Energy & Fuels 2007, 21, 2637-2641
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An Example of Alkalization of SiO2 in a Blast Furnace Coke S. S. Gornostayev,*,† P. A. Tanskanen,† E.-P. Heikkinen,† O. Kerkkonen,‡ and J. J. Ha¨rkki† Laboratory of Process Metallurgy, UniVersity of Oulu, P.O. Box 4300, Oulu, 90014, Finland, and Ruukki Oyj, P.O. Box 93, Raahe, 92101, Finland ReceiVed March 11, 2007. ReVised Manuscript ReceiVed May 27, 2007
Scanning electron microscopy and an electron-microprobe analysis of a sample of blast furnace (BF) coke have revealed alkalization (5.64 wt % Na2O + K2O) and Al saturation (17.28 wt % Al2O3) of SiO2 by BF gases. The K/Naat value of 1.15 in the new phase (alteration zone) reflects close atomic proportions of the elements and suggests that the abilities to incorporate K and Na during the process are almost equal. This Al saturation and alkalization of SiO2 indicates an active role for Al along with alkali metals in BF gases. The average width of the altered area in the SiO2 grain is about 10 µm, which suggests that SiO2 particles of that size can be transformed fully to the new phase, provided that at least one of their faces is open to an external pore (surface of the coke) or internal pore with circulating BF gases. The grains that exceed 10 µm can only be partly altered, which means that smaller SiO2 grains can incorporate more alkali metals and Al (during their transformation to the Al and alkali-bearing phase) than a similar volume of SiO2 concentrated in larger grains. Thermodynamic calculations for 100 gsolid/100 ggas and temperatures 800-1800 °C have shown that the BF gases have very little or no effect on the alkalization of SiO2. If the alteration process described in this paper proves to be a generalized phenomenon in blast furnace cokes, then the addition of fine-grained quartz to the surface of the coke before charging a BF can be useful for removing of some of the Al and alkali from the BF gases and reduce coke degradation by alkalis, or at least improve its properties until the temperature reaches approximately 2000 °C.
Introduction Metallurgical coke is made from a mix of several types of coal that include various inorganic compounds (minerals), by heating the mix to ∼1100-1200 °C in coke batteries. Taking into account the data collected by Finkelman1 that coals contain more than 125 mineral species, one can suggest that the hightemperature conditions involved in coking processes should cause a wide range of phase transformations and lead to the formation of a number of new phases, commonly known as “ashes”, the “generic term for the various solid products of the coal consumption process”.2 The amount and nature of the inorganic compounds in metallurgical coke are regarded as factors having a significant effect on coke properties and on its behavior in the blast furnace (BF).3-7 Nevertheless, data on the mineral phases in a coke in the BF are still scarce, and the behavior of these phases during * Author to whom correspondence should be addressed. E-mail address:
[email protected]. † University of Oulu. ‡ Ruukki Oyj. (1) Finkelman, R. B. Modes of occurrence of trace elements in coal. Open-File Rep. - U. S. Geol. SurV. 1981, 322, 81-99. (2) Ghosh, S. K. Understanding thermal coal ash behavior. Min. Eng. (Littleton, CO, U. S.) 1985, 37, 158-162. (3) Tsikarev, D. A. The effect of mineral additives to coal charges on the properties of blast furnace coke. Koks Khim. 1993, 4, 30-32. (4) Kerkkonen, O. Influence of ash reactions on feed coke degradation in the blast furnace. Coke Making Int. 1997, 9, 34-41. (5) Gornostayev, S. S.; Kerkkonen, O.; Ha¨rkki, J. J. Importance of mineralogical data for influencing properties of coke: A reference on SiO2 polymorphs. Steel Res. Int. 2006, 77, 770-773. (6) Hilding, T.; Gupta, S.; Sahajwalla, V.; Bjo¨rkman, B.; Wikstro¨m, J.O. Degradation Behaviour of a High CSR Coke in an Experimental Blast Furnace: Effect of Carbon Structure and Alkali Reactions. ISIJ Int. 2005, 7, 1041-1050.
coke consumption processes in the BF is still poorly understood. In particular, the influence of alkali-bearing minerals was reported as less certain6 than, for example, the catalytic effect of iron.7-9 The changes that take place in the mineral phases of BF coke can be regarded as both structural and chemical. The structural changes, in particular,4 include the expansion of meta-clays and swelling and balling-up of aluminosilicates, while the chemical changes are more complex and include mineral phase decomposition, alkalization, and the formation of new crystalline and glass phases. It has been reported that alkalization causes saturation of the “meta-illite” and “meta-kaolinite” by K and Na, at least in the cohesive and stagnant zones, and can lead4 to the appearance of phases close in composition to leucite, KAlSi2O6; kalsilite, KAlSiO4; and nepheline, [Na,K]AlSiO4. No information has been available so far on the “aluminization” of mineral phases in BF coke. Quartz is one of the most abundant phases among the minerals that can be found in coals, and for that reason, investigation of its behavior is essential for an understanding of the mineralrelated properties of coke. We have recently discussed the importance of mineralogical data for influencing properties of coke with respect to SiO2 polymorphs.5 It was suggested that a high amount of free quartz (especially in a coarse-grained form) (7) Gupta, S.; Dubikova, M.; French, D.; Sahajwalla, V. Characterization of the Origin and Distribution of the Minerals and Phases in Metallurgical Cokes. Energy Fuels 2007, 21, 303-313. (8) Gornostayev, S.; Ha¨rkki, J. Graphite crystals in blast furnace coke. Carbon 2007, 45, 1145-1151. (9) Feng, B.; Bhatia, S. K.; Barry, J. C. Structural ordering of coal char during heat treatment and its impact on reactivity. Carbon 2002, 40, 481486. (10) Heaney, P. J. Structure and Chemistry of the low-pressure silica polymorphs. ReV. Miner. 1994, 29, 1-40.
10.1021/ef070125q CCC: $37.00 © 2007 American Chemical Society Published on Web 07/31/2007
2638 Energy & Fuels, Vol. 21, No. 5, 2007
GornostayeV et al.
Figure 1. BSE image (Jeol JXA-8200) of a SiO2 grain in the blast furnace coke (plates A and B) and interpretation of its alteration to an Al and alkali-bearing phase (Plate C). c1, c2, and c3: endpoints of cracks (see text for details). Scale bar: (A) -100 µm; (B) -10 µm.
Figure 2. X-ray mapping for K, Na, and Al. Dark rounded spots on K and Na maps are points of EPMA analysis (Table 1).
in a coal blend can be considered a negative factor for the physical properties (strength) of a coke. The best way to study the process of SiO2 alteration by BF gases is to find a grain with gradual changes, where one can see the “original” (unaltered) part and the part affected by the gas phase. Such grains must be located on pore walls or cracks to be able to react with the gas phase. However, many of the mineral phase grains in a coke can be found surrounded by the coke matrix. The ratio of grains on pore walls/grains in a coke matrix can vary, for example, depending on the amount and the size of the pores. The grains, which are open to pores or cracks, are, in many cases, heavily altered, and it is very difficult to identify the primary phase. There are also many other mineral phases that occur in a coke. For that reason, not all of the grains, which are found on pore walls or cracks, will be represented by SiO2. So, partly altered grains of SiO2 are unique fingerprints of alteration (transformation) features of this phase under the BF conditions, and they should be carefully documented even on a single case basis. We describe in this paper a particular case of chemical changes of SiO2 to an Al and alkali-bearing phase by the BF gases and the capacity of the new phase with respect to Na and K. Samples and Methods The sample for this study was taken from a drill core obtained from the tuyere zone of an operating BF at the Ruukki Steel Works, Finland, using a mobile tuyere rig. The original location of the piece of BF coke was 150 cm from the tuyere level. The details of the tuyere drilling were similar to those reported earlier.11 A plate 5 mm thick was cut from the central part of the sample, and a 22 mm circle was drilled to obtain a rounded plate, which was mounted in a 1-in. plastic holder and filled with epoxy for the preparation of a polished section. The polished section was first studied under an optical microscope in reflected light and then with a Jeol JSM-6400 scanning electron microscope (SEM) equipped (11) 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, Sept. 15-17, 2004; Association for Iron & Steel Technology: Warrendale, PA, 2004; Vol. 1, pp 469-481.
with an energy-dispersive spectrometer (EDS) and INCA software at the Institute of Electron Optics, University of Oulu, Finland. The EDS data were used for identification and preliminary assessment of the composition and homogeneity of the mineral phases. Finally, a selected mineral grain was studied with a Jeol JXA-8200 Electron Probe Microanalyzer (EPMA) equipped with five wavelengthdispersive spectrometers (WDSs). The conditions for the WDS analyses were as follows: accelerating voltage 15 kV, probe current 15 nA, and beam diameter 1-5 µm. Both natural minerals and pure metals were used as standards. All of the analyses were normalized to 100% of Na2O, K2O, MgO, CaO, MnO, Al2O3, TiO2, FeO, ZnO, and SiO2 for statistical calculations, comparison, and plotting.
Results and Discussion A number of mineral phases of varying size (Figure 1A) and composition were observed in the coke matrix of the sample. Some of them were further away from pores, while others had one of their sides open to the pores or extensive cracks (Figure 1B) and consequently were accessible for interaction with circulating BF gases. The latest mineral phases are of particular importance, since they can be used to assess the extent (depth) of interaction of BF gases with a particular mineral phase and can also be useful for quantitative estimations of such interaction, that is, of the capacity of a given mineral phase to retain elements circulating in the gas phase and for qualitative assessment of the gas-phase composition. The grain shown in Figure 1B has two areas which can be distinguished one from the other by the intensity of their gray color. In the case of a scanning electron microscope image, it means that the lighter area that is open to the crack (external, exposed part of the grain) contains heavier elements than the internal part of the grain. The boundary between the parts is not sharp, but it is nevertheless quite clear. X-ray mapping (Figure 2) has shown the presence of relatively high amounts of K, Na, and Al in the external part of the grain. The WDS analyses revealed that the internal part of the grain (Table 1, analyses 1-4, 18) is composed of SiO2 with small amounts of Na, K, Al, Fe, Ca, Mn, Ti, V, Cr, and Zn (close to
An Example of Alkalization of SiO2
Energy & Fuels, Vol. 21, No. 5, 2007 2639 Table 1. Normalized Data of Electron Microprobe Analysis (wt %)a
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 min (5-17) max (5-17) av. (5-17)
Na2O
K 2O
MgO
CaO
MnO
Al2O3
TiO2
FeO
ZnO
SiO2
K2O + Na2O
0.00 0.00 0.04 0.01 2.35 2.05 1.93 2.48 2.68 2.42 2.89 1.99 1.64 1.81 1.65 1.37 1.82 0.02 1.37 2.89 2.05
0.01 0.01 0.02 0.02 4.10 4.07 3.96 4.03 4.27 4.31 4.38 4.33 2.95 2.90 2.97 2.94 3.12 0.00 2.90 4.38 3.58
0.00 0.00 0.00 0.00 0.25 0.19 0.17 0.26 0.40 0.20 0.29 0.32 0.25 0.23 0.40 0.30 0.45 0.00 0.17 0.45 0.31
0.00 0.01 0.00 0.02 0.03 0.02 0.02 0.00 0.07 0.03 0.03 0.03 0.03 0.00 0.03 0.03 0.07 0.00 0.00 0.07 0.04
0.02 0.00 0.00 0.02 0.00 0.00 0.00 0.00 0.03 0.07 0.01 0.03 0.00 0.00 0.00 0.01 0.00 0.00 0.00 0.07 0.02
0.00 0.00 0.01 0.01 16.65 15.10 14.14 16.32 17.52 16.15 17.58 17.84 16.45 16.89 18.92 16.38 19.09 0.04 14.14 19.09 17.28
0.00 0.02 0.01 0.02 0.05 0.00 0.00 0.02 0.02 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.02 0.00
0.00 0.01 0.02 0.04 0.03 0.05 0.02 0.05 0.00 0.06 0.00 0.06 0.08 0.04 0.04 0.05 0.05 0.04 0.00 0.08 0.04
0.02 0.11 0.12 0.07 0.02 0.00 0.00 0.02 0.04 0.00 0.00 0.07 0.00 0.00 0.00 0.00 0.00 0.02 0.00 0.07 0.02
99.95 99.84 99.78 99.79 76.52 78.53 79.76 76.81 74.97 76.77 74.81 75.33 78.60 78.13 75.98 78.91 75.39 99.87 74.81 78.91 76.60
0.01 0.01 0.06 0.02 6.45 6.11 5.89 6.51 6.95 6.73 7.27 6.32 4.59 4.71 4.62 4.31 4.94 0.02 4.31 7.27 5.64
a Jeol JXA-8200 electron microprobe (Institute of Electron Optics, University of Oulu, Finland). Conditions of the analysis: accelerating voltage 15 kV, probe current 15 nA, beam diameter 1-5 µm. Both natural minerals and pure metals were used as standards.
the detection limit in some cases). A number of other grains of SiO2 from the same sample, but encapsulated in the coke matrix, were also analyzed. The amount of impurities in these grains was similar to those observed in an unaltered (internal) part of the grain listed in Table 1. Given the fact that the temperature level in a tuyere zone exceeds 1500 °C, the SiO2 must be represented here by cristobalite.5 The amounts of the above impurities in this zone of the grain were within the range of those reported for some terrestrial and extraterrestrial tridymites and cristobalites.12 This is in agreement with the well-known fact that “quartz has a very low tolerance for the incorporation of impurities because its void space is fairly cramped”.10 It means that any SiO2 polymorph (tridymite, cristobalite) produced from quartz in a tuyere zone should have a very low amount of impurities. The external part of the grain which is open to the pore and has a light gray appearance in the image is marked as Si-Al-K-Na-O in Figure 1C and contains much greater amounts of Na2O (1.37-2.89 wt %; average, 2.05), K2O (2.90-4.38 wt %; average, 3.58), and Al2O3 (14.14-19.09 wt %; average, 17.28), as seen in Table 1, analyses 5-17. The alkali total in the Si-Al-K-Na-O area is quite high (4.317.27 wt %) with an average of 5.64 wt %, but this is only a third of the amount of Al2O3. The ratio Al/(Na + K)at is 2.38. The amount of potassium in this part of the grain is higher than that of sodium with an average K2O/Na2O (wt %) ratio of about 1.75, while K/Naat is 1.15, reflecting almost equal atomic proportions of the two elements. The average composition of the Si-Al-K-Na-O area (Table 1, analyses 5-17) in the phase diagram13 K2O-Al2O3SiO2 (Figure 3) falls into the field of stability of mullite, a mineral that has the ideal formula Al6Si4O13, and as the field of stability of mullite in the Na2O-Al2O3-SiO2 system13 is quite similar, the phase also falls into that. The fact that the K/Naat ratio is close to 1 may suggest that the abilities of SiO2 to incorporate potassium and sodium during its transformation to the new phase are almost equal. This diagram also shows (12) Schneider, H. Chemical composition of tridymite and cristobalite from volcanic and meteoritic rocks. Neues Jahrb. Mineral., Monatsh. 1986, 10, 433-444. (13) Osborn, B. F.; Muan, A. Phase Equilibrium Diagrams of Oxide Systems, Plate 5; The American Ceramic Society and the E. Orton Jr. Ceramic Foundation: Columbus, OH, 1960.
Figure 3. Composition of the Al and alkali-bearing phase in BF coke in the K2O-Al2O3-SiO2 system. The compositional fields and temperatures (centigrade) are from ref 13, Figure 407. Solid square: average of analyses 5-17 in Table 1.
that the alkali retention capacity of the new phase can be somewhat higher, with its maximum level at the invariant point at 1315 °C (Figure 3). Any further increase in potassium concentration in the system at this temperature should cause the crystallization of leucite, KAlSi2O6. Since this grain, and especially its external face that is open to the pore, has no direct contact with the other mineral particles, it seems that the appearance of the Si-Al-K-Na-O phase may be related to alteration of the primary phase of SiO2 composition by circulating BF gases alone. The presence of potassium and sodium in a BF gas phase is a widely known phenomenon4 and has great implications for coke properties and the behavior of coke in a BF, while the existence of Al in the BF gases has not been widely reported. The other interesting feature of the grain is the presence of cracks (Figure 1 B,C), which are most likely temperature-related, since they have an irregular (dendrite-like) appearance and have no extensive linear parts that would indicate mechanical origin. These cracks were obviously formed before the appearance of the Si-Al-K-Na-O zone, since they end up in the SiO2 area
2640 Energy & Fuels, Vol. 21, No. 5, 2007
and do not cross the boundary between the phases (see the interpretation of Figure 1C, where some endpoints of cracks are marked as c1, c2, and c3). The latter may suggest that the cracks were healed during the formation of the new phase, which could happen only if the related host phase was in a molten state. The melting point of cristobalite is 1713 °C,14 so the temperature here should have been above that. Nevertheless, experimental data15-17 and thermodynamic calculations18 have shown that the melting point of SiO2 can be somewhat lower in the presence of K and Na. Anyway, the composition of the new phase plotted on a K2O-Al2O3-SiO2 diagram (Figure 3) is above the 1600 °C isotherm and gives a temperature estimate somewhere between 1700 and 1800 °C. The formation of the new phase under conditions of rising temperature may have taken place in two stages:
GornostayeV et al.
Figure 4. The fraction of liquid phases (%) in the Al and alkali-bearing system (Table 1, average of analyses 5-17) at 800-1800 oC. Solid line: inert atmosphere (Ar). Dashed line: BF gas.
SiO2 + (K,Na)2O(from a gas phase) f K-Na-Si-Omelt (1) K-Na-Si-Omelt + AlxOy(from a gas phase) f Si-Al-K-Na-Omelt (2) When the coke sample was removed from the BF, cooling of the Si-Al-K-Na-O melt caused the formation of the respective glass phase (glass 1). There is also a possibility that two phases may form upon fast cooling (this is the case when the drill core is removed from the BF): tiny crystals of mullite (Al6Si4O13) and another interstitial glass phase depleted in Al and enriched in K and Na (glass 2). The microprobe analysis in this case would actually represent a mix of the two phases. Nevertheless, the SEM studies did not reveal the presence of crystals in the Si-Al-K-Na-O zone. In either case (formation of glass 1 or mullite + glass 2), the bulk system is enriched in Al and alkalis in comparison with the primary phase (SiO2). There is no information at present on the form in which Al occurs in a gas phase of a tuyere zone. We suggest that it can form tiny crystals by the evaporation of a refractory liquid followed by the rapid crystallization of some AlxOy phase. Such a mechanism was recently referred to in the formation of spinel (FeAl2O4) in the blast furnace coke.19 The temperature estimates with the phase diagram (Figure 3) were made, however, without consideration of the possible influence of circulating BF gases. In order to resolve this issue, thermodynamic calculations with the FactSage 5.4.1 program20 were made for temperatures 800-1800 °C. The initial composition of the system was as follows: 100 g of the solid phase (Table 1, average of analyses 5-17) and 100 g of the BF gas with 44% CO, 4% H2, and 52% N2.21 The calculations were (14) Brunk, F. Refractory materials; Vulkan-Verlag: Essen, Germany, 1997; pp 39-44. (15) Kracek, F. C. The System Sodium Oxide-Silica. J. Phys. Chem. 1930, 34, 1583-1598. (16) Kracek, F. C. The Ternary System, K2SiO3-Na2SiO3-SiO2. J. Phys. Chem. 1932, 36, 2529-2542. (17) Kracek, F. C.; Bowen, N. L.; Morey, G. W. Equilibrium Relations and Factors Influencing Their Determination in the System K2SiO3-SiO2. J. Phys. Chem. 1937, 41, 1183-1193. (18) Yazhenskikh, E.; Hack, K.; Mu¨ller, M. Critical thermodynamic evaluation of oxide systems relevant to fuel ashes and slags. Part 1: Alkali oxide-silica systems. Calphad 2006, 30, 270-276. (19) Gornostayev, S.; Ha¨rkki, J. Spinel crystals in tuyere coke. Metall. Mater. Trans. B 2005, 36 (2), 303-305. (20) Bale, C. W.; Chartrand, P.; Degterov, S. A.; Eriksson, G.; Hack, K.; Ben Mahfoud, R.; Melancon, J.; Pelton, A. D.; Petersen, S. FactSage Thermochemical Software and Databases. Calphad 2002, 26, 189-228. (21) Quinn, G.; Faraj, B.; Callcott, R.; Callcott, T. Elucidation of the Effects of Minerals on Coke BehaViour in the Blast Furnace; Australian Coal Association Research Program, Project C10054, Australian Coal Research Limited: Brisbane, Australia, 2002; pp 1-83.
Figure 5. Transformation of SiO2 to the Al and alkali-bearing phase. Note: (1) SiO2 grains of varying size and morphology; (2) Si-AlK-Na-O zone in SiO2; (3) pores, (a) external and (b) internal.
also made for an inert atmosphere (Ar). The temperatures of solidus are about 914 °C for the inert atmosphere and 926 °C in the presence of the BF gas phase. The first substantial increase of a liquid phase happens at about 985 °C in the inert atmosphere and ∼975 °C in the presence of the BF gas phase. After that, the curves are almost identical (Figure 4), which suggests that the BF gases have very little or no effect on the system under the referred to temperatures and solid/gas ratio. The width of the alteration zone (Si-Al-K-Na-O) in the middle of the edge exposed to the pore is between 7 and 10 µm, and it is ∼10-15 µm along its contact with the coke matrix on the left side of the image (Figure 1B). Thus, the average width of the Si-Al-K-Na-O area is about 10 µm. This observation allows us to suggest that SiO2 grains of such size can be transformed fully to the new phase under the conditions referred to here, provided that at least one of their faces is open to an external pore (surface of the coke) or an internal pore with circulating BF gases (Figure 1C). Other grains that exceed 10 µm in size can only be partly altered (Figure 1C). These data are summarized in Figure 5. One important consequence of this feature is that the smaller the SiO2 grains we have in a coke, the better (more fully) they
An Example of Alkalization of SiO2
will react with the circulating alkali and Al. In other words, small SiO2 grains can incorporate more alkali metals and Al in a BF coke (during their transformation to the Si-Al-K-Na-O phase) than a similar volume of SiO2 concentrated in larger grains. From this point of view, one may consider that the addition of fine-grained quartz to the surface of the coke before charging the BF can remove some of the Al, K, and Na from the BF gases and can reduce coke degradation by alkalis in a BF, or at least can improve its properties up to ∼2000 °C when the composition of the system is in the field of stability of corundum. In this case, the system is capable of retaining a certain amount of Na and K as it comes from the K2O-Al2O3SiO2 phase diagram (Figure 3). It is obvious that the quartzbearing rocks (e.g., quartzites) intended for utilization must be as free from impurities as possible, and they must be checked for alkali minerals (e.g., micas) before possible use. The latter can also be taken to refer to the overall burden material content in general. The role of Al is also very important, since Al and alkalis can remain together in the Si-Al-K-Na-O melt for longer (under higher temperatures) than alkalis alone in the Si-KNa-O melt, as can be seen in the phase diagram (Figure 3). Thus, the addition of an Al-bearing phase to the feed coke may also be useful from this point of view. This might be needed in cokes with a high K + Na content (not the case with Ruukki’s coke) and when the initial Al/(K + Na) ratio in the coke ash is very low. The alkali-free clay-forming mineral kaolinite, Al4(OH)8[Si4O10], for example, could be considered for testing for this purpose. Preliminary dehydroxylation of the kaolinite
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(removal of the (OH)- component) at about 500 °C22 with subsequent crushing could perhaps prove essential. Conclusions During the transformation of SiO2 to the Si-Al-K-Na-O phase in a BF coke, the new phase can retain at least 5.64 wt % of Na2O + K2O at about 1700-1800 °C, and its abilities to incorporate potassium and sodium during the process are almost equal. The transformation of SiO2 to the new phase reflects an active role for Al along with alkali metals in circulating BF gases. Thermodynamic calculations for 100 gsolid/100 ggas and temperatures 800-1800 °C have shown that the BF gases have very little or no effect on the alkalization of SiO2. We suggest that, if the alteration process described in this paper proves to be a generalized phenomenon in blast furnace cokes, then the addition of fine-grained quartz to the surface of the coke before charging the BF can be useful for removing of some of the Al and alkali from the BF gases and will as a result reduce coke degradation by alkalis, or at least improve the properties of the coke until the temperature reaches approximately 2000 °C. Acknowledgment. This research was funded by the Academy of Finland. Mr. T. Kokkonen is thanked for preparing the sample. Editorial handling was provided by Prof. L. J. Broadbelt and Mrs. H. Price. EF070125Q (22) McConville, C. J.; Lee, W. E. Microstructural Development on Firing Illite and Smectite Clays Compared with that in Kaolinite. J. Am. Ceram. Soc. 2005, 88, 2267-2276.
