Sciencemadness Discussion Board

Filtering the impossible to filter: calcium sulfate and similar

macckone - 26-8-2022 at 11:50

This isn't exactly a beginner question.
More of a how do you do it question.

We have all had to deal with the almost impossible to filter precipiate of a double displacement with calcium sulfate as the waste product.

I have tried vaccum cleaner bags, nylons, even a french press.

1) a lot of liquid is entrained in the solid
2) the filter either lets through a lot of calcium sulfate or it clogs or both

so how do you deal with it?

I hate it is an acceptable answer.

karolus28 - 26-8-2022 at 12:22

I've heard of people putting "sand, coarse silica, alumina, even shredded paper" into the suspension and then filtering, not sure how it works.

B(a)P - 26-8-2022 at 12:24

I typically use standard filter paper and the biggest funnel I have. Then it is down to the passage of a lot of time and the acceptance that I will loose yield because as you point out a lot of liquid remains entrained in the solid. Vacuum filtration with a buchner in my experience takes only slightly less time if you have a lot of very fine solid material and I don't like running my pump for extended periods. Decanting off as much liquid as possible after the solid has settled for a period of time (sometimes days) is generally the best way to go IMHO. If I am not concerned about yield I will often not even bother with the remaining liquid and solid after initial decanting.

I have tried the following, but with little success:
- using a kitchen sieve lined with paper towel to try and increasing filtration surface area.
- making vessels from inverted 2 L milk containers with holes in an inverted screwed on lid lined with filter paper. The idea here was to build up some head to drive the liquid through.

Great topic! I look forward to hearing the solution to removing barium sulfate from acids that attack organic material while preserving yield.

Rainwater - 26-8-2022 at 13:17

In industry, a belt filter is used in combination with heat. Basicly providing unlimited surface area

UC235 - 26-8-2022 at 17:24

Add a whole bunch of celite/diatomaceous earth (acid washed if maintaining pH is important. The cheap (pool) stuff is alkaline). Flip initial filtrate until clear. Wash cake out thoroughly.

Mostly I filter lots of horrible sticky, tarry protein glop.

[Edited on 27-8-2022 by UC235]

PirateDocBrown - 26-8-2022 at 19:35

Start out with as dilute solutions as is practical.

draculic acid69 - 26-8-2022 at 20:45

This may or may not help but a cotton t-shirt (preferably white) can act as a quick coarse filter if solution is not too far from neutral pH and it can be twisted and squished to recover solutions

[Edited on 27-8-2022 by draculic acid69]

Metacelsus - 27-8-2022 at 04:38

Perhaps you could centrifuge instead of filtering?

Gammatron - 2-9-2022 at 13:51

I wonder if you could use a large sep funnel. Since the particles are so small and always remain somewhat suspended even after settling I don't expect it would clog. I've also had success using a pump or siphon too.

WGTR - 2-9-2022 at 15:04

When I made graphene oxide (this is completely impossible to bulk filter), I loaded the suspended samples into dialysis tubing and stirred them for several days in a large beaker until the rinse water was neutral. I did the same thing when successfully producing amorphous titania, that did not separate out in the centrifuge even.

Of course, that was when I was trying to keep the precipitate, not discard it…but a little DE does help on filter paper in those cases. A little is just enough. A bit more is way too much.

The counterintuitive thing is that fine particles will clog coarse filter paper. For fine precipitates you need the finest filter paper. Things like a thin layer of DE keeps these fine particles from getting into the pores of the filter.

[Edited on 22-09-02 by WGTR]