Sustainable Recovery of Cu and Sn from Problematic Global Waste

Research Article pubs.acs.org/journal/ascecg

Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Sustainable Recovery of Cu and Sn from Problematic Global Waste: Exploring Value from Waste Printed Circuit Boards Rumana Hossain,* Rasoul Khayyam Nekouei, Irshad Mansuri, and Veena Sahajwalla Centre for Sustainable Materials Research and Technology (SMaRT@UNSW), School of Materials Science and Engineering, University of New South Wales (UNSW), Sydney, NSW 2052, Australia

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S Supporting Information *

ABSTRACT: There is a continuous quest for “mining” electronic waste for valuable materials. Currently, e-waste is either disposed of in landfills or transported often to underdeveloped nations where the materials are either disassembled mechanically or incinerated. These processes, unfortunately, can result in toxic pollutants, like dioxins, furans, and lead, contaminating air, soil, and water. In this study, a new pathway, based on thermal micronizing (TM), is used for the controlled transformation of metals present in waste printed circuit boards (PCBs) into value-added tin and copper-based alloys. This fast heating process in a reducing atmosphere recovered tin-based alloy at 500 °C and copper-based alloy at 1000 °C from the waste PCB. Low-temperature heat treatment, short heating times, and decrease in process steps made it possible to remove the lead with tin-based alloy effectively. This prevented the lead evaporating to the atmosphere or diffusing in solid copper of waste PCBs due to the low solubility of lead in copper. After removing the tin-based alloy, the thermal transformation of waste PCBs at 1000 °C facilitated the formation of copper-based alloy droplets, leaving behind the PCB residue. Tin-based alloy is known to be soldering material, and copper is known to be a highly conductive element in electrical applications. Recovered tin-based alloy demonstrated an increased ultimate tensile strength of ∼30% compared to a standard Sn-9%Zn solder alloy. The recovered copper demonstrated ∼80% of IACS (International Annealed Copper Standard) conductivity which makes it favorable for application in various electrical appliances. This simple, low-cost approach opens the opportunity for e-waste processing in the industrial or informal sectors with minimal toxic emissions and encouraging safety. KEYWORDS: Selective thermal transformation, Printed circuit board, Hazardous emission, Metal recovery, Value-added materials

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INTRODUCTION E-waste generation is increasing at 4%−5% annually making it the fastest growing solid waste stream in the world. Approximately 41.8 million tons of e-waste was generated worldwide in 2014,1 and the generation of e-waste in 2018 is expected to be around 50 million tons.2 Printed circuit boards (PCBs), which form an essential component of electronic devices, have become a new environmental challenge. The ever-growing demand for metals, depleting mineral resources, costs and environmental impacts associated with mining, and the exploitation of natural resources have made e-waste a valuable source. While the rates for recycling are still quite low, there is an urgent need to take a critical look at the current ewaste recycling techniques and to optimize the recovery of metals and other valuable products. Embedded within the printed circuits boards (PCBs) are many valuable materials with an encouraging market value for recovery, such as a mixture of polymers, ceramics, and metals as part of the boards, circuitry, components, solders, and connectors.3−5 The metal content of waste PCBs can be as high as 20−30%.6,7 However, PCBs are particularly complex as © XXXX American Chemical Society

they also contain hazardous substances like lead and cadmium.8 The integration of various materials present in ewaste is an obstacle for recycling.9 PCBs contain 10%−25% of copper by weight,7,10−15 compared to up to 3% in mined copper ore.15 The amount of copper and tin found in one ton of PCBs is 10 to 20 and 4 to 6 times more than that available in one metric ton of their natural ores.7,16 In mining, it is possible to achieve a copper concentration of 2%−30% after two preliminary processes of beneficiation and froth flotation in a primary copper production.17 Elements used for soldering electronic components, such as tin, zinc, and lead, melt at very low temperatures and are hence is very energy efficient to recover from the PCBs compared to producing this material from primary sources.7 The recovery of the solder material is essential to recover copper in pure and homogeneous form because the tin-based PCB solder material melts at a lower temperature and diffuses Received: September 13, 2018 Revised: December 5, 2018

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DOI: 10.1021/acssuschemeng.8b04657 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering

Figure 1. Sn−Zn, Sn−Pb, and Cu−Sn binary phase diagram.s41

recycling process which are expensive or infeasible for commercial application.35−37 To capture such hazardous emissions, extensive gas cleaning systems and filters have become integral components of e-waste recycling in developed countries;38,39 nevertheless, these processes are not in use in developing countries due to inadequate legislation, cost constraints, and technological barriers. This study investigates the direct transformation of toxic ewaste, namely, waste PCBs, into value-added tin and copperbased alloys via thermal micronizing (TM),40 which is applicable in the formal/industrial and informal sectors/ developing countries. In this technique, by utilizing short heating times enabled by selective thermal transformation, we can minimize the lead emission and dioxins formation without any additional mechanism. The transformation mechanism has been investigated by analyzing the samples after TM at different temperatures. Low-temperature heat treatment, short heating times, and reductions in process steps are expected to reduce energy requirements significantly and pollution associated with the handling and processing of e-waste, hence, promoting the sustainable synthesis of high purity advanced materials.

into the copper to form some intermetallic compound. Taken together, it is more economical and sustainable to recover copper, tin, and associated base metals from e-waste compared to extracting from the natural mineral.7 The recovery of metal from waste materials is referred to as “urban mining”,18,19 deriving value from waste and simultaneously reducing pressure on landfills. Solder, composed of mainly tin, lead, and zinc, is used to electrically connect various components on the surface of PCBs. These metals are not harmful in their solid state. However, when they encounter liquid waste streams and/or sewage, they turn into hazardous liquid solutions which could be absorbed by the soil and contaminate the water.20 Eventually, this affects plants and human organs by creating hormonal or genetic disorders.21,22 Melting and evaporation of the solder metal are also toxic to the environment.20 E-waste recycling methods used in the informal sector release micronsized particles loaded with toxic elements such as zinc, copper, nickel, lead, cadmium, etc.; hence, they have a detrimental effect on human health.23 This is an alarming situation in developing countries where poor recycling techniques generate high levels of environmental pollution for both ecosystems and the population living near the recycling areas.24,25 A study on pyrolysis of e-waste indicated that more than 90% of the lead content of e-waste was lost as emissions during heating to 750 °C.26 Commercial recycling of e-waste is currently carried out at an operating temperature of 1250 °C with multiple processing steps for the recovery of metal.27 Lead (Pb) is a continual and inevitable environmental pollution that has a detrimental effect on neurology, hematology, immunology.28,29 Even a low level of Pb exposure can lead to intelligence and behavioral issues in children.30 Moreover, it can deteriorate the DNA in PC12 cells and mouse blood cells.31 The extended heating period also promotes harmful emissions during e-waste processing and poses serious threats to health and the environment.32 Another significant concern about e-waste recycling is the dioxins formation triggering pollution in the environment. The adverse effects of these chemicals on human health have been proven for many years. Dioxins are formed during the combustion of wastes and escape into the environment via exhaust gases from incinerators.33 The dioxin formation from the waste incineration occurs at temperatures above 450 °C and reduces significantly at temperatures above 850 °C.34 Plenty of studies have been conducted to overcome the challenges with the capture of hazardous metals in the e-waste

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EXPERIMENTAL SECTION

Material. Multilayer waste PCBs supplied by Reverse E-waste, NSW, Australia were used in this research. Hazardous and metallic pieces and nonmetallic parts of the PCBs such as the steel CPU case, capacitors, and large plastic parts, which are readily separable and readily recyclable, were detached from the boards.4 PCBs were cut into fragments of approximately 2−3 cm in size for heat treatment. Heat Treatment Method. The binary phase diagram (Figure 1) indicates pure tin melts at 232 °C. The inclusion of copper can increase the thermal capacity of tin and hence will raise its melting temperature. In contrast, a small amount of zinc (above ∼10%) and lead (above ∼30%) will decrease its melting temperature. To recover the solder material in a homogeneous form without the evaporation of metals, such as lead, tin, zinc, etc., our selected temperature range was 450−800 °C for 5−10 min time. In contrast, copper has an elevated melting temperature compared to tin (1084 °C, Figure 1). The impurities, such as tin, zinc, lead, can reduce the melting point.41 If the molten tin or lead diffuses in the solid copper, it could lower the melting temperature of copper. The binary phase diagrams of coppertin, lead, or zincdepict that some copper alloys are stable at much lower temperatures. Nevertheless, we recovered a substantial amount of solder material to get a comparatively pure form of copper and retain the homogeneity. Thus, solder material can only diffuse into the outer surface of copper resulting in a miniscule amount of alloy formation. After optimizing the temperature and time for the solder material at 500 °C for 5 min, we selected the experimental B


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Figure 2. Heat treatment conditions for metal recovery from a printed circuit board.

Figure 3. Flowchart of the experimental procedures. Group-1 is for characterization of the base material. Group-2 is for recovery and characterization of the tin-based solder alloy. Group-3 is for recovery and characterization of copper from waste PCB after recovering the tin-based solder alloy at 500 °C. temperature range for copper recovery from 950 to 1250 °C for 10 min. This temperature range can avoid dioxin formation.34 At this temperature range, the plastics are decomposed completely, and there could be 10%−15% fixed carbon remaining in the residue. The optimum temperature for copper recovery was found 1000 °C. According to the Ellingham diagram, this temperature is not enough for the reduction of the metal oxides present in the PCBs by the fixed carbon and minimizes the chances of impurities in the copper alloy. To support our selected operating temperatures and times for metal recovery, we investigated the TM conditions (Figure 2) and

formation of metals from waste PCB on a small scale using an infrared (IR) furnace SVF17-SP coupled with an advanced camera. The heat source, infrared rays, are radiated from halogen lamps which are located at the bottom focal point in a gold-coated elliptic chamber. The sample is positioned at the top focal point in the ellipsoidal mirror. For efficient control of the temperature, the sample sits above the thermocouple. It was evident from the IR furnace observation that the formation of the tin-based metal droplet is possible at temperatures as low as 450 °C. After 800 °C, the copper part of the PCB starts melting partially due to the diffusion of tin in the C


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Table 1. ICP-OES Analysis for Chemical Composition of Metallic Parts of PCBs Used in This Research Worka Element

Cu

Sn

Zn

Fe

Ni

Pb

Ti

Al

Si

Total metal

wt. %

24.63 ± 0.2

4.4 ± 0.2

1.29 ± 0.1

3.30 ± 0.2

0.41 ± 0.2

0.43 ± 0.1

0.15

1.97

0.01

36.59 ± 0.7

a

The wt. % of total metal is from the total mass of PCBs.

Figure 4. Tin and copper-based alloys recovered after thermal transformation at 500 and 1000 °C. copper. Also, time plays a vital role in this diffusion process; increasing heat treatment time enhances the diffusion process, which is a hindrance for recovery of tin-based and copper-based metal separately. After analyzing the information from an IR furnace, a two-step TM was carried out in the horizontal tube furnace with argon (Ar) gas flowing at 1 L min−1, and the gas inlet was controlled by a gas flowmeter throughout the experiment. For the first step, the temperature was applied from 450 to 800 °C, and for the second step, the temperature was applied from 950 to 1250 °C. In each case, TM was carried out for 10 min. The evolution of gases at these temperatures and times was measured using an infrared gas analyzer. Figure 3 demonstrates the details of the experimental methods used in this research. Characterization Method. The tin- and copper-based alloys obtained from our experiments were characterized by the PerkinElmer OPTIMA 7300 inductively coupled plasma−optical emission spectroscopy (ICP-OES). The metal samples were digested/extracted with a mixed acid (HCL + HNO3) followed by open digestion for ICP-OES. A defined weight of metal samples (0.5 g) was digested with a mixed acid (30%HCL + 70%HNO3) for ICP-OES. The sample was heated with the acid until visually dissolved (2−3 h) at 90 °C, and the quantity of the solution was 10 mL. After performing ICPMS, the sample was analyzed by ICP OES.42 Thermogravimetric analysis (TGA) was carried out in the PerkinElmer simultaneous thermal analyzer STA-8000 with a 20 °C min−1 heating rate. After collecting the copper and tin alloy, standard metallographic grinding and polishing methods43 were used to prepare the surface of the samples for X-ray analysis.44 The PANalytical Xpert multipurpose Xray diffraction system was used for obtaining X-ray diffractograms for 2θ = 10° to 130°, and the data were processed by HighScore Plus software. A thermo ESCALAB250Xi X-ray photoelectron spectrometer was used for performing XPS on the tin- and copper-based products. The X-ray source was monochromated Al Kα. For the quantitative mapping, the elemental concentration of the structure was measured by electron probe microanalysis (WDS, JEOL JXA8500F). EPMA elemental maps were acquired of an area encompassing the polished surface of the recovered metal at a beam energy of 10 kV, 40 nA current with a dwell time of 30 ms per pixel, and a step size of 0.1 μm. The creation of the concentration

maps was done through pixel-by-pixel ZAF correction, normalized and referring to standards for each element. Maps on secondary standards were done using the same analytical conditions as the samples for quality control. Average values of these standards were found to be in the usual analytical range for EPMA (i.e., ± 2% relative). A commercial grade solder alloy, KappAloy9, 45 having a composition similar to our recovered tin-based metal, was selected to evaluate the mechanical property. After TM, the tin-based metal droplets are separated from PCB and then remelted and cast with a constant thickness. Specimens for the tensile test were prepared according to an ASTM standard.46 Tensile tests with an initial strain rate of 3 × 10−4 s−1 were performed at room temperature. Each datum was the average of three test results. To evaluate the electrical property of the recovered copper from PCB, a standard pure copper was chosen. The current−voltage curves were measured through a Kiethley Sourcemeter 2400 for both Cu metals. To measure the resistance, a two-point probe was used with an identical distance for all the experiments. Ten measurements were carried out for each copper sample. Conductivity (σ) can be calculated from eq 1 if the resistance (R) is known. i L y σ = jjj zzz k AR {

(1)

where A = area of the sample, and L = length of the sample.

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RESULTS AND DISCUSSION Characterization of Raw Material. PCBs are a complex mixture of different components which typically contain metals, oxides, and plastics. The PCBs used in this research have multilayers, and the tin-based solder is used to connect the different components to the polymer-based board. Table 1 summarizes the various percentages of the most dominant and critical elements within these PCBs before TM by using the ICP-OES analysis. Copper was found in a significant quantity compared to the other base metals present in PCBs. Selective Recovery of Metal Droplets. The tin-based alloy was recovered as metal droplets (Figure 4) via TM of D


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ACS Sustainable Chemistry & Engineering Table 2. Percentage of Metallic Content of Tin-Based Metal Droplet Recovered at Various Temperaturesa Temperature (°C) 450 500 600 700 800

Cu (%) 0.4 1.8 6.1 7.6 11.5

± 0.05 ± 0.3 ± 2.1 ± 2.7 ± 3.1

Sn (%) 83 86 81.2 79.2 74.4

± 2.5 ± 2.9 ± 4.1 ± 2.4 ± 2.8

Zn (%) 8.2 8.8 8.8 9.1 9.9

Fe (%)

Ni (%)

Pb (%)

Ti (%)

Al (%)

Si (%)

0.02 0.01 0.01 0.01 0.02

0.03 0.03 0.03 0.03 0.03

3 ± 0.1 3.1 ± 0.1 2.1 ± 0.5 0.3 0.1

0.04 0.01 0.02 0.03 0.03

0.03 0.01 0.01 0.02 0.02

0.01 0.01 0.01 0.01 0.01

± 1.1 ± 0.9 ± 1.3 ± 1.5 ± 1.6

Total recovery (%) 2.1 3.4 3.3 3.5 3.6

± 0.3 ± 0.2 ± 0.3 ± 0.5 ± 0.5

a

The % of total recovery is from the total mass of PCBs.

Table 3. Percentage of Metallic Content of Copper-Based Metal Droplet Recovered at Various Temperaturesa Temperature (°C) 950 1000 1100 1200

Cu (%) 85.8 93.3 91.1 92.6

±2 ± 2.5 ±3 ±2

Sn (%) 9.3 4.1 4.2 4

± 2.5 ± 1.5 ± 1.1 ± 1.3

Zn (%) 1.1 1.2 1.1 1.4

± 0.5 ± 0.3 ± 0.3 ± 0.3

Fe (%) 0.6 1.1 2.7 0.8

Ni (%)

± 0.2 ± 0.1 ± 1.0 ± 0.1

0.8 1.6 1.1 1.2

± 0.1 ± 0.2 ± 0.1 ± 0.1

Pb (%)

Ti (%)

Al (%)

Si (%)

0.1 0.1 0.1 0.1

0.11 0.13 0.12 0.12

0.2 0.2 0.2 0.2

0.01 0.01 0.01 0.01

Total recovery (%) 15.1 21.9 21.8 20.9

± 3.5 ± 2.5 ± 2.1 ± 2.3

a

The % of total recovery is from the total mass of PCBs.

Figure 5. (a) Evaluation of gases during thermal transformation. (b) Weight loss of the PCB board during the heat treatment process.

substantial amount of copper remains within the residues with trace amounts of Ni, Fe, Al, Sn, Zn, and other elements. The physical separation of the copper elements is not feasible since it is firmly embedded within the glass fiber layers with heterogeneous composition, structures, and sizes. To achieve the homogeneous composition and structure of copper metal, we performed TM on PCB residues in an inert atmosphere (argon gas) from temperatures of 950−1200 °C for 10 min. This fast and selective transformation mechanism produced the copper-rich metallic droplets as shown in Figure 4. The droplets were chemically analyzed using the ICP technique. These results demonstrate that TM of the sample PCBs at different temperatures do not have much effect on the percentage of copper extracted after 1000 °C (Table 3). The diffusion of tin into the copper subsequently was triggered at elevated temperature and time due to the high solubility of tin in copper.40,41 The fast heating process at the first step of TM prevented this diffusion. Lead was also present in the initial material which is separated out at the first step of TM with the tin-based metallic droplets due to the low solubility of lead in copper. The plastics in the waste generated a reducing and nonoxidizing environment and avoided the metallic droplets from oxidation, while carbon minimized agglomeration.40

PCBs in an inert atmosphere (argon gas) for temperatures ranging from 450 to 800 °C for 10 min. At relatively higher temperatures, the droplets agglomerate and grow larger. ICP analysis depicted that the product contained tin in the range of 75%−86% with zinc, copper, and lead for the temperature range of 450−800 °C (Table 2). After 500 °C, the elevated temperature did not bring about an appreciable change in the tin content of the product. At this temperature, the tin-based metal exhibits a pure form compared to the alloy recovered at higher temperatures (Table 2). At elevated temperature, tin and zinc start defusing in the copper sheet, and local melting of copper commences. Hence, its recovery at 500 °C counters the use of higher temperature and thus lowers the energy input. The percentage of tin and zinc was found as high as 86% and 8.8% with 3% lead and 1.8% copper at 500 °C. This recovered alloy (where the % of total recovery represents the mass recovered from the total mass of the PCB) is compatible with the standard soldering alloy because of its good wettability, strength, and corrosion resistance (which could be possible for the zinc and copper content in tin). The droplets of tin-based alloy formed at the surface of PCBs within a very short time (5−10 min) and could be removed very easily, i.e., manually after rapid cooling. After recovering the metal droplets of tin-based metal at 500 °C, a E


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Figure 6. X-ray diffractogram of recovered metal: (a) 500 °C tin-based metal and (b) 900 °C Cu-based metal.

metallic phase to a certain extent. The loss of copper as volatiles became significant only at high temperatures (more than 1250 °C).47 The diffusion of molten lead and tin into solid copper is likely to be much slower than their corresponding diffusion in molten copper. Moreover, the operating time is optimized to avoid the diffusion process. These observations suggest that it may be possible to untangle the overall complexity of waste PCBs to a great extent by choosing appropriate operating temperatures, and their wellknown material properties could be achieved by optimal material recovery for commercial application. Characterization of Synthesized Metal Droplets. To further study the recovered tin-based metal from 500 °C and copper-based metal from 1000 °C, in-depth XRD, XPS, and EPMA-WDS analyses were carried out. These analytical methods could help us to know the phases, compositional distribution, and homogeneity of the structure and presence of metal oxides in the recovered metals. X-ray Diffraction Analysis. X-ray diffraction patterns reveal clear peaks of tin, tin−zinc alloy, and tin−zinc−lead alloy (Figure 6a). The absence of a metal oxides peak provides clear evidence of unoxidized tin and tin−zinc alloy. The presence of zinc in the tin gives superior mechanical and corrosion properties. These types of alloys are reported to have better corrosion resistance than pure zinc or pure tin particularly in high humidity conditions and are also stated to be superior to other expensive coating elements, such as cadmium, in marine environments.48 This tin−zinc-based solder is suitable for electronics components. Zinc into tin is also attributed to providing mechanical integrity to electronics packaging.49,50 Another X-ray diffraction pattern shows the presence of tin in the copper (Figure 6b). Tin is detrimental for the conductivity of pure copper. According to the thermal transformation profile of PCB (Figure 2) and evaluation from the phase diagram (Figure 1), the reaction time and temperature was not adequate to form any homogeneous alloy of copper and tin. However, a minuscule amount of the Cu10Sn3 compound was present in the recovered metal which formed due to the diffusion of molten tin into copper during TM at the first step (Figure 2). Instead of acting as impurities,

In conventional e-waste processing, toxic elements, such as lead, tend to escape the heat-treated residue at a high temperature, resulting in toxic emissions. The fast-thermal transformation process used in this study enables the potentially toxic elements to be safely locked within the tinbased metallic droplets, as indicated in Table 2. The table shows a substantial percentage of lead (around 3%) at the tinbased metal which indicates that lead did not go to the gas phase and escape the residue. Mechanism. To better understand the selective and fast TM of the PCB samples at 500 °C, the evolution of gases at this temperature was measured using an IR gas analyzer. As shown in Figure 5a, at 500 °C, most of the plastics in the PCB samples undergo the degradation process between 25 and 200 s. During this rapid transformation, organic/polymeric components break into lower weight molecules and higher weight molecules (Table S2). The high concentration of carbonaceous gases, such as CO and CH4, indicate that the produced gases generate a reducing atmosphere.40 The degradation of polymeric/organic materials of PCBs produces carbon and hydrogen which will react with oxides of PCBs and oxygen of the atmosphere. These phenomena will reduce the oxidation of the metal during the TM process. Figure 5b depicts the TGA results for heat-treated PCB during TM at different temperatures. As shown in Figure 5b, the PCB samples undergo a major degradation step at around 300 °C. During heating, the mass loss of the PCB samples increased sharply to about 300 °C, and this slope decreased slightly at 350 °C. Beyond 450 °C, the mass of the PCB samples remains constant approximately. The thermal degradation of polymers, ceramics, and metals occurred in different temperature regimes. Hence, the degradation of these constituents did not interfere much with each other. While the polymer degradation was complete by 350−400 °C, the segregation of metals from waste PCBs occurred predominantly in the temperature regime of 450− 1200 °C. Ceramics and other oxides were still stable at this temperature range and tended to separate out as residual fractions. Low melting point metals, such as tin, zinc, and lead, melt at a much lower temperatures (around 232−420 °C), and the comparatively high melting point of copper kept it in the F


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Figure 7. Characteristic XPS for tin-based metal: (a) Sn, (b) Zn, and (c) Pb peaks.

Figure 8. Characteristic XPS for recovered copper: (a) Cu 2P and (b) Sn 3d peaks.

Figure 9. Quantitative mapping of tin-based metal obtained at 500 °C by EPMA.

this can improve the functionality of the copper by providing extra hardness and strength.51 It can, therefore, be used for

electrical conductivity applications in highly corrosive environments such as chemical and marine industries.52 G


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Figure 10. Quantitative mapping of Cu-based metal obtained at 1000 °C by EPMA.

XPS Analysis. To support the ICP and XRD studies, XPS was carried out on a thermo ESCALAB250Xi X-ray photoelectron spectrometer using monochromated Al Kα as the Xray source. XPS analysis also confirmed the absence of metal oxides in the product. We investigated the tin-based metal using XPS (Figure 7). The peak at 493.3 eV is recognized as pure tin.53 The binding of 484.90 eV for Sn 3d5/2 is almost equal to the literature value of pure Sn (485.0 eV)54 without any oxidation. The Zn 2p3/2 peak at 1021.90 (Figure 7b) attributes to pure zinc which is entirely devoid of oxides. XPS analysis of the copper metal confirmed the absence of copper oxides in the copper metal droplet obtained at 1000 °C. A characteristic copper 2p3/2 peak was obtained in the XPS spectra at binding energy 932.68 eV (Figure 8a). Satellite peaks of Cu(I) and Cu(II) oxides were absent which occur at 945 and 943 eV, respectively.53 The peak occurring at 952.5 eV belongs to Cu 2p1/2 which appears due to spin−orbit splitting of the copper 2p orbitals. Thus, XPS analysis has confirmed that copper is present in its pure metallic form in the final product and is entirely free of oxides. The peak at 493.3 eV is attributed to tin55 (Figure 8b). EPMA-WDS Analysis. In-depth EPMA-WDS investigations were carried out on the tin- and copper-rich metals recovered after heat treatments. Acquiring precise, quantitative, elemental analyses of material at very small spot sizes, as little as 1−2 μm, is possible by wavelength-dispersive spectroscopy (WDS). The spatial scale of analysis, combined with the ability to create detailed images of the sample, made it possible to analyze the recovered materials’ complex chemical variation (if there is any) within the matrix. Figure 9 clearly shows the homogeneous distribution of tin with a low concentration of zinc and lead. A very tiny copper-rich region (indicated by red arrow) was also revealed. The darker spots were exposed as a zinc-rich region within the tin matrix (indicated by red circle). A very low concentration of lead was revealed without any significant segregation.

Figure 10 shows an EPMA-WDS analysis of a copper-rich alloy recovered after 10 min of heat treatment at 1000 °C. Copper is distributed homogeneously and is overlaid by a small amount of concentrated tin-rich particles indicated by blue circles. The metallic phase was generated in the form of metal droplets while that remaining was the nonmetallic phase in the form of a carbonaceous/slag residue. The elemental analysis of the resulting two types of metal droplets showed a concentration of high purity tin-based and copper-based metals. The optimized temperature for the tin-based product was determined at 500 °C. The percent of tin starts decreasing after 500 °C. Due to its high solubility in copper, tin starts diffusing. In contrast, lead has limited solubility in solid copper, so that it could be removed successfully with the tin-based product at a lower temperature. The ICP analysis and EPMAWDS mapping results in the current study were found to be constant as mentioned in Tables 2 and 3. Our results from ICP analysis, XRD, XPS, and EPMA-WDS mappings have clearly shown that tin and copper could be recovered from waste PCBs at two different steps of TM. Although small PCB pieces (1−2 cm) were used in these laboratory investigations due to the small size of the furnace, large-sized PCB pieces can be inserted within the industrialscale furnace, while the mechanical crushing or powdering of PCBs was not necessary which reduces the operating cost. In a previous study, a copper-based product was synthesized from PCBs at 500 °C where PCB chips were kept at 300 °C for 30 min. Nonetheless, the authors did not mention anything about the soldering material of PCBs. In our study, we demonstrated the successful recovery of copper and the soldering material of PCBs which is the major concern and a matter of interest for a growing body of research.56 The soldering material is used on the top of the PCBs to join several components with it. The copper is used in a sheet form within PCBs, layer by layer, and the glass fibers and epoxy/ H


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Figure 11. Tensile stress−strain curve and quantitative analysis of the tensile test of standard solder alloy and recovered tin.

Figure 12. Current−voltage graphs of (a) Cu−Sn alloy recovered at 1000 °C and (b) standard copper.

another step to be recovered effectively. At higher temperature (950 °C or more), the copper melts and comes out from the carbonaceous/glass fiber layer, and after the rapid cooling, it was collected as metal droplets. We captured a substantial percentage of Pd and Zn with the solder material through the TM process which is economically and environmentally sustainable. The composition of the residue did not have Zn or Pb which is very promising. We assessed the properties of the both the recovered solder material and copper which exhibit superior properties. Properties of Recovered Metal Droplets. Mechanical Properties of Tin Obtained at 500 °C. Tin−zin-based metals and alloys have recently been considered as candidates for solder material because of low melting points and excellent mechanical properties.57 In order to evaluate the mechanical properties of the recovered solder material, tensile tests were carried out, as depicted in Figure 11. For the recovered tinbased sample, the ultimate tensile strength (σu) and total elongation (δ) were 44 MPa and 43%. On the other hand, considering the standard Sn−9 wt. % Zn solder alloy, σu and δ were 31.5 MPa and 55%, respectively. The curves represent average results of tensile tests carried out with three different specimens.

raisin/plastics are used between every layer. It was observed by the in situ observation of the heat treatment process that the soldering material melts at 200−250 °C. When Sn−Zn−Pbbased soldering alloys are in the liquid phase, they have very good wettability with Cu. If we keep the PCB over this temperature range for a longer time, liquid soldering alloy gets enough time to go through the thin and porous layers of glass fibers and spread over the Cu sheets. It will decrease the purity of copper, and it can even form some intermetallic compounds at lower temperatures (such as Cu3Sn, Cu6Sn5, Cu41Sn11, Cu10Sn3, etc.) and increase the impurities in copper, resulting in inhomogeneity of the recovered copper. The molten solder alloy also will mix with the residue, and later, it can seep into the environment. Moreover, the rapid thermal transformation results in the degradation of organic components and melting of the solder alloy of PCBs simultaneously. This transformation process is very fast, and there is insufficient time for the soldering material to go through plastics and glass fibers and spread over the pure copper of PCBs. As the plastic part of the surface of PCBs decomposed, the solder material is separated easily from PCBs, but the copper foils are still entangled with the carbonaceous material and glass fibers of PCBs which need I



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It is evident from the tensile testing that the combination of the alloying elements led to an increase in the strength of the recovered solder material of around 30%. However, a slight decrease in ductility occurred by this increased tensile strength. Electrical Property of Copper Obtained at 1000 °C. Copper is an excellent electrical conductor and used for numerous electrical applications. To understand the quality of the recovered copper at 1000 °C, we compared it with a standard pure copper available in the market. The current− voltage (I−V) plots gave the resistance for standard copper and the recovered copper metal as 1.489 and 1.505 Ω, respectively, which was the same in dimension and thickness (Figure 12). This result indicates that our recovered copper could act as a standard copper source for electrical applications. The corresponding electrical conductivity for the standard and recovered copper was very close: 4.84 × 107 ± 0.068 and 4.82 × 107 ± 0.08 S m−1, respectively. According to the International Annealed Copper Standard (IACS), the conductivity of the recovered copper is ∼80% of the IACS standard. This type of copper is suitable for various electrical appliances, such as switches.58,59 The best way to increase the electrical conductivity of copper is to decrease the impurity levels. The existence of impurities and all common alloying elements, except for silver, decrease the electrical and thermal conductivity of copper. However, the impurities like zinc and tin enhance the mechanical properties of copper.51 The extracted metal is also promising for marine application as this type of metal exhibits very high corrosion resistance under seawater.52

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.8b04657. Composition of residues after 500 and 1000 °C. Residues were digested with mix acid (HCL+HNO3) and analyzed semiquantitatively by ICP OES. List of compounds found in waste PCBs at 360 °C by GC-MS. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Rumana Hossain: 0000-0002-5585-931X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support for this research was provided by the Australian Research Council through Laureate Fellowship FL140100215. We gratefully acknowledge the technical support provided by the Mark Wainwright Analytical Centre in the UNSW Australia also Mr. Anirban Ghose for editing the manuscript.

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REFERENCES

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CONCLUSION

This investigation has shown that thermal-micronizing-based technologies can prevent the formation of hazardous constituents and their adverse effect on the environment during the processing of e-waste. This study has shown that it is possible to optimize the recovery of metals form problematic e-waste, especially waste printed circuit boards, through a judicious choice of operating temperatures and heating times at a low cost and simple technique. The thermal transformation below the melting point of copper and short heating times were found to play important roles in limiting the diffusion of lead and tin in the copper-rich fractions and enables recovery of metals with low melting points. Low levels of impurities present in recovered metals are likely to reduce its further refining commercial applications. Tin-based product without copper fraction was recovered in the first step of heat treatment (500 °C, 5−10 min). The impurities in tin-based alloys enhanced the tensile strength compared to the same kind of commercial solder alloy. Thermal transformation at 1000 °C was also quite promising for the recovery of high purity copper. The copper-based product had lead, tin, and zinc levels as low as 0.1% and 2.3% and 1.1%, respectively. The impurity levels in the recovered copper were found to increase with increasing temperatures and heating times. The recovered copper possessed electrical conductivity corresponding to ∼80% of the standard pure copper. This approach of metal recovery from spent printed circuit boards offers new solutions for the transformation of waste materials into value and reducing waste pollution. J


Research Article


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Sustainable Recovery of Cu and Sn from Problematic Global Waste

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