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Texture and graphitization behaviour of fluid coke
Carbon, 1974, Vol.
12, pp. 639-643.
Pergamon
TEXTURE
Press.
AND
Printed
in Great
Britain
GRAPHITIZATION OF FLUID COKE
M. INAGAKI, Faculty of Engineering,
Y. TAMAI,
BEHAVIOUR
S. NAKA
Nagoya University, Chikusa-ku, Nagoya 464, Japan and K. KAMIYA
Faculty of Engineering,
Mie University, Tsu, Mie-ken, Japan
(Received 25 March 1974) Abstract-The texture, its changes with heat-treatments and with oxidation, and the graphitization behaviour of the fluid coke were investigated using polarized light microscopes, scanning electron microscopes and X-ray diffraction. The fluid coke as received had the homogeneous particle size of 0.1-0.2 mm. It consisted of two kinds of particles in almost equal amounts: one type with relatively smooth surfaces and showing crossed extinction contours for cross cuts, and the second type with very rough surfaces and radial extinction contours. In the former ones, the graphite-like layers seem to align approximately concentrically like that of onions. In the latter ones, regions with large graphite-like layers are highly oriented but those with small layers orient randomly being arranged like in a chrysanthemum flower. Formation of cracks in the particles heat-treated under normal pressure was observed and is explained as due to th: texture. The graphitizability of the coke is very poor, co-spacing decreasing only tP 6.78 A after 2780”C-treatment under normal pressure and only to 6.82-6.84 A after 2000°C-treatment under 5 kbar. The characteristic textures of the particles, onion- and chrysanthemum-like are still retained after heat-treatment under 5 kbar. 1. INTRODUCTION Fluid
coke
material
has
not
for carbon
products
poor graphitizability sulfur, although the strong.
been
under used
as a raw
because
of its
and large content of material is dense and
texture In
However,
the recent interest in isotropic and high-density carbon products might make the fluid coke usable as raw materials, gilsonite
like
it happened
coke[l].
We
have
in the
case
investigated
changes
that
graphitizes onion-like
retained[5].
present work, the with heat-treatment, were
texture, and
its the
investigated
2. EXPERIMENTAL The
fluid coke used (Philips
a homogeneous
particle
Oil Co.) was of
size of 0.1-0.2
mm.
Some chemical data supplied by the supplier were as follows: sulfur content: l*S%, ash: 0.5% and volatile matter: 4.5%. Heat-treatments of the fluid carried out in a graphite tube 639
12. NC>.GC
being the
however
only partly characteristic
of the
5 kbar [2-61. Many carbon materials, including non-graphitizing carbons such as phenolformaldehyde resin char[3] and carbon beads[4], were found to graphitize at such relatively low temperature as 1500”-1600°C
Vol
We found
graphitization behaviour for the fluid coke.
graphitization behaviour of various carbon materials under pressures as high as
CARBON
5 kbar[2-41.
the gilsonite coke under 5 kbar, its
coke were furnace at
640
M. INAGAKI
various temperatures 1840-2780°C with 1 hr holding time in nitrogen gas atmosphere of normal pressure, and also in a simple piston-cylinder apparatus at type lOOO-2000°C for 1 hr under pressure of 5 kbar. The heat-treatments under 5 kbar were done for fluid coke samples with various particle sizes; the as-received one with the particle sizes of O*l-O-2 mm, the one with about 44-74 pm (pufverized in an agate mortor) and also with about 5-25 pm (pulverized in a vibrating ball mill). For the fluid cokes thus heat-treated, the texture was examined by scanning eiectron and polarized light microscopes. The cospacing E0 and the apparent crystallite size L, were determined from (002) diffraction line by the standard procedure. The bulk density was measured for the samples which sintered in heat-treatment under 5 kbar. For the cokes heat-treated at 2400°C under normal pressure, the change of texture resulting from oxidation in a KnCr20,H2S0, solution at 80°C was studied mostly using scanning electron microscope.
et al.
Fig. 1. Scanning electron micrographs of the original (as received) fluid coke.
3. RESULTS 3.1 Texture of original coke (as received) The observations with a scanning electron microscope show the presence of two kinds of particles of very different appearance (Fig. 1): some having relatively smooth surface and others very rough. These two kinds of particles were found to be in almost equal amounts in our lot. Under optical microscope, these two kinds of particles show very different textures. The former ones (smooth surface) show crossed extinction contours (particle A in Fig. 2) and cracks which are roughly parallel to the surface. The latter ones (rough surface), on the other hand, have many contours arranged radially (particle B in Fig. 2). Some particles show two centers of radially-arranged contours, and are probably coalesced pairs of particles. The orientation of anisotropic graphitelike layers in such particles was deduced from
Fig. 2. Optical micrographs of the original fluid coke under crossed nicols. the movement of extinction contours with rotation of the polarized microscope stage and from the change in retardation when a gypsum test-plate was inserted. The results are shown schematically in Fig. 3. For the first type of particles (smooth surface and crossed extinction contours), the graphite-like layers align 11 o anoroximatelv , concentricallv like in an
TEXTURE AND GRAPHITIZATION BEHAVIOUR OF FLUID COKE
(a)
641
(b)
Fig. 3. Schematic representation of the texture of the two types of particles of the fluid coke (as received). onion. In the second (rough surface and radially-arranged contours), however, the parts where the graphite-like layers are highly oriented are arranged like petals in a chrysanthemum flower. The graphite-like layers in these petals are relatively large, but must be strongly bent because the retardation changes sharply in the same petal by only a slight rotation of the stage. The parts in between the petals consist of small graphitelike layers randomly-o~ented and look isotunder the microscope with lowropic magnification. 3.2 Changes of texture with heat-treatment The changes in texture with heattreatment under normal pressure are pretty characteristic for each kind of particles. Most of the particles with onion-like texture break up after heat-treatment at 2’780°C (Fig. 4). Many cup-shaped fragments are observed. Concentric long cracks are formed in the heat-treatment (Fig. 5a). The particles with chrysanthemum-like texture seem to undergo deformations and the roughness of the surface is smoothed out a Iittle. Cracks are formed mostly in the highly-oriented parts in the form of semicircles (Fig. 5b). Large cracks toward the center are observed for all of these particles after a 2780”C-treatment. The difference in texture between the two kinds of particles is also shown in the oxidation of the 2400”C-treated coke. After oxidation in K&r 20 7--H 2SOS solution for 10 hr, many of the particles with onion-like
Fig. 4. Scanning electron micrographs of the fluid coke heat-treated under normal pressure. (a) HTT 1840°C (b) HTT 2780°C. texture broke into pieces and cup-shaped fragments were observed. However particles with chrysanthemum-like texture showed not so much change. Even after heat-treatment at 2000°C under 5 kbar, the particles of the two kinds retain their characteristics in texture, as shown by polarized light micrograph (Fig. 6).
M. INAGAKI
642
et al.
Fig. 6. Optical micrograph under crossed nichols of the fluid coke heat-treated at 2000°C under 5 kbar.
6.7550 HTT.
‘C (b)
- PO0
.._J9+-?~;~-_ w -*\. 0
Fig. 5. Optical micrographs of the fluid coke heat-treated at 2780°C under normal pressure.
‘”
z\
6.80
3.3
Graphitization
behaviour
Under normal pressure, E, decreases and L, increases with the increase in heattreatment temperature (Fig. 7a). However, the changes in Co and L, are very small, for instance, they change to 6.78 and 200& respectively, after heat-treatment at 278O”C, whereas after heat-treatment at 28OO”C, most graphitizing carbons, such as the needle-like cokes, are completely graphitized; Co decreases down to 6.72 A and L, increases up to about 800 A. The fluid coke was also found to
-m-z
6&50
.
a 44-74/m
.
o
5-25/m
-_o
_ _Y
.A
$-A .-•=?--,-.-
‘-.-¤_
l-e-e_ I 2000
I 1500
HTT,
‘C
Fig. 7. Changes in the G-spacing Co and in the apparent crystallite size L, of the fluid coke with heat-treatment temperature HTT: (a) under normal pressure, (b) under 5 kbar. have were
large amount of lattice strains, which measured from half widths of the 002
and 004 lines, even after heat-treatment at high temperatures. Under 5 kbar, E0 decreases only to 6.82 0
6.84
A and
L, increases
only to 150 - 170 A
TEXTURE
AND GRAPHITIZATION
after 2OOO”Gtreatment (Fig. 7b). For smaller particle sizes of the coke the larger values of G are obtained, that is, a lower degree of graphitization is found. For the polyvinylchloride
coke,
for
example,
Z0 decreases
down to 6.72 A above 17OO”C, which is about the same as for the same coke heat-treated above
2700°C
under
normal
pressure.
In
case of the fluid coke, however, a lesser degree of graphitization was obtained after heat-treatments above 1700°C under 5 kbar than above 2700°C under normal pressure. The bulk density of the sintered cakes of the coke was 1*70-l-85 g/cm’. The change in bulk density of the cakes with heat-treatment temperature was not clearly seen. The cakes made from the coke with the particle size of 5-25 pm seem to have a little larger bulk density than those from the cokes with larger sizes. No increase in graphitization degree was obtained by pre-heat-treatment of the asreceived coke at 1840°C under normal pressure before the heat-treatments under 5 kbar. 4. DISCUSSION
The texture of the original fluid coke is very characteristic, particularly that of the particles with the chrysanthemum-like texture. The onion-like texture observed on the fluid coke is very similar to that of the gilsonite coke, except that the particles of the latter are connected with each other. Formation of cracks in the particles in the heat-treatment under normal pressure is closely related to the texture of each individual particle, that is, to the orientation of graphite-like layers in the particle. In particles with the onion-like texture, large cracks are formed mostly concentrically, in other words, oriented
roughly
parallel
graphite-like
the fracture fragments.
to the concentricallylayers,
of the particles
and they lead to into cup-shaped
In the particles with the chrysanthemum-like texture, on the other hand, cracks are also formed in the highlyoriented petal areas, but they are unable to
BEHAVIOUR
OF FLUID
COKE
64
connect with cracks in the neighboring petals across the strong randomly-oriented isotropic regions. Therefore, cracks cannot propagate throughout the particle and only the semicircle small cracks are formed in the petal areas. The large stress accumulation which is probably caused by the lack of free contraction of the particle, might become released by the formation of large cracks which are shown in Fig. 5. It is surprising that the fluid coke does not graphitize under 5 kbar, although most carbon materials do graphitize under this pressure. This may be attributed to either its characteristic texture or to a large content of sulfur. The finely powdered coke has a rather lower degree of graphitization than the as-received coke. The coke which is pre-heattreated under normal pressure does not show a better graphitizability than the original one. The reason why the fluid coke does not graphitize under pressure is not clear, but it is noteworthy that the carbon blacks, the gilsonite coke and the fluid coke have a similar texture of the particles, (although the scales are widely different) and all of them do not graphitize easily under 5 kbar [5,6].
Acknowledgements-The authors are indebted to the Toyo Carbon Co. Ltd. for supplying the fluid coke and also to the Government Industrial Research Institute at Nagoya for making the scanning electron microscopes available to this work.
REFERENCES 1. Engle G. B. and Eatherly W. P., High Temp.High Press. 4, 119 (1972). 2. Noda T., Kamiya K. and Inagaki M., Bull. Chem. Sot. Jafian 41, 485 (1968). 3. Kamiya K., Inagaki M., Mizutani M. and Noda T., ibid, 41, 2169 (1968). 4. Inagaki M., Tamai Y., Kamiya K. and Naka S., Ceram. Bull 52, 856 (1973). 5. Inagaki M., Tamai Y. and Naka S., ~~~~~0 1973 [No. 751, 118. 6. Inagaki M., Hayashi S., Kamiya K. and Naka S., High Temp.-Nigh Press. 8, 355 (1971).
12, pp. 639-643.
Pergamon
TEXTURE
Press.
AND
Printed
in Great
Britain
GRAPHITIZATION OF FLUID COKE
M. INAGAKI, Faculty of Engineering,
Y. TAMAI,
BEHAVIOUR
S. NAKA
Nagoya University, Chikusa-ku, Nagoya 464, Japan and K. KAMIYA
Faculty of Engineering,
Mie University, Tsu, Mie-ken, Japan
(Received 25 March 1974) Abstract-The texture, its changes with heat-treatments and with oxidation, and the graphitization behaviour of the fluid coke were investigated using polarized light microscopes, scanning electron microscopes and X-ray diffraction. The fluid coke as received had the homogeneous particle size of 0.1-0.2 mm. It consisted of two kinds of particles in almost equal amounts: one type with relatively smooth surfaces and showing crossed extinction contours for cross cuts, and the second type with very rough surfaces and radial extinction contours. In the former ones, the graphite-like layers seem to align approximately concentrically like that of onions. In the latter ones, regions with large graphite-like layers are highly oriented but those with small layers orient randomly being arranged like in a chrysanthemum flower. Formation of cracks in the particles heat-treated under normal pressure was observed and is explained as due to th: texture. The graphitizability of the coke is very poor, co-spacing decreasing only tP 6.78 A after 2780”C-treatment under normal pressure and only to 6.82-6.84 A after 2000°C-treatment under 5 kbar. The characteristic textures of the particles, onion- and chrysanthemum-like are still retained after heat-treatment under 5 kbar. 1. INTRODUCTION Fluid
coke
material
has
not
for carbon
products
poor graphitizability sulfur, although the strong.
been
under used
as a raw
because
of its
and large content of material is dense and
texture In
However,
the recent interest in isotropic and high-density carbon products might make the fluid coke usable as raw materials, gilsonite
like
it happened
coke[l].
We
have
in the
case
investigated
changes
that
graphitizes onion-like
retained[5].
present work, the with heat-treatment, were
texture, and
its the
investigated
2. EXPERIMENTAL The
fluid coke used (Philips
a homogeneous
particle
Oil Co.) was of
size of 0.1-0.2
mm.
Some chemical data supplied by the supplier were as follows: sulfur content: l*S%, ash: 0.5% and volatile matter: 4.5%. Heat-treatments of the fluid carried out in a graphite tube 639
12. NC>.GC
being the
however
only partly characteristic
of the
5 kbar [2-61. Many carbon materials, including non-graphitizing carbons such as phenolformaldehyde resin char[3] and carbon beads[4], were found to graphitize at such relatively low temperature as 1500”-1600°C
Vol
We found
graphitization behaviour for the fluid coke.
graphitization behaviour of various carbon materials under pressures as high as
CARBON
5 kbar[2-41.
the gilsonite coke under 5 kbar, its
coke were furnace at
640
M. INAGAKI
various temperatures 1840-2780°C with 1 hr holding time in nitrogen gas atmosphere of normal pressure, and also in a simple piston-cylinder apparatus at type lOOO-2000°C for 1 hr under pressure of 5 kbar. The heat-treatments under 5 kbar were done for fluid coke samples with various particle sizes; the as-received one with the particle sizes of O*l-O-2 mm, the one with about 44-74 pm (pufverized in an agate mortor) and also with about 5-25 pm (pulverized in a vibrating ball mill). For the fluid cokes thus heat-treated, the texture was examined by scanning eiectron and polarized light microscopes. The cospacing E0 and the apparent crystallite size L, were determined from (002) diffraction line by the standard procedure. The bulk density was measured for the samples which sintered in heat-treatment under 5 kbar. For the cokes heat-treated at 2400°C under normal pressure, the change of texture resulting from oxidation in a KnCr20,H2S0, solution at 80°C was studied mostly using scanning electron microscope.
et al.
Fig. 1. Scanning electron micrographs of the original (as received) fluid coke.
3. RESULTS 3.1 Texture of original coke (as received) The observations with a scanning electron microscope show the presence of two kinds of particles of very different appearance (Fig. 1): some having relatively smooth surface and others very rough. These two kinds of particles were found to be in almost equal amounts in our lot. Under optical microscope, these two kinds of particles show very different textures. The former ones (smooth surface) show crossed extinction contours (particle A in Fig. 2) and cracks which are roughly parallel to the surface. The latter ones (rough surface), on the other hand, have many contours arranged radially (particle B in Fig. 2). Some particles show two centers of radially-arranged contours, and are probably coalesced pairs of particles. The orientation of anisotropic graphitelike layers in such particles was deduced from
Fig. 2. Optical micrographs of the original fluid coke under crossed nicols. the movement of extinction contours with rotation of the polarized microscope stage and from the change in retardation when a gypsum test-plate was inserted. The results are shown schematically in Fig. 3. For the first type of particles (smooth surface and crossed extinction contours), the graphite-like layers align 11 o anoroximatelv , concentricallv like in an
TEXTURE AND GRAPHITIZATION BEHAVIOUR OF FLUID COKE
(a)
641
(b)
Fig. 3. Schematic representation of the texture of the two types of particles of the fluid coke (as received). onion. In the second (rough surface and radially-arranged contours), however, the parts where the graphite-like layers are highly oriented are arranged like petals in a chrysanthemum flower. The graphite-like layers in these petals are relatively large, but must be strongly bent because the retardation changes sharply in the same petal by only a slight rotation of the stage. The parts in between the petals consist of small graphitelike layers randomly-o~ented and look isotunder the microscope with lowropic magnification. 3.2 Changes of texture with heat-treatment The changes in texture with heattreatment under normal pressure are pretty characteristic for each kind of particles. Most of the particles with onion-like texture break up after heat-treatment at 2’780°C (Fig. 4). Many cup-shaped fragments are observed. Concentric long cracks are formed in the heat-treatment (Fig. 5a). The particles with chrysanthemum-like texture seem to undergo deformations and the roughness of the surface is smoothed out a Iittle. Cracks are formed mostly in the highly-oriented parts in the form of semicircles (Fig. 5b). Large cracks toward the center are observed for all of these particles after a 2780”C-treatment. The difference in texture between the two kinds of particles is also shown in the oxidation of the 2400”C-treated coke. After oxidation in K&r 20 7--H 2SOS solution for 10 hr, many of the particles with onion-like
Fig. 4. Scanning electron micrographs of the fluid coke heat-treated under normal pressure. (a) HTT 1840°C (b) HTT 2780°C. texture broke into pieces and cup-shaped fragments were observed. However particles with chrysanthemum-like texture showed not so much change. Even after heat-treatment at 2000°C under 5 kbar, the particles of the two kinds retain their characteristics in texture, as shown by polarized light micrograph (Fig. 6).
M. INAGAKI
642
et al.
Fig. 6. Optical micrograph under crossed nichols of the fluid coke heat-treated at 2000°C under 5 kbar.
6.7550 HTT.
‘C (b)
- PO0
.._J9+-?~;~-_ w -*\. 0
Fig. 5. Optical micrographs of the fluid coke heat-treated at 2780°C under normal pressure.
‘”
z\
6.80
3.3
Graphitization
behaviour
Under normal pressure, E, decreases and L, increases with the increase in heattreatment temperature (Fig. 7a). However, the changes in Co and L, are very small, for instance, they change to 6.78 and 200& respectively, after heat-treatment at 278O”C, whereas after heat-treatment at 28OO”C, most graphitizing carbons, such as the needle-like cokes, are completely graphitized; Co decreases down to 6.72 A and L, increases up to about 800 A. The fluid coke was also found to
-m-z
6&50
.
a 44-74/m
.
o
5-25/m
-_o
_ _Y
.A
$-A .-•=?--,-.-
‘-.-¤_
l-e-e_ I 2000
I 1500
HTT,
‘C
Fig. 7. Changes in the G-spacing Co and in the apparent crystallite size L, of the fluid coke with heat-treatment temperature HTT: (a) under normal pressure, (b) under 5 kbar. have were
large amount of lattice strains, which measured from half widths of the 002
and 004 lines, even after heat-treatment at high temperatures. Under 5 kbar, E0 decreases only to 6.82 0
6.84
A and
L, increases
only to 150 - 170 A
TEXTURE
AND GRAPHITIZATION
after 2OOO”Gtreatment (Fig. 7b). For smaller particle sizes of the coke the larger values of G are obtained, that is, a lower degree of graphitization is found. For the polyvinylchloride
coke,
for
example,
Z0 decreases
down to 6.72 A above 17OO”C, which is about the same as for the same coke heat-treated above
2700°C
under
normal
pressure.
In
case of the fluid coke, however, a lesser degree of graphitization was obtained after heat-treatments above 1700°C under 5 kbar than above 2700°C under normal pressure. The bulk density of the sintered cakes of the coke was 1*70-l-85 g/cm’. The change in bulk density of the cakes with heat-treatment temperature was not clearly seen. The cakes made from the coke with the particle size of 5-25 pm seem to have a little larger bulk density than those from the cokes with larger sizes. No increase in graphitization degree was obtained by pre-heat-treatment of the asreceived coke at 1840°C under normal pressure before the heat-treatments under 5 kbar. 4. DISCUSSION
The texture of the original fluid coke is very characteristic, particularly that of the particles with the chrysanthemum-like texture. The onion-like texture observed on the fluid coke is very similar to that of the gilsonite coke, except that the particles of the latter are connected with each other. Formation of cracks in the particles in the heat-treatment under normal pressure is closely related to the texture of each individual particle, that is, to the orientation of graphite-like layers in the particle. In particles with the onion-like texture, large cracks are formed mostly concentrically, in other words, oriented
roughly
parallel
graphite-like
the fracture fragments.
to the concentricallylayers,
of the particles
and they lead to into cup-shaped
In the particles with the chrysanthemum-like texture, on the other hand, cracks are also formed in the highlyoriented petal areas, but they are unable to
BEHAVIOUR
OF FLUID
COKE
64
connect with cracks in the neighboring petals across the strong randomly-oriented isotropic regions. Therefore, cracks cannot propagate throughout the particle and only the semicircle small cracks are formed in the petal areas. The large stress accumulation which is probably caused by the lack of free contraction of the particle, might become released by the formation of large cracks which are shown in Fig. 5. It is surprising that the fluid coke does not graphitize under 5 kbar, although most carbon materials do graphitize under this pressure. This may be attributed to either its characteristic texture or to a large content of sulfur. The finely powdered coke has a rather lower degree of graphitization than the as-received coke. The coke which is pre-heattreated under normal pressure does not show a better graphitizability than the original one. The reason why the fluid coke does not graphitize under pressure is not clear, but it is noteworthy that the carbon blacks, the gilsonite coke and the fluid coke have a similar texture of the particles, (although the scales are widely different) and all of them do not graphitize easily under 5 kbar [5,6].
Acknowledgements-The authors are indebted to the Toyo Carbon Co. Ltd. for supplying the fluid coke and also to the Government Industrial Research Institute at Nagoya for making the scanning electron microscopes available to this work.
REFERENCES 1. Engle G. B. and Eatherly W. P., High Temp.High Press. 4, 119 (1972). 2. Noda T., Kamiya K. and Inagaki M., Bull. Chem. Sot. Jafian 41, 485 (1968). 3. Kamiya K., Inagaki M., Mizutani M. and Noda T., ibid, 41, 2169 (1968). 4. Inagaki M., Tamai Y., Kamiya K. and Naka S., Ceram. Bull 52, 856 (1973). 5. Inagaki M., Tamai Y. and Naka S., ~~~~~0 1973 [No. 751, 118. 6. Inagaki M., Hayashi S., Kamiya K. and Naka S., High Temp.-Nigh Press. 8, 355 (1971).