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Characterization of the Origin and Distribution of the Minerals and Phases in Metallurgical Cokes Sushil Gupta,* Maria Dubikova, David French,† and Veena Sahajwalla School of Materials Science and Engineering, The UniVersity of New South Wales, Sydney, NSW 2052, Australia ReceiVed August 29, 2006. ReVised Manuscript ReceiVed October 18, 2006
Three industrial metallurgical cokes were examined using X-ray diffraction (XRD) and scanning electron microscopy combined with energy dispersive X-ray analysis (SEM/EDS). The study highlighted the difficulties and implications of identifying the inherent crystalline mineral phases in cokes using XRD such that increasing the ashing temperature led to the formation of anhydrite and destruction of metallic iron: microwave plasma ashing resulted in minimal alteration of the original coke mineralogy apart from the formation of bassanite and possibly jarosite. A preliminary scheme to characterize coke minerals is presented such that, physically, minerals can be classified as fine (1000 µm); chemically, minerals can be grouped as refractory, semirefractory, and reactive, while on the basis of distribution they can be described as discrete, disseminated, or pore inclusions. Quartz, cristobalite, mullite, and high melting point Al-silicates were found to be the predominant refractory phases while low melting point Al-silicates, e.g., containing high fluxing elements such as K, and Fe were the main semirefractory phases present in all cokes. A variety of iron containing phases including pyrrhotite, troilite, iron oxides, metallic iron, and iron silicates were also invariably present in all cokes while calcium phases were found to occur as sulfide, silicates, and phosphates. In general, iron and calcium phases can be categorized as reactive phases with few exceptions such as oldhamite (CaS). The study highlighted that most of the cokes possess a similar mineralogy, with the main distinction being in their relative abundance, particle size, and nature of distribution in the coke matrix. The study provides a basis to develop a mechanistic understanding of the influence of minerals on coke reactivity and strength at high temperatures.
Introduction Coke plays multiple roles in a blast furnace, acting as a fuel, reducing agent, and support for maintaining the bed permeability. Recently, there has been an increasing trend to reduce the reliance on coke in a blast furnace. Due to the strong effect of coal and coke properties on blast furnace performance, pulverized coal injection (PCI) and coke gasification have been most extensively studied topics.1 At low coke rate operations, coke quality issues become more critical, as less coke is available to supply reducing gases and provide sufficient physical support and residence time in the blast furnace is longer.1 Consequently, coke would experience longer periods of mechanical, thermal, and chemical stresses and, consequently, degrade more. Coke fines influence the liquid and gas permeability as well as metal/slag drainage conditions as a consequence of their accumulation at lower regions in a blast furnace. Therefore, blast furnace fuel properties would also influence the mechanisms of fines generation and subsequent assimilation. Conventionally, coke quality is characterized in terms of coking coal rank, oxide ash analysis, and maceral composition, coke strength, and coke reactivity.2 * Corresponding author. Tel.: + 61-2-93854433. Fax: 61-2-93855956. E-mail:
[email protected]. † CSIRO Energy Technology, Lucas Heights Science and Technology Center, Bangor, 2234, Australia. (1) Ishii, K. AdVanced PulVerised Coal Injection Technology and Blast Furnace Operation; Pergamon Publishers: Elmsford, NY, 2000. (2) Best, M. H.; Burgo, J. A.; Valia H. S. Effect of coke strength after reaction (CSR) on blast furnace performance. In Proceedings of the 61st Ironmaking Conference, Nashville, TN, March 10-13; ISS: Warendale, PA, 2002; pp 213-239.
Recently, advanced approaches have been used for characterizing coke properties such as atomic carbon structure and mineralogy, which could influence coke weakening and fines generation.3,4 Coke minerals could affect coke strength and reactivity in many ways, e.g., crystallization of minerals at high temperatures in an operating blast furnace were shown to be accompanied by the formation of cracks and weak spots in the coke matrix5 while at low temperatures coke reactivity was catalyzed by crystalline phases of iron.6 Therefore, due to the strong relationship between the coke carbon characteristics and mineral matter, minerals are expected to affect coke gasification as well as graphitization. The current understanding of the inorganic matter of coke is largely based on oxide analysis of coke ash, which ignores the fact that the inorganic elements are mostly present in different mineral forms, of varying size, textural association, and location within the coke. Therefore, there is a need to develop a comprehensive understanding of the true nature of coke inorganic matter by characterizing all aspects of coke minerals, which is the main aim of this study. In order to understand the origin of coke minerals, a brief description of the thermal behavior of the major mineral constituents of coals is provided. Physical and chemical properties of coke minerals and their (3) Gupta, S.; Sahajwalla, V.; Burgo, J.; Chaubal, P.; Youmans, T. Metall. Mater. Trans. B 2005, 36, 386-394. (4) Hilding, T.; Gupta, S.; Sahajwalla, V.; Bjo¨rkman, B.; Wikstro¨m, J. ISIJ Int. 2005, 45 (7), 1041-1050. (5) Gornostayev, S.; Ha¨rkki J. Fuel 2006, 85, 1047-1051. (6) Grigore, M.; Sakurovs R.; French D.; Sahajwalla, V. ISIJ Int. 2006, 46 (4), 503-512.
10.1021/ef060437d CCC: $37.00 © 2007 American Chemical Society Published on Web 12/15/2006
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distribution were characterized using X-ray diffraction and a scanning electron microscope (SEM) in order to develop a systematic approach to characterize coke minerals. Thermal Behavior of Coal Minerals. Inorganic matter in coal occurs mainly as crystalline or amorphous mineral grains chemically associated with the organic matter as either ionexchanged or as metallo-organics. The transformation of coal minerals including the impact of mineral distribution, size, and chemical nature have been extensively studied primarily in order to assess the impact of ash related issues on the thermal performance of power systems.7-15 Accordingly, mineral matter of pulverized coals is often further differentiated in two categories, namely, included and excluded minerals; the former are those which are intimately associated with carbon matter while excluded minerals are those which show no or negligible association with the organic matter of coal particles. In power systems, included minerals often experience relatively higher temperatures and evaporate more easily due to the proximity of carbon compared to excluded minerals.8,9 Included minerals also melt and coalesce to form dense ash particles, e.g., included clays and quartz react with pyrite or calcite to form low melting point aluminosilicate.7 Under combustion conditions, the vaporization of silica and calcium is strongly influenced by their mode of occurrence10-11 while Na and K species are more likely to vaporize irrespective of their mode of occurrence.11 The size distribution would influence the kinetics of mineral reactions, e.g., a large mineral grain is less reactive than a smaller one, as only the exterior surface participates in the reactions forming a thin sticky external layer.12 Fine silica in coal evaporates easily,13 and thermodynamically finer quartz and clay could retain greater amount of alkalis.14 The quartz phase of silica is believed to be less reactive compared to the silica present in the clay phase such as illite which can react with other minerals and coalesce.10 The chemical nature of minerals also influences the mechanisms of their physical transformation, e.g., pyritic iron is slowly oxidized without fragmentation, whereas ankerite or siderite is more likely to fragment.7,9,10 Calcium and magnesium when present in illite are expected to react more easily and display greater fluxing action compared to their carbonate phase.7,10 In the carbonization process, often minerals are relatively larger in size and undergo a prolonged heat treatment in the absence of air such that the local reacting environment could be highly reducing. Therefore, under coking conditions, unlike pulverized coal, most coal minerals can be described as being included in organic matter. A recent study provides a detailed description of coal mineral transformations at a range of temperatures.15 However, few studies have reported mineral (7) Benson, S. A.; Jones, M. L.; Harb J. N. Fundamentals of Coal Combustion; Smoot, L. D., Ed.; Elsevier Publishers: New York, 1993; Chapter 4. (8) Srinivasachar, S.; Helble, J. J.; Boni, A. A. Prog. Energy Comb. Sci. 1990, 16 (4), 281-292. (9) Srinivasachar, S.; Helble, J. J.; Boni, A. A.; Shah, N.; Huffman, G. P.; Huggins, F. E. Prog. Energy Comb. Sci. 1990, 16 (4), 293-302. (10) Raask, E. Mineral Impurities in Coal Combustion; Hemisphere Publishing: Bristol, PA, 1985. (11) Bryers, R. W. Prog. Energy Comb. Sci. 1996, 22, 29-120. (12) Wibberley, L. J.; Wall, T. F. Fuel 1982, 61, 93-99. (13) Boni, A. A.; Beer, J. M.; Bryers, R. W.; Flagan, R. G.; Helble, J. J.; Huffman, G. P.; Huggins, F. E.; Peterson, T. W.; Sarofim, A. F.; Srinivasachar, S.; Wendt, J. O. J. US DOE Report No. DE-AC2286PC90751, 1990. (14) Gupta, R. P.; Wall, T. F. BehaVior of Refractory Oxides During Combustion of High Si/Ca/Mg Coals; University of Newcastle: Newcastle, Australia, 1995. (15) Vassileva, C. G.; Vassilev S. V. Fuel Process. Technol. 2005, 86 (12-13), 1297-1333.
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transformations during the coking process,16-19 most of which are based on conventional oxide analysis of bulk coke rather than the size and distribution of mineral phases. Therefore, there is clear lack of understanding of coke minerals particularly their nature of distribution in the coke matrix. Due to the complex interrelationship between mineral size, chemistry, and distribution and coke properties such as reactivity and strength, it is difficult to divide minerals into clearly distinct groups. However, on the basis of thermal stability, fusibility behavior, and particularly the possible impact of coke minerals on coke reactivity, the parent coal minerals can be broadly described in three groups. Refractory/Least Reactive Phases. Quartz and clay minerals such as kaolinite (Al2O3‚2SiO2‚2H2O) are the most common and often abundant minerals found in coal. Kaolinite decomposes to metakaolinite between 823 and 873 K,20,21 which remains stable up to 1273 K. On further heating at higher temperatures, metakaolinite decomposes to a spinel form of γ-Al2O3 or mullite.22 After heating at a temperature more than 1473 K, the γ-Al2O3, which could be present in dehydrated kaolinite, disappears with the formations of mullite and cristobalite.23 Mullite and cristobalite are the major stable phases up to 1673 K, while upon heating further up to 1773 K, the tetragonal modification of silica to cristobalite persists along with mullite.21 The frequent occurrence of a wide range of cations such as Ca, Fe, Mg, K, and Na in clay minerals can also affect their thermal behavior, e.g., the presence of iron accelerates mullitization; potassium inhibits mullitization while calcium is not known have any significant impact.24 In general, kaoline derivatives in coke when present in isolation are of a refractory nature; however, due to a high specific surface area, metakaolinte could react with other minerals and their decomposition products by providing a framework for the formation of a range of silicates and Al-silicates including anorthite, pyroxenes, melilites, kalsilite, and nepheline.18,25,26 Quartz (SiO2) contents of the coal inorganic matter vary from 30 to 60%. With increasing temperature, quartz could transform to several polymorphs of distinctively different structures and thermal stabilities.10,23 Quartz transforms to tridymite in a temperature range from 1043 to 1743 K and to cristobalite at a higher temperature of 1743 K.23 Compared to quartz, both tridymite and cristobalite are open structures of layers of sixmembered rings of SiO4 tetrahedra. The kinetics of quartz transformations is generally slow and is associated with slight (16) Mahoney, M.; Rogers, H.; Andriopoulos, N.; Gupta, R. ACARP Project Report C9059; Australia, 2002. (17) Price, J. T.; Iliffe, M. J.; Khan, M. A.; Gransden, J. F. Minerals in coal and high temperature properties of coke. In Proceedings of the 53rd Ironmaking Conference, Chicago, IL, March 20-23; ISS: Warrendale, PA, 1994; pp 79-87. (18) Kerkkonen, O.; Mattila, E.; Heiniemi R. The correlation between reactivity and ash mineralogy of coke. In Proceedings of the 55th Ironmaking Conference, Pittsburgh, PA, March 24-27; ISS: Warendale, PA, 1996; pp 275-282. (19) Vallova, S.; Slovak, V.; Lesko J. J. Therm. Anal. Calorim. 2003, 71 (3), 875-881. (20) Deer, W. A.; Howie, R. A.; Zussman, J. An Introduction to the Rock-Forming Minerals; Longman Scientific & Technical: Essex, UK, 1992. (21) O’Gorman, J. V.; Walker P. L. Fuel 1973, 52, 71. (22) Watt, J. D. Br. Coal Utilisat. Res. Assoc. 1959, XXIII (2); Monthly Bulletin. (23) Todor, D. N. Thermal Analysis of Minerals; Abacus Press: Kent, UK, 1976; p 102. (24) Parmelee; Rodriguez. J. Am. Ceram. Soc. 1942, 25, 1. (25) Falcone, K.; Schobert, H. H. Mineral Matter and Ash in Coal; Vorres, K. S., Ed.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986; Vol. 301, pp 114-126. (26) Ford, W. F. The effect of heat on ceramics; Institute of Ceramics Textbook Series; Maclaren & Sons. Ltd.: London, 1967; p 117.
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Table 1. Proximate Analysis of the Three Cokes (Dry Basis) code
volatile matter
fixed carbon
ash content
sulfur
coke A coke B coke C
1.2 0.5 2.0
88.8 91.0 88.9
10.0 8.5 9.1
0.54 0.45 0.69
Table 2. Chemical Composition of Coke Ash Expressed as Oxides Shown as weight percent, S-Free Basis coals
SiO2
Al2O3 Fe2O3 CaO MgO K2O Na2O TiO2 P2O5
coke A 52.65 31.61 coke B 53.37 28.03 coke C 48.63 27.94
8.62 9.73 16.22
2.39 3.58 1.59
0.77 1.15 0.93
0.74 1.22 1.90
0.75 0.92 0.74
1.62 1.71 1.64
0.84 0.28 0.41
expansion due to lower density of tridymite and cristobalite,26,27 depending on the chemical environment and proximity of other phases.10 Under severe reducing environments, the refractory nature of quartz or its polymorphs can be compromised due to reaction with adjacent pyrite or carbon phase particularly at higher temperatures due to carbothermal reduction of silica and molten silicate formation. Solid-state reactions could also occur even at a relatively low temperature.28-30 The presence of tridymite and cristobalite in coke has been reported before.16 Quartz, kaolinite, or feldspar are not believed to have any significant effect in modifying the coke reactivity.18 However, during CSR (coke strength after reaction) test conditions, cracks were observed around large clays mineral, which implied that these minerals could have a mechanical impact on coke strength.18 Semirefractory/Moderate Reactive Phases. Illite and other swelling clays are considered as semirefractory coal minerals as these are not significantly changed under coking conditions but become reactive at higher temperatures, depending on the reacting environments and the presence of impurities. Illite is commonly observed in bituminous coals some of which contain more than 25 wt % of total mineral matter.10 Illite has a variable composition which can be most suitably expressed as (Ky(Al4)(Si8-y, Aly)O20(OH)4), but other cations such as Na, Mg, and Fe are often present.20 Illite decomposes exothermically in the temperature range of 1023-1173 K, resulting in the formation of spinels of increasing size with increasing temperature21 while mullite formation continues from 1373 to 1673 K.28,29 In general, natural illite contains smectite interlayers, which shows a different behavior at temperatures less than 1173 K, while compositions closer to that of muscovite could raise the melting temperature by approximately 373 K.20 Illite transforms to the glassy phase in a temperature range between 1200 and 1375 K,31 which may be attributed to the fluxing effect of potassium and ferrous iron which are often present.8,9 At higher temperatures, illite is also known to show swelling and pore formation.32 Illite derivatives have been reported in coke,16 and due to their swelling nature, they could have a greater effect on the coke reactivity when compared to quartz and kaolinite derivatives in coke.18 Reactive Phases. Calcite, pyrite, siderite, and gypsum are other minerals commonly found in coals. In coking coals, generally calcium occurs as calcite while generally potassium is believed to occur in illite.10 Pyrite phases are transformed to (27) Mitchell, R. S.; Gluskoter, H. J. Fuel, 1976, 55, 90. (28) Grim, R. E.; Bradely, W. F. Am. Mineral. 1948, 33, 50. (29) Grim, R. E. Applied clay mineralogy; McGraw-Hill: New York, 1962; p 422. (30) Vassilev, S. S.; Kitano, K.; Takeda S.; Tsure T. Fuel Process. Technol. 1995, 45, 27. (31) Segnit, Fr.; Anderson, C. A. Trans. Br. Ceram. Soc. 1972, 71, 85. (32) Hubbard, F. H.; McGill, R. J.; Dhir, R. K.; Ellis, M. S. Am. Mineral. 1984, 48, 251.
Figure 1. (a) XRD patterns of raw coke A and the 643 and 873 K ash samples: (Q) quartz; (Cr) cristobalite; (M) mullite; (P) pyrrhotite; (H) hematite; (Fe) metallic iron; (C) corundum; (A) anhydrite; (R) rutile. (b and c) Similar XRD spectra of coke B and C, respectively.
a number of phases including metallic iron and oxide phases,33 while calcite decomposes to either join with sulfur related from pyrite or reacts with quartz and clay phases to form silicate phases. Magnitude of Ca and Fe phases in coke will be dependent on the nature of location of the occurrence of these minerals in parent coals. Transformation of sulfide and carbonate phases of iron and calcium will also depend on the reaction environment resulting in the formation of a wide range of phases such as hematite, jarosite, and anhydrite, etc.15 In an operating blast furnace, the presence of recirculating alkalis could also combine with coke constituents to form intermediate compounds which may also behave as reactive mineral phases and are often difficult to identify and quantify.16,34 (33) Huffman, G. P.; Huggins, F. E. Mineral Matter and Ash in Coal; Vorres, K. S., Ed.; ACS Symposium Series; American Chemical Society: Washington, D.C., 1986; Vol. 301, pp 110-113.
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Figure 2. Comparison of XRD patterns of raw coke and its low temperature ash (LTA) sample. Table 3. Major Minerals in LTA Specimens of Cokes (weight percent) as Measured by SIROQUANT mineral crystalline (%) amorphous quartz cristobalite rutile mullite pyrrhotite wuestite fluorapatite wollastonite calcite anhydrite bassanitea jarositea a
formula
coke A
coke B
coke C
SiO2 SiO2 TiO2 Al6Si2O13 Fe1-xS FeO Ca5(PO4)3F CaSiO3 CaCO3 CaSO4 CaSO4‚0.5H2O KFeSO4
18 82 9.2 1.1 0.0 71.3 0.0 0.0 0.0 12.1 2.9 2.9 0.6 0.0
39 61 25.7 2.3 0.0 61.4 2.6 0.0 0.0 0.0 0.0 0.0 3.6 4.4
20 80 56.7 0.0 3.0 19.9 2.0 0.5 4.0 0.0 0.0 4.0 6.0 4.0
Minerals formed during low temperature ashing.
Experimental Materials. Metallurgical cokes were obtained from three largescale operating blast furnaces from the USA. All cokes had relatively high coke strength after reaction (CSR > 60) values. Proximate analysis of the cokes, including the ash content, is provided in Table 1. Sulfur content of the cokes was measured separately using a LECO analyzer. X-ray Diffraction (XRD) Analysis. The crystalline mineral phases of the cokes were identified and quantified using X-ray diffraction analysis. The identification of the mineral phases in coke is difficult because of the high percentage of carbon (about 90%), which produces peaks with high intensity and reduces the intensity of the mineral phases through dilution. Therefore, the organic matter of coke is removed by oxidation at different temperatures. Mineralrich coke specimens were prepared by slow combustion of coke in air at 873 and 643 K in air in a muffle furnace. Each coke piece was crushed to powder for XRD examination using a Siemens D5000 powder diffractometer at the University of New South Wales. The XRD data was acquired in a two theta range from 2 to 80° using Cu KR radiation, a step size of 0.02, and counting 1 s per step. SIROQUANT. Mineral-rich specimens of cokes were also prepared using radio frequency oxygen plasma ashing at much lower temperatures of 393 K. This ash will be referred as low temperature ash (LTA) in the subsequent discussion. The XRD diffractograms of the LTA samples were obtained with a Philips PW1050 goniometer at CSIRO using Co KR radiation at 45 kV and 30 mA, with step scans over the two theta range from 3 to 90°, a step interval of 0.04°, and a 10 s count time per step. SIROQUANT which uses the full-profile Rietveld method for refining the shape of a calculated XRD pattern against the profile of a measured pattern35 was used to quantify the minerals in the ash. The mineralphase quantification error was typically less than 0.3% in the LTA sample.6 (34) Chan, B. K. C.; Thomas, K. M.; Marsh, H. Carbon, 1993, 31, 1071. (35) Taylor, J. C. Powder Diffr. 1991, 6, 2.
Scanning Electron Microscope (SEM) Analysis. A field emission scanning electron microscope (Hitachi S4500) was used to examine the physical nature of the coke minerals and their distribution in the coke matrix. A small lump specimen of coke was embedded in epoxy and polished for SEM analysis. Qualitative element identification and semiquantitative analyses of the observed minerals were obtained using a LINK ISIS 200 energy dispersive X-ray (EDX or EDS) micro-analysis system. The EDX analyses were performed on particles larger than 5 µm by operating the SEM at an accelerating voltage of 20 kV. Approximately 20-40 spots were analyzed in each coke sample. The intensity ratio of the various constituent elements present in the EDS spectra were also used to identify the chemical composition of some of the mineral phases.
Results and Discussion The influence of the inorganic matter of coke on coke properties is often estimated on the basis of bulk ash analysis. The ash content was about 10% in coke A, 8.5% in coke B, and 9% in coke C, all less than the 10% typically found in metallurgical cokes. Elemental composition is indicated as their respective oxides in Table 2. The alumina and silica contents of all three cokes are similar, but coke C has significantly more iron than the other two cokes, is slightly enriched in potassium, and is correspondingly depleted in calcium. Coke B contains higher calcium and magnesium and lower phosphorus levels than the other two cokes. Coke A has relatively higher phosphorus content than other two cokes. Oxide analysis of coke ash inherently implies that the inorganic matter is homogeneous dispersed throughout the coke and provides no indication of either the mineral assemblage or distribution. Due to the physical and the chemical heterogeneity of the inorganic matter and its distribution within the carbon matrix, no single analytical technique is sufficient to characterize all aspects of coke mineralogy. XRD was used to identify and quantify the crystalline phases while the SEM analysis was used to examine the physical and chemical distribution of the various minerals. Identification and Quantification of Crystalline Phases. Figure 1 shows the XRD patterns of cokes and their mineralrich specimens. Quartz (Q) and mullite (M) peaks were clearly visible in the XRD spectra of each coke and are well-known to occur in cokes.6 Clay minerals of parent coking coals such as kaolinite, illite, and smectite are well-known to decompose to the mullite phase.29 Figure 1c shows that the mullite peak is the least obvious in the XRD pattern of raw coke C when compared to other raw cokes; however, mullite peaks can be clearly seen in the 643 and 873 K ash spectra of coke C. The cristobalite peaks were most clearly visible in the raw coke B (Figure 1b). Tridymite, another polymorph of silica, thermally stable up to 1743 K,20 was not observed in any of the three cokes unlike in a previously reported study.18 The absence of tridymite implies that cristobalite was attributed to the clay decomposition rather than quartz transformation. The XRD patterns of all the raw cokes contain small peaks indicating the presence of metallic iron (Fe). Other iron phases, such as iron oxides, silicate, and sulfides phases, often believed to occur in coke33 could not be positively identified in the XRD spectra of the raw cokes. On the other hand, the XRD patterns of the low-temperature coke ashes show peaks which can be attributed to quartz (Q), mullite (M), hematite (H), and anhydrite (Figure 1). With an increased ashing temperature, the intensity of the metallic iron peak decreased and was difficult to observe in the diffractograms of the 873 K ash of cokes A and B (Figure 1a and b) and completely absent in the coke C ash samples (Figure 1c). The 873 K ash of coke C indicated the presence of
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Figure 3. SEM images illustrating the heterogeneity of size and distribution of coke minerals: (a and b) coke C; (c and d) coke B, and (e and f) coke A. Bright spots, gray areas, and darker phases show minerals, the carbon matrix, and pores, respectively. Typical agglomerate (b), disseminated (c and d), and pore minerals (f) can also be seen. Table 4. Summary of Mineral Characterization Scheme physical basis
thermal/chemical basis
agglomerate (>100-1000 µm) coarse (50-100 µm)
refractory (least reactive)
discrete/cluster
semirefractory (moderate reactive) reactive
disseminated
fine (
