Perspective Cite This: ACS Sustainable Chem. Eng. 2017, 5, 9586-9598
pubs.acs.org/journal/ascecg
Recycling Tin from Electronic Waste: A Problem That Needs More Attention Congren Yang,† Quanyin Tan,† Lili Liu,‡ Qingyin Dong,‡ and Jinhui Li*,† †
State Key Joint Laboratory of Environment Simulation and Pollution Control, School of Environment, Tsinghua University, Beijing 100084, China ‡ Basel Convention Regional Centre for Asia and the Pacific, Beijing 100084, China ABSTRACT: The rapid generation of electronic waste (ewaste) has become a global problem owing to its potential environmental pollution and human-health risk, especially from informal recycling in developing countries. In 2014, however, only ∼15.5% of the total global e-waste was formally treated by national take-back programs. Waste printed circuit boards (PCBs) are an integral part of e-waste, and they contain many valuable metal resources. Most recycling from waste PCBs has focused on metals like Au, platinum group metals, and Cu, which have high economic value, but tin also makes up a large proportion of the metal in waste PCBs. Over the past decade, ∼44% of the refined tin has been used as solder in the electronics industry each year. Although current global tin reserves can meet the short-term demand, for long-term sustainable development, recycling tin from secondary resources, especially from e-waste, is essential. For addressing the shortage of mineral resources and conserving energy, tin recycling from ewaste needs more attention. KEYWORDS: E-waste, Waste PCBs, Tin, Recycling, Sustainability
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INTRODUCTION Waste electrical and electronic equipment (WEEE or e-waste) is a rapidly increasing waste stream, and coping with it is a complex problem. The worldwide amount of e-waste generation was estimated to be ∼45.7 million tons (Mt) in 2016 and is expected to be ∼49.8 Mt by 2018. China is the largest producer of e-waste, and its e-waste generation was estimated to be ∼10.4 Mt in 2016 and is expected to be ∼12.9 Mt by 2018.1−3 E-waste contains not only a significant amount of valuable metals (e.g., Cu, Fe, Al, Au, Ag, and Pd) but also many hazardous substances [e.g., Pb, Cr, Cd, polybrominated diphenyl ethers (PBDEs), polybrominated biphenyls (PBBs)].4−6 The rapid generation of e-waste has become a global issue owing to its potential for environmental pollution and human-health risks, especially when it is informally recycled in developing countries.7−10 Over the past several decades, informal recycling techniques like simple heating, open burning, and leaching with aqua regia have caused serious environmental and human-health problems.11−13 In an attempt to solve the ewaste problem, many countries and regions, such as the European Union (EU), China, Japan, and South Korea, have established a series of legislations and regulations.1,14−16 Waste printed circuit boards (PCBs) are an integral part of ewaste (∼4 wt %).17,18 Formally recycling metals from waste PCBs will not only address the shortage of mineral resources but also reduce environmental pollution and human-health risks. Therefore, many environmentally sound processes have © 2017 American Chemical Society
been developed for recovering valuable metals from waste PCBs: for example, physical separation based on differences in electrical19−21 and density22−24 properties, froth flotation based on differences in surface properties,23,25 pyrometallurgy,26,27 hydrometallurgy,28−31 biohydrometallurgy,32−35 and supercritical fluid.36−38 Desoldering and removing electronic components (ECs) was the first step in recovering valuable metals from waste PCBs.39 Tin is an essential metal for the electronics industry. During PCB manufacturing, tin solder is plated onto the copper surface as an etch resistant, and ECs like chips, resistors, capacitors, expansion slots, and so forth are mounted onto the surface of the PCBs with tin solder. In this study, we investigated world reserves, mining production, smelter production, and commercial applications of tin, summarized the various technologies available for the recovery of tin from waste PCBs, and analyzed the current status of recycling tin from e-waste.
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GLOBAL TIN RESERVES, PRODUCTION, AND APPLICATION Global tin reserves declined from 6,100,000 tons in 2005 to 4,700,000 tons in 2016 (Figure 1A). Estimated tin reserves have remained at ∼4,800,000 tons over the last five years even Received: August 21, 2017 Revised: September 10, 2017 Published: September 13, 2017 9586
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Figure 1. World tin reserves (A) and mine production (B). Data sources: Reserves: United States Geological Survey (USGS);40 mine production: USGS.40,41
though ∼266,000 tons are mined per year; reserve amounts are dynamic because, although they may be reduced as ore is mined, they may also be increased as new deposits are discovered or as economic variables and/or new technology improve the economic feasibility of further exploiting current deposits. In 2016, the majority of tin reserves were in China (24%), Indonesia (17%), Brazil (15%), Bolivia (9%), Australia (8%), and Russia (7%). Global tin mine production reached peak value (more than 301,000 tons) in 2007 (Figure 1B). Production was 280,000 tons in 2016; China was the leading producer (36% of world output), followed by Indonesia (20%), Burma (12%), Brazil (9%), Bolivia (7%), and Peru (6%) (Figure 1B). There are ∼23 different types of naturally occurring tin minerals, but only cassiterite is economically feasible to mine and recover. The grade of tin in ore is ∼0.2−1.5%.42−45 Although the content of tin in waste PCBs is ∼1−6%, the average value is ∼4%.44−47 Cassiterite is concentrated from tin ore using jigs, spirals, shaking tables, centrifugal concentrators, and froth flotation. Approximately 50−60% of cassiterite is recovered using gravity concentration. The fine particles are lost in the gravity tailings, but these can be recovered with centrifugal concentrators and froth flotation. The overall recovery rate of cassiterite is over 80%.42,43,48 Metallic tin is obtained from cassiterite concentrate via smelting (using ausmelt furnace, reverberatory furnace, electric furnace, etc.) followed by refining (using pyrometallurgy, electrolysis, etc.).49 The flowsheet of the recovery process of tin from ore is shown in Figure 2. Figure 3 shows the applications of refined tin worldwide. Global refined tin use reached peak value (∼372,700 tons) in 2007. Most refined tin is used for solder in electronics (44.1%), solder in industrial applications (8.8%), tin platings (16.4%), chemicals (13.9%), brass and bronze (5.5%), and float glass (2.1%). Refined tin use in 2010 was similar to that in 2007. In 2014, global refined tin use was estimated to be 358,500 tons, distributed thusly: solders-electronic (43.5%), solders-industrial (8%), tin platings (14.7%), chemicals (15.5%), lead acid batteries (7.3%), brass and bronze (5.2%), and float glass (2%). Sn-Pb solder (such as 63Sn-37Pb solder) has been used extensively in the electronics industry. However, lead is restricted for use in electrical and electronic equipment by legislation and regulations like Restriction of the use of certain hazardous substances in electrical and electronic equipment, Ordinance on Management of Prevention and Control of Pollution
Figure 2. Process of recovering tin from ore.
f rom Electronic and Information Products14,50 because it is harmful to the environment and human health.51−54 Lead-free solders, such as Sn-Cu, Sn-Ag, and Sn-Ag-Cu alloys, have therefore been developed.55
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RECYCLING TIN FROM WASTE PCBS Overall energy consumption can be reduced by recycling metal from end-of-life products (Table 1).59−63 For example, the electricity consumption for aluminum production from bauxite is almost 35-times higher than the consumption for recycling aluminum from end-of-life products, and the fuel consumption is more than 2.5-times higher.64 Furthermore, recycling metal from waste can significantly reduce the mine production. In 9587
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Figure 3. Applications of refined tin worldwide. Data sources: International Tin Research Institute (ITRI).56−58
Zhou et al.72 The waste PCBs were first pyrolyzed at 600 °C for 30 min, and the pyrolysis oil and gas were obtained, which can be used as fuel or chemical feedstock after treatment. Then, the solder was recovered via centrifugal separation when the pyrolysis residue was heated to 400 °C under a vacuum. These researchers also proposed another method for recovering solder: heating the waste PCBs to 240 °C using diesel oil as a heating medium then separating the solder via centrifuge.73 Park et al.74 used an infrared heater to heat waste PCBs to 250 °C, and a maximum 94% of ECs were removed from the waste PCBs, but they did not mention the recovery of solder. Wang et al.75 developed an automatic system at the pilot-scale for disassembling waste PCBs and recycling solder with heated air. The solder was completely removed from the waste PCBs at a temperature of 265 ± 5 °C. Zeng et al.39 used water-soluble ionic liquid [BMIm]BF4 (1butyl-3-methylimidazolium tetrafluoroborate) as a heating medium to dismantle ECs and recover tin solder from waste PCBs; nearly 90% of the ECs were removed from the waste PCBs at 250 °C, and high-purity solder was obtained from the water-soluble ionic liquid. Simultaneously, the water-soluble ionic liquid could be reused. Moreover, only minute amounts of benzene and methylbenzene were generated during the disassembling process. A set of pilot equipment using watersoluble ionic liquid for dismantling waste PCBs was also designed. The equipment consisted of a waste PCB feeding system, a water-soluble ionic liquid spraying system, a waste PCB washing system, a water-soluble ionic liquid recycling system, a system for separating and collecting ECs and bare boards, and an air releasing system. The dismantling of waste PCBs using an ionic liquid of [EMIM]BF4 (1-ethyl-3methylimizadolium tetrafluoroborate) was also investigated by
Table 1. Recycled Metals Energy Savings over Ore66,67 metal
energy savings (%)
Cu Pb Zn Al Fe
85 65 60 95 74
2016, the metallic lead production and usage were 11,144,000 and 11,121,000 tons, respectively, but only 4,721,000 tons of lead were mined.65 Approximately 44% of refined tin has been used as solder in electronics manufacturing over the past decade. Thus, recycling tin from e-waste is necessary for both the sustainable development of the electrical and electronics industry and for reduced energy consumption. Waste PCBs are integral parts of e-waste, containing wire boards and electronic components (ECs). The ECs are mounted onto the surface of the PCBs with tin solder. The compositions and melting points of solders are shown in Table 2. Sn-Pb (63Sn-37Pb, 60Sn-40Pb), Sn-Cu (99.3Sn-0.7Cu), SnAg (96.5Sn-3.5Ag), and Sn-Ag-Cu (95.5Sn-3.8Ag-0.7Cu, 95.54Ag-0.5Cu, 95.5Sn-3.9Ag-0.6Cu, 96.5Sn-3Ag-0.5Cu, 98.5Sn1Ag-0.5Cu) solder are widely used in the electronics industry; the melting points of these tin solders are in the range of 183− 227 °C. Thermal Treatment. To recover the tin solder and dismantle the ECs from waste PCBs, the waste PCBs should be heated to a temperature above the melting point of tin solder. Infrared heaters, electric heaters, and liquid-medium heating are the most commonly used for this purpose. A technique consisting of vacuum pyrolysis of waste PCBs, followed by centrifugal separation of solder, was proposed by 9588
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SnO2; however, the SnO2 can be dissolved in hydrochloric acid solution. Thus, Castro and Martins83 used a solution consisting of 3.0 mol/L HCl and 1.0 mol/L HNO3 to leach Sn and Cu from waste PCB powder, and the Sn and Cu extraction percentages were 98 and 93%, respectively. Finally, 85.8% of Sn and 34.3% of Cu were recovered by neutralizing the leaching solution using NaOH via precipitation. In a 1 mol/L HCl solution, ∼90% of Sn was leached from the waste PCBs with a particle size of −3 mm at 80 °C.84,85 Moreover, the waste PCBs with a particle size of −8 mm were first pyrolyzed at 900 °C under a nitrogen atmosphere, followed by leaching of Sn from the pyrolyzed sample investigated in a 1 mol/L HCl solution at 80 °C. Nearly 95% of Sn was leached out.85 Zhang et al.86 reported a selective dissolution of tin−lead solder from waste PCBs. Almost 100% of the solder was dissolved selectively with 2.5 mol/L HBF4 and 0.4 mol/L H2O2 at 20 °C. The standard electrode potentials of H2O2, Sn, and Pb are 1.776, −0.136, and −0.126 V, respectively (eqs 1−3). The differences of the standard electrode potentials between H2O2 and Sn and H2O2 and Pb are 1.912 and 1.902 V, respectively (eqs 4 and 5). Thus, Sn and Pb can be oxidized by H2O2 at room temperature. For Cu, the standard electrode potential is 0.337 V (eq 6). The Cu can also be oxidized by H2O2 at room temperature, because the difference in the standard electrode potential between H2O2 and Cu is 1.439 V (eq 7). Actually, however, very little Cu was leached out for two reasons: galvanic corrosion and the displacement reaction between Cu2+ and both Sn and Pb. In the galvanic corrosion process, the metal with the higher electrode potential acts as a cathode, whereas the metal with the lower electrode potential acts as an anode. Therefore, the dissolution rate of the metal with the lower potential (anode) increases, but the dissolution rate of the metal with the higher potential (cathode) decreases.87 As for the displacement reactions between Cu2+ and Sn or Pb (eqs 8 and 9), the standard electrode potential of Cu is far more than that of either Sn or Pb. Similar, the tin− lead solder can be dissolved selectively with 3.5 mol/L methanesulfonic acid (MSA) and 0.5 mol/L H2O2.88
Table 2. Composition and Melting Points of Various Solders55,68−71 alloy In-Bi-Sn In-Bi Bi-In-Sn Bi-In In-Sn Bi-Sn Sn-Bi-In Sn-Zn-In-Bi Sn-In-Ag Sn-In-Zn Sn-Pb Sn-Zn-Bi Sn-Zn Sn-In-Ag Sn-Ag-Bi Sn-Bi-Ag Sn-Ag-Bi Sn-Bi-Ag Sn-Ag-Cu-Sb Sn-Ag-Zn Sn-Ag-Cu
Sn-Au Sn-Ag Sn-Cu Sn-Sb Sn-Ag-Sb Sn-Sb Au-Sn
composition
melting range (°C)
51In-32.5Bi-16.5Sn 66.3In-33Bi 57Bi-26In-17Sn 54.02Bi-29.68In-16.3Sn 67Bi-33In 52In-48Sn 50In-50Sn 58Bi-42Sn 70Sn-20Bi-10In 86.5Sn-5.5Zn-4.5In-3.5Bi 77.2Sn-20In-2.8Ag 83.6Sn-8.8In-7.6Zn 63Sn-37Sn 60Sn-40Pb 89Sn-8Zn3Bi 91Sn-9Zn 86.9Sn-10In-3.1Ag 93.5Sn-3.5Ag-3Bi 95Sn-2Bi-3Ag 91.8Sn-3.4Ag-4.8Bi 91Sn-7.5Bi-2Ag 96.7Sn-2Ag-0.8Cu-0.5Sb 95.5Sn-3.5Ag-1Zn 95.5Sn-3.8Ag-0.7Cu 95.5Sn-4Ag-0.5Cu 95.5Sn-3.9Ag-0.6Cu 96.5Sn-3Ag-0.5Cu 93.6Sn-4.7Ag-1.7Cu 98.5Sn-1Ag-0.5Cu 90Sn-10Au 96.5Sn-3.5Ag 98Sn-2Ag 99.3Sn-0.7Cu 99Sn-1Sb 97Sn-3Sb 65Sn-25Ag-10Sb 95Sn-5Sb 80Au-20Sn
60 (eutectic) 72 (eutectic) 79 (eutectic) 81 (eutectic) 109 (eutectic) 118 (eutectic) 118−125 138 (eutectic) 143−193 174 175 181 183 (eutectic) 183 (near eutectic) 189−199 198.5 (eutectic) 204 206 210 211 212 216 217 217 (near eutectic) 217 217 217 217 217 217 221 (eutectic) 221−226 227 (eutectic) 232 232 233 235 280 (eutectic)
Zhu et al. The solder was removed at 240 °C with a stirring speed of 150 rpm and a desoldering time of 10 min. Hydrometallurgy. Hydrometallurgical technology is widely used to recover tin from waste PCBs owing to its low operating temperature. HNO3 and HCl are most commonly used. Mecucci and Scott78 achieved the recycling of Cu, Pb, and Sn from waste PCBs. The waste PCBs were first crushed to 2.5 mm2; then, the valuable metals, e.g., Cu, Pb, Sn, Zn, and Ni, were leached from the crushed waste PCBs with HNO3. It is well established in the literature that Sn reacts with HNO3, which converts it into an insoluble SnO2 because hydrolysis of Sn4+ to Sn(OH)4 occurs very easily.79−81 The SnO2 was then separated from the leaching solution via filtration. The insoluble SnO2 was dissolved in 1.5 mol/L HCl followed by electrodeposition of Sn. Cu and Pb were also recycled using electrodeposition. In another study, essentially all of the Pb was selectively leached from waste PCBs with 0.2 mol/L HNO3 at 90 °C for 45 min, and then 98.74% of Sn was leached with 3.5 mol/L HCl at 90 °C for 120 min.82 The tin can be oxidized to Sn4+ by nitric acid, which then converts it into an insoluble 76,77
H 2O2 + 2H+ + 2e− = 2H 2Oφ10 = +1.776 V
(1)
Sn 2 + + 2e− = Snφ20 = −0.136 V
(2)
Pb2 + + 2e− = Pbφ30 = −0.126 V
(3)
H 2O2 + Sn + 2H+ = Sn 2 + + 2H 2OΔφ1 = +1.912 V (4)
H 2O2 + Pb + 2H+ = Pb2 + + 2H 2OΔφ2 = +1.902 V (5)
Cu
2+
−
+ 2e =
Cuφ40
= + 0.337 V
(6)
H 2O2 + Cu + 2H+ = Cu 2 + + 2H 2OΔφ3 = + 1.439 V (7)
Sn + Cu 2 + = Sn 2 + + Cu Pb + Cu
2+
2+
= Pb
+ Cu
(8) (9)
89
Yang et al. proposed a closed-loop process for recycling tin from multimetal powder originating from waste PCBs. The results showed that 99% of the tin was leached out with SnCl4 and HCl at 60−90 °C, and the tin was then recovered from the 9589
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ACS Sustainable Chemistry & Engineering Table 3. Summary of Various Technologies for Recycling Tin from Waste PCBs technological process
product
Thermal Treatment 1. vacuum pyrolysis of waste PCBs at 600 °C (30 min) → heating pyrolysis residue to solder 400 °C (vacuum) → centrifugal separation of solder (1200 rpm, 10 min) 2. heating waste PCBs to 250 °C with infrared heater solder 3. heating waste PCBs to 265 ± 5 °C with heated air
solder
4. heating waste PCBs in diesel oil (240 °C) → centrifugal separation of solder (1400 solder rpm, 6 min) 5. heating waste PCBs via [BMIm]BF4 (250 °C, 45 rpm, 12 min) → condensation → solder solid−liquid separation → high-purity solder 6. heating waste PCBs via [EMIM+][BF4−] (240 °C, 150 rpm, 10 min) → solder condensation → solid−liquid separation → solder Hydrometallurgy 7. leaching solder with 2 mol/L HNO3 → dissolution of SnO2 precipitate with 1.5 metallic tin mol/L HCl → electrodeposition 8. selectively leaching Pb with 0.2 mol/L HNO3 (90 °C, 45 min) → leaching Sn with 3.5 mol/L HCl (90 °C, 120 min) 9. leaching waste PCB powder with 3.0 mol/L HCl + 1.0 mol/L HNO3 → SnO2 precipitation precipitate 10. leaching tin from waste PCBs (−3 mm) with 1 mol/L HCl (80 °C, 180 min) 11. pyrolysis of waste PCBs (−8 mm) at 900 °C → leaching tin from pyrolysis residue with 1 mol/L HCl (80 °C, 180 min) 12. selectively leaching solder with 2.5 mol/L HBF4 and 0.4 mol/L H2O2 (20 °C, 35 min) 13. selectively leaching solder with 3.5 mol/L MSA and 0.5 mol/L H2O2 (20 °C, 45 min) 14. leaching tin with SnCl4 and HCl (60 to 90 °C) → purification → metallic tin electrodeposition 15. leaching waste PCBs with spent TSS (S/L = 1:5−1:8, 2 h) → precipitation of Sn SnO2 precipitate Crushing−Separation 16. crushing → flotation → Sn-rich products Sn-rich products 17. crushing → electrostatic separation → mixed metallic particles → vacuum Sn-rich metallurgy → Sn-rich materials materials a
tin recovery a
b
a
a
b
a
environmental impact high energy POPs high energy POPs high energy POPs high energy
ref
consumption, heavy metals,
72
consumption, heavy metals,
74
consumption, heavy metals,
75
consumption, heavy metals
73
high energy consumption, heavy metals, organic pollutants high energy consumption, heavy metals, organic pollutants
39 76,77
NOx, HCl, Cl2, heavy metal wastewater
78
98.74%
NOx, HCl, heavy metal wastewater
82
84.1%
NOx, HCl, heavy metal wastewater
83
∼90% ∼95%
HCl, heavy metal wastewater high energy consumption, heavy metals, POPs, HCl, heavy metal wastewater heavy metal wastewater
84 85
∼100% ∼100% 99% 86−97%
86
organic pollutants, heavy metal wastewater HCl, Cl2, heavy metal wastewater
89
NOx, heavy metal wastewater
81
wastewater
25
high energy consumption
88
96,97
The solder was completely separated. bWith 90−94% of the ECs removed.
NaOH, but a large amount of NaOH is consumed because of its high nitric acid concentration. Nevertheless, many environmentally sound processes for recycling HNO3 and metals have been developed.90−95 Recently, a novel technique for coprocessing waste PCBs and spent TSS at room temperature was developed by Yang et al.81 Approximately 87% of the Sn− Pb solder, 30% of the Cu, 29% of the Fe, and 78% of the Zn was leached from waste PCBs with spent TSS after 2 h. Approximately 99% of the Sn, Pb, Fe, Cu, and Zn were recovered from the leaching solution by step precipitation in the following order: Sn → Pb → Fe → Cu → Zn. At the same time, more than 87% of the ECs were removed from the waste PCBs. This proposed environmentally friendly process has substantial advantages over traditional recovery methods of heating waste PCBs in terms of both material recovery and energy efficiency. Other Methods. Estrada-Ruiz et al.25 developed a technology for separating the metallic and nonmetallic fractions from waste PCBs. The waste PCBs were first crushed to −250 μm. The metallic fraction was then separated from the nonmetallic portion by froth flotation using a flotation column with a superficial air velocity of 0.4 cm/s, taking advantage of the difference in surface properties between the nonmetals and metals: the nonmetallic portion is hydrophobic whereas the metallic portion is hydrophilic. Hence, the metals were separated from the nonmetallic portion as concentrate and
purified solution by electrodeposition. During the leaching process, Sn was oxidized by SnCl4 according to eq 10. The leachate was purified to remove impurities such as Cu2+ and Pb2+, for electrowinning. During the electrowinning process, Sn2+ in the purified solution was reduced to Sn at the cathode (eq 2). Simultaneously, H+ was reduced to H2 at the cathode (eq 11), causing a decrease in current efficiency. At the anode, Sn2+ was oxidized to Sn4+ (eq 12), whereas Cl− may have been oxidized to Cl2 (eq 13). After electrowinning, the anode and cathode waste solution was reused for leaching tin from the multimetal powders.
Sn + SnCl4 = 2SnCl 2
(10)
2H+ + 2e− = H 2↑
(11)
Sn 2 + = Sn 4 + + 2e−
(12)
2Cl− = Cl 2 ↑ + 2e−
(13)
Tin stripping is one of the key processes of PCB manufacturing. During the production of PCBs, a large volume of spent tin stripping solution (spent TSS), containing ∼5% Sn with ∼6 g/L of Cu, ∼10 g/L of Fe, and ∼4 mol/L HNO3, is produced. Most of the Sn in the spent TSS presents as insoluble hydrated SnO2. Currently, valuable metal in spent TSS is usually recycled via neutralization-precipitation with 9590
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Figure 4. World tin smelter production (A), primary tin smelter production (B), and secondary tin smelter production (C) (Data source: USGS41).
Hydrometallurgical technology is therefore widely used to recover tin from waste PCBs because of the low operating temperature. However, many pollutants, such as NOx, HCl, Cl2, and so forth, could be released into the environment, and a large volume of heavy metal wastewater will be produced. Like the thermal treatment, pollution control technology needs to be developed too, but there are few studies on the pollutants release and control in the process of hydrometallurgy. Furthermore, coprocessing hazardous waste may be a good way to disassemble waste PCBs and recycle valuable metals. Recently, hazardous wastes have been coprocessed in cement kilns at the industrial scale, and coprocessing methods have also been developed for CRT funnel glass and lead concentrates.103,104
tailings, respectively. The metallic fraction consisted of 7.91% C, 12.27% O, 0.44% Al, 52.57% Sn, and 26.81% Pb. Mechanical−physical recycling processes have also been widely used to recycle metal from waste PCBs. The waste PCBs are first crushed, and then the metallic fraction is separated from the nonmetallic portion by electrostatic separation, obtaining a product containing various metals (e.g., Cu, Pb, Sn, Zn). The pure metal can be separated from the mixed metallic particles with vacuum metallurgy.96 For example, Pb is first separated from Cu-rich particles containing Cu, Pb, and Sn at 1123 K under 0.1−1 Pa. Then, an aggregate Cu-Sn intermetallic compound is formed during the separation of Pb from Cu-rich particles using vacuum metallurgy. The aggregate Sn-rich materials are then further separated from Cu particles by sieving.97 Environmental Impact. Various technologies for recycling tin from waste PCBs are summarized in Table 3. Heating is the most effective way to disassemble waste PCBs and recover solder. The melting points of tin solders are in the range of 183−280 °C. The recovered solder is a good resource of tin for refining. However, several studies have indicated that polychlorinated dibenzo-p-dioxin and dibenzofurans (PCDD/ Fs) and polybrominated dibenzo-p-dioxin and dibenzofurans (PBDD/Fs) are formed during the thermal processing of waste PCBs at temperatures between 250 and 400 °C.98,99 Heavy metals (e.g., Cu, Pb, Cd, and Cr) and persistent organic pollutants (POPs) have been found in e-waste recycling site.9,100−102 Therefore, pollution control technology also needs to be developed.
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TIN LONG-TERM SUSTAINABLE SUPPLY Waste PCBs are the most valuable part of e-waste; they contain a significant mass of valuable metals, accounting for approximately 40% of the metal recovery value of e-waste.18 In 2014, approximately 51% of the refined tin was used in the electronics industry and in lead acid batteries. The amount of tin in e-waste in 2014 was estimated at 35% of the mined metal in the same year.18 In 2016, world tin reserves and mine production were estimated at 4,700,000 and 280,000 tons, respectively.40 At these rates, current reserves of tin will be depleted in approximately 16 years. For long-term sustainable development, recycling tin from secondary sources will be essential. According to USGS statistics,41 world tin smelter production increased from 344,000 tons in 2005 to 349,000 tons in 2015 with approximately 94% of these amounts 9591
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Figure 5. Cumulative tin mine production and circulability values for tin under different e-waste collection rates (A and B) and content of tin in waste PCBs (C and D).
developed countries was illegally exported to developing countries like China and India.63 In the European Union, approximately 40% of the e-waste was collected and treated by official take-back systems owing to a series of legislations and regulations, e.g., Directive on Waste Electrical and Electronic Equipment (WEEE directive).1 Simultaneously, 8% (0.7 Mt) of the total e-waste (such as phones, USB sticks, lamps, etc.) was thrown into the waste bin. Approximately 12% of the e-waste generated in the United States was collected by official takeback systems, but Namias109 reported that the United States did not process any of this e-waste: 50−80% of it was illegally exported to developing countries such as China, India, and Pakistan, and the remainder was processed via pyrometallurgical processing at copper smelters in Western Europe and Canada. Approximately 24% of the e-waste generated in Japan was collected by official take-back systems. In China, approximately 28% of the e-waste generated was officially collected and treated; although there are a total of 109 formal ewaste dismantling enterprises in China, informal collection and recycling of e-waste still play a major role. For some small equipment like waste mobile phones, the recycling rate is even lower.110,111 Tan et al.112 reported that less than 20% of the old mobile phones are collected, and more than half are stored in
produced by primary tin smelters (Figure 4A). The majority of primary tin smelters were in China, Indonesia, Peru, and Thailand (Figure 4B). Most of the secondary tin smelters were in the United States and Belgium. In the United States in 2015, approximately 12,000 tons of tin was recycled from old scrap, metal in products having reached their end-of-life,105,106 and new scrap, originating from a fabrication or manufacturing process105,106 (Figure 4C), accounting for approximately 30% of apparent consumption. Most of the tin was recovered from old scrap at detinning plants and secondary nonferrous metalprocessing plants.107 ITRI survey results showed that, in recent years, the global refined tin production was approximately 330,000−370,000 tons per year; the mine production was approximately 270,000−310,000 tons per year. The difference between the two filled by some 50,000−70,000 tons per year of secondary refined tin production.108 The proportion of secondary refined tin production is only approximately 17%. According to Balde et al.,3 the worldwide amount of e-waste generation was approximately 41.8 Mt in 2014. The regions and countries with the highest e-waste generation were the European Union, China, United States, and Japan, where 11.6, 8.53, 7.1, and 2.2 Mt of e-waste were generated, respectively. At the same time, almost 50% of the e-waste generated by the 9592
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Figure 6. Proposed scenario for tin sustainable supply.
reduced by recycling tin from waste PCBs. On the basis of the above hypotheses, the future global mine production per year (Qmine/year) can be estimated with the equation
homes. In summary, only around 15.5% (approximately 6.5 Mt) of the e-waste generated at the global scale in 2014 was formally treated by national take-back programs. The economic value of Au, platinum group metals (PGMs), Cu, Sn, and Ag present in waste PCBs constitutes 59, 15, 15, 7, and 4% of their values, respectively.18 Compared with recycled Au, PGMs, and Cu from waste PCBs, the economic value of tin is lower,18,112,113 and recovering these more valuable metals from e-waste has had the highest priority, especially in the informal recycling sector.112,114 Surprisingly, most of these metals in waste mobile phones, such as Au (88−92%), Pd (88%), and Ag (89%), are discarded.115,116 This disregard for even the more valuable metals, combined with the comparatively lower value of tin, can explain the low rate of tin recycling from e-waste. However, as is clear from the recycling practices in the European Union, there is a proven benefit from recycling e-waste given the sound legislation and regulations there.1,14,15 The effect of the e-waste collection rate by official take-back systems on global tin long-term sustainable supply was evaluated in this study under the following assumptions: (1) the worldwide amount of e-waste generation was approximately 45.7 Mt in 2016, and the annual growth rate of e-waste was set to 4.5% based on Baldé et al.;3 (2) the e-waste collection rate (CR) by official take-back systems was set to 15.5, 40, 60, 80, and 100%; (3) the average weight of the waste PCBs in e-waste was 4%, and the average value of the tin content (CTin) in waste PCBs was 4%; (4) the tin recovery rate, also called the recycling process efficiency rate (RR), was set to 85% based on Table 3; (5) world tin reserves were estimated at 4,700,000 tons in 2016, and the future global mine production was assumed to be 270,000 tons per year based on the annual average values over the past decade (Figure 1B). The tin mine production can be
Q (year)(CR, CTin) = 270,000 − 45.7 × 1,000,000 × (1 + 4.5%)(year − 2016) × CR × 4% × C Tin × 85%(tons) (14)
The cumulative tin mine production under different e-waste collection rates can be estimated with the equation year
∑ Q (year)(CR,C 2016
Tin)
≤ 4, 700, 000(tons) (15)
The cumulative tin mine production amounts under different e-waste collection rates and contents of tin in waste PCBs are shown in Figure 5. The circulability (CM) defined by Sun et al.117 was also presented in Figure 5, where CM = R/(R + M), such that R is the amount of tin recycling from waste PCBs each year, M is the amount of tin mine production each year, and R + M equals to 270,000 tons per year. When CM = 0, it indicates that the tin to meet the consumption demand is only supplied by mining; when CM = 1, it indicates that the tin to meet the consumption demand is only supplied by recycling from waste PCBs. All the recycling scenarios showed that the recovery of tin from e-waste could significantly reduce the tin mine production and extend the service life of the mine, especially when a large amount of e-waste was generated and a high e-waste collection rate was achieved. Therefore, a proposed scenario for tin sustainable supply is shown in Figure 6. 9593
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Biographies
CONCLUSIONS AND PERSPECTIVES
The rapid generation of e-waste has become a global problem owing to its potential for environmental pollution and humanhealth risks, especially when it is informally recycled in developing countries. Waste PCBs are an integral part of ewaste, and they contain many valuable metal resources. Throughout the past decade, approximately 44% of the refined tin has been used as solder in the electronics industry each year. In 2014, the amount of tin in e-waste was estimated to be approximately 35% of the amount of metal mined in that year. Current reserves of tin will be depleted in approximately 16 years. For long-term sustainable development, therefore, recycling tin from secondary sources (especially from ewaste) will be essential. Many environmentally sound processes have been developed for recycling tin from waste PCBs. In actuality, however, the worldwide amount of e-waste generation was approximately 41.8 Mt in 2014, but only around 15.5% (approximately 6.5 Mt) of the e-waste generated at the global scale in 2014 was formally treated by national take-back programs. On the other hand, compared with Au, PGMs, and Cu recycled from waste PCBs, the economic value of tin is low. These can explain the low tin recycling rate from e-waste. Nevertheless, there is a clear benefit to recycling e-waste; where there is a sound legislation and regulations system, as in the European Union, recovering tin from e-waste can significantly reduce the tin mine production and extend the service life of mines. Therefore, to address the shortage of mineral resources and conserve energy, sound e-waste legislation and regulations must be established to improve the collection rate of e-waste by official take-back systems. Simultaneously, green technology for recycling of tin from e-waste needs to be developed. Recycling tin from e-waste will play a significant role in tin long-term sustainable supply, but recycling tin will depend on the balance between the revenue of recycling tin from e-waste and the falling concentrations of tin found in end-of-life products as a result of development of science and technology.
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Dr. Congren Yang is a postdoctoral fellow at the School of Environment of Tsinghua University (Beijing, China). He received his B.Sc., M.Sc., and Ph.D. in minerals processing engineering from the School of Minerals Processing and Bioengineering, Central South University (Changsha, China). His research interest focuses on mineral processing, hydrometallurgy, electronic waste treatment, and metals recycling and sustainability. He has published over 20 peerreviewed journal articles.
Dr. Quanyin Tan received his diploma and Ph.D. degree in Environmental Science and Engineering from Tsinghua University (Beijing, China). Presently, he is a postdoctoral fellow of School of Environment, Tsinghua University, and his interests focus on waste management, technologies for critical and valuable metals recycling from wastes, and the sustainability and life cycle environmental impacts of these elements.
AUTHOR INFORMATION
Corresponding Author
*Room 804, Sino-Italian Environmental and Energy-Efficient Building, School of Environment, Tsinghua University, Haidian District, Beijing 100084, China. E-mail:
[email protected]. cn. Tel.: +86-10-62794143. Fax: +86-10-62772048. ORCID
Congren Yang: 0000-0001-7040-2302 Jinhui Li: 0000-0001-7819-478X Notes
Dr. Lili Liu is a senior program officer of Basel Convention Regional Centre for Asia and the Pacific. She received her B.Sc. degree in
The authors declare no competing financial interest. 9594
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Chemistry, M.Sc. degree in Organic Chemistry, and Ph.D. in Environmental Chemistry. Her research interest is circular economy and urban mining, environmental technology and risk assessment, ewaste policy and management, solid waste treatment, and disposal technology. She has led approximately 50 projects related to solid waste treatment, policy, and management. She has published over 30 articles and obtained 15 patents.
REFERENCES
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Qingyin Dong is a program officer of convention implementation support branch at Basel Convention Regional Centre for Asia and the Pacific. He is an engineer with experience in solid waste management and governance, recycling technologies, and demonstration. His current research focuses on developing new governance models on hazardous waste from social resources in China.
Dr. Jinhui Li is a professor in the School of Environment of Tsinghua University, executive director of Basel Convention Regional Centre for Asia and the Pacific, and director of Circular Economy Branch of Chinese Society of Environmental Sciences. He obtained his B.Sc. in 1987, M.Sc. in 1990, and Ph.D. in environmental chemistry in 1997. His research includes circular economy and urban mining, policy and management of solid waste and hazardous waste, resource technology for e-waste and hazardous waste, and environmental risk assessment. He won the second prize of National Science and Technology Progress Award. He has published over 300 articles and chaired for decades the International Conference on Waste Management and Technology (ICWMT).
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ACKNOWLEDGMENTS
This work was supported by the National Key Technology R&D Program (2014BAC03B04). We also thank Drs. Xianlai Zeng and Abhishek Kumar Awasthi for their valuable advice. 9595
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