Environ. Sci. Technol. 2010, 44, 1418–1423
Environmental Friendly Automatic Line for Recovering Metal from Waste Printed Circuit Boards JIA LI AND ZHENMING XU* School of Environmental Science and Engineering Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, People’s Republic of China
Received October 23, 2009. Revised manuscript received November 30, 2009. Accepted January 7, 2010.
methods to treat waste PCBs (7, 8), but most of them are still only lab-scale. They ignore the problem of energy consumption, production capacity, and technology rationality which are important factors for industrial applications. New recycling technology on the industrial -application scale is needed to replace the crude processing and prevent further environmental pollution. The traditional fluid production line has been adopted by some of local processors. It is better than the highly polluting methods, but still not good enough. As shown in Figure 1 (a), the traditional fluid bed production line generates a large quantity of wastewater. Some metal cannot be separated from nonmetal powders and stays in the wastewater. The metal recovery rate R is R)
The technology industrialization was the final goal of the research. A set of automatic line without negative impact to environment for recycling waste printed circuit boards (PCB) in industry-scale was investigated in this study. The independent technologies were integrated and many problems in the process of technology industrialization were solved. The whole technology contained four parts: multiple scarping, material screening, multiple-roll corona electrostatic separator, and dust precipitation. The output of this automatic line reached 600 kg/handtherecoveryrateofcopperreached95%.Afterseparation, the metal and nonmetal products were totally reused. Compared with other production lines (traditional fluid bed production line and processing from developed countries), the automatic line has lower energy consumption and better technology rationality. The cost of this line was in acceptable level for local processors.
Introduction A large number of electronic waste (e-waste) has been generated following the dramatically increased demand for electronics products. The quantity of e-waste in China is more than 100 million tons each year, meanwhile a large number of foreign e-waste flows to China every year, which includes large numbers of waste printed circuit boards (PCB). Because the metal resources (1) and the economic profit drive, lots of backyards and small workshops joined the market of recycling waste PCBs. They use simple and severely polluting methods (2) to extract metals from waste PCBs. The salary paid for workers is only $9-15 (U.S.) per day (the work time is 8 h per day), and the only protective equipment for them are gloves. The crude processing of e-waste has become one of the major contributors of polybrominated diphenyl ethers (PBDEs) and polychlorinated dibenzo-pdioxins and dibenzofurans (PCDD/Fs) to the terrestrial environment (3). The backyards and small workshops become emission points of hazardous gases and pungent odor, which seriously impact the normal life of local people. Through communicating with local operators of small workshops, most of them worry about the local environment and they want to use pollution-free processing. But they have two major problems: (1) the technology of pollution-free processing from developed countries is too expensive to use (4-6); (2) pollution-free processing in China is still in infancy. Many researchers have done lots of work to find suitable * Corresponding author phone: +86 21 54747495; fax: +86 21 54747495; e-mail:
[email protected]. 1418
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Mr × 100% Mc
(1)
Where Mr is the mass of metal recovery from waste PCBs and Mc is the mass of metal in raw waste PCBs. The metal recovery rate of the traditional fluid bed production line is only 60-70%. About 30% of the metals are wasted after processing. So the workers always manually deliver the nonmetal powders to the fluid bed many times. The energy and human power are wasted during the process of repeated fluid bed separation. As shown in Figure 1 (b), the waste nonmetal powders are mounded into a hill outdoors after processing. Through erosion of wind and rain, the nonmetal powders damage the surrounding environment. Even the properties of nonmetal powders are damaged during the process of soaking and cannot be used again (9). The traditional fluid bed production line must be replaced by other methods. The objective of this paper is to establish an environmentally friendly automatic line on an industry-scale and suitable for local processors. Designing Ideas. The researchers have indicated that a method of mechanical scraping combined with corona electrostatic separation (CES) (7) is one of the environmentally friendly technologies used to recover resources and products from waste PCBs. But there are still some problems with this type of technology being used in industry: (1) the waste PCBs are comprised of reinforced resin and metal parts. Using general scraping machines cannot provide good metal stripping; (2) the particle size distribution of waste PCBs is continuous after scraping. Fine particles will impact the process of separation; (3) the balance between metal purity and production capacity cannot both be satisfied by using single-roll CES; (4) dust is generated during scraping and material delivering. Lu et al. (10) has studied the pulverization characteristics and energy consumption analysis during the process of scraping waste PCBs. Recently, Xu et al. (11-14) studied the effects of operating parameters of CES and properties of metallic metals (such as particle shapes and particle sizes) on the efficiency of CES during the process of recycling waste PCBs. The computer simulation was also used to optimize the process of recycling waste PCBs by CES (15-18). However, the above studies are independent of fundamental research and technologies and still stayed on the lab-scale. The technology industrialization is the final goal of the research. Based on the above research, a set of automatic lines without a negative impact to the environment for recycling metal from waste PCBs in industry-scale was investigated in this study. The independent technologies were integrated, and many problems in the process of technology industrialization were solved. The whole technology contains four parts: (A) 10.1021/es903242t
2010 American Chemical Society
Published on Web 01/21/2010
FIGURE 1. Traditional fluid bed production line, (a) wastewater is generated and some metals are waste with nonmetal powders, (b) nonmetal powders cannot be recycled after fluid bed. multiple scraping part, (B) material screening part, (C) multiple-roll CES part, and (D) dust precipitation part, as shown in Figure 2. The above problems would be solved by this automatic line.
Materials and Methods In order to accurately evaluate the metal recovery rate of automatic line, a total of 4500 kg of waste PCBs was collected from a local PCBs factory. The weight content of copper of materials was about 30%. The largest size of materials was 600 × 100 × 5 mm, and the smallest size was 200 × 20 × 2 mm. The proportions of metal in the final products (metals, media, nonmetal, and dust) were found via measuring the solution quantity with aqua regia.
Results and Discussion Part A: Multiple Scraping. The scraping process is an essential part of the mechanical method for treating waste PCBs (6). The waste PCBs are a mixture of woven glass reinforced with resin and metal. They have high hardness and tenacity, so general scraping machines cannot provide good metal stripping without producing waste energy. Twostep crushing is a suitable method to scraping waste PCBs (7), and its energy consumption can be reduced by changing the structure. According to the previous study (10), the experimental equation for energy consumption is
(
A ) 2340 · m
1 1 D2 D1
)
(1)
Where m is the mass of waste PCBs, D1 is the average size of PCB particles before scraping, and D2 is the average size of PCB particles after scraping. So the energy consumption is decided by the average size of particles. The scraping part contains one feed conveyer, one shredder, and two hammer grinders, as shown in Figure 2 (b). The vertical structure of shredder and hammer grinder-1 is good for material delivering and reduces the dust during the process of scraping. The diameters of the screen holes in the hammer grinder were set based on eq 1 to save energy. In the scraping part, a shredder is used before hammer grinders to reduce the average size of waste PCBs. Utilizing the differential rotation in interactions between two axes with square knives, the shredder is designed to shred sizable waste PCBs into small pieces (50 × 50 mm). Then the materials are further scraped to 5 × 5 mm pieces by hammer grinder-1. After hammer grinder-2, the particle size is below
2.5 mm, and the size distribution of scraped waste PCBs is continuous. Under the premise of 500 kg/h production capacity, the energy consumption of the scraping part is less than 100 kW. The process of scraping released vast energy and increased the temperature inside crushing machine. The high temperature environment not only accelerates the abrasion of scraping machines, but also increases the possibility of PCB pyrolysis (8). To avoid the above problems, the rotating speed of knives in hammer grinders must be controlled. But slowing the speed too much does not fully complete the scraping of waste PCBs (10). So the rotating speed of knives in hammer grinders was controlled to 1800 rpm. The circulating water system is also fixed on the hammer grinders. Then the temperature of materials from scraping machine was controlled to 80 °C when the machines were running at full capacity. Part B: Material Screening. In order to save energy, the size of screen hole of hammer grinder-2 is designed to Φ 2.5 mm. The size distribution of scraped waste PCBs from hammer grinder-2 is continuous. As a previous paper (11) mentioned, the particle size of scraped PCBs should be limited to the range of 0.6 and 1.2 mm in industrial applications. The fine nonmetal particles brought negative impacts on the electrostatic separation (19). The crude particles (size: +1.2 mm) were not completely liberated after scraping (7). In order to achieve high metal recycling ratio, the size of particles must be suitable (size: +0.6-1.2 mm) before they are fed to electrostatic separators. Part B is used to ensure that the right size particles are sent to the next process, as shown in Figure 2 (a). Part B contains two cyclone and one vibrating screen, as shown in Figure 2 (b). The continuous particles are from hammer grinder-2. They are delivered to cyclone-1 by high speed air (the blast volume of air is appropriately 3000 pa). The materials come into rectangle-shaped pipes along the tangential direction to generate circular motion in the cyclone. Under the effect of centrifugal force, the large particles are thrown to the wall and collect at the bottom of the cyclone. However, the small particles are discharged by the center discharge pipe, as shown in Figure 3. After the process of cyclone separating, the fine nonmetal particles are almost delivered to the cyclone-2, the crude particles (size: +1.2 mm) and suitable particles (size: +0.6-1.2 mm) are delivered to the vibrating screen, as shown in Figure 3. The vibrating screen screen mesh is made by wire netting, and the blinding chance from VOL. 44, NO. 4, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. Whole automatic line for recycling waste PCBs in industry-scale, (a) flow process chart, (b) engineering schematic plan. irregular shape of particulate system is reduced to minimum. The size of the screen holes in the vibrating screen is 1.2 × 1.2 mm, so the crude particles are delivered back to hammer grinder-2 by screw conveyor-2, as shown in Figure 2. Then the suitable particles are fed to 6-roll electrostatic separator for recovering metals and nonmetal particles. After cyclone-1, the fine nonmetal particles are treated by cyclone-2. Some of them are collected by cyclone-2, and the dust is collected by a bag-type collector. Part C: Multiple-Roll CES. Compared with other methods, the electrostatic separator has advantages of saving energy and no impact to the environment. Although, the optimization for the electrostatic separation has been studied (17, 20), there are still some problems limiting the traditional single roll electrostatic separator to satisfy the industrial application. There are three problems: (1) the balance between metal purity and production capacity (16); (2) the impact from fine nonmetal particles during the separating process (11); (3) 1420
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the further treatment for media material (mixture of metal and nonmetal particles after separating). Conception of multiseparation has been widely applied in industries of mining and smelting. Based on research of a 2-roll electrostatic separator (21), a 6-roll electrostatic separator (triple-step separator) was used in this automatic line, as shown in Figure 2. The mixture particles (scraped PCBs from vibrating screen) are equally distributed into two parts. Each part was processed by three vertical rotating rolls. These three rolls had different operating parameters. The three major influencing factors to separating are construction of electrodes, position of separator board, and rotating speed of roll, as shown in Figure 4. The major factors are two types, as shown in Table 1. The corona enhancement type of electrodes increases the purity of metal, and the electrostatic enhancement type of electrodes increases the output of metal. According the trajectories of metal particles (18), the position of separator board
FIGURE 3. Schematic of material screening system.
FIGURE 4. Internal construction of triple-step electrostatic separator.
TABLE 1. Operating Parameters of Triple Step Electrostatic Separator construction of electrodes parameter-1 corona enhancement parameter-2 electrostatic enhancement parameter-3 electrostatic enhancement
position of separator board highmetal purity highmetal output highmetal purity
rotating speed n high speed low speed low speed
could be adjusted to get high purity or high output of metal. As shown in Table 1, three groups of operating parameters are set for solving the three major problems of traditional single roll electrostatic separator. Parameter-1. The output of the whole system is decided by roll-1, as shown in Figure 5. So the feeding rate of roll-1 is higher than the other two rolls. The rotating speed of roll-1 must be set to a higher speed to avoid multilayer materials
(11). Some of the nonmetal particles were not saturated with the charging value and detached the roll under high rotating speed (15). So the construction of electrode is set to corona enhancement to enhance the charging value of nonmetal particles, and the position of separator board was set to high metal purity to block nonmetal particles. The separation of roll-1 is material crude separation. The nonmetal particles from roll-1 were directly delivered to roll-3 to decrease the impact of fine nonmetal particles. Most of metal and media were delivered to roll-2 to have fine separation. Parameter-2. The metal content of mixture particles which was fed to roll-2 increased significantly and the impact from fine nonmetal particles decreased. So the separation of roll-2 is a metal collecting process. The electrostatic enhanced construction is good for metal collecting. The position of the separator board was set to high metal output to get maximum metal collection. Low rotating speed is good for the charging of nonmetal particles and increased the purity of metal. Most metal was collected by roll-2, and the mixture of media and nonmetal particles were delivered to roll-3, as shown in Figure 5. Parameter-3. The nonmetal particles from roll-1, roll-2, and media from roll-2 were fed to roll-3. The feeding of roll-3 was almost entirely nonmetal particles. The separation of roll-3 was nonmetal collecting process. In order to decrease the metal loss and increase the metal collecting ratio of whole system, the construction of electrodes was set to electrostatic enhancement and rotating speed was low. The media material from roll-3 was delivered back to roll-1. Compared with traditional single roll separator, the triplestep separator achieved that (1) the purity and output of metal were both high; (2) the impact from fine nonmetal particles was reduced to smallest; (3) the media material was further treated. Part D: Dust Precipitation. The dry process would generate lot of dust during the process of scraping and material delivering. The dust was comprised of resin and fiber, and some of it was flying in the air. The flying dust not only impacted the work environment but also was harmful for workers. The bag-type dust collector was used to precipitate the dust from process of scraping and material delivering. As shown in Figure 2 (b), the pulse pumps were fixed on the top of dust collector to clean the bags and move the dust to the collector. Through dry process, the dust contained its original properties. So it could be used to replace VOL. 44, NO. 4, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 5. Flow of triple-step electrostatic separation.
TABLE 2. Weight Content of Final Product after Automatic Line waste PCBs metal powder collector from 6-roll CES media powder collector from 6-roll CES nonmetal powder collector from 6-roll CES nonmetal powder collector from cyclone -2 nonmetal powder collector from dust collector a
weight content 100%
metal 30%
nonmetal 70%
28.4%
27.7%
0.7%
3.3%
0.9%
2.4%
35%
0.7%a
34.3%
15%
a
0.3%
14.7%
18.3%
0.4%a
17.9%
The metals cannot be recovered by automatic line.
wood flour in the production of phenolic molding compound (PMC) (9). Evaluation of Automatic Line. The material used in pilotscale production was 4500 kg. The output of this automatic line can reach 600 kg/hour and the recovery rate of copper can reach 95%. The metal contents of each final products was shown in Table 2. The weight content of waste metal was about 1.4% during the process for recovering waste PCBs (30% metal content). The media powder from 6-roll CES was sent to belt conveyor-2 (Figure 2). Because the quantity of media powder was small and the powder was dry, it was easier to have twice the separation than from traditional fluid bed production line. The materials coming out of the separators were metal powders and nonmetal powders. Because the complex species of PCBs, the compositions of metals always contained copper, zinc, lead, etc. The mixture metals could be further separated and purified by vacuum metallurgy (22). Because of the dry process of separation, the nonmetal powders maintained their original properties. They could be made into a kind of nonmetallic plate (23) and be used to replace wood flour in the production of phenolic molding compound (PMC) (9). They also could be reused as a new 1422
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TABLE 3. Comparison of Three Kinds of PCBs Mechanical Production Lines traditional fluid bed process from automatic production line developed countries line output (ton/hour) power (KW) metal recovery rate operators environmental effects equipment cost ($) maintenance cost ($/ton) cost ($/ton) gross profit ($/ton)
0.3 200 90%
0.3 130 >90%
10 waste water
4 None
4 none
30 k 4.95
700 k 2.95a
100 k 2.95
1422.45 17.55
1490.85 129.15
1363.65 256.35
a The maintenance cost of the process from developed countries was assumed to be the same with the automatic line.
modifier to improve the performance of asphalt (24). Then the metal and nonmetal products would be totally reused. The comparison of three kinds of PCBs mechanical production lines is showed in Table 3. In order to make an economic evaluation of them, it was assumed that (A) the cost of raw material (waste PCBs) is $1285 (U.S.) per ton and the content of copper in raw material was about 30%; (B) the salary of the operator is $1.62 (U.S.) per hour; (C) the service life of automatic line is 10 years and the work time is 8 h per day; (D) the cost of electric power was $0.116 (U.S.) per kilowatt-hour (in shanghai); (E) the price of waste copper was assumed to $6000 (U.S.) per ton; (F) the maintenance cost includes wear and tear of the scraping machine and maintenance of bag house filters. Then the cost and gross profit of each line can be computed as follows:
cost ) A +
B C + + output 10 × 365 × 24 × 0.3 power D× +F output
gross profit ) E × metal recovery rate × 0.3 - cost
(2)
(9) (10)
(3)
As shown in Table 3, the traditional fluid bed production line has low “metal recovery rate”, so its gross rate was lowest. The process from developed countries has high power, so its gross profit was lower than the automatic line and the equipment cost was too high for local processors. Compared with other production lines, the automatic line has lower energy consumption and better technology rationality.
(11)
Acknowledgments
(14)
This project was supported by the National High Technology Research and Development Program of China (863 program 2006AA06Z364), Program for New Century Excellent Talents in University, Research Fund for the Doctoral Program of Higher Education (20090073120041), Program for Energy Saver and Exhaust Reducer of Shanghai (09dz1204404) and Shanghai Natural Science Foundation (10ZR1415900).
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