Fyndium
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Proper way to commit hot filtration?
What is the proper procedure to commit hot filtration? Is it always necessary to over-dilute the solution to prevent it from crystallizing to the
filter and clogging it?
What I've usually done to reduce losses in general is to pour some boiling hot solvent through a filter first to a secondary flask, and then quickly
switch the filter to receiver flask and immediately pour the solution through. This works great when the saturation is not too great, but if it is
even near supersaturation, it will usually always result in clogging the filter.
There are filters with mantles for circulating hot or cool medium to keep the filter at desired temp, but these are extraordinarily expensive and
scarce.
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BromicAcid
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No, it is not always necessary to have your solution dilute when doing a hot filtration. I mean, for small amounts you can get away with most
anything providing the stars align just right. I've gotten away with passing plenty of concentrated solutions through a Buckner funnel without it
plugging up. But you know, for large amounts of stuff to filter when you start pulling vacuum on it to complete the filtration things can go a little
sideways. Suddenly you're ripping off solvent, causing evaporative cooling and concentration and stuff starts crashing out and plugging. I mean, if
you're going to do it right, pre-heating the filter with a filter mantle (a heating mantle in the shape of the filter) is a good start but even more
important is to accomplish the filtration with pressure. This can mean using a sintered glass frit with a ST neck and throwing some pressure on it
once everything has been charged to the frit or it can mean using an encapsulated filter like for the brewing industry and transferring through it
from one flask to another. Obviously you don't throw a lot of pressure on flasks, less than 5 PSIG so the you need some way to regulate things unless
you're lucky enough to be working in a lab, but usually with a pressure filtration you can throw things though a filter licked-split like and get it
done quickly without the issues of evaporative cooling.
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Fyndium
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Posts: 1192
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I've been using small Chinese membrane pumps for vacuum filtrations, they generate probably something around 100-300mbar of vacuum at max, which is
quite decent for filtrations and allows for use of erlenmeyers, but nowhere near enough to start stripping solvents. For some filtrations, I usually
do dual suction, first strip the majority with the small pump, and then attach a high vacuum pump and suction the mass to dryness.
The clogging seems like a chain reaction, when some precipitate occurs, it will cause the flow to cease, resulting in energy loss and further
precipitation, and once this vicious cycle starts, it's impossible to stop by no other means than to flush the filter with pure hot solvent to
dissolve the precipitate. I've had good success with filtering near-saturated solutions that allow for some temp drop before precipitation, but
saturated solutions will fail without exception. The filter should be heated with mantle you mentioned to hotter temp than the solution, so it would
carry on.
Diluting solutions usually is not too harmful, but it causes a lot more work because it needs to be re-concentrated after crystals have been collected
to minimize yield losses. I sometimes intentionally over-dilute solutions for crystallizations and perform it multi-step, because it seems to produce
crystals of higher purity and size, and for these hot filtration is not an issue.
For example, one procedure of recrystallization of synthesis product is to first place it in an erlenmeyer, add solvent, heat it to near reflux, and
then pour it through a filter to another erlenmeyer, and cool it to crystallize the product. If it is heated too long and too hot, it
can supersaturate and clog the filter, so it it can be a bit tricky to find the sweet spot initially. This solvent is then returned through a filter
to the original erlenmeyer, and some extra solvent is used to rinse the second flask, the crystals and the filter, and the process is repeated until
all of the product is recrystallized. More product can be added as it dissolves and leaves the cycle, but obviously eventually the solvent will
collect enough of the impurities to not work anymore, hence the final cycle can be cooled down to -30C to crash out any product remaining. It is also
beneficial because the already dissolved product will cause the solvent to yield out all of the excess crystals, and very small yield losses can be
achieved, and more work can be put on the last cycle to recover the final dissolved crop.
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