energies Article Plasma Technology for Phosphogypsum Treatment Imed Ghiloufi 1,2, *, Miqad S. Albishi 3 , Ahmed A. Alharbi 3 and Ibrahim A. AlShunaifi 3   Citation: Ghiloufi, I.; Albishi, M.S.; Alharbi, A.A.; AlShunaifi, I.A. Plasma Technology for Phosphogypsum Treatment. Energies 2021, 14, 5813. https://doi.org/10.3390/en14185813 Academic Editor: Marek Sciazko Received: 17 June 2021 Accepted: 19 August 2021 Published: 14 September 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 Department of Physics, College of Sciences, Imam Mohammad Ibn Saud Islamic University (IMSIU), Riyadh 11623, Saudi Arabia 2 Laboratory of Physics of Materials and Nanomaterials Applied at Environment (LaPhyMNE), Faculty of Sciences in Gabes, Gabes University, Gabes 6072, Tunisia 3 Energy and Water Research Institute, King Abdulaziz City for Science and Technology (KACST), Riyadh 11442, Saudi Arabia; malbishi@kacst.edu.sa (M.S.A.); ahalharbi@kacst.edu.sa (A.A.A.); ialshunaifi@kacst.edu.sa (I.A.A.) * Correspondence: ghiloufimed@yahoo.fr Abstract: The phosphate industry generates a large amount of waste called phosphogypsum (PG). Generally, this waste is discharged without any treatment, and it causes considerable environmental problems. Hence, the objective of this study is the treatment of phosphate waste using thermal plasma technology. First, the waste is characterized using different techniques, such as X-ray fluorescence (XRF), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and inductively coupled plasma (ICP). Such characterization shows that the waste contains different toxic elements, such as heavy metals, fluorine, chlorine, sulfur, and phosphorus. For this reason, a plasma reactor is used to separate toxic elements from metals, such as silicon, aluminum, and magnesium, with a pyrolysis/combustion plasma system. In this work, the influence of different parameters, such as time of treatment and plasma current, on the volatility of toxic elements is studied. The obtained results show that after 40 min of treatment and at a plasma current of 160 A, the phosphogypsum completely melts, and the most toxic elements, namely Pb, Cd, V, Cr, and As, are completely vaporized. Keywords: thermal plasma; characterization techniques; waste treatment; phosphogypsum 1. Introduction Phosphorus is an essential trace element (in the form of salts) for many living or- ganisms, as well as for plants when in the form of phosphate. For example, it is used in biological processes such as energy transfer, and it is also an important element of fertilizers in agriculture production [1–5]. The production of phosphate is increasing annually around the world due to rapid global development and the increase in energy demand. During the production of phosphoric acid, the phosphate industry pollutes the environment because it generates a large amount of waste called phosphogypsum (PG) [6–10]. Studies show that the production of PG around the world is about 250 million tons per year [11]. The waste treated in this study was collected from the phosphate industry in Gabes (Tunisia), which uses phosphate rocks (phosphorite). These phosphorites were used to produce phosphoric acid. To produce 1 ton of phosphoric acid, this process generates 5 tons of phosphogypsum (PG) [9]. Phosphogypsum contains several pollutants, such as heavy metals, fluorine, phosphorus, and radioelements [7,9,10,12–16]. All of the waste (PG, industrial sludge, and waste water) is directly discharged into the sea through a canal, with a considerable environmental impact [17,18]. PG causes huge environmental problems because most of this waste is discharged without any treatment, and the use of PG treatment methods is limited around the world. Among the methods of valorization and treatment of PG is the use of sodium fluoride (NaF) to transform the waste into sodium sulfate and pure calcium fluoride. This technique allows 15 million tons of PG to be transformed into sodium sulfate (12.383 million tons) Energies 2021, 14, 5813. https://doi.org/10.3390/en14185813 https://www.mdpi.com/journal/energies