1. The XRF determination of sulphur in iron oxide.
M. Cowleya and J.L. Fischer
a Sasol Technology R&D, P.O. Box 1, Sasolburg, 1947, South Africa.
address:
Introduction
Alternative techniques
Advantages of XRF
Several available for sulphur determination, but iron oxide
Non-destructive
Elemental content identified and quantified
matrix difficult
Detection limit for our existing method on S analyser is not suitable (0.02 mass% S)
Ideal for solid matrices
Analysis range:
major: > 1%
ICP-OES/MS – difficult to digest solids without S loss
minor: %
trace: < 1000 mg/kg
ICP-MS – severe mass
Iron ore samples can be prepared as pressed pellets or fused beads [1,2]
interference on S making
low mg/kg measurements
impossible
2. Experimental
Sulphur standards were prepared as pressed powders
Iron oxide powder and sulphur compound (0.28% S in coal) milled for minutes using a planetary mill (ZrO2 bowl) to obtain a fine powder.
Wax (3 mass%) was added and milled for an additional 30 seconds.
Pressed in Al cup at 25 tons for 30s to produce a sample with a smooth surface.
A sample height of 6mm was obtained.
Blank sample was prepared in the same way without adding sulphur.
XRF measurements were performed on a Panalytical Axios Petro WD instrument (Table 1).
The standards and SRM (NIST SRM 690; wt% S in iron ore) were measured ten times.
Table 1 Instrument and Method parameters
Sample preparation required to:
Homogenise sample [3]
Minimise voids
Achieved by grinding (20-30µm) followed by fusion or pressing pellets
Tube target
Power
Voltage
Current
Collimator
Tube filter
Detector
Crystal Rh
2.4 kW
25 kV
120 mA
300µm
None
Gas flow
Ge 111
Analysis medium
S Line
Scanning angle
Offset Bg1
PHD LL
PHD UL
Count time peak
Count
Time Bg1
Vacuum Kα
°2θ
°2θ 35 69
50s
20 s
Fusion
Pressed pellets
+ No matrix matching required
+ Fast and simple
- Volatility of S
+ Good repeatability for S measurements [4]
- Carbon content of sample
- Availability of fusion apparatus
- Prone to matrix effects (requires close matrix matching)
3. Results and discussion
A relatively wide range of standards were chosen to allow the use of one calibration for samples with unknown S concentration. (Calibration curve Fig.1)
A linear response was obtained over the range of sulphur standards used.
The presence of S in the iron oxide used to prepare the standards contributed to the intercept being unequal to zero. (Fig. 1 ○ )
An alternative method of measuring the S content was not available. The standard addition approach was taken to estimate the S content of the starting material. An experimental value of 23 mg/kg was obtained. (Figure 1 )
Adding more standards to the lower end of the curve is not practical due to the S content in the blank sample.
The average [S] for the SRM measurements was 34.1 mg/kg. The standard deviation was calculated according to Eq.3 [5] as mg/kg.
From the trueness data (Table 2), no bias was found in the method.
Figure 1. Calibration for S measurement.
Eq. 3
4. Conclusions
A calibration curve for determining sulphur in an iron oxide was
obtained by preparing in-house standards of the same matrix.
Corrections were made for the presence of the analyte S in the blank sample.
Measurement of a 0.03 g/kg SRM (S in iron ore) gave an acceptable
result in terms of trueness and precision.
None of the available methods are able to measure S at the low level required.
The detection limit is an order of magnitude lower than the method
currently used.
The method was found to be convenient and fit for purpose for the application.
Figure 2 . Trueness – Measurement of SRM
5. References
1. Analysis of major and minor elements in iron ores prepared as pressed powder pellets. PANalytical
application note.
2. ISO : Iron ores -- Determination of various elements by X-ray fluorescence spectrometry –
Part 1: Comprehensive procedure.
Willis, J.P., et al.. XRF in the workplace. A guide to practical XRF spectrometry., 2011, PANalytical
Australia, 6-1.
4. Gagnon, J-P. and Slim, F. World Cement, April
5. Miller, J.N, and Miller, J.C. Statistics for Analytical Chemistry, Ch4.
6. Acknowledgements
Sasol Technology for this opportunity, Willemien van Schalkwyk for assistance with sample preparation.