Method and apparatus for producing self-baking carbon electrode

This invention relates to a method for continuous production of a self-baking carbon electrode in direct connection with the smelting furnace wherein the electrode is consumed. Blocks of a first unbaked carbonaceous electrode paste are supplied to a curing chamber arranged at the upper end of the electrode, which curing chamber is open at its top and at its bottom and has an inner cross section corresponding to the cross section of the electrode which is to be produced, blocks of the first unbaked carbonaceous paste having a smaller diameter than the inner diameter of the curing chamber, supplying a second particulate unbaked carbonaceous electrode paste to the annulus between the curing chamber and the blocks of the first unbaked carbonaceous electrode paste, second electrode paste comprising a binder which cures at a lower temperature than the first carbonaceous paste by heating means arranged on the curing chamber. The second carbonaceous electrode paste thereby forms a cured shell about the central blocks of the first carbonaceous electrode paste. The central unbaked blocks of the first carbonaceous electrode paste are then baked into a solid carbon electrode together with the cured shell by the heat generated in the area of electric current supply to the electrode. The invention further relates to an apparatus for production of such electrodes.

TECHNICAL FIELD 
The present invention relates to a method for producing a self-baking 
carbon electrode for the use in electric smelting furnaces. The invention 
further relates to an apparatus for production of such electrodes. 
BACKGROUND ART 
Conventional self-baking electrodes comprise a vertical arranged electrode 
casing normally made from steel, extending through an opening in the 
furnace roof or hood. The upper end of the electrode casing is open in 
order to allow addition of unbaked carbonaceous electrode paste which upon 
heating softens and melts and is thereafter baked into a solid carbon 
electrode due to heat evolved in the paste in the area of supply of the 
electric operating current to the electrode. As the electrode is consumed 
in the furnace the electrode is lowered and new sections of casing are 
installed on the top of the electrode column and further unbaked electrode 
paste is added. 
Conventional electrodes of this type are equipped with inner, vertical 
metallic ribs affixed to the inner surface of the electrode casing which 
ribs extend radially towards the center of the electrode. When a new 
section of electrode casing is installed at the top of the electrode 
column, the ribs are welded to the ribs in the casing below in order to 
obtain continuous ribs in the vertical direction. The ribs serve as a 
reinforcement for the baked electrode and to conduct electric current and 
heat radially into the electrode paste during the baking process. To 
compensate for the consumption of the electrode, the electrode is lowered 
downwardly into the furnace by means of electrode holding and slipping 
means. 
When conventional electrodes of this type are used, the electrode casing 
and the inner ribs melt when the electrode is being consumed in the 
furnace. The metal content of the casing and the ribs is thus transferred 
to the product produced in the smelting furnace. As the electrode casing 
and the inner ribs usually are made from steel, such conventional 
self-baking electrodes can not be used for electric smelting furnaces for 
the production of silicon or for the production of ferro-silicon having a 
high silicon content, as the iron content in the produced product will 
become unacceptably high. 
Through the years a number of modifications of the above described 
conventional self-baking electrode with casing and steel ribs have been 
proposed in order to avoid contamination of produced silicon with iron 
from the casing and the steel ribs. 
Thus in Norwegian patent No. 149451 it is disclosed a self-baking electrode 
wherein an electrode paste with a tar-based binder contained in a casing 
having no inner vertical ribs is baked above the area where electric 
operation current is supplied to the electrode and where the casing is 
removed after baking of the electrode, but before the electrode reaches 
the area where electric operating current is supplied to the electrode. In 
this way a casing and rib free electrode can be produced. This kind of 
electrode has been used in smelting furnaces for the production of 
silicon, but has the disadvantage compared to conventional prebaked 
electrodes that it needs costly apparatuses for baking of the electrode as 
the electrode in the area of baking has to be heated to a temperature in 
the range of 700.degree.-1000.degree. C. Further, as gases containing 
polyaromatic hydro-carbon compounds (PAH) evolve during baking, the 
apparatus has to be equipped with means for collecting and destroying the 
PAH compounds. Finally, it has to be arranged devices for removal of the 
casing after the electrode has been baked. 
U.S. Pat. No. 4,692,929 discloses a self-baking electrode which is useful 
in the production of silicon. The electrode comprises a permanent metal 
casing having no inner ribs and a support structure for the electrode 
comprising carbon fibers, where the electrode paste is baked about the 
support structure and where the baked electrode is held by the support 
structure. This electrode has the disadvantage that separate holding means 
have to be arranged above the top of the electrode in order to hold the 
electrode by means of the support structure made from carbon fibers. 
U.S. Pat. No. 4,575,856 discloses a self-baking electrode having a 
permanent casing having no inner ribs where the electrode paste is baked 
about a central graphite core and where the electrode is held by the 
graphite core. This electrode has the same disadvantage as the electrode 
disclosed in U.S. Pat. No. 4,692,929, but in addition the graphite core is 
subjected to breakage when the electrode is subjected to horizontal 
forces. 
The above mentioned methods for producing self-baking electrodes having no 
inner metal ribs all have the disadvantage that they can not be used for 
electrodes having a diameter above about 1.2 m without a substantially 
increased risk of electrode breakage. In contrast, conventional 
self-baking electrodes may have a diameter of up to 2.0 m. 
In the production of all the above mentioned types of carbon electrodes it 
is used a carbonaceous electrode paste comprising a particulate solid 
carbon material, preferably anthracite, and a tar-based binder. This 
electrode paste is solid at room temperature. Upon heating, the paste 
starts to soften at a temperature in the range of 50.degree.-150.degree. 
C. as the tar-based binder starts to melt at this temperature. Upon 
further heating to about 500.degree. C. the paste starts to bake, and a 
complete baking to a solid carbonaceous body takes place at a temperature 
above about 800.degree. C. 
DISCLOSURE OF INVENTION 
In spite of the above mentioned methods and apparatuses for production of 
self-baking electrodes in order to avoid iron contamination of the product 
which is produced in the furnace, there is still a need for a reliable 
method and apparatus for production of self-baking carbon electrodes 
whereby the disadvantages of the known methods can be overcome. 
Accordingly, the present invention relates to a method for continuous 
production of a self-baking carbon electrode in direct connection with the 
smelting furnace wherein the electrode is consumed, said method being 
characterized in that blocks of a first unbaked carbonaceous electrode 
paste are supplied to a curing chamber arranged at the upper end of the 
electrode, which curing chamber is open at its top and at its bottom and 
has an inner cross-section corresponding to the cross-section of the 
electrode which is to be produced, said blocks of the first unbaked 
carbonaceous paste having a smaller diameter than the inner diameter of 
the curing chamber, supplying a second particulate unbaked carbonaceous 
electrode paste to the annulus between the curing chamber and the blocks 
of the first unbaked carbonaceous electrode paste, said second electrode 
paste comprising a binder which cures at a lower temperature than the 
first carbonaceous electrode paste, heating and curing the second 
carbonaceous paste by means of heating means arranged on the curing 
chamber, whereby the second carbonaceous electrode paste forms a cured 
shell about the central blocks of the first carbonaceous electrode paste, 
and that the central unbaked blocks of the first carbonaceous electrode 
paste are baked into a solid carbon electrode together with the cured 
shell by means of the heat generated in the area of electric current 
supply to the electrode. 
In order to form the annulus between the curing chamber and the blocks of 
the first unbaked electrode paste, cylinder-shaped blocks of the first 
unbaked electrode paste are preferably supplied, but blocks having another 
cross-section than circular cross-section, such as blocks having oval, 
quadratic or rectangular cross-sections can also be used. 
According to a preferred embodiment the blocks of this first carbonaceous 
electrode paste contain a tar-based binder, while the second carbonaceous 
electrode paste contains a resin-based binder which cures at a temperature 
below 500.degree. C. By heating of the second carbonaceous paste to the 
curing temperature, the first electrode paste containing the tar-based 
binder will remain substantially unaffected. 
By the method of the present invention, during curing of the second 
carbonaceous electrode paste in the area of the curing chamber, a cured 
shell of the second carbonaceous paste, which shell has a sufficient 
strength to allow the electrode to be held and slipped by means of 
conventional electrode holding and slipping equipment when the electrode 
enters below the curing chamber. The cured shell of the second 
carbonaceous electrode paste will further have a sufficient electric and 
thermal conductivity in order to supply electric current via conventional 
current supply means which are used for self-baking carbon electrodes. In 
the area of electric current supply, the cured shell of the second 
electrode paste will then be baked at a high temperature at the same time 
as the blocks of the first electrode paste are baked into solid carbon. A 
monolithic solid carbon electrode is thereby formed in the area of current 
supply. 
The thickness of the cured shell of the second electrode paste is adjusted 
according to the electrode diameter with an increased shell thickness with 
increased electrode diameter. It is, however, preferred that the cured 
shell of the second electrode paste is formed has a minimum thickness of 1 
cm. The cured shell has, however, normally a thickness of at least 5 cm 
and preferably above 10 cm. 
According to another embodiment, the present invention relates to an 
apparatus for continuously production of a self-baking electrode in direct 
connection with a smelting furnace wherein the electrode is being 
consumed, the apparatus comprising holding and slipping means for the 
electrode and means for supplying electric operating current to the 
electrode, said apparatus being characterized in that it further comprises 
a curing chamber arranged at the upper end of the electrode, which curing 
chamber has an open top and an open bottom and has an inner cross-section 
corresponding to the cross-section of the electrode to be produced, which 
curing chamber is affixed to the electrode holding- and slipping means and 
is equipped with heating means for heating the curing chamber to a 
temperature sufficiently high to provide a cured shell of electrode paste 
on the inside of the curing chamber. 
According to a preferred embodiment the heating means comprises at least 
two separate heating means arranged vertically in relation to each other. 
According to another preferred embodiment the heating means comprises a 
plurality of electric resistance heating elements. 
The curing chamber is affixed to the electrode holding- and slipping means. 
Thus by slipping of the electrode the electrode is moved down through the 
curing chamber. The curing chamber is preferably affixed to the electrode 
holding- and slipping means in such a way that the distance between the 
curing chamber and the electrode holding and slipping means is kept 
constant. This gives a simple and reliable design which needs little 
maintenance. In some cases it may be of advantage to affix the curing 
chamber to the electrode holding- and slipping means in such a way that 
the distance between the lower end of the curing chamber and the electrode 
holding- and slipping means can be adjusted. This can be done by affixing 
the curing chamber by means of rails comprising hydraulic or pneumatic 
cylinders. 
The curing chamber can be made from any material which can be used at a 
temperature above 500.degree. C. The curing chamber is preferably made 
from a metal such as steel, or from a ceramic material. As ceramic 
material it is preferred to use ceramic materials having high thermal 
conductivity. 
In order to prevent sticking of electrode paste to the inside of the curing 
chamber, the inside of the curing chamber can be lined with a suitable 
material in order to reduce sticking and friction between the inside of 
the curing chamber and the second electrode paste. Examples of such 
material are polytetrafluretylene, silicones, ceramic lining and polished 
steel. 
The method and the apparatus according to the present invention show a 
number of advantages compared to conventional self-baking electrodes and 
also compared to other prior art self-baking electrodes. The produced 
electrodes give no contamination from electrode casing or ribs and can 
therefore be used in production of silicon and other products where iron 
would contaminate the products. The cured shell of the second electrode 
paste gives a stable outer part of the electrode without causing problems 
such as inconstant material properties caused by segregation which 
conventionally takes place in electrodes which are based on electrode 
paste containing only tar-based binder. The cured shell of the second 
electrode paste further gives an improved safety against so-called soft 
paste electrode breakage compared to the steel casing used in connection 
with conventional self-baking electrodes. As the blocks of the first 
electrode paste do not melt and bake until they reach the area of electric 
current supply to the electrode, the electrode will be closed above the 
area where the first electrode paste melts. The gases including PAH 
compounds, which evolve during baking of the first electrode paste will 
thus not escape to the environment. PAH pollution its thereby avoided by 
the method of the present invention. 
The thickness of the cured shell of the second electrode paste can be 
adjusted according to the electrode diameter, the kind of furnace and the 
current density and can be optimalized for each electrode. This adjustment 
is made by selecting a proper diameter of the blocks of the first 
electrode paste. 
A further substantial advantage of the present invention is that there are 
no requirements with respect to the flow properties of the first electrode 
paste, and the first electrode paste can therefore be selected to give 
optimum properties of the baked electrode without the need to pay 
attention to the flow properties of the paste. For tar-based electrode 
paste, the amount of binder in the paste can thus be reduced.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
FIG. 1 shows an electrode 1 in an electric smelting furnace 2. The smelting 
furnace 2 is equipped with a smoke hood 3 and the charge level in the 
furnace 2 is indicated by reference numeral 4. Contact clamps for supply 
of electric current to the furnace are schematically shown by reference 
numeral 5. The contact clamps 5 are pressed against the electrode by means 
of a pressure ring 6. The contact clamps 5 and the pressure ring 6 are in 
a conventional way equipped with internal channels for circulation of a 
cooling fluid. The contact clamps 5 are via rods 7 suspended from an 
electrode frame 8. 
The electrode frame 8 is in an conventional way suspended in the furnace 
building by means of hydraulic electrode regulation cylinders 13 and 14. 
On the electrode frame 8 there is arranged electrode holding- and slipping 
rings 9,10 for the electrode 1. The upper electrode holding- and slipping 
ring 9 can be moved in the vertical direction by means of hydraulic or 
pneumatic cylinders 11 and 12. 
A curing chamber 17 is affixed to the upper electrode holding- and slipping 
ring 9 by means of a number of rails 15,16. The curing chamber 17 thus 
constitutes the top of the electrode column. The curing chamber 17 is open 
at its top and at its bottom and has an inner cross-section corresponding 
to the cross-section of the electrode to be produced. When the holding- 
and slipping ring 9 is released from the electrode 1 and lifted by means 
of the cylinders 11, 12, the curing chamber 17 will be lifted relative to 
the electrode. When the holding- and slipping ring 9 is reconnected to the 
electrode 1 in its upper position and moved downwardly by means of the 
cylinders 11,12 and with the holding- and slipping ring 10 released from 
the electrode, the electrode 1 together with the curing chamber 17 will be 
moved downwards in vertical direction. In the same way as for conventional 
electrodes the slipping is effected in order to move the electrode 
downwards at the same rate as the electrode is being consumed in the 
smelting furnace 2. Alternatively the curing chamber 17 can be affixed to 
the electrode frame 8. Also in this case slipping of the electrode will 
move the electrode downwards in relation to the curing chamber 17. 
The curing chamber 17 is equipped with a heating means 18. The heating 
means 18 preferably comprises a number of independent sections as shown in 
FIG. 1 where the temperature for each section can be regulated independent 
from the other sections. In the embodiment shown in FIG. 1 the heating 
means 18 comprises four sections, but the number of sections can be more 
or less than four. The heating means 18 comprises preferably one or more 
electric resistance heating elements, but other kind of heating means can 
be used such as for example induction heating, convection heating, gas 
firing and others. 
For production of the electrodes according to the present invention 
cylindrical shaped blocks 19 of the first unbaked electrode paste in the 
center of the electrode are preferably used. The blocks 19 of the first 
electrode paste are placed one upon the other in the center of the curing 
chamber 17. There is, however, no need for exact centering of one block 
relative to the other. Further, there is no need to affix the individual 
blocks 19 to each other. The blocks 19 of the first electrode paste have a 
diameter which is less than the inner diameter of the curing chamber 17, 
whereby an annulus is formed between the curing chamber 17 and the blocks 
19 of the first electrode paste. 
The blocks 19 of the first electrode paste are preferably made from an 
electrode paste comprising a tar-based binder. 
A second electrode paste 20 containing a binder which cures at a lower 
temperature then the first electrode paste is supplied to the annulus 
between the blocks 19 of the first electrode paste and the curing chamber 
17. The second electrode paste 20 is supplied in the forms of particles, 
paste or briquettes. 
The second electrode paste 20 is heated by means of the heating means 18 to 
such a temperature that the second electrode paste is cured while the 
blocks 19 of the first electrode paste remains substantially unaffected. A 
cured shell 21 of the second electrode paste 20 is thereby formed about 
the blocks 19 of the first electrode paste. As the electrode is being 
consumed in the smelting furnace 2, the electrode 1 is being slipped 
downwards by means of the holding- and slipping rings 9, 10, and as the 
curing chamber 17 is affixed to the electrode frame 8, the cured shell 21 
of the second electrode paste 20 is moved out of the lower end of the 
curing chamber 17 as the electrode is slipped. 
The cured shell 21 has a sufficient strength to hold the electrode by means 
of the holdings and slipping rings 9,10. 
When the electrode enters the area of the contact clamps 5 where electric 
operating current is supplied to the electrode, the cured shell 21 of the 
second electrode paste 20 will be heated and conduct heat radially into 
the electrode. The blocks 19 of the first electrode paste will thereby 
melt and form a liquid phase 22 which is then baked into solid carbon. In 
this area the finished baked electrode is produced. 
As the blocks 19 of the first electrode paste are melted and baked in the 
area of the contact clamps 5, PAH containing gases which evolve during the 
baking will not be able to escape to the environment outside of the 
electrode. By use of the present invention the environmental problem of 
PAH containing gases is thereby eliminated. 
As set out above, the heating means 18 preferably comprises a number of 
heating elements with separate temperature regulation. The temperature is 
then regulated in order to have the lowest temperature in the highest 
arranged heating element and the highest temperature in the lowest 
arranged heating element. 
By use of a second electrode paste 20 comprising a novolac resin binder 
with a curing temperature of about 400.degree. C. and by the use of four 
heating elements, the temperature in the individual heating elements may 
advantageously be adjusted in such a way that the temperature is 
regulated, from the upper to the lower heating elements within the range 
of 50.degree.-100.degree. C., 100.degree.-200.degree. C., 
200.degree.-300.degree. C. and 300.degree.-400.degree. C. 
In this way a gradual heating of the second electrode paste 20 is obtained 
and ensures that a cured shell 21 of the second electrode paste 20 has 
been formed when the electrode moves out from the curing chamber 17. The 
blocks 19 of the first electrode paste are substantially unaffected during 
the heating in the curing chamber 17 as the temperature only will provide 
a local softening on the surface of the blocks 19. The blocks 19 will 
thereby maintain their shape and provide a formwork for the formation of 
the cured shell 21 of the second electrode paste 20. 
In FIG. 3 there is shown a second embodiment of the apparatus according to 
the present invention. Parts on FIG. 3 corresponding to parts on FIG. 1 
have been given the same reference numerals. 
The apparatus shown in FIG. 3 only differs from the apparatus shown in FIG. 
1 in that the curing chamber 17 is adjustably affixed to the holding- and 
slipping ring 9. In the apparatus shown in FIG. 3 the curing chamber 17 is 
affixed to the holding- and slipping ring 9 by means of hydraulic or 
pneumatic cylinders 23, 24. The distance between the lower end of the 
curing chamber 17 and the holding- and slipping ring 9 can be adjusted by 
movement of the cylinders 23, 24. This can be of advantage when the 
electrode consumption is high, such as for example in connection with an 
electrode breakage in the smelting furnace. An additional length of 
electrode can then be slipped down by reducing the distance between the 
lower end of the curing chamber 17 and the holding- and slipping ring 9 by 
means of the cylinders 23, 24. 
In normal electrode operation, the temperature in each heating element will 
be kept substantially constant. In abnormal electrode operation such as 
for example in connection with high electrode consumption rate, the 
temperature can be increased in order to increase the curing rate of the 
second electrode paste 20. 
The electrode produced according to the present invention can be installed 
in smelting furnaces where conventional self-baking electrodes are used 
today and also in furnaces using prebaked carbon electrodes of graphite 
electrodes, as existing holding- and slipping equipment and electric 
current supply means can be used without modifications.