New Technology for Separating Resin Powder and Fiberglass Powder

Article pubs.acs.org/est

New Technology for Separating Resin Powder and Fiberglass Powder from Fiberglass−Resin Powder of Waste Printed Circuit Boards Jia Li,* Bei Gao, and Zhenming Xu School of Environmental Science and Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, People’s Republic of China

ABSTRACT: New recycling technologies have been developed lately to enhance the value of the fiberglass powder−resin powder fraction (FRP) from waste printed circuit boards. The definite aim of the present paper is to present some novel methods that use the image forces for the separation of the resin powder and fiberglass powder generated from FRP during the corona electrostatic separating process. The particle shape charactization and particle trajectory simulation were performed on samples of mixed non-metallic particles. The simulation results pointed out that particles of resin powder and particles of fiberglass powder had different detach trajectories at the conditions of the same size and certain device parameters. An experiment carried out using a corona electrostatic separator validated the possibility of sorting these particles based on the differences in their shape characteristics. The differences in the physical properties of the different types of particles provided the technical basis for the development of electrostatic separation technologies for the recycling industry.

1. INTRODUCTION

After recycling metal particles from the mixture of metal and non-metal particles, large amounts of fiberglass powder−resin powder fraction (FRP) with a low value are generated during the recycling process.20 Usually the FRP consisting of glass fibers and resin powder would be disposed in such ways as incineration, landfill, destructive distillation, chemical dilution, and physical recycling. The first four ways of non-metal particle disposal all have their own shortcomings. Incineration would generate toxic compounds, such as dioxin, which is considered a carcinogen.21 An inappropriate landfill would cause heavy metal residual in non-metal powders to contaminate soil and underground water, let alone its considerable cost. The high energy and economic cost of destructive distillation impeded its industrial application. The chemical dilution method is still being developed. Therefore, physical recycling is comparably more promising. Recent studies revealed that FRP could also be recycled and used in industries with the physical recycling method. Guo and Xu presented four potential applications of FPR from WPCBs as packing material in phenolic molding compounds (PMC),22 wood plastic compounds (WPC),23

Printed circuit boards (PCBs) are fundamental components of all kinds of electronic and electrical equipment. With the increasing production of waste electronic and electrical equipment (WEEE) in recent years, the amount of waste printed circuit boards (WPCBs) has been climbing drastically.1 Although it is not well-known, WPCBs contain both valuable and toxic components. The inappropriate handling ways create serious pollution upon disposal.2 Then, the recycling of WPCBs has drawn the attention of plenty of researchers because of the economic value of its metal fragments.3,4 Several technologies of recycling WPCBs have been developed,5−11 among which the corona electrostatic separation (CES) method has been regarded as one of the most promising and environmentally friendly technologies for industrial applications.12 Figure 1 shows the classic automatic line for recycling WPCBs, which has been installed and applied in most of the professional recycling plants in China.13 The raw materials of this automatic line are WPCBs, and the products of this automatic line are the metal fraction and non-metal contents. Studies regarding recycling metal contents have been conducted thoroughly, and many relevant papers have been published in the past decade.14−19 However, little attention has been paid to the nonmetal fragments of WPCBs. © 2014 American Chemical Society

Received: Revised: Accepted: Published: 5171

December March 16, March 28, March 28,

20, 2013 2014 2014 2014

dx.doi.org/10.1021/es405679n | Environ. Sci. Technol. 2014, 48, 5171−5178

Environmental Science & Technology

Article

Figure 1. Classic automatic line for recycling WPCBs in industrial applications.

non-metallic plate (NMP),24 non-metal-modified asphalt (NMA),25 which are all common manufacturing materials with broad markets. In Guo’s work, he identified two types of FRP,20 particles of resin powder and particles of fiberglass powder. The fiberglass powder can improve the performance of PMC, WPC, NMP, and NMA significantly in terms of a sharpy notched impact strength and thermal deformation temperature, while the resin powder cannot contribute such improvements as fiberglass powder.24,25 Therefore, it would greatly improve the performance of PMC, WPC, NMP, etc. manufacturing material if resin powder could be removed from the FRP. By separating fiberglass powder and resin powder, not only would the performance of manufacturing material be improved because of the higher purity of fiberglass powder but resin powder can also be used in other applications. However, the traditional method of buoyancy separation cannot be applied to this case because of the close density of the two types of non-metals. The aim of this work is to introduce a new method of separating fiberglass powder and resin powder by CES with the assistance of computer simulation. Electrostatic separation of metals and non-metals from WPCBs has been thoroughly studied the past several years. The roll-type CES has been proven as an effective and efficient device for electrostatic separation. The CES has been used in agriculture to clean seeds and mining to separate minerals.26−30 However, the research about separating different non-conducting materials by CES is still in its infancy. In this study, a MATLAB program was designed to simulate the process of separating PTR and PGF based on the theoretical model in CES.

Figure 2. Diagram of the roll-type corona electrostatic separator.

geometric parameters. Thus, some FRP particles detach from the rotating roll if the co-acting forces satisfy certain conditions, while other FRP particles do not detach the roll until they counter the brush behind the roll. Below the CES, there is a collecting bin, which consists of 10 sub-bins, each 5 cm wide. The center of the roll is 30 cm, right above the number 5 sub-bin. 2.2. FRP Characterization. As shown in Figure 1, the FPR powder originated from an electronic waste treatment plant in Shanghai, China. After being screened, particles of +0.45−0.6 mm were chosen to be the experiment samples in this study. After the multi-scraping process (Figure 1), the resin powder and fiberglass powder were milled to different fractions. As shown in Figure 3, the color of fiberglass powder is white and light green and the color of resin powder is yellow. They have similar densities and completely different shapes. The geometric measurements were performed with the help of the Olympus IX71 inverted microscope. Figure 4 shows the image captured by the microscope. According to the different structures and compositions, the resin powder and fiberglass powder have

2. MATERIALS AND METHODS 2.1. Principle of Separation. Figure 2 shows the principle of operation of CES. The FRP particles are supplied from the feeder and fell on a rotating roll, which is also a grounded electrode. Particles will then be subjected to “‘ion bombardment”’ (corona discharge) and charged when passing the ionization zone generated by the corona electrode. The charged FRP particles are “pinned” by the electric image force to the rotating roll, and two types of FRP particles will be influenced by different image forces because of their significantly different 5172



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2.3. Trajectory Model of FRP. Computer simulation is a convenient tool for new technology developments. The trajectory model of the conductive particle has been used for improving the yield of CES.32 However, the research about the trajectory model for the non-conductive particle is not well considered and far from being established. On the basis of the results of characterization of FRP and “trajectory model of conductive particle”, the trajectory model of FRP was established. Usually, the study of the trajectory model consists of electric field simulation, force analysis, and programming. The electric field simulation has been established.32 E = f1 (U , L , R1 , R 2 , α) E = f2 (U , L , R1 , R 2 , x , y)

(1)

As shown in Figure 5, before detaching from the roll, non-metal particles are affected by gravity force Fg, electric force Fe, image force Fi, air resistance force Fr, and centrifugal force Fc. Once all of the forces satisfy the following equation, particles will detach from the roll: Fc − (Fi + Fe + Fg cos(α)) = 0 Figure 3. Fiberglass powder−resin powder.

(2)

After detaching from the roll, particles are then affected by gravity force Fg, electric force Fe, and air resistance force Fr, as also shown in Figure 5. Considering that the shape of nonmetal particles will also have impact on force analysis and particle trajectories, force formulas are to be revised to reflect the different geometric conditions of resin powder and fiberglass powder. Because the charging value of the particle is correlated with its surface,33 the surface area equivalent radius was employed to replace the spherical radius. Therefore, the surface area equivalent radius is

significantly different shapes under the same size. As shown in Figure 4, the non-metal fraction has different shape characteristics after the process of mechanical scraping. The shape of resin powder is a plate (Figure 4a), and the shape of fiberglass powder is a stripe (Figure 4b). The resin powder is from an epoxy glass fiber laminate, and the fiberglass powder is from a phenolic paper laminate.22−24 For the purpose of simplifying the calculation, plate-like resin powder was approximated to a cylinder, while stripe-like fiberglass powder was approximated to cuboids. The true density was measured with the pycnometer method. On the basis of the results shown in Table 1, the density of both nonmetals is similar to each other. Fiberglass powder is thinner than resin powder (H1 > H2). Because the distance between the centroid of particles and the surface of the rotating roll has significant impact on image force, the fiberglass powder is more likely to be influenced by a bigger image force than resin powder.31

rs =

S /π 2

(3)

where S denotes the surface area of certain particles. In the case of fiberglass powder, the particle is approximated to a cuboid; thus, its surface area is S = 2(lw + wh + hl)

(4)

Figure 4. Geometric measurements of fiberglass powder and resin powder with the Olympus IX71 inverted microscope.

Table 1. Characterization of FRP of +0.45−0.6 mm from WPCBs FRP

shape

approximation

source

density (kg/m3)

measurements

resin powder fiberglass powder

plate stripe

cylinder cuboid

phenolic paper laminate epoxy glass fiber laminate

1548.56 1941.64

R = 0.226 mm; H1 = 0.052 mm L = 1.401 mm; W = 0.204 mm; H2 = 0.034 mm

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Figure 7. Calibration of landing coordinates and detachment angles.

include the detachment angle and landing coordinate. The parameters of simulation were shown Table 2. The landing coordinate is employed to characterize trajectories and detaching conditions of particles, as shown in Figure 7. The distance between the center of the roll and the top of collecting boxes was set to 30 cm. The coordinate origin point of the collection box is directly below the center of the rotating roll.

Figure 5. Force analysis of non-metal particles in real-time embedded systems (RTES).

while in the case of resin powder, the particle is approximated to a cylinder, then its surface area is S = 2πr 2 + 2πrh

(5)

3. RESULTS AND DISCUSSION 3.1. Simulation Results of Non-metal Particles. Unlike metal particles from WPCBs, non-metal particles are pinned to the rotating rolls firmly, which usually require a very high rotating speed or a brush to make particles detach from the roll. In previous work of our research,32 the landing coordinate is employed to characterize trajectories and detaching conditions of metal particles. However, in the case of non-metal particles, only the landing coordinate is not enough. Experimental data showed that, under normal rotating speed (n < 100 rpm) of the roll, both types of non-metal particles had the same trajectories because both particles did not detach the roll until countering the brush. As shown in Figure 8a, when the rotating speed n increases, at a certain value (n = 240 rpm), resin powder starts to fall from the roll in advance; however, the fiberglass powder still adheres

With the electric field strength calculation and force analysis above, a MATLAB program was written to simulate the trajectories of non-metal particles. MATLAB is an advanced tool with powerful functions for modern scientific research. With the MATLAB program of trajectories of non-metal particles, the experimental process can be shortened, which greatly increases the efficiency of research. The first step of this program is to input rotating speed n, applied corona voltage U, particle radius r0, and the shape of non-metal particles. The detachment angle is calculated after the computation of the electric field strength and force analysis. With the calculated detachment angle and other initial values of variables of particles, the trajectories are obtained by the computation of the electric field strength at the particle location, force analysis, etc. The whole process is shown in Figure 6. The output data

Figure 6. Flowchart of the program of trajectories of non-metal particles.

Table 2. Parameters for Computer Simulation parameter

U (kV)

R2 (m)

α (deg)

r0 (mm)

L (m)

H (m)

R1 (m)

value

15

0.019

45

+0.45−0.6

0.171

0.3

0.114

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Figure 8. Trajectories of non-metal particles from WPCBs under different rotating speeds: (a) n = 240 rpm and (b) n = 820 rpm.

Figure 9. Critical speed of non-metal particles under different shapes: (a) stripe non-metals (fiberglass powder) and (b) plate non-metals (resin powder).

showed that the resin powder detached easier from the rotating roll compared to fiberglass powder. The results of landing coordinates of non-metals under different rotating speeds are shown in Figure 10. The landing coordinate of non-metal particles had a very radical change as the rotating speed of the roll increases. The landing coordinate of fiberglass powder (Figure 10a) was more impacted by the rotating speed than the resin powder (Figure10b) because of the high centrifugal force under the high rotating speed. From the results of simulation, the critical rotating speed is a more important expression of detaching conditions of nonmetal particles than the landing coordinate. Unlike the metal particle model,32 it is usually hard to compare the landing

on the surface of the rotating roll. The simulating result showed the possibility of the separation of these two types of non-metal particles. As the rotating speed continues to increase, fiberglass powder detaches from the roll after resin powder, as shown in Figure 8b. The critical rotating speed is the rotating speed of the roll at which the majority of non-metals fall off the roll before they encounter the brush behind the roll.33 The critical speed of non-metal particles under different shapes is shown in Figure 8. As shown in Figure 9, the critical speed of the roll is n2 = 820 rpm for fiberglass powder, under the selected setting of parameters. The critical speed of the roll is n2 = 240 rpm of resin powder, under the same setting of parameters. The results 5175



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Figure 10. Landing coordinates of non-metals of different shapes: (a) stripe non-metals (fiberglass powder) and (b) plate non-metals (resin powder).

Figure 11. Experimental results of separating non-metal particles from WPCBs under the actual critical speed of each particle.

Figure 12. Experimental results of separating non-metal particles from WPCBs.

coordinates under the same rotating speed, unless the rotating speed is bigger than both critical rotating speeds. If the rotating speed is smaller than the critical rotating speed, the landing coordinates are just identical. However, the landing coordinate should also be employed when evaluating the detaching conditions. It can also serve as a reference characterization when verifying the simulation with experimental results. 3.2. Experiment Results of Separating Non-metal Particles. If at a certain rotating speed of roll, a certain percentage of non-metal particles falls off the roll in advance, then the rotating speed is defined as the actual critical speed. As is shown in Figure 11, the same percentage of 87% of two types of non-metal particles falls off the roll in advance at different rotating speeds, resin powder at n′ = 170 rpm and fiberglass at n′ = 200 rpm. That is to say, the actual critical speed of these two non-metals was different. This phenomenon was coordinated with the simulation result. The actual critical speed difference makes it possible to separate these two types of non-metals. When the rotating roll speed is set to the actual critical speed of the resin powder, the majority of the resin powder may fall off the roll, while the fiberglass may still cling to the roll until it is brushed off. Figure 12 shows the results of separating resin powder and fiberglass powder from WPCBs. At the rotating speed of n = 160 rpm,

two types of non-metals were separated for the fact that the majority of fiberglass powder was collected in the number 1 collecting box and the majority of resin powder was collected in number 5 and 6 collecting boxes. It can be concluded that fiberglass powder and resin powder were successfully separated, which means that non-metal particles can be potentially separated according to their different shapes as long as their image forces have significant differences. Beside, the experimental results also showed the potential of applying this separating method for the pretreatment of manufacturing material, such as PMC, WPC, NMP, and NMA (Figure 13). However, the value of the critical rotating speed did not agree with the simulation results. This problem is most likely related to the charging model of non-metal particles. The critical rotating speed is mainly decided by the image force (Fi) and centrifugal force (Fc) of non-metal particles. While the computation of the centrifugal force should be right, the image force is then the possible problem. The image force is mostly affected by the electrical charge, which non-metal particles carry. However, because of the experimental limitations, the exact value of the charge that non-metal particles carry cannot be measured, as well as its exact decaying curves measured by a direct method. Therefore, it is hard to verify the accuracy of the charging model, which was used in simulation. 5176



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Figure 13. Application of products of WPCB electrostatic separation. (7) Zhu, P.; Chen, Y.; Wang, L. Y.; Zhou, M.; Zhou, J. The separation of waste printed circuit board by dissolving bromine epoxy resin using organic solvent. Waste Manage. 2013, 33, 484−488. (8) Jaeryeong, L.; Youngjin, K.; Jaechun, L. Disassembly and physical separation of electric/electronic components layered in printed circuit boards (PCB). J. Hazard. Mater. 2012, 241−242, 387−394. (9) Zhu, P.; Chen, Y.; Wanga, L. Y.; Qian, G. Y.; Zhou, M.; Zhou, J. A new technology for separation and recovery of materials from waste printed circuit boards by dissolving bromine epoxy resins using ionic liquid. J. Hazard. Mater. 2012, 239−240, 270−278. (10) Yihui, Z.; Keqiang, Q. A new technology for recycling materials from waste printed circuit boards. J. Hazard. Mater. 2010, 175, 823− 828. (11) Zhiyuan, H.; Fengchun, X.; Yang, M. Ultrasonic recovery of copper and iron through the simultaneous utilization of printed circuit boards (PCB) spent acid etching solution and PCB waste sludge. J. Hazard. Mater. 2011, 185, 155−161. (12) Li, J.; Xu, Z. M. Environmental friendly automatic line for recovering metal from waste printed circuit boards. Environ. Sci. Technol. 2010, 44, 1418−1423. (13) Zhou, L.; Xu, Z. M. Response to waste electrical and electronic equipments in China: Legislation, recycling system, and advanced integrated process. Environ. Sci. Technol. 2012, 46, 4713−4724. (14) Zhan, L.; Xu, Z. M. Application of vacuum metallurgy to separate pure metal from mixed metallic particles of crushed waste printed circuit board scraps. Environ. Sci. Technol. 2008, 42, 7676− 7681. (15) Zhan, L.; Xu, Z. M. Separating and recycling metals from mixed metallic particles of crushed electronic wastes by vacuum metallurgy. Environ. Sci. Technol. 2009, 43, 7074−7078. (16) Zhan, L.; Xu, Z. M. Separating and recovering Pb from copperrich particles of crushed waste printed circuit boards by evaporation and condensation. Environ. Sci. Technol. 2011, 45, 5359−5365. (17) Zhan, L.; Xu, Z. M. Separating criterion of Pb, Cd, Bi and Zn from metallic particles of crushed electronic wastes by vacuum evaporation. Sep. Sci. Technol. 2012, 47, 913−919. (18) Zhan, L.; Qiu, Z. L.; Xu, Z. M. Separating zinc from copper and zinc mixed particles using vacuum sublimation. Sep. Purif. Technol. 2009, 68, 397−402. (19) Li, J.; Xu, Z. M.; Zhou, Y. H. Theoretic model and computer simulation of separating mixture metal particles from waste printed circuit board by electrostatic separator. J. Hazard. Mater. 2008, 153 (3), 1308−1313. (20) Guo, J.; Jiang, Y.; Hu, X. F.; Xu, Z. Volatile organic compounds and metal leaching from composite products made from fiberglass− resin portion of printed circuit board waste. Environ. Sci. Technol. 2012, 46, 1028−34. (21) Leung, A. O.; Luksemburg, W. J.; Wong, A. S.; Wong, M. H. Spatial distribution of polybrominated diphenyl ethers and polychlorinated

In the future research, the charging mechanism is to be studied, so that the charging model of non-metals and the charge-decaying curve can be revised. Therefore, the simulation program can be more accurate and employed to optimize the parameters of electrostatic separators to separate non-metals of different shapes better, which would for sure improve the development of the non-metal particles from WPCB physical recycling application.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project was supported in part by the National Natural Science Foundation of China (51008192), the National High Technology Research and Development Program of China (863 program 2012AA063206), the Shanghai Natural Science Foundation (10ZR1415900) and Shanghai Cooperative Centre for WEEE Recycling (ZF1224-12). The authors are grateful to the reviewers who helped them improve the paper by many pertinent comments and suggestions.

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REFERENCES

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New Technology for Separating Resin Powder and Fiberglass Powder

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