The effect of particle size polydispersity on the explosibility characteristics of aluminum dust

An experimental investigation was carried out on the influences of dust concentration, particle size distribution and humidity on aluminum dust explosion. Tests were mainly conducted thanks to a 20 L explosion sphere. The effect of humidity was studied by storing the aluminum particles at constant relative humidity until the sorption equilibrium or by introducing water vapour in the explosion vessel. The tested particles sizes ranged from a volume median diameter of 7 to 42 μm and the dust concentrations were up to 3000 g m−3. Among other results, the strong influence of the particle size was pointed out, especially when the Sauter mean diameter is considered. These results stressed the predominance of the specific surface area on the mass median particle diameter. The effect of water on aluminum dust explosion was decoupled: on the one hand, when water adsorption occurs, hydrogen generation leads to an increase of the explosion severity; on the other hand, when the explosion of dried aluminum powder occurs in a humid atmosphere, the inhibiting effect of humidity is put forward. A model based on mass and heat balances, assuming a shrinking core model with chemical reaction limitation, leads to a satisfactory representation of the pressure evolution during the dust explosion.

Experimental investigation and modelling of aluminum dusts explosions in the 20 L sphere

An insight into ignition factors and mechanisms of magnesium based materials: A review

Explosion characteristics of micron- and nano-size magnesium powders were determined using CSIR-CBRI 20-L Sphere, Hartmann apparatus and Godbert-Greenwald furnace to study influence of particle size reduction to nano-range on these. The explosion parameters investigated are: 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) and effect of reduced oxygen level on explosion severity. Magnesium particle sizes are: 125, 74, 38, 22, 10 and 1 μm; and 400, 200, 150, 100, 50 and 30 nm. Experimental results indicate significant increase in explosion severity (Pmax: 7–14 bar, KSt: 98–510 bar·m/s) as particle size decreases from 125 to 1 μm, it is maximum for 400 nm (Pmax: 14.6 bar, KSt: 528 bar·m/s) and decreases with further decrease of particle size to nano-range 200–30 nm (Pmax: 12.4–9.4 bar, KSt: 460–262 bar·m/s) as it is affected by agglomeration of nano-particles. MEC decreases from 160 to 30 g/m3 on decreasing particle size from 125 to 1 μm, its value is 30 g/m3 for 400 and 200 nm and 20 g/m3 for further decrease in nano-range (150–30 nm). MIE reduces from 120 to 2 mJ on decreasing the particle size from 125 to 1 μm, its value is 1 mJ for 400, 200, 150 nm size and <1 mJ for 50 and 30 nm. Minimum ignition temperature is 600 °C for 125 μm magnesium, it varies between 570 and 450 °C for sizes 38–1 μm and 400–350 °C for size range 400–30 nm. Magnesium powders in nano-range (30–200 nm) explode less violently than micron-range powder. However, likelihood of explosion increases significantly for nano-range magnesium. LOC is 5% for magnesium size range 125–38 μm, 4% for 22–1 μm, 3% for 400 nm, 4% for 200, 150 and 100 nm, and 5% for 50 and 30 nm. Reduction in oxygen levels to 9% results in decrease in Pmax and KSt by a factor of 2–3 and 4–5, respectively, for micron as well as nano-sizes. The experimental data presented will be useful for industries producing or handling similar size range micron- and nano-magnesium in order to evaluate explosibility of their magnesium powders and propose/design adequate safety measures.

Explosion characteristics of micron- and nano-size magnesium powders

In 2005, an investigation conducted on three nanoscale attrition millers in Taiwan revealed that all three had undergone metal nanopowder explosions in the past. This research was aimed at determining the relationships between the particle diameter of an aluminum nanopowder and its maximum explosion pressure (P(subscript max)), maximum rate of pressure rise ((dP/dt) (subscript max)), minimum explosion concentration (MEC), and minimum ignition energy (MIE) by 20 liter apparatus and 1.2-L Hartmann apparatus. The results revealed that 35-nm aluminum powder has a P(subscript max) of 7.3 bar and deflagaration index (KSt) of 349 bar•m/s, in 100-nm aluminum powder, P(subscript max) of 12.5 bar and KSt of 296 bar•m/s and 40-μm aluminum powder, P(subscript max) of 5.9 bar and KSt of 77 bar•m/s. The value of (dP/dt) (subscript max) for the 35-nm aluminum powder is 4.5 times that for the 40-μm aluminum powder. The 35-nm, 100-nm, and 40-μm powders have MEC values of 40, 50, and 35 g/m^3, respectively. The 35-nm and 100-nm powders both have MIEs less than 1 mJ, while the 40-μm powder has an MIE of 59.7 mJ.

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Explosion Characteristics of Aluminum Nanopowders

The authors investigated the ignitability of aluminium and magnesium dusts that are generated during the shredding of post-consumer waste. The relations between particle size and the minimum explosive concentration, the minimum ignition energy, the ignition temperature of the dust clouds, etc. the relation between of oxygen concentration and dust explosion, the effect of inert substances on dust explosion, etc. were studied experimentally. The minimum explosive concentration increased exponentially with particle size. The minimum explosive concentrations of the sample dusts were about 170 g/m 3 (aluminium: 0–8 μm) and 90 g/m 3 (magnesium: 0–20 μm). The minimum ignition energy tended to increase with particle size. It was about 6 mJ for the aluminium samples and 4 mJ for the magnesium samples. The ignition temperature of dust clouds was about 750 °C for aluminium and about 520 °C for magnesium. The lowest concentrations of oxygen to produce a dust explosion were about 10% for aluminium and about 8% for magnesium. A large mixing ratio (more than about 50%) of calcium oxide or calcium carbonate was necessary to decrease the explosibility of magnesium dust. The experimental data obtained in the present investigation will be useful for evaluating the explosibility of aluminium and magnesium dusts generated in metal recycling operations and thus for enhancing the safety of recycling plants.

H. Ohya

Papers

Ignitability characteristics of aluminium and magnesium dusts that are generated during the shredding of post-consumer wastes