Fundamental Parameters (FP) EDXRF Analysis of Low Sodium Coal

When coal is mined, processed and burnt, process monitoring and control are critical in order to achieve efficient and quality final product. A rapid, simple, and precise technique for analyzing samples is essential during different stages of the coal production cycle, especially for analyzing the sodium content (Na2O). The presence of high sodium content, when combined with silicon and aluminum, or sulfur, can lead to the formation of sodium alumina-silicate or sodium sulfate compounds. During burning high sodium coals, the formation of such compounds can result in fouling of the cyclone or heat exchange surfaces.

The NEX CG energy dispersive X-ray fluorescence (EDXRF) analyzer from Rigaku can address the challenges in low sodium coal analysis. Using secondary excitation and polarization, the system provides unprecedented sensitivity, with best-in-class detection limits for light elements such as sodium. It is capable of reliably measuring elements from sodium to uranium. It is simple enough to be operated by non-technical users, yet robust enough for research and development applications. This article demonstrates the ability of the NEX CG EDXRF analyzer to analyze coal, with special focus on Na2O measurement, utilizing the Fundamental Parameters (FP) approach with Matching Library.

Experimental Setup

The Rigaku NEX CG EDXRF Analyzer

Figure 1. The Rigaku NEX CG EDXRF Analyzer

Model: Rigaku NEX CG
Detector: High performance SDD
X-ray tube: 50 W Pd-anode
Excitation: Indirect with polarization
Options: Light Element Optimization
Targets: Multiple
Analysis Time: 1400 sec
Environment: Helium Purge or Vacuum
Standard: 15-position Sample Tray (32 mm)

Sample Preparation

Each sample was prepared by grinding the material to a homogeneous dry powder of <200 mesh (~75 µm grain size) using a ball mill. The powdered sample was then pressed into pellets using a hydraulic press at a pressure of 20 tons. The combination of hydraulically pressed pellets and Rigaku's Light Element Optimization target enabled achieving optimal sensitivity to measure Na2O.

Rigaku RPF-SQX Fundamental Parameters (FP)

Rigaku's RPF-SQX Pellet Template was used to develop a Fundamental Parameters (FP) method. The RPF-SQX technique employs a sophisticated scattering FP program, which deconvolutes spectral peaks automatically and simulates the sample matrix utilizing fundamental XRF equations. The selection of the scattering FP template was for the estimation and better simulation of the non-measureable components (H to F) of the sample matrix. Scattering FP uses the ratio of Compton/Thomson scatter for determination of the average atomic number (Z) of the matrix, thus providing an estimate of the percentage of the sample that is not possible to measure and yielding highly precise analytical results for the remaining elements that are measureable. This, in turn, provides a semi-quantitative measurement of elemental concentrations without using a large set of known assayed calibration standards.

Moreover, three assayed field samples were used to create a matrix-specific Matching Library in order to obtain better analytical results. It is simple to create the Matching Library, which can be used along with the standard FP library to obtain better model of the coal matrix in order to optimize the calculation of concentration results.

The Rigaku NEX CG EDXRF spectrometer combined with polarization targets, indirect excitation with secondary targets, and a high-performance SDD provides a robust and versatile analysis tool with a user-friendly software interface. Unlike traditional EDXRF systems, the use of indirect excitation eliminates almost all the background, thus providing spectra with a very high characteristic signal-to-noise ratio. This, in turn, enables a higher level of accuracy and much lower detection limits utilizing FP methods.

RPF-SQX Results

Using a 3-sample Matching Library, the RPF-SQX results are obtained for four field samples of coal, as listed in the following tables:

Sample ID: 268         Units: Mass%
Compound RPF- SQX Result Given Value % Dev
Na2O 0.105 0.098 7.2
MgO 0.257 0.275 -6.5
AhO3 0.885 0.930 -4.9
SiO2 1.76 1.85 -4.8
P2O5 0.0332 -- --
SO3 0.296 0.324 -8.6
K2O 0.0188 0.018 4.4
CaO 1.34 1.41 -4.5
TiO2 0.0898 0.0960 -6.5
Fe2O3 0.297 0.313 -5.3
SrO 0.0175 -- --
BaO 0.0261 -- --
Sample ID: 977         Units: Mass%
Compound RPF- SQX Result Given Value % Dev
Na2O 0.142 0.134 5.8
MgO 0.362 0.389 -7.0
Al2O3 1.20 1.29 -7.4
SiO2 2.64 2.80 -5.6
P2O5 0.0309 -- --
SO3 0.722 0.781 -7.5
K2O 0.0334 0.034 -1.7
CaO 1.51 1.42 6.5
TiO2 0.0901 0.0820 9.9
Fe2O3 0.444 0.470 -5.5
SrO 0.0257 -- --
BaO 0.0310 -- --
Sample ID: 265         Units: Mass%
Compound RPF- SQX Result Given Value % Dev
Na2O 0.100 0.099 1.4
MgO 0.253 0.279 -9.2
Al2O3 0.916 1.003 -8.7
SiO2 1.86 2.04 -8.9
P2O5 0.0348 -- --
SO3 0.289 0.326 -11.3
K2O 0.0198 0.022 -10.0
CaO 1.34 1.42 -5.2
TiO2 0.0941 0.1010 -6.8
Fe2O3 0.295 0.321 -8.2
SrO 0.0179 -- --
BaO 0.0246 -- --
Sample ID: 211         Units: Mass%
Compound RPF- SQX Result Given Value % Dev
Na2O 0.169 0.168 0.5
MgO 0.249 0.268 -7.1
Al2O3 0.947 1.022 -7.3
SiO2 1.80 1.99 -9.7
P2O5 0.0467 -- --
SO3 0.262 0.288 -9.1
K2O 0.0237 0.026 -8.9
CaO 1.27 1.37 -7.6
TiO2 0.0813 0.0890 -8.7
Fe2O3 0.286 0.312 -8.3
SrO 0.0196 -- --
BaO 0.0290 -- --

Instrument Repeatability

To demonstrate instrument precision or repeatability, select samples containing low and high Na2O were repeatedly analyzed for five times in static position. The average analytical value and relevant statistics are listed in the following tables:

Sample ID: 265         Units: Mass%
Compound Average Value STD DEV RSD (%)
Na2O 0.100 0.003 3.4
MgO 0.257 0.004 1.6
Al2O3 0.930 0.005 0.6
SiO2 1.89 0.004 0.2
P2O5 0.0347 0.0004 1.1
SO3 0.295 0.0003 0.1
K2O 0.0215 0.002 10.2
CaO 1.35 0.006 0.4
TiO2 0.0955 0.001 1.3
Fe2O3 0.299 0.001 0.3
SrO 0.018 0.0001 0.4
BaO 0.0254 0.001 3.5
Sample ID: 211         Units: Mass%
Compound Average Value STD DEV RSD (%)
Na2O 0.161 0.007 4.4
MgO 0.250 0.007 3.0
Al2O3 0.952 0.007 0.8
SiO2 1.81 0.010 0.5
P2O5 0.0473 0.001 1.2
SO3 0.266 0.001 0.4
K2O 0.0239 0.001 5.7
CaO 1.29 0.007 0.5
TiO2 0.0833 0.001 1.0
Fe2O3 0.292 0.001 0.4
SrO 0.0199 0.0001 0.7
BaO 0.0296 0.001 3.6

Qualitative Analysis

Clean isolation of the respective peaks of interest was revealed by spectral analysis of each of the samples. Select spectra are overlaid as shown in Figure 2 through Figure 5.

Select samples on the Light Element Optimization (LEO) secondary target

Figure 2. Select samples on the Light Element Optimization (LEO) secondary target

Select samples on the RX9 (HOPG polarizer) secondary target

Figure 3. Select samples on the RX9 (HOPG polarizer) secondary target

Select samples on the Cu secondary target

Figure 4. Select samples on the Cu secondary target

Select samples on the Mo secondary target

Figure 5. Select samples on the Mo secondary target

Conclusion

The use of the Rigaku RPF-SQX FP method was able to achieve outstanding semi- quantitative analysis. It was possible to model and subtract any remaining background from the calculations using RPF-SQX. The automatic deconvolution of the spectral peaks was made possible by utilizing a sophisticated method of modeling the entire spectrum using theoretical calculations, which include X-ray matrix absorption/enhancement corrections. By comparing the calculated spectrum with the measured spectrum, the model was repeatedly refined until the calculations converge in order to get highly precise FP results when compared to traditional methods used for EDXRF analysis.

The intuitive software and Rigaku FP templates facilitates further refining of the matrix models and further optimization of FP accuracy through the creation of a Matching Library, which enables the operator to measure and record one or more known samples as illustrations of the actual coal matrix. The Matching Library then automatically refines the calculations of theoretical intensities and matrix effects in order to yield analytical results within the range of 10% relative or better. From the results, it is evident that the Rigaku NEX CG EDXRF spectrometer is an ideal solution for the analysis and quality control of coal.

This information has been sourced, reviewed and adapted from materials provided by Rigaku Corporation.

For more information on this source, please visit Rigaku Corporation.

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