Fluorite or fluorspar, a mineral composed of calcium and fluorine, is used in a number of industrial applications, namely in the production of aluminum, fluorine, fluorocarbons, and hydrofluoric acid. It also serves as flux in the manufacture of steels, lowering energy costs by reducing the melting temperature of the steel. The chemical formula of fluorite is CaF2.
Since the purity of fluorite can vary greatly, analyzing the mineral rapidly and reliably is highly critical. Analytical techniques such as titration or ion chromatography are often used for determining fluorite, but these wet chemical methods require digestion of samples with acids. This, in turn, increases the time and cost of the analysis.
Conversely, X-ray fluorescence analysis (XRF) does not require sample digestion, making XRF the ideal analytical technique for grade control of industrial minerals. XRF analysis is straightforward, accurate, precise and rapid due to its simple sample preparation requirements. Energy dispersive X-ray fluorescence spectrometers (EDXRF) are the most commonly used instruments of the two major types of XRF spectrometer for product grade control.
However, conventional EDXRF spectrometers have very poor sensitivity for light elements due to the absorption of low energy fluorescence from elements such as magnesium (Mg, Ka1 = 1.3keV) and fluorine (F, Kα1 = 0.677keV) by thick detector windows.
However, the EDXRF spectrometer S2 RANGER with the XFlash® LE detector has far superior capabilities thanks to the combination of modern detector technology, thin stabilized entrance windows, and high power direct excitation. The analysis of light elements, especially fluorine, using the S2 RANGER with XFlash® LE detector and Ag target X-ray tube, is discussed in this article.
An S2 RANGER with the XFlash® LE silicon drift detector (SDD) was used to perform the measurements. The system is an all-in-one benchtop system equipped with a user-friendly touchscreen interface, TouchControlTM (Figure 1). The XFlash® LE silicon drift detector features an ultrathin, high transmission entrance window, which greatly optimizes the light element sensitivity.
Figure 1. S2 RANGER EDXRF spectrometer
The Ag target X-ray tube was used in place of the standard Pd target X-ray tube to eliminate overlays with tube lines, thereby providing optimum results for these elements. A typical spectrum of a sample containing calcium fluoride acquired with the Ag target X-ray tube is shown in Figure 2. The spectrum shows the clearly resolved F signal at 0.677keV, revealing the intensity increase with each standard.
Figure 2. Spectrum of a typical fluorspar sample in the low energy range of 0.45 to 0.85keV
The experiment involved the preparation of samples as pressed pellets. For each preparation, 11g samples were milled with four grinding aid pellets from Polysius. A 40mm diameter pellet was produced using the automatic press APM from Polysius and an applied pressure of 150kN.
The low energy and high energy lines were excited using two measurement regions. The detailed measurement parameters are summarized in Table 1. All measurements were carried out under vacuum. It took 4-5 minutes for the overall processing per sample which includes sample handling, evacuating the sample chamber and actual counting time for the measurement.
Table 1. Measurement parameters for the different elements
||Tube voltage [kV]
||Tube current [µA]
||Measurement time [s]
|F, Mg, Si
||500 µm Al
Using a set of six in-house standards, calibration for Ca, F, SiO2 and Mg was carried out for quantitative XRF measurements. An independent analytical technique has verified the chemical composition of the standards. The element concentrations of the different standards are presented in Table 2. The calibration curve for F, Mg and Ca is presented in Figures 3 to 5, respectively.
Table 2. Element concentrations of the different standards
Figure 3. Calibration curve for F
Figure 4. Calibration curve for Mg
Figure 5. Calibration curve for Ca
The calibration of the instrument with standards for the compounds Ca, F, SiO2 and Mg was carried out. However, other compounds such as CaF2 or CaCO3 were also determined for an appropriate process control.
It is possible to determine these compounds with minimal additional efforts. A muffle furnace is used to determine the loss on ignition (LOI) of the samples at 950°C. This content is automatically converted into the CO3 concentration with the instrument software.
The CaF2 concentration is estimated from the measured F concentration and the difference of Ca total and CaF2 is used to calculate CaCO3 concentration. Table 3 summarizes the chemical composition for different samples.
Table 3. Chemical composition of various samples
The repeatability for fluorine for a fluorspar sample is graphically represented in Figure 6. The red lines represent the standard deviation of the measurements. It is possible to define the threshold values within the instrument software for each element, thereby helping users to determine ‘out-of-spec’ samples.
Figure 6. Repeatability for the sample B shown for F
Conventional EDXRF finds it difficult to determine light elements such as fluorine. However, the S2 RANGER can determine these light elements very accurately and precisely, thanks to its direct excitation technique, the ultrathin high transmission window, and the excellent resolution of the SDD.
The results clearly demonstrate the superior analytical performance of S2 RANGER equipped with XFlash LE and its applicability for fluorspar monitoring in process and quality control.
This information has been sourced, reviewed and adapted from materials provided by Bruker AXS Inc.
For more information on this source, please visit Bruker AXS Inc.