Rapid Analysis of Limestone Using EDXRF

It is a well-known fact that in the industrial minerals sector, mined raw material and processed limestone must be monitored on a routine basis in order to ensure constant composition and quality of the final products. The accuracy needed is on the order of 0.1 wt. % for the MgO content. This requirement is quite high since MgO in limestone produces low fluorescence yield and heavier elements cause high absorption of X-rays in the matrix. This report explains the analytical performance of the S2 RANGER with XFlash detector for limestone samples, including the light element magnesium.

Instrumentation

The S2 RANGER is a complete bench top energy- dispersive X-ray fluorescence (EDXRF) spectrometer having TouchControl, which is a user-friendly touch screen interface. The system needs only electrical power. Input count rates of 100,000 cps are easily processed with its distinct XFlash detector while ensuring low dead time, excellent resolution and high signal stability. The design of the S2 RANGER also includes a vacuum pump in the instrument’s cabinet, enabling the best possible performance for this application.

Sample Preparation

Limestone samples were prepared as pressed powder pellets without a binding agent. About 10 g of the milled powder were pressed in aluminum cups of 40 mm in diameter on a bed of 2.5 g of boric acid while applying a pressure of 8 tons for 20 s. This preparation is possible with any commercially available pellet press.

Measurement Parameters

Measurements were performed on an S2 RANGER with XFlash detector, in vacuum with a 30 mm sample holder mask. Table 1 lists the measurement conditions used.

Table 1. Measurement Conditions

Voltage [kV] Current [ìA] Beam Filter Live Time [s] Elements Analyzed
20 130 none 100 Mg, Al, Si, Ca, Mn, Fe

This is a rapid method with a measurement time of only 100 s and a constant current of 130 ìA. The XFlash detector has an excellent resolution and a count rate stability due to which these conditions were selected. Overall detector count rate was very much above 100,000 cps. The overall analysis time including sample handling, evacuation of the sample chamber and actual counting time for the measurement was 4:2 minutes. Vacuum was below 0.1 mbar for each sample.

Calibration

The superior energy resolution of the XFlash detector separated the spectral lines of Al, Si and Mg.

Figure 1 shows the low-energy range of the spectrum from a typical limestone sample.

Spectrum of a typical limestone sample including Pd Lá1 tube line

Figure 1. Spectrum of a typical limestone sample including Pd Lá1 tube line

A set of 12 reference samples was used to set up calibrations for the compounds MgO, Al2O3, SiO2, CaO, Mn3O4 and Fe2O3. No matrix absorption correction was applied. Figure 2 shows the calibration curve for MgO.

Calibration curve for MgO

Figure 2. Calibration curve for MgO

Measurement Accuracy

Measurement of a single limestone pellet was repeated 13 times against this calibration. Each time, the sample was unloaded and reloaded between the measurements. Table 3 demonstrates the impressive constancy of the results obtained with the S2 RANGER.

Table 2. Details of calibration data

  Concentration Range [wt. %] Calibration standard deviation 1 ó [wt. %]
MgO 0.90 – 3.87 0.1032
Al2O3 0.07 – 4.65 0.0914
SiO2 0.33 – 13.30 0.6778
CaO 78.80 – 98.51 0.5383
Mn3O4 0.015 – 0.280 0.0048
Fe2O3 0.068 – 1.000 0.0206

The required accuracy of 0.1 wt.% is easily achieved. In order to clearly explain the limits of the EDXRF method, a repetition test with the same sample was performed with an increased measurement time of 1000 s. The results are shown in Table 3.

Table 3. Precision of the measurement (100 s) of main components in limestone

Time 100 s MgO [wt. %] Al2O3 [wt. %] SiO2 [wt. %] CaO [wt. %] Mn3O4 [wt. %] Fe2O3 [wt. %]
Rep-1 0.8423 0.4795 1.5988 53.6814 0.0371 0.2547
Rep-2 0.7691 0.4643 1.5956 53.7039 0.0339 0.2536
Rep-3 0.8565 0.4690 1.6013 53.6829 0.0379 0.2571
Rep-4 0.8322 0.4579 1.5827 53.6866 0.0342 0.2491
Rep-5 0.8419 0.4610 1.6084 53.7157 0.0351 0.2517
Rep-6 0.9180 0.4777 1.6202 53.7394 0.0349 0.2526
Rep-7 0.8553 0.4453 1.5973 53.7738 0.0360 0.2613
Rep-8 0.8533 0.4678 1.6026 53.6778 0.0394 0.2601
Rep-9 0.7953 0.4646 1.5887 53.7326 0.0345 0.2601
Rep-10 0.9509 0.4997 1.6049 53.7178 0.0375 0.2592
Rep-11 1.0649 0.4865 1.6136 53.7308 0.0367 0.2591
Rep-12 0.9178 0.4689 1.6006 53.7753 0.0367 0.2578
Rep-13 0.9031 0.4661 1.5924 53.7110 0.0375 0.2570
Average 0.8770 0.4699 1.6005 53.7176 0.0363 0.2564
Abs.Std.Dev. 0.0729 0.0131 0.0096 0.0312 0.0016 0.0036
Rel.Std.Dev. 8.31 2.78 0.60 0.06 4.39 1.41

Table 4 proves the impressive signal stability of the instrument. In the case of MgO, the error of measurement was only 0.016 %.

Table 4. Precision of main components in limestone with 1000 s measurement time

Time 100 s MgO [wt. %] Al2O3 [wt. %] SiO2 [wt. %] CaO [wt. %] Mn3O4 [wt. %] Fe2O3 [wt. %]
Rep-1 0.8697 0.4650 1.5879 53.5054 0.0368 0.2564
Rep-2 0.8571 0.4710 1.5857 53.4575 0.0377 0.2565
Rep-3 0.8764 0.4725 1.5979 53.5355 0.0371 0.2574
Rep-4 0.8593 0.4686 1.5863 53.5471 0.0365 0.2583
Rep-5 0.8693 0.4755 1.5930 53.5471 0.0369 0.2574
Rep-6 0.8751 0.4709 1.5948 53.5022 0.0376 0.2570
Rep-7 0.8830 0.4729 1.5903 53.5639 0.0378 0.2595
Rep-8 0.8857 0.4728 1.6007 53.5389 0.0368 0.2569
Rep-9 0.8883 0.4700 1.5970 53.5233 0.0369 0.2579
Rep-10 0.9055 0.4726 1.5926 53.5042 0.0373 0.2557
Rep-11 0.9064 0.4728 1.5944 53.4928 0.0363 0.2585
Rep-12 0.8970 0.4703 1.5921 53.4968 0.0364 0.2556
Average 0.8811 0.4712 1.5927 53.5179 0.0370 0.2573
Abs.Std.Dev. 0.0157 0.0025 0.0044 0.0287 0.0005 0.0011
Rel.Std.Dev. 1.78 0.54 0.28 0.05 1.30 0.43

Conclusions

The special characteristics of the XFlash detector ensure that rapid monitoring of the main element composition of limestone including MgO in the regular routine is feasible with the benchtop EDXRF spectrometer S2 RANGER. High accuracy is achievable due to the low dead time and high usable overall count rate of the XFlash detector. The comprehensive design with system controller and TouchControl as well as the vacuum pump in the benchtop instrument allows applications like this to be performed outside a clean laboratory.

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.

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