Page 41 - Dust Explosions in the Process Industries

14 Dust Explosions in the Process Industries

Friction is a process of fairly long duration whereby objects are rubbed against each
other and heat is gradually accumulated.This produces hot surfaces, and in some cases
inflammation;for example, when an elevator or conveyor belt is slipping.
Impact is a short-duration interaction between two solid bodies under conditions of
large transient mechanical forces. Small fragmentsof solid material may be torn off, and
if made of metal, they may start burning in airdue to the initial heat absorbedin the impact
process. In addition, local “hot spots” may be generated at the points of impact. In some
cases, the impact may occur repeatedly at one specific point; for example, when a fixed
object inside a bucket elevator is repeatedly hit by the buckets. This may gradually gen-
erate a hot spot of sufficient size and temperature to ignite the dust cloud directly.
A practical mechanical impact situation is illustrated in Figure 1.11.A steel bolt acci-
dentally enters the top of a large concrete silo during filling of the silo with corn starch.
The bolt falls into the nearly empty silo and hits the concrete wall near the silo bottom
at a velocity of 25-30 ds. Visible sparks are generated. A dense, explosible cloud of
corn starch occupies the region where the impact occurs. Is ignition of the cloud prob-
able? This problem is discussed in detail in Chapter 5, but it should be indicated at this
point that ignition by simple impact, where steel is the metal component, seems less
likely than believed by many in the past. However, if the metal had been titanium or zir-
conium, ignition could have occurred in this situation.

Figure 1 .I 1 A steel bolt falls into a tall silo for corn and collides
with the concrete silo wall at high velocity. Can the steel sparks
generated initiate an explosion in the corn starch cloud in the silo?

The thermite reaction (2A1f Fe203-+Al,03 + 2Fe + heat) is often mentioned as a
potential ignition source from impact involving aluminum and rust. However, if a lump
of normal soft aluminum collides with a rusty steel surface, a thermite reaction will not
necessarily take place. In fact, due to the softness of the aluminum, the result is often
just a thin smear of aluminum on top of the rust. However,if this sandwichof aluminum
and rust is given a hard blow by a third object, a thermite flash capable of igniting dust
clouds can easily be produced. The same applies to a rusty surface that has been painted
with aluminumpaint, if the pigment content of the paint is comparativelyhigh. (Further
information is given in Section 1.4.2.6 and Chapters 5, 7, and 9).