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Energy & Fuels 2006, 20, 2580-2585
Chemical Constraints on Fly Ash Glass Compositions John H. Brindle*,†,‡ and Michael J. McCarthy§ Comerton Studio, Comerton House, Drumoig, Leuchars, Fife, KY16 0BX, Scotland, United Kingdom, and DiVision of CiVil Engineering, UniVersity of Dundee, DD1 4HN, United Kingdom ReceiVed June 28, 2006. ReVised Manuscript ReceiVed August 9, 2006
The major oxide content and mineralogy of 75 European fly ashes were examined, and the major element composition of the glass phase was obtained for each. Correlation of compositional trends with the glass content of the ash was explored. Alkali content was deduced to have a major influence on glass formation, and this in turn could be related to the probable chemistry of clay minerals in the source coals. Maximal glass content corresponded to high aluminum content in the glass, and this is in accordance with the theoretical mechanism of formation of aluminosilicate glasses, in which network-modifying oxides are required to promote tetrahedral coordination of aluminum in glass chain structures. Iron oxide was found to substitute for alkali oxides where the latter were deficient, and some indications of preferred eutectic compositions were found. The work suggests that the proportion of the glass phase in the ash can be predicted from the coal mineralogy and that the utility of a given ash for processing into geopolymers or zeolites is determined by its source.
Introduction Fly ash is produced in pulverized-coal-fired boilers at power stations. It mainly consists of aluminosilicate glass microspheres, whose pozzolanic properties have long been recognized by the concrete construction industry. Indeed, substantial quantities of fly ash are used annually as an adjunct to cement in concrete production. More recently, the use of fly ash as a cement in its own right has been the subject of worldwide interest. This involves the reaction of the glass phase with sodium hydroxide or other alkalis to release aluminosilicate monomers from the glass structure1, and under suitable conditions, these form inert zeolites which give a solid crystalline matrix. Research is also in progress into the use of fly ash as a source material for artificial zeolites (e.g., ref 2). These are potent ion-exchange materials and find application in wastewater cleanup, in catalysis, and as molecular sieves. It does not seem possible to relate the glass content of fly ash to its performance3. However, processes involving the zeolitization of fly ash are dependent on glass content and composition2,4,5. The factors controlling fly ash glass content * Corresponding author tel.: (+44) 1382 541 437; e-mail: jbrind@ balanus.fsnet.co.uk. † Comerton Studio. ‡ Formerly of Division of Civil Engineering, University of Dundee. § University of Dundee. (1) Buchwald, A.; Kaps, C.; Hohmann, M. Alkali-Activated Binders and Pozzolan Cement Binders - Complete Binder Reaction or Two Sides of the Same Story? Proceedings of the 11th International Conference on the Chemistry of Cement, Durban, South Africa, 2003; Tech Book International: New Delhi, India, 2003; pp 1238-1246. (2) Andres, J. M.; Ferrer, P.; Querol, X.; Plana, F.; Uman˜a, J. C. Zeolitisation of Fly Ashes Using Microwaves: Process Optimisation. 1999 International Ash Utilization Symposium; Center for Applied Energy Research, University of Kentucky: Lexington, KY, 1999; paper #94, Web link: http://www.flyash.info/1999/94/and.pdf (accessed July 2006). (3) Dhir, R. K.; Jones, M. R.; Munday, J. G. L.; Hubbard, F. H. Physical Characterisation of UK Pulverized-Fuel Ashed for Use in Concrete. Mag. Concr. Res. 1985, 37 (131), 75-87. (4) Kriegel, R.; Buchwald, A. Amorpher Anteil und Reaktivita¨t von Flugaschen. Proc. 15th IbauSil, Weimar, Germany, September 24-27, 2003; FA Fingerinstitut fu¨r Baustoffkunde, Bauhaus-Universita¨t Weimar, Germany, 2003; pp 1-0977-1-0987.
and composition are not understood in detail. Models developed for coals from a single provenance tend to be inapplicable to other coals.6,7 Furthermore, while the dependence on the source has been described, it has not been quantified. It has been shown8 that the glass content of ashes derived from low-rank coals (i.e., those subjected to low temperatures and shallow burial depths during their formation) differs from those from high-rank coals (high temperatures and deep burial). The same researchers found that both quartz and crystalline iron oxide contents could be related to those in the parent coal. On the basis of nine Australian coals, it was concluded8 that the factors controlling the variable proportion of glass measured “probably included” the mineralogy and inorganic geochemistry of the feed coals. Concomitant with variation in the coal rank is variation in the mineralogy of the clay minerals found in the coal. A mixture of clay minerals is usually present in coal, and much depends on the original source material: for example, beds or partings of kaolinite derived from volcanic ash fall events may be juxtaposed with clays derived from the subaerial weathering of distant continental material. In comparable depositional settings, however, low-rank coals tend to be richer in smectite and kaolinite, whose alkali content is relatively low.9 With increasing (5) Font, O.; Moreno, N.; Diez, S.; Querol, X.; Lo´pez-Soler, A.; Coca, P.; Pen˜a, F. G. Differential Behaviour of Combustion and Gasification FlyAsh from Puertollano Power Plants (Spain) for Zeolite Synthesis and Silica Extraction. New Products: 2005 “World of Coal Ash” Conference; Center for Applied Energy Research, University of Kentucky: Lexington, KY, 2005. Web link: www.flyash.info/2005/152/fon.pdf (accessed July 2006). (6) Barta, L. E.; Toquan, M. A.; Beer, J. M.; Sarofim, A. F. Prediction of Fly Ash Size and Chemical Composition Distributions: The Random Coalescence Model. 24th Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1992; pp 1135-1144. (7) Juniper, L.; Huber, G.; Jak, E.; Creelman, R.; Wall, T. Making the Ash Fusion Test Useful. ACARP Project C8050: End-of-Grant Report; Ultra-Systems Technology Web site: http://www.ultrasys.com.au/downloads/ resUsefulAFT.pdf (accessed June 2006). (8) Ward, C. R.; French, D. Relation Between Coal and Fly-Ash Mineralogy, Based on Quantitative X-Ray Diffraction Methods. Fuel 2006, in press. (9) Tucker, M. E. Sedimentary Petrology; Blackwell Science: Oxford, U. K., 1991; pp 200-207.
10.1021/ef0603028 CCC: $33.50 © 2006 American Chemical Society Published on Web 09/22/2006
Chemical Constraints on Fly Ash Glass Compositions
rank, mixed-layer illite-smectite clays form a larger proportion, with increasing interlayer cation content and an increasing tendency for K to replace Na and Ca in interlayer sites. Other aluminosilicates found in higher-rank coals include chlorite, in which Mg and Fe are predominant, and micas. In addition, clastic material containing silica and alumina, such as quartz and feldspar grains, are variably present. There is extensive literature on the diagenetic alteration of clay minerals, and the current consensus is that this is thermally mediated in the case of smectite-to-illitic phases. According to Bruvoll et al.,10 the diagenesis of smectite-bearing sediments occurs in distinct phases, with the dissolution of smectite and precipitation of quartz producing mixed-layer illite-smectites rather abruptly between 60 and 120 °C. The alteration of smectite to illite-smectite in authigenic clays associated with Pennsylvanian anthracite is inferred by Daniels and Altaner.11 This differs in style between the coal matrix and one of the joint sets found in the deposit and may in the latter case be associated with basinal fluid flow, which supplied heat to the system. Guthrie et al.12 correlated clay mineralogy, Ku¨bler’s illite crystallinity index, and vitrinite reflectance in Carboniferous strata in the Ouachita mountains and proposed that illite crystallinity could be used quantitatively in the estimation of levels of thermal maturity. Srodon,13 investigating the relationship between thermal history and the percentage of smectite in illite-smectite (%S) in Oligocene shales, found that the 110-120 °C %S isotherm coincided with the line of total resetting of apatite fission track ages (125 °C), confirming the utility of %S as a palaeogeothermometer in this environment. The aluminosilicate content of the glass in fly ash is derived from precursor clay minerals: broadly speaking, these decompose on heating to quartz, mullite, or spinel with liquid.14,15 The solid phases are refractory and to some extent survive heating between the decomposition temperature (typically 900-1000 °C) and the maximum flame temperature (typically 1400-1500 °C). Crystalline mullite, iron oxides, and quartz or cristobalite are found enclosed in the spherical glass particles constituting much of the fly ash; other derived minerals such as melilite and portlandite are associated with high Ca coals (e.g., ref 16). This study examines the relationships between ash glass content and glass major oxide chemistry, with reference to the whole ash chemistry and the inferred mineralogy of the source coal. A detailed examination of some compositional trends, which are relevant to the utility of fly ash in geopolymer and zeolite synthesis, is considered. (10) Bruvoll, M.; Jahren, J. S.; Aagaard, P. Smectite Illitization in Organic Rich Shale Offshore Mid-Norway. Illite Diagenesis a Quarter-Century after Hower et al., 41st annual meeting, Clay Minerals Society, Richland, Washington, 2004. (11) Daniels, C. J.; Altaner, S. P. Clay Mineral Authigenesis in Coal and Shale from the Anthracite Region, Pennsylvania. Am. Miner. 1990, 75 (7-8), 825-839. (12) Guthrie, J. M.; Houseknecht, D. W.; Johns, W. D. Relationships among Vitrinite Reflectance, Illite Crystallinity and Organic Geochemistry in Carboniferous Strata, Ouachita Mountains, Oklahoma and Arkansas. Am. Assoc. Pet. Geol. Bull. 2001, 70 (1), 26-33. (13) Srodon, J. John Hower Was Right: Illite-Smectite as a Geothermometer Illite Diagenesis a Quarter-Century after Hower et al., 41st Annual Meeting, Clay Minerals Society, Richland, Washington, 2004. (14) Deer, W. A.; Howie, R. A.; Zussman, J. An Introduction to the Rock-Forming Minerals, 2nd ed.; Longman Scientific and Technical: Harlow, U. K., 1992; p 698. (15) Araujo, J. H.; da Silva, N. F.; Acchar, W.; Gomes, U. U. Thermal Decomposition of Illite. Mater. Res. 2004, 7 (2), 2004. (16) Vassilev, S. V.; Vassileva, C. G.; Karayigit, Y. B.; Alastuey, A.; Querol, X. Phase-Mineral and Chemical Composition of Fractions Recovered from Composite Fly Ashes at the Soma Power Station, Turkey. Int. J. Coal Geol. 2005, 61, 65-85
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Material Data Sets Used for the Investigation The fly ash data used in this study were obtained from routine analyses undertaken in the course of several studies on UK and European fly ash at the University of Dundee.18-22 In these, XRF major oxide content and XRD quantitative mineralogy were recorded for all materials. A wide range of fly ash properties and chemistry is represented by the data; and one subset of the data also includes some ashes derived from co-fuelling with organic wastes. The ashes are generally low in calcium, and crystalline CaO was detected by XRD in only two samples, both derived from cocombustion. The 75 analysed ashes were derived from five source groups: (1) Blended-coal-biomass co-combustion ashes. Six commercialscale co-fuelled ashes and three coal-only control ashes, loss on ignition (LOI) range 4.4 to 12.2%, 45 µm sieve retention 21 to 34%.18 (2) Eight primary samples, of which two provided additional sets of classified ash within specified fineness ranges. LOI (eight primary samples 3.6 to 8.0%; 45 µm sieve retention 2.8 to 41.5%.19 (3) Fly ashes from four combustor sources. LOI 3.3 to 12.0%, µm sieve retention 7.7 to 52.5%.20 (4) Stockpiled (conditioned) and dry fly ashes from UK combustors, including samples at different locations in individual repositories. LOI, 2.2 to 10.6%, µm sieve retention 4.8 to 36.6%.21 (5) Samples from five sources, obtained during the early 1990s, including ashes blended from the output of four separate precipitators at one of the source combustors. LOI, 1.8 to 4.8%, µm sieve retention 1.6 to 23.0%.22
Test Procedures Glass content was calculated by subtraction of the crystalline content, identified by quantitative XRD from the total. Because the greater part of the carbon content appeared to be detected by XRD as amorphous material, the LOI was also subtracted from the XRD total. This may be an imperfect approximation, because there exists a crystalline carbon diffraction peak which is hard to distinguish from the principal peak for quartz. However, for all but the cocombustion fly ashes, the XRD angle data were not available for inspection. The fly ashes used provided a near-continuous range of glass contents between 40% and 90%, with considerable overlap between sample groups. Ashes containing between 70 and 75% glass were poorly represented, however. The proportion of SiO2 in the glass phase was calculated by subtracting the SiO2 content of the mullite and quartz, determined by XRD, from the major oxide assay obtained from XRF, and glass CaO, Al2O3, and Fe2O3 contents were similarly calculated from XRD lime, mullite, and iron oxide contents, respectively. By contrast with XRD, the XRF analyses uniformly gave total oxide contents below 100%, and this was due to the inability of the apparatus to detect elemental carbon. LOI was accordingly added to the raw XRF totals, which then approached 100% in most cases. No minerals containing Na, K, or Mg were found by XRD, and although substitution of these elements in the structures of those minerals (17) Hoffman, J.; Hower, J. Clay Mineral Assemblages as Low Grade Metamorphic Geothermometers: Application to the Thrust-Faulted Disturbed Belt of Montana, USA. Spec. Publ. - Soc. Econ. Paleontol. Mineral. 1979, 26, 55-79. (18) McCarthy, M. J.; Dhir, R. K.; Csetenyi, L. J.; Brindle, J. H. CoCombustion Fly Ash for Use in Concrete Construction. Paper 166, World of Coal Ash, Lexington, Kentucky, April 2005. (19) Dhir, R. K.; McCarthy, M. J.; Magee, B. J. Impact of BS EN 450 PFA on Concrete Construction in the UK. Constr. Building Mater. 1998, 12 (1), 59-74. (20) McCarthy, M. J.; Dhir, R. K.; Halliday, J. E.; Wibow, A. Role of PFA Quality and Conditioning in Minimising ASR in Concrete. Mag. Concr. Res. 2006, 58 (1), 49-61. (21) McCarthy, M. J.; Tittle, P. A. J.; Dhir, R. K. Characterisation of Conditioned PFA for Use as a Cement Component in Concrete. Mag. Concr. Res. 1999, 51 (3), 191-206. (22) Dhir, R. K.; Jones, M. R.; Seneviratne, A. M. G. Diffusion of Chlorides into Concrete: Influence of PFA Quality. Cem. Concr. Res. 1991, 21 (6), 1102-1123.
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Figure 1. Relation of the total Si/Al molar oxide ratio in ash to ash glass content (% mass). (Indicative trendlines only; one anomalous point, from pilot-scale combustor, not shown.)
Figure 2. Relationship of M+ oxides to the Si/Al oxide ratio of ash. The “step” occurs at Si/Al ) 3, corresponding to the inferred change from smectite to mixed-layer clays in the source (four points omitted).
identified is known to occur, quantification was not possible on the basis of the data available, and it was assumed for the purposes of the work that these were present in the glass phase alone.
Results and Discussion Silica/Alumina Ratios of Total Ash and of Glass Content Only. A clear relationship between the melt content and total ash Si/Al molar ratio was found. This resolved into three broad visible trends by the total ash Si/total ash Al ratio, and high glass content (>75%) ashes corresponded to those on the highSi/Al trends (B-B and C-C) (Figure 1). The tendency in most of the ashes considered was for crystalline mullite significantly to exceed quartz content. The majority of the few ashes for which this did not hold contained 10%) exceeded mullite content in two of them, as well as in the sample omitted from the plot, while the third contained 9% quartz and 10% mullite. Their alkali and calcium content, however, was not exceptional. Their relatively low glass content (∼70%) probably represents an additional trend for ashes derived from a low-alumina/high-silica geological source. Figure 2 shows that K and Na oxides increase sharply for Si/Al > 3.0, again coincident with the higher trends. There is apparently some fundamental difference in the initial chemistry of the ash sources. Variation in the depth and temperature of burial affects the composition of coal clay minerals, which have been proposed as geothermometers for this reason. The sequence smectite-(mixed-layer clay + illite 1M + quartz + chlorite)-(illite M2 + quartz + chlorite) has been proposed,9 with the first transition taking place over the temperature range 65-120 °C and the second above 160 °C. An increase in the ratio of free silica to clay mineral is implied
Brindle and McCarthy
Figure 3. Relation of the Si/Al molar oxide ratio in glass to ash glass content (% mass). (One anomalous point not shown.)
by both transitions, but the probable presence of clastic silica in the coal complicates this picture. Hoffman and Hower17 proposed that two structural transitions take place: the first, at ca. 100 °C, represents the change from random to ordered interstratification of illite and smectite layers, and the second, at ca. 170 °C, is indicative of a progression from the nearestneighbor to longer-range ordering of the layers. Smectites (and smectitic layers in mixed-layer clays) typically contain Na and Ca as interlayer cations, while illites are by definition potassic. Figure 2 shows that both Na and K are relatively low below a Si/Al oxide ratio of 3.0, above which both increase abruptly. The assignment of trend B-B in Figure 1 to the appearance of ordered illite-smectite is at least plausible, and trend C-C may reflect a second structural transition, with further cation replacement taking place. In contrast to Figure 1, the molar Si/Al oxide ratio in the glass phase (Figure 3), determined by subtracting the Al and Si content of the crystalline mullite and quartz, revealed by XRD, from the XRF total, falls on a broadly linear trend as the glass content rises, from approximately 5.0 (40% glass) to 3.4 (90% glass). The compositions of the glass and the total assay (cf. Figure 1) converge above an ∼85% glass content. The highest glass content is associated with the lowest Si/Al ratio in the glass. This is the inverse of what might be expected from a total melt of the ash constituents. Apparently, the prior Si/Al ratio does not determine the ratio in the glass. The possibility of variation in combustor conditions was investigated as a possible cause. The glass Si/Al ratio varied similarly with the glass content for all ash sources, with conspicuous overlap between the groups occurring. No distinction could be made between the groups on the basis of this measure. However, it was not felt that variation attributable to different combustion conditions contributed significantly to the anomaly. The provenance of the ashes indicated that variations in melt cation chemistry were more likely to be responsible. Melt Chemistry. The process of glass formation in the context of pulverized-fuel combustion involves the following: (1) the decomposition of the aluminosilicate minerals in the flame zone to (typically) quartz and mullite and the initial liquid, which occurs exothermically above 900 °C; (2) the melting of the mineral components to liquid droplets on the surface of the fuel particle, and their eventual coalescence into larger, free droplets;6 and (3) the rapid chilling of the liquid as it leaves the flame zone and enters the exhaust stream, which is cooled by heat transfer to the boiler waterwall. According to the accepted structural theory of glass formation,23 silicate glasses, and their precursor liquids, contain chains of SiO4 tetrahedra. These are interconnected to form three-
Chemical Constraints on Fly Ash Glass Compositions
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Figure 4. Interdependence of M+, M2+ oxides, Al2O3, and SiO2 and glass content. (SiO2 divided by 2 for clarity of plot.)
dimensional polymeric structures. The addition of alkali oxides (M2O or MO) reduces the “interconnectedness” of the polymer by introducing nonbridging oxygens coordinated to the alkali cation. In the case of M2O, termination of two chains is possible, and some physical separation of the two cations is implicit. The melting point of the liquid is reduced as a result of partial depolymerization. In the case of MO, the nonbridging oxygens remain coordinated to a single cation, and a measure of interconnectedness remains. While the melting point is reduced, MO is less effective than M2O in this respect. Alkali and alkalineearth oxides are described as network-modifying oxides (NMOs). Aluminum oxide can also participate in glass formation. In the presence of NMOs, it can coordinate tetrahedrally in the same fashion as Si4+, because its charge deficit is balanced in the unit Al3+-M+-O4. An aluminosilicate glass in which Al2O3 e M2O contains mixed chains of aluminum and silicon oxide tetrahedra, but excess Al must be tricoordinated and may then act as a NMO itself. Aluminum oxide cannot form a glass alone, but only in coordination with an NMO, and is termed an intermediate oxide. According to Pauling’s packing rule, both Fe2+ and Fe3+ prefer octahedral coordination and would be expected to participate in glass formation solely as NMOs. Crystalline Fe chain silicates tend to have lower melting points than their Mg and Ca analogues. The residence time of an ash particle in the combustor flame zone is limited, typically to 4-5 s. While the extent of melting is partially a function of the rate of heat transfer to the particle during this short intervalsand hence its maximum temperatures it is also at least partially dependent on the melt chemistry of the ash. Relationship of NMOs and Glass Content. A clear relationship was found between the sum of Na, K oxides, and the Al2O3 content of the glass phase, expressed as molar M over molar total glass assay, and plotted against the glass content (Figure 4). The converse was found when the sum of Ca and Mg was plotted against the glass content. The gradient of the linear leastsquares fit to the K + Na plot is ∼-0.5 times that for Ca + Mg. The gradient of the trend for Al2O3 is almost identical to that of K + Na. The depletion of 2(Ca + Mg) is accompanied by the addition of Al + (K, Na) to the system. This in turn suggests that the removal of M2+ as a NMO (associated solely with SiO4 tetrahedra) is accompanied by the addition of AlM+-O4 tetrahedral units, and the increase in glass content associated with this results from a decrease in M2+-chain cross-linking. (23) Zachariasen, W. H. The Atomic Arrangement in Glass. J. Chem. Soc. 1932, 54, 3841.
Figure 5. Molar K2O vs CaO vs Al2O3 in the glass phase. (Glass content distinguished by symbols.)
Figure 6. Scatter plot of molar M2+/total glass NMO versus Fe/total glass NMO, indicating preferred glass stoichiometries. Symbols as in Figure 5.
The vertical lines in Figure 4 match those in Figure 1. In the region 50-60% glass, where a low illite content is postulated, the SiO2 content declines from approximately 70 to 66%. Above 60% glass, with the entry of substantial K and Na, the SiO2 content of the glass is constant at around 66%. It was also found that the sum (Na + K + Fe) exhibited a similar gradient to that of (Na + K) alone, and that the sum (Na + K)2O + (Ca + Mg)O + Fe2O3 approximately equalled the Al2O3 content of the glass. Plotting Al against the Ca and K oxide content in the glass phase (Figure 5) shows two distinguishable populations related to glass content. These are the axes providing the clearest indication of this effect; although (Na + K) plotted against Al and (Ca + Mg) oxides gave a similar picture. High-glass-content (>75%) ashes cluster around Al ) 75%; K ) 15%; Ca ) 15%, while low-glass-content ashes are centered on Al ) 63%; K ) 7%; Ca ) 30%, with some plots of intermediate chemistry and generally high glass content. The role of Fe in glass formation was found to be complex. Fe levels in the glass phase were, in general, low compared to Na + K, highly variable, and not related to Al. However, linear trends were observed in a scatter plot of (Ca + Mg)O/total NMO against FeO/total NMO (Figure 6). Total NMO was taken to be (Na + K)2O + (Ca + Mg + Fe)O; the measure used is moles per gram of glass. The same broad groups of glass content as those used in Figure 5 are indicated.
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(Na + K) oxide content per 100 g of ash. Maxima of around 90% SiO2 and 80% Al2O3 are approached for (Na + K) oxide content in excess of ca. 0.06 mol/100 g of ash. Conclusions
Figure 7. Dependence of ratio of network-forming oxides in glass to total NFO upon alkali content. Table 1. Preferred Stoichiometric Trends in Figure 6a
a
trend
G: gradient (d M2+/d Fe)
H: constant
A B C D
-1.6 -1.17 -1.38 -1.39
1.10 0.90 0.82 0.61
“Eyeball” linear fits to data: y ) Gx + H.
The trends appear to be determined by M2O content, with continuous substitution taking place principally between FeO and MO. Compositional ranges could be inferred from these trends:
trend A: (FeO, MO)0.95f0.9, (M2O)0.05f0.1 trend B: (FeO, MO)0.9f0.85, (M2O)0.1f0.15 trend C: (FeO)x, (MO)0.82-1.38x, (M2O)0.38x+0.18 trend D: (FeO, MO)0.6, (M2O)0.4 For trend C, mutual substitution between all three groups occurred, and the suggested formula is based on the unit FeO. It will be noted that the constant H for trend A is 1.1, greater than the sum (1.0) of components. Hence (Ca + Mg) varies positively with (Na + K) for this trend alone. There is some interdependence between all NMO groups. Low-glass-content ashes plot principally on trends A and B and intermediate content on C (and its possible high Fe continuation), while the majority of ashes with glass >80% plot close to trend D. It was noted that low-Fe members of trend D were richer in glass than samples of similar MO content, higher Fe, and low M2O on trends A and B, and the principal control on glass content appeared to be M2O. Crystalline Fe in the ash was conspicuously in excess of the levels found in the glass of the high-glass-content (>75%) ashes on trend D, while, as already noted, all other NMOs were present only in the glass phase. The tentative conclusion was drawn that the assimilation of Fe in the glass phase depends on there being a deficit of other network-modifying cations in the original fuel. It was found that both Si and Al were progressively incorporated in the melt phase as the (Na + K) content increased, the relationship being approximately logarithmic (Figure 7). The least-squares fit to the logarithm used may give an overestimate of maximum Al levels for predictive purposes. Here, the ratio of the percentage of each oxide in the glass to that in the XRF total assay is plotted against the calculated molar
The ratio of Si to Al in the glass phase is not correlatable with the ratio in the ash as a whole. It is correlatable with alkali metal content in the glass, and no crystalline minerals containing substantial alkali metal were identified in any of the analyses carried out. Unless alkali metals were lost during combustion as volatiles, all of those originally present entered the glass phase. The amount of melt formed during the finite residence time of the ash in the flame zone, within a temperature range which is constrained by the need for efficient combustion, is plausibly determined by the Na and K content of the fuel. The Si/Al ratio in the ash as a whole shows trends which are suggestive of depositional or diagenetic mechanisms in the source coal, and the rapid increase in Na + K content coincides with a trend boundary, implying a simultaneous increase in interlayer cation content in the precursor clays, associated with a total Si/Al greater than 3.0. On this basis, higher-rank coals would be expected to provide a higher glass content. High levels of Ca and Mg relative to alkalis are associated with a higher melting point and less-voluminous glass phases. These are also lower in Al. While MO is stoichiometrically equivalent to M2O in terms of coordinating Al(M)O4 tetrahedra in the melt, this looks rather as if the addition of MO to SiO4 tetrahedra (as chain termination or nonbridging groups) is preferred to the incorporation of Al(1/2M)O4. Ca is more typically found in smectite interlayers than in illite, and the Ca and Mg content of a coal may also reflect the presence of nonaluminosilicate minerals such as calcite, dolomite, or talc. When M2+O and M+2O are equimolar in the glass, the glass content appears reliably to attain 85% in the ashes studied. It should be stressed that these are all probably derived from Carboniferous basinal or deltaic coals and that different provenances, with high clastic and low clay inputs relative to these, may contain low levels of both cation groups, which would not permit an approach to complete melting. The role of Fe in glass formation is analogous to that of M2+, for which it substitutes continuously on four compositional trends. The amount of glass formed rises slightly for Fe-rich members of the two highest M2+ trends. The trends may reflect eutectic melt compositions in which initial deficiencies of the other cations are filled by Fe: the highest glass contents are associated with high crystalline Fe, and Fe in the melt appears to be low if the M+ content is high. A weak inverse correlation was generally found between crystalline and glass-phase Fe in the ashes. Crystalline Fe oxides were present in all ashes. Much of the Fe found in coal is in the form of pyrite, and it is not an essential component of most clay minerals. The prediction of glass content on the basis of coal chemistry would appear to be possible if it can be assumed that all of the NMOs present enter the glass phase and are not lost in volatile form. Processes based on the direct conversion of the ash glass phase to crude zeolitic products, as well as those involving the isolation of zeolites by alkali leaching and subsequent curing, are favored by a high alumina content in the glass and a high (>75%) glass content. These coincide in ashes derived from high-(Na + K) European coals, and these in turn are probably the product of relatively high burial temperatures and diagenetic
Chemical Constraints on Fly Ash Glass Compositions
mechanisms affecting the cation content of the clay minerals present. The presence of iron in the source material may also be undesirable. Iron content in the glass is not necessarily supplied by the precursor clay minerals but is incorporated in melts deficient in other cations derived from the clays, to an extent for the most part determined by the Na + K content.
Energy & Fuels, Vol. 20, No. 6, 2006 2585 Acknowledgment. The authors acknowledge Professor R. K. Dhir (University of Dundee) for his input on many of the projects from which the data, considered in the work described in this paper, were obtained. We are grateful for the helpful advice given by staff at the Centre for Applied Energy Research, University of Kentucky. EF0603028
