Note: Descriptions are shown in the official language in which they were submitted.
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CARBON TILES WITH REFRACTORY COATING FOR USE
AT ELEVATED TEMPERATURE
Field of the Invention
The invention relates to the protection with a
refractory coating of carbon tiles, in particular for use
in oxidising and/or corrosive environments.
Backaround of the Invention
The production, purification or recycling of metals,
such as aluminium or steel, is usually carried out at
high temperature in very aggressive environments, in
particular in molten metal, molten electrolyte and/or
corrosive gas. Therefore, the materials used for the
manufacture of components exposed to such environments
must be thermally and chemically stable.
Graphite and other carbonaceous materials are
commonly used for components, especially conductive
components. Unfortunately, carbon components do not
resist oxidation and/or corrosion and must be
periodically replaced.
Several proposals have been made to reduce wear of
carbon components in such technologies to achieve a
higher operation efficiency, reduce pollution and the
costs of operation.
For the purification of molten metals, in particular
molten aluminium, by the injection of a flux removing
impurities towards the surface of the molten metal, it
has been proposed to coat carbon components which are
exposed to the molten metal with a coating of refractory
material as disclosed in WO00/63630 (Holz/Duruz).
During the electrowinning of metals, such as
aluminium, some components are exposed to molten
electrolyte, molten metal and corrosive gases. In
conventional aluminium production cells these components
are still made of consumable carbonaceous materials.
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
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which contacts the molten constituents and corrosive
gases. Conductor bars connected to the negative pole of a
direct current source are embedded in the carbon cathode
forming the cell bottom floor. The cathode is usually an
anthracite based carbon lining made of prebaked cathode
blocks, joined with a ramming mixture of anthracite, coke
and coal tar, or with glue.
The use of titanium diboride and other RHM current-
conducting elements in electrolytic aluminium production
cells has been contemplated for long time and is
described in US Patents Nos 2,915,442, 3,028,324,
3,215,615, 3,324,876, 3,330,756, 3,156,639, 3,274,093 and
3,400,061. Despite extensive efforts and the potential
advantages of having surfaces of titanium diboride at the
cell cathode bottom, such propositions have not been
commercially adopted by the aluminium industry.
It has long been recognised that it would be
desirable to make (or coat or cover) the cathode of an
aluminium electrowinning cell with a refractory boride
such as titanium diboride that would render the cathode
surface wettable to molten aluminium which in turn would
lead to a series of advantages. Many difficulties were
encountered in producing refractory boride coatings which
meet up to the rigorous conditions in an aluminium
electrowinning cell. Nevertheless, as described in the
following patents, such coatings on carbon bodies have
recently been introduced.
W001/42168 (de Nora/Duruz) and W001/42531
(Nguyen/Duruz/de Nora) disclose applying a protective
coating of a refractory boride such as titanium diboride
to a carbon component of an aluminium electrowinning
cell, by applying thereto a slurry of particulate boride
in a colloid in several layers.
US Patents 4,333,813 and 4,341,611 (both in the name
of Kaplan) disclose an aluminium electrowinning cell with
a carbon cathode bottom covered with tiles having a
refractory hard metal surface, e.g. made of TiB2.
WO 98/17842 (Sekhar/Duruz/Liu) discloses inter-alia
tiles that can be fitted together to form an aluminium
production cell. The tiles can be made of a carbon
substrate covered with a refractory boride coating
applied from a colloidal slurry containing a particulate
of the refractory boride. To obtain improved coatings
without mud-cracks certain organic additives can be added
to the colloidal slurry such as polyvinyl alcohol,
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polyacrylic acid, hydroxy propyl methyl cellulose,
polyethylene glycol, ethylene glycol, butyl benzyl
phthalate and ammonium polymethacrylate.
Summary of the Invention
The invention relates to a method of bonding a
protective coating on a wear-exposed surface of a carbon
tile comprising applying onto the wear-exposed surface of
the carbon tile at least one layer of a slurry of
particulate refractory material and/or a heat convertible
precursor thereof suspended in a colloidal and/or
inorganic polymeric carrier comprising a dispersion of
colloidal particles and/or a solution of inorganic
polymeric particles of binding metal oxide and/or a heat
convertible precursor of the binding oxide; and heat
treating the slurry-applied layers) to consolidate the
particulate refractory material in the binding metal
oxide to form said protective coating.
According to the invention a hydrophobic (organic)
carbon compound, usually a carbon monomer or polymer,
having a hydrophilic substituent is used in the slurry as
a bonding agent that bonds the binding metal oxide to the
carbon tile by being bonded to the wear-exposed surface
of the carbon tile and by having its hydrophilic
substituent bonded to the binding metal oxide.
The bonding between the hydrophilic substituent and
the (hydrophilic) colloidal and/or inorganic polymeric
particles is of electrostatic nature. In the case of a
slurry of TiB2 particles suspended but not dispersed in a
colloidal alumina carrier containing polyvinyl alcohol as
a (hydrophobic) carbon compound with hydrophilic
substituents, the bonding of the constituents is as
follows
( - CH2 - CH - CHI - CH - CH2 - CH
OH OH OH
A1 - O - Al TiB2
O 0
The alcohol groups (-OH) of the polyvinyl alcohol
chain interact with the hydrophilic constituents, i.e.
alumina and titanium diboride, whereas the hydrophobic
hydrocarbon sites (-CH- and -CH2-) of the polyvinyl
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alcohol chain are absorbed by the surface of the
(hydrophobic) carbon tile and secure the layer thereon.
During heat treatment the carbon compounds) usually
decomposes and the hydrophilic substituents may evaporate
leaving the colloidal and/or inorganic polymeric
particles in intimate contact with the carbon remaining
from the carbon compound.
suitable hydrophilic substituents of carbon
compounds) may be selected from -OH, -S03Na and -COON,
and combinations thereof. The carbon compounds) can have
a carbon/hydrophilic substituent ratio in the range of 2
to 4. For example, the carbon compounds) is/are selected
from ethylene glycol, hexanol, polyvinyl alcohol,
polyvinyl acetate, polyacrylic acid, hydroxy propyl
methyl cellulose and ammonium polymethacrylate and
mixtures thereof.
Usually, the tiles are from 10 to 120 cm long, in
particular from 10 to 50 cm long, from 10 to 50 cm wide
and from 0.5 to 5 cm thick.
In a preferred embodiment the protective coating
comprises on the layers) containing the hydrophobic
carbon compound (hereinafter called the "bonding layer")
one or more hydrophilic layers) of refractory material
which are substantially free of any elemental carbon or
organic carbon compound.
The attachment between the bonding layers) and the
hydrophilic layers) is ensured by the affinity between
hydrophilic constituents, e.g. ceramics, present in the
layers.
It follows that the or each hydrophilic layer on the
bonding layers) is well bonded to the carbon tile
without containing at its surface organic carbon material
that could react during use to form carbides or other
compounds, in particular aluminium carbide when exposed
to aluminium, thereby damaging the protective coating and
impairing its electrical conductivity.
Conveniently, the or each hydrophilic layer is also
applied from a slurry of particulate refractory material
and/or a heat convertible precursor thereof suspended in
a colloidal and/or polymeric carrier comprising a
dispersion of particles of a binding metal oxide and/or a
heat convertible precursor thereof.
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The protective coating, in particular the bonding
and/or the hydrophilic layer(s), may comprise a binding
metal oxide selected from lithia, beryllium oxide,
magnesia, alumina, silica, titania, vanadium oxide,
chromium oxide, magnesia, iron oxide, nickel oxide,
gallium oxide, yttria, zirconia, niobium oxide,
molybdenum oxide, ruthenia, indium oxide, tin oxide,
tantalum oxide, tungsten oxide, thallium oxide, ceria,
hafnia and thoria and precursors thereof. In particular,
the slurry can contain colloidal particles selected from
lithia, beryllium oxide, magnesia, alumina, silica,
titania, vanadium oxide, chromium oxide, manganese oxide,
iron oxide, gallium oxide, yttria, zirconia, niobium
oxide, molybdenum oxide, ruthenia, indium oxide, tin
oxide, tantalum oxide, tungsten oxide, thallium oxide,
ceria, hafnia and thoria, and precursors thereof, all in
the form of colloids; and/or hydrophilic inorganic
polymeric particles selected from lithia, beryllium
oxide, alumina, silica, titania, chromium oxide, iron
oxide, nickel oxide, gallium oxide, zirconia, niobium
oxide, ruthenia, indium oxide, tin oxide, hafnia,
tantalum oxide, ceria and thoria, and precursors thereof,
all in the form of inorganic polymers.
Usually, the hydrophilic colloidal particles are
made of a heat stable ceramic, e.g. oxide, or a precursor
thereof in the form of a metal salt (e.g. hydroxide) and
have a generally spherical or polyhedral shape of
submicronic dimensions, typically having a diameter
between 10 and 100 nanometer, and are dispersed in an
aqueous dispersion liquid. The hydrophilic inorganic
polymeric particles are also made of precursors of heat
stable ceramic such as oxides (e.g. in the form of
hydrolysed metal salts), and are in the form of molecular
chains of submicronic length, typically form 1 to 100 nm
long, dissolved in a solution. The magnitude of these
dimensions distinguishes colloids/inorganic polymers from
bulk systems in the following way: (a) an extremely large
surface area and (b) a significant percentage of
molecules reside in the surface of colloidal/polymeric
systems. Up to 40~ of the molecules may reside at the
surface of the colloidal particles and up to 100% of the
molecules may reside at the surface of the polymeric
particle.
The protective coating, in particular the bonding
and/or the hydrophilic layer(s), usually comprises a
refractory material selected from borides, silicides,
nitrides, oxynitrides, carbides, oxycarbides, phosphides,
oxides, aluminides, of titanium, zirconium, hafnium,
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vanadium, silicon, niobium, tantalum, nickel, molybdenum
and iron.
It is preferable to choose particle size below 100
microns for the non-dispersed refractory particles and,
when employing combinations of non-dispersed refractory
particles, to choose particle sizes which are varied such
that the packing of particles is optimised. For example
when choosing a composition containing mostly SiC and
some MoSi~ as non-dispersed particles it is preferable to
choose the particle size of the MoSi~ much smaller (at
least three times smaller) than the SiC. Generally, the
ratio of the particle sizes will be in the range from 2:1
to 5:1, preferably about 3:1, for instance with large
particles in the range 15 to 30 micrometers and small
particles in the range 5 to 10 micrometers.
Advantageously, the slurry (-ies) producing the
protective coating, in particular the bonding and/or the
hydrophilic layer(s), comprises) a reinforcing metal
oxide which reacts with the bonding metal oxide and an
integral oxide film on the particles of refractory metal
compound to form a mixed oxide matrix embedding the
refractory metal compound particles. Suitable
combinations of refractory metal compounds, bonding metal
oxides and reinforcing metal oxides are disclosed in
W001/42531 (Nguyen/Duruz/de Nora).
The protective coating, in particular the bonding
and/or the hydrophilic layer(s), can comprise an
aluminium-wetting metal oxide, such as oxides of iron,
copper, cobalt, nickel, zinc or manganese, which when
exposed to molten aluminium reacts therewith to produce
aluminium oxide and the metal of the aluminium-wetting
metal oxide, as disclosed in W001/42168 (de Nora/Duruz).
When the protective coating has hydrophilic and
bonding layers, the hydrophilic layers) preferably
comprises) the aluminium-wetting oxide, the bonding
layers) being made of aluminium-repellent material
substantially free of any aluminium-wetting metal oxide.
In this way, the protective coating can be well wetted by
molten aluminium without exposing the carbon tile to
aluminium which would react with the carbon tile to form
aluminium carbide and impair the adherence of the
protective coating.
Preferably the carbon tile is made of graphite or
graphitised carbon material.
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The carbon tile can comprise a bottom surface which
is not covered with the protective coating.
Another aspect of the invention relates to a slurry
for forming a protective coating of particulate
refractory material consolidated in a binding metal oxide
upon application and heat treatment of the slurry on a
wear-exposed surface of a carbon tile for use in an
aluminium electrowinning cell. The slurry comprises the
particulate refractory material and/or a heat convertible
precursor thereof suspended in a colloidal and/or
inorganic polymeric carrier comprising a dispersion of
colloidal particles and/or a solution of inorganic
polymeric particles of binding metal oxide and/or a heat
convertible precursor of the binding oxide.
More particularly the invention relates to the use
in such a slurry of a hydrophobic carbon compound having
a hydrophilic substituent as an agent for bonding the
binding metal oxide to the carbon tile by bonding the
hydrophobic carbon compound to the wear-exposed surface
of the carbon tile and by bonding its hydrophilic
substituent to the binding metal oxide.
Another aspect of the invention relates to a tile
that comprises a carbon substrate having a wear-exposed
surface covered with a protective coating that comprises:
at least one bonding layer of particulate refractory
material consolidated in a binding metal oxide formed on
the wear-exposed surface of the carbon substrate from a
heat-treated slurry that comprises the particulate
refractory material and/or a heat convertible precursor
thereof suspended in a colloidal and/or inorganic
polymeric carrier Comprising a dispersion of colloidal
particles and/or a solution of inorganic polymeric
particles of binding metal oxide and/or a heat
convertible precursor of the binding oxide; and one or
more outer layers of refractory ceramic material.
According to the invention, the outer layers)
is/are hydrophilic and substantially free of any
elemental carbon or organic carbon compound, and the
bonding layers) further comprises) dispersed particles
of elemental carbon and/or a carbon compound derived from
a hydrophobic carbon compound having a hydrophilic
substituent that is present is the slurry prior to heat
treatment and that bonds the binding metal oxide to the
carbon tile by being bonded to the wear-exposed surface
of the carbon substrate and by having its hydrophilic
substituent bonded to the binding metal oxide.
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A further aspect of the invention concerns an
apparatus for the production, purification or recycling
of a metal in a molten state. The apparatus comprises at
least one component which is protected against a high
temperature environment that is oxidising and/or
corrosive, and gaseous and/or molten, by at least one
tile as described above.
The apparatus can be a cell for the electrowinning
of a metal from a compound thereof dissolved in a molten
electrolyte. In this case, the tile protects a cell
component against the electrowon metal, the molten
electrolyte and/or cell gases.
In one embodiment, the apparatus is an aluminium
electrowinning cell, the tile protecting the cell
component against at least one of molten aluminium,
molten electrolyte and anodically produced gases. The
hydrophilic layers) of the tile may form a catholic
surface which can be aluminium-wettable and optionally
drained.
In another embodiment, the apparatus is an arc
furnace for the recycling of steel, the tile protecting a
component against oxidising gas and/or molten steel.
In a further embodiment, the apparatus is a device
for the purification of a molten metal, in particular
molten aluminium, magnesium, iron or steel, by the
injection of a purifying fluid into the molten metal to
remove impurities towards the surface thereof, the tile
protecting a component of the device against the molten
metal to be purified, the purifying fluid and/or
impurities of the molten metal.
As opposed to direct application of the slurry of
refractory material to a component, e.g. a cathode
bottom, of an apparatus followed by in-situ heat-
treatment to form the coating, the application of the
slurry to a tile permits consolidation by heat-treatment
of the slurry in a controlled environment on the tile
before use of the coating in the apparatus.
The invention also relates to a method of producing,
purifying or recycling a metal in one the above mentioned
apparatus. The method comprises using the tile described
above to protect a component of the apparatus against a
high temperature environment which is oxidising and/or
corrosive, and gaseous and/or molten.
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For the electrowinning of metal, the method
comprises electrolysing a metal compound, in particular
alumina, in a molten electrolyte of a metal
electrowinning cell to produce the metal, in particular
aluminium, of the compound cathodically and gas
anodically, and using the tile to protect a cell
component against at least one of electrolyte,
cathodically produced metal and anodically produced gas.
Brief Description of the Drawings
Embodiments of the invention will now be described
by way of example with reference to the accompanying
schematic drawings, wherein:
- Figure 1 shows a schematic cross-sectional view of
an aluminium production cell having its cathode bottom
and sidewalls lined with aluminium-wettable tiles
according to the invention;
- Figure 2 schematically shows an arc electrode
furnace having sidewalls and bottom lined with the tiles
according to the invention;
- Figure 3 shows an apparatus for the purification
of a molten metal having sidewalls and bottom lined with
the tiles according to the invention;
- Figure 3a is an enlarged schematic sectional view
of part of a stirrer shown in Figure 3; and
- Figure 4 schematically shows a variation of the
stirrer shown in Figure 3.
Detailed Description
Aluminium Electrowinnin~_Cell_,
Figure 1 shows an aluminium electrowinning cell
comprising a series of metal-based anodes 5 having
operative surfaces 6 suspended over drained generally
horizontal cathode surface 21 in a fluoride-containing
molten electrolyte 42 containing dissolved alumina.
The anodes 5 can be of the type disclosed in
WO00/40781 or WO00/40782 (both de Nora) and made of the
materials disclosed in any one of the following
references: W099/36591 and W099/36592 (both in the name of
de Nora), W099/36593 and W099/36594 (both in the name of
de Nora/Duruz), WO00/06800 (Duruz/de Nora), WO00/06801
(de Nora/Duruz), W000/06802 and W000/06803 (both in the
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name of Duruz/de Nora/Crottaz), W000/06804 (Crottaz/
Duruz), W000/06805, W000/40783 and W001/42534 (all in the
name of de Nora/Duruz), and W001/42536 (Duruz/Nguyen/
de Nora), and W001/43208 (Duruz/de Nora).
The anodes may be coated with a protective layer of
one or more cerium compounds, in particular cerium
oxyfluoride. The protective layers can be maintained by
maintaining an amount of cerium species in the
electrolyte. Further details of such coatings and cell
operation therewith can be found in the above mentioned
US Patents 4,614,5&9 (Duruz/Derivaz/Debely/Adorian),
4,680,094, 4,683,,037 (both in the name of, Duruz) and
4,966,674 (Bannochie/Sheriff).
The drained cathode surface 21 is formed according
to the invention by tiles 20A which have their upper face
coated with a bonding layer and an (outer) hydrophilic
aluminium-wettable layer (free of any elemental carbon or
organic carbon compound) according to the invention and a
temporary start-up layer as taught in Example 4 below.
The tiles 20A are placed side-by-side on a series of
carbon cathode blocks 15 extending in pairs arranged end-
to-end across the cell. As shown in Figure 1, each
cathode block 15 is covered by a tile 20A that extends
from the side of the cell to the centre of the cell.
Alternatively, a plurality of smaller tiles placed side-
by-side across the cell could be used to cover such a
cathode block.
The cathode blocks 15 comprise, embedded in recesses
located in their bottom surfaces, current supply bars 22
of steel or other conductive material for connection to
an external electric current supply.
The drained cathode surface 2.1 is divided by a
central aluminium collection groove 26 located in or
between pairs of cathode blocks 15 arranged end-to-end
across the cell. The tiles 20A preferably cover a
substantial part of the groove 26 to maximise the surface
area of the aluminium-wettable cathode surface 21. The
tiles 20A leave only a vertical opening, e.g. a gap
between two tiles 20A across the cell, above the groove
26 sufficient to let product aluminium drain from the
aluminium-wettable cathode surface 21 through the small
vertical opening into the aluminium collection groove 26.
This vertical opening can also be used to access groove
26 for the tapping of molten aluminium.
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The cell comprises carbonaceous sidewalls 16 exposed
to molten electrolyte 42 and to the environment above the
molten electrolyte 42, These sidewalls 16 are protected
against the molten electrolyte 42 and the environment
above the molten electrolyte with tiles 20B according to
the invention.
In operation of the cell illustrated in Figure 1,
alumina dissolved in the molten electrolyte 42 at a
temperature of 750° to 960°C is electrolysed between the
anodes 5 and the cathode blocks l5 to produce gas on the
operative anodes surfaces 6 and molten aluminium on the
aluminium-wettable drained cathode tiles 20A.
The cathodically-produced molten aluminium flows on
the drained cathode surface 21 into the aluminium
collection groove 26 for subsequent tapping.
Figure 1 shows a specific aluminium electrowinning
cell by way of example. It is evident that many
alternatives, modifications, and variations will be
apparent to those skilled in the art,
For instance the cell may have a sloping cathode
bottom, as disclosed in W099/02764 (de Nora/Duruz), and
optionally one or more aluminium collection reservoirs
across the cell, each intersecting the aluminium
collection groove to divide the drained cathode surface
into four quadrants as described in WO00/63463 (de Nora).
The cell's cathodic tiles may be covered with a
shallow or deep pool of aluminium as disclosed in US
Patent 5,651,874 (de Nora/Sekhar).
Arc Furnace:
The arc furnace shown in Figure 2 comprises three
consumable electrodes 15A arranged in a triangular
relationship. For clarity, the distance between the
electrodes 15A as shown in Figure 2 has been
proportionally increased with respect to the furnace.
Typically, the electrodes 15A have a diameter between 200
and 500 mm and can be spaced by a distance corresponding
to about their diameter. The bottom and sidewalls 45 of
the furnace are protected with coated carbon tiles 20C
according to the invention.
The electrodes 15A are connected to an electrical
power supply (not shown) and suspended from an electrode
positioning system above the cell which is arranged to
adjust their height.
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The consumable electrodes 15A are made of a carbon
substrate laterally coated with a protective layer 20
protecting the carbon substrate from oxidising gas. The
bottom of electrodes 15A which is consumed during
operation and constitutes the electrodes' operative
surface is uncoated. The protective layer 151 protects
only the electrodes' lateral faces against premature
oxidation. Suitable protective layers are disclosed in
W001/42168 (de Nora/Duru~), W001/42531 (Nguyen/Duruz/de
Nora) and PCT/IB01/00949 (Nguyen/de Nora).
The electrodes 15A dip in an iron source 41, usually
containing iron oxide or oxidised iron, such as scrap
iron, scrap steel and pig iron. Preferably, the iron
source 41 further comprises reductants selected from
gaseous hydrogen, gaseous carbon monoxide or solid carbon
bearing reductants. The reductants may also comprise non-
iron minerals known as gangue which include silica,
alumina, magnesia and lime.
The iron source 41 floats on a pool of liquid iron
or steel 40 resulting from the recycling of the iron
source 41. The iron source 41 and the liquid iron or
steel 40 contact the tiles 20C.
During use, a three phase AC current is passed
through electrodes 15A, which directly reduces iron from
the iron source 41. The reduced iron is then collected in
the iron or steel pool 40. The gangue contained in the
reduced iron is separated from the iron by melting and
flotation forming a slag (not shown) which is removed,
for example through one or more apertures (not shown)
located on the tiles 20C and sidewalls of the arc furnace
at the level of the slag.
The pool of iron or steel 40 is periodically or
continuously tapped for instance through an aperture (not
shown) located in the bottom of the arc furnace, through
the tiles 20C.
Molten_Metal_Purification-Apparatus:_
The molten metal purification apparatus partly shown
in Figure 3 comprises a vessel 45 containing molten metal
40', such as molten aluminium, to be purified. A
rotatable stirrer 10 made of carbon-based material, such
as graphite, is partly immersed in the molten metal 40'
and is arranged to rotate therein. The sidewalls and
bottom of the vessel 45 are protected with coated carbon
tiles 20D according to the invention.
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The stirrer 10 comprises a shaft 11 whose upper part
is engaged with a rotary drive and support structure 30
which holds and rotates the stirrer 10. The lower part of
shaft 11 is carbon-based and dips in the molten metal 40'
contained in vessel 45. At the lower end of the shaft 11
is a rotor 13 provided with flanges or other protuberances
for stirring the molten metal 40'.
Inside the shaft 11, along its length, is an axial
duct 12, as shown in Figure 3a, which is connected at the
stirrer's upper end through a flexible tube 35 to a gas
supply (not shown), for instance a gas reservoir provided
with a gas gate leading to the flexible tube 35.
The axial duct 12 is arranged to supply a fluid to
the rotor 13. The rotor 13 comprises a plurality of
apertures connected to the internal duct 12 for injecting
the gas into the molten metal 40', as shown by arrows 51.
The lower part of the shaft 11, i.e. the immersed
part and the interface region at or about the meltline 14
of the shaft, as well as the rotor 13 are coated a
protective layer 111 which improves the resistance to
erosion, oxidation and/or corrosion of the stirrer during
operation.
As shown in Figure 3, the upper part of shaft 11 is
protected against oxidation and/or corrosion with a
protective layer 112. The upper part of the carbon-based
shaft 11 is coated with a thin layer of refractory
material 112 protection against oxidation and corrosion,
whereas the layer 111 protecting the immersed part of the
shaft 11 and the rotor 13 is a thicker layer of refractory
material against erosion, oxidation and corrosion.
During operation of the apparatus shown in Figure 3,
a reactive or non-reactive fluid, in particular a gas 50
alone or a flux, such as a halide, nitrogen and/or argon,
is injected into the molten metal 40' contained in the
vessel 45 through the flexible tube 35 and stirrer 10
which dips in the molten metal 40'. The tiles 20C protect
the sidewalls and bottom of the vessel 45 against molten
metal, the reactive or non-reactive fluid and air.
The stirrer 10 is rotated at a speed of about 100 to
500 RPM so that the injected gas 50 is dispersed
throughout the molten metal in finely divided gas
bubbles. The dispersed gas bubbles 50, with or without
reaction, remove impurities present in the molten metal
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40' towards its surface, from where the impurities may be
separated thus purifying the molten metal.
The stirrer 10 schematically shown in Figure 4 dips
in a molten metal bath 40' and comprises a shaft 11 and a
rotor 13. The stirrer 10 may be of any type, for example
similar to the stirrer shown in Figure 3 or of
conventional design as known from the prior art. The
rotor 13 of stirrer 10 may be a high-shear rotor or a
pump action rotor.
In Figure 4, instead of coating the entire shaft 11
and rotor 13, parts of the stirrer 10 liable to erosion
are selectively coated with a protective layer.
The interface portion at and about the meltline 14 of
the carbon-based Lower part of the shaft 22 is coated with
a refractory interface layer 113, for instance over a
length of up to half that of the shaft 11. Excellent
results have been obtained with a layer over a third of
shaft 11. However, the length of layer 113 could be a
quarter of the length of shaft 11 or even less, depending
on the design of stirrer 10 and the operating conditions.
In addition to the interface portion of such
stirrers, other areas may be liable to erode, again
depending on the design and operating conditions of the
stirrers. The schematically shown stirrer 10 in Figure 4
illustrates further coated surfaces which are particularly
exposed to erosion. The lower end of the shaft 11 adjacent
to the rotor 13 is covered with a protective layer 113.
The lateral surface of rotor 13 is covered with a
protective layer, 131 and the bottom surface of the rotor
13 is coated with a protective layer 132.
For each specific stirrer design, the layer or
different protective layers on different parts of the
stirrer, such as layers 113, 114, 131 and 132 shown in
Figure 4, may be adapted as a function of the expected
lifetime of the stirrer. For optimal use, the amount and
location of such layers can be so balanced that they each
have approximately the same lifetime.
Suitable coatings for protecting the shaft 11 and the
rotor 13 are described in W000/63630 (Holz/Duruz),
W001/42168 (de Nora/Duruz), W001/42531 (Nguyen/Duruz/
de Nora) and PCT/IB01/00949 (Nguyen/de Nora).
In an alternative embodiment (not shown), the layer
on such stirrers may be continuous as illustrated in
Figure 3 but with a graded thickness or composition so as
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to adapt the resistance against erosion to the intensity
of wear of each part of the stirrer, thereby combining the
advantages of the different layers shown in Figure 4.
Various modifications can be made to the apparatus
shown in Figures 3, 3a and 4, For instance, the shaft
shown in Figure 3 may be modified so as to consist of an
assembly whose non-immersed part is made of a material
other than carbon-based, such as a metal and/or a ceramic,
which is resistant to oxidation and corrosion and which,
therefore, does not need any protective layer, whereas the
immersed part of the shaft is made of carbon-based
material protected with a protective layer. Such a
composite shaft would preferably be designed to permit
disassembly of the immersed and non-immersed parts so the
immersed part can be replaced when worn.
Likewise, a carbon-based non-immersed part of the
shaft may be protected from oxidation and corrosion with
a layer and/or impregnation of a phosphate of aluminium,
in particular applied in the form of a compound selected
from monoaluminium phosphate, aluminium phosphate,
aluminium polyphosphate, aluminium metaphosphate, and
mixtures thereof. It is also possible to protect the non-
immersed part of the shaft with a layer and/or
impregnation of a boron compound, such as a compound
selected from boron oxide, boric acid and tetraboric acid
as disclosed in US Patent 5,486,278 (Manganiello/Duruz/
Belld) and in c~-pending application W097/26626 (de Nora/
Duruz/Berclaz).
In a modification, a protective layer may simply be
applied to any part of the stirrer in contact with the
molten metal, to be protected against erosion, oxidation
and/or corrosion during operation.
Further details of such a purification or treatment
apparatus are disclosed in WO00/63630 (Holz/Duruz).
The invention will be further described in the
following examples.
Example 1
A slurry for use on a carbon tile in accordance with
the invention was prepared as follows.
A refractory hard metal boride consisting of 47.5 g
surface-oxidised particulate spherical TiB~ (-325 mesh)
having a Ti02 surface film and a particulate reinforcing
metal oxide in the form of 2.5 g Ti02 (-325 mesh) were
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stirred and suspended in a colloidal carrier consisting
of 20 ml colloidal A1203 (NYACOL~ Al-20, a milky liquid
with a colloidal particle size of about 40 to 60
manometer) to form an colloidal slurry.
After the particulate titanium diboride and oxide
had been suspended in the colloidal carrier, an amount of
1 ml of an aqueous solution containing 15 weight%
polyvinyl alcohol (PVA), a hydrophobic carbon polymer
comprising hydrophilic substituents (-OH), was added to
the colloidal slurry.
This slurry produces upon heat treatment an oxide
matrix of titanium-aluminium mixed oxide from the
reaction of the colloidal oxide A1203 and Ti02 present as
suspended oxide particles and oxide film covering the
suspended TiB2 particles intimately mixed with carbon
from the hydrophobic carbon polymer. The oxide matrix
contains and bonds TiB~ particles.
This slurry is suitable for the manufacture of a
composite coating as described in Example 4.
Example 1a
The constituents of the slurry of Example 1 may be
changed as shown in the following Table in which each
line represents possible combinations of constituents
which are combined with one or more carbon compounds in
the form of hydrophobic carbon monomers and/or polymers
that comprise hydrophilic substituents, such as ethylene
glycol, hexanol, polyvinyl alcohol, polyvinyl acetate,
polyacrylic acid, hydroxy propyl methyl cellulose and
ammonium polymethacrylate.
Colloidal and/or Suspended Suspended Surface-
Polymeric Oxides Reinforcing Metal Oxidised
Oxides Refractory Metal
Com ounds
A1~03 Ti02, Mg0 or Si02 TiB2, SiC, TiC or
TiN
Ti02 A1203 or Mg0 SiC or SiN
Si02 A1203 or Mg0 TiB~, TiC or TiN
In a variation, the suspended refractory metal
compound does not need to be surface oxidised and the
suspended reinforcing metal oxide may be replaced by the
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suspended surface-oxidised refractory metal compounds in
the same weight percentage.
Example 2
A slurry for producing a hydrophilic layer free of
organic carbon (elemental or in the form of a compound)
which can be bonded through a bonding layer to a carbon
tile in accordance with the invention was prepared by
suspending 2.5 g particulate Fe203 (-325 mesh), a
refractory hard metal boride consisting of 92.5 g
20 particulate needle-shaped surface-oxidised TiB2 (-325
mesh) having a TiO~ surface oxide film, and 2.5 g
particulate Ti02 (-325 mesh) in a colloid consisting of a
combination of two grades of colloidal A1203, namely 28
ml of a first grade of colloidal A1203 (NYACOL~ Al-20, a
milky liquid with a colloidal particle size of about 40
to 60 nanometer) and 24 ml of a second grade of colloidal
A1203 (CONDEA~ 10/2 Sol, a clear, opalescent liquid with
a colloidal particle size of about 10 to 30 nanometer).
This slurry produces upon heat treatment a matrix of
mixed oxides consisting of titanium-aluminium mixed oxide
and a small amount of iron-titanium-aluminium mixed oxide
from the reaction of Ti02, Fe203 and A1203. This matrix
contains and bonds the TiB2 and Fe203 particles.
This slurry is suitable fox the manufacture of a
composite coating as described in Example 4.
Example 2a
Example 2's slurry composition consists of Fe203
and a reaction mixture made of the colloid (A1203), the
suspended refractory metal boride (TiB2) the suspended
metal oxide (Ti02). This Example can be modified by
completely or partly substituting Fe~03 with partly
oxidised or oxides of copper and/or nickel, and/or by
varying the composition of the reaction mixture as in
Example 1a.
Example 3
A further slurry for producing a highly aluminium-
wettable hydrophilic start-up .layer free of organic
carbon which can be bonded through a bonding layer to a
carbon tile in accordance with the invention, was
prepared as follows. An amount of 60 g of surface
oxidised copper particles (-325 mesh) was suspended in a
carrier consisting of 13 ml of colloidal A1~03 (7 ml
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NYACOL~ Al-20, a milky liquid with a colloidal particle
size of about 40 to 60 nanometer and 6 ml CONDEA~ 1012
Sol, a clear, opalescent liquid with a colloidal particle
size of about 10 to 30 nanometer).
Upon heat treatment the slurry produces an alumina
matrix containing and bonding the oxidised copper
particles.
As a modification, oxidised or partly oxidised
particles of nickel and/or iron may be used to substitute
in part or completely the oxidised copper particles in
colloidal alumina (CONDEA 25/5 with a pH > 7).
Example 4
Three graphite tiles for use on a cathode bottom of
a drained-cathode cell for the production of aluminium
were covered with a mufti-layer coating produced from the
slurries of Examples 1, 2 and 3 as follows:
First, a bonding layer having a thickness of about
100 micron was painted onto the exposed surface of the
graphite plate from the slurry of Example 1. The bonding
layer was allowed to dry for 30 minutes.
The bonding layer was covered with 8 permanent
aluminium-wettable hydrophilic layers obtained by
painting layers of the slurry of Example 2. Each applied
hydrophilic layer was allowed to dry for 30 minutes
before application of the next layer. The 8 layers had a
cumulated thickness of about 1.8 mm.
The permanent aluminium-wettable hydrophilic layers
were then covered with a temporary start-up layer
obtained by painting one layer of the slurry of Example
3. The start-up layer had a thickness of about 100 to 150
micron.
The coating formed by the bonding layer, the
permanent aluminium-wettable hydrophilic layers and the
temporary start-up layer on the tile was allowed to dry
for 24 hours.
Two of the three coated tiles were then covered with
an aluminium sheet having a thickness of about 1.5 cm and
heated in an oven at a temperature of about 850-900°C
in air.
The first coated tile was extracted from the oven
after 30 minutes and allowed to cool down to ambient
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temperature. Examination of a cross-section of the
coating showed that aluminium had infiltrated the start-
up layer so that the coating was superficially wetted by
molten aluminium. No reaction between aluminium and iron
oxide had yet taken place. The bonding layer was
intimately bonded to the (hydrophobic) carbon tile.
The second coated tile was extracted from the oven
after 24 hours and allowed to cool down to ambient
temperature. Examination of a cross-section of the
coating showed that aluminium had infiltrated the start
up layer and the permanent aluminium-wettable hydrophilic
layers. Part of the aluminium had reacted with Fe203 to
form A1203 and Fe metal. Aluminium infiltration had been
stopped on the bonding layer for lack of oxide reactable
with aluminium.
The aluminium metal infiltration into the start-up
layer and the permanent aluminium-wettable hydrophilic
layers enhanced the conductivity of the coating. At
ambient temperature, the perpendicular electrical
resistance through the coating was less than 1 ohm after
infiltration versus more than 500 ohm before
infiltration.
The coatings on both tiles showed a continuous
matrix of titanium-aluminium mixed oxides between the
bonding layer and the permanent aluminium-wettable
hydrophilic layers which guarantees an excellent
adherence between the layers. In both cases the particles
of TiB2 had not been oxidised by the heat treatment and
wettability of the coating by aluminium was very good.
The angle of wettability was less than 10 deg.
The third coated carbon cathode was used in an
aluminium production drained cell as follows:
The graphite tile with the dried coating according
to the invention was placed on the cell cathode bottom
and covered with a 1.5 cm thick sheet of aluminium. The
cell was heated to a temperature of about 850-900°C by
passing an electrical current between the cathode and
facing anodes through carbon powder. Other start-up
heating procedures could also have been used, e.g. using
gas burners to generate heat.
After 30 minutes at 850-900°C, the start-up coating
was superficially wetted by molten aluminium which
constitutes a barrier against damaging fluoride-based
molten electrolyte constituents, such as sodium
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compounds, and a cryolite based electrolyte was filled
into the cell.
The cell was further heated to 960°C at which
temperature the cell was operated under an electrolysis
current density of 0.8 A/cm2 to produce aluminium under
conventional steady state conditions.
Example 5
Any of the layers of Examples 1 to 4 can be modified
using inorganic polymeric carriers, such as the polymeric
solutions (A) and (B) prepared as set out below, in
replacement of the layer's colloidal carriers.
(A) An amount of 150 g of Fe(N03)3.9 H20 was heated
to dissolve the salt in its own water of crystallisation
to form a solution containing 29 g Fe203. The solution
was heated to 120°C and 18.9 g of magnesium hydroxy-
carbonate dissolved in the hot solution to form 7.5 g Mg0
in form of an inorganic polymer together with Fe203
suitable for use as an inorganic polymeric carrier.
(B) An amount of 100 g of Cr(N03)3.9 H20 was heated
to dissolve the salt in its own water of crystallisation
to form a solution containing 19 g Cr203. The solution
was heated to 120°C and 12.5 g of magnesium-hydroxy
carbonate containing the equivalent of 5.0 g Mg0 was
added. Upon stirring a solution was obtained in the form
of an anion-deficient polymer mixture with a density of
approximately 1.5 g/cm3 suitable to act as an inorganic
polymeric carrier.
