Very simple and everyday products can cause large flares due to a phenomenon called a dust explosion. This, which consists of the rapid combustion of fine particles suspended in the air, has been responsible for major disasters in silos, carpentry shops and mills around the world.
That is why it is better not to do certain experiments at home, always use protections if we jump into it and know the risks, of which there are many examples.
One of them dates from May 2, 1878, the day that what was then one of the largest mills in the world, the Washburn A Mill, which stood on the banks of the Mississippi, in the city of Minneapolis, blew up into the air. Eighteen people were killed and four buildings adjacent to the collapsed structure fell. A simple spark produced by the friction of two millstones unleashed the catastrophe: it reacted with the enormous amount of dust from the milling of tons and tons of wheat.
After the event, many large and small mills and sawmills were renovated throughout the United States to prevent the accumulation of flour dust. Ventilation systems and other precautionary devices were installed to reduce the concentration in the air of fine particles that could ignite in contact with accidental sparks. For a dust explosion to occur, it is enough for the oxygen in the air – which works as an oxidant – and an ignition source to start the chemical reaction to coincide.
An outbreak of this type usually has a primary and a secondary phase . In the first, a small amount of dust burns extremely quickly, generating a large fireball. This deflagration creates a high pressure wave that raises the dust settled throughout the room, and from there arises a cloud of fine particles much larger than the pre-existing one. As this second wave of dust ignites, a secondary explosion is produced much more powerful than the initial one.
Many everyday products – starch, powdered milk, cocoa, coffee … – can star in a good explosion of this kind. Why, if these materials do not usually burn when we put a lit match on them? For them to bundle, the key is that the particles that compose them are very fine and are surrounded by oxygen. By having a large contact surface compared to a proportionally very small mass, each dust particle ignites easily, without requiring too much energy.
For these fragments to ignite, they must be less than 500 microns (µm) in diameter. Let’s remember: a micrometer is one thousandth of a millimeter. Powdered wheat flour particles, for example, reach a diameter of 50 µm.
Lycopodium clavatum is a very common plant. Its spores are yellow in color and have an average diameter of only 25 µm. This product was used in the flashes of the first cameras. Its hydrophobic properties and structure made the inventors take advantage of it to develop various prototypes. French chemist Joseph Nicéphore Niépce added Lycopodium powder to the fuel of one of the first internal combustion engines in history , on which he worked with his brother Claude in the early 19th century. In 1938, the American physicist, inventor and businessman Chester Carlson used it in his first experiments with xerography, a technique that is the basis of printers.
Today, Lycopodium powder is added as a lubricant in latex gloves and condoms. Its pyrotechnic power also works in fireworks, spectacular magic tricks or film and theater special effects.
Every year there are hundreds of dust explosions in the world that kill dozens of people . On June 27, 2015, one occurred in a water park in Taiwan, during a party that imitated Holi, the festival of colors celebrated in India, in which large quantities of dyed powders are thrown into the air that impregnate to the participants. In this case, powdered cornstarch was used that caught fire from a spark: the fire lasted 40 seconds, killed 15 people and caused burns of varying severity to 508.
Serve as a reminder that we are facing an experiment that must be done in a laboratory, with extreme caution and with the appropriate protections.
