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kilowatt
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I did some experiments tonight with reducing silica sand in a small (roughly 4kg of CaCl2) test cell. At first I tried containing the silica in a
stainless steel mesh basket around the cathode. The silica appeared to dissolve quite readily, and when I applied power from a battery charger set at
6V, the initial current was about 40A. After a short time the current dropped off to zero. After shutting off the power and letting the cell rest
for a few seconds, the initial current was again about 20A, but dropped to zero within barely a second. This could be repeated over and over. I
concluded that perhaps silicon was plating out onto the cathode and forming an insulating layer, which would dissolve once power was removed. I
attempted to get around this problem by doping the silica with boron from boric oxide, added via boric acid directly into the melt. For this second
attempt I added roughly 100g of silica and 50g of boric acid directly into the melt and used a free hanging cathode. Upon addition of the boric acid,
the melt outgassed HCl and emitted a brown smoke until all the water had been driven off leaving boric oxide. The smoke evolution did not stop even
after nearly an hour, but I attempted the run anyway once the bubbling had stopped. I am not sure what reactions were going on in the melt, perhaps
some free calcium reducing the oxides or also reacting with air or water from the boric acid. The results of the electrolysis were the same, only
this time conduction ceased even faster upon applying power - only a brief spike of about 20A before the current would drop to zero, and this could be
repeated by letting the cell rest for about a minute. Keeping the cell operating for some time did not seem to develop a significantly thick layer of
silicon.
If my assumption that a film of silicon forms on the cathode and stops conduction, why would doping it with boron not make it more conductive? Would
a high level of a metal such as iron from iron oxide help? I did not think silicon was even soluble in fused CaCl2. Are there any other explanations
for this phenomenon?
The mind cannot decide the truth; it can only find the truth.
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bereal511
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Hmmm, would you mind taking a picture of your setup? Have you tried another oxide of something more conductive to see if in fact, the metal was
actually plating onto the cathode? I'm not sure how you put together your cell, but perhaps the chlorine evolved at the anode is forming layers of
insoluble (in calcium chloride) metal chlorides. What did you use for your anode?
The idea of silica dissolving calcium chloride seems, well, counter-intuitive. I also was not aware that that was even possible, considering there
have been experiments done with the FFC Cambridge process to reduce silicon dioxide to silicon, as I'm sure you've come across.
If you are doing another run, you might want to try adding a small portion of calcium oxide to the melt so you don't have to produce an initial phase
of chlorine before having the cyclic CaO <-> Ca reaction occurring after oxide reduction. I don't know how much that would help, but I will
have to try it myself later in the week. Still tweaking up my old cell.
As an adolescent I aspired to lasting fame, I craved factual certainty, and I thirsted for a meaningful vision of human life -- so I became a
scientist. This is like becoming an archbishop so you can meet girls.
-- Matt Cartmill
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kilowatt
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I have a picture at http://chrisf.4hv.org/projects/chem/FFC_Cambridge/HPIM1471.J... which I have not bothered resizing for the forum. I have run MgO in this cell,
and I got notable droplets of Mg metal at the cathode. I ran it until the Mg was depleted. As for chlorine, there never was any produced by this
cell. The calcium chloride I started with was low grade and has some water content, to partial hydrolysis during fusion actually produced some lime
(you could smell it outgassing HCl when I was fusing it for the first time). It would have been quite notable had any chlorine ever been evolved.
However I did use a graphite anode. A jacket of CCl4 would be rather insulating. A jacket of CO or any gas also would be however. The anode gas
production dropped to nearly nothing when the current went low. Perhaps a small anode area was the limiting factor here?
I cannot think of anything else.
Edit: This appears to be an anode related problem, as when reversing the electrodes to make the steel rod the anode, the cell had fine conduction,
drawing about 30A and depositing lumps of Fe/Si/B alloy onto the cathode. I then changed out the graphite rod with a new one and used it again as the
anode. Again I had good conduction for a short time and got good deposits on the cathode, but it soon ceased.
[Edited on 9-11-2008 by kilowatt]
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12AX7
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Sounds like the anode is being passivated then. Boron carbide perhaps?
Interesting structure on that, er, is that the anode, or the anode when you used it as a cathode?
Tim
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kilowatt
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That buildup was the cathodic deposit when I used the original anode as the cathode, and vice-versa. It was more iron-rich than the similar deposit I
got on the steel cathode when I switched them back around, but otherwise looked very similar.
This conduction loss happened when I was running straight silica too remember. I don't know how the anode could become passivated with silicon or
boron carbide, because silicon and boron are attracted to the cathode, and the anode is obviously a highly oxidizing environment.
The mind cannot decide the truth; it can only find the truth.
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12AX7
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Silicate and borate anions, I'm thinking. Both are present as neutral salts (SiO2, B2O3), but there will probably be some ionization, no? I wonder
how much though...
Crude SiC should be conductive anyway, so that doesn't help too much. Maybe SiO2 is simply accumulating, gumming it up?
Tim
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kilowatt
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I had the melt saturated with silica, the excess existing as a glassy phase with boric oxide at the bottom of the vessel. Both the anode and cathode
were directly above this, and perhaps dipping into it (you could feel thick glass at the bottom if you stirred the electrodes). Do you find it likely
that running the cell at less-than-saturated with silica would help, or perhaps just not having the anode so close to the reserve silica? I can't
think of much else that could be done. Recall though that the steel anode appeared to work indefinitely (though it was losing iron into the melt
which in turn ended up alloyed at the cathode).
The mind cannot decide the truth; it can only find the truth.
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kilowatt
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I was wondering if bereal511 ever continued his experiments with the cell. I would be most interested to hear more if anyone has continued dabbling
with this.
I have been preparing to make an improved small cell out of some stainless steel which I recently obtained, which is electrically heated and has a
fully functional design (cooled gas/light metal traps, bottom drain, thermocouple, quick change electrodes, argon feed, etc) just as the large cell I
have planned only smaller. The melt is not very exposed to air in this design which, according to what I have read in articles on the process, should
help a lot with corrosion, and should eliminate the continual HCl outgassing problem I have experienced before.
The mind cannot decide the truth; it can only find the truth.
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Texium
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Thread Moved
19-11-2023 at 12:49 |
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