metalresearcher
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CaSO4 + Al powder + Zn snippets: like Thermite
Today I mixed 40g CaSO4 (burnt-out crumbled, i.e. anhydrous plasterboard) with 20g Al powder and some snippets Zn metal.
I ignited with KClO3 + sugar and it was a quick (and very hot) deflagration as the thin steel container collapsed. The green Zn flames are clearly
visible and the Zn metal evaporates and burns quickly as it cannot withstand the high temperatures.
See the video (at the end in 1:10 slow motion).
http://www.metallab.net/jwplayer/video.php?f=/clips/Plaster%...
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PHILOU Zrealone
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Known themite like reaction.
Quite dangerous because of the gaseous products...SO2
Nice video...like a volcano especially in slow motion...You really need to improve your ignition system   because burning your fingers is not friendly
nor Professional 
PH Z (PHILOU Zrealone)
"Physic is all what never works; Chemistry is all what stinks and explodes!"-"Life that deadly disease, sexually transmitted."(W.Allen)
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blogfast25
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Careful with the slag: contains quite a bit of CaS. Combined with water that means H2S. Dispose off properly!
SO2? There's should be very little of that. Never smelled any when I did it.
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metalresearcher
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I know that. Already after a few minutes it stinks to H2S. So I put it into a plastic bag as soon it is cooled off.
During the experiment I wore a gas mask due to SO2 appearing.
Strange that SO2 appears as the reaction should be:
8 Al + 3 CaSO4 => 4 Al2O3 + 3 CaS.
Maybe the high temperatures decompose some CaSO4.
Both results appear as a liquid due to the high temperatures. I can see that by seeing the frozen slag.
[Edited on 2016-8-28 by metalresearcher]
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PHILOU Zrealone
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Quote: Originally posted by blogfast25  | Careful with the slag: contains quite a bit of CaS. Combined with water that means H2S. Dispose off properly!
SO2? There's should be very little of that. Never smelled any when I did it. |
All info is already provided here into a former Metalresearcher tread
CaSO4 is decomposed upon strong heating into CaO and SO3.
SO3 above 1000°C exist as SO2(g) and 1/2 O2(g).
So Al will first react with the free preheated O2 then maybe with some SO2 (but most will escape unless strongly confined)...and also directly with
CaSO4 (melts at 1450°C) but Al may also react with CaO (and CaS if formed then leading to Ca(g) and Al2S3).
Decomposition of metal sulfates for SO2 production
[Edited on 28-8-2016 by PHILOU Zrealone]
PH Z (PHILOU Zrealone)
"Physic is all what never works; Chemistry is all what stinks and explodes!"-"Life that deadly disease, sexually transmitted."(W.Allen)
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RogueRose
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| Quote: Originally posted by metalresearcher | Today I mixed 40g CaSO4 (burnt-out crumbled, i.e. anhydrous plasterboard) with 20g Al powder and some snippets Zn metal.
I ignited with KClO3 + sugar and it was a quick (and very hot) deflagration as the thin steel container collapsed. The green Zn flames are clearly
visible and the Zn metal evaporates and burns quickly as it cannot withstand the high temperatures.
See the video (at the end in 1:10 slow motion).
http://www.metallab.net/jwplayer/video.php?f=/clips/Plaster%...
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Has anyone gotten this video to work?
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PHILOU Zrealone
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Worked fine for me under internet explorer...was black for a few 10-15 seconds before it started.
PH Z (PHILOU Zrealone)
"Physic is all what never works; Chemistry is all what stinks and explodes!"-"Life that deadly disease, sexually transmitted."(W.Allen)
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blogfast25
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With HoF of - 476 kJ/mole and - 651 kJ/mole resp. for CaS and Al2S3 respectively, the reduction of CaS by Al is basically
impossible.
I don't believe any SO2 escapes either: it would suppress the HoR of the reaction too much... Perhaps a bit in highly non-stoichiometric
conditions.
I've used stoichio mixtures of CaSO4 + Al as heat boosters in titanium thermite: that would not work if SO2 was being released.
[Edited on 28-8-2016 by blogfast25]
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PHILOU Zrealone
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| Quote: Originally posted by blogfast25 |
With HoF of - 476 kJ/mole and - 651 kJ/mole resp. for CaS and Al2S3 respectively, the reduction of CaS by Al is basically
impossible.
I don't believe any SO2 escapes either: it would suppress the HoR of the reaction too much... Perhaps a bit in highly non-stoichiometric
conditions.
I've used stoichio mixtures of CaSO4 + Al as heat boosters in titanium thermite: that would not work if SO2 was being released.
[Edited on 28-8-2016 by blogfast25] |
Maybe. Only experiment will tell if above 1500°C (probably arround 2000°C) the reaction between CaS and Al is favourable or not...depends on
enthropy also.
If no gas as you claim...how the hell can the flame project so much metallic sparks above 4-5 times the height of the recipient while it contains only
1/3 of powder.
-The mix is dry mix and the plaster is also anhydrous...--> not water
-Trapped gas between the grains? I doubt it.
-The tiny pieces of Zn...Possible, I have seen Zn-S rockets and excess Zn makes a green-grey flame (observed at the end of the video).
Metalresearcher confirms the smell of SO2 upon CaSO4 thermal decomposition and into this thermite.
Maybe does it come from CaS...hydrated somehow then setting H2S free that burns into H2O and SO2.
PH Z (PHILOU Zrealone)
"Physic is all what never works; Chemistry is all what stinks and explodes!"-"Life that deadly disease, sexually transmitted."(W.Allen)
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blogfast25
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| Quote: Originally posted by PHILOU Zrealone |
If no gas as you claim...how the hell can the flame project so much metallic sparks above 4-5 times the height of the recipient while it contains only
1/3 of powder.
-The mix is dry mix and the plaster is also anhydrous...--> not water
-Trapped gas between the grains? I doubt it.
-The tiny pieces of Zn...Possible, I have seen Zn-S rockets and excess Zn makes a green-grey flame (observed at the end of the video).
Metalresearcher confirms the smell of SO2 upon CaSO4 thermal decomposition and into this thermite.
Maybe does it come from CaS...hydrated somehow then setting H2S free that burns into H2O and SO2. |
With thermites there is ALWAYS some gas/smoke, that doesn't prove it is SO2.
The reaction:
CaSO4 + 2/3 Al ===> CaO + 1/3 Al2O3 + SO2
... is the only one that can explain SO2 and is poor in heat generation (pitiful, in fact): while that plaster + Al is one of the hottest
'burning' mixtures available to hobbyists.
No, what might be happening is that in Metalresearcher's 'bucket chemistry' an excess of CaSO4 is present in the mix and undergoes
a little decomposition due to T > 2000 C.
Using Nernst it's possible to estimate the partial pressure of SO2 in function of temperature for:
CaSO4(s) < === > CaO(s) + SO3(g)
$$K=\frac{a_{CaS} \times a_{SO_3}}{a_{CaSO_4}}$$
Because of CaSO4's very high Free Energy of Formation, that K is very small but does increase with T. I might have a shot at estimating
pSO3...
Pyrolysis of ionic sulphates has been a past route to SO3 of course but CaSO4 would be an ill-advised choice for that purpose.
[Edited on 29-8-2016 by blogfast25]
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metalresearcher
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| Quote: Originally posted by blogfast25 |
The reaction:
CaSO4 + 2/3 Al ===> CaO + 1/3 Al2O3 + SO2
... is the only one that can explain SO2 and is poor in heat generation (pitiful, in fact): while that plaster + Al is one of the hottest
'burning' mixtures available to hobbyists.
No, what might be happening is that in Metalresearcher's 'bucket chemistry' an excess of CaSO4 is present in the
mix and undergoes a little decomposition due to T > 2000 C.
Using Nernst it's possible to estimate the partial pressure of SO2 in function of temperature for:
CaSO4(s) < === > CaO(s) + SO3(g)
$$K=\frac{a_{CaS} \times a_{SO_3}}{a_{CaSO_4}}$$
Because of CaSO4's very high Free Energy of Formation, that K is very small but does increase with T. I might have a shot at estimating
pSO3...
Pyrolysis of ionic sulphates has been a past route to SO3 of course but CaSO4 would be an ill-advised choice for that purpose.
[Edited on 29-8-2016 by blogfast25] |
Well, an excess of CaSO4 is not probable as I weighed 40g of CaSO4, probably impure as it is burned out plasterboard remains and at least 20g of Al
powder (purchased) and 10...20g Zn, so stoichiometrically the Al is in excess.
The theory that the intense heat (1500-2000 C as guessing the white glow and the collapsing container) decomposed (small) parts of the CaSO4, hence I
smelled SO2 (but not much). The extreme heat also explains the sparks escaping far from the reaction vessel, well visible in the slow motion part of
the video. The quick overheating of the Zn also makes lots of sparks (of burning Zn ?) as solid or liquid Zn cannot exist at such temperatures.
And CaS is surely formed as, despite the dry weather, I smelled H2S only a few minutes after the experiment due to attracting moisture.
[Edited on 2016-8-29 by metalresearcher]
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