Graphitization system method and apparatus

Amorphous carbon bodies forming a lengthwise series horizontal string or column 26 are converted to graphite by the Joule effect in a semi-cylindrical metal shell 20 with a refractory lining 22 and containing particulate thermal insulation medium 28. When the conversion to graphite is complete the insulation medium is dumped through the bottom of the metal shell into a hopper 50 and transferred for re-use while hot. Air pollution control is facilitated and energy and capital costs are lowered by specialization of equipment, retention of sensible heat and shorter cycle times, allowing operation with less equipment and higher production rates.

DESCRIPTION 
BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a method and apparatus for carrying out the 
production of graphite electrodes and other graphitized bodies. 
A great variety of electrochemical and metallurgical processes are carried 
out with the use of carbon and graphite electrodes. In this context it 
should be understood that the word carbon denotes the amorphous form of 
carbon and graphite denotes the multilayered hexagonal crystalline form of 
carbon. 
Carbon and graphite electrodes are used in many electrochemical processes, 
including the production of magnesium, chlorine, iodine, phosphorus, 
steel, and the production of aluminum in Hall cells. 
Carbon electrodes consist of the essentially amorphous carbon from 
petroleum coke which has been calcined, ground, classified by size, mixed 
with a binder, and bound in a matrix of amorphous carbon derived from the 
binder after baking at temperatures of approximately 700.degree. to 
1100.degree. C. in a baking furnace. Graphite electrodes are produced from 
the carbon forms by placing them in an Acheson furnace and in recent years 
in a Lengthwise Graphitization (LWG) type furnace and heating them to a 
temperature between 2500.degree. to 3000.degree. C., which converts the 
amorphous form of carbon to the crystalline graphite, and vaporizes most 
of the impurities present in the original carbon, including most metals 
and sulfur compounds. 
Graphite, compared to amorphous carbon, has much higher electrical and 
thermal conductivity, lower coefficient of thermal expansion (CTE), 
superior ductility and vastly superior thermal shock resistance at the 
operating temperatures of the electric arc steel furnace. These physical 
properties are uniquely valuable in the electric furnace, with its need 
for high electrical currents, and the need to resist the mechanical and 
thermal shock suffered by the electrodes from the falling scrap, 
fluctuations in metal and electrode level, and generally high thermal 
stresses. Consequently, graphite is universally used as an electrode in 
the electric arc melting of steel. 
The production of graphite electrodes from the so-called carbon electrodes 
has traditionally been carried out in the Acheson furnace in which the 
electrodes are typically placed in a transverse orientation to the flow of 
the electrical current, and surrounded by a resistor medium, thereby 
causing the current to pass alternately through tiers of electrodes and 
resistor media, the latter being typically metallurgical or petroleum 
coke. The Acheson process is of such ancient vintage and so well known as 
not to require any further description. The LWG process, although also 
very old, is less well known and has been practiced on a commercial scale 
only in recent years. The LWG process is carried out by arranging the 
carbon electrodes in a continuous column with an electrical connection at 
each end of the column. See U.S. Pat. No. 1,029,121 Heroult, Cl. 13/7, 
June 11, 1912 and U.S. Pat. No. 4,015,068, Vohler, Mar. 29, 1977 Cl. 13/7. 
In the LWG process, the electrodes themselves form the dominant path for 
the heating current, with one or both of the ends of the column subjected 
to a mechanical or hydrostatic pressure source in order to keep the 
connection tight under expansion or contraction of the column during the 
heating cycle. Vohler does not use a packing medium, but discloses a metal 
shell with a felt liner as insulation. 
The LWG process has many advantages over the Acheson process. The energy 
efficiency is much higher, as the material is heated directly instead of 
indirectly, and the cycle time for the process is much faster taking 
typically less than 12 hours as compared to 60 to 120 hours for the 
Acheson process. 
SUMMARY OF THE INVENTION 
One of the persistent problems encountered in the graphitization process 
has been the handling of the hot stock and packing medium. In the Acheson 
process the packing medium between the electrodes is also the conducting 
medium and must be well packed and then removed during the unloading step. 
In industry this has been handled by mechanical loaders and unloaders such 
as clam shell buckets and front end loaders. 
The LWG process uses the packing medium primarily as heat insulation in 
contrast to the Acheson process. Its handling has also been a problem and 
has been done in the past with mechanical loaders, either clam shell or 
pneumatic suction devices. The hot medium typically must be removed from 
the furnace, transported to another location, cooled, rescreened and 
resized for reuse. This process has proved to be one of the more time 
consuming and troublesome aspects of the LWG process as well as a source 
of severe air pollution with the clam bucket type operation. 
My invention is a process and apparatus for production of graphite 
articles, particularly large electrodes, by the LWG process, and comprises 
a U shaped open top furnace shell fabricated from metal with a 
cast-in-refractory lining. The furnace is composed of several shell 
modules with each module electrically isolated to localize any electrical 
leakage to the shell. Each section is also suspended by a system of 
flexible or sliding support brackets to allow for differential thermal 
expansion between the anchored center and the free ends of each section. 
The joints between the furnace sections termed as the expansion joint can 
be of various designs, preferably a refractory lined U-shaped metal insert 
positioned in the contoured nests of adjacent section ends to contain the 
packing medium, allow for expansion, and serve as electrical isolation 
gaps. The apparatus is well adapted for handling the packing medium by 
gravity unloading of the medium from the shell enclosing the medium and 
column, by means of suitable valves or slide gates incorporated at 
openings or dump ports located at the bottom of the furnace shells, into 
storage bins or hoppers located beneath the dump ports, all of which are 
in continuous connection or association with one another and well suited 
to rapidly and easily facilitate handling of the medium for reuse. By this 
means the labor and energy involved in handling the medium and the 
possibility of damage to the furnace and to the electrodes are minimized. 
Time is also saved, the heat energy in the medium may be transferred and 
conserved, and the problems with gaseous and particulate emissions are 
minimized. 
The LWG furnace is well suited to a movable arrangement by means of wheels 
travelling on tracks from station to station or by means of an independent 
vehicle called a transporter to move the furnace from station to station. 
By this movable furnace arrangement, each unit operation such as loading, 
firing, cooling and unloading can be carried out in separate stations thus 
enhancing the control of particulate and gaseous emissions, greatly 
reducing labor as well as reducing the possibility of damage to the 
furnace. 
Gravity unloading of the packing medium can also be carried out in a 
stationary or non-movable furnace arrangement where the storage bins or 
hoppers are in a movable arrangement thus moving beneath the dump ports 
for the unloading of the medium and moved out for the packing or reloading 
of the same furnace or another furnace. 
A third scheme with the gravity unloading furnace can utilize a conveyor 
arrangement beneath the dump ports to remove the medium and convey to a 
central point for reloading of bins for reuse. 
The installation and operation of an LWG apparatus using a metal shell also 
presents the problem of thermal expansion of the shell. The longitudinal 
expansion is accomodated by a system of sliding support brackets between 
sections in the shell, and by flexible support members allowing lateral 
movement at the ends of each section. 
The apparatus of the invention preferably uses DC from a rectiformer as the 
energy source. Each section of the shell is electrically isolated from the 
adjoining sections and from the structural framework, in order to localize 
any electrical short to the shell through the packing medium and the shell 
insulation. By this means, if an electrically weak spot develops in the 
insulating refractory allowing current to leak from the electrode column, 
the leak is isolated and does not short out the entire furnace. 
The apparatus as actually used is comprised of two of the U-shaped furnaces 
side-by-side in the supporting structure, making a horizontal U-shaped 
path for the current. The power heads at the end of the furnace nearest 
the rectiformer are of positive and negative polarity with a shunt at the 
opposite end and carry the total current load through the furnace. 
In producing graphite electrodes in the apparatus of the invention, a 
number of the shaped baked carbon bodies are laid end-to-end placed in a 
bed of particulate insulation medium, forming a horizontally placed column 
between the two power heads. This is preferably accomplished by use of the 
apparatus shown herein in FIGS. 8 and 9 and further described in my 
co-pending application Ser. No. 315,161, filed Oct. 26, 1981. 
The movable furnace arrangement is a distinct break with past 
graphitization practice. The electrodes are fired in the furnace at a 
firing station, then when power is cut off, the furnace is moved to a 
separate cooling area, then to a dump and re-load station. As soon as the 
power is cut off and the furnace moved to the cooling area, another loaded 
furnace is placed in the firing station and power applied. 
The specific advantages found in this arrangement of the apparatus include 
a lower capital cost due to the use of one firing station serving a 
plurality of furnaces instead of only one as in current practice. In 
particular, a simpler electrical bus system is used giving considerable 
savings in capital and operating expenses. Each station, the firing, 
cooling, and dump and re-load, is equipped with the necessary air 
pollution control equipment for that operation. By concentrating each 
function in one area, capital and operating costs are lowered, and in 
particular, control of air pollution is facilitated. 
Further advantages are found in the better mechanization of the total 
process, in effect using an assembly line concept for faster turnaround 
time, lower labor costs, and less exposure of the operators to heat and 
air pollutants. The metal shell and refractory liner are not designed to 
hold heat, rather to conduct and dissipate it while isolating the furnace 
electrically. When firing the furnace, the heat lost by the electrodes is 
slowly conducted away by the insulation medium. The shell and liner remain 
relatively cool because of the thickness of the insulation with the 
resultant low total heat conductivity. In typical practice the peak of the 
heat wave will only reach the refractory liner several hours after the 
electrodes are graphitized and the electrical power is cut off. After a 
predeterminal cooling period, the electrodes are removed from the furnace 
by means of a stock extractor. After the electrodes are removed and the 
medium dumped into the hoppers, the high thermal conductivity of the 
refractory liner and shell allow it to cool down relatively quickly, 
principally by radiation, to alleviate problems due to high temperatures. 
The use of a steel shell makes the movable construction and bottom dump 
features possible, and is a key element in the total invention. 
When firing is finished and power disconnected, a transporter car equipped 
with lifting devices is moved into place under the furnace, it is raised 
off the piers, and moved to the cooling station. After cooling the 
electrodes to about 1500.degree. to 1700.degree. C., the furnace is moved 
to the dump and reload station and a chute car placed under the furnace in 
alignment with the dump gates over the hoppers. The medium is dumped, the 
electrodes removed, the furnace partially filled with insulation medium, 
and a fresh column of pre-baked electrodes is placed in the furnace. A 
hopper of insulating medium is discharged into the furnace, covering the 
column, and the furnace is then moved to the firing station. After the 
furnace is removed from the dump and re-load station, the filled hoppers 
may be removed by crane to a storage area and empty hoppers placed in 
position for the next furnace dump, or the same hot medium may be 
immediately re-used. 
The particulate insulation is described as a sized grade of calcined 
petroleum coke fines recovered from the settling chamber of a rotary kiln 
calcining installation. When raw petroleum coke is calcined at 
temperatures of about 1200.degree. to 1400.degree. C. to remove volatiles 
and convert the physical structure to the harder and denser calcined coke, 
a small fraction is degraded to particulate matter which is too fine for 
use as is and has, in the past, been burned or allowed to dissipate into 
the atmosphere as particulates. Recovery of this fraction is now mandated 
for abatement of air pollution and economics. Other grades of particulate 
insulation such as crushed baked scrap may also be used, or metallurgical 
coke made from coal. 
DESCRIPTION OF THE PREFERRED EMBODIMENT 
The residual particulate media in a recently unloaded graphitizing furnace 
is leveled and compacted by a vibratory device to form a firm bed for the 
electrode column. The vibratory device may be a plate or tongs connected 
to a vibrator and inserted into the insulation bed. 
The column of baked carbon electrodes is next positioned in the furnace on 
the partially filled bed of insulation medium and aligned with the head 
electrodes positioned at each end of the furnace. The furnace may still be 
quite hot, on the order of several hundred degrees Celsius. Pressure is 
applied to the ends by the hydraulic cylinders and the remainder of the 
charge of the insulation medium is then dumped into the furnace from the 
overhead hoppers, with a "pants-leg" or inverted Y-shaped chute directing 
the flow of medium along both sides of the column. 
During loading of the insulation medium, each layer is vibratorily 
compacted to insure that the column is firmly supported against vertical 
and lateral movement. An uncompacted layer of insulation is then placed 
over the column. This completes the furnace loading stage. 
The furnace is next transported to the firing station by a transporter 
means. Hydrostatic pressure of about 1.7.times.10.sup.5 Pa (25 P.S.I.) is 
maintained by the use of a self-contained hydraulic system including pumps 
and controls. 
At the firing station the power head electrodes are connected to the 
current source and the hydraulic pressure on the electrodes is increased 
typically from about 1.7.times.10.sup.5 Pa (25 P.S.I.) to a 
6.9.times.10.sup.6 Pa (200 P.S.I.). The pressure used on the electrode 
column will vary with column length, longer columns requiring higher 
pressure, and whether one or both electrical power heads are hydraulically 
powered. 
The current is applied, heating the column of electrodes rapidly by the 
Joule effect to the required graphitization temperature, usually from 
2400.degree.-2800.degree. C., sometimes as high as 3000.degree. C., taking 
approximately 4 to 12 hours, until the graphitization process is 
completed. The power is turned off, the furnace moved to a cooling station 
and the electrodes allowed to cool. When the electrodes have reached 
approximately 1500.degree. C.-1700.degree. C., the furnace is moved to the 
dump and re-load station and the transporter is replaced by a chute car 
with ducts leading from the dumping gates to the hoppers below. The 
electrodes are unloaded by a grab (stock extractor), the insulation medium 
is dumped at a weighted average temperature of from 700.degree. to 
1100.degree. C. into the hoppers, and the furnace loaded with another 
electrode string and insulation charge. The chute car is removed and the 
furnace is transported back to the firing station. 
After dumping the insulation medium to the hoppers, and removal of the 
hoppers, the hoppers may be moved by crane to storage. It is preferable to 
recycle the hot medium, which has a temperature in the range of 
approximately 600.degree. to 1100.degree. C., immediately for re-use, 
retaining its heat and thereby conserving electrical energy. It had 
previously been standard practice in the industry to cool and re-screen 
insulation media between graphitization runs; however, I have found that 
this is not necessary. 
I have also found that air pollution is lowered when transferring the 
medium while hot. The finer mesh particles which would normally be 
air-borne are oxidized during the hot dumping step rather than dispersed, 
and the amount of CO and SO.sub.2 evolved is minimal. 
When transferring the insulating medium, the retained heat also enables me 
to bring the next electrode string to its conversion temperature quicker, 
effectively saving both operating and capital expenses by producing more 
electrodes in the same time period with lower power requirements. I have 
found that the hot handling of the medium results in a small percentage 
loss of the medium to combustion and to the dust collector, typically 5% 
or less per furnace cycle. 
When transferring the hot medium, the hopper may be used as the sole vessel 
for the dump and recharging operation or the medium may be transferred to 
a separate bin for re-charging the next furnace.

DESCRIPTION OF THE DRAWINGS 
FIG. 1 is a cross-sectional view of the LWG furnace. The steel shell 20 is 
insulated with refractory liner 22, preferably a high alumina 
cast-in-place refractory with anchors 23, although a masonry type may also 
be used. Framework 24 supports the furnace by elastic brackets 32, which 
are plates or I-beam sections welded to the shell 20 and framework 24, the 
web of the bracket being flexible to accomodate differential thermal 
expansion of the shell between adjacent brackets. The outer two vertical 
members of the framework 24 are elastic in the longitudinal direction to 
allow movement of the ends of the shell segments caused by thermal 
expansion. Bottom support brackets 34 are sliding I-beam sections welded 
to the shell and sliding on plates 33 to allow for axial movement and an 
arrangement of isolation pads 35 and anchoring brackets 36 is utilized to 
secure each individual furnace shell section to the subframe 37 while 
isolating the shell from ground. The column of electrodes 26 is embedded 
in the insulating medium 28. 
FIG. 2 is a side elevation of a LWG furnace shell assembly 38 showing the 
isolation pads 35, the shell support frame 24, dumping ports 72 equipped 
with shutoff gates 52 and discharge pipes 90 (both shown in FIG. 6), and 
substructure 37. 
FIG. 3 is a top view of an LWG furnace shell assembly 38 showing steel 
shell 20 and elastic supports 32. 
FIG. 4 is a cross-sectional view of the chute car 44 with 2 sets of chutes 
46, one set leading from each leg of the furnace to the center. During 
operation, after the furnace is brought to the dump and re-load station, 
the transporter car is removed and the chute car 44 is moved on rails 48 
under the furnace so that the chutes 46 are aligned with the discharge 
pipes 90 of the shell assembly and the hoppers 50 below. The hot 
insulation medium is then dumped into the hoppers. 
FIG. 5 shows a reloading operation with a hopper 50 above the furnace, a 
reloading pantleg chute 74 over the electrode column 26 held in stock 
loading truss 60 (described in more detail in my co-pending application 
Ser. No. 315,161 filed Oct. 26, 1981). With the pantleg chute it is 
possible to load the insulation medium uniformly along both sides of the 
electrode string. It also shows the chute car 44 with chutes 46 in place 
over the hopper 50 in the hopper pit 76. The hopper may also be insulated 
to lower the operative temperature of the hopper wall which is typically 
of hot rolled steel plate and further conserve and retain the sensible 
heat in the insulating medium. A heat shield 86 can be used to protect the 
operators from radiant heat while unloading the column. 
FIG. 6 is a cross-sectional view of the LWG furnace with transporter car 82 
in place over hopper 50. Hydraulic jacks 78 lift the furnace support 
structure 37 from piers 80 for transport to the next working station. 
Transporter car 82 runs on the same rails as chute car 44 (not shown). 
FIG. 7 shows the expansion joint arrangement which provides a tight seal 
between adjacent shell assemblies 38 to contain the particulate insulation 
medium while accomodating differential thermal expansion between adjacent 
shell sections 20. Shell 20 has a contoured nest 21 in the refractory 
liner 22, a U-shaped insert assembly 54 comprising refractory 55, 
preferably the same castable refractory used for liner 22, anchors 62, and 
a flexible ceramic fiber gasket 64 such as materials known as 
Fiberfrax.RTM., Kaowool.RTM., or similar alumina-silica fibers. Stiffeners 
66 strengthen the steel liner 56. This structure allows for thermal 
expansion of the shell segments relative to each other while maintaining a 
tight mechanical seal and electrical isolation between shell assemblies 
38. 
FIG. 8 is a cross-section of a furnace shell 20, electrode column 26, 
column loading truss 60, crane attachment 68, crane hook 74, and one or 
more chain slings 76 holding each electrode. 
FIG. 9 is a longitudinal cross-section of the furnace with column loading 
truss 60 holding the electrode column 26 by chain slings 76 in place for 
the column loading operation. 
FIG. 10 is a flow sheet showing the transfer of the furnace from firing 
station A to cooling area B, dump and re-load station C and back to firing 
station A. 
FIG. 11 illustrates the components to support, anchor and electrically 
isolate the shell support frame 24 from the furnace subframe 37. 
Insulation pad 35 carries the weight of the furnace while anchoring 
bracket 36 holds the support frame 24 in place through pressure applied to 
top insulator 92. Bushings 94 and coating 95 provide additional dielectric 
protection against shorting which may be caused by dust build-up.