EDXRF Measurement of Sulfur in Gypsum

Gypsum (calcium sulfate dihydrate) is a highly useful industrial mineral mainly sought after for its high sulfur content. An essential use of gypsum is in finished Portland cement to prevent flash-setting of concrete and also to establish drying properties in wet environments. Other applications are in fertilizers, dry wall and soil conditioners.

It is important to closely monitor the sulfur content of gypsum as it is processed to make sure that the products are formulated well and have good quality. To meet the industry needs, Rigaku offers NEX QC, a simple and versatile benchtop EDXRF analyzer for the analysis of sulfur in gypsum. The measurement of sulfur in gypsum is shown below.

Figure 1.

Instrument Configuration

Model: Rigaku NEX QC
X-ray tube: 50kV 4W Ag-anode
Detector: Semiconductor
Sample Type: Gypsum
Film: Mylar
Analysis Time: 120 sec
Environment: Air
Optional: Autosampler, Manual Sample Press

Figure 2.

Sample Preparation

First a gypsum sample is pulverized and ground to a dry homogenous powder less than 200 mesh or less than 75 µm particle size. Around 8 g of powder is placed in a 32 mm sample cup and tap-packed gently by tapping the cup on the hard, clean surface. Alternately, the sample can be compressed with 250 inch- pounds torque using the manual sample press to ensure even and consistent compaction of the samples.

Calibration

Empirical calibration was developed using a set of six assayed standards ranging between 12-17% S.

Table 1.

Element: S
Units: %
SEE: 0.064
Correlation: 0.999071
Sample I.D. Standard Value Calculated Value
STD-1 12.2 12.22
STD-2 13.7 13.68
STD-3 14.5 14.43
STD-4 14.9 14.91
STD-5 15.4 15.48
STD-6 17 16.98

Correlation Plot

The correlation plot is shown in Figure 3

Figure 3.

Standard Sample Recovery and Precision

The low and high S content calibration standards were measured 10 consecutive times in a static position to show effective recovery and analytical precision (repeatability). Analytical results and precision data are reported below using an analysis time of 120 sec per measurement.

Table 2.

S in Gypsum       Units: Mass%
Sample ID Assay Value NEX QC Average Value STD DEV % Relative
STD-1 12.2 12.18 0.02 0.2
STD-6 17 16.95 0.03 0.3

Empirical Detection Limit

The empirical method was used for determining detection limits in a clean matrix. In the empirical method, 10 repeat analyses of blank boric acid powder sample are taken with the sample in static position and the standard deviation (a) is determined. The Lower Limit of Detection (LLD) is then defined as 3ct. The typical LLD is reported here for S in gypsum using an analysis time of 120 sec per measurement.

Table 3.

Element Calibration Range Empirical LLD Analysis Time
S 12 - 17% 0.02% 120 sec

Detection limits are determined in part by the calibration range and analysis time. Lower LLD for S can be achieved by calibrating at lower sulfur concentration ranges or increasing the analysis time of each measurement.

Conclusion

The data shows that NEX QC offers excellent results for the measurement of sulfur in gypsum, without the need for helium purge. Given complete assay values, the calcium content can also be determined as well as other elements in the gypsum. Simple, modern touch screen interface allows for reliable and efficient measurement protocols to meet the analytical needs at the quarry, at the cement plant when testing gypsum and finished cement, as well as during the manufacturing of other gypsum products.

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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