Extraction of Copper and Zinc from Waste Printed Circuit Boards

recycling

Technical Note Extraction of Copper and Zinc from Waste Printed Circuit Boards

Ayorinde Emmanuel Ajiboye 1,2,*, Folorunsho Emmanuel Olasehinde 1, Ojo Albert Adebayo 1, Olubode John Ajayi 1, Malay Kumar Ghosh 2 and Suddhasatwa Basu 2 1 Department of Chemistry Federal University of Technology, 340252-Akure, Nigeria 2 CSIR-Institute of Minerals and Materials Technology, Bhubaneswar 751013, India * Correspondence: [email protected]; Tel.: +23-4803-7304-122; Fax: +33-1427-321-20

Received: 31 May 2019; Accepted: 23 August 2019; Published: 27 August 2019

Abstract: The recovery of valuable metals from waste printed circuit boards (WPCBs) is crucial in order to harness their economic resources, and prevents potential environmental contamination. However, selective extraction of Cu and Zn, and the co-extraction of other metals as impurities at ambient temperature using selected lixiviants such as HCl, H2SO4, HNO3, triﬂuoromethanesulfonic acid (TFMS), NaOH, and mixtures of NaCl and CuCl2 was studied. It is shown that the extraction eﬃciencies of all the metals increased with increases in lixiviant concentrations. High selectivity of Cu and Zn toward Fe were achieved in dilute H2SO4, HNO3, TFMS, and 0.5 M NaCl + 0.1 M CuCl2, and low dissolution of Pb (<5%) was observed in all H2SO4 lixiviants. Almost 100% Zn extraction using NaOH lixiviants without trace of other metals was achieved. Therefore, 0.5 M NaCl + 0.5 M CuCl2, 1.0 M HNO3, 0.5 M H2SO4, and 1.0 M TFMS showed high extraction selectivity toward Cu and Zn with low chemical consumption, and produced pregnant leach solution rich in Cu and Zn, as well as residue containing Fe, Ni, and other metals.

Keywords: extraction; copper; zinc; ambient temperature; lixiviants

1. Introduction Presently, one type of waste with great environmental concern and economic value is electronic waste [1]. Although most metals are base metals such as copper, iron, nickel, tin, lead, aluminum, and zinc, a significant amount of attention has been expended on the recovery of precious metals, including gold, silver, and palladium [2]. Copper is widely used as a major interconnecting material in electronic industries, owing to its good electrical and thermal conductivity [3]. In recent years, several techniques have been employed for recycling activities, such as pyro-metallurgy [4], hydrometallurgy [5], bio-hydrometallurgy [6], etc. Various hydrometallurgical methods for the recovery of metals from printed circuit boards (PCBs) have been developed using leaching reagents such as acids, bases, and salts. Meanwhile, hydrometallurgical techniques [7], leaching [8], kinetic studies [9], and bioleaching of precious metals from waste electrical and electronic equipment (WEEE) using bacteria [6,10,11] have been attempted and reported in the literature. On average, typical PCBs contain about 7.0% Fe, 27.0% Cu, 2.0% Al, 1.5% Zn, 0.5% Ni, 2000 ppm Ag, and 80 ppm of Au [12]. The leaching of metals from the PCBs by acids produces pregnant leach solutions containing several metals, including iron, which have been identified as the major problem in the electro-winning or electrochemical deposition of metals. For instance, in the electro-winning of Cu, it has been reported that Fe concentrations >30 ppm reduce current efficiency and increase the energy consumption in addition to changing the morphology of the cathodic Cu product due to Fe ions being oxidized from ferrous (Fe2+) to ferric (Fe3+) at the anode, and conversely reduce from Fe3+ to Fe2+ at the cathode, using up the required current and energy for the deposition of copper [13,14].

Recycling 2019, 4, 36; doi:10.3390/recycling4030036 www.mdpi.com/journal/recycling Recycling 2019, 4, 36 2 of 13

However, in order to avoid iron leaching into solution, the selective recovery of Cu using ammoniacal solutions and salts remains the most common and effective established method in hydrometallurgy [15]. The extraction of metals from ammonia solutions seems to be more complicated as compared to acidic environments [16,17]. The alkalinity of the solution and the free ammonia concentration affect the route of extraction [17]. However, copper-pregnant leached solutions require a further preceding process (solvent extraction) due to the complexation of Cu with ammonia, and require a solvent to push out the copper (Equations (1) and (2)), as this process has been reported to be ineffective [16,18].

M2+ + 4NH M (NH ) 2+ (1) 3→ 3 4 Cu (NH ) 2+ + 2HA (o) CuA (o) + 2NH + + 2NH (2) 3 4 (aq) → 2 4 (aq) 3(aq) With proper understanding of the principle of economic, eco-friendly, and eﬃciency (EEE) in the recycling of valuable metals from waste materials, the application of many further unit operations will make the recovery cumbersome and costly. Therefore, this study was carried out to investigate the selective extraction of Cu and Zn from waste printed circuit boards (WPCBs) using some selected lixiviants at ambient temperatures to produce pregnant leach solutions rich in Cu and Zn and residues rich in Fe and Ni, with other metals for further recovery. The lixiviants investigated for this purpose were: hydrochloric acid (0.5–5.0 M HCl), sulfuric acid (0.5–4.0 M H2SO4), nitric acid (1.0–5.0 M HNO3), triﬂuoromethanesulfonic acid (1.0–2.5 M TFMS), sodium hydroxide (1.0–5.0 M NaOH), and mixtures of sodium chloride and copper (II) chloride salts (0.5 M NaCl + 0.1 M CuCl2, 0.5 M NaCl + 0.5 M CuCl2, 4.5 M NaCl + 0.1 M CuCl2, and 4.5 M NaCl + 1.0 M CuCl2).

2. Materials and Methods

2.1. Sample Preparation Discarded WEEE for this research work was collected from waste store at the CSIR-Institute of Minerals and Materials Technology, Bhubaneswar, India, and unwanted attachments such as cords, wires, and plastic coverings were detached manually. Attached electronic components (ECs) on PCBs were removed by de-soldering using a hot air gun (Black & Decker KX 1800, Beijing, China). The pulverized sample obtained was screened using a sieve for particle size distribution, while the coarse sample was returned into the mill to obtain powder samples 100 µm. ≤ Material Pretreatment by Roasting For each batch of roasting, 100 g of pulverized WPCBs were roasted in a muﬄe furnace (Nabertherm Furnace, Lilienthal, Germany) at 700 ◦C for 2 h without any addition of roasting agent to convert metals into their oxide form and remove organic materials. The gas passed through aluminum oxynitride tube connected at the top corner of the furnace into an alkaline solution/distilled water chamber tank, where it was absorbed. Then, the roasted sample obtained was powdered using a mortar and pestle, sieved with 75-µm mesh, and stored in an airtight container.

2.2. Characterization The particle size distribution (PSD) of the sample (Figure1) was obtained by a Mastersizer 2000 laser diffraction particle size analyzer with a Scirocco 2000 Dry Powder Feeder, both manufactured by Malvern Instruments (Malvern, UK). The sample was characterized for elemental phases present (Figure2) using X-ray diffraction (X’Pert Pro-PAN Analytical X-Ray Diffractometer (Model: EMPYREAN)) operating at an anode current of 40 mA at 45 kV with Cu Kα radiation, by Rietveld refinement method using High Score Plus software. SEM-EDX analysis for the morphological and phase identifications (Figure3) was performed with an EVO 18 scanning electron microscope (Carl Zeisis, Munich, Germany) and E1220 EDAX (Element Company, Pleasanton, CA, USA) with Team software. Tungsten filament was used Recycling 2019, 4, 36 3 of 13

as a cathode, the acceleration voltage (EHT voltage) was 20 kV, and the working distance (WD) was Recycling8.5Recycling mm. 2019 2019, 4,, x4 , x 3 of3 14of 14

FigureFigureFigure 1. 1. The1.The The particle particle particle size size size distribution distribution distribution of of ofthe the the powdered powdered powdered printed printed printed circuit circuit circuit board board board (PCB) (PCB) (PCB) sample. sample. sample.

1200012000 Zn ZOnO

CuOCuO 10000 SnO 10000 SnO2 2 PbO PbO CuFe O 80008000 CuFe2O24 4 Fe O Fe2O23 3

60006000 ZnSiO ZnSiO3 3 Ni (TiO ) Ni (TiO3) 3 4000

Intensity (Counts) 4000 Intensity (Counts)

q 2000 2000 l l

0 0 00 102030405060708090100110 102030405060708090100110 2θ2 degreeθ degree

FigureFigureFigure 2. 2. The2.The The observed observed observed X-ray X-ray X-ray diffraction di diffractionﬀraction (XRD) (XRD) (XRD) pattern pattern pattern of of ofpowdered powdered powdered PCBs PCBs PCBs sample. sample. sample.

Recycling 2019, 4, 36 4 of 13 Recycling 2019, 4, x 4 of 14

Element Average spot weight (%) O K 34.93 AlK 4.57 SiK 16.24 PbM 0.67 SnL 3.18 CaK 9.13 TiK 0.93 CrK 0.33 FeK 3.95 YbL 0.92 NiK 0.11 CuK 20.93 Recycling 2019, 4, x 4 of 14 OsL 1.62 Element Average spot PtL 1 weight (%) O K 34.93 AuL 1.5 AlK 4.57 SiK 16.24 PbM 0.67 SnL 3.18 CaK 9.13 TiK 0.93 CrK 0.33 FeK 3.95 YbL 0.92 NiK 0.11 CuK 20.93 OsL 1.62 PtL 1 AuL 1.5

Figure 3. The scanning electron microscopy (SEM-EDX) of PCBs powder sample.

Chemical Analysis

ChemicalFigure analysis 3. The ofscanning WPCBs electron powder microscopy was performed(SEM-EDX) of after PCBs digestion powder sample. using aqua regia. The concentration ofFigure tin 3. (Sn) The scanning in the electron solution microscopy (SEM-EDX) was analyzed of PCBs powder bysample. inductively coupled plasma optical Chemicalemission Analysis spectroscopyChemical Analysis (ICP-OES, Thermo Scientiﬁc, iCAP 7600 Duo SERIES, Waltham, MA, USA), while the concentrationChemical analysis of otherof WPCBs metals powder was was perf analyzedormed after digestion using using Atomic aqua regia. Absorption The Spectrophotometer Chemicalconcentration analysis of tin of (Sn) WPCBs in the solution powder was analyzed was by inductively perf ormedcoupled plasma after optical digestion emission using aqua regia. The concentration(AAS, AA200,spectroscopy of Perkin tin (Sn) (ICP-OES, Elmer, in the Thermo Shelton, solution Scientific, CT, was iCAP USA) 7600analyz Duo (Table SERIES,ed by1 Waltham,). inductively MA, USA), coupledwhile the plasma optical emission concentration of other metals was analyzed using Atomic Absorption Spectrophotometer (AAS, spectroscopy AA200,(ICP-OES, Perkin Elmer, Thermo Shelton, CT, Scientific, USA) (Table 1). iCAP 7600 Duo SERIES, Waltham, MA, USA), while the Table 1. Composition of metals in the waste printed circuit boards (WPCBs) powder sample ( 75 µm). concentration ofTable other 1. Composition metals of metals was in the analyzedwaste printed circuit using boards (WPCBs) Atomic powder Absorptionsample (-75 µm). Spectrophotometer− (AAS, AA200, Perkin Elmer, Shelton, CT, USA) (Table Elements1). ElementsWeight (%) Weight (%) Fe 11.118 Cu Fe41.64 11.118 Table 1. Composition of metals in the waste printedNi circuitCu boards2.28 (WPCBs) 41.64 powder sample (-75 µm). Recycling 2019, 4, x NiElements 5 of 2.2814 Al 5.05 Weight (%) ZnTi Al1.790.18Fe 5.0511.118 Pb Zn1.09 1.79 Cu 41.64 Sn Pb1.63 1.09 SnNi 1.63 2.28

Ti 0.18

2.3. Experimental Studies Al 5.05 All chemicals and reagents used were of analytical grade, with the specifications given in Table 2. The extraction of metals from powder WPCBs by the lixiviants was conductedZn at ambient 1.79 temperature (25 °C) for 12 h using a 0.5 L Erlenmeyer glass reactor connected to a thermostatic water bath without any external oxidation or the addition of an oxidizing agent. MixingPb of the solutions 1.09 was achieved using an MS-H280-pro magnetic stirrer (SCILOGEX, Rocky Hill, CT, USA) at 500 rpm. The solid–liquid ratio employed was 0.05 (pulp density: 50 g/L). In Snorder to evaluate the 1.63 extraction efficiencies of Cu, Zn, and other metals (Equation (1)), the resulting liquors were filtered after leaching and the filtrate was analyzed by inductively couple plasma-optical emission spectroscopy (ICP-OES) and atomic absorption spectrophotometry. Extraction efficiency (%) = ( Cm × 10000)/(P × Xm), (3) where Cm is the concentration of metals in the leached solution in grams per liter (g/L); P is the pulp density of the sample in percentage (%; i.e., g/100 mL); and Xm is the metal content in the original sample (%; 1%= 10,000 g/L).

Table 2. Detailed information of the lixiviants used for the extraction studies.

Solutions Concentrations Chemical Strength Manufacturer 0.5 M 1.5 M Percent purity: 35.4%, specific gravity: Hydrochloric Acid (HCl) 2.5 M 1.18 g/cm3, percent weight: 36.46 Finar Chemical 4.5 M g/mole 5.0 M 0.5 M Percent purity: 98.0%, specific gravity: 1.5 M Merck Specialities Sulfuric Acid (H2SO4) 1.84 g/cm3, percent weight: 98.08 2.5 M Private g/mole 4.0 M 1.0 M Percent purity: 69.0%, specific gravity: HiMedia 2.5 M Nitric Acid (HNO3) 1.42g/cm3, percent weight: 63.01 Laboratories Pvt. 3.5 M g/mole Ltd. 5.0 M Trifluoromethanesulfonic 1.0 M

Acid (TFMS) 2.5 M 1.0 M Sodium Hydroxide 2.0 M NaOH 5.0 M 0.5 M NaCl + 0.1M CuCl2 4.5M NaCl + 0.5 M

Copper (II) Chloride, pH CuCl2 CuCl2 2H2O 1 4.5 M NaCl + 0.1 M

CuCl2 4.5 M NaCl + 1.0 M CuCl2 Recycling 2019, 4, 36 5 of 13

2.3. Experimental Studies All chemicals and reagents used were of analytical grade, with the specifications given in Table2. The extraction of metals from powder WPCBs by the lixiviants was conducted at ambient temperature (25 ◦C) for 12 h using a 0.5 L Erlenmeyer glass reactor connected to a thermostatic water bath without any external oxidation or the addition of an oxidizing agent. Mixing of the solutions was achieved using an MS-H280-pro magnetic stirrer (SCILOGEX, Rocky Hill, CT, USA) at 500 rpm. The solid–liquid ratio employed was 0.05 (pulp density: 50 g/L). In order to evaluate the extraction efficiencies of Cu, Zn, and other metals (Equation (1)), the resulting liquors were filtered after leaching and the filtrate was analyzed by inductively couple plasma-optical emission spectroscopy (ICP-OES) and atomic absorption spectrophotometry.

Extraction eﬃciency (%) = (Cm 10000)/(P Xm), (3) × × where Cm is the concentration of metals in the leached solution in grams per liter (g/L); P is the pulp density of the sample in percentage (%; i.e., g/100 mL); and Xm is the metal content in the original sample (%; 1%= 10,000 g/L).

3. Results and Discussion

Table 2. Detailed information of the lixiviants used for the extraction studies.

Solutions Concentrations Chemical Strength Manufacturer 0.5 M Percent purity: 35.4%, 1.5 M specific gravity: 1.18 Hydrochloric Acid (HCl) 2.5 M Finar Chemical g/cm3, percent weight: 4.5 M 36.46 g/mole 5.0 M 0.5 M Percent purity: 98.0%, 1.5 M specific gravity: 1.84 Merck Specialities Sulfuric Acid (H SO ) 2 4 2.5 M g/cm3, percent weight: Private 4.0 M 98.08 g/mole 1.0 M Percent purity: 69.0%, 2.5 M specific gravity: HiMedia Laboratories Nitric Acid (HNO ) 3 3.5 M 1.42g/cm3, percent Pvt. Ltd. 5.0 M weight: 63.01 g/mole Trifluoromethanesulfonic 1.0 M Acid (TFMS) 2.5 M 1.0 M Sodium Hydroxide 2.0 M NaOH 5.0 M

0.5 M NaCl + 0.1M CuCl2 4.5M NaCl + 0.5 M CuCl2 Copper (II) Chloride, pH 1 CuCl2 2H2O 4.5 M NaCl + 0.1 M CuCl2 4.5 M NaCl + 1.0 M CuCl2

3.1. Physical and Chemical Characterization of Pulverized WPCBs According to the mineral analysis results of the powdered WPCBs sample (Figure1), the main mineral phases in the sample of particle size D50 (75 µm) were: CuO, Ni (TiO3), NiCuO, ZnO, Zn2SiO4, SnO, CuFe2O3, and FeMn2O4. It is noteworthy that nickel formed a stable phase with TiO3 and CuO, while iron was in oxide form. Additionally, zinc silicate and oxide were presents in the sample. It is expected that higher extraction of Cu and Zn will be achieved due to the fact that most oxides of metal are readily dissolve in dilute acid. Lower extraction of Ni is expected because of its stable metallic phase except in a concentrated acidic environment. In Table1, elemental analyses revealed that the sample mainly contained Cu (41.64%), Zn (1.79%), Ni (1.8%), Fe (11.12%), Pb (2.09%), Al (5.05%), Recycling 2019, 4, 36 6 of 13 and Sn (1.63%). These results reveal that Cu and Fe were the major metals in the WPCBs. Therefore, the degree of selective extraction of Cu and Zn with the co-extraction of Fe, Ni, Al Pb, and Sn using selected lixiviants at ambient temperature will be discussed in detail in the following paragraphs.

3.2. Extraction in Hydrochloric Acid We studied the extraction of Cu and Zn, and the co-extraction of other metals from the pulverized WPCBs powder using varying concentrations of HCl lixiviant at ambient temperature for 12 h. However, the results presented in Figure4 show that the extraction e fficiency of Cu increased rapidly with increases in the concentration of HCl from 0.5 to 2.5 M. That is, the efficiency of Cu increased from 65.1% to 80.1% when the HCl concentration increased from 0.5 to 2.5 M. Further increases in the concentration from 2.5 to 5.0 M HCl had a negligible effect, as the efficiencies somewhat decreased because of incomplete dissociation equilibrium due to insufficient hydration. On the other hand, an increase in the concentration of HCl was observed to have an insignificant effect on the extraction efficiencies of Zn, with 58.5% to 62.3% extracted when the HCl concentration was increased from 0.5 to 5.0 M, respectively. Except for Ni, the co-extraction of other metals (Fe, Ni, Al, Pb, and Sn) was observed with extraction efficiencies increasing proportionately with increases in HCl concentrations. The exception of Ni is due to stable phase with titanium oxides as Ni(TiO3), which is known for its high corrosion resistance [19]. It has been reported that higher concentration of HCl or temperature is required to achieve higher Sn extraction [20]. However, high extraction efficiency of Sn was observed when the concentration of HCl was increased from 2.5 to 4.5 M. Our results are in tandem with the report of [21], which indicated that the rate of dissolution of Sn increases with increasing hydrogen concentration. A further increase in the concentration to 5.0 M showed slight decrease in the extraction efficiency. This was as a result of formation of black surface film due to increasing hydrogen ion concentration at higher HCl concentration [21]. Therefore, it is important to note that the extraction of powdered PCBs sample by HCl lixiviant is not suitable for generating pregnant leach solution (PLS) that is rich in Cu and Zn since higher

Recyclingdissolution 2019, 4 of, x other metals as impurities were observed. 7 of 14

100 Cu 90 Zn Ni 80 Fe Al 70 Pb Sn 60

50 40

% Extraction % 30 20 10 0 0.5 M 1.5 M 2.5 M 4.5 M 5.0 M HCl FigureFigure 4. 4. ComparativeComparative extraction extraction efficiencies efficiencies of of Cu Cu and and Zn, Zn, and and the the co-extraction co-extraction of of other other metals metals from from WPCBsWPCBs using using HCl HCl lixiviant, lixiviant, 50 50 g/L g/L pulp pulp density, density, 25 °C,◦C, 300 rpm, and 12 h. 3.3. Extraction in Sulfuric Acid 3.3. Extraction in Sulfuric Acid The degree of extraction of Cu and Zn, and the co-extraction of other metals from pulverized The degree of extraction of Cu and Zn, and the co-extraction of other metals from pulverized powdered WPCBs using different concentrations of H2SO4 lixiviant at ambient temperature for 12 h powdered WPCBs using different concentrations of H2SO4 lixiviant at ambient temperature for 12 h was also studied. In contrast to the results of HCl lixiviants, a high extraction efficiency of Zn was was also studied. In contrast to the results of HCl lixiviants, a high extraction efficiency of Zn was achieved in the H2SO4 lixiviant investigated (Figure 5). This behavior was as result of the high corrosion potentials of H2SO4 (−0.98 mV.SCE) as compared to HCl (−1.00 mV.SCE) [22]. Yoshida reported similar results [23]. Further slight increase in the extraction efficiencies of Zn was observed when the concentration of H2SO4 lixiviant increased to 4.0 M. The low extraction of Pb was due to non-solubility in sulfuric acid, as reported in [24]. Proportionate increases in the extraction efficiency of iron with respect to increases in concentration of H2SO4 lixiviant were observed. In comparison to results of HCl lixiviants, the extraction efficiencies of iron and Sn were considerably low. This was as a result of the lower pKa of H2SO4 as compared to HCl. Thus, the pKa for HCl and H2SO4 were found to be −6.3 and −3.0, respectively. This implies that HCl dissociates faster than H2SO4 [25]. Additionally, the lower extraction of Fe in all the H2SO4 treatments was because the main phase of Fe was copper- magnetite (CuFe2O3), as it has been reported that temperature has a greater effect on the dissolution of magnetite with sulfuric acid than the acid concentration [26]. Generally, the dissolution of powdered WPCBs in 0.5 M H2SO4 would produce PLS rich in Cu and Zn, with low concentrations of Fe and other metals. Recycling 2019, 4, 36 7 of 13

achieved in the H2SO4 lixiviant investigated (Figure5). This behavior was as result of the high corrosion potentials of H SO ( 0.98 mV.SCE) as compared to HCl ( 1.00 mV.SCE) [22]. Yoshida 2 4 − − reported similar results [23]. Further slight increase in the extraction efficiencies of Zn was observed when the concentration of H2SO4 lixiviant increased to 4.0 M. The low extraction of Pb was due to non-solubility in sulfuric acid, as reported in [24]. Proportionate increases in the extraction efficiency of iron with respect to increases in concentration of H2SO4 lixiviant were observed. In comparison to results of HCl lixiviants, the extraction efficiencies of iron and Sn were considerably low. This was as a result of the lower pKa of H2SO4 as compared to HCl. Thus, the pKa for HCl and H2SO4 were found to be 6.3 and 3.0, respectively. This implies that HCl dissociates faster than H SO [25]. − − 2 4 Additionally, the lower extraction of Fe in all the H2SO4 treatments was because the main phase of Fe was copper-magnetite (CuFe2O3), as it has been reported that temperature has a greater effect on the dissolution of magnetite with sulfuric acid than the acid concentration [26]. Generally, the dissolution of powdered WPCBs in 0.5 M H2SO4 would produce PLS rich in Cu and Zn, with low concentrations ofRecycling Fe and 2019 other, 4, x metals. 8 of 14

100 Cu Zn 90 Ni Fe 80 Al Pb 70 Sn 60

50 40

% Extraction % 30 20

10 0 0.5 M 1.5 M 2.5 M 4.0 M H SO 2 4 FigureFigure 5. 5.Comparative Comparative extraction extraction e ffiefficienciesciencies of of Cu Cu and and Zn, Zn, and and the the co-extraction co-extraction of of other other metals metals from from WPCBsWPCBs using using H H2SO2SO4 4lixiviant, lixiviant, 50 50 g /g/LL pulp pulp density, density, 25 25◦C, °C, 300 300 rpm, rpm, and and 12 12 h. h. 3.4. Extraction Study in Nitric Acid 3.4. Extraction Study in Nitric Acid Although nitric acid is well-known for its strong oxidizing power [27], we studied its efficiency for Although nitric acid is well-known for its strong oxidizing power [27], we studied its efficiency the extraction of Cu and Zn, and the co-extraction of other metals from powdered WPCBs at ambient for the extraction of Cu and Zn, and the co-extraction of other metals from powdered WPCBs at temperature (Figure6). It was observed that the extraction e fficiency of Cu increased slowly with ambient temperature (Figure 6). It was observed that the extraction efficiency of Cu increased slowly respect to increasing concentration from 1.0 to 3.5 M. A further increase in the concentration to 4.0 M with respect to increasing concentration from 1.0 to 3.5 M. A further increase in the concentration to did not have much effect on the extraction efficiency. Conversely, a high extraction efficiency of Zn was 4.0 M did not have much effect on the extraction efficiency. Conversely, a high extraction efficiency also achieved in HNO3 lixiviants, except 4.0 M HNO3, where the extraction of Zn was found to have of Zn was also achieved in HNO3 lixiviants, except 4.0 M HNO3, where the extraction of Zn was found decreased. The decreased in the dissolution efficiency might be a result of the poor dissociation of to have decreased. The decreased in the dissolution efficiency might be a result of the poor HNO3 at high concentration. dissociation of HNO3 at high concentration. The co-extraction of other metals was also explored. It was found that their extraction efficiencies The co-extraction of other metals was also explored. It was found that their extraction efficiencies increased with increasing in the concentrations of HNO3 lixiviant. The extraction efficiencies of Ni and increased with increasing in the concentrations of HNO3 lixiviant. The extraction efficiencies of Ni Fe increased from 5.1% and 6.7% to 40% and 15.8%, respectively, when the concentration of HNO and Fe increased from 5.1% and 6.7% to 40% and 15.8%, respectively, when the concentration 3of lixiviant increased from 0.5 to 4.0 M. The high extraction efficiency of Ni was due to the oxidizing HNO3 lixiviant increased from 0.5 to 4.0 M. The high extraction efficiency of Ni was due to the power of HNO3. However, 1.0 and 2.5 M HNO3 lixiviants were observed to yield PLSs with 60% and oxidizing power of HNO3. However, 1.0 and 2.5 M HNO3 lixiviants were observed to yield PLSs with 60% and 70% Cu and Zn, respectively. Additionally, the co-extraction of Fe and Ni in the PLS was observed to be <7.0%. Recycling 2019, 4, 36 8 of 13

70%Recycling Cu and 2019 Zn,, 4, respectively.x Additionally, the co-extraction of Fe and Ni in the PLS was observed9 to of 14 be <7.0%.

100 Cu Zn 90 Ni Fe 80 Al Pb Sn 70 60

50 40

% Extraction% 30 20 10 0 1.0 M 2.5 M 3.5 M 5.0 M HNO 3 FigureFigure 6. Comparative 6. Comparative extraction extraction eﬃ efficienciesciencies of Cuof Cu and and Zn, Zn, and and the the co-extraction co-extraction of otherof other metals metals from from

WPCBsWPCBs using using HNO HNO3 lixiviant,3 lixiviant, 50 g50/L g/L pulp pulp density, density, 25 ◦25C, °C, 300 300 rpm, rpm, and and 12 h.12 h. 3.5. Extraction in Trifluoromethanesulfonic Acid (TFMS) 3.5. Extraction in Trifluoromethanesulfonic Acid (TFMS) Organic acids (e.g., some sulfonic acids) are a class of strong acids with readily (bio)degradable Organic acids (e.g., some sulfonic acids) are a class of strong acids with readily (bio)degradable conjugate bases that have been demonstrated as generally less environmentally persistent than mineral conjugate bases that have been demonstrated as generally less environmentally persistent than acids such as sulfuric acid [28]. mineral acids such as sulfuric acid [28]. Methanesulfonic acid, CH3SO3H (pKa = 1.9), has been demonstrated as being highly efficient Methanesulfonic acid, CH3SO3H (pKa −= −1.9), has been demonstrated as being highly efficient for the dissolution of a number of different heavy metals via the formation of soluble methanesulfonate for the dissolution of a number of different heavy metals via the formation of soluble complexes [28]. Yet, TFMS with a pKa of 15 is also a derivative and has not been employed for the methanesulfonate complexes [28]. Yet, −TFMS with a pKa of−15 is also a derivative and has not been leaching of metals. However, a high extraction of Cu and Zn was achieved, although the extraction employed for the leaching of metals. However, a high extraction of Cu and Zn was achieved, efficiencies were almost the same when the concentration of TFMS increased from 1.0 to 2.5 M although the extraction efficiencies were almost the same when the concentration of TFMS increased (Figure7). Additionally, the extraction e fficiencies of Ni and Fe were observed to be <10% in 1.0 M from 1.0 to 2.5 M (Figure 7). Additionally, the extraction efficiencies of Ni and Fe were observed to TFMS lixiviant, while that of Fe increased to 18.5% when the concentration of TFMS was increased be <10% in 1.0 M TFMS lixiviant, while that of Fe increased to 18.5% when the concentration of TFMS from 1.0 to 2.5 M. The achieved Cu and Zn extraction of >65% was due to the fact that TFMS as was increased from 1.0 to 2.5 M. The achieved Cu and Zn extraction of >65% was due to the fact that organic acid exhibits almost the same strength as perchloric acids in all solvents [29]. As compared TFMS as organic acid exhibits almost the same strength as perchloric acids in all solvents [29]. As to H2SO4 lixiviant, it is interesting to note that 50% Pb was extracted in 1.0 M TFMS, while the compared to H2SO4 lixiviant, it is interesting to note that 50% Pb was extracted in 1.0 M TFMS, while efficiency decreased to 42.1% when the concentration increased to 2.5 M TFMS. The extractions of Al the efficiency decreased to 42.1% when the concentration increased to 2.5 M TFMS.2 The extractions1 and Sn were observed to be 20%. TFMS has a comparative conductivity of 348.0 S cm mol− [30] to of Al and Sn were observed to be 20%. TFMS has a comparative conductivity of 348.0 S cm2 mol−1[30] that of hydrochloric acid (346.1 S cm2 mol 1) and sulfuric acid (444.9 S cm2 mol 1), and higher than 2− −1 2 − −1 to that of hydrochloric acid (346.12 S cm1 mol ) and sulfuric acid (444.9 S cm mol ), and higher than methanesulfonic acid (299.6 S cm mol− ) reported for the efficient recovery of metals from solution methanesulfonic acid (299.6 S cm2 mol−1) reported for the efficient recovery of metals from solution using electro-winning [30]. The lower extraction efficiency of Cu and Zn was due to the incomplete using electro-winning [30]. The lower extraction efficiency of Cu and Zn was due to the incomplete dissociation of TFMS in aqueous solution, irrespective of the concentration as a result of its sulfonic dissociation of TFMS in aqueous solution, irrespective of the concentration as a result of its sulfonic end-group [31]. Therefore, TFMS can be used for the selective extraction of Cu and Zn, owing to end-group [31]. Therefore, TFMS can be used for the selective extraction of Cu and Zn, owing to aforementioned advantages and having co-extracted minimal metals as impurities. aforementioned advantages and having co-extracted minimal metals as impurities. Recycling 2019, 4, 36 9 of 13 Recycling 2019, 4, x 10 of 14

Recycling 2019, 4, x 10 of 14 100 Cu 100 90 Zn Cu Ni 90 Zn 80 Fe Ni Al 8070 Fe Pb Al 70 Sn 60 Pb 6050 Sn 5040

% Extraction 4030

% Extraction 3020 2010 100 0 1.0 M 2.5 M TFMS 1.0 M 2.5 M FigureFigure 7. Comparative 7. Comparative extraction extraction effi efficienciesciencies ofof CuCu and TFMS Zn, Zn, and and the the co-extraction co-extraction of other of other metals metals from from WPCBsFigureWPCBs using 7. using Comparative TFMS TFMS lixiviant, lixiviant, extraction 50 50 g/ Lg/Lefficiencies pulp pulp density, density, of Cu 25and25 ◦°C,C, Zn, 300 and rpm, rpm, the andco-extraction and 12 12 h. h. of other metals from WPCBs using TFMS lixiviant, 50 g/L pulp density, 25 °C, 300 rpm, and 12 h. 3.6. Extraction3.6. Extraction in Chloride in Chloride Media Media 3.6. Extraction in Chloride Media FigureFigure88 demonstrates demonstrates that that 33.5% 33.5% Cu Cu extraction extraction eefficiencyfficiency in in 0.5 0.5 M M NaCl NaCl + 0.1+ 0.1M CuCl M CuCl2 lixiviant2 lixiviant was achieved. When the concentration of CuCl2 in the lixiviant was increased i.e. (0.5 M NaCl + 0.5 was achieved.Figure8 When demonstrates the concentration that 33.5% ofCu CuClextraction2 in the efficiency lixiviant in 0.5 was M increased NaCl + 0.1 i.e., M CuCl (0.52 M lixiviant NaCl + 0.5 M CuCl2), the Cu extraction efficiency decreased drastically while relative increase in the extraction M CuClwas2 ),achieved. the Cu extractionWhen the concentration efficiency decreased of CuCl2 in drastically the lixiviant while was increased relative increase i.e. (0.5 M in NaCl the extraction + 0.5 efficiency Zn was achieved. Further increases in the concentration of CuCl2 to 1.0 M and NaCl to 4.5 M CuCl2), the Cu extraction efficiency decreased drastically while relative increase in the extraction efficiency Zn was achieved. Further increases in the concentration of CuCl2 to 1.0 M and NaCl to M (4.5 M NaCl + 1.0 M CuCl2) resulted in further decrease of Cu extraction efficiency (2.0%).The efficiency Zn was achieved. Further increases in the concentration of CuCl2 to 1.0 M and NaCl to 4.5 4.5 M (4.5 M NaCl + 1.0 M CuCl2) resulted in further decrease of Cu extraction efficiency (2.0%).The highest Cu selective extraction (33.0%), observed in 0.5 M NaCl + 0.1 M CuCl2 lixiviant was due to M (4.5 M NaCl + 1.0 M CuCl2) resulted in further decrease of Cu extraction efficiency (2.0%).The highestchloride Cu selective ions stabilized extraction Cu (II) (33.0%), ions, thereby observed increasing in 0.5 MCu NaClsolubility.+ 0.1 It Mcan CuCl be noticed2 lixiviant that wasthe due highest Cu selective extraction (33.0%), observed in 0.5 M NaCl + 0.1 M CuCl2 lixiviant was due to to chloride ions stabilized Cu (II) ions, thereby increasing Cu solubility. It can be noticed that the chlorideminimum ions concentration stabilized ofCu Na (II)Cl ions,solution thereby (0.1 M) increasing had an effect Cu onsolubility. the dissolution It can beof Cu.noticed Carneiro that andthe minimumminimumLeao (2007) concentration concentration also observed of NaClof Naa beneficialCl solution solution effect (0.1 (0.1 M) ofM) NaCl had an anon effect echalcopyriteffect on on the the dissolution residue dissolution leaching of Cu. of Cu.Carneiro using Carneiro FeCl and3. and However, the effect was less pronounced [32]. LeaoLeao (2007) (2007) also also observed observed a beneficiala beneficial e effectffect ofof NaClNaCl on on chalcopyrite chalcopyrite residue residue leaching leaching using using FeCl3 FeCl. 3. However,However, the etheffect effect was was less less pronounced pronounced [ 32[32].]. 100

10090 Cu Zn Cu 9080 Ni ZnFe 8070 NiAl FePb 7060 AlSn Pb 6050 Sn 5040

% Extraction 4030

% Extraction 3020

2010 100 0.5 M +0.1 M 0.5 M + 0.5 M 4.5 M + 0.1 M 4.5 M + 1.0 M 0 0.5 M NaCl+ 0.5 M + CuCl 4.5 M + 1.0 M 0.5 M +0.1 M 4.5 M 2+ 0.1 M NaCl + CuCl 2 Figure 8. Comparative extraction eﬃciencies of Cu and Zn, and the co-extraction of other metals from

WPCBs using NaCl + CuCl2 lixiviant, 50 g/L pulp density, 25 ◦C, 300 rpm, and 12 h. Recycling 2019, 4, x 11 of 14

Figure 8. Comparative extraction efficiencies of Cu and Zn, and the co-extraction of other metals from

WPCBs using NaCl + CuCl2 lixiviant, 50 g/L pulp density, 25 °C, 300 rpm, and 12 h.

On the other hand, a Zn extraction efficiency of 45.0% was achieved with 0.5 M NaCl + 0.5 M CuCl2 lixiviant, while 30% was achieved in other chloride lixiviants. Furthermore, a trace of Fe was observed in chloride media. That is, Fe (III) forms FeCl2+ and Fe3+ at lower Cl− concentrations, whereas FeCl2+ is formed at higher chloride concentrations [33–35]. There was no extraction of Fe in lixiviants containing 0.5 M NaCl, while low extraction efficiency was achieved by increasing the concentration to 4.5 M NaCl. Overall, the obtained results show that Fe dissolution was strongly related to the solution pH. At pH < 2, iron is known to remain soluble [36]. This is supported by Equations (4)–(7) below [37]. The extraction of Ni and Pb increased with increased chloride lixiviants.

Fe + H2O = Fe (OH)ads + H+ (4)

Fe + Cl− = Fe (Cl−) ads (5)

Recycling 2019, 4, 36 Fe (OH)ads + Fe (Cl−) ads = Fe + FeOH+ (Cl−) + 2e− 10 of(6) 13

FeOH+ + H+ = Fe2+aq + H2O (7) On the other hand, a Zn extraction efficiency of 45.0% was achieved with 0.5 M NaCl + 0.5 M CuCl lixiviant, while 30% was achieved in other chloride lixiviants. Furthermore, a trace of Fe was 3.7. Extraction2 in Hydroxide Media 2+ 3+ observed in chloride media. That is, Fe (III) forms FeCl and Fe at lower Cl− concentrations, whereas + FeCl2Theis selective formed at extraction higher chloride of Cu concentrationsand Zn, and the [33 –co-extraction35]. There was of noother extraction metals offrom Fe in powdered lixiviants WPCBscontaining sample 0.5 M was NaCl, carried while lowout extractionat ambient e ffitemperciencyature was achievedusing different by increasing concentrations the concentration of NaOH to lixiviants4.5 M NaCl. (Figure Overall, 9). the obtained results show that Fe dissolution was strongly related to the solution pH. AtThe pH highest< 2, iron Zn is knownextraction to remain efficiency soluble was [36 achi]. Thiseved is supportedin NaOH bylixiviants, Equations and (4)–(7) the belowextraction [37]. efficiencyThe extraction increased of Ni andwith Pb increasing increased withconcentratio increasedn from chloride 1.0 lixiviants.to 3.0 M. A further increase in the concentration had a negligible effect on the extraction efficiency. Zinc was selectively dissolved over + Cu with traces of Pb and Sn as impurities.Fe + H However,2O = Fe (OH) Zn adsis amphoteric,+ H it can act as an acid and react(4) with a strong base such as NaOH to form sodium zincate (Equation (8). The same factor applied to Sn, although its dissolution efficiency wasFe + ≤10%.Cl− = Fe (Cl−) ads (5) + Fe (OH)adsZnO + HFe2O (Cl + −NaOH) ads = →FeNaZn(OH)+ FeOH 3 (Cl+ H−2) + 2e− (8)(6) + + 2+ According to the XRD resultsFeOH of the+ powderedH = Fe aqWPCBs+ H2 Osample, ZnO and ZnSiO4 were the (7) predominant phases (Figure 1). The highest extraction of Zn as NaZn(OH)3 in NaOH lixiviants was supported3.7. Extraction by the in Hydroxide report by Media the literature that ZnO solid exists in the equilibrium state in the low- concentrationThe selective alkali extraction regions ofand Cu the and solubility Zn, and the of co-extractionzinc oxide is of almost other metalsinvariable from with powdered temperature. WPCBs Whensample alkali was concentration carried out at was ambient increased, temperature there was using suddenly different shift concentrations in the equilibrium of NaOH as reactant lixiviants ZnO reacted(Figure9 with). hydroxide to form product NaZn(OH)3, which indicated higher dissolution of Zn [39].

100 Cu 90 Zn Ni 80 Fe Al 70 Pb Sn 60

50 40

% Extraction 30

20 10 0 1.0 M 3.0 M 5.0 M NaOH . Figure 9. Comparative extraction eﬃciencies of Cu and Zn, and the co-extraction of other metals from

WPCBs using NaOH lixiviant, 50 g/L pulp density, 25 ◦C, 300 rpm, and 12 h.

The highest Zn extraction efficiency was achieved in NaOH lixiviants, and the extraction efficiency increased with increasing concentration from 1.0 to 3.0 M. A further increase in the concentration had a negligible effect on the extraction efficiency. Zinc was selectively dissolved over Cu with traces of Pb and Sn as impurities. However, Zn is amphoteric, it can act as an acid and react with a strong base such as NaOH to form sodium zincate (Equation (8). The same factor applied to Sn, although its dissolution efficiency was 10%. ≤

ZnO + H O + NaOH NaZn(OH) + H (8) 2 → 3 2 According to the XRD results of the powdered WPCBs sample, ZnO and ZnSiO4 were the predominant phases (Figure1). The highest extraction of Zn as NaZn(OH) 3 in NaOH lixiviants was supported by the report by the literature that ZnO solid exists in the equilibrium state in the low-concentration alkali regions and the solubility of zinc oxide is almost invariable with temperature. Recycling 2019, 4, 36 11 of 13 Recycling 2019, 4, x 12 of 14

WhenFigure alkali 9. Comparative concentration extraction was increased, efficiencies there of Cu was and suddenly Zn, and the shift co-extraction in the equilibrium of other metals as reactant from ZnO reactedWPCBs with using hydroxide NaOH lixiviant, to form 50 product g/L pulp NaZn(OH) density, 253 ,°C, which 300 rpm, indicated and 12 higherh. dissolution of Zn [37].

4.4. Selectivity Selectivity Studies Studies ThisThis experiment experiment was was purposely purposely conducted conducted to to obtain obtain suitable suitable lixiviants lixiviants that that would would exhibit exhibit high high selectivityselectivity for for Cu Cu and and Zn Zn while while retaining retaining Fe Fe and and Ni Ni with with other other metals metals in in the the residues residues for for further further hydrometallurgicalhydrometallurgical recovery. recovery. Since Since the the extraction extraction of ofother other metals metals was was obse observedrved to to increase increase with with increasesincreases in in the the concentration, concentration, extraction extraction efficienci efficiencieses of of metals metals with with dilute dilute lixiviants lixiviants were were used used to to calculatecalculate the the selectivity selectivity ratio, ratio, as as tabulated tabulated in in Figure Figure 10.10. Additionally, aa selectivityselectivity ratioratio ofof 5 5 was was set set as asthe the minimum minimum standard standard ratio ratio for for the the lixiviants lixiviants to beto judgedbe judged as easffective effective for thefor selectivethe selective extraction extraction of Cu ofand Cu Znand towards Zn towards Fe and Fe Ni. and It isNi. shown It is shown in Figure in Figure10 that the10 that selected the selected dilute HCl dilute lixiviants HCl lixiviants had selectivity had selectivityratios below ratios 5. Therefore, below 5. HClTherefore, lixiviant HCl exhibit lixiviant poor selectivity,exhibit poor as theselectivity, leach solution as the obtained leach solution contains obtainedhigher othercontains metals higher as impurities other metals which as might impurities require which a tedious might subsequent require a purification tedious subsequent process to purificationrecover. It isprocess interesting to recover. to note It thatis interesting Cu/Fe and to Zn note/Fe that ratios Cu/Fe using and both Zn/Fe 0.5 andratios 1.5 using M H bothSO 0.5were and5, 2 4 ≥ 1.5while M H Cu2SO/Ni4 were and Zn≥5,/Ni while were Cu/Ni10. Anand excellent Zn/Ni were selectivity ≥10. An ratio excellent ( 10) was selectivity achieved ratio by 1.0 (≥ M10) HNO was ≥ ≥ 3 achievedand 0.5 Mby+ 1.00.1 M M HNO NaCl3 and+ CuCl 0.5 Mlixiviants. + 0.1 M NaCl Similarly, + CuCl 2.52 lixiviants. M HNO oSimilarly,ffered moderate 2.5 M HNO selectivity3 offered ( 5) 2 3 ≥ moderatefor both Cuselectivity and Zn ( against≥5) for both Fe and Cu Ni. and In Zn TFMS against lixiviants, Fe and selectivityNi. In TFMS ratios lixiviants, ( 5) for selectivity Cu/Fe and ratios Zn/Fe ≥ (≥were5) for achieved Cu/Fe and using Zn/Fe 1.0 were M. achieved using 1.0 M.

Cu/Fe 30 Cu/Ni Zn/Fe 25 Zn/Ni

10 Selectivity ratio Selectivity

0 1M 5M 0. 0. M M M M 0 M 5 M + + 0 M 5 M 0.5 1.5 .5 .5 1. 2. M M 1. 2. 0 1 0.5 0.5 HNO HCl H SO TFMS NaCl + CuCl 3 2 4 2 FigureFigure 10. 10. PlotPlot of ofselectivity selectivity ratios ratios for for the the selected selected dilute dilute lixiviants. lixiviants.

5. Conclusions 5. Conclusion ThisThis study study demonstrated demonstrated thatthat CuCu and and Zn Zn could could be selectivelybe selectively extracted extracted from from WPCBs WPCBs using dilutedusing dilutedlixiviants lixiviants at longer at longer time. Thetime. extraction The extraction capacities capacities of each of lixiviant each lixiviant were investigated were investigated at diﬀerent at differentconcentrations concentrations with constant with constant time, mixing time, mixing intensity, intensity, and solid–liquid and solid–liquid ratio. Theratio. following The following can be canconcluded: be concluded: • CuCu and and Zn Zn were were extracted extracted with with other other metals metals in dilutedin diluted HCl HCl lixiviant, lixiviant, which which resulted resulted in ain low a • selectivitylow selectivity ratio and ratio made and HCl made lixiviants HCl lixiviant performs perform poorly forpoorly selective for selective extraction. extraction. • DilutedDiluted H SOH2SO, HNO4, HNO, and3, TFMSand TFMS lixiviants lixiviants performed performed excellently, excellently, with the highestwith the selectivity highest of • 2 4 3 Cuselectivity and Zn toward of Cu Fe.and Zn toward Fe. • TheThe selective selective extraction extraction of of Zn Zn without without the the extractionextraction ofof other metals was was achieved achieved in in NaOH NaOH • lixiviantslixiviants because because Zn Zn acted acted as as an an amphoteric amphoteric oxide. oxide. TheThe high selectivity of of Zn Zn toward toward Fe Fe was was achieved in 0.5 M NaCl + 0.5M CuCl2 lixiviant. achieved in 0.5 M NaCl + 0.5M CuCl2 lixiviant. • Selectivity of Cu and Zn toward Fe in 0.5 M NaCl + 0.1 M CuCl2 lixiviant was obtained. As Selectivity of Cu and Zn toward Fe in 0.5 M NaCl + 0.1 M CuCl2 lixiviant was obtained. As the • the concentration of CuCl2 increased to 0.5 M, the Cu extraction efficiency decreased concentration of CuCl2 increased to 0.5 M, the Cu extraction eﬃciency decreased drastically. drastically. Therefore,Therefore, diluted diluted H H2SO2SO4,4 HNO, HNO3, 3TFMS,, TFMS, and and 0.5 0.5 M M NaCl NaCl + +0.10.1 M M CuCl CuCl2 produced2 produced pregnant pregnant leach leach solutionssolutions rich rich in in Cu Cu and and Zn Zn with with traces traces of of other other metals. metals. This This work work also also provides provides a afoundation foundation which which Recycling 2019, 4, 36 12 of 13 substantiates further research into the performance of TFMS for the extraction of metals’ value, since its application has not been reported previously.

Author Contributions: Conceptualization, A.E.A., O.A.A., O.J.A.; Methodology, A.E.A.; Formal Analysis, M.K.G., A.E.A.; Investigation, A.E.A.; Original draft A.E.A., F.E.O.; Review and Editing, M.K.G.; Supervision, S.B. and M.K.G.; Project administration, S.B. Funding: This research received no external funding. Acknowledgments: The authors gratefully acknowledge the Council of Scientific and Industrial Research (CSIR) India, as well as The World Academy of Science (TWAS), Italy, for the award of Doctoral fellowship to Ajiboye Emmanuel Ayorinde. Conflicts of Interest: The authors declare no conflict of interest.

References

1. UNEP. E-Waste Inventory Assessment Manual; UNEP: Nairobi, Kenya, 2007; Volume 1. 2. Sheng, P.P.; Etsell, T.H. Recovery of gold from computer circuit boards. Waste Manag. Resour. 2007, 25, 380–383. [CrossRef] 3. Nguyen, N.V.; Lee, J.; Jha, M.K.; Yoo, K.; Jeong, J. Copper recovery from low concentration waste solution using Dowex G-26 resin. Hydrometallurgy 2009, 97, 237–242. [CrossRef] 4. Ebin, B.; Isik, M.I. Pyrometallurgical Processes for the Recovery of Metals from WEEE. In WEEE Recycling; Elsevier: Amsterdam, The Netherlands, 2006; pp. 107–137. 5. Tuncuk, A.; Stazi, V.; Akcil, A.; Yazici, E.Y.; Deveci, H. Aqueous metal recovery techniques from e-scrap: Hydrometallurgy in recycling. Miner. Eng. 2012, 25, 28201337. [CrossRef] 6. Bryan, C.G.; Watkin, E.L.; McCredden, T.J.; Wong, Z.R.; Harrison, S.T.L.; Kaksonen, A.H. The use of pyrite as a source of lixiviant in the bioleaching of electronic waste. Hydrometallurgy 2015, 152, 33–43. [CrossRef] 7. Kumar, M.; Lee, J.C.; Kim, M.S.; Jeong, J.; Yoo, K. Leaching of metals from waste printed circuit boards (WPCBs) using sulfuric and nitric acids. Environ. Eng. Manag. J. 2014, 13, 2601–2607. 8. Grause, G.; Ishibashi, J.; Kameda, T.; Bhaskar, T.; Yoshioka, T. Kinetic studies of the decomposition of ﬂame retardant containing high-impact polystyrene. Polym. Degrad. Stab. 2010, 95, 1129–1137. [CrossRef] 9. Arshadi, M.; Mousavi, S.M. Simultaneous recovery of Ni and Cu from computer-printed circuit boards using bioleaching: Statistical evaluation and optimization. Bioresour. Technol. 2014, 174, 233–242. [CrossRef] 10. Natarajan, G.; Ting, Y.P. Pretreatment of e-waste and mutation of alkali-tolerant cyanogenic bacteria promote gold biorecovery. Bioresour. Technol. 2014, 152, 80–85. [CrossRef] 11. Saidan, M.; Brown, B.; Valix, M. Leaching of electronic waste using biometabolised acids. Chin. J. Chem. Eng. 2012, 20, 530–534. [CrossRef] 12. Cui, J.; Zhang, L. Metallurgy Recovery of Metals from Electronic Waste: A Review. J. Hazard. Mater. 2008, 158, 228–256. [CrossRef] 13. Das, S.C.; Gopala, P.; Krishna, D. Eﬀect of Fe3+ during copper electrowinning at higher current density. Int. J. Miner. Process. 1996, 46, 91–105. [CrossRef] 14. Shaw, D.R.; Dreisinger, D.B.; Lancaster, T.; Richmond, G.D.; Tomlinson, M. The commercialization of the FENIX iron control system for purifying copper electrowinning electrolytes. J. Mater. 2004, 38–41. [CrossRef] 15. Ochromowicz, K.; Jeziorek, M.; Wejman, K. Copper (II) Extraction from ammonia leach solution. Physicochem. Probl. Miner. Process. 2014, 50, 327–335. 16. Jergensen, G.V. Copper Leaching, Solvent Extraction and Electrowinning Technology; SME Inc.: Littleton, CO, USA, 1999; pp. 267–268. 17. Miller, J.D.; Haung, H.H.; Pereira, E.F. Equilibrium and kinetics of copper extraction from ammonia solutions by hydroxyoximes with particular emphasis on transport phenomena. In Process and Fundamental Considerations of Selected Hydrometallurgical Systems; Society of Mining Engineers of American Institute of Mining, Metallurgical, and Petroleum Engineers: Englewood, CO, USA, 1981; pp. 221–241. 18. Radmehr, V.; Koleini, S.M.J.; Khalesi, M.R.; Mohammadi, M.R.T. Ammonia leaching in the copper industry: A Review. In Proceedings of the International Mineral Processing Congress (IMPC), New Delhi, India, 23–28 September 2012; p. 487. Recycling 2019, 4, 36 13 of 13

19. Gehrmann, B.; Hattendorf, H.; Kolb-Telips, A.; Kramer, W.; Mottgen, W. Corrosion behavior of soft magnetic iron-nickel alloys. Mater. Corrossion 2004, 48, 535–541. [CrossRef] 20. Ranitović,M.; Kamberović,Ž.; Korać,M.; Jovanović,N.; Mihjalović,A. Hydrometallurgical Recovery of tin and lead from waste printed circuit boards (WPCBs): Limitations and opportunities. Metalurgija 2016, 55, 153–156. 21. Lui, W.K. Dissolution of Tin in Hydrochloric Acid. Electronic Theses and Dissertations 6311. Master’s Thesis, University of Windsor, Windsor, ON, Canada, 1962. 22. Zhang, X.G. Corrosion and Electrochemistry of Zinc; Springer: Berlin, Germany, 1996. 23. Yoshida, T. Leaching of Zinc Oxide in Acidic Solution. Materials Transactions 2003, 44, 2489–2493. [CrossRef] 24. Rusen, A.; Sunkar, A.S.; Topkaya, Y. Zinc and lead extraction from Cinkur leach residues by using hydrometallurgy method. Hydrometallurgy 2008, 93, 45–50. [CrossRef] 25. Koltrly, S.; Sucha, L. Handbook of Chemical Equilibria in Analytical Chemistry; Ellis Horwood Ltd.: Chichester, UK, 1985. 26. Salmimies, R.; Mannila, M.; Kallas, J.; Häkkinen, A.H. Acidic dissolution of magnetite: experimental study on the effects of acid concentration and temperature. Clays Clay Miner. 2011, 59, 136–146. [CrossRef] 27. Clarke, S.I.; Mazzafro, W.J. Nitric Acid; Wiley Online Library: Hoboken, NJ, USA, 2005. [CrossRef] 28. Michael, D.G.; Min, W.; Thomas, B.; Patrick, J. Environmental benefits of methanesulfonic acid: comparative properties and advantages. Green Chem. 1999, 1, 127–140. 29. Posternak, A.G.; Garlyauskayte, R.Y.; Polovinko, V.V.; Yagupolskii, M.L.; Yagupolskii, Y.L. New kinds of organic superacids. Bis (perfluoroalkylsulfonylimino)-trifluoromethanesulfonic acids and their trimethylsilyl esters. Org. Biomol. Chem. 2008.[CrossRef] 30. Borra, C.R.; Pontikes, Y.; Binnemans, K.; Van Gerven, T. Leaching of rare earths from bauxite residue (red mud). Miner. Eng. 2015, 76, 20–27. [CrossRef] 31. Sampoli, M.; Marziano, N.C. Tortato, C. Dissociation of Tritluoromethanesutfonic Acid in Aqueous Solutions by Raman Spectroscopy. J. Phys. Chem. 1989, 93, 7252–7257. [CrossRef] 32. Carneiro, M.F.C.; Leao, V.A. The role of sodium chloride on surface properties of chalcopyrite leached with ferric sulphate. Hydrometallurgy 2007, 87, 73–82. [CrossRef] 33. Ashurst, K.G. The Thermodynamics of the Formation of Chlorocomplexes of Iron (III), Cobalt (II), Iron (II), Manganese (II) in Perchlorate Medium; National Institute of Metallurgy: Randburg, South Africa, 1976; Volume 1820, pp. 1–43. 34. Peek, E.M.; Van Weert, G. Chloride metallurgy. In Proceedings of the 32nd Annual Hydrometallurgy Meeting and International Conference of the Practice and Theory of Chloride/Metal interaction, Montreal, QC, Canada, 19–23 October 2002; Volume 23, pp. 760–780.

35. Misawa, T. The thermodynamics consideration for Fe-H2O system at 25 ◦C. Corros. Sci. 1973, 13, 659–676. [CrossRef] 36. Darwish, N.A.; Hilbert, F.; Lorenz, W.J.; Rosswag, H. The inﬂuence of chloride ions on the kinetics of iron dissolution. Electrochim. Acta 1973, 18, 421–425. [CrossRef] 37. Chen, A.; Xu, D.; Chen, X.; Zhang, W.; Liu, X. Measurements of zinc oxide solubility in sodium hydroxide

solution from 25 to 100 ◦C. Trans. Nonferrous Met. Soc. China 2012, 22, 1513–1516. [CrossRef]

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