  Citation: Samal, S.; Blanco, I. An Overview of Thermal Plasma Arc Systems for Treatment of Various Wastes in Recovery of Metals. Materials 2022, 15, 683. https:// doi.org/10.3390/ma15020683 Academic Editors: Igor Cretescu and Shinichi Tashiro Received: 12 October 2021 Accepted: 14 January 2022 Published: 17 January 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/). materials Review An Overview of Thermal Plasma Arc Systems for Treatment of Various Wastes in Recovery of Metals Sneha Samal 1, * and Ignazio Blanco 2 1 FZU-Institute of Physics of Czech Academy of Science, Prague 8, Na Slovance 1999/2, 18221 Prague, Czech Republic 2 Department of Civil Engineering and Architecture, University of Catania and UdR-Catania Consorzio INSTM, Viale Andrea Doria 6, 95125 Catania, Italy; iblanco@unict.it * Correspondence: samal@fzu.cz; Tel.: +420-266052361 Abstract: Thermal plasma systems are being used for the recovery of metals from complex waste and minerals. The latter contain multiphase metals in various forms that are extremely tedious to separate. Thermal plasma arc melts the waste and minerals for qualitative plasma products for powder industries. In this overview, we briefly report a description of the various thermal plasma systems and their uses in recovering metal from metal-containing materials in the form of waste or minerals. Various plasma arc systems, such as transferred, nontransferred, and extended arc, have enabled the development of an efficient and environmentally friendly way to recover valuable metals from industrial wastes such as red mud and minerals such as ilmenite. Keywords: thermal plasma; metal; recycling; waste; minerals 1. Introduction Metal-containing materials fall into various categories, from waste materials to min- erals. Metals are hidden resources in the waste materials generated from the day-to-day industrial production of human needs [1–3], such as metal-containing waste generated from electronic products, metal plants, minerals sectors, and metal-containing soils and minerals [4,5]. To utilize this waste in a valuable way, both hydro- and pyrometallurgical treatments have been proposed in industrial sectors. However, hydrometallurgical treat- ments create acidic and alkaline residual threats to the environment; as a result, the focus has been shifted more significantly toward pyrometallurgical routes. Thermal plasma is an efficient way to treat metal-containing waste materials. Plasma, generated in a highly ionized state, operates with a high energy density at high temperature. Such a system could enable the degradation of organic compounds from electronic waste materials into elemen- tal ions and atoms, with end-products including gases [6–8]. Plasma waste treatment falls into various categories, such as plasma gasification and vitrification of hazardous wastes into slags. The high temperatures of the plasma melt organic and inorganic pollutants into reduced slag. The slag generated by plasma technology is very stable, less harmful, and easy to handle for further treatment. However, volatile metals do not remain in slag during plasma melting of waste material. In plasma reduction, high temperature acts as a boon for soil-containing metals in multiphase forms; for example, waste from the bauxite industry, known as red mud, and ilmenite. The different recovery methods of the valuable metals using various processes by thermal plasma are summarized in Table 1 [9–15]. Plasma is classified into two broad categories: high and low temperature. The first cat- egory comprises a completely ionized system in which the temperature reaches 10 6 –10 7 ◦ C and above, whilst low-temperature plasma exhibits partially ionized plasma with tem- peratures falling in the range of 10 1 –10 5 ◦ C. Low-temperature plasma is divided in two categories, thermal and nonthermal, based on the thermodynamic equilibrium temperature. Thermal plasma combines arc plasma, combustion plasma, and high-frequency plasma Materials 2022, 15, 683. https://doi.org/10.3390/ma15020683 https://www.mdpi.com/journal/materials