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Using low pressure compressed air to filter process liquids from process solids in plastic vessel Answered

Hello all,

This is my first post here, and after many trails proving a concept method i am ready to scale up, but require a little guidance for safety purposes. This process is happening at a lab scale with the need to make a jump form 10 gal to 60 gallon per batch.

The problem:

We are currently chemically processing concrete construction fines (roughly a gradation of sand and fine particles between 20 micron & 5 mm) with a water-based chemical. We are using a standard concrete mixer to process, and then filter the solids from the liquids using #50 mesh bags. Then we let the ultra-fines settle and decant as much of the remaining liquid as possible. With this process both the liquids AND the solids are of great value, so any valuable chemical in the liquids is important to get out of the solids with as much efficiency as possible.

The valuable chemical in the liquid easily volatilizes and is fairly corrosive, so there is a need to keep everything contained and in CHEAP non-corrosive materials as much as possible. From this i was thinking doing batches in 55 gallon plastic top sealing drums and using a drum mixing machine to stand-in for the concrete mix. This way everything is sealed and can be stored until separation.

Then i started thinking about filtration... we use kegs around the lab to process certain chemicals and started to think about putting a filter on the end of the stand pipe inside a corney keg and using 2 psi of air to force the liquid out form the solid. At first i used #100 poly mesh and wrapped the liquid outlet and though it took a while i was able to get a high degree of the liquid out, more than we ever have in gravity method. So then i made a filter using 1 micron mesh and wrapped it in #100 mesh and through a mass of very fine particles mixed with water. Again, it worked wonderfully, though i had to increase pressure to 20 psi to force more water out of the smaller fines.

Now i have started testing on plastic vessels, YES i know this can be dangerous, however i am working at very low pressures. I have a 7 gallon screw-top lid bucket (not the pop-on kind) and have mimicked a keg setup to see if i can force water out of an exit pipe, and it worked. I am about to make the jump of trying to process actual solid/liquid mixture with a similar filtered outlet.

Am i going in a completely wrong direction?

Can this be scaled to a 55 gallon drum under 5 psi or similar low pressure?

Because i am allowing an outlet (liquid), is this less dangerous than actually pressuring a drum with no outlet (i realize the filter itself with create back pressure)?

Are there pieces of process equipment that are made of cheap non-corrosive materials, can be used on a drum mixer, AND can be pressurized moderatly?

Many thanks!


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Jack A Lopez
Jack A Lopez

12 months ago

You call them "plastic vessels."

Yet, if you look more closely at the hieroglyphics printed on these artifacts, you will discover the name of the plastic, revealed in the symbols printed there, and very often big plastic drums, plastic buckets too, made for shipping chemicals, are made from HDPE (high density polyethylene), also known by its recycling resin code, number 2.

The best thing about HDPE, in my opinion, is its chemical resistance. HDPE is the Honey Badger of chemical resistance, almost.

It has good mechanical strength too, if you can find it thick enough. Some discarded containers are thicker than others. You know, a bottle that used to contain dollar-store bleach, can be almost paper thin. In contrast, a 5 gallon, HDPE bottle, formerly containing commercial car-wash detergent, is several millimeters thick, and much more sturdy.

I have a bunch of those things, and they come with threaded fittings and gaskets on them, that strongly suggest, at least to my eyes, the whole bottle is made to withstand internal pressure, at least a little bit. I have to admit I have never seen them connected to car-wash hardware. I pull these things out of dumpsters. Presumably it is some car-wash busines in my town who is putting them there.

I am guessing you can find pictures of these artifacts, just by doing an image search, but if you want to see some of the ones I have, I suppose could take a picture of some of the ones in my collection. I have seen approximately two sizes so far: boxy shaped 5 gallon (20 liter?) and cylindrical shaped 10 gallon (40 liter?)

What else? The pressure vessel for a typical sprayer, for garden chemicals, this is also made of thick, sturdy HDPE.

I think the only drawback to HDPE, is it loses that mechanical strength at higher temperatures, like roughly around 100 C, the normal boiling point of water.

Or maybe lower than that, around 50 C, which is still some pretty hot water.

I mean, we can kind of glean some things from these "chemical compatibility" pages, like this one:


Where we see little footnotes, especially for plastics, about temperature, e.g.

Note 1-Satisfactory to 72F (22C)
note 2-Satisfactory to 120F (48C)

Also my intuition is telling me that a HDPE bottle, under pressure, is not going to fail by shattering int shrapnel, as though it were made out of glass.

I am guessing the failure would more closely resemble the failure of a chewing gum bubble. That is to say, I would kind of expect to see expansion, followed by a big rip in one side where it gets thinnest and weakest first.

Of course, ultimately the responsibility for safety at your R-and-D lab, or work site, is yours.

What cliche or proverb is appropriate here? I dunno. Perhaps: Proceed with caution.

I think that is usually good advice.


12 months ago

I prefer to go the other way around ;)
Look up "vacuum filtration" ;)
Assuming you already have means of removing most of the liquid:
Buy or make a suitably sized vacuum filter - if in doubt go one or two sizes bigger in size if possible.
As you are only dealing with limited amounts of liquid you can use a stainless steel keg or similar vessel under the vacuum filter funnel - as long as it is able to survive a bigger negative pressure.

How much vacuum you need depends on how well the solids can hold on to the liquids and how fine the solids are.
In most cases a good venturi nozzle running from the tap will do.
I like to do some fun stuff with herbs and spices.
Using a good vacuum cleaner will give plenty of suction and allows you the use on the large scale you need.
Thinking in the region of industrial wet/dry vacuum cleaner.
If you want to recover even more liquid and get an almost dry product consider buying a 2 stage rotary vane vacuum pump as used in the HVAC/refrigaration business - you can buy them online for under 100 bucks.
As you are dealing with quite dangerous stuff by the looks of it I would opt for a commercail model rather than some cheap one from Fleabuy or the Amazonas ;)
This gives you the option to have it filled right away with a silicone based oil.
Otherwise the required cleaning process to swap a new pump with standard oil already filled in will be very time consuming.
If in doubt and you want to go cheaper ask for the pump to be delivered dry and without any oil.
Use silicone based oil and follow the recommendations of the oil manufacturer in terms when or how often to change the oil.