Accurate Elemental Analysis of Rare Earths Using WDXRF and Fusion

Introduction

Rare Earth Elements (REEs) comprise a group of 17 chemically similar elements, including the fifteen lanthanides (La–Lu), scandium (Sc), and yttrium (Y). These elements play a critical role in the energy transition, aerospace, artificial intelligence (AI), defense, and many other high-technology applications. Given their high economic value, accurate determination of REE composition is essential throughout the mining value chain – from ores and concentrates to the production of high-purity rare earth compounds and metals.

Inductively Coupled Plasma (ICP)-based methods are widely used for the elemental analysis of REEs in geological materials. However, the determination of elements present at major and minor concentration levels typically requires significant sample dilution, introducing additional sources of uncertainty that can compromise analytical accuracy and reproducibility. Furthermore, ICP methods require complete sample digestion using strong acids, involve labor-intensive sample preparation, and incur ongoing operational costs associated with argon gas, reagents, consumables, and frequent instrument recalibration.

In contrast, wavelength-dispersive X-ray fluorescence (WDXRF) enables direct analysis of solid samples with minimal sample preparation, providing a safer, simpler, and more cost-effective analytical workflow. Analysis of fused beads delivers accurate and highly reproducible results across both minor and major concentration levels, making WDXRF an attractive alternative for routine analysis.

Despite these advantages, the analysis of REEs by WDXRF presents two major challenges: the limited availability of suitable reference materials (RMs) and the presence of complex spectral interferences. More than 200 spectral line overlaps can occur among the REEs, making accurate quantification difficult and increasing the need for suitable reference materials to address line overlaps and achieve accurate calibration.

To address these challenges, Malvern Panalytical has developed an innovative WDXRF application solution that enables the determination of all rare earth elements in a single measurement, excluding promethium (Pm), which is not naturally present in economically relevant concentrations due to its radioactive decay. This solution is suitable for rare earth ores, concentrates, processing-related materials, and downstream products such as magnets. In this application note, we demonstrate the accurate and reproducible elemental analysis of rare earth ores using this solution, in combination with the Zetium WDXRF spectrometer and fused beads prepared using the FORJ fusion instrument.

WROXI REE Application Package

The WROXI REE package is manufactured at our ISO 17025 and ISO 17034 accredited Nottingham facility. It consists of 25 synthetic and line-overlap-free calibration standards, an optimized fusion recipe, and a dedicated XRF application template. Each standard is supplied as approximately 10 g of powder under an inert atmosphere, except ThO₂ and U₃O₈, which are provided as fused beads.

Combined with the WROXI BASE and WROXI PRO packages (containing 15 and 12 synthetic standards, respectively), the three WROXI application packages provide coverage for a total of 42 elements in oxide powders, as shown in Figure 1 and Table 1. Elements included in the WROXI REE package are highlighted in red. The table also lists the maximum concentration of each oxide in the synthetic standard powders, representing the highest calibration point on WDXRF. The minimum reportable concentration corresponds to the limit of quantification (LOQ), which is typically in the range of 200 to 400 ppm for these oxides.

Instrument

In this study, a Malvern Panalytical FORJ fusion instrument and a Zetium WDXRF spectrometer were used. The Zetium configuration is detailed in Table 2. Depending on specific analytical requirements, alternative configuration options and additional features may be selected.

Sample Preparation and Measurement

Specimens of calibration standards and validation certified reference materials were prepared as 32 mm diameter fused beads with lithium borate fusion technique. A dilution ratio of 1 mass part of a sample to 10 mass parts of lithium borate flux was used with an integrated non-wetting agent. Fusion of beads was done at 1050°C with the FORJ fusion instrument. Total fusion time of “cold-to-cold” operation cycle takes about 25 minutes. Proven fusion recipes are also available for LeNeo and Eagon2 fusion instruments.

The typical measurement time on 4kW WDXRF Zetium takes approximately 45 mins for 42 elements, in a single measurement.

Calibration Results

Examples of the calibrations obtained for Nd, Sm, and Yb are shown in Figures 2-4. The calibration parameters, including the correlation coefficient (R²), root mean square (RMS), and K-factor values for lanthanide REEs, Th, and U oxides, are summarized in Table 3. The accuracy of the calibrations is evaluated using the R², RMS (absolute error), and K-factor (weighted error) values. The lower the RMS and K-factor values, the better the calibration.

*Unlike the RMS value, the K-factor is independent of the calibrated concentration. Therefore, it gives a more reliable indication of calibration quality. The lower the K-value, the better the calibration, with values below 0.05 indicative of good calibrations.

Validation of Calibration Accuracy

The calibration accuracy, or method trueness, was validated using commercially certified reference materials MBH-REE-ORE1-22-P, OREAS 463 and OREAS 465, as shown in Tables 4-6. For each certified reference material, two fused beads were prepared following the WROXI REE fusion recipe and measured against the WROXI REE calibration. The measured results showed excellent agreement with the certified values, demonstrating the high accuracy of the WROXI REE method.

Conclusion

This study demonstrates the high analytical accuracy of Malvern Panalytical's WROXI REE application solution for the determination of rare earth elements across a broad range of major and minor element concentrations. Using the Zetium WDXRF spectrometer in combination with the FORJ fusion instrument, the solution provides a simple, robust, and acid-free method for the quantification of all rare earth elements in a single measurement, excluding Pm. The package is suitable for the analysis of rare earth ores, concentrates, process-related materials, and downstream products such as magnets.