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kilowatt
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Advice needed for HF synthesis
With my metallurgical interests peaking, I will be utilizing many fluoride chemicals. I have found neutral ammonium fluoride to be the cheapest
available molar source of (relatively) easily extracted fluoride, and since I can buy it I will be using it as the starting point for all fluoride
chemistry whether acid or neutral.
For preparing nearly anhydrous hydrogen fluoride, I intend to react concentrated sulfuric acid (supplied by an addition funnel) directly with dry
ammonium fluoride in a PTFE reaction vessel. This way, ammonium sulfate could be recovered which is easily decomposed with distillation in a pressure
vessel to reclaim H2SO4 and anhydrous ammonia. Sulfate will thus ultimately not be lost to an ultra stable salt, and I can reuse it.
A PTFE tube coming from the side of the primary reaction vessel would form an upward warm condenser section to condense sulfuric acid vapor back down
into the vessel. The tube would then curve over to a downward cold condenser section which would condense anhydrous HF into a chilled PTFE storage
bottle. This will pretty much only be done as needed, in the amount needed, as storing the dangerous chemical long term would be an unneccesary
hassle.
Does the anhydrous NH4F + H2SO4 reaction require any sort of heating (like the NaCl + H2SO4 reaction for example) or will it work with cold dry
reagents? I have concerns about the stability of the resultant ammonium sulfate in this exothermic reaction as it has a rather low decomposition
temperature. I do not want any ammonium present in the final product. Hopefully I am not missing any safety concerns with this process.
The mind cannot decide the truth; it can only find the truth.
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12AX7
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Go for ammonium bisulfate, it's an acidic mixture after all.
Don't know what kind of heat you'd need. HF, like most acids, probably dissolves in H2SO4 a good bit.
Be sure to run offgasses into alkali or something with calcium, to neutralize the "chemical tiger".
Tim
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kilowatt
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If it doesn't take too much heat to go to ammonium sulfate, wouldn't the bisulfate stochiometry just mean more H2SO4 for the HF to dissolve in,
requiring higher temperature to distill it out? I guess either one should work and allow for full H2SO4 recovery, though.
I plan to use an aqueous ammonia scrubber for the processes so I can recover any would-be lost fluoride easily, without rigorous recrystallization.
[Edited on 26-1-2008 by kilowatt]
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not_important
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While a water solution of ammonium sulfate will lose ammonia on boiling, getting all the way to NH3 and H2SO4 via thermal methods isn't too easy.
There tends to be internal redox reactions, resulting in SO2, H2O, N2, and even NOx production.
with a BP of circa 320 C I doubt you'll be boiling much H2SO4.
Note that polyethene and polypropene are resistant to aqueous and anhydrous HF up to about 40 C.
Hopefully you read the other HF related threads here, in particular the safety aspects.
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kilowatt
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If I could get a decent yield of H2SO4 back I would be quite happy. The ammonia is secondary to that really, since I have quite a stockpile of other
ammonium salts. On the other hand SO3 is easier to synthesize from every day materials, and cheaper/easier to buy than NH3, but for now I have
plenty. SO2 or NOx impurities should form ammonium salts which I could separate out anyhow. Are there any better methods for recovering the H2SO4
and ammonia? What about electrolysis with a titanium anode?
I have read through the ammonium fluoride related material here and some of the HF stuff, and have checked out the MSDS's and other safety related
documents for both. These are among the most serious chemicals I have dealt with so far; I have never handled them before and I would like to be
quite prepared.
Polyethylene will do for temporary storage or use, but in general I'm not satisfied with simply "resistant" materials. Maybe for dilute HF only.
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MagicJigPipe
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I would buy some calcium gluconate gel or a wash specifically designed for HF burns before doing anything with anhydrous or concentrated HF. It's one
of the few chemicals that can kill with relatively small skin exposure. Not to mention the intense pain that is caused (from what I've read).
Ooooh.... Just reading this makes me cringe.
"The patients suffered from severe pain. However, the pain was relieved after treatment with calcium gluconate. 6 out of the 7 cases were men. At the
accidental exposures, all the patients had been engaged in washing or cleaning work and received burns on their hands and/or fingers. In one case, the
forearm was also involved. During such work, all the patients had used rubber gloves, but the gloves had pinholes. For the treatments, 4% calcium
gluconate jelly was applied in 5 cases and 4 of 7 were subcutaneously injected with 8.5% calcium gluconate. The involved nails were removed in 5
cases. It is concluded that physicians should provide calcium gluconate jelly and subcutaneous injections to treat an HF burn and should not hesitate
to remove the involved nails..."
"My boss was exposed to HF on his hand some years ago. The stuff doesn't
cause pain or visible burning right away. But 12 hrs. later you feel it.
It can cause very deep tissue burns that take a long time to heal and it
can attack your bones. My boss says that it was the most intense pain he
had ever felt in his life. He fainted from the pain of the burn."
Gross huh? I'm not trying to scare you. Just, safety cannot be emphasized enough with this stuff. Sorry for those of you with weak constitutions.
[Edited on 27-1-2008 by MagicJigPipe]
![hf-burn[1].jpg - 67kB](http://www.sciencemadness.org/talk/files.php?pid=116221&aid=4689 "hf-burn[1].jpg - 67kB")
"There must be no barriers to freedom of inquiry ... There is no place for dogma in science. The scientist is free, and must be free to ask any
question, to doubt any assertion, to seek for any evidence, to correct any errors. ... We know that the only way to avoid error is to detect it and
that the only way to detect it is to be free to inquire. And we know that as long as men are free to ask what they must, free to say what they think,
free to think what they will, freedom can never be lost, and science can never regress." -J. Robert Oppenheimer
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kilowatt
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Yeah, I am well aware of those properties and treatments. I also saw a prior thread some time ago where that image was posted. I'm not sure why you
felt it necessary to post it here.
Anyways safety stuff like that is a big part of why I want to gather advice from anyone here who has dealt with hydrogen fluoride. I happen to have
some shoulder length lead lined hypalon gloves, which I purchased for cheap a couple years ago for an isolation glove box, anyhow. Chemical burns are
not my main concern here as I will be well protected; inhalation is, hence the scrubbers and such. I have in my lesser days been sprayed with boiling
red fuming nitric acid when my still clogged and and burst open, and realize that even that painful experience does not compare to a HF related
accident.
So... back to the chemistry...
[Edited on 27-1-2008 by kilowatt]
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YT2095
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I think if it were Mine and I just wanted the pure HF, I`d convert the Ammonium fluoride into calcium fluoride and drive off the Ammonia.
you now have a Very stable near insoluble salt.
when you add H2SO4 to this you will get your pure HF and calcium sulphate (also nearly insoluble).
the 2 will be Very easy to separate.
I wouldn`t bother trying to reclaim any sulphuric acid from it either.
\"In a world full of wonders mankind has managed to invent boredom\" - Death
Twinkies don\'t have a shelf life. They have a half-life! -Caine (a friend of mine)
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not_important
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Calcium sulfate is roughly 100 times more soluble than calcium fluoride, and the sulfates solubility is increased by the presence of strong acids
including sulfuric. Ignoring the CaF2 aspect, you'd get about 2 grams of CaSO4 per liter in solution.
On top of that a reaction that has an insoluble compound reacting to form another insoluble compound is asking for an incomplete reaction. Drop a few
marble chips into some dilute HCl, and into some dilute H2SO4, and tell me which reacts faster.
A problem with HF burns beyond simple corrosive effects is the toxic action of fluoride. The tying up of calcium throws the body out of adjustment,
causing much grief.
One of my bottles of aqueous HF is a 30 year old polythene one, never opened, that I picked up in a sale of surplus property. As it is still sold in
HDPE containers, I'd not worry much in using such to store homemade HF. It is suggested to double container bottles of HF, mine is stored in a HDPE
box, which sits with several other such boxes holding bottles of volatile acids in a large covered PE tub with a layer of agricultural lime on the
bottom.
Use neoprene, nitrile or vinyl gloves, not latex, and have the calcium based treatment paste at hand.
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YT2095
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with Chips sure, but a fine powder? a little extra time isn`t all that significant offset against the safety aspect.
and although the calcium sulphate is more soluble, it`s also VERY stable and will not come over when distiled.
but again, as I said, it`s a Personal choice and the route I would take myself if I wanted pure HF.
[Edited on 27-1-2008 by YT2095]
\"In a world full of wonders mankind has managed to invent boredom\" - Death
Twinkies don\'t have a shelf life. They have a half-life! -Caine (a friend of mine)
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Nicodem
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| Quote: | Originally posted by kilowatt
I have found neutral ammonium fluoride to be the cheapest available molar source of (relatively) easily extracted fluoride, and since I can buy it I
will be using it as the starting point for all fluoride chemistry whether acid or neutral. |
I don't know where you buy your ammonium fluoride from, but hydrogen fluoride (~70%) is generally several times cheaper on weight and, of course even
more so, on molar basis.
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kilowatt
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It all depends on availability to the individual. I can't easily just buy a bottle of HF at any concentration. If I could, I would just as soon keep
the NH4F around for its own sake. Nitric acid also runs cheaper than milk in the industry, but most of us cannot procure it directly without paying
an exorbitant sum.
I have even realized the possibility of thermally decomposing R-134a (1,1,1,2 tetrafluoroethylene) at >300°C with steam in an aluminum heat
exchanger leading into an alkali scrubber to yield sodium and aluminum fluorides. That process would be about the cheapest way for a typical
individual to procure fluoride, but it is more involved and far more dangerous (fluorophosgene is produced as an intermediate ) than just buying a salt to acidify, and is only a little cheaper per mole assuming
a good yield. At $0.86/mole shipped the ammonium fluoride may seem expensive (thats $43/kg of HF) but I think it's about as good as I'm going to get.
R-134a runs about $0.60/mole of fluoride and a bit cheaper if you buy it in the big cylinders. Maybe I'll get into that method someday when I have
better means.
If only I could just get mineral fluorite at the price industry pays.
Anyways My alternative to direct NH4F acidification would normally be to convert it to sodium fluoride, collecting the ammonia during said conversion.
Also soluble, much moreso than calcium, but far more stable than ammonium anyhow. Upon further thought though, maybe copper fluoride would be a good
choice, from cupric hydroxide. I'm not sure which oxidation state would tend to form but I assume Cu(II). This way the sulfate should be stable and
not come over, but still could be decomposed thermally to reclaim SO3.
[Edited on 27-1-2008 by kilowatt]
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not_important
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| Quote: | Originally posted by kilowatt
If only I could just get mineral fluorite at the price industry pays.
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Pottery supply - cryolite (synthetic) and sometimes NaF. Boil cryolite with excess strong aqueous NaOH, then dilute a bit and treat with CO2 to get
Al(OH)3 ppt and a solution of NaF and Na2CO3.
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microcosmicus
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My pottery supplier sells the mineral fluorite (a.k.a. fluorspar) at $1.16/lb bulk,
which sounds reasonable. Smaller quantities than a 25-lb bag will
cost more for repackaging, but not prohibitive either --- $1.80 for a pound jar.
Yes, they also have cryolite, but that costs more than twice as much for the same
weight. Taking molecular weights into account, a pound of fluorite will yield about
as much fluorine as a pound of cryolite, so go with the fluorite and spend the
money you save on antidote.
[Edited on 28-1-2008 by microcosmicus]
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kilowatt
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Oh, wow. I just checked out some ceramic suppliers for chemicals. They are awesome. They have lots of stuff at good prices. Thanks.
The mind cannot decide the truth; it can only find the truth.
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kilowatt
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Update: Here's what I've got going now. The aparattus consists of a teflon tube with a tapered teflon plug at the lower end (permanently sealed in
by heating the tube to expand it, cooling the plug to contract it, hammering it in and letting it equalize). The tube contains fluorspar, and a PE
squeeze bottle above is filled with concentrated H2SO4. HF gas should be emitted from the side tube and condensed in the ice filled condenser at the
right, to drip into the collection bottle in an ice bath. The bottom of the reactor goes down into a can of boiling water over a bunsen burner.
I am just running the aparattus for the first time and it doesn't really seem to be making any HF. I am trying to condense anhydrous HF for now, and
I am not getting any condensate. I have placed a glass test tube over the collection bottle which is showing no signs of etching. Perhaps 100°C is
not hot enough to either allow the reaction to proceed, or to remove HF from the system. I may need to try a hotter oil bath for the reactor, or I
may simply not have added enough H2SO4 yet (I have thus far only added a few mL; I am doing this slowly) to displace enough HF to get condensate.
I will update this post as the process proceeds or doesn't proceed.
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MagicJigPipe
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Nice! How much did that run you? I think you should try adding more acid and raising the temp to at least 150C. It can't hurt, right? Are you sure
your H2SO4 is >98%? Also, I believe I read somewhere that HF can polymerize to a solid because of extensive hydrogen bonding (for some reason that
doesn't sound right but I know I've read it somewhere although a quick google search turned up nothing).
I've always wondered if storing anhydrous HF is safe at room temperature. Does anyone know the vapor pressure at room temp? According to the MSDS
the VP at 2.5C is 400mmHg (7.73psi) but that's expected as it's below the boiling point. Apparently calculating vapor pressure for HF is difficult.
Does anyone have access to this article? It would be great if someone could post it.
http://dx.doi.org/10.1016/S0378-3812(01)00572-6
(You'll have to copy and paste this)
[Edited on 9-3-2008 by MagicJigPipe]
"There must be no barriers to freedom of inquiry ... There is no place for dogma in science. The scientist is free, and must be free to ask any
question, to doubt any assertion, to seek for any evidence, to correct any errors. ... We know that the only way to avoid error is to detect it and
that the only way to detect it is to be free to inquire. And we know that as long as men are free to ask what they must, free to say what they think,
free to think what they will, freedom can never be lost, and science can never regress." -J. Robert Oppenheimer
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carbonic
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So how exactly do you fit your super-safe lead lined gloves OVER that backyard moonshine still? Since you're so well versed in HF safety, you
understand that it burns the calcium out of your nerve ending first so you don't notice 3rd degree chemical burns...
But do your neighbors know that?
Do they know not to breathe the toxic fog from your uncontrolled reaction as it comes over their fence? Now Im not your mother here, but attempting
to making bulk HF this close to residence that are not your own is a bit irresponsible. Maybe you should give THEM some calcium gluconate because I
doubt it's you that's going to get hurt when you go inside to take a piss with boiling sulfuric and HF bubbling out back.
[Edited on 9-3-2008 by carbonic]
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kilowatt
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For one thing, this isn't an uncontrolled reaction, do you think I am just dumping a bunch of sulfuric acid and fluorspar together in a bucket and
cooking it in my back yard? Calling this uncontrolled and a "backyard moonshine still" seems like a pretty blatant attempt at being insulting. There
is no toxic fog. Also it sure as hell isn't "bulk." There is enough calcium fluoride in the reactor to generate about 17g of anhydrous HF, and it is
done over the course of hours. If that was all released into the air it would amount to about 19L of gas, definitely of no concern to the neighbors.
Do you think they just sit there waiting for me to go inside for a minute so they can run over to drink and huff my volatile chemicals?
Where do you even get off making such statements? Where do you get these ideas? You said yourself you aren't my mother, but you're acting far more
overbearing than any parent. Do you have some personal problem with me that you see the need to blow my projects far out of proportion in a pointless
attempt to ridicule them? Your post is completely unhelpful and distracting, misguided, and an insult to my intelligence and responsibility. I
suggest it be deleted. People like you are the reason home chemists have such a hard time finding chemicals. In fact, maybe if it weren't for people
like you blowing shit out of proportion and trying to protect everyone from themselves, I could just buy my own hydrofluoric acid. Go find someone
else (preferably a mod or admin) to give shit to; I'm not the only one here that distills and handles toxic volatiles in or near their home and
neighbors.
[Edited on 10-3-2008 by kilowatt]
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garage chemist
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In industry they mix stochiometric amounts of CaF2 and conc. H2SO4 into a slurry and use this as the reactor feed.
The CaF2/H2SO4 mixture doesn't emit HF gas at room temperature, and upon heating the gas evolution is slow and endothermic, so stopping the heating
stops the gas evolution as well.
So I would charge the reactor with premixed fluorite and H2SO4 instead of slowly dripping in the H2SO4, which brings the risk of bad mixing.
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kilowatt
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Thanks. One of my concerns (which I have encountered before with other liquid into solid additions especially acidifications) was slow or poor mixing
as the reactor vessel in this case is tall and narrow, which was really the only way I could build it. If I charge it with premixed slurry any idea
what sort of temperatures I will start to get HF coming over at? 150°C perhaps? I don't have a direct way to measure temperature inside, and can
only control the temperature of the double boiler outside. The thick teflon wall has very high thermal resistance. I don't want it to come over too
fast though either as I suspect it is quite hard to condense.
[Edited on 10-3-2008 by kilowatt]
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not_important
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Here are two older books that contain instructions for making HF, the procedures could be adapted and updated. One example might be on a little large
scale than you are interested in, but still informative. I notice that one has the retort being heated over a naked flame, implying a fair degree of
heat.
http://books.google.com/books?id=qAgtAAAAIAAJ&pg=PA151&a...
http://books.google.com/books?id=TNIAAAAAYAAJ&pg=PA185&a...
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kilowatt
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Ok, tomorrow I will run the rig in an oil bath with pre-mixed slurry. If I don't get anything coming over before approaching the max working temp of
the PTFE tube (260°C or so) I will go to a steel retort like they mentioned. I'm planning to make about 50-100g of HF overall (which would require
several loads with this small teflon vessel) and dilute to 50% for my precious metal work. Prior to dilution I plan to redistill it with the teflon
tube packed with teflon turnings as a reflux column, as the powdered fluorspar I got from a ceramics supplier no doubt contains significant impurity.
The mind cannot decide the truth; it can only find the truth.
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garage chemist
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According to Ullmann, the reaction of CaF2 with H2SO4 takes place between 100 and 300°C.
So you indeed need an oil bath.
The mixture changes consistency during the reaction several times:
0-40%: liquid, pasty, then sticky and corrosive
40-70%: powdered
70-80%: pasty, sticky, corrosive
80-100%: powdered
The percentages indicate the extent of reaction that has happened, e.g. the amount of reactants that have reacted.
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carbonic
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Hey KW,
Didn't mean to get you all fired up. Would you consider sealing the output vessel with a 2-hole stopper, whose second hole could have just a piece of
tubing into a bucket of ice water to act as gas trap?
That would take care of 90% of my concerns (escaping gas).
As for the control of the reaction, I wonder if there is some way to more closely monitor the heat. If the H2SO4 is dripping slowly enough - I leave
it up to you.
Bottom line, HF is nasty. It's not really people like me (warning naysayers) that give home chem a bad name, but rather the opposite case.(when
something goes wrong). I should not have insulted your rig, and I only wanted to suggestion extra precaution. I stick to that opinion.
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