When we hear explosives, we commonly think of chemicals such as nitroglycerin or gunpowder. But an explosion is essentially just a rapid release of energy and expansion of gases due to combustion. This means that if something is combustible – that is, it can burn – it has the ability to explode. However, certain conditions have to be met so that the combustion can be sudden and rapid.
Because of this principle, it is possible to explode dust or other powdered substances. If the dust is combustible (such as flour or coal), suspended in the air at enough concentration in a confined space, with sufficient oxygen and an ignition source, a dust explosion can occur. For example, coal dust explosions are a common threat to miners and there have been countless incidents where mills and factories have exploded with great fury due to a dust cloud forming from grain, flour, powdered milk and sawdust etcetera.
The reason why dust will explode in such a condition but not when it a pile of dust is lit with fire is that in a dust cloud, every dust particle is in contact with air. This massively increases the surface area of the dust relative to its mass, meaning there more reactions can occur. In this case, the reaction is oxidisation of the substance, leading to combustion. This is why the finer the dust or powder, the greater the risk of an explosion. Even substances such as wood or aluminium can create an explosion when finely powdered enough.
The greatest casualty due to a dust explosion was in 1942 when a coal dust explosion in a Chinese mine killed 1549 miners. Another famous case was in 1878 when the Washburn “A” Mill in Minneapolis exploded, leading to the destruction of the world’s largest grain mill and five other mills. This reduced the city’s grain production to almost one-half.