Sciencemadness Discussion Board

Celite alternatives for filtering - MgCO3 & CaSO4?

RogueRose - 14-4-2018 at 04:14

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?

Ozone - 14-4-2018 at 05:49

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

RogueRose - 14-4-2018 at 06:10

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.

AvBaeyer - 14-4-2018 at 18:07

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