the_grungler
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Looking for Re-usable crucible for molten sodium hydroxide / silicate
Hi all,
I recently refined a small batch of sand into silica, and I'm thinking about running larger batches now that I have the process down. I'm looking for
a crucible that can handle molten sodium hydroxide, sand, and the eventual sodium silicate it produces at >310C. I used a stainless steel cooking
pan in the first run, but it very quickly became a stained steel pan, and then upon later use it became a rust pan. This isn't ideal!! I've heard good
things about graphite, but I worry about other reductions happening with the hydroxide. If I don't go above the 320C mark, would it really be the end
of the world to use graphite? Or should I look for something else?
Thanks so much!
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Keras
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I’m not sure why your stainless steel crucible eroded. I have done several fusion reactions in mine, most recently to make indigo in several steps
from anthranilic acid, and it is still almost shiny new. It is important to clean it every time with a slightly abrasive paste such as the 'Pink
Stuff', or clay 'stones' or even sodium bicarbonate spread on a sponge. This way, you’ll remove the stains as they appear and the surface of your
crucible should remain shiny.
It is also possible that your stainless steel is not really stainless, or of a low quality steel. You can look for pure nickel crucibles, but they are
more expensive.
I have no experience with graphite crucibles, but I’m pretty sure they can’t be used outside closed ovens (and especially not on gas burners). I
use my crucible in a standard heating mantle (or sometimes over a stove burner) that I cover with an aluminium foil to keep the heat from radiating as
much as possible. The high heat conduction and thinness of the walls makes it possible, while graphite crucibles are both very thick and relatively
non-heat conducting.
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macckone
Dispenser of practical lab wisdom
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Graphite should handle 320C fine. High quality stainless steel handles sodium hydroxide well. Nickel will as well. But cheap steel in the form of
soup and vegetable cans are literally free with dinner. So even though they are single use they are serviceable.
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the_grungler
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I used a seemingly high-end stainless steel pan I bought for about $40 at the local Target. I think the high temperature got to it, because at the end
it looked... bad. Here's the before and the after. No amount of scrubbing, soap, water, HCl, or baking soda did anything to help. Suppose they call it "caustic soda" for a reason.
I ended up using the same pan for boiling off the water content in the silicic acid later on in the process, but most of the damage had already been
done. Nickel crucibles seem expensive but if I can get away with a soup can for a use or two, I'll give it a shot. My current landlord is not fond of
the idea of lighting a camp stove of this size anywhere near my apartment, so I'll have to figure that out later.
Thanks for the responses!
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MrDoctor
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if nickel is expensive then why not just electroplate a layer of nickel you can reclaim it once a hole forms and rust makes its way in? i had a
similar experience too processing PET, and it took so little lye to blacken the pot, but it was a very cheap pot. industrially they still just use a
specific stainless alloy im pretty sure.
Many aqueous reactions also are conducted in alkali resistant stainless steel.
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