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Minimum ignition energies of pure magnesium powders due to electrostatic discharges and nitrogen's effect
Journal of Loss Prevention in the Process Industries 41 (2016) 144e146
Contents lists available at ScienceDirect
Journal of Loss Prevention in the Process Industries journal homepage: www.elsevier.com/locate/jlp
Short communication
Minimum ignition energies of pure magnesium powders due to electrostatic discharges and nitrogen's effect Kwangseok Choi a, *, Hitoshi Sakasai b, Koujirou Nishimura c a
Japan National Institute of Occupational Safety and Health, 1-4-6 Umezono, Kiyose, Tokyo 204-0024, Japan Yamaishi Metal Co., LTD., 791 Kimagase, Noda, Chiba 270-0222, Japan c Technology Institution of Industrial Safety, 2-16-26 Hirosedai, Sayama-shi, Saitama 350-1328, Japan b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 August 2015 Received in revised form 10 March 2016 Accepted 11 March 2016 Available online 17 March 2016
This paper experimentally investigated the relation between the minimum ignition energy (MIE) of magnesium powders as well as the effect of inert nitrogen (N2) on the MIE. The modified Hartmann vertical-tube apparatus and four kinds of different-sized pure magnesium powders (median particle size, D50; 28.1 mme89.8 mm) were used in this study. The MIE of the most sensitive magnesium powder was 4 mJ, which was affected by the powder particle size (D50; 28.1 mm). The MIE of magnesium powder increased with an increase in the N2 concentration for the inerting technique. The magnesium dust explosion with an electrostatic discharge of 1000 mJ was suppressed completely at an N2 concentration range of more than 98%. The experimental data presented in this paper will be useful for preventing magnesium dust explosions generated from electrostatic discharges. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Electrostatic discharges Minimum ignition energy Magnesium Nitrogen Dust explosions
1. Introduction Magnesium has been widely used in industrial processes because of its light weight and high mechanical strength. However, magnesium powder is combustible and explosive. In fact, Kuai et al., 2011 reported on a severe magnesium dust explosion that occurred in the industrial process. Magnesium powder is also considered an electrostatic hazardous material. The safe handling of magnesium powder requires exact and reliable data on ignitibility due to electrostatic discharges (ESD). The present study focuses quantitatively on the MIE of several pure magnesium powders in normal air. On the other hand, explosion venting technology is widely used to prevent and mitigate dust explosions. However, it is not effective for fine magnesium dust because of extremely high explosion intensity (Li et al., 2009). Inerting technology was recommended as a means of preventing magnesium dust explosions [R.K. Eckhoff]. This paper also shows clearly and specifically the relationship between the N2 concentration for the inerting technique and the MIE of magnesium powders. It should be noted here that the MIE of polymer and aluminum powders and the inerting effect of N2 were
* Corresponding author. E-mail address: [email protected] (K. Choi). http://dx.doi.org/10.1016/j.jlp.2016.03.008 0950-4230/© 2016 Elsevier Ltd. All rights reserved.
introduced in our previous studies (Choi et al., 2015 (a, b)). The purpose of the present paper is to communicate necessary new information regarding ESD risk assessment that will be important to individuals working in the area of accident prevention and mitigation.
2. Experimental 2.1. MIE apparatus and method The MIE apparatus and methods used for this paper are the same as those used in our previous studies (Choi et al., 2015 (a, b)). The Hartmann vertical-tube (1.2 L) apparatus (MIKE-3) was used for the MIE test of magnesium powder (BSI, 2002). For the safety of the operator, the main range of the magnesium dust concentrations was limited to 0.25 kg/m3 to 1.25 kg/m3. Half of the glass on the front door was changed to metal mesh in order to release high pressure due to magnesium dust explosions. For MIE measurements, the ESD was triggered under a discharge circuit consisting of the following: electrode spacing, 6 mm; electrode material and diameter, 2 mm of tungsten; inductance in the discharge circuit, 1 mH; ignition delay time, 120 ms. In this paper, the energy value W (J) of ESD was measured using the MIKE-3 apparatus software and was checked in every ignition
K. Choi et al. / Journal of Loss Prevention in the Process Industries 41 (2016) 144e146
experiment by the spark monitoring system. Observations were made until the lowest possible energy at which flame propagates through dust clouds was reached under constant testing conditions (10 times). On the other hand, to test the influence of the N2 concentration on the MIE of magnesium powder, the gas mixture (O2/N2) supply part was added to the MIE apparatus, and the standard process was changed. Two holes, 4 mm in diameter, were in the explosion vent panel mounted on the explosion vessel roof. One was used to fill up the explosion vessel with the desired gas mixture (O2/N2); the other was used to measure the oxygen concentration in the explosion vessel using the oxygen analyzer. Thus, we know the N2 concentration in the tested air. The air venting system located above the MIE apparatus was turned off to stabilize the N2 concentration in the explosion vessel during testing. The procedure for a routine test at a certain dust concentration was as follows: a) Set the desired concentration of O2 in a gas-mixing tank by adjusting the pressure gauges for both the normal air and the N2 supply lines. b) Place the sample powder at the bottom inside the explosion vessel to be tested. c) Fix the air tube, which is connected to a gas-mixing tank 5 cm away from the bottom of the explosion vessel, and fix the other air tube, which is connected to an oxygen analyzer 5 cm away from the top of the explosion vessel. d) Fill an explosion vessel with the desired gas mixture (O2/N2). e) Disperse the sample powder by means of the desired gas mixture stored in the reservoir. f) Trigger an electrostatic spark to ignite the dust. g) If ignition is obtained, the test is carried out again after lowering the energy level. If ignition is not obtained, the test is repeated 10 times under the same conditions. All tests were carried out at room temperature, 23 ± 5 C, and a relative humidity of 40 ± 5% RH. 2.2. Sample powders Four kinds of magnesium powders were used in this experiment, abbreviated in this paper as Mg-1, Mg-2, Mg-3, and Mg-4. All of the kinds of magnesium were supplied by the same company (Yamaishi Metal Co., LTD). These samples were pure magnesium (Mg > 99.90%, Al &0.01%, Si & 0.01%, Mn & 0.01%, Fe & 0.01%, Zn & 0.05%, Cu & 0.005%). SEM imaging of Mg-1 (Fig. 1) indicated an irregular morphology. The others had similar shapes but
Fig. 1. SEM image of the magnesium powder (Mg-1) used in this study.
145
different sizes. Before any tests were carried out, the powder sample was dried in a desiccator at 23 C ± 5 C for 24 h. 3. Results and discussion Fig. 2 shows the ignition energy as a function of the dust concentration of different-sized magnesium powders. Each data point in the figure indicates the lowest energy at which ignition occurred with 10 successive attempts to ignite the dust. Mg-3 at the 0.25 kg/ m3 and 0.50 kg/m3 marks did not explode with a maximum energy of 1000 mJ, and Mg-4 did not explode at any of the dust concentration. As a result, the ignition sensibilities of the sample magnesium powders due to ESD were in the following order: Mg-1, Mg-2, Mg-3, and Mg-4. The Mg-1 powder, especially, was so sensitive that an ESD of 10 mJ could easily ignite it at all dust concentrations, except 0.25 kg/m3. The MIE is usually quoted as a range: no ignition (W1) < MIE < ignition (W2). W1 is the lower value that represents the highest energy at which no ignition occurs. W2 is the higher value that represents the lowest energy at which the dust is ignited. The statistical minimum ignition energy (MIEs) value can be estimated by using the probability of ignition as follows (BSI, 2002):
MIEs ¼ 10^ðlog W2 I½W2 $log W2 log W1 =ðNI þ IÞ½W2 þ 1Þ; (1) where I is number of tests with ignition at energy W2, and (NI þ I) is the total number of tests at energy W2. The MIEs of each magnesium powder is shown in Table 1. The particle sizes of the magnesium samples were determined by using a laser diffraction system (dry type, Nikkiso Co., Ltd., LDSA-1400A). The particle size distribution of each magnesium sample is also shown in Table 1. The results shown in Table 1 clearly indicate that particle size influences the ignitability of magnesium due to ESD. Metal particles will melt, evaporate, and burn as discrete entities as described by Nifuku et al. (2007); thus, dust clouds with smaller particles will ignite and explode more easily. The MIEs of Mg-1 powder with D50; 28.1 mm was 4 mJ, which was the most sensitive powder used in this study. This result is in good agreement with the data that the
Fig. 2. Ignition energy as a function of the dust concentration of different-sized magnesium powders (Mg-3 at the 0.25 kg/m3 and 0.50 kg/m3 marks did not explode with a 1000 mJ ESD, and Mg-4 did not explode at any dust concentration).
146
K. Choi et al. / Journal of Loss Prevention in the Process Industries 41 (2016) 144e146
Table 1 MIEs and particle size distribution of magnesium powders used in this study. Magnesium sample
Mg-1 Mg-2 Mg-3 Mg-4
MIEs [mJ]
4 55 71 >1000
Particle size distribution [mm] D10
D50
D90
14.1 32.8 41.1 54.8
28.1 58.5 71.9 89.8
48.8 85.2 102 123
D10, 50 and 90 are defined as the particle size at 10, 50 and 90 cumulative weight percent, respectively.
the industrial process. Finally, the MIEs exceeded 1000 mJ when the N2 concentration was 98%. This result shows that the inerting effects of N2 were quite different from those of the polymer powders and aluminum powder obtained from our previous studies (Choi et al., 2015 (a, b)). When the MIEs of the polymer and aluminum powders exceeded 100 mJ, the N2 concentrations were 83% and 90%, respectively. Magnesium powder is more explosive than polymer and aluminum powders, in terms of the inerting effect of N2. This result may be related to chemical reactions with magnesium and nitrogen (Li et al., 2009) and the high thermal decomposition heat. However, Nifuku et al. found that the minimum oxygen concentration (MOC) was approximately 8% volume (or N2: 92%) for magnesium with nitrogen as the dilutant while Li et al. (2009). found it to be 6.8%. This point merits further discussion. This paper suggests that an N2 concentration of 97% effectively prevents dust explosions that are the result of ESD in the industrial process with magnesium (Mg-1) powder; ideally, when the safety margin is considered, N2 concentration should be increased to more than 98% for the inerting technique. 4. Conclusions The minimum ignition energies of pure magnesium powders due to ESD and N2's effect were investigated in an effort to find information for preventing dust explosions. The following results were obtained:
Fig. 3. MIEs of magnesium (Mg-1) and lycopodium powders as functions of different N2 concentrations in the test air.
MIE of particle sizes 20e37 mm was 5 mJ and in the case of the particle size 22 mm, 4 mJ, reported by Nifuku et al. (2007), and Mittal (2014) respectively. In addition, if the particle size is smaller than Mg-1, the MIE may decrease even further. In fact, when the particle size was 6 mm, its MIE was below 2 mJ as reported by Gang et al. (2008). On the other hand, it is well known that when the oxygen content of the atmosphere is reduced by mixing inert gas with the air, both the ignition sensitivity and the explosion violence of the dust cloud are reduced (Eckhoff, 2004). As the focus of this paper, the effect of the N2 concentration in the tested air on the MIE of Mg1, which was the most sensitive magnesium powder, was quantitatively investigated. The range of the Mg-1 dust concentration was from 0.25 kg/m3 to 1.25 kg/m3. The experimental results are shown in Fig. 3. The MIEs of lycopodium, which is a calibration powder for ignition tests, is also shown in Fig. 3 (Choi et al., 2015 (a)). In the case of lycopodium, the MIEs value, 12 mJ in normal air, generally agrees with the defined value of the BSI standards of 5 mJe15 mJ. Data in the test affected the N2 concentration in the test air in a way generally similar to that described by Eckhoff (2004). Thus, the validity of the modified MIE apparatus used in this experiment was confirmed for determining the MIE of dust and Nitrogen's Effect. In the case of magnesium (Mg-1), the MIEs changed to 17 mJ at the 95% N2 concentration mark, but the electrostatic hazard level remained high. When the N2 concentration was 97%, the MIEs suddenly became 200 mJ, which does not occur easily with ESD in
(1) The MIE of the most sensitive magnesium powder (particle size 28.1 mm) was 4 mJ. The powder particle size had a significant relation to the MIE of magnesium powders. (2) The MIE of magnesium increased with an increase in the N2 concentration in the tested air. Especially, when the N2 concentration was the 97%, the MIE of magnesium (Mg-1) exceeded 200 mJ, which does not occur easily in industrial processes. (3) Magnesium dust explosions were suppressed completely at the N2 concentration range of more than 98%. References British Standards Institution (BSI, 2002. BS EN 13821, Potentially Explosive Atmospheres- Explosion Prevention and Protection- Determination of Minimum Ignition Energy of Dust/air Mixtures. Choi, K., Choi, K., Nishimura, K., 2015a. Experimental study on the influence of the nitrogen concentration in the air on the minimum ignition energies of combustible powders due to electrostatic discharges. J. Loss Prev. Process Ind. 34, 163e166. Choi, K., Sakasai, H., Nishimura, K., 2015b. Experimental study on the ignitability of pure aluminum powders due to electrostatic discharges and nitrogen's effect. J. Loss Prev. Process Ind. 35, 232e235. Eckhoff, Rolf K., 2004. Partial inerting e an additional degree of freedom in dust explosion protection. J. Loss Prev. Process Ind. 17, 187e193. Gang, L., Chunmiao, Y., Peihong, A., Baozhi, C., 2008. Experiment-based fire and explosion risk analysis for powdered magnesium production methods,. J. Loss Prev. Process Ind. 21, 461e465. Kuai, N., Li, J., Chen, Z., Huang, W., Yuan, J., Xu, W., 2011. Experiment-based investigations of magnesium dust explosion characteristics. J. Loss Prev. Process Ind. 24, 302e313. Li, G., Yuan, C.M., Fu, Y., Zhong, Y.p., Chen, B.Z., 2009. Inerting of magnesium dust cloud with Ar, N2 and Co2. J. Hazard. Mater. 170, 180e183. Mittal, M., 2014. Explosion characteristic of micron- and nano-size magnesium powders. J. Loss Prev. Process Ind. 27, 55e64. Nifuku, M., Koyanaka, S., Ohya, H., Barre, C., Hatori, M., Fujiwara, s., Horiguchi, S., Sochet, I., 2007. Ignitability characteristics of aluminium and magnesium dusts that are generated during the shredding of post-consumer wastes. J. Loss Prev. Process Ind. 20, 322e329.
Contents lists available at ScienceDirect
Journal of Loss Prevention in the Process Industries journal homepage: www.elsevier.com/locate/jlp
Short communication
Minimum ignition energies of pure magnesium powders due to electrostatic discharges and nitrogen's effect Kwangseok Choi a, *, Hitoshi Sakasai b, Koujirou Nishimura c a
Japan National Institute of Occupational Safety and Health, 1-4-6 Umezono, Kiyose, Tokyo 204-0024, Japan Yamaishi Metal Co., LTD., 791 Kimagase, Noda, Chiba 270-0222, Japan c Technology Institution of Industrial Safety, 2-16-26 Hirosedai, Sayama-shi, Saitama 350-1328, Japan b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 August 2015 Received in revised form 10 March 2016 Accepted 11 March 2016 Available online 17 March 2016
This paper experimentally investigated the relation between the minimum ignition energy (MIE) of magnesium powders as well as the effect of inert nitrogen (N2) on the MIE. The modified Hartmann vertical-tube apparatus and four kinds of different-sized pure magnesium powders (median particle size, D50; 28.1 mme89.8 mm) were used in this study. The MIE of the most sensitive magnesium powder was 4 mJ, which was affected by the powder particle size (D50; 28.1 mm). The MIE of magnesium powder increased with an increase in the N2 concentration for the inerting technique. The magnesium dust explosion with an electrostatic discharge of 1000 mJ was suppressed completely at an N2 concentration range of more than 98%. The experimental data presented in this paper will be useful for preventing magnesium dust explosions generated from electrostatic discharges. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Electrostatic discharges Minimum ignition energy Magnesium Nitrogen Dust explosions
1. Introduction Magnesium has been widely used in industrial processes because of its light weight and high mechanical strength. However, magnesium powder is combustible and explosive. In fact, Kuai et al., 2011 reported on a severe magnesium dust explosion that occurred in the industrial process. Magnesium powder is also considered an electrostatic hazardous material. The safe handling of magnesium powder requires exact and reliable data on ignitibility due to electrostatic discharges (ESD). The present study focuses quantitatively on the MIE of several pure magnesium powders in normal air. On the other hand, explosion venting technology is widely used to prevent and mitigate dust explosions. However, it is not effective for fine magnesium dust because of extremely high explosion intensity (Li et al., 2009). Inerting technology was recommended as a means of preventing magnesium dust explosions [R.K. Eckhoff]. This paper also shows clearly and specifically the relationship between the N2 concentration for the inerting technique and the MIE of magnesium powders. It should be noted here that the MIE of polymer and aluminum powders and the inerting effect of N2 were
* Corresponding author. E-mail address: [email protected] (K. Choi). http://dx.doi.org/10.1016/j.jlp.2016.03.008 0950-4230/© 2016 Elsevier Ltd. All rights reserved.
introduced in our previous studies (Choi et al., 2015 (a, b)). The purpose of the present paper is to communicate necessary new information regarding ESD risk assessment that will be important to individuals working in the area of accident prevention and mitigation.
2. Experimental 2.1. MIE apparatus and method The MIE apparatus and methods used for this paper are the same as those used in our previous studies (Choi et al., 2015 (a, b)). The Hartmann vertical-tube (1.2 L) apparatus (MIKE-3) was used for the MIE test of magnesium powder (BSI, 2002). For the safety of the operator, the main range of the magnesium dust concentrations was limited to 0.25 kg/m3 to 1.25 kg/m3. Half of the glass on the front door was changed to metal mesh in order to release high pressure due to magnesium dust explosions. For MIE measurements, the ESD was triggered under a discharge circuit consisting of the following: electrode spacing, 6 mm; electrode material and diameter, 2 mm of tungsten; inductance in the discharge circuit, 1 mH; ignition delay time, 120 ms. In this paper, the energy value W (J) of ESD was measured using the MIKE-3 apparatus software and was checked in every ignition
K. Choi et al. / Journal of Loss Prevention in the Process Industries 41 (2016) 144e146
experiment by the spark monitoring system. Observations were made until the lowest possible energy at which flame propagates through dust clouds was reached under constant testing conditions (10 times). On the other hand, to test the influence of the N2 concentration on the MIE of magnesium powder, the gas mixture (O2/N2) supply part was added to the MIE apparatus, and the standard process was changed. Two holes, 4 mm in diameter, were in the explosion vent panel mounted on the explosion vessel roof. One was used to fill up the explosion vessel with the desired gas mixture (O2/N2); the other was used to measure the oxygen concentration in the explosion vessel using the oxygen analyzer. Thus, we know the N2 concentration in the tested air. The air venting system located above the MIE apparatus was turned off to stabilize the N2 concentration in the explosion vessel during testing. The procedure for a routine test at a certain dust concentration was as follows: a) Set the desired concentration of O2 in a gas-mixing tank by adjusting the pressure gauges for both the normal air and the N2 supply lines. b) Place the sample powder at the bottom inside the explosion vessel to be tested. c) Fix the air tube, which is connected to a gas-mixing tank 5 cm away from the bottom of the explosion vessel, and fix the other air tube, which is connected to an oxygen analyzer 5 cm away from the top of the explosion vessel. d) Fill an explosion vessel with the desired gas mixture (O2/N2). e) Disperse the sample powder by means of the desired gas mixture stored in the reservoir. f) Trigger an electrostatic spark to ignite the dust. g) If ignition is obtained, the test is carried out again after lowering the energy level. If ignition is not obtained, the test is repeated 10 times under the same conditions. All tests were carried out at room temperature, 23 ± 5 C, and a relative humidity of 40 ± 5% RH. 2.2. Sample powders Four kinds of magnesium powders were used in this experiment, abbreviated in this paper as Mg-1, Mg-2, Mg-3, and Mg-4. All of the kinds of magnesium were supplied by the same company (Yamaishi Metal Co., LTD). These samples were pure magnesium (Mg > 99.90%, Al &0.01%, Si & 0.01%, Mn & 0.01%, Fe & 0.01%, Zn & 0.05%, Cu & 0.005%). SEM imaging of Mg-1 (Fig. 1) indicated an irregular morphology. The others had similar shapes but
Fig. 1. SEM image of the magnesium powder (Mg-1) used in this study.
145
different sizes. Before any tests were carried out, the powder sample was dried in a desiccator at 23 C ± 5 C for 24 h. 3. Results and discussion Fig. 2 shows the ignition energy as a function of the dust concentration of different-sized magnesium powders. Each data point in the figure indicates the lowest energy at which ignition occurred with 10 successive attempts to ignite the dust. Mg-3 at the 0.25 kg/ m3 and 0.50 kg/m3 marks did not explode with a maximum energy of 1000 mJ, and Mg-4 did not explode at any of the dust concentration. As a result, the ignition sensibilities of the sample magnesium powders due to ESD were in the following order: Mg-1, Mg-2, Mg-3, and Mg-4. The Mg-1 powder, especially, was so sensitive that an ESD of 10 mJ could easily ignite it at all dust concentrations, except 0.25 kg/m3. The MIE is usually quoted as a range: no ignition (W1) < MIE < ignition (W2). W1 is the lower value that represents the highest energy at which no ignition occurs. W2 is the higher value that represents the lowest energy at which the dust is ignited. The statistical minimum ignition energy (MIEs) value can be estimated by using the probability of ignition as follows (BSI, 2002):
MIEs ¼ 10^ðlog W2 I½W2 $log W2 log W1 =ðNI þ IÞ½W2 þ 1Þ; (1) where I is number of tests with ignition at energy W2, and (NI þ I) is the total number of tests at energy W2. The MIEs of each magnesium powder is shown in Table 1. The particle sizes of the magnesium samples were determined by using a laser diffraction system (dry type, Nikkiso Co., Ltd., LDSA-1400A). The particle size distribution of each magnesium sample is also shown in Table 1. The results shown in Table 1 clearly indicate that particle size influences the ignitability of magnesium due to ESD. Metal particles will melt, evaporate, and burn as discrete entities as described by Nifuku et al. (2007); thus, dust clouds with smaller particles will ignite and explode more easily. The MIEs of Mg-1 powder with D50; 28.1 mm was 4 mJ, which was the most sensitive powder used in this study. This result is in good agreement with the data that the
Fig. 2. Ignition energy as a function of the dust concentration of different-sized magnesium powders (Mg-3 at the 0.25 kg/m3 and 0.50 kg/m3 marks did not explode with a 1000 mJ ESD, and Mg-4 did not explode at any dust concentration).
146
K. Choi et al. / Journal of Loss Prevention in the Process Industries 41 (2016) 144e146
Table 1 MIEs and particle size distribution of magnesium powders used in this study. Magnesium sample
Mg-1 Mg-2 Mg-3 Mg-4
MIEs [mJ]
4 55 71 >1000
Particle size distribution [mm] D10
D50
D90
14.1 32.8 41.1 54.8
28.1 58.5 71.9 89.8
48.8 85.2 102 123
D10, 50 and 90 are defined as the particle size at 10, 50 and 90 cumulative weight percent, respectively.
the industrial process. Finally, the MIEs exceeded 1000 mJ when the N2 concentration was 98%. This result shows that the inerting effects of N2 were quite different from those of the polymer powders and aluminum powder obtained from our previous studies (Choi et al., 2015 (a, b)). When the MIEs of the polymer and aluminum powders exceeded 100 mJ, the N2 concentrations were 83% and 90%, respectively. Magnesium powder is more explosive than polymer and aluminum powders, in terms of the inerting effect of N2. This result may be related to chemical reactions with magnesium and nitrogen (Li et al., 2009) and the high thermal decomposition heat. However, Nifuku et al. found that the minimum oxygen concentration (MOC) was approximately 8% volume (or N2: 92%) for magnesium with nitrogen as the dilutant while Li et al. (2009). found it to be 6.8%. This point merits further discussion. This paper suggests that an N2 concentration of 97% effectively prevents dust explosions that are the result of ESD in the industrial process with magnesium (Mg-1) powder; ideally, when the safety margin is considered, N2 concentration should be increased to more than 98% for the inerting technique. 4. Conclusions The minimum ignition energies of pure magnesium powders due to ESD and N2's effect were investigated in an effort to find information for preventing dust explosions. The following results were obtained:
Fig. 3. MIEs of magnesium (Mg-1) and lycopodium powders as functions of different N2 concentrations in the test air.
MIE of particle sizes 20e37 mm was 5 mJ and in the case of the particle size 22 mm, 4 mJ, reported by Nifuku et al. (2007), and Mittal (2014) respectively. In addition, if the particle size is smaller than Mg-1, the MIE may decrease even further. In fact, when the particle size was 6 mm, its MIE was below 2 mJ as reported by Gang et al. (2008). On the other hand, it is well known that when the oxygen content of the atmosphere is reduced by mixing inert gas with the air, both the ignition sensitivity and the explosion violence of the dust cloud are reduced (Eckhoff, 2004). As the focus of this paper, the effect of the N2 concentration in the tested air on the MIE of Mg1, which was the most sensitive magnesium powder, was quantitatively investigated. The range of the Mg-1 dust concentration was from 0.25 kg/m3 to 1.25 kg/m3. The experimental results are shown in Fig. 3. The MIEs of lycopodium, which is a calibration powder for ignition tests, is also shown in Fig. 3 (Choi et al., 2015 (a)). In the case of lycopodium, the MIEs value, 12 mJ in normal air, generally agrees with the defined value of the BSI standards of 5 mJe15 mJ. Data in the test affected the N2 concentration in the test air in a way generally similar to that described by Eckhoff (2004). Thus, the validity of the modified MIE apparatus used in this experiment was confirmed for determining the MIE of dust and Nitrogen's Effect. In the case of magnesium (Mg-1), the MIEs changed to 17 mJ at the 95% N2 concentration mark, but the electrostatic hazard level remained high. When the N2 concentration was 97%, the MIEs suddenly became 200 mJ, which does not occur easily with ESD in
(1) The MIE of the most sensitive magnesium powder (particle size 28.1 mm) was 4 mJ. The powder particle size had a significant relation to the MIE of magnesium powders. (2) The MIE of magnesium increased with an increase in the N2 concentration in the tested air. Especially, when the N2 concentration was the 97%, the MIE of magnesium (Mg-1) exceeded 200 mJ, which does not occur easily in industrial processes. (3) Magnesium dust explosions were suppressed completely at the N2 concentration range of more than 98%. References British Standards Institution (BSI, 2002. BS EN 13821, Potentially Explosive Atmospheres- Explosion Prevention and Protection- Determination of Minimum Ignition Energy of Dust/air Mixtures. Choi, K., Choi, K., Nishimura, K., 2015a. Experimental study on the influence of the nitrogen concentration in the air on the minimum ignition energies of combustible powders due to electrostatic discharges. J. Loss Prev. Process Ind. 34, 163e166. Choi, K., Sakasai, H., Nishimura, K., 2015b. Experimental study on the ignitability of pure aluminum powders due to electrostatic discharges and nitrogen's effect. J. Loss Prev. Process Ind. 35, 232e235. Eckhoff, Rolf K., 2004. Partial inerting e an additional degree of freedom in dust explosion protection. J. Loss Prev. Process Ind. 17, 187e193. Gang, L., Chunmiao, Y., Peihong, A., Baozhi, C., 2008. Experiment-based fire and explosion risk analysis for powdered magnesium production methods,. J. Loss Prev. Process Ind. 21, 461e465. Kuai, N., Li, J., Chen, Z., Huang, W., Yuan, J., Xu, W., 2011. Experiment-based investigations of magnesium dust explosion characteristics. J. Loss Prev. Process Ind. 24, 302e313. Li, G., Yuan, C.M., Fu, Y., Zhong, Y.p., Chen, B.Z., 2009. Inerting of magnesium dust cloud with Ar, N2 and Co2. J. Hazard. Mater. 170, 180e183. Mittal, M., 2014. Explosion characteristic of micron- and nano-size magnesium powders. J. Loss Prev. Process Ind. 27, 55e64. Nifuku, M., Koyanaka, S., Ohya, H., Barre, C., Hatori, M., Fujiwara, s., Horiguchi, S., Sochet, I., 2007. Ignitability characteristics of aluminium and magnesium dusts that are generated during the shredding of post-consumer wastes. J. Loss Prev. Process Ind. 20, 322e329.