U.S. patent application number 11/103092 was filed with the patent office on 2005-09-08 for metal-based anodes for aluminium electrowinning cells.
Invention is credited to De Nora, Vittorio, Duruz, Jean-Jacques, Nguyen, Thinh T..
Application Number | 20050194066 11/103092 |
Document ID | / |
Family ID | 34910335 |
Filed Date | 2005-09-08 |
United States Patent
Application |
20050194066 |
Kind Code |
A1 |
Duruz, Jean-Jacques ; et
al. |
September 8, 2005 |
Metal-based anodes for aluminium electrowinning cells
Abstract
An anode of a cell for the electrowinning of aluminium comprises
a nickel-iron alloy substrate having an openly porous nickel metal
rich outer portion whose surface is electrochemically active. The
outer portion is optionally covered with an external integral
nickel-iron oxide containing surface layer which adheres to the
nickel metal rich outer portion of the nickel-iron alloy and which
in use is pervious to molten electrolyte. During use, the nickel
metal rich outer portion contains cavities some or all of which are
partly or completely filled with iron and nickel compounds, in
particular oxides, fluorides and oxyfluorides.
Inventors: |
Duruz, Jean-Jacques;
(Geneva, CH) ; Nguyen, Thinh T.; (Onex, CH)
; De Nora, Vittorio; (Nassau, BS) |
Correspondence
Address: |
J. R. Deshmukh
458 Cherry Hill Rd.
Princeton
NJ
08540
US
|
Family ID: |
34910335 |
Appl. No.: |
11/103092 |
Filed: |
April 11, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11103092 |
Apr 11, 2005 |
|
|
|
PCT/IB00/01812 |
Dec 6, 2000 |
|
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Current U.S.
Class: |
148/241 ;
204/293; 205/385; 420/435 |
Current CPC
Class: |
C22C 19/07 20130101;
C25C 3/12 20130101 |
Class at
Publication: |
148/241 ;
204/293; 205/385; 420/435 |
International
Class: |
C22C 019/07; C25C
003/12 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 9, 1999 |
WO |
PCT/IB99/01976 |
Oct 16, 2000 |
WO |
PCT/IB00/01481 |
Claims
1-53. (canceled)
54. An anode of a cell for the electrowinning of aluminium from
alumina dissolved in a fluoride-containing molten electrolyte, said
anode comprising a cobalt-iron alloy having an openly porous cobalt
rich outer portion which consists predominantly of cobalt metal and
whose surface constitutes an electrochemically-active anode surface
of high active surface area, the openly porous cobalt-rich outer
portion having a vermicular porosity obtainable by removal of at
least part of the iron from the cobalt-iron alloy.
55. The anode of claim 54, wherein the cobalt-iron alloy is further
alloyed with nickel, cobalt being predominant over nickel.
56. The anode of claim 54, wherein the cobalt rich openly porous
outer portion contains pores which are partly or completely filled
with iron and cobalt compounds.
57. The anode of claim 55, wherein the pores have an average
diameter of up to 5 micron and an average length of up to 30
micron.
58. The anode of claim 55, wherein the openly porous cobalt-rich
outer portion has a thin integral oxide film which underlies the
electrochemically active anode surface.
59. The anode of claim 58, wherein said oxide film has a thickness
of less than 1 micron.
60. The anode of claim 54, which is covered with a thick external
integral cobalt-iron containing oxide layer which adheres to the
openly porous outer portion and which is pervious to molten
electrolyte.
61. The anode of claim 60, wherein the external integral oxide
layer has a thickness of less than 50 micron, in particular from 5
to 30 micron.
62. The anode of claim 60, wherein said external integral oxide
layer comprises iron-rich cobalt-iron oxide.
63. The anode of claim 62, wherein said external integral oxide
layer comprises cobalt-ferrite.
64. The anode of claim 63, wherein the cobalt-ferrite of said
external integral oxide surface layer contains non-stoichiometric
cobalt-ferrite having an excess of iron or cobalt, and/or an oxygen
deficiency.
65. The anode of claim 54, wherein the cobalt-iron alloy comprises
a non-porous inner portion.
66. The anode of claim 65, wherein the non-porous inner portion has
a Ni/Fe atomic ratio below 1 before use.
67. The anode of claim 54, wherein the cobalt-rich openly porous
outer portion has a Ni/Fe atomic ratio of at least 1, in particular
from 1 to 4, before use.
68. The anode of claim 54, wherein the cobalt rich openly porous
outer portion has a decreasing concentration of iron metal towards
its outermost part.
69. The anode of claim 68, wherein the outermost part of the openly
porous cobalt rich outer portion comprises cobalt metal and iron
metal in an Co/Fe atomic ratio of more than 3.
70. The anode of claim 54, wherein the cobalt-iron alloy comprises
cobalt metal and iron metal in a total amount of at least 65 weight
%, in particular at least 80 weight %, preferably at least 90
weight % of the alloy.
71. The anode of claim 70, wherein the cobalt-iron alloy comprises
at least one further metal selected from chromium, copper, nickel,
silicon, titanium, tantalum, tungsten, vanadium, zirconium,
yttrium, molybdenum, manganese and niobium in a total amount of up
to 10 weight % of the alloy.
72. The anode of claim 70, wherein the cobalt-iron alloy comprises
at least one catalyst selected from iridium, palladium, platinum,
rhodium, ruthenium, tin or zinc metals, Mischmetals and their
oxides and metals of the Lanthanide series and their oxides as well
as mixtures and compounds thereof, in a total amount of up to 5
weight % of the alloy.
73. The anode of claim 70, wherein the cobalt-iron alloy comprises
aluminium in an amount less than 20 weight %, in particular less
than 10 weight %, preferably from 1 to 5 or even 6 weight % of the
alloy.
74. The anode of claim 54, comprising a core made of an
electronically conductive material, such as metals, alloys,
intermetallics, cermets and conductive ceramics, which is covered
with the cobalt-iron alloy.
75. The anode of claim 74, wherein the core is a non-porous cobalt
rich cobalt-iron alloy.
76. A method of manufacturing an anode according to claim 54 for
use in a cell for the electrowinning of aluminium, comprising
forming the cobalt-rich openly porous outer portion which consists
predominantly of cobalt metal by providing a cobalt-iron alloy
having an outer portion and selectively removing at least part of
the iron from the outer portion.
77. The method of claim 76, wherein the cobalt-rich openly porous
outer portion is formed by selectively removing iron from a cobalt
iron alloy by electrolytic dissolution.
78. The method of claim 76, wherein the cobalt-rich openly porous
outer portion is formed by selectively oxidising and diffusing iron
from a cobalt-iron alloy.
79. The method of claim 78, wherein an external integral
cobalt-iron oxide containing layer pervious to molten electrolyte
is formed from the diffused oxidised iron rather than cobalt, the
oxide surface layer adhering to the openly porous cobalt rich outer
portion, the oxidation of the cobalt-iron alloy comprising one or
more steps at a temperature of 800.degree. to 1200.degree. C. for
up to 60 hours in an oxidising atmosphere.
80. The method of claim 78, wherein the cobalt-iron alloy is
oxidised in an oxidising atmosphere for 0.5 to 10 hours.
81. The method of claim 79, wherein the oxidising atmosphere
consists of oxygen or a mixture of oxygen and one or more inert
gases having an oxygen content of at least 10 molar/o of the
mixture.
82. The method of claim 79, wherein the oxidising atmosphere is
air.
83. The method of claim 78, comprising oxidising the cobalt-iron
alloy at a temperature of 1050.degree. to 1150.degree. C.
84. The method of claim 78, comprising subjecting the cobalt-iron
alloy to a thermal-mechanical treatment to modify its
microstructure before oxidation.
85. The method of claim 78, comprising casting the cobalt-iron
alloy with additives to provide a microstructure for enhancing
oxidation.
86. The method of claim 81, wherein oxidation in the oxidising
atmosphere is followed by a heat treatment in an inert atmosphere
at a temperature of 800.degree. to 1200.degree. C. for up to 60
hours.
87. The method of claim 77, wherein the selective removal of iron,
in particular by oxidation in the oxidising atmosphere, is carried
out partly before use of the anode and is continued in-situ by iron
dissolution at electrolysis start-up.
88. The method of claim 76, comprising forming a cobalt-iron alloy
layer on a core made of an electronically conductive material, such
as a cobalt-rich cobalt-iron alloy.
89. The method of claim 88, comprising depositing cobalt and iron
metal on the core.
90. The method of claim 88, comprising depositing cobalt and iron
compounds on the core and then reducing the compounds.
91. The method of claim 90, wherein the cobalt and iron compounds
are Fe(OH).sub.2 and Co(OH).sub.2 which are reduced in a hydrogen
atmosphere to form an openly porous cobalt-iron alloy layer.
92. The method of claim 88, comprising co-depositing cobalt and
iron and/or compounds thereof onto the core.
93. The method of claim 88, comprising depositing at least one
layer of iron and/or an iron compound and at least one layer of
cobalt and/or a cobalt compound onto the core, and then
interdiffusing the layers.
94. The method of claim 88, comprising depositing electrolytically
or chemically at least one of cobalt, iron and compounds thereof
onto the core.
95. The method of claim 88, comprising arc spraying or plasma
spraying at least one of cobalt, iron and compounds thereof onto
the core.
96. The method of claim 88, comprising applying at least one of
cobalt, iron and compounds thereof by painting, dipping or spraying
onto the core.
97. The method of claim 76, wherein the cobalt-rich openly porous
outer portion is formed by sintering a powder precursor.
98. A cell for the electrowinning of aluminium from alumina
dissolved in a fluoride-containing molten electrolyte, the cell
comprising at least one anode as defined in claim 54 facing and
spaced from at least one cathode.
99. A method of producing aluminium in a cell according to claim 98
containing alumina dissolved in a molten electrolyte, the method
comprising passing an ionic current in the molten electrolyte
between the cathode(s) and the electrochemically active surface
layer of the anode(s), thereby evolving at the anode(s) oxygen gas
derived from the dissolved alumina and producing aluminium on the
cathode(s).
100. The method of claim 99, wherein at least part of the iron
rather than cobalt of the cobalt-rich openly porous outer portion
of at least one anode is selectively removed by electrolytic
dissolution in-situ.
101. The method of claim 99, wherein at least part of the iron
rather than cobalt of the cobalt-rich openly porous outer portion
of at least one anode is selectively removed by oxidising said
outer portion in-situ by atomic and/or molecular oxygen formed on
the electrochemically active surface until the electrochemically
active surface forms a barrier impervious to oxygen.
102. The method of claim 99, comprising permanently and uniformly
substantially saturating the molten electrolyte with alumina and
species of at least one major metal present in the cobalt-rich
openly porous outer portion of the anode(s) to inhibit dissolution
of the anode(s).
103. The method of claim 102, wherein the cell is operated with the
molten electrolyte at a temperature sufficiently low to limit the
solubility of said major metal species thereby limiting the
contamination of the product aluminium to an acceptable level.
104. The method of claim 99, wherein the cell is operated with the
molten electrolyte at a temperature from 730.degree. to 910.degree.
C.
105. The method of claim 99, wherein aluminium is produced on an
aluminium-wettable cathode, in particular a drained cathode.
Description
FIELD OF THE INVENTION
[0001] 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, methods for
their fabrication, and electrowinning cells containing such anodes
and their use to produce aluminium.
BACKGROUND ART
[0002] The technology for the production of aluminium by the
electrolysis of alumina, dissolved in molten cryolite, at
temperatures around 950.degree. C. is more than one hundred years
old. This process, conceived almost simultaneously by Hall and
Hroult, has not evolved as many other electrochemical
processes.
[0003] 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 CO.sub.2 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.
[0004] Using metal anodes in aluminium electrowinning cells would
drastically improve the aluminium process by reducing pollution and
the cost of aluminium production.
[0005] U.S. Pat. No. 4,374,050 (Ray) discloses inert anodes made of
specific multiple metal compounds which are produced by mixing
powders of the metals or their compounds in given ratios followed
by pressing and sintering, or alternatively by plasma spraying the
powders onto an anode substrate. The possibility of obtaining the
specific metal compounds from an alloy containing the metals is
mentioned.
[0006] U.S. Pat. No. 4,614,569 (Duruz/Derivaz/Debely/Adorian)
describes non-carbon 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 by the
addition 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 the gaseous oxygen
but not from the nascent monoatomic oxygen.
[0007] 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, U.S. Pat. Nos. 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 oxygen barrier layer. However,
full protection of the alloy substrate was difficult to
achieve.
[0008] U.S. Pat. No. 5,510,008 (Sekhar/Liu/Duruz) discloses an
anode made from an inhomogeneous porous metallic body obtained by
micropyretically reacting a metal powder mixture of nickel, iron,
aluminium and optionally copper. The porous metal is anodically
polarised in-situ to form a dense iron-rich oxide outer portion
whose surface is electrochemically active. Bath materials such as
cryolite which may penetrate the porous metallic body during
formation of the oxide layer become sealed off from the electrolyte
and from the active outer surface of the anode where electrolysis
takes place, and remain inert inside the electrochemically-inactive
inner metallic part of the anode.
[0009] PCT publication WO00/06803 (Duruz/de Nora/Crottaz) discloses
an anode produced from a nickel-iron alloy which is surface
oxidised to form a coherent and adherent outer iron oxide-based
layer whose surface is electrochemically active surface. Oxidation
is carried out in air for 5 to 100 hours at a temperature of
750.degree. to 1150.degree. C., in particular at
850.degree.-950.degree. C. for 24 hours or at 1150.degree. C. for
72 hours, to grow the coherent outer oxide layer from the alloy and
to a thickness from about 100 to 300 micron. This oxidation
depletes the outer part of the alloy in iron metal and produces
therein inclusions of iron oxide. The coherent and adherent outer
iron oxide-based layer reduces the diffusion of oxygen and prevents
electrolyte circulation to the alloy underneath so that oxidation
of ions from the bath is confined on the electrochemically active
surface of the oxide layer.
[0010] 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 for
commercial aluminium production because their lifetime must still
be increased.
OBJECTS OF THE INVENTION
[0011] 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 has a long life.
[0012] A further object of the invention is to provide an aluminium
electrowinning anode material with a surface having a high
electrochemical activity for the oxidation of oxygen ions and the
formation of bimolecular gaseous oxygen and a low solubility in the
electrolyte.
[0013] Yet another object of the invention is to provide an
improved anode for the electrowinning of aluminium which is made of
readily available material(s).
[0014] 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.
SUMMARY OF THE INVENTION
[0015] The invention relates to an anode of a cell for the
electrowinning of aluminium from alumina dissolved in a
fluoride-containing molten electrolyte. The anode comprises a
nickel-iron alloy having an openly porous nickel rich outer portion
which consists predominantly of nickel metal and whose surface
constitutes an electrochemically-active anode surface of high
active surface area, the openly porous nickel-rich outer portion
being obtainable by removal of at least part of the iron from the
nickel-iron alloy.
[0016] Cermet anodes which have been described in the past in
relation to aluminium production have an oxide content which forms
the major phase of the anode. Such anodes have an overall
electrical conductivity which is higher than that of solid ceramic
anodes but insufficient for industrial commercial production.
Moreover, the uniformly distributed metallic phase is exposed to
dissolution into the electrolyte.
[0017] Conversely, anodes predominantly made of metal and protected
with a thick oxide outer layer (about 0.1 to 1 mm), e.g. as
disclosed in U.S. Pat. No. 5,510,008 (Sekhar/Liu/Duruz) and
WO00/06803 (Duruz/de Nora/Crottaz), have a higher conductivity and
longer life because the metal is normally shielded from the bath
and resists dissolution therein. However, in case such a thick
oxide layer is damaged, molten electrolyte may penetrate into
cracks between the metallic inner part and the oxide layer. The
surfaces of the crack would then form a dipole between the metallic
inner anode part and the oxide layer, causing electrolytic
dissolution of the metallic inner part into the electrolyte
contained in the crack and corrosion of the metallic anode part
underneath the thick oxide layer.
[0018] The anode of the present invention provides a solution to
this problem. Instead of being covered with a thick protective
oxide layer, during use the metallic inner anode part contacts or
virtually contacts circulating molten electrolyte. As opposed to
prior art anodes, the electrolyte close to the openly porous anode
portion, typically at a distance of less than 10 micron, is
continuously replenished with Dissolved alumina. The electrolysis
current does not dissolve the anode. Instead the entire
electrolysis current passed at the anode surface is used for the
electrolysis of alumina by oxidising oxygen-containing ions
directly on the active surfaces or by firstly oxidising
fluorine-containing ions that subsequently react with
oxygen-containing ions, as described in PCT/IB99/01976 (Duruz/de
Nora).
[0019] Furthermore, the overall electrical conductivity of the
metal anode according to the present invention is substantially
higher than that of anodes covered with a thick oxide protective
layer or made of bulk oxide.
[0020] In addition, the open porosity of the nickel-metal rich
outer part provides an electrochemically active surface of high
surface area. Hence, the anode can be operated with an apparent
high electrolysis current while having a low effective current
density at the anode's electrochemically active surface which makes
it suitable for use in an electrolyte at reduced temperature
containing a limited concentration of dissolved alumina.
[0021] The metal phase underlying the electrochemically active
surface layer of the anode forms a matrix containing a minor amount
of metal compound inclusions. Such inclusions can include oxides
resulting from a pre-oxidation treatment in an oxidising atmosphere
before use, or from oxidation during use, as well as fluorides and
oxyfluorides resulting from use. This matrix confers an overall
high electrical conductivity to the anode.
[0022] After an oxidation treatment before use, the inclusions may
be iron-rich nickel-iron oxides, typically containing oxidised iron
and oxidised nickel in an Fe/Ni atomic ratio above 2.
[0023] The nickel rich openly porous outer portion may contain
pores, in particular round or elongated cavities in different
configurations such as a vermicular configuration, which are partly
or completely filled with iron and nickel compounds. The pores may
have an average diameter of up to 5 micron and an average length of
up to 30 micron.
[0024] The anode may be covered with two different types of oxide
layers.
[0025] The first possible type of oxide layer is a thin integral
oxide film, in particular having a thickness of less than 1 micron,
which extends at the surface of the openly porous nickel-rich outer
portion and along its pores and which underlies the
electrochemically active anode surface.
[0026] The second possible type of oxide layer is a thicker
integral nickel-iron containing oxide layer external but adhering
to the openly porous outer portion and pervious to molten
electrolyte, as mentioned above. The external integral oxide layer
of the invention is usually thin, preferably having a thickness of
less than 50 micron, in particular from 5 to 20 or even 30
micron.
[0027] Such an external integral oxide layer offers the advantage
of limiting the width of possible pores and/or cracks present in
the surface layer to a small size, usually below about a tenth of
the thickness of the surface layer. When a small pore and/or crack
is filled with molten electrolyte, the electrochemical potential
difference in the molten electrolyte across the pore and/or crack
is below the reduction-oxidation potential of any metal oxide of
the surface layer present in the molten electrolyte contained in
the pore and/or crack. Therefore, such a surface layer cannot be
dissolved by electrolysis of its constituents within the pores
and/or cracks.
[0028] On the other hand, the external integral oxide layer may be
sufficiently electrically conductive to be electrochemically active
and contribute to the oxidation of ions. Nevertheless, given the
respective electrical resistivity of the external oxide layer and
the electrolyte, it is believed that oxidation of ions
predominantly takes place on the electrochemically active surface
of the openly porous nickel-rich outer portion.
[0029] As mentioned above, the thinness of the external integral
oxide layer permits circulation of electrolyte to the openly porous
outer portion. When monoatomic oxygen evolved during electrolysis
or resulting from dissolution in the electrolyte of biatomic
molecular oxygen possibly reaches nickel metal instead of iron
metal of the nickel metal rich outer portion, the nickel metal is
oxidised to passive nickel oxide on the surface of the nickel metal
rich outer portion. However, the presence of oxygen near the metal
of the openly porous nickel-metal rich outer portion can be
minimised by oxidising fluoride-containing ions instead of oxygen
ions at the electrochemically active surface, as discussed in
greater detail in the Examples and in PCT/IB99/01976 (Duruz/de
Nora).
[0030] The external integral oxide layer usually comprises
iron-rich nickel-iron oxide, such as nickel-ferrite, in particular
non-stoichiometric nickel-ferrite. For instance, the external
integral oxide layer may comprise nickel-ferrite having an excess
of iron or nickel and/or an oxygen-deficiency.
[0031] The nickel-iron alloy may further comprise a non-porous
oxide-free inner portion.
[0032] Before use, the anode can have an overall Ni/Fe atomic ratio
below 1. Alternatively, it may be of at least 1, in particular from
1 to 4. For example, the inner portion of the anode may have a
Ni/Fe atomic ratio below 1 and the outer portion of the anode may
have a Ni/Fe atomic ratio from 1 to 4.
[0033] Usually the nickel rich openly porous outer portion has a
decreasing concentration of iron metal towards its outermost part.
This outermost part may comprise nickel metal and iron metal in an
Ni/Fe atomic ratio of about 3 or more.
[0034] The nickel-iron alloy usually comprises nickel metal and
iron metal in a total amount of at least 65 weight %, usually at
least 80, 90 or 95 weight %, of the alloy, and further alloying
metals in an amount of up to 35 weight %, in particular up to 5, 10
or 20 weight %, of the alloy. Minor amounts of further elements,
such as carbon, boron, sulphur, phosphorus or nitrogen, may be
present in the nickel-iron alloy, usually in a total amount which
does not exceed 2 weight % of the alloy.
[0035] For example, the nickel-iron alloy can comprise at least one
further metal selected from chromium, copper, cobalt, silicon,
titanium, tantalum, tungsten, vanadium, zirconium, yttrium,
molybdenum, manganese and niobium in a total amount of up to 5 or
10 weight % of the alloy. The nickel-iron alloy may also comprise
at least one catalyst selected from iridium, palladium, platinum,
rhodium, ruthenium, tin or zinc metals, Mischmetals and their
oxides and metals of the Lanthanide series and their oxides as well
as mixtures and compounds thereof, in a total amount of up to 5
weight % of the alloy. Furthermore, the nickel-iron alloy may
comprise aluminium in an amount of up to 5, 10 or 20 weight % of
the alloy. The aluminium may form an intermetallic compound with
nickel which is known to be mechanically and chemically well
resistant.
[0036] The anode of the invention may comprise a inner core made of
an electronically conductive material, such as metals, alloys,
intermetallics, cermets and conductive ceramics, which inner core
is covered with the nickel-iron alloy. In particular, the inner
core may comprise at least one metal selected from copper,
chromium, nickel, cobalt, iron, aluminium, hafnium, molybdenum,
niobium, silicon, tantalum, tungsten, vanadium, yttrium and
zirconium, and combinations and compounds thereof. For instance,
the core may consist of an alloy comprising 10 to 30 weight % of
chromium, 55 to 90 weight % of at least one of nickel, cobalt
and/or iron and up to 15 weight % of at least one of aluminium,
hafnium, molybdenum, niobium, silicon, tantalum, tungsten,
vanadium, yttrium and zirconium.
[0037] In one embodiment, the inner core is a non-porous nickel
rich nickel-iron alloy, having a nickel/iron weight ratio that is
close to or higher than the nickel/iron weight ratio of the openly
porous nickel rich outer portion, for example from 1 to 4 or
higher, in particular above 3. Thus, during use, little or no iron
diffuses from the inner core.
[0038] Another aspect of the invention relates to a method of
manufacturing an anode as described above. The method comprises
forming the nickel-rich openly porous outer portion which consists
predominantly of nickel metal by: providing a nickel-iron alloy
having an outer portion and selectively removing at least part of
the iron from the outer portion; or providing particles of a
nickel-iron alloy precursor and agglomerating these particles into
an alloy with an openly porous outer portion.
[0039] When the anode is produced from a nickel-iron alloy, at
least part of the iron rather than nickel can be selectively
removed therefrom by electrolytic dissolution to form the
nickel-rich openly porous outer portion of the nickel-iron
alloy.
[0040] Alternatively, at least part of the iron rather than nickel
of the nickel-iron alloy may be selectively oxidised and diffused
therefrom to form the openly porous outer portion of the
nickel-iron alloy. An external integral nickel-iron oxide
containing layer pervious to molten electrolyte is usually formed
from the diffused oxide surface layer on the openly porous nickel
metal rich outer portion. The oxidation of the nickel-iron alloy
may comprise one or more steps at a temperature of 800.degree. to
1200.degree. C., in particular 1050.degree. to 1150.degree. C., for
up to 60 hours in an oxidising atmosphere. Preferably, the
nickel-iron alloy substrate is oxidised in an oxidising atmosphere
for a short period of time, such as 0.5 to 5 or even 10 hours. The
oxidising atmosphere may contain 10 to 100 molar % oxygen and the
balance one or more inert gases, such as argon. Conveniently, the
oxidising atmosphere can be air.
[0041] In order to obtain a microstructure of the nickel-iron alloy
giving upon oxidation an optimal electrochemically active surface
layer on an optimal nickel metal rich outer portion, the
nickel-iron alloy may be subjected to a thermal-mechanical
treatment for modifying its microstructure before oxidation.
Alternatively, it may be cast, before oxidation, with known casting
additives.
[0042] Furthermore, the oxidation of the nickel-iron alloy in an
oxidising atmosphere may be followed by a heat treatment in an
inert atmosphere at a temperature of 800.degree. to 1200.degree. C.
for up to 60 hours. The selective removal of iron, in particular by
oxidation in an oxidising atmosphere, can be carried out before use
of the anode, then continued by iron dissolution in-situ at the
beginning of electrolysis.
[0043] As mentioned above, the nickel-iron alloy layer may be
formed on an inner core made of an electronically conductive
material, such as a nickel-rich nickel-iron alloy inner core.
Nickel and iron metal may be deposited as such onto the inner core,
or compounds of nickel and iron may be deposited on the inner core
and then reduced, for example one or more layers of Fe(OH).sub.2
and Ni(OH).sub.2 are deposited onto the core, e.g. as a colloidal
slurry, and reduced in a hydrogen atmosphere to form an openly
porous nickel-iron alloy layer. Nickel and iron and/or compounds
thereof may be co-deposited onto the inner core or deposited
separately in different layers which are then interdiffused, e.g.
by heat treatment. This heat treatment may take place in an inert
atmosphere such as argon, if the nickel and iron are applied as
metals, or in a reducing atmosphere such as hydrogen, if nickel and
iron compounds are applied onto the inner core. The nickel and iron
metals and/or compounds may be deposited by electrolytic or
chemical deposition, arc or plasma spraying, painting, dipping or
spraying.
[0044] When the anode is manufactured by providing particles of a
nickel-iron alloy precursor of the openly-porous outer portion,
these particles may be agglomerated by reactive or non-reactive
sintering.
[0045] A further aspect of the invention concerns a cell for the
electrowinning of aluminium from alumina dissolved in a
fluoride-containing molten electrolyte. The cell according to the
invention comprises at least one anode as described above which
faces and is spaced from at least one cathode.
[0046] The invention also relates to a method of producing
aluminium in such a cell. The method comprises passing an ionic
current in the molten electrolyte between the cathode(s) and the
electrochemically active surface layer of the anode(s), thereby
evolving at the anode(s) oxygen gas derived from the dissolved
alumina and producing aluminium on the cathode(s).
[0047] At least part of the iron rather than nickel of the
nickel-rich openly porous outer portion of the anode(s) may be
selectively removed by electrolytic dissolution in-situ.
[0048] At the beginning of electrolysis, at least part of the iron
rather than nickel of the nickel-rich openly porous outer portion
of the anode(s) may be removed by oxidising the outer portion
in-situ by atomic and/or molecular oxygen formed on the
electrochemically active surface until the electrochemically active
surface forms an impervious barrier to oxygen.
[0049] Advantageously, the method includes substantially saturating
the molten electrolyte with alumina and species of at least one
major metal, usually iron and/or nickel, present in the nickel-rich
openly porous outer portion of the anode(s) to inhibit dissolution
of the anode(s). The molten electrolyte may be operated at a
temperature sufficiently low to limit the solubility of the major
metal species thereby limiting the contamination of the product
aluminium to an acceptable level.
[0050] A "major metal" refers to a metal which is present in the
surface of the metal-based anode, in an amount of at least 25
atomic % of the total amount of metal present in the surface of the
metal based anode.
[0051] The cell can be operated with the molten electrolyte at a
temperature from 730.degree. to 910.degree. C., in articular below
870.degree. C.
[0052] As disclosed in PCT/IB99/01976 (Duruz/de Nora), the
electrolyte may contain AlF.sub.3 in such a high concentration that
fluorine-containing ions predominantly rather than oxygen ions are
oxidised on the electrochemically active surfaces, however, only
oxygen is evolved, the evolved oxygen being derived from the
dissolved alumina present near the electrochemically active anode
surfaces.
[0053] Preferably, aluminium is produced on an aluminium-wettable
cathode, in particular on a drained cathode, for instance as
disclosed in U.S. Pat. No. 5,683,559 (de Nora) or in PCT
application WO99/02764 (de Nora/Duruz).
[0054] In a modification, the nickel of the nickel-iron alloy, in
particular of the openly porous outer portion, is wholly or
predominantly substituted by cobalt.
DETAILED DESCRIPTION
[0055] The invention will be further described in the following
Examples:
EXAMPLE 1
[0056] Anode Preparation:
[0057] An anode according to the invention was made by
pre-oxidising in air at 1100.degree. C. for 1 hour a substrate of a
nickel-iron alloy consisting of 60 weight % nickel and 40 weight %
iron, whereby an external integral oxide layer was formed on the
alloy.
[0058] The surface-oxidised anode was cut perpendicularly to the
anode operative surface and the resulting section of the anode was
subjected to microscopic examination.
[0059] The anode before use had an openly porous nickel metal rich
outer portion having a thickness of up to 10-15 micron. This outer
portion was covered with the external integral oxide layer that was
made of iron-rich nickel-iron oxide and had a thickness of up to
10-20 micron. The openly porous outer portion was made of an
iron-depleted nickel-iron alloy containing generally round cavities
filled with iron-rich nickel-iron oxide inclusions and having a
diameter of about 2 to 5 micron. The nickel-iron alloy of the outer
portion contained about 75 weight % nickel.
[0060] Underneath the openly porous outer portion, the nickel-iron
alloy had remained substantially unchanged.
EXAMPLE 2
[0061] Electrolysis Testing:
[0062] An anode prepared as in Example 1 was tested in an aluminium
electrowinning cell containing a molten electrolyte at 870.degree.
C. consisting essentially of NaF and AlF.sub.3 in a weight ratio
NaF/AlF.sub.3 of about 0.7 to 0.8, i.e. an excess of AlF.sub.3 in
addition to cryolite of about 26 to 30 weight % of the electrolyte,
and approximately 3 weight % alumina. The alumina concentration was
maintained at a substantially constant level throughout the test by
adding alumina at a rate adjusted to compensate the cathodic
aluminium reduction. The test was run at a current density of about
0.6 A/cm.sup.2 which generally corresponds to a current density of
less than about 0.06 A/cm.sup.2 on the surface of the pores. The
electrical potential of the anode remained substantially constant
at 4.2 volts throughout the test.
[0063] During electrolysis aluminium was cathodically produced
while fluorine and/or fluorine-containing ions, such as aluminium
oxyfluoride ions, rather than oxygen ions were oxidised on the
nickel-iron anodes. However, only oxygen was evolved which was
derived from the dissolved alumina present near the anodes.
[0064] After 72 hours, electrolysis was interrupted and the anode
was extracted from the cell. The external dimensions of the anode
had remained unchanged during the test and the anode showed no
signs of damage.
[0065] The anode was cut perpendicularly to the anode operative
surface and the resulting section of the used anode was subjected
to microscopic examination, as in Example 1.
[0066] It was observed that the anode had an electrochemically
active surface covered with a discontinuous, non-adherent,
macroporous iron oxide external layer of the order of 100 to 500
micron thick, hereinafter called the "excess iron oxide layer". The
excess iron oxide layer was pervious to and contained molten
electrolyte, indicating that it had been formed during
electrolysis.
[0067] The excess iron oxide layer resulted from the excess of iron
contained in the portion of the nickel-iron alloy underlying the
electrochemically active surface and which diffuses therethrough.
In other words, the excess iron oxide layer resulted from an iron
migration from inside to outside the anode during the beginning of
electrolysis.
[0068] Such an excess iron oxide layer has no or little
electrochemical activity. It slowly diffuses and dissolves into the
electrolyte until the portion of the anode underlying the
electrochemically active surface reaches an iron content of about
15-20 weight % corresponding to an equilibrium under the operating
conditions at which iron ceases to diffuse, and thereafter the iron
oxide layer continues to dissolve into the electrolyte.
[0069] The anode's aforementioned openly porous outer portion had
been transformed during electrolysis. Its thickness had grown from
10-20 micron to about 300 to 500 micron and the cavities had also
grown in size to vermicular form but were only partly filled with
iron and nickel compounds, in particular oxides and fluorides or
oxyfluorides. No electrolyte was detected in the cavities and no
sign of corrosion appeared throughout the anode.
[0070] Underneath the outer portion, the nickel-iron alloy had
remained unchanged.
[0071] The shape and external dimensions of the anode had remained
unchanged after electrolysis which demonstrated stability of this
anode structure under the operating conditions in the molten
electrolyte.
[0072] In another test a similar anode was operated under the same
conditions for several hundred hours at a substantially constant
current and cell voltage which demonstrated the long anode life
compared to known non-carbon anodes.
EXAMPLE 3
[0073] Anode Preparation:
[0074] Another anode according to the invention was prepared by
coating a nickel-rich nickel-iron alloy substrate with a layer of
nickel-iron alloy richer in iron, and heat treating this coated
substrate. The alloy substrate consisted of 80 weight % nickel and
20 weight % iron. The alloy layer consisted of about 50 weight %
nickel and 50 weight % iron.
[0075] The alloy layer was electrodeposited onto the alloy
substrate using an appropriate electroplating bath prepared by
dissolving the following constituents in deionised water at a
temperature of about 50.degree. C.:
1 a. Nickel sulfate hydrate (NiSO.sub.4 .multidot. 7 H.sub.2O): 130
g/l b. Nickel chloride hydrate (NiCl.sub.2.6 H.sub.2 .multidot. O):
90 g/l c. Ferrous sulfate hydrate (FeSO.sub.4.78 H.sub.2 .multidot.
O): 52 g/l d. Boric acid H.sub.3BO.sub.3: 49 g/l e.
5-Sulfo-salicylic acid hydrate (C.sub.7H.sub.6O.sub.6S .multidot. 2
H.sub.2O): 5 g/l f. o-Benzoic acid sulfimide Sodium salt hydrate
3.5 g/l (C.sub.7H.sub.4NaO.sub.3S.aq): g. 1-Undecanesulfonic acid
Sodium salt (C.sub.11H.sub.23NaO.sub.3S): 3.5 g/l
[0076] To assist dissolution, the constituents were stirred in the
deionised water.
[0077] The alloy layer was electrodeposited onto the cathodically
polarised alloy substrate from a nickel-iron alloy anode consisting
of 50 weight % nickel and 50 weight % iron, immersed in the
electroplating bath at a temperature of 50 to 55.degree. C. After 4
hours electrodeposition at a cathodic current density of 0.060
A/cm.sup.2, the deposited layer had an average thickness of about
250 to 280 micron with an average composition of 47.5 weight %
nickel and 52.5 weight % iron.
[0078] After deposition, the coated alloy substrate was surface
oxidised at 1100.degree. C. in air for 1 hour and cooled to room
temperature. The surface-oxidised anode was then cut
perpendicularly to the anode operative surface and the resulting
section of the anode was subjected to microscopic examination as in
Example 1.
[0079] It was observed that the external anode surface was covered
with iron-rich nickel-iron oxides over a thickness of about 20 to
25 micron.
[0080] The alloy layer had an iron-depleted nickel-iron alloy
openly porous outer portion with a thickness of about 50 micron,
this outer portion containing generally round iron-rich nickel-iron
oxide inclusions in a nickel-iron alloy containing about 70 to 75
weight % nickel metal. The inclusions had a diameter of about 2 to
5 micron. Underneath this outer part, the composition of the alloy
layer had remained substantially unchanged.
[0081] Some minor interdiffusion of iron was also observed at the
interface between the alloy layer and the alloy substrate enhancing
the adherence of the layer on the substrate.
EXAMPLE 4
[0082] Electrolysis Testing:
[0083] An anode prepared as in Example 3 was tested in an aluminium
electrowinning cell as in Example 2 except that the electrolyte
contained approximately 4 weight % alumina and that the anode was
tested during 75 hours.
[0084] During electrolysis aluminium was produced and oxygen
evolved. The anode when inspected showed no signs of having been
subjected to the usual type of oxidation/passivation mechanisms
observed with prior art process. This lead to the conclusion that
predominantly fluorine and/or fluorine-containing ions, such as
aluminium oxyfluoride ions, rather than oxygen ions were oxidised
on the nickel-iron anodes. However, only oxygen as evolved which
was derived from the dissolved alumina resent near the anodes.
[0085] After electrolysis the anode was extracted from the cell and
examined.
[0086] The external surfaces of the anode were crust free and its
external dimensions were practically unchanged. No sign of damage
was visible.
[0087] The anode was cut perpendicularly to the operative surface
and the resulting section of the anode was subjected to the
microscopic examination as in Example 1.
[0088] It was observed that the anode surface was covered with an
iron rich oxide over a thickness of less than 25 to 50 micron. The
thinness of this oxide layer attested the fact that the anode had
not, or only marginally, been exposed to nascent monoatomic oxygen,
hence that the oxidation process of fluorine-containing ions was
predominant over the process of oxygen ions.
[0089] The anode's openly porous outer portion (depleted in iron
metal) had grown from 50 to about 250 micron containing mainly
empty pores. The pores were vermicular with a length limited to the
thickness of the overall alloy layer and a diameter of about 10
micron. The openly porous outer portion was further depleted in
iron metal and had a composition of about 75 weight % nickel and 25
weight % iron.
[0090] The structure and composition of the alloy substrate had
remained substantially unchanged, with the exception of empty pores
of random shape having a size of about 5 to 10 micron that were
located at the substrate/layer interface and up to a depth of 100
to 150 micron. The empty pores resulted from the internal oxidation
and diffusion towards the anode's surface of iron during
electrolysis.
EXAMPLE 5
[0091] Anode Preparation:
[0092] A metallic anode consisting of an alloy of 70 weight %
nickel and 30 weight % iron was conditioned to be suitable for
electrolysis according to the invention by anodic polarisation in
an electrolytic cell. The electrolytic cell contained a molten
electrolyte at 850.degree. C. consisting essentially of NaF and
AlF.sub.3 in a weight ratio NaF/AlF.sub.3 of about 0.7 to 0.8, i.e.
an excess of AlF.sub.3 in addition to cryolite of about 26 to 30
weight % of the electrolyte. The electrolyte contained no alumina
other than that present as impurity in the added AlF.sub.3 making
about 2 weight % of the electrolyte.
[0093] Before immersion into the electrolyte, the anode was
pre-heated for 0.5 hour over the cell to a temperature of about
750.degree. C.
[0094] After immersion into the conditioning electrolyte, the anode
was polarised at an initial current density of about 0.06-0.1
A/cm.sup.2 which decreased over time to less than about 0.01
A/cm.sup.2. The cell voltage was about 2.2 volt and the anode
potential was below 2 volt. Thus, substantially no oxygen could be
evolved during polarisation. The current passed during polarisation
was essentially due to selective anodic dissolution of iron present
at and close to the surface of the anode.
[0095] After 24 hours, polarisation was interrupted and the anode
was extracted from the cell. The external dimensions of the anode
had remained unchanged and was covered with black oxide.
[0096] This conditioned anode was ready to be used for the
production of aluminium according to the invention. The anode's
composition was ascertained by cutting it perpendicular to the
operative surface and the resulting section of the anode was
subjected to the microscopic examination, as in Example 1.
[0097] It was observed that the anode surface was covered with a
very thin film of iron-rich oxide having a thickness of less than 1
micron. Underneath, the anode had an iron-depleted nickel-iron
alloy openly porous outer portion which had an average thickness of
100 to 150 micron. This outer alloy portion had vermicular pores
with a diameter of 10 to 30 micron that were empty except for small
oxide inclusions.
[0098] The average metal composition of the openly porous outer
portion was about 80 weight % nickel and 20 weight % iron. Below
the openly porous outer portion, the initial nickel-iron alloy
composition had remained substantially unchanged.
[0099] In a variation of this Example, the composition of the anode
can be changed. For instance, the starting alloy contains 30 weight
% nickel and 70 weight % iron or 80 weight % nickel and 20 weight %
iron.
[0100] A coated substrate as described in Example 3 can also be
conditioned to form an anode suitable far the production of
aluminium according to the invention by dissolving part of the iron
of the anode as described in Example 5.
[0101] All or part of the nickel content of the anodes of Examples
1, 3 and 5 can be replaced by cobalt.
EXAMPLE 6
[0102] Electrolysis Testing:
[0103] An anode as prepared in Example 5 was used in an aluminium
electrowinning cell containing a molten electrolyte as described in
Example 4.
[0104] As in Example 4, during electrolysis aluminium was produced
and oxygen evolved. The anode inspection also led to the conclusion
that fluorine-containing ions predominantly rather than oxygen ions
were oxidised on the anode surface.
[0105] After 75 hours, electrolysis was interrupted and the anode
was extracted from the cell. The external surfaces of the anode
were crust free and its external dimensions were practically
unchanged. No sign of damage was visible.
[0106] The anode was cut perpendicularly to the operative surface
and the resulting section of the anode was subjected to the
microscopic examination as in Example 1.
[0107] It was observed that the anode surface was covered with a
iron rich oxide over a thickness of less than 25 to 50 micron. The
anode surface was covered by a very thin film of iron-rich oxide
having a thickness of less than 100 micron, which indicated that
the iron depletion during electrolysis was less than for a
pre-oxidised anode as in Example 2.
[0108] The anode's openly porous outer portion had grown from 150
micron to about 500 to 750 micron and contained pores that were
substantially empty in their majority. Below this openly porous
outer portion, the alloy composition had remained unchanged.
EXAMPLE 7
[0109] Anode Construction and Electrolysis Testing:
[0110] An anode having an active structure of 210 mm diameter was
made of three concentric rings spaced from one another by gaps of 6
mm. The rings had a generally triangular cross-section with a base
of about 19 mm and were connected to one another and to a central
vertical current supply rod by six members extending radially from
the vertical rod and equally spaced apart from one another around
the vertical rod. The gaps were covered with chimneys for guiding
the escape of anodically evolved gas to promote the circulation of
electrolyte and enhance the dissolution of alumina in the
electrolyte as disclosed in PCT publication WO00/40781 (de
Nora).
[0111] The anode and the chimneys were made from cast nickel-iron
alloy containing 50 weight % nickel and 50 weight % iron that was
heat treated as in Example 1. The anode was then tested in a
laboratory scale cell containing an electrolyte as described in
Example 2 except that it contained approximately 4 weight %
alumina.
[0112] During the test, a current of approximately 280 A was passed
through the anode at an apparent current density of about 0.8
A/cm.sup.2 on the apparent surface of the anode which generally
corresponds 1 to a current density of less than about 0.08
A/cm.sup.2 on the surface of the columnar pores of the anode. The
electrical potential of the anode remained substantially constant
at approximately 4.2 volts throughout the test.
[0113] The electrolyte was periodically replenished with alumina to
maintain the alumina content in the electrolyte close to
saturation. Every 100 seconds an amount of about 5 g of fine
alumina powder was fed to the electrolyte. The alumina feed was
periodically adjusted to the alumina consumption based on the
cathode efficiency, which was about 67%.
[0114] As in Examples 4 and 6, during electrolysis aluminium was
produced and oxygen evolved. The anode inspection also led to the
conclusion that fluorine-containing ions predominantly rather than
oxygen ions were oxidised on the anode surface.
[0115] After more than 1000 hours, i.e. 42 days, electrolysis was
interrupted and the anode was extracted from the cell and allowed
to cool. The external dimensions of the anode had not been
substantially modified during the test but the anode was covered
with iron-rich oxide and bath. The anode showed no sign of
damage.
[0116] The anode was cut perpendicularly to the anode operative
surface and the resulting section of a ring of the active structure
was subjected to microscopic examination, as in Example 1.
[0117] It was observed that the openly porous outer alloy portion
had grown inside the anode ring to a depth of about 7 mm leaving
only an inner portion of about 5 mm diameter unchanged, i.e.
consisting of a non-porous alloy of 50 weight % nickel and 50
weight % iron. The openly porous outer portion of the anode had a
concentration of nickel varying from 85 to 90 weight % at the anode
surface to 70 to 75 weight % nickel close to the non-porous inner
portion, the balance being iron. The iron depletion in the openly
porous outer portion corresponded about to the accumulation of iron
present as oxide on the surface of the anode, which indicated that
the iron oxide had not substantially dissolved into the electrolyte
during the test.
Summary of Examples
[0118] In summary, the analysis of the anodes tested in all the
above Examples showed that, at equal anode current, the oxidation
rate of nickel-alloy anodes was between about 20 and 100 times
smaller than the oxidation rate under conventional conditions in
which the oxidation of oxygen ions is the sole or the predominant
mechanism occurring at the surface of the anode, so in the above
described Examples the nickel-alloy anodes should last several
thousand hours, whereas in a normal cryolite electrolyte the anodes
last less than 50 hours.
[0119] It is believed that the greatly reduced oxidation of iron at
the anode surface under the present electrolysis conditions can
have two causes. The first possible cause of oxidation is exposure
to nascent oxygen produced by the oxidation of oxygen ions at the
anode surface which may marginally occur in parallel to the
oxidation of fluorine-containing ions and which might represent
less than 1% of the overall oxidation mechanism at the anode
surface. The second cause of oxidation is exposure to dissolved
molecular oxygen which is marginally present in the electrolyte at
a theoretical pressure of about 10.sup.-10 atm under the test
conditions.
[0120] If the surface of nickel-iron alloy anodes described above
were exposed to a significant oxygen concentration in the
electrolyte, the nickel of the anode would be rapidly oxidised into
NiO which would passivate the anode and prevent electrolysis. The
absence of such oxidation/passivation confirms that no or
substantially no oxygen ions are oxidised at the surface of the
nickel-alloy anodes.
[0121] In addition, the presence of sodium-free fluorides, such as
nickel, iron and aluminium fluorides and oxyfluorides, was observed
in the pores of the tested anodes. This indicates that not
electrolyte but fluorine or fluorides from the active anode surface
penetrated into these pores, and confirms that the mechanism of
oxidation of fluorine-containing ions took place at the surface of
the anodes.
* * * * *