US3287247A - Electrolytic cell for the production of aluminum - Google Patents

Description

Nov. 22, 1966 DEWEY 3,287,247

ELECTROLYTIC CELL FOR THE PRODUCTION OF ALUMINUM Filed July 24, 1962 5 Sheets-Sheet 1 INVENTOR JOHN L DEWEY J. L. DEWEY Nov. 22, 1966 ELECTROLYTIC CELL FOR THE PRODUCTION 0F ALUMINUM Filed July 24, 1962 5 Sheets-Sheet 2 l6 -5i/ican nitride banded silicon carbide Refractory mixture of cryo/i/e and a/um/na- 24 w my WW 1% L N H m Nov. 22, 1966 J. DEWEY 3,287,247

ELECTROLYTIC CELL FOR THE PRODUCTION OF ALUMINUM Filed July 24, 1962 5 Sheets-Sheet 5 United States Patent M 3,287,247 ELECTROLYTIC CELL FOR THE PRODUCTION OF ALUMINUM John L. Dewey, Florence, Ala., assignor to Reynolds Metals Company, Richmond, Va., a corporation of Delaware Filed July 24, 1962, Ser. No. 215,234 3 Claims. (Cl. 204-243) This application is a continuation-in-part of application Serial No. 7,681, filed February 9, 1960, now abandoned.

This invention relates to the manufacture of aluminum by the reduction of alumina in an electrolytic cell, and particularly to an improved electrode construction for use therein and for other purposes.

The production of aluminum by the time-honored Hall reduction process comprises dissolving alumina in molten cryolite, within an electrolytic cell, and withdrawing the molten aluminum which is deposited at the cathode. A reduction cell (or pot) for this purpose includes a pot shell, usually constructed of mild steel, a suitable lining (typically carbon) for containing the molten bath constituents, and heat-insulating material between the shell and the lining. In addition, the conventional arrangement uses a carbon anode, suspended within the cavity formed by the lining. The carbon lining and pad of molten aluminum overlying it function as the cathode, and iron bars are embedded within the carbon to establish the necessary electrical connection.

Carbon has been used for the lining material principally because it is a good conductor and because cryolite does not dissolve carbon. However, a profound change in this aspect of the operation has been introduced by applicants copending application Serial No. 847,594 (now US. Patent 3,093,570). That application concerns cell lining material which consists essentially of a high melting mixture of refractory oxide and a cryolite of the general composition X AlF where X designates at least one alkali metal selected from the group consisting of sodium, potassium, lithium, rubidium and cesium; and one embodiment thereof is a cell lining which consists essentially of 20 to 75% alumina, the balance substantially cryolite. The advantages of such a construction are fully discussed in the copending application. It is the object of the present invention to provide an electrode system especially suited for use in conjunction therewith. Since the lining material referred to has insulating properties, the electrode must contact the metal pad. It is then preferable that the electrode be able to accommodate higher current densities than before, and also have a much greater resistance to corrosion and deterioration.

The use of various refractory hard metals has been suggested for this purpose. That term is intended, in the present circumstances, to encompass the class of compounds composed of carbides, nitrides, borides and silicides of the transition metals of the Fourth through the Sixth Groups. (Such usage of the term has been recognized; see Refractory Hard Metals, by Schwarzkopf and Kiefier, MacMillan Company, New York, 1953).

However, the results of a number of experiments with titanium diboride have disclosed a marked susceptibility of that material to be attacked by substances present in the cell. While those portions of the collector bars which protruded into the molten metal pad were not adversely affected, large areas of corrosion were observed on those portions seemingly shielded within the lining of the cell. Apparently the embedded portions were not protected sufficiently to exclude from the surface of the bar small amounts of aluminum and sodium metals. Inspection indicated that the corrosion comprised a conversion of titanium diboride to aluminum boride and titanium. Beyond 3,287,247 Patented Nov. 22, 1966 this, the mechanism of the phenomenon has not been greatly elucidated. However, it is believed to result from the propensity of the lining materials (particularly those which contain carbon and those which become saturated with sodium metal or other alkali metal constituents of the bath) to conduct electrical current. Thus, it may be that current can flow from the bar to other areas of the cell, thereby rendering anodic those portions of the bar within the lining and constituting the bar, in part, a sacrificial anode; or there may be other complex electrolytic action adjacent the bar surface.

While being noted for resistance to molten aluminum, it has become evident that refractory hard metal is subject to attack by other substances present in the lining of a reduction cell.

We have found, as the instant teaching discloses, that this problem can be overcome by encasing the electrode with materials which will simultaneously minimize the ingress of aluminum to the bar surface and minimize current leakage away from the bar.

Furthermore, upon the exclusion of corrosive substances, it is no longer neessary that the refractory hard metal be used in the areas so protected; but, instead, a less expensive material such as iron can advantageously be substituted for a significant portion of the collector bar.

Another problem regarding collector bars for alumina reduction arises from the fact that the cells may have to be shut down from time to time. Since certain of the more practicable refractory hard metals are inherently fragile and would be expected to break or crack under the contraction forces of a freezing and cooling metal pad, the entire expensive array of collector bars would have to be replaced after every shutdown. However, in accordance with the present invention, proper shaping and dimensioning of the bars in the area of protrusion into the metal pad can obviate this problem. When the upper surfaces are so shaped that the protruding surfaces are convex and smoothly curved, the metal pad (which wets the collector only with difliculty when molten and does not adhere strongly thereto when frozen) will tend to slide over and along the rounded surface as it freezes and cools and thus subject the fragile metal to a minimum of stress. Furthermore, the stress to which the collector is then subjected is insufficient to cause damage. It is to be understood that this shaping principle is independent of the previously de scribed shielding principle, although the two may function cooperatively to advantage.

For a better understanding of the invention and its objects, advantages and details, reference is now made to the present preferred embodiments of the invention which are shown, for purposes of illustration only, in the accompanying drawings. In the drawings:

FIGURE 1 is a plan view of an electrolytic alumina reduction cell, showing several possible dispositions of the electrodes;

FIGURE 2 is a cross-section on line 2-2 of FIG- URE 1;

FIGURE 3 is a cross-section on line 3-3 of FIG- URE 1;

FIGURE 4 is a detailed cross-sectional view of an electrode construction, according to the present invention; and

FIGURES 58 are alternate embodiments of the collector element of the electrode.

In FIGURES 1 and 2 there is shown a reduction cell having an electrode 9 disposed in bottom entry relationship. The cell utilizes the invention of the copending application previously referred to, and includes a steel pot shell 6 and insulating side walls 7. The bathcontacting lining 24 is composed of a suitable refractory material, preferably a high-melting mixture of refractory oxide and cryolite. The curved contact surface of electrode 9 extends into the electrolytic bath 8, or at least into the pad of molten aluminum. Other possible placements of electrodes are also shown inFIGS. 1-3, such as the spaced bottom-entry bars having contact surfaces 10'. Side-entry electrodes are also shown, extending laterally through the refractory lining and the pot shell, and having contact surfaces 10".

Referring now to FIGURE 4, a generally hemispherical cap of titanium diboride 10 is joined to a high-purity iron ,bar 12. This may be accomplished, for example, by

use of an intermediate layer of molybdenum 14. The

molybdenum is bonded to both the cap and the bar and Surrounding the bar 12 is a corrosion-resistant shield A in the form of a sleeve 16. In the present preferred embodiment of the invention, the sleeve 16 is made of silicon nitride bonded silicon carbide (having the trade name Refrax). An end of this sleeve fits into an annular recess in the titanium diboride cap, and the other end is supported by the pot shell 18. A short collar 19 of sintered aluminum oxide is inserted between the shell and the sleeve to provide electrical insulation and to reduce the loss of heat from the cell. The sleeve 16 may be only in contact with the cap 10, or it may be bonded to the cap, to assure a fluid-tight connection.

.The bar 12 is spaced from the sleeve 16 by a resilient gasket 15, preferably a sheet material of alumina and silica, having a melting point in excess of 3000 F. (e.g., Fiberfrax). This gasket permits the iron bar 12 to expand radially under the influence of heat without subjecting the sleeve 16 to critical stresses, while at the same time providing a measure of protection against the ingress of molten aluminum which is corrosive to the iron material.

The bar 12 extends through an opening in the pot shell 18 for a distance of about 6 inches and terminates in a cast aluminum cap 20 of such size as to permit the welding thereto of suflicient flexible electrical conductors to carry the current to the external bus system (not shown). This permits the bottom of the bar to be in a floating relationship relative to the fixed top of the bar. Thus, the bar may expand and contract with variations of temperature without inducing undesirable bar stress. The bar assembly is advantageously supported by springs 22 adapted to carry the bulk of the weight of the bar and to maintain stress in the bar 12 of such low order as to prevent excessive high-temperature creep in the neighborhood of its junction with the molybdenum layer. The spring in turn is reacted upon a convenient portion of the cell structure or its support. Also pro- .vided are stainless steel bellows 21 to prevent ingress of air while assuring free movement of the bar under'expansion and contraction.

An electrode suitable for collection of approximately 10,000 amperes has been found to require a cap of about 10-inch diameter, having 2-4 inch exposure above the. dining; Under. these conditions,- a voltage drop not in excess of 200 millivolts, between the metal pad and the flexible connection to the bus bar, can be expected.

lybdenum layer was joined to the iron by means of a sleeves.

Also, it creates a more tortuous path against ingress of the corrosive materials to the sleeve interior.

FIGURES 6 and 7 illustrate possible dispositions of the sleeve 16 relative to a collector bar formed completely of refractory hard metal, but shielded and shaped according to the present invention. In FIGURE 6, there is shown a bar having integral .cap portion 30. A similar arrangement is shown in FIGURE 7, but the integral cap 40 overlaps the end of sleeve 16 to provide additional sealing means. If desired, the sleeve and bar may be bonded about the periphery of their juncture, either internally or externally of the sleeve.

The embodiment of FIGURE 8 is similar to'that of FIGURE 4, but the iron bar has an aligning tip 12a extending into the cap, and the intermediate layer 14 is shown as a washer. Also, the cap 50 has a. body portion that extendswell into the sleeve. The tip serves to align the bar within the sleeve and thus obviates the need for a gasket therebetween.

The sleeve 12 of Refrax material has shown excellent resistance to attack in the instant environment. However, those skilled in the art will recognize that there are other materials which are suitable for the purpose. The performance of various materials in this regard is indicated by the following examples, which are included for purposes of illustration and are not to be regarded as limiting: 7

Example 1 Six possible sleeve materials were tested in Pot 3B, 3 10,000-ampere cell having an alumina-cryolite bottom lining constructed in accordance with the invention disclosed and claimed in copending application Serial No. 847,594 (now U.S. Patent 3,093,570). Ten electrodes were employed, using two-inch diameter by eighteen-inch long diboride bars. The lower end of each bar was capped with an aluminum casting, and sleeves of different compositions surrounded the remainder of each bar up to lining-bath interface. type shown in FIGURE 6 was employed, except that the 'bar was entirely cylindrical. The various sleeve materials used are indicated below:

, Sleeve Material Bar No.

Top 4 Inches of Sleeve Lower Portion of Sleeve Granh ire Boron nitride 3- Retrax.

Boron nitride MgO (see Note 1). Refrax.

graphite sleeves, all but the top three inches of the Refrax sleeves, and to the Alundum and steel sleeves,

After 168 days of cell operation, corrosion was minor in all bars except the one with the steel sleeve (the diameter of bar No. 1 was decreased from 2.0 to 1.85 inches in a region just below the steel sleeve). Minor pitting corrosion was found on bars 4, 5, 6 and 8 in the vicinity of the lower sleeves (diameter decrease up to 0.1 inch).

The graphite sleeveswere in good condition afterthe test and there were no signsof bar corrosion .under these Upper portions of one BN sleeve (bar No. 2) had cracked off; and the remainder .of the BN sleeves were cracked in several places and darkened on the exterior, although they were not swollen and did not appear to have been attacked chemically.

A sleeve-bar arrangement of the The Reflax sleeves were swollen in the upper portion of the lining. The protective glaze appeared to have prevented sleeve oxidation, but some localized cracking was noted.

The magnesia sleeve on bar No. was partially dissolved, but under the sleeve the bar was in good condition. Bar No. 1, however, was attacked beneath the magnesia sleeve (the steel sleeve was directly above this sleeve). The Alundum sleeve on bar No. 5 was swollen and cracked ,to some extent, but that on bar No. 10 was in good condition and appeared to have protected the bar.

The steel sleeves were electricallyinsulated from the cast aluminum cap on the bars by a one-inch ring of A1 0 All graphite and steel sleeves were coated with the protective glaze used' in Example I. A A annular space between the bars and sleeves was filled with an alumina-cryolite mixture similar to the cell bottom lining composition (about 70% alumina--30% cryolite).

It was found that aluminum had penetrated along many of the bars, although very little corrosion was noted. The maximum corrosive effect which could be determined was a band about three inches below the metal pad-lining interface on bar No. 6, where the diameter was decreased from 2.0" to 1.96 (cross-section decrease of 4%). The top six inches of this bar had no sleeve and the lower sleeve was steel. The corroded area was just above the top of the steel sleeve.

Large amounts of sodium had collected in the vicinity of the steel sleeve locations generally, and the steel had disappeared.

All the other sleeves had cracked somewhat, and all but the BN sleeves had swollen in the area where the cell lining was fused.

In general, the results of the tests in Examples I and 11 indicated that Refrax and graphite sleeves afford superior protection compared to Alundum, boron nitride, magnesia and zirconia.

Example III Pot 3A was operated with diboride bars of the same composition as those used in the preceding examples, and with loose fitting sleeves of silicon nitride bonded silicon carbide. A A gap between each bar and its sleeve was filled with silicon nitride powder on five bars and with titanium diboride powder on the other five bars.

After 120 days of cell operation, bar corrosion was found to be very slight (not more than about 0.02" reduction in diameter). The corrosion was in the form of pits occurring about half-way between the top and bottom of the cell lining in a region where the bar was contained in the sleeve.

The sleeve material itself had not been coated with a protective glaze and was found to be oxidized near the bottom of the lining where the alumina-cryolite mixture was still loose powder; also, the sleeves were swollen to nearly twice the original thickness at the upper ends where the alumina-cryolite lining had fused. X-ray examination of the swollen portions showed a content of about 30% soduim, 2% aluminum'and minor amounts of sodium-aluminum fluorides.

Example-IV Pot 20'was operated with four bars to evaluate two other sleeve materials. A coating of molybdenuma'bo'ut 3 mils thick was applied to two titanium diboride bars; and a sleeve of calcium aluminate bonded'tabular alumina was cast around a third 'bar. Pre-fused alumina-cyrolite material was used as the'cell lining in a region covering about one square foot around the bars, and the remainder of the bottom lining was alumina-cryolite mixture as in the preceding examples.

After 28 days, all of the bars were slightly corroded (up to 12% area decrease), but the calcium aluminate bonded alumina sleeve had disintegrated and more aluminum was found around this bar than any of the others.

High-purity iron is the preferred material for the shielded -bar due to its well known ability to withstand attack by alkali metals and their oxides, materials which areknown to be present in reduction cells adjacent the collector bar. Other's-uitable materials which are electrically conductive and which have a high-resistance to corrosion include the 18-8 types of stainless steel, nickel, chromium, etc. The iron is superior to the extent of its relatively low price and better electrical conductivity as compared to these other materials. The bar and cap may be formed entirely of refractory hard metal (see FIG- URES 6 and 7), in one or more sections, of the same or different materials. The iron bar is used only for economy, and such use is independent of the shielding and the shaping principles. Furthemore, the'bellows and spring arrangement of FIGURE 4 may 'be unnecessary where a metallic bar is not substituted, since at least some of the refractory hard metals have relatively slight expansion coefficients; thus the bus system may lie within the pot shell. Furthermore, it is not essential that the gasket 15 be used, as s-ufiicient clearance between the bar and the sleeve can accomplish the desired result (see FIGURE 8). When the cap and bar are made entirely of refractory hard metal, without substitution of iron or the like, it may be unnecessary to provide such clearance between the bar and sleeve since the expansion coeflicients are more similar (see FIGURES 6 and 7).

When the highest strength diboride materials are used (such as those pure titanium diboride materials having an apparent specific gravity in excess of four, or a composition of chromium-titanium diboride with an apparent specific gravity well in excess of 4), a satisfactory configuration for the collector bar is a generally cylindrical rod, provided its protrusion into the metal pad does not exceed one-half its diameter. It is not necessary that the bar protrude above the lining surface at all, as long as sufiicient surface area of the bar is exposed to afford the desired current collection. For a bar of other than circular cross-section, an equivalent protrustion would be less than one-half its minimum width. Such a rod is still subject to the disadvantages of (1) placing a greater stress on the collector than if the protruding end were curved according to the preferred practice of the invention, and (2) requiring greater quantities of the expensive material for the same ampere collection.

In connection with forming the cast aluminum cap 20 about the base of a bar composed entirely of refractory hard metal, it has been found advantageous to first mold an aluminum sleeve about the bar at a temperature of about 1200 C. Then the partially formed cap may be coated with zinc, and the remaining portion of the cap 20 cast thereabout. Such a procedure assures better bonding between the refractory hard metal and the aluminum.

Finally, the base of cap 10 can be placed orthogonally to the 'bar axis, as shown in FIGURES 4-8, but need not be; see 10" in FIGURE 2. Particularly in the case of side-entry bars, the desired angularity may be obtained by canting the cap relative to the bar.

While present preferred embodiments of the invention have been illustrated and described, it will be recognized that the invention may be otherwise variously embodied and practiced within the scope of the following claims.

What is claimed is:

1. An electrolytic cell for the reduction of alumina to produce aluminum, comprising a pot shell, a lining of electrically insulative refractory material within said shell to contain a bath of dissolved alumina and an underlying pad of molten aluminum, a composite electrode having a stem portion comprising a metal bar which terminates inwardly of the cell within said lining in close proximity to the pad of molten aluminum, and having a refractory hard metal current collector attached to the inner terminal end of said stem portion, said collector projecting through the lining to be at least partially exposed to the pad of molten aluminum in the cell, an annular sleeve around said bar and spacedtherefrom, said current collector contacting the adjacent end of said sleeve to form a seal therebetween, said sleeve serving to block ingress of molten contents of the cell along the bar surfaces inwardly of said lining, the opposite end of the bar from said collector extending beyond said sleeve and throughan opening in the shell, and means adjacent said opening for excluding air from said sleeve.

2. Apparatus according to claim 1, in which said current collector has a convex configuration where so exposed, whereby the cell may be intermittenly shut down without damage to the refractory hard metal due to freezing and cooling of the molten aluminum.

3. In an electrolytic cell for the production of aluminum, having a chamber adapted to contain the molten contents of the cell including a pad of molten aluminum, and having a bottom lining of the cell defining said chamber and providing an interior surface disposed in contact with substantially the entire lower surface of said molten contents, the improvement comprising; an

electrode entering the cell through said interior surface of the lining, having a stem portion composedvof dation by a surface coating composed of cryolite in phosphoric acid.

References Cited by the Examiner UNITED STATES PATENTS 2,915,442 12/1959 Lewis 20467 3,028,324 4/1962 Ranslcy 20467 3,088,906 '5/1963' Hurd 204243 3,093,570 6/1963 =Dewey 204243 WINSTON A. DOUGLAS, Primary Examiner.

JOHN H. MACK, ALLEN B. CURTIS, H. S. WIL- LIAMS, Assistant Examiners;

Claims (2)

1. AN ELECTROLYTIC CELL FOR THE REDUCTION OF ALUMINA TO PRODUCE ALUMINUM, COMPRISING A POT SHELL, A LINING OF ELECTRICALLY INSULATIVE REFRACTORY MATERIAL WITHIN SAID SHELL TO CONTAIN A BATH OF DISSOLVED ALUMINA AND AN UNDERLYING PAD OF MOLTEN ALUMINUM, A COMPOSITE ELECTRODE HAVING A STEM PORTION COMPRISING A METAL BAR WHICH TERMINATES INWARDLY OF THE CELL WITHIN SAID LINING IN CLOSE PROXIMITY TO THE PAD OF MOLTEN ALUMINUM, AND HAVING A REFRACTORY HARD METAL CURRENT COLLECTOR ATTACHED TO THE INNER TERMINAL END OF SAID STEM PORTION, SAID COLLECTOR PROJECTING THROUGH THE LINING TO BE AT LEAST PARTIALLY EXPOSED TO THE PAD OF MOLTEN ALUMINUM IN THE CELL, AN ANNULAR SLEEVE AROUND SAID BAR AND SPACED THEREFROM, SAID CURRENT COLLECTOR CONTRACTING THE ADJACENT END OF SAID SLEEVE TO FORM A SEAL THEREBETWEEN, SAID SLEEVE SERVING TO BLOCK INGRESS OF MOLTEN CONTENTS OF THE CELL ALONG THE BAR SURFACES INWARDLY OF SAID LINING, THE OPPOSITE END OF THE BAR FROM SAID COLLECTOR EXTENDING BEYOND SAID SLEEVE AND THROUGH AN OPENING IN THE SHELL, AND MEANS ADJACENT SAID OPENING FOR EXCLUDING AIR FROM SAID SLEEVE.

3. IN AN ELECTROLYTIC CELL FOR THE PRODUCTION OF ALUMINUM, HAVING A CHAMBER ADAPTED TO CONTAIN THE MOLTEN CONTENTS OF THE CELL INCLUDING A PAD OF MOLTEN ALUMINUM, AND HAVING A BOTTOM LINING OF THE CELL DEFINING SAID CHAMBER AND PROVIDING AN INTERIOR SURFACE DISPOSED IN CONTACT WITH SUBSTANTIALLY THE ENTIRE LOWER SURFACE OF SAID MOLTEN CONTENTS, THE IMPROVEMENT COMPRISING: AN ELECTRODE ENTERING THE CELL THROUGH SAID INTERIOR SURFACE OF THE LINING, HAVING A STEM PORTION COMPOSED OF AN ELEMENTAL METAL OR ALLOY THEREOF TERMINATING INWARDLY OF THE CELL WITHIN SAID LINING IN CLOSE PROXIMITY TO THE PAD OF MOLTEN ALUMINUM, SAID ELECTRODE ALSO HAVING A REFRACTORY HARD METAL END PORTION PROVIDING A SURFACE IN CONTACT WITH SAID MOLTEN CONTENTS OF THE CELL; AND A PROTECTIVE SLEEVE AROUND THE ELECTRODE TO BLOCK INGRESS OF MOLTEN CONTENTS ALONG THE ELECTRODE SURFACES INWARDLY OF SAID LINING, SAID SLEEVE BEING PROTECTED AGAINST OXIDATION BY A SURFACE COATING COMPOSED OF CRYOLITE IN PHOSPHORIC ACID.

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