From: glhurst@onr.com (Gerald L. Hurst)
Newsgroups: sci.engr.chem
Subject: Re: HF/H2o
Date: 19 Feb 1996 21:34:19 GMT

In article <gibson7-1902960835210001@ip217.msp.primenet.com>,
gibson7@primenet.com (C. W. Gibson) says:

>In article <4g8958$mo9@oslo-nntp.eunet.no>, enholm@login.eunet.no (Morten
>Enholm) wrote:
>
>>Can any one adivice me how to get the "H2o" out of "HF" (HYDROFLUORIC ACID)?
>>My problem is that the HF attaks metal an i want it only to attack GLASS,
>>i think the trick is to get the H2o out of it but how ?,
>>In Norway we can only bye 48% HF the rest is H2o.
>>Please Help
>
>Because HF is a halogenated acid, it will attack most metals, especially
>those containing iron. The only safe way to handle HF is by the use of
>plastic, most likely Teflon.

I am surprised that you cannot buy pure HF as a bottled, liquified
gas. I never had any trouble obtaining the substance in lab
cylinders in either the U.S. or England and I literally used
tons of the material in conjunction with the production of
elementary fluorine in the U.S.

Dry HF can be used in monel, SS and copper equipment and even 
in glass (if it is truly bone dry). Of course, once the attack
on glass begins with a trace of water the reaction produces 
more and you wind up with a trap full of SiF4.

HF loves water, so you might decide instead tosimply generate 
HF as you need it by reacting NaF with H2SO4. If you like, you 
can capture and store the HF by passing it over NaF to form NaHF2, 
which will give up the HF on heating.

You'll have to verify this idea, but I believe  NaF will also 
extract HF from HF/H20 to form NaHF2, a white solid. If you 
carefully dry the material (or run the extraction from the
gas phase) you can regenerate the HF by simply heating the
double salt you have formed. It is well known that NaHF2 
precipitates when an excess of 40% HF is mixed with NaOH.

I used a large stainless steel box filled with baffled trays
of pure NaF to remove HF from the effluent fluorine from an AEC
fluorine cell. I actually used two of these boxes, which were 
wound with nichrome wire. While one was absorbing HF from the
product stream, the other was being heated and purged to drive
off the HF to a sodium carbonate scrubbing tower where it formed
NaF.  

The entire pilot plant was subject to contact with HF, but it 
gave us no trouble in dry form using monel, 316 SS or copper
reactors, tubing and valves. I also built a laboratory vacuum
system of monel and copper which handled dry HF nicely, but
never as nicely as other gases in glass systems since you
can't see through monel and the surface layer of metal fluorides
is a great holder of residual traces of whatever gases are being
manipulated.

Jerry (Ico)