US3186927A - Process and apparatus for the continuous feeding of electrolytic aluminum cells - Google Patents

Description

June 1965 G. MANTOVANELLO 7 PROCESS AND APPARATUS FOR THE CONTINUOUS FEEDING OF ELECTROLYTIC ALUMINUM CELLS Filed April 20. 1960 5 Sheets-Sheet 1 FIG.I

June 1955 G. MANTOVANELLO 3,186,927

PROCESS AND APPARATUS FOR THE CONTINUOUS FEEDING OF ELECTROLYTIC ALUMINUM CELLS Filed April 20. 1960 i5 Sheets-Sheet 2 June 1, 1965 G. MANTOVANELLO PROCESS AND APPARATUS FOR THE CONTINUOUS FEEDI OF ELECTROLYTIC ALUMINUM CELLS 5 Sheets-Sheet 3 Filed April 20. 1960 FIG. 5

J1me 1965 s. MANTOVANELLO 3,186,927

PROCESS AND APPARATUS FOR THE CONTINUOUS FEEDING OF ELECTROLYTIC ALUMINUM CELLS Filed April 20. 1960 5 Sheets-Sheet 4 a i i 51: 1 111 Em l 1 E :NH N Q EM 3 I- l a; L y 5; N s; I ll :I Q Ill u -1 :gl W M II Hf ih 5f 4 g J1me 1955 G. MANTOVANELLO 3,186,927

PROCESS AND APPARATUS FOR THE CONTINUOUS FEEDING OF ELECTROLYTIC ALUMINUM CELLS Filed April 20, 1960 5 Sheets-Sheet 5 United States Patent 3,186,927 PRGEIESS AND APPARATUd FQR THE CONTENU- GUS FEEDING GE ELEQTRQELYTIC ALUMENUM CELLS Giovanni Mantovanello, Bolzano, Etaly, assignor to lviontecatini Societa Generate per llndustria Mineraria e Chi-mica, Milan, Italy Filed Apr. 20, 1960, Ser. No. 23,421 Claims priority, application Italy, Apr. 24, 1959, 6,964/ 59 4 Claims. (Cl. 2194-67 My invention relates to a process and apparatus for the continuous feeding of cells for fused bath electrolysis of aluminum oxide, and more particularly to a method and means for adjusting the location at which the oxide is introduced through the crust into the bath.

It is known that in operating cells for the production of aluminum by the electrolytic reduction of alumina in a bath of fused cryolite or fluorides, the feeding of alumina is generally carried out in a discontinuous manner at the time at which the anod-ic effect occurs. Ordinarily the feeding is carried out by breaking the frozen top crust of the bath with a tool and introducing a predetermined charge of alumina previously loaded onto said crust.

The anodic effect occurs in electrolytic cells when the concentration of alumina dissolved in the fused bath drops below a certain critical limit; so that the resistance to the passage of electric current increases and the voltage of the cell, which in normal operation is generally between 4 and volts, rises to values of about to volts.

With such a discontinuous feeding operation the concentration of aluminum oxide in the fused electrolytic bath varies continuously between a maximum and a minimum, from the instant when the alumina i introduced immediately after occurrence of the anodic effect, to the instant when the anodic effect reappears. This causes higher consumption of energy and smaller current elficiency than would be attainable if the composition of the bath could be kept constant by introducing the alumina into the bath continuously.

An attempt has been made in the past to feed the alumina into the cell at a time prior to the occurrence of the anodic effect, taking into account the time necessary for the electrolysis of the alumina solution. This method is unsatisfactory because at the feeding moment the pre vious charge of alumina may not yet be fully dissolved, and thus the anodic effect may happen earlier than foreseen; and also because the charge of alumina introduced out of time may not dissolve because it exceeds the solubility of alumina in the fused bath at the working temperature and hence may precipitate to the bottom of the cell, thus causing irregular operation.

The best Way of overcoming these deficiencies is to introduce the alumina continuously into the fused bath in a quantity corresponding to that being electrolyzed in the cell, so as to maintain a constant percentage of dissolved alumina. However, most of the methods so far proposed have failed to produce a useful result because they involved simply dropping the alumina onto the surface of the bath. Even if such gravity feeding is performed at the hottest or heat-insulated points of the cell, the entrance of alumina, pre-heated or not, is opposed by the formation of a top crust on the bath, so that intervention from the outside is necessary. At best, this method can provide a semi-continuous feed but remains affected by the abovedescribed disadvantages.

U.S. Patent No. 2,713,024 of G. Mantovanello, issued July 12, 1955, discloses a process for continuous feeding of aluminum oxide into electrolytic cells for the production of metallic aluminum. In that disclosure, the aluminum oxide is continuously introduced through a top crust assess? Patented June 1, i965 into the .bath of the cell by mechanical thrust means, such as by a screw-type thrust feeder, under a continuous pressure sufficient to overcome the mechanical resistance of the superficial crust so that the alumina is forced to penetrate in a continuous and regular manner into the molten hath portion.

Although the process disclosed in the above-mentioned patent is quite satisfactory, certain difi'icult'ies may arise which can be troublesome in maintaining a continuous operation. Among these difficulties are:

(1) An accumulation of alumina may occur over the top crust covering the electrolytic bath, in the formation of heaps of the alumina feed material, either over the crust or at the exit of the screwatype thrust feeder, or other thrust device. This drawback occurs when the terminal portion of the thrust feeder is placed at a height of more than a few centimeters, such as about 4 centimeters, over the molten bath.

(2) I am-min g may occur in the screw-type feeder which introduces the alumina into the hath. This inconvenience is experienced when a part of the screw-type thrust feeder is immersed into the molten bath, which then tends to solidify in contact with the screw feeder, thereby blocking the feeder assembly.

(3) Irregular consumption of the anodes may occur. When .the alumina is fed into the furnace always at the same location, namely in the immediate proximity of the electrode, this tends to cool down the electrode at the feed point owing to the continuous arrival of the cold alumina. The cooled-down part of the anode has a higher electrical resistance, hence it conducts less current and undergoes a burning-off below its normal rate. As a consequence, a formation of irregularities takes place at the electrode bottom.

it is therefore an object of the present invention to provide a method and means for eliminating the danger of the above-mentioned inconveniences occurring.

it is a more specific object of the present invention to secure a continuous and regular operation of electrolytic aluminum cells in a simple manner, and to provide means for adjusting the point of introduction of the powdered aluminum oxide into the bath.

To these ends, and in accordance with a feature of my invention, such adjustment is effected during the course of the feeding operation, by displacing, both in height and in plan, the point of introduction of the powdered material {aluminum oxide) being fed into the bath of the electrolytic cell; and suitable means are provided to effect such adjustment or displacement by means of horizontal translation of the feeding assembly along the sides of the cell electrode. Devices are also provided for vertical translation of the feeding assembly independent of those for horizontal translation, and these translatory displacements are preferably carried out by apparatus comprising essentially worm-type couplings or by guide tracks.

The above-mentioned and further objects, features, and many of the attendant advantages of this invention will be more readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawing, wherein:

FIG. 1 is a simplified view in longitudinal section through an electrolysis furnace and anode, showing a device according to the invention for the suspension of the feeder mechanism from the anode casing and its arrangement for horizontal and vertical shifting movement.

FIG. 2 is an enlarged view partly in section, of a portion of FIG. 1, showing the suspension device in greater detail.

FIG. 3 is an end view, partly in section, of the device of FIG. 2.

FIG. 4 is a fragmentary end view, similar to FIG. 3, but of another embodiment of the suspension device for moving the feeder mechanism transversely.

FIG. 5 is an enlarged perspective view of details of the embodiment of FIG. 4; and

FIGS. 6 and 7 illustrate how separate carriages may be movably mounted on each side of the anode casing, FIG. 7 being an end view taken in the direction VIIVII of FIG. 6.

The electric furnace cell shown in FIG. 1 has the cavity of its lined vessel 33 filled with a quantity of a fused fluoride bath 24. Partially submerged in the bath is an electrode 31 enclosed within an anode casing 7. The bath is supplied with comminuted alumina or aluminum oxide from a tank or feed hopper 5. The alumina is discharged from the hopper 5 by means of a screw-type measuring device 6 into a pipe 6a and is injected into the bath 24 by means of a screw-type thrust feeder I. The feeder assembly is actuated by an electric motor 3, which drives the measuring device 6 and the screw 25 through a gear box 4 and through a power transmission chain or coupling 29. A metallic feed pipe 30 containing the screw 25 extends down to a height h (FIG. 3) in the order of several centimeters above the highest level of the surface of the bath 24, taking into account the fluctuations in bath level. Such a spacing h, of about 4 centimeters, is sufiicient to prevent contamination of the bath by the pipe 30 and is also small enough for the screw (or other pressure or thrust means) to impose on the feed material the necessary thrust pressure for penetrating through the bath crust.

The level of the molten bath 24 may vary during the operation of the electrolytic cell for various reasons, including variations in temperature, rise of the cell bottom, etc. To make allowance for such rise in bath level, the pipe 30 containing the screw 25 is made adjustable in height or elevation. In addition to such adjustment in elevation, adjustment of the feeding location may also be made in plan, i.e., in a longitudinal or transverse direction or both.

According to the embodiment of FIGS. 1-3 the continuous feeding assembly, comprising the motor 3, the reduction gear 4, the hopper 5 for the alumina, the measuring screw feeder 6, and the screw-type thrust feeder 1, are all mounted on a supporting frame or carriage 2. However, the frame 2 is not fixed to the anode casing 7, but according to the invention is supported by four small wheels 8, journalled on frame 2 and mounted so as to roll within two guide rails 9 so that the entire feeding assembly is carried along on carriage 2. The respective ends of guide rails 9 are vertically adjustable in height and are supported by two worm-gear type raising and lowering devices comprising worm screws 10 engageable for up and down movement within worm gears or nuts 11. A hand wheel 12 is fixedly attached to each Worm gear 11, which is supported on the upper surface of a guide journal 10a in which worm 10 is guided for vertical up and down movement. Journals 10a are fixedly mounted within respective brackets 13 which are fastened to the anode casing 7. Asuspension member 14 serves to connect beam 9 with the worm 10. Thus, joint rotation of the left and right hand wheels 12 causes the respective ends of the guide rails 9 to be raised or lowered, thus raising or lowering the entire feeder assembly suspended by frame'2 from beam 9. In this manner, the terminal or feeding end of the screw-type thrust feeder 1 can be easily adjusted to optimum level, so as to follow the variations in the'level of the rnoltenbath 24 during the operation of the electrolytic cell; adjusting the left and right hand Wheels 12 relative to each other, the guide rails 9 can be inclined-or tilted in one Moreover, by separately or opening at the bottom of thrust feeder 1 can be varied at will along the electrode 31, by moving the feedingdevice carriage along the guide rail 9.

According to another embodiment, illustrated in FIGS. 4 and 5, transverse shifting of the carriage is also possible. In this embodiment, the journals 10a, instead of being fixedly mounted within brackets 13, as in FIGS. 1-3, are movable, in a direction transverse to that of rails 9, along rails 18 which are fixedly mounted on brackets 13. Wheels '16 ride captively within channel members 18 and wheels 17 guide the member 10a and prevent twisting thereof by riding along the outer edge of transverse rails 18. In FIG. 5 the worm is shown as being prevented from movement downward by a collar 19 which prevents worm 10 from moving downwardly but allows it to be rotatable with respect to journal 10a. The worm nut 11' rises up or down on the worm 10' as the handwheel 1-2 is. turned, and carries with it the support member '14 and the pivot studs 15, from which the end of beam 9 is suspended.

V In this manner, as above described, conditions necessary for regular and continuous operation of the cell can be realized. For such regular operation and continuous feeding it is preferable that the terminal part of the screwtype thrust feeder I :be located at about four centimeters above the level of the molten bath (distance 11), and that the feeding location not always be in the same place. Since, as above mentioned, the level of the molten bath varies during the operation of the electrolytic cell for various reasons, these conditions can be realized and achieved by the apparatus and method above described.

As shown in FIGS. 6 and 7 two separate carriages 2 may be mounted on opposite sides of the electrode casing 7 for feeding the alumina into different portions of the bath 24.

It will be obvious to those skilled in the art, upon a study of this disclosure, that my invention permits of various modifications with respect to details not critical to the invention proper and hence can be given embodiments other than particularly illustrated and described herein, without departing from the essential features of the invention as set forth in the claims annexed hereto.

I claim:

1. In the production of metallic aluminum by the electrolytic reduction of alumina in a bath of fused electrolyte, the process which comprises maintaining the bath at crust-forming temperature conditions, continuously feeding alumina at a predetermined variable location through the crust into the bath at a feed'pressure sufficient to penetrate into said bath through said crust while simultaneously varying said location over the area of the bath surface in a substantially continuous manner during the electrolytic reduction operation, and varying said feeding location in height during said operation and in accordance with changes in elevation of said bath surface, to thereby prevent excessive accumulations of alumina at the feeding location and to avoid submersion of the feeding location into the bath.

2. In the production of metallic aluminum by the electrolytic reduction of alumina in a :bath of fused electrolyte', the process which comprises maintaining the bath at crust-forming temperature conditions, continuously feeding alumina'at a predetermined variable location through the crust into the bath at a feed pressure sufficient to penetrate into said bath through said crust while simultaneously varying said location over the area of the bath surface in a substantially continuous manner during the electrolytic reduction operation, and continuously varying said feeding location both in elevation and in plan trolytic reduction of alumina in a bath of fused electrolyte, the process which comprises maintaining the bath at crust-forming temperature conditions, feeding a continuous column of alumina through the crust into the bath at a predetermined variable location closely adjacent to the outside of the electrolysis electrode While simultaneously varying said location over the area of the bath surface in a substantially continuous manner during the electrolytic reduction operation, applying to the column a substantially continuous feed pressure sufficient to penetrate the crust, and continuously varying the position of said predetermined location during said operation and in accordance with bath conditions to prevent excessive accumulations of alumina at any one location and to avoid submersion of the feeding location into the bath.

4. A process according to claim 3, the position of said predetermined location being varied in accordance with the temperature of the bath in the vicinity of said location.

References (lite-cl by the Examiner UNITED STATES PATENTS 1,930,195 10/33 Eigenheer 204-245 2,713,024- 7/55 Mantovanello 204-67 FOREIGN PATENTS JOHN R. SPECK, JOSEPH REBOLD, JOHN H. MACK,

Examiners.

Claims (1)

1. IN THE PRODUCTION OF METALLIC ALUMINUM BY THE ELECTROLYTIC REDUCTION OF ALUMINA IN A BATH OF FUSED ELECTROLYTE, THE PROCESS WHICH COMPRISES MAINTAINING THE BATH AT CRUST-FORMING TEMPERATURE CONDITIONS, CONTINOUSLY FEEDING ALUMINA AT A PREDETERMINED VARIABLE LOCATION THROUGH THE CRUST INTO THE BATH AT A FEED PRESSURE SUFFICIENT TO PENETRATE INTO SAID BATH THROUGH SAID CRUST WHILE SIMULTANEOUSLY VARYING SAID LOCATION OVER THE AREA OF THE BATH SURFACE IN A SUBSTANTIALLY CONTINUOUS MANNER DURING THE ELECTROLYTIC REDUCTION OPERATION, AND VARYING SAID FEEDING LOCATION IN HEIGHT DURING SAID OPERATION AND IN ACCORDANCE WITH CHANGES IN ELEVATION OF SAID BATH SURFACE, TO THEREBY PREVENT EXCESSIVE ACCUMULATIONS OF ALUMINA

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