Explosion characteristics of micron- and nano-size magnesium powders
Introduction
Magnesium is being used extensively in military as well as aeronautics, chemical and metallurgical industries. Powdered and granulated magnesium have recently undergone rapid development to be used as metal-based fuels in propellants, explosives and pyrotechnics, thermite and incendiary devices. Magnesium powder manufacturing processes include cutting, milling, emulsification, vortex milling, atomization milling, mechanical attrition milling and airflow milling producing a range from micron- to nano-size magnesium powders. There exists high potential fire and explosion risk in magnesium powder manufacturing plants and related facilities using other metalworking operations like casting, grinding, machining and welding. High purity and ultrafine powders are currently attracting more attention than in the past and an increasing range of materials including magnesium are being produced as nano-powders composed of particles in size range from about 1 to 100 nm for use in industrial and research fields. Information on explosion characteristics of magnesium powder is necessary to predict the likelihood and severity of explosions and design explosion prevention and mitigation measures. Literature studies concerning the evaluation of flammability and explosion risks of micron-sized powders do not enable to evaluate the fire and explosion risk probabilities and gravities of nano-powders. It is desirable that the explosion characteristics of nano-powders be determined using the standard apparatus and procedures already employed for assessing dust explosion hazards. Comparison with data for micron-scale powders of the same materials will allow knowledge of particle size effects to be extended into the nano-size range.
There exist some information on explosion characteristics – maximum explosion pressure (Pmax), maximum rate of pressure rise ((dP/dt)max), dust explosibility index (KSt), minimum explosible concentration (MEC), minimum ignition energy (MIE), minimum ignition temperature (MIT), limiting oxygen concentration (LOC) – of micron scale magnesium powders (Benson, 2012, BIA, 1997, Cashdollar and Zlochower, 2007, Eckhoff, 1991, Jacobson et al., 1964, Kuai et al., 2011, Li et al., 2009, Li et al., 2008, NFPA, 2012, Nifuku et al., 2007; Roser, 1998) based on experiments using standard set-ups having same or different volumes and configurations. The literature data are summarized in Table 1. The explosion severity data in the work reported by Jacobson et al. (1964) were measured in 1.2-L Hartmann apparatus while those from others work are using either 20-L or 1 m3 chamber. The values of various ignition and explosion parameters for micron scale magnesium powder reported by different workers are quiet different even for the closer particle size-range. For example values of explosion severity data – Pmax and KSt for 30 μm magnesium are 10.8 bar and 400 bar·m/s (Roser, 1998) and those for 28 μm are 17.5 bar and 508 bar·m/s (BIA, 1997, Eckhoff, 1991). The variation of values of explosion characteristics obtained by the laboratories reflects the difficult nature of the magnesium dust samples testing in respect of uniform experimental conditions.
For nano-range particles, there have been number of dust explosion studies performed since 2002 (Benson, 2012, Boilard et al., 2013, Bouillard et al., 2010, Dufaud et al., 2011, Eckhoff, 2011, Holbrow et al., 2010, Kwok et al., 2002, Pritchard, 2004, Vignes et al., 2009, Worsfold et al., 2012, Wu et al., 2009, Wu et al., 2010). Kwok et al. (2002) studied nano-aluminium explosion characteristics. Using 20-L Sphere, Vignes et al. (2009) investigated explosion data of carbon black, carbon nanotubes and aluminium; Wu et al., 2009, Wu et al., 2010 studied explosibility of aluminium, titanium and iron; and Holbrow et al. (2010) determined explosion characteristics of different nano-materials: iron, copper, silicon carbide, zinc oxide, iron oxide, zirconium oxide, zinc, multiwalled carbon nanotubes, carbon monofibres, aluminium, carbon black, etc. in specially designed 2-L explosion chamber. Boilard et al. (2013) studied explosibility of micron and nano-size titanium and Dufaud et al. (2011) studied aluminium, zinc, carbon nanotubes and carbon black. The knowledge of explosion data of nano-powders is still limited. The recent edition of NFPA 484 (NFPA, 2012) covering the unique hazards associated with metal fine dusts and powders, does not include the risks of metal dust combustibility at nano-scale.
To the author's best knowledge, there exists no data on explosion characteristics of nano-size magnesium. The reliability of standard experimental method designed for micron-size dust explosions has been questioned due to various factors influencing behaviour of nano-size dust cloud during experiments (Boilard et al., 2013, Vignes et al., 2009). In the standard 20-L sphere, powder is dispersed using compressed air. With nano-powders, their large surface to volume ratio means that many of them are spontaneously flammable on contact with air, or surface oxidation alters their properties. Hence equipment/procedure that avoids oxidation until the point of ignition is required.
The present investigation was undertaken to measure the ignition sensitivity and explosion severity parameters of magnesium powder in relation to particle size in both the micron- and nano-size ranges and compare the results to examine whether production, handling and use of magnesium nano-powders can present an enhanced dust explosion risk compared to that of micron-size powders. Standard dust explosion equipments were used for the study. Some modifications were incorporated in experimental procedure to minimize the problems faced by earlier investigators.
Section snippets
Experimental procedures
The explosion characteristics measured for micron- and nano-size ranges of magnesium powders include maximum explosion pressure, Pmax, maximum rate of pressure rise (dP/dt)max, minimum explosible concentration, MEC, minimum ignition energy, MIE, minimum ignition temperature, MIT, limiting oxygen concentration, LOC, and influence of reduced oxygen level on explosion severity data using CSIR-CBRI 20-L Sphere, Hartmann apparatus and Godbert-Greenwald furnace (Mittal, 1996, 2012).
The magnesium
Explosion severity data
Experiments for explosion severity measurement were conducted for dust concentration range 20–2000 g/m3 for nano-size particles (30, 50, 100, 150, 200 and 400 nm) and for 30–2500 g/m3 for micron-size particles (1, 10, 22, 38, 74 and 125 μm). Pressure-time curves were recorded for all the experiments. The experimental curves were analysed and values of Pex, (dP/dt)ex and (dP/dt)ex·V1/3were estimated for each experiment. The values of Pmax and KSt corresponding to optimum dust concentration were
Conclusion
The experimental results presented in this paper indicate that explosion severity of magnesium nano-powders (size < 200 nm) is lower than that of micron-powders, but the likelihood of explosion is quite high due to very low ignition energies of magnesium nano-powders. Relatively low quantities and very low spark energies can cause nano-magnesium to ignite leading to severe explosions. The magnesium dust with nano-size range can self-ignite under the appropriate conditions during laboratory
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