557378
research-article2014
WMR0010.1177/0734242X14557378Waste Management & ResearchZhan and Xu
Original Article
Assessment of heavy metals exposure, noise and thermal safety in the ambiance of a vacuum metallurgy separation system for recycling heavy metals from crushed e-wastes
Waste Management & Research 2014, Vol. 32(12) 1247–1253 © The Author(s) 2014 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/0734242X14557378 wmr.sagepub.com
Lu Zhan1 and Zhenming Xu2
Abstract Vacuum metallurgy separation (VMS) is a technically feasible method to recover Pb, Cd and other heavy metals from crushed e-wastes. To further determine the environmental impacts and safety of this method, heavy metals exposure, noise and thermal safety in the ambiance of a vacuum metallurgy separation system are evaluated in this article. The mass concentrations of total suspended particulate (TSP) and PM10 are 0.1503 and 0.0973 mg m-3 near the facilities. The concentrations of Pb, Cd and Sn in TSP samples are 0.0104, 0.1283 and 0.0961 μg m-3, respectively. Health risk assessments show that the hazard index of Pb is 3.25 × 10-1 and that of Cd is 1.09 × 10-1. Carcinogenic risk of Cd through inhalation is 1.08 × 10-5. The values of the hazard index and risk indicate that Pb and Cd will not cause non-cancerous effects or carcinogenic risk on workers. The noise sources are mainly the mechanical vacuum pump and the water cooling pump. Both of them have the noise levels below 80 dB (A). The thermal safety assessment shows that the temperatures of the vacuum metallurgy separation system surface are all below 303 K after adopting the circulated water cooling and heat insulation measures. This study provides the environmental information of the vacuum metallurgy separation system, which is of assistance to promote the industrialisation of vacuum metallurgy separation for recovering heavy metals from e-wastes. Keywords Heavy metals exposure, noise, thermal safety, risk assessment, e-wastes
Introduction With technological innovation and intense marketing, large amounts of electric and electronic equipment wastes (e-wastes) are generated with both toxic and valuable materials in them. It is estimated that 20–50 million tonnes of e-wastes are generated around the world each year. China, the largest dumping ground of e-wastes, accommodates more than70% e-wastes all over the world (Bruke, 2007; Huang et al., 2009). Some villages in China as famous e-wastes destinations are facing serious environmental pollutions as a result of the backyard operations adopted. Open sky incineration and acid leaching have resulted dramatically in aquatic, terrestrial and atmospheric pollutions (Deng et al., 2006; Fu et al., 2008; Leung et al., 2008). The blood lead and cadmium levels of the children living there are far over the normal standards (Zheng et al., 2008). Many scholars have done a lot of research and mainly focus on the extraction of valuable metals like Au, Ag and Cu from various kinds of e-wastes (Hagelüken, 2006; Park and Fray, 2009; Zhang and Zhang, 2013). Although the heavy metal contamination has been reported a lot, the recycling of heavy metals like Pb, Cd and Zn in e-wastes has not attracted enough attention because of their
nominal prices. However, the resource utilisation of heavy metals in e-wastes is just the way to clear up heavy metal contamination completely and thoroughly. Recently, several qualified factories in China adopt advanced recycling technologies to recover metals from televisions, refrigerators, printed circuit boards and other e-wastes (Li and Xu, 2010; Li et al., 2009; Xue et al., 2013). Metal-rich particles containing Pb, Cd, Cu, Sn and other metals are recycled after dismantling, crushing, magnetic separating and corona-electrostatic separating. However, it is still a mixture of
1Shanghai
Key Lab for Urban Ecological Processes and EcoRestoration, School of Ecological and Environmental Science, East China Normal University, Shanghai, People’s Republic of China 2School of Environmental Science and Engineering, Shanghai Jiao Tong University, Shanghai, People’s Republic of China Corresponding author: Lu Zhan, Shanghai Key Lab for Urban Ecological Processes and EcoRestoration, School of Ecological and Environmental Science, East China Normal University, 500 Dong Chuan Road, Shanghai, People’s Republic of China. Emails: [email protected]
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Figure 1. The VMS system for recycling heavy metals from crushed e-wastes.
various metals, and the heavy metals like Pb and Cd are still waiting for a satisfactory recycling method. Vacuum metallurgy separation (VMS) has been applied for separating and recovering metals with high vapour pressures successfully (Chen et al., 2009; Li et al., 2012; Smeets and Fray, 1991). The mixture of metals can be separated from each other as a result of their different vapour pressures, and then the pure metal can be recycled by condensation under different conditions. It is also verified as a technically feasible method to recover Pb and Cd from metal-rich particles of crushed e-wastes (Zhan and Xu, 2008, 2009, 2011). However, the environmental impacts and safety of the VMS system are still not researched clearly. During the VMS process, whether the vapour of the processing objects (Pb, Cd and other heavy metals) may leak into the workshop owing to incomplete condensation or uninterrupted pumping, the data about the potential risk are unavailable. Exposure to heavy metals, especially Pb and Cd adhering on the particles, will probably result in significant health effects to the workers (Bi et al., 2010; Gullett et al., 2007; US EPA, 1992). Thus, the concentrations of heavy metals in air should be of concern. The noise of a vacuum pump motor and water pump may cause hearing damage. The injury caused by exposure to a high noise level has been recognised for many years (Ahmed et al., 2001; Rachiotis et al., 2006). Many anti-noise laws and ordinances have been issued by many countries (Piccolo et al., 2005). Therefore, this article will estimate the noise exposure level in the ambiance of a VMS system. VMS is processed at high temperatures (873–1273 K), and the oil diffusion pump starts working at a temperature of more than 523 K. The surface temperatures
of the furnace and other corresponding facilities may result in potential scald risks. Consequently, the surface temperatures of the whole VMS system are monitored, and the thermal risk is evaluated. In this study, heavy metals (Pb, Cd and Sn) in total suspended particulate (TSP) and PM10 are monitored and evaluated for chronic risk of the recycling workers. Meanwhile, the potential safety problems about the exposure of noise and thermal safety are also studied. This article is expected to provide a complete assessment about the potential risks of the VMS system.
Materials and methods VMS system The VMS system is applied to further separate and recover metal-rich particles from crushed e-wastes that are obtained after crushing and electrostatic separating. Heavy metals (especially Pb and Cd) are evaporated and condensed during the VMS process. It is operated under an environment of high vacuum (0.01–10 Pa) and high temperature (873–1273 K). As shown in Figure 1, the processing conditions are realised by the combination of a resistance furnace, a vacuum pump team and other supporting facilities. The vacuum chamber is evacuated by means of a two-stage pumping system consisting of a mechanical vacuum pump and an oil diffusion pump. The operating procedure is as follows: (1) metal-rich particles are fed into the graphite crucible; (2) after reaching the set vacuum degree, the heating process starts; (3) the vacuum pump team keeps working until all the metals with high vapour pressures are evaporated and condensed.
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Monitoring of heavy metals Sample collection and analysis. A middle volume air sampler (flow rate of 0.120 m3 min-1) was applied to collect TSP and PM10 samples during the VMS process. The measuring procedures were standardised with HJ 618-2011 (determination of atmospheric articles PM10 and PM2.5 in ambient air by gravimetric method) and GB/T 15432-1995 (ambient air – determination of total suspended particulates – gravimetric method). Two sampling points were set. One was located 2 m away from the resistance furnace where workers frequently operated the facilities, 1.5 m above the ground, which is the breathing height (the position of noses). The other sampling point was located in the exhaust outlet of mechanical pump. Meanwhile, the concentrations of TSP and PM10 outside the workshop were detected as the background value. The actual values of TSP, PM10 and the heavy meals adhered on the particles may be affected by the operating conditions, such as heating temperature, residence time, vacuum pressure and so on. Therefore, three samples of the experiments with different operating conditions were collected at each sampling point, and the mean concentrations of TSP and PM10 were calculated. After the necessary sample pretreatment, all samples were digested according to the United States Environmental Protection Agency (US EPA) method (Deng et al., 2006; US EPA, 1999). Because the main separated and recovered metals by VMS were Cd and Pb of solder (Pb–Sn alloy), the concentrations of Pb, Cd and Sn in TSP and PM were monitored and determined by an inductively coupled plasma-atomic emission spectrometry (ICP-AES, IRIS-advantage 1000, THERMO, US). All the relative standard deviation (RSD) is less than 2%. Health risk assessment of heavy metals. Risk assessment models of US EPA are applied to evaluate the health risk of heavy metals in the air of the workshop (US EPA, 1992). The operator is exposed to heavy metals through three pathways: ingestion, inhalation and dermal contact. The average daily dose (mg (kg·d)-1) contacted or absorbed through ingestion (ADDing), inhalation (ADDinh) and dermal contact (ADDderm) can be expressed as follows (US EPA, 1997):
ADDing
IngR × ED (1) =C× BW × AT
ADDinh = C ×
ADDderm = C ×
InhR × ED (2) BW × AT
SA × SL × ABS × ED (3) BW × AT
where C is the metal concentration in the TSP or PM10 sample (mg kg-1). For ingestion, the intake rate (IngR) of dust is 100 mg d-1 for adults. For inhalation, the intake rate (InhR) for male is 15.2 m3 d-1. For dermal contact, the exposed skin area
(SA) is given as 1150 cm2, the skin adherence factor (SL) is 0.2 mg (cm2 d)-1, the dermal absorption factor (ABS) is 0.001 (Baptista and Miguel, 2005), the average body weight (BW) of Chinese people is 60 kg for adults (Chang et al., 2009). The exposure duration (ED) is calculated by working days (250 days per year) timing the service life (10 years). The average time (AT) is 3650 days. Among the three metals studied, Cd is recognised to be carcinogenic. Because the slope factors (SF) for carcinogenic risk through ingestion and dermal contact are not given by the US EPA, this study only considers carcinogenic risk resulting from inhalation. For carcinogens, the lifetime average daily dose (LADD) is calculated as (US EPA, 1997).
LADDinh = C ×
InhR × ED (4) BW × AT
where acronyms represent the same variables as in eq. (1)–(3), except AT = 70 × 365 d. The non-carcinogenic risks of a single heavy metal can be indicated as hazard quotient (HQ), and hazard index (HI) represents the total non-carcinogenic risks of different heavy metals through the above-mentioned three ways. HQ and HI can be expressed as follows (US EPA, 1992):
HQing =
ADDing (5) RfDing
HQinh =
ADDinh (6) RfDinh
HQderm =
ADDderm (7) RfDderm
HI =
∑ HQ (8) i
where the reference dose (RfD, mg (kg·d)-1) the estimate of daily exposure below which adverse non-cancer health effects are unlikely. If HQ 10, it indicates high chronic risk (US EPA, 1997). For carcinogen metals, the cancer risk level can be expressed through LADD multiplying by the slope factor (SF), as shown in equation (9) (Baptista and Miguel, 2005; US EPA, 1997):
Risk = LADD × SF (9)
where Risk indicates the probability of cancer happening. SF ([mg/(kg·d)]-1) is the maximum carcinogenic probability when
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Table 1. TSP and PM10 of three different sites. Monitoring sites
TSP, mg m-3
TSP/TSP* %
PM10, mg m-3
PM10/PM10* %
PM10/TSP, %
Site I: 2 m away from the furnace Site II: exhaust outlet background
0.1503 0.0948 0.0853
50.1 31.6 28.4
0.0973 0.0591 0.0494
64.9 39.4 32.9
64.7 62.3 57.9
TSP* and PM10* are the accepted emission limits according to GB 3095-2012, which are 0.3 mg/m-3 and 0.15 mg m-3, respectively.
Table 2. The concentrations of Pb, Cd and Sn in TSP and PM10 of three different sites. Site I: 2 m away from the furnace
Site II: exhaust outlet
Background
TSP
PM10
TSP
PM10
TSP
PM10
Cd, μg m-3
0.0104
0.0002
0.0031
0.0002
Pb, μg m-3 Sn, μg m-3
0.1283 0.0961
0.0782 0.0205
0.0975 0.0194
Below detection limit 0.0025 0.0139
Below detection limit 0.0004 0.0062
the human body is exposed to a fixed dose of a certain heavy metal. The threshold value of Risk is 10-4 (US EPA, 1997).
Monitoring of noise and temperature An integrated sound level meter (AR824) was employed to monitor noise levels of the VMS system in its running state. The measuring procedures were standardised with GBZ/T 189.8– 2007 (measurement of noise in the workplace). An area 2 m away from the facilities and 1.5 m above the ground was the noise monitoring point. The motors of the cooling water pump and mechanical vacuum pump were the main monitoring objects. At each monitoring point, the noise level was measured three times and the mean value was calculated. The procedures of measuring the temperatures of the VMS system were standardised with GBZ/T 189.7-2007 (measurement of heat stress in the workplace). The errors for measuring the temperature were mainly caused by different operating conditions. For the temperature of each facility, it was measured three times under different heating temperatures and residence time. Then, the mean values were obtained and adopted.
Results and discussion Assessment of heavy metals exposure Distribution characteristics of heavy metals in TSP and PM10 . TSP and PM10 of the two sites as well as the background values are shown in Table 1. According to the standards of indoor air quality standard of China (GB 3095-2012), the limit of the TSP daily mean concentration is 0.3 mg m-3, and that of PM10 is 0.15 mg m--3. Compared with the accepted emission limits, the percentages of the measured values (TSP and PM10) are shown in Table 1. All of the monitoring values are below the limits of the corresponding requirements. Compared with the primitive treatments, TSP and PM10 concentrations in VMS workshop are much lower.
0.0014 0.0081
Table 1 shows that values of Site I are higher than those of Site II and background. The ratios of PM10 to TSP mass concentration in the workshop are 64.7% and 62.3%, respectively. It means that particles below 10 μm diameter account for a greater proportion of the particles below 100 μm diameter, which may pose health risks to workers. Metal-rich particles with non-uniform sizes (smaller than 0.1 mm) were obtained during the processes of crushing and corona electrostatic separating. When the metalrich particles containing Pb, Cd, Sn, etc., are fed into the furnace, some dust may be raised and cause the risks of heavy metal exposure. Therefore, a suction hood is suggested to be set above the furnace and the pumps, ensuring effective dust removal. Additionally, wearing dustproof masks and gloves is suggested to prevent the possible ingestion, inhalation and dermal contact of particles or heavy metals. As shown in Table 2, Pb, Cd and Sn concentrations of the three monitoring points are all far below the emission limits, which are 0.006, 0.04 and 0.24 mg m-3 according to the integrated emission standard of air pollutants of China (GB 16297-1996). Figure 2 shows the heavy metals distribution in TSP and PM10 of the two sites. The concentrations of Pb and Sn are much higher than those of Cd for all the samples. The feeding materials treated by the VMS system usually contain a certain quantity of solders, while Cd accounts for a very small proportion of the metal-rich particles from crushed e-wastes. Therefore, the compositions of the treated materials can explain the reason for the distribution of the heavy metals. Pd, Cd and Sn concentrations in TSP and PM10 of Site I are higher than those of Site II and background, which is in accordance with the sequences of TSP and PM10. Because the pressure in the furnace (0.01–10 Pa) is much lower than the atmospheric pressure, the gaseous metals cannot leak out of the furnace. The differences between values of Site II and the background are not obvious. It can be deduced that the exhausting of the mechanical pump will not bring uncondensed gaseous metals.
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Table 4. A-weighted noise level of different monitoring sites.
Figure 2. Heavy metals distribution in TSP and PM10 of Site I and Site II.
the monitoring sites
Noise level dB (A)
Mechanical pump Cooling water pump Background
71.2 69.5 55.4
for the workers during the operating process. Table 3 also shows the LADD and Risk for Cd through inhalation. The LADDinh of Cd is 1.71 × 10-6, and the Risk is 1.08 × 10-5, which is lower than the risk threshold value (10-4). It indicates that the carcinogenic risk on workers is relatively slight in the ambience of the VMS system.
Assessment of noise exposure Table 3. Risk assessments of Pb and Cd in the TSP near the VMS system for each exposure pathway.
Average daily dose Reference dose Hazard quotients Carcinogenic risk
ADDing,mg (kg·d)-1 ADDinh,mg (kg·d)-1 ADDderm,mg (kg·d)-1 RfDing,mg (kg·d)-1 RfDinh,mg (kg·d)-1 RfDderm,mg (kg·d)-1 HQing HQinh HQderm HI LADDinh,mg (kg·d)-1 SFinh, mg/(kg·d)]-1 Risk
Pb
Cd
9.74 × 10-4 1.48 × 10-4 2.24 × 10-6 3.50 × 10-3 3.52 × 10-3 5.25 × 10-4 2.78 × 10-1 4.21 × 10-2 4.27 × 10-3 3.25 × 10-1 – – –
7.90 × 10-5 1.20 × 10-5 1.82 × 10-7 1.00 × 10-3 1.00 × 10-3 1.00 × 10-5 7.90 × 10-2 1.20 × 10-2 1.82 × 10-2 1.09 × 10-1 1.71 × 10-6 6.30 1.08 × 10-5
Noise may result in many problems to the workers, including hearing loss, stress, high blood pressure, sleep loss and so on. The corresponding noise level is listed in Table 4. The values are all smaller than 90 dB (A), which is the permissible limit for the equivalent continuous sound level of the Occupational Safety and Health Standards (warning level) (Schomer, 2001). According to the literature (Xiang et al., 2007), there is no problems of hearing diseases even if workers work for 20 years in the environment of 80 dB (A). The noise mainly comes from the motors used in the mechanical pump and water cooling pump. Though it is below the limit, an acoustic hood is designed around the motors to reduce the noise level to the maximum extent. Meanwhile, it is suggested that the workers should wear earplugs during the operating process.
Assessment of thermal safety Heavy metal risk assessment. Organic tin is the most dangerous forms of tin for humans (Graceli et al., 2013). Tin will be less dangerous to human health, therefore, the health assessment of Sn is not referred in this section. However, Pb and Cd are toxic heavy metals, which can bring huge damage to the human nervous system, kidney, bone and immune system. The risk assessments of Pb and Cd in the TSP in the ambience of the VMS system through each exposure pathway are assessed by the model of the health risk assessment of EPA (US). The assessments results are presented in Table 3. The average daily doses (ADD) for Pb and Cd are both smaller than the corresponding reference doses. The HQ of Pb and Cd by ingestion, inhalation and dermal absorption are all smaller than 1 (safety level). The hazard indexes (HI) for both Pb and Cd are also smaller than 1, which indicates that the concentrations of Pb and Cd in the ambience of the VMS system will not bring non-carcinogenic risks to the workers. As shown in Table 3, the risks of each metal through different exposure pathways are not the same. HQing is obviously higher than HQinh and HQderm for both Pb and Cd. Therefore, ingestion of dust may pose a likely potential risk to the workers. Wearing dustproof masks is strongly suggested
Proceeding at a high temperature, the potential thermal risk may exist around the VMS system. The inadequate containment action may lead to uncomfortableness or scald the workers. The hygiene standard for industry and enterprise design (GBZ12010) requires that the mean temperature of the high temperature facility surface cannot exceed 313 K and the transient temperature cannot exceed 333 K. The temperatures of the furnace, oil diffusion pump and mechanical pump are monitored and the results are summarised in Table 5. The inner temperature of the vacuum furnace is usually between 873–1273 K. In order to achieve excellent heat preservation, the heater in the furnace is surrounded by insulation materials in order to insulate the heat diffusion. In addition, the furnace wall is cooled by movable water as shown in Figure 1, thus the temperature of the furnace surface is decreased to 288 ±5 K. The process of vacuum exhaust by an oil diffusion pump can be divided into three steps, as follows. First, the boiling oil generates a high speed jet and directs the vapour through a jet assembly. Then, the boiling oil condenses when hitting the cool outer cooled shell of the diffusion pump. Third, the residual gas in the furnace flows out through the diffusion pump outlet. The oil
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Table 5. The monitored temperatures of the VMS system surface. VMS system surface
Ex ante temperature (K)
Measures taken
Ex post temperature (K)
Resistance furnace Oil diffusion pump body Electric stove of oil diffusion pump Mechanical pump
1073 ±200 523 ±5 523 ±10
Water cooling Water cooling Surrounding insulation materials Water cooling
288 ±5 288 ±5 303 ±5
333 ±5
diffusion pump is worked at a temperature of about 523 K. The silicone oil is heated by an electric stove located on the bottom of the pump. The outside of the diffusion pump is cooled using a cooling water jacket. The monitored temperature of the oil diffusion pump body is 288 ±5 K, while the bottom electric stove has a high temperature with 523 ±10 K. It exceeds the required value and may pose potential thermal risks. Therefore, heat insulation materials are used to surround the out layer of the electric stove, and water cooling jackets are designed for the oil diffusion pump. During the operating process, the workers should wear the heat resistance working gloves. Thus, the injuries can be avoided even if there is an unintentional contact. The mechanical pump as the forepump of the oil diffusion pump works at room temperature. However, the uninterrupted exhausting makes the temperature of the mechanical oil higher than the room temperature, as shown in Table 5. Thus, the mechanical pump with the water-cooled system is suggested in order to avoid the potential thermal injury.
Conclusions VMS is applied for recycling heavy metals from crushed e-wastes, and it is proceeded at a high temperature. Therefore, the heavy metals exposure, motor noise and thermal safety during the VMS process are evaluated. Owing to the minus pressure, the gaseous metals cannot leak out of the furnace. The mass concentrations of TSP and PM10 in the ambience of the VMS system are all below the limit of daily mean concentration required of GB 3095-2012. The concentrations of metals are not elevated in the exhaust outlet of the VMS system, and the exhausting of the mechanical pump will not bring uncondensed gaseous metals. Health risk assessments of Pb and Cd show that HI of Pb is 3.25 × 10-1 and that of Cd is 1.09 × 10-1, and carcinogenic Risk of Cd through inhalation is 1.08 × 10-5. The values of HI and Risk indicate that there is no non-cancerous effects or carcinogenic risk on workers in the ambience of the VMS system. The motors of the mechanical vacuum pump and the water cooling pump are the noise sources of the VMS system. Both of them have a noise level below 80 dB (A). After applying the water cooling system, the surface temperatures of the resistance furnace, the oil diffusion pump and the mechanical pump are all below 288 K. Surrounded by the heat insulation materials, the
288 ±5
surface temperature of the electric stove on the bottom of the oil diffusion pump is decreased from 523 K to 303 K. The evaluations about the heavy metals exposure, motor noise and thermal safety during the VMS process reveal that VMS is not only technically feasible but also environmentally friendly for recovering heavy metals from e-wastes.
Declaration of conflicting interests The authors declare that there is no conflict of interest.
Funding This project was partly supported by the National Natural Science Foundation of China (51178262, 41130525), Shanghai Natural Science Foundation (12ZR1443300) and the Fundamental Research Funds for the Central Universities.
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research-article2014
WMR0010.1177/0734242X14557378Waste Management & ResearchZhan and Xu
Original Article
Assessment of heavy metals exposure, noise and thermal safety in the ambiance of a vacuum metallurgy separation system for recycling heavy metals from crushed e-wastes
Waste Management & Research 2014, Vol. 32(12) 1247–1253 © The Author(s) 2014 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/0734242X14557378 wmr.sagepub.com
Lu Zhan1 and Zhenming Xu2
Abstract Vacuum metallurgy separation (VMS) is a technically feasible method to recover Pb, Cd and other heavy metals from crushed e-wastes. To further determine the environmental impacts and safety of this method, heavy metals exposure, noise and thermal safety in the ambiance of a vacuum metallurgy separation system are evaluated in this article. The mass concentrations of total suspended particulate (TSP) and PM10 are 0.1503 and 0.0973 mg m-3 near the facilities. The concentrations of Pb, Cd and Sn in TSP samples are 0.0104, 0.1283 and 0.0961 μg m-3, respectively. Health risk assessments show that the hazard index of Pb is 3.25 × 10-1 and that of Cd is 1.09 × 10-1. Carcinogenic risk of Cd through inhalation is 1.08 × 10-5. The values of the hazard index and risk indicate that Pb and Cd will not cause non-cancerous effects or carcinogenic risk on workers. The noise sources are mainly the mechanical vacuum pump and the water cooling pump. Both of them have the noise levels below 80 dB (A). The thermal safety assessment shows that the temperatures of the vacuum metallurgy separation system surface are all below 303 K after adopting the circulated water cooling and heat insulation measures. This study provides the environmental information of the vacuum metallurgy separation system, which is of assistance to promote the industrialisation of vacuum metallurgy separation for recovering heavy metals from e-wastes. Keywords Heavy metals exposure, noise, thermal safety, risk assessment, e-wastes
Introduction With technological innovation and intense marketing, large amounts of electric and electronic equipment wastes (e-wastes) are generated with both toxic and valuable materials in them. It is estimated that 20–50 million tonnes of e-wastes are generated around the world each year. China, the largest dumping ground of e-wastes, accommodates more than70% e-wastes all over the world (Bruke, 2007; Huang et al., 2009). Some villages in China as famous e-wastes destinations are facing serious environmental pollutions as a result of the backyard operations adopted. Open sky incineration and acid leaching have resulted dramatically in aquatic, terrestrial and atmospheric pollutions (Deng et al., 2006; Fu et al., 2008; Leung et al., 2008). The blood lead and cadmium levels of the children living there are far over the normal standards (Zheng et al., 2008). Many scholars have done a lot of research and mainly focus on the extraction of valuable metals like Au, Ag and Cu from various kinds of e-wastes (Hagelüken, 2006; Park and Fray, 2009; Zhang and Zhang, 2013). Although the heavy metal contamination has been reported a lot, the recycling of heavy metals like Pb, Cd and Zn in e-wastes has not attracted enough attention because of their
nominal prices. However, the resource utilisation of heavy metals in e-wastes is just the way to clear up heavy metal contamination completely and thoroughly. Recently, several qualified factories in China adopt advanced recycling technologies to recover metals from televisions, refrigerators, printed circuit boards and other e-wastes (Li and Xu, 2010; Li et al., 2009; Xue et al., 2013). Metal-rich particles containing Pb, Cd, Cu, Sn and other metals are recycled after dismantling, crushing, magnetic separating and corona-electrostatic separating. However, it is still a mixture of
1Shanghai
Key Lab for Urban Ecological Processes and EcoRestoration, School of Ecological and Environmental Science, East China Normal University, Shanghai, People’s Republic of China 2School of Environmental Science and Engineering, Shanghai Jiao Tong University, Shanghai, People’s Republic of China Corresponding author: Lu Zhan, Shanghai Key Lab for Urban Ecological Processes and EcoRestoration, School of Ecological and Environmental Science, East China Normal University, 500 Dong Chuan Road, Shanghai, People’s Republic of China. Emails: [email protected]
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Figure 1. The VMS system for recycling heavy metals from crushed e-wastes.
various metals, and the heavy metals like Pb and Cd are still waiting for a satisfactory recycling method. Vacuum metallurgy separation (VMS) has been applied for separating and recovering metals with high vapour pressures successfully (Chen et al., 2009; Li et al., 2012; Smeets and Fray, 1991). The mixture of metals can be separated from each other as a result of their different vapour pressures, and then the pure metal can be recycled by condensation under different conditions. It is also verified as a technically feasible method to recover Pb and Cd from metal-rich particles of crushed e-wastes (Zhan and Xu, 2008, 2009, 2011). However, the environmental impacts and safety of the VMS system are still not researched clearly. During the VMS process, whether the vapour of the processing objects (Pb, Cd and other heavy metals) may leak into the workshop owing to incomplete condensation or uninterrupted pumping, the data about the potential risk are unavailable. Exposure to heavy metals, especially Pb and Cd adhering on the particles, will probably result in significant health effects to the workers (Bi et al., 2010; Gullett et al., 2007; US EPA, 1992). Thus, the concentrations of heavy metals in air should be of concern. The noise of a vacuum pump motor and water pump may cause hearing damage. The injury caused by exposure to a high noise level has been recognised for many years (Ahmed et al., 2001; Rachiotis et al., 2006). Many anti-noise laws and ordinances have been issued by many countries (Piccolo et al., 2005). Therefore, this article will estimate the noise exposure level in the ambiance of a VMS system. VMS is processed at high temperatures (873–1273 K), and the oil diffusion pump starts working at a temperature of more than 523 K. The surface temperatures
of the furnace and other corresponding facilities may result in potential scald risks. Consequently, the surface temperatures of the whole VMS system are monitored, and the thermal risk is evaluated. In this study, heavy metals (Pb, Cd and Sn) in total suspended particulate (TSP) and PM10 are monitored and evaluated for chronic risk of the recycling workers. Meanwhile, the potential safety problems about the exposure of noise and thermal safety are also studied. This article is expected to provide a complete assessment about the potential risks of the VMS system.
Materials and methods VMS system The VMS system is applied to further separate and recover metal-rich particles from crushed e-wastes that are obtained after crushing and electrostatic separating. Heavy metals (especially Pb and Cd) are evaporated and condensed during the VMS process. It is operated under an environment of high vacuum (0.01–10 Pa) and high temperature (873–1273 K). As shown in Figure 1, the processing conditions are realised by the combination of a resistance furnace, a vacuum pump team and other supporting facilities. The vacuum chamber is evacuated by means of a two-stage pumping system consisting of a mechanical vacuum pump and an oil diffusion pump. The operating procedure is as follows: (1) metal-rich particles are fed into the graphite crucible; (2) after reaching the set vacuum degree, the heating process starts; (3) the vacuum pump team keeps working until all the metals with high vapour pressures are evaporated and condensed.
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Monitoring of heavy metals Sample collection and analysis. A middle volume air sampler (flow rate of 0.120 m3 min-1) was applied to collect TSP and PM10 samples during the VMS process. The measuring procedures were standardised with HJ 618-2011 (determination of atmospheric articles PM10 and PM2.5 in ambient air by gravimetric method) and GB/T 15432-1995 (ambient air – determination of total suspended particulates – gravimetric method). Two sampling points were set. One was located 2 m away from the resistance furnace where workers frequently operated the facilities, 1.5 m above the ground, which is the breathing height (the position of noses). The other sampling point was located in the exhaust outlet of mechanical pump. Meanwhile, the concentrations of TSP and PM10 outside the workshop were detected as the background value. The actual values of TSP, PM10 and the heavy meals adhered on the particles may be affected by the operating conditions, such as heating temperature, residence time, vacuum pressure and so on. Therefore, three samples of the experiments with different operating conditions were collected at each sampling point, and the mean concentrations of TSP and PM10 were calculated. After the necessary sample pretreatment, all samples were digested according to the United States Environmental Protection Agency (US EPA) method (Deng et al., 2006; US EPA, 1999). Because the main separated and recovered metals by VMS were Cd and Pb of solder (Pb–Sn alloy), the concentrations of Pb, Cd and Sn in TSP and PM were monitored and determined by an inductively coupled plasma-atomic emission spectrometry (ICP-AES, IRIS-advantage 1000, THERMO, US). All the relative standard deviation (RSD) is less than 2%. Health risk assessment of heavy metals. Risk assessment models of US EPA are applied to evaluate the health risk of heavy metals in the air of the workshop (US EPA, 1992). The operator is exposed to heavy metals through three pathways: ingestion, inhalation and dermal contact. The average daily dose (mg (kg·d)-1) contacted or absorbed through ingestion (ADDing), inhalation (ADDinh) and dermal contact (ADDderm) can be expressed as follows (US EPA, 1997):
ADDing
IngR × ED (1) =C× BW × AT
ADDinh = C ×
ADDderm = C ×
InhR × ED (2) BW × AT
SA × SL × ABS × ED (3) BW × AT
where C is the metal concentration in the TSP or PM10 sample (mg kg-1). For ingestion, the intake rate (IngR) of dust is 100 mg d-1 for adults. For inhalation, the intake rate (InhR) for male is 15.2 m3 d-1. For dermal contact, the exposed skin area
(SA) is given as 1150 cm2, the skin adherence factor (SL) is 0.2 mg (cm2 d)-1, the dermal absorption factor (ABS) is 0.001 (Baptista and Miguel, 2005), the average body weight (BW) of Chinese people is 60 kg for adults (Chang et al., 2009). The exposure duration (ED) is calculated by working days (250 days per year) timing the service life (10 years). The average time (AT) is 3650 days. Among the three metals studied, Cd is recognised to be carcinogenic. Because the slope factors (SF) for carcinogenic risk through ingestion and dermal contact are not given by the US EPA, this study only considers carcinogenic risk resulting from inhalation. For carcinogens, the lifetime average daily dose (LADD) is calculated as (US EPA, 1997).
LADDinh = C ×
InhR × ED (4) BW × AT
where acronyms represent the same variables as in eq. (1)–(3), except AT = 70 × 365 d. The non-carcinogenic risks of a single heavy metal can be indicated as hazard quotient (HQ), and hazard index (HI) represents the total non-carcinogenic risks of different heavy metals through the above-mentioned three ways. HQ and HI can be expressed as follows (US EPA, 1992):
HQing =
ADDing (5) RfDing
HQinh =
ADDinh (6) RfDinh
HQderm =
ADDderm (7) RfDderm
HI =
∑ HQ (8) i
where the reference dose (RfD, mg (kg·d)-1) the estimate of daily exposure below which adverse non-cancer health effects are unlikely. If HQ 10, it indicates high chronic risk (US EPA, 1997). For carcinogen metals, the cancer risk level can be expressed through LADD multiplying by the slope factor (SF), as shown in equation (9) (Baptista and Miguel, 2005; US EPA, 1997):
Risk = LADD × SF (9)
where Risk indicates the probability of cancer happening. SF ([mg/(kg·d)]-1) is the maximum carcinogenic probability when
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Table 1. TSP and PM10 of three different sites. Monitoring sites
TSP, mg m-3
TSP/TSP* %
PM10, mg m-3
PM10/PM10* %
PM10/TSP, %
Site I: 2 m away from the furnace Site II: exhaust outlet background
0.1503 0.0948 0.0853
50.1 31.6 28.4
0.0973 0.0591 0.0494
64.9 39.4 32.9
64.7 62.3 57.9
TSP* and PM10* are the accepted emission limits according to GB 3095-2012, which are 0.3 mg/m-3 and 0.15 mg m-3, respectively.
Table 2. The concentrations of Pb, Cd and Sn in TSP and PM10 of three different sites. Site I: 2 m away from the furnace
Site II: exhaust outlet
Background
TSP
PM10
TSP
PM10
TSP
PM10
Cd, μg m-3
0.0104
0.0002
0.0031
0.0002
Pb, μg m-3 Sn, μg m-3
0.1283 0.0961
0.0782 0.0205
0.0975 0.0194
Below detection limit 0.0025 0.0139
Below detection limit 0.0004 0.0062
the human body is exposed to a fixed dose of a certain heavy metal. The threshold value of Risk is 10-4 (US EPA, 1997).
Monitoring of noise and temperature An integrated sound level meter (AR824) was employed to monitor noise levels of the VMS system in its running state. The measuring procedures were standardised with GBZ/T 189.8– 2007 (measurement of noise in the workplace). An area 2 m away from the facilities and 1.5 m above the ground was the noise monitoring point. The motors of the cooling water pump and mechanical vacuum pump were the main monitoring objects. At each monitoring point, the noise level was measured three times and the mean value was calculated. The procedures of measuring the temperatures of the VMS system were standardised with GBZ/T 189.7-2007 (measurement of heat stress in the workplace). The errors for measuring the temperature were mainly caused by different operating conditions. For the temperature of each facility, it was measured three times under different heating temperatures and residence time. Then, the mean values were obtained and adopted.
Results and discussion Assessment of heavy metals exposure Distribution characteristics of heavy metals in TSP and PM10 . TSP and PM10 of the two sites as well as the background values are shown in Table 1. According to the standards of indoor air quality standard of China (GB 3095-2012), the limit of the TSP daily mean concentration is 0.3 mg m-3, and that of PM10 is 0.15 mg m--3. Compared with the accepted emission limits, the percentages of the measured values (TSP and PM10) are shown in Table 1. All of the monitoring values are below the limits of the corresponding requirements. Compared with the primitive treatments, TSP and PM10 concentrations in VMS workshop are much lower.
0.0014 0.0081
Table 1 shows that values of Site I are higher than those of Site II and background. The ratios of PM10 to TSP mass concentration in the workshop are 64.7% and 62.3%, respectively. It means that particles below 10 μm diameter account for a greater proportion of the particles below 100 μm diameter, which may pose health risks to workers. Metal-rich particles with non-uniform sizes (smaller than 0.1 mm) were obtained during the processes of crushing and corona electrostatic separating. When the metalrich particles containing Pb, Cd, Sn, etc., are fed into the furnace, some dust may be raised and cause the risks of heavy metal exposure. Therefore, a suction hood is suggested to be set above the furnace and the pumps, ensuring effective dust removal. Additionally, wearing dustproof masks and gloves is suggested to prevent the possible ingestion, inhalation and dermal contact of particles or heavy metals. As shown in Table 2, Pb, Cd and Sn concentrations of the three monitoring points are all far below the emission limits, which are 0.006, 0.04 and 0.24 mg m-3 according to the integrated emission standard of air pollutants of China (GB 16297-1996). Figure 2 shows the heavy metals distribution in TSP and PM10 of the two sites. The concentrations of Pb and Sn are much higher than those of Cd for all the samples. The feeding materials treated by the VMS system usually contain a certain quantity of solders, while Cd accounts for a very small proportion of the metal-rich particles from crushed e-wastes. Therefore, the compositions of the treated materials can explain the reason for the distribution of the heavy metals. Pd, Cd and Sn concentrations in TSP and PM10 of Site I are higher than those of Site II and background, which is in accordance with the sequences of TSP and PM10. Because the pressure in the furnace (0.01–10 Pa) is much lower than the atmospheric pressure, the gaseous metals cannot leak out of the furnace. The differences between values of Site II and the background are not obvious. It can be deduced that the exhausting of the mechanical pump will not bring uncondensed gaseous metals.
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Table 4. A-weighted noise level of different monitoring sites.
Figure 2. Heavy metals distribution in TSP and PM10 of Site I and Site II.
the monitoring sites
Noise level dB (A)
Mechanical pump Cooling water pump Background
71.2 69.5 55.4
for the workers during the operating process. Table 3 also shows the LADD and Risk for Cd through inhalation. The LADDinh of Cd is 1.71 × 10-6, and the Risk is 1.08 × 10-5, which is lower than the risk threshold value (10-4). It indicates that the carcinogenic risk on workers is relatively slight in the ambience of the VMS system.
Assessment of noise exposure Table 3. Risk assessments of Pb and Cd in the TSP near the VMS system for each exposure pathway.
Average daily dose Reference dose Hazard quotients Carcinogenic risk
ADDing,mg (kg·d)-1 ADDinh,mg (kg·d)-1 ADDderm,mg (kg·d)-1 RfDing,mg (kg·d)-1 RfDinh,mg (kg·d)-1 RfDderm,mg (kg·d)-1 HQing HQinh HQderm HI LADDinh,mg (kg·d)-1 SFinh, mg/(kg·d)]-1 Risk
Pb
Cd
9.74 × 10-4 1.48 × 10-4 2.24 × 10-6 3.50 × 10-3 3.52 × 10-3 5.25 × 10-4 2.78 × 10-1 4.21 × 10-2 4.27 × 10-3 3.25 × 10-1 – – –
7.90 × 10-5 1.20 × 10-5 1.82 × 10-7 1.00 × 10-3 1.00 × 10-3 1.00 × 10-5 7.90 × 10-2 1.20 × 10-2 1.82 × 10-2 1.09 × 10-1 1.71 × 10-6 6.30 1.08 × 10-5
Noise may result in many problems to the workers, including hearing loss, stress, high blood pressure, sleep loss and so on. The corresponding noise level is listed in Table 4. The values are all smaller than 90 dB (A), which is the permissible limit for the equivalent continuous sound level of the Occupational Safety and Health Standards (warning level) (Schomer, 2001). According to the literature (Xiang et al., 2007), there is no problems of hearing diseases even if workers work for 20 years in the environment of 80 dB (A). The noise mainly comes from the motors used in the mechanical pump and water cooling pump. Though it is below the limit, an acoustic hood is designed around the motors to reduce the noise level to the maximum extent. Meanwhile, it is suggested that the workers should wear earplugs during the operating process.
Assessment of thermal safety Heavy metal risk assessment. Organic tin is the most dangerous forms of tin for humans (Graceli et al., 2013). Tin will be less dangerous to human health, therefore, the health assessment of Sn is not referred in this section. However, Pb and Cd are toxic heavy metals, which can bring huge damage to the human nervous system, kidney, bone and immune system. The risk assessments of Pb and Cd in the TSP in the ambience of the VMS system through each exposure pathway are assessed by the model of the health risk assessment of EPA (US). The assessments results are presented in Table 3. The average daily doses (ADD) for Pb and Cd are both smaller than the corresponding reference doses. The HQ of Pb and Cd by ingestion, inhalation and dermal absorption are all smaller than 1 (safety level). The hazard indexes (HI) for both Pb and Cd are also smaller than 1, which indicates that the concentrations of Pb and Cd in the ambience of the VMS system will not bring non-carcinogenic risks to the workers. As shown in Table 3, the risks of each metal through different exposure pathways are not the same. HQing is obviously higher than HQinh and HQderm for both Pb and Cd. Therefore, ingestion of dust may pose a likely potential risk to the workers. Wearing dustproof masks is strongly suggested
Proceeding at a high temperature, the potential thermal risk may exist around the VMS system. The inadequate containment action may lead to uncomfortableness or scald the workers. The hygiene standard for industry and enterprise design (GBZ12010) requires that the mean temperature of the high temperature facility surface cannot exceed 313 K and the transient temperature cannot exceed 333 K. The temperatures of the furnace, oil diffusion pump and mechanical pump are monitored and the results are summarised in Table 5. The inner temperature of the vacuum furnace is usually between 873–1273 K. In order to achieve excellent heat preservation, the heater in the furnace is surrounded by insulation materials in order to insulate the heat diffusion. In addition, the furnace wall is cooled by movable water as shown in Figure 1, thus the temperature of the furnace surface is decreased to 288 ±5 K. The process of vacuum exhaust by an oil diffusion pump can be divided into three steps, as follows. First, the boiling oil generates a high speed jet and directs the vapour through a jet assembly. Then, the boiling oil condenses when hitting the cool outer cooled shell of the diffusion pump. Third, the residual gas in the furnace flows out through the diffusion pump outlet. The oil
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Table 5. The monitored temperatures of the VMS system surface. VMS system surface
Ex ante temperature (K)
Measures taken
Ex post temperature (K)
Resistance furnace Oil diffusion pump body Electric stove of oil diffusion pump Mechanical pump
1073 ±200 523 ±5 523 ±10
Water cooling Water cooling Surrounding insulation materials Water cooling
288 ±5 288 ±5 303 ±5
333 ±5
diffusion pump is worked at a temperature of about 523 K. The silicone oil is heated by an electric stove located on the bottom of the pump. The outside of the diffusion pump is cooled using a cooling water jacket. The monitored temperature of the oil diffusion pump body is 288 ±5 K, while the bottom electric stove has a high temperature with 523 ±10 K. It exceeds the required value and may pose potential thermal risks. Therefore, heat insulation materials are used to surround the out layer of the electric stove, and water cooling jackets are designed for the oil diffusion pump. During the operating process, the workers should wear the heat resistance working gloves. Thus, the injuries can be avoided even if there is an unintentional contact. The mechanical pump as the forepump of the oil diffusion pump works at room temperature. However, the uninterrupted exhausting makes the temperature of the mechanical oil higher than the room temperature, as shown in Table 5. Thus, the mechanical pump with the water-cooled system is suggested in order to avoid the potential thermal injury.
Conclusions VMS is applied for recycling heavy metals from crushed e-wastes, and it is proceeded at a high temperature. Therefore, the heavy metals exposure, motor noise and thermal safety during the VMS process are evaluated. Owing to the minus pressure, the gaseous metals cannot leak out of the furnace. The mass concentrations of TSP and PM10 in the ambience of the VMS system are all below the limit of daily mean concentration required of GB 3095-2012. The concentrations of metals are not elevated in the exhaust outlet of the VMS system, and the exhausting of the mechanical pump will not bring uncondensed gaseous metals. Health risk assessments of Pb and Cd show that HI of Pb is 3.25 × 10-1 and that of Cd is 1.09 × 10-1, and carcinogenic Risk of Cd through inhalation is 1.08 × 10-5. The values of HI and Risk indicate that there is no non-cancerous effects or carcinogenic risk on workers in the ambience of the VMS system. The motors of the mechanical vacuum pump and the water cooling pump are the noise sources of the VMS system. Both of them have a noise level below 80 dB (A). After applying the water cooling system, the surface temperatures of the resistance furnace, the oil diffusion pump and the mechanical pump are all below 288 K. Surrounded by the heat insulation materials, the
288 ±5
surface temperature of the electric stove on the bottom of the oil diffusion pump is decreased from 523 K to 303 K. The evaluations about the heavy metals exposure, motor noise and thermal safety during the VMS process reveal that VMS is not only technically feasible but also environmentally friendly for recovering heavy metals from e-wastes.
Declaration of conflicting interests The authors declare that there is no conflict of interest.
Funding This project was partly supported by the National Natural Science Foundation of China (51178262, 41130525), Shanghai Natural Science Foundation (12ZR1443300) and the Fundamental Research Funds for the Central Universities.
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