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Dust Explosion - A hazard outside the GHS hazard category

Posted Date:2023/12/29

Any flammable substance or mixture whose fine particle form may pose a dust explosion hazard in an oxidizing medium such as air. Risk assessment may be required for many substances, mixtures or solid materials, not just for the various categories of flammable solids in accordance with Chapter 2.7 of the GHS.

The following points out the situations where flammable dust may be present and therefore the risk of dust explosion should be considered.

Flammable dust: fine solid particles of a substance or mixture that diffuse in air or other oxidizing media and are prone to fire or explosion under ignition conditions.

Identification of flammable dust

Is there any data, evidence or experience to confirm that the solid is flammable dust?

Clear evidence of a flammable dust can be obtained from publicly published accident reports related to the substance, mixture or solid material in question. Similarly, if experience shows that the substance, mixture or solid material in powder form is flammable, a risk of dust explosion can be presumed. If a substance, mixture, or solid material is not classified as flammable, it is still possible to form an explosive dust-air mixture. In particular, any organic or metallic material that is handled in powder form, or which may form powder during processing, shall be presumed to be a flammable dust unless there is clear evidence to the contrary.

Is the solid completely oxidized?

When a solid substance or mixture is completely oxidized, such as silica, no further combustion occurs. Therefore, this solid substance or mixture will not be ignited even if it comes into contact with the ignition source. However, if a solid substance or mixture is not completely oxidized, it may burn upon contact with an ignition source.

Does the solid contain particles with a nominal size ≤ 500 μm?

When evaluating materials related to Box 3, users should consider whether the material contains fine particles that may be released under normal or foreseeable conditions of use.

In assessing particle sizes associated with dust explosion risk, only fine particles with a particle size ≤ 500 μm were relevant, 3 even if the median particle size for the entire sample was greater than 500 μm. Therefore, when assessing the risk of forming an explosive dust environment, it is necessary to consider the dust composition itself, rather than the mixture of coarse and fine particles. However, it is not possible to define the lower concentration limit at which dust particles in a solid do not contribute to this risk (for example, as a percentage of the weight of the particles contained), so small amounts of fine particles are also relevant.

Is it possible to form particles of nominal size ≤ 500 μm during delivery and transportation operations?

In this part of the flow chart, the solids presented do not contain particles smaller than 500 μm. This form is not flammable dust. However, the solid is not fully oxidized and fine particles may form during supply and handling. Therefore, such conditions should be closely examined, especially foreseeable effects that may lead to the formation of fine particles, such as mechanical stresses such as grinding during transport or handling operations, or the drying of wet materials. If these effects cannot be ruled out, expert advice should be sought.

Do you choose to test the explosive properties of solid dust?

If dust explosive tests are carried out, they should be carried out in accordance with recognized and proven test standards, such as those listed in the GHS. If the solid test is carried out, and the solid used does not contain particles ≤ 500 μm, it should be ground to meet the purpose of the dust explosive test.

Do the results show that the solid is a flammable dust?

Characteristics such as particle size, chemical properties, moisture content, shape, and surface changes (e.g., oxidation, coating, activation, passivation) can affect explosive behavior. Standard tests determine whether a dust can actually form an explosive mixture with air.