redj85
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spontaneous combustion with ZnO
Hello, I was hoping you could please help me or orient me in the right direction to find an answer to my problem. The company I work for produces
brass alloys in a furnace and one of our byproducts is zinc oxide dust. We sell this zinc oxide to other manufacturers. The last shipment we sent on a
truck combusted, it burned out of the plastic bag, burned the wooden pallet it was on and it burned a hole in the wooden bed on the truck. This has
only happened twice in the 30 yr span of the company. We keep our zinc oxide for two weeks before we sell it, to make sure it has cooled down, we also
use two plastic bags, closed off with tape to avoid contact with moisture. Based on a past profile done on our material, some of the other elements
found in our zinc oxide dust are: H2O 1.92%, Cd 0.13%, Cu 0.66%, Fe 0.17%, Pb 7.43%, Mn 0.01%, K 0.36%, Na 1.18%, Ca 0.06%, Mg 0.01%, Al 0.02% Do you
have any idea what could have caused this reaction? Can you please suggest a source I can look into for more information and answers? We don’t want
to risk shipping this substance if there is a chance it will combust and put people in danger.
Thank you very much!
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aga
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What kind of Plastic, and is that the same plastic that you normally use ?
Clearly something changed, so it's a straightforward fault-finding excercise.
Presumably you have data from the first ignition event to analyse as well.
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Artemus Gordon
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I sure as heck hope your company is not the source for my sunscreen!
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blogfast25
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@redj85:
Is your ZnO byproduct the result of oxidised Zn vapours generated during the alloying process? Zn's boiling point is much lower than that of Cu, so a
high temperature molten brass alloy would have some Zn vapour pressure.
In contact with air that could form ZnO.
But free, unoxidised Zn powder in your ZnO product could cause the problem. And it would not easily show up in an elemental analysis either.
[Edited on 10-5-2016 by blogfast25]
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hissingnoise
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| Quote: | | Based on a past profile done on our material, some of the other elements found in our zinc oxide dust are: H2O 1.92%, Cd 0.13%, Cu 0.66%, Fe 0.17%, Pb
7.43%, Mn 0.01%, K 0.36%, Na 1.18%, Ca 0.06%, Mg 0.01%, Al 0.02%. |
Though their quantities are small, the most likely culprits here may be Al and Mg?
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woelen
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I go with blogfast25's answer. Any metal, also Mg and Al, which are present as oxide cannot contribute to combustion. Only free non-oxidized metal can
in some way produce enough heat when brought in contact with air.
Is the ZnO really ZnO, or is it deficient in oxygen? What color does your ZnO have. Frequently, materials being deficient in oxygen have very
different colors. E.g. WO3 is pale yellow, but even when it is 1% deficient in oxygen, it is very dark blue. Maybe a similar thing is true for ZnO, I
do not have personal experience with that.
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AJKOER
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Given the rarity of the reaction, a few infrequent events occuring together may be responsible.
One scenario, a bag containing the oxide mix is waiting outside. It may have been torn allowing access to oxygen and excess moisture especially in
months with higher rainfall. Further, assuming it was sitting on top of the pile, add sunlight/heating. It is then loaded into the truck and sits on
the floor where it is subject to pressure and friction from a rough ride which could produce static electricity.
Many possible chemical routes. Likely the action of Zn (per prior comments) on H20 with heat/light liberating H2. A spark from static electricity
ignites the H2, and starts a thermite reaction.
Another scenario, a nail in the floor of the truck causes leakage. The oxide mix interacts with residual chemicals (pool chemicals, for example) from
a prior run.
Solution: place plastic oxide mix bag in a cardboard box to avoid punctures, light and friction exposure.
[Edited on 10-5-2016 by AJKOER]
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blogfast25
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Quote: Originally posted by woelen  |
Is the ZnO really ZnO, or is it deficient in oxygen? What color does your ZnO have. Frequently, materials being deficient in oxygen have very
different colors. E.g. WO3 is pale yellow, but even when it is 1% deficient in oxygen, it is very dark blue. Maybe a similar thing is true for ZnO, I
do not have personal experience with that. |
If this is so, then free oxygen (air) in the powder, heat from friction during transport and perhaps some catalytic effect from a contaminant
(lead?) might cause the occasional 'spontaneous', exothermic reaction.
In that case I see no real other solution than to subject the product to an additional roasting to ensure all Zn is present as ZnO. Not something you
want to do with a cheap and cheerful by-product but there you go.
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Cryolite
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| Quote: Originally posted by woelen | I go with blogfast25's answer. Any metal, also Mg and Al, which are present as oxide cannot contribute to combustion. Only free non-oxidized metal can
in some way produce enough heat when brought in contact with air.
Is the ZnO really ZnO, or is it deficient in oxygen? What color does your ZnO have. Frequently, materials being deficient in oxygen have very
different colors. E.g. WO3 is pale yellow, but even when it is 1% deficient in oxygen, it is very dark blue. Maybe a similar thing is true for ZnO, I
do not have personal experience with that. |
Zinc oxide does indeed have a different color when oxygen deficient, and this is very easy to demonstrate: just take some zinc oxide and heat it with
a blowtorch. The high temperatures knock some oxygen out of the crystal lattice, forming a very nice golden yellow compound. Unfortunately, oxygen
deficient ZnO reoxygenates at room temperature forming the boring white powder again, so this is rather unlikely...
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blogfast25
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| Quote: Originally posted by Cryolite |
Zinc oxide does indeed have a different color when oxygen deficient, and this is very easy to demonstrate: just take some zinc oxide and heat it with
a blowtorch. The high temperatures knock some oxygen out of the crystal lattice, forming a very nice golden yellow compound. Unfortunately, oxygen
deficient ZnO reoxygenates at room temperature forming the boring white powder again, so this is rather unlikely... |
Considering the relatively low Enthalpy of Formation of ZnO, some dissociation of ZnO at high temperature sounds very plausible.
But I'm suggesting something slightly differently: that some of the Zn vapours during alloying didn't oxidise quite completely to ZnO.
[Edited on 10-5-2016 by blogfast25]
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hissingnoise
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| Quote: | | But I'm suggesting something slightly differently: that some of the Zn vapours during alloying didn't oxidise quite completely to ZnO.
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Indeed, as woelen said, it seems more likely than unoxidised Mg or Al in the mix . . . ?
[Edited on 10-5-2016 by hissingnoise]
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Morgan
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TIdbits
"This is a list of common brass alloys, their chemical compositions and the uses of the different types of brass."
http://chemistry.about.com/od/alloys/a/Brass-Alloys.htm
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AJKOER
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Here is a source http://pubs.acs.org/doi/abs/10.1021/j100883a026 indicating that ZnO can increase the photo-sensitivity of Lead salts in wetted mixtures with ZnO
via a postulated process described by some as "intramolecular electrolysis" (wow!?) similar to silver halide decomposition under photolysis.
This could be a source of O2 and Pb in the case of PbO. Also, the author notes that the usual chemical action of ZnO/H2O with heavy metals is the
formation of basic salts, which I suspect would be generally stronger in fire suppression (and not ignition!).
Other metal (or non-metal) oxides contained in the mix could similarly be photo-sensitived by the ZnO leading to perhaps increased localized oxygen
content or the introduction of a zero valence metal (or hydrogen from water, sulfur from any sulfides, chlorine from any chlorides,,....).
So, we need a torn bag, rain exposure followed by strong sunlight and a rough road.
[Edit] More interesting is the apparent fact that the above path best explains why the conflagration could be restricted to a single bag! If, as
argued previously by many, including myself, that metallic Zn is likely present always as a consequence of processing, along with sufficient air, the
single bag ignition should have been sufficient to ignite others (assuming there was more than one bag in the historical record of shipments that
resulted in a fire), increasing the observed magnitude of the event. If not, then only zero valence metal was produced in a single bag along with
oxygen, as this path ostensibly professes!
[Edited on 11-5-2016 by AJKOER]
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