Mitigating fire and explosion hazards of powdered metals

Although a lay person's common experience is that metals do not typically burn, many metals when present as a fine powder present a fire and explosion hazard. According to the National Fire Protection Association (NFPA) Standard on Combustible Metals (NFPA 484), ‘[a]ny metal in a fine enough form can be combustible and/or explosible’. A good example is the combustibility of iron dust, when present as a fine dust that has not been oxidized.

It is the small particle size or large surface area to volume ratio of a combustible dust that differentiates it from a bulk combustible solid. The combustion rate of bulk solids is typically limited by the surface area of the solid in contact with the oxidizer (air), and so as the particle size of a material decreases, the specific surface area increases and combustion rate per mass of material increases. This concept is illustrated in Fig. 1, which is based upon Eckhoff . A log of wood is difficult to ignite and burns relatively slowly. If the log is chopped into kindling, it is easier to ignite and burns more rapidly. In the extreme, if the log is ground up into a fine dust and dispersed in air, it is much easier to ignite and burns very rapidly. A flame propagating through a cloud of fine combustible dust dispersed in air is in some ways similar to a flame propagating through a mixture of flammable gas or vapor and air.