Sulaiman
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Filtering hot saturated solutions - ideas wanted
I'm purifying 1kg of potassium nitrate by re-crystallisation,
to keep volumes manageable I'm working at up to 110C,
I tried to filter the hot solution through folded filter paper in a glass funnel,
failed due to almost instant blocking of the funnel caused by the funnel cooling the saturated solution,
even though I pre-heated the funnel and filter with boiling water.
Nothing new.
I want a heated filter ...
Pumping hot liquids is not my first choice,
As I hope to access my existing perforated porcelain Buchner funnel I'd like something complimentary to it
for general chemistry work at the 100ml to 1000ml scale.
Any cheap and easy suggestions ?
CAUTION : Hobby Chemist, not Professional or even Amateur
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violet sin
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Flood light? We used them (3) for a hot plate for silkscreen transfer printing odd objects, unless I'm mixing up the purpose. I know it got smoking
hot just pointing at an aluminum plate on tall legs.
I bet the filters would have no prob warming up with a low wattage flood light pointed in that direction. Saves you the trouble of having to get
affordable metal of sufficient resistance and nichrome, or shattering a glass one with uneven heating? Another long day on this end, so apologies if
that's not helpful.
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Sulaiman
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I'll definitely consider that - nice novel (to me) idea - thanks.
Any more?
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karlos³
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You could heat the funnel with a heatgun during the filtration maybe?
verrückt und wissenschaftlich
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Sulaiman
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I'd like to have a heat gun,
this may 'justify' it 
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SWIM
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When I filtered hot saturated potassium nitrate solution I used a paper towel.
Don't know if these things are similar the world over, but in the US they are very porous compared to filter paper.
I was about to write, "Maybe the higher flow rate helped keep my filter hot." when it occurred to me that my success may have been due to the fact
that I used a large polypropylene kitchen funnel instead of a glass lab funnel.
Those things would absorb less heat from the solution than glass because of lower mass and heat conductivity perhaps. It may be what made the
difference.
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Fulmen
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The answer should be obvious: Insulate and preheat the funnel by running boiling water through it first. Though you can never filter a saturated
solution due to the solubility curve, any cooling will instantly cause some precipitation. The max concentration will depend on the setup, I prefer to
err on the side of caution as a clogged filter is far more work than an excess of water. Just recycle the filtrate for this step to eliminate losses,
it's better to do the crystallization in a separate step.
We're not banging rocks together here. We know how to put a man back together.
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Sulaiman
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I could not decide on any new equipment so I decided to work at lower concentrations
- filtering now successful.
Now for some boiling down...
I'll keep looking for a long term solution.
Thanks for all of the suggestions!
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Fyndium
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If there were a filter that had a heating element, either on the sides, or even inside, saturated solutions could be filtered. Technically, it
shouldn't be too impossible to make. There are heating mantles for filters, too, and one could be constructed.
When I filtered hot, almost saturated solutions, I used to first attach it to flask one, pour in large amount of basically boiling solvent, in this
case ethanol, filter it through, quickly switch it into flask 2 and immediately pour in the solution.
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rockyit98
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heated chamber. can make one for few bucks. use fiber glass insulating board with aluminum refection foil. cheap temperature regulator with halogen
bulb 250w or 500W, depends on size.
"A mind is a terrible thing to lose"-Meisner
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Dr.Bob
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The make porcelin buchner funnels which are jacketed for heating or cooling, I have a few of them, about 20-25 mm diameter, which can be heated with
steam (from the days when all labs had that plumbed in) or hot water or cooled with a chiller. I think I have also seen a few jacketed glass frits
before as well, which were smaller. If not for the weight of the Buchners I would offer to sell you one cheap, but the shipping to the UK would be
awfel, as they weight about 4-6 pounds each. You might be able to find one on an European sale site.
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Fulmen
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That sounds like a very elegant (and quite possibly expensive) solution. Although I do have free access to TIG and 316 sheet metal at the moment...
We're not banging rocks together here. We know how to put a man back together.
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Dr.Bob
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Here are some examples of these:
https://www.amazon.com/Chemglass-Filter-Buchner-Jacketed-Por...
https://www.amazon.com/LG-7110-116-Borosilicate-Jacketed-Por...
Not cheap, but they are sometimes on Ebay cheaper. The porcelin ones are now hard to find, but also exist. I have a coupld of them left.
Bob
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