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Castner process for making Sodium metal

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Backyard Skunkworks

Sun Nov 30 2008, 08:04PM

Registered Member #1262 Joined: Fri Jan 25 2008, 05:22AM
Location: Maryland, USA
Posts: 451

Well I've been brainstorming a bit about ways of making sodium metal, and I ran across the Castner process of electrolyzing molten NaOH into H2 and O2 gas along with Sodium metal. I was wondering if anyone on 4hv had done this on a small scale already, and how feasible it is.

I had the idea of simplifying things somewhat by starting with a small "seed" amount of molten NaOH, then heating it by applying higher voltage, and adding more NaOH which subsequently becomes molten via heat transfer.

I was thinking of doing the entire experiment under an N2+CO2 environment, would this work?

...

Sun Nov 30 2008, 08:37PM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

There are several threads about Chris F making a huge downs cell for that purpose. Try searching...

Backyard Skunkworks

Sun Nov 30 2008, 09:56PM

Registered Member #1262 Joined: Fri Jan 25 2008, 05:22AM
Location: Maryland, USA
Posts: 451

Yeah, I know about Chris' downs cell

, but he was using a higher heat process involving salts.

I know it would be quite similar, except the castner process only needs the NaOH to be around 330C, and no halogens are formed (obviously), thus making it somewhat of a different (and easier?) beast.

Chris

Mon Dec 01 2008, 09:45AM

Registered Member #8 Joined: Thu Feb 02 2006, 04:34AM
Location: Harlowton, MT, United States
Posts: 214

I know a guy on another forum (sciencemadness) who made a successful Castner cell. He used stainless steel for the body and it was electrically heated if I remember right. It worked well enough and he got significant amounts (several lbs) of sodium. However, IMO they are trickier than a downs cell even though they use lower temperature, as well as more expensive to operate because of the more expensive feedstock. This is because of the narrow temperature range they must operate in to minimize the solubility of sodium in the electrolyte, and formation of sodium hydride. The bottom area of the cell around the electrode feedthroughs is kept solid, so a temperature differential is maintained. The cell is supposed to run just a few degrees above the melting point of NaOH. Many people have had problems when trying to run these cells at too high a temperature; my understanding is pure sodium will simply not come out if it is too hot. Anode corrosion is also severe with any high temperature electrolysis that releases oxygen. Ideally an anode of tin oxide (either coated or sintered SnO2 with other additives) is used, or a consumable carbon anode may be used. In the latter case you will have to beware of carbon monoxide. However if you don't have a good way to deal with hot chlorine, this may be preferable to the downs process. There are several possibilities for getting rid of or absorbing chlorine though, such as running it into an iron (II) solution to make FeCl3, or similar aqueous reactions. It is also useful for making anhydrous metal chlorides.

As for my downs cell, I have abandoned it in light of my experiments with modified FFC Cambridge and similar calcothermic reduction cells. I realized I can extract sodium from oxide sources with that perfectly well (albeit less efficiently), along with other more useful materials. For example scrap soda-lime glass would yield sodium (vapor) and silicon and/or calcium silicide in such a cell. In my current situation it would be silly to make a large dedicated sodium cell when I don't have much demand for sodium but do have demand for other metals made with a process that can also yield sodium. The FFC Cambridge process like the Castner process does not emit any free halogen.

newbie

Mon Dec 01 2008, 04:37PM

Registered Member #1825 Joined: Wed Nov 26 2008, 06:44PM
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CO2 may be a bad choice as you may lose much of the NaOH to carbonate

Chris

Mon Dec 01 2008, 05:20PM

Registered Member #8 Joined: Thu Feb 02 2006, 04:34AM
Location: Harlowton, MT, United States
Posts: 214

Yeah you definitely want to use an argon environment everywhere the thing isn't hydraulically filled. Metallic sodium will react perfectly well with CO2, and most reactive metals also react perfectly well with nitrogen to form nitrides (I *think* sodium is an exception under these conditions but am not sure). The products will be sodium oxide or carbonate and sodium carbide.

Backyard Skunkworks

Mon Dec 01 2008, 06:42PM

Registered Member #1262 Joined: Fri Jan 25 2008, 05:22AM
Location: Maryland, USA
Posts: 451

So I'm getting the impression that a Castner cell is pretty finicky about temperature regulation. I think I might be able to pull off the correct temperature differential, however I was wondering... If the entire cell were a bit too hot (lets say 350C), could you still get some impure sodium mixed with the hydroxide? (and if you ran the cell until the hydroxide was exhausted, could you get pure sodium metal anyway?)

Using carbon for the electrodes seems to be the easiest. I had the idea of using N2/CO2 mix for the filler gas because its so easy to make (simply combust something organic until it will not burn), could I use a rough vacuum for this?

I was only thinking of making a few grams (don't care about purity) to start with, enough to lob into a bucket for some fireworks

Chris

Tue Dec 02 2008, 12:31AM

Registered Member #8 Joined: Thu Feb 02 2006, 04:34AM
Location: Harlowton, MT, United States
Posts: 214

If you are only making a few grams, you could just make sodium amalgam in an aqueous cell with a mercury cathode, then distill the mercury out.

I wouldn't use any CO2 in any case, it will react, but nitrogen may work. A vacuum will be fine. I don't know what the "maximum" temperature you could run a Castner cell is. I suggest experiments.

Dragon64

Tue Dec 02 2008, 03:03AM

Registered Member #1438 Joined: Sat Apr 12 2008, 12:57AM
Location: Canada
Posts: 218

I had this question for awhile.

Why don't we use a MOT to create the temperature needed with the arc, and at the same time, connect the MOT to a carbon and iron electrode? It would be sort of like a dual system?

Backyard Skunkworks

Tue Dec 02 2008, 05:17AM

Registered Member #1262 Joined: Fri Jan 25 2008, 05:22AM
Location: Maryland, USA
Posts: 451

hydraliskdragon, I think the main reason is because an MOT only puts out about half an amp to an amp. It's a bit after midnight and I don't feel like looking up electrochemistry, but you only need a few volts at most to begin decomposing the lye, and the reaction rate should be proportional to the current. Therefore, its a lot better (and safer) to use a low voltage high current supply to decompose the lye; and either the same supply, or some other method to heat the mixture.

Basically what I'm trying to say is that an MOT would produce a lot of heat alright, but it won't cause much in the way of a reaction.

Chris, mercury is out of the question for this (and any other) project for me. Once I have some free time (after college apps in a couple weeks), I should be able to start "playing around" with this, using a rough vacuum and the best temperature regulation I can design.

Would a good design for such a cell involve a big "heat sink" on the bottom being water cooled, along with some heat source (maybe electrically internal?) near the top?

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