US3562136A - Igneous-electrolysis multicell furnaces,for the protection of the inner layer in contact with molten salts - Google Patents

Abstract

DESCRIBED IS A FURNACE FOR THE ELECTROLYSIS IN MOLTEN BATH OF ELECTROLYTICALLY AGGRESIVE SALTS, IN PARTICULAR A MULTICELL FURNACE HAVING BIPOLAR ELECTRODES, FOR THE PRODUCTION OF ALUMINUM OR OF MAGNESIUM, HAVING AN LECTROLYSIS VAT THE BOTTOM AND THE WALLS WHEREOF COMPRISE A LAYER OF ELECTRICALLY CONDUCTING MATERIAL, IN PARTICULAR OR CAR BONACEOUS MATERIAL, INTERNALLY (THAT IS ON THE BATH SIDE) COATED WITH A LAYER OF REFRACTORY MATERIAL SUBSTANTIALLY INSULATING THE ELECTRICAL CURRENT. IN ORDER TO ELIMINATE OR REDUCE THE BYPASS OF ELECTRIC CURRENT THROUGH THE LAYER OF SAID REFRACTORY MATERIAL AND THE INCREASE OF CORROSION RESULTING THEREFROM ON SAID BOTTOM AND ON SAID WALLS BY THE MOLTEN MASS OF THE BATH PENETRATING INTO THE RESPECTIVE POROSITIES OR CRACKS, THE FURNACE IS CHARACTERIZED IN THAT THE VAT LAYER OR LAYERS, BEING BETTER ELECTRICAL CONDUCTORS, ARE MADE DISCONTINUOUS ALONG THE DIMENSION CORRESPONDING TO THE DIRECTION OF THE ELECTROLYSIS CURRENT, BY AT LEAST ONE SEPARATING BAFFLE, DIAPHRAGM OR TRANSVERSE LAYER MADE OF PRACTICALLY ELECTRONISULATING REFRACTORY MATERIAL THUS DIVIDING THE VAT INTO PORTIONS. THE TERMINAL ELECTRODES ARE ELECTRICALLY CONNECTED WITH THE CORRESPONDING VAT PORTIONS MADE OF CONDUCTING, E.G. CARBONACEOUS MATERIAL, BY MEANS OUTSIDE THE ELECTROLYTICAL BATH.

Description

Feb. 9, 1971 Filed March 25, 1968 (3, D VARDA ETAL 3,562,136

IGNEOUS-ELECTROLYSIS MULTICELL FURNACES, FOR THE PROTECTION OF THE INNER LAYER IN CONTACT WITH MOLTEN SALTS 5 Sheets-Sheet 1 16 '18 25 20 22 23 23 14 17 19\ 21 24 15 l J 1 1 7 7 '[fl l u 1 1 l u 26/ \1 a U Q 1 s 7 a 9 10 11 12 1a z Z 24 n I r i r 24 L I I I I /A/k/ 4 .AA

I/l IL \T 3 4 14 17 9 38 19 31 21 I 23 2a 16 2a 1a 2032 as 22 15 INVENTORS:

Quay e Mia 6L DE VAR ET AL 3,562,136

G. IGNEOUS-ELECTROLYS MULT LL FURNACES, FOR THE PROTE IO F TH NNER LAYER IN CO CT WITH I TEN SALTS Filed March 25, 1968 3 Sheets-Sheet 2 Feb. 9, 1971 FIG. 2

Feb. 9, 1971 DE VARDA ETAL 3,562,136

' IGNEOUS-ELECTROLYSIS MULTICELL FURNACES, FOR THE PROTECTION OF THE INNER LAYER IN CONTACT WITH MOLTEN SALTS Filed March 25, 1968 3 Sheets-Sheet 3 FIG.3

INVENTORS: um 7w 0U n Mr ac n u ml mm M M m m United States Patent US. Cl. 204243 10 Claims ABSTRACT OF THE DISCLOSURE Described is a furnace for the electrolysis in molten bath of electrolytically aggressive salts, in particular a multicell furnace having bipolar electrodes, for the production of aluminum or of magnesium, having an electrolysis vat the bottom and the walls whereof comprise a layer of electrically conducting material, in particular of carbonaceous material, internally (that is on the bath side) coated with a layer of refractory material substantially insulating the electrical current. In order to eliminate or reduce the bypass of electric current through the layer of said refractory material and the increase of corrosion resulting therefrom on said bottom and on said Walls by the molten mass of the bath penetrating into the respective porosities or cracks, the furnace is characterized in that the vat layer or layers, being better electrical conductors, are made discontinuous along the dimension corresponding to the direction of the electrolysis current, by at least one separating baflle, diaphragm or transverse layer made of practically electroinsulating refractory material thus dividing the vat into portions. The terminal electrodes are electrically connected with the corresponding vat portions made of conducting, e.g. carbonaceous material, by means outside the electrolytical bath.

Our invention relates to the vats for containing the molten bath in electrolysis multicell furnaces used for instance in the production of aluminum or magnesium. The invention is directed to prevention of corrosion of the refractory walls in direct contact with the molten bath and/ or molten metal.

More particularly, our invention relates to the vats of electrolysis multicell furnaces having suspended electrodes, internally coated with a layer of a special refractory material being a poor electrical current conductor. This special refractory material may, for instance, consist of silicon nitride bonded silicon carbide, e.g. the commercial products Refrax and Christolon. Backing said special refractory material is a layer of carbonaceous material having much higher electric conductivity than the special refractory material. This carbonaceous material layer is to secure the vat tightness, i.e. to prevent leakage from the vat of the molten bath and/or molten metal which might seep through either the pores of said special material or through the joints or any cracks therein.

In these furnaces, going outwardly from the layer of carbonaceous material, are one or more layers of heatinsulating material, all of which is contained in an iron shell. It is known that the thickness of the individual vat layers of carbon or of heat-insulating materials is calculated by taking into account heat dispersion. In conventional furnace vats, the freezing isotherm surface of the molten bath must, if possible, be contained within a zone which for practical purposes is sufiiciently immune against destruction by the action of the seeped bath. No serious inconveniences are met, if and until said freezing surface remains wholly contained within the layer of carbonaceous material. That means, one achieves what is technically called a thermal block. This is a blocking by freezing 5 the advance of the infiltration, when molten bath arrives at a vat zone with an isotherm corresponding to the freezing temperature of the bath.

In the multicell furnaces having suspended electrodes, operating with an electrolysis bath composed of molten salts, such as for instance cryolite baths in the electrolysis of aluminum oxide, for the production of metallic aluminum, the following two problems, amongst others, give rise to serious drawbacks and surprisingly are resolved or greatly attenuated by the present invention:

The first problem relates to the protection of the refractory layer in direct contact with the molten bath. The second problem relates to the possibility of short-circuiting the operation of a multicell furnace having suspended electrode. In these furnaces a considerable part of the electrical current feeding the furnace passes as parasitic current through the vat walls as follows: terminal anodebath-refractory layer internally coating the vat-carbon vat (zone close to the terminal anode)carbon vat (zone close to the terminal cathode)-refractory material-bath-terminal cathode.

It is known that certain special refractory materials resist quite well the action of molten salts, for instance molten cryolite at 900960 C. However, in an electrolysis furnace where the chemical action is accompanied by an electrochemical action, there occurs, especially in multicell furnaces, a deep attack of the refractory material by more or less intense corrosion, up to the disgregation or complete destruction of the special refractory material in the zones more exposed to the electrochemical action of the electrical current. The higher the temeprature, the more rapidly these deleterious phenomena occur. The chemical composition of the bath may contribute to aggravating this great inconvenience.

In what follows, reference will be made in particular to multicell electrolysis furnaces having suspended electrodes, for the production of aluminum, but without limitation thereto of the present invention, since it can be applied to all the multicell furnaces presenting analogous conditions regardless of the electrolytical molten bath and the metal produced.

FIGS. 1 to 3 of the drawing show schematically in plan view and in sections along lines AA and BB, respectively, an embodiment according to the present invention, of a multicell furnace having suspended electrodes, with seven cells, for the electrolysis of aluminum oxide.

The special refractory material in contact with molten bath, under the operating conditions, soaks up fluoridecontaining bath and becomes electrically conductive. Although the electrical resistivity of the thus soaked refractory materialis considerably higher than the resistivity of the electrolytical bath, a part of the electrolysis current is shunted by the internal vat coating consisting of special refractory material.

This shunting phenomenon is greatly aggravated by the presence of the carbon layer backing the special refractory material, as carbon is a good electrical conductor. The coating of special refractory material which should act as an insulating screen fails whereby a strong parasitic current passes through the refractory layer into the carbon layer partially attacking and destroying those zones of special refractory material which are next to the bipolar electrodes. Those zones of special refractory material next to the terminal electrodes, and particularly to the terminal cathode, are more strongly attacked. The latter zone of the vat, in fact, will thus function as anode. The zones where the electrical current enters and comes out, passing from the bath into the refractory material or from the refractory material into the bath, are subject to localized, often deep attacks of the refractory material itself. A remarkable part of the electrolysis current introduced into the multicell furnace is thus wasted through this parasitic shunt, i.e., through the vat layers made of special refractory material and of carbon.

Of course, the portion of current shunted through the special refractory material and the carbon of the vat increases as the distance between the walls of refractory material and the terminal electrodes becomes smaller and the number of the cells and the amperage with which the furnace is fed becomes greater.

The corrosion effects grow by geometrical progression with the increase of intensity of the thus shunted current. When examining the fiow of the above parasitic current, one can see that also this current passes at least twice through the cryolitic molten bath, giving rise to the formation of parasitic secondary cells which, while perhaps producing aluminum, do so with local destructive effects on the vat of refractory material, particularly in the zones on the faces of the special refractory material, which are next to the terminal electrodes, especially next to the terminal cathode. Moreover, per unit consumption of electrical energy becomes high, due to the fact that a considerable part of the electrolysis current is shunted, as above described, instead of regularly flowing through the row of the single cells of the muticell furnace.

We have found that it is possible to eliminate either completely or at least substantially the above drawbacks, by very simple expedients which can be easily achieved. More precisely and surprisingly, we use a combination of expedients which apparently would have opposite and destructive effects.

According to our invention, the carbon vat is divided into slices by transverse separating layers of refractory material which interrupt the electrical continuity between the carbonaceous material at the back of the special refractory material. The effect of such division can be improved in that the slices of carbon vat resulting thereby, or at least those corresponding, by their position, to the terminal electrodes, are connected outside the bath to the electrodes by good external electrical conductors which we shall call external shunts as used hereinbelow.

In a preferred embodiment, the number of the carbon slices corresponds to that of the electrodes, or of the cells, and the external shunt is provided not only for the two terminal electrodes but also for the intermediate bipolar electrodes. Combining these two features, which would appear to increase at least the causes of eddy current losses, results, however, in a stopping blockage of the corrosion on the inner face of the refractory coating, on the contrary, as well as a reduction. of the current loss through the vat. The external shunt, in fact, provides equipotentiality of the electrodes and vat slices in correspondence with each cell.

In this way, we almost completely eliminate the parasitic electrical current which formerly entered from the bath into the special refractory material next to the terminal anode and left the special refractory material towards the bath in the zones next to the terminal cathode. Moreover, this modest current which will continue to pass through the vat (from the vat zones next to the terminal anode, to the vat zones next to the terminal cathode), will no longer enter into or come out from the special refractory layer on the bath side, but on the carbon side. It is thus possible to eliminate conspicuous phenomena of destructive corrosion, which the special refractory material formerly underwent in the multicell furnaces.

In multicell furnaces of the kind to which the present invention refers, collecting pits for the molten aluminum produced by the electrolysis are provided on the vat bottom, preferably in correspondence with the various cells or, better, in the interelectrodic spaces. These pits, depending on the nature of the electrolytical process, may be completely independent and constructed in a way that the separating sectors contact the lower part of the respective overlying anodes-cathodes. In the case of the electrolysis for aluminum, the pits may be at a certain distance from the anodes or cathodes, e.g., 35 cm., so as to allow the free circulation of the bath between one pit and the next. The pits can be formed by small walls or partitions made of special refractory material, and in particular can be connected with or be a part of the aforesaid separating layers (partitions) or baffles (diaphragms) of special refractory material which establish the discontinuity of the carbon vat, according to the present invention.

The material of the bafiles or separating layers can be the same as that of the inner coating, or may be different.

, In the latter case, it must be kept in mind that the material of the baffles (diaphragms) or separating layers should be a better electrical insulating material than the inner coating (special refractory material as defined above) of the vat, whereas it may be less resistant to attack of the bath and less resistant to heat. For instance, electrocast alumina can be used.

Therefore, when using our invention, the electrolysis current entering the furnace through the current-carrying studs of the terminal electrode which functions as anode, and exiting from the current-carrying studs which are connected to the multicell furnace on the side of the terminal cathode, is shunted only partially through the refractory vat, which is coated externally with a layer of discontinuous carbonaceous material, i.e. subdivided by the said partitions made of refractory material. The current shunted through the conductors outside the bath enters the refractory material of the walls; partially soaked with bath and therefore, in turn, to a certain extent a conductor of the electrolysis current; on the side opposite to that of the bath, following a coarse which goes from the anodic zone, i.e. from the terminal anode, to the cathodic zone, i.e. to the terminal cathode, and comes out from the refractory material through the adjacent layer of carbonaceous material and not through the bath itself.

Moreover, owing to the resistivity of the refractory wall which is impregnated with bath, which resistivity is a multiple of the electrical resistivity of the bath alone, the thus shunted or parasitic current represents only a small fraction of that fed to the furnace, whereas most of the current, when passing through the cells of the furnace, performs the electrolysis of the aluminum.

Having set forth its general nature, the invention will be best understood from the more detailed description hereinafter which refers to the accompanying drawings. Although the drawings illustrate one arrangement of apparatus in which the process of this invention may be practiced, it is not intended to limit the invention to the particular apparatus or material described.

EXAMPLE In a furance with seven cells and 5 ka., built according to the present invention, the current shunted through the layers of refractory material (which in this specific case was Refrax) and of carbon, was of the order of 5% on the furnace feeding current. The Refrax refractory materials in contact with the molten bath, after several months of operation, were found, on dismantling the furnace, to have been attacked on the surface only to a depth of from 1 to 2 mm.

In all the figures, the external iron shell 1 encloses the layers of insulating material 2, of carbonaceous material 3, of special refractory material 4. All these layers form the vat of the multicell furnace. This vat is filled with molten cryolite bath 5, in which are suspended the electrodes of carbonaceous material 6, 7, 8, 9, 10, 11, 12 and 13, by a device known per se and not shown, from the top and immersed in the bath. The electrolysis current enters through proper current-carrying studs 14 into the terminal anode 6 and after having run through the row of the seven cells, comes out from the terminal cathode 13 through current-carrying studs 15. The layer of refractory material 4; and often also the part, next to the bath, or baflles 16, 17, 18, 19, 20, 21 and 22, made of refractory material, too; is impregnated with bath, since also the special refractory materials usually have a certain degree of porosity. In the carbonaceous Zones, separated from each other by partitions 16, 17, 18, 19, 20, 21 and 22, are fixed iron studs 23 protruding upwards, connected by flexible electrical conductors 24 with the studs fixed on electrodes 6, 7, 8, 9, 10, 11, 12 and 13. These studs may coincide, as in the drawing, with the current-carrying studs 14 and 15 for the terminal electrodes 6 and 13, respec tively. The studs of the intermediate electrodes are generically indicated by 25.

The external conductors 24; which electrically shortcircuit the carbon zone 26 of the anodic head of the vat with the terminal anode 6, and electrically short-circuit the carbon zone 27 of the cathodic head of the vat with the cathode 13, and analogously, optionally short-circuit electrically also the intermediate electrodes 7, 8, 9, 10, 11, 12, with the corresponding zones or intermediate carbon slices 28, 29, 30, 31, 32, 33, respectively; can be connected with the electrodes by means of studs of the same electrodes. They also may be the current-carrying studs for the terminal electrodes or can be either individual studs, or studs acting simultaneously as suspension organs of the same electrodes, on one side, and corresponding studs provided on the carbonaceous zones or slices of the vat, on the other side, preferably on both longitudinal sides of the furnace.

The seven-cell furnace illustrated in the figures has eight zones of carbonaceous material which sheathe the outside of as many zones of the special-refractory layer in contact with the molten bath as correspond to the eight electrodes suspended in the same bath.

The wall partitions 34 divide the furnace bottom into pits 35, for the collection of the produced aluminum.

The present invention makes it possible to direct the freezing isotherm of the bath to a zone consisting of carbonaceous materials of low cost and practically immune against destruction by action of the infiltrated bath. Such a zone of sufficient width is created by rendering equipotential the electrodes and the corresponding vat portions, controlling at, the same time the current losses through the vat, and thereby also the formation of Joule heat generated by said current in the carbon, thanks to the transversal partitions made of refractory material.

Under the scope of the present invention fall the obvious variants and the obvious constructive and functional equivalents, either for instance as concerns the number and the position of the separating bafiies (diaphragms) of refractory material, or for instance as regards the choice of the special refractory material or its substitution by another material, or of the means for giving the electrodes and vat portions the same electric potential. Thus, for instance, there may be one or more intermediate electrodes, or one or more intermediate vat portions, not connected in this Way.

We claim:

1. Furnace for the electrolysis in molten bath of electrolytically aggressive salts, said furnace being a multicell furnace having bipolar electrodes, having an electrolysis vat with the bottom and the walls comprised of a layer of electrically conducting material, coated on the bath side or inside with a layer of refractory material substantially insulating the electrical current, wherein the vat layer, being a better electrical conductor, is made discontinuous along the dimension corresponding to the direction of the electrolysis current, by at least one separating ballle made of practically electroinsulating refractory material dividing the vat into portions.

2. The furnace of claim 1, wherein the terminal electrodes are electrically connected with the corresponding vat portions made of conducting material, by means which are outside the electrolytical bath, whereby the passage of electric current through the layer of said refractory material and the increase of corrosion resulting therefrom on said'bottom and on said walls by the molten mass of the bath penetrating into the respective porosities or cracks are at least substantially reduced.

3. The furnace of claim 1, wherein the layer of practically insulating refractory material internally coating the vat is made of silicon nitride bonded silicon carbide.

4. The furnace of claim 1, wherein the practically electroinsulating refractory material of said separating baflles is silicon nitride bonded silicon carbide.

5. The furnace of claim 1, wherein the electrically conducting material of the vat is carbonaceous.

6. The furnace of claim 1, wherein the practically electroinsulating refractory material of said separating baffies is electrocast A1 0 7. The furnace of claim 1, wherein the vat portions correspond approximately with the electrodes.

8. The furnace of claim 2, wherein at least one intermediate electrode is connected with vat portions.

9. Multicell furnace accordingto claim 1, wherein the separating baffles of refractory material constitute, on the vat bottom, separating walls or partitions either forming or delimiting pits on the vat bottom, in correspondence with the single electrolysis cells of the multicell furnace, to collect the metal produced in said cells.

10. The furnace of claim 2, wherein the electrically connecting means outside the bath are flexible connectors attached to studs provided on the vat portions made of conducting material and, respectively, to the electrodes.

References Cited UNITED STATES PATENTS 3,321,392 5/1967 McMinn et a1 204-243 JOHN H. MACK, Primary Examiner D. R. VALENTINE, Assistant Examiner US. Cl. X.R. 204-244

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