for
its dimensional integrity.
The following examples are offered to further il-
lustrate the invention.
EXAMPLE 1
Titanium diboride was coated onto Diamonite alumina
balls. These balls were supplied by Diamonite Products
Manufacturing Incorporated and were comprised of approxi-
; mately 1 to 3 percent silicondioxide and the balance of alumina.
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These balls were first etched in a molten salt mixture of
49 percent NaOH, 49 percent KOH, and 2 percent NaF at
180C for approximately 1 hour. Following ethcing, these
balls were solvent degreased and coated with titanium by
immersion in a molten salt mixture of 203.6 grams of KCl,
165.2 grams of NaCl, 15.2 grams of CaC12 and 16.7 grams of
TiH2. Coating was conducted at approximately 1000C for
four hours.
The balls were then washed and dried. Following
10 drying the balls were packed into a boron power bed and
boridized using well known techniques at 1000C for 48 hours
in an argon atmosphere scrubbed of residual oxygen by pas-
sage over hot titanium. Following boridization, the balls
were placed in a ball mill including alumina beads and
15 agitated to remove excess surface boron from the balls
by abrasion.
All of the titanium diboride coated samples showed
good surface conductivity and titanium diboride adhesion.
The balls were then each placed in an alumina crucible
20 with 30 grams of aluminum and 3-5 grams cryolite. The
crucible was evacuated and heated to 1000C for 4-8 weeks.
While under heat the crucible was maintained under an argon
purge, the argon being scrubbed of oxygen by passage over
hot titanium at 800-900C.
Removed from the crucible, the balls were inspected
and found to be wetted by aliminum. Only extremely limited
grain boundry corrosion was noted in a TiB2 coating that
averaged only 10-20 micrometers in thickness. Additionally,
trace amounts of titanium were found in the alumina crucible,
30 primarily in the pores. These pores were also found to be
at least partially wetted by aluminum with a small quantity
of the aluminum being found in pores of the alumina
crucible. Specifically with respect to the ba]ls, the
interface between the TiB2 coating and the alumina sub-
35 strate was found to be intact, and no evidence of grain
.
~Z~3378~L
boundary corrosion of TiB2 was observed. In the balls, a
contrast gradation was observed in the alumina substrate
which was attributed to filling of the pores with aluminum.
Infiltration of pores within Al2O3 balls by aluminum was
made possible by increasing the wettablility of Al2O3. The
coating it is believed acted as a source of surfactants.
It is important to recognize that coating permits instant-
aneous wetting of the surface but the action of the sur-
factants results in aluminum infiltration of pores.
EXAMPLE 2
Example 1 was repeated with the exception that the
balls were not solvent degreased. The results were
essentially identical.
EXAMPLE 3
The following alumina objects were etched for 15
minutes at 300C in a mixture of 392 grams of NaOH, 392
grams of KOH, and 16 grams of NaF:
cubes of alumina honeycomb, the cubes being
approximately 2-1/2 centimenters per edge;
cross sections of alumina tubes 4 centimeters
in diameter by approximately 3 centimeters in
height; and
1 centimeter diameter alumina balls similar
to those in Example 1.
The etched aluminum materials were rinsed in distilled
water and stored in methyl alcohol. Each was then coated
with titanium for four hours at approximately 1000C under
; an argon atmosphere scrubbed of oxygen by passage over hot
titanium in a bath comprising 796 grams of KCl, 640 grams
of NaCl, 59 grams of CaCl, and 65 grams of TiH2.
Following cooling, the excess salt was washed from
~ 233?7B~
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the alumina objects using hot water and the objects were
stored under methyl alcohol. Each of the objects was then
boridized by packing the materials in amorphous boron
and heating to approximately 1000C in well known manner
for approximatley 48 hours again under an oxygen free argon
environment.
These treated cubes, tube sections and balls were
then ready for electrolytic cell testing. Each of the
honeycomb cubes and balls was individually placed within
one of the treated alumina tube sections. A larger diameter
untreated alumina section also three centimeters in height
was then placed around the treated alumina tube section.
Both tube sections were then filled with aluminum beads.
A12O3 powder was packed around the outside of the untreated
tube section to contain aluminum during melting. The
aluminum beads were then melted so as to encapsulate the
honeycomb cube and the ring or the ball and its containing
ring. The untreated large diameter aluminum tube section
was broken away after cooling.
The honeycombs and balls surrounded by the treated
alumina tube sections and encased in aluminum were sub-
jected to 10 hour polarization tests. Each honeycomb or
ball in its alumina tube section was placed on a carbon
disc 6 centimeters in diameter by 0.7 centimeters thick
resting on a 6 centimeter diameter alumina pallet positioned
in the bottom of a 750 milliliter alumina crucible. A
molybdenum rod encased in boron nitride was employed as a
cathodic current feeder connecting to the carbon disc and
alumina pallet to the catnode of a source of electrical
current. The cell was completed by positioning a carbon
cylinder 3 centimeters in diameter and 3-1/2 centimeters
in length into the crucible for employ as an anode. The
cell was charged with 600 grams of 10 percent alumina in
cryolite. 4.81 amperes were passed between anode and cathode
~ 35 for 10 hours. 3 centimeters of molten aluminum was main-
:`
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tained in the cell so that the honeycombs or balls remained
immersed at all times.
After cooling, each aluminum cathode was disassembled
and the coated alumina honecomb or ball was examined. For
each cube or ball, the surrounding alumina tube section had
fractured~ Examination of the honeycomb revealed that
thealuminum surrounding the honecomb had protected the
aluminum honeycomb from attack while under polarizationO
During the tests 4.8 grams of anode was consumed,
the cell voltage was approximately 2.47 volts, and the
spacing between the anode and the aluminum cryolite inter-
face within the cell was 2.5 to 2.7 centimeters.
EXAMPLE 4
Example 3 was repeated except that provision was made
for draining aluminum from the crucible as it formed so
that the honeycomb or balls were bathed in cryolite, the
carbon disc was replaced with a titanium diboride disc of
equal dimension, and the honeycomb or balls were placed
directly on the titanium diboride disc without benefit of
the surrounding treated tube section. The honeycomb or
balls were each encased in aluminum upon insertion into
the cell. That aluminum melted upon cell start-up and
was withdrawn from the crucible. The cryolite charged
to the cell was electrolyzed to produce molten aluminum
under electrolysis conditions identical to Example 3
except that the anode was maintained at approximately 2.5
centimeters distance from upper portions of the honevcomb
or ball as arranged in the crucible cell.
After ten hours each cell was cooled and each
honeycomb or ball was removed for examination. These
oojects, notwithstanding their direct contact with molten
cryolite during electrolysis, were found to have a 100 to
500 micron film of aluminum upon all surfaces. The
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alumina substrates of each honeycomb or ball were not
attacked.
EXAMPLE 5
_ _
A cylindrical solid section of AlB12 was split long-
itudinally to yield a solid half cylinder. The half
cylinder was degreased with propanol. The degreased half
of the cylinder was immersed in a mixture of 20.36 grams
KCl, 16.52 grams NaCl, 1.52 grams CaCl2, and 2 grams of
titanium hydride at approximately 1000C under an argon
inerted atmosphere scurbbed of oxygen by passage over hot
titanium. Immersion was continued for four hours. The
half cylinder was then boridized in a manner identical to
that of Example 1. Upon inspection, a 15 micron coating
of titanium diboride was found to be present on the surface
of the half cylinder.
The half cylinder was placed in a 750 milliliter
alumina crucible, containing a titanium diboride ring
filled with aluminum. The half cylinder was inserted into
the ring so that a portion of the half cylinder protruded
above the aluminum contained within the titanium diboride
ring. The balance of the crucible was filled with cryolite.
The crucible was heated to 1000C for 2 hours. After 2
hours the treated half cylinder was found to be coated
uniformly with aluminum, even those portions protruding
from the titanium diboride half cylinder into cryolite
floating atop molten aluminum contained in the TiB2 ring.
EX~PLE 6
Example 5 was repeated using a half cylinder of BN
with essentially identical results.
While a preferred embodiment of the invention has
been shown and described in detail, it should be apparent
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that various modifications, alterations, or adjustments
may be made from the embodiment as shown without departing
from the scope of the claims followingO
