Note: Descriptions are shown in the official language in which they were submitted.
,
. CA 02800855 2012-11-27
WO 2011/151302 Al
CARBON BODY, METHOD FOR PRODUCING A CARBON BODY AND USE
THEREOF
The present invention relates to a carbon body, a
method for producing a carbon body and use thereof.
Carbon bodies are used as components in the chemical
industry, where they are often exposed to aggressive
chemicals and high temperatures. The requirements of
these components are high and they have a limited
lifetime.
Cathodes made of nongraphitic carbon and graphite are
used as the bottom lining of aluminum electrolysis
cells, for example. These materials combine a very good
electrical conductivity with a high thermal stability
and chemical resistance. Graphitized cathodes in
particular are suitable for modern high-amperage cells
because of their excellent electrical conductivity.
However, these cells are subject to severe erosion
during operation as anthracite-based cathodes, for
example. The erosion is concentrated at the ends of the
cathodes, where a high current density prevails,
resulting in the development of a W-shaped profile.
Mechanical influences play a not insignificant role
because the molten aluminum layer on the cathode
surface is constantly in movement due to the high
magnetic fields. Furthermore, chemical attacks occur
due to the components of the electrolysis bath. Both
types of erosion limit the lifetime of the cathode
blocks and thus of the electrolysis cell.
A cathode in which this problem is to be avoided is
described in FR 2,821,365. This cathode comprises a
carbon product that is reactive with sodium even after
a treatment at a temperature above 2400 C. The sodium
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comes from cryolite, which is typically added to the
electrolysis bath.
Blast furnace bricks are also manufactured from monographitic
carbon or graphite. The erosion of blast furnace bricks
preferentially in the tap hole area of the blast furnace is a
technical problem with blast furnaces for iron production.
Here in particular the molten crude iron excessively attacks
the graphite and carbon lining both chemically and
mechanically.
There is thus the problem that the carbon body is attacked by
chemical and mechanical erosion, which therefore shortens its
lifetime.
The present invention relates to a carbon body which has a long
lifetime.
In one product aspect, the invention relates to a carbon body
that is produced by burning a mixture containing at least coke,
wherein the coke has a degree of graphitization according to
Maire and Mehring of 0.50 or less after a thermal treatment of
the coke at 2800 C, calculated from the average layer spacing
c/2, and wherein the grain of the coke is greater than 0.5 mm.
In one process aspect, the invention relates to a method for
producing a carbon body comprising: (a) mixing: (i) coke with
anthracite, graphite or a mixture thereof, (ii) at least one
binder selected from the group consisting of a petroleum-based
binder, a carbon-based binder, a synthetic resin-based binder
and a mixture thereof, and (iii) optionally, an additive; (b)
shaping the mixture from step (a) to a predetermined shape; and
=
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(c) burning the shaped mixture from step (b) and, optionally,
graphitizing the resultant burned molded body, wherein the coke
has a degree of graphitization according to Maire and Mehring
of 0.50 or less after a thermal treatment of the coke at
2800 C, calculated from the average layer spacing c/2, and
wherein the coke has a grain of greater than 0.5 mm.
According to the invention, a carbon body produced by burning a
mixture that contains at least coke is made available, the coke
being a coke with a low graphitizability.
A coke with a low graphatizability has a high hardness and
abrasion resistance even after being exposed to a high
temperature in the production process, which is up to 3000 C,
for example. Due to the addition of coke with low
graphitizability, the abrasion resistance of the surface of the
carbon body is increased in comparison with traditional carbon
bodies without the addition of the poorly graphitizable coke
according to the invention. Coke with a low graphitizability
acts in
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particular as an agent for producing a high abrasion
resistance.
The coke with a low graphitizability preferably
produces a higher bulk density of the carbon body in
comparison with traditional carbon bodies.
The coke is preferably a coke with a spherical
morphology. Due to its approximately spherical
geometry, the coke increases the flowability of the
mass in shaping the carbon body. The carbon body
therefore advantageously has a higher bulk density than
traditional carbon bodies. The carbon body therefore
preferably also has a high wear resistance.
In addition to the coke, which has a low
graphitizability according to the invention, the carbon
body is preferably produced using anthracite, graphite
and/or traditional coke, for example, petroleum coke or
coal tar pitch coke, at least one binder, for example,
from the group of petroleum-based or carbon-based
binders such as tar, petroleum pitch, coal tar pitch,
bitumen or a phenolic resin or furan resin and optional
additives such as carbon fibers. The starting materials
listed above may have different grain sizes. Recipes
comprising the aforementioned starting materials for
traditional carbon bodies, for example, a cathode block
or a blast furnace are known. With carbon bodies that
traditionally contain petroleum coke and/or coal tar
pitch coke, preferably at least some of the petroleum
coke and/or coal tar pitch coke is replaced by the coke
with a low graphitizability.
In a preferred embodiment, the degree of graphitization
according to Maire and Mehring is max. 0.5 or less
after a thermal treatment of the poorly graphitizable
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coke at 2800 C, calculated from the average layer
spacing c/2. During graphitization of the carbon body
at a temperature up to 3000 C, the coke forms very
little or no graphite structure and therefore retains
its abrasion resistance and its high hardness.
In another preferred embodiment, the grain of the coke
with a low graphitizability is greater than 0.2 mm,
preferably greater than 0.5 mm. If the carbon body is
to have a predetermined conductivity, the coke with a
low graphitizability will be present in the carbon body
in an amount of at most 25% by weight, based on the dry
mixture. More preferably the coke is present in the
carbon body in an amount of 10% to 20% by weight, based
on the dry mixture.
Because of its spherical morphology, the coke with a
low graphitizability is a molding aid with which a
higher bulk density of the carbon body can be achieved
in comparison with a traditional carbon body. In a
preferred embodiment, the coke has a spherical to
slightly ellipsoidal shape. Furthermore, it preferably
has an onion skin structure which is consistent with
its low graphitizability. In the sense of the present
invention, the term "onion skin structure" denotes a
multilayer structure comprising and inner layer with a
spherical to ellipsoidal shape, which is covered
completely or partially by at least one intermediate
layer and one outer layer. The agent for creating a
high bulk density and a high abrasion resistance is
especially preferably a coke with a low
graphitizability, a high hardness and a spherical
structure composed like an onion skin.
The coke with a spherical to ellipsoidal shape
preferably has a length/diameter ratio of 1 to 10,
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preferably 1 to 5, more preferably 1 to 3. The more the
shape of the coke approximates a spherical structure,
the better is the flowability of the mass and the
better are the mechanical properties of the carbon
body.
The coke used according to the invention is especially
preferably hard and highly isotropic, difficultly
graphitizable to nongraphitizable and has a low
porosity and a low specific surface area, for example,
in the range of 10 to 40 m2/g, more preferably 20 to
30 m2/g. However, the average layer spacing d002 of the
coke, which can be determined by x-ray diffraction, is
preferably 0.340 to 0.344 nm (this corresponds to a
degree of graphitization of 0.0 to 0.5 according to
Maire and Mehring) is no less than 0.339 nm after a
thermal treatment at 2800 C. The apparent stacking
height Lc is preferably less than 20 nm after a thermal
treatment at 2800 C.
A specific representative of this coke with a low
graphitizability is a coke obtained as a byproduct in
the production of unsaturated hydrocarbons, in
particular acetylene. The coke with a low
graphitizability used according to the invention can be
obtained in particular from crude oil fractions or
steam cracking residues, which are used in quenching
reaction gas in the synthesis of unsaturated
hydrocarbons (acetylene), wherein the quenching
oil/carbon black mixture is sent to a coker, which is
heated to approximately 500 C. Volatile components of
the quenching oil evaporated in the coker, from the
bottom of which the coke can be removed. A fine-grained
onion-skin coke, which has a high purity in addition to
having the properties described above and has little to
no ash and mineral content, is obtained in this way.
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The coke preferably has a carbon content of at least
96% by weight and an ash content of at most 0.05% by
weight, preferably at most 0.01% by weight.
A method for producing acetylene in which such a coke
is obtained as a byproduct is described in DE 29 47 005
Al, for example. In this process, the coke with a low
graphitizability as described above is produced from
the quenching oil. To form the body according to the
invention, petroleum coke in particular is at least
partially replaced by the coke with a low
graphitizability according to the invention in a
traditional composition of the carbon body comprising
petroleum coke.
The coke with a low graphitizability especially has a
high purity and contains little to no ash and mineral
content. However, the coke with a low graphitizability
may also contain ash and minerals and may be less pure.
The purity of the coke depends on the purity of the
quenching oil used. Coke is usually a solid with a high
carbon content and in the nongraphitic state and is
produced by pyrolysis of organic material which has
passed through a liquid or liquid crystalline state at
least partially during the carbonization process.
Presumably the carbon black particles prevent the
development of an undisturbed liquid phase (mesophase)
and supply a coke with a high hardness and poor
graphitizability. Therefore, coke obtained from the gas
quenching process can be graphitized only slightly by
thermal treatment at temperatures above 2200 C. After a
heat treatment at 2800 C, the average layer spacing
c/2, as determined from the x-ray diffraction
interference d002, is 0.34 nm or more, and the
crystallite size in the c direction L, is less than
20 nm and the crystallite size Lano is less than 50 nm,
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preferably less than 40 nm. The coke used according to
the invention especially preferably has a high hardness
and a poor graphitizability, and the average layer
spacing c/2 is greater than or equal to 0.34 nm after a
thermal treatment at 2800 C.
As a rule, coke produced in this way is obtained in
spherical particles of a few micrometers to a few
millimeters. The coke with a low graphitizability used
according to the invention preferably has grains larger
than 0.2 mm, preferably greater than 0.5 mm. The
preferred grain can be determined by screening and then
suitable fractionation of the coke, for example. In a
preferred embodiment, the coke with a spherical
morphology has a BET surface area of 20 to 40 m2/g. It
has a very low porosity.
The coke with a structure like an onion skin may also
have at least one extra substance in its structure. One
example of this is the incorporation of carbon black
particles such as those necessarily formed in acetylene
synthesis.
In addition or as an alternative to the coke derived
from the production of acetylene, coke from the fluid
coking/flexicoking process (Exxon Mobil) may also be
used according to the present invention as a coke with
a low graphitizability. A coke obtainable from the
fluid coking process also has a low graphitizability.
Furthermore, it also has a spherical to ellipsoidal
shape and is constructed by the onion skin principle.
In comparison with the coke described above, which is
obtained as a byproduct in the production of acetylene,
this has a higher ash content. The x-ray structural
data given above also applies to this coke.
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In addition or as an alternative, a so-called shot coke
(translated into German approximately as "scrap coke")
from the "delayed coking process" can also be used in
the sense of the present invention as coke with a
spherical morphology. The limits of the x-ray
structural data given above are to be modified in this
case because of the somewhat better graphitizability.
After a temperature treatment at 2800 C, the average
layer spacing should be greater than 0.338 nm and the
crystallite size in the c direction should be less than
30 nm. The abrasion resistance of the graphitized
carbon body does not entirely achieve that of the coke
variants described above. However, improved flowability
of the mass is also obtained because of the spherical
morphology of this shot coke and a carbon body of a
high bulk density is achieved.
The carbon body is preferably a cathode block. In the
case of cathode blocks, a distinction is made between
an amorphous cathode block, a graphitized cathode block
and a graphitic cathode block, depending on the raw
material used and/or the production process. In a
preferred embodiment, the carbon body is a graphitized
cathode block. The carbon body is suitable as a cathode
block because of its high abrasion resistance, hardness
and conductivity.
In an alternative embodiment, the carbon body is
preferably a graphitized blast furnace brick. The
carbon body according to the invention can handle the
thermal and mechanical loads of a blast furnace brick,
in particular a blast furnace brick, which serves as
the lining of a blast furnace for iron production.
Molten iron in the blast furnace hardly penetrates into
such bricks, so there is only minor wear on the bricks.
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The method according to the invention for producing a
carbon body includes the steps of mixing anthracite,
graphite or traditional coke, for example, petroleum
coke or coal tar pitch coke or mixtures thereof, at
least one petroleum or coal-based binder and optionally
at least one synthetic resin-based binder and
optionally any mixtures of the aforementioned binders
as well as optionally additional additives and at least
one agent for creating a high bulk density, wherein the
agent for creating a high bulk density is coke with a
spherical morphology, then the mixture is shaped to
form the predetermined shape, the shaped mixture is
burned and then the burned mixture is optionally
graphitized.
In the method according to the invention, at least one
binder from the group of petroleum- or coal-based
binders such as tar, pitch, bitumen or a phenolic resin
or a furan resin is used.
The additives may be carbon nanofibers or carbon
fibers, for example.
In the method according to the invention, the starting
materials that are used to produce the carbon body are
used in the respective desired grain(s). A starting
material may optionally be screened prior to use. All
the predetermined starting materials are mixed
together, optionally under the influence of temperature
and optionally kneaded. Then the resulting mixture is
shaped and compacted. The shaping and compacting may be
performed, for example, by extrusion, pressing or
vibration molding, i.e., shaking in vacuo. Then the
shaped body is fired. Next the carbon body may
optionally be subjected to graphitization. Then the
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carbon body may be processed to yield the desired
dimensions of its final shape.
The burning temperature in the embodiment without a
subsequent graphitization surface is preferably 1100 C
to 1500 C. If the carbon body is subjected to a
graphitization, the burning temperature is preferably
in the range of 700 C to 1100 C and the temperature in
graphitization is in the range of 2000 C to 3000 C. The
carbon body may be impregnated and burned again, before
or after graphitization. Graphitization is preferably
performed according to the Acheson graphitization
process, more preferably according to the Length Wise
Graphitization (LWG) process (Castner process).
In the method according to the invention, the coke with
a low graphitizability used in this process is obtained
from a quenching oil, which is used in quenching the
reaction gas in the synthesis of unsaturated
hydrocarbons, in particular acetylene (so-called
acetylene coke). The coke preferably has a carbon
content of at least 96% by weight and an ash content of
at most 0.05% by weight, preferably at most 0.01% by
weight. Spheroidal coke from the fluid/flexicoking
process is an alternative to a coke from acetylene
synthesis. Another alternative to a coke acetylene
synthesis is shot coke from the delayed coking process.
The two types of coke mentioned first are poorly
graphitizable hard coke for which the x-ray structural
data given above are applicable.
The coke with a low graphitizability, which is obtained
in the production of acetylene as a byproduct, can be
used in the method according to the invention in the
form in which it is obtained from the process described
above as disclosed in DE 29 47 005 Al. As an
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alternative, the coke may be thermally pretreated
before being used in the method according to the
invention. The thermal pretreatment comprises
calcination, i.e., a heat treatment of the coke at a
temperature in the range between 700 C and 1600 C,
preferably 1000 C to 1500 C, more preferably 1100 C to
1300 C, optionally under a reducing atmosphere. Such a
treatment leads in particular to evaporation of water,
volatile flammable substances such as hydrocarbons,
e.g., methane, carbon monoxide and/or hydrogen.
In a preferred embodiment, the coke with a spherical
morphology and with a degree of graphitization
according to Maire and Mehring after a thermal
treatment of the coke at 2800 C, calculated from the
average layer spacing c/2 of 0.5 or less, is used in
the method according to the inventtion. The coke with a
spherical morphology and a low graphitizability is an
agent for creating a high bulk density and a high
abrasion resistance.
The quantity of coke used with a spherical morphology
is preferably at most 25% by weight, more preferably
10% to 20% by weight, based on the dry mixture. The
coke with a spherical morphology with a grain larger
than 0.2 mm, more preferably larger than 0.5 mm is
preferably used.
The carbon body according to the invention can be used
in a wide range. In particular because of its high bulk
density, high abrasion resistance, high wear
resistance, chemical inertia and high thermal
stability, it is used as a component in machinery,
chemical equipment or heat exchangers in the field of
process technology, for example. In addition, because
of its properties described above, the carbon body
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according to the invention is used as an electrode or
lining of components in the production of substances
which are manufactured under relatively aggressive
conditions such as exposure to aggressive chemicals or
high temperatures.
In a preferred embodiment, the carbon body according to
the invention is used at a cathode block in an
electrolysis cell for producing aluminum. By replacing
a part of the traditional petroleum or coal tar pitch
coke in traditional cathode block recipes with the
special almost spherical hard coke which has a low
graphitizability, a cathode block with a higher bulk
density in comparison with traditional cathode block is
obtained. Furthermore, the abrasion resistance of the
cathode block surface is increased in comparison with
that of traditional cathode block surfaces. Due to the
higher bulk density and greater abrasion resistance,
such a cathode is readily capable of withstanding
corrosion during electrolysis to produce aluminum due
to chemical stresses and in particular mechanical
stresses.
Alternatively, the carbon body is preferably used as a
blast furnace brick in a blast furnace for iron
production. Because of the higher bulk density and
higher abrasion resistance in comparison with
traditional blast furnace bricks, the carbon body
according to the invention as a blast furnace brick can
withstand the mechanical stresses and thermal wear. The
carbon body according to the invention is suitable in
particular for use in the tap hole area of a blast
furnace for iron production.
In another preferred embodiment, the carbon body
according to the invention is used as an electrode in
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carbothermal reduction processes. For example, the
carbon body according to the invention is used in
carbothermal production of silicon in which silicon
dioxide is reduced to silicon.
In addition, the carbon body according to the invention
is preferably used as an electrode in electrothermal
reduction processes or as a lining for a glass furnace,
for example, for producing aluminum, titanium, silicon,
iron, iron alloys, phosphorus, glass or cement and also
as a shaping tool and/or as a lining for melting and
holding crucibles as well as spouts and runout channels
in the aforementioned regions.
The carbon body according to the invention may also be
used as an anode in electrolytic production of various
substances. Examples of this include an anode for the
production of fluorine, which is required for the
production of uranium hexafluoride in particular, an
anode for the production of magnesium, sodium, lithium
(melt-flow electrolysis) or an anode in chloralkali
electrolysis.
Examples of other applications of the carbon body
according to the invention include a heating pipe
and/or ring, a plate heating element, a degassing pipe
and/or a gas distributor system for nonferrous metal
melting, a gasket, a diamond tool, a nozzle for high-
voltage switches, graphite spheres for a pebble bed
reactor, a component in the field of strand casting,
compression casting, spin casting or railway wheel
casting such as, for example, a casting mold, a solder
or glass melting mold for use in the production of
semiconductor housings, glass bushings and soldered
joints.
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The carbon body according to the invention is also used
in the field of process technology. The carbon body
according to the invention may be used as a component
of a heat exchanger, for example, as a pipe bundle heat
exchanger, a pipe tray, plate heat exchanger or gasket.
In addition, the carbon body according to the invention
may be used as a column, for example, in synthesis of
acids, e.g., HC1 synthesis, as a protective tube, a
screen plate, a tunnel plate, a bell plate, a liquid
distributor, a pore reactor, a pump or a rupture disk.
