A common situation we run into when visiting our customer’s locations is that they will have combustible dust throughout the facility, but not even realize that it is an explosion hazard. Most often, this is when there is some sort of metal dust produced, and the idea of metal being explosive sounds so bizarre it isn’t even considered. However, many metals do become combustible when they are ground into small enough sizes, despite being safe around fire as a solid mass.
There are two main forces that work together to create a metal dust explosion able to occur: oxidation and surface area.
When metals react with Oxygen they can oxidize, which is commonly seen on the surface of solid pieces of metal as rust. Some metals, such as aluminum and zinc, create a protective skin that prevents further oxidation instead of rusting. When the metal is in a solid mass, this oxidation occurs slowly since only the small surface area of the outside is exposed to oxygen, and the inside metal is prevented from oxidizing.
An important result of the chemical reaction created by oxidation is the production of heat. On large pieces of metal the heat produced by the slow oxidation is easily dissipated throughout the rest of the metal as well as the surrounding air, and never builds up to a noticeable temperature.
When the metal is shaved or ground down into dust there is now significantly more surface area for the oxidation reactions to occur. Since the reaction is now occurring to more metal at once and there is not the large mass to dissipate the heat, the reaction can occur much faster than if the metal was solid, creating larger amounts of heat. The heat has the added effect of making the reaction occur even faster, which is how some piles of metal dust can spontaneously combust.
As we know from the fire triangle, fire needs oxygen to burn. Just like how we wouldn’t be able to get a solid log of wood to burn with just a match, but a pile of sawdust would explode, metal behaves the same way with the added effect of the oxidation reaction.
When they come in contact with a heat source the small shavings of metal dust are able to easily become hot enough to ignite, since the heat does not have anywhere to dissipate. This heat causes the high surface area of the metal to rapidly oxidize by burning, which consumes the oxygen that is present.