Article pubs.acs.org/EF
Characterization of Ferrospheres Recovered from High-Calcium Fly Ash Olga M. Sharonova, Natalia N. Anshits, Marina A. Fedorchak, Anatoly M. Zhizhaev, and Alexander G. Anshits*
Downloaded by TEXAS A&M INTL UNIV on August 31, 2015 | http://pubs.acs.org Publication Date (Web): August 4, 2015 | doi: 10.1021/acs.energyfuels.5b01618
Institute of Chemistry and Chemical Technology, Siberian Branch of the Russian Academy of Sciences, Akademgorodok 50/24, Krasnoyarsk 660036, Russia ABSTRACT: Eight fractions of ferrospheres in a range of sizes from 0.4 to 0.02 mm recovered from high-calcium fly ash have been studied. The major component composition of obtained fractions can be described by two linear regression equations, [CaO] = 54.50 − 0.54[FeO] and [SiO2] = 27.71 − 0.29[FeO] with the correlation coefficients of −0.96 and −0.88, respectively. On the basis of SEM-EDS study of the structure of 540 ferrospheres, it was found that the fraction −0.04 + 0.032 mm contains the individual globules with block-like, plate-like, and dendritic structures in concentrations of 60, 10−13, and 13−15%, respectively. The block-like globules containing 94−95 wt % FeO mainly consist of intergrown blocks (“single-block type”) of the ferrospinel which is subjected to partial martitization in regions with CaO content more than 0.9 wt %. The composition of the local sites of the plate-like globules containing 79−90 wt % FeO and 3.5−14.0 wt % CaO are characterized by the general dependence [CaO] = 87.4 − 0.93[FeO] with the correlation coefficient −0.96. These globules consist of the fragments of the “core−shell” type with the size ranging from 3 to 6 μm. The composition of the core with the block-like structure corresponds to a region of ferrospinel crystallization on the phase diagram FexOy−CaO system. The composition of the shell with a plate-like structure corresponds to a region of the crystallization of the Fe2O3, CaFe2O4, and CaFe4O7 phases. The composition of the local sites of the dendritic individual globules containing ∼90 wt % FeO and 4.8−5.5 wt % SiO2 are characterized by the general dependence [SiO2] = 61.3 − 0.63[FeO] with the correlation coefficient −0.94. It was shown that the structure of ferrospinel aggregates depends on the concentrations of Al2O3 and MgO.
■
(ferricalsialic) according to Vassilev classification.17 The formation of ferrospheres occurs in the carbon matrix,18 which provides a reducing medium, when iron exists in the form of FeO, that interacts with other mineral components and forms melts of low-temperature eutectics.19,20 In the composition range under investigation, an increase in the iron concentration leads to a 20 times decrease in the viscosity of the melt.21 This determines the change in the main morphological type of globules in the following sequence: porous (foam-like), glass-like, fine-grained, dendritic, skeletal−dendritic, and coarse-grained (blocklike).15,16 In the general dependence of the major component composition of narrow fractions, there are two regions that differ in the relationships of the major components Fe2O3, SiO2, Al2O3, CaO, compositions of iron-containing phases, and their microstructural characteristics. For the Fe2O3 content ranging from 30 to 80 wt %, the major component composition of ferrospheres corresponds to the two regression equations [SiO2] = 65.71 − 0.71[Fe2O3] and [Al2O3] = 24.92 − 0.26[Fe2O3] with the correlation coefficients −0.99 and −0.97, respectively. These ferrospheres are characterized by the formation of the aluminum magnesium ferrite spinel, the content of which increases monotonically with an increase in the iron concentration. The lattice parameter of the ferrospinel also increases in the range from 8.3440 to 8.3897 Å. On this basis, it was concluded that, in this composition range, the morphology of globules and the
INTRODUCTION About 25−30 million tons of coal ash and slag are produced annually in Russia; their use is less than 15%, and the rest comes to the ash ponds, increasing the negative impact on the environment.1 The use of concentrates of microspherical components and their narrow fractions with certain characteristics as the target products can significantly extend the scope of utilization of fly ash from industrial coal combustion.2,3 In particular the narrow fractions of clean ferrospheres separated from industrial coal combustion fly ashes sufficiently are often used as effective catalysts for deep oxidation4−7 and oxidative coupling of methane (OCM),7−9 thermolysis of heavy oil and petroleum residue,10,11 magnetic carriers for the isolation of recombinant proteins,12,13 and composite sorbents.14 The properties of microspherical functional materials in each particular case are determined by their composition, morphology of globules, crystallite size, and microstructure of active phases. In particular, it was demonstrated that narrow fractions of ferrospheres containing 87.5 wt % Fe2O3 and 2.0 wt % MnO are catalysts for the oxidative coupling of methane,8,9 whereas narrow fractions of ferrospheres with a lower iron concentration are effective catalysts for the CH4 deep oxidation.4−7 In order to elucidate causes of various catalytic actions of ferrospheres in the deep oxidation and OCM reactions, we studied the relationship of the composition, morphology of globules, the microstructure of iron-containing phases, and catalytic properties of narrow fractions of ferrospheres with a Fe2O3 content in the range from 30 to 92 wt %,7,15,16 which were separated from four types of fly ashes S (sialic), FS (ferrisialic), CS (calsialic), and FCS © 2015 American Chemical Society
Received: May 21, 2015 Revised: July 17, 2015 Published: July 20, 2015 5404
DOI: 10.1021/acs.energyfuels.5b01618 Energy Fuels 2015, 29, 5404−5414
Article
Energy & Fuels Table 1. Characteristics for Fractions of Ferrospheres
Downloaded by TEXAS A&M INTL UNIV on August 31, 2015 | http://pubs.acs.org Publication Date (Web): August 4, 2015 | doi: 10.1021/acs.energyfuels.5b01618
chemical composition (wt %) fractions (mm)
yield (wt %)
bulk density (g/cm3)
SiO2
Al2O3
FeO
CaO
MgO
SO3
Na2O
K2O
TiO2
Mb
−0.4 + 0.2 −0.2 + 0.16 −0.16 + 0.1 −0.1 + 0.063 −0.063 + 0.05 −0.05 + 0.04 −0.04 + 0.32 −0.032 + 0.02
23.1 17.2 31.4 21.0 0.8 2.3 3.2 1.0
1.87 1.93 2.33 2.44 2.57 2.55 2.65 2.64
4.30 2.68 1.37 1.42 1.48 1.78 1.53 1.33
2.04 1.30 0.90 1.00 1.12
