Metal Fires

[Tom Sutton]

I have received the following enquiry and would suggest Sodium Chloride Dry Powder as the solution but I need confirmation or a slagging off depending how near I am to the solution.

“Please would you help .........
 
Class D fires (i.e. metals in the form of powder or swarf) require Specialist Powder fire extinguishers.
 
Please would you advise which type of extinguisher you would recommend for solid metal sodium (which is normally kept immersed in oil, to prevent spontaneous combustion)?
 
The quantity involved is small, i.e. less than 1kg.”

[slubberdegullion]

tw

I recall ternary euclectic chlorides for this sort of risk.  But this is specialist stuff and I would probe deeper and enquire:

1. Why do they think they will need such extinguishing medium (i.e. what scenarios do they think might lead to their requirement)?
2. Why are they, themselves, not already familiar with the correct extinguishing medium??

I would discuss the requirements with a company found by searching on t.e.c. as mentioned above.

Stu

[kurnal]

Interesting question. I cut and pasted the following from wikipedia. Have no personal knowledge to add though.

Extreme care is required in handling elemental/metallic sodium. Sodium is potentially explosive in water (depending on quantity) and is a caustic poison, since it is rapidly converted to sodium hydroxide on contact with moisture. The powdered form may combust spontaneously in air or oxygen. Sodium must be stored either in an inert (oxygen and moisture free) atmosphere (such as nitrogen or argon), or under a liquid hydrocarbon such as mineral oil or kerosene.

The reaction of sodium and water is a familiar one in chemistry labs, and is reasonably safe if amounts of sodium smaller than a pencil eraser are used and the reaction is done behind a plastic shield by people wearing eye protection. However, the sodium-water reaction does not scale up well, and is treacherous when larger amounts of sodium are used. Larger pieces of sodium melt under the heat of the reaction, and the molten ball of metal is buoyed up by hydrogen and may appear to be stably reacting with water, until splashing covers more of the reaction mass, causing thermal runaway and an explosion which scatters molten sodium, lye solution, and sometimes flame. (18.5 g explosion [1]) This behavior is unpredictable, and among the alkali metals it is usually sodium which invites this surprise phenomenon, because lithium is not reactive enough to do it, and potassium is so reactive that chemistry students are not tempted to try the reaction with larger potassium pieces.

Sodium is much more reactive than magnesium; a reactivity which can be further enhanced due to sodium's much lower melting point. When sodium catches fire in air (as opposed to just the hydrogen gas generated from water by means of its reaction with sodium) it more easily produces temperatures high enough to melt the sodium, exposing more of its surface to the air and spreading the fire.

Few common fire extinguishers work on sodium fires. Water, of course, exacerbates sodium fires, as do water-based foams. CO2 and Halon are often ineffective on sodium fires, which reignite when the extinguisher dissipates. Among the very few materials effective on a sodium fire are Pyromet and Met-L-X. Pyromet is a NaCl/(NH4)2HPO4 mix, with flow/anti-clump agents. It smothers the fire, drains away heat, and melts to form an impermeable crust. This is the standard dry-powder canister fire extinguisher for all classes of fires. Met-L-X is mostly sodium chloride, NaCl, with approximately 5% Saran plastic as a crust-former, and flow/anti-clumping agents. It is most commonly hand-applied, with a scoop. Other extreme fire extinguishing materials include [[Lith+]], a graphite based dry powder with an organophosphate flame retardant; and [[Na+]], a Na2CO3-based material.

Because of the reaction scale problems discussed above, disposing of large quantities of sodium (more than 10 to 100 grams) must be done through a licensed hazardous materials disposer. Smaller quantities may be broken up and neutralized carefully with ethanol (which has a much slower reaction than water), or even methanol (where the reaction is more rapid than ethanol's but still less than in water), but care should nevertheless be taken, as the caustic products from the ethanol or methanol reaction are just as hazardous to eyes and skin as those from water. After the alcohol reaction appears complete, and all pieces of reaction debris have been broken up or dissolved, a mixture of alcohol and water, then pure water, may then be carefully used for a final cleaning. This should be allowed to stand a few minutes until the reaction products are diluted more thoroughly and flushed down the drain. The purpose of the final water soak and wash of any reaction mass which may contain sodium is to ensure that alcohol does not carry unreacted sodium into the sink trap, where a water reaction may generate hydrogen in the trap space which can then be potentially ignited, causing a confined sink trap explosion

[Tom Sutton]

Stu I understand from a previous submission on this forum TEC is a none starter as it is toxic and you are unable to purchase it in the UK but is available in India.
1. & 2. the only information I got is in the above submission.

Kurnal the problem with wikipedia has an American an slant and when it comes to UK products it some times not very helpful. However I did note "Met-L-X is mostly sodium chloride, NaCl" which is the road I went down. From some research I did I found the American Navy go for two extinguishing mediums which are copper powder and sodium chloride (NaCl), Salt being the cheapest I went for that and found two interesting links.

Check out http://www.canonfire.co.uk/pages/extl2m28.htm   http://www.ucl.ac.uk/efd/maintenance/fire/documents/UCLFire_TN_033.pdf which seemed to be what I was looking for but I need to find a UK provider.

[patrickhamblin]

Try Chubb - they have a Class D Powder Pyromet extinguisher, stated to be suitable for class D fires involving metals like lithium, magnesium, sodium or aluminium (powder or swarf).

[Goodsparks]

Bucket of dry, soft sand would sound reasonable for a small quantity, that is what most secondary school labs seem to use anyway.

AnthonyB will set you straight when he reads the post :-)

[Tom Sutton]

I eventually found Chubb's Class D Powder Pyromet extinguisher and another supplier aswell.

[AnthonyB]

Most UK class D extinguishers are graphite based & originate from Chubb's Chinese factory (via Chubb, Thomas Glover or Firechief), although Amerex use Sodium Chloride (being US)

For 1 kilo a single 9 kilo Class D extinguisher should cope, although as a specialist high risk a spare is advisable. Dry sand, kept bagged until required (or indeed bulb plastic bagged Sodium Chloride [jumbo catering packs of table salt ] is a more low tech solution as long as you can  ensure it will be totally dry - you also need scoops and shovels to apply it or throw the bags directly onto the metal

Blatantly copied & pasted from my earlier submissions on Powders.......

(Paste 1)

TEC was created by UKAEA decades ago for uranium fires, is highly toxic as it's contents include barium chloride and isn't seen in use much. If you really want some they still make it in India.

Class D Powders extinguishers are readily available and have been for years, and the formulations usually fall into three types - graphite, sodium chloride & copper.

You simply need a typical Class D extinguisher which we supply as does Chubb and any company selling Amerex, Gloria, or Thomas Glover extinguishers which will have low velocity applicator. To see one in use there is a free streaming video on Chubbs site showing the results of using normal extinguishers and then the correct use of their Pyromet Class D unit.

Expect to pay £150 - £500 a unit depending on who you buy them from

Or you could buy the agent bagged to throw on manually of course.

The manufacturers of the well known (in the trade) Cetrimax Powders state that their high grade ABC powder is approved for class d fires if use with a class d extinguisher body (i.e. low velocity applicator) but this is only tested on magnesium

(Paste 2)

Class D powders are designed for fires in flammable metals and three main types are in use-

-  Sodium Chloride for certain Class D fires, those involving alkali metals such as sodium and potassium, also zirconium, uranium and powdered aluminium, extinguishes a metal fire by fusing to form a crust. This excludes oxygen from the surface of the molten metal; a carbonaceous rafting agent prevents the powder from sinking into the surface of molten metal
- Copper extinguishing agent specially developed by the U.S. Navy for fighting lithium and lithium alloy fires. The copper compound smothers the fire and provides an excellent heat sink for dissipating heat. Copper powder has been found to be superior to all other known fire extinguishing agents for lithium.
- Graphite - has a high heat absorption & crusts over the metal, similar to Sodium Chloride in use

In older times:
- Ternary Eutectic Chloride, developed by UKAEA for uranium fires, which works similarly to Sodium Choride. It is extremely toxic.

[Tom Sutton]

AB I well remember your first submission and my reply was in accordance with that advice. I also found a UK supplier that had NaCl based extinguishers most probably imported from the States. http://www.canonfire.co.uk/pages/extl2m28.htm Being a shrinking violet I just need assurance.

[Ken Taylor]

For laboratory purposes the Schools Science Service recommend smothering with mineral absorbent or anhydrous sodium carbonate (soda ash) and say that, if a fire extinguisher is used at all, it must be of a dry powder type.

[AnthonyB]

Just goes to show not to listen totally to the Schools Science Service - they are totally spot on with the soda ash (if dry) but not the dry powder bit as they are probably unaware of the varying types.

If you want to see why normal powder isn't much use (or water, foam, co2 for that matter) go to Chubb's web site & watch the Pyromet video

[Ken Taylor]

I suppose (to be fair) they do say 'anhydrous' expecting science technicians to understand this as meaning 'dry' - and then add the words 'at all' with regard to extinguisher use - presumably on the basis that if there is a need to be using one of the extinguishers in the lab the only one not likely to substantially increase the risk is likely to be the DP. Personally, I like the continued use of sand fire buckets in labs and the like as providing a relatively inert medium for both extinguishment and absorbing chemical spillage. In terms of risk assessment, anyone using chemicals needs to be prepared with whatever extinguishant or 'antidote' is required for both the chemicals and their potential products.

[Pete M]

Sand has been proven time and time again as one of the best methods for extinguishing metal fires (and cigarettes!) hence their persistance in science labs.

Sand, being mainly SO2 provides a suffocating blanket to most fires and is still recommended today for science labs in schools.

One project I worked on in South Africa (a fertilizer plant using basic, massive sulphur) would only use sand or copious amounts of additional sulphur to extinguish a fire.  The main combustible was sulphur.

[Ken Taylor]

Incidentally, the Chubb video is interesting. The pyromet seemed to be basically gently applying a pile of powder onto the burning metal to cover it until it had cooled sufficiently to prevent re-ignition when the metal was exposed. I would have liked to see the effect of the DP being discharged downward onto the burning metal to form a pile (eg like laying a foam blanket) rather than at a 45 deg angle thereby blowing the metal away. Water-reactive metals are often present in school labs and I somehow doubt whether Pyromet extinguishers will be added extensively while they still have access to their sand buckets.

[Tom Sutton]

I can see why Ken, the cost of sand over a special dry powder is no contest and for small amounts it is adequate. It was used widely during world war two to extinguish incendiary bombs which were basically magnesium fires.

An important consideration is it must be applied with a long handled shovel or applicator to prevent injury to the operator.

It should also be stored in dry conditions because of its reaction with water. I have seen an inch cube of sodium thrown into a pond and the explosion was very spectacular.