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Registered Member #8
Joined: Thu Feb 02 2006, 04:34AM
Location: Harlowton, MT, United States
Posts: 214
I plan to use the electrolysis of molten alkali metal nitrates to produce lithium, sodium, and potassium metal and nitric acid in my mini fused salt cell, which can be seen in my Mini Fused Salt Cell thread. I have some serious safety issues with the cell especially during shutdown, as I don't want to be left with a cell full of solidified alkali metal when I'm done (the salt might even crack the quartz tubes during crystallization), and I'm quite sure that molten alkali metal coming in contact with said metal nitrate such as exists in the major part of the cell bath would be incredibly bad. I shudder to think how violently they would react. The cell functions by a transition from sodium nitrate to nitrite to a mixture of sodium oxide and sodium metal as you approach the cathode. Details can be found in US Patent 517001.
During shut down one option is to simply electrolyze the cell down until it's pretty much empty. As the bath level goes below the hydraulically filled quartz tube of the metal collector, the molten metal will spill out on top of the bath. This would surely cause an explosion unless an inert material floated above the bath, on top of which the metal would in turn float, under a layer of argon. For lithium, this is no problem as regular paraffin wax should be suitable. As liquid sodium and potassium are both more dense than any paraffins, they would sink below this layer and come in contact with the nitrate. The requirement for a shielding later for these metals would be something with a specific gravity between approximately 1 and 2, a melting point lower than 300°C and a boiling point above 350°C (preferably not too volatile at that temperature either). The substance must be unreactive toward alkali metal nitrates and alkali metals alike, and insoluble with both as well. Some kind of haloparaffin would probably work, but that is not something that is even remotely likely to be obtainable. Another possibility is pyrene or other polycyclic compounds which might be isolated from asphalt. I am not entirely sure that any of these will work in the presence of fused sodium nitrite, though. I'm not sure if something like hexachlorobenzene would be less flammable.
Another shut down option would be to quickly draw the fused salt high up into (or even through) the outlet tube under suction and then let the thing solidify, hoping the crystallization does not crack the tubes. Once it was solidified it could then be washed out safely as all constituents are water soluble. Unfortunately this includes, to a degree, the boric oxide seals used to hold the cathode collection tube in the cathode block (don't worry, no alkali metal can reach this area during operation to reduce the boric oxide). I guess I'd rather have the tubes crack from crystallization and deal with washing out and rebuilding the whole cell than I would blow up my lab and deal with Class D fires if the first method is unsuccessful.
The liquid shield layer on top of the bath would still be nice in case any other part of the collection system fails during operation and metal is spilled into the bath.
Any other ideas how I can run this thing? I'm not sure if I will have problems with the nitrate attacking the stainless steel screen in the cell either.
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Darling Process
