Characterization of Fly Ash Cenospheres Produced from the

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Characterization of Fly Ash Cenospheres Produced from the Combustion of Ekibastuz Coal Elena V. Fomenko,† Natalia N. Anshits,† Nataly G. Vasilieva,† Olga A. Mikhaylova,† Elena S. Rogovenko,† Anatoliy M. Zhizhaev,† and Alexander G. Anshits*,†,‡ †

Institute of Chemistry and Chemical Technology, Akademgorodok 50/24, Krasnoyarsk, 660036 Russia Siberian Federal University, Svobodny pr. 79, Krasnoyarsk, 660041 Russia

‡

ABSTRACT: The relationship between the chemical composition and shell structure of cenospheres with a low bulk density of 0.40−0.45 g/cm3 and a high Al2O3 content of 33−38 wt % has been systematically studied. It was established that the composition of the narrow fractions of cenospheres can be described by the general regression equation SiO2/Al2O3 = 4.34 − 0.08[Al2O3] with the correlation coefficient of r = −0.99. The phase composition includes the glass phase (57−73 wt %), mullite (25−40 wt %), and quartz (1.2−2.5 wt %). An increase in the Al2O3 content leads to an increase in the size of particles and the porosity of their shells. In the obtained fractions of cenospheres, there are two types of globules: spherical globules with a singlering structure and foamy globules with a network structure. It is established that the composition of individual particles with a network structure localized in the range of the Al2O3 content from 43 wt % to 51 wt % and can be described by the regression equation SiO2/Al2O3 = 2.71 − 0.04[Al2O3] with the correlation coefficient of r = −0.97. The framework of these particles permeated by mullite microcrystallites and coated with a nanoscale surface film. The structure-forming mineral precursor of these particles is kaolinite. The gross composition of the shell of individual globules with a single-ring structure localized in the range of the Al2O3 content from 26 wt % to 42 wt % and can be described by the general regression equation SiO2/Al2O3 = 4.71− 0.09[Al2O3] with the correlation coefficient of r = −0.98. The outer and inner surfaces of the shell are covered by large and small in-plane localized mullite crystals hidden by the nanoscale film. The spherical shape and their crystalline framework are formed from the illite melt with inclusions of products of the thermal conversion of other mineral forms.

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INTRODUCTION

sensitizers capable of replacing expensive hollow synthetic microspheres.27 A criterion for the applicability of narrow fractions of cenospheres as functional materials in each particular case is the fulfillment of the requirements for the composition and structure of the shells of globules. In particular, important criteria for the applicability of cenospheres as precursors of mineral-like phases for the immobilization of the radionuclides 137Cs and 90Sr are the ratio SiO2/Al2O3 and the porosity of the shells of the cenospheres.19,28 Compositions of the aluminosilicate minerals, which are the proper phases of Cs+ and Sr2+ (pollucite, Cssanidine, Sr-anorthite) or can isomorphically include Cs+ and/or Sr2+, i.e., low-modulus zeolites (NaA, NaX, NaP, analcime), feldspars (albite, orthoclase, anorthite), and feldspathoids (nepheline), correspond to the SiO2/Al2O3 ratio of 1.2−3.5. Therefore, the key problem in the use of cenospheres in the process of the extraction of radionuclides and subsequent directional crystallization of phases of the specified structural type is the preparation of the cenosphere fractions with the SiO2/ Al2O3 ratio of 1.2−3.5. A criterion of the applicability of cenospheres as emulsion explosive sensitizers based on ammonium nitrate is the size of globules and the thickness of continuous shells with a single-ring structure.27 In the design and preparation of microspherical glass-crystalline materials for highly selective separation of helium, the determining factors

In the process of generating power from coal, large quantities of coal combustion products (CCPs) are produced. According to the various estimates, combustion of coal in the Russia alone generates ∼25 million tons of CCPs per year but utilization rate does not exceed 15%.1 The complex composition of CCPs, including the fly ash, has proven to be a barrier to its bulk utilization in many fields. Extraction of concentrates of microspherical components with specific characteristics from fly ashes of variable composition provides wider opportunities for the multicomponent use of fly ashes produced from coal combustion.2,3 The unique properties of cenospheres, namely, their low density, sphericity of particles, and high strength and nontoxicity, make them useful for a variety of fields of applications. Thus, concentrates of cenospheres are used as fillers of lightweight composite materials, such as concretes,4,5 polymers and resins,6−8 and metal alloys.9,10 Also, they are been studied for the production of ceramic composite foams with different properties.11,12 In recent years, new functional materials have been developed based on a detailed characterization of narrow fractions of cenospheres with specific composition and structure.13,14 Among these materials are high-selectivity microspherical membranes for diffusion separation of gases,15−17 effective composite sorbents,18,19 including those capable of operating in corrosive medium,20,21 magnetically controlled encapsulated pH-sensitive spin probes for the examination of biological objects,22 supported catalyst systems,23−26 and emulsion explosive © XXXX American Chemical Society

Received: May 6, 2015 Revised: July 1, 2015

A

DOI: 10.1021/acs.energyfuels.5b01022 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels are the shell thickness and the content of crystalline phases.15 Our previous study16 indicated that, as the content of the mullite phase in the shells of the cenospheres increases in the range from 4 wt % to 48 wt %, the helium permeability coefficient at 25 °C increases by 2 orders of magnitude and exceeds, by approximately the same value, the corresponding coefficient for synthetic hollow glass microspheres 3 M Scotchlite K37 glass bubbles. For the cenospheres containing 48 wt % mullite and 14 wt % cristobalite, the separation factors of the helium−hydrogen and helium−neon mixtures at a temperature of 280 °C are equal to 32 and 221, respectively, which are considerably higher than the corresponding values for polymeric membrane materials He/ H2 = 1−4 (ref 29) and He/Ne = 5 (ref 30). At present, in the literature, there has been no clear understanding of how the composition and structural features of glass-ceramic materials based on the cenospheres affect the properties of new functional materials. Therefore, the targeted preparation of effective functional materials requires a systematic investigation of the relationship between the composition, structure, and properties of glass-crystalline shells of the cenospheres. The purpose of this work was to study the relationship between the composition and structure of narrow fractions of cenospheres with a low bulk density and a high aluminum oxide content, which were separated from fly ashes produced from the combustion of Ekibastuz coals.

Scheme 1. Block Diagram Showing the Technological Stages of the Separation of the Cenosphere Concentrate into Narrow Fractions

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EXPERIMENTAL SECTION Preparation of Narrow Fractions of Cenospheres. The raw materials used for the preparation of narrow fractions of the nonmagnetic nonperforated cenospheres with a low bulk density were concentrate of fly ash cenospheres produced from the pulverized combustion of Ekibastuz coal of the SS grade in boiler furnaces PK-39-2, P-57-2, and P-57-3 with the average (over the boiler furnace height) temperature of 1520−1550 °C at the Reftinskaya Thermal Power Plant (Russia). The type of the original ash according to the Standard Specification for Coal Fly Ash (ASTM C618)31 corresponds to Class F. The particle size distribution of the original cenosphere concentrate, determined on the laser diffraction sensor Sympatec Helos & Rodos (Sympatec GmbH, Germany), was as follows: d10 = 40 μm, d50 = 111 μm, d90 = 209 μm, and d99 = 282 μm. The chemical composition of the cenosphere concentrate determined by the chemical analysis method32 was as follows (wt %): SiO2, 55.20; Al2O3, 37.65; Fe2O3, 2.20; CaO, 1.47; MgO, 1.05; SO3, 0.36; Na2O, 0.32; K2O, 0.40; MnO, 0.04. The loss on ignition (LOI) was 0.94 wt %. The separation of narrow fractions of the cenospheres was performed according to the technological scheme (Scheme 1), including stages of magnetic separation and grain-size classification with the subsequent hydrostatic separation from perforated and destroyed globules. At the stage of the magnetic separation, the concentrate of cenospheres was subjected to the separation using a Model 138T electromagnetic separator (Russia) at a magnetic field strength of 10.55 kOe. As a result, two products were obtained: the magnetic and nonmagnetic cenospheres, which were designated as MR and HMR, respectively. The nonmagnetic product, labeled HMR, then was separated on a VP-S/220 vibrodrive (Russia) equipped with a set of standard sieves with meshes 0.25, 0.2, 0.16, 0.125, 0.1, 0.08, 0.071, 0.063, 0.05, and 0.04 mm in size. In order to separate out the perforated cenospheres and fragment of broken globules,

the obtained fractions were subjected to further hydrostatic separation after the preliminary evacuation. Characterization Methods. The chemical and phase compositions, bulk density, the particle size distribution, the average diameter of the globules, the thickness of the globule shells, as well as the content of globules of a particular morphological type were determined for each narrow fraction of cenospheres. These characteristics should meet the basic criteria for the applicability of narrow fractions of cenospheres as functional materials. Details of the techniques used for the determination of these parameters, including the control of the purity of isolated fractions, were described in previous papers.13,14 The outer surface of the cenospheres was examined on Axioskop Imager D1 optical microscope equipped with an AxioCam MRc5 color digital video camera (Carl Zeiss, Germany). The obtained narrow fractions of cenospheres with a particular chemical composition contain three main morphological types of globules: network-structure globules, single-ringstructure globules with a solid shell, and single-ring-structure globules with a porous shell. The contents of globules of different morphological types were determined using the specially developed computer program “Msphere” for the processing of digital optical images of at least 5000 globules in each fraction.14 The relationship of the composition and structure of individual globules was studied on powder samples and polished sections of cenospheres of the narrow fractions HMR −0.063 + 0.05 mm and HMR −0.25 + 0.2 mm. The samples were fixed with a dusting powder on a conductive carbon adhesive tape secured on a flat substrate prepared from a Duopur poly(methyl methacrylate) resin with a diameter of 30 mm and a thickness of 1−3 mm. Taking into account the high fragility of the cenospheres shell, their polished sections were prepared according to the specially developed technique.33 At the first stage, the cenospheres were fixed in an Epofix epoxy resin with B


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Figure 1. SEM images of the cenospheres in regions of the quantitative analysis of the shell compositions: (a) outer surface of the globule; (b) polished section of the ring-structure globule with the designation of the analysis points; (c) polished section of the network-structure globule with the designation of analysis points; and (d) distribution map of elements in a thick shell of the globule shown later in this work in Figure 8a.

determination of the gross composition of polished sections and local regions of the intersections of the diameters with the globule shells (Figures 1b and 1c); and the study of heterogeneous regions of the polished sections, which were identified by the elemental mapping (Figure 1d). These studies were carried out using Model TM-1000 and TM-3000 SEM systems (Hitachi, Japan) that were equipped with a microanalysis system including a Bruker Quantax 70 energy-dispersive X-ray spectrometer with a Model XFlash 430H detector. The data accumulation time was determined by the quality of the spectrum assembly, which made it possible to perform the quantitative data processing, and it was found to be at least 10 min. The contents of the elements were recalculated in terms of the corresponding oxides, and their sum was normalized to 100%.

the subsequent polishing on a Struers Discoplan-TS precision cutting and polishing machine. During the polishing process, the cross sections were repeatedly examined by scanning electron microscopy (SEM) (Model TM-1000, Hitachi, Japan) to monitor the size of the particles in the residual layer. The thickness of the residual layer after polishing on the first stage was equal to 3/4 of the diameter of the cenospheres. Then, the cenospheres on the substrate were again covered by an Epofix epoxy resin layer, polished by Carborundum paper (P2000, P1200, and P1000) to a residual layer thickness equal to one-half of the diameter of the globules. The finish polishing of the sections of the fractions was carried out using 3 M suspensions with grain sizes of 0.080 and 0.032 μm on a rubber polishing wheel. The conductive coating on the surface of the powder samples and polished sections was formed by the plasma spraying of a ∼50-nm-thick platinum layer with the use of an Emitech K575XD Turbo spraying coater. The composition and structure of the globules were studied by SEM−EDS method in several variants: the examination of local regions of the surface of the globules (Figure 1a); the

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RESULTS AND DISCUSSION Aluminosilicate Composition of the Globules of Narrow Fractions with a High Al2O3 Content. The formation of cenospheres with different structures is affected by the chemical and phase compositions of mineral inclusions of C


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Energy & Fuels Table 1. Yielda and Chemical Composition of Narrow Fractions of Low Bulk Density Cenospheres Composition (wt %)

a

cenosphere fraction

yield

LOI

SiO2

Al2O3

Fe2O3

CaO

MgO

Na2O

K2O

SO3

P2O5

SiO2/ Al2O3

HMR −0.25 + 0.2 HMR −0.2 + 0.16 HMR −0.16 + 0.125 HMR−0.125 + 0.1 HMR −0.1 + 0.08 HMR −0.08 + 0.071 HMR−0.071 + 0.063 HMR −0.063 + 0.05 HMR −0.05 + 0.04

8.1 10.5 14.0 11.0 7.9 2.6 1.3 1.4 0.4

0.39 0.15 0.41 0.45 0.27 0.50 0.44 0.32 0.52

56.18 57.29 57.73 58.30 59.09 59.90 60.50 61.24 61.13

38.08 37.04 37.35 36.57 36.21 34.90 34.70 33.55 33.03

1.68 1.82 1.33 1.06 0.98 1.10 0.90 1.12 1.22

1.62 1.48 0.97 1.23 1.22 1.03 0.93 0.97 1.36

1.17 1.11 1.17 1.06 1.25 1.41 1.32 0.96 1.26

0.29 0.33 0.40 0.44 0.37 0.45 0.45 0.50 0.56

0.39 0.40 0.42 0.48 0.46 0.55 0.55 0.60 0.60

0.28 0.22 0.13 0.18 0.17 0.06 0.08 0.30 0.20

0.05 0.06 0.06 0.06 0.05 0.09 0.04 0.05 0.07

1.48 1.55 1.55 1.59 1.63 1.72 1.74 1.83 1.85

In terms of initial concentrate.

Table 2. Physical Characteristics and Phase Composition of the Cenosphere Fractions Physical Characteristics 3

Phase Composition (wt %)

cenosphere fraction

bulk density (g/cm )

average diameter (μm)

apparent thickness (μm)

mullite

quartz

calcite

glass phase

HMR −0.25 + 0.2 HMR −0.2 + 0.16 HMR −0.16 + 0.125 HMR −0.125 + 0.1 HMR −0.1 + 0.08 HMR −0.08 + 0.071 HMR −0.071 + 0.063 HMR −0.063 + 0.05 HMR −0.05 + 0.04

0.44 0.45 0.44 0.43 0.42 0.42 0.42 0.40 0.41

225 183 143 112 91 76 67 58 47

12.7 10.6 8.0 6.2 4.9 4.1 3.6 2.9 2.4

40.3 39.1 38.1 38.2 37.4 34.7 32.6 30.1 25.3

2.4 2.5 1.5 1.8 1.5 1.5 1.1 1.3 1.2

0.1 0.1 0.2 0.1 0.1 0.3 0.2 0.2 0.4

57.2 58.3 60.2 59.9 61.0 63.5 66.1 68.4 73.1

and quartz phases increase in the ranges of 33−38 wt %, 25−40 wt %, and 1.2−2.5 wt %, respectively, whereas the content of the glass phase decreases in the range from 73 wt % to 57 wt %. The aluminosilicate composition of the cenosphere fractions can be described by the general regression equation SiO2/Al2O3 = 4.34 − 0.08[Al2O3] with the correlation coefficient of r = −0.99 (Figure 2). Note that the aluminosilicate composition of the

the initial coal, the conditions of coal combustion, and thermochemical conversion of coal mineral forms. An important characteristic of the composition of the cenospheres is the SiO2/ Al2O3 ratio, which characterizes the aluminosilicate minerals of the coal from which the cenospheres are formed.13,34 The thermal destruction of aluminosilicates at temperatures in the range of 400−800 °C leads to the transformation of all their constituent oxides into an activated amorphous state. This fundamentally important point was considered in detail in the literature,35,36 because, from the amorphous state, the subsequent phase transformations in the ash part of the fuel, including the melting, occur within a fraction of a second. The main crystalline phase of the thermochemical conversion of aluminosilicate minerals is the mullite phase. After the calcination under the same conditions, the theoretically possible yield of the mullite phase is higher for kaolinite clays (55%−64%), compared to hydromicaceous minerals, such as illite (35%−46%) and montmorillonite (15%−31%).37 According to literature data,38−40 the predominant mineral components of coals from the Ekibastuz Basin are clay minerals of the kaolinite group with the SiO2/Al2O3 ratio of 1.2 and quartz. The total content of kaolinite and quartz is ∼82%. There are also hydromicas of the illite type with a SiO2/Al2O3 ratio of 1.4, feldspars with a SiO2/Al2O3 ratio of 1.2−3.5, as well as calcite and dolomite as impurity phases. Narrow fractions of the nonmagnetic nonperforated cenospheres with a low bulk density of 0.40−0.45 g/cm3, and a low Fe2O3 content of 0.9−1.8 wt % were separated from the cenosphere concentrate produced from the combustion of Ekibastuz coals. The chemical, phase composition, bulk density, average diameter, apparent thickness of the globule shells and the yield of the products are presented in Tables 1 and 2. In particular, the obtained results have demonstrated that, as the size of the fractions increases, the contents of Al2O3, the mullite

Figure 2. Dependence of the SiO2/Al2O3 ratio on the Al2O3 content for the narrow fractions of cenospheres (Table 1), together with literature data.33,41,42

cenosphere fractions from Australian33,41 and Indian power stations42 also corresponds to the trend of observed dependence (Figure 2). It can mean that the formation of cenospheres in the aforementioned cases occur from the same mineral form of initial coals. According to our previous study,13,14 the aluminosilicate composition of narrow fractions of the cenospheres with an D


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Figure 3. Optical images of cenospheres: (a) globules of the fraction HMR −0.063 + 0.05 mm (transmitted light); (b) globules of the fraction HMR −0.25 + 0.2 mm (reflected light); (c, d) globules with crystallites of the fraction HMR −0.063 + 0.05 mm (transmitted light).

temperatures below 1400 °C, the majority of kaolinite grains retain their original size and shape. After the combustion at 1400 °C, kaolinite may form a honeycomb (network) structure. The association of quartz with mullite derived from kaolinite is also obvious in ash particles. Thus, the formation of foamy particles with a variable density occurs according to the general scheme including several stages. At the first stage, the dehydroxylation of kaolinite is accompanied by the formation of particles with foamy morphology. The next stage includes their coalescence with each other and with quartz particles, which leads to their contamination by quartz and to an increase in the SiO2/Al2O3 ratio and the bulk density of the fractions. The final stage determining the morphology is the stage of rapid cooling of the particles. Simultaneously, at temperatures of 600−1200 °C, these processes are accompanied by the phase transformation of kaolinite through the amorphous state (metakaolinite) with the formation of mullite.47 Cristobalite as a product of the high-temperature (1150−1250 °C) transformation of kaolinite47 is not observed in the cenospheres with an Al2O3 content of >33 wt %. It is formed at the content of 14 wt % only after the subsequent exposure of the cenospheres for 3 h at a temperature of 1100 °C.16 Morphology of the Globules of Narrow Fractions with a High Al2O3 Content. The structure and content of different morphology globules were studied by optical microscopy and SEM. The optical microscopy study of the morphology of the globules revealed that small-sized fractions with an Al2O3 content of 33−34 wt % are dominated by globules of the ideal spherical shape with different degrees of porosity of their shells (Figure 3a,

Al2O3 content of 19−33 wt % and mullite phase content (1.5−12 wt %) produced from the combustion of Kuznetsk coals is described by the general regression equation SiO2/Al2O3 = 5.54 − 0.12[Al2O3] with the correlation coefficient of r = −0.99. It is supposed that these cenospheres are formed through the inflation of molten hydromica particles with quartz inclusions. The significant differences in the aluminosilicate compositions of the fractions with high (33−40 wt %) and low (19−33 wt %) Al2O3 contents clearly indicate that these cenospheres are formed from different mineral precursors. In turn, a high content of kaolinite in the initial coals allows suggesting that it is the structure-forming precursor of cenospheres with high Al2O3 content. The transformation of kaolinite particles during a rapid heating for 0.2−2.0 s at temperatures of 900−1250 °C, followed by rapid cooling, leads to the formation of foam particles.43 The porous structure formed at the stage of the dehydroxylation of kaolinite is retained due to a rapid cooling of the particles. This point of view is also followed by the authors,44 who earlier observed the formation of foam particles upon coal combustion. According to data,45 the combustion of the sub-bituminous coal containing 60%−70% kaolinite, 10%−20% siderite, and 5%−15% quartz is accompanied by the coalescence of kaolinite-derived particles with each other and with quartz particles. This leads to the formation of large particles of irregular shape, including regions of low and high porosities. The regions with a low porosity have a high SiO2 content, whereas the regions with a high porosity correspond to metakaolinite. In the Australian coals,46 kaolinite exists as a pure mineral and in an associated form with quartz. At E


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shells were also revealed in the SEM image of low bulk density fractions of cenospheres with an Al2O3 content of 35−41 wt %, separated from fly ash from an Indian power plant.42 Particles of similar structure with deviations from the sphericity are also formed during the foaming of the pyroplastic state of kaolinites.47 The analysis of the data presented in Tables 1 and 2 demonstrated that the average size of the globules and the thickness of their shells increase as the Al2O3 content increases. In this case, the dependence of the average thickness of the shell on the Al2O3 content exhibits a breakpoint at ∼36 wt % Al2O3 (Figures 4a and 4b), which corresponds both to the fraction with the maximum amount of cenospheres with single-ring structure and porous shells and the minimum amount of foamy particles (Figure 4c). An increase in the Al2O3 content above 36 wt % leads to a drastic increase in the concentration of foamy particles and to a monotonic decrease in the concentration of globules with porous shells (see Tables 1 and 3, as well as Figure 4c). The investigation of the structure of the globules in the concentrates of cenospheres with an Al2O3 content of 24−30 wt %4,48,49 and in individual fractions with an Al2O3 content of 21−31 wt %34,50 revealed that they are also dominated by spherical globules with different degrees of porosity of their shells, whereas the globules with a network structure were not observed. In our previous studies,13,14 the content of globules with different degrees of porosity of their shells in fractions of the cenospheres with an Al2O3 content of 19−33 wt %, which were produced from the combustion of Kuznetsk coals, was quantitatively determined using SEM and optical microscopy. In particular, it was shown that the average thickness and porosity of the globule shells, as well as the sizes of the globules, decrease

Table 3). It can be seen that, on the surface of the majority of these globules, there are needlelike crystals (see Figures 3c and Table 3. Content of Globules of Different Morphological Type in the Cenosphere Fractions Content (particle %) cenosphere fraction HMR −0.25 + 0.2 HMR −0.2 + 0.16 HMR −0.16 + 0.125 HMR −0.125 + 0.1 HMR −0.1 + 0.08 HMR −0.08 + 0.071 HMR −0.071 + 0.063 HMR −0.063 + 0.05 HMR −0.05 + 0.04

network

single-ring with solid single-ring with porous shell shell

57 45 25

1 1 2

42 54 73

21 4 2

3 8 14

76 88 84

1

22

77

0

26

74

0

51

49

3d). Large-sized fractions of the cenospheres with the Al2O3 content of >36 wt % are characterized by a sharp increase in the concentration of foamy particles with a significant deviation from the sphericity (Table 3, Figure 3b). A similar increase in the concentration of foamy particles with a network structure in large-sized fractions was observed for cenospheres with a high Al2O3 content separated from fly ash of Australian power stations.33 Globules of irregular spherical shape with foamed

Figure 4. Dependences of (a) shell thickness, (b) average diameter, and (c) content of globules of different morphological type on the Al2O3 content in narrow fractions of cenospheres. F


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Figure 5. SEM images of (a−c) outer surface of the cenospheres of the fraction HMR −0.063 + 0.05 mm with microprobe analysis regions and (d) nanoscale surface film.

surface of spherical globules and on the surface of particles with a network structure (Figure 6). The analysis of the SEM images demonstrated that the size of crystallites on the outer surface of spherical globules is significantly larger (Figures 6a and 6b) than that on their inner surface (Figures 6c and 6d). These differences can be associated with the heterogeneous composition of the shells of the cenospheres. In particular, the analysis of the distribution of Si, Al, and Fe over the thickness of the shells of the cenospheres with an Al2O3 content of 30.4 wt % and a SiO2/ Al2O3 ratio of 1.75 revealed that these elements have different distribution profiles.37 In most cases, the silicon concentration increases from the edges toward the center of the shell, whereas the aluminum concentration in the same direction decreases. The aluminum concentration on the outer surface of the shell is higher than that on the inner surface. On this basis, we can conclude that the larger size of the crystallites on the outer surface of the shell is determined by a higher Al2O3 content in the melt. A comparison of the SEM images of the globules with singlering and network structures demonstrated that, on the surface of the particles with a network structure, there are small mullite crystallites (Figure 6f), whose sizes are comparable to the sizes of crystallites on the inner surface of the single-ring globules (Figure 6d). As can be seen from Figures 6g and 6h, the microcrystalline mullite also penetrate through the entire volume of the shell. The local regions with large mullite crystals observed on the surface

with an increase in the Al2O3 content. This indicates an inverse dependence, compared to the obtained cenospheres with a high Al2O3 content (Figure 4). The SEM−EDS study of the relationship between the composition and structure of local regions of individual cenospheres of the fraction −0.063 + 0.05 mm demonstrated that the porosity of the globule shell increases with an increase in the Al2O3 content (Figures 5a−5c). However, in the case under consideration, on the surface of these globules, there are no needlelike crystals, which are seen on the optical images (Figures 3c, 3d). The observed differences can be associated with the presence of a film covering needlelike crystals (Figure 5d), which was found earlier on the surface of cenospheres.13,51 The simultaneous presence of a 30- to 50-nm-thick surface film and needlelike crystals in-plane localized on the surface was observed for cenospheres with the Al2O3 content of 33 wt % separated from fly ashes produced from the combustion of Kuznetsk coals.15 The formation of this type of crystals is explained by the crystallization of the melt with a high Al2O3 content, and the precipitation of needlelike mullite crystals on the surface of the globules.15,52 It was also shown that the surface film can be removed by the etching cenospheres with an aqueous solutions of HF until mullite crystallites manifest themselves on the surface of the globules. The etching of the fraction HMR −0.063 + 0.05 mm under conditions similar to those used in a previous study15 made it possible to reveal needlelike mullite crystallites on the G


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Figure 6. SEM images of the cenospheres of the fractions (a−d) HMR −0.063 + 0.05 mm and (e−h) HMR −0.25 + 0.2 mm after the removal of the surface film with HF acid etching: (a, e) single globule; (b, f) regions of the outer surface of the globule; and (c, d, g, f) regions of the inner surface of the destroyed globules.

can be explained, as in the case of single-ring globules, by a higher local content of Al2O3. The presented results demonstrate that the cenospheres have a complex structure consisting of a nanoscale surface film and a glass-crystalline shell permeated

with mullite crystallites. This structure provides a structural stability of the cenospheres over a wide temperature range. In particular, because of the in-plane localization of crystallites on the surface of the shells, the globules acquire additional impact H


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Figure 7. SEM images of polished sections of individual globules of the HMR −0.063 + 0.05 mm cenospheres (labeling of the globules corresponds to that described by Table 4).

strength, as in the case of glass-ceramic structures.53 As a result, the cenospheres have an additional advantage in the development of functional materials, such as microspherical membranes,15,16 composite sorbents,18,19 and supported catalysts.23−26 Composition of Individual Globules with Single-Ring and Network Structures. The investigation of the relationship between the composition and the structure of individual globules of different morphological types was performed on polished sections of the fractions HMR −0.063 + 0.05 mm and HMR −0.25 + 0.2 mm. As an example, Figures 7 and 8 show the SEM images of small and large globules with increasing Al2O3 content.

The gross compositions of polished sections of the shells of individual globules with single-ring and network structures in both fractions are presented in Table 4. SEM images illustrate that, with an increase in the Al2O3 content, the structure of the globules changes. The single-ring structure of the globules with small and large sizes is retained at an Al2O3 content ranging from 26 wt % to 42 wt % (see Figures 7a−7c and 8a−8c, as well as Table 4). An increase in the Al2O3 content from 42 wt % to 51 wt % (Figures 7 and 8) first leads to the formation of spheres with two large cavities (Figures 7d and 8d); then, the number of cavities increases and, at the Al2O3 content above 46−47 wt %, the particles acquire a network structure (Figures 7f, 8e, and 8f). I


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Figure 8. SEM images of polished sections of individual globules of the HMR −0.25 + 0.2 mm cenospheres (labeling of the globules corresponds to that described by Table 4).

The dependence SiO2/Al2O3 = f(Al2O3) for the aluminosilicate gross composition of polished sections of globules with the single-ring and network structures of both factions is shown in Figure 9. For the cenospheres with a single-ring structure, this dependence is described by the general regression equation SiO2/Al2O3 = 4.71 − 0.09[Al2O3] with the correlation coefficient of r = −0.98 at the Al2O3 content ranging from 26 wt % to 42 wt %. For the globules of both fractions with a network structure, this dependence is described by the general regression equation SiO2/Al2O3 = 2.71 − 0.04[Al2O3] with the correlation coefficient of r = −0.91 at the Al2O3 content ranging from 43 wt % to 51 wt %. In order to elucidate the influence of foreign inclusions on

the general dependence SiO2/Al2O3 = f(Al2O3) for the globules with single-ring and network structures, we performed the statistical analysis of the compositions of local regions of the intersections of the diameters with the globule shells (see Figures 1b and 1c). Figure 10 shows the dependence SiO2/Al2O3 = f(Al2O3) for local regions of globules with both types of the structure of the fraction HMR −0.25 + 0.2 mm. The regression equation for local regions of network-structure particles (except for the globules with a transition structure, Figure 8d) has the form SiO2/Al2O3 = 2.71 − 0.04[Al2O3] with the correlation coefficient of r = −0.98. This equation is completely identical to the functional relationship for the gross composition of smallJ


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Table 4. Gross Compositions of the Shells of Individual Cenospheres, According to the Electron Microprobe Analysis Data: (a) HMR −0.063 + 0.05 mm and (b) HMR −0.25 + 0.2 mm (a) HMR −0.063 + 0.05 mm Composition (wt %) No.

SiO2

Al2O3

K2O

7a 7b 7c

67.46 59.70 51.77

25.73 35.84 42.16

0.82 1.21 0.22

7d 7e 7f 7−4 7−7

49.47 49.73 46.00 52.23 47.81

43.71 45.72 47.31 42.95 46.04

0.48 0.48 0.37 0.40 0.40

Na2O

MgO

Single-Ring Structure 0.89 0.42 0.88 0.49 0.74 0.49 Network Structure 0.97 0.60 0.62 0.41 0.11

Characterization of Fly Ash Cenospheres Produced from the

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