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Puffing inhibitors for coal tar based needle coke
Carbon Vol. 24. No 4. pp 397-401. Primed in Great Britain.
1986
ooO8-6223/86 $3.00+ .oO 6 1986 Pergamon Journals Ltd.
PUFFING INHIBITORS FOR COAL TAR BASED NEEDLE COKE K. FWMOTO, M. SATO and M. YAMADA Nippon Steel Corporation, R & D Laboratories-I, 1618 Ida, Nakahara-ku, Kawasaki 21 I, Japan
R. YAMASH~TAand K. SHIBATA Nippon Steel Chemical Co., Ltd., 13-16 Ginza, %chome, Chuo-ku, Tokyo 104, Japan (Received 2 August 1985; in revisedform 26 September 1985) Abstract-The efficiency of various puffing inhibitors has been studied systematically using carbon samples prepared from coal tar based needle coke. Puffing was reduced by the addition of nickel and cobalt oxides. The bulk density of moulded blocks subjected to graphitization was increased also by the addition of these oxides, because of their catalytic ability for carbonization. No sulfide was observed during the graphitization process, implying that puffing inhibition is not necessarily due to the reaction of metals with sulfur compounds in cokes. The interlayer spacing of needle cokes mixed with effective puffing inhibitors was considerably decreased during graphitization. The action of puffing inhibitors seems to change the timing between the gas evolution and the softening of carbon bodies. Key Words--Coke,
1.
puffing, nickel oxide, cobalt oxide, sulfur. interlayer spacing
INTRODUCTION
It is generally known that an irreversible volume expansion, the so-called puffing, takes place during the graphitization of carbon products. Puffing decreases bulk density, strength and electrical and thermal conductivities of the products, and gives rise often to the formation of cracks. In the case of petroleum coke based product, puffing is ascribed to a sudden release of sulfurous gases from the coke taking place between 1400 and 22OO”C, and these troubles have successfully been removed by use of low-sulfur needle coke and/or by adding appropriate puffing inhibitors, such as iron oxide, together with binder. Puffing is also dependent upon the structure of coke; non-graphitizable carbons like amorphous coke are less susceptible to puffing than graphitizable carbons like needle coke[ l-51. On the other hand, the previous papers of the authors[6,7] have revealed that the puffing of coal tar based needle coke (pitch coke) is not only caused by sulfur but is correlated rather to the nitrogen content and the structure of coke. Therefore, puffing inhibitors for petroleum coke are not necessarily effective for pitch coke. Morris et a1.[8] reported that the puffing of pitch coke can be inhibited by the use of chromium oxide. The object of the present study is to seek out the effective puffing inhibitors for pitch coke as well as to elucidate the puffing inhibition mechanism. 2. EXPERIMENTAL 2.1 Materials Carbon specimens were prepared by using LPC-U of Nippon Steel Chemical Co., Ltd. as pitch coke filler and coal tar pitch of Nippon Steel Corporation for graphite electrode as binder. Characteristics of LPC-U are sum-
marized in Table 1. Commercially available metal oxides with iron, nickel cobalt, chromium, molybdenum, manganese and tungsten were examined as puffing inhibitors without further purification. 2.2 Experimental procedure Green test pieces 100 mm long and 20 mm in diameter were prepared from a standard mixture of calcined pitch coke (65%), binder pitch (35%) and a certain amount of metal oxide hot pressed in an electrically heated steel die at 10 KN cmm2and lOYC, and were followingly baked in a reducing atmosphere by coke particles at 900°C for 6 hrs. Particle size of coke was 1 mm for 60% and l2.5 mm for 40%. The extent of puffing was measured with a push-rod-type diatometer heated from room temperature up to 2600°C with a rate of 10°C min-’ in argon atmosphere. The linear expansion was measured in the direction perpendicular to the moulding direction, viz. parallel to the particle alignment imposed through moulding. The apparatus is illustrated elsewhere[‘l]. Figure 1 shows a typical puffing curve. The puffing curve is defined as the linear expansion ranging from the second inflection point to the maximum point at 2600°C. The puffing occurs mainly between around 1700 and 2200°C. Weight loss of these green test pieces in the temperature range up to 1450°C was measured by a thermal gravimetric analyzer, where heating rate was also 10°C min-‘. The binding energy of sulfur compounds in coke was measured by using an X-ray photoelectron spectrometer (Shimadzu Corporation, ESCA 750). Changes in the mean interlayer distance during graphitization was determined by a high temperature X-ray diffractometer (Rigaku Corporation, 2500°C). The samples (2.5 x 7.0 x 70 mm) were cut out of the baked test pieces and internally heated by an electric current. The
397
K. FUIIMOTOet al.
398
2.5 t
Temperature t”CI
Fig.
I. Puffing curve and definition of puffing.
Table 2. Effect of puffing inhibitors on puffing and bulk density
6 D
: Bulk
1.30
I .442
1.46
1.439
Density
c& 5.0 f f 4 ‘O.O E f 15.0
*--____
200
400
600
600
KXKI
1200
TemperaluretC) Fig. 2. TGA curves of green mould test pieces.
MOO
Puffing inhibitors for coal tar based needle coke
Table 3. Metal compounds at Additive
IComads.
Fe,O,
t
Fe
Fez%
’
-
’
’
1
\
1230 ’ -
lee0
1030
w
WC
’
-. 1400
WC,
I
’
-1340
Cr3C2
WZC
’
’ 1180
Cr,C,
W%
’
-lax,
Ni
Cr2%
’
Table 4. Effect of additives on sulfur content change
high temperature
lam
FeC NIO
’
399
‘1350 !
‘1620
1110’
duction temperatures of the metal oxides; nickel, iron, tungsten and chromium oxides were reduced around 470, 820, 950 and 1300°C. respectively, and the weight losses of the samples of these temperatures were wholly due to the elimination of oxygen. It was confirmed by this experiment that the change of BD during the baking process depends on the kind of metal oxides added, reflecting the difference in the catalytic ability of them for carbonization.
I
1160’ 1030 1200
1403
160.3
1600
1
Temperature C’C)
heating rate was around 20°C min- ‘. Interlayer distance (0.5 c) was calculated from the (002) diffraction peaks.
3. RESULTS AND DISCUSSION 3.1 Extent of puffing and bulk density Effects of various inhibitors on puffing and bulk density (hereafter BD) are shown in Table 2, the additive content being I wt%. It is noted that puffing is reduced significantly by the addition of chromium, nickel and cobalt oxides, but BD of the sample containing chromium oxide after graphitization is much lower than BD of those containing nickel and/or cobalt oxides. Iron, manganese. tungsten, molybdenum and titanium oxides do not reduce the puffing at all. Since one of the objects of puffing inhibition is to increase BD as well as to enhance the strength of graphite products, just nickel and cobalt oxides are considerable to be effective inhibitors for pitch coke from a practical point of view. The reason why BD of baked specimens is increased by the addition of nickel and cobalt oxides may be ascribed to their catalytic ability for carbonization[9]. In order to obtain more detailed information about the baking process, a thermal gravimetric analysis of green test pieces was conducted. In this experiment, however, the amount of metal oxides added was increased to 10 wt% of each green test piece for seeking more accurately the catalytic ability of metals. It is noted that in Fig. 2 the weight loss of the sample containing nickel oxide is smaller than those of the other samples at 1450°C. This behavior is consistent with the aforesaid catalytic function of nickel compound increasing the carbon yield of binder pitch. The weight loss of a sample mixed with iron oxide is rather larger than that with no additive. This unexpected result seems to come from the elimination of oxygen from iron oxide taking place so actively at about 820°C. where is noted that the amount of metal oxide added is as much as 10 wt%. The differential curves in Fig. 2 show re-
3.2 State of metal oxides at high temperature 6ya and 6tani[ lo] have studied the catalytic graphitization of carbons by various metals, where nickel, cobalt and iron were found to remain as metallic state but tungsten and chromium were converted to their carbides. An X-ray diffraction analysis was made to determine how the state of metal oxides in our specimens changes at high temperatures. The results for the specimens whose metal content was IO wt% of the green test pieces are shown in Table 3. The metal oxides added to green test pieces were reduced to various states during the heat treatment, and the diffraction peaks were not observed in the temperature ranges other than marked in Table 3. implying that the metal compounds are melted and dispersed over the carbon body at high temperatures. The reduction of nickel oxide, most effective puffing inhibitor, takes place at relatively low temperature. It is essential for the effective inhibition that the melt, dispersion and puffing occur in the same temperature range. The formation of chromium and tungsten carbides during the heating agrees with observation of 6ya and 6tani. 3.3 Behavior @sulfur in coke In the case of petroleum coke, the addition of certain metal compounds, particularly iron oxide, is known to inhibit or eliminate puffing by reacting with sulfur so as to form sulfide. The sulfur content was measured of our coal-tar pitch based specimens for deciding whether sulfur reacts with metal compounds or not during the heating. The results obtained are shown in Table 4. After heated up to HTTs indicated in the table with a rate of 10°C min ’ and were kept each time for IO min. samples were quenched and then examined to the analysis. The
Table 5. Binding energy of sulfur Observed HTT
Reference
900%
1600
1600
2000
2200
2400
FeS
FeS w
Bonding Energ), 1643”
164.2
164.2
164.2
164.1
164.3
162.0
161.2 164.2
K. FUJIMOTOet al.
400
3.4 Change of metal content The metal content in carbon samples stepwise heattreated up to 2200°C was measured using an atomic absorption spectrometer, where 1 wt% of metal oxide was added to each green test piece. As shown in Fig. 3, all metals examined tend to decrease gradually with heating, but still remain enough to act as puffing inhibitors. All metals except tungsten evaporate rapidly above 2000°C. a
A o
f
A
Cr Fe
t 1
I
Baked
I
4400
I 1600
I
I
I
I
I
1800 2000
I
I
2200
HTT Co0 Fig. 3. Change of metal content.
amount added to the green test piece was 1 wt%. As compared with the reference sample, however, the difference is too little to make final judgement in this way. Thus, in order to obtain more precise informations about the sulfide formation through the heat treatment, the binding energy of sulfur compounds was measured by means of X-ray photoelectron spectroscopy. In this experiment, 3 wt% of iron oxide was added to the green samples to intensify the signal. The binding energy evaluated of sulfur compounds in these cokes ranges from 164.1 to 164.3 eV as shown in Table 5. These values agree with that of thiophene rather than ferrous sulfide and are not changed by heating. Such a result indicates that sulfur in pitch cokes does not react with metal to form sulfide during the graphitization.
3.5 Change of inter-layerspacing The irreversible expansion of the interlayer spacing (0.5 C) of baked specimens was measured in the temperature range up to 2400°C using a high temperature Xray diffractometer. Results are shown in Fig. 4(a) and (b). In this experiment, 10 wt% of metal oxides was added to each green test piece so as to produce more pronounced effect. Similar measurement was conducted of the reversible thermal expansion of an artificial graphite for the sake of comparison. It will be seen that 0.5 C of artificial graphite increases linearly in proportion to temperature, while those of the baked test pieces expand irreversibly and show an inflection at about 1700°C. Brandstaedler and Fitzer[ 1I] have studied the graphitization process of petroleum and pitch cokes by means of in situ X-ray diffraction and dilatometry. They claim that the lattice shrinkage is strictly correlated to bulk expansion. Wagner and Letizia[ 121 have demonstrated that puffing of carbon artifacts during graphitization starts rather in the pregraphitization stage, where the carbon structure softens due to crystal alignment in C-direction.
3.65
3.651
’
I
I
I
Additive None
1400
1600
1800
2000
2200
Additive
Mn02
0 None l Fe203
w03
A
Cr203
0
NiO
3.50 -
2400
345
’ 1400 ’ ’ 1600 ’ ’ 1800 ’ ’ ’ ’ ’ ’ ’ ’ 2OCO 2200 2400
Tempemture t?Z)
Temperature t’C)
(a) Fig. 4.(a) Influence
of additives
I
(b) on mean interlayer spacing. spacing.
(b) Influence
of additives
on mean interlayer
_
401
Puffing inhibitors for coal tar based needle coke
On these bases, puffing is concluded to be an intercrystalline phenomenon. Mochida and Marsh[S] have reported that with increasing HTT gaseous products are generated so as to build up a pressure giving rise to convoluted to parallel transformation of structure. In the case of a petroleum needle coke, the sulfurous gases are evolved over the temperature range just coincident with that where puffing occurs as pointed out by Whittaker and Grindstaffl21. The present authors]71 have studied the puffing behavior of coal tar based needle coke using a high temperature thermal gravimetric analyzer and a mass spectrographic gas analyzer, whereby nitrogen was found to evolve mainly at puffing temperatures. Accordingly, there is no doubt that puffing in needle coke is generally caused by the pressures due to the gas evolution from the softened carbon bodies. Therefore, puffing can be inhibited by changing the timing between the gas evolution and the softening of carbon bodies. Figure 4 shows that the addition of effective puffing inhibitors such as chromium and/or nickel oxides gives rise to remarkable lowering not only of the interlayer specing of baked test pieces but of the curve slope in the temperature range below 1700°C where puffing is initiated. On the other hand, tungsten and molybdenum oxides affect not much. Transition metals have catalytic ability in general for graphitization, and the catalytic function of chromium oxide and nickel compounds has been reported by Mochida et al. [ 131 and Gtani et al. [ 141, respectively. As compared with the behavior of artificial graphite just showing linear expansion, the lower slope exhibited by the baked test pieces below about 1700°C may be taken as an evidence for occurrence of the lattice shrink-
age and the softening of structure at these temperatures. In other words, the softening temperature is lowered by the addition of effective puffing inhibitor. On the other hand, the sulfur content in the heat treatment process is not changed by the addition of metal oxides as shown in Table 4; i.e. puffing inhibitors do not change the gas evolution temperature. Therefore, the action of puffing inhibitors seems to change the timing of the softening of carbon bodies from the gas evolution.
REFERENCES
I. F. M. Collins, Proceedings of 1st and 2nd Conferences on Carbon, Buffalo, New York, 177 (1956). 2. M. P. Whittaker and L. 1. Grindstaff, Carbon 7,615 (1969). 3. E. Fitzer and H-P. Tanoschek, High Temp.-High Pressures 9, 243 (1977). 4. E. Fiter and S. Weisenburger, Rev. Ini. Hauies. Temp. Refract. 12, 69 (1975). 5. 1. Mochida and H. Marsh, 17th Biennial Conference on Carbon, Extended Abstracts 216 (1985). 6 K. Fujimoto, K. Ohtsuka, M. Iwasa and R. Yamashita, High Temp.-High Pressures 16, 617 (1984). 7 K. Fujimoto, M. Yamada, M. Iwasa and H. Nagino. High Temp.-High Pressures 16, 669 (1984). 8 E. G. Morris, K. W. Tucker and L. A. Joo, 16th Biennial Conference on Carbon, Extended Abstracrs 595 ( 1983). 9. P. T. K. Baker and P. S. Harris, Chemistry and Physics of Carbon (Edited by P. L. Walker Jr. and J. S. Thrower) Vol. 14, Chapter 2, Decker, New York (1978). 10. A. Oya and S. Otani, Carbon 17, 131 (1979). 11 W. Brandstaedler and E. Fitzer, Proceedings 4th London Inrernarional Carbon and Graphite Conferences 71 (1982). 12. M. H. Wagner and 1. Letizia, 17th Biennial Conference on Carbon, Extended Abstracts 29 (1985). 13 1. Mochida, R. Ohtsubo and K. Takeshita. Carbon 18, 117 (1980). 14 S. Otani, A. Gya and I. Akagami. Carbon 13, 353 (1975).
1986
ooO8-6223/86 $3.00+ .oO 6 1986 Pergamon Journals Ltd.
PUFFING INHIBITORS FOR COAL TAR BASED NEEDLE COKE K. FWMOTO, M. SATO and M. YAMADA Nippon Steel Corporation, R & D Laboratories-I, 1618 Ida, Nakahara-ku, Kawasaki 21 I, Japan
R. YAMASH~TAand K. SHIBATA Nippon Steel Chemical Co., Ltd., 13-16 Ginza, %chome, Chuo-ku, Tokyo 104, Japan (Received 2 August 1985; in revisedform 26 September 1985) Abstract-The efficiency of various puffing inhibitors has been studied systematically using carbon samples prepared from coal tar based needle coke. Puffing was reduced by the addition of nickel and cobalt oxides. The bulk density of moulded blocks subjected to graphitization was increased also by the addition of these oxides, because of their catalytic ability for carbonization. No sulfide was observed during the graphitization process, implying that puffing inhibition is not necessarily due to the reaction of metals with sulfur compounds in cokes. The interlayer spacing of needle cokes mixed with effective puffing inhibitors was considerably decreased during graphitization. The action of puffing inhibitors seems to change the timing between the gas evolution and the softening of carbon bodies. Key Words--Coke,
1.
puffing, nickel oxide, cobalt oxide, sulfur. interlayer spacing
INTRODUCTION
It is generally known that an irreversible volume expansion, the so-called puffing, takes place during the graphitization of carbon products. Puffing decreases bulk density, strength and electrical and thermal conductivities of the products, and gives rise often to the formation of cracks. In the case of petroleum coke based product, puffing is ascribed to a sudden release of sulfurous gases from the coke taking place between 1400 and 22OO”C, and these troubles have successfully been removed by use of low-sulfur needle coke and/or by adding appropriate puffing inhibitors, such as iron oxide, together with binder. Puffing is also dependent upon the structure of coke; non-graphitizable carbons like amorphous coke are less susceptible to puffing than graphitizable carbons like needle coke[ l-51. On the other hand, the previous papers of the authors[6,7] have revealed that the puffing of coal tar based needle coke (pitch coke) is not only caused by sulfur but is correlated rather to the nitrogen content and the structure of coke. Therefore, puffing inhibitors for petroleum coke are not necessarily effective for pitch coke. Morris et a1.[8] reported that the puffing of pitch coke can be inhibited by the use of chromium oxide. The object of the present study is to seek out the effective puffing inhibitors for pitch coke as well as to elucidate the puffing inhibition mechanism. 2. EXPERIMENTAL 2.1 Materials Carbon specimens were prepared by using LPC-U of Nippon Steel Chemical Co., Ltd. as pitch coke filler and coal tar pitch of Nippon Steel Corporation for graphite electrode as binder. Characteristics of LPC-U are sum-
marized in Table 1. Commercially available metal oxides with iron, nickel cobalt, chromium, molybdenum, manganese and tungsten were examined as puffing inhibitors without further purification. 2.2 Experimental procedure Green test pieces 100 mm long and 20 mm in diameter were prepared from a standard mixture of calcined pitch coke (65%), binder pitch (35%) and a certain amount of metal oxide hot pressed in an electrically heated steel die at 10 KN cmm2and lOYC, and were followingly baked in a reducing atmosphere by coke particles at 900°C for 6 hrs. Particle size of coke was 1 mm for 60% and l2.5 mm for 40%. The extent of puffing was measured with a push-rod-type diatometer heated from room temperature up to 2600°C with a rate of 10°C min-’ in argon atmosphere. The linear expansion was measured in the direction perpendicular to the moulding direction, viz. parallel to the particle alignment imposed through moulding. The apparatus is illustrated elsewhere[‘l]. Figure 1 shows a typical puffing curve. The puffing curve is defined as the linear expansion ranging from the second inflection point to the maximum point at 2600°C. The puffing occurs mainly between around 1700 and 2200°C. Weight loss of these green test pieces in the temperature range up to 1450°C was measured by a thermal gravimetric analyzer, where heating rate was also 10°C min-‘. The binding energy of sulfur compounds in coke was measured by using an X-ray photoelectron spectrometer (Shimadzu Corporation, ESCA 750). Changes in the mean interlayer distance during graphitization was determined by a high temperature X-ray diffractometer (Rigaku Corporation, 2500°C). The samples (2.5 x 7.0 x 70 mm) were cut out of the baked test pieces and internally heated by an electric current. The
397
K. FUIIMOTOet al.
398
2.5 t
Temperature t”CI
Fig.
I. Puffing curve and definition of puffing.
Table 2. Effect of puffing inhibitors on puffing and bulk density
6 D
: Bulk
1.30
I .442
1.46
1.439
Density
c& 5.0 f f 4 ‘O.O E f 15.0
*--____
200
400
600
600
KXKI
1200
TemperaluretC) Fig. 2. TGA curves of green mould test pieces.
MOO
Puffing inhibitors for coal tar based needle coke
Table 3. Metal compounds at Additive
IComads.
Fe,O,
t
Fe
Fez%
’
-
’
’
1
\
1230 ’ -
lee0
1030
w
WC
’
-. 1400
WC,
I
’
-1340
Cr3C2
WZC
’
’ 1180
Cr,C,
W%
’
-lax,
Ni
Cr2%
’
Table 4. Effect of additives on sulfur content change
high temperature
lam
FeC NIO
’
399
‘1350 !
‘1620
1110’
duction temperatures of the metal oxides; nickel, iron, tungsten and chromium oxides were reduced around 470, 820, 950 and 1300°C. respectively, and the weight losses of the samples of these temperatures were wholly due to the elimination of oxygen. It was confirmed by this experiment that the change of BD during the baking process depends on the kind of metal oxides added, reflecting the difference in the catalytic ability of them for carbonization.
I
1160’ 1030 1200
1403
160.3
1600
1
Temperature C’C)
heating rate was around 20°C min- ‘. Interlayer distance (0.5 c) was calculated from the (002) diffraction peaks.
3. RESULTS AND DISCUSSION 3.1 Extent of puffing and bulk density Effects of various inhibitors on puffing and bulk density (hereafter BD) are shown in Table 2, the additive content being I wt%. It is noted that puffing is reduced significantly by the addition of chromium, nickel and cobalt oxides, but BD of the sample containing chromium oxide after graphitization is much lower than BD of those containing nickel and/or cobalt oxides. Iron, manganese. tungsten, molybdenum and titanium oxides do not reduce the puffing at all. Since one of the objects of puffing inhibition is to increase BD as well as to enhance the strength of graphite products, just nickel and cobalt oxides are considerable to be effective inhibitors for pitch coke from a practical point of view. The reason why BD of baked specimens is increased by the addition of nickel and cobalt oxides may be ascribed to their catalytic ability for carbonization[9]. In order to obtain more detailed information about the baking process, a thermal gravimetric analysis of green test pieces was conducted. In this experiment, however, the amount of metal oxides added was increased to 10 wt% of each green test piece for seeking more accurately the catalytic ability of metals. It is noted that in Fig. 2 the weight loss of the sample containing nickel oxide is smaller than those of the other samples at 1450°C. This behavior is consistent with the aforesaid catalytic function of nickel compound increasing the carbon yield of binder pitch. The weight loss of a sample mixed with iron oxide is rather larger than that with no additive. This unexpected result seems to come from the elimination of oxygen from iron oxide taking place so actively at about 820°C. where is noted that the amount of metal oxide added is as much as 10 wt%. The differential curves in Fig. 2 show re-
3.2 State of metal oxides at high temperature 6ya and 6tani[ lo] have studied the catalytic graphitization of carbons by various metals, where nickel, cobalt and iron were found to remain as metallic state but tungsten and chromium were converted to their carbides. An X-ray diffraction analysis was made to determine how the state of metal oxides in our specimens changes at high temperatures. The results for the specimens whose metal content was IO wt% of the green test pieces are shown in Table 3. The metal oxides added to green test pieces were reduced to various states during the heat treatment, and the diffraction peaks were not observed in the temperature ranges other than marked in Table 3. implying that the metal compounds are melted and dispersed over the carbon body at high temperatures. The reduction of nickel oxide, most effective puffing inhibitor, takes place at relatively low temperature. It is essential for the effective inhibition that the melt, dispersion and puffing occur in the same temperature range. The formation of chromium and tungsten carbides during the heating agrees with observation of 6ya and 6tani. 3.3 Behavior @sulfur in coke In the case of petroleum coke, the addition of certain metal compounds, particularly iron oxide, is known to inhibit or eliminate puffing by reacting with sulfur so as to form sulfide. The sulfur content was measured of our coal-tar pitch based specimens for deciding whether sulfur reacts with metal compounds or not during the heating. The results obtained are shown in Table 4. After heated up to HTTs indicated in the table with a rate of 10°C min ’ and were kept each time for IO min. samples were quenched and then examined to the analysis. The
Table 5. Binding energy of sulfur Observed HTT
Reference
900%
1600
1600
2000
2200
2400
FeS
FeS w
Bonding Energ), 1643”
164.2
164.2
164.2
164.1
164.3
162.0
161.2 164.2
K. FUJIMOTOet al.
400
3.4 Change of metal content The metal content in carbon samples stepwise heattreated up to 2200°C was measured using an atomic absorption spectrometer, where 1 wt% of metal oxide was added to each green test piece. As shown in Fig. 3, all metals examined tend to decrease gradually with heating, but still remain enough to act as puffing inhibitors. All metals except tungsten evaporate rapidly above 2000°C. a
A o
f
A
Cr Fe
t 1
I
Baked
I
4400
I 1600
I
I
I
I
I
1800 2000
I
I
2200
HTT Co0 Fig. 3. Change of metal content.
amount added to the green test piece was 1 wt%. As compared with the reference sample, however, the difference is too little to make final judgement in this way. Thus, in order to obtain more precise informations about the sulfide formation through the heat treatment, the binding energy of sulfur compounds was measured by means of X-ray photoelectron spectroscopy. In this experiment, 3 wt% of iron oxide was added to the green samples to intensify the signal. The binding energy evaluated of sulfur compounds in these cokes ranges from 164.1 to 164.3 eV as shown in Table 5. These values agree with that of thiophene rather than ferrous sulfide and are not changed by heating. Such a result indicates that sulfur in pitch cokes does not react with metal to form sulfide during the graphitization.
3.5 Change of inter-layerspacing The irreversible expansion of the interlayer spacing (0.5 C) of baked specimens was measured in the temperature range up to 2400°C using a high temperature Xray diffractometer. Results are shown in Fig. 4(a) and (b). In this experiment, 10 wt% of metal oxides was added to each green test piece so as to produce more pronounced effect. Similar measurement was conducted of the reversible thermal expansion of an artificial graphite for the sake of comparison. It will be seen that 0.5 C of artificial graphite increases linearly in proportion to temperature, while those of the baked test pieces expand irreversibly and show an inflection at about 1700°C. Brandstaedler and Fitzer[ 1I] have studied the graphitization process of petroleum and pitch cokes by means of in situ X-ray diffraction and dilatometry. They claim that the lattice shrinkage is strictly correlated to bulk expansion. Wagner and Letizia[ 121 have demonstrated that puffing of carbon artifacts during graphitization starts rather in the pregraphitization stage, where the carbon structure softens due to crystal alignment in C-direction.
3.65
3.651
’
I
I
I
Additive None
1400
1600
1800
2000
2200
Additive
Mn02
0 None l Fe203
w03
A
Cr203
0
NiO
3.50 -
2400
345
’ 1400 ’ ’ 1600 ’ ’ 1800 ’ ’ ’ ’ ’ ’ ’ ’ 2OCO 2200 2400
Tempemture t?Z)
Temperature t’C)
(a) Fig. 4.(a) Influence
of additives
I
(b) on mean interlayer spacing. spacing.
(b) Influence
of additives
on mean interlayer
_
401
Puffing inhibitors for coal tar based needle coke
On these bases, puffing is concluded to be an intercrystalline phenomenon. Mochida and Marsh[S] have reported that with increasing HTT gaseous products are generated so as to build up a pressure giving rise to convoluted to parallel transformation of structure. In the case of a petroleum needle coke, the sulfurous gases are evolved over the temperature range just coincident with that where puffing occurs as pointed out by Whittaker and Grindstaffl21. The present authors]71 have studied the puffing behavior of coal tar based needle coke using a high temperature thermal gravimetric analyzer and a mass spectrographic gas analyzer, whereby nitrogen was found to evolve mainly at puffing temperatures. Accordingly, there is no doubt that puffing in needle coke is generally caused by the pressures due to the gas evolution from the softened carbon bodies. Therefore, puffing can be inhibited by changing the timing between the gas evolution and the softening of carbon bodies. Figure 4 shows that the addition of effective puffing inhibitors such as chromium and/or nickel oxides gives rise to remarkable lowering not only of the interlayer specing of baked test pieces but of the curve slope in the temperature range below 1700°C where puffing is initiated. On the other hand, tungsten and molybdenum oxides affect not much. Transition metals have catalytic ability in general for graphitization, and the catalytic function of chromium oxide and nickel compounds has been reported by Mochida et al. [ 131 and Gtani et al. [ 141, respectively. As compared with the behavior of artificial graphite just showing linear expansion, the lower slope exhibited by the baked test pieces below about 1700°C may be taken as an evidence for occurrence of the lattice shrink-
age and the softening of structure at these temperatures. In other words, the softening temperature is lowered by the addition of effective puffing inhibitor. On the other hand, the sulfur content in the heat treatment process is not changed by the addition of metal oxides as shown in Table 4; i.e. puffing inhibitors do not change the gas evolution temperature. Therefore, the action of puffing inhibitors seems to change the timing of the softening of carbon bodies from the gas evolution.
REFERENCES
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