Composition–Structure Relationship and Routes of Formation of Blocklike Ferrospheres Produced by Pulverized Combustion of Two Coal Types

Abstract

The composition−structure relationship of blocklike ferrospheres (FSs) isolated from fly ash produced during the combustion of two different types of coal was studied systematically by scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy. Monoblock globules were shown to consist of large sintered crystallites of Mg, Mn ferrospinel, which are formed from excluded siderite particles containing isomorphic impurities of magnesium and manganese carbonates. The common groups of globules for which the gross composition of polished sections corresponds to the general equations for the relationship of the concentrations SiO 2 = f(Al 2 O 3 ) and CaO = f(SiO 2 ) were highlighted from FSs of two series. These globules are formed during the thermochemical transformation of associates of siderite, quartz, calcite, and anorthite, which have a silicate modulus of SiO 2 /Al 2 O 3 equal to 1.18, which corresponds to the coefficients in the general equations of the relationship SiO 2 = f(Al 2 O 3 ). SEM analysis of polished cross-sections of the globules of selected FS groups demonstrates that the crystallite size of ferrospinel decreases, while the content of the glass phase increases with the declining FeO concentration in individual globules. The crystallite size and shape are found to depend on the size of the local melt area where the concentration of spinel-forming oxides is >85 wt %. The observed increase in the glass-phase content is attributed to the broadening of the liquation zone in the FeO−Fe 2 O 3 −SiO 2 system as the oxidative potential increases and to the higher content of [Fe 3+ O 2 ] − and [Fe 3+2 O 5 ] 4− ferrite complexes in calcium-rich melts.

Remediation of Azure A Dye from Aqueous Solution by Using Surface-Modified Coal Fly Ash Extracted Ferrospheres by Mineral Acids and Toxicity Assessment

“…The particles in Figure 2(b) are smooth in texture while all others have a rough texture. The rough surface FS [40] have either angular-shaped particles or spherule depositions [41]. These ferrous-rich concentrates on the surface of FS provide magnetic properties [42].…”

Emerging trends in the recovery of ferrospheres and plerospheres from coal fly ash waste and their emerging applications in environmental cleanup

“…Several investigators have used CFA ferrospheres as an adsorbent for the removal of dyes and other inorganic pollutants from wastewater. Fomenko (1998) demonstrated the application of FS extracted from TPPs as a catalyst for the high-temperature oxidative conversion of CH 4 (Anshits et al, 2021).…”

“…In the last decade, ferrospheres have been used widely in ceramics, research, and other industries in comparison to plerospheres due to their easy recovery, wider applications in catalysis, etc. Contrary to ferrospheres, very limited information is available on plerospheres (Anshits et al, 2021). These structurally varied spherical particles have different structural and elemental properties and have started gaining importance in various applications (Goodarzi and Sanei, 2009).…”

Physicochemical Properties of Fe-Bearing Phases from Commercial Colombian Coal Ash

“…Sokol et al (2002) attributed skeletal and dentritic microtextures to the crystallization under drastic supercooling and related the high catalytic activity of these morphotypes to the high-index facets formed during the process, which are absent in slow-forming natural crystals. However, recent systematic research conducted in narrow fractions of magnetic concentrates correlating major elements and specific microtextures through SEM/EDS analysis suggests that specific minerals may act as precursors of the studied microtextures; e.g., illite is the precursor of skeletal-dendritic microtextures [42,43,46].…”

“…The Fe-bearing morphotypes can be easily recovered from coal ashes as magnetic concentrates (MC), usually yielding between 0.5 to 18.1 wt.%, with a Fe content ranging from approximately 20 to 88% [23,24,27,29,40,41]. Narrower fractions of the MC with stable composition and reproducible magnetic properties can be obtained via beneficiation flow processes, which may include size-classification, hydrodynamics, and density separation [9,35,36,[42][43][44][45][46][47]. Ferrospheres are expected to occur in low amounts in ashes derived from commercial coal since pyrite (main Fe-bearing mineral in coals) contents are reduced as pre-combustion S-emissions control measure.…”