U.S. patent application number 10/684649 was filed with the patent office on 2004-04-22 for method of making an inert anode for electrolytic reduction of metal oxides.
Invention is credited to Bengali, Abid, Cheetham, Jeffrey K., Meissner, David C., Musat, Jeffrey B., Srivastava, Ashvin.
Application Number | 20040074625 10/684649 |
Document ID | / |
Family ID | 32180501 |
Filed Date | 2004-04-22 |
United States Patent
Application |
20040074625 |
Kind Code |
A1 |
Musat, Jeffrey B. ; et
al. |
April 22, 2004 |
Method of making an inert anode for electrolytic reduction of metal
oxides
Abstract
A method for producing a non-carbon inert anode for metal oxide
electrolytic reduction by coating a metal, cermet, or ceramic
substrate with a molten metal oxide compound of a ferrite and at
least one divalent metal selected from iron, nickel, manganese,
magnesium, and cobalt. The coated anode is protected from attack by
the liberated oxygen and the solute in the electrolysis of metals,
such as aluminum, magnesium, lithium, or calcium. Apparatus for
carrying out the method, and the resulting product are also
disclosed.
Inventors: |
Musat, Jeffrey B.; (Canton,
OH) ; Meissner, David C.; (Charlotte, NC) ;
Srivastava, Ashvin; (Sarasota, FL) ; Cheetham,
Jeffrey K.; (N. Canton, OH) ; Bengali, Abid;
(Naperville, IL) |
Correspondence
Address: |
DOUGHERTY, CLEMENTS & HOFER
1901 ROXBOROUGH ROAD
SUITE300
CHARLOTTE
NC
28211
US
|
Family ID: |
32180501 |
Appl. No.: |
10/684649 |
Filed: |
October 14, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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|
10684649 |
Oct 14, 2003 |
|
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|
10641635 |
Aug 15, 2003 |
|
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60420240 |
Oct 22, 2002 |
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Current U.S.
Class: |
164/138 ;
164/72 |
Current CPC
Class: |
C25C 7/02 20130101; C25C
3/12 20130101 |
Class at
Publication: |
164/138 ;
164/072 |
International
Class: |
B22C 003/00 |
Claims
what is claimed is:
1. A method for producing an inert anode for metal oxide
electrolytic reduction, comprising the steps of: providing a
substrate consisting of a metal, a cermet, or a ceramic material;
forming a molten metal oxide compound comprising ferrite and at
least one divalent metal selected from the group consisting of
iron, nickel, manganese, magnesium, and cobalt; and coating said
substrate with said molten metal oxide compound to form an inert
anode.
2. A method according to claim 1, wherein said substrate is
oxidized, whereby it promotes adherence of said molten metal oxide
compound.
3. A method according to claim 1 further comprising attaching an
electrical connector to said anode substrate.
4. A method according to claim 1 wherein said substrate is a metal
selected from the group consisting of iron, nickel, cobalt,
chromium, copper, manganese, magnesium, or an alloy thereof.
5. A method according to claim 1 wherein said substrate is a cermet
selected from the group consisting of nickel ferrite in combination
with silver, copper, nickel, copper-silver alloy, nickel-copper
alloy, or nickel-copper-silver alloy.
6. A method according to claim 1 wherein said substrate is a nickel
ferrite ceramic material.
7. A method according to claim 6, wherein said ceramic further
includes at least one metal ion selected from the group consisting
of manganese, magnesium, and cobalt.
8. A method according to claim 1 wherein said substrate is
bi-metallic.
9. A method according to claim 8 wherein said bi-metallic substrate
is an iron base metal and nickel or nickel alloy.
10. A method according to claim 8 wherein said bi-metallic
substrate is iron-nickel-chromium alloy and nickel.
11. A method according to claim 1, wherein said molten metal oxide
compound is formed by melting oxides of iron and at least one other
metal, or by oxidizing iron and at least one other metal to form a
molten ferrite.
12. A method according to claim 1, wherein said step of coating
said substrate is carried out by spray atomization, immersion of
said substrate in a bath of said molten metal oxide compound, or by
pouring said molten metal oxide compound onto said substrate.
13. A method according to claim 1, wherein said substrate has a
surface, said surface being provided with raised or indented
portions.
14. A method according to claim 13, wherein said surface is
provided with knurls, dimples, or a waffle pattern.
15. A method according to claim 1, wherein said molten metal oxide
compound includes a metal dispersed therein, said metal being
selected from the group consisting of silver, copper, nickel,
copper-silver alloy, nickel-copper alloy, or nickel-copper-silver
alloy.
16. A method according to claim 1, further comprising adding a
dopant to the molten metal oxide compound.
17. A method according to claim 16, wherein the dopant is selected
from the group consisting of zinc, cobalt, or lithium
compounds.
18. A method according to claim 17 wherein the compounds are
oxides.
19. A method according to claim 17 wherein the compounds are
carbonates or sulfides.
20. A method according to claim 1, further comprising a
post-coating heat treatment of the coated anode in an
oxygen-containing atmosphere.
21. A method according to claim 20 wherein said post-coating heat
treatment is an anneal of the coated anode.
22. A method according to claim 20 wherein said post-coating heat
treatment is a phase composition adjustment comprising: soaking the
anode at a temperature of from 1000.degree. C. to 1400.degree. C.
for a sufficient time to oxidize any remaining metallic nickel and
metallic iron.
23. A method according to claim 22 further comprising slow cooling
the anode to a temperature of from 100 to 400.degree. C. lower than
said soaking temperature.
24. A method according to claim 23 further comprising soaking said
anode in an oxygen-containing gas for a second period of time at a
temperature to which the anode has been slow cooled, for final
phase composition adjustment and microstructure adjustment.
25. A method according to claim 23 further comprising hot isostatic
pressing of the anode at a temperature of at least 1000.degree. C.
and a pressure of at least 1360 bar for a period of from about 4 to
about 8 hours.
26. A method according to claim 24 further comprising hot isostatic
pressing of the anode at a temperature of at least 1000.degree. C.
and a pressure of at least 1360 bar for a period of from about 4 to
about 8 hours.
27. A method for producing an inert anode for metal oxide
electrolytic reduction, comprising the steps of: providing a
substrate consisting of a metal, a cermet, or a ceramic material;
feeding at least one compound selected from the group consisting of
nickel oxides, iron oxides, nickel ferrite, iron sulfides, nickel
sulfides, iron carbonates, nickel carbonates, or mixtures thereof,
to a melting vessel; melting the compounds and forming molten
nickel ferrite; discharging molten nickel ferrite from the melting
vessel at a temperature sufficient to maintain the molten nickel
ferrite in the molten state; adding a dopant to the nickel ferrite
to form a molten mixture; and coating said substrate with said
molten mixture to form an inert anode.
28. A method according to claim 27, wherein the dopant is selected
from the group consisting of zinc, cobalt, or lithium
compounds.
29. A method according to claim 28 wherein the dopant compounds are
oxides.
30. A method according to claim 28 wherein the dopant compounds are
carbonates or sulfides.
31. A method for producing an inert anode for metal oxide
electrolytic reduction, comprising the steps of: providing a
substrate consisting of a metal, a cermet, or a ceramic material;
feeding metallic iron and metallic nickel in solid form to an
oxidizing reactor; melting and oxidizing the iron and nickel and
forming molten nickel ferrite; discharging molten nickel ferrite
from the oxidizing reactor at a temperature sufficient to maintain
the molten nickel ferrite in the molten state; adding a dopant to
increase electrical conductivity to the nickel ferrite and to form
a molten mixture; and coating said substrate with said molten
mixture to form an inert anode.
32. A method according to claim 31, wherein the dopant is selected
from the group consisting of zinc, cobalt, or lithium
compounds.
33. A method according to claim 32 wherein the dopant compounds are
oxides, carbonates or sulfides.
34. Apparatus for producing an inert anode for metal oxide
electrolytic reduction, comprising: means for producing or
providing a substrate; means for forming a molten metal oxide; and
means for coating said substrate with said metal oxide to form said
anode.
35. Apparatus according to claim 34, wherein said means for forming
a molten metal oxide is an oxidizing reactor.
36. Apparatus according to claim 34, wherein said means for forming
a molten metal oxide is a melting vessel.
37. Apparatus according to claim 34, wherein said means for coating
said substrate is a spray atomizer.
38. Apparatus according to claim 34, wherein said means for coating
said substrate is a bath of molten oxide, and means for immersing
said substrate in said bath.
39. Apparatus according to claim 34, wherein said means for coating
said substrate is a container for said molten oxide compound, and
means for pouring said molten oxide compound over said
substrate.
40. Apparatus according to claim 34 further comprising means for
cooling said substrate during coating of said substrate.
41. Apparatus according to claim 40 wherein said means for cooling
is water cooling or forced air cooling.
42. An inert anode product for metal oxide electrolytic reduction
made by the method of claim 1.
43. An inert anode product for metal oxide electrolytic reduction
made by the method of claim 27.
44. An inert anode product for metal oxide electrolytic reduction
made by the method of claim 31.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application is a continuation-in-part of our co-pending
U.S. patent application Ser. No. 10/641,635, filed Aug. 15, 2003,
and also claims the benefit of U.S. Provisional Patent Application
Serial No. 60/420,240, filed Oct. 22, 2002.
FIELD OF THE INVENTION
[0002] The present invention relates to a method and apparatus for
producing an inert anode for metal oxide electrolytic reduction,
and more particularly relates to a means for protecting an anode
from attack by the electrolyte and liberated oxygen during the
electrolysis of alumina to aluminum.
BACKGROUND OF THE INVENTION
[0003] Since the advent of the electrolytic reduction process for
producing aluminum, the anodes used have been made of carbon which
is consumed during the electrolytic reduction process. In the more
recent past (20 years) there has been an effort to produce an inert
anode or electrode that is not consumed during reduction. Metal
anodes, ceramic anodes, and cermet anodes have been proposed for
development. Metal anodes are currently being developed in addition
to the development work being conducted on cermet anodes of the
electrolyte cell, which generally operates at a temperature of
about 950.degree. C. Metal anodes are attacked by the cryolite
electrolyte and by the oxygen evolved at the anode. The current
development effort is focused on developing coatings to protect the
metal anode or at least to slow down the rate of attack to
manageable levels. Metal oxide coatings have been proposed. These
coatings are applied to the metal anode by metal oxide slurry
addition, by plasma spray coating of a metal oxide or metal
followed by an oxidation step, or by the self-forming of an oxide
of the actual anode metal during electrolysis. The oxide coatings
formed tend to be somewhat porous to the oxygen evolved at the
anode, which then continues to attack or oxidize the metal
substrate of the anode. This attack or oxidation reduces the life
of the anode. Some of the coatings applied may be applied in
multiple layers, which in turn requires complex and costly
processing steps.
DESCRIPTION OF THE PRIOR ART
[0004] Applicant is aware of the following U.S. Patents concerning
non-carbon, metal based anodes for electrolytic reduction of
aluminum:
1 U.S. Pat. No. Inventor Title 6,436,274 DeNora et al. SLOW
CONSUMABLE NON- CARBON METAL-BASED ANODES FOR ALUMINUM PRODUCTION
CELLS 6,379,526 DeNora et al. NON-CARBON METAL-BASED ANODES FOR
ALUMINUM PRODUCTION CELLS 6,372,099 Duruz et al. CELLS FOR THE
ELECTROWINNING OF ALUMINUM HAVING DIMENSIONALLY STABLE METAL- BASED
ANODES 6,361,681 DeNora et al. SLURRY FOR COATING NON- CARBON
METAL-BASED ANODES FOR METAL PRODUCTION CELLS 6,248,227 DeNora et
al. SLOW CONSUMABLE NON- CARBON METAL-BASED ANODES FOR ALUMINUM
PRODUCTION CELLS 6,113,758 DeNora et al. POROUS NON-CARBON METAL-
BASED ANODES FOR ALUMINUM PRODUCTION CELLS 6,103,090 DeNora et al.
ELECTROCATALYTICALLY ACTIVE NON-CARBON METAL- BASED ANODES FOR
ALUMINUM PRODUCTION CELLS 6,077,415 Duruz et al. MULTI-LAYER
NON-CARBON METAL-BASED ANODES FOR ALUMINUM PRODUCTION CELLS AND
METHOD 5,904,828 Sekhar et al. STABLE ANODES FOR ALUMINUM
PRODUCTION CELLS 5,510,008 Sekhar et al. STABLE ANODES FOR ALUMINUM
PRODUCTION CELLS
SUMMARY OF THE INVENTION
[0005] The invention provides a method for producing a non-carbon
inert anode for metal oxide electrolytic reduction, such as
reduction of aluminum, by feeding metallic iron and metallic nickel
in solid form to an oxidizing reactor; oxidizing the iron and
nickel and forming molten nickel ferrite; then coating an anode
substrate with a thin coating of the molten ferrite, or other
desired molten oxide coating. Molten nickel ferrite can be also
formed by melting solid nickel ferrite or by melting oxides of iron
and nickel. Metal containing compounds, especially oxides, may be
added to dope the ferrite, which increases the electrical
conductivity of the ferrite.
[0006] The inert anode has a ceramic portion, which is solidified
from a molten metal oxide bath and a substrate onto which the
ceramic portion is solidified. The ceramic portion may also include
a metal component that is dispersed into the ceramic bath prior to
solidification. The ceramic material is coated onto a separately
fabricated substrate to form an inert anode. The substrate can be a
metal, cermet, or ceramic material. Use of this method is simpler
and more cost efficient than the current state of the art of inert
metal anode manufacture.
[0007] The invention also comprises apparatus for producing an
inert anode for metal oxide electrolytic reduction, the apparatus
comprising: an oxidizing reactor; means for feeding metallic iron
and metallic nickel to the oxidizing reactor; a ladle or tundish
positioned for receiving molten metal oxide from the reactor; and
means for discharging molten metal oxide from the ladle or tundish
onto the substrate by spray atomization, by pouring over the
substrate, or by discharging into a vessel into which the substrate
may be dipped into the molten metal oxide. The substrate is
advantageously cooled during dipping or spray coating to keep the
substrate from melting. Suitable cooling is effected by water
cooling or by air being blown against the substrate.
[0008] The invention also comprises the product of the method, a
non-carbon, inert, metal-based anode for metal oxide electrolytic
reduction comprising a nickel ferrite coating on a metal, cermet or
ceramic substrate.
OBJECTS OF THE INVENTION
[0009] The principal object of the present invention is to provide
a process for the manufacture of inert anodes that is simpler and
more cost efficient than the current state of the art of metal
based anode manufacture.
[0010] Another object of the invention is to produce an inert anode
that has as good or better properties of conductivity, strength,
resistance to attack by electrolyte and oxygen in a metal oxide
reduction process than metal based anodes.
[0011] Another object of the invention is to provide an inert anode
for electrolytic reduction of metal oxides.
[0012] A further object of this invention is to provide apparatus
for the manufacture of ceramic coated inert anodes.
[0013] Another object of this invention is to provide a ceramic
type inert anode made from a ferrite that may be used as an anode
in the brine electrolysis process in the chlor-alkali industry.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] The foregoing and other objects will become more readily
apparent by referring to the following detailed description and the
appended drawings in which:
[0015] FIG. 1 is a cross-section of an inert anode produced
according to the invention.
[0016] FIG. 2 is a cross-section of an alternative inert anode
according to the invention.
[0017] FIG. 3 is a cross-section of another alternative inert anode
according to the invention.
[0018] FIG. 4 is a cross-section of a further alternative inert
anode, having a generally metal substrate according to the
invention.
[0019] FIG. 5 is a schematic diagram of the method and apparatus of
a preferred embodiment of the invention.
[0020] FIG. 6 is a schematic diagram of the method and apparatus of
another alternative embodiment of the invention which utilizes a
melting furnace rather than an oxidizing furnace.
[0021] FIG. 7 is a schematic diagram of an alternative embodiment
of the invention utilizing a variation of feed materials to the
oxidizing furnace.
[0022] FIG. 8 is a schematic diagram of an alternative embodiment
of the invention utilizing spray coating of the anode
substrate.
[0023] FIG. 9 is a plan view of an alternative substrate having a
grid configuration.
[0024] FIG. 10 is an elevation view of the substrate of FIG. 9.
DETAILED DESCRIPTION
[0025] In the invented process, a molten metal oxide compound is
formed by melting oxides or by oxidizing iron and other metal(s) to
form a molten ferrite of the general formula
A.sub.(x)B.sub.(1-X)Fe.sub.2O.sub.4.
[0026] where A & B are divalent metal ions such as Mg, Ni, Mn,
Co, Fe; and x can vary from 0 to 1.0.
[0027] Alternatively, the molten oxide can be formed by any desired
combination of melting and oxidation. The molten oxide is then
applied to the substrate of the anode. This can be achieved by
dipping the anode substrate into the molten oxide, or by pouring
the molten oxide in a controlled manner over the surface of the
anode substrate, or by spray atomizing of the molten oxide onto the
surface of the substrate. The surface of the substrate may have a
raised and/or dimpled or knurled surface to aid in the adherence of
the molten coating on the metal substrate. The metal surface of the
anode substrate may also be oxidized to aid in the adherence of the
molten coating. This oxide layer or coating acts as a controlled
interfacial layer assisting in bonding the molten oxide to the
substrate. Varying time, temperature and oxygen content of the
oxidizing atmosphere will control the thickness of the oxide layer
or coating. The substrate may be cooled during dipping or spray
coating to keep the substrate from melting. After the molten
coating is applied, the coated anode is allowed to cool in a
controlled manner to avoid delaminating of the coating from the
substrate and/or formation of cracks in the coating.
[0028] In a preferred embodiment, metallic iron and metallic nickel
in briquet form are fed to an oxidizing reactor where the iron and
nickel are melted and oxidized by oxygen. The iron and nickel are
fed into the reactor in a molar ratio of
Fe/Ni=2/1.
[0029] A molten nickel ferrite of formula Ni Fe.sub.2O.sub.4 is
formed.
[0030] It is also possible to utilize a molar ratio of Fe/Ni of
greater than 2/1 to produce a mixture of nickel ferrite (Ni
Fe.sub.2O.sub.4) and iron ferrite (Fe.sub.3O.sub.4). It is also
possible to operate with a molar ratio of Fe/Ni less than 2 in
order to produce a nickel ferrite plus excess nickel oxide
(NiO).
[0031] The molten nickel ferrite is discharged from the oxidizing
reactor at a temperature of at least 1660.degree. C., which is
sufficient to maintain it in the molten state. The molten nickel
ferrite is discharged into a holding vessel such as a ladle or
hearth, wherein it forms a molten bath. The ladle can be heated to
prevent the molten mixture from solidifying. Dopants may be added
to the ladle. Suitable dopants include zinc, cobalt, or lithium
compounds, which are preferably added as oxides. Alternatively,
dopants can be added in metal sulfide or carbonate form, but they
will be present in the final product as oxides. The metal substrate
is dipped into the bath and coated with the molten nickel ferrite.
The substrate may be repeatedly dipped into the molten bath to
increase and control the thickness and other properties of the
coating. An electrical connector can be affixed to the anode before
or after coating. The finished product is a coated inert anode of
correct shape with an electrical connector attached. The connector
can be any desired electrically conductive material, such as
copper, nickel, or nickel-iron.
[0032] As shown in FIG. 1, the product is an inert anode 10 having
a substrate 12, an electrical connector 14 affixed thereto, and a
coating of nickel ferrite 16 thereon.
[0033] The coated anode of FIG. 2 has a hollow configuration, the
substrate 12a preferably being cylindrical and closed at the bottom
to form recess 30 therein. The coating 16 may be applied by dipping
or spraying. An electrical connector can be affixed to the
substrate, such as by welding.
[0034] The anode 10 of FIG. 3 has wires or rods 18 of the same
material as the substrate depending therefrom to promote adherence
of the coating material 16. Alternatively, the wires or rods can be
any material from which the substrate can be made, as described
above.
[0035] FIG. 4 shows a metal wire substrate, the wires 20 forming
the substrate, the wires also forming the conductor at their upper
ends 22. The upper ends of the wires may be connected to a terminal
head by swaging and/or soldering. A guide block 24 may be provided
to space the wires which depend downwardly therethrough prior to
dipping them into a metal oxide bath. A gap 26 is shown between the
guide block and the coating 16, but in practice there is little or
no gap. The wires, which are preferably copper, comprise about five
to twenty-five percent (5-25%) of the finished anode by weight,
preferably about ten to twenty percent (10-20%).
[0036] The electrical connector 20 is preferably an INCONEL rod
embedded in or connected to the substrate. Alternatively the
connector can be another conductive metal or alloy, or it can be
the same composition as the substrate, as described above.
[0037] After coating and cooling, the anode may be post-heat
treated. This treatment may be an annealing step carried out in an
oxygen-containing atmosphere. This post heat treatment is for
stress relief, phase composition adjustment, as required, and final
microstructure adjustment. The post heat-treatment step may include
a soak at a temperature of from 1000.degree. C. to 1400.degree. C.
in an oxygen-containing gas to further oxidize any remaining
metallic nickel and metallic iron. This first soak may then be
followed by a slow cooling to a reduced temperature 100 to
400.degree. C. less than the temperature of the first soak, and
then a second soak in an oxygen-containing gas at the reduced
temperature. The second soak causes phase composition adjustment
and microstructure adjustment. The overall reaction taking place in
the post-heat treatment step may be represented by the following
reaction:
Ni.sub.xFe.sub.1-x+Ni.sub.yFe.sub.1-YO+Ni.sub.zFe.sub.3-zO.sub.4+O.sub.2=N-
iFe.sub.2O.sub.4
[0038] An alternative post treatment is, after the slow cooling, or
after the second soak, hot isostatic pressing of the coated anode
at a temperature of at least 1000.degree. C. and a pressure of at
least 1360 bar for a period of from about 4 to about 8 hours.
[0039] In a preferred embodiment shown in FIG. 5, metallic iron and
metallic nickel in briquet form from source 40 are fed to an
oxidizing reactor 42 wherein the iron and nickel are melted and
oxidized by oxygen from source 44. The iron and nickel are fed into
the reactor in a molar ratio of
Fe/Ni=2/1.
[0040] A molten nickel ferrite of formula Ni Fe.sub.2O.sub.4 is
formed.
[0041] It is possible to utilize a molar ratio of Fe/Ni greater
than 2/1 to produce a mixture of nickel ferrite (Ni
Fe.sub.2O.sub.4) and iron ferrite (Fe.sub.3O.sub.4). It is also
possible to operate with a molar ratio of Fe/Ni less than 2 in
order to produce a nickel ferrite plus excess nickel oxide
(NiO).
[0042] The molten nickel ferrite is discharged from the oxidizing
reactor 42 through outlet 46 at a temperature sufficient to
maintain it in the molten state plus sufficient superheat to melt
any dopants being added thereto. The molten nickel ferrite
discharges into a receiving and holding vessel 48 such as a tundish
or ladle. Dopants 50, such as zinc oxide, cobalt oxide, or lithium
oxide, can be added and mixed into the molten nickel ferrite in the
holding vessel wherein the dopants are melted. The molten oxide may
be stirred with an oxygen containing gas 52 through inlet 54 or
through injector 56, and any remaining metal is oxidized. The
dopant that is added can be in the form of powder or larger
particles that are readily melted. The ladle or tundish 48 can be
heated to prevent the molten metal oxide compound from solidifying.
The molten compound is discharged from the tundish 48 into a molten
bath container for coating of substrate 12. A substrate is dipped
in the molten bath to form a coated anode. The substrate is
advantageously cooled, as shown by injecting cooling water into the
interior 30 of by the substrate. Alternatively, the substrate can
be cooled by forced air cooling.
[0043] It is also to be understood that a ceramic-coated inert
anode made in accordance with the disclosed process may be used in
other electrolytic reduction processes besides aluminum such as
magnesium, lithium, or calcium reduction.
Alternative Embodiments
[0044] The substrate 12 may be metal, cermet or a ceramic. If
metal, it may be a solid unit, or it may incorporate multiple rods
18 (FIG. 3) or wires 20 (FIG. 4), which may be embedded therein and
depend downwardly therefrom. The coating 16 adheres to and
surrounds these rods or wires, which promote cooling of the
coating. The substrate may have a raised surface, or raised
portions, such as a waffle pattern, knurls, or dimples which may be
raised or indented. The substrate may be oxidized, if desired. All
of these promote adherence of the coating to the substrate.
[0045] The coating may be a ceramic, or a combination of a ceramic
and dispersed metal. The ceramic may be a metal oxide, preferably
NiFe.sub.2O.sub.4, but can be a ferrite of nickel, manganese,
magnesium, cobalt, aluminum, or a combination thereof.
[0046] If the substrate is a ceramic or a cermet, the coating is
generally thicker than if the substrate is metal.
[0047] The metal substrate could be a bi-metallic material, the
metals having different coefficients of expansion. The coefficient
of expansion of the metal surface on which the coating is to be
applied is matched closely to that of the coating material.
Suitable bi-metallics include: iron-nickel-chromium alloy and
nickel; and an iron base metal and nickel or nickel alloy.
[0048] The molten ceramic bath can be formed by melting oxides of
metal in a gas fired furnace, an electric furnace or a combination
gas/electric furnace.
[0049] It is also to be understood that the invented inert anode
made from a ferrite may be used in other electrolytic reduction
processes besides aluminum, such as electrolytic reduction of
magnesium, lithium, or calcium. It is also understood that a
ceramic type inert anode made from a ferrite may be used as an
anode in the brine electrolysis process in the chlor-alkali
industry.
[0050] Thickness of the coating on the substrate can be controlled
by repeating the selected process technique until the desired
coating is obtained.
[0051] In the alternative embodiment shown in FIG. 6, solid nickel
ferrite 70, or a mixture of nickel ferrite, nickel oxide and iron
oxide (or hematite), or iron-containing and nickel-containing
compounds 72 are melted in a melting furnace or vessel 74, which
need not be an oxidizing vessel, to form molten nickel ferrite. The
melting vessel can be a gas fired furnace, induction furnace, or
electric arc furnace.
[0052] In the alternative embodiment shown in FIG. 7, mostly
metallic iron and nickel 40 are fed to reactor 82 along with some
iron oxide and/or nickel oxide, which are melted and oxidized to
form molten nickel ferrite. There is sufficient exothermic heat
available from the oxidation of nickel and iron to allow the use of
nickel oxide and iron oxide as feed materials to the reactor. The
molten nickel ferrite is then discharged into a ladle or tundish
and further treated, then discharged into a container into which a
substrate is dipped to form a coated anode. The substrate is
advantageously cooled by injecting cooling water into the
substrate. Alternatively, the substrate can be cooled by forced air
cooling, i.e., blowing of air onto the substrate. In this
embodiment, solid nickel oxide 84, or iron oxide 86, or a mixture
of nickel oxide and iron oxide 88 is introduced to and melted in an
oxidizing reactor or vessel 82, to form molten nickel ferrite.
Spent anodes may be utilized as part of the oxide feed
material.
[0053] In the alternative embodiment of FIG. 8, metallic iron and
metallic nickel from source 40 are fed to oxidizing reactor 42
wherein the iron and nickel are melted and oxidized by oxygen from
source 44. The molten nickel ferrite is discharged from the
oxidizing reactor 42 into a receiving and holding vessel 48 such as
a tundish or ladle. Dopants can be added and mixed into the molten
nickel ferrite in the holding vessel 48. The molten oxide compound
is discharged into a container and atomized in a spray atomizer 66,
then sprayed onto the substrate to form a coated anode 10. It is
advantageous to cool the substrate while the coating is being
applied.
[0054] In a further alternative substrate shown in FIGS. 9 and 10,
the substrate is a grid 90 of rods 92, 94, which are fixed into
position, preferably by welding. A connector can be welded to the
rod grid prior to coating of the substrate with the molten oxide
compound. This type of substrate can be coated by dipping or
spraying.
Summary of the Achievement of the Objects of the Invention
[0055] From the foregoing, it is readily apparent that we have
invented an improved process for the manufacture of inert anodes
for electrolytic reduction of metal oxides that is simpler and more
cost efficient than the current state of the art of anode
manufacture, an anode product that has as good or better properties
of conductivity, strength, resistance to attack by the electrolyte
than metal based anodes, and apparatus for the manufacture of inert
anodes.
[0056] It is to be understood that the foregoing description and
specific embodiments are merely illustrative of the best mode of
the invention and the principles thereof, and that various
modifications and additions may be made to the apparatus by those
skilled in the art, without departing from the spirit and scope of
this invention, which is therefore understood to be limited only by
the scope of the appended claims.
* * * * *