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Sieve for Molten Precious Metals

Sieve for Molten Precious Metals

Sieve for Molten Precious Metals

(OP)
Hi eng-tippers,

So my hobby is turning my wife's and my sister's sketches into actual fine jewelry. It's fun as heck!

I've been refining scrap gold from everything you can imagine, and also occasionally we hit a chunk or some flakes in a stream with a pan, just like the forty-niners. NOTE: The stream is a different state than my house, so don't come looking.

Anyway, the acids stink and they're a pain to work with, but I deal with that. I also have an environmental conscience that forces me to deal properly with spent fluids.

If my stuff is already known to be reasonably pure, though, I have fun smelting it and pouring it into shot in a home-made crucible and fixture. A frustration is that I am not good with a skimming spoon. I either leave too much debris in the melt or I allow too much gold to harden on the spoon, or I spend too much time staring into the little pot and reduce myself to a puddle of sweat.

What I'd like is a sieve with the smallest holes that will let gold pass through. It would look something like a small tea strainer. I understand the concept of surface tension, but the terms and calculations are like alien beings to me. I don't know how to calculate the range of hole sizes in my proposed sieve, and my considerable Google kung-fu has failed me.

I WOULD like to learn how to calculate same, if I can. A reference would be great.

Otherwise, if I had a rough idea of hole or mesh size for gold, silver, and aluminum, I'd be set. One day I'll get into the hotter-melting stuff like platinum, but no need right now.

Summing up, with other questions I have:

Is my sieve idea practical, or am I all wet?

If practical, what mesh or hole sizes will do the trick?

Does temperature of the melt make a difference? If hotter makes it runnier, I can do that to some extent.

Is there a surface prep for the sieve metal that might help?

Is one sieve metal better than another?

Thanks to anyone who can give me ideas.

Best to you,

Goober Dave

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RE: Sieve for Molten Precious Metals

like these?

http://www.aliexpress.com/item/2pcs-New-Porcelain-...

http://www.thomassci.com/Supplies/Crucibles/_/PORC...

Your sieve would need to be made from ceramic (fiberglass cloth would probably not work, but a pure silica quartz fiber cloth might), not stainless or similar nickel/iron/cobalt alloys - their melt points are too close to that of gold and silver, and both will tend to stick to the metals.

Yes, hotter melt gives you more pouring time before the alloy starts to freeze, i.e. hang up in your filter/funnel/sprues and runners.

If there is dross in your gold, you need to refine it more (soak it in nitric acid?).

Dunno about the silver and aluminum.

RE: Sieve for Molten Precious Metals

(OP)
Thanks btrueblood, I didn't know about the sticking problem. Those look like nice ceramic filters too. I'll be trying out a sample soon.

I always have to process more after the shot is made, unless I've melted down something very pure. Nasty acids. I'm thinking of investing in a fume hood so I don't have to go outside so much.

Best to you,

Goober Dave

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RE: Sieve for Molten Precious Metals

(OP)
That's correct, KENAT.

Best to you,

Goober Dave

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RE: Sieve for Molten Precious Metals

(OP)
A rake-type thing might help. It made me thing of raking peas into a spoon when I was very young.

I'd one day like to automate the skimming / straining process once I get the manual method squared away. The primary problem I have is actually arthritis. My hands aren't shaky, but finger movement isn't as precise as it used to be.

Best to you,

Goober Dave

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RE: Sieve for Molten Precious Metals

Will it stick to the back of the spoon and solidify a thin layer containing the impurities? If not, use a colder spoon? Can of compressed air sprayed upside down is a quick and easy chiller.

RE: Sieve for Molten Precious Metals

(OP)
That's possible, 1gibson -- I'm open to any experiment. I have in the past, when the crud looked to be extra-fine, let the gold solidify and removed the stuff by buffing.

Best to you,

Goober Dave

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RE: Sieve for Molten Precious Metals

Isn't the most common method to remove dirt: melting the gold in some fluxing material that will absorb the dirt, then letting the flux solidify and breaking the gold out?

RE: Sieve for Molten Precious Metals

Dave,

Take a look at LEE casting equipment (used for molding bullets, check here: http://leeprecision.com/bullet-casting/electric-me...). They have an inexpensive bottom pour pot (I use it for 230 gr bullets for my .45) that keeps the dross away from your pour. Don't know if it will reach the temps you are looking for (I can get about 1100 F), and it may have more holdup than you want, but it may be an option.

Regards,

Matt

Quality, quantity, cost. Pick two.

RE: Sieve for Molten Precious Metals

(OP)
Compositepro: That sounds fun, I have never heard of it. I'll be doing research. The most common method for hobby-level folks is powerful acids to dissolve all the non-gold (aqua regia), neutalizing with urea, filtering, and then precipitating the gold out of the solution. Makes it very pure. But stinky and dangerous. I'd rather play with fire bigsmile.

MatthewL, thanks. 1100F is a bit too cool, I have to work at around 2000F. I'll check out the bullet mold things though, I can take a shape and use different materials if needed. leeprecision.com seems to be offline right now, though.

Best to you,

Goober Dave

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RE: Sieve for Molten Precious Metals

I have some silver clad copper I'd love to separate and sell as scrap, any good method for that?

RE: Sieve for Molten Precious Metals

(OP)
Gee, good question ivymike.

Getting the pure silver part is about the same as getting gold out of scraps or alloys. Dissolve the whole thing in acid, then precipitate out the silver and melt it into a lump. I don't know how to recover the copper from the remaining solution, though. Maybe electrolysis?

Best to you,

Goober Dave

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RE: Sieve for Molten Precious Metals

In the precious metals lab I used to work at we extracted precious metals by the "fire assay" method I believe Compositepro alluded to (melting with flux and lead oxide). Be aware that it winds up with the precious metals in an allow with lead, and the lead is subsequently volatilized. Not especially environmentally friendly. But more environmentally friendly than the mercury almalgam method that was used in the 1800s. There are other methods out there too. None are particularly safe for a household user. When you use acid, are you using aqua regia? If so, you do know that the fumes produced contain chlorine gas and nitrogen dioxide (brown fumes), right? Chlorine gas is bad enough, but according to the Merck Index, nitrogen dioxide is:

"One of the most insidious gases. Inflammation of lungs may cause only slight pain or pass unnoticed, but the resulting edema several days later may cause death. 100 ppm is dangerous for even a short exposure, and 200 ppm may be fatal."

Sorry, I don't have an answer to your question about a sieve.

RE: Sieve for Molten Precious Metals

(OP)
Thanks for the safety tip, though --

Yes, I do the aqua regia portion in a fume hood in my work shop, which is not connected to the house. I built the fume hood myself, so I naturally way oversized the fan. It vents well.

Best to you,

Goober Dave

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