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Most woodworkers know that a spark can cause an explosion in air filled with fine wood dust. Other hazards, less well-known, can exist for a number of other finely divided materials; the explosive destruction of flour mills is one example.The flammable properties of steel wool show how this works. Steel wool is like kindling. You could hold a match to a nail and never get action. But touch the match to fine steel wool and it starts burning immediately.
A recent accident suffered by a British Columbia craftsman shows a danger that may lurk in the use of sander/grinders. First, a bit of background: thermite has long been a source of intense heat for welding large items such as railway rails and bridge sections. In wartime it was the primary constituent for incendiary bombs. Thermite is composed of powdered aluminum and iron oxide.The aluminum acts as a reducing agent; when it reacts with the iron oxide, the result is molten iron, aluminum oxide and large amounts of heat.
The incident in which the craftsman was injured began when his son used his belt sander to remove the heads of some aluminum pop rivets.This loaded the sander belt with fine aluminum particles and deposited more on the surface of the grinder stand. When he then used the sander to smooth the edge of a piece of rusted angle iron, the conditions were right for existence of what was, in effect, a small amount of thermite. A brilliant two-foot diameter fireball erupted, then instantly vanished, leaving dense white smoke with little odor. The intense heat blistered his hands and arms and removed a good bit of his hair and mustache. Fortunately he was wearing eye protection, so has fully recovered.
The message here is that any time you are grinding aluminum, be sure to clean the belt or wheel thoroughly before moving on to other metals, as well as cleaning up any accumulation on or around the sander/grinder.
