Separating and Recovering Pb from Copper-Rich Particles of Crushed

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Separating and Recovering Pb from Copper-Rich Particles of Crushed Waste Printed Circuit Boards by Evaporation and Condensation Lu Zhan and Zhenming Xu* School of Environmental Science and Engineering, Shanghai Jiao Tong University, 800 Dong chuan Road, Shanghai, People’s Republic of China

bS Supporting Information ABSTRACT: Waste printed circuit boards (WPCBs) are treated by crushing and electrostatic separation to obtain the copper-rich particles. However, the copper-rich particles contain a certain content of solder, which may cause Pb contamination if improperly treated. The separation behaviors of Pb from single solder and solder mixed with Cu particles under vacuum are studied in this work. Due to the presence of Cu particles in the copper-rich particles, it becomes much easier to separate Pb from mixed particles than from single solder. On the basis of the experiments, the rules and phenomena different from previous studies are concluded, including the multilayer evaporation effect, the formation of CuSn intermetallic compound and so on. Mechanisms of these phenomena are also explored. Pb is separated and recovered from copper-rich particles of crushed WPCBs at 1123 K for 90 min under 0.11 Pa. The metals including Cu, Pb, Sn in WPCBs are all efficiently recovered. This work enriches separating rules for recovering Pb by evaporation and condensation, and also points out an efficient and promising method for recovering toxic heavy metals from WPCBs.

’ INTRODUCTION Due to technological advances and the obsolescence of electronic equipment on the market, the amount of e-waste is growing increasingly quickly. It is estimated that 2050 million tons of e-waste are generated around the world each year.1,2 Printed circuit boards (PCBs) are important parts of electronic equipment, accounting for approximately 3% of the total generated e-waste.3,4 Metals such as copper, tin, lead, iron, and precious metals constitute about 30% of the PCBs by weight (copper: ∼16%, solder: ∼4%, iron and ferrite: ∼3%, nickel: ∼2%), whereas nonmetals constitute the remaining 70%.5 However, the backyard operations in Asia, including open sky incineration and acid leaching, are worrisome because of their adverse effects on the environment. Many toxic ingredients such as lead contained in e-wastes may enter into aquatic and terrestrial ecosystems and even into the atmosphere, which has resulted in the elevation of blood lead levels of children.69 Much work has been done to deal with these urgent phenomena. Pyrometallurgical, hydrometallurgical, biological, and mechanicalphysical processes mainly place emphasis on the recovery of precious metals.1020 For e-wastes with very low concentrations of precious metals, most of the existing methods focus on recovering copper in them. However, the heavy metals with high vapor pressure such as Pb, which are also toxic, are wasted as metal oxides and enter into dust or slag during traditional smelting. r 2011 American Chemical Society

Previous research was carried out on mechanical two-step crushing, corona electrostatic separation, and the recovery of the nonmetals.21,22 However, mixed metallic particles obtained from these processes are still mixtures of various metals. To increase the purity of copper and recover the metals with high vapor pressure, literature 23,24 reported the feasibility and the separation criteria of separating mixed metallic particles by vacuum metallurgy separation (VMS). VMS does not need secondary offgas or wastewater treatment, and is believed to be an environmentally friendly method. In most cases, the mixed metallic particles contain a certain content of solder, which may lead to Pb contamination without proper treatment. In order to recover Pb and eliminate Pb contamination, this work further studies the separation behavior of Pb from copper-rich particles of waste printed circuit boards (WPCBs) by VMS. This work investigates the rules and parameters of separating Pb from solder and solder mixed with Cu particles. Since a majority of the Cu particles are present, some rules and phenomena are different from previous studies. 23,24 Then, the flowchart of recovering Pb from copper-rich particles of crushed WPCBs is Received: January 20, 2011 Accepted: May 9, 2011 Revised: May 8, 2011 Published: May 19, 2011 5359

dx.doi.org/10.1021/es200243f | Environ. Sci. Technol. 2011, 45, 5359–5365

Environmental Science & Technology

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Figure 1. Curve of the Pb separation efficiency from solder and solder-copper mixed particles (a), and XRD patterns of original solder (a) and the solder treated at 1123 K for 60 min (b) and 90 min (c).

given. Energy cost and economic assessment are also analyzed. This work greatly enriches the separating rules for recovering metals (especially Pb) by VMS, and also provides a promising method for recovering toxic heavy metals from WPCBs.

’ EXPERIMENTAL SECTION 1. Study on the Separation Behavior of Pb from CopperRich Particles. Two categories of materials were adopted to

study the rules and parameters of separating Pb from copper-rich particles. The first was particles of solder Pb40Sn60. The other was solder and copper mixed particles with the copper content of 95 wt %.24 The sizes of particles (both solder and copper particles) adopted were between 0.08 and 1.2 mm according to literature.21 The tests with solder particles were conducted in 1 g scale. The tests with solder and copper particles were conducted in 2 g scale and the layer height of feeding materials was 1.5 mm. The experimental data were repeated three times. In this work, an exploratory vacuum resistance furnace (as shown in Figure S1 in the Supporting Information, SI) was fabricated and used. The furnace wall with a movable watercooler was fabricated from a stainless steel plate. The vacuum chamber was evacuated by means of a two-stage pumping system consisting of a mechanical vacuum pump and an oil diffusion pump. All of the experiments were carried out under a dynamic vacuum level between 0.1 and 1 Pa. Power supply to the furnace remained uninterrupted, predicting excellent temperature stability. The heating rate was 20 °C/min in all cases in this work. The experimental procedure was as follows: (1) Different feeding materials were put into the furnace. (2) After reaching the set vacuum degree of 1 Pa, the heating process started. (3) When the furnace cooled down to an ambient temperature, the ceramic crucible containing residue was weighed and the separation efficiency of Pb calculated. After the vacuum separation processing, the output materials were characterized and analyzed by X-ray diffraction (XRD, D8 ADVANCE, BRUKER, Germany) with Cu KR radiation, operated at 40 kV and 25 mA, and Inductively coupled plasma (ICP, IRIS-Advantage 1000, THERMO, U.S.). 1.1. Experiments with Solder Particles. As shown in Figure S2 of the SI, the vapor pressures of Pb and Sn at the same temperature

are quite different. According to the principle of VMS, Pb with high vapor pressure and low boiling point can be separated from Sn by VMS. Then, Pb can be recovered through condensation. Solder particles were heated for 30 min at 873, 973, 1073 and 1123 K, respectively. The Pb separation efficiency range is 4.751.3 wt %, as shown in Figure 1a. With the heating time at 1123 K prolonged, the separation efficiency is 57.8 wt % for 60 min and 76.6 wt % for 90 min. The output materials were analyzed by XRD as shown in Figure 1bb and 1bc. Figure 1ba is the pattern of the original solder. By comparing the diffraction peaks with ICDD card No.040686, it can be seen that Pb still exists in the residue alloy. According to calculation, 23.4 wt % of the initial Pb in solder is left when heating for 90 min. By ICP examination, Pb left accounts for 13.5 wt % in the residue that has excluded the vaporized Pb. It can be concluded that Pb in the solder cannot be separated completely under the conditions of this study. Pb content in the PbSn alloy plays an important role in separating Pb. As shown in Table S1 of the SI, it presents the vapor pressures of Pb in PbSn alloy with different Pb contents at different temperatures. Table S1 of the SI shows that Pb vapor pressure decreases with a decrease in Pb content in PbSn alloy. Meanwhile, the evaporation rate of Pb also decreases. During the process of vacuum separation, Pb content in the residue alloy decreases with an increase in the heating time. The vapor pressure and evaporation rate of Pb become lower and lower. As a result, Pb in the solder cannot be separated completely in the given heating time (90 min). 1.2. Experiments with Solder and Copper Particles. Copper is in the majority in WPCBs.10 Therefore, it is necessary to study the separation behavior of Pb from solder in the presence of a large amount of Cu particles. Similarly, the solder and copper mixed particles were heated for 30 min at 873, 973, 1073 and 1123 K, respectively. As shown in Figure 1a, Pb separation efficiencies are obviously higher than those of solder, when the solder was mechanically mixed with copper particles. The Pb separation efficiency range is 13.998.5 wt %. After the process of VMS at 1123 K, the Pb content left in the solder and copper mixed particles decreases from 2 wt % to less than 0.03 wt % by the examination of ICP. XRD was conducted to identify phases in the materials remaining in the crucible and the condensation materials. 5360

dx.doi.org/10.1021/es200243f |Environ. Sci. Technol. 2011, 45, 5359–5365


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Figure 2. The solder and copper mixed particles before separating (a) and after separating (b, c), and XRD patterns of the output materials (d, e).

Figure 3. The models of separating Pb from solder and solder-copper mixed particles.

By comparing the major diffraction peaks with ICDD card No. 040836 and 040686, it indicates that the incompact materials left in the crucible are pure Cu (Figure 2d) and the collected prills are pure Pb (Figure 2e). When the Pb vapor came into the condensation room, the Pb vapor became supersaturated at a lower temperature. Due to the poor wetting ability between Pb and Fe, the supersaturated Pb vapor was coagulated as prills on the condenser made of iron, as shown in Figure 2c. Pb in the solder and copper mixed particles can be separated almost completely. It becomes much easier to separate Pb from the mixed particles (solder mixed with copper particles) than from single solder. Two reasons are presented as follows. 1.2.1. Multilayer Evaporation Effect. During the process of VMS, the solder particles melt first when the temperature is above 456 K which is the melting point of solder. The solder particles are prone to diffuse with each other due to the good diffusion ability of solder. This diffusion process will not stop until all of the particles diffuse together, as shown in Figure 3.

Figure 4. CuSn intermetallic formed on the bottom of crucible (a), among Cu particles (b), its micrograph (c), and the corresponding XRD pattern (d).

Therefore, the large specific surface areas of solder particles which are beneficial to Pb evaporation disappear. However, the solder particles mixed with an amount of copper particles are dispersed from each other. Just like the multilayer evaporation, the solder is laid on different layers of the whole feeding materials, which has a positive dispersive effect on the Pb evaporation. The copper particles prevent the diffusion of solder 5361



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Figure 5. Pb separation efficiency from Cu-solder mixed particles (a) and Pb evaporation rate (b), and flowchart of separating Pb from WPCBs.

particles, which enables solder particles to possess the advantages of large specific surface areas. Therefore, larger evaporation surface areas bring higher separation efficiency due to the effect of multilayer evaporation. 1.2.2. Formation of CuSn Intermetallic. During the process of separating Pb from solder and copper mixed particles, a few aggregates with bronzy color were found as shown in Figure 4a,b. Figure 4c shows the micrograph of CuSn intermetallic. XRD was conducted to identify phases in the aggregates. As shown in Figure 4d, the XRD data reveal the presence of CuSn intermetallic, including Cu327.92Sn88.08, Cu13.7Sn and CuSn. By comparing the major diffraction peaks with ICDD card Nos. 300511, 656821, and 653434, the peaks at about 43°, 45° and 50° are overlapped to some extent, which indicates that the bronzy aggregate is a mixture of these three kinds of CuSn intermetallic. Solder was distributed in the Cu-solder mixed particles randomly. Due to the uneven distribution of solder particles, two appearances of CuSn intermetallic were observed. Some melting solder passed through the gaps among copper particles and assembled on the bottom of the crucible (Figure 4a). Other solder particles did not pass through the gaps, tin of these solder may form CuSn intermetallic among copper particles

(Figure 4b). Because of the good wetting ability between solder and Cu, solder spread on the Cu particles. Tin of liquid solder diffused into Cu particles. CuSn intermetallic was formed on the surface of copper particles. As a result of the formation of CuSn intermetallic, the content of Sn in solder decreases and the Pb percentage increases. The more content of Sn integrates with Cu, the larger Pb percentage in the residue alloy becomes. A larger vapor pressure and a higher evaporation rate of Pb can be achieved. Therefore, it is much easier to separate Pb from the Cu and solder mixed particles. 1.2.3. Blocking Effect of the Copper Particles. Sections 1.2.1 and 1.2.2 state the positive dispersive effect of copper particles on improving Pb separation efficiency. However, there also exists a negative blocking effect of copper particles, which means that the Cu particles will block the Pb evaporation when the layer height attains to a certain value. Tests were carried out with three different heights of sample layer ranging from 3.5 to 8 mm. The heating time range was 30120 min at 1123 K. The specified data of separation efficiency under each condition is plotted in Figure 5a. As shown in Figure 5a, Pb separation efficiency increases gradually with an increase in heating time and decreases with an increase in the 5362



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Figure 6. The mass balance after VMS and sieving.

layer height. It demonstrates that the blocking effect of Cu particles becomes more and more dominant with the layer height increasing. The blocking effect of Cu particles is caused by many times of collision between the atoms of Pb vapor and Cu particles, which leads to less opportunity of Pb atoms leaving from the copper-rich particles and results in the fall in Pb separation efficiency. In order to study the blocking effect, Pb evaporation rate per layer height under each condition is calculated and plotted in Figure 5b. The evaporation rate per layer height decreases with an increase in the layer height and the heating time. The fitting curves of the data are shown in Figure 5b. The fitting curve equations under different layer heights are listed as follows: h ¼ 3:5mm

y ¼ 0:133x0:82817

R 2 ¼ 0:997

h ¼ 6mm

y ¼ 0:021x0:48234

R 2 ¼ 0:991

h ¼ 8mm

y ¼ 0:034x0:56266

R 2 ¼ 0:986

electrostatic separator was used to separate the metals from the nonmetals.21,22 The copper-rich particles were separated and recovered by VMS under the optimum operating conditions mentioned above (heating for 90 min at 1123 K under 0.11 Pa). The separation experiment was conducted in 174 g scale with a layer height of 5 mm. After the process of VMS, Pb separated from the copper-rich particles was condensed as Pb prills as shown in Figure 5g. The output materials could be divided into two parts by sieving. The first is the impact particles and the other is the aggregate materials with bronzy color. The pictures show that most of copper particles do not melt during VMS processing, different from traditional vacuum separation that requires input materials to melt first and then realizes separation.25 Therefore, VMS studied in this work requires lower temperature to separate Pb and consumes less energy from the view of heat supply. The components of the copper-rich particles before VMS as well as that of the output materials are shown in Table S2 of the SI. The content of Pb left in the output materials is reduced significantly. Figure 6 shows the mass balance of the copper-rich particles after VMS and sieving. The aggregate materials account for 9.85 wt % of the total output materials. Meanwhile, the aggregates contain a certain content of Sn, which play an important part of enrichment of Sn. It is suggested that these aggregates could be sent to batch CuSn alloy. To sum up, the metals including Cu, Pb, Sn in the WPCBs are all recovered efficiently. 3. Economic Assessment. On the basis of the experiments mentioned above and the parameters obtained, a furnace with an energy consumption of about 600 kWh/t and a productivity of 0.6 t/h could be designed. Figure S3 of the SI shows the detailed powers per tonne over different periods. The industrial electricity charge in Shanghai is $0.15/kWh. The average annual value (201035 to 201134) of the materials is shown in Figure S4 of the SI and used in the economic assessment. During the trading process, the prices indicate per t of pure copper. The other metals in the original copper-rich particles are regarded as impurities and are not counted during the trading. After VMS, the output materials contain three parts as shown in Figure 6. Assume that the recovery ratio of the three parts is the same as the experiment mentioned above. Detailed calculations about the revenues are listed as follows. The revenues do not factor in the cost of the capital equipment, transporting, or labor. E ¼ Eheating þ Esoaking þ Eevacuating

From the fitting curve equations, the evaporation rates of all the samples tend to be zero with an increase in heating time. Therefore, a further increase in the heating time does not show any significant increase in the evaporation rate or the Pb separation efficiency. As a result, 90 min is selected as the optimum heating time. In view of the effect of layer height, the optimum height range should be 3.5 6 mm so that the separation efficiency of Pb could be above 95 wt %. To improve the throughput, a furnace with multiple evaporating dishes can be developed with the adoption of multievaporation. 2. Flowchart of Separating Pb from WPCBs. The test with WPCBs after dismantling and removing electronic elements was carried out to separate and recover Pb in the residual solder attached on WPCBs, as shown in Figure 5. WPCBs were first pulverized in a process consisting of a coarse crushing step and a fine-pulverizing step. The two-step crushing can strip metal from the base plates of WPCBs. Then, an

¼ Pheating t heating þ Psoaking t soaking þ Pevacuating t evacuating ¼ 283 0:75 þ 85 1:5 þ 79 3:25 ¼ 596:5ðkWh=tÞ C ¼ E $0:15 ¼ $89:5 V o ¼ 1t Po $7561:0=t ¼ 1t 93wt% $7561:0=t ¼ $7031:7 R copper ¼ 1t Ocopper Pcopper $8394:9=t ¼ 1t

151:9 100% 99:22wt% $8394:9=t 174

¼ $7271:5 5363


Environmental Science & Technology R aggregates ¼ 1t Oaggregates Paggregates $4999:3=t ¼ 1t

16:6 100% 66:15wt% $4999:3=t 174

¼ $315:5 R Pb ¼ 1t OPb $2096:7=t 4 100% $2096:7=t 174 ¼ $48:2

¼ 1t

R t ¼ R copper þ R aggregates þ R Pb ¼ $7271:5 þ $315:5 þ $48:2 ¼ $7635:2 R ¼ R t V o C ¼ $7635:2 $7031:7 $89:5 ¼ $514 Above all, the revenues after accounting for energy costs can be about $ 514/t according to the market quotation in China. Therefore, this method is economically viable to deal with the copper-rich particles from WPCBs. More importantly, it is believed that Pb contamination caused cannot be merely measured by money once the toxic heavy metals come into human bodies or the environment, as shown in the literature.69 The elimination of Pb contamination will have a profound significance from a social and environmental point of view. In short, besides realizing the resource recovery maximally, the most important point of this work lies in avoiding the Pb contamination. This method can also be applied to separate and recover the toxic heavy metals (Pb, Cd, Hg) in other e-wastes such as the cathode ray tube and so on. The contamination caused by these heavy metals could be eliminated to a considerable extent by evaporation and condensation.

’ APPENDIX A ’ ASSOCIATED CONTENT

bS Supporting Information. Schematic illustration of the furnace body; description of the relationship between vapor pressure and temperature; table summarizing Pb vapor pressure with different Pb content in PbSn alloy; table showing components of the copper-rich particles from crushed WPCBs before and after VMS; the graph of detailed powers per t over different periods; the graph of the average annual value of the materials. This material is available free of charge via the Internet at http://pubs.acs.org. ’ AUTHOR INFORMATION Corresponding Author

*Phone: þ86 21 54747495; fax: þ86 21 54747495; e-mail: [email protected].

’ ACKNOWLEDGMENT This project was supported by the National High Technology Research and Development Program of China (863 program 2009AA06Z318), the National Natural Science Foundation of China (21077071), Shanghai Tongji Gao TingYao Environmental Science & Technology Development Foundation, and Scholarship Award for Excellent Doctoral Student granted by Ministry

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of Education. Moreover, the authors would also like to thank Rao Qunli in Shanghai Jiao Tong University, PRC, for his efforts in the aspect of X-ray diffraction.

’ NOMENCLATURE E energy consumption, kWh/t P power, kW/t t time, h C energy cost the value of the original copper-rich particles (purity of Vo 93 wt %) without treating by VMS Rcopper the revenue of copper particles recovered (purity of 99.22 wt %) Raggregates the revenue of the aggregates recovered (purity of 66.15 wt %) the revenue of Pb recovered RPb O the recovery ratio P the purity of copper the revenue after treating by VMS Rt R the revenues after accounting for energy costs ’ REFERENCES (1) Widmer, R.; Oswald-Krapf, H.; Sinha-Khetriwasl, D.; Schnellmann, M.; Boni, H. Global perspectives on e-waste. Environ. Impact Assess. Rev. 2005, 25, 436–458. (2) Bruke, M. The gadget scrap heap. Chem. World 2007, 6, 45–48. (3) Basdere, B.; Seliger, G. Disassembly factories for electrical and electronic products to recover resources in product and material cycles. Environ. Sci. Technol. 2003, 37, 5354–5362. (4) Bernardes, A.; Bohlinger, I.; Rodriguez, D.; Milbrandt, H.; Wuth, W. Recycling of Printed Circuit Boards by Melting with Oxidising/Reducing Top Blowing Process; TMS Annual Meeting: Orlando, FL, 1997, pp363375. (5) Goosey, M.; Kellner, R. Recycling technologies for the treatment of end of life printed circuit boards (PCBs). Circuit World 2003, 29, 33–37. (6) Deng, W. J.; Louie, P. K. K.; Liu, W. K.; Bi, X. H.; Fu, J. M.; Wong, M. H. Atmospheric levels and cytotoxicity of PAHs and heavy metals in TSP and PM2.5 at an electronic waste recycling site in southeast China. Atmos. Environ. 2006, 40, 6945–6955. (7) Leung, A. O. W.; Duzgoren-Aydin, N. S.; Cheung, K. C.; Wong, M. H. Heavy metals concentrations of surface dust from e-waste recycling and its human health implications in southeast China. Environ. Sci. Technol. 2008, 42, 2674–2680. (8) Zheng, L.; Wu, K.; Li, Y.; Qi, Z.; Han, D.; Zhang, B.; Gu, C.; Chen, G.; Liu, J.; Chen, S.; et al. Blood lead and cadmium levels and relevant factors among children from an e-waste recycling town in China. Environ. Res. 2008, 108, 15–20. (9) Fu, J.; Zhou, Q.; Liu, J.; Liu, W.; Wang, T.; Zhang, Q.; Jiang, G. High levels of heavy metals in rice (Oryza sativa L.) from a typical E-waste recycling area in southeast China and its potential risk to human health. Chemosphere 2008, 71, 1269–1275. (10) Hagel€uken, C. Recycling of electronic scrap at Umicore’s integrated metals smelter and refinery. World Metallurgy 2006, 59, 152–161. (11) Lehner, T. Integrated Recycling of Non-Ferrous Metals at Boliden Ltd. Ronnskar Smelter; IEEE International Symposium on Electronics & the Environment: Oak Brook, IL, 1998; pp 4247. (12) Park, Y. J.; Fray, D. J. Recovery of high purity precious metals from printed circuit boards. J. Hazard. Mater. 2009, 164, 1152–1158. (13) Veit, H. M.; Bernardes, A. M.; Ferreira, J. Z.; Tenorio, J. A. S.; Malfatti, C.d.F. Recovery of copper from printed circuit boards scraps by mechanical processing and electrometallurgy. J. Hazard. Mater. 2006, 137, 1704–1709. 5364



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Separating and Recovering Pb from Copper-Rich Particles of Crushed

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