Coal carbonization and coke anisotropy: some comments

Short communications

Coal carbonization

and coke anisotropy:

some comments

John W. Patrick, Malcolm J. Reynolds and Alan Walker The British

Carbonization

(Received

15 July

Research

Association,

Chesterfield,

Derbyshire

S42

WS,

UK

1982)

Previously published work is presented in modified form to emphasise that, except under special circumstances, spherical mesophase units have not been observed during coal carbonization and that the viscosity of the fluid phase, considered so important during pitch carbonization, has no strong bearing on the size of anisotropic units found in metallurgical coke. (Keywords:

coal; carbonization;

coke)

The processes by which graphitizing carbons are formed from a wide range of carbonaceous materials, particularly pitches, is now well documented.’ -3 An essential feature is the appearance, during the fluid stage, of spherical mesophase units, the nucleation, growth and coalescence of which determine the size and shape of the anisotropic units in the resultant carbon. Mesophase coalescence, favoured by low viscosity of the fluid phase, and the subsequent re-alignment of the lamellar molecules of which they are composed leads eventually to an extensive lamelliform morphology, perhaps distorted, in the restricted coalescence, variously carbon. However, associated with low aromaticity of the pitch, high viscosity of the fluid phase or the presence of inert particles or certain foreign atoms, leads either to a mere compressing together of mesophase units or to coalescence with little re-alignment of lamellar molecules. In either case, under polarized light, the carbon consists of randomly oriented anisotropic areas and is described as exhibiting mosaic anisotropy. The vitrinite-derived components of carbonized coals exhibit anisotropy varying from mozaic to flow-type,4v5 the latter being consistent with extensively aligned structures. As a result it frequently seems to be assumed6 that the behaviour ofcoal during carbonization conforms closely to the pattern of behaviour exhibited by carbonizing pitches. The object of this Communication is to reappraise previously published data on the carbonization of vitrains to emphasize that, except under spherical mesophase units have special circumstances,6.7 not been observed during coal carbonization, and further, to point out other important respects in which carbonization of coal differs from that of pitch. Full experimental details have been given in previous publications.4*5 Briefly, hand-picked vitrains from singleseam UK coals, crushed to < 600 !lrn, were carbonized at to temperatures within the plastic a rate of 5K min-’ range and to 1OOOYZ.After cooling, crushing, embedding in resin and polishing, products were examined under polarized light at an overall magnification of x 1500. Components were classified into categories ranging from isotropic through mozaics of varying size to flow-type anisotropy, and quantified by point-counting. Micrographs illustrating the appearance of the components have been published previously.4*5 The variation of the anisotropic composition with heattreatment temperature, during the carbonization of live vitrains selected from the available data, is given in Figures I and 2. The reflectances of the vitrains and the @16-2361/83/010131-02$3.00 @1983 Butterworth & Co. (Publishers) Ltd

class of coal from which they originate are given in the captions to the Figures. The carbonization behaviour of vitrains can be divided conveniently into two groups depending on their reflectance. Those with reflectances in the range 1.55-1.80% give cokes exhibiting flow-type anisotropy whereas cokes from vitrains within the 0% 1.46% reflectance range exhibit mosaic and/or granular flow anisotropy. Flow-type anisotropy results from either the direct conversion of the basic anisotropy of the vitrain (Figure la) or from a type of fine mosaic unit formed initially (Figure lb). In the second group of vitrains, fine mosaic units, formed from the isotropic matrix, either retain their identity into the semicoke (Figure 2c) or act as intermediaries in the formation of larger mosaic entities (Figures 2a and b). In view of the marked difference in size

a

k

l\

Temperature

(VI)

Figure 1 Anisotropic development during the carbonization of (a) Ffaldau (CRC 203, R, max 1.76%); and (b) Tilmanstone (CRC, 204, R, max 1.5%) vitrains. 0, Basic anisotry; 0, fine mozaic anisotropy; n , granular flow anisotropy; 0, flow-type anisotropy

FUEL, 1983, Vol 62, January

131

Short communications

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.

c

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c

400

450

500

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550

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Figure 2 Anisotropic development during the carbonization of (a) Snowdown (CRC 301 a, R, max 1.36%); (b) Shuttle Eye (CRC 401, R, max 0.92%); and (c) Manton (CRC 501, R, max 0.85%) vitrains. 0, Isotropic; 0, fine mozaic anisotropy; n , medium and coarse mosaic anisotropy; IJ, granular flow anisotropy

and shape of the entities formed directly from them, there is some doubt whether the fine mosaic units in the two groups of vitrains are identical. A behaviour intermediate between those of vitrains in the two groups specified, in that appreciable amounts of both mosaic and flow-type anisotropy are developed, has been observed only for two vitrains with reflectances of 1.54 and 1.467:. It should be stressed that at no stage in the carbonization of these or any other coals under the conditions described have individual mesophase spheres, similar to those observed in carbonizing pitch, been observed. This statement does not exclude the possibility that such units are present in a size range below the limit of resolution of the optical microscope. Furthermore, at intermediate temperatures, although anisotropic and isotropic components are present in a char, they are not finely and mutually distributed but exist in discrete areas. This is understandable if each vitrain particle, even if fused to another, reacts homogeneously becoming anisotropic at a specific temperature. Also the variation of anisotropic content with heat-treatment temperature would then be related to the spread of reactivities of particles within the

132

FUEL, 1983, Vol 62, Januaw

sample. As a result of this discrete area effect the growth of anisotropic units, noted particularly during carbonization of Snowdown and Shuttle Eye vitrains, does not appear to involve the incorporation of additional isotropic material into individual mozaic units. That this is true is apparent from Figure 26 from which it is evident that a large quantity of fine mozaic material is converted into medium mozaics when only a small amount of isotropic material is present. No Gieseler fluidity data are available for the particular vitrains considered. However, from a knowledge of their rank it is possible to conclude that the order of decreasing fluidity of the carbonizing mass would be Shuttle Eye, Snowdown, Manton, Tilmanstone, Ffaldau. If this order is compared with the anisotropic composition of the semicokes evident from the Figures then it is clear that low viscosity within the carbonizing mass is not associated with larger sized anisotropic units in the semicoke. Indeed Ffaldau vitrain is unlikely to exhibit any discernable fluidity in a Gieseler plastometer, and no evidence of any mesophase units of mosaic intermediates was observed on carbonization, yet its semicoke consists predominantly of flow-type anisotropic components characteristic of extensive basal layer ordering. A plausible explanation is that the vitrain contains coal molecules of a size, shape and spatial arrangement such that relatively little change is necessary before large anisotropic entities become evident. It seems likely, therefore, that the formation of flow-type anisotropy in high rank vitrains is associated primarily with the nature of the vitrains, possibly with their aromaticity. In contrast to the behaviour of the high rank vitrains, the cokes from those in the reflectance range 0.8-1.4x contain only anisotropic mozaic units of various sizes, yet maximum Gieseler viscosities are exhibited by vitrains within this reflectance range. Growth from one mozaic size to another does take place but no evidence has been observed of the transformation of larger mozaic units into flow-type structures. This effect cannot be explained in terms of the viscosity of the fluid phase. It would appear, therefore, that these lower rank vitrains differ inherently from those in the higher reflectance group. Whether the reduced aromaticity or increased oxygen content associated with decreasing coal rank is a factor 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. REFERENCES Brooks, J. D. and Taylor, G. H. ‘Chemistry and Physics of Carbon’, (Ed. P. L. Walker, Jr.), Edward Arnold, London, 1968, Vol. 4, p. 243 White, J. L. Proyress in Solid State Chemistry 1974, 9, 59 Marsh, H. Proc. 4th London International Carbon and Graphite Conference, Society of Chemical Industry, London, 1976, p. 2 Patrick, J. W., Reynolds, M. J. and Shaw, F. H. Fuel 1973, 52, 198 Patrick, J. W., Reynolds, M. J. and Shaw, F. H. Fuel 1979, 58, 501 Inter alia Marsh, H. and Smith, J. ‘Analytical methods for coal and coal products’, (Ed. C. Karr, Jr.), Academic Press, New York, 1978, Vol. II, p. 371 Fricl, J. J., Mehta, S., Mitchell, G. D. and Karpinski, J. M. Fuel 1980, 59, 611