U.S. patent application number 11/442721 was filed with the patent office on 2007-12-06 for anode for use in aluminum producing electrolytic cell.
This patent application is currently assigned to Northwest Aluminum Technologies. Invention is credited to James R. Ballinger, Robert J. Barnett.
Application Number | 20070278107 11/442721 |
Document ID | / |
Family ID | 38788837 |
Filed Date | 2007-12-06 |
United States Patent
Application |
20070278107 |
Kind Code |
A1 |
Barnett; Robert J. ; et
al. |
December 6, 2007 |
Anode for use in aluminum producing electrolytic cell
Abstract
A method of producing aluminum in an electrolytic cell
containing alumina dissolved in an electrolyte, the method
comprising the steps of providing a molten salt electrolyte at a
temperature of less than 900.degree. C. having alumina dissolved
therein in an electrolytic cell having a liner for containing the
electrolyte, the liner having a bottom and walls extending upwardly
from said bottom. A plurality of non-consumable Cu--Ni--Fe--Sn
anodes and cathodes are disposed in a vertical direction in the
electrolyte, the cathodes having a plate configuration and the
anodes having a flat configuration to compliment the cathodes. The
anodes contain apertures therethrough to permit flow of electrolyte
through the apertures to provide alumina-enriched electrolyte
between the anodes and the cathodes. Electrical current is passed
through the anodes and through the electrolyte to the cathodes,
depositing aluminum at the cathodes and producing gas at the
anodes.
Inventors: |
Barnett; Robert J.;
(Goldendale, WA) ; Ballinger; James R.; (The
Dalles, OR) |
Correspondence
Address: |
ANDREW ALEXANDER & ASSOCIATES
3124 KIPP AVENUE, P.O. BOX 2038
LOWER BURRELL
PA
15068
US
|
Assignee: |
Northwest Aluminum
Technologies
|
Family ID: |
38788837 |
Appl. No.: |
11/442721 |
Filed: |
May 30, 2006 |
Current U.S.
Class: |
205/372 ;
204/243.1; 204/293 |
Current CPC
Class: |
C25C 3/12 20130101; C25C
3/06 20130101 |
Class at
Publication: |
205/372 ;
204/293; 204/243.1 |
International
Class: |
C25C 3/06 20060101
C25C003/06; C25B 11/04 20060101 C25B011/04; C25C 3/08 20060101
C25C003/08 |
Claims
1. A method of producing aluminum in an electrolytic cell
containing: alumina dissolved in an electrolyte, the method
comprising the steps of; (a) providing a molten salt electrolyte at
a temperature of less than 900.degree. C. having alumina dissolved
therein in an electrolytic cell having a liner for containing the
electrolyte, said liner having a bottom and walls extending
upwardly from said bottom; (b) providing a plurality of
substantially non-consumable anodes and cathodes disposed in a
generally vertical direction in said electrolyte, said anodes
comprised of a Cu--Ni--Fe--Sn alloy; and (c) passing electrical
current through said anodes and through said electrolyte to said
cathodes for purposes of electrolysis, depositing aluminum at said
cathodes and producing gas at said anodes.
2. The method in accordance with claim 1 wherein said anode
comprises 10 to 70 wt. % Cu, 15 to 50 wt. % Ni, and 1 to 15 wt. %
Sn, the remainder Fe.
3. The method in accordance with claim 1 wherein said anode
comprises 10 to 20 wt. % Cu, 20 to 30 wt. % Ni, and 2 to 5 wt. %
Sn, the remainder Fe.
4. The method in accordance with claim 1 wherein said anode
comprises 10 to 20 wt. % Cu, 20 to 30 wt. % Ni, and 2 to 5 wt. %
Sn, and 50 to 70 wt. % Fe.
5. The method in accordance with claim 1 wherein said electrolyte
is comprised of one or more alkali metal fluorides.
6. The method in accordance with claim 1 wherein said electrolyte
is comprised of one or more alkali metal fluorides and aluminum
fluoride.
7. The method in accordance with claim 1 including maintaining said
electrolyte in a temperature range of about 660.degree. to 8
60.degree. C.
8. The method in accordance with claim 1 wherein said electrolyte
has a melting point in the range of 715.degree. to 860.degree.
C.
9. The method in accordance with claim 1 including passing an
electric current through said cell at a current density in the
range of 0.1 to 1.5 A/cm
10. The method in accordance with claim 9 wherein said cathodes are
selected from the group consisting of titanium diboride, zirconium
diboride, titanium carbide, zirconium carbide and molybdenum.
11. The method in accordance with claim 1 wherein said anodes and
cathodes have planar surfaces arranged in a vertical orientation in
said electrolyte and wherein said anodes and cathodes are arranged
in alternating relationship.
12. The method in accordance with claim 1 including adding alumina
to said cell on a substantially continuous basis.
13. The method in accordance with claim 1 including collecting
aluminum from said cathode in the bottom of said cell.
14. The method in accordance with claim 1 including maintaining
alumina in said electrolyte in a range of 2 to 6 wt. %.
15. A method of producing aluminum in an electrolytic cell
containing alumina dissolved in an electrolyte, the method
comprising the steps of; (a) providing a molten salt electrolyte
having a melting point in the range of 715.degree. to 900.degree.
C. and having alumina dissolved therein in an electrolytic cell
having a liner for containing the electrolyte, said liner having a
bottom and walls extending upwardly from said bottom; (b) providing
a plurality of anodes and cathodes disposed in a generally vertical
direction in said electrolyte, said anodes comprised of 10 to 70
wt. % Cu, 15 to 50 wt. % Ni, 1 to 15 wt. % Sn, the remainder Fe,
said cathodes having a planar surface disposed opposite an anode
planar surface, said cathodes' and said anodes' planar surfaces
defining a region therebetween; and (c) passing electrical current
through said anodes and through said electrolyte to said cathodes,
depositing aluminum at said cathodes and producing gas at said
anodes.
16. A method of producing aluminum in an electrolytic cell
containing alumina dissolved in an electrolyte, the method
comprising the steps of; (a) providing a molten salt electrolyte
having alumina dissolved therein in an electrolytic cell having a
liner for containing the electrolyte, said liner having a bottom
and walls extending upwardly from said bottom; (b) adding alumina
to said electrolyte on a continuous basis to provide an
alumina-enriched electrolyte; (c) providing a plurality of
substantially non-consumable anodes and cathodes disposed in said
electrolyte, said anodes comprised of an alloy containing
Cu--Ni--Sn, the remainder Fe; (d) flowing alumina-enriched
electrolyte between said anodes and said cathodes; and (e) passing
electrical current through said anodes and through said electrolyte
to said cathodes, depositing aluminum at said cathodes and
producing gas at said anodes.
17. An improved anode for use in an electrolytic cell for producing
aluminum from alumina dissolved in a molten salt electrolyte
contained in said cell wherein aluminum is deposited at the
cathode, oxygen is produced at the anode when electric current is
passed through the cell, said cell containing at least one cathode
and one anode disposed in said electrolyte, said anode comprised of
an alloy, said cathode having a surface, said anode having a
surface for disposing opposite said cathode surface to provide an
anode-cathode distance defining a region between said anode and
said cathode surfaces, said anode comprised of a Cu--Ni--Fe--Sn
alloy.
18. The anode in accordance with claim 17 wherein said anode is
comprised of 10 to 70 wt. % Cu, 15 to 60 wt. % Ni, 1 to 15 wt. %
Sn, the remainder Fe, incidental elements and impurities.
19. The anode in accordance with claim 17 wherein said anode is
comprised of 10 to 20 wt. % Cu, 20 to 30 wt. % Ni, and 2 to 5 wt. %
Sn, the remainder Fe, incidental elements and impurities.
20. The anode in accordance with claim 17 wherein said anode is
comprised of 10 to 70 wt. % Cu, 15 to 50 wt. % Ni, 1 to 15 wt. %
Sn, and 15 to 75 wt. % Fe, incidental elements and impurities.
21. In an improved method of producing aluminum in an electrolytic
cell containing alumina dissolved in an electrolyte wherein a
molten salt electrolyte is maintained at a temperature of less than
900.degree. C., the electrolyte having alumina dissolved therein,
and alumina add to the electrolyte on a continuous basis to provide
alumina-enriched electrolyte, and wherein a plurality of
non-consumable anodes and cathodes are disposed in a vertical
direction in said electrolyte, said cathodes having a flat surface,
the improved method comprising; (a) providing anodes having a
planar surface disposed opposite the flat surface of the cathode to
define a region between the cathode flat surface and the planar
surface of the anode, said anodes comprised of a Cu--Ni--Fe--Sn
alloy; (b) passing electrical current through said anodes and
through said electrolyte to said cathodes, depositing aluminum at
said cathodes and producing gas at said anodes.
22. An electrolytic cell for producing aluminum from alumina
dissolved in an electrolyte, the cell comprised of; (a) a liner for
containing the electrolyte, the liner having a bottom and walls
extending upwardly from said bottom and means for adding alumina to
said cell to provide alumina-enriched electrolyte; (b) a plurality
of non-consumable anodes and cathodes disposed in said electrolyte
contained in said cell, said anodes comprised of a Cu--Ni--Fe--Sn
alloy, said cathodes having a cathode surface, said anodes having
an anode surface disposed from said cathode surface to define a
region between said anode and cathode to permit flow of electrolyte
therethrough to provide alumina-enriched electrolyte to said region
between said anodes and said cathodes; and (c) means for passing
electrical current through said anodes and through said electrolyte
to said cathodes for producing aluminum at said cathode and gas at
said anodes.
23. The cell in accordance with claim 22 wherein said cathode
surface is a planar surface.
24. The cell in accordance with claim 22 wherein said anode surface
is a planar surface.
25. The cell in accordance with claim 22 wherein said anode and
said cathode have an active area ratio anode to cathode in the
range of 1.1:1 to 5:1.
26. The anode in accordance with claim 22 wherein said anode is
comprised of 10 to 20 wt. % Cu, 20 to 30 wt. % Ni, and 2 to 5 wt. %
Sn, the remainder Fe, incidental elements and impurities.
27. The anode in accordance with claim 22 wherein said anode is
comprised of 10 to 70 wt. % Cu, 15 to 50 wt. % Ni, 1 to 15 wt. %
Sn, and 15 to 75 wt. % Fe, incidental elements and impurities.
28. The cell in accordance with claim 22 wherein the anodes have
two planar surfaces, each planar surface disposed opposite a
surface of said cathode, permitting flow of alumina-enriched
electrolyte to the region between said anodes and said
cathodes.
29. An anode having increased anode active surface area for use in
an electrolytic cell for producing aluminum from alumina dissolved
in a molten salt electrolyte contained in the cell, the cell
containing at least one cathode and one anode disposed in said
electrolyte, said anode comprised of a Cu--Ni--Fe--Sn alloy, said
cathode having a surface, the anode having a first surface for
disposing opposite said cathode surface to provide a controlled
anode-cathode distance defining a region between said anode and
said cathode surfaces.
30. The anode in accordance with claim 29 wherein said anode
surface is a planar surface.
31. The anode in accordance with claim 29 wherein said first
surface of said anode is a planar surface.
32. The anode in accordance with claim 29 wherein said anode is
comprised of 10 to 20 wt. % Cu, 20 to 30 wt. % Ni, and 2 to 5 wt. %
Sn, the remainder Fe, incidental elements and impurities.
33. The anode in accordance with claim 29 wherein said anode is
comprised of 10 to 70 wt. % Cu, 15 to 50 wt. % Ni, 1 to 15 wt. %
Sn, and 15 to 75 wt. % Fe, incidental elements and impurities.
Description
BACKGROUND OF THE INVENTION
[0001] This invention relates to aluminum and more particularly it
relates to an improved anode for use in the electrolytic production
of aluminum from alumina dissolved in a molten salt
electrolyte.
[0002] There is great interest in using an inert anode in an
electrolytic cell for the production of aluminum from alumina
dissolved in the molten salt electrolyte. By definition, the anode
should not be reactive with the molten salt electrolyte or oxygen
generated at the anode during operation. Anodes of this general
type are disclosed in U.S. Pat. Nos. 4,592,812; 4,865,701;
5,006,209; 5,284,562; 6,558,525; 6,723,222; 6,800,191; 6,811,676
and 6,837,982. Examples of other such anodes of this type are
comprised of a cermet. For example, U.S. Pat. No. 4,399,008
discloses a composition suitable for fabricating into an inert
electrode for use in the electrolytic production of metal from a
metal compound dissolved in a molten salt. The electrode comprises
at least two metal oxides combined to provide a combination metal
oxide.
[0003] Also, U.S. Pat. No. 5,284,562 discloses an oxidation
resistant, non consumable anode for use in the electrolytic
reduction of alumina to aluminum, which has a composition
comprising copper, nickel and iron. The anode is part of an
electrolytic reduction cell comprising a vessel having an interior
lined with metal which has the same composition as the anode. The
electrolyte is preferably composed of a eutectic of AlF and either
(a) NaF or (b) primarily NaF with some of the NaF replaced by an
equivalent molar amount of KF or KF and LiF.
[0004] Other anodes of this type are disclosed in U.S. Pat. Nos.
3,943,048; 3,957,600; 4,049,887; 4,529,494; 4,620,905; 4,865,701;
4,871,438; 4,956,068; 4,960,494; 4,999,097; 5,006,209; 5,069,771;
5,637,239; 5,667,649; 5,725,744; and 5,993,637.
[0005] Anodes used for electrolysis take different forms. For
example, U.S. Pat. No. 3,300,396 discloses electroplating
techniques and anode assemblies for electroplating wherein the
anode pieces are contained in a titanium basket which is
permanently deployed in the plating tank.
[0006] U.S. Pat. No. 3,558,464 discloses novel anodes for use in
electrolytic cells having generally vertical slots in the lower
portion of the anodes which are open at the bottom of the anode and
closed at the ends of the slots with a plurality of gas conducting
channels connecting the top of the slots with the upper surface of
the anode. The cathodes of the cells are the liquid mercury anode
type.
[0007] U.S. Pat. No. 5,391,285 discloses an adjustable plating cell
for uniform bump plating of semiconductor wafers wherein an
apparatus plates metal bumps of uniform height on one surface of a
semiconductor wafer (32). A plating tank (12) contains the plating
solution. The plating solution is filtered (16) and pumped (14)
through an inlet (22) to an anode plate (24) within plating cell
(20). The anode plate has a solid center area to block direct
in-line passage of the plating solution, and concentric rings of
openings closer to its perimeter to pass the plating solution.
[0008] U.S. Pat. No. 5,532,086 discloses an anode for use in an
electrochemical cell comprising a current collector layer having a
thickness less than about 10 mils, and desirably less than about 4
mils, and a rigid support extending adjacent one side of the
current collector layer so that the current collector layer is
sandwiched between the anodic layer of the anode and the rigid
support. The rigid support maintains the current collector layer in
the original configuration of the current collector layer during
discharge and recharge cycles of the cell. A cell containing the
anode is also disclosed. The rigid support for the anode current
collector can be mounted in the electrochemical cell case so as to
allow for the release from the cell of gas produced at the
anode.
[0009] U.S. Pat. No. 6,099,711 discloses a method for the
electrolytic deposition of metal coatings, in particular of copper
coatings with certain physical-mechanical and optical properties
and uniform coating thickness. According to known methods using
soluble anodes and applying direct current, only uneven metal
distribution can be attained on complex shaped workpieces. By using
a pulse current or pulse voltage method, the problem of the
coatings being of varying thickness at various places on the
workpiece surfaces can indeed be reduced.
[0010] U.S. Pat. No. 6,113,759 discloses an anode assembly includes
a perforated anode and an electrical contact assembly attached to
the anode. A perforated anode holder holds the anode. The anode
holder includes perforations at least in a bottom wall such that
plating solution may flow through perforations in the anode holder
and perforations in the anode. An anode isolator separates the
anode and a cathode. The anode isolator includes at least one
curvilinear surface. The contact assembly includes a closed or
substantially closed cylinder member of titanium or titanium alloy,
a copper lining or disk disposed within the cylinder, and a
titanium or titanium alloy post fixed and in electrical engagement
with the lining or disk.
[0011] U.S. Pat. No. 6,251,251 discloses an anode assembly
including a perforated anode. A perforated anode holder holds the
anode. The anode holder includes perforations at least in a bottom
wall such that plating solution may flow through perforations in
the anode holder and perforations in the anode. An anode isolator
separates the anode and a cathode. The anode isolator includes at
least one curvilinear surface.
[0012] In spite of these disclosures, there is still a great need
for a process utilizing a low temperature electrolytic cell for the
production of aluminum using an improved anode and anode
design.
SUMMARY OF THE INVENTION
[0013] It is an object of the present invention to provide an
improved method for producing aluminum from alumina in an
electrolytic cell.
[0014] It is another object of the invention to provide an improved
method for producing aluminum from alumina in an electrolytic cell
employing inert or nonconsumable anodes requiring reduced voltage
and having a highly stable surface oxide layer.
[0015] It is still another object of the invention to provide an
improved method for supplying alumina saturated electrolyte to the
active surface of the anode in an electrolytic cell for producing
aluminum.
[0016] And, it is another object of the invention to provide a
method of operating an electrolytic cell employing improved inert
anodes for producing aluminum from alumina by using an improved
method of flowing alumina saturated electrolyte to the anode
surface.
[0017] And, yet it is another object of the invention to provide a
new inert anode and alloy therefor for the electrolytic production
of aluminum, the anode requiring reduced voltage.
[0018] These and other objects will become apparent from the
specification, claims and drawings appended hereto.
[0019] In accordance with these objects, there is provided a method
of producing aluminum in an electrolytic cell containing alumina
dissolved in an electrolyte, the method comprising the steps of
providing a molten salt electrolyte at a temperature of less than
900.degree. C. having alumina dissolved therein in an electrolytic
cell having a liner for containing the electrolyte, the liner
having a bottom and walls extending upwardly from said bottom. A
plurality of non-consumable anodes and cathodes are disposed in a
vertical direction in the electrolyte. The anodes are comprised of
a Cu--Ni--Fe--Sn alloy having long-term stability and which can
operate on reduced voltage while reducing alumina to aluminum. The
anode may contain apertures therethrough to permit flow of
electrolyte through the apertures to provide alumina-enriched
electrolyte between the anodes and the cathodes. Electrical current
is passed through the anodes and through the electrolyte to the
cathodes, depositing aluminum at the cathodes and producing gas at
the anodes.
[0020] The invention includes an improved anode for use in an
electrolytic cell for producing aluminum from alumina dissolved in
a molten salt electrolyte contained in the cell. The cell contains
at least one cathode and one anode disposed in the electrolyte
defining a region between the electrodes, the cathode having a flat
surface. The improved anode is comprised of a Cu--Ni--Fe--Sn alloy
which can operate on reduced voltage resulting from reduced
resistance of the surface oxide layer. The anode has long term
stability of the surface oxide layer and thus produces excellent
purity aluminum. It has a substantially flat surface configuration
for disposing opposite the cathode surface to provide an
anode-cathode distance defining a region between said anode and
said cathode surfaces. When apertures are used in the anode, it
permits flow of electrolyte through the apertures to provide
alumina-enriched electrolyte in the region between the anodes and
the cathodes.
[0021] The anodes of the present invention may be fabricated by
casting a Cu--Ni--Fe--Sn melt of the desired composition.
[0022] In addition, there is provided a method of producing
aluminum in an electrolytic cell comprising the steps of providing
molten electrolyte in an electrolytic cell, said cell having
alumina dissolved in the electrolyte. Anodes and cathodes are
provided in the cell, the anodes comprised of the aforesaid
Cu--Ni--Fe--Sn alloys, incidental elements and impurities. Electric
current is passed between anodes and cathodes in the cell and
aluminum is formed at the cathodes.
[0023] The invention further includes an electrolytic cell for
producing aluminum from alumina dissolved in an electrolyte, the
cell comprised of a liner for containing the electrolyte, the liner
having a bottom and walls extending upwardly from the bottom. A
plurality of non-consumable anodes and cathodes are disposed in the
electrolyte contained in the cell. The cathodes have a plate
configuration having a cathode surface and the anodes having a
first surface and second flat surface disposed from the cathode
surface to define a region between the anode and cathode. The
anodes may contain apertures extending from the first surface to
the second flat surface to permit flow of electrolyte therethrough
to provide alumina-enriched electrolyte between the anodes and the
cathodes. Means are provided for passing electrical current through
the anodes and through the electrolyte to the cathodes for
producing aluminum at the cathode and gas at the anodes.
[0024] Thus, an anode which is comprised of a Cu--Ni--Fe--Sn alloy
is provided for use in an electrolytic cell for producing aluminum
from alumina dissolved in a molten salt electrolyte contained in
the cell. The cell contains at least one cathode and one anode
disposed in the electrolyte, the cathode having a planar surface.
The anode has a substantially flat first surface for disposing
opposite the cathode planar surface to provide a controlled
anode-cathode distance defining a region between the anode and the
cathode surfaces. The anode has a second surface disposed opposite
the first surface to provide the anode with a thickness dimension.
Apertures can extend from the first surface of the anode to the
second surface, the apertures defined by a wall of the anode, the
wall can provide additional anode active surface area during
electrolysis of the alumina in the cell.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] FIG. 1 is a cross-sectional view of a test electrolytic cell
employed in testing.
[0026] FIG. 2 is a schematic of an anode useful in the
invention.
[0027] FIG. 3 is another view of the anode of FIG. 2.
[0028] FIG. 4 is a cross-sectional view along the line A-A of FIG.
3.
[0029] FIG. 5 is a schematic of another embodiment of the
invention.
[0030] FIG. 6 is schematic of yet another embodiment of the
invention.
[0031] FIG. 7 is a cross section of an electrolytic cell in
accordance with the invention.
[0032] FIG. 8 is a cross-sectional view of an anode in FIG. 7 along
the line B-B.
[0033] FIG. 9 is a perspective view of the anode used in FIG.
7.
[0034] FIG. 10 is a cross-sectional view illustrating a cylindrical
cell having a central cathode surrounded by a cylindrical
anode.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0035] The subject invention includes an electrolytic cell for the
production of aluminum from alumina dissolved in a molten salt
electrolyte. Preferably, the molten electrolyte is maintained at a
temperature of less than 900.degree. C. However, electrolytes such
as cryolite may be used at higher temperatures, e.g., 925.degree.
to 975.degree. C. Further, preferably, the alumina is added to the
cell on a continuous basis to ensure a controlled supply of alumina
during electrolysis. The electrolytic cell of the invention employs
anodes and cathodes. In the process of the invention, electric
current is passed from the anode through the molten electrolyte to
cathode reducing alumina to aluminum and depositing the aluminum at
the cathode. While the cathodes are preferably comprised of
titanium diboride, it will be understood that the cathodes can be
comprised of any suitable material that is substantially inert to
the molten aluminum at operating temperatures. Such materials can
include zirconium boride, molybdenum, tungsten, titanium carbide
and zirconium carbide.
[0036] By the use of the terms inert or non-consumable is meant
that the anodes are resistant to attack by molten electrolyte and
do not react or become consumed in the same manner as carbon anodes
in a Hall-Heroult type cell. As fabricated, the metal anode is
substantially free of metal oxides. A metal, non-consumable anode
for use in the cell in accordance with the invention is comprised
of nickel, copper, iron and tin. The metal anode can contain about
10 to 70 wt. % Cu, 15 to 50 wt. % Ni, 1 to 15 wt. % Sn, the
remainder comprising iron. A preferred anode consists essentially
of 10-20 wt. % Cu, 20 to 30 wt. % Ni, 2 to 5 wt. % Sn, remainder
iron. Typical non-consumable anodes can have compositions in the
range of 10 to 20 wt. % Cu, 20 to 30 wt. % Ni, 2 to 5 wt. % Sn and
50 to 70 wt. % Fe. The ranges set forth herein are intended to
include all the numbers within the range as if specifically set
forth.
[0037] The composition employing Cu--Ni--Sn--Fe has advantages
compared to prior anodes comprised of only Cu--Ni--Fe. That is, the
Cu--Ni--Sn--Fe can operate at a reduced voltage because of the
reduced surface oxide layer resistance. Further, the use of tin
permits increased usage of lower cost iron as the primary component
of the anode, resulting in lower cost anodes. Both Cu and Ni can be
reduced, still allowing the production of high purity aluminum
metal at a stable cell voltage. Another advantage includes
increased long term stability of the surface oxide layer of the
Cu--Ni--Sn--Fe anode. That is, the oxide layer of Cu--Ni--Sn--Fe
exhibits more stability than the oxide layer of Cu--Ni--Fe in the
electrolytic cell. This results in higher metal purity and longer
term life of the anode. For example, metal purity can reach 99.9
wt. % aluminum or commercial purity, even after 500 hours of
operation. Another benefit of this composition results in improved
corrosion resistance at the cell bath interface that eliminates the
need for extra protection at the boundary layer.
[0038] Inert anodes in accordance with the invention may be cast
from a melt of an alloy having the desired composition or the
anodes may be fabricated from powders of the individual components
mixed in the desired proportions. The powders are then sintered or
melted to form the anode.
[0039] The electrolytic cell can have an operating temperature less
than 900.degree. C. and typically in the range of 660.degree. C. to
about 8 60.degree. C. The cell can employ electrolytes comprised of
NaF+AlF.sub.3, KF+NaF+AlF.sub.3, or NF+AlF.sub.3. More broadly, the
cell can use electrolytes that contain one or more alkali metal
fluorides and at least one metal fluoride, e.g., aluminum fluoride,
and use a combination of fluorides as long as such baths or
electrolytes operate at less than about 900.degree. C. For example,
the electrolyte can comprise NaF and AlF.sub.3. That is, the bath
can comprise 62 to 53 mol. % NaF and 38 to 47 mol. % AIF.sub.3.
[0040] Referring now to FIG. 1, there is shown a schematic of a
laboratory electrolytic cell 10 used for electrolytically reducing
alumina to aluminum, in accordance with the invention. Cell 10 is
comprised of an alumina crucible, a metal crucible, or any other
suitable material 12 containing anodes 14 of the invention and
cathode 16. A molten salt electrolyte 18 also is provided in cell
10. Cell 10 is sealed with a cover 2. Anodes 14 and cathode 16 are
suspended through lid 2 from a superstructure (not shown) and
connected to bus bars above the cell. Anodes 14 and cathode 16 are
in the form of vertical plates with an anode on each side of the
cathode. The cathode used in the test cell was TiB.sub.2 and the
anodes were comprised of an Ni--Cu--Sn--Fe alloy having 20 wt. %
Ni, 18 wt. % Cu, 2 wt. % Sn and 60 wt. % Fe. The molten salt
electrolyte was comprised of 38.89 wt. % sodium fluoride and 61.11
wt. % aluminum fluoride. For tests, typically the molten
electrolyte was maintained below 900.degree. C. and normally in the
range of 730.degree. to 860.degree. C. When the cell is operated,
aluminum is deposited at the cathode and collects in a pool 20. If
the crucible 12 is comprised of metal, then an insulated reservoir
21 is required to collect molten aluminum 20. If crucible 12 is
comprised of refractory, then molten aluminum can collect on the
bottom of the cell, as illustrated in FIG. 7.
[0041] The present invention has the advantage that it efficiently
provides an alumina enriched molten electrolyte to active surface 8
of anodes 14. That is, molten salt electrolyte has certain flow
patterns within cell 10 and alumina particles 26 are added to
surface 22 of the electrolyte from hopper 24. In the embodiment
illustrated in FIG. 1, molten electrolyte is shown flowing in a
downward direction adjacent walls 4 and 6 of cell 10 and in an
upwardly direction adjacent cathode surfaces 28 and 30. The lift or
upward direction movement of the molten electrolyte is caused in
part by the evolution of gases such as oxygen gas at the active
anode surface.
[0042] In the present invention, apertures 32 are provided in
anodes 14 to permit flow of alumina-enriched electrolyte to be
quickly available at active surfaces 8 of anodes 14. Thus, during
operation of cell 10, molten electrolyte flows downwardly adjacent
walls 4 and 6 and simultaneously therewith flows through holes or
apertures 32 supplying alumina laden or enriched electrolyte to
anode active surfaces 8. This has the advantage of minimizing
starvation of alumina at the active surface of the anode resulting
in greater stability of the anode. That is, in using a conventional
anode in cell 10 of FIG. 1, molten electrolyte has to traverse to
the bottom or ends of the anode before providing dissolved alumina
for reduction. Thus, it will be appreciated gradations of
concentrations of alumina can occur with conventional planar anodes
and in commercial cells the distance along the surface of the anode
can vary significantly, adversely affecting operation of the cell
and the integrity of the anodes. That is, at the center, for
example, of the anode surface there can be starvation of available
alumina, thus subjecting the anode surface to reduction, defeating
the inert quality desired.
[0043] The apertures provided in anodes 14 have another benefit.
That is, depending on the number of apertures and the thickness of
the anode, the apertures may contribute to the active surface area
of the anode. The ratio of anode active surface to cathode active
surface can range from 1:1 to 5:1. It will be understood that the
wall of anode material defining apertures 32 can contribute to
anode active surface 8. Further it will be seen in FIGS. 1, 2 and 3
that apertures 32 have a cylindrical shape. However, other shapes
such as square or oval, for example, are contemplated. Further,
apertures 32 can have a fluted or funnel shape. That is, aperture
32 can increase in diameter from one side of the anode to the
other, e.g., from the non-active surface to the active surface. The
active surface of the anode is the surface opposite the cathode
surface and can include the wall defining apertures 32. While only
one hopper 24 is shown projecting through lid or cover 2, it will
be understood that a number of hoppers can be used to introduce
alumina to the melt.
[0044] FIG. 2 is a dimensional view of anode 14 in accordance with
the invention, illustrating apertures 32 provided in orderly manner
across the thickness of anode 14 from surface 8 to surface 9. The
apertures can be formed by any convenient manner such as by casting
or drilling. Further, the apertures can have a diameter from about
1/8 inch to about 1 inch, depending on the size of the anode being
used.
[0045] FIG. 3 is a perspective view of one face or surface of the
anode and apertures 32 provided therein. FIG. 4 is a
cross-sectional view along the line A-A of FIG. 3, illustrating
apertures extending from surface 9 to surface 8 to permit the free
flow of alumina-enriched, molten electrolyte through the anode to
the active surface which is surface 8 and can include wall 34
defining aperture 32 in FIG. 1.
[0046] Alumina useful in the cell can be any alumina that is
comprised of finely divided particles. Usually, the alumina has a
particle size in the range of about 1 to 100 .mu.m.
[0047] In the present invention, the cell can be operated at a
current density in the range of 0.1 to 1.5 A/cm.sup.2 with almost
zero increase in voltage after the first 100 hours of cell
operation while the electrolyte is maintained at a temperature in
the range of 660.degree. to 860.degree. C. A preferred current
density is in the range of about 0.4 to 1.3 A/cm.sup.2. The lower
melting point of the bath (compared to the Hall cell bath which is
above 950.degree. C.) permits the use of lower cell temperatures,
e.g., 730.degree. to 860.degree. C. reduces corrosion of the anodes
and cathodes.
[0048] The anodes and cathodes in the cell can be spaced to provide
an anode-cathode distance as low as 1/2 inch. That is, the
anode-cathode distance is the distance between anode surface 8 and
cathode surface 28 or 30.
[0049] Further, in a commercial cell thermal insulation can be
provided around liner or crucible and on the lid in an amount
sufficient to ensure that the cell can be operated without a frozen
crust and frozen side walls. However, in certain instances, it
maybe desirable to permit freezing of bath on the sidewalls to
provide for sidewall protection.
[0050] While the anodes of the invention have been described with
apertures 32 being provided as cylindrical openings as shown in
FIGS. 1, 2 and 3, it is believed that any means or opening that
permits or improves the flow of alumina-enriched electrolyte to the
region between the cathode surface and anode surface can be used.
Thus, for example, an anode of the invention is shown in FIG. 5
wherein apertures 32 maybe provided as slots 40 which extend
substantially vertically from a bottom wall 42 to a top wall 44.
Slots 40 are defined by walls 46 and 48. As noted earlier with
respect to apertures 32, slots 40 permit flow of alumina-enriched
molten electrolyte to the region between the anode and cathode
surfaces and thus efficiently provides alumina at the active
surfaces for electrolysis purposes and thus the efficiency of the
cell is enhanced, permitting the use of higher current
densities.
[0051] While the apertures 32 or slots 40 are shown in FIG. 5
extending substantially vertically, it should be understood that
apertures 32 can take the form of horizontal slots 50 as shown in
FIG. 6. Thus, the apertures may be provided as horizontal slots 50
defined by walls 52 and 54. As noted earlier, slots 50 permit flow
of alumina enriched molten electrolyte to the region between the
anode and cathode active surfaces for purposes of electrolysis.
Thus, as aluminum ions are removed from the electrolyte and
deposited at the cathode as aluminum metal, the apertures
immediately provide a supply of alumina-enriched electrolyte for
electrolysis. Further, the active surface of anode is increased by
the wall defining the slot depending on the thickness of the anode,
as explained earlier with respect to circular shaped apertures. It
should be noted that in FIGS. 5 and 6, the slots do not have to
extend fully from top to bottom or from side to side as shown but
may be comprised of a series of short slots which may be formed
randomly in the anode to complement flow of alumina-enriched
electrolyte between the active surface of the anode and cathode. It
will be appreciated that different size apertures can be used in an
anode whether they are slots or circles. Further, it will be
appreciated that the invention includes utilizing apertures in an
anode which provides the shortest distance for alumina-enriched
electrolyte to the region between the active surfaces of the anode
and the cathode.
[0052] While the anode and cathode surfaces have been depicted as
being flat, such surfaces can be curved or corrugated. One surface
or both surfaces can be curved or corrugated preferably to provide
a uniform distance between anode and cathode active surface. For
example, the anode can take the form of a cylinder 100, FIG. 10,
with the appropriate apertures provided therein to flow electrolyte
into the region 102 between cathode 104 which is illustrated in the
form of a post.
[0053] When multiple anodes and cathodes are used as in a
commercial cell, an improved design of anode can be used having
active faces which are continuously supplied with alumina-enriched
electrolyte. In FIG. 7, there is illustrated an improved
electrolytic cell 10' having multiple anodes 14' and cathodes 16'.
Multiple hoppers 24' can be used to feed alumina 26 on a continuous
basis to electrolyte 18 wherein the alumina is efficiently digested
in the molten electrolyte. The cell can be comprised of a metal
shell 12' having sides 60 and bottom 62. When the shell is not
active, i.e., anodic, the outside or end anode can have the
configuration shown in FIG. 1 for anodes and also illustrated in
FIG. 7 as 14'. Cathodes 16' can also have the same configuration as
illustrated in FIG. 1 and shown in FIG. 7 as 16. Also, as
illustrated in FIGS. 1 and 7, the cathode can be longer than anode
14 and 14' extending towards molten aluminum 20. However, cathode
16 can be sufficiently short in order to avoid contacting molten
aluminum 20. In such design, current is removed from the cathode
above lid 2, for example. However, cathode 16 can be designed to
remove current through bus (not shown) at the bottom of the cell.
Further, cathode 16 can be mounted or positioned in the bottom of
the cell and current removed through bottom bus.
[0054] In accordance with the invention shown in FIG. 7, anode 14'
is designed as a hollow anode in order to provide two active
surfaces 64 and 66. Hollow anodes 14' are provided with apertures
32' to facilitate flow of alumina-enriched molten electrolyte to
the region of the cell between active surfaces of anode 14' and
cathode 16. As illustrated in FIG. 7, electrolyte flow is in an
upward direction between anode- and cathode-active surfaces and in
a generally down direction in side hollow anode 14'. In FIG. 7, as
molten electrolyte flows downwardly in hollow anode 14' electrolyte
escapes into the region between anode and cathode active surface to
provide or add alumina-enriched electrolyte as it is depleted
during electrolysis. It will be appreciated that apertures may be
sized from top to bottom to facilitate flow therethrough as desired
during electrolysis.
[0055] A cross section of hollow anode 14' along the line B-B in
FIG. 7 is shown in FIG. 8. Cross section illustrated in FIG. 8
shows apertures 32' for flowing alumina-enriched electrolyte from
inside or hollow 70 to the region between the anode-active surface
and the cathode-active surface. Hollow 70 is defined by sides 63
and ends 72 and 74. It will be appreciated that ends 72 and 74 may
be eliminated and spacers (not shown) used to maintain hollow
70.
[0056] FIG. 9 is a dimensional view of hollow anode 14' showing
stub 76 which may be used for supporting anode plates 63 and 65 in
cell 10'. Anode 14', as shown in FIG. 9, is comprised of plates 63
and 65 which are separated sufficiently to permit location of stub
76 therebetween for purposes of supporting the anode of the cell.
It will be seen that plates 63 and 65 are provided with apertures
or holes 32' which, as noted, permit flow of electrolyte from
hollow 70 or inside anode 14' to active surfaces 64 and 66. In the
embodiment of anode 14', sides 74 and 72 may be provided to contain
electrolyte and force flow of electrolyte through apertures 32'.
Further, from the description of FIG. 7 it will be noted that
molten electrolyte enters at the top or opening between plates 63
and 65 and flows downwardly and outwardly through apertures
32'.
[0057] The following examples are still further illustrative of the
invention.
EXAMPLE 1
[0058] This invention was tested in a 150 A cell having the
configuration shown in FIG. 1 with alumina added to the cell
substantially continuously. The cell comprised an alumina ceramic
container. Within the ceramic container was placed a vertical
cathode suspended through the lid of the container and connected to
a bus bar. On either side of the cathode, two anodes were
positioned or suspended through the lid and connected to bus bar.
The anodes were 4 inches by 5 inches by 3/8 inch thick. Each anode
was drilled to provide 112 holes 1/4 inch in diameter. The anodes
were comprised of 18 wt. % Cu, 20 wt. % Ni, 2 wt. % Sn and 60 wt. %
Fe, and the cathode was TiB.sub.2. The cell contained a molten salt
bath comprised of 37.93 wt. % sodium fluoride and 62.07 wt. %
aluminum fluoride. The top of the cell was sealed with an
insulating lid and the cell was maintained at an operating
temperature of 770.degree. to 780.degree. C. which was above the
melting point of the salt bath and the aluminum metal. The alumina
fed to the cell had a particle size of about 100 .mu.m or less and
was effectively ingested by the circulation of the bath in the cell
during operation. The cell was operated at a current density of up
to 0.53 amp/cm.sup.2 for a period of 264 hours. Aluminum deposited
at the cathode drained downwardly to the bottom of the cell and was
removed periodically. Oxygen gas evolved at the active face of the
anode provided a generally upward movement of the bath in the
regions between the anodes and the cathode. The bath had a
generally downward movement between the anode and the wall of the
container. Oxygen was removed from the cell through feed tube of
the alumina. The apertures provided in the anodes permitted
alumina-rich electrolyte to more effectively reach the active
regions of the electrodes without the need to travel to the bottom
of the anode and then to the surface of the electrolyte to get
replenished. That is, the anodes permitted a more effective method
for feeding alumina-enriched electrolyte to the active region
between anode and the cathode and for replenishing the electrolyte
with alumina. The anodes were used for 264 hours without any
appearance of blistering or significant corrosion. The cell voltage
averaged about 4.17 volts with a one-inch ACD and a current density
of 0.53 amps/cm.sup.2. The aluminum metal had a purity equal to or
better than commercially pure aluminum.
EXAMPLE 2
[0059] A second test was run for 503 hours. The composition of the
anode was the same as in Example 1. The operating temperature was
800.degree. C., the current density was 0.53 amps/cm.sup.2, and the
ACD was one inch. The average voltage for the entire run was 4.08
volts and was steady at the completion of the run. Smelter grade
alumina was used. The melt purity was 99.83% for the entire test.
This included impurities from the alumina.
[0060] The use of the new anode results in reduced voltage due to
reduced surface oxide layer resistance compared to anodes using
only Cu--Ni--Fe. Further, the new anode permits increased usage of
lower cost iron as the primary alloying component. Also, compared
to a Cu--Ni--Fe anode, the use of Cu--Ni--Fe--Sn results in long
term stabililty of the surface oxide layer. In addition, the new
Cu--Ni--Fe--Sn anode results in better metal purity. Furthermore,
the Cu--Ni--Fe--Sn anode results in improved corrosion resistance
at the cell bath interface, eliminating the need for extra
protection at the cell bath interface.
[0061] Having described the presently preferred embodiments, it is
to be understood that the invention may be otherwise embodied
within the scope of the appended claims.
* * * * *