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.
