Recovery of precious metals using nickel sulfide fire assay collection: problems at nanogram per gram concentrations

Abstract

The NiS fire assay is widely used for the separation and concentration of PGE and Au from geological materials. The effect of collector mass on the recovery of PGE and Au in a Pt ore grade (µg g^–1) sample and a komatiite sample which contained <10 ng g^–1 of PGE are reported. The PGE and Au recoveries were not a function of collector mass for the ore grade sample. The recovery of Ir, Os and Au from the komatiite sample did not significantly depend on the mass of the collector used but Ru, Rh, Pd, and Pt recoveries were reproducible and dependent upon the collector mass. From these recoveries, estimates of the partition coefficients (K_d) were calculated for the sulfide and silicate melt phases. The K_d values were also estimated from direct analysis of the two phases. The recoveries of the precious metals were not always independent of the collector-to-flux-ratio. Also, the K_d values were found to depend upon the PGE concentrations. The K_d values obtained for an ore-grade sample were 4 to 103-fold higher than those obtained for a sample which contained < 10 ng g^–1 PGE concentrations. Thus, when determining PGE concentrations at low levels using NiS fire assay collection, significant biases may occur.