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.

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.

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.