Recovery of copper and magnetite from copper slag using concentrated solar power (CSP)

metals Article Recovery of Copper and Magnetite from Copper Slag Using Concentrated Solar Power (CSP) Daniel Fernández-González 1,* , Janusz Prazuch 2,Íñigo Ruiz-Bustinza 3, Carmen González-Gasca 4, Cristian Gómez-Rodríguez 5and Luis Felipe Verdeja 6   Citation: Fernández-González, D.; Prazuch, J.; Ruiz-Bustinza, Í.; González-Gasca, C.; Gómez-Rodríguez, C.; Verdeja, L.F. Recovery of Copper and Magnetite from Copper Slag Using Concentrated Solar Power (CSP). Metals 2021,11, 1032. https:// doi.org/10.3390/met11071032 Academic Editor: Fernando Castro Received: 31 May 2021 Accepted: 24 June 2021 Published: 27 June 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. 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/). 1Nanomaterials and Nanotechnology Research Center (CINN-CSIC), Universidad de Oviedo (UO), Principado de Asturias (PA), Avda. de la Vega, 4-6, 33940 El Entrego, Spain 2Department of Physical Chemistry and Modelling, Faculty of Materials Science and Ceramics, AGH University of Science and Technology, 30-059 Krakow, Poland; [email protected] 3 Departamento de Ingeniería Geológica y Minera, Escuela Técnica Superior de Ingenieros de Minas y Energía, Universidad Politécnica de Madrid, 28003 Madrid, Spain; [email protected] 4Vice-Rector’s Office, Universidad Internacional de Valencia, 46002 Valencia, Spain; [email protected] 5Departamento de Mecánica, Facultad de Ingeniería, Campus Coatzacoalcos, Universidad Veracruzana, Av. Universidad km 7.5 Col. Santa Isabel, Coatzacoalcos 6535, Veracruz, Mexico; [email protected] 6Department of Materials Science and Metallurgical Engineering, Oviedo School of Mines, Energy and Materials, University of Oviedo, 33004 Oviedo/Uviéu, Spain; [email protected] *Correspondence: [email protected] Abstract: On the one hand, copper slag is nowadays a waste in copper pyrometallurgy despite the significant quantities of iron (>40 wt. %) and copper (1 to 2 wt. %). On the other hand, solar energy, when properly concentrated, offers great potential in high-temperature processes. Therefore, concentrated solar power (CSP) could be used in the treatment of copper slag to transform fayalite into magnetite and copper sulfides and oxides into copper nodules. This is the objective of this paper. The results show that fayalite was partially decomposed into magnetite and silica. Moreover, copper nodules (65–85 wt. % Cu) were identified in the treated samples, while the initial slag, analyzed by X-ray diffraction, X-ray fluorescence, and SEM-EDX, did not show the presence of metallic copper. Finally, the treated copper slag was crushed and grinded down to 40 µ m, and two fractions were obtained by magnetic separation. The magnetic fraction (85%) was mainly comprised of magnetite, while the non-magnetic fraction (15%) had 5–10 wt. % Cu. Considering the experimental results, 7.5–18 kg Cu/t slag might be recovered from the slag. A preliminary economic analysis, considering the current copper price, indicates that only the recovery of copper could represent a significant economic benefit (>30 € /t slag). Therefore, CSP might be a potential candidate for the treatment of copper slag to recover copper and iron. Keywords: concentrated solar power (CSP); copper slag; copper; environment; sustainable metallurgy; solar energy 1. Introduction Copper is one of the most mined metals on the Earth due to the applications of this metal in different fields. However, a lot of tailing material, currently unprocessed, is generated in the process, and provides a huge opportunity to use concentrated solar power (CSP). The pyrometallurgical technique is the most important to produce copper, and the smelting conversion process is the most widely used within this route [ 1 ]. Thus, 80% of the copper is currently produced by the concentration, smelting, and refining of sulfide ores [ 2 ], which include chalcopyrite (CuFeS 2 ), bornite (Cu 5 FeS 4 ), and chalcocite (Cu 2 S). Different products are generated in the fusion conversion process (Figure 1). These include matte, which is heavy and contains most of the copper as sulfide (it is later treated in the Metals 2021,11, 1032. https://doi.org/10.3390/met11071032 https://www.mdpi.com/journal/metals Metals 2021,11, 1032 2 of 18 converter to obtain blister copper, and finally metallic copper is produced after fire refining and electrowinning) and slag, which contains most of the iron as fayalite. Metals 2021, 11, x FOR PEER REVIEW 2 of 18 include matte, which is heavy and contains most of the copper as sulfide (it is later treated in the converter to obtain blister copper, and finally metallic copper is produced after fire refining and electrowinning) and slag, which contains most of the iron as fayalite. Figure 1. Flow diagram of the smelting conversion process of copper. Two slags are produced in the smelting conversion process: first, in the smelting furnace, and second in the converter. However, these slags contain (apart from fayalite and magnetite, which are the main phases) significant quantities of copper (sulfide (matte) dragged or trapped by the slag, or oxide associated with other oxides of the slag [1,3]). The slags are reprocessed in the smelting furnace or in the slag cleaning furnace (electric furnaces, where carbon and pyrite are used as additives) to recover some of the copper. Finally, the final slag is disposed of in a controlled landfill when the copper concentration is approximately in the range 1 to 2 wt. % Cu. Copper slags might be considered a residue/by-product with great potential due to both the copper (1–2 wt. % Cu) and iron (>40 wt. % [4–10]) contents as a secondary resource for metal recovery [11]. A total of 20 Mt of primary copper are produced worldwide, and this involves 45 Mt of slag generated in the process (2.2–3 tons of slag/ton copper [2,12,13]), which would represent >20 Mt Fe and 0.5–1 Mt Cu yearly sent to controlled landfills. Therefore, copper slag represents an environmental impact [4]: risks of heavy metals lixiviation, a visual impact, and sometimes occupation of cultivable areas. Researchers have tried to find a use for copper slags, but a massive industrial utilization has still not been found [14]. Copper slags have been used as abrasives (polishing and cleaning) for metallic structures [15,16] and mainly in the building industry: concrete manufactured with copper slag [5]; copper slags as fine particles in concrete manufacturing [13]; copper slag as a replacement for the sand in cements [9]; copper slag as a filler in glass– epoxy composites [17]; and copper slag as a construction material in bituminous pavements [18]. Research has also focused on the recovery of iron from the slag: by using coke as reductant of the copper oxide and the magnetite [19]; by modifying the molten slag with the purpose of promoting the mineralization of recoverable mineral phases and inducing the growth of the mineral phases [20]; by using a method based on coal, Direct Reduction Iron (DRI), and magnetic separation [21]; by means of a process based on Figure 1. Flow diagram of the smelting conversion process of copper. Two slags are produced in the smelting conversion process: first, in the smelting furnace, and second in the converter. However, these slags contain (apart from fayalite and magnetite, which are the main phases) significant quantities of copper (sulfide (matte) dragged or trapped by the slag, or oxide associated with other oxides of the slag [ 1 , 3 ]). The slags are reprocessed in the smelting furnace or in the slag cleaning furnace (electric furnaces, where carbon and pyrite are used as additives) to recover some of the copper. Finally, the final slag is disposed of in a controlled landfill when the copper concentration is approximately in the range 1 to 2 wt. % Cu. Copper slags might be considered a residue/by-product with great potential due to both the copper (1–2 wt. % Cu) and iron (>40 wt. % [ 4 – 10 ]) contents as a secondary resource for metal recovery [ 11 ]. A total of 20 Mt of primary copper are produced worldwide, and this involves 45 Mt of slag generated in the process (2.2–3 tons of slag/ton copper [ 2 , 12 , 13 ]), which would represent >20 Mt Fe and 0.5–1 Mt Cu yearly sent to controlled landfills. Therefore, copper slag represents an environmental impact [ 4 ]: risks of heavy metals lixiviation, a visual impact, and sometimes occupation of cultivable areas. Researchers have tried to find a use for copper slags, but a massive industrial utilization has still not been found [ 14 ]. Copper slags have been used as abrasives (polishing and cleaning) for metallic structures [ 15 , 16 ] and mainly in the building industry: concrete manufactured with copper slag [ 5 ]; copper slags as fine particles in concrete manufacturing [ 13 ]; copper slag as a replacement for the sand in cements [ 9 ]; copper slag as a filler in glass–epoxy composites [ 17 ]; and copper slag as a construction material in bituminous pavements [ 18 ]. Research has also focused on the recovery of iron from the slag: by using coke as reductant of the copper oxide and the magnetite [ 19 ]; by modifying the molten slag with the purpose of promoting the mineralization of recoverable mineral phases and inducing the growth of the mineral phases [ 20 ]; by using a method based on coal, Direct Reduction Iron (DRI), and magnetic separation [ 21 ]; by means of a process based on aluminothermic reduction [ 22 ]; by reduction in an electric furnace with the objective of obtaining a Cu-Pb-Fe alloy [ 23 ]; by carbothermic reduction to transform the copper slag into pig iron and glassy material [ 24 ]; Metals 2021,11, 1032 3 of 18 by irradiation with a microwave as a support of the carbothermal method [ 25 ]; or by reduction with coke powders and magnetic separation [26]. Solar energy has been widely used in materials science and metallurgy [ 27 ]. The main types of research in the field of metallurgy carried out with concentrated solar power (CSP) are listed in Table 1. Table 1. Main types of research in the field of metallurgy and mineral processing using CSP (abbreviations: OSF, Odeillo Solar Furnace; PSI, Paul Scherrer Institute; WIS, Weizmann Institute of Science; UKR, Institute for Problems of Materials Science of NAS of Ukraine; PSA, Plataforma Solar de Almería). Material Process Temperature Installation Researcher Si Dissociation of Si3N4by carbothermal reduction of SiO 2 under an N2atmosphere >1400 ◦C OSF Jean P. Murray, Gilles Flamant, Carolyn J. Roos [28] Si Carbothermic reduction of SiO 2 under vacuum conditions at a high temperature 1725–2000 ◦C PSI Peter G. Loutzenhiser, Ozan Tuerk, Aldo Steinfeld [29] Al Production of aluminum via carbothermal reduction >2000 ◦C PSI and OSF Jean P. Murray [30–32] Al Production of aluminum using both electricity and heat generated using solar energy ≃1000 ◦C UKR Y. M. Lytvynenko [33] Zn Production of zinc via CSP to be used in water and carbon dioxide splitting (scaled up to demonstration plant scale [ 34 ]) >1750 ◦CMainly at the PSI, OSF, and WIS E. A. Fletcher, R. D. Palumbo, T. Osinga, M. Epstein A. Steinfeld, C. Wieckert, L. Schunck, W. Villasmil, E. Koepft, and others [35–45] Fe, Mn, Cd Treatment of different materials containing iron >1100 ◦CMainly at the PSI, PSA, OSF, and WIS F. Sibieude, A. Steinfeld, E. A. Fletcher, I. Ruiz-Bustinza, J. Mochón and others [46–49] Concentrated solar power (CSP) has also been used in materials processing and nonmetallic materials. The use of CSP in these latter fields can be found in Fernández-González et al. [ 27 ]. The application of CSP in materials science and metallurgy has been widely investigated by our research group. The following fields can be mentioned: synthesis of calcium aluminates [ 50 ], iron metallurgy [ 46 , 51 , 52 ], the treatment of Basic Oxygen Furnace (BOF) slag [ 53 ], the production of silicomanganese [ 54 ], and transformations in the Ca-Si-O system [55]. The utilization of concentrated solar power (CSP) to treat copper slag is proposed in this paper. Since most of the copper is produced in mines located in the west of the American continent (from Arizona (United States) to Chile), where high values of solar radiation are measured, the use of CSP might have great potential to recover copper and iron from slags. Figure 2shows a flow-sheet of the process to examine Cu tailings using CSP. The main results are schematically indicated. Therefore, the aims of the research are: to transform the fayalite into magnetite, as iron could be collected from the slag as magnetite using magnetic methods; and to transform copper oxide and copper matte (the occluded matte in the slag) into metallic copper and concentrate it. Metals 2021,11, 1032 4 of 18 Metals 2021, 11, x FOR PEER REVIEW 4 of 18 Figure 2. Flow-sheet for the process to examine Cu tailings using CSP. 2. Materials and Methods 2.1. Materials Original copper slag from the slag cleaning furnace was used in the experiments (Figure 1). It was homogenized, milled, and screened into four granulometric fractions: >4 mm, 2–4 mm, 1–2 mm, and <1 mm, and only the last three were considered to facilitate chemical reactions. They were mixed in equal parts (33.3%). The chemical composition of the slag (Table 2) was obtained by X-ray fluorescence. X-ray fluorescence measurements were performed with a wavelength dispersive X-ray fluorescence spectrometer (Axios, PANalytical) equipped with a Rh-anode X-ray tube with a maximum power of 4 kW. All samples were measured in a vacuum with a 15–50 eV energy resolution. For quantitative analysis of the spectra, the PANalytical standardless analysis package Omnian was used. Table 2. Elemental analysis of the copper slag (wt. %) determined by X-ray fluorescence. Fe O Si Al Cu Ca Na S Others 42.82 36.15 10.36 3.24 1.84 1.76 0.99 0.52 2.32 Original copper slag was studied by X-ray diffraction of powders. X-ray diffraction measurements were conducted with an Empyrean PANalytical diffractometer using kα1 and kα2 radiation from a Cu anode. All measurements were performed with the Bragg– Brentano setup at room temperature with a 0.006° step size in the 5–90° 2θ scanning range and 145 s of measurement time for each step. Data analysis and the peak profile fitting were carried out using XPowder 01.02 (Database PDF2 (70 to 0.94)). The quantitative analysis of the crystalline phases was also performed using the software XPowder12 Ver. 01.02. The main crystalline phase in the slag was fayalite (Fe2SiO4), 85.80 ± 1.30% in the quantitative analysis. Other important crystalline phases were magnetite (Fe3O4) and copper–iron oxide (cuprospinel, CuFe2O4, with Cu2+ and Fe3+), 7.90 ± 1.60% and 6.20 ± 1.80%, respectively, in the quantitative analysis. 2.2. Experimental Procedure Tests were performed in a vertical axis 1.5 kW solar furnace located in Font RomeuOdeillo-Via (PROMES-CNRS laboratory (Procédés, Matériaux et Énergie Solaire—Centre National de la Recherche Scientifique), Font Romeu-Odeillo-Via, France). The operation Figure 2. Flow-sheet for the process to examine Cu tailings using CSP. 2. Materials and Methods 2.1. Materials Original copper slag from the slag cleaning furnace was used in the experiments (Figure 1). It was homogenized, milled, and screened into four granulometric fractions: >4 mm, 2–4 mm, 1–2 mm, and <1 mm, and only the last three were considered to facilitate chemical reactions. They were mixed in equal parts (33.3%). The chemical composition of the slag (Table 2) was obtained by X-ray fluorescence. X-ray fluorescence measurements were performed with a wavelength dispersive X-ray fluorescence spectrometer (Axios, PANalytical) equipped with a Rh-anode X-ray tube with a maximum power of 4 kW. All samples were measured in a vacuum with a 15–50 eV energy resolution. For quantitative analysis of the spectra, the PANalytical standardless analysis package Omnian was used. Table 2. Elemental analysis of the copper slag (wt. %) determined by X-ray fluorescence. Fe O Si Al Cu Ca Na S Others 42.82 36.15 10.36 3.24 1.84 1.76 0.99 0.52 2.32 Original copper slag was studied by X-ray diffraction of powders. X-ray diffraction measurements were conducted with an Empyrean PANalytical diffractometer using k α 1 and k α 2 radiation from a Cu anode. All measurements were performed with the Bragg– Brentano setup at room temperature with a 0.006 ◦ step size in the 5–90 ◦ 2 θ scanning range and 145 s of measurement time for each step. Data analysis and the peak profile fitting were carried out using XPowder 01.02 (Database PDF2 (70 to 0.94)). The quantitative analysis of the crystalline phases was also performed using the software XPowder12 Ver. 01.02. The main crystalline phase in the slag was fayalite (Fe 2 SiO 4 ), 85.80 ± 1.30% in the quantitative analysis. Other important crystalline phases were magnetite (Fe 3 O 4 ) and copper–iron oxide (cuprospinel, CuFe 2 O 4 , with Cu 2+ and Fe 3+ ), 7.90 ± 1.60% and 6.20 ± 1.80%, respectively, in the quantitative analysis. 2.2. Experimental Procedure Tests were performed in a vertical axis 1.5 kW solar furnace located in Font RomeuOdeillo-Via (PROMES-CNRS laboratory (Procédés, Matériaux et Énergie Solaire—Centre National de la Recherche Scientifique), Font Romeu-Odeillo-Via, France). The operation of the solar furnace is based on making solar radiation converge on a small surface (12–15 mm Metals 2021,11, 1032 5 of 18 in diameter), called the focal point, using optical systems (or mirrors). In the case of this furnace (Figure 3), a heliostat directs sun radiation towards a parabolic concentrator (2.0 m in diameter), which makes radiation converge at the focal point, where the experimental device is located. Metals 2021, 11, x FOR PEER REVIEW 5 of 18 of the solar furnace is based on making solar radiation converge on a small surface (12–15 mm in diameter), called the focal point, using optical systems (or mirrors). In the case of this furnace (Figure 3), a heliostat directs sun radiation towards a parabolic concentrator (2.0 m in diameter), which makes radiation converge at the focal point, where the experimental device is located. Figure 3. Scheme of the equipment used in the experiments. This solar furnace allows for a maximum concentration of 15,000 times the incident radiation. It is possible to control the value of the power applied to the sample by a shutter (Figure 3). This way, the value of the power can be calculated with Equation (1). Power (W) = Incident radiation (W/m2) × 1.5 × ShOp/100, (1) where ShOp is the shutter opening value, which is 0 when the shutter is completely closed and 100 when it is totally open. Fast heating and cooling rates are usual in solar furnaces. This allows us to obtain metastable phases even at room temperature. The temperature was controlled by a type K thermocouple (cromel-alumel thermocouple). It was located at half height outside of the crucible. The maximum temperature inside of the crucible (in the range 1700–1900 °C, calculated by Finite Element Method (FEM)-based software) is reached when the temperature stabilizes at approximately 600 ± 30 °C in this thermocouple. The sample was held at the maximum temperature for 10 min. Both the temperature and time were enough to perform the experiments according to the thermodynamic calculations (theoretical) and, thus, achieve the decomposition of the fayalite into magnetite and silica, and the formation of metallic copper nodules from copper oxide (and from the occluded matte) [56]. From the thermodynamics point of view, the decomposition of the fayalite is possible in an oxidizing environment, while obtaining metallic copper is favorable under reductant conditions [56]. Both processes compete between them and it is not possible to complete them in a single step. Reductant agents were not used during the experiments. However, the addition of reductant agents would favor the obtaining of Figure 3. Scheme of the equipment used in the experiments. This solar furnace allows for a maximum concentration of 15,000 times the incident radiation. It is possible to control the value of the power applied to the sample by a shutter (Figure 3). This way, the value of the power can be calculated with Equation (1). Power (W) = Incident radiation (W/m2)×1.5 ×ShOp/100, (1) where ShOp is the shutter opening value, which is 0 when the shutter is completely closed and 100 when it is totally open. Fast heating and cooling rates are usual in solar furnaces. This allows us to obtain metastable phases even at room temperature. The temperature was controlled by a type K thermocouple (cromel-alumel thermocouple). It was located at half height outside of the crucible. The maximum temperature inside of the crucible (in the range 1700–1900 ◦ C, calculated by Finite Element Method (FEM)-based software) is reached when the temperature stabilizes at approximately 600 ± 30 ◦ C in this thermocouple. The sample was held at the maximum temperature for 10 min. Both the temperature and time were enough to perform the experiments according to the thermodynamic calculations (theoretical) and, thus, achieve the decomposition of the fayalite into magnetite and silica, and the formation of metallic copper nodules from copper oxide (and from the occluded matte) [ 56 ]. From the thermodynamics point of view, the decomposition of the fayalite is possible in an oxidizing environment, while obtaining metallic copper is favorable under reductant conditions [ 56 ]. Both processes compete between them and it is not possible to complete them in a single step. Reductant agents were not used during the experiments. However, the addition of reductant agents would favor the obtaining of metallic copper. In this case, the decomposition of the fayalite into magnetite and silica would be less favorable. Future research might focus on the addition of wastes from other metallurgies to promote Metals 2021,11, 1032 6 of 18 the formation of copper nodules or fayalite decomposition. An option is the utilization of blast furnace powders. This by-product is rich in carbon, which would promote reductant conditions, and rich in iron, which would increase the iron content in the slag. Crucibles of tabular alumina (55 mm in height, 30 mm in upper diameter, 25 mm in lower diameter, and 3 mm in thickness) were used in the experiments. Samples were located under a glass chamber as in Figure 3. No special atmosphere was used in the experiments. However, the glass chamber was connected to a pump to facilitate the extraction of the gases generated in the process. This ensured a pressure of 0.85 atm inside the chamber. Seven experiments were carried out in total. The conditions are collected in Table 3. The duration of the experiments was controlled by the temperature in the thermocouple T1. Samples were held at the maximum temperature for 10 min once this temperature had been reached. Except for the samples CuSin3 and Cusinbonus, the rest of the samples were subjected to similar values of power. Table 3. Conditions used in the experiments. Sample Lime Shutter Opening Time (min) Average Incident Radiation (W/m2)Power (W) CuSin1 No 80 20 941.2 1129 CuSin2 No 88 25 955.8 1262 CuSin3 No 41 30 915.5 563 Cusinbonus No 60 23 867.8 781 CuCon1 (stop in the middle of the experiment) Yes 92 - 938 1294 CuCon2 Yes 100 15 939.5 1409 CuCon3 Yes 84 20 973 1226 3. Results 3.1. Macroscopic Analysis Samples were subjected to visual observation. Some remarkable questions can be deduced from this analysis. The average dimensions of the volume affected by the beam of CSP were: 19 mm in diameter and 13 mm in depth. This layer of treated material (material that completely melted during the process) blocks the heating until melting of all the material available in the crucible. Therefore, the slag remains unreacted below the above-indicated layer. On another note, the sample was clearly magnetic after the treatment with CSP. The initial slag was not magnetic (fayalite was the main constituent of the initial slag, which is not magnetic), so the magnetite content increased during the process and became the main constituent of the final product. On another note, elemental sulfur (greenish yellow) is identified in the face located in contact with the unaffected material. It is the product of the thermal decomposition of the occluded matte (copper sulfides). In this context, Winkel studied the thermal decomposition of copper sulfides under concentrated irradiation [ 57 ]. He observed that metallic copper, iron sulfide, and elemental sulfur can be obtained after the treatment of copper concentrates at a high temperature under an inert atmosphere. To a certain extent, copper slag always contains a certain amount of copper sulfides as occluded matte (a binary mixture of Cu2S and FeS). The elemental analysis of the slag used in our experiments indicates 0.52 wt. % S, which is combined with iron and copper. Moreover, our experiments were performed under an ambient atmosphere impoverished in oxygen promoted by the utilization of the glass chamber and the pump. Additionally, the layer of molten slag blocks the circulation of gases (oxygen) through the slag and there is both a high temperature and an inert atmosphere in part of the charge. Under these conditions, occluded matte can decompose and give copper, as in Winkel’s work [ 57 ]. This explains the presence of metallic copper in the final samples (apart from the decomposition of the copper oxides under reductant Metals 2021,11, 1032 7 of 18 conditions). It is necessary to consider that this mechanism is very limited due to the small proportion of occluded matte (there was around 0.52 wt. % S in the initial sample). 3.2. SEM-EDX Samples were also analyzed using a scanning electron microscope and point analysis. The objective of this analysis was to check for both the presence of copper and the increase in magnetite in the final samples that resulted from the treatment with solar energy. Copper nodules were identified using this technique (65–85 wt. % Cu). Magnetite was also identified, the same as fayalite. Four representative SEM-EDX analyses of the treated samples are shown in Figure 4. Metals 2021, 11, x FOR PEER REVIEW 7 of 18 and an inert atmosphere in part of the charge. Under these conditions, occluded matte can decompose and give copper, as in Winkel’s work [57]. This explains the presence of metallic copper in the final samples (apart from the decomposition of the copper oxides under reductant conditions). It is necessary to consider that this mechanism is very limited due to the small proportion of occluded matte (there was around 0.52 wt. % S in the initial sample). 3.2. SEM-EDX Samples were also analyzed using a scanning electron microscope and point analysis. The objective of this analysis was to check for both the presence of copper and the increase in magnetite in the final samples that resulted from the treatment with solar energy. Copper nodules were identified using this technique (65–85 wt. % Cu). Magnetite was also identified, the same as fayalite. Four representative SEM-EDX analyses of the treated samples are shown in Figure 4. A representative zone of the sample CusinBonus is observed in Figure 4a. Point 1 corresponds to a copper-rich nodule (79.12 wt. % Cu; 9.66 wt. % Fe; 5.02 wt. % O); point 2 shows magnetite; while point 3 and point 4 represent non-decomposed fayalite. It was observed that the presence of copper-rich nodules causes the impoverishment in copper of the surrounding areas. A representative zone of the sample CuCon3 can be observed in Figure 4b. Point 1 corresponds to copper-rich nodules (79.03 wt. % Cu; 7.63 wt. % Fe; 5.72 wt. % O; 0.33 wt. % S) and point 2 indicates a ferrite of calcium and silicon. In the case of the sample CuCon1 (Figure 4c), it is possible to check that point 1 corresponds to the fayalite, point 2 represents the magnetite, while point 3 represents the copper-enriched nodule (80.51 wt. % Cu; 10.01 wt. % Fe; 3.71 wt. % O). Sample CuSin1 (Figure 4d) was also analyzed using SEM-EDX. Point 1 represents copper nodules (82.45 wt. % Cu; 8.98 wt. % Fe; 2.91 wt. % O), point 2 represents magnetite, while point 3 represents non-decomposed fayalite. Figure 4. (a) SEM-EDX sample identified as CusinBonus; (b) SEM-EDX sample identified as CuCon3; (c) SEM-EDX sample identified as CuCon1; (d) SEM-EDX sample identified as CuSin1. Figure 4. ( a ) SEM-EDX sample identified as CusinBonus; ( b ) SEM-EDX sample identified as CuCon3; (c) SEM-EDX sample identified as CuCon1; (d) SEM-EDX sample identified as CuSin1. A representative zone of the sample CusinBonus is observed in Figure 4a. Point 1 corresponds to a copper-rich nodule (79.12 wt. % Cu; 9.66 wt. % Fe; 5.02 wt. % O); point 2 shows magnetite; while point 3 and point 4 represent non-decomposed fayalite. It was observed that the presence of copper-rich nodules causes the impoverishment in copper of the surrounding areas. A representative zone of the sample CuCon3 can be observed in Figure 4b. Point 1 corresponds to copper-rich nodules (79.03 wt. % Cu; 7.63 wt. % Fe; 5.72 wt. % O; 0.33 wt. % S) and point 2 indicates a ferrite of calcium and silicon. In the case of the sample CuCon1 (Figure 4c), it is possible to check that point 1 corresponds to the fayalite, point 2 represents the magnetite, while point 3 represents the copper-enriched nodule (80.51 wt. % Cu; 10.01 wt. % Fe; 3.71 wt. % O). Sample CuSin1 (Figure 4d) was also analyzed using SEM-EDX. Point 1 represents copper nodules (82.45 wt. % Cu; 8.98 wt. % Fe; 2.91 wt. % O), point 2 represents magnetite, while point 3 represents non-decomposed fayalite. Metals 2021,11, 1032 8 of 18 The formation of magnetite is also significative in the experiments due to the decomposition of the fayalite (into magnetite and silica). Therefore, oxidizing conditions were also verified during the experiments because magnetite was clearly formed. Figure 5shows a large region of the sample where magnetite was massively formed. Points 1, 2, 3, and 5 (40–50 wt. % Fe, 25–29 wt. % O) represent iron oxides. The well-developed tetrahedrons of magnetite in the region represented by point 3 can clearly be observed. Magnetite refers in this manuscript to a series of phases belonging to the iron spinel as magnesioferrite (MgFe 2 O 4 ), magnetite (FeFe 2 O 4 , where one Fe is +2 and two Fe’s are +3, respectively), and maghemite ( γ -Fe 2 O 3 , Fe (II)-deficient magnetite). Cuprospinel (CuFe 2 O 4 ) would also belong to this series of iron spinel. Point 4 corresponds to the non-decomposed fayalite (22.72 wt. % Si, 36.52 wt. % O, 11.58 wt. % Si). Metals 2021, 11, x FOR PEER REVIEW 8 of 18 The formation of magnetite is also significative in the experiments due to the decomposition of the fayalite (into magnetite and silica). Therefore, oxidizing conditions were also verified during the experiments because magnetite was clearly formed. Figure 5 shows a large region of the sample where magnetite was massively formed. Points 1, 2, 3, and 5 (40–50 wt. % Fe, 25–29 wt. % O) represent iron oxides. The well-developed tetrahedrons of magnetite in the region represented by point 3 can clearly be observed. Magnetite refers in this manuscript to a series of phases belonging to the iron spinel as magnesioferrite (MgFe2O4), magnetite (FeFe2O4, where one Fe is +2 and two Fe’s are +3, respectively), and maghemite (γ-Fe2O3, Fe (II)-deficient magnetite). Cuprospinel (CuFe2O4) would also belong to this series of iron spinel. Point 4 corresponds to the non-decomposed fayalite (22.72 wt. % Si, 36.52 wt. % O, 11.58 wt. % Si). Figure 5. SEM-EDX sample identified as CuCon2. 3.3. Size of the Nodules Strictly, copper nodules are not formed. It would be more convenient to talk about nodules enriched in copper (65–85 wt.% Cu; 5–10 wt. % Fe; 2–10 wt. % O; <3 wt. % S). The size of the nodules was determined using the scanning electron microscope in different sections and SEM images obtained from crushed final specimens. Most of the nodules had a size between 5 and 10 μm (see Figure 6). However, individual copper nodules with a larger size were detected in some samples (between 20 and 35 μm). The size of the nodules will define the grinding size for the subsequent separation. Figures 7–9 show fields of copper nodules formed during the treatment of the copper slag with CSP. It is possible to see that reductant conditions were also promoted during the experiments since copper nodules are formed. The presence of magnetite formed during the decomposition of the fayalite promotes an environment with reductant potential; this ensures the formation of the copper nodules and prevents during the cooling the reversion of the process. It is also necessary to consider that some of the copper nodules are formed because of the occluded matte decomposition according to the mechanism proposed by Winkel [57]. The clustering mechanism is evident (Figures 7–9) because many copper nodules are formed in the same field. However, the growth mechanism did not take place because these nodules did not merge to form larger nodules. This is a consequence of the short duration of the treatment, the viscosity of the liquid phase (due to the presence of magnetite), and the high cooling rate. Figure 5. SEM-EDX sample identified as CuCon2. 3.3. Size of the Nodules Strictly, copper nodules are not formed. It would be more convenient to talk about nodules enriched in copper (65–85 wt. % Cu; 5–10 wt. % Fe; 2–10 wt. % O; <3 wt. % S). The size of the nodules was determined using the scanning electron microscope in different sections and SEM images obtained from crushed final specimens. Most of the nodules had a size between 5 and 10 µ m (see Figure 6). However, individual copper nodules with a larger size were detected in some samples (between 20 and 35 µ m). The size of the nodules will define the grinding size for the subsequent separation. Figures 7–9show fields of copper nodules formed during the treatment of the copper slag with CSP. It is possible to see that reductant conditions were also promoted during the experiments since copper nodules are formed. The presence of magnetite formed during the decomposition of the fayalite promotes an environment with reductant potential; this ensures the formation of the copper nodules and prevents during the cooling the reversion of the process. It is also necessary to consider that some of the copper nodules are formed because of the occluded matte decomposition according to the mechanism proposed by Winkel [ 57 ]. The clustering mechanism is evident (Figures 7–9) because many copper nodules are formed in the same field. However, the growth mechanism did not take place because these nodules did not merge to form larger nodules. This is a consequence of the short duration of the treatment, the viscosity of the liquid phase (due to the presence of magnetite), and the high cooling rate. Metals 2021,11, 1032 9 of 18 Metals 2021, 11, x FOR PEER REVIEW 9 of 18 Figure 6. SEM-EDX image of a copper nodule. Figure 7. SEM-EDX sample identified as CuCon1. Figure 8. SEM-EDX sample identified as CuCon3. Figure 6. SEM-EDX image of a copper nodule. Metals 2021, 11, x FOR PEER REVIEW 9 of 18 Figure 6. SEM-EDX image of a copper nodule. Figure 7. SEM-EDX sample identified as CuCon1. Figure 8. SEM-EDX sample identified as CuCon3. Figure 7. SEM-EDX sample identified as CuCon1. Metals 2021, 11, x FOR PEER REVIEW 9 of 18 Figure 6. SEM-EDX image of a copper nodule. Figure 7. SEM-EDX sample identified as CuCon1. Figure 8. SEM-EDX sample identified as CuCon3. Figure 8. SEM-EDX sample identified as CuCon3. Metals 2021,11, 1032 16 of 18 A preliminary economic analysis, considering the current copper price, indicates that only the recovery of copper could represent a significant economic benefit (>30 € /t slag). Therefore, CSP might be a potential candidate for the treatment of copper slags to recover copper and iron, although further research is still required to think about the scalation of the process up to the industrial level. Author Contributions: D.F.-G. and L.F.V. designed the experiments; D.F.-G., Í.R.-B., and C.G.-G. performed the experiments; D.F.-G. and J.P. analyzed the results; D.F.-G. and L.F.V. interpreted the results; D.F.-G. and C.G.-R. wrote the manuscript. All authors have read and agreed to the published version of the manuscript. Funding: Financial support from the Access to Research Infrastructures activity in the 7th Framework Program of the EU (SFERA 2 Grant Agreement No. 312643) is gratefully acknowledged and the use of the facilities and researchers/technology experts. This research was also supported by a Juan de la Cierva Formación grant from the Spanish Ministry of Science and Innovation (MCINN) to Daniel Fernández-González (FJC2019-041139-I). Acknowledgments: The authors want to acknowledge the X-ray Diffraction Laboratory at the Faculty of Materials Science and Ceramics of AGH University of Science and Technology (Head of the laboratory, Bartosz Handke) in Krakow for X-ray diffraction and X-ray fluorescence measurements. Conflicts of Interest: The authors declare no conflict of interest. References 1. Sancho, J.P.; Verdeja, L.F.; Ballester, A. Metalurgia Extractiva. Volumen II. Procesos de Obtención, 1st ed.; Síntesis: Madrid, Spain, 2000. 2. Fan, Y.; Shibata, E.; Iizuka, A.; Nakamura, T. Crystallization behaviors of copper smelter slag studied using time-temperaturetransformation diagram. Mater. Trans. 2014,55, 958–963. [CrossRef] 3. Davenport, W.G.; King, M.; Schlesinger, M.; Biswas, A.K. Extractive Metallurgy of Copper, 4th ed.; Pergamon-Elsevier Science Ltd.: Oxford, UK, 2002. 4. Nazer, A.; Pavez, O.; Rojas, F.; Aguilar, C. Una revisión de los usos de las escorias de cobre. In Proceedings of the IBEROMET XI. X CONAMET/SAM, Viña del Mar, Chile, 2–5 November 2010. 5. Coursol, P.; Cardona, N.; Mackey, P.; Bell, S.; Davis, B. Minimization of copper losses in copper smelting slag during electric furnace treatment. JOM 2012,64, 1305–1313. [CrossRef] 6. Cardona, N.; Coursol, P.; Vargas, J.; Parra, R. Physical chemistry of copper smelting slags and copper losses at the Paipote smelter. Part 1-Thermodynamic modelling. Can. Metall. Quart. 2011,50, 318–329. [CrossRef] 7. Cardona, N.; Coursol, P.; Vargas, J.; Parra, R. Physical chemistry of copper smelting slags and copper losses at the Paipote smelter. Part 2-Characterization of industrial slags. Can. Metall. Quart. 2011,50, 330–340. [CrossRef] 8. Madheswaran, C.K.; Ambily, P.S.; Dattatreaya, J.K.; Rajamane, N.P. Studies on use of copper slag as replacement material for river sand in building constructions. J. Inst. Eng. Ser. A 2014,95, 169–177. [CrossRef] 9. Nazer, A.; Pavez, O.; Rojas, F. Use of copper slag in cement mortar. REM Rev. Esc. Minas 2012,65, 87–91. [CrossRef] 10. Shi, C.; Meyer, C.; Behnood, A. Utilization of copper slag in cement and concrete. Resour. Conserv. Recy. 2008 ,52, 1115–1120. [CrossRef] 11. Potysz, A.; Van Hullebusch, E.D.; Kierczak, J.; Grybos, M.; Lens, P.N.L.; Guibaud, G. Copper metallurgy slagsCurrent knowledge and fate: A review. Crit. Rev. Env. Sci. Tec. 2015,45, 2424–2488. [CrossRef] 12. Palacios, J.; Sánchez, M. Wastes as resources: Update on recovery of valuable metals from copper slags. Miner. Process. Extr. Metall. 2011,120, 218–223. [CrossRef] 13. Cendoya, P. Efecto en la resistencia de las escorias de fundición de cobre como agregado fino en el comportamiento resistente del hormigón. Ingeniare Rev. Chil. Ing. 2009,17, 85–94. [CrossRef] 14. Murari, K.; Siddique, R.; Jain, K.K. Use of waste copper slag, a sustainable material. J. Mater. Cycles. Waste Manag. 2015 ,17, 13–26. [CrossRef] 15. Kambham, K.; Sangameswaran, S.; Datar, S.R.; Kura, B. Copper slag: Optimization of productivity and consumption for cleaner production in dry abrasive blasting. J. Clean Prod. 2007,15, 465–473. [CrossRef] 16. Jiménez-Padilla, B. Armado de Tuberías. FMEC0108, 1st ed.; IC Editorial: Antequera, Spain, 2014. 17. Biswas, S.; Satapathy, A. Use of copper slag in glass-epoxy composites for improved wear resistance. Waste Manag. Res. 2010 ,28, 615–625. [CrossRef] 18. Pundhir, N.K.S.; Kamaraj, C.; Nanda, P.K. Use of copper slag as construction material in bituminous pavements. J. Sci. Ind. Res. India. 2005,64, 997–1002. 19. Busolic, D.; Parada, F.; Parra, R.; Sánchez, M.; Palacios, J.; Hino, M. Recovery of iron from copper flash smelting slags. Miner. Process. Extr. Metall. 2011,120, 32–36. [CrossRef] Metals 2021,11, 1032 17 of 18 20. Guo, Z.; Zhu, D.; Pan, J.; Wu, T.; Zhang, F. Improving beneficiation of copper and iron from copper slag by modifying the molten copper slag. Metals 2016,6, 86. [CrossRef] 21. Xian-Lin, Z.; De-Qing, Z.; Jian, P.; Teng-Jiao, W. Utilization of waste copper slag to produce directly reduced iron for weathering resistant steel. ISIJ Int. 2015,55, 1347–1352. [CrossRef] 22. Heo, J.H.; Chung, Y.; Park, J.H. Recovery of iron and removal of hazardous elements from waste copper slag via a novel aluminothermic smelting reduction (ASR) process. J. Clean. Prod. 2016,137, 777–787. [CrossRef] 23. Siwiec, G.; Sozanska, M.; Blacha, L.; Smalcerz, A. Behaviour of iron during reduction of slag obtained from copper flash smelting. Metabk 2015,54, 113–115. 24. Sarfo, P.; Wyss, G.; Ma, G.; Das, A.; Young, C. Carbothermal reduction of copper smelter slag for recycling into pig iron and glass. Miner. Eng. 2017,107, 8–19. [CrossRef] 25. Liao, Y.; Zhou, J.; Huang, F. Separating and recycling of Fe, Cu, Zn from dumped copper slag by microwave irradiation assisted carbothermic method. J. Residuals Sci. Technool. 2016,13, S155–S160. [CrossRef] 26. Li, K.; Ping, S.; Wang, H.; Ni, W. Recovery of iron from copper slag by deep reduction and magnetic beneficiation. Int. J. Min. Met. Mater. 2013,20, 1035–1041. [CrossRef] 27. Fernández-González, D.; Ruiz-Bustinza, I.; González-Gasca, C.; Piñuela-Noval, J.; Mochón-Castaños, J.; Sancho-Gorostiaga, J.; Verdeja, L.F. Concentrated solar energy applications in materials science and metallurgy. Sol. Energy 2018 ,170, 520–540. [CrossRef] 28. Murray, J.P.; Flamant, G.; Roos, C.J. Silicon and solar-grade silicon production by solar dissociation of Si 3 N 4 .Sol. Energy 2006 ,80, 1349–1354. [CrossRef] 29. Loutzenhiser, P.G.; Tuerk, O.; Steinfeld, A. Production of Si by vacuum carbothermal reduction of SiO 2 using concentrated solar energy. JOM 2010,62, 49–54. [CrossRef] 30. Murray, J.P. Aluminum production using high-temperature solar process heat. Sol. Energy 1999,66, 133–142. [CrossRef] 31. Murray, J.P. Aluminum-silicon carbothermal reduction using high-temperature solar process heat. In Proceedings of the 128th TMS Annual Meeting, San Diego, CA, USA, 28 February–4 March 1999. 32. Murray, J.P. Solar production of aluminum by direct reduction: Preliminary results for two processes. J. Sol. Energy Eng. 2001 ,123, 125–132. [CrossRef] 33. Lytvynenko, Y.M. Obtaining aluminum by the electrolysis with the solar radiation using. Appl. Sol. Energy 2013 ,49, 4–6. [CrossRef] 34. Epstein, M.; Olalde, G.; Santén, S.; Steinfeld, A.; Wieckert, C. Towards the industrial solar carbothermal production of zinc. J. Sol. Energ. 2008,130, 014501–014504. [CrossRef] 35. Fletcher, E.A.; Noring, J.E. High temperature solar electrothermal processing—Zinc from zinc oxide. Energy 1983 ,8, 247–254. [CrossRef] 36. Fletcher, E.A.; Macdonald, F.J.; Kunnerth, D. High temperature solar electrothermal processing—II. Zinc from zinc oxide. Energy 1985,10, 1255–1272. [CrossRef] 37. Palumbo, R.D.; Fletcher, E.A. High temperature solar electrothermal processing—III. Zinc from zinc oxide at 1200–1675K using a non-consumable anode. Energy 1988,13, 319–332. [CrossRef] 38. Osinga, T.; Frommherz, U.; Steinfeld, A.; Wieckert, C. Experimental investigation of the solar carbothermic reduction of ZnO using a two-cavity solar reactor. J. Sol. Energ. 2004,126, 633–637. [CrossRef] 39. Epstein, M.; Ehrensberger, K.; Yogev, A. Ferro-reduction of ZnO using concentrated solar energy. Energy 2004 ,29, 745–756. [CrossRef] 40. Steinfeld, A.; Brack, M.; Meier, A.; Weidenkaff, A.; Wuillemin, D. A solar chemical reactor for co-production of zinc and synthesis gas. Energy 1998,23, 803–814. [CrossRef] 41. Wieckert, C.; Palumbo, R.; Frommherz, U. A two-cavity reactor for solar chemical processes: Heat transfer model and application to carbothermic reduction of ZnO. Energy 2004,29, 771–787. [CrossRef] 42. Wieckert, C.; Frommherz, U.; Kräupl, S.; Guillot, E.; Olalde, G.; Epstein, M.; Santén, S.; Osinga, T.; Steinfeld, A. A 300 kW solar chemical pilot plant for the carbothermic production of zinc. J. Sol. Energy 2006,129, 190–196. [CrossRef] 43. Schunk, L.O.; Lipinski, W.; Steinfeld, A. Heat transfer model of a solar receiver-reactor for the thermal dissociation of ZnOExperimental validation at 10 kW and scale-up to 1 MW. Chem. Eng. J. 2009,150, 502–508. [CrossRef] 44. Villasmil, W.; Brkic, M.; Wuillemin, D.; Meier, A.; Steinfeld, A. Pilot scale demonstration of a 100-kWth solar thermochemical plant for the thermal dissociation of ZnO. J. Sol. Energy 2013,136, 011016–0111027. [CrossRef] 45. Koepft, E.; Villasmil, W.; Meier, A. Pilot-scale solar reactor operation and characterization for fuel production via the Zn/ZnO thermochemical cycle. Appl. Energy 2016,165, 1004–1023. [CrossRef] 46. Ruiz-Bustinza, I.; Cañadas, I.; Rodríguez, J.; Mochón, J.; Verdeja, L.F.; García-Carcedo, F.; Vázquez, A. Magnetite production from steel wastes with concentrated solar energy. Steel Res. Int. 2013,84, 207–217. [CrossRef] 47. Sibieude, F.; Ducarroir, M.; Tofighi, A.; Ambriz, J. High temperature experiments with a solar furnace: The decomposition of Fe3O4, Mn3O4, CdO. Int. J. Hydrogen Energy 1982,7, 79–88. [CrossRef] 48. Steinfeld, A.; Fletcher, E.A. Theoretical and experimental investigation of the carbothermic reduction of Fe 2 O 3 using solar energy. Energy 1991,16, 1011–1019. [CrossRef] Metals 2021,11, 1032 18 of 18 49. Steinfeld, A.; Kuhn, P.; Karni, J. High-temperature solar thermochemistry: Production of iron and synthesis gas by Fe 3 O 4 - reduction with methane. Energy 1993,18, 239–249. [CrossRef] 50. Fernández-González, D.; Prazuch, J.; Ruiz-Bustinza, I.; González-Gasca, C.; Piñuela-Noval, J.; Verdeja, L.F. Solar synthesis of calcium aluminates. Sol. Energy 2018,171, 658–666. [CrossRef] 51. Mochón, J.; Ruiz-Bustinza, Í.; Vázquez, A.; Fernández, D.; Ayala, J.M.; Barbés, M.F.; Verdeja, L.F. Transformations in the ironmanganese-oxygen-carbon system resulted from treatment of solar energy with high concentration. Steel Res. Int. 2014 ,85, 1469–1476. [CrossRef] 52. Fernández-González, D.; Prazuch, J.; Ruiz-Bustinza, I.; González-Gasca, C.; Piñuela-Noval, J.; Verdeja, L.F. Iron Metallurgy via Concentrated Solar Energy. Metals-Basel 2018,8, 873. [CrossRef] 53. Fernández-González, D.; Prazuch, J.; Ruiz-Bustinza, I.; González-Gasca, C.; Piñuela-Noval, J.; Verdeja, L.F. The treatment of Basic Oxygen Furnace (BOF) slag with concentrated solar energy. Sol. Energy 2019,180, 372–382. [CrossRef] 54. Fernández-González, D.; Prazuch, J.; Ruiz-Bustinza, I.; González-Gasca, C.; Piñuela-Noval, J.; Verdeja, L.F. Transformations in the Mn-O-Si system using concentrated solar energy. Sol. Energy 2019,184, 148–152. [CrossRef] 55. Fernández-González, D.; Prazuch, J.; Ruiz-Bustinza, I.; González-Gasca, C.; Piñuela-Noval, J.; Verdeja, L.F. Transformations in the Si-O-Ca system: Silicon-calcium via solar energy. Sol. Energy 2019,181, 414–423. [CrossRef] 56. Fernández-González, D.; Prazuch, J.; Ruiz-Bustinza, I.; González-Gasca, C.; Gómez-Rodríguez, C.; Verdeja, L.F. Treatment of copper slag with concentrated solar energy. In Proceedings of the Environmental Safety-Non-Energy and Raw Materials, I International Conference on Engineering Materials, Safety, Environment and Technology, Zielona Gora, Poland, 3–4 June 2020; Gabryelewicz, I., Wedrychowicz, M., Eds.; University of Zielona Gora: Zielona Gora, Poland, 2021; pp. 39–59. 57. Winkel, H.E. Thermal Decomposition of Copper Sulfides under Concentrated Irradiation. Ph.D. Thesis, Swiss Federal Institute of Technology Zurich, Zurich, Switzerland, 2006. 58. Alcock, C.B. Principles of Pyrometallurgy; Academic Press: London, UK, 1976; pp. 15–16. 59. Ballester, A.; Verdeja, L.F.; Sancho, J.P. Metalurgia Extractiva. Volumen I. Fundamentos, 1st ed.; Síntesis: Madrid, Spain, 2000. 60. Richardson, F.D.; Jeffers, J.H.E. The Ellingham diagram for metal oxides. In Introduction to Metallurgical Thermodynamics; Hemisphere Publishing Corporation: San Francisco, CA, USA, 1981. 61. Verdeja, L.F.; Sancho, J.P.; Ballester, A. Refractory and Ceramic Materials, 1st ed.; Síntesis: Madrid, Spain, 2014. 62. Pero-Sanz, J.A.; Quintana, M.J.; Verdeja, L.F. Solidification and Solid-state Transformations of Metals and Alloys, 1st ed.; Elsevier: Boston, MA, USA, 2017. 63. Flamant, G.; Ferriere, A.; Laplaze, D.; Monty, D. Solar processing of materials: Opportunities and new frontiers. Sol. Energy 1999 , 66, 117–132. [CrossRef] 64. Alvarez, M.A. Environmental Impact of Plastics. J. Mater. Ed. 2018,40, 119–124. 65. Dobiszewska, M. Waste Materials Used in Making Mortar and Concrete. J. Mater. Ed. 2017,39, 133–156. 66. Fernández-González, D.; Sancho-Gorostiaga, J.; Piñuela-Noval, J.; Verdeja, L.F. Anodic Lodes and Scrapings as a Source of Electrolytic Manganese. Metals 2018,8, 162. [CrossRef] 67. Ordiales, M.; Iglesias, J.; Fernández-González, D.; Sancho-Gorostiaga, J.; Fuentes, A.; Verdeja, L.F. Cold agglomeration of Ultrafine Oxidized Dust (UOD) from ferromanganese and silicomanganese industrial process. Metals 2016,6, 203. [CrossRef] 68. Fernández-González, D.; Piñuela-Noval, J.; Verdeja, L.F. Silicomanganese and ferromanganese slags treated with concentrated solar energy. Proceedings 2018,2, 1450. [CrossRef] 69. Fernández-González, D. Aplicaciones de la Energía Solar Concentrada en Metalurgia y Ciencia de los Materiales. Ph.D. Thesis, Department of Materials Science and Metallurgical Engineering, University of Oviedo, Oviedo/Uviéu, Asturias, Spain, September 2019.