US2959528A - Method of rapidly starting closed multicell electrolytic furnaces - Google Patents

Description

G. DE VARDA METHOD OF RAPIDLY STARTING CLOSED Nov. 8, 1960 MULTICELL ELECTROLYTIC FURNACES 5 Sheets-Sheet 2 Filed Jan. 28, 1958 l I l Nov. 8, 1960 G. DE vARDA 2,959,528

METHOD OF RAPIDLY STARTING CLOSED MULTICELL ELECTROLYTIC FURNAcEs Filed Jan. 28, 1958 3 Sheets-Sheet 3 United States Patent Office Patented Nov. 8, 196,0

METHOD OF RAPIDLY STARTING CLOSED MUL- TICELL ELECTROLYTIC F URNACES Giuseppe de Varda, Milan, Italy, assignor to Montecatini, Societa Generale per llndustria Mineraria e Chimica, a corporation of Italy Filed Jan. 28, 1958, Ser. No. 711,660

Claims priority, application Italy Jan. 31, 1957 12 Claims. (Cl. 204-67) This invention relates to a process and apparatus for the comparatively quick starting of multicell electrolytic furnaces, particularly those employing fused salt baths contained in inclined or vertical individual cells. It especially relates to the starting of abattery, or closed necklace chain or circuit, of closed electrolytic cells employed to produce aluminum by electrolysis of alumina at temperatures, for example, of from 900 to 1000 C., in a fused fluorinated salt bath, such as cryolite.

. A divisional application directed to the apparatus was iiled on October 20, 1959, as Serial No 847,538.

A main purpose of the present invention is to provide a method and apparatus for facilitating the rapid starting of multicell, or necklace, furnaces of the types described in the G. de Varda applications Serial No. 587,985, tiled May 29, 1956; Serial No. 711,577, filed January 28, 1958, and Serial No. 706,077, tiled December 30,1957. The starting of such multicell, or necklace-type electrolytic furnaces, equipped with anodic restoring layers, and with devices for continuously removing the aluminum produced, is possible by adapting known methods. For example, it is possible to introduce, while the furnace is empty, transportable resistors into the individual interelectrodic gaps, as well as into the upper terminal chamber, lower terminal chamber, and alumina feed chamber etc. of the necklace-type furnace. Heating is effected by passing current through mobile resistors until the interior of the furnace has attained the desired temperature, for example 900 to 950 C., whereafter the resistors are removed. Fused bath is then fed into all the cells and the anodic restoring layers are set in place, if needed. The electrolysis process is then at once started by applying voltage to the terminals of the furnace. A method of this type, however, presents inconveniences, since the electrolysis process cannot be started until all the cells of the necklace-type circuit have been made ready. These cells may be quite numerous, thirty for example. Making them ready requires time, so that the fluid bath first fed into the cells may .tend to solidify. This interrupts the passage of current through the furnace and eventually blocks all the furnace cells. In such case it is hardly possible either to heat the cells to complete bath uidifcation, or to discharge their contents. The only thing that remains to be done is to scrap the cells when the solidification of the bath is terminated,

The danger of blocking is also present when it is necessary to start a necklacetype furnace which is equippcd With continuously self-restoring electrodes, and with overow devices for continuously removing the aluminum from the cells as soon as it is produced.

l The said inconveniences, and also others, are eliminated by adopting the apparatus and starting procedure hereinafter described.

" An illustrative example of a preferred convenient embodiment of the present invention is constituted by and explained with respect to the accompanying drawings, in

Fig. 1 is a longitudinal section taken at yI-I of Fig.2, of a number of adjacent cells of a closed bath circuit, electrolysis furnace for aluminum production, with provision for continuous tapping of aluminum, and having xed and stationary electrodic blocks provided with juxtaposed self-restoring anodes, said blocks modified in accordance with the present invention;

. Fig. 2 is a composite horizontal sectional view taken at different intermediate levels, at A-Aof Fig. 1;

Fig. 3 comprises in part a top view and in part a horizontal section, being taken at B-B of Fig. 1;

Fig 4 is a vertical transverse section, at C-C of Fig. 1;

Fig. 5 is a vertical section, at D-D of Fig. 4, taken parallel and close to its longitudinal axis.

Fig. 6 is a vertical section at line F-F of Fig. 1; Fig. 7 is a vertical section at line E-E of Fig. 3.

The liquid levels indicated in the live figures are those attained at the termination of the furnace starting operation.

The internal cavities or surfaces of the furnace n contact with the bath are, as is known, protected by a refractory lining, for instance of magnesium oxide. The latter may be treated for instance as described in my application Serial No. 705,374, tiled December 26, 1957, to make them practically impermeable by the bath maintaining however an electric resistivityV higher than that of the fused bath and or by other methods.

Reference may be made to the above-mentioned copending applications for more complete descriptions of the furnace elements and of the assembled furnace. i

In Fig. l is shown a number of bipolar, upwardly downwardly extending, inclined bipolar electrodic structures. In the normal operation of the furnace electrolysis current is introduced through a left end terminal electrode (not shown) constituting an anode electrodic structure. The current thence passes serially through the intermediate bipolar electrodic structures as indicated by arrows 14 of Fig. l, and serially through the respective upwardly-downwardly extending intermediate electrolysis gaps, to a right end terminal electrode (not shown) constituting a xed and stationary cathode electrode. The arrow at 10, in Fig. 1, indicates the counter-flow of the bath through the conduits 10 passing through the magnesium blocks 4 described below. These features are shown in my prior application Serial No. 587,985; The upwardly-facing inclined surfaces of the stationary part of the bipolar electrodic structures provide, in the form shown in the drawing, the cathodically active electrode surfaces,

This stationary and fixed part of the bipolar electrode comprises a number of superposed, or stacked, carbon (preferably graphite) blocks or horizontal layers 2,3, 23 and 24, surmounted by a magnesium oxide block4', which in turn is surmounted by heat-insulating members 8. Bridging the top of each electrolysis gap is a supiporting piece of magnesium oxide, over which is placed the heat insulative cover plate 9. Refractory, chimney-shaped, conduit structures 6 extend through the outer heat-insulated wall 81. Conduits 6 are provided with removable cover 5.

As described in my application Serial No. 706.077, filed December 30, 1957, the anodic surfaces comprise, or are provided with, automatically and continuously renewing anode surface elements or structure, which may comprise a single block 1 of prebaked carbonaceous electrode ma: terial, or an anodic assembly (not shown) of superposed blocks of the same material. The block 1 continuously descends, as the lower tip, resting upon the surface of ledge v20, is electrolytically consumed. The block is guided downwardly adjacent to the surface of thestationed parts 2, 3, 23, 24 by the chimney 6.

The bottom wall 15 (Figl l) and the lateral walls 31 (Figs. 2, 3, 4) are each provided with an inner lining 22, 39 of magnesium oxide. Ledges 20 and ribs 25 are part of lining 22. Wall 15 and also lateral walls 31 are of or contain heat-insulating material, such as porosite. A horizontal groove 18 (Fig. 1) is formed in the lower face of each ledge 20, providing blind, horizontal channels. As shown in Fig. 4, the horizontal channels 18, which are provided with heat-insulating covers, serve to permit introduction of mobile resistors 42. Blind vertical channels 33 (Figs, 2, 3, 4 and 7) are formed between wall 31 and lining 39, into which vertical channels other mobile resistors 27 may be inserted. Each cell may be provided with at least one horizontal and one vertical blind channel. The resistors may be connected with one another. They are connected to a source of power and serve for heating the furnace in the various preparatory procedures, required, for example, for the various stages of impermeabilization with pitch (about 200 C.) and subsequent cooking (about 700 C.) and for the various starting stages (700 C. to 950 C.) of said furnace. The heat generated by the individual, mobile and transportable resistors, which are, for ex- .ample of 3 kw rating each, is transmitted through the refractory of magnesium oxide. The transmission is fairly good since its coefficient of heat conductivity is of the order to 0.006 cal./l/l cm./1 cm2/1 C.

The two graphite lower layers 23 and 24 of the bipolar electrode have reduced thickness, for example each one is 2 to 4 cm. thick, the upper layers 2 and 3 being, as indicated in Fig. l, of greater thicknesses. After the stage or stages of mperrneabilization of the internal MgO refractory lining with pitch, according to application Serial No. 705,374, the various layers 23, 24, 2 and 3 of the stationary part of the bipolar electrodes are placed into the individual cells. As shown in Fig. 2, the layers are provided with lateral projections to enable them to slide in the grooves 12. These grooves are formed in the sides of the inner walls 39 and 44 of the furnace, and also in the two smalls walls 13 (Fig. 2) which divide each bipolar electrode into three substantially equal portions. Walls 13 also divide the active electrodic surfaces into three cathodic zones and into three anodic zones.

Upon the upper face of each layer of electrodic graphite there is spread a veil or coating of alumina to reduce, to a minimum, electric contact with the overlying electrodic layer. For example (Figl 1) face 21 of layer 23 is so coated before placing layer 24 on layer 23. The lower face of each layer has a recess 25, and the upper face has a corresponding projection 26, so the superimposed layers t into each other. This is also the case with respect to the bottom non-conductive refractory ledge 20 and the upper refractory block 4, provided with recess 11. These indentations have, among other purposes, the function of securing good alignment of the layers of the graphite electrode, and also the required parallelism with the sliding plane 7 of the anodic restoring assembly 1 overlying the stationary part of the bipolar electrode.

In this way the easy and the perfect mechanical workability of graphite is utilized, by means of said indentations, to allow without inconveniences the net negligible thermal expansions and contractions of the non-conductive refractory, The indentations maintain, to a significant extent, a barrier against the by-passage of bath from one cell to the next. The passage should take place, at least prevailingly, through conduits 10 (Fig. 2) provided in non-conductive refractory blocks 4.

The ledges 20 under the electrodes, and also the side walls and the bottoms 22 of the lower chambers serving to collect the metal, should be entirely proof, that is impervious with respect to the fused metal.

. Finally in the upper terminal chamber (not shown) at the head of the necklace of cells (see the reference to Serial No. 587,985 below), as well as in the lower terminal chamber (not shown) at the end of said circuit there are provided, at the top, electric resistors, preferably fixed, and also hatchways leading towards the outside to effect rapid control of liquid levels.

Electric resistors are also applied in the upper portion of the alumina feed chambers. These chambers, as well as the terminal chambers, are not shown in the drawings.

Furnace starting procedure After its walls have been impermeabilized to the bath, which contains fused fluorinated compounds, such as cryolite, the furnace is completed by assembling therein, as already described, the stationary graphite parts of the bipolar electrodic structures, subdivided into various horizontal layers, and also the terminal electrodes (not shown) provided with metal conductors coming out preferably at the top of the furnace and connected by metal with the furnace terminals. Upper blocks of refractory 4, through which pass the channels 10, are placed thereto. The top insulation 8 and 9 for said blocks and for the open bath surface is put in place, and also the chimney-shaped members 6.

Now the layers 1 constituting the restoring anodes are introduced into the empty and cold furnace and the correct position of the insulating pads 9 is made certain of, after which the covers 5 above the chimneys are carefully closed. The first stage of starting is now begun by feeding current to the flying or mobile resistors accommodated in the blind-bottom, horizontal channels 18 and the vertical channels 33, in the base and in the side walls respectively of the furnace, as well as the resistors (not shown) in the vertical tapping pockets 35 provided in the inner longitudinal dividing walls 44 of the furnace.

Upon heating gradually, the closed furnace is brought up to a temperature of about 700 C. It is best or convenient to provide an atmosphere of inert gas, for example by means of a stream of nitrogen, in order to avoid oxidation of the impermeabilizing coke contained in the pores of the non conductive refractory linings and parts 16, 22, 4, 39, 44.

At this time, the mobile resistors accommodated in the tapping pockets 35 are removed, and at once thereafter such a quantity of molten aluminum is poured into said pockets, that in the cell, and in the tapping pocket in communication therewith, it is above the level 20 (Fig. 1) of the MgO seat on which each stack of graphite bipolar electrodic layer confining the respective oells rests. However, although surpassing said level 20, it is convenient or best that the height of the metal should not be above the level 21 (Fig. 1) of the interstice between the lowest graphite layer and the immediately overlying one. In this way each individual graphite stack, or at least the lower layer thereof, is in practice short-circuited in the necklace circuit, with respect to the adjacent ones.

This short-circuiting operation can be carried out in a limited time even if the number of cells is increased. A1- though the resistors have been previously removed from the tapping pockets in order to enable the introduction of fused aluminum, it is neither necessary nor convenient to cut off current from the transportable resistors remaining accommodated in the external walls. Having thus placed in short circuit all the cells of the circuit, one may now feed current to the furnace terminals.

The resistance of the path in the molten aluminum in each cell is negligible in practice. It is sensible, however, at the aluminum, graphite interfacial contact surfaces, and in the path through the individual base layers of the bipolar electrodes.

If new amperages equal to those of the full potentiality or capacity of the furnace in normal operation, corresponding for instance to 0.5 ampere for one square centimeter of active electrodic surface in a cell in normal operation, are made to pass through the furnace, the` power absorbed will be, for example, IAO of that of normal furnace operation.

At this time the passage to the second starting stage takes place, the furnace being kept closed, and While operating, for example, with a nitrogen atmosphere in the furnace. 'I'he maximum available or absorbable magnitude of direct current is made to pass through the cells, in short circuit, while the series resistors disposed in the external furnace walls are being energized at maximum. By proceeding in this way the inner furnace temperature increases gradually from 700 to about 900 to 950 C. Feeding of bath fluid to the high chamber (not shown) at the head of the necklace of cells, is then begun. A uid bath rich in A1203 is introduced into the closed circuit (Fig. 5) through the pocket at 43 and the channel 46 which constitutes the necessary connection between the two terminal chambers employed. Note Serial No. 587,985, tiled May 29, 1956, in this relation. The bath passes rapidly from the high chamber into the iirst contiguous cell, that is the one which, through conduit 17 (Fig. 5) is in communication with the pocket 35 provided with the overflow mouth 45, and from that one into the second one, and so forth, passing also through the alumina feed chambers to arrive at the last cell, the lowest one of the necklace. It then discharges into the low terminal chamber side with the high one.

This operation takes place in a comparatively limited time, for instance in one or two hours, without any need for disconnecting either the main furnace current or the one passing through the removable resistors.

As the fluid bath iills the electrolysis gap, of a given cell, the level 16 (Fig. l) of the metal in the cell lowers, until the anodic and the cathodic surfaces of the cell vare no longer in short circuit with each other, since the bath has completely displaced the metal from the interelectrodic gap. It is convenient, however, that the quantity of aluminum initially added should be such that when the cell lls with bath liquid the level of the metal in the cell should not descend below the level 19 sothat bat-h liquid will not be introduced into the pocket 35 (Fi-g. 5) which should lill up with molten metal.

Consequently, the voltage drop in the individual cell increases until it reaches values sutlicient to start the electrolytic process. The direct current amperage passing through the furnace as a Whole tends to diminish, so that it may be convenient to adjust the current feeding the furnace to keep it constant.

The beginning of the electrolysis in all the cells of the circuit, which are no longer short-circuited by the metal because filled with bath liquid, marks the beginning of the third and last stage of the furnace starting operation. This stage comprises starting the device for lifting the bath from the low terminal chamber to the high terminal chamber. This can be done by means of an oscillating ladle as described in application Serial No. 670,785, led July 9, 1957, by G. Calabria, or by means of a graphite-pump as described in application Serial No. 705,373, filed December 26, 1957, of De Pava, as well as by other known devices.

Also the measuring devices for all of the alumina feeding stations must be started. It is necessary to thereafter adjust and check the delivery of the bath-lifting device. It is also necessary to check each cell with respect to the attaining of the temperature limits employed in normal operation. If provided, the conventional device for discharging the electrolytic gases from the upper chamber should conveniently now be put to operation. Finally, current may be shut otf the circuit of the mobile resistors, which, when conveniently cooled, can be removed from the blind bottom channels, and utilized, for instance, in subsequent operations for starting other furnaces. The channels should be preferably filled, for example with A1203 powder, carefully closed and sealed with the plugs 41 and 42, after which the furnace may be considered completely started and by now put to normal productive operation. Channels 18 and'33 are? termed blind channels because the molten liquids in the order of magnitude of 700 C. means the range of` about 630 C. to 770 C. p

The term necklace of cells signifies cells arranged to form a complete, closed circuit arranged for cyclic tiow of bath liquid in the circuit, the electric current passing around the necklace serially through the cells, the bath liquid also passing serially, preferably in the oppo site direction, while aluminum oxide is fed into the furnace at least at one point in the liquid circuit, and is carried to the cells by the cyclicly flowing bath liquid. Preferably the necklace is flattened, the cells being -arranged at oppo' site sides of an internal wall longitudinally dividing the furnace. This arrangement, and the placing of the various tapping pockets, channels, feed chambers, etc. withinl the heat-insulating structure of the furnace, provides advantageous heat economy and facilitates the various operations.

I claim:

1. In a furnace process of producing aluminum by electrolysis of alumina in an enclosed fused salt bath, in which process electric current is passed from an anodic carbon surface through an upwardly-downwardly extending electrolysis gap containing alumina and said fused salt -and thence through a cathodic carbon surface, said gap and said surfaces forming a cell, and in which process the aluminum produced is collected below, the improvement in the method of starting said process bei fore feeding bath liquid into the cell, comprising disposing heating devices in the furnace, but outside the cel-l, to raise the temperature in a iirst stage, said temperature being below electrolysis operating temperature, and .pouring molten aluminum into the furnace, in a quantity sufficient to short-circuit the electrodic carbon surfaces of said cell and passing electric current through the short-circuited cell to raise the temperature still further in a subsequent stage.

2. In a furnace process of producing aluminum by electrolysis of alumina in an enclosed fused salt bath, in which process electric current is passed through an upwardly-downwardly extending consumable anodic carbon surface, through an upwardly-downwardly extending electrolysis gap containing alumina and said fused salt, and thence through an opposite stationary yand permanent cathodic active graphite surface, said gap and said surfaces forming a cell, and in which process the aluminum produced is collected below the salt bath, the improvement in the method of starting said process, before feeding bath liquid into the cell, comprising disposing heating devices in the furnace but outside the cell, to raise the temperature in a iirst stage, said temperature being below electrolysis operating temperature, introduc- :ing molten aluminum into the furnace, in a quantity sucient to short-circuit, in the heated furnace, said opposite electrodic carbon and graphite surfaces of said cell and passing electric current through the short-circuited cell to raise the temperature still further. in a subsequent starting stage, the cell remaining enclosed during the said starting stages.

3. The process of claim 2, in which the temperature is raised to the order of magnitude of about 700 C. in the rst stage and of about 950 C. in the subsequent stage of heating.

4. The process of claim 2 in which an inert atmosphere is maintained in the furnace during at least the iirst two starting stages, the electric non conductive refractory furnace lining having been previously impregnated with pitch which has been successively cokied.

5. The process of claim 2, the amount of molten aluminum so introduced being suiiicient only to cover a limited lower part of the opposed anodic and cathodic surfaces.

6. A process for starting a multicell furnace employed for electrolytic aluminum production, said furnace hav ing a closed necklace of cells and providing a closed fused salt-bath circulation path contained therein, and having the cells comprising electrodic carbons including self-restoring anodic surface assemblies, characterized in that said starting is carried out, while the furnace cells are closed, by heating in a rst stage by means of auxiliary electric resistors arranged externally of the empty cells, until all the cells and chambers of the furnace have attained temperatures of the order of magnitude of at least about 700 C., and by heating in a subsequent stage to a temperature of the order of magnitude of at least about 950 C., by pouring in the course of the subsequent stage, into the individual aluminum tapping pockets with which each individual cell of the furnace is provided, a quantity of fused aluminum sufficient to bridge and shortcircuit with one another the electrodic carbons belonging to the cells, and subsequently applying voltage to the furnace to pass an electric current through the cells of the furnace, continuing the electrical heating of the furnace, then introducing fused bath at at least one point of the top part of the furnace until the bath and the aluminum in all the cells and chambers of the furnace hasattained the required operating levels and While thus eliminating the aluminum bridges between the electrodes contemporaneously and gradually increasing the voltage of the electrolysis furnace until the operating voltage is attained, circulating the bath, feeding in measured amounts of alumina, discharging electrolytic gases, shutting olf the current from the auxiliary resistors, and initiating the normal electrolysis process.

7. A process according to claim 6, characterized in that the auxiliary resistors are at least in part removable and utilizable for the starting of other furnaces.

8. A process according to claim 6, characterized in that there occurs a gradual increase of voltage drop during and following the addition of bath, the operation being carried on in such a way as to prevent the amperage from descending below that required for normal operation.

9. A process according to claim 6, characterized in that the device lfor lifting the bath is` actuated before the bath, having reached the individual cells and chambers of the necklace-type circuit, surpasses therein sensibly the levels of normal operation, including that of the lower terminal chamber wherein said device is accommodated and operating.

10. A process according to claim 6, characterized ,in that the feeding in of alumina is actuated a short time after the bath has reached the zone where the alumina is fed.

l1. The process of claim 6, the auxiliary resistors being inserted into the tapping pockets, and energized, and being removed therefrom prior to introducing fused aluminum into lthe pockets, said introduction being in the course of the said subsequent heating stage.

12. The process defined in claim 6, the quantity of aluminum introduced into the individual tapping pockets being such that the level in the pocket and that in the corresponding cell are not more than 4slightly above the lower ends of the electrodic carbons, which carbons extend in an upward-downward direction.

References Cited in the le of this patent UNITED STATES PATENTS 2,593,741 Ferrand Apr. 22, 1952 2,742,414 Itoh Apr. 17, 1956 2,761,830 Kibby sept. 4, 1956 2,804,429 Wieugei Aug. 27, 1957 -hm H).

Claims (1)

1. IN A FURNACE PROCESS OF PRODUCING ALUMINUM BY ELECTROLYSIS OF ALUMINA IN AN ENCLOSED FUSED SALT BATH, IN WHICH PROCESS ELECTRIC CURRENT IS PASSED FROM AN ANODIC CARBON SURFACE THROUGH AN UPWARDLY-DOWNWARDLY EXTENDING ELECTROLYSIS GAP CONTAINING ALUMINA AND SAID FUSED SALT AND THENCE THROUGH A CATHODIC CARBON SURFACE, SAID GAP AND SAID SURFACES FORMING A CELL, AND IN WHICH PROCESS THE ALUMINUM PRODUCED IS COLLECTED BELOW, THE IMPROVEMENT IN THE METHOD OF STARTING SAID PROCESS BEFORE FEEDING BATH LIQUID INTO THE CELL, COMPRISING DISPOSING HEATING DEVICES IN THE FURNACE, BUT OUTSIDE THE CELL, TO RAISE THE TEMPERATURE IN A FIRST STAGE, SAID TEMPERATURE BEING BELOW ELECTROLYSIS OPERATING TEMPERATURE, AND POURING MOLTEN ALUMINUM INTO THE FURNACE, IN A QUANTITY SUFFICIENT TO SHORT-CIRCUIT THE ELECTRODE CARBON SURFACES OF SAID CELL AND PASSING ELECTRIC CURRENT THROUGH THE SHORT-CIRCUITED CELL TO RAISE THE TEMPERATURE STILL FURTHER IN A SUBSEQUENT STAGE.

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