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Loptr
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Quote: Originally posted by Raid  | | Is there a way of recovering the P2O5 after the oleum synthesis? Maybe like drying it at a very high temp or something? |
It's not a reversible reaction as the oxygen is incorporated into the chemical structure.
"Question everything generally thought to be obvious." - Dieter Rams
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Keras
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| Quote: Originally posted by Raid | | Is there a way of recovering the P2O5 after the oleum synthesis? Maybe like drying it at a very high temp or something? |
As far as I know, no. You can’t get back from phosphoric acid to phosphorus pentoxide. As you heat, the acid transforms into others forms (e.g.
pyrophosphoric acid) and then polymerises. At a pinch, you maybe able to recover phosphorus, but that needs a special apparatus since phosphoric acids
attack glass at high temperature (and metal also – so you'd need clay or something).
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Tsjerk
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In theory; yes. In practice; check out the lengthy elemental phosphorus thread...
You could separate the phosphate from the sulfate, reduce to phosphorus, oxidize to P2O5. But none of those steps is going to be easy.
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Raid
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hmm, I see.
Thanks for the info!
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clearly_not_atara
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I'm sure it isn't easy, but I've found a couple of reports that aluminum dihydrogen phosphate dehydrates to the metaphosphate Al(PO3)3 around 800-900
C and releases P2O5 around 1000-1200 C to leave AlPO4. So it seems like you theoretically might be able to avoid reducing phosphorus. As far as I can
tell, this is not used in practice. The temperature control is probably tricky, but it's the only direct "synthesis" of P2O5 I've seen in the
literature. Interestingly, if the partial pressure of oxygen is low, the gas released is actually P2O3 + O2.
The "azeotropic point" of metaphosphoric acid is at about 860 C and is slightly enriched in P2O5 compared to the nominal formula HPO3. But
metaphosphoric acid is incredibly corrosive and at these temperatures would be even more so. So this is not useful.
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BromicAcid
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But the real question is, do you need to go back? There are multiple threads around here on using the meta phosphoric acids and pyro phosphoric acids
as dehydrating agents:
One example
http://www.sciencemadness.org/talk/viewthread.php?tid=4409
It's even specifically called out that it can dehydrate sulfuric acid to sulfur trioxide. And would be recyclable (with some effort).
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Loptr
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This was my thought exactly.
What about sodium pyrosulfate? Would that also be an option?
"Question everything generally thought to be obvious." - Dieter Rams
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BromicAcid
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A cursory glance shows it could be. Take your sodium sulfate, treat with stoichiometric sulfuric acid to make the bisulfate then strip down,
decompose to the pyrosulfate. Then workup to make your sulfur trioxide and take the residues and repeat. Or course that is a gross simplification
but it's a reasonable start.
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