From: ILPI Support <info**At_Symbol_Here**ILPI.COM>
Subject: Re: [DCHAS-L] Pyrophoric gases
Date: Wed, 19 May 2021 18:07:20 -0400
Reply-To: ACS Division of Chemical Health and Safety <DCHAS-L**At_Symbol_Here**Princeton.EDU>
Message-ID: 0143F34C-BAF1-4443-B2E9-07AA3E644C9D**At_Symbol_Here**

A lot to chew on there.  We sell Class D agents so between my product knowledge and various mid-incident calls over the years, I can hit a few key points.

Class D extinguishers primarily work on solids by forming a thick blanket that excludes air which is why they are applied in depth with a soft-flow wand or scoop/shovel depending on the scale.  However, there are additional factors involved. For sodium-chloride based agents (trade name Met-L-X, Super D), the material will heat-cake or even form a molten crust which denies oxygen to the fire even better than the powder in-depth. While Lith-X,a graphite based agent, does not cake or melt, it cools the solid significantly because graphite conducts heat as well as or better than some metals:  Copper metal, which was developed as a Class D agent by the US Navy to combat lithium fires, not only conducts heat away, but can act as a heat sink to pull heat from the central fire and is the only known lithium fire fighting agent which will cling to vertical surfaces.

A preferred agent for Class D liquids such as alkylaluminums and lithium alkyls is Met-L-Kyl, which is a dry chemical agent that is designed to extinguish the flame in depth while absorbing the remaining fuel. We have some details on our web site. As you can see, it's horrendously expensive and, curiously, is NOT recommended for regular Class D metals such as magnesium, sodium, potassium etc.  It's a proprietary blend and the current SDS lists no hazardous ingredients. If you look around at older SDS's and those outside the US, you find that it's 60-70% sodium hydrogen carbonate and 30-35% silica gel with some free flowing agents (talc, silicon dioxide, magnesium stearate) thrown in.

Burning phosphorous can not be extinguished.  Other than letting it burn, all you can do is bury it somewhere and hope nobody digs it up because it will reignite. Although you can tinker around with trying to quench it with copper salts, this is not a practical firefighting method.  For a fascinating HazMat discussion see 

Dry sand can often be a solution to industrial scale flammable metal fires, although in many instances it may just be better to let it burn itself out. There are spectacular videos out there of water used on magnesium and titanium fires with catastrophic results. ABC or BC agents on these just feed the fire.  Most industrial places that catch fire rarely have Class D agents on hand and, if they do, they are of insufficient quantity to fight a major fire - you need to have several inches of agent over the entire burning surface.  The smallest Class D extinguisher made is a 30 lb handheld NaCl unit and that can handle only about three to five square feet of burning metal (half that for NaK alloy).  You'd need dumptruck loads of agent to fight an industrial fire and the expense would be mind-blowing.

As to gases, well, as your deep dive has shown, Dave, it's a lot like a fire company responding to a derailed freight train with tankers full of burning butane.  There's really no point in trying to fight the fire - just get away and let it burn itself out.  In the case of phosphine, by doing nothing you're burning off something that is highly toxic and replacing it with with a phosphoric acid fog.  And for silane, you're making silica. I'll take those trades any day.

Rob Toreki

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On May 19, 2021, at 4:16 PM, David C. Finster <dfinster**At_Symbol_Here**WITTENBERG.EDU> wrote:

I am working on a document about pyrophorics and how to extinguish fires involving them.
I have read that:  for pyrophoric solids (and for active metal fires involving water - which "behave" like pyrophorics) one can use a Type D extinguisher.  This makes sense to me since the Type D material will largely exclude air and probably cool the solid (a bit).  For liquids, a Type ABC extinguisher is recommended, and this makes sense to me since the ABC agent will cover the liquid (excluding air) and reduce vaporization.
For pyrophoric gases, the situation is less clear.  The 8th edition of the GHS Purple Book does not have pyrophoric gases as a hazard category.  (There are pyrophoric solids and pyrophoric liquids/solutions.)   The hazard statements for a "pyrophoric gas" will include H220 (extremely flammable gas) and H250 (catches fire spontaneously if exposed to air).
I have poked around a bit using phosphine as the test case.
The Linde SDS for phosphine (phosphine-ph3-safety-data-sheet-sds-p4643.pdf ( revised in 8/31/18 says:  "Suitable extinguishing media : Carbon dioxide, Dry chemical, Water spray or fog".  This is shocking to me since none of these materials would adequately eliminate a leg of the fire triangle (or tetrahedron).  On a lucky day, I can imagine that CO2 could momentarily displace enough air to extinguish a small volume of the burning gas but assuming that the gas is leaking from some system, it would reappear and catch fire. The SDS continues, (colored fonts added by me):
DANGER! Toxic, flammable, corrosive, liquid and gas under pressure.   If venting or leaking gas catches fire, do not extinguish flames. Flammable vapors may spread from leak, creating an explosive reignition hazard. Vapors can be ignited by pilot lights, other flames, smoking, sparks, heaters, electrical equipment, static discharge, or other ignition sources at locations distant from product handling point. Explosive atmospheres may linger. Before entering an area, especially a confined area, check the atmosphere with an appropriate device.
The phrase in red seems appropriate to me.  Important, too, since the natural intuition for chemists and emergency responders is to put out the fire 
The section in green seems entirely inappropriate since no "ignition source" is needed for a pyrophoric substance.
As a former firefighter the phrase in blue is not helpful since only 0% oxygen would be safe, and this is not realistic at any fire scene. 
FYI, the 2021 DOT Emergency Response Guide lists phosphine with a Guide #119.  It says:
Small Fire 
• Dry chemical, CO2, water spray or alcohol-resistant foam.
Large Fire 
• Water spray, fog or alcohol-resistant foam. 
• FOR CHLOROSILANES, DO NOT USE WATER; use AFFF alcohol-resistant medium-expansion foam.
• If it can be done safely, move undamaged containers away from the area around the fire.
• Damaged cylinders should be handled only by specialists
I found two useful documents from the University of Nebraska-Lincoln:
For (pyrophoric) gases, close the valve at the cylinder if it is safe to do so. If not, evacuate the area. Most burning pyrophoric gases are difficult to extinguish. Silane is reported to be impossible to extinguish.
DO NOT use carbon dioxide or water fire extinguishers as these types of extinguishers can actually enhance the combustion of some pyrophoric compounds.
(Phosphine might not be applicable for this statement.)
It seems that the best remedy for a burning gas (presumably arising from a leak in a system) is to isolate the leak or close the valve - if these steps are even possible.
Perhaps in some situations there is no way to stop the fire except to let it burn itself when the pyrophoric gas concentration is exhausted.  Best to be working in a hood, I imagine.  (Preferably an uncluttered hood not storing bottles of flammable liquids!)
Additional thoughts are welcome.

David C. Finster
Professor Emeritus, Department of Chemistry
Wittenberg University

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