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Comment by joering2 | Hacker News Reader

Props to Fire Dept. for having some decent response code enforcement - I mean you would never put off a molten metal; it would burn forever, and while you add water to it, it would only expand. AFAIK the only way to put it off is with sand.

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Class D fire extinguishers can be used to put out metal fires. They are expensive however and not all fire departments have access to them.

Class D fire extinguishers are used at air bases in my country to put out plane fires since our fighter jets are made out of a magnesium alloy which can burn.

I was curious as to the reason why water or CO2 can't be used to put off a metal fire. The TL;DR is that with a metal fire the high temperatures cause the H2O and CO2 molecules to be unstable enough that the metal can attract the oxygen molecules to fuel the ongoing fire rendering them useless as a fire suppressors. H2O is even worse as once the oxygen is split from the water molecule you are left with hydrogen which will combust and explosion might cause the molten fire to spread further. This doesn't happen with wood burning as the attraction to the oxygen molecules from the carbon being burnt is the same or stronger than the attraction from the CO2 or H2O molecules.

Do you think pure nitrogen would work in this situation?

You'd need tons. It's plausible but I don't think it's practical. Also, magnesium and titanium can burn in pure nitrogen, so it's not a panacea.

Probably not. Nitrogen is roughly the same density as air. It would just get dispersed into the atmosphere before it could do much, unless you had insane amounts of it.

Nitrogen wouldn't work in this situation. In fact nitrogen and aluminum will burn, forming AlN. Normally this doesn't happen because of Aluminum readily forms a "rust" layer in the presence of oxygen, and there isn't enough heat. Molten aluminum in a pure nitrogen environment wouldn't have that layer, and there likely would be enough heat.

Out of curiosity, what would happen if you quickly submerged the burning metal into a very large body of water?

Fire on the metal increases then ‘explosion’ the size of which depends on the metal type and quantity obviously https://youtu.be/tj7S_DNFgEU

Also alkali metals is fun

https://youtu.be/I8tOtZKpi04

> very large

It depends on how large, what temperature, and what type of metal. If there's enough latent heat in the quantity of metal to flash boil a significant fraction of the water, you're going to have a bad time. If you're literally dropping it in the ocean on the other hand, then you'll get a brief but impressive geyser and the fire will go out.

(Unless this is an alkali metal we're talking about. In that case you'll just get a "vigorous reaction" of a size proportional to the quantity of metal. Ex: https://www.youtube.com/watch?v=cc6W6SptLvU)

Another big concern is the water turning to steam. It expands 1,000x. 2 cubic centimeters of water expanding to a 2 liter bottle worth of steam. My facility had its own quick response fire unit (production employees who went through fire and rescue training), and I believe the company helped fund a ladder truck for the town of ~600 people which is adjacent to a decent metropolitan area. Fast response is vital to minimize danger to people and damage to the facility.

Factories have been leveled by water hitting molten aluminum. The Hackaday link below does a great job summarizing the dangers.

https://hackaday.com/2020/12/30/water-and-molten-aluminium-i...

Explosion links:

https://youtu.be/sIJmwF1Qs7M

https://youtu.be/jvddrln900Q (last ~20 seconds)

Source: Used to work at a very large aluminum facility.

Aluminum is also used as fuel in solid rocket propellants.

https://en.m.wikipedia.org/wiki/Ammonium_perchlorate_composi...

We did this in HS chem class, with a block of dry ice that we cut in half and hollowed out. Then we coiled up some magnesium ribbon so it would perch in the hole, lit it with a MAPP gas torch, and closed the block. The magnesium continued to burn long after any lingering air in the hollow could've been consumed. After a few minutes the magnesium was gone and we opened the block, and there was black carbon dust in the bottom of the hollow.

And that introduced the conversation about using the right extinguisher for the job!

> H2O is even worse as once the oxygen is split from the water molecule you are left with hydrogen which will combust and explosion might cause the molten fire to spread even farther

Yup, this is why you never douse an oil fire (say in the kitchen) with water. The fire will erupt! Seen it happen live. Fortunately the damage wasn't bad. One third-degree burn.

A kitchen oil fire is a different effect. They're not hot enough to disassociate water molecules. What happens typically is that the water floats the oil, and the fire, remaining on top of the oil, spreads more rapidly.

Another effect is when the fire (and typically, the cooking vessel) are hot enough to vaporize the water. This rapid vaporization spreads oil droplets rapidly, allowing much more of the oil to burn at once. It can appear explosive, even though it's just rapidly-expanding steam spreading fine oil droplets.

I think "rapidly expanding and exothermic" is equivalent to "explosive" at least functionally.

I'd agree that it's explosive. I suspect they were trying to make a distinction between a detonation and a deflagration. A detonation is an explosion with a supersonic shock wave. A deflagration is an explosion without such a shock wave. An oil fire getting water dumped on it is a deflagration, but almost certainly not a detonation. A metal fire getting water dumped on it can result in a detonation. Detonations are vastly more destructive in general than deflagrations.

Exactly that. Thank you. You know better words for it than I do :)

Yeah in the case of water on burning kitchen oil it just makes a big fuel-air bomb if the oil is atomized enough. Kinda like one of these:

https://www.youtube.com/watch?v=i20zvZ-3MMw

Note the initial dispersion of some atomized material followed by a triggered explosion some time later.

Can't wait for Elon's tweets praising CA public service.

> I mean you would never put off a molten metal; it would burn forever

I doubt that, if it burned forever you could use it to heat water and spin a turbine and produce free power. If you meant “forever” as “a long time”, then sure :P

It would eventually cool down after removing the source of heat keeping the metal molten, which are electric resistive heating elements (assuming it works like an aluminum smelter).

It would burn until all the metal has reacted. I think the point is that due to its chemical properties what you're using to try to "extinguish" the fire is actually fueling the reaction so not forever but you're certainly not helping.

Oh, absolutely. Water would exacerbate things (as you said in your sibling comment), I was just being a smartass about the word “forever” :P

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