Zollner, Dieter (Erlangen, DT)
Koziol, Konrad (Rothenbach/Pegnitz, DT)
Reichelt, Bernhard (Nurnberg-Laufamholz, DT)
Lippert, Wolfgang (Schwabach, DT)
What we claim is
1. A high output electrode of carbon material, especially carbon electrode and graphite electrode, which includes a titanium-boron combination consisting of at least one of the compounds TiB and TiB2 of from 1 to 8 percent to thereby improve the oxidation resistance, electrical load ability, and electric arc stability for light arc furnace means used in steel making and a remaining part of the electrode amounting to 80 percent and more consists of graphite.
The high output operation of electric arc furnaces during which transformer outputs of approximately from 400 to 500 kVA/t are employed require graphite electrodes of high power transmitting ability. The electric conductivity of the graphite material had, similar to the thermal shock resistance and the oxidation resistance, to be adapted to the increasing current densities and temperature loads. This was done by employing oil cokes of ever higher grade and very good graphitizing behavior, of higher graphitizing temperatures and additional pitch impregnation which require an additional furnace process for the ordinary finishing step for a post-coking. All of these steps are expensive and raise the costs of the manufacturing process of the electrode. In addition thereto, the impregnation of the electrode, which is necessary for lowering the specific electric resistance of the graphite material, frequently brings about an increased liability to form tears in the graphite electrode or to decrease the resistance of the electrode to breaking.
Of great importance in this connection for an economic operation of the high output electric arc furnace is an as uniform as possible current withdrawal from the supply network without any special reaction by the network. Due to the operation with relatively short light arc, it is possible to reduce the flickering, but it is not possible completely to eliminate the same. Moreover, when operating the light arc at a low voltage and high current intensity, a greater wear of the tip of the electrode takes place than is the case in the reversed instance. Therefore, attempts have been made to cause the arc to burn in a more stable manner by the employment of hollow electrodes with and without the supply of gases stabilizing the arc. These attempts have been successful. This method, however, has the drawback that it requires a drilled or very dense electrode which at any rate is more expensive than the heretofore customary solid electrode. When operating with gases, additional costs are incurred which could not be justified in spite of the metallurgical advantages of this method.
It is, therefore, an object of the present invention to provide means for increasing the electric load of the electrode and for reducing its liability to oxidation, while maintaining its resistance against tears.
It is also an object of the present invention to provide means for stabilizing the light arc in order to eliminate the flickering for all practical purposes.
These and other objects and advantages of the invention will appear more clearly from the following specification.
By the addition of or impregnation with suitable substances during the manufacturing process of the graphite electrode, the finished graphite electrodes will contain titanium borides which will bring about a considerable reduction in the electric resistance of the electrode while increasing the resistance of the electrode against oxidation and while stabilizing the light arc when the electrode is being used in the electric arc furnace. The titanium borides may be added already directly to the mixture of the raw material which mixture customarily consists of a granular mixture of oil coke, tar and pitch. The said titanium borides will, while unchanged during the manufacturing process of the electrode, become effective only when the electrode is used in the electric arc furnace.
Two borides of the titanium are known, namely titanium monoboride TiB, and the titanium diboride TiB2. In addition thereto, the titanium is able to absorb considerable quantities of boron in solid solution so that also this type has the advantages inherent to the present invention. A certain proportion of titanium carbide which may form during the graphitizing process at boundary surface reactions of the added particles with the carbon or graphite will have no disturbing or interfering influence. The total content in titanium-boron compounds may amount to 20 percent, but preferably will be from 1 to 8 percent.
The introduction of the borides into the graphite electrode may be effected in different ways, namely:
1. By admixing reaction components to the starting material during the finishing process.
2. By impregnating reaction components and introducing the same into the boron electrode prior to graphitizing the latter. The titanium boron compounds will then form during the graphitizing process from a temperature of 1,300° C. upwards.
3. By admixing titanium borides to the starting mixture. This method may also be applied with the electrodes which are still to be graphitized. This method is particularly advantageous with carbon electrodes which are not to be graphitized as they are employed, for instance, in connection with the arc air method (Fugenhobelverfahren). Also in this instance the same problems are encountered as with large electrodes in the electric arc furnace. The extreme high current intensities which are employed with this cutting-blowing-method require increasing electric conductivity and a higher resistance against oxidation. In view of the stabilization of the electric light arc, a more favorable working condition can be obtained.
With the admixture to the completed electrode, titanium-boron-alloys, titanium monoboride and titanium diboride may be employed.
When admixing the reaction components, there exists the possibility of varying the admixture in conformity with the expected or desired reactions. Thus, for instance, TiO2 and B2 O3 may be admixed to the electrode raw mixture, and in this instance during the graphitizing process there will be obtained
TiO2 + B2 O3 + 5 C -- TiB2 + 5 CO,
or B4 C, titanium and B2 O3 are added in which instance during the graphitizing process the following reaction is obtained
7 Ti +3 B4 C + B2 O3 -- 7 TiB + 3 CO.
These methods may serve as examples.
The introduction of titanium components and boron components into an already burned electrode may be effected also by impregnation with titanium silicon compounds and boron organic compounds in organic solution with a subsequent dilution of the solution, for instance, by humidification so that a disintegration of such organic compounds will occur. Such disintegration may also be carried out in a purely thermal way.
There may now be set forth two examples for preparing the mixture for an electrode according to the invention.
7% by weight of oil coke with a granular size of from 4 to 8 mm
14% by weight of oil coke with a granular size of from 2 to 4 mm
18% by weight of oil coke with a granular size of from 0.5 to 2 mm
53.5% by weight of oil coke with a granular size of less than 0.5 mm
4% by weight of TiO2 dust with a granular size of less than 0.1 mm
3.5% by weight of B2 O with a granular size of less than 0.5 mm
To the above mentioned dry starting mixture there were added 25 percent by weight of pitch with a softening or fusion point of 82° C.
The thus obtained product was then further processed in a manner customary with the manufacture of carbon or graphite electrodes, i.e., by mixing, pressing, glowing and graphetizing.
26.5% by weight of oil coke with a granular size of from 4 to 8 mm
20.5% by weight of oil coke with a granular size of from 2 to 4 mm
21.3% by weight of oil coke with a granular size of from 0.5 to 2 mm
25 % by weight of oil coke with a granular size of less than 0.5 mm
6.7% by weight of TiB2 dust with a granular size of less than 0.1 mm.
To the above mentioned starting mixture there was added 23.5 percent by weight of pitch or insulating asphalt with a softening or fusion point of 82° C.
The thus obtained product was then further processed in a manner customary with the manufacture of carbon or graphite electrodes, by mixing, pressing, glowing, and graphetizing, which steps have no harmful effect upon the stabilizing effect of TiO2.
Electrodes produced in conformity with the present invention can absorb a considerably higher electric load, have a higher resistance against oxidation and are characterized by a great stability of the light arc, which is in contrast to normal carbon or graphite electrodes.
It is, of course, to be understood that the present invention is, by no means, limited to the particular examples set forth above but also comprises any modifications within the scope of the appended claims. Thus, while e.g., in Example I TiO2 2 dust with a granular size of less than 0.1 mm has been mentioned, the granular size of TiO2 may advantageously be less than 60/l,000 mm.