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Author: Subject: Preparation of elemental phosphorus
clearly_not_atara
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[*] posted on 17-4-2021 at 07:04


I don't know what you mean by "small scale". It's hard enough to deal with electricity at high temperatures, and even a kilo of MgCl2 isn't breaking anybody's budget. You need all kinds of insulation and temperature control, so it's simpler to set up the equipment to hold around a liter of molten salts anyway. At that scale, you get a few grams of P per run, which really isn't bad.

The big advantage of not running a "bomb" process like carbo/metallothermal reduction is that the damage to equipment and clean-up should be much less.




[Edited on 04-20-1969 by clearly_not_atara]
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[*] posted on 27-4-2021 at 05:55


You know, maybe setting the system up so that you occasionally top it off a mixture of chloride and phosphate would allow you to keep going indefinitely. Also, I found two papers that give a pretty good idea: https://www.sciencedirect.com/science/article/pii/S092702481... for the euctic, and https://pubs.rsc.org/en/content/articlelanding/2009/gc/b9063... for the electrostablility of the euctic. The first paper basically says that a choline chloride euctic works. The second paper I found a bootleg copy of online. Any thoughts? Looks promising to me.

Attachment: Greenchemistrypublished.pdf (210kB)
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[*] posted on 27-4-2021 at 06:18


Atara, if you do the math: 2 percent solution of Ca3(PO4)2 in 1 Kg molten salt = 20 grams Ca3(PO4)2 = .0645 mols. With 100 percent recovery of phosphorus that's 1.9 grams per run. What I mean by small scale is 1 Kg. Or 10 Kg for that matter. Not to mention even if one gets this to work you have to deal with gaseous WP.



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[*] posted on 27-4-2021 at 09:04


njl: You've used a density of 1 for the molten salt (I said "liter", you said "kilogram"), which is only a little wrong when you're dealing with organic chemicals, but the density of alkali chloride salts is about 2-2.5, which raises your raw numbers to about 4 grams. There was also a proposal of using (NaPO3)x as the phosphorus source instead of CaPO4, which may significantly increase the phosphorus content (unclear).

For amateur-feasibility, we are already forced to improvise because 850 C is a difficult temperature. It is also plausible that the reaction doesn't work at all at lower temperatures. But while we're on the subject, the KCl-LiCl eutectic is shockingly low: 352 C!
https://pubs.acs.org/doi/pdf/10.1021/ja01538a001

In this context, it's worth remembering that elemental phosphorus is a precursor to various reagents and catalysts that support difficult reactions under mild conditions in high yield on delicate substrates. A few grams of phosphorus is all you need to make a lot of PPh3-ligated catalysts, for example.

The gaseous WP thing is a moot point because all practical phosphorus refinement techniques give a gaseous product. Electrolysis should give WP at a lower temperature and in higher purity than carbothermal reduction.

Triflic Acid: Neither of these papers shows anything about choline chloride being effective at the potentials required for phosphate reduction...




[Edited on 04-20-1969 by clearly_not_atara]
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[*] posted on 27-4-2021 at 09:06


Is no one thinking about the euctic :mad: Why would it be gaseous WP if it is a euctic. WP boils at 280C, and with a bit of work a euctic could be made so that it is liquid below this temp. Also, constantly topping up the CaCl2 with Ca3(PO4)2 would let you get out more than 1.9g per kg. Even then, this could be played around with to get a better ratio of chloride to phosphate, by trying different cations like sodium. I think that this is worth trying. I'll post when I get around to trying this.



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[*] posted on 27-4-2021 at 10:14


See ref for LiCl-KCl eutectic production reference:

https://iopscience.iop.org/article/10.1149/1.2428639/meta?ca...

352 C is pretty good; it's not at all difficult to condense something coming over at 400ish. I think that getting the working temperature safely under 500 C opens up a lot of doors, because a lot of equipment is rated to around 500 C.

The truth is that if the phosphorus product weren't gaseous you'd have a whole host of new problems, like whether it passivates the cathode, reacts with the membrane, forms complex dissolved salts, floats, sinks, foams, etc.




[Edited on 04-20-1969 by clearly_not_atara]
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[*] posted on 27-4-2021 at 10:29


I don't know, I'm thinking that, assuming that the P floats, we could just have an inverted test tube over the cathode, like in a hofmann voltmeter. Airtight and to remove the white P you just flood with water and remove the test tube. On the other hand, if it sinks, just flip the test tube over and the white P will collect under the euctic, still airtight and easy to collect.

[Edited on 27-4-2021 by Triflic Acid]




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[*] posted on 27-4-2021 at 10:37


Yes, I think it would be necessary to remove the phosphorus as it forms by distilling it off. But this means that the cell and distillation apparatus must be well sealed against the atmosphere and evacuated. That's the problem with this batch process, there's no convenient way to periodically add phosphate without disassembling the entire setup. Unlike in a retort where the atmosphere within the apparatus is consumed (leaving an inert space for distillation) this design reintroduces oxygen with every batch (which again have a yield of maximum 1.9 grams WP). There is a lot of room for improvement, but I can't see how this would be practical until at least the yield per batch is optimized.



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