RogueRose
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Celite alternatives for filtering - MgCO3 & CaSO4?
I've never bought celite but have made something similar by washing fresh water diatomaceous earth (the white kind) with HCl and removing the larger
pieces (that which doesn't pass through coffee filter) and keeping the fine & large parts seperated then washing with fresh water. This left
extremely fine DE which is pretty close to celite. The problem is that it seems to be a little difficult to filter with even with vacuum and is
extremely slow. It is also a little time consuming and can be a PITA to make if you don't have it already.
I recently worked with MgCO3 and CaSO4 with vacuum filtration and a buchner funnel. Both compounds were made with a double replacement reaction (slow
addition of the "solvent" compound") which resulted in extremely fine particles probably as small, if not smaller than the celite or DE fines I have.
It seems like it would work just as well as celite when filtering and both are minimally soluble in water (CaSO4 about 2g/L and MgCO3 .2g/L @ 20 deg C
and .06g/L @ 100C - so MgCO3 is about 1/10 to 1/32 as soluble as CaSO4 - so a better choice). There are many solvents in which both are totally
insoluble, ammonia being one and many alcohols.
BaSO4 might be another option which is even less soluble, but also less likely to be available or easily made in the lab & that which is available
may not be as fine as one would hope for this kind of filtration.
Has anyone ever used a compound like this to aid in filtration of extremely fine particles?
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Ozone
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When this kind of thing happens, I'll usually pass my filtrate through at least once more--the fines build up a cake that tightens the filter. Of
course, if your filter cake is blinded (e.g. compacted and impermeable, as with fine clay) then you're screwed.
O3
-Anyone who never made a mistake never tried anything new.
--Albert Einstein
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RogueRose
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Quote: Originally posted by Ozone  | When this kind of thing happens, I'll usually pass my filtrate through at least once more--the fines build up a cake that tightens the filter. Of
course, if your filter cake is blinded (e.g. compacted and impermeable, as with fine clay) then you're screwed.
O3 |
Agreed with the multi-pass method. Many people have mentioned that copper hydroxide and even CuO is difficult to remove from solution as it passes
though even fine filters. I found that one of the only methods that removes this is the multi-pass as the oxide or hydroxide builds up on the top
layer of the filter and acts to improve the effectiveness of the filter. It is noticeably slower sometimes by a factor of 10-20 times slower so I put
it in a corner for a few days and allow it to drip through. It is also best to have a more concentrated solution (less liquid) if you are dealing with
filtering something this slow.
I'm going to try the MgCO3 method in the next few days and compare it to my homemade celite. One thing both seem to have in common is "fracturing" of
the filter cake when all the liquid has been pulled through, then the cake tends to split basically down the middle, so if using anything like celite
or MgCO3, you have to be aware of the liquid level passing though and add more before it has pulled all the liquid out of the filter material (before
it fractures). IDK if there is any way to prevent this or not.
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AvBaeyer
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One of the techniques for using celite is to first make a pad of celite in the buchner funnel and tamp it down tightly while under vacuum. Typically
this layer will be about 1/2 inch thick. Ensure that the edges are tight against the funnel to prevent leakage. Then slurry the material to be
filtered with additional celite. Finally, pour the slurried mixture onto the celite in the funnel while pulling a vacuum. This should provide a clear
filtrate without significant slow down of the filtering process. This process does require a good aspirator vacuum.
I learned this from and old process chemist years ago. I use this method all the time to filter solutions containing decolorizing carbon and other
fine materials with no pass through of carbon or celite. Actually I use food grade diatomaceous earth which works just fine.
AvB
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