# Pressure vs. Vacuum



## dtectr (Mar 28, 2011)

Some of the older chemistry texts discuss _pressure_ filtering, as well as the more common _vacuum_ filtering. 
My question(s) is/are:
If the pressure was controlled, i.e. not so much as to puncture the filter paper, would it accomplish the same thing as vacuum filtering? Or are there inherent downsides that would make it ineffective for our uses?

any feedback, including experiences with the methods welcomed.
thanks
dtectr


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## rusty (Mar 28, 2011)

Search for "filter press" you will find that the commercial refiner uses them. Harold_V had one.


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## dtectr (Mar 28, 2011)

rusty said:


> Search for "filter press" you will find that the commercial refiner uses them. Harold_V had one.


thanks for the quick response.
dtectr


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## Barren Realms 007 (Mar 28, 2011)

You can find small ones for sale online if you search.


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## Harold_V (Mar 29, 2011)

Barren Realms 007 said:


> You can find small ones for sale online if you search.


If you're handling solutions of great value, don't turn to a common filter press. In order to isolate the solution, a filter press should use "gasketed" plates. Most filter presses allow the filtered solution to drain from the exterior of the plates, whereas a gasketed press has ducts within the press head and plates, directing solution to a discharge pipe. Plates for the common filter press are generally made of cast iron, whereas a gasketed filter press intended for use with acidic solutions has plates made of polypropylene. 

Do not confuse a filter press with vacuum filtration. Filter presses can handle a huge volume of solution, whereas a vacuum filter is restricted to the size of the single filter element. They are also operated at pressures greater than atmospheric pressure, unlike vacuum filtration. 

Note the O rings that are a part of the plates in the picture, below. They are a part of the gasketed plate system. The large one that frames the filter ( as well as keeping the filter trapped on the plate) seals the solution from escaping from the press when it is closed with the hydraulic ram. The press was closed under ten tons of pressure. The plates are fed from the center, with the fluid passing through the round filtered area, and routed to the four corners, where you see small O rings. They are the discharge ports of the press. This press was used to filter gold/silver cyanide solution from finely crushed ore. Note the staining on the walls from solution splashes. 

Harold


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## Barren Realms 007 (Mar 29, 2011)

Harold_V said:


> Barren Realms 007 said:
> 
> 
> > You can find small ones for sale online if you search.
> ...



Yes that was the one I was referring to Harold. I used the same desighn as you have there only on a much larger scale. If memory serves me right the plate's were 3' square in size.


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## husker4515 (Jul 18, 2011)

Harold, Just wondering who was the manufacturer of the your filter press?


Harold_V said:


> Barren Realms 007 said:
> 
> 
> > You can find small ones for sale online if you search.
> ...


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## Barren Realms 007 (Jul 18, 2011)

husker4515 said:


> Harold, Just wondering who was the manufacturer of the your filter press?
> 
> 
> Harold_V said:
> ...



He made his himself.


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## Bellien (Jul 20, 2011)

dtectr said:



> Some of the older chemistry texts discuss _pressure_ filtering, as well as the more common _vacuum_ filtering.
> My question(s) is/are:
> If the pressure was controlled, i.e. not so much as to puncture the filter paper, would it accomplish the same thing as vacuum filtering? Or are there inherent downsides that would make it ineffective for our uses?
> 
> ...




Vacuum filtration is a form of pressure filtration, it's limited by the difference between the strength of your vacuum and ambient air pressure. If you weren't thinking of a filter press but rather a pressure vessel with a regular filter, unless you are exceeding atmospheric pressure you are dealing with essentially the same thing.


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## dtectr (Jul 21, 2011)

Bellien said:


> dtectr said:
> 
> 
> > Some of the older chemistry texts discuss _pressure_ filtering, as well as the more common _vacuum_ filtering.
> ...


I am unfamiliar with a pressure vessel - do you have a picture or diagram that could help me?
Thanks
dtectr


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## Bellien (Jul 21, 2011)

Certainly, I apologize if my reply was ambiguous. 

In the simplest form a pressure filter is a container that can be pressurized which has a exit hole covered by a filter. The 'pressure vessel' is just the container which is able to be pressurized. 

To the best of my knowledge the dangers of compressed air over hydrostatic pressure, the complexity of gasketing and a host of other issues have led to pressure filters being largely abandoned in anything other then niche applications where they are used to purify a liquid in a contentious process, for example city water supplies may use a sand filter. 

In most applications filter presses like Harold, Rusty, and Barren described are superior for all the reasons they have already mentioned.

Here are some hopefully helpful diagrams/pictures 

http://www.thewatertreatments.com/water-filters-filtration/pressure-filter-water-filtration

http://chestofbooks.com/food/beverages/A-Treatise-On-Beverages/Globe-Pressure-Water-Filter.html

This thread on ScienceMadness has some more information, but I personally wouldn't be caught with anything made of PVC when compressed air is part of the equation.

http://www.sciencemadness.org/talk/viewthread.php?tid=9630


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## skippy (Jul 21, 2011)

I have been using a pressure filter. I used a 6 foot section of 4" pvc sewer pipe. I fitted a hose barb to a cap, with a hose and a bicycle tube valve on it. This cap is glued on the pipe. Then I take another cap that I drilled a dozen 1/8" holes in. In this I put a layer of polyester felt cut into a circle, then pack a half an inch of damp toilet paper onto the felt layer. I then pack some toilet paper around the edge of the 1/2" layer of toilet paper. I do this to prevent the liquid from slipping around the packed toilet paper bed. 
To operate the filter, I tip the apparatus upside down, filling it with the liquid to be filtered, and then I cap it with the filtering cap. The cap is sealed and held on by a rubber drain splice with stainless band clamps . Then tip the pipe upside down and start filtering - but tip it nice and slow and gently so the movement of the solution doesn't dislodge the filter. 
I typically pull the bicycle tube valve from the hose and let the head do the work at first, and after the head sinks, fit the valve and apply some air pressure. Not much! I'd say keep it to no more than 3 PSI or so. I took it to six and had the cap shoot off, which is a potential danger to using this setup. Keep the end in a plastic garbage can in case, as you wouldn't want to blast your solution everywhere. I am really liking it for filtering solutions with small to moderate amounts of crap in them that just doesn't want to settle.


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