US 2526876 A
Description (OCR text may contain errors)
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ITTO/PN Y J SEJERSTED METHOD OF HANDLING CONTINUOUS ELECTRODES IIIIHIH l MHHlllHllHlllllllllIllHHl l .1 a l A K Oct. 24, 1950 Patented 0a. 24, 1950 METHOD OF HANDLING CONTINUOUS ELECTRODES Johannes Seiersted, Roa, Norway, assignor to Elektrokemisk A/S, Oslo, Norway, a corporation of Norway A plication April 30, 1949, Serial No. 90,594
In Norway May 8, 1948 5 Claims. 1
This application relates to methods for handling continuous electrodes of the so-called Soderberg type and particularly to electrodes used for low voltage electrolytic work as in the case of aluminium furnaces.
Electrodes of this type are made by putting so-called paste into a casing having an internal shape corresponding to the shape of the desired electrode. The paste is a mixture of carbonaceous material such as coke, and a binder. Ordinarily the binder has a melting point somewhat above normal atmospheric temperature so that the paste is solid at room temperature. The upper part of the electrode (either at the top or a short distance down in the electrode mass) is hot enough so that the binder of the paste melts and ordinarily the paste flows without the necessity of tamping to take the shape of the electrode. The nature of a type of paste commonly employed is described in U. S. Patent No. 1,670,052.
becomes hotter, the more volatile ingredients in' the binder are first distilled oil so that the mass which was originally solid and then liquefied, again solidifies. Substantial solidity is reached some time before the binder is fully carbonized and in this stage the particles of carbonaceous matter are held bound together in solid form by substantially non-liquid tars. While in this state the electrode mass will hold its shape but it still has sufficient flexibility so that an iron bar can be driven into its side. At this stage the temperature of the mass will be below 425 C. and ordinarily will be somewhat lower, say between 300 C. and 350 C.
As the temperature still further rises, the tars undergo cracking and chemical decomposition until finally a mass is obtained which is substantially fully carbonized. This final baking begins to take place at about 425 C. and therefore is substantially complete when the mass becomes incandescent.
In past years it was customary to form the casing for the electrode from separate bars which moved down with the electrode mass and which could be removed from around the electrode when it has finally become carbonized and was approaching the bath of metal subjected to the action of the electrode. Such a construction is 2 illustrated, for example, in Torchet Patent No. 2,073,356 and Legeron Patent No. 2,169,563.
In more recent years it has been proposed that the casing be made fixed and that the electrode slide through the casing and be supported by studs or contact members passing vertically down from the top into the electrode mass (see, for example, Manfredini Patent No. 2,224,739). One difilculty with this system has been that the electrode which is only moved intermittently tends to stick to the casing. At first this sticking may not be serious but gradually the amount of adhering baked paste increases until a point is reached where the sliding of the electrode mass through the permanent casing is substantially hindered. This, of course, causes difliculties in the operation of the furnace, for the only way of removing the adhered paste is to remove a portion of the permanent casing and clean it. Such an interference with production is extremely expensive.
The present invention is dependent upon my discovery that the tendency of the electrode mass to bake onto the metal of the casing occurs after the mass has solidified and during the stage when the tars are finally carbonized. Taking advantage of this discovery, I have developed a method of handling continuous electrodes in a furnace for producing aluminium or the like which comprises putting a paste mass into a casing having an internal section of the shape desired for the electrode. Such mass is then brought into a zone where it will melt to assume the shape of the electrode and is gradually moved downward into a zone hot enough to cause the mass, or at least its outer portion, to solidify but not to be fully carbonized. The temperature at this stage is between 300 C. and about 400 C. When the mass is in this condition and. before the temperature of its outer shell adjacent the casing exceeds 425 C. it is moved downwardly out of the casing, the bottom edge of which is held above the bath a suflicient distance so that the heat of the furnace does not cause the outer portion of the electrode mass to reach the carbonizing temperature while within the casing. Ordinarily the temperature of the mass toward the center is hotter than around the outside so that the carbonizing zone will be higher in the middle than toward the outer edges. However, the phenomena which we are here discussing take place at the outer part of the electrode mass and therefore it is the outer temperature of the mass which is critical. Ordinarily the bottom of the casing should be at least 15-20 centimeters above the bath or somewhat higher, depending on details of design.
After emerging from this casing the electrode is lowered further into a, hotter zone where it is progressively heated until it becomes incandescent and carbonized fully. In this latter part of the operation, the sides of the electrode are not supported and this permits the decomposition products and vapors resulting from the cracking of the tars to escape outwardly through the sides of the electrode mass, and entirely prevents the mass from baking onto the casing. The electrode finally enters the furnace bath in which it functions, which has a temperature of about 950 C.
If the mass at the temperature of incandescence were left exposed to the air, it would tend to burn and corrode, causing very substantial loss. I find, however, that by supplying a memper forming a confined space surrounding the bottom part of the electrode (but out of contact with its surface) and by maintaining a non-oxidizing atmosphere in such space, combustion and corrosion of the sides of the electrode are prevented. In this connection I have found that combustion of the tar vapors or their cracking products and gases that evolve from the exposed faces of the electrode will ordinarily be suflicient to consume oxidizing gases such as C02 as may be present and thus maintain a sufficiently nonoxidizing atmosphere in the enclosed space. Provision should be made so that the products of combustion can be withdrawn as desired.
When the casing is so constructed that it is held about 15 to 30 centimeters above the level of the bath in the furnace, the temperature at the bottom of the casing will be above about 300 C., a temperature necessary for solidifying but below the critical temperature of 425 C. which should not be exceeded. Ordinarily when an electrode is lowered within the casing to compensate for the normal consumption which takes place within the bath, the amount of lowering which is done at one time is only from 2 to 5 millimeters. In an aluminium furnace it is customary periodically for the aluminium to be tapped off and at this time a larger movement of the electrode is necessary. The depth of aluminium tapped off is from 3 to 5 centimeters and would ordinarily not exceed 7 centimeters so that it is safe to assume that there is no necessity of lowering the electrode more than about 7.5 centimeters. Within this amount of movement there is virtually no danger of a non-solidified portion of the electrode mass coming down below the bottom of the electrode easin into an area where it will be unsupported on its sides.
This system contemplates the use of vertical studs. One of the difficulties found with the use of vertical studs has been the tendency of the electrodes to crack. The cracks ordinarily pass through the spaces in which the vertical studs are located. If such cracks run longitudinally of the electrode (which ordinarily is made with a length much greater than its width) there is a tendency for the gases generated at the bottom surface of the electrode to be caught by such cracks. These gases consist largely of CO2, and there is a tendency for the CO2 to react with the carbon of the electrode to form CO, thus tending to increase the size of the cracks. Such cracks also mean a higher voltage drop in the electrode and should therefore be avoided.
It has been found that if the cracks run transversely from the outer ring of the electrodes these take care of the shrinkage of the electrode mass and there i relatively little tendency for the longitudinal cracks to form. The transverse cracks running out longitudinally from the outer line of contact studs do relatively little harm as the gas escapes rapidly from them into the nearby gas-collecting ducts so that there is relatively little consumption of the electrode mass. In order to direct the crack formation in this transverse direction it is desirable to space the electrode supports or studs further apart from each other longitudinally than the distance between these studs and the outer wall of the electrode. Heretofore this placing of the studs near the outer electrode wall tended to emphasize the danger of having the electrode mass stick to the casing (since it caused localized heating). However, by the use of my present invention where the electrode mass emerges from the casin before the final carbonization, this tendency to stick is done away with and this permits the vertical studs to be located quite close to the outer margin of the electrode mass. For example it is quite feasible to locate the studs so that the distance in the horizontal plane between the center points of the studs longitudinally is at least 33% greater than the distance from the center line of the outer row of studs to the outer margin of the electrode mass. This spacin has previously been indicated as for example in Patent No. 2,224,739, but heretofore no particular reason for this spacing had been stated nor was any construction known in which it could be employed to the best advantage.
Further details of my invention can readily be understood from the example illustrated in the accompanying drawings in which Fig. 1 shows a transverse section of an electric aluminium furnace embodying my invention; Fig. 2 shows a transverse section of one-half of a similar furnace illustrating a modified form of construction and Fig. 3 is a plan view of the electrode showing the location of the electrode studs.
In Fig. l the numeral I designates an electric aluminium furnace; 2 is the molten bath and 3 is the electrode provided with internal vertical contact studs 4 which extend down into the lower part of the electrode 3 where they are anchored in the carbonized mass. The numeral 5 indicates that portion of the electrode 3 which is unbaked and pasty, and 6 is the portion of the electrode where a sullicient amount of volatile matter has been driven out of the paste so that the mass in this zone will be solid but not fully baked. The approximate line of demarcation between portion 5 and portion 6 of the electrode mass is indicated by a dot and dash line. I is the lower part of the electrode which is fully coked or carbonized and the approximate line of separation between the zone 1 and zone 5 is indicated by a dotted line. It is understood that these zones are not sharp and definite but are only approximated by the lines indicated.
The numeral I 5 indicates the casing for the electrode which is suspended from the usual superstructure (not shown) in any conventional manner. The interior of this casing will ordinarily be rectangular with its length substantially greater than its width as indicated in Fig. 3. Since the electrode mass will slide smoothly within the casing I5 without any danger of sticking, it has been found that the height of this casing need not be as great as in previous constructions of this type.
The numeral 9 indicates a member which serves to form a gas-collecting duct or space around the lower portion of the electrode mass. This is preferably connected to the casing by a gas-tight sand lock I and at its lower end it is imbedded as at H in alumina which acts as a gas lock. Gases which occur within the space defined by member 9 may be led away through pipe II.
In Fig. 2 the space around the lower part of the electrode is formed by a member I6 connected to the casing l5 by the sand lock l0 and to the furnace pot by the sand lock [3. Openings are formed in the top of member I6 and these are provided with removable covers H which connect with the member It by appropriate sand locks. These covers can readily be removed when additional alumina must be added to the furnace or when the crust has to be broken down, or other work done.
While I have indicated the temperatures of the various zones which are critical to the operation of this invention, these zones may also be checked by mechanical means. Specifically it will be found that the mass in zone 1 is baked so hard that one cannot drive a metal bar into it without cracking it. On the other hand, zone 6, while suiliciently solid so that it does not fiow, is soft enough so that a metal bar can be driven into it.
In Fig. 3 I indicate the spacing of the electrode studs 4 relative to the casing I5. It will be noted as stated above, that thedistance between the center point of the studs longitudinally and taken in a horizontal plane is at least 33% greater than the distance from the center line of the outer row of studs to the outer margin of the electrode mass. By this arrangement if any shrinkage cracks occur, they will tend to run transversely out to the sides of the casing iather than longitudinally.
It is understood that the examples given are intended only as illustrations and that my in vention may be modified in many particulars.
What I claim is:
1. The method of handling a, continuous elecwhere the mass melts to assume such shape, r
moving the mass downwardly within and in contact with such casing into a zone hot enough to cause the outer portion of the mass to solidify but not to be fully carbonized, moving the mass downwardly out of the casing while in such partly baked condition and into a hotter zone where the mass will no longer be in contact with the casing and will become incandescent and can carbonize before entering the bath where it functions, maintaining a substantially gas-tight seal between the fixed casing and the surface of the bath surrounding the lower part of the electrode whereby admission of air into the area surrounding the exposed portion of the electrode is substantially prevented and generated gases enter such area, and withdrawin portions of such gases from such area through an escape pipe while a substantially non-oxidizing atmosphere is maintained around that portion of the electrode which extends from the bottom of the casing to the surface of such bath to prevent combustion of the exposed face of the electrode.
2. A method as specified in claim 1 in which the temperature of the outer portion of the electrode mass as it emerges from the casing is between 300 C. and 425 C.
3. A method as specified in claim 1 in which the electrode mass as it emerges from the casing is soft enough so that an iron bar can be driven into it without; causing it to crack.
4. A method of handling a continuous electrode in an aluminium furnace which comprises shaping the electrode and baking it at a temperature of between 300 C. and 400 C. in a fixed casing whereby the outer portion of the electrode is solidified but not fully carbonized, withdrawing it downwardly from the casing at a temperature not in excess of 425 C., passing the electrode with its sides substantially unsupported downwardly towards and into the furnace bath while raising the temperature progressively to incandescent temperature whereby the electrode is fully carbonized, and maintaining a substantially gas-tight seal between the fixed casing and the lower surface of the bath surrounding the lower part; of the electrode whereby admission of air into the area surrounding the exposed portion of the electrode is substantially prevented and generated gases enter such area, and withdrawing portions of such gas from such area through an escape pipe so that a substantially non-oxidizing atmosphere around the electrode while passing it down from the fixed casing to the furnace bath.
5. A method as specified in claim 1 in which the electrode mass is supported by vertical studs anchored in the carbonized portion of the electrode which studs are positioned so that the distance in a horizontal plane between the center point of the studs longitudinally is greater than the distance from the center line of the outer row of studs to the outer margin of the electrode mass whereby shrinkage cracks will tend to run out from the studs to the sides of the electrode rather than longitudinally.
REFERENCES CITED The following references are of record in the file of this patent:
UNITED STATES PATENTS Number Name Date 1,441,037 Soderberg Jan. 2, 1923 1,657,948 Westly Jan. 31 1928 2,193,434 Sem Mar. 12, 1940 2,224,739 Mandfredinl Dec. 10, 1940 2,243,096 Hardin May 27, 1941 2,330,576 Hagerup-Larssen Sept. 28, 1943 2,338,936 Hagerup-Larssen Jan. 11, 1944 2,495,148 Tanberg Jan. 17, 1950 FOREIGN PATENTS Number Country Date 52,456 Netherlands Apr. 16, 1942 227,451 Switzerland Sept. 1, 1943 608,475 Great Britain Sept. 15, 1948 OTHER REFERENCES Journal of the Electrochemical Society; vol. 94, No. 5, November 1948, pages 220-231.