Based on use of IOSH cyclone, MOUDI sampling devices and SMPS, this study attempted to evaluate respirable dust and NP concentrations at three different ENP-related workplaces. Particle size distributions and chemical compositions were also measured. At the bagging areas of the nano-CB manufacturing plant, a diesel forklift was used for bag transportation leading to higher NP mass concentration than the other two workplaces where no diesel forklifts were used. The highest RPM concentration was observed at the nano-SiO2
epoxy molding compound plant due to higher NP emission rate and ineffective ventilation system at this location. The MMADs at these workplaces were supermicron rather than nano-sized, which was also confirmed by chemical analysis of the collected particles and the results of the rotating drum testing. Number concentrations of NPs maintained near background level unless there were material handling activities or if the diesel forklifts were used in the workplaces. The TEM analysis showed that nano-sized particles indeed existed in large number. However both the field exposure data and results of rotation drum test showed that although NP number concentration was much higher than that of supermicron particles, its mass concentration was much lower due to agglomeration of NP into micron-sized particles. Therefore, it is suggested that both number and mass concentrations of nano-sized and respirable particles be considered when assessing the exposure of nanopowder-related workplaces. Number concentration measured by real-time instrument can reflect temporary variation and relate to the working scenario. In most exposure limits for chemicals are based on mass, thus it may support a reference for NP and respirable particle field exposure.
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Table 1 Characteristic of the workplaces Site
code Workplaces Process description
Table 2 Particle concentrations at the workplaces
Workplaces Nano-SiO2 epoxy molding compound plant
(nm) 4611±1671 6146±1700 5232±1767
GSD 2.4±0.3 2.3±0.2 2.7±0.6
NMD
(nm) 85.0±32.8 183.9±65.8* 55.5±7.2
GSD 2.7±0.6 1.6±0.1* 2.8±0.3
GSD: geometric standard deviation; *Another NMD mode at 35.4±4.6 nm (GSD=2.6±0.6) PM0.1: particle mass concentration for diameter less than 100 nm
RPM: respirable particle mass concentration for diameter less than 4 m
Table 3 Chemical composition analysis at different workplaces
Unit: g/m3
PM0.1 SiO2 RPM SiO2
Nano-SiO2
epoxy molding compound
plant 2.8±1.4 N.D. 1963±1051 904±390
PM0.1 OC EC RPM OC EC
Nano-CB manufacturing
plant 10.0±3.4 2.4±1.7 4.3±1.9 159±52 39.1±15.8 45.0±20.0
PM0.1 CaCO3 RPM CaCO3
Nano-CaCO3
manufacturing
plant 1.5±0.6 N.D. 154±74 66.9±44.9
N.D.: not detectable
MDL=2.23 g/m3 for SiO2, 0.18 g/m3 for OC, 0.03 g/m3 for EC, and 0.002 g/m3 for CaCO3
Table 4 Comparison of different dispersion methods and field exposure data
Powder Nano-SiO2 Nano-CB Nano-CaCO3
Apparent density (g/cm3) 0.04 0.31 0.464
Usage amount (mg) 3.15 9.32 11.98
MMAD (m) 1.71 1.33 0.94
SSPD
GSD 2.47 2.54 2.05
Usage amount (g) 1.37 10.85 16.25
MMAD (m) 5.49 7.53 7.02
Rotating drum
GSD 3.62 2.47 2.28
MMAD (m) 4.61 6.15 5.23
Field
GSD 2.4 2.3 2.7
Weighting
Figure 1 Schematic diagram of the three workplaces (a) Mixing area of the nano-SiO2
epoxy molding compound plant; (b) Bagging area of the nano-CB manufacturing plant;
(c) Bagging area of the nano-CaCO3 manufacturing plant
(a)
Figure 2 Experimental setup of dustiness test (a) standard rotating drum tester with a modified sampling train (b) SSPD
10 100 1000 10000 100000 Aerodynamic diameter (nm)
0 1000 2000 3000 4000
dM /d lo gD p ( g/ m
3)
Nano-SiO2 epoxy molding compound plant Nano-CB manufacturing plant
Nano-CaCO3 manufacturing plant
Figure 3 Particle mass distributions measured by MOUDI at different workplaces
11:09 12:40 14:10 15:50 10:10 11:44 13:14 14:44 16:14 11:03 12:33 14:03 15:33
Nano-SiO2 epoxy molding compound plant
10:14 11:45 13:15 14:45 15:35 16:05 9:02 10:32 12:02 13:32 15:02
Time
9:27 10:57 12:27 13:57 15:27 10:43 12:13 13:43 15:13
Time
Figure 4 Number concentrations with sampling time measured by SMPS
Figure 5 Particle number distributions measured by SMPS at different workplaces
(a)
(b)
Figure 6 Particle number distributions measured by SMPS and APS for rotating drum test (a) 2-min average (b) 30-min average
Figure 7 Particle mass distributions measured by MOUDI for rotating drum test
(a)
(b)
Figure 8 Dispersed size distributions of different powders based on unit mass (a) SSPD (b) rotating drum
Figure 9 Dispersed mass distributions of different powders based on unit powder mass (a) SSPD (b) rotating drum
(a) Nano-SiO2 (b) Nano-CB
(c) Nano-CaCO3
Figure 10 TEM pictures (a) Nano-SiO2 (b) Nano-CB (c) Nano-CaCO3
APPENDIX
Spatial RPM concentration distributionsThe RPM concentration spatial distributions at different workplaces are shown in Fig. A1.
At Site A, the RPM concentration was in the range of 1534~2422 g/m3. More closer to the nano-SiO2 loading port, higher RPM concentration was measured. In this study, there was no IOSH cyclone located near the coarse SiO2 loading port. The highest RPM concentration occurred at this location was because more coarse SiO2 was fed into the ball mill. At site B and C, the RPM concentrations ranged from 191~204 g/m3 and 87~310 g/m3, respectively, were much lower than Site A due to more effective ventilation control. Only the left-most bagging machine of Site B and the right-most bagging machine of Site C were used during the sampling period. That’s why the RPM concentrations were the highest near these bagging machines.
Temporal number concentration variation with sampling time
The total number concentrations dispersed by the SSPD and rotating drum tester are presented in Fig. A2 (a) and (b), which show the concentration dispersed by the SSPD was more stable than the rotating drum during the 30-min period due to different mechanisms. In the result of rotating drum, the decaying rate of nano-CaCO3 was much faster than nano-CB and nano-SiO2 due to its higher apparent densities and larger diameter. Especially, the apparent density of nano-SiO2 was much less than others, this could account for the concentration was more stable, even it has a slight increasing trend.
The total number concentrations based on unit mass of powders by the SSPD and rotating drum are shown in Fig. A3(a) and (b), which show the ranking of dispersed number concentration based on unit mass of powders was similar to those without considering powder usage amount.
1500
Figure A 1 Spatial RPM concentration distributions at different workplaces (a) mixing area of the nano-SiO2 epoxy molding compound plant (b) bagging area of the nano-CB manufacturing plant (c) bagging area of the nano-CaCO3 manufacturing plant
(a)
(b)
Figure A 2 Total particle number concentrations versus time (a) SSPD (b) rotating drum
(a)
(b)
Figure A 3 Total particle number concentrations versus sampling time (a) SSPD (b) rotating drum, based on per unit mass of dispersed powders