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Copper leaching from waste printed circuit boards using typical acidic ionic liquids recovery of e-wastes’ surplus value
Waste Management 78 (2018) 191–197
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Waste Management journal homepage: www.elsevier.com/locate/wasman
Copper leaching from waste printed circuit boards using typical acidic ionic liquids recovery of e-wastes’ surplus value Ding-jun Zhang ⇑, Li Dong, Yong-tong Li, Yanfei Wu, Ying-xia Ma, Bin Yang State Key Laboratory of Advanced Processing and Recycling of Non-ferrous Metals, Lanzhou University of Technology, Lanzhou 730050, China
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Article history: Received 26 March 2018 Revised 14 May 2018 Accepted 19 May 2018
Keywords: WPCBs Copper recovery Leaching rate Ionic liquids 1-Carboxymethyl-3-methylimidazolium bisulfate ([CM-MIM][HSO4]) Hydrometallurgical separation
a b s t r a c t In this study, using several types of acidic ionic liquids as the leaching reagents, the leaching behaviors of the copper present in waste printed circuit boards (WPCBs) were investigated. The effects of various parameters on the copper leaching rate were studied, such as the particle size of the shredded WPCB, type of ionic liquid used, hydrogen peroxide dosage, solid-to-liquid ratio, leaching temperature, and leaching time. The experimental results showed that the copper leaching rate increases continuously when the powder particle size is increased from 0.071 to 0.500 mm. Moreover, the copper leaching rate also increases with an increase in the leaching temperature. In contrast, the leaching rate first increases and then decreases with increases in the leaching time, hydrogen peroxide dosage, and solid-to-liquid ratio. The optimal conditions that provided a 98.31% copper leaching rate were: particle size >0.500 mm, 8.5 mL 90% (v/v) ionic liquid, 1.5 mL 30% hydrogen peroxide, solid-to-liquid ratio of 1/20, leaching temperature of 80 °C, and leaching time of 2 h. Ó 2018 Elsevier Ltd. All rights reserved.
In recent years, with the advancement of technological innovation and the acceleration of the replacement of electronic appliances, more and more electronic waste has been created. The world’s output in 2017 was 40 million tons; this not only harms the environment but also causes human health issues. Thus, solving the issues of electronic waste, especially the recycling of waste CPUs, has become an urgent environmental and economic requirement. With regard to the process of recovering metals from utilized circuit boards, we specifically studied the recovery of metallic copper, and compared the use of ionic liquids, mechanical methods, fire, biological methods, and supercritical fluid methods in terms of yield and recovery efficiency, differences in environmental protection, toxicity, and reaction conditions. Experimental results show that the fastest copper recovery time from waste circuit boards was 2.5 h by using ionic liquids. Furthermore, dioxins are produced by the fire method, the biological method requires of the use of toxic bacteria, and the supercritical fluid method has very strict requirements on temperature and pressure. In comparison, the ionic liquid reaction conditions are relatively mild and easy to implement. ⇑ Corresponding author. E-mail addresses: [email protected] (D.-j. Zhang), [email protected] (L. Dong), [email protected] (Y.-t. Li), [email protected] (Y. Wu), [email protected] (Y.-x. Ma), [email protected] (B. Yang). https://doi.org/10.1016/j.wasman.2018.05.036 0956-053X/Ó 2018 Elsevier Ltd. All rights reserved.
Ionic liquids have several useful properties: (1) wide liquid range, from below or near room temperature to above 300 °C, with high thermal and chemical stability; (2) low vapor pressure and no volatilization means that evaporation does not occur during storage and use; (3) can be reused many times; (4) eliminates the production of toxic volatile organic compounds (VOCs); (5) High conductivity and large electrochemical window mean that they can be used as electrolytes for the study of many substances; (6) the design of anions and cations can be used to adjust its solubility to inorganic substances, water, organics, and polymers; and (7) a large degree of polarity controllability, low viscosity, high density, and the ability to form two or more systems, means they are suitable for separation of solvents or constitute a new system of reaction-separation coupling. In this study, the leaching rate was 94.30% when the solidliquid ratio was 1/20, leaching time was 2 h, and temperature was 80 °C. The ionic liquid consumption was relatively lower than pyrometallurgical separation, the time was shorter, the efficiency was higher, and involved lower energy consumption. The study is of great significance for industrial production.
1. Introduction Printed circuit boards can cause serious environmental damage after disposal (Rüsßen and Topçu, 2017). Waste Printed Circuit
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Boards (WPCBs) present a continuous growth trend with an increase rate of 17–25% per year, and a total of 65.4 million tons of waste electric and electronic equipment (WEEE) are generated annually worldwide (Tatariants et al., 2017). WPCBs account for about 3% by weight of the total amount of electronic waste, bring a global challenge for the environment because of their complex and hazardous components (Wang et al., 2018). WPCBs contain large amounts of metal fractions, such as Cu 20%, Fe 8%, Ni 2%, Sn 4%, Pb 1%, Al 2%, Zn 1%, Sb 0.4%, Au 500 g/t, Ag 1000 g/t, and Pd 50 g/t, and include epoxy resin, glass fibers, ceramics and other nonmetallic fractions which embody large numbers of brominated flame retardants and other harmful substances (Li et al., 2014; Liu et al., 2017; Zhou et al., 2016). Therefore, the recycling of WPCBs is an immediate problem due to serious threats to human health and the surrounding environment (Debnath et al., 2016). The high economic value of metal has undoubtedly contributed in attracting more attraction to this issue. Thus, many recycling methods have been successively utilized to reclaim metals from WPCBs, such as mechanical separation, pyrometallurgical separation, bioleaching separation (Zhu et al., 2011), supercritical fluid separation, and hydrometallurgical separation (Leite et al., 2017). Mechanical separation has low efficiency in recovering precious metals because of the purification step required after obtaining the metal mixtures (Cui and Zhang, 2008; Huang et al., 2014); as a result, mechanical separation is usually used as the pretreatment process. Pyrometallurgical separation became the main method used for a long time, but was eventually eliminated due to toxic gases, dioxins, furans, and other poisonous substances produced during combustion (Birloaga and Vegliò, 2016). The third common method is bioleaching separation, which is a promising alternative to extract metals from WPCBs (Davris et al., 2018). However, the presence of nonmetallic fractions encouraged bacterial toxicity during the leaching process, which reduced the leaching rate (Chen et al., 2015b; Ilyas et al., 2010). Furthermore, longer leaching time and specific bacterial species that were harder to obtain prevented the promotion of this method. Another method uses supercritical fluids with negligible low surface tension, high diffusivity, low viscosity, and ability to dissolve inorganic salts (Liu et al., 2016). The disadvantage is that the method requires higher critical temperature and pressure which may result in significant reactor corrosion and higher energy consumption (Yousef et al., 2018). It should be noted that the use of ionic liquids to leach copper from waste circuit boards is more environmentally friendly than pyrometal recovery of copper. Furthermore, it is more efficient than mechanical recovery of copper metal that achieves 79.54% yield of the heavy liquid separation process to recover copper, which is generally lower than the 90–99.84% yield acquired via ionic liquid. Additionally, its toxicity is lower than the biological method, which typically recovers only 76.59%. At the same time, its reaction conditions are milder than the supercritical method and the required reaction time at 2 h is shorter than the supercritical method that requires 54 h. Room temperature ionic liquids (RTILs) are essentially in the liquid state at low temperatures (Kim et al., 2018), and typically contain organic cations and inorganic or organic anions. Ionic liquids have a wide liquid temperature range and many other unique properties such as negligible volatility, low vapor pressure, high thermal stability (<400 °C), high electrical conductivity, and wide electrochemical windows (Zhu et al., 2016). As a result, ionic liquids are increasingly employed in fields including biochemical process, organic synthesis processes (Zhu et al., 2012), separation process, catalytic reaction, and materials engineering (Ma et al., 2012). Previous study showed that ionic liquids can usually be reused more than 10 times (Zhang et al., 2013). Thus, ionic liquids are very promising as an environmentally friendly reagent for leaching metals from WPCBs (Dong et al., 2009).
The first step in dismantling WPCBs is to remove the solder. Commonly, the IL 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM]BF4) (Bar-Cohen et al., 2017) was used as the heating medium to recycle the valuable solder material from WPCBs (Zeng et al., 2013). Furthermore, 1-carboxymethyl-3-methylimidazolium bisulfate ([CM-MIM]HSO4) can be used as an alternative IL for recycling metals from WPCBs under different conditions, such as different leaching times, leaching temperatures, solid–liquid ratios, WPCB powder particle sizes (Zhang et al., 2018), and hydrogen peroxide dosages. 2. Materials and experimental methods 2.1. Preparation of WPCBs and removal of solder The WPCBs with electronic components mounted on them were pretreated as shown in Fig. 1. They were first cut into small pieces with dimensions of approximately 50 50 mm using a cutting machine. Then, these pieces were shredded using a cutting mill and sieved into different fractions using standard sieves: F1 < 0.071 mm, 0.071 < F2 < 0.100 mm, 0.100 < F3 < 0.250 mm, 0.250 < F4 < 0.500 mm, and F5 > 0.500 mm. Next, the sieved parts were dried at 105 °C for 24 h. As is shown in Fig. 2, the process of dismantling the WPCBs was as follows. The IL [BMIM]BF4 was heated to 200 °C. Then, while keeping the temperature uniform, a certain mass of the cut WPCB pieces was added to the IL, which was stirred with a mechanical stirrer at a rate of 150 rpm until all the solder points in the WPCBs had melted. As a result, owing to gravity and the centrifugal force, the molten solder dropped from the circuit board, resulting in the separation of the circuit board substrate and the electronic components. The main parameters for the recovery of the solder from the WPCBs were temperature of 200 °C, mechanical stirring speed of 150 rpm, and melting time of 3 min. 2.2. Leaching experiments After recycling the solder, the next step was crushing the WPCBs into powders, followed by leaching the metals using ILs.
Fig. 1. Schematic of process for recovering metals from WPCBs using ILs.
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system, as can also be seen from the results of its spectrum analysis. Further, the presence of small voids on the surface was due to the penetration of the solvent molecules or other organic molecules during the pyrolysis process and the difference in the shrinkage rates of the different materials. As shown in Table 1, in addition to containing 58.288% Sn and 6.422% Pb, it also consisted of 21.867% Cu, 11.238% Fe, and trace amounts of other metals; as a result, how to further separate tin and copper is an urgent problem to be solved.
3.2. Effect of ionic liquid used
Fig. 2. Process for separating solders from the WPCBs using [BMIM]BF4.
All the leaching experiments were carried out in a 100-mL glass round-bottomed flask that was stirred with a magnet at a constant rate at a uniform temperature. An aqueous solution of the IL ([CMMIM]HSO4, laboratory-made reagent) (concentration (v/v) of 90%) was used as the leaching agent, while hydrogen peroxide (30%) was used as the oxidant (Chen et al., 2015a). A certain volume was removed from the post-reaction leaching solution, and an 80% hydrazine hydrate solution was added to it in the volume ratio of 1:1 for reduction. Then, the reduced copper was washed 3–5 times after centrifugation and vacuum dried at 80–120 °C for 8–12 h. Eventually, the copper leaching rate was calculated as:
Copper leaching rate ¼ m=m0 100%
ð1Þ
where m0 is the mass of total copper in WPCBs specimens and m is the mass of copper extracted into leaching solution. Additionally, recovery of the separated ionic liquids was carried out by rotary evaporation later. 3. Results and discussion 3.1. Characterization of the solder As can be seen from Fig. 3, the surface of the solder recovered was macroscopically smooth. However, backscattered imaging revealed that the images of the solder surface consisted of bright and dark areas (Fig. 3a), and that relatively small voids were present on the surface (Fig. 3b). The reason for the presence of the areas with the bright and dark contrasts is that the recovered solder material contained different metals, i.e., it was a heterogeneous
As is shown in Fig. 4, the copper leaching rate was 89.19% when the IL ([CM-MIM]HSO4) was used, decreasing to 26.16% in the case of the hydrometallurgical process, in which case sulfuric acid of the same concentration was used. Furthermore, the copper leaching rate decreased significantly when ([BMIM]HSO4), 1-propane sulfonate-3-methylimidazolium bisulfate ([PS-MIM]HSO4), and 1,10 -hexyl-3,30 -dicarboxymethylimidazolium bisulfate ([C6(di-AcIM)]HSO4) were used. In addition, the copper leaching rate was even lower, at 10%, when 1,3-dicarboxymethylimidazolium bisulfate ([di-Ac-IM]HSO4) was used. There are several reasons for these results. First, using an acid-functionalized IL is equivalent to substituting the hydrogen ion in the sulfuric acid molecule with an imidazolium cation and thereby reducing the concentration of hydrogen ions involved in the reaction. In addition, the volume of the imidazole cation greatly enhances the affinity of the anion towards it, resulting in the hydrogen ions in the solution being primarily concentrated on the surface of the imidazole cation. Owing to these factors, the copper leaching rates of the ILs were higher than that of the conventional hydrometallurgical method. The copper recovery rates of ILs 1# and 2# were similar (Fig. 4), indicating that the introduction of a sulfonic acid group does not result in a synergistic effect with the potent hydrogen sulfate ions to lead to an increase in the leaching rate. The introduction of the sulfonic acid group increases the affinity of the imidazolium ions with respect to the free hydrogen ions in the solution, reducing the leaching rate of copper under the same time conditions. The results for ILs 3# and 4# (Fig. 4) indicated that the presence of a second carboxylic acid group reduced the leaching rate by 47.63%. This was because the introduction of the carboxylic acid group enhanced hydrogen bonding in the acid-functionalized ILs and reduced the concentration of hydrogen ions, thus decreasing the strength of the acid participating in the reactions and, consequently, the leaching rate. Finally, comparing the results for IL 4# with those for IL 5# (Fig. 4), it can be concluded that the presence of a double cationic
Fig. 3. Electron probe transmission map images of solder recovered using [BMIM]BF4.
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D.-j. Zhang et al. / Waste Management 78 (2018) 191–197 Table 1 Contents of major elements in solder recovered with [BMIM][BF4]. ELE.
Wt (%)
Mol (%)
Sn Cu Fe Pb Ca Sb La
58.288 21.867 11.238 6.422 0.96 0.748 0.478
44.606 31.256 18.277 2.815 2.176 0.558 0.312
Fig. 5. Effect of [CM-MIM]HSO4 concentration on rate of metal leaching from WPCBs.
3.4. Effect of WPCB powder particle size
Fig. 4. Rates of metal leaching from WPCBs using various ILs: 1#-[BMIM]HSO4; 2#[PS-MIM]HSO4; 3#-[CM-MIM]HSO4; 4#-[di-Ac-IM]HSO4; and 5#-[C6(di-Ac-IM)] HSO4.
As shown in Fig. 6, for WPCB powder particle sizes less than 0.250 mm, the copper leaching rate increased with an increase in the particle size. The maximum metal yield was 94.30% when the particle size was 0.100–0.250 mm. In addition, for particle sizes greater than 0.250 mm, the copper leaching rate decreased. The copper leaching rate increases with the decrease of particle size because the surface area per unit mass is increased. However, the reduction of particle size below a critical level would increase the particle–particle collision and impose severe attrition (Zhu et al., 2011), hindering the leaching liquid permeating through the fine WPCB powders. 3.5. Effect of hydrogen peroxide dosage
structure increased the metal leaching rate by 33%. The reason for this phenomenon is as follows. A relatively long alkyl chain links the two imidazolium cations; this increases the volume of the IL molecule and reduces the number of hydrogen bonds per unit area, as well as the number of hydrogen bonds formed by the imidazolium cations. Thus, the hydrogen ion concentration in the reaction system is higher for IL 5#, and hence, the corresponding leaching rate was higher.
As shown in Fig. 7, the copper leaching rate increased initially and then decreased with an increase in the oxidant content. When the oxidant content was less than 15%, the copper leaching rate increased with an increase in the oxidant content. For instance, for an oxidant content of 15%, the metal yield was 79.55%. In contrast, when the oxidant content was more than 15%, the copper leaching rate decreased with increases in the oxidant content. This is because the oxidant mainly assists the acid in the solution in
3.3. Effect of ionic liquid concentration As is shown in Fig. 5, the copper leaching rate increased with an increase in the IL concentration. When the acid-functionalized IL concentration was less than 70%, the copper leaching rate increased significantly. However, for acid-functionalized IL concentrations of more than 70%, the copper leaching rate did not vary significantly. This is probably because when the concentration was more than 70%, the number of hydrogen bonds within the acid-functionalized IL increased. However, the number of free hydrogen ions did not increase in proportion to the increase in the IL concentration, and therefore, the copper leaching rate increased slowly. Hence, the IL concentration should be chosen with care to ensure a higher leaching rate. Another explanation is that depending on the function of [CM-MIM]HSO4 and the overall reaction for copper dissolution, the mechanism can be expressed as:
Cu+2Hþ +H2 O2 !Cu2þ +2H2 O
ð2Þ
The increase of the copper recovery could be attributed to the increasing acidity and dissolved oxygen concentration.
Fig. 6. Effect of particle size of WPCB powder on rate of metal leaching from WPCBs (F1 < 0.071 mm; 0.071 < F2 < 0.100 mm; 0.100 < F3 < 0.250 mm; 0.250 < F4 < 0.500 mm; and F5 > 0.500 mm).
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rate tended to be steady. From the point of view of the thermodynamics of the chemical reaction, as the reaction progresses, the concentrations of the hydrogen ions and the oxidant decrease, and the reaction tends towards completion:
1 ð1 xÞ1=3 ¼ kc t
ð3Þ
1 2=3x ð1 xÞ2=3 ¼ kd t
ð4Þ
where x refers to the fraction reacted, t is the react time, kc and kd are the rate constants calculated from Eqs. (3) and (4), respectively. 3.7. Effect of solid-to-liquid ratio
Fig. 7. Effect of hydrogen peroxide dosage on rate of metal leaching from WPCBs.
accelerating the oxidation of the copper in the WPCBs into ions, which can subsequently be leached. When the oxidant content was too high, it combined with the copper ions in the solution, forming copper peroxide. Together, the copper peroxide, the oxidant, and the hydrogen ions resulted in the formation of copper ions, water, and oxygen, thus reducing the hydrogen content in the reaction solution. Further, the oxygen produced by the above reaction also accumulated on the surfaces of the WPCB particles, thus slowing the reaction and even preventing it from proceeding. Another theory is that H2O2 firstly reacts with copper to generate CuO, followed by functioning with ILs resulting in the formation of CuSO4. However, when hydrogen peroxide is excessive, the ionic liquid is simultaneously oxidized leading to lower the copper yield.
As is shown in Fig. 9, for solid-to-liquid ratio values greater than 1:20, the copper leaching rate decreased with an increase in the ratio. In particular, when the solid-to-liquid ratio was 1:20, the copper leaching rate was 74.53%. On the contrary, for solid-toliquid ratio values of less than 1:20, the copper leaching rate increased slightly with an increase in the ratio. A decrease in the solid-to-liquid ratio for the leaching process is equivalent to an increase in the volume of the reaction system. As a result, the WPCB particles are better submerged in the reaction solution, and the reaction solution and the particles are in complete contact with each other, thus increasing the mass transfer efficiency and hence the leaching rate. Meanwhile, when the solid-to-liquid ratio
3.6. Effect of leaching time As is revealed in Fig. 8, with an increase in the leaching time, the copper leaching rate first increased sharply and then tended to be steady gradually. When the leaching time was 90–120 min, the copper leaching rate increased rapidly. For the metal dissolution kinetics, if the metal particles were considered as spherical particles, the leaching process can be described with the shrinking core model. The step with the highest resistance is the rate controlling step, meanwhile, the surface reaction control model (3) and diffusion control model (4) can both explain this phenomenon. In contrast, for leaching periods longer than 120 min, the copper leaching
Fig. 8. Effect of leaching time on rate of metal leaching from WPCBs.
Fig. 9. Effect of solid-to-liquid ratio on rate of metal leaching from WPCBs.
Fig. 10. Effect of leaching temperature on rate of metal leaching from WPCBs.
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Fig. 11. Backscattered morphological images of (a) WPCB powder and (b) leached metals.
is too low, although the mass transfer efficiency is increased, it is equivalent to decreasing the concentration of the WPCB powder in the reaction system. Consequently, the yield remains unchanged or even decreases to some degree. Additionally, when the solid-toliquid ratio is too high, lower heat transfer and transmission efficiency leads to lower leaching rate.
3.8. Effect of leaching temperature As is depicted in Fig. 10, the copper leaching rate increased sharply initially with an increase in the leaching temperature, and then increased gently. For a leaching temperature of 80 °C, the copper leaching rate was 70.99%. The reaction rate of the leaching reaction satisfies the Arrhenius equation: k = Aexp(Ea/RT). As the reaction temperature is increased, the reaction rate also increases gradually. The mechanism of copper ions leaching rate increasing still needs further exploration.
3.9. Backscattered topographic images and energy spectrum analysis As shown in Fig. 11, the results of backscattering imaging of the WPCB powders (a) and the leached metals (b), and an energy spectrum analysis of the leached metals (Table 2) were in keeping with expectations. As can be seen in Fig. 11(a), the energy spectrum indicated that the WPCB powder contained large amounts of carbon and oxygen, as well as a small amount of sulfur. This can primarily be attributed to the epoxy resin and other additives used in the PCBs. The WPCBs also contained several metallic elements, including Cu, Al, Ti, Ba, Ca, and La. As can be seen in Fig. 11(b), after being centrifuged and dried, the copper particles obtained after the reduction process were large and difficult to crush further. During the reduction process, the copper formed clusters at the atomic level, resulting in copper particles. As is demonstrated in Table 2, the concentrations of the leached copper was 98.330%; consequently, copper is the most valuable metal leached from this WPCB.
4. Conclusions When the concentration of the IL ([CM-MIM][HSO4]) used was increased from 10% to 90%, the leaching rate improved continuously, increasing from 60.96% to 69.32%. In addition, the particle size had a determining effect on the copper leaching rate, which increased from 17.21% to 94.30% when the powder particle size was increased from less than 0.071 mm to 0.100–0.250 mm. It was observed that the copper leaching rate increased initially and then decreased with increases in the hydrogen peroxide dosage, solid-to-liquid ratio, and leaching time. Further, when the temperature was increased from 40 to 80 °C, the leaching rate increased from 29.50% to 78.66%, which is an improvement of 167%. The copper leaching rate was as high as 98.305% which was higher than other leaching methods when 0.5 g of the WPCB powder was leached under optimal conditions, i.e., using 8.5 mL of a 90% (v/v) IL solution and 1.5 mL 30% hydrogen peroxide at a solid-to-liquid ratio of 1/20 at 80 °C for 2 h. We also found that the recovery rate of ionic liquid by rotary evaporation was generally above 97%, and thus could be useful to reduce cost. In conclusion, leaching copper from the WPCBs using ionic liquids is a more environmentally friendly and efficient method for the ecosystem and industries. Acknowledgments The authors are grateful for support of the National Natural Science Foundation of China (NSFC, 51663013) and State Key Laboratory of Advanced Processing and Recycling of Non-ferrous Metals, Lanzhou University of Technology. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.wasman.2018.05. 036. References
Table 2 Contents of major elements in metal leached with ionic liquid. ELE.
Wt (%)
Mol (%)
Cu Al Ca La
98.330 0.695 0.327 0.058
96.547 1.607 0.509 0.027
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Contents lists available at ScienceDirect
Waste Management journal homepage: www.elsevier.com/locate/wasman
Copper leaching from waste printed circuit boards using typical acidic ionic liquids recovery of e-wastes’ surplus value Ding-jun Zhang ⇑, Li Dong, Yong-tong Li, Yanfei Wu, Ying-xia Ma, Bin Yang State Key Laboratory of Advanced Processing and Recycling of Non-ferrous Metals, Lanzhou University of Technology, Lanzhou 730050, China
a r t i c l e
i n f o
Article history: Received 26 March 2018 Revised 14 May 2018 Accepted 19 May 2018
Keywords: WPCBs Copper recovery Leaching rate Ionic liquids 1-Carboxymethyl-3-methylimidazolium bisulfate ([CM-MIM][HSO4]) Hydrometallurgical separation
a b s t r a c t In this study, using several types of acidic ionic liquids as the leaching reagents, the leaching behaviors of the copper present in waste printed circuit boards (WPCBs) were investigated. The effects of various parameters on the copper leaching rate were studied, such as the particle size of the shredded WPCB, type of ionic liquid used, hydrogen peroxide dosage, solid-to-liquid ratio, leaching temperature, and leaching time. The experimental results showed that the copper leaching rate increases continuously when the powder particle size is increased from 0.071 to 0.500 mm. Moreover, the copper leaching rate also increases with an increase in the leaching temperature. In contrast, the leaching rate first increases and then decreases with increases in the leaching time, hydrogen peroxide dosage, and solid-to-liquid ratio. The optimal conditions that provided a 98.31% copper leaching rate were: particle size >0.500 mm, 8.5 mL 90% (v/v) ionic liquid, 1.5 mL 30% hydrogen peroxide, solid-to-liquid ratio of 1/20, leaching temperature of 80 °C, and leaching time of 2 h. Ó 2018 Elsevier Ltd. All rights reserved.
In recent years, with the advancement of technological innovation and the acceleration of the replacement of electronic appliances, more and more electronic waste has been created. The world’s output in 2017 was 40 million tons; this not only harms the environment but also causes human health issues. Thus, solving the issues of electronic waste, especially the recycling of waste CPUs, has become an urgent environmental and economic requirement. With regard to the process of recovering metals from utilized circuit boards, we specifically studied the recovery of metallic copper, and compared the use of ionic liquids, mechanical methods, fire, biological methods, and supercritical fluid methods in terms of yield and recovery efficiency, differences in environmental protection, toxicity, and reaction conditions. Experimental results show that the fastest copper recovery time from waste circuit boards was 2.5 h by using ionic liquids. Furthermore, dioxins are produced by the fire method, the biological method requires of the use of toxic bacteria, and the supercritical fluid method has very strict requirements on temperature and pressure. In comparison, the ionic liquid reaction conditions are relatively mild and easy to implement. ⇑ Corresponding author. E-mail addresses: [email protected] (D.-j. Zhang), [email protected] (L. Dong), [email protected] (Y.-t. Li), [email protected] (Y. Wu), [email protected] (Y.-x. Ma), [email protected] (B. Yang). https://doi.org/10.1016/j.wasman.2018.05.036 0956-053X/Ó 2018 Elsevier Ltd. All rights reserved.
Ionic liquids have several useful properties: (1) wide liquid range, from below or near room temperature to above 300 °C, with high thermal and chemical stability; (2) low vapor pressure and no volatilization means that evaporation does not occur during storage and use; (3) can be reused many times; (4) eliminates the production of toxic volatile organic compounds (VOCs); (5) High conductivity and large electrochemical window mean that they can be used as electrolytes for the study of many substances; (6) the design of anions and cations can be used to adjust its solubility to inorganic substances, water, organics, and polymers; and (7) a large degree of polarity controllability, low viscosity, high density, and the ability to form two or more systems, means they are suitable for separation of solvents or constitute a new system of reaction-separation coupling. In this study, the leaching rate was 94.30% when the solidliquid ratio was 1/20, leaching time was 2 h, and temperature was 80 °C. The ionic liquid consumption was relatively lower than pyrometallurgical separation, the time was shorter, the efficiency was higher, and involved lower energy consumption. The study is of great significance for industrial production.
1. Introduction Printed circuit boards can cause serious environmental damage after disposal (Rüsßen and Topçu, 2017). Waste Printed Circuit
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Boards (WPCBs) present a continuous growth trend with an increase rate of 17–25% per year, and a total of 65.4 million tons of waste electric and electronic equipment (WEEE) are generated annually worldwide (Tatariants et al., 2017). WPCBs account for about 3% by weight of the total amount of electronic waste, bring a global challenge for the environment because of their complex and hazardous components (Wang et al., 2018). WPCBs contain large amounts of metal fractions, such as Cu 20%, Fe 8%, Ni 2%, Sn 4%, Pb 1%, Al 2%, Zn 1%, Sb 0.4%, Au 500 g/t, Ag 1000 g/t, and Pd 50 g/t, and include epoxy resin, glass fibers, ceramics and other nonmetallic fractions which embody large numbers of brominated flame retardants and other harmful substances (Li et al., 2014; Liu et al., 2017; Zhou et al., 2016). Therefore, the recycling of WPCBs is an immediate problem due to serious threats to human health and the surrounding environment (Debnath et al., 2016). The high economic value of metal has undoubtedly contributed in attracting more attraction to this issue. Thus, many recycling methods have been successively utilized to reclaim metals from WPCBs, such as mechanical separation, pyrometallurgical separation, bioleaching separation (Zhu et al., 2011), supercritical fluid separation, and hydrometallurgical separation (Leite et al., 2017). Mechanical separation has low efficiency in recovering precious metals because of the purification step required after obtaining the metal mixtures (Cui and Zhang, 2008; Huang et al., 2014); as a result, mechanical separation is usually used as the pretreatment process. Pyrometallurgical separation became the main method used for a long time, but was eventually eliminated due to toxic gases, dioxins, furans, and other poisonous substances produced during combustion (Birloaga and Vegliò, 2016). The third common method is bioleaching separation, which is a promising alternative to extract metals from WPCBs (Davris et al., 2018). However, the presence of nonmetallic fractions encouraged bacterial toxicity during the leaching process, which reduced the leaching rate (Chen et al., 2015b; Ilyas et al., 2010). Furthermore, longer leaching time and specific bacterial species that were harder to obtain prevented the promotion of this method. Another method uses supercritical fluids with negligible low surface tension, high diffusivity, low viscosity, and ability to dissolve inorganic salts (Liu et al., 2016). The disadvantage is that the method requires higher critical temperature and pressure which may result in significant reactor corrosion and higher energy consumption (Yousef et al., 2018). It should be noted that the use of ionic liquids to leach copper from waste circuit boards is more environmentally friendly than pyrometal recovery of copper. Furthermore, it is more efficient than mechanical recovery of copper metal that achieves 79.54% yield of the heavy liquid separation process to recover copper, which is generally lower than the 90–99.84% yield acquired via ionic liquid. Additionally, its toxicity is lower than the biological method, which typically recovers only 76.59%. At the same time, its reaction conditions are milder than the supercritical method and the required reaction time at 2 h is shorter than the supercritical method that requires 54 h. Room temperature ionic liquids (RTILs) are essentially in the liquid state at low temperatures (Kim et al., 2018), and typically contain organic cations and inorganic or organic anions. Ionic liquids have a wide liquid temperature range and many other unique properties such as negligible volatility, low vapor pressure, high thermal stability (<400 °C), high electrical conductivity, and wide electrochemical windows (Zhu et al., 2016). As a result, ionic liquids are increasingly employed in fields including biochemical process, organic synthesis processes (Zhu et al., 2012), separation process, catalytic reaction, and materials engineering (Ma et al., 2012). Previous study showed that ionic liquids can usually be reused more than 10 times (Zhang et al., 2013). Thus, ionic liquids are very promising as an environmentally friendly reagent for leaching metals from WPCBs (Dong et al., 2009).
The first step in dismantling WPCBs is to remove the solder. Commonly, the IL 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM]BF4) (Bar-Cohen et al., 2017) was used as the heating medium to recycle the valuable solder material from WPCBs (Zeng et al., 2013). Furthermore, 1-carboxymethyl-3-methylimidazolium bisulfate ([CM-MIM]HSO4) can be used as an alternative IL for recycling metals from WPCBs under different conditions, such as different leaching times, leaching temperatures, solid–liquid ratios, WPCB powder particle sizes (Zhang et al., 2018), and hydrogen peroxide dosages. 2. Materials and experimental methods 2.1. Preparation of WPCBs and removal of solder The WPCBs with electronic components mounted on them were pretreated as shown in Fig. 1. They were first cut into small pieces with dimensions of approximately 50 50 mm using a cutting machine. Then, these pieces were shredded using a cutting mill and sieved into different fractions using standard sieves: F1 < 0.071 mm, 0.071 < F2 < 0.100 mm, 0.100 < F3 < 0.250 mm, 0.250 < F4 < 0.500 mm, and F5 > 0.500 mm. Next, the sieved parts were dried at 105 °C for 24 h. As is shown in Fig. 2, the process of dismantling the WPCBs was as follows. The IL [BMIM]BF4 was heated to 200 °C. Then, while keeping the temperature uniform, a certain mass of the cut WPCB pieces was added to the IL, which was stirred with a mechanical stirrer at a rate of 150 rpm until all the solder points in the WPCBs had melted. As a result, owing to gravity and the centrifugal force, the molten solder dropped from the circuit board, resulting in the separation of the circuit board substrate and the electronic components. The main parameters for the recovery of the solder from the WPCBs were temperature of 200 °C, mechanical stirring speed of 150 rpm, and melting time of 3 min. 2.2. Leaching experiments After recycling the solder, the next step was crushing the WPCBs into powders, followed by leaching the metals using ILs.
Fig. 1. Schematic of process for recovering metals from WPCBs using ILs.
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system, as can also be seen from the results of its spectrum analysis. Further, the presence of small voids on the surface was due to the penetration of the solvent molecules or other organic molecules during the pyrolysis process and the difference in the shrinkage rates of the different materials. As shown in Table 1, in addition to containing 58.288% Sn and 6.422% Pb, it also consisted of 21.867% Cu, 11.238% Fe, and trace amounts of other metals; as a result, how to further separate tin and copper is an urgent problem to be solved.
3.2. Effect of ionic liquid used
Fig. 2. Process for separating solders from the WPCBs using [BMIM]BF4.
All the leaching experiments were carried out in a 100-mL glass round-bottomed flask that was stirred with a magnet at a constant rate at a uniform temperature. An aqueous solution of the IL ([CMMIM]HSO4, laboratory-made reagent) (concentration (v/v) of 90%) was used as the leaching agent, while hydrogen peroxide (30%) was used as the oxidant (Chen et al., 2015a). A certain volume was removed from the post-reaction leaching solution, and an 80% hydrazine hydrate solution was added to it in the volume ratio of 1:1 for reduction. Then, the reduced copper was washed 3–5 times after centrifugation and vacuum dried at 80–120 °C for 8–12 h. Eventually, the copper leaching rate was calculated as:
Copper leaching rate ¼ m=m0 100%
ð1Þ
where m0 is the mass of total copper in WPCBs specimens and m is the mass of copper extracted into leaching solution. Additionally, recovery of the separated ionic liquids was carried out by rotary evaporation later. 3. Results and discussion 3.1. Characterization of the solder As can be seen from Fig. 3, the surface of the solder recovered was macroscopically smooth. However, backscattered imaging revealed that the images of the solder surface consisted of bright and dark areas (Fig. 3a), and that relatively small voids were present on the surface (Fig. 3b). The reason for the presence of the areas with the bright and dark contrasts is that the recovered solder material contained different metals, i.e., it was a heterogeneous
As is shown in Fig. 4, the copper leaching rate was 89.19% when the IL ([CM-MIM]HSO4) was used, decreasing to 26.16% in the case of the hydrometallurgical process, in which case sulfuric acid of the same concentration was used. Furthermore, the copper leaching rate decreased significantly when ([BMIM]HSO4), 1-propane sulfonate-3-methylimidazolium bisulfate ([PS-MIM]HSO4), and 1,10 -hexyl-3,30 -dicarboxymethylimidazolium bisulfate ([C6(di-AcIM)]HSO4) were used. In addition, the copper leaching rate was even lower, at 10%, when 1,3-dicarboxymethylimidazolium bisulfate ([di-Ac-IM]HSO4) was used. There are several reasons for these results. First, using an acid-functionalized IL is equivalent to substituting the hydrogen ion in the sulfuric acid molecule with an imidazolium cation and thereby reducing the concentration of hydrogen ions involved in the reaction. In addition, the volume of the imidazole cation greatly enhances the affinity of the anion towards it, resulting in the hydrogen ions in the solution being primarily concentrated on the surface of the imidazole cation. Owing to these factors, the copper leaching rates of the ILs were higher than that of the conventional hydrometallurgical method. The copper recovery rates of ILs 1# and 2# were similar (Fig. 4), indicating that the introduction of a sulfonic acid group does not result in a synergistic effect with the potent hydrogen sulfate ions to lead to an increase in the leaching rate. The introduction of the sulfonic acid group increases the affinity of the imidazolium ions with respect to the free hydrogen ions in the solution, reducing the leaching rate of copper under the same time conditions. The results for ILs 3# and 4# (Fig. 4) indicated that the presence of a second carboxylic acid group reduced the leaching rate by 47.63%. This was because the introduction of the carboxylic acid group enhanced hydrogen bonding in the acid-functionalized ILs and reduced the concentration of hydrogen ions, thus decreasing the strength of the acid participating in the reactions and, consequently, the leaching rate. Finally, comparing the results for IL 4# with those for IL 5# (Fig. 4), it can be concluded that the presence of a double cationic
Fig. 3. Electron probe transmission map images of solder recovered using [BMIM]BF4.
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D.-j. Zhang et al. / Waste Management 78 (2018) 191–197 Table 1 Contents of major elements in solder recovered with [BMIM][BF4]. ELE.
Wt (%)
Mol (%)
Sn Cu Fe Pb Ca Sb La
58.288 21.867 11.238 6.422 0.96 0.748 0.478
44.606 31.256 18.277 2.815 2.176 0.558 0.312
Fig. 5. Effect of [CM-MIM]HSO4 concentration on rate of metal leaching from WPCBs.
3.4. Effect of WPCB powder particle size
Fig. 4. Rates of metal leaching from WPCBs using various ILs: 1#-[BMIM]HSO4; 2#[PS-MIM]HSO4; 3#-[CM-MIM]HSO4; 4#-[di-Ac-IM]HSO4; and 5#-[C6(di-Ac-IM)] HSO4.
As shown in Fig. 6, for WPCB powder particle sizes less than 0.250 mm, the copper leaching rate increased with an increase in the particle size. The maximum metal yield was 94.30% when the particle size was 0.100–0.250 mm. In addition, for particle sizes greater than 0.250 mm, the copper leaching rate decreased. The copper leaching rate increases with the decrease of particle size because the surface area per unit mass is increased. However, the reduction of particle size below a critical level would increase the particle–particle collision and impose severe attrition (Zhu et al., 2011), hindering the leaching liquid permeating through the fine WPCB powders. 3.5. Effect of hydrogen peroxide dosage
structure increased the metal leaching rate by 33%. The reason for this phenomenon is as follows. A relatively long alkyl chain links the two imidazolium cations; this increases the volume of the IL molecule and reduces the number of hydrogen bonds per unit area, as well as the number of hydrogen bonds formed by the imidazolium cations. Thus, the hydrogen ion concentration in the reaction system is higher for IL 5#, and hence, the corresponding leaching rate was higher.
As shown in Fig. 7, the copper leaching rate increased initially and then decreased with an increase in the oxidant content. When the oxidant content was less than 15%, the copper leaching rate increased with an increase in the oxidant content. For instance, for an oxidant content of 15%, the metal yield was 79.55%. In contrast, when the oxidant content was more than 15%, the copper leaching rate decreased with increases in the oxidant content. This is because the oxidant mainly assists the acid in the solution in
3.3. Effect of ionic liquid concentration As is shown in Fig. 5, the copper leaching rate increased with an increase in the IL concentration. When the acid-functionalized IL concentration was less than 70%, the copper leaching rate increased significantly. However, for acid-functionalized IL concentrations of more than 70%, the copper leaching rate did not vary significantly. This is probably because when the concentration was more than 70%, the number of hydrogen bonds within the acid-functionalized IL increased. However, the number of free hydrogen ions did not increase in proportion to the increase in the IL concentration, and therefore, the copper leaching rate increased slowly. Hence, the IL concentration should be chosen with care to ensure a higher leaching rate. Another explanation is that depending on the function of [CM-MIM]HSO4 and the overall reaction for copper dissolution, the mechanism can be expressed as:
Cu+2Hþ +H2 O2 !Cu2þ +2H2 O
ð2Þ
The increase of the copper recovery could be attributed to the increasing acidity and dissolved oxygen concentration.
Fig. 6. Effect of particle size of WPCB powder on rate of metal leaching from WPCBs (F1 < 0.071 mm; 0.071 < F2 < 0.100 mm; 0.100 < F3 < 0.250 mm; 0.250 < F4 < 0.500 mm; and F5 > 0.500 mm).
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rate tended to be steady. From the point of view of the thermodynamics of the chemical reaction, as the reaction progresses, the concentrations of the hydrogen ions and the oxidant decrease, and the reaction tends towards completion:
1 ð1 xÞ1=3 ¼ kc t
ð3Þ
1 2=3x ð1 xÞ2=3 ¼ kd t
ð4Þ
where x refers to the fraction reacted, t is the react time, kc and kd are the rate constants calculated from Eqs. (3) and (4), respectively. 3.7. Effect of solid-to-liquid ratio
Fig. 7. Effect of hydrogen peroxide dosage on rate of metal leaching from WPCBs.
accelerating the oxidation of the copper in the WPCBs into ions, which can subsequently be leached. When the oxidant content was too high, it combined with the copper ions in the solution, forming copper peroxide. Together, the copper peroxide, the oxidant, and the hydrogen ions resulted in the formation of copper ions, water, and oxygen, thus reducing the hydrogen content in the reaction solution. Further, the oxygen produced by the above reaction also accumulated on the surfaces of the WPCB particles, thus slowing the reaction and even preventing it from proceeding. Another theory is that H2O2 firstly reacts with copper to generate CuO, followed by functioning with ILs resulting in the formation of CuSO4. However, when hydrogen peroxide is excessive, the ionic liquid is simultaneously oxidized leading to lower the copper yield.
As is shown in Fig. 9, for solid-to-liquid ratio values greater than 1:20, the copper leaching rate decreased with an increase in the ratio. In particular, when the solid-to-liquid ratio was 1:20, the copper leaching rate was 74.53%. On the contrary, for solid-toliquid ratio values of less than 1:20, the copper leaching rate increased slightly with an increase in the ratio. A decrease in the solid-to-liquid ratio for the leaching process is equivalent to an increase in the volume of the reaction system. As a result, the WPCB particles are better submerged in the reaction solution, and the reaction solution and the particles are in complete contact with each other, thus increasing the mass transfer efficiency and hence the leaching rate. Meanwhile, when the solid-to-liquid ratio
3.6. Effect of leaching time As is revealed in Fig. 8, with an increase in the leaching time, the copper leaching rate first increased sharply and then tended to be steady gradually. When the leaching time was 90–120 min, the copper leaching rate increased rapidly. For the metal dissolution kinetics, if the metal particles were considered as spherical particles, the leaching process can be described with the shrinking core model. The step with the highest resistance is the rate controlling step, meanwhile, the surface reaction control model (3) and diffusion control model (4) can both explain this phenomenon. In contrast, for leaching periods longer than 120 min, the copper leaching
Fig. 8. Effect of leaching time on rate of metal leaching from WPCBs.
Fig. 9. Effect of solid-to-liquid ratio on rate of metal leaching from WPCBs.
Fig. 10. Effect of leaching temperature on rate of metal leaching from WPCBs.
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Fig. 11. Backscattered morphological images of (a) WPCB powder and (b) leached metals.
is too low, although the mass transfer efficiency is increased, it is equivalent to decreasing the concentration of the WPCB powder in the reaction system. Consequently, the yield remains unchanged or even decreases to some degree. Additionally, when the solid-toliquid ratio is too high, lower heat transfer and transmission efficiency leads to lower leaching rate.
3.8. Effect of leaching temperature As is depicted in Fig. 10, the copper leaching rate increased sharply initially with an increase in the leaching temperature, and then increased gently. For a leaching temperature of 80 °C, the copper leaching rate was 70.99%. The reaction rate of the leaching reaction satisfies the Arrhenius equation: k = Aexp(Ea/RT). As the reaction temperature is increased, the reaction rate also increases gradually. The mechanism of copper ions leaching rate increasing still needs further exploration.
3.9. Backscattered topographic images and energy spectrum analysis As shown in Fig. 11, the results of backscattering imaging of the WPCB powders (a) and the leached metals (b), and an energy spectrum analysis of the leached metals (Table 2) were in keeping with expectations. As can be seen in Fig. 11(a), the energy spectrum indicated that the WPCB powder contained large amounts of carbon and oxygen, as well as a small amount of sulfur. This can primarily be attributed to the epoxy resin and other additives used in the PCBs. The WPCBs also contained several metallic elements, including Cu, Al, Ti, Ba, Ca, and La. As can be seen in Fig. 11(b), after being centrifuged and dried, the copper particles obtained after the reduction process were large and difficult to crush further. During the reduction process, the copper formed clusters at the atomic level, resulting in copper particles. As is demonstrated in Table 2, the concentrations of the leached copper was 98.330%; consequently, copper is the most valuable metal leached from this WPCB.
4. Conclusions When the concentration of the IL ([CM-MIM][HSO4]) used was increased from 10% to 90%, the leaching rate improved continuously, increasing from 60.96% to 69.32%. In addition, the particle size had a determining effect on the copper leaching rate, which increased from 17.21% to 94.30% when the powder particle size was increased from less than 0.071 mm to 0.100–0.250 mm. It was observed that the copper leaching rate increased initially and then decreased with increases in the hydrogen peroxide dosage, solid-to-liquid ratio, and leaching time. Further, when the temperature was increased from 40 to 80 °C, the leaching rate increased from 29.50% to 78.66%, which is an improvement of 167%. The copper leaching rate was as high as 98.305% which was higher than other leaching methods when 0.5 g of the WPCB powder was leached under optimal conditions, i.e., using 8.5 mL of a 90% (v/v) IL solution and 1.5 mL 30% hydrogen peroxide at a solid-to-liquid ratio of 1/20 at 80 °C for 2 h. We also found that the recovery rate of ionic liquid by rotary evaporation was generally above 97%, and thus could be useful to reduce cost. In conclusion, leaching copper from the WPCBs using ionic liquids is a more environmentally friendly and efficient method for the ecosystem and industries. Acknowledgments The authors are grateful for support of the National Natural Science Foundation of China (NSFC, 51663013) and State Key Laboratory of Advanced Processing and Recycling of Non-ferrous Metals, Lanzhou University of Technology. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.wasman.2018.05. 036. References
Table 2 Contents of major elements in metal leached with ionic liquid. ELE.
Wt (%)
Mol (%)
Cu Al Ca La
98.330 0.695 0.327 0.058
96.547 1.607 0.509 0.027
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