US3303119A - Metal shathed carbon electrode - Google Patents

Description

Feb. 7, 1967 M. B. DELL 3,303,119

METAL SHEATHED CARBON ELECTRODE Filed June 25, 1962 ALUMINUM FOIL H I MW Mi "WW2 2 ANODE CARBON ANODE CARBON MAST/c INVENTOR.

M. BENJA MIN DELL United States Patent 3,303,119 METAL SHEATHED CARBON ELECTRODE Manuel Benjamin Dell, Pittsburgh, Pa, assignor to Aluminum Company of America, Pittsburgh, Pa., 21 corporation of Pennsylvania Filed June 25, 1962, Ser. No. 210,284 5 Claims. ((11. 204-290) This application is a continuation-in-part of my similarly entitled application Serial No. 193,683, filed May 10, 1962, now abandoned.

This invention relates to metal sheathed carbon, especially foil sheathed carbon anodes for use in an electrolytic cell for the roduction of aluminum, and to mastic compositions especially adapted for adhering metal foil to baked carbon. More specifically, this invention relates to mastic compositions useful for foil sheathing of baked carbon anodes to protect them against severe air burning when subsequently, during operation of a cell of the above type, they are exposed to and maintained at elevated temperatures gradually approaching the melting point of the aluminum and finally exceeding that temperature.

In the smelting of aluminum by electrochemical decomposition of alumina dissolved in a molten electrolyte, a conventional electrolytic cell comprises, in general, a steel shell provided with a carbon lining forming the cell cavity and serving as the cathode. Insulating material is generally used between the carbon lining and the shell. In one conventional arrangement, current carrying bus bars are supported above the cavity of the cell, and one or more carbon anodes hang from these bus bars and dip into the electrolyte.

In operation, a mixture of alumina and cryolite (usually with one or more other fluorides) is provided in the cell cavity, and an electric current is passed through the cell. The resistance of the alumina-cryolite charge to the passage of current produces sufficient heat to fuse the same, and form a molten electrolyte or bath, which may then be considered as a solution of alumina in molten cryolite. Aluminum is electrolyzed from the solution, depositing as a molten layer on the cathode, while oxygen passes to the anode. A crust of frozen electrolyte forms on the surface of the bath (which is usually at a temperature of about 970 C.) and this crust is usually covered over with some undissolved alumina.

The anode carbon is exposed to elevated temperatures on the order of 400 to 500 C. near its top and to electrolyte temperature at the bottom. The oxygen passing to the anode carbon reacts with the hot anode carbon to form carbon dioxide (which to some extent is subsequently reduced to carbon monoxide by the hot carbon). Operating data confirm that approximately 0.4 pound of carbon per pound of aluminum metal produced is necessarily consumed in this manner. Accommodation for this loss is made by lowering periodically anodes (taller than are required at the outset of operations) usually with only a portion of the anode at any time being submerged in the electrolyte. As the anode carbon is consumed, the anode is lowered into the bath by mechanical or automatic means.

The bonnet and upper side surfaces of the anode, referred to hereinafter as simply the head, protrude above the electrolytic bath during at least the initial period of operation. The anode carbon becomes heated as described, and the head (being exposed to the oxygen of the air) is subject to oxidation, this action being generally referred to as air burning. This adds substantially to the carbon consumption. The average net anode carbon consumption, which of course is somewhat dependent on the size, shape and quality of the anode carbon will as ice a general rule be not less than 0.5 pound of carbon per pound of aluminum metal produced (approximately 0.1 pound greater than that directly consumed by oxygen coming from electrolysis). Net anode carbon consumption, as used herein, refers to the total pounds of baked anode carbon introduced into the cell less the weight of the unconsumed anode carbon butt removed from the cell when replacing anodes.

To reduce air burning, the conventional practice is to cover the head of the anode carbon with a blanket of alumina and solidified electrolyte mixed therewith generally referred to as an alumina blanket (or ore blanket). The depth of such a blanket must obviously be somewhat limited. In order to permit covering the side walls of the anode as well as possible, the height of the anode employed in the cell is usually restricted below that which might otherwise be used. In other words, the upper side Walls of very tall anodes cannot be afforded desired protection against air burning by means of an alumina blanket. Moreover, the alumina blanket is somewhat permeable in any case and air therefore reaches the anode carbon through it, with some inevitable air burning of the anode carbon so protected.

The power input to the cell usually exceeds that required for the electrochemical decomposition of the alumina, and maintains the electrolyte molten. Under steady operating conditions, excess heat is allowed to dissipate from the cell to maintain optimum operating temperature. Since the conventional alumina blanket acts as aninsulating medium, some heat which might escape from the cell by radiation from the anode is partially restrained from doing so. This limits metal production from the cell, since current input must be reduced accordingly, and this means that the amount of aluminum metal produced is proportionally reduced. It is desirable, however, to increase the current, to produce a corresponding increase in metal production, if the additional heat can be dissipated as by eliminating or reducing the alumina blanket.

While efforts have been made in the past to protect anode carbons from air burning by aluminum sheaths, it has been found that such sheaths are wholly inadequate unless intimately and tightly adhered to the baked carbon surface. Adhesives previously thought to be most suitable (and inexpensive) for adhering aluminum foil to anode-carbons are of the inorganic type, but such adhesives have the disadvantage that contaminants are introduced into the cell, i.e. the molten bath, the anode carbon butts (which are reclaimed), and the aluminum produced.

It is therefore a general object of this invention to provide a mastic adhesive composition for adhering aluminum foil to baked carbon which will withstand elevated temperatures and which, when used in connection with anode carbon for aluminum production, will not introduce inorganic contaminants.

Another general object of this invention is to provide a baked carbon anode, for use in an electrolytic cell for the production of aluminum, having an improved sheathing for substantially reducing anode carbon consumption resulting from air burning.

It is an advantage of the invention that its use obviates the need for maintaining an alumina blanket over the head of the anode, thereby rendering economical the use of taller carbon anodes and further permitting increased current input to the cell without modifying conventional cell design or installed facilities.

The mastic composition of this invention is one especially adapted for adhering aluminum foil to baked anode carbon subsequently to be exposed to and maintained at elevated temperatures approaching the melting point of the aluminum. It should contain, as the essential binder component, about 40 to 50 percent by weight, preferably 42 to 47 percent, bituminous tar or pitch or mixtures thereof. Such binder material should have a softening point less than about 110 C., conveniently about 40 C. Aromatic bituminous materials, of the characteristic just mentioned such as crude or refined coal tar, coal tar pitch, petroleum pitch or mixtures thereof, may be used most advantageously.

As the essential aggregate component, the mastic composition should contain about 60 to 50 percent by weight, preferably 58 to 53 percent coal, coke or mixtures thereof. This aggregate material should be finely divided; the bulk of it should pass a 100 mesh screen, preferably a 200 mesh screen. Some larger particles are not objectionable, but substantially all of the aggregate should pass a 28 mesh screen (Tyler scale). While aggregates comprised of cokes and/or low-volatile coals such as anthracite may be employed advantageously for many purposes, the preferred aggregate is calcined petroleum coke. Since calcined petroleum coke has a low mineral matter content, it will not introduce substantial amounts of contaminants into the bath and metal being produced when used as aggregate in a mastic for foil sheathing of carbons employed in an aluminum production cell. Finely divided coke as obtained from the precipitators or dust collectors of aluminum industry carbon plants is also a useful aggregate for this purpose.

The proportion of binder tar or pitch mixed with aggregate may be adjusted within the aforesaid weight ranges, and the most suitable proportion may readily be determined, with the particular components employed, so

as to obtain high bond strength and optimum spreadability at a temperature above the softening point of the bituminous material.

Since the mastic composition as described above will be inherently somewhat stiff (even at somewhat elevated temperature), it may be conveniently cut back with about 1 to 10 percent, preferably about 3 to 5 percent, of an organic solvent for the binder tar or pitch. Aromatic hydrocarbons such as high flash coal tar :naphtha and xylenes may be used for brush, spray or roller coating applications, and preferably hydrocarbon solvents having boiling points of about 130 to 140 C. are used permitting convenient application of the solvent-diluted mastic composition at a temperature above the softening point of the binder tar or pitch and below the boiling point of the solvent.

The mastic composition is most effective as an adhesive between aluminum and baked carbon when applied in a coating weight of about 0.3'to 1.0 gram per square inch of surface (solvent-free weight basis). 7

It has been found in accordance with the present invention that baked carbon anodes may be afforded effective resistance to air burning, during operation of an aluminum reduction cell, by means of a substantially airtight sheath comprised of aluminum foil tightly and intimately bonded to the surfaces of the anode by an airexcluding adhesive stratum of a mastic composition as described herein. The foil sheath is conveniently made to extend over the bonnet or top surface of the anode and down along the upper side surfaces (preferably for a predetermined distance down the side so that the foil sheath covers only that portion of the anode that is to be initially exposed to the atmosphere). The foil sheath may be applied in sections, or more desirably as a single, unitary sheet, and any overlapping margins bonded in place. Aluminum foil as used herein refers to aluminum in sheet form up to about 0.006 inch thick, and foil about 0.001 to 0.006 inch thick is preferred.

It is important in effecting a reduction in air burning that the aluminum foil sheath be in intimate contact with the surfaces of the anode; otherwise air may diffuse under the metal sheath in the channels between grains of carbon and cause burning. To insure a tight sheath, impervious to air, aluminum foil, preferably in an annealed or soft temper, is bonded to the anode surface by the mastic composition. Foil in a hard temper has a tendency to crinkle or crease, and consequently may not as readily conform in an airtight manner to the anode surfaces. Preferably the mastic is applied to the foil surface, and the foil then applied to the anode. Alternatively, the anode surfaces may be coated with the mastic, both anode and foil being hot, and the foil then applied. The mastic will resist the high temperature conditions created during operation of the electroyltic cell, so as to retain a good airtight bond. In addition, the aluminum foil and mastic should not contain elements or compounds which are detrimental to cell operations or which may cause undesirable or excessive contamination of the anode carbon, the electrolyte, or the aluminum metal produced. The mastic meets these conditions, and the aluminum foil recommended in practice of our invention should be of relatively high purity,

preferably of not less than 99.45% purity. The aluminum foil sheath should have a thickness of not less than about 0.001 inch. A sheath oflesser thickness may be easily torn or ripped, either during application of the foil to the anode or during installation of the anode in the cell or in operation thereof. Employing a sheath of greater thickness than 0.006 inch is neither necessary nor desirable, in that a substantially thicker sheath is required to attain any further noticeable improvement against air burning while use thereof may only result in an excessive and uneconomical use of the metal, recovery of which may be small. Aluminum foil-paper laminates may also be employed as sheathing materials where desired.

It will be observed that the aluminum foil sheath obviates the need for maintaining an alumina blanket over the anode. Consequently, with a reduced alumina blanket or substantially none at all, a substantial amount of heat produced during operation of the cell will be radiated from the sheathed anode and dissipated into the surround-1 ing atmosphere. To maintain the bath temperature, additional current input to the cell may be employed, and therefore the amount of aluminum metal produced per day is increased Without modifying an existing cell design or installed facilities.

The heavy alumina blanket required to protect the anodes from air burning according to conventional practice results in an accumulation of frozen electrolyte and aluminabetween the anodes and sides of the cell, commonly referred to as high backs. A reduced alumina blanket therefore eliminates high backs from the cell, thus making installation or setting of the anodes in the cell considerably easier. The invention further renders it economical to employ taller anodes in the reduction cell, for example anodes 23 inches in height as compared to the more normal 18 inch anode. Equally important,

the taller anodes may be utilized without altering conventional existing facilities.

For a better understanding of our invention reference 7 is made herein to the accompanying figures where FIG- URE 1 is a perspective view of a typical baked carbon anode having an aluminum foil sheath to protect its upper surfaces against severe air burning; FIGURE 2 is a fragmentary cross-sectional view taken on line 22 of FIGURE 1. The thickness of the foil sheath and mastic coating is somewhat exaggerated to illustrate the manner of adhesively bonding the sheath to the anode surface.

The invention may further be illustrated by considering the following detailed practices fortypical baked carbon anodes for use in aluminum smelting cells. It was found desirable, when'using mastic cut back with solvent, to have the anodes at a temperature below 100 C. Mastic coating weights of about 0.375, 0.5, 0.75 and 1.0 gram per square inch of surface were evaluated. In one series of tests the mastic composition was 44 percent by weight coal tar pitch having an C. softening point, and 56 percent 28 mesh carbon plant collector dust. This was suitable for application at 40 C. by spreading. In

another series 44 percent coal tar pitch having a 40 C. softening point was used and 56 percent 200 mesh calcined petroleum coke, cut back with about 3% highflash coal tar solvent naptha (140 C. boiling point), was applied at about 130 C. by spraying, Generally, 1l450 plain foil 0.002 inch thick was used for sheathing, being applied manually and pressed onto the baked carbon surface with a hand roller. In all of these cases the foil adhered well to the anode carbon at room temperature and at elevated temperatures (such as 550 C., conveniently used as an evaluation temperature), and in operating tests resulted in reduced air burning and increased aluminum production which the reduced alumina blankets thus made possible.

Having thus described my invention, I claim:

1. In a baked carbon anode adapted for use in an electrolytic cell for the production of aluminum from alumina dissolved in a molten electrolyte and having over its bonnet and upper side surfaces, a substantially airtight sheath comprised of aluminum foil 0.001 to 0.006 inch in thickness, bonded to the aforesaid surfaces by an adhesive, the improvement in said anode which consists in said aluminum foil sheath being tightly and intimately bonded to said baked carbon anode surfaces by a mastic composition consisting essentially of I about 40 to 50 percent by weight bituminous material having a softening point less than about 110 C. and selected from the group consisting of tar, pitch and mixtures thereof, and about 60 to 50 percent by Weight aggregate selected from the group consisting of low volatile coal, coke and mixtures thereof, the bulk of which passes a 100 mesh screen and substantially all of which passes a 28 mesh screen (Tyler scale),

said composition being present in an amount of about 0.3 to 1.0 gram per square inch of said sheathed surfaces,

said anode being characterized in use by being so protected by said sheath against air burning as to obviate the need for the conventional alumina blanket over its upper side surfaces, without introduction into the cell of materials contaminating the aluminum produced.

2. In a baked carbon anode as set forth in claim 1, the further improvement in which said bituminous material is coal tar pitch having a softening point of about 40 C. and the amount thereof is about 42 to 47 percent by weight, and said aggregate is calcined petroleum coke the bulk of which passes a 200 mesh screen, and the amount thereof is about 58 to 53 percent by weight.

3. A carbon electrode having a metal foil sheath adhered to its surface by a mastic composition consisting essentially of about 40 to 50 percent by weight bituminous material having a softening point less than about 110 C. and selected from the group consisting of tar, pitch and mixtures thereof, and about 60 to 50 percent by weight aggregate selected from the group consisting of low volatile coal, coke and mixtures thereof, the bulk of which passes a mesh screen and substantially all of which passes a 28 mesh screen (Tyler scale),

said composition being spreadable as a mastic at a temperature above the softening point of said bituminous material.

4. A carbon electrode adapted to be exposed to elevated temperatures approaching the melting point of aluminum and having a substantially airtight sheath comprised of aluminum foil about 0.001 to 0.006 inch in thickness bonded tightly and intimately to its surface by a mastic composition consisting essentially of about 40 to 50 percent by weight bituminous material having a softening point less than about C. and selected from the group consisting of tar, pitch and mixtures thereof, and about 60 to 50 percent by weight aggregate selected from the group consisting of low volatile coal, coke and mixtures thereof, the bulk of which passes a 100 mesh screen and substantially all of which passes a 28 mesh screen (Tyler scale),

said composition being spreadable as a mastic at a temperature above the softening point of said bituminous material. 5. A carbon electrode as set forth in claim 4 wherein said bituminous material is coal tar pitch having a softening point of about 40 C. and the amount thereof is about 42 to 47 percent by weight, and

said aggregate is calcined petroleum coke the bulk of which passes a 200 mesh screen, and the amount thereof is about 58 to 53 percent by weight.

References Cited by the Examiner UNITED STATES PATENTS 2,500,208 3/1950 Shea 106-284 2,683,107 7/1954 Juel 106--284 2,890,128 6/1959 Bushong et al. 106-284 3,060,115 10/1962 Haupin et al. 204290 JOHN H. MACK, Primary Examiner.

D. JORDAN, Assistant Examiner.

Claims (1)

1. IN A BAKED CARBON ANODE ADAPTED FOR USE IN AN ELECTROLYTIC CELL FOR THE PRODUCTION OF ALUMINUM FROM ALUMINA DISSOLVED IN A MOLTEN ELECROLYTE AND HAVING OVER ITS BONNET AND UPPER SIDE SURFACES, A SUBSTANTIALLY AIRTIGHT SHEATH COMPRISED OF ALUMINUM FOIL 0.001 TO 0.006 INCH IN THICKNESS, BONDED TO THE AFORESAID SURFACES BY AN ADHESIVE, THE IMPROVEMENT IN SAID ANODE WHICH CONSISTS IN SAID ALUMINUM FOIL SHEATH BEING TIGHTLY AND INTIMATELY BONDED TO SAID BAKED CARBON ANODE SURFACES BY A MASTIC COMPOSITION CONSISTING ESSENTIALLY OF ABOUT 40 TO 50 PERCENT BY WEIGHT BITUMINOUS MATERIAL HAVING A SOFTENING POINT LESS THAN ABOUT 110*C. AND SELECTED FROM THE GROUP CONSISTING OF TAR, PITCH AND MIXTURES THEREOF, AND ABOUT 60 TO 50 PERCENT BY WEIGHT AGGREGATE SELECTED FROM THE GROUP CONSISTING OF LOW VOLATILE COAL, COKE AND MIXTURES THEREOF, THE BULK OF WHICH PASSES A 100 MESH SCREEN AND SUBSTANTIALLY ALL OF WHICH PASSES A 28 MESH SCREEN (TYLER SCALE), SAID COMPOSITION BEING PRESENT IN AN AMOUNT OF ABOUT 0.3 TO 1.0 GRAM PER SQUARE INCH OF SAID SHEATHED SURFACES, SAID ANODE BEING CHARACTERIZED IN USE BY BEING SO PROTECTED BY SAID SHEATH AGAINST AIR BURNING AS TO OBVIATE THE NEED FOR THE CONVENTIONAL ALUMINA BLANKET OVER ITS UPPER SIDE SURFACES, WITHOUT INTRODUCTION INTO THE CELL OF MATERIALS CONTAMINATING THE ALUMINUM PRODUCED.

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