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Metallus
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[*] posted on 9-9-2013 at 09:46
Questions about thermite


Hi there

I was lurking around and found some discrepancies here and there about the thermite mixtures, ratios etc so I decided to gather all my questions in one topic:

1) The stechiometry of the reaction:
2Al + Fe2O3 ----> 2Fe + Al2O3

for 100g of Al (n = 100g/27 = 3,7mol), I'd need 3,7/2 mol of Fe2O3, meaning 1,85*159,69 = 295,42g. The ratio is more like 3:1 oxide-metal rather than 8:3 as somewhere else specified. Am I overlooking something? Is an excess of one of the two more favorable? Why?

2) Granulometry: the finer the powders are, the faster the reaction will be. However, if ultra fine powders are mixed, the reaction might be too fast and spread the reagents all around, therefore losing its "firepower". What are the "optimal" granumoletries for the two powders, the best compromise? I would be pleased if you could give exact numbers.

3) Would 100micron Fe2O3 powder and non-atomized Al work well, or would they give out a faint pitiful thermite reaction? What about 400mesh Fe2O3?

4) Ignition: several times I see the magnesium strips mentioned, but there is something off with what's said and what I've learned from books. First off, shouldn't pure magnesium burn when exposed to O2? Shouldn't it oxidize immediately to MgO? How can it be that they make rolls of a metal akin to sodium and it's even "safe"? Or perhaps those strips aren't pure magnesium but some metal alloy containing magnesium?

Secondly, I read that a lighter isn't enough to ignite it but you need a blowtorch: isn't there an alternative way of igniting the strip (that should then ignite the thermite mixture)? If I put some black powder on the top of a strip, would the temperature released from it be enough to ignite the strip? Would KNO3 + sugar be enough for it? Or perhaps KClO3 + sugar?

5) Ignition via KMnO4 and glycerin: do you really need "big" amounts of it, like 1 teaspoon of permanganate and few drops of glycerin, or even some mg will be enough? I mean, KMnO4 costs :D

5) Ignition via something else: do you know any other viable ways of igniting thermite mixture? Magnesium strips, KNO3/KClO3 + sugar, KMnO4 + glycerin, Black powder have already been mentioned; anything else?


Thanks for your attention, hoping you can solve these doubts of mine. Sadly where I live, if you don't order things online, you can't easily find them and if you do, they are horrible quality. This leads me to be very careful with money and so I'm trying to gather the most information possible before attempting my experiences.



Edited

[Edited on 9-9-2013 by Metallus]
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[*] posted on 9-9-2013 at 10:38


I think you're thinking of potassium permanganate and glycerin, not acetone. Acetone and KMnO4 do not react without the addition of sulfuric acid before the acetone, which creates horrifically dangerous manganese heptoxide. Way overkill for igniting thermite. Glycerin, on the other hand, does not require an initiation with sulfuric acid, and has a delayed burn time, which gives you some time to get away from your thermite.
I've had a lot of trouble igniting thermite. The mix that worked was very fine (I'd guess 400 mesh, if not finer) bright red iron (III) oxide, homemade aluminum powder from foil (~200 mesh?), a sulfur-aluminum charge on top and a small permanganate-glycerin mix on top of that. I think the sulfur and aluminum is what did it, as that was the only thing I changed.
Magnesium is a reactive metal, true, but not quite on the level with sodium: For instance, it does not react with water at room temperature, and the strips are fairly stable. They can be ignited with a blowtorch or chemical ignition.
KNO3 or KClO3 + sugar are, from what I've heard, good and reliable mixes. I believe they also need to be initiated with sulfuric acid to work properly, but do not pose the danger that Mn2O7 does.

If I'm wrong on any of this, other members can feel free to correct me - this is a mix of research and personal experience.

[Edited on 9-9-2013 by elementcollector1]




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[*] posted on 9-9-2013 at 11:58


regarding Granulometry:

It depends also on the amount of mixture you are using. For large amounts, a coarses particle size will work fine. For very small amounts, smaller particles are better. I can't give you 'exact' particle sizes. 400 mesh Fe<sub>2</sub>O<sub>3</sub> will definately work. Fe<sub>2</sub>O<sub>3</sub> can be easily ground with a mortar and pestle to a consistency that works well (messy though!). I have always used pottery-grade Fe<sub>2</sub>O<sub>3</sub> which is cheap and works great. I don't know the exact mesh size but it is quite fine.

Regarding magnesium strip and igniting it:

Firstly, magnesium is not nearly as reactive as sodium. It would corrode though (like calcium, barium and strontium which are more comparable in that respect), if it were not for a thin coating of MgO that forms on its surface upon exposure to air/oxygen. This protects magnesium metal againts further oxidation. For this reason, it can be used as a structural metal (you can find allow that are mostly Mg in various things like lawnmowers, engine blocks, pencil sharpeners).

I doubt you can reliable ignite Mg strip with the mixtures you mention. They burn so quickly that not much time is available to transfer the energy to the magnesium. A hot and slow mixture would work better.

Regarding alternative means of ignition:

I have used sparklers reliably. They are based on a barium nitrate/aluminium/iron mixture that burns very hot. I do like to run the sparkler through a hole in a sheet of cardboard, however, to prevent sparks from falling in the thermite and igniting it prematurely (although I have tested whether these sparks would ignite a bit of thermite once which did not work, so I don't think the risk is very great).




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[*] posted on 9-9-2013 at 12:05


Ye sorry, I meant KMnO4 + glycerin, I'll edit. The thing is that I've been reading the whole day about acetone peroxide and, later in the day, I had to move 20L of acetone in lab so the word "acetone" kinda got stuck in my head.

I've worked with Mn2O7 for a whole year, trying to figure out the "safest" ways to use it. Meh, even a little metal filament with the top imbued in that compound was enough to start fire on a little acetone (just to stay in theme :D) pond, quite outstanding (PS: Do not worry, I always work with very few mg and just 2-3 drops of H2SO4. As stated before, KMnO4 costs :D, can't go to waste). By the way, acetone reacts with KMnO4 even in KOH at 150°C, with the difference that MnO4- gets reduced to MnO4-- in a very non violent way.

Anyways, how did you grind aluminium foil in fine powder? I think a normal grinder would overheat and the blades get ruined... Also, could you tell me more about this aluminium-sulfur charge? Never tried it out, it's new to me.

Thanks for your answer



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[*] posted on 9-9-2013 at 12:44


The aluminum-sulfur charge stinks both before and after ignition, can be lit with a blowtorch, and is relatively stable. If you attempt to wash the resultant slag for any reason, I hope you like the smell of H2S...
For my aluminum, I use a coffee grinder on foil, adding foil until I can't add any more and grinding, and then repeating the process. This gives a dull gray powder, which can be boiled in isopropanol to make it a bit brighter.




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[*] posted on 9-9-2013 at 13:10


I find that a stoichiometric mix of aluminum powder and sodium chlorate works well to ignite thermite. Don't have the reagents too finely powdered, as you will then make flash powder. Also, I would be wary about storing the mixture for any amount of time. The reason I use it is because both reagents are very easy to make at home, and the mixture is easy to ignite.



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[*] posted on 9-9-2013 at 13:10


Can it be lit via other ways, like KClO3/KNO3 + sugar or blowtorch is the only way for everything?
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[*] posted on 9-9-2013 at 13:41


KClO3 and sugar, KClO3 and coarse Al or Fe2O3 and MgAl will all reliably ignite Fe2O3/Al thermite. All three mixtures can be ignited by a flame or fuse.

I have experimented a bit with different particle sizes, and the burn speed will increase down to submicron sizes. I have only had problems with incomplete burning with CuO based thermites.
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[*] posted on 10-9-2013 at 08:01


Thanks for the fast answers, they were much appreciated.

One more thing: in the mesh scale, which granulometry would it be better to start from in order to get a very good thermite? 400 mesh was already confirmed to be very good but what about 100 mesh? Is that too "big"? Is there an "optimal minimum"? Like "start from 200++ mesh" for very good thermite?

Also, what does exactly change? I mean, the reaction will probably be slower with a 100 mesh powder compared to a 400 mesh, but how much slower? Is it noticeably slower? Also, will it be able to reach 2k °C? Does it lose too much in terms of functionality? Again, I'm asking for a confirm because I need to know what granulometry I should avoid in order not to waste money. Or perhaps I'm just overthinking and even thick particles react anyways with decent results?

The application is small scale: circa 3-4ml by volume (I'm anal when it comes to chemicals: they cost a lot and I experiment very often, so I can't exactly use 300g of products everytime or I'd run out of money in no time).

Thanks for your attention
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[*] posted on 10-9-2013 at 09:31


i would definately say the potential of the decently slow oxidizer:sugar has been totally downrated..
you just take your strip, put a straw down over it, to this you just add some hot glue and when it has cooled down you add your KNO3/KClO3/whatever Sugar mix and you can ignite this by a normal lighter..
if you can however mold a still soft piece of rocket candy around it, i would say theres no way it will fail.. ofcourse im talking about it being sticked properly on the magnesium strip and not the most ignorant and k3wlish way (:
it does however burn at around 1500*C




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[*] posted on 10-9-2013 at 09:33


@Metallus
In my experience, thermite mixtures below 10 grams don't work too well; it needs a certain critical mass. CuO/Al thermite is an exception, however. The reaction is quite vigorous, even at small scales. It's more of a flash powder than a thermite!

[Edited on 10-9-2013 by Cheddite Cheese]




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[*] posted on 10-9-2013 at 11:32


Thermite can be made with aluminium powder of pretty much any particle size. It will get harder and harder to light with increasing particle size though. I successfully tested a two gram batch of pure, 1 µm Fe2O3 and 50 µm (280 mesh) Al once. It was very difficult to light, even with magnesium ribbon, but did burn to completion. I wouldn't recommend using Al coarser than 300 mesh on the scale you will work on.

The effects of different particle sizes can be seen in this video: http://www.youtube.com/watch?v=61gImPPFRrk
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[*] posted on 10-9-2013 at 11:48


My powders were much coarser, about 90 micron, so that's probably why my small batches failed miserably.



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[*] posted on 10-9-2013 at 13:59


That '1 micron thermite' is amazing. I never knew that Fe<sub>2</sub>O<sub>3</sub>-based thermites could be that fast!



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[*] posted on 10-9-2013 at 15:25


Just found this german blackhead aluminium which is a "darker" version of the aluminium powder: would it be viable for these purposes?
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[*] posted on 10-9-2013 at 18:41


I think so, although I don't have personal experience. Blackhead is just aluminum powder that's been milled with a few percent charcoal to inhibit oxidation.



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[*] posted on 10-9-2013 at 20:38


All of these methods of ignition are way to expensive, elaborate and smelly. Just use a sparkler! It works fine for me



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[*] posted on 10-9-2013 at 22:17


I have not tried german dark with Fe2O3, but I have talked to someone who has. He said that it burned as slowly as with 70 micron Al. This is most likely due to the increased oxide content and the charcoal.
I would recommend this kind of aluminium or similar: http://www.skylighter.com/mall/product-details.asp?id=1 (atomized, 23 - 48 micron)
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[*] posted on 11-9-2013 at 03:41


Thank you very much, that was very much appreciated.

One last thing: safety. When igniting thermite, what are the obvious precautions to take? I saw some video where small scale thermite seems to actually explode rather than simply burn. Also, the radius of the molten iron spread all around seemed quite large in some cases so ye, how long should be the fuse in order to run a way (since the fuse is variable, how much time to I need to "run away") ? Is it preferable to ignite it inside some kind of container rather than in the open air? Are there any kind of gloves that would give me the time to take them off before molten iron pierces through them?

Thanks again for the answers, you have all been very helpful :)
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[*] posted on 11-9-2013 at 08:57


Quote: Originally posted by Metallus  
Are there any kind of gloves that would give me the time to take them off before molten iron pierces through them?


You could try asbestos gloves, if you can find any.




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[*] posted on 12-9-2013 at 01:52


I think the dangers of asbestos gloves would outweigh the potential benefits. Using a modicum of caution will ensure you don't get molten iron on your hands. The chances of an accidental ignition are pretty much negligible.



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[*] posted on 12-9-2013 at 06:31


Welders leathers are enough for protection from a few drops of flying molten Iron.

But having the sense to use remote ignition would be better.




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[*] posted on 12-9-2013 at 23:01


Completely off-topic: Bert is that a terry pratchet quote I detect in your signature?



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[*] posted on 19-9-2013 at 06:29


My ignition method for thermite is a combination of two other methods. I first make the thermite into a pile with a depression at the top, like a little volcano. To this I add potassium permanganate to fill the 'crater', usually about the size of a dime. Then I take a short strip of magnesium ribbon and push that down through the permanganate, so it extends down into the thermite. I then add glycerin. This reacts with the KMnO4, which ignites the Mg, which lights the thermite. Seems complex, but I like it because it has a lot of benefits: 1) Glycerin ignition gives plenty of time to get away (about 20 seconds usually). 2) It only fails very rarely. 3) You don't need a blowtorch or other tools.
I've tried KMnO4/glycerin and Mg ribbon separately, but in my experience they can be unreliable. Permanganate doesn't seem to burn hot enough, and Mg ribbon tends to fall apart and die out before it burns down to the level of the thermite.

As for precautions, it depends on the type and amount of thermite you are working with. Regular old iron oxide thermite doesn't throw sparks all that far, but copper oxide can almost be explosive (as mentioned before). For any composition, personally I would stay at least 15 feet away. Also I recommend wearing sunglasses, since thermites produce very bright, UV-rich light.

My experience with particle size is interesting. As others have said, smaller particles lead to faster reaction rates. I started out with 80 mesh aluminum, the consistency of fine(ish) sand. I tried a chromium(III) oxide thermite with this, and never got it to work. It would sputter a bit, but never ignite the whole charge. I switched to 400 mesh Al, and the reaction went perfectly. I can't comment on optimum particle sizes, but in my experience the finer the better!
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[*] posted on 23-12-2014 at 18:55


I have managed to ignite Thermite with just a match, as can be seen here.
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