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Registered Member #396
Joined: Wed Apr 19 2006, 12:55AM
Location: Pittsburgh, PA
Posts: 176
I picked up a small clay graphite crucible (same as the first one pictured here to use in a small electric kiln I have. I've always wanted to melt and pour some sodium chloride, so I filled up the crucible with rock salt (halite) and heated it up (to around 1500F).
Upon melting, the salt began to smoke quite vigorously, but I assumed this was because of impurities burning off, so I poured the salt into a metal container where it solidified. I did spill a bit on the side of the crucible because I wasn't used to the low surface tension of the salt when compared with aluminum, etc. I was hoping to be done here.
But I was left with a crucible apparently impregnated with salt, which was still smoking heavily. So I put it back into the kiln, hoping to burn of whatever was smoking. A coating of something started growing on the outside of the crucible, so I scraped it off. It promptly grew back, so I scraped it again . . . eventually that stopped.
I was getting pretty angry by the point so I allowed everything to cool, then just washed the crucible with water, hoping to dissolve the salt left in it. After thoroughly drying it, and firing it according to the instructions to rid it of any moisture, I heated it up to around 1700F (925C), and of cource it began smoking again.
The smoke is white in color and smells like chlorine. It "condenses" on the vent hole of the kiln and tastes salty in the air.
What could this be and how can I get rid of it? I'm a bit weary of soaking my crucible in water for any significant period of time. There is no way I could have reached the boiling point of NaCl or even any of the common impurities of rock salt (KCl, CaSO4). The vapor pressure of NaCl at this approximate temperature is only 1 mm Hg. Maybe the salt reacted with some of the "clay" components of the crucible? There is still some hard residue of something on the outside.
I really should have just used regular table salt . . .
Registered Member #8
Joined: Thu Feb 02 2006, 04:34AM
Location: Harlowton, MT, United States
Posts: 214
I do not think you are dealing with any impurities here.
Molten salts are fairly volatile; the smoke was probably salt vapor. Molten calcium chloride actually reacts with water in the air and especially flame exhaust to form HCl and CaO but I have never experienced this with sodium chloride when I have melted it. As such calcium chloride melts smoke much more vigorously than NaCl melts. I can say though that fused NaCl is volatile and smells salty and acrid, and it may well be that the vapor can react with air or water vapor. Also fused salts have incredibly low surface tension and are very "thin" and capable of seeping through the smallest pores. Porous things that can hold water often cannot hold fused salts. Your clay graphite crucible is likely porous enough that it can soak through. Not only that but fused salts are an excellent solvent for nearly all metal oxides and other minerals including the clay portion of your crucible. The growth you were getting on the outside was probably material from the clay being deposited on the outside by the NaCl as it soaked through.
Registered Member #396
Joined: Wed Apr 19 2006, 12:55AM
Location: Pittsburgh, PA
Posts: 176
Chris wrote ...
I do not think you are dealing with any impurities here.
Molten salts are fairly volatile; the smoke was probably salt vapor. Molten calcium chloride actually reacts with water in the air and especially flame exhaust to form HCl and CaO but I have never experienced this with sodium chloride when I have melted it. As such calcium chloride melts smoke much more vigorously than NaCl melts. I can say though that fused NaCl is volatile and smells salty and acrid, and it may well be that the vapor can react with air or water vapor. Also fused salts have incredibly low surface tension and are very "thin" and capable of seeping through the smallest pores. Porous things that can hold water often cannot hold fused salts. Your clay graphite crucible is likely porous enough that it can soak through. Not only that but fused salts are an excellent solvent for nearly all metal oxides and other minerals including the clay portion of your crucible. The growth you were getting on the outside was probably material from the clay being deposited on the outside by the NaCl as it soaked through.
Thanks - that sounds spot on. When you're taking the proper safety precautions, learning from experience is probably the best way - I was very surprised at how low the surface tension of the molten NaCl was.
When you melted NaCl what sort of crucible did you use? Pure graphite?
Registered Member #8
Joined: Thu Feb 02 2006, 04:34AM
Location: Harlowton, MT, United States
Posts: 214
I have only melted such salts in steel vessels, since I had heard beforehand that they could dissolve and/or soak through any ceramic crucible. I have experienced its low surface tension firsthand though by observing it seep through welds which are normally quite capable of holding water. For the remainder my fused salt endeavors I shall be using vessels which are internally plated with a heavy layer of sulfamate nickel.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
I have no personal experience of using microwave kilns, but it should be possible to make one for rather less than the commercial types, such as these:
Registered Member #8
Joined: Thu Feb 02 2006, 04:34AM
Location: Harlowton, MT, United States
Posts: 214
Well you can often get large graphite stock on ebay, which is an excellent microwave absorber and should hold the salt. Drill it out and insulate with some kaowool and you'd be good to go. However I don't think a microwave kiln would be particularly useful for Billybobjoe's endeavors since I assume he needs direct access to the melt for whatever he is doing.
Registered Member #396
Joined: Wed Apr 19 2006, 12:55AM
Location: Pittsburgh, PA
Posts: 176
Chris wrote ...
I have experienced its low surface tension firsthand though by observing it seep through welds which are normally quite capable of holding water. For the remainder my fused salt endeavors I shall be using vessels which are internally plated with a heavy layer of sulfamate nickel.
That is very impressive. I wish I would have known that a few weeks ago . . .
Anyhow, I already have a small electric kiln - this experiment didn't have any specific purpose. It was really "for fun" and for the sake of melting salt. Had I known it was going to ruin my crucible I probably wouldn't have even attempted it (and I likely won't again any time in the future). Interesting stuff though - especially those microwave kilns.
Registered Member #1188
Joined: Sun Dec 23 2007, 12:18AM
Location: Ventura CA USA
Posts: 2
A late post, rock salt is not pure NaCl, it contains significant quantities of magnesium salts and phosphates. The propensity for alkaline salts to dissolve metallic oxides in fusion is well documented. We are actually using alkaline fusion to dissolve alumina and have been successful. I have had good luck with silicon carbide crucibles, but would avoid clay based crucibles, whether partially graphite or not. I have had good luck with pure graphite crucibles in a vacuum, but ate up a pure nickel crucible like it was not there. Tricky business this...
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Melting rock salt in a clay-graphite crucible
