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Registered Member #1497
Joined: Thu May 22 2008, 05:24AM
Location: Toronto, Ontario, Canada
Posts: 801
One thing you may want to consider is a method to mix the internal contents, if the process is somewhat temperature sensitive and only works within a certain range, you should make sure that the vat temperature is somewhat even. For a heat source something that minimizes the hysteresis (ie: electrical heat source vs external flame transfering heat through the container) would be better if you want to regulate the temperature with with a PID controller or just a simple thermocouple + relay circuit.
Maybe rewind a MOT for low voltage high current and experiment to see just how much voltage you actually need to get the reaction going.
Registered Member #1262
Joined: Fri Jan 25 2008, 05:22AM
Location: Maryland, USA
Posts: 451
aonomus wrote ...
One thing you may want to consider is a method to mix the internal contents, if the process is somewhat temperature sensitive and only works within a certain range, you should make sure that the vat temperature is somewhat even. For a heat source something that minimizes the hysteresis (ie: electrical heat source vs external flame transfering heat through the container) would be better if you want to regulate the temperature with with a PID controller or just a simple thermocouple + relay circuit.
Maybe rewind a MOT for low voltage high current and experiment to see just how much voltage you actually need to get the reaction going.
The thing is, ideally the bottom of the Caster cell is supposed to be slightly cooler than the top (enough to solidify the NaOH). A thermocouple and relay may turn out to be the easiest, or it may turn out that a constant heat source works. To know that, I'll need to start trying things...
Would ceramic work as a container for this, or do I need a steel reaction vessel? And I'm guessing glass+330C NaOH = No go?
Registered Member #8
Joined: Thu Feb 02 2006, 04:34AM
Location: Harlowton, MT, United States
Posts: 214
Would ceramic work as a container for this, or do I need a steel reaction vessel?
Absolutely not; you would be hard pressed to find a ceramic that is not dissolved by fused NaOH, much less one that is not reduced by the solution containing metallic sodium. A steel or inconel vessel is used; the latter would allow for higher purity while the former would be much cheaper and still suitable. Ceramic or even glass feedthroughs may be used for at the bottom where the NaOH is kept solid; it is kept solid simply for the reason that it will not dissolve the feedthroughs (at least not very fast) in this state.
aonomus: stirring the electrolyte may be a poor idea because it should maintain an ion concentration differential between the anode and cathode. In other words the cathode region should be sodium-rich while the anode region should be oxygen-rich, and you would not want to screw this up by stirring. The temperature should remain fairly even by adjusting the electrolysis current which adds heat in a fairly even manner. A smaller cell may be even easier to keep at a constant temperature throughout, even though it is more difficult (per volume) to heat in general than a large cell.
Anyways the Castner process is quite old school, and sodium hydroxide is not the best electrolyte. It is highly undesirable to produce hydrogen along with sodium, as I mentioned before they react quite well to form the pyrophoric sodium hydride salt at lower temperatures (not sure which exactly) if the hydrogen is not removed quickly. Of course this could be useful too if you did intentionally mix them, since sodium hydride is useful and very reactive stuff and a precursor to sodium borohydride. NaOH is also expensive, corrosive and hard to handle, and absorbs moisture, but I don't need to tell you any of that. If your main reason for choosing the Castner process over the Downs process is the lack of halogen release, you may wish to consider a more modern eutectic electrolyte containing raw sodium oxide or sodium carbonate dissolved in a mixture sodium, calcium, potassium, or (harder to find) barium salts. This would no doubt be lots cheaper in the long run, and only oxygen is liberated from electrolysis systems using mixed oxide and chloride. The exact choice would be up to you, what temperatures you can attain and what you can get. All of these that I have looked at would run at somewhat higher temperature, just over 600°C, but you would not have to worry so much about regulation. Excess solid soda or lime would simply collect at the bottom of the cell (it would be added there initially) to form a solid layer or plug. A suspended electrode arrangement could be used to allow for an entirely fused electrolyte too.
Registered Member #1262
Joined: Fri Jan 25 2008, 05:22AM
Location: Maryland, USA
Posts: 451
One of the things I really like about the Castner process is the lower temperature required. I know how nasty lye can be, I can easily imagine how much nastier molten lye is. As far as overall cost per pound of sodium, it's not a big issue; any home process has to be orders of magnitude cheaper than ordering the stuff from a chem supplier (and easier too).
So the design I'm at right now is... A steel container holding a few ounces of lye, inside a ceramic container surrounded by bricks. I would initially heat the lye container with a burner until the lye liquifies, then turn on a rough vacuum system and apply ~6V at up to several amps across two graphite electrodes. The H2 and O2 gases would be evacuated, the molten sodium and molten lye would mix; and after running the cell long enough, all the lye would be exhausted, leaving (sort of) pure molten sodium metal, which I would allow to cool inside the vacuum, and upon reaching room temp, flood with mineral oil for storeage. I would wear full body protective equipment for the whole thing, monitor it from afar, and have the entire setup on a "gallows floor" over a tub of ice water inside an enclosure incase I get cold feet.
Registered Member #8
Joined: Thu Feb 02 2006, 04:34AM
Location: Harlowton, MT, United States
Posts: 214
If you try to run it until much of the lye is depleted, you will effectively have a short across your power supply by the time you get close. The sodium would be boiling hot just from the current before you'd build a very high average concentration, drawing perhaps hundreds of amps even with just a few ounces, and you would not be able to cool it fast enough. These sodium cells (any type of fused salt electrolytic cell really) are meant to run at a more or less continuous concentration, that is, you add more electrolyte as that which is there is depleted, and maintain the same concentration throughout the run. Otherwise, they will not work, it will just short out the electrolyte. You really need a somewhat divided cell to get *any* sodium, as in an anode area where the gases come out, and a cathode area where sodium collects. You could run a batch process where you just shut the cell down after some sodium has floated to the top and let it cool, but you would still need to keep the electrodes separate, and you can forget about running your electrolyte to depletion of more than a few percent. In other words you will need a much larger amount of lye than metallic sodium in the system if you run a batch process as opposed to a continuous one. With only a few ounces of lye you might get a fraction of a gram of sodium out before things started to go haywire without adding in fresh electrolyte. I suggest allowing for at least some means to replenish the lye as sodium forms. It is completely necessary that your system has sodium coming out as a distinct phase from the electrolyte, as you will not be able to separate them or reach more than a few percent sodium in a mixed phase.
Registered Member #8
Joined: Thu Feb 02 2006, 04:34AM
Location: Harlowton, MT, United States
Posts: 214
The voltage would soon fall down to useless levels if you used a current limited supply.
By "divided" cell I simply mean like a collector of some sort over the electrodes, something to keep the products from recombining, and enough distance between the electrodes to keep the products separate. Many cells use a metal screen between the electrodes too.
Extracting the sodium can be done with a riser pipe or just by letting it collect over the cathode until you shut the cell down and let it cool.
Registered Member #1134
Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351
I once made sodium metal on a small scale.
I used a 3 inch diameter stainless steel pot to contain the NaOH.
An old battery carbon was used as the anode, and a loop of copper wire as the cathode. A car battery was used as the power source, and the pot was heated with a blowlamp.
The copper wire loop was simply dipped in the molten solution, and when the sodium formed, it was trapped in the loop by surface tension. The loop could then be quickly withdrawn, and tapped on the rim of a beaker filled with paraffin oil, the drop of sill molten sodium would fall into the oil, where it was safe, and the process repeated.
It was tedious, but it worked. Interestingly, if the temperature of the NaOH is too high, the sodium metal will dissolve back into it, so proper temperature control is necessary. The NaOH is at the correct temp, when it has just become molten.
Yes, using N2 or CO2 to disperse the gases, would be a very good idea. The hydrogen produced explodes when globs of molten sodium ignite in the oxygen produced. Wear heavy gloves and FULL FACE protection. I had a glob of molten hydroxide spit out and get behind the googles. and hit me in the eyelid, and some got into my eye, it was VERY painful. Fortunately I was working right by the sink, so I still have both eyes
Registered Member #1262
Joined: Fri Jan 25 2008, 05:22AM
Location: Maryland, USA
Posts: 451
plazmatron wrote ...
I once made sodium metal on a small scale.
I used a 3 inch diameter stainless steel pot to contain the NaOH.
An old battery carbon was used as the anode, and a loop of copper wire as the cathode. A car battery was used as the power source, and the pot was heated with a blowlamp.
The copper wire loop was simply dipped in the molten solution, and when the sodium formed, it was trapped in the loop by surface tension. The loop could then be quickly withdrawn, and tapped on the rim of a beaker filled with paraffin oil, the drop of sill molten sodium would fall into the oil, where it was safe, and the process repeated.
It was tedious, but it worked. Interestingly, if the temperature of the NaOH is too high, the sodium metal will dissolve back into it, so proper temperature control is necessary. The NaOH is at the correct temp, when it has just become molten.
Yes, using N2 or CO2 to disperse the gases, would be a very good idea. The hydrogen produced explodes when globs of molten sodium ignite in the oxygen produced. Wear heavy gloves and FULL FACE protection. I had a glob of molten hydroxide spit out and get behind the googles. and hit me in the eyelid, and some got into my eye, it was VERY painful. Fortunately I was working right by the sink, so I still have both eyes
Leslie
Sounds like a pretty interesting idea, but I want something that can be fully remote controlled. That way, at worst I'd end up damaging the concrete in my basement and losing a pound of lye.
At this point I'm really leaning toward running it as a batch process, even if the yield is only a few grams
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Castner process for making Sodium metal
