Sciencemadness Discussion Board - Questions about thermite

Questions about thermite

Metallus - 9-9-2013 at 09:46

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

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]

elementcollector1 - 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]

phlogiston - 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 Fe2O3 will definately work. Fe2O3 can be easily ground with a mortar and pestle to a consistency that works well (messy though!). I have always used pottery-grade Fe2O3 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).

Metallus - 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

) 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

, 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

elementcollector1 - 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.

Metacelsus - 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.

Metallus - 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?

Dornier 335A - 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.

Metallus - 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

Fantasma4500 - 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

Metacelsus - 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]

Dornier 335A - 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

Metacelsus - 10-9-2013 at 11:48

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

phlogiston - 10-9-2013 at 13:59

That '1 micron thermite' is amazing. I never knew that Fe2O3-based thermites could be that fast!

Metallus - 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?

Metacelsus - 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.

Oscilllator - 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

Dornier 335A - 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)

Metallus - 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

Metacelsus - 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.

Oscilllator - 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.

Bert - 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.

Oscilllator - 12-9-2013 at 23:01

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

MrHomeScientist - 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!

DFliyerz - 23-12-2014 at 18:55

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

roXefeller - 24-12-2014 at 04:53

You didn't mention it in the youtube details, what was the composition that you demonstrated?

DFliyerz - 24-12-2014 at 12:13

Quote: Originally posted by roXefeller

You didn't mention it in the youtube details, what was the composition that you demonstrated?

2.93 parts Iron(III) Oxide, 1 part powdered aluminum (lots of aluminum oxide too, from the powdering proccess). I did a 76.3% Iron Oxide and 27.3% Aluminum (logic, right?) mixture earlier, which burned a little more intensely, but both left lots of iron oxide over. You can see the instructions where I got the first mixture here.

Conor579 - 21-8-2015 at 20:45

I'm wondering if Thermite can be activated with the use of Sulfur.

[Edited on 22-8-2015 by Conor579]

blogfast25 - 22-8-2015 at 05:20

Quote: Originally posted by Conor579

I'm wondering if Thermite can be activated with the use of Sulfur.

[Edited on 22-8-2015 by Conor579]

What do you understand by 'activated'?

iloveloli - 24-9-2015 at 06:39

is it posible that i use PbO2/Mg and ZnO2/Mg to do thermite?

blogfast25 - 24-9-2015 at 06:47

Quote: Originally posted by iloveloli

is it posible that i use PbO2/Mg and ZnO2/Mg to do thermite?

DO NOT DO THIS: these would be dangerous flash powders. And imagine spewing lead all over the place...

crystal grower - 9-2-2016 at 10:09

Hello, I want to make thermite from B2O3 + Al. The thing is that I dont have magnesium strips.
Is it possible to start this reaction only with potassium permanganate and glycerin?
Or would be sulfur with aluminium better ?
And second question, what granularities should I use if I dont want to make a flash powder (I want to collect elemental boron from it so explosion would be bad).
Thanks for answer.

blogfast25 - 9-2-2016 at 15:42

Quote: Originally posted by crystal grower

Hello, I want to make thermite from B2O3 + Al. The thing is that I dont have magnesium strips.
Is it possible to start this reaction only with potassium permanganate and glycerin?
Or would be sulfur with aluminium better ?
And second question, what granularities should I use if I dont want to make a flash powder (I want to collect elemental boron from it so explosion would be bad).
Thanks for answer.

There's a preparation in Georg Brauer, I think.

http://library.sciencemadness.org/library/books/brauer_ocr.p...

KMnO₄ + glycerine should work OK.

These themites aren't flash powders, so not much to worry there.

j_sum1 - 9-2-2016 at 16:02

Quote: Originally posted by crystal grower

Al powder will work ok, but I recommend Mg powder for this one. That is because it allows you to recover the elemental boron afterwards simply by dissolving the MgO in hydrochloric acid.
MrHomeScientist has a good video of the procedure.

Ignition is not too difficult. I used KMnO4/glycerine with a strip of Mg but the extra Mg was probably overkill.

blogfast25 - 9-2-2016 at 16:46

https://www.youtube.com/watch?v=0QBCyOrjR2o

One problem with the reduction:

B₂O₃ + 3 Mg === > 2 B + 3 MgO

... is that the Enthalpy of Reaction is only - 531 kJ/mol of B₂O₃, not enough to reach the MP of MgO (2852 C), so element-slag separation is quite poor.

[Edited on 10-2-2016 by blogfast25]

Tdep - 9-2-2016 at 17:12

I have tried B2O3 and Al thermite before and it really does not want to burn. I had real difficulty lighting it, and I had to heat boost it just to get it to burn (I think I used sulfur.... it was two years ago). I was actually trying to get aluminium boride to try and produce diborane, and do a similar reaction to the one where people make silane and watch it combust in air. But I got nothing on addition of the heat boosted thermite to acid. The slag was ugly so I didn't bother recovering any elemental boron, not even sure if it was present.

blogfast25 - 9-2-2016 at 17:54

Quote: Originally posted by Tdep

I have tried B2O3 and Al thermite before and it really does not want to burn. I had real difficulty lighting it, and I had to heat boost it just to get it to burn (I think I used sulfur.... it was two years ago). I was actually trying to get aluminium boride to try and produce diborane, and do a similar reaction to the one where people make silane and watch it combust in air. But I got nothing on addition of the heat boosted thermite to acid. The slag was ugly so I didn't bother recovering any elemental boron, not even sure if it was present.

Yes, I believe S + Al is used as a heat booster in Al/B₂O₃ thermites.

An interesting thing to try would be Al/B₂O₃, heat boosted with nitrate or chlorate and Al.

Excess Al then produces borides like AlB₂ and AlB₁₂, or so I've read. Never tried it myself.

Crude B, strongly heated with Mg probably also yields borides.

[Edited on 10-2-2016 by blogfast25]

j_sum1 - 9-2-2016 at 19:00

Quote: Originally posted by blogfast25

https://www.youtube.com/watch?v=0QBCyOrjR2o

One problem with the reduction:

B₂O₃ + 3 Mg === > 2 B + 3 MgO

... is that the Enthalpy of Reaction is only - 531 kJ/mol of B₂O₃, not enough to reach the MP of MgO (2852 C), so element-slag separation is quite poor.

[Edited on 10-2-2016 by blogfast25]

That agrees with my findings. However, my goal was to dissolve the MgO in acid and leave boron powder and so element-slag separation was not really an issue.

blogfast25 - 9-2-2016 at 19:23

Quote: Originally posted by j_sum1

That agrees with my findings. However, my goal was to dissolve the MgO in acid and leave boron powder and so element-slag separation was not really an issue.

This is where a boric oxide/Al reaction, boosted with nitrate or chlorate could be an interesting proposition: there really isn't much of a problem to shoot past the relevant MPs. Just a basic guess would be that 0.2 mol of KClO₃ + 0.4 mol Al (per mol of boric oxide) would be enough to achieve about 2500 C, if not more.

Take the Enthalpy of the KClO₃ booster reaction to be about - 1300 kJ/mol of KClO₃, vaporisation of KCl included, so each 0.1 mol adds roughly - 130 kJ.

[Edited on 10-2-2016 by blogfast25]

j_sum1 - 9-2-2016 at 21:05

That could turn out to be cleaner. Just a question. What kind of temperature must be reached before crystalline boron occurs?

crystal grower - 10-2-2016 at 05:23

Thanks for every answer.
In the brauer book is written that you need 110g of boron trioxide , 115g Mg etc. my question is if it is possible to make the reaction only with few grams of each substance ??

blogfast25 - 10-2-2016 at 07:07

Quote: Originally posted by j_sum1

That could turn out to be cleaner. Just a question. What kind of temperature must be reached before crystalline boron occurs?

Not sure. Obviously going over the MP would help.

Quote: Originally posted by crystal grower

Thanks for every answer.
In the brauer book is written that you need 110g of boron trioxide , 115g Mg etc. my question is if it is possible to make the reaction only with few grams of each substance ??

I'd ballpark at no less than 50 g of mixture.

[Edited on 10-2-2016 by blogfast25]

woelen - 10-2-2016 at 07:59

Be careful with homemade boron oxide derived from boric acid. From experience I known that boron oxide is surprizingly difficult to obtain in a pure state. If you heat boric acid, then initially quite some water is split off, but removing the last amount of water is hard. H3BO3 can easily be converted to something with empirical formula HBO2, but driving off the last remains of water to obtain real B2O3 requires very strong heating. Also keep in mind that B2O3 is very hygroscopic and easily picks up water again. I once did the reaction with fine Mg-powder and when this is done with partially dehydrated boric acid, then the reaction is very violent, much more so than when pure B2O3 is used. I do not have personal experience with the Al-based reaction, but I can imagine that with this reaction a similar problem may exist.

blogfast25 - 10-2-2016 at 09:01

Quote: Originally posted by woelen

H3BO3 can easily be converted to something with empirical formula HBO2, but driving off the last remains of water to obtain real B2O3 requires very strong heating.

Yes, that's correct and often overlooked.