An advanced analytical assessment of rare earth element concentration, distribution, speciation, crystallography and solid-state chemistry in fly ash

Ilemona C Okeme*, Peter George Martin, Christopher P Jones, Richard A. Crane, Theophilus Ile Ojonimi, Konstantin Ignatyev, Thomas Bligh Scott

*Corresponding author for this work

Research output: Contribution to journalArticle (Academic Journal)peer-review

Abstract

Fly ash represents a promising alternative source of rare earth elements (REE). However, information on REE containing mineral phases and their association with other fly ash components, vital for REE recovery from fly ash, is currently lacking. Herein, the mass fraction, distribution, crystallography and solid-state chemistry of REE, U and Th in Nigerian simulated fly ash samples were characterised using a range of laboratory and synchrotron x-ray based analytical techniques to underpin future extraction methodologies. Inductively coupled plasma mass spectrometry following full-acid digest of forty-five samples revealed recoverable average total REE content which ranged between 442 mgkg-1and 625 mgkg-1, comprising over 30wt% of the critical REE Nd, Eu, Tb, Dy, Y and Er. These REE within the fly ash samples were found to be most frequently associated with discrete monazite, xenotime and Y-bearing zircon mineral particles, with the former the most detected, which could be beneficiated through gravity separation. Analysis of monazite particles isolated from the composite samples through a complimentary suite of analytical synchrotron radiation techniques revealed a core-shell pattern, with the shell rich in colocalised Ce, Nd and La, and the core enrich in both U and Th. Ce in monazite was found to exist in a mixed trivalent and tetravalent oxidation state, with the monazite structure amorphized due to the high temperature combustion process. Such results demonstrate the strong co-association and physical distribution of REE, U and Th within monazite in fly ash; knowledge of which can subsequently be used to optimise or develop a more selective, cost-effective and environmentally friendly solvent extraction methodology, by targeting the strongly colocalised and surface bound REE in fly ash monazite particles.
Original languageEnglish
Article number105950
Number of pages11
JournalSpectrochimica Acta Part B: Atomic Spectroscopy
Volume177
Early online date12 Aug 2020
DOIs
Publication statusPublished - 1 Mar 2021

Bibliographical note

Funding Information:
The authors wish to thank Diamond Light Source for rapid access to beamline I18 (SP22876) that contributed to the results presented here. We would like to thank Sharon Uren (Senior Analytical Technician, Camborne School of Mines, University of Exeter) for performing the total acid digestion followed by ICP-MS analysis. We also wish to thank the Commonwealth Scholarship Commission for funding the PhD studentship. The SEM used in this work to conduct imaging and EDS analysis was purchased following funding by the Engineering and Physical Sciences Research Council (EPSRC), (Reference: EP/K040340/1).

Funding Information:
The authors wish to thank Diamond Light Source for rapid access to beamline I18 (SP22876) that contributed to the results presented here. We would like to thank Sharon Uren (Senior Analytical Technician, Camborne School of Mines, University of Exeter) for performing the total acid digestion followed by ICP-MS analysis. We also wish to thank the Commonwealth Scholarship Commission for funding the PhD studentship.

Funding Information:
The SEM used in this work to conduct imaging and EDS analysis was purchased following funding by the Engineering and Physical Sciences Research Council (EPSRC) , (Reference: EP/K040340/1 ).

Publisher Copyright:
© 2020 Elsevier B.V.

Keywords

  • Nigeria
  • fly ash
  • rare earth elements
  • monazite
  • synchrotron radiation

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