US3619465A

US3619465A - Method for operating self-baking electrodes

Info

Publication number: US3619465A
Application number: US882811A
Authority: US
Inventors: Mario Cavigli
Original assignee: Montedison SpA
Current assignee: Montedison SpA
Priority date: 1968-12-09
Filing date: 1969-12-05
Publication date: 1971-11-09
Anticipated expiration: 1988-11-09
Also published as: NO123094B; FR2025666A1; DE1961351A1; CA918210A

Abstract

Improved results in operating self-baking electrodes are achieved by surrounding that portion of the shell which contains the solid and the molten raw, electrode-forming paste, with a stream of a hot gas, while maintaining its temperature at at least the softening point of the raw paste.

Description

United States Patent Inventor Appl. No.

Filed Patented Assignee Priority Mario Cavigli Mestre, Italy Dec. 5, 1969 Nov. 9, 1971 Montecatini Edison S.p.A. Milan, Italy Dec. 9, 1968 Italy METHOD FOR OPERATING SELF-BAKING ELECTRODES 9 Claims, 1 Drawing Fig.

U.S. Cl 13/18 Int. Cl 05b 7/06 [50] FieldolSearch 13/18; 3 l 3/327 [56] References Cited UNITED STATES PATENTS l,75l,l77 3/1930 Sem,etal. 13/18 2,495,!48 1/1950 Tanberg l3/l8 Primary Examiner-Bernard A. Gilheany Assistant Examiner-R. N. Envall, Jr. Attorney-Hubbell, Cohen & Stiefel ABSTRACT: Improved results in operating self-baking electrodes are achieved by surrounding that portion of the shell which contains the solid and the molten raw, electrode-forming paste, with a stream ofa hot gas, while maintaining its temperature at at least the softening point of the raw paste.

METHOD FOR OPERATING SELF-BAKING ELECTRODES BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for operating selfbaking electrodes. More particularly, this invention relates to a method for operating self-baking electrodes especially suitable for submerged-arc furnaces. This invention also relates to an apparatus for carrying out said method.

2. Description of the Prior Art Self-baking electrodes have been known and used for a long time, for producing metals, alloys, etc., by electrometallurgical processes.

It is well known that a self-baking electrode substantially comprises a metallic, peripherally extending, vertical shell and a carbon mass contained therein. The electrode is fed electrode-forming its top end with a raw electrode-forming paste, made up of pieces of calcined coal of various particle sizes mixed with a binder, usually pitch. As a result of the heat developed by the furnace and of the Joule heat due to the resistance encountered by the current while flowing through the electrode, the electrode-forming paste undergoes a process of gradual transformation. The carbon mass may be schematically subdivided, from the top downward, into four zones. Thus, in the first or top zone, where the temperature is lower than about 100 C., the paste is in the solid state. In the second zone, wherein the temperature is generally between about 100 and 300 C. (depending on the characteristics of the raw paste), the paste takes on the characteristics of a liquid phase, the viscosity thereof gradually decreases downward; in this zone the paste is called molten. In the third zone, wherein the temperature is generally between about 300 and 700 C., the paste is in its baking stage. The tarry and pitchy substances decompose and distill; the electrode-forming paste then gradually changes into a tough, compact carbon mass highly suitable for carrying the electrode current.

In the fourth or lowest zone, where the temperature is in excess of 700 C., the electrode is baked.

As the baked electrode is gradually lowered into the furnace in order to compensate for its wear, the transformation process extends to new portions of the electrode; a new portion of molten paste enters the baking stage and a new portion of solid paste goes over into the molten state.

it is also well known that this self-baking carbon mass is contained in a shell, usually a metallic shell, which is generally provided with a metallic internal reinforcing structure. One purpose of said structure is to support the weight of the baked electrode as well as of the overlaying electrode-forming paste.

In a widely used type of self-baking electrode, this internal reinforcing structure consists of fins or other similarly suitable means which are attached to and extend radially and inwardly from the inner walls of the shell. Aside from performing the above-mentioned supporting action, this structure is also useful for introducing the current into the electrode. As the electrode wears out during the process the whole electrode structure (shell, reinforcing structure and carbon electrode) is lowered into the furnace. However, this type of electrode presents, as a disadvantage, the substantial consumption of the metallic structure of the electrode which in turn causes contamination of the substance (e.g., metal or alloy) to be produced.

This drawback has been overcome by the self-baking electrode disclosed in Italian Pat. No. 606,568. In this type of electrode, the internal reinforcing structure is not in contact with the shell, but, on the contrary, it is electrically insulated from it.

This structure is vertically displaceable with respect to the shell, and its movement allows the electrode mass to slip along the shell. It is therefore possible to lower the baked electrode in the furnace without, at the same time, having to lower the shell, thereby reducing the consumption thereof and hence substantially decreasing the amount of impurities introduced into the furnace.

Another common drawback of self-baking electrodes is their low electrical conductivity as compared to that shown by conventional prebaked electrodes.

Self-baking electrodes possess satisfactory electrical conductivities of from about 0.0l0 to 0.012 mho.m.mm.". Such values of the electrical conductivity allow the use of the current densities of up to 5-6.5 a./cm.*. However, the conductivity of prebaked amorphous carbon electrodes amounts to 0.020 mho.m.mm.", and it is therefor quite evident that the conductivity of the self-baking electrodes should be improved in order to attain an increase of the output capacity of the furnaces.

The conductivity of the self-baking electrodes and the properties related thereto, such as, e.g., density and mechanical resistance, are furthermore not constant, since the process of conversion of the raw paste into a baked paste occurs in an irregular manner. This depends in particular on the often sudden variations of the heat fed to the paste. In the case of flashings, the furnace feeds to the electrode too much heat, while in other instances too little.

Aside from the influence of the furnace itself, the room temperature is subject to frequent variations, due to the working of the surrounding furnaces, the air currents, and the changes of the outside temperature.

It has been found that these variations exert a harmful action in the raw electrode zone. Thus, following the temporary overheating of the shell, the raw paste in contact with this shell softens (at temperatures generally around 75 C., depending on the characteristics of the paste). When the temperature drops, the paste solidifies again and sticks to the inner walls of the shell, thus hindering the downward sliding of the raw paste inside the shell. in fact, the paste descends irregularly and discontinuously, and sometimes there may even be formed archlike configurations transversely of the electrode so that the paste is supported by the shell.

In the case of the self-baking electrode disclosed in the above mentioned Italian Pat. No. 606,568, in which the carbon electrode slips downward with respect to its shell, it happens that the sliding is often further hindered. It has been found that the obstacle is often caused by the previously mentioned sticking of pieces of raw paste to the inner walls of the shell. Therefore, as it is conceived presently, self-baking electrodes do not fully and satisfactorily comply with the requirements of the various types of processings which are carried out in the furnaces. An electrode capable of giving satisfactory results in the case of processings at high-intensity currents, such as the preparation of Fe-Mn and Fe-Si-Mn alloys, may yield unsatisfactory results in the case of processings at low-intensity currents, such as the preparation of Fe-Cr alloys, inasmuch as the quantity of heat supplied by the Joule effect may become insufficient to ensure a suitable baking of the electrode.

SUMMARY OF THE INVENTION 1 have now found an improved method for operating selfbaking electrodes, especially for those electrodes which are particularly suitable for submerged-arc furnaces. l have also found an apparatus for carrying out such a method. In accordance with my invention, this method comprises surrounding that part of the shell encompassing the layers of solid and molten raw electrode-forming paste with a stream of hot has, usually air, while maintaining its temperature at a value such that the thickness of the molten layer is of from about 4 to 5 times the length of the major transverse dimension of the electrode.

By "major transverse dimension of the electrode, I mean the maximum dimension of the section of largest area perpendicular to the longitudinal axis of the electrode. Since usually the shape of these electrodes is cylindrical, such dimension is their diameter.

By constantly maintaining the temperature of the gas at a value at least equal to the softening point of the raw paste, a regular sliding down of the raw paste towards the melting zone is ensured, inasmuch as the hot air maintains the shell constantly at such a temperature as to practically eliminate any adhesion of pieces of raw paste to the inner wall as well as the formation of archlike configurations.

On the other hand, discontinuous or continuous increases of the temperature of the gases above the softening point enables one to speed up, if desired, the melting process of the paste so as to maintain constant the thickness of the layer of molten paste at a value of four to five times the value of the diameter of the electrode.

When a specific ratio, for instance 4.2, has been chosen for a particular electrode, it is preferred to carry out the above mentioned temperature increase in such a way as to maintain this ratio constant, rather than permitting the ratio to oscillate within the above-indicated range (4-5).

By maintaining the thickness of the liquid layer within the above specified range, the baking process is thereby facilitated, inasmuch as the greater pressure due to the liquid head increases the compactness of the baked paste and facilitates the downward flowing out of the distillation gases through the pores of the baked electrode, wherein, as a result of the high temperatures, the gases decompose, filling up these pores with finely divided carbon particles. In this way, the electric conductivity is increased and the density and the mechanical characteristics of the electrode are highly improved as well.

When the heat due to the furnace and to the Joule effect drops for a longer or shorter period, thus tending to cause a decrease of the height of the liquid head, the hot airstream is fed at a higher temperature, in order to compensate for the heat deficiency, thereby ensuring a suitable head under the layer of solid paste. Air may also be used pennanently as an auxiliary source of heat, particularly in processings at low-intensity currents, where the heat generated by the Joule effect is lower.

As already explained, the softening point of the electrodeforming paste usually varies, depending on the type of the paste itself, between about 45 and 80 C. Often the softening point is about 75 C. When the temperature is increased beyond this point, it may be varied within a wide range, depending on the extent of the necessary supply of heat. In certain cases it may be sufficient to raise the temperature by just a few degrees e.g., from l to C.). Often the temperature increase may be higher, and in certain cases it may even exceed 100' C. (with respect to the softening point). In general however, the gases are not heated to temperatures exceeding 200 C. Preferably the minimum temperature of the gases is greater than the softening point; often such temperature is about 90 C., while the increases above this minimum are discontinuous and are such that the maximum temperature of the gases will not exceed about 160 C.

I have surprisingly found that my method is highly suitable for improving both the electrical characteristics (such as, e.g., the conductivity which is generally increased by about 10-15 percent) and the physical and mechanical characteristics (such as, e.g., the density and the mechanical resistance) of the self-baking electrodes, as well as for ensuring a greater uniformity of such properties.

Furthermore, I have found that my method allows a perfect adaptation of the electrodes to each change of processing in the furnace, and particularly that is ensures an equal performance of the electrodes both in the processes at high-intensity currents as well as in processes at low-intensity currents.

I have also found that my method facilitates the sliding down of the electrode-forming paste in the shell of electrodes with differential sliding.

BRIEF DESCRIPTION OF THE DRAWING The FIG. is a diagrammatic illustration of an apparatus for carrying out the method of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS According to a preferred embodiment of this invention, the temperature of the air is regulated with reference to the temperature measured in the electrode itself. The measurement is preferably carried out along the axis of the electrode, and preferably in a zone where the mass is molten or where, in any event, the mass is desired to be molten.

It is particularly convenient to carry out the measurement in a zone close to the upper limit surface of the paste layer that is desired to be maintained in the molten state. The position of said upper surface may be easily determined, since generally the lower surface of the molten layer substantially coincides with that part of the electrode which is in contact with the upper end of the current-carrying plates.

It is preferable that the measured temperature be maintained at a preselected value, or within a preselected range to which corresponds a suitable head of molten paste. When the temperature is too low, air at higher temperature is introduced.

When, on the contrary, the measured temperature is such that there is a suitable liquid head, the air may be introduced at a temperature equal to or slightly higher than that of the softening point of the raw paste.

Referring now to FIG. 1 in detail, there is shown a self-baking electrode device suitable for carrying out the method in accordance with my invention.

A cylindrical shell 5 contains the carbon mass 16 consisting of a layer of solid raw paste 1, defined by

limit surfaces

17 and 18, a layer of molten paste 2, defined by limit surfaces 18 and I9, a baking zone 3, defined by

limit surfaces

19 and 20, and portion of the baked electrode 4. The current-carrying plate 6, and the metal ring 7, which clasps the plates against shell 5, connect the electrode to the external electrical circuit, not shown in FIG. 1. For the sake of simplicity, the internal reinforcing structure of the electrode has not been represented.

The part of the shell 5 encompassing the solid paste zone 1 and the molten paste zone 2 is surrounded by a cylindrical jacket 8 concentric to the shell itself. A stream of hot air is caused to circulate in the interspace 9, defined by the external wall of shell 5 and jacket 8. The gaseous stream is caused to circulate by means of a fan-blower l2 and, before entering the interspace 9 through conduit 10, is heated up into the heating device 13. The upper end 21 and lower end 22 ofjacket 8 obviously need not coincide exactly with the upper limit 17 of the zone of solid paste 1 and with the inferior limit 19 of mol ten paste 2 respectively, provided that these zones are always completely subjected to the action of the hot airstream. In fact, there is no objection to the lower end 22 of the jacket 8 coinciding with the zone of the paste under baking, while the upper end 21 may obviously extend well beyond the upper level 17 of the solid paste.

Preferably the jacket ends towards its lower part at a short distance 10-20 cm.) from the current-carrying plates. The raw paste is loaded into the shell in such a way as not to way beyond the upper level 21 of jacket 8.

In order to allow the displacement of the electrode and of its shell, which are consumable, with respect to the jacket, which is a fixed structure, jacket 8 is not fastened to shell 5, the interspace 9 is open, preferably at the bottom, while at the upper end the interspace is sealed by a packing l l, for instance made of asbestos rope, which allows the electrode, when necessary, to move with respect to the jacket. The interspace is of such a size as to ensure a suitable outflow of the air and an efficient heat exchange with the shell. For instance, in the case of an electrode of 1,000 mm. diameter, the jacket may be 5-7 meters long, while the interspace may have a width of 50-100 mm.

A temperature-measuring device 14 is immersed in the electrode along the axis thereof. Thermal signals are transmitted through a thermoelectric converter and an electric amplifier (not shown in FIG. 1) to the servocontrol mechanism 15, which is connected with the heating device 13.

The servocontrol mechanism may consist, for instance, of relays or electropneumatic devices.

The point of measurement of the temperature may be fixed, or the temperature measuring device may be disposed along the axis of the electrode in order to explore a certain zone of it.

If the point of measurement is fixed, the measurement is preferably taken at the upper end of the layer of paste that one wishes to maintain in molten state.

The servocontrol mechanism is regulated in such a way that the temperature of the air introduced into the interspace is always at least equal to the softening point of the raw paste.

The following example is presented to further illustrate my invention.

EXAMPLE described previously and illustrated in FIG. 1.

The interspace 9 had a thickness of 60 mm. The flow rate of the air was of about 3,000 m./hr. The temperature of the electrode was measured by means of a thermocouple, the lower end thereof being dipped at about 4.5 meters from the upper edge of the current-feeding plates 6, thus maintaining a liquid head of equal length.

The servocontrol mechanism was a relay. It was regulated in such a way as to maintain a temperature of 95 C. in the measurement zone and in such a way as to maintain the air at a temperature not lower than 90 C.

The temperature of the air in general ranged between 90 and 160 C.

The electrode was worded under these conditions for 12 months. During the whole of this period it was possible to work with a current density of about 7.0 a./cm. Therefore, the electric conductivity of the electrode had increased by about 13 percent.

To this increased conductivity there evidently corresponded a greater density. On the other part, the electrode, during all this period of operation, proved to have better mechanical characteristics.

The capacity of the furnace was increased by about percent.

The main aspects of my invention as well as the advantages thereof can be summarized as follows:

1. It is possible to maintain the layer of molten paste at an optimum and constant thickness, thereby improving both the mechanical and the electrical characteristics of the baked electrode;

2. In these types of processings, in which the heat supplied to the electrode by the furnace and by the Joule eifect is normally sufficient to ensure an adequate liquid head, provided that a regular slipping down of the raw paste is ensured, the method of this invention allows one to quickly and effectively compensate the temporary deficiencies that may occur in the supply of heat. This aspect of the invention is particularly useful in processings at high-intensity currents and at low-voltage, such as for instance, in the production of calcium carbide and of ferromanganese and ferro-silicium-manganese alloys. In such processings it is sufficient, in general, to maintain the air temperature, for instance, at a minimum of about 90 C. with only occasional rises, for instance, to about 120 C.,

3. In those types of processings where the heat supplied by the Joule effect is relatively low, the function of heat supply may be boosted, thus supplying air at a higher temperature in a more or less continuous manner. This is the case of the processings at low current intensity and high voltage, such as the ferro-chromium production.

In such a case, the temperature will usually be maintained, for instance, at a minimum of C. with occasional rises to 200 C.

4. The method of this invention easily allows one to adapt the operation of the electrode to any change in the processing in the furnace.

5. The method of this invention results in further advantages with respect to the operating of electrodes with differential slippage as disclosed in Italian Pat. No. 606,568: heating the shell in that portion thereof surrounding the solid paste zone considerably lowers the friction coefficient between paste and shell, thus facilitating the slippage of the paste with respect to the shell. Furthermore, a particular operation is facilitated which is made possible by the differential slippage: when it is useful to speed up the baking of the electrode, one can in fact lower it into the furnace together with its shell; when the baking process of the a portion of the electrode is completed (and, thus, said portion no longer need be contained by the shell), the shell alone may be lifted again. This operation is obviously facilitated by the lesser friction occurring between the raw paste and the shell.

Modifications and variations can, of course, be made without departing from the spirit and scope of this invention.

Having thus described my invention, what I desire to secure by Letters Patent and hereby claim is:

1. In a method for operating a vertically extending self-baking electrode, said electrode comprising a vertically extending shell and a carbon mass contained therein, said carbon mass comprising two abutting vertically stacked layers of the electrode-forming paste, the upper layer of said two layers being a layer of solid raw electrode-forming paste and the lower of said two layers being a layer of molten, raw electrode-forming paste, the improvement comprising surrounding that portion of said shell encompassing said layers of solid and molten paste with a stream of hot gas, determining the temperature of the electrode at a preselected location therein, and controlling the temperature of said gas in response to said determined temperature to maintain the thickness of the molten layer from about four to five times the length of the major transverse dimension of the electrode.

2. The method of claim 1 wherein the temperature of said gas is from about the softening point of the electrode-forming paste of about 200 C.

3. The method of claim 1, further comprising regulating the temperature of said gas at a value such as to maintain the temperature of a preselected portion of the electrode within a preselected range of temperatures, said preselected portion comprising said preselected location.

4. The method of claim 7 wherein the preselected portion of the electrode is maintained at a preselected temperature.

5. The method of claim 3, wherein the preselected portion of the electrode is a portion of the molten layer of the electrode-forming paste.

6. The method of claim 5, wherein said portion of the molten layer of the electrode-forming paste is in the upper zone of said molten layer.

7. A self-baking electrode structure comprising:

a. a heating jacket;

b. a peripherally continuous vertically extending shell disposed within said heating jacket for containing electrode-forming paste and the upper portion of a baked electrode mass therewithin in vertically movable relation relative to said jacket, said paste being normally solid and being progressively convertible from solid paste to molten paste and from molten paste to baked electrode mass as said paste moves downwardly of said jacket, said solid and molten electrode-forming paste and said baked electrode mass being in substantially segregated vertically stacked abutting layers, the vertical extent of said jacket being substantially equal to the projected vertical extent of said solid and molten layers;

c. means for supplying heating fluid to said jacket for heating the electrode paste therewithin; and

means for slidably mounting said shell within said jacket,

whereby said shell may be moved vertically relative to said jacket.

9. The electrode structure of claim 7, wherein said temperature measuring means is a thermoelectric means and said connecting means includes a servocontrol means responsive to said thermoelectric means.

POW) UNITED STATES PATENT OFFICE 's/sq) CERTIFICATE OF CORRECTION Patent No. 3 19,4 5 D te November 9L 1971 r Mario Cavigli It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 50, "One' should read The Column 2, line 7, "mm. should read mm.

Column 2 line 11, "1nm. should read mun Column 2 lino ll "therefor" should read therefore Column 2 line 66 "has should read gas Colrmn 3 line 16 increase" should read increases Column 3 line 47, e.g. should read (e.g.

Column 3 line 66 'is should read it Column 4, line 55 "Way", second occurrence, should read extend Column 5, line 7, before "molten state" insert the Column 5, line 39 "worded" should read worked Column 6, line 17, (first occurrence) delete "the".

Column 6 line (Claim 2 line 3) of should read to UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3, 19 ,455 D t November 9 1971 Inventor( Mario Cavigli PAGE 2 It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 6 line 51, (Claim 4 line 1) 7" should read 3 Column 7, line 5, after "means" insert for operating said heating means Signed and sealed this 27th day of June 1 97 (SEAL) Attest:

v '1 1 w 1 ED JARD MmLhTCHER JH. ROBLRI f OTTbbriA Atte sting Officer commissloner of Patents

Claims

2. The method of claim 1 wherein the temperature of said gas is from about the softening point of the electrode-forming paste of about 200* C.

7. A self-baking electrode structure comprising: a. a heating jacket; b. a peripherally continuous vertically extending shell disposed within said heating jacket for containing electrode-forming paste and the upper portion of a baked electrode mass therewithin in vertically movable relation relative to said jacket, said paste being normally solid and being progressively convertible from solid paste to molten paste and from molten paste to baked electrode mass as said paste moves downwardly of said jacket, said solid and molten electrode-forming paste and said baked electrode mass being in substantially segregated vertically stacked abutting layers, the vertical extent of said jacket being substantially equal to the projected vertical extent of said solid and molten layers; c. means for supplying heating fluid to said jacket for heating the electrode paste therewithin; and d. means for controlling the temperature of said heating fluid comprising means for measuring the temperature at the projected position of said molten layer, means for heating said fluid, and means for connecting said temperature measuring means to said heating means in response to the measured temperature to maintain the height of said molten layer equal to about 4 to 5 times the maximum horizontal dimension thereof.

8. The electrode structure of claim 7, further comprising means for slidably mounting said shell within said jacket, whereby said shell may be moved vertically relative to said jacket.