Metals ground to a particle size of less than 50-100 µm are capable of igniting and exploding even at ambient temperatures below 20 °C. This is due to exothermic reactions during oxidation processes. Fine metal particles react faster with the same oxygen and release a considerable amount of heat, which can lead to an explosion.
A bit of statistics
Metal dust explosions happen every year. In Russia alone, there have been at least two such incidents in the media in the last six months.
Explosions of metal dust are almost always accompanied by casualties and fatalities. This is due to the great force of the blast wave and the dispersion of burning molten particles.
Examples of accidents caused by metal dust explosions:
- August 2014 — Aluminium dust explosion at an auto parts factory in Kunshan, Jiangsu province, China. Seventy-five people were killed, of whom 25 died in hospital. Witnesses said the burning dust adhered to clothing and skin, causing severe burns. About 185 people were injured. The cause: a safety violation while polishing aluminium rims.
- December 2021 — An aluminium dust explosion at the AIDC aerospace facility in the Xitun area of Taichung, Taiwan. One person died and six others were injured to varying degrees of severity.
- October 2022 — An explosion due to the ignition of aluminium dust at the Forte Prom GmbH radiator plant in Volgograd, Russia. One person died and three others received burns of varying degrees of severity. The cause was said to be a failure in the machine software, which led to equipment malfunctions.
- January 2023 — An aluminium dust outbreak and fire at the Moliren plant in Roshal, Moscow Region, Russia. Two people were injured. The fire affected 200 sq m.
- March 2023 — Fire at the Dicastal North America aluminium disc manufacturer's plant in Greenville, Michigan, USA. One worker was injured - he suffered severe burns. The fire originated near a blast furnace melting aluminium and magnesium for wheels.
Investigations into accidents show that the main causes of metal dust explosions are often:
- non-compliance with safety procedures during metal processing;
- Inadequate staff qualifications;
- Inadequate work organisation
- Equipment malfunctions including inadequate maintenance, repair and replacement times.
Metal dust is an explosive and fire hazard. This fact must be remembered by all owners and employees of facilities where it may be present: plants, mining and processing plants, scrap metal and non-ferrous metal recycling facilities.
What are the hazards of metal dust?
To understand the hazards you need to know the parameter " Lower Explosive Limit" (LEL). It indicates the minimum level of combustible dust in the air at which an explosion or fire occurs.
LEL is influenced by the type of substance and the dispersity. The finer the particles, the less dust is needed to cause an explosion. Fine particles oxidise more quickly, releasing large amounts of heat and detonating. It is worth bearing in mind that some metals do not need oxygen for oxidation. They can also ignite in reaction with nitrogen, carbon dioxide.
LELs of organic dusts are usually known and are available in publicly available reference books. For metal dusts there are less studies, and for fine dusts only 'narrow experts' know about them.
Take aluminium powder as an example. Its LEL is stated in the state standard as 40 g/m3. All that has a lower threshold of 65 g/m3, refers to explosive substances, while below 15 g/m3 - to the first class of hazard. LEL of fine aluminium powder with a particle size of less than 63 μm can be as low as 15 g/m3. These test data are given in the GESTIS-DUST-EX database.
Powdered aluminium in suspension with air, i.e. as an aerosol, is explosive. In the form of aerogel, i.e. as a residue, it is flammable. On contact with a source of ignition, aluminium aerosols explode. The shock wave lifts the aerogels into the air, creating "fuel" for more explosions. All this takes a few seconds, causing devastating consequences.
Causes of metal dust
Dust arises from various metalworking processes:
- drilling, etc.
As metal dust is harmful to human health, dust extraction is a must in the design of the metallurgical plant. Dust extraction equipment may be stationary, mobile or as a cartridge.
Compact dust extractors can be placed close to workplaces with turning, milling, grinding and sanding machines. Fixed aspiration systems in aluminium smelters are calculated individually, depending on the number of dust sources and dust types.
If the dust collectors are not maintained in good time, they, too, become a source of danger. Dust concentrations in filters and cyclones in suction systems can reach explosive limits and cause an explosion.
Accidents in metalworking plants cannot be completely ruled out, but prevention and explosion protection measures can be taken.
Explosion protection in the construction and operation of an aluminium smelter
European directives classify metal working as a hazardous production facility. The owners of such facilities are required to implement measures to prevent accidents and to contain and eliminate their consequences.
These typically include:
- Explosion-proof safety valves must be installed in process spaces and gas ducts where uncontrolled processes leading to an explosion can occur.
- In aluminium powder production rooms the possibility of sparks must be excluded. The floors in the said rooms shall be made of spark-free materials.
- All tools and containers used when handling aluminium powder shall be made of spark-free materials.
- To avoid oxidation, self-ignition and explosion of aluminium powder, no damp or wet areas shall be used for production and storage of aluminium powder.
- Dust should be cleaned from the floors and equipment in the silos, conveyor galleries and packaging department on a regular basis, and from walls, ceilings and steel structures once a week.
The safety measures are tailored to the individual operating conditions:
- explosion protection in the design of the aluminium plant;
- retrofitting of equipment and installation of explosion protection devices for an existing plant.
Experts determine the flammability parameters for a particular metal dust and classify the explosion risk zones in order to determine the protection class of the respective equipment. Sometimes this requires a 'controlled explosion' which can only be carried out in a special laboratory environment. You can contact the ATEX.CENTER specialists to solve these problems.