Hydrometallurgical processing of Platreef flotation concentrate

Abstract

Platinum producers are evaluating hydrometallurgical process routes to extract platinum group metals (PGM) and base metals (BM) from flotation concentrate that are alternative to conventional pyrometallurgical matte smelting treatment of flotation concentrates. This approach would attract several potential benefits to the cost-effective processing of lower-grade and high-chromium concentrates. Ivanhoe Nickel and Platinum Ltd. wished to investigate this option on samples arising from their Platreef project in the northern limb of the Bushveld Complex in South Africa, from concentrate containing 1.5 g/t Pt, 2.5 g/t Pd and 0.5 g/t Au, as well as 1.5% Ni and 1.2% Cu. Other PGMs assayed rather low grades in the concentrate (Rh 128 ppb; Ru 153 ppb; Os 18 ppb; Ir 31 ppb) so the focus was on Pt, Pd and Au in this study. Some twenty-two hydrometallurgical process flowsheet variants were considered for evaluation in a testwork programme combining several conventional and proprietary unit process technologies. The testwork outcomes suggested that a hydrometallurgical flowsheet alternative to smelting (the patented Kell Process) is potentially a practical, environmentally benign and energy-efficient approach for treating low-grade Platreef concentrate with relatively low risk. This process broadly comprises selective leaching of base metals by pressure oxidation, followed by leaching of PGMs after a roasting pretreatment step shown to render PGMs and particularly Pt amenable to leaching. The process embodies separation of the value metals (PGM and base metals) into separate chloride and sulfate solution streams, respectively, and impurity elements (Fe, Al and gangue) into a tailings product. Key elemental deportments from the un-optimized process were Ni 96%, Co 94%, Cu 94%, Pt 96%, Pd 94% and Au 87%. Preliminary work indicated that further improvements in value metal recoveries, impurity separations and reagent consumptions may be gained from further optimization of the individual unit processes. Moreover, the recycle of key reagents uses existing proven technologies, rendering the process more economically and environmentally favourable. Initial assessment suggests that the Kell Process should offer both economic and environmental benefits over conventional pyrometallurgical matte smelting treatment of Platreef flotation concentrate.