Citation: Melo, E.; Hernández, M.-C.; Benavente, O.; Quezada, V. Selenium Dissolution from Decopperized Anode Slimes in ClO − /OH − Media. Minerals 2022, 12, 1228. https:// doi.org/10.3390/min12101228 Academic Editors: Kenneth N. Han and William Skinner Received: 5 September 2022 Accepted: 23 September 2022 Published: 28 September 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 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/). minerals Article Selenium Dissolution from Decopperized Anode Slimes in ClO - /OH - Media Evelyn Melo *, María-Cecilia Hernández , Oscar Benavente and Víctor Quezada Departamento de Ingeniería Metalúrgica y Minas, Universidad Católica del Norte, Antofagasta 1270709, Chile * Correspondence: emelo@ucn.cl Abstract: About 90% of selenium is obtained from treating copper anode slimes, which are a by- product of copper electrorefining. Selenium has been traditionally obtained by the pyrometallurgic treatment of anode slimes, which has been effective in recovery. However, in pyrometallurgical processes, there are increasingly strict environmental regulations. Hydrometallurgical treatments have been proposed to totally or partially replace conventional methods, some of which are in the developmental stage, while others are being used at the industrial scale. The selenium present in anode slimes is in the form of silver and copper selenides. This article proposes a hydrometallurgy alternative to recover selenium from decopperized anode slimes generated by a copper electrorefining plant in Chile by an alkaline-oxidizing leaching media (ClO − /OH − ). The Taguchi experimental design was used to assess the effects of temperature, reagent concentration, and pH over time. The results indicated that the optimal selenium dissolution of 90% was achieved at pH 11.5, 45 ◦ C, and 0.54 M of ClO − . According to the SEM/EDX characterization of the solid leaching residue, the undissolved percentage of selenium is due to the generation of a layer of AgCl around the selenium particles that hinders the effective diffusion of the reagent. Keywords: copper anode slimes; oxidative leaching; sodium hypochlorite; selenium; leaching 1. Introduction Selenium is found in most rocks and soils in the earth’s crust. It is an essential material with a wide range of applications in medicine and the production of food, semiconductors, sensors, and photochemical devices. Selenium is rarely found in its native state, 90% being obtained from copper anode slimes, which are a byproduct of the copper electrorefining process [1–6]. During electrolysis, copper dissolved from the anode is deposited on the cathode. However, other elements in copper anodes, such as gold, silver, arsenic, antimony, selenium, tellurium, bismuth, lead, iron, and nickel are released from the anode. Some of these are soluble and accumulate in the solution, while the insoluble elements sink to the bottom of the cell and form raw anode slime [7], which is collected periodically from the bottom of the electrolytic cell for subsequent treatment [8,9]. Table 1 shows the ranges of the chemical elements in typical anodic slimes from refineries such as the Canadian Copper Refinery; Outokumpu (Finland); Saganoseki (Japan); Balkhash Mining-Metallurgical Combine (Kazakhstan); Kovoguty Krompakhi (Slovakia, closed); La Caridad Copper Refinery (Mexico); Jinchuan (China); Baiyin (China); and Ronnskar (Sweden) [10–14]. The elemental contents vary widely, depending on the origin of the anode slime. Selenium is currently recovered from anode slimes by hydrometallurgical and py- rometallurgical processes [5,15,16]. Although the traditional pyrometallurgical method recovers selenium effectively, there are difficulties relating to high energy costs and increas- ingly strict environmental regulations [1,3,6,17,18]. Therefore, new alternatives have been Minerals 2022, 12, 1228. https://doi.org/10.3390/min12101228 https://www.mdpi.com/journal/minerals