REE induced defects in minerals: A Spectroscopic Study – PhD Thesis by Nicky Horsburgh

Nicky Horsburgh’s PhD thesis has recently been accepted by the University of St Andrews following her successful defence, so congratulations to Dr Horsburgh. The purpose of the work was to explore the luminescence of rare earth-bearing minerals as a tool for smart sorting. She was able to show that most rare-earth ore minerals can be identified by their luminescence. She focussed on light emitted during X-irradiation (called X-Ray Excited Optical Luminescence, XEOL, or RadioLuminescence, RL) since XRF and XRT are already tested methods for smart sorting. XEOL could form another channel alongside XRF or XRT data.

The abstract of her thesis is given below. The thesis itself is under embargo to allow her to publish her work but we are happy to hear from anyone interested in trying to take these ideas further into production. Supplementary data from the thesis will also be made available at the end of the embargo period.

Contour equivalent of the same data as left. The unusual discontinuities that cross the graph left to right are Thermoluminescence emissions.

Nicky Horsburgh at her poster on the Smart Sorting of Rare Earth-bearing Ore

Dr Horsburgh has now taken up a position as lecturer in the School of Earth & Environmental Sciences at the University of St Andrews.

Abstract

This thesis examines the luminescence and mineral physics of Rare Earth Element (REE) bearing minerals as a precursor to developing smart sorting tools for critical metals used in low-carbon technologies. I characterise luminescence responses of complex zirconosilicates; eudialyte (Na₁₅Ca₆(Fe²⁺,Mn²⁺)₃Zr₃[Si₂₅O₇₃](O,OH,H₂O)₃(OH,Cl)₂), wöhlerite (NaCa₂(Zr,Nb)(Si₂O₇)(O,OH,F)₂) and catapleiite (Na₂Zr(Si₃O₉) · 2H₂O). Fluorite was included as it is commonly associated with REE ores and displays strong REE luminescence. Its behaviour provides key insights into REE substitution into ionic minerals.

X-ray Excited Optical Luminescence (XEOL) and Thermoluminescence (TL) measurements were taken  from 20 to 673 K. Fluorite responses result from a balance of intrinsic luminescence and REE substituents and evidence for REE and defect coupling. Thermoluminescence indicates the presence of electron traps and the coupling of these traps to lanthanide emissions show that the defect and the lanthanide are clustered in physical space. The absence of changes in TL for different lanthanides shows that energy is passed efficiently between rare earths, indicating that the REE are clustered.

The zirconosilicates all show increased intensity in XEOL response below 150 K. Cryogenic emissions are interpreted as originating from the host mineral. There are 3 shared features: UV (~280 nm) paramagnetic oxygen or oxygen vacancy; blue (440 nm) Al-O^–-O /Ti centres; and REE. Wöhlerite and eudialyte show Fe³⁺ band (~708 nm) and wöhlerite displays broad emission attributed to Mn²⁺. Eudialyte shows two additional responses; UV (~320 nm) tentatively assigned to Na migration and UV/blue (~400 nm) potentially associated with charge balances associated with the coupled substitution of Al³⁺. Eudialyte shows little emission at room temperature, this is attributed to quenching from Fe²⁺. Emission from eudialyte above room temperature is attributed to alteration minerals such as catapleiite and potentially to inclusions of luminescent primary mineral phases.

I demonstrate that smart sorting could be a valuable beneficiation tool for REE minerals.

The work has attracted coverage in the journal Ancient TL and in the newsletter of the Global Rare Earth Industries Association.  The project also benefited from samples loaned from the National Museum of Scotland, the Hunterian Museum in Glasgow and the Mineralogical and Petrological Museum of Norway.

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