Sciencemadness Discussion Board

Unanticipated extraction problem.

j_sum1 - 24-4-2021 at 21:19

I recently received a pile of chems from a fellow SM member (very generous) who was (partially?) decommisioning his lab.

Amongst the goodies was a 100g packet of iron oxide and some aluminium powder. The obvious thing was to treat yhe kids to a backyard thermite on a beautiful Autumn afternoon. I thought it a little strange that the seal was broken on the packet and that only a coiple of grams had been used: unusual for Fe2O3, but I did not give it another thought as I weighed out a stoichiometric amount of aluminium powder.

Both reagents were added to a zip-seal plastic bag for mixing. It was a few minutes later that I noticed the bag was warm and filling with gas. Evidently, it was not iron oxide but something very similar in appearance muslabelled, presumably to ease the shipping process some time in the past.

I now have an intimate mixture of red phosphorus, aluminium, and whatever reaction products, which presumably resulted from moisture in the reagents.

My question is, what is the best way to recover the unreacted red phosphorus (or some other useful product) from this mix. I do have some ideas, but I thought I would float the problem here first.

Answer is in the body of your question

Panache - 25-4-2021 at 01:32

Quote: Originally posted by j_sum1  
I recently received a pile of chems from a fellow SM member (very generous) who was (partially?) decommisioning his lab.

Amongst the goodies was a 100g packet of iron oxide and some aluminium powder. The obvious thing was to treat yhe kids to a backyard thermite on a beautiful Autumn afternoon. I thought it a little strange that the seal was broken on the packet and that only a coiple of grams had been used: unusual for Fe2O3, but I did not give it another thought as I weighed out a stoichiometric amount of aluminium powder.

Both reagents were added to a zip-seal plastic bag for mixing. It was a few minutes later that I noticed the bag was warm and filling with gas. Evidently, it was not iron oxide but something very similar in appearance muslabelled, presumably to ease the shipping process some time in the past.

I now have an intimate mixture of red phosphorus, aluminium, and whatever reaction products, which presumably resulted from moisture in the reagents.

My question is, what is the best way to recover the unreacted red phosphorus (or some other useful product) from this mix. I do have some ideas, but I thought I would float the problem here first.

Float the red phos. You’ll need to do it in a hot solution of BaSO4. I was also thinking a ki solution

unionised - 25-4-2021 at 01:32

To some extent the LA and P will have reacted which means that you have a phosphide.
It's going to be difficult to stop that producing phosphine if you attempt to react the mixture with anything.

Your best bet is probably to wrap the stuff in a few sheets of crumpled newspaper and set fire to it outside.

Amos - 25-4-2021 at 05:43

Quote: Originally posted by Panache  
You’ll need to do it in a hot solution of BaSO4. I was also thinking a ki solution


Surely you know that you cannot form an aqueous solution of barium sulfate, right? Flotation wouldn't be a half bad idea but it sounds like his aluminium powder is pretty reactive to moisture, so it would at the very least be very messy, and any evolution of gas will undoubtedly complicate things as well.

If you're absolutely positive that it's red phosphorus, it shouldn't be reacting with aluminium at room temperature, should it? I've seen the mixture react only hesitantly at first when presented with a match flame. I would sooner assume only your aluminum is reacting with water on its own, but maybe somebody will prove me wrong. If you don't want to keep the aluminum, I was thinking you could add it to very cold, dilute sodium hydroxide solution a little at a time and stir until all of it has reacted. Then just filter out the phosphorus.

j_sum1 - 25-4-2021 at 15:13

NaOH wash was my best thought too. But I do need to do some diagnostics first to check that it is red P.

It was one of those scary moments. I had my mixture heating up rapidly and got to the point where it melted a hole n the plastic bag. It was also effusing some gas. Totally unexpected. I assumed the worst and quickly dropped it at the back of the yard and monitored the temperature from a distance with an infra red thermometer. I certainly did not want a vigorous reaction at that scale in my hand!

The person I got it from has messaged me and says that to the best of his knolwedge it was properly labelled. So it may be a false alarm. I have done many thermites using the same Al powder and have never had it heat up like that.

Anyway, it seems to be stable now. I won't get to it today: much work to do. But I will do a couple of tests to confirm the identity of the material. If it is iron oxide then I have a thermite mix ready to go. If it is RP then it would have been slightly damp and I really would like to know what reaction occurred and would like to recover what I can. But I am not at all keen on producing phosphine. (Understatement.)

arkoma - 25-4-2021 at 16:37

Phosphine is indeed scary. We used aluminum phosphide pellets in the grain bins on the farm. Would hyperventilate inside the empty bin, hold my breath while i uncapped that shit, and make a quick circle dumping the pellets out and then out the bin. Remember the smell.........although the absolute worst thing I have ever smelled on the farm is rotten soybeans. Right up there with a decomposing body.

draculic acid69 - 26-4-2021 at 02:38

Quote: Originally posted by unionised  
To some extent the LA and P will have reacted which means that you have a phosphide.
It's going to be difficult to stop that producing phosphine if you attempt to react the mixture with anything.

Your best bet is probably to wrap the stuff in a few sheets of crumpled newspaper and set fire to it outside.

Yeah this is a real dangerous possibility any moisture may release phosphine
Which is super nasty. Fire is probably the best idea

j_sum1 - 6-5-2021 at 03:03

Update time.

I did a little experimenting to discover that it was not red P but in actual fact Fe2O3 as labelled.
I guess there must have been some moisture and maybe some other contaminants (chloride?) to cause it to exotherm when Al powder was added.
Anyway, I do not regret my caution. Unexpected reactions at scale are not something I really enjoy and the consequences if it had been phosphorus were pretty significant.

A week after initially planned it went off in a pretty nice thermite. Kids and grndparent were suitably impressed. And I got to investigate the effect of adding a little flux to the mix. So, all happy.