Fly ash, a byproduct from coal-fired thermal power plants, is a pozzolanic material recognized as a valuable resource that can be added to construction materials to form cementitious compounds when in contact with water. Cenospheres, hollow, spherically shaped particles that are mostly open-pore type in nature, are one of the most important value-added materials or subproducts that mix with fly ash. This is due to the distinctive properties of cenospheres, such as being lightweight, good flowability, chemically inertness, good insulation, high compressive strength, and low thermal conductivity, which enable them to be widely used in many industrial applications. Cenospheres have become a highly in-demand material as a filler or additive in many specialized applications, such as in lightweight cement, polymeric composites, automotive brake rotors, and differential covers, mullite-coated diesel engine components, and electromagnetic shielding and energy absorption applications. Applications for cenospheres as a construction material have been found, such as in lightweight thermal insulation composite, lightweight sound-absorbing structural material with cenosphere-reinforced cement and asphalt concrete, and lightweight concrete. It has been recently reported that cenospheres are used as an additive or filler in polymer concrete matrix for manufacturing composite beams and composite railway sleepers. With their spherical and hollow morphologies, cenospheres are particularly promising, with high resistance to crack propagation.
The density of cenospheres varies from 0.2 g/cc up to 2.6 g/cc. The availability of such low-density (<1 g/cc) cenospheres is quite low, as they constitute a small fraction of about 0.3%–1.5% by weight in fly ash. A higher density up to 2.9 g/cc could possibly be found for the cenosphere type containing heavy iron oxides in the silica matrix of the spheres. The particle size of cenospheres is much larger than fly ash particles and can range from 5 µm to 500 µm in diameter. The shell thickness varies from 2 µm to 30 µm. Shape, size, and density are the three major characteristics putting cenospheres in high demand. Cenospheres are of a similar chemical composition to fly ash, with their mineralogical composition depending on the geological features of the coal source and the reactions occurring as a result of combustion conditions that determine how the minerals fuse and form the solid hollow aluminosilicate spheres. The main minerals in fly ash are quartz, mullite, hematite, magnetite, and calcite, while the crystalline phases consist of gypsum, aluminum oxides, chlorite, feldspars, iron-bearing oxides, spinel (FeAl2O4), mullite. Cenospheres formed with a high content of mullite could be used to form cementitious compounds to enhance durability in concrete work.
The existing methods of cenosphere separation are classified into two groups: wet separation and dry separation. The wet separation mechanism is based on the differences between the density of solid particles and a liquid medium: with this process, cenospheres can be recovered from fly ash through a gravity settling separation-the sink-float method. The separation efficiency of wet separation is based on the natural buoyancy of cenospheres in the medium, the concentration of feeding particles, the surface topology, and porosity of particles, and refinement cycles. However, the efficiency is limited by the influence of the dense mass of fly ash, which prevents particles lighter than water from escaping the agglomeration, consequently hindering the rise of the lighter particles to the surface. The advantage of the wet separation method is the capability of obtaining low-density, intact cenospheres directly from the separation process: nonetheless, the availability of land and water is a major concern for large-scale production. Another problem is the issue of the dissolution of hazardous materials into water sources. A further drawback is a need for an additional drying step before further use. In terms of the material quality (particularly for class C fly ash with a high calcium content), crystals are formed on the particle surface and then become hardened in the drying stage, consequently limiting their use for further applications. Dry separation is an alternative method aiming to overcome the problems of wet separation. With this method, the chemical composition remains unchanged. In addition, there is no drying stage needed, thus avoiding energy consumption issues: nonetheless, this method requires advanced technology involving pneumatic separation, such as an air classifier, to efficiently classify the particles. Air classification is an operation that separates dispersed solid particles based on their differences in size, geometrical shape, and density in the air stream.
Post time: Feb-27-2023