X-ray fluorescence spectroscopy (XRF) is an established analytical technique in the building materials industry for monitoring and controlling the chemical composition of the raw materials used in the production of clinkers and, ultimately, cements. Continuous analysis of the raw mix allows process adjustments, ensuring product quality and consistency, while maximizing kiln efficiency and minimizing CO2 emissions. XRF is a safe, cost-effective and highly reliable method and does not require daily re-calibration.
This application note demonstrates that the Epsilon 4, a benchtop energy dispersive X-ray fluorescence spectrometer, is more than capable of analyzing Na2O, MgO, Al2O3, SiO2, SO3, Cl, K2O, CaO, TiO2 and Fe2O3 in raw mix samples in 3 minutes, while complying with the Chinese GB/T19140-2003 and GB/T176-2008 testing methods. GB/T19140-2003 specifies the limits for calibration accuracy, while GB/T176-2008 specifies the limits for repeatability of the analysis of compounds in raw mix samples.
Measurements were performed using a Malvern Panalytical Epsilon 4 EDXRF spectrometer, equipped with a 10 W, 50 kV silver (Ag) anode X-ray tube, 6 software-selectable filters, a helium purge facility to improve the light-element performance, a high-resolution SDD silicon drift detector, a sample spinner, and a 10-position removable sample tray for batch analysis.
The raw mix reference standards were analyzed as pressed powder pellets. The sample preparation technique is easily mastered, safe and relatively inexpensive. Five grams of raw mix sample were mixed with 0.5 gram of binder (wax) and pressed to pellets of 30 mm in diameter at 20 ton and 30 s dwell time.
Eleven commercially available Chinese raw mix reference standards were used to set up the calibrations. Two different measurement conditions were used, each optimizing the excitation of a group of elements (see Table 1). The total measurement time per standard was only 3 minutes.
Table 1. Measurement conditions
Figures 1, 2 and 3 show the resulting calibration graphs for Na2O, Al2O3, and CaO in raw mix samples, respectively. The graphs show very good correlation between the certified concentrations and the measured intensities. Detailed calibration results for all analyzed elements in the raw mix standards are listed in Table 2. The RMS (root mean square) value is equivalent to 1 sigma standard deviation.
From Table 2 it can be observed that the RMS values are well within the limits specified by the GB/T19140-2003 norm.
Table 2. Calibration details
Figure 1. Calibration graph for Na2O in raw mix
Precision and LSF
To test the instrument precision, one pressed pellet sample was measured 20 times consecutively. The average concentration and RMS value for all elements are presented in Table 3, demonstrating excellent precision. Table 3 also demonstrates that the RMS values are well within the limits specified by the GB/T176-2008 norm.
In cement manufacturing, the lime saturation factor (LSF) is used to control the composition of the raw mix. Using this fast XRF method, the absolute uncertainty on the LSF value is even 0.05. The formula used is the following:
Figure 2. Calibration graph for Al2O3 in raw mix
Table 3. Repeatability results
The results clearly demonstrate the capability of Epsilon 4 for the fast analysis of raw mix in compliance with the Chinese GB/T19140-2003 and GB/ T176-2008 testing methods. The high resolution and outstanding sensitivity of the SDD silicon drift detector combined with powerful software deconvolution algorithms make it possible to quantify 10 compounds in raw mix in 3 minutes. Excellent results have been obtained for the calibrations and repeatability test. Furthermore, the low uncertainty on the LSF value demonstrates that the Epsilon 4 is an ideal instrument for fast raw mix analysis.