The extraction of Pt, Pd and Au from an alkaline cyanide simulated heap leachate by granular activated carbon C.N. Mpinga a , S.M. Bradshaw a,⇑ , G. Akdogan a , C.A. Snyders a , J.J. Eksteen b a Department of Processing Engineering, Stellenbosch University, Private Bag X1, Matieland 7602, South Africa b Department of Metallurgical Engineering, Western Australian School of Mines, Curtin University, GPO Box U1987, Perth, WA 6845, Australia article info Article history: Received 18 June 2013 Accepted 8 September 2013 Available online 3 October 2013 Keywords: Precious metals Adsorption Granular activated carbon (GAC) Platreef 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 acti- vated 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. AuðCNÞ  2 > PtðCNÞ 2 4 > PdðCNÞ 2 4 > NiðCNÞ 2 4 > CuðCNÞ 2 3 It was shown that adsorption rates of precious metals within the first 60 min 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 2 h. The kinetics of Cu adsorption were slower, with less than 30% being recovered within the 120 min 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 2 h and 10 discontinuous loading cycles, the loading capacity of the activated carbon for PMs was observed to be 0.64, 0.66, 0.17 mg of Pt, Pd and Au/g of carbon, respectively. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Approximately 75% of the world’s platinum production and 35% of the world’s palladium production in 2011 originated from the Bushveld Igneous Complex in South Africa. The most important platinum group metal (PGM) reefs mined are the Merensky, Upper Group Two (UG2) and Platreef. Ore grades range from 3 to 8 g PGM/t, with associated nickel and copper in the 0.1% to 0.2% range present mainly as sulphides (Kyriakakis, 2005). The platinum group metals were initially recovered from high-grade concentrate by the traditional matte-smelting technique. The pyrometallurgi- cal route has significant environmental impacts. The concentrate- smelt-refine route results in approximately 144 kg CO 2 equivalents and 0.45 kg SO 2 per ton of ore milled, and consumes roughly 753 m 3 H 2 O(AngloAmerican, 2011). However high-grade precious metal reserves have been dimin- ished and the remaining reserves contain low-grade ores associ- ated with high chromite grades (in the case of UG2) or high pyrrhotite content (in the case of Platreef), which invariably leads to high smelting costs (low-grade) and smelter integrity risks (due to the chromite). Hence, many studies have been recently focused on the extraction of precious metals from low-grade ore/concen- trate due to both increasing industrial need for these metals and their limited sources (see, e.g. Liddell and Adams, 2012). Most re- cently, a low-cost hydrometallurgical process, consisting of a heap bioleach process to first extract the base metals (BMs), followed by a caustic rinse of the residue material and a heap cyanidation 0892-6875/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.mineng.2013.09.001 ⇑ Corresponding author. Tel.: +27 21 808 4493; fax: +27 21 808 2059. E-mail address: smb@sun.ac.za (S.M. Bradshaw). Minerals Engineering 55 (2014) 11–17 Contents lists available at ScienceDirect Minerals Engineering journal homepage: www.elsevier.com/locate/mineng