Dust classes that generally have the highest Dust Explosion Class of 3 are: Thermites which involve a metal powder fuel plus a metal powder oxide. Fuel examples are Al, Mg, Ti, Zn, Si, and B. Oxidizers include boron oxide, silicon oxide, chrome oxide, manganese oxide, iron oxide, copper oxide, and lead oxide.
Particularly flammable metal dusts per NFPA-484 include all alkali metals. Some of the most flammable are Al, Mg, Ti, Ta, Nb, and Zr.
Other Class 3 Dust Explosion Clases include: Metal Reactive pairs that can autoignite require two metal powders that produce intermetallic plus heat. Examples of reactive pairs are: Al+Ni, Al+B, Zr+B, Si+Zr, Ni+Si, Ce+Si, Al+Ti, Ti+B, Ti+C, Co+Si, Mn+Si, and Si+Ti. Reactive metal pairs do not require oxygen for combustion and will almost always react to competition before they can be extinguished.
Metal dusts can behave much differently in comparison to organic dusts if a cloud is ignited. The flame temperature of some metal dusts can exceed 3,500°C, which is greater than a 1,000°C higher than an organic flame. This can result in high explosion maximum pressures. Also, the maximum pressure rate of rise, as well as the flame speed, can greatly exceed that found with organic fuels. Taken all together, these factors can make it much more challenging to control a dust explosion involving metal dust.
Aluminum metal has been used in the propellant, explosive and pyrotechnic (PEP) industries for slightly more than 100 years. Aluminum powder is often added to explosives to provide an additional incendiary effect that results from the massive release of energy as the aluminum reacts with oxygen to form the stable aluminum oxide. This reaction yields more than 225 kcal per mole. Aluminized solid propellants utilize this substantial heat release, and after World War II large rocket boosters took advantage of this fact by using aluminum as an additive. Aluminized propellants are still currently used in the solid rocket boosters for the NASA Space Shuttle. For comparative purposes only, 1 gram of aluminum dust has about the same explosive energy as 0.7 grams of TNT.
Vacuuming a combustible dust presents a fire and explosion hazard in the vacuum cleaner. A Dust Cloud is present inside the vacuum that creates a static charge due to particle motion requiring the use of special procedures.
Bulk dusts are required to be removed by using conductive and non-sparking brushes and dust pans prior to vacuuming residual dusts. Mineral Oil Immersion Vacuums that are properly bonded and grounded with appurtenances that are also rated as conductive and anti-sparking must be utilized.