Physical factors that lead to the explosion of metal powders

image_pdfimage_print

Influence of metal powder particle size and surface area

The particle size and surface area of dust cloud can determine whether it will ignite or not, or determine whether it will spread the combustion. When the particle size is reduced, the metal dust cloud will be easy to ignite and has fast reaction rate at the same time. Generally, when the particle size is reduced to less than 149m (100 meshes), the metal powder is easy to ignite and the pressure rise rate will become fast when the blast occurs. Provided that the small particle size of metal powder compounds is easy to cause explosion, the large and heavy particles may deposit in the dust cloud. While the residual dust cloud will have much larger explosibility than the original powder mixture.

flake aluminum nanopowder

Influence of metal powder particle shape

As the surface-to-volume ratios of leafing aluminum powders and magnesium powders are both bigger than other particle shapes, they are much easier to burn than other shapes of atomized aluminum powders and magnesium powders. In addition, leafing aluminum powders and magnesium powders are easy to break into fine powders and have intensive quadratic effect, in which many crisp metal particles (powders) will generate fine particles that can form the dust cloud, thus the quadratic effect will occur under this condition.

The particle concentration of dust cloud is the main factor that leads to the explosion. Once the powder particles and air are mixed homogeneously as well as the powder particles are suspended in the dust cloud (slow deposition), the explosion will be spread. Electrostatic force can also influence the ability of particle retention in the symmetric suspension. According to the feature of materials, the electrostatic force that generated when the dust cloud rises will hinder or help the dispersion of dust in the air.

Influence of gathered static charge

The composition of particle surface will determine whether the particle is gathered into static charge or inherently has free ions on the particle surface. For example, after coating stearic acid on the surface of aluminum particle, free ions will be generated on the surface, which will repulse mutually in the suspension and look for the irons that have opposite charges in the gas area. Regarding the electrostatic property of atomized aluminum powders, it tends to remain neutral and will deposit fast.

Due to the disturbance of dust cloud, the spreading rate of explosion will be accelerated as well as the total destructiveness will be intensified. The disturbance of dust may start from the explosion initiation of the inside dust cloud, or may be caused by the processing equipments that provide large energy for the particles from the external. Generally speaking, when the disturbance increases, the explosion pressure and pressure rise rate will both increase.

The dust cloud appearance will influence the explosive force

Dust cloud that exists in the small container will have high explosion pressure rise rate due to the multiple reflections of pressure output wave (the reflection will enhance the strength of pressure wave and accelerate the reaction). The size of dust cloud will have effect on the expanding scope of explosion, while the shape of dust cloud will have effect on the main direction of explosion extension. In conclusion, the chemical and physical parameters of powders will both influence its sensitivity to ignition, the rising of powders and the trend of forming dust cloud as well as the possibility of generating secondary explosion.