Citation: Mhandu, T.J.; Park, I.; Jeon, S.; Hamatsu, S.; Elakneswaran, Y.; Ito, M.; Hiroyoshi, N. A Pretreatment of Refractory Gold Ores Containing Sulfide Minerals to Improve Gold Leaching by Ammonium Thiosulfate: A Model Experiment Using Gold Powder and Arsenic-Bearing Sulfide Minerals. Metals 2023, 13, 1357. https://doi.org/10.3390/ met13081357 Academic Editor: Jean François Blais Received: 29 June 2023 Revised: 24 July 2023 Accepted: 26 July 2023 Published: 28 July 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). metals Article A Pretreatment of Refractory Gold Ores Containing Sulfide Minerals to Improve Gold Leaching by Ammonium Thiosulfate: A Model Experiment Using Gold Powder and Arsenic-Bearing Sulfide Minerals Takunda Joseph Mhandu 1, *, Ilhwan Park 2, * , Sanghee Jeon 3 , Sohta Hamatsu 1 , Yogarajah Elakneswaran 2 , Mayumi Ito 2 and Naoki Hiroyoshi 2 1 Division of Sustainable Resource Engineering, Graduate School of Engineering, Hokkaido University, Sapporo 060-8628, Japan; sohta1415@gmail.com 2 Division of Sustainable Resource Engineering, Faculty of Engineering, Hokkaido University, Sapporo 060-8628, Japan; elakneswaran@eng.hokudai.ac.jp (Y.E.); itomayu@eng.hokudai.ac.jp (M.I.); hiroyosi@eng.hokudai.ac.jp (N.H.) 3 Department of Earth Resources Engineering and Environmental Science, Faculty of International Resources Science, Akita University, Akita 010-0852, Japan; jeon@gipc.akita-u.ac.jp * Correspondence: takundajoseph.mhandu.i9@elms.hokudai.ac.jp (T.J.M.); i-park@eng.hokudai.ac.jp (I.P.) Abstract: The use of thiosulfate to extract gold from refractory ores is promising because of its non-toxicity and high selectivity. Sulfide minerals (i.e., pyrite, arsenopyrite, chalcopyrite), major gold carriers in refractory gold ores, however, hinder gold extraction due to the high consumption of a lixiviant. In this study, a new method to improve gold extraction from sulfide bearing gold ores is proposed based on the model experiments using a mixture of gold powder and arsenopyrite-bearing sulfide (HAsBS) ore. The effects of HAsBS ore on gold leaching in ammonium thiosulfate solutions were investigated, and it was found that gold extraction in the presence of HAsBS ore was suppressed because of the unwanted decomposition of thiosulfate on the surface of sulfide minerals. To improve gold extraction in the presence of the sulfide minerals, this study investigated the effects of the pretreatment of HAsBS ore using ammonium solutions containing cupric ions and confirmed that HAsBS ore was oxidized in the pretreatment and its surface was covered by the oxidation products. As a result, thiosulfate consumption was minimized in the subsequent gold leaching step using ammonium thiosulfate, resulting in an improvement in gold extraction from 10% to 79%. Keywords: gold; arsenopyrite; ammonium thiosulfate; cyanide; leaching; refractory ores 1. Introduction Gold (Au) is an important metal in the livelihood of humans and is currently used for jewelry (53%), financial purposes (i.e., investment and central banks) (39%), and electric and electronic equipment (6%), and the remaining 2% is used for other applications such as medical and pharmaceutical products [1–3]. This demand for gold can be sustained by discovering new high-grade gold deposits, but finding the high-grade deposits has become difficult. Wilson et al. (2022) reported that there has been a decrease in the gold grades in global gold deposits from 10 g/t in the 1970s [4] to less than 1.5 g/t [5] currently, of which the deposits being discovered have more unwanted materials that are difficult to process. Hence, the gold mining industry has been driven by the demand for gold to find ways to process lower grade and refractory gold deposits over the past 50 years [6,7]. In these refractory ores, gold commonly occurs as free particles of gold and electrum, with the inclusion of these phases in the sulfide mineral matrix [8]. The encapsulation of gold in sulfide matrices as well as its association with other minerals (e.g., arsenopyrite (FeAsS), chalcopyrite (CuFeS 2 ), and pyrite (FeS 2 )) renders these gold ores refractory. For the past Metals 2023, 13, 1357. https://doi.org/10.3390/met13081357 https://www.mdpi.com/journal/metals