Electrode for arc furnaces

Electrode for arc furnaces, more particularly for electrosteel production, also using scrap, comprising a detachable top portion (5) of metal and a replaceable bottom portion (6) of material which is consumed, where appropriate only slowly, having substantially cylindrical shape and being interconnected by a screw nipple (1) or the like and the top portion has a liquid cooling device with a header duct (2) and a return duct (3) and an inner part (16) and an outer part (17) of the top portion are constructed so as to be detachable from each other, so that the inner part (16) contains the liquid conducting chamber with header duct and return duct and the outer part (17) surrounds only a portion of the inner part (16). The electrodes have a reliable coolant conducting system, are easy to maintain and can be simply repaired in the event of mechanical damage while minimizing electrode down times.

This invention relates to an electrode for arc furnaces comprising a 
detachable top portion of metal and a replaceable bottom portion of 
consumable or only slowly consumable material, substantially of 
cylindrical shape and connected to each other by means of a screw nipple 
or the like, the top portion having a liquid cooling means with a header 
duct and a return duct. 
Such electrodes, for example as disclosed in the German Auslegeschrift No. 
27 39 483, offer an advantage over conventional carbon electrodes, in that 
only the electrode tip is consumable and requires replacement. The 
remaining part of the electrode, namely the liquid cooled electrode 
holder, can be used for a longer period of time. 
When used in an arc furnace, particularly an arc furnace in which scrap is 
melted, such electrodes are exposed to substantial stresses. Damage to the 
electrode, for example in the region coated with a material of high 
temperature stability, or in the liquid cooled top part of the electrode, 
can occur simply when the electrode moves into the furnace. Furthermore, 
there is the risk of arc breakthrough between the top metal shank, which 
conducts the both electrical current and the coolant liquid, and the 
metallic contents of the arc furnace. Sliding of scrap into the melt also 
provides mechanical stresses which combined with other factors can lead to 
electrode failure as well as to discharge of the coolant fluid into the 
arc furnace, and thereby to explosions. 
Liquid cooled arc electrodes in which the electrode tip also consists of 
non-consumable material but is constructed from liquid cooled metal, are 
particularly exposed to these risks. Such electrodes have been disclosed, 
for example in German Offenlegungsschrift No. 15 65 208 and U.S. Pat. No. 
3,689,740 wherein the arc is guided over the electrode tip by means of 
magnetic fields or adequate velocity. Owing to the risk of short circuit 
when the electrodes enter or due to the tendency to sustain damage when 
the charge is melted, it was not possible for such electrodes to be 
accepted for use in arc furnaces in which scrap is also melted. 
British Patent Specification No. 1 223 162 therefore proposed the use of 
liquid cooled metal shanks with a consumable part, the metal shank is 
provided with a ceramic protective coating. Belgian Patent Specification 
No. 867 876 also describes such an electrode in which water conducting 
tubes are embedded in a compound of refractory material. 
European Patent Application No. 79302809.3 also describes a ceramically 
protected electrode in which the liquid cooling system extends centrally 
in the metal shank. Graphite rods, the fracture or errosion of which can 
be monitored by the pressure of gas which flows around the rods, are 
inserted into such shanks. Although this construction of the metal shank 
facilitates monitoring of mechanical damage, the construction of the 
entire electrode is relatively complex and actually mechanical damage 
occurring to the metal shank can be remedied generally only with a 
substantial effort after removal of the entire electrode. 
The German Auslegeschrift No. 27 39 483 also describes an electrode of the 
initially mentioned kind in which liquid cooling is ensured inter alia by 
annular ducts which are directly guided on the external wall. In this 
system, special attention has been given to ensure that the liquid return 
adjoins directly on the external surface line of the metal shank so that 
the external wall of the metal shank also represents the internal wall of 
the return duct. To facilitate maintenance and inspection it is finally 
possible to remove the entire inner part from the outer part of the top 
portion. To this end it is necessary to release the screw fasteners of a 
ring flange and to lift out the internal structure after shutting down the 
supply of liquid and emptying the cooling system. However, in the event of 
damage in the region of the top portion, this electrode does not permit 
any rapid and relatively simple means of repair. Furthermore, mechanical 
damage of the top portion or as a result of short circuits leads directly 
to water breakthrough as a result of the externally disposed annular ducts 
and return ducts and in some cases leads to explosions associated with 
such a defect. 
It is the object of the invention to provide a reliably operating electrode 
which is easy to maintain and less trouble prone. It is to be particularly 
easy to instal or to dismantle for inspection. Escape of cooling fluid is 
to be avoided in the event of mechanical damage of the electrode and rapid 
simple repair is to be possible while minimizing the down times. 
This problem is solved by an electrode of the initially mentioned kind in 
which an inner part and an outer part of the top portion are constructed 
so as to be detachable each from the other, the inner part including a 
liquid cooling means having header and return ducts, the outer part only 
partially surrounds the inner part. 
The outer part performs as a terminal electrode and can consist of the same 
metal or metal alloy as comprises the inner part. Cooling ports or the 
like can be provided in the outer part. It is also possible to provide the 
outer part with retaining bores, for example for biasingly guiding and 
supporting insulating protective layers which are disposed below. 
In a preferred embodiment of the electrode according to the invention the 
inner part is only partially surrounded by the outer part so that the 
metal shank in its entirety can be formed to include a top region of 
larger diameter and a bottom region of smaller diameter. The inner part of 
such an electrode can be protected by an insulating layer of high 
temperature stability, for example advantageously adjoining the outer part 
and extending downwardly to a point near the screw nipple or the like or 
beyond such screw nipple to a partial covering, usually small, of the 
consumable part. The insulating layer of high temperature stability can 
consist of a ceramic material but also can consist of graphite coated with 
a ceramic material. It is particularly advantageous if the insulating 
coating comprises a solid moulding, for example a coated individual 
graphite tube or a series of parts or segments which are self-supporting, 
in an abutment, for example in accordance with a tongue and groove system, 
and are movable in the direction of the electrode axis. 
In the preferred embodiment of the electrode, in which a partial top region 
of the inner part, more particularly in the region of the lateral current 
supply means, is surrounded, it is not usually necessary to cover the 
outer part additionally with a ceramic, insulating coating. This will 
however depend on the dimensions of the height of the outer part in 
relation to the inner part and can be determined in accordance with the 
use and purpose of the electrode. 
The inner part of the electrode extends to a nipple connection by means of 
which the top metallic portion and the consumable bottom portion are 
threadably interconnected. The liquid cooling device of the inner part, 
extending axially therein, is advantageously extended to the screw nipple, 
since this threadable interconnection can be exposed to particular heat 
stresses depending on the material used in forming such a screw nipple. 
The connection between the inner and outer part can be effected in 
different ways. The connecting line usually extends parallel with a 
longitudinal electrode axis. For example, the detachable connection can be 
obtained by screwthreading or by appropriate fitting of the parts. It is 
particularly preferred for the inner part to be constructed as a register 
member of conical or taper form and for sections of the outer and of the 
inner parts, where appropriate, and to have additional screwthreading. 
Connecting jaws can be attached to the outer part, for example by means of 
pocket or retaining means to which the current supply for the electrode is 
connected. Pockets, in which graphite plates or segments are introduced to 
supply current, are attached to the outer part in a preferred embodiment 
of the invention. 
The inventive construction of the electrode achieves a series of 
advantages. By virtue of the water duct system being guided in the inner 
part, this system remains intact even if the outer part is mechanically 
damaged. In the event of damage of the outer region of the top portion it 
is therefore not necessary to interrupt the supply of cooling liquid, to 
empty the electrode etc. The simple detachability of the outer portion 
enables this to be readily exchanged as a component in the event of damage 
while more conventional constructions call for complete repair of the 
metal shank or its replacement. The lateral current supply, for example 
via graphite contact jaws or segments, which are attached, for example in 
retaining pockets formed in the outer part, dispenses with the need for 
removal of the electrode in its entirety on the busbar in the event of 
defects in the region of the internally disposed liquid duct system, since 
the internal part alone may be detached. By constructing the top region as 
two parts, one of larger diameter and one of smaller diameter it is 
possible for the insulating protective layer of high temperature stability 
to be connected in a particularly compact and convenient form and it is 
not necessary in addition to protect the outer part in insulating manner, 
if this is confined to the current supply means.

FIG. 1 shows the basic construction of the electrode comprising the top 
portion 5 and the bottom portion 6, which are interconnected by a screw 
nipple 1. Fluid is supplied through a central header duct 2 and the 
coolant liquid is again discharged via the return ducts 3. The 
illustrations clearly show that the cooling system is guided in the 
internal part 16 which the outer part 17 surrounds. 
Some of the preferred means of connecting the inner part 16 and the outer 
or upper part 17 as a register member, where appropriate with additional 
part screwthreading, can be seen particularly by reference to FIGS. 2 and 
3. Pins 9 or the like can be guided by means of bores 8 to retain an 
insulating coating 4 via the spring 10 on an abutment 7. The insulating 
part can be additionally secured by retaining means 14. Cooling ports 15 
are shown in the outer part while connecting jaws 18, for example of 
graphite, are shown on the outside. These jaws can be secured in retaining 
means or pockets 19 which are attached to the outer edge of the metal 
shank.