The dominance of China in the rare earths industry has forced the USA, Russia, European Union, Japan, Australia, and other counties to formulate policies and implement strategies to ensure security and sustainability of rare earth elements (REE) to reduce dependency on Chinese supplies. In 2020 the European Union’s report on critical raw materials resilience noted that light rare-earth elements (LREE) and heavy rare-earth elements (HREE) had the highest supply risk of all economically important raw material, Figure 1 (European Commission, 2020).
Figure 1. European Commission (2020) Economic importance and supply risk results of 2020 criticality assessment.
What are Rare Earth Elements?
Rare earth elements are a group of 16 elements in the periodic table, namely the lanthanides (atomic number 57 to 71) plus yttrium (Figure 2). These elements are divided into LREE (La, Ce, Pr, Nd, Pm, Sm) and HREE (Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Y) based on a comparison of their atomic radius and charge. Promethium (Pm) effectively does not occur in nature.
Figure 2. Classification of rare earth elements (REE): light rare earth elements (LREE) in blue and heavy rare earth elements (HREE) in orange (after Morin-Ka, 2018).
Commonly used REE abbreviations:
REE = Rare Earth Elements; RE = Rare Earth; REM = Rare Earth Metals; REO = Rare Earth Oxides; REY = Rare Earth elements and Yttrium; LREE = Light Rare Earth Elements (Sc, La, Ce, Pr, Nd); MREE = Medium Rare Earth Elements (Sm, Eu, Gd, Y, Tb, Dy); HREE = Heavy Rare Earth Elements (Ho, Er, Tm, Yb, Lu); TREO = Total Rare Earth Oxide; TREM = Total Rare Earth Metal.
Why are REE in demand?
A significant shift from carbon based energy sources towards renewable clean energy is driving the increased demand of REE. These rare-earth metals are fundamental for the development of clean energy such as REE magnets for wind turbines, and rechargeable batteries for hybrid cars. They also have extensive and established applications in electronics, telecommunications, chemical catalysis, metallurgy, and medical imaging.
As a result of their broad use, the demand for REE has increased considerably over the past few decades (Figure 3). The broad and expanding demand for these critical raw materials has seen an increase in activity in the REE discovery and development space over the last 12 months.
Figure 3. Global rare earth production and demand and (inset) the distribution of global rare earth production and consumption in 2015 (adapted from Zhou et al 2017).
REE Deposit Types
Deposits of REE occur in diverse, generally uncommon geologic settings. There are two styles of REE deposits; magmatic and sedimentary. Their distributions is shown in Figure 4 with the majority of production, especially HREE, dominated by ion adsorption clays from China.
1) Magmatic rare earth deposits
2) Sedimentary rare earth deposits
Figure 4. World map showing locations of active or recently active rare-earth-element (REE) mines and ongoing advanced exploration projects (from Van Gosen et al 2017).
Portable Spectral Solutions and Applications for Rare Earth Elements
Portable Spectral Solutions are non-destructive, rapid, transportable, and economical.
Visible Near Infrared (VNIR) and Short Wave Infrared (SWIR)
Studies have demonstrated that REE have specific absorption features in the VNIR spectral range (Turner et al., 2016). These features are identifiable by SWIR technologies along with the host mineral phases and common gangue minerals (e.g., clays).
Fourier-transform infrared (FTIR)
FTIR is an analytical technique that uses either absorbed or transmitted spectra in the Infra-Red region of a substance to determine its molecular conformation, also known as molecular “fingerprint” of a substance. Li et al (2020) demonstrated that halloysite could be differentiated from kaolinite in FTIR spectra and the proportion of kaolinite progressively increases in the upper saprolite and lower pedolith, with the kaolinite-group minerals becoming more crystalline.
Potable X-Ray Fluorescence (pXRF)
Portable-XRF instrumentation has offered a lot of promise in the past, however typical instruments are inadequate, reporting REE with low accuracies and an incomplete suite of elements. Portable Spectral Services (PSS) known for its innovation (e.g., Brand and Brand, 2017) have developed algorithms to accurately provide the full suite of REE close to or below crystal abundance (Table 1); the Rare Earth Element Index (RE2I).
Table 1. Rare Earth Elements, spectral lines and detection limits for the Portable Spectral Services Rare Earth Element Index also known as RE2I
REE and the Application of pXRF
Since the introduction of the silicon drift detector (SDD) there have been significant advancements to the pXRF tube, detector, geometry, and electronics, as well as data analysis software that have improved the elemental range, detection limits and speed of measurement for pXRF instruments. Additionally, these advancements have significantly expanded the breadth of calibrations and data analysis techniques available to enhance user experience in the field.
NEW S1 TITAN graphene window family
The new Bruker S1 TITAN portable XRF family includes the S1 TITAN 800 model with the latest graphene window SDD and a 50 kV X-ray tube with an active area of 20mm2. This new model continues to demonstrate quality and longevity thanks to the Bruker patented TITAN Detector Shield, which keeps the detector safe and protected from costly punctures. Additional features include the ability to connect via bluetooth, WiFi and USB. The S1 TITAN along with Bruker CounterTop XRF (CTX) and TRACER 5g allow fast real-time measurements, ideal for all analytical challenges including REE exploration and delineation.
Figure 5. The Bruker family of portable XRF instruments (L-R) S1 TITAN, TRACER 5, CTX.
Bruker’s advancements in pXRF technology along with PSS ability to provide custom calibrations has enabled the development of the RE2I calibration with compelling results (Figure 6 and 7), enabling the instrument to display the results is real time (Figure 8).
Figure 6. Scatter plots of Y and Dy showing known value (x axis) vs the Rare Earth Element Index [RE2I] (y axis).
Figure 7. Dysprosium (Dy) showing laboratory results (x-axis) vs pXRF Rare Earth Element Index [RE2I] (y-axis).
Figure 8. Screen shot of the S1 TITAN 800 showing the results of the Rare Earth Element Index (RE2I) calibration including the ability to have the TREO value calculated from the pXRF results and displayed on the screen.
For further information on the Rare Earth Element Index (RE2I) calibration contact Portable Spectral Services.
Brand, N., & Brand, C. (2017, July 31– August 4). Detecting the Undetectable: Lithium by Portable XRF [Paper presentation, S-3]. 2017, 66th Annual Denver X-Ray Conference, Big Sky, Montana, USA. https://www.portaspecs.com/wp-content/uploads/2020/09/DXC_17_BRAND.pdf.
European Commission. (2020, September 3). COMMUNICATION FROM THE COMMISSION TO THE EUROPEAN PARLIAMENT, THE COUNCIL, THE EUROPEAN ECONOMIC AND SOCIAL COMMITTEE AND THE COMMITTEE OF THE REGIONS: Critical Raw Materials Resilience: Charting a Path towards greater Security and Sustainability. [Report]. COM(2020) 474 final, Brussels. https://ec.europa.eu/docsroom/documents/42849
Li, M. Y. H., & Zhou Meifu, Z. M. (2020). The role of clay minerals in formation of the regolith-hosted heavy rare earth element deposits. The American Mineralogist, 105 (1), 92–108. https://doi.org/10.2138/am-2020-7061
Morin-Ka, S. (2018). Detection and distinction of rare earth elements using hyperspectral technologies. In Geological Survey of Western Australia. Report (187), (pp. 16).
Turner, D. J., Rivard, B., & Groat, L. A. (2016). Visible and short-wave infrared reflectance spectroscopy of REE phosphate minerals. The American Mineralogist, 101 (10), 2264–2278. https://doi.org/10.2138/am-2016-5692
Van Gosen, B.S., Verplanck, P.L., Seal, R.R., II, Long, K.R., & Gambogi, J. (2017). Rare-Earth Elements. In Schulz, K.J., DeYoung, J.H., Jr., Seal, R.R., II, and Bradley, D.C., eds., & Geological Survey (U.S.) (issuing body). Critical Mineral Resources of the United States: Economic and Environmental Geology and Prospects for Future Supply. (pp. O1– O31), Professional Paper 1802, Reston, Virginia U.S. Geological Survey. https://doi.org/10.3133/pp1802O.
Zhou, B., Li, Z., & Chen, C. (2017). Global Potential of Rare Earth Resources and Rare Earth Demand from Clean Technologies. Minerals (Basel), 7 (11), 203–. https://doi.org/10.3390/min7110203
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