A Composition–Structure Relationship of Skeletal-Dendritic

Jun 16, 2019 - The structure–composition relationship of skeletal-dendritic ferrospheres (FSs) isolated from fly ash from the coal and lignite combu...
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Article Cite This: Energy Fuels 2019, 33, 6788−6796

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Composition−Structure Relationship of Skeletal−Dendritic Ferrospheres Formed during Industrial Combustion of Lignite and Coal Natalia N. Anshits,† Marina A. Fedorchak,† Anatoliy M. Zhizhaev,† and Alexander G. Anshits*,†,‡ †

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Institute of Chemistry and Chemical Technology SB RAS, Federal Research Center “Krasnoyarsk Science Center SB RAS”, Akademgorodok 50/24, Krasnoyarsk 660036, Russia ‡ Siberian Federal University, Svobodnyi pr. 79, Krasnoyarsk 660041, Russia ABSTRACT: The structure−composition relationship of skeletal−dendritic ferrospheres (FSs) isolated from fly ash from the coal and lignite combustion has been studied systematically by scanning electron microscopy (SEM) and energy-dispersive Xray spectroscopy. It is shown that illite is the aluminosilicate precursor determining the structure of globules in both cases. The formation of skeletal−dendritic globules occurs due to the “seed” of Al, Mg-ferrospinel that is formed in the thermochemical conversion of illite from initial coals. The dependence CaO = f(SiO2) that reflects the influence of glass-forming components reveals six groups of FSs, the composition of which is represented by linear regression equations. An analysis of SEM images of polished sections from six globule groups reveals that an increase in the concentration of glass-forming components in all groups is accompanied by gradual changes in the structure of globules, from the coarse-grained crystalline skeletal type to the finecrystalline dendritic type with a high content of the glass phase. The observed change in the structure is explained by expansion of the liquation region in the FeO−Fe2O3−SiO2 system, a rise in the oxidation potential, an increase in the proportion of ferrite complexes [Fe3+O2]− and [Fe23+O5]4− in high-calcium melts, and a decrease in the concentration of ferrospinel-forming Fe2+ and Fe3+ ions.

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INTRODUCTION

neous globules, FSs with a mixed structure, comprising simultaneously fragments of the mentioned types, porous FSs with one or several large cavities,9,11,12,14,20,21 and plerospheres with an outer shell filled with spheres, crystallites, and smaller carbon particles.9,11,12,17,18 The yield of magnetic fractions from fly ashes produced by combustion of different coals is 0.5−18%; the iron content in them varies in a range of 20−88 wt %; the globule distribution maximum occurs in a range of 40−150 μm.9−14 In particular, the highest yield of blocklike and platelike FSs was shown to occur upon combustion of high-calcium lignite.22 Skeletal− dendritic globules are more commonly found in coal combustion ashes.9,12,23 Investigation of the composition− structure relationship of individual skeletal and dendritic FSs revealed the differences in their composition and the iron oxidation level in ferrospinel phases.11,15,16,19,23 The heterogeneity of the composition of the local globules areas11,15,19,23 and the general relationship between the component concentrations of (SiO2) = f(FeO)19 have been established. However, despite the wide prevalence of FSs and their study, the reasons determining the formation of their several morphological types are not established still. In the last decade, fly ash components have been used as functional materials.24−27 In particular, narrow FS fractions17,18 of constant composition with reproducible magnetic properties28 are used as catalysts for deep oxidation,29 oxidative coupling of methane,30−32 and thermolysis of heavy oil and

The formation of ash particles during pulverized coal combustion involves several thermochemical processes, such as included mineral coalescence,1−3 char fragmentation,4 and excluded mineral fragmentation.5,6 The composition, morphology, and size distribution of particles are determined by a combination of the mentioned processes and depend on coal combustion conditions and characteristics of coal’s mineral components.3,7−9 Ferrospheres (FSs) are some of the most common components of fly ashes. The globular structure of FSs forms in a reducing environment of the carbon matrix in the course of thermochemical conversions of iron-containing and aluminosilicate minerals of the initial coal, which leads to the formation of droplets of high iron melts with a complex composition (FeO−SiO2−Al2O3−CaO−MgO) and to partial crystallization of individual phases during their cooling.9−14 There is a clear correlation between the content of FSs in fly ashes and the content of iron in different initial coals,9 which indicates involvement of all known iron-containing forms of the initial coal in the FS formation. Variations in the chemical and phase compositions of FSs from different ashes are associated with the presence of several morphological types of globules and their different contents in narrow fractions. In particular, studies on the composition and properties of FSs concentrates from different ash types and their narrow fractions have most often reported homogeneous globules of blocklike, skeletal, dendritic, and platelike structures, which differ in size and shape of crystallites from iron-containing phases, and glass-phase concentration,9,11−19 as well as porous (foamy) globules with a relatively low iron content.14,18,19 Narrow fractions contain, along with homoge© 2019 American Chemical Society

Received: April 8, 2019 Revised: May 28, 2019 Published: June 16, 2019 6788

DOI: 10.1021/acs.energyfuels.9b01077 Energy Fuels 2019, 33, 6788−6796

Article

Energy & Fuels residues,33 as well as magnetic carriers for affinity sorbents in protein separation.34 The criterion for the applicability of ferrospheres as functional materials in each particular case is their compliance with a specific composition and microstructure of the active iron-containing phases,31,32 which is determined by one of the morphological types of globules. To find the most promising sources for the separation of narrow fractions of ferrospheres with a high content of a certain morphological type of globules, active in a particular process, it is important to have a general idea of their formation routes. This paper presents the results of a systematic study of a relationship between the major components composition of individual skeletal−dendritic FSs separated from coal and lignite combustion fly ashes, studying the features of their formation routes, and the nature of mineral precursors that determine their structure.

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Figure 1. SEM image of a polished section of a skeletal globule with indications of analyzed local areas.

Al2O3 (5.0 wt %), the total content of which is 99.1 wt %. The phase composition of this fraction includes ferrospinel (65.4 wt %), hematite (5.9 wt %), and an amorphous phase (26.8 wt %). The unit cell parameters of ferrospinel and hematite are a = 8.3851(2) Å and b = 5.0322(5) Å, c = 13.730(2) Å, respectively. The observed values are significantly lower than the parameters of stoichiometric oxides Fe3O4 and Fe2O3, which indicates partial replacement of the Fe2+ and Fe3+ cations by cations with smaller ionic radii, Mg2+ and Al3+. An analysis of SEM images of ∼500 globules of initial fraction B −0.05 mm allowed establishing the contents of globules of skeletal−dendritic (Figure 2a,b), foamy (porous) (Figure 2c), and blocklike (Figure 2d) structures, which are 50, 22, and 11%, respectively. The content of platelike FSs is less than 1% (Figure 2e). Along with the mentioned FSs, there are plerospheres (10%) and globules (7%) with a mixed structure, which include simultaneously fragments of the mentioned types and/or internal cavities. It should be noted that the content of these FSs differs significantly from the −0.04 + 0.032 mm fraction of FSs separated from high-calcium lignite ashes.22 This fraction includes blocklike, skeletal−dendritic, and platelike globules at concentrations of 58, 16, and 10%, respectively, and does not contain porous globules. The presence of skeletal−dendritic globules in the narrow-fractions composition recovered from the fly ash of the industrial combustion of coal (Figure 2a,b), high-calcium lignite,22 and other different coals9,11,12,15,23 may testify in favor of the general route of their formation with the participation mineral precursors similar in composition. Composition−Structure Relationship of Skeletal− Dendritic FSs. An analysis of the gross composition of polished sections of skeletal−dendritic globules (Table 1) demonstrates that a decrease in the FeO concentration from 85 to 38 wt % is accompanied by an increase in the concentrations of SiO2, Al2O3, and CaO in the ranges of 2.6− 37.6, 2.7−15.5, and 1.2−15 wt %, respectively. The total content of these oxides in globules is 87−98 wt %. The effect of composition on the structure of individual FSs was studied using the following dependencies: SiO2 = f(FeO), which characterizes the iron-silicate basis; SiO2 = f(Al2O3), which allows identifying the nature of aluminosilicate precursors involved in the formation of FSs; and CaO = f(SiO2), which reflects the relationship of two glass-forming components. The dependence SiO2 = f(FeO) for the gross composition of skeletal−dendritic globules reveals two main groups of FSs (Figure 3) whose composition is described by linear regression equations SiO2 = 47.41 − 0.53[FeO] (1) and SiO2 = 54.04 −

EXPERIMENTAL SECTION

A −0.05 mm FS fraction was recovered from a concentrate of pilotscale magnetic separation of fly ash from pulverized combustion of Dand DG (hvBb)-grade coal (the Tugnuysky coal region of the OlonShibirsky deposit) with a mean ash content of 23 wt %. The combustion has been performed in E-160-2.4 boiler furnaces with a temperature in the flame core zone of about 1100 °C and dry ash removal units at the Ulan-Ude Thermal Power Station-2. Fly ash was sampled from a cyclone battery. Narrow fractions were obtaining by classifying the magnetic concentrate by size, followed by purification of separated products from nonmagnetic impurities in a pulsating ascending water stream. Detailed information on the methods used for separation of the −0.05 mm fraction of series B with a Fe2O3 content of 78.4 wt %, analysis of its chemical and phase composition, and microstructural characteristics of iron-containing phases has been described earlier.17,35 A −0.05 mm FS fraction of the S series was recovered from a concentrate of pilot-scale magnetic separation of fly ash from the pulverized combustion of B2 (sub-C) grade lignite with a mean ash content of 7 wt % in boiler furnaces P-67 with solid slag removal and the average (over the boiler furnace height) temperature of 1350− 1450 °C at the Berezovskaya Power Plant. Detailed information about the methods used to separate narrow fractions and determine their chemical and phase compositions and other parameters has been described earlier.17,18 The −0.04 + 0.032 mm size fraction was obtained by additional sieving of the −0.05 size fraction of series S containing 89.12 wt % Fe2O3 and was characterized previously.19 To investigate the structure and composition of individual globules, we used polished sections of FSs produced by their fixation in epoxy resin, followed by grinding, polishing, and deposing a platinum layer of thickness about 20 nm. Polished sections of individual FSs were analyzed using a TM-3000 scanning electron microscope (SEM, Hitachi) equipped with an energy-dispersive X-ray spectrometer with a Flash 430 H detector at an accelerating voltage of 15 kV in a mapping mode. The quality of the spectrum assembly determined the data accumulation time, which exceeded 10 min. This allowed us to determine the composition of individual globules quantitatively. The gross composition of the entire cross-sectional surface and compositions of local areas with a diameter of 4 μm were determined on perpendicular diameters of the globule (Figure 1). All elements were calculated as appropriate oxides, and iron was calculated as a FeO.

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RESULTS AND DISCUSSION To study the relationship between the major components’ composition and structure of individual FSs of skeletal− dendritic type, a narrow fraction of −0.05 mm series B was used, recovered from the fly ash of Tugnuysky coal combustion.17,35 The major components of this fraction are Fe2O3 (78.4 wt %), SiO2 (9.4 wt %), CaO (6.2 wt %), and 6789

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Figure 2. SEM images of skeletal (a), dendritic (b), foamy (porous) (c), blocklike (d), and platelike (e) globules of the initial fraction B −0.05 mm.

Table 1. Chemical Gross Composition (wt %) of Polished Sections of Skeletal−Dendritic Globules Produced by Combustion of Tugnuysky Coal (Series B) globule

SiO2

Al2O3

FeO

CaO

MgO

Na2O

K2O

TiO2

MnO

8222 8204 1262 7906 8202 8212 8201 1264 8203 1275 9024 9537 1271 3968 3972 9539 1267

2.64 9.77 13.43 11.47 6.11 12.31 12.15 11.24 18.22 17.83 20.15 26.28 23.78 31.67 37.63 27.36 35.28

2.69 2.94 6.05 4.67 5.02 5.32 2.97 5.47 7.20 6.10 13.80 11.59 8.07 13.71 11.74 15.47 8.23

84.95 79.03 77.42 75.64 75.28 71.48 70.92 68.09 64.56 56.36 50.82 49.71 43.46 39.68 38.68 38.07 38.66

5.16 2.96 1.19 2.98 5.89 4.85 6.30 7.66 5.04 13.57 9.19 2.16 14.96 8.28 3.86 10.62 5.05

2.67 2.47 0.69 2.22 6.66 4.45 6.26 6.13 3.06 4.30 3.18 2.41 7.74 4.15 4.88 3.68 10.77

0.62 0.53 0.80 0.57 0.45 0.73 0.45 0.77 0.27 0.50 0.98 0.86 0.71 0.90 1.11 0.49 0.58

0.07 0.07 0.05 0.18 0.11 0.09 0.10 0.13 0.05 0.11 0.42 5.36 0.19 0.35 0.50 0.53 0.82

0.08 0.09 0.07 0.41 0.08 0.22 0.09 0.20 0.05 0.08 0.72 0.62 0.38 0.37 0.55 3.20 0.28

0.87 1.98 0.00 1.55 0.22 0.33 0.44 0.06 1.50 0.78 0.11 0.65 0.45 0.71 0.87 0.27 0.00

The gross composition of globules 1262, 1267, and 3972 obeys the equation [SiO2] = 59.44 − 0.59[FeO], r = −0.99. The values of coefficients in this equation are similar to those in the equation SiO2 = 58.45 − 0.60[FeO] that reflects the composition of skeletal−dendritic FSs in the second group, which are recovered from high-calcium lignite ash (series S).36 Figure 3 shows gross compositions of polished sections of globules from the B and S series, which are described by the general regression eq 3 [SiO2] = 59.96 − 0.61[FeO], r = −1.00; with allowance for local areas, the general equation becomes (3a) [SiO2] = 60.59 − 0.62[FeO], r = −0.99 (Figure 3). The presence of several relationship equations SiO2 = f(FeO) and significant concentrations of Al2O3 in the FSs composition may indicate that not only quartz but also other aluminosilicate mineral forms of the initial coal are involved in

0.56[FeO] (2), with the correlation coefficient (r) being equal to −1.00 in both cases. By analogy with data of the papers,19,36 we may suggest that globules in each of these groups are formed from small ironsilicate fragments whose composition also obeys these equations. To test this suggestion, compositions of local areas with a diameter of 4 μm, which are located on perpendicular diameters of globule sections, are depicted on the plot (1) (Figure 1) as an example. With allowance for local areas, the composition is described by the general regression eq 1a [SiO2] = 47.35 − 0.53[FeO], r = −0.99. A wide range of variations in the composition of local areas indicates the absence of melt droplet homogeneity. Melt inhomogeneity is also observed in the second group, the composition of which, with allowance for local areas, obeys the eq 2a [SiO2] = 53.87 − 0.56[FeO], r = −0.99 (Figure 3). 6790

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Figure 4. Dependence of the SiO2 content on the Al2O3 content for FSs of series B and S.

Figure 3. Dependence of the SiO2 content on the FeO content for FSs of series B and S.

their formation. The established general equation of the relationship SiO2 = f(FeO) for FSs of two different ashes may indicate a common route for the formation of not only the iron-silicate base but also the whole globule. In this case, the relationship of the major components composition of certain globule groups from two FSs series may be represented by the same regression equations SiO2 = f(Al2O3) and CaO = f(SiO2). To test this suggestion, the dependence SiO2 = f(Al2O3) of the composition of studied FSs (Table 1) and globules separated from high-calcium lignite ashes (Table 2)36 is presented in Figure 4, which reveals the nature of the aluminosilicate precursor. The dependence may be related to three main groups of FSs, including globules of two series (Figure 4), the gross composition of which is represented by three linear regression equations [SiO2 ] = −0.76 + 1.37[Al 2O3] , r = 0.97

[SiO2 ] = 4.62 + 1.41[Al 2O3], r = 0.98

(2)

[SiO2 ] = 9.23 + 1.39[Al 2O3], r = 0.98

(3)

The formation of these globules involves an aluminosilicate mineral form with similar values (1.37−1.41) of the silicate modulus SiO2/Al2O3, as evidenced by the coefficients in eqs 1−3 (Figure 4). The three globule groups differ in the amount for silica involving in their formation. According to the constant term of the equation, the amounts of silica in the second and third groups amount to 4.62 and 9.23 wt %, respectively. The general equations SiO2 = f(Al2O3) for FSs of the two series, which are produced by combustion of two different coals, indicate the formation of FSs via similar routes with involvement of the same aluminosilicate precursors that may control their structure. It may be supposed that skeletal− dendritic FSs not included in the three main groups (Figure 4)

(1)

Table 2. Chemical Gross Composition (wt %) of Polished Sections of Skeletal−Dendritic Globules Produced by Combustion of Lignite (Series S)36 globule

SiO2

Al2O3

FeO

CaO

MgO

Na2O

K2O

TiO2

MnO

2786 6709 2788 2315 8498 6725 2181 6731 2322 2800 6730 2327 8500 2318 2321 8509 2247 2186 2185 1670 2199

2.19 2.95 1.60 4.79 5.55 3.74 4.73 5.80 6.66 11.35 8.20 12.32 8.03 12.35 17.48 16.20 17.38 22.26 43.70 37.52 36.60

1.69 2.01 1.88 3.61 0.90 0.85 4.58 5.04 1.70 1.26 7.28 8.67 2.37 1.30 6.64 6.26 6.02 21.75 1.67 3.70 11.03

93.72 92.58 91.39 90.08 89.83 89.31 85.62 85.17 85.17 79.33 77.94 76.54 75.13 67.19 64.59 57.73 52.96 42.05 40.76 38.19 35.68

0.68 0.91 2.67 0.40 1.82 4.09 3.32 2.20 4.25 5.51 4.87 1.05 11.25 16.26 6.45 12.15 7.42 8.54 10.95 17.19 10.21

0.31 0.47 1.10 0.10 0.45 0.87 0.54 0.47 0.82 0.86 0.77 0.19 1.41 1.74 0.26 5.14 2.69 1.04 1.50 2.17 2.76

0.58 0.62 0.49 0.35 0.81 0.50 0.52 0.52 0.49 0.70 0.34 0.47 0.57 0.31 2.63 0.51 0.81 3.64 1.06 0.68 1.38

0.01 0.00 0.01 0.04 0.04 0.03 0.05 0.04 0.03 0.05 0.01 0.02 0.07 0.00 0.85 0.05 0.24 0.64 0.12 0.14 1.26

0.00 0.00 0.05 0.01 0.11 0.13 0.00 0.09 0.16 0.07 0.09 0.03 0.39 0.11 0.41 1.24 10.77 0.05 0.05 0.31 0.95

0.66 0.46 0.64 0.51 0.37 0.46 0.54 0.58 0.48 0.63 0.47 0.53 0.44 0.57 0.41 0.28 1.56 0.00 0.00 0.00 0.00

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SEM images of the first group include globules of both series (Figure 6), with the FeO content being in a range of 38.7−90.1 wt %; the globule composition is represented by the regression eq 1 [CaO] = −0.2 + 0.1[SiO2], r = 0.99 (Figure 5). The second group comprises globules of both series (Figure 7), with the FeO content being in a range of 64.6−89.8 wt %; the globule composition is represented by the regression eq 2 [CaO] = 0.98 + 0.19[SiO2], r = 0.95.

are also formed with involvement of the same aluminosilicate precursor and a larger amount of SiO2 compared to the third group. The composition and content of glass phase is another important factor determining the structure of FSs. The dependence CaO = f(SiO2) (Figure 5) that reflects a

Figure 5. Dependence of the CaO content on the SiO2 content for FSs of series B and S. Figure 7. SEM images of FSs of series B and S, corresponding to the equation [CaO] = 0.98 + 0.19[SiO2] (Figure 5).

relationship between two glass-forming components reveals six groups that include almost all FSs of the B (Table 1) and S (Table 2) series. The selected groups include different numbers of globules of the B and S series, with the FeO content in the globules being very different. In particular, approximately half of globules of the S series have a high FeO content (85.2−94.4 wt %) (Table 2), while FSs of the B series (Table 1) and the second half of globules of the S series contain FeO in a range of 35.7−79 wt %. Figures 6−10 show

The third group includes globules of both series (Figure 8) with the FeO content in the range of 39.7−91.4 wt %; the globule composition is represented by the regression eq 3 [CaO] = 2.39 + 0.20[SiO2], r = 1.00.

Figure 8. SEM images of FSs of series B and S, corresponding to the equation [CaO] = 2.39 + 0.20[SiO2] (Figure 5). Figure 6. SEM images of FSs of series B and S corresponding to the equation [CaO] = −0.20 + 0.10[SiO2] (Figure 5).

The fourth group, unlike the first three, includes only globules of the S series with the FeO content ranging from 35.7 to 89.3 wt % (Figure 9); the globule composition is represented by the eq 4 [CaO] = 3.30 + 0.20[SiO2], r = 0.99, and coincides with a similar equation reported in ref 36. This group comprises ∼30% of the studied globules of the S series (Table 2). Probably, lignite combustion leads to preferred formation of globules of this group compared to FSs of other groups.

SEM images of polished sections of six globule groups, which characterize changes in their structure as the SiO2 and CaO contents increase and the FeO concentration decreases. The globule number is indicated in the left corner of the SEM image, and the globule series, S or B, is indicated in the right corner. 6792

DOI: 10.1021/acs.energyfuels.9b01077 Energy Fuels 2019, 33, 6788−6796

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groups have similar skeletal structure with large ferrospinel crystallites (Figures 6−11). An increase in the content of glass-

Figure 9. SEM images of FSs of series S corresponding to the equation [CaO] = 3.30 + 0.20[SiO2] (Figure 5).

The fifth group consists only of globules of the B series (Figure 10), with the FeO content being in the range of 38.1−

Figure 11. SEM images of FSs of B and S series, corresponding to the equation [CaO] = 9.49 + 0.21[SiO2] (Figure 5).

forming components and a decrease in the FeO content in all groups lead to a consistent change in the globule structure, from a skeletal type to skeletal−dendritic and dendritic finegrained crystalline types, with a high content of the glass phase. Transition from coarse-grained skeletal globules to fine-grained skeletal−dendritic ones (globules B8203, S2321, B9024, S8509, and B1275) in different groups occurs at FeO and SiO2 contents of 56.4−64.5 and 16−18 wt %, respectively, against the background of increasing CaO concentration in the range of 5−13.6 wt % (Figure 5; Tables 1 and 2). The subsequent transition to dendritic globules with a high content of the glass phase in different groups (globules B3972, B3968, S2185, S2199, B9539, and S1670) (Figure 5; Tables 1 and 2) occurs at FeO and SiO2 contents of 35.7−40.8 and 27.4−43.7 wt %, respectively, as the CaO concentration is increased in the range of 4−17.2 wt %. Thus, an analysis of the relationship between the major components composition and the structure of polished sections revealed a general effect of the composition on the structure of FSs produced by industrial combustion of coal and lignite, which may indicate similar or identical routes of their formation. Routes of Skeletal−Dendritic FSs Formation. To elucidate the nature of precursors involved in the formation of FSs, the composition of mineral components of the initial coals and the features of their thermochemical conversion should be analyzed. Initial Tugnuysky coals of grade D, DG with low sulfur (0.2−0.7%) and phosphorous (0.03%) contents, include fine scattered syngenetic minerals, such as marcasite, pyrite (FeS2), siderite (FeCO3), and chalcopyrite (CuFeS2). Epigenetic calcite (CaCO3), chalcedony (a microcrystalline quartz species), gypsum (CaSO4·2H2O), pyrite, siderite, and ferric and ferrous compounds form films and pseudomorphs on mineral and organic remains.37,38 The dominant mineral form of iron in the initial lignite B2 is pyrite that accounts for up to 20 wt % of the mineral content of coal. Pyrite inclusions (40−100 μm) closely associated with the organic coal mass are very abundant, with the iron (in the form of pyrite) content in the inclusions amounting to 50%. Complex Ca, Mg, and Fe humates contain up to 40% of calcium of the mineral component.39

Figure 10. SEM images of FSs of series B corresponding to the equation [CaO] = 4.72 + 0.22[SiO2] (Figure 5).

84.9 wt %; the globule composition is represented by the eq 5 [CaO] = 4.72 + 0.22[SiO2], r = 0.99 (Figure 5). This group includes ∼30% of globules of the B series, which means their preferred formation during combustion of the initial coal. The sixth group includes globules of both series (Figure 11), with the FeO content being in the range of 38.2−75.1 wt %, whose composition obeys the eq 6 [CaO] = 9.49 + 0.21[SiO2], r = 0.98 (Figure 5). From the analysis of the composition of the ferrospheres of two series, it follows that the globules of the series B have systematically higher concentrations of MgO, 0.7−10.8 wt %, compared to FSs from lignite in which the concentration of MgO is 0.2−5.1 wt % (Tables 1 and 2). In addition, FSs from coal are preferably formed with a higher content of CaO as evidenced by the values of the free term in the equations CaO = 4.72 + 0.22[SiO2] and CaO = 3.3 + 0.2[SiO2] characterizing the composition of 30% of globules from coal and lignite, respectively (Figure 5). A general analysis of the relationship between the composition and structure of FSs shows that globules with high iron content in the range of 75−91 wt % FeO in six main 6793

DOI: 10.1021/acs.energyfuels.9b01077 Energy Fuels 2019, 33, 6788−6796

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Energy & Fuels

of FeO and SiO2 in the ranges of 64.5−56.4 and 16−18 wt %, respectively, (Figures 6−11). If we suppose that melt droplets with different FeO contents stay the same time in an oxidizing atmosphere, then the degree of iron oxidation increases as the iron concentration decreases. Therefore, the observed change in the structure and an increase in the glass-phase content may be explained by extension of the liquation region in the FeO− Fe2O3−SiO2 system as the oxidation potential rises.43 Subsequent expansion of the liquation region in the FeO− Fe2O3−SiO2 system43 with a decrease in the FeO content and an increase in the SiO2 content to 35−40 and 24−43.7 wt %, respectively, explains transition to dendritic globules with a high content of the glass phase. An additional influence on this process is exerted by an increase in the CaO concentration, from 4 to 17 wt %, in globules of the six groups (Figures 6−11; Tables 1 and 2). An increase in the glass phase of well-studied high-calcium industrial slags is explained by an increase in the fraction of ferrite complexes [Fe3+O2]− and [Fe23+O5]4− in the melt.44,45 A decrease in the concentration of spinel-forming Fe2+ and Fe3+ ions in oxidized high-calcium melts, along with a low concentration of Mg2+ (Tables 1 and 2), leads to the formation of dendritic FSs with a high content of the glass phase. To elucidate the role of aluminosilicate precursors in the formation of skeletal−dendritic FSs of the B and S series, it is necessary to analyze their composition in the initial coals and the features of their thermochemical conversion. The composition of an aluminosilicate component of the initial Tugnuysky coals is represented by small crystals of quartz, feldspar, pyroxenes, as well as kaolinite and hydromica (illite) in a finely dispersed state.38 In the initial B2 lignite, the silicate component includes, along with feldspar, quartz, kaolinite, illite, and montmorillonite, the total content of which reaches 20−30 wt %.46 Of the listed mineral forms of initial coals, the SiO2/Al2O3 ratio amounts to 1.39 only for illite,9,47 which coincides with the value of the coefficient in eq 3 and is close to the 1.37 and 1.41 values of coefficients in eqs 1 and 2, respectively, of the SiO2 = f(Al2O3) dependence (Figure 4). Therefore, it may be concluded that illite is involved, as an aluminosilicate precursor, in the formation of three common groups of skeletal−dendritic FSs during coal and lignite combustion. Hydromicas of the illite group belong to layered minerals with the general formula K1−x+y(Al,Fe3+)2−y(Mg, Fe2+)ySi3+xAl1−xO10(OH)2. Their thermochemical conversion at 940−1000 °C results in the glass phase, quartz, and iron− aluminum spinel formation.9,40 In particular, the spinel content in products of thermal decomposition of illite-type hydromica at 950 °C was shown to amount to 30%.9 Obviously, the formed Al−Mg ferrospinel crystallites act as a “seed” during crystallization of melt droplets differing in their composition. Therefore, the “seed” in the form of Al−Mg ferrospinel crystallites is a key factor determining crystallization of melt microdroplets toward skeletal−dendritic FSs in the FeO− SiO2−Al2O3−CaO system. An analysis of SEM images of FSs that are not included in the six groups makes it possible to establish violations of the general tendency to change the structure depending on the composition. The character of changes in the structure of globules B8201, S2247, and S2186 (Figure 12), localized between the trends of the fourth and fifth groups of compositions (Figure 5) must correspond to the nature of the structure change in the row of the fourth group S2800, S2321, and S2199 (Figure 9). Globule S2247 that contains

An important role in the formation of FSs of both series may be played by associations of iron-containing precursors (pyrite and siderite) with quartz, calcite, or aluminosilicate minerals, which are present in different coals and significantly facilitate coalescence of spatially localized products resulting from the thermochemical conversion of mineral components of the initial coal.40 The oxidative transformation of pyrite and its associations with aluminosilicate and calcium-containing mineral forms of both coals proceeds in several successive stages with involvement of low-temperature eutectics (∼940 °C) of the FeS−FeO system, which include iron oxysulfide complexes.5,40 These oxysulfide complexes react further with finely dispersed products of the thermochemical conversion of mineral precursors with the formation of melt droplets in the FeO− SiO2−Al2O3−CaO system with partial crystallization of individual phases during their cooling, which leads to the formation of FSs.9,11,12,17,19 The formation of FSs of the S series with high (85.2−94 wt %) and low (38.7−79.3 wt %) FeO contents (Table 2) can be explaining by the general scheme of the particles transformation of excluded and included pyrite. In particular,5 combustion of coals containing large particles of excluded pyrite was demonstrated to be accompanied by particles fragmentation with the formation of magnetite particles half as large as the initial pyrite. The transformation of large excluded pyrite particles of lignite due to this process should lead to the formation of globules of the S series with a high FeO content (Table 2) and large skeletal ferrospinel crystals (Figures 6−10). During coal combustion, small included pyrite particles can coalesce with products of decomposition of other mineral forms, forming iron-aluminosilicate melt droplets with a low FeO content (Table 2).5 Combustion of Tugnuysky coals containing fine pyrite and siderite impregnation37,38 results in the formation of only globules with a FeO content of 38.1−79 wt %. Only one globule B8222 contains 85 wt % of FeO (Table 1). The composition of the products and the rate of decomposition of siderite depend on the temperature and partial pressures of O2, CO2, and CO. Iron oxides (FeO and partially Fe3O4) form in the carbon matrix under reducing conditions at 1000−1200 °C. The subsequent interaction of FeO with finely dispersed thermochemical conversion products of aluminosilicates and calcite with the formation of melt drops occurs at high temperatures in the system FeO− silicate−CaO that include low-temperature eutectics.41 For example, the eutectic point is 1070 °C in the FeO− CaSi2Al2O8−SiO2 system, 1105 °C in the Ca3Si3O9−olivine system, 1115 °C in the FeO−CaFeSiO4 system, and 1177 °C in the FeO−Fe2SiO4 system.11,41,42 Given these facts, we may conclude that the formation of FSs from pyrite, siderite, and other mineral precursors occurs due to thermochemical conversion in the common FeO−SiO2−Al2O3−CaO system with involvement of the same low-temperature eutectics. Only primary stages of the FeO formation differ; this is the oxidation stage in the case of pyrite and the decarbonation stage in the case of siderite. This is confirmed by general equations of the composition relationship SiO2 = f(FeO), SiO2 = f(Al2O3), and CaO = f(SiO2) (Figures 3−5) for two series of FSs and by a general composition effect on the globule structure. As shown in the previous section, a monotonous decrease in the FeO content and an increase in glass-forming components lead to transition of the skeletal structure to the fine-crystalline skeletal−dendritic structure in all groups at the concentration 6794

DOI: 10.1021/acs.energyfuels.9b01077 Energy Fuels 2019, 33, 6788−6796

Article

Energy & Fuels

The dependence CaO = f(SiO2) that reflects the influence of glass-forming components reveals six groups of FSs, the composition of which is represented by linear regression equations. An analysis of SEM images of polished sections from six globule groups reveals that an increase in the concentration of glass-forming components in all groups is accompanied by gradual changes in the structure of globules, from the coarse-grained crystalline skeletal type to the finecrystalline dendritic type with a high content of the glass phase. The observed change in the structure is explained by expansion of the liquation region in the FeO−Fe2O3−SiO2 system, a rise in the oxidation potential, an increase in the proportion of ferrite complexes [Fe3+O2]− and [Fe23+O5]4− in high-calcium melts, and a decrease in the concentration of ferrospinelforming Fe2+ and Fe3+ ions. A high content, up to 11 wt %, of spinel-forming components, TiO2 and MgO, in individual globules disrupts the general pattern of structural changes in FSs via expansion of the crystallization field of spinel structures at high oxidation potentials.

Figure 12. SEM image of FS 2247 with failure of the general trend in globule structure changes compared to globules 8201 and 2186.

10.8 wt % TiO2 is out of the general trend toward an increase in the glass-phase content (Table 2). In this case, solid solutions Fe2−x2+Fe2x3+Ti1−xO4 are formed in the Fe3O4− Fe2TiO4 system at a high TiO2 concentration and at temperature of 600−1400 °C,43 which expand the crystallization field of ferrospinels at high oxidation potentials. In the row of B3972, B1267, and S2199 globules (Figure 13) with close contents of FeO (35.7−38.7 wt %) and SiO2 (35.3−

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +7 391 205 19 43. Fax: +7 391 249 41 08. ORCID

Alexander G. Anshits: 0000-0002-5259-0319 Notes

The authors declare no competing financial interest.

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Figure 13. SEM images of FSs with close contents of FeO and SiO2.

ACKNOWLEDGMENTS This work was supported by the Siberian Branch of the Russian Academy of Sciences (basic research project no. V.45.3.3) and performed at the Institute of Chemistry and Chemical Technology, Siberian Branch, Russian Academy of Science, Federal Research Center “Krasnoyarsk Science Center SB RAS”.

37.6 wt %) (Tables 1 and 2), there should be a close content of the glass phase. Globule B1267 is out of the general trend due to a high MgO concentration of 10.8 wt % in the globule melt. The ability of Mg2+ cations to occupy both positions in the spinel, with a preference for the octahedral one,48 provides an increased concentration of spinel-forming components at local points of dendritic ferrospinel crystallite formation.

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CONCLUSIONS A systematic SEM−energy-dispersive X-ray spectroscopy study of the relationship between the composition and structure of skeletal−dendritic FSs recovered from fly ashes produced by coal and lignite combustion revealed the common routes of their formation and the features of a mineral precursor influence on the FS structure. The groups of globules for which the gross composition of polished sections corresponds to the general equations of the relationship between the concentrations of SiO2 = f(FeO), SiO2 = f(Al2O3), and CaO = f(SiO2) were highlighted from the two series FSs. The studied FSs were formed from melt droplets of the common FeO−SiO2− Al2O3−CaO system. The formation of melt droplets occurs due to sequential transformation of dispersed products from thermal conversion of mineral precursor associates: pyrite, quartz, and Ca; Al-humates in the case of lignite; and pyrite, siderite, quartz, and calcite in the case of coal. In both cases, the aluminosilicate precursor determining the structure of globules is illite with the silicate modulus SiO2/Al2O3 equal to 1.39, as indicated by the coefficients values in the general equations SiO2 = f(Al2O3). Crystallization of skeletal− dendritic ferrospinel globules occurs due to the Al, Mgferrospinel “seed” that is formed in the thermochemical conversion of illite from initial coals.

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DOI: 10.1021/acs.energyfuels.9b01077 Energy Fuels 2019, 33, 6788−6796