US3161579A - Electrolytic cell for the production of aluminum - Google Patents

Description

Dec. 15, 1964 .1. HENRY ETAL 3,161,579

ELECTROLYTIC CELL FOR THE PRODUCTION OF ALUMINUM Filed March 6, 1961 2 Sheets-Sheet 1 FUSED SALT l5 MOLTEN ALUM|NUN:4

MOLTEN l3 M ZELREFRACTORY HARD METAL IN VEN TORS JACK L. HENRY BY WILLIAM A. KLEMM 9 1964 i J. HENRY ETAL 3,

ELECTROLYTIC CELL FOR THE PRODUCTION OF ALUMINUM Filed March 6, 1961 2 Sheets-Sheet 2 D.C. 2 (I A i E 11 35 o.c.+ 0.c.+ [34 Q L; i

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INVENTORS JACK L. HENRY BY WILLIAM A. KLEMM United States Patent 3,161,579 ELECTRQLYTHQ CELL lFtOR THE PRGDUQJ'EEQ N 63F ALUlt HNUM .laclr L. Henry, Los Altos, and William A. Klernm, Morita Vista, Calif., assignors to Kaiser-Aluminum tlhernh call Corporation, Galdanti, Qaliii, a corporation of Delaware Filed Mar. 6, 1961, Ser. No. 93,539

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This invention relates in general to electrolytic cells for the production of aluminum. More particularly, the

invention relates to a new and improved electrolytic aluminum reduction cell design utilizing refractory hard metal current-conducting bodies.

As used herein in the specification and the claims the expression refractory hard metals refers to materials which possess a low electrical resistivity, a low solubility in molten aluminum and molten electrolyte under cell operating conditions, are wettable by molten aluminum under cell operating conditions, and have good stability under conditions existing at the cathode of a reduction cell. The preferred refractory hard metal material for at least that portion of the surface of such element in contact with the molten aluminum consists essentially of at least one of the materials selected from the group consisting of the carbides and borides of titanium, zirconium, tantalum and niobium, and mixtures thereof. Such materials being found to exhibit all or substantially all of the above properties.

The expression consisting essentially as used here inafter in the specification and the claims means that the refractory hard metal material referred to above does not contain other substances in amounts sufficient to materially aifect the desired characteristics of the material, al-

though other substances may be present in minor amounts which do not materially affect such desirable characteristics, for example, small proportions of oxygen, nitrogen, titanium, nitride, and iron in the refractory hard metal subtance.

Conventionally, carbon cathodes as well as carbon ano'des have been used in the electrolytic reduction cells for the production of aluminium. Cathodes made of carbon possess a number of disadvantages particularly irl'regard to their use in electrolytic reduction cells for the production of aluminum. Carbon cathodic elements are subject to penetration by fused bath electrolyte and molten metal and exhibit undesirable swelling. In addition carbon cathodes are not wetted by molten aluminum under'cell operating conditions and have relatively high electrical resistance. Furthermore, the cathode should be rugged and possess the necessary strength to permit its handling and use without excessive cracking, breaking or chipping. The use of carbon cathodes, therefore, has contributed significantly to the cost of aluminium production.

ments are:

(1) Horizontal side entering wherein the refractory gdblfilb Patented Dec. 15, 1964 hard metal elements extend horizontally through the vertical side walls of the reduction cell and-project at their interior extremities into the molten aluminum layer;

(2) Top entering refractory hard metal elements wherein the elements are'disposed along the sides of the cavity or chamber of the cell and enter into the cell at the solidified crust and molten electrolyte and terminate at their extremities a short distance above the base of the ing the elements constitute one significant problem encountered when using refractory hard metal elements. The furnace design should be such that'the refractory hardmetal elements are subjected to the minimum amount of mechanical stress caused by distortions in the furnace lining structure and in furnace operation. Furthermore, such elements which are to be subjected to the abuse in mechanical handling and stress imposed of the magnitude found in electrolytic reduction cell operation, are relatively difficult to fabricate.

Another problem with refractory hard metal elements used as suggested by the above mentioned French patent is the necessity of maintaining thecathod-ic elements and the cathode metal layer'in direct contact with each other. If the heat generation in'a particular section of the cell cavity is reduced below normal, a ledge of frozen electrolyte may grow on the walls and bottom of the cavity around that section, and this ledge may envelop and cover any elements near :it, thereby breaking contact between the elements and the metal layer. To prevent the interruption of contact between the elements and the metal it is necessary to prevent the muck (a loose mixture of undissolved alumina and frozen electrolyte which sinks through the cathode metal layer) from building up under the cathode metal layer and disrupting the cathode surface thereby interferring with the necessary flow of current. The muck build-up is prevented by periodically raking and redispersing the muck so formed. Cell design involving horizontal side entering elements as in (1) above have exhibited difiiculties with muck under the cathode metal layer and with excessive ledging of frozen bath enveloping the cathode elements. Further, relatively long pieces of refractory hard metal material are required in all three above arrangements (horizontal side entering, top entering and bottom entering elements) because of the thick insulation and cell lining of the side and bottom necessary in conventional reduction furnaces V to contain the molten cryolite bath and aluminum through which the elements must pass. In cells with top entering cathodic elements along the side walls of the cell (2 above), the elements may interfere with normal cell operation particularly with breaking in of the crust at the sides of the cavity. Further, insertion'of the bars through the crust entails additional risk of breakage of to a minimum amount of mechanical stress.

the refractory hard metal cathodic elements which would thereby necessitate replacement of the cathodic elements. In cells with bottom entering cathodic elements (3 above), the elements are very susceptible to damage during raking. In addition to the above-mentioned disadvantage of undue lengths required to penetrate the cell insulation and lining, it should also be noted that the fabrication of refractory hard metal elements in the lengths required for top entering, horizontal side entering and bottom entering designs as discussed above, is relatively difficult and very expensive.

Because these refractory hard metals are inherently expensive, one of the major problems in designing reduction cells employing refractory hard metal current-conducting elements is that of being able to utilize a minimum possible amount of the refractory hard metal material and still accomplish the maximum advantages of using these materials as cathodic elements. In addition, the inherent fragility of these refractory hard metal materials poses a problem in furnace design. The design of furnaces should be such that refractory hard metal elements are subjected The susceptibility of refractory hard metal to oxidation and corrosion by electrolyte imposes further restriction on furnace design to provide a minimum oxidation of the elements. The refractory hard metal current-conducting elements should also be removed from the work area where mechanical abuse is unavoidable.

All of the above disadvantages accompanying the use of refractory hard metal elements may be avoided while still securing the greatly improved electrolytic cell operation resulting from the use of refractory hard metal current-conducting elements by this novel cell design. According to the invention there is provided an electrolytic cell for the production of aluminum having side Walls and a bottom floor, at least one refractory wall structure dividing said cell into at least one cathode current collecting chamber and at least one electrolytic reduction chamber, said electrolytic reduction chamber adapted to contain a molten aluminum layer in the lower portion thereof and a body of fused salt electrolyte above said molten aluminum layer and in contact therewith, an anode member disposed at least partially in said electrolytic reduction chamber and in contact with the fused salt electrolyte, molten aluminum at least partially filling said cathode current collecting chamber and maintained out of physical contact with the molten aluminum and fused salt electrolyte in said electrolytic reduction chamber by said refractory wall structure, a cathode member at least partially disposed in said cathode current collecting chamber and in contact with the molten aluminum contained therein, said refractory wall structure comprising substantially nonelectrically conductive refractory material and including current conducting refractory hard metal bodies disposed at the lower portion thereof in such a manner as to provide an electrical path between the molten aluminum in the electrolytic reduction chamber and the molten aluminum in the cathode current collecting chamber.

Various objects and advantages of the instant invention will be apparent from the ensuing description thereof.

The invention generally comprises an electrolytic cell having at least two chambers or compartments, at least one cathode current collecting chamber and at least one electrolytic reduction chamber. A refractory dividing wall structure, which may be constructed primarily of a suitable refractory brick, separates the current collecting chamber from the reduction chamber. The cathode current collecting chamber contains molten aluminum metal or alloy which is kept entirely free of the electrolyte bath by the dividing wall and becomes a current collecting well. At least one refractory hard metal body in the form of a brick or a half brick is incorporated in the refractory dividing wall. Cathodic current leads, which may also be of refractory hard metal, are electrically connected to tr e cathode bus and are inserted and immersed into the molten aluminum metal in the current collecting well.

l Thus, the metal in the well is electrically connected, without physical contact, to the metal bath in the main portion of the electrolytic reduction cell through the refractory hard metal bricks in the refractory dividing wall.

This combination makes possible the use of short lengths of refractory hard metal material for the current conducting bodies in the refractory wall. In addition, the cathodic current leads contacting the molten aluminum in the Well are maintained out of the portion of the electrolytic cell where they would be exposed to relatively high temperatures. Suitable connections with aluminum, or other metal, caps and cathode bus connecting means can be made in lower temperature regions where conditions are less severe thereby further minimizing structural failures and chance of cell stoppage. The metal in the well is maintained at a considerably lower temperature than the cell, i.e., 660950 (1., thus further serving to reduce the rate of corrosion of refractory hard metal current leads. The refractory hard metal bricks or half bricks incorporated into the refractory dividing wall are extremely rugged and far less subject to thermal shock and mechanical abuse by virtue of the more compact form.

The problem of muck and ledge is readily eliminated in the cell design of the instant invention due to the easy accessibility of the refractory hard metal brick surfaces to mechanical tools. Muck and/ or ledge could be removed mechanically with bars, rakes or specially designed tools without fear of damage to the refractory hard metal body. Furthermore, the refractory hard metal bricks or half bricks can be incorporated into the dividing wall in accordance with optimum thermal design to insure localization of heat generation. The cooler metal in the well free of the electrolyte bath provides an excellent collector from which the current is removed to the cathode bus.

In the accompanying drawings are illustrated one preferred embodiment of the instant invention as applied to aluminum reduction cells.

In the drawings:

FIG. 1 is a fragmentary longitudinal vertical view partly in section of one embodiment of an electrolytic cell which is suitable for carrying out the invention and showing the position of the refractory wall structure with respect to the cathode current collecting chamber and electrolytic reduction chamber;

FIG. 2 is a front elevational view of the refractory wall structure shown in FIGURE 1 and depicts the disposition of the refractroy hard metal bodies within the wall.

FIG. 3 is a fragmentary plan View of the cell of FIG- URE l and shows the disposition of the refractory wall and current collecting well within the illustrated cell.

FIG. 4 is a fragmentary plan view of an electrolytic cell illustrating another embodiment of the invention wherein the refractory Wall structure containing the refractory hard metal current conducting bodies is in the form of a shell-like structure.

FIG. 1 which is a fragmentary longitudinal vertical view partly in section of an elongated aluminum reduction cell suitable for practice of the invention shows an electrolytic reduction cell 10, generally comprising a metal shell 11, e.g., of steel, within which is disposed an insulating lining 12, which can be of any desired insulating material such as alumina, bauxite, clay or aluminum silicate brick. Within the insulation 12 is disposed refractory cell lining 13, which can be of any desired material for example, carbon, alumina, fused alumina, silicon carbide, silicon nitride, bonded silicon carbide or other desired materials. Most commonly, the lining is made up of a plurality of carbon blocks or is a rammed carbon mixture or a combination of a rammed carbon mixture for the bottom or floor of the lining with side and end walls constructed of blocks of carbon. Alternatively, the side and end walls can be constructed of silicon carbide bricks or other suitable material. The lining 13 defines a cavity or chamber Within which is disposed a pool or layer of molten aluminum 14. The molten aluminum layer 14 is normally rebricks or half bricks 22.

ferred to as a metal pad. Also disposed within the chamber and in contact with the aluminum layer 14 is a body or layer 15 of molten electrolyte, e.g., cryolite. The molten electrolyte bath 15 is covered by solid crust layer 16 which consists essentially of frozen electrolyte constituents and additional alumina. As alumina is consumed in electrolyte 15 the frozen crust is broken and more alumina is fed into the electrolyte. Disposedat least partially within the chamber and partially immersed in electrolyte layer 15 are prebaked carbon anodes 17. Although prebaked carbon anodes are shown in the embodiment, either prebaked or the self-baking anodes known in the art may be employed in the invention. Anode i7 is connected by suitable means, not shown, to the positive pole of a source of electrolyzing current. Ledge Zli which is an extension .of crust 16 consists of frozen electrolyte constituents and provides protection to the refractory wall structure 21,

which may be of any suitable refractory brick, from attack of 'molten aluminum and molten electrolyte.

.Within refractory dividing wall 21 are disposed a limited number of refractory hard metal bodies in the form of in the embodiment shown in FIGURES l and 3, dividing wall 21 is spaced from a side- .Wall 3.0 in such a manner as, to provide a well I'll. Molten aluminum, or alloy, at least partially fills the well 31 and refractoryhardmetal current leads 32 are at least partially immersed in the molten aluminum bath in well ill. Re-

fractory hardmetal current leads 32 are capped with a suitable capping metal 33, e.g., aluminum, which serves to connect the refractory hard metal current lead to a flexible connecting means 34, forming capped current lead assembly 40, which is electrically connected to the cathode .bus 35. When thus assembled the anode i7 is electrically connected to the cathode bus through the electrolyte 15, the metal bath i4, refractory hard metal bodies 22 in the dividing wall, current collector bath 31, refractory hard metal current lead 32, capping 33, and flexible con necting means 34. Hence, the metal in thewell and the metal pad are electrically connected without being in physical contact.

FIG. 2 is a front elevational view of the refractory wall structure 21 composed primarily of refractory bricks 23 and incorporating a limited number of refractory hard 'A number of capped current lead assemblies are depicted withinthe current collecting chamber 24, however,

any suitable number of leads may be employed. Flex connecting means 34 are shown electrically connecting capped current lead assemblies 4d to cathode bus 35.

FIG. 4 depicts an embodiment wherein the refractory wall is a shell-like structure having three sides and forming with a portion of a side wall of the cell ill, a

current collecting chamber 24 which is separated from reduction chamber 25. Flex connecting means 34 electrically connect cappedcurrent lead assemblies 4% to cath- 'ode bus 35.

The current lead 32, which may also be of refractory hard rrietal material, may be joined to an aluminum cond'uctor member 33 to facilitate the attachment of a flexible connecting means enabling the refractory hard metal element 32 to be electrically connected to the cathode bus "35. This procedure is referred to as capping the refractory hard metal element. The elements maybe capped in any convenient manner; one satisfactory method being disclosed in the copending application of lack L. Henry, SN. 729,621, filed April 21, 1958, now US. Patent No.

. 3,100,338. This method generally comprises a cleaning of the refractory hard metal member at the portion of the surface where the joint is to be made, preheating the refractory hard metal member to a temperature above the melting point of aluminum, contacting the refractory hard metal member While at said preheating temperature with a molten flux consisting essentially of the fluorides of sodium, aluminum and lithium, and sodium chloride, and then contacting the refactory hard metal member with molten aluminum after which the refractory hard metal member and the aluminum are allowed to cool. The joint which is formed between the refractory hard metal member and the aluminum has superior mechanical strength and electrical conductivity characteristics. Flexible connecting means 34 are preferably comprised of multiple leaves of aluminum. One method of connecting the flex to a refractory hard metal element isby welding the end of the flex to the metal cap which has been cast on to the end of the bar, another method is to set one end of the flex which has beenheated into the molten aluminum during the capping'operation, theconnection is then allowed to cool in the same manner.

I A number of significant advantages are achieved by the inventive cell design above described. Bylplacing the refractory hard metal b'odies within the dividing wall the use of elements protruding into the electrolytic cellis avoided. The problem of growth of muck deposits under the cathode is substantially reduced since all sides of the cells are free of protruding refractory hard metal elements and are therefore open to efficient raking.

In the embodiment illustrated in FIG. 1 the alumina may be fed to the cells in the conve ntional manner by breaking crust anywhere in the cell but without the danger' of breakage of protruding refractory hard metal elements. The employment of a current-collecting well refractory hard metal material which can be used in this novel cell design are characterized by relatively simple a fabrication methods which yield superior current conducting elements. Prior cell designs utilized refractory hard metal elements of large configurations and considerable lengths. In contrast to this, the refractory hard metal elements used in this invention are relatively short bodies which lend themselves to easier fabrication by hot pressing or cold forming followed by sintering. If desired after initial fabrication, the bodies could additionally undergo a form of heat treatment to relieve stresses, f or example, a post sintering operation within the same sintering furcording to the invention permits the use of refractory hard metal elements capable of production by fabrication techniques which offer significant economic advantages over the manufacture of longer bodies and result in very rugged shapes. i M

Although the embodiment depicted in FIG. 1 and discussed above, involves the use of one well and one refractory wall structure, it should be expressly understood that electrolytic cells according to this invention can employ a plurality of current-collecting wells and refractory dividing walls depending upon the size and intended capacity of the electrolytic reduction cell. Further, While the cathode current collecting chamber has beendescribed in PEG. 1 as being connected across two opposite side walls, the cathode may be located anywhere in the electrolytic cell as, for example, a shell-like structure wherein the refractory wall is three-sided and attached to a portion of the side wall which serves as the fourth side of the It is apparent, therefore, that the cell design acti current collecting well so formed (FIG. 4). Moreover, the refractory wall structure may extend across to any two side walls.

In the event the cathode is disposed away from a side wall, a refractory wall structure of any suitable configuration, such as a tube or cylinder, may be employed to isolate the cathode current collecting chamber from the electrolytic reduction chamber thus forming a current collecting well containing molten aluminum which facilitates the electrical connection from the anode through the refractory hard metal current conducting bodies in the refractory wall structure as described above. It is clear, therefore, that the invention is not limited to the illustrative embodiment presented and that various changes may be made without departing from the spirit and scope thereof, the invention being limited only as defined in the following claims wherein, what is claimed is:

1. In an electrolytic cell for the production of aluminum having side walls and a bottom floor, at least one refractory wall structure dividing said cell into at least one electrolytic reduction chamber and at least one cathode current collecting chamber, said refractory wall structure comprising substantially non-electrically conductive refractory material and including at least one current conducting refractory hard metal body disposed in the wall and adapted to pass electric current from an anode in the electrolytic reduction chamber to a current lead in the cathode current collecting chamber.

2. An electrolytic cell as in claim 1 wherein the current lead in the cathode current collecting chamber is comprised of refractory hard metal material.

3. An electrolytic cell for the production of aluminum having side walls and a bottom floor, at least one refractory wall structure dividing said cell into at least one reduction chamber and at least one current collecting chamber, said reduction chamber adapted to contain a molten aluminum layer confined in said reduction chamber in the lower portion thereof and a body of fused salt electrolyte above said molten aluminum layer and in contact therewith, said refractory wall structure comprising substantially non-electrically conductive refractory material and including current conducting refractory hard metal bodies adapted to pass electric current from an anode in the electrolytic reduction chamber to a current lead in the cathode current collecting chamber.

4. An electrolytic cell for the production of aluminum havingside walls and a bottom floor, said cell having at least one shell-like refractory wall structure dividing said cell into at least one electrolytic reduction chamber and at least one cathode current collecting chamber, said shell-like refractory wall structure being on contact with at least a portion of a side wall and the bottom floor and open at the upper end, said shell-like refractory Wall structure comprising substantially non-electrically conductive refractory material and including current :conducting refractory hard metal bodies adapted to pass electric current from an anode in the electrolytic reduction chamber to a current lead in the cathode current collecting chamber.

5. An electrolytic cell as in claim 4 wherein the current lead in the cathode current collecting chamber is comprised of refractory hard metal material.

6. An electrolytic reduction cell for the production of aluminum having side walls and a bottom fioor, at least one refractory wall means dividing said cell into at least one electrolytic reduction chamber and at least one cathode current collecting chamber, said reduction chamber adapted to contain a molten aluminum layer in the lower portion thereof and a body of fused salt electrolyte above said molten aluminum layer and in contact therewith, said cathode current collecting chamber containing a body of molten aluminum separate and apart from said last mentioned molten aluminum layer, the molten aluminum in said cathode current collecting chamber being maintained out of physical contact with said molten aluminum and fused electrolyte in said reduction chamber by said refractory wall means, said refractory wall means comprising substantially non-electrically conductive refractory material and at least one current-conducting refractory hard metal body adapted to provide an electrical path between the molten aluminum in the reduction chamber and the molten aluminum in the cathode current collecting chamber.

7. An electrolytic cell for the production of aluminum having side walls and a bottom floor, at least one shelllilie refractory wall structure dividing said cell into at least one reduction chamber and at least one cathode current collecting chamber, said cathode current collecting chamber being defined by said shell-like structure in contact with at least a portion of a side wall and bottom floor, said reduction chamber adapted to contain a molten aluminum layer in the lower portion thereof and a body of fused salt electrolyte above said molten aluminum layer and in contact therewith, molten aluminum separate and apart from said last mentioned molten aluminum layer at least partially filling said cathode chamber and maintained out of physical contact with said molten aluminum and electrolyte in the anode chamher by said shell-like refractory wall structure, said shelllike refractory wall structure comprising substantially non-electrically conductive refractory material and current-conducting refractory hard metal bodies adapted to provide an electrical path between the molten aluminum in the reduction chamber and molten aluminum in the cathode current-collecting chamber.

8. An electrolytic cell for the production of aluminum having side walls and a bottom floor, at least one refractory wall structure dividing said cell into at least one cathode current collecting chamber and at least one reduction chamber, said reduction chamber adapted to contain a molten aluminum layer in the lower portion thereof and a body of fused salt electrolyte above said molten aluminum layer and in contact therewith, at least one anode member disposed at least partially in said reduction chamber and in contact with the fused salt electrolyte, molten aluminum separate and apart from said last mentioled molten aluminum layer at least partially filling said cathode current collecting chamber and maintained out of physical contact with the molten aluminum and fused salt electrolyte in said reduction chamber by said refractory wall structure, a current lead at least partially disposed in said cathode current collecting chamber and in contact with the molten aluminum contained therein, said refractory Wall structure comprising substantially non-electrically conductive refractory material and including current-conducting refractory hard metal bodies disposed at the lower portion thereof in such a manner as to provide an electrical path between the molten aluminum in the reduction chamber and the molten aluminum in the cathode current-collecting chamher.

9. An electrolytic cell as in claim 8 wherein the current lead in the cathode current collecting chamber comprises refractory hard metal material.

10. An electrolytic cell for the production of aluminum having side Walls and a bottom floor, at least one shell-like refractory wall structure dividing said cell into at least one cathode current collecting chamber and at least one reduction chamber, said cathode current collecting chamber being defined by said shell-like refractory wall structure in contact with at least a portion of a side wall and bottom floor, said reduction chamber adapted to contain a molten aluminum layer in the lower portion thereof and a body of fused salt electrolyte above said molten aluminum layer in contact therewith, molten aluminum separate and apart from said last mentioned molten aluminum layer at least partially filling said cathode current collecting chamber and maintained out of physical contact with the molten material in said reduc- 9 tion chamber by said shell-like refractory wall structure, at least one anode member disposed at least partially said reduction chamber and in contact with said fused salt electrolyte, at least one current lead disposed at least partially in said cathode current-collecting chamber and in contact with said molten aluminum contained therein, said shell-like refractory wall structure comprising substantially non-electrically conductive refractory material and current conducting refractory hard metal bodies disposed in the wall in such a manner as to provide an electrical path between the molten aluminum in the reduction chamber and the molten aluminum in the cathode current-collecting chamber.

References Cited in the file of this patent UNITED STATES PATENTS 1,741,469 Long Dec. 31, 1929 2,512,206 Holden et a1. June 20, 1950 2,915,442 Lewis Dec. 1, 1959 FOREIGN PATENTS 201,350 Switzerland Nov. 30, 1938 257,787 Switzerland Apr. 16, 1949

Claims (1)

1. IN AN ELECTROLYTIC CELL FOR THE PRODUCTION OF ALUMINUM HAVING SIDE WALLS AND A BOTTOM FLOOR, AT LEAST ONE REFRACTORY WALL STURCTURE DIVIDING SAID CELL INTO AT LEAST ONE ELECTROLYTIC REDUCTION CHAMBER AND AT LEAST ONE CATHODE CURRENT COLLECTING CHAMBER, SAID REFRACTORY WALL STRUCTURE COMPRISING SUBSTANTIALLY NON-ELECTRICALLY CONDUCTIVE REFRACTORY MATERIAL AND INCLUDING AT LEAST ONE CURRENT CONDUCTING REFRACTORY HARD METAL BODY DISPOSED IN THE WALL AND ADAPTED TO PASS ELECTRIC CURRENT FROM AN ANODE IN THE ELECTROLYTIC REDUCTION CHAMBER TO A CURRENT LEAD IN THE CATHODE CURRENT COLLECTING CHAMBER.

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