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Author: Subject: Electrolytic DEplating of silver from copper?
jgourlay
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[*] posted on 12-10-2009 at 05:54
Electrolytic DEplating of silver from copper?


Gents: I have a whack of silver plated copper bar. I want to melt this bar down and toss in a teaspoon of silicon powder to make silicon bronze.

However, I'm concerned the silver plating may adversely affect the alloy properties either by causing difficulties in casting (narrowing pour temperature range, gas uptake, blah blah) or by throwing off the color (this is art, after all).

Could you find gentlemen walk me through deplating the copper? Preferably in a way that can recover the silver?
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Picric-A
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[*] posted on 12-10-2009 at 06:59


Best thing to do it buy an electric sander and use that.
There are no chemical means which can be used which will remove the Ag without removing the Cu.
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jgourlay
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[*] posted on 12-10-2009 at 07:06


Thanks!
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watson.fawkes
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[*] posted on 12-10-2009 at 07:30


Quote: Originally posted by Picric-A  
Best thing to do it buy an electric sander and use that.
There are no chemical means which can be used which will remove the Ag without removing the Cu.
Using mechanical means is a good suggestion. If you want to recover the silver, and I assume "whack" means enough to consider, then think about taking it off with a single-point machine scraper. You can do that over a bin where it will fall in, rather than fly all over the place with a sander. If you don't have scraper, it's a standard blacksmith-type job from an old file, requiring only a torch and a bench grinder.
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jgourlay
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[*] posted on 12-10-2009 at 08:09


Yup. I've got the next best thing. I can chuck the pieces into a lathe and "face them off".

I'm really surprised there isn't a chemical means, however. I thought if you could plate, you could also deplate.
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12AX7
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[*] posted on 12-10-2009 at 08:59


Well, you can certainly drop it in a bucket of nitric acid and take it out when it looks coppery.

The danger is that silver will plate out on the copper, but maybe it'll stay in solution as long as it's acidic enough?

In a KNO3/NaNO3 solution, you might electrostrip it. Since it's neutral, it won't matter as much that copper displaces silver -- the silver will precipitate as Ag2O (or does AgOH exist?).

There may be complexes which change the reduction potential sufficiently to prevent plating. There may also be precipitations possible, like adding a little HCl to the HNO3 bath in the hopes that AgCl precipitates (rather than passivates, mind you). Maybe you can precipitate Ag2O2 without it being reduced or bubbling off O2?

Tim




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watson.fawkes
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[*] posted on 12-10-2009 at 09:09


Quote: Originally posted by jgourlay  
Yup. I've got the next best thing. I can chuck the pieces into a lathe and "face them off".

I'm really surprised there isn't a chemical means, however. I thought if you could plate, you could also deplate.
Yeah, rig up an old bedsheet over the lathe ways and just catch all the turnings. This is more or less what they do on a metalsmith's bench, where cutting work is always done over a catcher, so as to be able to recover the scrap.

You could certainly deplate. But figuring out the right way may be more work than it's worth to you.
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merrlin
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[*] posted on 12-10-2009 at 09:30


Ag is considerably more soluble in liquid Pb (2.5% eutectic at 304C°) than copper. Also, silver oxide is unstable in air at about 200 C°. You might try using molten lead to dissolve the surface coating. If the copper surface oxidation leads to dewetting, it would simplify the removal of the lead/silver alloy. Others here are better qualified than I to comment on recovery of silver from a silver/lead alloy, although I would favor electrolysis.
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Mossydie
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[*] posted on 12-10-2009 at 10:01


Could it be done using Mercury as in the quicksilver method of purifying silver?
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[*] posted on 12-10-2009 at 10:06


I2/KI==>Ag layer will change to AgI BECAUSE OF VERY small KSP which can be solved in KI sol.
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[*] posted on 12-10-2009 at 10:10


or KCN and H2O2 till Ag layer is striped and color of surface changes.
Ag as cyanide complex can not be reduced by Cu ,i believe,u can check reaction E'
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Picric-A
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[*] posted on 12-10-2009 at 11:39


Quote: Originally posted by 12AX7  
Well, you can certainly drop it in a bucket of nitric acid and take it out when it looks coppery.

The danger is that silver will plate out on the copper, but maybe it'll stay in solution as long as it's acidic enough?


Tim


Thats the danger i considered and thought it would be easier to remove it by physical means.
It would also be problematic because Cu dissolves MUCH more readily that Ag so as soon as you get copper comming through that point will be attacked instead of Ag.
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jgourlay
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[*] posted on 12-10-2009 at 11:43


Picric: yep, this is rod. So at each end is a nice shiny copper face....
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[*] posted on 12-10-2009 at 11:58


What about using cyanide, which dissolves Ag and Au? This would be in contention if the quantity of Ag-plated Cu bar pieces or lengths that you have is simply too large for you to go about manually scraping or grinding the Ag off.

As for using aqueous HNO3, which dissolves both Ag and Cu, you could dip each of the rod-shaped pieces (using tongs coated with a material resistant to HNO3) in it until the Ag plating is no longer visible, being replaced by the color of Cu metal. The unplated ends of each rod could be masked from the HNO3 using a wax or high-viscosity grease. Each deplated Cu piece, after this treatment, can be taken out and washed with water. After this, the Ag(I) in solution with a smaller amount of Cu(II) could be purified by some means, involving either electrolysis at a controled voltage, or precipitation of one or the other (probably of Ag(I) as AgCl, which is very insoluble whereas CuCl2 is soluble).

[Edited on 12-10-09 by JohnWW]
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jgourlay
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[*] posted on 12-10-2009 at 12:09


Cynanide...she scares me!
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Picric-A
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[*] posted on 12-10-2009 at 13:36


Why on earth owuld you even think about using cyanide when you coulkd simply sand the Ag off?!?! :O
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jgourlay
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[*] posted on 12-10-2009 at 13:38


That's what I was thinking! :o
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12AX7
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[*] posted on 12-10-2009 at 20:30


Quote: Originally posted by merrlin  
Ag is considerably more soluble in liquid Pb (2.5% eutectic at 304C°) than copper.


Ah yes, they'll all dissolve quite excellently in solder, anything with tin, lead or zinc. New problem: now you have tinned copper instead of silvered copper. :P 10% HNO3 will take that off easily enough, but...

Another off-the-wall option: heat it to about 700C under reducing atmosphere for an hour and let the silver diffuse into the copper. You'll get a silver-rich layer, which may still be silvery or at least brassy, but it won't be silver plating anymore. This assumes you simply don't want the plating and don't care about recovery.

What kind of plating is it, anyway? A few grams of silver isn't worth anything. A few ounces is worth a few bucks. It would take a *lot* of heavy plating to be worthwhile.

Tim




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Picric-A
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[*] posted on 12-10-2009 at 23:08


He wants the pure Cu to make Cu/Si alloy so making a Cu/Ag allow would defeat the point of the layer.
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[*] posted on 29-7-2010 at 10:21


I apologize for dredging up an old topic.

Here's a common way to selectively strip silver plating from copper. This is used in the plating industry to remove faulty silver plating, so that the copper parts can be re-plated. It consists of 95%, by volume, concentrated sulfuric acid plus 5%, by volume, concentrated nitric acid - no water. The books recommend heating it to about 190F, but that can be dangerous and isn't usually necessary if the silver is thin - it just takes longer at room temp. If the stripping is going to take awhile, I would cover the container well to prevent the H2SO4 from absorbing moisture from the air. Any water in the solution will produce at least a little attack on the copper.

If there is nickel plating between the Ag and Cu, it can be removed, after stripping the Ag and rinsing the part well, with a nasty fumey solution of (all by volume) 49.6% H2SO4, 24.6% HNO3, 1.5% HCl, and 24.6% water, at 20-25C.

Another common method to remove nickel plating from copper is a solution of 50-60% (by vol) conc. H2SO4 + 3% (by vol) glycerin. Run as the anode at 20-25C and 3-6 volts using lead cathodes.

To remove the silver from the H2SO4/HNO3 solution, you can first dilute it considerably (say, down to 20%) and then precipitate the silver, as AgCl. The AgCl can be reduced to silver metal using various methods - one of the most common is the use of NaOH and a sugar (many use light Karo syrup). However, I doubt if the silver will be worth messing with. The silver is probably not over .0002" thick and, even at that thickness, there is only about 2 cents worth of silver per square inch of silver plated area, at an $18 Ag market.

The electroplating industry is a wealth of information for the chemical or electrolytic stripping of most any coating from most any base. They also sell many proprietary solutions for this purpose.




[Edited on 30-7-2010 by goldsilverpro]
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densest
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[*] posted on 30-7-2010 at 11:24


@goldsilverpro - are there similar solutions for removing gold from copper? I'm trying to avoid cyanide. I tried a thiosulfate based reagent from fragmentary information with occasional unreproduceable success and a lot of failures.
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[*] posted on 31-7-2010 at 05:29


Quote: Originally posted by densest  
@goldsilverpro - are there similar solutions for removing gold from copper? I'm trying to avoid cyanide. I tried a thiosulfate based reagent from fragmentary information with occasional unreproduceable success and a lot of failures.


Here are two non-cyanide methods that I have worked with:

Au can be selectively stripped from Cu using I2/KI. This solution is expensive but, it can be regenerated and re-used many times, as in this patent:
http://www.google.com/patents/about?id=oLZ7AAAAEBAJ&dq=39575...

A more common method is to make the part(s) the anode in a concentrated sulfuric acid solution containing about 3-5% water, by volume. I usually use mild steel cathodes. When the Au has all been stripped, the amperage will decrease to near-zero. All of the gold forms a metallic powder and settles to the bottom. After siphoning off the tops, the solution can be re-used many times. I once had a 50 gal solution that I used daily for about 2 years. It was in a mild steel tank and I used the tank as the cathode. Here's the original patent.
http://www.google.com/patents/about?id=L3FVAAAAEBAJ&dq=21858...

The H2SO4 stripping method has been covered, over and over, in great detail, on this forum.
http://www.goldrefiningforum.com/phpBB3/

There are a couple of videos describing it on this site:
http://www.goldrecovery.us/

All in all, though, cyanide/oxidizer methods are, by far, the best. They can be home-brew using such things as NaCN/H2O2 or NaCN/m-NBSS (ludigol). However, these rely on there being a nickel layer between the Au and Cu, to prevent attack on the Cu. Also, without this Ni layer, dissolved Au will tend to cement (immersion plate) back onto the exposed Cu. Luckily, most all modern Au plating has this Ni layer (or, sometimes, Pd) as a barrier to prevent the migration of Cu into the Au layer. After stripping, the Au is simply precipitated with 325 mesh zinc dust.

There are expensive proprietary CN mixtures (e.g., Techni-strip AU) which don't attack Cu (Ni layer, or not), but precipitating the Au is much more difficult. A precipitating method that works fairly well, using sodium dithionite (sodium hydrosulfite) is described on this thread, by Till_Eulenspiegel.
http://www.sciencemadness.org/talk/viewthread.php?tid=5700&p...

Gold plated on a non-copper substrate, such as Kovar, can be made the anode and easily stripped in a NaCN (say, 40g/l), NaOH (about 15g/l) room temp solution, at about 4-6 volts. The gold is subsequently plated onto a passivated stainless steel cathode.

Many hobbyist refiners dissolve the copper base with 50/50 nitric or conc. HCl/3% H2O2 and then treat the Au residues with aqua regia. However, due to the large amounts of acid needed and waste generated, these methods are generally inferior to any method that will selectively strip the gold without attacking the Cu.

Although it may be futile, I wouldn't yet give up on the thiosulfate. Although I haven't had the time to work with it, I've always been attracted to its possibilities. And, the chemicals needed are cheap, safe, and readily available, which would separate it from other known stripping methods. A thiosulfate stripper that works as well as cyanide could revolutionize the industry.


[Edited on 31-7-2010 by goldsilverpro]
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[*] posted on 31-7-2010 at 11:19


The I2/KI method is interesting. Of course, buying iodine is impossible in the US without DEA paperwork :mad: One patent referring to the one you mentioned says that gold reduced from an iodide solution tends to be in a colloidal form. Dunno if the reducing agent and conditions affect that a lot. Still, colloidal gold is pretty easy to capture.

One thing that I didn't know about the thiosulfate process is the need for a high oxygen partial pressure - various patents mention using 15-100 psi oxygen. Others use BrO3 or other non-chlorine oxidants. I didn't see H2O2 listed anywhere - I think it destroys the S2O3 on contact.

A plating patent (got it somewhere, I'll post the # if anyone is interested) uses a benzene sulfinic acid to stabilize the thiosulfate in the presence of oxidizers. That works - I tried it. The failures I had possibly related to insufficient oxidizer in solution. If the box of electronic scrap surfaces again I'll give a couple of tries. I've got an oilless oxygen compressor which would give me 100 psi from my oxygen extractor output.
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