US3090744A - Electrolytic furnace for producing aluminum having a crust breaking apparatus - Google Patents

Description

May 21, 1963 P. MULLER ETAL 3,9 0,744

ELECTROLYTIC FURNACE FOR PRODUCING ALUMINUM HAVING A CRUST BREAKING APPARATUS 5 Sheets-Sheet 1 Filed Dec. 27, 1950 r m m TEMN N m m U m WM w 1 5 {A Mr 22 a w M/ May 21, W63 P. MULLER ETAL 3,090,744

ELECTROLYTIC FURNACE FOR PRODUCING ALUMINUM HAVING A CRUST BREAKING APPARATUS 5 Sheets-Sheet 5 Filed Dec. 27, 1960 INVENTORSJ P404 MULAE/P J ,x4/v/v$ Abs 44777 BY #05527 MTf/VE F f P. MULLER ET AX. TIC FU May 21, 1963 ELECTROL Y RNACE FOR PRODUCING ALUMINUM HAVING A CRUST BREAKING APPARATUS 5 Sheets-Sheet 4 Filed Dec. 27, 1960 A TORNE S- 3,090, 744 LUMINUM US May 21, 1963 P. MULLER EIAL ELECTROLYTIC FURNACE FOR PRODUCING A HAVING A CRUST BREAKING APPARAT Filed Dec. 27, 1960 5 Sheets-Sheet 5 Elite States Patent 3,690,744 ELECTROLYTIC FURNACE FUR PRQDUCENG ALUMINUM HAVlNG A CRUST BREAKING APPARATUS Paul Miiller, Kusnacht, Zurich, Switzerland, and Johannes Schmitt and Hubert Wittner, Rheinfelden, Baden, Germany, assignors to Aluminium-Industrie-Alstien-Gesellschaft, Chippis, Switzerland, a joint-stock company of Switzerland Filed Dec. 27, 1960, Ser. No. 78,346 Claims priority, application Switzerland Dec. 29, 19:9 (Ilaims. (Cl. 294-243) In continuous attempts to improve the efl'iciency of the production of aluminum by electrolysis in furnaces with fused fluoride salt baths the furnaces have been built in recent decades to work with ever-increasing current densities and have been more and more mechanized.

Thus in furnaces with self-baking anodes, the so-called Siiderberg anodes, the conveying of the fresh carbon mass to the furnace and its application to the anode has been extensively mechanized. Likewise in furnaces with pre-baked anodes, the conveying of fresh anodes to the furnaces and of the anode residues to be removed from the furnace has been more and more mechanized.

Various devices for withdrawing lateral or vertical current-supply bolts from or inserting them in self-baking anodes have also been developed.

The breaking of the crust above the melt, as well as the supply of fresh alumina after the breaking and rehardening of the surface of the melt especially has been extensively mechanized.

The crust, upon which the alumina is charged from time to time, is broken when anode polarization, the socalled anode effect, occurs. This effect is made apparent by the considerable increase in voltage necessary to maintain the operating current. When the anode effect occurs the crust, on which further alumina has been placed, is broken and the fresh alumina is stirred into the melt. The crust may also be broken one or more times for the addition of more alumina when no anode effect is observed, so as to increase the time interval between successive anode effects.

For some decades crust-breakers operated by compressed air have generally been used. These are movable, involve the use of manpower and serve to break the crusts of furnaces in succession.

After the breaking of the crust and as soon as the surface of the melt has solidified once more, alumina is applied. Today the alumina is mostly mechanically conveyed to hoppers over the furnace, from which it passes onto the crust.

In spite of the use of these mechanical aids for breaking the crust and applying the alumina, the work involved in these operations is still comparatively high in com parison to that required for charging Siiderberg anodes, the interchange of prebaked block anodes and the removal and insertion of current-supply studs. The reason for this is that the crusts of the furnaces must be broken comparatively frequently (about every 2 to 4 hours) and correspondingly frequently charged with alumina, and that man power is still used today for the control of both the crust-breaking machines and the filling and discharging of the alumina hoppers.

In the furnace with pre-baked carbon electrodes the crust is broken at the edge of the melt, and also between the individual electrodes, so that the melt is well stirred. In furnaces with Siiderberg electrodes the electrode covers the whole central area of the melt, so that the electrolyte layer is less accessible and can only be attacked around the periphery of the electrode, where the crust is always very thick and solid. Because of this, it has 3,69%,744 Patented May 21, 1963 been proposed to make working openings in a Stiderberg electrode or to make this in two parts semi-circular in cross-section, which are separated from each other by a gap.

Furnaces are also known in which two rows each composed of pre-baked anode blocks pushed closely together are so arranged that a longitudinal gap remains between the rows in the longitudinal axis of the furnace, and above this gap there are alumina containers from which batches of alumina can be released onto the crust by manual operation of a handle. At each end there are rollers from which long rods with bent ends extend through the gap as far as the middle of the furnace, these rods being moved manually to break the crust. Thus the crustbreaking and the supply of alumina are manually controlled.

Although comparatively favourable operating results are obtained, these types of furnaces show substantial disadvantages. The work required to break the crust is particularly high. The gap between the anode rows must be kept comparatively narrow, in order to avoid the risk that two independent pools of metal form in the furnace. As a result the amount of alumina which enters the furnace melt after each breaking operation must also be kept relatively small.

Although in furnaces with pre-baked anode blocks, manual crust-breaking takes place often, say every two hours, as well as when the anode effect occurs, it often happens that the alumina is unevenly supplied over the length of the space between the two rows of anodes. Thus it often happens that pieces of crust too heavily heaped up with alumina enter the furnace melt. The density of a furnace melt with a high content of alumina is greater than that of the liquid metal, and the pieces of crust with a high alumina content therefore sink to the bottom in the gaps between the two anode rows, where in time they form an exceedingly hard, electrically insulating bottom coating. As a result the current flowing from the anodes into the melt is forced to the sides of the furnace, the furnace begins to go hollow, i.e. the electrolyte coating that has solidified on the pot walls and finally also the carbon lining are hollowed out in their lower part, and the current efficiency usually falls considerably in time; the highest current efiiciency obtainable may be no more than 87%.

To avoid these disadvantages these furnaces are often provided with particularly thin carbon linings and the distance between each anode and carbon lining is kept as small as possible. In this way the sides of the bath are heavily cooled, the carbon linings become coated with a thin solidified crust of melt, which is a good electrical insulator and which causes the current to flow from the anodes at first predominantly vertically. However, with the formation of the bottom coating deposit in the middle of the furnace the lateral deviation of the current at the outer anode corners increases with the time, so that the carbon linings are nevertheless comparatively quickly hollowed out. Because of this phenomenon the cathode pots of these furnaces must be exchanged'or renewed frequently, that is to say at least every 1 /2 years, if the furnace is to work satisfactorily. In contrast the life of the cathode pots of furnaces the outer sides of which are normally lined is from three to four years.

It also has been proposed to make the breaking of the crust automatic. Thus German patent specification No. 895,379 discloses a rotary roller with working teeth of heat, resistant material such as quartz, special ceramic compositions or the like, in the middle of the furnace. At each anode effect the increase in voltage causes a driving motor to set the roller in motion. This apparatus, however, occupies a very large space in the middle of the furnace. In addition at least one tooth always remains immersed in the melt when the apparatus is not working, so that the roller must consist of a material that resists very strongly corrosive fluoride elec trolytes at the working temperature of about 990 C. Such materials are not only very expensive, but also brittle.

I According to our invention the anode area of the furnace is divided into two parts with a gap between them and a mechanically operated crust-breaker works substantially vertically in and extends over the whole length of the gap. The anodes may be of the usual Stiderberg type or self-baking Soderberg anodes or may be prebaked and by the anode area we mean the horiiontal surface covered by the anodes together with any spaces between them. The invention is particularly applicable to furnaces of elongated rectangular or elliptical shape, and preferably the gap extends along the longitudinal centre line.

The crust breaker preferably comprises one or' more beams extending throughout the gap and having downwardly directed teeth.

Preferably also the supports and operating mechanism or mechanisms for the crust-breaker are situated on the sides of the furnace outside the anode area. This not only protects them from the furnace Waste gases and the heat but also leaves the gap over the crust-breaker free for the installation of exhaust gas collecting devices and advantageously also one or more supply hoppers for alumina.

The crust breaker should be so constructed and mounted that not only is the crust surface broken, but also the resulting crust fragments are thrust into the liquid bath. In the breaking process the alumina lying on the crust falls into the melt so that the concentration of alumina in the melt is increased. After breaking of the crust an exactly measured amount of alumina may be discharged from the hopper or hoppers preferably under automatic control, onto the reformed crust.

Some furnaces according to our invention are shown in the accompanying drawings in which:

FIGURE 1 is a side view, partly in section, of a furnace with pre-baked carbon anodes;

FIGURE 2 is a similar view of a furnace with Stiderberg anodes and lateral current-supply bolts;

FIGURE 3 is a similar view of a furnace with Sederberg anodes and vertical current-supply bolts;

FIGURE 4 is a central longitudinal section through a furnace, showing one form of crust-breaking device; and

FIGURE 5 is a section similar to FIGURE 4, showing an alternative form of crust-breaker, but not showing the anode in detail.

The electrolytic furnaces shown in the drawings each comprises a pot 1, which is lined with refractory bricks 2 and 3. Cathodic current-supply bars 4 are embedded in the carbon lining 3. The pot contains a layer 5 of separated molten aluminum and a bath 6 of molten fluoride, the surface of which solidifies to a crust 7. The anode part of the furnace is supported on a framework 8 which includes upper cross-beams carrying lifting mechanism 9, operated by a motor 14) through gearing 11, cross-shafts 12 and longitudinal shafts 13. This lifting mechanism 9 actuates lifting and lowering shafts 14-, which carry an anode carrier 15 and anodic conductors 16. Anode rods 17 fixed to the anode carrier 15 are connected to the conductors 16, and to current-supply bolts or studs 20 (FIGURE 1), 21 (FIGURE 2) or 22 (FIGURE 3) embedded in anodes 13 (FIGURE 1) or 19 (FIGURES 2 and 3), and thus both support the anode or anodes and supply electric current. By raising or lowering the anode carrier '15 through the motor 16 the anodes can be adjusted to such a height that the desired potential difference exists between the anodes and the cathodic bottom of the pct.

In the furnace shown in FIGURE 1 the anodes 18 are prebaked blocks arranged in two rows running in the longitudinal direction of the furnace to leave a gap 23 between them. The width of this gap is preferably from 8 to 28 inches. The anode area is therefore divided in two halves and a crust-breaker is mounted to work in the gap 23 between them. This crust-breaker consists of two beams 24 and 25 which carry crushing teeth 26. The beams of the crust-breaker are movable vertically and in the drawings they are shown in the working position. When the crust breaker is working one beam lies in its highest position while the other is in its lowest position; the position is subsequently reversed, as is shown in broken lines. When the crust is not being broken both beams are held in the raised position, so that the tips of the teeth do not lie permanently in the strongly corrosive electrolyte melt. Alternatively both the beams 2 and 25 may be simultaneously raised and lowered in operation.

An alumina hopper 27 lies above the central gap 23 and has bottom outlets 28 which can be opened or shut as desired.

The furnaces shown in FIGURES 2 and 3 each have two self-baking anodes 18 separated from one another longitudinally to leave a gap 23. The anodes are surrounded by a casing 29, partly double-walled, which is suspended from the frame-work 8 by rod 39 and forms a gas-collecting hood 31 around the anode area. In the furnace shown in FIGURE 2 the current-supply studs 21 enter the anodes laterally, and to allow them to move downwards with the anode vertical slits are provided in the casing 29. With the exception of its upper part, which forms a continuous gas-collecting channel, the anode casing is therefore not continuous, but consists of individual box-like vertical chambers in which the exhaust gases flow upwards into the upper channel and between which the studs 21 are arranged. In FIGURE 2 one side wall of such a vertical chamber is shown behind the studs 21. In the furnace shown in FIGURE 3 the current-supply bolts 22 are arranged vertically.

The alumina hopper 27 (shown in FIGURES 2 and 3) discharges into two passages 36 at the sides of the gap 23. This hopper has a double bottom for the admission of compressed air to fiuidise the alumina, the upper bottom consisting of porous'plates 32. When compressed air is admitted into the space between the Ibottom through pipes 33 and jets 34, it passes through the porous plates 32 and loosens the alumina in the hopper, so that the alumina flows freely through perforated walls 35 and out through the passages 36.

The crust-breakers used in the invention can take various forms, two of which .are shown in FIGURES 4 and 5.

The crust-breaker shown in FIGURE 4 consists of two lever-like beams extending over and beyond both ends of the anode area, but only the beam 25 is shown, This beam 25 is carried at one end of the furnace by a pivot shaft 37 mounted in the furnace frame and connected at its other end to a ram 38 which can be hydraulically or pneumatically operated and which is carried by a flange 39 on the furnace frame. The beam 25 carries teeth 26 only on that half of the beam remote from the shaft 37, since the other half, near the shaft 37, has only a relatively small stroke. The beam is slightly bent at its centre so that the teeth may lie in a substantially horizontal plane when the beam is lowered and hence can thrust the crust into the molten electrolyte to an even depth. A stop 49 prevents the teeth from dipping too far into the bath. The beam 25 is shown in dotted lines in its raised or rest position. A catch 41 is provided to latch the beam in this position. The second beam 24 is identical to the beam 25, but is pivoted at the opposite end of the furnace so that its teeth break the crust below that part of the beam 25 which does not carry teeth.

The gap between the anode is closed at its ends by a metal hood 42 to collect the exhaust gases escaping between the two anodes. The exhaust gases are burnt in the chimney 43 to which air is supplied and are led to a purification plant. The discharge outlets of the gascollecting hoods 31 also discharge into this chimney 43. It is however also possible to install a collecting channel corresponding to the collecting channel 31 and thus leave the gap 23 open at both ends so that, if necessary, the inner walls of the anode can be kept cooler.

The crustbrea'ker shown in FIGURE 5 has a straight beam 25 extending over and projecting beyond both ends of the whole anode area, and this is connected at each end to a ram 38. This beam 25 can thus be moved vertically over its whole length through the same distance and is therefore provided with teeth 26 over its whole useful length. The anode part of the furnace is merely outlined in FIGURE 5.

The crust-breakers shown in FIGURES 4 and 5 can be used in any of the furnaces shown in FIGURES 1, 2 and 3.

With the driving mechanism shown in FIGURES 4 and 5 the crushing teeth can be moved practically purely vertically. The beam may also be mounted for movement across or along the gap between the anodes in order better to agitate the bath in addition to breaking the crust. For this purpose an eccentric drive may for example be used in combination with the ram or each ram to produce three-dimensional movement of the teeth.

The voltage in the furnace can be controlled by known methods, for example by regulating the eifective voltage to the estimated voltage. The voltage is regulated by adjusting the distance between the anodes and the bottom of the cathode pot.

The furnaces according to our invention are particularly suitable for automatic control by a programme controller. This controller may control the movements of the crustbreakers, the addition of the alumina and the regulation of the furnace potential, in particular re-adjustment of the estimated potential after the breaking of the crust.

In operation, the sequence of events is as follows: When an anode effect occurs, the crust-breaker is set in motion and the crust is broken into pieces by the crushing teeth. There should be enough teeth on the crustbreaker to ensure that the crust is broken into small fragments. When the crust has been broken and the alumina on it dissolved in the melt the voltage across the electrodes is adjusted once more to the desired value. As soon as a crust has again formed more alumina is discharged from the hoppers. The amount of alumina discharged is controlled as accurately as possible to ensure the desired concentration in the electrolyte. The alumina should be separated evenly over the crust to facilitate an even distribution of it in the electrolyte, and to avoid local high concentrations in the electrolyte. Early application of the layer of alumina to the crust helps to prevent the crust from cooling, thus keeping it relatively soft, and preheats the alumina before it enters the molten electrolyte.

The driving mechanism of the crust-breaking device may be set in motion and stopped under manual control by a push-button system, but we prefer to control the operation automatically by means of a programme controller. The programme controller advantageously controls the time intervals between individual breaking operations, the duration of each breaking operation, (cg. /23 min), the time interval between the end of the breaking operation and the beginning of the alumina addition (e.g. /2 to 5 min), the duration of the flow of alumina from the hoppers, and also the furnace voltage. This cycle of operations may conveniently be set in motion as a result of the increase in voltage through the furnace voltage controller when an anode efiect occurs. The cycle is conveniently repeated at intervals of e.g. 30 min., 1 hour, 1 /2 hours or 2 hours, the furnace voltage also being adjusted each time. It is generally desirable that an anode effect should occur at least once a day, and if one does not occur naturally, it maybe induced through the programme controller by reducing or shutting off the alumina additions. On the other hand, if anode efiects become too frequent the controller automatically increases the amount of alumina added to raise its concentration in the molten electrolyte. For safety, however, each furnace should be provided with push-button controls in addition to automatic control.

Although it is possible to break the crust and add alumina continuously during the electrolytic process, such a procedure would entail considerable additional wear on the apparatus (in particular corrosion of the crustbreaker), but by repeating the cycle of operations as frequently as every /2 to 2 hours fluctuations of temperature of the molten electrolyte bath can be substantially reduced. Thus by automatic control of furnaces accord ing to the invention we find that it is possible to maintain the bath temperature and the concentration of alumina in the bath within narrow limits, within which optimum results can be obtained.

The alumina concentration in the electrolyte should be at least 3% by weight, and preferably at least 3.5%. Although it is possible to increase the concentration of alumina to 8 to 10%, we obtain the best results at concentration of 3.5 to 5%, at which concentration we have obtained current yield values of 90 to 96%.

We have found that the specific energy consumption can be reduced by about 0.5 to l kWh/kg. aluminum in furnaces according to our invention. The manual labour involved in all operations concerned with working furnaces can also be reduced by about 25 to 50%, depending on the type of furnace used.

According to a further improvement of our invention, the side lining of the pot is not made by damming the usual carbonaceous mass, but by damming a mixture of 40 to 85 percent (of weight) powdered silicon carbide, 45 to 7 percent coke powder and 15 to 8 percent pitch, which mixture has a very low conductivity for heat and for electric current. Preferably We use a mixture of the following composition:

Percent of weight Powdered silicon carbide -80 Powdered coke 15-10 Medium-hard pitch 1510 Very suitable is a mixture of percent silicon carbide, 14 percent coke and 11 percent medium-hard pitch. Of course, the mixture may contain supplemental substances, if the same do not impair the results.

The particle size of the silicon carbide should be between 0 and about 6 mm., the particle size of the coke between 0 and about 22 mm. The components are mixed suitably at a temperature between 50 and 250 C.

in a furnace provided with the side linings containing silicon carbide it is no more necessary that the same become coated with solidified crust of electrolyte, as these silicon carbide containing linings have a very low electric conductivity and do not become impregnated with electrolyte. The said linings are not attacked by the electrolyte and remain warm because of their low heat conductivity. Therefore the horizontal size of the electrolyte melt does not change during the operation of the furnace. This is particularly advantageous with the furnaces according to our invention.

What we claim is:

1. In an electrolytic furnace for the production of aluminum with the use of a molten salt bath, the combination comprising a frame, anodes for said furnace, means supporting said anodes from said frame in two rows separated by a gap therebetween extending along the full length of the region occupied by said anodes and directly above the area where a crust is formed on the electrolyte, and, a crust breaker comprising one or more substantially hori zontal beams extending in and along substantially the full length of said gap and provided with substantially vertically downwardly directed teeth distributed substantially along the full length of said gap, and means supported from said frame for operating each beam substantially vertically at least at one end through successive strokes, comprising means for moving said beam during each stroke of a cycle from a position in which the teeth on the beam are located above the crust downwardly to a position in which the teeth perforate and break the crust and for moving the beam during the next stroke of the cycle upwardly to the first-mentioned position, and means for bringing the beam to rest in said first-mentioned position.

2. In an electrolytic furnace for the production of aluminum from alumina with the use of a molten salt bath, the combination comprising a frame, anodes for said furnace, means supporting said anode in two rows separated by a gap therebetween extending along the full,

length of the region occupied by said anodes, and a crust breaker comprising two horizontal beams inthe form of levers extending side by side in and along substantially the full length of said gap and provided with substantially vertically downwardly directed teeth distributed substantially along the full length of said gap, each of said beams being pivoted at one end, and means supported on said frame and connected to the other end of the beam for operating each beam substantially vertically at said other end through successive strokes from a position in which the teeth on the beam are located above the crust to a position in which the teeth perforate and break the crust and then back upwardly to said first-mentioned position, in which latter position the beam may be brought to rest.

3. In an electrolytic furnace for the production of aluminum from alumina with the use of a molten salt bath, the combination comprising a frame, anodes for said furnace, means supporting said anode in two rows separated by a gap therebetween extending along the full length of the region occupied by said anodes, and a crust breaker comprising two horizontal beams in the form of levers extending side by side in and along substantially the full length of said gap and provided with substantially vertically downwardly directed teeth distributed substantially along the. full length of said gap, each of said beams being pivoted at one end, and means supported on said frame and connected to the other end of the beam for operating each beam substantially vertically at said other end through successive strokes from a position in which the teeth on the beam are located above the crust to a position in "which the teeth perforate and break the crust and then back upwardly to said first-mentioned position,-

4. In an electrolytic furnace for the production of aluminum with the use of a molten salt bath, the combination comprising a frame, anodes for said furnace, means supporting said anodes from said frame in two rows separated by a gap therebetween extending along the full length of the region occupied by said anodes, anda crust breaker comprising one or more substantially horizontal beams extending in and along substantially the full length of said gap and provided with substantially vertically downwardly directed teeth distributed substantially along the [full length of said gap, and means supported on said frame and connected to both ends of each beam for operating each beam substantially vertically through successive strokes from a position in which the teeth on the beam are located above the crust to a' position in which the teeth perforate and break the crust and then back upwardly to said first-mentioned position, in which latter position the beam may be brought to rest. 7

5. In an electrolytic furnace for the production of aluminum with the use of a molten salt bath, the combination comprising a frame, anodes for said furnace, means supporting said anodes from said frame in two rows separated by a gap therebetween extending along the full length of the region occupied by said anodes from one end of said region to the other, and a crust breaker comprising one or more substantially horizontal beams extending in and along substantially the full length of said gap and provided with substantially vertically downwardly directed teeth distributed substantially along the full length of said gap, means located beyond said anode region and beyond said gap for operating each beam substantially vertically at least at one end through successive strokes from a position in which the teeth on the beam are located above the crust to a position in which the teeth perforate and break the crust and then backwardly to said first-mentioned position, andim'eans for bringing the beam to rest in said first-mentioned position.

References Cited in the file of this patent UNITED STATES PATENTS 2,713,024 Mautovanello July 12, 1955 FOREIGN PATENTS 631,358 France Dec. 19, 1927 45,694 Norway Oct. 15, 1928 895,379 Germany Nov. 2, 1953 530,221 Italy July 6, 1955 193,619 Austria Nov. 25, 1957

Claims (1)

1. IN AN ELECTROLYTIC FURNANCE FOR THE PRODUCTION OF ALUMINUM WITH THE USE OF A MOLTEN SALT BATH, THE COMBINATION COMPRISING A FRAME, ANODES FOR SAID FURNANCE, MEANS SUPPORTING SAID ANODES FROM SAID FRAME IN TWO ROWS SEPARATED BY A GAP THEREBETWEEN EXTENDING ALONG THE FULL LENGTH OF THE REGION OCCUPIED BY SAID ANODES AND DIRECTLY ABOVE THE AREA WHERE A CRUST IS FORMED ON THE ELECTROCLYTE, AN A CRUST BREAKER COMPRISING ONE OR MORE SUBSTANTIALLY HORIZONTAL BEAMS EXTENDING IN AND ALONG SUBSTANTIALLY THE FULL LENGTH OF SAID GAP AND PROVIDED WITH SUBSTANTIALLY VERTICALLY DOWNWARDLY DIRECTED TEETH DISTRIBUTED SUBSTANTIALLY ALONG THE FULL LENGTH OF SAID GAP, AND MEANS SUPPORTED FROM SAID FRAME FOR OPERATING EACH BEAM SUBSTANTIALLY VERTICALLY AT LEAST AT ONE END THROUGH SUCCESSIVE STROKES, COMPRISING MEANS FOR MOVING SAID BEAM DURING EACH STROKE OF A CYCLE FROM A POSITION IN WHICH THE TEETH ON THE BEAM ARE LOCATED ABOVE THE CRUST DOWNWARDLY TO A POSITION IN WHICH THE TEETH PERFORATE AND BREAK THE CRUST AND FOR MOVING THE BEAM DURING THE NEXT STROKE OF THE

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