Characterization of Fly Ash Cenospheres Produced from the Combustion of Ekibastuz Coal

Abstract

The relationship between the chemical composition and shell structure of cenospheres with a low bulk density of 0.40−0.45 g/cm 3 and a high Al 2 O 3 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 SiO 2 /Al 2 O 3 = 4.34 − 0.08[Al 2 O 3 ] 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 Al 2 O 3 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 Al 2 O 3 content from 43 wt % to 51 wt % and can be described by the regression equation SiO 2 /Al 2 O 3 = 2.71 − 0.04[Al 2 O 3 ] 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 Al 2 O 3 content from 26 wt % to 42 wt % and can be described by the general regression equation SiO 2 /Al 2 O 3 = 4.71− 0.09[Al 2 O 3 ] 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. ■ INTRODUCTIONIn 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 d...

Synthesis and structure of analcime and analcime-zirconia composite derived from coal fly ash cenospheres

“…In the case of the Zr involving synthesis, the zirconium salt added into the sodium alkaline solution is hydrolyzed resulting in [ZrO x (OH) 4À2x $yH 2 O] n [39] and decreasing the concentration of active OH À groups and, accordingly, NaAlO 2 . The change of NaAlO 2 content in the alkaline solution of the lower NaOH concentration can be due also to kinetic factors of dissolution of CFAC glass which is inhomogeneous in chemical composition and contains regions enriched in Si, Al or other elements [40]. XRD peaks of ZrO 2 were not observed for the analcime-zirconia composite.…”

The Effect of Fly Ash Microspheres on the Pore Structure of Concrete

“…The grain size of the microspheres generally ranges from 20 to 300 µm. Their chemical composition is similar to the composition of the fly ash [10,11]. The microspheres are different from other types of the combustion waste: low density (0.2-0.8 g/cm 3 ), high mechanical strength (210-350 kg/cm 2 ), low thermal conductivity (0.1-0.2 W/m•K), and high thermal stability (sintering usually occurs at 1000-1450 • C) [5,8].…”

“…In addition, they have been studied for the production of ceramic composite foams with different properties. The fly ash microspheres are currently used as lightweight filler, which improves the thermal insulation properties of mortars and concretes based on mineral binders [10,11,16,17].…”

Experimental investigation on foamed concrete produced using a planetary ball mill

“…These are small, hollow spheres (see Figure 1) with an average diameter of 30-350 m. The low density and high mechanical strength values (20-100 MPa), thermal insulation and acoustic properties, inertness against acids and alkalis promotes cenospheres application in different sectors (Krämer, Kowald, & Trettin, 2015;Fomenko & Anshits, 2015). To the FC mixes the polycarboxylate based superplastifying chemical admixture, produced by "Stachema", was added.…”