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Influence of carbonization conditions on the development of different types of optical anisotropy in cokes
Carbon,1975,Vol.13,pp. 509-514. Pergamon Press. Printed inGreatBritain
INFLUENCE OF CARBONIZATION CONDITIONS ON THE DEVELOPMENT OF DIFFERENT TYPES OF OPTICAL ANISOTROPY IN COKES? J. W. PATRICK, M. J. REYNOLDSand F. H. SHAW British Carbonization Research Association, Chesterfield, Derbyshire, England (Received 3 July 1975)
Abstract-The vitrain components of a series of coal samples were carbonized at temperatures from 400to 1000°Cat different rates of heating ranging from 0.5 to lO”K/minand utilizing soaking times up to 24 hr. Polished specimens prepared from the carbonized products were examined microscopically under polarized light in order to determine the proportions of the various types of optical anisotropy present in them. The variations in heating rate and soaking time were found to exert little significant influence on the anisotropy developed in high-temperaturecokes. But in semicokes produced at carbonization temperatures within the plastic range the influence of the carbonization conditions was much more pronounced with the effects being inter-related. Decreasing the heating rate or increasing the soaking time led to the optical anisotropy generally becoming detectable at lower carbonization temperatures. Fast heating rates caused an increase in the rate of transformation of the fine-grain mosaic anisotropy into coarser-grained types of anisotropy and increased soaking time led to enhanced anisotropic development in the semicokes produced at temperatures within the plastic range. The type of anisotropy developed in cokes is closely related to the release of volatile matter and the plasticity developed during carbonization and the conclusion is drawn that the balance between these factors controls the extent of the anisotropic development. 1.
INTRODUCTION
character of coke was first describedtl] many years ago and it is over 20 yr since a description was given[2] of the different anisotropic structures found in cokes and the relation of these structures to the starting material. More recently attempts were made[3] to relate the various optically anisotropic structures to the formation, growth and coalescence of anisotropic spherical bodies exemplified by those observed during the carbonization of pitch and a wide range of materials which develop a pitch-like fluidity on carbonization [4]. Following this line of approach we previously investigated[5] the role of the spherical mesophase in the formation of the different anisotropic structures in cokes produced from a wide range of coals. The conclusion was drawn that the process of formation and growth of spherical bodies leading to mosaic structures such as occurs during the carbonization of pitch, is of little significance in the carbonization of British coals under normal industrial conditions, but the anisotropic components found in coke can be classified according to their appearance and the grain size of the granular mosaics. From this was developed a quantitative means of characterizing cokes produced from different coals in terms of the types and amounts of anisotropy developed. However, the carbonization conditions used, such as heating rate and time of soaking at the final carbonization temperature, were arbitrarily chosen and the development of optical anisotropy is known [2] to be dependent on the carbonization conditions. Hence the purpose of the studies described in this paper was to document more fully the influence of carbonization conditions on the The optically
anisotropic
iParts of this paper formed the basis of that presented at the 11th Biennial Conference on Carbon, Gatlinburg, Tennessee, 1973.
development of the different anisotropic structures with reference to the characterization of cokes. 2. EXPERIMENTAL F’ROCEDURE The vitrain components from a series of coals of different rank were heated to various temperatures between 400 and 1000°Cat heating rates between 0.5 and lO”K/min and subjected to soaking times up to 24 hr. The resultant cokes and semicokes were subsequently examined by optical microscopy to determine the types and amounts of optical anisotropy present. 2.1 Coals used The petrographic and chemical analyses of the handpicked vitrains are given in Tables 1 and 2 respectively. The coals used ranged from a coking steam coal with a vitrain of mean maximum reflectance 1.75% and carbon content of over 90% (d.a.f.b.) to a medium caking coal whose vitrain had a mean maximum reflectance of 0.84% and carbon content of 85% (d.a.f.b.). The vitrains were composed largely of the vitrinite maceral. Table 1. Petrographic analyses of vitrain samples
Table 2. Chemical analyses of vitrain samples
J. W. PATRICKet al.
510 2.2 Carbonization
procedure
The vitrain samples were ground to less than 600pm particle size and 3-4 g of the ground material were carbonized in a silica boat in an electrically heated silica tube furnace through which a stream of oxygen-free nitrogen was passed. The selected final temperature, heating rate and soaking time were closely controlled by automated temperature-programming equipment. The solid products of the carbonizations were allowed to cool in nitrogen before being crushed to less than 600pm particle size. 2.3 Microscopic examination The examination of the carbonized vitrains by optical microscopy was carried out using the procedure described in detail previously[S]. In brief, polished sections were prepared by pelletizing the ground cokes mixed with a small amount of epoxy resin and mounting the pellets in epoxy resin blocks for polishing. The coke surfaces were examined using a polarizing microscope with a ~95 objective giving an overall magnificationof x 1425.The microscopically identifiable components were divided into several categories from isotropic material to mosaic type anisotropy of various grain sizes and to a flow-type anisotropy, this latter term referring only to the appearance and not to any measured fluidity. The grain size of the various mosaics was estimated at 0.3 pm nominal diameter for the fine-grain, 0.7 Frn for the medium-grain and 1.3 pm for the coarse-grain. The anisotropy identified in the high-rank coal v&rain was described as basic anisotropy and appeared relatively featureless. The proportions of the different anisotropic components were determined using the point-count technique and were based on 300 point counls, the total error in
m_
0 Britannia vitrain Sacriston vitrain l
480
460.
. /’
440 420~’
,/
/’
/’
:/ 0
/’ 0
/ /’
Heating rate, K/min
Fig. 1. Influence of heating rate on the carbonization temperature at which anisotropy became detectable in high-rank vitrains (no soaking at final temperature). 4. Britannio vitrain
b. Sacriston vitrain
100 80 60 40 20 0
the determinationsbeing about -cS%. 3. EXPERIMENTAL RESULTS It is convenient to consider the results in two sections dealing with the influence of heating rate on the one hand and soaking time at the final carbonization temperature on the other. 3.1 Influence of heating rate With no soaking time at the final carbonization temperature, the development of optical anisotropy was strongly influenced by variations in the heating rate between 0.5 and lO”K/min, the magnitude of the effect also being dependent on the rank of the coal. The highest rank vitmin examined, that from the Britannia coal, was anisotropic and on carbonization, this basic anisotropy was converted directly to flow type anisotropy. The remaining vitrains were, under our experimental conditions, optically isotropic and the anisotropy developed on carbonization appeared initially as a fine-grain mosaic. With vitrains of high carbon content an increase in heating rate led to the anisotropy making its initial appearance at a higher carbonization temperature as shown in Fig. 1. At the higher heating rates however, the fine-grained mosaic anisotropy had a shorter existence in that, on continued carbonization, it was transformed into progressively coarser-grained anisotropy over a smaller range of temperature than if carbonized at a slow heating rate. This type of behaviour applies for both high rank and medium rank coal vitrains as shown by the results given in Figs. 2 and 3. It is also evident from those figures that, of the cokes carbonized to lOOO”C, those prepared at a high rate of heating had an enhanced proportion of the coarser grained anisotropy. This pattern of behaviour, i.e. a low heating rate leading to a low carbonization temperature at which anisotropy
Carbonization tomperolurq°C
q Basic anisotropy q Isotropic q Fin+grain mosaic q Medium-grain mosaic rm]Coarse-mosaicOFlow-type H
Coarse mosaic/flow
Fig. 2. Influence of heating rate on development of anisotropy during carbonization of high-rank vitrains (no soaking at final temperature).
was first detected but slow subsequent development of fine-grained mosaic anisotropy into coarser-grained material was, with some slight modification, also followed by the lower-rank vitrains. As shown by Fig. 4, the pattern was partially obscured because the coals remained completely isotropic at all carbonization temperatures when heated at the very low heating rate of O*S”K/min. Differences associated with a change in heating rate from 5 to lO’K/min also appeared to be progressively less marked with decreasing rank. The high rank Sacriston vitrain was used to examine the influence of heating rate when the final carbonization temperature was maintained for 4 hr. At this soaking time the heating rate appeared to exert little influence on the carbonization temperature at which anisotropy was first detected as shown in Fig. 5 but the anisotropic
Influence of carbonization conditions on the development of different types of optical anisotropy in cokes b.Cortonwood vltrain
a. Houqhton vitmin
100
87.2 %C
e7+%c
511
80 60
100 80
40 2
60 40 20 0
h 8 r; zT z P
100 80 60 40 20
Carbonization temperoture,°C Carbonization temperature,‘C El •III
q Fine-grainmosaic@/Medium-grain mosaic Coarse mosaic q Coarse mowic/flowm Flow-type
0
Isotropic
Fig. 3. Influence of heating rate on development of anisotropy during carbonization of medium-rank vitrains (no soaking at final temperature). a.Wearmouthvitmin 04.7 %c
b.Markhumvitmin e5o%c
100 00 60 40 20 0
Carbonization tomperature,‘C 0
Isotropic
a
Fine-grain mosaic
q
Medium-gmin mosaic •ul
Coarse mosaic
Fig. 4. Influence of heating rate on development of anisotropy during carbonization of medium/low-rank vitrains (no soaking at finaltemperature).
development still occurred more readily at the faster heating rate. 3.2 Influence of soaking time The results described in the previous section for the Sacriston vitrain indicate the influence which soaking time at the final carbonization temperature has on the
iml
q Fine-grainmosaic q Medium-gminmosaic Coarse mosaic EI Coarse mosaic/flowq Flow-type Isotropic
Fig. 5. Influence of heating rate on anisotropy developed during carbonization of Sacriston vitrain using a 4 hr soaking time at the final temperature.
development of optical anisotropy. A comparison of Figs. 2(b) and 5 suggests that an increase in the soaking time from 0 to 4 hr caused little change in the anisotropy developed in cokes produced at carbonization temperatures above about 500°C or indeed in that developed at lower temperatures at the slowest heating rate used. At the faster heating rates however increasing the soaking time enabled the development of anisotropy to take place at lower carbonization temperatures. A similar effect was also obtained at the slowest heating rate used, namely O*S”K/min,when the soaking time was increased to 24 hr, anisotropy becoming detectable at 380°C (cf. 400°C with a 4 hr soaking time and 420°C with no soaking). For the other vitrains carbonized at S”K/min, a comparison of the anisotropy developed with no soaking time and with a 4 hr soaking time is shown in Figs. 6 and 7. In common with the Sacriston vitrain, all of the vitrains carbonized at temperatures within the plastic range showed evidence of enhanced anisotropy with the longer soaking time, but with little significant change at the higher carbonization temperatures. A more detailed examination was made of the influence of soaking time up to 4 hr at carbonization temperatures near to those at which maximum rate of development of anisotropy occurred (cf. [l], Fig. 9) and the results obtained are given in Fig. 8. The dependence of the development of anisotropy at these low carbonization temperatures on the soaking time is evident but it is also apparent that generally there are only relatively small changes when the soaking time is increased beyond l-2 hr. In one instance, using the Sacriston vitrain, the soaking time was extended to 12hr and the result suggested that there was a possibility of a gradual but slow development of the anisotropy with extended soaking times. It may be observed that the anisotropy
512
J.
W.
PATRICKef al.
4 h soaking
No soaking
a. Britannia- 480
“C
b. Sacriston-420’C
a. Britannia vitrain
100 80 60 40 20 0
60 40 20 0
Om?
00 Carbonization temperature,*C &
Basic anisotropy 0
Isotropic @Fine-groin
q
Medium-grain mosaic
mosaic
q Coarse-mosaicmFlow-type
Fig. 6. Influence
of soaking time on development of anisotropy in high- and medium-rank vitrains carbonized at YKlmin.
too
No roakina 4 h soakina a.Wearmouthvitroin ”
60 60 <
40
!j ”
20 0
‘% 0 100 b ,o .2
b. Markham vitrain
a0 60 40 20 0 zEU!” 49Pfrn_
8
E?8%!00 ttqqvl0
Carbonization temperature,‘C
q Isotropic m Fine-grain mosaic q Medium-grain mosaic q Coarse mosaic Fig. 7. Muence of soaking time on development of anisotropy in medium/low rank vitrains carbonized at S”K/min.
developed in this series of tests sometimes differed from that produced in the previously described tests but these differences can be attributed to the interval of time between the tests leading to some oxidation of the coal vitrains. 4. DISCUSSION
The results obtained in this investigation provide further confirmatory evidence that the development of optical anisotropy during the carbonization of vitrains follows the pattern described earlier [5]. It is also evident that variation of carbonization conditions may influence the pattern of the development of anisotropy. However these changes are primarily concentrated in the low
-cum*
Or?? 00 Soaking time, hours
-cum
9
q Basic anisotropy 0 Isotropic q Fine-grain mosaic q Medium-grain mosaicq Coarse mosaicq Flow-type Fig. 8. Dependence of anisotropic development in vitrains carbonized at 5”Klmin to selected temperatures within the plastic range, on soaking time at the carbonization temperature.
temperature region of the carbonization process, i.e. at temperatures within the plastic range, so that anisotropy developed in high temperature cokes is largely independent of variations within a wide range of carbonization conditions, such as heating rate and soaking time at the final carbonization temperature. As regards the characterization of coke anisotropy in relation to the parent coal, the conclusions drawn previously{51 are evidently still valid and the choice of a heating rate of 5Wmin and a soaking time of 4 hr was a reasonable one in order to obtain the anisotropic development representative of that required for the purpose of coke characterization. The effects of heating rate and soaking time are also inter-related. This is clearly demonstrated by the results obtained for the Sacriston vitrain (Figs. 2 and 5) where dependence on the heating rate of the carbonization temperature at which anisotropy was first observed was eliminated by increasing the soaking time from 0 to 4 hr, presumably as a consequence of increasing the time available for structural ordering to take place. The data obtained for the semicokes provide information relevant to considerations of the processes responsible for the development of anisotropy occurring during the plastic stage of the carbonization process. Again, in accord with the previous work, no evidence was found at any stage of the carbonization process under any of the conditions used, for the presence of the spherical mesophase observed as the precursor to the mosaic anisotropy produced in the carbonization of pitch and pitch-like materials [4]. Under the carbonization conditions we have used it is clear that the formation of anisotropic-spherical bodies plays no significant part in the develobment of structural order associated with the anisotropy developed in vitrains during the latter stages of the plastic range.
Influence of carbonization conditions on the development of different types of optical anisotropy in cokes In the earlier work[S] the progression of the anisotropy from fine-grain mosaic to a mosaic of coarser grain and in some instances to a flow-type anisotropy was identified with increasing structural order. Such a process could be expected to be facilitated by increasing the time for the ordering to take place, and the appearance of anisotropy at a lower carbonization temperature when a slow heating rate was used is an indication of this. The elimination of this difference in the temperature at which anisotropy was first detected by utilising long soaking times, and indeed all the observed effects of varying the soaking time are further indications of the time dependence of the ordering process, a feature described by Sanada et al.[31. However the influence of rapid heating rates in increasing the rate of transformation of the fine-grained into coarser-grained anisotropy cannot be explained on this basis. The previous studies[5] demonstrated the significance of volatile matter in the development of anisotropy and this provides an indication of the probable explanation of the behaviour described. The rate of decomposition is increased at faster heating rates[6] although the total percentage loss of volatile matter is lower[7]. Also the fluidity is considerably increased by increased heating rates[8] so that it is feasible that the enhanced ordering may result directly from either the increased fluidity or from the retention of a greater quantity of the appropriate molecular species acting either as a plasticising component or as a reactive component causing a chemical change in the anisotropic entities. The failure of coking coals to soften and fuse to a coherent coke when carbonized under vacuum when the rate of loss of volatile matter is at a maximum, and conversely the promotion of caking by heating under pressure when weight loss is at a minimum[8] can be cited as further indications of the probable importance of volatile matter retention in the formation of opticallyanisotropic structures. It may also be observed however the the wide variations in the anisotropic development of coking-steam coals and prime-coking coals appear to be associated with comparatively small differences in volatile matter content. The degree of structural ordering obtained thus appears to be dependent on the balance attained between, on the one hand, the loss of volatile matter necessary before the molecular rearrangements can take place, and, on the other hand, the retention of sufficient fluidity and/or the appropriate molecular constitutents. In this context it should be noted that the development of optical anisotropy during the plastic stage of coal carbonization can be likened to a liquid-crystal system and Marsh[9] has developed a theory of coke formation in terms of a process of liquid-crystal development, from observations of the formation, growth and coalescence of spherical anisotropic liquid crystals during the carbonization of pitch-like materials and in the carbonization under high pressure of many other organic substances [lo]. Although we have been unable to identify spherical mesophase, as such, during the carbonization of coal under conditions comparable to those used industrially, we find that prime-coking coals such as Sacriston Victoria, form during carbonization a wider range of anisotropic species
513
than other coals. If this feature is identifiable with an optimum capability for liquid crystal formation, then our findings are in accord with the Marsh hypothesis. The significance of having present in the system the required degree of fluidity and the appropriate constituent molecular species is recognised and the observations made in this report can be accounted for in terms of the effects on the liquid-crystal growth of the volatile matter lost from the carbonizing system. The degree of fluidity and the exact nature of the molecular species required for optimum liquid-crystal or anisotropic development are however still matters of some speculation. 5. CONCLUSIONS The carbonization of v&rains as described in this report led to the development of anisotropy, dependent on carbonization temperature and coal rank, in genera1 agreement with that found previously[5]. The anisotropy developed in high temperature cokes was substantially unaffected by wide variations in heating rate and soaking time at the final carbonization temperature, only very slow heating rates of the order of O*S”K/minhaving any significantly different effect. The result of these observations was therefore to confirm the validity of the conclusions drawn from the earlier study[5] concerning the use of optical anisotropy as a means of characterising coke. For semicokes prepared at temperatures within the plastic range, the influence of soaking time and heating rate was much more pronounced. At slow heating rates the anisotropy appeared at a lower carbonization temperature than at high heating rates but the inter-relation of the effects of heating rate and soaking time meant that this difference could be eliminated for example by coupling a long soaking time to a fast heating rate. The transformation of the fine-grain mosaic anisotropy formed initially, into the coarser-grained varieties of anisotropy proceeded more rapidly at the higher heating rates. Increasing the soaking time also facilitated this development of anisotropy in the semicokes. The significance of the coal volatile matter in relation to the anisotropy developed was noted in the previous study [5] and the influence of the carbonization conditions can be accounted for in similar terms. It is suggested that the loss of volatile matter and the retention of fluidity are two of the main factors controlling the extent to which the various anisotropic types are developed, these factors attaining optimum conditions for the development of anisotropy in the prime-coking coals. Acknowledgements-This investigation formed part of the fundamental research programme of the British Carbonization Research Association and we thank the Director of the Association for permission to publish. We also acknowledge the co-operation of those membersof the Association’sstaffwho have assisted in the work. REFERENCES
1. RamdohrP., Arch. Eisenhuttenw. 609 (1928). 2. Abramski C. and Mackowsky M-Th., In Hundbuch der Mikroskipie in der Tecknik (edited by H. Freund), Vol. 2, part 1, pp. 31l-340. Umschau-Verlag, Frankfurt (1952). 3. Sugimura H. et al., J. Fuel Sot. Japan 48, 920 (1969); 49, 744
514
.I. W. PATRICKet al.
(1970);Kimura H. et al., I. Fuel Sot. Japan 49, 752 (1970); Sanada Y. et al., Fuel, Lond. 52, 143 (1973). 4. Brooks J. D. and Taylor G. H., In Chemistry and Physics of Carbon (edited by P. L. Walker, Jr.), Vol. 4, ._ pp. 243-286. Edward Arnold, iondon (1%8). 5. Patrick J. W.. Revnolds M. J. and Shaw F. H.. Fuel. Lond. 52. 198 (1973). ’ 6. Howard H., In Chemistry of Coal Utihkation (edited by H. H.
Lowry), Supplementary vol., p. 367.Wiley, New York (1%3). 7. Van Krevelen D. W., In Coal, p. 226. Elsevier, Amsterdam (1961). 8. Loison R. et al., In Chemistry of Coal Utilization (edited bv H. H. Lowry), Supplementari v&., pp. 150-201.‘Wiley, fiew York (1963). 9. Marsh‘H., Fuel, Land. 52, 205 (1973). 10. Marsh H. et al., Fuel, Land. 52, 234; 243; 253 (1973).
INFLUENCE OF CARBONIZATION CONDITIONS ON THE DEVELOPMENT OF DIFFERENT TYPES OF OPTICAL ANISOTROPY IN COKES? J. W. PATRICK, M. J. REYNOLDSand F. H. SHAW British Carbonization Research Association, Chesterfield, Derbyshire, England (Received 3 July 1975)
Abstract-The vitrain components of a series of coal samples were carbonized at temperatures from 400to 1000°Cat different rates of heating ranging from 0.5 to lO”K/minand utilizing soaking times up to 24 hr. Polished specimens prepared from the carbonized products were examined microscopically under polarized light in order to determine the proportions of the various types of optical anisotropy present in them. The variations in heating rate and soaking time were found to exert little significant influence on the anisotropy developed in high-temperaturecokes. But in semicokes produced at carbonization temperatures within the plastic range the influence of the carbonization conditions was much more pronounced with the effects being inter-related. Decreasing the heating rate or increasing the soaking time led to the optical anisotropy generally becoming detectable at lower carbonization temperatures. Fast heating rates caused an increase in the rate of transformation of the fine-grain mosaic anisotropy into coarser-grained types of anisotropy and increased soaking time led to enhanced anisotropic development in the semicokes produced at temperatures within the plastic range. The type of anisotropy developed in cokes is closely related to the release of volatile matter and the plasticity developed during carbonization and the conclusion is drawn that the balance between these factors controls the extent of the anisotropic development. 1.
INTRODUCTION
character of coke was first describedtl] many years ago and it is over 20 yr since a description was given[2] of the different anisotropic structures found in cokes and the relation of these structures to the starting material. More recently attempts were made[3] to relate the various optically anisotropic structures to the formation, growth and coalescence of anisotropic spherical bodies exemplified by those observed during the carbonization of pitch and a wide range of materials which develop a pitch-like fluidity on carbonization [4]. Following this line of approach we previously investigated[5] the role of the spherical mesophase in the formation of the different anisotropic structures in cokes produced from a wide range of coals. The conclusion was drawn that the process of formation and growth of spherical bodies leading to mosaic structures such as occurs during the carbonization of pitch, is of little significance in the carbonization of British coals under normal industrial conditions, but the anisotropic components found in coke can be classified according to their appearance and the grain size of the granular mosaics. From this was developed a quantitative means of characterizing cokes produced from different coals in terms of the types and amounts of anisotropy developed. However, the carbonization conditions used, such as heating rate and time of soaking at the final carbonization temperature, were arbitrarily chosen and the development of optical anisotropy is known [2] to be dependent on the carbonization conditions. Hence the purpose of the studies described in this paper was to document more fully the influence of carbonization conditions on the The optically
anisotropic
iParts of this paper formed the basis of that presented at the 11th Biennial Conference on Carbon, Gatlinburg, Tennessee, 1973.
development of the different anisotropic structures with reference to the characterization of cokes. 2. EXPERIMENTAL F’ROCEDURE The vitrain components from a series of coals of different rank were heated to various temperatures between 400 and 1000°Cat heating rates between 0.5 and lO”K/min and subjected to soaking times up to 24 hr. The resultant cokes and semicokes were subsequently examined by optical microscopy to determine the types and amounts of optical anisotropy present. 2.1 Coals used The petrographic and chemical analyses of the handpicked vitrains are given in Tables 1 and 2 respectively. The coals used ranged from a coking steam coal with a vitrain of mean maximum reflectance 1.75% and carbon content of over 90% (d.a.f.b.) to a medium caking coal whose vitrain had a mean maximum reflectance of 0.84% and carbon content of 85% (d.a.f.b.). The vitrains were composed largely of the vitrinite maceral. Table 1. Petrographic analyses of vitrain samples
Table 2. Chemical analyses of vitrain samples
J. W. PATRICKet al.
510 2.2 Carbonization
procedure
The vitrain samples were ground to less than 600pm particle size and 3-4 g of the ground material were carbonized in a silica boat in an electrically heated silica tube furnace through which a stream of oxygen-free nitrogen was passed. The selected final temperature, heating rate and soaking time were closely controlled by automated temperature-programming equipment. The solid products of the carbonizations were allowed to cool in nitrogen before being crushed to less than 600pm particle size. 2.3 Microscopic examination The examination of the carbonized vitrains by optical microscopy was carried out using the procedure described in detail previously[S]. In brief, polished sections were prepared by pelletizing the ground cokes mixed with a small amount of epoxy resin and mounting the pellets in epoxy resin blocks for polishing. The coke surfaces were examined using a polarizing microscope with a ~95 objective giving an overall magnificationof x 1425.The microscopically identifiable components were divided into several categories from isotropic material to mosaic type anisotropy of various grain sizes and to a flow-type anisotropy, this latter term referring only to the appearance and not to any measured fluidity. The grain size of the various mosaics was estimated at 0.3 pm nominal diameter for the fine-grain, 0.7 Frn for the medium-grain and 1.3 pm for the coarse-grain. The anisotropy identified in the high-rank coal v&rain was described as basic anisotropy and appeared relatively featureless. The proportions of the different anisotropic components were determined using the point-count technique and were based on 300 point counls, the total error in
m_
0 Britannia vitrain Sacriston vitrain l
480
460.
. /’
440 420~’
,/
/’
/’
:/ 0
/’ 0
/ /’
Heating rate, K/min
Fig. 1. Influence of heating rate on the carbonization temperature at which anisotropy became detectable in high-rank vitrains (no soaking at final temperature). 4. Britannio vitrain
b. Sacriston vitrain
100 80 60 40 20 0
the determinationsbeing about -cS%. 3. EXPERIMENTAL RESULTS It is convenient to consider the results in two sections dealing with the influence of heating rate on the one hand and soaking time at the final carbonization temperature on the other. 3.1 Influence of heating rate With no soaking time at the final carbonization temperature, the development of optical anisotropy was strongly influenced by variations in the heating rate between 0.5 and lO”K/min, the magnitude of the effect also being dependent on the rank of the coal. The highest rank vitmin examined, that from the Britannia coal, was anisotropic and on carbonization, this basic anisotropy was converted directly to flow type anisotropy. The remaining vitrains were, under our experimental conditions, optically isotropic and the anisotropy developed on carbonization appeared initially as a fine-grain mosaic. With vitrains of high carbon content an increase in heating rate led to the anisotropy making its initial appearance at a higher carbonization temperature as shown in Fig. 1. At the higher heating rates however, the fine-grained mosaic anisotropy had a shorter existence in that, on continued carbonization, it was transformed into progressively coarser-grained anisotropy over a smaller range of temperature than if carbonized at a slow heating rate. This type of behaviour applies for both high rank and medium rank coal vitrains as shown by the results given in Figs. 2 and 3. It is also evident from those figures that, of the cokes carbonized to lOOO”C, those prepared at a high rate of heating had an enhanced proportion of the coarser grained anisotropy. This pattern of behaviour, i.e. a low heating rate leading to a low carbonization temperature at which anisotropy
Carbonization tomperolurq°C
q Basic anisotropy q Isotropic q Fin+grain mosaic q Medium-grain mosaic rm]Coarse-mosaicOFlow-type H
Coarse mosaic/flow
Fig. 2. Influence of heating rate on development of anisotropy during carbonization of high-rank vitrains (no soaking at final temperature).
was first detected but slow subsequent development of fine-grained mosaic anisotropy into coarser-grained material was, with some slight modification, also followed by the lower-rank vitrains. As shown by Fig. 4, the pattern was partially obscured because the coals remained completely isotropic at all carbonization temperatures when heated at the very low heating rate of O*S”K/min. Differences associated with a change in heating rate from 5 to lO’K/min also appeared to be progressively less marked with decreasing rank. The high rank Sacriston vitrain was used to examine the influence of heating rate when the final carbonization temperature was maintained for 4 hr. At this soaking time the heating rate appeared to exert little influence on the carbonization temperature at which anisotropy was first detected as shown in Fig. 5 but the anisotropic
Influence of carbonization conditions on the development of different types of optical anisotropy in cokes b.Cortonwood vltrain
a. Houqhton vitmin
100
87.2 %C
e7+%c
511
80 60
100 80
40 2
60 40 20 0
h 8 r; zT z P
100 80 60 40 20
Carbonization temperoture,°C Carbonization temperature,‘C El •III
q Fine-grainmosaic@/Medium-grain mosaic Coarse mosaic q Coarse mowic/flowm Flow-type
0
Isotropic
Fig. 3. Influence of heating rate on development of anisotropy during carbonization of medium-rank vitrains (no soaking at final temperature). a.Wearmouthvitmin 04.7 %c
b.Markhumvitmin e5o%c
100 00 60 40 20 0
Carbonization tomperature,‘C 0
Isotropic
a
Fine-grain mosaic
q
Medium-gmin mosaic •ul
Coarse mosaic
Fig. 4. Influence of heating rate on development of anisotropy during carbonization of medium/low-rank vitrains (no soaking at finaltemperature).
development still occurred more readily at the faster heating rate. 3.2 Influence of soaking time The results described in the previous section for the Sacriston vitrain indicate the influence which soaking time at the final carbonization temperature has on the
iml
q Fine-grainmosaic q Medium-gminmosaic Coarse mosaic EI Coarse mosaic/flowq Flow-type Isotropic
Fig. 5. Influence of heating rate on anisotropy developed during carbonization of Sacriston vitrain using a 4 hr soaking time at the final temperature.
development of optical anisotropy. A comparison of Figs. 2(b) and 5 suggests that an increase in the soaking time from 0 to 4 hr caused little change in the anisotropy developed in cokes produced at carbonization temperatures above about 500°C or indeed in that developed at lower temperatures at the slowest heating rate used. At the faster heating rates however increasing the soaking time enabled the development of anisotropy to take place at lower carbonization temperatures. A similar effect was also obtained at the slowest heating rate used, namely O*S”K/min,when the soaking time was increased to 24 hr, anisotropy becoming detectable at 380°C (cf. 400°C with a 4 hr soaking time and 420°C with no soaking). For the other vitrains carbonized at S”K/min, a comparison of the anisotropy developed with no soaking time and with a 4 hr soaking time is shown in Figs. 6 and 7. In common with the Sacriston vitrain, all of the vitrains carbonized at temperatures within the plastic range showed evidence of enhanced anisotropy with the longer soaking time, but with little significant change at the higher carbonization temperatures. A more detailed examination was made of the influence of soaking time up to 4 hr at carbonization temperatures near to those at which maximum rate of development of anisotropy occurred (cf. [l], Fig. 9) and the results obtained are given in Fig. 8. The dependence of the development of anisotropy at these low carbonization temperatures on the soaking time is evident but it is also apparent that generally there are only relatively small changes when the soaking time is increased beyond l-2 hr. In one instance, using the Sacriston vitrain, the soaking time was extended to 12hr and the result suggested that there was a possibility of a gradual but slow development of the anisotropy with extended soaking times. It may be observed that the anisotropy
512
J.
W.
PATRICKef al.
4 h soaking
No soaking
a. Britannia- 480
“C
b. Sacriston-420’C
a. Britannia vitrain
100 80 60 40 20 0
60 40 20 0
Om?
00 Carbonization temperature,*C &
Basic anisotropy 0
Isotropic @Fine-groin
q
Medium-grain mosaic
mosaic
q Coarse-mosaicmFlow-type
Fig. 6. Influence
of soaking time on development of anisotropy in high- and medium-rank vitrains carbonized at YKlmin.
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!j ”
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a0 60 40 20 0 zEU!” 49Pfrn_
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q Isotropic m Fine-grain mosaic q Medium-grain mosaic q Coarse mosaic Fig. 7. Muence of soaking time on development of anisotropy in medium/low rank vitrains carbonized at S”K/min.
developed in this series of tests sometimes differed from that produced in the previously described tests but these differences can be attributed to the interval of time between the tests leading to some oxidation of the coal vitrains. 4. DISCUSSION
The results obtained in this investigation provide further confirmatory evidence that the development of optical anisotropy during the carbonization of vitrains follows the pattern described earlier [5]. It is also evident that variation of carbonization conditions may influence the pattern of the development of anisotropy. However these changes are primarily concentrated in the low
-cum*
Or?? 00 Soaking time, hours
-cum
9
q Basic anisotropy 0 Isotropic q Fine-grain mosaic q Medium-grain mosaicq Coarse mosaicq Flow-type Fig. 8. Dependence of anisotropic development in vitrains carbonized at 5”Klmin to selected temperatures within the plastic range, on soaking time at the carbonization temperature.
temperature region of the carbonization process, i.e. at temperatures within the plastic range, so that anisotropy developed in high temperature cokes is largely independent of variations within a wide range of carbonization conditions, such as heating rate and soaking time at the final carbonization temperature. As regards the characterization of coke anisotropy in relation to the parent coal, the conclusions drawn previously{51 are evidently still valid and the choice of a heating rate of 5Wmin and a soaking time of 4 hr was a reasonable one in order to obtain the anisotropic development representative of that required for the purpose of coke characterization. The effects of heating rate and soaking time are also inter-related. This is clearly demonstrated by the results obtained for the Sacriston vitrain (Figs. 2 and 5) where dependence on the heating rate of the carbonization temperature at which anisotropy was first observed was eliminated by increasing the soaking time from 0 to 4 hr, presumably as a consequence of increasing the time available for structural ordering to take place. The data obtained for the semicokes provide information relevant to considerations of the processes responsible for the development of anisotropy occurring during the plastic stage of the carbonization process. Again, in accord with the previous work, no evidence was found at any stage of the carbonization process under any of the conditions used, for the presence of the spherical mesophase observed as the precursor to the mosaic anisotropy produced in the carbonization of pitch and pitch-like materials [4]. Under the carbonization conditions we have used it is clear that the formation of anisotropic-spherical bodies plays no significant part in the develobment of structural order associated with the anisotropy developed in vitrains during the latter stages of the plastic range.
Influence of carbonization conditions on the development of different types of optical anisotropy in cokes In the earlier work[S] the progression of the anisotropy from fine-grain mosaic to a mosaic of coarser grain and in some instances to a flow-type anisotropy was identified with increasing structural order. Such a process could be expected to be facilitated by increasing the time for the ordering to take place, and the appearance of anisotropy at a lower carbonization temperature when a slow heating rate was used is an indication of this. The elimination of this difference in the temperature at which anisotropy was first detected by utilising long soaking times, and indeed all the observed effects of varying the soaking time are further indications of the time dependence of the ordering process, a feature described by Sanada et al.[31. However the influence of rapid heating rates in increasing the rate of transformation of the fine-grained into coarser-grained anisotropy cannot be explained on this basis. The previous studies[5] demonstrated the significance of volatile matter in the development of anisotropy and this provides an indication of the probable explanation of the behaviour described. The rate of decomposition is increased at faster heating rates[6] although the total percentage loss of volatile matter is lower[7]. Also the fluidity is considerably increased by increased heating rates[8] so that it is feasible that the enhanced ordering may result directly from either the increased fluidity or from the retention of a greater quantity of the appropriate molecular species acting either as a plasticising component or as a reactive component causing a chemical change in the anisotropic entities. The failure of coking coals to soften and fuse to a coherent coke when carbonized under vacuum when the rate of loss of volatile matter is at a maximum, and conversely the promotion of caking by heating under pressure when weight loss is at a minimum[8] can be cited as further indications of the probable importance of volatile matter retention in the formation of opticallyanisotropic structures. It may also be observed however the the wide variations in the anisotropic development of coking-steam coals and prime-coking coals appear to be associated with comparatively small differences in volatile matter content. The degree of structural ordering obtained thus appears to be dependent on the balance attained between, on the one hand, the loss of volatile matter necessary before the molecular rearrangements can take place, and, on the other hand, the retention of sufficient fluidity and/or the appropriate molecular constitutents. In this context it should be noted that the development of optical anisotropy during the plastic stage of coal carbonization can be likened to a liquid-crystal system and Marsh[9] has developed a theory of coke formation in terms of a process of liquid-crystal development, from observations of the formation, growth and coalescence of spherical anisotropic liquid crystals during the carbonization of pitch-like materials and in the carbonization under high pressure of many other organic substances [lo]. Although we have been unable to identify spherical mesophase, as such, during the carbonization of coal under conditions comparable to those used industrially, we find that prime-coking coals such as Sacriston Victoria, form during carbonization a wider range of anisotropic species
513
than other coals. If this feature is identifiable with an optimum capability for liquid crystal formation, then our findings are in accord with the Marsh hypothesis. The significance of having present in the system the required degree of fluidity and the appropriate constituent molecular species is recognised and the observations made in this report can be accounted for in terms of the effects on the liquid-crystal growth of the volatile matter lost from the carbonizing system. The degree of fluidity and the exact nature of the molecular species required for optimum liquid-crystal or anisotropic development are however still matters of some speculation. 5. CONCLUSIONS The carbonization of v&rains as described in this report led to the development of anisotropy, dependent on carbonization temperature and coal rank, in genera1 agreement with that found previously[5]. The anisotropy developed in high temperature cokes was substantially unaffected by wide variations in heating rate and soaking time at the final carbonization temperature, only very slow heating rates of the order of O*S”K/minhaving any significantly different effect. The result of these observations was therefore to confirm the validity of the conclusions drawn from the earlier study[5] concerning the use of optical anisotropy as a means of characterising coke. For semicokes prepared at temperatures within the plastic range, the influence of soaking time and heating rate was much more pronounced. At slow heating rates the anisotropy appeared at a lower carbonization temperature than at high heating rates but the inter-relation of the effects of heating rate and soaking time meant that this difference could be eliminated for example by coupling a long soaking time to a fast heating rate. The transformation of the fine-grain mosaic anisotropy formed initially, into the coarser-grained varieties of anisotropy proceeded more rapidly at the higher heating rates. Increasing the soaking time also facilitated this development of anisotropy in the semicokes. The significance of the coal volatile matter in relation to the anisotropy developed was noted in the previous study [5] and the influence of the carbonization conditions can be accounted for in similar terms. It is suggested that the loss of volatile matter and the retention of fluidity are two of the main factors controlling the extent to which the various anisotropic types are developed, these factors attaining optimum conditions for the development of anisotropy in the prime-coking coals. Acknowledgements-This investigation formed part of the fundamental research programme of the British Carbonization Research Association and we thank the Director of the Association for permission to publish. We also acknowledge the co-operation of those membersof the Association’sstaffwho have assisted in the work. REFERENCES
1. RamdohrP., Arch. Eisenhuttenw. 609 (1928). 2. Abramski C. and Mackowsky M-Th., In Hundbuch der Mikroskipie in der Tecknik (edited by H. Freund), Vol. 2, part 1, pp. 31l-340. Umschau-Verlag, Frankfurt (1952). 3. Sugimura H. et al., J. Fuel Sot. Japan 48, 920 (1969); 49, 744
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(1970);Kimura H. et al., I. Fuel Sot. Japan 49, 752 (1970); Sanada Y. et al., Fuel, Lond. 52, 143 (1973). 4. Brooks J. D. and Taylor G. H., In Chemistry and Physics of Carbon (edited by P. L. Walker, Jr.), Vol. 4, ._ pp. 243-286. Edward Arnold, iondon (1%8). 5. Patrick J. W.. Revnolds M. J. and Shaw F. H.. Fuel. Lond. 52. 198 (1973). ’ 6. Howard H., In Chemistry of Coal Utihkation (edited by H. H.
Lowry), Supplementary vol., p. 367.Wiley, New York (1%3). 7. Van Krevelen D. W., In Coal, p. 226. Elsevier, Amsterdam (1961). 8. Loison R. et al., In Chemistry of Coal Utilization (edited bv H. H. Lowry), Supplementari v&., pp. 150-201.‘Wiley, fiew York (1963). 9. Marsh‘H., Fuel, Land. 52, 205 (1973). 10. Marsh H. et al., Fuel, Land. 52, 234; 243; 253 (1973).