Fast 2D Laser-Induced Fluorescence Spectroscopy Mapping of Rare Earth Elements in Rock Samples

Abstract

Due to the rapidly increasing use of energy-efficient technologies, the need for complex materials containing rare earth elements (REEs) is steadily growing. The high demand for REEs requires the exploration of new mineral deposits of these valuable elements, as recovery by recycling is still very low. Easy-to-deploy sensor technologies featuring high sensitivity to REEs are required to overcome limitations by traditional techniques, such as X-ray fluorescence. We demonstrate the ability of laser-induced fluorescence (LIF) to detect REEs rapidly in relevant geological samples. We introduce two-dimensional LIF mapping to scan rock samples from two Namibian REE deposits and cross-validate the obtained results by employing mineral liberation analysis (MLA) and hyperspectral imaging (HSI). Technique-specific parameters, such as acquisition speed, spatial resolution, and detection limits, are discussed and compared to established analysis methods. We also focus on the attribution of REE occurrences to mineralogical features, which may be helpful for the further geological interpretation of a deposit. This study sets the basis for the development of a combined mapping sensor for HSI and 2D LIF measurements, which could be used for drill-core logging in REE exploration, as well as in recovery plants.

A spectral library for laser-induced fluorescence analysis as a tool for rare earth element identification

“…The two emission bands represent 4 F 3/2 -4 I 11/2 and 4 F 3/2 -4 I 9/2 transitions, respectively (e.g. Reisfeld et al, 1996;Gaft et al, 2005;Czaja et al, 2012;Shalapska et al, 2014). Emission bands at shorter wavelengths (< 850 nm; Gaft et al, 2005) were not observed during measurements and hence are not recorded in our phosphate reference spectra.…”

“…In general, it overlaps with the characteristic Ce 3+ emission band around 360 nm that is associated with a 5d-4f transition (e.g. Reisfeld et al, 1996;Gaft et al, 2005;Shalapska et al, 2014). Several studies report broadened Ce-related peaks and discuss reasons behind them; for example, Shalapska et al (2014) identified a peak doublet with the peak distance dependent on the site symmetry in the crystal lattice, and Chen et al (2014) observed broadened emission in the range between 310 and 410 nm resulting from mixed phases of a monoclinic and hexagonal lattice.…”

“…Reisfeld et al, 1996;Gaft et al, 2005;Shalapska et al, 2014). Several studies report broadened Ce-related peaks and discuss reasons behind them; for example, Shalapska et al (2014) identified a peak doublet with the peak distance dependent on the site symmetry in the crystal lattice, and Chen et al (2014) observed broadened emission in the range between 310 and 410 nm resulting from mixed phases of a monoclinic and hexagonal lattice. The Ce-related emission band appears less broad and shifts towards lower wavelengths in the two non-luminescent samples of LuPO 4 and YPO 4 (grey lines in Fig.…”

“…However, the mapping application cases focused on general measures of emission intensities or their ratios rather than on specific features in LiF spectra. Other studies emphasise the high potential of LiF for REE detection in natural rocks (Reisfeld et al, 1996;Gaft et al, 2005;Lorenz et al, 2019;Seidel et al, 2019). Despite the promising studies, the method is not yet deployed in the raw material sector.…”

Upconversion Fluorescence in Naturally Occurring Calcium Fluoride

“…Fluorescence spectroscopy is a technique wherein incoming photons excite optical centers within a material, which then emit a specific spectrum as the centers return to the ground state. [17][18] In solids, optical centers can be ion inclusions in the host, or defects in the solid matrix. Optical centers are not ionized, unlike for XRF, and so electrons in excited states are still bound to the host mineral.…”