Metallurgical Cokes Tutorial | Petrographic Atlas | SIU

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Crelling's Petrographic Atlas of Coals and Carbons

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Coke Petrography

The carbonization/coking of coals produces cokes that exhibit a variety of microscopic textures whose optical behavior in polarized light aids in their characterization. The word texture relates to the carbons' optical properties, while structure relates to the amount and size of coke pores and walls. Many properties of carbons such as their graphitizability, their electrical resistivity, reactivity to CO2 at elevated temperatures and strength are related to the optical properties of the coke carbon forms.

Cokes consist of binder and filler phases. Binder phase carbons are produced from vitrinite, exinite, resinite, and reactive semifusinite which soften during carbonization. Filler phase carbon forms are derived from inert semifusinite, fusinite, possibly micrinite, macrinite and inertodetrinite macerals which do not soften appreciably during carbonization. Each vitrinoid type which represents 0.1% reflectance ranging from V-7 through V-18 or 19 produce a distinguishable coke carbon form. The relation of coal rank based on vitrinite reflectance to coke carbon forms in the binder phase is shown in Table I.


The binder phase carbons from high volatile coals consist of isotropic, incipient and circular forms. Isotropic carbon is derived form vitrinoid V-Type 7 or lower and remains pink to purple when the specimen is rotated using polarized light and a tint plate. Incipient anisotropic carbon is produced from vitrinoid V-Type 8 having domain sizes of <0.5 micron; they remain pink to purple and appear faintly textured when rotated in polarized light. Circular anisotropic domains are derived from vitrinoid V-Types 9, 10 and 11; they are relatively circular in outline and increase in size from 0.5 to 2.0 microns as the V-type increases. Some systems refer to circular anisotropic domains as granular.

The binder phase carbons produced from medium volatile coals that contain vitrinoid V-Types 12, 13 and 14 are lenticular in shape having widths that range from 1.0 to 12.0 microns, with a length (L) to width (W) ratio of 2 to 4. Some systems refer to lenticular domains as leaflet. The fine, medium and coarse categories closely correspond to V-Types 12, 13 and 14.

The binder phase carbons derived form low volatile coals consist of ribbon-like domains which are produced from V-Types 15, 16, 17 and 18. The domain widths range from 2.0 microns to >25 microns and have length to width rations of L > 4W. The fine, medium and coarse domains approximately correspond to V-Types 15, 16 and 17 plus. The interference colors range from pink or red to blue. Ribbon shaped carbon domains are sometimes called flow structure.

The blend proportions of coking blends can be approximated by point counting the carbons in coke samples and assigning them to the V-type or coal rank that produced the carbon forms, even if the point-count falls on inerts or filler phase carbon. The count sheet that can be used to record the binder phase carbon is shown in Table II. Data from coke carbon form analysis is corrected for yield in estimating the coal blend used to produce the coke.


Filler phase carbons are derived from organic and inorganic inerts, as well as oxidized and brecciated coals, as shown in Table I and II. The filler phase carbons from organic inerts are sized into categories of <10 microns, 10 to 50 microns and >50 microns, as shown in Table II. The inorganic inerts are allocated to categories of <50 and >50 microns. Pyrite-derived minerals are sometimes counted separately. Non-Coking vitrinoids which are derived from coals that are either too high or too low in rank to coke, are also counted separately.

Depositional carbons are produced from gases and vapors which are cracked to solid carbon as they encounter the hot coke surfaces. They are separated into sooty, spherulitic and pyrolytic subcategories. They may serve to indicate thermal conditions in the coke ovens and definitely decrease the coke's reactivity to CO2. Additive carbons such as coke breeze, anthracite and petroleum coke, which are carbon extenders and antifissurants, act to decrease porosity and are counted separately. Binder phase additives such as pitch and tar are counted separately if they can be recognized. Green coke or under-carbonized coke is counted as an indicator of coke that is incompletely carbonized. Burnt coke is one indicator of stickers and pushing delays.

The results of continuous binder phase and filler phase analysis are combined to form the petrographic analysis of the coke. This information is useful in estimating reactivity, strength, oven wall pressure problems, coke pushing problems, excessive carbon accumulations, over heated or under heated ovens, etc. In general, isotropic carbon forms are the most reactive to CO2, while reactivity decreases from circular to lenticular carbon forms and increases for ribbon carbon forms.


Relation of Coal Rank (Reflectance) to Coke Carbon Forms in Binder Phase
Bituminous Coal Rank High Volatile Medium Volatile Low Volatile
Vitrinoid Type 7 or lower 8 9 10 11 12 13 14 15 16 17
Appearance in Polarized Light Isotropic Incipient Circular Anisotropic (Granular) Lenticular Anisotropic (Leaflet) Ribbon Anisotropic (Flow)
Domain Size Diameter, Micron 0 0-5 Fine 0.5-1.0 Medium 1.0-1.5 Coarse 1.5-2.0 Fine 1.0-3.0 Medium 3.0-8.0 Coarse 8.0-12.0 Fine 2.0-12.0 Medium 12.0-25.0 Coarse +25.0
Length to Width Ratio 0 0.5 L=W L=W L<2W L>2W L=2W To 4W L<4W L>4W But Usually >10W
Color Pink/Purple Pink/Purple and Yellow or Green Range From Pink/Purple to Yellow, Green, and Blue

Total Carbons With Composite Binder, Organic Inerts, Inorganic Inerts, and Miscellaneous Materials in Filler Phase
Binder Phase Organic Inerts Inorganic Inerts Miscellaneous Inerts Additives Depositional Carbons Miscellaneous Non-Carbons Other Miscellaneous Materials
Fine <10mu Medium 10-50 Coarse +50 Fine <50 Coarse +50 Oxidized
Too High or Too Low in Rank to Coke
Petroleum Coke
Melts or Slag


Gray, R. J. and De Vanney, K. F., 1986, Coke carbon forms: microscopic classification and industrial applications: Int. J. Coal Geol., v.6, p.277-297.

Gray, R. J., 1989, Coal to Coke conversion: in Marsh, Harry, ed., Introduction to Carbon Science, Butterworths, London, p.285-321.

Gray, Ralph J., 1991, Some petrographic applications to coal, coke, and carbons: Org. Geochem., v. 17, no. 4, p.535-555