Combining Absorption and Emission Spectroscopy for the Detection and Characterization of Rare Earth Elements


Combining Absorption and Emission Spectroscopy for the Detection and Characterization of Rare Earth Elements

Jakob, S.; Fuchs, M.; Gloaguen, R.

In the last decade, the fast development of technology and high-tech industry distinctly increased the demand of Rare Earth Elements (REEs). The combination with the globally strongly concentrated distribution of production sites classifies REEs as critical raw materials and raises the need for the exploration of complex deposits with lower concentrations or remote locations. Spectroscopic methods are the key for an advanced, fast and non-invasive approach to reduce the economic and ecologic costs of REE characterization, not only within exploration, but along the whole raw material value chain. 
Currently, the research of spectral detection and characterization of REEs is concentrated on absorption spectroscopy. Although a considerable amount of REEs can be detected by their characteristic reflectance spectrum, the commonly low intensity of characteristic absorption limits its applicability for robust detection and characterization to a few REEs, such as Neodymium and Samarium.
In the past decades, studies were conducted to characterize REE crystals by their fluorescence properties. In contrast to absorption spectroscopy, an emission is induced in the sample using a laser with a defined excitation wavelength to maximize the response that depends on the investigated material. With emission spectroscopy, a broad set of REEs can be characterized, but still, the attribution of emission features is challenging, as it depends on crystal structure and experimental parameters.
Hereby, we propose a new approach for combining absorption and emission spectroscopy to characterize REEs and overcome the limitations of the single method. For that, we first investigated single REE crystal standards with different bindings using absorption as well as emission spectroscopy. The results can be used to create a library or decision routine for the detection of REE using combined absorption and emission spectroscopy. We will then test the proposed method on natural REE bearing samples, which are additionally characterized chemically and mineralogically to provide a proper validation. Hereby, the influence of the mineral matrix, natural crystal structure and mixed REE composition can be estimated and overcome by using lasers with different stimulation wavelengths in the UV and visible range of the spectrum. Absorption spectroscopy is conducted by point measurements with a reflectance spectrometer as well as with hyperspectral cameras. We developed the tools for processing and the analysis of the spectral data to ensure a fast and robust interpretation of the spectral features.
First results show the detectability of Dy, Er, Ho, Nd, Pr, Sm, and Tm with absorption and the detection of Er, Eu, Ho, Nd, Pr, Tb, and Yb with emission spectroscopy. Additionally, for REEs having spectral signatures in both cases, absorption features overprint the emission spectra within the broad fluorescence signal of the mineral matrix. They coincide with known and measured absorption features of the specific REEs. This, the integration of both features remarkably increases the detectability and the robustness of detection for those elements.

  • Lecture (Conference)
    10th EARSeL SIG Imaging Spectroscopy Workshop, 19.-21.04.2017, Zürich, Switzerland

Permalink: https://www.hzdr.de/publications/Publ-25955
Publ.-Id: 25955