US 3624261 A
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United States Patent Inventor Marlo Cavigll Mestre (Venice), Italy Appl. No. 63,251 Filed Aug. 12, 1970 Patented Nov. 30, 1971 Assignee Montecatlni Edison S.p.A. Milan, Italy Priority Aug. 22, 1969 Italy 21 150 A/69 SELF-BAKING ELECTRODE STRUCTURE AND METHOD OF OPERATING SAME  Field of Search 13/18  References Cited UNITED STATES PATENTS 1,751,177 3/1930 Sem etal. 13/18 3,513,245 5/1970 Sullivan 13/18 X Primary Examiner-Bernard A. Gilheany Assistant ExaminerR. N. Envall, Jr. AuomeyHubbell, Cohen & Stiefel ABSTRACT: improved results in operating self-baking electrodes are achieved by constantly maintaining a bar made of an electrically conductive material well immersed in a carbon mass contained within a metal shell, measuring the voltage drop between the bar and the metal shell and measuring a ratio between this voltage drop and the electrode current.
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INvIsN'n m l/n jw [7 BY 1 v I I ATTORNE SELF -BAKING ELECTRODE STRUCTURE AND METHOD OF OPERATING SAME 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 continuously measuring the baking degree of selfbaking 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 instance in electrical submerged arc furnaces for producing metals, alloys, etc.
It is well known that a self-baking electrode substantially comprises a metallic, peripherally extending, vertical shell and a carbon mass contained therein. This metallic shell is generally provided with a metallic internal reinforcing structure. One of the purposes of the reinforcing structure is to support the weight of the carbon mass. The electrode is fed at its top end with a raw electrode-forming paste, made up of pieces of calcined coal of various particle size 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 current), 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 l 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 increasing downwardly. In this zone the paste may be termed 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 metal conductors carrying the electric current to the electrode must be fitted to the baked part of the carbon mass, that is, to the conducting portion of the self-baking electrode.
When, during the furnace operation, the electrode is periodically lowered into the furnace, the electrical conductors must be shifted to a new electrode zone. This shifting of the electrode with respect to the electric conductors (which is usually referred to as the slipping of the electrode) is in general carried out periodically. The slipping, for instance the daily slipping, of an electrode must of course compensate for the wear out of the electrode during the same period of time. The slipping frequency and the thereto related length of each slipping depends on the baking degree of the electrode-forming paste, since one must avoid having the unbaked or partially baked paste carry the current as it is a poor electrical conductor. In such a case the electric current would meet, in fact, a strong resistance to its flow and, if the electrode current were not decreased (with a consequential loss of power) until normal baking conditions are restored, the excessive heat produced by the' joule effect would cause a deterioration of the electrode. Such deterioration may lead to the breakage of the electrode itself. The measuring of the baking degree of an electrode has, up to now, presented considerable difficulties.
In fact, one usually must rely on an uncertain evaluation by sight of the temperature of the electrode in the zone of the electric contacts. This evaluation of the temperature is, however, made difficult due to the presence of the contacts themselves and becomes altogether impossible when other equipment screens off this zone, as is the case, for instance, in closed furnaces.
In practice the operator relies on a cautious procedure of frequent and limited slipping of the electrodes, which involves, however, burdensome work and still does not eliminate the danger of breakages of the electrode when the baking is at less than the nonnal degree.
SUMMARY OF THE INVENTION I have now found an improved method for operating selfbaking electrodes, especially for those electrodes which are particularly suitable for submerged arc furnaces. More particularly, I have found a method for continuously measuring the baking degree of self-baking electrodes. I have also found an apparatus for carrying out such a method. In accordance with my invention, this method comprises:
a. constantly maintaining a bar made of an electrically conductive material inside the carbon mass of the electrode, the lower part of this bar being immersed in the baked zone of the carbon mass so as to ensure a good and lasting electrical contact with that zone;
b. measuring the voltage drop between the conductive bar and the metal shell; and
c. measuring the ratio between the voltage drop and the electrode current.
In fact, I have discovered that the value of the ratio thus obtained (hereinbelow referred to as index C" or more briefly C), is a measure of the baking degree of the electrode.
More particularly, the value of index C is inversely proportional to the baking degree of the electrode inasmuch as high values of C correspond to low values of the baking degree and, thus, in practice to situations in which the electrode should not be slipped and in which the current density is or should be reduced.
In order of relationship, which defines an inverse proportionality between C and the baking degree, has been chosen instead of a direct proportionality because, at a constant electrode current, the measurement of the index C may be simplified into a simple voltage measurement.
Thus, I have found that my invention provides a method which overcomes the drawbacks and uncertainties of the above-mentioned empirical evaluation methods of the prior art. I have, in fact, found that my invention makes possible continuous and reliable information on the baking degree of the electrode forming paste in a self-baking electrode, and thereby enables an operator to know how much the electrode may be slipped with respect to the electrical contacts without having to decrease the electrode current and therefore the output capacity of the furnace without running the risk of breakages.
I havealso found that my invention allows a furnace operator to continuously monitor the baking process of the raw electrode-forming paste (for instance, at the startup of a new furnace or after a long shutdown of a furnace) through the constant indication of the progress of the baking, up to the attainment of a normal baking degree.
BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagrammatic illustration of a self-baking electrode and of an apparatus for carrying out the method of the present invention.
abnormal, which may occur while operating a self-baking electrode.
DESCRIPTION OF THE PREFERRED EMBODIMENTS 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 metal shell I contains a carbon mass M consisting of an upper layer 2 of solid raw paste, a second layer 3 of molten paste, a paste baking zone 4 and section 5 of baked electrode. The current-carrying plates 6 and conductors 7 connect the electrode to the external electrical circuit not shown in FIG. 1. The carbon mass is of the type that is well known in the prior art. Specifically, it is commonly prepared in the form of a paste made up of finely divided particles of calcined coal mixed with a binder, preferably pitch. In this state the carbon mass is what has been termed above alayer 2 of solid raw paste. Clearly, as the paste is subjected to the heat from the joule effect of electric current passing through the electrode structure it is progressively converted from solid raw paste to molten paste and ultimately to a baked electrode.
For the sake of simplicity, the well-known internal reinforcing structure of the electrode has not been illustrated nor has the furnace to which the electrode is applied. These are thoroughly conventional.
A metal bar 8 is disposed within the carbon mass M, preferably along the longitudinal axis of the electrode throughout the length thereof. In order to maintain bar Sin its axial position inside the carbon mass M, tension rods (not shown in FIG. 1) made of an insulating material such as wood or Bakelite may be employed to fix conductive bar 8 to the previously mentioned conventional reinforcing structure of the electrode. The lower portion'of the conductive bar 8 is well immersed in the baked zone 5 of the carbon mass so as to ensure a good electrical contact with this zone.
In order to facilitate this electric contact, the bar material, for instance a metal, is selected to have the highest possible melting point that is compatible with the requirement that it must be consumed together with the electrode. The choice of such a material, which depends on the type of process for which the electrode shall be used, is well within the ability of the skilled art worker. In order to facilitate electric contact between the bar and the carbon mass, the conductive bar is preferably shaped to offer a widesurface of contact with the baked carbon mass, for instance in the shape of a strap or ribbon or any other equivalent shape easily determined by the skilled art worker.
As already indicated, the conductive bar is intended to be consumed together with the carbon electrode and must be therefore periodically restored by adding a new length of bar. This addition may be carried out simply by welding a new length of bar to the upper end of the conductive bar, although other connecting procedures may be employed.
A device 9 for measuring the voltage drop between bar 8 and shell I, is connected by means of conductors 10 and 11 to the upper ends of bar 8 and shell 1.
The measuring of the voltage drop is preferably carried out between shell 1 and bar 8, inasmuch as when measuring between bar 8 and the current carrying plates 6, the contact resistance between shell 1 and current-carrying plates 6 would decrease the sensitivity of the method. To the same device 9, the measuring of the electrode current is conveyed, as by conductors I2 and 13.
The following Example is presented to further illustrate my invention.
EXAMPLE Reference is made to the apparatus of FIG. 1. In a threephase electrical furnace producing calcium carbide, the electrodes have a diameter of 950 mm. and were delta connected and supplied with alternating current through a delta-delta transformer. The electrodes have an outside shell made of a steel sheet and an inside reinforcing structure consisting of six fins welded radially to the inner wall of the outside shell. On the axis of each electrode there was arranged a steel strap 30 mm. wide and 2mm. thick.
Device 9 was a double moving element recording ohmmeter.
Conductors l0 and 11, suitably insulated and shielded, are connected to the upper ends of the central bar 8 and of the metal shell 1 and to the input terminals of the recording ohmmeter. The same meter 9 was used to measure the electrode current, suitably derived from a current transformer on the primary side of the current supply to the transformer of the furnace. This is not necessary to the invention and is not shown in FIG. 1 for simplicitys sake.
Ohmmeter 9 recorded the ratio between the two quantities introduced, that is, the potential between shell 1 and bar 9, and the electrode current. The electrode current of the nor mal operation of the furnace was 45,000 A. Voltage drops detected between shell 1 and bar 8 varied from about 0.2 to about 0.8 v., depending on the degree of baking of the carbon mass layer 5, and on the electrode current.
In a constant electrode current run, wherein the current density was 45,000 A., a 0.2 v. voltage drop was found to correspond to an excellent baking degree and consequently the slipping of the electrode was made possible.
For conveniences sake, the scale of the recording device was calibrated so that the ratio 0.2/45,000 was made to correspond to an index C value of 1. Index C detected by the recording device, varied between about I and about 2.3 depending on the baking degree of the electrode. Values of index C equal to or lower than 1 correspond to an excellent baking degree which allowed the slipping of the electrodes. Conversely, values of index C larger than 1, correspond to increasingly low baking degrees and have an increased risk of breakages. It is quite evident that the relationship between C=l and the ratio 0.2/45,000 is valid only for the particular example given. Other relationships of shell-bar potential to electrode current may be assigned the arbitrary C value of 1 when a different electrode structure and furnace are employed.
Thus, if the characteristics of the electrode (such as for instance diameter, electrode-fonning paste composition, shall and fins shape, current supplying conductors type, and so on) are changed, the index C figures corresponding respectively to excellent and poor baking conditions, will also change. While operating at a fixed and constant electrode current, after having determined the voltage drop figure corresponding to the best baking conditions, the scale of the recording device will be recalibrated and the value 1 of index C will be made to correspond to this new ratio Voltage drop corresponding to the best baking conditions Elec uqis 91 3595 9- In FIG. 2 the typical graph of C is shown. The electrode was slipped down 2 cm. every hour without varying the electrode current. Each slipping was clearly pointed out by a sudden increase of C which immediately dropped back to values equal to about 1.5 times that preceding the slippage. Thereafter, in the course of 1 hour, the value of c resumed its normal value 1 thereby indicating the baking of a portion of the electrode equal to the portion slipped down.
FIG. 3 shows the graph of C during the period of time immediately subsequent to the time period graphed in FIG. 2. The electrode, after the repeated slippages shown in FIG. 2, was very long; slippage was therefore discontinued for ll hours. During this period the value of C dropped below unity, stabilizing itself around 0.8 and thus indicating a baking degree higher than the normal one.
Lastly, FIG. 4 illustrates a dangerous situation in which the electrode was slipped 8 cm. at the 10th hour and again at the llth hour, before the value of C was restored to normal, and then the electrode was caused to be slipped 4 cm. Index C, after the usual fast peak, returned towards the figure 2.2, while the electrode, loaded at the normal current of 45,000 A, demonstrated a serious risk of breakage which compelled the immediate reduction of electrode current to 20,000 A., with a consequential reduction in the output capacity of the furnace.
Variations can, of course, be made without departing from the spirit and scope of the invention.
Having thus described the invention, what is desired to be secured by Letters Patent and hereby claimed is:
l. A method for continuously measuring the baking degree of a self-baking electrode, having a current flowing therethrough, said electrode comprising a peripherally continuous vertically extending shell, a carbon mass contained therein, said carbon mass comprising an upper layer of unbaked electrode-forming paste and a lower layer of baked electrode mass, said layers being in vertically stacked relation, said method comprising: providing a bar of electrically conductive material inside said carbon mass, said bar extending into both the unbaked electrode forming paste and the baked mass so as to enable an electrical contact between said bar and said carbon mass:
measuring a voltage drop between said shell and said bar;
measuring a ratio between said voltage drop and said electrode current in order to determine the baking degree of layer of baked electrode material, said layers being in a vertically stacked relation;
b. a bar made of an electrically conductive material disposed inside said carbon mass and extending into both the unbaked electrode-forming paste and the baked material;
c. means for measuring a voltage drop between said bar and said shell comprising means for connecting said voltage drop measuring means to said bar and said shell; and
d. means for measuring an electrode current.
3. The electrode structure of claim 2, wherein said bar is vertically positioned.
4. The electrode structure of claim 3, wherein said bar is positioned along the longitudinal axis of said electrode structure.
5. The electrode structure of claim 4, wherein said electrically conductive material is a metal.
6. The electrode structure of claim 5, wherein said metal is consumable contemporaneously with the carbon mass.
7. The electrode structure of claim 6, wherein said bar is shaped in the form of a flat strip.
8. A method in accordance with claim 1 wherein said measured ratio is inversely proportional to said baking degree, said baking degree being determined in accordance with said ratio.
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