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Cyrus
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Toldjya so!
It does react. But I never got enough heat going for the silicon to fuse into globules- the silicon or silicide or whatever what formed stayed
intimately mixed with the rest of the reaction products and leftover sand. By the way, dissolving the products in HCl did produce silane as far as I
know. (syntheticish fruity smelling explosive gas). I never figured out how to seperate the unknown amount of Si powder formed from the SiO2
easily... I hope your method works.
Cyrus
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blogfast25
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A sulfur-free silicon thermite
I'm sure many of you here have experimented with thermite reactions, including one of the harder reactions, that of silicon dioxide and aluminium to
produce silicon metal(loid) and alumina.
Although the reaction 3 SiO<sub>2</sub> + 4 Al ---> 3 Si + 2 Al<sub>2</sub>O<sub>3</sub> is thermodynamically
favourable (ΔH < 0), this reaction does not propagate by itself, presumably because the heat of reaction isn't enough to overcome the
activation energy it needs. Straight mixes of a silica source and aluminium powder therefore fizzle out or can't be ignited.
To make the reaction self-sustainable, the most used method by backyard scientists (and fellow travellers) is to add a booster mix of aluminium powder
and sulfur, which reacts according to 2 Al + 3 S ---> Al<sub>2</sub>S<sub>3</sub> with great development of heat (ΔH
≈ -5.3 kJ/g of stoichiometric mix). This heat provides the missing activation energy and makes the reduction of silica to silicon with Al
self-sustaining.
A typical sulfur boosted silicon thermite mixture is silica/Al/S = 100/111/133 (9:10:12) but I've also successfully used mixes much lower in S, such
as 100/72/21.
Apart from yielding a self-sustaining reaction, S-boosted silicon thermites have also other advantages:
• Quite easy to ignite, using Mg ribbon (e.g.).
• The resulting slag is a mix of alumina and aluminium sulfide. The much lower MP of the sulfide (around 1,100 C) causes the slag mix to be more
fluid than pure alumina, which freezes at around 2,000 C. This greatly helps slag/metal separation, as the slag/metal mix remains liquid longer,
allowing it to collect in the bottom of the crucible and the metal to coalesce out. And a mix of alumina and aluminium sulfide is also much softer
than pure, fused alumina, making the slag easier to break up mechanically.
• The alumina/aluminium sulphide slag mix reacts readily with water through hydrolysis of the sulphide:
Al<sub>2</sub>S<sub>3</sub> + 6 H<sub>2</sub>O ---> 2 Al(OH)<sub>3</sub> + 3
H<sub>2</sub>S. This breaks up the slag into a (stinky) hydrated alumina slurry (or mud), giving easy access to the metal globules.
You can find an example of a 300 g S-boosted silicon thermite at this blog post of mine.
But that's the good news and there's some bad news too: the aluminium sulfide is so prone to hydrolysis, that even the newly fused slag positively
reeks of H<sub>2</sub>S, in plain English: rotten eggs. Needless to say, adding water or a mineral acid to it, seriously aggravates the
problem. Not only does H<sub>2</sub>S stink terribly, it's also toxic and it's perceptible even in trace amounts.
(Tip: if you're going to treat an alumina/aluminium slag mix with water, use bleach instead of pure water: the hypochlorite will convert much of the
H<sub>2</sub>S to elemental sulfur, which is even recoverable).
So much for the long-winded intro, I guess.
In a nutshell, I got so fed up with the smell of rotten eggs, I decided to try and replace the S-booster mix with a sulfur-free system. I chose to
investigate a potassium chlorate/Al mix, which reacts according to 2 Al + KClO<sub>3</sub> --->
Al<sub>2</sub>O<sub>3</sub> + KCl with an estimated ΔH ≈ 9.5 kJ/g (of stoichio mix). I had used such mixes before
for lighting thermites.
Initial tests with a silica/Al/KClO<sub>3</sub> = 100/72/27 mix showed clearly that the reaction proceeded self-sustainingly and that Si
metal was formed, in a hard, porous alumina matrix. I gradually stepped up the amount of booster mix to 100/84/57 and later 100/96/81, to find that
progressively more of the slag ends up at the bottom of the crucible (I used mostly 20 g mini batches for the development work) because of the
increasingly high peak temperatures during the reactions.
Much of this development can be followed here and on subsequent pages at the ABYMC forum (where I post as Gert from England).
The main problem remains slag/metal separation, both in situ and after the reaction products have cooled down: the pure, fused alumina
freezes up quickly into a very, very hard mass. 32 w% HCl doesn't even begin to dent it and forget about mechanical separation: this stuff is HARD!
I then proceeded to test calcium fluoride (Fluorite, CaF<sub>2</sub> as
a potential flux, at 20 w% added to the promising 100/96/81 formulation (this then became 100/96/81/55 - with 55 the CaF<sub>2</sub> . Although it made a world of difference in the sense that much larger globules of Si
metal form, the slag remains extremely hard and insensitive to HCl. Another test at 40 w% CaF<sub>2</sub> showed that at that level the
reaction was being slowed down, probably due to adding so much inert material, and hence slag metal separation deteriorated again due to lower peak
temperatures.
In the mean time I've tried borax (without success) and a glassy flux formulation (designed for fluxing copper or bronze) without positive results
either.
My main hope now lies with Cryolite (Na<sub>3</sub>AlF<sub>6</sub> which has a lower melting point than CaF<sub>2</sub> (about 1,000 against about 1,400 C) and I'm waiting for a delivery of
this stuff.
Other ideas that have been floated include reducing the radiative losses by covering the crucible partially, thereby reaching even higher peak
temperatures and postponing the freeze.
Any useful ideas or experiences would be appreciated... 
[Edited on 11-3-2008 by blogfast25]
[Edited on 11-3-2008 by blogfast25]
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ScienceGeek
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Very interesting post. It seems you been busy lately 
My experience is the same as yours. Hate the smell of rotten eggs in the morning after having done a Silicon Oxide thermite.
I think all your ideas are/was promising, and using Cryolite does not seem like a bad one either.
Unfortunately, my experience with Thermite is limited. Just wanted to say: Nice research!
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chloric1
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Glad someone tried the KClO3/Al booster. I had seen this in a scholar article about obtaining pure vanadium from calcium metavanadate. The mixture
had to be heated to like 600 or 800°C before it could be ignited. Two suggestions stated using a booster of KClO3/Al or V2O5/Al. This is very
simular to the most modern form of titanium isolation where titania is mixed with molten CaCl2 and electrolyzed to yield pure Ti via an active Ca
intermediate. The elemental Ca intermediate is also the active reducer in vanadium and tungsten by aluminothermic reactions.
In leu of the calcium factor, why not try calcium metasilicate mixed with aluminum and a KClO3/Al booster additive. You might try additional Calcium
silicate to obtain calcium aluminosilicate which is a major ingredient to concrete. It will still be hard but perhaps more brittle.
Fellow molecular manipulator
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chemoleo
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I'm not sure what the advantage of this is, after all you end up with a monstrously hard and inert korundum slag from which it is very hard
to seperate elemental Si... whilst the sulfur method just wastes a bit of cheap S and somewhat more Al (which considering our quantities) is also
relatively cheap...
I notice you are into metal casting, what purpose did you have in mind for the Si?
[Edited on 12-3-2008 by chemoleo]
Never Stop to Begin, and Never Begin to Stop...
Tolerance is good. But not with the intolerant! (Wilhelm Busch)
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not_important
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A potential, meaning I don't know if it happens to a significant extent, problem with using fluorides is:
2CaF2 + 5 Si => 2 CaSi2 + SiF4 (g)
2CaF2 + 3 Si => 2 CaSi + SiF4 (g)
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blogfast25
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| Quote: | Originally posted by chemoleo
I'm not sure what the advantage of this is, after all you end up with a monstrously hard and inert korundum slag from which it is very hard
to seperate elemental Si... whilst the sulfur method just wastes a bit of cheap S and somewhat more Al (which considering our quantities) is also
relatively cheap...
I notice you are into metal casting, what purpose did you have in mind for the Si?
[Edited on 12-3-2008 by chemoleo] |
chemoleo, the main purpose here is to get rid of the S, so as to avoid the horribly smelling H<sub>2</sub>S. Those who've
run S-boosted thermites repeatedly will probably concur that that gas is a problem, especially when running larger thermites.
Optimising the flux system should yield a considerably softer slag. That's the objective of the investigation...
The connection to metal casting was an investigation into the possibility of producing Si/Al master alloys by means of thermite reactions.
Chlorate-boosted thermites also have potential for other difficult thermites, notably TiO<sub>2</sub> or
B<sub>2</sub>O<sub>3</sub> and possibly others that are hard to light and/or sustain. Good burns have been obtained both with
titania and boron trioxide. But in the case of titania, no metal was observed.
| Quote: | Originally posted by not_important
A potential, meaning I don't know if it happens to a significant extent, problem with using fluorides is:
2CaF2 + 5 Si => 2 CaSi2 + SiF4 (g)
2CaF2 + 3 Si => 2 CaSi + SiF4 (g)
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No, this does not appear to be a problem. CaF<sub>2</sub> is too stable for reduction by Si or Al.
| Quote: | Originally posted by chloric1
In leu of the calcium factor, why not try calcium metasilicate mixed with aluminum and a KClO3/Al booster additive. You might try additional Calcium
silicate to obtain calcium aluminosilicate which is a major ingredient to concrete. It will still be hard but perhaps more brittle.
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chloric, Al would reduce Ca silicate to Si and CaO, as it does CaSO<sub>4</sub> (to alumina, Al sulphide and CaO). CaO
has an even higher MP than alumina! 
[Edited on 12-3-2008 by blogfast25]
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chloric1
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Yes but CaO is soluble in dilute HCl and Si is not. Even if you still have aluminum oxide slag, I would think it would be easier to work with after
leaching calcium out. Hot NaOH will probably dissolve the alumina or you could add the mix to hot concentrated sulfuric acid to get aluminum sulfate.
I think you need magnesium or calcium to get titanium metal. Magnesium with titania and 10 or 15% potassium perchlorate/Mg mixed in would probably
do it.
Fellow molecular manipulator
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blogfast25
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| Quote: | Originally posted by chloric1
Yes but CaO is soluble in dilute HCl and Si is not. Even if you still have aluminum oxide slag, I would think it would be easier to work with after
leaching calcium out. Hot NaOH will probably dissolve the alumina or you could add the mix to hot concentrated sulfuric acid to get aluminum sulfate.
I think you need magnesium or calcium to get titanium metal. Magnesium with titania and 10 or 15% potassium perchlorate/Mg mixed in would probably
do it. |
chloric:
I did in fact run a test using CaSO<sub>4</sub> as a booster (AND chlorate):
3 CaSO<sub>4</sub> + 8 Al ---> 3 CaS + 4 Al<sub>2</sub>O<sub>3</sub> + ΔH
CaS also reacts with HCl and I could smell the H<sub>2</sub>S. But the slag wasn't appreciably easier to handle...
Hot NaOH would dissolve the slag, given time and temperature, unfortunately it also dissolves Si, because the metalloid is amphoteric (like Al).
Hot 70 - 80% sulfuric acid would probably also dissolve the slag (again, given time and temp.) but it's hard to get hold of.
Regarding TiO<sub>2</sub>, thermodynamically the reduction with Al is possible (but that doesn't mean it works in practice). Several
Internet reports report a burning TiO<sub>2</sub>/Al mixture, I've managed to get it to burn with chlorate booster and also with magnalium
(50/50 Mg/Al) without booster. A work in progress, I'd say.
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chief
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Quote: Originally posted by 12AX7  | Sounds like the trick is to make stoichiometric mixes of Al+S and Al+SiO2 + CaF2 (about 10% as flux, it is fluorite after all!) then blend
different proportions of the two mixtures to see which will sustain. Or other things... lead oxide may be a candidate, as it does not form silicides.
Tim |
I did this 2 days ago, but not with sulfur ...
==> Instead I used stoechiometric MnO2-Al-thermite for the exothermicity and ignition ...
==> and SiO2-Al-thermite for the Si ...
50-50-mixtures of the both thermites do burn, and quite hot ...
Results still are in the furnace, will have a look these days ... (abused one of my old furnaces, for thermal insulation and thereby slower cooling
... )
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Random
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There is a video on youtube about extracting silicon from chips.
http://www.youtube.com/user/Naravoslovje#p/u/24/-H4rt8q1XtY
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blogfast25
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The method Chemoleo describes here:
http://www.sciencemadness.org/talk/viewthread.php?tid=2030#p...
... I've used many times. It's probably the easiest way for a backyard chemist to prepare some relatively pure silicon.
Boron is prepared in much the same way, although one obtains AlB12. There's a detailed procedure in Brauer's 'Inorganic Preparative Chemistry' (see
library).
I'm not sure B2O3 would actually colour a fire work green: the spectrum of B contains a strong green line but B2O3 is very stable and wouldn't
dissociate much in fire work conditions. AlB12 may be better suited for that purpose. Worth trying!
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a_bab
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Just stumbled across this reaction while studying the silicone carbide (which itself has a fascinating manufacturing process, plus it looks great  ). It is claimed that silicon carbide is decomposed to silicon by the silicon
dioxide:
SiC+SiO2--->2Si + CO2
It looks too good to be true. Even if electric arc is needed it still looks nice giving the "clean" reaction and the "OTC-bility of the reagents". It
may need less then 1000 degrees C.
Does anyone else heard about this? All I could find was about thin silicon films that could be formed on the surface of SiO2 crystals using this
reaction.
[Edited on 9-7-2010 by a_bab]
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blogfast25
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| Quote: Originally posted by a_bab |
It looks too good to be true. Even if electric arc is needed it still looks nice giving the "clean" reaction and the "OTC-bility of the reagents". It
may need less then 1000 degrees C.
[Edited on 9-7-2010 by a_bab] |
Sure does. Where did you stumble on it?
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a_bab
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An old russian chemistry book. It was just a note but it attracted my attention.
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The WiZard is In
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| Quote: Originally posted by chemoleo | Remember this post in the Exotic thermites thread?
| Quote: | Silicon Dioxide thermite
Today this was tried, in stoichiometric proportions, i.e. 54 g Al and 180 g SiO2, according to 3 SiO2 + 2 Al --> 3 Si + 2 Al2O3.
[snip]
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Lateron I found that 200 mesh Al and superdry SiO2 powder would still not ignite, not with a torch, nor with NaClO3/Al or sparklers.
S.C. Wack then posted this | Quote: | | Sulfur. Schlessinger writes of 90 g sand, 100 g Al powder, and 120 g S in a crucible which is in sand. |
And this I did! All of the SiO2/sand and Al was combined, and the adjusted amount of S added, and filled into a flowerpot.
This indeed was ignitable by sparklers, and would burn with intense white light for many minutes! Very pyrotechnic and pretty.
The resulting glowing mass was allowed to cool, and left overnight in the wet grass - and the next day this had completely disintegrated into grey
squishy powder.... with lumps in them. Guess what they were - silicon !! Crystalline at that (see pic), one can see the crystal faces glittering here
and there. It does not dissolve in HCl, and is shiny on the surface! Ideally I'd have subjected it to HF treatment but didnt deem it worth it.
Pretty eh?
I suspect the larger agglomerates of Si were possible becuase the reaction was fairly large, giving finely divided Si time to agglomerate at high
white heat. So the smaller the reaction, the harder it will be to produce large chunks of Si!
One word of warning though - an enormous amount of Al2S3 is produced - which happily reacts with H2O forming tons of H2S. Unfortunately I left a small
test amount in front of a window, which completely reeked out the room overnight by air/H2S being blown in!
[Edited on 12-5-2005 by chemoleo] |
Mellor sez 6:149
K.A. Kühne used the thermite process with a mixture of 360 parts
silica, and 400 parts of aluminium ; he also used a mixture of
silica , 36 parts ; aluminium 40 parts ; and sulphur , 50 parts
in a fireclay crucible. Holleman and Sliper used a modification of
this process. Watts recommended using cryolite or felspar as a
flux. Gröppel, a basic aluminium silicate.
K.A. Kühne, German patent D.R.P. 147871, 1902 ; 179403, 1905.
Holleman and Sliper, Rec, Trav. Chim. Pays-Bas, 23. 381, 1904.
Watts, An Investigation of the Borides and Silicides, Madison, WI,
1906
Gröppel ?
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blogfast25
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This is indeed one of the old ways of producing technical grade Si. Good enough for alloying purposes I'd imagine: most impurities (apart from some
Al) will simply surface as dross...
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The WiZard is In
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| Quote: Originally posted by Cyrus | I just finished a little test reaction.
2Mg + SiO2 -> Si + 2MgO
10.00 grams Mg in the form of turnings (about 4 g) and larger
chunks, which I figured would melt as soon as the reaction got
going.
[snip]
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This from Watt's Dictionary of Chemistry. 1902. If found it
in my print edition and then checked Google.com/books
from which this was cut.
Preparation. — Amorphous silicon.— 1. An intimate mixture of 10
g. Mg powder and 40 g. thoroughly dry sand is placed in a
testtube, of fairly thick glass, c. 2-3 cms. diameter and c. 15 cms.
long; the tube is heated throughout by a large flame, and then
the lower part is very strongly heated, when reduction quickly
occurs. If the tube is gradually moved downwards so that one part
is strongly heated after another, the whole of the SiO, is reduced
in a few minutes. The contents of the tube are shaken out,
pulverised, and treated with HCl Aq (1:2), the solid being added to
the acid in small successive quantities (to prevent explosion from
evolution of SiH4) (Gattermann, B. 22, 186).
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condennnsa
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how do you guys make your SiO2 powders for these experiments? I thought of using expanded perlite, since it's 70-75% SiO2 (wikipedia) and it's really
soft, easily pulverised with a coffee grinder. would it work?
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The WiZard is In
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| Quote: Originally posted by condennnsa | | how do you guys make your SiO2 powders for these experiments? I thought of using expanded perlite, since it's 70-75% SiO2 (wikipedia) and it's really
soft, easily pulverised with a coffee grinder. would it work? |
-----
I wonder if Cab-O-Sil (fumed silica), a really-rally fine powder
wouldn't work. Excepting you would have to compact it.
White quartz sand reduced to a powder in a ball mill
or some such perhaps ....
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a_bab
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What about the very OTC silicagel? Easy tu pulverize, although I woudn't stay while doing it.
Also, some large electrical fuses (>100 amps) contain pure white fine silicon dioxide.
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blogfast25
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Cab-O-Sil ('fumed' or 'pyrogenic' silica) works: slightly wet it, then dry it: it compacts no end, while staying very fine.
There's also pottery silica or sand. Grind down your sand in a granite mortar and pestle of maybe a ball grinder... Even ground glass works but it's
not pure SiO2, of course...
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condennnsa
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What about dissolving an aluminum alloy of high Si content in hydrochloric acid? Al, Mg, Fe should go in solution , while silicon is not attacked by
HCl as far as I know... ?
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12332123
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@condennnsa
While pure silicon may not be attacked by HCl, I very much suspect that it would form silane if reacted in the form of an alloy.
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blogfast25
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If the alloy containes silicide(s) (it ain't necessarily so) then you get: metal silicide + acid ---> Metal + silane
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