This invention relates to the art of making carbon electrodes for
use in the steel and other industries, and particularly to a process for preparing
an impregnating pitch for impregnating carbon electrodes, said impregnating pitch
being made by the sequential employment of two specific steps before distillation
-- the centrifugation of a coal tar to remove large particles of quinoline insoluble
materials, and the milling of the centrifuged coal tar to reduce the sizes of the
remaining quinoline insoluble particles. Contrary to prevailing assumptions, a
coal-tar pitch having relatively high quinoline insolubles is thus found to be
useful as an impregnating pitch for carbon electrodes.
Background of the Invention
The commercial carbon industry manufactures graphite electrodes that
are used in electric-arc steelmaking furnaces. These carbon artifacts must carry
large electric currents in the steel melting processes. The desirable characteristics
of these carbon electrodes are high density, high modulus of elasticity, high electrical
conductivily and high flexural strength.
Such electrodes are typically made by mixing petroleum coke with
coal-tar pitch having a high solids content including many particles greater than
10 microns, known as binder pitch. The mix is extruded to form a cylinder known
as a "green form", which is baked at 900°-1300°C to volatilize and remove non-carbonaceous
material. When the green form is baked, it is transformed from a product which
contains about 95% carbon to one which contains greater than 99% carbon. During
the baking process, some of the organic compounds are destructively distilled,
resulting in carbon deposition in the form. As the vaporized materials vacate
their specific locations and exit the form, they produce a porous and channeled
structure, resulting in a reduced density and reduced capacity of the form for
carrying current. Impregnating pitches are used to fill the pores and channels
to increase the carbon density of the form and thus improve the current carrying
capacities of the electrode. After impregnation, the form is baked again and then
graphitized at temperatures as high as 3000°C.
In the prior art, impregnation required a pitch having a low content
of solids greater than about 1 micron in size. Petroleum pitch has been most frequently
used in the past because it is relatively free of solid particles; if coal-tar
pitch is used, it must have a low solids content to pass the filterability test.
Solids content of coal-tar pitch is generally expressed in terms of quinoline
insolubles, or "QI", because the particulate matter in coal-tar pitch is largely
particles of coal, coke and carbon, which are insoluble in quinoline, while the
balance of the pitch is soluble.
More particularly, there are five characteristics normally used to
guide the choice of a coal-tar impregnating pitch. These are:
- 1. Softening point, usually as measured by ASTM D3104. This test gives an indication
of pitch viscosity at impregnating conditions.
- 2. Quinoline Insolubles, (QI), usually as measured by ASTM D2318. This test
provides a measure of the coal, coke, and carbon particles in the pitch as well
as any liquid crystals that may have formed if the pitch was heat-treated.
- 3. Ash, usually as measured by ASTM D2415. This test gives an indication of
materials that may be left in the electrode that may catalyze carbon loss under
ultimate use conditions.
- 4. Coking value, usually as measured by ASTM D2416. This test gives an indication
of how much in-situ carbon will be deposited from the impregnating pitch in the
- 5. Rate of filtration and filterability index as measured by any suitable process,
which may be similar to that described by Couderc et al in U.S. Patent 4,997,542,
column 1, lines 40-65, Generally, filterability indices of 2.5g2/min. or greater
are considered acceptable for an impregnant.
Because of its extremely low solids content and high filterability
index, petroleum pitch is normally utilized as the impregnating pitch. However,
petroleum pitch has a lower in-situ carbon yield than coal-tar pitch and yields
a more non-uniform deposition of its carbon. Coal-tar pitch generally has a lower
filterability index because of the coal and coke particles contained therein and
is usually subjected to the expensive step of solids removal to make a suitable
The present invention enables the economic use of coal-tar pitch
as the impregnant for green form electrodes.
As mentioned above, it has been known in the past to use petroleum
pitch as an impregnant for carbon electrodes. See U.S. Patents 4,961,837 and 4,277,324.
These patents of course do not address the problem solved by applicant, which is
to prepare a coal-tar pitch economically for such use.
The basic objective of the Couderc et al patent mentioned above (U.S.
Patent 4,997,542) is to make a pitch having minimal QI. The present invention
has as its object the opposite, in the sense that the quinoline insoluble materials
are preserved in the pitch insofar as possible or practical. Couderc et al employ
a thermal treatment and flash distillation, and do not centrifuge as does the
A relatively simple centrifugation of coal tar is shown by Bernet
et al in U.S. Patent 4,036,603. While the description says the liquid product
is "substantially solid-free" (column 1, line 54), no use is suggested for it,
and very likely it would be unsuitable as an impregnation pitch because of residual
particles greater than one micron.
Boodman et al, in U.S. Patent 4,436,615, prepare a coal-tar pitch
which is proposed for making electrodes. They filter as well as centrifuge, and
optionally distill liquids from the separation steps to make a product suggested
for impregnating graphite electrodes (column 3, line 68 - column 4, line 1).
Mori et al, in U.S. Patent 4,640,761, use a heat-treating step prior
to centrifugation to cause aggregation of relatively small particles of quinoline
insolubles so they can be more easily removed; in Mori et al U.S. Patent 4,986,895,
two centrifugation steps are used with heat treatment between them to cause aggregation
of the smaller quinoline insolubles to facilitate centrifugation.
A low QI impregnating pitch is made by Chu et al in U.S. Patent 4,664,774.
They use an oxidation system with no resemblance to applicant's.
The only reference of which I am aware utilizing milling actually
mills coal-tar pitch rather than coal tar. This is Japanese Patent 63,130,697
(June 2, 1988), which made a pitch capable of impregnating graphite electrodes
having a porosity of 17%. The process is not like applicant's, which combines
the steps of centrifugation and milling.
Summary of the Invention
Unlike many workers in the art who want to completely remove the
quinoline insolubles from the pitch, applicant tolerates a significant amount (1
to 15 wt.%) of quinoline insolubles, and is able to do so because of the important
milling step after centrifugation. Applicant's process comprises centrifuging
a coal tar to remove particles greater in size than about 15 microns and milling
the remainder to achieve a product suitable for green form impregnation, having
a QI of at least about 3 wt.% which is due to the presence of solid particles
having an average size no greater than about 1 micron. The milled material is then
distilled to produce a coal-tar pitch useful for impregnating carbon electrodes.
Detailed Description of the Invention
The centrifuging can be conducted in any suitable centrifuge of the
type which will cause a separation between the large and small particle size solids
materials. A solid-bowl type centrifuge is preferred.
The viscosity of the coal tar during centrifuging is maintained by
controlling the temperature of said coal tar and/or the amount and type of diluent
mixed with said coal tar. Desirable diluents, if used, include lighter fractions
of coal tar, such as creosote. The viscosity of the coal tar during centrifugation
is preferably maintained below about 400 SUS (Saybolt Universal Seconds), and
more preferably between 100 and 200 SUS. The viscosity of the coal tar during centrifugation
may also be controlled by varying temperature. Preferably the coal tar temperature
is maintained between 60°C (140°F) and 163°C (325°F), and more preferably between
93°C (200°F) and 149°C (300°F).
The small particle size material generally has an average size of
less than about 10 microns, whereas the large particle size solids generally has
an average particle size greater than about 10 microns. The speed of the centrifuge,
residence time, and other conditions will be varied depending upon the type of
coal tar, viscosity of the coal tar, and other characteristics of the coal tar
in order to get the desired separation. The centrifuge should be operated to produce
an acceleration of at least 1000 times that of the earth's gravity.
After centrifugation, the centrate is transferred to a mill. The
mill is of a type wherein a vessel containing grinding media having diameters
of 0.4 to 5 millimeters is equipped with a suitable motor driven rotor for agitation.
Such a mill is sold by Epworth Manufacturing Co., Inc. The effluent from the mill
is distilled conventionally to produce an impregnating pitch of the desired softening
point. The centrate from the centrifuge is transferred to the mill (or series
of mills) which is then operated continuously or intermittently to grind the tar,
until the solids contained in the tar are reduced to less than 1 micron in diameter.
I have found that the process is far more efficient than otherwise
if the grinding media have diameters no greater than 1 millimeter in the final
stage of grinding.
Examples of my process follow:
Debenzolized coal tar at 96°C (205°F) was fed to a solid-bowl centrifuge
at 190 l (50 gallons) per minute. The centrifuge was operated to produce an acceleration
2100 times that of earth's gravity at the bowl wall. The yield of centrate was
96.3 volume %. Analysis of the feed and products are as follows:
Quinoline Insolubles, wt.%
A sample of the centrate was milled in a one-gallon Mini-Lab SWMILL
made by Epworth Manufacturing Co., Inc. of South Haven, MI. The mill was operated
at 2500 rpm. Equal volumes of centrate and 0.8 mm diameter steel shot were charged
to the mill. The centrifuged coal tar was milled for 12 hours while controlling
the outside of the milling chamber to approximately 80°C. At the end of the run,
creosote was added to the mix of media and tar to facilitate straining the media
from the tar. The amount of creosote added was 10 wt.% of the milled tar.
The media-free milled tar and creosote were subjected to a simple
side-arm distillation at 100 mm Hg absolute overhead pressure and a final pot
temperature of 335°C to produce a pitch with a Mettler softening point of 109.9°C.
This pitch was then tested for filterability at 225°C and a filterability index
of >10,000 g2/min. was obtained.