Evaluation of the Nickel Sulphide Bead Method of Fire-assay for the Platinum-Group Elements using Neutron Activation Analysis

Abstract

Fire-assay has been implemented as a preconcentration step before neutron activation analysis to determine the platinum-group elements (PGE) and gold in representative samples (50g). The procedure is based on the established nickel sulphide bead method, with lithium tetraborate in the flux and a fusion temperature of 1000°C. When the analytical technique was tested by measuring the PGE in reference materials SARM 7 and PTC-1, the data agreed with recommended values.

The method was modified for chromitites both by the addition of sodium hydroxide to the flux and by raising the temperature to 1150°C. The accuracy of the modified technique was checked with a MINTEK ‘in house’ chromite standard, 2/77, and the concentrations of the PGE measured agreed with the preferred values. Replicates of two chromites, UG2 and LG6, were analysed to measure the analytical reproducibility. Results confirmed that the variations were acceptable. Concentrations of the PGE in the blanks were insignificant compared to those in the samples but high gold blanks may be the source of some variation in the results. A second source of error could be losses during dissolution.

It is concluded that, even though the method appears to give satisfactory results for high concentrations of the PGE, there may be significant problems due to losses and reagent blanks when it is applied to samples with lower values. There is an urgent need for geological reference materials, particularly chromites, containing 0·1–1·0 ppm total PGE, to check analytical procedures. Meanwhile, in the absence of such reference samples, it is proposed that the use of radiochemical techniques may be the only way to ensure that accurate data are obtained for samples containing low concentrations of the PGE.