Note: Descriptions are shown in the official language in which they were submitted.
~ .lZ79~8 ~
Background of the Invention
1. Field of the Invention
This invention relates to the manufacture of
graphite electrodes ~rom petroleum coke, and more parti-
cularly to the inhibition of "puffing" of coke during
graphitization of premium coke electrodes. These elec-
trodes, primarily used in the electric furnace steel
making process, must meet rigid specifications. Much
of the petroleum coke presently available commercially
has a tendency to expand rapidly and irreversibly during
the heat treatment required as part of the electrode
manufacturing process. This expansion is commonly re-
ferred to as "puffing". When the puffing phenomenon oc-
curs to too great an extent, the electrode is ren-
dered useless.
2. Prior Art
The reduction or elimination of the puffing
phenomenon has been accomplished in the past by adding
a puffing inhibitor to a blend of calcincd petroleum
coke and binder pitch prior to extruding and baking the
electrode.
A method of controlling puffinq comprising
adding sodium carbonate prior to extruding the elec-
trode is descxibed in U.S. Patent No. 2,814,076.
A method of con-trolliny puffin~ by addition
of iron oxide prior to extruding and baking an electrode
is descri~ed in U.S Patents Nos. 3,5~"745 and
3,624,231.
- ~1~98B
A method of blending petroleum coke with an
oxygen-containing compound of iron, calcium, aluminum
or manganese to inhibit puffing during graphitization
is described in U.S. Patent No. 3,842,165.
The addition of a titanium or zirconium com-
pound with conventional puffing inhibitors is described
in U.S. Patent No. 3,563,705; and the addition of calcium
cyanamid as a puffiny inhibitor is disclosed in U.S.
Patent No. 3,642,962.
A method of producing a low sulfur coke com-
prising addition of an iron compound and a metal chloride,
either to a coker feedstoc~ or subsequent to the coking
operation, is described in U.S. Patent No. 3,873,427. The
process described in that patent contemplates addition of
a large amount, such as from 3 to 25 weight percent, of
an iron compound along with a metal chloride, and the
resulting low sulfur coke is not suitable for electrode
manufacture because of the high level of impurities in-
troduced during the desulfurization step.
Conventional commercial yraphite electrode
manufacture presently utilizes the addition of from 0.5
to 3.0 percent iron oxide mixed with co~e and binder
pitch prior to extrusion and baking of the electrode.
This proce~ure has been ~uite effective in controlling
puffing of the electrode during the graphitization step.
It is an object of the present invention to
pxo~ide an improved process for reducing or eliminating
puffiny during manufacture of gra~hite electrodes.
It is a further object of the invention to
llZ7988
pro~desuch a process which results in an electrode having
a lower level of impurities than results from conventional
methods of manufacture.
The present invention provides a method of produ-
cing premium delayed petroleum coke wherein a coking feed-
stock having a sulfur content high enough to produce a puf-
fing coke is heated in a coking furnace and then charged to
a coking drum maintained at premium coking conditions to
produce premium coke, the improvement wherein an effective
amount of a puffing inhibitor is added to said feedstock
prior to charging the feedstock to the coking drum.
In accordance with the present invention, electrode
puffing is controlled by addition of a small amount of a puf-
fing inhibitor to a coker feedstock prior to introducing the
feedstock to a coking drum. The addition of from 0.005 to
1.0 percent by weight based on weight of coker feedstock,
of an inhibitor such as iron oxide to a coker feedstock has
been found to inhibit puffing as effectively as does the
addition of a much larger amount of puffing inhibitor to a
blend of calcined coke and binder pitch in accordance with
conventional procedure.
As used herein, the term "premium coke" is used
to described delayed petroleum coke which is suitable for
manufacture of large graphite electric arc steel furnace
electrodes.
~ n order for coke to be marketa~le as premium coke,
it must meet certain specifications as to composition and
physical properties. An important specification is the
linear coefficient of thermal expansion (CTE). One method
of determining the CTE of a coke is the optical lever method
in which the expansion of a bar formed of graphitized coke
is measured over a temperature range of 30 to g8C. The CTE
measured in this manner should be less than 7.0 x 10 /C,
and in many cases .................. ~
~Z79~
must be less than 5.0 x 10 7/oC in order to meet customer
requirements. The CTE of an actual large electrode made
from such coke is normally slightly higher than that of
the graphite test rod.
The conventional process for making premium
coke comprises introducing a premium coker feedstock or
feedstocks such as thermal tar, decant oil from a fluid
catalytic cracking unit, pyrolysis tar or the like to a
coker furnace where the feedstock is heated to coking
temperature, generally from 425 to 540C, Eollowed by
introduction of the heated feedstock to a delayed coking
drum maintained at typical premium coking conditions of
from 440 to 470C and 2.5 to 7 kg/cm2. The coke product
is removed from the coking drum, and is typically cal-
cined in a rotary kiln at a temperature of from 1100 to
1500C to remove Yolatile material therefrom. The cal-
cined coke is then sizc~ to a desired size distribution
such as from 200 mesh material to about 3 or 4 mesh mate-
rial, blended with a binder pitch and extruded into the
desired electrode configuration. The thus-formed material
is then baked at a temperature of from 800 to 1200C to
car~onize the binder pitch and to give the extruded
electrode strength. Finally, the baked electrode is
graphitized by heating to a temperature oE from 2500 to
2S 3000C to produce thc graphitizod clectrodc. The graphi-
tized electrode is then machined to specifications for
use in an electric arc steel furnace.
~ccording to present co~nercial practice, pre-
mium coke having an amount of sulfur insufficient to
1~,2 7988 .~
cause puffing can be graphitized as described above with-
out addition of a puffing inhibitor. Premium coke ha~ing
an amount of sulfur high enough to cause puffing generally
is treated with from 0.5 to 3.~ weight percent iron oxide,
depending on the amount of sulfur and the degree of puf-
fing, added to a mixture of calcined coke and binder pitch
prior to extrusion and baking. Premium coke having less
than 0.25 weight percent sulfur normally does not require
a puffing inhibitor. Premium co~e having up to about 0.7
weight percent sulfur may require an inhibitor, and prem-
ium coke having more than that amount almost always needs
an inhibitor.
According to the present invention, the above
procedure is modified by adding the puffing inhibitor to
lS the coker feedstock prior to introduction of the feed-
stock to the coking drum, and by utilizing a much lower
amount of the inhibitor. The exact operation of the
inhibitor is not clearly unders~ood, b~t it is apparent
that when the inhibitor is added directly to the coker
feedstock prior to formation of the coke particles, the
inhibitor is distributed throughout the coke particles
more or less uniformly, whereas according to con~entional
techniques for adding the inhibitor, the inhihitor can
only contact the outer surface of the previously formed
2~ co~e particles.
The preferred inhibitors in accordance with
the in~ention are oxygen-containing cc,mpounds o~ iron,
calcium, aluminum and manyanese. Of these, iron oxide
is the preferred matelial. Inhihitors il~ so~id particle
form can be added to the coker feedstock by forming a
slurry of the inhibitor particles with a portion of the
feedstock and injecting it at the discharge of the fur-
nace charge pump Any conventional puffing inhibitor
S can be used in the invention, including oxygen-containing
compounds of iron, calcium, aluminum and manganese, with
or without a titanium or zirconium compound. The oxides,
hydroxides and carbonates of these elements all have
utility as puffing inhibitors. Ferric oxide is widely
used because of its availability and low price. ~inely
divided iron powder may also be used.
In accordance with one embodiment of the in-
vention, the puffing inhibitors are added in hydrocarbon-
soluble form. Acetyl acetonates of metals are soluble
in hydrocarbons and can be used as a source of the in-
hibiting metal. Another soluble iron compound which is
suitable is ferrocene. The use of soluble compounds as
a source of the inhibitor assures maximurn uniformity of
dispersion, and also ~liminates the problems associated
with presence of particulate material in the piping.
The in~ention is applicable in the formation
of graphitized electrodes fro~l coke containin~ an amount
of sulfur sufficient to cause an undesirable amount of
puffing. The amount of inhibitor needed is dependent to
some extent upon the sulfur leve} in the feedstock, and
varies from about 50 parts pex million to about 1.0
percent by weight of the inhibiting metal in the coker
feedstock. The level of inhibitor in the coke is de-
pendent on the coke yield. For example, if a feedstock
1~7988
makes 30 weight percent coke, the metal inhibitor in
the coke would be expected to be about three times as
much as the level in the feedstock, as the inhibitor
tends to concentrate in the coke rather than in the
volatile material going from the coke drum as vapor.
The impro~ed results obtained by operation
in accordance with the invention are illustrated in
the following example.
Example I
In this example, various amounts of iron oxide
were added to a blend of binder pitch and previously
formed coke particles. The amount of expansion of the
resulting material during heat treatment is shown in
Table l below.
_able 1
i ppm Iron Added ~egree of Puffing
as Ferric Oxide (Percent of 1ength)
0 7.0
300 7.0
700 6~2
1100 5 9
2500 4 4
In another run where the conditions were the
same as for the experiments listed in Table l, a coker
feedstock to which iron oxide had been added was coked,
and the resulting coke with the iron distributed there-
through was ~lended with binder pitch and tested in the
same manner as the samples to which the iron oxide had
been added later. The resulting coke contained 1260
1127988
parts per million of iron and showed an expansion of
only 2.4 percent, whereas, as can be seen from Table l
above, addition of 2500 parts per million iron after
the coke was formed produced an electrode which exhi-
bited 4.4 percent expansion. Thus, it can be seen
from the above example that operation in accordance
with this invention provides reduced puffing compared
to operation with a larger amount of inhibitor added
after the coke has been formed.
To meet most specifications, it is generally
required that the expansion during graphitization be
of the order of l.0 percent or less, and preferably an
expansion of near zero is obtained. The required re-
duction or elimination of puffing can be obtained
according to this invention utilizing a much lower
amount of inhibitor than is required according to con-
ventional practice.
The process accordin~ to this invention in-
volves addition of from 50 parts per million to l.0
percent by weight of a puffing inhibitor to a coker
feedstock. Preferably, from 500 to 5000 parts per million
of the puffing inhibitor, based on the amount of metal in
the inhibitor compound, is added to the feedstock.
The point of addition of the inhibitor may be
anywhere upstream from the coking drum, but preferably is
at the discharge of the coker furnace charge pump to mini-
mize materials handling problems, particularly when a
particulate inhibitor is utilized.
It is essential in accordance with the invention
l~Z7988
that an effective amount of a puffing inhibitor be added
to a coking feedstock prior to forming coke in the coke
drum. When this is done, reduction or elimination of puf-
fing is obtained with a smaller amount of inhibitor than
is required according to conventional practice, The fore-
going detailed description of the preferred embodiments of
the invention is intended to be exemplary, rather than
limiting, and numerous variations and modifications will
be apparent to those skilled in the art upon consideration
of the foregoing disclosure.
--10-
