US 3714002 A
Improvements in the construction and 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.
Claims available in
Description (OCR text may contain errors)
Kibby 1 ,lan.30,1973
 ALUMINA REDUCTION CELL AND IMPROVED ANODE SYSTEM THEREIN 75 lnventor: R6665 M. KibB'ifibitibj'Aia.
 Assignee: Reynolds Metals Company,
 Filed: Sept. 2, 1970  Appl. No.: 68,939
 U.S. Cl. .l ..204/67, 204/245, 204/247, 204/284  Int. Cl. ..C22d 3/12, C22d 3/02  Field of Search ..204/67, 243-7;
[561 References Cited 7 V UNITED STATES PATENTS 2,917,441 12/1959 Donald ..204/247 X 3,322,658 5/1967 Sem 1.
3,192,140 6/1965 Zorzenoni ..204/247 X 3,243,364 3/1966 Kittlaus et al. ..204/247 3,551,308 12/1970 Capitaine et al. ..204/067 3,582,483 6/1971 Sem ..204/243 R FOREIGN PATENTS OR APPLICATIONS 689,398 6/1964 Canada ..204/247 124,627 8/1969 U.S.S.R.... ..204/67 183,953 6/1966 U.S.S.R........ ..204/245 45,694 10/1928 Norway ..204/243 R Primary Examiner-John H. Mack Assistant ExaminerD. A. Valentine AttorneyGlenn, Palmer, Lyne, Gibbs & Thompson  ABSTRACT Improvements in the construction and 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.
2% Claims? DiaiiiTg'Figiii-"s PATENTEDJMI 30 I973 sum 10F 3' (BERT INVENTOR R KI ATTO EYS I PATENTEDJAN 30 I975 sum 2 or 3 mvsmon ROBERT M. KIBBY ATTORNEYS PATENTEDJAHBO I975 SHEEF 3 OF 3 H65 Mafia? ATTORNEYS ALUMINA REDUCTION CELL AND IMPROVED ANODE SYSTEM THEREIN 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.
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, 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 prebaked 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 selfbaking 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 isavoided 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 ex-' ample, 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, andthe greater total bottom area of an array of prebaked 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 selfbaking 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 a total anode cross-sectional area comparable to that of pre-bake cells and runningthe larger self-baking anodes at a current density comparable to that conventional for prebaked 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 having a continuous electrode, 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, as well as accessory apparatus for feeding and gas collection purposes.
More particularly, the present invention concerns self-baking and other forms of continuous anodes having a plurality of spacedly adjacent anode passageways having their lower outlet ends located inside the outer periphery of the anode working surface. Anode passageways in accordance with the invention preferably are arranged and disposed so that no point on the outer peripheral edge of the anode working surface is farther removed from the nearest passageway outlet than three-fourths the anode width. In this respect, the term anode width" refers to the narrowest outer cross-sectional dimension of an anode in the plane of its working surface. Some passageways may be used only for feeding and others for gas removal, or some or all may be used for both purposes; however, I prefer to provide a plurality of passageways for feeding purposes in order to achieve more uniform distribution of alumina in the bath, particularly in the region of the bath underlying the anode working surface.
As regards anode configurations of substantially rectangular cross-section in a horizontal plane, furthermore, especially those having a longer dimension lengthwise of the cell than their width across the cell, the anode arrangement includes passageways which are spaced lengthwise of the cell (i.e. along its longer dimension), and which preferably are disposed to satisfy the additional constraint that the distance between their adjacent outlet ends is no greater than three-fourths the anode width across the cell.
In a preferred anode construction, the passageway outlets are so distributed that every point on the anode working surface is no farther removed from the nearest outlet than three-fourths the anode width (as abovedefined).
It will be apparent that the foregoing arrangements are effective to reduce the paths of travel for feeding alumina beneath the anode and for venting anode reaction gases along the anode working surface. In addition, gas collection is readily accomplished by using hood means associated with said anode passageways or using supplemental hood means along the sides of the anode, or both.
By placing the aforementioned continuous anode or anodes in close proximity 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, an anode system of the type described herein 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 lbs/day for each square foot of floor area can be achieved.
The method aspects of the present invention include feeding alumina into the bath through the anode passageways, and maintaining a blanket of alumina on the crust adjacent exposed surfaces of the anode to protect the anode carbon against air burning. It can be seen, of course, that feeding at spaced locations within the outer periphery of the anode can be utilized to achieve better distribution of alumina beneath the anode than would be the case using feed sites located along the sides of the cell. Feeding through the anode also makes it possible to place the anode closer to the sidewalls of the cell, so as to achieve a higher ratio of anode to pot shell area 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 and around the anode. For one reason, as previously noted, this crust may be used to support a blanket of alumina adjoining the anode for protection against air burning. Since crust may also form at the lower outlet ends of the anode passageways, a system of plungers or other means are provided for maintaining holes through the crust adjacent the passageway outlets and the alumina requirements of the cell are preferably introduced through such passageways and holes rather than by breading massive portions of the crust outwardly of the anode. Anode reaction gases or a portion thereof may also be released from the cell through these holes and the adjoining anode passageways.
Most of the anode gases (from 50 to percent) are released at the sides of the anodes through enclosures which occupy a small portion of the peripheral space between the anode and the pot sidewall. Plunger means are provided to maintain communication between the gas collecting enclosures and the space beneath the pot crust, without disturbing the gas seal between the enclosures formed by frozen electrolyte and the blanket of ore resting on the frozen electrolyte. In addition, the practice of the present invention preferably includes feeding alumina into the bath at frequent intervals and at spaced locations; but may involve feeding at one location and then another, rather than feeding simultaneously at different locations. A further benefit of this feeding arrangement is that it allows for maintenance ofa blanket of alumina on the crust between the anode and adjacent sidewall of the cell to protect the anode 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.
The invention may be practiced with anodes formed of Soderberg paste, continuous cemented prebaked carbon, or continuous cemented semi-baked carbon.
VERTICAL PIN SODERBERG In the case of a vertical pin Soderberg pot employing a fixed steel casing around the anode, the present invention may include provision for supplemental collection of anode gases by means of a hood or other similar enclosure attached to the anode casing, which occupies a substantially smaller portion of the anode periphery than the usual complete skirts. With such a hood arrangement, plunger means are provided for maintaining 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 also be introduced through an inlet into the enclosure and onto the crust therein.
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 I to 5 minutes. In this way the crust never forms inside the gas collection enclosures to an extent that it offers high resistance to breakage and lighter mechani-' described generally above, it then becomes unnecessary to routinely break the crust that forms between the sidewalls of the pot and the anode. 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 ordinarily required breaking the crust outside the periphery of the anode or the anode gas skirt for 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.
SIDE PIN SODERBERG For use with a side pin Soderberg anode, for example, a supplemental gas collection enclosure may be mounted-on thepot 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 thepot and also between the enclosure and adjacent side of the anode. Within the enclosure, holes are provided through the crust for removing the anode gases evolved from the cell.
CONTINUOUS CEMENTED BLOCKANODES block anodes that no portion of them has to be removed routinely. This means that the pot crust does not have to be broken for routine anode replacement, thus eliminating a troublesome source of fume from the pot. Continuous prebaked electrodes have the further advantage over pre-baked block anodes that no portion of the carbon is returned to the carbon plant for recycling. This means that, with continuous prebaked anodes, electrolyte materials are not returned to the carbon plant where they adversely affect refractory life in baking furnaces and add to the difficulty of scrubbing baking furnace gases efficiently.
The continuous semi-baked anodes have the advantages of the continuous pre-baked and have the further advantage that final baking temperature is in the range 400 to 600 C. as opposed to l,I00'C. for
fully pre-baked blocks. This means that baking furnaces can bemuch smaller and less expensive to build and maintain. It also means that tunnel kilns can be used advantageously.
Continuous pre-baked anodes in which the blocks have been fully pre-baked (I,l00 C.) before addition to the pot use short anode contact pins, cemented into holes which have been drilled into the baked anode. The distance electric current must travel from these pins to the working surface of the anode is greater than for side pin Soderberg pots with the same anode width. This results in higher voltage losses for the continuous pre-baked anode. This disadvantage gets more severe as one widens the anode to obtain the benefits achievable with center feeding as taught in this invention.
Semi-baked slabs have the advantage over fully baked slabs that long pins, typically as long as are used for Soderberg anodes, can be formed into the green blocks and left in the blocks during the furnace treatment to drive off pitch and establish a rigid block structure.
Typical Forms of Anode, Feed, and Gas Vent Locations Illustrating the Invention with Single In cells having continuous pre-baked or semibaked anodes, the gas collection enclosure may be supported as in the case with the gas collection and feeding systems 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 anode gases can be collected from this hood. Here again it is not necessary to disturb the crust and ore blanket cover between and around the carbon anodes during at least a major portion of the operating cycle,thereby reducing the occasions for the attack of air on the anode carbon and avoiding the necessity of disturbing the pot crust outside the enclosure, either for purposes of feeding'the pot or removing gas.
In the various anode systems described it will be advantageousto provide means of access to the area inside the supplemental gas collection hoods. In the case of a hood mounted from 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.
For purposes of further comparison it may be noted that typical conventional side pin self-baking anode systems employing single anodes carry up to about amps for each square inch of anode bottom surface, and about 2.5 amps 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 amps for each square inch of anode bottom area, and about 4 amps for each square inch of pot shell bottom area.
In contrast, pots equipped with a continuous anode system and operated in accordance with the present invention may carry about 7.5 amps 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 passageways through a self-baking anode 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 particularly as regards the inclusion of anode passageways spaced lengthwise of the cell and distributing the passageways outlets substantially uniformly over the anode working surface.
For further comparison, typical side pin Soderberg pots with anodes carrying 80,000 amps, for example, are hooded to capture hydrocarbon and fluoride gases for scrubbing at a rate of about 5,000 cfm per pot. In contrast, continuous anodes, carrying the same current whether Soderberg, semibaked, or pre-baked employing this invention capture the fluoride-bearing gases and most of the gases resulting from decomposition of pitch binder with a movement of about 500 cfm per pot.
.Thus, scrubbing can be much more efficient with a given investment, because more concentrated gases are scrubbed and less air is handled to accomplish the removal of potential air pollutants.
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) showing a Soderberg cell arranged for operation in accordance with the invention;
FIG. 2 is a similar perspective view of a reduction cell having a continuous cemented slab anode and associated apparatus of the present invention; and
FIGS. 3 and 4 are detailed sectional views of portions of the apparatus shown in FIGS. 1 and 2, respectively.
FIG. 1 illustrates an embodiment of the invention in which the cell comprises a vertical pin Soderberg electrode arranged for center venting and feeding of alumina, as well as for supplementary peripheral venting of anode gases. The pot shell 10, cathode lining 12 and cathode collector bars 13 are conventional. The anode is formed in a permanent casing 14, supported by rods 15 from suitable frame members (not shown), to allow freedom of movement upwardly when the anode is raised, while providing support for the casing in its lowermost position.
Anode pins 19 are clamped for electrical and mechanical connection by clamps 20 to movable bus 21. Supplementary clamps 22 also connect the anode pins to the bus through a flexible connector 23. The combination of clamps 20 and 22 permits periodic upward movement of the bus 21 with respect to the pins 19 without interruption of current flow to the pins.
Each of the alumina feeders 16 (later detailed in connection with FIG. 3) has tubular casing means extending approximately to the level of the bake zone of the carbon to provide a passageway through the anode, and further includes plunger means adapted to penetrate any crust that may form within the anode passageway at its lower outlet end adjacent the bath. The feeder means are arranged to receive alumina from an ore bin 17, as hereinafter described more fully.
The feeder assemblies, which also include means for removing reaction gases liberated through the anode passageways, are spaced along the centerline of the anode at distances such that no point on the anode working surface is farther removed from a point of feeding than three-fourths the anode width.
Supplementary gas collection enclosures 30 are mounted to the permanent casing 14 in such a way that they are supported with their lower edges about 4 to 6 inches above the level of liquid electrolyte in the pot and low enough to be enclosed within the ore blanket 31 which covers the electrolyte normally freezing between the anode casing and the adjacent sidewall of the cell. Plunger means 32 are provided to maintain an opening in the crust within the gas enclosure 30.
Gas collection tubes 33 convey evolved gases from the enclosures 30 to a scrubbing system.
Secondary shields 34 are provided to capture any hydrocarbon fume that is evolved during operations to replace pins 19. This system provides the opportunity to capture and scrub concentrated fluoride gases separately from the dilute hydrocarbon-bearing gases captured by secondary hoods 34. The feeding and fume collection arrangement described makes it possible to feed alumina to the cell and collect gases from beneath the anode without disrupting the ore cover 31. We have found that it is sufficient to have relatively small enclosures 30 adequately spaced around the cell instead of complete gas collection skirts as previously practiced in vertical pin Soderberg pots. Servicing of the anode with respect to anode pin replacement and paste addition is as normally practiced in vertical pin Soderberg operations.
FIG. 3 illustrates in detail the feeder/gas collection mechanism 16 of FIG. 1. A steel sleeve 41 is supported in the ore bin 17 by the brace members 44. That sleeve has lateral openings 45 to admit alumina from the bin. Within sleeve 41 is a rotatable sleeve 46 resting upon a fixed support ring 47 which is attached to sleeve 41. After the installation of sleeve 46, a collar 48 is installed to which is affixed a lever arm 49. Sleeve 46 supports a transverse plate 50 incorporating outlet holes 51 for passage of alumina and a central opening 52 for the breaker rod 53. The breaker rod is attached to piston 54 which operates in cylinder 55, and an inner sleeve 56 is attached to the bottom of cylinder 55. Both cylinder 55 and sleeve 56 are mounted so that they cannot rotate.
Sleeve 56-supports a lower plate 57 having holes 58 for passage of alumina. Sleeve 46 has lateral openings 59 for passage of alumina. In this arrangement the entire assembly comprising the cylinder 55, sleeve 56 and sleeve 46 can be placed within sleeve 41 after which the collar 48 is affixed.
Tube 59 admits air for the downward thrust of the breaker rod 53. Tube 60 provides air for the return stroke.
The sleeve 46 can be rotated by means of an air cylinder (not shown) acting against the lever 49. The inlet holes 45 and outlet holes 58 are so oriented that when holes 45 are open, holes 58 are closed. This arrangement provides a metering chamber in the annular space between sleeves 46 and 56 to hold a charge of alumina until it is dumped. In order to dump ore, lever 49 is activated to close port 45 and open port 58. On the return stroke port 58 is closed, port 45 is opened and a new charge of ore is measured out. The action which dumps ore is a rotating of sleeve 46 within sleeve 41. I
The collar 48 shaped so that leakage of alumina can be stopped due to its angle of repose, without requiring a tight fit. Operation of thebreaker rod 53 is conveniently controlled through a timer mechanism (not shown) separate. from the mechanism which.
operates the ore dump control lever 49. While it is possible to make these two actions coincident, it is contemplated that under most conditions they will be performed independently. For example the breaker rod may be activated once every five to ten minutes and the ore dump mechanism may be activated once everyhalf hour.
An anode sleeve 61 is provided which fits loosely into the chamber 63. Such an exhaust port would be adjustedto discharge 'airfrom the upper chamber 'of the cylinder whenthe piston 54,reaches the port position. Alternatively, the plunger shaft 53 may be threadedthrough the piston 54 and extend outwardly at the top of the cylinder for adjustment purposes.
FIG, 2 illustrates an alternate embodiment of the in-v vention in which the cell comprises a continuous, cemented, semi-baked anode with side pin electrical contacts, center venting andfeeding of alumina, and peripheral venting of concentrated anode gases. Pot shell 10, lining 12, and cathode collectors 13 are conventional. The anode 74 is comprised of extruded or rammedcarbon blocks 75 which have been baked to a temperature sufficient to develop their structural strength and drive off the pitch volatiles in the furnace. This temperature is between about 400and about 600 C, depending upon the mix used in forming the carbon block. The baking can be donein a tunnel kiln under conditions such that a goodportion of the evolved pitch is condensed and availablefor re-use in carbon forming operations. The carbon blocks 75 are cemented together by carbonaceous paste at the interface 76, and, as the anode is consumed and the blocks are lowered, their baking is completed in place, but without evolution of pitch fume except for that present in the cementing paste, and any additional paste used to seal around the ore feeding breakers as hereinafter discussed.
Anode contact pins 77 are long, typically of the length used for side pin Soderbergs. They are inserted in the blocks before the blocks are placed in the baking furnace. Since the bocks are not baked above 600 C, the steel pins are not affected by the temperatures in the baking furnace. The pins are later removed in accordance with conventional side pin practice. Pins 77 are connected through conductors 78: to the anode bus 21, using clamps 80. Pin changing and connector raising procedures are as typically performed in side pin Soderberg operations.
The ore feeders 81 are sealed to the anode blocks by means of carbonaceous paste or mixtures of alumina and cryolite. Each feeder (later described in detail with respect to FIG. 4) has an activating mechanism for its plunger 83, and includes ore inlet tube 84. To provide access for additional blocks 75 as the anode is consumed, the space above the blocks is left open and the ore bin normally used is replaced by conveyor means 85. When blocks are to be replaced, ore feeders 81 are disengaged and raised to a new higher position to accommodate the added height of the new blocks, and are re-sealed to the anode in this position to prevent flow of gas between the casing of the feeder and the adjacent carbon.
The ore feeders are so located that no point on the workingsurface of the anode is farther removed from a point of venting and ore feeding than three-fourths the anode width. Gas collection tubes 86 are provided to remove that portion of anode gases evolved through the anode at the feed sites. Supplementary gas collection enclosures 87 are suspended above the electrolyte at a distance of about 4 to 6 inches by means of cantilever arm 88 supported by pivot and block 89. Breaker bars 91 operate inwardly of the enclosures 87 to maintain an access hole through the crust, admitting gases from beneath the, crust to the enclosures, from which theyare withdrawn byway of tube 92. The breaker 91 is height-adjustable, so that its lowest point of movement is above immersion in the electrolyte. By this action, cryolite does not freeze and adhere to the breaker bar, and the hole it cuts through the crust is approximately equal in shape to the cross-section of the bar. Sufficient vent locations are provided to approximately match the alumina feed, locations. Concentrated fluoride gases are collected through tubes 86 and 92, and conveyed to scrubbers. The volume of gas which must be collected under this system in order to properly vent the pot is on the order of one-tenth the volume of gas collected in a system with general hooding around the anode as would be practiced in conventional side pin Soderberg pots. The concentration of fluorides is correspondingly greater and therefore the efficiency of the scrubbers is greater for a given amount of gas movement horsepower installed.
The gas collection enclosure 87 is sealed in the alu- -mina blanket 31 which rests upon the crust that normally forms over the electrolyte. A complete ore blanket is maintained under all routine conditions such that alumina is banked up against the electrode to prevent air burning, and this blanket is not disturbed for the purposes of gas venting or alumina feeding.
It can be seen that the embodiment of the invention as illustrated in FIG. 2 can be practiced also for fully pre-baked carbon slabs, under which conditions it has been found more practical to drill holes in the carbon for the insertion of short anode pins as opposed to the insertion before baking of long anode pins as in the case where semibaked carbons are used.
It can further be seen that the feeder and gas collection elements illustrated in FIG. 2 can be practiced in conventional side pin Soderberg pots.
Turning next to FIG. 4, the feeder/venting mechanism 81 will be described in greater detail.
FIG. 4 illustrates an alternative breaker and ore feeding arrangement as provided in the reduction cell of FIG. 2. In FIG. 4, cylinder 101 is fixed to a beam 102 supported by the cells superstructure. Anode sleeve 104 slides over cylinder 101 and is set into a recess 105 of the prebaked block 75 after the block is installed. Carbon paste or alumina 107 is used to make a gas seal between the sleeve 104 and the anode block. Piston 108 operates plunger 83which is preferably height adjustable (as previously discussed) so that the plunger does not touch the bath in its lowest position. Alumina is transported through duct 85 from which it discharges to metering device 111, and then through duct 84 to the anode. The shaft of the metering device is turned by a motor and associated clock mechanism (not shown), and piston 108 is operated by air from a valve control which is responsive to a separate actuating mechanism. The two control mechanisms may be set to give coincident operation of ore metering device 111 and plunger 83; however, in most cases, they would be operated independently as previously discussed in connection with FIG. 3. Tube 86 is provided to receive anode gases passed outwardly of the cell through the anode passageway.
With respect to the anode blocks 75 semicircular holes are formed in the side of each anode segment to provide for putting the segments in place around the feeder device.
As the electrode is consumed sleeve 104 slides down over sleeve101. When it comes time to place a new block over anode block 75, sleeve 104 is disengaged from the sealing medium 107 and raised. After the new block is in place, the sleeve is set back down in the recess provided and rescaled.
While the presently preferred practices of the 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.
What is claimed is:
1. In an alumina reduction cell having a bath of molten electrolyte containing dissolved alumina, a crust of solidified electrolyte overlying the bath and means for passing electric current through the bath, the improvement comprising:
a. a continuous anode having a plurality of tubular passageways extending downwardly through the anode at spaced locations within its transverse cross-section;
b. means for breaking an opening through any crust formed in said passageways at their lower outlet ends adjacent the anode working surface; and c. means for introducing alumina into the bath through said passageways.
2. Apparatus according to claim 1 including means for collecting gases evolved from the cell through said anode passageways.
. 3. Apparatus according to claim 2 in which said passageways are so disposed that no point on the outer periphery of the anode working surface is further removed from the nearest passageway outlet than three-fourths the anode width.
4. Apparatus according to claim 1 including an anode having passageways which are spaced lengthwise of the cell.
5. Apparatus according to claim 4 having anode passageways spaced lengthwise of the cell along its centerline.
6. Apparatus according to claim 4 in which said anode is longer lengthwise of the cell than its width across the cell.
7. Apparatus according to claim 1 in which said passageways are so disposed that every point on the anode working surface is no further removed from the nearest passageway outlet than three-fourths the anode width.
8. Apparatus according to claim 1 in which the aggregate anode cross-sectional area constitutes more than 50 percent of the floor area occupied by the cell.
9. Apparatus according to claim 1 in which said anode comprises carbonaceous blocks which are baked prior to installation to eliminate substantially all hydrocarbon fumes and to develop sufficient structural strength to support themselves in service in the anode.
10. Apparatus according to claim 9 in which said carbonaceous blocks are baked at a temperature between 400 and 600 C.
l 1. Apparatus according to claim 10 in which electrical contact pins are inserted in the anode blocks before baking.
12. In an alumina reduction cell having a bath of molten electrolyte containing dissolved alumina, a selfbaking carbon anode including a baked lower portion adjacent the bath and a softer upper portion, and a crust of solidified electrolyte overlying the bath, the improvement comprising:
a. tubular casing means in at least said upper portion of the anode to provide interior passageways extending downwardly through the anode; and
b. plunger means operable inwardly of said passageways to prevent obstructions therein due to crust formations at the lower outlet ends thereof adjacent the anode working surface.
13. Apparatus according to claim 12 including feeder means for introducing alumina into the anode passageways, said plunger means being arranged for movement between a lower position to produce an access'opening through any crust formed at said lower outlet end of each anode passageway and an upper position allowing alumina to flow into the bath through said access opening.
14. Apparatus according to claim 12 in which said passageways and associated plunger means are so disposed that no point on the outer periphery of the anode working surface is farther removed from the nearest access opening than three-fourths the anode width.
15. Apparatus according to claim 14 including means for collecting gases evolved from the cell through said passageways.
16. In the operation of an alumina reduction cell having an anode, a bath of molten electrolyte containing dissolved alumina and a crust of solidified electrolyte overlying the bath, the method which comprises:
providing a continuous anode having a plurality of interior passageways extending downwardly through the anode, said passageways having their lower outlet ends disposed adjacent the anode working surface;
feeding alumina passageways;
maintaining a blanket of alumina and supporting crust around the anode to provide a substantially gas-tight cover over the bath as said feeding proceeds;
breaking an opening through any crust formed within said passageways at the lower outletends thereof; and
collecting gases evolved from the cell through said passageways, whereby said feeding and gascollecting operations are carried out without disrupting the main body of said crust outwardly of the anode.
17. The method of claim 16 which includes providing access openings through the crust at spaced locations outwardly of the anode, said openings being spaced apart sufficiently to preserve the integrity of the crust into the bath through said between openings; and collecting gases evolved from the cell through said openings in the crust,
18. The method of claim 16 which includes providing a continuous anode in which said passageways are so disposed that no point on the outer periphery of the anode working surface is farther removed from the nearest passageway outlet than three-fourths the anode width.
19. The method of claim 16 which includes providing a continuous anode having passageways which are spaced lengthwise of the cell.
20. The method of claim 16 which includes providing a continuous anode which is longer lengthwise of the cell than its width across the cell.
21. The method of claim 16 which includes providing a continuous anode in which said passageways are so disposed that every point on the anode working surface is no farther removed from the nearest passageway outlet than three-fourths the anode width.
22. The method of claim 16 in which said feeding comprises passing alumina into the bath from the lower outlet end of one or more of said passageways but includes repeated feeding at spaced locations.
23. The method of claim 22 in which said feeding occurs at intervals and in amounts sufficient substantially to compensate for depletion of alumina from the bath.
24. In an alumina reduction cell having a bath of molten electrolyte containing dissolved alumina, a crust of solidified electrolyte overlying the bath and an anode for passing electric current through the bath, the improvement comprising: l
a. a continuous anode having a plurality of interior passageways extending downwardly through the anode, said passageways being so disposed that no point on the outer periphery of the anode working surface is farther removed from the nearest of said passageways at its lower outlet end than threefourths the anode width;
b. means for collecting reaction gases of the cell through openings in the crust outwardly of the anode at spaced locations which are separated sufficiently to preserve integrity of the crust as a substantially gas-tight cover over the bath;
c, means for feeding alumina into the bath through said interior passageways of the anode, including means for breaking an opening through any crust formed within said passageways at their lower outlet ends adjacent the bath; and
(1. means for collecting reaction gases evolved from the cell through said anode passageways.