The extraction of Pt, Pd and Au from an alkaline cyanide simulated heap leachate by granular activated carbon

Abstract

A two-stage heap leach process to extract base and precious metals from the Platreef low-grade ores is currently being investigated industrially. As part of the investigation, and by analogy with current gold recovery practices, the present study investigates the preferential adsorption of precious metals (Pt, Pd and Au) over base metals (Cu, Ni and Fe) from an alkaline cyanide medium, by means of granular activated carbon. Experiments were designed statistically to optimise the process parameters using synthetic alkaline cyanide solutions similar in composition to those expected from plant leach solutions. Precious metal adsorption efficiency was studied in terms of process kinetics and recovery as a function of solution pH, and the concentrations of copper, nickel, free cyanide, thiocyanate, precious metals (Pt, Pd and Au) and activated carbon. Results of kinetic experiments demonstrated that the adsorption of PGM and Au was effective and rapid. Based on their distribution coefficients, the affinity of activated carbon for metal ions follows the selectivity sequence expressed below. Au(CN)2->Pt(CN)42->Pd(CN)42->Ni(CN)42->Cu(CN)32-It was shown that adsorption rates of precious metals within the first 60min were very high, giving more than 90% extraction. Among the different adsorption parameters, nickel concentration had the most influential effect on the adsorption process followed by the adsorbent concentration. Adsorption of Ni was found to proceed at approximately the same rate as the precious metals, showing a recovery of approximately 90% in 2h. The kinetics of Cu adsorption were slower, with less than 30% being recovered within the 120min period. This suggests that the co-adsorption of Cu can be minimised by shortening the residence time. Adsorption of Fe was found to be less than 5%, while the recovery of Rh was negligibly small. The effect of thiocyanate ion concentration was not as important as the effect of free cyanide ion concentration but still had some influence. Under optimal (best) conditions, for a load cycle time of 2h and 10 discontinuous loading cycles, the loading capacity of the activated carbon for PMs was observed to be 0.64, 0.66, 0.17mg of Pt, Pd and Au/g of carbon, respectively.