In powder metallurgy technology, there are two main forms of explosion caused by explosive materials:
（1）metal powder Particles suspended in the air form dust clouds.Dust clouds are usually generated when processing metal powders.Once the dust cloud forms an appropriate mixing concentration and is ignited by the energy from metal powder or external kindling, the dust cloud will explode.
(2) Explosion caused by gas mixture.Some gases and air can form explosive mixtures in a large concentration range.[1]
Explosion conditions
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Usually, a large amount of metal powder will dissolve when it is locally heated, or if there is enough air, local combustion will occur.However, if the metal powder is mixed with airactivityIf it is extremely large, the combustion rate can be fully accelerated, so that the metal particles are suspended in the air, forming dust clouds in front of the combustion surface.Due to external disturbance or under appropriate atmosphere conditions, the dust cloud will ignite and explode.
Metals or mixtures of metals and other materials may react by themselves or by external fire（Metal spark, local impact energy or flame).ThermitA type of mixture, such as aluminum plus iron oxide, is an example of such an active mixture.The reaction between the components can be accelerated and the combustible particles can be thrown into the air at an extremely fast speed.At the same time, the unreacted mixture particles are dispersed in the air to form a dust cloud, which will lead to an explosion hazard.
When processing and processing metal powders, dust clouds are usually generated. Once the dust clouds form a suitable mixture and are ignited by energy from metal powders or external kindling, the dust clouds will explode.Continuous explosion may occur because the dust accumulated in the area will be thrown into the air during the first explosion. Once ignited, it will cause secondary explosion or combustion, which will cause more catastrophic damage.The following "Explosion Conditions" summarizes the conditions required for dust explosion and second explosion or ignition:
Explosion conditions
In any dust explosion, the pressure increases significantly and can last for a long time (the pressure can be as high as 1380kPa in a limited space and can last for several milliseconds).Although the explosion pressure value (even in the limited space) is not particularly high, the impact on the building structural materials may be destructive, such as making concretesiloAnd particlesMaterial elevatorFracture and failure.Similarly, the potential danger of metal dust cloud explosion is also great.[2]
influence factor
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Chemical and physical factors
(1) Chemical factors
The chemical composition and large surface area on the surface of metal powder or metal powder mixture may cause the sensitivity to spontaneous combustion, or constitute the danger of expansion by external kindling.
Special chemical treatment or formation of protective oxide film can change the explosiveness of metal powder.For example, very fine flake powders of unprotected lubricant coatings are particularly sensitive to ignition.Because the active gas from powder processing or combustion reaction and trapped in the metal powderThermal performanceVery poor, may reach very high temperature, these flammable gases may ignite, thus igniting the dust cloud.
Some metal powders can produce extremely highactivityOnce potassium, rubidium and cesium are mixed with waterOxidantWhen contacted, it burns rapidly and suddenly.Some metals will produce flammable gas or vapor (such as hydrogen) in the presence of moisture. If there is dust cloud, there is an explosion risk.Metal powders such as magnesium, aluminum, and zinc all react with water and may self heat enough to cause local combustion.In addition, some metal powders, such as aluminum, may react violently with halohydrocarbons or other chemicals due to oxygen free oxidation/reduction reaction.
(2) Physical factors
Many physical properties, such asgranularityandSurface area, which determines whether the dust cloud will ignite or whether it will spread once ignited.When the particle size decreases, the metal dust cloud becomes easier to ignite and the reaction rate is faster.Generally, when the particle size is reduced to below 149 μ m (100 meshes), the metal powder mixture is easy to ignite, and the rate of pressure rise is faster during explosion.If the more explosive component in the metal powder mixture is the one with smaller particle size, then the larger and heavier particles may fall from the formed dust cloud, and the dust cloud left behind is much more explosive than the original powder mixture.
Grain shapeIt may also affect.For example, the ratio of surface area to volume of flake aluminum powder and magnesium powder is larger than that of other particle shapes, so they are much easier to burn than atomized aluminum powder and magnesium powder of other shapes.Moreover, flaky aluminum powder or magnesium powder is easy to be broken into finer particles, resulting in a strong secondary effect.Brittle metal particles (powders) will probably form finer particles of dust clouds and have secondary effects.
ParticulateconcentrationIs the main determinant of the explosion.To expand the explosion, dust particles and air must form a relatively uniform mixture. At the same time, powder particles must be suspended in the dust cloud (slowly falling). Electrostatic force also affects the ability of particles to stay in the uniform suspension.According to the characteristics of materialsElectrostatic forceMay hinder or contribute to the dispersion of dust in the air.The composition of the particle surface determines that the particles are electrostatic charges on the surface.[2]
Atmospheric conditions
The atmosphere in which the dust rises greatly affects the explosion characteristics of the dust,oxygen contentIt can determine whether the metal is on fire and explodes or whether the combustion expands;Some metals require very little oxygen to burn, while others burn only when the oxygen content is high.
Some metal powders (such as lithium, magnesium and hafnium) that burn in the dust cloud react with gas (such as carbon dioxide) rather than air. The presence of flammable gases and vapors, which can significantly reduce the explosion limit, strongly affects the parameters of combustion and explosion.[2]
preventive measure
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The danger caused by the equipment is that the dust will gather near the heating, exhaust and air conditioning equipment, which is easy to generate high temperature.The equipment generating dust cloud is too close to other equipment in the production process, which is conducive to explosion expansion.Dust concentration in dust collection system (especially pipeline system) is easy to reach dangerous level.These hazards can be eliminated by sealing the equipment to prevent leakage and according to the dust collector used for the equipment. The cleaning and maintenance plan of the equipment should be sufficient to prevent dust layer accumulation.
Although no more hazards will occur after these protective measures are taken, the dust collection system must be designed to handle special metal powders safely.For example, during the production of aluminum, water vapor and water shall not accumulate in the collected aluminum dust, otherwise spontaneous combustion may occur in the dust collection chamber.In addition, do not usecompressed airThe nozzle can remove the accumulated dust, because it will only produce more dust clouds, so the explosion-proof air circulation system can only be used for cleaning operations.[2]
