Measuring Sulfur Content of Coal Using EDXRF

Pyrite (Fe2S) is a key mineral present in most coal, which contains different amounts of sulfur based on how much iron and sulfur combined in the peat when it transformed into coal, and the wetness of the climate in the coal formation. When coal is burnt, the metal equipment can get corroded rapidly due to the presence of higher sulfur content. Moreover, sulfur itself is an environmental pollutant when it burns. Hence, there is a requirement for a simple, yet rapid and robust tool to measure the sulfur content of coal for the characterization and value of the coal, from mining to industrial use.

The NEX QC benchtop energy dispersive X-ray fluorescence (EDXRF) analyzer from Rigaku is the ideal solution to analyze sulfur and other elements in coal. This article demonstrates the ability of the Rigaku NEX QC EDXRF analyzer to measure sulfur content in coal.

Experimental Setup

Model: Rigaku NEX QC

Detector: Semiconductor

X-ray tube: 4 W Ag-anode

Sample Type: Coal (powder)

Film: Mylar

Analysis Time: 300 sec; 240 sec for S and 60 sec for Ca, Ti, Fe

Environment: Air

Options: Autosampler, Manual Sample Press

Sample Preparation

Samples were prepared by grinding coal to a homogeneous dry powder of <200 mesh (<75µm particle size), followed by transferring and compressing roughly 8 g of powder into a 32 mm sample cup by applying 250 inch-pounds torque through the manual sample press in order to ensure homogenous compaction.

Calibration

A suite of assayed coal powder standards was used to develop an empirical calibration. The overlap compensation was created between S and Cl peaks by measuring Cl. Differences in matrix absorption/enhancement effects were compensated by enabling influence coefficients (alpha corrections) through the measurement of Ca, Ti and Fe. The calibration results are shown below:

Element: S Units: % Std Error of Est: 0.01623
Correlation: 0.99972
Sample I.D. Standard Value Calculated Value
STD 1 0.40 0.405
STD 2 0.72 0.729
STD 3 0.76 0.737
STD 4 0.87 0.879
STD 5 1.40 1.393
STD 6 1.73 1.724
STD 7 1.82 1.841
STD 8 1.91 1.913
STD 9 2.09 2.081
STD 10 2.61 2.608

Precision

Select samples were picked from the suite of calibration standards in order to illustrate repeatability or precision o f the instrument. Each sample was repeatedly measured in static position for 10 times, utilizing a total analysis time of 300 seconds per measurement. The repeatability results are listed below:

Element: S      Units: %
Sample ID Standard Value Average Value Std Dev % Relative
STD 1 0.40 0.412 0.003 0.8
STD 6 1.40 1.434 0.009 0.6
STD 11 2.61 2.614 0.011 0.4

Typical Detection Limits

Using the empirical approach, boric acid powder containing no sulfur was repeatedly analyzed for 10 times in order to calculate standard deviations for determining the lower limit of detection (LLD). The LLD was set as three folds the standard deviation. The measurement time was 240 seconds for the sulfur acquisition condition.

S LLD @ 240 sec
40 ppm

Results

The capability of the Rigaku NEX QC to perform multi-element analysis was demonstrated by measuring Cl to offset its effects on the sulfur reading. The measurement of Si was performed, although its matrix effects were trivial. The measurements of heavier elements Fe, Ti and Ca were also performed to offset their impact on the sulfur reading, thus providing corrections for variations in matrix effects among samples and coal types. To measure two or more different coal types, developing separate calibrations for each coal type can help achieve optimum accuracy.

Conclusion

The results clearly demonstrate the capability of the Rigaku NEX QC EDXRF analyzer to rapidly and accurately measure sulfur content in coal, without using helium purge. Simple yet advanced touch screen interface provides reliable and efficient measurement protocols to address the analytical challenges throughout different industries.

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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