I have quite a bit of experience remediating alkali & reactive metals, and as Steven stated for what you are proposing to demonstrate, i.e. the small quantities - there is a simpler solution to not “lugging” around a heavy Class D Powder – fire extinguisher. Class ABC, or Class BC Dry Chemical fire extinguishers use “dry chemicals” and Class D “extinguishers” use Dry Powders. Yes, there is a distinction between the two.
Some overarching safety points from the previous email chain reference this subject.
1. As Steven states you can purchase Dry Powder rated for Mg – Magnesium fires from suppliers like Ansul. In our work with small and large quantities of these reactive metals at various National Laboratories, we have used both. You can use a scoop to carefully apply the Dry Powder onto the burning mass – see pictures I have attached from one of our projects at a National Laboratory showing such a scoop and the LITH-X dry powder in a pail we had on site to extinguisher incidental Lithium fires. We also had lots of the “heavy” Class-D extinguishers as well.
2. In my experience, unless an incompatible oxidizing material is applied such as water H20, these metal fires are quiescent fires, meaning that they are generally not “vigorous combustion events” unless you introduce the oxygen enhancing material. Which is the exact point of your demo. The O2 in the Dry Ice (Solid CO2) accelerates the combustion dramatically (albeit locally with small quantities such as your ribbon of Mg). If water is applied the resulting reaction can be explosive – such as the Mg fire that killed a responding firefighter in Wisconsin when he applied water to a large amount of burning Mg in a dumpster a few years ago.
3. The Mg burns at an extremely high temperature, which is the very reason the H20 (or the CO2, or whatever) releases H and O2, or C and O2 – just what you want in a combustion event! So please take safety and PPE precautions that the demonstrator - and the students cannot touch even the “cooling” mass.
4. The use of Dry Chemical fire extinguishers. Class ABC or Class BC extinguishers should be strictly AVOIDED. The Monoammonium Phosphate or Monoammonium Sulfate (a.k.a. “Foray”) found in some ABC Dry Chemical agents; and the Potassium Bicarbonate (a.k.a. “Purple-K) found in some BC Dry Chemical agents, may release toxic gases or actually greatly accelerate the fire, when applied to metal fires of high temperature.
5. Obviously all foam. AFFF, “wet agent” and “wet” Class-K fire extinguishers agents are completely incompatible because of their water content (some AFFF foam extinguishers contain as much as 94% to 99% is water).
6. No, please do not use a Carbon Dioxide (a.k.a. CO2) fire extinguisher on a metal fire event, it will greatly accelerate the fire (remember the C and the O2 byproducts are just as flammable). You are using solid CO2 to demonstrate the accelerating effects on the Mg fire, a liquefied gas state of CO2 will accomplish the same acceleration. (Lots of YouTube video clips on this). Also do NOT use a HALON, halon substitute, or related “clean agent” fire extinguisher on these fires as they can greatly accelerate the fire as well. ANSUL states that HALON may also react with certain metals on fire.
7. As with any combustion demonstration be sure all flammables are well out of the area of the demonstration.
8. As with any combustion demonstration be sure to wear FRC (flame resistive clothing and PPE) to protect yourself.
9. Know where the closest safety shower is located and test it (unless it alarms) to be sure clear tepid water comes out. I found one specialty chemical manufacturer client had their safety shower feed by a live steam line! (which told me how often they were not testing it). One of my heartaches in the UCLA incident was that apparently her lab mates did not think to immediately use a fire extinguisher on her burning clothing and/or drag her immediately into a safety shower to extinguish smoldering clothing and cool her burns – instead they apparently tried splashing water on her from a sink (per C&EN articles).
10. I agree with Steven – purchase a manageable amount of Class-D Powder appropriate for Magnesium based fires (Class-D Dry Powders are formulated for differing metal fires - Magnesium, Sodium, Potassium, Lithium, Titanium and Sodium-Potassium alloy fires; so please be sure to obtain the Magnesium compatible version of the Class-D Powder). The container of the Powder should be moisture resistant as well. Use a metal scoop to gently apply the powder.
11. ANSUL (one of the manufacturers of Class D dry powders), has an excellent white paper at https://www.ansul.com/en/us/DocMedia/F-98172.pdf entitled “FIGHTING FIRES WITH DRY POWDER AGENTS”, which should be very helpful as an overview to this discussion. Additionally, ANSUL has an excellent Technical Bulletin at https://www.ansul.com/en/us/DocMedia/F-8086.pdf discussing in particular Metal Alkyl fire events.
12. If you do choose a hand portable Class D fire extinguisher, consider securing some professional training on the use of these specialized extinguishers. Class D extinguishers use, in most cases, completely different application techniques than dry chemical hand portable extinguishers - as you are trying essentially to gently “bury” the burning metal with the expelling agent. One of the precautions that ANSUL brings forth is the issue of re-ignition as a large amount of residual heat will be present.
13. One last 2 cents bit here. If you do try to extinguish the Mg-CO2 reaction (the CO2 as dry ice), remember the burning Mg will still be “fed” oxygen from the dry ice underneath the combusting mass. Until you can surround the mass totally with applied Class D powder the Mg will continue to combust.
14. Have good and safe demonstrations! Don’t become a “headline”, or the object of a Chemical Safety Board investigation. Let’s fire them up – their minds, not their clothes (sorry).
I hope these points are helpful Debbie.
All the Best,
If you cannot see the photos in the text I have attached the doc and pdf as well.
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We currently do a magnesium/dry ice demonstration in which about a meter of magnesium ribbon, closely coiled, is put into a well drilled into a block of dry ice. The ribbon is ignited using an electric lighter and another block of dry ice is slid over the magnesium. Cue spectacular bright flamage, sparks, and a glowing chunk of dry ice. One of my personal faves.
We’ve defaulted to having a Class D fire extinguisher on site for this particular demo. All of the other demos that produce flame can be managed with an ABC dry chem extinguisher.
But the Class D extinguisher is heavy (>75 pounds) and difficult to use. The fire extinguisher maintenance folks don’t want to maintain it, our on-site fire department would rather we not have it and would probably just let the metal fire burn itself out. I’ve also done a bit of digging and ABC dry chem will work on a magnesium fire, perhaps not as effectively as the Class D but it’ll work.
I’m inclined to stop hauling this thing around – it’s a back injury/workers comp claim waiting to happen – in favor of having just our ABC dry chem extinguisher on site.
What do you all think?
Debbie M. Decker, CCHO, ACS Fellow
Department of Chemistry
University of California, Davis
3467 Chemistry Annex
1 Shields Ave.
Davis, CA 95616
(530)754-7964 (T)/(530)304-6728 (cell)
Birkett's hypothesis: "Any chemical reaction
that proceeds smoothly under normal conditions,
can proceed violently in the presence of an idiot."
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