Contents lists available at ScienceDirect Minerals Engineering journal homepage: www.elsevier.com/locate/mineng Gold passivation by sulfur species: A molecular picture Yasin Zia, Sima Mohammadnejad ⁎ , Mahmood Abdollahy Department of Mining, Tarbiat Modares University, P.O. Box 14115-143, Tehran, Iran ARTICLEINFO Keywords: Sulfur species Passivation Gold DFT Cyanide Thiosulfate ABSTRACT The role of sulfur species in the passivation of gold surface in cyanide and thiosulfate solutions was in- vestigated using density functional theory (DFT). The interaction energy between various sulfur complexes as well as deposited sulfur allotropes with the gold surface were computed and the contribution of each specie to the formation of a passivated layer were compared. For this purpose, a new parameter was introduced to explain and understand the tendency of each sulfur complex to interact with the gold surface from a molecular point of view. It was observed that the ratio of the hydration to the binding energy (H/B) of each sulfur specie can be well correlated with the calculated interaction energy with the gold surface. This parameter could be effectively used to predict the interaction of each specie with a substance, only based on the structural characteristics of the specie. It has been also demonstrated that the theoretically calculated energies of in- teractions of sulfur complexes with the gold surface are well correlated with experimentally reported data. The results showed that S, S 2- , polysulfides, tetrathionate, and sulfite species affect the gold leaching process directly through the passivation of the gold surface. Sulfate, dithionate, trithionate, and thiocyanate do not contribute to the passivation effect, although they indirectly affect the gold leaching process by consuming the oxygen and cyanide required to dissolve gold in the cyanidation process. The sulfur allotropes forming on the gold surface also passivate the gold surface and limit the access of the lixiviants required for the gold leaching process. The results could shed light on the mechanism of passivation of gold surface by sulfur species in gold leaching and this effect can be controlled by inhibiting the formation of main sulfur contributors in passivation process. 1. Introduction It is well known that sulfide minerals slow down the rate of the gold cyanidation by forming either a protective layer or surface products, or a combination of both on the gold surface (Rees and van Deventer, 2000). The addition of trace amounts of sulfides to the cyanide solution hinders the gold leaching process dramatically (Jeffrey and Breuer, 2000). The formation of a passive layer on the gold surface has been used to explain this effect (Wierse et al., 1978; Kondos et al., 1995). This explanation seems inconsistent with some other reports, as high concentrations of sulfur in the solution do not affect the gold leaching process (Lorenzen and van Deventer, 1992). On the other hand, due to the environmental concerns regarding cyanide, thiosulfate has been introduced as the most promising alter- native for the gold recovery (Muir and Aylmore, 2004). However, one challenge in using thiosulfate as the lixiviant is the degradation of thiosulfate and the formation of a passive layer on the gold surface (Jeffrey, 2008). It has also been demonstrated that thiosulfate leaching of gold is not effective in sulfide ores (Lorenzen and van Deventer, 1992). Various thiosulfate degradation products, such as sulfide ions, polymeric sulfurs, and polythionates have been identified as the main contributors to the passivated film forming on the gold surface (Baron, 2013). Many studies have been conducted on the passivation effect and the role played by sulfur ions in the gold leaching solutions. It is generally accepted that the partial dissolution of sulfide minerals or the decom- position of sulfur-containing leaching agents (thiosulfate) hinder the gold leaching process by forming a passive layer (Bas, 2015; Lorenzen and Van Deventer, 1992; Petruk, 2000). However, the mechanism of the passivation effect and the manner in which sulfides or their oxi- dation products interact with the leaching process of gold are still little understood. Furthermore, the contribution of different sulfur species and the mechanism of their interaction with the gold surface are un- clear. Here, using an ab-initio quantum modeling method, the mechanism of the gold surface passivation by sulfur species was studied. For this purpose, the interactions of various sulfur ions and deposited allotropes with the gold surface were investigated. The results explain the existing https://doi.org/10.1016/j.mineng.2019.02.009 Received 18 July 2018; Received in revised form 15 January 2019; Accepted 3 February 2019 ⁎ Corresponding author. E-mail address: sima.mnejad@modares.ac.ir (S. Mohammadnejad). Minerals Engineering 134 (2019) 215–221 0892-6875/ © 2019 Published by Elsevier Ltd. T