Determination of the equilibrium and film diffusion constants of the platinum cyanide anions during the elution from activated carbon C.A. Snyders a,⇑ , S.M. Bradshaw a , G. Akdogan a , J.J. Eksteen b a Department of Processing Engineering, Stellenbosch University, Private Bag X1, Matieland 7602, South Africa b Department of Metallurgical Engineering and Mining Engineering, Western Australian School of Mines, Curtin University, GPO Box U1987, Perth 6845, Western Australia article info Article history: Received 14 April 2015 Revised 29 June 2015 Accepted 1 July 2015 Keywords: Platinum Elution Activated carbon Model Cyanide abstract Various leach approaches have been developed for the alkaline cyanide leaching of platinum group metals (PGMs) from ores, concentrates and from secondary wastes such as spent automobile catalytic converters. Recently, the adsorption and elution of PGM cyanide complexes onto activated carbon has also been demonstrated and is similar to the activated carbon process for gold. The stripping of adsorbed tetracyanoplatinate(II) ([Pt(CN) 4 ] 2 ) from activated carbon consists of a two-step batch process which involves the pre-treatment of the metal-loaded activated carbon with a relatively strong sodium cyanide and sodium hydroxide solution prior to the elution step with de-ionized water at 80 °C. This paper mod- els the elution and describes the mechanism of platinum elution from activated carbon. The equilibrium and film transfer constants were determined by fitting the model to experimental results and then val- idated by comparing to additional experiments. It was found that the rate of release of the platinum ions is governed by the amount of platinum and sodium on the activated carbon and the concentrations of these ions in the bulk of the liquid. It was found that the adsorption-elution of the single component sys- tem can be adequately described by using a modified Freundlich isotherm and a mass transfer diffusion equation. The rate of platinum elution interchangeably depends on the equilibrium of the Pt ions at the carbon-liquid interface and the mass transfer of these Pt ions from the interface to the bulk liquid. As both of these rate limiting factors were found to depend on the sodium concentration, the dominant platinum elution rate limiting factor shifts as the sodium concentrations change as the elution progresses. The elu- tion process could be modelled accurately and it is shown that the second step (elution) of the two-step batch process (pre-treatment and elution) can be broken down into a sequence of four consecutive stages. The benefits of fundamentally understanding this process can ultimately lead to improved elution, better process control, shorter elution times, smaller elution columns or assist in the development of a continues elution process. Ó 2015 Elsevier Ltd. All rights reserved. 1. Introduction Numerous direct leach technologies, including a number of cyanide-based leach processes has been reviewed by Mpinga et al. (2015). The leaching of Platinum Group Metals (PGMs) with cyanide, in a manner similar to that used for gold extraction, has been investigated as an alternative PGM process option that is less energy intensive and better suited to the extraction of complex, lower grade ores (Mwase et al., 2012a,b, 2014a,b; Eksteen et al., 2012, 2014). After showing that direct cementation using Merrill-Crowe technology was unsuitable for PGMs (Mpinga et al., 2014b), carbon adsorption and consequently elution (Snyders et al., 2012, 2013, 2014a,b; Mpinga et al., 2014a) and ion exchange (Schoeman et al., 2012) were considered as technolo- gies for recovery and concentration upgrading from solution. Two of the major steps of the process, i.e. the adsorption of the metal cyanide complexes onto activated carbon and the elution of these complexes into a more concentrated stream, were investigated and the effects of operating variables on process performance were determined (Mpinga et al., 2014a; Snyders et al., 2013). Successful elution of the metal complexes from the adsorbent into a sufficiently concentrated solution is critical in the development of this process and it was indeed only following the development of a suitable elution process that the now universal carbon in pulp process for gold recovery became commercially viable. In a previous paper by the authors (Snyders et al., 2013) adsorp- tion experiments were performed on a pregnant alkaline leach http://dx.doi.org/10.1016/j.mineng.2015.07.001 0892-6875/Ó 2015 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. E-mail address: nsnyders@sun.ac.za (C.A. Snyders). Minerals Engineering 80 (2015) 57–68 Contents lists available at ScienceDirect Minerals Engineering journal homepage: www.elsevier.com/locate/mineng