U.S. patent application number 11/123768 was filed with the patent office on 2005-11-17 for graphite electrode for electrothermic reduction furnaces, electrode column, and method of producing graphite electrodes.
This patent application is currently assigned to SGL Carbon AG. Invention is credited to Daimer, Johann.
Application Number | 20050254545 11/123768 |
Document ID | / |
Family ID | 34967904 |
Filed Date | 2005-11-17 |
United States Patent
Application |
20050254545 |
Kind Code |
A1 |
Daimer, Johann |
November 17, 2005 |
Graphite electrode for electrothermic reduction furnaces, electrode
column, and method of producing graphite electrodes
Abstract
A graphite electrode for an electrothermic reduction furnace is
formed from anode grade coke and graphitized at a graphitization
temperature below 2700.degree. C. The resulting electrode is
particularly suited for carbothermal reduction of alumina. It has
an iron content of about 0.05% by weight, a specific electrical
resistivity of above 5 .mu.Ohm.multidot.m, and a thermal
conductivity of less than 150 W/m.multidot.K. The graphite
electrode is manufactured by first mixing calcined anode coke with
a coal-tar pitch binder, and a green electrode is formed from the
mixture at a temperature close to the softening point of the pitch
binder. The green electrode is then baked to carbonize the pitch
binder to solid coke. The resultant carbonized electrode, after
further optional processing is then graphitized at a temperature
below 2700.degree. C. for a time sufficient to cause the carbon
atoms in the carbonized electrode to organize into the crystalline
structure of graphite.
Inventors: |
Daimer, Johann;
(Morfelden-Walldorf, DE) |
Correspondence
Address: |
LERNER AND GREENBERG, PA
P O BOX 2480
HOLLYWOOD
FL
33022-2480
US
|
Assignee: |
SGL Carbon AG
|
Family ID: |
34967904 |
Appl. No.: |
11/123768 |
Filed: |
May 5, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60570984 |
May 12, 2004 |
|
|
|
Current U.S.
Class: |
373/54 ; 201/5;
252/502; 373/88 |
Current CPC
Class: |
C25C 7/025 20130101;
C04B 2235/661 20130101; C04B 2235/72 20130101; C04B 38/0058
20130101; C04B 2235/5427 20130101; H05B 7/09 20130101; C04B 38/0058
20130101; C04B 2235/602 20130101; C04B 35/522 20130101; C04B 35/83
20130101; C04B 35/62675 20130101; C04B 35/522 20130101; C04B 35/532
20130101; C04B 35/521 20130101; C04B 2235/6021 20130101; C25C 3/125
20130101; C04B 2235/6562 20130101; C04B 2235/9607 20130101; C04B
2235/656 20130101; C04B 2235/77 20130101 |
Class at
Publication: |
373/054 ;
373/088; 252/502; 201/005 |
International
Class: |
H05B 003/60; C10L
005/02 |
Claims
I claim:
1. In an electrothermic reduction furnace, a graphite electrode
comprising a shaped graphite electrode body formed from anode grade
coke, graphitized at a graphitization temperature below
2700.degree. C., and having an iron content of less than 0.1% by
weight.
2. The graphite electrode according to claim 1, wherein said
electrode body has a specific electrical resistivity of above 5
.mu.Ohm.multidot.m and a thermal conductivity of less than 150
W/m.multidot.K.
3. The graphite electrode according to claim 1, wherein said
electrode body has an iron content of approximately 0.05% by
weight.
4. The graphite electrode according to claim 1 configured for an
electrothermic reduction furnace for producing one of aluminum,
titanium, silicon, ferroalloys, and phosphorous.
5. The graphite electrode according to claim 1, which further
comprises an amount of carbon nanofibers incorporated in said
electrode body for increasing a mechanical strength and adjusting a
coefficient of thermal expansion thereof.
6. The graphite electrode according to claim 1, which further
comprises an amount of carbon fibers incorporated in said electrode
body for increasing a mechanical strength and adjusting a
coefficient of thermal expansion thereof.
7. The graphite electrode according to claim 1, wherein said anode
grade coke has a mean particle size of approximately 5 to
approximately 10 mm.
8. The graphite electrode according to claim 7, wherein said mean
particle size is between 5 and 7 mm.
9. In a reactor for direct carbothermic reduction of alumina, the
carbon electrode according to claim 1.
10. An intermediate product in the production of a graphite
electrode, comprising: particles of anode grade coke having a mean
particle size of between 5 and 10 mm and an ash content of less
than 0.5% mixed with a pitch binder and formed into a green
electrode to be baked and graphitized to form a graphite
electrode.
11. In combination with the graphite electrode according to claim
1, a graphite pin formed of anode grade coke, graphitized at a
graphitization temperature below 2700.degree. C., having an iron
content of less than 0.1% by weight, and being formed to mate with
said graphite electrode body to form an electrode column.
12. In a self-baking composite electrode for an electrothermic
reduction furnace, the graphite electrode according to claim 1
disposed to form a central column of the self-baking composite
electrode.
13. A method of producing a graphite electrode, which comprises:
providing calcined anode coke with an average particle size of 5 to
10 mm and mixing the anode coke with a coal-tar pitch binder to
form a mixture; forming an electrode body from the mixture to form
a green electrode at a temperature in a vicinity of a softening
point of the pitch binder; baking the green electrode at a
temperature of between approximately 700.degree. C. and
approximately 1100.degree. C., to carbonize the pitch binder to
solid coke, to form a carbonized electrode; graphitizing the
carbonized electrode with a heat treatment at a final temperature
between 2100.degree. C. to 2700.degree. C. for a time sufficient to
cause carbon atoms in the carbonized electrode to organize into a
crystalline structure of graphite.
14. The method according to claim 13, which comprises graphitizing
at a temperature of between 2200.degree. C. to 2500.degree. C.
15. The method according to claim 13, which comprises baking the
green electrode at a temperature between 800.degree. C. and
1000.degree. C.
16. The method according to claim 13, which comprises baking the
green electrode in a relative absence of air at a heating rate of
approximately 1 K to approximately 5 K per hour to the final
temperature.
17. The method according to claim 13, which comprises, after the
baking, impregnating the electrode at least one time with coal tar
or petroleum pitch for depositing additional pitch coke in open
pores of the electrode, and following each impregnating step with
an additional baking step.
18. The method according to claim 13, which adding oils or other
lubricants into the mixture and forming the green electrode by
extrusion.
19. The method according to claim 13, which comprises forming the
green electrode by molding in a forming mold or by vibromolding in
an agitated mold.
20. The method according to claim 13, which comprises adding a
relatively low proportion of carbon fibers or carbon nanofibers
into the mixture for forming the green electrode.
21. The method according to claim 13, which further comprises
machining the graphitized electrode formed in the graphitizing step
to provide a final form of the graphite electrode.
22. The method according to claim 13, which comprises providing the
calcined anode coke with an average particle size of 5 to 7 mm.
23. A method of producing a graphite electrode column, which
comprises producing a plurality of graphitized electrodes with the
method according to claim 13, producing a nipple configured to mesh
with the graphitized electrodes, and connecting the electrodes and
the nipple to form a graphite electrode column.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims the benefit under 35 U.S.C. .sctn.
119 (e), of copending U.S. Provisional Application No. 60/570,984,
filed May 12, 2004.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to graphite electrodes for
electrothermic reduction furnaces, in particular for the production
of aluminum, titanium, silicon, ferroalloys, phosphorous. The
invention also pertains to a method of producing such graphite
electrodes.
[0004] 2. Description of the Related Art
[0005] For a century the aluminum industry has relied on the
Hall-Heroult process for aluminum smelting. In comparison with
processes used to produce competing materials, such as steel and
plastics, the process is energy-intensive and costly. Hence,
alternative aluminum production processes have been sought.
[0006] One such alternative is the process referred to as direct
carbothermic reduction of alumina. As described in U.S. Pat. No.
2,974,032 (Grunert et al.) the process, which can be summarized
with the overall reaction
Al.sub.2O.sub.3+3C=2Al+3CO (1)
[0007] takes place, or can be made to take place, in two steps:
2Al.sub.2O.sub.3+9C=Al.sub.4C.sub.3+6CO (2)
Al.sub.4C.sub.3+Al.sub.2O.sub.3=6Al+3CO (3).
[0008] Reaction (2) takes place at temperatures between 1900 and
2000.degree. C. The actual aluminum producing reaction (3) takes
place at temperatures of 2200.degree. C. and above; the reaction
rate increases with increasing temperature. In addition to the
species stated in reactions (2) and (3), volatile Al species
including Al.sub.2O are formed in reactions (2) and (3) and are
carried away with the off gas. Unless recovered, these volatile
species represent a loss in the yield of aluminum. Both reactions
(2) and (3) are endothermic.
[0009] Various attempts have been made to develop efficient
production technology for the direct carbothermic reduction of
alumina (cf. Marshall Bruno, Light Metals 2003, TMS (The Minerals,
Metals & Materials Society) 2003). U.S. Pat. No. 3,607,221
(Kibby) describes a process in which all products quickly vaporize
to essentially only gaseous aluminum and CO, containing the
vaporous mixture with a layer of liquid aluminum at a temperature
sufficiently low that the vapor pressure of the liquid aluminum is
less than the partial pressure of the aluminum vapor in contact
with it and sufficiently high to prevent the reaction of carbon
monoxide and aluminum and recovering the substantially pure
aluminum.
[0010] Other patents relating to carbothermic reduction to produce
aluminum include U.S. Pat. No. 4,486,229 (Troup et al.) and U.S.
Pat. No. 4,491,472 (Stevenson et al.). Dual reaction zones are
described in U.S. Pat. No. 4,099,959 (Dewing et al.). More recent
efforts by Alcoa and Elkem led to a novel two-compartment reactor
design as described in U.S. Pat. No. 6,440,193 (Johansen et
al.).
[0011] In the two-compartment reactor, reaction (2) is
substantially confined to a low-temperature compartment. The molten
bath of Al.sub.4C.sub.3 and Al.sub.2O.sub.3 flows under an
underflow partition wall into a high-temperature compartment, where
reaction (3) takes-place. The thus generated aluminum forms a layer
on the top of a molten slag layer and is tapped from the
high-temperature compartment. The off-gases from the
low-temperature compartment and from the high-temperature
compartment, which contain Al vapor and volatile Al.sub.2O are
reacted in a separate vapor recovery units to form Al.sub.4C.sub.3,
which is re-injected into the low-temperature compartment. The
energy necessary to maintain the temperature in the low-temperature
compartment can be provided by way of high intensity resistance
heating such as through graphite electrodes submerged into the
molten bath. Similarly, the energy necessary to maintain the
temperature in the high-temperature compartment can be provided by
a plurality of pairs of electrodes substantially horizontally
arranged in the sidewalls of that compartment of the reaction
vessel.
[0012] With the exception of aluminum production, electrothermic
reduction of various metals and also non-metals, such as titanium,
silicon, ferroalloys, as well as phosphorous, are well-established
industrial processes. Due to the relatively low current densities,
ranging from 6 to 10 A/cm.sup.2, in many of these processes
self-baking carbon electrodes (also called "Soderberg electrodes")
are being used.
[0013] The use of self-baking carbon electrodes has been known for
a long time (see U.S. Pat. Nos. 1,440,724 and 1,441,037 to
Soderberg). Self-baking carbon electrodes basically consist of a
pasty mixture of carbon-containing material such as anthracite,
coke, tar, and pitch, which is filled into a metal casing held in
position within an electric arc furnace by way of contact shoes and
a suspension/sliding device. The application of high electric
currents plus the heat of the arc struck by the electrode during
the furnace operation develops sufficient heat to melt the material
filled into the casing and form a paste, then cokify the so-formed
paste, and finally bale the electrode. In accordance with its
consumption rate the electrode is lowered stepwise, a new casing
sheet is joined to the upper part, the casing is filled with the
mixture, and the middle section is baked. In a variation, the
electrode may be partly baked at a low temperature of about
600.degree. to 700.degree. C. In the context of the Soderberg
electrode, the lower part of the steel casing dissolves in the bath
of molten metal, thus injecting iron into the bath. To avoid this
contamination by iron, several solutions have been proposed, which
all consist of mechanically detaching the electrode and the steel
casing so that the electrode can be caused to slide without the
steel casing.
[0014] U.S. Pat. No. 6,635,198 (Vatland et al.) describes a method
for the continuous production of self-baking composite electrodes
utilizing sectioned metallic casings. Each new section of casing is
mounted upon the section of casing below without applying welding
or other means to rigidly affix the section to each other. As the
sections of casing are not rigidly affixed to each other by welding
or the like, it is easy to remove the casing after the electrode
has been baked.
[0015] Another solution is a mounting configuration as described in
U.S. Pat. No. 4,575,856 (Persson) which involves supporting the
weight of the electrode by means of a column formed from pre-baked
carbon or graphite electrodes being enclosed by the baked paste,
both the column and the paste being consumed at the same time.
[0016] Modern electric arc furnaces for steel production are
operated at current densities in excess of 25 A/cm.sup.2 and thus
require highly conductive graphite electrodes. To achieve
electrical resistivities below 10 .mu.Ohm m, such graphite
electrodes are produced using well-ordered needle cokes and they
are graphitized at temperatures above 3000.degree. C. The use of
costly needle coke and the high electricity costs for
graphitization bar such electrodes from being used in low-power
electric furnaces that are used for producing non-steel materials.
Furthermore, iron oxides are added to the electrode raw material
mixture to inhibit puffing (caused by the release of sulfur from
its bond with carbon inside the coke particles). Hence, the
increased iron content can contaminate the melt and cause high
electrode erosion in melt furnace atmospheres that are rich in CO,
such as in the case of carbothermic reduction of alumina.
SUMMARY OF THE INVENTION
[0017] It is accordingly an object of the invention to provide a
graphite electrode for electrothermic reduction furnaces, in
particular for the production of aluminum, titanium, silicon,
ferroalloys, and phosphorous, as well as a production method for
such electrodes and electrode columns, which overcome the
above-mentioned disadvantages of the heretofore-known devices and
methods of this general type and which provide for graphite
electrodes that do not contaminate the melt with iron, which can be
used in melt furnace atmospheres that are rich in CO, and which are
economical to produce.
[0018] With the foregoing and other objects in view there is
provided, in accordance with the invention, a graphite electrode
for an electrothermic reduction furnace comprising a shaped
graphite electrode body formed from anode grade coke, graphitized
at a graphitization temperature below 2700.degree. C., and having
an iron content of less than 0.1% by weight, and preferably about
0.05% by weight.
[0019] In accordance with an added feature of the invention, the
electrode has a specific electrical resistivity of above 5
pOhm.multidot.m and a thermal conductivity of less than 150
W/m.multidot.K.
[0020] The graphite electrode is particularly suited for an
electrothermic reduction furnace for producing aluminum, titanium,
silicon, ferroalloys, or phosphorous. Specific emphasis is placed
on the direct carbothermal reduction of alumina.
[0021] In accordance with an additional feature of the invention,
an amount of carbon nanofibers and/or carbon fibers is incorporated
in the electrode body for increasing a mechanical strength and
adjusting a coefficient of thermal expansion thereof.
[0022] In accordance with another feature of the invention, the
anode grade coke has a mean particle size of approximately 5 to 10
mm, and preferably between 5 and 7 mm.
[0023] An intermediate product in the production of the graphite
electrode comprising particles of anode grade coke having a mean
particle size of between 5 and 10 mm and an ash content of less
than 0.5% mixed with a coal tar pitch binder and formed into a
green electrode to be baked and graphitized to form the graphite
electrode.
[0024] With the above and other objects in view there is also
provided, in accordance with the invention, a graphite pin formed
of anode grade coke, graphitized at a graphitization temperature
below 2700.degree. C., having an iron content of less than 0.1% by
weight. The graphite pin is formed to mate with the graphite
electrode body to form an electrode column.
[0025] In accordance with a further feature of the invention, the
above-summarized graphite electrode is disposed to form a central
column of a self-baking composite electrode in an electrothermic
reduction furnace.
[0026] With the above and other objects in view there is also
provided, in accordance with the invention, a method of producing a
graphite electrode. The method comprises the following method
steps:
[0027] providing calcined anode coke with an average particle size
of 5 to 10 mm and mixing the anode coke with a coal-tar pitch
binder to form a mixture;
[0028] forming an electrode body from the mixture to form a green
electrode at a temperature in a vicinity of a softening point of
the pitch binder;
[0029] baking the green electrode at a temperature of between
approximately 700.degree. C. and approximately 1100.degree. C., to
carbonize the pitch binder to solid coke, to form a carbonized
electrode;
[0030] graphitizing the carbonized electrode with a heat treatment
at a final temperature between 2100.degree. C. to 2700.degree. C.
for a time sufficient to cause carbon atoms in the carbonized
electrode to organize into a crystalline structure of graphite.
[0031] Preferably, the graphitization temperature is between
2200.degree. C. to 2500.degree. C. and the green electrode is bake
at a temperature between 800.degree. C. and 1000.degree. C. It is
further preferred to bake the green electrode in a relative absence
of air at a heating rate of approximately 1 K to approximately 5 K
per hour to the final temperature.
[0032] In accordance with again an added feature of the invention,
the electrode may be impregnating at least one time with coal tar
or petroleum pitch after baking. This deposits additional pitch
coke in open pores of the electrode. Ech impregnating step is
followed with an additional baking step.
[0033] The green electrode may be formed by extrusion. In that
case, it is advantageous to add to the mixture oils or other
lubricants to aid in the extrusion throughput. Alternatively, the
green electrode may be formed by molding in a conventional forming
mold or by vibromolding in an agitated mold.
[0034] In accordance with again an additional feature of the
invention, the graphitized electrode formed in the graphitizing
step is machined to provide a final form of the graphite
electrode.
[0035] In accordance with a concomitant feature of the invention, a
plurality of graphite electrodes as outline are formed, one or more
nipples are formed substantially in the same process sequence and
such that the nipples and the electrodes can mesh, and the
electrodes and a nipple are connected to form a graphite electrode
column.
[0036] In sum, the invention provides for graphite electrodes for
electrothermic reduction furnaces, in particular for the production
of aluminum, titanium, silicon, ferroalloys as well as phosphorous.
The electrodes are produced using anode grade coke and
graphitization temperatures below 2700.degree. C.
[0037] The invention also provides for the utilization of graphite
pins to be mated with the above-summarized graphite electrodes to
form electrode columns. The pins are preferably produced in the
same manner as the electrodes of this invention. In this way, the
pins (also referred to as nipples) have the same characteristics,
such as CTE and mechanical properties, as the electrodes.
[0038] The novel electrodes lend themselves very favorably in their
utilization as central columns for self-baking composite electrodes
for electrothermic reduction furnaces.
[0039] Other features which are considered as characteristic for
the invention are set forth in the appended claims.
[0040] Although the invention is illustrated and described herein
as embodied in a graphite electrode for electrothermic reduction
and a production method, it is nevertheless not intended to be
limited to the details shown, since various modifications and
structural changes may be made therein without departing from the
spirit of the invention and within the scope and range of
equivalents of the claims.
[0041] The construction of the invention, however, together with
additional objects and advantages thereof will be best understood
from the following description of the specific examples and
embodiments of the invention.
DETAILED DESCRIPTION OF A BEST MODE EXEMPLARY EMBODIMENT
[0042] The first step in the production of graphite electrodes
comprises combining calcined coke and pitch. As noted above,
graphite electrodes for steel production are produced using
well-ordered needle cokes that are characterized by a coefficient
of thermal expansion (CTE) of 0.3-1.0.times.10.sup.4 K.sup.-1,
anisotropy of thermal expansion of 1.8, and they possess a coarse
fibrous microstructure. According to this invention, the graphite
electrodes for electrothermic reduction furnaces are produced using
anode coke. Anode cokes have a CTE above 1.2.times.10.sup.6
K.sup.-1, an anisotropy of thermal expansion of 1.5 and a mosaic
microstructure. These cokes are very pure. They have an ash content
of less than 0.3%. They are readily available at a significantly
lower cost than needle cokes and they are used in large quantities
for the production of carbon anodes for the Hall-Heroult aluminum
smelting process.
[0043] The crushed, sized and milled calcined anode coke is mixed
with a coal-tar pitch. The particle size of the calcined coke is
selected according to the end use of the electrode. Generally, in
graphite electrodes for use in processing steel, particles up to
about 25 millimeters (mm) in average diameter are employed in the
blend. For the graphite electrodes of this invention, an average
particle size of 5 to 10 mm, more preferably of 5 to 7 mm, is
appropriate. Other ingredients that may be incorporated into the
blend at low levels include carbon nanofibers or carbon fibers to
provide additional mechanical strength or to adjust the CTE of the
final electrode as well as oils or other lubricants to facilitate
extrusion of the blend.
[0044] After mixing calcined coke and pitch binder, the electrode
body is formed (or shaped) either by extrusion though a die or
molded in conventional forming molds or vibromolded in agitated
molds to form a so-called green electrode. The forming step is
conducted at a temperature close to the softening point of the
pitch, which is usually about 100.degree. C. or higher. Although
the die or mold can form the electrode in substantially final form
and size, machining of the finished electrode is usually needed, at
the very least to provide threads or other recesses, which may be
required to mate with a pin or nipple to from an electrode column.
The circumference of the graphite electrodes of this invention may
be rectangular or circular.
[0045] The green electrode is then baked at a temperature of
between about 700.degree. C. and about 1100.degree. C., more
preferably between about 800.degree. C. and about 1000.degree. C.,
to carbonize the pitch binder to solid coke, to give the electrode
permanency of form, high mechanical strength, good thermal
conductivity, and comparatively low electrical resistance. The
baking step is carried out in the relative absence of air at a
heating rate of about 1 K to about 5 K per hour to the final
temperature. After baking, the electrode may be impregnated one or
more times with coal tar or petroleum pitch, or other types of
pitches known in the industry, to deposit additional pitch coke in
any open pores of the electrode. Each impregnation is then followed
by an additional baking step. Preferably the electrode is only
impregnated one time with such pitch.
[0046] After baking, the electrode--referred to at this stage as a
carbonized electrode--is then graphitized by heat treatment at a
final temperature between 2100.degree. C. to 2700.degree. C., more
preferably between 2200.degree. C. to 2500.degree. C., for a time
sufficient to cause the carbon atoms in the calcined coke and pitch
coke binder to transform from a poorly ordered state into the
crystalline structure of graphite. Because of the purity of the
anode coke, the comparably low graphitization temperatures are
sufficient to reach the required final electrode ash contents. In
the case of graphite electrodes for steel production,
graphitization is performed at a temperature of between about
2700.degree. C. and about 3200.degree. C. At these
high-temperatures, all elements other than carbon are volatilized
and escape as vapors. The time required for maintenance at the
graphitization temperature is no more than about 12 hours,
preferably about 30 min to about 3 hours. Graphitization can be
performed in Acheson furnaces or in lengthwise graphitization (LWG)
furnaces, the latter can also be operated in a continuous mode.
After graphitization is completed, the finished electrode can be
cut to size and then machined or otherwise formed into its final
configuration.
[0047] The finished electrodes can be mounted in electrothermic
reduction furnaces as single-piece electrodes, as electrode
bundles, or they can be continuously supplied as electrode columns
joined by graphite pins.
[0048] In the latter case, the electrode has typically an internal
section that is axially tapered from an end to a lengthwise middle
portion to receive a graphite pin, and then threads are machined
into the tapered portion of the electrode, to permit mating with
corresponding threads of the pin, to form the electrode column.
Given its nature, the graphite permits machining to a high degree
of tolerance, thus permitting a strong connection between the pin
and the electrode.
[0049] The graphite pins used to join electrode columns can be
substantially the same pins as used for electrode columns for steel
production or, more preferably, are produced in the same manner as
the graphite electrodes of this invention. In the latter case, the
pins would have similar properties to the electrodes which is
advantageous for preventing cracking of the electrode column due to
uneven thermal expansion of electrodes and pins. However, the pins
have to resist heavier mechanical load than the electrodes. To
achieve the required mechanical properties, yet to have thermal
expansion behavior matching that of the electrodes, typically the
raw material mixture of the pins is somewhat altered while the
processing sequence remains the same as described for the
electrodes.
[0050] Further, the electrodes and pins can be equipped with means
to prevent loosening of the electrode column during operation, such
as holes or recesses containing binder pitch or other means.
[0051] An additional embodiment of this invention is the
utilization of graphite electrodes as described above as central
columns for self-baking composite electrodes for electrothermic
reduction furnaces. As described in U.S. Pat. No. 4,575,856
(Persson), in order to avoid iron contamination, Soderberg-type
electrodes can be produced as composite electrodes consisting of a
carbon or graphite electrode core column embedded in Soderberg
paste. Using conventional graphite electrodes for
steel-manufacturing would, however, increase costs as well as iron
contamination. Furthermore, it was determined that the nature of
the bond between the graphite and the paste baked into the graphite
consist of interpenetration of the paste at their surface of
contact. Conventional graphite electrodes for steel-manufacturing
typically have low open porosity at about 15% or below. Hence, the
surface contact with the Soderberg paste would be limited.
[0052] In contrast, graphite electrodes produced as described above
provide an economic way to manufacture such self-baking composite
electrodes for electrothermic reduction furnaces having low iron
content and having an intimate surface contact between the graphite
core column and the Soderberg paste.
[0053] A further object of this invention is to provide a process
to manufacture anode grade coke-based electrodes for electrothermic
reduction furnaces using a self-baking carbon electrode
manufacturing sequence followed by graphitization at temperatures
below 2700.degree. C.
[0054] As described above, conventional self-baking electrodes
comprise a vertically disposed cylindrical metal casing which
extends downwardly through an opening in the roof of an
electrothermic reduction furnace. The upper end of the casing is
open to permit the insertion of a carbonaceous paste-like material
which first melts and then cures to a solid state as it passes
downwardly through the casing as a result of heat which is
conducted upwardly from the cured portion of the electrode
extending below the lower end of the casing. Such paste may be
made, for example, by calcining anthracite or petroleum or asphalt
cokes which is then mixed with a bonding material such as pitch or
tar.
[0055] According to this embodiment of the invention, in a first
step, a self-baking carbon electrode is produced in a similar
manner by using a paste composed of calcined anode grade coke and
pitch. Instead of feeding the electrode directly into the
electrothermic reduction furnace, it is, if necessary, detached
from its metal casing and graphitized at a final temperature
between 2100.degree. C. to 2700.degree. C., more preferably between
2200.degree. C. to 2500.degree. C. The graphitization step can be
carried out in a separate graphitization furnace, such as an
Acheson furnace or an LWG furnace, or in a continuous-mode
graphitization furnace which is ideally located between the
self-baking unit and the electrothermic reduction furnace.
[0056] The electrodes prepared in accordance with the present
invention offer numerous advantages over the art. For
electrothermic reduction furnaces, they are an economical
alternative to high-temperature graphite electrodes for steel
production and, at the same time, provide a high purity alternative
to Soderberg electrodes. Further, they can be manufactured using
several routes which are essentially based on existing
manufacturing equipment.
[0057] The following examples are presented to further illustrate
and explain the present invention and should not be viewed as
limiting in any regard. Unless otherwise indicated, all parts and
percentages are by weight, and are based on the weight of the
product at the particular stage in processing indicated.
EXAMPLE 1
[0058] 85% anode coke having an average particle size of 6 mm and
15% coal-tar pitch were mixed in an intense mixer at 150.degree. C.
The mixture was then cooled and extruded to about 600 mm
diameter.times. about 2400 mm long green electrodes. The green
electrodes were processed as described above. The physical
properties of these electrodes (GE etectrothermic) compared to
those of graphite electrodes for steel production (GE steel) as
well as Soderberg electrodes are shown below.
1 Electrode type GE.sub.electrothermic GE.sub.steel Soderberg Bulk
Density (g/cm.sup.3) 1.62 1.75 1.38 Open Porosity (%) 25 16 34
Specific electrical (.mu.Ohm m) 11 4.5 29 resisivity Thermal
Conductivity (W/mK) 100 180 8 Iron content (%) 0.05 0.2 >1
[0059] Due to the lower graphitization temperatures, the graphite
electrodes of this invention (GE.sub.electrothermic) have a higher
specific electrical resistivity and lower thermal conductivity
compared to those of graphite electrodes for steel production
(GE.sub.steel). This renders them suitable with regard to the
requirements of the electrothermic reduction furnaces having
comparably low current densities. Besides the significant cost
advantage, the graphite electrodes of this invention excel in their
high purity specifically with respect to their iron content. The
common Soderberg electrodes can cause contamination of the
electrothermic melt, especially with iron. Furthermore, their
relatively poor electrical as well as thermal conductivity, a
compared to graphite electrodes, also has adverse effects on the
energy consumption during smelting operations.
EXAMPLE 2
[0060] 80% anode coke having an average particle size of 6 mm and
20% coal-tar pitch were mixed in an intense mixer at 150.degree.
C., cooled, and extruded to about 330 mm diameter.times. about 2100
mm long green cylindrical bodies. The green cylindrical bodies were
processed as the electrodes as described above. After
graphitization, out of each cylindrical body 3 graphite pins were
machined, having a double-conical shape with threaded surface to
mate with the electrode threads. The physical properties of both,
pins and corresponding electrodes, essentially matched each other.
The assembled electrode column did not crack under thermal
stress.
[0061] The above description is intended to enable the person
skilled in the art to practice the invention. It is not intended to
detail all of the possible variations and modifications that will
become apparent to the skilled worker upon reading the description.
It is intended, however, that all such modifications and variations
be included within the scope of the invention that is defined by
the following claims. The claims are intended to cover the
indicated elements and steps in any arrangement or sequence that is
effective to meet the objectives intended for the invention, unless
the context specifically indicates the contrary.
* * * * *