Preliminary Study of Hydrometallurgical Extraction of Silver from Selected E-Waste

© 2020 by the authors. Submitted for possible open access publication under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). Authors’ affiliations and addresses: 1 Slovak University of Technology in Bratislava, Faculty of Materials Science and Technology in Trnava, Institute of Integrated Safety, Jána Bottu Street, number 2781/25, 917 24 Trnava, Slovakia e-mail: igor.wachter@stuba.sk


Introduction
E-waste has been a major segment of the waste produced in the past decades (Widmer et al., 2005) (Behnamfard et al., 2013). In recent years a huge growth in the use of many information and communication technology products has been observed. In the European Union, approximately 8 million tonnes of e-waste are generated every year with an annual increase of 3 -5 % ( Drechsel, 2006) while approximately 20 -50 million metric tonnes of e-waste are generated worldwide (Petraniková, 2008). There are also growing concerns about the e-waste generated in developed countries due to the lack of infrastructure for environmentally sound management of e-waste. The reasons for decreasing life span of electrical and electronic devices are as follows (Akcil et al., 2015, Ivanova et al., 2018: • Incoming of highly advanced and technically skilled devices/equipment at a lower price and more features.
• Rapid growth in the lifestyle of human beings with modern facilities having user-friendly electrical and electronic equipment. • Stiff competition amongst individuals to use and small enterprises and industries to produce and sell the best products made on advanced technologies.
WEEE (Waste of electric-electronic equipment) contains a variety (>1000) of organic and inorganic substances with its composition depending largely on the type, manufacturer and age of the equipment (Table 1). WEEE can contain up to 61% metals and 21% plastics (Widmer et al., 2005, Biały et al., 2019, Biały et al., 2019a. Polyethylene, polypropylene, polyesters and polycarbonates are typical plastic components (Gramatyka et al., 2007, Sviatskii et al., 2020. Many of the materials such as chlorinated and brominated substances, toxic metals, photoactive and biologically active materials, acids, plastics and plastic additives present in WEEE are highly toxic. E-waste encompasses valuable metals, alongside numerous dangerous materials. An enormous number of dangerous metals (Cd, Hg, Pb, Cr) from e-waste may contribute to increasing the toxicity levels of the ecosystem (Qu et al., 2019). E-waste material in the environment may increase the exposure risk of hazardous materials. Serious pollution of groundwater and human health could be associated with these hazardous materials. One of the important routes to enter toxic chemicals from e-waste to the human body is the soil-crop-food pathway. Toxicity, negative environmental impact, as well as financial reimbursements from e-waste, are necessitated the need for metal recovery from e-waste. The utilization of e-waste could be a potential secondary source of precious and base metals (Otto et al., 2018). Without knowing the hurtful impact, E-waste has been discarded in the open wellsprings of water bodies, the agricultural land, and open landfills by unconscious social people. For the open disposal of the e-waste containing toxic substances in water bodies and landfills pollutes the groundwater (Romaric et al., 2019), as shown in Fig. 1.  (Ashiq et al., 2019) E-waste from such equipment contains many toxic elements such as lead, mercury, cadmium, nickel, chromium, etc., which has an adverse impact on our environment. Moreover, e-waste also contains many valuable metals, such as gold, silver, platinum, and palladium (Tripathi et al., 2012). The proper management of discarded electronic devices is an emerging issue for solid waste professionals throughout the world because of the large growth of the waste stream, and the content of toxic metals in them, most notably heavy metals such as lead (Jang and Townsend, 2006). Harmful effects of various metals present in the electronic waste on human health are summarized in Table 2. Currently, e-waste recycling focuses mainly on mechanical approaches, pyro-metallurgy, bio-metallurgy, and hydro-metallurgy (Hsu et al., 2019;Liu et al., 2019). The reason for preference of hydro-metallurgy over pyro-metallurgy is because of low or no gas emission compared to pyro process which releases toxic gases (dioxins/furans) and volatile metals, dust, Cl 2 , Br 2 , SO 2 and CO 2 together with others Pb, Hg, Cr 6+ , Cd, flame retardants. No dust or low dust generation, low energy consumption, high recovery rate, no slag generation except few plastics, and easy working conditions (Ni et al., 2013;Tue et al., 2013;Zhang et al., 2012). According to these studies (Andrews et al., 2000;Cui and Zhang, 2008), hydro-metallurgy could be preferred over pyro-metallurgy for the recovery of precious metals such as gold, silver, and platinum.  t  2011  2012  2013  2014  2015  2016  2017  2018  2019  Industrial  15804  14012  14332  13984  14189  15250  16087  15909  15891  …of which  photovoltaics  2127  1711  1571  1505  1683  2914  3166  2877  3070   Photography  1916  1633  1425  1356  1281  1176  1092  1064  1048  Jewellery  5045  4952  5819  6018  6302  5885  6106  6317  6261  Silverware  1291  1247  1421  1630  1760  1627  1795  2034  1860  Net physical  investment  8460  7490  9334  8790  9654  6653  4858  5154  5788   TOTAL  32515  30590  33246  31775  33187  30963  30005  30739  30848   Table 4 shows the development in global silver supply by the sources During the last decade (2011 -2019). Global recycling edged higher last year (2019), up 1.3 % to 5,284t. Volumes from industrial end-uses, the biggest source of scrap, rose 2 % to the highest level this decade (The Silver Institute, 2020). Spent electronic equipment consists of several components in the form of metals and multicomponent elements. The base metals include iron, aluminum, nickel, zinc, selenium, indium, and gallium. The noble metals can be divided into copper, palladium, or gold, silver. Hazardous substances that can be found in spent electronic equipment include mercury, beryllium, lead, arsenic, cadmium, antimony and plastics, glass, and ceramics. Depending on many factors, such as the age of the device, manufacturer, or the type of equipment, the content of the individual electronic component in the waste is mixed . Spent electronic equipment consists of several components in the form of metals and multicomponent elements. The base metals include iron, aluminum, nickel, zinc, selenium, indium, and gallium. The noble metals can be divided into copper, palladium, or gold, silver. Hazardous substances that can be found in spent electronic equipment include mercury, beryllium, lead, arsenic, cadmium, antimony, and plastics . Table 2 shows the selected material composition of electronic devices. A decisive impact on the value of electronic scrap has the content of precious metals, although iron and plastic are dominant components, and a seemingly small content of precious metals in different electronic devices (<0.5 %). Although iron and plastic are dominant components, in terms of weight, a seemingly small content of precious metals in different electronic devices (<0.5 %), constitutes about the electronic scrap value (Table 4) (Fornalczyk and Saternus, 2013). Analysing only computer equipment and mobile phones, this share is 3 % of the world's production for Ag, 4 % for Au, and 16 % for Pd. In 2019 the global silver demand was 30848 metric tonnes (Table 3). The highest demand is in long-termed achieved in the industry, which also includes the electronics and IT sector.
More than ten times higher purity of precious metals in waste printed circuit boards compared to the rich ore content attract the attention to extract noble metal from e-waste. Hence, the extraction of noble metals (Au, Pt, Pd, Ta, Te, Ge, Se) from e-waste should be given major priorities. However, the realization of recycling should be the basis of maximum recovery and minimum negative impact on the environment (Islam et al., 2020). This paper discusses the extraction of silver from used electronic relays via a simple hydrometallurgical process. The objective was to determine the relative amount of Ag recoverable from this type of waste.

Material and Methods
Chemical leaching (Fig. 2) involves leaching either by using acid or ligand supported complexation. Chemical leaching of metals from E-waste can also be done by utilizing various inorganic-acids.

Fig. 2. Types of hydrometallurgical techniques
In this paper, two sets of electrical relays were used to extract silver by hydrometallurgical acidic techniques. The difference in the two sets was in the used precipitation of the silver from the pregnant leach solution. The first set consisted of seven commercially used electrical relays (Allen-Bradley 700-HA33Z2-3) containing nine silver contacts made of silver, copper, nickel, and aluminum. The weight of the whole relay was 86.74 grams. The experimental procedure consisted of six main steps. All of the used chemicals were purchased from CentralChem s.r.o. The used chemicals were of analytical grade. After collecting the raw material, the relays were manually disassembled, and the silver-containing contacts were collected in a beaker. To remove all of the base and redundant metals except silver, the contacts were treated by concentrated hydrochloride acid + 35 % hydrogen peroxide (100 ml + 5 ml). The beaker was placed on a heating plate to accelerate the reaction of metals removal. Chemicals were added and heated until no visible signs of reaction were observed, and only silvery metal was present. The contacts were then washed three times with distilled water, and in the next step, concentrated nitric acid with distilled water (50 ml + 50 ml) were added to the beaker with the contacts. After two hours, the contacts were completely dissolved in the solution in the form of silver(I) nitrate (AgNO 3 ). To precipitate the silver(I) nitrate from the solution, a concentrated hydrochloric acid was used to produce AgCl (silver(I) chloride). After three washings with boiling distilled water, sodium hydroxide (NaOH) was added to convert AgCl to silver(II) oxide (Ag 2 O). As a final step, the silver oxide dust was placed into a melting dish and melted at a temperature of 980 °C to form a single bead of silver using a muffle furnace. Analysis of the metal was performed using a scanning electron microscope (SEM) and energy dispersive spectrum (EDS) analysis. The main individual steps and corresponding reactions of the procedure are shown in Table 6. In the second stage of the research, ten electrical relays, each containing nine contacts made of silver, copper, nickel, and aluminum were used. The weight of the whole relay was 86.74 grams. The experimental procedure consisted of four main steps. All chemicals (analytical grade) were purchased from CentralChem s.r.o. Contacts were mechanically trimmed from relays, put in a beaker, and treated by a mixture of concentrated HCl and 35 % H 2 O 2 (100 ml + 5 ml) to remove all the base metals. The solution was heated by a heating plate to increase the reaction speed. After the removal of all metals, the residual silver coatings were washed by distilled water. In the next step, concentrated HNO 3 with distilled water (ratio 1:1) was used to dissolve the silver. After two hours, all silver was completely dissolved, forming silver(I) nitrate (AgNO 3 ). In this case, pure metallic silver was obtained from the solution by cementing on a solid copper cylinder. Obtained silver was washed by hot distilled water three times and subsequently melted into one bead. The main individual steps and corresponding reactions of the procedure are shown in Table 7. H2O2 + Cu + 2 HCl → CuCl2 + 2 H2O H2O2 + Ni + 2HCl → NiCl2 + 2H2O 3 H2O2 + 2 Al + 6 HCl → 2 AlCl3 + 6 H2O 3.
Cementing on the copper cylinder AgNO3 + Cu0 → Ag0 + CuNO3 The summarised flowchart of the processes is shown in the Figure 4.

Results and Discussion
Researchers have indicated that effective recovery of precious metals from e-waste is feasible (Ficeriová et al., 2011(Ficeriová et al., , 2005Ficeriová and Baláž, 2010). To illustrate, printed circuit board (PCB) of a PC can contain up to 20 % Cu and 250 g/ton Au, which are significantly high, i.e. 25-250-fold for gold and 20-40-fold for copper when compared with gold ores (1 -10 g/ton Au) and copper ores (0.5-1 % Cu), respectively. Recycling turns WEEE into a secondary resource allowing the recovery and reuse of metals and non-metals contained and mitigating the environmental impact of WEEE (Cui and Zhang, 2008;Havlík et al., 2010;Yazici et al., 2010).
The recovery yield of silver obtained from the first set of electrical relays reached 0.44 %. In each of seven electrical relays, there are nine silver-containing contacts made of silver, copper, nickel, and aluminum. The total weight of one whole relay was 86.744 g (seven relays weight 607.21 g). After mechanical removal of the nine silver-bearing contacts, its weight was 2.892 g (63 pieces of contacts weight 182.196 g). After acid -peroxide bath, the contacts weighed 3.981 g. In total, 2.667 g of silver was extracted from the material by this process. Electrical relay mainly consists of circuitry, plastic, and base metals, which makes it easily recyclable e-waste. This kind of e-waste becomes a valuable material for further processing of basic mechanical separation. Therefore, an estimated concentration of silver in this type of relay is 4400 g/t.
The recovery yield of silver obtained from the second set of electrical relays reached 0.54 %. The total weight of ten relays was 921.35 g. The total weight of separated contacts (90 pieces) was approximately 25.4 g. In total, 4.79 g of silver was extracted from the material by this process, which is 5200 g/t (from the total weight). Cui et al. (2008) determined the amount of Ag in different e-waste samples to be comparable or lower than in keyboards. Li et al. (2019), reported that CPU sockets also contain silver (431 g/t) with a lower yield. For the mining sector, the silver reserves are divided into known reserves and hidden reserves and stratified into four levels of ore quality: 1. Rich silver is labeled as extra high quality (10,000 -6000 g/t), 2. High grade (1100 -800 g/t), 3. Low grade (100 -80 g/t) and 4. Ultra-low grade (below 10 -8 g/t), which means this type of e-waste could be considered to be an extra high-quality source of silver (Sverdrup et al., 2014). As shown in Table 8, copper and precious metals contribute invariably and extensively to the economic potential of all WEEE. Analysis of the metal beads was performed using a scanning electron microscope (SEM) and energy dispersive spectrum (EDS) analysis ( Figure 5). EDS (Energy Dispersive Spectrum) analysis was used to determine the chemical composition and concentration of individual elements. It was performed by the energydispersive X-ray spectroscopy analyzer, which was a part of the scanning electron microscope of JEOL JSM 7600 Ftype. The topography of silver beads was observed at an accelerating voltage of 20 kV, current 2 nA, and a working distance of approximately 15 mm. The chemical composition of silver bead was investigated by software INCA.  Table 9 shows the results of EDS analysis. Elemental analysis showed that the purity of obtained silver beads was 88.45 % (1 st set) and 95.65 % ) (2 nd set). Because only the surface of the sample is analyzed by this method, it is possible that the sample has a higher purity under the top layer. The presence of oxygen may be caused by the formation of silver oxide, which is produced under normal conditions when silver reacts with the oxygen present in the air. The presence of carbon may be explained by the impurities on the melting cupel from the previous melting. The magnesium also comes from the cupel, which is made of pure and compressed magnesium oxide (MgO). The waste solution containing dissolved metals from the process was cemented according to the electronegativity series, and as a result, almost zero waste was produced during the whole extraction process.

Conclusion
At present mechanical and hydrometallurgical separation technologies has a relatively high recovery rate of precious metals, although these methods have not been adopted by countries with low GDP because of its complexity and high economic cost. Hence, recycling this kind of e-waste can both decrease the pressure on natural resources and reduce environmental contamination. This paper presented a simple silver extracting method from electrical relays and determined the quantity of recoverable precious metal. EDS analyses showed the purity of the obtained metal. The advantages of the presented hydrometallurgical method were its costeffectiveness, environmental friendliness, and time-efficiency. We demonstrated that discarded electrical relays contain an appreciable quantity of silver with high economic potential. Using this method, it is possible to extract 4400 g/t -5200 g/t of silver from commercially used electrical relays (Allen-Bradley 700-HA33Z2-3) with 88.45 % purity (1 st set) and 95.65 % purity (2 nd set). The procedure should be further studied for different aspects and for different e-wastes to help with negative impacts on the environment.