Separation of coal combustion residue for critical element extraction and other bulk uses

Abstract

The demand for critical and rare earth elements is surging and coal combustion residue could be an alternate source of critical elements. Data on the concentration of critical and rare earth elements (REYs) in different size fractions of fly ash would help in segregation of the ash. This study was conducted with the objective of examining the possibility of separation of coal ash into a size fraction useful for element extraction and the rest for bulk uses like cement, concrete, landfill, roads, embankments, etc. The concentration of critical elements, their partitioning in different size ash particles (>500 to <25 μm), and their chemical association were determined for a coal fly ash sample from Talcher, India. The total REY concentration in the ash varied between 440 and 529 mg kg⁻¹, wherein the contents were relatively higher for Nd (75–103 mg kg⁻¹) followed by Ce (58.3–88.7 mg kg⁻¹), La (41.6–80.3 mg kg⁻¹), Sm (39.0–79.3 mg kg⁻¹), and Y (38.4–49.3 mg kg⁻¹). The REY outlook coefficient of the raw ash (1.03) is more than 0.7 and accordingly this fly ash can be considered as an interesting source of rare earth elements. This factor was further enhanced to 2.3 in the coarse ash particles of size > 250 μm. Sequential extraction showed that most of the rare and critical elements are associated with the alumino-silicate matrix. The Al₂O₃ content of this ash is relatively high (25%), so there is scope for co-extraction of Al along with the rare earth elements. The ash disposal and utilization policy should consider the separation and preservation of the coarse ash fraction (>250 μm) for the extraction of critical and rare earth elements.