Sciencemadness Discussion Board

Filtering hot saturated solutions - ideas wanted

Sulaiman - 16-9-2021 at 00:38

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 ?

violet sin - 16-9-2021 at 01:32

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.

Sulaiman - 16-9-2021 at 02:16

I'll definitely consider that - nice novel (to me) idea - thanks.

Any more?

karlosĀ³ - 16-9-2021 at 05:00

You could heat the funnel with a heatgun during the filtration maybe?

Sulaiman - 16-9-2021 at 06:23

I'd like to have a heat gun,
this may 'justify' it :)

SWIM - 16-9-2021 at 10:06

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.

Fulmen - 16-9-2021 at 11:43

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.

Sulaiman - 16-9-2021 at 20:48

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!

Fyndium - 17-9-2021 at 00:19

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.

rockyit98 - 17-9-2021 at 04:15

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.

Dr.Bob - 17-9-2021 at 07:09

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.

Fulmen - 17-9-2021 at 10:46

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...

Dr.Bob - 18-9-2021 at 18:36

Here are some examples of these:

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.