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blogfast25
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Quote: Originally posted by Upsilon  |
Honestly I think I'll go with the carbothermic. I was worried about carbon monoxide but then I remembered that it would simply burn immediately after
being produced to form carbon dioxide. Besides, the alumina would probably be too hard to melt in the arc furnace (especially since I'll probably be
using alumina brick), plus the carbothermic reduction should leave nothing but Si metal behind so I don't have to deal with slag (in reality though, I
expect there to be a noticeable amount of carbon left over). |
On of the problems you could be having is to protect the Si from air oxygen since as you need very high temperature to conduct that reduction.
[Edited on 15-10-2015 by blogfast25]
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Upsilon
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| Quote: Originally posted by blogfast25 |
On of the problems could be having to protect the Si from air oxygen sinceas you need very high temperature to conduct that reduction.
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That too; I'll try it out in air first to see just how bad the silicon is attacked. If it's too bad then I'll carry it out in a vacuum or inert gas
environment.
But I'm getting ahead of myself. I won't be able to make the arc furnace for a while yet and the vacuum chamber is even further off (probably around
December-January). For now I'll see what else I can do. I'll be trying the TiO2 thermite this weekend and possibly the lithium nitrate electrolysis. I
think I have a pretty solid plan for lithium; my diagram is still floating around up there somewhere. I found this Kapton tape stuff that's designed
for high temperature so I'll be using that to insulate my wires. As long as borosilicate glass can withstand the temperature of lithium nitrate, I'll
be good to go.
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blogfast25
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Electrolysis of a LiCl/KCl eutectic (MP about 300 C), not very well executed:
https://www.youtube.com/watch?v=lGs-4bUM5Gk
The best source of information on lab based preparation of Li is found in "Small-Scale Synthesis of Laboratory Reagents with Reaction Modeling" by
Leonid Lerner:
http://www.twirpx.com/file/1618910/
From which it can be downloaded, I think.
Leonid Lerner was a prolific poster here, as 'Len1', before he left for greener pastures.
His method uses LiCl and a graphite crucible as cathode.
Several members here have a free version of the book (as *.pdf) so it shouldn't be too hard to get if you want it.
[Edited on 14-10-2015 by blogfast25]
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Upsilon
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That's pretty interesting, I guess that works because the potassium metal produced is so soluble in its molten salt? Also, is there a way to predict
the melting point of mixtures like there?
Though I still think I'll use lithium nitrate since it melts even lower than that mixture (enough so to be conducted under mineral oil where the
lithium won't be exposed to air as well as being safe for regular glassware).
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blogfast25
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See also my edit to my last post.
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j_sum1
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I could. I do need to get some CaSO4. Last several visits to the hardware store have had me looking closely at the plaster section wondering how
much CaSO4 is present and how much carbonate, hydroxide, magnesium and iron compounds and others -- and how much do these impurities and additions
matter. With multiple products and no detailed list of ingredients I have walked away without buying anything.
I already have sulfur. (Or sulphur!)
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blogfast25
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| Quote: Originally posted by j_sum1 | | Last several visits to the hardware store have had me looking closely at the plaster section wondering how much CaSO4 is present and how much
carbonate, hydroxide, magnesium and iron compounds and others -- and how much do these impurities and additions matter. |
I believe no-frills wall filler plaster is almost 100 % CaSO4 hydrate. More sophisticated brands may contain some CaO/Ca(OH)2 because its affects
setting behaviour, 'allegedly'. A friend of mine used waste plaster boards from a skip (building site), ground them down and semi-calcined it, no
problem.
Carbonates, if any, are of course easy to detect.
I've also used black board chalk sticks, some of which are CaSO4, some CaCO3, not hard to tell which is which, of course.
[Edited on 15-10-2015 by blogfast25]
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blogfast25
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| Quote: Originally posted by Upsilon |
That's pretty interesting, I guess that works because the potassium metal produced is so soluble in its molten salt? Also, is there a way to predict
the melting point of mixtures like there?
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No, the K<sup>+</sup> ions don't get reduced at all.
Eutectics? Just look for the various eutectic databases, e.g.:
http://www.crct.polymtl.ca/fact/documentation/FTsalt/FTsalt_...
[Edited on 15-10-2015 by blogfast25]
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Upsilon
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Why is that? Is it because the lithium reduction is more favorable? If so, then woudn't K+ start to be reduced at some point when the Li+
concentration drops low enough to where K+ reduction is more favorable? Or is this not what happens at all?
Also, I would like to go off on a tangent about arc furnace design for a little bit, if anyone would like to help me out. I want to have an extremely
high-heat design capable of 2500C+. Thus I decided not to use alumina brick and instead will use a silicon carbide crucible. I would like to be able
to create a small pour spout in the crucible to easily pour molten material, but given the hardness of silicon carbide mechanical means of doing this
are probably not ideal. Perhaps I could use an acetylene torch to do this? Also, how well would tungsten wire work for arc electrodes as opposed to
graphite? Tungsten wire would be easier to work with than graphite, plus graphite electrodes burn out quickly due to reaction with oxygen. I feel like
tungsten would have a similar problem though, oxidizing too quickly in air at extremely high temperatures
[Edited on 15-10-2015 by Upsilon]
[Edited on 15-10-2015 by Upsilon]
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blogfast25
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| Quote: Originally posted by Upsilon |
Why is that? Is it because the lithium reduction is more favorable? If so, then woudn't K+ start to be reduced at some point when the Li+
concentration drops low enough to where K+ reduction is more favorable? Or is this not what happens at all?
Also, I would like to go off on a tangent about arc furnace design for a little bit, if anyone would like to help me out. I want to have an extremely
high-heat design capable of 2500C+. Thus I decided not to use alumina brick and instead will use a silicon carbide crucible. I would like to be able
to create a small pour spout in the crucible to easily pour molten material, but given the hardness of silicon carbide mechanical means of doing this
are probably not ideal. Perhaps I could use an acetylene torch to do this? Also, how well would tungsten wire work for arc electrodes as opposed to
graphite? Tungsten wire would be easier to work with than graphite, plus graphite electrodes burn out quickly due to reaction with oxygen. I feel like
tungsten would have a similar problem though, oxidizing too quickly in air at extremely high temperatures
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Yes. Li is reduced first. On prolonged operation you'd need to top up the melt with LiCl, if only to maintain the eutectic composition.
Most use carbon I think because of it's high MP and because although it does burn off, at least it leaves no trace. W would give WO3. I'm sure in some
instances W electrodes are used though.
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MrHomeScientist
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Back to the subject of silicon thermites, I've posted a nicer video of the method that produced larger amounts (though not in nice little spheres like
my blog post you linked to). The video link is at the top of the post, but in case you missed it: https://www.youtube.com/watch?v=73YmP_JSrlU
It's my second most popular video, but not really the best quality IMO.
Also re: plaster quality, I had some experience with that in another of my blog posts, in the pursuit of titanium: http://thehomescientist.blogspot.com/2012/09/titanium-thermi...
That brand (DAP) had a 15% - 25% impurity of CaCO<sub>3</sub>, but that's easily remedied by adding sulfuric acid until the bubbling
stops. Perhaps cheapo 'wal-mart' brand plaster would be more pure for our purposes.
I'm interested to try the silicon thermite sans sulfur. That one's always nasty.
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blogfast25
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My formulation for TiO2/Al/KClO3/CaF2 was the direct result of research into a sulphur-free SiO2 thermite. I tried replacing the S + Al system by
KClO3 + Al system.
Although that worked well, there where problems with coalescence of the Si, of which I found nice chunks completely stuck in rock-hard alumina.
Impossible to remove them from there. This coalescence problem is largely due to the similar densities of molten Si and molten alumina.
I then used that same formulation with great success by simply replacing, mol for mol the SiO2 with TiO2.
I would therefore suggest to try the TiO2/Al/CaSO4/CaF2 formulation but substituting TiO2 with SiO2 (mol for mol). That should work because the Heats
of Formation of TiO2 and SiO2 are very similar.
Using CaSO4 has one advantage: the slag is a mixture of CaS and Al2O3 and a lot softer than pure Al2O3.
[Edited on 15-10-2015 by blogfast25]
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Upsilon
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Well since I plan on carrying out these arc furnace experiments in an inert atmosphere, I think I'll use tungsten for that anyway. Of course graphite
electrodes wouldn't decompose in this situation either, but like I said tungsten wire would be easier to work with.
The issue with this is that the ionization voltage for argon, for example, is much lower than that of regular air. Does this translate to a colder
arc, though?
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blogfast25
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| Quote: Originally posted by Upsilon |
The issue with this is that the ionization voltage for argon, for example, is much lower than that of regular air. Does this translate to a colder
arc, though? |
What do you mean by ionization voltage here?
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Upsilon
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There's a real term for it but it has slipped my mind. It's the voltage required to break down the substance into an ionized plasma state that can
conduct electricity.
EDIT: "Dielectric strength" is the term I'm referring to.
[Edited on 16-10-2015 by Upsilon]
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blogfast25
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Yes, that's the right term. And Ar has lower dielectric strength than air? Well, I never knew that!
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Upsilon
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Apparently so; search "Dielectric gas" on Wikipedia and you'll get a nice table of dielectric strengths of various gases proportionate to air. Argon
has 1/5 the dielectric strength of air, while Neon is only 1/50! Makes sense why they use neon as opposed to other noble gases in signs. On the other
end of the spectrum is diamond - 1333 times more difficult to break down than air, at a whopping 4 gigavolts per meter!
[Edited on 16-10-2015 by Upsilon]
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Upsilon
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Just checked on that lithium nitrate/citrate disaster again; the calcium citrate had precipitated out and formed a beaker full of something
frighteningly similar to sour cream. I could not resist playing with it before dumping it out 
[Edited on 16-10-2015 by Upsilon]
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blogfast25
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| Quote: Originally posted by Upsilon | Just checked on that lithium nitrate/citrate disaster again; the calcium citrate had precipitated out and formed a beaker full of something
frighteningly similar to sour cream. I could not resist playing with it before dumping it out
[Edited on 16-10-2015 by Upsilon] |
That's your calcium citrate: compounds like that often don't neatly crystallise.
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aga
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Sounds horribly familiar.
So much can 'go wrong' in even the simplest syntheses.
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Upsilon
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Yup, even though it did eventually precipitate, it would have been a nightmare to try and extract any lithium nitrate from that mess. I tried again
with direct reaction of calcium nitrate and lithium carbonate and it worked much better.
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Upsilon
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Ok, so I did more research and learned that the LiNO3 electrolysis will not work because the nitrate ions in the melt will react with lithium metal.
However, I also found that a LiNO3+KNO3 eutectic melts at around 120C! This means that solid lithium will be evolved; this solid lithium will form a
protective layer insoluble in the melt that will protect it from further oxidation.
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blogfast25
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| Quote: Originally posted by Upsilon | | Ok, so I did more research and learned that the LiNO3 electrolysis will not work because the nitrate ions in the melt will react with lithium metal.
However, I also found that a LiNO3+KNO3 eutectic melts at around 120C! This means that solid lithium will be evolved; this solid lithium will form a
protective layer insoluble in the melt that will protect it from further oxidation. |
What makes you think solid Li will not be attacked by nitrate ions?
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Upsilon
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Take a look here:
http://www.sciencemadness.org/talk/viewthread.php?tid=26524&...
Someone on that thread actually did this. Apparently because the protective oxidized layer of lithium is insoluble in the molten mixture and prevents
further attack.
EDIT: Sorry, linked the wrong page. It's actually the page before that one. Here is what it says:
Even if lithium/sodium/potassium/etc reacts with propylene carbonate (or any other electrolyte/solvent), it may still be useful for isolating these
alkali metals. For example, I've electrolyzed a eutectic mix of LiNO3 and KNO3 at 125-150C, and isolated small amounts of lithium that way. You'd
think that lithium would react vigorously with nitrates, and it does. However, lithium is a solid below 180C. The oxide formed on the surface of the
lithium is a fast ion conductor for Li+, and is insoluble in the nitrate melt. This protects the bulk of the lithium from the electrolyte. The problem
that I had with this setup was that I didn't have a platinum anode. Pretty much anything else gets destroyed by the nitrates. Also, the molten salt
has to be very anhydrous, or the oxide layer weakens.
What I don't understand, though, is that even though the nitrate attacks the metal, why does it matter in the long run? Since nitrate is constantly
being removed at the anode, eventually it should run out of nitrate ions to react with the lithium, right?
[Edited on 17-10-2015 by Upsilon]
[Edited on 17-10-2015 by Upsilon]
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blogfast25
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| Quote: Originally posted by Upsilon |
What I don't understand, though, is that even though the nitrate attacks the metal, why does it matter in the long run? Since nitrate is constantly
being removed at the anode, eventually it should run out of nitrate ions to react with the lithium, right?
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Yeah, like when the beaker is empty: for each reduced Li<sup>+</sup> a nitrate ion must be oxidised! 
I did tell you someone had tried this before, I just couldn't find the reference.
Search for related posts with 'WGTR' as author and you might find what he did precisely.
Remember: molten nitrates will oxidise anything that's oxidisable.
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