VeritasC&E
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Highest recommended temperature working with molten salts and gases in glass?
What would you recommend as a maximum temperature when working with solid/molten/gaseous (non-alkaline) salts in borosilicate glassware? (with up to
full vacuum, and with the use of gas streams for reaction/sublimation/distillation) This taking account for moderately well externally insulated
apparatus.
Borosilicate melts >1260C, is considered to soften >825C, anneals at >560C, and has a maximum "working temperature" of <500C. So the
recommendation should be somewhere below that point, and preferably come from experience.
Factors of importance are:
⤑ Not contaminating the product with metals from the glass, nor contaminating the glass with the product in ways that can't be cleaned out or would
affect the physical properties of the glassware after use.
⤑ That the physical properties or swelling of the glass not be changed to the point that things get permanently stuck or that the glassware
fails/implodes due to increased softness
⤑ Safety: My biggest fear would be to crack the glass due to internal fluctuations of temperature by gas flow / evaporation / condensation /
internal material heterogenicity with products in different phases that may locally differentially insulate or cool the glassware from inside.
The margin for temperature shock in borosilicate glassware is fairly large for lower temperatures, but I believe large fluctuations are much easier to
accidentally achieve at higher temperatures. Also by personal experience there's quite a difference between theoretical heat shock limits and
practical reality: While borosilicate in theory can tolerate temperature shocks of 160C, more than once have I been surprised breaking pieces
containing aqueous solutions close to BP, when, for instance, confidently crashing microcrystals of a product in an ice bath after purification. This
happened to me many times with many pieces of glassware from different manufacturers, including the most expensive ones and the thinnest walled, in
more or less surprising contexts (luckily this never happened to me while doing something really dangerous; it somehow always happens in more or less
trivial moments).
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BromicAcid
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https://www.aceglass.com/dpro/kb_article.php?ref=4347-TFBN-1...
In summary, 200-230°C continuous service, up to 400°C for short term (measured in minutes not hours).
[Edited on 7/31/2022 by BromicAcid]
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DraconicAcid
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I have a fond memory of my undergraduate project involving molten salts. I tried drying a lithium/sodium chloride mix under vacuum at 450 oC, since
that was below the melting point of the mixture. I wound up with a hilariously deflated Schlenk tube, which was the only memorable part of my final
presentation. Sadly, it disintegrated a few months later due to ion exchange with the lithium salts.
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
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VeritasC&E
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Dang, that's much lower than I expected! I was expecting something in the four hundreds. This really shows the gap between theoretical values and
practical realities.
What alternative inert materials are there for apparatus working at these temperatures? (distillation, sublimation, etc)
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VeritasC&E
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Quote: Originally posted by DraconicAcid  | | I have a fond memory of my undergraduate project involving molten salts. I tried drying a lithium/sodium chloride mix under vacuum at 450 oC, since
that was below the melting point of the mixture. I wound up with a hilariously deflated Schlenk tube, which was the only memorable part of my final
presentation. Sadly, it disintegrated a few months later due to ion exchange with the lithium salts. |
It's also under quoted "maximum working temperature" of 500C. I myself would have confidently gone for it, especially for drying.
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DraconicAcid
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It was 30 years ago. Don't take my temperature quote seriously.
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
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VeritasC&E
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Well one thing to think about is that glassware carries away so much heat (especialy for large pieces / during heating / or when
evaporatings/subliming large amounts of product per second) that when we are working at a usable temperature of 250C we are in many cases already very
locally hitting the 500C "maximum working temperature" at some contact areas with the heat source.
In addition to the obvious external insulation of the apparatus (which increases internal thermal homogeneity to the limit of the inedvitable internal
dynamics of the process at hand), we might "safely" carry out works at higher temperatures by:
1) Using the maximum available relevant surface area for heating, while using, beyond a certain temperature, only liquid baths with high thermal
conductivity.
2) Carrying out the process in as small apparatus as possible.
3) Carrying out the process with smaller batches of product (horizontal vs vertical scaling).
[Edited on 1-8-2022 by VeritasC&E]
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WGTR
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I don't have an exact numerical figure for temperature, but I did make a small quantity of white phosphorous in a borosilicate test tube under vacuum.
It used the usual mix of aluminum powder, sodium hexametaphosphate, etc. Soda lime glass was not robust enough to handle the heat. Boro was barely
enough. The reaction would proceed slowly with red heat, just as the test tube would begin slowly collapsing under vacuum. As a reward for my
efforts the result was a thin ring of yellowish/red phosphorus that spontaneously burst into flames when I let a bit of air inside the tube.
Anyway, one way to get a bit of extra temperature range out of boro under vacuum is to apply vacuum to both sides of the glass, inside and out.
Secondary containment would be needed around the outside, but only mild vacuum around the outside is needed to prevent collapse. Heating at that
point could be internal with resistance wire or inductive heating.
After many tries of heating molten salts in glassware over the years, the longevity of the glassware depends strongly on the molten salt. When I
dissolved titania into molten sodium bisulfate (a sodium salt that is strongly acidic), the borosilicate tubes were usually reusable. Alkaline melts
would always reduce the glassware to a single-use item as they would readily crack upon cooling.
Another thing is that under vacuum you don't have convective heat losses, so your temperature differentials across the glass can be less. Since you
mentioned vacuum earlier, if the process allows it then this can be used to an advantage.
[Edited on 22-08-01 by WGTR]
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VeritasC&E
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| Quote: Originally posted by WGTR | I don't have an exact numerical figure for temperature, but I did make a small quantity of white phosphorous in a borosilicate test tube under vacuum.
It used the usual mix of aluminum powder, sodium hexametaphosphate, etc. Soda lime glass was not robust enough to handle the heat. Boro was barely
enough. The reaction would proceed slowly with red heat, just as the test tube would begin slowly collapsing under vacuum. As a reward for my
efforts the result was a thin ring of yellowish/red phosphorus that spontaneously burst into flames when I let a bit of air inside the tube.
Anyway, one way to get a bit of extra temperature range out of boro under vacuum is to apply vacuum to both sides of the glass, inside and out.
Secondary containment would be needed around the outside, but only mild vacuum around the outside is needed to prevent collapse. Heating at that
point could be internal with resistance wire or inductive heating.
After many tries of heating molten salts in glassware over the years, the longevity of the glassware depends strongly on the molten salt. When I
dissolved titania into molten sodium bisulfate (a sodium salt that is strongly acidic), the borosilicate tubes were usually reusable. Alkaline melts
would always reduce the glassware to a single-use item as they would readily crack upon cooling.
Another thing is that under vacuum you don't have convective heat losses, so your temperature differentials across the glass can be less. Since you
mentioned vacuum earlier, if the process allows it then this can be used to an advantage.
[Edited on 22-08-01 by WGTR] |
That's a really smart thing actually. The entire apparatus being in vacuum solves most of the external temperature gradient issue. Then only remain
the internal gradient issue which can be somewhat lessened by working with small quantities at the time and trying to work at low flow rates (in the
case of sublimation / distillation).
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pneumatician
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| Quote: Originally posted by BromicAcid |
Dang, that's much lower than I expected! I was expecting something in the four hundreds. This really shows the gap between theoretical values and
practical realities.
What alternative inert materials are there for apparatus working at these temperatures? (distillation, sublimation, etc) |
quartz??? porcelain??? clay??? you need to search a guy with the abilities to make this in clay & porcelain, in quartz is more easy to find guys
working with it but more expensive of course.
this guy make their own "stoneware".
https://www.youtube.com/watch?v=Fd6pTY7BLHQ
I read crucibles made with bone ashes are megaresistant but I need to search info of "how to".
I don't understand for what reason quartz is not the standard in labs and/or much more common and more cheap and at disposal in shops... inclusive if
you are working only in the 100-150 ºC range your peacefulness of mind is much more :-D
mass production lower the price/unit
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WGTR
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| Quote: Originally posted by VeritasC&E |
That's a really smart thing actually. The entire apparatus being in vacuum solves most of the external temperature gradient issue. Then only remain
the internal gradient issue which can be somewhat lessened by working with small quantities at the time and trying to work at low flow rates (in the
case of sublimation / distillation). |
Thank you. Since you mentioned working in small quantities there is also another possibility. Picture a small glass "thimble" containing your
sample. This is placed on a small piece of...insulatiive firebrick...that is placed inside a considerably larger glass tube/container. The inside of
this larger container is put under vacuum. The inner "thimble" is surrounded with a steel ring that also sits on the firebrick. This is used as an
inductive heating element. The steel is also under vacuum, and much closer to the sample than to the outer container.
The entire container can be suspended through the middle of an inductive heating coil. Both the coil and glass container can be submerged in DI water
to keep them cool. It sounds silly putting high voltage into water, but it does work fine so long as DI or distilled water is chosen and not tap
water. I have done this before. Just don't put your hand in the water when it's on... The water also keeps the outer glass container cool, while
things inside get very hot. A "cold plate" can be positioned over the open end of the sample holder. It doesn't have to be actually cold, just cold
enough for your sample to condense under vacuum. In research an actual cold plate attached to a helium refrigeration unit is often used, but if the
thermal mass of the collector plate is sufficient and the sample size is small, then it's possible that no active cooling would be needed. It mainly
depends on your sample.
[Edited on 22-08-03 by WGTR]
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BromicAcid
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This will also be an issue, ion exchange does happen at elevated temperatures, I remember that glassware can be treated with potassium salts at higher
temps to make a tougher glassware surface.
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clearly_not_atara
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| Quote: | | I don't understand for what reason quartz is not the standard in labs and/or much more common and more cheap and at disposal in shops..
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https://highlyeducatedti.com/blogs/information/thermal-shock...
"It is a popular fallacy among consumers of quartz (fused quartz) to equate its thermal shock resistance to its tensile strength. In reality this
is far from the truth. Quartz actually has significantly less mechanical strength than glass (borosilicate). So why does everyone believe it to be
stronger? We believe this erroneous thought process is derived from the extreme temperature differentiation (thermal shock) the fused quartz can
withstand that borosilicate can not. Users that torch quartz know that it would be detrimental to torch their glass rig, so the impression is that
quartz is strong while glass is fragile. "
"Many users that break a quartz product from an accidental drop, are under the impression that the quartz should have had more tensile strength
because it is quartz. These same users would probably be surprised to know that quartz is significantly more likely to shatter on impact than
borosilicate would be in the same scenario. This is because of the different mechanical properties between fused quartz and borosilicate (glass).
"
[Edited on 04-20-1969 by clearly_not_atara]
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pneumatician
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hey guy, maybe you play soccer with your flask full of molten salts or boiling liquids? :-) so not is this assertation pointlessness a fallacy? in an
average lab use is better boro or quartz glass???
I don't enter in flask wall thickness because this is sci-ficction, but in boro you can find a good glass thickness and some others thin like paper.
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Texium
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Ok pneumo, so if that doesn’t convince you, what about the fact that quartz requires higher temperature and greater skill to work? It can’t be
produced with the same ease and efficiency as borosilicate glass, so even if we were to assume economies of scale would reduce the price, it would
still cost more than boro. Additionally, repairs would still be very expensive and be less accessible since it requires specialized equipment and
someone with the skill to work quartz. And because of that reduced tensile strength, it’s not unreasonable to expect that repairs will be needed
more often, even if you aren’t playing soccer with your flasks. 
I’d also argue that in an “average lab” borosilicate is superior to quartz. Working with molten salts is not something that’s done in an
“average lab” so it’s not a good example of a typical use. In an organic chemistry lab, 99% of the reactions that are run will be under 200°C,
and at least half of them may be under 100°C. Quartz would just be a waste of money in those circumstances.
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pneumatician
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Call me a fucking perfectionism texiom...
who repairs boro glass??? Is a waste of money... I need to repair one and the price is more expensive than buy another piece brand new. And when a
piece of boro glass need to be in repair, one, the guy is a bit silly and break a expensive custom made new piece or two, the X guy don't known or
call and ask how many money cost to repair their old piece that due to wear and tear it will not last much longer :-D
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