Citation: Serga, V.; Zarkov, A.; Shishkin, A.; Melnichuks, M.; Pankratov, V. Investigation of the Impact of Electrochemical Hydrochlorination Process Parameters on the Efficiency of Noble (Au, Ag) and Base Metals Leaching from Computer Printed Circuit Boards. Metals 2024, 14, 65. https://doi.org/10.3390/ met14010065 Academic Editors: Bernd Friedrich, Petros E. Tsakiridis and Alexander Birich Received: 8 November 2023 Revised: 23 December 2023 Accepted: 3 January 2024 Published: 5 January 2024 Copyright: © 2024 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/). metals Article Investigation of the Impact of Electrochemical Hydrochlorination Process Parameters on the Efficiency of Noble (Au, Ag) and Base Metals Leaching from Computer Printed Circuit Boards Vera Serga 1 , Aleksej Zarkov 2,3 , Andrei Shishkin 4,5,* , Maksims Melnichuks 1 and Vladimir Pankratov 2 1 Institute of Materials and Surface Engineering, Faculty of Materials Science and Applied Chemistry, Riga Technical University, 3/7 P. Valdena Street, LV-1048 Riga, Latvia; vera.serga@rtu.lv (V.S.); maksims.melnicuks@rtu.lv (M.M.) 2 Institute of Solid State Physics, University of Latvia, 8 Kengaraga Street, LV-1063 Riga, Latvia; aleksej.zarkov@chf.vu.lt (A.Z.); vladimirs.pankratovs@cfi.lu.lv (V.P.) 3 Institute of Chemistry, Vilnius University, 24 Naugarduko Street, LT-03225 Vilnius, Lithuania 4 ZTF Aerkom SIA, 32 Miera Street., Salaspils, Salaspils nov., LV-2169 Riga, Latvia 5 Laboratory of Ecological Solutions and Sustainable Development of Materials, Institute of General Chemical Engineering, Faculty of Materials Science and Applied Chemistry, Riga Technical University, 3/7 P. Valdena Street, LV-1048 Riga, Latvia * Correspondence: andrejs.siskins@rtu.lv Abstract: The development of environmentally friendly and energy-saving processes for recycling electronic waste (e-waste) is still relevant today. The research presented in this work relates to hydrometallurgy, namely, the electrochemical leaching of metals from e-waste under the action of alternating current (AC) into hydrochloric acid solutions of electrolytes, and can be used for leaching both noble and non-ferrous metals from secondary raw materials. The main object of the study was disintegrator-crushed mixed computer PCBs metal-rich powders with a particle size (d) of <90 µm. The impact of such leaching process parameters as temperature (Tel) and composition of the electrolyte solution, AC density (i) on the electrodes, experiment duration (tex) while maintaining a constant electrolyte temperature (60 ◦C, 70 ◦C, and 80 ◦C) on the metal (Au, Ag, Cu, Al, Ni, Pb, Sn, Ti, Zn, and Fe) leaching efficiency has been studied. In addition, under similar experimental conditions, but without external control of Tel, the kinetics of metal leaching from raw material powders obtained via PCBs single and double crushing in a disintegrator has been also presented. Comparison of raw material powders obtained from different batches of the source material showed both the variability of its chemical composition and the different kinetics of Au and Ag leaching under the same experimental conditions. The optimal conditions for pretreatment of the raw material obtained by single crushing in a disintegrator (CHCl = 6 mol·L−1, i = 0.88 A·cm−2, tex = 1 h, solid- to-liquid ratio—8.6 g·L−1 and without external control of Tel) were determined. It has been shown that this electrochemical pretreatment is accompanied by transition of only base metals into the electrolyte solution, making it possible to significantly reduce their concentration in the final solution. Under pretreatment conditions, the following degree of metal leaching (RMe) has been established: RCu = 98.2%, RAl = 62.8%, RNi = 53.4%, RPb = 93.2%, RSn = 98.0%, RTi = 88.5%, RZn = 61.6%, and RFe = 78.8%. As a result of a subsequent two-hour electrochemical treatment of a solid residue, the degree of leaching of gold and silver was 73.6% and 86.7%, respectively. The presented results provide a broader understanding of the possibility of using the proposed electrochemical hydrochlorination method for noble and base metals leaching from waste PCBs. The novelty and practical value of this research is a validation of the developed technology in laboratory conditions using the real batch of the PCBs. This approach may also be useful to researchers involved in the recycling of other types of secondary raw materials. Keywords: electrochemical leaching; alternating current; mixed printed circuit boards; base metals; gold; silver Metals 2024, 14, 65. https://doi.org/10.3390/met14010065 https://www.mdpi.com/journal/metals Metals 2024, 14, 65 2 of 25 1. Introduction Vast quantities of waste electrical and electronic equipment (WEEE) have been increas- ingly produced in recent years. Based on recent data from the European Union statistics institute [1] the quantity of electrical and electronic equipment introduced into the EU market saw a progression from 7.6 million tons in 2012 to a peak of 13.5 million tons in 2021. Notably, in 2013, the recorded amount hit its lowest point at 7.3 million tons. Throughout the entire period spanning from 2012 to 2021, the influx of EEE in the market exhibited a 77.1% increase. Correspondingly, the total collected WEEE augmented from 3.0 million tons in 2012 to 4.9 million tons in 2021, marking a rise of 65.1%. Figure 1 reflects the statistical data for small equipment and small IT and telecommunication equipment in absolute value in tons, by country, in the 2019–2021 period. Metals 2024, 14, x FOR PEER REVIEW  2  of  26  Keywords: electrochemical leaching; alternating current; mixed printed circuit boards; base metals;  gold; silver    1. Introduction Vast quantities of waste electrical and electronic equipment  (WEEE) have been  in- creasingly produced in recent years. Based on recent data from the European Union sta- tistics institute [1] the quantity of electrical and electronic equipment introduced into the  EU market saw a progression from 7.6 million tons in 2012 to a peak of 13.5 million tons  in 2021. Notably,  in 2013,  the  recorded amount hit  its  lowest point at 7.3 million  tons.  Throughout the entire period spanning from 2012 to 2021, the influx of EEE in the market  exhib ted a 77.1% increase. Correspondingly, the total collected WEEE augmented fr m  3.0 milli n tons  n 2012 to 4.9 million tons   2021, marking a rise of 65. %. Figure 1 reflect   the statistica  data for sm ll equipment and small IT a d telecommunicatio  equipm nt  in ab olute value in tons, by country, in the 2019–2021 period.  Figure 1. Waste electronic equipment (small equipment with no external dimension greater than 50  cm and small IT and telecommunications equipment with no external dimension greater than 50  cm) waste collection statistics. Reprinted/Adapted from Ref. [2]. Printed circuit boards (PCBs) are estimated to account for 3 to 5% of total WEEE by weight [3]. The composition of PCBs includes such non-conductive materials as a lami- nate, including a polymer binder (epoxy or phenolic resins) and reinforcing fillers (glass,  paper, polyester fiber  fabrics), as well  as  ceramics  and plastics  located  in  components  mounted to PCBs. The metal (conductive) part contains both base metals (copper, alumi- num, tin, lead, zinc, nickel, and iron) and expensive noble metals (gold, silver, and palla- dium) [4,5]. Therefore, efficient recycling of waste PCBs is important both for conserving  metal resources and preventing environmental pollution.  In PCBs recycling, the predominant commercial process is the pyrometallurgical pro- cess,  which  focuses  on  the  recovery  of  precious  metals  and  copper  [6,7]  The  main  1 10 100 1,000 10,000 100,000 1,000,000 Lie ch te ns te in Cy pr us M al ta Ic el an d Lu xe m bo ur g Es to ni a La tv ia Lit hu an ia Cr oa tia Sl ov en ia Gr ee ce Ire la nd Ro m an ia Bu lg ar ia Sl ov ak ia Po rt ug al Fi nl an d Hu ng ar y Cz ec hi a De nm ar k Sw ed en Au st ria No rw ay Be lg iu m Ne th er la nd s Sp ai n Ita ly Po la nd Fr an ce Ge rm an y To ns 2019 2020 2021 Figure 1. te electronic equipment (small equipmen with no external d mension greater than 50 cm and small IT and telecommunications equipment with no external dimension greater than 50 cm) waste collection statistics. Reprinted/Adapted from Ref. [2]. Printed circuit boards (PCBs) are estimated to account for 3 to 5% of total WEEE by weight [3]. The composition of PCBs includes such non-conductive materials as a laminate, including a polymer binder (epoxy or phenolic resins) and reinforcing fillers (glass, paper, polyester fiber fabrics), as well as ceramics and plastics located in components mounted to PCBs. The metal (conductive) part contains both base metals (copper, aluminum, tin, lead, zinc, nickel, and iron) and expensive noble metals (gold, silver, and palladium) [4,5]. Therefore, efficient recycling of waste PCBs is important both for conserving metal resources and preventing environmental pollution. In PCBs recycling, the predominant commercial process is the pyrometallurgical process, which focuses on the recovery of precious metals and copper [6,7] The main disadvantages of this technology are high energy consumption, environmental hazards, insufficient selectivity, and the need for additional operations (usually hydrometallurgical). Hydrometallurgy for recycling e-waste is a sustainable alternative to pyrometallurgy. The hydrometallurgical process involves two main steps: transferring metals from the solid matrix into the aqueous phase (leaching) and separating the target metals from the produced solution. To separate and concentrate target metals from leaching solutions, liquid–liquid extraction, ion exchange, and precipitation are most often used in hydromet- allurgical processes [7,8]. Before leaching precious metals from waste PCBs, pretreatment Metals 2024, 14, 65 3 of 25 is usually performed, including physical (crushing and various operations to enrich the metal component of scrap: pneumatic separation, magnetic separation, screening, eddy current separation, and electrostatic separation) and chemical pretreatments (with mineral acids) [9]. It is noted that, as a result of physical pretreatment, the precious metals present in the raw material in small or trace quantities are partially lost both in the mass of metal dust and plastic particles [3,10]. Cyanides, thiosulfates, thiourea, and halides are used in hydrometallurgical techniques [9,11]. The mechanisms of the reactions of precious metal leaching from PCBs with these reagents, and progress in hydrometallurgical technologies, are presented in a number of reviews published in the last decade [12–17]. Cyanide (sodium or potassium), as a gold leaching agent, dominates in the mining industry due to its high leaching efficiency, low consumption, and its ability to carry out the process in an alkaline environment [18,19]. However, this reagent is extremely toxic. In terms of toxicity and environmental impact, the use of chelating agents, including thiosulfates (sodium and ammonia) [20] and thiourea [21] to leach precious metals from PCBs is less hazardous. The use of these reagents makes it possible to achieve a high degree of leaching of Au and Ag. However, both hydrometallurgical processes using thiosulfate and thiourea require large amounts of reagent due to the slow kinetics of thiosulfate and the low stability of thiourea [22]. Thiourea has a higher dissolution rate of gold than cyanide [23]. This reagent is considered as an alternative to cyanide for the leaching of gold from refractory ores [24]. Research results on the use of thiourea for leaching gold from waste PCBs [21,25,26] and different types of random-access memory (RAM) sticks [11] are also reported. The selectivity of the leaching of gold by thiourea increases in the absence of base metals. Therefore, when leaching this metal from e-waste, pre-leaching of base metals is mandatory. PCBs include different components containing Au: microchips (processors, RAM) and electric contacts. Typically, in gold leaching studies, separate gold-rich components are used. In real cases, waste PCBs containing gold only in non-dismantled components are used for recycling. The use of thiourea as a leaching agent of Au from such PCBs, in our opinion, has the following disadvantages: (i) part of the gold-plated contacts mounted on PCBs are located under a layer of solder, which inevitably leads to the loss of valuable metal and (ii) the selectivity of gold leaching is difficult to control during the process. The use of the preliminary leaching of base metals, on the one hand, can lead to an increase in the Au leaching efficiency and, on the other hand, to losses of valuable metal at the stages of its subsequent filtering and washing. The chlorine method (blowing chlorine through molten unrefined gold) and the hydrochlorination method (chlorination in hydrochloric acid solution) are used for gold and platinum group metal refining, respectively [27]. The use of halogen/halide systems (chlorine, bromine, iodine/chloride, bromide, iodide) to dissolve gold preceded cyanidation [28]. Gold forms both Au(I) and Au(III) complexes with chloride, bromide and iodide, depending on the concentration of the solution. However, among the halides, only chlorine/chloride was used on an industrial scale [29]. The use of chlorine instead of oxygen as an oxidizing agent, and chloride instead of cyanide as a complexing agent, significantly increases the rate of gold dissolution [28,30]. However, chlorine leaching of gold is more difficult to use than cyanide leaching for two main reasons: (i) special stainless steel and rubber-lined equipment is required that is resistant to highly aggressive acidic and oxidizing conditions; (ii) chlorine gas is a highly toxic substance, and therefore, both during storage and when working with it, special safety measures must be observed. Therefore, in situ production of chlorine is proposed to minimize the risks associated with the use of chlorine in chloride leaching systems [31,32]. The main advantage of this approach is that there is no need for its separate production, storage, and handling [31]. The kinetics of the hydrochlorination of pure gold with molecular chlorine, generated in situ as a result of the chemical reaction between sodium hypochlorite and hydrochloric acid, has been studied by various authors [32]. The presented results show that the rate of Metals 2024, 14, 65 4 of 25 metal dissolution is mainly determined by the concentration of the trichloride ion (Cl3−) in the solution. To avoid chlorine emissions into the atmosphere during the hydrochlorination of secondary raw materials, it is also preferable to conduct a process with low chlorine consumption to improve the environmental friendliness of the process. To implement the hydrochlorination process for leaching metals from PCBs, the authors [33–37] proposed a different approach. In these studies, chlorine was produced in a separate electrochemical cell with an anion exchange membrane via electrolysis of an HCl solution and supplied to a leaching reactor. In this case, the products of dissolution of Cl2(gas.) in the leaching solution such as: Cl2(aq), Cl3−, and HClO acted as metal oxidizer [38]. The leaching behavior of base metals, such as Cu, Zn, Pb, Sn [33], and Au only [34], and the mixture of the metals: Ag, Au, Cu, Pb, Pd, and Sn [35] from shredded PCB’s, have been detailed studied. Within the framework of our proposed technique [39,40], the process of generat- ing chlorine and the leaching of metals from PCBs are combined in one reactor. More- over, chlorine is produced under conditions of non-stationary electrolysis of hydrochloric acid solutions, using alternating current (AC). An innovative high-energy semi-industrial disintegration-milling system was used to crush the PCBs [41]. The main idea that guided us when choosing the disintegrator and hydrochlorination scheme was to minimize the loss of noble metals at the stages of physical pretreatment of the source material (computer PCBs) and ensure the completeness of their transfer into solution at the leaching stage. The transfer of all metals into the solution within the framework of the proposed approach makes it possible, on the one hand, to obtain a solid residue free of metals, which creates the possibility of its further use and does not create problems with its disposal, which is important from an environmental point of view. On the other hand, the resulting electrolyte solutions can be further used for complex recycling, because they contain not only expensive noble metals, but also non-ferrous metals. As a result of the proposed process, all metals contained in the PCBs powder trans- ferred into the electrolyte solution. The main feature of produced multicomponent solutions is not only the low content of noble metals in the background of the high content of base metals, but also a high concentration of hydrochloric acid. Therefore, at the stage of metal separation, special requirements are imposed on sorbents and extractants: selectivity in relation to the target metal and stability in a strongly acidic environment. Gold (III) halide complexes are easily extracted from acidic solutions with various types of known organic extractants. But selective extraction is easily achieved only from weakly acidic solutions [42]. Currently, effective and economical methods for the selective extraction of noble metals, based on the use of nitrogen- and sulfur-containing sorbents, are being developed. It has been demonstrated that the S,N-rich MOF (metal–organic framework) has a high adsorption capacity, fast adsorption rate, and good selectivity under acidic conditions for gold recovery [43]. To isolate noble metals from acidic chloride solutions obtained as a result of the leaching of computer PCBs in aqua regia, mesoporous carbons and sulfur- impregnated mesoporous carbon have been proposed. It has been shown that the efficiency of Au(III) adsorption in the concentration range of from 1 to 5 M HCl is noticeably higher than for Pd(II) and Pt(IV) [44]. In addition, for the selective adsorption of precious metals from hydrochloric acid solutions, it has been proposed to use such sorbents as nanosized Fe3O4@SiO2-NH2 [45]. One of the most common methods for removing iron (III) from chloride solutions is its hydrolytic precipitation [46]. However, when recycling acidic solutions to avoid acid neutralization, the liqui–liquid extraction method is used. The possibility of selective extraction of Fe3+ ions from acidic chloride solutions (CHCl > 6 mol·L−1) with 2-octanone in the presence of macro amounts of nickel chloride as well as the trace impurities of Co and Cu has been demonstrated [47]. In turn, the results presented in the paper [48] showed that the use of a solution of hydrazides of Versatic acids (fraction C15–C19) in kerosene Metals 2024, 14, 65 5 of 25 as an extractant with the addition of tributyl phosphate (TBP, 10%) makes it possible to selectively extract copper in the presence of Fe3+, Ni2+, and Co2+ at CHCl > 1 mol·L−1. It should be noted that studies on the isolation of metals from the resulting electrolyte solutions are not the subject of the studies presented in this work, and will be carried out in the future at the next stage of research. The purpose of this work is to study the influence of the most important parameters of the metal leaching process from fine raw material powders obtained from disintegrator- crushed mixed computer PCBs at a controlled electrolyte temperature within the previously proposed electrochemical hydrochlorination technique [39]. Continuation of the search for optimal process conditions for the most complete leaching of metals from the raw materials under study and the production of electrolyte solutions suitable for further recovery of valuable metals is also presented. 2. Materials and Methods Preliminary preparation of raw materials (mixed computer PCBs) includes: PCB dismantling; double crushing in a hammer mill; single or double crushing using the high-energy semi-industrial disintegration-milling system (disintegrator DSL-350, Tallinn University of Technology, Tallinn, Estonia) [41]; and subsequent sieving using a FRITSCH ANALYSETTE 3 PRO Vibratory Sieve Shaker (FRITSCH GmbH, Weimar, Germany). As a result, the finest fraction of the obtained powders with a particle size (d) < 90 µm was selected as a research material. The experimental setup is represented in Figure 2—a common view Figure 2a. It consisted of an alternating current (AC) circuit (Figure 2b) and an electrochemical cell. The AC circuit included a step-down transformer, a laboratory autotransformer, and a rheostat, as well as an AC ammeter and AC voltmeter to measure current and voltage, respectively (Figure 2b) [39]. Graphite rods (d = 8 mm) were used as electrodes. During the electrochemical leaching process, the required current value was maintained constant. A polypropylene reactor—cell 1 (Figure 2c) and a water-jacketed glass reactor—cell 2 (Figure 2d) were used as an electrochemical cell. Cell 1 was used in experiments without adjusting the electrolyte temperature (at ambient temperature) during the electrochemical process. When studying the influence of parameters of the electrochemical process on the efficiency of metal leaching, experiments were carried out in cell 2 while maintaining a constant electrolyte temperature. The choice of temperature was based on the maximum temperature that the electrolyte reaches during the electrochemical process at a given current density (i) in the absence of external control of electrolyte temperature. During experiments, the electrolyte temperature (Tel) was measured using a chromel–alumel thermocouple. Powders of the raw material were used in the form of a dispersed (solid) phase in the electrolyte solution (liquid phase). The solid-to-liquid (S/L) ratio in the suspension was 8.6 g·L−1. In all experiments, the volume of the electrolyte was 350 mL, and the concentration of the hydrochloric acid was 4, 6, and 8 mol·L−1. After filling the reactor with the initial components (PCB powder and electrolyte solution) to create a suspension, the mixture was stirred at a speed of 1200 rpm for 5 min, then the stirring speed was reduced to 700 rpm, and the power was turned on. To produce chlorine, an alternating current (AC) of industrial frequency (50 Hz) with a density (i) of 0.21 A·cm−2, 0.42 A·cm−2, 0.63 A·cm−2, 0.84 A·cm−2, and 0.88 A·cm−2 was used. The duration of the experiments (tex) varied from 0.5 to 6 h. When carrying out a two-stage leaching process, chemical or electrochemical pretreatment of the raw material was first carried out. Then, after removing the leach solution and adding a fresh portion of electrolyte to the wet residue of the raw material, electrochemical leaching was performed. Chemical pretreatment of the raw material powder was carried out at a constant electrolyte temperature (Tel = 70 ◦C) under conditions of stirring of reaction mixture for an hour. Electrochemical pretreatment was also carried out for an hour under the same experimental conditions as the subsequent electrochemical leaching process. Metals 2024, 14, 65 6 of 25 Metals 2024, 14, x FOR PEER REVIEW  6  of  26         (c)  (d)  Figure 2. Experimental setup: a common view without power supply and control (a), AC circuit,  where Tr1—laboratory autotransformer, R1—rheostat, Tr2—step-down transformer, V—AC volt- meter, A—AC ammeter (b), and schematic representation of electrochemical cells: polypropylene  cell—cell 1 (c) and a water-jacketed glass cell—cell 2 (d). The (b) section is Reprinted/Adapted from  Ref. [39] (CC BY Open Access, MDPI).  A polypropylene reactor—cell 1 (Figure 2c) and a water-jacketed glass reactor—cell  2 (Figure 2d) were used as an electrochemical cell. Cell 1 was used in experiments without  adjusting the electrolyte temperature (at ambient temperature) during the electrochemical  process. When studying the influence of parameters of the electrochemical process on the  efficiency of metal leaching, experiments were carried out in cell 2 while maintaining a  constant electrolyte temperature. The choice of temperature was based on the maximum  temperature that the electrolyte reaches during the electrochemical process at a given cur- rent density (i) in the absence of external control of electrolyte temperature. During exper- iments, the electrolyte temperature (Tel) was measured using a chromel–alumel thermo- couple. Powders of the raw material were used in the form of a dispersed (solid) phase in  the electrolyte solution (liquid phase). The solid-to-liquid (S/L) ratio in the suspension was  8.6 g·L−1. In all experiments, the volume of the electrolyte was 350 mL, and the concentra- tion of the hydrochloric acid was 4, 6, and 8 mol·L−1. After filling the reactor with the initial  components (PCB powder and electrolyte solution) to create a suspension,  the mixture  was stirred at a speed of 1200 rpm for 5 min, then the stirring speed was reduced to 700  rpm, and the power was turned on. To produce chlorine, an alternating current (AC) of  industrial frequency (50 Hz) with a density (i) of 0.21 A·cm−2, 0.42 A·cm−2, 0.63 A·cm−2, 0.84  A·cm−2, and 0.88 A·cm−2 was used. The duration of the experiments (tex) varied from 0.5 to  6 h. When carrying out a two-stage leaching process, chemical or electrochemical pretreat- ment of the raw material was first carried out. Then, after removing the leach solution and  adding a fresh portion of electrolyte to the wet residue of the raw material, electrochemical  leaching was performed. Chemical pretreatment of the raw material powder was carried  out at a constant electrolyte temperature (Tel = 70 °C) under conditions of stirring of reac- tion mixture for an hour. Electrochemical pretreatment was also carried out for an hour  under the same experimental conditions as the subsequent electrochemical leaching pro- cess.  To establish the metal content in the raw material, the chemical treatment (leaching)  of three representative samples (0.500 g each) was carried out. This treatment  included  sequentially boiling the sample in a solution of HCl (1:1) and two portions of aqua regia  until a wet residue was formed. During the boiling process in aqua regia, HNO3 excess  was removed by adding several portions of concentrated HCl. The resulting wet residue  was transferred to the filter with a 3 mol·L−1 HCl solution and washed accurately. Filtrate  was diluted to a volume of 100 mL with 3 mol·L−1 HCl solution. Quantitative determina- tion of the metals in the prepared solutions was carried out by inductively coupled plasma  optical emission spectrometry (ICP-OES, Perkin Elmer Optima 7000 DV ICP-OES, Perkin  Figure 2. Experimental setup: a common view without power supply and control (a), AC circuit, where Tr1—laboratory autotransformer, R1—rheostat, Tr2—step-down transformer, V—AC voltmeter, A—AC ammeter (b), nd sch matic repr sentation of electrochemical cells: polypropyl ne cell—cell 1 (c) and a water-jacketed glass cell—cell 2 (d). The (b) section is Reprinted/Adapted from Ref. [39] (CC BY Open Access, MDPI). To stablish the met l content in the raw material, the chemical treatment (leaching) of three representative samples (0.500 g each) was c rried out. This treatment included sequentially boiling the sample in a solution of HCl (1:1) and two portions of aqua regia until a wet residue was formed. During the boiling process in aqua regia, HNO3 excess was removed by adding several portions of concentrated HCl. The resulting wet residue was transferred to the filter with a 3 mol·L−1 HCl solution and washed accurately. Filtrate was diluted to a volume of 100 mL with 3 mol·L−1 HCl solution. Quantitative determination of the metals in the prepared solutions was carried out by inductively coupled plasma optical emission spectrometry (ICP-OES, Perkin Elmer Optima 7000 DV ICP-OES, Perkin Elmer Inc., Waltham, MA, USA). Based on the results of the analysis, an average concentration of each metal under study was calculated. Solid residues after leaching were washed with distilled water to pH 5–6, dried at a temperature of 105 ◦C in one hour, and weighed. ICP-OES was also used for the quantitative determination of metals under study in electrolyte solutions obtained as a result of electrochemical leaching. The efficiency of electrochemical leaching of metals from raw materials was evaluated by the degree of metal leaching (RMe), which was defined as the ratio of the quantity of metal transferred into the electrolyte solution (Mel) to the quantity of etal contained in the raw material (MPCBs), as demonstrated in Equation (1): RMe (%) = (Mel/MPCBs) × 100% (1) The content of metals in the raw material (MPCBs) was calculated from the results of the ICP-OES analysis of the solutions obtained from the chemical leaching of a represen- Metals 2024, 14, 65 7 of 25 tative sample of the raw material. The quantity of metals in the solid residue (Ms) after electrochemical leaching of the raw material was estimated using Equation (2): Ms = MPCBs − Mel (2) 3. Results and Discussion 3.1. Raw Material Characterization and Kinetics of Electrochemical Leaching of Metals The main object of the research was the raw material 1 powder fraction with a particle size < 90 µm prepared by single crushing in a disintegrator of mixed computer PCBs. The percentage of both metals in this raw material, based on the results of ICP-OES analysis of the solutions prepared by chemical leaching and obtained after chemical leaching of solid residue, is presented in Table 1. The metal content in double-crushed mixed computer PCBs from the same batch of source material (raw material 2) and single-crushed mixed computer PCBs from another batch of mixed PCBs (raw material 3) are also shown. Table 1. Content of metals in the raw material fraction with a particle size < 90 µm. Raw Material Powder Fraction d < 90 µm Component, Content Raw Material 1 Raw Material 2 Raw Material 3 [39] Fe, (%) 33.38 12.37 7.56 Sn, (%) 2.82 2.52 4.76 Al, (%) 1.74 3.56 1.31 Cu, (%) 1.42 1.57 1.40 Zn, (%) 0.53 0.43 2.00 Pb, (%) 1.07 0.77 7.50 Ni, (%) 1.10 0.53 1.03 Ti, (%) 1.54 0.48 1.74 Sb, ppm 510 1790 1365 Mn, ppm n/a 2 760 1698 Cr, ppm 504 144 188 Co, ppm 902 140 224 V, ppm 74 46 29 Mo, ppm n/a 2 n/a 2 17 Ag, ppm 1734 906 8450 Pd, ppm BDL 1 BDL 1 1550 Au, ppm 504 606 824 Solid residue, % 34.1 60.4 45.8 1 BDL—below the detectable limit. 2 n/a—not analyzed. The results obtained show that the content of both metals in the raw material powders and the solid residue obtained as a result of their leaching differ significantly. Thus, the composition of the disintegrator-crushed raw material fractions with a particle size d < 90 µm depends on both the PCB batch and the number of times it is crushed. Moreover, it should be noted that the content of individual metals in PCBs, given in the literature [49], varies widely. It is associated not only with the methods of preliminary grinding of the raw material, but also with the sampling and leaching method for transferring metals into a solution for quantitative analysis. Under the same conditions of electrochemical leaching, the kinetics of metal leaching from these raw material powders was studied. All experiments were carried out without external regulation of the electrolyte temperature (at ambient conditions). The presented results (Figure 3, Table 1) show that the leaching kinetics from the studied powders of such active metals as Sn is almost identical: complete metal leaching is achieved with an experiment duration of 0.5 h. The leaching efficiency for Pb and Al depends on the metal content in the raw material: with increasing metal content in the raw material, the time for complete leaching also increases. Moreover, the complete leaching of Metals 2024, 14, 65 8 of 25 Al from raw material 2, with the highest content, was not observed under the conditions of a 5 h experiment. Metals 2024, 14, x FOR PEER REVIEW 8 of 26 The results obtained show that the content of both metals in the raw material pow- ders and the solid residue obtained as a result of their leaching differ significantly. Thus, the composition of the disintegrator-crushed raw material fractions with a particle size d < 90 µm depends on both the PCB batch and the number of times it is crushed. Moreo- ver, it should be noted that the content of individual metals in PCBs, given in the literature [49], varies widely. It is associated not only with the methods of preliminary grinding of the raw material, but also with the sampling and leaching method for transferring metals into a solution for quantitative analysis. Under the same conditions of electrochemical leaching, the kinetics of metal leaching from these raw material powders was studied. All experiments were carried out without external regulation of the electrolyte temperature (at ambient conditions). The presented results (Figure 3, Table 1) show that the leaching kinetics from the studied powders of such active metals as Sn is almost identical: complete metal leaching is achieved with an experiment duration of 0.5 h. The leaching efficiency for Pb and Al depends on the metal content in the raw material: with increasing metal content in the raw material, the time for complete leaching also increases. Moreover, the complete leach- ing of Al from raw material 2, with the highest content, was not observed under the con- ditions of a 5 h experiment. (A) (a) (b) (c) 0 20 40 60 80 100 0 1 2 3 4 5 6 R , % tex, h Cu Fe Sn 0 20 40 60 80 100 0 1 2 3 4 5 6 R , % tex, h Al Ni Pb Ti Zn 0 20 40 60 80 100 0 1 2 3 4 5 6 R , % tex, h Au Ag Metals 2024, 14, x FOR PEER REVIEW  9  of  26     (B)      (a)  (b)    (c)  (C)      (a)  (b)      (c)  (d)  Figure 3. Impact of experiment duration on the degree of metal leaching from raw material pow- ders: (A) is raw material 1: for Cu, Fe and Sn (a); for Al, Ni, Pb, Ti and Zn (b); and for Au and Ag (c).  (B) is raw material 2 for Cu, Ni, Al and Fe (a); for Pb, Sn, Ti and Zn (b); and for Au and Ag (c). (C) is  0 20 40 60 80 100 0 1 2 3 4 5 R, % tex, h Cu Ni Al Fe 0 20 40 60 80 100 0 1 2 3 4 5 R, % tex, h Pb Sn Ti Zn 0 20 40 60 80 100 0 1 2 3 4 5 R, % tex, h Au Ag Figure 3. Cont. Metals 2024, 14, 65 9 of 25 Metals 2024, 14, x FOR PEER REVIEW 9  of  26  (B) (a) (b)  (c)  (C)  (a)  (b)  (c)  (d)  Figure 3. Impact of experiment duration on the degree of metal leaching from raw material pow- ders: (A) is raw material 1: for Cu, Fe and Sn (a); for Al, Ni, Pb, Ti and Zn (b); and for Au and Ag (c).  (B) is raw material 2 for Cu, Ni, Al and Fe (a); for Pb, Sn, Ti and Zn (b); and for Au and Ag (c). (C) is  0 20 40 60 80 100 0 1 2 3 4 5 R, % tex, h Cu Ni Al Fe 0 20 40 60 80 100 0 1 2 3 4 5 R, % tex, h Pb Sn Ti Zn 0 20 40 60 80 100 0 1 2 3 4 5 R, % tex, h Au Ag Figure 3. Impact of experiment duration on the degree of metal leaching from raw material powders: (A) is raw material 1: for Cu, Fe and Sn (a); for Al, Ni, Pb, Ti and Zn (b); and for Au and Ag (c). (B) is raw material 2 for Cu, Ni, Al and Fe (a); for Pb, Sn, Ti and Zn (b); and for Au and Ag (c). (C) is raw material 3 for Sn, Zn Fe and Pb (a); for Al, Cu, Ni, Ti and Sb (b); for Mn, Cr, Co and V (c); and for Au, Pd and Ag (d). Experimental conditions: cell 1, CHCl = 6 mol·L−1, i = 0.88 A·cm−2, S/L = 8.6 g·L−1 Reprinted/Adapted from Refs. [39,40]. In the case of Ti, complete metal leaching was achieved with an experiment duration of 5 h for raw material 1 (Figure 3(Ab)) and raw material 3 (Figure 3(Cb)). The efficiency of metal leaching from raw material 2, with the lowest content, was only 71.6% (tex = 5 h). The maximum degree of Zn leaching after 5 h of the experiment was 80.1% when using raw material 1. From the raw material 2 and the raw material 3, less effective metal leaching was observed—63.6% and 30.8%, respectively. The same regularity was established for Ni leaching (tex = 5 h): RNi = 100% from raw material 1, RNi = 72.5% from raw material 2, and RNi = 42.6% from raw material 3. Iron was almost completely leached from raw material 1 and raw material 2 with experiment durations of 1.5 h and 2 h, respectively. In the case of raw material 3, the degree of leaching was significantly lower and RFe = 50.0%. After 2 h of electrochemical leaching, Cu was leached completely from raw material 1 (RCu = 100%). From raw material 2 and raw material 3, 97.2% and 86.3% of metal was leached, respectively. Complete leaching of Ag was observed with experiment durations of 3 h, 2 h, and 1 h (Figure 3(Ac,Bc,Cd)) from raw material 1, raw material 2, and raw material 3, respectively. Au was completely leached from raw material 2 at tex = 5 h; from raw material 3 the maximum degree of metal leaching was 86.3% at tex = 4 h; and from raw material 1—77.5% at tex = 6 h. The observed differences in the kinetics of the leaching of gold and silver from the materials studied are probably associated both with the mass content of the active metals relative to Au and Ag, and with the location of thr noble metals in the microparticles of the raw material. Thus, from the presented data, it is clear that the composition of the raw material most significantly affects the leaching of such metals as Au, Ag, Ni, Zn, and Fe. Metals 2024, 14, 65 10 of 25 3.2. Impact of Electrochemical Process Parameters on the Degree of Metal Leaching The passage of AC through the system causes the heating of the electrolyte solution. Moreover, the temperature of the electrolyte depends both on the current density and the duration of the experiment [39,40]. Therefore, when studying the influence of such process parameters as the composition and temperature of the electrolyte and the current density on the efficiency of metal leaching from the raw material, experiments were carried out while maintaining the electrolyte at a constant temperature. 3.2.1. Electrolyte Composition Study of the impact of the concentration of hydrochloric acid in the electrolyte solution on the degree of metal leaching from raw material 1 was carried out at a constant electrolyte temperature (Tel) of 80 ◦C. The results obtained showed (Figure 4) that the increase in hydrochloric acid con- centration in the electrolyte solution from 4 mol·L−1 to 8 mol·L−1 leads to a significant increase in the degree of Ni, Zn, and Ag leaching. The complete leaching of Cu is achieved at CHCl = 6 mol·L−1. It should be noted that the presence of gold in the obtained electrolyte solutions was not established under these experimental conditions. Metals 2024, 14, x FOR PEER REVIEW  11  of  26       Figure 4. Impact of HCl concentration on the degree of metal leaching. Experimental conditions: cell  2, raw material 1; i = 0.84 A·cm−2; tex = 1 h; S/L = 8.6 g·L−1; Tel = 80 °C.  To reduce the acidity of the electrolyte solution, hydrochloric acid solutions with the  addition of sodium chloride were also used. Figure 5a,b demonstrate the results of these  studies.  For comparison, the results of the electrochemical leaching of metals into pure hy- drochloric acid solutions (without NaCl) with the same concentration of chloride ions are  also presented.    (a)    (b)  94.1 86.3 63.9 77.5 69.1 54.9 100 100 32.4 83.5 20.5 0 0 20 40 60 80 100 8 M 6 M 4 M R, % Zn Ni Cu Ag 0 100 32.4 54.9 99.2 97.3 63.9 98.2 12.7 73.7 60.1 78.3 100 100 63.8 66.8 100 0 20 40 60 80 100 Ag Al Cu Ni Pb Sn Ti Zn Fe R, % 4 M HCl 3 M NaCl + 1 M HCl 20.5 99.1 69.1 100 100 98.7 86.3 100 0 100 100 90.4 20.1 100 98.4 0 20 40 60 80 100 Ag Al Cu Ni Pb Sn Ti Zn Fe R, % 6 M HCl 5 M HCl + 1 M NaCl Figure 4. Impact of HCl concentration on the degree of metal leaching. Experimental conditions: cell 2, raw material 1; i = 0.84 A·cm−2; tex = 1 h; S/L = 8.6 g·L−1; Tel = 80 ◦C. To reduce the acidity of the electrolyte solution, hydrochloric acid solutions with the addition of sodium chloride were also used. Figure 5a,b demonstrate the results of these studies. For comparison, the results of the electro hemi al leaching of metals into pure hy- drochloric acid solutions (wi hout NaCl) with th sam conce tration of chloride ions are also pres nted. Figure 5a demonstrates that the complete leaching of Al and Sn was achieved at CHCl = 4 mol·L−1. The other metals (Pb, Ti, and Fe) are characterized by a high degree of leaching at this concentration of hydrochloric acid in the electrolyte solution, but their complete leaching from the raw material was not achieved. At the same concentration of chloride ions (4 mol·L−1) in the electrolyte solutions, the presence of sodium chloride leads to a decrease in the degree of Al and Ti leaching and an increase in the degree of Cu and Ni leaching. In the case of other metals, this effect is less significant. The presence of sodium chloride (CNaCl = 1 mol·L−1) in a hydrochloric acid solution with a concentration of chloride ions of 6 mol·L−1 leads to an increase in the degree of Ni and Zn leaching (Figure 5b). At the same time, a significant decrease in the efficiency of Pb leaching is observed. This is probably due to the peculiarities of the formation of lead chloride complexes in this environment [50]. Silver is leached i a small amou t (20.5%) into the hydrochloric acid electrolyte in the absence of NaCl and does not leach at all in the presence of sodium chloride. The degree of the leaching of other metals is characterized by high values and practically does not depend on the composition of the electrolyte. The presence of gold in the studied electrolytes was not established. Metals 2024, 14, 65 11 of 25 Metals 2024, 14, x FOR PEER REVIEW  11  of  26       Figure 4. Impact of HCl concentration on the degree of metal leaching. Experimental conditions: cell  2, raw material 1; i = 0.84 A·cm−2; tex = 1 h; S/L = 8.6 g·L−1; Tel = 80 °C.  To reduce the acidity of the electrolyte solution, hydrochloric acid solutions with the  addition of sodium chloride were also used. Figure 5a,b demonstrate the results of these  studies.  For comparison, the results of the electrochemical leaching of metals into pure hy- drochloric acid solutions (without NaCl) with the same concentration of chloride ions are  also presented.    (a)    (b)  94.1 86.3 63.9 77.5 69.1 54.9 100 100 32.4 83.5 20.5 0 0 20 40 60 80 100 8 M 6 M 4 M R, % Zn Ni Cu Ag 0 100 32.4 54.9 99.2 97.3 63.9 98.2 12.7 73.7 60.1 78.3 100 100 63.8 66.8 100 0 20 40 60 80 100 Ag Al Cu Ni Pb Sn Ti Zn Fe R, % 4 M HCl 3 M NaCl + 1 M HCl 20.5 99.1 69.1 100 100 98.7 86.3 100 0 100 100 90.4 20.1 100 98.4 0 20 40 60 80 100 Ag Al Cu Ni Pb Sn Ti Zn Fe R, % 6 M HCl 5 M HCl + 1 M NaCl Figure 5. Impact of the presence of NaCl in hydrochloric acid electrolyte on the degree of metal leaching: CNaCl = 3 mol·L−1 in 1 mol·L−1 HCl solution (a); CNaCl = 1 mol·L−1 in 5 mol·L−1 HCl solution (b). Experimental conditions: cell 2; raw material 1; tex = 1 h, i = 0.84 A·cm−2; S/L = 8.6 g·L−1; Tel = 80 ◦C. 3.2.2. Current Density and Electrolyte Temperature The influence of AC superimposition on the system was studied at electrolyte temper- atures of 60 ◦C, 70 ◦C, and 80 ◦C (Figure 6). The presented results show that an increase in electrolyte temperature in the absence of current leads to an increase in the degree of leaching of almost all of the studied metals. At i = 0.63 A·cm−2, an increase in the electrolyte temperature leads to an increase in the degree of Ni, Zn, Fe, and Al leaching. Effective leaching of Sn, Pb, Ti, and Cu was observed at 60 ◦C and a further increase in the electrolyte temperature has practically no positive effect. At the same electrolyte temperature, the effect of electrical current superimposition on the system was most noticeable in the leaching of Cu, Ni, and Zn. The transition of Au and Ag into the electrolyte solution under experimental conditions was not established. At an electrolyte temperature of 70 ◦C, the effect of electrical current superimposition with a higher value (I = 0.84 A·cm−2) on the system was also studied. Metals 2024, 14, 65 12 of 25 Metals 2024, 14, x FOR PEER REVIEW  12  of  26     Figure 5. Impact of the presence of NaCl  in hydrochloric acid electrolyte on  the degree of metal  leaching: CNaCl = 3 M·L−1  in 1 M·L−1 HCl solution  (a); CNaCl = 1 M·L−1  in 5 M·L−1 HCl solution  (b).  Experimental conditions: cell 2; raw material 1; tex = 1 h, i = 0.84 A·cm−2; S/L = 8.6 g·L−1; Tel = 80 °C.  Figure 5a demonstrates that the complete leaching of Al and Sn was achieved at CHCl  = 4 mol·L−1. The other metals (Pb, Ti, and Fe) are characterized by a high degree of leaching  at this concentration of hydrochloric acid in the electrolyte solution, but their complete  leaching from the raw material was not achieved. At the same concentration of chloride  ions  (4 mol·L−1)  in  the electrolyte solutions,  the presence of sodium chloride  leads  to a  decrease in the degree of Al and Ti leaching and an increase in the degree of Cu and Ni  leaching. In the case of other metals, this effect is less significant. The presence of sodium  chloride (CNaCl = 1 mol·L−1) in a hydrochloric acid solution with a concentration of chloride  ions of 6 mol·L−1 leads to an increase in the degree of Ni and Zn leaching (Figure 5b). At  the same time, a significant decrease in the efficiency of Pb leaching is observed. This is  probably due to the peculiarities of the formation of lead chloride complexes in this envi- ronment [50]. Silver is leached in a small amount (20.5%) into the hydrochloric acid elec- trolyte in the absence of NaCl and does not leach at all in the presence of sodium chloride.  The degree of the leaching of other metals is characterized by high values and practically  does not depend on the composition of the electrolyte. The presence of gold in the studied  electrolytes was not established.    3.2.2. Current Density and Electrolyte Temperature  The influence of AC superimposition on the system was studied at electrolyte tem- peratures of 60 °C, 70 °C, and 80 °C (Figure 6).    (a)      89.0 92.5 63.7 93.4 93.9 90.6 78.0 88.0 100 84.0 66.6 100 100 96.9 82.8 83.2 0 20 40 60 80 100 Al Cu Ni Pb Sn Ti Zn Fe R, % i = 0.63 A·cm⁻² i = 0Metals 2024, 14, x FOR PEER REVIEW  13  of  26       (b)    (c)  Figure 6. Impact of current superimposition on the degree of metal leaching. Experimental condi- tions: cell 2; raw material; tex = 1 h; S/L = 8.6 g·L−1; CHCl = 6 mol·L−1, (a)—Tel = 80 °C, (b)—Tel = 70 °C  and (c)—Tel = 60 °C.  The presented results show that an increase in electrolyte temperature in the absence  of current leads to an increase in the degree of leaching of almost all of the studied metals.  At i = 0.63 A·cm−2, an  increase  in the electrolyte temperature  leads to an  increase in the  degree of Ni, Zn, Fe, and Al leaching. Effective leaching of Sn, Pb, Ti, and Cu was observed  at 60 °C and a further increase in the electrolyte temperature has practically no positive  effect. At the same electrolyte temperature, the effect of electrical current superimposition  on the system was most noticeable in the leaching of Cu, Ni, and Zn. The transition of Au  and Ag into the electrolyte solution under experimental conditions was not established.  At an electrolyte temperature of 70 °C, the effect of electrical current superimposition  with a higher value (I = 0.84 A·cm−2) on the system was also studied.  The results obtained show (Figure 7) that electrical current superimposition leads to  an increase in the leaching efficiency of all metals, except for Al and Ti, compared to the  experiment without current superposition on the system. The leaching of Au and Ag into  the electrolyte solution under experimental conditions was also not observed. Compari- son of the results obtained at different current densities (Figures 6 and 7) shows that in- creasing the current density promotes more efficient leaching of almost all metals from  the  raw material. An  increase  in  the current density  leads  to promotion of  the current  90.6 84.3 55.9 94.3 97.8 92.9 70.1 80.3 91.8 65.0 48.7 94.9 97.7 94.7 64.0 80.2 0 20 40 60 80 100 Al Cu Ni Pb Sn Ti Zn Fe R, % i = 0.63 A·cm⁻² i = 0 76.8 94.8 46.9 93.8 99.9 90.7 61.1 63.9 78.1 65.1 36.8 94.6 100 92.3 55.7 71.4 0 20 40 60 80 100 Al Cu Ni Pb Sn Ti Zn Fe R, % i = 0.63 A·cm⁻² i = 0 Figure 6. Impact of current superimposition on the degree of metal leaching. Experimental conditions: cell 2; raw material; tex = 1 h; S/L = 8.6 g·L−1; CHCl = 6 mol·L−1, (a)—Tel = 80 ◦C, (b)—Tel = 70 ◦C and (c)—Tel = 60 ◦C. The results obtained show (Figure 7) that electrical current superimposition leads to an increase in the leaching efficiency of all metals, except for Al and Ti, compared to the experiment without current superposition on the system. The leaching of Au and Ag into the electrolyte solution under experimental conditions was also not observed. Comparison of the results obtained at different current densities (Figures 6 and 7) shows that increasing the current density promotes more efficient leaching of almost all metals from the raw Metals 2024, 14, 65 13 of 25 material. An increase in the current density leads to promotion of the current efficiency of chlorine and, as a consequence, to an increase in the concentration of the oxidizing agent in the electrolyte solution [36]. Metals 2024, 14, x FOR PEER REVIEW  14  of  26     efficiency of chlorine and, as a consequence, to an increase in the concentration of the ox- idizing agent in the electrolyte solution [36].    Figure 7.  Impact of alternating current superimposition on  the degree of metal  leaching. Experi- mental conditions: cell 2; raw material 1; S/L = 8.6 g·L−1; CHCl = 6 mol·L−1; tex = 1 h; Tel = 70 °C.  The results of studies on the effect of current density on the efficiency of metal leach- ing at different electrolyte temperatures (60 °C, 70 °C and 80 °C) are presented in Figure  8.      (a)  (b)    (c)  90.2 88.7 57.9 96.9 100 92.8 71.8 85.291.8 65.0 48.7 94.9 97.7 94.7 64.0 80.2 0 20 40 60 80 100 Al Cu Ni Pb Sn Ti Zn Fe R , % i = 0.84 A·cm⁻² i = 0 0 20 40 60 80 100 0 0.2 0.4 0.6 0.8 1 R, % i, A·cm⁻² Zn Ni Cu Ti Fe Ag 0 20 40 60 80 100 0 0.2 0.4 0.6 0.8 1 R, % i, A·cm⁻² Cu Ni Ti Zn Fe 0 20 40 60 80 100 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 R, % i, A·cm⁻² Cu Ni Ti Zn Fe Figure 7. Impact of alternating current superimposition on the degree of metal leaching. Experimental conditions: cell 2; raw material 1; S/L = 8.6 g·L−1; CHCl = 6 mol·L−1; tex = 1 h; Tel = 70 ◦C. The results of studies on the effect of current density on the efficiency of metal leaching at different electrolyte temperatures (60 ◦C, 70 ◦C and 80 ◦C) are presented in Figure 8. Metals 2024, 14, x FOR PEER REVIEW  14  of  26     efficiency of chlorine and, as a consequence, to an increase in the concentration of the ox- idizing agent in the electrolyte solution [36].    Figure 7.  Impact of alternating current superimposition on  the degree of metal  leaching. Experi- mental conditions: cell 2; raw material 1; S/L = 8.6 g·L−1; CHCl = 6 mol·L−1; tex = 1 h; Tel = 70 °C.  The results of studies on the eff ct of current density on the efficiency of metal leach- ing at  ifferent electrolyte temperatures (60 °C, 70 °C and 80 °C) are presented in Figure  8.      (a)  (b)    (c)  90.2 88.7 57.9 96.9 100 92.8 71.8 85.291.8 65.0 48.7 94.9 97.7 94.7 64.0 80.2 0 20 40 60 80 100 Al Cu Ni Pb Sn Ti Zn Fe R , % i = 0.84 A·cm⁻² i = 0 0 20 40 60 80 100 0 0.2 0.4 0.6 0.8 1 R, % i, A·cm⁻² Zn Ni Cu Ti Fe Ag 0 20 40 60 80 100 0 0.2 0.4 0.6 0.8 1 R, % i, A·cm⁻² Cu Ni Ti Zn Fe 0 20 40 60 80 100 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 R, % i, A·cm⁻² Cu Ni Ti Zn Fe Figure 8. Impact of current density on the degree of metal leaching. Experimental conditions: cell 2; raw material 1; tex = 1 h; S/L = 8.6 g·L−1; CHCl = 6 mol·L−1; (a)—Tel = 80 ◦C, (b)—Tel = 70 ◦C, (c)—Tel = 60 ◦C. At an electrolyte temperature of 80 ◦C, an increase in the degree of leaching of Cu and Ag begins at i > 0.42 A·cm−2, and of other metals at i > 0.63 A·cm−2 (Figure 8a). With a Metals 2024, 14, 65 14 of 25 decrease in the electrolyte temperature (Tel = 70 ◦C), an increase in current density leads to a gradual increase in the degree of Ni, Cu, and Zn leaching and practically does not affect the leaching of other metals. At a temperature of 60 ◦C and i > 0.42, an increase in the degree of leaching of Ni, Cu, and Zn was also observed. Moreover, in the case of Cu, this effect was most significant. It should be noted that Ag leaching is observed only at the electrolyte temperature of 80 ◦C at i > 0.42 A·cm−2. The leaching of Au into the electrolyte solution under experimental conditions was not observed. Thus, the presented results show that an increase in electrolyte temperature from 60 ◦C to 80 ◦C, both in the absence of electrical current and under electrical current superimposi- tion on the system, leads to an increase in the degree of leaching of almost all of the studied metals. An increase in current density has the most significant effect on the efficiency of leaching of all metals at an electrolyte temperature of 80 ◦C. However, the transition of Au into the electrolyte solution under experimental conditions was not established. For comparison, hydrochloric acid solutions with the addition of sodium chloride were used as an electrolyte in the experiments at an electrolyte temperature of 80 ◦C (Figures 9 and 10). Metals 2024, 14, x FOR PEER REVIEW  15  of  26  Figure 8. Impact of current density on the degree of metal leaching. Experimental conditions: cell 2;  raw material 1; tex = 1 h; S/L = 8.6 g·L−1; CHCl = 6 mol·L−1; (a)—Tel = 80 °C, (b)—Tel = 70 °C, (c)—Tel = 60  °C.  At an electrolyte temperature of 80 °C, an increase in the degree of leaching of Cu  and Ag begins at i > 0.42 A·cm−2, and of other metals at i > 0.63 A·cm−2 (Figure 8a). With a  decrease in the electrolyte temperature (Tel = 70 °C), an increase in current density leads to  a gradual increase in the degree of Ni, Cu, and Zn leaching and practically does not affect  the  leaching of other metals. At a  temperature of 60 °C and  i > 0.42, an  increase  in  the  degree of leaching of Ni, Cu, and Zn was also observed. Moreover, in the case of Cu, this  effect was most significant. It should be noted  that Ag leaching  is observed only at the  electrolyte temperature of 80 °C at i > 0.42 A·cm−2. The leaching of Au into the electrolyte  solution under experimental conditions was not observed.  Thus, the presented results show that an increase in electrolyte temperature from 60  °C to 80 °C, both in the absence of electrical current and under electrical current superim- position on the system, leads to an increase in the degree of leaching of almost all of the  studied metals. An increase in current density has the most significant effect on the effi- ciency of leaching of all metals at an electrolyte temperature of 80 °C. However, the tran- sition of Au  into  the electrolyte solution under experimental conditions was not estab- lished.  For comparison, hydrochloric acid solutions with  the addition of sodium chloride  were used as an electroly e i  the experiments at  n electrolyte temperature of 80 °C (Fig- ures 9 and 10).    Figure 9 demonstrates results obtained for  lectrolyte with a total content of chloride  ions CСl− = 6 mol·L−1.  Figure 9. Impact of current density on the degree of metal leaching. Experimental conditions: cell 2;  raw material 1; tex = 1 h; S/L = 8.6 g·L−1; electrolyte composition: CHCl = 5 mol·L−1, CNaCl = 1 mol·L−1; Tel  = 80 °C.  0 20 40 60 80 100 0 0.2 0.4 0.6 0.8 1 R, % i, A·cm⁻² Cu Ni Pb Zn Fe 0 20 40 60 80 100 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 R, % i, A·cm⁻² Ag Al Cu Ni Ti Zn Fe Fig re 9. Impact of current density on the degree of metal leaching. Experimental conditions: cell 2; raw material 1; tex = 1 h; S/L = 8.6 g·L−1; electrolyte composition: CHCl = 5 mol·L−1, CNaCl = 1 mol·L−1; Tel = 80 ◦C. Metals 2024, 14, x FOR PEER REVIEW  15  of  26  Figure 8. Impact of current density on the degree of metal leaching. Experimental conditions: cell 2;  raw material 1; tex = 1 h; S/L = 8.6 g·L−1; CHCl = 6 mol·L−1; (a)—Tel = 80 °C, (b)—Tel = 70 °C, (c)—Tel = 60  °C.  At an electrolyte temperature of 80 °C, an increase in the degree of leaching of Cu  and Ag begins at i > 0.42 A·cm−2, and of other metals at i > 0.63 A·cm−2 (Figure 8a). With a  decrease in the electrolyte temperature (Tel = 70 °C), an increase in current density leads to  a gradual increase in the degree of Ni, Cu, and Zn leaching and practically does not affect  the  leaching of other metals. At a  temperature of 60 °C and  i > 0.42, an  increase  in  the  degree of leaching of Ni, Cu, and Zn was also observed. Moreover, in the case of Cu, this  effect was most significant. It should be noted  that Ag leaching  is observed only at the  electrolyte temperature of 80 °C at i > 0.42 A·cm−2. The leaching of Au into the electrolyte  solution under experimental conditions was not observed.  Thus, the presented results show that an increase in electrolyte temperature from 60  °C to 80 °C, both in the absence of electrical current and under electrical current superim- position on the system, leads to an increase in the degree of leaching of almost all of the  studied metals. An increase in current density has the most significant effect on the effi- ciency of leaching of all metals at an electrolyte temperature of 80 °C. However, the tran- sition of Au  into  the electrolyte solution under experimental conditions was not estab- lished.  For comparison, hydrochloric acid solutions with  the addition of sodium chloride  were used as an electrolyte in the experiments at an electrolyte temperature of 80 °C (Fig- ures 9 and 10).    Figure 9 demonstrates results obtained for electrolyte with a total content of chloride  ions CСl− = 6 mol·L−1.  Figure 9. Impact of current density on the degree of metal leaching. Experimental conditions: cell 2;  raw materi l 1; tex = 1 h; S/L = 8.6 g·L−1; electrolyte composition: CHCl = 5 m l·L−1, CNaCl = 1 mol·L−1; Tel  = 80 °C.  0 20 40 60 80 100 0 0.2 0.4 0.6 0.8 1 R, % i, A·cm⁻² Cu Ni Pb Zn Fe 0 20 40 60 80 100 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 R, % i, A·cm⁻² Ag Al Cu Ni Ti Zn Fe Figure 10. Impact of current density on the degree of metal leaching. Experimental conditions: cell 1; raw material 1; tex = 1 h; S/L = 8.6 g·L−1; electrolyte composition: CHCl = 1 mol·L−1, CNaCl = 3 mol·L−1; Tel = 80 ◦C. Figure 9 demonstrates results obtained for electrolyte with a total content of chloride ions CCl− = 6 mol·L−1. It should be noted that the presence of NaCl in the electrolyte solution does not affect the leaching efficiency of Al, Sn, and Ti: the degree of metal leaching reaches 100% in the absence of electrical current. That is why the results for these metals are not shown in Figure 9. Comparison of the results presented in Figures 9 and 10 demonstrates that the addition of NaCl into hydrochloric acid electrolyte has practically no effect on the efficiency of Fe Metals 2024, 14, 65 15 of 25 leaching under studied range of current densities. At the same time, the leaching efficiency of Zn, Cu, and Ni increases: complete leaching of Zn was achieved at i = 0.42 A·cm−2 instead of RZn = 86.3% at i = 0.84 A·cm−2; complete leaching of Cu was observed at a lower current density (i = 42 A·cm−2 instead of i = 0.84 A·cm−2) and an increase in the degree of leaching of Ni at i = 0.84 A·cm−2 from 69.1% to 90.4% was achieved. The maximum leaching of Fe, Cu, and Zn into the NaCl-containing electrolyte was observed at i = 0.42 A·cm−2. However, the presence of sodium chloride significantly decreases the efficiency of Pb leaching. An increase in current density leads only to a slight increase in the degree of Pb leaching. The maximum value of RPb reaches only 21.7% at i = 0.63 A·cm−2. The transition of Ag and Au into the electrolyte solution with NaCl additive was not established. A similar study, but with a higher content of sodium chloride (CNaCl = 3 mol·L−1) with a lower total content of chloride ions (CCl− = 4 mol·L−1) was carried out (Figure 10). From the presented data, it can be seen that the active transition of copper into the electrolyte solution and the onset of silver leaching occurs at a higher current density (i = 0.63 A·cm−2) compared to the hydrochloric acid electrolyte (Figure 8a). The degree of Pb and Sn leaching reaches 100% in the absence of current. That is why the content of these elements in the electrolyte solutions obtained as a result of electrochemical leaching was not determined. 3.2.3. Experiment Duration Studies of the effect of experiment duration on the efficiency of metal leaching were carried out at a constant electrolyte temperature of 70 ◦C. The results obtained show (Figure 11) that an increase in the duration of the experiment from 1 h to 3 h leads to an increase in the degree of all metal leaching, except for Sn (the metal is completely leached at the end of a one-hour experiment). Metals 2024, 14, x FOR PEER REVIEW  16  of  26     Figure 10. Impact of current density on the degree of metal leaching. Experimental conditions: cell  1; raw material 1; tex = 1 h; S/L = 8.6 g·L−1; electrolyte composition: CHCl = 1 mol·L−1, CNaCl = 3 mol·L−1;  Tel = 80 °C.  It should be noted that the presence of NaCl in the electrolyte solution does not affect  the leaching efficiency of Al, Sn, and Ti: the degree of metal leaching reaches 100% in the  absence of electrical current. That  is why  the results  for  these metals are not shown  in  Figure 9.    Comparison of the results presented in Figures 9 and 10 demonstrates that the addi- tion of NaCl into hydrochloric acid electrolyte has practically no effect on the efficiency of  Fe leaching under studied range of current densities. At the same time, the leaching effi- ciency of Zn, Cu, and Ni increases: complete leaching of Zn was achieved at i = 0.42 A·cm−2  instead of RZn = 86.3% at i = 0.84 A·cm−2; complete leaching of Cu was observed at a lower  current density (i = 42 A·cm−2 instead of i = 0.84 A·cm−2) and an increase in the degree of  leaching of Ni at i   0.84 A·cm−2 from 69.1% to 90.4% was achieved. The maximum leaching  of Fe, Cu, and Zn  into the NaCl-co taining electrolyte was observed at  i = 0.42 A·cm−2.  However,  the presence of  sodium  chloride  significantly decreases  the  efficiency of Pb  leaching. An increase in current density leads only to a slight increase in the degree of Pb  leaching. The maximum value of RPb reaches only 21.7% at i = 0.63 A·cm−2. The transition  of Ag and Au into the electrolyte solutio  with NaCl additive was not established.  A similar study, but with a higher content of s dium chloride (CNaCl = 3 mol·L−1) with  a lower total content of chloride ions (CСl− = 4 mol·L−1) was carried out (Figure 10).  From the presented data, it can be seen that the active transition of copper into the  electrolyte solution and the onset of silver leaching occurs at a higher current density (i =  0.63 A·cm−2) compared to the hydrochloric acid electrolyte (Figure 8a). The degree of Pb  and Sn leaching reaches 100% in the absence of current. That is why the content of these  elements in the electrolyte solutions obtained as a result of electrochemical leaching was  not determined.  3.2.3. Experiment Duration  Studies of the effect of experiment duration on the efficiency of metal leaching were  carried out at a constant electrolyte temperature of 70 °C.  The results obtained show (Figure 11) that an increase in the duration of the experi- ment from 1 h to 3 h leads to an increase in the degree of all metal leaching, except for Sn  (the metal is completely leached at the end of a one-hour experiment).  The kinetics of metal  leaching  from  raw material 1 was also studied at a constant  electrolyte temperature of 70 °C, both without power supply (Figure 12) and under con- ditions of alternating current superimposition (Figure 13).    Figure 11. Impact of experiment duration on the degree of metal leaching. Experimental conditions:  cell 2; raw material 1; S/L = 8.6 g·L−1; i = 0.84 A·cm−2; CHCl = 6 mol·L−1; Tel = 70 °C.  0 0 90.2 88.7 57.9 96.9 100 92.8 71.8 85.2 7.9 60.6 96.9 100 94.9 100 96.1 83.3 100 0 20 40 60 80 100 Au Ag Al Cu Ni Pb Sn Ti Zn Fe R , % t = 1 h t = 3 h Figure 11. Impact of experiment duration on the degree of metal leaching. Experimental conditions: cell 2; raw material 1; S/L = 8.6 g·L−1; i = 0.84 A·cm−2; CHCl = 6 mol·L−1; Tel = 70 ◦C. The kinetics of metal leaching from raw material 1 was also studied at a constant elec- trolyte temperature of 70 ◦C, both without power supply (Figure 12) and under conditions of alternating current superimposition (Figure 13). The comparison of the results obtained shows (Figures 12 and 13) that current super- imposition has practically no effect on the efficiency of Sn, Ti, Pb, and Zn leaching, but leads to an increase in the degree of leaching of such metals as Ni, Cu, and Ag. Under similar experimental conditions (tex = 3 h) in the absence of current, RAg = 14.4% and RNi = 73.0%, and in conditions of current superimposition, RAg = 69.6% and RNi = 95.4%. As a result of electrochemical leaching after 1.5 h of the experiment, RCu = 96.5%, and chemical leaching—RCu = 91.8% after 2 h of the experiment. In addition, under current superimposition, slow leaching of Au begins after 0.5 h of the experiment, and after 3 h, RAu achieves only 5.1%. Metals 2024, 14, 65 16 of 25 Metals 2024, 14, x FOR PEER REVIEW  17  of  26       Figure 12. Impact of experiment duration on the degree of metal leaching. Experimental conditions:  cell 2; raw material 1; S/L = 8.6 g·L−1; i = 0; CHCl = 6 mol·L−1; Tel = 70 °C.    Figure 13. Impact of experiment duration on the degree of metal leaching. Experimental conditions:  cell 2; raw material 1; S/L = 8.6 g·L−1; i = 0.63 A·cm−2; CHCl = 6 mol·L−1; Tel = 70 °C.  The comparison of the results obtained shows (Figures 12 and 13) that current super- imposition has practically no effect on the efficiency of Sn, Ti, Pb, and Zn leaching, but  leads to an  increase  in the degree of leaching of such metals as Ni, Cu, and Ag. Under  similar experimental conditions (tex = 3 h) in the absence of current, RAg = 14.4% and RNi =  73.0%, and in conditions of current superimposition, RAg = 69.6% and RNi = 95.4%. As a  result of electrochemical leaching after 1.5 h of the experiment, RCu = 96.5%, and chemical  leaching—RCu = 91.8% after 2 h of the experiment. In addition, under current superimpo- sition, slow leaching of Au begins after 0.5 h of the experiment, and after 3 h, RAu achieves  only 5.1%.  Thus, in the process of electrochemical hydrochlorination, high current density and  concentration of chloride ions in the electrolyte solution, as well as elevated temperature,  contribute to the high efficiency of the leaching of base metals. In addition, leaching time  also affects the completeness of base metal leaching.    Unfortunately, a correct comparative analysis of the results presented above with the  results of studies on  the hydrochlorination  leaching of base metals presented by other  authors [33–36] is difficult. This is due to both the different metal content in the materials  under study and the particle size of the crushed PCBs (from 0.15 mm to 5.0 mm), as well  as the concentration of chloride ions in the leaching solution, its temperature, and experi- ment duration.  3.3. Impact of Chemical/Electrochemical Pretreatment on the Efficiency of Metal Leaching    Two methods were used for the pretreatment of  the raw material, keeping experi- ment time at 1 h, based on the results (Figures 3(Ac) and 12), where Ag and Au leaching  into solution was not determined.    0 20 40 60 80 100 0 0.5 1 1.5 2 2.5 3 R, % t, h Ag Al Cu Ni Pb Sn Ti Zn Fe 0 20 40 60 80 100 0 0.5 1 1.5 2 2.5 3 R, % t, h Au Ag Al Cu Ni Pb Sn Ti Zn Fe Figure 12. Impact of experiment duration on the degree of metal leaching. Experimental conditions: cell 2; raw material 1; S/L = 8.6 g·L−1; i = 0; CHCl = 6 mol·L−1; Tel = 70 ◦C. Metals 2024, 14, x FOR PEER REVIEW  17  of  26       Figure 12. Impact of experiment duration on the degree of metal leaching. Experimental conditions:  cell 2; raw material 1; S/L = 8.6 g·L−1; i = 0; CHCl = 6 mol·L−1; Tel = 70 °C.    Figure 13. Impact of experiment duration on the degree of metal leaching. Experimental conditions:  cell 2; raw material 1; S/L = 8.6 g·L−1; i = 0.63 A·cm−2; CHCl = 6 mol·L−1; Tel = 70 °C.  The comparison of the results obtained shows (Figures 12 and 13) that current super- imposition has practically no effect on the efficiency of Sn, Ti, Pb, and Zn leaching, but  leads to an increase  in the degree of leaching of such metals as Ni, Cu, and Ag. Under  similar experimental conditions (tex = 3 h) in the absence of current, RAg = 14.4% and RNi =  73.0%, and in conditions of current superimposition, RAg = 69.6% and RNi = 95.4%. As a  result of electrochemical leaching after 1.5 h of the experiment, RCu = 96.5%, and chemical  leaching—RCu = 91.8% after 2 h of the experiment. In addition, under current superimpo- sition, slow leaching of Au begins after 0.5 h of the experiment, and after 3 h, RAu achieves  only 5.1%.  Thus, in the process of electrochemical hydrochlorination, high current density and  concentration of chloride ions in the electrolyte solution, as well as elevated temperature,  contribute to the high efficiency of the leaching of base metals. In addition, leaching time  also affects the completeness of base metal leaching.    Unfortunately, a correct comparative analysis of the results presented above with the  results of studies on  the hydrochlorination  leaching of base metals presented by other  authors [33–36] is difficult. This is due to both the different metal content in the materials  under study and the particle size of the crushed PCBs (from 0.15 mm to 5.0 mm), as well  as the concentration of chloride ions in the leaching solution, its temperature, and experi- ment duration.  3.3. Impact of Chemical/Electrochemical Pretreatment on the Efficiency of Metal Leaching    Two methods were used for the pretreatment of  the raw material, keeping experi- ment time at 1 h, based on the results (Figures 3(Ac) and 12), where Ag and Au leaching  into solution was not determined.    0 20 40 60 80 100 0 0.5 1 1.5 2 2.5 3 R, % t, h Ag Al Cu Ni Pb Sn Ti Zn Fe 0 20 40 60 80 100 0 0.5 1 1.5 2 2.5 3 R, % t, h Au Ag Al Cu Ni Pb Sn Ti Zn Fe Figure 13. Impact of experiment duration on the degree of metal leaching. Experimental conditions: cell 2; raw material 1; S/L = 8.6 g·L−1; i = 0.63 A·cm−2; CHCl = 6 mol·L−1; Tel = 70 ◦C. Thus, in the process of electr chemical hydrochlorination, high current density and concentration of chlorid ions in the electrolyte solution, as well as elevated temp ature, contribute to the high efficiency of the leac ing of base metals. In addition, leaching time also affects the completeness of ba e metal leachi g. Unfortunately, a correct comparative analysis of th results presented above with the results of st dies on th hydroc lorinat on leaching of base metals presented by other authors [33–36] is diffic lt. This is due to both the different metal content in the materials under study and the particle size of the crushed PCBs (from 0.15 mm to 5.0 mm), as well as the concentration of chloride ions in the leaching solution, its temperature, and experiment duration. 3.3. Impact of Che ical/Electrochemical Pretreatment on the Efficiency of Metal Leaching Two methods were used for the pretreatment of the raw material, keeping experiment time at 1 h, based on the results (Figures 3(Ac) and 12), where Ag and Au leaching into solution was not etermined. • Electrochemical pret eatment—pr treatment 1. Experim ntal conditions: polypropy- lene cell, CHCl = 6 M, i = 0.88 A·cm−2, tex= 1 h, S/L = 8.6 g/L. The treatment was carried out without regulation of the electrolyte temperature (Tel) and during the process Tel increased fro 20 ◦C to 68 ◦C; • Che ical pretr atment—pretreatm nt 2. Experimental conditions: glass c ll, CHCl = 6 M, i = 0, tex= 1 h, S/L = 8.6 g/L. The pretreatment was carrie o t at a constant e ectrolyte temperature (Tel = 70 ◦C). The results obtained show (Figure 14) that the use of electrochemical pretreatment of raw material 1 makes it possible to significantly intensify the process of gold leaching at an experiment duration of ≥1 h. It should be noted that, as a result of the use of Metals 2024, 14, 65 17 of 25 chemical pretreatment (pretreatment 2), the leaching of gold under similar conditions of the electrochemical process was not observed. Metals 2024, 14, x FOR PEER REVIEW  18  of  26      Electrochemical  pretreatment—pretreatment  1. Experimental  conditions:  polypro- pylene cell, CHCl = 6 M, i = 0.88 A·cm−2, tex= 1 h, S/L = 8.6 g/L. The treatment was carried  out without regulation of the electrolyte temperature (Tel) and during the process Tel  increased from 20 °C to 68 °C;     Chemical pretreatment—pretreatment 2. Experimental conditions: glass cell, CHCl = 6  M, i = 0, tex= 1 h, S/L = 8.6 g/L. The pretreatment was carried out at a constant electro- lyte temperature (Tel = 70 °C).  The results obtained show (Figure 14) that the use of electrochemical pretreatment of  raw material 1 makes it possible to significantly intensify the process of gold leaching at  an experiment duration of ≥1 h. It should be noted that, as a result of the use of chemical  pretreatment (pretreatment 2), the leaching of gold under similar conditions of the elec- trochemical process was not observed.    Figure 14. Impact of pretreatment 1 of raw material 1 on the efficiency of Au electrochemical leach- ing. Experimental conditions: cell 1, fresh portion of electrolyte (CHCl = 6 M), i = 0.88 A·cm−2.  Figure15 demonstrates that electrochemical pretreatment significantly increases the  degree of leaching of Ag in the process of electrochemical leaching. Silver leaching under  similar conditions of the electrochemical process is less effective using chemical pretreat- ment of the raw material (pretreatment 2) in comparison with pretreatment 1. After 0.5 h  of the experiment, the degree of Ag leaching was 1.6%, and after 1 h—62.6%. According  to  the  results obtained  (Figures 14 and 15),  the  leaching efficiencies of Au and Ag as a  result of the electrochemical leaching process (tex= 2 h, i = 0.88 A⸱cm−2) using pretreatment  1 of raw material 1 were 73.6% and 86.7%, respectively.      Figure 14. I act of retreat ent 1 of ra aterial 1 on the efficiency of Au electrochemical leaching. Experim ntal conditions: cell 1, fresh portion of el ctrolyte (CHCl ), i = 0. 8 A·cm−2. i 15 demonstrates that lectrochemical p treatment s gnificantly increas s the degree f Ag in the process of lectrochemical leaching. Silver leachi g under si il i s of the electrochemical proces is less effective using chemical pretreat- ment of t e r t e t 2) in comparison with pretreatment 1. After 0.5 h of the experiment, the degree of Ag leaching was 1. , ft r 62.6%. According to the results obtained (Figures 14 and 15), the leaching efficiencies of Au and Ag as a result of the electrochemical leaching process (tex= 2 h, i = 0.88 A·cm−2) using pretreat ent 1 of raw material 1 were 73.6% and 86.7%, respectively. Metals 2024, 14, x FOR PEER REVIEW  18  of  26      Electrochemical  pretreatment—pretreatment  1. Experimental  conditions:  polypro- pylene cell, CHCl = 6 M, i = 0.88 A·cm−2, tex= 1 h, S/L = 8.6 g/L. The treatment was carried  out without regulation of the electrolyte temperature (Tel) and during the process Tel  increased from 20 °C to 68 °C;     Chemical pretreatment—pretreatment 2. Experimental conditions: glass cell, CHCl = 6  M, i = 0, tex= 1 h, S/L = 8.6 g/L. The pretreatment was carried out at a constant electro- lyte temperature (Tel = 70 °C).  The results obtained show (Figure 14) that the use of electrochemical pretreatment of  raw material 1 makes it possible to significantly intensify the process of gold leaching at  an experiment duration of ≥1 h. It should be noted that, as a result of the use of chemical  pretreatment (pretreatment 2), the leaching of gold under similar conditions of the elec- trochemical process was not observed.    Figure 14. Impact of pretreatment 1 of raw material 1 on the efficiency of Au electrochemical leach- ing. Experimental conditions: cell 1, fresh portion of electrolyte (CHCl = 6 M), i = 0.88 A·cm−2.  Figure15 demonstrates that electrochemical pretreatment significantly increases the  degree of leaching of Ag in the process of electrochemical leaching. Silver leaching under  similar conditions of the electrochemical process is less effective using chemical pretreat- ment of the raw material (pretreatment 2) in comparison with pretreatment 1. After 0.5 h  of the experiment, the degree of Ag leaching was 1.6%, and after 1 h—62.6%. According  to  the  results obtained  (Figures 14 and 15),  the  leaching efficiencies of Au and Ag as a  result of the electrochemical leaching process (tex= 2 h, i = 0.88 A⸱cm−2) using pretreatment  1 of raw material 1 were 73.6% and 86.7%, respectively.      Figure 15. Impact of electrochemical pretreatment of raw material 1 on the efficiency of Ag electro- chemical leaching. Experimental conditions: cell 1; pretreatment 1 of raw material 1; fresh portion of electrolyte (CHCl = 6 M), i = 0.88 A·cm−2. The results of the study of the effect of pretreatment (chemical and electrochemical) on the efficiency of leaching of base metals and Fe are presented in Figure 16. From the presented data, it is clear that pretreatment 2 resulted in more efficient leaching of almost all base metals compared to pretreatment 1, except for Cu. Copper was leached more effectively with pretreatment 1. It should be noted that, at the stage of both pretreatments of raw material 1, the leaching of noble metals was not observed, which makes it possible to avoid their losses at this stage. Metals 2024, 14, 65 18 of 25 Metals 2024, 14, x FOR PEER REVIEW  19  of  26     Figure 15. Impact of electrochemical pretreatment of raw material 1 on the efficiency of Ag electro- chemical leaching. Experimental conditions: cell 1; pretreatment 1 of raw material 1; fresh portion  of electrolyte (CHCl = 6 M), i = 0.88 A·cm−2.  The results of the study of the effect of pretreatment (chemical and electrochemical)  on the efficiency of leaching of base metals and Fe are presented in Figure 16.    Figure 16. Impact of the pretreatment of raw material 1 on the degree of base metal leaching. Exper- imental conditions: cell 1, CHCl = 6 M, tex = 1 h.  From  the presented data,  it  is  clear  that pretreatment 2  resulted  in more  efficient  leaching of almost all base metals compared to pretreatment 1, except for Cu. Copper was  leached more effectively with pretreatment 1. It should be noted that, at the stage of both  pretreatments of raw material 1, the  leaching of noble metals was not observed, which  makes it possible to avoid their losses at this stage.  The concentration of the metals under study in the solutions obtained, both as result  of  electrochemical pretreatment  (stage 1) and  the  subsequent  electrochemical  leaching  (stage 2), are presented in Table 2. Additionally, the table presents the estimated quantities  (Equation (2)) of each metal in the solid residue after the completion of each stage of the  process. Moreover,  the results obtained  for solid  residue after stage 2 are  less accurate  compared to stage 1, due to incomplete removal of the electrolyte solution after pretreat- ment (stage 1).    Table 2. Concentration of metals in the electrolyte solution and residual amounts of metals in the  solid residue after electrochemical leaching.  Me  Leaching  Chemical  Electrochemical  Stage 1  Stage 2  Metal Concentration in  Leach Solution,    CMe, mg/L  Metal Concentration in  Electrolyte Solution,    CMe, mg/L  Metal Content in Solid  Residue,    mMe, mg  Metal Concentration in  Electrolyte Solution,    CMe, mg/L  Metal Content in  Solid Residue,    mMe, mg  Au  4.320  0  1.512  3.18  ~0.4  Ag  14.86  0  5.202  12.89  ~0.7  Pd  BDL 1  BDL 1  -  BDL 1  -  Al  149.2  96.25  18.52  27.74  ~8.8  Cu  121.4  115.1  2.189  6.300  0  Ni  94.34  43.61  17.75  38.96  ~4.1  Pb  91.34  85.29  2.117  5.020  ~0.4  Sn  242.1  231.3  3.765  3.940  ~2.4  Ti  132.4  116.7  5.481  9.150  ~2.3  Zn  45.60  27.93  6.189  2.000  ~5.5  62.8 98.2 53.4 93.2 98.0 88.5 61.6 78.8 84.9 77.4 54.7 99.9 100 96.7 71.8 81.6 0 20 40 60 80 100 Al Cu Ni Pb Sn Ti Zn Fe R, % pretreatment 1 pretreatment 2 Figure 16. Impact of the pretreatment of raw material 1 on the degree of base metal leaching. Experimental conditions: cell 1, CHCl = 6 M, tex = 1 h. The concentration of the metals under study in the solutions obtained, both as result of electrochemical pretreatment (stage 1) and the subsequent electrochemical leaching (stage 2), are presented in Table 2. Additionally, the table presents the estimated quantities (Equation (2)) of each metal in the solid residue after the completion of each stage of the process. Moreover, the results obtained for solid residue after stage 2 are less accurate com- pared to stage 1, due to incomplete removal of the electrol te solution after pretreatment (stage 1). Table 2. Concentration of metals in the lectrolyt ution and residual amounts of metals in the solid residue after electrochemical leaching. Me Leaching Chemical Electrochemical Stage 1 Stage 2 Metal Concentration in Leach Solution, CMe, mg/L Metal Concentration in Electrolyte Solution, CMe, mg/L Metal Content in Solid Residue, mMe, mg Metal Concentration in Electrolyte Solution, CMe, mg/L Metal Content in Solid Residue, mMe, mg Au 4.320 0 1.512 3.18 ~0.4 Ag 14.86 0 5.202 12.89 ~0.7 Pd BDL 1 BDL 1 - BDL 1 - Al 149.2 96.25 18.52 27.74 ~8.8 Cu 121.4 115.1 2.189 6.300 0 Ni 94.34 43.61 17.75 38.96 ~4.1 Pb 91.34 85.29 2.117 5.020 ~0.4 Sn 242.1 231.3 3.765 3.940 ~2.4 Ti 132.4 116.7 5.481 9.150 ~2.3 Zn 45.60 27.93 6.189 2.000 ~5.5 Fe 2861 2024 293.0 837.0 0 1 BDL—below the detectable limit. For comparison, Table 3 presents the same characteristics of the electrochemical leach- ing process, obtained under similar experimental conditions, but without the pretreatment stage of the raw material. Data for a 6 h experiment are also provided. The presented data (Tables 2 and 3, Figures 14 and 15) show that the usage of an elec- trochemical pretreatment stage for raw material 1 makes it possible not only to significantly increase the efficiency of Au and Ag leaching, but also to reduce the content of base metals in the final electrolyte solution. Thus, a gold concentration in the final electrolyte solution in a two-stage electrochemical process (with pretreatment) is almost an order of magnitude higher than in a one-stage process (without pretreatment). In the case of silver, this effect is less pronounced. At the same time, the content of base metals in the solution is significantly lower in comparison with the one-stage experiment. Metals 2024, 14, 65 19 of 25 Table 3. Concentration of metals in the electrolyte solution and residual amounts of metals in the solid residue after electrochemical leaching at different experiment duration. Me Leaching Chemical Electrochemical Experiment Duration tex = 2 h tex = 6 h Metal Concentration in Leach Solution, CMe, mg/L Metal Concentration in Electrolyte Solution, CMe, mg/L Metal Content in Solid Residue, mMe, mg Metal Concentration in Electrolyte Solution, CMe, mg/L Metal Content in Solid Residue, mMe, mg Au 4.320 0.350 1.389 3.35 0.339 Ag 14.86 8.820 0 14.87 0 Pd BDL 1 BDL 1 - BDL 1 - Al 149.2 141.7 2.611 149.3 0 Cu 121.4 121.5 0 121.5 0 Ni 94.34 86.85 2.620 94.34 0 Pb 91.34 91.36 0 91.35 0 Sn 242.1 242.3 0 242.2 0 Ti 132.4 126.8 1.946 132.5 0 Zn 45.60 32.29 4.654 43.17 0.850 Fe 2861 2861 0 2862 0 1 BDL—below the detectable limit. Increasing the duration of a one-stage experiment leads to an increase in the con- centration of all metals, except for those metals whose concentration reaches maximum values within 2 h of the electrochemical process (Cu, Pb, Sn, and Fe). As a result of a 6 h experiment, complete leaching of all metals under study is achieved, except for Au and Zn (Table 3). At the same time, the concentration of Au and Ag in the electrolyte solution is only slightly higher than under the conditions of a 2 h two-stage experiment (Tables 2 and 3). Thus, for raw material 1, pretreatment 1 is recommended, both to increase the degree of leaching of gold and silver, and to reduce the content of base metals in the final electrolyte solution. But to increase the completeness of leaching of noble metals from the raw material, an increase in the duration of the experiment is required. The S. Ilyas et al. [37] also carried out preliminary leaching of base metals followed by hydrochlorination of the dry PCBs residue. Under the experimental conditions (hy- drochlorination in a leaching reactor with an external supply of chlorine), the degree of gold leaching was more than 99% within 75 min of the experiment. It should be noted that solder removed PCBs with a lower gold content (286 ppm) was used in this study. However, after the finishing of the leaching process using this technique, subsequent neutralization of the excess chlorine is required. 3.4. Electrolyte Reuse in Metal Leaching The possibility of electrolyte reuse in the process of electrochemical leaching of metals without temperature control of the electrolyte was studied (Figure 17). The results of the study showed that the electrolyte reuse leads to the complete leaching of Al, Cu, Pb, Sn, and Ti and a slight increase in the efficiency of Zn, Ni, and Fe leaching from a fresh portion of raw material 1. At the same time, a decrease in the electrochemical leaching efficiency of Ag and Au was observed. The results of electrolyte reuse in the process of chemical leaching (without current superimposition) showed (Figure 18) that a decrease in the efficiency of leaching of all metals, except for Sn, was observed. Moreover, this effect is manifested most significantly in the case of Ag, Au, Al, Cu, and Ni. Thus, the results obtained showed that the electrolyte, after electrochemical leaching, can be reused for metal leaching for a fresh portion of the raw material. Moreover, base metals are leached more efficiently when the electrolyte is reused in an electrochemical leaching process compared to a chemical leaching process. A decrease in the leaching efficiency of Au and Ag was observed in both leaching processes. Metals 2024, 14, 65 20 of 25 Metals 2024, 14, x FOR PEER REVIEW  21  of  26     However, after the finishing of the leaching process using this technique, subsequent neu- tralization of the excess chlorine is required.    3.4. Electrolyte Reuse in Metal Leaching  The possibility of electrolyte reuse in the process of electrochemical leaching of met- als without temperature control of the electrolyte was studied (Figure 17).    Figure 17. The efficiency of metal leaching under the conditions of a two-stage experiment. Experi- mental conditions: polypropylene cell, loading option 1; raw material 1; tex = 3 h. Stage 1—S/L = 8.6  g·L−1; CHCl = 6 mol·L−1, i = 0.88 A·cm−2; stage 2—fresh portion of raw material 1, filtered electrolyte  after stage 1, S/L = 9.4 g·L−1, i = 1.0 A·cm−2.  The results of the study showed that the electrolyte reuse leads to the complete leach- ing of Al, Cu, Pb, Sn, and Ti and a slight increase in the efficiency of Zn, Ni, and Fe leaching  from a fresh portion of raw material 1. At the same time, a decrease in the electrochemical  leaching efficiency of Ag and Au was observed.  The results of electrolyte reuse in the process of chemical leaching (without current  superimposition) showed  (Figure 18)  that a decrease  in  the efficiency of  leaching of all  metals, except for Sn, was observed. Moreover, this effect is manifested most significantly  in the case of Ag, Au, Al, Cu, and Ni,. Thus, the results obtained showed that the electro- lyte, after electrochemical leaching, can be reused for metal leaching for a fresh portion of  the raw material. Moreover, base metals are leached more efficiently when the electrolyte  is reused in an electrochemical leaching process compared to a chemical leaching process.  A decrease  in  the  leaching efficiency of Au and Ag was observed  in both  leaching pro- cesses.    Figure 18. The efficiency of metal leaching under the conditions of a two-stage experiment. Experi- mental conditions: polypropylene cell, loading option 1; raw material 1. Stage 1—S/L = 8.6 g·L−1; CHCl  12.7 71.4 100 100 91.0 100 100 100 79.1 92.9 3.4 68.1 95.5 86.3 96.3 0 20 40 60 80 100 Au Ag Al Cu Ni Pb Sn Ti Zn Fe R, % Stage 1 Stage 2 15.0 99.6 98.6 99.3 84.9 96.2 98.2 93.1 66.6 100 7.3 0 38.1 19.4 37.2 90.3 64.6 58.3 61.5 0 20 40 60 80 100 Au Ag Al Cu Ni Pb Sn Ti Zn Fe R, % stage 1 stage 2 Figure 17. The efficiency of metal leaching under the conditions of a two-stage experiment. Experimental conditions: polypropylene cell, loading option 1; raw material 1; tex = 3 h. Stage 1—S/L = 8.6 g·L−1; CHCl = 6 mol·L−1, i = 0.88 A·cm−2; s age 2—fr sh portion of raw ma- teri l 1, filtered electrolyte after stage 1, S/L = 9.4 g·L−1, i = 1.0 A·cm−2. Metals 2024, 14, x FOR PEER REVIEW  21  of  26     However, after the finishing of the leaching process using this technique, subsequent neu- tralization of the excess chlorine is required.    3.4. Electrolyte Reuse in Metal Leaching  The possibility of electrolyte reuse in the process of electrochemical leaching of met- als without temperature control of the electrolyte was studied (Figure 17).    Figure 17. The efficiency of metal leaching under the conditions of a two-stage experiment. Experi- mental conditions: polypropylene cell, loading option 1; raw material 1; tex = 3 h. Stage 1—S/L = 8.6  g·L−1; CHCl = 6 mol·L−1, i = 0.88 A·cm−2; stage 2—fresh portion of raw material 1, filtered electrolyte  after stage 1, S/L = 9.4 g·L−1, i = 1.0 A·cm−2.  The results of the study showed that the electrolyte reuse leads to the complete leach- ing of Al, Cu, Pb, Sn, and Ti and a slight increase in the efficiency of Zn, Ni, and Fe leaching  from a fresh portion of raw material 1. At the same time, a decrease in the electrochemical  leaching efficiency of Ag and Au was observed.  The results of electrolyte reuse in the process of chemical leaching (without current  superimposition) showed  (Figure 18)  that a decrease  in  the efficiency of  leaching of all  metals, except for Sn, was observed. Moreover, this effect is manifested most significantly  in the case of Ag, Au, Al, Cu, and Ni,. Thus, the results obtained showed that the electro- lyte, after electrochemical leaching, can be reused for metal leaching for a fresh portion of  the raw mat rial. Moreover, base metals are leache  more efficiently when the  lectrolyte  is reused in an electrochemical l aching pr cess compared to   chemical leaching process. A decrease  in  the  leaching efficiency of Au and Ag was observed  in both  l aching pro- cesses.    Figure 18. The efficiency of metal leaching under the conditions of a two-stage experiment. Experi- mental conditions: polypropylene cell, loading option 1; raw material 1. Stage 1—S/L = 8.6 g·L−1; CHCl  12.7 71.4 100 100 91.0 100 100 100 79.1 92.9 3.4 68.1 95.5 86.3 96.3 0 20 40 60 80 100 Au Ag A Cu Ni Pb Sn Ti Zn Fe R, % Stage 1 Stage 2 15.0 99.6 98.6 99.3 84.9 96.2 98.2 93.1 66.6 100 7.3 0 38.1 19.4 37.2 90.3 64.6 58.3 61.5 0 20 40 60 80 100 Au Ag Al Cu Ni Pb Sn Ti Zn Fe R, % stage 1 stage 2 Figure 18. The efficiency of metal leaching under the conditions of a two-stage experiment. Experi- mental conditions: polypropylene cell, loading option 1; raw material 1. Stage 1—S/L = 8.6 g·L−1; CHCl = 6 mol·L−1, i = 0.88 A·cm−2; tex = 3 h; stage 2—fresh portion of raw material 1, filtered electrolyte after stage 1, S/L = 9.0 g·L−1, i = 0; tex = 20 h without stirring of the reaction mixture. 3.5. Quantitative Determination of Metals in Solid Residue Residual amounts of metals in the solid residue after electrochemical leaching were assessed via the results of ICP-OES analysis of both the solution obtained as a result of chemical treatment of the solid residue (leach solution) and of the resulting electrolyte solution. Figure 19 shows the results on the leaching efficiency of Au, Ni, and Zn from raw material 1. The choice of these metals is due to the fact that all other metals under study are almost completely leached at this stage of electrochemical leaching. Metals 2024, 14, x FOR PEER REVIEW  22  of  26  = 6 mol·L−1, i = 0.88 A·cm−2 ; tex = 3 h; stage 2—fresh portion of raw material 1, filtered electrolyte after  stage 1, S/L = 9.0 g·L−1, i = 0; tex = 20 h without stirring of the reaction mixture.  3.5. Quantita ve D termination of Metals in Solid Resi ue  esidual amounts of metals in the solid residu  after elect ochemi al leaching were  assessed via the result  of ICP-OES analysis of both the solution obtained as a result of  chemical  treatment of  the solid residue  (leach solution) and of  the resulting electrolyt   solution. Figure 19 shows the results on the leaching efficiency of Au, Ni, and Zn from raw  material 1. The choice of these metals is due to the fact that all other metals under study  are almost completely leached at this stage of electrochemical leaching.  Figure 19. The degree of metal  leaching:  1—electrochemical  leaching of  raw material 1  (experi- mental conditions: polypropylene cell, loading option 1, CHCl = 6 M, i = 0.88 A·cm⁻², tex = 3 h, S/L = 8,6  g·L−1); 2—chemical leaching of the solid residue produced from the electrochemical process.  From the presented data (Figure 19), it is clear that the total degree of Ni, Zn, and Au  leaching from the raw material does not reach 100%. Moreover, in the case of gold, RAu is  only 16.2%. This is probably due to the loss of fine particles from these metals, especially  Au, at the stages of preparation of the solid residue (washing of the filter, removal from  the filter after drying, grinding  in a mortar). Thus,  to evaluate  the effectiveness of  the  leaching process  and  the  residual  amounts of metals  in  the  solid  residue,  it  is  recom- mended to use ICP-OES for quantitative determination of metals directly in the electrolyte  solution, but not in the solution obtained as a result of the chemical leaching of solid resi- due.  3.6. Study of Metal Leaching Process Reproducibility  To assess  the reproducibility of  the results of  the  leaching of metals  from  the raw  material into an electrolyte solution, six series of experiments were carried out under the  same experimental conditions in each series. The experimental conditions for each series  are presented in Table 4.    Raw material 1 and raw material 2 were used as research objects. The results of the  ICP-OES analysis of  the obtained electrolyte solutions and  the values of  the root mean  square deviation (RMSD,  𝑆௫̅) of the results obtained for each metal in the experimental  series are presented in Table 5.  Table 4. Experimental conditions in all series: cell 1; S/L = 8.6 g/L; CHCl = 6 M; i = 0.88 A⸱cm−2.  Exp. Series Nº  Experimental Conditions  Number of Exp.  Raw Material  Exp. Duration, tex, h  1  3  1  1  2  4  1  3  3  3  1  5  4  3  2  1  15.0 84.9 66.6 1.2 4.1 13.7 0 20 40 60 80 100 Au Ni Zn R, % 1 2 Figure 19. The degree of metal leaching: 1—electrochemical leaching of raw material 1 (experimental conditions: polypropylene cell, loading option 1, CHCl = 6 M, i = 0.88 A·cm−2, tex = 3 h, S/L = 8.6 g·L−1); 2—chemical leaching of the solid residue produced from the electrochemical process. Metals 2024, 14, 65 21 of 25 From the presented data (Figure 19), it is clear that the total degree of Ni, Zn, and Au leaching from the raw material does not reach 100%. Moreover, in the case of gold, RAu is only 16.2%. This is probably due to the loss of fine particles from these metals, especially Au, at the stages of preparation of the solid residue (washing of the filter, removal from the filter after drying, grinding in a mortar). Thus, to evaluate the effectiveness of the leaching process and the residual amounts of metals in the solid residue, it is recommended to use ICP-OES for quantitative determination of metals directly in the electrolyte solution, but not in the solution obtained as a result of the chemical leaching of solid residue. 3.6. Study of Metal Leaching Process Reproducibility To assess the reproducibility of the results of the leaching of metals from the raw material into an electrolyte solution, six series of experiments were carried out under the same experimental conditions in each series. The experimental conditions for each series are presented in Table 4. Table 4. Experimental conditions in all series: cell 1; S/L = 8.6 g/L; CHCl = 6 M; i = 0.88 A·cm−2. Exp. Series No Experimental Conditions Number of Exp. Raw Material Exp. Duration, tex, h 1 3 1 1 2 4 1 3 3 3 1 5 4 3 2 1 5 3 2 2 6 3 2 5 Raw material 1 and raw material 2 were used as research objects. The results of the ICP-OES analysis of the obtained electrolyte solutions and the values of the root mean square deviation (RMSD, Sx) of the results obtained for each metal in the experimental series are presented in Table 5. Table 5. The average amount of each metal leached into the electrolyte solution under the conditions of the experimental series. Me Average Metal Content in Electrolyte Solution, ¯ mMe, mg Exp. Series 1 Exp. Series 2 Exp. Series 3 Exp. Series 4 Exp. Series 5 Exp. Series 6 Au 0 0.209 ± 0.020 0.801 ± 0.075 0 0.554 ± 0.340 2.022 ± 0.050 Ag 0 3.986 ± 0.401 3.814 ± 0.220 1.668 ± 0.274 2.949 ± 0.166 3.306 ± 0.280 Al 32.771 ± 0.610 52.602 ± 0.605 51.280 ± 0.780 72.042 ± 0.765 89.460 ± 3.040 86.310 ± 0.423 Cu 41.720 ± 0.720 42.893 ± 0.614 46.120 ± 0.270 46.877 ± 0.260 45.768 ± 1.288 40.413 ± 0.244 Ni 17.617 ± 1.360 29.159 ± 0.419 31.960 ± 0.400 12.025 ± 0.721 13.012 ± 0.454 11.516 ± 0.146 Pb 29.808 ± 0.190 31.608 ± 0.343 33.130 ± 0.400 23.211 ± 0.140 22.502 ± 0.845 19.109 ± 0.209 Sn 83.055 ± 1.260 84.626 ± 0.825 88.840 ± 1.180 76.988 ± 0.375 73.780 ± 3.261 61.577 ± 0.427 Ti 41.008 ± 0.120 45.855 ± 0.931 46.630 ± 0.470 10.935 ± 0.051 11.202 ± 0.339 10.414 ± 0.017 Zn 9.828 ± 0.150 11.544 ± 0.436 12.720 ± 0.031 9.093 ± 0.111 8.940 ± 0.260 8.222 ± 0.177 Fe 788.900 ± 45.130 960.15 ± 17.934 1150.70 ± 27.16 297.700 ± 16.145 375.900 ± 20.127 356.533 ± 1.345 Analysis of the data presented in Table 5 shows that the most reproducible results (standard deviation ≤ 5%) were obtained for Cu in all experimental series. The repro- ducibility of the results for Au and Ag is significantly worse (red font in Table 5). The most reliable results for Au were obtained in experimental series No. 2, No. 3, and No. 6, and for Ag in series No. 3 and No. 5. In the case of such active metals as Pb, Sn, Ti, and Zn, the most reliable results were obtained in all series of experiments, as well as for iron, except for series No. 1. In the case of Ni, the least reliable results were obtained in series No. 1 and No. 4 (red font in Table 5). Based on the presented data, it is clear that Metals 2024, 14, 65 22 of 25 the best reproducibility of results for all metals was obtained in experimental series No. 3. In addition, in experimental series No. 2 and 6, a high reproducibility was also obtained for all metals, except for Ag, and in series No. 5, for all metals except for Au. It should be noted that in these experimental series, different raw materials were used (raw material 1 and raw material 2). Thus, the reproducibility of the results of electrochemical hydrochlorination of disinteg rator-crushed PCB powders with a particle size < 90 µm generally depends on four factors: (i) on the non-metallic part (solid residue Table 1); (ii) the studied metal content; (iii) on the homogeneity of the raw material sample under study, and (iv) on the accuracy of the analytical determination of elements (especially of Ag) in the electrolyte solutions using ICP-OES. 4. Conclusions The results obtained show that at Tel = 80 ◦C, i = 0.84 A·cm−2, S/L = 8.6 g·L−1 and an experiment duration of 1 h, an increase in hydrochloric acid concentration from 4 mol·L−1 to 8 mol·L−1 leads to a significant increase in the degree of Ni, Zn, and Ag leaching from single-crushed mixed PCBs. At the same time, the complete leaching of Cu was achieved at CHCl = 6 mol·L−1. An increase in electrolyte (CHCl = 6 M) temperature from 60 ◦C to 80 ◦C, at an ex- periment duration of 1 h in the absence of current, leads to an increase in the degree of leaching of almost all base metals and Fe. At i = 0.63 A·cm−2, an increase in the electrolyte temperature leads to an increase in the degree of Ni, Zn, Fe, and Al leaching but practically does not affect the degree of Cu, Ti, Pb, and Sn leaching. In turn, an increase in AC density from 0.21 A·cm−2 to 0.84 A·cm−2 has the most significant effect on the efficiency of leaching of all base metals at an electrolyte temperature of 80 ◦C. However, under the conditions of all experiments carried out at constant electrolyte temperatures and with a duration of 1 h, the transition of Au into the electrolyte solution was not established. Comparison of the results on the kinetics of metal leaching into electrolyte solution with Tel = 70 ◦C in the absence AC and under AC superimposition (i = 0.63 A·cm−2) showed that the current superimposition has practically no effect on the efficiency of Sn, Ti, Pb, and Zn leaching, but leads to an increase in the degree of leaching of such metals as Ni, Cu, and Ag. With an experiment duration of 3 h in the absence of current RAg = 14.4% and RNi = 73.0%, and under conditions of current superimposition RAg = 69.6% and RNi = 95.4%. As a result of electrochemical leaching after 1.5 h of experiment, RCu = 96.5%, and for chemical leaching—RCu = 91.8% after 2 h of experiment. In addition, under current super- imposition, slow leaching of Au begins at ~0.5 h from the beginning of the experiment and, after 3 h, RAu achieves only 5.1%. Comparative study of the metal leaching kinetics from raw material powders under the same experimental conditions (CHCl = 6 mol L−1, i = 0.88 A·cm−2, S/L = 8.6 g·L−1, without electrolyte temperature control) showed that after 2 h of the electrochemical process, Cu was leached completely (RCu = 100%) from single-crushed PCBs. Complete leaching of Ag was observed with experiment durations of 3 h and 2 h from the single-crushed PCBs and the double-crushed PCBs, respectively. Au was completely leached from the double-crushed PCBs at tex = 5 h, and from the single-crushed PCBs the maximum degree of metal leaching was 77.5% at tex = 6 h. It was found that, as the result of a two-stage experiment (one-hour pretreatment at i = 0.88 A·cm−2 and a subsequent two-hour electrochemical process), the degree of Au and Ag leaching from single-crushed PCBs was 73.6% and 86.7%, respectively. As a result, an electrolyte solution of the following composition (mg·L−1): Au—3.18; Ag—12.89; Al—27.74; Cu—9.58; Ni—38.96; Pb—5.02; Sn—3.94; Ti—9.15; Zn—2.00; Fe—977.1 was obtained. In the absence of pretreatment of this raw material under similar experimental conditions, the degree of leaching of Au and Ag was 8.1% and 59.3%, respectively. As a result of this process, an electrolyte solution of composition (mg·L−1): Au—0.35; Ag— Metals 2024, 14, 65 23 of 25 8.82; Al—141.7; Cu—126.1; Ni—86.85; Pb—94.25; Sn—257.0; Ti—126.8; Zn—32.29; Fe— 3251.0 was obtained. Thus, the use of a one-hour electrochemical pretreatment in a two-stage experiment makes it possible to increase the content of gold and silver and significantly reduce the content of background (base) metals in the final electrolyte solution. This in turn will simplify the scheme for the further extraction of noble metals. Author Contributions: Conceptualization, V.S.; methodology, V.S.; formal analysis, V.S. and M.M.; investigation, V.S., M.M., A.Z. and A.S.; resources, V.P.; data curation, A.Z., M.M. and A.S.; writing— original draft preparation, V.S., M.M. and A.S.; writing—review and editing, A.Z., V.S. and A.S.; visualization, A.S., A.Z. and M.M.; supervision, V.P. and V.S.; project administration, V.P.; funding acquisition, V.P. All authors have read and agreed to the published version of the manuscript. Funding: This research was supported by ERDF project no. 1.1.1.1/20/A/139 “Development of sustainable recycling technology of electronic scrap for precious and non-ferrous metals extraction”. The project was co-financed by REACT-EU funding to mitigate the effects of the pandemic crisis. The article was published with financial support from the Riga Technical University Research Support Fund. This research was also supported by the Institute of Solid State Physics, University of Latvia, as the Center of Excellence has received funding from the European Union’s Horizon 2020 Framework Program H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART2. Data Availability Statement: The data presented in this study are available in article. Conflicts of Interest: Author Andrei Shishkin was employed by the company ZTF Aerkom SIA. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. References 1. European Commission Eurostat, Waste Statistics—Electrical and Electronic Equipment. Available online: https://ec.europa. eu/eurostat/statistics-explained/index.php?title=Waste_statistics_-_electrical_and_electronic_equipment#Further_Eurostat_ information (accessed on 7 November 2023). 2. European Commission Waste Electrical and Electronic Equipment (WEEE) by Waste Management Operations. Available online: https://ec.europa.eu/eurostat/databrowser/view/env_waseleeos__custom_8337649/default/table?lang=en (accessed on 7 November 2023). 3. Chatterjee, S.; Kumar, K. Effective Electronic Waste Management and Recycling Process Involving Formal and Non-Formal Sectors. Int. J. Phys. Sci. 2009, 4, 893–905. 4. Bizzo, W.; Figueiredo, R.; de Andrade, V. Characterization of Printed Circuit Boards for Metal and Energy Recovery after Milling and Mechanical Separation. Materials 2014, 7, 4555–4566. [CrossRef] 5. Li, J.; Shrivastava, P.; Gao, Z.; Zhang, H.-C. Printed Circuit Board Recycling: A State-of-the-Art Survey. IEEE Trans. Electron. Packag. Manuf. 2004, 27, 33–42. [CrossRef] 6. Cui, J.; Zhang, L. Metallurgical Recovery of Metals from Electronic Waste: A Review. J. Hazard. Mater. 2008, 158, 228–256. [CrossRef] [PubMed] 7. Gulliani, S.; Volpe, M.; Messineo, A.; Volpe, R. Recovery of Metals and Valuable Chemicals from Waste Electric and Electronic Materials: A Critical Review of Existing Technologies. RSC Sustain. 2023, 1, 1085–1108. [CrossRef] 8. Tunsu, C.; Retegan, T. Hydrometallurgical Processes for the Recovery of Metals from WEEE. In WEEE Recycling; Elsevier: Amsterdam, The Netherlands, 2016; pp. 139–175. 9. Zhang, Y.; Liu, S.; Xie, H.; Zeng, X.; Li, J. Current Status on Leaching Precious Metals from Waste Printed Circuit Boards. Procedia Environ. Sci. 2012, 16, 560–568. [CrossRef] 10. Chancerel, P.; Meskers, C.E.M.; Hagelüken, C.; Rotter, V.S. Assessment of Precious Metal Flows During Preprocessing of Waste Electrical and Electronic Equipment. J. Ind. Ecol. 2009, 13, 791–810. [CrossRef] 11. Birich, A.; Gao, Z.; Vrucak, D.; Friedrich, B. Sensitivity of Gold Lixiviants for Metal Impurities in Leaching of RAM Printed Circuit Boards. Metals 2023, 13, 969. [CrossRef] 12. Sethurajan, M.; van Hullebusch, E.D.; Fontana, D.; Akcil, A.; Deveci, H.; Batinic, B.; Leal, J.P.; Gasche, T.A.; Ali Kucuker, M.; Kuchta, K.; et al. Recent Advances on Hydrometallurgical Recovery of Critical and Precious Elements from End of Life Electronic Wastes—A Review. Crit. Rev. Environ. Sci. Technol. 2019, 49, 212–275. [CrossRef] 13. Xolo, L.; Moleko-Boyce, P.; Makelane, H.; Faleni, N.; Tshentu, Z.R. Status of Recovery of Strategic Metals from Spent Secondary Products. Minerals 2021, 11, 673. [CrossRef] 14. Wu, Z.; Yuan, W.; Li, J.; Wang, X.; Liu, L.; Wang, J. A Critical Review on the Recycling of Copper and Precious Metals from Waste Printed Circuit Boards Using Hydrometallurgy. Front. Environ. Sci. Eng. 2017, 11, 8. [CrossRef] Metals 2024, 14, 65 24 of 25 15. Islam, A.; Ahmed, T.; Awual, M.R.; Rahman, A.; Sultana, M.; Aziz, A.A.; Monir, M.U.; Teo, S.H.; Hasan, M. Advances in Sustainable Approaches to Recover Metals from E-Waste-A Review. J. Clean. Prod. 2020, 244, 118815. [CrossRef] 16. Van Yken, J.; Boxall, N.J.; Cheng, K.Y.; Nikoloski, A.N.; Moheimani, N.R.; Kaksonen, A.H. E-Waste Recycling and Resource Recovery: A Review on Technologies, Barriers and Enablers with a Focus on Oceania. Metals 2021, 11, 1313. [CrossRef] 17. Zhang, L.; Xu, Z. A Review of Current Progress of Recycling Technologies for Metals from Waste Electrical and Electronic Equipment. J. Clean. Prod. 2016, 127, 19–36. [CrossRef] 18. Gökelma, M.; Birich, A.; Stopic, S.; Friedrich, B. A Review on Alternative Gold Recovery Re-Agents to Cyanide. J. Mater. Sci. Chem. Eng. 2016, 04, 8–17. [CrossRef] 19. Akcil, A.; Erust, C.; Gahan, C.S.; Ozgun, M.; Sahin, M.; Tuncuk, A. Precious Metal Recovery from Waste Printed Circuit Boards Using Cyanide and Non-Cyanide Lixiviants—A Review. Waste Manag. 2015, 45, 258–271. [CrossRef] [PubMed] 20. Ha, V.H.; Lee, J.; Huynh, T.H.; Jeong, J.; Pandey, B.D. Optimizing the Thiosulfate Leaching of Gold from Printed Circuit Boards of Discarded Mobile Phone. Hydrometallurgy 2014, 149, 118–126. [CrossRef] 21. Birloaga, I.; Vegliò, F. Study of Multi-Step Hydrometallurgical Methods to Extract the Valuable Content of Gold, Silver and Copper from Waste Printed Circuit Boards. J. Environ. Chem. Eng. 2016, 4, 20–29. [CrossRef] 22. Hsu, E.; Barmak, K.; West, A.C.; Park, A.-H.A. Advancements in the Treatment and Processing of Electronic Waste with Sustainability: A Review of Metal Extraction and Recovery Technologies. Green Chem. 2019, 21, 919–936. [CrossRef] 23. Swaminathan, C.; Pyke, P.; Johnston, R.F. Reagent Trends in the Gold Extraction Industry. Miner. Eng. 1993, 6, 1–16. [CrossRef] 24. Xu, R.; Nan, X.; Meng, F.; Li, Q.; Chen, X.; Yang, Y.; Xu, B.; Jiang, T. Analysis and Prediction of the Thiourea Gold Leaching Process Using Grey Relational Analysis and Artificial Neural Networks. Minerals 2020, 10, 811. [CrossRef] 25. Deng, C.; Xiang, P.; Liu, L.; Huang, Y. Study on Gold Extraction Process of Printed Circuit Board Based on Thiourea Method. J. Phys. Conf. Ser. 2020, 1549, 032093. [CrossRef] 26. Ray, D.A.; Baniasadi, M.; Graves, J.E.; Greenwood, A.; Farnaud, S. Thiourea Leaching: An Update on a Sustainable Approach for Gold Recovery from E-Waste. J. Sustain. Met. 2022, 8, 597–612. [CrossRef] 27. Maslenitsky, I.N.; Chugaev, L.V.; Borbat, V.F.; Nikitin, M.V.; Strizhko, L.S. Metallurgy of Noble Metals. In Metallurgy; Chugaev, L.V., Ed.; Metallurgy: Moscow, Russia, 1987; pp. 315–321. 28. La Brooy, S.R.; Linge, H.G.; Walker, G.S. Review of Gold Extraction from Ores. Miner. Eng. 1994, 7, 1213–1241. [CrossRef] 29. Dönmez, B.; Sevim, F.; Çolak, S. A Study on Recovery of Gold from Decopperized Anode Slime. Chem. Eng. Technol. 2001, 24, 91–95. [CrossRef] 30. Filmer, A.O.; Lawrence, P.R.; Hoffman, W.A. A Comparison of Cyanide, Thiourea, and Chlorine as Lixiviants for Gold. In Gold Mining, Metallurgy and Geology, Kalgoorlie; Australasian Institute of Mining and Metallurgy: Melbourne, Australia, 1984; pp. 279–287. 31. Herreros, O.; Quiroz, R.; Viñals, J. Dissolution Kinetics of Copper, White Metal and Natural Chalcocite in Cl2/Cl− Media. Hydrometallurgy 1999, 51, 345–357. [CrossRef] 32. Viñals, J.; Núñez, C.; Herreros, O. Kinetics of the Aqueous Chlorination of Gold in Suspended Particles. Hydrometallurgy 1995, 38, 125–147. [CrossRef] 33. Kim, E.; Kim, M.; Lee, J.; Jeong, J.; Pandey, B.D. Leaching Kinetics of Copper from Waste Printed Circuit Boards by Electro- Generated Chlorine in HCl Solution. Hydrometallurgy 2011, 107, 124–132. [CrossRef] 34. Kim, E.; Kim, M.; Lee, J.; Pandey, B.D. Selective Recovery of Gold from Waste Mobile Phone PCBs by Hydrometallurgical Process. J. Hazard. Mater. 2011, 198, 206–215. [CrossRef] 35. Pilone, D.; Kelsall, G.H. Metal Recovery from Electronic Scrap by Leaching and Electrowinning IV. In Electrometallurgy and Environmental Hydrometallurgy; John Wiley & Sons, Inc.: Hoboken, NJ, USA, 2013; pp. 1565–1575. 36. Zhao, J.; Liu, Z.; He, C.; Yang, Y.; Li, J.; Fujita, T.; Wang, G.; Shen, F. Improved Leaching of Cu, Sn, Pb, Zn, and Al from Waste Printed Circuit Boards by Electro-Generated Cl2 in HCl Solution. Waste Manag. 2022, 153, 386–396. [CrossRef] 37. Ilyas, S.; Srivastava, R.R.; Kim, H. Gold Recovery from Secondary Waste of PCBs by Electro-Cl2 Leaching in Brine Solution and Solvo-Chemical Separation with Tri-Butyl Phosphate. J. Clean. Prod. 2021, 295, 126389. [CrossRef] 38. Kim, M.; Lee, J.; Park, S.; Jeong, J.; Kumar, V. Dissolution Behaviour of Platinum by Electro-Generated Chlorine in Hydrochloric Acid Solution. J. Chem. Technol. Biotechnol. 2013, 88, 1212–1219. [CrossRef] 39. Serga, V.; Zarkov, A.; Blumbergs, E.; Shishkin, A.; Baronins, J.; Elsts, E.; Pankratov, V. Leaching of Gold and Copper from Printed Circuit Boards under the Alternating Current Action in Hydrochloric Acid Electrolytes. Metals 2022, 12, 1953. [CrossRef] 40. Serga, V.; Zarkov, A.; Shishkin, A.; Elsts, E.; Melnichuks, M.; Maiorov, M.; Blumbergs, E.; Pankratov, V. Study of Metal Leaching from Printed Circuit Boards by Improved Electrochemical Hydrochlorination Technique Using Alternating Current. Metals 2023, 13, 662. [CrossRef] 41. Blumbergs, E.; Serga, V.; Shishkin, A.; Goljandin, D.; Shishko, A.; Zemcenkovs, V.; Markus, K.; Baronins, J.; Pankratov, V. Selective Disintegration–Milling to Obtain Metal-Rich Particle Fractions from E-Waste. Metals 2022, 12, 1468. [CrossRef] 42. Zolotov, Y.A.; Iofa, B.Z.; Chuchalin, L.K. Extraction of Metal Halide Complexes; Nauka: Moscow, Russia, 1973. (In Russian) 43. Wang, B.; Ma, Y.; Xu, W.; Tang, K. A Novel S,N-Rich MOF for Efficient Recovery of Au(III): Performance and Mechanism. J. Hazard. Mater. 2023, 451, 131051. [CrossRef] [PubMed] 44. Zalupski, P.R.; McDowell, R.; Dutech, G. The Adsorption of Gold, Palladium, and Platinum from Acidic Chloride Solutions on Mesoporous Carbons. Solvent Extr. Ion Exch. 2014, 32, 737–748. [CrossRef] Metals 2024, 14, 65 25 of 25 45. Geng, Y.; Li, J.; Lu, W.; Wang, N.; Xiang, Z.; Yang, Y. Au(III), Pd(II) and Pt(IV) Adsorption on Amino-Functionalized Magnetic Sorbents: Behaviors and Cycling Separation Routines. Chem. Eng. J. 2019, 381, 122627. [CrossRef] 46. Cohen, B.; Shipley, D.S.; Tong, A.R.; Casaroli, S.J.G.; Petrie, J.G. Precipitation of Iron from Concentrated Chloride Solutions: Literature Observations, Challenges and Preliminary Experimental Results. Miner. Eng. 2005, 18, 1344–1347. [CrossRef] 47. Kasikov, A.G.; Sokolov, A.Y.; Glukhovskaja, I.V. Extraction of Iron {III) from Chloride Solutions with Aliphatic Ketones. Russ. J. Appl. Chem. 2005, 18, 1344–1347. (In Russian) [CrossRef] 48. Chekanova, L.G.; Vaulina, V.N.; Kharitonova, A.V.; Radushev, A.V.; Trukhinov, D.K. Hydrazides of Versatic Acids (Fraction C15–C19) Are the Extraction Reagents of Non-Ferrous Metals from Acid and Ammonia Solutions. Russ. J. Appl. Chem. 2019, 9, 227–239. (In Russian) 49. van Yken, J.; Cheng, K.Y.; Boxall, N.J.; Sheedy, C.; Nikoloski, A.N.; Moheimani, N.R.; Kaksonen, A.H. A Comparison of Methods for the Characterisation of Waste-Printed Circuit Boards. Metals 2021, 11, 1935. [CrossRef] 50. Polyansky, N.G. Analytical Chemistry of Elements. Lead; Nauka: Moscow, Russia, 1986. Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.