US3501386A - Apparatus and process for the reduction of aluminum - Google Patents

Description

March 17, 1970 I A. F; JOHNSON I 3,501,386

APPARATUS AND PROCESS FOR THE REDUCTION OF ALUMINUM Filed May 17, 1966 5 Sheets-Sheet 1 ercryolite Layer Lower Cryolite 7 Layer M t I F INVENTOR Molten eu og Aluminum Arthur F. Johnson BY F M 7am I ATTORNEYS March 17, 1970 A. F. JOHNSON APPARATUS AND PROCESS FOR THE REDUCTION OF ALUMINUM 5 Sheets-Sheet l Filed May 17, 1966 INVENTOR Arthur E Johnson BY W fl uuak, WM,WMW

ATTORNEYS APPARATUS AND PROCESS FOR THE REDUCTION OF ALUMINUM Filed May 17, 1966 March 17, 1970 A. F. JOHNSON 5 Sheets-Sheet 4 FIG. 6

FIG. 7

Molten Aluminum L Metdl F09 In Layer Electrolyte k Molten Electrolyte Loyer INVENTOR Arthur F. Johnson BY fwd WM 72 M M ATTORNEYS APPARATUS AND PROCESS FOR THE REDUCTION OF ALUMINUM 5 Sheets-Sheet 5 FIG. 8

Filed May 17, 1966 llll , Arthur E Johnson 3 BY M, aha/1%,

United States Patent US. Cl. 204-67 4 Claims This invention relates to the electrolytic reduction of aluminum and has for an object certain improvements in the composition and shape of carbon anodes in the process of reduction of molten aluminum from aluminum oxide dissolved in a fused electrolyte as, for example, fused fluorides. The invention provides improved anodes, an arrangement of improved anodes and a process for the operation of the reduction cells. The invention also provides an apparatus for operating groups of anodes in unison for the purpose of adjusting the slope of the anodic faces of the anodes. It is an object of the invention to provide for periodic setting at the most efficient angular and spatial positions of the anodes in reduction cells to improve the Faraday ampere efficiency and lower the consumption of electrical power in the production of aluminum.

The invention is in part based on my conception that the carbon dioxide gas evolved at the anodic face of the electrodes may be expeditiously removed from beneath the anodes with a minimum of reoxidation of molten aluminum resulting in an increase in the ampere efficiency. When aluminum is produced by electrolysis of a fused electrolyte as, for example, in the Hall process, the following reaction is considered to be the primary chemical equation occurring at the anode:

EQUATION 1 2A12O3 30 4A1 3002 Molecular formula weights 204 36 108 132 From the above equation and Faradays law one may derive that a Hall cell would produce 0.093 milligram of aluminum per second at 100% a-mpere efficiency and consequently (by molecular formula combining weights) 0.114 milligram of CO gas at the anodic face. One may compute by the gas laws that this weight of gas is equivalent to about 0.264 cubic centimeter of CO per ampere per second at 980 C. cell operating temperature. Since such cells commonly operate at about 1 ampere per square centimeter anode current density with prebaked electrodes and about 4.5 centimeters average anodecathode spacing, this means that the relatively flat and narrow space enclosed between anode and molten aluminum cathode (hereinafter designated A-C spaced) is filled 0264/45 or about 6% each second with gas which displaces the molten electrolyte and thus blankets the anodic face area with an appreciable amount of an electrical non-conductor. In commercial practice the CO gas evolved according to Equation No. 1 is partly reduced (without change in volume) by a reoxidation of the molten aluminum that forms the molten cathode layer underlaying the cryolite. This reduces the ampere eificiency commonly obtained in commercial practice to 83% to 90% and probably according to the following chemical equation:

EQUATION 2 Patented Mar. 17, 1970 Molten cryolite as commonly used in aluminum reduction and when free of contamination may be quite transparent, but when molten aluminum underlies the cryolite a fog rises from the molten aluminum and disperses in the cryolite making it cloudy in the vicinity of the molten aluminum. It is my belief that this fog is finely divided aluminum which is reoxidized according to Equation No. 2 to aluminum oxide which redissolves in the 'molten cryolite. Tests indicate that the bubble flow from the anodic face of conventional anodes is towards all edges of the anodes and particularly the longitudinal parallel edges. It is my conception that the bubbles drag the layer next to the molten aluminum, which contains the greatest amount of aluminum fog upward against the anodic face where the CO gas is capable of reoxidizing this finely divided aluminum with great rapidity and .by the reaction shown in Equation No. 2. This results in ampere inefficiency which the invention aims to overcome.

In one aspect of my invention I shape the anodes in the process of manufacture so that they will rid themselves of anode gases from their initial use onward. At least a part of the under faces of the anodes of this invention have an upward slope which cause bubbles, impelled by the weight of fused electrolytes which they displace, to escape quickly from the AC space. This removal of CO bubbles increases the effective area of the anodic face and also reduces the tendency to reoxidize the aluminum fog. The dissipation of gas bubbles reduces the resistance to the passage of electric current from the anodes to the cathode through the molten electrolyte.

In one embodiment of the invention the anodes are arranged in a manner so that the anodic faces are at the same angular direction with the horizontal to cause the anodic gases to escape in such a direction that they will not draw the aluminum fog off the molten aluminum cathode upward into contact with the anodic gases. Generally the desired direction will be the exit at the shortest distance towards one but not both of the longitudinal sides and sometimes also towards one or both ends if resulting electrolyte circulation is found particularly advantageous.

Where prebaked anodes are used the preferred direction of gas bubble removal is usually in the same direction under a large part if not all the electrodes in any one row. Preferably all the electrodes in a row will have their longitudinal sides parallel so the gas bubbles will have the shortest distance across the electrode face to travel to reach the edge, and the electrolyte flow induced by the bubble flow under one anode will continue smoothly on under the next anode in the row to assist in speeding bubble movement thereunder without the electrolyte sweeping the metal fog upward to mix with the anode gas.

Confining the bubbles in narrow slots between adjacent anodes or in narrow slots between an anode and the side lining of the cavity which contains the molten electrolyte effects a pumping action as the bubbles rise vertically from the anodes. By means of the Pohle air lift principle the electrolyte is made to circulate in a preferred pattern to give increased electrical efficiencies. The temporary positive flow of anode gases attained by forming or setting the anodes with an upward slope may thus be made to persist even though the anodes tend to be gradually leveled off horizontally on their anodic faces at points which are closer to the molten cathode than others because more current travels to those points. The desired upward slope of the anodes may be achieved by making the anodes less dense or have greater elec trical conductivity in the direction towards which slope is desired.

The invention also provides means for suspending the anodes in the electrolyte so that their under surfaces all slope upwardly, preferably in the same direction, and it is advantageous to suspend or hold the anodes by a tilting appartus constructed and arranged so that the entire group of anodes in the cell may be tilted, first in one direction and then in the opposite direction to maintain an upward slope. As the anodes are consumed by electrolysis they tend to become flat and the tilting should be at such intervals as to maintain an effective upward slope.

The tilting operation breaks the crust over the molten electrolyte fusion between and around the anodes and facilitates the feed of fresh alumina into the fusion. The alumina content of the electrolyte is thereby maintained at the relatively high level known to result in improved efficiency of aluminum production.

In the accompanying drawings:

FIG. 1 is an isometric view of one form of prebaked carbon anode of the invention;

FIG. 2 is an isometric view of another form of prebaked carbon anode of the invention;

FIG. 3 is an isometric view illustrating a compartmented molding box to feed an hydraulic press for pressing the shape of anode shown in FIG. 1;

FIG. 4 is an isometric view of another shape of pre baked anode of the invention;

FIG. 5 is a vertical longitudinal section through an aluminum reduction cell illustrating an embodiment of anode supporting and tilting apparatus;

FIG. 6 is a plan view of the circulation pattern of electrolyte obtainable underneath two rows of the anodes of FIG. 5;

FIG. 7 is a vertical longitudinal section through one row of the anodes of FIG. 6 showing in elevation the electrolyte circulation pattern obtained.

FIG. 8 is a plan view of the circulation pattern of electrolyte obtainable underneath anodes of the type of FIG. 4;

FIG. 9 is a vertical longitudinal section at 99 of FIG. 8 showing in elevation the electrolyte circulation pattern obtained, and

FIG. 10 is a vertical cross section through a Soderberg anode constructed according to this invention.

- Conventional aluminum reduction anodes as manufactured have a flat bottom which constitutes a horizontal anodic face during electrolysis. After a day or two of operation the anode tends to wear to an upward curved shape towards the bottom edges probably due to the greater amount of gas accumulating in the flat center lessening the current there so the carbon is not consumed there as fast as at the edges. This invention overcomes the haphazard uncontrollable wearing away of the anodes by constructing them initially to have the desired upward slope and provides means for maintaining the slope resulting in uniform predictable operation.

FIG. 1 illustrates an anode under surface slope which is an improvement over conventional anodes because, from its initial operation in an aluminum reduction cell, it immediately relieves the anodic face of bubbles as soon as it starts to draw current and removes these bubbles in a fairly short average path by conducting them towards the longitudinal sides 6 and 7 equidistant from the vertical axis a of the anode. The carbon anode 1 has a steel stub 2 attached to the'copper or aluminum anode rod 3 by a bolted connection, and is also attached by pressing and welding to the fiat steel plate 4 held in the carbon by cast iron 5 which is poured in a cavity 8 between the steel plate 4 and the anode 1. Alternately a graphite paste joint compound may be used to electrically and mechanically attach the steel plate 4 to the anode. The anode is rectangular in shape but is symmetrical about the axis a. A portion of the anode face BCFG may be left fiat to facilitate handling as on a conveyor. The distance BC or FG may be as little as 2 inches or as much as 8 inches to give upright stability, and in any case the center of gravity of the anode should be midway between the longitudinal center of the flat face. The angle 6 that AB makes with the horizontal or extension of BC may vary from 0.5 to 20 or more. However, with too high an angle the average A-C distance is increased too much to justify the faster removal of gas bubbles and the lower average specific resistance through the A-C space taken as a whole with the result the reduction voltage is increased. The range of from 1 to 10 will usually be found best to cause the gas bubbles to escape by buoyancy along the upward sloping anode faces ABGH anl CDEF. The bubbles will set up a circulation pattern of the electrolyte as shown by the arrows in FIG. 1 and a strong pumping action will be eifected by the bubbles particularly after they reach the edges AH and DE from which they rise vertically through perhaps 6 inches of molten electrolyte thus providing an appreciable amount of pumping energy. This pumping effect is particularly strong if the vertical longitudinal faces DENM and AHOL are close enough to adjacent electrodes or to the electrolyte cavity wall to form a slot less than 4 inches wide and preferably less than 1 inch wide in which the bubbles rise. Moreover, once the pumping up the vertical slot has started it tends to perpetuate the electrolyte circulation even after the anodic face has worn rather flat due to its consumption in the electrolytic process faster at points closer to the molten metal cathode.

The anode 1 of FIG. 2 has a stub 2, steel plate 4 and cast iron 5 in the cafity 8, longitudinal sides 6 and 7 as in FIG. 1, and vertical axis b. The anode face ACFH is horizontally fiat and larger than the upward sloping area DEF'C' to permit upright stability during manufacture and storage. The upward step CC'FF may be made so that an upward slope will be induced on the face ACFH after the electrode is used in a cell for a day or two. The anode of FIG. 2 effects circulation of the electrolyte in one direction across the anodic face as shown by the arrows so there is no tendency for the metal fog to be swept off the molten metal and up towards the bubbles moving upward across the anode face. It is important to note that the use of a multiplicity ofthe anodes of FIG. 2 can result in a flow pattern of electrolyte similar to that illustrated in plan in FIG. 6 and in vertical section in FIG. 7 if the electrodes are properly oriented in the cell. In this way the flow of electrolyte set up by one electrode tends to persist under the adjacent electrodes avoiding the upward turbulence produced in the anode of FIG. 1.

FIG. 3 is an example of an anode premolding box for feeding the mold of an hydraulic press (not shown) which is shaped to produce an anode with the configuration of FIG. 1. The premolding box is made of steel plates forming compartments 9, 10 and 11. In forming the anode a carbon-pitch mixture which bakes to a certain density is fed into compartment 10 and a mixture which bakes to a lesser density is fed into 9 and 11. Then the premolding box is pushed over the mold, as in conventional practice, so that the carbon-pitch mixtures drop into the uncompartmented mold of the hydraulic press. When the dies of the hydraulic press approach each other from the top and bottom of the mold, the carbon-pitch mixtures produce an anode which, when baked and used in a Hall-type cell, will tend to be consumed more slowly in the middle area (corresponding to compartment 10) than on the anode faces of the side areas (corresponding to compartments 9 and 11). In this way an anode is produced that tends to maintain the shape of FIG. 1 on the anode face throughout the life of the anode of one or two weeks. The two different mixtures can be kept from intermixing in the press box by any suitable means as by removable dividers, vibrators, or the like.

The premolding box of FIG. 3 may be shaped to pro duce the anode of FIG. 2 which has a predisposition to continue to slope in the same direction as originally formed. The use of a carbon of greater electrical conductivity in the area where it is desired to have faster consumption will have the same effect as the use of carbon of lower density. Likewise the location of the plate 4 of FIG. 2 closer to the longitudinal edge MN than to the opposite top edge LO will tend to make the anode face CDEF wear faster than face ACFH. Exposing the anode of FIG. 2 to a higher temperature of baking on face DMNE than ALOH will usually predispose it to the slope desired.

FIG. 4 represents another anode shape that may be made by the above methods. It has a slope on the anode face CDEF upward towards part of one longitudinal side 13 since the face GBH'E slopes towards end 14. The flat portions of the anode bottom ACHF and HGBB are retained merely to give the anode upright stability during manufacture and storage. This type of anode may be utilized to produce an electrolyte circulation pattern similar to that illustrated in plan in FIG. 8 and in vertical section in FIG. 9. Such a pattern may be advisable Where the magnetic circulation of the molten aluminum in the cell excessively reinforces the flow pattern illustrated in FIG. 6 so that sideways diversions are indicated to lessen the speed of travel of the electrolyte. Likewise the diverse pattern of FIGS. 8 and 9 produced by the anodes of FIG. 4 may occasionally be better suited in some cell designs for thoroughly and continuously mixing the electrolyte ingredients and particularly the alumina which is periodically added to the electrolyte after it has been preheated on the crust of the frozen surface of the electrolyte between the anodes.

FIG. 5 illustrates by way of example how conventional anodes with a flat base may be used in this invention by tilting them when suspended in the cell so that the anode faces slope upward from the horizontal to the end that an electrolyte circulation pattern similar to that illustrated in FIG. 6 and FIG. 7 may be attained.

The apparatus illustrated in FIG. 5 comprises a reduction cell consisting of a metal shell 15 having the usual carbon lining 16 the interior vessel of which has a layer of molten aluminum 17 and an overlying fusion 18 in which several anodes are suspended. The fusion has a crust 19 on which the feed of aluminum oxide 20 rests. The cell supports rigid end posts 22 and 23 which support the-horizontal superstructure 24. Motorized jacks 25 and 26 are secured to the superstructure 24 and each of these has a depending hanger rod 27 and 28 which supports the horizontal anode bus 29.

The anodes are connected to iron or steel stubs 32 each of which is connected to an aluminum or copper anode rod 33 which is secured to the bus bar by a clamp 34 which permits upward and downward movement of the anodes. The superstructure 24 has pairs of rollers 35 which bear on the opposite sides of the anode rods to guide them in both their upward and downward movement and their sidewise shifting as will presently be described.

The clamp 36 has a frame 37 on which are mounted guide rollers 38, 39, 40 and 41 which permit the clamp to be moved upward and downward on the post 22. The anode bus 29 has one end secured to the motorized screw 42 which can shift the ends horizontally backward and forward to tilt the anodes. In this tilting movement the anode rods 33 pivot on the rollers 35. The clamp 36 may be secured to the post 22 by any suitable means.

In operation of the apparatus of FIG. 5 the anodes which may be of conventional design are suspended in the cell with all the anodes hanging in a vertical position. Then the motorized screw 42 is actuated so as to move the anode bus 29 horizontally to a position similar to that illustrated. This movement may be controlled to effect any slope desired. Usually only a small inclination from the horizontal is required on the anode faces to positively and expeditiously rid the anode faces of gas, particularly where the induced electrolyte circulation is cooperative as illustrated in FIGS. 6 and 7. To understand how sensitive the gas flow is to the inclination of the anode face from horizontal one has only to observe the flow of a bubble in a carpenters level when it is inclined from a horizontal position.

Depending on the slope upward from the horizontal imposed on the anodic faces the cell may be operated for, say, 1 to 24 hours during which time the edges of the anodes dipping closest to the molten aluminum cathode will tend to wear off flat decreasing the slope and lessening the speed of bubble removal. Then the motorized screw 42 is reversed in direction to tilt the anodes in the opposite direction to slope the anodic faces in the opposite direction so that the gas flow and electrolyte flow direction are reversed. Depending on how frequent the reversals are made and how slight the inclination of anode faces used, it is possible to operate a cell with a relatively flat anode even though the anodic gases are predominantly removed along only one edge of each electrode at one time. The result is that a relatively low average A-C distance may be maintained (and thus low voltage per cell) while at the same time the ampere efficiency is high due to minimum reoxidation of aluminum by the anodic gases contacting aluminum fog.

If the change of slope is made at rather frequent intervals of one half hour to four hours and upon at least a portion of the anodes in a cell, it is possible to add most of the alumina requirements of the cell through the cracks made in the crust of electrolyte when the anode tilting operation produces shearing action on the crust between and around the anodes. Conventionally the alumina is added to the electrolyte crust after it has formed where it reposes and preheats as a powder or as a portion of the frozen crust until the crust is broken when the powdered portion drops into and replenishes the molten electrolyte alumina content. By tilting the electrodes in an oscillating fashion over a period of minutes each time the anode slope is reversed the powdered portion of the alumina and also that solidified in the crust may be churned or stirred into the molten electrolyte. The stirring action may also be accomplished, at least in part, by actuating the motorized jacks 25 and 26 for raising and lowering the electrodes.

It is often preferable in the practice of this invention to agitate the electrodes in only a portion of the cell at a time or even one electrode at a time since by this means a thicker layer of alumina may be kept preheating on the crust than if the entire surface of the crust is broken every hour or two. In this manner less heat is allowed to escape from the crust both on account of the thicker cover of alumina and there is less loss of heat due to radiation. In the practice of the invention where substantially molten cryolite is used as the electrolyte alumina contents between 3% and 7% may be found most suitable with the higher alumina contents giving better electrical efliciency perhaps due to a mass action effect being exerted in Equation No. 2. Cryolite containing aluminum fluoride in sufficient excess to produce a so-called acid bath while dissolving less alumina produces greater electrical efficiency in the practice of this invention thus a higher ampere efliciency or lower kilowatt hours per pound or both may be realized with relatively high alumina content and acid bath and more frequent stirrings of alumina into the bath. Since adding more alumina to the electrolyte than it will dissolve results in alumina settling to the metal layer and forming an electrical nonconducting sludge and overheating and loss of cell efficiency, the mechanized addition of alumina provided by this invention, combined with the extensive electrolyte circulation gives important results in the cell operation.

FIGS. 6, 7, 8 and 9 are the plan and sectional views of electrolyte circulation patterns previously discussed. The pattern illustrated in FIGS. 6 and 7 is usually preferred to that in FIGS. 8 and 9 since a more cooperative movement of electrolyte is effected and needed to distribute alumina uniformly. Since the viscosity of molten aluminum compared to molten electrolyte is about 1.5:4.8, or about one-third, electromagnetic circulation of the metal does not affect the electrolyte excessively unless there is electromagnetic heaping of the metal'which causes waves pushing the more viscous electrolyte layer along with the molten aluminum which underlays it.

FIG. 10 is a cross section of an example of a Soderberg electrode utilizing this invention. The steel walls 45 and 46 are the anode outer casing which remain fixed in space by being attached to the anode superstructure (not shown) and the inner casing (also fixed in space) is 47 and 48. Between 47 and 48 is fed a carbon-pitch paste mixture which bakes to a relatively dense carbon or one which has relatively poor electrical conductivity, or both compared to the paste added to the outer compartments between 45 and 57 and between 46 and 48. The line at which the two different pastes are baked to solid electrode is 50. In this way an electrode shape somewhat similar to that in FIG. 1 may be produced since the more dense central portion of the anode 51 between casing 47 and 48 will burn oif slower. In a similar manner the anode configuration of FIG. 2 may be produced as a Soderberg anode by grading the carbon from very dense on one outside compartment to less dense in the middle compartment. Likewise slots may be formed in the edges of the least dense edge to facilitate gas removal there via the slots. However, the preferred method of operating the Soderberg type of electrodes is similar to the method illustrated with prebaked electrodes in the discussion of FIG. 5. In this process all the paste composition in the outer casing 45, 46 may be the same, no inner compartment 47, 48 being needed, so the entire construction is similar to conventional Soderberg construction. With this conventional anode, the anodes are tilted so that the anode faces slope from 0.5 to 10 from the horizontal, usually 1 to 3. This tilting may be done by lifting anode rods 52 and 53 relative to anode rods 54 and 55 for one period of operation lasting perhaps 1 to 24 hours and then reversing the tilt by lifting rod 54 and 55 relative to rods 52 and 53. In this way the Soderberg anode face may be burned off relatively flat thus securing a low average A-C distance across the width of the anode while at the same time securing a positive gas removal from one longitudinal face or the other without the tendency to draw metal fog up against the anode face where it would reoxidize. In contrast, this reoxidation actually happens with conventional operation where gas exits in haphazard directions without directed flow and usually from both longitudinal sides as illustrated in FIG. 1 but is equally applicable to the arrangement of FIG. conventionally operated.

In case of an electrolytic aluminum reduction cell with a single Soderberg anode the including of the anode so as to draw all the gas from one longitudinal side induces an electrolyte circulation from one side edge of the cell bath cavity to the other Without the electrolyte circulation doubling back on itself in the narrow A-C space and thus impeding itself with eddy currents as happens with conventional operation. The electrolyte in conventional operation must move inward from both opposite longitu: dinal anode sides next to the molten metal (see FIG. 1) in order to move along the anodic face outward towards the two opposite longitudinal sides where it is pushed by the rising gas bubbles. Thus, in conventional operation electrolyte circulation is dampened by the movement of the lower layer of electrolyte inward from the opposite longitudinal sides while the upper layer of electrolyte moves outward. Dampening the'electrolyte circulation and hence bubble removal is particularly disadvantageously with Soderberg anodes as these may be four feet to six feet or more wide compared to one-third these widths in the case of the usual prebaked electrodes. The comparatively long average time it takes for the anode gas bubbles on a wide Soderberg electrode to reach the anode edge results in an appreciable part of the AC space being occupied by anodic gases which displace the electrolyte and increase the electrical resistance across the A-C space perhaps 20% to 40% depending on a great variety of conditions. In any case the conventional Soderberg anode current density usually runs 0.75 ampere per square centimeter or less without achieving as low power consumption per pound of aluminum as conventional prebaked electrodes operated at higher current densities of 1.0 amperes per square centimeter or more.

By speeding bubble flow with less impedance to electrolyte flow this invention may be used to greatly improve Soderberg anode operation. In existing Soderberg aluminum reduction potlines the advantages of this invention may be realized in very simple fashion by merely providing means of raising one longitudinal edge or preferably both longitudinal edges of the anode. This may be done as described above or by several other means including by way of example tilting the anode superstructure by jacks mounted either on top of its supporting legs or on the bottom of them. Since one side of the Soderberg anode is raised only as much as the other is lowered, the amount of power required for tilting is only that needed to shear the crust frozen on the top of the electrolyte bath between the anode and the walls of the cell.

It will usually be found that the width of the Soderberg anode may be increased since the gas is rapidly removed and the distance from the anode to the walls of the cell no longer needs to be as wide as for conventional cells which require breaking by crowbars or pneumatic jackhammers. The crust in the cell of this invention is made thinner by better electrolyte circulation and the periodic tilting which breaks the crust and stirs in the alumina automatically. Actually extending the longitudinal wall of the Soderberg anode as close as four inches or less from the walls of the cell Walls stimulates the pumping action of the anodic gases by the Pohle air lift principle. In this way conventional Soderberg potlines may be converted to potlines of this invention with increased capacity for aluminum production due to the greater Width of the anodes as well as the higher Faraday ampere efiiciency made possible.

I claim:

1. In the aluminum reduction from fused electrolytes in which an oxide of aluminum is dissolved and in which at least one anode having an active under surface is employed, the process which comprises raising one longitudinal edge of said under surface of an anode while lowering the opposite longitudinal edge of the surface an amount sufiicient to cause gas bubbles to be evolved in the direction of the raised longitudinal edge and maintaining the edge raised for a period at least until the lowered under surface is flattened with respect to the horizontal by electrolysis. Y

2. The process of claim 1 in which the direction of anode slope is periodically reversed as often as necessary to keep most of the anode gas evolving along one longitudinal anode edge.

3. The process of claim 1 in which the anode is tilted back and forth several times in succession each time that the slope is reversed so as to break the crust formed over the fusion and around the anode suspended therein causing preheated aluminum oxide onthe crust to enter the molten fusion.

4. The process of claim 1 in which a plurality of anodes are employed and in which the slope of the ac- 9 10 tive under surface of each anode as formed by the raised 3,192,140 6/1965 Zorzenoni 204-67 longitudinal edge thereof is in the same direction. 3,202,600 8/1965 Ransley 204243 XR References Cited JOHN H. MACK, Primary Examiner UNITED STATES PATENTS 5 D. R. VALENTINE, Assistant Examiner 2061 146 11/1936 Ferrand 204--222 U.S. Cl. X.R. 2,480,474 8/1949 Johnson 20467 204-247 3,067,124 12/1962 De Pava 204243 XR zg gg UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION patent 3, 501, 386 Dated March 17, 1970 J Inventor(s) A. F. Johnson It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 4 lihe 2]. "an 1" should. read --and- Column 4 line 39 "cafity" should read --cavi1;y--

Column 7 line 26 "57" v should-read -47-- Column 7 line 63 "including" should read -inc:lining- SIGNED AND SEALED JUL 2 81970 @EAL) Anew mmdnflew m Ir. WILLIAM 1! sum JR g o l om flomissione r of Patents

Claims (1)

1. IN THE ALUMINUM REDUCTION FROM FUSED ELECTROLYTES IN WHICH AN OXIDE OF ALUMINUM IS DISSOLVED AND IN WHICH AT LEAST ONE ANODE HAVING AN ACTIVE UNDER SURFACE IS EMPLOYED, THE PROCESS WHICH COMPRISES RAISING ONE LONGITUDINAL EDGE OF SAID UNDER SURFACE OF AN ANODE WHILE LOWERING THE OPPOSITE LONGITUDINAL EDGE OF THE SURFACE AN AMOUNT SUFFICIENT TO CAUSE AGAS BUBBLES TO BE EVOLVED IN THE DIRECTION OF THE RAISED LONGITUDINAL EDGE AND MAINTAINING THE EDGE RAISED FOR A PERIOD AT LEAST UNTIL THE LOWERED UNDER SURFACE IS FLATTENED WITH RESPECT TO THE HORIZONTAL BY ELECTROLYSIS.

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