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Author: Subject: Mg metal from MgO
kilowatt
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[*] posted on 22-10-2008 at 15:44


The reason I would be interested in electrolyzing MgCl2 or such to Mg is not necessarily for a cheap source of Mg, but rather for a source that I know is pure, for making high quality alloys. As such I would want to avoid contamination from metallic cell walls and such. I wonder if a fused quartz crucible would be suitable (these are actually not that expensive from a few companies), or if it would simply be converted to the volatile SiCl4 and contaminate the melt with silicon. I am going to assume a graphite or silicon carbide crucible would be rapidly dissolved and given off as CCl4 or SiCl4. I wonder though how corrosive that low melting MgCl2/KCl eutectic really is to steel though. Remember Mg metal has been industrially produced since long before there were such things as monel or inconel, and also long before aluminum was industrially produced.

Getting the anhydrous MgCl2 seems to be the most difficult part. We all know that it can be made under circulating HCl gas but this method is slow and somewhat bothersome in my experience. I found one patent which details another method here, but it is still involved. I do wonder about using direct chlorination of MgO using carbon though; this reaction is used to produce plenty of "non-textbook" or semi-covalent chlorides like titanium and similar chlorides, and BCl3. The lattice energies of MgO and MgCl2 lead me to suspect that this reaction is possible, but not nearly as favorable as the related reactions with less ionic chlorides.




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[*] posted on 22-10-2008 at 16:23


Well, to get Anhydrous Magnesium chloride from MgO couldn't you just make Magnesium Sulfate with sulfuric acid and then fuse that with anhydrous calcium chloride? I don't know how you'd ever separate them, unless you could pour the molten MgCl2 off of the solid CaSO4, or if MgCl2 was soluble in absolute ethanol or something like that. However, I doubt that the formed Calcium Sulfate would interfere many reactions.

[Edited on 22-10-2008 by kclo4]
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[*] posted on 22-10-2008 at 17:50


I don't think SiO2 is a problem. It wouldn't be chlorinated, but it will dissolve some and be reduced, leading to a silicon impurity. For graphite and SiC (if they are suitable, which I would think so, but who knows), they must be nonporous. Your average graphite and graphite-bonded materials are quite porous and will not hold a molten salt bath at all.

Maybe that could be an advantage. You could fuse the CaCl2 + MgSO4 metathesis reaction in a steel crucible, break it up and melt it in a graphite crucible held above a nonporous crucible. Who needs filters when you've got porous ceramics!

I wonder what the solubility of CaSO4 is in MgCl2...

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[*] posted on 22-10-2008 at 21:34


You might want to look into the Pidgeon process for making magnesium. Basically it reacts MgO with ferrosilicon under vacuum with external heating of the reaction mix, the elemental magnesium distilling out and being condensed. This give quite pure magnesium, due to the distillation and lack of other volatile materials save for contaminates in the ferrosilicon. Conceivably the heat could be provided internal the the reaction vessel using an electric arc.
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[*] posted on 22-10-2008 at 21:46


Obtaining the ferrosilicon (really silicon or any alloy will work, but still) would be the major problem with the pidgeon process. It would also require a fully enclosed distillation setup that can handle the temperature without reacting with magnesium.



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[*] posted on 22-10-2008 at 23:24


Steel, with MgO as a liner in the reaction area, should work. Magnesium boils a bit under 1100 C at 1 atmosphere, around 850 C at 50 mmHg.

Silicon works well, ferrosilicon is cheaper, at 1500 euro/mt. The main contaminates, Ca and Al, are not a serious problem in this application.

And it gets around making anhydrous MgCl2, which is a distinctly corrosive process, as is the electrolysis itself.

Alternatively you might go for directly producing an alloy of magnesium and another wanted metal from their oxides, using the FCC Cambridge process. That requires CaCl2, but it is easier to dehydrate and the presence of some CaO is allowable. The alloy would have to be analyzed for exact Mg content, and added to the remaing alloy metals. true.
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[*] posted on 23-10-2008 at 01:00


FeSi or Si may be that cheap, but it's not something I can find at the local scrapyard. Silicon is extremely difficult to extract from silica or silicates. I would not consider the process unless I had a source of this stuff, and I would be very surprised if there was one. I have searched extensively for sources of metallurgical grade silicon and have never found any.:(

An FCC Cambridge melt could possibly be prepared which would run below the melting point of magnesium metal. If the metal could be kept solid, this would minimize complications. Such a melt may consist of a eutectic CaCl2/NaCl or CaCl2/LiCl system. Anhydrous CaCl2 is readily available as an ice melter and is fairly cheap, but since none of the melt salts are consumed their cost is of little importance. However I suspect it is nearly as corrosive as MgCl2; though industrial downs cells made of steel or iron can contain it, in my experience with these sorts of melts there is contamination. All things considered, I have been wanting to try the FCC Cambridge process for titanium and other metals.




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[*] posted on 23-10-2008 at 08:32


While chasing down references above, I found that it's FFC, not FCC. Here's Wikipedia on the FFC Cambridge process.

Now, as to the Pidgeon process, since we're already talking about foundry territory, it seems feasible to make ferrosilicon in a small arc furnace. Certainly easier than reducing to silicon at much higher temperatures. Given that the feedstock is sand, coke, and scrap iron, there's no shortage of materials.

While I've put no work into it as yet, I'm quite interested it developing a small-batch, vacuum-capable, refractory reaction vessel and distillation head system. There are all sorts of useful syntheses that use this basic setup: mercury smelting and refining, phosphorus production, Pidgeon process for magnesium. If there's interest, I'd suggest a separate thread.

I'd recommend looking into skull melting also, since magnesia is both refractory and a reactant. A skull melter uses a temperature gradient that crosses a solid/liquid phase transition to form a crucible out of the reactant. It's used commercially for making cubic zirconia crystals (imitation diamond), since there's no practical refractory from which to make a crucible that will hold molten zirconia. The skull melter uses a heat source on the inside and cooling pipes on the outside. The typical heat source is RF induction, but arc can also be used. I have been unable to find information both readily-available and reliable on building induction power supplies. Carbon arc should suffice, however, for the Pidgeon process.
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[*] posted on 23-10-2008 at 08:46


Quote:
Originally posted by watson.fawkes
I have been unable to find information both readily-available and reliable on building induction power supplies.


http://webpages.charter.net/dawill/tmoranwms/Elec_IndHeat6.h... and such.
http://www.richieburnett.co.uk/indheat.html

If you really want a good system, you're better off buying one. If you're really comitted to building one, it's, well, possible.

Tim




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watson.fawkes
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[*] posted on 23-10-2008 at 09:14


Quote:
Originally posted by 12AX7
http://webpages.charter.net/dawill/tmoranwms/Elec_IndHeat6.h... and such.
http://www.richieburnett.co.uk/indheat.html

If you really want a good system, you're better off buying one. If you're really comitted to building one, it's, well, possible.
That's the readily-available information. I had already seen both sites, and I don't consider them particularly reliable for my purposes. I'm looking for solid engineering data, not hobbyist-level explanations. Both these projects are tabletop scale, and I'm looking at microscale industry, say, 10 kg charges.

And if I really wanted to buy one, would I be hanging out on this forum?
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[*] posted on 23-10-2008 at 09:25


Well, a power supply for "10 kg charges" is still ballpark tabletop size, not much bigger than a computer, or microwave oven maybe (although commercial units may be cabinet sized, they have a comfortable amount of extra space inside). It's the same as what's shown above, just scaled up a bit. So there's not really much more to it (...which is to say, you'll still blow plenty of transistors trying..). Could you be more specific about engineering data? I have much more information than is on my website, we could continue this privately if interested.

We now return you to your regularly scheduled thread...

Tim




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[*] posted on 24-10-2008 at 00:45


Argh - some for trhe FFC typing brain fart.

I don't think you want to use carbon electrodes for the Pidgeon process as they will introduce carbon into the magnesium.
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[*] posted on 24-10-2008 at 23:01


FFC, yes, my bad too. But at least we are all familiar enough with the process to know what we mean.:P



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[*] posted on 25-10-2008 at 07:54


Magnesium does not form a carbide (under these conditions; I believe an acetylide can be prepared, but it's not thermodynamically stable like CaC2), and I don't think carbon dissolves in it at all. The main problem with carbon is, MgO(s/l) + C(s) <--> Mg(g) + CO(g) is strongly in favor of the left side. The only way to get stuff on the right side is nonequilibrium quenching of the vapors. As far as I know, the "crown" of magnesium thus produced has quite excellent purity, little if any carbon.

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[*] posted on 25-10-2008 at 08:02


It's not carbide formation I was referring to, but the very equation you show. Using carbon electrodes is likely to get some CO forming, which will react with metallic magnesium in the cooler section, contaminating it with finely dispersed carbon and MgO. The reports on the carbon based production method seemed to consider that an important problem, getting the cooling rates needed to avoid it might be a bit difficult.
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