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Cenospheres Recovery from Fly Ash
Coal fly ash (CFA) is a major global pollutant produced by thermal power plants during the generation of electricity. A significant amount of coal fly ash is dumped every year in the near vicinity of the thermal power plants, resulting in the spoilage of agricultural land. CFA has numerous value-added structural elements, such as cenospheres, plerospheres, ferrospheres, and carbon particles. Cenospheres are spherical-shaped solid-filled particles, formed during the combustion of coal in thermal power plants. They are lightweight, have high mechanical strength, and are rich in Al-Si particles. Due to cenospheres’ low weight and high mechanical strength, they are widely used as ceramic/nanoceramics material, fireproofing material, and in nanocomposites.
Coal fly ash (CFA) is one of the major pollutants of the 20th and 21st centuries. It has drawn global attention regarding its safe management, optimized generation, and utilization . Globally, a million tons of CFA are generated per year, in thermal power plants (TPPs), from the burning of pulverized coal, during the production of electricity . The huge amount of generation of CFA leads to air, water, and soil pollution. Every year, a huge amount of CFA is dumped in the near vicinity of the TPPs, in fly ash ponds, which ultimately leads to water and land pollution. These fly ashes contain heavy metals, which can leach out once they contact water . Besides heavy metals, CFA also contains minerals, such as magnetite, calcite, mullite , cristobalite, and silica . These minerals contribute to the silica (40–60%), alumina (20–40%), and ferrous (5–15%) that rely on the source and types of coal used .
2. Morphological Properties of CFA Extracted Cenospheres
Based on the morphology, viscosity of liquid composition of slag, and elemental composition, some investigators have classified the CS into two broad types—namely, nonmagnetic CS and magnetic CS. The formation of magnetic CS depend upon the melts of A-Si and Fe-Al-Si . High viscosity type cenospheric particles have more tendency to trap gases and increase in diameter of the particle, resulting in the thinner shell with the complete absence of pores .
Certain cenospheres have high Fe deposition on their surface that exhibit magnetic properties, while others are mainly rich in Al and Si (along with Na and Ca). In numerous pieces of literature, CS with small Fe deposition have been widely in the petroleum cracking, catalysis, ceramic, nanocomposites, and steel or iron-based industries .
Based on some physical parameters, i.e., texture, shell thickness, and shape, cenospheric particles are either transparent, grey, and dark type . The former type of particle is thin-shelled, smooth-surfaced, grey type are translucent, porous, thick, and rough-surfaced . While the dark-type CS have numerous perforations on their surface, their porosity is much greater than the grey CS. Dark CS scatters the light completely, which is passed through them and acts as an opaque particle. Ferrous-rich CS can be divided into two types: One having a porous shell, while the other with a spotty nonuniform surface, containing grey, black, brown, and white inclusions, along with dark black or brown spheres with a bright and smooth surface . The Al content contributes a valuable role in the average size of a nonmagnetic CS, i.e., more Al equals higher mullite, and ultimately results in a decrease in the average diameter .
CS can be divided into two main groups, based on their magnetic features, which are acquired by the presence of Fe2O3/Fe3O4 minerals in the shell . The shell of CS is a complex, layered structure (which is covered by a nano-size film externally), and internally  (which is 30–50 nm thick and contains Fe2O3). When the ferrous oxide percentage varies from 3 and 4 wt.% in the alumino-silicate glass, then two types of Fe3+ are present, i.e., single ions and nanoparticles with a diameter of 3 to 5 nm. These particles consist of a superparamagnetic phase with a spinel structure whose sublattices are diamagnetically diluted with Mg2+ and Al3+ ions. CS, having about 7 wt.% of Fe2O3 content, also involves a magnetic phase, based on defective magnetite .
Magnetic CS have heterogeneous regions of ferrospinels on their outer surface. An increase in the concentration of iron increases the crystallite size of the ferrospinel phase in the magnetic CS, and decreases the degree of iron substitution (Mg and Al) .
In general, CS size falls in the range of 20–300 microns, with a shell thickness of 1–18 microns, but the average size is 5–500 microns . In comparison to the CFA particles, CS are much larger, and can vary from 5 to 500 µm .
3. Properties of Cenospheres Recovered from CFA
4. Methods Used to Recovery Cenospheres from CFA
The recovery of CS from CFA is possible by both dry- and wet-based methods . As the name suggests, the wet method involves a liquid media like water and or any organic solvent, whereas the dry-based method utilizes either air stream or size-based screening. Among all the techniques for recovery of CS from CFA most common methods are magnetic separation, sedimentation, flotation, or sink float method . All these recovery methods are carried out in a separation tank which contains water (1 g/cc) and acetone (0.789 g/cc), which is fitted with agitation or stirring, and consequently, heavier particles settle down, while lighter particles float at the top. The dry-based method mainly uses air classifiers to separate CS particles .
5. Applications of Cenospheres in Ceramics and Environmental Cleanup
This entry is adapted from 10.3390/cryst11091067
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