The Recycling of Coal Fly Ash: A Review on Sustainable Developments and Economic Considerations

Abstract

The recycling and utilization opportunities for coal fly ash (CFA) have increased in the past two decades. However, limited commercialization of the material is still reported, while disposal and management remain major concerns. CFA utilization is currently commercially feasible in the building and construction industry. Other alternative uses that are being explored involve the extraction of valuable metals and the purification of wastewater. The CFA-produced adsorbent material utilized in wastewater purification processes should be able to generate water that meets the legal quality requirements for reutilization in alternative applications. On the other hand, in the recovery of metallic components such as smelter-grade alumina, high recovery and high purity products are only achievable through the processing of CFA using expensive and energy—intensive processes. Furthermore, most of the current CFA recycling processes tend to generate secondary solid residues (SSR), which can cause environmental pollution, thus requiring further downstream processing. In this context, this paper reviews and discusses current research on CFA recycling methods, challenges and opportunities associated with resource recovery from CFA, and the acceptability of the value-added products, and it therefore proposes sustainable processes for CFA utilization. This review further suggests that to successfully compete with bauxite for production of smelter-grade alumina, other saleable value-added products such as Ti, Fe and the REEs should be recovered by engineering an integrated process design. The generated SSR in each process must also be characterized, recycled and re-used to reduce waste production and advance the circular economy concept. The review concludes that for CFA to become considered as a more attractive commercial resource, there is need for its complete and holistic utilization in high volumes and in different applications to offset its low value.

Improving Physical and Chemical Properties of Saline Soils with Fly Ash Saline and Alkaline Amendment Materials

“…The application of fly ash as a building and high-value-added recycling material has been previously reported [30][31][32][33]. In addition, several research works demonstrate the use of fly ash in agriculture as fertilizer to promote crop growth, acidic soils, and to remediate heavy metal pollution [34,35].…”

Influence of additive contents on the properties of SiC ceramic membranes and their performance in oil‐water separation

“…24 Research on the utilization of fly ash has opened various applications areas, mainly in the production of cement and concrete, backfilling, advanced value-added products such as synthetic zeolite, glass, glass-ceramics, porous ceramics, and composite catalysts, etc. [24][25][26][27] The application of SiC membranes has received recent attention from the scientific community for water cleaning and liquid separation processes. Continuous efforts are going on for reduction of fabrication costs for use of SiC membrane at an affordable cost for air and water filtration applications.…”

“…Coal fly ash is considered as the world's fifth‐largest waste by‐product derived from thermal power plants and causes serious problems of storage and environmental pollution 24 . Research on the utilization of fly ash has opened various applications areas, mainly in the production of cement and concrete, backfilling, advanced value‐added products such as synthetic zeolite, glass, glass–ceramics, porous ceramics, and composite catalysts, etc 24–27 …”

The state of Fe3+ in the C–F–A–S–H system with varying Fe/Si and Ca/Si ratios

“…This provides a novel prospect for the high-valued utilization of massive Fe-rich solid wastes. These wastes also have a high Fe content (typically around 34 wt% for red mud, 13 30 wt% for iron slag, 12 and 25 wt% for coal fly ash 14 ), which usually exist in the forms of hematite, goethite, and magnetite. Substantial amounts of Fe shape geopolymerization in a manner that remains elusive.…”