Preliminary Study on Gold Recovery from High Grade E-Waste by Thiourea Leaching and Electrowinning

minerals

Article Preliminary Study on Gold Recovery from High Grade E-Waste by Thiourea Leaching and Electrowinning

Nicolò Maria Ippolito *, Ionela Birloaga, Francesco Ferella , Marcello Centofanti and Francesco Vegliò

Department of Industrial Engineering, Information and Economy, University of L’Aquila, Monteluco di Roio, 67100 L’Aquila, Italy; [email protected] (I.B.); [email protected] (F.F.); [email protected] (M.C.); [email protected] (F.V.) * Correspondence: [email protected]

Abstract: The present paper is focused on the extraction of gold from high-grade e-waste, i.e., spent electronic connectors and plates, by leaching and electrowinning. These connectors are usually made up of an alloy covered by a layer of gold; sometimes, in some of them, a plastic part is also present. The applied leaching system consisted of an acid solution of diluted sulfuric acid (0.2 mol/L) with thiourea (20 g/L) as a reagent and ferric sulfate (21.8 g/L) as an oxidant. This system was applied on three different high-grade e-waste, namely: (1) Connectors with the partial gold-plated surface (Au concentration—1139 mg/kg); (2) different types of connectors with some of which with completely gold-plated surface (Au concentration—590 mg/kg); and (3) connectors and plates with the completely gold-plated surface (Au concentration—7900 mg/kg). Gold dissolution yields of 52, 94, and 49% were achieved from the ﬁrst, second, and third samples, respectively. About 95% of Au recovery was achieved after 1.5 h of electrowinning at a current efﬁciency of only 4.06% and current consumption of 3.02 kWh/kg of Au from the leach solution of the third sample.

Citation: Ippolito, N.M.; Birloaga, I.; Ferella, F.; Centofanti, M.; Vegliò, F. Keywords: gold; e-waste; connector; thiourea; electrowinning Preliminary Study on Gold Recovery from High Grade E-Waste by Thiourea Leaching and Electrowinning. Minerals 2021, 11, 1. Introduction 235. https://doi.org/10.3390/ The exponential growth of waste electronic and electric equipment (WEEE) has in- min11030235 creased over the years, especially in the last twenty years. This is mainly due to the great development of the technology relevant to personal computers (PCs), mobile phones, Academic Editor: Nomvano Mketo tablets, and, generally speaking, electronic equipment whose replacements with more advanced devices are very fast. Received: 18 January 2021 Recycling of WEEE implies several advantages, in particular, the recovery and reuse of Accepted: 22 February 2021 Published: 25 February 2021 metals and materials that can be re-introduced in the manufacturing chain of new electric and electronic equipment (EEE) or other devices, in the perspective of the new concept of

Publisher’s Note: MDPI stays neutral the circular economy. There is also an additional environmental benefit, represented by with regard to jurisdictional claims in avoiding the landfilling practices that can release heavy metals in groundwater tables due published maps and institutional affil- to the leaching action of rainwater [1]. iations. EEE contain several metals of interest, like base (Cu, Fe, Pb, Sn) and precious (Au, Ag, Pd) metals. The latter ones are abundant in motherboards, connectors, and integrated circuit pins. Many of these components are assembled in printed circuit boards (PCBs) that, however, contain around 70%wt of plastic-composite materials like epoxy resins, glass fibers, and brominated compounds used as flame retardants [2]. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. A double stage leaching with H2SO4 and H2O2 to extract Cu and Sn and a second stage 3+ This article is an open access article with SC(NH2)2/Fe /H2SO4 to dissolve Au and Ag was proposed by Birloaga et al. [3,4]. distributed under the terms and Among hydrometallurgical processes developed for spent PCBs, those using thiourea or conditions of the Creative Commons other chemical mixtures like HCl/H2O2/CH3COOH [5] are the most used, as they do not Attribution (CC BY) license (https:// involve very hazardous and toxic compounds. creativecommons.org/licenses/by/ Several other processes were investigated to recover metals from PCBs [6–14], but, 4.0/). considering the circular approach of the full recycling concept, even the recovery of the

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non-metallic fraction of PCBs was studied [10]. Debromination is a necessary treatment before any other recycling process. The research activity’s development goes towards the circular economy approach of the full recycling of spent devices [15–17]. Furthermore, the recovery of the plastic fraction limits the spread of microplastics, which is another current environmental concern. Thermal treatments are also applied to remove the plastic fraction and concentrate the metal fraction into the char [18], which hence undergoes hydrometallurgical treatments to dissolve, separate, and refine the most valuable and concentrated metals. The hydrothermal, as the thermal ones, also have the important role of detoxification of the materials, as PCBs contain brominated flame retardants that have to be removed from flue gas [19]. The hydrothermal treatment is a thermochemical conversion process, in the presence of water, at high temperature (200–375 ◦C) and high-pressure (up to 220 bar) [20]. It produces a liquid fraction containing organic compounds generated from the decomposition of plastics and a metal residue. Moreover, waste PCBs are the sole valuable WEEE whose recycling is profitable [21], without any environmental fee [14]. Considering the technical aspects, PCBs are usually crushed and milled after the removal of special components, like small aluminum heat exchangers, capacitors, transistors, etc. The crushing pre-treatments, unfortunately, cause the loss of a considerable amount of gold and other metals of interest that are pulverized and sucked by the dedusting systems. Nevertheless, such concentrated powder can be recovered in cyclones or bag filters and treated in the hydrometallurgical section of the plant. One of the systems foresees the leaching of whole or large pieces of PCBs. Jadhav and Hocheng [22] removed the epoxy solder mask of PCBs by 10 mol/L NaOH solution and thus leached the large pieces by different acids, both organic and mineral. The dissolution of Au, Ag, Pd, and Cu was quantitative, although at least 24 h are required to achieve such a result. Other authors tried the biometallurgical approach using Acidithiobacillus ferrooxidans to maximize the dissolution of Cu, Fe, and Ni: 100% of the first two metals were dissolved, whereas the dissolution yield of Ni was only 54% [23]. An integrated process used the combination of ammonium thiosulfate and Lactobacillus acidophilus for selective biosorption of gold from the contact point of PCBs. This novel leaching-sorption method allows 85% of gold recovery [24]. Other authors concentrated on base metals contained in PCBs, increasing the circular approach using spent solutions from PCB manufacturing processes to recover Cu, Sn, and Pb [25]. In the past, the most used and efficient method for the extraction of gold from ores was cyanidation. One study even tested this process on PCBs. 10 g/L sodium cyanide solution at 40 ◦C is able to dissolve 95% of gold in 15 min. Gold was recovered from the leachate by adsorption onto charcoal [26]. There are some electric and electronic components that are much richer in precious metals over the household WEEE. These components like connectors, processors, integrated circuits, microchips, etc., coming from special equipment such as army and space applications, contains more Au, Ag, and Pd per mass unit. The connectors are used in several applications depending on their requirements; they are made of different materials. The choice of base, insulation, and interface (plating) materials at the beginning of the connector design process essentially define the connector characteristics, determining how well the overall device or system will operate. Copper is the most used base material in the manufacturing of both input and output connectors. However, for certain connectors, copper is applied as a base alloy (i.e., brass, bronze, and aluminum bronze with nickel) or as underplating on base materials (steel, brass, aluminum zinc, and copper-beryllium) or as underplating for final plating (solder coatings and tin- nickel plate). Beryllium copper alloy (CuBe2) represents an alternative to pure or even silver-bearing copper, as it has a better corrosionresistance and has better machinability than copper, but not as good as brass. Minerals 2021, 11, 235 3 of 16

However, as this alloy has a higher price, its use is limited to the following connector components: Contact sockets (electrical contacts) and spring elements. Bronze is the alloy between Cu and Sn, including some phosphorous and zinc complexes. It is considered a suitable alternative to other copper alloys, particularly when the connector technical speciﬁcations do not mandate CuBe2. Generally, this alloy is used for the production of the following connectors parts: large contact sockets, resilient contacts, and outer conductors. Brass is the alloy between copper and zinc and is normally plated either with gold, silver, or SUCOPLATE. This alloy is used in the housings, outer conductors, and contact pins of connectors. Stainless steel is used for applications that require a hard metal, i.e., the outer conductor. Generally, it is used as a base material for bodies and outer contacts. Concerning plating materials, gold can be deposited on copper or nickel. However, in certain applications (military, space, etc.), the designer might want to apply a thicker layer of gold as the base plate. To minimize the cost of the connector, the thickness of the gold plating is usually kept to a minimum. Even if gold is corrosion-resistant, with the minimization of thickness, the risk of contamination (because of high porosity) and wear-through is detected. Therefore, the use of the gold plating variation SUCOPRO is considered a suitable alternative. Usually, connectors’ gold-plated components are inner conductors, springs, and generally the bodies and outer conductors of PCB connectors (only for soldering). SUCOPRO is a thin gold plating layer with a nickel-phosphorus alloy (13% phosphorus) underlayer. Its main advantages are excellent wear and corrosion resistance, low contact resistance, very high strength of solder joints without embrittlement, and excellent wettability/solderability. Silver can be plated on base materials of most ferrous and non-ferrous metals and is found as a plated layer of the following connectors/cable components: conductors, contacts, and sleeves [27]. Besides these base and plating materials, various plastics and rubbers are used for the manufacturing of connectors. Polyethylene (PE) is used for turned insulators, wire, and cable insulation, cable jacket material, corrugated cables, and packaging. Polytetraﬂuo- roethylene (PTFE) is used for insulators, gaskets, and anti-adhesive coatings. Polyether- Etherketone (PEEK) is used for radiation-resistant wire insulation, wire coating, and insulators. Polyphenylene oxide (PPO) is used for thermal and electrical insulator parts, switch housings, covers. Silicone rubber is used for gaskets. All these characteristics of connectors are important to determine the best way of treatment of the spent connectors. In this paper, coaxial connectors constituted by brass as the base material, gold as a plating material, and PTFE as insulating were chemically characterized and studied in order to identify a technical and economically feasible process to recover industrial interest metals, focusing on gold for his higher economic value. In the present paper, the application of the thiourea acidic leaching and electrowinning process for Au recovery from high-grade e-waste is presented. This study shows the preliminary results obtained in a wide experimental campaign whose aim is to design and optimize a recycling process for gold-rich materials.

2. Materials and Methods 2.1. Characterization of the Connectors Three samples of high-grade e-waste were provided by a company located in central Italy (Arezzo, Italy). The samples underwent acid digestion with aqua regia (HCl:HNO3 = 3:1) with a solid to liquid ratio (S/L) of 1:20 % wt/vol with Ethos Up Milestone microwave digestion system (Milestone Srl, Sorisole, Italy). The adopted digestion program included 20 min isotherm at 220 ◦C. The whole program lasted 2 h, a sufficient time for solubilization of the metal fraction: in this way, only the plastic fraction remained as residue. The solid residue obtained by the digestion was only the PTFE component. The acid solution was diluted to 50 mL with deionized water in a calibrated flask and stored to determine metals by inductively coupled plasma optical emission spectrometry (ICP-OES 5100) (Agilent Technologies, Cernusco sul Naviglio, Italy). To have a reasonable estimation of the content of the base and precious metals of interest, twenty replications were carried out on first Minerals 2021, 11, x FOR PEER REVIEW 4 of 16

Minerals 2021, 11, 235 4 of 16 was diluted to 50 mL with deionized water in a calibrated flask and stored to determine metals by inductively coupled plasma optical emission spectrometry (ICP-OES 5100) sample.(Agilent The Technologies, average concentration Cernusco sul and Naviglio, the standard Italy). deviation To have ofa reasonable Au, Ag, Pd, estimation Cu, Zn, Pb, of andthe Sncontent were of determined the base and (Table precious1). metals of interest, twenty replications were carried out on first sample. The average concentration and the standard deviation of Au, Ag, Pd, TableCu, Zn, 1. Average Pb, and (Avg.) Sn were concentration determined and (Table standard 1). deviation (Std. Dev.) of metals in connectors, sample 1. Table 1. Average (Avg.) concentration and standard deviation (Std. Dev.) of metals in connectors, sample 1. Metal Au Ag Pd Cu Zn Pb Sn Metal Cu Zn Pb Sn Au (mg/kg)Concentration Ag (mg/kg)(mg/kg) (mg/kg) Pd (mg/kg)(mg/kg) (%wt) (%wt) (%wt) (mg/kg) Concentration (%wt) (%wt) (%wt) (mg/kg) Avg. 1139 2417 15.4 53.5 32.7 2.2 385 Avg. 1139 Std. Dev. 2417 317 707 15.4 3.953.5 1.4 32.7 1.0 2.2 0.3 385 321 Std. Dev. 317 707 3.9 1.4 1.0 0.3 321

The ﬁrst sample (Figure1) is visually composed of a plastic component covered by The first sample (Figure 1) is visually composed of a plastic component covered by brass and the metallic part covered by gold. brass and the metallic part covered by gold.

Figure 1. First sample of connectors with partial gold-plated surface. Figure 1. First sample of connectors with partial gold-plated surface.

AsAs shown shown in in Figure Figure2, 2, the the second second sample, sample, constituted constituted by differentby differe typesnt types of connectors, of connect- Minerals 2021, 11, x FOR PEER REVIEWors, some of which are totally plated with Au and present a plastic part within connectors5 of 16 some of which are totally plated with Au and present a plastic part within connectors structure.structure. The third sample, Figure 3, is constituted by connectors and plates with the completely gold-plated surface. X-ray diffraction analysis were performed on the solid product obtained by electrowinning as confirmation of the results. PANalytical XPert PRO diffractometer (Mal- vern Products, Rome, Italy) equipped with XCelarator detector, and Cu X-ray source was used. The diffraction pattern is registered in the range of 10–70° 2Theta and 0.026° steps.

Figure 2. Second sample of connectors. Figure 2. Second sample of connectors.

Figure 3. Third sample of connector and plated with the completely gold-plated surface.

2.2. Leaching Procedure The thiourea leaching system was tested on all three samples. The leaching mechanism of gold and silver leaching by thiourea in sulphuric medium and in presence of Fe3+ ions is expressed by the following reactions [28]:

Au + 2 CS(NH2)2 + Fe3+ → Au[CS(NH2)2]2+ + Fe2+ (1)

Ag + 3 CS(NH2)2 + Fe3+ → Au[CS(NH2)2]3+ + Fe2+ (2) During this process, the oxidation of thiourea to formamidine disulfide (FDS) takes place (Equation (3)). This reagent is not stable in acid solution and oxidation conditions. Therefore, FDS decomposes irreversible to thiourea, cyanamide, and sulphur (Equation (4)).

2 CSN2H4 + 2 Fe3+ = 2 SCN2H3 + 2 Fe2+ + 2 H+ (3)

2 SCN2H3 = SCN2H4 + NH2CN + S (4) Besides, if thiourea is not added in excess by ferric ion concentration, the reactive is lost according to the following reactions:

Fe3+ + SCN2H4 + SO42− = (FeSO4SCN2H4)+ (5)

Fe3+ + SCN2H4 + SO42− = (FeSO4SCN2H4)3+ (6)

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The third sample, Figure3, is constituted by connectors and plates with the completely gold-plated surface. Figure 2. Second sample of connectors.

FigureFigure 3. 3.Third Third sample sample of of connector connector and and plated plated with with the the completely completely gold-plated gold-plated surface. surface.

2.2. X-rayLeaching diffraction Procedure analysis were performed on the solid product obtained by electrowinning as conﬁrmation of the results. PANalytical XPert PRO diffractometer (Malvern The thiourea leaching system was tested on all three samples. The leaching mecha- Products, Rome, Italy) equipped with XCelarator detector, and Cu X-ray source was used. nism of gold and silver leaching by thiourea in sulphuric medium and in presence of Fe3+ The diffraction pattern is registered in the range of 10–70◦ 2Theta and 0.026◦ steps. ions is expressed by the following reactions [28]: 2.2. Leaching Procedure Au + 2 CS(NH2)2 + Fe3+ → Au[CS(NH2)2]2+ + Fe2+ (1) The thiourea leaching system was tested on all three samples. The leaching mechanism 3+ of gold and silver leachingAg + 3 CS(NH by thiourea2)2 + Fe in3+ sulphuric → Au[CS(NH medium2)2]3 and+ + Fe in2+ presence of Fe ions(2) is expressed by the following reactions [28]: During this process, the oxidation of thiourea to formamidine disulfide (FDS) takes 3+ + 2+ place (Equation (3)).Au This + 2 CS(NHreagent2) 2is+ not Fe stable→ Au[CS(NH in acid solution2)2]2 +and Fe oxidation conditions.(1) Therefore, FDS decomposes irreversible to thiourea, cyanamide, and sulphur (Equation 3+ + 2+ (4)). Ag + 3 CS(NH2)2 + Fe → Au[CS(NH2)2]3 + Fe (2)

During this process,2 CSN the2H oxidation4 + 2 Fe3+ = of 2 thioureaSCN2H3 + to 2 formamidineFe2+ + 2 H+ disulﬁde (FDS) takes(3) place (Equation (3)). This reagent is not stable in acid solution and oxidation conditions. Therefore, FDS decomposes2 irreversibleSCN2H3 = SCN to thiourea,2H4 + NH cyanamide,2CN + S and sulphur (Equation (4)). (4)

3+ 2+ + Besides, if thiourea2 CSN is 2notH4 +added 2 Fe in= excess 2 SCN by2H 3ferric+ 2 Fe ion concentration,+ 2 H the reactive(3) is lost according to the following reactions: 2 SCN2H3 = SCN2H4 + NH2CN + S (4) Fe3+ + SCN2H4 + SO42− = (FeSO4SCN2H4)+ (5) Besides, if thiourea is not added in excess by ferric ion concentration, the reactive is lost according to the followingFe3+ + SCN reactions:2H4 + SO42− = (FeSO4SCN2H4)3+ (6)

3+ 2− + Fe + SCN2H4 + SO4 = (FeSO4SCN2H4) (5)

3+ 2− 3+ Fe + SCN2H4 + SO4 = (FeSO4SCN2H4) (6) 3+ 2− + Fe + 2SCN2H4 + SO4 = ((FeSO4SCN2H4)2) (7) Fe3+ ions are used as oxidant and thiourea as a complexing agent, whereas sulphuric acid assures that the pH remains acidic, avoiding iron precipitation. This treatment was chosen as the precious metals are on the connectors’ surface, so the competition against copper in consuming thiourea is greatly reduced. The runs for the first sample were carried out in 250 mL flasks, under magnetic stirring (300 rpm) with 0.2 mol/L H2SO4 solution, 20 g/L of thiourea, and 21.8 g/L of Fe2(SO4)3. The S/L ratio was 10% wt/vol, and the reaction time was investigated up to 1.5 h, at the end of which the solution was filtered by a vacuum pump and a 0.45 µm paper filter and Minerals 2021, 11, 235 6 of 16

stored for the analysis. These experimental conditions were chosen after the optimization was achieved after several tests carried out with PCBs [1–3]. The runs on the completely gold-plated connectors and plates (second sample) were carried out with (second sample) (100 rpm) and without magnetic stirring (third sample) for 1 h using the concentration of the above-presented reagents. Metals’ concentration was determined by ICP-OES, and the dissolution yields were determined as a percentage of the content determined by acid digestion [29].

2.3. Electrowinning Procedure During the electrowinning process into an acid solution of thiourea, gold and silver are reduced to cathode according to the following reactions:

2+ − 0 Au(CSN2H4) + 2 e = Au + 2 CSN2H4 (8)

3+ − 0 Ag(CSN2H4) + 3 e = Ag + 3 CSN2H4 (9) Besides, as the thioureation process was performed in the presence of ferric ion, the resulted formamidine disulﬁde, at controlled cathode potential may be reduced back to thiourea according to the following reaction:

+ − 2 SCN2H3 + 2 H + 2 e = 2 CSN2H4 (10)

At the anode occurs the water decomposition reaction:

+ − 2 H2O = O2 + 4 H + 4e (11)

Most of all scientific works have been conducted on solutions obtained by stripping of gold from activated carbon with solutions of diluted sulfuric acid containing the only thiourea as reagent [30]; immersion of both gold and silver complexes within acidic thiourea solution [31]; dissolution of metallic gold within the solution of acidic thiourea with a small excess of hydrogen peroxide and with/without the presence of alcohol [32]. The authors have shown that both precious metals can be recovered efficiently from their solutions by this technology within these papers. However, it was shown in many articles that the current efficiency of the process is very low for the recovery of both elements. This is due to the parasitic reactions (e.g., reduction of hydrogen to the cathode and partial degradation of thiourea to anode) during the process. The solution obtained by thiourea leaching from the first sample with gold and silver concentration of 20 mg/L and 250 mg/L, respectively, was subjected to electrowinning in order to recover gold and silver as metals. Since the current efficiency decreases with the increase of current density for gold electrowinning from thiourea solutions [33], a value of 40 A/m2 of current density was adopted. The other operative conditions that allowed maximizing the gold recovery were the following: cylindrical graphite electrodes with a surface area of 6.8 cm2, room temperature, 100 rpm of stirring, and cell potential of 0.9 V. The recovery of gold and silver was investigated as a function of time carrying out the chemical analysis on the solution. For other electrowinning tests (second sample), the influence of current-voltage pa- rameters was studied. Runs were conducted in galvanic conditions at different current intensities (75, 100, and 200 mA) and corresponding cell potential (0.9, 1.2, and 1.5 V) on the solution achieved after treatment of connectors of the second sample. 100 mL of solution and two cylindrical electrodes of graphite with a surface area of 26.12 cm2 each were used for all three runs. The experiments were performed under continuous magnetic stirring (100 rpm) for 1.5 to 3 h at room temperature at 2 cm distance between the electrodes and 41% of electrodes surface area immersed within the solution. The optimal conditions of electrowinning runs of the second sample were also tested for an electrowinning test with a cathode of copper and graphite as the anode of the same dimensions. Moreover, the same Minerals 2021, 11, x FOR PEER REVIEW 7 of 16

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intensities (75, 100, and 200 mA) and corresponding cell potential (0.9, 1.2, and 1.5 V) on theintensities solution achieved(75, 100, andafter 200 treatment mA) and of correspond connectorsing of thecell secondpotential sample. (0.9, 1.2, 100 and mL 1.5 of V)so- on intensities (75, 100, and 200 mA) and corresponding cell potential (0.9, 1.2, and 1.5 V) on lutionintensitiesthe solutionand two (75, achieved cylindrical 100, and after 200 electrodes treatmentmA) and of correspondof graphite connectors withing ofcell a thesurface potential second area (0.9,sample. of 1.2,26.12 100and cm mL 1.52 each ofV) so-on the solution achieved after treatment of connectors of the second sample. 100 mL of so- werethelution usedsolution and for two allachieved threecylindrical runs.after electrodestreatmentThe experiments ofof connectorsgraphite were withperformed of the a surface second under areasample. continuous of 26.12 100 mLcm mag- 2of each solution and two cylindrical electrodes of graphite with a surface area of 26.12 cm2 each neticlutionwere stirring used and (100fortwo all cylindricalrpm) three for runs. 1.5 electrodes to The 3 h experiments at roomof graphite temperature were with performed a atsurface 2 cm under distancearea ofcontinuous 26.12between cm 2the mag-each were used for all three runs. The experiments were performed under continuous mag- electrodeswerenetic stirringused and for 41% (100 all of threerpm) electrodes runs.for 1.5 The surfaceto 3experiments h at area room immersed temperature were performed within at the 2 cm undersolution. distance continuous The between optimal mag- the netic stirring (100 rpm) for 1.5 to 3 h at room temperature at 2 cm distance between the conditionsneticelectrodes stirring of and electrowinning (100 41% rpm) of electrodes for 1.5runs to surfaceof3 hthe at room seareacond immersedtemperature sample withinwere at 2also thecm solution.distancetested for betweenThe an optimalelec- the Minerals 2021, 11, 235 electrodes and 41% of electrodes surface area immersed within the solution. The 7optimal of 16 trowinningelectrodesconditions test andof withelectrowinning 41% a of cathode electrodes ofruns copper surface of the an area dse graphitecond immersed sample as thewithin were anode the also ofsolution. thetested same Thefor dimen- anoptimal elec- conditions of electrowinning runs of the second sample were also tested for an elec- sions.conditionstrowinning Moreover, oftest electrowinning thewith same a cathode optimal runs of conditionscopper of the ansed condof graphite electrowinning sample as thewere anode werealso of testedadopted the same for for an dimen- theelec- trowinning test with a cathode of copper and graphite as the anode of the same dimen- treatmenttrowinningsions. Moreover, of solution test with the achieved asame cathode optimal after of copperleaching conditions an ofd thegraphite of thirdelectrowinning assample the anode with were theof the adoptedgraphite same dimen-forelec- the sions. Moreover, the same optimal conditions of electrowinning were adopted for the optimaltrodes.sions.treatment conditions Moreover, of solution of the electrowinning sameachieved optimal after were conditionsleaching adopted of of forthe electrowinning thethird treatment sample of withwere solution the adopted graphite achieved for elec- the treatment of solution achieved after leaching of the third sample with the graphite elec- aftertreatmenttrodes. leachingA laboratory of of solution the rectifier third achieved sample (EA-PSwith after3016-20 the leaching graphiteB) wi thof an electrodes.the output third samplepower ofwith 0–16 the V graphiteand output electrodes. currenttrodes.A laboratoryA of laboratory 0–20 A rectiﬁerwas rectifier used (EA-PS for (EA-PS these 3016-20 3016-20tests.B) withB) wi anth an output output power power of 0–16of 0–16 V and V and output output A laboratory rectifier (EA-PS 3016-20 B) with an output power of 0–16 V and output currentcurrentA A ofsampling laboratory 0–20of 0–20 A wasofA wastherectifier used solutionused for (EA-PS for these ofthese tests.each3016-20 tests. test B) was wi thperformed an output at power predetermined of 0–16 V and time output in- current of 0–20 A was used for these tests. tervalscurrentAsampling Aand samplingof then 0–20 analyzed of A the ofwas the solution used bysolution ICP-OES for of these each of foreach tests. test Au wastest and was performed Ag performedcontent. at The predetermined at thiourea predetermined concentration time inter-time in- A sampling of the solution of each test was performed at predetermined time in- valsdeterminationtervals andA then andsampling analyzedthen within analyzed of solutionthe by solution ICP-OES by ICP-OESachieved of for each Au forafte andtestAur both and Agwas content.Ag leachingperformed content. The and Theat thiourea electrowinningpredetermined thiourea concentration concentration proce-time in- tervals and then analyzed by ICP-OES for Au and Ag content. The thiourea concentration determinationdurestervalsdetermination was and performed then within within analyzed solution by solutiontitration by achieved ICP-OES withachieved afterKIO for 3both afteAuand rand potato leachingboth Ag leaching content.starch and electrowinningas andThe indicator thioureaelectrowinning [34]. proceduresconcentration proce- determination within solution achieved after both leaching and electrowinning proce- wasdeterminationdures performed was performed by within titration bysolution withtitration KIO achieved 3withand KIO potato afte3 andr starchboth potato leaching as starch indicator and as indicator[ 34electrowinning]. [34]. proce- dures was performed by titration with KIO3 and potato starch as indicator [34]. 3. Resultsdures was and performed Discussion by titration with KIO3 and potato starch as indicator [34]. 3. Results3. Results and and Discussion Discussion 3.1.3. Characterization Results and Discussion of the Connectors 3.1.3. Characterization Results and Discussion of the Connectors 3.1.The Characterization concentration of of the the Connectors foremost metals of sample 1 is listed in Table 1. 3.1.The Characterization concentration of of the the Connectors foremost metals of sample 1 is listed in Table1. 3.1. CharacterizationThe concentration of the of Connectors the foremost metals of sample 1 is listed in Table 1. TheThe results concentration showed that of athe gold foremost average metals concentration of sample of 1 1139 is listed mg/kg in Table was determined, 1. TheThe results concentration showed ofthat the a foremost gold average metals concentration of sample 1 is of listed 1139 in mg/kg Table 1.was deter- the main components were copper and zinc: 53.5% and 32.7%, respectively. With regard to mined, Thethe mainresults components showed that were a goldcopper average and zinc: concentration 53.5% and 32.7%,of 1139 respectively. mg/kg was With deter- the otherThe precious results metals showed 2417 that mg/kg a gold of silver average and 15.4concentration mg/kg of palladiumof 1139 mg/kg were detected.was deter- regardmined, Theto thethe results mainother componentsshowed precious that metals were a gold 2417 copper average mg/kg and concentrationzinc:of silver 53.5% and and 15.4of 32.7%, 1139 mg/kg mg/kgrespectively. of palladium was deter- With Figuremined,4 shows the main the connectors components before were and copper after an thed chemicalzinc: 53.5% digestion. and 32.7%, respectively. With weremined,regard detected. tothe the main Figure other components 4precious shows the metalswere connectors copper 2417 mg/kgan befored zinc: of and silver53.5% after and the 32.7%,15.4chemical mg/kg respectively. digestion. of palladium With regard to the other precious metals 2417 mg/kg of silver and 15.4 mg/kg of palladium regardwere detected. to the other Figure precious 4 shows metals the connectors 2417 mg/kg before of silverand after and the 15.4 chemical mg/kg digestion.of palladium were detected. Figure 4 shows the connectors before and after the chemical digestion. were detected. Figure 4 shows the connectors before and after the chemical digestion.

FigureFigure 4.4. SampleSample 11 beforebefore (right)(right) andand afterafter (left)(left) acidacid digestion.digestion. Figure 4. Sample 1 before (right) and after (left) acid digestion. Figure 4. Sample 1 before (right) and after (left) acid digestion. FigureGoldGold 4. content contentSample of1of before differentdifferent (right) typetype and ofof after connectorsconnectors (left) acid (sample (sample digestion. 2) 2) were were reported reported in in Table Table2. 2. Gold content of different type of connectors (sample 2) were reported in Table 2. Gold content of different type of connectors (sample 2) were reported in Table 2. Table 2. GoldAu content content within of different different typestype of of connectorsconnectors (sample(sample (sample 2).2). 2) were reported in Table 2. Table 2. Au content within different types of connectors (sample 2). Type of Connector TableandType Its 2. of WeightAu Connector content within and different Photographic types of connectors Aspect (sample Au 2). Concentration (mg/kg) Table 2. Au content within different Photographictypes of connectors Aspect (sample 2). Au Concentration (mg/kg) Type of Connector and ItsIts Weight Weight Photographic Aspect Au Concentration (mg/kg) 1—largeType ofround Connector connector—13.22 and Its Weight g Photographic Aspect Au Concentration885 (mg/kg) Type of Connector and Its Weight Photographic Aspect Au Concentration (mg/kg) 1—large round 1—large round connector—13.22 g 885885 1—large round connector—13.22connector—13.22 g g 885 2—thin and long connector—2.06 g 446.7 1—large round connector—13.22 g 885

2—thin and long 3—medium2—thin long and and long thin connector—2.06 connector—1.77 g g 524.7446.7446.7 2—thin and long connector—2.06connector—2.06 g g 446.7 2—thin and long connector—2.06 g 446.7 4—long and very thin connector—0.84 g 726 3—medium long and thin3—medium connector—1.77 long and thing 524.7 3—medium long and thin connector—1.77 g 524.7524.7 3—medium long and thin connector—1.77 g g 524.7 Minerals 2021, 11, x FOR PEER REVIEW 8 of 16 Minerals 2021, 11, x FOR PEER REVIEW 8 of 16 4—long and very thin connector—0.84 g 726 4—long and very thin connector—0.844—long and very thing 726 4—long and very thin connector—0.84 g 726726 connector—0.84 g

5—squared connector—3.305—squared connector—3.30 g g 440440 5—squared connector—3.30 g 440

6—round and medium6—round long connector—2.05 and medium long g 370 6—round and medium long connector—2.05 g 370370 connector—2.05 g

7—round and short connector—1.17 g 370

A gold average of 590 mg/kg was obtained by mixing all the above-presented connectors. Concerning the plates and the connectors with the completely gold-plated surface of the third samples a gold content of 7900 mg/kg was determined.

3.2. Thiourea Leaching of Gold from the First Sample The results of the replicated thiourea leaching tests for the first sample were reported in Table 3. Au dissolution yield was 31% only after 1 h of reaction, whereas those of Ag and Cu were 76% and below 1%, respectively. The result was rather unsatisfying, so another test was carried out applying a counter-current leaching process with four stages: every stage, fresh reagents were added, according to the concentrations indicated in Section 2.2, to treat the same connectors sample. After the third stage, the total Au dissolution yield was 52%, whereas 95% and 5% of Ag and Cu were leached, respectively.

Table 3. Gold and silver cumulative dissolution yields as a function of stage number. Table 3. Gold and silver cumulative dissolution yields as a function of stage number.

Stage Number Au Extraction (%) Ag Extraction (%) 1 30.6 75.9 2 44.5 94.7 3 51.6 95.1 4 52.7 95.1

3.3. Thiourea Leaching of Gold from Second and Third Samples The second sample with 590 mg/kg of Au was obtained by mixing all the types of connectors that were previously subjected to chemical attack, in order to evaluate the efficiency of thiourea leaching. The third sample, with a gold content of 7900 mg/kg was also leached within the conditions presented within Section 2.2. The recovery yields are reported in Table 4.

Table 4. Gold content within leaching solution and recovery. Table 4. Gold content within leaching solution and recovery.

Photographic Aspect of Solid Sample Au Recovery (%) Residue

2nd sample 93.9

3rd sample 49.0

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5—squared connector—3.30 g 440 5—squared connector—3.30Table 2. Cont. g 440 5—squared connector—3.30 g 440 Type of Connector and Photographic Aspect Au Concentration (mg/kg) 6—round6—round and andmedium medium long long connector—2.05 connector—2.05Its Weight g g 370 370

6—round and medium long connector—2.05 g 370 7—round and short 7—round7—round and andshort short connector—1.17 connector—1.17 g g 370 370370 connector—1.17 g 7—round and short connector—1.17 g 370

A gold average of 590 mg/kg was obtained by mixing all the above-presented con- AA gold gold average average of 590590 mg/kgmg/kg waswas obtained obtained by by mixing mixing all theall the above-presented above-presented connectors. connectors. nectors.A Concerninggold average the of plates590 mg/kg and the was connectors obtained withby mixing the completely all the above-presented gold-plated surface con- of Concerning the plates and the connectors with the completely gold-plated surface of nectors.theConcerning third samples the a plates gold content and the of connectors 7900 mg/kg with was the determined. completely gold-plated surface of the thirdConcerningthe third samples samples the a goldplates a gold content and content the of connectors 7900 of 7900 mg/kg mg/kg with was the wasdetermined. completely determined. gold-plated surface of the3.2. third Thiourea samples Leaching a gold of content Gold from of 7900 the First mg/kg Sample was determined. 3.2. Thiourea Leaching of Gold from the First Sample 3.2. ThioureaThe results Leaching of the of Gold replicated from the thiourea First Sample leaching tests for the ﬁrst sample were reported 3.2.in Thiourea TableThe The3results. AuLeaching results dissolution of the ofof Gold replicatedthe yield fromreplicated was the thiourea First 31% thiourea onlySample leac after hingleac 1 hhing tests of reaction, tests for thefor whereas firstthe firstsample those sample ofwere Ag were andre- re- portedCuTheported were in results Table 76% in Table and3.of Au the below3. dissolution Aureplicated dissolution 1%, respectively. yieldthiourea yield was wasleac The31%hing 31%resultonly onlytests after was after for1 rather h theof 1 hreaction, unsatisfying, firstof reaction, sample whereas whereaswere so another those re- those portedoftest Agof was andinAg Table carriedandCu were Cu3. Au outwere 76% dissolution applying 76%and andbelow a yield counter-currentbelow 1%, was respectively.1%, 31% respectively. only leaching afterThe Theresult1 process h ofresult wasreaction, with wasrather four ratherwhereas unsatisfying, stages: unsatisfying, those every ofso stage,Ag anotherso and another fresh Cu test reagentswere testwas 76% wascarried were and carried added, belowout applyingout according1%, applying respectively. a counter-current to a the counter-current concentrations The result leaching was leaching indicated rather process processunsatisfying, in with Section with four 2.2 four, sostages: toanother treatstages: every the test every samestage, was stage, connectors freshcarried fresh reagents out reagents sample. applying were were adde a counter-currentadded, accordingd, according to theleaching to concentrationsthe concentrations process with indicated indicatedfour stages:in Sectionin everySection 2.2, stage, to2.2, treat tofresh treat the reagents samethe same connectors were connectors adde sample.d, accordingsample. to the concentrations indicated in TableSectionAfter 3. AfterGold2.2, the to andthirdthe treat silverthird stage, the cumulativestage, same the total theconnectors total dissolutionAu dissolution Au sample. dissolution yields yield as a yield function was was52%, of 52%, stagewhereas whereas number. 95% 95%and and5% of 5% of Ag and Cu were leached, respectively. Ag andAfter Cu Stagethe were third Number leached, stage, therespectively. total Au Audissolution Extraction yield (%) was 52%, whereas Ag Extraction 95% and (%) 5% of Ag and Cu were leached, respectively. Table 3. Gold and silver cumulative dissolution yields as a function of stage number. Table 3. Gold and1 silver cumulative dissolution yields 30.6 as a function of stage number. 75.9 Table 3. Gold and 2silver cumulative dissolution yields 44.5 as a function of stage number. 94.7 StageStage Number Number Au Extraction Au Extraction (%) (%) Ag Extraction Ag Extraction (%) (%) 3 51.6 95.1 Stage Number1 1 Au Extraction30.6 30.6 (%) Ag Extraction 75.9 75.9 (%) 12 42 30.644.5 44.5 52.7 75.9 94.7 95.1 94.7 23 3 44.551.6 51.6 94.7 95.1 95.1 After3 the4 third4 stage, the total Au51.652.7 dissolution 52.7 yield was 52%, whereas 95.1 95.1 95.195% and 5% of Ag and Cu4 were leached, respectively.52.7 95.1 3.3. Thiourea3.3. Thiourea Leaching Leaching of Gold of Gold from fromSecond Second and Thirdand Third Samples Samples 3.3. Thiourea Leaching of Gold from Second and Third Samples 3.3. ThioureaThe Thesecond Leaching second sample ofsample Gold with from with 590 Second mg/kg590 mg/kg and of Third Au of wasAu Samples wasobtained obtained by mixing by mixing all the all typesthe types of of The second sample with 590 mg/kg of Au was obtained by mixing all the types of connectorsTheconnectors second that thatweresample were previously with previously 590 subjected mg/kg subjected of to Au chemical towas chemical obtained attack, attack, byin ordermixing in order to allevaluate to the evaluate types the of ef-the ef- connectors that were previously subjected to chemical attack, in order to evaluate the connectorsficiencyficiency of thatthiourea of werethiourea leaching.previously leaching. The subjected Thethird third sample, to sample,chemical with with attack,a gold a gold incontent order content ofto 7900evaluate of 7900 mg/kg themg/kg was ef- was efﬁciency of thiourea leaching. The third sample, with a gold content of 7900 mg/kg was ficiencyalso alsoleached of leached thiourea within within leaching.the conditionsthe conditions The third presented presentedsample, within with within Sectiona gold Section content2.2. The2.2. ofTherecovery 7900 recovery mg/kg yields yields was are are also leached within the conditions presented within Section 2.2. The recovery yields are alsoreported leachedreported in Tablewithin in Table 4. the 4. conditions presented within Section 2.2. The recovery yields are reported in Table4. reported in Table 4. Table 4. Gold content within leaching solution and recovery. Table 4. Gold content within leaching solution and recovery. Table 4. Gold content within leaching solution and recovery. Table 4. Gold content within leaching solution and recovery. Photographic Aspect of Solid Sample PhotographicPhotographic Aspect Aspect of of Solid Au Recovery (%) SampleSample AuAu Recovery Recovery (%) (%) PhotographicSolidResidue ResidueAspectResidue of Solid Sample Au Recovery (%) Residue 2nd2nd 2ndsample sample 93.993.9 93.9

2nd sample 93.9

3rd sample3rd sample 49.0 49.0 3rd3rd sample sample 49.049.0

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The result of the leaching of completely gold-plated surface connectors demonstrates that this leaching process can efﬁciently recover about 94% of the gold content of these wastes. This fact demonstrates the suitability of this leaching system for gold dissolution. Not complete dissolution is achieved due to the fact that once the Au layer is dissolved, the copper metal substrate is exposed to leaching media (the sample has a copper content of 80%). This base metal determines both Au cementation and the degradation of thiourea to elemental sulfur that rapidly passivates the precious metal surface and prevents the gold dissolution [1–3]. Besides Au, 80.8 mg/L of Ag were detected within the leaching solution of the second sample. A relatively high gold dissolution was also achieved from the third sample. This was probably due to the very large content of Au within the solid material. So far, there is no data about the amount of Au that can be dissolved with this leaching system. Moreover, no silver was detected within the leaching solution of this sample. This may be due to the fact that the gold layer was not completely dissolved, and, therefore, the substrate of the silver layer was not exposed to the leaching environment. The determined concentration of thiourea had a value of 12.2 and 12.5 g/L for the solution of leaching of the second and third samples, respectively. The high consumption of thiourea is due to its oxidation by ferric ion to formamidine disulﬁde. This complex is not stable in acidic solutions and decomposes irreversibly, producing elemental sulphur and cyanamide. Both solutions have been further subjected to the electrowinning process. The pH of all solutions was measured after each test using an electrode.

3.4. Electrowinning 3.4.1. Electrowinning Test on Solution of First Sample Leaching Gold and silver were recovered from the solution obtained by the ﬁrst stage of thiourea leaching by electrodeposition. In Table5, the results of gold and silver deposition as a function of time were reported.

Table 5. Gold and silver recovery as a function of time by electrowinning.

Time of Reaction Au Recovery Ag Recovery h (%) (%) 0.25 38.1 18.1 0.50 50.4 25.0 0.75 55.9 32.5 1.00 62.4 36.9 1.25 68.7 46.2 1.50 72.8 53.5 1.75 79.0 62.2 2.00 79.0 62.2

After 1 h of the process, the maximum difference between gold and silver recoveries was observed based on the obtained results. Hence, this time was selected in order to obtain a product with a higher gold grade. The current efﬁciency for gold was 10%, due to the low concentration of gold in the solution, and the average energy consumption was 12 kWh/kg of gold. The recovered metallic powder was dissolved by aqua regia to determine the chemical composition: Ag 85.7%, Au 11.8%, Cu 1.4%, and traces of other elements.

3.4.2. Electrowinning Tests on Solutions of Second and Third Samples Leaching As is presented within Section 2.3, three runs at different current intensities were performed on the solution obtained from the second sample treatment. The achieved recovery degrees for Au and silver are reported in Figures5 and6. Minerals 2021, 11, 235 10 of 16 MineralsMinerals 20212021,, 1111,, xx FORFOR PEERPEER REVIEWREVIEW 1010 ofof 1616

Figure 5. Evolution of Au concentration and recovery from solution at various current intensities Figure 5.FigureEvolution 5. Evolution of Au concentration of Au concentration and recovery and recovery from solution from solution at various at currentvarious intensitiescurrent intensities vs. process time. vs.vs. processprocess time.time.

Figure 6. Evolution of Ag concentration and recovery from solution at various current intensities Figure 6.FigureEvolution 6. Evolution of Ag concentration of Ag concentration and recovery and recovery from solution from solution at various at currentvarious intensitiescurrent intensities vs. process time. vs.vs. processprocess time.time. As is shown in Figure5, the process yield for gold increased linearly with time in the AsAs isis shownshowncurrent inin FigureFigure limited 5,5, thethe regime processprocess as yield peryield Faraday’s forfor goldgold increasedincreased law only linearly forlinearly runs withwith conducted timetime inin at thethe 75 and 100 mA. currentcurrent limitedlimitedAfter regimeregime 1 h as ofas per process,per Faraday’sFaraday’s about lawlaw 88 and onlyonly 86% forfor of runsruns gold conductedconducted recovery wereatat 7575 achievedandand 100100 mA.mA. for the treatments AfterAfter 11 hh ofof process,process,performed aboutabout at 8888 75 andand and 86%86% 100 ofof mA, goldgold respectively. recoveryrecovery werewere There achievedachieved was noticed forfor thethe in treatmentstreatments both of these runs that performedperformed atat 7575once andand the 100100 mass mA,mA, transport respectively.respectively. limited TherTher regimeee waswas was noticednoticed approached, inin bothboth ofof the thesethese rate runsruns of increase thatthat was slower, onceonce thethe massmass eventransporttransport stopped limitedlimited for regime theregime run waswas conducted aapproached,pproached, at 100 thethe mA. raterate This ofof increaseincrease may be was explainedwas slower,slower, by the fact that eveneven stoppedstopped forforthe thethe increase runrun conductedconducted of current atat intensity 100100 mA.mA. determines ThisThis maymay bebe the explainedexplained evolution byby of thethe hydrogen factfact thatthat at thethe the cathode and increaseincrease ofof currentcurrentpossible intensityintensity partial determinesdetermines oxidation of thethe thiourea evolutionevolution at the ofof anode. hydrogenhydrogen Moreover, atat thethe at cathodecathode 200 mA, andand the kinetics of the possiblepossible partialpartialprocess oxidationoxidation for the ofof thiourea goldthiourea reduction atat thethe wasanode.anode. much Moreover,Moreover, slower, and atat 200200 only mA,mA, 60% thethe of kinetics goldkinetics was ofof recovered from thethe processprocess forforthe thethe solution goldgold reductionreduction after three waswas hours muchmuch of slower,slower, the process. andand onlyonly For 60%60% silver, ofof goldgold only waswas within recoveredrecovered the experiment with fromfrom thethe solutionsolution100 afterafter mA therethreethree washourshours a linearofof thethe increaseprocprocess.ess. ofForFor recovery silver,silver, onlyonly yield withinwithin till one thethe hour experimentexperiment of the process. It was withwith 100100 mAmA therethere waswas aa linearlinear increaseincrease ofof recorecoveryvery yieldyield tilltill oneone hourhour ofof thethe process.process. ItIt

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noticed after this process time that silver tends to be dissolved again within the solution. wasThe noticed other two after experiments this process have time shown that silver that thetends applied to be dissolved conditions again ensure within only the about solution.50% of The Ag recovery,other two even experiments after 3 h have of the shown process. that This the mayapplied be dueconditions to the resistanceensure only of aboutthe Ag 50% deposit, of Ag whichrecovery, does even not after allow 3 h the of the electrons process. to This transfer may from be due the to cathode the resistance to the ofsilver-thiourea the Ag deposit, complex. which does not allow the electrons to transfer from the cathode to the silver-thioureaThe determined complex. values of thiourea within solutions after each run have shown that at 75 mA,The the determined thiourea wasvalues regenerated of thiourea within within the solutions solution after at a percentageeach run have of 10.65%.shown that For atthe 75 other mA, runs,the thiourea there was was noticed regenerated a diminishing within the of thioureasolution concentrationat a percentage with of 10.65%. 5.73% (test For thewith other 100 mA)runs, andthere 13.11% was noticed (200 mA). a diminishing These results of thiourea are a confirmation concentration of the with possibility 5.73% (test to withregenerate 100 mA) thiourea and 13.11% by reduction (200 mA). of FDSThese content results of are the a solutionconfirmation and also of the for possibility the thiourea to regeneratedegradation thiourea by increasing by reductio currentn of intensity. FDS content of the solution and also for the thiourea degradationFurthermore, by increasing the current current efficiencies intensity. for both Au and Ag recovery and specific energy consumptionFurthermore, for each the current run were efficiencies calculated. for The both current Au and efficiency Ag recovery of an and electrochemical specific en- process is given as the ratio of electrical charge used to form the desired product over the ergy consumption for each run were calculated. The current efficiency of an electro- total charge passed during the electrochemical process. This is given by the following chemical process is given as the ratio of electrical charge used to form the desired product equation based on Faraday’s Law: over the total charge passed during the electrochemical process. This is given by the following equation based on Faraday’s Law:φ = m·n·F/q (12) ϕ = m∙n∙F/q (12) where m is the molar amount of starting material (mole), n is the number of electrons whereinvolved m inis the electrodemolar amount reaction, of starting F is the mate Faradayrial (mole), constant n (96485is the C/mol),number of and electrons q is the involvedcharge passed in the (C electrode·s). reaction, F is the Faraday constant (96485 C/mol), and q is the chargeThe passed specific (C∙ energys). consumption of an electrochemical process relates to the electrical energyThe consumed specific energy per mass consumption of product, of and an is el givenectrochemical as follows: process relates to the electrical energy consumed per mass of product, and is given as follows: Es = −n·F·ECELL/(φ·M) (kWh/kg) (13) Es = −n∙F∙ECELL/(ϕ∙M) (kWh/kg) (13) where n is the number of electrons involved in the electrode reaction, F is the Faraday constant (96485 C/mol), E is the cell voltage (V), φ is the current efficiency (%), and constant (96485 C/mol), ECELLCELL is the cell voltage (V), ϕ is the current efficiency (%), and M isM the is the mass mass of ofproduct product (kg). (kg). The The determined determined current current efficiencies efficiencies and and specific specific energy consumptionsconsumptions vs. process process time for Au and Ag are shown in Figures 7 7 and and8 .8.

Figure 7. Current efficiency and specific energy consumption at different current intensity vs. time for Au. Figure 7. Current efﬁciency and speciﬁc energy consumption at different current intensity vs. time for Au.

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Figure 8. Current efficiency and specific energy consumption at different current intensity vs. time for Ag. Figure 8. Current efﬁciency and speciﬁc energy consumption at different current intensity vs. time for Ag.

FromFrom Figures Figures 7 andand8 8,, it it can can be be inferred inferred that that the the current current efficiency efficiency was was very very low low for for bothboth elements’ elements’ recovery. recovery. The The highest highest current current ef efficienciesficiencies were were achieved achieved within within the the first first 15 min15 min of reaction: of reaction: 1.6, 1.6,1.4 and 1.4 and0.76% 0.76% for Au for and Au and4.48, 4.48,3.67 and 3.67 1.84% and 1.84% for Ag for during Ag during the runs the performedruns performed at 75, at100 75, and 100 200 and mA, 200 respectively. mA, respectively. Moreover, Moreover, as the as experimental the experimental runs runspro- ceededproceeded to the to point the point where where over over 85% 85% (1 h (1of hreaction of reaction for runs for runs conducted conducted at 75 at mA 75 mAand and100 mA)100 mA) of gold of gold was was recovered recovered from from the the solution, solution, overall overall current current efficiency efficiency decreased decreased to 0.55%0.55% and 0.79%, respectively.respectively. For For the the other other run, run, where where the the recovery recovery yields yields of both of both precious pre- ciousmetals metals were were lower lower than inthan the in other the other two experiments,two experiments, the current the current efficiency efficiency after after 2.5 h 2.5 (at hthe (at largest the largest recovery recovery yield yield of Au of and Au Ag and of Ag this of test), this thetest), current the current efficiency efficiency was decreased was de- creasedto 0.1%. to For 0.1%. the For energy the energy consumption, consumptio whichn, variedwhich varied between betw 18–200een 18–200 kWh/kg kWh/kg of metal of metalrecovered recovered (depending (depending on process on process time time and appliedand applied current current intensity), intensity), it is it clear is clear that that the theoptimal optimal condition condition for Aufor recoveryAu recovery was achievedwas achieved during during the experiment the experiment with the with lowest the lowestlevel of level current of current intensity intensity (75 mA). (75 AboutmA). About 94% of 94% Au of was Au recovered was recovered from from solution solution after after1.5 h 1.5 of reactionh of reaction and specific and specific energy energy consumption consumption of 18 kWh/kgof 18 kWh/kg of Au. of Instead, Au. Instead, for better for betterefficiency efficiency of Ag recovery,of Ag recovery, a larger a current larger current intensity intensity is required. is required. About 83% About of Ag 83% recovery of Ag recoverywas obtained was obtained after one after hour one of process hour of at proc 100ess mA at with 100 amA current with efficiencya current ofefficiency 1.65% and of 1.65%current and consumption current consum of 18.04ption kWh/kg. of 18.04 kWh/kg. TheThe efficiency efficiency of of process process conditions conditions that that were were observed observed as as optimal for for Au recovery waswas tested onon thethe samesame leaching leaching solution solution using using a coppera copper electrode electrode and and a graphite a graphite electrode elec- trodeas anode. as anode. The achieved The achieved results results for Au for and Au Ag and recovery Ag recovery are shown are shown in Tables in Tables6 and 76. and 7.

Table 6. DataTable on 6.AuData recovery on Au by recovery electrowinning by electrowinning with Cu cathode with Cu and cathode graphite and anode. graphite anode.

Time of Reaction Energy Time of Concentration Current Concentration (mg/L) Recovery (%) Current EfRecoveryficiency (%) (%) Energy ConsumptionConsumption (KWh/kg) (h) Reaction (h) (mg/L) Efﬁciency (%) 0 58.9 0.00 0.00 0 (KWh/kg) 0.25 38.0 035.40 58.91.51 0.00 0.00 8.10 0 0.50 22.7 0.2561.50 38.01.31 35.40 1.51 9.32 8.10 0.75 14.1 0.5075.98 22.71.08 61.50 1.31 11.32 9.32 1.00 9.3 0.7584.26 14.10.90 75.98 1.08 13.61 11.32 1.25 5.9 1.0089.93 9.30.77 84.26 0.90 15.94 13.61 1.50 5.9 89.95 0.64 19.12 1.25 5.9 89.93 0.77 15.94 1.75 4.2 92.86 0.57 21.61 1.50 5.9 89.95 0.64 19.12 2.00 12.3 79.14 0.42 28.98 2.25 10.4 82.36 0.39 31.33

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Table 6. Cont.

Energy Time of Concentration Current Recovery (%) Consumption Reaction (h) (mg/L) Efﬁciency (%) (KWh/kg) 1.75 4.2 92.86 0.57 21.61 2.00 12.3 79.14 0.42 28.98 2.25 10.4 82.36 0.39 31.33 2.50 9.6 83.77 0.36 34.22 2.75 11.6 80.37 0.31 39.24 3.00 12.6 78.55 0.28 43.80

Table 7. Data on Ag recovery by electrowinning with Cu cathode and graphite anode.

Energy Time of Concentration Current Recovery (%) Consumption Reaction (h) (mg/L) Efﬁciency (%) (KWh/kg) 0 80.8 0.00 0 0 0.25 53.8 33.37 3.57 6.26 0.50 33.7 58.30 3.12 7.16 0.75 24.4 69.79 2.49 8.98 1.00 18.6 76.93 2.06 10.86 1.25 15.2 81.23 1.74 12.86 1.50 9.7 87.98 1.57 14.24 1.75 33.5 58.58 0.90 24.95 2.00 32.5 59.72 0.80 27.98 2.25 33.5 58.54 0.70 32.11 2.50 38.8 51.93 0.56 40.22 2.75 40.5 49.85 0.49 46.08 3.00 42.2 47.77 0.43 52.46

The results presented within these Tables reveal that both precious metals have the same recovery yields (89.95% for Au and 87.98%for Ag) after 1.5 h of reaction. This is probably also due to the cathode of copper metal that favors the cementation of both metals. The main inconvenience in using this electrode’s metal is its susceptibility to react with thiourea and decompose it. At the end of the process, the concentration of thiourea was decreased by about 20%. Similar to other experiments, the current efficiency was low, and the specific current consumption was 12 kWh/kg. It is worth mentioning that the pH of both leaching solutions (1.2) has not suffered considerable modification during the electrowinning process; the measured pH values of the solution after electrowinning were within the range of 1–1.3. Gold electrowinning from the third sample’s solution was performed for 1.5 h with 75 mA and 0.9 V. 18.2 mg/L of gold were detected within the final solution at the end of the experiment and 13.1 g/L of thiourea. Accordingly, the adopted process conditions were suitable for both Au recovery from solution and to avoid degradation of thiourea. In order to determine the gold content within the achieved deposit, this was manually removed from the cathode, weighted, and then digested with aqua regia. The deposited gold is shown in Table8, a similar gold deposit was achieved by Urbansky et al. [32]. MineralsMinerals 2021 2021, 11,, x11 FOR, x FOR PEER PEER REVIEW REVIEW 13 of13 16of 16

2.502.50 9.6 9.6 83.7783.77 0.360.36 34.22 34.22 2.752.75 11.611.6 80.3780.37 0.310.31 39.24 39.24 3.003.00 12.612.6 78.5578.55 0.280.28 43.80 43.80

TableTable 7. Data 7. Data on Agon Agrecovery recovery by electrowinningby electrowinning with with Cu Cucathode cathode and and graphite graphite anode. anode.

TimeTime of Reactionof Reaction ConcentrationConcentration (mg/L) (mg/L) Recovery Recovery (%) (%) Current Current Efficiency Efficiency (%) (%) Energy Energy Consumption Consumption (KWh/kg) (KWh/kg) (h) (h) 0 0 80.880.8 0.000.00 0 0 0 0 0.250.25 53.853.8 33.3733.37 3.573.57 6.26 6.26 0.500.50 33.733.7 58.3058.30 3.123.12 7.16 7.16 0.750.75 24.424.4 69.7969.79 2.492.49 8.98 8.98 1.001.00 18.618.6 76.9376.93 2.062.06 10.86 10.86 1.251.25 15.215.2 81.2381.23 1.741.74 12.86 12.86 1.501.50 9.7 9.7 87.9887.98 1.571.57 14.24 14.24 1.751.75 33.533.5 58.5858.58 0.900.90 24.95 24.95 2.002.00 32.532.5 59.7259.72 0.800.80 27.98 27.98 2.252.25 33.533.5 58.5458.54 0.700.70 32.11 32.11 2.502.50 38.838.8 51.9351.93 0.560.56 40.22 40.22 2.752.75 40.540.5 49.8549.85 0.490.49 46.08 46.08 3.003.00 42.242.2 47.7747.77 0.430.43 52.46 52.46

TheThe results results presented presented within within these these Tables Tables reveal reveal that that both both precious precious metals metals have have the the samesame recovery recovery yields yields (89.95% (89.95% for forAu Au and and 87.9 87.98%for8%for Ag) Ag) after after 1.5 1.5h of h reaction.of reaction. This This is is probablyprobably also also due due to theto the cathode cathode of copperof copper metal metal that that favors favors the the cementation cementation of bothof both metals.metals. The The main main inconvenience inconvenience in using in using this this electrode’s electrode’s metal metal is its is susceptibilityits susceptibility to react to react withwith thiourea thiourea and and decompose decompose it. Atit. Atthe the end end of theof the process, process, the the concentration concentration of thioureaof thiourea waswas decreased decreased by byabout about 20%. 20%. Similar Similar to otheto other experiments,r experiments, the the current current efficiency efficiency was was low,low, and and the the specific specific current current consumption consumption was was 12 kWh/kg.12 kWh/kg. It isIt worth is worth mentioning mentioning that that the the pH pH of bothof both leaching leaching solutions solutions (1.2) (1.2) has has not not suffered suffered considerableconsiderable modification modification during during the the electrow electrowinninginning process; process; the the me measuredasured pH pH values values of of thethe solution solution after after electrowinning electrowinning were were within within the the range range of 1–1.3.of 1–1.3. GoldGold electrowinning electrowinning from from the the third third sample sample’s solution’s solution was was performed performed for for1.5 1.5h with h with 75 mA75 mA and and 0.9 0.9V. 18.2V. 18.2 mg/L mg/L of goldof gold were were detected detected within within the the final final solution solution at theat the end end of of thethe experiment experiment and and 13.1 13.1 g/L g/L of thiourea.of thiourea. Accordingly, Accordingly, the the adopted adopted process process conditions conditions werewere suitable suitable for forboth both Au Au recovery recovery from from solution solution and and to avoidto avoid degradation degradation of thiourea.of thiourea. In orderIn order to determineto determine the the gold gold content content within within the the achieved achieved deposit, deposit, this this was was manually manually Minerals 2021, 11, 235 14 of 16 removedremoved from from the the cathode, cathode, weighted, weighted, and and then then digested digested with with aqua aqua regia. regia. The The deposited deposited goldgold is shown is shown in Tablein Table 8, a 8, similar a similar gold gold deposit deposit was was achieved achieved by byUrbansky Urbansky et al. et [32].al. [32].

Table 8. PhotographicTableTable 8. aspectPhotographic 8. Photographic of gold aspect deposit aspect of on gold of graphite gold deposi deposi chatodet ont graphiteon andgraphite purity. chatode chatode and and purity. purity.

CathodeCathodeCathode after after after Gold Gold EW EWEW Recovered Recovered Recovered Gold—58.9 Gold—58.9 mg mg mg Purity Purity Purity

57% 57%57%

Minerals 2021, 11, x FOR PEER REVIEW 14 of 16

GoldGold content content of of 57% 57% was was determined determined within within the the solid solid product. product. No No copper copper or or other other elementelement was was detected detected within within the the solid solid product. product. The The rest rest of of the the material material is is carbon carbon from from the the graphitegraphite cathode cathode that that was was detached detached during during the the manual manual removal removal of of Au Au from from the the electrode electrode surface. X-ray diffraction analysis, as shown in Figure9, conﬁrmed that only gold and surface. X-ray diffraction analysis, as shown in Figure 9, confirmed that only gold and carboncarbon were were detected. detected.

FigureFigure 9. 9.X-ray X-ray diffraction diffraction pattern pattern of of solid solid product product obtained obtained by by electrowinning. electrowinning.

AccordingAccording to to the the analysis analysis of of the the solution, solution, the the final final recovery recovery yield yield of Au of was Au 95%.was The95%. determinedThe determined current current efficiency efficiency after 1.5 after h was 1.5 4.06%, h was and 4.06%, the specificand the energy specific consumption energy con- wassumption 3.02 kWh/kg was 3.02 of gold.kWh/kg The of consumption gold. The co ofnsumption energy was of considerablyenergy was reducedconsiderably due tore- theduced much due higher to the Au much content higher within Au thecontent leaching within solution. the leaching The technology solution. mayThe betechnology further improvedmay be further to achieve improved the almost to achieve complete the recovery almost ofcomplete Au and recovery Ag from thisof Au type and of e-waste.Ag from this type of e-waste. 4. Conclusions 4. ConclusionsThis paper showed the preliminary results obtained in the extraction of gold from militaryThis equipment paper showed connectors. the preliminary These electronic results componentsobtained in the are extraction used for special of gold appli- from cations,military and equipment thus have connectors. to guarantee These long-life electronic reliability components and duration. are used There for special was shown appli- thatcations, thiourea and acidthus leaching, have to muchguarantee less pollutinglong-life thanreliability traditional and duration. cyanide processes, There was may shown be successfullythat thiourea applied acid leaching, on completely much gold-plated less polluting components. than traditional Precious cyanide metals processes, are selectively may dissolvedbe successfully by using applied a single on stagecompletely of leaching. gold-plated The best components. gold dissolution Precious yield metals was nearly are se- 94%.lectively Gold dissolved was recovered by using from a single the solution stage of by leaching. electrowinning The best process gold dissolution with high recoveryyield was yields;nearly a 94%. recovery Gold of was 95% recovered on graphite from cathode the solution was achieved by electrowinning after 1.5 h ofprocess reaction with with high a currentrecovery consumption yields; a recovery of 3.02 kWh/kg.of 95% on Thegraphi finalte solidcathode product was achieved has a purity after of 1.5 57 h %, of since reac- only carbon was detected as impurity; a thermal treatment can increase the grade. tion with a current consumption of 3.02 kWh/kg. The final solid product has a purity of 57 %, since only carbon was detected as impurity; a thermal treatment can increase the grade.

Author Contributions: Conceptualization, I.B. and N.M.I.; methodology, I.B.; validation, F.V.; analyses, I.B., N.M.I., and M.C.; resources, F.V.; data curation, I.B. and N.M.I..; writing—original draft preparation, F.F., I.B., and N.M.I.; writing—review and editing, I.B. N.M.I., and F.F.; supervision, F.V.; project administration, F.V.; funding acquisition, F.V. All authors have read and agreed to the published version of the manuscript. Funding: This work has received funding from the European Union’s Horizon 2020 research and innovation program under grant agreement No 760792. In any case, the present work cannot be considered as an official position of the supporting organization, but it reports just the point of view of the authors.

Minerals 2021, 11, 235 15 of 16

Author Contributions: Conceptualization, I.B. and N.M.I.; methodology, I.B.; validation, F.V.; analyses, I.B., N.M.I., and M.C.; resources, F.V.; data curation, I.B. and N.M.I.; writing—original draft preparation, F.F., I.B., and N.M.I.; writing—review and editing, I.B., N.M.I., and F.F.; supervision, F.V.; project administration, F.V.; funding acquisition, F.V. All authors have read and agreed to the published version of the manuscript. Funding: This work has received funding from the European Union’s Horizon 2020 research and innovation program under grant agreement No 760792. In any case, the present work cannot be considered as an official position of the supporting organization, but it reports just the point of view of the authors. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Not applicable. Conflicts of Interest: The authors declare no conflict of interest.

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