US3730859A - Multicell furnaces for the production of aluminum by electrolysis - Google Patents

Abstract

A MULTICELL FURNACE HAVING INCLINED BI-POLAR ELECTRODES FOR ELECTROLYTIC PRODUCTION OF ALUMINUM. A TERMINAL CATHODE IS SITUATED OVER A METAL COLLECTION PIT IN THE SLOPED VAT BOTTOM. THE CATHODE EXTENDS INTO THE COLLECTION PIT WHEREBY IT CONTACTS THE MOLTEN METAL.

Description

y 1973 G. DE VARDA 3,730,859

MULTICELL FURNACES FOR THE PRODUCTION OF ALUMINUM BY ELECTROLYSIS Filed June 26, 1970 fig 5; z,

United States Patent 3,730 859 MULTICELL FURNACES FOR THE PRODUCTION OF ALUMINUM BY ELECTROLYSIS Giuseppe dc Varda, Milan, Italy, assignor of a fractional part interest to Montecatini Edison S.p.A., Milan, Italy Filed June 26, 1970, Ser. No. 50,050 Claims priority, application Italy, June 30, 1969, 18,962/ 69 Int. Cl. C22d 3/12, 3/02 U.S. Cl. 204-67 9 Claims ABSTRACT OF THE DISCLOSURE whereby it contacts the molten metal.

Multicell furnaces having inclined bi-polar electrodes for the production of aluminum by electrolysis, have already been described.

It is known that these bi-polar electrodes are made of a carbon material and that they are suspended in a molten fluoride bath (U.S. Pat. 3,178,363, issued Apr. 13, 1965). The molten cryolitic bath is contained in a vat having an outside iron lining, which is lined on its inner surface by a solid and electrically highly insulating material, such as for instance, shaped pieces made of siliconnitride-bonded silicon carbide, or of fused alumina, or of pure cryolite, etc.

It is also known that it is highly desirable that the bottom of said vat be step-shaped or at least sloping toward one or two grooves or pits for collecting the molten aluminum produced during the electrolytic process, coassigned U.S. application Ser. No. 809,852, filed Mar. 24, 1969 now Pat. No. 3,647,673.

Since the specific weight of liquid aluminum is only slightly higher (2.3) than that of the molten bath (2.1), the metal produced by electrolysis in the various cells of the furnace tends to collect in the lowest part of the stepped down vat bottom, namely in the above-mentioned pits. These pits for collecting aluminum tend, however, to become encrusted with solidified bath. This shortcoming depends on the bath composition and may happen even at not very low temperatures ranging, for instance, between 920 and 930 C.

Indeed if the molten bath in the various cells, crossed by the electrolytic current, for instance, is kept at a temperature of 960 C., the lower layers of the bath, located above the vat bottom but beneath the electrode system, are a somewhat lower temperature. The temperature reaches a minimum value in those layers of the bath,

which contact the solid vat bottom or the liquid aluminum collected in the pits. If the temperature of these lower layers, for some reason becomes too low the bath first thickens and then freezes or solidifies. This bath thickening and/ or bath freezing, may in actual practice sometimes occur in an almost irreversible way.

When the drawback of the frozen bath occurs, the downward flow of the aluminum, produced by electrolysis in the several cells of the multicell furnace, is hampered and the aluminum may even find its natural receptacle (pits) clogged up. As a result, the produced aluminum spreads over a large portion of the step-shaped bottom surface of the vat, thus forming an almost continuous aluminum layer. On this layer, both cathodic and anodic zones may form, which are crossed by eddy currents that by-pass the overhanging carbon electrodes system. The eddy currents exert a harmful influence upon the electric current efiiciency of the multicell furnace.

3,730,859 Patented May 1, 1973 It is also known that several methods have been disclosed for keeping up with the temperature of the lower layers of the molten bath in order to avoid or to restrain the bath thickening or solidifying over the vat bottom and especially its crust forming tendency within the pits. These pits account for the zone of the vat bottom surface farthest away from the electrode system and therefore from the heat supplying cells. This is exactly the opposite of what happens in the conventional horizontal single cell furnaces.

For instance, coassigned U.S. Pat. application, Ser. No. 859,790 filed Sept. 22, 1969, now U.S. Pat. No. 3,666,654 it has been proposed of supplying additional heat to the pits for collecting molten aluminum by electric resistors placed externally to the groove (i.e. resistors sandwiched between layers made of an inert solid material just under the groove), such resistors consisting of fibrous graphite in the form of fabrics, tapes or felts.

These heating devices, however, have the drawback of their brittleness. Furthermore, they are subjected not only to the seepage of molten bath, but also sometimes to that of molten metal. The latter seepage destroys the utility of those resistors, by short-circuiting them with their source of electric power. Several means for shielding these external resistors have been devised, experimented and disclosed. These means, however, are not always easy to carry out and usually are complicated and rather expensive.

The present invention, surprisingly, overcoming the foregoing drawbacks, allows very simply and inexpensively, to keep the pit for collecting aluminum at least at a temperature which is slightly above the critical temperature of the bath. Thus incrustation of the pit bottom or even obstruction of the whole pit by bath thickening is avoided.

During the run of the multicell furnace, the cryolitic bath may have temperatures ranging between 930 and 980 C. It is well known that the bath transfers heat very poorly from top to bottom. From this point of view, its behavior does not substantially differ from that of water at temperatures ranging between 0 and C. It is indeed known that in a glass test tube full of water and heated only at the top, some particles of ice on the bottom, may at least for some time very well coexist with a layer of boiling water on the top.

When the multicell furnace is in regular operation, the calories that keep the furnace at its running temperature are produced almost exclusively in the interelectrodic spaces of the various cells as a consequence of the ohmic resistance that the bath opposes to the flow of the electrolysis current. The carbon electrodes have a much higher heat conductivity than that of the bath and therefore heat propagates easily in. such electrodes in any direction and also toward the bottom. It is known that in multicell furnaces having suspended electrodes, immersed in a cryolitic bath, and having a vat bottom sloping down towards the aluminum pit, such electrodes purposely do not reach the step-shaped vat bottom, in order not to short-circuit the multicell furnace or at least not to increase considerably the by-pass of idle electric current outside the single cells.

The present invention, overcoming a technical prejudice, consists in constantly keeping the lower end of the terminal cathode (or terminal cathodes) and of no other cathode, permanently immersed in the molten aluminum contained in the underlying pit for collecting aluminum. In such a way, a good heat transfer from the terminal cathode to the metal collected in the pit is secured, the metal, in turn, being an excellent heat conductor toward the neighboring zones of the bath and of the vat bottom, thus dispelling or preventing the formation and floating of dangerous aluminum layers below the suspended carbon electrodes of the multicell furnace.

It is, however, needed to design the furnace with the step-shaped vat bottom so as to maintain this contact between the terminal cathode and the aluminum collected in the pit, even after the periodical tapping of the aluminum has occurred. Just this time interval needed for the tapping operation often is among the critical periods wherein the pit and its surroundings tend to become more easily encrusted. The present invention, however, is not confined to teach that it is highly desirable to create and maintain a wide lasting contact between the terminal cathode and the aluminum collected in the underlying pit. Sometimes, it is needed to supply an additional amount of calories to the terminal cathode. This happens every time the calories supplied from the various cells (calories produced by the current of electrolysis) to the lower bath layers (located between the carbon electrodes and the vat bottom) become insufficient (for instance as a consequence of a transitory cooling of the bath due to the electrode renewal, furnace starting operation, overall current interruption and so on) for keeping said lower layers above the critical temperature. The current supply connecting bars or nipples (metal conductors through which the electric current leaves the furnace) which penetrate from above into the terminal carbon-cathode, are highly suitable for this purpose. As it is well known, the contact surface iron/ carbon shows a noticeable ohmic resistance to the passage of the electric current. Such an electric resistance obviously generates a certain amount of calories.

If, while the total amount of electric current (Amps) flowing through the furnace is kept constant, one or more of the cathodic current supply connecting bars or nipples are disconnected from the bus bar carrying the overall electric current out of the furnace, the overall active contact surface iron/ cathodic carbon will be reduced. Consequently, the overall voltage drop due to the contact iron/ carbon (of those current supply connecting bars still inserted in the electric circuit), will increase as well as the calories developed in the inner part of the terminal cathode. Because of the carbon cathode being a good heat conductor, a large amount of these calories will flow downwardly, provided that the aluminum collected in the pit be at a lower temperature with respect to the temperature of the non-heated up terminal cathode (and also with respect to temperature of the electrolytic bath in the cells).

As soon as the usual thermal condition between the pit and the bath in the cells is restored, the cathodic current supply connecting bars are reconnected to the bus bar, by means of simple known operations.

The figure represents schematically a longitudinal section of a multicell furnace with a stepped bottom in accordance with my invention.

This and other characteristics of my invention will be now more evidenced by an example with reference to the drawing, given purely for the purpose of example and having no limiting nature.

Referring now to the figure in detail, there is shown a symmetrical-type multicell furnace, which comprises eight spaced apart suspended and inclined bi-polar electrodes 5, two terminal anodes 8 and terminal cathode 3. In such embodiment of my invention, namely in this symmetrical-type of multicell furnace, terminal cathode 3 is placed in central position with respect to the other electrodes. It should be clearly understood that there is no contradiction whatsoever for the cathode 3 being terminal yet being placed in a central position. In fact, terminal electrode is understood to mean such electrodes have either only cathode or only anode active surfaces, whereas bi-polar electrodes possess both anode and cathode active surfaces. Bi-polar electrodes 5, terminal anodes 8 and terminal cathode 3 define ten (five-l-five) cells 7, wherein electrolytic decomposition of alumina takes place.

Steps 1 descend from the ends of the furnace towards the central zone of the same, where a single collecting pit 2 is placed for the collecting and the tapping of the molten aluminum produced in the individual cells 7. Cathode 3 is axially perforated so that the introduction of a suitable tapping apparatus (not shown in the figure) through the hole 13 into the collecting pit 2, is possible. In accordance with my invention, in order to avoid the danger of the electrolytic bath 6 solidification and clogging of the collecting pit 2, terminal cathode 3 extends downwardly well inside collecting pit 2. It is within the ordinary skill of the planning engineer to select the dimensions of the pit and the distance between bottom 4 of cathode 3 and the pit bottom 9 so that, even immediately after a tapping operation, the residual aluminum is sufficient to assure a wide and lasting contact between the aluminum left in the pit 9 and the bottom 4 of cathode 3.

Such furnace is operated at a current intensity of l0,000+l0,000=20,000 amperes and at a voltage of 16 volts. The usual cathodic voltage drop may be maintained for instance between 0.3 and 0.5 volt. Under such conditions, during a usual run, the terminal cathode alone will develop a heating ohmic power of 6-10 kw. However, it is very easy to markedly increase the power during the comparatively short time interval, wherein it is needed to supply additional heat to the aluminum collecting pit. It is enough to disconnect one or more of the cathodic current supply connecting bars 10 by means of switches 11 during the desired time interval. If the same operation is applied to a half of the actual cathodic nipples, the corresponding increase of ohmic calories supplied to the terminal cathode will amount to about further 6-10 kwh. In such a way, the thermal source located in the terminal cathode is almost doubled. The production of these additional calories is advantageous first of all to the carbon terminal cathode and secondly to the underlying pit for collecting aluminum, such aluminum being in direct and lasting contact with the cathodic carbon electrode, which in its turn is partially immersed in said aluminum.

It is among the outstanding advantages obtainable through my invention, that it is no longer necessary to resort to a strong increase of the current intensity, even though for limited periods of time and provided that the installed rectifiers have the capacity to provide such a tremendous increase of the Amperes, whenever it is desired to dissolve or to prevent incrustations on the bottom of the multicell furnace. Such sharp, even though short, increases of current intensity result in harmful increases of the electrodic current density, in deterioration and shorter life, of electrolytic baths and electrodes, and lastly in a decrease of current efiiciency. Furthermore, they do not often achieve the desired result, because this larger amount of ohmic heat supplied by means of the current intensity increases does not reach, or only partially and insufficiently reaches, the lower bath layers and the inner part of the aluminum collecting pit, and cannot therefore, develop therein a suitable heating up action, for getting rid of said incrustations, by dissolving the bath solidified in the pit itself.

I claim:

1. In a multicell furnace for producing aluminum by electrolysis of alumina dissolved in molten fluoride baths, such furnace comprising a bath containing vat, lined on its inner surface by a solid and electrically poorly conducting material, and a plurality of internally positioned bi-polar electrodes and terminal monopolar electrodes, such electrodes adopted to be suspended in the bath, inclined and made of a carbonaceous material, such furnace further comprising at least one terminal cathode and a stepped shaped vat bottom sloping down toward at least one pit for collecting the aluminum produced by electrolysis, the improvement which comprises a pit being under said terminal cathode, said terminal cathode being well inside said pit so that when said furnace is in operation a good and permanent contact of the carbonaceous material of said terminal cathode and the aluminum collected in the pit is obtained, whereby a suitable heat transfer from the carbon cathode to the molten aluminum collecting into the pit and to the vat bottom is obtained, and means for switching electric contacts between the carbonaceous material of said terminal cathode and metallic electric leads imbedded in said terminal cathode.

2. In the multicell furnace of claim 1, wherein the terminal cathode has a plurality of metallic current sup ply connecting bars and nipples, and at least one of said connecting bars is arranged in such a manner that it may be easily electrically connected with and disconnected from a bus bar.

3. The multicell furnace of claim 1, wherein said aluminum collecting pit is located in a central position with respect to the stepped shaped vat bottom.

4. The multicell furnace of claim 1, wherein said pit for collecting aluminum is of such dimensions and capacity that during operation, even after the periodical tapping of the aluminum produced by the furnace, a direct contact between the terminal carbonaceous material electrode and the residual aluminum in said pit is still maintained.

5. The multicell furnace of claim 4, wherein said carbonaceous terminal cathode is traversed from the top to the bottom with a tapping hole.

6. The multicell furnace of claim 5, wherein said terminal cathode is so shaped as to have at least its bottom part more enlarged than its upper part, said enlarged part of said cathode being so located in said collecting pit as to prevent direct contact of said cathode with the walls and the bottom of said pit.

7. The multicell furnace of claim 1, wherein all the electrodes and the stepped bottom vat are disposed symmetrically with respect to a central vertical transversal section of the furnace.

8. A multicell furnace for producing aluminum by electrolysis of alumina dissolved in molten fluoride baths, such furnace comprising a bath containing vat, said vat lined on its inner surface by a solid and electrically poorly conducting material, a plurality of internally positioned bipolar electrodes and terminal monopolar electrodes suspended in the bath, said electrodes being inclined and made of a carbon material, said furnace further comprising at least one terminal cathode and a stepped shaped vat bottom sloping down toward at least one pit for collecting the aluminum produced by electrolysis, said pit being located under a terminal cathode in a central position with respect to the bottom of the vat, said terminal cathode extending well inside of said pit whereby a good and lasting contact of the carbon material of the terminal cathode and the molten aluminum collected in the pit is realized during furnace operation, said terminal cathode being provided with a plurality of metallic current supply connecting bars and nipples adoptable for tempoarry disconnection and reconnection whereby, the temperature of the carbonaceous cathode and the temperature of the molten aluminum collected in the pit beneath said cathode and of the bath in its neighborhood may be at higher temperatures with respect to the critical temperature of freezing or thickening of the cryolitic bath.

9. A multicell furnace for producing aluminum by electrolysis of alumina dissolved in molten fluoride baths, such furnace comprising a bath containing vat, said vat being lined on its inner surface by a solid and electrically poorly conductive material, a plurality of internally positioned bipolar electrodes and terminal monopolar electrodes, said electrodes being suspended in the bath, inclined and made of a carbonaceous material, said furnace further comprising a stepped shaped vat bottom sloping down toward at least a pit for collecting the aluminum produced by electrolysis, said pit being located beneath a terminal cathode at the bottom of the vat.

References Cited UNITED STATES PATENTS 3,502,553 3/1970 Gruber 204-67 3,400,061 9/ 1968 Lewis et a1. 204-243 X 3,475,314 10/1969 Johnston 204-243 R 3,314,876 4/ 1967 Ransley 204-294 X 3,578,580 5/1971 Schmidt-Hatting et a].

JOHN H. MACK, Primary Examiner D. R. VALENTINE, Assistant Examiner US. Cl. X.R. 204-244, 245

Pd-IOSO Dated Patent No.

Inventofls) GIUSEPPE de VARDA t n P. t a p W d O 8 1 ml m I u m e o C 3 S a S v a O d a G t e C 1 Q E I I n O i C S V. I 10 a e 3 I. n: e 3.10 a

e I I O a f I t e n c E t a a h P C S d I Q a t s; t n m I G d C 1 a S S 1 C t 3 Ti h t d n a In the heading to the printed specification, line 8,

"18,962/69" shouldiread Signed and sealed this 20th day of November 1973.

' (SEAL) Attest RENE D. 'IEG'IMEYER Acting Commissioner of Patents R P me C 6 1 T 1 ma 0' F so M n 1 Dt RS mm P6 E A

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