Birdy_407 - 11-10-2004 at 14:36
After searching for quite a while, I've found a reference to a russian patent, (from 1938) where the preparation of KF via fusion of CaF2, K2CO3
and SiO2 is described.
Unfortunately, I neither have access to this patent nor could I translate it from Russian.
A similar problem has been discussed here: http://www.sciencemadness.org/talk/viewthread.php?tid=881, yet it deals with the preparation of NaF.
So my question is: Does anybody have a more quantitative description of the process (which temperature for how long, how much SiO2, excess of CaF2
needed?, etc...)?
BromicAcid - 11-10-2004 at 15:26
CaF2 + K2CO3 + SiO2 ---> 2KF + CaSiO3 + CO2
Would most definitely be favored by high temperatures, i.e., >800C possibly higher. Probably with a stoichiometric quantity of each ingredient, if
SiO2 were in excess it might hamper heat conductivity, K2CO3 in excess means more contaminates in your final product. Excess CaF2 might lead to
incomplete reductions and excess insulation in the reaction, so go for a regular 1:1:1 molar ratio.
Heat to high temp without access to oxygen but allowing gasses to exit. Keep at high temperature for about 2 hours. Cool to room temp and pulverize
the mass. Dissolve in water. CaF2 is insoluble, CaSiO3 is pretty much insoluble, CaCO3 is insoluble, SiO2 is insoluble, your only contaminates
should be K2SiO3 (minor) and K2CO3 (might be significant). So you will have a solution of KF, K2CO3, and K2SiO3 left. Test for silicates, test for
carbonates, destroy each in the manner of your choosing.
Evaporate and recrystalize your KF, careful poisonous (as you already know). The melt itself may well eat a number of crucible types so be careful.
garage chemist - 12-10-2004 at 08:49
This reaction is VERY interesting, I've never heard of it before.
I thought that alkali fluorides were always made from HF and the hydroxide/ carbonate, the HF being made from H2SO4 and CaF2 in a special distillation
apparatus made of lead (or lead coated on the inside).
But there is still the problem of obtaining CaF2, at least for me.
However, I've got another possibility.
If I am lucky, maybe I could get a bottle of 48% hydrofluoric acid. Then I would put a few 100 milliliters of it in a plastic container (drinking cup)
and add Na2CO3 until it doesn't fizz anymore.
Sounds fun and safe, yes? 
I would have already tried before to get the HF, but the (true) horror stories of persons being killed by getting a few drops of it on their hands
prevented me from doing it- up to now.
Does anyone have good advice on the safe handling of HF?
I know that I have to get some expensive gloves, put on a leather apron and wear full face protection (motorcycle helmet).
Something that especially worries me are the very tiny, invisible drops of liquid which get splashed around when a gas is liberated from a liquid.
I've dissolved silver in HNO3 before and I had lots of tiny black spots on my hands after this. I could use NaOH instead, but this generates heat
and poses the risk of flash boiling, which would be fatal. It could also deform the plastic container.
Are there other dangers of HF which I've not listed here?
Also, does neutral NaF attack glass when hot (when evaporating the water from the NaF)?
blip - 12-10-2004 at 16:07
I might try Na<sub>2</sub>CO<sub>3</sub> + CaF<sub>2</sub> <s> ></s> 2 NaF +
CaCO<sub>3</sub> myself with some calcium fluoride I have, I would imagine CaCO3 is less soluble than CaF<sub>2</sub> though
the reaction may take a while to complete. I actually got the CaF<sub>2</sub> from my grandparents' house after they died in brown
glass pill bottles, though I have no idea why they would have it in pill form along with others containing
Na<sub>2</sub>SO<sub>4</sub> etc. Obviously if you want KF use K<sub>2</sub>CO<sub>3</sub> instead.
JohnWW - 12-10-2004 at 16:35
Are you talking about an aqueous reaction? If so, it would not get far if at all, because CaF2 is only very slighly soluble in water, 0.0016 parts/100
parts of water at 20ºC, and I very much doubt that Na2CO3 could be induced to react with it in saqueous solution. The two forms of CaCO3, aragonite
and calcite, are soluble in water to the extent of only 0.0012 and 0.0014 parts/100 parts water, practically the same as for CaF2, so any possibility
of the lesser solubility of CaCO3 driving the reacton would be infinitesimal.
Birdy_407 - 13-10-2004 at 12:09
Thanks BromicAcid for your input. Are 800C (or 900) really sufficient? CaF2 will not melt before 1400C. Further, I'm a bit unsure how to destroy
the K2CO3 carbonate contamination. Addition of perchloric acid would result in the formation of KClO4, which is not very soluble, but the whole
procedure is then no longer OTC. Are there any better alternatives?
The melting pot material is the next problem. Would a graphite crucible be adequate?
Theoretic - 13-10-2004 at 13:11
Yes, I believe a graphite crucible will withstand molten fluorides/carbonates, although hot HF fumes in contact with air or molten alkali hydroxides
also in contact with air might eat it.
BromicAcid - 13-10-2004 at 15:34
800 - 900 C was a ballpark guess, but it's probably not going to be melting the CaF2 that is going to be the necessary action. CaF2 is a flux,
carbonates are used to lower melting points, I would guess that the mixture probably melts before any of the individual components due to eutectic
formation. Might even separate out for you into layers upon cooling. Probably just heat it as high as possible for a few hours and see what you end
up with.
If you want to get rid of carbonate, you can buy weak solutions of HF OTC for cleaning rims on cars and other assorted purposes, only about 2 - 4 %
but if you achieve a good conversion to potassium fluoride then the weak acid solution could be added till bubbling stops to neutralize any remaining
carbonate. It would also precipitate any potentially soluble calcium from soluiton.
Graphite will hold up, in theory, if it's not too hot for nickel to physically stand that should hold up too.
[Edited on 10/13/2004 by BromicAcid]