minerals Review Metal Sulfide Precipitation: Recent Breakthroughs and Future Outlooks Humberto Estay 1, * , Lorena Barros 1 and Elizabeth Troncoso 2,3   Citation: Estay, H.; Barros, L.; Troncoso, E. Metal Sulfide Precipitation: Recent Breakthroughs and Future Outlooks. Minerals 2021, 11, 1385. https://doi.org/10.3390/ min11121385 Academic Editor: Yul Roh Received: 15 November 2021 Accepted: 6 December 2021 Published: 8 December 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 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/). 1 Advanced Mining Technology Center (AMTC), University of Chile, Santiago 8370451, Chile; lorena.barros@amtc.cl 2 Department of Chemistry, Universidad Tecnológica Metropolitana, Santiago 7800003, Chile; elizabeth.troncoso@utem.cl 3 Programa Institucional de Fomento a la Investigación, Desarrollo e Innovación (PIDi), Universidad Tecnológica Metropolitana, Santiago 8940577, Chile * Correspondence: humberto.estay@amtc.cl Abstract: The interest in metal sulfide precipitation has recently increased given its capacity to effi- ciently recover several metals and metalloids from different aqueous sources, including wastewaters and hydrometallurgical solutions. This article reviews recent studies about metal sulfide precipi- tation, considering that the most relevant review article on the topic was published in 2010. Thus, our review emphasizes and focuses on the overall process and its main unit operations. This study follows the flow diagram definition, discussing the recent progress in the application of this process on different aqueous matrices to recover/remove diverse metals/metalloids from them, in addition to kinetic reaction and reactor types, different sulfide sources, precipitate behavior, improvements in solid–liquid separation, and future perspectives. The features included in this review are: operational conditions in terms of pH and Eh to perform a selective recovery of different metals contained in an aqueous source, the aggregation/colloidal behavior of precipitates, new materials for controlling sul- fide release, and novel solid–liquid separation processes based on membrane filtration. It is therefore relevant that the direct production of nanoparticles (Nps) from this method could potentially become a future research approach with important implications on unit operations, which could possibly expand to several applications. Keywords: metal sulfide precipitation; critical metals; precipitates; AMD; metal recovery; wastewa- ter treatment 1. Introduction Metal sulfide precipitation is studied because it is a process that is applied to recover or remove metals and metalloids from industrial effluents or hydrometallurgical leachates. This process uses a sulfide source, typically H 2 S, Na 2 S, CaS, (NH 4 ) 2 S, or NaHS to react with a cation contained in an aqueous solution, according to the reaction mechanism described in Equations (1)–(4) [1]. Metal sulfides have low solubility (Table 1) with respect to other precipitates such as hydroxides [2]. This characteristic is very attractive for environmental purposes, particularly when a dangerous heavy metal is removed and disposed of safely, because it is less likely to be leached in a wide pH range. In addition, a high pK sp value translates into a favorable tendency for Equations (3)–(4) to form products, showing a high conversion and, consequently, an efficient precipitation process. Metal sulfide precipitation is naturally appealing for research, which can include the removal of potentially toxic elements (metals and metalloids) from industrial effluents, such as acid mine drainage (AMD) or copper smelter wastewater, and for recovering valuable metals from leachates of hydrometallurgical plants treating ores, wastes, or Minerals 2021, 11, 1385. https://doi.org/10.3390/min11121385 https://www.mdpi.com/journal/minerals