- Email: [email protected]
Graphite crystals in blast furnace coke
Carbon 45 (2007) 1145–1151 www.elsevier.com/locate/carbon
Graphite crystals in blast furnace coke Stanislav S. Gornostayev *, Jouko J. Ha¨rkki Laboratory of Process Metallurgy, University of Oulu, P.O. Box 4300, 90014 Oulu, Finland Received 15 November 2006; accepted 26 February 2007 Available 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 Sibearing iron and other inorganic compounds. It occurred in foliated intergrowths and rose-like aggregates of euhedral to subhedral crystals of various size (10 lm–0.7 mm) and morphologies, the generic forms that can be seen in the crystals being basal pinacoid, hexagonal prism and hexagonal dipyramid. The largest crystals have a flake-like habit and are represented by a combination of a basal pinacoid and a hexagonal prism. Graphite crystals that have an ideal hexagonal prism in their cross-section are rarer than distorted forms. There are twins on the dypiramidal composition planes and twins produced by rotation on [0 0 0 1]. The major deformations in the crystals, i.e. detaching, bending and rolling of (0 0 0 1) planes, may be referred to exfoliation phenomena caused by potassium and sodium from circulating BF gases and enforced by the BF gas flows. The formation of graphite crystals on a surface of BF coke, especially those of flakelike morphology with an undeveloped hexagonal prism, possibly decreases its reactivity and can be accompanied by the generation of fines. The graphitic coating on a surface of BF coke can act as a lubricant to facilitate coke pieces sliding over each other thus affecting the 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 reducing gases, including CO; (3) a carburisation agent for the hot metal, to give it the required properties, and (4) a structural support to provide permeability for liquid phase drainage and upward flow of the blast furnace gases [1]. It is charged into 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 mix to 1100 C in coke batteries (ovens) that consist of two heated walls spaced about 40–45 cm apart. Reactivity and strength are the most important parameters that characterize the quality of the coke, and these are in turn dependent on the amounts and properties of organic (maceralderived) carbon-based components [1–4] and inorganic matter (minerals) [5–9] inherited from the primary coals *
Corresponding author. Fax: +358 8 553 2339. E-mail address: [email protected].fi (S.S. Gornostayev).
0008-6223/$ - see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2007.02.033
and serve to determine the behaviour of the coke in the BF. The evolution of the carbon structure of coke at temperatures comparable to those in the upper parts of a BF have been discussed recently based on the results of experiments carried out in a horizontal laboratory-scale tube furnace [10]. The conditions of formation and subsequent degradation and deformation of graphite crystals can be reflected in their size and morphology, as documented in a number of studies of natural [11–20] and synthetic [21–23] graphite. It is mentioned that well formed natural and laboratorygrown graphite crystals are uncommon and that they rarely exceed a few millimetres in size [24]. Conventional graphite forms hexagonal (dihexagonal–dipyramidal class – 6/m2/ m2/m) crystals, with weak bonding between the graphite layers. The hexagonal nature of the crystal arises because graphite exists as a stack of ‘‘sheets’’ of carbon atoms parallel to {0 0 0 1}, each sheet having a hexagonal arrangement of atoms. Graphite crystals in the natural environment usually possess a scaly (flake-like), platy, tabular or columnar habit [11–20]. In addition to its unique
1146
S.S. Gornostayev, J.J. Ha¨rkki / Carbon 45 (2007) 1145–1151
technological properties, graphite is also known as one of the few single-mineral geothermometers [25], a feature that could make it useful for temperature estimations in systems and processes where direct measurements are not possible. Growing energy demands and shortage of resources require new approaches and methods for production of highly effective cokes. The latest is impossible without an understanding of fundamentals of coking and coke consumption processes, which includes detailed observation, documentation and careful interpretations of various phenomena that occur in materials taken from different stages of these processes and comparable laboratory-scale experiments. Very little is known as yet about carbon behaviour in a coke after it is charged into a BF (such coke is often referred to as BF coke), where it exposed to high temperature and takes part in various solid-to-solid, solid-to-melt and solid-to-gas reactions that occur inside the BF. The scarcity of data on BF coke in general and on the appearance of graphite in it in particular [26], is mostly due to the obvious difficulties encountered in obtaining samples from inside a working BF. This paper represents the description of the mode of occurrence and morphological features of graphite crystals in sample of coke taken from inside a working BF, from the tuyere zone, where air flows into the BF and where temperature exceeds 2000 C. 2. Materials and methods The sample was selected from a drill core obtained from the tuyere zone 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 coke was 35 cm from the tuyere level. The details of the tuyere drilling were similar 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 conditions (no cooling water was used in sawing), preserving one original surface. The piece was then fixed to a glass plate of 28 · 48 mm that fits into the specimen holder of a scanning electron microscope (SEM). A few particles were also selected from the sample surface for the preparation of polished sections. The dry-cut section and the polished sections were examined preliminarily under a stereo microscope and optical microscope and then with a JEOL JSM-6400 SEM equipped with an energy dispersive spectrometer (EDS) and operated with the INCA and SemAfore analytical and image processing software packages.
3. Results and discussion Investigations of the dry-cut section with a stereomicroscope have shown that the porous surface of the sample hosts numerous particles of different size, shape and optical properties (Figs. 1–4). There are relatively large segregations of slag phases (qualitative EDS data), which were met in a few locations (Fig. 1C). The most abundant particles, which can be observed by stereomicroscope, are of 0.1–0.4 mm in diameter (Fig. 1A). They have a rounded or drop-like shape and possess a dull metallic luster. The particles often have a sharp contact with the coke matrix and look ‘‘submersed’’ into the matrix (Fig. 1A). The surface of these particles has geometric facets bounded by ridges of triangular cross section (Fig. 1B). These carbon
Fig. 1. Appearance, size and morphology of various particles on the surface of the BF coke. A and C – stereomicroscope; B – SEM Jeol JSM6400. See text for details.
(EDS-data) facets look similar to the graphitic shells synthesized on top of millimeter-sized spherical transition metal cores in high vacuum [28]. Some of these particles also 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 the sample and used for the preparation of polished sections. EDS analyses (Table 1) have shown that they are composed of Si-bearing (4.01–7.30 wt.%) iron with traces of Mn, V and P. So, these rounded particles seem to be drops
S.S. Gornostayev, J.J. Ha¨rkki / Carbon 45 (2007) 1145–1151
1147
Fig. 2. Appearance, size and morphology of graphite crystals. SEM Jeol JSM-6400. See text for details.
of molten iron descending from the cohesive zone of the BF and 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 crystals. Such particles were observed in several separate places (spots) located a few millimetres apart in different parts of the sample. Two of them (Fig. 2A and B) located 9–10 mm apart are discussed below. These particles have no geometric facets, but their surface in many places is covered by the inorganic shells containing Ca, Al and Si (qualitative EDS data on an unpolished surface). A clear oxygen peak was also detected in the EDS spectrum. The particles also look
submersed into the coke matrix and host occasional graphite 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. 2A and B) of euhedral to subhedral crystals of varying size and morphology (Fig. 2C–K). The crystals themselves were found on the coke matrix in close association with various inorganic compounds, which were located in interstices between the crystals (Fig. 5). X-ray mapping (Fig. 5) has revealed that the interstitial phase can be presented either by Fe (Fig. 5A) or by a complex Ca–Fe–Si–O phase (Fig. 5B). The later also contains traces of Al and K (qual-
1148
S.S. Gornostayev, J.J. Ha¨rkki / Carbon 45 (2007) 1145–1151
Fig. 4. Deformations in graphite crystals.
Fig. 3. Twinning in graphite crystals.
itative EDS data). One of the EDS analyses on an unpolished surface on the top of one particle (Fig. 2B) gave the following concentrations (wt.%, normalized, carbon was not measured): Fe – 97.07, Ca – 0.24, Si – 0.15, O – 2.53. So, the particles surrounded by the reaction zone composed of graphite crystals, seem to be also droplets of molten iron, but they were probably mixed with (covered by) the spatially associated mineral compounds from the coke matrix. There are also other small particles, which were observed attached to the coke surface and to the graphite crystals. These particles can be seen under high magnifications with a SEM (Figs. 2C–K, 3 and 4). Qualitative EDS analyses has indicated that they composed of aluminosilicate (±Ca, K and Na) phases. The nature and the compo-
Table 1 EDS analysis of some particles from the BF coke surfacea
1 2 3
Si
P
V
Mn
Fe
Sum
4.74 7.30 4.01
– 0.32 –
0.56 1.24 0.35
0.81 0.58 0.70
94.55 89.79 94.81
100.66 99.22 99.88
a Note: JEOL JSM-6400. Particle index: 1 – 30424109-35b2; 2 – 30424109-35b3; 3 – 30424109-35b4.
sition of various inorganic compounds in the BF coke and on its surface have been also discussed in our earlier studies [6–9]. The generic forms that can be clearly seen in the crystals from the contact zone are basal pinacoid {0 0 0 1} (Fig. 2C– K; Fig. 3, Fig. 4A and B), hexagonal prism f1 0 1 0g
S.S. Gornostayev, J.J. Ha¨rkki / Carbon 45 (2007) 1145–1151
1149
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 Jeol JSM-6400. Scale bars: A – 40 lm; B – 100 lm.
(Fig. 2F–K) and hexagonal dipyramid f1 0 1 1g. The dipyramid can be seen in a twinned crystal (Fig. 3A), which is discussed below. The combination of basal pinacoid and hexagonal prism (Figs. 2C and 4A) seems to be more frequent than any other combination and it was observed in crystals of varying size (from 10 lm to 0.7 mm), although the hexagonal prism in many crystals is not well developed (Fig. 2C). The crystals which combine basal pinacoid and hexagonal prism have a flake-like (Fig. 2C), platy (Fig. 4A) or tabular habit. The flake-like crystals, which are about 0.5–0.7 mm in their greatest dimension, along the basal pinacoid, greatly exceed all the other forms in size. The combination of basal pinacoid and hexagonal dipyramid (Fig. 2D and E) is present in crystals which measure about 50–70 lm along the (0 0 0 1) plane. They have a platy (Fig. 2D) or in rare cases an elongated-columnar habit (Fig. 2E), which has been observed to be a rare morphology in natural graphite [13,16]. The combination of basal pinacoid, hexagonal prism and hexagonal dipyramid is found in crystals with a platy (Fig. 2F and G) or tabular (Fig. 2I–K) habit, while graphite crystals that have an equidimensional 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 subordinate or even missing, reflecting less-than-ideal growth conditions. It seems likely that the crystals of flake-like morphology with undeveloped hexagonal prism (Fig. 2C) and their foliated intergrowths (Fig. 3C), which have larger surface area than the columnar crystals (Fig. 2E), can form graphitic cover (‘‘shield’’) on a surface of BF coke. The fact that ‘‘The carbon structure of coke. . . have a significant influence on the coke behavior in a BF such that highly ordered
coke displayed lower reactivity. . .’’ [10] combined with these data may suggest, that in the case of favourable conditions for graphite crystallization, the graphitic shield can cover larger area of surface of BF coke (Fig. 6) thus preventing it from reactions with gases circulating in a BF.
Fig. 6. Generalized scheme for occurrence of graphitic coating on a surface of BF coke (cross-section). (1) A – rounded particles of Si-bearing iron covered by graphitic shells; B – particles of Si-bearing iron surrounded by zones of graphite crystals; (2) graphite crystals and their aggregates; (3) segregations of slag phases; (4) Internal pores (white) in a coke matrix (black).
1150
S.S. Gornostayev, J.J. Ha¨rkki / Carbon 45 (2007) 1145–1151
One consequence of the symmetry of the internal structure of crystals is the possibility of formation of twins. Twinned crystals have two or more parts, in which the crystal lattice of one part is differently but symmetrically oriented to that of the next. The mechanisms by which twinned crystals can be formed include growth (for penetration and contact twins), transformation and deformation [29]. The following types of twins have been identified in the graphite-bearing spots studied here: one formed on fh 0 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 the other related to rotation on [0 0 0 1] (Fig. 3C), producing contact twins, as observed in large flake-like or plate-like crystals. It should be mentioned, that the intensity of coke graphitization with temperature was found to depend on iron content of a coke [10]. The assessment of the mechanism for the graphite crystallization in the BF coke based on the experiments with the feed coke and almost 100% pure iron was made by Wang et al. [26] including data of other investigators [30,31] on the molten iron and vanadium carbide catalyzed graphitization process. It was summarized that ‘‘The iron melt at first penetrates the surrounding carbon matrix through carbon dissolution–precipitation sequences leaving behind the well ordered graphitic carbons. As the penetration front moves further into the carbon the coke sample transforms into well ordered graphitic carbon. Since the penetration of iron melt follows the graphitization front, the precipitated small graphite crystals would be transformed in later recrystallization steps into large flakes’’ [26]. The last referred paper contains no details on the graphite morphology and has no discussion on possible influence of graphite on the processes in the BF, but among many other interesting data it reports that the synthetic graphite has been formed in the feed coke samples at temperatures ca. 1500 C that is well below normal graphitization temperatures due to the catalytic effect of molten iron. It seems, that the model proposed for the graphite crystallization in the experiment [26], can be applied, at least partly (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 to explain the reason for the graphite appearance near certain particles of molten iron and lack of it around the others as well as the graphite association with the Ca–Fe–Si–O phase (Fig. 5B). We suggest, that one of the possible explanations could be related to the properties of the coke matrix themselves, i.e. various maceral-derived components inherited from the coking coal may have reacted differently with the iron droplets and other inorganic compounds. The reason for the graphite appearance near the Ca–Fe–Si–O phase (Fig. 5B) can be explained by the presence of Fe in the phase and, probably, by the impurities of other metals, i.e., Ni and Mn, which also have catalytic effect for graphite crystallization [32]. These metals (Ni and Mn) were not
detected by the X-ray mapping (Fig. 5B) probably due to low sensitivity of the method, but they are common constituents of the mineral phases of the BF coke samples from the tuyere drillings at Ruukki Steel Works analyzed by CAMECA SX-50 electron microprobe (WDS-method) [33]. Graphite has low modulus of elasticity, but its high lubricity [34] allows easy sliding of the graphene layers across one another. For that reason, the graphitic coating on a surface of BF coke can act as a lubricant to facilitate the coke pieces sliding over each other thus affecting mechanical stability of the coke cone in a BF. Furthermore, crystallization of graphite in a tuyere level can be accompanied by the generation of coke-related fines in this area. This suggestion can be supported by data from experiments on coke annealing carried out in a horizontal laboratory-scale tube furnace, which established that ‘‘a higher ordered carbon structure provided a greater amount of fines. . .’’ [10], where the coke fines are attributed to the particles (fractions from 63 to +450 lm were discussed) originated from coke, which were found in the BF exhaust dust. The major deformations found in the crystals are detaching, bending and rolling of the (0 0 0 1) planes (Fig. 4A–C). Such deformations in graphite may be attributed to exfoliation, and are considered to be the result of a phase transition involving vaporization of the intercalate in the graphite [19,20]. The major intercalates in the BF graphite were probably potassium and sodium, which are abundant in BF gases. The process of exfoliation has probably caused the dismembering of the crystals to micro and nanosheets [20] and, then, the fragile graphite crystals may have been deformed by the BF gas flows. In some of the crystals this concerns only tiny, submicron-thick sheets (Fig. 4A and B), as noted elsewhere [20], while in others a whole graphite crystal can be dismembered into several parts (yielding a stack of sheets) and then they can be bent (Fig. 4C). The dismembering of the crystals can be behind the generation of the smallest fraction (comparable to the crystal size) of fines, when particles of graphite, especially those of flake-like morphology detached from the larger crystals, can be easy captured by the circulating BF gas flows thus contributing the total amount of the BF-generated dust. 4. Concluding remarks The BF coke on tuyere level contains graphite crystals of various size and morphologies. They occur in randomly distributed spots and are associated with droplets of iron with varying Si content and other inorganic compounds. Alkali, which circulate in a BF gases, may have causes subsequent exfoliation of the crystals enforced by the BF gas flows. The formation of graphite crystals on a surface of BF coke, especially those of flake-like morphology with undeveloped hexagonal prism probably decreases its reactivity and can be accompanied by fines generation in a tuyere level. The graphitic cover on a surface of BF coke
S.S. Gornostayev, J.J. Ha¨rkki / Carbon 45 (2007) 1145–1151
can act as a lubricant to facilitate the coke pieces to slide along each other and thus affect mechanical stability of coke cone in a BF. The data presented in this study can be useful for assessing coke behaviour in a BF. It will be essential to perform further detailed investigations on graphite formation and its evolution in a coke as well as on the nature and behaviour of associated inorganic compounds. This is needed for better understanding of major reactions in a BF and for revealing typomorphic features of graphite crystals that can serve as an indicator of particular coke composition and BF environment. Acknowledgements This research was funded by the Academy of Finland. Mr. T. Kokkonen is thanked for the samples preparation. Anonymous referees have provided valuable comments which greatly improved clarity and quality of the manuscript. References [1] Andriopoulos N, Loo CE, Dukino R, McGuire SJ. Micro-properties of Australian coking coals. ISIJ Int 2003;3:1528–37. [2] Yamaoka H, Suyama S. Prediction model of coke strength after gasification reaction. ISIJ Int 2003;43:338–47. [3] Kawakami M, Karato T, Takenaka T, Yokoyama S. Structure analysis of coke, wood charcoal and bamboo charcoal by Raman spectroscopy and their reaction rate with CO2. ISIJ Int 2005;45:1027–34. [4] Sharma R, Dash PS, Banerjee PK, Kumar D. Effect of coke microtextural and coal petrographic properties on coke strength characteristics. ISIJ Int 2005;45:1820–7. [5] Todoschuk TW, Price JP, Gransden JF. Development of coke strength after reaction (CSR) at Dofasco. Iron Steel Technol 2004;March:73–84. [6] Gornostayev S, Ha¨rkki J. Spinel crystals in tuyere coke. Metall Mater Trans 2005;36B-2:303–5. [7] Gornostayev S, Ha¨rkki J. Mineral matter crystallization and crack formation in tuyere coke. Fuel 2006;85:1047–51. [8] Gornostayev SS, Kerkkonen O, Ha¨rkki JJ. Importance of mineralogical data for influencing properties of coke: a reference on SiO2 polymorphs. Steel Res Int 2006;77:770–3. [9] Gornostayev S, Ha¨rkki J. Mechanism of physical transformations of mineral matter in the blast furnace coke with reference to its reactivity and strength. Energy Fuels 2006;20:2632–5. [10] Gupta S, Sahajwalla V, Burgo J, Chaubal P, Youmans T. Carbon structure of coke at high temperatures and its influence on coke fines in blast furnace dust. Metall Mater Trans 2005;36B:385–94. [11] Palache C. Graphite. Am Miner 1941;26:709–17. [12] Freise EJ, Kelly A. Twinning in graphite. Proc R Soc London 1961;64-A:269–76.
1151
[13] Jaszczak JA. Famous graphite crystals from Sterling Hill, New Jersey. The Picking Table 1994;35:6–11. [14] Jaszczak JA. Unusual graphite crystals from the lime crest quarry, Sparta New Jersey. Rocks Miner 1997;72:330–4. [15] Luque FJ, Pasteris JD, Wopenka B, Rodas M, Barrenechea JF. Natural fluid-deposited graphite: mineralogical characteristics and mechanisms of formation. Am J Sci 1998;298:471–98. [16] Kvasnitsa VN, Yatsenko VG, Jaszczak JA. Disclinations in unusual graphite crystals from anorthosites of Ukraine. Can Mineral 1999;37:951–60. [17] Rakovan J, Jaszczak JA. Multiple length scale growth spirals on metamorphic graphite {0 0 1} surfaces studied by atomic force microscopy. Am Miner 2002;87:17–24. [18] Jaszczak JA, Robinson GW, Dimovski S, Gogotsi Y. Naturally occurring graphite cones. Carbon 2003;41:2085–92. [19] Anderson SH, Chung DDL. Exfoliation of intercalated graphite. Carbon 1984;22:253–63. [20] Chen G, Wu D, Weng W, Wu C. Exfoliation of graphite flake and its nanocomposites. Carbon 2003;41:579–625. [21] Gogotsi Y, Dimovski S, Libera JA. Conical crystals of graphite. Carbon 2002;40:2263–7. [22] Dimovski S, Nikitin A, Ye H, Gogotsi Y. Synthesis of graphite by chlorination of iron carbide at moderate temperatures. J Mater Chem 2004;14:238–43. [23] Okuno H, Palnichenko A, Despres J-F, Issi J-P, Charlier J-C. Synthesis of graphite polyhedral crystals using a combustion flame method. Carbon 2005;43:692–7. [24] Rotkin S, Gogotsi Y. Analysis of non-planar graphitic structures: from arched edge planes of graphite crystals to nanotubes. Mat Res Innov 2002;5:191–200. [25] Luque FJ, Barrenechea JF, Rodas M. Graphite geothermometry in low and high temperature regimes; two case studies. Geol Mag 1993;130:501–11. [26] Wang W, Thomas KM, Poultney RM, Willmers RR. Iron catalysed graphitisation in the blast furnace. Carbon 1995;33:1525–35. [27] Kerkkonen O. Tuyere Drilling Coke Sample Data from Rautaruukki’s Blast Furnaces No. 1 and 2. AISTech 2004; Iron & Steel Technology Conference Proceedings, Nashville, TN, USA, September 15–17, 2004;I:469–81. [28] Winder SM, Liu D, Bender JW. Synthesis and characterization of compound-curved graphite. Carbon 2006;44:3037–42. [29] Nesse WD. Introduction to mineralogy. New York: Oxford University Press; 2000. p. 442. [30] Fitzer E, Kegel B. The reaction of vanadium carbide melt with ordered and disordered carbon. Carbon 1968;6:433–6. [31] Weisweiler W, Subramanian N, Terwiesch B. Catalytic influence of metal melts on the graphitization of monolithic glasslike carbon. Carbon 1971;9:755–8. [32] Sevilla M, Fuertes AB. Catalytic graphitization of templated mesoporous carbons. Carbon 2006;44:468–74. [33] Gornostayev S, Kerkkonen O, Ha¨rkki J. Occurrence and composition of some mineral phases in the tuyere coke. ISIJ Int 2005;45:1–7. [34] Dienwiebel M, Verhoeven GS, Pradeep N, Frenken JWM, Heimberg JA, Zandbergen HW. Superlubricity of graphite. Phys Rev Lett 2004;92:126101-1–4.
Graphite crystals in blast furnace coke Stanislav S. Gornostayev *, Jouko J. Ha¨rkki Laboratory of Process Metallurgy, University of Oulu, P.O. Box 4300, 90014 Oulu, Finland Received 15 November 2006; accepted 26 February 2007 Available 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 Sibearing iron and other inorganic compounds. It occurred in foliated intergrowths and rose-like aggregates of euhedral to subhedral crystals of various size (10 lm–0.7 mm) and morphologies, the generic forms that can be seen in the crystals being basal pinacoid, hexagonal prism and hexagonal dipyramid. The largest crystals have a flake-like habit and are represented by a combination of a basal pinacoid and a hexagonal prism. Graphite crystals that have an ideal hexagonal prism in their cross-section are rarer than distorted forms. There are twins on the dypiramidal composition planes and twins produced by rotation on [0 0 0 1]. The major deformations in the crystals, i.e. detaching, bending and rolling of (0 0 0 1) planes, may be referred to exfoliation phenomena caused by potassium and sodium from circulating BF gases and enforced by the BF gas flows. The formation of graphite crystals on a surface of BF coke, especially those of flakelike morphology with an undeveloped hexagonal prism, possibly decreases its reactivity and can be accompanied by the generation of fines. The graphitic coating on a surface of BF coke can act as a lubricant to facilitate coke pieces sliding over each other thus affecting the 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 reducing gases, including CO; (3) a carburisation agent for the hot metal, to give it the required properties, and (4) a structural support to provide permeability for liquid phase drainage and upward flow of the blast furnace gases [1]. It is charged into 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 mix to 1100 C in coke batteries (ovens) that consist of two heated walls spaced about 40–45 cm apart. Reactivity and strength are the most important parameters that characterize the quality of the coke, and these are in turn dependent on the amounts and properties of organic (maceralderived) carbon-based components [1–4] and inorganic matter (minerals) [5–9] inherited from the primary coals *
Corresponding author. Fax: +358 8 553 2339. E-mail address: [email protected].fi (S.S. Gornostayev).
0008-6223/$ - see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2007.02.033
and serve to determine the behaviour of the coke in the BF. The evolution of the carbon structure of coke at temperatures comparable to those in the upper parts of a BF have been discussed recently based on the results of experiments carried out in a horizontal laboratory-scale tube furnace [10]. The conditions of formation and subsequent degradation and deformation of graphite crystals can be reflected in their size and morphology, as documented in a number of studies of natural [11–20] and synthetic [21–23] graphite. It is mentioned that well formed natural and laboratorygrown graphite crystals are uncommon and that they rarely exceed a few millimetres in size [24]. Conventional graphite forms hexagonal (dihexagonal–dipyramidal class – 6/m2/ m2/m) crystals, with weak bonding between the graphite layers. The hexagonal nature of the crystal arises because graphite exists as a stack of ‘‘sheets’’ of carbon atoms parallel to {0 0 0 1}, each sheet having a hexagonal arrangement of atoms. Graphite crystals in the natural environment usually possess a scaly (flake-like), platy, tabular or columnar habit [11–20]. In addition to its unique
1146
S.S. Gornostayev, J.J. Ha¨rkki / Carbon 45 (2007) 1145–1151
technological properties, graphite is also known as one of the few single-mineral geothermometers [25], a feature that could make it useful for temperature estimations in systems and processes where direct measurements are not possible. Growing energy demands and shortage of resources require new approaches and methods for production of highly effective cokes. The latest is impossible without an understanding of fundamentals of coking and coke consumption processes, which includes detailed observation, documentation and careful interpretations of various phenomena that occur in materials taken from different stages of these processes and comparable laboratory-scale experiments. Very little is known as yet about carbon behaviour in a coke after it is charged into a BF (such coke is often referred to as BF coke), where it exposed to high temperature and takes part in various solid-to-solid, solid-to-melt and solid-to-gas reactions that occur inside the BF. The scarcity of data on BF coke in general and on the appearance of graphite in it in particular [26], is mostly due to the obvious difficulties encountered in obtaining samples from inside a working BF. This paper represents the description of the mode of occurrence and morphological features of graphite crystals in sample of coke taken from inside a working BF, from the tuyere zone, where air flows into the BF and where temperature exceeds 2000 C. 2. Materials and methods The sample was selected from a drill core obtained from the tuyere zone 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 coke was 35 cm from the tuyere level. The details of the tuyere drilling were similar 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 conditions (no cooling water was used in sawing), preserving one original surface. The piece was then fixed to a glass plate of 28 · 48 mm that fits into the specimen holder of a scanning electron microscope (SEM). A few particles were also selected from the sample surface for the preparation of polished sections. The dry-cut section and the polished sections were examined preliminarily under a stereo microscope and optical microscope and then with a JEOL JSM-6400 SEM equipped with an energy dispersive spectrometer (EDS) and operated with the INCA and SemAfore analytical and image processing software packages.
3. Results and discussion Investigations of the dry-cut section with a stereomicroscope have shown that the porous surface of the sample hosts numerous particles of different size, shape and optical properties (Figs. 1–4). There are relatively large segregations of slag phases (qualitative EDS data), which were met in a few locations (Fig. 1C). The most abundant particles, which can be observed by stereomicroscope, are of 0.1–0.4 mm in diameter (Fig. 1A). They have a rounded or drop-like shape and possess a dull metallic luster. The particles often have a sharp contact with the coke matrix and look ‘‘submersed’’ into the matrix (Fig. 1A). The surface of these particles has geometric facets bounded by ridges of triangular cross section (Fig. 1B). These carbon
Fig. 1. Appearance, size and morphology of various particles on the surface of the BF coke. A and C – stereomicroscope; B – SEM Jeol JSM6400. See text for details.
(EDS-data) facets look similar to the graphitic shells synthesized on top of millimeter-sized spherical transition metal cores in high vacuum [28]. Some of these particles also 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 the sample and used for the preparation of polished sections. EDS analyses (Table 1) have shown that they are composed of Si-bearing (4.01–7.30 wt.%) iron with traces of Mn, V and P. So, these rounded particles seem to be drops
S.S. Gornostayev, J.J. Ha¨rkki / Carbon 45 (2007) 1145–1151
1147
Fig. 2. Appearance, size and morphology of graphite crystals. SEM Jeol JSM-6400. See text for details.
of molten iron descending from the cohesive zone of the BF and 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 crystals. Such particles were observed in several separate places (spots) located a few millimetres apart in different parts of the sample. Two of them (Fig. 2A and B) located 9–10 mm apart are discussed below. These particles have no geometric facets, but their surface in many places is covered by the inorganic shells containing Ca, Al and Si (qualitative EDS data on an unpolished surface). A clear oxygen peak was also detected in the EDS spectrum. The particles also look
submersed into the coke matrix and host occasional graphite 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. 2A and B) of euhedral to subhedral crystals of varying size and morphology (Fig. 2C–K). The crystals themselves were found on the coke matrix in close association with various inorganic compounds, which were located in interstices between the crystals (Fig. 5). X-ray mapping (Fig. 5) has revealed that the interstitial phase can be presented either by Fe (Fig. 5A) or by a complex Ca–Fe–Si–O phase (Fig. 5B). The later also contains traces of Al and K (qual-
1148
S.S. Gornostayev, J.J. Ha¨rkki / Carbon 45 (2007) 1145–1151
Fig. 4. Deformations in graphite crystals.
Fig. 3. Twinning in graphite crystals.
itative EDS data). One of the EDS analyses on an unpolished surface on the top of one particle (Fig. 2B) gave the following concentrations (wt.%, normalized, carbon was not measured): Fe – 97.07, Ca – 0.24, Si – 0.15, O – 2.53. So, the particles surrounded by the reaction zone composed of graphite crystals, seem to be also droplets of molten iron, but they were probably mixed with (covered by) the spatially associated mineral compounds from the coke matrix. There are also other small particles, which were observed attached to the coke surface and to the graphite crystals. These particles can be seen under high magnifications with a SEM (Figs. 2C–K, 3 and 4). Qualitative EDS analyses has indicated that they composed of aluminosilicate (±Ca, K and Na) phases. The nature and the compo-
Table 1 EDS analysis of some particles from the BF coke surfacea
1 2 3
Si
P
V
Mn
Fe
Sum
4.74 7.30 4.01
– 0.32 –
0.56 1.24 0.35
0.81 0.58 0.70
94.55 89.79 94.81
100.66 99.22 99.88
a Note: JEOL JSM-6400. Particle index: 1 – 30424109-35b2; 2 – 30424109-35b3; 3 – 30424109-35b4.
sition of various inorganic compounds in the BF coke and on its surface have been also discussed in our earlier studies [6–9]. The generic forms that can be clearly seen in the crystals from the contact zone are basal pinacoid {0 0 0 1} (Fig. 2C– K; Fig. 3, Fig. 4A and B), hexagonal prism f1 0 1 0g
S.S. Gornostayev, J.J. Ha¨rkki / Carbon 45 (2007) 1145–1151
1149
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 Jeol JSM-6400. Scale bars: A – 40 lm; B – 100 lm.
(Fig. 2F–K) and hexagonal dipyramid f1 0 1 1g. The dipyramid can be seen in a twinned crystal (Fig. 3A), which is discussed below. The combination of basal pinacoid and hexagonal prism (Figs. 2C and 4A) seems to be more frequent than any other combination and it was observed in crystals of varying size (from 10 lm to 0.7 mm), although the hexagonal prism in many crystals is not well developed (Fig. 2C). The crystals which combine basal pinacoid and hexagonal prism have a flake-like (Fig. 2C), platy (Fig. 4A) or tabular habit. The flake-like crystals, which are about 0.5–0.7 mm in their greatest dimension, along the basal pinacoid, greatly exceed all the other forms in size. The combination of basal pinacoid and hexagonal dipyramid (Fig. 2D and E) is present in crystals which measure about 50–70 lm along the (0 0 0 1) plane. They have a platy (Fig. 2D) or in rare cases an elongated-columnar habit (Fig. 2E), which has been observed to be a rare morphology in natural graphite [13,16]. The combination of basal pinacoid, hexagonal prism and hexagonal dipyramid is found in crystals with a platy (Fig. 2F and G) or tabular (Fig. 2I–K) habit, while graphite crystals that have an equidimensional 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 subordinate or even missing, reflecting less-than-ideal growth conditions. It seems likely that the crystals of flake-like morphology with undeveloped hexagonal prism (Fig. 2C) and their foliated intergrowths (Fig. 3C), which have larger surface area than the columnar crystals (Fig. 2E), can form graphitic cover (‘‘shield’’) on a surface of BF coke. The fact that ‘‘The carbon structure of coke. . . have a significant influence on the coke behavior in a BF such that highly ordered
coke displayed lower reactivity. . .’’ [10] combined with these data may suggest, that in the case of favourable conditions for graphite crystallization, the graphitic shield can cover larger area of surface of BF coke (Fig. 6) thus preventing it from reactions with gases circulating in a BF.
Fig. 6. Generalized scheme for occurrence of graphitic coating on a surface of BF coke (cross-section). (1) A – rounded particles of Si-bearing iron covered by graphitic shells; B – particles of Si-bearing iron surrounded by zones of graphite crystals; (2) graphite crystals and their aggregates; (3) segregations of slag phases; (4) Internal pores (white) in a coke matrix (black).
1150
S.S. Gornostayev, J.J. Ha¨rkki / Carbon 45 (2007) 1145–1151
One consequence of the symmetry of the internal structure of crystals is the possibility of formation of twins. Twinned crystals have two or more parts, in which the crystal lattice of one part is differently but symmetrically oriented to that of the next. The mechanisms by which twinned crystals can be formed include growth (for penetration and contact twins), transformation and deformation [29]. The following types of twins have been identified in the graphite-bearing spots studied here: one formed on fh 0 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 the other related to rotation on [0 0 0 1] (Fig. 3C), producing contact twins, as observed in large flake-like or plate-like crystals. It should be mentioned, that the intensity of coke graphitization with temperature was found to depend on iron content of a coke [10]. The assessment of the mechanism for the graphite crystallization in the BF coke based on the experiments with the feed coke and almost 100% pure iron was made by Wang et al. [26] including data of other investigators [30,31] on the molten iron and vanadium carbide catalyzed graphitization process. It was summarized that ‘‘The iron melt at first penetrates the surrounding carbon matrix through carbon dissolution–precipitation sequences leaving behind the well ordered graphitic carbons. As the penetration front moves further into the carbon the coke sample transforms into well ordered graphitic carbon. Since the penetration of iron melt follows the graphitization front, the precipitated small graphite crystals would be transformed in later recrystallization steps into large flakes’’ [26]. The last referred paper contains no details on the graphite morphology and has no discussion on possible influence of graphite on the processes in the BF, but among many other interesting data it reports that the synthetic graphite has been formed in the feed coke samples at temperatures ca. 1500 C that is well below normal graphitization temperatures due to the catalytic effect of molten iron. It seems, that the model proposed for the graphite crystallization in the experiment [26], can be applied, at least partly (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 to explain the reason for the graphite appearance near certain particles of molten iron and lack of it around the others as well as the graphite association with the Ca–Fe–Si–O phase (Fig. 5B). We suggest, that one of the possible explanations could be related to the properties of the coke matrix themselves, i.e. various maceral-derived components inherited from the coking coal may have reacted differently with the iron droplets and other inorganic compounds. The reason for the graphite appearance near the Ca–Fe–Si–O phase (Fig. 5B) can be explained by the presence of Fe in the phase and, probably, by the impurities of other metals, i.e., Ni and Mn, which also have catalytic effect for graphite crystallization [32]. These metals (Ni and Mn) were not
detected by the X-ray mapping (Fig. 5B) probably due to low sensitivity of the method, but they are common constituents of the mineral phases of the BF coke samples from the tuyere drillings at Ruukki Steel Works analyzed by CAMECA SX-50 electron microprobe (WDS-method) [33]. Graphite has low modulus of elasticity, but its high lubricity [34] allows easy sliding of the graphene layers across one another. For that reason, the graphitic coating on a surface of BF coke can act as a lubricant to facilitate the coke pieces sliding over each other thus affecting mechanical stability of the coke cone in a BF. Furthermore, crystallization of graphite in a tuyere level can be accompanied by the generation of coke-related fines in this area. This suggestion can be supported by data from experiments on coke annealing carried out in a horizontal laboratory-scale tube furnace, which established that ‘‘a higher ordered carbon structure provided a greater amount of fines. . .’’ [10], where the coke fines are attributed to the particles (fractions from 63 to +450 lm were discussed) originated from coke, which were found in the BF exhaust dust. The major deformations found in the crystals are detaching, bending and rolling of the (0 0 0 1) planes (Fig. 4A–C). Such deformations in graphite may be attributed to exfoliation, and are considered to be the result of a phase transition involving vaporization of the intercalate in the graphite [19,20]. The major intercalates in the BF graphite were probably potassium and sodium, which are abundant in BF gases. The process of exfoliation has probably caused the dismembering of the crystals to micro and nanosheets [20] and, then, the fragile graphite crystals may have been deformed by the BF gas flows. In some of the crystals this concerns only tiny, submicron-thick sheets (Fig. 4A and B), as noted elsewhere [20], while in others a whole graphite crystal can be dismembered into several parts (yielding a stack of sheets) and then they can be bent (Fig. 4C). The dismembering of the crystals can be behind the generation of the smallest fraction (comparable to the crystal size) of fines, when particles of graphite, especially those of flake-like morphology detached from the larger crystals, can be easy captured by the circulating BF gas flows thus contributing the total amount of the BF-generated dust. 4. Concluding remarks The BF coke on tuyere level contains graphite crystals of various size and morphologies. They occur in randomly distributed spots and are associated with droplets of iron with varying Si content and other inorganic compounds. Alkali, which circulate in a BF gases, may have causes subsequent exfoliation of the crystals enforced by the BF gas flows. The formation of graphite crystals on a surface of BF coke, especially those of flake-like morphology with undeveloped hexagonal prism probably decreases its reactivity and can be accompanied by fines generation in a tuyere level. The graphitic cover on a surface of BF coke
S.S. Gornostayev, J.J. Ha¨rkki / Carbon 45 (2007) 1145–1151
can act as a lubricant to facilitate the coke pieces to slide along each other and thus affect mechanical stability of coke cone in a BF. The data presented in this study can be useful for assessing coke behaviour in a BF. It will be essential to perform further detailed investigations on graphite formation and its evolution in a coke as well as on the nature and behaviour of associated inorganic compounds. This is needed for better understanding of major reactions in a BF and for revealing typomorphic features of graphite crystals that can serve as an indicator of particular coke composition and BF environment. Acknowledgements This research was funded by the Academy of Finland. Mr. T. Kokkonen is thanked for the samples preparation. Anonymous referees have provided valuable comments which greatly improved clarity and quality of the manuscript. References [1] Andriopoulos N, Loo CE, Dukino R, McGuire SJ. Micro-properties of Australian coking coals. ISIJ Int 2003;3:1528–37. [2] Yamaoka H, Suyama S. Prediction model of coke strength after gasification reaction. ISIJ Int 2003;43:338–47. [3] Kawakami M, Karato T, Takenaka T, Yokoyama S. Structure analysis of coke, wood charcoal and bamboo charcoal by Raman spectroscopy and their reaction rate with CO2. ISIJ Int 2005;45:1027–34. [4] Sharma R, Dash PS, Banerjee PK, Kumar D. Effect of coke microtextural and coal petrographic properties on coke strength characteristics. ISIJ Int 2005;45:1820–7. [5] Todoschuk TW, Price JP, Gransden JF. Development of coke strength after reaction (CSR) at Dofasco. Iron Steel Technol 2004;March:73–84. [6] Gornostayev S, Ha¨rkki J. Spinel crystals in tuyere coke. Metall Mater Trans 2005;36B-2:303–5. [7] Gornostayev S, Ha¨rkki J. Mineral matter crystallization and crack formation in tuyere coke. Fuel 2006;85:1047–51. [8] Gornostayev SS, Kerkkonen O, Ha¨rkki JJ. Importance of mineralogical data for influencing properties of coke: a reference on SiO2 polymorphs. Steel Res Int 2006;77:770–3. [9] Gornostayev S, Ha¨rkki J. Mechanism of physical transformations of mineral matter in the blast furnace coke with reference to its reactivity and strength. Energy Fuels 2006;20:2632–5. [10] Gupta S, Sahajwalla V, Burgo J, Chaubal P, Youmans T. Carbon structure of coke at high temperatures and its influence on coke fines in blast furnace dust. Metall Mater Trans 2005;36B:385–94. [11] Palache C. Graphite. Am Miner 1941;26:709–17. [12] Freise EJ, Kelly A. Twinning in graphite. Proc R Soc London 1961;64-A:269–76.
1151
[13] Jaszczak JA. Famous graphite crystals from Sterling Hill, New Jersey. The Picking Table 1994;35:6–11. [14] Jaszczak JA. Unusual graphite crystals from the lime crest quarry, Sparta New Jersey. Rocks Miner 1997;72:330–4. [15] Luque FJ, Pasteris JD, Wopenka B, Rodas M, Barrenechea JF. Natural fluid-deposited graphite: mineralogical characteristics and mechanisms of formation. Am J Sci 1998;298:471–98. [16] Kvasnitsa VN, Yatsenko VG, Jaszczak JA. Disclinations in unusual graphite crystals from anorthosites of Ukraine. Can Mineral 1999;37:951–60. [17] Rakovan J, Jaszczak JA. Multiple length scale growth spirals on metamorphic graphite {0 0 1} surfaces studied by atomic force microscopy. Am Miner 2002;87:17–24. [18] Jaszczak JA, Robinson GW, Dimovski S, Gogotsi Y. Naturally occurring graphite cones. Carbon 2003;41:2085–92. [19] Anderson SH, Chung DDL. Exfoliation of intercalated graphite. Carbon 1984;22:253–63. [20] Chen G, Wu D, Weng W, Wu C. Exfoliation of graphite flake and its nanocomposites. Carbon 2003;41:579–625. [21] Gogotsi Y, Dimovski S, Libera JA. Conical crystals of graphite. Carbon 2002;40:2263–7. [22] Dimovski S, Nikitin A, Ye H, Gogotsi Y. Synthesis of graphite by chlorination of iron carbide at moderate temperatures. J Mater Chem 2004;14:238–43. [23] Okuno H, Palnichenko A, Despres J-F, Issi J-P, Charlier J-C. Synthesis of graphite polyhedral crystals using a combustion flame method. Carbon 2005;43:692–7. [24] Rotkin S, Gogotsi Y. Analysis of non-planar graphitic structures: from arched edge planes of graphite crystals to nanotubes. Mat Res Innov 2002;5:191–200. [25] Luque FJ, Barrenechea JF, Rodas M. Graphite geothermometry in low and high temperature regimes; two case studies. Geol Mag 1993;130:501–11. [26] Wang W, Thomas KM, Poultney RM, Willmers RR. Iron catalysed graphitisation in the blast furnace. Carbon 1995;33:1525–35. [27] Kerkkonen O. Tuyere Drilling Coke Sample Data from Rautaruukki’s Blast Furnaces No. 1 and 2. AISTech 2004; Iron & Steel Technology Conference Proceedings, Nashville, TN, USA, September 15–17, 2004;I:469–81. [28] Winder SM, Liu D, Bender JW. Synthesis and characterization of compound-curved graphite. Carbon 2006;44:3037–42. [29] Nesse WD. Introduction to mineralogy. New York: Oxford University Press; 2000. p. 442. [30] Fitzer E, Kegel B. The reaction of vanadium carbide melt with ordered and disordered carbon. Carbon 1968;6:433–6. [31] Weisweiler W, Subramanian N, Terwiesch B. Catalytic influence of metal melts on the graphitization of monolithic glasslike carbon. Carbon 1971;9:755–8. [32] Sevilla M, Fuertes AB. Catalytic graphitization of templated mesoporous carbons. Carbon 2006;44:468–74. [33] Gornostayev S, Kerkkonen O, Ha¨rkki J. Occurrence and composition of some mineral phases in the tuyere coke. ISIJ Int 2005;45:1–7. [34] Dienwiebel M, Verhoeven GS, Pradeep N, Frenken JWM, Heimberg JA, Zandbergen HW. Superlubricity of graphite. Phys Rev Lett 2004;92:126101-1–4.