Pin for connecting carbon electrodes and process therefor

Details Pin for connecting carbon electrodes and process therefor Pin for connecting carbon electrodes and process therefor

2001-04-18

2002-08-27

Jones, Deborah (Department: 1775)

Stock material or miscellaneous articles

Self-sustaining carbon mass or layer with impregnant or...

Reexamination Certificate

active

06440563

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a pin for connecting carbon electrodes, and a process for preparing the inventive pin. More particularly, the invention concerns a pin for connecting carbon electrodes, such as graphite electrodes, formed by processing a blend of calcined coke, pitch and carbon fibers derived from mesophase pitch.
BACKGROUND ART
Carbon electrodes, especially graphite electrodes, are used in the steel industry to melt the metals and other ingredients used to form steel in electrothermal furnaces. The heat needed to melt metals is generated by passing current through a plurality of electrodes, usually three, and forming an arc between the electrodes and the metal. Currents in excess of 100,000 amperes are often used. The resulting high temperature melts the metals and other ingredients. Generally, the electrodes used in steel furnaces each consist of electrode columns, that is, a series of individual electrodes joined to form a single column. In this way, as electrodes are depleted during the thermal process, replacement electrodes can be joined to the column to maintain the length of the column extending into the furnace.
Generally, electrodes are joined into columns via a pin (sometimes referred to as a nipple) that functions to join the ends of adjoining electrodes. Typically, the pin takes the form of opposed male threaded sections, with at least one end of the electrodes comprising female threaded sections capable of mating with the male threaded section of the pin. Thus, when each of the opposing male threaded sections of a pin are threaded into female threaded sections in the ends of two electrodes, those electrodes become joined into an electrode column. Commonly, the joined ends of the adjoining electrodes, and the pin therebetween, are referred to in the art as a joint.
Given the extreme thermal stress that the joint (and indeed the electrode column as a whole) undergoes, mechanical factors such as thermal expansion must be carefully balanced to avoid damage or destruction of the electrode column or individual electrodes. For instance, longitudinal (i.e., along the length of the pin/electrode/electrode column) thermal expansion of the pin, especially at a greater rate than that of the electrodes, can force the joint apart, reducing effectiveness of the electrode column. A certain amount of transverse (i.e., across the diameter of the pin/electrode/electrode column) thermal expansion of the pin in excess of that of the electrodes may be desirable to form a firm connection between pin and electrode; however, if the transverse thermal expansion of the pin greatly exceeds that of the electrode, damage to the electrode may result, in the form of cracking or splitting. Again, this can result in reduced effectiveness of the electrode column, or even destruction of the column if the damage is so severe that a joint fails. Thus, control of the thermal expansion of a pin, in both the longitudinal and transverse directions, is of paramount importance.
There have been references to the use of mesophase pitch-based carbon fibers to improve specific properties of bulk graphite products such as electrodes. For instance, Singer, in U.S. Pat. No. 4,005,183, describes the production of mesophase pitch-based fibers and states that, because of their low electrical resistivity, these fibers can be employed as filler material in the production of graphite electrodes. In British Patent 1,526,809 to Lewis and Singer, 50% to 80% by weight of carbon fibers are added to 20% to 50% by weight of pitch binder and then extruded to form a carbon artifact that can be graphitized. The resulting article exhibits relatively low longitudinal thermal expansion.
In U.S. Pat. No. 4,998,709, Griffin et al. attempt to address the problems caused by excessive longitudinal thermal expansion of electrode pins by preparing a graphite nipple (i.e., pin) with mesophase pitch-based carbon fibers included in the extrusion blend. The carbon fibers used by Griffin et al. have a Young's modulus of greater than 55×10
6
pounds per square inch (psi), and are present in the blend at about 8 to 20 weight percent. The blend is extruded, baked, and then graphitized for from about 5 to 14 days to produce the nipple. Although nipples produced by the Griffin et al. process show a decrease in the coefficient of thermal expansion (CTE) in the longitudinal direction, they also show an undesirable increase in CTE in the transverse direction, an increase in electrical resistivity and a decrease in the modulus of rupture. In addition, the graphitizing time is extremely long compared with times that would be advantageous for commercial production.
What is desired, therefore, is a pin for connecting carbon electrodes, the pin having reduced CTE in the longitudinal direction as compared with art-conventional pins, without sacrificing transverse CTE or resistivity and modulus of rupture. Especially desirable is such a pin that is prepared by a process that does not require 5 days of graphitization. It is also highly desirable to achieve these property benefits without using high quantities of expensive materials.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a process for preparing pins for connecting carbon electrodes.
It is another object of the present invention to provide a process for preparing pins for connecting carbon electrodes, the pins having reduced longitudinal coefficient of thermal expansion as compared to art-conventional pins.
It is yet another object of the present invention to provide a process for preparing pins for connecting carbon electrodes, the pins having reduced longitudinal coefficient of thermal expansion as compared to art-conventional pins, without substantial sacrifice of transverse CTE or resistivity while also increasing the modulus of rupture.
It is still another object of the present invention to provide a process for preparing pins for connecting carbon electrodes, the pins having reduced longitudinal coefficient of thermal expansion as compared to art-conventional pins, wherein the process requires graphitization times significantly shorter than 5 days.
These objects and others that will become apparent to the artisan upon review of the following description can be accomplished by providing a process for preparing pins for connecting carbon electrodes, the process including combining calcined coke, a liquid pitch binder and carbon fibers derived from mesophase pitch to form a pinstock blend; extruding the pinstock blend to form a green pinstock; baking the green pinstock to form a carbonized pinstock; and graphitizing the carbonized pinstock by heating to a temperature of at least about 2500° C. and maintaining it at that temperature for no more than about 18 hours.
In the inventive process, the carbon fibers are preferably present at a level of about 0.5 to about 5 parts by weight of carbon fibers per 100 parts by weight of calcined coke, or at about 0.4% to about 4.0% by weight of the total mix components, have a Young's modulus after graphitization of no more than about 40×10
6
psi, an average diameter of about 6 to about 15 microns, and a length of about ⅙ inch to about 1 inch. Most advantageously, the carbon fibers are added to the pinstock blend as bundles, each bundle containing from about 2000 to about 20,000 fibers. The baking of the green pinstock preferably takes place at a temperature of up to about 700 to about 1000° C. in a non-oxidizing or reducing environment, and graphitization is more preferably at a temperature of from about 2500 to about 3400° C.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
As noted above, pins for connecting graphite electrodes can be fabricated by first combining calcined coke, pitch and mesophase pitch-based carbon fibers into a pinstock blend. More specifically, crushed, sized and milled calcined petroleum coke is mixed with a coal-tar pitch binder to form the blend. The particle size of the calcined coke is selected according to the end use of the electrode, and is

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