Author Description

Because the values for PGM + Au (ie, platinum, palladium, rhodium, and gold) and for gold alone, and the precision with which they are determined by fire assay, have been shown to vary considerably between South African laboratories, and because small differences in these values can represent millions of rands, it is essential that the accepted method of analysis, fire assay, should give accurate and precise values. In an investigation into the fire-assay procedure it was found that a reducing atmosphere should be maintained throughout the fusion step (which results in near-stoichiometric recoveries of lead and higher recoveries of gold and of total platinum, palladium, rhodium, and gold) and that the additions of litharge should be smaller than those normally used at the Council for Mineral Technology (Mintek) provided the sample contains little or no nickel and the ration of litharge to reducing agent is between 9 and 12. Under these controlled conditions, the recoveries of PGM + Au in silicate and chromite ores from the Merensky Reef and of gold in Witwatersrand ores are approximately 5, 16, and 9 per cent higher respectively than the accepted values. A revised laboratory method is detailed in an appendix.