Page 166 - Dust Explosions in the Process Industries

Dust Explosions:An Overview 739

A high-speed automatic isolation valve was installed in the duct between the filter and
the other parts of the process, and the filter enclosure was equipped with a vent. Detectors
for airflow and pressure were integrated in the interlocking system.
In plants producing very fine aluminum and magnesium powders, extensive gas inert-
ing is necessary. For aluminum, nitrogen is normally suitable as inert gas, whereas a rare
gas (helium or argon) is required for magnesium. However, to enable the particle sur-
face to become oxidized and thus avoid extreme reactivity when the powder or dust is
later exposed to air, a certain fraction of oxygen, normally between 3 and 5 vol%, should
remain in the inerting gas. The National Fire Protection Association (1987) discussed
inerting and other necessary measures more extensively.
Eckhoff and Alfert (1988) reviewed the influence of particle size on the ignitability
and explosibility properties of aluminum powders.

1.5.3.7
Silicon, Silicon Alloys, and Other Metals
As indicated by Table 1.1in Section 1.1.2, silicon dust has the potential to generate nearly
the same explosion strength as aluminum dust of the same particle size. This has been
confirmed in practice. Fine silicon dust has given rise to catastrophic explosions in pro-
duction and handling plants (see Chapter 2). Like magnesium and aluminum dust clouds,
clouds of silicon in air burn at a very high temperature, and thermal radiation from the
burning cloud represents a severe threat to personnel.
If silicon is alloyed with iron, ignitability and explosibility is generally reduced as the
iron content increases. On the other hand, the presence of magnesium in silicon alloys
significantly increases the explosion hazard. In particular, the minimum electric spark
ignition energy drops significantly if the magnesium content approaches 5-10 wt% or
more. In general, understanding the influence of various alloy compounds on the ignitabil-
ity anid explosibility of silicon alloys is incomplete, and specific investigation is often
required.
Eckhoff et al. (1986) investigated the ignitability and explosibility of silicon dust
clouds in air and confirmed that the minimum electric spark ignition energy decreases
and the explosion violence increases systematically with decreasing particle size.
However, very fine powders and dusts of particle sizes in the range of 1 pm and even
smaller may be difficult to disperse completely into primary particles and therefore
behave as If they were coarser. This can complicate the correlation of primary particle
size with ignitability and explosibility data (see Chapters 3 and 9 for further details on
dust dispersion).
In manganese and ferromanganese, flashes that can initiate flame propagation in dust
clouds are easily produced by mechanical impact of lumps of the material or in cmsh-
ing operations. (This particular feature has also been observed with ferro-silicon-
magnesium.) Clouds of fine manganese dust in the aircan have very low minimum electric
spark ignition energies, on the order of 1 mJ. On the other hand, flame propagation in
clouds in the air of dusts of manganese and manganese alloys is comparatively slow and
the flame temperature comparatively low. Qian Qiyong, Wang Taisheng, and Xiao Hechai
(1987) studied how dust explosions and fires in the cyclone separator of a ferromanganese
milling plant could be prevented, despite unavoidable flashes in the crushing and milling
units. As part of the work, they also studied ignition of layers of ferromanganese dusts