Page 57 - Dust Explosions in the Process Industries

30 Dust Explosions in the Process Industries

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Figure 1.23 Influence of specific surface area of
aluminum dust on the maximum rate of pressure
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For metals, in particular those at the top of Table 1.1, the limiting particle size, below
which the ignition sensitivity and explosion violence no longer increase, is considerably
smaller than for most organic materials. This is because these metals do not devolatilize
or pyrolyze, but melt, evaporate, and burn as discrete entities (see Chapter 4). Figure 1.23
shows how the combustion rate of clouds of aluminum dust in air increases systemati-
cally with the specific surface area of the dust, in agreement with the trend in Figure 1.17.
However, the range of specific surface areas in Figure 1.23 is more than 10 times that
of Figure 1.17. For aluminum, a specific surface area of 6.5 m2/g corresponds to mono-
sized spheres of diameter 0.34 pm, or flakes of thickness 0.11 pm, which is a more-likely
particle shape for the most violently exploding powders in Figure 1.23.
Figure 1.24 shows a comparatively coarse atomized aluminum powder of specific sur-
face area only 0.045 m2/g, and Figure 1.25 shows a fine aluminum flake powder. Note
that the maximum rate of pressure rise of 2600 barb found for this powder in the 1.2
liter Hartmann bomb is not comparable to the values in Figure 1.23. This is due to dif-
ferent degrees of turbulence, degrees of dispersion into primary particles, and vessel
volumes.

Figure 1.25 Scanning electron microscope
Figure 1.24 Scanning electron microscope picture of aluminum flakes of thickness < 1 pn.
picture of atomized aluminum: typical par- Minimum ignition energy < 1-2 mJ: (dP/dt),,,
tide size 50 pm, minimum ignition energy in Hartmann bomb 2600 bar/s (Courtesy of
3000 m] (Courtesy of W. C. Wedberg). W. C. Wedberg).