Optical anisotropy of carbonized coking- and caking-coal vitrains

The purpose of this work was to characterize in detail the optical anisotropy formed during carbonization of the range of coals used in the coking industry, the ultimate objective being to attain a better understanding of the coking process. Vitrains hand-picked from a series of coking and caking coals were carbonized to various temperatures between 380 and 1000 °C. The semicokes and cokes so produced were examined by polarized-light microscopy to determine the proportions of the different types of optical anisotropy developed during carbonization. The results demonstrated that coals normally grouped within one class of the coal classification system used by the National Coal Board can lead to cokes which are significantly different in terms of their optical anisotropy. The process of the anisotropic development during carbonization can be explained generally in terms of loss of volatile matter, variations in viscosity of the plastic mass, and distortion of ordered phases by the pressure of evolving gases. Differences in carbonization behaviour as judged by the coke anisotropy can be attributed to differences in the ‘molecular-structure’ of the parent coal. In this respect the oxygen in the coal is considered to be of primary significance.     

Carbonization of coal blends: mesophase formation and coke properties

Laboratory investigations of strength of cokes from blends of coals incorporating pitch were supported by 7 kg trials. The stronger cokes showed a greater interaction between coal and pitch to produce an interface component of anisotropic mozaics which is relatively resistant to crack propagation. The process whereby coal is transformed into coke includes the formation of a fluid zone in which develop nematic liquid crystals and anisotropic carbon which is an essential component of metallurgical coke. Strength, thermal and oxidation resistance of coke can be discussed in terms of the size and shape of the anisotropic carbon which constitutes the optical texture of pore-wall material of coke. Coals of different rank form cokes with different optical textures. Blending procedures of non-caking, caking and coking coals involve the interactions of components of the blend to form mesophase and optical texture. Petroleum pitches used as additives are effective in modifying the carbonization process because of an ability to participate in hydrogen transfer reactions.     

Influence of carbonization conditions on the development of different types of optical anisotropy in cokes

The vitrain components of a series of coal samples were carbonized at temperatures from 400 to 1000°C at different rates of heating ranging from 0.5 to 10°K/min and 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-temperature cokes. 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.     

Structure of cokes from coking coal vitrites

The coking process of vitrites and thermobitumens separated from vitrites was examined; structural X-ray and microscopic examinations of the cokes obtained were carried out. A correlation between reflectance distribution of vitrites and microscopic structure of their cokes was found.An increase in the structural ordering of the cokes from vitrites, passing from cokes of gas coal to cokes of orthocoking coals, is observed. It is accompanied by an increase of the optical anisotropy of the resultant cokes; this anisotropy first appears in coke from gas-coaking coal.The cokes from the thermobitumens are lower ordered than the cokes from parent vitrites but all these cokes are partially or entirely optically anisotropic.Total removal of the thermobitumens from coals deprives the cokes from the residues after the extraction of any optical anisotropy.     

Co-carbonization of hard coals and coal extracts 2. The co-carbonization of medium- and high-rank coals with extracts

Studies on the influence of anthracene coal extracts on the carbonization process of medium- and high-rank coals were undertaken. Extracts from flame coal (Int. Class. 900) and gas-coking coal (Int. Class. 632) were used as additives. The blends prepared from the examined coals and the extracts exhibited better coking properties than the parent coals. The addition of extract to the coals gave an increase in the microstrength of the resultant cokes. The effects of co-carbonization of coking coals with extracts were increases in the size of the optical texture as well as in the degree of structural ordering of cokes. In the co-carbonization of semicoking coal with addition of coal extracts, a reduction in the size of the anisotropic units and a decrease in the crystallite height of cokes were observed. No modification of the basic anisotropy of coke from anthracite by coal extract was observed. With increasing extract content in anthracite/extract blends there was an increase in the degree of structural ordering of co-carbonization products. Extract addition was unable to modify the behaviour of fusinite. Based on the results of investigation of the influence of coal extracts on the carbonization of different-rank coals, a division of coals according to the modification of the optical texture of coke is given.     

Structure in cokes from coals of different rank

A range of bituminous coals has been carbonized to 1273 K. Polished surfaces of the solid products, carbons or cokes, are examined for optical texture by optical microscopy. Fracture surfaces of the carbons are examined by scanning electron microscopy (SEM). The carbon from the lowest rank coal (NCB Code No. 702) is isotropic and fracture surfaces are featureless. Carbons from coals of ranks 602, 502 are optically isotropic but fracture surfaces are granular (size 0.1–0.2 μm), indicating small growth units of mesophase. In the carbon/coke from a 401 coal, the anisotropic optical texture and grain size are both ≈0.5–10 μm diameter. Coke from a coking coal (301a, 301b) has a layered structure extending in units of at least 20 μm diameter with sub-structures ~ 1.5 μm within the layers, indicating perhaps that the bedding anisotropy of these coals is not totally lost in the fluid phase of carbonization. The carbons from the higher rank coals have the bedding anisotropy of the parent coal. The combined techniques of optical microscopy and SEM (both before and after etching of the fracture surfaces of coke in chromic acid solution) reveal useful detail of structure in carbons/cokes and of the mechanism of carbonization of coking coals.     

Bonding of solid additives in cokes from coals: a microscopy study

Cortonwood Silkstone (NCB class 401) and Betteshanger (NCB class 301 a/204) coals were co-carbonized with solid additives such as anthracite, coke breeze, green and calcined petroleum cokes. The resultant carbonization products (cokes) were examined by optical microscopy and SEM was used to investigate polished surfaces etched by chromic acid and fracture surfaces. For both coals only the anthracite and green petroleum coke become bonded to the coal cokes. This probably results from softening and interaction of interfaces of the anthracite and green coke with the fluid coal via a mechanism of hydrogenating solvolysis during the carbonization process. The coke breeze and calcined petroleum cokes were interlocked into the matrix of coal coke.     

Structure of the cokes obtained from the group components separated from coking coal vitrites by selective thermosolvolysis

The examination of the structure of cokes obtained from extracts separated from preheated vitrites of coking coals by progressive and continuous extraction with chloroform was carried out. The structural ordering (interplanar spacing and crystallite dimensions) of the cokes depends on the rank of the parent vitrites but it does not depend on the degree of extraction. The occurrence of optical anisotropy in cokes from the extracts is connected with both the rank of the parent vitrite and the degree of extraction. In the formation of the optical anisotropic structure during the carbonization of coking coal vitrites, the part of the extract which is of small size, which partially undergoes decomposition, is an important factor.     

Carbonization and liquid-crystal (mesophase) development. Part 4. Carbonization of coal-tar pitches and coals of increasing rank

Coal-tar pitches, from coals of different rank and with various quinoline-insoluble contents, were carbonized under pressure (67 to 200 MN m⁻²) to maximum temperatures of 923 K. The resultant cokes were examined by optical and scanning electron microscopy in terms of size and shape of anisotropic structures within the coke. Natural quinoline-insolubles and carbon blacks both destroyed growth of the mesophase and development of anisotropy. Graphite particles (<10 μm) promoted growth and coalescence of the mesophase. Fourteen coals, of carbon content 77 to 91 wt%, VM 41 to 26%, were similarly carbonized under pressure. In the lower-rank coals no microscopically resolvable anisotropic mesophase was produced, but at a carbon content of 85% anisotropic units 1–2 μm in diameter were detected, increasing in size at a carbon content of 90% to 5 μm diameter. Results are discussed in terms of the origins of anisotropic mosaics observed in cokes, their variation in size with coal rank, and their significance in the carbonization of coal.     

Carbonization and liquid-crystal (mesophase) development. 13. Kinetic considerations of the co-carbonizations of coals with organic additives

Five coals, of rank from an anthracite to a non-caking coal, have been carbonized singly and also cocarbonized with decacyclene, mixing ratio 7:3, in the temperature range 648 K to 823 K, heating at 10 K min^?1, with various soak times. The objective of the study is to derive the basic factors which influence the kinetics of formation of mesophase and anisotropic coke. Accordingly, resultant cokes were polished and surfaces examined by reflected polarized light in an optical microscope. The size, shape and extent of anisotropic development is discussed in terms of the conditions of carbonization and the rank of coal. In these systems a somewhat larger optical texture results in cokes produced at the higher carbonization temperatures. The temperature of onset of growth of anisotropic carbon in co-carbonizations was below that of either the coal or the decacyclene. Reactivities are evidently modified. The origins, growth and coalescence of growth units of anisotropic carbon in these cocarbonizations of coals with decacyclene are demonstrated.     

Evaluation of several petroleum residues as the needle coke feedstock using a tube bomb

Nine feedstocks were carbonized at 500°C for 4 h in a tube bomb into lump cokes which can be evaluated in terms of their optical anisotropy, CTE and bulk density. The properties of some cokes were found comparable to those of commercial ones. Variable carbonization pressure allows more precise evaluation of feedstocks, since higher pressure was generally preferable for the production of better needle coke. The participation of the lighter fraction which is more emphasized under higher pressure is suggested to be very critical for the formation of needle coke. The reactivity of the feedstock measured by the coke formation in the carbonization for 1 h appears to reflect the quality of the resultant coke. The most reactive fraction may form the porous coke in the initial stage of the carbonization, separating from the matrix which can mediate the carbonization reaction.     

Structural study of cokes using optical microscopy and X-ray diffraction

Cokes exhibiting a range of optical texture from isotropie to anisotropic domains > 60 μm diameter were examined by X-ray diffraction. The variation of an optical texture index (OTI) with crystallite height and interlayer spacing was studied. The OTI varies little with the X-ray parameters for cokes whose optical texture is larger than medium-grained (1.5–5.0 μm) mosaic anisotropy. For cokes of smaller optical texture there is a sharp decrease in crystallite height and an increase in interlayer spacing. These results are discussed in terms of fluid mesophase removing defects in cokes of optical texture of size of coarsegrained mosaics and larger. The cokes of smaller optical texture are formed from less fluid mesophase which does not coalesce. Defects therefore remain in this anisotropic carbon of the coke so reducing crystallographic order.     

Gasification studies of cokes from coals. The effects of carbonization pressure on optical texture and porosity

Ten coals were carbonized under various pressures (4 kPa, normal pressure and 10 MPa). Optical textures and physical structures of resultant cokes were monitored. The extent of optical anisotropy increased greatly with increasing carbonization pressure, such a trend being more pronounced with the lower-rank coals. Physical structure was also influenced by carbonization pressure. Gasification reactivities of the cokes with carbon dioxide and steam (1200 °C) were studied with respect to their optical anisotropy and physical structure. Gasification reactivities of optical textures were estimated using both the point-counting technique and regression analysis. The reactivities of cokes with the same optical texture produced from the same parent coal were similar. However, there were considerable differences when compared with cokes from different parent coals. Although the values estimated by regression analyses are consistent with those obtained by point-counting, except for the leaflet and inert textures, the physical locations of respective textures can be important in quantitative discussions of their reactivities.     

E.s.r. study of formation of coke texture

The optical coke texture is a major factor in the high-temperature properties of metallurgical coke but the mechanism of formation of coke texture is not established yet. By studying the carbonization process during the formation of optically anisotropic coke with an e.s.r. spectrometer fitted with a high temperature cavity a relation between the concentration of the free-spins during carbonization and coke texture has been inferred for some organic compounds used as the model materials and for coals covering a range of coal rank.     

Carbonization and liquid-crystal (mesophase) development. 9. The co-carbonization of vitrains with Ashland A200 petroleum pitch

Vitrains from a wide range of ranks of coals were carbonized singly and also co-carbonized (HTT 1273 K) with 25% of Ashland A200 petroleum pitch. Polished surfaces of the resultant cokes were examined for optical texture in a polarizing-light optical microscope using a half-wave retarder plate to produce interference colours. For the anthracites, there is no modification of either component during co-carbonization. The growth of optical texture from the A200 pitch is not affected. For all caking vitrains the optical texture of coke from the blend system is extensively modified when compared to the optical texture of coke from the vitrain. For the low-rank non-caking vitrains the isotropic coke becomes totally or partially anisotropic in co-carbonization. The mechanism of modification of the optical texture of resultant cokes is related to the formation of nematic liquid crystals, mesophase and the semi-coke. It is not considered that the chemistry of pyrolysis is modified on cocarbonization of the vitrain and pitch.     

Optical properties of vitrinite carbonized at different pressures

The effect of pressure on the optical properties of cokes from a medium volatile bituminous coal (carbon = 87.9 wt% daf), some carbonized at atmospheric pressure and others under hydraulic pressure (21–310 MPa), over temperatures ranging from 350 to 600 °C at 50 °C intervals, has been studied. The cokes formed at atmospheric pressure developed fine grained mosaics, while medium-flow type mosaics formed in coke carbonized under hydraulic pressure. The thermal decomposition stage began at lower temperatures with increasing hydraulic pressure, resulting in a prolonged devolatilization phase for coke formed at a pressure of 21 MPa. Hence the fluidity of samples carbonized under pressure decreases with increasing hydraulic pressure. Pressure promotes the optical anisotropy apparent from the level of bireflectance. The reflectance of coke formed at atmospheric pressure is higher than that of cokes carbonized under hydraulic pressure, perhaps due to the inhibitory effect of entrapped volatile matter during carbonization under hydraulic pressure. The morphological features of vitirinite carbonized under pressure resemble those of coals naturally affected by heat.     

Carbonization of coals to produce anisotropic cokes: 3. Evaluation of low-rank bituminous and subbituminous coals by single and co-carbonizations with pitch

Single carbonizations and co-carbonizations of 17 low-rank bituminous and subbituminous coals have been studied to evaluate their suitability as sources of blast furnace coke in terms of pore-wall profile and anisotropic development within the cokes. Co-carbonizations suggest the possible use of low-rank coals which from single carbonizations would not have been considered suitable. To evaluate semi-quantitatively the coke quality, two structural characteristics of the cokes produced by single and co-carbonizations are graded on a scale of 1 to 5. Overall assessments for each coal are plotted against the atomic H/C and 0/C ratios of the original coals. Although there are a few exceptions, coals with similar assessments are located in the same region of the plot, indicating that, to a first approximation, the H/C and 0/C ratios are suitable indicators of the single and co-carbonization properties of a coal. The presence of cations in the coal appears to be an additional factor influencing the carbonization properties and may explain the exceptional behaviour of some coals. Removal of these cations by pretreatment of the coals improves the carbonization properties.     

Characteristics and carbonization behaviors of coal extracts

N-methyl pyrrolidone (NMP) raw coal extract (EXT), hydrogenated coal extract (HEXT) and the blend of EXT and HEXT in NMP (BLD), from two bituminous coals, were studied. The extracts were carbonized in both tube-bomb and a temperature programmable furnace. Elemental analysis, FTIR spectroscopy and optical microscopy techniques were employed to characterize the extracts and the carbonization residues. It was found that the extracts resembled petroleum-derived pitches in the hydrogen content and (C/H)_atomic ratio. Higher oxygen and nitrogen contents differentiated the coal extracts from commercial petroleum pitch. More carbon and hydrogen, and lesser oxygen and sulfur differentiated HEXT from EXT. The ratios of integrated IR band intensity for aromatic and aliphatic CH stretching indicate that the relative content of aliphatic hydrogen in EXT is higher than in HEXT. HEXT contains comparatively more aromatic hydrogen, a feature necessary for thermal stability and fluidity during carbonization. BLD materials are at a place somewhere in between. Kinetic modeling of the aliphatic hydrogen change during carbonization reveals that EXT has high carbonization rate and low apparent activation energy. This can be related to the optical texture size of carbonization residues. The residues made from EXTs exhibited fine mosaic optical texture and limited mesophase development. HEXTs were readily converted into highly anisotropic coke. BLDs produced carbonization residues with intermediate properties. Extracts with similar activation energies produced similar residues in the same coal series. The degree or extent of anisotropy displayed by the carbonization residues was found to be dependent on the relative distribution of aromatic and aliphatic hydrogen.     

Carbonizations under pressure of chloroformsoluble material of coking vitrinite

Chloroform-soluble material from a Polish coking coal was prepared by extracting the coal after heating it initially to 673 K for 15 min. The soluble material was carbonized in sealed gold tubes to 800 K and 873 K at 200 MPa pressure and to 800 K, 1000 K and 1200 K at atmospheric pressure. Coke morphology was assessed by SEM with optical texture (micro-texture) being assessed by polarizedlight microscopy of polished surfaces. Cokes from the pressure carbonizations (100 wt % yield) showed coalescing spherule morphology, 10 to 20 μm diameter, and are totally anisotropic. The material normally lost as volatiles thus contributes totally to the formation of mesophase and anisotropic coke. Coke from the carbonization of the soluble material at one atmospheric pressure (41 wt % yield) is composed mainly of anisotropic fine-grained mozaics in the range 1–5 μm. Carbonization under pressure extends the range of sizes of optical texture in cokes from this chloroform-soluble material. Applications may exist in graphite manufacture.