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A new calcining technology for manufacturing of coke with lower thermal expansion coefficient
Cohn Vol. 19. No. 5, pp. 347-352. 1981 Prinled in Great Britain.
@X&5223/81/050347WSUZ.CQ/0 @ 1981 Pergamon Press Ltd.
A NEW CALCINING TECHNOLOGY FOR MANUFACTURING OF COKE WITH LOWER THERMAL EXPANSION COEFFICIENT M. KAKUTAand H. TANAKA Osaka Research Laboratory, Koa Oil Company 2-1, Takasago Takaishi-shi, Osaka 592, Japan
J. SATO and K. NOGUCHI Koa Oil Company 6-2, Ohte-Machi 2-Chome, Chiyoda-ku, Tokyo 100,Japan (Received 9 February 1981) Abstract-The low thermal expansion coefficient is an important factor in determining the quality of calcined coke for the manufacturing of artificial graphite electrodes. A new calcining technology has been developed to lower the thermal expansion coefficient considerably less than the traditional methods at the calcining stage of green coke. The results obtained from calcining various types of green coke revealed that this technology is effective towards all types of green coke. From the experimental results, it was found that the development of unique microcracks appeared in the coke after calcination, affected by the heating pattern applied by the new calcination method. It was pointed out that these microcracks absorbed and relaxed the expansion of crystallites to result in a reduction of the thermal expansion coefficient of the coke.
1. INTRODUCTION Calcined coke for artificial graphite electrode calls for a
high quality standard, and in this context, coke with a low thermal expansion coefficient is drawing attention as a product befitting such requirement. So far, calcining process was simply considered as a heat treatment process at a temperature ranging from 1300 to 1400°Cto ensure that the calcined coke has the properties appropriate for production of graphite electrodes, and that the properties of calcined coke are strongly dependent on the manufacturing conditions of the green coke&51. Consequently, various manufacturing methods were developed to gain a low thermal expansion coefficient property, all centered on producing a needle coke at the coker. Thus, development of new technologies for calcining process was primarily aimed at improving the efficiency and economy of the operation. Our attention was focussed on this calcining process, and upon investigating the structural changes of green coke, related to the calcining conditions and characteristics of calcined coke [6,7]. Then a new calcining method was developed reducing the thermal expansion coefficient of calcined coke by appropriately controlling the heating pattern during calcination[l,,8,9]. The new calcining process has been named the two-stage calcining process based on the peculiar method adopted. In this study, an outline of this new calcining method and results of observations related to the major causes of a reduction of the thermal expansion coefficient are stated. 2. EXPERIMENTAL 2.1 Materials
As starting materials, five different types of green coke (A, B, C, D and E) were employed for the testing. Green
coke was crushed and particles of 3-4 Tyler mesh sizes were selected. Properties of these green cokes are given in Table 1. From the properties of these testing materials and also by microscopic observation results, Coke A was defined as low sulfur regular grade petroleum coke, and Coke B as petroleum needle coke mainly composed of fibrous texture, Coke C of high sulfur content, Coke D of high ash content and Coke E as coke derived from coal. Cokes C, D and E had both fibrous and mosaic mixed textures. These green coke materials possessing different properties and characteristics were employed as test samples for calcining. 2.2 Calcining technology For the purpose of evaluating the effects of the new calcining method, calcining tests were conducted in two different ways, namely, the new and traditional methods, by utilizing the rotary Kiln type electric furnace. As shown in Fig. 1, the traditional calcining process involved a one stage process in the calciner at a peak temperature of 1300 to 1400°Cand after maintaining this temperature for a while, it is then cooled off to the room temperature. On the other hand, the new calcining process was adopted a two stage processing where green coke was calcined initially at a temperature from 600 to 9WC, then cooled off and then re-calcined at a temperature of about 13W14OO”C which was the same level as that of the traditional method. 2.3 Experimental techniques The volatile matters and ash of the green cokes were measured by Japanese Industrial Standards (JIS) M 8812, and the sulfur was obtained by JIS M 8813. The properties of the calcined coke samples were determined by measuring the thermal expansion coefficient, apparent 347
348
M. KAKUTA et al. Table I. Properties of green cokes
Sample
Volatile
Name
Matter (wt%)
Elemental
Real
Coke B
7.6
10.6
93.8 3.3
89.0 3.2
Analysis c H
(WW) (wt%)
N
(wt%)
1 .l
1.l
s
(WW
0.73
5.7
Density
1.39
(g/cm3)
Ash
(wt%)
Time
Tlmc
(Hr)
1.22
0 11
differential dilatometer using quartz as the standard material. The heating rate were lO”C/min up to designated temperature under argon gas atmosphere. Green carbon bodies were prepared by both molding (rod; 2x2x8cm3) and extrusion (rod; 2cm dia. and 11 cm length) ways from a mixture of appropriately sized calcined coke and coal tar binder pitch. The rods were then baked at a slow rate to 1000°C in an electric furnace and kept at this temperature for 1 hr. The extruded rods were further graphitized at 2800°C for 0.5 hr under an argon flow. The thermal expansion coefficient was measured in the direction perpendicular to the molding direction and parallel to the extruding direction, and thus parallel to the coke particle alignment. Test pieces were obtained by cutting into cylinders of 5 mm dia. and 50 mm length after heating the rods. Bending strength and Young’s modulus of the graphitized rods w.ere determined by JIS R 7202.
(Hr)
New
Method
-l-
1.41
1.35
1.38 0.06
0.05
/Tmditional
Coke C
Method
Fig. 1. Simplified calcination profile.
densityt, real density+ and porosity was calculated from values of the apparent and real densities. Distribution of pore size was determined by the mercury porosimeter. Interlayer spacings and apparent crystalline sizes were obtained by X-ray diffraction method[6]. Texture of coke sample was observed by a polarized light microscope and structure of the sample by a scanning electron microscope. Test pieces were prepared from green cokes A and B by cutting them into specimens of 5 mm square and 50 mm length. Dimensional changes during calcination of these test pieces were obtained by examining the
3. RESULTS AND DISCUSSION
3.1 Effects of the new calcining process Table 2 shows the results on properties of calcined cokes A, B, C and D by the new and traditional methods. Regardless of the green coke type, the thermal expansion coefficient of calcined coke by the new method indicated a lower values than that of traditional calcination method. As for porosity, it is revealed that the calcined cokes by the new method have a higher value, regardless of the coke type, than those processed under the traditional method.
tTest sample of about 30g (3.5-4 Tyler mesh) was measured by pycnometer method by substituting water. SAbout log (200 Tyler mesh under) of test sample was measured by pycnometer method by substituting n-butyl alcohol.
Table 2. Properties of calcined cokes Sample
Name
Coke
A
Coke
6
Coke
C
Coke
D
Method
New
Traditional
New
Traditional
New
Traditional
New
Tradltlonal
CTE’
@66/*c,
1.5
1.9
1.2
1.6
2.1
2.5
2.0
2.4
Real
Density ig/cr&
2.094
2.088
2.095
2.092
2.051
2.040
2.092
2.082
40.3
35.8
37.0
33.1
39.1
36.3
43.6
41.4
Calcining
Porosity ATE
Thermal
(%) expansion
coeffnc~ent
3OC-1OOC
Test
pfetes
Molded
weces
1OOO’CHT
5mmdla
5Omm
length
A new calcining technology for manufacturing
349
of coke with lower thermal expansion coefficient
The properties of extruded graphite rods obtained from calcined cokes A and B by two different calcining methods are shown in Table 3. The thermal expansion coefficient of graphitized pieces gained by the new method also revealed a lower values. This indicates that even after graphitization, the effect of the new calcining method is still maintained. In terms of test results on strengths, no significant difference was observed between the calcined coke experimentally manufactured by the two different methods. That is to say, no ill effect will be given on the mechanical strengths of electrodes when this material is processed to graphite electrodes. Table 4 shows the X-ray parameters obtained from the cokes by two different calcining methods. Interlayer spacing (dOO2)and apparent crystalline size (Lc) observed by X-ray diffractometry indicated no particular difference between those processed under the new method and the traditional method. Furthermore, no specific difference was also found on these samples even after graphitization. From these facts it can be said that adoption of the new calcining method would not lead to adverse effects on the development and rearrangement of coke crystallines. From the study of the new calcining method, the following facts were revealed [9]: (1) During the primary processing, the thermal expansion coefficient of calcined coke changes, and that treatment at the range of 700-900°C is effective. (2) After the completion of the primary processing, it is necessary to cool the coke down to room temperature. No significant effects result from continued calcining process from the primary to secondary processing without this cooling.
And also the thermal expansion is not affected by the change of cooling rate at new calcining process. (3) Thermal expansion coefficient of calcining coke will remain almost unchanged even if the primary processing is repeated. Although the effects of the atmosphere and heating rate during calcination can be recognized on the density of calcined coke, the effects on the thermal expansion coefficient were seen to be minimal[l]. 3.2 Controlling
factors
on
the
thermal
As seen from Fig. 2, comparison of the records on pore size distribution obtained from the mercury porosimeter revealed that those calcined by the new method maintained a larger number of pores between 1 and 60pm, regardless of the coke type, than those processed under the traditional calcining method. Observation results on calcined cokes A and B by the scanning electron microscope showed that microcracks equivalent to the pore sizes of l-60 Fm had conspicuously developed and increased in the cokes treated under the new calcination method (Fig. 3). The same phenomena were observed for calcined cokes C, D and E. It is said that the major factors of the thermal expansion coefficients of calcined coke are the degree of preferred orientation of the crystallites and void structure [lO-121. For example, the thermal expansion coefficient is low for needle coke because it is strongly affected by the preferred orientation of its crystallites. Void structure has the function of absorbing and relaxing expansion of the crystallites. From the test results shown in Tables 2 and 3 and Figs. 2 and 3, it is observed that
Table 3. Properties of extruded graphite rods Sample
-I-
Name
Calclning
Method
CTE*
(XlP
Bulk Density
“Cl
(9 cn;)
Bendlng Strengthckg
Coke A
cm)
I
of X-ray
Sample
Name
Cbining
Method
Calcined
Coke dcux (b, Lc
Graphitiied
&I
1.52
I .53
I .52
I .52
II5
I IO
I I5
105
CAR Vol. 19.No. LB
parameters
Coke
740
3ooc
I .o
0.8
810
Test laeces,Exti”ded
740
Dlece5 2800CHT Smmd\a XSOrnrn lengthl
obtained from the cokes manufactured
A
I
I Coke
B
by two different methods
I Coke
C
Cok’
New
h#timai
New
Tmdilii
New
Wditiil
New
3.444
3.440
3.447
3.446
3.447
3.446
3.442
38
40
38
40
42
41
38
3.366
3.366
3.365
3.364
3.365
3.365
3.366
840
860
1000
1000
460
460
810
Coke
dooz &) Lc
I
Traditional
1.2
.’ CTE Thermalexpans10n coeff~clent130C Table 4. Comparison
New
B
1.0
790
Young s Modulus (kg mm)
Coke
TraditioMl
New ~-
(&I
expansion
coeficient
I
M. KAKUTA at al.
350
specific microcracks developed conspicuously, while the thermal expansion coefficient was reduced considerably in the new calcining process. It is considered that the development of these microcracks under the new calcining method had contributed to effective reduction of the thermal expansion coefficient. Dimensional changes of green coke during calcination were measured between each heat treatment, utilizing
Pore
Diameter
----+--Traditional
#athod
v
Method
New
(pm f
Fig. 2. Pore size distribution of mercury penetrable pore volume in calcined cokes A, B, C, D and E.
Coke B
&ooo 500
a01
oil Pore
1
10
Diammtor
11
(pm)
Coke C
heating patterns of the new and traditional calcining processes. An example of such measurements is given in Figs. 4 and 5 concerning Cokes A and B. While coke material continued to expand until it reached about 6OO”C,rapid contractions started at a temperature of above 600°C. Specifically, under the new calcining method a considerable contraction was observed during the peak temperature period of the primary processing, followed by some contraction during the cooling stage of the primary processing. During the secondary processing, a slight expansion was observed during heating, followed by a contraction up to the final calcining temperature. On the other hand, dimensional changes observed on coke processed under the traditional calcining method indicated contraction from 600°C to the final calcining temperature, showing that the process of dimensiona changes differs between the new and traditional calcining processes. From these facts it is conceived that development and increase of microcracks under the new calcining method was derived from special distribution of stress within the coke caused by the expansion and contraction by this heat treatment patterns. 4. CONCLUSION
hooo B 4 232000 1000
0.01
0.1 Pot*
1 Diamot*f
10 (pm)
10
By adoption of the new calcining technology for the calcining process, it is possible to reduce the thermal expansion coefficient to below those obtained from the ~aditionai calcining methods. Major cause for reduction of the thermal expansion coefficient was the development of unique microcracks of sizes between 1 and 60~~rn within coke processed under the new calcining process. These microcracks were found to have functioned as voids to absorb and relax the expansion of crystallites. Also, such development of microcracks under the new calcining process seems to have been caused by specific distribution of stress within the coke arising from significant expansion and contraction following the heat treatment patterns. The development of these microcracks may have some effect in terms of mechanical strength when these cokes
A new caicining technology for manufacturing
351
of coke with lower thernral expansion coefficient
3
E
352
M. KAKUTA et al. Tempemture (X 10°C) 20 40 60 80 100 120 140 2.0 1;o 0 -1 .o
8
-2.0
k -3.0 \ 7 -4.0 -5.0 -6.0 -7.0 -8.0 -9.0 -10.0
Coke A __*_._ Traditional Method iiz Method
Temperatature(‘C)
0 Coke 0
-1 .o % -2.0 % - -3.0 \ 3 -4.0
Fig. 5. Dimensional changes of coke during cooling process of primary step and heating process of secondary step under new calcining process.
-5.0
REFERENCES
-6.0
1. K. Yoshimura, U.S. Pat. 4, 100,265 (1978). 2. R. Thomas, Hydrocarbon Processing, p. 97. Gulf Publishing Company, Houston, Texas (1975). 3. C. L. Mantel], Carbon and Graphite Handbook. Interscience, New York (1%8). 4. E. Kurami, J. Japan Petrol. Inst. U(5) 366 (1973). 5. U.S. Pat. 2.775549 (1956)and 2922,755 (1960). 6. M. Kakuta, M. Kohriki, T. Tano and Y. Sanada, Tanso No. 95 135(1978). 7. M. Kakuta and H. Tanaka, 3rd International Carbon Conf., Baden-Eaden, Preprints, p. 406 (1980). 8. K. Noguchi, 117 Committee of the Japan Society for Promotion of Science, No. 152(1978). 9. M. Kakuta, 117 Committee of the Japan Society for Promotion of Science, No. 153(1979). 10. S. Mrozowski, Proc. 1st and 2nd Carbon Conf., p. 31. Pergamon Press, Oxford (1956). 11. F. M. Collins, Proc. 1st and 2nd Carbon Conf., p. 177. Pergamon Press, Oxford (1956). 12. J. Okada, Proc. 4th Carbon Conf., pp. 547, 553. Pergamon Press, Oxford (l%O).
-7.0 -8.0 -9.0 -10.0
Coke B __*_. TradItional Method iii Method
Fig. 4. Dimensional changes of coke during calcination. are processed into artificial graphite electrodes. However, no significant difference in strength between
graphitized test pieces under the new and traditional method was found. Acknowledgements-The authors wish to thank Messrs. T. Tano, T. Nakagawa and M. Minohata, Koa oil company Osaka research laboratory, for their experimental assistance.
@X&5223/81/050347WSUZ.CQ/0 @ 1981 Pergamon Press Ltd.
A NEW CALCINING TECHNOLOGY FOR MANUFACTURING OF COKE WITH LOWER THERMAL EXPANSION COEFFICIENT M. KAKUTAand H. TANAKA Osaka Research Laboratory, Koa Oil Company 2-1, Takasago Takaishi-shi, Osaka 592, Japan
J. SATO and K. NOGUCHI Koa Oil Company 6-2, Ohte-Machi 2-Chome, Chiyoda-ku, Tokyo 100,Japan (Received 9 February 1981) Abstract-The low thermal expansion coefficient is an important factor in determining the quality of calcined coke for the manufacturing of artificial graphite electrodes. A new calcining technology has been developed to lower the thermal expansion coefficient considerably less than the traditional methods at the calcining stage of green coke. The results obtained from calcining various types of green coke revealed that this technology is effective towards all types of green coke. From the experimental results, it was found that the development of unique microcracks appeared in the coke after calcination, affected by the heating pattern applied by the new calcination method. It was pointed out that these microcracks absorbed and relaxed the expansion of crystallites to result in a reduction of the thermal expansion coefficient of the coke.
1. INTRODUCTION Calcined coke for artificial graphite electrode calls for a
high quality standard, and in this context, coke with a low thermal expansion coefficient is drawing attention as a product befitting such requirement. So far, calcining process was simply considered as a heat treatment process at a temperature ranging from 1300 to 1400°Cto ensure that the calcined coke has the properties appropriate for production of graphite electrodes, and that the properties of calcined coke are strongly dependent on the manufacturing conditions of the green coke&51. Consequently, various manufacturing methods were developed to gain a low thermal expansion coefficient property, all centered on producing a needle coke at the coker. Thus, development of new technologies for calcining process was primarily aimed at improving the efficiency and economy of the operation. Our attention was focussed on this calcining process, and upon investigating the structural changes of green coke, related to the calcining conditions and characteristics of calcined coke [6,7]. Then a new calcining method was developed reducing the thermal expansion coefficient of calcined coke by appropriately controlling the heating pattern during calcination[l,,8,9]. The new calcining process has been named the two-stage calcining process based on the peculiar method adopted. In this study, an outline of this new calcining method and results of observations related to the major causes of a reduction of the thermal expansion coefficient are stated. 2. EXPERIMENTAL 2.1 Materials
As starting materials, five different types of green coke (A, B, C, D and E) were employed for the testing. Green
coke was crushed and particles of 3-4 Tyler mesh sizes were selected. Properties of these green cokes are given in Table 1. From the properties of these testing materials and also by microscopic observation results, Coke A was defined as low sulfur regular grade petroleum coke, and Coke B as petroleum needle coke mainly composed of fibrous texture, Coke C of high sulfur content, Coke D of high ash content and Coke E as coke derived from coal. Cokes C, D and E had both fibrous and mosaic mixed textures. These green coke materials possessing different properties and characteristics were employed as test samples for calcining. 2.2 Calcining technology For the purpose of evaluating the effects of the new calcining method, calcining tests were conducted in two different ways, namely, the new and traditional methods, by utilizing the rotary Kiln type electric furnace. As shown in Fig. 1, the traditional calcining process involved a one stage process in the calciner at a peak temperature of 1300 to 1400°Cand after maintaining this temperature for a while, it is then cooled off to the room temperature. On the other hand, the new calcining process was adopted a two stage processing where green coke was calcined initially at a temperature from 600 to 9WC, then cooled off and then re-calcined at a temperature of about 13W14OO”C which was the same level as that of the traditional method. 2.3 Experimental techniques The volatile matters and ash of the green cokes were measured by Japanese Industrial Standards (JIS) M 8812, and the sulfur was obtained by JIS M 8813. The properties of the calcined coke samples were determined by measuring the thermal expansion coefficient, apparent 347
348
M. KAKUTA et al. Table I. Properties of green cokes
Sample
Volatile
Name
Matter (wt%)
Elemental
Real
Coke B
7.6
10.6
93.8 3.3
89.0 3.2
Analysis c H
(WW) (wt%)
N
(wt%)
1 .l
1.l
s
(WW
0.73
5.7
Density
1.39
(g/cm3)
Ash
(wt%)
Time
Tlmc
(Hr)
1.22
0 11
differential dilatometer using quartz as the standard material. The heating rate were lO”C/min up to designated temperature under argon gas atmosphere. Green carbon bodies were prepared by both molding (rod; 2x2x8cm3) and extrusion (rod; 2cm dia. and 11 cm length) ways from a mixture of appropriately sized calcined coke and coal tar binder pitch. The rods were then baked at a slow rate to 1000°C in an electric furnace and kept at this temperature for 1 hr. The extruded rods were further graphitized at 2800°C for 0.5 hr under an argon flow. The thermal expansion coefficient was measured in the direction perpendicular to the molding direction and parallel to the extruding direction, and thus parallel to the coke particle alignment. Test pieces were obtained by cutting into cylinders of 5 mm dia. and 50 mm length after heating the rods. Bending strength and Young’s modulus of the graphitized rods w.ere determined by JIS R 7202.
(Hr)
New
Method
-l-
1.41
1.35
1.38 0.06
0.05
/Tmditional
Coke C
Method
Fig. 1. Simplified calcination profile.
densityt, real density+ and porosity was calculated from values of the apparent and real densities. Distribution of pore size was determined by the mercury porosimeter. Interlayer spacings and apparent crystalline sizes were obtained by X-ray diffraction method[6]. Texture of coke sample was observed by a polarized light microscope and structure of the sample by a scanning electron microscope. Test pieces were prepared from green cokes A and B by cutting them into specimens of 5 mm square and 50 mm length. Dimensional changes during calcination of these test pieces were obtained by examining the
3. RESULTS AND DISCUSSION
3.1 Effects of the new calcining process Table 2 shows the results on properties of calcined cokes A, B, C and D by the new and traditional methods. Regardless of the green coke type, the thermal expansion coefficient of calcined coke by the new method indicated a lower values than that of traditional calcination method. As for porosity, it is revealed that the calcined cokes by the new method have a higher value, regardless of the coke type, than those processed under the traditional method.
tTest sample of about 30g (3.5-4 Tyler mesh) was measured by pycnometer method by substituting water. SAbout log (200 Tyler mesh under) of test sample was measured by pycnometer method by substituting n-butyl alcohol.
Table 2. Properties of calcined cokes Sample
Name
Coke
A
Coke
6
Coke
C
Coke
D
Method
New
Traditional
New
Traditional
New
Traditional
New
Tradltlonal
CTE’
@66/*c,
1.5
1.9
1.2
1.6
2.1
2.5
2.0
2.4
Real
Density ig/cr&
2.094
2.088
2.095
2.092
2.051
2.040
2.092
2.082
40.3
35.8
37.0
33.1
39.1
36.3
43.6
41.4
Calcining
Porosity ATE
Thermal
(%) expansion
coeffnc~ent
3OC-1OOC
Test
pfetes
Molded
weces
1OOO’CHT
5mmdla
5Omm
length
A new calcining technology for manufacturing
349
of coke with lower thermal expansion coefficient
The properties of extruded graphite rods obtained from calcined cokes A and B by two different calcining methods are shown in Table 3. The thermal expansion coefficient of graphitized pieces gained by the new method also revealed a lower values. This indicates that even after graphitization, the effect of the new calcining method is still maintained. In terms of test results on strengths, no significant difference was observed between the calcined coke experimentally manufactured by the two different methods. That is to say, no ill effect will be given on the mechanical strengths of electrodes when this material is processed to graphite electrodes. Table 4 shows the X-ray parameters obtained from the cokes by two different calcining methods. Interlayer spacing (dOO2)and apparent crystalline size (Lc) observed by X-ray diffractometry indicated no particular difference between those processed under the new method and the traditional method. Furthermore, no specific difference was also found on these samples even after graphitization. From these facts it can be said that adoption of the new calcining method would not lead to adverse effects on the development and rearrangement of coke crystallines. From the study of the new calcining method, the following facts were revealed [9]: (1) During the primary processing, the thermal expansion coefficient of calcined coke changes, and that treatment at the range of 700-900°C is effective. (2) After the completion of the primary processing, it is necessary to cool the coke down to room temperature. No significant effects result from continued calcining process from the primary to secondary processing without this cooling.
And also the thermal expansion is not affected by the change of cooling rate at new calcining process. (3) Thermal expansion coefficient of calcining coke will remain almost unchanged even if the primary processing is repeated. Although the effects of the atmosphere and heating rate during calcination can be recognized on the density of calcined coke, the effects on the thermal expansion coefficient were seen to be minimal[l]. 3.2 Controlling
factors
on
the
thermal
As seen from Fig. 2, comparison of the records on pore size distribution obtained from the mercury porosimeter revealed that those calcined by the new method maintained a larger number of pores between 1 and 60pm, regardless of the coke type, than those processed under the traditional calcining method. Observation results on calcined cokes A and B by the scanning electron microscope showed that microcracks equivalent to the pore sizes of l-60 Fm had conspicuously developed and increased in the cokes treated under the new calcination method (Fig. 3). The same phenomena were observed for calcined cokes C, D and E. It is said that the major factors of the thermal expansion coefficients of calcined coke are the degree of preferred orientation of the crystallites and void structure [lO-121. For example, the thermal expansion coefficient is low for needle coke because it is strongly affected by the preferred orientation of its crystallites. Void structure has the function of absorbing and relaxing expansion of the crystallites. From the test results shown in Tables 2 and 3 and Figs. 2 and 3, it is observed that
Table 3. Properties of extruded graphite rods Sample
-I-
Name
Calclning
Method
CTE*
(XlP
Bulk Density
“Cl
(9 cn;)
Bendlng Strengthckg
Coke A
cm)
I
of X-ray
Sample
Name
Cbining
Method
Calcined
Coke dcux (b, Lc
Graphitiied
&I
1.52
I .53
I .52
I .52
II5
I IO
I I5
105
CAR Vol. 19.No. LB
parameters
Coke
740
3ooc
I .o
0.8
810
Test laeces,Exti”ded
740
Dlece5 2800CHT Smmd\a XSOrnrn lengthl
obtained from the cokes manufactured
A
I
I Coke
B
by two different methods
I Coke
C
Cok’
New
h#timai
New
Tmdilii
New
Wditiil
New
3.444
3.440
3.447
3.446
3.447
3.446
3.442
38
40
38
40
42
41
38
3.366
3.366
3.365
3.364
3.365
3.365
3.366
840
860
1000
1000
460
460
810
Coke
dooz &) Lc
I
Traditional
1.2
.’ CTE Thermalexpans10n coeff~clent130C Table 4. Comparison
New
B
1.0
790
Young s Modulus (kg mm)
Coke
TraditioMl
New ~-
(&I
expansion
coeficient
I
M. KAKUTA at al.
350
specific microcracks developed conspicuously, while the thermal expansion coefficient was reduced considerably in the new calcining process. It is considered that the development of these microcracks under the new calcining method had contributed to effective reduction of the thermal expansion coefficient. Dimensional changes of green coke during calcination were measured between each heat treatment, utilizing
Pore
Diameter
----+--Traditional
#athod
v
Method
New
(pm f
Fig. 2. Pore size distribution of mercury penetrable pore volume in calcined cokes A, B, C, D and E.
Coke B
&ooo 500
a01
oil Pore
1
10
Diammtor
11
(pm)
Coke C
heating patterns of the new and traditional calcining processes. An example of such measurements is given in Figs. 4 and 5 concerning Cokes A and B. While coke material continued to expand until it reached about 6OO”C,rapid contractions started at a temperature of above 600°C. Specifically, under the new calcining method a considerable contraction was observed during the peak temperature period of the primary processing, followed by some contraction during the cooling stage of the primary processing. During the secondary processing, a slight expansion was observed during heating, followed by a contraction up to the final calcining temperature. On the other hand, dimensional changes observed on coke processed under the traditional calcining method indicated contraction from 600°C to the final calcining temperature, showing that the process of dimensiona changes differs between the new and traditional calcining processes. From these facts it is conceived that development and increase of microcracks under the new calcining method was derived from special distribution of stress within the coke caused by the expansion and contraction by this heat treatment patterns. 4. CONCLUSION
hooo B 4 232000 1000
0.01
0.1 Pot*
1 Diamot*f
10 (pm)
10
By adoption of the new calcining technology for the calcining process, it is possible to reduce the thermal expansion coefficient to below those obtained from the ~aditionai calcining methods. Major cause for reduction of the thermal expansion coefficient was the development of unique microcracks of sizes between 1 and 60~~rn within coke processed under the new calcining process. These microcracks were found to have functioned as voids to absorb and relax the expansion of crystallites. Also, such development of microcracks under the new calcining process seems to have been caused by specific distribution of stress within the coke arising from significant expansion and contraction following the heat treatment patterns. The development of these microcracks may have some effect in terms of mechanical strength when these cokes
A new caicining technology for manufacturing
351
of coke with lower thernral expansion coefficient
3
E
352
M. KAKUTA et al. Tempemture (X 10°C) 20 40 60 80 100 120 140 2.0 1;o 0 -1 .o
8
-2.0
k -3.0 \ 7 -4.0 -5.0 -6.0 -7.0 -8.0 -9.0 -10.0
Coke A __*_._ Traditional Method iiz Method
Temperatature(‘C)
0 Coke 0
-1 .o % -2.0 % - -3.0 \ 3 -4.0
Fig. 5. Dimensional changes of coke during cooling process of primary step and heating process of secondary step under new calcining process.
-5.0
REFERENCES
-6.0
1. K. Yoshimura, U.S. Pat. 4, 100,265 (1978). 2. R. Thomas, Hydrocarbon Processing, p. 97. Gulf Publishing Company, Houston, Texas (1975). 3. C. L. Mantel], Carbon and Graphite Handbook. Interscience, New York (1%8). 4. E. Kurami, J. Japan Petrol. Inst. U(5) 366 (1973). 5. U.S. Pat. 2.775549 (1956)and 2922,755 (1960). 6. M. Kakuta, M. Kohriki, T. Tano and Y. Sanada, Tanso No. 95 135(1978). 7. M. Kakuta and H. Tanaka, 3rd International Carbon Conf., Baden-Eaden, Preprints, p. 406 (1980). 8. K. Noguchi, 117 Committee of the Japan Society for Promotion of Science, No. 152(1978). 9. M. Kakuta, 117 Committee of the Japan Society for Promotion of Science, No. 153(1979). 10. S. Mrozowski, Proc. 1st and 2nd Carbon Conf., p. 31. Pergamon Press, Oxford (1956). 11. F. M. Collins, Proc. 1st and 2nd Carbon Conf., p. 177. Pergamon Press, Oxford (1956). 12. J. Okada, Proc. 4th Carbon Conf., pp. 547, 553. Pergamon Press, Oxford (l%O).
-7.0 -8.0 -9.0 -10.0
Coke B __*_. TradItional Method iii Method
Fig. 4. Dimensional changes of coke during calcination. are processed into artificial graphite electrodes. However, no significant difference in strength between
graphitized test pieces under the new and traditional method was found. Acknowledgements-The authors wish to thank Messrs. T. Tano, T. Nakagawa and M. Minohata, Koa oil company Osaka research laboratory, for their experimental assistance.