Graphyt Crystals in Blast Furnace Coke

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Carbon 45 (2007) 1145–1151

Graphite crystals in blast furnace coke

Stanislav S. Gornostayev *, Jouko J. Harkki

Laboratory of Process Metallurgy, University of Oulu, P.O. Box 4300, 90014 Oulu, Finland

Received 15 November 2006; accepted 26 February 2007Available online 4 March 2007

Abstract

Graphite was found at several spots on the porous surface of a sample of blast furnace (BF) coke in association with droplets of Si-bearing iron and other inorganic compounds. It occurred in foliated intergrowths and rose-like aggregates of euhedral to subhedral crys-tals of various size (10 lm–0.7 mm) and morphologies, the generic forms that can be seen in the crystals being basal pinacoid, hexagonalprism and hexagonal dipyramid. The largest crystals have a flake-like habit and are represented by a combination of a basal pinacoid anda hexagonal prism. Graphite crystals that have an ideal hexagonal prism in their cross-section are rarer than distorted forms. There aretwins on the dypiramidal composition planes and twins produced by rotation on [0001]. The major deformations in the crystals, i.e.detaching, bending and rolling of (0001) planes, may be referred to exfoliation phenomena caused by potassium and sodium from cir-culating BF gases and enforced by the BF gas flows. The formation of graphite crystals on a surface of BF coke, especially those of flake-like morphology with an undeveloped hexagonal prism, possibly decreases its reactivity and can be accompanied by the generation offines. The graphitic coating on a surface of BF coke can act as a lubricant to facilitate coke pieces sliding over each other thus affectingthe mechanical stability of the coke cone in a BF.� 2007 Elsevier Ltd. All rights reserved.

1. Introduction

Coke is a key material for blast furnace (BF) operation,acting as (1) a fuel; (2) a reductant, to generate reducinggases, including CO; (3) a carburisation agent for the hotmetal, to give it the required properties, and (4) a structuralsupport to provide permeability for liquid phase drainageand upward flow of the blast furnace gases [1]. It is chargedinto the BF along with the iron ores and various fluxes.Metallurgical coke is made from a mix of several coals,preferably of low inorganic content, by heating the mixto 1100 �C in coke batteries (ovens) that consist of twoheated walls spaced about 40–45 cm apart. Reactivity andstrength are the most important parameters that character-ize the quality of the coke, and these are in turn dependenton the amounts and properties of organic (maceral-derived) carbon-based components [1–4] and inorganicmatter (minerals) [5–9] inherited from the primary coals

0008-6223/$ - see front matter � 2007 Elsevier Ltd. All rights reserved.doi:10.1016/j.carbon.2007.02.033

* Corresponding author. Fax: +358 8 553 2339.E-mail address: [email protected] (S.S. Gornostayev).

and serve to determine the behaviour of the coke in theBF. The evolution of the carbon structure of coke at tem-peratures comparable to those in the upper parts of a BFhave been discussed recently based on the results of exper-iments carried out in a horizontal laboratory-scale tubefurnace [10].

The conditions of formation and subsequent degrada-tion and deformation of graphite crystals can be reflectedin their size and morphology, as documented in a numberof studies of natural [11–20] and synthetic [21–23] graphite.It is mentioned that well formed natural and laboratory-grown graphite crystals are uncommon and that they rarelyexceed a few millimetres in size [24]. Conventional graphiteforms hexagonal (dihexagonal–dipyramidal class – 6/m2/m2/m) crystals, with weak bonding between the graphitelayers. The hexagonal nature of the crystal arises becausegraphite exists as a stack of ‘‘sheets’’ of carbon atoms par-allel to {0001}, each sheet having a hexagonal arrange-ment of atoms. Graphite crystals in the naturalenvironment usually possess a scaly (flake-like), platy, tab-ular or columnar habit [11–20]. In addition to its unique

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1146 S.S. Gornostayev, J.J. Harkki / Carbon 45 (2007) 1145–1151

technological properties, graphite is also known as one ofthe few single-mineral geothermometers [25], a feature thatcould make it useful for temperature estimations in systemsand processes where direct measurements are not possible.

Growing energy demands and shortage of resourcesrequire new approaches and methods for production ofhighly effective cokes. The latest is impossible without anunderstanding of fundamentals of coking and coke con-sumption processes, which includes detailed observation,documentation and careful interpretations of various phe-nomena that occur in materials taken from different stagesof these processes and comparable laboratory-scale exper-iments. Very little is known as yet about carbon behaviourin a coke after it is charged into a BF (such coke is oftenreferred to as BF coke), where it exposed to high tempera-ture and takes part in various solid-to-solid, solid-to-meltand solid-to-gas reactions that occur inside the BF. Thescarcity of data on BF coke in general and on the appear-ance of graphite in it in particular [26], is mostly due to theobvious difficulties encountered in obtaining samples frominside a working BF. This paper represents the descriptionof the mode of occurrence and morphological features ofgraphite crystals in sample of coke taken from inside aworking BF, from the tuyere zone, where air flows intothe BF and where temperature exceeds 2000 �C.

2. Materials and methods

The sample was selected from a drill core obtained from the tuyerezone of an operating BF at the Ruukki Steel Works in Raahe, Finland,using a mobile tuyere rig. The original location of the piece of BF cokewas 35 cm from the tuyere level. The details of the tuyere drilling were sim-ilar to those reported by Kerkkonen [27]. A piece about 30 mm long,23 mm wide and 5–7 mm thick was cut from the sample under dry condi-tions (no cooling water was used in sawing), preserving one original sur-face. The piece was then fixed to a glass plate of 28 · 48 mm that fitsinto the specimen holder of a scanning electron microscope (SEM). Afew particles were also selected from the sample surface for the prepara-tion of polished sections. The dry-cut section and the polished sectionswere examined preliminarily under a stereo microscope and optical micro-scope and then with a JEOL JSM-6400 SEM equipped with an energy dis-persive spectrometer (EDS) and operated with the INCA and SemAforeanalytical and image processing software packages.

Fig. 1. Appearance, size and morphology of various particles on thesurface of the BF coke. A and C – stereomicroscope; B – SEM Jeol JSM-6400. See text for details.

3. Results and discussion

Investigations of the dry-cut section with a stereomicro-scope have shown that the porous surface of the samplehosts numerous particles of different size, shape and opticalproperties (Figs. 1–4). There are relatively large segrega-tions of slag phases (qualitative EDS data), which weremet in a few locations (Fig. 1C). The most abundant parti-cles, which can be observed by stereomicroscope, are of0.1–0.4 mm in diameter (Fig. 1A). They have a roundedor drop-like shape and possess a dull metallic luster. Theparticles often have a sharp contact with the coke matrixand look ‘‘submersed’’ into the matrix (Fig. 1A). The sur-face of these particles has geometric facets bounded byridges of triangular cross section (Fig. 1B). These carbon

(EDS-data) facets look similar to the graphitic shells syn-thesized on top of millimeter-sized spherical transitionmetal cores in high vacuum [28]. Some of these particlesalso carry occasional graphite crystals on their surface.

In order to investigate the composition of such particles,a few of them (3–5 mm in size) were detached from thesample and used for the preparation of polished sections.EDS analyses (Table 1) have shown that they are com-posed of Si-bearing (4.01–7.30 wt.%) iron with traces ofMn, V and P. So, these rounded particles seem to be drops

Fig. 2. Appearance, size and morphology of graphite crystals. SEM Jeol JSM-6400. See text for details.

S.S. Gornostayev, J.J. Harkki / Carbon 45 (2007) 1145–1151 1147

of molten iron descending from the cohesive zone of the BFand they are external in respect of BF coke.

There are larger particles (2–5 mm) surrounded by a‘‘contact’’ zone (0.5–1.5 mm), which is full of graphite crys-tals. Such particles were observed in several separate places(spots) located a few millimetres apart in different parts ofthe sample. Two of them (Fig. 2A and B) located 9–10 mmapart are discussed below. These particles have no geomet-ric facets, but their surface in many places is covered by theinorganic shells containing Ca, Al and Si (qualitative EDSdata on an unpolished surface). A clear oxygen peak wasalso detected in the EDS spectrum. The particles also look

submersed into the coke matrix and host occasional graph-ite crystals on their surface (Fig. 2A and B).

The graphite in the contact zone occurs in foliated(Fig. 3C) intergrowths and rose-like aggregates (Fig. 2Aand B) of euhedral to subhedral crystals of varying sizeand morphology (Fig. 2C–K). The crystals themselves werefound on the coke matrix in close association with variousinorganic compounds, which were located in intersticesbetween the crystals (Fig. 5). X-ray mapping (Fig. 5) hasrevealed that the interstitial phase can be presented eitherby Fe (Fig. 5A) or by a complex Ca–Fe–Si–O phase(Fig. 5B). The later also contains traces of Al and K (qual-

Fig. 3. Twinning in graphite crystals. Fig. 4. Deformations in graphite crystals.

Table 1EDS analysis of some particles from the BF coke surfacea

Si P V Mn Fe Sum

1 4.74 – 0.56 0.81 94.55 100.662 7.30 0.32 1.24 0.58 89.79 99.223 4.01 – 0.35 0.70 94.81 99.88

a Note: JEOL JSM-6400. Particle index: 1 – 30424109-35b2; 2 –30424109-35b3; 3 – 30424109-35b4.


itative EDS data). One of the EDS analyses on an unpol-ished surface on the top of one particle (Fig. 2B) gavethe following concentrations (wt.%, normalized, carbonwas not measured): Fe – 97.07, Ca – 0.24, Si – 0.15, O –2.53. So, the particles surrounded by the reaction zonecomposed of graphite crystals, seem to be also dropletsof molten iron, but they were probably mixed with (cov-ered by) the spatially associated mineral compounds fromthe coke matrix.

There are also other small particles, which wereobserved attached to the coke surface and to the graphitecrystals. These particles can be seen under high magnifica-tions with a SEM (Figs. 2C–K, 3 and 4). Qualitative EDSanalyses has indicated that they composed of aluminosili-cate (±Ca, K and Na) phases. The nature and the compo-

sition of various inorganic compounds in the BF coke andon its surface have been also discussed in our earlier studies[6–9].

The generic forms that can be clearly seen in the crystalsfrom the contact zone are basal pinacoid {0001} (Fig. 2C–K; Fig. 3, Fig. 4A and B), hexagonal prism f10�10g

Fig. 5. X-ray maps for Fe (A: no elements other than Fe were detected) and Ca, Fe and Si (B) of two separate areas in graphite-bearing spots. SEM JeolJSM-6400. Scale bars: A – 40 lm; B – 100 lm.

Fig. 6. Generalized scheme for occurrence of graphitic coating on asurface of BF coke (cross-section). (1) A – rounded particles of Si-bearingiron covered by graphitic shells; B – particles of Si-bearing ironsurrounded by zones of graphite crystals; (2) graphite crystals and theiraggregates; (3) segregations of slag phases; (4) Internal pores (white) in acoke matrix (black).


(Fig. 2F–K) and hexagonal dipyramid f10�11g. The dipyr-amid can be seen in a twinned crystal (Fig. 3A), which isdiscussed below. The combination of basal pinacoid andhexagonal prism (Figs. 2C and 4A) seems to be more fre-quent than any other combination and it was observed incrystals of varying size (from 10 lm to 0.7 mm), althoughthe hexagonal prism in many crystals is not well developed(Fig. 2C). The crystals which combine basal pinacoid andhexagonal prism have a flake-like (Fig. 2C), platy(Fig. 4A) or tabular habit. The flake-like crystals, whichare about 0.5–0.7 mm in their greatest dimension, alongthe basal pinacoid, greatly exceed all the other forms insize. The combination of basal pinacoid and hexagonaldipyramid (Fig. 2D and E) is present in crystals which mea-sure about 50–70 lm along the (0001) plane. They have aplaty (Fig. 2D) or in rare cases an elongated-columnarhabit (Fig. 2E), which has been observed to be a rare mor-phology in natural graphite [13,16]. The combination ofbasal pinacoid, hexagonal prism and hexagonal dipyramidis found in crystals with a platy (Fig. 2F and G) or tabular(Fig. 2I–K) habit, while graphite crystals that have an equi-dimensional hexagonal prism in their cross-section(Fig. 2F–H) are much rarer than the distorted forms(Fig. 2C,E and I–K). In the latest forms some faces are sub-ordinate or even missing, reflecting less-than-ideal growthconditions.

It seems likely that the crystals of flake-like morphologywith undeveloped hexagonal prism (Fig. 2C) and their foli-ated intergrowths (Fig. 3C), which have larger surface areathan the columnar crystals (Fig. 2E), can form graphiticcover (‘‘shield’’) on a surface of BF coke. The fact that‘‘The carbon structure of coke. . . have a significant influ-ence on the coke behavior in a BF such that highly ordered

coke displayed lower reactivity. . .’’ [10] combined withthese data may suggest, that in the case of favourable con-ditions for graphite crystallization, the graphitic shield cancover larger area of surface of BF coke (Fig. 6) thus pre-venting it from reactions with gases circulating in a BF.


One consequence of the symmetry of the internal struc-ture of crystals is the possibility of formation of twins.Twinned crystals have two or more parts, in which thecrystal lattice of one part is differently but symmetricallyoriented to that of the next. The mechanisms by whichtwinned crystals can be formed include growth (for pene-tration and contact twins), transformation and deforma-tion [29]. The following types of twins have beenidentified in the graphite-bearing spots studied here: oneformed on fh0�h lg dypiramidal composition planes(Fig. 3A and B), yielding twinned crystals (contact twins)of size about 100 lm and 15–20 lm, respectively, and theother related to rotation on [0001] (Fig. 3C), producingcontact twins, as observed in large flake-like or plate-likecrystals.

It should be mentioned, that the intensity of coke graph-itization with temperature was found to depend on ironcontent of a coke [10]. The assessment of the mechanismfor the graphite crystallization in the BF coke based onthe experiments with the feed coke and almost 100% pureiron was made by Wang et al. [26] including data of otherinvestigators [30,31] on the molten iron and vanadium car-bide catalyzed graphitization process. It was summarizedthat ‘‘The iron melt at first penetrates the surrounding car-bon matrix through carbon dissolution–precipitationsequences leaving behind the well ordered graphitic car-bons. As the penetration front moves further into the car-bon the coke sample transforms into well ordered graphiticcarbon. Since the penetration of iron melt follows thegraphitization front, the precipitated small graphite crys-tals would be transformed in later recrystallization stepsinto large flakes’’ [26]. The last referred paper containsno details on the graphite morphology and has no discus-sion on possible influence of graphite on the processes inthe BF, but among many other interesting data it reportsthat the synthetic graphite has been formed in the feed cokesamples at temperatures ca. 1500 �C that is well below nor-mal graphitization temperatures due to the catalytic effectof molten iron.

It seems, that the model proposed for the graphite crys-tallization in the experiment [26], can be applied, at leastpartly (Fig. 5A, iron-catalyzed graphite crystallization),to the sample from the real process discussed in this study.Nevertheless, further research will be needed in order toexplain the reason for the graphite appearance near certainparticles of molten iron and lack of it around the others aswell as the graphite association with the Ca–Fe–Si–O phase(Fig. 5B). We suggest, that one of the possible explanationscould be related to the properties of the coke matrix them-selves, i.e. various maceral-derived components inheritedfrom the coking coal may have reacted differently withthe iron droplets and other inorganic compounds. The rea-son for the graphite appearance near the Ca–Fe–Si–Ophase (Fig. 5B) can be explained by the presence of Fe inthe phase and, probably, by the impurities of other metals,i.e., Ni and Mn, which also have catalytic effect for graph-ite crystallization [32]. These metals (Ni and Mn) were not

detected by the X-ray mapping (Fig. 5B) probably due tolow sensitivity of the method, but they are common constit-uents of the mineral phases of the BF coke samples fromthe tuyere drillings at Ruukki Steel Works analyzed byCAMECA SX-50 electron microprobe (WDS-method)[33].

Graphite has low modulus of elasticity, but its highlubricity [34] allows easy sliding of the graphene layersacross one another. For that reason, the graphitic coatingon a surface of BF coke can act as a lubricant to facilitatethe coke pieces sliding over each other thus affectingmechanical stability of the coke cone in a BF. Further-more, crystallization of graphite in a tuyere level can beaccompanied by the generation of coke-related fines in thisarea. This suggestion can be supported by data from exper-iments on coke annealing carried out in a horizontal labo-ratory-scale tube furnace, which established that ‘‘a higherordered carbon structure provided a greater amount offines. . .’’ [10], where the coke fines are attributed to the par-ticles (fractions from 63 to +450 lm were discussed) origi-nated from coke, which were found in the BF exhaust dust.

The major deformations found in the crystals aredetaching, bending and rolling of the (00 01) planes(Fig. 4A–C). Such deformations in graphite may be attrib-uted to exfoliation, and are considered to be the result of aphase transition involving vaporization of the intercalate inthe graphite [19,20]. The major intercalates in the BFgraphite were probably potassium and sodium, which areabundant in BF gases. The process of exfoliation has prob-ably caused the dismembering of the crystals to micro andnanosheets [20] and, then, the fragile graphite crystals mayhave been deformed by the BF gas flows. In some of thecrystals this concerns only tiny, submicron-thick sheets(Fig. 4A and B), as noted elsewhere [20], while in othersa whole graphite crystal can be dismembered into severalparts (yielding a stack of sheets) and then they can be bent(Fig. 4C). The dismembering of the crystals can be behindthe generation of the smallest fraction (comparable to thecrystal size) of fines, when particles of graphite, especiallythose of flake-like morphology detached from the largercrystals, can be easy captured by the circulating BF gasflows thus contributing the total amount of the BF-gener-ated dust.

4. Concluding remarks

The BF coke on tuyere level contains graphite crystals ofvarious size and morphologies. They occur in randomlydistributed spots and are associated with droplets of ironwith varying Si content and other inorganic compounds.Alkali, which circulate in a BF gases, may have causes sub-sequent exfoliation of the crystals enforced by the BF gasflows. The formation of graphite crystals on a surface ofBF coke, especially those of flake-like morphology withundeveloped hexagonal prism probably decreases its reac-tivity and can be accompanied by fines generation in atuyere level. The graphitic cover on a surface of BF coke


can act as a lubricant to facilitate the coke pieces to slidealong each other and thus affect mechanical stability ofcoke cone in a BF.

The data presented in this study can be useful for assess-ing coke behaviour in a BF. It will be essential to performfurther detailed investigations on graphite formation andits evolution in a coke as well as on the nature and behav-iour of associated inorganic compounds. This is needed forbetter understanding of major reactions in a BF and forrevealing typomorphic features of graphite crystals thatcan serve as an indicator of particular coke compositionand BF environment.

Acknowledgements

This research was funded by the Academy of Finland. Mr.T. Kokkonen is thanked for the samples preparation. Anon-ymous referees have provided valuable comments whichgreatly improved clarity and quality of the manuscript.

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Graphyt Crystals in Blast Furnace Coke

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