U.S. patent application number 10/461695 was filed with the patent office on 2003-11-13 for insulating lid for aluminum production cells.
Invention is credited to Kozarek, Robert L., Slaugenhaupt, Michael L..
Application Number | 20030209426 10/461695 |
Document ID | / |
Family ID | 29401835 |
Filed Date | 2003-11-13 |
United States Patent
Application |
20030209426 |
Kind Code |
A1 |
Slaugenhaupt, Michael L. ;
et al. |
November 13, 2003 |
Insulating lid for aluminum production cells
Abstract
An aluminum production cell includes an inert anode and an
insulating lid comprising alumina and at least one metal fluoride.
The insulating lid preferably comprises about 35-90 wt. % of a
mixture of sodium fluoride and aluminum fluoride and about 10-65
wt. % alumina.
Inventors: |
Slaugenhaupt, Michael L.;
(Apollo, PA) ; Kozarek, Robert L.; (Apollo,
PA) |
Correspondence
Address: |
ALCOA INC
ALCOA TECHNICAL CENTER
100 TECHNICAL DRIVE
ALCOA CENTER
PA
15069-0001
US
|
Family ID: |
29401835 |
Appl. No.: |
10/461695 |
Filed: |
June 12, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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10461695 |
Jun 12, 2003 |
|
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09732716 |
Dec 8, 2000 |
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Current U.S.
Class: |
204/247.4 ;
205/379 |
Current CPC
Class: |
C25C 3/085 20130101 |
Class at
Publication: |
204/247.4 ;
205/379 |
International
Class: |
C25C 003/08 |
Claims
What is claimed is:
1. An aluminum production cell comprising at least one inert anode,
a chamber containing a molten salt bath contacting said inert
anode, a cathode spaced from said inert anode, and an insulating
lid covering said chamber said insulating lid comprising a
preformed refractory block containing about 10-65 wt. % alumina and
about 35-90 wt. % of at least one metal fluoride selected from
sodium fluoride, aluminum fluoride, and mixtures thereof.
2. The aluminum production cell of claim 1, wherein alumina
comprises about 25-50 wt. % of the preformed refractory block.
3. The aluminum production cell of claim 1, wherein said insulating
block comprises sodium fluoride and aluminum fluoride.
4. The aluminum production cell of claim 1, wherein said insulating
block comprises sodium fluoride and aluminum fluoride in an
NaF:AlF.sub.3 weight ratio of about 1:1 to about 2:1.
5. The aluminum production cell of claim 4, wherein the
NaF:AlF.sub.3 weight ratio is about 1.5:1.
6. The aluminum production cell of claim 1, wherein said refractory
block further comprises at least one metal fluoride selected from
calcium fluoride and magnesium fluoride.
7. The aluminum production cell of claim 1, wherein said inert
anode comprises a ceramic or cermet material.
8. The aluminum production cell of claim 1, wherein said inert
anode comprises nickel oxide and iron oxide.
9. The aluminum production cell of claim 1, further comprising
loose insulation around a portion of the preformed insulating
block.
10. The aluminum production cell of claim 9, further comprising at
least one additional layer of insulating material above the
preformed insulating block.
11. The aluminum production cell of claim 1, wherein the preformed
insulating block is positioned above at least one anode of said
cell.
12. A process for producing aluminum in a cell comprising at least
one inert anode, a chamber containing a molten salt bath containing
alumina, a cathode spaced from said inert anode, and an insulating
lid covering said chamber, said process comprising (a) passing an
electric current between said anode and said cathode, thereby to
produce aluminum at the cathode, and (b) conserving heat in said
chamber by providing said insulating lid with a composition
comprising about 10-65 wt. % alumina and about 35-90 wt. % of at
least one metal fluoride.
13. The process of claim 12, wherein said insulating lid comprises
at least one metal fluoride selected from sodium fluoride, aluminum
fluoride, magnesium sluoride, calcium fluoride, and mixtures
thereof.
14. The process of claim 12, wherein said insulating lid comprises
a preformed refractory block comprising about 50-75 wt. % of a
mixture of sodium fluoride and aluminum fluoride and about 25-50
wt. % alumina.
15. The process of claim 14, wherein said mixture comprises NaF and
AlF.sub.3 in an NaF:AlF.sub.3 weight ratio of about 1:1 to about
2:1.
Description
PENDING RELATED APPLICATION
[0001] This application is a continuation-in-part of Slaugenhaupt
et al. U.S. Ser. No. 09/732,716, filed Dec. 8, 2000.
FIELD OF THE INVENTION
[0002] The present invention relates to aluminum production cells
and, more particularly, relates to insulating lids for aluminum
production cells having inert anodes.
BACKGROUND INFORMATION
[0003] The energy requirements and cost efficiencies of aluminum
smelting cells can be significantly reduced with the use of inert,
non-consumable and dimensionally stable anodes. Replacement of
traditional consumable carbon anodes with inert anodes allows a
highly productive cell design to be utilized, thereby reducing
capital costs. Significant environmental benefits are also possible
because inert anodes produce essentially no CO.sub.2 or CF.sub.4
emissions.
[0004] The successful retrofit of conventional consumable anode
Hall aluminum production cells with inert anodes requires extremely
stable cell operation with respect to bath chemistry and heat
balance. To achieve cell stability, several operating parameters
must be controlled. The reduced capacity of inert anode cells to
produce heat in comparison with conventional cells having
consumable carbon anodes requires heat losses to be minimized in
order to maintain an adequate heat balance and stable operation.
The amount of alumina dissolved in the bath must be controlled
within a very narrow range, e.g., about 7 to 8 percent, which is
essential for low corrosion rates of the inert anodes and to
prevent deposits of excess alumina on the cathode surface. At an
operating voltage equivalent to a conventional carbon anode cell,
an inert anode cell will generate less heat due to a high
decomposition potential. Control of the cell temperature is crucial
because the alumina saturation level of the bath is proportional to
the bath temperature. Temperature control is also crucial to the
formation and stability of the protection layer of frozen bath or
ledge along the sidewalls of the cell. In a conventional Hall cell,
this ledge prevents horizontal electrical currents between the
anode and cathode as well as protecting the sidewall lining
material from erosion. In a cell with inert anodes, this ledge may
also protect the inert anodes from reduction by carbonaceous
material of the cell lining.
[0005] A need exists for increased insulation in inert anode
aluminum production cells in order to reduce heat losses from the
cells. The present invention has been developed in view of the
foregoing.
SUMMARY OF THE INVENTION
[0006] The present invention improves the operating stability of an
inert anode aluminum production cell by controlling the heat loss
through a top insulating cover. The top cover is the primary area
that heat loss can be regulated during normal operation. Reduction
in heat loss is preferably obtained by utilizing a multiple layer
insulation system on top of the inert anodes. The initial layer of
insulation preferably comprises fabricated blocks of a cryolite and
alumina mixture. These blocks, which are resistant to attack from
the bath fumes, may shield and support subsequent layers of high
temperature insulation. To further protect the upper layers of
insulation, a semi-gas-tight barrier may be created by filling
voids between the blocks with loose refractory material, such as
coarse tabular alumina choked by a smaller particle size alumina
material. The heat balance of the cell may be maintained, e.g., by
adding or removing layers of insulation from the top of the
cell.
[0007] An additional application for the preformed insulating
blocks of the present invention is during startup of an inert anode
cell. The blocks may be used as an inner sidewall between a carbon
cell lining and the anodes. This inert lining provides horizontal
electrical insulation and protect the anodes from carbothermic
reduction until a permanent cryolite ledge is formed.
[0008] An aspect of the present invention is to provide an aluminum
production cell insulating lid assembly comprising at least one
preformed refractory block comprising Al.sub.2O.sub.3 and at least
one material selected from NaF, AlF.sub.3, CaF.sub.2 and
MgF.sub.2.
[0009] Another aspect of the present invention is to provide an
aluminum production cell including a cathode, at least one anode,
and an insulating lid above the cathode and anode(s). The
insulating lid comprises at least one preformed refractory block
including Al.sub.2O.sub.3 and at least one material selected from
NaF, AlF.sub.3, CaF.sub.2 and MgF.sub.2.
[0010] These and other aspects of the present invention will be
more apparent from the following description.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] FIG. 1 is a partially schematic sectional side view of an
inert anode aluminum production cell including an insulating lid in
accordance with an embodiment of the present invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0012] FIG. 1 illustrates an insulating lid 10 of an aluminum
production cell 11 in accordance with an embodiment of the present
invention. The insulating lid 10 includes preformed refractory
blocks 12 which are positioned above inert anodes 14. The inert
anodes 14 may be made of a ceramic material as described in Dimilia
et al. U.S. Ser. No. 10/291,874, filed Nov. 8, 2002 or in Weirauch
et al. U.S. Ser. No. 10/291,874 filed Nov. 9, 2002; or a cermet
material as described in Ray et al. U.S. Pat. No. 6,423,204 issued
Jul. 23, 2002. The disclosures of the aforesaid patent applications
and patent are incorporated herein by reference.
[0013] The preformed refractory blocks 12 preferably comprise from
about 35 to about 90 weight percent of at least one metal fluoride
selected from sodium fluoride, aluminum fluoride, and mixtures
thereof and from about 10 to about 65 weight percent
Al.sub.2O.sub.3. Compositions comprising mixtures of sodium
fluoride and aluminum fluoride together with alumina are preferred.
More preferably, the preformed refractory block comprises about 50
to 75 weight percent of a NaF/AlF.sub.3 mixture, and the
Al.sub.2O.sub.3 comprises from about 25 to about 50 weight percent.
The weight ratio of NaF:AlF.sub.3 preferably ranges from about 1:1
to about 2:1. A particularly preferred weight ratio of
NaF:AlF.sub.3 is about 1.5:1. The NaF and AlF.sub.3 may be provided
separately or together, e.g., in the form of synthetic cryolite. In
addition to, or in place of, the NaF and AlF.sub.3, other
materials, such as calcium aluminate, CaF.sub.2, and MgF.sub.2, may
be used. For example, the refractory block material may comprise
crushed bath held together with a suitable amount of binder, such
as sodium aluminum tetrafluoride or alumina cement.
[0014] The preformed insulating blocks 12 may be formed by mixing,
e.g., the Al.sub.2O.sub.3, NaF/AlF.sub.3 mixture and binder
components in the desired weight ratios, followed by pressing and
heating to a temperature sufficient to bind the block together,
e.g., 1,000.degree. C. The thickness and shape of the preformed
insulating blocks 12 may be selected in order to provide sufficient
structural integrity and heat insulating properties. For example,
thicknesses of from less than 1 inch to greater than 1 foot may be
used.
[0015] After the preformed refractory blocks 12 are positioned
above the inert anodes 14, spaces around the preformed refractory
blocks 12 may be packed with loose insulation 16, such as
particles, fibers, and the like. For example, the loose insulation
may comprise tabular alumina, crushed bath and/or alumina
particles. As a particular example, the loose insulation may
comprise from 75 to 90 weight percent ESP dust and from 10 to 25
weight percent crushed bath.
[0016] Additional layers of insulation 18 may be positioned above
the preformed refractory blocks 12, as shown in FIG. 1. The
additional insulation 18 may include alumina particles, ceramic
fiber, fiberfrax, fiberglass, or any other suitable material.
[0017] In addition to their use as insulating lids, the preformed
refractory blocks of the present invention may be used as sidewall
liners of the cell. FIG. 1 illustrates a preformed sidewall block
20 positioned inside the sidewall 22 of the cell. The sidewall
block 20 may be the same composition as the insulating blocks 12.
In this case, the preformed sidewall insulation 20 may act as a
sacrificial material which may be at least partially consumed
during operation of the cell.
[0018] The aluminum production cell 11 also includes a chamber 30
containing a molten salt bath 32 comprising cryolite and dissolved
alumina; and a cathode 34 spaced from the inert anodes 14. When an
electric current passes between the inert anodes 14 and the cathode
34, aluminum 36 is produced at the cathode 34 and oxygen is
produced at the anodes 14.
[0019] Whereas particular embodiments of this invention have been
described above for purposes of illustration, it will be evident to
those skilled in the art that numerous variations of the details of
the present invention may be made without departing from the
invention as defined in the appended claims.
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