iron catalysed graphitisation in the blast furnace

Pergamon

0008-6223 (95) 00097-6

Carbon Vol. 33, No. 11, pp. 1525-1535,1995 Copyright 0 1995 Elsevier Science Ltd

Printed in Great Britain. All rights reserved 000%6223/95 $9.50 + 0.00

IRON CATALYSED GRAPHITISATION IN THE BLAST FURNACE

W. WANG a K. M. THOMAS,*’ R. M. POULTNEY~ and R. R. WILLMERS* “Northern Carbon Reiearch Laboratories, Department of Chemistry, Bedson Building, The University of

Newcastle upon Tyne, Newcastle upon Tyne NE1 7RU, U.K. bBritish Steel Technical, Teesside Laboratories, P.O. Box 11, Grangetown, Middlesbrough, Cleveland

TS6 6UB, U.K.

(Received 4 November 1994; accepted in revisedform 18 March 1995)

Abstract-The paper describes the study of iron catalysed graphitisation in a set of coke samples extracted from a blast furnace, by means of optical microscopy, X-ray diffraction (XRD) and Raman microprobe techniques. The direct link obtained between optical microscopy and Raman spectroscopy in the Raman microprobe proved useful in linking morphology to molecular structure. The results show that catalytic graphitisation takes place at temperatures well below the temperature where graphitisation would normally be expected to occur. The mechanism of graphitisation probably involves dissolution and precipitation processes. The most noticeable spectral difference between the graphite produced by the catalytic process and the coke is the resolution of the second order Raman G’ band of the former into an overlapping doublet, whereas the latter only has a single symmetric peak. These spectral changes are associated with the decrease in the dooz values and the development of graphitic structure. The results suggest that temperature, amount of iron in contact with the coke and residence time of the coke in the hottest regions of the furnace are important factors in the catalytic graphitisation process. The use of XRD and Raman spectroscopy for the characterisation of the thermal history of blast furnace coke samples is discussed.

Key Words-Coke, catalytic graphitisation, raman microprobe, blast furnace.

1. INTRODUCTION

It is well established that certain metals have a catalytic influence on the rate and degree of graphitisation of carbon materials. There have been numerous studies[ l-lo] on catalytic graphitisation covering topics such as the mechanisms of catalytic graphitisation, the role of temperature, types of carbon raw material, type and amount of catalyst added, as well as the effects of the particle size of the catalysts. Previous studies [ 4,5] have identified four main types of catalytic graphitisation process. These processes are listed as: (1) the formation of graphitic carbon (G effect) through the dissolution of carbon into the catalyst followed by precipitation as graphite. The formation of the G component could also be achieved through the formation and decomposition of carbide intermediates. (2) The T, effect is the formation of more ordered turbostratic carbon from a non- graphitising parent carbon by a finely-divided catalyst. (3) A more homogeneous catalytic graphitisation occurs if the parent carbon is heated with a very finely divided catalyst such as vaporised metals. (4) The reaction of metal vapours such as Ca and Mg with charcoal can also lead to the formation of a large amount of graphitisable carbon (7’,, component).

The catalytic effect of iron on the graphitisation of carbon is well documented[9-161. The formation of graphite crystals from melting iron has been demonstrated by a number of investigators[9-141.

*To whom all correspondence should be addressed.

This is believed to involve the super-saturation of amorphous carbon in iron, which on cooling crystal- lises to form graphite. Yamada[14] heat-treated a mixture of iron with a carbon and the development of the two (002) X-ray diffraction peaks was monitored. The sharper peak (at higher scattering angle) developed as a result of graphitisation while the other peak was attributed to the carbon raw materials. There was also evidence for catalytic graphitisation proceeding via the formation and decomposition of the iron carbide cementite. Acheson [ 151 showed that heating iron or one of its compounds with amorphous carbon led to the formation of cementite (Fe&), which decomposed on further heating to give graphite. Oberlin et ~I.[161 found that heating a non-graphitising carbon in the presence of iron transformed it into a mixture of graphite and a graphitisable phase. It was believed that iron acted to form an unstable phase (presumably carbide), which then decomposed to give iron globules and a shell of graphitisable carbon surrounding the iron.

The structure of carbonaceous materials depends on the nature of the precursor material and the thermal history. The transformation of carbon structure with heat treatment may be monitored by employing various techniques, such as X-ray diffraction, electron microscopy, optical microscopy and Raman spectroscopy, depending on the heat treatment temperature. Raman microprobe spectroscopy is a technique which combines an optical microscope and a Raman spectrometer allowing the Raman spectra to be obtained with high spatial resolution

1525

1526 W. WANG et al.

(- 1 pm). Hence structural features identified by optical microscopy can be characterised in terms of their molecular structure using Raman spectroscopy. This provides a direct link between morphology and structure. The Raman spectrum of crystalline graphite consists of a single strong first order line at - 1580 cm-i (designated as the G band) due to the E2, vibrational mode and a second order doublet G; and G; at -2695 cm-’ and - 2735 cm-i plus weak bands at - 2440 cm-’ and -3250cm-‘. For imperfect graphite and disordered carbons, additional lines appear at - 1360 cm-i (D), - 1620 cm-’ (D') and sometimes at -2960 cm-i (D") as a result of the relaxation of symmetry selection rules. The second order Raman bands at - 2695 cm- ’ and - 2735 cm i merge into a single symmetrical G’ band at - 2710 cm-‘, which is considerably broadened with increasing structural disorder [ 17-231. It is known that the first order Raman spectrum is especi- ally sensitive to the extent of the two dimensional graphitic ordering. This structural change may be monitored by the variation in the frequency and width of the Raman bands and by measuring the ratio of the intensities of the 1360cm-’ (D) and 1580 cm-’ (G) lines. The second order Raman spectrum is most sensitive to the graphitisation process, which gives rise to the three dimensional ordering of hexagonal graphite. The final stages of the graphitisation process are characterised by the resolution of the second order Raman line G’ at 2710 cm-’ into a doublet G; and GZ at -2695 cm-i and 2735 cm-‘121,221.

The present paper presents the results of Raman microprobe spectroscopy and X-ray diffraction studies of a set of coke samples extracted from a blast furnace. The main aim of the study was to investigate the distribution of the catalytically formed graphite in the blast furnace using Raman microprobe spectroscopy and to compare and contrast these results with the Raman spectral features of the non-catalysed coke. The results are discussed in relation to the heat- treatment temperature, the possible effects of iron particle sizes and the use of X-ray diffraction and Raman microprobe spectroscopy for monitoring the thermal history of coke samples obtained from a blast furnace.

2. EXPERIMENTAL

2.1 Materials used Five coke samples, obtained from an operating

blast furnace using a drill core technique[24], were studied in the present investigation. Schematic diagrams illustrating blast furnace structure and the sampling positions are presented in Fig. 1. Samples A-D were collected from the bosh, raceway, bird’s nest and deadman regions, respectively, in the furnace. Sample E is immediately adjacent to sample D and the XRD and Raman microprobe results of the two samples are very similar. Therefore, the sample was not studied in detail. Sample F was also collected

from the deadman region of the furnace, but farther away from the blast than sample E. Blast furnace feed coke was used in the graphitisation experiment.

2.2 Optical microscopy Polished resin-impregnated blocks were made

from each coke sample and examined in reflected light in an optical microscope. The textural relation- ships between iron, graphite and non-graphitised coke were noted. Photomicrographs were taken from areas of interest for detailed examination by Raman microprobe spectroscopy.

2.3 Raman microprobe spectroscopy The polished blocks of the blast furnace samples

were examined employing a Spex 1401 double mono- chromator fitted with a BGSC microscope equipped with a Nikon x 40/0.65 air objective. The Raman signal was detected using a RCA31034A photomulti- plier in conjunction with a Spectr Acq photon count- ing and spectrometer computer control system. The Raman spectra were obtained using the 488 nm line of a Spectra Physics 2016 argon ion laser. The slit width used gave a spectral resolution of 8 cm-‘. Spectra were obtained from the average of two spectra with 2 cm-’ steps and 5 seconds’ count time. The Raman spectra were curve-resolved using the computer software program Origin supplied by Microcal. This program provided the Raman band width and intensity data.

2.4 X-ray diffraction X-ray diffraction (XRD) measurements of coke

crystallite parameters (interlayer spacing, dooz, and crystallite stack height, 1,) were made to assess the degree of ordering of the graphitic carbon layers associated with the heat treatment temperature of blast furnace cokes. Heat treatment temperature standards prepared by heating a typical feed coke at temperatures between 1500 and 2300°C were used to generate calibration curves for the X-ray diffractometer [ 251.

Representative samples of coke from the different zones along the drill core were lightly ground, and then subjected to cobalt K, radiation in a Phillips 1830 X-ray diffractometer. The carbon peak data were analysed and deconvoluted using Phillips APD1700 software, since initial examination of the XRD traces had indicated that more than one peak could be identified. Positions, d-spacings, intensities and full widths at half maximum intensity (FWHM) of the carbon peaks were determined. A more detailed description of the method can be found else- where[25].

2.5 Experimental catalytic graphitisation qf metallurgical coke

In order to determine if metallurgical coke can be graphitised in the presence of iron at temperatures around 1500-1600°C an experiment was carried out under controlled conditions in a laboratory furnace.

Iron catalysed graphitisation 1527

Physical Raceway

Deadman Coke

Cohesive Zone

fj=$ns goyes

;iz;r$fp

(Deadman)

Raceway Region

Hot Blast

Hearth

A B c D ,E, F Sample --c----)~-cc - Positions

0 1 2 3m

q Bosh&Iaceway~Bird’s NestnDeadman

Fig. 1. Schematic diagrams of the blast furnace showing the zones around the raceway and the relative positions of the drill core samples.

A sample of blast furnace feed coke, which had been carbonised at temperatures less than 1200°C was mixed in a crucible with iron. The coke did not contain any graphite prior to the experiment and the iron was almost 100% pure, from an electrolytic source. The coke and iron were then heated to 1600°C in nitrogen to partially simulate the conditions occur- ring at this temperature in a blast furnace. The mixture was held at 1600°C for 1 hour and then the temperature was reduced to 1500°C for a further soak time of 1 hour before being quenched by helium. After removal from the furnace, the material was examined by optical microscopy to check for the formation of graphite, before being analysed by XRD and Raman microprobe spectroscopy.

3. RESULTS

3.1 Optical microscopy Microscopic examination of the drill-core coke

samples shows several modes of the graphitisation processes. Figure 2(a) is a photomicrograph taken from sample A showing the occurrence of large graphite flakes (Al) with well ordered structure inside a large iron globule together with the well crystallized graphite around the edge of the iron particle. Similar graphite particles are also common in samples B, C and D. Sample F is different in that it does not contain large iron particles in which recrystallized graphite may occur. Figure 2(b) shows a photomicrograph of the recrystallized graphitic carbon with finer

iron inclusions immediately adjacent to the large iron particle shown in Fig. 2(a). The A2 crystallites do not show the euhedral, hexagonal morphology of well crystallized graphite, but are lathlike and clearly more ordered than the surrounding coke. It is noticeable that the crystallite size of the graphitic carbon decreases with increasing distance from the iron. The same phenomenon has been observed by other investigators for the molten iron and vanadium carbide catalysed graphitisation processes[9,10]. The following mechanism has been proposed for the process[26]. 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 graphitisation front, the precipitated small graphite crystals would be transformed in later recrystallisation steps into large flakes. Eventually, as the iron particles become progressively smaller, the extent of graphitisation appears to decrease.

3.2 X-ray diflraction Table 1 presents the results of the X-ray diffraction

studies of the coke samples. Figure 3 shows that the (002) diffraction peaks of the coke samples are clearly asymmetric and can be resolved into two strong narrow peaks superimposed on a low intensity broad peak. The multiphase graphitisation process is well-

1528 W. WANG et ul

Fig. 2. Photomicrographs showing the association of graphitic carbon with iron: (top) graphite (Al) inside an iron globule; (bottom) graphitic carbon (A2) associated with fine iron particles.

documented[ 141 for the iron catalysed graphitisation process. Since a great proportion of the blast furnace coke samples are graphitisable carbon, it is proposed that the three peaks are as follows: peak 1, the broad low intensity peak, is attributed to disordered carbon in the coke; peak 2 is observed at low scattering angle and corresponds to the peak due to heat- treatment; and peak 3, at the highest scattering angle, is due to the graphite formed by the catalytic graphitisation. Only a small amount of synthetically graphi-

tised material was available for XRD analysis of G15, so it was not possible to separate the broad low intensity disordered carbon peak 1. However, the clear presence of peaks 2 and 3 confirms that iron catalyses the growth of graphite in coke oven at temperatures as low as 1500-1600°C. It is clear from Fig, 3 that in all these samples the peak due to heat- treatment is the predominant feature, and the catalytically formed graphitic carbon only accounts for a small proportion of the total peak. There is also clear

Iron catalysed graphitisation

Table 1. XRD results for the blast furnace cokes

1529

doo2 (nm) FWHM (“) L (nm)

Samples Peak 1 Peak 2 Peak 3 Peak 1 Peak 2 Peak 3 Peak 1 Peak 2 Peak 3

A 0.3505 0.3420 0.3366 2.97 0.68 0.18 3.3 14.6 56.0 B 0.3500 0.3410 0.3366 2.68 0.57 0.16 3.7 17.6 63.0 C 0.3503 0.3411 0.3365 2.37 0.61 0.17 4.2 16.3 59.5 D 0.3508 0.3422 0.3367 2.98 0.82 0.25 3.3 12.2 39.8 F 0.3520 0.3436 0.3371 4.11 1.31 0.47 2.4 7.6 21.4

Peak 1, non-graphitising carbon; Peak 2, due to heat treatment; Peak 3, catalytic graphitisation.

26 28 30 32 34

20

Fig. 3. XRD profiles for the blast furnace cokes.

evidence that for the carbon collected from different regions in the blast furnace, the intensity of the (002) diffraction peak varies dramatically. The raceway sample peak has the smallest full width at half maximum intensity, which increases in the order: raceway > bird’s nest > bosh > deadman samples. Accompanying this there is also a considerable broad- ening of the diffraction peaks and a slight gradual shift of the peak positions. The peak of the catalytically formed graphitic carbon behaves in a similar manner. It is apparent that the peak for the raceway sample is the highest and it decreases gradually, being barely detectable for the second deadman sample (F). Table 1 shows that the crystallite height of the catalytically formed graphitic carbons range from over 60 nm for sample B to approximately 40 nm for D, whereas in sample F the I, value is 20nm. At the same time, the interlayer spacing of the catalytically formed carbons increases from 0.3366 to 0.3370nm from B to F. The corresponding crystallite heights of the carbons due to heat treatment are smaller and decrease from 17.6 to 7.6 nm in the order of B, C, A, D, F. The corresponding interlayer spacing d,,oz also increases from 0.3410 to 0.3436 nm in the same order. The crystallite height 1, and interlayer spacing of the non-graphitising carbon vary little among the samples and are typically below 4 nm and greater than 0.35 nm, respectively, indicating a high degree of structural disorder.

3.3 Raman spectral data The first and second order Raman spectra and

associated parameters of all the samples studied are presented in Figs 4-l and Tables 2 and 3. Figure 4 shows the first and second order Raman spectra of graphitic carbons inside iron (Al), and associated

with fine iron particles (A2) and the coke away from any visible iron particles (A3) in the bosh sample. Table 2 shows that the ID/Z&RI) ratios are rather similar for the coke and the graphitic carbons with the carbon inside the iron having a slightly higher

RI value. The positions from which the Raman spectra Al and A2 are taken are shown in photo-

micrographs in Fig. 2(a) and (b). Overall, the ratios are higher than those reported for well ordered graphitic carbons. The frequency and the width at half maximum intensity of the disorder induced D line, Av,, vary slightly for the various particles. However, the half maximum intensity width of the Z& lines,

Avo, for the three spectra show significant differences. The average Avo value for the graphitic carbons was 21 cm-‘, whereas that for the coke is 27 cm-‘. It is known that the width of both D and G bands decreases with increasing heat-treatment temperature to -2300 K. At even higher temperatures Avn tends

to level off at -40 cm-‘, whereas Avo continues to

decrease to a minimum value of - 20 cm-’ for graphite. The spectral features of the carbons at the edge of large iron particles are almost identical to that of

the carbon inside the iron.

An examination of Fig. 4 shows that the second order Raman spectra of Al and A2 are clearly asymmetric, whereas that of the coke sample (A3) is

a single symmetric peak at 2720 cm-‘. The latter is surprisingly intense bearing in mind that the first order Raman spectrum of the particle is weak. The

second order Raman spectra of Al and A2 can be resolved into two overlapping bands at -2705 and -2735 cm-‘, typical of well ordered graphitic car-

bons. It is interesting to note that the second order Raman spectrum of A2 has a rather flat top (see Fig. 4) suggesting that the graphitic carbon associated with the finely divided iron particles is intermediate in structure between the graphite in the iron and the coke samples. This point is further illustrated by the higher Z,‘,/Z,‘, (R4) ratio and by the broader Gr

peak of this particle compared with the graphite inside the iron (Table 3).

1530 w. WANG t?t d.

L , I

1200 1300

0

0. I. ,I. 1.1.1’ t

2600 2650 2700 2750 2800 2650 2900 22

1400 1500 16 1700

Raman shift (cm-‘)

Fig. 4. First and second order Raman spectra for

I-

-

I’

the bosh coke

- t , a I7rn 2el-J 27ml


Fig. 5. First and second order Raman spectra for the first deadman coke

The Raman spectral features of the raceway spectrum for the carbon inside the iron. Table 2

sample are rather similar to those of the bosh sample shows that the width at half maximum intensity for

but show a more well defined second order Raman the disorder induced D bands (Avo) is similar


J L

L F2

i

J

1 vm 1303 ,600 --A 2600 *Kc


Fig. 6. First and second order Raman spectra for the second deadman coke

2.b 2503

hi-w J I; Fl

G’

1400 15bo


I

1600 1700

2600 I 1 I

2700 Raman shift (cm-‘)

2800 2900

Fig. 7. First and second order Raman spectra of a laboratory-synthesised graphite, catalytically formed at 1500°C.

1532 W. WANG et al.

Table 2. First order Raman spectral data for the blast furnace cokes

D G D’

VDI Avol VGi Av,/ VLV/ AVIS/ Samples cm-’ cm-’ R, cm-’ cm-’ cm ’ cm-l R,

Al 1361 37 1.58 1582 21 1622 13 A2 1355 35 1.30 1578 21 1617 16 A3 1357 32 1.30 1582 27 1618 48 Bl 1357 38 1.36 1578 20 1618 9 B2 1356 36 1.07 1578 22 1617 18 B3 1359 31 1.27 1583 26 1621 23 Cl 1363 40 0.67 1583 19 1624 12 c2 1359 36 1.35 1582 23 1621 15 Dl 1357 36 0.89 1578 20 1618 10 D2 1357 32 1.08 1581 22 1621 17 D3 1356 48 1.59 1583 46 1615 21 F1 1357 39 0.98 1580 26 1619 27 F2 1355 47 1.37 1580 33 1615 28 F3 1356 71 1.52 1588 64 G15 1358 41 0.92 1578 19 1620 9

0.12 0.10 0.12 0.05 0.08 0.17 0.03 0.11 0.04 0.10 0.12 0.15 0.27

0.03

R,=I,lI,; R,=I,,/IG

Table 3. Second order Raman spectral data for the blast furnace cokes

G’ G,, G,,

VIZ,! Av,,l v,,,i Av,~v vC?;# AvG2,; Samples cm-’ cm ’ R, cm-r cm-’ R, cm-’ cm ’

Al A2 A3 Bl B2 B3 Cl c2 Dl D2 D3 Fl F2 F3 Cl5

~ 2706 49 1.15 2738 38 ~ 2707 62 3.02 2737 32

2721 48 3.50 - 2701 50 1.02 2734 36 2708 47 1.90 2738 30

2716 53 1.01 ~- ~ 2709 54 1.46 2741 33

2719 44 1.28 ~ _

2703 53 1.80 2736 31 2717 47 1.47 - 2708 71 0.65 ~~ 2718 36 1.81 - 2716 48 0.63 - 2705 137 0.46 -

2702 45 0.87 2736 34

R,=I,,/I,; R,=Ic;,,,tI,.,.

(35k4 cm-‘) for spectra Bl, B2 and B3, whereas higher Avc and Avn’ values are recorded for the coke (B3) compared with the carbon inside the iron (Bl) and that associated with finely divided iron particles

(B2). As for the bosh sample, the coke (B3) in the

raceway sample has a single symmetric peak G’ centered at 2716cm-r with a Av,, of 53cm-‘, whereas Bl and B2 give rise to asymmetric second order spectra. The latter is indicative of the development of a three-dimensional graphitic structure. The second order Raman spectrum of the graphitic carbon (B2) associated with the fine iron particles is also rather flat at the top and the recorded R, ratio is higher than that of Bl.

The first and second order Raman spectral data of a carbon flake inside a large iron globule (Cl) and a coke particle (C2) from the bird’s nest sample are given in Tables 2 and 3. The variation in Raman

band intensities is illustrated by the 1,/I, ratios in which the R, value is the lowest for Cl among all the Raman spectra recorded for the blast furnace samples.

Table 3 shows that the second order Raman spectral features for the coke particles from the bosh, raceway and the bird’s nest samples are rather similar. Apart from A3 having a higher I,‘/I, ratio (R3), the second order Raman spectra of the coke samples all peak at -272Ok4 cm-‘, with half maximum intensity widths of 45555 cm-‘. It is also apparent that the Raman band positions, half widths as well as the R, ratios for the graphitic carbons associated with iron, are also comparable for these samples.

The first and second order Raman spectra of the various particles from the first sample from the deadman region are shown in Fig. 5. It is apparent that the Raman spectra of the carbon inside an iron particle (Dl ) and the large coke particle (D3) differ


substantially. D2 is taken from a carbon particle

immediately adjacent to a large iron particle. Table 2 shows that RI and Avo are significantly higher for D3 than for Dl and D2.

Figure 5 also shows that although Dl has an asymmetric second order Raman spectrum, the carbon (D2) near the iron particle only gives an intense and relatively narrow symmetric single peak. This is in contrast with the samples discussed pre- viously in which the carbons associated with iron all gave rise to asymmetric second order Raman spectra. The large coke particle gives rise to a less intense and broader second order Raman spectrum (D3) indicating greater disorder in the carbon compared with the other areas studied in the sample.

Sample F is also from the deadman region in the blast furnace and is the furthest sample from the furnace wall investigated in this study. Figure 6 presents the first and second order Raman spectra of carbon particles associated with fine iron (Fl), coke (F2) and a relatively large solid coke grain derived from fusinite maceral present in the coal (F3). Apparently, the G band is significantly stronger than the D band for the spectrum of Fl, whereas comparable D and G bands are recorded for the coke (F2). The large coke grain (F3) gives rise to a first order Raman spectrum which is different from those of the other regions studied in the sample. The disorder- induced D band is significantly more intense than the G band, and the D’ band is so intense that it is impossible to separate the D’ and the G band. The peak position of the combined G and D' band has shifted to 1588 cm-‘. Table 2 also shows that both Av, and Av, are significantly higher for spectrum F3 than for Fl and F2. It is noticeable from Fig. 6 that the second order spectrum of the carbon associated with iron (Fl) is significantly narrower and more intense than those of F2 and F3. The large coke grain gives rise to a barely detectable, very weak and broad second order Raman spectrum, which is typical of disordered carbon derived at relatively low temperature from poorly graphitised coke of fusinite origin[27].

4. DISCUSSION

When studying the operation of the blast furnace, the temperatures in the various regions are an important consideration. Consequently, it is useful to assess the temperatures observed in the samples by characterisation of the coke structure. Similar studies[28-311 on cokes taken from the British Gas Lurgi slagging gasifier have shown that Raman microprobe spectroscopy, X-ray diffraction and optical microscopy are useful techniques for the characterisation of coke thermal history. Previous studies [ 321 of the maximum temperatures experienced by these samples suggested that there are significant temperature variations among the samples. According to XRD determination the raceway sample (B) has the highest temperature (215O”C), followed

by the bird’s nest sample (2085°C) and the bosh sample (1990°C). The two deadman samples (D and F) have temperatures of approximately 1860 and 162O”C, respectively. The XRD results in Table 1 show that for the catalytically formed graphitic carbons the measured 1, values decrease from over 60 nm for the raceway sample to approximately 20 nm for the deadman sample, corresponding with the temperature variation in the blast furnace. The decrease in the degree of ordering of the graphite with decreasing temperature is also evident in the increase of the interlayer spacing, dooz. With increasing temperature, the (002) diffraction peaks of the coke also become progressively narrower and stronger with a slight shift of the peak position to higher diffraction angles. Since the bulk of the coke samples is made up of graphitising carbons, the results show that heat- treatment in the blast furnace to temperatures significantly higher (1600-2100°C) than those used in carbonisation (900-1200°C) has resulted in significant increases in the structural ordering of the cokes.

The Raman microprobe results in Table 2 show that for the raceway and bosh samples the R, values do not differ significantly between the coke and the graphite. In the bird’s nest and the deadman regions of the furnace the RI values are significantly higher for the coke than for the graphitic carbons (Table 2). The R, values are on average higher than would be expected for well ordered carbons [ 20,33,34]. Techniques for the assessment of thermal history, developed in previous studies[28-311, of coke samples in a gasifier were applied to the Av, values in Table 2. Results for samples A3, B3 and C2 suggest that these cokes have attained a maximum HTT of over 2OOO”C, in agreement with those from the XRD studies[32]. The Av, values for the two deadman samples (D3 and F3) are significantly higher, indicating lower HTT values. It is also clear that the Av, values for the catalytically formed graphitic carbons are in the range of 32-40cm-‘, implying that a temperature of over 2000°C has been attained. This is clearly not a true representation of the thermal history for samples D and F in view of the XRD and Raman results discussed above for the coke. Previous studies[ 18,341 on iron-containing glassy carbon materials prepared by co-polymerisation of furfuryl alcohol (FA) and vinyl ferrocene (VF) showed that catalytic carbonisation may be effective at temperatures as low as 700°C. The structure formed at this temperature from the iron-containing glassy carbon was similar to that of the pure glassy carbon heat treated to 27OO”C, as characterised by their similar Av, and I,/Io ratios. In that study the concentration of iron was significantly higher and the iron was homogeneously distributed in the carbon. In this work the iron concentration was considerably lower and exists as discrete particles. The study indicates that caution should be applied when interpreting XRD results without deconvoluting peaks for different types of carbon and Raman microprobe results for the study of the thermal history of the

1534 W. WANG et al.

coke samples, when catalytic graphitisation has section. This supports the above conclusion relating

occurred. to contact time and crystallisation.

Figures 4 and 5 and the data presented in Table 3 show that the second order Raman spectra of graphitic carbons are clearly asymmetric and can be resolved into two overlapping bands at -2705 and 2735 cm-‘. The differences in frequencies of the doublet are in the range of 29933 cm-’ smaller than that of natural graphite, where a difference of approximately 45 cm -r is expected. The R, ratios are also significantly higher than that for natural graphite[35]. The higher R, ratios and less well resolved G; and G; bands and the higher G’r intensity all appear to support the finding that further development of the graphitic structure is possible, although the graphite is more ordered than the corresponding coke carbon. The reason for this is possibly the short contact time between the coke and the iron and the rapid cooling experienced by the sample after being withdrawn from the furnace. Figure 7 presents the first and second order Raman spectra of the graphite G15, which was formed synthetically from the reaction of blast furnace feed coke with iron in a laboratory furnace at 1500°C. Small graphite crystals have formed as long, thin flakes in the metal (Fig. 8). XRD study on this sample shows the presence of two (002) peaks corresponding to the coke and the catalytically formed graphitic carbon, respectively. The former has an interlayer spacing of 0.3434 nm and a 1, value of 6.5 nm, whereas the latter has a d,,, of 0.3356 nm and a I, of 37 nm. Tables 2 and 3 show that both the R, and R, ratios are lower for this graphite than most of the graphitic carbons reported in the previous

Previous studies[21,22] have shown that the resolution of the second order Raman band at - 2700 cm-’ from a single band to the doublet occurs

at d,,, of -0.338 nm. An examination of the XRD results in Table 1 suggests that the catalytically formed graphitic carbons all have doa2 values below 0.338 nm. The detection of the asymmetric second order Raman spectra from these carbons supports previous studies. The failure to obtain a graphite second order Raman doublet from sample F is consis- tent with the XRD diffraction profile, which shows that only a very small amount of this graphitic carbon has been formed in this sample. This is probably due to the lower temperature and to the very low concentration of iron in the deadman region of the furnace.

5. CONCLUSIONS

The results show that graphite has been formed in the coke samples at temperatures well below normal graphitisation temperatures due to the catalytic effects of molten iron in the blast furnace. The laboratory experiment under controlled conditions confirmed that graphite formation can occur in relatively short periods of time at temperatures as low as 1500°C in the presence of iron. XRD studies show that the formation involves a multiphase graphitisation process, which is common for iron catalytic graphitisation. The formation of graphite from the coke is believed to involve the dissolution and precipitation mechanism. With increasing heat treatment

Fig. 8. Photomicrograph showing synthetically catalysed graphite GlS around iron globules on the surface of the coke.


temperature, the crystallite height 1, increases and the 8. H. N. Murty, D. L. Biederman and E. A. Heintz. Carbon

interlayer spacing dooz decreases for both the catalyti- 11, 163 (1973).

tally formed graphite and the coke. The results of 9. E. Fitzer and B. Kegel, Carbon 6, 433 (1968).

10. W. Weisweiler, N. Subramanian and B. Terwiesch, Raman microprobe studies agree with those obtained Carbon 9, 755 (1971). from XRD. The dooZ and 1, values and the higher 11. S. B. Austerman, in Chemistry and Physics of Carbon

Raman R, and R, intensity ratios for the graphitic (Edited by P. L. Walker Jr), Vol. 4, p. 137 (1968).

carbons indicate that the graphitisation process is 12. S. B. Austerman, S. M. Myron and J. W. Wagner, Carbon

incomplete. The most noticeable spectral difference 5, 549 (1967).

13. Y. Hishiyama, A. Ono and T. Tsuzuka, Carbon 6, 203 between the graphite and that of the coke is the (1968).

resolution of the second order Raman G’ band of the 14. I. Yamada, Symp. Carbon, Tokyo, Session III-21 (1964).

former into an overlapping doublet, whereas the 15. E. G. Acheson, U.S. Patents, 568323 (1896), 616974

latter only has a single symmetric peak in the second (1899), 645285 (1900).

16. A. Oberlin and J. P. Rouchv, Carbon 9, 39 (1971). order Raman spectrum. The results suggest that 17. F. Tuinstra and J. L. Koenig, J. Chem. Phys. 53, 1126

temperature, amount of iron in contact with the coke (1970~ 18. and residence time of the coke in the furnace are

important factors in the catalytic graphitisation process. The direct link obtained between optical microscopy and Raman spectroscopy in the Raman microprobe instrument has proved useful in linking morphology to molecular structure. Caution must be exercised if the XRD and Raman results are used for the characterisation of the thermal history of the coke samples in regions of high iron concentration.

19.

20.

21.

k ~~

M. &kamizo, R. Kammereck and P. L. Walker Jr, Carbon 12, 259 (1974). R. Vidano and D. B. Fischbach, J. Am. &ram. Sot. 61, 13 (1978). P. R. Vidano, PhD Thesis, University of Washington, p. 257 (1980).

Acknowledgements-The authors would like to thank the European Coal and Steel Community for supporting the project under contract 7220-EB 842 and British Steel for permission to publish the paper.

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iron catalysed graphitisation in the blast furnace

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