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Explosive characteristics of nanometer and micrometer aluminum-powder
Mining Science and Technology (China) 21 (2011) 661–666
Contents lists available at SciVerse ScienceDirect
Mining Science and Technology (China) journal homepage: www.elsevier.com/locate/mstc
Explosive characteristics of nanometer and micrometer aluminum-powder Jiang Bingyou a,b,⇑, Lin Baiquan a,b, Shi Shulei c, Zhu Chuanjie a,b, Li Wenxia a,b a
Faculty of Safety Engineering, China University of Mining & Technology, Xuzhou 221116, China State Key Laboratory of Coal Resources and Mine Safety, China University of Mining & Technology, Xuzhou 221008, China c School of Management, Xuzhou Normal University, Xuzhou 221009, China b
a r t i c l e
i n f o
Article history: Received 24 January 2011 Received in revised form 22 February 2011 Accepted 20 March 2011 Available online 2 November 2011 Keywords: Nano-aluminum powder Explosion pressure Rate of pressure rise Peak overpressure
a b s t r a c t The explosive characteristics of aluminum powder have great significance in preventing and controlling aluminum-dust explosion accidents, especially the nano-aluminum powder. The explosion characteristics of 100 nm and 75 lm aluminum powders were investigated by using a 20 L spherical explosion cavity and a horizontal pipe whose cross-section area is 80 mm 80 mm and length is 8 m. The results show that the maximum explosion pressure and its rising rate of 100 nm aluminum powder gradually increase with increasing concentration of aluminum-powder at the beginning. When aluminum-powder concentration is 1 kg/m3, the maximum explosion pressure reaches its maximum, and then gradually decreases. While when the concentration is 1.25 kg/m3, the maximum rate of pressure rise obtains its maximum, and then decreases. After 100 nm aluminum powder is exploded in pipes, the peak overpressure of blast wave first decreases and then increases to the maximum at a distance of 298 cm from the ignition source, and then gradually decreases. The most violent concentration is about 0.4 kg/m3 which is lower than 0.8 kg/m3 of 75 lm aluminum powder, so 100 nm aluminum powders are more easily exploded. The change laws of maximum explosion pressure, maximum rate of pressure rise and blast-wave peak overpressure of 100 nm aluminum powders with concentration are similar to those of 75 lm aluminum powders, but these values are much higher than 75 lm aluminum powders under the same concentration, so the aluminum-powders explosion of 100 nm will produce more harms. In the process of production, storage and transportation of aluminum powder, some relevant preventive measures can be taken to reduce the loss caused by aluminum-dust explosion according to nano-aluminum dust. Ó 2011 Published by Elsevier B.V. on behalf of China University of Mining & Technology.
1. Introduction Aluminum powder is an important industrial raw material and widely used in pigments, paints, fireworks, metallurgy, aircraft and ship manufacturing. With the development of science and technology, new material of nano-aluminum powder emerges as the times require, and more and more attention is being paid by people in the production and life. In the process of production, storage and transportation of aluminum powder, it will generate a lot of dust which will create dust explosion easily, thus causing casualties and property loss. Nano-aluminum powder is a special aluminum-powder whose particle size decreases to nanometer level for common one, and it not only has the characteristics of metal aluminum-powder, but also the characteristics of nano-material. And because of its smaller particle size, more damages will be caused [1]. Therefore, the explosion characteristics of aluminum powder have great significance in preventing and controlling
⇑ Corresponding author. Tel.: +86 15996953831. E-mail address: [email protected] (B. Jiang).
aluminum-dust explosion accident, especially the nano-aluminum powder. Domestic and foreign scholars carried out extensive research on the explosion characteristics of aluminum powder. Zheng and Wang studied the explosion characteristics of aluminum powders whose particle sizes were in the range of 10–40 lm by using 1.3 and 1.2 L Hartmann bomb, and they obtained that the ignition delay time had obvious effect on the maximum explosion pressure and its rising rate of aluminum powder, maximum explosion pressure and its rising rate increased with the decreasing particle size of dust, and there was an optimum dust concentration which makes these parameters maximum [2,3]. Zhang tested the lower explosion limits of aluminum powders whose particle sizes were in the range of 6–17 lm by using Hartmann device, and there was an optimum particle size which makes the lower explosion limits minimum under certain conditions [4]. Dufaud obtained that when particle sizes decreased from 42 to 7 lm, the maximum explosion pressure and its rising rate gradually increased by studying the explosive characteristics of aluminum powder in 20 L spheroid [5]. Ding et al. investigated the influence of dispersioninduced turbulence, aluminum-powder concentrations, particle sizes and oxygen concentrations in gas phase on the explosive
1674-5264/$ - see front matter Ó 2011 Published by Elsevier B.V. on behalf of China University of Mining & Technology. doi:10.1016/j.mstc.2011.10.006
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characteristics of aluminum powder whose particle sizes were in the range of 3–30 lm in 20 L spherical closed vessel [6–9]. Tan et al. studied the explosion characteristics of aluminum powder whose particle sizes were in the range of 6–100 lm in horizontal pipe, and found that the smaller particle sizes of aluminum powder, the higher maximum explosion pressure and its rising rate, and with the ignition delay time increasing, the maximum explosion pressure and its rising rate increased and then decreased [10–14]. The above research results are mainly to the aluminum powders of micrometer level, and the previous studies about nano-aluminum powders were mainly focused on their preparation mechanism and inner properties, however, the researches on their explosion characteristics are less [15–17]. Lin and Li studied the explosion characteristics of several nano-aluminum powders by using 20 L spheroid, but they did not detailedly compare the differences of nano-aluminum powder and micron aluminum-powder, and they did not study the explosion characteristics of nano-aluminum powder in horizontal pipe [18]. This paper does research on the maximum explosion pressure and its rising rate of nano-aluminum powder in 20 L spherical explosion cavity and the blast-wave peak overpressure of nano-aluminum powder in horizontal pipe, and compares with micron aluminum-powder. In order to reveal the explosion characteristics of nanometer and micrometer aluminum-powders in spherical cavity and horizontal pipe, and provide some suggestions for the prevention and treatment of aluminumdust explosion. 2. Experimental Aluminum powders, 100 nm and 75 lm were selected to make experiments, the ambient temperature was 8–20 °C, humidity was about 35%, initial pressure was standard atmospheric pressure and initial oxygen content was 20.6%. Using 20 L spherical cavity and horizontal pipe to make the experiments, the specific experimental systems are shown as follows: 2.1. Spherical explosion system The ontology of 20 L spherical explosion device is a spherical test cavity, as shown in Fig. 1 [19,20]. The cavity is made up by two layers of stainless steel plates, and the sandwich can be cooled by cooling water whose cooling temperature is less than 25 °C. The
spheroid top has the electrodes used for ignition, the bottom has the dust inlet, and the sides have the entrance for compressed air or oxygen and a view port. The instrument has a control unit which can control the internal pressure of spheroid, vacuum degree and the time from blowing dust to ignition. The maximum working pressure is 4 MPa, and the time interval of sampling is less than 0.2 ms. Aluminum powders were ignited by self-made device, and the ignition energy is 10 kJ. The data acquisition system is mainly composed of pressure sensor and acquisition system of PCI21710 and the system can achieve the acquisition and processing functions of pressure signal [21]. The boost-pressure characteristics of dust explosion are the same as gas explosion in closed vessel. There are two parameters to measure the intensity of dust explosion, one is maximum explosion pressure pmax, and the other one is maximum rate of pressure rise (dp/dt)max. The maximum rate of pressure rise is the maximum slope of explosive boost-pressure curve, and it is usually determined by the maximum ratio of pressure increment dp and time increment dt [18]. 2.2. Pipe explosion system Pipe explosion experimental system is shown in Fig. 2, and mainly composed of the explosion pipes, pumping system (vacuum pump), valve system, explosion ignition device, explosion pressure measurement system and dynamic data acquisition system. The explosion pipes are 80 mm 80 mm square tubes welded by 16 Mn steel plates whose thickness is 12 mm. The total length of pipes is 8 m, their withstand pressure are more than 20 MPa, and the terminal is closed. Pipes have some holes used for installation of pressure sensor and ignition device, and have several valves used for pumping vacuum, air inlet and connecting vacuum meter and so on. The output signals of pressure sensors are voltage signals whose units are mV, so the overpressure values whose units are MPa cannot be obtained directly. Through the calibration of various pressure sensors by using piston gauge, the corresponding relations between voltage signals and overpressure values of pressure sensors can be found, and the overpressure values whose units are MPa can be outputted directly by inputting related parameters into the acquisition system. The explosion ignition source is a 10 kJ chemical igniter, and its quality is 2.4 g. The chemical igniter is detonated by using a battery whose voltage is 5 V, and then the aluminum-dust clouds are detonated by this chemical igniter. There is a switch between the battery and the ignition device in order to ensure safety. The dynamic data acquisition system of TST6300 is based on the Ethernet interface and it is a new type of portable instrument by applying the current results of embedded technology and network technology. It has 32 channels which can meet the demand of data acquisition speed of lm-level. Dynamic data acquisition system integrating the signal conditioning, power supply, data acquisition, data storage, data communication, and data analysis and treatment for one body can complete all work of non-electrical quantity measurement except sensors [22]. 3. Results and discussion 3.1. Maximum explosion pressure and its rising rate in spherical cavity
Fig. 1. 20 L spherical explosion system [19]. 1. Sealing cover; 2. outer layer of jacket; 3. inner layer of jacket; 4. vacuum meter; 5. circulating water inlet; 6. mechanical bi-directional valve; 7. base; 8. view port; 9. vacuum pumping port; 10. dispersive nozzle; 11. powder storage tank; 12. electro connecting pressure gauge; 13. pressure sensor; 14. circulating water export; 15. safety limit switch; 16. spark lever; 17. electromagnetic valve.
It is well know, dust concentration has great effects on the maximum explosion pressure and its rising rate in dust explosion of non-nanometer level. Beginning a certain concentration, the maximum explosion pressure initially increases as the dust concentration is increasing, when dust concentration reaches a certain
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Fig. 2. Explosion system of horizontal pipe.
value, the maximum explosion pressure tends to the maximum, and when dust concentration exceeds a certain value, the maximum explosion pressure begins to decrease with the increasing concentration. The variation law of maximum rate of pressure rise with dust concentration is similar to maximum explosion pressure [23]. Fig. 3 show the typical explosion pressure curves of aluminum powder in 20 L spherical cavity. Fig. 4 is the maximum explosion pressure of 100 nm and 75 lm aluminum powders, and the aluminum-powder concentration can be expressed by the ratio of flappy qualities of aluminum powders and spheroid volume in 20 L spherical cavity. From Fig. 4 we can see, the corresponding maximum explosion pressure of 100 nm aluminum powder to the concentration of 0.06 kg/m3 is 0.33 MPa, while the maximum explosion pressure of 75 lm aluminum powders is only 0.18 MPa. When the concentration was increased, the maximum explosion pressure of 100 nm and 75 lm aluminum powders gradually increased. When the concentration was 1 kg/m3, the maximum explosion pressure of 100 nm aluminum powders reached the maximum value of 0.79 MPa, while that of 75 lm aluminum-powders was only 0.61 MPa. Then the maximum explosion pressure of both two types of aluminum powders gradually decreased, and when the concentration was 1.7 kg/m3, the maximum explosion pressure of 100 nm aluminum-powders decreased to 0.61 MPa, at the same time, the maximum explosion pressure of 75 lm aluminum powders was 0.47 MPa. The variation law of maximum explosion pressure of 100 nm aluminum powders with dust concentration is similar to that of 75 lm aluminum powders, but these maximum pressures of 100 nm aluminum powders are higher than 75 lm aluminum powders under the same concentration. Fig. 5 show the maximum rate of pressure rise of 100 nm and 75 lm aluminum powders in 20 L spherical cavity. It can be seen, beginning the concentration of 0.06 kg/m3, the maximum rates of explosion pressure rise increased with the increasing dust concentration, and the increasing extent of the rising rate of 100 nm aluminum powders was much higher than 75 lm aluminum powder.
Fig. 4. Influence of aluminum-powder concentration on maximum explosion pressure.
Fig. 5. Influence of aluminum-powder concentration on maximum rate of explosion pressure rise.
When the concentration was 1 kg/m3, the maximum rate of pressure rise of 75 lm aluminum powders reached the maximum
Fig. 3. Explosion pressure curves of aluminum powder in 20 L cavity.
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value of 9.76 MPa/s, and when the concentration was 1.25 kg/m3, the rising rate of 100 nm aluminum powders obtained the maximum value of 34.16 MPa/s which is much larger than 75 lm aluminum powder. Then the maximum rates of pressure rise decreased with the increasing concentration. The variation law of maximum rate of pressure rise of 100 nm aluminum powders with concentration is similar to maximum explosion pressure, and this variation law is similar to that of 75 lm aluminum powders. Combining Figs. 4 and 5, the maximum explosion pressure and its rising rate of 100 nm aluminum-powders are much higher than 75 lm aluminum-powders under the same concentration, so more harms will be produced by 100 nm aluminum-powders explosion than 75 lm aluminum-powders. 3.2. Peak overpressure of blast wave in straight pipe Fig. 6 is the typical explosion pressure curves measured by pressure sensors. Fig. 7 show the explosion peak overpressure curves of 100 nm and 75 lm aluminum powders in straight pipes, and the concentration of aluminum powder can be expressed by the ratio of flappy qualities of aluminum powders and pipes volume in straight pipes whose length is 8 m and cross-section area pf 80 mm 80 mm. Taking 100 nm aluminum powder whose concentration is 0.4 kg/m3 and 75 lm aluminum powder whose concentration is 0.8 kg/m3 as examples, the explosion peak overpressure of 100 nm aluminum powder at the distance of 44 cm from ignition source is 557.57 kPa after the aluminum powder exploding in pipes, while that of 75 lm aluminum powder is only 61.09 kPa, which is far less than that of 100 nm aluminum powder. Then blast wave moves away from ignition source, and the peak overpressure decreases to the minimum at the distance of 168 cm from ignition source because the energy is not replenished in time. The minimum peak overpressure of 100 nm aluminum powder is 504.85 kPa, while that of 75 lm aluminum powder is 60.736 kPa. Soon when the front aluminum powder is participated in explosion, the peak overpressure increases rapidly, and reaches the maximum at the distance of 298 cm from ignition source. The maximum value of 100 nm aluminum powder is 726.62 kPa, while that of 75 lm aluminum is only 62.308 kPa. The gradually decrease of the peak overpressure was caused by some factors such as aluminum-powders consumption, pipe resistance, and friction loss. Considering the existence of reflection effect of pipe terminal, the peak overpressure near the end will depend on the comprehensive effect of these factors. The peak overpressure of 100 nm aluminum powder is 574.97 kPa at the distance of 766 cm from ignition point, while that of 75 lm aluminum powder is 62.102 kPa, which is far less than that of 100 nm aluminum powder. The variation law of peak overpressures of various concentrations is the same, and the variation law of peak overpressures of 100 nm aluminum powders is similar to that of 75 lm aluminum powders.
From the above we can see, peak overpressures of aluminumpowder explosion of 100 nm and 75 lm obtain their maximums at the distance of 298 cm from the ignition source. Drawing the maximum peak overpressures of this position of 100 nm and 75 lm aluminum powder in Fig. 8, we can study the influence of concentration on maximum peak overpressures of aluminum powder explosion in pipes. From Fig. 8a we know, concentration has great effect on the explosion peak overpressure of 100 nm aluminum-powder. With the increasing concentration, the maximum peak overpressure increases at the beginning, and reaches the maximum value when the concentration is 0.4 kg/m3, then maximum peak overpressure gradually decreases when exceeding 0.4 kg/m3, so the most violent concentration is about 0.4 kg/m3. From Fig. 8b we can see, the variation law of peak overpressure of 75 lm aluminum-powder explosion when concentration is similar to 100 nm, but the most violent concentration of 75 lm aluminum-powders is about 0.8 kg/m3 which is higher than that of 100 nm aluminum-powders, so 100 nm aluminum powder is easier to be exploded than 75 lm aluminum powder. The influence of concentration on explosion peak overpressure of aluminum powder in pipes is the same as in 20 L spheroid. From Figs. 7 and 8 we can see that the explosion peak overpressures of 100 nm aluminum powders are higher than 75 lm aluminum powders under the same position and concentration, and the maximum peak overpressure of 100 nm aluminum powders is 726.62 kPa which is much larger than 62.308 kPa of 75 lm aluminum powders. The peak overpressures of 100 nm aluminum powders are between the ranges of 39–750 kPa, while that of 75 lm aluminum powders are between the ranges of 25–65 kPa, so more harms will be produced by 100 nm aluminum-powders explosion than 75 lm aluminum-powders. 3.3. Reasons analysis In order to compare the difference of surface morphology of nanometer and micrometer aluminum powders, Scanning Electron Microscopy (SEM) was used to scan 100 nm and 75 lm aluminum powder as shown in Fig. 9. From Fig. 9 we can clearly see: the particles of 100 nm aluminum powders are spherical and their shapes are like heaped beans. Among them there are also some larger particles, which may be the oxidation products. The morphologies of 75 lm aluminum powders are completely different from those of 100 nm aluminum powders, and their shapes are squamous. There are some agglomerations formed among aluminum powder, and it is difficult to determine their average particle sizes. The activity of nano-aluminum powder is much higher than the normal one [24]. Firstly, nano-aluminum powders are produced in vacuum or inert gas media, so contaminations from the environment is not easy. Meanwhile, metal aluminum is vaporized into
Fig. 6. Explosion pressure curves determined by sensor in straight pipe.
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Fig. 7. Peak overpressure curves of aluminum powder.
Fig. 8. Influence of aluminum-powder concentration on maximum explosion peak overpressure.
From the above analysis we can see, the activity of 100 nm aluminum powder is higher than 75 lm aluminum powder, and it has a higher affinity for oxygen, so it will be easier to be burned or exploded. The nano-aluminum powders have smaller particle sizes, larger specific surface area of particle, more oxygen adsorptions, and burn more sufficient than the common one, so the harms will be more. 4. Conclusions
Fig. 9. Morphology comparison of 100 nm and 75 lm aluminum-powder (enlargement factor: 200).
metal steam, which also makes the separation of aluminum and impurities in raw materials. Therefore, the prepared nano-aluminum powder has high purity, good crystallization structure, controllable particle size and large specific surface area, while the preparation of common aluminum-powder is simple, and has low cost, low purity, uneven particle distribution and large particle size. Secondly, the affinity of nano-aluminum powder for oxygen is very strong, and it is difficult to prevent the formation of dense oxide on the surface either during production or detection. In addition, the oxidation reaction temperatures of nano-aluminum powders are lower, and the oxidation weight gain of nano-aluminum powder starts at the temperatures of 371 °C, while that of common aluminum powder at the temperatures of 556 °C [25].
(1) Aluminum-powder concentration has great effect on the maximum explosion pressure and its rising rate of 100 nm aluminum-powder. Beginning with the concentration of 0.06 kg/m3, the maximum explosion pressure and its rising rate gradually increase with the increasing concentration. When the concentration is 1 kg/m3, the maximum explosion pressure reaches its maximum, and then gradually decreases. While the concentration is 1.25 kg/m3, the maximum rate of explosion pressure rise obtains its maximum, and then decreases. The variation law of maximum rate of pressure rise with concentration is similar to maximum explosion pressure. (2) After 100 nm aluminum powder explosion in the pipes, the peak overpressure of blast wave decreases to the minimum, and soon increases rapidly until to the maximum at the distance of 298 cm from ignition source, and then the peak overpressure gradually decreases. The peak overpressure near the closed end will depend on the comprehensive effect of aluminum-powders qualities participated in explosion, pipe resistance, friction loss, reflection effect of pipe terminal and so on.
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(3) The change of laws of maximum explosion pressure, maximum rate of pressure rise and blast-wave peak overpressure of 100 nm aluminum powders with concentration are similar to those of 75 lm aluminum powders, and the effect of particle sizes on them is smaller. The conclusions obtained in horizontal pipes are similar to those in 20 L spheroid. (4) The most violent concentration of 100 nm aluminum-powder explosion in pipes is about 0.4 kg/m3, which is lower than 0.8 kg/m3 of 75 lm aluminum powder, and the maximum explosion pressure, maximum rate of pressure rise and blast-wave peak overpressure of 100 nm aluminum powders are much higher than 75 lm aluminum powders under the same concentration. So the 100 nm aluminum powder is more easily exploded, and more harm will be produced than 75 lm aluminum powder. The main reasons are that the particle sizes of nano-aluminum powders are smaller than common ones, the specific surface area of particle is larger, the oxygen adsorptions are many more, and the particle burning is more sufficient.
Acknowledgments This work was partly supported by the Major State Basic Research Development Program of China (No. 2011CB201205) and the Open Fund Program of the State Key Laboratory of Explosion Science and Technology (No. KFJJ10-19M). References [1] Li F. Experimental study of aluminum dust explosion with different diameter. Shanxi Chem Ind 2008;28(6):8–11 (in Chinese). [2] Zheng DG. Experimental studies on the characteristics of dust explosion in closed rooms and relief calculation. Dalian: Dalian University of Technology; 2005 (in Chinese). [3] Wang F. Comparative study on explosive dust and industrial dust explosion characteristics. Taiyuan: North University of China; 2009 (in Chinese). [4] Zhang LF. Determination of minimum explosible concentration of flammable dust. Mech Eng Autom 2009(5):97–8 (in Chinese). [5] Dufaud O, Traoré M, Perrin L, Chazelet S, Thomas D. Experimental investigation and modeling of aluminum dusts explosions in the 20 L sphere. J Loss Prev Process Ind 2010;23(2):226–36. [6] Ding DY, Pu YK, Yuan SX, Tang MJ, Wang BL. Experimental investigation of characteristics of aluminum dust explosions. Explosion Shock Waves 1993;13(1):32–40 (in Chinese).
[7] Pu YK, Yuan SX, Ding DY, Wang BL, Tang MJ. Investigation of explosion characteristics in fine aluminum dust-air mixtures. Explosion Shock Waves 1993;13(3):193–204 (in Chinese). [8] Ding DY, Fan BC, Tang MJ. Numerical analysis of characteristics of aluminum dust explosions in closed spherical chamber. J Comput Phys 1992;9(4):377–81 (in Chinese). [9] Ding DY, Fan BC, Tang MJ, Wang BL. Numerical simulation of aluminum dust explosion in a closed sphere. J East China Inst Technol 1993(1):70–5 (in Chinese). [10] Tan YX, Huo XD, Yu CJ. Explosion pressure test of different-sized aluminum powder in horizontal pipeline equipment. China Saf Sci J 2008;18(12):80–3 (in Chinese). [11] Yu CJ, Tan YX, Lu X, Liu QK. Effect of the ignition delay time on the explosion pressure of aluminum dust. J North Univ China 2009;30(3):257–60 (in Chinese). [12] Liu Q. Research on control system part of horizontal pipeline combustible dust explosion device. Taiyuan: North University of China; 2008 (in Chinese). [13] Zheng CF. Experimental investigation of shock waves induce dust explosion. Taiyuan: North University of China; 2008 (in Chinese). [14] Bo T. Study on explosion index of combustible dust detonated by shock wave. Taiyuan: North University of China; 2009 (in Chinese). [15] Jobin KA, Nilesh JV, Chakravarthy SR, Sarathi R. Understanding the mechanism of nano-aluminum particle formation by wire explosion process using optical emission technique. J Quant Spectrosc Radiat Transfer 2010;111(17– 18):2509–16. [16] Sarathi R, Sindhu TK, Chakravarthy SR. Generation of nano aluminum powder through wire explosion process and its characterization. Mater Charact 2007;58(2):148–55. [17] Sarathi R, Sindhu TK, Chakravarthy SR. Impact of binary gas on nanoaluminum particle formation through wire explosion process. Mater Lett 2007;61(8–9):1823–6. [18] Li WX, Lin BQ, Wei WJ, Lu HL. Experimental study on the explosive characteristics of nano-aluminum powder. J China Univ Min Technol 2010;39(4):475–9 (in Chinese). [19] Li WX. Experimental study on the dynamic characteristics of aluminum explosion in confined space. Xuzhou: China University of Mining & Technology; 2010 (in Chinese). [20] Zhao HY. Principles of gas and dust explosion. Beijing: Beijing Institute of Technology Press; 1995 (in Chinese). [21] Li ZH, Lin BQ, Zhang LJ, Pan SK, He YH. Effects of hydrogen production on gas explosion. J China Univ Min Technol 2008;37(2):147–51 (in Chinese). [22] Jiang BY, Lin BQ, Zhu CJ, Lu ZG, Shen CM. Influence of uniform electric field on flame transmission and blast wave in gas explosion. In: Proceedings of 2010 (Shenyang) international colloquium on safety science and technology. Shenyang: Northeastern University Press; 2010. p. 267–72 (in Chinese). [23] Chen WH, Song SZ, Hu YT, Liu RH, Wang C, Zhang ZC. Studies on the behaviors of dust explosion of aluminum and RDX powders. Energ Mater 2003;11(2):91–3 (in Chinese). [24] Fan JH, Zhang K, Wu JY, Ma Y. Study on the activity and shelf-life prediction of nano-aluminium powder. Energ Mater 2004;12(4):239–42 (in Chinese). [25] Zhang K. Preparation, characterization and applications of functionality inorganics/organics composite particles. Chengdu: Sichuan University; 2005 (in Chinese).
Contents lists available at SciVerse ScienceDirect
Mining Science and Technology (China) journal homepage: www.elsevier.com/locate/mstc
Explosive characteristics of nanometer and micrometer aluminum-powder Jiang Bingyou a,b,⇑, Lin Baiquan a,b, Shi Shulei c, Zhu Chuanjie a,b, Li Wenxia a,b a
Faculty of Safety Engineering, China University of Mining & Technology, Xuzhou 221116, China State Key Laboratory of Coal Resources and Mine Safety, China University of Mining & Technology, Xuzhou 221008, China c School of Management, Xuzhou Normal University, Xuzhou 221009, China b
a r t i c l e
i n f o
Article history: Received 24 January 2011 Received in revised form 22 February 2011 Accepted 20 March 2011 Available online 2 November 2011 Keywords: Nano-aluminum powder Explosion pressure Rate of pressure rise Peak overpressure
a b s t r a c t The explosive characteristics of aluminum powder have great significance in preventing and controlling aluminum-dust explosion accidents, especially the nano-aluminum powder. The explosion characteristics of 100 nm and 75 lm aluminum powders were investigated by using a 20 L spherical explosion cavity and a horizontal pipe whose cross-section area is 80 mm 80 mm and length is 8 m. The results show that the maximum explosion pressure and its rising rate of 100 nm aluminum powder gradually increase with increasing concentration of aluminum-powder at the beginning. When aluminum-powder concentration is 1 kg/m3, the maximum explosion pressure reaches its maximum, and then gradually decreases. While when the concentration is 1.25 kg/m3, the maximum rate of pressure rise obtains its maximum, and then decreases. After 100 nm aluminum powder is exploded in pipes, the peak overpressure of blast wave first decreases and then increases to the maximum at a distance of 298 cm from the ignition source, and then gradually decreases. The most violent concentration is about 0.4 kg/m3 which is lower than 0.8 kg/m3 of 75 lm aluminum powder, so 100 nm aluminum powders are more easily exploded. The change laws of maximum explosion pressure, maximum rate of pressure rise and blast-wave peak overpressure of 100 nm aluminum powders with concentration are similar to those of 75 lm aluminum powders, but these values are much higher than 75 lm aluminum powders under the same concentration, so the aluminum-powders explosion of 100 nm will produce more harms. In the process of production, storage and transportation of aluminum powder, some relevant preventive measures can be taken to reduce the loss caused by aluminum-dust explosion according to nano-aluminum dust. Ó 2011 Published by Elsevier B.V. on behalf of China University of Mining & Technology.
1. Introduction Aluminum powder is an important industrial raw material and widely used in pigments, paints, fireworks, metallurgy, aircraft and ship manufacturing. With the development of science and technology, new material of nano-aluminum powder emerges as the times require, and more and more attention is being paid by people in the production and life. In the process of production, storage and transportation of aluminum powder, it will generate a lot of dust which will create dust explosion easily, thus causing casualties and property loss. Nano-aluminum powder is a special aluminum-powder whose particle size decreases to nanometer level for common one, and it not only has the characteristics of metal aluminum-powder, but also the characteristics of nano-material. And because of its smaller particle size, more damages will be caused [1]. Therefore, the explosion characteristics of aluminum powder have great significance in preventing and controlling
⇑ Corresponding author. Tel.: +86 15996953831. E-mail address: [email protected] (B. Jiang).
aluminum-dust explosion accident, especially the nano-aluminum powder. Domestic and foreign scholars carried out extensive research on the explosion characteristics of aluminum powder. Zheng and Wang studied the explosion characteristics of aluminum powders whose particle sizes were in the range of 10–40 lm by using 1.3 and 1.2 L Hartmann bomb, and they obtained that the ignition delay time had obvious effect on the maximum explosion pressure and its rising rate of aluminum powder, maximum explosion pressure and its rising rate increased with the decreasing particle size of dust, and there was an optimum dust concentration which makes these parameters maximum [2,3]. Zhang tested the lower explosion limits of aluminum powders whose particle sizes were in the range of 6–17 lm by using Hartmann device, and there was an optimum particle size which makes the lower explosion limits minimum under certain conditions [4]. Dufaud obtained that when particle sizes decreased from 42 to 7 lm, the maximum explosion pressure and its rising rate gradually increased by studying the explosive characteristics of aluminum powder in 20 L spheroid [5]. Ding et al. investigated the influence of dispersioninduced turbulence, aluminum-powder concentrations, particle sizes and oxygen concentrations in gas phase on the explosive
1674-5264/$ - see front matter Ó 2011 Published by Elsevier B.V. on behalf of China University of Mining & Technology. doi:10.1016/j.mstc.2011.10.006
662
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characteristics of aluminum powder whose particle sizes were in the range of 3–30 lm in 20 L spherical closed vessel [6–9]. Tan et al. studied the explosion characteristics of aluminum powder whose particle sizes were in the range of 6–100 lm in horizontal pipe, and found that the smaller particle sizes of aluminum powder, the higher maximum explosion pressure and its rising rate, and with the ignition delay time increasing, the maximum explosion pressure and its rising rate increased and then decreased [10–14]. The above research results are mainly to the aluminum powders of micrometer level, and the previous studies about nano-aluminum powders were mainly focused on their preparation mechanism and inner properties, however, the researches on their explosion characteristics are less [15–17]. Lin and Li studied the explosion characteristics of several nano-aluminum powders by using 20 L spheroid, but they did not detailedly compare the differences of nano-aluminum powder and micron aluminum-powder, and they did not study the explosion characteristics of nano-aluminum powder in horizontal pipe [18]. This paper does research on the maximum explosion pressure and its rising rate of nano-aluminum powder in 20 L spherical explosion cavity and the blast-wave peak overpressure of nano-aluminum powder in horizontal pipe, and compares with micron aluminum-powder. In order to reveal the explosion characteristics of nanometer and micrometer aluminum-powders in spherical cavity and horizontal pipe, and provide some suggestions for the prevention and treatment of aluminumdust explosion. 2. Experimental Aluminum powders, 100 nm and 75 lm were selected to make experiments, the ambient temperature was 8–20 °C, humidity was about 35%, initial pressure was standard atmospheric pressure and initial oxygen content was 20.6%. Using 20 L spherical cavity and horizontal pipe to make the experiments, the specific experimental systems are shown as follows: 2.1. Spherical explosion system The ontology of 20 L spherical explosion device is a spherical test cavity, as shown in Fig. 1 [19,20]. The cavity is made up by two layers of stainless steel plates, and the sandwich can be cooled by cooling water whose cooling temperature is less than 25 °C. The
spheroid top has the electrodes used for ignition, the bottom has the dust inlet, and the sides have the entrance for compressed air or oxygen and a view port. The instrument has a control unit which can control the internal pressure of spheroid, vacuum degree and the time from blowing dust to ignition. The maximum working pressure is 4 MPa, and the time interval of sampling is less than 0.2 ms. Aluminum powders were ignited by self-made device, and the ignition energy is 10 kJ. The data acquisition system is mainly composed of pressure sensor and acquisition system of PCI21710 and the system can achieve the acquisition and processing functions of pressure signal [21]. The boost-pressure characteristics of dust explosion are the same as gas explosion in closed vessel. There are two parameters to measure the intensity of dust explosion, one is maximum explosion pressure pmax, and the other one is maximum rate of pressure rise (dp/dt)max. The maximum rate of pressure rise is the maximum slope of explosive boost-pressure curve, and it is usually determined by the maximum ratio of pressure increment dp and time increment dt [18]. 2.2. Pipe explosion system Pipe explosion experimental system is shown in Fig. 2, and mainly composed of the explosion pipes, pumping system (vacuum pump), valve system, explosion ignition device, explosion pressure measurement system and dynamic data acquisition system. The explosion pipes are 80 mm 80 mm square tubes welded by 16 Mn steel plates whose thickness is 12 mm. The total length of pipes is 8 m, their withstand pressure are more than 20 MPa, and the terminal is closed. Pipes have some holes used for installation of pressure sensor and ignition device, and have several valves used for pumping vacuum, air inlet and connecting vacuum meter and so on. The output signals of pressure sensors are voltage signals whose units are mV, so the overpressure values whose units are MPa cannot be obtained directly. Through the calibration of various pressure sensors by using piston gauge, the corresponding relations between voltage signals and overpressure values of pressure sensors can be found, and the overpressure values whose units are MPa can be outputted directly by inputting related parameters into the acquisition system. The explosion ignition source is a 10 kJ chemical igniter, and its quality is 2.4 g. The chemical igniter is detonated by using a battery whose voltage is 5 V, and then the aluminum-dust clouds are detonated by this chemical igniter. There is a switch between the battery and the ignition device in order to ensure safety. The dynamic data acquisition system of TST6300 is based on the Ethernet interface and it is a new type of portable instrument by applying the current results of embedded technology and network technology. It has 32 channels which can meet the demand of data acquisition speed of lm-level. Dynamic data acquisition system integrating the signal conditioning, power supply, data acquisition, data storage, data communication, and data analysis and treatment for one body can complete all work of non-electrical quantity measurement except sensors [22]. 3. Results and discussion 3.1. Maximum explosion pressure and its rising rate in spherical cavity
Fig. 1. 20 L spherical explosion system [19]. 1. Sealing cover; 2. outer layer of jacket; 3. inner layer of jacket; 4. vacuum meter; 5. circulating water inlet; 6. mechanical bi-directional valve; 7. base; 8. view port; 9. vacuum pumping port; 10. dispersive nozzle; 11. powder storage tank; 12. electro connecting pressure gauge; 13. pressure sensor; 14. circulating water export; 15. safety limit switch; 16. spark lever; 17. electromagnetic valve.
It is well know, dust concentration has great effects on the maximum explosion pressure and its rising rate in dust explosion of non-nanometer level. Beginning a certain concentration, the maximum explosion pressure initially increases as the dust concentration is increasing, when dust concentration reaches a certain
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Fig. 2. Explosion system of horizontal pipe.
value, the maximum explosion pressure tends to the maximum, and when dust concentration exceeds a certain value, the maximum explosion pressure begins to decrease with the increasing concentration. The variation law of maximum rate of pressure rise with dust concentration is similar to maximum explosion pressure [23]. Fig. 3 show the typical explosion pressure curves of aluminum powder in 20 L spherical cavity. Fig. 4 is the maximum explosion pressure of 100 nm and 75 lm aluminum powders, and the aluminum-powder concentration can be expressed by the ratio of flappy qualities of aluminum powders and spheroid volume in 20 L spherical cavity. From Fig. 4 we can see, the corresponding maximum explosion pressure of 100 nm aluminum powder to the concentration of 0.06 kg/m3 is 0.33 MPa, while the maximum explosion pressure of 75 lm aluminum powders is only 0.18 MPa. When the concentration was increased, the maximum explosion pressure of 100 nm and 75 lm aluminum powders gradually increased. When the concentration was 1 kg/m3, the maximum explosion pressure of 100 nm aluminum powders reached the maximum value of 0.79 MPa, while that of 75 lm aluminum-powders was only 0.61 MPa. Then the maximum explosion pressure of both two types of aluminum powders gradually decreased, and when the concentration was 1.7 kg/m3, the maximum explosion pressure of 100 nm aluminum-powders decreased to 0.61 MPa, at the same time, the maximum explosion pressure of 75 lm aluminum powders was 0.47 MPa. The variation law of maximum explosion pressure of 100 nm aluminum powders with dust concentration is similar to that of 75 lm aluminum powders, but these maximum pressures of 100 nm aluminum powders are higher than 75 lm aluminum powders under the same concentration. Fig. 5 show the maximum rate of pressure rise of 100 nm and 75 lm aluminum powders in 20 L spherical cavity. It can be seen, beginning the concentration of 0.06 kg/m3, the maximum rates of explosion pressure rise increased with the increasing dust concentration, and the increasing extent of the rising rate of 100 nm aluminum powders was much higher than 75 lm aluminum powder.
Fig. 4. Influence of aluminum-powder concentration on maximum explosion pressure.
Fig. 5. Influence of aluminum-powder concentration on maximum rate of explosion pressure rise.
When the concentration was 1 kg/m3, the maximum rate of pressure rise of 75 lm aluminum powders reached the maximum
Fig. 3. Explosion pressure curves of aluminum powder in 20 L cavity.
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value of 9.76 MPa/s, and when the concentration was 1.25 kg/m3, the rising rate of 100 nm aluminum powders obtained the maximum value of 34.16 MPa/s which is much larger than 75 lm aluminum powder. Then the maximum rates of pressure rise decreased with the increasing concentration. The variation law of maximum rate of pressure rise of 100 nm aluminum powders with concentration is similar to maximum explosion pressure, and this variation law is similar to that of 75 lm aluminum powders. Combining Figs. 4 and 5, the maximum explosion pressure and its rising rate of 100 nm aluminum-powders are much higher than 75 lm aluminum-powders under the same concentration, so more harms will be produced by 100 nm aluminum-powders explosion than 75 lm aluminum-powders. 3.2. Peak overpressure of blast wave in straight pipe Fig. 6 is the typical explosion pressure curves measured by pressure sensors. Fig. 7 show the explosion peak overpressure curves of 100 nm and 75 lm aluminum powders in straight pipes, and the concentration of aluminum powder can be expressed by the ratio of flappy qualities of aluminum powders and pipes volume in straight pipes whose length is 8 m and cross-section area pf 80 mm 80 mm. Taking 100 nm aluminum powder whose concentration is 0.4 kg/m3 and 75 lm aluminum powder whose concentration is 0.8 kg/m3 as examples, the explosion peak overpressure of 100 nm aluminum powder at the distance of 44 cm from ignition source is 557.57 kPa after the aluminum powder exploding in pipes, while that of 75 lm aluminum powder is only 61.09 kPa, which is far less than that of 100 nm aluminum powder. Then blast wave moves away from ignition source, and the peak overpressure decreases to the minimum at the distance of 168 cm from ignition source because the energy is not replenished in time. The minimum peak overpressure of 100 nm aluminum powder is 504.85 kPa, while that of 75 lm aluminum powder is 60.736 kPa. Soon when the front aluminum powder is participated in explosion, the peak overpressure increases rapidly, and reaches the maximum at the distance of 298 cm from ignition source. The maximum value of 100 nm aluminum powder is 726.62 kPa, while that of 75 lm aluminum is only 62.308 kPa. The gradually decrease of the peak overpressure was caused by some factors such as aluminum-powders consumption, pipe resistance, and friction loss. Considering the existence of reflection effect of pipe terminal, the peak overpressure near the end will depend on the comprehensive effect of these factors. The peak overpressure of 100 nm aluminum powder is 574.97 kPa at the distance of 766 cm from ignition point, while that of 75 lm aluminum powder is 62.102 kPa, which is far less than that of 100 nm aluminum powder. The variation law of peak overpressures of various concentrations is the same, and the variation law of peak overpressures of 100 nm aluminum powders is similar to that of 75 lm aluminum powders.
From the above we can see, peak overpressures of aluminumpowder explosion of 100 nm and 75 lm obtain their maximums at the distance of 298 cm from the ignition source. Drawing the maximum peak overpressures of this position of 100 nm and 75 lm aluminum powder in Fig. 8, we can study the influence of concentration on maximum peak overpressures of aluminum powder explosion in pipes. From Fig. 8a we know, concentration has great effect on the explosion peak overpressure of 100 nm aluminum-powder. With the increasing concentration, the maximum peak overpressure increases at the beginning, and reaches the maximum value when the concentration is 0.4 kg/m3, then maximum peak overpressure gradually decreases when exceeding 0.4 kg/m3, so the most violent concentration is about 0.4 kg/m3. From Fig. 8b we can see, the variation law of peak overpressure of 75 lm aluminum-powder explosion when concentration is similar to 100 nm, but the most violent concentration of 75 lm aluminum-powders is about 0.8 kg/m3 which is higher than that of 100 nm aluminum-powders, so 100 nm aluminum powder is easier to be exploded than 75 lm aluminum powder. The influence of concentration on explosion peak overpressure of aluminum powder in pipes is the same as in 20 L spheroid. From Figs. 7 and 8 we can see that the explosion peak overpressures of 100 nm aluminum powders are higher than 75 lm aluminum powders under the same position and concentration, and the maximum peak overpressure of 100 nm aluminum powders is 726.62 kPa which is much larger than 62.308 kPa of 75 lm aluminum powders. The peak overpressures of 100 nm aluminum powders are between the ranges of 39–750 kPa, while that of 75 lm aluminum powders are between the ranges of 25–65 kPa, so more harms will be produced by 100 nm aluminum-powders explosion than 75 lm aluminum-powders. 3.3. Reasons analysis In order to compare the difference of surface morphology of nanometer and micrometer aluminum powders, Scanning Electron Microscopy (SEM) was used to scan 100 nm and 75 lm aluminum powder as shown in Fig. 9. From Fig. 9 we can clearly see: the particles of 100 nm aluminum powders are spherical and their shapes are like heaped beans. Among them there are also some larger particles, which may be the oxidation products. The morphologies of 75 lm aluminum powders are completely different from those of 100 nm aluminum powders, and their shapes are squamous. There are some agglomerations formed among aluminum powder, and it is difficult to determine their average particle sizes. The activity of nano-aluminum powder is much higher than the normal one [24]. Firstly, nano-aluminum powders are produced in vacuum or inert gas media, so contaminations from the environment is not easy. Meanwhile, metal aluminum is vaporized into
Fig. 6. Explosion pressure curves determined by sensor in straight pipe.
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Fig. 7. Peak overpressure curves of aluminum powder.
Fig. 8. Influence of aluminum-powder concentration on maximum explosion peak overpressure.
From the above analysis we can see, the activity of 100 nm aluminum powder is higher than 75 lm aluminum powder, and it has a higher affinity for oxygen, so it will be easier to be burned or exploded. The nano-aluminum powders have smaller particle sizes, larger specific surface area of particle, more oxygen adsorptions, and burn more sufficient than the common one, so the harms will be more. 4. Conclusions
Fig. 9. Morphology comparison of 100 nm and 75 lm aluminum-powder (enlargement factor: 200).
metal steam, which also makes the separation of aluminum and impurities in raw materials. Therefore, the prepared nano-aluminum powder has high purity, good crystallization structure, controllable particle size and large specific surface area, while the preparation of common aluminum-powder is simple, and has low cost, low purity, uneven particle distribution and large particle size. Secondly, the affinity of nano-aluminum powder for oxygen is very strong, and it is difficult to prevent the formation of dense oxide on the surface either during production or detection. In addition, the oxidation reaction temperatures of nano-aluminum powders are lower, and the oxidation weight gain of nano-aluminum powder starts at the temperatures of 371 °C, while that of common aluminum powder at the temperatures of 556 °C [25].
(1) Aluminum-powder concentration has great effect on the maximum explosion pressure and its rising rate of 100 nm aluminum-powder. Beginning with the concentration of 0.06 kg/m3, the maximum explosion pressure and its rising rate gradually increase with the increasing concentration. When the concentration is 1 kg/m3, the maximum explosion pressure reaches its maximum, and then gradually decreases. While the concentration is 1.25 kg/m3, the maximum rate of explosion pressure rise obtains its maximum, and then decreases. The variation law of maximum rate of pressure rise with concentration is similar to maximum explosion pressure. (2) After 100 nm aluminum powder explosion in the pipes, the peak overpressure of blast wave decreases to the minimum, and soon increases rapidly until to the maximum at the distance of 298 cm from ignition source, and then the peak overpressure gradually decreases. The peak overpressure near the closed end will depend on the comprehensive effect of aluminum-powders qualities participated in explosion, pipe resistance, friction loss, reflection effect of pipe terminal and so on.
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(3) The change of laws of maximum explosion pressure, maximum rate of pressure rise and blast-wave peak overpressure of 100 nm aluminum powders with concentration are similar to those of 75 lm aluminum powders, and the effect of particle sizes on them is smaller. The conclusions obtained in horizontal pipes are similar to those in 20 L spheroid. (4) The most violent concentration of 100 nm aluminum-powder explosion in pipes is about 0.4 kg/m3, which is lower than 0.8 kg/m3 of 75 lm aluminum powder, and the maximum explosion pressure, maximum rate of pressure rise and blast-wave peak overpressure of 100 nm aluminum powders are much higher than 75 lm aluminum powders under the same concentration. So the 100 nm aluminum powder is more easily exploded, and more harm will be produced than 75 lm aluminum powder. The main reasons are that the particle sizes of nano-aluminum powders are smaller than common ones, the specific surface area of particle is larger, the oxygen adsorptions are many more, and the particle burning is more sufficient.
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