Article pubs.acs.org/EF
Composition and Morphology of Fly Ash Cenospheres Produced from the Combustion of Kuznetsk Coal Elena V. Fomenko,*,† Natalia N. Anshits,†,‡ Leonid A. Solovyov,† Olga A. Mikhaylova,† and Alexander G. Anshits†,‡ †
Institute of Chemistry and Chemical Technology, Siberian Branch of the Russian Academy of Sciences, Akademgorodok 50/24, Krasnoyarsk 660036, Russia ‡ Siberian Federal University, Svobodny pr. 79, Krasnoyarsk, 660041 Russia ABSTRACT: The chemical and phase composition, morphology, and shell structure of narrow fractions of nonmagnetic and magnetic low-density nonperforated cenospheres separated from fly ash concentrates produced via the pulverized combustion of coal from the Kuznetsk Basin (Russia) have been studied. Narrow fractions of nonmagnetic cenospheres contain 2.6−3.5 wt % Fe2O3 and include globules with a uniform smooth or relief surface and shells with different degrees of porosity. For nonmagnetic cenospheres, the aluminum concentration increase leads to an increase in the mullite phase content and a decrease in the average sphere diameter, glass-crystalline shell thickness and porosity, and the crystalline quartz content. The quartz phase comprises two modifications with different lattice parameters. The narrow fractions of magnetic cenospheres contain 3−21 wt % Fe2O3 and include globules with thick porous shells covered by heterogeneous regions of ferrospinel on their outer surface. In magnetic cenospheres, an increase in the iron concentration leads to an increase in the ferrospinel phase content and crystallite size, accompanied by a decrease in the degree of substitution of magnesium and aluminum for iron.
■
INTRODUCTION Hollow aluminosilicate microspheres with a low bulk density of 0.2−0.8 g/cm3, namely, cenospheres, are one of the microspherical components of fly ash produced from the pulverized combustion of coals. The content of cenospheres in fly ash varies over a rather wide range from 0.01 to 4.80 wt % and, in most cases, amounts to 0.3−1.5 wt %.1−4 The separation of cenosphere concentrates from fly ash in the aqueous media is based on the density difference between the cenospheres and other fly ash components. In the case of the hydraulic ash removal, the separation is carried out with the use of hydraulic structures of the ash dump.1 The vast majority of studies carried out so far in this field have been devoted to the determination of the granulometric, chemical, and phase compositions of fly ash cenosphere concentrates produced from the combustion of different types of coals1,5−8 as well as the investigation of the morphology and composition of individual globules of such concentrates by the methods of scanning electron microscopy and electron microprobe analysis.9,10 In particular, it has been shown that the diameter of globules in cenosphere concentrates varies over a wide range from 5 to 500 μm, with the particle size distribution maximum in the interval from 50 to 250 μm (for different sources).1,11 The cenosphere shell thickness in this case varies from 2 to 30 μm and reaches 5−10% of the globule diameter.1,3,12 The major chemical components of cenosphere concentrates are silicon, aluminum, and iron. Their content in cenospheres from different sources varies significantly. In general, the major component composition of cenosphere concentrates varies in the ranges of 50−65 wt % SiO2, 20−37 wt % Al2O3, and 1−11 wt % Fe2O3.1,3,5−8,11 © XXXX American Chemical Society
In accordance with quantitative X-ray powder diffraction analysis, cenosphere concentrates are multiphase systems consisting of a glass phase of essentially aluminosilicate composition (from 29 to 90 wt %) and crystalline phases of quartz and mullite.13 Additionally, cenosphere concentrates contain small amounts of cristobalite, magnetite, hematite, calcite, potassium feldspar, and hydromica.1,6,9,11,13 It has been found that cenosphere concentrates produced from different types of ashes contain globules of various morphological types: cenospheres with smooth or relief surfaces and shells with different degrees of porosity, thinwalled cenospheres with solid shells and smooth surfaces,9,11,14−16 cenospheres with porous shells,9,11,14,17 highly porous cenospheres with relief surfaces,11 and cenospheres with perforated shells.11 There are also globules that include smallersized particles and belong to the morphological-type “plerospheres”,12,15,18,19 globules with a network structure,7,18 and globules containing smallest mineral crystals enclosed in aluminosilicate glass shells, namely, dermaspheres.15,20 It has also been shown that the outer and inner surfaces of cenospheres are covered by a nanoscale film with a thickness of 30−50 nm.21 The stabilization of composition and, correspondingly, properties of cenospheres, owing to the differences in physical characteristics of individual globules (size, density, and magnetic properties), makes it possible to use them instead of expensive synthetic microspheres in the fabrication of new functional materials. In particular, narrow fractions of cenospheres have been used in the fabrication of microspherical Received: April 25, 2013 Revised: June 25, 2013
A
dx.doi.org/10.1021/ef400754c | Energy Fuels XXXX, XXX, XXX−XXX
Energy & Fuels
Article
Table 1. Physical Characteristics, Phase Composition, and Values of SiO2/Al2O3 Ratio for the Cenosphere Fractionsa physical characteristics cenosphere fractions
bulk density (g/cm3)
average diameter (μm)
phase composition (wt %)
apperent thickness (μm)
quartz quartz 1 (Al,Si)O2 calcite
ferrospinel
mullite
glass phase
SiO2/Al2O3
H−0.05 + 0.04 H−0.063 + 0.05 H−0.071 + 0.063 H−0.1 + 0.071 HMH−0.1 + 0.071 HMH−0.125 + 0.1 HMH−0.16 + 0.125
0.34 0.34 0.33 0.35 0.36 0.35 0.38
49 59 68 78 88 116 147
nonmagnetic cenosphere fractions 2.1 3.5 3.5 2.5 1.2 1.8 2.8 4.4 4.4 3.4 5.6 5.6 3.9 2.7 2.9 5.0 2.6 3.4 6.9 3.2 3.5
MH 9.33−0.18 + 0.08 MH 8.15−0.18 + 0.08 MH 5.08−0.18 + 0.08 MH 2.2−0.18 + 0.08 MH 1.18−0.18 + 0.08 MH 0.41−0.18 + 0.08 MH 9.33−0.5 + 0.25 MH 8.15−0.5 + 0.25 MH 5.08−0.5 + 0.25 MH 2.2−0.5 + 0.25 MH 1.18−0.5 + 0.25 MH 0.8−0.5 + 0.25 MH 0.48−0.5 + 0.25
0.39
142
magnetic cenosphere fractions 6.9 1.4 4.3
0.4
