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Recovery of Metals from Printed Circuit Boards By Means of Electrostatic Separation

Recovery of Metals from Printed Circuit Boards By Means of Electrostatic Separation Without the use of appropriate recycling technologies, the growing amount of electronic waste in the world can be a threat to the development of new technologies, and in the case of improper waste management, may have a negative impact on the environment. This is due to the fact that this waste contains large amounts of valuable metals and toxic polymers. Therefore, it should be recycled in accordance with the assumptions of the circular economy. The methods of mechanical recovery of metals from electronic waste, including printed circuits, may be widely used in the future by waste management companies as well as metal production and processing com- panies. That is why, a well-known and easily applicable electrostatic separation (ES) method was used to recover metals from printed circuit boards. The grain class of 0.32 - 0.10 mm, obtained after grinding the boards, was fed to a separator. Feed and separation products were analyzed by means of ICP-AES, SEM/EDS and XRD. The con- centrate yield obtained after electrostatic separation amounted to 32.3% of the feed. Its density was 11.1 g/cc. Out of the 91.44% elements identified in the concentrate, over 90% were metals. XRD, SEM observations and EDS analysis confirmed the presence of non-metallic materials in the concentrate. This relatively high content of im- purities indicates the need to grind printed circuit board into grain classes smaller than 0.32-0.10 mm. Key words: electrostatic separation, metals recovery, PCB, SEM, XRD INTRODUCTION In order to obtain accurate test results and eliminate po- The production of Waste Electrical and Electronic Equi- tential measurement errors, the following analysis met- pment (WEEE) is growing at an alarming rate. In 2016, hods were used: X-ray Powder Diffraction (XRD), Scanning 44.7 million metric tonnes of WEEE were generated, but Electron Microscopy (SEM) with the Energy Dispersive is expected to increase to 55 million metric tonnes by Spectroscopy (EDS) system and Inductively Coupled Pla- 2021 [5, 25]. People can process them, degrading the sma atomic emission spectroscopy (ICP-AES). As a result environment to a greater or lesser extent [24]. Effective of the tests, non-metallic and metallic parts were separa- management of WEEE has become a global problem, be- ted from PCB. cause in the event of improper management and recyc- ling, they can have a significantly impact on the environ- LITERATURE REVIEW ment. The basic element of the construction of most WEEE are Considering environmental protection, depleting of metal PCB which contain about 70% of non-metallic parts, such deposits and economic benefit, environmentally friendly as fiberglass, epoxy resin, polyester, woven glass, as well and high-efficiency methods of recovering metals from as 30% of metallic parts [2]. It is difficult to determine the printed circuit boards (PCB) should be sought. Basically, type and amount of metals in PCB. It can be estimated the methods of recovering metals from PCB are divided that a PCB contains about 16% Cu, 3% Fe, 3% Sn, 2% Pb, into physical and chemical [15]. Since chemical methods 1% Zn 0.05% Au, 0.03% Ag, 0.01% Pd and others metals usually have a negative impact on the environment, the such as Cr, Na, Cd, Mo, Ti, Co [26, 27]. authors of the study focused on one of the physical met- In ES, grains placed in an electric field are separated as a hods, i.e. electrostatic separation (ES) [15, 23, 30]. result of differences in the ability to accumulate electric The aim of the article was to assess the efficiency of metal charges on grain surfaces [9]. The scheme of the electro- recovery from PCB using ES. The article contains the re- static drum separator used in the study is shown in Fig. 1. sults of the tests on the recovery of metals from grinded PCB with a grain size of 0.1-0.32 mm, using an ES. 214 Management Systems in Production Engineering 2020, Volume 28, Issue 4 According to the authors, Kozłowski et al. and Franke and Suponik, grinding can be carried out in a knife mill [6, 11]. Table 1 Densities and electrical properties of selected metals and plastics Density, Electrical conductivity, Material 6 -1 -1 g/cc 10 Ω m Gold Au 19.30 44.35 Lead Pb 11.30 4.74 Silver Ag 10.50 61.84 Copper Cu 8.96 58.41 Iron Fe 7.87 10.13 Silicone Si 2.33 0.04 Density, Electrical resistivity, Material g/cc 10 Ω m Fiberglass rein- FRP 1.80-2.00 10 forced plastics PET vs. Polyesters 1.31-1.39 1-1.4 × 10 PBT Polypropylene PP 0.90 10 Source: [3, 21, 28]. Fig. 1 Scheme of electrostatic drum separator: METHODS 1 – feed container, 2 – vibrating feeder, 3 – electrode, 4 – drum, Preparation for electrostatic separation 5 – brush, 6 – partition, 7 – conductors container (concentrate), PCB from personal computers, hard disks, graphic cards 8 – non-conductors container (waste), 9 – grains with good and RAMs were used in this study. The way of preparing electrical conductivity, 10 – complex grains folded with metals and grinding electronic waste is presented in the paper and non-metals, 11 – grains with weak electrical conductivity written by Franke and Suponik [6]. The knife mill manu- factured by TESTCHEM was used to grind the PCB. The ro- Placing the grain that has accumulated electric charge in tation speed of mill was 2815 rpm. The blades used were the electric field induces the electric field force. The value made of hardened steel and perforated sieve with a mesh of the resultant force depends on the value of the electric size of 2 mm. Four grain classes were obtained from the field force in which the grain is located. The surface elec- grinded material: 2.00-0.56 mm, 0.56-0.32 mm, 0.32-0.10 tric charge is generated on the surface of any material, mm and < 0.10 mm. The grain class of 0.32-0.10 mm was and depends on time and the type of material. Materials 40% of the total. This was a feed for the electrostatic se- with high electrical conductivity (metals) quickly get rid of parator. Results for the grain class of 0.56-0.32 were pre- the accumulated electrical charge [9]. However, the elec- sented in the paper by Franke and Suponik [6]. So far, re- trostatic force is not the only one acting on the grain du- maining grain classes have not been tested for the follo- ring the separation process. There are also (in the electro- wing reasons: in the grain class of 2.00-0.56 mm there static drum separator): gravity force, image forces and were significant connections of metals with non-metals centrifugal force. The resultant force acting on well-con- parts that reduce the purity of the concentrate, while for ductive grains is directed outwards, contrary to grains grains lower than 0.1 mm, the damage of electrode trigge- with low conductivity (non-metals) [1]. red by high risk of spark discharge [16] can occurred. In Consequently, the performance of the electrostatic drum addition, the aggregation effect may appear for this class, separator is mainly dependent on the electrical conduc- which may also affect the efficiency of separation [13, 14]. tivity of the grain, as well as the grain size and its density However, despite this, it is planned that the efficiency of [9]. Electrical conductivity of selected metals, the values electrostatic separation will be tested for grain size < 0.1 of electrical resistance of plastic materials, and their den- mm. sities are shown in Table 1 Based on the experimental research carried out by the au- Electrostatic separation thors of the paper and the literature review, it can be The drum separator used in the study allows to change concluded that purity of the concentrate is most impacted three operating parameters. As a result of the experimen- by the size of grain. According to Niu et al, Dascalescu et tal research, the following parameters were used: shaft al. and Hogzhou, changes in parameters such as voltage rotation speed 100 rpm, electrical voltage at the electrode and rotational speed do not significantly affect the purity 17 kV and distance of the electrode from the shaft 0.03 m. of the concentrate [4, 18, 19]. That is why the choice of the method and device for crushing PCB is very important. Plastics Metals D. FRANKE et al. – Recovery of Metals from Printed Circuit Boards… 215 Product analysis Table 3 Elemental concentrations in the feed and in ES products: The feed and products obtained from ES were digested A – this study, B – study by Guo et al. for a similar grain class and the concentrations of the elements were measured [10], “-” no data with the JY 2000 spectrometer (by Yobin-Yvon) using the Content of the element [%] in ICP-AES method. The source of induction was a plasma Element Feed Concentrate Waste torch coupled with a frequency generator of 40.68 MHz. A B A B A B Furthermore in the feed, concentrate and waste phase Al 3.33 1.51 1.89 2.63 0 0.93 composition have been determined on the basis of the X- Si 15.6 - 5.15 - 0.0989 - ray diffraction measurements, performed with the Pana- K 0.0589 - 0.00980 - 0 - lytical X’Pert Pro MPD diffractometer, utilizing filtered ra- Ca 8.99 - 1.11 - 0.0095 - diation of a copper-anode lamp (λKα 0.154 nm). The Mg 0.0045 - 0.00890 1.23 0.00055 0.28 diffraction lines were recorded in the Bragg-Brentano Mn 0.0355 - 0.10 - 0 - geometry, using the step-scanning method by means of a Fe 0.3821 1.38 0.93 3.74 0 0.19 Ni 0.185 0.28 0.85 0.75 0 0.039 PIXcell 3D detector on the diffracted beam axis, in the an- Cu 19.5 27.08 59.70 72.81 1.22 3.99 gle range from 20-95° [20] (1 step 0.05°, count time per Zn 0.25 0.79 1.09 2.12 0 0.11 step 120 s). The diffractograms obtained were analyzed Br 13.8 - 2.98 - 0.00055 - with the use of Panalytical High Score Plus software with Ag 0.1415 0.0019 0.4996 - 0 - the PAN-ICSD database. Au 0.0019 0.0069 0.0101 - 0 - The morphology of the feed and products from ES, as well Sn 2.38 3.23 7.83 9.63 0.0045 0.01 as the chemical composition in microareas, were analyzed Ba 2.2 - 1.27 - 0.0075 - by means of the Zeiss Supra 35 high resolution electron Pb 1.95 2.44 8.00 9.63 0 0.12 microscope, equipped with EDAX EDS chemical analysis Totality based on this study 68.81 91.44 1.34 system. (A) Totality based RESULTS AND DISCUSSION on study by 36.72 99.99 5.65 As a result of ES, the grinded PCB with grain size of 0.32- Guo et al. (B) 0.10 mm were separated into concentrate and waste. The concentrate was about 1/3 of the mass of the tested sam- An example of connection of metal parts with plastics is ple (Table 2), what confirms the average metal content in shown in Fig. 4, while Table 4 presents the results of the PCB ranging from 20% to 40%, assessed by authors such chemical analysis. On the other hand, the non-metallic as Kumar et al., Bizzo et al., Burat et al. and Wu et al. [8, elements could have penetrated into the concentrate as 17, 26, 27]. The waste was 2/3 of the mass. A high concen- a result of imperfections in the separation process. This trate density of about 11 g/cc indicates high separation issue should be checked in further studies. A similar pro- efficiency, while waste density of 3 g/cc may indicate the blem concerned waste. Over 1% of copper was found in penetration of metal parts into the waste. The analysis of this group of products. Probably, the reason for contami- the ferromagnetic content shows that the waste did not nation by copper was the layered construction of the PCB. contain ferromagnetic parts, in contrast to the concen- According to Tatariants et al. and LaDou, some very thin trate, which had the ferromagnetic content of 0.3% (see elements consists of several layers, and the segments re- Table 2). sponsible for connecting them together are often made of copper [12, 20]. It can be assumed that, if the PCB were Table 2 grinded to smaller fractions, this element would not pe- The results of ES netrate into non-metals. Content Density Yield Guo et al. [10] (see Table 3) received a cleaner concen- of ferromagnetics Product of product, of product, trate from the ES of a similar grain class. But in their ana- in product, g/cc % lyzes, they did not take into account such elements as Si, Feed 5.4 - 0 Ca, Br, Ba neither in feed nor in the product of ES. Waste 3.0 67.7 0 The creation of a semiproduct chamber in the electrosta- Concentrate 11.1 32.3 0.3 tic separator can improve the efficiency of metal recovery. Metals mechanically bonded to plastics or glass can be fo- The results of measurements carried out in the ICP-AES of und in this product. They could be ground again to sepa- the feed, concentrate and waste products are presented rate metals from plastics. Then this product could be se- in Table 3. parated again. Out of the 91.44% elements identified in the concentrate, The concentrate contained the following valuable metals: over 90% were metals. Si and Br content was over 8%. Cu, Pb, Sn, Al, Zn, Ni, Ag, Au. The amount of the metals They form a lead-barium borosilicate glass on PCB. This identified depends on the date of production, the manu- relatively high content of impurities indicates that PCB ne- facturer or the quality of the PCB and the type of the com- eds to be ground into grain classes smaller than 0.32-0.10 ponents used [22]. As provided by Bizzo et al., over the mm. In this way, metals would be free of impurities. These years PCB have had various metal contents i.e. Cu 12-28%, elements were probably mechanically bonded to metals. 216 Management Systems in Production Engineering 2020, Volume 28, Issue 4 Al 1.7-7%, Pb 1-3%, Zn 0.08-2.7%, Ag 79-3300 ppm, Au 29- The SEM analysis of the concentrate (Fig. 3 and 4) showed 11200 ppm [27]. the presence of mainly metal particles with a small amo- To determine the morphology of the feed and products unt of non-metallic materials, such as glass fiber, poly- obtained from the ES, SEM observations and chemical mers, and ceramics, which were not separated from the analysis in micro-regions, by means of energy-dispersive metallic particles in the milling process. These metal par- X-ray spectroscopy (EDS) were performed. Imaging of the ticles with various geometry and dimensions approx. 300- tested samples using the backscatter electron detection 400 μm (a few particles of the order of 800 μm were also technique (QBSD) (Fig. 2 and 3), allowed to investigate the observed) were characterized by different chemical com- morphology. position, even within one particle, which was demonstra- ted by means of the chemical composition analysis in mi- cro-areas (Fig. 4 and Tab. 4). Fig. 2 Image of the feed (QBSD SEM) Fig. 3 Image of the concentrate from ES (QBSD SEM) The contrast obtained in these pictures is a result of diffe- rences in the chemical composition. The areas containing elements with a high atomic number are clearly brighter compared to the areas consisting of lower Z-number ele- ments. In the tested feed sample (Fig. 2), both metallic particles of various shapes and dimensions mostly in the range of 100 to 400 μm, as well as many fragments of non- metallic fibers and particles, were observed. In many ca- ses, these non-metallic particles are bonded with metal, which may be due to the PCB production process, in which thin films of good electrical conductivity metals (mainly Cu and Sn, Au, Ag, Pt) are applied on a glass fiber and epoxy laminate [7, 29, 31]. This may create difficulties in the ES Fig. 4 Images of the concentrate obtained from ES with marked process, leading to "contamination" of the metallic pro- points of chemical microanalysis duct with non-metallic particles. D. FRANKE et al. – Recovery of Metals from Printed Circuit Boards… 217 Table 4 The results of the XRD (qualitative phase analysis) of the Results of chemical composition microanalysis for points feed and concentrate and waste products are presented shown in Figure 4 in Fig. 5. For the feed sample, diffraction lines from metal- Point of analysis/Concentration [% at.] lic phases (Cu, Sn, Pb, CuSn) and oxides phases SiO and BaO were recorded. The same phases were indicated in the waste sample, while the intensity of lines obtained 1 2 3 4 5 6 7 8 9 from metallic phases significantly decreased, which indi- cates a much lower volume share of these phases. It can be assumed, that these are mainly the residues of small Cu 9.71 49.43 38.47 83.39 88.37 38.62 - - 3.84 metal fragments which, combined with larger non-metal- Sn 16.76 0.73 3.04 - - - 100 - 1.07 lic particles of PCB, got into the waste during the separa- tion process. On the diffractogram obtained from the con- Ni 5.19 30.20 - - - 2.71 - 37.59 - centrate sample, only the diffraction lines from Cu, Sn, Pb, Au 68.35 2.65 - - - - - - - CuSn metallic phases were identified. However, the pre- sence of other metallic phases in a lower volume share O - 5.25 30.41 11.66 - 25.23 - - 38.19 being under detection limit cannot be excluded, as well as with this method it is difficult to identify the small amo- Al - 7.05 23.81 3.56 - 29.28 - - 22.33 unts of amorphous phases (polymers, glass). Si - 2.69 4.27 1.39 - 0.7 - 2.24 26.55 CONCLUSION Pb - 2.01 - - 11.63 - - - - As a result of the research analysis, it can be concluded that the products obtained from the ES were contamina- Ti - - - - - 1.56 - - 0.38 ted. Based on the ICP analysis, approximately 91% of me- P - - - - - 0.81 - - - tals were identified in the concentrate. These were Cu, in the largest amount (ca. 60%), and then Pb , Sn, Si, Br, Al, K - - - - - 0.5 - - - Ba, Ca, Zn and small amounts of Fe, Ni, Ag, Mn, Au, K and Mo - - - - - - - 0.72 1.28 Mg. It can be assumed that the maximum of 9% of the mass was contaminated. The EDS analysis, as well as the Ag - - - - - - - 1.45 - ICP-AES, confirmed appearance of these elements: Cu, Sn, Ni, Au, Al, Si, Pb, K, Ag, Mn, Fe and Br. Quantitative analy- Mn - - - - - - - 0.67 - sis was difficult to perform for both methods. The authors Fe - - - - - - - 57.35 - used a larger amount of material in ICP than in EDS, in which only microscopic survey was carried out. The XRD Br - - - - - - - - 5.28 analysis revealed that the concentrate contained mainly Cu, Sn, Pb, CuSn metallic phases, as well as small amounts of oxides phases such as SiO2 and BaO. Fig. 5 X-ray diffraction patterns of feed (blue line), concentrate (red line) and waste (green line) Element 218 Management Systems in Production Engineering 2020, Volume 28, Issue 4 [10] J. Guo, J. Guo, Z. Xu. “Recycling of non-metallic fractions The SEM analysis of the concentrate showed the presence from waste printed circuit boards: A review.” Journal of of mainly metal particles with a small amount of non-me- Hazardous materials, vol. 168(2-3), pp. 567-590, 2009. tallic materials, such as glass fiber, polymers, and cera- [11] J. Kozłowski, W. Mikłasz, D. Lewandowski and H. Czyżyk, mics, which were not separated from the metallic partic- “Research on hazardous waste - management part I”, Ar- les in the milling process. These metal particles, with va- chives of Waste Management and Environmental Protec- rious geometry and dimensions, were characterized by tion, vol. 15, no. 2, pp. 69-76, 2013. different chemical compositions, even within a single par- [12] J. LaDou. “Printed circuit board industry” International Jo- ticle. urnal of Hygiene and Environmental Health, vol.209 (3), The analyzes of the waste indicated that the small amo- pp. 211-219, 2006. [13] J. Li, Q. Zhou, Z. Xu, “Real-time monitoring system for im- unts of metallic phases were in the waste sample. They proving corona electrostatic separation in the process of were mainly Cu (ca. 1%) but also Ca, Mg, Sn, Ba in smaller recovering waste printed circuit boards”, Waste Manag quantities. Presumably, they were mainly the residues of Res, vol. 32, no. 12, pp. 1227-1234, 2014. small metal fragments which, combined with larger non- [14] J. Li, Z. Xu, and Y. Zhou, “Application of corona discharge metallic particles of PCB, got into the waste during the se- and electrostatic force to separate metals and nonmetals paration process. from crushed particles of waste printed circuit boards”, Jo- In conclusion, the results of the research confirmed that urnal of Electrostatics, vol. 65, no. 4, pp. 233-238, 2007. the efficiency of metal recovery for the grain class of 0.32- [15] J. Sohaili, S.K. Muniyandi, S.S. Mohamad. “A review on 0.10 mm was still insufficient. It is reasonable to optimize printed circuit board recycling technology.” Journal of Emerging Trends in Engineering and Applied Sciences, vol. the separation process for significantly smaller grains in 3(1), pp. 12-18, 2012. subsequent works. Consideration should also be given to [16] J. Wu, J. Li, and Z. Xu, “Electrostatic Separation for Re- extending the separator with an additional receiver for covering Metals and Nonmetals from Waste Printed semi products, i.e. for grains containing both metals and Circuit Board: Problems and Improvements”, Environ. Sci. non-metals. Technol., vol. 42, no. 14, pp. 5272-5276, 2008. [17] J. Wu, J. Li, Z. Xu. “Electrostatic separation for multi-size REFERENCES granule of crushed printed circuit board waste using two- [1] A. Cieśla, W. Kraszewski, M. Skowron, A. Surowiak, P. 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Dawid Franke ORCID ID: 0000-0002-5522-6889 Silesian University of Technology Faculty of Mining, Safety Engineering and Industrial Automation Akademicka Str., 44-100 Gliwice, Poland e-mail: dawid.franke@polsl.pl Tomasz Suponik ORCID ID: 0000-0002-4251-4275 Silesian University of Technology Faculty of Mining, Safety Engineering and Industrial Automation Akademicka Str., 44-100 Gliwice, Poland e-mail: tomasz.suponik@polsl.pl Paweł M. Nuckowski ORCID ID: 0000-0002-2606-0525 Silesian University of Technology Faculty of Mechanical Engineering Department of Engineering Materials and Biomaterials 18A Konarskiego Str., 44-100 Gliwice, Poland e-mail: pawel.nuckowski@polsl.pl Klaudiusz Gołombek ORCID ID: 0000-0001-5188-1950 Silesian University of Technology Faculty of Mechanical Engineering Department of Engineering Materials and Biomaterials 18A Konarskiego Str., 44-100 Gliwice, Poland e-mail: klaudiusz.golombek@polsl.pl Kamila Hyra ORCID ID: 0000-0002-9533-0066 Silesian University of Technology Faculty of Mechanical Engineering Department of Engineering Materials and Biomaterials 18A Konarskiego Str., 44-100 Gliwice, Poland e-mail: kamilahyra.65@gmail.com http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Management Systems in Production Engineering de Gruyter

Recovery of Metals from Printed Circuit Boards By Means of Electrostatic Separation

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Abstract

Without the use of appropriate recycling technologies, the growing amount of electronic waste in the world can be a threat to the development of new technologies, and in the case of improper waste management, may have a negative impact on the environment. This is due to the fact that this waste contains large amounts of valuable metals and toxic polymers. Therefore, it should be recycled in accordance with the assumptions of the circular economy. The methods of mechanical recovery of metals from electronic waste, including printed circuits, may be widely used in the future by waste management companies as well as metal production and processing com- panies. That is why, a well-known and easily applicable electrostatic separation (ES) method was used to recover metals from printed circuit boards. The grain class of 0.32 - 0.10 mm, obtained after grinding the boards, was fed to a separator. Feed and separation products were analyzed by means of ICP-AES, SEM/EDS and XRD. The con- centrate yield obtained after electrostatic separation amounted to 32.3% of the feed. Its density was 11.1 g/cc. Out of the 91.44% elements identified in the concentrate, over 90% were metals. XRD, SEM observations and EDS analysis confirmed the presence of non-metallic materials in the concentrate. This relatively high content of im- purities indicates the need to grind printed circuit board into grain classes smaller than 0.32-0.10 mm. Key words: electrostatic separation, metals recovery, PCB, SEM, XRD INTRODUCTION In order to obtain accurate test results and eliminate po- The production of Waste Electrical and Electronic Equi- tential measurement errors, the following analysis met- pment (WEEE) is growing at an alarming rate. In 2016, hods were used: X-ray Powder Diffraction (XRD), Scanning 44.7 million metric tonnes of WEEE were generated, but Electron Microscopy (SEM) with the Energy Dispersive is expected to increase to 55 million metric tonnes by Spectroscopy (EDS) system and Inductively Coupled Pla- 2021 [5, 25]. People can process them, degrading the sma atomic emission spectroscopy (ICP-AES). As a result environment to a greater or lesser extent [24]. Effective of the tests, non-metallic and metallic parts were separa- management of WEEE has become a global problem, be- ted from PCB. cause in the event of improper management and recyc- ling, they can have a significantly impact on the environ- LITERATURE REVIEW ment. The basic element of the construction of most WEEE are Considering environmental protection, depleting of metal PCB which contain about 70% of non-metallic parts, such deposits and economic benefit, environmentally friendly as fiberglass, epoxy resin, polyester, woven glass, as well and high-efficiency methods of recovering metals from as 30% of metallic parts [2]. It is difficult to determine the printed circuit boards (PCB) should be sought. Basically, type and amount of metals in PCB. It can be estimated the methods of recovering metals from PCB are divided that a PCB contains about 16% Cu, 3% Fe, 3% Sn, 2% Pb, into physical and chemical [15]. Since chemical methods 1% Zn 0.05% Au, 0.03% Ag, 0.01% Pd and others metals usually have a negative impact on the environment, the such as Cr, Na, Cd, Mo, Ti, Co [26, 27]. authors of the study focused on one of the physical met- In ES, grains placed in an electric field are separated as a hods, i.e. electrostatic separation (ES) [15, 23, 30]. result of differences in the ability to accumulate electric The aim of the article was to assess the efficiency of metal charges on grain surfaces [9]. The scheme of the electro- recovery from PCB using ES. The article contains the re- static drum separator used in the study is shown in Fig. 1. sults of the tests on the recovery of metals from grinded PCB with a grain size of 0.1-0.32 mm, using an ES. 214 Management Systems in Production Engineering 2020, Volume 28, Issue 4 According to the authors, Kozłowski et al. and Franke and Suponik, grinding can be carried out in a knife mill [6, 11]. Table 1 Densities and electrical properties of selected metals and plastics Density, Electrical conductivity, Material 6 -1 -1 g/cc 10 Ω m Gold Au 19.30 44.35 Lead Pb 11.30 4.74 Silver Ag 10.50 61.84 Copper Cu 8.96 58.41 Iron Fe 7.87 10.13 Silicone Si 2.33 0.04 Density, Electrical resistivity, Material g/cc 10 Ω m Fiberglass rein- FRP 1.80-2.00 10 forced plastics PET vs. Polyesters 1.31-1.39 1-1.4 × 10 PBT Polypropylene PP 0.90 10 Source: [3, 21, 28]. Fig. 1 Scheme of electrostatic drum separator: METHODS 1 – feed container, 2 – vibrating feeder, 3 – electrode, 4 – drum, Preparation for electrostatic separation 5 – brush, 6 – partition, 7 – conductors container (concentrate), PCB from personal computers, hard disks, graphic cards 8 – non-conductors container (waste), 9 – grains with good and RAMs were used in this study. The way of preparing electrical conductivity, 10 – complex grains folded with metals and grinding electronic waste is presented in the paper and non-metals, 11 – grains with weak electrical conductivity written by Franke and Suponik [6]. The knife mill manu- factured by TESTCHEM was used to grind the PCB. The ro- Placing the grain that has accumulated electric charge in tation speed of mill was 2815 rpm. The blades used were the electric field induces the electric field force. The value made of hardened steel and perforated sieve with a mesh of the resultant force depends on the value of the electric size of 2 mm. Four grain classes were obtained from the field force in which the grain is located. The surface elec- grinded material: 2.00-0.56 mm, 0.56-0.32 mm, 0.32-0.10 tric charge is generated on the surface of any material, mm and < 0.10 mm. The grain class of 0.32-0.10 mm was and depends on time and the type of material. Materials 40% of the total. This was a feed for the electrostatic se- with high electrical conductivity (metals) quickly get rid of parator. Results for the grain class of 0.56-0.32 were pre- the accumulated electrical charge [9]. However, the elec- sented in the paper by Franke and Suponik [6]. So far, re- trostatic force is not the only one acting on the grain du- maining grain classes have not been tested for the follo- ring the separation process. There are also (in the electro- wing reasons: in the grain class of 2.00-0.56 mm there static drum separator): gravity force, image forces and were significant connections of metals with non-metals centrifugal force. The resultant force acting on well-con- parts that reduce the purity of the concentrate, while for ductive grains is directed outwards, contrary to grains grains lower than 0.1 mm, the damage of electrode trigge- with low conductivity (non-metals) [1]. red by high risk of spark discharge [16] can occurred. In Consequently, the performance of the electrostatic drum addition, the aggregation effect may appear for this class, separator is mainly dependent on the electrical conduc- which may also affect the efficiency of separation [13, 14]. tivity of the grain, as well as the grain size and its density However, despite this, it is planned that the efficiency of [9]. Electrical conductivity of selected metals, the values electrostatic separation will be tested for grain size < 0.1 of electrical resistance of plastic materials, and their den- mm. sities are shown in Table 1 Based on the experimental research carried out by the au- Electrostatic separation thors of the paper and the literature review, it can be The drum separator used in the study allows to change concluded that purity of the concentrate is most impacted three operating parameters. As a result of the experimen- by the size of grain. According to Niu et al, Dascalescu et tal research, the following parameters were used: shaft al. and Hogzhou, changes in parameters such as voltage rotation speed 100 rpm, electrical voltage at the electrode and rotational speed do not significantly affect the purity 17 kV and distance of the electrode from the shaft 0.03 m. of the concentrate [4, 18, 19]. That is why the choice of the method and device for crushing PCB is very important. Plastics Metals D. FRANKE et al. – Recovery of Metals from Printed Circuit Boards… 215 Product analysis Table 3 Elemental concentrations in the feed and in ES products: The feed and products obtained from ES were digested A – this study, B – study by Guo et al. for a similar grain class and the concentrations of the elements were measured [10], “-” no data with the JY 2000 spectrometer (by Yobin-Yvon) using the Content of the element [%] in ICP-AES method. The source of induction was a plasma Element Feed Concentrate Waste torch coupled with a frequency generator of 40.68 MHz. A B A B A B Furthermore in the feed, concentrate and waste phase Al 3.33 1.51 1.89 2.63 0 0.93 composition have been determined on the basis of the X- Si 15.6 - 5.15 - 0.0989 - ray diffraction measurements, performed with the Pana- K 0.0589 - 0.00980 - 0 - lytical X’Pert Pro MPD diffractometer, utilizing filtered ra- Ca 8.99 - 1.11 - 0.0095 - diation of a copper-anode lamp (λKα 0.154 nm). The Mg 0.0045 - 0.00890 1.23 0.00055 0.28 diffraction lines were recorded in the Bragg-Brentano Mn 0.0355 - 0.10 - 0 - geometry, using the step-scanning method by means of a Fe 0.3821 1.38 0.93 3.74 0 0.19 Ni 0.185 0.28 0.85 0.75 0 0.039 PIXcell 3D detector on the diffracted beam axis, in the an- Cu 19.5 27.08 59.70 72.81 1.22 3.99 gle range from 20-95° [20] (1 step 0.05°, count time per Zn 0.25 0.79 1.09 2.12 0 0.11 step 120 s). The diffractograms obtained were analyzed Br 13.8 - 2.98 - 0.00055 - with the use of Panalytical High Score Plus software with Ag 0.1415 0.0019 0.4996 - 0 - the PAN-ICSD database. Au 0.0019 0.0069 0.0101 - 0 - The morphology of the feed and products from ES, as well Sn 2.38 3.23 7.83 9.63 0.0045 0.01 as the chemical composition in microareas, were analyzed Ba 2.2 - 1.27 - 0.0075 - by means of the Zeiss Supra 35 high resolution electron Pb 1.95 2.44 8.00 9.63 0 0.12 microscope, equipped with EDAX EDS chemical analysis Totality based on this study 68.81 91.44 1.34 system. (A) Totality based RESULTS AND DISCUSSION on study by 36.72 99.99 5.65 As a result of ES, the grinded PCB with grain size of 0.32- Guo et al. (B) 0.10 mm were separated into concentrate and waste. The concentrate was about 1/3 of the mass of the tested sam- An example of connection of metal parts with plastics is ple (Table 2), what confirms the average metal content in shown in Fig. 4, while Table 4 presents the results of the PCB ranging from 20% to 40%, assessed by authors such chemical analysis. On the other hand, the non-metallic as Kumar et al., Bizzo et al., Burat et al. and Wu et al. [8, elements could have penetrated into the concentrate as 17, 26, 27]. The waste was 2/3 of the mass. A high concen- a result of imperfections in the separation process. This trate density of about 11 g/cc indicates high separation issue should be checked in further studies. A similar pro- efficiency, while waste density of 3 g/cc may indicate the blem concerned waste. Over 1% of copper was found in penetration of metal parts into the waste. The analysis of this group of products. Probably, the reason for contami- the ferromagnetic content shows that the waste did not nation by copper was the layered construction of the PCB. contain ferromagnetic parts, in contrast to the concen- According to Tatariants et al. and LaDou, some very thin trate, which had the ferromagnetic content of 0.3% (see elements consists of several layers, and the segments re- Table 2). sponsible for connecting them together are often made of copper [12, 20]. It can be assumed that, if the PCB were Table 2 grinded to smaller fractions, this element would not pe- The results of ES netrate into non-metals. Content Density Yield Guo et al. [10] (see Table 3) received a cleaner concen- of ferromagnetics Product of product, of product, trate from the ES of a similar grain class. But in their ana- in product, g/cc % lyzes, they did not take into account such elements as Si, Feed 5.4 - 0 Ca, Br, Ba neither in feed nor in the product of ES. Waste 3.0 67.7 0 The creation of a semiproduct chamber in the electrosta- Concentrate 11.1 32.3 0.3 tic separator can improve the efficiency of metal recovery. Metals mechanically bonded to plastics or glass can be fo- The results of measurements carried out in the ICP-AES of und in this product. They could be ground again to sepa- the feed, concentrate and waste products are presented rate metals from plastics. Then this product could be se- in Table 3. parated again. Out of the 91.44% elements identified in the concentrate, The concentrate contained the following valuable metals: over 90% were metals. Si and Br content was over 8%. Cu, Pb, Sn, Al, Zn, Ni, Ag, Au. The amount of the metals They form a lead-barium borosilicate glass on PCB. This identified depends on the date of production, the manu- relatively high content of impurities indicates that PCB ne- facturer or the quality of the PCB and the type of the com- eds to be ground into grain classes smaller than 0.32-0.10 ponents used [22]. As provided by Bizzo et al., over the mm. In this way, metals would be free of impurities. These years PCB have had various metal contents i.e. Cu 12-28%, elements were probably mechanically bonded to metals. 216 Management Systems in Production Engineering 2020, Volume 28, Issue 4 Al 1.7-7%, Pb 1-3%, Zn 0.08-2.7%, Ag 79-3300 ppm, Au 29- The SEM analysis of the concentrate (Fig. 3 and 4) showed 11200 ppm [27]. the presence of mainly metal particles with a small amo- To determine the morphology of the feed and products unt of non-metallic materials, such as glass fiber, poly- obtained from the ES, SEM observations and chemical mers, and ceramics, which were not separated from the analysis in micro-regions, by means of energy-dispersive metallic particles in the milling process. These metal par- X-ray spectroscopy (EDS) were performed. Imaging of the ticles with various geometry and dimensions approx. 300- tested samples using the backscatter electron detection 400 μm (a few particles of the order of 800 μm were also technique (QBSD) (Fig. 2 and 3), allowed to investigate the observed) were characterized by different chemical com- morphology. position, even within one particle, which was demonstra- ted by means of the chemical composition analysis in mi- cro-areas (Fig. 4 and Tab. 4). Fig. 2 Image of the feed (QBSD SEM) Fig. 3 Image of the concentrate from ES (QBSD SEM) The contrast obtained in these pictures is a result of diffe- rences in the chemical composition. The areas containing elements with a high atomic number are clearly brighter compared to the areas consisting of lower Z-number ele- ments. In the tested feed sample (Fig. 2), both metallic particles of various shapes and dimensions mostly in the range of 100 to 400 μm, as well as many fragments of non- metallic fibers and particles, were observed. In many ca- ses, these non-metallic particles are bonded with metal, which may be due to the PCB production process, in which thin films of good electrical conductivity metals (mainly Cu and Sn, Au, Ag, Pt) are applied on a glass fiber and epoxy laminate [7, 29, 31]. This may create difficulties in the ES Fig. 4 Images of the concentrate obtained from ES with marked process, leading to "contamination" of the metallic pro- points of chemical microanalysis duct with non-metallic particles. D. FRANKE et al. – Recovery of Metals from Printed Circuit Boards… 217 Table 4 The results of the XRD (qualitative phase analysis) of the Results of chemical composition microanalysis for points feed and concentrate and waste products are presented shown in Figure 4 in Fig. 5. For the feed sample, diffraction lines from metal- Point of analysis/Concentration [% at.] lic phases (Cu, Sn, Pb, CuSn) and oxides phases SiO and BaO were recorded. The same phases were indicated in the waste sample, while the intensity of lines obtained 1 2 3 4 5 6 7 8 9 from metallic phases significantly decreased, which indi- cates a much lower volume share of these phases. It can be assumed, that these are mainly the residues of small Cu 9.71 49.43 38.47 83.39 88.37 38.62 - - 3.84 metal fragments which, combined with larger non-metal- Sn 16.76 0.73 3.04 - - - 100 - 1.07 lic particles of PCB, got into the waste during the separa- tion process. On the diffractogram obtained from the con- Ni 5.19 30.20 - - - 2.71 - 37.59 - centrate sample, only the diffraction lines from Cu, Sn, Pb, Au 68.35 2.65 - - - - - - - CuSn metallic phases were identified. However, the pre- sence of other metallic phases in a lower volume share O - 5.25 30.41 11.66 - 25.23 - - 38.19 being under detection limit cannot be excluded, as well as with this method it is difficult to identify the small amo- Al - 7.05 23.81 3.56 - 29.28 - - 22.33 unts of amorphous phases (polymers, glass). Si - 2.69 4.27 1.39 - 0.7 - 2.24 26.55 CONCLUSION Pb - 2.01 - - 11.63 - - - - As a result of the research analysis, it can be concluded that the products obtained from the ES were contamina- Ti - - - - - 1.56 - - 0.38 ted. Based on the ICP analysis, approximately 91% of me- P - - - - - 0.81 - - - tals were identified in the concentrate. These were Cu, in the largest amount (ca. 60%), and then Pb , Sn, Si, Br, Al, K - - - - - 0.5 - - - Ba, Ca, Zn and small amounts of Fe, Ni, Ag, Mn, Au, K and Mo - - - - - - - 0.72 1.28 Mg. It can be assumed that the maximum of 9% of the mass was contaminated. The EDS analysis, as well as the Ag - - - - - - - 1.45 - ICP-AES, confirmed appearance of these elements: Cu, Sn, Ni, Au, Al, Si, Pb, K, Ag, Mn, Fe and Br. Quantitative analy- Mn - - - - - - - 0.67 - sis was difficult to perform for both methods. The authors Fe - - - - - - - 57.35 - used a larger amount of material in ICP than in EDS, in which only microscopic survey was carried out. The XRD Br - - - - - - - - 5.28 analysis revealed that the concentrate contained mainly Cu, Sn, Pb, CuSn metallic phases, as well as small amounts of oxides phases such as SiO2 and BaO. Fig. 5 X-ray diffraction patterns of feed (blue line), concentrate (red line) and waste (green line) Element 218 Management Systems in Production Engineering 2020, Volume 28, Issue 4 [10] J. Guo, J. Guo, Z. Xu. “Recycling of non-metallic fractions The SEM analysis of the concentrate showed the presence from waste printed circuit boards: A review.” Journal of of mainly metal particles with a small amount of non-me- Hazardous materials, vol. 168(2-3), pp. 567-590, 2009. tallic materials, such as glass fiber, polymers, and cera- [11] J. Kozłowski, W. Mikłasz, D. Lewandowski and H. Czyżyk, mics, which were not separated from the metallic partic- “Research on hazardous waste - management part I”, Ar- les in the milling process. These metal particles, with va- chives of Waste Management and Environmental Protec- rious geometry and dimensions, were characterized by tion, vol. 15, no. 2, pp. 69-76, 2013. different chemical compositions, even within a single par- [12] J. LaDou. “Printed circuit board industry” International Jo- ticle. urnal of Hygiene and Environmental Health, vol.209 (3), The analyzes of the waste indicated that the small amo- pp. 211-219, 2006. [13] J. Li, Q. Zhou, Z. Xu, “Real-time monitoring system for im- unts of metallic phases were in the waste sample. They proving corona electrostatic separation in the process of were mainly Cu (ca. 1%) but also Ca, Mg, Sn, Ba in smaller recovering waste printed circuit boards”, Waste Manag quantities. Presumably, they were mainly the residues of Res, vol. 32, no. 12, pp. 1227-1234, 2014. small metal fragments which, combined with larger non- [14] J. Li, Z. Xu, and Y. Zhou, “Application of corona discharge metallic particles of PCB, got into the waste during the se- and electrostatic force to separate metals and nonmetals paration process. from crushed particles of waste printed circuit boards”, Jo- In conclusion, the results of the research confirmed that urnal of Electrostatics, vol. 65, no. 4, pp. 233-238, 2007. the efficiency of metal recovery for the grain class of 0.32- [15] J. Sohaili, S.K. Muniyandi, S.S. Mohamad. “A review on 0.10 mm was still insufficient. 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Dawid Franke ORCID ID: 0000-0002-5522-6889 Silesian University of Technology Faculty of Mining, Safety Engineering and Industrial Automation Akademicka Str., 44-100 Gliwice, Poland e-mail: dawid.franke@polsl.pl Tomasz Suponik ORCID ID: 0000-0002-4251-4275 Silesian University of Technology Faculty of Mining, Safety Engineering and Industrial Automation Akademicka Str., 44-100 Gliwice, Poland e-mail: tomasz.suponik@polsl.pl Paweł M. Nuckowski ORCID ID: 0000-0002-2606-0525 Silesian University of Technology Faculty of Mechanical Engineering Department of Engineering Materials and Biomaterials 18A Konarskiego Str., 44-100 Gliwice, Poland e-mail: pawel.nuckowski@polsl.pl Klaudiusz Gołombek ORCID ID: 0000-0001-5188-1950 Silesian University of Technology Faculty of Mechanical Engineering Department of Engineering Materials and Biomaterials 18A Konarskiego Str., 44-100 Gliwice, Poland e-mail: klaudiusz.golombek@polsl.pl Kamila Hyra ORCID ID: 0000-0002-9533-0066 Silesian University of Technology Faculty of Mechanical Engineering Department of Engineering Materials and Biomaterials 18A Konarskiego Str., 44-100 Gliwice, Poland e-mail: kamilahyra.65@gmail.com

Journal

Management Systems in Production Engineeringde Gruyter

Published: Dec 1, 2020

Keywords: electrostatic separation; metals recovery; PCB; SEM; XRD

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