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Course MgAl/Potassium Nitrate composition heating up


Kalifireworks

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Posted (edited)

Hello everyone,

lately I have been confronted with a issue that i would never expect, just for context I have some decent knowledge in this hobby.

 

The Issue is I have been Developing some Comps and came up with a Composition consisting of the following:

x% Potassium Nitrate

x% Charcoal

x% Sulfur

x% MgAl 40-70 Mesh (pretty course)

x% Dextrin

 

This composition has been wetted with 3% Water and pressed with arround 7500 PSI into paper tubes. Once pressed they gradualy get more and more hot, until Probably arround 50°C (122 Fahrenheit) when it stops and cools down again, I didnt notice any Ammonia smell only a slight sulfury paper smell.

 

Same issue also happened with a very similar comp with the addition of Iron oxide and a bit finer 70 Mesh magnalium (different vendor). This time wetted with 2% moisture, they got significantly less hot but definately noticably warm (Measured 28°C (83 Fahrenheit)).

 

Really confused and scared to keep using this comp in this Setup Im hoping somebody here has a Idea what could cause this reaction, litterature has told me that MgAl is less prone to reactions with water than Magnesium or Aluminium. Especially since there is only 2-3% water in this mix.

 

Is it due to the Course size and high pressure and thus high density? 

 

 

 

Edited by Kalifireworks
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Sounds like a typical nitrate/Al reaction.

Where are you located & what's the source(s) of your MgAl? If the alloy is Al-heavy (like aircraft MgAl...) and not the 'typical' 50:50 used in fireworking, it may be more prone to nitrate reactions in the presence of water.

Heating up is rather surprising given the low concentration of added water. Boric acid addition impairs nitrate-Al reactions but may directly attack Mg. In a small test batch, if adding boric acid 1% stops the comp from heating up during/after pressing, then you might infer that your MgAl is Al-heavy.

Just spitballing here.

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Theoretically magnelium is less prone to react with water but I do have experienced issue of getting star composition heating up many times specially green star batch which uses barium nitrate magnelium sulphur parlon dextrin.

Thats the reason why two varieties of magnelium is sold in india.

One is uncoated another id coated magnelium.

If you have coated magnelium give a try with this magnelium.

It is coated with potassium dichromate as per suppliers discussion.

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2 minutes ago, Zumber said:

Thats the reason why two varieties of magnelium is sold in india.

One is uncoated another id coated magnelium.

 

Interesting. In the US at least, I've never seen fireworking MgAl that is dichromate treated. I haven't had any issues with barium nitrate comps heating up with MgAl, but if the comp has parlon in the recipe then I'm probably going to omit the dextrin and bind with acetone and not water anyways. The dissolved parlon probably provides additional protection to MgAl surfaces.

Shimizu's FAST, Table 12, indicates similar reactivity of both barium nitrate and KNO3 with untreated metals. Slight rxn with Al at cooler temps but prone to runaway rxns at higher temps (especially with water present)--the typical nitrate/Al rxn. Slow rxn with MgAl (50:50) and considerable nitrate rxn with Mg, hence, why pure Mg often needs surface passivation treatment.

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Yeah...now a day magnelium is not of superior quality and we hardly get 50:50 magnelium rather more aluminium than magnesium....only reputed industry truly provides 50:50 with a bit high price but its worth irrespective of price.

Acetone parlon is good idea but its expensive and we rely on rolled stars only .....our batch is higher 10 kg plus at a time.....

 

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Posted (edited)

The Magnalium is a 50/50 Alloy.

It is Both from trustful pyrotechnical chemical suppliers, one from Germany, the other from poland. The very course Magnalium is dark and looks like it oxidized plenty, the 70 mesh Magnalium is really Shiny Silver like it was manufactured yesterday. 

 

Either way both Magnaliums heated up, so the Natural oxide layer doesnt seem to matter at all. I dont know if the powders are coated, but probably not.

I never had any issues when using 230 Mesh MgAl, but I also never pressed any comps with it to such immense densities.

Maybe the reaction is minimal and just has to do with the high density the comp is pressed to, again I never felt any warming up of the powdered comp, only couple second after pressing it it starts to get hotter and hotter

Edited by Kalifireworks
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  • 2 weeks later...
On 5/30/2024 at 11:35 AM, Kalifireworks said:

The Magnalium is a 50/50 Alloy.

Technically, magnalium isn't a true alloy, but an intermetallic mix. A true alloy has it's own unique properties, distinct from the parent metals.

WSM B)

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I would check if your potassium nitrate is alkaline, which has been the case with some agricultural grades. A recrystallization will fix this problem if you dont have other sources. 

Some types of charcoal can also create unwanted reactions with mgal, likely because of alkaline ash content. 

A third possibility is if the high pressure which can actually crush the coarse mgal grains and cause oxidation of the freshly exposed metal surfaces. 7500psi is quite insane and unnecessary much for a simple tubular effect I think, unless if the effect is a rocket of some kind. This problem has been reported by another pyrotechnist dry pressing a comp with coarse MgAl. 

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The Potassium nitrate is Nitroparis, made in spain for fireworks.

 

I use the high pressure so those inserts survive even beeing shot out of blank firearms. Together with the dextrin the pressed composition get Rock hard.

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1 hour ago, Kalifireworks said:

The Potassium nitrate is Nitroparis, made in spain for fireworks.

 

I use the high pressure so those inserts survive even beeing shot out of blank firearms. Together with the dextrin the pressed composition get Rock hard.

Ok I understand the need for a higher density then. 

But from what I read in your post nr 6 in this thread, I am quite certain you are having a reaction caused by crushing of the coarse MgAl grains under high pressure. The MgAl is likely oxidized because of the increased surface resulting from the crushing under pressure. There is no solution to this other than avoiding the use of coarse MgAl in this application, I think. 

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