Is Aluminum Dust Flammable? (A Comprehensive Overview)

Is Aluminum Dust Flammable?

Aluminum dust (or powder) has a high explosion hazard and it can act as a flammable material under certain conditions. Aluminum dust is also a combustible material.

Unlike aluminum in bulk or solid form, aluminum in dust form poses a high risk of explosions, especially when suspended in the air.

Aluminum dust can have different particle sizes and shapes. They can be classified in granular or flakes. Each class has specific particle size and shapes. There are differences in their behavior under naked flames or sparks exposure.

Aluminum can also form flammable alloys with magnesium and other metals.

A large number of incidents in the past were caused by aluminum or other metals in dust form.

The Combustion Reaction of Aluminum (Alº)

Chemically, aluminum is the same whether in bulk or dust form. In both forms aluminum has an oxidation number of zero and the atoms of aluminum are bound to each other by metallic bonds (that is, they share the valency electrons through the atoms).

The combustion reaction (a reaction in which a substance is oxidized by an oxidizing agent) is the event that leads to the release of heat. Whether such an reaction will produce enough heat to generate fire depends on many factors.

The main factor is the amount of energy released by the combustion reaction per unit of time. The combustion reaction of aluminum dust is the same for bulk aluminum. The main difference is that aluminum dust reacts far more quickly than bulk aluminum.

Since the combustion of aluminum dust proceeds so fast, heating of the system takes place very fast which can lead to more aluminum being oxidized (more aluminum going through a combustion reaction).

Why Is Aluminum Dust Flammable and Bulk Aluminum Not?

As mentioned above, the combustion reaction is the same for both. The difference lies in the speed of the reaction, which is much higher for aluminum dust than for bulk aluminum.

The main factor for this difference in speed is the difference in the surface area. The higher the surface area the higher the reaction rate will be. That is because more atoms are exposed to the oxidizer.

The smaller the particle size the higher the surface area and the faster is the combustion speed and more energy will be released by the combustion reaction.

Some Aluminum Chemical Hazards

Aluminum reacts with acids to form aluminum salts and hydrogen gas (H2, highly flammable gas).

Under specific circumstances aluminum also reacts with moisture or alcohols to form hydrogen gas.

Minimum Explosible Concentration (MEC) for Metal Dust

MEC is the concentration of a combustible dust cloud below which it cannot sustain or propagate a flame.

Unlike many gases and vapors (which have known flammability limits), dust explosive limits are very hard to properly assess. Therefore, explosive limits for dust often have significant differences between different experiments even when attempting to use the same procedures.

Obtaining uniform data on and reproducible dispersion of dust from one experiment to another is nearly impossible.

Metal Dust Explosion (or Deflagration) Prevention Measures

The methods involve reduction of at least one of the elements necessary for deflagration to occur, these elements are: ignition source, a fuel, oxygen (or another strong oxidizing agent), dispersion of the aluminum dust.

Oxygen Concentration Reduction Approach

This method is also called inerting, it consists of the addition of an inert gas in the ambient in order to decrease the concentration of oxygen in the air. Therefore decreasing the chance for ignition and subsequent deflagration to occur.

Possible inert gases include argon (Ar), Helium (He) or nitrogen (N2). Nitrogen gas can react dangerously with magnesium (Mgº) at high temperatures. Carbon dioxide can react with aluminum and should not be used. 

The most common techniques for the insertion of an inert gas are blanketing and purging. Blanketing (or padding) the space above the metal dust is blanketed with the inert gas. In purging, the inert gas is injected into the vessel prior to the addition of the metal.

Reduction of the Aluminum Dust (Combustible Concentration Reduction)

This technique relies on maintaining the concentration of the combustible (in this case aluminum dust) at levels below the minimum explosible concentration). For that to be accomplished, constant and reliable monitoring of the dust is essential. 

It is recommended that the metal dust concentration remains under 25% of the MEC in systems that are not continuously monitored. In systems that are continuously monitored, the metal dust concentration can go to as high as 60% of the MEC.

In facilities that use ventilation systems, it is important to make sure to avoid entrainment of hazardous concentrations of the exhausted air to enter back in the facility by the intake systems of fresh air.

Theoretically, in the cases that the concentration of dust is reliably monitored and kept below the MEC, the dust will not pose danger for explosions. However, such an ideal scenario is very impractical given the high difficulty to accurately and continuously assess the dust concentration.

The concentration of dust in the air may substantially oscillate during normal operations. Additionally, dust may deposit in equipment surfaces, in such cases the dust may eventually be suspended in the air depending on the activities being performed.

Overall, monitoring and reduction of the combustible (aluminum dust) has the potential to be very effective in diminishing explosion hazards, however this technique is considerably difficult to implement with precision.

Explosion Mitigation Measures for Metal Dust Workplaces

In workplaces that involve metal powder or dusts, explosion mitigation measures are necessary.

Some well-known mitigation measures are: pressure containment, deflagration venting and explosion suppression. At times they can be implemented in parallel.

Pressure Containment Technique in Metal Dusts Involved Workplaces

This technique is used for protection against explosions, it relies on equipment or vessel and appurtenances designed to withstand, without rupturing, the maximum overpressure expected in case of an explosion.

These appurtenances are connected to the vessel which has the possibility to suffer pressure from an incidental explosion. These appurtenances can be ducts and vents and other connections to the vessel.

The vessel can be designed to allow permanent deformation but not rupturing (explosion-proof shock resistant), or the vessel can be designed to prevent both deformation and rupture.

Deflagration Venting

The deflagration venting technique main feature is called explosion vent, this vent has the purpose of acting as an escape route for release of pressure and flame that are generated by an explosion.

The venting must take place in a quick manner in order to prevent pressure build up within the vessel, pipes and ducts or the building being protected.

protection measure that involves the provision of a predesigned escape route (called an explosion vent) for the release of the pressure and flame generated during an explosion quickly enough to prevent a dangerous buildup of pressure within the vessel, pipes and ducts, or building that is being protected.

The deflagration venting can be implemented for the protection of a variety of industrial equipment such as bins, bucket elevators, cyclones, dryers, dust collectors, mills and buildings.

In the case of buildings, vents can be used to protect their structural integrity in the event of an explosion. The installation of deflagration venting in buildings is typically done by employing metal panels on the exterior walls of the building to represent the weakest point of the structure.

This way, in the case of an explosion (or deflagration), the deflagration vents should be the only part of the structure that will rupture while the remainder of the building will be protected from damage.

Explosion Suppression in Metal Dust Involved Workplaces

Explosion suppression is intended to provide protection against explosions by detection and suppression of an eventual explosion during the initial stages in order to avoid building-up of pressure.

Aluminum Element General Properties

Some general properties of pure aluminum:

PropertyValue
Atomic weight26.98 au
Density2.7 g/cm3
MalleabilityHighly malleable
Electrical conductivity38,000,000 S/m
Thermal conductivity235 W/k.m
Melting point660 ºC

Aluminum in either dust or bulk form exposed to air gradually forms aluminum oxide (Al2O3) on its layer which gives off a more opaque and whitish appearance to the metal.

Aluminum Dust Properties

Aluminum flakes are produced by stamping the metal at normal temperatures. Granulated aluminum is produced from molten aluminum. Both these types can have variable sizes.

A particularly fine powder in flaked form is called pyro powder and it is composed of particles with diameters of 1 micrometer or lower.

Conclusion

This article provided insights into some properties of aluminum dust. It also provided some information regarding prevention and mitigation methods in workplaces that involve metal dusts.

Frequently Asked Questions (FAQ): Is Aluminum Dust Flammable?

Are aluminum shavings flammable?

Yes, aluminum shavings are considered a type of  aluminum dust, which is a flammable material, even though aluminum in bulk form is not flammable. Both aluminum in dust and in bulk form are cobustible.

What happens to aluminum in a fire?

In the case of bulk aluminum, ultimately, what will happen to aluminum in a fire depends on the fire temperature, the time of exposure to the fire and the purity of the aluminum. Pure aluminum will melt at its melting temperature of 660 ºC. 

Ordinary flames can have up to 1000 ºC, however the heat from a flame is not efficiently transferred to aluminum. So, under ordinary conditions aluminum will withstand exposure to a fire.

If the aluminum in question is in dust form, that is, in very small particles, exposure to fire will likely cause the aluminum to burst in flames. The more aluminum in this form the more energy will be released, making this a very dangerous experiment.

What happens when aluminum is heated?

Bulk aluminum will melt at 660 ºC. Aluminum dust will also melt at this temperature. However, aluminum dust has a much higher risk of igniting when heating than bulk aluminum.

Will aluminum melt in a fire pit?

Most common fire pits will have difficulty to heat aluminum up to its melting point. But it is possible.

Is aluminum flame resistant?

Aluminum in bulk form such as in sheets or bars does withstand some amount of heat. Long exposures to flames or short exposure to high temperature fires can cause aluminum to melt.

What temperatures can aluminum withstand?

Pure aluminum starts to lose its rigidity at around 200 ºC.

References

https://www.nfpa.org/Assets/files/AboutTheCodes/484/NFPA651-1998.pdf (Accessed on April 25th, 2022)

https://www.nfpa.org/codes-and-standards/all-codes-and-standards/list-of-codes-and-standards/detail?code=654 (Accessed on April 25th, 2022)

Martin P. Clouthier, Luc Cormier, Marc Hodapp, Damilare Ogungbemide, Chapter Seven – Dust explosions: Safety measures, Editor(s): Paul R. Amyotte, Faisal I. Khan, Methods in Chemical Process Safety, Elsevier, Volume 3, 2019, Pages 193-228, https://doi.org/10.1016/bs.mcps.2019.05.001.

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