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Mica

Crucible for molten salt

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Posted (edited)

Hey all

I'm working on an odd project right now that involves melting down sodium chloride (salt), as well as other salts (like potassium carbonate). However, I need to buy a crucible. I came across this post from another forum that led me to be very cautious in buying the right kind of crucible.
https://4hv.org/e107_plugins/forum/forum_viewtopic.php?67080

Apparently, molten salt has a low surface tension, leading it to seep through cracks and pours that would otherwise hold water. So clay-graphite crucibles are out. I'm thinking fused quartz. Would this be a viable solution? What other kinds of crucible would work?

Thanks
Mica

Edited by Mica
Forgot to add something

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Crucible material is not your only problem

Quote
What happens when sodium chloride melts?
When it melts, sodium chloride undergoes electrolysis, which involves conduction of electricity because of the movement and discharge of the ions. In the process, sodium and chlorine are produced. This is a chemical change rather than a physical process.5 Jun 2019

 

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Posted (edited)

So what you're saying is that when the molten salt cools down, and recoagulates, it's no longer sodium chloride as such? Sodium and chlorine ions are produced? Are these ions lost to the atmosphere or do they simply become free-floating in the matter? 

I'm still willing to do this and so any help in figuring out the best crucible would be most appreciated.

Thanks

Edited by Mica

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@studiot said that when you will pass current through molten sodium chloride, there will be produced metallic sodium and chlorine gas.. But you did not say you will be passing current through it, but it is plausible thing which you maybe omitted in your OP.

 

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No, I'm just melting it. But it looks like the ions still form when it's in a molten state. Which is fine, btw

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8 hours ago, Sensei said:

@studiot said that when you will pass current through molten sodium chloride, there will be produced metallic sodium and chlorine gas.. But you did not say you will be passing current through it, but it is plausible thing which you maybe omitted in your OP.

 

 

8 hours ago, Mica said:

No, I'm just melting it. But it looks like the ions still form when it's in a molten state. Which is fine, btw

Sensei said it better than the website I linked to.

Sodium Chloride
MP 801oC

Sodium
MP 98oC
BP 883oC

Chlorine
BP -34oC

So if you just 'heat it in a crucible'  to 800oC +  you will drive off the chlorine as a rapidly expanding highly corrosive gas.
The sodium will be just liquid but close to its boiling point so will also be exterting substantial vapour pressure.

 

So you will need more than just a crucible in a furnace.

My comment was about safety.

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3 hours ago, studiot said:

So if you just 'heat it in a crucible'  to 800oC +  you will drive off the chlorine as a rapidly expanding highly corrosive gas.

No you won't.

 

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14 hours ago, studiot said:

Crucible material is not your only problem

Quote
What happens when sodium chloride melts?
When it melts, sodium chloride undergoes electrolysis, which involves conduction of electricity because of the movement and discharge of the ions. In the process, sodium and chlorine are produced. This is a chemical change rather than a physical process.5 Jun 2019

 

I can't believe what I am reading. But I suppose you get what you pay for. 

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Quote

I can't believe what I am reading. But I suppose you get what you pay for. 

That's what I'm thinking too. There's a whole series of videos of people melting salt on youtube. You don't see anyone keeling over from chlorine gas. It appears I'm not going to get any help here

 

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How could salt have a boiling point of 883C  if it fell apart when it melts at 801?

 

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7 hours ago, Mica said:

It appears I'm not going to get any help here

Yes, I’m sorry about that. I’m sure there is someone who could help without copying arrant nonsense from some website. 

I have no real expertise, not having done any practical chemistry for more than 40 years ... but you can get glazed crucibles. I would have thought that would be adequate. As long as it is new and undamaged (If cracks are problem)

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11 hours ago, John Cuthber said:

How could salt have a boiling point of 883C  if it fell apart when it melts at 801?

 

Who said it does ?

 

I see that someone who has not the guts to say why does not like my genuine attempt to help.

 

I do apologise to Mica that it was not the best written of links.
I did wonder why they talked about electrolysis, but what did they say that was factually incorrect ?

In any event here is a better link.

https://courses.lumenlearning.com/boundless-chemistry/chapter/electrolysis/

They clearly describe the industrial process of decomposing molten sodium chloride into metallic sodium and chlorine by electrolysis.
The melting is achieved by heating, not electrolysis.

OK so you are not going to electrolyse it so the question remains

Does sodium chloride ionise when it melts ?

Well most alkali halides apparantly do.

If molten sodium chloride does not ionise (ie decompose on melting) how does it conduct electricity and what species are present in the melt?

The was a study published in 1931 by Guthrie and Nance

https://pubs.rsc.org/-/content/articlelanding/1931/tf/tf9312700228/unauth#!divAbstract

It contains a discussion of differences of opinion from various authorities about how much chlorine is released.
They mostly used a platinum crucible.

Wikipedia shows pictures of melting halides on a charcoal block and blowpipe, a common past chemistry technique.

 

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30 minutes ago, studiot said:

Who said it does ?

Oops!

Boils at 1465, but the point still stands.

It has a boiling point, so it doesn't decompose.
 

31 minutes ago, studiot said:

I did wonder why they talked about electrolysis, but what did they say that was factually incorrect ?

Yes, notably, if it decomposed to form chlorine, it wouldn't have a boiling point.

 

32 minutes ago, studiot said:

Does sodium chloride ionise when it melts ?

A matter of definition, but the solid is composed of a lattice of ions.
It's already ionised as a solid.

32 minutes ago, studiot said:

Well most alkali halides apparantly do.

To whom is this apparent?

 

 

33 minutes ago, studiot said:

If molten sodium chloride does not ionise (ie decompose on melting) how does it conduct electricity and what species are present in the melt?

It's ionised as a solid and as a liquid.
In the solid form, the ions are not mobile.

In some other compounds, even the solid conducts by the movement of ions.

https://en.wikipedia.org/wiki/Rubidium_silver_iodide

https://en.wikipedia.org/wiki/Silver_iodide

(Above 420K)

 

36 minutes ago, studiot said:

I see that someone who has not the guts to say why does not like my genuine attempt to help.

Because it's factually incorrect.

 

On 6/1/2020 at 9:38 AM, studiot said:

My comment was about safety.

Scaremongering, (deliberate or accidental) doesn't help anyone.

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45 minutes ago, John Cuthber said:

It has a boiling point, so it doesn't decompose.

 

45 minutes ago, John Cuthber said:

It's already ionised as a solid.

So what melts and what boils?

So what is the liquid and gas comprised of?

I already asked that question about the liquid and you have suggested it must be ions, in which case it is decomposed.

 

You can't use the argument for one thing and not the other.

 

49 minutes ago, John Cuthber said:

Scaremongering, (deliberate or accidental) doesn't help anyone.

You can you use your professional reputatuion to guarantee that no chlorine or sodium vapour is released.

Did you read the RSC archive article?

How does chlorine possess a vapour pressure if none is released?

How does sodium possess a vapour pressure if none is released?

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1 hour ago, studiot said:

I see that someone who has not the guts to say why does not like my genuine attempt to help.

There are no rules that say people have to announce that they have voted on a post, nor any rules about why they should vote a post up or down. It is nothing to do with "guts".

However, voting down a post that quotes, with no comment, a blatantly false (or, at least, appallingly badly written) statement seems entirely reasonable to me.

 

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11 hours ago, studiot said:

So what is the liquid and gas comprised of?

I already asked that question about the liquid and you have suggested it must be ions, in which case it is decomposed.

Ionically bonded solids, such as NaCl, when melted to a liquid, are variously called liquid electrolytes, ionic melts/fluids, fused/liquid salts, or ionic glasses.
Some of the nomenclature gives clues as to the make-up.

Last time I studied any Chemistry was 1977. I am not a chemist, I just play one, at work.
( and I don't give demerits )

 

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Posted (edited)
22 hours ago, studiot said:

I already asked that question about the liquid and you have suggested it must be ions, in which case it is decomposed.

No. Decomposed NaCl would have formula 2Na and Cl2

See on the other example CaCO3 (which really decomposes). It has positive ion Ca2+ and negative ion CO32- but after heating, it reaches decomposition temperature >= 840 C and molecule decomposes into CaO and CO2. CaO is still in ionic form Ca2+ and O2-.

 

Edited by Sensei

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