Page 167 - Dust Explosions in the Process Industries

140 Dust Explosions in the Process Industries

on a hot plate. Even for a layer thickness of only 2 mm, the minimum ignition temper-
ature was as low as 320°C.
Allenbach (1984) proposed a special system for classifying the fire and explosion
hazards associated with dusts of various metals and ferro alloys in industrial plants.
He introduced three combustibility classes based on observation of the flame devel-
opment in clouds of freshly ground <44 pm dust fractions in a laboratory-scaleexplosion
vessel:
Class 1. Very active: Very violent flame propagation.
Class 2. Active: Quite fast flame propagation.
Class 3. Combustible: Slow propagation of weak flame.
The hazard of a particular powder or dust was evaluated by combining the flamma-
bility class of the ground <44 pm dust sample and the actual particle size of the prod-
uct. Allenbach provided a list of the combustibilityclass ratings of a wide range of ferro
alloys and other metals. All listed calcium alloys and most alloys containing aluminum
and magnesium were of Class 1. The other metals and alloys tested, including boron
alloys, chromium and chromium alloys, manganese and its alloys, and silicon and sili-
con alloys were of Class 2 or Class 3.
Allenbach did not provide sufficientinformationabout the experimental apparatus and
procedure to permit further evaluation of his proposed classification system.
Ma et al. (1987) and Xiao et al. (1987)produced kinetic data and mathematical models
for the oxidation of calcium silicon alloys, which may prove useful in future modeling
of dust explosions involving these materials.

1.5.3.8
Miscellaneous Powders and Dusts

Baklygin and Nikitina (1978) investigated the minimum explosible dust concentration
and minimum ignition temperature(dust layer) of various dust mixtures generated in the
mixing plant of the Moscow Tyre Works.
Gehring, Friesenhahn, and Rindner (1978) studied the explosiveness of clouds in air
of dust of a propellant containing 84% nitrocellulose, 10% dinitrotoluene, 5% dibutyl-
phthalate, and 1% diphenylamine. For a <75 pm fraction of this particularpropellant, the
minimum explosibledust concentrationin air was 100-200 g/m3,whereas the minimum
electric spark ignition energy of dust clouds was about 150 mJ. This means that, when
dispersed as clouds in air, such materials exhibit ignitability and explosibility proper-
ties similar to, or even less severe than, those of normal organic solid fuels like starch
and proteins of the same particle size. However, the pressure and temperature waves
generated by the initial dust explosion may in some cases initiate more hazardous sec-
ondary exothermal reactions in adjacent condensed propellant deposits.
The fire and dust explosion hazard connected with mine blasting of oil shale has been
considered by several authors, including Cashdollar, Hertzberg, and Conti (1984);
Richmond and Beitel (1984); Weiss, Cashdollar, and Sapko (1985, 1986); Miron and
Lazzara (1985); Sapko, Weiss, and Cashdollar (1986); and Hertzberg and Cashdollar
(1988). Karim, Bardon, and Hanafi (1979), in a more basic investigation, studiedthe com-
bustion of oil sand fragments in hot, flowing, oxidizing gas.