Improved Platinum‐Group Element Extraction by NiS Fire Assay from Chromitite Ore Samples Using a Flux Containing Sodium Metaphosphate

Abstract

Many chromite‐rich rocks contain relatively high concentrations of the platinum‐group elements (PGE). In many cases, the phases carrying PGE occur as either platinum‐group minerals (PGM) or as base metal sulfides in solid solution in sulfides. In some cases, such as the UG‐2 unit of the Bushveld Complex, the PGM are occluded inside chromite grains. Chromites are notably difficult to dissolve in most fluxes and if the chromite contains some PGM the possibility exists that not all the PGE will be recovered during fusion. In this work, shortcomings in published methods of analysis based on the nickel sulfide fire assay procedure were investigated and a new procedure developed based on the addition of sodium metaphosphate to the fusion mixture. Optimum composition of the fusion mixture was found to be 10 g sodium metaphosphate and 9 g silica to 10 g sample, 15 g sodium carbonate, 30 g lithium tetraborate, 7.5 g nickel and 4.5 g sulfur to achieve complete dissolution of chromite grains. The new flux mixture was evaluated by the analysis of reference material CHR‐Pt+ (which is known to contain PGM inside chromite grains) and no undissolved chromite grains were found in the glassy slag. Analysis of the nickel sulfide beads from this fire assay using neutron activation analysis showed similar results for Rh and Ru when compared with published conventional true (or accepted) values, while Au, Ir, Os, Pd and Pt values determined here were 10 to 30% higher than the corresponding published conventional true values. It was concluded that the addition of sodium metaphosphate improved chromite dissolution in the flux and appears to improve PGE recovery.

Rôle des liquides sulfurés dans la formation des minéralisations riches en éléments du groupe du platine :

“…The PGE and Au in the silicate rocks were determined by Ni-sulphide fire assay followed by instrumental neutron activation analysis (INAA) on 10 g of sample using a slightly modified version of the formulas. Because of the difficulty of dissolving chromitites in the standard flux mixtures, the flux mixture for the chromitites was modified using the formula of Bédard and Barnes (2004). Results from our laboratory are in agreement with the certified values for the international reference material AMIS0007, which is a sample of Merensky Reef (Table 3.4).…”

Platinum-group elements in the UG1 and UG2 chromitites, and the Bastard reef, at Impala platinum mine, western Bushveld Complex, South Africa: Evidence for late magmatic cumulate instability and reef constitution

“…Because of the difficulty of dissolving chromites in the standard fire assay flux . mixtures, the flux mixture for the chromitites was modified using the formula of Bédard and Barnes (2004). Copper was determined by atomic absorption spectrophotometry.…”

Comparison between Nickel‐Sulfur Fire Assay Te Co‐precipitation and Isotope Dilution with High‐Pressure Asher Acid Digestion for the Determination of Platinum‐Group Elements, Rhenium and Gold

“…Analytical techniques: the nickelsulfur fire assay technique, followed by SnCl 2 AE2H 2 O-Te co-precipitation (NiS-FA) Fire assay is a neoclassical technique for PGE determination in geological materials and many variants of the nickel-sulfur fire assay technique (NiS-FA) exist (e.g., Robert et al 1971, Asif and Parry 1989, Bédard and Barnes 2004. NiS-FA is fast, simple, flexible and, in most cases, uses more than 10 g of sample to reduce the risk of erratic results due to the presence of native gold particles or platinum-group minerals, known as the nugget effect.…”