With regard to physical state, dusts belong to the solid materials, but their physical and chemical properties differ from those of those same materials in compact form. It is known that industrial accidents and catastrophes are caused by dust explosions. Materials that are non-combustible in their usual form, such as metals, may initiate an explosion in the form of dust mixed with air when affected by any ignition source, even of low energy. The hazard of an explosion also exists with dusts of combustible materials.
Dust can be an explosion hazard not only when floating in the air, but also when settled. In layers of dust, heat may accumulate, and slow burning may develop in the inside as a result of the increased ability of particles to react and their lower thermal conductivity. Then the dust may be stirred up by flashes, and the possibility of dust explosion will grow.
Floating particles in fine distribution present a more severe hazard. Similar to the explosion properties of combustible gases and vapours, dusts also have a special range of air-dust concentration in which an explosion may occur. The lower and upper limit values of explosion concentration and the width of concentration range depend on the size and distribution of particles. If the dust concentration exceeds the highest concentration leading to an explosion, a portion of the dust is not destroyed by fire and absorbs heat, and as a consequence the explosion pressure developed remains below the maximum. The moisture content of air also influences the occurrence of an explosion. At higher humidity, the ignition temperature of the cloud of dust will increase in proportion with the heat quantity necessary for the evaporation of humidity. If an inert foreign dust is mixed in a cloud of dust, the explosivity of the dust-air mixture will be reduced. The effect will be the same if inert gases are mixed in the mixture of air and dust, because the oxygen concentration necessary for burning will be lower.
Experience has shown that all the ignition sources, even of minimum ignition energy, are capable of igniting dust clouds (open flames, electric arc, mechanical or electrostatic spark, hot surfaces, etc.). According to test results obtained in laboratory, the energy demand for ignition of dust clouds is 20 to 40 times higher than in the case of mixtures of combustible vapour and air. The factors that influence the explosion hazard for settled dusts are the physical and thermal engineering properties of the dust layer, the glowing temperature of the dust and the ignition properties of the decomposition products released by the dust layer.