US2761830A - Wiring arrangement for a series of electrolytic cells - Google Patents

Description

P 1956 M. KIBBY 2,761,830

WIRING ARRANGEMENT FOR A SERIES OF ELECTROLYTIC CELLS Filed March 22, 1952 2 Sheets-Sheet 1 I INVENTOR 9055/?7/14 K/BBY ATTORNEYS Sept. 4, 1956 R. M. KIBBY 2,761,830

WIRING ARRANGEMENT FOR A SERIES OF ELECTROLYTIC CELLS Filed March 22, 1952 2 Sheets-Sheet 2 INVENTOR fioamv M K155) BY 4% PM M /\7C- M ATTORNEYS WIRING ARRANGEMENT FOR A SERIES OF ELECTRDLYTIC QELLS Robert M. Kibby, Troutrlale, Greg, assignor to Reynolds Metals Company, Richmond, Va, a corporation oi Delaware Application March 22, 1952, Serial No. 278,073

' 4 Claims. (Cl. 204*244) My invention relates to a wiring arrangement which is particularly adapted for connecting a series of electrolytic cells or pots in which metallic aluminum is produced.

More particularly, I have discovered a unique arrangement of electrical connections for a series of electrolytic cells or pots in which metallic aluminum is produced, my arrangement providing distinct advantages over the conventional wiring arrangements heretofore utilized, particularly when applied to a series of cells designed to carry current loads in excess of 50,000 amperes.

In commercial practice, aluminum is manufactured by the electrolysis of a bath which comprises a solution of alumina in fused cryolite. The voltage drop across one cell is small, usually about 6 volts, and hence a large number of cells are connected in series in order that electrical generating machines of standard voltages may be employed. On the other hand, the amount of current passing through each cell is very large, being up to about 50,000 amperes in actual practice.

In electrolytic cells for the production of aluminum, both the anode and the cathode are conventionally composed essentially of carbon. In the larger cells, for example, those of 50,000 amperes capacity, both the anode and the cathode naturally must have a large surface area in order that current densities will be at a reasonably low figure. Further, because oxygen at high temperature is released at the anode, the anode is gradually consumed.

Due to these factors, the art conventionally utilizes anodes of the Soderberg or self-bahin. type, such anodes being formed by feeding a plastic, carbonaceous mixture to a bottomless mold which is positioned at some distance vertically above the bath. This mixture is gradually fed downwardly as the lower part of the anode is consumed, and the mixture in its downward passage gradually becomes baked into a hard condition as it approaches the high temperature bath. As is apparent, the use of selfbaking anodes is advantageous in that they are continuous and also in that at the same time they eliminate the necessity for handling pro-baked electrodes of large size, which are cumbersome.

The art has also paid particular attention to the horizontal, cross-sectional configuration of self-baking anodes, and as a result has found that such anodes, at least in the larger sizes, should have a cross-section which is substantially rectangular, rather than round or square. That is, the anode should be greater in horizontal length along its side than along its end.

There are several sound technical reasons for this. In the first place, oxygen is formed at the interface between the anode and the bath. This oxygen, as well as the gases which are produced by its reaction with the carbon anode at the high temperature existing in the bath, has the effect of blanketing the anode, thereby reducing the area of contact between the anode and the bath and increasing the voltage drop across the cell. Therefore, it is advisable to provide the oxygen and the gases produced therefrom with the shortest possible route for escaping from the underside of the anode. In the second place,

2,761,830 Patented Sept. 4,1955

the oxygen formed at the anode reacts with the anode, as has been just mentioned, resulting in consumption of the anode. The cost of anodes is also an important item in aluminum production, the ratio of the weight of anode carbon consumed to the weight of aluminum produced generally being about 0.6, so that here again it is advisable to provide the shortest possible path for oxygen to escape from the underside of the anode, thereby reducing the amount of time during which the oxygen and the anode can react and hence reducing the anode consumption. 7

Moreover, in operating a cell, the anode should be in close proximity to the layer of molten aluminum lying on the bottom of the cell, in order to provide a low voltage drop. However, the molten aluminum is only slightly more dense than the cryolite bath, and excessive agitation in the bath can cause globules of molten aluminum to rise and contact the anode. This agitation can be caused by the oxygen formed at the anode and also by the gases formed by the reaction of the oxygen with the anode, so that for this reason also it is advisable to provide the shortest possible path for gases to escape from beneath the anode, and thereby avoid short circuiting of the cell caused by globules of molten aluminum separated from the molten aluminum layer by excessive agitation and also avoid oxidation of aluminum metal. There are also other reasons why it is technically desirable to use an anode of substantially rectangular cross-section, and this type of cross-section naturally influences the horizontal configuration of the cathode, in view of the fact that the current passes from the anode to the cathode of the cell.

Means must also be provided for supplying the electric current to the anode and for removing it from the cathode. One conventional method for supplying current to the anode utilizes steel anode pins which are driven into each of the long sides of the anode to form horizontal rows of pins, each horizontal row on a given side of the anode having a corresponding horizontal row on the opposite side of the anode at the same horizontal level. These pins, through flexible leads, usually made of copper, are connected with two adjacent anode bus bars, one anode bus bar being positioned horizontally at each side of the anode.

Similarly steel bars embedded within the cathode are used to conduct the current from the cathode to cathode bus bars which are located on the long sides of the cathode and which are connected to the anode bus bars of the next cell down the line of cells.

The art has found that it is necessary to provide means for supporting the self-baking anode, which weighs a considerable amount in the present, large capacity cells, and also for lowering the anode gradually into the bath as the anode is consumed. One conventional method employed in the art to do this has been to use the anode pins for supporting the anode. These pins, in turn, are supported by the use of several horizontal courses of steel channels, which encase the anode on its four vertical sides, the anode pins being inserted into the anode through suitable holes formed in the webs of the channel.

For further information concerning self-baking anodes, anode pins and channels which encase the anode in its downward passage towards the bath, reference is made to the drawings which constitute a part of the disclosure of United States Patent No. 2,169,563 to Legeron. In the specific apparatus there illustrated a slight variation is shown, since the means for supplying current to the anode pins also serve to support the pins, which support the channels as Well as the anode.

In the past, it has been common practice, in the operation of cells of 50,000 or less amperes capacity, to arrange the series of cells with adjacent cells having their shorter ends facing each other. The high positive potential from the power source is connected to the pair of adjacent ends of the two anode bus bars of the first cell in the series at the end which does not have an adjacent cell. One of the cathode bars of the first cell is connected at the other end of the first cell to the nearest anode bus bar of the next adjacent cell in the line, and the other cathode bus bar of the first cell is connected, at that end of the first cell but by a different connection, to the near est anode bus bar of the next adjacent cell in the line. In other words, on the intermediate pots the cathode bus on the right side of the pot connects by a lead to the anode bus on the right side of the next pot, and the left hand cathode bus of the first pot connects by a separate lead to the left anode bus of the next pot. This arrange ment is repeated through the series of cells. The outside pair of ends of the cathode bus bars of the last cell in the series are connected together and attached to the negative voltage lead of the power source.

This type of wiring arrangement for the series of cells gives satisfactory service in cells having a capacity of up to about 50,000 amperes. However the art has repeatedly striven to build cells of larger and larger sizes, the reason underlying this being explained as follows:

Since cryolite is a salt which has a very high melting point, the molten bath is necessarily maintained at a very high temperature, usually of the order of 1000 C. Because of the very high temperature at which the bath is maintained, the loss in sensible heat from the bath is necessarily very large. This heat is supplied to the cell in the form of electrical energy, and this means that the electrical energy efficiency of the cell is low, the amount of electrical energy consumed per amount of aluminum produced being large. The cost of the electrical energy used in producing metallic aluminum is a very important item, and the art has therefore striven to improve the energy efiiciency of the cell. Thus, the art has turned to the expedient of increasing the size of the cell, the reason being that cells of larger size have a proportionately smaller surface area from which heat can be dissipated.

The wiring arrangement heretofore described in this specification is not a satisfactory one, however, when applied to cells having a capacity of the order of 125,000 am-peres. The reason for this is that, when such wiring arrangement is used, the layer of molten aluminum does not lie fiat on the cathode, but instead the upper surface of the molten aluminum is tilted to a considerable extent, assuming a decided angle with the horizontal. In addition, the position of the aluminum layer does not remain constant during the normal operation of the cell. This behavior of the layer of molten aluminum leads to somewhat erratic and therefore inefiicient cell operation, and is an undesirable result.

The metal build up at one end of each pot can be greatly reduced by feeding current in and taking it out at the four corners of the anode and the four corners of the cathode bus bars, respectively, rather than connecting the series of cells as previously indicated. I beli ve that the cause of the metal build up is due to the fact that the current traveling through the cathode buses and the anode buses in a given cell in the end to end current feed arrangement is always in the same direction, resulting in a strong magnetic field which influences the current through the electrolyte in the pot in the same manner that the armature of an electric motor is caused to turn, thus tending to shift the molten aluminum toward one end of the pot. By feeding the current to all four corners of the anode and by withdrawing it from all four corners of the cathode, the resultant electric fields generated by the bus bars are reoriented to cancel each other out and thus do not cause the motor action encountered in the end to end arrangern-ent.

When working with extremely high currents, the length of electrical connections between pots represents an important limitation on the size of the pots because of the amount of metal required for such connections. Therefore, in order to obtain the advantages of four corner feed, the most economical method of lining up the series of pots is with their long sides parallel. This arrangement I shall hereafter refer to as the side to side series as opposed to the end to end series first described.

Such four corner side to side feed systems have been previously employed in the operation of cells of 50,000 or less ampere capacity. Although a single power supply is customarily used, the feeding arrangement is divided between the ends of the cells, that is a positive lead is connected to two adjacent ends of the two anode bus bars of the first cell and a second positive lead from the power source is connected to the other two adjacent ends of the two anode bus bars of the first cell. Each pair of adjacent ends of the two cathode bus bars of the first cell is connected by a single lead to each pair of adjacent ends of the two anode bus bars of the next cell downstream in the series of cells, the two leads from the cathodes of the first cell to the anodes of the next adjacent cell being connected paralleling each other, rather than crossing each other. This arrangement is repeated down the series of cells until the final cell of the series each pair of adjacent ends of the cathode bus bars is connected to the negative side of the power supply.

In order to obtain electrical balance between each pair of adjacent cells in the series, it is desirable that the voltage drops between any one cathode bus bar of a given cell and either anode bus bar of the next cell downstream be equal. Under the side to side arrangement just described, such a result is not obtained unless, in each pair of adjacent cells, each pair of adjacent ends of the two cathode bus bars of the upstream cell are connected together and each pair of adjacent ends of the two anode bus bars of the downstream cell are connected together. The lead connecting each pair of cathodes to the next pair of anodes taps such cathode and anode bridging connections at the middle of each.

It is, of course, obvious that such electrical balance is highly desirable and, in fact, necessary to prevent shorting out one side of a cell.

My invention is a unique method of connecting a side to side series of cells employing four corner feed which achieves a considerable reduction in the required amount of metal in the electrical leads between each adjacent pair of cells and yet provides a balanced operation comparable to that of the system just described.

In accordance with the wiring arrangement of my invention, electrically adjacent pairs of cells in the series of cells have the ends of one cathode bus bar of one cell connected by a pair of electrical leads to the ends of one anode bus bar of the next electrically adjacent cell in the series, and the ends of the second cathode bus bar of the first cell are connected by a second pair of electrical leads to the ends of the second anode bus bar of the second cell. That is, the two leads from a given cathode bus bar of the first cell are connected to only one of the anode bus bars of the second cell and the two leads from the other cathode bus bar of the first cell are connected only to the other anode bus bar of the second cell. This arrangement is repeated down the series of cells.

The arrangement of my invention differs from the last prior arrangement in that it reduces the electrical leads connecting each pair of adjacent cells by an amount of conductor material approximately equal to that of a bus conducting the entire current passing through. the cells through a physical length equal to just less than the shorter width of each cell. lvicrcovcr, the wiring arrangement acco ding to my invention isolates the anode bus bars and the cathode bus bars of each cell, other than those of the first and last cells in the series, with attendant reduction of the tendency of one-half of a cell to short out the other half through local variations in internal resistance. Electrical balance is nevertheless desirable and is obtained either by increasing the crosssectional area of the longer of the two pairs of leads between two adjacent cells to equalize the resistances of each pair, or by using a more resistive conductor material in the shorter pair, or both.

For a more complete understanding of the nature of the present invention, reference is made to the accompanying drawings in which:

Fig. 1 is a simplified horizontal view of the end of a cell in cross section;

Fig. 2 is a simplified horizontal view of one of the long sides of a cell;

Fig. 3 is the wiring arrangement of the present invention.

Referring particularly to Figs. 1 and 2, the numeral 1 represents the self-baking anode which is supported by anode pins 2 driven into the anode while it is still in the plastic or unbaked condition. The anode pins in the lowest course of channels 3 are supported by those channels, which in turn are supported vertically from the superstructure of the cell in a conventional manner not shown. Suitable flexible leads 4 form an electrical con nection between the anode pins in the lowest course and the anode bus bar 5.

Still referring to Figs. 1 and 2, the numerals 6 and 7, respectively, represent the bath and the layer of molten aluminum, both of which are supported by the cathode 8, most of which is positioned below "the floor level 9. Inserted in the cathode are cathode pins 10, which serve to connect the cathode with the cathode bus bars 11 through leads 12.

Referring now to Fig. 3, a line of cells 13, 14, 15, 16 and so forth, is arranged electrically in series, 13 representing the first cell in the line and about 100 cells usually being arranged in a given line in actual practice.

It will be noted that the current is supplied to the line of cells from four different leads 33, 34, 35 and 36. One lead 33 is connected to a first anode bus bar at end 37, another lead 34 is connected to the adjacent anode bus bar at end 38, another lead 35 is connected to the second anode bus bar at end 39 and the fourth lead 36 is connected to the first anode bus bar at end 40, adja cent to end 39. Thus, the two leads '33 and 36 are connected to the same anode bus bar of cell 13, and the other two leads 34 and 35 are connected to the other anode bus bar.

Still referring to Figure 3, one cathode bus bar of cell 13 at end 43 is connected to one anode bus bar of cell 14 at end 44. The other end 45 of the first cathode bus bar is connected to the other end 46 of the first anode bus bar of cell 14. The second cathode bus bar of cell 13 at end '47 is connected to end 48 of the second anode bus bar of cell 14 and the other end 49 of the second cathode bus bar is connected to the other end 50 of the second anode bus bar of cell 14. Here again, the two ends 43 and 45 from a given cathode bus bar of cell 13 are connected to one anode bus bar of cell 14, and the other two ends 47 and 49 from the other cathode bus bar of cell 13 are connected to the other anode bus bar of cell 14. This arrangement is repeated down the line of cells, as shown.

I claim:

1. In a series of electrolytic cells suitable for use in the production of aluminum by the electrolysis of a bath of molten cryolite having alumina dissolved therein, in which each cell is provided with a rectangular cathode and with a downwardly feeding, self-baking, rectangular anode, and in which said cells are disposed in a fourcorner feed, side by side arrangement, the improvement in connecting a pair of physically adjacent cells electrically in series which comprises four isolated electrical 'leads interconnecting the four corners of the cathode of the more positive of the pair of cells with the correspond ingly disposed four corners of the anode of the more negative of the pair of cells, each said lead interconnecting a different single cathode corner and a different single anode corner, whereby each corner of said electrodes is fed current independently.

2. In a series of electrolytic cells suitable for use in the production of aluminum by the electrolysis of a bath of molten cryolite having alumina dissolved therein, in which each cell is provided with a. carbonaceous cathode and with a downwardly feeding, self-baking carbonaceous anode, the electrolytically active surfaces of which electrodes are substantially rectangular and of greater length than width and are situated in horizontal planes, the anode surface of said cell being directly above and similarly oriented to the cathode surface, the electrical current to each anode being supplied by a pair of horizontal anode bus bars, one of said anode bus bars being situated adjacent and parallel to and running the length of one of the long sides of the anode and the other of said anode bus bars being situated adjacent and parallel to and running the length of the other long side of the anode, the electrical current for each cathode being removed through a pair of horizontal cathode bus bars, one of said cathode bus bars being situated adjacent and parallel to and running the length of one long side of the cathode and the other of said cathode bus bars being situated adjacent and parallel to and running the length of the other long side of the cathode, the anode bus bars of each cell being parallel to the cathode bus bars of such cell, said series of cells being disposed in a row with a long side of each cell parallel to and facing a long side of a physically and electrically adjacent cell, the improvement in connecting a pair of physically adjacent cells electrically in series which comprises four isolated electrical leads interconnecting the four ends of the cathode bus bars of the electrically more positive of the pair of cells with the correspondingly disposed four ends of the anode bus bars of the electrically more negative of the cells, the electrical resistance of said four leads being equalized whereby complete electrical balance of current distribution to the four corners of said electrodes is provided and each corner of said electrodes is fed current independently.

3. A series of electrolytic cells suitable for use in the production of aluminum according to claim 2 in which the length of at least one of said four isolated electrical leads is greater than the length of at least one other of said leads and in which the cross-section of said longer lead is greater than the cross-section of said shorter lead to equalize the electrical resistance of said leads.

4. A series of electrolytic cells suitable for use in the production of aluminum according to claim 2 in which the length of at least one of said four isolated leads is greater than the length of at least one other of said lead-s, and in which said shorter lead is made of more resistive conductor material than said longer lead to equalize the electrical resistance of said leads.

References Cited in the file of this patent UNITED STATES PATENTS 1,255,197 Malm Feb. 5, 1918 1,265,551 Thomson May 7, 1918 FOREIGN PATENTS 601.873 Great Britain May 13, 1948

Claims (1)

1. IN A SERIES OF ELECTROLYTIC CELLS SUITABLE FOR USE IN THE PRODUCTION OF ALUMINUM BY THE ELECTROLYSIS OF A BATH OF MOLTEN CRYOLITE HAVING ALUMINA DISSOLVED THEREIN, IN WHICH EACH CELL IS PROVIDED WITH A RECTANGULAR CATHODE AND WITH A DOWNWARDLY FEEDING, SELF-BAKING, RECTANGULAR ANODE, AND IN WHICH SAID CELLS ARE DIPOSED IN A FOURCORNER FEED, SIDE BY SIDE ARRANGEMENT, THE IMPROVEMENT IN CONNECTING A PAIR OF PHYSICALLY ADJACENT CELLS ELECTRICALLY IN SERIES WHICH COMPRISES FOUR ISOLATED ELECTRICAL LEADS INTERCONNECTING THE FOUR CORNERS OF THE CATHODE OF THE MORE POSITIVE OF THE PAIR OF CELLS WITH THE CORRESPONDING INGLY DISPOSED FOUR CORNERS OF THE ANODE OF THE MORE NEGATIVE OF THE PAIR OF CELLS, EACH SAID LEAD INTERCONNECTING A DIFFERENT SINGLE CATHODE CORNER AND A DIFFERENT SINGLE ANODE CORNER, WHEREBY EACH CORNER OF SAID ELECTRODES IS FED CURRENT INDEPENDENTLY.

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