1. Introduction
The European Union unites the waste issue with actual policies for the use of material resources in the “Thematic Strategy on waste prevention and recycling” [1] and in the “Thematic Strategy on the use of natural resources” [2], aiming to decouple the use of resources from the production of waste and the associated negative effects on the environment. In addition, the application of the concept of “waste hierarchy” is encouraged, which classifies the different waste management options from the best to the least good for the environment. Priority is given to the prevention of waste generation, the minimization of the amount and degree of danger, followed by reuse, recycling, energy recovery and, finally, disposal by incineration or storage. In applying the waste hierarchy, the options that produce the best overall environmental result should be adopted. This may require that certain specific waste streams move away from the hierarchy; this is determined by analyzing the entire life cycle regarding the global effects of the generation and management of that waste. Waste policies can primarily reduce three types of pressure on the environment: emissions from waste treatment/disposal facilities, irrational exploitation of natural resources, and air pollution and greenhouse gas emissions caused by the consumption of energy and fuels in the waste management process.
Electrical and electronic equipment waste represents the waste of equipment that operates on the basis of electric current or electromagnetic fields and the equipment for generating, transporting and measuring these currents and fields, intended for use at a voltage lower than or equal to 1000 volts alternating current and 1500 volts direct current. The main objectives of Directive 2002/96/EC regarding electrical and electronic equipment waste—WEEE [3]—with subsequent amendments and additions are: 1. preventing the production of electrical and electronic equipment waste, as well as their reuse, recycling and other forms of valorization, so as to reduce the volume of disposed waste; and 2. improving the performance regarding the environment and the health of the population of producers, importers, distributors, consumers and especially of the economic operators who collect, treat, recycle, valorize or dispose of WEEE.
According to the latest UN’s General E-Waste Monitor, in 2022, 62 million tons of E-waste were generated globally. The quantity of e-waste is expected to rise to 82 million tons by 2030 [4]. Starting with 2016, the European Union will oblige member countries to collect, recycle and reuse at least 85% of used electronic equipment. Currently, each member of the European Union is obliged to recycle 4 kg/inhabitant of waste from WEEE [5].
In general, WEEE is composed of metals (40%), plastic materials (30%) and refractory oxides (30%) [6,7]. Regarding metals, WEEE generally contains average quantities of copper (20%), iron (8%), tin (4%), nickel (2%), lead (2%), zinc (1%), silver (0.02%), gold (0.1%) and palladium (0.005%) [8,9]. Polyethylene, polypropylene, polyesters and polycarbonates typically constitute plastic components.
In the European Union, the disposal of WEEE is closely controlled by Directive 2003/108 [10] of the European Commission, which requires that WEEE be collected selectively, broken down separately into each type of material (e.g., plastics and metals) and then recycled. The current technologies for recycling electrical waste and electronic equipment create an important advantage for the recovery of metallic components. Separating metals from WEEE is a simpler task, as each respective metal can be easily identified and dismantled, being in a quasi-pure state without being mixed with other components. On the other hand, the separation of thermoplastic components of WEEE is also relatively facile, involving mainly packaging or support items for other components; here, it is also possible that some plastic types are partially mixed, making their separation and recycling more difficult. The most problematic subject, with severe limitation regarding recycling, is represented by the content of non-metallic components. The non-metallic section of electronic waste—especially printed circuit boards (PCBs) with scrap electronic components—represents one of the major problems facing consumer societies in Europe, Japan, and North America. Such components are practically inseparable from thermorigid organic materials, traces of precious metals, semiconductors as crystals and metal oxides. However, the recycling of non-metallic components of WEEE is particularly problematic because they also contain special additives, such as heavy metals (Hg, Pb, Cd and hexavalent chromium) and halogenated flame retardants, which have a negative impact on the environment (according to restrictions from Directive 2002/95/CE—RoHS, [11]). In combination with plastic, halogen compounds may form volatile metal halides, which further perform a catalytic effect with the formation of dioxins and furans, dangerous for people and the environment, [12].
Along with the established raw materials to be recovered from non-metallic components of WEEE, research was oriented with priority in the direction of the recovery of precious metals, towards sustainable business [13,14]. But in the last 20 years, electronic technology has advanced with new types of electronic components with better integration and very limited use of precious metals, so the problem related to the recycling of scrap electronic components remains conceptually and technologically unsolved and has rarely been targeted by the scientific literature. In analyzing the evolution of electronic technology, one can identify the following: 1. electronic equipment produced before the 1990s, where there is a significantly large number of passive components (capacitors, resistors, coils with ferrimagnetic cores, etc.) of a volume comparable to that of the active components (such as diodes/transistors and some integrated circuits of large areas), with a below-average degree of integration, all disposed of on PCBs of large surfaces, with a high quantity of soldering alloys; 2. electronic equipment produced between the 1990s and 2010s, with a higher degree of integration, where there is a significantly large number of integrated-circuit-type active components, and passive ones are present with less than 30% of the volume of active ones, but are still of a large volume, all disposed of on PCBs of smaller surfaces, eventually within a predefined spatial architecture, with soldering alloys still largely being used; and 3. electronic equipment produced after the 2010s, with a very high degree of integration, miniaturized integrated circuits—microprocessors predominating, with a negligible volume of passive components and negligible PCB areas—based on new methods of soldering, without alloys. As the years pass, it is becoming more evident that WEEE recycling technologies must keep up with technological development, i.e., with new concepts of electronic technology, because classic waste with large amounts of metals and thermoplastic components present before the 2000s is on its way to becoming extinct. Accordingly, the legislative Decree n. 49 of 2014 represents the reference legislation of EC regarding Waste Electrical and Electronic Equipment towards implementing the European Directive 2018/849, present in the Circular Economy Package [15].
The primary issue with PCB recycling is their complicated structure and material combination. Actual technologies presume either thermal processing, chemical treatment, or mechanical non-thermal processing, including disassembly, separation and shredding techniques. The actual purpose of PCB recycling is to recover approximately 99% of precious and scarce metals, mainly Cu, Ag and Au, but this also depends on their purity and amount. Other fractions are ignored and/or sent either to damping or to high-temperature thermal processing for energy purposes.
This paper’s novelty lies in showcasing the technology used to produce electromagnetically active powders by selectively recycling electronic waste, specifically scrap electronic components—as detailed in [16]—aligning with current electronic technology and the principles of the circular economy. These powders could replace expensive conductive materials currently used in EMC/EMI composites, coatings, and paints. This study aimed to show how sintered materials could be used as more affordable electromagnetic shielding systems by utilizing powders from the new recycling concept.
2. Materials and Methods
2.1. Manufacturing and Characterization Equipment
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Sintering was performed by use of the Spark plasma sintering furnace HHP D (FCT Systeme Gmbh, Rauenstein, Germany).
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A simultaneous thermal analyzer—Thermogravimetry (TG)/Differential Scanning Calorimetry (DSC) type STA 449 F3 Jupiter, (NETZSCH, Selb, Germany)—allowed the determination of mass variations and thermal changes for different types of materials, including inhomogeneous materials.
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Hydrostatic density was determined utilizing XS204 Analytical Balance, characterized by the following specifications: maximum capacity of 220 g, precision of 0.1 mg, linearity of 0.2 mg, internal calibration, equipped with a density kit for solids and liquids, and an RS 232 interface. The measurements were conducted at a temperature of 25 °C.
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Chemical analysis was performed by use of the XRF spectrometer model WD-XRF S8 TIGER-1 kW (Bruker AXS GmbH, Berlin, Germany).
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Structural characterization was carried out by X-ray diffraction (XRD) using CuKα radiation (λ = 0.154 nm) with Ni filter Bruker AXS D8 Advance (Bruker AXS, Billerica, MA, USA). Diffraction patterns were recorded at room temperature in Bragg–Brentano geometry at an angle 2θ from 20° to 65° at a rate of 0.6°/min (2θ)/min.
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Scanning electron microscopy (SEM) was performed with a field emission and focused ion beam scanning electron microscope (SEM) model Tescan Lyra III XMU (Brno-Kohoutovice, Czech Republic).
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Shore hardness tests were performed with a common Microdurometer Vickers FM700 (Future-Tech Corp, Tokyo, Japan).
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Dielectric analysis was performed via broadband dialectic spectroscopy, by use of a Turn Key Dielectric Spectrometer BDS 40BDS (frequency band 3 μHz–3 GHz), with variable temperature control (Novocontrol Gmbh, Montabaur, Germany).
2.2. Powder Manufacturing Process
The manufacturing process starts with the selective dismantling and selection of PCBs with electronic components (integrated circuits, but with the presence of minoritarian quantities of diodes, metallic film resistors and ceramic capacitors). Initially, the PCBs were submitted to an extraction process of copper wires, ferritic elements, metallic carcasses and radiators, and most of the solder alloys, etc. These kinds of PCB scraps constitute the non-metallic WEEE category that is considered non-recyclable. The respective scraps were submitted to dimensional reduction till reaching 3 × 3 cm2, performed with a mini-breaker, and further to a succession of grinding processes, as suggested in Figure 1, which were carried out with two types of mills, the RETSCH SM 2000 knife cutting mill and the RETSCH RS100 vibrating disc mill (both from Retsch Gmbh, Haan, Germany). The first milling process can be adjusted by changing the grinding size by the use of interchangeable sieves with different dimensions, to be adapted according to the type and technological age of the PCBs to be processed. After the first milling process of 10 min was completed., the resulting granules were passed through a vibrating sieve to separate the fraction with a size greater than 0.8 mm (basically till a maximum of 2.5 mm), a fraction practically formed by pieces of PCBs without a significant quantity of electronic components (eventually with some pieces of capsule of such components). This first fraction can be processed by classical chemical recycling technologies in order to recover Cu [17,18], and it is not the purpose of our study. The second fraction, with a size smaller than 0.8 mm, represents the concentrated part, containing in large proportion the electronic components, and this was submitted to a second milling process for 15–20 min. The new powder, with a particle size of up to 10 microns, is presented in Figure 2. This powder was finally submitted to magnetic separation to select the magnetic fractions from non-magnetic fractions (the magnetic fraction may include Fe, Ni, but also NiO, Cr2O3 and ferrite powders), performed using a Carpco MIH 111-5 laboratory magnetic separator (IMSC Group, Jacksonville, FL, USA), using an average magnetic field of 6000 G. This dry separator was considered the most adequate for the type of powder to be processed. Further, it was submitted to a final electrostatic separation performed by a EHTP 111-15 laboratory electrostatic separator (Sepor Inc., Los Angeles, CA, USA) to remove most of the metallic particles from the remaining non-magnetic fraction. The following parameters were set: ionizing electrode: distance to rotor = 25 cm; static electrode: distance to rotor = 25 cm; rotor speed: 80 rpm; and high tension average value: 45 kV.
The separation processes results and yield values are similar to the ones described, e.g., in [19], and the average fraction distribution was, in our case, of about 13% of the magnetic fraction, about 23% of the metallic non-magnetic fraction, and about 64% of remaining powder requiring further analysis. We note that some small quantities of non-magnetic metallic powders were still trained by a magnetic separator (mainly Cu and Sn), and some residual magnetic powders were present in the fraction that resulted after electrostatic separation (e.g., Fe2O3). We note further that some quantities of metallic and magnetic components remained in the powder of study after the separation processes. Accordingly, it is difficult at this moment to evaluate a precise recovery rate of the separation processes, but it is estimated to be over 97% for all metals, and notably, about 99% for Cu.
Hence, after the separation processes were finalized, we obtained three main final fractions: magnetic particles (Fe, Ni, etc., with residues of ferritic powders), those associated with metallic non-magnetic particles (Cu, Sn, Pb, Au, Ag, etc.)—which both have a classical route of valorization—depending on their concentration and purity, and the powder of non-metallic particles (of our direct interest, which will be analyzed further), as presented in Figure 1.
The second step was related to the sintering of the powder material (4 g of powder per sample) in order to obtain solid samples with a diameter of 12 mm and a thickness between 2 and 3 mm. The spark plasma sintering process was performed within a spark plasma sintering furnace. The direct heating of the mold–sample–piston system allowed high temperature-increase speeds (50–130 °C/min) and short sintering times, in the order of tens of minutes. The equipment is presented in Figure 3, and the resulting samples are shown in Figure 4. The mold and the sample were heated by the direct passage of a pulsating electric current of low voltage, which propagated through the piston–sample–mold system, with cycle times in the order of several minutes. The processing data were pre-selected; in the case of Figure 3, a thermal cycle of 15 min is presented, with a maximal temperature exposure equivalent to 1000 °C for 5 min.
Above 600 °C, the mixture of heterogeneous (inhomogeneous) powders went through a pre-sintering process, finalized at 800 °C with slight compaction.
3. Results and Discussion
The tests were performed on both powders and sintered disks from respective powders. Three types of powders were analyzed, i.e., powder after fine grinding (P1), after magnetic separation (P2) and after electrostatic separation (P3).
3.1. Thermal Analysis for Powders
The results are presented in Figure 5, Figure 6 and Figure 7.
For P1, Figure 5, four thermal processes were identified in the studied temperature range 25–900° C.
Process I (green TG line)—mass loss (on the green curve—TG): 10.88% (25–400 °C), 9.85% (400–48 0 °C), 7.80% (480–780 °C), 6.89% (720–780 °C), and 2.47% (780–800 °C); the total mass loss in the studied temperature range 25–800 °C was 37.89%.
Process II (blue line—DSC)—mass loss at 84.8 °C and an endothermic process of melting of thermoplastic polymers at 182 °C.
Process III (dotted green DTG line)—a two-step oxidation reaction is observed (exothermic process), possibly due to the presence of oxygen in the material, with a minimum at 260 °C and a maximum at 290.7 °C.
Process IV—includes the decomposition of the products resulting from the delamination of printed PCBs based on epoxy resin. During these decomposition processes, enthalpies with high values of 37.23 J/g, 86.93 J/g and 103.9 J/g appeared.
For P2, Figure 6, several processes of decomposition were also recorded.
Process I (green TG line)—mass loss (on the green curve—TG): 1.66% (25–300 °C), 8.54% (300–420 °C), 9.57% (420–500 °C), 3.4% (500–640 °C), 8.93% (640–780 °C) and 3.45% (780–800 °C); the total mass loss over the studied temperature range 25–800 °C was 35.55%.
Process II (blue line—DSC)—mass loss at 84.8 °C and an endothermic melting process of thermoplastic polymers at 182 °C.
Process III (dotted green DTG line)—a two-step oxidation reaction is observed (exothermic process), possibly due to the presence of oxygen in the material, with a minimum at 262.8 °C and a max. at approx. 280 °C.
Process IV—includes the decomposition of the products resulting from the delamination of printed PCBs based on epoxy resin. During these decomposition processes, enthalpies with values of 40.41 J/g and 11.04 J/g appeared.
In the case of P3, Figure 7, four thermal processes may be identified:
Process I (green line TG)—loss of water (bound and unbound). It is found that three losses of water occur: 11.07% (25–350 °C), 2.49% (350–460 °C) and 5.09% (460–800 °C); the total mass loss in the studied temperature range 25–800 °C was 18.65%.
Process II (blue line—DSC)—includes a mass loss corresponding to a temperature of 84.8 °C and an endothermic process of melting thermoplastic polymers at 182 °C.
Process III (dotted green DTG line)—a two-step oxidation reaction is observed (exothermic process), possibly caused by the presence of oxygen in the material, with a maximum of 290.7 °C.
Process IV—includes the decomposition of the products resulting from the delamination of printed PCBs based on epoxy resin. During these decomposition processes, enthalpies with high values of 37.23 J/g, 86.93 J/g and 103.9 J/g appeared.
3.2. Evaluation of Hydrostatic Density of Powders before Sintering Process
The freely poured density was determined according to the standard ISO 3923-2 [20], when the powder flowed freely in a collector cylinder with volume of 4.5167 cm3. The results are presented in Table 1 as an average of five measurements (eliminating the lowest and highest values), with a standard deviation of under 1% [20]. Table 1 offers a preliminary view of the sintered samples’ characteristics following the separation processes.
The highest diminution of density was noticed after the magnetic separation process, where magnetic fractions of higher mass were excluded from the powder.
3.3. Chemical Analysis of Powders via Spectrometry with X-ray Fluorescence—XRF
The results are presented in Table 2, Table 3 and Table 4 and Figure 8, Figure 9 and Figure 10.
As regards P1, it was found that the composition was a mixture of complex structures based on metallic oxides (mainly of Fe, Si, Cu, Ca Pb, Sn, etc.) and some traces of metals (mainly Ag); the concentrations of metals are given in Table 2. The total concentration of metallic compounds was approximately 98.32%. The remainder is expected to be polymeric components, as it comprised integrated circuits (thermoplastic) and PCBs (mainly epoxy-derived resins). XRF equipment cannot identify carbon compounds.
As regards P2, it was found that the powder consisted of a mixture of complex structures based on metallic oxides (mainly of Ca, Cu, Si, Sn, Fe, etc.) and metallic traces; the concentrations of metals are given in Table 3. The total concentration of metallic compounds was approximately 83.42%, the remainder being a polymeric component. It is obvious that the concentrations of certain metals (and related metallic oxides) with magnetic properties, mainly Fe, Ni, and Cr, were significantly reduced by magnetic separation.
As regards P3, it was found that the powder consisted of a mixture of complex structures based on metallic oxides (mainly of Ca, Cu, Si, Sn, Pb, Ba, Zn, etc.); the concentrations of metals are given in Table 4. The total concentration of metallic compounds was approximately 78,65%. The remainder is expected to be a polymeric component. It was noticed that the main separation of metallic oxides is assured by magnetic separation, and following the metallic compound separation, the concentration of polymeric components progressively increased. This aspect is in line with the thermal analysis. More details regarding the composition of samples P1–P3 are presented in Section 3.4.
Please note that the results presented above cannot be directly correlated with the results from powder separation processes, because the results from Table 2, Table 3 and Table 4 refer to the samples that were sintered, when most metals reacted to become oxides and related complexes.
A comparative overview of the variation of powder components along the separation processes is presented in Figure 11. As we can see, a significant reduction in Fe, Ni and Cr oxides is noticed, occurring mainly after magnetic separation. The relative stationary percentages of Sn, Pb and Ag are not related to weak performance of the electrostatic separation, but to the fact that the remaining compounds after each separation step constituted a lower quantity of remaining powder.
3.4. X-ray Diffraction (XRD) Analysis of Disks
Analysis via X-ray diffraction (XRD) of sintered disks from the powders P1–P3 is presented in Figure 12, Figure 13 and Figure 14. The results largely confirmed the composition presented in Table 2, Table 3 and Table 4, obtained by spectrometry with X-ray fluorescence. After sintering, some new complex inorganic compounds were formed, based on some metallic ions. As the samples contained many elements, the peaks related to XRD analysis overlap, and therefore, the diffractograms are highly loaded.
P1 (Figure 12) and P2 (Figure 13) contained mixed compounds based mainly on Ca, Si, Pb, Sn, Cu and Fe. Three common crystallographic phases were clearly identified in these samples: Ca2SiO4, Ca2PbO4 and Sn (in a tetragonal crystallization system). P1 additionally contained a crystalline phase based on ZnMnFe (Zn0.41Mn0.5Fe1.83O4, 2θ = 35.2°). From the analysis of the diffractograms of P1 and P2, it can be seen that the peaks of sample P1 have a higher intensity than those for sample P2, a fact to be further confirmed by the calculation of the degree of crystallinity, as presented in Table 5.
As regards the disk from P3, Figure 14, the presence of carbon can be noticed, due to the larger quantity of polymeric residues, reduced to residual carbon, fixed within the inorganic complexes. Accordingly, a very high intensity of the C peak can be observed (2θ = 26.5°) in relation to the other peaks, explained by texturing (preferential orientation in the 0 0 4 direction). In addition, crystallographic phases related to some types of compounds based on Ca, Si, Sn, Fe, Ba, Br, etc., were highlighted.
Compared to P2 and P3, P1 (Figure 15) showed the highest degree of crystallinity (59%) (Table 5).
3.5. Evaluation of Vickers Hardness of Disks
Vickers hardness results are presented in Table 6 as an average of five measurements (eliminating the lowest and highest values), with a standard deviation under 5%.
The hardness seems to diminish with the increase in the concentration of the polymeric component. Accordingly, an increase in disks fragility was noticed when a lower percentage of metallic compounds was present.
The results are in line with and can be justified by the results from Section 3.2, this time referring to sintered powders. Sintered materials coming from powders with higher density present a higher hardness.
3.6. SEM Images and Evaluation of the Chemical Composition of Disks Carried Out by the Use of the EDS Probe
SEM images of samples with powders P1–P3 (at 5.000 magnitude) and related analysis (eight areas taken into account) are presented in Figure 16, Figure 17 and Figure 18. A reasonable homogeneity of the investigated areas was noticed, meaning that the powders are relatively uniform in composition, and the sintering process was fairly performed. On the other hand, by analyzing the SEM images, it is obvious that the opacity (darker color) and granule dimensions increase with the increase in the quantity of polymeric residues, e.g., in P2 and P3, i.e., due to the presence of more residual carbon, fixed within the inorganic complexes.
When analyzing the centralized values for the concentration of each element in Figure 16, Figure 17 and Figure 18, it is obvious that these results are in line with chemical analyses presented above, i.e., the concentrations of Fe, Cr and Ni are clearly diminishing, mainly by magnetic separation, when comparing the results for P1 with P2, and further with P3.
Analyzing the maximum values obtained for each component element of the P1–P3 disk samples for the areas taken into account, it can be observed, in Figure 16, Figure 17 and Figure 18:
−. The elements Al, Si, Ca, Cu, Mg, Fe, Cr, Ni, Sn and Pb were identified in all disk samples in all areas, e.g., Al (with a maximum percentage of 44.59% in the case of area 2 of P2, a value close to the maximum also being identified in the case of area 5 of P1, namely 44.52%); Si (with a maximum percentage of 47.42% in the case of area 2 of P3); Cr (with a maximum percentage of 18.9% in the case of area 3 of P1); Fe (with a maximum percentage of 68.09% in the case of area 5 of P1, and 62.37% in the case of area 7 of P1); Ni (with a maximum percentage of 19.96% in the case of area 2 of P1); and Cu (with a percentage of 9.98% in the case of area 1 of P3).
−. There are elements that appear only in limited areas: Ti, with a maximum percentage of 5.86% in the case of area 2 of P2; Zn, with a maximum percentage of 8.43% in the case of area 1 of P3; K, which appears only in the case of, e.g., area 1 of P2 with a percentage of 0.48%; Mn, which appears only in the case of, e.g., area 3 of P1 in a percentage of 0.85%; Al, which appears only in the case of, e.g., area 2 of P3 with a percentage of 0.63%; and Br, with a maximum percentage of 5.57% in the case of area 1 of P1.
−. The presence of precious metals is also found, such as the following: Ag, in all areas, with a minimum percentage of 0.59%, e.g., in P1—area 1, and a maximum percentage of 1.63% in P3—area 2; Pt, only in limited areas, with a maximum percentage of 0.89%, e.g., in the case of P1—area 2; and Au, in many areas, with a maximum percentage of 0.96%, e.g., in the case of P3—area 5.
3.7. Dielectric Tests
Dielectric analysis for the three types of disks is presented in Figure 19, Figure 20 and Figure 21.
The characteristics taken into account were dielectric permittivity, dielectric loss factor (tangent delta) and conductivity. The highest values of permittivity and loss factor were reached by P1, followed by P2. The variation in the dielectric characteristics with temperature for P1 is specific to the composites with interfacial–ionic polarization processes, i.e., an increase with temperature at lower frequencies, followed by a progressive decrease with temperature at higher frequencies, explained by the initial activation of polarization of orientation, followed by a saturation process (Figure 19). At temperatures over 85 °C, due to the agitation of electric charges, the permittivity increases again. As regards P2 and P3 (Figure 20 and Figure 21), the evolution of the dielectric loss factor is different from that of P1 due to the presence of more residues of polymeric components and an increased quantity of residual carbon, which is conductive matter. The polarization in this case is more interfacial–dipolar, and we noticed an important increase with temperature, especially at lower frequencies for the permittivity, where the interfacial effect is predominant. On the other hand, an increase at lower frequencies and a decrease at higher frequencies was noticed at higher temperatures for the dielectric loss factor, an aspect that confirms the influence of dipolar polarization, mainly for P3. As regards the conductivity values, they are not so different between the three samples, but P3 presents slightly higher values due to the presence of a larger content of residual carbon.
Taking into account all the results presented above, especially the values of the dielectric parameters, one can notice the economic value of such powders as additives in composites for electromagnetic shielding purposes. They can efficiently substitute scarce raw materials actually used as additives in composites, coatings and paints, mainly due to their high permittivity (above 7.5 for P1 and above 6 for P2 and P3, in all frequency domains) and dielectric loss factor (above 0.2 in all cases, in all frequency domains).
End-of-life mobile phones, smart devices, laptops and tablets constitute one of the fastest growing electrical and electronic equipment waste streams in the world (e.g., about 5.3 billion mobile phones became waste in 2022 only). EC recommended new strategies for improving the rate of return and recycling of used and waste mobile phones, tablets and laptops [21]. Unfortunately, the actual circuit board processing from WEEE, [22,23,24] is still not selective, i.e., is performed only for precious metal recovery by melting entire PCBs or chemical treatment/leaching in acid solutions, all of which are polluting technologies that lead to a large quantity of non-recyclable waste, under which circumstances many valuable components are lost. In [25], a brief calculus related to the global WEEE recycling estimated an increase of 3 million new job opportunities per year under the circumstances that the environmental load (i.e., the cost required to offset the environmental impacts) was estimated up to 9 USD/kg, so new approaches must be taken into account aimed at reducing the related carbon emissions.
It was estimated that between 2014 and 2020, embodied GHG emissions generated from information and communications technology (ICT) devices increased by 53%, up to 580 million metric tons (MMT) of CO2 emitted in 2020. That is why the purpose of the research was also to outline the technological evolution in the domain of ICT devices. This presumes less metal use, better integration of electronic components and other technologies for connecting (eventually soldering) the electronic parts. The way we approached the PCB selection and primary dismantling of some classical components, along with ferritic and metallic powders separation, is in line with newer electronic technologies, which progressively eliminates such components. The importance of integrating in fabrication circuits made of such powders with special electromagnetic features lies also in the fact that statistics show that nowadays, the recovery of electronic parts comprises about 20% of WEEE, and recovered metals from WEEE constitute under 9%, from which only 3–4% come from electronic parts (wt%), even if the recovery rate is high, as presented above. Consequently, over 90% of electronic part scrap represents the origin of the powder studied in the paper, up to now destined to be dumped.
The technology described in this paper allows, beyond the classical recovery of useful metals, here with an average recovery rate of over 97% (e.g., in line with the data presented in [26]), a potential exploitation of remaining powder as raw material for sintered electromagnetic devices. The use of such semiconducting fillers for, e.g., hybrid materials for electromagnetic shielding—in fact, the use of electromagnetically active inorganic matters—is meeting rising interest nowadays [27]. According to the achieved features of the sintered discs from the P3 residual powder, described above, mainly the dielectric ones, the following applications may become possible, to be fabricated by spark plasma sintering: hard composites with high electromagnetic shielding efficiency, as in [28]; electromagnetic interference and RFID absorbers and related gaskets, as in [29,30]; microwave absorbers, as in [31]; radar emission absorbers, as in [32]; high-k dielectric devices; high-Q dielectric resonators; and other related devices for electromagnetic applications.
Another important application of the powders is as ingredients/pigments in electromagnetic shielding paints and primers, due mainly to the actual pressure of reducing the quantity of scarce metal powders (mainly Ag, Cu and Ni), nanocarbon or ferrite powders, which are common ingredients in such paints [33]. A homologue application, but with lower technical impact, related to electromagnetic shielding systems in building areas, is presented in [34], along with a preliminary life cycle analysis.
The recent literature generally addresses the environmental impact assessment of WEEE and management strategies towards industrially integrating WEEE [35,36,37], but without offering global sustainable solutions, other than metal recovery, as in [24,25,38], or eventually waste plastic recovery, as in [34,39]. The application of circular economy practices in WEEE management represents a new scientific trend, as presented, e.g., in [40,41,42,43,44,45,46], but in all actual publications, the concept is described too generally, without a clear example of technological circuits to be recommended, and practically recommending sending non-metallic residual powders to dumps after metal extraction.
We present below a practical analysis of the technological sustainability of the studied powders derived from WEEE, in terms of a circular economy [47]. The final purpose would be “Zero Waste Recycling of PCBs”, a concept preliminary described in [48]. The principle of this concept is shown in Figure 22.
The technological stages follow the classical recycling circuit, i.e., electronic waste collection and dismantling (of large components, metallic parts as radiators, etc., coils/transformers, cables, etc.) till the remaining PCBs have mostly small electronic components, first-stage shredding of PCBs (with separation of large items without components—sent to preliminary recovery of metals such as Cu, Sn, Pb, and Al), second-stage shredding of PCBs to create a powder, described as P1 in this paper; magnetic separation of the powder, described as P2 in this paper (resulting in a mixture of Fe, Ni and magnetic powders as metallic oxides); and electrostatic separation of the residual powder, described as P3 in this paper (resulting in a mixture of metals such as Cu, Sn, Pb, Ag, and Au). In classical technologies, the residual powder is sent to a dump, but in our case, it follows the concept of the circular economy, being used for innovative high-value products: as ingredients for paints with electrical applications, or for advanced hard components with specialized electromagnetic features after a sintering process. These paints may follow the already-established recycling process under the circular economy concept, as described in [49]. The hard components are self-recyclable by the same technology, being suitable for specialized shredding processes and reintegrated within residual powder, because they have the same composition. In this way, the concept is ‘cradle to cradle’ and tends to zero-waste recycling within a closed-loop system in order to optimize resource efficiency. The proposed model of re-using metal and non-metal fractions of PCBs may lead to new business models for residual powder integration within electromagnetic components technologies, and new markets for the respective components. Benefits of the proposed recycling scheme include financial and environmental gains, since large quantities of powders do not go to landfills, and a reduced chemical pollution of WEEE, initially leading to harming soil and aquatic life.
When producing new electromagnetic shielding systems from powders from recycled WEEE components, only 10% of the original CO2 emissions are released in the process. Accordingly, we estimate that the technology described in the paper could be able to reduce, by a minimum of 15%, the embodied GHG emissions generated from ICT devices by advanced recycling under the circular economy concept, and reduce by 90% the carbon footprint related to the processing of inorganic ingredients to be used in composites or paints for electromagnetic shielding purposes, without speaking about their economic benefit, under the circumstances that the powders obtained by recycling are at least 10 times cheaper than virgin raw materials.
By tending to the desiderates of “no net emissions of greenhouse gases by 2050” and of “economic growth decoupled from scarce resource use”, the proposed technology is also in line with The European Green Deal strategy [50].
4. Conclusions
This paper presents a technological approach for obtaining electromagnetically active powders and related sintered components by the selective recycling of electronic PCB waste, in particular scrap electronic components, in line with actual electronic technology and under the circular economy concept.
PCB scraps were submitted to a succession of grinding processes, followed by progressive magnetic and electrostatic separation, resulting in two final fractions: metallic particles (with residues of ferritic powders) and non-magnetic/non-metallic particles including different metallic oxides. Three types of powders were analyzed, i.e., powder after fine grinding, after magnetic separation and after electrostatic separation. Finally, the powder material was processed within a spark plasma sintering furnace in order to obtain solid samples with a diameter of 12 mm and a thickness between 2 and 3 mm.
The results of the thermal analysis outlined for all samples specific decomposition processes, at temperatures depending on the nature of the residual metals/metallic oxides.
The chemical analysis of powders, via spectrometry with X-ray fluorescence (XRF), emphasized the presence of a mixture of metal oxides (mainly CaO, Fe2O3, CuO, Cr2O3, SiO2, SnO2, NiO, ZrO2, PbO, and ZnO) and traces of metals (mainly Ag), with concentrations diminishing along with the purification process, from 98.32% after fine grinding till 78,65% after electrostatic separation.
The EDS analysis revealed that there was a relatively uniform composition of disks in all analyzed areas, the sintering process being effectively performed. By analyzing the SEM images, it is noticed that the opacity (darker color) and granule dimensions increase with the increase in the quantity of polymeric residues, mainly in the samples from powders after electrostatic separation, due to the presence of more residual carbon, fixed within the inorganic complexes.
The most important analysis was related to dielectric parameters, mainly permittivity and loss factor. It was concluded that the powders obtained by the proposed technology could efficiently substitute scarce raw materials actually used as additives in composites, coatings and paints, mainly due to their high permittivity (above 6 in all frequency domains) and dielectric loss factor (above 0.2 in all cases, in all frequency domains).
The technology described in the paper allows the recovery of precious metals, but also allows the recovery of powders with special electromagnetic features, under the circumstances that electronic parts in all comprise about 20%, and recovered metals from WEEE comprise under 9%, from which only 3–4% is from the electronic parts (wt%). Accordingly, we estimate that the technology described in this paper is a sustainable one, as it could be able to reduce by a minimum of 15% the embodied GHG emissions generated from ICT devices by advanced recycling under the circular economy concept, and reduce by 90% the carbon footprint related to the processing of inorganic ingredients to be used in composites or paints for electromagnetic shielding purposes, without speaking about their economic benefit, under the circumstances that the powders obtained by recycling are at least 10 times cheaper than the homologous virgin raw materials.
Conceptualization, C.S.; methodology, R.C.C., C.S. and M.A.; validation, R.C.C., C.S. and M.A.; formal analysis, M.A. and C.S.; investigation, R.C.C., C.S. and M.A.; data curation, R.C.C., C.S. and M.A.; writing—original draft preparation, C.S. and R.C.C.; writing—review and editing, C.S.; visualization, R.C.C., C.S. and M.A.; supervision, C.S. and R.C.C. All authors have read and agreed to the published version of the manuscript.
The data are included in this study.
The authors declare no conflicts of interest.
Footnotes
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Enlarge this image.Figure 1. Technological flow for obtaining non-metallic powder from processed PCBs.
Enlarge this image.Figure 2. Image of non-metallic powder/fraction with particle size of up to 10 microns.
Enlarge this image.Figure 3. Spark plasma sintering equipment and template with heating and cooling zones.
Hydrostatic density of powders.
| Sample | ρ [g/cm3] |
|---|---|
| P1 | 0.6630 |
| P2 | 0.6338 |
| P3 | 0.6242 |
Composition of P1.
| Element | Concentration (%) |
|---|---|
| Fe | 17.96 |
| Si | 17.62 |
| Br | 17.21 |
| Cu | 13.46 |
| Ca | 8.91 |
| Pb | 8.28 |
| Sn | 7.37 |
| Ba | 3.20 |
| Ni | 1.76 |
| Zr | 1.55 |
| Zn | 1.51 |
| Cr | 1.17 |
Composition of powder P2.
| Element | Concentration (%) |
|---|---|
| Ca | 25.16 |
| Fe | 15.72 |
| Cu | 13.91 |
| Si | 12.69 |
| Sn | 7.93 |
| Pb | 7.68 |
| Ba | 3.39 |
| Br | 3.10 |
| Cr | 2.68 |
| Zn | 2.61 |
| Mn | 1.88 |
| Ni | 1.57 |
| Zr | 0.69 |
| Sr | 0.61 |
| Ag | 0.38 |
Composition of P3.
| Element | Concentration (%) |
|---|---|
| Ca | 27.19 |
| Cu | 14.55 |
| Si | 13.43 |
| Sn | 9.93 |
| Fe | 9.30 |
| Pb | 8.84 |
| Ba | 3.80 |
| Br | 3.42 |
| Cr | 3.07 |
| Zn | 2.31 |
| Ni | 1.59 |
| Zr | 0.88 |
| Sr | 0.70 |
| Ag | 0.49 |
| K | 0.36 |
| Mn | 0.14 |
Crystallinity degree of samples.
| Sample | Crystallinity Degree [%] |
|---|---|
| P1 | 59 |
| P2 | 35.4 |
| P3 | 16.8 |
Vickers hardness results.
| Sample | HV 0.1/10 [kgf/mm2] |
|---|---|
| P1 | 15.91 |
| P2 | 13.64 |
| P3 | 11.28 |
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