Method development and validation for the analysis of silicon, phosphorus, sulphur, chromium and metal oxides in charge chrome ferro-alloy by x-ray fluorescence spectroscopy

Van Niekerk, Ferdinand
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High-carbon ferrochrome, also known as charge chrome can be used in the production of stainless steel of various grades. For charge chrome to be used as a raw material for stainless steel production, it is essential to know the chemical composition of the alloy. Charge chrome is currently analysed by X-ray fluorescence spectroscopy (XRF) in the form of a button sample and the slags (oxides forming during the reduction process in charge chrome production) are analysed in the form of a briquette. The main aim during this study was to develop an XRF method for the analysis of charge chrome as a powder briquette and to develop a method for the analysis of the oxides. During XRF analysis interference effects such as spectral overlaps and matrix effects (mainly in the form of absorption) must be compensated for to ensure accurate analysis. The general composition of the sample matrices, especially with regard to the oxides, is known. The possibility of matrix matching between calibration standards and samples was investigated to see if the necessity of corrections in terms of overlaps and interferences can be eliminated. After setting up calibration lines using production samples analysed by an alternative validated analytical technique (Inductively Coupled Plasma – Optical Emission Spectroscopy) for the elements and oxides (indirectly as elements), no corrections on the calibration lines were made using relevant mathematical correction algorithms. The method development phase was followed by a complete validation of all the necessary parameters to ensure an accurate XRF analytical technique. The validation of the technique showed that the method is capable of yielding accurate and trustworthy results. This confirmed the theory that matrix matching between calibration standards and samples can compensate for the necessity to make any corrections for spectral overlaps and spectral interferences. The preparation of samples as powder briquettes was also investigated to determine the optimum conditions for sample preparation. The main parameter studied was the influence of particle size on analysis. The optimum sample preparation conditions were determined and confirmed by validating the analytical results obtained when the powder briquette was analysed using the validated XRF method. After research, the conclusions were made that charge chrome can be analysed in briquette form and that matrix matching between calibration standards and samples eliminates the need for any correction with regard to spectral overlaps and matrix effects. The newly developed and validated methods for the analysis of Si, P, S and Cr in charge chrome metal and the oxide content in charge chrome slag will be implemented to assist in the daily routine analysis of charge chrome production samples.