Sciencemadness Discussion Board

Ammonium nitrate and zinc: Why so sensitive?

Chisholm - 9-7-2017 at 07:22

In Forensic Investigation of Explosives, 2nd Edition (2012), one finds this tidbit:

"Mixtures of ammonium nitrate and zinc have been known to detonate when heated well below the melting point of ammonium nitrate. Many fatal accidents in the mining industry have been attributed to mixtures of AN and zinc. This is the only ammonium nitrate mixture the author knows of that is so unstable."

While I haven't actually had a detonation, sub-gram-level experiments* I performed comparing powdered AN+ icing sugar in aluminium foil (cookoff temperature: 180ºC) with powdered AN + zinc powder in aluminium foil revealed that the zinc mixture begins smoking with less heat, and produces more copious vapor with less effort.

*Both used stoichiometric ratios of oxidizer to fuel.

Most fuels and/or additives that destabilize AN do so for well-explained reasons: chlorates and permanganates promote metathetical formation of dangerously sensitive compounds; sulfur and icing sugar have low melting/decomposition points; copper powder forms an ammine complex in the presence of trace moisture; and so on.

The fact that the author mentioned above is Dr. Kirk Yeager (chief explosives technician for the Federal Bureau of Investigation), and that even he didn't have an explanation (at least, as of 2012), is curious.

If ammine complexes of zinc nitrate were primary explosives, you'd think they would be mentioned somewhere. Especially given that ammine complexation is mentioned on the very same page as the reason for the sensitivity of AN+Cu mixtures.

So what is the precise reason for the sensitivity?

PHILOU Zrealone - 9-7-2017 at 10:29

I have typed a good reply to your request...but when pushing onto the "post reply" button...it is all gone lost into the electronic internet space...

I hate those SMF bugs :-(...

I don't feel the mood like retyping all now ...so maybe another day...

Fulmen - 9-7-2017 at 13:25

I remember playing with AN/Zn/NH4Cl as a novelty as it will ignite from a drop of water. Even got a nasty burn when a freshly prepared mixture self ignited within minutes and sprayed my hand with molten slag. So be careful, these mixtures have zero shelf life.

clearly_not_atara - 9-7-2017 at 17:58

Zn2+ is a Lewis acid so maybe it pisses off the nitrate anion once even a tiny bit forms

PHILOU Zrealone - 12-7-2017 at 13:21

Quote: Originally posted by Chisholm  
In Forensic Investigation of Explosives, 2nd Edition (2012), one finds this tidbit:

"Mixtures of ammonium nitrate and zinc have been known to detonate when heated well below the melting point of ammonium nitrate. Many fatal accidents in the mining industry have been attributed to mixtures of AN and zinc. This is the only ammonium nitrate mixture the author knows of that is so unstable."

While I haven't actually had a detonation, sub-gram-level experiments* I performed comparing powdered AN+ icing sugar in aluminium foil (cookoff temperature: 180ºC) with powdered AN + zinc powder in aluminium foil revealed that the zinc mixture begins smoking with less heat, and produces more copious vapor with less effort.

*Both used stoichiometric ratios of oxidizer to fuel.

Most fuels and/or additives that destabilize AN do so for well-explained reasons: chlorates and permanganates promote metathetical formation of dangerously sensitive compounds; sulfur and icing sugar have low melting/decomposition points; copper powder forms an ammine complex in the presence of trace moisture; and so on.

The fact that the author mentioned above is Dr. Kirk Yeager (chief explosives technician for the Federal Bureau of Investigation), and that even he didn't have an explanation (at least, as of 2012), is curious.

If ammine complexes of zinc nitrate were primary explosives, you'd think they would be mentioned somewhere. Especially given that ammine complexation is mentioned on the very same page as the reason for the sensitivity of AN+Cu mixtures.

So what is the precise reason for the sensitivity?

There are a few known fact that may point out towards usual suspects working alone or as a team...
---------------------------------------------------
1°) It is known that NH4NO3/NH4Cl/Zn mix catches fire spontaneously when moist; a single water drop will start the reaction.
The NH4Cl is sufficiently acidic to be very corrosive towards the metallic Zn surface...
2 NH4Cl + Zn --> ZnCl2 + H2 + 2 NH3 + heat
ZnCl2 + 4 NH3 --> Zn(NH3)4Cl2 + heat

2°) NaCl may also sensitize the NH4NO3/Zn mix...
The halide anion is a corrosion catalyst for metals
Discretely NH4Cl is formed...
NaCl + NH4NO3 <==> NaNO3 + NH4Cl

3°) Zn/S is used for some rocket propellant...due to reaction heat the Zn and S both melt and turn into a gas and burn blue into the outlet.
S melt arround 115°C and Zn arround 420°C.
S boils arround 445°C while Zn arround 907°C.
So into the NH4NO3/Zn mix the Zn will vapourise into the hot burning mix and turn the mix to be even more intimately mixed.

4°) Zn is a strong reductor while NO3(-) is an oxydizer...
Depending onto the pH one may get Zn(2+) and...
NO2(-)
N2
NH2OH
NH3

5°) NH4NO3 is also quite acidic and reactive towards some reductive metals when moist (Al, Mg, Zn, ...)...but less so than NH4Cl...; so even if chloride anion is absent (as NaCl or NH4Cl), the heating will take care of the activation energy barrier and allow for the speeding up of the surfacial oxydoredox
---------------------------------------------------

From the above points 1°) to 5°) one may conclude that into a mix of Zn/NH4NO3...one may find the following explosive compounds:
*Zn(NH3)4(NO3)2 (moderate sensitivity and stable)
*Zn(NH3)4(NO2)2 (probably sensitive and unstable - decomposing on its own)
*Zn(NH2OH)4(NO3)2 (or Zn(NH2OH)2(NO3)2) (probably sensitive and stable)
Two or four NH2OH into the complex depending if NH2-OH complexate like NH3 or like NH2-NH2 thus as monodentate or bidentate ligand...
*Zn(NH2OH)4(NO2)2 (or Zn(NH2OH)2(NO2)2) (probably sensitive and unstable - decomposing on its own)
*NH4NO2 (sensitive and unstable - decomposing on its own)
*HONH3NO3 (sensitive and stable)
*HONH3NO2 (probably sensitive and unstable -decomposing on its own)

Many of the above compounds are powerful HE (high explosives) with VOD (velocity of detonations) ranking from 4 to 8 km/s...and some decompose on their own generating heat and moisture...
This explains that a single drop of water is able to start a vigorous reaction when NH4Cl is present...and propagate it very fast...because the reaction releases even more water and heat...the reaction is thus auto-catalytic...
HONH3NO2 --> N2 + 2 H2O + 1/2 O2
NH4NO3 --> N2 + 2 H2O + 1/2 O2
NH4NO2 --> N2 + 2 H2O

In the absence of NH4Cl or Cl(-), the heating afwords some moisture migration and micro-melting zones where the reactions starts...even if the overal mass is not macroscopically melting.

The above speculations would need further study to confirm what is effectively happening...but the truth must not be far from the depicted story...


[Edited on 12-7-2017 by PHILOU Zrealone]

Chisholm - 14-7-2017 at 05:47

Quote: Originally posted by PHILOU Zrealone  
Quote: Originally posted by Chisholm  
In Forensic Investigation of Explosives, 2nd Edition (2012), one finds this tidbit:

"Mixtures of ammonium nitrate and zinc have been known to detonate when heated well below the melting point of ammonium nitrate. Many fatal accidents in the mining industry have been attributed to mixtures of AN and zinc. This is the only ammonium nitrate mixture the author knows of that is so unstable."

While I haven't actually had a detonation, sub-gram-level experiments* I performed comparing powdered AN+ icing sugar in aluminium foil (cookoff temperature: 180ºC) with powdered AN + zinc powder in aluminium foil revealed that the zinc mixture begins smoking with less heat, and produces more copious vapor with less effort.

*Both used stoichiometric ratios of oxidizer to fuel.

Most fuels and/or additives that destabilize AN do so for well-explained reasons: chlorates and permanganates promote metathetical formation of dangerously sensitive compounds; sulfur and icing sugar have low melting/decomposition points; copper powder forms an ammine complex in the presence of trace moisture; and so on.

The fact that the author mentioned above is Dr. Kirk Yeager (chief explosives technician for the Federal Bureau of Investigation), and that even he didn't have an explanation (at least, as of 2012), is curious.

If ammine complexes of zinc nitrate were primary explosives, you'd think they would be mentioned somewhere. Especially given that ammine complexation is mentioned on the very same page as the reason for the sensitivity of AN+Cu mixtures.

So what is the precise reason for the sensitivity?

There are a few known fact that may point out towards usual suspects working alone or as a team...
---------------------------------------------------
1°) It is known that NH4NO3/NH4Cl/Zn mix catches fire spontaneously when moist; a single water drop will start the reaction.
The NH4Cl is sufficiently acidic to be very corrosive towards the metallic Zn surface...
2 NH4Cl + Zn --> ZnCl2 + H2 + 2 NH3 + heat
ZnCl2 + 4 NH3 --> Zn(NH3)4Cl2 + heat

2°) NaCl may also sensitize the NH4NO3/Zn mix...
The halide anion is a corrosion catalyst for metals
Discretely NH4Cl is formed...
NaCl + NH4NO3 <==> NaNO3 + NH4Cl

3°) Zn/S is used for some rocket propellant...due to reaction heat the Zn and S both melt and turn into a gas and burn blue into the outlet.
S melt arround 115°C and Zn arround 420°C.
S boils arround 445°C while Zn arround 907°C.
So into the NH4NO3/Zn mix the Zn will vapourise into the hot burning mix and turn the mix to be even more intimately mixed.

4°) Zn is a strong reductor while NO3(-) is an oxydizer...
Depending onto the pH one may get Zn(2+) and...
NO2(-)
N2
NH2OH
NH3

5°) NH4NO3 is also quite acidic and reactive towards some reductive metals when moist (Al, Mg, Zn, ...)...but less so than NH4Cl...; so even if chloride anion is absent (as NaCl or NH4Cl), the heating will take care of the activation energy barrier and allow for the speeding up of the surfacial oxydoredox
---------------------------------------------------

From the above points 1°) to 5°) one may conclude that into a mix of Zn/NH4NO3...one may find the following explosive compounds:
*Zn(NH3)4(NO3)2 (moderate sensitivity and stable)
*Zn(NH3)4(NO2)2 (probably sensitive and unstable - decomposing on its own)
*Zn(NH2OH)4(NO3)2 (or Zn(NH2OH)2(NO3)2) (probably sensitive and stable)
Two or four NH2OH into the complex depending if NH2-OH complexate like NH3 or like NH2-NH2 thus as monodentate or bidentate ligand...
*Zn(NH2OH)4(NO2)2 (or Zn(NH2OH)2(NO2)2) (probably sensitive and unstable - decomposing on its own)
*NH4NO2 (sensitive and unstable - decomposing on its own)
*HONH3NO3 (sensitive and stable)
*HONH3NO2 (probably sensitive and unstable -decomposing on its own)

Many of the above compounds are powerful HE (high explosives) with VOD (velocity of detonations) ranking from 4 to 8 km/s...and some decompose on their own generating heat and moisture...
This explains that a single drop of water is able to start a vigorous reaction when NH4Cl is present...and propagate it very fast...because the reaction releases even more water and heat...the reaction is thus auto-catalytic...
HONH3NO2 --> N2 + 2 H2O + 1/2 O2
NH4NO3 --> N2 + 2 H2O + 1/2 O2
NH4NO2 --> N2 + 2 H2O

In the absence of NH4Cl or Cl(-), the heating afwords some moisture migration and micro-melting zones where the reactions starts...even if the overal mass is not macroscopically melting.

The above speculations would need further study to confirm what is effectively happening...but the truth must not be far from the depicted story...


[Edited on 12-7-2017 by PHILOU Zrealone]


So ammine complexation almost certainly plays some role, as I suspected. That would fit the overall picture, where other reducing metals such as magnesium and aluminium—while sensitizing the AN and producing mixtures that can be detonated by strong shock—produce mixtures less prone to detonating from heat, and which require the AN to begin melting (app. 169.6ºC) before such a reaction occurs.

Melgar - 4-8-2017 at 08:50

Quote: Originally posted by PHILOU Zrealone  
I have typed a good reply to your request...but when pushing onto the "post reply" button...it is all gone lost into the electronic internet space...

I hate those SMF bugs :-(...

I don't feel the mood like retyping all now ...so maybe another day...

Get the "Lazarus" extension if you use either Chrome or Firefox. It saves all the data you ever type into text boxes, so you can bring it back from the dead when a web page craps out on you.

PHILOU Zrealone - 4-8-2017 at 11:27

Quote: Originally posted by Melgar  
Quote: Originally posted by PHILOU Zrealone  
I have typed a good reply to your request...but when pushing onto the "post reply" button...it is all gone lost into the electronic internet space...

I hate those SMF bugs :-(...

I don't feel the mood like retyping all now ...so maybe another day...

Get the "Lazarus" extension if you use either Chrome or Firefox. It saves all the data you ever type into text boxes, so you can bring it back from the dead when a web page craps out on you.

Thank you Melgar for that valuable information :D:):P...

woelen - 7-8-2017 at 13:12

There are more mixes with the property that adding a little water sets of the mix. The water is essential for setting off, by dissolving the soluble reactant and allowing the concentrated solution to react with the metal in the mix. This reaction produces sufficient heat to have the remaining solid mix reacting as well, with fire and smoke.

Some other examples:
- Mix of finely ground AgNO3 plus Mg-powder. The dry mix is stable, but even adding a tiny amount of water leads to ignition and a bright white flash of fire, with molten silver droplets spraying around. If you want to try this, do not use more than a few tenths of a gram of this mix.
- Mix of crushed I2 and Mg powder. Adding a small drop of water leads to puffs of violet iodine vapor and if you use a somewhat larger pile of the mix (a few grams), you may get ignition of the mix.

AJKOER - 27-9-2017 at 10:52

Here is my take on the possible reaction mechanics.

First, as occurs in the case of the metal Aluminum (see, for example, equation (3.7) in a doctoral thesis from 2008, "Alkaline dissolution of aluminum: surface chemistry and subsurface interfacial phenomena", by Saikat Adhikari, link: https://www.google.com/url?sa=t&source=web&rct=j&...), I would argue similarly with zinc, the creation of the metal hydroxide directly from the metal, proceeds with the release of electrons per the reaction:

Zn + 2 OH- → Zn(OH)2 + 2 e-

Note, the reaction is charged balanced. Based on the above reaction, if we introduce a source of H+ from say:

NH4+ = NH3 + H+

Upon adding H+ to each side, in neutral to acidic conditions:

Zn + 2 H2O → Zn(OH)2 + 2 (H+ + e-)
H+ + e- → •H
•H + •H → H2

And also, a further possible reaction in the presence of nitrate, with either prehydrated or totally solvated electrons, being reported as readily scavenged by nitrate:

e(p)-/e(aq)- + NO3- + H2O -> NO2 + 2OH- (Source: see eq. (5) in JAERI-Conf 95-003, "5. 6 Radiolysis of Concentrated Nitric Acid Solutions R. Nagaishi" by P.Y. Jiang, et al, link: https://www.google.com/url?sa=t&source=web&rct=j&... )

Or, one could rewrite the above reaction upon adding H+ to both sides as:

(H+ + e(p)-/e(aq)-) + NO3- + H2O → OH- + •NO2 + H2O

Or, equivalently, after eliminating water, one could expressed the above in the form of a radical based charge transfer/oxygen extraction reaction (since H+ + e- = •H):

•H + NO3- → OH- + •NO2

(and, I suspect similarly the process continues with say any created nitrite from a water interaction with NO2, then forming correspondingly nitric oxide:

•H + NO2- → OH- + •NO )

Finally, the creation of the very unstable ammonium nitrite :

2NO2 + 2(NH3•H2O) → NH4NO2 + NH4NO3 + H2O (Source: https://chemiday.com/en/reaction/3-1-0-8045)

Or, a gas phase reaction with NH3, NO, NO2 and water vapor at RT, creating a dry ammonium nitrite powder:

2 NO2 + 2 NO + 4 NH3 --> 2 NH4NO2 + 2 H2O + 2 N2 (Source: see http://www.sciencemadness.org/talk/viewthread.php?tid=23807&...)

which in the presence of zinc powder and heat likely detonates.

[Edited on 28-9-2017 by AJKOER]