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Author: Subject: Silicon Tetrafluoride - "Sand Gas"
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[*] posted on 26-11-2012 at 12:37
Silicon Tetrafluoride - "Sand Gas"


I found an old article relating to fluorides (see here http://www.sciencemadness.org/talk/viewthread.php?tid=8 ) and in it there is an experiment in making silicon tetrafluoride, they call it "sand gas" because it becomes silicic acid on contact with water.

The vague instructions call for equal amounts of a fluoride (CaF, NaF, etc) and sand, as well as con. sulfuric acid.

I wanted to try and make some of this silicon tetrafluoride, it turns out it is a very neat experiment but I had to abort it due to excessive foaming and fumes...

I collected 100ml of the gas and then let some flow into a glass of water. Instantly a skin of silicic acid appeared on the surface of the water and walls of the glass.

I stopped the experiment because there were more fumes than expected and the fear of fluorides got the best of me. I stored some of the gas in a plastic container and if I wet a glass rod and stick it into the atmosphere within the container it almost instantly solidifies due to a thick silicic acid skin.

A fun experiment but be careful...


Opted to use calcium fluoride
photo 1.JPG - 104kB

Calcium fluoride and builders sand, mixed with sulfuric acid added. The mixture was slowly bubbling even without heat.
photo 2.JPG - 92kB

After "dumping" the fumes into a glass of water...
photo3.JPG - 114kB

The article where this "safe" idea came from... See full 4 pages at the link I posted above.
image4.jpg - 428kB

[Edited on 26-11-2012 by Mailinmypocket]
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[*] posted on 26-11-2012 at 13:25


Good job on replicating and documenting an experiment from Scientific American.
I think SiF4 is a unique substance as it reacts to produce a rigid gel upon contact with water. The gel is so rigid that bubbling SiF4 gas into water actually produces a "tube" extending to the surface that prevents its complete reaction with water. Can you think of any other substance that does this?
This, together with its very simple production and the fact that it actually doesn't attack glass, gives potential for spectacular experiments.




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[*] posted on 26-11-2012 at 14:47


Quote: Originally posted by garage chemist  
Good job on replicating and documenting an experiment from Scientific American.
I think SiF4 is a unique substance as it reacts to produce a rigid gel upon contact with water. The gel is so rigid that bubbling SiF4 gas into water actually produces a "tube" extending to the surface that prevents its complete reaction with water. Can you think of any other substance that does this?
This, together with its very simple production and the fact that it actually doesn't attack glass, gives potential for spectacular experiments.


It really is amazing how the test tube is completely unharmed. When I first started to see white fumes of silicic acid (and perhaps HF?) it took me a moment to grab the PE jar and start collecting the gas. During this time the gas exiting the tube started to create icicle-like formations at the end of the exit tube! Unfortunately I forgot to take pictures of those.

This material seems to have lots of potential as far as experiments go, it seems easy to store in a very dry container too.

I was in the process of making an apparatus to have the gas bubble vertically up through water to try and create tubes of silicic acid. After making the perfect piece of glass tube for the job I managed to jam myself in the palm with it while trying to stick it into a stopper. After picking bits of glass out of my hand I wasn't in the mood to try the experiment again (today anyways ;) )

A nice small, but deep hand stab, huzzah! What a stupid mistake.
stab.JPG - 108kB

[Edited on 26-11-2012 by Mailinmypocket]
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[*] posted on 26-11-2012 at 15:02


Ouch. Are you using rubber stoppers or do you have Quickfit joint adapters?

I cannot help but wonder what it would look like to open a compressed gas cylinder of SiF4 and throw it into a shallow lake.
Branched tubes of silica would grow from the cylinder outwards towards the surface of the water. A chemical garden with gas instead of salts, and in extra rapid mode!




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[*] posted on 26-11-2012 at 15:08


Quote: Originally posted by garage chemist  
Ouch. Are you using rubber stoppers or do you have Quickfit joint adapters?

I cannot help but wonder what it would look like to open a compressed gas cylinder of SiF4 and throw it into a shallow lake.
Branched tubes of silica would grow from the cylinder outwards towards the surface of the water. A chemical garden with gas instead of salts, and in extra rapid mode!


For this I was using rubber stoppers, too afraid to use fluorides in my Quickfit stuff, even SiF4. Next time I will just be more careful and probably bubble it from below a layer of mercury in a column so that the tube doesn't block with silicic acid. The rubber stopper was completely unaffected after the experiment. Only white deposits that you can wipe off by hand. Pisses me off because I had the perfect tube for the job :P

If you were to grow them (using compressed SiF4) using aqueous solutions of fluorescent dyes such as rhodamine or fluorescein you could create some cool formations. Depending on how solid they are.

[Edited on 26-11-2012 by Mailinmypocket]

[Edited on 26-11-2012 by Mailinmypocket]
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[*] posted on 27-11-2012 at 06:24


Periodicvideos on Youtube has a good video about your mishap, Mailinmypocket, if you hadn't seen it. They call that injury the "mark of the chemist" :)

<iframe sandbox width="560" height="315" src="http://www.youtube.com/embed/Wn9qxPKkRDw" frameborder="0" allowfullscreen></iframe>
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[*] posted on 27-11-2012 at 07:27


I was thinking of that periodicvideo while applying pressure to the wound haha! I guess I am now marked.

This morning I wanted to play with this strange gas again. Instead of just letting the tube exit into open air I used a piece of bent glass tube (broken from last night) and placed it so that the tube was just above the water surface.

The mix was used with a higher proportion of sand to discourage any HF from attacking the test tube instead. As soon as the gas evolution started the beaker fogged up and a gel like layer started growing quite quickly on the surface of the water like an island. Not the whole surface of the water became covered though so the SiF4 seems to be highly attracted to the water. It doesn't seem to float around in the beaker much.

Draping a very wet paper towel over the beaker completely prevented any fumes from escaping and at the end of the experiment the paper had a layer of snow like deposit on it.

Another interesting observation is if a jar of the gas is suddenly inverted over water and kept about 5mm from the surface, the surface instantly solidifies and makes a crackly sound!


My crude set up
photo1.JPG - 87kB photo 2.JPG - 77kB

Water surface as seen from below and above, after there was no more SiF4 being generated.
photo 3.JPG - 83kB photo 4.JPG - 58kB

This time the layer was so thick it could easily be skimmed off. The consistency was similar to jell-o
photo 5.JPG - 71kB
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[*] posted on 27-11-2012 at 07:41


This all looks very very interesting. I, however, have seen this kind of behavior also with SiH4. I once made SiH4 and bubbled that gas in an inverted test tube above water. SiH4 does not react with water at once, but when I came back to the test tube a few days later, I saw that the water inside was covered by a thin milky layer which was strong enough to keep the test tube nearly horizontal before the layer broke apart. But the SiF4 experiments seem to be more impressive (SiH4 is impressive in another way, because of its self-igniting properties, as soon as it comes in contact with air).



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[*] posted on 27-11-2012 at 07:57


Quote: Originally posted by woelen  
This all looks very very interesting. I, however, have seen this kind of behavior also with SiH4. I once made SiH4 and bubbled that gas in an inverted test tube above water. SiH4 does not react with water at once, but when I came back to the test tube a few days later, I saw that the water inside was covered by a thin milky layer which was strong enough to keep the test tube nearly horizontal before the layer broke apart. But the SiF4 experiments seem to be more impressive (SiH4 is impressive in another way, because of its self-igniting properties, as soon as it comes in contact with air).


Yes I have seen your page on silane along with the image of the said skin over the water in the test tube. I am thinking that this is the same skin of silicic acid as is produced from SiF4, a quick read about SiH4 on Wikipedia revealed this in the "safety" section:

"In addition, contact with eyes may form silicic acid with resultant irritation"

So it might be possible that both gases react with water to form the same substance, SiF4 just seems like it does it faster.

I want to try two more experiments; one in which I slowly release the gas from a tube positioned over a steaming beaker of water to see if I can get "growths" on the end of the tube due to the high water content of the surrounding atmosphere (although the rising steam may just carry the gas up with it.) Second, I would like to add an acid base indicator to the water to see what effects the reaction has on the water. I am expecting to see acid forming near the surface due to HF.

All in all a fun stuff to experiment with, and no glassware damage :)


[Edited on 27-11-2012 by Mailinmypocket]
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[*] posted on 27-11-2012 at 08:55


Quote: Originally posted by MrHomeScientist  
They call that injury the "mark of the chemist" :)

this injury (eerily exactly described in the video) actually stopped me experimenting for a long time. i'm relieved to hear now it's actually a very common mark among chemists and to be worn with pride =D
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[*] posted on 27-11-2012 at 10:03


I just hope people here understand what a nasty little chemical SiF4 really is: hydrolysis generates HF. Beware!

See this SiF4 MSDS and start shivering:

http://www.orcbs.msu.edu/msds/LINDE_MSDS/pdf/061.pdf

[Edited on 27-11-2012 by blogfast25]




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[*] posted on 27-11-2012 at 10:32


Quote: Originally posted by blogfast25  
I just hope people here understand what a nasty little chemical SiF4 really is: hydrolysis generates HF. Beware!

See this SiF4 MSDS and start shivering:

http://www.orcbs.msu.edu/msds/LINDE_MSDS/pdf/061.pdf

[Edited on 27-11-2012 by blogfast25]


Absolutely - The first time I ran this experiment I stopped it right away because of the fumes that are VERY noticeable. The old papers don't highlight much about safety I must admit. As soon as I caught a whiff of what smelled exactly like conc HCL fumes I left the room, stopped heating and took the apparatus outside. Just smelling that pungent odor and knowing that it IS NOT HCl got the adrenaline flowing.

Draping a very wet paper towel over whatever vessel you plan on venting the gas into seems to work quite well, no fugitive fumes at all. Still, be careful. I rarely experiment with fluorides, let alone volatile ones that hydrolyse to HF and if inhaled would deposit silicic acid in your lungs, along with the HF exposure.

Unfortunately, although the gas does not affect dry glass, it does affect wet glass. The beaker I used in the previous experiment has dried after cleaning and there is a white ring where the water level was :( It would seem that due to the hydrolysis to HF, if there is a thin coating of water enough is produced in high enough concentrations in those areas to etch the glass. Do not do this with glass you like... This was a brand new beaker.

sif4.png - 41kB

[Edited on 27-11-2012 by Mailinmypocket]

[Edited on 27-11-2012 by Mailinmypocket]
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[*] posted on 27-11-2012 at 13:14


SiF4 doesn't form HF upon reaction with water.
The reaction goes:

3 SiF4 + 2 H2O ---> 2 H2SiF6 + SiO2

The white ring on your beaker must be adhering silica, because H2SiF6 does not noticeably attack glass. It's also not nearly as dangerous as HF, though it's a much stronger acid.
SiF4 is still very toxic upon inhalation, but not because of HF formation.






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[*] posted on 27-11-2012 at 13:16


Quote: Originally posted by garage chemist  
SiF4 doesn't form HF upon reaction with water.
The reaction goes:

3 SiF4 + 2 H2O ---> 2 H2SiF6 + SiO2

The white ring on your beaker must be adhering silica, because H2SiF6 does not noticeably attack glass. It's also not nearly as dangerous as HF, though it's a much stronger acid.
SiF4 is still very toxic upon inhalation, but not because of HF formation.




You sure? I have tried using steel wool and it still won't come off. Upon hydrolysis wouldn't the HF formed have the ability to perhaps etch the glass a bit? How do you suggest I remove this stuff if otherwise?

[Edited on 27-11-2012 by Mailinmypocket]
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[*] posted on 27-11-2012 at 13:27


Try NaOH solution, warm or hot if necessary.

How would it be possible for HF to exist in the presence of finely divided silicic acid, which the hydrolysis of SiF4 produces? It's not possible, and there simply isn't any free HF present in the hydrolysis products of SiF4.




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[*] posted on 27-11-2012 at 14:17


Quote: Originally posted by garage chemist  
SiF4 doesn't form HF upon reaction with water.
The reaction goes:

3 SiF4 + 2 H2O ---> 2 H2SiF6 + SiO2

The white ring on your beaker must be adhering silica, because H2SiF6 does not noticeably attack glass. It's also not nearly as dangerous as HF, though it's a much stronger acid.
SiF4 is still very toxic upon inhalation, but not because of HF formation.




I think you’re oversimplifying. My A.F.Holleman (‘Inorganic Chemistry’) mentions very clearly that dilute solutions of H2SiF6 on mild heating contain much more HF in their vapours than than they do SiF4. That shows that H2SiF6 really exists as an equilibrium:

H2SiF6 < === > 2 HF + SiF4

According to the same source, a solution of 13.3 % H2SiF6 has a vapour composition of 2 HF + 1 SiF4 (2:1 molar ratio). For those reasons, H2SiF6 is difficult to isolate although a solid hydrate H2SiF6.2H2O (MP = 19 C) does appear to exist.

Hexafluorosilicates are also ‘imperfect complexes’: their anions are partly dissociated, similar to hexachlorostannates (for instance).

SiF4 and H2SiF6 may be less dangerous than HF but remain seriously dangerous chemicals.



[Edited on 27-11-2012 by blogfast25]




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[*] posted on 27-11-2012 at 18:56


All of this sounds very similar to the prodcution of superphosphate and the flouride byproducts

below is an edited extract from a literature review of fluoride managment during SSP production

"The exact chemistry of the reaction between sulphuric acid and phosphate rock is not precisely understood due to the unknown composition of phosphate rock. The simplified reaction of fluorapatite and sulphuric acid is commonly used as a good representation of this process. In this process sulphuric acid reacts with the fluorapatite to form phosphoric acid, calcium sulphate and a fluoride based compound. The phosphoric acid goes on to react with another fluorapatite molecule to form monocalcium phosphate and an additional fluoride compound. This process can be seen below;

2{3[Ca3(PO4)].CaF2} + 18H2SO4 → 18CaSO4 + 12H3PO4 + 2CaF2
3[Ca3(PO4)].CaF2 + 12H3PO4 → 9Ca(H2PO4)2 + CaF2
3CaF2 + 3H2SO4 → 3CaSO4 + 6HF
9Ca(H2PO4)2 + 9H2O → 3Ca(H2PO4)2.H2O
21CaSO4 + 42H2O → 6CaSO4.2H2O

3[Ca3(PO4)].CaF2 + 7H2SO4 + 17H2O → 3Ca(H2PO4)2.H2O + 7CaSO4.2H2O + 2HF

The current mechanism of fluoride evolution is the production of hydrogen fluoride from the reaction of fluorapatite and sulphuric acid. It has then been suggested that the hydrogen fluoride gas is initially converted into hydrofluoric acid. A reaction then occurs with the silicates in the phosphate rock to form silicon tetrafluoride. This reaction can be seen in the 2 equaitons below where M2SiO3 represents the silica based compounds.

6HF → 3H2F2
M2SiO3 + 3H2F2 → SiF4 + M2F2 + 3H2O

there is only partial volatilisation of fluoride and it is predominantly in the form of silicon tetrafluoride. This is assuming there is a high rate of conversion from hydrogen fluoride to hydrofluoric acid

The established set of reactions with hydrogen fluoride as an intermediate product has been put under dispute. It was found the reaction between concentrated hydrofluoric acid and silicates are usually very slow; therefore, there should be more hydrofluoric acid present in the gas evolved. It was even found that hydrofluoric acid reacts with the fluorapatite to form available phosphate rather than with the silica to enhance the volatilisation of fluorine as seen below. These results also coincide with experiments completed where quartz flour and other silicates were added to low silica phosphate rocks where it was found that there was no increase in the fluorine volatilisation. Also with experiments where 12% hydrogen fluoride was directly added to the process and there was no increase in fluoride evolution observed. It has been said it is more likely that silicon tetrafluoride is directly produced but there are currently no reaction pathways which detail this process

Ca10(PO4)6F2 + 10H2F2 → 6H3PO4 + 10CaF2 + 2HF

An experiment was completed which showed that the addition of calcium fluosilicate increases the evolution of the total fluoride evolution. The percentage of total fluoride evolution remained very consistent between 35.4-35.9%. This indicates that the level of fluoride evolution is directly related to the presence of calcium flurosilicate compounds. It was concluded that fluorine evolution behaves like hexafluorosilicic acid and not like hydrofluoric acid.


Fluoride evolution is the main by-product from the production of superphospahte. The majority of the fluoride is evolved from the initial reaction of phosphate rock with sulphuric acid in the form of silicon tetrafluoride. The silicon tetrafluoride vapours are then run through a wet scrubbing system in the presence of excess water. It is generally accepted that the silicon tetrafluoride is hydrolysed to form HFA and silicon dioxide. This reaction has even been shown to take place within the reaction chamber as there is an excess of water vapour present with the silicon tetrafluoride. There has been a wide variety of different reactions which have been shown to form the HFA and silica based products as seen in below. The first three reactions agree on the formation of HFA but differ in the type of silica molecule that will be formed. Three different types of silica have been hypothesised to form they are silicon dioxide (SiO2), hydrated silicon dioxide (SiO2.2H2O) and silicon hydroxide (Si(OH)4) below. Most workers have cited at least one of these reactions but no experimental evidence has been produced.

3SiF4 + 2H2O → 2H2SiF6 + SiO2
3SiF4 + 4H2O → 2H2SiF6 + SiO2.2H2O
3SiF4 + 4H2O → 2H2SiF6 + Si(OH)4

It has also been noted that when silicon levels are high fluodisilicic acid (H2SiF6.SiF4) is formed in equilibrium with HFA in solution. In this case the silicon tetrafluoride is thought to be hydrolysed the reaction expressed below.

(3+2n)SiF4 + 2H2O → 2H2SiF6.nSiF4 + SiO2

Any of these reactions are possible. There has been insufficient experimental work completed; therefore, it can not be said which ones take place. It has even been hypothesised that each of the reactions will occur depending on the reaction conditions, such as, temperature, pressure, pH and many others. A significant amount of work has been completed on the gaseous phase reaction between silicon tetrafluoride and water which is often used in the production of high purity amorphous silicon and silicon dioxide fibres . It was stated that other chemicals besides HFA are formed during the hydrolysis of SiF4. These include hexafluorodisiloxane (SiOF6) and perfluorotrisiloxane (Si3O2F8). Mass spectrometer readings showed that the compound SiF3OSiF2OSiF3 was present. The Majority of this research is not applicable to the wet scrubbing system used in the production of SSP. This is because reactions are completed at temperatures often above 200°C and are completed with an excess of silicon tetrafluoride and extremely low concentration of oxygen and hydrogen. There was a study completed that examined the gaseous phase reaction between silicon tetrafluoride and water at atmospheric conditions. This was completed because of the potential for the formation of hydrogen fluoride which poses a safety risk. This work may relate to the conditions found above the reaction of phosphate rock and sulphuric acid. The reactions put forward are shown below. The results from this work concluded that hydrogen fluoride is not produced during the exposure of silicon tetrafluoride to humid air. An unidentified hydrolysed compound was produced.

4SiF4 + 2H2O → 2(F3Si)2O + 4HF
2SiF4 + 2H2O → H2SiF6 + SiO2 + 2HF
SiF4 + 2H2O → SiO2 + 4HF

A solution is produced from the hydrolysation of silicon tetrafluoride during the production of Superphosphate this is mainly comprised of HFA (H2SiF6) and silicon dioxide (SiO2) in solution with water. It has been shown by several workers to be an inaccurate simplification. This is because the scrubber liquor has been found to have a fluoride to silica ratio (F:Si) of 4.7-5.35. The ratio is significantly lower than the F:Si ratio of 6 of pure HFA. Several workers have stated that the low ratio is due to the formation fluodisilicic acid (H2SiF6.SiF4) which is present in a state of equilibrium with the HFA compound. The equilibrium position is mainly influenced by the concentration of fluoride ions. This hypothesis does hold as the properties and composition of the solution act as a mixture of HFA and hydrated SiF4.2H2O. It has also been proposed that the species SiF5- is in equilibrium with SiF4 and SiO2. It has also been stated that the HFA may be present in either mono-, di-, and tetra-, forms of hydration. A significant amount of work has been completed relating to the vapour pressure and vapour phase composition of scrubber liquor solution and pure HFA. Several experiments were completed to examine the volatility of HFA. It was found that when HFA was heated a distillate of higher concentration could be produced. It was identified that this could be due to the volatilisation of HFA or the decomposition by means of the reaction shown below. Additional experiments were completed where the HFA was again heated, but the vapour produced were passed through glass fibre which would adsorb the hydrogen fluoride, if it was present. In this experiment no distillate was produced regardless of the condenser temperature. A similar experiment was then completed where the humid air was added to the receiver vessel and a distillate containing HFA was again formed. It was therefore concluded from these experiments that HFA is a non-volatile acid similar to sulphuric acid, and cannot exist under ordinary conditions in the vapour state. This means pure HFA cannot be isolated as it decomposes to silicon tetrafluoride and hydrogen fluoride.

H2SiF6 → SiF4 + 2HF

It was also noted in the experiments that vessels which contained HFA became etched. This was most pronounced at the surface line of the liquid but there was consistent etching below the liquid level. The etching faded about an inch above the surface of the solution. There was little to no etching in further vessels which were connected up. It was hypothesised that this process occurred due to the decomposition of HFA to hydrofluoric acid and silicon tetrafluoride. The hydrofluoric acid rapidly went on to react with the silicon in the glass to form silicon tetrafluoride and water. This result coincides with the observation that when scrubber liquors contain high levels of silica the vapour phase will be rich in silicon tetrafluoride. This is compared to the solutions used inother experiments, which were low in silica, so formed elevated levels of hydrofluoric acid that went on to react with the silica in the glass. These results are significant to the process because this explains the relatively low levels hydrogen fluoride found within the fluoride vapour evolved from the phosphate rock and sulphuric acid. This is due to the reaction of hydrogen fluoride with the silica which is of great abundance within the scrubber liquor.

The affect of variations in the concentration of HFA were also examined. It was found that the maximum practical concentration of HFA within an industrial scrubbing system was between 22-25%. Above 25-26 wt% it was found that there was a sudden marked change in the character of the suspended silica. It was found to act as a stiff foam causing rapid expansion in volume. This significantly reduces the efficiency of fluoride scrubbing to a point where there can be overflow between vessels and a significant reduction in fluoride absorption capacity occurs. Foaming was only found to happen in a scrubbing system, as it was not found when HFA was concentrated 60.92% at room temperature (20-22°C). It was found that HFA was stable in this form for long periods of time.


The reaction of HFA and sulphuric acid has been of significant interest since recycling of scrubber liquor was developed. This was developed as a fluoride management process that reincorporated fluoride, which was initially present within the phosphate rock back into the finally product. It was found that this resulted in significant re-evolution of fluoride within the process. This is directly attributed to the reaction between sulphuric acid and HFA due to a clear relationship between several sulphuric acid based factors. Influencing factors included the concentration of sulphuric acid and proximity of sulphuric acid injection to HFA injection.
There were two main reaction pathways which have been attributed to the model where sulphuric acid is the driving factor for the re-evolution of fluoride from HFA. This can be seen in the Equations below.

H2SiF6 + 6H2SO4 → 6HSO3F + SiO2 + 4H2O
6HSO3F + 6H2O → 6H2SO4 + 6HF
4HF + SiO2 → SiF4 + 2H2O
H2SiF6 + 2H2SO4 → 2HSO3F + SiF4 + 2H2O
4HSO3F + SiO2 + 2H2O → SiF4 + 4H2SO4

The overall generalised process of sulphuric acid catalysis of HFA decomposition can be seen in the Equation below.

H2SiF6 + 6H2SO4 → 6H2SO4 + 2HF + SiF4

It should be noted that these reactions are purely theoretical and can be rearranged to give the same final product. In all of these reactions sulphuric acid only acts as a catalyst in the formation of hydrogen fluoride and silicon tetrafluoride from HFA.

Several experiments have been completed on this process with varying results. It has been found that the level of fluoride evolution can vary between 75 and 100% as the fluoride to silica ratio is decreased from 6:1 to 4:1. This relates back to what is seen in the reactions above where they can both proceed to completion in the presence of excess silica. It is more likely to follow the reactions seen in Equations below

H2SiF6 + 2H2SO4 → 2HSO3F + SiF4 + 2H2O (23)
4HSO3F + SiO2 + 2H2O → SiF4 + 4H2SO4

as the evolution of hydrogen fluoride in the absence of silica is not observed. There is also the alternative possibility that the main reason for high fluoride re-evolution is due to the heat which is released with the mixing of the HFA and sulphuric acid. It should also be noted that none of the reactions explain the repeated observation of the formation of voluminous white solid. The solid flows like a gel at the mixing tee of the sulphuric acid and HFA which suggests a silica or calcium sulphate compound. Considering the amount of research that has been completed on the recycling of HFA, there is relativity little known about the reactions that govern this process.


It is common practice to recycled HFA back into the reaction of phosphate rock and sulphuric acid. This reaction acts as a means of locking the fluoride into a solid state which is of negligible environmental effect. It also supplements the use of sulphuric acid as an active hydrogen donating molecule. Two different reactions between phosphate rock and HFA have been hypothesised. It has been suggested by two separate workers that calcium hexafluorosilicate, phosphoric acid and hydrogen fluoride are the produced from the reaction between phosphate rock and HFA as seen in the Equation below. An alternative theory suggests that calcium fluoride, phosphoric acid and silicon dioxide are formed from this reaction as seen in the other Equation below.

3[Ca3(PO4)2].CaF2 + 10H2SiF6 → 10CaSiF6 + 6H3PO4 + 2HF
3[Ca3(PO4)2].CaF2 + 3H2SiF6 + 6H2O → 10CaF2 + 6H3PO4 + 3SiO2

Both of the above reactions produce phosphoric acid and a compound that traps the fluoride within the solid state. This coincides with the observations made that the addition of HFA acts to supplement the action of the sulphuric acid as an active hydrogen donating compound to aid in the acidulation of the phosphate rock. It has also been found that approximately 25% of the recycled fluoride is contained within the final SSP as a solid compound.
Several workers have found that the solid fluoride based product was in the form of calcium fluoride. It was also found by another worker that when phosphate rock was reacted with the scrubbing liquor, only 85% of the stoichiometric portion of the rock could be added before an extremely voluminous precipitation of silica was formed. It was found that there is a clear reduction in the vapour pressure of fluoride above scrubber liquor with the increased proportion of phosphate rock. All of these observations indicate that the reaction shown in the Equation below is the most likely to occur. It was stated that further analysis is required to be completed on the reaction between phosphate rock and HFA.

3[Ca3(PO4)2].CaF2 + 3H2SiF6 + 6H2O → 10CaF2 + 6H3PO4 + 3SiO2"

hopefully some of this information may be usefull for you current or future experiments

[Edited on 28-11-2012 by feacetech]
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[*] posted on 27-11-2012 at 23:48


I have to agree with blogfast25. Hexafluorosilicate does form HF under certain conditions and it is capable of attacking glass. Not very strongly, but still noticeable. I heated an acidified solution of (NH4)2SiF6 in a test tube and afterwards, the test tube is slightly frosty when it becomes dry.
So, I can imagine that putting SiF4 in a glass beaker above some water may result in damage. This is the main reason why I still did not do these experiments, they sound fascinating, but I value my glassware too much. I'll wait until I have some old small glass household jar or something like that which may be discarded after the experiments. I also have a few thin glass tubes, used for all kinds of gas-experiments and I also am quite hesitant to use these tubes for experiments with SiF4.




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[*] posted on 29-11-2012 at 12:47


Quote: Originally posted by feacetech  
All of this sounds very similar to the prodcution of superphosphate and the flouride byproducts

below is an edited extract from a literature review of fluoride managment during SSP production

"The exact chemistry of the reaction between sulphuric acid and phosphate rock is not precisely understood due to the unknown composition of phosphate rock. The simplified reaction of fluorapatite and sulphuric acid is commonly used as a good representation of this process. In this process sulphuric acid reacts with the fluorapatite to form phosphoric acid, calcium sulphate and a fluoride based compound. The phosphoric acid goes on to react with another fluorapatite molecule to form monocalcium phosphate and an additional fluoride compound. This process can be seen below;

2{3[Ca3(PO4)].CaF2} + 18H2SO4 → 18CaSO4 + 12H3PO4 + 2CaF2
3[Ca3(PO4)].CaF2 + 12H3PO4 → 9Ca(H2PO4)2 + CaF2
3CaF2 + 3H2SO4 → 3CaSO4 + 6HF
9Ca(H2PO4)2 + 9H2O → 3Ca(H2PO4)2.H2O
21CaSO4 + 42H2O → 6CaSO4.2H2O

3[Ca3(PO4)].CaF2 + 7H2SO4 + 17H2O → 3Ca(H2PO4)2.H2O + 7CaSO4.2H2O + 2HF

The current mechanism of fluoride evolution is the production of hydrogen fluoride from the reaction of fluorapatite and sulphuric acid. It has then been suggested that the hydrogen fluoride gas is initially converted into hydrofluoric acid. A reaction then occurs with the silicates in the phosphate rock to form silicon tetrafluoride. This reaction can be seen in the 2 equaitons below where M2SiO3 represents the silica based compounds.

6HF → 3H2F2
M2SiO3 + 3H2F2 → SiF4 + M2F2 + 3H2O

there is only partial volatilisation of fluoride and it is predominantly in the form of silicon tetrafluoride. This is assuming there is a high rate of conversion from hydrogen fluoride to hydrofluoric acid

The established set of reactions with hydrogen fluoride as an intermediate product has been put under dispute. It was found the reaction between concentrated hydrofluoric acid and silicates are usually very slow; therefore, there should be more hydrofluoric acid present in the gas evolved. It was even found that hydrofluoric acid reacts with the fluorapatite to form available phosphate rather than with the silica to enhance the volatilisation of fluorine as seen below. These results also coincide with experiments completed where quartz flour and other silicates were added to low silica phosphate rocks where it was found that there was no increase in the fluorine volatilisation. Also with experiments where 12% hydrogen fluoride was directly added to the process and there was no increase in fluoride evolution observed. It has been said it is more likely that silicon tetrafluoride is directly produced but there are currently no reaction pathways which detail this process

Ca10(PO4)6F2 + 10H2F2 → 6H3PO4 + 10CaF2 + 2HF

An experiment was completed which showed that the addition of calcium fluosilicate increases the evolution of the total fluoride evolution. The percentage of total fluoride evolution remained very consistent between 35.4-35.9%. This indicates that the level of fluoride evolution is directly related to the presence of calcium flurosilicate compounds. It was concluded that fluorine evolution behaves like hexafluorosilicic acid and not like hydrofluoric acid.


Fluoride evolution is the main by-product from the production of superphospahte. The majority of the fluoride is evolved from the initial reaction of phosphate rock with sulphuric acid in the form of silicon tetrafluoride. The silicon tetrafluoride vapours are then run through a wet scrubbing system in the presence of excess water. It is generally accepted that the silicon tetrafluoride is hydrolysed to form HFA and silicon dioxide. This reaction has even been shown to take place within the reaction chamber as there is an excess of water vapour present with the silicon tetrafluoride. There has been a wide variety of different reactions which have been shown to form the HFA and silica based products as seen in below. The first three reactions agree on the formation of HFA but differ in the type of silica molecule that will be formed. Three different types of silica have been hypothesised to form they are silicon dioxide (SiO2), hydrated silicon dioxide (SiO2.2H2O) and silicon hydroxide (Si(OH)4) below. Most workers have cited at least one of these reactions but no experimental evidence has been produced.

3SiF4 + 2H2O → 2H2SiF6 + SiO2
3SiF4 + 4H2O → 2H2SiF6 + SiO2.2H2O
3SiF4 + 4H2O → 2H2SiF6 + Si(OH)4

It has also been noted that when silicon levels are high fluodisilicic acid (H2SiF6.SiF4) is formed in equilibrium with HFA in solution. In this case the silicon tetrafluoride is thought to be hydrolysed the reaction expressed below.

(3+2n)SiF4 + 2H2O → 2H2SiF6.nSiF4 + SiO2

Any of these reactions are possible. There has been insufficient experimental work completed; therefore, it can not be said which ones take place. It has even been hypothesised that each of the reactions will occur depending on the reaction conditions, such as, temperature, pressure, pH and many others. A significant amount of work has been completed on the gaseous phase reaction between silicon tetrafluoride and water which is often used in the production of high purity amorphous silicon and silicon dioxide fibres . It was stated that other chemicals besides HFA are formed during the hydrolysis of SiF4. These include hexafluorodisiloxane (SiOF6) and perfluorotrisiloxane (Si3O2F8). Mass spectrometer readings showed that the compound SiF3OSiF2OSiF3 was present. The Majority of this research is not applicable to the wet scrubbing system used in the production of SSP. This is because reactions are completed at temperatures often above 200°C and are completed with an excess of silicon tetrafluoride and extremely low concentration of oxygen and hydrogen. There was a study completed that examined the gaseous phase reaction between silicon tetrafluoride and water at atmospheric conditions. This was completed because of the potential for the formation of hydrogen fluoride which poses a safety risk. This work may relate to the conditions found above the reaction of phosphate rock and sulphuric acid. The reactions put forward are shown below. The results from this work concluded that hydrogen fluoride is not produced during the exposure of silicon tetrafluoride to humid air. An unidentified hydrolysed compound was produced.

4SiF4 + 2H2O → 2(F3Si)2O + 4HF
2SiF4 + 2H2O → H2SiF6 + SiO2 + 2HF
SiF4 + 2H2O → SiO2 + 4HF

A solution is produced from the hydrolysation of silicon tetrafluoride during the production of Superphosphate this is mainly comprised of HFA (H2SiF6) and silicon dioxide (SiO2) in solution with water. It has been shown by several workers to be an inaccurate simplification. This is because the scrubber liquor has been found to have a fluoride to silica ratio (F:Si) of 4.7-5.35. The ratio is significantly lower than the F:Si ratio of 6 of pure HFA. Several workers have stated that the low ratio is due to the formation fluodisilicic acid (H2SiF6.SiF4) which is present in a state of equilibrium with the HFA compound. The equilibrium position is mainly influenced by the concentration of fluoride ions. This hypothesis does hold as the properties and composition of the solution act as a mixture of HFA and hydrated SiF4.2H2O. It has also been proposed that the species SiF5- is in equilibrium with SiF4 and SiO2. It has also been stated that the HFA may be present in either mono-, di-, and tetra-, forms of hydration. A significant amount of work has been completed relating to the vapour pressure and vapour phase composition of scrubber liquor solution and pure HFA. Several experiments were completed to examine the volatility of HFA. It was found that when HFA was heated a distillate of higher concentration could be produced. It was identified that this could be due to the volatilisation of HFA or the decomposition by means of the reaction shown below. Additional experiments were completed where the HFA was again heated, but the vapour produced were passed through glass fibre which would adsorb the hydrogen fluoride, if it was present. In this experiment no distillate was produced regardless of the condenser temperature. A similar experiment was then completed where the humid air was added to the receiver vessel and a distillate containing HFA was again formed. It was therefore concluded from these experiments that HFA is a non-volatile acid similar to sulphuric acid, and cannot exist under ordinary conditions in the vapour state. This means pure HFA cannot be isolated as it decomposes to silicon tetrafluoride and hydrogen fluoride.

H2SiF6 → SiF4 + 2HF

It was also noted in the experiments that vessels which contained HFA became etched. This was most pronounced at the surface line of the liquid but there was consistent etching below the liquid level. The etching faded about an inch above the surface of the solution. There was little to no etching in further vessels which were connected up. It was hypothesised that this process occurred due to the decomposition of HFA to hydrofluoric acid and silicon tetrafluoride. The hydrofluoric acid rapidly went on to react with the silicon in the glass to form silicon tetrafluoride and water. This result coincides with the observation that when scrubber liquors contain high levels of silica the vapour phase will be rich in silicon tetrafluoride. This is compared to the solutions used inother experiments, which were low in silica, so formed elevated levels of hydrofluoric acid that went on to react with the silica in the glass. These results are significant to the process because this explains the relatively low levels hydrogen fluoride found within the fluoride vapour evolved from the phosphate rock and sulphuric acid. This is due to the reaction of hydrogen fluoride with the silica which is of great abundance within the scrubber liquor.

The affect of variations in the concentration of HFA were also examined. It was found that the maximum practical concentration of HFA within an industrial scrubbing system was between 22-25%. Above 25-26 wt% it was found that there was a sudden marked change in the character of the suspended silica. It was found to act as a stiff foam causing rapid expansion in volume. This significantly reduces the efficiency of fluoride scrubbing to a point where there can be overflow between vessels and a significant reduction in fluoride absorption capacity occurs. Foaming was only found to happen in a scrubbing system, as it was not found when HFA was concentrated 60.92% at room temperature (20-22°C). It was found that HFA was stable in this form for long periods of time.


The reaction of HFA and sulphuric acid has been of significant interest since recycling of scrubber liquor was developed. This was developed as a fluoride management process that reincorporated fluoride, which was initially present within the phosphate rock back into the finally product. It was found that this resulted in significant re-evolution of fluoride within the process. This is directly attributed to the reaction between sulphuric acid and HFA due to a clear relationship between several sulphuric acid based factors. Influencing factors included the concentration of sulphuric acid and proximity of sulphuric acid injection to HFA injection.
There were two main reaction pathways which have been attributed to the model where sulphuric acid is the driving factor for the re-evolution of fluoride from HFA. This can be seen in the Equations below.

H2SiF6 + 6H2SO4 → 6HSO3F + SiO2 + 4H2O
6HSO3F + 6H2O → 6H2SO4 + 6HF
4HF + SiO2 → SiF4 + 2H2O
H2SiF6 + 2H2SO4 → 2HSO3F + SiF4 + 2H2O
4HSO3F + SiO2 + 2H2O → SiF4 + 4H2SO4

The overall generalised process of sulphuric acid catalysis of HFA decomposition can be seen in the Equation below.

H2SiF6 + 6H2SO4 → 6H2SO4 + 2HF + SiF4

It should be noted that these reactions are purely theoretical and can be rearranged to give the same final product. In all of these reactions sulphuric acid only acts as a catalyst in the formation of hydrogen fluoride and silicon tetrafluoride from HFA.

Several experiments have been completed on this process with varying results. It has been found that the level of fluoride evolution can vary between 75 and 100% as the fluoride to silica ratio is decreased from 6:1 to 4:1. This relates back to what is seen in the reactions above where they can both proceed to completion in the presence of excess silica. It is more likely to follow the reactions seen in Equations below

H2SiF6 + 2H2SO4 → 2HSO3F + SiF4 + 2H2O (23)
4HSO3F + SiO2 + 2H2O → SiF4 + 4H2SO4

as the evolution of hydrogen fluoride in the absence of silica is not observed. There is also the alternative possibility that the main reason for high fluoride re-evolution is due to the heat which is released with the mixing of the HFA and sulphuric acid. It should also be noted that none of the reactions explain the repeated observation of the formation of voluminous white solid. The solid flows like a gel at the mixing tee of the sulphuric acid and HFA which suggests a silica or calcium sulphate compound. Considering the amount of research that has been completed on the recycling of HFA, there is relativity little known about the reactions that govern this process.


It is common practice to recycled HFA back into the reaction of phosphate rock and sulphuric acid. This reaction acts as a means of locking the fluoride into a solid state which is of negligible environmental effect. It also supplements the use of sulphuric acid as an active hydrogen donating molecule. Two different reactions between phosphate rock and HFA have been hypothesised. It has been suggested by two separate workers that calcium hexafluorosilicate, phosphoric acid and hydrogen fluoride are the produced from the reaction between phosphate rock and HFA as seen in the Equation below. An alternative theory suggests that calcium fluoride, phosphoric acid and silicon dioxide are formed from this reaction as seen in the other Equation below.

3[Ca3(PO4)2].CaF2 + 10H2SiF6 → 10CaSiF6 + 6H3PO4 + 2HF
3[Ca3(PO4)2].CaF2 + 3H2SiF6 + 6H2O → 10CaF2 + 6H3PO4 + 3SiO2

Both of the above reactions produce phosphoric acid and a compound that traps the fluoride within the solid state. This coincides with the observations made that the addition of HFA acts to supplement the action of the sulphuric acid as an active hydrogen donating compound to aid in the acidulation of the phosphate rock. It has also been found that approximately 25% of the recycled fluoride is contained within the final SSP as a solid compound.
Several workers have found that the solid fluoride based product was in the form of calcium fluoride. It was also found by another worker that when phosphate rock was reacted with the scrubbing liquor, only 85% of the stoichiometric portion of the rock could be added before an extremely voluminous precipitation of silica was formed. It was found that there is a clear reduction in the vapour pressure of fluoride above scrubber liquor with the increased proportion of phosphate rock. All of these observations indicate that the reaction shown in the Equation below is the most likely to occur. It was stated that further analysis is required to be completed on the reaction between phosphate rock and HFA.

3[Ca3(PO4)2].CaF2 + 3H2SiF6 + 6H2O → 10CaF2 + 6H3PO4 + 3SiO2"

hopefully some of this information may be usefull for you current or future experiments

[Edited on 28-11-2012 by feacetech]


Thank you for providing all this information! I will actually print this and add it to my notebook for the next time I experiment with SiF4.

I have done another experiment, I moistened a microscope slide with a solution of bromothymol blue and a very small amount of surfactant triton-x to avoid droplets from forming. This slide was placed into a plastic sample container into which the SiF4 was directed into. The indicator turned yellow as quickly as a layer of silicic acid formed. I was able to wipe off the crusty layer of yellowish silicic acid, but the slide remains fogged after drying. It has a feeling similar to a chalkboard when scratched with my fingernail. So evidently acid is indeed formed, and if there are very thin layers of water it seems that the HF levels are high enough to etch the surface of the glass.

Still, this is an intriguing material, I will be building a better suited set up to experiment with. I wonder how its solubility might be in non-aqueous solvents?
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[*] posted on 6-7-2013 at 03:54


Quote: Originally posted by Mailinmypocket  
I wonder how its solubility might be in non-aqueous solvents?

CRC handbook says soluble in abs. alcohol and HF.
Insoluble in ether.

Feacetech, thanks for posting that info, it is interesting. I was wandering if the unbalanced equations are some kind of ancient notation that we don't use anymore?, e.g (+ many more):
Quote: Originally posted by feacetech  

21CaSO4 + 42H2O → 6CaSO4.2H2O



[Edited on 6-7-2013 by phlogiston]




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[*] posted on 14-7-2013 at 16:52


Quote: Originally posted by garage chemist  
The reaction goes:

3 SiF4 + 2 H2O ---> 2 H2SiF6 + SiO2

That is basically true, but just a slight little correction:

3 SiF4 + 8 H2O --> 4 H3O+ + 2 SiF62- + Si(OH)4

:D


I had an idea to react SiF4 with CaCl2 or CaBr2 in a molten ZnCl2 flux. It would be hazardous, and probably impractical, but it would be an interesting route to SiCl4 without first having to make elemental silicon.

SiF4 + 2 CaBr2 --> 2 CaF2 + SiBr4

CaF2 is completely insoluble, and has a high entropy of formation, so it would drive the reaction forward. ZnCl2 has a relatively low melting point, at 275-292 °C. It may be a little difficult to prepare since the anhydrous form cannot be obtained by simply heating the hydrate.

[Edited on 15-7-2013 by AndersHoveland]
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