Calcination and graphitization behaviour of cokes from Ql-free coal-derived pitches

Calcination cokes from Jacek

Machnikowski

and graphitization QI-free coal-derived and Lucjan

behaviour pitches

of

Wajzer

Institute of Chemistry and Technology

and Coal, Technical

Wroclaw, (Received

1993)

of Petroleum Gda/iska 7/9, 50-344 Wroclaw, Poland 9 November 1992; revised 15 September

University

of

Coal tars and coal liquefaction products are possible feedstocks for anisotropic carbon material production. Semi-coke samples were produced from coal tar pitch, CTP 101, and coal hydrogenation pitch, CHP 88, prepared in a laboratory scale from QI-free parent liquids. Changes in the structure and properties of the semi-cokes, during two stage calcination and graphitization treatment up to 27OO”C, were evaluated. Both pitches produced highly anisotropic semi-cokes, slightly differing in the distribution of microconstituents revealed by optical texture. The calcined coke from coal tar pitch CTP 101 was distinguished by higher real density and porosity, lower heteroatom (N +0) content and microstrength, and slightly higher graphitizability. The increase of heating rate in the temperature range 500-850°C from 5 to 100 K min- ’ induced negligible alteration in the properties of calcined cokes. The more intensive evolution of nitrogen on heating between 1300 and 1700°C of the CHP 88 coke was associated with larger development of porosity in the coke grains, corresponding to greater puffing. The lower extent of puffing favours the QI-free coal tar rather than the coal hydrogenation product as a feedstock for electrode coke production. (Keywords: coal; reactivity; structure)

The suitability of coke as a precursor of polycrystalline graphite material is determined primarily during the carbonization step. Fluid phase pyrolysis of selected petroleum or coal-derived parent materials enables the creation, via mesophase mechanisms, of highly oriented needlelike texture of the resultant coke’,‘. The optical texture established on the resohdification of the plastic mesophase is practically unaltered following heat treatment, except for fissuring due to shrinkage stresses3. The optical texture of green coke determines to a great extent the properties of the calcined and graphitized products. An additional effect frequently associated with the graphitization of needle cokes is an irreversible thermal expansion (puffing) of coke grains resulting in a decrease of bulk density, strength and conductivity of graphite artifacts. Generally, the puffing is generated by the evolution of residual heteroatoms on treatment within the temperature range 1600-2200°C when the coke matter softens due to crystallite rearrangement4. Relatively well recognized puffing of petroleum cokes is induced by sulfur release, and can be effectively inhibited by the addition of iron oxide5-‘. Coal tar pitch-based needle cokes show anomalous puffing behaviour. In that case, the extent of puffing is correlated rather with the nitrogen content, which is typically two to four times higher than in petroleum cokes 9 l1 . Denitrogenation of coal tar pitch effectively reduces the puffing of resultant cokes12. Differences in the porous structure and crystallinity’3,‘4 as well as in refractoriness of nitrogen’, are also believed to contribute to differences in puffing behaviour of coal tar pitch coke and petroleum coke. Recently, the products of direct hydrogenation of coal have been reported as precursors of highly anisotropic

0016-2361:94/0610957C15 3.1 1994 Butterworth-Heinemann

Ltd

coke15-i7. The objective of this study is, therefore, to elucidate the calcination and graphitization behaviour of the coal hydrogenation pitch coke in comparison to QI-free coal tar pitch coke, with special attention focused on puffing characteristics.

EXPERIMENTAL Materials Materials used were pitches prepared on a laboratory scale from QI-free coal tar (CTP 101) and QI-free product of coal hydrogenation (CHP 88). The details concerning pitch preparation and characteristics have been given elsewhere17~‘8. Table I summarizes the basic analytical data on the pitches. Procedure The semi-coke samples were prepared from the parent pitches by soaking for 3 h at 480°C (CTP 101) or 450°C (CHP 88) with a following temperature rise to 500°C and 1 h soaking. The heating rate was 5 K min- ‘. The carbonization procedure has been described in a previous work17. A two stage calcining process was used in the study as a method recommended to produce coke of reduced thermal expansion coefficient’9-21. Semi-coke samples, ground below 3.15 mm, were calcined initially for 1 h at 85O”C, cooled off and re-calcined at 1300°C with 2 h soak. The first stage treatment was performed at two different heating rates, 5 and 100 K min-‘, using a vertical electrical furnace and a stainless steel retort of 26 mm diameter. The conditions (SSOC, 1 h soak) ensured that

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Calcination Table 1

and graphitization

Characteristics

of cokes: J. Machniko

of pitches

Determination

CTPlOl

CHP88

Softening point, KS (“C) Ash content (wt%) Quinoline insolubles (wt%) Coking value (wt%) Solvent analysis (wt%) Oils (hexane solubles) Asphaltenes (HI/TS) Preasphaltenes (TI/PS) Pyridine insolubles (PI) Elemental composition (wt%) Cda’ Hd”’ Nd‘?’ S&J

101

88

Od?f d,ff

(C/W,, (N+S+ Hydrogen

wa,

0.11 0.1 39.0

0.12 0.1 36.3

21.0 55.0 21.1 2.9

32.5 44.3 22.2 1.0

92.9 4.1 0.6 0.4 1.4 1.65 0.018

91.1 5.1 1.3 0.2 1.7 1.33 0.027

81.9 16.1 0.8 0.8 0.4

59.5 23.0 7.9 6.2 3.4

atom distribution

H,, HZ H, H, H;

devolatilization and shrinkage could be almost completed during this stage3. The final calcination was carried out in a graphite tube resistance furnace” with a heating rate of 20 K min-‘. The calcined cokes were ground to prepare size fractions of do.2 and 0.5-1.5 mm, and were graphitized at 1700,220O and 2700°C with a heating rate of 20Kmin-’ and 2 h soak. All heat treatments were performed in an argon atmosphere. Analyses The elemental composition of cokes was determined using standard methods of chemical analysis. Real density was measured using cokes ground below 0.2 mm by pycnometric displacement of ethanol. The apparent density of cokes was measured for the size fraction 0.5-1.5 mm by displacing mercury under vacuum. The porosity was calculated from the real and apparent density data. The increase of porosity on heat treatment of the calcined cokes was used for the assessment of puffing extent. The evaluation of coke microstrength was performed according to the method described by Ragan and Marsh3. Test duration was standardized at 150 rev min- ‘. The optical texture of cokes was examined under a Neophot 2 polarized reflected light microscope. The anisotropic content of the semi-cokes was determined quantitatively from the polished surface using a point counting technique. Detailed procedures have been described in a previous paper”. X-ray measurements of the graphitization products were carried out using a URD 5 diffractometer. Structural parameters, interlayer spacing d, Oz and apparent crystallite sizes, L, and L,, were determined according to the procedure described previouslyz3. In addition the extent of the three-dimensional ordering, L I 12r was calculated from the half-height 1 12 line width3. RESULTS

AND

DISCUSSION

Characteristics of semi-cokes The analytical data of semi-cokes

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coal

tar

wski and 1. Wajzer pitch CTP 101 and coal hydrogenation pitch CHP 88 are given in Table 2. More aromatic CTP 101 gives a considerably higher coke yield. The elementary composition of the semi-cokes reflects the differences occurring in respective pitch constitution. In particular, higher oxygen and nitrogen content in CHP 88 semi-coke is meaningful. Oxygen retained in the semi-coke matter acts as a cross-linking agent. Good correlation between oxygen content at this stage of treatment and coke graphitizability is reportedz4. The significance of the nitrogen content is related mostly to its role in puffing generation’-Il. Distinctly higher microstrength of CHP 88 semi-coke, in spite of its increased porosity, seems to have a structural reason, and corresponds to a harder, more cross-linked structure. This suggestion is supported by characteristics of the parent pitches (Table 1). Cross-linkage creation in the carbonization product is related to the number of heteroatoms and alkyl side chains in the parent pitch’*2s, here larger in CHP 88. Both the pitches produce cokes with similar highly anisotropic optical texture. Detailed microscopic analysis (Table 3) indicates that the CHP 88 semi-coke texture is distinguishable by a higher proportion of anisometric constituents, short fibrous and fine fibrous and lower content of large isometric units. This can be explained by some restrictions in the growth of mesophase units coincident with a high propensity to deformation of the bulk mesophase; the latter being related to the extent of volatiles evolved just before resolidification26. Figures 1 and 2 are optical micrographs of calcined cokes from CTP 101 and CHP 88. Table 2

Characteristics

of semi-cokes

Determination

CTP 101

CHP 88

Carbonization yield (wt%) Moisture (wt%) Ash (wt%) Volatile matter (wt%) El;xytal composition (wt%)

61.5 0.40 0.26 5.85

51.5 0.79 0.21 7.05

94.8 3.15 0.48 0.23 1.3 1.434 1.369 4.5 2.8

93.8 3.27 0.75 0.16 2.0 1.445 1.353 6.4 12.1

Hd”’ N&7’ Sda’ @?f dlff Real density (g cm 3) Apparent density (g cmm3) Porosity (X) Microstrength, R, (wt%) y Determined microstrength

Table 3

as weight per cent of coke with grain size unaltered test, i.e. > 600 pm

Microscopic

Microconstituent

analysis

of semi-cokes

during

(~01%)

CTP 101

CHP

0.7

0.3

Isometric, including: Mosaics Large mosaics Small domains Domains

36.7 4.6 14.3 12.6 5.2

34.6 6.3 17.5 8.5 2.3

Anisometric, including: Flow domains Short fibrous Coarse fibrous Fine fibrous

62.6 13.9 5.8 39.2 3.7

65.1 10.5 7.8 36.0 10.8

Disordered

88

Calcination Effect of heating

rate on the properties

of calcined

and graphitization

cokes

Table 4 gives some characteristics of calcined cokes from CTP 101 and CHP 88, produced at heating rates ’ during the first stage of calcination. of5and lOOKmin_ During this stage, major removal of volatiles (77%) accompanied by partial crushing of coke grains occurs. The heating rate appears to have a slight effect on the weight loss and the extent of the crushing, with the exception of a surprisingly high loss of the large grains during slow heating of CHP 88 semi-coke. For both the cokes, fast heating results in a distinct increase of microstrength and a limited growth of real density of the calcined cokes.

of cokes: J. Machnikowski

Variation in semi-coke properties graphitization

and L. Wajzer

during

calcination

and

Figure 3 presents the changes in elemental composition of semi-cokes from CTP 101 and CHP 88 upon heat treatment up to 1700°C. As expected, the hydrogen content decreases gradually with temperature to a residual amount in the 1700°C cokes. No heteroatomic evolution is observed on treatment to 850°C. Oxygen is released to a great extent during the second stage of calcination, in contrast to sulfur which is completely refractory to the treatment at 1300°C. The nitrogen content decreases gradually on heating above 850°C to about 0.2% at 1700°C. It seems significant that CTP 101 coke loses a majority of its nitrogen (63%) during the second stage of calcination. The data are, in general, consistent with earlier reports concerning heteroatomic refractoriness in petroleum and coal tar pitch cokes, and confirm that the profile of heteroatom evolution is a specific characteristic of a given coke4,‘0V27. The variation in density and porosity on calcination and graphitization treatment of semi-cokes from CTP 101 and CHP 88 are presented in Figure 4. The two stage

3.5 \ 3.0 ii ‘,

Figure I

Optical

texture

of the calcined

coke from CTP

101

500

700

900

1100

1300

1500

1700

HTT (“C)

Figure 2

Table 4

Optical

texture

Effect of heating

Sample

HTT (“C)

of the calcined

coke from CHP

rate on the properties

Heating (K min

rate ‘)

88

of calcined

Weight (wt%)

Figure 3 Variation in elemental composition of semi-cokes from CTP 101 and CHP 88 with heat treatment temperature (HTT): 0, 0, H; 0, n , N; A, A, S; V, v, Odin. Open points and solid lines refer to CTP 101; filled points and dashed lines refer to CHP 88

loss

cokes Thermal resistance index” (%)

Microstrength, pwl%)

Real density (g cm-s)

(gem-7

Apparent

density

Porosity (%)

CTP 101

850

5

5.1

CTP 101

850

100

5.6

7.8

CHP 88

850

5

1.2

13.1

CHP 88

850

100

6.8

7.1

CTP 101

1300

5120

1.6

3.3

65.0

2.175

1.861

14.4

CTP 101

1300

100/20

1.6

3.7

72.5

2.186

1.856

15.1

CHP

88

1300

2.2

3.6

67.5

2.129

1.867

12.3

CHP

88

1300

2.0

1.9

17.5

2.135

1.867

12.5

> 3 mm following

heat treatment

y Determined

5120 loo/20

as weight loss of size fractions

7.8

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Calcination

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20

18

16

and L. Wajzer

enhanced expansion of grains as a result of porosity development. The variation of microstrength of semi-cokes from CTP 101 and CHP 88 with heat treatment temperature (Figure 6) seems to result from chemical and structural transformations typical of graphitizing cokes. The increase of the R, index to a maximum value in the calcined coke occurs as an effect of devolatilization, resulting in the densification and creation of numerous linkages between structural units. A minimum microstrength at 1700°C corresponds to softening of the carbon material at the beginning of graphitization. A final

100

10

a 80 6

4 500

900

1300

1700

2100

2500

Figure 4 Variation in density and porosity of semi-cokes from CTP 101 and CHP 88 with heat treatment temperature (HTT): 0, 0, real density; 0, n , apparent density; A, A, porosity. Open points and solid lines refer to CTP 101; filled points and dashed lines refer to CHP 88

calcination produces cokes of high real density, especially when CTP 101 is used as a starting material. In this case, only a very slight further increase of the density occurs on graphitization heating to 2700°C. Needle puffing cokes, based on petroleum” and coal tar13, show prepuffing, or a decrease in real density on treatment above 12OO”C, preceding a decrease in apparent density associated with coke grain expansion. This prepuffing, which is attributed to the development of closed porosity 28, is not observed here. The minimum apparent density occurs in both cokes studied when heated at 1700°C but is distinctly greater in that from coal hydrogenation. It is well established that gas evolution, inducing puffing, results in the development of porosity, mostly of diameter in the 0.1-l pm range5s2’. Figure 4 demonstrates that, in semi-cokes from CTP 101 and CHP 88, an increase of porosity is associated with the temperature increase up to 1700°C. Relevant to the puffing behaviour is the development of porosity on calcined coke which is distinctly greater for CHP 88 coke (8.7%) than for CTP 101 coke (4.5%) and similar to that observed in petroleum regular puffing coke28. In addition to the total nitrogen content in semi-coke, the ease of its evolution seems to be a factor influencing the puffing behaviour of the coal-derived cokes. Figure 5 demonstrates that CTP 101 loses, on calcination at 1300°C more nitrogen (60%) than CHP 88 (40%), and produces at that stage of treatment coke with better developed porosity (Figure 4). It is assumed that both the early nitrogen removal and porosity development result in diminishing the internal pressure of evolving gases when graphitization begins and the structure softens (about 1700°C). In contrast, in the case of the CHP 88 coke, the coincidence of intensive nitrogen and sulfur evolution in this most sensitive region can lead to the

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* T)

60

5 z

HTT(OC)

960

3

6

m .s

40

.[rr &

20

0

t

I

900

I

I

I

I

1100

1300

1500

1700

HTT

("Cl

Figure 5 Kinetics of nitrogen and sulfur evolution on heat treatment of semi-coke from CTP 101 and CHP 88: 0, W, N; A, A, S. Open points and solid line refer to CTP 101; filled points and dashed line refer to CHP 88

80 1 70

60

10

0’ 500

1 900

2 1300

p

HTT

’ 1700

j 2100

t 2500

('Cl

Figure 6 Variation in microstrength of semi-cokes from CTP 101 and CHP 88 with heat treatment temperature (HTT): 0, CTP 101 coke; 0, CHP 88 coke

Calcination Table 5 Relevant properties of the green, cokes from coal-derived pitches

calcined

and graphitization

and graphitized

88

CTP

Green coke (SOOC) Heteroatom content (wt%) Nitrogen content (wt%) Mosaic microconstituents (vol.%) Anisometric microconstituents (vol.%)

2.0 0.48 18.9 62.6

2.9 0.75 23.8 65.1

Calcined coke (13OO‘C) Nitrogen content (wt%) Real density (g cmm3) Porosity (%) Puffing extent” (%)

0.22 2.186 15.1 4.5

0.52 2.135 12.6 8.7

2.193

2.187

0.3362 44 70 9.0

0.3363 38 65 8.3

Graphitized coke (27OO“C) Real density (g cmm3) Structural ordering: d 002 (nm) L, (nm) L, (nm) L I , z (nm) “Determined temperature

as an increase of porosity rise from 1300 to 1700-C

with

the

101

CHP

Description

heat

treatment

This work was performed as part of the Central Research Programme 01.16 sponsored by the Polish Academy of Sciences. REFERENCES 1

2 3 4

5 6 7

9

10

11 12 13

14

the characteristics of the green, calcined and graphitized cokes from the coal tar pitch and coal hydrogenation pitch, which are most relevant in evaluation of the materials as filler for polycrystalline graphite artifact manufacture. A lower extent of puffing, assessed from the increase in porosity following calcined coke treatment, and slightly higher graphitizability distinguishes the coke from the QI-free coal tar pitch, CTP 101, as a preferable material for application. Enhanced puffing of the coke from the coal hydrogenation pitch, CHP 88, can be correlated with the higher content and retarded release of nitrogen from the coke structure, resulting in accumulation of sulfur and nitrogen evolution within the puffing temperature range. Slightly lower graphitizability of coke corresponds with a greater proportion of less ordered mosaic constituents in the optical texture of semi-coke from CHP 88, originating in the parent pitch constitution, i.e. increased heteroatom content and decreased aromaticity. One of the primary requirements for a filler coke is a low coefficient of thermal expansion (CTE), which is related to the thermal shock resistance of the resultant graphite electrodes. It is well established29.30 that the CTE value of coke is closely related to its optical texture, including the size and orientation of anisotropic units.

and L. Wajzer

ACKNOWLEDGEMENTS

Evaluation of suitability of semi-cokesfrom CTP IO1 and CHP 88 for polycrystalline graphite manufacture Table 5 summarizes

J. Machnikowski

The comparable anisotropic content of semi-cokes from CTP 101 and CHP 88 indicates that similar values should be obtained from CTE measurements.

8

decrease in microstrength is associated with the threedimensional ordering development as cross-linkages are gradually removed. Both cokes show very similar profiles of microstrength variation; however, at all the stages of treatment the microstrength of hydrogenized coal coke is higher than that of corresponding products from coal tar pitch, in spite of greater porosity in most cases. Apparently, the structural characteristics of carbon material and possibly the pore size distribution, but not total pore volume, have a decisive effect on the coke microstrength.

of cokes:

15 16 17 I8 19 20

21

22 23 24 25

26 27 28

29 30

Marsh, H. and Walker, P. L. Jr in ‘Chemistry and Physics of Carbon’, (Eds P. L. Walker and P. A. Thrower), Vol 15, Marcel Dekker, New York, 1979, p. 228 Zimmer, J. E. and White, J. L. Adv. Liquid Crystals 1982,5,157 Ragan, S. and Marsh, H. J. Mater. Sci. 1983, 18, 3695 Heintz, E. A. in ‘Proceedings of the Fifth London International Carbon and Graphite Conference’, Vol. 2, Society of Chemical Industry, London, 1978, p. 575 Whittaker, M. P. and Grindstaff, L. J. Carbon 1969, 7. 615 Letizia. L. High Temp.-High Pressure 1977, 9, 291 and 297 Fitzer, E., Janoschek, K.-H. and Kochling, K.-H. in ‘Proceedings of the Fifth London International Carbon and Graphite Conference’, Vol. 2, Society of Chemical Industry, London, 1978, p. 621 Fujimoto, K., Mochida, I., Todo, Y., Oyama, T.. Yamashita, R. and Marsh, H. Carbon 1989, 27, 909 Morris, E. G., Tucker, K. W. and Joo, L. A. in ‘Extended Abstracts of the 16th Biennial Conference on Carbon, San Diego’, American Carbon Society, San Diego, 1983, p. 595 Fitzer, E., Kompalik, D. and Wormer, 0. in ‘Proceedings, Carbon ‘86, Fourth International Carbon Conference’, DKG, Baden-Baden, 1986, p. 116 Fujimoto, K., Sato, M., Yamada, M., Yamashita, R. and Shibata, K. Carbon 1986, 24, 397 Mochida, I., Fei, Y. Q., Sakanishi, K., Korai, Y., Usuba, H. and Miura, K. Carbon 1992, 30, 241 Wagner, M. H. and Wilhelmi, G. in ‘Proceedings, Carbon ‘86, Fourth International Carbon Conference’, DKG, Baden-Baden, 1986, p. I1 3 Letizia, I. and Wagner, M. H. in ‘Extended Abstracts of the 16th Biennial Carbon Conference, San Diego’, American Chemical Society, San Diego, 1983, p. 593 Pilch-Kowalczyk, A., Paidziorek, T. and Pawlowski, M. Fuel 1989, 68, 631 Rusin, E. Fuel 1989, 68, 929 Machnikowski, J., Petryniak, J., Rusin, E. and Pietrzok, B. Carbon 1991, 29, 371 Machnikowski, J. Koks Smoh Gaz 1990, 35, 215 Kakuta, M., Tanaka, H., Sato, J. and Noguchi, K. Carbon 1981, 19, 347 Kakuta, M., Yamasaki, H., Tanaka, H., Sato, J. and Noguchi, K. in ‘Petroleum Derived Carbons’, ACS Symposium Series 303, American Chemical Society, Washington DC, 1986, p. 179 Lindhout, I., Downing, R. S. and Geiger, F. J. A. in ‘Extended Abstracts of the International Carbon Conference’, (Eds B. McEnaney and T. J. Mays), Institute of Physics, Newcastleupon-Tyne, 1988, p. 380 Jasienko, S., Kidawa, H., Sobocinski, T. and Dusza, M. Chemia Stosowanu 1974, 18, 533 Jasienko, S. and Machnikowski, J. Carbon 1981, 19, 205 Oberlin, A. Carbon 1984, 22, 521 Fitzer, E., Mueller, K. and Schaefer, W. in ‘Chemistry and Physics of Carbon’. (Ed. P. L. Walker. Jr). Vol. 7. Marcel Dekker. New York, 1971, p. 237 Mochida, I., Oyama, T. and Korai, Y. Carbon 1988, 26, 49 Millet, J.. Millet, J. and Vivares, A. J. Chim. Phys. 1963,60,533 Machnikowski, J., Wajzer, L. and Jasienko, S. in ‘Extended Abstracts of the 16th Biennial Carbon Conference, San Diego’, American Carbon Society, San Diego, 1983, n. 597 Mochida, I., Korai, Y., Fujitsu, H., byama, T. and Nesumi, Y. Carbon 1987, 25, 259 Mochida, I., Korai, Y. and Oyama, T. Carbon 1987, 25, 273

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