Strategic routes in valorising pre-processed coal fly ash to promote circular economy

Abstract

Coal is a well-known for power generation, particularly in developing countries, due to its favourable economic benefits. Its combustion generates large quantities of waste by-products including coal fly ash (CFA), bottom ash, and noxious gases. These waste components challenge global movements towards sustainability and exploring ways to manage these by-products is of marked importance. Single-component separation of CFA is often reported in the literature, leaving out means for comprehensive utility (i.e., multi-component extraction), which challenges the circular economy aspects of CFA utilisation. To this end, we identify the requirement for multi-stage processing of CFA and herein, a thorough account of the opportunities associated with pre-processing of CFA is presented. As a result, we recognise washing cycles as a primary pre-processing strategy that paves the way for various downstream value-added products, namely cenospheres, fertiliser precursors, critical elements and zeolite precursors. Furthermore, to validate our premise, we propose a process flow to recover these valuable components through a preliminary experimental approach. These experiments concentrated on synthesising zeolites for wastewater treatment and the remaining solid (top and bottom) and liquid phases showed promise for extracting valuable components. Notably, the solution was enriched in fertiliser ions and the floating solid fraction was populated with cenospheres. While observing potential pathways for critical elements recovery and recycling the spent zeolites, our approach conforms to the sustainability and circular economy of CFA with cleaner chemical engineering process strategies with further studies being recommended to appraise its scalability and efficiency improvements.