US3607713A - Anode for production of aluminum metal - Google Patents

Abstract

Carbon anode for use in aluminum extraction is attached to a metallic stub by means of a carbon-rich furfuryl alcohol-derived resin mortar and a flat perforated metal plate which has a plurality of perforations, the sidewalls of which taper upwardly and outwardly.

Description

United States Patent [56] References Cited UNITED STATES PATENTS 12/1933 Camescasse..................

[72] inventors Lloyd H. Brown Crystal Lake;

David D. Watson, Barrington, both of 111. 822,416

204/294 X 204/294 X 204/243 X 204/290 R X [21 App]. No.

[22] Filed May 7, 1969 [45] Patented Sept. 21, 1971 [73] Assignee The Quaker Oats Company Chicago, Ill.

Assistant Examiner-D. R. Valentine Attorneys-Milton C. Hansen and Donnei Rudd [54] ANODE FOR PRODUCTION OF ALUMINUM METAL 9 Claims, 13 Drawing Figs.

[52] US. 204/290 R,

204/297, 156/292 ABSTRACT: Carbon anode for use in aluminum extraction is [51] Int. B0lk 3/04, attached to a metallic stub by means of a carbon-rich furfuryl B32b 31/20 alcohol-derived resin mortar and a flat perforated metal plate which has a plurality of perforations, the sidewalls of which taper upwardly and outwardly.

[50] Field of 204/290 R,

PATENTEDSEP21|97| SHEET 1 BF 3 INVENTORS LLOYD H. BROWN DA W0 0. WATSON By W. (I M Attorney ATE IEUsa-zmn 6501.713

snmaora' If-- INVEMT'ORS LLOYD H. BROWN DA W0 0. WATSON Attorney ANODE FOR PRODUCTION OF ALUMINUM METAL BACKGROUND OF THE INVENTION Aluminum is extracted from aluminum oxide, dissolved in a molten cryolite bath, by electrolysis, the cathode being a molten aluminum layer lying on the bottom carbon lining of a cell, the anode or anodes being made of carbon blocks and being suspended in the bath from above. The carbon anodes are made from blocks of bound carbon, e.g., 80+ percent carbon bound with furfural alcohol and/or pitch which are subsequently carbonized to provide an anode block which is substantially all carbon. The carbon anodes are connected at the positive pole of the cell with the lower end extending into the bath to within about 2 inches of the molten metal surface. The electrical resistance of the bath develops heat as the current passes through it, and the rate of heat development with a given current therefore depends on the length of the current path in the bath. Hence, it is necessary that the anode-cathode distance be adjustable for that reason.

Another reason that it is necessary that the anode-cathode distance be adjustable is the fact that the passage of direct current through the cryolite alumina bath leads to a decomposition of the alumina, the aluminum being deposited on the cathode, and oxygen being deposited on the carbon anode. Because of the high temperatures involved, the deposition of oxygen at the anode leads to the gradual consumption of the anode by the oxygen. Thus, it is for this reason also that the anode-cathode distance must be adjustable. In order to provide an electrical connection with the carbon anode, and in order to provide a dependable mechanical handle by which to readjust the position of the anode in the bath as it is being consumed, and by which to readjust the position of the anode in order to vary the heat input by varying the anode-cathode separation distance, it is necessary to fix a metal stub or heavy rod to the carbon electrode. conventionally, the metal stub has been placed in a cavity in the top of the carbon electrode and molten iron cast around the stub, thereby joining the stub to the carbon electrode.

Several disadvantages are inherent in the anode-stub structure of the prior art. After a portion of the electrode has been consumed the stub which is imbedded into the electrode comes close to the path, and risk of contamination results, and use must be terminated. After the anode block had been used as long as it could have been used, a portion of the electrode remained unconsumed. The remaining portion of anode is preferred to as the butt." Periodically it is necessary to replace the butt with a new anode. The carbonaceous materials are conventionally removed from the butt by breaking up the butt, grinding the pieces and reusing the ground carbonaceous material in the manufacture of new electrodes. The cast iron remaining in contact with the stub must be broken away from the stub before the stub can be reused. This is a difficult task. However, it is necessary that the cast iron be removed in order to provide adequate clearance between the stub and the cavity wall when the stub is placed in its new seat in the new carbon anode. The use of the cast iron involves a risk of contaminating the recovered carbon, and thus eventually the aluminum product, with iron. It is an object of this invention to eliminate the use of cast iron as the means of accomplishing the direct carbon-anode-stub joint in the potting of aluminum anodes. It is a further object of this invention to provide an anode structure which permits more efficient utilization of a greater portion of all of the carbon in the anodes, and which does not require that a metallic stub penetrate the aluminum anode in order to achieve adequate strength and electrical conductivity. It is also an object of this invention to provide a structure which is easily fabricated.

SUMMARY OF THE INVENTION In accordance with this invention a carbon block is attached to a stub by means of a flat perforated plate abutting one face of the block. A layer of catalyzed carbon or graphite-rich furfural alcohol-derived mortar fixes the plate to the carbon block. In a preferred method of manufacture of the improved plate-block structure of this invention, the mortar layer is permitted to green-cure to provide adequate strength and is subsequently coked either in a conventional coking cycle during the carbonization of the block, or in use in an aluminumextraction cell.

DESIGNATION OF THE FIGURES OF THE DRAWING FIG. 1 is a partially cutaway perspective view showing one embodiment of the improved anode-stub structure of this inventlon.

FIG. 2 is a partially cutaway perspective view showing an anode-plate structure.

FIG. 3 is a fragmentary perspective view illustrating use of a plurality of sectioned plates in accordance with this invention.

FIG. 4 is an elevational view showing the perforations in a plate which is fixed to the carbon electrode in accordance with this invention.

FIG. 5 is a cross-sectional view taken along the line 55 of FIG. 4.

FIG. 6 is a plan view of a portion of plate which can be fixed to the anode in accordance with this invention.

FIG. 7 is a cross-sectional view taken approximately along the line 7-7 of FIG. 6.

FIG. 8 is a fragmentary plan view illustrating still another plate which can be fixed to the anode in accordance with this invention.

FIG. 9 is a cross-sectional view taken approximately alon the line 9-9 of FIG. 8.

FIG. 10 is a fragmentary plan view of the top of another preferred plate in accordance with this invention.

FIG. 11 is a cross-sectional view taken approximately along the line 11-11 in FIG. 10.

FIG. 12 is a fragmentary cross-sectional view taken approximately along the line 12-12 of FIG. 1.

FIG. 13 is a fragmentary cross-sectional view taken approximately through the same line as FIG. 12, except that an alternative heat shield structure is employed in that embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Although the following disclosure offered for public dissemination in return for the grant of a patent is detailed to ensure adequacy and aid understanding, this is not intended to prejudice that purpose of a patent which is to cover each new inventive concept therein no matter how others may later disguise it by variations in form or additions or further improvements. The claims at the end hereof as intended as the chief aid toward this purpose, as it is these that meet the requirements of pointing out the parts, improvements or combinations in which the inventive concepts are found.

Throughout this application the term parts refers to parts by weight and the term percent, refers to percent by weight. All temperatures referred to herein are expressed in degrees Fahrenheit.

Thus, in accordance with this invention, anode-stub structure, generally 10, includes stub 11 and carbon block anode l2. Stub 11 is attached to carbon block anode 12 in the embodiments shown in FIG. 1 by means of mortar layer 15, mortar plate 16, stub plate 17. Stub 11 is secured to stub plate 17 by any conventional means in the embodiment illustrated in FIG. 1. For example, stub plate 17 and collar 20 are joined by welds 21. Stub 11 is positioned within the well 23 formed by collar 20 and is spaced apart from stub plate 17. Molten iron 22 is then poured around stub 11 to fill the remaining portion of the well 23 within collar 20. Grooves 25 are provided to greater insure permanence of the joint. Use of the cast iron in this manner provides great uniformity of passage of electrical current from stub 11 into stub plate 17. This cast iron joint can be made permanent since replacement of carbon anode 12 does not require breaking this joint when this invention is used.

It is preferred that the anode-stub structure 11 include ceramic heat shield 27, or, carbon heat shield 28 as shown in FIG. 13. These structures 27, 28 also serve to protect the metal plate from electrical contact with the baths. Direct plate-bath contact must be avoided since this leads to plate damage, and bath contamination with iron. The details of the structure of FIG. 1 are further developed in FIGS. 4, 5,- and 12. Upon consideration of FIGS. 4 and 5 it will be appreciated that plate 16 includes a large number of openings 30 passing completely therethrough. Openings or perforations 30 have sidewalls 31 which taper upwardly and outwardly. As used herein the term upwardly is intended to refer to the general direction from the carbon anode toward the stub. While the preferred configuration of perforations 30 is circular, as illustrated in FIGS. 4 and 5, openings 30 can be rectangular or elongated as illustrated in FIGS. 6, 7, 8 and 9. Plate 16 is also provided with holes 39 to permit the passage of bolts 37 therethrough. Plate 17 is provided with oversized holes 40 through which bolt 37 passes. Washer 42 and nut 43 complete fastening means for securing anode 12 to stub plate 17. Heat shield 27 is provided with relatively large openings 45 to permit washer 42 to bear directly against the top face of plate 17 In accordance with this invention plate 17 is preferably secured to carbon anode 12 by means of mortar layer 15, and the mortar layer is permitted to cure before completion of the attachment of anode 12 to stub 11. However, in those embodiments in which a structure equivalent to plate 16 is directly secured, e.g., by welding, to stub plate 17, and in which openings 30 do not extend through stub plate 17, it is essential that stub plate 17 be equipped with vents e.g., grooves 48 (see FIG. 1), which are arranged to pass over the openings 30 to provide venting of mortar 50, confined within sidewalls 31 and also to provide venting for the actual mortar layer 15. It is not essential that sidewalls 31' of openings 30 extend from bottom face 52 to top face 53 of mortar plate 55, as illustrated in FIG. 11. However, it is essential that recesses 57 be vented e.g., by holes 59 extending through top face 53. Thus, in accordance with this invention, it is absolutely essential that recesses 57 or holes 30 be provided with vents 59 to carry off volatile decomposition products, and volatiles from mortar 50 and mortar layer 15. Without such venting inadequate strength is achieved in the resulting structure. Alternatively plate 16 can be welded directly to stub 11.

Although the illustrated embodiments show stub 11 secured to stub plate 17, and stub plate 17 in turn secured to mortar plate 16 it is contemplated that stub plate 17 be provided with openings 30 tapered sidewalls 31 and the various venting means described hereinbefore, and mortar layer 15 be directly secured to thus structured stub plate 17. Although nuts 37 and bolts 43 are shown for the purpose of illustration it is to be appreciated that any conventional fastening means can be utilized. For example, it is contemplated that nuts and bolts be welded prior to installation in aluminum cell (not shown) and that the fastening means be severed by a cutting torch prior to replacement of anodes 12. Also, it is contemplated that as an alternative, plate 17 be secured to plate 16 by welding, e.g., along the outer edge line 62 between plate 17 and 16. It is essential that excellent electrical connection be provided, however. Thus, plate 17 is preferrably in tight physical, face-toface abutment with plate 16.

It is preferred that relatively small plate portions 65, which correspond to mortar plate 16, be utilized as illustrated in FIG. 3. Thus, each plate 65 includes a plurality of openings 30 and other structural elements as illustrated as even before in connection with plates 16. However, utilization of a large number of separate plates 65 instead of single plate 16 provides the advantage of permitting different degrees of expansion of stub plate 17 and anode 12 at elevated temperatures. Thus, bolts 37' which pass through oversized openings 40 and plate 17 provide for at least a limited amount of freedom of movement within opening 40. In any assembly of an embodiment in which plate 16 is fastened into face-to-face abutment with stub plate 17, subsequent to attachment of plates 16 to anode 12, it is highly desirable to scrape the top face of plate 16 with a trowel before the mortar green cures to remove any excess mortar 50 extending therethrough. (It is also highly desirable to similarly scrape plates 65 before the mortar green cures.) [t is also highly desirable when segmented plates 65 are utilized to use straight edge, e.g., lumber, to assure proper alignment of the faces of plates 65 to thus optimize the face-to-face abutment of top face of segments 65 with the lower face of plate 17.

EXAMPLE I A primer was prepared from the following ingredients: furfuryl alcohol monomer-free polymer, 300 parts; 15% P- toluene-sulfonic acid (in 50-50 acetone-water), 30 parts; and graphite flour, parts. The resin referred to above was a viscous furfuryl alcohol-formaldehyde resin (2: 1 molar ratio of furfuryl alcohol: formaldehyde) in an admixture including 25% by weight furfural solvent. The resin-catalyst-graphite ingredients were thoroughly mixed together and the resulting admixture was found to have a potlife of about 25 minutes at room temperature, and was found to cure as a film on a metal surface in about 15 minutes to the tack-free stage. A flat plate having dimensions approximately equal to the cross-sectional dimensions of the carbon anode for an aluminum extraction cell is welded perpendicularly to the end of an aluminum cell stub. The plate is provided with a large number of spaced holes approximately one-half inch in diameter at the lower face. The sidewalls of the holes taper outwardly and upwardly. The primer prepared as described above is applied in a thin film to the surfaces of the plate including the walls of the holes. After approximately one-half hour the thin film is completely set and the plate is ready for affixing to the carbon anode by means of the mortar described hereinafter. The mortar mixture of this example is prepared from the following ingredients: Graphite (8+2O mesh), 350 parts; graphite (35+65b mesh), 350 parts; furfuryl alcohol-derived resin in furfural as described previously in this example, 300 parts; 50% P-toluene-sulfonic acid (SO-50 acetone-water), 12 parts. The resulting admixture was found to be a mortar entirely suitable for use in accordance with this invention. The mortar prepared as described herein as applied in a thin layer to the entire end of an aluminum cell anode carbon block. The end is substantially perpendicular to the axis of the anode. The precoated plate is pressed against the thus-coated end of the anode block preferably at about 10 p.s.i. during green cure, permitting only a thin film of mortar to remain between the plate and the end of the anode. Some mortar is forced through each of the holes in the plate and remains in small mounds above each of the perforations. However, before cure the mortar is slightly fluid, and each of the resulting mounds of mortar flows radially from the center of the hole a short distance outwardly against the tapered sidewalls of the hole and furthermore remain in a rounded rivetlike head above the plate. The mortar cures into a solid mass and a strong bond is observed between the carbon anode and the metallic plate. The joint between the stub plate and the carbon anode is found to be sufficiently electrically conductive to permit the assembled structure to be used in an aluminum cell as an anode. The resulting assembly, when used in an aluminum cell, is heated to an extremely high temperature and the mortar layer is found to coke out. After the mortar layer is coked out the electrical conductivity of the mortar joints increased dramatically, e.g. about l0fold or higher.

When the resulting assembly is used as an anode in an aluminum extraction cell it is observed that since no portion of the stub metal extends downwardly into the mass of the anode carbon, the life of the anode is thus extended. A greater amount of the anode carbon can be used during the electrolysis cycle before the unit must be replaced. Moreover carbon remaining attached to the plate can be flattened, e.g., by grinding wheel, and a new anode can be attached directly to the butt remaining attached to the plate. This attachment is made by use of a special carbon or graphite-filled furfuryl alcohol-derived mortar, e.g., the mortar admixed previously in this example. It is further noted that during the use cycle the exposed edges of the carbon anode are subject to a certain amount of air burn." When, in accordance with the preferred embodiments of this invention the butt portion remaining affixed to the stub plate is ground flat and bonded to the new carbon anode using a high graphite furfuryl alcohol-derived mortar as described and defined herein, it is also preferred that the edge portions of the butt which had been lost by air burn be restored by the use of additional mortar which can be applied above the newly affixed anode and around the circum ference of the old butt by use ofa trowel.

Another advantage in the use of this invention results from the unusually large surface of metal which is exposed to and is in direct electrical contact with the carbon electrode. Thus, in accordance with this preferred embodiment there is far greater uniform distribution of electrical current into the electrode than is possible using the imbedded stub configurations of the prior art.

Hence, in addition to the advantage of more uniform current distribution, the novel configuration of this invention permits more complete utilization of the electrode inasmuch as the greater portion of the carbon can be utilized before the replacement of the butt is required, and inasmuch as the remaining portion of the butt sometimes can be bonded directly to the new carbon anode. In the latter instance, therefore, 100% of the new anode portion can be utilized before replacement is necessary. Nonetheless, after replacement becomes necessary the butt can be knocked off the stub plate, should that appear desirable, and the plate can be reaffixed to a new anode in accordance with this invention, without the necessity of removing cast iron joint connecting the stub to the anode structure.

Generally speaking, the mortar which is useful in the structure of this invention includes approximately 70% carbon (preferably graphite) the balance being a binder and binder catalyst. Generally speaking, -35% of the mortar can be binder, with the range 28-33% being the preferred range. The binder can include a substantial amount of pitch, formaldehyde, or other ingredients well known for their compatibility with furfuryl alcohol-derived or furfural-derived resins. However, it has been our experience that resin containing a substantial level i.e., more than 1% of urea is not useful in accordance with this invention.

At least 25% of the binder must be a resin selected from the furfuryl-alcohol-derived resin, furfural-derived resin, or other thermosetting resin which includes at least 33% of materials derived from the furan ring. Thus furfural-phenol resins having furfural-phenol molar rations from 1:1 to 2:1 can be used in accordance with this invention. Also, furfural-keton resins can be used in nondeleterious amounts, It is preferred however, that substantially all of the binder be derived from furfuryl alcohol, and it is more preferred that the furfuryl alcohol resins by furfuryl alcohol monomerfree liquid resins.

Generally speaking, any strong acid can be used as the catalyst for the furan resins used in the mortar in accordance with this invention. However, a preferred catalyst is P-toluenesulfonic acid, more preferably that acid dissolved in a 50-50 volatile organic solvent-water mixture.

As indicated above, we have found that, generally speaking, subjecting the plate and anode to pressure during the greencure, i.e., the initial cure of the mortar, results in substantially increased conductivity. Moreover, we have found that curing the primer at substantially room temperature i.e., produces extremely great increase in conductivity as compared to curing the primer at elevated temperature, e.g. 350 F. However, we have found that curing the primer at temperatures up to 200 F. does not have a significant adverse effect on the electrical conductivity of the resulting joints.

As an alternative to the electrical carbonization described herein, the carbon block-binder, prior to carbonization and the plate, and green-cured mortar can be subjected to carbonization conditions in a normal coking cycle used in anode' block manufacture.

Therefore, by use of the term carbon block" we intend to include finished, completely carbonized block, as well as an anode block predominately carbon e.g., or more carbon bound with furfuryl alcohol resin, and/or pitch, or other suitable binder.

Therefore We claim:

1. ln an aluminum cell anode-stub structure having a carbon block and a stub joined therewith, the improvement in which: the structure includes a flat plate having perforations with sidewalls tapering upwardly and outwardly; and a carbon-rich furfuryl alcohol-derived mortar layer; the plate abutting a top face of the carbon anode, and being secured thereto by said mortar layer, said mortar being catalyzed, said mortar extending into said perforations.

2. in an aluminum cell anode-stub structure having a carbon block anode and a stub joined therewith, the improvement in which: the structure includes a flat plate secured perpendicularly to the end of the stub; a plurality of metal anode plates having perforations therethrough, said anode plates being fixed to a top face of the carbon block by a layer of carbonrich furfuryl-derived mortar, said mortar extending into said perforations, said anode plates being spaced apart from each other on the top face of the electrode", means for securing the anode plates against the stub plate in direct face-to-face abutmerit.

3. The improvement of claim 2 in which means for securing said anode plates to the stub plates includes means for compensating for difference in thermal expansion of stub plate and anode block.

4. The improvements of claims 1 and 2 in which the stub plate is covered by a ceramic heat shield.

5. A method of manufacture of an aluminum cell anodestub structure from a carbon anode block and a stub comprising: (1) affixing a flat plate perpendicularly to the end of a stub, the resulting structure thereby having a plate perpendicular to the axis of the stub, the plate having a plurality of perforations extending upwardly and outwardly into the plate, and including means for venting the perforations through the stub plate; (2) applying a layer of graphite-rich furfural alcohol-derived mortar to a face of the block; (3) pressing the stub plate into the layer of mortar, sufficient mortar and sufficient pressure being applied to move a portion of the mortar into the perforation; (4) permitting the mortar to green cure.

6. The method of claim 5 which includes the step: (5) passing electrical current through the green-cured anode-stub structure resulting from step 4 above, said electrical current being in sufficient quantity to generate enough heat to carbonize the mortar layer.

7. A method of manufacture of an aluminum cell anodestub structure from a carbon anode block and a stub, comprising: l) fixing a flat stub plate to the stub, the plate being perpendicular to the axis of the stub; (2) applying to a top face of the anode block a layer of graphite-rich furfuryl alcoholderived mortar (3) pressing an anode plate into the layer of mortar, said anode plate including perforations having sidewalls which taper upwardly and outwardly, said perforations being vented through said anode plate, sufficient mortar and sufficient pressure being applied to force mortar into the perforations, (4) permitting the mortar to green cure (5) and, securing the anode plate, to the stub plate by a mechanical electrically conductive connection.

8. The method of claim 7 which includes the step: (6) passing electrical current through the assembly resulting from step 5 in sufficient quantity to generate enough heat to carbonize the mortar layer.

9. An article of manufacture for use in an anode-stub structure in a cell for extracting aluminum metal from a cryolite bath, the article including. a carbon anode block, and a metallic anode plate secured to a top face thereof; the anode plate including a plurality of perforations extending through the plate, sidewalls of the perforations extending upwardly and outwardly; means for securing the anode plate to the cell stub; said anode plate being secured to the carbon block by a mortar layer, said mortar layer being a catalyzed carbon-rich furfuryl alcohol-derived mortar extending into said perforations.

Claims (8)

1. 2. In an aluminum cell anode-stub structure having a carbon block anode and a stub joined therewith, the improvement in which: the structure includes a flat plate secured perpendicularly to the end of the stub; a plurality of metal anode plates having perforations therethrough, said anode plates being fixed to a top face of the carbon block by a layer of carbon-rich furfuryl-derived mortar, said mortar extending into said perforations, said anode plates being spaced apart from each other on the top face of the electrode; means for securing the anode plates against the stub plate in direct face-to-face abutment.
2. 3. The improvement of claim 2 in which means for securing said anode plates to the stub plates includes means for compensating for difference in thermal expansion of stub plate and anode block.
3. 4. The improvements of claims 1 and 2 in which the stub plate is covered by a ceramic heat shield.
4. 5. A method of manufacture of an aluminum cell anode-stub structure from a carbon anode block and a stub comprising: (1) affixing a flat plate perpendicularly to the end of a stub, the resulting structure thereby having a plate perpendiCular to the axis of the stub, the plate having a plurality of perforations extending upwardly and outwardly into the plate, and including means for venting the perforations through the stub plate; (2) applying a layer of graphite-rich furfural alcohol-derived mortar to a face of the block; (3) pressing the stub plate into the layer of mortar, sufficient mortar and sufficient pressure being applied to move a portion of the mortar into the perforation; (4) permitting the mortar to green cure.
5. 6. The method of claim 5 which includes the step: (5) passing electrical current through the green-cured anode-stub structure resulting from step 4 above, said electrical current being in sufficient quantity to generate enough heat to carbonize the mortar layer.
6. 7. A method of manufacture of an aluminum cell anode-stub structure from a carbon anode block and a stub, comprising: (1) fixing a flat stub plate to the stub, the plate being perpendicular to the axis of the stub; (2) applying to a top face of the anode block a layer of graphite-rich furfuryl alcohol-derived mortar (3) pressing an anode plate into the layer of mortar, said anode plate including perforations having sidewalls which taper upwardly and outwardly, said perforations being vented through said anode plate, sufficient mortar and sufficient pressure being applied to force mortar into the perforations, (4) permitting the mortar to green cure (5) and, securing the anode plate, to the stub plate by a mechanical electrically conductive connection.
7. 8. The method of claim 7 which includes the step: (6) passing electrical current through the assembly resulting from step 5 in sufficient quantity to generate enough heat to carbonize the mortar layer.
8. 9. An article of manufacture for use in an anode-stub structure in a cell for extracting aluminum metal from a cryolite bath, the article including: a carbon anode block, and a metallic anode plate secured to a top face thereof; the anode plate including a plurality of perforations extending through the plate, sidewalls of the perforations extending upwardly and outwardly; means for securing the anode plate to the cell stub; said anode plate being secured to the carbon block by a mortar layer, said mortar layer being a catalyzed carbon-rich furfuryl alcohol-derived mortar extending into said perforations.

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