Note: Descriptions are shown in the official language in which they were submitted.
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HIGH-STRENGTH LOW-ALLOY STEEL ANODES FOR
ALUMINIUM ELECTROWINNING CELLS
Field of the Invention
This invention relates to non-carbon, metal-based,
anodes for use in cells for the electrowinning of
aluminium from alumina dissolved in a fluoride-containing
molten electrolyte such as cryolite, and to methods for
their fabrication, as well as to electrowinning cells
containing such anodes and their use to produce aluminium.
Background Art
The technology for the production of aluminium by
the electrolysis of alumina, dissolved in molten cryolite,
at temperatures around 950°C is more than one hundred
years old.
This process conceived almost simultaneously by
Hall and Heroult, has not evolved as many other
electrochemical processes.
The anodes are still made of carbonaceous material
and must be replaced every few weeks . During electrolysis
the oxygen which should evolve on the anode surface
combines with the carbon to form polluting C02 and small
amounts of CO and fluorine-containing dangerous gases. The
actual consumption of the anode is as much as 450 Kg/Ton
of aluminium produced which is more than 1/3 higher than
the theoretical amount of 333 Kg/Ton.
Using metal anodes in aluminium electrowinning
cells would drastically improve the aluminium process by
reducing pollution and the cost of aluminium production.
US Patent 4,999,097 (Sadoway) describes anodes for
conventional aluminium electrowinning cells provided with
an oxide coating containing at least one oxide of
zirconium, hafnium, thorium and uranium. To prevent
consumption of the anode, the bath is saturated with the
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materials that form the coating. However, these coatings
are poorly conductive and have not been used.
US Patent 4,504,369 (Keller) discloses a method of
producing aluminium in a conventional cell using anodes
whose dissolution into the electrolytic bath is reduced by
adding anode constituent materials into the electrolyte,
allowing slow dissolution of the anode. However, this
method is impractical because it would lead to a
contamination of the product aluminium by the anode
constituent materials which is considerably above the
acceptable level in industrial production.
US Patent 4,614,569 (Duruz/Derivaz/Debely/Adorian)
describes metal anodes for aluminium electrowinning coated
with a protective coating of cerium oxyfluoride, formed
in-situ in the cell or pre-applied, this coating being
maintained during electrolysis by the addition of small
amounts of a cerium compound to the molten cryolite
electrolyte. This made it possible to have a protection of
the surface from the electrolyte attack and to a certain
extent from gaseous oxygen but not from nascent monoatomic
oxygen.
EP Patent application 0 306 100 (Nyguen/Lazouni/
Doan) describes anodes composed of a chromium, nickel,
cobalt and/or iron based substrate covered with an oxygen
barrier layer and a ceramic coating of nickel, copper
and/or manganese oxide which may be further covered with
an in-situ formed protective cerium oxyfluoride layer.
Likewise, US Patents 5,069,771, 4,960,494 and 4,956,068
(all Nyguen/Lazouni/Doan) disclose aluminium production
anodes with an oxidised copper-nickel surface on an alloy
substrate with a protective barrier layer. However, full
protection of the alloy substrate was difficult to
achieve.
Metal or metal-based anodes are highly desirable in
aluminium electrowinning cells instead of carbon-based
anodes. Many attempts were made to use metallic anodes for
aluminium production, however they were never adopted by
the aluminium industry because they had a short life and
contaminated the aluminium produced.
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Obiects of the Invention
A major object of the invention is to provide an
anode for aluminium electrowinning which has no carbon so
as to eliminate carbon-generated pollution and increase
the anode life.
A further object of the invention is to provide an
aluminium electrowinning anode material with a surface
having a high electrochemical activity and a low
solubility in the electrolyte.
Another object of the invention is to provide an
anode for the electrowinning of aluminium which is covered
with an electrochemically active layer with limited ionic
conductivity for oxygen ions.
Yet another object of the invention is to provide
an anode for the electrowinning of aluminium which is made
of readily available material(s).
An important object of the invention is to
substantially reduce the solubility of the surface layer
of an aluminium electrowinning anode, thereby maintaining
the anode dimensionally stable.
Yet another object of the invention is to provide
operating conditions for an aluminium electrowinning cell
under which the contamination of the product aluminium is
limited.
Summarv of the Invention
This invention is based on the observation that
low-carbon high-strength low-alloy (HSLA) steels such as
Cor-TenTM even at high temperature form under oxidising
conditions an iron oxide-based surface layer which is
dense, electrically conductive, electrochemically active
for oxygen evolution and, as opposed to oxide layers
formed on standard steels or other iron alloys, is highly
adherent and less exposed to delamination and limits
diffusion of ionic, monoatomic and molecular oxygen.
HSLA steels are used for their strength and
resistance to atmospheric corrosion especially at lower
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temperatures (below 0°C) in different areas of technology
such as civil engineering (bridges, dock walls, sea walls,
piping), architecture (buildings, frames) and mechanical
engineering (welded/bolted/riveted structures, car and
railway industry, high pressure vessels). However, these
HSLA steels have never been proposed for applications at
high temperature, especially under oxidising or corrosive
conditions, in particular in cells for the electrowinning
of aluminium.
It has been found that the iron oxide-based
surface layer formed on the surface of a HSLA steel under
oxidising conditions limits also at elevated temperatures
the diffusion of oxygen oxidising the surface of the HSLA
steel. Thus, diffusion of oxygen through the surface layer
decreases with an increasing thickness thereof.
If the HSLA steel is exposed to an oxidising
environment which maintains or preserves the surface
layer, the iron oxide-based surface layer grows until its
thickness constitutes a sufficient barrier to oxygen and
then remains dimensionally stable. If the HSLA steel is
exposed to an environment promoting dissolution or
delamination of the surface layer, the rate of formation
of the iron oxide-based surface layer (by oxidation of the
surface of the HSLA steel) reaches the rate of dissolution
or delamination of the surface layer after a transitional
period during which the surface layer grows or decreases
to reach an equilibrium thickness in the specific
environment.
Anodes and Manufacture
The invention relates in particular to an anode of
a cell for the electrowinning of aluminium from alumina
dissolved in a fluoride-containing molten electrolyte.
This anode comprises a low-carbon high-strength low-alloy
(HSLA) steel body or layer whose surface is oxidised to
form a coherent and adherent outer iron oxide-based layer
the surface of which is electrochemically active for the
evolution of oxygen. The iron oxide-based layer has a low
solubility in the molten electrolyte. The thickness of the
iron oxide-based layer is such as to reduce or prevent
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diffusion of oxygen from the electrochemically active
surface into the steel body or layer during use.
During steady operation the reduced rate of
diffusion through the oxide-based layer can be such that
oxygen only diffuses into the steel body or layer in a
controlled manner without significant increase of the
thickness of the oxide-based layer.
High-strength low-alloy (HSLA) steel designates a
group of low-carbon steels (typically up to 0.5 weighto
carbon of the total) that contain small amounts of
alloying elements. These steels have better mechanical
properties and sometimes better corrosion resistance than
carbon steels.
The surface of the high-strength low-alloy steel
body or layer may be oxidised in an electrolytic cell or
in an oxidising atmosphere, in particular a relatively
pure oxygen atmosphere. For instance the surface of the
high-strength low-alloy steel body or layer may be
oxidised in a first electrolytic cell and then transferred
to an aluminium production cell. In an electrolytic cell
oxidation would typically last 5 to 15 hours at 800 to
1000°C. Oxidation may also take place in air or in oxygen
for 5 to 25 hours at 750 to 1150°C before electrolysis.
In order to prevent thermal shocks causing
mechanical stresses, a high-strength low-alloy steel body
or layer may be tempered or annealed after pre-oxidation.
Alternatively, the high-strength low-alloy steel body or
layer may be maintained at elevated temperature after pre
oxidation until immersion into the molten electrolyte of
an aluminium production cell.
The high-strength low-alloy steel body or layer
may comprise 94 to 98 weight% iron and carbon, the
remaining constituents being one or more further metals
selected from chromium, copper, nickel, silicon, titanium,
tantalum, tungsten, vanadium, zirconium, aluminium,
molybdenum, manganese and niobium, and possibly small
amounts of at least one additive selected from boron,
sulfur, phosphorus and nitrogen.
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In one embodiment, the anode comprises a layer of
high-strength low-alloy steel on an oxidation resistant
metallic core. The layer of high-strength low-alloy steel
may be applied on the metallic core before or after
formation of the outer iron oxide-based layer.
The metallic core is preferably electrically
highly conductive and may be made of copper or a copper
alloy. To enhance the mechanical properties, the metallic
core may contain minor amounts of at least one oxide, such
as alumina, hafnia, yttria and/or zirconia. Furthermore,
to enhance oxidation resistance, the metallic core may be
coated with at least one metal selected from nickel,
chromium, cobalt, iron, aluminium, hafnium, manganese,
molybdenum, niobium, silicon, tantalum, titanium,
tungsten, vanadium, yttrium and zirconium, and alloys,
intermetallic compounds and combinations thereof.
The metallic core may be coated with an oxygen
barrier layer of chromium and/or niobium.
The layer of high-strength low-alloy steel may be
plasma sprayed, arc sprayed, slurry-applied or
electrodeposi''~Eed onto the metallic core. Alternatively, to
enhance adhesion, the high-strength low-alloy steel layer
may be bonded to the metallic core through at least one
intermediate layer, in particular a film of silver,
typically 0.1 to 10 micron thick, which is in intimate and
continuous contact with the metallic core and with the
steel layer, and/or at least one layer of nickel and/or
copper.
The invention also relates to a bipolar electrode
of a cell for the electrowinning of aluminium from alumina
dissolved in a fluoride-containing electrolyte, comprising
on its anodic side an anode as described above.
The high strength low allow (HSLA) steel body can
also be bonded or connected to an electrically conductive
anode structure of special design as disclosed in
WO00/40781 and WO00/40782
(both in the name of de
Nora ) .
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One aspect of the invention is an anode precursor
comprising a low-carbon high-strength low-alloy (HSLA)
steel body or layer and which can be converted into a
fully manufactured anode as described above by oxidising
the surface of the steel body or layer to form the
coherent and adherent outer iron oxide-based layer.
Another aspect of the invention is a method of
manufacturing an anode as described above comprising:
- providing a low-carbon high-strength low-alloy (HSLA)
steel body or layer; and
- oxidising the surface of the high-strength low-alloy
steel body or layer to form the coherent and adherent
outer iron oxide-based layer the surface of which is
electrochemically active for the evolution of oxygen.
Cells and Aluminium Production
A further aspect of the invention is a cell for
the electrowinning of aluminium from alumina dissolved in
a fluoride-containing molten electrolyte comprising at
least one anode having a low-carbon high-strength low-
alloy (HSLA) steel body or layer and an electrochemically
active outer iron oxide-based layer whose surface is
electrochemically active, as described above.
During normal operation the electrochemically
active layer of the or each anode may be progressively
further formed by surface oxidation of the steel body or
layer by controlled oxygen diffusion through the
electrochemically active layer, and progressively
dissolved into the electrolyte at the electrolyte/anode
interface, the rate of formation of the outer iron oxide-
based layer being substantially equal to its rate of
dissolution into the electrolyte.
However, it has been observed that this type of
anode may be maintained dimensionally stable under
specific cell operating conditions.
In known processes, even the least soluble anode
material releases excessive amounts of constituents into
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the bath, which leads to an excessive contamination of the
product aluminium. For example, the concentration of
nickel (a frequent component of proposed metal-based
anodes) found in aluminium produced in small scale tests
at conventional cell operating temperatures is typically
comprised between 800 and 2000 ppm, i.e. 4 to 10 times the
maximum acceptable level which is 200 ppm.
Iron oxides and in particular hematite (Fe203) have
a higher solubility than nickel in molten electrolyte.
However, in industrial production the contamination
tolerance of the product aluminium by iron is also much
higher (up to 2000 ppm) than for other metal impurities.
Solubility is an intrinsic property of anode
materials and cannot be changed otherwise than by
modifying the electrolyte composition and/or the operating
temperature of a cell.
Small scale tests utilising a NiFe204/Cu cermet
anode and operating under steady conditions were carried
out to establish the concentration of iron in molten
electrolyte and in the product aluminium under different
operating conditions.
In the case of iron oxide, it has been found that
lowering the temperature of the electrolyte deacreases
considerably the solubility of iron species. This effect
can surprisingly be exploited to produce a major impact on
cell operation by limiting the contamination of the
product aluminium by iron.
Thus, it has been found that when the operating
temperature of the cell is reduced below the temperature
of conventional cells (950-970°C) an anode covered with an
outer layer of iron oxide can be made dimensionally stable
by maintaining a concentration of iron species and alumina
in the molten electrolyte sufficient to reduce or suppress
the dissolution of the iron-oxide layer, the concentration
of iron species being low enough not to exceed the
commercial acceptable level of iron in the product
aluminium.
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The presence of dissolved alumina in the electrolyte
at the anode surface has a limiting effect on the
dissolution of iron from the anode into the electrolyte,
which reduces the concentration of iron species necessary
to substantially stop dissolution of iron from the anode.
Therefore, anodes according to the invention may
be kept dimensionally stable by maintaining a sufficient
amount of dissolved alumina and iron species in the
electrolyte to reduce or prevent dissolution of the outer
oxide layer.
The cell should be operated at a sufficiently low
temperature to limit the solubility of iron species in the
electrolyte, thereby limiting contamination of the product
aluminium by constituents of the outer iron oxide-based
layer of the anodes) to a commercially acceptable level.
When the cell is operated with a fluoride-based
melt the operating temperature of the electrolyte should
be below 910°C, usually from 730 to 870°C.
The amount of iron species and alumina dissolved
in the electrolyte preventing dissolution of the iron
oxide-based outside surface layer of the or each anode
should be such that the product aluminium is contaminated
by no more than 2000 ppm iron, preferably by no more than
1000 ppm iron, and even more preferably by no more than
500 ppm iron.
Usually the iron species are intermittently fed
into the electrolyte, for instance together with alumina,
to maintain the amount of iron species in the electrolyte
constant which, at the operating temperature, prevents the
dissolution of the iron oxide-based outside surface layer
of the anodes. However, the iron species can also be
continuously fed, for instance by dissolving a sacrificial
electrode which continuously feeds the iron species into
the electrolyte.
The iron species may be fed in the form of iron
metal and/or an iron compound, in particular iron oxide,
iron fluoride, iron oxyfluoride and/or an iron-aluminium
alloy.
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Advantageously, the cell may comprise an aluminium-
wettable cathode which can be a drained cathode on which
aluminium is produced and from which it continuously
drains, as described in US Patent 5,651,874 (de
Nora/Sekhar) and 5,683,559 (de Nora).
Usually, the cell is in a monopolar, mufti-monopolar
or in a bipolar configuration. The bipolar cell comprises
a terminal cathode facing a terminal anode and
thereinbetween at least one bipolar electrode, the
anodes) described above forming the anodic side of the or
each bipolar electrode and/or of the terminal anode.
In such a bipolar cell an electric current is passed
from the surface of the terminal cathode to the surface of
the terminal anode as ionic current in the electrolyte and
as electronic current through the bipolar electrodes,
thereby producing aluminium on each cathode surface and
oxygen on each anode surface.
Preferably, the cell comprises means to improve the
circulation of the electrolyte between the anodes and
facing cathodes and/or means to facilitate dissolution of
alumina in tile electrolyte. Such means can for instance be
provided by the geometry of the cell as described in co-
pending application W099/41429 (de Nora/Duruz) or by
periodically moving the anodes as described in co-pending
application W099/41430 (Duruz%Bellb).
Yet another aspect of the invention is a method of
producing aluminium in a cell as described above. The
method comprises dissolving alumina in the electrolyte and
passing an ionic electric current between the
electrochemically active surface of the anodes) and the
surface of the cathode(s), thereby producing aluminium on
the cathode surfaces) and oxygen on the anode surface(s).
Yet a further aspect of the invention is a method
of manufacturing an anode and producing aluminium in an
electrolytic cell comprising inserting an anode precursor
as described above into the electrolyte of an electrolytic
cell and forming the iron oxide-based layer to produce a
fully manufactured anode, and producing oxygen on the
surface of the iron oxide-based layer and aluminium on a
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facing cathode in the same (or nearly the same) or in a
different electrolyte.
The thus-produced anode may then be transferred
from the electrolytic cell in which it was produced to an
aluminium electrowinning cell. Alternatively the
composition of the electrolyte in which the anode was
produced can be suitably modified, for instance by
dissolving alumina and optionally iron species, and
electrolysis continued in the same cell to produce
aluminium.
Detailed Description
The invention will be further described in the
following Examples:
Example 1
Electrolysis was carried out in a laboratory scale
cell equipped with an anode according to the invention.
The anode was made with a Cor-TenTM type low-carbon
high-strength (HSLA) steel containing niobium, titanium,
chromium and copper in a total amount of less than 4
weighto. The anode was pre-oxidised in air at about 1050°C
for 15 hours for the formation of a dense hematite-based
outer layer.
The anode was then tested in a fluoride-containing
molten electrolyte at 850°C and at a current density of
about 0.7 A/cm2. The electrolyte contained cryolite and 15
weighto excess of AlF3, approximately 3 weighto alumina
and approximately 200 ppm iron species obtained from the
dissolution of iron oxide thereby surely saturating the
electrolyte with iron species and inhibiting dissolution
of the hematite-based anode surface layer.
To maintain the concentration of dissolved alumina
in the electrolyte, fresh alumina was periodically fed
into the cell. The alumina feed contained sufficient iron
oxide so as to replace the iron which had been deposited
into the product aluminium, thereby maintaining the
concentration of iron in the electrolyte at the limit of
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solubility and preventing dissolution of the hematite-
based anode surface layer.
After 140 hours electrolysis was interrupted and
the anode extracted. Upon cooling the anode was examined
externally and in cross-section. No corrosion was observed
at or near the surface of the anode.
The produced aluminium was also analysed and
showed an iron contamination of about 700 ppm which is
below the tolerated iron contamination in commercial
aluminium production.
Example 2
As in Example 1, aluminium was produced in a
laboratory scale cell equipped with an anode according to
the invention.
The anode was made with a low-carbon high-strength
(HSLA) steel containing manganese 0.4 weighto, niobium
0.02 weighto, molybdenum 0.02 weight%, copper 0.3 weighto,
nickel 0.45 weighto and chromium 0.8 weighto. The anode
was pre-oxidised in air at about 850°C for 12 hours to
form a dense hematite-based outer layer.
The anode was then tested under similar conditions
as in Example 1 and the test results were similar.
Example 3
As in Example 1, aluminium was produced in a
laboratory scale cell equipped with an anode according to
the invention.
The anode was made with a low-carbon high-strength
(HSLA) steel containing nickel, copper and silicon in a
total amount of less than 1 . 5 weight o . The anode was pre-
oxidised in air at about 850°C for 12 hours to form a
dense hematite-based outer layer.
The anode was then tested under similar conditions
as in Example 1 and the test results were similar.
