U.S. patent number 3,673,075 [Application Number 05/031,591] was granted by the patent office on 1972-06-27 for alumina reduction system.
This patent grant is currently assigned to Reynolds Metals Company. Invention is credited to Robert M. Kibby.
United States Patent |
3,673,075 |
Kibby |
June 27, 1972 |
ALUMINA REDUCTION SYSTEM
Abstract
Improved techniques in the operation of alumina reduction cells,
particularly as regards procedures and equipment for feeding
alumina into the bath of such a cell and for collecting and
removing anode reaction gases; and reduction cells of improved
design and construction.
Inventors: |
Kibby; Robert M. (Florence,
AL) |
Assignee: |
Reynolds Metals Company
(Richmond, VA)
|
Family
ID: |
21860339 |
Appl.
No.: |
05/031,591 |
Filed: |
April 24, 1970 |
Current U.S.
Class: |
204/245;
204/247 |
Current CPC
Class: |
C25C
3/22 (20130101); C25C 3/08 (20130101); C25C
3/14 (20130101) |
Current International
Class: |
C25C
3/00 (20060101); C25C 3/22 (20060101); C25C
3/08 (20060101); C25C 3/14 (20060101); C22d
003/02 (); C22d 003/12 () |
Field of
Search: |
;204/67,243-247 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
1,495,653 |
|
Aug 1967 |
|
FR |
|
204,595 |
|
Jan 1968 |
|
SU |
|
Primary Examiner: Mack; John H.
Assistant Examiner: Valentine; D. R.
Claims
What is claimed is:
1. An alumina reduction cell having a bath of molten electrolyte
containing dissolved alumina, and a blanket of alumina and
supporting crust of solidified electrolyte overlying the bath, said
cell comprising sidewalls extending lengthwise of the cell, an
electrode system including an anode suspended between said
sidewalls, hood means engaging said blanket of alumina and defining
an enclosure adjacent the anode for collecting anode reaction gases
and means for feeding alumina into the bath at spaced locations
including plunger means operable inwardly of said hood means to
provide an opening through the crust and means for supplying
alumina adjacent said opening in each of said spaced locations,
whereby the cell is adapted for feeding without breaking the crust
outwardly of said hood means and for collecting concentrated anode
reaction gases as said feeding proceeds.
2. An alumina reduction cell according to claim 1 having laterally
spaced anodes, wherein said hood and feeding means are disposed
between said anodes.
3. An alumina reduction cell according to claim 1 wherein said hood
and feeding means are disposed between the anode and an adjacent
sidewall of the cell.
4. An alumina reduction cell according to claim 1 including feeding
means between the anode and an adjacent sidewall at spaced
locations lengthwise of the cell, said hood means being pivotably
supported adjacent said sidewall on the same side of the cell.
5. An alumina reduction cell according to claim 1 including an
anode having a plurality of continuous anode segments, and
individual hood and feeding means for each of said anode
segments.
6. An alumina reduction cell according to claim 1 wherein said
plunger means are so arranged that no point on the anode working
surface is farther removed horizontally from the nearest of said
feed locations than three-fourths the overall anode width across
the cell between said sidewalls.
7. In an alumina reduction cell having an anode, a bath of molten
electrolyte containing dissolved alumina, and a blanket of alumina
and supporting crust of solidified electrolyte overlying the bath,
the improvement which comprises:
hood means associated with said crust and blanket, and defining
therewith a gas collection enclosure to receive anode reaction
gases of the cell; and
means for feeding alumina into the bath from inside the gas
collection enclosure at spaced locations which are separated
sufficiently to preserve integrity of the crust between adjacent
locations and outwardly of said hood means as said feeding
proceeds, including means for introducing alumina into said hood
means adjacent said feeding means in each of said spaced
locations.
8. The improvement of claim 7 in which said feeding means are
provided at spaced feed sites which are so disposed that no point
on the anode working surface is farther removed horizontally from
the nearest feed site than three-fourths the overall anode width
across the cell.
9. In an alumina reduction cell having an anode, a bath of molten
electrolyte containing dissolved alumina, and a blanket of alumina
and supporting crust of solidified electrolyte overlying the bath,
the improvement which comprises:
an anode having a plurality of continuous anode segments including
permanent casing means for each segment;
hood means associated with said crust and blanket, and defining
therewith a gas collection enclosure to receive anode reaction
gases of the cell;
plunger means operable inwardly of said hood means to provide
openings through the crust for feeding alumina into the bath and
for releasing anode reaction gases into said hood means, said
plunger means being so arranged that no point on the anode working
surface is farther removed horizontally from the nearest of said
plunger means than three-fourths the overall anode width across the
cell;
said casing means peripherally enclosing the upper side surfaces of
the anode segments and their lower surfaces being enclosed within
said blanket of alumina for protection against air burning; and
said crust and blanket providing a substantially gas-tight cover
over the bath whereby said hood means are effective for collecting
concentrated anode reaction gases as said feeding proceeds.
Description
BACKGROUND OF THE INVENTION
Alumina reduction cells have traditionally employed anode systems
of two different types: pre-baked carbon blocks arrayed in the pot
for individual height adjustment and replacement, and Soderberg or
self-baking anodes in which a large mass of carbonaceous material,
typically a mixture of pitch and coke, is supported in a casing
over the cell. In the latter system, the heat of the operating cell
causes progressive baking of the anode. Then, as the bottom of the
anode is consumed during electrolysis, the anode is lowered and
more pitch and coke mixture is added to the top of the anode to
provide a replacement for the bottom portion consumed.
Soderberg type cells are further divided into classes which have
either top entry electrical contact pins or side pin
connections.
In pots using prebaked blocks, the anodes are traditionally
arranged in two rows, sometimes with a longitudinal center aisle
between them, in order to provide the opportunity conveniently to
feed alumina into the bath by means of mechanical devices for
breaking the crust along the aisle between anode rows. Alumina from
an ore hopper or other suitable supply can then be admitted to the
bath through the resultant massive opening in the crust.
With respect to collection of anode gases evolved during operation
of alumina reduction cells, vertical pin Soderberg pots have
employed fixed steel casings around the anode upon which have been
mounted skirts extending over the bath to collect a concentrated
form of the gases evolved from the reduction cell in operation. The
major portion of the pitch content of the Soderberg anode migrates
downwardly through the anode during operation and is evolved and
collected in the skirts. It is then possible to withdraw the gases
from the skirts, burn the pitch fumes to a large extent, and remove
a concentrated gas from the pot area to scrubbers where the
fluoride effluents can be efficiently removed. On the other hand,
pre-baked block pots, and side pin Soderberg pots have not
conventionally used such means to collect concentrated gases.
Instead, enclosures are made around the entire anode. These
enclosures must be opened during feeding operations and anode
servicing operations, and it has generally been necessary to remove
dilute gases from the pot area resulting in more expensive
equipment to treat the gases efficiently.
At the same level of power consumption (kilowatt hours per pound),
pots provided with replaceable pre-baked block anode systems
typically carry more than 1.5 times the amperage per square foot of
pot shell bottom area as do pots provided with conventional
self-baking anode systems and make more than 1.5 times the amount
of aluminum.
With so clear-cut an advantage favoring pre-bakes, one might
question why self-baking anode systems are used at all. In large
measure, the answer lies in the capital investment which must be
made for carbon presses and anode baking furnaces if a pre-baked
anode system is to be employed, but which is avoided if a
self-baking anode system is used. Once conventional self-baking
anode system pot lines are built and operating, their capacity
could be increased by investing in press and furnace equipment and
converting to a replaceable pre-baked block anode system. However,
it would be unusual for the gain in output so obtainable to exceed
that which could be obtained by investing the same amount in added
pot lines using conventional self-baking anodes.
In a conventional self-baking anode system, a rectangular anode
having a horizontal cross section of, for example, 63 inch width by
200 inch length is suspended in a pot whose lining circuits the
anode with a spacing of about 25 inches from each of the sides and
ends of the anode and the pot is run at an anode current density up
to about 5 or 6 amps per square inch.
The greater capacity of pot lines equipped with replaceable
pre-baked block anodes stems from the greater anode current density
at which such anodes are able to run, typically about 8.5 amps per
square inch, and the greater total bottom area of an array of
pre-baked anodes as conventionally used, compared to the
corresponding bottom area of self-baking anodes, as conventionally
used, in pots occupying the same floor area.
The questions which may logically be asked at this point are: If
one wished to avoid the expense of presses and furnaces, yet obtain
for a planned or existing self-baking anode system pot line the
capacity which it would have if equipped with a pre-baked anode
system, why not merely make the self-baking anodes larger, so they
occupy more of the pot cross-sectional area, and why not run these
larger anodes at an increased current density?
The answers are founded upon some fundamental considerations.
During the reduction of alumina to aluminum about one-half pound of
anode carbon is consumed, for each pound of aluminum produced.
Gases, largely carbon dioxide, are formed at the anode as the
carbon is consumed. To the extent that these gases experience
difficulty in exiting from the vicinity of the region between the
anode and cathode, the capacity of the cell or its efficiency in
producing aluminum is reduced.
Further, as alumina is reduced to aluminum more alumina must be
added. To the extent that it is difficult to supply alumina to the
pot and for the alumina to work under the anode to utilize the
whole bottom area of the anode, operating costs are increased and
the efficiency of the pot in producing aluminum is reduced.
Accordingly, merely increasing the size of conventional self-baking
anodes to achieve an anode cross-sectional area comparable to that
of pre-bake cells and running the larger self-baking anode at a
current density comparable to that conventional for pre-baked anode
systems has been impractical because: (a) the gas venting path
along the anode working surface would be increased, (b) the ore
travel path counter current to the flow of gas would be increased
and (c) it would be difficult to provide for feeding ore into the
pot by conventional means at the resulting narrow regions left
between the larger anode and the pot lining. In addition, as
regards the removable side channel casing construction commonly
used with side entry Soderberg cells, there would be increased
danger of excessive air burning of the anode due to the higher
anode current density.
SUMMARY OF THE INVENTION
The present invention is directed to improvements in the operation
of alumina reduction cells, including those of conventional design,
and particularly as regards techniques for feeding alumina into the
bath of a reduction cell and procedures for collecting and removing
anode reaction gases. It further relates to improved anode
constructions in alumina reduction cells, including divided
continuous anode configurations having a plurality of continuous
anode segments, for example, and anode systems having permanent
casing means, as well as accessory apparatus for feeding and gas
collection purposes. Also involved is an improved method and cell
construction for feeding alumina into the bath of an electrolytic
reduction cell having a carbon anode system, regardless of the type
of anode employed, or for removing and collecting anode gases, or
both, while still providing adequate protection against air burning
of the anode carbon.
In a preferred construction, the cell includes a divided continuous
anode system having at least four anode segments. Such an
arrangement effectively reduces the paths of travel for feeding
alumina beneath the anode and for venting anode reaction gases
along the anode working surface. In addition, feeding is readily
accomplished either between the anode segments or along the sides
of the cell.
By placing the continuous anode segments close to the sidewalls of
the cell, furthermore, preferably at a clearance of a foot or less,
it becomes possible to increase the total anode area and
consequently to improve the rate of production of aluminum to a
value more typical for cells using pre-baked anode blocks. In
particular, the continuous anode segments may be arranged to
provide an anode loading of the cell in which the aggregate anode
cross-sectional area constitutes more than 50 percent of the floor
area occupied by the cell; and then a production rate exceeding 7
lb./day for each square foot of floor area can be achieved.
The method aspects of the present invention, which may be practiced
apart from the particular divided anode system, include feeding
alumina into the bath through individual holes in the crust at
spaced locations which are separated sufficiently to preserve the
integrity of the crust between holes, and maintaining a blanket of
alumina on the crust adjacent exposed surfaces of the anode to
protect the anode carbon against air burning. These spaced feed
locations may be established along the sides of the cell outwardly
from the anode, or some may be placed inside the outer periphery of
the anode system, such as between anode segments of a divided
anode. It can be seen, of course, that feeding at spaced locations
along the longitudinal centerline of the cell can be utilized to
achieve substantially the same results, in terms of the ease of
introducing alumina beneath the anode, as would be the case using
twice as many feed locations along opposite sides of the cell.
Feeding between laterally spaced anode segments also makes it
possible to place the segments closer to the sidewalls of the cell,
so as to achieve a higher anode loading and consequent increase in
productivity.
The feeding of alumina into the bath as operation of the cell
proceeds is preferably carried out with the least possible
disruption of the usual crust of solidified electrolyte which forms
over the bath. For one reason, as previously noted, this crust may
be used to support a blanket of alumina adjoining the anode
segments for protection against air burning. Accordingly, a system
of plungers or other means may be provided for opening holes
through the crust in limited areas and the alumina requirements of
the cell are preferably introduced through such holes at spaced
locations, including in the case of the previously mentioned
divided continuous anode arrangement, at least one such location
for each two anode segments, rather than by breaking massive
portions of the crust. Anode reaction gases may also be released
from the cell through these holes.
In addition, the practice of the present invention preferably
includes feeding alumina into the bath at frequent intervals of 20
minutes or less, ordinarily about 1 to 5 minutes. This may be
accomplished, for example, by employing plungers spaced as much as
4 to 6 feet apart, and so disposed relative to the anode that no
point on the anode working surface is farther removed from the
nearest feed site, as measured in a plane essentially parallel to
that to that surface, then three-fourths the overall anode width
across the cell. An additional benefit of this arrangement is that
it allows for maintenance of a blanket of alumina on the crust to
protect the anode segments against air burning.
Side entry pins, top entry pins or a combination of the two may be
used for establishing electrical connection between the bus flexes
and the anode. At present, top entry pins are preferred in this
regard.
In the case of a vertical pin Soderberg pot employing a fixed steel
casing around the anode, the present invention provides for
collection of anode gases by means of a hood or other similar
enclosure which occupies a substantially smaller portion of the
anode periphery than the usual complete skirts. With such a hood
arrangement, feeding of alumina into the bath is advantageously
accomplished by providing plunger means for opening holes through
the crust within the gas collection enclosures. These plungers are
conveniently activated by a common beam traversing the length of
the anode and moved by pneumatic or mechanical means at the ends of
the pot. Alumina may then be introduced by way of an inlet into the
enclosure.
We find it advantageous both from the standpoint of efficient pot
operation and from the standpoint of minimizing the hardware needed
to break the crust, to actuate the plungers frequently; for
example, once every 1 to 5 minutes. In this way the crust never
forms to an extent that it offers high resistance to breakage and
lighter mechanical equipment can be used than would be the case if
the crust were broken, for example, only once every 2 hours. Under
these conditions, pneumatic cylinder activators for the breakers
can be replaced with mechanical cam type activators at an economic
advantage.
With a vertical pin continuous anode system, and employing the gas
collection and feeding techniques described generally above, it
then becomes unnecessary to routinely break the crust that forms
between the sidewalls of the pot and the anode, or the crust that
forms between adjacent anode segments. Alumina can be placed upon
this crust in deep blanket to protect the anode carbon from attack
by air and increase the effective area of carbon presented for
electrolysis, thereby improving the efficiency of the cell and its
ability to carry high amperage. In contrast, former methods of
collecting gas required that the crust outside the periphery of the
anode or the anode gas skirt be broken for the purposes of feeding
alumina. This action destroyed the effectiveness of the gas skirts
and permitted air to attack the anode, reducing its area and
causing carbon particles to fall off into the electrolyte where
they caused a reduction in the efficiency of the cell.
For use with a side pin Soderberg anode, for example, the gas
collection enclosure may be mounted on the pot shell, so that its
bottom edge is spaced from the liquid electrolyte, in the region
between the anode and the sidewall, by a distance which is
sufficient distance to avoid consumption of the materials from
which the enclosure is made, but close enough to the electrolyte
that a crust of frozen electrolyte naturally forms between the
enclosure and adjacent sidewalls of the pot and also between the
enclosure and adjacent side of the anode. Within the enclosure,
holes are provided through the crust for feeding alumina into the
bath and for removing the anode gases evolved from the cell.
In cells having prebaked block anodes and employing center aisle
feeding, the gas collection enclosure may be supported from the
anode structure and extend downwardly to a position where it is
spaced from the liquid electrolyte but close enough to be sealed by
the frozen crust forming between the rows of blocks. As in the case
with the gas collection and feeding enclosure previously mentioned
for the side pin Soderberg pot, plunger means may be associated
with the hood-like enclosure to maintain communication between the
space below the crust and the gas collecting hood, so that
concentrated anode gases can be collected from this hood and
alumina can be fed to the pot through an inlet in the hood. Here
again it is not necessary to disturb the crust and ore blanket
cover between and over the carbon blocks during at least a major
portion of the operating cycle, thereby reducing the occasions for
the attack of air on the carbon blocks and avoiding the necessity
of disturbing the pot crust outside the enclosure for purposes of
feeding the pot. It will be understood, however, that in the case
of a cell having replaceable pre-baked anode blocks the crust cover
is disturbed to some extent whenever the used anode blocks are
removed from the pot and new blocks are put in their place. Such an
operation, however, does not require removal of the gas collection
hood.
In all three anode systems described it will be advantageous to
provide means of access to the area inside the gas collection hood.
In the case of a hood mounted to a permanent anode casing, as in a
vertical pin Soderberg cell, it is convenient to provide sliding
access doors. In the case of a hood for the side pin Soderberg pot,
either hinged doors may be provided or else the hood itself may be
hinged to the deckplate of the pot. With pre-baked blocks provision
may be made to elevate the hood or portions thereof above the
electrolyte to an extent sufficient for working tools to be used in
this area under abnormal conditions.
Because the crust is not broken routinely outside the hood, except
as regards the necessary periodic replacement of pre-baked carbon
blocks, it is possible to maintain a substantially continuous and
deeper blanket of alumina around the cell, thereby maintaining a
more effective gas seal and avoiding air burning of the anode
carbon.
It will be appreciated that there are various advantages to putting
permanent casings even around continuous prebaked anodes so that
gas enclosures may be mounted on these casings as in the case of
the vertical pin Soderberg. However, an enclosure of the type
described for the side pin Soderberg pot may be used with
continuous prebaked anodes which do not have such casings.
For purposes of further comparison it may be noted that typical
conventional side pin self-baking anode systems carry up to about 5
amperes for each square inch of anode bottom surface, and about 2.5
amperes for each square inch of shell bottom area (which provides
an estimate of aluminum reduction plant capacity per unit area of
plant floor space).
Typical pots using pre-baked block anodes carry up to about 8.5
amperes for each square inch of anode bottom area, and about 4
amperes for each square inch of pot shell bottom area.
In contrast, pots equipped with a divided continuous anode system
and operated in accordance with the present invention may carry
about 7.5 amperes for each square inch of anode bottom area, and
may be arranged to occupy a substantially greater part of the
available shell area than conventional Soderberg anodes. Thus,
although alumina reduction cells having laterally divided
self-baking anodes have been known for some time, the art has not
previously recognized how to achieve full benefits in the
construction and operation of such cells.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention is further discussed herein with reference to the
drawings wherein the presently preferred embodiments are shown. The
specifics illustrated in the drawings are intended to exemplify,
rather than limit, aspects of the invention as defined in the
claims.
FIG. 1 is a perspective view (partly in section) of an alumina
reduction cell or pot which is provided with an array of
permanently cased, continuous anodes. Some conventional or
repetitive parts not necessary to an understanding of the invention
have been eliminated for clarity in illustrating the parts
shown;
FIG. 2 is a cross-sectional transverse elevation of the pot of FIG.
1;
FIGS. 3 and 4 are transverse sections similar to FIG. 1, but
concerning other embodiments; and
FIGS. 5A, 5B and 5C are schematic plan views showing various
feeding arrangements.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The pot 10 includes a shell 12, typically of steel, having
conventional insulation 14 and cathode 16. A fixed frame 18,
supported upon the pot shell, extends over the pot. The frame 18 is
conventional and could be configured and supported in other
suitable ways than that shown. Collector bars for the cathode are
shown at 22 in FIG. 1.
In a preferred embodiment of the anode, as shown in the drawings,
there are at least four continuous anode segments 24a, 24b, 24c,
24d (collectively referred to by the numeral 24) arranged in a
rectangular array with sides of segments 24a and 24b transversely
adjacent one another toward the near end of the pot (FIG. 1
orientation) and with the sides of segments 24c and 24d
transversely adjacent one another toward the far end of the pot;
one end of segment 24a lies adjacent one end of segment 24d and one
end of segment 24b lies adjacent one end of segment 24c. This
configuration provides an aisle along the longitudinal centerline
of the pot, and a transverse aisle across the pot between each pair
of anode segments.
Each segment of anode 24 is provided with a perimetrical casing 29,
for instance of steel plate providing an integral structure
consisting of sidewalls, but no top or bottom. The facing sides of
the casings 29 of transversely adjacent anode segments or the
facing ends of the casings 29 of the longitudinally adjacent anode
segments, or both, may be provided with cooling chambers (not
shown), also of welded steel plate connected around the respective
casing corners and supplied with coolant, such as air, through
suitable conduits. This assists in the formation and maintenance of
a crust 30 over the bath between the anode segments. Under the
usual operating conditions, a crust may be maintained in these
locations without need for such artificial cooling. For purposes of
facilitating ore feeding and other types of pot working, and to
help crust formation, the longitudinal center aisle should be at
least about 10 inches wide.
The casings 29 may be height-adjustably suspended from the fixed
frame 18 by conventional mounting means (not shown) which allow the
casings to be individually raised and lowered. Ordinarily, however,
it is sufficient to provide adjustable support means limiting the
downward travel of the casing, leaving the casing free to move
upwardly when the anode is raised for voltage adjustment.
In the drawings, a pad of molten aluminum 34 is shown reposing on
the cathode lining 16. Over the molten aluminum lies the fused
electrolyte 38 above which a crust 30 comprising frozen electrolyte
and alumina is formed between the pot sidewalls and the anode
segments where the anode carbon is exposed below casings 29. An ore
blanket comprising particulate alumina 42 is banked against the
anode casings upon the crust and is preferably deep enough to
largely cut off exposure to the air of the carbon anode segments to
prevent air burning.
Along and above the longitudinal aisle between laterally spaced
anode segments, within the overall perimeter of the anode array, a
longitudinally elongated ore hopper 44 is mounted upon the frame
18.
Electrical connection to the anode segments, as shown, is via
vertical pins 50 which enter the carbonaceous anode material from
the top. The anode pins are supported by adjustable hangers
relative to the fixed frame 18 in any of a number of ways known in
the art.
In the embodiment of FIGS. 1 and 2, hood means 52 are mounted on
the outer sides of the anode casings 29 to provide an enclosed
chamber for collecting anode gases evolved from the cell. Each hood
has an inlet tube 54 for introducing alumina from the ore hopper
44; a gas outlet 56; and a plunger rod 58 operable from outside the
hood to maintain an opening in the crust for feeding alumina into
the bath of molten electrolyte. Outlet means are also provided at
the foot of the ore hopper for maintaining a blanket of alumina on
the crust along the longitudinal aisle, between the laterally
separated anode casings.
Two or more of the plungers 58 may be arranged for conjoint or
sequential actuation, as by operation of a mechanical cam or
eccentric mechanism. Each plunger is preferably disposed or made
adjustable to allow for penetration of the crust without contacting
the bath. As to the hood means 52, it is further contemplated in
accordance with the invention that more than one plunger may be
located within the same hood means or, in other words, the hood
means may be elongated to embrace several plungers.
The individual anode segments shown are substantially rectangular
in horizontal section, and each is preferably proportioned so that
its ratio of orthogonal dimensions (i.e. the ratio of the sides of
such section) is less than 5:1, typically between about 2:1 and
about 3:1.
In the operation of this cell configuration, a substantially
continuous cover of crust and overlying ore blanket is maintained
over the bath for at least a major portion of the cycle between
successive tapping operations to remove accumulated aluminum
produced in the cell.
Supplemental crust breaking equipment may also be used for purposes
of exposing the bath surface when necessary to skim or burn carbon
particles which may be floating on the bath.
The embodiments of FIG. 3 and FIG. 4 are similar except that each
shows a pre-baked anode system without permanent casing means. The
hood means 52 in FIG. 3 are pivotably mounted on the pot shell. In
FIG. 4, the hood means are mounted between the laterally spaced
anode segments, which may be preferred if maximum anode area is
desired, since the anode segments may extend closer to the side
walls.
FIGS. 5A, 5B and 5C summarize schematically various feeding
arrangements contemplated, for multiple anodes and large Soderberg
anodes. Feed sites in FIG. 5A, representing the location of plunger
means 58, for example, are designated by reference letters M at
positions between each continuous anode segment and an adjacent
sidewall of the cell, and by reference letters N between anode
segments. Series N or M may be employed alternatively or both may
be provided. Depending on the size and cross-sectional
configuration of the anode segments, the feed sites may be located
about midway of each segment, as shown, or at other positions such
as near or in an aisle between segments.
As shown in FIG. 5B for a single self-baking anode, the feed sites
R are preferably arranged to assure that no point on the anode
working surface is farther removed from the nearest feed site than
three-fourths the overall anode width across the cell. FIG. 5C
shows a similar arrangement of feed sites S for a conventional
anode system having multiple pre-baked carbon blocks.
Regardless of the anode configuration and the location of plunger
means 58, and whether or not associated casing means 29 or hood
means 52 are provided, it will be appreciated that certain of the
openings through the crust may be employed for withdrawing anode
gases and others for feeding aluminum into the bath, or each
opening can serve both purposes.
In Table I there is presented in the upper portion of the table, a
consideration of four typical single Soderberg anode pots of
different sizes, when operated at two levels of power
consumption.
TABLE I
Single Anode A B C D Anode width 51 63 79 82 Working Space * 52 52
52 52 Side Lining * 29 29 29 29 Shell Width 132 144 160 163 Anode
length 129 216 274 388 Working Space * 32 32 32 32 End Lining *, 31
31 31 31 Shell length 192 279 337 451 Anode Area 6,580 13,600
21,600 31,800 Shell Area 25,300 40,200 53,900 73,500 S/A ratio 3.9
2.95 2.49 2.31 Anode CD at 8.0 6.4 5.8 5.1 5.0 Kwh/- Current at 8.0
42.0 79.0 111.0 159. Kwh/-KA Anode CD at 7.0 5.1 4.75 4.3 4.25
Kwh/- Current at 7.0 33.6 64.6 92.9 135.2 Kwh/- KA Multiple Anode
Anode Width 74 86 102 105 (overall) Center Aisle 9 9 9 9 Working
Space * 20 20 20 20 Side Lining * 29 29 29 29 Shell Width 132 144
160 163 Anode Length 120 207 265 370 (overall) Space between 9 9 9
18 Anode segments Working Space * 32 32 32 32 End Lining * 31 31 31
31 Shell Length 192 279 337 451 Anode Area 8,800 17,800 27,000
38,900 Shell Area 25,300 40,200 53,800 73,500 S/A ratio 2.84 2.26
1.99 1.90 Anode CD at 8.0 7.2 6.8 6.4 6.4 Kwh/- Current at 8.0 63.9
121.0 172.8 249.0 Kwh/- Production increase 52% 53% 56% 57% Anode
CD at 7.0 5.5 5.3 5.1 5.1 Kwh/- Current at 7.0 48.8 94.3 137.7
198.4 Kwh/- Production increase 45% 50% 48% 47%
In the lower portion of the table the productivity of the same pots
upon conversion to operation in accordance with the present
invention, including installation of divided continuous anodes, is
presented. The table indicates that, for the same levels of power
consumption as used in the upper portion of the table, operating in
accordance with the present invention can add about 50 percent to
the productivity (current-carrying capacity) of the cell for a wide
range of cell sizes.
By way of summary, it can be seen that the present invention not
only provides an improved continuous anode system as regards ease
of venting anode reaction gases along the anode working surface, by
subdividing the anode into spacedly adjacent anode segments,
preferably at least four such segments, but these segments may be
arranged to achieve an anode area at least about 20 percent greater
than would ordinarily be installed with a single continuous anode
into the same pot shell.
In order to accommodate increased anode current density, permanent
anode casing means are further provided and a blanket of alumina
adjoining the casing means is used to protect against air
burning.
The feeding of alumina into the bath as operation of the cell
proceeds is controlled to minimize disruption of the crust, which
supports a protective blanket of alumina. Feeding is accomplished
frequently throughout the period between successive tapping
operations to remove accumulated aluminum, preferably at several
locations in order to achieve uniform distribution of alumina in
the bath. Plunger means used for opening holes through the crust at
spaced locations for feeding and for releasing anode reaction gases
may be disposed between adjacent rows of anode segments or between
such segments and adjacent sidewalls of the cell. The feed and gas
release openings in the crust may be enclosed by hood means
effective to collect evolved gases, and the associated plunger
means are operable from outside the gas collection enclosure.
In accordance with the method aspects of the present invention,
furthermore, the anode system and feeding technique enable
operation of the cell at increased anode current density to achieve
increased rates of production.
The term "continuous anode" is used herein with reference to anodes
of the type in which consumption of anode material (e.g. carbon) at
the working surface is compensated by adding more material, whether
as a plastic mix or in the form of blocks or slabs, rather than by
removing the anode butt and substituting a replacement anode. This
includes therefore not only conventional self-baking anodes, but
also those comprising a pack of pre-baked components.
Such terms as "length," "width," and "side" are used in a general
or relative sense, with length ordinarily referring to the longer
dimension (and width the shorter dimension) of a cell or other
rectangular element.
The term "permanent casing" includes any unitary casing structure
for an anode, in contrast to conventional side pin Soderberg
casings having multiple channels, the lowermost of which is
periodically removed and relocated. One aspect of using a permanent
casing is that the anode carbon moves downwardly relative to the
casing.
While present preferred embodiments and practices of the present
invention have been illustrated, described and discussed, it will
be apparent that the invention may be otherwise variously embodied
and practiced within the scope of the following claims.
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