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Alkaline Desilication–Induced Matrix Densification and Rare Earth Element Retention in Coal Fly Ash

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Author Affiliations

  • 1NTPC Energy Technology Research Alliance (NETRA), Plot No. E-3, Ecotech-II, Udyog Vihar, Greater Noida -201306, Uttar Pradesh, India
  • 2NTPC Energy Technology Research Alliance (NETRA), Plot No. E-3, Ecotech-II, Udyog Vihar, Greater Noida -201306, Uttar Pradesh, India
  • 3NTPC Energy Technology Research Alliance (NETRA), Plot No. E-3, Ecotech-II, Udyog Vihar, Greater Noida -201306, Uttar Pradesh, India
  • 4NTPC Energy Technology Research Alliance (NETRA), Plot No. E-3, Ecotech-II, Udyog Vihar, Greater Noida -201306, Uttar Pradesh, India
  • 5NTPC Energy Technology Research Alliance (NETRA), Plot No. E-3, Ecotech-II, Udyog Vihar, Greater Noida -201306, Uttar Pradesh, India
  • 6NTPC Energy Technology Research Alliance (NETRA), Plot No. E-3, Ecotech-II, Udyog Vihar, Greater Noida -201306, Uttar Pradesh, India
  • 7NTPC Energy Technology Research Alliance (NETRA), Plot No. E-3, Ecotech-II, Udyog Vihar, Greater Noida -201306, Uttar Pradesh, India
  • 8NTPC Energy Technology Research Alliance (NETRA), Plot No. E-3, Ecotech-II, Udyog Vihar, Greater Noida -201306, Uttar Pradesh, India

Res. J. Recent Sci., Volume 15, Issue (3), Pages 8-15, July,2 (2026)

Abstract

Coal fly ash (CFA), an aluminosilicate-rich by-product obtained after coal combustion, is a potential secondary resource for rare earth elements (REEs). This study evaluates a matrix-engineering approach based on alkaline desilication–induced densification to restructure the host framework while retaining REEs in the solid phase. Magnetic separation followed by controlled treatment with 4 M NaOH (solid-to-liquid ratio 1:5) was applied to CFA from an Indian thermal power plant. Major oxides were analyzed by ED-XRF and 18 REEs were quantified using ICP–MS. Magnetic separation removed only 3.68% of the total mass, indicating limited association of REEs with strongly magnetic iron phases. Alkaline treatment achieved >92% silica removal, reducing SiO₂ from 58.55 wt.% to 4.41 wt.% and enriching Al₂O₃ to 85.00 wt.% in the bottom-settled fraction. The slurry stratified into density-driven layers, with the alumina-rich residue representing a densified structural phase. Despite extensive desilication, REEs remained predominantly in the solid phase. The abundance pattern (Ce > La > Nd > Y > Sc > Dy) was preserved across treated fractions, and critical elements such as Nd and Dy were retained in the densified residue. Observed decreases in ppm values reflect matrix mass redistribution rather than significant alkaline solubilization. The process is best described as alkaline desilication–induced matrix densification with stratified REE retention. This acid-minimizing strategy upgrades CFA into an alumina-enriched precursor suitable for downstream targeted REE recovery, supporting sustainable resource utilization.

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