- Email: [email protected]
Carbonization of coal-tar pitches: effect of rank of parent coal
Short communications
Carbonization
of coal-tar
pitches:
John W. Patrick, Malcolm J. Reynolds
effect
of rank of parent
coal
and Alan Walker
The British Carbonization Research Association, Chesterfield, Derbyshire, S42 WS, UK (Received 15 June 1982; revised 4 October 1982)
The object of the present work was to investigate whether the rank of coal from which the coal-tar pitches were obtained had any bearing on their carbonization behaviour. Using six laboratory produced pitches it is shown that the behaviour during the mesophase temperature range, and consequently the size of the anisotropic flow structures in the pitch coke, vary with the rank of the parent coal. (Keywords: coal; carbonization;
pitch)
Optically anisotropic carbons are formed from a wide range of carbonaceous materials which fuse during carbonization, the essential feature being the formation of spherical mesophase units in the isotropic fluid phase.’ The nucleation, growth and coalescence of these units determines the size of the anisotropic entities in the carbon.’ Except under special circumstances, spherical mesophase units have not been observed during coal carbonization. Nevertheless, metallurgical cokes are composed largely of anisotropic units whose size and/or shape are dependent upon the rank of the coal carbonized.3 For coal-tar pitches no corresponding systematic investigation has been reported of the influence of the rank of coal from which they were derived on their carbonization behaviour. Accordingly, such a study, involving laboratory-prepared pitches, was carried out as part of the present investigation of the factors influencing the optical anisotropy of metallurgical cokes. In the preparation of the pitches, tars were first obtained from coal heated at a rate of 5°C min- ’ to 600°C in an inclined-tube furnace. Released volatile matter was swept from the hot zone by a flow of O,-free nitrogen, and tars were condensed at the lower water-cooled end of the tube. Tars were converted to pitches by gradual heating to 280-300°C and allowing to cool. Pitches were obtained from six coals ranging in rank from a steam coking coal (NCB class 204, International class 344) to a weakly caking coal (NCB class 702, International class 722). The pitches are identified by class number of the coal from which they originated. Complete analytical data for the coals will be given in a later publication. Due to the method of preparation, the pitches are likely to differ from commercially produced pitches, particularly in quinolineinsoluble content. The pitches were carbonized in an O,-free nitrogen atmosphere in an open silica thimble in a tube furnace. They were heated at a rate of 5°C min-’ to temperatures within the range 35CL46O”C and to lOOO”C, with a 4 h soaking period. Cooled samples were crushed, embedded in epoxy resin and polished before being examined, at magnifications of x 150 and x 600, under polarized light with polars crossed to within 2” of extinction. Preliminary examination of the heat-treated pitches revealed the following general pattern. The first visible Change was the appearance of small anisotropic points in the isotropic matrix. These grew to various sizes before coalescing. When all the isotropic material was eliminated flow structures remained. The term ‘flow structures’ is used to describe anisotropic components similar to those
QO16-2361/83/010129-02S3.00 61983 Butterworth & Co. (Publishers)
Ltd
observed in cokes from higher rank coals.3 However, the flow patterns of the 1OOO“C pitch cokes differed in appearance when studied at low magnification in that the degree of distortion of the structure varied and pitch cokes from coals of lower rank appeared to exhibit mosaic anisotropy. In the materials heated to intermediate temperatures, a fine granular isotropic material was also present but this was not observed in the 1000°C cokes. Variations in detailed behaviour of the pitches were observed and to quantify the effects, the pitch cokes were analysed using a point-counting technique involving 300 counts. At each position the visible structure was classified as either isotropic, spherical mesophase within one of three size ranges, coalescing mesophase (distorted spheres) or flow structures. For the six pitches studied, the conversion of isotropic to anisotropic structures is illustrated in Figure 1 from which it is evident that the length of the mesophase range, i.e. the difference in temperature between that at which anistropic entities were first noted and that where isotropic material was completely eliminated, varies considerably depending on the rank of coal from which the pitch was obtained. In fact, the pitches seem to fall into two groups. The first comprises those obtained from the three higher rank coals, and described as 204, 301b and 300/400 in Figure 1, which all exhibit mesophase ranges
400 Temperature,
425
450
“C
Figure 7 Influence of coal rank and temperature on the development of optical anisotropy during carbonization of coaltar pitches. Pitches from: l , 204 coal; 0, 301 b coal; n , 300/400 coal; 0, 401 coal; A, 502 coal; x, 702 coal
FUEL, 1983, Vol 62, January
129
Short communications
1 3016
1
’
30
20
0 702
30
20
10
Photomicrographs illustrating variation in appearance of 1 OOO’C pitch cokes. Pitches obtained from: (a) 204; (b) 301 b; (c) 300/400; (d) 401; (e) 502; and (f) 702 class coals
Figure 3
0 375
425
400
Temperature,
“C
Effect of rank of parent coal on mesophase range behaviour of coal-tar pitches. Spherical mesophase sizes: l , < 12 pm; 0, 12-60 pm; W, > 60 pm Figure 2
>7o”C. The pitches from the three lower rank coals in NCB classes 401,502 and 702 did not exhibit mesophase ranges > 50°C. Also, whereas at no stage in the carbonization of the pitches from the lower rank coals were spherical mesophase units > 60 pm observed, 5-10 % of such units were found in pitches from higher rank coals when heated to temperatures within the mesophase range. The behaviour of one coal from each group is compared in Figure 2. The appearance of the six 1000°C pitch cokes are illustrated in Figure 3, the pitch cokes being identified by the rank of parent coal. All the pitch cokes were composed of Row-type components but those obtained from the pitches from the two coals of lowest rank appeared to exhibit mosaic anisotropy when viewed at a magnification of x 100. In the remaining pitch cokes the flow structures were much larger but they exhibited different degrees of distortion with the coke from the 300/400 pitch having the flattest appearance. Comparison of the appearance of the 1000°C pitch cokes with their behaviour observed at lower temperatures during their development shows that the larger flow patterns are generally associated with greater amounts of the largest sphere size during the mesophase range. In industrial usage, particularly for anode production in the aluminium industry, the type of anisotropic entities
130
FUEL, 1983, Vol 62, January
in the pitch coke strongly influences the anode’s performance.4 It is known that the presence of inert quinolineinsoluble particles has an effect on the anisotropy present in the carbonized pitch, and now the present work demonstrates that the rank of coal from which the pitch was obtained also has a marked influence. In addition to the presence of non-fusing particles, the presence of certain foreign atoms, e.g. sulphur or oxygen, low aromaticity of the pitch or high viscosity of the fluid phase have been invoked to explain the low degree of mesophase coalescence responsible for the short-order flow structures noted in the 1000°C cokes obtained from the 502 and 702 pitches. The precise factor responsible for the findings of the present study has not been established. ACKNOWLEDGEMENTS This investigation formed part of the fundamental research programme of the British Carbonization Research Association. The authors thank the Director of the Association for permission to publish and acknowledge the co-operation of other members of the Association’s staff. REFERENCES Marsh, H. and Walker, P. L., Jr. ‘Chemistry and Physics of Carbon’, Vol. 15, (Eds. P. L. Walker and P. A. Thrower), Dekker, New York, 1979, p. 229 Brooks, J. D. and Taylor, G. H. ‘Chemistry and Physics of Carbon’, Vol. 4, (Ed. P. L. Walker), Arnold. London. 1968. D. 243 Patrick; J. W., Reynolds,‘M. J. and Shaw, 6. H. Fuel 1973, 52, 198 Jones, S. S. and Hildebrandt, R. D. ‘Light Metals’, AIME 1975, Vol. 1, p. 291
Carbonization
of coal-tar
pitches:
John W. Patrick, Malcolm J. Reynolds
effect
of rank of parent
coal
and Alan Walker
The British Carbonization Research Association, Chesterfield, Derbyshire, S42 WS, UK (Received 15 June 1982; revised 4 October 1982)
The object of the present work was to investigate whether the rank of coal from which the coal-tar pitches were obtained had any bearing on their carbonization behaviour. Using six laboratory produced pitches it is shown that the behaviour during the mesophase temperature range, and consequently the size of the anisotropic flow structures in the pitch coke, vary with the rank of the parent coal. (Keywords: coal; carbonization;
pitch)
Optically anisotropic carbons are formed from a wide range of carbonaceous materials which fuse during carbonization, the essential feature being the formation of spherical mesophase units in the isotropic fluid phase.’ The nucleation, growth and coalescence of these units determines the size of the anisotropic entities in the carbon.’ Except under special circumstances, spherical mesophase units have not been observed during coal carbonization. Nevertheless, metallurgical cokes are composed largely of anisotropic units whose size and/or shape are dependent upon the rank of the coal carbonized.3 For coal-tar pitches no corresponding systematic investigation has been reported of the influence of the rank of coal from which they were derived on their carbonization behaviour. Accordingly, such a study, involving laboratory-prepared pitches, was carried out as part of the present investigation of the factors influencing the optical anisotropy of metallurgical cokes. In the preparation of the pitches, tars were first obtained from coal heated at a rate of 5°C min- ’ to 600°C in an inclined-tube furnace. Released volatile matter was swept from the hot zone by a flow of O,-free nitrogen, and tars were condensed at the lower water-cooled end of the tube. Tars were converted to pitches by gradual heating to 280-300°C and allowing to cool. Pitches were obtained from six coals ranging in rank from a steam coking coal (NCB class 204, International class 344) to a weakly caking coal (NCB class 702, International class 722). The pitches are identified by class number of the coal from which they originated. Complete analytical data for the coals will be given in a later publication. Due to the method of preparation, the pitches are likely to differ from commercially produced pitches, particularly in quinolineinsoluble content. The pitches were carbonized in an O,-free nitrogen atmosphere in an open silica thimble in a tube furnace. They were heated at a rate of 5°C min-’ to temperatures within the range 35CL46O”C and to lOOO”C, with a 4 h soaking period. Cooled samples were crushed, embedded in epoxy resin and polished before being examined, at magnifications of x 150 and x 600, under polarized light with polars crossed to within 2” of extinction. Preliminary examination of the heat-treated pitches revealed the following general pattern. The first visible Change was the appearance of small anisotropic points in the isotropic matrix. These grew to various sizes before coalescing. When all the isotropic material was eliminated flow structures remained. The term ‘flow structures’ is used to describe anisotropic components similar to those
QO16-2361/83/010129-02S3.00 61983 Butterworth & Co. (Publishers)
Ltd
observed in cokes from higher rank coals.3 However, the flow patterns of the 1OOO“C pitch cokes differed in appearance when studied at low magnification in that the degree of distortion of the structure varied and pitch cokes from coals of lower rank appeared to exhibit mosaic anisotropy. In the materials heated to intermediate temperatures, a fine granular isotropic material was also present but this was not observed in the 1000°C cokes. Variations in detailed behaviour of the pitches were observed and to quantify the effects, the pitch cokes were analysed using a point-counting technique involving 300 counts. At each position the visible structure was classified as either isotropic, spherical mesophase within one of three size ranges, coalescing mesophase (distorted spheres) or flow structures. For the six pitches studied, the conversion of isotropic to anisotropic structures is illustrated in Figure 1 from which it is evident that the length of the mesophase range, i.e. the difference in temperature between that at which anistropic entities were first noted and that where isotropic material was completely eliminated, varies considerably depending on the rank of coal from which the pitch was obtained. In fact, the pitches seem to fall into two groups. The first comprises those obtained from the three higher rank coals, and described as 204, 301b and 300/400 in Figure 1, which all exhibit mesophase ranges
400 Temperature,
425
450
“C
Figure 7 Influence of coal rank and temperature on the development of optical anisotropy during carbonization of coaltar pitches. Pitches from: l , 204 coal; 0, 301 b coal; n , 300/400 coal; 0, 401 coal; A, 502 coal; x, 702 coal
FUEL, 1983, Vol 62, January
129
Short communications
1 3016
1
’
30
20
0 702
30
20
10
Photomicrographs illustrating variation in appearance of 1 OOO’C pitch cokes. Pitches obtained from: (a) 204; (b) 301 b; (c) 300/400; (d) 401; (e) 502; and (f) 702 class coals
Figure 3
0 375
425
400
Temperature,
“C
Effect of rank of parent coal on mesophase range behaviour of coal-tar pitches. Spherical mesophase sizes: l , < 12 pm; 0, 12-60 pm; W, > 60 pm Figure 2
>7o”C. The pitches from the three lower rank coals in NCB classes 401,502 and 702 did not exhibit mesophase ranges > 50°C. Also, whereas at no stage in the carbonization of the pitches from the lower rank coals were spherical mesophase units > 60 pm observed, 5-10 % of such units were found in pitches from higher rank coals when heated to temperatures within the mesophase range. The behaviour of one coal from each group is compared in Figure 2. The appearance of the six 1000°C pitch cokes are illustrated in Figure 3, the pitch cokes being identified by the rank of parent coal. All the pitch cokes were composed of Row-type components but those obtained from the pitches from the two coals of lowest rank appeared to exhibit mosaic anisotropy when viewed at a magnification of x 100. In the remaining pitch cokes the flow structures were much larger but they exhibited different degrees of distortion with the coke from the 300/400 pitch having the flattest appearance. Comparison of the appearance of the 1000°C pitch cokes with their behaviour observed at lower temperatures during their development shows that the larger flow patterns are generally associated with greater amounts of the largest sphere size during the mesophase range. In industrial usage, particularly for anode production in the aluminium industry, the type of anisotropic entities
130
FUEL, 1983, Vol 62, January
in the pitch coke strongly influences the anode’s performance.4 It is known that the presence of inert quinolineinsoluble particles has an effect on the anisotropy present in the carbonized pitch, and now the present work demonstrates that the rank of coal from which the pitch was obtained also has a marked influence. In addition to the presence of non-fusing particles, the presence of certain foreign atoms, e.g. sulphur or oxygen, low aromaticity of the pitch or high viscosity of the fluid phase have been invoked to explain the low degree of mesophase coalescence responsible for the short-order flow structures noted in the 1000°C cokes obtained from the 502 and 702 pitches. The precise factor responsible for the findings of the present study has not been established. ACKNOWLEDGEMENTS This investigation formed part of the fundamental research programme of the British Carbonization Research Association. The authors thank the Director of the Association for permission to publish and acknowledge the co-operation of other members of the Association’s staff. REFERENCES Marsh, H. and Walker, P. L., Jr. ‘Chemistry and Physics of Carbon’, Vol. 15, (Eds. P. L. Walker and P. A. Thrower), Dekker, New York, 1979, p. 229 Brooks, J. D. and Taylor, G. H. ‘Chemistry and Physics of Carbon’, Vol. 4, (Ed. P. L. Walker), Arnold. London. 1968. D. 243 Patrick; J. W., Reynolds,‘M. J. and Shaw, 6. H. Fuel 1973, 52, 198 Jones, S. S. and Hildebrandt, R. D. ‘Light Metals’, AIME 1975, Vol. 1, p. 291