This application note demonstrates that the Epsilon 4 – a benchtop energy dispersive X-ray fluorescence (EDXRF) spectrometer – is exceedingly capable of assessing the purity of fluorite mineral samples that are prepared as fused beads.
Fluorite (also called fluorspar) is the mineral form of calcium fluoride, CaF2 and is a widely occurring mineral found in large deposits. Fluorite is used as a flux in iron and aluminium smelting and in the production of certain glasses and enamels. The purest grades of fluorite are also a source of fluoride for the production of hydrofluoric acid, which is in turn used in the production of fluorine-containing fine chemicals and optically clear transparent fluorite lenses. In order to determine the individual applications for fluorite, the purity of the mineral must be quantified in an accurate way. Wet-chemical methods, including multiple digestion, precipitation and titration steps are traditionally employed for fluorite grading. These methods are notoriously time-consuming
and require the use of harmful chemicals by skilled laboratory staff. EDXRF analysis provides a useful alternative for classifying fluorite samples in a much faster, safer and cost-effective way.
Measurements were performed using an Epsilon 4 EDXRF spectrometer, equipped with a 15W, 50 kV rhodium (Rh) anode X-ray tube, 6 software selectable filters, a helium purge system to improve the light-element performance, a high-resolution SDDUltra silicon drift detector, a sample spinner to obtain more homogeneous results, and a 10-position removable sample tray for automated batch analysis.
Fused beads of 40 mm in diameter were prepared by fusing 0.9 g fluorite sample with 9 g of flux in a fully automatic Claisse Eagon 2 fusion machine. A dedicated fusion recipe was developed to retain fluorine in the beads at high temperatures. All standards were prepared in duplicates and measured in the Epsilon 4 spectrometer.
Five commercially available Certified Reference Materials (CRM) (NCS DC) and 3 standards from Malvern Panalytical’s wide-range oxide WROXI package were used to set up calibrations for F, SiO2, S, K2O, Ca, BaO and Fe2O3 in fluorite samples. Three different measurement conditions were used, each one optimized to excite a group of elements (Table 1). The total measurement time was 13 minutes per standard. Figure 1 shows the XRF spectra around the fluorine Kα peak in the five NCS DC standards with visible differences in peak size between the different standards, which contain different concentrations of fluorine.
Table 1. Measurement conditions
Figure 1. XRF spectra of fluorine in the five NCS DC standards, prepared as fused beads
Accurate calibration results
Figures 2 and 3 show calibration graphs obtained for fluorine and iron oxide using the fluorite and WROXI standards and they show excellent correlation between the certified concentrations and the measured intensities. Detailed calibration results for all analyzed elements in the standards are listed in Table 2. The light-element performance of the new SDDUltra detector contributes largely to the improved analysis capabilities for fluorine. The lower limits of detection (LLD), listed in Table 2 are based on the measurement times for each of the measurement conditions (Table 1).
Table 2. Calibration results
Figure 2. Calibration graph of fluorine in fluorite fused bead standards, measured in duplicates
Figure 3. Calibration graph of iron oxide in fluorite fused bead standards, measured in duplicates
One fluorite standard (NCS DC 14022) was measured 20 times consecutively to test the instrument stability. The certified concentration of the standard, the average concentration of the measurements, the RMS (1 sigma standard deviation) and the relative RMS of the repeated measurements for all the elements are shown in Table 3. With the exception of K2O, all compounds show a relative RMS of better than 3 %. The high relative RMS value for K2O is due to the low concentration of the compound in the sample.
Table 1. Results of the repeatability test with fluorite standard NCS DC 14022, prepared as a fused bead
The results clearly demonstrate the capability of Epsilon 4 for the analysis of F, SiO2, S, K2O, Ca, BaO and Fe2O3 in fluorite (fluorspar, CaF2) samples. Excellent results have been obtained for the calibrations and lower limit of detection. Furthermore, the repeatability experiment demonstrates that the Epsilon 4 is an ideal instrument for purity check of fluorite samples. The high resolution and outstanding sensitivity of the SDDUltra silicon drift detector and powerful deconvolution algorithms within the Epsilon 4 software, contribute to the success of this experiment.