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Energy & Fuels 2006, 20, 2632-2635
Mechanism of Physical Transformations of Mineral Matter in the Blast Furnace Coke with Reference to Its Reactivity and Strength Stanislav S. Gornostayev* and Jouko J. Ha¨rkki Laboratory of Process Metallurgy, UniVersity of Oulu, P.O. BOX 4300, Oulu 90014, Finland ReceiVed April 4, 2006. ReVised Manuscript ReceiVed August 9, 2006
Examinations of polished and dry cut sections of feed and tuyere coke revealed some possible mechanisms for the physical influence of mineral compounds on the reactivity and strength of coke. It was observed that rounded particles of mineral phases that are exposed to the pore walls and surface of coke at high temperature create an inorganic cover, thus reducing the surface available for gas-solid reactions. The particles of mineral matter that have a low melting point and viscosity can affect the coke at earlier stages in the blast furnace process, acting in the upper parts of the blast furnace (BF). The temperature-driven redistribution of mineral phases within the coke matrix probably leads to the creation of weak spots and in general to anisotropy in its properties, thus reducing its strength.
Introduction Reaction efficiency is very important in a blast furnace (BF), since it allows a decrease in the amount of reducing agent (coke) required for producing pig iron and affects the volume of CO and CO2 emitted. The reactivity of the coke, which is one of the principal factors in this, depends not only on the properties and content of the various maceral-derived components but also on inorganic (mineral) compounds, often referred to as ashes. Naito et al.,1 for example, have reported that an improvement in reaction efficiency in the BF can be achieved by reducing the temperature of the thermal reserve zone through the use of highly reactive coke, and Nomura et al.2 state that, “The addition of Ca compounds to coal before carbonization was found to considerably increase the reactivity of the coke at a low temperature range in the thermal reserve zone of a blast furnace. Furthermore it was proved that strong, highly reactive ‘lump’ form coke could be produced by adding a Ca-rich non-caking coal and adjusting the coal blend composition.” It should also be noted that the reactivity of coke not only is a function of the ability of any given carbon texture to react at a gas-solid surface or of the presence of additives, as referred to above, but it also depends on the properties of the surfaces themselves, which include “physical” features such as the number and size of pores (i.e., surface area) and, as will be discussed in this paper, the number of mineral particles exposed to the surface and pore walls of the coke. The strength of the coke, another important factor that affects BF operation, is determined by it microstructure and texture,3,4 the thickness of the pore walls, pore size distribution, pore density, and nature of the maceral-derived components being * To whom correspondence should be addressed. E-mail address:
[email protected]. (1) Naito, M.; Okamoto, A.; Yamaguchi, K.; Yamaguchi, T.; Inoue, Y. Tetsu to Hagane´ 2001, 87, 357-364. (2) Nomura, S.; Ayukawa, H.; Kitaguchi, H.; Tahara, T.; Matsuzaki, S.; Naito, M.; Koizumi, S.; Ogata, Y.; Nakayama, T.; Abe, T. ISIJ Int. 2005, 45, 316-324. (3) Andriopoulos, N.; Loo, C. E.; Dukino, R.; Mcguire, S. J. ISIJ Int. 2003, 43, 1528-1537. (4) Sharma, R.; Dash, P. S.; Banerjee, P. K.; Kumar, D. ISIJ Int. 2005, 45, 1820-1827.
among the key parameters that determine the strength values. As we have reported earlier,5 mineral phase crystallization can be accompanied by the formation of cracks in the coke matrix, thus detracting from its strength. We also discuss here the temperature-driven redistribution of mineral phases within a coke matrix, which may affect the strength of the coke. Samples and Methods Two sets of samples obtained from the Ruukki Steel Works in Raahe, Finland, were used. The first set was selected from the stockpile of feed coke, and the second set was taken from drill cores approximately 2.5 m in length obtained from the tuyere zone of an operating BF using a mobile tuyere rig. The details of tuyere drilling have been reported by Kerkkonen.6 The exact location of each sample referred is given in the figure captions, where the last two or three digits after the dash correspond to the distance in centimeters from the tuyere level. Each set of samples was used to prepare sections of two types. For the first, pieces of coke were sliced into plates of 5-7 mm thickness, after which 22 mm circles were drilled to obtain disks, which were then mounted in plastic holders and filled with epoxy for the preparation of polished sections. For the second type, the samples were cut into pieces about 35-40 mm long, 20-25 mm wide, and 5-7 mm thick under dry conditions (no cooling water was used in sawing), preserving one original surface. The pieces were then fixed to glass plates. The polished and dry-cut sections were examined preliminarily under an optical microscope and stereomicroscope and, then, with a JEOL JSM-6400 scanning electron microscope (SEM) equipped with an energy dispersive spectrometer (EDS) and a JEOL JXA-8200 electron microprobe (EPMA), to observe, identify, and analyze the mineral phases and collect digital electron images.
Results and Discussion Examination of the coke samples showed that the particles of mineral matter varied considerably in size and shape and (5) Gornostayev, S.; Ha¨rkki, J. Fuel 2006, 85, 1047-1051. (6) 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, Sept. 15-17, 2004; Vol. I, pp 469-481.
10.1021/ef060147x CCC: $33.50 © 2006 American Chemical Society Published on Web 09/16/2006
Mechanism of Transformations of Mineral Matter
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Figure 1. Distribution of mineral phase particles in a coke. Back-scattered electron (BSE) pictures of polished (A, C, and Esstudied with a JEOL JXA-8200) and dry-cut (B, D, Fsstudied with a JEOL JSM-6400) sections. (A and B) Feed coke. (C-F) Tuyere coke. Samples of coke: (A) c21-301005; (B) c4-09. Samples of tuyere coke: (C) 130303203-150 (see Table 1 for microprobe analyses of spots 1, 2, and 3); (D) 130303203-70; (E) 130303203-150; (F) 130303203-20.
that they were distributed irregularly within the coke matrix (Figure 1A and B). The particles were often polygonal, representing the parental shape of either the natural crystals or their aggregates, which had been broken mechanically during crushing of the coal. The particle sizes and the variations in their distribution were mostly related to the fact that any given coke sample will contain a random number of pieces of different coal types that were mechanically mixed during blending. Thus,
a coke blend is a much more heterogeneous “nonequilibrium” system than the original coal. Examinations of polished sections of the tuyere coke (Figure 1C and E) demonstrated that the mineral phases varied in size, shape, degree of alteration, and mode of occurrence from sample to sample and even within a single sample. In particular, one of the most notable differences between the samples and between sites within a single sample is the number of particles
2634 Energy & Fuels, Vol. 20, No. 6, 2006
GornostayeV and Ha¨rkki
Table 1. Composition of Some Mineral Particles from the Tuyere Cokea
1 2 3
Na2O
K2O
Cr2O3
V2O3
MgO
CaO
MnO
NiO
Al2O3
TiO2
FeO
ZnO
SiO2
total
1.55 1.68 1.62
7.84 9.37 8.08
0.00 0.00 0.03
0.00 0.00 0.03
0.13 0.17 0.12
0.13 0.17 0.11
0.03 0.04 0.01
0.06 0.00 0.00
12.96 9.82 13.33
0.13 0.15 0.15
0.02 0.08 0.03
0.00 0.05 0.02
76.04 76.56 75.49
98.88 98.07 99.01
a This wavelength dispersive spectrometry (WDS) analysis was performed on a JEOL JXA-8200 electron microprobe at the University of Oulu. The conditions were as follows: 15 kV, 1.3 nA, beam diameter 1 µm, sample 130303203-150; see Figure 1C for spot locations.
of mineral matter around the pores and in the coke matrix. Also, the surface of the tuyere coke hosts varying amounts of mineral particles, as was observed in the dry-cut samples (Figure 1D and F). As we have reported earlier,5,7,8 the mineral phases in tuyere coke form new compounds, ranging from complex aluminosilicates ((K, Na, and Ca) to binary phases, including oxides, silicides, sulfides, and phosphides. The dominant aluminosilicates occur mainly as spherules of 1-50 µm in diameter and irregularly shaped segregations. Transformation of the original mineral particles to spherules and irregular slag segregations takes place in the coke during the rise in temperature, when the original mineral phases undergo various transformations and finally melt, forming the shapes noted above. Under certain conditions, they can be transformed to crystals of corundum and spinel.5,8 This crystal formation can cause the generation of cracks in a coke matrix, thus lowering its strength, but the preceding formation of spherules does not in itself destroy the coke matrix.8 At relatively “moderate” temperatures, molten particles of mineral matter gradually move toward one to another to form large particles, leaving empty spaces in their original locations. The latter create weak spots in the coke matrix comparable to those reported by Kerkkonen9 with reference to the decomposition of mineral phases. Such spots could reduce the ability of the coke to withstand mechanical movement. The migration of these chemically and physically changed particles is directed toward the pores, and it is here that their further enlargement and partial outcropping to the pore walls takes place. At this stage, the coke sample is characterized by the coexistence of “new” and “old” mineral phases, which reflects the nature of the parental mineral phases since they have different temperatures of phase transformation and melting. Such a situation is demonstrated in Figure 1C and D, with a low/moderate concentration of mineral matter around the pores and on the sample surface. The new particles cover only some parts of the pore walls, and the value of the ratio surface of pore walls/ surface of mineral particles on pore walls can be relatively high. Another distribution of mineral matter around the pores and value for the above ratio were observed in the samples pictured in Figure 1E and F, where the amount of mineral matter on the pore walls and on the sample surface is very high and the value of the ratio surface of pore walls/surface of mineral particles on pore walls is much lower than in Figure 1C and D. Under such circumstances, the surface available for gas-solid (coke) interaction is very much smaller, which has a direct influence on the efficiency of the coke as a reduction agent. The formation of weak spots could increase at this stage, too. Differences in particle concentrations on the pore walls within a single piece of coke can be caused by the nature of the mineral phases, a suggestion supported by the electron microprobe analyses (Table 1) of particles 1, 2, and 3 marked in Figure (7) Gornostayev, S.; Kerkkonen, O.; Ha¨rkki, J. ISIJ Int. 2005, 45, 1-7. (8) Gornostayev, S.; Ha¨rkki, J. Metall. Mater. Trans. 2005, 36B, 303305. (9) Kerkkonen, O. Coke Making Int. 1997, 9-2, 34-41.
Figure 2. Generalized model for the evolution and redistribution of mineral matter in feed and tuyere coke. The large circles in A, B, and C correspond to pores. (A) Feed coke with polygonal crystals (triangles, rhombs, and rectangles) of mineral phases or their aggregates mechanically broken during coal crushing. (B) Coexistence of new (circles, ellipses) and old (triangles, rhombs, and rectangles) mineral particles within one sample at moderate temperatures. (C) Redistribution of mineral particles at higher temperatures. See text for details.
1C. As can be seen in this figure, particle 2 has primary crystalline outlines (edges) and is located inside the coke matrix just near the pore that hosts spherules 1 and 3, which represent new phases. The differences in composition can be clearly seen by comparing, the K/Naat and Si/Alat ratios, for example, which are 3.33, 3.68, and 3.28 and 4.98, 6.62, and 4.81, respectively,
Mechanism of Transformations of Mineral Matter
for particles 1, 2, and 3. The new, rounded particles 1 and 3 have lower K/Naat and Si/Alat ratios than the crystal-like phase (particle 2), which probably represents a partly altered member of a feldspar groups(K,Na)[AlSi3O8]. On the other hand, the reasons for the different concentrations of mineral particles around the pores and on the surface of the coke in samples taken from different parts of the tuyere zone can be explained by the nature of the mineral phases and temperature fluctuations between the sample locations. It should be mentioned that the referred analyses were obtained on mineral phases located inside the coke sample. So, the probability of potential contribution of recirculated alkalis to K/Naat ratio changes for this particular case is not very high. However, the possibility of influence of recirculated alkalis should be kept in mind when interpreting the results of analyses obtained on mineral phases located close to a coke surface. The mode of occurrence and the relationships of inorganic compounds with the coke matrix shown in Figure 1 have been also observed in a number of other samples, and they seem to be common for the tuyere coke. A generalized model for the evolution and redistribution of mineral matter in feed and tuyere coke is given in Figure 2, referring to the actual observations shown in Figure 1 and outlined below. As we noted above, the polygonal crystals and their aggregates of “primary” mineral particles (Figures 1A, 1B, and 2A) undergo the first changes at moderate temperatures, to form compounds of new shapes and compositions (Figures 1C, 1D, and 2B). At this stage, the new and old mineral phases coexist within one sample. As the rise in temperature continues, it causes further growth of the existing new particles and leads to further removal of disseminated primary mineral matter from the coke matrix. The particles that were close to the external part of the hosting piece of coke move toward the surface and finally cover it with a veneer of spherules (Figures 1F and 2C). The particles located further away from the surface of the piece of coke group around the nearest pores (Figures 1E and 2C). At this stage, the coke becomes even more inhomogeneous and gradually transforms into a “two-phase” system: (1) mineral matter and (2) a mineral-free carbon matrix (Figure 2C). Such
Energy & Fuels, Vol. 20, No. 6, 2006 2635
an inhomogeneity can cause anisotropy of its properties, which, in addition to the formation of weak spots, reduces the strength of the coke and consequently its ability to play a role of providing structural support. Nevertheless, temperature is not the only factor that may cause differences in the behavior of mineral matter in a coke. It is obvious that the extent of the transformation of mineral phases and their redistribution within a piece of coke at a given temperature is governed by the properties of the mineral phases themselves, i.e., by whether they reach the melting point and, if molten, by the viscosity of the molten material. The particles with a lower melting temperature will appear on the pore walls and on the surface of the piece of coke at the early stages of coke consumption and will consequently cause the earlier appearance of weak spots. The less viscous melt can probably migrate for longer distances within the coke, thus providing more material for covering the pore walls and the surface of the piece of coke. Conclusions The mineral phases in a coke can physically affect its reactivity by covering the pore walls and coke surface, thus reducing the surface available for gas-solid reactions. The particles of mineral matter that have a low melting point and viscosity can affect the coke at earlier stages in the blast furnace process (in the upper parts of the BF). Temperature-driven redistribution of mineral phases within a coke matrix probably leads to the creation of weak spots in the coke matrix and, in general, to anisotropy in the properties that reduce its strength. Acknowledgment. This research was funded by the Academy of Finland. Mr. T. Kokkonen is thanked for preparing the samples. We are grateful to Dr. K. Heina¨nen and Mr. O. Kerkkonen for helpful discussions. We thank two anonymous referees for their comments. Editorial handling was provided by Prof. L. J. Broadbelt and Mrs. H. Price. EF060147X
