# Filter media for large buchner.



## HAuCl4 (Oct 13, 2010)

A client has asked me to make him a large funnel 1 meter diameter give or take, and I do not know where to order good and economical filter media for same. 

He is in the silver refining business, processing 5,000 oz lots one at a time weekly with sulphuric acid and then dropping the AgCl with salt, etc.

Any suggestions for a durable filter media I can purchase for use and re-use?. The exact size of the funnel will depend on what's available.

Cheers.


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## nickvc (Oct 13, 2010)

4metals will know the answer to this one I bet as he's used to doing things on the grand scale,I vaguely remember mention of large re usable filters in a post some time back but can't remember who it was that posted.....going senile I think. :roll:
I'll see if I can find it and let you know.


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## nickvc (Oct 13, 2010)

I seem to recall the filters were made of polypropylene and were very tough... still can't think who brought the idea up.


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## 4metals (Oct 13, 2010)

bel art table top buchner funnels. Buchner table top funnels made by Bel Art. Some have a removable plate made of porous polypropylene. They can and do clog, but are well protected by a sheet of S&S sharkskin filter paper on top, which slows the clogging.




Standard production item up to 1 meter diameter.


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## HAuCl4 (Oct 13, 2010)

Thanks 4metals. 

Is there a place that sells the filter media by itself instead of complete funnels?. Something re-usable if possible. i.e.: not paper.


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## 4metals (Oct 13, 2010)

They may sell the porous plates separately, in fact they do as I've bought extras. But why not buy the whole thing, unless you're a kick ass plastic welder their work is top shelf. Bel Art Labs New Jersey USA


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## nickvc (Oct 13, 2010)

4metals said:


> bel art table top buchner funnels. Buchner table top funnels made by Bel Art. Some have a removable plate made of porous polypropylene. They can and do clog, but are well protected by a sheet of S&S sharkskin filter paper on top, which slows the clogging.
> 
> 
> 
> Standard production item up to 1 meter diameter.


There you go I knew 4metals would know.


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## HAuCl4 (Oct 13, 2010)

Cool. Thanks 4metals again. 8)


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## goldsilverpro (Oct 13, 2010)

I owned an 18" or 20" Bel-Art filter funnel, about 20 years ago, and was very disappointed with it. My problem was the difficulty in getting a good seal with the paper. The holes were too near the edge and the damned thing always seemed to leak. I would hope they've improved the design since then. All they had to do was keep the holes about 3/4" away from the edge of the plate. Damn, I hated that thing.


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## 4metals (Oct 13, 2010)

Well they're certainly not like the Coors porcelain tabletops. If you get the Bel Art model with the sintered poly propylene plate the plate is actually the filter. By covering it with a sheet of filter paper it lasts longer. The ones they claim to be for refining have a removable plate so you can clean it out, I never liked them. (It was a trade off between getting a good seal and the ability to clean under the plate) 

The ceramic Coors tabletops have a concave bottom so they drain completely and no removable plate is needed. I have one of them that is 32 CM in diameter, they're only made on custom order now. Years ago I had a 100 CM filter as well. 

The best filter to media seal is had using a porcelain funnel and the rigid Alpha Media Filter Pad. These pads are not recycle-able and the question posed was for a recyclable media. Recycle-able doesn't always mean cost effective. These filter pads swell when wet and fit snugly to start, they wold seal well in a Bel Art funnel as well, they are custom die cut to fit specific filter diameters.


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## HAuCl4 (Oct 14, 2010)

Like these?

http://danddfiltration.com/semiconductors/123.html

edit: Will sulphuric acid cook the cellulose filters?. The paper filters I have tried simply disintegrate. The only half satisfactory experiment I have in smaller scale is with fiberglass cloth (coarse filtering) and the "plug filter" that I learned here using fiberglass wool.

How about those carbon or ceramic based filters?. Too slow?.

The paper filters will work OK for the AgCl filtering, when I dilute the solution a lot, which is normal.


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## HAuCl4 (Oct 14, 2010)

4metals said:


> The best filter to media seal is had using a porcelain funnel and the rigid Alpha Media Filter Pad. These pads are not recycle-able and the question posed was for a recyclable media. Recycle-able doesn't always mean cost effective.



I can get porcelain/stoneware in that size cooked and ready for a few hundred dollars. That could be a solution for a careful worker, but these guys work like neanderthals eating elephant steaks. Thinking normal steel painted with polyurethane with fiberglass lining for the funnel and receiver with a gasket in between the removable top (filter) and the receiver (bottom). A little more expensive than the porcelain, but very sturdy even for these guys.

Still open on the filter media. :roll:


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## goldsilverpro (Oct 14, 2010)

4metals said:


> The ones they claim to be for refining have a removable plate so you can clean it out, I never liked them. (It was a trade off between getting a good seal and the ability to clean under the plate)



Correction to my last post. Thanks for jogging my memory. Thinking back, that was the same type I had. It wasn't the hole placement that caused the problem. It was that damned removable plate. The plate went to the very edge of the funnel chamber and there was a slight gap - very poor design. That meant that you had to use oversize paper, which always ended up with folds in the paper where it came up the sides. This caused air gaps and, unless the paper was hugely oversize, it was almost impossible to seal it without leaks. You had to keep messing with it. I always thought they should have made the removable plate like those on the large Coors tabletops. In those, the removable plate starts about 1/2", or so, in from the edge of the bottom of the funnel chamber. That way, you could use exact size paper that sits flat on the bottom and it would totally seal the gap.

One place I worked had several 50 gal SS tanks for dissolving the silver braze from jet engine stators using nitric acid. They then precipitated the silver as a chloride. Next, they let it settle, siphoned off most of the solution, and filtered and rinsed the silver chloride in a 30" dia (approx.), 300 series SS vacuum filter of excellent design. It stood on SS legs. All the SS was at least 1/8" thick. It consisted of 3 separate parts. The bottom part, the vacuum chamber, had a valve at the bottom to drain off the filtered solution and a vacuum connection near the top. A plate, with a bunch of 1" holes in it, fit perfectly in a groove (or, maybe, on a few recessed cross supports welded inside the top of the vacuum chamber - I don't remember exactly how this was done) on top of the vacuum chamber. The filter paper, which was at least 1/8" thick and came in rolls, was cut oversize (square - not round) and placed on the perforated plate - the paper hung over the sides a bit. On top of the paper, a 30" dia SS ring (funnel chamber), about 12" tall, was placed. The ring had about 8 screw down clamps mounted around the outside, which were then tightened (in a similar sequence as tire lug nuts) to seal everything. 

The only problem was that, if the vacuum was shut off before all the solution had drained through, the solution would wick out horizontally, to the edges of the paper, and drip on the floor. I found that out the hard way. However, the good thing about this was that it started me thinking about designing a wick filter, which I've discussed several times on this forum. Several of my inventions have resulted from screw-ups.

A filter of similar design could probably be constructed easily from thick polypro, using woodworking tools (a table saw and maybe a router, e.g.) and a plastic welder (inexpensive and easy to learn to use). To eliminate having to heat and bend the plastic into circular shapes, the filter could probably be made square much more simply.


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## HAuCl4 (Oct 14, 2010)

A couple or more good ideas for the design. Thanks GSP. I'll make a quick drawing and post it for your comments/criticisms after I get the final thing in my head with the proper media. I'll probably stick with normal steel instead of SS. Low budget and client doesn't care one way or the other.


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## goldsilverpro (Oct 14, 2010)

Mild steel would work but I would think it would tend to corrode if even a trace of either HNO3 or HCl were present. Even with the SS, we were patient and very careful to not overadd the HCl when dropping the AgCl. Unlike SS, mild steel is also attacked by weak HNO3.


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## goldsilverpro (Oct 14, 2010)

Some further thoughts and ideas. 

If the bottom plastic vacuum chamber were square, depending on the size, it would probably require a thickness of at least 1/2" to prevent collapse. Mathematically, I would think a round chamber would be less susceptible to collapse.

They make plastic pipe in very large diameters (I saw one reference of 10' dia) and most of what I could find was made of PVC or PE, both of which should be weldable. Most of the larger pipe was corrugated, which wouldn't work, but I did find a reference to some that was corrugated on the outside and smooth on the inside. That could work and the corrugation would add strength against collapse. Of course, the wall would still have to be thick enough to weld.

Were this my project, I would first look around for short scrap lengths of large diameter plastic pipe with at least 1/2" wall thickness (and maybe outside corrugation). Maybe, from large companies that lay drainage pipe. The bottom and top cut surfaces of the vacuum chamber would have to made flat. For the top chamber, only the bottom end would have to be made flat. Weld a thick plastic sheet base, of the same type plastic, on the bottom. Weld some heavy cross braces in the inside top of the vacuum chamber, recessed so that the removable perforated plate would be level with, or a fraction of an inch below, the top. You could probably use several web clamps to clamp the whole thing together. The filter media should be thick (I would guess from 1/8" to 1/4") to compress and get a good seal. The drain and vac fittings in the vac chamber could be welded on or, threaded bung-type fittings with inert washers or gaskets (I would probably use Buna-N, Teflon, or other chemical resistant material for the washers) could probably be used.

Plastic welding can be farmed out, or, you could buy or rent a plastic welder and learn to do it yourself (fairly short learning curve). The welding rod and the 2 pieces you're welding together all have to be made from the same type of plastic. The welder is simply a hot air nozzle that melts the pieces you are joining and the rod simultaneously. The round welding rod is held in proper position by a tube on the nozzle. The more you pay for the welder, the better control you have (pressure, temperature, etc.). Some are self-contained, with a blower built-in, and some, I think, can use inert gases from a bottle.

I know this is supposed to be low-budget but you're pretty limited to the materials you can use. About the only things I can think of are plastic or stainless. If it were made strong enough, I would prefer plastic. Mild steel is a poor choice unless you can first convert the silver chloride to silver oxide (with 100% efficiency) using NaOH. You might be able to coat the inside of the chambers with several coats of something like liquid polyester boat resin, but that could be iffy. I have used boat resin on wooden silver cells and it held up pretty well to the very weak nitric.

To purchase a ready made new vacuum filter, of the size you want, would cost a ton of money. I may be wrong, but I'm thinking that that crappy little 20" Bel-Art filter cost me $700 in the late 80s.


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## 4metals (Oct 14, 2010)

I re-read the original question and realized I missed the sulfuric acid part. Then I'm thinking, why sulfuric then to silver chloride, then a silver chloride reduction and then a silver cell to clean it up. He must be refining the silver cartridges used for silver photo recovery. 

If that is the case, why isn't he just melting the cartridges in a reverberatory furnace and fluxing off the base metals. Environmental Products makes a furnace just for that, http://environ-products.com/ then all you need to do is put the bars in a silver cell. Pyrrometallurgy can be fun too!


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## goldsilverpro (Oct 14, 2010)

I had assumed he was dissolving impure silver in hot concentrated sulfuric.


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## HAuCl4 (Oct 15, 2010)

Interesting link 4metals. But no. These guys are processing scrap silver from all sources, and they want a filter and a filter they'll get...after they pay my fee and I move on!. 

If they want something else, they can pay more fees. I got to eat too. I understand that they smuggle the 99% ingots to evade local taxes and thieves, and resell to a refiner, but have not asked probing questions...

Don't ask, don't tell!. :shock:


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