Evaluation and improvement of the nickel sulphide fire assay method with ICP-MS finish, for analysis of rocks for the precious metals

Frimpong, Augustine (1992) Evaluation and improvement of the nickel sulphide fire assay method with ICP-MS finish, for analysis of rocks for the precious metals. Masters thesis, Memorial University of Newfoundland.

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Abstract

This thesis examines the NiS fire assay, incorporating Te coprecipitation with ICP-MS quantification, procedure for the determination of the Platinum Group Elements (PGE) and gold (Au) in geological materials, with the aim of improving the method. The general analytical methods for the determination of the precious metals are reviewed. -- The sensitivity of an analytical procedure is much influenced by the choice of the determinative step. Inductively coupled plasma mass spectroscopy (ICP-MS) offers a precise, accurate method with low limits of detection for quantification of the PGE and Au. However, inherent in the ICP-MS are problems such as ionic interferences, memory, matrix and drift. The matrix effects of HCI, HNO₄ and Te on the sensitivity of the precious metals using ICP-MS were studied. Within the concentration range examined, the sensitivities of the internal standards (Cd and TI), PGE and Au were found to depend most strongly on the concentration of HCI. The reduction of HCI concentration from 0.6M to 0.2M provides about 20% enhancement in the sensitivities of the internal standards, and the PGE. -- The effects of reducing the quantity of Ni and S used in the NiS fire assay method on the recovery of PGE and Au was investigated in a Pt ore grade sample and a low ppb level sample. In the Pt ore grade sample, the recovery of the precious metals was found not to depend on the mass of Ni used as collector. However, the recovery of Ru, Rh, Pd and Pt in the low ppb level sample was found to be a function of the collector mass used, while the recovery of Os, Ir and Au in the low ppb level sample was not. To explain this behaviour of the PGE, a model equation was derived from the mass balance equation of PGE between the sulphide and silicate liquids, and the bulk partitioning coefficient equation. From this equation, the partition coefficients (D) of the PGE were determined. The D values were also independently determined by analysis of the PGE and Au contents of the sulphide and silicate phases. The D values were functions of PGE concentrations. The D values obtained for a Pt ore grade sample were 4 to 103 times higher than those for the low ppb level sample. D values for Ni, Cu and Cr are also reported. -- To improve PGE recovery in low ppb samples and to lower reagent blanks, further studies were undertaken. The NiS bead was dissolved in a closed vessel with a smaller amount of HCI, and the quantity of Te (and SnCI₂) used for the coprecipitation of PGE was also reduced significantly to lessen the total dissolved solids in the analytical solution. -- An improved method has been developed, which offers good precision and acceptable accuracy for the determination of PGE and Au concentration in geological materials. The reagent blank, matrix effect due to HCI, cost of chemicals and sample processing time have been reduced.

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